The actin-spectrin submembrane scaffold restricts endocytosis along proximal axons

Abstract

Neuronal clathrin-mediated endocytosis has unique features in compartments such as dendrites and presynaptic boutons, but how membrane and extracellular components are internalized along the axon shaft remains poorly known. Here we focused on clathrin-coated structures and endocytosis along the axon initial segment (AIS), and their relationship to the periodic actin-spectrin scaffold that lines the axonal plasma membrane. Super-resolution optical microscopy, platinum replica electron microscopy, and their correlative combination on cultured hippocampal neurons reveal that in the AIS, clathrin-coated pits form on bare membrane patches, [~]300 nm circular areas devoid of spectrin mesh and lined by actin filaments we termed "clearings". In fibroblasts and the proximal axon of neurons, spectrin depletion and drug-induced scaffold disorganization increase clathrin-coated pit formation. However, the presence of clathrin-coated pits at the AIS is not directly linked to actual endocytosis: using cargo uptake and live-cell imaging experiments, we find that most AIS clathrin-coated pits are long-lived and immobile within the spectrin mesh clearings. Direct perturbation of the spectrin scaffold as well as elevated neuronal activity could induce endocytosis downstream of clathrin pit formation, showing that spectrin clearings are structures responsible for regulated endocytosis at the AIS.