Organization of Myelinated Axons by Caspr and Caspr2 Requires the Cytoskeletal Adapter Protein 4.1B

Abstract

Caspr and Caspr2 regulate the formation of distinct axonal domains around the nodes of Ranvier. Caspr is required for the generation of a membrane barrier at the paranodal junction (PNJ), whereas Caspr2 serves as a membrane scaffold that clusters Kv1 channels at the juxtaparanodal region (JXP). Both Caspr and Caspr2 interact with protein 4.1B, which may link the paranodal and juxtaparanodal adhesion complexes to the axonal cytoskeleton. To determine the role of protein 4.1B in the function of Caspr proteins, we examined the ability of transgenic Caspr and Caspr2 mutants lacking their 4.1-binding sequence (d4.1) to restore Kv1 channel clustering in Caspr-and Caspr2-null mice, respectively. We found that Caspr-d4.1 was localized to the PNJ and is able to recruit the paranodal adhesion complex components contactin and NF155 to this site. Nevertheless, in axons expressing Caspr-d4.1, Kv1 channels were often detected at paranodes, suggesting that the interaction of Caspr with protein 4.1B is necessary for the generation of an efficient membrane barrier at the PNJ. We also found that the Caspr2-d4.1 transgene did not accumulate at the JXP, even though it was targeted to the axon, demonstrating that the interaction with protein 4.1B is required for the accumulation of Caspr2 and Kv1 channels at the juxtaparanodal axonal membrane. In accordance, we show that Caspr2 and Kv1 channels are not clustered at the JXP in 4.1B-null mice. Our results thus underscore the functional importance of protein 4.1B in the organization of peripheral myelinated axons.

Candidate autism gene screen identifies critical role for cell-adhesion molecule CASPR2 in dendritic arborization and spine development

“…In mature neurons, CASPR2 organizes a juxtaparanodal complex in the nodes of Ranvier by intracellular binding to protein 4.1 and MPP-type PDZ-domain proteins, which, in turn, recruit potassium channels and other proteins (38,39). However, the effects of the CASPR2 KD on dendritic arborization are probably not mediated by changes in ion channel distribution that influence neuronal excitability because the CASPR2 KD did not change action potential generation (Fig.…”

Kainate Receptor Post-translational Modifications Differentially Regulate Association with 4.1N to Control Activity-dependent Receptor Endocytosis

“…Other members of the 4.1 protein family are obvious candidates because these proteins also associate kainate receptors in heterologous expression systems (data not shown). 4.1 protein isoforms are localized to distinct subcellular compartments, including axons (50,51), and may play distinct roles in localizing kainate receptors to presynaptic terminals, where they function as autoreceptors to regulate neurotransmitter release (52). GluK3-containing receptors require dendritic endocytosis for their polarized expression at presynaptic terminals (53), raising the possibility that specific 4.1 isoforms are important for axonal targeting of these receptors by regulating their endocytosis.…”

The Kv1-associated molecules TAG-1 and Caspr2 are selectively targeted to the axon initial segment in hippocampal neurons

“…The role of Caspr2 and TAG-1 has been well established at the juxtaparanodes of myelinated axons where they mediate axo-glial contacts and induce the clustering of Kv1.1 and Kv1.2 to control the internodal resting potential (Poliak et al, 2003;Traka et al, 2003). The intracellular protein 4.1B (also known as EPB41L3), which binds Caspr2 is required for assembling the juxtaparanodal scaffold (Buttermore et al, 2011;Cifuentes-Diaz et al, 2011;Einheber et al, 2013;Horresh et al, 2010). In contrast to juxtaparanodes, Caspr2, and TAG-1, although present at the AIS are dispensable for the recruitment of Kv1 channels there (Duflocq et al, 2011;Inda et al, 2006;Ogawa et al, 2008).…”