The Axon Initial Segment, 50Years Later: A Nexus for Neuronal Organization and Function.

The axon initial segment is a highly specialized neuronal compartment, identified almost 50years ago by the pioneers of electron microscopy. Located in the first 50μm of the axon, it contains unique cytoskeletal features and concentrates a repertoire of specific scaffold and membrane proteins that assembles just after axon determination. The axon initial segment (AIS) supports two crucial physiological functions of the mature neuron: first, it generates and shapes the action potential. Second, it separates the cell body from the axon, preserving the molecular identity of each compartment. In addition to a diffusion barrier restricting membrane proteins and lipids exchange, an intracellular filter has been proposed that could selectively exclude somatodendritic vesicles and recruit axonal cargoes. Finally, the AIS scaffold is capable of morphological plasticity during development or in response to network activity. These changes directly impact the neuron excitability, allowing an adaptive and homeostatic response. These plastic electrogenic properties, as well as the regulation of protein transport to and from the axon, may have important implications in several neuropathological contexts where the AIS structure is altered. Fifty years after its first characterization, the AIS thus emerges as a nexus for both neuronal organization and physiology.