title: Functional specialization of the axon initial segment by isoform-specific sodium channel targeting.
abstract: Voltage-dependent sodium channels cluster at high density at axon initial segments, where propagating action potentials are thought to arise, and at nodes of Ranvier. Here, we show that the sodium channel Na(v)1.6 is precisely localized at initial segments of retinal ganglion cells (RGCs), whereas a different isoform, Na(v)1.2, is found in the neighboring unmyelinated axon. During development, initial segments first expressed Na(v)1.2, and Na(v)1.6 appeared later, approximately in parallel with the onset of repetitive RGC firing. In Shiverer mice, Na(v)1.6 localization at the initial segment was unaffected, although Na(v)1.6 expression was severely disrupted in the aberrantly myelinated optic nerve. Targeting or retention of Na(v)1.6 requires molecular interactions that normally occur only at initial segments and nodes of Ranvier. Expression at nodes but not initial segments exhibits an additional requirement for intact myelination. Because of their high density at the initial segment, Na(v)1.6 channels may be crucial in determining neuronal firing properties.
authors.affiliation: Department of Neurobiology and Behavior, State University of New York, Stony Brook, New York 11794, USA.
authors.forename: Tatiana
authors.initials: T
authors.lastname: Boiko
authors.forename: Audra
authors.initials: A
authors.lastname: Van Wart
authors.forename: John H
authors.initials: JH
authors.lastname: Caldwell
authors.forename: S Rock
authors.initials: SR
authors.lastname: Levinson
authors.forename: James S
authors.initials: JS
authors.lastname: Trimmer
authors.forename: Gary
authors.initials: G
authors.lastname: Matthews
mesh_terms: D000818:Animals; D017487:Ankyrins / Q000096:biosynthesis; D001369:Axons / Q000378:metabolism*; D015815:Cell Adhesion Molecules / Q000096:biosynthesis; D002454:Cell Differentiation / Q000502:physiology; D005455:Fluorescent Antibody Technique; D007150:Immunohistochemistry; D051379:Mice; D008818:Mice, Neurologic Mutants; D009186:Myelin Sheath / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D062551:NAV1.2 Voltage-Gated Sodium Channel; D009414:Nerve Growth Factors / Q000096:biosynthesis; D009419:Nerve Tissue Proteins / Q000096:biosynthesis; D009900:Optic Nerve / Q000473:pathology; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012160:Retina / Q000166:cytology; D012165:Retinal Ganglion Cells / Q000166:cytology / Q000378:metabolism; D015222:Sodium Channels / Q000096:biosynthesis* / Q000235:genetics
chemical_list: C094681:Ank3 protein, mouse; C490614:Ank3 protein, rat; D017487:Ankyrins; D015815:Cell Adhesion Molecules; D062551:NAV1.2 Voltage-Gated Sodium Channel; D009414:Nerve Growth Factors; D009419:Nerve Tissue Proteins; C504593:Nfasc protein, mouse; C504594:Nfasc protein, rat; D020033:Protein Isoforms; C568253:Scn2A protein, rat; C000654500:Scn2a protein, mouse; D015222:Sodium Channels