Calcium-dependent cytoskeletal collapse and recovery of axons after partial laser ablation

Abstract

Traumatic stretch or crush injury to axons causes widespread and often irreversible damage to the axonal cytoskeleton, in which calcium-mediated breakdown is known to play a central role. Unlike complete transection, where recovery must proceed through formation of a new growth cone, milder injury can disrupt the axonal cytoskeleton while leaving the plasma membrane intact. How the cytoskeleton fails, and how it can recover, under these conditions remains unclear. Here we address this using a partial laser-ablation method that damages the cytoskeleton and evokes a calcium transient while preserving membrane continuity. We show that the ensuing cytoskeletal retraction is set by a mechanical balance between acto-myosin contractility and microtubule stability: stabilizing microtubules or inhibiting acto-myosin contractility suppresses retraction. Moreover, chelating extracellular calcium mitigates degeneration and, in a subset of axons, permits complete recovery. We also show that microtubules and actin filaments show distinct loss and recovery dynamics and provide a hypothesis for the "burning-fuse" -like depolymerization of the microtubule bundle. These findings provide insights into how the axonal cytoskeleton collapses and recovers after injury and suggest strategies for mitigating damage.