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

Gap-Junctional communication between developing Drosophila muscles is essential for their normal development

Recent experiments have demonstrated that a family of proteins, known as the innexins, are structural components of invertebrate gap junctions. The shaking-B (shak-B) locus of Drosophila encodes two members of this emerging family, Shak-B(lethal) and Shak-B(neural). This study focuses on the role of Shak-B gap junctions in the development of embryonic and larval muscle. During embryogenesis, shak-B transcripts are expressed in a subset of the somatic muscles; expression is strong in ventral oblique muscles (VO4-6) but only weak in ventral longitudinals (VL3 and 4). Carboxyfluorescein injected into VO4 of wild-type early stage 16 embryos spreads, via gap junctions, to label adjacent muscles, including VL3 and 4. In shak-B2 embryos (in which the shak-B(neural) function is disrupted), dye injected into VO4 fails to spread into other muscles. In the first instar larva, when dye coupling between muscles is no longer present, another effect of the shak-B2 mutation is revealed by whole-cell voltage clamp. In a calcium-free saline, only two voltage-activated potassium currents are present in wild-type muscles; a fast IA and a slow IK current. In shak-B2 larvae, these two currents are significantly reduced in magnitude in VO4 and 5, but remain normal in VL3. Expression of shak-B(neural) in a shak-B2 background fully rescues both dye coupling in embryonic muscle and whole-cell currents in first instar VO4 and 5. Our observations show that Shak-B(neural) is one of a set of embryonic gap-junction proteins, and that it is required for the normal temporal development of potassium currents in some larval muscles.

The genetics of ivermectin resistance in Caenorhabditis elegans

The ability of organisms to evolve resistance threatens the effectiveness of every antibiotic drug. We show that in the nematode Caenorhabditis elegans, simultaneous mutation of three genes, avr-14, avr-15, and glc-1, encoding glutamate-gated chloride channel (GluCl) alpha-type subunits confers high-level resistance to the antiparasitic drug ivermectin. In contrast, mutating any two channel genes confers modest or no resistance. We propose a model in which ivermectin sensitivity in C. elegans is mediated by genes affecting parallel genetic pathways defined by the family of GluCl genes. The sensitivity of these pathways is further modulated by unc-7, unc-9, and the Dyf (dye filling defective) genes, which alter the structure of the nervous system. Our results suggest that the evolution of drug resistance can be slowed by targeting antibiotic drugs to several members of a multigene family.

Nested transcripts of gap junction gene have distinct expression patterns

The shaking B locus (shakB, or Passover) codes for structural molecules of gap junctions in Drosophila. This report describes the complex set of transcripts from the shakB locus. A nested set of five transcripts is described. The transcripts share 3' exons, but each has its own 5' exon. The transcripts are arrayed as a series in the genomic DNA stretching over 60 kb. The 5' end of each successive transcript lies further proximal on the chromosome. Each new transcript shares all the 3' exons with the one preceding it, but adds one or two more 5' exons. The different transcripts are expressed in a wide variety of locations in the nervous system and in non-neural tissues. Some tissues express more than one transcript, and the expression pattern of each is developmentally regulated. Within the adult central nervous system (CNS), these transcripts have an expression pattern that is restricted to the giant fiber system (GFS). The GFS is a small set of neurons which mediates the visually induced escape jump. shakB is required for function of the GFS electrical synapses. The transcript previously defined as active in the giant fiber is not, in fact, expressed in that cell. Instead, we find that another transcript, shakB(N3), and perhaps shakB(N4) as well, is expressed in the GFS; this transcript is not expressed elsewhere in the adult CNS. Two other transcripts, shakB(N1) and shakB(N2), are expressed in the optic lamina but not elsewhere in the CNS. This expression pattern explains the neurophysiological and behavioral defects in escape exhibited in mutants of shakB.

Deficient assembly and function of gap junctions in Trf1, a trafficking mutant of the human liver-derived cell line HuH-7

The Trf1 cell line, selected from the human hepatoma cell line HuH-7, manifests altered trafficking of various plasma membrane proteins. In particular, there is a striking loss of State 2 asialoglycoprotein receptors. This cell line is shown here to also manifest defects in function and assembly of gap junctions comprising connexin43 (Cx43). No alteration of Cx43 expression or phosphorylation was apparent. Nevertheless, immunostaining of Cx43 revealed that fewer and smaller gap junctions were present at appositional membrane areas in Trf1 cells as compared with parental HuH-7. This correlated with a significant attenuation in gap junction-mediated communication between Trf1 cells as demonstrated by markedly decreased dye transfer and their reduced ability to propagate mechanically evoked Ca(2+) waves. Isoelectric focusing (IEF) of Cx43 in HuH-7 cells indicated that the pIs of this protein were significantly lower than that predicted from its amino acid sequence; no differences in pI were evident in Cx43 from Trf1 cells and the HuH-7 cell line. The effects of the Trf1 mutation on assembly and function of gap junctions indicate that this mutation influences trafficking of Cx43. Connexins differ in several respects from other membrane proteins thus far analyzed in Trf1 mutants: gap junctions localize exclusively to the lateral cell surface; they are not glycoproteins; and they do not play a role in endocytic pathways. The disruption of trafficking of Cx43 by this mutation suggests that the Trf1 phenotype is a defect at a common point along the trafficking pathway of cell-surface proteins, irrespective of their ultimate destination on the cell surface or their glycosylation profile.

Molecular determinants of electrical rectification of single channel conductance in gap junctions formed by connexins 26 and 32

The fully open state of heterotypic gap junction channels formed by pairing cells expressing connexin 32 (Cx32) with those expressing connexin 26 (Cx26) rectifies in a way that cannot be predicted from the current-voltage (I-V) relation of either homotypic channel. Using a molecular genetic analysis, we demonstrate that charged amino acids positioned in the amino terminus (M1 and D2) and first extracellular loop (E42) are major determinants of the current-voltage relation of the fully open state of homotypic and heterotypic channels formed by Cx26 and Cx32. The observed I-V relations of wild-type and mutant channels were closely approximated by those obtained with the electrodiffusive model of Chen and Eisenberg (Chen, D., and R. Eisenberg. 1993. Biophys. J. 64:1405-1421), which solves the Poisson-Nernst-Plank equations in one dimension using charge distribution models inferred from the molecular analyses. The rectification of the Cx32/Cx26 heterotypic channel results from the asymmetry in the number and position of charged residues. The model required the incorporation of a partial charge located near the channel surface to approximate the linear I-V relation observed for the Cx32*Cx26E1 homotypic channel. The best candidate amino acid providing this partial charge is the conserved tryptophan residue (W3). Incorporation of the partial charge of residue W3 and the negative charge of the Cx32E41 residue into the charge profile used in the Poisson-Nernst-Plank model of homotypic Cx32 and heterotypic Cx26/Cx32 channels resulted in I-V relations that closely resembled the observed I-V relations of these channels. We further demonstrate that some channel substates rectify. We suggest that the conformational changes associated with transjunctional voltage (V(j))-dependent gating to these substates involves a narrowing of the cytoplasmic entry of the channel that increases the electrostatic effect of charges in the amino terminus. The rectification that is observed in the Cx32/Cx26 heterotypic channel is similar although less steep than that reported for some rectifying electrical synapses. We propose that a similar electrostatic mechanism, which results in rectification through the open and substates of heterotypic channels, is sufficient to explain the properties of steeply rectifying electrical synapses.

Ultrastructural features of the adult hermaphrodite gonad of Caenorhabditis elegans: relations between the germ line and soma

Genetic and embryological experiments have established the Caenorhabditis elegans adult hermaphrodite gonad as a paradigm for studying the control of germline development and the role of soma-germline interactions. We describe ultrastructural features relating to essential germline events and the soma-germline interactions upon which they depend, as revealed by electron and fluorescence microscopy. Gap junctions were observed between oocytes and proximal gonadal sheath cells that contract to ovulate the oocyte. These gap junctions must be evanescent since individual oocytes lose contact with sheath cells when they are ovulated. In addition, proximal sheath cells are coupled to each other by gap junctions. Within proximal sheath cells, actin/myosin bundles are anchored to the plasma membrane at plaque-like structures we have termed hemi-adherens junctions, which in turn are closely associated with the gonadal basal lamina. Gap junctions and hemi-adherens junctions are likely to function in the coordinated series of contractions required to ovulate the mature oocyte. Proximal sheath cells are fenestrated with multiple small pores forming conduits from the gonadal basal lamina to the surface of the oocyte, passing through the sheath cell. In most instances where pores occur, extracellular yolk particles penetrate the gonadal basal lamina to directly touch the underlying oocytes. Membrane-bounded yolk granules were generally not found in the sheath cytoplasm by either electron microscopy or fluorescence microscopy. Electron microscopic immunocytochemistry was used to confirm and characterize the appearance of yolk protein in cytoplasmic organelles within the oocyte and in free particles in the pseudocoelom. The primary route of yolk transport apparently proceeds from the intestine into the pseudocoelom, then through sheath pores to the surface of the oocyte, where endocytosis occurs. Scanning electron microscopy was used to directly visualize the distal tip cell which extends tentacle-like processes that directly contact distal germ cells. These distal tip cell processes are likely to play a critical role in promoting germline mitosis. Scanning electron microscopy also revealed thin filopodia extending from the distal sheath cells. Distal sheath filopodia were also visualized using a green fluorescent protein reporter gene fusion and confocal microscopy. Distal sheath filopodia may function to stretch the sheath over the distal arm.

Innexin-3 forms connexin-like intercellular channels

Innexins comprise a large family of genes that are believed to encode invertebrate gap junction channel-forming proteins. However, only two Drosophila innexins have been directly tested for the ability to form intercellular channels and only one of those was active. Here we tested the ability of Caenorhabditis elegans family members INX-3 and EAT-5 to form intercellular channels between paired Xenopus oocytes. We show that expression of INX-3 but not EAT-5, induces electrical coupling between the oocyte pairs. In addition, analysis of INX-3 voltage and pH gating reveals a striking degree of conservation in the functional properties of connexin and innnexin channels. These data strongly support the idea that innexin genes encode intercellular channels.

Drosophila has several genes for gap junction proteins

The Innexin gene family forms gap junctions in invertebrates. Many genes in this family have been identified in Caenorhabditis elegans, but only two in Drosophila. We have used PCR techniques to identify three new members of this family from Drosophila. These are designated pas-related proteins (prp) 6, 7, and 33. The putative proteins coded by these new genes show 25-35% identity and 39-66% similarity to other Drosophila innexins and share a similar hydrophobicity profile. The genes form two small clusters on the X-chromosome, with three of the genes sitting within 10kb of each other. The closeness in sequence and location suggests an evolutionary origin of these genes via local duplication. In situ hybridization shows expression in the CNS, gut and epidermis. Each gene has a distinct pattern of expression in different tissues at different developmental times. However, parts of the expression patterns overlap, especially for prp33 and ogre which may be expressed from the same transcriptional enhancers. This suggest that the Prp33 and Ogre proteins may join in forming heteromeric gap junction channels.

Genetic diseases and gene knockouts reveal diverse connexin functions

Intercellular channels present in gap junctions allow cells to share small molecules and thus coordinate a wide range of behaviors. Remarkably, although junctions provide similar functions in all multicellular organisms, vertebrates and invertebrates use unrelated gene families to encode these channels. The recent identification of the invertebrate innexin family opens up powerful genetic systems to studies of intercellular communication. At the same time, new information on the physiological roles of vertebrate connexins has emerged from genetic studies. Mutations in connexin genes underlie a variety of human diseases, including deafness, demyelinating neuropathies, and lens cataracts. In addition, gene targeting of connexins in mice has provided new insights into connexin function and the significance of connexin diversity.

Neurobiology of the Caenorhabditis elegans genome

Neurotransmitter receptors, neurotransmitter synthesis and release pathways, and heterotrimeric GTP-binding protein (G protein)-coupled second messenger pathways are highly conserved between Caenorhabditis elegans and mammals, but gap junctions and chemosensory receptors have independent origins in vertebrates and nematodes. Most ion channels are similar to vertebrate channels but there are no predicted voltage-activated sodium channels. The C. elegans genome encodes at least 80 potassium channels, 90 neurotransmitter-gated ion channels, 50 peptide receptors, and up to 1000 orphan receptors that may be chemoreceptors. For many gene families, C. elegans has both conventional members and divergent outliers with weak homology to known genes; these outliers may provide insights into previously unknown functions of conserved protein families.

C. elegans neuroscience: genetics to genome

From their earliest experiments, researchers using Caenorhabditis elegans have been interested in the role of genes in the development and function of the nervous system. As the C. elegans Genome Project completes the genomic sequence, we review the accomplishments of these researchers and the impact that the Genome Project has bad on their research. We also speculate on future directions in this research that are enabled by the efforts of the Genome Project.

Neuronal coincidence detection by voltage-sensitive electrical synapses

Coincidence detection is important for functions as diverse as Hebbian learning, binaural localization, and visual attention. We show here that extremely precise coincidence detection is a natural consequence of the normal function of rectifying electrical synapses. Such synapses open to bidirectional current flow when presynaptic cells depolarize relative to their postsynaptic targets and remain open until well after completion of presynaptic spikes. When multiple input neurons fire simultaneously, the synaptic currents sum effectively and produce a large excitatory postsynaptic potential. However, when some inputs are delayed relative to the rest, their contributions are reduced because the early excitatory postsynaptic potential retards the opening of additional voltage-sensitive synapses, and the late synaptic currents are shunted by already opened junctions. These mechanisms account for the ability of the lateral giant neurons of crayfish to sum synchronous inputs, but not inputs separated by only 100 microsec. This coincidence detection enables crayfish to produce reflex escape responses only to very abrupt mechanical stimuli. In light of recent evidence that electrical synapses are common in the mammalian central nervous system, the mechanisms of coincidence detection described here may be widely used in many systems.

The shaking B gene in Drosophila regulates the number of gap junctions between photoreceptor terminals in the lamina

The molecular structure of insect gap junctions differs from that in vertebrates, and in Drosophila is possibly encoded by the shaking B (= Passover) locus. shaking B2 is a null allele that acts in the nervous system. In the shakB2 mutant, one site of action are gap junctions between photoreceptor terminals in the cartridges of the lamina, beneath the compound eye, which we assayed from the number of close-apposition profiles in thin-section EM. The number of profiles in the Canton-S (C-S) wild type is about 0.5 per cartridge per section in distal and mid-lamina depths, and significantly less, about one quarter this value, closer to the brain, in the proximal lamina. In shakB2, there are fewer profiles, approximately one quarter the number of appositions in distal and mid-lamina depths as in C-S, and their number does not differ significantly from those at the proximal depth in either the mutant or wild type. Thus mutant action is associated with a reduced number of appositions at distal and mid-lamina depths. We propose that R1-R6 gap junctions are partitioned into at least two strata, proximal and distal, and that two populations of gap junctions exist, one extending throughout the lamina that does not require shakB, and a second at distal and mid-depth levels, which does. The number of gap junctions is reduced in mutant shakB2, and surviving appositions at distal and middle lamina depths possibly have wider clefts than in C-S. Gap junctions are reduced equally between all R1-R6 terminals, so the two different types of junction proposed, shakB2- and non-shakB2-dependent, can apparently express in a single receptor terminal.