Local Ca2+ coupling between mitochondria and sarcoplasmic reticulum following depolarization in guinea pig urinary bladder smooth muscle cells

Optogenetic Calcium Ion Influx in Myoblasts and Myotubes by Near-Infrared Light Using Upconversion Nanoparticles

Bioactuators made of cultured skeletal muscle cells are generally driven by electrical or visible light stimuli. Among these, the technology to control skeletal muscle consisting of myoblasts genetically engineered to express photoreceptor proteins with visible light is very promising, as there is no risk of cell contamination by electrodes, and the skeletal muscle bioactuator can be operated remotely. However, due to the low biopermeability of visible light, it can only be applied to thin skeletal muscle films, making it difficult to realize high-power bioactuators consisting of thick skeletal muscle. To solve this problem, it is desirable to realize thick skeletal muscle bioactuators that can be driven by near-infrared (NIR) light, to which living tissue is highly permeable. In this study, as a promising first step, upconversion nanoparticles (UCNPs) capable of converting NIR light into blue light were bound to C2C12 myoblasts expressing the photoreceptor protein channelrhodopsin-2 (ChR2), and the myoblasts calcium ion (Ca2+) influx was remotely manipulated by NIR light exposure. UCNP-bound myoblasts and UCNP-bound differentiated myotubes were exposed to NIR light, and the intracellular Ca2+ concentrations were measured and compared to myoblasts exposed to blue light. Exposure of the UCNP-bound cells to NIR light was found to be more efficient than exposure to blue light in terms of stimulating Ca2+ influx.

Mitofusin 1 and 2 differentially regulate mitochondrial function underlying Ca2+ signaling and proliferation in rat aortic smooth muscle cells

Mitochondria play a substantial role in cytosolic Ca²⁺ buffering and energy metabolism. We recently demonstrated that mitofusin 2 (Mfn2) regulated Ca²⁺ signaling by tethering mitochondria and sarcoplasmic reticulum (SR), and thus, facilitated mitochondrial function and the proliferation of vascular smooth muscle cells (VSMCs). However, the physiological role of mitofusin 1 (Mfn1) on Ca²⁺ signaling and mitochondrial function remains unclear. Herein, the roles of Mfn1 and Mfn2 in mitochondrial function underlying Ca²⁺ signaling, ATP production, and cell proliferation were examined in rat aortic smooth muscle A10 cells. Following an arginine vasopressin-induced increase in cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt), Mfn2 siRNA (siMfn2) reduced cytosolic Ca²⁺ removal and mitochondrial Ca²⁺ uptake. However, Mfn1 siRNA (siMfn1) attenuated mitochondrial Ca²⁺ uptake, facilitated Ca²⁺ removal from mitochondria, and resulted in increased [Ca²⁺]_cyt, which was mediated by the downregulation of mitochondrial Ca²⁺ uniporter (MCU) expression and the upregulation of mitochondrial Na⁺/Ca²⁺ exchanger (NCLX) expression. Furthermore, siMfn1 increased the mitochondrial membrane potential, ATP production by adenine nucleotide translocase (ANT), and cell proliferation, whereas siMfn2 exhibited the opposite responses. In conclusion, Mfn1 modulates the expressions of MCU, NCLX, and ANT, and Mfn2 tethers mitochondria to SR, which demonstrates their different mitochondrial functions for Ca²⁺ signaling, ATP production, and the proliferation of VSMCs.

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 (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 Ca²⁺ signaling by tethering mitochondria to SR, thereby regulating ATP production and proliferation of VSMCs.

Reciprocal Relationship between Ca2+ Signaling and Ca2+-Gated Ion Channels as a Potential Target for Drug Discovery

Cellular Ca²⁺ signaling functions as one of the most common second messengers of various signal transduction pathways in cells and mediates a number of physiological roles in a cell-type dependent manner. Ca²⁺ signaling also regulates more general and fundamental cellular activities, including cell proliferation and apoptosis. Among ion channels, Ca²⁺-permeable channels in the plasma membrane as well as endo- and sarcoplasmic reticulum membranes play important roles in Ca²⁺ signaling by directly contributing to the influx of Ca²⁺ from extracellular spaces or its release from storage sites, respectively. Furthermore, Ca²⁺-gated ion channels in the plasma membrane often crosstalk reciprocally with Ca²⁺ signals and are central to the regulation of cellular functions. This review focuses on the physiological and pharmacological impact of i) Ca²⁺-gated ion channels as an apparatus for the conversion of cellular Ca²⁺ signals to intercellularly propagative electrical signals and ii) the opposite feedback regulation of Ca²⁺ signaling by Ca²⁺-gated ion channel activities in excitable and non-excitable cells.