The pattern and mechanism of mitochondrial transport in axons

Mitochondria in nerve axons display motility behavior that is as distinctive as their metabolic function. Unlike many other classes of organelles, mitochondria undergo net movement that is the sum of movements in both the anterograde and retrograde directions, and their net velocity is strongly influenced by their recruitment between stationary and motile states. They recently became the first specific class of organelle shown to be capable of moving along either microtubule or F-actin tracks in the axon, indicating that they probably use a diversity of molecular motors. Although we still know relatively little about how the movement of specific classes of axonal organelles is coordinated with their function in the neuron, in the case of mitochondria it is at least clear that their transport delivers them to regions of the neuron where ATP consumption is likely to be high, and disperses them when local energy needs change. In addition, although mitochondria contain both anterograde and retrograde motor activities, the modulation of their motility necessary to achieve these redistributions seems to rely largely upon regulation of the anterograde motor activity alone. A further element in the regulation of their motility and distribution is the apparent "docking" of mitochondria to microtubules or neurofilaments, a phenomenon which may serve to stabilize their distribution once regulated motility has moved them to appropriate sites. This review considers the current state of knowledge in these areas with an emphasis on the pattern of regulation of motility and how it underlies the role of mitochondria as the aerobic ATP source of the neuron.