Advancing age leads to an accumulation of senescent endothelial cells (ECs) within arteries. Senescent cells have undergone permanent cell cycle arrest, are pro-oxidative and pro-inflammatory, and therefore represent a likely cause of age-related EC dysfunction. Yet, the molecular mechanisms and physiological consequences of EC senescence remain incompletely understood. Telomeres are repeat DNA sequences that cap chromosomes. Telomeres shorten with each cell division and are highly susceptible to oxidative damage. When telomeres become critically short or damaged, they become uncapped, which activates the DNA damage response and leads to cellular senescence. Here, we tested the hypothesis that aging results in EC telomere uncapping that induces senescence, leading to physiological hallmarks of aging. To assess whether aging results in EC telomere uncapping, we compared ECs from young (~3 mo) and old (~27 mo) mice. Aging resulted in ~4-fold greater EC telomere uncapping (p<0.001, Figure 1A). To determine if EC telomere uncapping induces senescence, we deleted the telomere capping protein, TRF2, in ECs of young (~3.5mo) mice (TRF2-ecKO). Compared to wildtype (WT) littermate controls, TRF2-ecKO mice had a ~78% reduction in TRF2 gene expression (p<0.0001). TRF2 deletion reduced EC division by ~47% (p<0.0001, Figure 1B), indicative of senescence. To examine the physiological consequences of EC senescence, we examined hallmarks of vascular aging including perfused microvascular density and endothelium-dependent dilation (EDD). TRF2-ecKO mice had an ~18% reduction in perfused mesenteric microvessels between 5-25 µM (p<0.001, Figure 1C). Likewise, TRF2-ecKO displayed ~26% reduction in mesenteric artery EDD compared to WT mice (p<0.05, Figure 1D). Furthermore, TRF2-ecKO arterial EDD was ameliorated by the superoxide scavenger, TEMPOL (p>0.05, Figure 1D). Endothelium-independent dilation to the exogenous nitric oxide donor sodium nitroprusside was not different between WT and TRF2-ecKO mice (p>0.05). These data suggest EC telomere uncapping leads to senescence that reduces perfused microvascular density, and to elevated oxidative stress that suppresses EDD, similar to advanced age. To assess metabolic and muscle function, we performed an intraperitoneal glucose tolerance test (GTT, 2g/kg body mass) and a forelimb grip strength test. TRF2-ecKO mice had ~21% greater area under the curve during GTT compared to WT mice (p<0.05, Figure 2A) as well as a ~14% reduction in grip strength (p<0.05, Figure2B). Taken together, these data provide evidence that aging results in EC telomere uncapping that induces senescence and age-related physiological dysfunction.© FASEB.