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Mitochondria are dynamic organelles that constantly undergo opposing fission and fusion events which impact many aspects of mitochondrial and cellular homeostasis including bioenergetic activity, calcium buffering and organelle transport. The large GTPase dynamin-related protein 1 (Drp1) acts as a mechanoenzyme to catalyze fission of mitochondria. Drp1 activity is regulated through a series of reversible posttranslational modifications. Phosphorylation of the conserved serine residue, S656, by cAMP dependent protein kinase A (PKA) acts as a master regulator of Drp1 activity. Two phosphatases oppose PKA by dephosporylating Drp1 S656, a mitochondrial isoform of protein phosphatase 2A and the calcium-calmodulin dependent phosphatase calcineurin (CaN). Here I report the characterization of a conserved CaN docking site on Drp1, an LxVP motif, just upstream of the Drp1 S656 site. Mutational modification of the Drp1 LxVP motif resulted in selective bidirectional modulation of formation of the CaN:Drp1 complex. Stability of the CaN:Drp1 LxVP motif mutant complexes was qualitatively described by affinity purification and quantitatively described by isothermal titration calorimetry. Stability of the CaN:Drp1 complex was found to directly correlate with Drp1 S656 dephosphorylation kinetics as demonstrated by studies conducted in vitro and in intact cells. Further, the CaN:Drp1 signaling axis was shown to shape basal mitochondrial morphology in a heterologous cell line system and in primary hippocampal neurons. Finally, disruption of the CaN:Drp1 signaling axis was found to protect neurons from oxygen-glucose deprivation, an in vitro model of ischemic injury. While these results suggest that the CaN:Drp1 signaling axis may be a potential target for neuroprotective therapeutic exploitation, the mechanism by which disruption of the CaN:Drp1 signaling axis specifically and mitochondrial elongation generally results in resistance to ischemic injury remains unknown.
Additional studies reported here demonstrate that mitochondrial fragmentation remains a prominent feature of injured neurons regardless of the fidelity of the CaN:Drp1 signaling axis. Mitochondrial fragmentation at the time of injury was found to occur in a Drp1-independent manner. Chronic mitochondrial elongation was also found to leave unaltered the ability of neurons to detoxify reactive oxygen species, buffer intracellular calcium and supply ATP for homeostatic function.