Transport of neurofilaments in growing axons requires microtubules but not actin filaments

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Abstract
Neurofilament (NF) polymers are conveyed from cell body to axon tip by slow axonal transport, and disruption of this process is implicated in several neuronal pathologies. This movement occurs in both anterograde and retrograde directions and is characterized by relatively rapid but brief movements of neurofilaments, interrupted by prolonged pauses. The present studies combine pharmacologic treatments that target actin filaments or microtubules with imaging of NF polymer transport in living axons to examine the dependence of neurofilament transport on these cytoskeletal systems. The heavy NF subunit tagged with green fluorescent protein was expressed in cultured sympathetic neurons to visualize NF transport. Depletion of axonal actin filaments by treatment with 5 μM latrunculin for 6 hr had no detectable effect on directionality or transport rate of NFs, but frequency of movement events was reduced from 1/3.1 min of imaging time to 1/4.9 min. Depolymerization of axonal microtubules using either 5 μM vinblastine for 3 hr or 5 μg/ml nocodazole for 4–6 hr profoundly suppressed neurofilament transport. In 92% of treated neurons, NF transport was undetected. These observations indicate that actin filaments are not required for neurofilament transport, although they may have subtle effects on neurofilament movements. In contrast, axonal transport of NFs requires microtubules, suggesting that anterograde and retrograde NF transport is powered by microtubule‐based motors.