Axonal transport of mitochondria along microtubules and F-actin in living vertebrate neurons.

TLDR

Microtubules are known to mediate organelle transport in axons, while recent squid axoplasm studies suggest actin microfilaments may also contribute. The study investigates the distinct roles of microtubules and actin microfilaments in mitochondrial transport by monitoring movements in chick sympathetic neurons with selective elimination of each cytoskeletal element. Researchers used cytoskeletal drugs—cytochalasin E to remove actin microfilaments and nocodazole or vinblastine to eliminate microtubules—in both developing neurons and established cultures to selectively disrupt each element and observe mitochondrial movement. Mitochondria moved normally along microtubule-rich neurites lacking actin but failed to enter microtubule-free neurites with actin; disrupting actin increased speed yet reduced net transport, disrupting microtubules slowed movement and favored retrograde bias, and loss of both elements abolished motility, showing that axonal organelle transport can occur along both microtubules and actin filaments but with distinct velocities and net directional outcomes.

Abstract

A large body of evidence indicates that microtubules (MTs) conduct organelle transport in axons, but recent studies on extruded squid axoplasm have suggested that actin microfilaments (MFs) may also play a role in this process. To investigate the separate contributions to transport of each class of cytoskeletal element in intact vertebrate axons, we have monitored mitochondrial movements in chick sympathetic neurons experimentally manipulated to eliminate MTs, MFs, or both. First, we grew neurons in the continuous presence of: (a) cytochalasin E to create neurites which had never contained MFs; or (b) nocodazole or vinblastine to produce neurites which had never contained MTs. Mitochondria moved bidirectionally at normal velocities along the length of neurites which contained MTs and lacked MFs, but did not even enter neurites grown without MTs but containing MFs. In a second approach, we treated established neuronal cultures with cytoskeletal drugs to disrupt either MTs or MFs in axons already containing mitochondria. In cytochalasin-treated cells, which retained MTs but lacked MFs, average mitochondrial velocity increased in both directions, but net directional transport decreased. In vinblastine-treated cells, which lacked MTs but retained essentially normal levels of MFs, mitochondria continued to move bidirectionally but the average mitochondrial velocity and excursion length were reduced for both directions of movement, and the mitochondria spent threefold as much time moving in the retrograde as in the anterograde direction, resulting in net retrograde transport. Treatment of established cultures with both drugs produced neurites lacking MTs and MFs but still rich in neurofilaments; these showed a striking absence of any mitochondrial motility. These data indicate that axonal organelle transport can occur along both MTs and MFs in vivo, but with different velocities and net transport properties.