Numerous research and development initiatives have emerged from the heterochronic parabiosis studies of the past decade or more, in which an old and a young mouse have their circulatory systems linked. Researchers have moved on from the initial experiments to the search for circulating factors in blood that change in ways that are harmful in aged individuals, and which might be adjusted to improve cell and tissue function. This area of research is one of many to explore the question of how much of degenerative aging is the result of (a) direct consequences of molecular damage versus (b) the result of inappropriate cellular reactions to the existence of damage, the latter mediated to some unknown degree by signaling carried in the bloodstream.
Is it possible to ignore the damage and extend healthy life just by suppressing the reactions to damage? It would be very strange if the answer were that this works comprehensively and damage never has to be repaired. Further, the consequences of any given form of underlying damage can be thought of as a network of diverse chains of cause and effect spreading from a single root: it will require far more work to identify and address all of these reactions to damage than to focus down on a means of repairing the damage. Still, and unfortunately, the concept of damage repair, striking at the root of aging, remains a comparatively unpopular strategy in the research community for some reason. Near all work on the treatment of aging is focused on tinkering with the downstream consequences of damage, and therefore probably a highly inefficient use of funds and time, even given the successes that arise.
One of the more noted scientific teams involved in parabiosis research here report on their recent work, opening this open access paper with a bold statement on the degree to which they believe aging to result from signaling changes, reactions to damage. They are focusing down on just a few signaling factors in the bloodstream, TGF-β and oxytocin, and finding ways to alter amounts in circulation in comparative isolation, without adjusting other factors as well. Given that earlier work on GDF-11 as circulating signal involved in cellular responses to aging has resulted in a great deal of ongoing research and at least one biotech startup, the results here seem interesting enough to drawn in funding for further, similar projects.
We hypothesize that altered intensities of a few morphogenic pathways account for most or all the phenotypes of aging. In heterochronic parabiosis, a young and old animal are surgically connected to share a common blood circulation. Experiments in mice showed this shared circulatory milieu restored tissue health and regeneration of the old partner; and at the same time, the young partner experienced a regenerative decline in a number of tissues. However, parabiosis is not clinically translatable and infusion of young blood or plasma into old mammals is controversial and fraught with multiple side-effects. Blood fractionation is typically cumbersome, and it is inherently complicated by the fact that the rejuvenative activities are likely to be contained in multiple molecularly different fractions. Plus, the assays for determining such activity are themselves complex, thus adding to the hurdles of a screen for active blood molecules. With these observations to consider, what would be the key set of molecular parameters that were changed by the blood heterochronicity and what would be the best translational way forward?
The changes that manifest with aging include altered cell metabolism, increased Reactive Oxygen Species (ROS), inflammation, senescence, and decline in immune function. However, from the viewpoint of tissue maintenance and regeneration, we postulated that these arise from changes in tissue growth and homeostasis and specifically in key signaling networks regulating stem cells and their differentiated niches. In support of this idea, pathway modifier-based approaches for the enhancement of aged tissue repair and maintenance have been reported, for example, by systemic delivery of OT which induces MAPK/pERK signaling, by forced activation of Notch-1, by antagonism of TGF-beta/pSmad signaling, or by antagonism of the Jak/Stat pathway.
The highest risk from modulating key cell-fate regulatory signaling pathways come from changing levels too far above or too far below normal healthy levels. Such drastic alterations result in severe multi-tissue side-effects. But high levels of a single modifier might be required to overcome the many age-specific molecular changes. For example, ectopic oxytocin (OT) might be needed at a considerably high dose to overcome age-elevated TGF-beta 1. And, the Alk5 inhibitor (Alk5i) of the TGF-beta receptor might be needed at high dose to overcome the lack of OT and other hormones with age.
Using a two-prong approach of simultaneously diminishing TGF-beta signaling and adding OT (which activates pERK via the oxytocin receptor (OTR)), we were able to reduce the required dose of Alk5i, shorten the duration of treatment and to achieve a more broad rejuvenation of the three germ-layer derivative tissues: brain, liver and muscle. And, we found that Alk5i+OT downregulated the number of cells that show an age-associated increase of the cyclin dependent kinase (CDK) inhibitor and marker of senescence, p16, thereby representing a pharmacological combination of two FDA approved drugs to normalize this checkpoint protein, which when chronically elevated negatively impacts tissue health.
Translationally, this study points toward a pharmacological approach to rapidly enhance the health and maintenance of multiple old tissues. Here we focused on a few key age-related parameters of the three germ layer tissues: neurogenesis and neuroinflammation of the brain, regeneration and fibrosis of the skeletal muscle and adiposity and fibrosis of the liver. In future work if would be interesting to study how these seemingly unrelated aging features become rapidly rejuvenated by A5i+OT, and if additional phenotypes, such as muscle innervation, neural plasticity, metabolism, etc. also become improved in old animals. The observed rejuvenating effects are at least as robust as, and act faster than, heterochronic parabiosis.