In airway smooth muscle (ASM) cells, excitation-contraction coupling is accomplished via a cascade of events that connect an elevation of cytosolic Ca2+ concentration ([Ca2+]cyt) with cross-bridge attachment and ATP-consuming mechanical work. Excitation-energy coupling is mediated by linkage of the elevation of [Ca2+]cyt to an increase in mitochondrial Ca2+ concentration, which in turn stimulates ATP production. Proximity of mitochondria to the sarcoplasmic reticulum (SR) and plasma membrane is thought to be an important mechanism to facilitate mitochondrial Ca2+ uptake. In this regard, mitochondrial movement in ASM cells may be key in establishing proximity. Mitochondria also move where ATP or Ca2+ buffering is needed. Mitochondrial movement is mediated through interactions with the Miro-Milton molecular complex, which couples mitochondria to kinesin motors at microtubules. We examined mitochondrial movement in human ASM cells and hypothesized that, at basal [Ca2+]cyt levels, mitochondrial movement is necessary to establish proximity of mitochondria to the SR and that, during the transient increase in [Ca2+]cyt induced by agonist stimulation, mitochondrial movement is reduced, thereby promoting transient mitochondrial Ca2+ uptake. We further hypothesized that airway inflammation disrupts basal mitochondrial movement via a reduction in Miro and Milton expression, thereby disrupting the ability of mitochondria to establish proximity to the SR and, thus, reducing transient mitochondrial Ca2+ uptake during agonist activation. The reduced proximity of mitochondria to the SR may affect establishment of transient "hot spots" of higher [Ca2+]cyt at the sites of SR Ca2+ release that are necessary for mitochondrial Ca2+ uptake via the mitochondrial Ca2+ uniporter.
Keywords: Milton; Miro; TNFα; airway; asthma; mitochondria.
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