Characterization of G-protein betagamma expression in inner ear

Calcium-dependent binding of HCN1 channel protein to hair cell stereociliary tip link protein protocadherin 15 CD3

The cytoplasmic amino terminus of HCN1, the primary full-length HCN isoform expressed in trout saccular hair cells, was found by yeast two-hybrid protocols to bind the cytoplasmic carboxyl-terminal domain of a protocadherin 15a-like protein. HCN1 was immunolocalized to discrete sites on saccular hair cell stereocilia, consistent with gradated distribution expected for tip link sites of protocadherin 15a. HCN1 message was also detected in cDNA libraries of rat cochlear inner and outer hair cells, and HCN1 protein was immunolocalized to cochlear hair cell stereocilia. As predicted by the trout hair cell model, the amino terminus of rat organ of Corti HCN1 was found by yeast two-hybrid analysis to bind the carboxyl terminus of protocadherin 15 CD3, a tip link protein implicated in mechanosensory transduction. Specific binding between HCN1 and protocadherin 15 CD3 was confirmed with pull-down assays and surface plasmon resonance analysis, both predicting dependence on Ca(2+). In the presence of calcium chelators, binding between HCN1 and protocadherin 15 CD3 was characterized by a K(D) = 2.39 x 10(-7) m. Ca(2+) at 26.5-68.0 microm promoted binding, with K(D) = 5.26 x 10(-8) m (at 61 microm Ca(2+)). Binding by deletion mutants of protocadherin 15 CD3 pointed to amino acids 158-179 (GenBank accession number XP_238200), with homology to the comparable region in trout hair cell protocadherin 15a-like protein, as necessary for binding to HCN1. Amino terminus binding of HCN1 to HCN1, hypothesized to underlie HCN1 channel formation, was also found to be Ca(2+)-dependent, although the binding was skewed toward a lower effective maximum [Ca(2+)] than for the HCN1 interaction with protocadherin 15 CD3. Competition may therefore exist in vivo between the two binding sites for HCN1, with binding of HCN1 to protocadherin 15 CD3 favored between 26.5 and 68 microm Ca(2+). Taken together, the evidence supports a role for HCN1 in mechanosensory transduction of inner ear hair cells.

Immunohistochemical localization of G protein betagamma subunits in the lateral wall of the rat cochlea

The role of G protein-mediated signal transduction in the production of endolymph, an extracellular fluid of unusual ionic composition, is beginning to be understood. The identity of Galpha subunits in the stria vascularis and the spiral ligament of the lateral wall of the cochlear duct is well established. However, little is known about the presence of betagamma subunits. This study used immunohistochemistry to investigate the distribution of G protein betagamma subunits in the lateral wall of the cochlea. Temporal bones of 6- to 8-week-old rats were fixed in 4% paraformaldehyde and 0.1% glutaraldehyde and processed for embedding in paraffin wax. The dewaxed, midmodiolar sections of the cochlea were incubated with subunit-specific polyclonal antibodies. The results show that the pattern of immunoreactivity varies for the G protein beta1-4 and gamma1-3, 5 and 7 subunits in the stria vascularis and spiral ligament. In the stria vascularis, immunoreactivity was detected for beta2, beta3, beta4, gamma1, gamma2 and gamma7 subunits. All five types of fibrocytes in the spiral ligament exhibited positive staining for gamma2 and gamma7. However, immunoreactivity for beta1-4 subunits was variable. Immunoreactivity for gamma3 and gamma5 subunits was not detected in the lateral cochlear wall. The expression pattern of G protein betagamma subunits in lateral wall provides a basis for interpreting the functions of G protein-coupled receptors in cochlear fluid homeostasis.

Diversity of Ca2+-activated K+ channel transcripts in inner ear hair cells

Hair cells express a complement of ion channels, representing shared and distinct channels that confer distinct electrophysiological signatures for each cell. This diversity is generated by the use of alternative splicing in the alpha subunit, formation of heterotetrameric channels, and combinatorial association with beta subunits. These channels are thought to play a role in the tonotopic gradient observed in the mammalian cochlea. Mouse Kcnma1 transcripts, 5' and 3' ESTs, and genomic sequences were examined for the utilization of alternative splicing in the mouse transcriptome. Comparative genomic analyses investigated the conservation of KCNMA1 splice sites. Genomes of mouse, rat, human, opossum, chicken, frog and zebrafish established that the exon-intron structure and mechanism of KCNMA1 alternative splicing were highly conserved with 6-7 splice sites being utilized. The murine Kcnma1 utilized 6 out of 7 potential splice sites. RT-PCR experiments using murine gene-specific oligonucleotide primers analyzed the scope and variety of Kcnma1 and Kcnmb1-4 expression profiles in the cochlea and inner ear hair cells. In the cochlea splice variants were present representing sites 3, 4, 6, and 7, while site 1 was insertionless and site 2 utilized only exon 10. However, site 5 was not present. Detection of KCNMA1 transcripts and protein exhibited a quantitative longitudinal gradient with a reciprocal gradient found between inner and outer hair cells. Differential expression was also observed in the usage of the long form of the carboxy-terminus tail. These results suggest that a diversity of splice variants exist in rodent cochlear hair cells and this diversity is similar to that observed for non-mammalian vertebrate hair cells, such as chicken and turtle.

Differential expression of KCNQ4 in inner hair cells and sensory neurons is the basis of progressive high-frequency hearing loss

Human KCNQ4 mutations known as DFNA2 cause non-syndromic, autosomal-dominant, progressive high-frequency hearing loss in which the cellular and molecular basis is unclear. We provide immunofluorescence data showing that Kcnq4 expression in the adult cochlea has both longitudinal (base to apex) and radial (inner to outer hair cells) gradients. The most intense labeling is in outer hair cells at the apex and in inner hair cells as well as spiral ganglion neurons at the base. Spatiotemporal expression studies show increasing intensity of KCNQ4 protein labeling from postnatal day 21 (P21) to P120 mice that is most apparent in inner hair cells of the middle turn. We have identified four alternative splice variants of Kcnq4 in mice. The alternative use of exons 9-11 produces three transcript variants (v1-v3), whereas the fourth variant (v4) skips all three exons; all variants have the same amino acid sequence at the C termini. Both reverse transcription-PCR and quantitative PCR analyses demonstrate that these variants have differential expression patterns along the length of the mouse organ of Corti and spiral ganglion neurons. Our expression data suggest that the primary defect leading to high-frequency loss in DFNA2 patients may be attributable to high levels of the dysfunctional Kcnq4_v3 variant in the spiral ganglion and inner hair cells in the basal hook region. Progressive hearing loss associated with aging may result from an increasing mutational load expansion toward the apex in inner hair cells and spiral ganglion neurons.

Molecular characterization of two novel splice variants of G alphai2 in the rat vestibular periphery

GTP binding proteins play an important role in mediating signals transduced across the cell membrane by membrane-bound receptors. We previously described a partial sequence, termed Galphai2vest, obtained from rat vestibular tissue that was nearly identical to rat Galphai2. Using an experimental strategy to further characterize Galphai2vest (GenBank accession number AF189020) and identify other possible Galphai2-related transcripts expressed in the rat vestibular periphery, we employed a RecA-based gene enrichment protocol in place of conventional library screening techniques. We identified two novel Galphai2 splice variants, Galphai2(a) (GenBank accession number AY899210) and Galphai2(b) (GenBank accession number AY899211), that have most of exons 8 and 9 deleted, and exons 5 through 9 deleted, respectively. In situ hybridization studies were completed to determine the differential expression of Galphai2 between the vestibular primary afferent neurons and the vestibular end organs. Computer modeling and predicted 3D conformation of the wild type Galphai2 and the two splice variants were completed to evaluate the changes associated with the Gbetagamma and GTP binding sites. These two novel alternatively spliced isoforms of Galphai2 putatively encode truncated proteins that could serve unique roles in the physiology of the vestibular neuroepithelium. Galphai2vest was found to be a processed pseudogene.

Analysis of differential gene expression in the cochlea and kidney of mouse by cDNA microarrays

Microarray hybridization analysis of gene expression in the cochlea and kidney suggest a relationship between these tissues at the genomic level, indicating the common gene expression, likely serving a common function in both the organs primarily maintaining ion transport, and implied previously from morphological, pharmaco-kinetic and teratogenic studies. The cDNAs of more than 100 genes listed on the hereditary hearing loss homepage were amplified as targets by RT-PCR and were hybridized with probes prepared from total RNA of the cochlea and the kidney. Thirteen of the genes analyzed showed altered fluorescence ratios of more than two logs. Of these, the expressions of 11 genes were over expressed and two were under expressed in the cochlea than in the kidney. Our data is the first report to corroborate the genomic similarities between these two important organs and may help to explain the somewhat similar response of these organs to certain therapeutic drugs.

Identification of unique transcripts from a mouse full-length, subtracted inner ear cDNA library

A small-scale full-length library construction approach was developed to facilitate production of a mouse full-length cDNA encyclopedia representing approximately 250 enriched, normalized, and/or subtracted cDNA libraries. One library produced using this approach was a subtracted adult mouse inner ear cDNA library (sIEa). The average size of the inserts was approximately 2.5 kb, with the majority ranging from 0.5 to 7.0 kb. From this library 22,574 sequence reads were obtained from 15,958 independent clones. Sequencing and chromosomal localization established 5240 clusters, with 1302 clusters being unique and 359 representing new ESTs. Our sIEa library contributed 56.1% of the 7773 nonredundant Unigene clusters associated with the four mouse inner ear libraries in the NCBI dbEST. Based on homologous chromosomal regions between human and mouse, we identified 1018 UniGene clusters associated with the deafness locus critical regions. Of these, 59 clusters were found only in our sIEa library and represented approximately 50% of the identified critical regions.

Cloning and Expression of a Small-Conductance Ca2+-Activated K+ Channel From the Mouse Cochlea: Coexpression with α9/α10 Acetylcholine Receptors

Functional interactions between ligand-gated, voltage-, and Ca2+-activated ion channels are essential to the properties of excitable cells and thus to the working of the nervous system. The outer hair cells in the mammalian cochlea receive efferent inputs from the brain stem through cholinergic nerve fibers that form synapses at their base. The acetylcholine released from these efferent fibers activates fast inhibitory postsynaptic currents mediated, to some extent, by small-conductance Ca2+-activated K+ channels (SK) that had not been cloned. Here we report the cloning, characterization, and expression of a complete SK2 cDNA from the mouse cochlea. The cDNAs of the mouse cochlea α9 and α10 acetylcholine receptors were also obtained, sequenced, and coexpressed with the SK2 channels. Human cultured cell lines transfected with SK2 yielded Ca2+-sensitive K+ current that was blocked by dequalinium chloride and apamin, known blockers of SK channels. Xenopus oocytes injected with SK2 in vitro transcribed RNA, under conditions where only outward K+ currents could be recorded, expressed an outward current that was sensitive to EGTA, dequalinium chloride, and apamin. In HEK-293 cells cotransfected with cochlear SK2 plus α9/α10 receptors, acetylcholine induced an inward current followed by a robust outward current. The results indicate that SK2 and the α9/α10 acetylcholine receptors are sufficient to partly recapitulate the native hair cell efferent synaptic response.

Cochlear whole mount in situ hybridization: identification of longitudinal and radial gradients

The morphology of the organ of Corti has a radial asymmetry and also changes longitudinally from base to apex. Cellular localization of transcripts within the inner ear has relied primarily on the use of sectioned tissue with in situ hybridization. However, radial and longitudinal gradients of expression are not readily recognized using sectioned tissue owing to problems in visualization of signals with varying intensities. Herein, we describe the use of whole mount in situ hybridization for identification of cochlear longitudinal and radial expression gradients in the neurosensory epithelium, hair cells. Not only can these hair cell gradients be shown in adult tissues, but also the developmental up-regulation and down-regulation of genes and their associated spatio-temporal expression patterns can be demonstrated.

Expression of subunits for the cAMP-sensitive 'olfactory' cyclic nucleotide-gated ion channel in the cochlea: implications for signal transduction

Cyclic nucleotide-gated (CNG) ion channels have been implicated as functioning in sensory transduction and in second-messenger modulation of synaptic neurotransmitter release. The olfactory, cAMP-sensitive CNG ion channel in vivo is considered to comprise the pore-forming CNG2 subunit together with CNG5 and CNG4.3 modulatory subunits. The expression of these 'olfactory' CNG subunit transcripts in microdissected subfractions of the rat cochlea and hair cell libraries has been investigated with RT-PCR. Unmodified transcripts of CNG2 were detected in the organ of Corti, lateral wall and spiral ganglion subfractions. CNG5 message was found in both the sensory organ of Corti and the non-sensory lateral wall subfractions but not in the spiral ganglion subfraction. The CNG5 sequence obtained for the organ of Corti fraction encompassed 78% of the olfactory CNG5 cDNA sequence. CNG5 message has also been detected in an inner hair cell cDNA library. In the lateral wall, unmodified CNG5 sequence was observed as well as truncated versions of CNG5 transcripts, one of which was also found in the rat brain. The truncated versions were characterized by deletions that resulted in a shift in reading frame and the premature appearance of a stop codon. The 'olfactory' CNG4.3 cDNA was amplified from all three subfractions. Within the cochlea, CNG2 immunoreactivity was selectively distributed in a pattern similar to that of adenylyl cyclase type I. Immunoreactivity to CNG2 has been localized to stereocilia of inner hair cells. CNG5 immunoreactivity was associated with stereocilia and lateral plasma membranes of outer hair cells. We conclude that transcripts necessary for a functional cAMP-sensitive CNG ion channel are present in the cochlea resulting from combinations of CNG2 with CNG5 and CNG4.3. Further, the localization of CNG2 and CNG5 immunoreactivity to hair cell stereocilia suggests a role for cAMP-sensitive CNG channels in hair cell signal transduction.

Receptor-G protein gamma specificity: gamma11 shows unique potency for A(1) adenosine and 5-HT(1A) receptors

G protein coupled receptors activate signal transducing guanine nucleotide-binding proteins (G proteins), which consist of an alpha subunit and a betagamma dimer. Whole cell studies have reported that receptors signal through specific betagamma subtypes. Membrane reconstitution studies with the adenosine A(1) and alpha(2A) adrenergic receptors have reached a similar conclusion. We aimed to test the generality of this finding by comparing the gamma subtype specificity for four G(i)-coupled receptors: alpha(2A) adrenergic; A1 adenosine (A(1)-R); 5-hydroxytryptamine(1A) (5-HT(1A)-R); mu opioid. Membranes were reconstituted with Galpha(i)(1) and five gamma subtypes (dimerized to beta1). Using a sensitive alpha-betagamma binding assay, we show that all recombinant betagamma (except beta1gamma1) had comparable affinity for alpha(i)(1). Using high affinity agonist binding as a measure of receptor-G protein coupling, betagamma-containing gamma11 was the most potent for A(1)-R and 5-HT(1A)-R (p < 0.05, one way ANOVA) while gamma7 was most potent for the other two receptors. gamma11 was 3-8-fold more potent for the A(1)-R than were the other gamma subtypes. Also, gamma11 was 2-8-fold more potent for A(1)-R than at the other receptors, suggesting a unique coupling specificity of the A(1)-R for gamma11. In contrast, the discrimination by receptors for the other betagamma subtypes (beta1 and gamma1, gamma2, gamma7, and gamma10) was limited (2-3-fold). Thus the exquisite betagamma specificity of individual receptors reported in whole cell studies may depend on in vivo mechanisms beyond direct receptor recognition of betagamma subtypes.

Differential distribution of the G protein gamma3 subunit in the developing zebrafish nervous system

G proteins play an essential role in the transduction and propagation of extracellular signals across the plasma membrane. It was once thought that the G protein alpha subunit was the sole regulator of intracellular molecules. The G protein betagamma complex is now recognized as participating in many signaling events. While screening a zebrafish cDNA library to identify members of the protein 4.1 superfamily (Kelly, G.M., Reversade, B., Biochem. Cell Biol. 75 (1997), 623), we fortuitously identified a clone that encodes a zebrafish G protein gamma subunit. The 666 nucleotides of the zebrafish G protein gamma subunit cDNA encodes a polypeptide of 75 amino acids with high degree of homology to human, bovine, rat and mouse gamma subunits. BLAST search analysis of GenBank revealed that the zebrafish gamma subunit is 93% identical and 97% similar to the mammalian gamma3 subunit. The gamma3 gene was mapped to the zebrafish linkage group 21, approximately 10.76 cRays from bf, a gene with sequence homology to the human properdin factor gene. RT-PCR and in situ hybridization analyses first detected gamma3 mRNA during late somitogenesis, where it was expressed preferentially in the Vth cranial nerve, the forebrain and in ventrolateral regions of the mid- and hindbrain including the spinal cord. The ability of the zebrafish gamma3 subunit to form a signaling heterodimeric complex with a beta subunit was tested using a human beta2 subunit. The gamma3 formed a heterodimer with beta2 and the complex was capable of binding calmodulin in a calcium-dependent manner. Overexpression of the beta2gamma3 complex in zebrafish embryos lead to the loss of dorsoanterior structures and heart defects, possibly owing to an up-regulation of mitogen-activated protein kinase activity and/or decline in protein kinase A signaling. Together, these data imply that a betagamma heterodimer plays a role in signal transduction events involving G protein coupled receptors and that these events occur in specific regions in the nervous system of the developing zebrafish.

New thoughts on the role of the beta-gamma subunit in G-protein signal transduction

Heterotrimeric G proteins are involved in numerous biological processes, where they mediate signal transduction from agonist-bound G-protein-coupled receptors to a variety of intracellular effector molecules and ion channels. G proteins consist of two signaling moieties: a GTP-bound alpha subunit and a beta-gamma heterodimer. The beta-gamma dimer, recently credited as a significant modulator of G-protein-mediated cellular responses, is postulated to be a major determinant of signaling fidelity between G-protein-coupled receptors and downstream effectors. In this review we have focused on the role of beta-gamma signaling and have included examples to demonstrate the heterogeneity in the heterodimer composition and its implications in signaling fidelity. We also present an overview of some of the effectors regulated by beta-gamma and draw attention to the fact that, although G proteins and their associated receptors play an instrumental role in development, there is rather limited information on beta-gamma signaling in embryogenesis.

Longitudinal gradients of KCNQ4 expression in spiral ganglion and cochlear hair cells correlate with progressive hearing loss in DFNA2

Mutations in the human KCNQ4 gene were recently found by Kubisch et al. [Cell 96 (1999) 437-446] to cause a non-syndromic, autosomal dominant, progressive hearing loss, DFNA2. The mouse Kcnq4 orthologue was previously localized to the outer hair cells (OHCs) of the inner ear, suggesting the pathophysiological effects were due to dysfunctional OHCs. Yet, OHC dysfunction does not provide a plausible explanation for the progressive nature of the frequency specific hearing loss. We have re-examined and extended the expression analyses of KCNQ4 in the murine inner ear using RT-PCR and whole mount in situ hybridization. Our results confirmed that the rat KCNQ4 orthologue is expressed in both inner and outer hair cells. Reciprocal longitudinal gradients were found in inner hair cells (IHCs) and OHCs. The strongest expression of KCNQ4 in IHCc was in the base of the cochlea and in the apex for OHCs. Similar to the IHCs, a basal to apical gradient was present in the spiral sensory neurons. IHCs mediate hearing via their afferent sensory neurons, whereas OHCs function as active cochlear amplifiers. The complete absence of OHCs leads only to severe sensitivity reduction, but not complete hearing loss. Our data suggest that the primary defect leading to initial high frequency loss and subsequent progressive hearing loss for all frequencies may be due to spiral ganglion and/or IHC dysfunction, rather than an OHC aberration.