Defective autophagy in vascular smooth muscle cells alters contractility and Ca2+ homeostasis in mice

Autophagy is an evolutionary preserved process that prevents the accumulation of unwanted cytosolic material through the formation of autophagosomes. Although autophagy has been extensively studied to understand its function in normal physiology, the role of vascular smooth muscle (SM) cell (VSMC) autophagy in Ca2+ mobilization and contraction remains poorly understood. Recent evidence shows that autophagy is involved in controlling contractile function and Ca2+ homeostasis in certain cell types. Therefore, autophagy might also regulate contractile capacity and Ca2+-mobilizing pathways in VSMCs. Contractility (organ chambers) and Ca2+ homeostasis (myograph) were investigated in aortic segments of 3.5-mo-old mice containing a SM cell-specific deletion of autophagy-related 7 (Atg7; Atg7(fl/fl) SM22 alpha-Cre(+) mice) and in segments of corresponding control mice (Atg7(+/+) SM22 alpha-Cre(+)). Our results indicate that voltage-gated Ca2+ channels (VGCCs) of Atg7(fl/fl) SM22 alpha-Cre(+) VSMCs were more sensitive to depolarization, independent of changes in resting membrane potential. Contractions elicited with K+ (50 mM) or the VGCC agonist BAY K8644 (100 nM) were significantly higher due to increased VGCC expression and activity. Interestingly, the sarcoplasmic reticulum of Atg7(fl/fl) SM22 alpha-Cre(+) VSMCs was enlarged, which, combined with increased sarco(endo) plasmic reticulum Ca2+-ATPase 2 expression and higher store-operated Ca2+ entry, promoted inositol 1,4,5-tris-phosphate-mediated contractions of Atg7(fl/fl) SM22 alpha-Cre(+) segments and maximized the Ca2+ storing capacity of the sarcoplasmic reticulum. Moreover, decreasedplasma membrane Ca2+-ATPase expression in Atg7(fl/fl) SM22 alpha-Cre(+) VSMCs hampered Ca2+ extrusion to the extracellular environment. Overall, our study indicates that defective autophagy in VSMCs leads to an imbalance between Ca2+ release/influx and Ca2+ reuptake/extrusion, resulting in higher basal Ca2+ concentrations and significant effects on vascular reactivity.