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

Mitochondria buffer cytosolic Ca 2+ increases following Ca 2+ influx from extracellular spaces, and Ca 2+ release from intracellular Ca 2+ store sites under physiological circumstances. Therefore, close contact of mitochondria with the sarcoplasmic reticulum (SR) is required for maintaining Ca 2+ homeostasis. Mitofusin 2 (Mfn2) localizes in both mitochondrial and SR membranes and is hypothesized to optimize the distance and Ca 2+ 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 Ca 2+ concentration (Ca 2+ cyt ) induced by 100 nM arginine vasopressin (AVP) was not affected by Mfn2 siRNA knockdown, whereas cytosolic Ca 2+ removal was slower after Mfn2 knockdown. Following the AVP-induced Ca 2+ cyt increase, mitochondrial Ca 2+ uptake and Ca 2+ refill into the SR were attenuated by Mfn2 knockdown. In addition, Mfn2-knockdown cells exhibited a loss of mitochondrial membrane potential (ΔΨ mito ) and lower ATP levels in mitochondria. Moreover, Mfn2 knockdown inhibited cell proliferation. In contrast, Mfn2 overexpression increased ΔΨ mito and cell growth. This study strongly suggests that Mfn2 is responsible for SR-mitochondria Ca 2+ signaling by tethering mitochondria to SR, thereby regulating ATP production and proliferation of VSMCs.