Cloning of a G protein-activated inwardly rectifying potassium channel from human cerebellum

A-type K+ channels encoded by Kv4.2, Kv4.3 and Kv1.4 differentially regulate intrinsic excitability of cortical pyramidal neurons

Rapidly activating and rapidly inactivating voltage-gated A-type K+ currents, IA, are key determinants of neuronal excitability and several studies suggest a critical role for the Kv4.2 pore-forming α subunit in the generation of IA channels in hippocampal and cortical pyramidal neurons. The experiments here demonstrate that Kv4.2, Kv4.3 and Kv1.4 all contribute to the generation of IA channels in mature cortical pyramidal (CP) neurons and that Kv4.2-, Kv4.3- and Kv1.4-encoded IA channels play distinct roles in regulating the intrinsic excitability and the firing properties of mature CP neurons. In vivo loss of Kv4.2, for example, alters the input resistances, current thresholds for action potential generation and action potential repolarization of mature CP neurons. Elimination of Kv4.3 also prolongs action potential duration, whereas the input resistances and the current thresholds for action potential generation in Kv4.3−/− and WT CP neurons are indistinguishable. In addition, although increased repetitive firing was observed in both Kv4.2−/− and Kv4.3−/− CP neurons, the increases in Kv4.2−/− CP neurons were observed in response to small, but not large, amplitude depolarizing current injections, whereas firing rates were higher in Kv4.3−/− CP neurons only with large amplitude current injections. In vivo loss of Kv1.4, in contrast, had minimal effects on the intrinsic excitability and the firing properties of mature CP neurons. Comparison of the effects of pharmacological blockade of Kv4-encoded currents in Kv1.4−/− and WT CP neurons, however, revealed that Kv1.4-encoded IA channels do contribute to controlling resting membrane potentials, the regulation of current thresholds for action potential generation and repetitive firing rates in mature CP neurons.

Involvement of inward rectifier and M-type currents in carbachol-induced epileptiform synchronization

Exposure to cholinergic agonists is a widely used paradigm to induce epileptogenesis in vivo and synchronous activity in brain slices maintained in vitro. However, the mechanisms underlying these effects remain unclear. Here, we used field potential recordings from the lateral entorhinal cortex in horizontal rat brain slices to explore whether two different K(+) currents regulated by muscarinic receptor activation, the inward rectifier (K(IR)) and the M-type (K(M)) currents, have a role in carbachol (CCh)-induced field activity, a prototypical model of cholinergic-dependent epileptiform synchronization. To establish whether K(IR) or K(M) blockade could replicate CCh effects, we exposed slices to blockers of these currents in the absence of CCh. K(IR) channel blockade with micromolar Ba(2+) concentrations induced interictal-like events with duration and frequency that were lower than those observed with CCh; by contrast, the K(M) blocker linopirdine was ineffective. Pre-treatment with Ba(2+) or linopirdine increased the duration of epileptiform discharges induced by subsequent application of CCh. Baclofen, a GABA(B) receptor agonist that activates K(IR), abolished CCh-induced field oscillations, an effect that was abrogated by the GABA(B) receptor antagonist CGP 55845, and prevented by Ba(2+). Finally, when applied after CCh, the K(M) activators flupirtine and retigabine shifted leftward the cumulative distribution of CCh-induced event duration; this effect was opposite to what seen during linopirdine application under similar experimental conditions. Overall, our findings suggest that K(IR) rather than K(M) plays a major regulatory role in controlling CCh-induced epileptiform synchronization.

Molecular dissection of I(A) in cortical pyramidal neurons reveals three distinct components encoded by Kv4.2, Kv4.3, and Kv1.4 alpha-subunits

The rapidly activating and inactivating voltage-gated K(+) (Kv) current, I(A), is broadly expressed in neurons and is a key regulator of action potential repolarization, repetitive firing, backpropagation (into dendrites) of action potentials, and responses to synaptic inputs. Interestingly, results from previous studies on a number of neuronal cell types, including hippocampal, cortical, and spinal neurons, suggest that macroscopic I(A) is composed of multiple components and that each component is likely encoded by distinct Kv channel alpha-subunits. The goals of the experiments presented here were to test this hypothesis and to determine the molecular identities of the Kv channel alpha-subunits that generate I(A) in cortical pyramidal neurons. Combining genetic disruption of individual Kv alpha-subunit genes with pharmacological approaches to block Kv currents selectively, the experiments here revealed that Kv1.4, Kv4.2, and Kv4.3 alpha-subunits encode distinct components of I(A) that together underlie the macroscopic I(A) in mouse (male and female) cortical pyramidal neurons. Recordings from neurons lacking both Kv4.2 and Kv4.3 (Kv4.2(-/-)/Kv4.3(-/-)) revealed that, although Kv1.4 encodes a minor component of I(A), the Kv1.4-encoded current was found in all the Kv4.2(-/-)/Kv4.3(-/-) cortical pyramidal neurons examined. Of the cortical pyramidal neurons lacking both Kv4.2 and Kv1.4, 90% expressed a Kv4.3-encoded I(A) larger in amplitude than the Kv1.4-encoded component. The experimental findings also demonstrate that the targeted deletion of the individual Kv alpha-subunits encoding components of I(A) results in electrical remodeling that is Kv alpha-subunit specific.

Mapping of partially overlapping de novo deletions across an autism susceptibility region (AUTS5) in two unrelated individuals affected by developmental delays with communication impairment

Autism is a neurodevelopmental disorder characterized by deficits in reciprocal social interaction and communication, and repetitive and stereotyped behaviors and interests. Previous genetic studies of autism have shown evidence of linkage to chromosomes 2q, 3q, 7q, 11p, 16p, and 17q. However, the complexity and heterogeneity of the disorder have limited the success of candidate gene studies. It is estimated that 5% of the autistic population carry structural chromosome abnormalities. This article describes the molecular cytogenetic characterization of two chromosome 2q deletions in unrelated individuals, one of whom lies in the autistic spectrum. Both patients are affected by developmental disorders with language delay and communication difficulties. Previous karyotype analyses described the deletions as [46,XX,del(2)(q24.1q24.2)dn]. Breakpoint refinement by FISH mapping revealed the two deletions to overlap by approximately 1.1Mb of chromosome 2q24.1, a region which contains just one gene—potassium inwardly rectifying channel, subfamily J, member 3 (KCNJ3). However, a mutation screen of this gene in 47 autistic probands indicated that coding variants in this gene are unlikely to underlie the linkage between autism and chromosome 2q. Nevertheless, it remains possible that variants in the flanking genes may underlie evidence of linkage at this locus.

G-protein inwardly rectifying potassium channel 1 (GIRK 1) gene expression correlates with tumor progression in non-small cell lung cancer

BACKGROUND

G-protein inwardly rectifying potassium channel 1 (GIRK1) is thought to play a role in cell proliferation in cancer, and GIRK1 gene expression level may define a more aggressive phenotype. We detected GIRK1 expression in tissue specimens from patients with non-small cell lung cancers (NSCLCs) and assessed their clinical characteristics.

METHODS

Using reverse transcription-polymerase chain reaction (RT-PCR) analyses, we quantified the expression of GIRK1 in 72 patients with NSCLCs to investigate the relationship between GIRK1 expression and clinicopathologic factors and prognosis.

RESULTS

In 72 NSCLC patients, 50 (69%) samples were evaluated as having high GIRK1 gene expression, and 22 (31%) were evaluated as having low GIRK1 gene expression. GIRK1 gene expression was significantly associated with lymph node metastasis, stage (p = 0.0194 for lymph node metastasis; p = 0.0207 for stage). The overall and stage I survival rates for patients with high GIRK1 gene expressed tumors was significantly worse than for those individuals whose tumors had low GIRK1 expression (p = 0.0004 for the overall group; p = 0.0376 for stage I).

CONCLUSIONS

These data indicate that GIRK1 may contribute to tumor progression and GIRK1 gene expression can serve as a useful prognostic marker in the overall and stage I NSCLCs.

Mutation analysis of the inwardly rectifying K(+) channels KCNJ6 (GIRK2) and KCNJ3 (GIRK1) in juvenile myoclonic epilepsy

Genetic factors play a major role in the etiology of idiopathic generalized epilepsy. However, in most syndromes, especially the common ones, multiple genetic factors seem to be involved. Mutations in K(+) channel genes have previously found to be associated with epilepsy both in humans and in mice. The weaver mice phenotype, characterized by ataxia, tremor, male infertility, and tonic-clonic seizures, is caused by a point mutation in the inwardly rectifier K(+) channel gene KCNJ6 (GIRK2). A knockout mouse model deprived of functional KCNJ6 protein is susceptible to spontaneous and provoked seizures without showing the histological signs of neuronal cell death found in the weaver mouse. Thus, the KCNJ6 gene seems to play an important role in seizure control. We therefore performed a mutation analysis of KCNJ6 and the related KCNJ3 gene in 38 patients with juvenile myoclonic epilepsy (JME). Two novel same-sense nucleotide exchanges were identified, but none of these changed the coding sequence. These results do not support a major role for the KCNJ6/KCNJ3 heteromeric receptor in the etiology of JME. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:8-11, 2000

Co-expression of human Kir3 subunits can yield channels with different functional properties

To date, no comprehensive study has been done on all combinations of the human homologues of the Kir3.0 channel family, and the human homologue of Kir3.3 has not yet been identified. To obtain support for the contention that most of the functional data on non-human Kir3.0 channels can be extrapolated to human channels, we have cloned the human homologues of the Kir3.0 family, including the yet unidentified human Kir3.3, and the human Kir4.1. The expression pattern of these channels in various human brain areas and peripheral tissues, analysed by Northern blot analysis, allows for the existence of various homomeric and heteromeric forms of human Kir3.0 channels. Expression studies of all possible combinations in Xenopus oocytes indicated that in homomeric Kir3.2c and heteromeric Kir3.1/3.2c channels mediate, in our studies, inward currents with largest amplitude of any other Kir3.0 channel combinations, followed by heteromeric Kir3.1/3.4 and homomeric Kir4.1 channels. Channel combinations which include Kir3.3 are detrimental to the formation of functional channels. The co-expression experiments with different Kir channel subunits indicate the selective formation of certain channel combinations, suggesting that channel specificity is not solely dependent on spatial and temporal regulation of Kir subunit expression.

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

Cardiac G protein-activated Kir (GIRK) channels may assemble as heterotetrameric polypeptides from two subunits, Kir3.1 and Kir3.4. For a functional comparison with native channels in the CNS we investigated all possible combinations of heteromeric channel formation from brain Kir3.1, Kir3.2, Kir3.3, and Kir3.4 subunits in mRNA-injected Xenopus oocytes. Analysis of macroscopic current amplitudes and channel gating kinetics indicated that individual subunits or combinations of Kir3.2, Kir3.3, and Kir3.4 formed functional channels ineffectively. Each of these subunits gave rise to prominent currents with distinct characteristics only in the presence of Kir3.1 subunits. Functional expression of concatemeric constructs between Kir3.1 and Kir3.2/3.4 subunits as well as coimmunoprecipitations with subunit-specific antibodies confirmed heteromeric channel formation. Mutational swapping between subunits of a single pore loop residue (Kir3.1F137S; Kir3.3S114F; a phenylalanine confers slow channel gating in Kir3.1 subunits) revealed that Kir3.1 subunits are an important constituent for native heteromeric channels and dominate their functional properties. However, homomeric channels from Kir3.1 subunits in vivo may not exist due to the spatial conflict of bulky phenylalanines in the pore structure.