Agonist-induced regulation of mitochondrial and endoplasmic reticulum motility

Using fluorescently tagged markers for organelles in conjunction with confocal microscopy, we have studied the effects of agonist-induced Ca²⁺ signals on the motility of mitochondria and the ER (endoplasmic reticulum). We observed that the muscarinic agonist carbachol produced a rapid, simultaneous and reversible cessation of the movements of both organelles, which was dependent on a rise in cytosolic Ca²⁺. This rise in Ca²⁺ was shown to cause a fall in cellular ATP levels, and the effect of carbachol on organelle movement could be mimicked by depleting ATP with metabolic inhibitors in the absence of any such rise in Ca²⁺. However, a Ca²⁺-sensing process independent of ATP appears also to be involved, because we identified conditions where the ATP depletion was blocked (by inhibitors of the Ca²⁺ pumps), but the organelle movements still ceased following a rise in cytosolic Ca²⁺. We conclude that the co-ordinated cessation of mitochondria and ER motility is a process regulated by the cytosolic concentration of both Ca²⁺ and ATP, and that these two parameters are likely to synergize to regulate the localization of the two organelles, and to facilitate the transfer of Ca²⁺ between them.