The node of Ranvier influences the in vivo axonal transport of mitochondria and signalling endosomes

Abstract

Efficient long-range axonal transport is essential for maintaining neuronal function, and perturbations in this process underlie severe neurological diseases. We have previously demonstrated that signalling endosomes are transported in vivo at comparable speeds across motor neurons (MNs) innervating different hindlimb muscles, as well as between forelimb and hindlimb peripheral nerves. In contrast, axonal transport is faster in MNs compared to sensory neurons innervating the same muscle. Found periodically across the myelin sheath, Nodes of Ranvier (NoR) are short uncovered axonal domains that facilitate action potential propagation. Currently, it remains unresolved how the distinct molecular structures of the NoR impact axonal transport dynamics. Here, using intravital time-lapse microscopy of sciatic nerves in live, anaesthetised mice, we assessed diverse organelle dynamics at the NoR. We first observed that axonal morphologies were similar between fast and slow MNs, and found that signalling endosomes and mitochondria accumulate on the distal side of the NoR in both motor neuron subtypes. Assessment of axonal transport of signalling endosomes and mitochondria revealed a decrease in velocity and increase in pausing as the organelles transit through the NoR, followed by an increase in speed in the adjacent intranodal region. Collectively, this study has established axonal transport dynamics of two independent organelles at the NoR in vivo, and has relevance for several pathologies affecting peripheral nerves and the NoR, such as peripheral neuropathy, motor neuron diseases, and/or multiple sclerosis.