Researchers here report on the presence of protein aggregates characteristic of Alzheimer’s disease in old chimpanzees. This may not result in any meaningful new lines of investigation, however, given that studies of this species are now heavily restricted. In principle, comparative biology studies using similar, related species can be useful in helping to understand specific mechanisms and cellular behaviors. In the case of Alzheimer’s disease, any of the benefits that might result from such a research program may well be overtaken by success in any one of the numerous forms of therapy currently under development. Comparative biology is a useful approach, but successfully removing a possible cause, such as one type of protein aggregate, and then observing the effects is even better as a source of new information.
Researchers have discovered tell-tale signs of Alzheimer’s disease in 20 elderly chimpanzee brains, rekindling a decades-old debate over whether humans are the only species that develop the debilitating condition. Whether chimps actually succumb to Alzheimer’s or are immune from symptoms despite having the key brain abnormalities is not clear. But either way, the work suggests that chimps could help scientists better understand the disease and how to fight it – if they could get permission to do such studies on these now-endangered animals.
A definitive diagnosis of Alzheimer’s includes dementia and two distortions in the brain: amyloid plaques, sticky accumulations of misfolded pieces of protein known as amyloid beta peptides; and neurofibrillary tangles, formed when proteins called tau clump into long filaments that twist around each other like ribbons. Many other primates including rhesus monkeys, baboons, and gorillas also acquire plaques with aging, but tau tangles are either absent in those species or don’t fully resemble those seen in humans. In the new study, and thanks to a newly founded center that collects brains from chimps that die at zoos or research centers, the team was able to examine the brains of 20 chimps aged 37 to 62 – the oldest recorded age for a chimp, roughly equivalent to a human at the age of 120. Of these chimps, 13 had amyloid plaques, and four also had the neurofibrillary tangles typical of more advanced stages of Alzheimer’s in humans.
But so far, only humans are known to show the Alzheimer’s trifecta of plaques, tangles, and dementia. The 20 chimps whose brains were studied had not been tested for cognitive or behavioral changes. As a result, “we can’t say these chimps had Alzheimer’s, but we can say for sure that they are the only other species with its pathologic hallmarks.” Some scientists aren’t persuaded that the chimp brains really do match those of human Alzheimer’s patients. In human brains, amyloid plaques are associated with neuron death, which wasn’t measured in the new study. The researchers plan to go back to the same chimp brains to calculate neuron death, but proving that chimps develop dementia will require research on living animals.