Clustering and Activity Tuning of Kv1 Channels in Myelinated Hippocampal Axons

Abstract

Precise localization of axonal ion channels is crucial for proper electrical and chemical functions of axons. In myelinated axons, Kv1 (Shaker) voltage-gated potassium (Kv) channels are clustered in the juxtaparanodal regions flanking the node of Ranvier. The clustering can be disrupted by deletion of various proteins in mice, including contactin-associated protein-like 2 (Caspr2) and transient axonal glycoprotein-1 (TAG-1), a glycosylphosphatidylinositol-anchored cell adhesion molecule. However, the mechanism and function of Kv1 juxtaparanodal clustering remain unclear. Here, using a new myelin coculture of hippocampal neurons and oligodendrocytes, we report that tyrosine phosphorylation plays a critical role in TAG-1-mediated clustering of axonal Kv1.2 channels. In the coculture, myelin specifically ensheathed axons but not dendrites of hippocampal neurons and clustered endogenous axonal Kv1.2 into internodes. The trans-homophilic interaction of TAG-1 was sufficient to position Kv1.2 clusters on axonal membranes in a neuron/HEK293 coculture. Mutating a tyrosine residue (Tyr 458 ) in the Kv1.2 C terminus or blocking tyrosine phosphorylation disrupted myelin-and TAG-1-mediated clustering of axonal Kv1.2. Furthermore, Kv1.2 voltage dependence and activation threshold were reduced by TAG-1 coexpression. This effect was eliminated by the Tyr 458 mutation or by cholesterol depletion. Taken together, our studies suggest that myelin regulates both trafficking and activity of Kv1 channels along hippocampal axons through TAG-1. Proper subcellular targeting of various Kv channels2 is critical for neuronal excitability and synaptic transmission. Molecular mechanisms underlying polarized axon-dendrite targeting of Kv channels have been extensively studied (1-10). In contrast, channel targeting in distinct membrane domains within axons is much less understood. Most axons in mammals are wrapped by layers of compact lipid membranes from myelinating glial cells, oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system. Myelinated axons form distinct membrane domains, such as nodes of Ranvier, paranodes, and juxtaparanodal (JXP), and internodal regions (11)(12)(13)(14). In myelinated axons of both CNS and peripheral nervous system, Kv1 channels are clustered into JXP regions, controlling the fidelity of action potential conduction (11,(15)(16)(17).Based on the findings that the JXP targeting of Kv1 channels was largely eliminated in both Caspr2 (contactin-associated protein-like 2) and TAG-1 (transient axonal glycoprotein-1) knock-out mice, it was hypothesized that Caspr2 interacts with TAG-1 and clusters Kv1 channels via a PDZ domain-containing protein (18,19). 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 K...

Altered Distribution of Juxtaparanodal Kv1.2 Subunits Mediates Peripheral Nerve Hyperexcitability in Type 2 Diabetes Mellitus

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α-Dendrotoxin-sensitive Kv1 channels contribute to conduction failure of polymodal nociceptive C-fibers from rat coccygeal nerve

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