Multiple Molecular Interactions Determine the Clustering of Caspr2 and Kv1 Channels in Myelinated Axons

Abstract

Clustering of Kv1 channels at the juxtaparanodal region (JXP) in myelinated axons depends on their association with the Caspr2/TAG-1 adhesion complex. The interaction between these channels and Caspr2 was suggested to depend on PDZ (PSD-95/Discs large/zona occludens-1) scaffolding proteins. Here, we show that at a subset of the JXP, PSD-93 colocalizes with Caspr2, K ϩ channels and its related protein postsynaptic density protein-95 (PSD-95). The localization of PSD-93 and PSD-95 depends on the presence of Caspr2, as both scaffolding proteins failed to accumulate at the JXP in mice lacking either Caspr2 or TAG-1. In contrast, Caspr2 and K ϩ channels still colocalized and associated in PSD-93, PSD-95 or double PSD-93/PSD-95 null mice. To directly evaluate the role of PDZ domain proteins in the function of Caspr2, we examined the ability of transgenic Caspr2 molecules lacking either their cytoplasmic domain (Caspr2dCT), or their PDZ-binding sequence (Caspr2dPDZ), to restore Kv1 channel clustering in Caspr2 null mice. We found that while Kv1 channels were distributed throughout internodes in nerves expressing Caspr2dCT, they were clustered at the JXP in axons expressing a full-length Caspr2 (Caspr2FL) or the Caspr2dPDZ transgene. Further proteomic analysis revealed that Caspr2 interacts with a distinct set of scaffolding proteins through its PDZ-and protein 4.1-binding sequences. These results demonstrate that while the molecular assembly of the JXP requires the cytoplasmic domain of Caspr2, its carboxy-terminal PDZ-binding motif is dispensable for Kv1 channel clustering. This mechanism is clearly distinct from the one operating at the axon initial segment, which requires PSD-93 for Kv1 channel clustering.

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.…”

Clustering and Activity Tuning of Kv1 Channels in Myelinated Hippocampal Axons

“…Actually, both TAG-1 and Caspr2 associate with Kv1 in brain lysates but not in the lysates of transfected cell lines (18,19), indicating that they do not directly interact with Kv1 channels. Furthermore, both PSD-93 and PSD-95 are dispensable for Kv1 JXP targeting (22), also suggesting that another mechanism is involved.…”

“…Whereas Caspr2 is expressed only in neurons, TAG-1 is expressed in both neurons and myelinating glial cells (19 -21). Caspr2 has a PDZ domain ligand at its C terminus, but deleting PDZ domain-containing proteins, such as PSD-95 (postsynaptic density-95) and/or PSD-93 (postsynaptic density-93), in mice had no effect on the Kv1 JXP targeting (22,23). A recent study suggests that the expression of TAG-1 in myelin cells is sufficient to rescue the JXP complex and phenotypes of TAG-1 knock-out mice (24), questioning the function of axonal TAG-1.…”

Genome-wide association study with DNA pooling identifies variants at CNTNAP2 associated with pseudoexfoliation syndrome

“…36,38 So far it has been suggested as a candidate gene for various neuropsychiatric disorders, [39][40][41][42][43][44][45] however little is known about its specific function and regulation. This protein contains multiple interaction domains such as laminin G, fibrinogen-like domains and epidermal growth factor (EGF)-repeat domains that are also found in members of the fibrillins and fibulins protein families.…”