Mitofusin 2 positively regulates Ca2+ signaling by tethering the sarcoplasmic reticulum and mitochondria in rat aortic smooth muscle cells

Mitochondria buffer cytosolic Ca2+ increases following Ca2+ influx from extracellular spaces, and Ca2+ release from intracellular Ca2+ store sites under physiological circumstances. Therefore, close contact of mitochondria with the sarcoplasmic reticulum (SR) is required for maintaining Ca2+ homeostasis. Mitofusin 2 (Mfn2) localizes in both mitochondrial and SR membranes and is hypothesized to optimize the distance and Ca2+ transfer between these organelles. However, the physiological significance of Mfn2 in vascular smooth muscle cells (VSMCs) is poorly understood. In the present study, the role of Mfn2 in the physical and functional couplings between SR and mitochondria was examined in rat aortic smooth muscle cells (rASMCs) by confocal and electron microscope imaging. When Mfn2 was knocked down using siRNA in rASMCs, the mean distance between these organelles was extended from 16.2 to 21.6 nm. The increase in the cytosolic Ca2+ concentration ([Ca2+]cyt) induced by 100 nM arginine vasopressin (AVP) was not affected by Mfn2 siRNA knockdown, whereas cytosolic Ca2+ removal was slower after Mfn2 knockdown. Following the AVP-induced [Ca2+]cyt increase, mitochondrial Ca2+ uptake and Ca2+ refill into the SR were attenuated by Mfn2 knockdown. In addition, Mfn2-knockdown cells exhibited a loss of mitochondrial membrane potential (ΔΨmito) and lower ATP levels in mitochondria. Moreover, Mfn2 knockdown inhibited cell proliferation. In contrast, Mfn2 overexpression increased ΔΨmito and cell growth. This study strongly suggests that Mfn2 is responsible for SR-mitochondria Ca2+ signaling by tethering mitochondria to SR, thereby regulating ATP production and proliferation of VSMCs.