Localization and targeting of voltage-dependent ion channels in mammalian central neurons

Abstract

The intrinsic electrical properties and the synaptic input-output relationships of neurons are governed by the action of voltage-dependent ion channels. The localization of specific populations of ion channels with distinct functional properties at discrete sites in neurons dramatically impacts excitability and synaptic transmission. Molecular cloning studies have revealed a large family of genes encoding voltage-dependent ion channel principal and auxiliary subunits, most of which are expressed in mammalian central neurons. Much recent effort has focused on determining which of these subunits coassemble into native neuronal channel complexes, and the cellular and subcellular distributions of these complexes, as a crucial step in understanding the contribution of these channels to specific aspects of neuronal function. Here we review progress made on recent studies aimed to determine the cellular and subcellular distribution of specific ion channel subunits in mammalian brain neurons using in situ hybridization and immunohistochemistry. We also discuss the repertoire of ion channel subunits in specific neuronal compartments and implications for neuronal physiology. Finally, we discuss the emerging mechanisms for determining the discrete subcellular distributions observed for many neuronal ion channels.

FIG. 1

Subunit composition and subcellular localization of K_v channel principal and auxiliary subunits in mammalian central neurons. Schematic representation of a single K_v α subunit, four of which (within the same subfamily) assemble to form functional K_v channels. Native channels also comprise auxiliary K_vβ (K_v1 subfamily), or KChIP and DPP-like (K_v4 family) subunits. Top middle box shows the classification, genetic nomenclature, and subcellular localization of K_v channel α subunits with well-characterized subcellular localization in mammalian central neurons. Top right box, the classification and genetic nomenclature of K_v channel principal subunits with unknown subcellular localization in mammalian brain. Bottom right box, classification of K_v channel auxiliary subunits expressed in mammalian central neurons, and their functional effects on K_v α subunits.

FIG. 2

Cellular and subcellular distribution of K_v channels in adult hippocampus. A-C, F: rat, D-E: mouse. A, Double immunofluorescence staining for K_v1.4 (red) and K_v2.1 (green). Note K_v1.4 staining in terminals fields of the medial perforant path in the middle molecular layer of the dentate gyrus, and mossy fiber axons and terminals in s. lucidum of CA3. B, Immunoperoxidase staining for K_v3.1b. C, Double immunofluorescence staining for K_v4.2 (green) and K_v4.3 (red) in dentate gyrus. Note uniform staining for both K_v4 α subunits in granule cell dendrites in molecular layer, and K_v4.3 staining in scattered interneurons. D, Double immunofluorescence staining for K_v7.2 (red) and parvalbumin (green) in CA3. White arrows depict K_v7.2-negative, and red arrow K_v7.2-positive neurons. MF: mossy fibers, sr: s. radiatum, sl: s. lucidum, sp: s. pyramidale, so: s. oriens. Reproduced with permission from reference (59). E, Double immunofluorescence staining for K_v7.2 (green) and DNA (DAPI, blue) in CA1. Reproduced with permission from reference (229). F, In situ hybridization of K_v11.1 (top panel) and K_v11.3 (bottom panel). DG: dentate gyrus. Reproduced with permission from reference (272).

FIG. 3

Subunit composition and subcellular localization of Na_v channel principal and auxiliary subunits in mammalian central neurons. Schematic representation of a single Na_v α subunit that forms macromolecular complexes with auxiliary Na_vβ subunits. Bottom left box, the classification, genetic nomenclature, and subcellular localization of mammalian brain Na_v channel principal α subunits. Bottom right box, classification of Na_vβ auxiliary subunits expressed in mammalian central neurons, and their functional effects on coexpressed Na_v α subunits.

FIG. 4

Cellular and subcellular distribution of Na_v channels in mammalian brain and retina. A, B: Immunoperoxidase staining of adult rat hippocampus. Reproduced with permission from reference (100). A, Na_v1.1. Note staining in somata and proximal dendrites of dentate granule cells, CA3-CA1 pyramidal cells and interneurons. B, Na_v 1.2. Note staining in the mossy fiber pathway within CA3 s. lucidum and in s. radiatum of CA1. C, Double immunofluorescence staining for Na_v1 channels (red) and spectrin IVβ (green) in the axonal initial segments (arrows) of pyramidal cells in human temporal neocortex. Reproduced with permission from reference (128). D, Double immunofluorescence staining for Na_v1.1 (red) and Na_v1.6 (green) in the axonal initial segments of retinal ganglion cells. Arrowhead denotes proximal portion of initial segment positive for Na_v1.1 Reproduced with permission from reference (342). E, Triple immunofluorescence staining for nodal Na_v1 channels (red), paranodal Caspr (green) and juxtaparanodal K_v1.2 (blue) in adult rat optic nerve. Image courtesy of Dr. Matthew N. Rasband.

FIG. 5

Subunit composition and subcellular localization of Ca_v channel principal and auxiliary subunits in mammalian central neurons. Schematic representation of a single Ca_v α₁ subunit that forms macromolecular complexes with auxiliary Ca_vβ, Ca_vα₂δ, and Ca_vγ subunits. Bottom left box, the classification, genetic nomenclature, and subcellular localization of mammalian brain Ca_v channel principal α₁ subunits. Bottom right box, classification of Ca_vβ, Ca_vα₂δ, and Ca_vγ auxiliary subunits expressed in mammalian central neurons, and their functional effects on coexpressed Na_v α subunits.

FIG. 6

Cellular and subcellular distribution of Ca_v channels in adult rat brain. A-F: Immunoperoxidase staining. G-K, Immunofluorescence staining. A, D-J: Hippocampus. B, C, K: Neocortex. A, Ca_v2.1. Note staining in terminals fields of the medial perforant path in the middle molecular layer of the dentate gyrus, and mossy fiber axons and terminals in s. lucidum of CA3. B, Higher magnification view of Ca_v2.1 staining in CA1. C, Higher magnification view of Ca_v2.1 staining in CA3, arrows indicate mossy fiber terminals, and arrowheads CA3 pyramidal cell somata. D, Ca_v1.3. Note staining in somata and proximal dendrites of dentate granule cells, CA3-CA1 pyramidal cells and interneurons. E, Higher magnification view of Ca_v1.3 staining in CA3-CA2. F, Much higher magnification view of Ca_v1.3 staining in CA3-CA2. Arrows: base of apical dendrites. Arrowheads: proximal dendrites. G-I: Ca_v3.1 (G), Ca_v3.2 (H), and Ca_v3.3 (I) staining in CA1. J, Ca_v3.3 staining in subiculum. K, Ca_v3.3 staining in neocortex. SP, s. pyramidale. SR, s. radiatum. Reproduced with permission from reference: A-C: (359); D-F: (116); G-I: (201).