Sequence-based exon prediction around the synaptophysin locus reveals a gene-rich area containing novel genes in human proximal Xp

The role of prickle proteins in vertebrate development and pathology

Prickle is an evolutionarily conserved family of proteins exclusively associated with planar cell polarity (PCP) signalling. This signalling pathway provides directional and positional cues to eukaryotic cells along the plane of an epithelial sheet, orthogonal to both apicobasal and left-right axes. Through studies in the fruit fly Drosophila, we have learned that PCP signalling is manifested by the spatial segregation of two protein complexes, namely Prickle/Vangl and Frizzled/Dishevelled. While Vangl, Frizzled, and Dishevelled proteins have been extensively studied, Prickle has been largely neglected. This is likely because its role in vertebrate development and pathologies is still being explored and is not yet fully understood. The current review aims to address this gap by summarizing our current knowledge on vertebrate Prickle proteins and to cover their broad versatility. Accumulating evidence suggests that Prickle is involved in many developmental events, contributes to homeostasis, and can cause diseases when its expression and signalling properties are deregulated. This review highlights the importance of Prickle in vertebrate development, discusses the implications of Prickle-dependent signalling in pathology, and points out the blind spots or potential links regarding Prickle, which could be studied further.

Draft Genome of the European Mouflon (Ovis orientalis musimon)

Mouflon (Ovis orientalis) with its huge and beautiful horns is considered as one of the ancestors of domesticated sheep. The European mouflon (Ovis orientalis musimon) is in the Asiatic mouflon (O. orientalis) clade. In order to provide novel genome information for mouflon, moreover promote genetic analysis of genus Ovis both domestic and wild, we propose to sequence the mouflon genome. We assembled the highly heterozygous mouflon genome based on Illumina HiSeq platform using the next-generation sequencing technology. Finally, the draft genome we accessed approximately 2.69 Gb (42.15% GC), while N50 sizes of contig and scaffold are 110.1 kb and 10.4 Mb, respectively. The contiguity of this assembly is obviously better than earlier versions. Further analyses predicted 20,814 protein-coding genes in the mouflon genome and 12,390 shared gene families among bovine species. It is estimated that the divergence time between O. orientalis musimon and Ovis aries was 7.6 million years ago. The draft mouflon genome assembly will provide data support and theoretical basis for various investigations of the genus Ovis species in future.

Photoreceptor degeneration in a new Cacna1f mutant mouse model

The Cacna1f gene encodes the α1F subunit of an L-type voltage-gated calcium channel, Cav1.4. In photoreceptor synaptic terminals, Cav1.4 channels mediate glutamate release and postsynaptic responses associated with visual signal transmission. We have discovered a new Cacna1f mutation in nob9 mice, which display more severe phenotypes than do nob2 mice. To characterize the nob9 phenotype at different ages, we examined the murine fundus, applied retinal optical coherence tomography, measured flash electroretinograms (ERGs) in vivo, and analyzed the retinal histology in vitro. After identifying the X-linked recessive inheritance trait, we sequenced Cacna1f as the candidate gene. Mutations in this gene were detected by polymerase chain reaction (PCR) and confirmed by restriction fragment length polymorphism. Morphologically, an early-onset of retinal disorder was detected, and the degeneration of the outer plexiform layers progressed rapidly. Moreover, the mutant mice showed drastically reduced scotopic ERGs with increasing age. In 14-month-old nob9 retinas, immunostaining of cone opsins demonstrated a reduction in the number of short-wavelength opsins (S-opsins) to 54% of wild-type levels, and almost no middle-wavelength opsins (M-opsins) were observed. No cone ERGs could be detected from residual cones, in which S-opsins abnormally migrated to inner segments of the photoreceptors. The mutations of the Cacna1f gene in nob9 mice involved both a single nucleotide G to A transition and a 10-nucleotide insertion, the latter resulting in a frame-shift mutation in exon 14.

Channeling Vision: CaV1.4-A Critical Link in Retinal Signal Transmission

Voltage-gated calcium channels (VGCC) are key to many biological functions. Entry of Ca2+ into cells is essential for initiating or modulating important processes such as secretion, cell motility, and gene transcription. In the retina and other neural tissues, one of the major roles of Ca2+-entry is to stimulate or regulate exocytosis of synaptic vesicles, without which synaptic transmission is impaired. This review will address the special properties of one L-type VGCC, CaV1.4, with particular emphasis on its role in transmission of visual signals from rod and cone photoreceptors (hereafter called "photoreceptors," to the exclusion of intrinsically photoreceptive retinal ganglion cells) to the second-order retinal neurons, and the pathological effects of mutations in the CACNA1F gene which codes for the pore-forming α1F subunit of CaV1.4.

Tweeters, Woofers and Horns: The Complex Orchestration of Calcium Currents in T Lymphocytes

Elevation of intracellular calcium ion (Ca²⁺) levels is a vital event that regulates T lymphocyte homeostasis, activation, proliferation, differentiation, and apoptosis. The mechanisms that regulate intracellular Ca²⁺ signaling in lymphocytes involve tightly controlled concinnity of multiple ion channels, membrane receptors, and signaling molecules. T cell receptor (TCR) engagement results in depletion of endoplasmic reticulum (ER) Ca²⁺ stores and subsequent sustained influx of extracellular Ca²⁺ through Ca²⁺ release-activated Ca²⁺ (CRAC) channels in the plasma membrane. This process termed store-operated Ca²⁺ entry (SOCE) involves the ER Ca²⁺ sensing molecule, STIM1, and a pore-forming plasma membrane protein, ORAI1. However, several other important Ca²⁺ channels that are instrumental in T cell function also exist. In this review, we discuss the role of additional Ca²⁺ channel families expressed on the plasma membrane of T cells that likely contribute to Ca²⁺ influx following TCR engagement, which include the TRP channels, the NMDA receptors, the P2X receptors, and the IP₃ receptors, with a focus on the voltage-dependent Ca²⁺ (Ca_V) channels.

Identification of a novel heterozygous missense mutation in the CACNA1F gene in a chinese family with retinitis pigmentosa by next generation sequencing

BACKGROUND

Retinitis pigmentosa (RP) is an inherited retinal degenerative disease, which is clinically and genetically heterogeneous, and the inheritance pattern is complex. In this study, we have intended to study the possible association of certain genes with X-linked RP (XLRP) in a Chinese family.

METHODS

A Chinese family with RP was recruited, and a total of seven individuals were enrolled in this genetic study. Genomic DNA was isolated from peripheral leukocytes, and used for the next generation sequencing (NGS).

RESULTS

The affected individual presented the clinical signs of XLRP. A heterozygous missense mutation (c.1555C>T, p.R519W) was identified by NGS in exon 13 of the CACNA1F gene on X chromosome, and was confirmed by Sanger sequencing. It showed perfect cosegregation with the disease in the family. The mutation at this position in the CACNA1F gene of RP was found novel by database searching.

CONCLUSION

By using NGS, we have found a novel heterozygous missense mutation (c.1555C>T, p.R519W) in CACNA1F gene, which is probably associated with XLRP. The findings might provide new insights into the cause and diagnosis of RP, and have implications for genetic counseling and clinical management in this family.

Spontaneous and nicotine-induced Ca2+ oscillations mediated by Ca2+ influx in rat pinealocytes

The pineal gland regulates circadian rhythm through the synthesis and secretion of melatonin. The rise of intracellular Ca(2+) concentration ([Ca(2+)]i) following nicotinic acetylcholine receptor (nAChR) stimulation due to parasympathetic nerve activity downregulates melatonin production. Important characteristics and roles of Ca(2+) mobilization due to nAChR stimulation remain to be clarified. We report here that spontaneous Ca(2+) oscillations can be observed in ∼15% of the pinealocytes in slice preparations from rat pineal glands when this dissociation procedure is done within 6 h from a dark-to-light change. The frequency and half-life of [Ca(2+)]i rise were 0.86 min(-1) and 19 s, respectively. Similar spontaneous Ca(2+) oscillations were recorded in 17% of rat pinealocytes that were primary cultured for several days. Simultaneous measurement of [Ca(2+)]i and membrane potential revealed that spontaneous Ca(2+) oscillations were triggered by periodic membrane depolarizations. Spontaneous Ca(2+) oscillations in cultured pinealocytes were abolished by extracellular Ca(2+) removal or application of nifedipine, a blocker of voltage-dependent Ca(2+) channel (VDCC). In contrast, blockers of intracellular Ca(2+)-release channels, 2-aminoethoxydiphenylborate and ryanodine, have no effect. Our results also reveal that, in 23% quiescent pinealocytes, Ca(2+) oscillations were observed following the withdrawal of nicotine. Norepinephrine-induced melatonin secretion from whole pineal glands was significantly decreased by the coapplication of acetylcholine (ACh). This inhibitory effect of ACh was attenuated by nifedipine. In conclusion, both spontaneous and evoked Ca(2+) oscillations are due to membrane depolarization following activation of VDCCs. This consists of VDCC α1F subunit, and the associated Ca(2+) influx can strongly regulate melatonin secretion in pineal glands.

Weft, warp, and weave: the intricate tapestry of calcium channels regulating T lymphocyte function

Calcium (Ca(2+)) is a universal second messenger important for T lymphocyte homeostasis, activation, proliferation, differentiation, and apoptosis. The events surrounding Ca(2+) mobilization in lymphocytes are tightly regulated and involve the coordination of diverse ion channels, membrane receptors, and signaling molecules. A mechanism termed store-operated Ca(2+) entry (SOCE), causes depletion of endoplasmic reticulum (ER) Ca(2+) stores following T cell receptor (TCR) engagement and triggers a sustained influx of extracellular Ca(2+) through Ca(2+) release-activated Ca(2+) (CRAC) channels in the plasma membrane. The ER Ca(2+) sensing molecule, stromal interaction molecule 1 (STIM1), and a pore-forming plasma membrane protein, ORAI1, have been identified as important mediators of SOCE. Here, we review the role of several additional families of Ca(2+) channels expressed on the plasma membrane of T cells that likely contribute to Ca(2+) influx following TCR engagement, particularly highlighting an important role for voltage-dependent Ca(2+) channels (CaV) in T lymphocyte biology.

Effect of transfection with PLP2 antisense oligonucleotides on gene expression of cadmium-treated MDA-MB231 breast cancer cells

Emerging evidence indicates that cadmium (Cd) is able to regulate gene expression, drastically affecting the pattern of transcriptional activity in human normal and pathological cells. We have already shown that exposure of MDA-MB231 breast cancer cells to 5 μM CdCl(2) for 96 h, apart from significantly affecting mitochondrial metabolism, induces modifications of the expression level of genes coding for members of stress response-, mitochondrial respiration-, MAP kinase-, NF-κB-, and apoptosis-related pathways. In the present study, we have expanded the knowledge on the biological effects of Cd-breast cancer cell interactions, indicating PLP2 (proteolipid protein-2) as a novel member of the list of Cd-upregulated genes by MDA-MB231 cancer cells and, through the application of transfection techniques with specific antisense oligonucleotides, we have demonstrated that such over-expression may be an upstream event to some of the changes of gene expression levels already observed in Cd-treated cells, thus unveiling new possible molecular relationship between PLP2 and genes linked to the stress and apoptotic responses.

Human skeletal muscle calcium channel alpha1S is expressed in the basal ganglia: distinctive expression pattern among L-type Ca2+ channels

Voltage-gated calcium channels (VGCCs) are essential molecules for neuronal function. VGCCs consist of five subunits, alpha1, alpha2, beta, gamma, and delta. Among the ten subtypes of the alpha1 subunit (alpha1A-I and S), expression of alpha1S was previously believed to be restricted to the skeletal muscle. We report here, however, that alpha1S is also expressed in human and rat central nervous system. First, we performed PCR screening for VGCC alpha1 subunits in human nervous system using degenerate primers, and identified alpha1S as well as all the eight alpha1 subunits with previously described expression. Intriguingly, alpha1S was selectively localized to the basal ganglia, particularly the caudate nucleus. In situ hybridization showed that alpha1S was expressed in medium-sized caudate neurons. Quantitative analysis using real time RT-PCR revealed a distinct pattern of alpha1S expression among L-type calcium channels. Furthermore, RT-PCR using laser-mediated manipulation of single cells suggested that human alpha1S was coexpressed with ryanodine receptors (RYRs) in GABAergic neurons. Our results suggest the potential relevance of alpha1S to dopaminergic signal transduction and calcium-induced calcium release in caudate neurons.

The function of Ca(2+) channel subtypes in exocytotic secretion: new perspectives from synaptic and non-synaptic release

By mediating the Ca(2+) influx that triggers exocytotic fusion, Ca(2+) channels play a central role in a wide range of secretory processes. Ca(2+) channels consist of a complex of protein subunits, including an alpha(1) subunit that constitutes the voltage-dependent Ca(2+)-selective membrane pore, and a group of auxiliary subunits, including beta, gamma, and alpha(2)-delta subunits, which modulate channel properties such as inactivation and channel targeting. Subtypes of Ca(2+) channels are constituted by different combinations of alpha(1) subunits (of which 10 have been identified) and auxiliary subunits, particularly beta (of which 4 have been identified). Activity-secretion coupling is determined not only by the biophysical properties of the channels involved, but also by the relationship between channels and the exocytotic apparatus, which may differ between fast and slow types of secretion. Colocalization of Ca(2+) channels at sites of fast release may depend on biochemical interactions between channels and exocytotic proteins. The aim of this article is to review recent work on Ca(2+) channel structure and function in exocytotic secretion. We discuss Ca(2+) channel involvement in selected types of secretion, including central neurotransmission, endocrine and neuroendocrine secretion, and transmission at graded potential synapses. Several different Ca(2+) channel subtypes are involved in these types of secretion, and their function is likely to involve a variety of relationships with the exocytotic apparatus. Elucidating the relationship between Ca(2+) channel structure and function is central to our understanding of the fundamental process of exocytotic secretion.

Alternative splicing in intracellular loop connecting domains II and III of the alpha 1 subunit of Cav1.2 Ca2+ channels predicts two-domain polypeptides with unique C-terminal tails

Novel splice variants of the alpha(1) subunit of the Ca(v)1.2 voltage-gated Ca(2+) channel were identified that predicted two truncated forms of the alpha(1) subunit comprising domains I and II generated by alternative splicing in the intracellular loop region linking domains II and III. In rabbit heart splice variant 1 (RH-1), exon 19 was deleted, which resulted in a reading frameshift of exon 20 with a premature termination codon and a novel 19-amino acid carboxyl-terminal tail. In the RH-2 variant, exons 17 and 18 were deleted, leading to a reading frameshift of exons 19 and 20 with a premature stop codon and a novel 62-amino acid carboxyl-terminal tail. RNase protection assays with RH-1 and RH-2 cRNA probes confirmed the expression in cardiac and neuronal tissue but not skeletal muscle. The deduced amino acid sequence from full-length cDNAs encoding the two variants predicted polypeptides of 99.0 and 99.2 kDa, which constituted domains I and II of the alpha(1) subunit of the Ca(v)1.2 channel. Antipeptide antibodies directed to sequences in the second intracellular loop between domains II and III identified the 240-kDa Ca(v)1.2 subunit in sarcolemmal and heavy sarcoplasmic reticulum (HSR) membranes and a 99-kDa polypeptide in the HSR. An antipeptide antibody raised against unique sequences in the RH-2 variant also identified a 99-kDa polypeptide in the HSR. These data reveal the expression of additional Ca(2+) channel structural units generated by alternative splicing of the Ca(v)1.2 gene.

Isolation and characterization of a calcium channel gene, Cacna1f, the murine orthologue of the gene for incomplete X-linked congenital stationary night blindness

The mutant L-type calcium channel alpha(1)-subunit gene, CACNA1F, was recently identified as the gene responsible for incomplete X-linked congenital stationary night blindness. The 6070-bp mRNA transcript is predicted to encode a 1977-amino-acid pore-forming protein with cytoplasmic amino- and carboxyl-termini separated by four homologous repeat domains, each consisting of six transmembrane segments. CACNA1F has been shown to be preferentially expressed in the retina, indicative of a specific functional role in visual processing. We have established the complete sequence of the murine orthologue of CACNA1F, namely Cacna1f. The total length of the mRNA transcript of the murine gene was established to be 6080 bp with an open reading frame that translates into a 1985-amino-acid protein. Cacna1f is highly homologous to the human sequence, with 90% identity at the amino acid level and almost perfect conservation between the functional domains. Furthermore, as in the human gene, the 3' end of the Cacna1f gene maps within 5 kb of the 5' end of the mouse synaptophysin gene in a region orthologous to Xp11.23. Using in situ hybridization, Cacna1f was found to be expressed in the inner and outer nuclear layers and the ganglion cell layer of the retina.

A transcript map of a 2-Mb BAC contig in the proximal portion of the mouse X chromosome and regional mapping of the scurfy mutation

A physical clone contig has been constructed, spanning 2 Mb on the proximal mouse X chromosome containing the mouse scurfy (sf) and tattered (Td) mutations. Extensive transcript mapping in this interval has identified 37 potential transcription units, including a number of novel genes, and 4 pseudogenes. These genes have been ordered by STS content and restriction mapping. Comparison of the transcript map to the corresponding region in human Xp11.23-p11.22 shows extensive homology, with complete conservation of gene order for loci in common between the two maps. Further, using a novel method to identify simple sequence length polymorphisms, we have developed a number of genetic markers, which has enabled the region containing the sf mutation to be narrowed to <300 kb. This contig has already allowed the cloning of the Td gene using a candidate gene approach and now serves as a starting point for the cloning of the sf mutation.

Spiny legs and prickled bodies: new insights and complexities in planar polarity establishment

Epithelial cells can be polarized along two axes, namely in the apical-basolateral axis and in the horizontal plane of the epithelium. Vertebrate examples of planar polarization include aspects of skin development or features in internal organs, such as the inner ear epithelium. In insects like Drosophila, adult cuticular structures show planar polarization. Studies on planar polarity in Drosophila have identified several genes that regulate this process. Notably, the Frizzled receptor and its signaling cascade provide an entry point to the molecular aspects of planar polarization. A recent study by Gubb et al.((1)) of the prickle locus, which encodes a cytoplasmic protein with three LIM domains, provides new insights and raises several interesting questions that can now be addressed. Pk might serve a scaffolding function involved in assembling a protein complex required for planar polarity establishment.

G protein modulation of N-type calcium channels is facilitated by physical interactions between syntaxin 1A and Gbetagamma

The direct modulation of N-type calcium channels by G protein betagamma subunits is considered a key factor in the regulation of neurotransmission. Some of the molecular determinants that govern the binding interaction of N-type channels and Gbetagamma have recently been identified (see, i.e., Zamponi, G. W., Bourinet, E., Nelson, D., Nargeot, J., and Snutch, T. P. (1997) Nature 385, 442-446); however, little is known about cellular mechanisms that modulate this interaction. Here we report that a protein of the presynaptic vesicle release complex, syntaxin 1A, mediates a crucial role in the tonic inhibition of N-type channels by Gbetagamma. When syntaxin 1A was coexpressed with (N-type) alpha(1B) + alpha(2)-delta + beta(1b) channels in tsA-201 cells, the channels underwent a 18 mV negative shift in half-inactivation potential, as well as a pronounced tonic G protein inhibition as assessed by its reversal by strong membrane depolarizations. This tonic inhibition was dramatically attenuated following incubation with botulinum toxin C, indicating that syntaxin 1A expression was indeed responsible for the enhanced G protein modulation. However, when G protein betagamma subunits were concomitantly coexpressed, the toxin became ineffective in removing G protein inhibition, suggesting that syntaxin 1A optimizes, rather than being required for G protein modulation of N-type channels. We also demonstrate that Gbetagamma physically binds to syntaxin 1A, and that syntaxin 1A can simultaneously interact with Gbetagamma and the synprint motif of the N-type channel II-III linker. Taken together, our experiments suggest a mechanism by which syntaxin 1A mediates a colocalization of G protein betagamma subunits and N-type calcium channels, thus resulting in more effective G protein coupling to, and regulation of, the channel. Thus, the interactions between syntaxin, G proteins, and N-type calcium channels are part of the structural specialization of the presynaptic terminal.

Determinants of voltage-dependent inactivation affect Mibefradil block of calcium channels

The voltage gated calcium channel family is a major target for a range of therapeutic drugs. Mibefradil (Ro 40-5967) belongs to a new chemical class of these molecules which differs from other Ca2+ antagonists by its ability to potently block T-type Ca2+ channels. However, this molecule has also been shown to inhibit other Ca2+ channel subtypes. To further analyze the mechanism governing the Ca2+ channel-Mibefradil interaction, we examined the effect of Mibefradil on various recombinant Ca2+ channels expressed in mammalian cells from their cloned cDNAs, using Ca2+ as the permeant ion at physiological concentration. Expression of alpha1A, alpha1C, and alpha1E in tsA 201 cells resulted in Ca2+ currents with functional characteristics closely related to those of their native counterparts. Mibefradil blocked alpha1A and alpha1E with a Kd comparable to that reported for T-type channels, but had a lower affinity (approximately 30-fold) for alpha1C. For each channel, inhibition by Mibefradil was consistent with high-affinity binding to the inactivated state. Modulation of the voltage-dependent inactivation properties by the nature of the coexpressed beta subunit or the alpha1 splice variant altered block at the Mibefradil receptor site. Therefore, we conclude that the tissue and sub-cellular localization of calcium channel subunits as well as their specific associations are essential parameters to understand the in vivo effects of Mibefradil.

Identification of three novel Ca(2+) channel gamma subunit genes reveals molecular diversification by tandem and chromosome duplication

Gene duplication is believed to be an important evolutionary mechanism for generating functional diversity within genomes. The accumulated products of ancient duplication events can be readily observed among the genes encoding voltage-dependent Ca(2+) ion channels. Ten paralogous genes have been identified that encode isoforms of the alpha(1) subunit, four that encode beta subunits, and three that encode alpha(2)delta subunits. Until recently, only a single gene encoding a muscle-specific isoform of the Ca(2+) channel gamma subunit (CACNG1) was known. Expression of a distantly related gene in the brain was subsequently demonstrated upon isolation of the Cacng2 gene, which is mutated in the mouse neurological mutant stargazer (stg). In this study, we sought to identify additional genes that encoded gamma subunits. Because gene duplication often generates paralogs that remain in close syntenic proximity (tandem duplication) or are copied onto related daughter chromosomes (chromosome or whole-genome duplication), we hypothesized that the known positions of CACNG1 and CACNG2 could be used to predict the likely locations of additional gamma subunit genes. Low-stringency genomic sequence analysis of targeted regions led to the identification of three novel Ca(2+) channel gamma subunit genes, CACNG3, CACNG4, and CACNG5, on chromosomes 16 and 17. These results demonstrate the value of genome evolution models for the identification of distantly related members of gene families.

A new neurological syndrome with mental retardation, choreoathetosis, and abnormal behavior maps to chromosome Xp11

Choreoathetosis is a major clinical feature in only a small number of hereditary neurological disorders. We define a new X-linked syndrome with a unique clinical picture characterized by mild mental retardation, choreoathetosis, and abnormal behavior. We mapped the disease in a four-generation pedigree to chromosome Xp11 by linkage analysis and defined a candidate region containing a number of genes possibly involved in neuronal signaling, including a potassium channel gene and a neuronal G protein-coupled receptor.

Voltage-gated ion channels and hereditary disease

By the introduction of technological advancement in methods of structural analysis, electronics, and recombinant DNA techniques, research in physiology has become molecular. Additionally, focus of interest has been moving away from classical physiology to become increasingly centered on mechanisms of disease. A wonderful example for this development, as evident by this review, is the field of ion channel research which would not be nearly as advanced had it not been for human diseases to clarify. It is for this reason that structure-function relationships and ion channel electrophysiology cannot be separated from the genetic and clinical description of ion channelopathies. Unique among reviews of this topic is that all known human hereditary diseases of voltage-gated ion channels are described covering various fields of medicine such as neurology (nocturnal frontal lobe epilepsy, benign neonatal convulsions, episodic ataxia, hemiplegic migraine, deafness, stationary night blindness), nephrology (X-linked recessive nephrolithiasis, Bartter), myology (hypokalemic and hyperkalemic periodic paralysis, myotonia congenita, paramyotonia, malignant hyperthermia), cardiology (LQT syndrome), and interesting parallels in mechanisms of disease emphasized. Likewise, all types of voltage-gated ion channels for cations (sodium, calcium, and potassium channels) and anions (chloride channels) are described together with all knowledge about pharmacology, structure, expression, isoforms, and encoding genes.

Single gene defects in mice: the role of voltage-dependent calcium channels in absence models

Nineteen genes encoding alpha1, beta, gamma, or alpha2delta voltage-dependent calcium channel subunits have been identified to date. Recent studies have found that three of these genes are mutated in mice with generalised cortical spike-wave discharges (models of human absence epilepsy), emphasising the importance of calcium channels in regulating the expression of this inherited seizure phenotype. The tottering (tg) locus encodes the calcium channel alpha1 subunit gene Cacna1a, lethargic (lh) encodes the beta subunit gene Cacnb4, and stargazer (stg) encodes the gamma subunit gene Cacng2. These calcium channel mutants should provide important insights into the basic mechanisms of neuronal synchronisation, and the genes may be considered candidates for involvement in similar human disorders. The mutant models offer an important opportunity to elucidate the molecular, developmental, and physiological mechanisms underlying one subtype of absence epilepsy. Since calcium channels are involved in numerous cellular functions, including proliferation and differentiation, membrane excitability, neurite outgrowth and synaptogenesis, signal transduction, and gene expression, their role in generating the absence epilepsy phenotype may be complex. A comparative analysis of channel function and neural excitability patterns in tottering, lethargic, and stargazer brain should be useful in identifying the common elements of calcium channel involvement in these absence models.

The balance between isoforms of the prickle LIM domain protein is critical for planar polarity in Drosophila imaginal discs

The tissue polarity mutants in Drosophila include a set of conserved gene products that appear to be involved in the control of cytoskeletal architecture. Here we show that the tissue polarity gene prickle (pk) encodes a protein with a triple LIM domain and a novel domain that is present in human, murine, and Caenorhabditis elegans homologs which we designate PET. Three transcripts have been identified, pk, pkM, and sple, encoding 93-, 100-, and 129-kD conceptual proteins, respectively. The three transcripts span 70 kb and share 6 exons that contain the conserved domains. The pk and sple transcripts are expressed with similar tissue-specific patterns but have qualitatively different activities. The phenotypes of pk mutants, and transgenic flies in which the different isoforms are overexpressed show that the balance between Pk and Sple is critical for the specification of planar polarity. In addition, these phenotypes suggest a tessellation model in which the alignment of wing hairs is dependent on cell shape and need not reflect fine-grained positional information. Lack of both pk and sple transcripts gives a phenotype affecting the whole body surface that is similar to those of dishevelled and frizzled (fz) suggesting a functional relationship between pk and fz signaling.

Voltage-dependent calcium channel mutations in neurological disease

Calcium ion channel mutations disrupt channel function and create recognizable disease phenotypes in the nervous system. The broad array of underlying cellular alterations is commensurate with the expanding genetic diversity of the voltage-gated calcium ion channel complex and its critical role in regulating cell function. Currently, 16 calcium channel genes are known, and mutations in 7 of these are associated with distinct inherited neurological disorders. These mutations provide new insight into the structure and function of the channels, and link specific subunits to cellular disease processes, including altered excitability, synaptic signaling, and cell death. Studies of mutant channel behavior, subunit interactions, and the differentiation of neural networks demonstrate unique patterns of downstream rearrangement. Developmental analysis of molecular plasticity in these mutants is a critical step to define the intervening mechanisms that translate aberrant ion channel behavior into the diverse clinical phenotypes observed.

Identification of an integration center for cross-talk between protein kinase C and G protein modulation of N-type calcium channels

The modulation of presynaptic calcium channel activity by second messengers provides a fine tuning mechanism for neurotransmitter release. In neurons, the activation of certain G protein-coupled receptors reduces N-type channel activity by approximately 60%. In contrast, activation of protein kinase C (PKC) results in an approximately 50% increase in N-type channel activity, and subsequent G protein inhibition is antagonized. Here, we describe the molecular determinants that control the dual effects of PKC-dependent phosphorylation. The double substitution of two adjacent PKC consensus sites in the calcium channel domain I-II linker (Thr422, Ser425) to alanines abolished both PKC-dependent up-regulation and the PKC-G protein cross-talk. The single substitution of Ser425 to glutamic acid abolished PKC up-regulation but had no effect on G protein modulation. Replacement of Thr422 with glutamic acid eliminated PKC-dependent up-regulation and mimicked the effects of PKC phosphorylation on G protein inhibition. Our data suggest that Thr422 mediates the antagonistic effect of PKC on G protein modulation, while phosphorylation of either Thr422 or Ser425 are sufficient to increase N-type channel activity. Thus, Thr422 serves as a molecular switch by which PKC is able to simultaneously trigger the up-regulation of channel activity and antagonize G protein inhibition.

Molecular characterization of a novel family of low voltage-activated, T-type, calcium channels

Low voltage-activated, T-type, calcium channels are thought to be involved in pacemaker activity, low threshold Ca2+ spikes, neuronal oscillations and resonance, and rebound burst firing. Mutations in T-type channel genes may be a contributing factor to neurological and cardiovascular disorders, such as epilepsy, arrhythmia, and hypertension. Due to the lack of selective blockers, little is known about their structure or molecular biology. This review discusses our recent findings on the cloning, chromosomal localization, and functional expression, of two novel channels, alpha1G and alpha1H. The biophysical properties of these cloned channels (distinctive voltage dependence, kinetics, and single channel conductance) demonstrates that these channels are members of the T-type Ca2+ channel family.

Loss-of-function mutations in a calcium-channel alpha1-subunit gene in Xp11.23 cause incomplete X-linked congenital stationary night blindness

X-linked congenital stationary night blindness (CSNB) is a recessive non-progressive retinal disorder characterized by night blindness, decreased visual acuity, myopia, nystagmus and strabismus. Two distinct clinical entities of X-linked CSNB have been proposed. Patients with complete CSNB show moderate to severe myopia, undetectable rod function and a normal cone response, whereas patients with incomplete CSNB show moderate myopia to hyperopia and subnormal but measurable rod and cone function. The electrophysiological and psychophysical features of these clinical entities suggest a defect in retinal neurotransmission. The apparent clinical heterogeneity in X-linked CSNB reflects the recently described genetic heterogeneity in which the locus for complete CSNB (CSNB1) was mapped to Xp11.4, and the locus for incomplete CSNB (CSNB2) was refined within Xp11.23 (ref. 5). A novel retina-specific gene mapping to the CSNB2 minimal region was characterized and found to have similarity to voltage-gated L-type calcium channel alpha1-subunit genes. Mutation analysis of this new alpha1-subunit gene, CACNA1F, in 20 families with incomplete CSNB revealed six different mutations that are all predicted to cause premature protein truncation. These findings establish that loss-of-function mutations in CACNA1F cause incomplete CSNB, making this disorder an example of a human channelopathy of the retina.

An L-type calcium-channel gene mutated in incomplete X-linked congenital stationary night blindness

The locus for the incomplete form of X-linked congenital stationary night blindness (CSNB2) maps to a 1.1-Mb region in Xp11.23 between markers DXS722 and DXS255. We identified a retina-specific calcium channel alpha1-subunit gene (CACNA1F) in this region, consisting of 48 exons encoding 1966 amino acids and showing high homology to L-type calcium channel alpha1-subunits. Mutation analysis in 13 families with CSNB2 revealed nine different mutations in 10 families, including three nonsense and one frameshift mutation. These data indicate that aberrations in a voltage-gated calcium channel, presumably causing a decrease in neurotransmitter release from photoreceptor presynaptic terminals, are a frequent cause of CSNB2.