Response of mitochondrial traffic to axon determination and differential branch growth

Mitochondria are concentrated in regions of the neuron where the demand for mitochondrial function is high, such as nodes of Ranvier, synapses, and active growth cones. Does mitochondrial transport respond to changes in neuronal energy consumption and architecture, or does it precede and perhaps predict them? We have used axon determination, elongation, and alternating branch growth in hippocampal neurons to analyze the cellular cues that control mitochondrial traffic. During the stage 2-3 transition, when one minor process becomes the axon and accelerates its growth, mitochondria do not uniformly cluster at the base of the prospective axon. There is increased entry of mitochondria into the nascent axon, but this does not require accumulation near the axon. After axonal elongation is under way, the mitochondrial density of the minor processes decreases. Axonal towing experiments showed that elongation alone does not result in transport of mitochondria into the axon; thus, cytoplasmic flow cannot explain the entry of mitochondria into growing axons. Analysis of mitochondrial transport during alternating growth of axonal branches showed that mitochondrial traffic responds to changes in growth through regulation of entry into, but not exit from, branches. Branch-towing experiments showed that this response is not caused by axonal elongation alone, nor does it require an active growth cone. We propose that mitochondrial traffic in axons responds to changes in axonal outgrowth, and that the mechanism by which sorting at branch points occurs is different from the mechanism responsible for concentrating mitochondria at the growth cone.