BetaII-Spectrin Gaps and Patches Emerge from the Patterned Assembly of the Actin/Spectrin Membrane Skeleton in Human Motor Neuron Axons

The actin/spectrin membrane-associated periodic skeleton (MPS), is a cytoskeletal structure that supports axonal integrity and function. Lower spinal motor neurons (MNs) are characterized by exceptionally long axons and are particularly susceptible to degeneration in a wide range of hereditary neuromuscular disorders, including amyotrophic lateral sclerosis (ALS). Using confocal and super-resolution imaging, we characterized the spatial distribution βII-spectrin and the assembly pattern of the MPS in human MN axons derived from induced pluripotent stem cells (iPSCs). We discovered a striking gap-and-patch pattern in the medial axon, where sharply demarcated βII-spectrin gaps alternate with patches containing a well-organized MPS. The pattern is acutely induced by the kinase inhibitor staurosporine and pharmacological inhibition of actin polymerization prevents patch formation, indicating a requirement for actin nucleation in MPS assembly. Our data supports a model in which spectrin incorporation into nascent MPS patches depletes neighboring regions, producing long-range gaps-and-patches patterns. Key words: actin, spectrin, MPS, cytoskeleton, motor neurons, iPSCs, staurosporine.