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

Abstract

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 Ca(2+) mobilization and contraction remains poorly understood. Recent evidence shows that autophagy is involved in controlling contractile function and Ca(2+) homeostasis in certain cell types. Therefore, autophagy might also regulate contractile capacity and Ca(2+)-mobilizing pathways in VSMCs. Contractility (organ chambers) and Ca(2+) 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) SM22α-Cre(+) mice) and in segments of corresponding control mice (Atg7(+/+) SM22α-Cre(+)). Our results indicate that voltage-gated Ca(2+) channels (VGCCs) of Atg7(fl/fl) SM22α-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) SM22α-Cre(+) VSMCs was enlarged, which, combined with increased sarco(endo)plasmic reticulum Ca(2+)-ATPase 2 expression and higher store-operated Ca(2+) entry, promoted inositol 1,4,5-trisphosphate-mediated contractions of Atg7(fl/fl) SM22α-Cre(+) segments and maximized the Ca(2+) storing capacity of the sarcoplasmic reticulum. Moreover, decreased plasma membrane Ca(2+)-ATPase expression in Atg7(fl/fl) SM22α-Cre(+) VSMCs hampered Ca(2+) extrusion to the extracellular environment. Overall, our study indicates that defective autophagy in VSMCs leads to an imbalance between Ca(2+) release/influx and Ca(2+) reuptake/extrusion, resulting in higher basal Ca(2+) concentrations and significant effects on vascular reactivity.

Contractile Behavior of Mouse Aorta Depends on SERCA2 Isoform Distribution: Effects of Replacing SERCA2a by SERCA2b

“…Furthermore, the spontaneous leakage of contractile Ca 2+ from the SR assessed by PE phasic contractions upon various duration of 0 mM Ca 2+ challenges consistently showed that the AUC of the phasic contraction was smaller in SERCA2 b/b mice suggestive for a lower contractile Ca 2+ content of the SR stores. Additionally, segments of SERCA2 b/b mice spontaneously lose their contractile Ca 2+ slower in function of the duration of the 0 mM Ca 2+ challenge, suggesting an altered ratio of Ca 2+ release and removal (Michiels et al, 2015).…”

Functional variants of the ATG7 gene promoter in acute myocardial infarction

“…Accumulating evidence suggests that ATG7 and autophagy play protective roles in cardiomyocytes, vascular endothelial cells, and smooth muscle cells (Michiels et al, 2015;Pattison et al, 2011;Torisu et al, 2016). Autophagy is an innate and potent process that protects cardiomyocytes from ischemic death during acute myocardial infarction in animal experiments (Orogo & Gustafsson, 2015).…”

“…In neonatal rat cardiomyocytes, ATG7 induces basal autophagy and attenuates the accumulation of misfolded proteins and aggregates (Pattison, Osinska, & Robbins, 2011). In vascular smooth muscle cells, ATG7 gene deletion alters contractility and Ca² + homeostasis (Michiels, Fransen, De Munck, De Meyer, & Martinet, 2015). In endothelial cells, ATG7 gene deletion results in higher levels of intracellular lipid accumulation (Torisu et al, 2016).…”

Protoporphyrin IX-mediated sonodynamic therapy promotes autophagy in vascular smooth muscle cells

“…Recently, Michiels et al , reported that PpIX-SDT promoted VSMC phenotype transformation from a dedifferentiated to a differentiated status (14), which partially explained why SDT inhibited in-stent restenosis in animal models (4). PpIX has low toxicity and a short dark period in the cells, as compared with other sonosensitizers (15,16).…”

“…PpIX has low toxicity and a short dark period in the cells, as compared with other sonosensitizers (15,16). In addition, Michiels et al (14) showed that PpIX-SDT had no effect on VSMC viability. Consistent with the above findings, we did not observe that PpIX-SDT altered the viability of VSMCs, despite having increased the ultrasound intensity to 1.0 W/cm 2 .…”