β spectrin-dependent and domain specific mechanisms for Na+ channel clustering

Abstract

Previously, we showed that a hierarchy of spectrin cytoskeletal proteins maintains nodal Na+ channels (Liu et al., 2020). Here, using mice lacking b1, b4, or b1/b4 spectrins, we show this hierarchy does not function at axon initial segments (AIS). Although b1 spectrin, together with AnkyrinR (AnkR), compensates for loss of nodal b4 spectrin, it cannot compensate at AIS. We show AnkR lacks the domain necessary for AIS localization. Whereas loss of b4 spectrin causes motor impairment and disrupts AIS, loss of b1 spectrin has no discernable effect on central nervous system structure or function. However, mice lacking both neuronal b1 and b4 spectrin show exacerbated nervous system dysfunction compared to mice lacking b1 or b4 spectrin alone, including profound disruption of AIS Na+ channel clustering, progressive loss of nodal Na+ channels, and seizures. These results further define the important role of AIS and nodal spectrins for nervous system function.