In neurons, mitochondria are synthesized in the cell body and transported down the axon towards the growth cone. But they can also undergo retrograde transport in the opposite direction. So what determines which way a mitochondrion goes? And how does it know where to stop? Kyle Miller and Michael Sheetz have used kymographic analyses to track movement of mitochondria along axons over several hours to tackle these questions (see ). They find that mitochondria are uniformly distributed in axons and tend to stop at the gaps between stationary mitochondria, speculating that the docked organelles release a signal that prevents other mitochondria stopping nearby. The authors then use inhibitors of the electron transport chain to demonstrate that the direction in which mitochondria move correlates with their mitochondrial potential: mitochondria that display high potentials move towards the growth cone, whereas those that are depolarized move towards the cell body. This presumably ensures that old or damaged mitochondria further down the axon are returned to the cell body for autophagy and efficiently replaced with new, fully functional mitochondria.
