Vascular Smooth Muscle Myosin 2 Filaments Dynamically Assemble and Stabilize During Induced Contractility

BACKGROUND: Vascular smooth muscle cells (SMCs) dynamically tune blood vessel diameter to regulate blood pressure, provide vessel wall structural integrity, and absorb shock on a beat-to-beat timescale. SMII (smooth muscle myosin 2) is the dominant motor protein driving SMC contraction. To function, SMII monomers dynamically assemble into filaments, which associate with the actin cytoskeleton to drive contractility. Precisely how SMII filaments assemble and exchange in living SMCs, however, both at steady state and during induced contractility, remains poorly defined. METHODS: We used a single-cell filament assembly assay to determine SMII assembly into filaments at steady state and upon induced contractility in rat aortic SMCs (A7r5) transiently expressing EGFP (enhanced green fluorescent protein)-tagged SM1A (SMII isoform 1A). We then used fluorescence recovery after photobleaching (FRAP) to characterize SMII exchange kinetics at steady state and upon induced contractility, and measured changes in force production using traction force microscopy. Finally, we developed a knock-in EGFP-SMII murine model to quantify SMII dynamics at endogenous expression in primary SMCs and intact arterioles. RESULTS: While predominantly filamentous at baseline, induced contraction rapidly increased SMII filament assembly. Fluorescence recovery after photobleaching revealed rapid SMII exchange kinetics, more similar to nonmuscle myosin II than striated myosin II, and induced contractility consistently stabilized SMII filaments. Super-resolution imaging revealed SMII and nonmuscle myosin II filament structures consistent with coassembly. Endogenous EGFP-SMII in primary SMCs and intact arterioles paralleled cell culture studies with similar baseline exchange kinetics and activation-dependent stabilization. CONCLUSIONS: Together, these data support a model in which SMII is surprisingly dynamic and coassembles with nonmuscle myosin II. Vascular SMC activation further increases SMII filament assembly while reducing filament exchange, consistent with stabilization of a dynamic SMII pool during force generation, thereby allowing cells to dynamically adapt their overall contractility in response to environmental conditions.