βIV Spectrins Are Essential for Membrane Stability and the Molecular Organization of Nodes of Ranvier

Abstract

High densities of sodium channels at nodes of Ranvier permit action potential conduction and depend on βIV spectrins, a family of scaffolding proteins linked to the cortical actin cytoskeleton. To investigate the molecular organization of nodes, we analyzedqv^3J“quivering” mice, whose βIV spectrins have a truncated proline-rich “specific” domain (SD) and lack the pleckstrin homology (PH) domain. Central nodes ofqv^3Jmice, which lack βIV spectrins, are significantly broader and have prominent vesicle-filled nodal membrane protrusions, whereas axon shape and neurofilament density are dramatically altered. PNSqv^3Jnodes, some with detectable βIV spectrins, are less affected. In contrast, a larger truncation of βIV spectrins inqv^4Jmice, deleting the SD, PH, and ankyrinG binding domains, causes βIV spectrins to be undetectable and causes dramatic changes, even in peripheral nodes. These results show that quivering mutations disrupt βIV spectrin retention and stability at nodes and that distinct protein domains regulate nodal structural integrity and molecular organization.