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- I’m Not Dead Yet as a Path to Calorie Restriction Mimetics
- Recent Research into the Details of Cellular Senescence
- Recent Examples of the Road to Pharmacological Enhancement of Muscle Regeneration
- Revisiting Whole Body Induced Cell Turnover as a Therapeutic Strategy
- Common Human Growth Hormone Receptor Variant Associated with Greater Longevity
- Latest Headlines from Fight Aging!
- Regeneration of Structured, Full-Thickness Skin, Including Hair Follicles
- Ghrelin Knockout Mice Eat Less But Fail to Live Longer as a Result
- Another Potential Approach to the Creation of Tissue Engineered Cartilage
- An Update on the Effects on PAPP-A Knockout on Longevity in Mice
- Measurable Amyloid Buildup Occurs Significantly Before Alzheimer’s Disease
- A Profile of Valter Longo’s Work on Fasting and Calorie Restriction
- Weak Evidence for Amino Acid Processing Dysfunction Theories of Sarcopenia
- Using Photosynthetic Microbes to Oxygenate Ischemic Tissue
- Mutant Dietary Bacteria as a Way to Explore Mechanisms of Aging in Nematodes
- Yeast Life Extension via Lithocholic Acid Provides Support for the Membrane Pacemaker Hypothesis of Aging
I’m Not Dead Yet as a Path to Calorie Restriction Mimetics
INDY, I’m Not Dead Yet, is one of the earliest of longevity-associated genes to be documented. Researchers uncovered its effects in flies at the turn of the century, something like half an eternity ago given the pace of modern biotechnology. Despite the rapid pace of progress in the field as a whole, the INDY gene is also an example of the extremely slow and incremental progression that is characteristic of any one specific line of research in molecular biochemistry. The open access paper I’ll point out today is a review of what is presently known of INDY in flies and mammals, information gathered over the what is now going on for two decades of work. The focus is on the ways in which the beneficial effects of reduced levels of INDY appear quite similar to those of calorie restriction – though clearly it is a complicated overlap, because trying both reduced levels of INDY and calorie restriction either has no effect or shortens life span. Regardless, anything that looks a lot like calorie restriction tends to be treated as a potential road to the development of calorie restriction mimetic drugs in this day and age.
Why is progress slow when you follow any one particular thread in aging research? Well, for one funding for aging research is small in comparison to other fields of medical research. Secondly, cellular biochemistry is enormously complex. It does in fact tend to take a few years even now for any one group to make a single connection in the complex web that is cellular metabolism. Just moving the focus of research into INDY from flies to mice took a long time, and it is still clearly in its early stages when considered in the context of the bigger picture of identifying targets, developing drugs, and producing clinical treatments. Mapping and tinkering with metabolism is a good thing in the long term, as gaining a full understanding of our cells is – and should be – the goal of the life sciences, but it certainly isn’t the fast road to meaningful interventions to slow or reverse the aging process. For that we need engineering approaches like SENS, turning what we already know of aging into repair treatments capable in principle of rejuvenation.
INDY – A New Link to Metabolic Regulation in Animals and Humans
The Indy (I’m Not Dead Yet) gene encodes the fly homolog of the mammalian SLC13A5 transporter of the tricarboxylic acid (TCA) cycle intermediates. Reduced expression of the Indy gene in flies and worms extends longevity in all but one study. INDY is expressed on the plasma membrane of metabolically active tissues. In flies INDY is predominantly expressed in the midgut, fat body, and oenocytes (fly liver). In humans, Indy mRNA is mainly expressed in the liver, less in the brain and testis, while small levels of Indy mRNA expression were found in the kidneys, thymus, ovaries, adipose tissue, stomach, and colon.
Decreased expression of Indy in worms, flies, mice, and rats alters metabolism in a manner similar to calorie restriction (CR). This is supported by similar phenotypes found in CR wild type flies and in Indy flies that were kept on a high calorie diet. These Indy flies have lower lipid levels, increased mitochondrial biogenesis, increased spontaneous physical activity and a reduction in components of the insulin-signaling pathway activity. Furthermore, Indy heterozygous flies laid more eggs during their life compared to controls. However, under CR condition, Indy heterozygous flies have reduced fecundity due to lower energy resource caused by the effect of reduced Indy on metabolism. Consistently, CR does not further extend longevity of long-lived Indy heterozygous flies and shortens longevity of Indy homozygous flies.
Preservation of intestinal stem cell (ISC) homeostasis has a key role in maintaining normal midgut function and contributes to extended health and longevity in flies. Changes in mitochondrial biogenesis found in the midgut of Indy flies, combined with increased antioxidant activity and reduced production of reactive oxygen species preserve ISC homeostasis and intestinal integrity in Indy flies. These changes maintain midgut function and mediate extended health and longevity of Indy flies.
Reduced activity of the Indy homologs in other organisms is associated with similar metabolic effects that mimic CR. siRNA mediated knockdown of Indy/CeNac2, the worm Indy homolog, results in worms that are smaller, have reduced lipid levels, and have extended longevity. Mammalian Indy (mIndy)-/- knockout mice are protected from the negative effects of aging or a high-fat diet on metabolism, which include hepatic fat accumulation, obesity, and insulin insensitivity. These mice have increased energy expenditure, reduced hepatic lipogenesis, increased mitochondrial biogenesis, and enhanced hepatic fatty acid (FA) oxidation. Whole-genome microarray studies comparing mIndy-/- and mIndy-/+ revealed that transcriptional changes found in the liver of mIndy-/- mice are 80% identical to changes found in the liver of CR mice. All of these findings confer that INDY reduction creates a state similar to CR.
In summary, reduction of Indy gene activity in flies and worms extends their health and longevity. Genetically reduced INDY expression has beneficial effects on metabolism and prevents diet-induced obesity in flies and mice, suggesting INDY as a target in the treatment of metabolic disorders in humans. By contrast, high levels of INDY are associated with negative effects on metabolism and health. Recently, increased hepatic levels of mINDY were linked to insulin resistance in obese humans. These findings illustrate both the relevance of the mIndy gene to human health and a highly conserved role for INDY in the metabolism of a broad range of species. Thus, mIndy has emerged as a novel target for the treatment of age- and diet-associated metabolic syndrome. The further development of mIndy inhibitors may additionally provide effective interventions targeting the debilitating health effects that are often associated with aging and will thereby allow a healthier life.
Recent Research into the Details of Cellular Senescence
Today’s papers are representative of present investigations that aim primarily to expand our knowledge of the details of cellular senescence. This is as opposed to efforts to immediately produce treatments that can address the impact of senescent cells on health. Senescent cells accumulate with age, and their presence is one of the root causes of degenerative aging. The most important work on cellular senescence at the moment is that aimed at selective destruction of these cells. The vast majority of cells that become senescent in our bodies, countless numbers day in and day out, are in fact already efficiently destroyed, either through programmed cell death or via the actions of the immune system. We’d all be much better off if the lingering remainder, the tiny fraction that evade this fate, were also removed.
That said, a sizable fraction of today’s research into cellular senescence aims to better understand the processes involved, or to intervene so as to reduce the harms caused by these cells, or reduce the number of cells that become senescent, or even attempt to reverse senescence, rather than destroy these cells after the fact. I have to think that this isn’t anywhere near as cost-effective a path forward when it comes to the development of practical therapies, in particular because destroying senescent cells effectively deals with the harms we don’t understand in addition to those we do – and mapping cellular biochemistry is a slow and expensive process. There are long years ahead for those who want to fully understand how senescent cells cause harm at the detail level, and destruction solves the problem now. It is nonetheless the case that more information is better than less information in the long term, and expanded knowledge may well lead to new targets for the development of selective cell destruction therapies – a point illustrated in the first of the two papers below.
How is it that a comparatively small number of senescent cells are so harmful? In old age, senescent cells are perhaps a few percent by number of the cells present in any given tissue, yet their presence strongly shapes the functional decline of that tissue. The answer is that they produce a potent mix of signal molecules that cause chronic inflammation, degrade the normal regenerative and tissue maintenance activities of stem cells and immune cells, induce fibrosis and other disruptions of the extracellular matrix structure important for normal tissue function, and reduce tissue elasticity, among other issues. There is evidence for many other contributions to disease progression, from calcification of arteries to buildup of fatty deposits in blood vessels to failing lung function. Ridding our bodies of these cells would produce rapid benefits in later life, a point already well illustrated in a number of animal studies.
p21 maintains senescent cell viability under persistent DNA damage response by restraining JNK and caspase signaling
Senescent cells present elevated activity of senescence-associated beta-galactosidase (SA-β-gal) and a persistent DNA damage response that distinguish them from other non-proliferating cell populations. In addition, senescent cells produce a variety of characteristic secreted factors, collectively termed the senescence-associated secretory phenotype (SASP), which reinforces senescence arrest in an autocrine manner and mediates immune surveillance of the senescent cells. With aging, however, senescent cells accumulate in the organism promoting local inflammation that drives tissue aging, tissue destruction, and potentially also tumorigenesis and metastasis in a cell non-autonomous manner. Recent studies have shown that elimination of senescent cells promotes stem cell proliferation and prolongs lifespan. Therefore, mechanisms that regulate the viability of senescent cells in tissues evidently play an important role in tissue homeostasis.
The senescence program is driven by a complex interplay of signaling pathways. To promote and support cell cycle arrest, p16INK4A (CDKN2A), accompanied by the p53 (TP53) target p21 (CDKN1A), inhibits cyclin-dependent kinases (CDKs), thereby preventing phosphorylation of the retinoblastoma protein (pRb) and thus in turn suppressing the expression of proliferation-associated genes. In addition, the nuclear factor kappa B protein complex (NF-κB) acts as a master regulator of SASP and therefore affects both the microenvironment of senescent cells and their immune surveillance.
Whereas mechanisms driving senescence have been extensively studied, the mechanisms allowing their prolonged retention in tissues are much less well characterized. Recently, the anti-apoptotic BCL-2 family members BCL-W, BCL-XL, and BCL-2 were shown to facilitate the resistance of senescent cells to apoptosis. However, the contribution of pathways that regulate the formation of senescent cells to the resistance of these cells to cell death has yet to be determined. On one hand, senescent cells cannot accumulate p53 protein to the levels required for apoptosis. On the other hand, the p53 target p21, via its ability to promote cell cycle inhibition, can protect some cells from apoptosis.
This effect might be governed by both p53-dependent and -independent upregulation of the pro-apoptotic protein BAX, or by activation of members of the tumor necrosis factor (TNF)-α family of death receptors, or by effects on DNA repair. We therefore set out to determine how p21 regulates the viability of senescent cells after DNA damage. We found that following p21 knockdown, senescent cells sustain multiple DNA lesions, leading to further activation of DNA damage response and NF-κB pathways. This activation was regulated by both TNF-α secretion and JNK activation, and it mediated senescent cell death in a caspase-dependent and JNK-dependent manner. Moreover, p21 knockout in mice led to the elimination of senescent cells from fibrotic scars in the liver and alleviated liver fibrosis. These results uncovered new mechanisms that control the fate of senescent cells.
MicroRNA Regulation of Oxidative Stress-Induced Cellular Senescence
In the past years, microRNAs (miRNAs) turned out to be important players in controlling aging and cellular senescence by regulating gene expression. Of note, a global decrease in miRNAs abundance was found in aging of different model organisms, suggesting aging-associated alteration of miRNAs biogenesis. In fact, aging-induced dysregulation of miRNAs biogenesis proteins is reported to promote aging and aging-associated pathologies. Among them, ribonuclease Dicer is most studied and a reduced level was reported in tissues of aged mice and rats, as well as in senescent cells.
Although the mechanisms of miRNA biogenesis have been intensively investigated in recent years, processes regulating miRNA stability remain to be explored. miRNAs have been generally considered as stable molecules with half-life of days long, while some miRNAs are actually short lived with half-life of no more than few hours. It is now clear that the absolute levels of mature miRNAs are also controlled by factors that directly affect stability. Whether miRNA stability changes during cellular senescence is so far, to our knowledge, not known. Further researches on miRNA stability and degradation mechanisms in cellular senescence and aging are needed to identify its impact on age-associated process and may provide potential new targets to interfere the process.
A number of miRNAs have been found to be differentially expressed in senescent cells or aged tissues and play a role in cellular senescence. Recently, miRNAs have been found extracellularly and function in intercellular communication upon taken up by recipient cells. The fact that circulating miRNAs are packed in the form of microvesicles protects them from degradation. The stability of miRNAs in the circulation and in body fluids, their tissue and disease specificity, and the easy and reliable quantification methods make them feasible as potential biomarkers. Several miRNAs detected in blood samples have been found in several studies to be associated with human aging. Further efforts are needed to identify consensus miRNA biomarkers not only as indicators of aging process and aging-associated disease but also as longevity predictors and eventually therapeutic approaches to modulate the aging process.
Although a relatively new field of research, miRNAs add substantial complexity to the regulation of aging processes and cellular senescence. On one side, a single miRNA can regulate the expression of hundreds of genes from different signaling pathways, which means the whole signaling network could be reset by modulating the expression of one single miRNA. In contrast, miRNAs as players of adaptive stress response could act both as promoters and inhibitors of senescence, depending on the type of stress, the cell or tissue where they are located, and the molecular context in which they play a role. Further efforts are needed to explore the modulatory role of miRNAs in cellular senescence.
Recent Examples of the Road to Pharmacological Enhancement of Muscle Regeneration
Numerous research teams are interested in finding ways to enhance muscle regeneration, and below find the publicity materials for a couple of different lines of research along these lines – researchers in search of specific mechanisms that might be amenable to change, and thus the potential foundation for a drug discovery program and therapies in the clinic. Enhanced muscle regeneration encompasses more than just faster and more comprehensive recovery from injury, as much the same set of mechanisms are also involved in the normal maintenance and growth of muscles. As I’m sure the audience here is well aware, muscle tissue weakens and diminishes with age, a condition known as sarcopenia. Researchers hope that enhancements to the processes of muscle repair will be able to at least partially compensate for the losses of age, or delay those losses somewhat, even though they fail to directly address the underlying reasons for this form of age-related decline.
There is considerable debate over the causes of sarcopenia, and as for most aspects of aging, it is not yet possible to draw a consensus line of cause and effect from the sort of root cause forms of cellular damage outlined in the SENS rejuvenation research proposals all the way to well known age-related diseases, passing through the better explored metabolic changes observed in aging along the way. There is a great deal of data for sarcopenia, however: it is a puzzle in which at least some of the pieces are already joined together, even if it isn’t quite settled as how these little islands of knowledge relate to one another in the bigger picture.
Changes in processing of amino acids appear important, as does the decline in diet and exercise in later life. Cellular senescence is implicated, as is becoming the case in many parts of the field now that more attention is being given to removal of senescent cells. Loss of mitochondrial function is also thought important, and there are related views involving loss of capillaries and nutrient supply in older tissue. Of late, there has been a fairly compelling argument to point to loss of stem cell activity as the primary lynchpin issue, though everything else just mentioned may well contribute to that loss. Everything is connected to everything else in cellular metabolism, and there are many angles from which the wise men can approach this elephant.
Inflammatory molecule essential to muscle regeneration in mice
Muscle stem cells usually nestle quietly along the muscle fibers. They spring into action when a muscle is damaged by trauma or overuse, dividing rapidly to generate enough muscle cells to repair the injury. But it’s not entirely clear what signals present in inflammation activate the stem cells. Prostaglandin E2, or PGE2, is a metabolite produced by immune cells that infiltrate the muscle fiber as well by the muscle tissue itself in response to injury. Anti-inflammatory treatments have been shown to adversely affect muscle recovery, but because they affect many different pathways, it’s been tough to identify who the real players are in muscle regeneration.
Researchers discovered a role for PGE2 in muscle repair by noting that its receptor was expressed at higher levels on stem cells shortly after injury. They found that muscle stem cells that had undergone injury displayed an increase in the expression of a gene encoding for a receptor called EP4, which binds to PGE2. Furthermore, they showed that the levels of PGE2 in the muscle tissue increased dramatically within a three-day period after injury, indicating it is a transient, naturally occurring immune modulator. “This transient pulse of PGE2 is a natural response to injury. When we tested the effect of a one-day exposure to PGE2 on muscle stem cells growing in culture, we saw a profound effect on the proliferation of the cells. One week after a single one-day exposure, the number of cells had increased sixfold compared with controls.”
After seeing what happened in laboratory-grown cells, researchers tested the effect of a single injection of PGE2 into the legs of the mice after injury. “When we gave mice a single shot of PGE2 directly to the muscle, it robustly affected muscle regeneration and even increased strength. Conversely, if we inhibited the ability of the muscle stem cells to respond to naturally produced PGE2 by blocking the expression of EP4 or by giving them a single dose of a nonsteroidal anti-inflammatory drug to suppress PGE2 production, the acquisition of strength was impeded.” The researchers next plan to test the effect of PGE2 on human muscle stem cells in the laboratory, and to study whether and how aging affects the stem cells’ response. Because PGE2 is approved by the Food and Drug Administration for use in the induction of labor, a path to the clinic could be relatively speedy.
Researchers Find Key to Muscle Regeneration
Earlier this year, researchers published findings showing that a nuclear receptor called REV-ERB is involved in lowering LDL cholesterol. They previously studied REV-ERB’s role in regulating mammals’ internal clocks. Now the researchers are uncovering REV-ERB’s role in muscle regeneration. Skeletal muscle comprises 40 to 50 percent of our total body mass and is essential for postural support, locomotion and breathing. With a high capacity for regeneration, skeletal muscle normally maintains muscle mass and function in response to minor injuries and normal wear and tear without much trouble. When injuries are severe – with more than 20 percent loss of muscle mass – normal muscle regeneration often cannot keep pace with the regenerative demands. In this scenario, the loss of skeletal muscle mass can trigger widespread fibrosis and loss of muscle function.
“Identifying new means of accelerating muscle regeneration has proved a daunting challenge. Therefore understanding the underlying mechanisms that regulate muscle cell regeneration and coordinate regenerative repair could provide future therapeutic options for stymieing the loss of muscle function in the traumatically injured.” A simplified version of muscle cells’ life-cycle looks like this: muscle stem cells produce myoblasts that will either reproduce (proliferate) or form muscle tissue (differentiate). Successful regeneration of skeletal muscle after traumatic injury depends on the replenishment of muscle fibers through elevated myoblast proliferation and differentiation.
The research team identified a mechanism through which REV-ERB may regulate gene expression pre and post muscle differentiation. “We demonstrate that REV-ERB can stimulate muscle regeneration upon acute muscle injury in an animal model. Our findings reveal that REV-ERB may be a potent therapeutic target for the treatment of a myriad of muscular disorders.”
Revisiting Whole Body Induced Cell Turnover as a Therapeutic Strategy
Last year some of the researchers associated with the Biogerontology Research Foundation proposed a class of therapy they call whole body induced cell turnover. I noticed a new paper and publicity materials on this topic today. In essence the goal is to augment the normal processes of cell turnover with therapies that remove and replace more cells than would normally be the case, thus clearing out the damage in those cells along the way. Since aging is caused by cell and tissue damage, in the ideal case this approach should act as a form of rejuvenation therapy. Obviously there are some limits here, such as areas of the brain where cells are storing the state of the mind, but in principle all other tissues are amenable to cell replacement. Beyond that, however, there is also the question of damage outside cells, such as waste compounds in tissue fluids and within the extracellular matrix structures that support cells. Further, consider signals propagated by damaged cells in one part of a tissue that affect normal cells elsewhere, such as the inflammation spurred by senescent cell signaling.
That said, it is nonetheless clear that the logical long-term direction for tissue engineering and regenerative medicine strategies must be to move towards more incremental, in-situ, small-scale replacement of damaged parts. Practical tissue engineering in the years immediately ahead will certainly start with tissue sections and organs grown in bioreactors and then transplanted into patients. Surgery is expensive and traumatic, however, and especially so in the old. In order to avoid those costs, all treatments will ultimately involve manipulation and repair of cell populations in the body, with the complexity baked into the therapeutics, and little human supervision of their progression required. This might be accomplished by delivery of signals, delivery of cells, or various other more sophisticated therapeutics, but there will be no surgery and no construction of tissue outside the body.
It is easy enough to point out the underpinnings of this trend, and theorize at the high level as to how to make it a reality, but the implementation details are of great importance – they are the whole of the story, in fact. Replacing cells in a way that is safe, and that also removes cellular damage rather than propagating it, isn’t a straightforward task with an obvious solution, for all that there are plenty of starting points in today’s biotechnology industry and research community. Strategies will likely vary considerably from tissue to tissue. It is likely that much more of the biochemistry of natural regenerative processes must be mapped and understood, so as to avoid interfering in counterproductive ways. Targeted cell destruction technologies must evolve into more sophisticated and discriminating forms. And so on. It is a big task and an expansive vision for the future of medicine.
Induced Cell Turnover: A proposed modality for in situ tissue regeneration and repair
Researchers originally proposed Induced Cell Turnover (ICT) in 2016. The proposed therapeutic modality would aim to coordinate the targeted ablation of endogenous cells with the administration of minimally-differentiated, hPSC-derived cells in a gradual and multi-phasic manner so as to extrinsically mediate the turnover and replacement of whole tissues and organs with stem-cell derived cells. In a new paper the authors refine the methodological underpinnings of the approach, take a closer look at potential complications and strategies for their deterrence, and analyze ICT in the context of regenerative medicine as an intervention for a broader range of conditions.
“One of the major hurdles limiting traditional cell therapies is low levels of engraftment and retention, which is caused in part by cells only being able to engraft at locations of existing cell loss, and by the fact that many of those vacancies have already become occupied by extracellular matrix (ECM) and fibroblasts (i.e. scar tissue) by the time the cells are administered, long after the actual occurrence of cell loss. The crux underlying ICT is to coordinate endogenous cell ablation (i.e. induced apoptosis) with replacement cell administration so as to manually vacate niches for new cells to engraft, coordinating these two events in space and time so as to minimize the ability for sites of cell loss to become occupied by ECM and fibroblasts. This would be done in a gradual manner so as to avoid acute tissue failure resulting from the transient absence of too many cells at any one time. While the notion of endogenous cell clearance prior to replacement cell administration has become routine for bone marrow transplants, it isn’t really on the horizon of researchers and clinicians working with solid tissues, and this is something we’d like to change.”
Cell-type and tissue-specific rates of induced turnover could be achieved using cell-type specific pro-apoptotic small molecule cocktails, peptide mimetics, and/or AAV-delivered suicide genes driven by cell-type specific promoters. Because these sites of ablation would still be “fresh” when replacement cells are administered, the presumption is that the patterns of ablation will make administered cells more likely to engraft where they should, in freshly vacated niches where the signals promoting cell migration and engraftment are still active. By varying the dose of cell-type targeted ablative agents, cell type and tissue-specific rates of induced turnover could be achieved, allowing for the rate and spatial distribution of turnover to be tuned to the size of the tissue in order to avoid ablating too many cells at once and inadvertently inducing acute tissue failure.
“ICT could theoretically enable the controlled turnover and rejuvenation of aged tissues. The technique is particularly applicable to tissues that are not amenable to growth ex vivo and implantation (as with solid organs) — such as the vascular, lymphatic, and nervous systems. The method relies upon targeted ablation of old, damaged and/or senescent cells, coupled with a titrated replacement with patient-derived semi-differentiated stem and progenitor cells. By gradually replacing the old cells with new cells, entire tissues can be replaced in situ. The body naturally turns over tissues, but not all tissues and perhaps not optimally.”
Induced Cell Turnover: A novel therapeutic modality for in situ tissue regeneration
Induced Cell Turnover (ICT) is a theoretical intervention in which the targeted ablation of damaged, diseased and/or nonfunctional cells is coupled with replacement by partially differentiated induced pluripotent stem cells in a gradual and multi-phasic manner. Tissue-specific ablation can be achieved using pro-apoptotic small molecule cocktails, peptide mimetics, and/or tissue-tropic AAV-delivered suicide genes driven by cell-type specific promoters. Replenishment with new cells can be mediated by systemic administration of cells engineered for homing, robustness, and even enhanced function and disease resistance. Otherwise, the controlled release of cells can be achieved using implanted biodegradable scaffolds, hydrogels, and polymer matrices. In theory, ICT would enable in situ tissue regeneration without the need for surgical transplantation of organs produced ex vivo, and addresses non-transplantable tissues (such as the vasculature, lymph nodes, skin and nervous system). We have outlined several complimentary strategies for overcoming barriers to ICT in an effort to stimulate further research at this promising interface of cell therapy, tissue engineering, and regenerative medicine.
Common Human Growth Hormone Receptor Variant Associated with Greater Longevity
The longest lived laboratory mice, more than a decade after the creation of the first lineage, are still those with impaired growth hormone signaling. That record has yet to be overtaken, and I suspect that it may well stand unbroken until the development of rejuvenation therapies based on damage repair is further advanced, and senescent cell clearance is joined by other types of therapy, their effects adding together. Thus growth hormone and growth hormone receptor genes in humans are one of the places to look for variants and mutations that might improve our understanding of how metabolism influences aging, and as a bonus for those interested in pharmacological or genetic adjustment of metabolism, may lead to ways to modestly slow aging.
With this in mind, attention has fallen on a rare human lineage with a growth hormone receptor mutation that produces dwarfism, the Laron syndrome population. Unfortunately it isn’t yet possible to say whether or not these individuals have any advantage over the rest of us when it comes to longevity. They do appear to be resistant to cancer and type 2 diabetes, though the evidence in support of that conclusion isn’t completely iron-clad at this point. People with Laron syndrome and, separately, people who practice calorie restriction for its health benefits can be compared with the same processes operating in mice. When doing so, we can see that human longevity is not enormously changed, while mouse longevity does increase by up to 70% for growth hormone disruption, and up to 40% for calorie restriction. That degree of gain is certainly not the case in humans. Evolution has delivered a much more plastic life span to short-lived mammals, responsive to environmental circumstances, and with a biochemistry capable of these large changes in the pace of aging.
Putting growth hormone signaling to one side for the moment, considerable effort has been devoted over the years to the broader search for human genetic variants that are associated with longevity. The consensus on genes and longevity is that individual variations in your genome have little influence over aging until later life, and from that point forward, the older and more damaged you get the more that genetic variation matters. Nonetheless, the search has found very few compelling associations. There is solid evidence for variants in APOE and FOXO3A, and less solid evidence for a few other variants such as in TXNRD1, but these are not large effects. Beyond this there are scores of other associations that are never replicated, showing up in only one study or one population, and again with small effects – by which I mean maybe you have a 1.5% chance of living to 100 instead of a 1% chance if you held one of these variants. The big picture is of hundreds or thousands of individually tiny effects; which of these genes and variants are more or less relevant varies widely between populations and individuals, and is very dependent on environmental factors.
That said, there are genetic variants with sizable effects on resistance to specific age-related disease, such as those in ASGR1 or ANGPTL4, both of which reduce blood cholesterol and cardiovascular disease risk. Nothing is published on their effects on longevity at this time, but give it time. Should we believe that there are human genetic variants that meaningfully increase life expectancy in the carriers based on what we’ve seen to date? The association studies with their poor catch of results suggest no. The existence of the variants mentioned above suggest maybe, but equally it is the case that aging has many facets. Being resistant – or even immune – to one thin facet, such as cardiovascular disease, is thought unlikely to do a great deal to overall longevity. It just means that something else gets you in the end, perhaps a couple of years later.
Returning to growth hormone metabolism, I see that researchers are claiming that a common human gene variant of growth hormone receptor, per their statistics, may result in a ten year difference in life expectancy. This is replicated in multiple study populations, but the effect appears only in men. It is interesting, but there is every reason to be cautious with this sort of very statistical genetic association study. I’d say read the paper and put it aside until someone replicates the result. Would a ten year gain from a growth hormone signaling genetic variant be surprising if this turns out in fact be the case? Maybe not, given what we know about the relative sizes of effects in mice and humans. Ten years is about on the outside end of variations that can plausibly exist in some numbers and yet blend in with broader population data, and the effects in mice are considerably larger on a relative basis.
The GH receptor exon 3 deletion is a marker of male-specific exceptional longevity associated with increased GH sensitivity and taller stature
Growth hormone (GH) and insulin-like growth factor (IGF) play a central role in development, differentiation, growth, and metabolism among divergent taxa. Dwarf individuals appear to live longer among many species, suggesting a role for the GH/IGF-1 axis in modulating aging and life span. A considerable body of in vitro experimental evidence also suggests an important role for the IGF axis in human longevity and aging-related processes in a tissue-specific manner. Furthermore, several studies in selected human populations lend support on the role of this axis in health and life span.
For instance, we have previously identified a cluster of functional mutations in the IGF-1 receptor in centenarians. We showed that Laron dwarfs, who are naturally short, have decreased prevalence of diabetes, cancer, and stroke, suggesting increased health span although life span in this small sample size cannot be determined accurately. Also, we previously established that centenarians with lower levels of IGF-1 had significantly longer survival. Clearly, individuals with severe GH deficiency have reduced life expectancy, suggesting that some GH is necessary for survival. On the other hand, interventional GH therapy in humans is commonly used to reverse age-related morbidities; hence, the kind of deficiency that will be most beneficial for health span and longevity needs to be further established.
GH production is decreased with age; however, it is never completely diminished. That said, there is accumulating evidence that GH may play a crucial role in modulating aging. Surprisingly, GH deficiency or diminished secretion has been linked to longevity phenotypes both in mice models and in humans with familial longevity. The GH receptor (GHR) gene has nine coding exons and consists of two common isoforms: (i) full-length GHR-flGHR and (ii) a shorter form with a deletion of exon 3, d3-GHR. The allele frequencies of these isoforms among human populations range from 68-90% for flGHR and 10-32% for d3-GHR.
Investigations of the effects of GHR isoforms on human health have provided mixed results. In two Genome Wide Association Studies (GWAS) based on single-nucleotide polymorphisms (SNPs), the GHR locus showed association with final height. However, to our knowledge, the association of d3-GHR with final height has not been examined, possibly because individuals with d3-GHR are expected to maintain normal GH action despite lower GH production. It is reasonable to hypothesize that increased GH sensitivity can also alter IGF-1 secretion and therefore regulate longevity. Given the potential role of the GH/IGF axis in longevity, we hypothesize that low IGF-1 levels will assure longevity of the d3-GHR carriers. To address this hypothesis, we genotyped the d3-GHR locus in four human cohorts with long-lived participants, and we tested its association with longevity-related phenotypes and stature with a relatively common GHR variation.
In Ashkenazi males, but not in females, a marked difference in allele frequency for the exon 3 deletion polymorphism (d3-GHR) was found between centenarian and control, as well as offspring and control groups. Whereas the male control group carried only 4% homozygote deletions, male offspring of centenarians and male centenarians carried 11 and 12%, respectively. We further validated these results in three independent cohorts – the Old Order Amish (OOA), Cardiovascular Health Study (CHS), and the French Long-Lived Study (FLLS). These results demonstrate a consistent relationship between homozygosity for the d3-GHR deletion allele and longevity among the cohorts studied. However, this observation was limited only to males; the frequency of d3-GHR deletion homozygosity among females did not differ with age in any of the cohorts studied. On average, d3/d3 homozygotes were 1 inch taller than the wild-type (WT) allele carriers and also showed lower serum IGF-1 levels. Multivariate regression analysis indicated that the presence of d3/d3 genotype adds approximately 10 years to life span.
It appears that deletion of the GHR gene exon 3 might have originated from complex genomic events taking place after the emergence of Old World monkeys, followed by homologous recombination between two retro-elements in Homo sapiens. Thereafter, it spread throughout the human clades to be present now in approximately 25% of Caucasian chromosomes. In centenarians, most IGF-1 regulation seems to respond to caloric and protein nutritional signals, not from GH. IGF-1 is not lower in carriers of d3-GHR during childhood, adolescence, and adulthood despite several reports showing that the GHR genotype may influence circulating IGF-1 under basal conditions. People with d3-GHR or fl-GHR alleles produce comparable amounts of circulating IGF-1. That said, we suspect people with d3-GHR alleles to have a decreased GH secretion.
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Regeneration of Structured, Full-Thickness Skin, Including Hair Follicles
The tissue engineering company PolarityTE is claiming regrowth of correctly structured skin in pigs, incorporating hair follicles and various glands. The press release and company website are light on some of the more interesting details, such as just how close to natural skin the end result is in this case, but we shouldn’t have to wait too long to find out more. Clinical trials are starting this year.
PolarityTE, Inc. today announced pre-clinical results demonstrating that the Company’s lead product, SkinTE, regenerated full-thickness, organized skin and hair follicles in third degree burn wounds. The findings represent the first known successful regeneration of skin and hair in full-thickness swine wound models, the standard animal model for human skin. The Company expects to initiate a human clinical trial evaluating the autologous homologous SkinTE construct in the third quarter of 2017. In pre-clinical models of full-thickness burns and wounds, SkinTE demonstrated scar-less healing, hair follicle growth, immediate complete wound coverage, and the progressive regeneration of all skin layers including epidermis, dermis and hypodermal layers.
“These findings using SkinTE demonstrate an entirely new and pragmatic system whereby Polarity has used autologous tissue to regenerate full-thickness skin, hair follicles and appendages for the treatment of burns and wounds. Our revolutionary approach to a new form of regenerative healing offers hope to both burn and wound patients, as well as medical providers who have not seen a significant advance in skin regeneration since the 1980s.” Swine models of burns and wounds are known to be predictive of results found in humans due to the unique similarities between swine and human skin. Of note, it is believed that swine skin may be more difficult to regenerate with all layers and appendages (hair and glands), as was done in the studies by PolarityTE, suggesting that the results of these studies may predict similar efficacy in human patients when clinical trials begin later this year.
Ghrelin Knockout Mice Eat Less But Fail to Live Longer as a Result
One of the many interesting but unresolved questions relating to calorie restriction and its beneficial effects on health and longevity is the role played by ghrelin. This hormone regulates appetite, but also has a range of other effects on metabolism. For example, it appears to be involved in immune function and inflammation. This sort of observation raises the question of the degree to which the full physiological experience of hunger is a necessary part of the benefits produced by calorie restriction. Researchers here take a first step in the exploration of this topic with a study of mice genetically engineered to lack ghrelin. The interesting portion of the data is that mice without ghrelin eat less, at least while young, but did not live longer, as is reliably the case in normal mice with a reduced calorie intake. I think that the authors head off in the wrong direction with a focus on AMPK, rather than exploring calorie restriction as an explanation for much of what they observed. There is no real discussion of why it might be that life span was not increased in ghrelin knockout mice, which seems to me the real question here.
In line with what is seen in humans during aging, here we show that old wild-type (WT) mice show an increase in body weight and fat mass, along with a significant decrease in muscle strength and endurance. Although ghrelin deletion (KO) in young animals on regular diet was previously shown not to have a significant effect on food intake, energy expenditure, or body weight, we show for the first time that ghrelin deletion significantly prevented body weight and fat mass gain in older mice while maintaining lean mass and muscle function when compared to wild-type age-matched animals.
As body weight gain develops as a result of energy imbalance, food intake and energy expenditure were studied in detail. Aging was associated with a decline in food intake, but also in spontaneous locomotor activity and total energy expenditure. Ghrelin deletion decreased food intake in young animals and partially prevented the decrease in energy expenditure seen with aging in WT mice. Given that the decrease in locomotor activity seen with aging was similar in WT and KO mice, we postulate that the difference in total energy expenditure between genotypes was primarily due to changes in resting energy expenditure. The data also suggest that a decrease in energy expenditure due to decrease locomotor activity and, perhaps also in resting energy expenditure, is the main variable driving the energy imbalance during aging in mice.
We found no differences in muscle mass or whole body lean mass between genotypes. Nevertheless, the decline in endurance and grip strength seen with aging in WT mice was also partially prevented by ghrelin deletion. In this study, we also show a significant increase in type IIa (fatigue resistant, more oxidative) fiber content with aging in KO compared to WT mice that is likely to be responsible for the increased endurance seen in KO aged animals. We postulate that this increase in type IIa, fatigue-resistant, oxidative fibers could have contributed to the increased energy expenditure and subsequently decreased fat mass seen in aged KO mice as skeletal muscle fibers are major contributors to resting energy expenditure.
At the molecular level, the age-related decreases in endurance and muscle strength were associated with downregulation of phospho-AMPK and its downstream mediators. These changes were partially prevented by ghrelin deletion. Previous studies have shown the importance of the AMPK pathway on improving endurance, and this finding suggests that AMPK modulation by ghrelin could contribute to the phenotype seen in our model of increased endurance and muscle strength. The interplay between ghrelin and AMPK is not well-understood, however. There are no previous reports of chronic effects of ghrelin or ghrelin blockade on AMPK activation; however, it is known that AMPK target genes are key to mitochondrial biogenesis, fatty acid oxidation, and energy expenditure. Taken together, the data are consistent with the hypothesis that AMPK modulation by ghrelin may contribute to ghrelin’s effects on muscle function, fat accumulation, and energy expenditure.
Another Potential Approach to the Creation of Tissue Engineered Cartilage
Cartilage is a comparatively simple, homogeneous tissue, and the medical community has a great deal of experience in treating cartilage injuries, so it is an obvious place to start for tissue engineers. Creating cartilage that has the correct load-bearing characteristics has proven to be a challenge, however: you can’t just put cartilage cells into a bioreactor on their own and expect to obtain anything other than a sloppy gel at the end of the day. Fortunately, a number of groups have made progress in recent years on this front, finding approaches to convince the cells involved to generate the suitably structured extracellular matrix needed to form a solid, high-strength tissue. The method described here is one of the more straightforward ones:
Biomedical engineers have created a lab-grown tissue similar to natural cartilage by giving it a bit of a stretch. The tissue, grown under tension but without a supporting scaffold, shows similar mechanical and biochemical properties to natural cartilage. Articular cartilage provides a smooth surface for our joints to move, but it can be damaged by trauma, disease or overuse. Once damaged, it does not regrow and is difficult to replace. Artificial cartilage that could be implanted into damaged joints would have great potential to help people regain mobility.
Natural cartilage is formed by cells called chondrocytes that stick together and produce a matrix of proteins and other molecules that solidifies into cartilage. Bioengineers have tried to create cartilage, and other materials, in the lab by growing cells on artificial scaffolds. More recently, they have turned to “scaffold-free” systems that better represent natural conditions. The research team grew human chondrocytes in a scaffold-free system, allowing the cells to self-assemble and stick together inside a specially designed device. Once the cells had assembled, they were put under tension – mildly stretched – over several days. They showed similar results using bovine cells as well. “As they were stretched, they became stiffer. We think of cartilage as being strong in compression, but putting it under tension has dramatic effects.”
The new material had a similar composition and mechanical properties to natural cartilage, the researchers found. It contains a mix of glycoproteins and collagen, with crosslinks between collagen strands giving strength to the material. Experiments with mice show that the lab-grown material can survive in a physiological environment. The next step is to put the lab-grown cartilage into a load-bearing joint, to see if it remains durable under stress. “The artificial cartilage that we engineer is fully biological with a structure akin to real cartilage. Most importantly, we believe that we have solved the complex problem of making tissues in the laboratory that are strong and stiff enough to take the extremely high loads encountered in joints such as the knee and hip.”
An Update on the Effects on PAPP-A Knockout on Longevity in Mice
Deletion of pregnancy-associated plasma protein-A (PAPP-A) is one of a number of genetic alterations that have been used to produced lineages of long-lived mice. For researchers interested in translating this sort of discovery into treatments that might modestly slow human aging, an important question is whether the mice live longer because this alteration was present throughout development and childhood, or whether the effects on life span are determined over the course of adult life. Only in the latter case would researchers be able to proceed with any confidence to work on the basis for a human treatment. To further investigate the basis for enhanced longevity in mice lacking PAPP-A, researchers have now used a form of gene therapy to delete PAPP-A in adult mice, an approach we should expect to see applied in the years ahead to all of the genetic approaches that extend life in mice. Here, they report on the results.
To date, the only known function of pregnancy-associated plasma protein-A (PAPP-A) is to enhance local insulin-like growth factor (IGF) availability for receptor activation through cleavage of inhibitory IGF binding proteins. As reduced IGF signaling has been shown to increase life span in a wide variety of species, we postulated that loss of PAPP-A would suppress IGF receptor signaling and extend life span. This was proven true in that both male and female PAPP-A knockout (KO) mice lived significantly longer than their wild-type littermates. The PAPP-A KO mice were also resistant to the development of several age-related diseases, such as atherosclerosis.
However, these mice were generated through homologous recombination in embryonic stem cells. To distinguish the impact of PAPP-A deficiency in the adult from that during fetal and early postnatal development, we developed a mouse model suitable for tamoxifen (Tam)-inducible, Cre recombinase-mediated excision of the PAPP-A gene. In an atherosclerosis-prone mouse model, Tam administration in adult mice inhibited established atherosclerotic plaque progression by 70%. In this study, we sought to answer the question of whether conditional reduction of PAPP-A gene expression in adult mice would result in extended life span.
Female mice homozygous for floxed PAPP-A (fPAPP-A) and either positive (pos) or negative (neg) for Tam-Cre were used in the life span study. fPAPP-A/neg and fPAPP-A/pos mice had similar weights at the start of the experiment and showed equivalent weight gain up to 17 months of age. We found that fPAPP-A/pos mice had a significant extension of life span. The median life span was increased by 21% for fPAPP-A/pos compared to fPAPP-A/neg mice. Mortality in life span quartiles indicates that the proportion of deaths of fPAPP-A/pos mice were lower than fPAPP-A/neg mice at young adult ages and higher than fPAPP-A/neg mice at older ages. This study is the first to show that downregulation of PAPP-A expression in adult mice can significantly extend life span.
Importantly, this beneficial longevity phenotype is distinct from the dwarfism of long-lived PAPP-A KO, Ames dwarf, Snell dwarf and growth hormone receptor (GHR) KO mice with germ-line mutations. Thus, downregulation of PAPP-A expression joins other treatment regimens, such as resveratrol, rapamycin and dietary restriction, which can extend life span when started in mice as adults. In a recent study, inducible knockdown of the GHR in young adult female mice increased maximal, but not median, life span. Tissue-specific PAPP-A KO models would provide insight into the tissues and organs that contribute to extended life span and healthspan.
Measurable Amyloid Buildup Occurs Significantly Before Alzheimer’s Disease
Named and formally recognized age-related diseases are the late stages of processes of damage that start much earlier in life. So it is never a surprise to see that specific forms of damage strongly associated with any one specific age-related disease can be detected in smaller amounts earlier in old age, and that the people with more of that damage have a higher risk of later exhibiting the disease state. In the case of the research materials noted here, the disease is Alzheimer’s, and the damage is accumulation of amyloid-β, a form of misfolded protein that accumulates in the brain. It and its surrounding halo of chemical interactions disrupt the correct function of brain cells, ultimately causing significant neurodegeneration.
The obvious solution here is to try to remove the amyloid, and in fact the Alzheimer’s research community has and continues to spend considerable effort on this goal. It is one of the few areas where mainstream aging research aligns with the goals of the SENS rejuvenation research programs: identify the root cause damage that produces differences between old and young tissue, and repair it. Sadly, safe and effective clearance of amyloid has proven to be far more challenging than hoped. The field is littered with failed attempts, largely forms of immunotherapy, and only in the past couple of years have there been signs of success in human trials. Nonetheless, removing amyloid, and then expanding efforts to other forms of repair therapy, is the only game in town if the goal is to cure age-related neurodegenerative disease rather than just slow it down it little.
Older adults with elevated levels of brain-clogging plaques – but otherwise normal cognition – experience faster mental decline suggestive of Alzheimer’s disease, according to a new study that looked at 10 years of data. Just about all researchers see amyloid plaques as a risk factor for Alzheimer’s. However, this study presents the toxic, sticky protein as part of the disease – the earliest precursor before symptoms arise. Notably, the incubation period with elevated amyloid plaques – the asymptomatic stage – can last longer than the dementia stage. “To have the greatest impact on the disease, we need to intervene against amyloid, the basic molecular cause, as early as possible.”
The researchers likened amyloid plaque in the brain to cholesterol in the blood. Both are warning signs with few outward manifestations until a catastrophic event occurs. Treating the symptoms can fend off the resulting malady – Alzheimer’s or a heart attack – the effects of which may be irreversible and too late to treat. The researchers hope that removing amyloid at the preclinical stage will slow the onset of Alzheimer’s or even stop it.
One in three people over 65 have elevated amyloid in the brain, and the study indicates that most people with elevated amyloid will progress to symptomatic Alzheimer’s within 10 years. The study uses 10 years of data from the Alzheimer’s Disease Neuroimaging Initiative, an exploration of the biomarkers that presage Alzheimer’s. Although elevated amyloid is associated with subsequent cognitive decline, the study did not prove a causal relationship. Researchers measured amyloid levels in 445 cognitively normal people via cerebrospinal fluid taps or positron emission tomography (PET) scans: 242 had normal amyloid levels and 202 had elevated amyloid levels. Cognitive tests were performed on the participants, who had an average age of 74. Although the observation period lasted 10 years, each participant, on average, was observed for three years. The maximum follow-up was 10 years.
The elevated amyloid group was older and less educated. Additionally, a larger proportion of this group carried at least one copy of the ApoE4 gene, which increases the odds that someone will develop Alzheimer’s. Based on global cognition scores, at the four-year mark, 32 percent of people with elevated amyloid had developed symptoms consistent with the early stage of Alzheimer’s disease. In comparison, only 15 percent of participants with normal amyloid showed a substantial decline in cognition. Analyzing a smaller sample size at year 10, researchers noted that 88 percent of people with elevated amyloid were projected to show significant mental decline based on global cognitive tests. Comparatively, just 29 percent of people with normal amyloid showed cognitive decline.
A Profile of Valter Longo’s Work on Fasting and Calorie Restriction
Valter Longo is one of the more recognizable names in calorie restriction research. Beyond the science, his most noteworthy recent achievement has been to figure out how to commercialize the research, pulling in for-profit funding by packaging low-calorie diets as a medical product. This has helped to fund a series of advances in quantifying the effects of reduced calorie intake and fasting in humans, in search of the 80/20 point for optimal benefits, and along the way generating new knowledge of the effects on the immune system and other important areas of cellular metabolism. One of the most interesting outcomes is the accumulation of evidence to suggest that low-calorie diets can be about as effective as fasting in our species, at least in the near term.
As an aside, the tale of Longo’s early work on the biochemistry of aging, provided in the article here, is illustrative of the degree to which the field was held back, both internally and by the rest of the research community. Aging research was a backwater, disrespected, lacking in funding. We could be ten to twenty years further ahead in treating aging as a medical condition than we are today, had the study of aging been taken as seriously as was the study of age-related diseases over the past fifty years. It is just one more example of the irrationality of the human condition that people are so enthused about research to treat heart disease, Alzheimer’s disease, and so on, but reject outright work on the root causes of these conditions. The only way to cure age-related disease is to control the processes of aging – everything else is just putting thin patch over the problem and hoping. We should be thankful that this era of deliberate repression of aging research has largely come to an end, thanks to persistent and outspoken advocacy by those within and without the research community, and the field now has a chance to grow in funding and support.
He knows he sounds like a snake-oil salesman. It’s not every day, after all, that a tenured professor at a prestigious university starts peddling a mail-order diet to melt away belly fat, rejuvenate worn-out cells, prevent diseases ranging from diabetes to cancer – and, for good measure, turn back the clock on aging. But biochemist Valter Longo is convinced that science is on his side. He now believes he’s developed a diet that may boost longevity – by mimicking the effect of periodic fasting. His approach stands out because he insists he can use certain combinations of nutrients to trick the body into thinking it’s fasting without actually being on a punishing, water-only diet.
Intrigued, we reviewed dozens of scientific studies and talked to a half-dozen aging and nutrition experts about fasting in general and Longo’s diet in particular. Our conclusion? Fasting does appear to boost health – certainly in mice, and preliminary evidence suggests it might do so in humans as well, at least in the short term. It’s not yet clear whether that’s because abstaining from food prompts cellular changes that promote longevity, as some scientists believe – or because it simply puts a brake on the abundant and ceaseless stream of calories we consume to the detriment of our health. Either way, it can be a powerful force.
Mice and rats on fasting regimes are slimmer, live longer, and stay smarter and physically stronger as they age. They resist tumors, inflammatory diseases, and the neurodegeneration that characterizes diseases like Parkinson’s and Alzheimer’s. They handily fight off infection and can even sprout new neurons. They don’t end up with diabetes, autoimmune disease, high cholesterol or fatty livers. Longo believes he knows why. Fasting, he and others argue, gives cells a break to rest, renew, rebuild themselves and, essentially, take out the trash as the body shifts from storing fat to burning it. They can’t do that when the body is constantly ingesting food, stockpiling excess calories and pushing cells and organs to exhaustion.
Of course, many exciting findings that hold true for lab mice don’t translate to more complex human biology. Small, short-term studies in humans do show that periodic fasting reduces weight, abdominal fat, cholesterol, and blood glucose, as well as proteins like C-reactive protein and IGF-1 that are linked to inflammatory diseases and cancer. But it’s not clear how long these effects last or whether they translate into any lasting clinical advantage – such as fewer heart attacks or longer lifespan.
In the 1990s Longo was growing frustrated with attempts to study longevity in humans, and even mice, without having adequate tools to drill down into the genetic mechanisms underlying aging. He transferred to a genetics lab focused on yeast, figuring that would let him study the mechanisms of aging in the simplest of organisms. Few people took his early results seriously. Studying aging was still considered flaky. And many scientists at the time were deeply skeptical that you could learn much about human biology by studying simple yeast. “If someone said, ‘What are you working on?’ we would say oxidative chemistry. You couldn’t say aging. That was viewed as a joke.” Convinced his work was important, Longo kept his head down and kept going.
In just a year, Longo was able to work out a genetic pathway to describe aging in yeast and show that food – proteins and sugars – could speed aging. It was 1994. “I was so excited, I thought people were going to say, ‘This is the discovery of the century.’ Of course, it was sent back – rejected.” He rewrote the paper and resubmitted. No luck. He couldn’t get any of the work published without taking out every last reference to aging. As years passed, other groups started publishing work detailing, as Longo had, specific aging pathways, first in worms and eventually in flies. “The frustrating thing is that we had all of these things figured out and no one was listening.”
Weak Evidence for Amino Acid Processing Dysfunction Theories of Sarcopenia
One of the theories relating to causes of sarcopenia, the characteristic loss of muscle mass and strength with age, is that it relates to dysfunction in the processing of amino acids such as leucine. There is evidence for leucine supplementation to help slow the progression of sarcopenia, for example. The research here adds more along these lines, though it seems the authors will have to run a redesigned study to see whether or not the cellular differences observed actually produce meaningful results over a longer period of time:
The loss of skeletal muscle mass and quality is common with aging. This loss highlights the development of sarcopenia, where diminished muscle mass and strength are major contributors engendering loss of independence and quality of life for older adults. Current research suggests that reductions in the ability to stimulate muscle protein synthesis and promote proliferation and differentiation of muscle satellite cells may be important contributors to the development of sarcopenia.
It is well known that exercise and the ingestion of essential amino acids (EAA), in particular the amino acid leucine, are important stimulators of muscle protein synthesis through activation of the mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway. While an anabolic resistance to the independent effects of exercise and EAA or protein is prevalent with aging, combining the two stimuli shows promise in combating sarcopenia via the ability for this combination to maximally stimulate mTORC1 and upregulate the translation initiation machinery. Indeed, we have recently demonstrated that provision of leucine-enriched EAA mixture following a bout of high-intensity resistance exercise (RE) stimulates mTORC1 and prolongs myofibrillar protein synthesis for up to 24 hours post-RE in the very same cohort of older men we examined in this study whereas, in the absence of EAA this mTORC1 response is blunted in older adults.
A host of evidence has suggested increased satellite cell (SC) activation and content following RE in human skeletal muscle, yet we and others have demonstrated a blunting of or a delayed ability to activate and increase the SC pool in older men compared with a younger cohort. EAA and leucine provision has been shown to upregulate SC activity via mTORC1. Therefore, we hypothesized that EAA ingestion, which we have previously shown to potently activate mTORC1 following an acute bout of leg resistance exercise, would enhance skeletal muscle satellite cell proliferative capacity and content in older men. We demonstrate that older men do not appear to increase skeletal muscle satellite cell content at 24 hours following heavy, high volume resistance exercise in the absence of EAA ingestion. However, when 10g of EAA is ingested one hour postexercise we found that MHC I myofiber satellite cell content displays obvious trends to be greater than when older men are not given postexercise EAA. Although this pattern is visually evident in MHC II myofibers and when all myofibers are pooled, the current data set did not reach statistical significance.
Using Photosynthetic Microbes to Oxygenate Ischemic Tissue
Ischemic injuries, in which insufficient oxygen is delivered to tissues, can occur in numerous ways, but heart attacks are among the most common, evident, and dangerous. A sizable branch of the research community works on ways to efficiently and quickly provide oxygen to the impacted tissues so as to reduce the long-term damage and speed recovery. At the end of this road lie permanent enhancements such as respirocyte nanomachinery that will provide hours of supplemental oxygen for all tissues, but for now researchers are still working on the first potential advances in emergency oxygen supplementation, such as the example noted here.
The use of photosynthetic microorganisms to provide much needed oxygen to damaged heart tissue could be a feasible approach to treating heart attacks. Recent research describes the injection of the cyanobacterium Synechococcus elongatus into ischemic heart muscles of live rats, where, in response to light exposure, the microbes produced oxygen and improved organ function. Photosynthetic organisms capture energy from sunlight and use it to convert carbon dioxide and water into carbohydrates for growth. The process creates a surplus of oxygen, which the organisms simply expel into the atmosphere, much to the delight of aerobic organisms such as humans. “One day I was thinking: what is the fundamental problem with a heart attack? It’s the absence of oxygen being delivered to the heart muscle. And, what in nature makes oxygen for us every single minute? Plants.”
“Cardiologists are always thinking about how to deliver more blood to ischemic heart tissue. But, if oxygen is the critical component, what if you could take a plant, or the photosynthetic mechanism of a plant, and put it right next to a heart cell?” To investigate this unusual idea, researchers took an equally unusual approach. “We started grinding up kale and spinach to isolate the chloroplasts and put them with heart cells.” But the results of these experiments were disappointing. “What we found is that chloroplasts do not like being outside of a plant cell. They’re not very stable.”
So, the team instead tried the photosynthetic unicellular microorganism S. elongatus. “We put them with heart cells in a dish and we found that they could live together and, when we shone light on them, they could produce oxygen.” The team then tested the idea in live rats. They gave the animals heart attacks and then injected their hearts with S. elongatus and exposed the hearts to light. After just 10 minutes, oxygen in the bacteria-containing and light-exposed hearts had risen approximately 25-fold compared with just a 3-fold rise in oxygen in bacteria-containing hearts kept in darkness. And by 45 minutes, left ventricle pressure and cardiac output had improved, suggesting increased heart contractility. The microbes also provided long-term improvements to heart function. Four weeks after rats were subjected to temporary cardiac ischemia (60 minutes) – during which the animals were, or were not, injected with S. elongatus and exposed to light – analyses of heart function revealed that recipients of the microbe treatment had significantly improved contractility compared with controls.
Mutant Dietary Bacteria as a Way to Explore Mechanisms of Aging in Nematodes
Researchers here outline a novel method of searching for longevity-related mechanisms in nematode worms: mutate the bacteria that the worms eat. The researchers worked their way through a selection of bacterial mutations, and along the way uncovered a few items of interest for further exploration. I think the leap made by press and publicly materials to supplements for human consumption containing mutated bacteria is getting far ahead of the science, however. This is, as of the moment, really only a demonstration of a new method of discovery in a commonly used laboratory species.
Scientists have identified bacterial genes and compounds that extend the life of and also slow down the progression of tumors and the accumulation of amyloid-beta, a compound associated with Alzheimer’s disease, in the laboratory worm C. elegans. “The scientific community is increasingly aware that our body’s interactions with the millions of microbes in our bodies, the microbiome, can influence many of our functions, such as cognitive and metabolic activities and aging. In this work we investigated whether the genetic composition of the microbiome might also be important for longevity.”
This question is difficult to explore in mammals due to technical challenges, so the researchers turned to the laboratory worm C. elegans, a transparent, simple organism that is as long as a pinhead and shares essential characteristics with human biology. During its 2 to 3 week long lifespan, the worm feeds on bacteria, develops into an adult, reproduces, and progressively ages, loses strength and health and dies. Many research laboratories around the world work with C. elegans to learn about basic biological processes.
Researchers employed a complete gene-deletion library of bacterium E. coli; a collection of E. coli, each lacking one of close to 4,000 genes. “We fed C. elegans each individual mutant bacteria and then looked at the worms’ life span. Of the nearly 4,000 bacterial genes we tested, 29, when deleted, increased the worms’ lifespan. Twelve of these bacterial mutants also protected the worms from tumor growth and accumulation of amyloid-beta.” Further experiments showed that some of the bacterial mutants increased longevity by acting on some of the worm’s known processes linked to aging. Other mutants encouraged longevity by over-producing the polysaccharide colanic acid. When the scientists provided purified colanic acid to C. elegans, the worms also lived longer. Colanic acid also showed similar effects in the laboratory fruit fly and in mammalian cells cultured in the lab.
Interestingly, the scientists found that colanic acid regulates the fusion-fission dynamics of mitochondria, the structures that provide the energy for the cell’s functions. “These findings are also interesting and have implications from the biological point of view in the way we understand host-microbe communication. Mitochondria seem to have evolved from bacteria that millions of years ago entered primitive cells. Our finding suggests that products from bacteria today can still chime in the communication between mitochondria in our cells. We think that this type of communication is very important and here we have provided the first evidence of this. Fully understanding microbe-mitochondria communication can help us understand at a deeper level the interactions between microbes and their hosts.”
Yeast Life Extension via Lithocholic Acid Provides Support for the Membrane Pacemaker Hypothesis of Aging
A fair amount of aging research starts in yeast (or worms, or flies) for reasons of cost, only later moving to mammals such as laboratory mice. A surprisingly large fraction of the cellular mechanisms relevant to aging are much the same in all of these species. Most of the ways in which metabolism determines natural variations in longevity, both between individuals and between species, were established very early in the evolution of cellular life. With this in mind, you might recall researchers demonstrating earlier this year that life span in yeast can be extended via provision of lithocholic acid. The open access paper here follows that with a consideration of the mechanisms involved: it appears to work via alteration of the composition of mitochondria, making them more resistant to damage.
The membrane pacemaker theory of aging puts mitochondrial composition front and center, based on comparisons of mitochondria between species with very different life spans. Longer-lived species tend to have mitochondria built out of more resilient lipids – though in this paper the researchers suggest that such differences in lipid composition are far more influential in the cell than simply a matter of damage resistance. Mitochondrial function and mitochondrial damage are any case considered to be very important in aging. Loss of mitochondrial activity is seen in many age-related diseases, and some forms of damage to mitochondria appear capable of creating dysfunctional cells that export harmful reactive molecules into the surrounding tissues. Restoring mitochondria to youthful function in the aged is an important component of rejuvenation research.
Mitochondria are indispensable for organismal physiology and health in all eukaryotes. The efficiencies with which these organelles generate the bulk of cellular ATP and make biosynthetic intermediates for amino acids, nucleotides, and lipids are known to deteriorate with age. Such age-related deterioration of mitochondrial functionality is the universal feature of aging in evolutionarily distant eukaryotic organisms.
A number of mechanisms underlie the essential roles of some traits of mitochondrial functionality in both modes of yeast aging. These traits in replicatively and chronologically aging yeast include mitochondrial electron transport chain and oxidative phosphorylation, membrane potential, reactive oxygen species (ROS) homeostasis, protein synthesis and proteostasis, iron-sulfur cluster formation, and synthesis of amino acids and NADPH. Until recently, it was unknown if such trait of mitochondrial functionality as the composition of mitochondrial membrane lipids can influence aging in yeast. Our recent studies have revealed that lithocholic bile acid (LCA) can delay the onset and decrease the rate of yeast chronological aging. We demonstrated that the robust geroprotective effect of exogenously added LCA is due to its ability to cause certain changes in lipid compositions of both mitochondrial membranes. These changes in mitochondrial membrane lipids enable mitochondria to establish and maintain an aging-delaying pattern of the entire cell.
This LCA-driven remodeling of mitochondrial lipidome triggers major changes in mitochondrial abundance and morphology and also alters mitochondrial proteome. These changes in the abundance, morphology, and protein composition of mitochondria lead to specific alterations in mitochondrial functionality. Our recent unpublished data indicate that the LCA-dependent alterations in mitochondrial lipidome, proteome, and morphology can also elicit changes in lipidomes of other organelles and in concentrations of a specific set of water-soluble metabolites. By sensing different aspects of mitochondrial functional state, a discrete set of ten transcription factors orchestrates a distinct transcriptional program for many nuclear genes. The denouement of this cascade of consecutive events is the establishment of a cellular pattern that delays the onset and slows the progression of yeast chronological aging.
Of note, the proposed mechanism here for how the LCA-dependent remodeling of mitochondrial lipidome in the yeast S. cerevisiae allows to establish an aging-delaying cellular pattern is reminiscent of the mechanism in which the mitochondrial unfolded protein response causes remodeling of the mitochondrial lipidome in the nematode C. elegans, and then triggers a cascade of events that institute an aging-delaying cellular pattern. Moreover, the essential role of mitochondrial lipid metabolism in defining the pace of yeast chronological aging further supports the notion that the vital role of lipid homeostasis in healthy aging has been conserved in eukaryotes.