Association and colocalization of the Kvbeta1 and Kvbeta2 beta-subunits with Kv1 alpha-subunits in mammalian brain K+ channel complexes

Abstract

The differential expression and association of cytoplasmic beta-subunits with pore-forming alpha-subunits may contribute significantly to the complexity and heterogeneity of voltage-gated K+ channels in excitable cells. Here we examined the association and colocalization of two mammalian beta-subunits, Kvbeta1 and Kvbeta2, with the K+ channel alpha-subunits Kv1.1, Kv1.2, Kv1.4, Kv1.6, and Kv2.1 in adult rat brain. Reciprocal coimmunoprecipitation experiments using subunit-specific antibodies indicated that Kvbeta1 and Kvbeta2 associate with all the Kv1 alpha-subunits examined, and with each other, but not with Kv2.1. A much larger portion of the total brain pool of Kv1-containing channel complexes was found associated with Kvbeta2 than with Kvbeta1. Single- and multiple-label immunohistochemical staining indicated that Kvbeta1 codistributes extensively with Kv1.1 and Kv1.4 in cortical interneurons, in the hippocampal perforant path and mossy fiber pathways, and in the globus pallidus and substantia nigra. Kvbeta2 codistributes extensively with Kv1.1 and Kv1.2 in all brain regions examined and was strikingly colocalized with these alpha-subunits in the juxtaparanodal region of nodes of Ranvier as well as in the axons and terminals of cerebellar basket cells. Taken together, these data provide a direct demonstration that Kvbeta1 and Kvbeta2 associate and colocalize with Kv1 alpha-subunits in native tissues and provide a biochemical and neuroanatomical basis for the differential contribution of Kv1 alpha- and beta-subunits to electrophysiologically diverse neuronal K+ currents.

Fig. 1.

Presence of Kvβ1 and Kvβ2 in rat brain K⁺ channel complexes. A detergent lysate of adult rat brain membranes (RBM; 60 μg) and aliquots of products of immunoprecipitation reactions from detergent extracts of 100 μg of RBM performed with the indicated rabbit polyclonal antibodies were size-fractionated by 12% SDS-PAGE. Samples were transferred to nitrocellulose and probed with affinity-purified rabbit anti-Kvβ1 polyclonal antibody (top panel) or mouse anti-Kvβ2 monoclonal antibody K17/70 (bottom panel). Bound antibody was detected by ECL and autoradiography for 15 min (top panel) or 5 min (bottom panel). Arrows on thetop panel point to the band resulting from detection of the rabbit IgG used in the immunoprecipitation reactions by the anti-rabbit secondary antibody used for immunoblotting (IgG) and the Kvβ1-specific band (Kvβ1). Rabbit IgG bands are not present in the bottom panel, because the immunoblot was developed with anti-mouse secondary antibodies.Kvβ2 arrow denotes the Kvβ2 β-subunit. Numbers at the left denoteM_r values of prestained molecular weight standards.

Fig. 2.

Presence of α-subunits in rat brain K⁺ channel complexes. A detergent lysate of adult rat brain membranes (RBM; 60 μg) and aliquots of products of immunoprecipitation reactions from detergent extracts of 100 μg of RBM performed with polyclonal antibodies specific for the indicated K⁺ channel α- and β-subunit polypeptides were size-fractionated by 9% SDS-PAGE. Samples were transferred to nitrocellulose and then probed with subunit-specific affinity-purified rabbit antibodies. The panels represent blots probed with the following antibodies and the respective exposure times for ECL and autoradiography: Kv1.1 (top left; 40 sec), Kv1.2 (top right; 5 min), Kv1.4 (middle left; 2 min) Kv1.6 (middle right; 40 sec), and Kv2.1 (bottom, 20 min). Bound antibody was detected by ECL and autoradiography. Arrows point to the respective α-subunit polypeptides; also visible are bands resulting from detection of the rabbit IgG used in the immunoprecipitation reactions by the anti-rabbit secondary antibody used for immunoblotting.Numbers at the left denote M_r values of prestained molecular weight standards.

Fig. 3.

Immunohistochemical localization of K⁺ channel α- and β-subunits in rat hippocampus.Arrows in B–D, F, and Gpoint to the band of immunoreactivity for Kv1.1, Kv1.2, Kv1.4, Kvβ1, and Kvβ2, respectively, in the middle third of the molecular layer of the dentate gyrus (DG). Arrowheads mark the boundaries between hippocampal subfields.

Fig. 4.

Triple-label immunfluorescence demonstrating the colocalization of Kv1.1 (A), Kv1.4 (B), and Kvβ1 (C) in the dentate gyrus. These three subunits are colocalized in the middle third of the molecular layer, in a pattern that overlaps with terminals of the medial perforant path. gc, Granule cell layer;it, inner third of the molecular layer;mt middle third of the molecular layer;ot, outer third of the molecular layer.

Fig. 5.

Triple-label immunfluorescence demonstrating the colocalization of KvB1 (A), Kv1.1 (B), and Kv1.4 (C) in the mossy fiber zone of CA3. These three subunits are colocalized at or near the terminals of the mossy fiber pathway. py, Stratum pyramidale; mf, mossy fiber zone.

Fig. 6.

Immunohistochemical localization of Kv1.1, Kv1.4, Kvβ1, and Kvβ2 (A–D, respectively) in the caudate nucleus (CPu), globus pallidus (GP;arrowheads), and pars reticulata of the substantia nigra (SNr). These four subunits are codistributed in the termination zones of neostriatal efferents to the globus pallidus and substantia nigra.

Fig. 7.

Immunohistochemical localization of K⁺ channel α- and β-subunits in the cerebellar cortex. Note the high density of immunoreactivity for Kv1.1, Kv1.2, and Kvβ2 in the terminals of cerebellar basket cells (arrows).

Fig. 8.

Triple-label immunfluorescence demonstrating the colocalization of Kv1.1 (A), Kv1.2 (B), and Kvβ2 (C) in cerebellar basket cell terminals. Arrows point to the terminal Pinceau of cerebellar basket cells, where all three subunits are colocalized.

Fig. 9.

Triple-label immunfluorescence demonstrating the colocalization of Kv1.1 (A), Kv1.2 (B), and Kvβ2 (C) at the juxtaparanodal region of nodes of Ranvier in the cerebellar white matter. Arrows point to the juxtaparanodal membrane of a node of Ranvier.