Binding of the G protein betagamma subunit to multiple regions of G protein-gated inward-rectifying K+ channels

We have previously shown that direct binding of the betagamma subunit of G protein (G betagamma) to both the N-terminal domain and the C-terminal domain of a cloned G protein-gated inward-rectifying K+ channel subunit, GIRK1, is important for channel activation. We have now further localized the G betagamma binding region in the N-terminal domain of GIRK1 to amino acids 34-86 and the G betagamma binding region in the C-terminal domain of GIRK1 to two separate fragments of amino acids 318-374 and amino acids 390-462. Of the four cloned mammalian GIRK subunits, GIRK1-4, GIRK1 and 4 form heteromeric K+ channels in the heart and similar channels in the brain include heteromultimers of GIRK1 and 2, and possibly other GIRK homomultimers and heteromultimers. We found that the N-terminal and the C-terminal domains of all four GIRKs bound G betagamma. The G betagamma binding activities for the C-terminal domains of GIRK2-4 were lower than that for the C-terminal domain of GIRK1. The higher G betagamma binding activity for the C-terminal domain of GIRK1 is due to amino acids 390-462 which are unique to GIRK1. We also found that the N-terminal and C-terminal domains of GIRKs interacted with each other, and the N-terminal domain of either GIRK1 or GIRK4 together with the C-terminal domain of GIRK1 exhibited much enhanced binding of G betagamma. These results are consistent with the idea that the N- and C-terminal domains of the cardiac G protein-gated K+ channel subunits may interact with each other to form higher affinity binding site(s) for G betagamma.

Receptor-regulated ion channels

Recent advances in the study of receptor-regulated ion channels include the cloning of the genes encoding three types of potassium channel that are favorite targets of receptors for transmitters and hormones. Studies of these channels have also provided a strong indication that G-protein betagamma subunits may gate ion channels via direct protein-protein interactions. Similarities between channel regulation by natriuretic peptides and channel regulation by secreted peptide products of the Alzheimer's beta-amyloid precursor protein offer hints for the existence of a receptor for the latter. There are also other novel examples of channel regulation in excitable and nonexcitable cells, including liver cells and blood cells.

Stabilization of ion selectivity filter by pore loop ion pairs in an inwardly rectifying potassium channel

Ion selectivity is critical for the biological functions of voltage-dependent cation channels and is achieved by specific ion binding to a pore region called the selectivity filter. In voltage-gated K+, Na+ and Ca2+ channels, the selectivity filter is formed by a short polypeptide loop (called the H5 or P region) between the fifth and sixth transmembrane segments, donated by each of the four subunits or internal homologous domains forming the channel. While mutagenesis studies on voltage-gated K+ channels have begun to shed light on the structural organization of this pore region, little is known about the physical and chemical interactions that maintain the structural stability of the selectivity filter. Here we show that in an inwardly rectifying K+ (IRK) channel, IRK1, short range interactions of an ion pair in the H5 pore loop are crucial for pore structure and ion permeation. The two residues, a glutamate and an arginine, appear to form exposed salt bridges in the tetrameric channel. Alteration or disruption of such ion pair interactions dramatically alters ion selectivity and permeation. Since this ion pair is conserved in all IRK channels, it may constitute a general mechanism for maintaining the stability of the pore structure in this channel superfamily.

Normal cerebellar development but susceptibility to seizures in mice lacking G protein-coupled, inwardly rectifying K+ channel GIRK2

G protein-gated, inwardly rectifying K+ channels (GIRK) are effectors of G protein-coupled receptors for neurotransmitters and hormones and may play an important role in the regulation of neuronal excitability. GIRK channels may be important in neurodevelopment, as suggested by the recent finding that a point mutation in the pore region of GIRK2 (G156S) is responsible for the weaver (wv) phenotype. The GIRK2 G156S gene gives rise to channels that exhibit a loss of K+ selectivity and may also exert dominant-negative effects on G(betagamma)-activated K+ currents. To investigate the physiological role of GIRK2, we generated mutant mice lacking GIRK2. Unlike wv/wv mutant mice, GIRK2 -/- mice are morphologically indistinguishable from wild-type mice, suggesting that the wv phenotype is likely due to abnormal GIRK2 function. Like wv/wv mice, GIRK2 -/- mice have much reduced GIRK1 expression in the brain. They also develop spontaneous seizures and are more susceptible to pharmacologically induced seizures using a gamma-aminobutyric acid antagonist. Moreover, wv/- mice exhibit much milder cerebellar abnormalities than wv/wv mice, indicating a dosage effect of the GIRK2 G156S mutation. Our results indicate that the weaver phenotypes arise from a gain-of-function mutation of GIRK2 and that GIRK1 and GIRK2 are important mediators of neuronal excitability in vivo.

Rho family GTP-binding proteins in growth cone signalling

Rho family GTP-binding proteins regulate various aspects of the actin cytoskeleton in a wide variety of organisms. Recent evidence suggests that they may also be important components of the signalling pathways that link the reception of extracellular cues to the regulation of the cytoskeleton in neuronal growth cones.

Reciprocal localization of Nod and kinesin fusion proteins indicates microtubule polarity in the Drosophila oocyte, epithelium, neuron and muscle

Polarization of the microtubule cytoskeleton is an early event in establishment of anterior-posterior polarity for the Drosophila oocyte. During stages 8–9 of oogenesis, when oskar mRNA is transported to the posterior pole of the oocyte, a fusion protein consisting of the plus-end-directed microtubule motor kinesin and beta-galactosidase (Kin:beta gal) similarly localizes to the posterior pole, thereby suggesting that plus ends of microtubules are pointed to the posterior. In this paper, we have substituted the motor domain of Kin:beta gal with the putative motor domain (head) from the kinesin-related protein Nod. In cells with defined microtubule polarity, the Nod:beta gal fusion protein is an in vivo minus-end reporter for microtubules. Nod:beta gal localizes to apical cytoplasm in epithelial cells and to the poles of mitotic spindles in dividing cells. In stage 8–10 oocytes, the Nod fusion localizes to the anterior margin, thus supporting the hypothesis that minus ends of microtubules at these stages are primarily at the anterior margin of the oocyte. The fusion protein also suggests a polarity to the microtubule cytoskeleton of dendrites and muscle fibers, as it accumulates at the ends of dendrites in the embryonic PNS and is excluded from terminal cytoplasm in embryonic muscle. Finally, the reciprocal in vivo localization of Nod:beta gal and Kin:beta gal suggests that the head of Nod may be a minus-end-directed motor.

Cloned potassium channels from eukaryotes and prokaryotes

Potassium channels contribute to the excitability of neurons and signaling in the nervous system. They arise from multiple gene families including one for voltage-gated potassium channels and one for inwardly rectifying potassium channels. Features of potassium permeation, channel gating and regulation, and subunit interaction have been analyzed. Potassium channels of similar design have been found in animals ranging from jellyfish to humans, as well as in plants, yeast, and bacteria. Structural similarities are evident for the pore-forming alpha subunits and for the beta subunits, which could potentially regulate channel activity according to the level of energy and/or reducing power of the cell.

Regions responsible for the assembly of inwardly rectifying potassium channels

Inwardly rectifying potassium channels have an important role in determining the resting potential of the cell. They are tetrameric proteins with two transmembrane segments (M1 and M2), a pore-forming loop (H5), a cytoplasmic N-terminal, and longer C-terminal domain. We have used biochemical and electrophysiological methods to identify regions required for homotypic interactions and those responsible for the incompatibility between IRK1 and two other members of the same subfamily (IRK2 and IRK3) and two members from other subfamilies (ROMK1 and 6.1 uK(ATP)). The data indicate that, in contrast to the voltage-gated class of potassium channel, the proximal C-terminus and the transmembrane segment M2 determine homo-and heteromultimerization and that heteromultimerization between members of the same or different subfamilies is case specific.

Heteromultimerization of G-protein-gated inwardly rectifying K+ channel proteins GIRK1 and GIRK2 and their altered expression in weaver brain

The weaver (wv) gene (GIRK2) is a member of the G-protein-gated inwardly rectifying potassium (GIRK) channel family, known effectors in the signal transduction pathway of neurotransmitters such as acetylcholine, dopamine, opioid peptides, and substance P in modulation of neurotransmitter release and neuronal excitability. GIRK2 immunoreactivity is found in but not limited to brain regions known to be affected in wv mice, such as the cerebellar granule cells and dopaminergic neurons in the substantia nigra pars compacta. It is also observed in the ventral tegmental area, hippocampus, cerebral cortex, and thalamus. GIRK2 and GIRK1, a related family member, have overlapping yet distinct distributions in rat and mouse brains. In regions where both channel proteins are expressed, such as the cerebral cortex, hippocampus, and cerebellum, they can be co-immunoprecipitated, indicating that they interact to form heteromeric channels in vivo. In the brain of the wv mouse, GIRK2 expression is decreased dramatically. In regions where GIRK1 and GIRK2 distributions overlap, both GIRK1 and GIRK2 expressions are severely disrupted, probably because of their co-assembly. The expression patterns of these GIRK channel subunits provide a basis for consideration of the machinery for neuronal signaling as well as the differential effects of the wv mutation in various neurons.

Neuronal type information encoded in the basic-helix-loop-helix domain of proneural genes

The proneural genes encode basic-helix-loop-helix (bHLH) proteins and promote the formation of distinct types of sensory organs. In Drosophila, two sets of proneural genes, atonal (ato) and members of the achaete-scute complex (ASC), are required for the formation of chordotonal (ch) organs and external sensory (es) organs, respectively. We assayed the production of sensory organs in transgenic flies expressing chimeric genes of ato and scute (sc), a member of ASC, and found that the information that specifies ch organs resides in the bHLH domain of ato; chimeras containing the b domain of ato and the HLH domain of sc also induced ch organ formation, but to a lesser extent than those containing the bHLH domain of ato. The b domains of ato and sc differ in seven residues. Mutations of these seven residues in the b domain of ato suggest that most or perhaps all of these residues are required for induction of ch organs. None of these seven residues is predicted to contact DNA directly by computer simulation using the structure of the myogenic factor MyoD as a model, implying that interaction of ato with other cofactors is likely to be involved in neuronal type specification.

frazzled encodes a Drosophila member of the DCC immunoglobulin subfamily and is required for CNS and motor axon guidance

We have identified a Drosophila member of the deleted in colorectal cancer (DCC) gene family. The frazzled gene encodes transmembrane proteins that contain four immunoglobulin C2 type domains, six fibronectin type III repeats, and a cytoplasmic domain of 278 amino acids. Like vertebrate members of the DCC family, Frazzled is expressed on axons in the embryonic central nervous system and on motor axons in the periphery. Frazzled is also expressed on epidermis and gut epithelium. Null mutants in frazzled are defective in axon guidance in the central nervous system and in motor axon guidance and targeting in the periphery. The phenotypes strongly resemble those of a deletion of the two Drosophila Netrin genes. We have rescued the frazzled CNS and motor axon defects by expressing Frazzled specifically in neurons; expression in target tissues does not rescue the phenotype. These data, together with vertebrate studies showing binding of DCC to netrin, suggest that Frazzled may function in vivo as a receptor or component of a receptor mediating Netrin-dependent axon guidance.

The Drosophila Numb protein inhibits signaling of the Notch receptor during cell-cell interaction in sensory organ lineage

Specification of unequal daughter cell fates in the Drosophila external sense organ lineage requires asymmetric localization of the intrinsic determinant Numb as well as cell-cell interactions mediated by the Delta ligand and Notch receptor. Previous genetic studies indicated that numb acts upstream of Notch, and biochemical studies revealed that Numb can bind Notch. For a functional assay of the action of Numb on Notch signaling, we expressed these proteins in cultured Drosophila cells and used nuclear translocation of Suppressor of Hairless [Su(H)] as a reporter for Notch activity. We found that Numb interfered with the ability of Notch to cause nuclear translocation of Su(H); both the C-terminal half of the phosphotyrosine binding domain and the C terminus of Numb are required to inhibit Notch. Overexpression of Numb during wing development, which is sensitive to Notch dosage, revealed that Numb is also able to inhibit the Notch receptor in vivo. In the external sense organ lineage, the phosphotyrosine binding domain of Numb was found to be essential for the function but not for asymmetric localization of Numb. Our results suggest that Numb determines daughter cell fates in the external sense organ lineage by inhibiting Notch signaling.

Role of inscuteable in orienting asymmetric cell divisions in Drosophila

Drosophila neuroblasts and epithelial cells in the procephalic neurogenic region divide perpendicular to the surface, and segregate the proteins Numb and Prospero into the basal daughter cell. We demonstrate here that orientation of the mitotic spindle and correct localization of Numb and Prospero in these cells require the inscuteable gene. Moreover, ectopic expression of inscuteable in other epithelial cells leads to spindle reorientation. The Inscuteable protein localizes to the apical cell cortex before mitosis, suggesting that Inscuteable functions in establishing polarity for asymmetric cell division.

Identification of a titratable lysine residue that determines sensitivity of kidney potassium channels (ROMK) to intracellular pH

Potassium (K+) homeostasis is controlled by the secretion of K+ ions across the apical membrane of renal collecting duct cells through a low-conductance inwardly rectifying K+ channel. The sensitivity of this channel to intracellular pH is particularly high and assumed to play a key role in K+ homeostasis. Recently, the apical K+ channel has been cloned (ROMK1,2,3 = Kir1.1a, Kir1.1b and Kir1.1c) and the pH dependence of ROMK1 was shown to resemble closely that of the native apical K+ channel. It is reported here that the steep pH dependence of ROMK channels is determined by a single amino acid residue located in the N-terminus close to the first hydrophobic segment M1. Changing lysine (K) at position 80 to methionine (M) removed the sensitivity of ROMK1 channels to intracellular pH. In pH-insensitive IRK1 channels, the reverse mutation (M84K) introduced dependence on intracellular pH similar to that of ROMK1 wild-type. A detailed mutation analysis suggests that a shift in the apparent pKalpha of K80 underlies the pH regulation of ROMK1 channels in the physiological pH range.

Control of daughter cell fates during asymmetric division: interaction of Numb and Notch

During development of the Drosophila peripheral nervous system, a sensory organ precursor (SOP) cell undergoes rounds of asymmetric divisions to generate four distinct cells of a sensory organ. Numb, a membrane-associated protein, is asymmetrically segregated into one daughter cell during SOP division and acts as an inherited determinant of cell fate. Here, we show that Notch, a transmembrane receptor mediated cell-cell communication, functions as a binary switch in cell fate specification during asymmetric divisions of the SOP and its daughter cells in embryogenesis. Moreover, numb negatively regulates Notch, probably through direct protein-protein interaction that requires the phosphotyrosine-binding (PTB) domain of Numb and either the RAM23 region or the very C-terminal end of Notch. Notch then positively regulates a transcription factor encoded by tramtrack (ttk). This leads to Ttk expression in the daughter cell that does not inherit Numb. Thus, the inherited determinant Numb bestows a bias in the machinery for cell-cell communication to allow the specification of distinct daughter cell fates.

Asymmetric localization of a mammalian numb homolog during mouse cortical neurogenesis

During Drosophila neurogenesis, differential segregation of Numb is necessary for daughter cells of asymmetric divisions to adopt distinct fates, at least partly by biasing the Notch-mediated cell-cell interaction. We have isolated a highly conserved mammalian homolog of Drosophila numb, m-numb. During mouse cortical neurogenesis, m-Numb is asymmetrically localized to the apical membrane of dividing ventricular neural progenitors. Depending upon the orientation of the cleavage plane, m-Numb may be distributed into one or both of the daughter cells. When expressed in Drosophila embryos, m-Numb is localized asymmetrically in dividing neural precursors and rescues the numb mutant phenotype. Furthermore, m-Numb can physically interact with mouse Notch1. We propose that some shared molecular mechanisms, both cell-intrinsic and cell-extrinsic, generate asymmetric cell divisions during neurogenesis of vertebrates and invertebrates.

Differential effects of the Rac GTPase on Purkinje cell axons and dendritic trunks and spines

Neurons contain distinct compartments including dendrites, dendritic spines, axons and synaptic terminals. The molecular mechanisms that generate and distinguish these compartments, although largely unknown, may involve the small GTPases Rac and Cdc42, which appear to regulate actin polymerization. Having shown that perturbations of Rac1 activity block the growth of axons but not dendrites of Drosophila neurons, we investigated whether this also applies to mammals by examining transgenic mice expressing constitutively active human Rac1 in Purkinje cells. We found that these mice were ataxic and had a reduction of Purkinje-cell axon terminals in the deep cerebellar nuclei, whereas the dendritic trees grew to normal height and branched extensively. Unexpectedly, the dendritic spines of Purkinje cells in developing and mature cerebella were much reduced in size but increased in number. These 'mini' spines often form supernumerary synapses. These differential effects of perturbing Rac1 activity indicate that there may be distinct mechanisms for the elaboration of axons, dendrites and dendritic spines.

Delta is a ventral to dorsal signal complementary to Serrate, another Notch ligand, in Drosophila wing formation

Wing margin formation in Drosophila requires the Notch receptor and, in the dorsal compartment, one of its ligands, Serrate. We provide evidence that Delta, the other known ligand for Notch, is also essential for this process. Delta is required in ventral cells at the dorsal/ventral compartment boundary, where its expression is specifically elevated in second-instar wing discs during wing margin formation. Moreover, ectopic Delta expression induces wingless, vestigial, and cut and causes adult wing tissue outgrowth in the dorsal compartment. The effect is mediated by Notch, because loss of Notch activity suppresses Delta-induced ectopic wing outgrowth. Whereas ectopic expression of Notch or the truncated activated Notch induces cut in both dorsal and ventral compartments, ectopic Delta expression induces cut only in the dorsal compartment and ectopic Serrate induces cut only in the ventral compartment. These observations indicate that Notch-expressing cells in a given compartment have different responses to Delta and Serrate. We propose that Delta and Serrate function as compartment-specific signals in the wing disc, to activate Notch and induce downstream genes required for wing formation.

Contributions of a negatively charged residue in the hydrophobic domain of the IRK1 inwardly rectifying K+ channel to K(+)-selective permeation

Inwardly rectifying K+ channels are highly selective for K+ ions and show strong interaction with ions in the pore. Both features are important for the physiological functions of these channels and pose intriguing mechanistic questions of ion permeation. The aspartate residue in the second putative transmembrane segment of the IRK1 inwardly rectifying K+ channel, previously implicated in inward rectification gating due to cytoplasmic Mg2+ and polyamine block, is found in this study to be crucial for the channel's ability to distinguish between K+ and Rb+ ions. Mutation of this residue also perturbs the interaction between the channel pore and the Sr2+ blocking ion. Our studies suggest that this aspartate residue contributes to a selectivity filter near the cytoplasmic end of the pore.

Functional effects of the mouse weaver mutation on G protein-gated inwardly rectifying K+ channels

The weaver mutation corresponds to a substitution of glycine to serine in the H5 region of a G protein-gated inwardly rectifying K+ channel gene (GIRK2). By studying mutant GIRK2 weaver homomultimeric channels and heteromultimeric channels comprised of GIRK2 weaver and GIRK1 in Xenopus oocytes, we found that GIRK2 weaver homomultimeric channels lose their selectivity for K+ ions, giving rise to inappropriate receptor-activated and basally active Na+ currents, whereas heteromultimers of GIRK2 weaver and GIRK1 appeared to have reduced current. Immunohistochemical localization indicates that GIRK2 and GIRK1 proteins are expressed in the cerebellar neurons of mice at postnatal day 4, at a time when these neurons normally undergo differentiation. Thus, the aberrant behavior of mutant GIRK2 weaver channels could affect the development of weaver mice in at least two distinct ways.

A strongly inwardly rectifying K+ channel that is sensitive to ATP

We have cloned an inwardly rectifying K+ channel from the hamster insulinoma cDNA library and shown that it is inhibited by cytoplasmic ATP. The channel is 90.97% identical to the IRK3 channels cloned from other species, and its mRNA is found primarily in the brain. When expressed in Xenopus oocytes, the channel displays strong inward rectification typical of inward rectifiers. The channel is inhibited reversibly by physiological concentrations of ATP via a mechanism that does not appear to involve ATP hydrolysis, as shown by studies of channels in excised inside-out membrane patches. This effect is antagonized by ADP, again in the physiological range, implying that this channel is sensitive to the index of metabolic state, i.e., the intracellular [ATP]/[ADP] ratio. This channel is different from previously known ATP-sensitive K+ channels, although it may also be stimulated by MgATP, as are other ATP-sensitive K+ channels. The potential physiological significance of these ATP-dependent regulations will be discussed.

Determination of the subunit stoichiometry of an inwardly rectifying potassium channel

Inwardly rectifying K+ channels are distantly related to their voltage-gated counterparts and possess a structural motif of only two putative transmembrane segments in each subunit. They are formed by the assembly of an unknown number of subunits. We have examined the subunit stoichiometry of a strongly rectifying K+ channel, IRK1, by linking together the coding sequence of three or four subunits and distinguishing channels with different numbers of subunits carrying a double mutation that alters inward rectification and single-channel properties. We find that IRK1 channels, like voltage-gated K+ channels, are tetrameric channels. Interestingly, the high sensitivity to Mg2+ and polyamines, cations that produce inward rectification by blocking the channel pore from the cytoplasmic side is largely retained in a channel containing only one wild-type subunit and three subunits bearing mutations that abolish high affinity Mg2+ and polyamine block.

Identification of structural elements involved in G protein gating of the GIRK1 potassium channel

Chimeras of GIRK1 and IRK1, a G protein-insensitive inward rectifier, are activated by coexpression of G beta gamma if they contain either the N-terminal or part of the C-terminal hydrophilic domain of GIRK1. The N-terminal domain of GIRK1 also facilitates the fast rates of activation and deactivation following m2 muscarinic receptor stimulation. The hydrophobic core of GIRK1 (M1-H5-M2) is important for determining the brief single-channel open times typical of GIRK1 but not important for determining G beta gamma sensitivity. Coexpression with CIR revealed that the gating properties associated with different GIRK1 domains could not have arisen from altered ability to form heteromultimers. These results implicate specific regions of GIRK1 in G protein activation and suggest that GIRK1 may be closely linked to the m2 muscarinic receptor-G protein complex.