A single heterochronic blood exchange reveals rapid inhibition of multiple tissues by old blood

• Justin Rebo,

• Melod Mehdipour,

• Ranveer Gathwala,

• Keith Causey,

• Yan Liu,

• Michael J. Conboy &

• Irina M. Conboy

Nature Communications volume 7, Article number: 13363 (2016) Cite this article

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Abstract Heterochronic parabiosis rejuvenates the performance of old tissue stem cells at some expense to the young, but whether this is through shared circulation or shared organs is unclear. Here we show that heterochronic blood exchange between young and old mice without sharing other organs, affects tissues within a few days, and leads to different outcomes than heterochronic parabiosis. Investigating muscle, liver and brain hippocampus, in the presence or absence of muscle injury, we find that, in many cases, the inhibitory effects of old blood are more pronounced than the benefits of young, and that peripheral tissue injury compounds the negative effects. We also explore mechanistic explanations, including the role of B2M and TGF-beta. We conclude that, compared with heterochronic parabiosis, heterochronic blood exchange in small animals is less invasive and enables better-controlled studies with more immediate translation to therapies for humans.

Introduction Technological developments and progress in our understanding of disease have eliminated many previously common sources of mortality. Along with our age shifting demographic, however, a host of new debilitating age-associated disorders have emerged. The medical costs of managing an aging world poses significant economic and social challenges and will ultimately require a long-term solution. One way to increase healthy longevity would be to rejuvenate the regenerative and repair capacity of aged tissues. Published work suggests restoring the circulatory environment of aged tissues back to a productive, young, composition may help to rapidly and broadly enhance the maintenance and repair of multiple old organs, combat degeneration and extend health span1,2,3,4,5,6,7,8. The effects seen by heterochronic parabiosis, the surgical joining of two animals of different ages, include rejuvenation of multiple tissues in the old partner, and are often simplistically assumed to be caused by the exchange of macromolecules found in plasma, however, parabiosis is far more complex. For example, old animals with young partners have, through the shared circulation, continuous access to the young organs, which regulate metabolic homeostasis, wound clearance and inflammation, and provide blood oxygenation to the animals3,5. Old mice attached to young animals also benefit from environmental enrichment and youthful pheromones, which may play a role in neuronal plasticity and neurogenesis9,10. The young parabiont partially maintains an additional aged body with deteriorating organs, chronic inflammation and skewed immune responses. Additionally, young and old organ systems have an opportunity to ‘adapt’ to prolonged sharing of circulatory milieus and thus change their local influences on resident stem cells. All of the above could contribute to the observed differences in regenerative responses. One conclusion from recent studies on heterochronic parabiosis is that the regenerative capacity of old tissue stem cells in all three germ layer derivatives can be enhanced by the young systemic milieu3,4,11,12,13. It is tempting to assume that young plasma has pro-regenerative factors, and indeed administration of young plasma to aged mice improved their cognition14. However, the effects of young blood plasma on stem cells in brain or other tissues have not been studied, and it remains to be discovered whether and which plasma factors would be active enough to influence neurogenesis or cognition at small doses when added to an aged circulation, and would be able to cross the blood–brain barrier to have positive or negative central effects. Thus far, only heterochronic parabiosis has been shown to enhance myogenesis, hepatogenesis, bone regrowth, neurogenesis, cognition and the numbers of dendritic spines in old mice. Most importantly, the positive effects of heterochronic parabiosis are robust for muscle, lesser for liver and marginal for neurogenesis; and a significant inhibition of even young tissue stem cells by the aged circulatory milieu takes place 2,4,5,8,11,15. In contrast to the permanent anastomosis of parabiosis, we developed a blood exchange system where animals are connected and disconnected at will, removing the influence of shared organs, adaptation to being joined and so on. The effects of heterochronic blood exchange were examined with respect to all three germ layer derivatives: injured-regenerating muscle, ongoing liver cell proliferation and adiposity, and in the brain, hippocampal neurogenesis, and this time in the presence versus absence of muscle injury. Most surprisingly, the onset of the influence of heterochronic blood exchange on myogenesis, neurogenesis and hepatogenesis turned out to be within a few days. Notably, the outcome of heterochronic blood exchange is also different from heterochronic parabiosis, particularly for neurogenesis where our results suggest that old blood is far more inhibitory to tissue health than that young blood is rejuvenative, and that peripheral tissue injury compounds the negative effects of old blood on young neurogenesis. Heterochronic blood exchange enhances old muscle repair without inhibition of young, and old hepatogenesis is improved and fibrosis and adiposity are decreased, while young hepatogenesis becomes diminished. Moreover, our studies demonstrate a rapid increase in beta-2 microglobulin (B2M) in young tissues by old blood; and this phenotype is not from elevated circulating B2M in old mice (as there is none), suggesting that another age-specific systemic molecule raises B2M in the young organs. Blood exchange in small animals enables well-controlled studies with more rapid translation for therapy for humans.