Abstract: Heterochronic parabiosis and serum transfer models have given evidence for the rejuvenating properties of young blood in old mice and equally potent deleterious effects of old blood in young mice. The critical factors or even tissues responsible for such effects remained elusive due, in part, to the complexity of the parabiosis models. Taking an organ-specific approach may lead to better understanding of how individual tissues contribute to the effects we have seen with parabiosis. Since the liver is a critical organ regulating whole-body metabolism and is the primary source of secreted proteins in circulation, it is an intriguing target to manipulate its biological age and determine resulting functional and molecular changes. To alter the biological age of the liver, we will utilize the heterochronic hepatocyte transplantation technique to incorporate young cells in the aged liver, and vice versa. Isochronic hepatocyte transplantation and non-surgical groups will serve as controls. We have generated preliminary data showing successful transplantation into both young and old livers and see whole-body effects on adiposity to validate efficacy of the model. We hypothesize the donor cells will engraft and influence the host liver to take on either, a more youthful or accelerated aging state based on the donor cell age. Moreover, we hypothesize that transplanted cells will contribute to the liver secretome and mimic profiles of the young and old plasma proteome. We will test these hypotheses through the following aims. Aim1 will test the hypothesis that transplanted cells will alter the host liver function, cellular landscape and biological age. We will measure clinical markers of liver function, including AST, ALT and BUN after heterochronic transplantation. In addition, we will use the recently developed 10xGenomics Xenium Spatial transcriptomic platform to determine how this procedure affects the cellular landscape of the liver, while using epigenetic clocks as an indices of biological age of the tissue. Aim2 will test the hypothesis that donor hepatocytes will alter host liver secretome dependent on the donor cell age. Hepatocytes-derived secreted proteins will be measured using the MetRS mouse model to label proteins in the plasma and compare the plasma proteome across groups. The proposed studies will fully test the hypothesis that donor cells will engraft into the host liver and influence tissue function, cellular phenotype and secretome, which may mimic the effects of heterochronic blood sharing.