In recent years, researchers have assembled a number of what appear to be important pieces of the puzzle when it comes to understanding regeneration and scarring. Why do mammals scar rather than regenerate like salamanders, and how do the exceptions to that rule function? Mutant MRL mice can heal small injuries without scarring, African spiny mice can regrow large sections of their skin without scarring, the liver can regrow sections of itself, and people can sometimes regenerate lost fingertips. It is of great interest to the medical community to come to a deeper understanding of the mechanisms of regeneration in our species and other mammals, as in principle anything that an MRL mouse can achieve in the healing of injury can be induced through suitable changes in the regulation of human regeneration. In principle, if fingertips can regenerate without scarring in some rare occasions, why can’t the root causes be identified and applied to larger injuries? A fair number of research groups have for years tackled various approaches to these questions, investigating the biochemistry of regeneration in a variety of mammalian lineages and other species capable of proficient regeneration.
A picture is beginning to emerge in which the activities of senescent cells and the immune cells called macrophages are the most important players. The final assembly and details of a theory that explains all of the observed variation in regeneration remains to be accomplished, but there is a good deal of evidence to speculate upon. For example, senescent cells are known to play a temporary role in wound healing; some of their signaling is important in this respect. One of the side-effects of the recent focus on removal of lingering senescent cells as a treatment for aging is that researchers have found wound healing to be impaired when these cells are constantly cleared. Senescent cells are created in wounded tissue and serve some transient purpose before destroying themselves; if they are removed before the healing process can get underway, this slows it down. Separately, researchers have found that salamanders, known for their ability to regenerate, have a much more efficient and energetic ability to create and then entirely clear out senescent cells during regeneration.
In salamanders, the clearance of senescent cells is accomplished by macrophages, and without their presence the process of efficient regeneration runs awry. This has been shown to be the case in zebrafish as well, another species capable of healing without scarring and regeneration of body parts. Macrophages respond to injuries in mammals, and play their part in regenerative processes. There is evidence to suggest that their activities can be improved upon – researchers have altered macrophage behavior to enhance nerve regeneration, for example. Similarly, and as is the case in the research noted below, there is good evidence for macrophages to be both beneficial and detrimental to healing depending on their characteristics; some spur regeneration, others spur scarring. Given that the evidence below makes proficient regeneration in African spiny mice look very much like proficient regeneration in salamanders and zebrafish, it now seems plausible that there is a lever in here somewhere that could be used to tilt mammalian regeneration in the direction of greater capacity and lesser degrees of scarring.
Researchers have discovered that macrophages, a type of immune cell that clears debris at injury sites during normal wound healing and helps produce scar tissue, are required for complex tissue regeneration in mammals. Their findings shed light on how immune cells might be harnessed to someday help stimulate tissue regeneration in humans. “With few examples to study, we know very little about how regeneration works in mammals; most of what we know about organ regeneration comes from studying invertebrates or from research in amphibians and fish. If we want to apply what we learn from basic regenerative biology to humans, it would be helpful to understand what cell types and molecules regulate regeneration in a mammal where it occurs naturally.”
Scientists have been trying to learn for years why some animals, like salamanders and zebrafish, are able to regrow body parts following injury, while others – like humans – can only produce scar tissue in response. Researchers learned nearly eight years ago that African spiny mice are one of the few mammalian models capable of complex tissue regeneration, making them particularly fascinating subjects. But what remained unclear was exactly how an identical injury in spiny mice and non-regenerating lab mice could produce dramatically different healing responses. The researchers decided to investigate how the inflammatory environment might differ between the regenerative response observed in spiny mice compared to the typical scarring response observed in lab mice. Although white blood cell profiles were the same in uninjured animals from both species, injury elicited different local responses. “Comparing spiny mice to common house mice, we discovered that subtypes of macrophages active during regeneration are different than those active during scarring.”
When the team looked at different types of macrophages in healing tissue they found that a pro-inflammatory type of macrophage was highly abundant during scarring, but very rare during regeneration. “Our findings imply that macrophage activation in our model favors regeneration. The next step is to identify the components of macrophage activation that are necessary for regeneration. Since we are actively developing clinically feasible therapies that selectively activate macrophages, identifying targetable components of macrophage activation opens new areas of discovery with real potential for improving tissue regeneration in humans.”
When an animal is injured, immune cells such as macrophages rush to the wounded site to clear debris and help repair the damage. Macrophages come in different forms and subtypes, and express different protein markers on their surface, depending on where in the body they reside. Few mammals can completely renew or regrow a damaged tissue – a process known as tissue regeneration. Instead, humans and most other mammals repair injuries by producing scar tissue, which has different properties compared to the original tissue it replaces. One exception is the African spiny mouse (Acomys cahirinus), which, unlike other rodents studied, can regrow skin and fur, nerves, muscles, and even cartilage. It has been shown that in highly regenerative animals such as salamanders and zebrafish, macrophages are necessary to initiate tissue regeneration. Documented cases of tissue regeneration in mammals are rare and therefore less understood. Until now, it was not clear why two species as closely related as spiny mice and house mice would heal identical injuries in different ways.
Here, we report how the two main orchestrators of inflammation, neutrophils and macrophages, respond to injury during regeneration in Acomys cahirinus compared to scarring in the house mouse (Mus musculus). Acomys and Mus exhibit the same circulating leukocyte profiles, and we demonstrate a robust acute inflammatory response in both species. We demonstrate higher neutrophil activity in the scarring system compared to higher reactive oxygen species (ROS) activity in the regenerative system. We show that macrophages between the two species display similar in vitro properties providing a comparable baseline prior to and following injury. We also observed distinct differences in the spatiotemporal distribution of macrophage subtypes during regeneration and scarring. Finally, depletion of macrophages, prior to and during injury, inhibited blastema formation and regeneration, thus demonstrating a necessity for these cells.
A popular hypothesis to explain why most mammals heal injuries with scar tissue is that they evolved a strong inflammatory and adaptive immune response that induces intense fibrosis in lieu of regeneration. Yet, the fact that some mammals exhibit epimorphic regeneration (e.g. rodent and primate digit tips, rabbit and spiny mice ear punches and skin) suggests that regeneration can occur despite a complex adaptive immune system. It is possible that macrophages provide an initiating signal for regeneration or remove subpopulations of local cells secreting inhibitory signals (e.g. senescent cells). In support of the first idea, ROS production has been suggested as an essential early signal for regeneration based on studies in zebrafish tail models of regeneration. Macrophages are a major source of ROS after injury, and we observed significantly stronger and prolonged ROS production during regeneration compared to scarring. In support of the idea that macrophages may limit inhibitory signals through selective removal of senescent cells, recent work in salamanders suggested that clearance of senescent cells is important for limb regeneration and persistence of senescent cells during liver regeneration leads to excessive fibrosis. Furthermore, the accumulation of senescent cells with age has been suggested to shorten lifespan, degrade tissue function, and increase the expression of pro-inflammatory cytokines in mammals. These and other studies suggest that proper clearance of senescent cells from damaged tissues may promote regenerative outcomes.