Rodriguez Morales, D.; Larcher, V.; Ruz Jurado, M.; Tombor, L.; Zanders, L.; Wagner, J. U. G.; Zeiher, A. M.; Kuppe, C.; John, D.; Schulz, M. H.; Dimmeler, S.

Background: Aging is a major, yet unmodifiable risk factor for cardiovascular diseases, leading to vascular alterations, increased cardiac fibrosis, and inflammation, all of which contribute to impaired cardiac function. However, the microenvironment inciting age-related alterations withing the multicellular architecture of the cardiac tissue is unknown. Methods: We investigated local microenvironments in aged mice hearts applying an integrative approach combining single-nucleus RNA sequencing and spatial transcriptomics in 12-week-old and 18-month-old mice. We defined distinct cardiac niches and studied changes in their cellular composition and functional characteristics. Results: Integration of spatial transcriptomics data across young and aged hearts allowed us to identify 11 cardiac niches, which were characterized by distinct cellular composition and functional signatures. Aging did not alter the overall proportions of cardiac niches but leads to distinct regional changes, particularly in the left ventricle. Whereas cardiomyocyte-enriched niches show disrupted circadian clock gene expression, vascular niches showed major changes in pro-inflammatory and pro-fibrotic signatures and altered cellular composition. We particularly identified larger vessel-associated cellular niches as key hotspots for activated fibroblasts and macrophages in aged hearts, with interactions of both cell types through the C3:C3ar1 axis. These niches were also enriched in senescence cells exhibiting high expression of immune evasion mechanisms that may impair senescent cell clearance. Conclusion: Our findings indicate that the microenvironment around the vasculature is particularly susceptible to age-related changes and serves as a primary site for inflammation-driven aging, so called \"inflammaging\". This study provides new insights into how aging reshapes cardiac cellular architecture, highlighting vessel-associated niches as potential therapeutic targets for age-related cardiac dysfunction.