Drosophila Shaking-B protein forms gap junctions in paired Xenopus oocytes

Ontogeny of flight initiation in the fly Drosophila melanogaster: implications for the giant fibre system

There are two modes of flight initiation in Drosophila melanogaster-escape and voluntary. Although the circuitry underlying escape is accounted for by the Giant fibre (GF) system, the system underlying voluntary flight initiation is unknown. The GF system is functionally complete before the adult fly ecloses, but immature adults initially fail to react to a stimulus known to reliably evoke escape in mature adults. This suggests that escape in early adulthood, approximately 2-h post-eclosion, is not automatically triggered by the hard-wired GF system. Indeed, we reveal that escape behaviour displays a staged emergence during the first hour post-eclosion, suggesting that the GF system is subject to declining levels of suppression. Voluntary flight initiations are not observed at all during the period when the GF system is released from its suppression, nor indeed for some time after. We addressed the question whether voluntary flight initiation requires the GF system by observing take-off in Shak-B ( 2 ) mutant flies, in which the GF system is defunct. While the escape response is severely impaired in these mutants, they displayed normal voluntary flight initiation. Thus, the escape mechanism is subject to developmental modulation following eclosion and the GF system does not underlie voluntary flight.

An innexin-dependent cell network establishes left-right neuronal asymmetry in C. elegans

Gap junctions are widespread in immature neuronal circuits, but their functional significance is poorly understood. We show here that a transient network formed by the innexin gap-junction protein NSY-5 coordinates left-right asymmetry in the developing nervous system of Caenorhabditis elegans. nsy-5 is required for the left and right AWC olfactory neurons to establish stochastic, asymmetric patterns of gene expression during embryogenesis. nsy-5-dependent gap junctions in the embryo transiently connect the AWC cell bodies with those of numerous other neurons. Both AWCs and several other classes of nsy-5-expressing neurons participate in signaling that coordinates left-right AWC asymmetry. The right AWC can respond to nsy-5 directly, but the left AWC requires nsy-5 function in multiple cells of the network. NSY-5 forms hemichannels and intercellular gap-junction channels in Xenopus oocytes, consistent with a combination of cell-intrinsic and network functions. These results provide insight into gap-junction activity in developing circuits.

Molecular basis of voltage dependence of connexin channels: An integrative appraisal

The importance of electrical and molecular signaling through connexin (Cx) channels is now widely recognized. The transfer of ions and other small molecules between adjacent cells is regulated by multiple stimuli, including voltage. Indeed, Cx channels typically exhibit complex voltage sensitivity. Most channels are sensitive to the voltage difference between the cell interiors (or transjunctional voltage, V(j)), while other channels are also sensitive to absolute inside-outside voltage (i.e., the membrane potential, V(m)). The first part of this review is focused on the description of the distinct forms of voltage sensitivity and the gating mechanisms that regulate hemichannel activity, both individually and as components of homotypic and heterotypic gap junctions. We then provide an up to date and precise picture of the molecular and structural aspects of how V(j) and V(m) are sensed, and how they, therefore, control channel opening and closing. Mutagenic strategies coupled with structural, biochemical and electrophysical studies are providing significant insights into how distinct forms of voltage dependence are brought about. The emerging picture indicates that Cx channels can undergo transitions between multiple conductance states driven by distinct voltage-gating mechanisms. Each hemichannel may contain a set of two V(j) gates, one fast and one slow, which mediate the transitions between the main open state to the residual state and to the fully closed state, respectively. Eventually, a V(m) gate regulates channel transitions between the open and closed states. Clusters of charged residues within separate domains of the Cx molecule have been identified as integral parts of the V(j) and V(m) sensors. The charges at the first positions of the amino terminal cytoplasmic domain determine the magnitude and polarity of the sensitivity to fast V(j)-gating, as well as contributing to the V(j)-rectifying properties of ion permeation. Additionally, important advances have been made in identifying the conformational rearrangements responsible for fast V(j)-gating transitions to the residual state in the Cx43 channel. These changes involve an intramolecular particle-receptor interaction between the carboxy terminal domain and the cytoplasmic loop.

Molecular modeling and mutagenesis of gap junction channels

Gap junction channels connect the cytoplasms of adjacent cells through the end-to-end docking of hexameric hemichannels called connexons. Each connexon is formed by a ring of 24 alpha-helices that are staggered by 30 degrees with respect to those in the apposed connexon. Current evidence suggests that the two connexons are docked by interdigitated, anti-parallel beta strands across the extracellular gap. The second extracellular loop, E2, guides selectivity in docking between connexons formed by different isoforms. There is considerably more sequence variability of the N-terminal portion of E2, suggesting that this region dictates connexon coupling. Mutagenesis, biochemical, dye-transfer and electrophysiological data, combined with computational studies, have suggested possible assignments for the four transmembrane alpha-helices within each subunit. Most current models assign M3 as the major pore-lining helix. Mapping of human mutations onto a C(alpha) model suggested that native helix packing is important for the formation of fully functional channels. Nevertheless, a mutant in which the M4 helix has been replaced with polyalanine is functional, suggesting that M4 is located on the perimeter of the channel. In spite of this substantial progress in understanding the structural biology of gap junction channels, an experimentally determined structure at atomic resolution will be essential to confirm these concepts.

Gap junctional communication in morphogenesis

Gap junctions permit the direct passage of small molecules from the cytosol of one cell to that of its neighbor, and thus form a system of cell-cell communication that exists alongside familiar secretion/receptor signaling. Because of the rich potential for regulation of junctional conductance, and directional and molecular gating (specificity), gap junctional communication (GJC) plays a crucial role in many aspects of normal tissue physiology. However, the most exciting role for GJC is in the regulation of information flow that takes place during embryonic development, regeneration, and tumor progression. The molecular mechanisms by which GJC establishes local and long-range instructive morphogenetic cues are just beginning to be understood. This review summarizes the current knowledge of the involvement of GJC in the patterning of both vertebrate and invertebrate systems and discusses in detail several morphogenetic systems in which the properties of this signaling have been molecularly characterized. One model consistent with existing data in the fields of vertebrate left-right patterning and anterior-posterior polarity in flatworm regeneration postulates electrophoretically guided movement of small molecule morphogens through long-range GJC paths. The discovery of mechanisms controlling embryonic and regenerative GJC-mediated signaling, and identification of the downstream targets of GJC-permeable molecules, represent exciting next areas of research in this fascinating field.

Innexins in the lobster stomatogastric nervous system: cloning, phylogenetic analysis, developmental changes and expression within adult identified dye and electrically coupled neurons

Gap junctions play a key role in the operation of neuronal networks by enabling direct electrical and metabolic communication between neurons. Suitable models to investigate their role in network operation and plasticity are invertebrate motor networks, which are built of comparatively few identified neurons, and can be examined throughout development; an excellent example is the lobster stomatogastric nervous system. In invertebrates, gap junctions are formed by proteins that belong to the innexin family. Here, we report the first molecular characterization of two crustacean innexins: the lobster Homarus gammarus innexin 1 (Hg-inx1) and 2 (Hg-inx2). Phylogenetic analysis reveals that innexin gene duplication occurred within the arthropod clade before the separation of insect and crustacean lineages. Using in situ hybridization, we find that each innexin is expressed within the adult and developing lobster stomatogastric nervous system and undergoes a marked down-regulation throughout development within the stomatogastric ganglion (STG). The number of innexin expressing neurons is significantly higher in the embryo than in the adult. By combining in situ hybridization, dye and electrical coupling experiments on identified neurons, we demonstrate that adult neurons that express at least one innexin are dye and electrically coupled with at least one other STG neuron. Finally, two STG neurons display no detectable amount of either innexin mRNAs but may express weak electrical coupling with other STG neurons, suggesting the existence of other forms of innexins. Altogether, we provide evidence that innexins are expressed within small neuronal networks built of dye and electrically coupled neurons and may be developmentally regulated.

Pannexin1 is expressed by neurons and glia but does not form functional gap junctions

Pannexins are a newly described family of proteins that may form gap junctions. We made antisera against mouse pannexin1 (Panx1). HeLa cells expressing Panx1 have cell surface labeling, but not gap junction plaques, and do not transfer small fluorescent dyes or neurobiotin in a scrape-loading assay. Neuro2a cells expressing Panx1 are not electrophysiologically coupled. Intracellular Panx1-immunoreactivity, but not gap junction plaques, is seen in cultured oligodendrocytes, astrocytes, and hippocampal neurons. Thus, at least in these mammalian cells lines, Panx1 does not form morphological or functional gap junctions, and it remains to be demonstrated that Panx1 forms gap junction-forming protein in the CNS.

The role of gap junctions in Caenorhabditis elegans oocyte maturation and fertilization

We have investigated the role of gap junctions in Caenorhabditis elegans oocyte maturation and fertilization. Gap junctions are observed between oocytes and the surrounding ovarian sheath cells in wild-type gonads. The sheath transcription factor CEH-18 is required to negatively regulate oocyte maturation, mitogen-activated protein kinase (MAPK) activation, and ovulation. Transmission electron microscopy (TEM) indicates that sheath/oocyte gap junctions are rare or absent in ceh-18(mg57) null mutant gonads. To test the hypothesis that gap junctions negatively regulate oocyte maturation, we performed an RNAi screen of innexin genes, which encode channel-forming proteins. Here we show that INX-14 and INX-22 are required in the female germ line to inhibit oocyte maturation, MAPK activation, and ovulation. Genetic analysis and TEM are consistent with INX-14 and INX-22 being components of sheath/oocyte gap junctions. Our results support the hypothesis that gap junctions maintain oocytes in meiotic prophase I when sperm are absent. We also implicate these channels in regulating sheath cell contractile activity and sperm recruitment to the spermatheca, the site of sperm storage and fertilization. Together with previous studies, our results help establish the C. elegans gonad as a model system for investigating the molecular mechanism(s) by which gap junctions regulate meiosis and fertilization.

Cell-cell communication beyond connexins: the pannexin channels

Direct cell-to-cell communication through specialized intercellular channels is a characteristic feature of virtually all multi-cellular organisms. The remarkable functional conservation of cell-to-cell coupling throughout the animal kingdom, however, is not matched at the molecular level of the structural protein components. Thus protostomes (including nematodes and flies) and deuterostomes (including all vertebrates) utilize two unrelated families of gap-junction genes, innexins and connexins, respectively. The recent discovery that pannexins, a novel group of proteins expressed by several organisms, are able to form intercellular channels has started a quest to understand their evolutionary relationship and functional contribution to cell communication in vivo. There are three pannexin genes in mammals, two of which are co-expressed in the developing and adult brain. Of note, pannexin1 can also form Ca2+-activated hemichannels that open at physiological extracellular Ca2+ concentrations and exhibit distinct pharmacological properties.

Analysis of two P-element enhancer-trap insertion lines that show expression in the giant fibre neuron of Drosophila melanogaster

The giant fibre system (GFS) of Drosophila is a simple neural circuit that mediates escape responses in adult flies. Here we report the initial characterization of two genes that are preferentially expressed in the GFS. Two P-element insertion lines, carrying the GAL4 transcriptional activator, were identified that exhibited pronounced expression in elements of the GFS and relatively low levels elsewhere within the adult central nervous system. Genomic DNA flanking the P-element insertion site was recovered from each of these lines, sequenced, and nearby transcripts identified and confirmed to exhibit GFS expression by in situ hybridization. This analysis revealed that these P-elements were in previously characterized genes. Line P[GAL4]-A307 has an insert in the gene short stop for which we have identified a novel transcript, while line P[GAL4]-141 has an insert in the transcription factor ken and barbie. Here we show that ken and barbie mutants have defects in escape behaviour, behavioural responses to visual stimuli and synaptic functions in the GFS. We have therefore revealed a neural role for a transcription factor that previously had no implicated neural function.

Electrical synapses--gap junctions in the brain

In the nervous system, interneuronal communication can occur via indirect or direct transmission. The mode of indirect communication involves chemical synapses, in which transmitters are released into the extracellular space to subsequently bind to the postsynaptic cell membrane. Direct communication is mediated by electrical synapses, and will be the focus of this review. The most prevalent group of electrical synapses are neuronal gap junctions (both terms are used interchangeably in this article), which directly connect the intracellular space of two cells by gap junction channels. The structural components of gap junction channels in the nervous system are connexin proteins, and, as recently identified, pannexin proteins. Connexin gap junction channels enable the intercellular, bidirectional transport of ions, metabolites, second messengers and other molecules smaller than 1 kD. More than 20 connexin genes have been found in the mouse and human genome. With the cloning of connexin36 (Cx36), a connexin protein with predominantly neuronal expression, the biochemical correlate of electrotonic transmission between neurons was identified. We outline the distribution of Cx36 as well as two other neuronal connexins (Cx57 and Cx45) in the nervous system, describing their spatial and temporal expression patterns. One focus in this review was the retina, as it shows many and diverse electrical synapses whose connexin components have been identified in fish and mammals. In view of the function of neuronal gap junctions, the network of inhibitory interneurons will be reviewed in detail, focussing on the hippocampus. Although in vivo data on pannexin proteins are still restricted to information on mRNA expression, electrophysiological data and the expression pattern in the nervous system have been included.

Low Conductance Gap Junctions Mediate Specific Electrical Coupling in Body-wall Muscle Cells of Caenorhabditis elegans

Invertebrate innexins and their mammalian homologues, the pannexins, are gap junction proteins. Although a large number of such proteins have been identified, few of the gap junctions that they form have been characterized to provide combined information of biophysical properties, coupling pattern, and molecular compositions. We adapted the dual whole cell voltage clamp technique to in situ analysis of electrical coupling in Caenorhabditis elegans body-wall muscle. We found that body-wall muscle cells were electrically coupled in a highly organized and specific pattern. The coupling was characterized by small (350 pS or less) junctional conductance (G(j)), which showed a bell-shaped relationship with junctional potential (V(j)) but was independent of membrane potential (V(m)). Injection of currents comparable to the junctional current (I(j)) into body-wall muscle cells caused significant depolarization, suggesting important functional relevance. The innexin UNC-9 appeared to be a key component of the gap junctions. Both Myc- and green fluorescent protein-tagged UNC-9 was localized to muscle intercellular junctions. G(j) was greatly inhibited in unc-9(fc16), a putative null mutant. Specific inhibition of UNC-9 function in muscle cells reduced locomotion velocity. Despite UNC-9 expression in both motor neurons and body-wall muscle cells, analyses of miniature and evoked postsynaptic currents in the unc-9 mutant showed normal neuromuscular transmission. These analyses provide a relatively detailed description of innexin-based gap junctions in a native tissue and suggest that innexin-based small conductance gap junctions can play an important role in processes such as locomotion.

Synaptic ultrastructure of Drosophila Johnston's organ axon terminals as revealed by an enhancer trap

The role of auditory circuitry is to decipher relevant information from acoustic signals. Acoustic parameters used by different insect species vary widely. All these auditory systems, however, share a common transducer: tympanal organs as well as the Drosophila flagellar ears use chordotonal organs as the auditory mechanoreceptors. We here describe the central neural projections of the Drosophila Johnston's organ (JO). These neurons, which represent the antennal auditory organ, terminate in the antennomechanosensory center. To ensure correct identification of these terminals we made use of a beta-galactosidase-expressing transgene that labels JO neurons specifically. Analysis of these projection pathways shows that parallel JO fibers display extensive contacts, including putative gap junctions. We find that the synaptic boutons show both chemical synaptic structures as well as putative gap junctions, indicating mixed synapses, and belong largely to the divergent type, with multiple small postsynaptic processes. The ultrastructure of JO fibers and synapses may indicate an ability to process temporally discretized acoustic information.

Heteromerization of innexin gap junction proteins regulates epithelial tissue organization in Drosophila

Gap junctions consist of clusters of intercellular channels, which enable direct cell-to-cell communication and adhesion in animals. Whereas deuterostomes, including all vertebrates, use members of the connexin and pannexin multiprotein families to assemble gap junction channels, protostomes such as Drosophila and Caenorhabditis elegans use members of the innexin protein family. The molecular composition of innexin-containing gap junctions and the functional significance of innexin oligomerization for development are largely unknown. Here, we report that heteromerization of Drosophila innexins 2 and 3 is crucial for epithelial organization and polarity of the embryonic epidermis. Both innexins colocalize in epithelial cell membranes. Innexin3 is mislocalized to the cytoplasm in innexin2 mutants and is recruited into ectopic expression domains defined by innexin2 misexpression. Conversely, RNA interference (RNAi) knockdown of innexin3 causes mislocalization of innexin2 and of DE-cadherin, causing cell polarity defects in the epidermis. Biochemical interaction studies, surface plasmon resonance analysis, transgenesis, and biochemical fractionation experiments demonstrate that both innexins interact via their C-terminal cytoplasmic domains during the assembly of heteromeric channels. Our data provide the first molecular and functional demonstration that innexin heteromerization occurs in vivo and reveal insight into a molecular mechanism by which innexins may oligomerize into heteromeric gap junction channels.

Making an escape: development and function of the Drosophila giant fibre system

Flies escape danger by jumping into the air and flying away. The giant fibre system (GFS) is the neural circuit that mediates this simple behavioural response to visual stimuli. The sensory signal is received by the giant fibre and relayed to the leg and wing muscle motorneurons. Many of the neurons in the Drosophila GFS are uniquely identifiable and amenable to cell biological, electrophysiological and genetic studies. Here we review the anatomy and development of this system and highlight its utility for studying many aspects of nervous system biology ranging from neural development and synaptic plasticity to the aetiology of neural disorder.

Intercellular communication: the Drosophila innexin multiprotein family of gap junction proteins

Gap junctions belong to the most conserved cellular structures in multicellular organisms, from Hydra to man. They contain tightly packed clusters of hydrophilic membrane channels connecting the cytoplasms of adjacent cells, thus allowing direct communication of cells and tissues through the diffusion of ions, metabolites, and cyclic nucleotides. Recent evidence suggests that gap junctions are constructed by three different families of four transmembrane proteins: the Connexins and the Innexins found in vertebrates and in invertebrates, respectively, and the Innexin-like Pannexins, which were recently discovered in humans. This article focuses on the Drosophila Innexin multiprotein family, which is comprised of eight members. We highlight common structural features and discuss recent findings that suggest close similarities in cellular distribution, function, and regulation of Drosophila Innexins and vertebrate gap junction proteins.

A possible flip-flop genetic mechanism for reciprocal gene expression

Innexins are a family of transmembrane proteins involved in the formation of gap junctions, specific intercellular channels, in invertebrates. Analyses of the entire innexin family during Drosophila melanogaster embryonic development shows the occurrence of complex and specific patterns of expression of the different genes. Innexins inx-2 and inx-7, in general, do not appear to exhibit extensive co-expression in different D. melanogaster cellular compartments. We propose here a new and robust mechanism, based on our analysis of the genomic organization of inx-2 and inx-7, that structurally justifies the reciprocal expression of genes.

Characterization of innexin gene expression and functional roles of gap-junctional communication in planarian regeneration

Planaria possess remarkable powers of regeneration. After bisection, one blastema regenerates a head, while the other forms a tail. The ability of previously-adjacent cells to adopt radically different fates could be due to long-range signaling allowing determination of position relative to, and the identity of, remaining tissue. However, this process is not understood at the molecular level. Following the hypothesis that gap-junctional communication (GJC) may underlie this signaling, we cloned and characterized the expression of the Innexin gene family during planarian regeneration. Planarian innexins fall into 3 groups according to both sequence and expression. The concordance between expression-based and phylogenetic grouping suggests diversification of 3 ancestral innexin genes into the large family of planarian innexins. Innexin expression was detected throughout the animal, as well as specifically in regeneration blastemas, consistent with a role in long-range signaling relevant to specification of blastema positional identity. Exposure to a GJC-blocking reagent which does not distinguish among gap junctions composed of different Innexin proteins (is not subject to compensation or redundancy) often resulted in bipolar (2-headed) animals. Taken together, the expression data and the respecification of the posterior blastema to an anteriorized fate by GJC loss-of-function suggest that innexin-based GJC mediates instructive signaling during regeneration.

Seizure suppression by shakB2, a gap junction mutation in Drosophila

Gap junction proteins mediate electrical synaptic transmission. In Drosophila, flies carrying null mutations in the shakB locus, such as shakB2, have behavioral and electrophysiological defects in the giant fiber (GF) system neurocircuit consistent with a loss of transmission at electrical synapses. The shakB2 mutation also affects seizure susceptibility. Mutant flies are especially seizure-resistant and have a high threshold to evoked seizures. In addition, in some double mutant combinations with "epilepsy" mutations, shakB2 appears to act as a seizure-suppressor mutation: shakB2 restores seizure susceptibility to the wild-type range in the double mutant. In double mutant combinations, shakB2 completely suppresses seizures caused by slamdance (sda), knockdown (kdn), and jitterbug (jbug) mutations. Seizures caused by easily shocked (eas) and technical knockout (tko) mutations are partially suppressed by shakB2. Seizures caused by bang-sensitive (bas2) and bang-senseless (bss1, bss2 alleles) mutations are not suppressed by shakB2. These results show the use of Drosophila as a model system for studying the kinds of genetic interactions responsible for seizure susceptibility, bringing us closer to unraveling the complexity of seizure disorders in humans.

Evolution of gap junction proteins--the pannexin alternative

Gap junctions provide one of the most common forms of intercellular communication. They are composed of membrane proteins that form a channel that is permeable to ions and small molecules, connecting the cytoplasm of adjacent cells. Gap junctions serve similar functions in all multicellular animals (Metazoa). Two unrelated protein families are involved in this function; connexins, which are found only in chordates, and pannexins, which are ubiquitous and present in both chordate and invertebrate genomes. The involvement of mammalian pannexins to gap junction formation was recently confirmed. Now it is necessary to consider the role of pannexins as an alternative to connexins in vertebrate intercellular communication.

Innexins: members of an evolutionarily conserved family of gap-junction proteins

Gap junctions are clusters of intercellular channels that provide cells, in all metazoan organisms, with a means of communicating directly with their neighbours. Surprisingly, two gene families have evolved to fulfil this fundamental, and highly conserved, function. In vertebrates, gap junctions are assembled from a large family of connexin proteins. Innexins were originally characterized as the structural components of gap junctions in Drosophila, an arthropod, and the nematode Caenorhabditis elegans. Since then, innexin homologues have been identified in representatives of the other major invertebrate phyla and in insect-associated viruses. Intriguingly, functional innexin homologues have also been found in vertebrate genomes. These studies have informed our understanding of the molecular evolution of gap junctions and have greatly expanded the numbers of model systems available for functional studies. Genetic manipulation of innexin function in relatively simple cellular systems should speed progress not only in defining the importance of gap junctions in a variety of biological processes but also in elucidating the mechanisms by which they act.

Molecular basis of gap junctional communication in the CNS of the leech Hirudo medicinalis

Gap junctions are intercellular channels that allow the passage of ions and small molecules between cells. In the nervous system, gap junctions mediate electrical coupling between neurons. Despite sharing a common topology and similar physiology, two unrelated gap junction protein families exist in the animal kingdom. Vertebrate gap junctions are formed by members of the connexin family, whereas invertebrate gap junctions are composed of innexin proteins. Here we report the cloning of two innexins from the leech Hirudo medicinalis. These innexins show a differential expression in the leech CNS: Hm-inx1 is expressed by every neuron in the CNS but not in glia, whereas Hm-inx2 is expressed in glia but not neurons. Heterologous expression in the paired Xenopus oocyte system demonstrated that both innexins are able to form functional homotypic gap junctions. Hm-inx1 forms channels that are not strongly gated. In contrast, Hm-inx2 forms channels that are highly voltage-dependent; these channels demonstrate properties resembling those of a double rectifier. In addition, Hm-inx1 and Hm-inx2 are able to cooperate to form heterotypic gap junctions in Xenopus oocytes. The behavior of these channels is primarily that predicted from the properties of the constituent hemichannels but also demonstrates evidence of an interaction between the two. This work represents the first demonstration of a functional gap junction protein from a Lophotrochozoan animal and supports the hypothesis that connexin-based communication is restricted to the deuterostome clade.

Electrical coupling and neuronal synchronization in the Mammalian brain

Certain neurons in the mammalian brain have long been known to be joined by gap junctions, which are the most common type of electrical synapse. More recently, cloning of neuron-specific connexins, increased capability of visualizing cells within brain tissue, labeling of cell types by transgenic methods, and generation of connexin knockouts have spurred a rapid increase in our knowledge of the role of gap junctions in neural activity. This article reviews the many subtleties of transmission mediated by gap junctions and the mechanisms whereby these junctions contribute to synchronous firing.

Gap junction channel protein innexin 2 is essential for epithelial morphogenesis in the Drosophila embryo

Direct communication of neighboring cells by gap junction channels is essential for the development of tissues and organs in the body. Whereas vertebrate gap junctions are composed of members of the connexin family of transmembrane proteins, in invertebrates gap junctions consist of Innexin channel proteins. Innexins display very low sequence homology to connexins. In addition, very little is known about their cellular role during developmental processes. In this report, we examined the function and the distribution of Drosophila Innexin 2 protein in embryonic epithelia. Both loss-of-function and gain-of-function innexin 2 mutants display severe developmental defects due to cell death and a failure of proper epithelial morphogenesis. Furthermore, immunohistochemical analyses using antibodies against the Innexins 1 and 2 indicate that the distribution of Innexin gap junction proteins to specific membrane domains is regulated by tissue specific factors. Finally, biochemical interaction studies together with genetic loss- and gain-of-function experiments provide evidence that Innexin 2 interacts with core proteins of adherens and septate junctions. This is the first study, to our knowledge, of cellular distribution and protein-protein interactions of an Innexin gap junctional channel protein in the developing epithelia of Drosophila.

Cellular distribution of innexin 1 and 2 gap junctional channel proteins in epithelia of the Drosophila embryo

Invertebrate gap junctions are composed of Innexin channel proteins that are structurally and functionally analogous to the connexins in vertebrates. In situ hybridization experiments have shown that most of the eight known innexin genes in Drosophila are expressed in a complex and overlapping temporal and spatial profile, with several members showing high levels of expression in developing epithelia of the embryo. To further study the cellular roles of Innexins, we have generated antibodies against Innexins 1 and 2 and studied their protein distribution in the developing embryo. We find that both Innexins are co-expressed in a number of epithelial tissues including the epidermis, the gut and the salivary glands. On the cellular level, we find both proteins localized to the membranes of epithelial cells. Immunohistochemical analysis using cell polarity markers indicates that Innexin 1 is predominantly localized to the baso-lateral domain of epithelial cells, basal to septate junctions. In contrast, we find a variable positioning of Innexin 2 along the apico-basal axis of epithelial cells depending on the type of tissue and organ. Our findings suggest that the distribution of Innexin channel proteins to specific membrane domains of epithelial cells is regulated by tissue specific factors during the development of epithelia in the fly embryo.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Regulation of intermuscular electrical coupling by the Caenorhabditis elegans innexin inx-6

The innexins represent a highly conserved protein family, the members of which make up the structural components of gap junctions in invertebrates. We have isolated and characterized a Caenorhabditis elegans gene inx-6 that encodes a new member of the innexin family required for the electrical coupling of pharyngeal muscles. inx-6(rr5) mutants complete embryogenesis without detectable abnormalities at restrictive temperature but fail to initiate postembryonic development after hatching. inx-6 is expressed in the pharynx at all larval stages, and an INX-6::GFP fusion protein showed a punctate expression pattern characteristic of gap junction proteins localized to plasma membrane plaques. Video recording and electropharyngeograms revealed that in inx-6(rr5) mutants the anterior pharyngeal (procorpus) muscles were electrically coupled to a lesser degree than the posterior metacorpus muscles, which caused a premature relaxation in the anterior pharynx and interfered with feeding. Dye-coupling experiments indicate that the gap junctions that link the procorpus to the metacorpus are functionally compromised in inx-6(rr5) mutants. We also show that another C. elegans innexin, EAT-5, can partially substitute for INX-6 function in vivo, underscoring their likely analogous function.

Characterization of Campoletis sonorensis ichnovirus unique segment B and excision locus structure

Polydnaviruses (PDVs) are segmented, symbiotic, double-stranded DNA viruses that are vertically transmitted as proviruses within the genomes of some parasitoid Hymenoptera. The PDV associated with the ichneumonid wasp Campoletis sonorensis (CsIV) consists of 24 non-redundant DNA segments varying in size from approximately 6 to 20 kbp. CsIV segment B, one of the smallest genome segments, was sequenced and the excision sites of the proviral segment were characterized. The segment B sequence was 83.2% non-coding with only two open reading frames (ORFs). Some non-coding sequences have similarities to database sequences and were likely pseudogenic, but most were unrelated to known nucleic acid or predicted protein sequences. One ORF, BHv0.9, encodes a member of the rep gene family and was expressed only in parasitized insects while transcription of the other ORF could not be detected. Previously, a third region of the segment was shown to hybridize to 0.6 and 1.2 kb poly A+ RNAs from female wasps during virus replication (Theilmann and Summers, 1988) but this region did not have an identifiable ORF in the determined sequence. In contrast to CsIV segment W, segment B had little repetitive sequence. The segment B proviral integration locus contains a 59 bp direct imperfect repeat. Further analyses of this integration locus demonstrated that segment B was excised from wasp genomic DNA with flanking sequences at the integration site rejoined after segment excision. The segment B "excision locus" retained one of the two copies of the 59 bp repeat sequence with the other repeat present in the excised segment. The data indicate that Ichnovirus segments have distinctive characteristics possibly reflecting functional co-evolution between the wasp and individual types of polydnavirus segments.

The Caenorhabditis elegans innexin INX-3 is localized to gap junctions and is essential for embryonic development

Innexins are the proposed structural components of gap junctions in invertebrates. Antibodies that specifically recognize the Caenorhabditis elegans innexin protein INX-3 were generated and used to examine the patterns of inx-3 gene expression and the subcellular sites of INX-3 localization. INX-3 is first detected in two-cell embryos, concentrated at the intercellular interface, and is expressed ubiquitously throughout the cellular proliferation phase of embryogenesis. During embryonic morphogenesis, INX-3 expression becomes more restricted. Postembryonically, INX-3 is expressed transiently in several cell types, while expression in the posterior pharynx persists throughout development. Through immuno-EM techniques, INX-3 was observed at gap junctions in the adult pharynx, providing supporting evidence that innexins are components of gap junctions. An inx-3 mutant was isolated through a combined genetic and immunocytochemical screen. Homozygous inx-3 mutants exhibit defects during embryonic morphogenesis. At the comma stage of early morphogenesis, variable numbers of cells are lost from the anterior of inx-3(lw68) mutants. A range of terminal defects is seen later in embryogenesis, including localized rupture of the hypodermis, failure of the midbody to elongate properly, abnormal contacts between hypodermal cells, and failure of the pharynx to attach to the anterior of the animal.

Gastrointestinal development in the Drosophila embryo requires the activity of innexin gap junction channel proteins

Cell to cell communication plays an essential role during pattern formation and morphogenesis of the diverse tissues and organs of the body. In invertebrates, such as the fruitfly Drosophila, the direct communication of closely apposed cells is mediated by gap junctions which are composed of oligomers of the innexin family of transmembrane channel proteins. Few data exist about the developmental role of the eight innexin genes which have been found in the Drosophila genome. We have investigated the role of the innexin 2 and ogre genes during gastrointestinal development of the fly embryo. Our findings suggest that innexins are involved in the formation of the proventriculus, an organ that develops at the foregut/midgut boundary by migration of primordial cells and subsequent infolding of epithelial tissue layers.

Innexins in C. elegans

Innexins are functionally analogous to the vertebrate connexins, and the innexin family of gap junction proteins has been identified in many invertebrates, including Drosophila and C. elegans. The genome sequencing project has identified 25 innexins in C. elegans. We are particularly interested in the roles that gap junctions may play in embryonic development and in wiring of the nervous system. To identify the particular C. elegans innexins that are involved in these processes, we are examining their expression patterns using specific antibodies and translational GFP fusions. In addition we are investigating mutant, RNAi and overexpression phenotypes for many of these genes. To date, we have generated specific antibodies to the non-conserved carboxyl termini of 5 innexins. We have constructed GFP translational fusions for 17 innexins and observed expression patterns for 13 of these genes. In total we have characterized expression patterns representing 14 innexins. Mutations have been identified in 5 of these genes, and at least 3 others have RNAi mutant phenotypes. Generalities emerging from our studies include: 1) most tissues and many individual cells express more than one innexin, 2) some innexins are expressed widely, while others are expressed in only a few cells, and 3) there is a potential for functional pairing of innexins.

Targeted expression of tetanus toxin: a new tool to study the neurobiology of behavior

Over the past few decades, the explosion of molecular genetic knowledge, particularly in the fruit fly Drosophila melanogaster, has led to the identification of a large number of genes, which, when mutated, directly or indirectly affect fly behavior. Beyond the genetic and molecular characterization of genes and their associated molecular pathways, recent advances in molecular genetics also have allowed the development of new tools dedicated more directly to the dissection of the neural bases for various behaviors. In particular, the conjunction of the development of two techniques--the enhancer-trap detection system and the targeted gene expression system, based on the yeast GAL4 transcription factor--has led to the development of the binary enhancer-trap P[GAL4] expression system, which allows the selective activation of any cloned gene in a wide variety of tissue- and cell-specific patterns. Thus, this development, in addition to allowing the anatomical characterization of neuronal circuitry, also allows, via the expression of tetanus toxin light chain (known to specifically block synaptic transmission), an investigation of the role of specific neurons in certain behaviors. Using this system of "toxigenetics," several forms of behavior--from those mediated by sensory systems, such as olfaction, mechanoreception, and vision, to those mediated by higher brain function, such as learning, memory and locomotion--have been studied. These studies aim to map neuronal circuitry underlying specific behaviors and thereby unravel relevant neurophysiological mechanisms. The advantage of this approach is that it is noninvasive and permits the investigation of behavior in the free moving animal. We review a number of behavioral studies that have successfully employed this toxigenetic approach, and we hope to persuade the reader that transgenic tetanus toxin light chain is a useful and appropriate tool for the armory of neuroethologists.

Gap junction proteins expressed during development are required for adult neural function in the Drosophila optic lamina

We provide evidence that gap junction proteins, expressed during development, are necessary for the formation of normally functioning connections in the Drosophila optic lamina. Flies with mutations in the gap junction genes (innexins), shakingB, and ogre have normal photoreceptor potentials but a defective response of the postsynaptic cells in the optic lamina. This is indicated by a reduction in, or absence of, transients in the electroretinogram. Ogre is required in the presynaptic retinal photoreceptors. ShakingB(N) is, at a minimum, required in postsynaptic lamina neurons. Transgenic expression of the appropriate innexins during pupal development (but not later) rescues connection defects. Transient gap junctions have been observed to precede chemical synapse formation and have been hypothesized to play a role in connectivity and synaptogenesis; however, no causal role has been demonstrated. Here we show that developmental gap junction genes can be required for normally functioning neural connections to form.

Gap junction proteins are not interchangeable in development of neural function in theDrosophilavisual system

Gap junctions (GJs) are composed of proteins from two distinct families. In vertebrates, GJs are composed of connexins; a connexin hexamer on one cell lines up with a hexamer on an apposing cell to form the intercellular channel. In invertebrates, GJs are composed of an unrelated protein family, the innexins. Different connexins have distinct properties that make them largely non-interchangeable in the animal. Innexins are also a large family with high sequence homology, and some functional differences have been reported. The biological implication of innexin differences, such as their ability to substitute for one another in the animal, has not been explored.

Recently, we showed that GJ proteins are necessary for the development of normal neural transmission in the Drosophila visual system. Mutations in either of two Drosophila GJ genes (innexins), shakB and ogre, lead to a loss of transients in the electroretinogram (ERG),which is indicative of a failure of the lamina to respond to retinal cell depolarization. Ogre is required presynaptically and shakB(N)postsynaptically. Both act during development.

Here we ask if innexins are interchangeable in their role of promoting normal neural development in flies. Specifically, we tested several innexins for their ability to rescue shakB2 and ogremutant ERGs and found that, by and large, innexins are not interchangeable. We mapped the protein regions required for this specificity by making molecular chimeras between shakB(N) and ogre and testing their ability to rescue both mutants. Each chimera rescued either shakB or ogre but never both. Sequences in the first half of each protein are necessary for functional specificity. Potentially crucial residues include a small number in the intracellular loop as well as a short stretch just N-terminal to the second transmembrane domain.

Temporary GJs, possibly between the retina and lamina, may play a role in final target selection and/or chemical synapse formation in the Drosophila visual system. In that case, specificity in GJ formation or function could contribute, directly or indirectly, to chemical synaptic specificity by regulating which neurons couple and what signals they exchange. Cells may couple only if their innexins can mate with each other. The partially overlapping expression patterns of several innexins make this `mix and match' model of GJ formation a possibility.

A germline-specific gap junction protein required for survival of differentiating early germ cells

Germ cells require intimate associations and signals from the surrounding somatic cells throughout gametogenesis. The zero population growth (zpg) locus of Drosophila encodes a germline-specific gap junction protein, Innexin 4, that is required for survival of differentiating early germ cells during gametogenesis in both sexes. Animals with a null mutation in zpg are viable but sterile and have tiny gonads. Adult zpg-null gonads contain small numbers of early germ cells, resembling stem cells or early spermatogonia or oogonia, but lack later stages of germ cell differentiation. In the male, Zpg protein localizes to the surface of spermatogonia, primarily on the sides adjacent to the somatic cyst cells. In the female, Zpg protein localizes to germ cell surfaces, both those adjacent to surrounding somatic cells and those adjacent to other germ cells. We propose that Zpg-containing gap junctional hemichannels in the germ cell plasma membrane may connect with hemichannels made of other innexin isoforms on adjacent somatic cells. Gap junctional intercellular communication via these channels may mediate passage of crucial small molecules or signals between germline and somatic support cells required for survival and differentiation of early germ cells in both sexes.

Gap junctions in Drosophila: developmental expression of the entire innexin gene family

Invertebrate gap junctions are composed of proteins called innexins and eight innexin encoding loci have been identified in the now complete genome sequence of Drosophila melanogaster. The intercellular channels formed by these proteins are multimeric and previous studies have shown that, in a heterologous expression system, homo- and hetero-oligomeric channels can form, each combination possessing different gating characteristics. Here we demonstrate that the innexins exhibit complex overlapping expression patterns during oogenesis, embryogenesis, imaginal wing disc development and central nervous system development and show that only certain combinations of innexin oligomerization are possible in vivo. This work forms an essential basis for future studies of innexin interactions in Drosophila and outlines the potential extent of gap-junction involvement in development.

Gene discovery by e-genetics: Drosophila odor and taste receptors

A new algorithm that examines DNA databases for proteins that have a particular structure, as opposed to a particular sequence, represents a novel`e-genetics' approach to gene discovery. The algorithm has successfully identified new G-protein-coupled receptors, which have a characteristic seven-transmembrane-domain structure, from the Drosophila genome database. In particular, it has revealed novel families of odor receptors and taste receptors, which had long eluded identification by other means. The two new gene families, the Or and Gr genes, are expressed in neurons of olfactory and taste sensilla and are highly divergent from all other known G-protein-coupled receptor genes. Modification of the algorithm should allow identification of other classes of multitransmembrane-domain protein.

Genetic suppression of seizure susceptibility in Drosophila

Despite the frequency of seizure disorders in the human population, the genetic and physiological basis for these defects has been difficult to resolve. Although many genetic defects that cause seizure susceptibility have been identified, the defects involve disparate biological processes, many of which are not neural specific. The large number and heterogeneous nature of the genes involved makes it difficult to understand the complex factors underlying the etiology of seizure disorders. Examining the effect known genetic mutations have on seizure susceptibility is one approach that may prove fruitful. This approach may be helpful both in understanding how different physiological processes affect seizure susceptibility and in identifying novel therapeutic treatments. In this study, we have taken advantage of Drosophila, a genetically tractable system, to identify factors that suppress seizure susceptibility. Of particular interest has been a group of Drosophila mutants, the bang-sensitive (BS) mutants, which are much more susceptible to seizures than wild type. The BS phenotypic class includes at least eight genes, including three examined in this study, bss, eas, and sda. Through the generation of double-mutant combinations with other well-characterized Drosophila mutants, the BS mutants are particularly useful for identifying genetic factors that suppress susceptibility to seizures. We have found that mutants affecting Na+ channels, mle(napts) and para, K+ channels, Sh, and electrical synapses, shak-B(2), can suppress seizures in the BS mutants. This is the first demonstration that these types of mutations can suppress the development of seizures in any organism. Reduced neuronal excitability may contribute to seizure suppression. The best suppressor, mle(napts), causes an increased stimulation threshold for the giant fiber (GF) consistent with a reduction in single neuron excitability that could underlie suppression of seizures. For some other double mutants with para and Sh(KS133), there are no GF threshold changes, but reduced excitability may also be indicated by a reduction in GF following frequency. These results demonstrate the utility of Drosophila as a model system for studying seizure susceptibility and identify physiological processes that modify seizure susceptibility.

Innexins get into the gap

Connexins were first identified in the 1970s as the molecular components of vertebrate gap junctions. Since then a large literature has accumulated on the cell and molecular biology of this multi-gene family culminating recently in the findings that connexin mutations are implicated in a variety of human diseases. Over two decades, the terms "connexin" and "gap junction" had become almost synonymous. In the last few years a second family of gap-junction genes, the innexins, has emerged. These have been shown to form intercellular channels in genetically tractable invertebrate organisms such as Drosophila melanogaster and Caenorhabditis elegans. The completed genomic sequences for the fly and worm allow identification of the full complement of innexin genes in these two organisms and provide valuable resources for genetic analyses of gap junction function.

Molecular determinants of membrane potential dependence in vertebrate gap junction channels

The conductance, g(j), of many gap junctions depends on voltage between the coupled cells (transjunctional voltage, V(j)) with little effect of the absolute membrane potential (V(m)) in the two cells; others show combined V(j) and V(m) dependence. We examined the molecular determinants of V(m) dependence by using rat connexin 43 expressed in paired Xenopus oocytes. These junctions have, in addition to V(j) dependence, V(m) dependence such that equal depolarization of both cells decreases g(j). The dependence of g(j) on V(m) was abolished by truncation of the C-terminal domain (CT) at residue 242 but not at 257. There are two charged residues between 242 and 257. In full-length Cx43, mutations neutralizing either one of these charges, Arg243Gln and Asp245Gln, decreased and increased V(m) dependence, respectively, suggesting that these residues are part of the V(m) sensor. Mutating both residues together abolished V(m) dependence, although there is no net change in charge. The neutralizing mutations, together or separately, had no effect on V(j) dependence. Thus, the voltage sensors must differ. However, V(j) gating was somewhat modulated by V(m), and V(m) gating was reduced when the V(j) gate was closed. These data suggest that the two forms of voltage dependence are mediated by separate but interacting domains.

Synaptogenesis in the giant-fibre system of Drosophila: interaction of the giant fibre and its major motorneuronal target

The tergotrochanteral (jump) motorneuron is a major synaptic target of the Giant Fibre in Drosophila. These two neurons are major components of the fly's Giant-Fibre escape system. Our previous work has described the development of the Giant Fibre in early metamorphosis and the involvement of the shaking-B locus in the formation of its electrical synapses. In the present study, we have investigated the development of the tergotrochanteral motorneuron and its electrical synapses by transforming Drosophila with a Gal4 fusion construct containing sequences largely upstream of, but including, the shaking-B(lethal) promoter. This construct drives reporter gene expression in the tergotrochanteral motorneuron and some other neurons. Expression of green fluorescent protein in the motorneuron allows visualization of its cell body and its subsequent intracellular staining with Lucifer Yellow. These preparations provide high-resolution data on motorneuron morphogenesis during the first half of pupal development. Dye-coupling reveals onset of gap-junction formation between the tergotrochanteral motorneuron and other neurons of the Giant-Fibre System. The medial dendrite of the tergotrochanteral motorneuron becomes dye-coupled to the peripheral synapsing interneurons between 28 and 32 hours after puparium formation. Dye-coupling between tergotrochanteral motorneuron and Giant Fibre is first seen at 42 hours after puparium formation. All dye coupling is abolished in a shaking-B(neural) mutant. To investigate any interactions between the Giant Fibre and the tergotroachanteral motorneuron, we arrested the growth of the motorneuron's medial neurite by targeted expression of a constitutively active form of Dcdc42. This results in the Giant Fibre remaining stranded at the midline, unable to make its characteristic bend. We conclude that Giant Fibre morphogenesis normally relies on fasciculation with its major motorneuronal target.

Two Drosophila innexins are expressed in overlapping domains and cooperate to form gap-junction channels

Members of the innexin protein family are structural components of invertebrate gap junctions and are analogous to vertebrate connexins. Here we investigate two Drosophila innexin genes, Dm-inx2 and Dm-inx3 and show that they are expressed in overlapping domains throughout embryogenesis, most notably in epidermal cells bordering each segment. We also explore the gap-junction-forming capabilities of the encoded proteins. In paired Xenopus oocytes, the injection of Dm-inx2 mRNA results in the formation of voltage-sensitive channels in only approximately 40% of cell pairs. In contrast, Dm-Inx3 never forms channels. Crucially, when both mRNAs are coexpressed, functional channels are formed reliably, and the electrophysiological properties of these channels distinguish them from those formed by Dm-Inx2 alone. We relate these in vitro data to in vivo studies. Ectopic expression of Dm-inx2 in vivo has limited effects on the viability of Drosophila, and animals ectopically expressing Dm-inx3 are unaffected. However, ectopic expression of both transcripts together severely reduces viability, presumably because of the formation of inappropriate gap junctions. We conclude that Dm-Inx2 and Dm-Inx3, which are expressed in overlapping domains during embryogenesis, can form oligomeric gap-junction channels.

Developmental expression and molecular characterization of two gap junction channel proteins expressed during embryogenesis in the grasshopper Schistocerca americana

Gap junctions are membrane channels that directly connect the cytoplasm of neighboring cells, allowing the exchange of ions and small molecules. Two analogous families of proteins, the connexins and innexins, are the channel-forming molecules in vertebrates and invertebrates, respectively. In order to study the role of gap junctions in the embryonic development of the nervous system, we searched for innexins in the grasshopper Schistocerca americana. Here we present the molecular cloning and sequence analysis of two novel innexins, G-Inx(1) and G-Inx(2), expressed during grasshopper embryonic development. The analysis of G-Inx(1) and G-Inx(2) proteins suggests they bear four transmembrane domains, which show strong conservation in members of the innexin family. The study of the phylogenetic relationships between members of the innexin family and the new grasshopper proteins suggests that G-Inx(1) is orthologous to the Drosophila 1(1)-ogre. However, G-Inx(2) seems to be a member of a new group of insect innexins. We used in situ hybridization with the G-Inx(1) and G-Inx(2) cDNA clones, and two polyclonal sera raised against different regions of G-Inx(1) to study the mRNA and protein expression patterns and the subcellular localization of the grasshopper innexins. G-Inx(1) is primarily expressed in the embryonic nervous system, in neural precursors and glial cells. In addition, a restricted stripe of epithelial cells in the developing limb, involved in the guidance of sensory growth cones, expresses G-Inx(1). G-Inx(2) expression is more widespread in the grasshopper embryo, but a restricted expression is found in a subset of neural precursors. The generally different but partially overlapping expression patterns of G-Inx(1) and G-Inx(2) supports the combinatorial character of gap junction formation in invertebrates, an essential property to generate specificity in this form of cell-cell communication.

Genes, gene knockouts, and mutations in the analysis of gap junctions

Gop junctions are cell junctions found between most cells and tissues. They contain membrane channels that mediate the cell-to-cell diffusion of ions, metabolites, and small cell signaling molecules. Cell-cell communication mediated by gap junctions has been proposed to have a variety of functions, including roles in regulating events in development, cell differentiation, and cell growth and proliferation. The analysis of these possibilities has been confounded by the fact that there are over a dozen connexin genes encoding polypeptides that make up vertebrate gap junctions. This complexity, coupled with the fact that most cells express multiple connexin isotypes, likely explains why recent studies using reverse genetic and genetic approaches to disrupt connexin gene function have yielded only limited insights into the physiological roles of gap junctions. Nevertheless, studies in vivo and in vitro together have provided evidence for gap junctions being involved in the regulation of cell metabolism, growth, and differentiation in restricted cell and tissue types. Surprisingly, studies in invertebrates suggest that their gap junctions are encoded not by connexins, but by a family of proteins referred to as innexins. Analysis of various Drosophila and C. elegans mutants suggest that innexins may be functional homologs to the connexins. However, whether innexins are the elusive invertebrate gap junction proteins or, rather, accessory proteins that facilitate gap junction formation remains an open question. Given the rapid progress being made in the cloning and functional analysis of gap junctions in many diverse species, confusion and difficulties with nomenclature are coming to a head in this rapidly expanding field. It may be timely to form a Nomenclature Committee to establish a uniform classification scheme for naming gap junction proteins.