Mitochondrial Motility During Vascular Smooth Muscle Proliferation

Abstract

Mitochondrial function is critical for multiple facets of cellular function, including ATP production and Ca2+ handling. Subcellular location of the organelle is important for function. For example, in smooth muscle, mitochondria modulate Ca2+ signals arising from the inositol-trisphosphate-sensitive channels (IP3R), even at the level of Ca2+ puffs, revealing a close physical relationship between IP3R and mitochondria. This relationship seems incompatible with rapid free movement of mitochondria-observed in several cell types. Here we report mitochondria in freshly-isolated smooth muscle cells lacked directed motion and Brownian-like movement was limited so that no displacement of individual organelles occurred. The movement did not change by disruption of actin polymerisation (latrunculin-B, 10 μM), inhibition of microtubule polymerisation (nocodazole, 10 μM) or removal of Ca2+ from the extra-cellular solution. In intact pressurised (40 mmHg) cerebral arteries, individual mitochondria within the smooth muscle were largely stationary; however in a small but significant number of cells mitochondria displayed directed movements. When these arteries were maintained in conditions which promote cell proliferation, an increased number of cells showing moving mitochondria and an increase in the extent of movement occurred. We postulated that those smooth muscle cells displaying motile mitochondria may be proliferative, as downregulation of the mitochondrial tether mitofusin-2 has been reported in proliferative vascular disease. In support, within days of allowing freshly-isolated cerebral artery smooth muscle cells to proliferate in culture, extensive mitochondrial motility developed. A spectrum of movements was observed: long- and short-distance directed movements, wiggling, looping, extension, retraction, and Brownian-like diffusion. Therefore, when isolated smooth muscle cells are allowed to proliferate, mitochondria switch from being static, with no observed motion (that suggests physical confinement), to being highly mobile and unconfined organelles. This increased mitochondrial mobility may contribute to the altered Ca2+ signalling observed in proliferative smooth muscle.