Glycogen synthase kinase-3β and the p25 activator of cyclin dependent kinase 5 increase pausing of mitochondria in neurons

The complex bi-directional axoplasmic transport of mitochondria is essential for proper metabolic functioning of neurons and is controlled by phosphorylation. We have investigated by time-lapse imaging the effects of increased expression of glycogen synthase kinase-3β (GSK-3β) and of the p25 activator of cyclin dependent kinase 5 on mitochondria movements in mammalian cortical neurons and in PC12 cells. Both GSK-3β and p25 increased the stationary behaviour of mitochondria in PC12 and in neurons, decreased their anterograde transport but did not affect the intrinsic velocities of mitochondria. The microtubule-associated tau proteins were more phosphorylated in GSK-3β and p25 transfected neurons, but ultrastructural observation showed that these cells still contained microtubules and nocodazole treatment further reduced residual mitochondria movements in GSK-3β or p25 transfected neurons, indicating that microtubule disruption was not the primary cause of increased mitochondrial stationary behaviour in GSK-3β or p25 transfected neurons. Our results suggest that increased expression of GSK-3β and p25 acted rather by decreasing the frequency of mitochondrial movements driven by molecular motors and that GSK-3β and p25 might regulate these transports by controlling the time that mitochondria spend pausing, rather than their velocities.