DrosophilaMiro Is Required for Both Anterograde and Retrograde Axonal Mitochondrial Transport

Abstract

Microtubule-based transport of mitochondria into dendrites and axons is vital for sustaining neuronal function. Transport along microtubule tracks proceeds in a series of plus and minus end-directed movements that are facilitated by kinesin and dynein motors. How the opposing movements are controlled to achieve effective transport over large distances remains unclear. Previous studies showed that the conserved mitochondrial GTPase Miro is required for mitochondrial transport into axons and dendrites and serves as a Ca 2ϩ sensor that controls mitochondrial mobility. 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 both anterograde and retrograde mitochondrial transport by selectively impairing kinesin-or dynein-mediated movements, depending on the direction of net transport. Net anterogradely transported mitochondria exhibited reduced kinesin-but normal dynein-mediated movements. Net retrogradely transported mitochondria exhibited much shorter dynein-mediated movements, whereas kinesin-mediated movements were minimally affected. 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 suggest 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.

Trafficking Kinesin Protein (TRAK)-mediated Transport of Mitochondria in Axons of Hippocampal Neurons

“…Furthermore, overexpression of Miro enhanced the fusion state of mitochondria under basal Ca 2ϩ concentrations, again reinforcing the concept of cross-talk between mitochondrial transport and fusion/fission mechanisms (31). Evidence suggests that Miro is not directly involved in fusion or fission but that it may modulate these processes (29,31). Because of the known association between Miro and TRAKs, this implicates a role for TRAKs in this modulation.…”

“…Association of TRAKs with dynein has been investigated by immunoprecipitations from brain extracts; however, no dynein was detected in anti-TRAK1/2 immune pellets (12). In Drosophila, however, it was found that Miro (and by implication TRAKs/Milton) was required for both anterograde and retrograde transport of axonal mitochondria (29). In investigating mediators of mitochondrial transport, some published reports do not discriminate between the anterograde and retrograde modes of transport, focusing only on the fact that there is an overall reduction in mobility (e.g.…”

Principles of the mitochondrial fusion and fission cycle in neurons

“…Our results support this hypothesis with quantitative data demonstrating that mitochondria entering fusion show twofold higher motility compared with their nonfusing sisters. Moreover, our results demonstrate that direct slowing of mitochondrial motility by suppressing mitochondrial Miro proteins [required for mitochondrial antero-and retrograde transport (Russo et al, 2009)] or by overexpressing the axonal docking protein syntaphilin inhibit the rate of mitochondrial fusion.…”

The axonal transport of mitochondria

“…The transport of mitochondria ( Fig. 1) is particularly important because mitochondrial functions are needed throughout the axon and their transport can be biased strongly in either direction, depending on whether axons grow or retract, and on specific signaling stimuli (Chada and Hollenbeck, 2004;Chang et al, 2006;Morris and Hollenbeck, 1993;Pilling et al, 2006;Russo et al, 2009).…”

“…During longdistance transport of mitochondria, continuous runs are interspersed with frequent brief pauses and direction changes; nevertheless, individual mitochondria are biased strongly toward runs in a primary direction (Misgeld et al, 2007;Morris and Hollenbeck, 1993;Pilling et al, 2006). For example, only 2-10% of the movement of mitochondria in axons in the intact Drosophila nervous system is in the 'reverse' direction for either anterograde or retrograde transport, and complete reversals in primary direction are rare (Barkus et al, 2008;Pilling et al, 2006;Russo et al, 2009;Shidara and Hollenbeck, 2010).…”