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Patients with end-stage renal disease (ESRD) have elevated circulating calcium (Ca) and phosphate (Pi), and exhibit accelerated progression of calcific aortic valve disease (CAVD). We hypothesized that matrix vesicles (MVs) initiate the calcification process in CAVD. Ca induced rat valve interstitial cells (VICs) calcification at 4.5mM (16.4-fold; p < 0.05) whereas Pi treatment alone had no effect. Ca (2.7 mM) and Pi (2.5 mM) synergistically induced calcium deposition (10.8-fold; p < 0.001) in VICs. Ca treatment increased the mRNA of the osteogenic markers Msx2, Runx2, and Alpl (p < 0.01). MVs were harvested by ultracentrifugation from VICs cultured with control or calcification media (containing 2.7mM Ca and 2.5mM Pi) for 16 hr. Proteomics analysis revealed the marked enrichment of exosomal proteins, including CD9, CD63, LAMP-1, and LAMP-2 and a concomitant up-regulation of the Annexin family of calciumbinding proteins. Of particular note Annexin VI was shown to be enriched in calcifying VICderived MVs (51.9-fold; p < 0.05). Through bioinformatic analysis using Ingenuity Pathway Analysis (IPA), the up-regulation of canonical signaling pathways relevant to cardiovascular function were identified in calcifying VIC-derived MVs, including aldosterone, Rho kinase, and metal binding. Further studies using human calcified valve tissue revealed the co-localization of Annexin VI with areas of MVs in the extracellular matrix by transmission electron microscopy (TEM). Together these findings highlight a critical role for VIC-derived MVs in CAVD. Furthermore, we identify calcium as a key driver of aortic valve calcification, which may directly underpin the increased susceptibility of ESRD patients to accelerated development of CAVD. [ABSTRACT FROM AUTHOR]