The weaver mutation changes the ion selectivity of the affected inwardly rectifying potassium channel GIRK2

Comparison of K+ Channel Families

K⁺ channels enable potassium to flow across the membrane with great selectivity. There are four K⁺ channel families: voltage-gated K (K_v), calcium-activated (K_Ca), inwardly rectifying K (K_ir), and two-pore domain potassium (K_2P) channels. All four K⁺ channels are formed by subunits assembling into a classic tetrameric (4x1P = 4P for the K_v, K_Ca, and K_ir channels) or tetramer-like (2x2P = 4P for the K_2P channels) architecture. These subunits can either be the same (homomers) or different (heteromers), conferring great diversity to these channels. They share a highly conserved selectivity filter within the pore but show different gating mechanisms adapted for their function. K⁺ channels play essential roles in controlling neuronal excitability by shaping action potentials, influencing the resting membrane potential, and responding to diverse physicochemical stimuli, such as a voltage change (K_v), intracellular calcium oscillations (K_Ca), cellular mediators (K_ir), or temperature (K_2P).

Screening Technologies for Inward Rectifier Potassium Channels: Discovery of New Blockers and Activators

K⁺ channels play a critical role in maintaining the normal electrical activity of excitable cells by setting the cell resting membrane potential and by determining the shape and duration of the action potential. In nonexcitable cells, K⁺ channels establish electrochemical gradients necessary for maintaining salt and volume homeostasis of body fluids. Inward rectifier K⁺ (Kir) channels typically conduct larger inward currents than outward currents, resulting in an inwardly rectifying current versus voltage relationship. This property of inward rectification results from the voltage-dependent block of the channels by intracellular polyvalent cations and makes these channels uniquely designed for maintaining the resting potential near the K⁺ equilibrium potential (E_K). The Kir family of channels consist of seven subfamilies of channels (Kir1.x through Kir7.x) that include the classic inward rectifier (Kir2.x) channel, the G-protein-gated inward rectifier K⁺ (GIRK) (Kir3.x), and the adenosine triphosphate (ATP)-sensitive (K_ATP) (Kir 6.x) channels as well as the renal Kir1.1 (ROMK), Kir4.1, and Kir7.1 channels. These channels not only function to regulate electrical/electrolyte transport activity, but also serve as effector molecules for G-protein-coupled receptors (GPCRs) and as molecular sensors for cell metabolism. Of significance, Kir channels represent promising pharmacological targets for treating a number of clinical conditions, including cardiac arrhythmias, anxiety, chronic pain, and hypertension. This review provides a brief background on the structure, function, and pharmacology of Kir channels and then focuses on describing and evaluating current high-throughput screening (HTS) technologies, such as membrane potential-sensitive fluorescent dye assays, ion flux measurements, and automated patch clamp systems used for Kir channel drug discovery.

Molecular bioelectricity: how endogenous voltage potentials control cell behavior and instruct pattern regulation in vivo

In addition to biochemical gradients and transcriptional networks, cell behavior is regulated by endogenous bioelectrical cues originating in the activity of ion channels and pumps, operating in a wide variety of cell types. Instructive signals mediated by changes in resting potential control proliferation, differentiation, cell shape, and apoptosis of stem, progenitor, and somatic cells. Of importance, however, cells are regulated not only by their own V_mem but also by the V_mem of their neighbors, forming networks via electrical synapses known as gap junctions. Spatiotemporal changes in V_mem distribution among nonneural somatic tissues regulate pattern formation and serve as signals that trigger limb regeneration, induce eye formation, set polarity of whole-body anatomical axes, and orchestrate craniofacial patterning. New tools for tracking and functionally altering V_mem gradients in vivo have identified novel roles for bioelectrical signaling and revealed the molecular pathways by which V_mem changes are transduced into cascades of downstream gene expression. Because channels and gap junctions are gated posttranslationally, bioelectrical networks have their own characteristic dynamics that do not reduce to molecular profiling of channel expression (although they couple functionally to transcriptional networks). The recent data provide an exciting opportunity to crack the bioelectric code, and learn to program cellular activity at the level of organs, not only cell types. The understanding of how patterning information is encoded in bioelectrical networks, which may require concepts from computational neuroscience, will have transformative implications for embryogenesis, regeneration, cancer, and synthetic bioengineering.

An inwardly rectifying K+ channel is required for patterning

Mutations that disrupt function of the human inwardly rectifying potassium channel KIR2.1 are associated with the craniofacial and digital defects of Andersen-Tawil Syndrome, but the contribution of Kir channels to development is undefined. Deletion of mouse Kir2.1 also causes cleft palate and digital defects. These defects are strikingly similar to phenotypes that result from disrupted TGFβ/BMP signaling. We use Drosophila melanogaster to show that a Kir2.1 homolog, Irk2, affects development by disrupting BMP signaling. Phenotypes of irk2 deficient lines, a mutant irk2 allele, irk2 siRNA and expression of a dominant-negative Irk2 subunit (Irk2DN) all demonstrate that Irk2 function is necessary for development of the adult wing. Compromised Irk2 function causes wing-patterning defects similar to those found when signaling through a Drosophila BMP homolog, Decapentaplegic (Dpp), is disrupted. To determine whether Irk2 plays a role in the Dpp pathway, we generated flies in which both Irk2 and Dpp functions are reduced. Irk2DN phenotypes are enhanced by decreased Dpp signaling. In wild-type flies, Dpp signaling can be detected in stripes along the anterior/posterior boundary of the larval imaginal wing disc. Reducing function of Irk2 with siRNA, an irk2 deletion, or expression of Irk2DN reduces the Dpp signal in the wing disc. As Irk channels contribute to Dpp signaling in flies, a similar role for Kir2.1 in BMP signaling may explain the morphological defects of Andersen-Tawil Syndrome and the Kir2.1 knockout mouse.

Structural basis for the selective permeability of channels made of communicating junction proteins

The open state(s) of gap junction channels is evident from their permeation by small ions in response to an applied intercellular (transjunctional/transchannel) voltage gradient. That an open channel allows variable amounts of current to transit from cell-to-cell in the face of a constant intercellular voltage difference indicates channel open/closing can be complete or partial. The physiological significance of such open state options is, arguably, the main concern of junctional regulation. Because gap junctions are permeable to many substances, it is sensible to inquire whether and how each open state influences the intercellular diffusion of molecules as valuable as, but less readily detected than current-carrying ions. Presumably, structural changes perceived as shifts in channel conductivity would significantly alter the transjunctional diffusion of molecules whose limiting diameter approximates the pore's limiting diameter. Moreover, changes in junctional permeability to some molecules might occur without evident changes in conductivity, either at macroscopic or single channel level. Open gap junction channels allow the exchange of cytoplasmic permeants between contacting cells by simple diffusion. The identity of such permeants, and the functional circumstances and consequences of their junctional exchange presently constitute the most urgent (and demanding) themes of the field. Here, we consider the necessity for regulating this exchange, the possible mechanism(s) and structural elements likely involved in such regulation, and how regulatory phenomena could be perceived as changes in chemical vs. electrical coupling; an overall reflection on our collective knowledge of junctional communication is then applied to suggest new avenues of research. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Sperm GIRK2-containing K+ inward rectifying channels participate in sperm capacitation and fertilization

The GIRK2-containing inward-rectifying K(+) ion channels have been implicated in mammalian spermatogenesis. While the Girk2 null mice are fertile, the male weaver transgenic mice carrying a gain-of-function mutation in the Girk2 gene are infertile. To establish the exact period of spermatogenesis affected by this mutation, we performed StaPut isolation and morphological characterization of the germ cells present in the weaver testis. Germ cells representing all periods of spermatogenesis were identified. However, no spermatozoa were present, suggesting that this mutation only affected the haploid phase of spermatogenesis. Real-time PCR studies performed on StaPut purified germ cells from wild-type mice indicated that the Girk2 transcripts were exclusively expressed in spermatids. Immunofluorescence studies of mouse and boar spermatids/spermatozoa localized the GIRK2 K(+) containing channels to the acrosomal region of the sperm plasma membrane. During porcine in vitro fertilization (IVF), GIRK2-containing channels remained associated with the acrosomal shroud following zona-induced acrosome reaction. Fertilization was blocked by tertiapin-Q (TQ), a specific inhibitor of GIRK channels, and by anti-GIRK2 antibodies. Altogether, studies in two different mammalian species point to a conserved mechanism by which the GIRK2 inward-rectifying K(+) ion channels support sperm function during fertilization.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 4: intercellular bridges, mitochondria, nuclear envelope, apoptosis, ubiquitination, membrane/voltage-gated channels, methylation/acetylation, and transcription factors

As germ cells divide and differentiate from spermatogonia to spermatozoa, they share a number of structural and functional features that are common to all generations of germ cells and these features are discussed herein. Germ cells are linked to one another by large intercellular bridges which serve to move molecules and even large organelles from the cytoplasm of one cell to another. Mitochondria take on different shapes and features and topographical arrangements to accommodate their specific needs during spermatogenesis. The nuclear envelope and pore complex also undergo extensive modifications concomitant with the development of germ cell generations. Apoptosis is an event that is normally triggered by germ cells and involves many proteins. It occurs to limit the germ cell pool and acts as a quality control mechanism. The ubiquitin pathway comprises enzymes that ubiquitinate as well as deubiquitinate target proteins and this pathway is present and functional in germ cells. Germ cells express many proteins involved in water balance and pH control as well as voltage-gated ion channel movement. In the nucleus, proteins undergo epigenetic modifications which include methylation, acetylation, and phosphorylation, with each of these modifications signaling changes in chromatin structure. Germ cells contain specialized transcription complexes that coordinate the differentiation program of spermatogenesis, and there are many male germ cell-specific differences in the components of this machinery. All of the above features of germ cells will be discussed along with the specific proteins/genes and abnormalities to fertility related to each topic.

Altered expression of neuronal nitric oxide synthase in weaver mutant mice

The weaver mouse represents the only genetic animal model of gradual nigrostriatal dopaminergic neurodegeneration which is proposed as a pathophysiological phenotype of Parkinson's disease. The aim of the present study was to analyze the nitric oxide and dopaminergic systems in selected brain regions of homozygous weaver mice at different postnatal ages corresponding to specific stages of the dopamine loss. Structural deficits were evaluated by quantification of tyrosine hydroxylase and neuronal nitric oxide synthase-immunostaining in the cortex, striatum, accumbens nuclei, subthalamic nuclei, ventral tegmental area, and substantia nigra compacta of 10-day, 1- and 2-month-old wild-type and weaver mutant mice. The results confirmed the progressive loss of dopamine during the postnatal development in the adult weaver mainly affecting the substantia nigra pars compacta, striatum, and subthalamic nucleus and slightly affecting the accumbens nuclei and ventral tegmental area. A general decrease in neuronal nitric oxide synthase-immunostaining with age was revealed in both the weaver and wild-type mice, with the decrease being most pronounced in the weaver. In contrast, there was an increase in the substantia nigra pars compacta nitric oxide synthase-immunostaining and a decrease mainly in the subthalamic and accumbens nuclei of the 2-month-old weaver mutant. The decrease in the expression of nNOS may bear functional significance related to the process of aging. DA neurons from the substantia nigra directly modulate the activity of subthalamic nucleus neurons, and their loss may contribute to the abnormal activity of subthalamic nucleus neurons. Although the functional significance of these changes is not clear, it may represent plastic compensating adjustments resulting from the loss of dopamine innervation, highlighting a possible role of nitric oxide in this process.

Association between KCNJ6 (GIRK2) gene polymorphisms and postoperative analgesic requirements after major abdominal surgery

Opioids are commonly used as effective analgesics for the treatment of acute and chronic pain. However, considerable individual differences have been widely observed in sensitivity to opioid analgesics. We focused on a G-protein-activated inwardly rectifying potassium (GIRK) channel subunit, GIRK2, that is an important molecule in opioid transmission. In our initial polymorphism search, a total of nine single-nucleotide polymorphisms (SNPs) were identified in the whole exon, 5′-flanking, and exon-intron boundary regions of the KCNJ6 gene encoding GIRK2. Among them, G-1250A and A1032G were selected as representative SNPs for further association studies. In an association study of 129 subjects who underwent major open abdominal surgery, the A/A genotype in the A1032G SNP and -1250G/1032A haplotype were significantly associated with increased postoperative analgesic requirements compared with other genotypes and haplotypes. The total dose (mean±SEM) of rescue analgesics converted to equivalent oral morphine doses was 20.45±9.27 mg, 10.84±2.24 mg, and 13.07±2.39 mg for the A/A, A/G, and G/G genotypes in the A1032G SNP, respectively. Additionally, KCNJ6 gene expression levels in the 1032A/A subjects were significantly decreased compared with the 1032A/G and 1032G/G subjects in a real-time quantitative PCR analysis using human brain tissues, suggesting that the 1032A/A subjects required more analgesics because of lower KCNJ6 gene expression levels and consequently insufficient analgesic effects. The results indicate that the A1032G SNP and G-1250A/A1032G haplotype could serve as markers that predict increased analgesic requirements. Our findings will provide valuable information for achieving satisfactory pain control and open new avenues for personalized pain treatment.

MPTP administration in mice changes the ratio of splice isoforms of fosB and rgs9

Most cases of Parkinson's disease (PD) are sporadic, suggesting an environmental influence on individuals affected by this neurodegenerative disorder. Environmental stresses often lead to changes in the regulation of splicing of pre-mRNA transcripts and this may lead to the pathogenesis of the disease. A 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)/probenecid mouse model was used to examine the changes in the splicing of the fosB and rgs9 transcripts. The ratio of DeltafosB/fosB transcript was decreased in the substantia nigra and unchanged in the striatum after acute MPTP treatment. The DeltafosB/fosB transcript ratio decreased initially and then increased in the striatum of chronically MPTP-treated animals due to different degrees of reduction for the splice variants over time, whereas the ratio was unchanged in the substantia nigra. The ratio of rgs9-2/rgs9-1 transcript decreased in the substantia nigra of mice after acute MPTP treatment and increased temporarily in the striatum after chronic MPTP treatment. There was an increase in the DeltaFosB/FosB and RGS9-2/RGS9-1 protein ratios 3 weeks and 3 days post-treatment, respectively, in chronically treated mice. The data indicate that the pattern of splice isoforms of fosB and rgs9 reflects the brain's immediate and long-term responses to the physiological stress associated with Parkinsonism.

Regional brain variations of cytochrome oxidase activity and motor coordination in Girk2Wv (Weaver) mutant mice

The Girk2(Wv) (weaver) phenotype, caused by a mutated inward rectifying potassium channel, is characterized by degeneration of cerebellar granule cell population as well as midbrain dopamine-containing cells of the nigrostriatal pathway. To investigate the regional brain metabolic consequences of this combined pathology, cytochrome oxidase (CO) activity was measured by histochemistry from brain regions of wild-type and homozygous Girk2(Wv) mutant mice and correlated with motor performances. CO activity of Girk2(Wv) mutants was abnormal in cerebellar cortex, dentate nucleus, and brainstem regions (medial and lateral vestibular nuclei, prepositus, superior colliculus, lateral cuneiform nucleus, and reticular nuclei) implicated in the gaze system. CO activity increased in midbrain dopaminergic regions after correcting for tissue density, regions with severe depletion of tyrosine hydroxylase activity. Forebrain regions were relatively spared in term of CO activity, except for subthalamic nucleus, lateral geniculate nucleus, and cortical eye field. Similarly to the Rora(sg) cerebellar mutant, metabolic alterations in cerebellar and vestibular regions were linearly correlated with poor motor coordination, underlining the sensitivity of these tests to cerebellar dysfunction.

Ultrastructural characterization of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced cell death in embryonic dopaminergic neurons

Developing neuronal populations undergo significant attrition by natural cell death. Dopaminergic neurons in the substantia nigra pars compacta undergo apoptosis during synaptogenesis. Following this time window, destruction of the anatomic target of dopaminergic neurons results in dopaminergic cell death but the morphology is no longer apoptotic. We describe ultrastructural changes that appear unique to dying embryonic dopaminergic neurons. In primary cultures of mesencephalon, death of dopaminergic neurons is triggered by activation of glutamate receptors sensitive to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and differs ultrastructurally from both neuronal apoptosis or typical excitotoxicity. AMPA causes morphological changes selectively in dopaminergic neurons, without affecting other neurons in the same culture dishes. Two hours after the onset of treatment swelling of Golgi complexes is apparent. At 3 h, dopaminergic neurons display loss of membrane asymmetry (coinciding with commitment to die), as well as nuclear membrane invagination, irregular aggregation of chromatin, and mitochondrial swelling. Nuclear changes continue to worsen until loss of cytoplasmic structures and cell death begins to occur after 12 h. These changes are different from those described in neurons undergoing either apoptosis or excitotoxic death, but are similar to ultrastructural changes observed in spontaneous death of dopaminergic neurons in the natural mutant weaver mouse.

The effect of GIRK2(wv) on neurite growth, protein expression, and viability in the CNS-derived neuronal cell line, CATH.A-differentiated

Death occurs in the homozygous mutant mouse weaver among several classes of neuron in cerebellum and ventral midbrain, because these neurons carry a mutation in the G protein-gated inwardly rectifying potassium channel, Girk2. GIRK2 is expressed in all neuronal types killed by wv in cerebellum and midbrain as well as in neurons elsewhere that suffer lesser consequences. GIRK2(wv) affects neurons postnatally, after proliferation, at the time of final differentiation. To assess the impact of GIRK2(wv) on neuronal development and viability, we introduced cDNA encoding wild-type and mutant channels into a variant of a CNS derived catecholamine cell line (Cath.a) known as Cath.a-differentiated. When cultured in serum-free medium, Cath.a-differentiated cells cease proliferation and undergo morphological differentiation, growing long neurites. Cath.a-differentiated cells do not express endogenous Girk channels. Transfection of GIRK2(wv) resulted in the death of Cath.a-differentiated cells, in a cDNA-concentration dependent manner. The highest concentration of Girk2(wv) cDNA caused loss of about half the cells, the next highest concentration one-third, and the least had no effect on viability. However, even the lowest concentration resulted in disruption of neurite outgrowth and reduced the protein products of co-transfected genes. High concentrations of MK801, which prevent Na(+) influx through the mutant channel, prevented death induced by GIRK2(wv). Cell death and disruption of neurite outgrowth were counteracted in GIRK2(wv)-expressing cells by the presence of an unrelated inwardly rectifying potassium channel, Kir2.3. These results are consistent with wv being a gain-of-function mutation, causing disruption of cellular homeostasis by mechanisms such as increased Na(+) influx and chronic depolarization which may in turn result in an excessive metabolic burden on the cell.

Insulin-like growth factor-I protects granule neurons from apoptosis and improves ataxia in weaver mice

Most cerebellar granule neurons in weaver mice undergo premature apoptosis during the first 3 postnatal weeks, subsequently leading to severe ataxia. The death of these granule neurons appears to result from a point mutation in the GIRK2 gene, which encodes a G protein-activated, inwardly rectifying K+ channel protein. Although the genetic defect was identified, the molecular mechanism by which the mutant K+ channel selectively attacks granule neurons in weaver mice is unclear. Before their demise, weaver granule neurons express abnormally high levels of insulin-like growth factor (IGF) binding protein 5 (IGFBP5). IGF-I is essential for the survival of cerebellar neurons during their differentiation. Because IGFBP5 has the capacity to block IGF-I activity, we hypothesized that reduced IGF-I availability resulting from excess IGFBP5 accelerates the apoptosis of weaver granule neurons. We found that, consistently with this hypothesis, exogenous IGF-I partially protected cultured weaver granule neurons from apoptosis by activating Akt and decreasing caspase-3 activity. To determine whether IGF-I protects granule neurons in vivo, we cross-bred weaver mice with transgenic mice that overexpress IGF-I in the cerebellum. The cerebellar volume was increased in weaver mice carrying the IGF-I transgene, predominantly because of an increased number of surviving granule neurons. The presence of the IGF-I transgene resulted in improved muscle strength and a reduction in ataxia, indicating that the surviving granule neurons are functionally integrated into the cerebellar neuronal circuitry. These results confirm our previous suggestion that a lack of IGF-I activity contributes to apoptosis of weaver granule neurons in vivo and supports IGF-I's potential therapeutic use in neurodegenerative disease.

Evolving potassium channels by means of yeast selection reveals structural elements important for selectivity

Potassium channels are widely distributed. To serve their physiological functions, such as neuronal signaling, control of insulin release, and regulation of heart rate and blood flow, it is essential that K+ channels allow K+ but not the smaller and more abundant Na+ ions to go through. The narrowest part of the channel pore, the selectivity filter formed by backbone carbonyls of the GYG-containing K+ channel signature sequence, approximates the hydration shell of K+ ions. However, the K+ channel signature sequence is not sufficient for K+ selectivity. To identify structural elements important for K+ selectivity, we randomly mutagenized the G protein-coupled inwardly rectifying potassium channel 3.2 (GIRK2) bearing the S177W mutation on the second transmembrane segment. This mutation confers constitutive channel activity but abolishes K+ selectivity and hence the channel's ability to complement the K+ transport deficiency of Deltatrk1Deltatrk2 mutant yeast. S177W-containing GIRK2 mutants that support yeast growth in low-K+ medium contain multiple suppressors, each partially restoring K+ selectivity to S177W-containing double mutants. These suppressors include mutations in the first transmembrane segment and the pore helix, likely exerting long-range actions to restore K+ selectivity, as well as a mutation of a second transmembrane segment residue facing the cytoplasmic half of the pore, below the selectivity filter. Some of these suppressors also affected channel gating (channel open time and opening frequency determined in single-channel analyses), revealing intriguing interplay between ion permeation and channel gating.

Differential activation of astrocytes and microglia during post-natal development of dopaminergic neuronal death in the weaver mouse

In order to understand the relationship between astrocytes, microglia and injured neurons, we studied the weaver mutant mouse. One of the main characteristics of this mutant is the progressive degeneration of the dopaminergic (DA) nigrostriatal pathway that starts around postnatal day 15 (P15), in the substantia nigra pars compacta (SNpc) and progresses until adult age (P60). In the present paper, we analysed the relationship between astroglial and microglial cells within DA neurons in the nigrostriatal system of homozygous weaver mice, at different postnatal ages corresponding to specific stages of the DA neuronal loss. The activation of astrocytes was found to be an early event in weaver DA denervation, appearing massively at the onset of DA neuronal loss in the SNpc at P15. Astrocytes remained activated in the adult brain even after the slowing down of the neuronal death process. Interestingly, in the ventral tegmental area, where no DA neuronal death could be detected, a profound, permanent astrogliosis was also observed in adult animals. In contrast, an activation of microglial cells was transiently observed in the SNpc but only at the postnatal age when maximal neuronal death was observed (P30). Lastly, in the striatum, where there was a massive loss of DA nerve terminals, neither astrogliosis nor microglial activation was detected. Hence, the reaction of astrocytes and microglial cells to progressive and spontaneous DA neuronal death showed different temporal kinetics, suggesting a different role for these two cell types in the DA neurodegenerative process in the weaver mouse.

Inhibition of insulin-like growth factor I activity contributes to the premature apoptosis of cerebellar granule neuron in weaver mutant mice: in vitro analysis

Evidence from transgenic mice and cultured cerebellar neurons supports an important role for insulin-like growth factor I (IGF-I) in the formation of cerebellar cytoarchitecture. To understand IGF-I's function during cerebellar development, we examined the involvement of IGF-I in the premature apoptosis of granule neurons derived from the cerebella of weaver (wv) mutant mice. Before their demise, wv granule neurons increased the expression and secretion of IGFBP5 in a gene dose-dependent manner. Because IGFBP5 may interfere with the interaction of IGF-I and its receptor, the abnormally high IGFBP5 levels in wv granule neurons suggest that a lack of IGF-I activation may contribute to their premature apoptosis. This hypothesis is supported by a gene dose-dependent decrease in IGF-I receptor (IGF-IR) phosphorylation. More importantly, there is a parallel gene dose-dependent decrease in Akt activity, which was inversely correlated with the activity levels of caspase 3. On the other hand, adding IGFBP5 antibody into culture media increased the survival of wv granule neurons, whereas adding IGFBP5 decreased the survival of wild-type granule neurons. To delineate the interaction between IGF-I and IGFBP5 on wv granule neurons, we examined neuronal survival after treating with IGF-I, des(1-3) IGF-I, or IGFBP5 antibody. At the same concentration, des(1-3) IGF-I was more effective than IGF-I in promoting survival, in increasing Akt activity, and in decreasing caspase 3 activity. These results indicate that IGF-I's actions on wv granule neurons are normally inhibited by excess IGFBP5, and sufficient IGF-I receptor activation rescues wv granule neurons via stimulating the Akt signaling pathway.

Effect of chronic treatment with riluzole on the nigrostriatal dopaminergic system in weaver mutant mice

The effects of a chronic treatment with the anti-glutamate and sodium channel modulating neuroprotective agent riluzole on the degeneration of dopamine-containing neurons were studied in the brain of weaver mutant mice. In these animals, as in Parkinson's disease, dopaminergic neurons of the nigro-striatal pathway undergo spontaneous and progressive cell death. Homozygous weaver mice were orally treated twice a day with either 8 mg/kg riluzole or placebo for 2 months. Quantification of tyrosine-hydroxylase and dopamine-transporter axonal immunostaining in the striatum revealed that riluzole significantly increased the density of striatal dopaminergic nerve terminals. These results suggest that riluzole protects dopaminergic processes in the weaver mice and/or promotes their neuroplasticity.

Cell death in weaver mouse cerebellum

Mice with the weaver mutation exhibit an uneven weave to their gait, ataxia, mild locomotor hyperactivity and, occasionally, tonic-clonic seizures. A single amino acid mutation in a G-protein coupled, inwardly rectifying K+ channel, GIRK2, gives rise to the symptoms seen in the weaver mice. Two areas of the brain are primarily affected. Cerebellar granule cell neurons die soon after birth and dopaminergic neurons are severely depleted in the substantia nigra. In this article we review recent studies of wild-type and mutant GIRK channels found in native cells or introduced into expression systems. We also review two models that explain some of the details leading to the neuronal cell death observed in weaver mice.

Topology of ionotropic glutamate receptors in brains of heterozygous and homozygous weaver mutant mice

In weaver mice, mutation of a G-protein inwardly rectifying K(+) channel leads to a cerebellar developmental anomaly characterized by granule and Purkinje cell loss and, in addition, degeneration of dopaminergic neurons. To evaluate other deficits, ionotropic glutamate receptors sensitive to N-methyl-D-aspartate (NMDA), amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and kainic acid (KA) were examined by autoradiography with [(3)H]MK-801, [(3)H]AMPA, and [(3)H]KA. These surveys were carried out in selected areas of cerebral cortex, hippocampus and related limbic regions, basal ganglia, thalamus, hypothalamus, brainstem, and cerebellum from heterozygous (wv/+) and homozygous (wv/wv) weaver mutants, and compared to wild-type (+/+) mice. In wv/+ and wv/wv mutants, NMDA receptor levels were lower in cortical areas, septum, hippocampus, subiculum, neostriatum, nucleus accumbens, superior colliculus, and in the cerebellar granular layer. Densities of KA receptors were lower in cortical areas, hippocampus, limbic system structures, neostriatum, nucleus accumbens, thalamus and hypothalamus, superior and inferior colliculi, and cerebellar cortex of wv/wv mutants. Levels of AMPA receptors in the weaver were higher than in +/+ mice, particularly in somatosensory and piriform cortices and periaqueductal gray of wv/+, and in somatosensory cortex, CA1 field of Ammon's horn and cerebellar granular layer of wv/wv. Abnormal developmental signals, aberrant cellular responses, or a distorted balance between neurotransmitter interactions may underlie such widespread and reciprocal glutamate receptor alterations, while in the case of cerebellar cortex, NMDA receptors are lacking due to a massive disappearance of cerebellar granule cells and some loss of Purkinje neurons.

Distribution of serotonin, its metabolites and 5-HT transporters in the neostriatum of Lurcher and weaver mutant mice

Serotonin (5-HT) uptake sites, or transporters, were measured in the neostriatum (caudate putamen) of wild type (+/+) mice and heterozygous (wv/+) and homozygous (wv/wv) weaver, as well as in heterozygous Lurcher (Lc/+) mutants. These topological surveys were carried out by quantitative ligand binding autoradiography using the uptake site antagonist [3H]-citalopram as a probe of innervation densities in four quadrants of the rostral neostriatum and in two halves of the caudal neostriatum. In addition, tissue concentrations of 5-HT, 5-hydroxyindole-3-acetic acid and 5-hydroxytryptophol were measured by high-performance liquid chromatography with electrochemical detection in these neostriatal divisions. In +/+ mice and in Lc/+ mutants there was a dorso-ventral gradient of increasing 5-HT levels, and they exhibited a similar heterogeneity of [3H]-citalopram labeling. In contrast, the gradients of 5-HT concentrations and [3H]-citalopram binding disappeared in the weaver mutants, suggesting a rearrangement of the 5-HT innervation. This reorganization of the 5-HT system in the neostriatum was more obvious in the wv/wv and is compatible with the hypothesis that the postnatal dopaminergic deficiencies that characterize weaver mutants lead to a sprouting of fibers and thus constitute a genetic model of dopaminergic denervation that leads to a 5-HT hyperinnervation.

Ion channel diseases of the central nervous system

In the last decade, advances in molecular genetics and cellular electrophysiology have increased our understanding of ion channel function. A number of diseases termed "channelopathies" have been discovered that are caused by ion channel dysfunction. Channelopathies can be caused by autoimmune, iatrogenic, toxic or genetic mechanisms. Mutations in genes encoding ion channel proteins that disrupt channel function are now the most commonly identified cause of channelopathies, perhaps because gene disruption is readily detected by the methods of molecular genetics. Ion channels are abundant in the central nervous system (CNS), but CNS channelopathies are rare; however, they overlap with some important neurological disorders, such as epilepsy, ataxia, migraine, schizophrenia, Alzheimer's disease and other neurodegenerative diseases. It is possible that more CNS channelopathies will be discovered when additional ion channels are characterized and the complex mechanisms of brain function are better understood. At present, increased knowledge of the identity, structure and function of ion channels is facilitating diagnosis and treatment of many channelopathies.

Distribution of glutamate receptors of the NMDA subtype in brains of heterozygous and homozygous weaver mutant mice

In weaver mice, mutation of an G-protein inwardly rectifying K+ channel leads to a cerebellar developmental anomaly characterized by granule and Purkinje cell loss and, in addition, degeneration of dopaminergic neurons. To evaluate other deficits, glutamate receptors sensitive to N-methyl-D-aspartate (NMDA) were examined by autoradiography with [3H]MK-801 in 36 brain regions from heterozygous (wv/+) and homozygous (wv/wv) weaver mutants, and compared to wild type (+/+) mice. In wv/+ and wv/wv mutants labelling decreased in cortical regions, septum, hippocampus, subiculum, neostriatum, nucleus accumbens, superior colliculus and in the cerebellar granular layer. The reductions in [3H]MK-801 binding were particularly specific in the cerebellar granular layer of wv/wv mutants, but an ubiquitous altered NMDA receptor topology was revealed in other brain regions. Abnormal developmental signals, or aberrant cellular responses, may underlie widespread NMDA receptor reductions, while in cerebellar cortex they could be lacking due to the massive loss of cerebellar granule cells.

G-protein mediated gating of inward-rectifier K+ channels

G-protein regulated inward-rectifier potassium channels (GIRK) are part of a superfamily of inward-rectifier K+ channels which includes seven family members. To date four GIRK subunits, designated GIRK1-4 (also designated Kir3.1-4), have been identified in mammals, and GIRK5 has been found in Xenopus oocytes. GIRK channels exist in vivo both as homotetramers and heterotetramers. In contrast to the other mammalian GIRK family members, GIRK1 can not form functional channels by itself and has to assemble with GIRK2, 3 or 4. As the name implies, GIRK channels are modulated by G-proteins; they are also modulated by phosphatidylinositol 4,5-bisphosphate, intracellular sodium, ethanol and mechanical stretch. Recently a family of GTPase activating proteins known as regulators of G-protein signaling were shown to be the missing link for the fast deactivation kinetics of GIRK channels in native cells, which contrast with the slow kinetics observed in heterologously expressed channels. GIRK1, 2 and 3 are highly abundant in brain, while GIRK4 has limited distribution. Here, GIRK1/2 seems to be the predominant heterotetramer. In general, neuronal GIRK channels are involved in the regulation of the excitability of neurons and may contribute to the resting potential. Interestingly, only the GIRK1 and 4 subunits are distributed in the atrial and sinoatrial node cells of the heart and are involved in the regulation of cardiac rate. Our main objective of this review is to assess the current understanding of the G-protein modulation of GIRK channels and their physiological importance in mammals.

Is there a role for potassium channel openers in neuronal ion channel disorders?

Malfunction in ion channels, due to mutations in genes encoding channel proteins or the presence of autoantibodies, are increasing being implicated in causing disease conditions, termed channelopathies. Dysfunction of potassium (K(+)) channels has been associated with the pathophysiology of a number of neurological, as well as peripheral, disorders (e.g., episodic ataxia, epilepsy, neuromyotonia, Parkinson's disease, congenital deafness, long QT syndrome). K(+) channels, which demonstrate a high degree of diversity and ubiquity, are fundamental in the control of membrane depolarisation and cell excitability. A common feature of K(+) channelopathies is a reduction or loss of membrane potential repolarisation. The identification of K(+) channel subtype specific openers will allow the recovery of the mechanism(s) responsible for counteraction of uncontrolled cellular depolarisation. Synthetic agents that demonstrate K(+) channel opening properties are available for a variety of K(+) channel subtypes (e.g., K(ATP), BK(Ca), GIRK and M-channel). This study reviews the realistic therapeutic potential that may be gained in a broad spectrum of clinical conditions by K(+) channel openers. K(+) channel openers would therefore identify dysfunctional K(+) channel as therapeutic targets for clinical benefit, in addition being able to modulate normally functioning K(+) channels to gain clinical management of pathophysiological events irrespective of the cause.

Insights from mouse models into the molecular basis of neurodegeneration

Thanks largely to cloning the genes for several neurodegenerative diseases over the past decade and the existence of mouse mutants, the molecular basis of neurodegeneration is finally beginning to yield some of its secrets. We discuss what has been learned about the pathogenesis of "triplet repeat" diseases through mouse models for spinocerebellar ataxia types 1 and 3 and Huntington disease, including the roles of nuclear aggregates and protein cleavage. We also discuss the neurologic phenotypes that arise from mutations in neurotransmitter receptors (lurcher mice) and ion channels (weaver, leaner, and tottering mice), drawing parallels between ischemic cell death and the neurodegeneration that occurs in the lurcher mouse. Finally, we discuss common mechanisms of cell death and lessons learned from these mouse models that might have broader relevance to other neurologic disorders.

Gain of function mutants: ion channels and G protein-coupled receptors

Many ion channels and receptors display striking phenotypes for gain-of-function mutations but milder phenotypes for null mutations. Gain of molecular function can have several mechanistic bases: selectivity changes, gating changes including constitutive activation and slowed inactivation, elimination of a subunit that enhances inactivation, decreased drug sensitivity, changes in regulation or trafficking of the channel, or induction of apoptosis. Decreased firing frequency can occur via increased function of K+ or Cl- channels. Channel mutants also cause gain-of-function syndromes at the cellular and circuit level; of these syndromes, the cardiac long-QT syndromes are explained in a more straightforward way than are the epilepsies. G protein-coupled receptors are also affected by activating mutations.

The weaver mutation reverses the function of dopamine and GABA in mouse dopaminergic neurons

In the present study, we characterized the intrinsic electrophysiological properties and the membrane currents activated by dopamine (DA) D(2) and GABA(B) receptors in midbrain dopaminergic neurons, maintained in vitro in a slice preparation, from wild-type and homozygous weaver (wv/wv) mice. By using patch-clamp techniques, we found that membrane potential, apparent input resistance, and spontaneous firing of wv/wv dopaminergic neurons were similar to those of dopamine-containing cells recorded from nonaffected (+/+) animals. More interestingly, the wv/wv neurons were excited rather than inhibited by dopamine and the GABA(B) agonist baclofen. This neurotransmitter-mediated excitation was attributable to the activation of a G-protein-gated inward current that reversed polarity at a membrane potential of approximately -30 mV. We suggest that the altered behavior of the receptor-operated wv G-protein-gated inwardly rectifying K(+) channel 2 (GIRK2) might be related to the selective degeneration of the dopaminergic neurons. In addition, the wv GIRK2 would not only suppress the autoreceptor-mediated feedback inhibition of DA release but could also establish a feedforward mechanism of DA release in the terminal fields.

Evidence of elevated intracellular calcium levels in weaver homozygote mice

1. A mutation in the G-protein-linked, inwardly rectifying K+ channel GIRK2 leads to the loss of cerebellar and dopaminergic mesencephalic neurons in weaver mice. The steps leading to cell death are not well understood but may involve constitutive influx of Na+ and Ca2+ into the neurons. 2. We found that resting [Ca2+]i was dramatically higher in cerebellar neurons from weaver mice compared to wild-type neurons. 3. High-K+ stimuli elicited much smaller changes in [Ca2+]i in weaver cerebellar neurons compared to wild-type neurons. 4. weaver cerebellar granule cells could be rescued from cell death by the GIRK2wv cationic channel blocker, QX-314. 5. QX-314 lowered resting intracellular Ca2+ levels in weaver cerebellar granule cells. 6. These results suggest that changes in resting [Ca2+]i levels and alterations in K+ channel function are most likely to contribute to the developmental abnormalities and increased cerebellar cell death observed in weaver mice.

Characterization of the selectivity filter of the epithelial sodium channel

The epithelial sodium channel (ENaC) is composed of three homologous subunits termed alpha, beta, and gamma. Previous studies suggest that selected residues within a hydrophobic region immediately preceding the second membrane-spanning domain of each subunit contribute to the conducting pore of ENaC. We probed the pore of mouse ENaC by systematically mutating all 24 amino acids within this putative pore region of the alpha-subunit to cysteine and co-expressing these mutants with wild type beta- and gamma-subunits of mouse ENaC in Xenopus laevis oocytes. Functional characteristics of these mutants were examined by two-electrode voltage clamp and single channel recording techniques. Two distinct domains were identified based on the functional changes associated with point mutations. An amino-terminal domain (alpha-Val(569)-alpha-Gly(579)) showed minimal changes in cation selectivity or amiloride sensitivity following cysteine substitution. In contrast, cysteine substitutions within the carboxyl-terminal domain (alpha-Ser(580)-alpha-Ser(592)) resulted in significant changes in cation selectivity and moderately altered amiloride sensitivity. The mutant channels containing alphaG587C or alphaS589C were permeable to K(+), and mutation of a GSS tract (positions alpha587-alpha589) to GYG resulted in a moderately K(+)-selective channel. Our results suggest that the C-terminal portion of the pore region within the alpha-subunit contributes to the selectivity filter of ENaC.

Distribution of dopamine, its metabolites, and D1 and D2 receptors in heterozygous and homozygous weaver mutant mice

In weaver mice, besides a postnatal cerebellar developmental anomaly probably caused by alterations of an inwardly rectifying K+ channel, there is a progressive loss of mesencephalic dopaminergic neurons. To further evaluate this deficit, endogenous dopamine and its metabolites were measured in 22 brain regions from heterozygous (wv/+) and homozygous (wv/wv) mutants, and compared to wild type (+/+) mice. In both wv/+ and wv/wv mutants there were profound dopamine depletions in all regions; these changes were accompanied by decreases in metabolites but with an increase of turnover indexes. Dopamine D1 and D2 receptors were examined by autoradiography, and their distribution was conserved. The results show that the dopaminergic deficit is widespread to all areas of innervation, and is probably compensated for by an increased turnover. Abnormal developmental growth signals, or aberrant cellular responses, may result in defective neurite formation of the midbrain dopaminergic neurons, leading to their postnatal death.

A cell cycle alteration precedes apoptosis of granule cell precursors in the weaver mouse cerebellum

A missense mutation in the gene coding for the G-protein-activated inwardly rectifying potassium (GIRK) channel, GIRK2, is responsible for apoptosis in the external germinal layer (EGL) of the cerebellum and a nonapoptotic death of midbrain dopaminergic neurons in the weaver (wv) mouse. Failure of axonogenesis and migration are considered to be the primary consequences of GIRK2 channel malfunction in the cerebellum. We investigated whether a disruption of the cell cycle precedes the failure of migration and axonogenesis and leads to massive apoptosis. To this end, immunohistochemistry and immunoblotting for PCNA, Cdk4, cyclin D, cyclin A, and the Cdk inhibitor p27/kip1, as well as in situ end-labeling for apoptotic DNA fragmentation, were applied to cerebella of P7-P21+/+, wv/+, and wv/wv mice. In +/+ and wv/+ mice, the expression of cell cycle proteins was limited to the outer, premigratory zone of the EGL. Antibodies to p27, a marker of cell differentiation, gave a reverse staining pattern. Due to migration delay, patches of p27-positive cells persisted in the outer EGL in P21 wv/+ mice. On the contrary, marked cell cycle up-regulation and absence of p27 occurred throughout the EGL at all ages in wv/wv mice, indicating an inability to switch off the cell cycle. Mitotic index evaluation showed that cell cycle activation was unrelated to proliferative events. Cell cycle proteins were not expressed in the substantia nigra, suggesting that nonapoptotic death of mature dopaminergic neurons is not preceded by abortive cell cycle re-entry. Our data show that abnormalities of the cell cycle in wv/wv cerebellum represent a major and early consequence of GIRK2 channel malfunction and may strongly influence the susceptibility of EGL cells to apoptosis. These observations may help in understanding the pathogenesis of human neurological channelopathies.

Girk2 expression in the ventral midbrain, cerebellum, and olfactory bulb and its relationship to the murine mutation weaver

The mouse mutant weaver exhibits developmental deficits and cell death in several neuronal classes. weaver is almost certainly a mutation in the potassium channel, Girk2. In some vulnerable neurons, including those in the midbrain, it is not known whether weaver expression is the primary defect, or whether deficits are secondary to weaver expression elsewhere. In wild-type mice, our results point to subsets of dopamine-containing cells of the midbrain as primary targets of weaver. In the midbrain, all Girk2-positive cells examined in A9 (substantia nigra), A10, and A8 (retrorubral nucleus) are tyrosine hydroxylase-positive. The expression of Girk2 varies among and within these regions. Girk2-positive cells are most numerous in the substantia nigra, pars compacta, a region badly affected in homozygous weavers; in this region, Girk2 expression is found in cell somata and dendrites. In addition, in homozygous weavers, the remaining neuronal processes in A9 (as well as A8) are stunted. Within A10, a region largely spared in weaver homozygotes, Girk2 expression is undetectable in the most medially placed nuclei and is present in the nuclei that border A9. In the cerebellum, Girk2 immunoreactivity was also found in somata and dendrites of populations vulnerable to weaver, including the deep cerebellar nuclei. In a region not previously known to be affected, the olfactory bulb, Girk2 protein is detectable only in processes. The expression of mutated Girk2 has consequences for the olfactory bulb where ectopic cells are present in the external plexiform layer of the homozygous weaver. Our results emphasize that the Girk2 mutation may act to alter the development and maintenance of cell processes and that defects may be present in all Girk2-containing regions in weaver mutants.

Endomorphins fully activate a cloned human mu opioid receptor

Endomorphins were recently identified as endogenous ligands with high selectivity for mu opioid receptors. We have characterized the ability of endomorphins to bind to and functionally activate the cloned human mu opioid receptor. Both endomorphin-1 and endomorphin-2 exhibited binding selectivity for the mu opioid receptor over the delta and kappa opioid receptors. Both agonists inhibited forskolin-stimulated increase of cAMP in a dose-dependent fashion. When the mu opioid receptor was coexpressed in Xenopus oocytes with G protein-activated K+ channels, application of either endomorphin activated an inward K+ current. This activation was dose-dependent and blocked by naloxone. Both endomorphins acted as full agonists with efficacy similar to that of [D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin (DAMGO). These data indicate that endomorphins act as full agonists at the human mu opioid receptor, capable of stimulating the receptor to inhibit the cAMP/adenylyl cyclase pathway and activate G-protein-activated inwardly rectifying potassium (GIRK) channels.

Apoptotic proteins Reaper and Grim induce stable inactivation in voltage-gated K+ channels

Drosophila genes reaper, grim, and head-involution-defective (hid) induce apoptosis in several cellular contexts. N-terminal sequences of these proteins are highly conserved and are similar to N-terminal inactivation domains of voltage-gated potassium (K+) channels. Synthetic Reaper and Grim N terminus peptides induced fast inactivation of Shaker-type K+ channels when applied to the cytoplasmic side of the channel that was qualitatively similar to the inactivation produced by other K+ channel inactivation particles. Mutations that reduce the apoptotic activity of Reaper also reduced the synthetic peptide's ability to induce channel inactivation, indicating that K+ channel inactivation correlated with apoptotic activity. Coexpression of Reaper RNA or direct injection of full length Reaper protein caused near irreversible block of the K+ channels. These results suggest that Reaper and Grim may participate in initiating apoptosis by stably blocking K+ channels.

Characterization of murine Girk2 transcript isoforms: structure and differential expression

A mutation in the G-protein-linked inwardly rectifying K+ channel 2 gene (Girk2) is the cause of the weaver mouse phenotype. We determined that the originally published Girk2 transcript is composed of five exons. The primary coding exon (designated exon 4a in our system) encodes over two-thirds of the protein. Five different full-length Girk2 transcript isoforms (designated Girk2-1, Girk2A-1, Girk2A-2, Girk2B, and Girk2C) originating from different transcriptional start sites and/or alternative splicing were isolated by cDNA RACE. Several of the transcripts were predicted to encode truncated proteins that may lack some of the G-proteincoupling sequence. Northern blotting and in situ hybridization studies with transcript-specific probes indicated that the transcripts were differentially expressed in both normal and weaver mice. All transcripts tested were expressed in the three major targets of action of the weaver mutation: cerebellum, substantia nigra, and testis. Two of the transcripts, Girk2A-1 and Girk2A-2, encode identical proteins and have a distinct pattern of expression in testis, which suggests that they are associated with specific stages of spermatogenesis. An additional transcript, Girk2D, appears to be brain-specific, not polyadenylated, and highly expressed in cerebellar granule cells.

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.

Cell death during development of testis and cerebellum in the mutant mouse weaver

The murine mutation weaver confers early death during development on cells in testes, cerebellum, and midbrain. The results reported here support the hypothesis that the action of weaver is intrinsic to testes and independent of Sertoli cells: germ cells are the only testicular cell type seen to die in weaver homozygotes, while Sertoli cell-dependent development of the blood testis barrier is normal. This report includes characterization of patterns of germ cell death and cerebellar granule cell death in homozygous weavers with respect to that seen during normal development by in situ end-labeling of DNA and high-magnification light microscopy. Comparison of the spatial distribution of dying cells in the weaver's cerebellum with that of dividing cells revealed disarray in the external germinal zone. The results show that cells vulnerable to weaver die by apoptotic and nonapoptotic mechanisms and indicate that weaver-induced cell death is not the consequence of extended naturally occurring developmental cell death, although their timing overlaps. Thus, although the death of cells in each region is likely to be caused by the same mutation, a base pair substitution in the G protein-coupled inwardly rectifying potassium channel 2 gene, the cell death program activated differs depending on cell type.

Developmental expression of the GIRK family of inward rectifying potassium channels: implications for abnormalities in the weaver mutant mouse

G-protein-gated inward rectifying potassium channels (GIRKs) are a newly identified gene family. These gene products are thought to form functional channels through the assembly of heteromeric subunits. Recently, it has been demonstrated that a point mutation in the GIRK2 gene, one of the GIRK family members, is the cause of the neurological and reproductive defects observed in the weaver (wv) mutant mouse. The mechanism(s) by which a single amino acid substitution in GIRK2 protein leads to the severe phenotypes in the wv / wv mouse is not fully understood. However, it implicates the importance of GIRK channels in neuronal development. To characterize the mRNA expression patterns of GIRK1-3 during mouse brain development we have used in situ hybridization analyses. We found that the expression of all three genes showed developmental regulation. In most areas that showed expression, the levels of GIRK1-3 transcripts reached their peak at around postnatal day 10 (P10). In general, GIRK1 showed the least fluctuation in its levels of expression during development, while dynamic changes were found with the levels of GIRK2 and GIRK3 transcripts. GIRK3 becomes the predominant inward rectifying K+-channel in the brain at later postnatal ages. In the CNS regions affected in the wv / wv mouse, GIRK2 is the predominant inward rectifying channel that is expressed. This suggests that the presence of the other subtypes are able to compensate for the mutated GIRK2 channel in weaver neurons that survive.

Signalling via the G protein-activated K+ channels

The inwardly rectifying K+ channels of the GIRK (Kir3) family, members of the superfamily of inwardly rectifying K+ channels (Kir), are important physiological tools to regulate excitability in heart and brain by neurotransmitters, and the only ion channels conclusively shown to be activated by a direct interaction with heterotrimeric G protein subunits. During the last decade, especially since their cloning in 1993, remarkable progress has been made in understanding the structure, mechanisms of gating, activation by G proteins, and modulation of these channels. However, much of the molecular details of structure and of gating by G protein subunits and other factors, mechanisms of modulation and desensitization, and determinants of specificity of coupling to G proteins, remain unknown. This review summarizes both the recent advances and the unresolved questions now on the agenda in GIRK studies.

Defective gamma-aminobutyric acid type B receptor-activated inwardly rectifying K+ currents in cerebellar granule cells isolated from weaver and Girk2 null mutant mice

Stimulation of inhibitory neurotransmitter receptors, such as gamma-aminobutyric acid type B (GABAB) receptors, activates G protein-gated inwardly rectifying K+ channels (GIRK) which, in turn, influence membrane excitability. Seizure activity has been reported in a Girk2 null mutant mouse lacking GIRK2 channels but showing normal cerebellar development as well as in the weaver mouse, which has mutated GIRK2 channels and shows abnormal development. To understand how the function of GIRK2 channels differs in these two mutant mice, we compared the G protein-activated inwardly rectifying K+ currents in cerebellar granule cells isolated from Girk2 null mutant and weaver mutant mice with those from wild-type mice. Activation of GABAB receptors in wild-type granule cells induced an inwardly rectifying K+ current, which was sensitive to pertussis toxin and inhibited by external Ba2+ ions. The amplitude of the GABAB receptor-activated current was severely attenuated in granule cells isolated from both weaver and Girk2 null mutant mice. By contrast, the G protein-gated inwardly rectifying current and possibly the agonist-independent basal current appeared to be less selective for K+ ions in weaver but not Girk2 null mutant granule cells. Our results support the hypothesis that a nonselective current leads to the weaver phenotype. The loss of GABAB receptor-activated GIRK current appears coincident with the absence of GIRK2 channel protein and the reduction of GIRK1 channel protein in the Girk2 null mutant mouse, suggesting that GABAB receptors couple to heteromultimers composed of GIRK1 and GIRK2 channel subunits.

Comparative expression of the inward rectifier K+ channel GIRK2 in the cerebellum of normal and weaver mutant mice

The main target for degeneration associated with the weaver mutation is the cerebellum. Expression of the GIRK2 mRNA and protein was studied in cerebellum of 12- and 22-day-old normal and weaver mice. In 12-day-old mice, GIRK2 is expressed at highest levels in the external granule layer (EGL) and in lower levels in the newly forming internal granule layer (IGL). In the weaver cerebellum, a high hybridization signal and dark immunostaining was observed in the EGL due to the higher density of non-migrated cells. In 22-day-old weaver cerebella, there are only few remaining granule cells existing as scattered cells within the IGL and molecular layer. GIRK2 is expressed in these neurons but the majority of cells expressing GIRK2 in these cerebella are Purkinje cells that are also affected by the weaver mutation (position, shape) but have not died. Normal cerebellar granule neurons but not homozygous mutant neurons in primary cultures and cerebellar slices of 8-day-old mice displayed inward rectifier K+ currents. Taken together, these findings suggest that cell loss in the weaver cerebellum is not directly related to a differential content of GIRK2 in the affected neurons during development. The lethal effect of the weaver mutation in specific neurons is probably due to a combination of the abnormal function of the inward rectifier K+ channels and other factors specific to the vulnerable neurons.

Atrophy and loss of dopaminergic mesencephalic neurons in heterozygous weaver mice

The phenotypic effect of the weaver mutation in the ventral midbrain of homozygous mutants is associated with the progressive loss of dopaminergic neurons. To discover whether the number of mesencephalic dopaminergic cells is altered in weaver heterozygotes (wv/+), we studied mice between 20 and 365 days of age. We counted tyrosine hydroxylase (TH)-immunopositive cells in the substantia nigra (SN), retrorubral nucleus (RRN), and ventral tegmental area (VTA), and measured cross-sectional areas of neuronal somata in the SN of wv/+ and age-matched wild-type controls (+/+). The number of TH-positive cells in the wv/+ ventral midbrain was on average 13% lower than normal. Cell loss was detected selectively in the SN (12%) and VTA (23%). The areas of somatic profiles in the wv/+ nigral neurons were on average reduced by 9.8%. The neuronal losses in the SN and VTA correlated with a 13.8% reduction in dopamine level in the ventral striatum in wv/+ mice at 14-16 months of age. Our findings imply that a single dose of the weaver gene in the mouse is associated with cellular damage leading to a chronic deficiency in the mesostriatal dopaminergic system.