Heterochronic parabiosis involves joining the circulatory system of two animals, one old, one young, in order to observe the results. At a high level, the older individual exhibits reversal of some aspects of aging, and the young individual exhibits acceleration of some aspects of aging. The details are complex, and still debated in many cases, however. Researchers see this phenomenon as one of the more effective paths forward to identifying the important age-related changes in the environment of signals generated by cells that find their way into the bloodstream. A more effective approach would be to repair the underlying damage that causes aging - and thus also causes signaling changes - but the technologies to achieve that goal barely exist yet. Of the needed approaches, only clearance of senescent cells via senolytic pharmaceuticals is both easily studied in the laboratory and producing a great deal of useful data.
Branching out from the initial focus on joined circulatory systems, there are numerous other possible approaches to mixing young and old signals and tissues. Groups are assessing the results of transfusions of blood or plasma from young to old, for example, with Alkahest and Ambrosia as two of the more public examples. There is mixed data for the effectiveness of this strategy in comparison to parabiosis, however. The nature of the interactions when blood is circulating through two bodies is significantly different from that of even regular transfusions, and that may be important. For example, what if outcomes depend upon young tissues reacting to signals present in old blood and stepping up beneficial activities in response?
Looking further afield, we might consider investigating the transplantation of organs and other large tissue sections. The organ donation and transplant industry is, in effect, an enormous natural experiment in what happens when tissues are placed into an older or younger environment. It further has the advantage of providing human data rather than animal data. What would we expect to happen when an old organ is placed into a younger body? We might expect a degree of functional rejuvenation, and that can be measured, and the details of the biochemistry assessed. Equally, we may expect that some of the damage of aging and consequent impairment of organ function will not be reverted. Human biochemistry doesn't appear to be capable of effectively clearing persistent cross-links that stiffen tissues, for example.
The logistics of obtaining data from this experiment are not quite straightforward, however. While tens of thousands of organ transplants take place every year, and there are at least hundreds of thousands of recipients still alive, tracking down past patients and connecting them reliably with medical records is an expensive proposition. Also, the more recent data is the more interesting data. The viable approach is thus to work with medical establishments for ongoing transplant procedures and the necessary followups. In this way a fair-sized study set and database could be accumulated in a year or two. The authors of this paper have made a start on such an effort, and it is interesting to see that the narrow slice of data they elected to survey shows little rejuvenating effect when old livers are transplanted into young recipients. There is, however, a negative impact when young livers are transplanted into old recipients.
The scarcity of human donor organs in terms of availability for transplants is a renowned problem. The high request of organs moves toward an increased use of marginal donors, including organs from old or very old donors usually transplanted into younger recipients. Within the context of orthotopic liver transplants, clinical evidence suggests that livers from aged donors (≥ 70 years) do have function and duration comparable to those achievable with livers from younger donors. Paradigmatic are the cases of 26 octogenarians livers being transplanted between 1998 and 2006, 15 patients out of 26 are currently alive and 2 of those organs being centenarians.
Our team was deeply involved in an Italian national project to collect biological data to answer the question - why livers from old donors may be successfully used for transplants. The first evidence was a relative low grade of aging signs of liver donors at histological and cytological level, also including the three major proteolytic activities of proteasome, comparing young and old livers. Further, we tried to investigate the epigenetic age-related modifications in terms of liver microRNAs (miRs). We discovered that at 60-70 years of chronological age, three miRs start to increase their expression level, i.e. miR-31-5p; miR-141-3p; miR-200c-3p, and we assumed such an increase as markers of aging in human liver. When a relatively young liver was transplanted into a relatively older recipient (Δ age-mismatch average: +27 years) the expression of these miRs significantly increased in the organ (follow up after graft at 15 ± 7 months). It is interesting that we were not able to document the reverse. Indeed, when a relatively old liver was transplanted into a relatively young recipient (Δ age-mismatch average: -17 years), the expression of the three above-mentioned miRs did not change (follow up after graft at 10 ± 2 months).
On the whole, these observations suggest that in the setting of liver transplantation the aging phenotype can be "transmitted/propagated" more easily than the young phenotype via the body microenvironment. Recently, we studied the above mentioned miRs using single-miR real time-RT qPCR on blood serum samples from 34 recipients stratified on the basis of donor liver chronological age. No difference was observed, thus suggesting that the phenomenon previously found was tightly related to the organ itself without miR-specific exocytosis and changes at circulating level, at least for the identified miRs.
The biological effect of donor and recipient age-mismatch is a topic rather neglected despite its great potential, biological and clinical interest. The possibility that a centenarian liver can still function properly may suggest not only the intrinsic peculiarity of this organ (slowed down ageing; regeneration phenomena), but also the interaction with the younger recipients. This interaction was previously demonstrated in heterochronic parabiosis experiments in mice models, but deep analyses need specifically in humans, aiming at explain the reason of the variability associated with the duration of transplant.