The role of endomembrane homeostasis in vascular calcification

Abstract Background Cardiovascular diseases are a leading cause of mortality, with cardiovascular calcification as a prominent predictor and contributor. Microcalcifications in thin fibrous caps of atherosclerotic plaques create biomechanical instability, leading to plaque vulnerability. Recent work indicated that cellular-derived extracellular vesicles (EVs) are pivotal for microcalcification nucleation. EVs can originate from the endolysosomal system, and FYVE-Type Zinc Finger Containing Phosphoinositide Kinase (PIKfyve), a lipid kinase, plays a key role in the endolysosomal maturation. Purpose We hypothesize that alterations in endomembrane homeostasis will modulate EV cargo and, thereby, vascular calcification. Methods and Results In calcified arteries and vascular smooth muscle cells (SMCs), we observed increased PIKfyve expression. Phosphatidylinositol 3-phosphate (PI3P) - the substrate of PIKfyve - was decreased in cellular membranes of calcifying SMCs (-40%) and recovered by Apilimod, a small molecule PIKfyve inhibitor. Apilimod prevented matrix mineralization and collagen network formation in calcifying SMCs (Fig. 1A) accompanied by reduced pro-collagen 1A1 secretion (-90%). Apilimod inhibited tissue non-specific alkaline phosphatase (TNAP) – an early marker of SMC calcification on mRNA, protein and activity level. PIKfyve silencing using siRNA showed similar results. Furthermore, Apilimod caused a dose-dependent increase of EV release assessed by nanoparticle tracking analysis (Fig. 1B). Analysis of the EV cargo demonstrated that Apilimod reduces TNAP protein abundance and activity (-60%) - a characteristic cargo that may determine EV calcification potential. Apilimod-induced EVs exhibited lower mineral content and reduced aggregation potential assessed by osteosense-based flow cytometry and turbidity assays. Next, we performed transcriptome and kinome analyses to discover the underlying mechanism of PIKfye-mediated SMC calcification. These data revealed a link to adipocyte-like differentiation and SMC phenotype specifying pathways. Validation supported increased expression of adipogenic transcription factors (CEBPA, PPARG),genes of the cholesterol and fatty acid metabolism pathways (FABP3, CD36) and enhanced fatty acid (C12) uptake (Fig. 1A) by Apilimod; while SMC-specific markers (ACTA2, SM22) and key SMC transcription factors (GATA6, MYOCD) were downregulated. Finally, in Ldlr-deficient mice fed a high-fat, high-cholesterol diet for 15 weeks, Apilimod application for 5 weeks increased PI3P levels (1.6-fold) and decreased vascular calcification (Fig. 1C), but did not alter plaque size or collagen content in atherosclerotic plaques. Conclusion Disrupting endolysosomal maturation with Apilimod promotes the release of EVs with reduced calcification potential and induces a phenotypic adaption towards adipocyte-like SMCs, causing reduced SMC calcification. Anti-arteriosclerotic effects of Apilimod in vivo remain to be further investigated.Figure