Miro's GTPase Domains Execute Anterograde and Retrograde Axonal Mitochondrial Transport and Control Morphology

Abstract

Microtubule-based mitochondrial transport into dendrites and axons is vital for sustaining neuronal function. Transport along microtubules proceeds in a series of plus- and minus-end directed movements facilitated by kinesin and dynein motors. How the opposing movements are controlled to achieve effective long distance transport remains unclear. Previous studies showed that the conserved mitochondrial GTPase Miro is required for mitochondrial transport into axons and dendrites. To directly examine Miro's significance for kinesin- and/or dynein-mediated mitochondrial motility, we live imaged movements of GFP-tagged mitochondria in larval Drosophila motor axons upon genetic manipulations of Miro. Loss of Drosophila Miro (dMiro) reduced the effectiveness of either antero- or retrograde mitochondrial transport by selectively impairing kinesin- or dynein-mediated movements, depending on the direction of net transport. In both cases, the duration of short stationary phases increased proportionally. Overexpression (OE) of dMiro also impaired the effectiveness of mitochondrial transport. Finally, loss and OE of dMiro altered the length of mitochondria in axons through a mechanistically separate pathway. We concluded that dMiro promotes effective antero- and retrograde mitochondrial transport by extending the processivity of kinesin and dynein motors according to a mitochondrion's programmed direction of transport. To determine how Miro achieves this control mechanistically, we introduced point mutations that render each GTPase either constitutively active or inactive. Expression of either first GTPase mutant impaired antero- (inactive) or retrograde motor movements (active) in a direction dependent manner. The active state of the second GTPase domain up-regulated the number of consecutive kinesin motions during anterograde transport but impaired kinesin transport biases while the inactive second GTPase state impaired transport in either direction. Together, these data suggest that Miro's first GTPase domain is major factor that controls the execution of either the antero- or retrograde directional program while Miro's second GTPase may provide a signal that supports or disfavors transport. In addition, the active state of the first and the second GTPase domain increased the length of stationary mitochondria but only the first GTPase domain modified motile mitochondrial lengths. Overexpression of these mutations generated opposing effects. We conclude that both domains control antero- and retrograde transport in a switch-like manner.