The popular science article I'll point out today does a fair job of following the past decade or so of work arising from heterochronic parabiosis, in which the circulatory systems of a young animal and an old animal are joined. The young animal exhibits some degree of accelerated aging, while the old animal exhibits some degree of rejuvenated function. The question all along has been why exactly this happens: what are the underlying mechanisms, and can they be replicated as a basis for therapy.
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The obvious first approach was to transfuse young donor blood into old recipients, as positive results would mean that the existing blood transfusion infrastructure could be used to provide a relatively low cost therapy to large number of older people. Unfortunately, this doesn't work. The results from animal studies and human trials indicate that if there are benefits, they are too small and unreliable to care about. There is something about parabiosis that isn't captured by transfusion.
Otherwise, initial research focused on factors in young blood that might be beneficial. This gave rise to the identification of GDF11 as one such factor, followed by considerable debate over whether this work was flawed, in parallel to the establishment of Elevian, a company that continues to work on therapies based on delivery or upregulation of GDF11. Researchers later provided compelling proof that the effect of beneficial factors in young blood is small in comparison to the effect of harmful factors in old blood, and from there identified TGF-β as one such problem factor. The suggestion here is that parabiosis works via a dilution of harmful factors, not via introduction of youthful factors.
Experiments in diluting blood in old animals with saline, while adding significant amounts of albumin because it cannot be diluted in the bloodstream without severe consequences, seemed to bear out this viewpoint. Animals exhibited similar benefits to those undergoing parabiosis. And yet, is the real signal in the albumin, and not in the dilution? Recently, researchers have delivered albumin to old animals without dilution of blood, and this also produces benefits. So is this a case of albumin becoming modified or damaged in increasing proportions in the bloodstream with advancing age, while cells are very sensitive to that form of damage? This is but the latest twist in this ongoing saga, so perhaps, or perhaps not. Give it a few years, and there may be another chapter to come.
Has the fountain of youth been in our blood all along?
In a series of studies over the last 15 years, researchers have shown that, when infused with blood from young mice, old ones heal faster, move quicker, think better, remember more. The experiments reverse almost every indicator of aging the teams have probed so far: It fixes signs of heart failure, improves bone healing, regrows pancreatic cells, and speeds spinal cord repair. It sounds sensational, almost like pseudoscience. It's some of the most provocative aging research in decades. These studies, which use a peculiar surgical method called parabiosis that turns mice into literal blood brothers, show that aging is not inevitable. It is not time's arrow. It's biology, and therefore something we could theoretically change.
Source: Fight Aging!