Subunit interactions in the assembly of neuronal Kir3.0 inwardly rectifying K+ channels

Alternative splicing generates a novel isoform of the rat metabotropic GABA(B)R1 receptor

Here we present a novel isoform of the metabotropic G-protein-coupled receptor for gamma-aminobutyric acid (GABA). The isoform, termed GABA(B)R1c (R1c), differs from the recently identified R1a and R1b receptors by an in-frame insertion of 31 amino acids between the second extracellular loop and the fifth transmembrane region. Analysis of the rat GABA(B)R1 gene demonstrates that the insertion is the result of an alternative splicing event within a 567-bp intron between exons 16 and 17. In situ hybridization in the rat brain shows a wide distribution of R1c transcripts and an overlap with the R1a and R1b transcripts. The highest mRNA levels are found in cerebellar Purkinje cells, cerebral cortex, thalamus and hippocampal CA1 and CA3 regions. Western blots and immunodetection of recombinant epitope-tagged receptors as well as [125I]CGP71872 photoaffinity labelling of cell membranes demonstrate that R1c is correctly expressed, although at a lower level than the previously identified isoforms. When coexpressed with the newly characterized GABA(B)R2, R1c functionally couples to G-protein-activated Kir3.1/3.2 channels in Xenopus oocytes and to PLC-activating chimeric G(alpha)qo subunits in HEK-293 cells with a similar EC50 for agonists. These data suggest that the R1c isoform represents a functional GABA(B)R in the rat brain.

GABA(B)-receptor subtypes assemble into functional heteromeric complexes

B-type receptors for the neurotransmitter GABA (gamma-aminobutyric acid) inhibit neuronal activity through G-protein-coupled second-messenger systems, which regulate the release of neurotransmitters and the activity of ion channels and adenylyl cyclase. Physiological and biochemical studies show that there are differences in drug efficiencies at different GABA(B) receptors, so it is expected that GABA(B)-receptor (GABA(B)R) subtypes exist. Two GABA(B)-receptor splice variants have been cloned (GABA(B)R1a and GABA(B)R1b), but native GABA(B) receptors and recombinant receptors showed unexplained differences in agonist-binding potencies. Moreover, the activation of presumed effector ion channels in heterologous cells expressing the recombinant receptors proved difficult. Here we describe a new GABA(B) receptor subtype, GABA(B)R2, which does not bind available GABA(B) antagonists with measurable potency. GABA(B)R1a, GABA(B)R1b and GABA(B)R2 alone do not activate Kir3-type potassium channels efficiently, but co-expression of these receptors yields a robust coupling to activation of Kir3 channels. We provide evidence for the assembly of heteromeric GABA(B) receptors in vivo and show that GABA(B)R2 and GABA(B)R1a/b proteins immunoprecipitate and localize together at dendritic spines. The heteromeric receptor complexes exhibit a significant increase in agonist- and partial-agonist-binding potencies as compared with individual receptors and probably represent the predominant native GABA(B) receptor. Heteromeric assembly among G-protein-coupled receptors has not, to our knowledge, been described before.

Human gamma-aminobutyric acid type B receptors are differentially expressed and regulate inwardly rectifying K+ channels

gamma-Aminobutyric acid type B receptors (GABABRs) are involved in the fine tuning of inhibitory synaptic transmission. Presynaptic GABABRs inhibit neurotransmitter release by down-regulating high-voltage activated Ca2+ channels, whereas postsynaptic GABABRs decrease neuronal excitability by activating a prominent inwardly rectifying K+ (Kir) conductance that underlies the late inhibitory postsynaptic potentials. Here we report the cloning and functional characterization of two human GABABRs, hGABABR1a (hR1a) and hGABABR1b (hR1b). These receptors closely match the pharmacological properties and molecular weights of the most abundant native GABABRs. We show that in transfected mammalian cells hR1a and hR1b can modulate heteromeric Kir3.1/3.2 and Kir3.1/3.4 channels. Heterologous expression therefore supports the notion that Kir3 channels are the postsynaptic effectors of GABABRs. Our data further demonstrate that in principle either of the cloned receptors could mediate inhibitory postsynaptic potentials. We find that in the cerebellum hR1a and hR1b transcripts are largely confined to granule and Purkinje cells, respectively. This finding supports a selective association of hR1b, and not hR1a, with postsynaptic Kir3 channels. The mapping of the GABABR1 gene to human chromosome 6p21.3, in the vicinity of a susceptibility locus (EJM1) for idiopathic generalized epilepsies, identifies a candidate gene for inherited forms of epilepsy.

Co-expression of P2X1 and P2X5 receptor subunits reveals a novel ATP-gated ion channel

P2X receptors are a family of ion channels gated by extracellular ATP. Each member of the family can form functional homomeric channels, but only P2X2 and P2X3 have been shown to combine to form a unique heteromeric channel. Data from in situ hybridization studies suggest that P2X1 and P2X5 may also co-assemble. In this study, we tested this hypothesis by expressing recombinant P2X1 and P2X5 receptor subunits either individually or together in human embryonic kidney 293 cells. In cells expressing the homomeric P2X1 receptor, 30 microM alpha,beta-methylene ATP (alpha,beta-me-ATP) evoked robust currents that completely desensitized in less than 1 sec, whereas alpha,beta-me-ATP failed to evoke current in cells expressing the homomeric P2X5 receptor. By contrast, alpha, beta-me-ATP evoked biphasic currents with a pronounced nondesensitizing plateau phase in cells that co-expressed both subunits. Further, the EC50 for alpha,beta-me-ATP was greater in cells expressing both P2X1 and P2X5 than in cells expressing P2X1 alone (5 and 1.6 microM, respectively). Heteromeric assembly was confirmed using a co-immunoprecipitation assay of epitope-tagged P2X1 and P2X5 subunits. In summary, this study provides biochemical and functional evidence of a novel channel formed by P2X subunit heteropolymerization. This finding suggests that heteromeric subunit assembly constitutes an important mechanism for generating functional diversity of ATP-mediated responses.

Subunit stoichiometry of a core conduction element in a cloned epithelial amiloride-sensitive Na+ channel

The molecular composition of a core conduction element formed by the alpha-subunit of cloned epithelial Na+ channels (ENaC) was studied in planar lipid bilayers. Two pairs of in vitro translated proteins were employed in combinatorial experiments: 1) wild-type (WT) and an N-terminally truncated alphaDeltaN-rENaC that displays accelerated kinetics (tauo = 32 +/- 13 ms, tauc = 42 +/- 11 ms), as compared with the WT channel (tauc1 = 18 +/- 8 ms, tauc2 = 252 +/- 31 ms, and tauo = 157 +/- 43 ms); and 2) WT and an amiloride binding mutant, alphaDelta278-283-rENaC. The channels that formed in a alphaWT:alphaDeltaN mixture fell into two groups: one with tauo and tauc that corresponded to those exhibited by the alphaDeltaN-rENaC alone, and another with a double-exponentially distributed closed time and a single-exponentially distributed open time that corresponded to the alphaWT-rENaC alone. Five channel subtypes with distinct sensitivities to amiloride were found in a 1alphaWT:1alphaDelta278-283 protein mixture. Statistical analyses of the distributions of channel phenotypes observed for either set of the WT:mutant combinations suggest a tetrameric organization of alpha-subunits as a minimal model for the core conduction element in ENaCs.

Identification of native atrial G-protein-regulated inwardly rectifying K+ (GIRK4) channel homomultimers

G-protein-regulated inwardly rectifying K+ (GIRK) channels play critical inhibitory roles throughout the nervous system, heart, and pancreas. They are believed to be heterotetramers consisting of GIRK1 (Kir3.1) and either GIRK2 (Kir3.2), GIRK3 (Kir3.3), or GIRK4 (Kir3.4) subunits. The GIRK1 subunit is hypothesized to be critical to form GIRK channels with normal channel kinetics based on heterologous expression studies. However, GIRK2 and GIRK3 proteins are present in areas of the brain where no GIRK1 has been detected. Here we demonstrate that GIRK tetramers lacking GIRK1 can be purified from bovine heart atria. We have found that only half of GIRK4 is purified as the GIRK1-GIRK4 heterotetramer, whereas the remaining GIRK4 forms a high molecular weight, SDS-resistant complex that does not contain GIRK1. These GIRK4 complexes, most likely GIRK4 homotetramers, were previously not seen because of their aberrant migration on SDS-polyacrylamide gels. We propose that all of GIRK1 and half of GIRK4 proteins in atria combine to form the heterotetramer IKACh, whereas the remaining GIRK4 forms a novel tetrameric complex. GIRK4 homotetramers form channels with unusual single channel behavior, and their contribution to native currents requires further investigation.

Asymmetrical contributions of subunit pore regions to ion selectivity in an inward rectifier K+ channel

We have investigated aspects of ion selectivity in K+ channels by functional expression of wild-type and mutant heteromultimeric G protein-coupled inward-rectifier K+ (GIRK) channels in Xenopus oocytes. Within the K+ channel pore (P) region signature sequence, a large number of point mutations in GIRK1 and GIRK4 subunits have been made at a key tyrosine residue--the "signature" tyrosine of the GYG. Studies of mutant GIRK1/GIRK4 heteromultimers reveal that the GIRK1 and GIRK4 subunits contribute asymmetrically to K+ selectivity. The signature tyrosine of GIRK1 can be mutated to many different residues while retaining selectivity; in contrast, the analogous position in GIRK4 must be tyrosine for maximum selectivity. Other residues of the P region also contribute to selectivity, and studies with GIRK1/GIRK4 chimeras reveal that an intact, heteromultimeric P region is necessary and sufficient for optimal K+ selectivity. We propose that the GIRK1 and GIRK4 P regions play roles similar to the two P regions of an emerging family of K+ channels whose subunits each have two P regions connected in tandem. We find different consequences between similar mutations in inward-rectifier and voltage-gated K+ channels, which suggests that the pore structures and selectivity mechanisms in the two classes of channel may not be identical. We confirm that GIRK4 subunits alone can form functional channels in oocytes, but we find that these channels are measurably permeable to Na2+ and Ca2+.

Pore mutation in a G-protein-gated inwardly rectifying K+ channel subunit causes loss of K+-dependent inhibition in weaver hippocampus

Weaver (wv) mice carry a point mutation in the pore region of a G-protein-gated inwardly rectifying K+ channel subunit (Kir3.2). wvKir3.2 conducts inward currents that may cause the loss of neurons in the cerebellum and substantia nigra. Although Kir3.2 is widely expressed in the CNS, significant morphological or physiological changes have not been reported for other brain areas. We studied the role of wvKir3.2 in hippocampal slices of young [postnatal day (P) 4-18] and adult wv/wv (>/=P24) mice, because protein levels of Kir 3. 1 and Kir3.2 appear to be normal in the first 3 postnatal weeks and only decrease thereafter. In disinhibited slices, the GABAB receptor agonist R-baclofen reduced burst activity in wv/wv mice but was much more potent in wild-type mice. Mean resting membrane potential, slope input resistance, and membrane time constant of CA3 neurons of adult wv/wv and wild-type mice were indistinguishable. However, R-baclofen or chloroadenosine did not induce K+ currents or any other conductance change in wv/wv mice. Moreover, electrical or chemical stimulation of inhibitory neurons did not evoke slow IPSPs in adult wv/wv mice. Only in a few cells of young wv/wv mice did GABAB receptor activation by R-baclofen or presynaptic stimulation induce small inward currents, which were likely caused by a Na+ ion influx through wvKir3.2 channels. The data show that the pore mutation in wvKir3.2 channels results in a hippocampal phenotype resembling Kir3.2-deficient mutants, although it is not associated with the occurrence of seizures.

Ontogeny of gene expression of Kir channel subunits in the rat

We report the detailed gene expression of all subunits within the Kir2 and Kir3 inwardly rectifying K+ channel subfamilies in the developing rat. Using in situ hybridization, onset of expression and cellular distribution of transcripts in embryonic and postnatal rat brains as well as in peripheral tissues is evaluated. Beginning at embryonic day 13 (E13), except "forebrain" Kir2.3 subunits which are absent from the body and brain until E21, all subunits appear with distinct and mainly nonoverlapping expression patterns. During ontogenic development, expression in the CNS becomes more widespread, leading to widely overlapping mRNA patterns as observed in the adult rat. Subunits are mainly found in regions of the developing brain that are also positive in the adult. Most subunits, in particular Kir3.2 and Kir3.4, are expressed transiently in distinct brain nuclei during ontogeny. Appearance of Kir transcripts is not generally related to the progressive and recessive phases during neurogenesis, but rather regulated differentially for each subunit and any specific group of neurons. It is demonstrated for the first time that several subunits, and most abundantly Kir2.2, are present early in the peripheral nervous system, i.e., in dorsal root-, sensory cranial-, and sympathetic ganglia. Also, of all subunits Kir3.3 is ubiquitously expressed in the entire embryonic nervous system and throughout the body. In summary, analysis of ontogenic Kir channel expression helps deciphering the importance of Kir channels (as exemplified for the defective weaver Kir3.2 gene) during proliferation, differentiation, and synaptogenesis in the CNS.

Number and stoichiometry of subunits in the native atrial G-protein-gated K+ channel, IKACh

The G-protein-regulated, inwardly rectifying K+ (GIRK) channels are critical for functions as diverse as heart rate modulation and neuronal post-synaptic inhibition. GIRK channels are distributed predominantly throughout the heart, brain, and pancreas. In recent years, GIRK channels have received a great deal of attention for their direct G-protein betagamma (Gbetagamma) regulation. Native cardiac IKACh is composed of GIRK1 and GIRK4 subunits (Krapivinsky, G., Gordon, E. A., Wickman, K. A., Velimirovic, B., Krapivinsky, L., and Clapham, D. E. (1995) Nature 374, 135-141). Here, we examine the quaternary structure of IKACh using a variety of complementary approaches. Complete cross-linking of purified atrial IKACh protein formed a single adduct with a total molecular weight that was most consistent with a tetramer. In addition, partial cross-linking of purified IKACh produced subsets of molecular weights consistent with monomers, dimers, trimers, and tetramers. Within the presumed protein dimers, GIRK1-GIRK1 and GIRK4-GIRK4 adducts were formed, indicating that the tetramer was composed of two GIRK1 and two GIRK4 subunits. This 1:1 GIRK1 to GIRK4 stoichiometry was confirmed by two independent means, including densitometry of both silver-stained and Western-blotted native atrial IKACh. Similar experimental results could potentially be obtained if GIRK1 and GIRK4 subunits assembled randomly as 2:2 and equally sized populations of 3:1 and 1:3 tetramers. We also show that GIRK subunits may form homotetramers in expression systems, although the evidence to date suggests that GIRK1 homotetramers are not functional. We conclude that the inwardly rectifying atrial K+ channel, IKACh, a prototypical GIRK channel, is a heterotetramer and is most likely composed of two GIRK1 subunits and two GIRK4 subunits.