Curving Actin Across Systems: Do Axonal Actin Rings Share Common Underlying Mechanisms?

Actin polymerization into curved and ring-shaped structures is an evolutionarily conserved mechanism that cells use to generate force, transmit tension, and maintain membrane architecture. In neurons, super-resolution imaging has revealed a striking periodic submembranous scaffold in axons, composed of circumferential actin rings interconnected by spectrin tetramers, known as the membrane periodic skeleton (MPS). This structure is widely conserved across species and neuronal subtypes and contributes to axonal integrity by reinforcing mechanical stability, organizing membrane proteins, regulating endocytosis, modulating axon caliber, and providing a transient platform for signaling. Although significant progress has been made in defining MPS functions, the molecular mechanisms governing the nucleation, assembly, and maintenance of axonal actin rings remain poorly understood. Here, we explore actin ring formation through comparison with three mechanistically informative systems: cytokinetic contractile rings, epithelial wound-edge purse strings, and adherens junction-associated actin belts. Across these contexts, ring assembly can be parsed into a series of conserved stages, including spatial confinement, membrane anchoring, actin nucleation and elongation, multivalent scaffolding, filament cross-linking and stabilization, regulated turnover, and force production. This comparative perspective uncovers shared design principles that may also apply to axons and offers a conceptual framework for dissecting the molecular basis of MPS nucleation, assembly, and long-term stability.