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

Orb2 enables rare-codon-enriched mRNA expression during Drosophila neuron differentiation

Regulation of codon optimality is an increasingly appreciated layer of cell- and tissue-specific protein expression control. Here, we use codon-modified reporters to show that differentiation of Drosophila neural stem cells into neurons enables protein expression from rare-codon-enriched genes. From a candidate screen, we identify the cytoplasmic polyadenylation element binding (CPEB) protein Orb2 as a positive regulator of rare-codon-dependent mRNA stability in neurons. Using RNA sequencing, we reveal that Orb2-upregulated mRNAs in the brain with abundant Orb2 binding sites have a rare-codon bias. From these Orb2-regulated mRNAs, we demonstrate that rare-codon enrichment is important for mRNA stability and social behavior function of the metabotropic glutamate receptor (mGluR). Our findings reveal a molecular mechanism by which neural stem cell differentiation shifts genetic code regulation to enable critical mRNA stability and protein expression.

Orb2 enables rare-codon-enriched mRNA expression during Drosophila neuron differentiation

Regulation of codon optimality is an increasingly appreciated layer of cell- and tissue-specific protein expression control. Here, we use codon-modified reporters to show that differentiation of Drosophila neural stem cells into neurons enables protein expression from rare-codon-enriched genes. From a candidate screen, we identify the cytoplasmic polyadenylation element binding (CPEB) protein Orb2 as a positive regulator of rare-codon-dependent expression in neurons. Using RNA sequencing, we reveal that Orb2-upregulated mRNAs in the brain with abundant Orb2 binding sites have a rare-codon bias. From these Orb2-regulated mRNAs, we demonstrate that rare-codon enrichment is important for expression control and social behavior function of the metabotropic glutamate receptor (mGluR). Our findings reveal a molecular mechanism by which neural stem cell differentiation shifts genetic code regulation to enable critical mRNA and protein expression.

Synaptic components are required for glioblastoma progression in Drosophila

Glioblastoma (GB) is the most aggressive, lethal and frequent primary brain tumor. It originates from glial cells and is characterized by rapid expansion through infiltration. GB cells interact with the microenvironment and healthy surrounding tissues, mostly neurons and vessels. GB cells project tumor microtubes (TMs) contact with neurons, and exchange signaling molecules related to Wingless/WNT, JNK, Insulin or Neuroligin-3 pathways. This cell to cell communication promotes GB expansion and neurodegeneration. Moreover, healthy neurons form glutamatergic functional synapses with GB cells which facilitate GB expansion and premature death in mouse GB xerograph models. Targeting signaling and synaptic components of GB progression may become a suitable strategy against glioblastoma. In a Drosophila GB model, we have determined the post-synaptic nature of GB cells with respect to neurons, and the contribution of post-synaptic genes expressed in GB cells to tumor progression. In addition, we document the presence of intratumoral synapses between GB cells, and the functional contribution of pre-synaptic genes to GB calcium dependent activity and expansion. Finally, we explore the relevance of synaptic genes in GB cells to the lifespan reduction caused by GB advance. Our results indicate that both presynaptic and postsynaptic proteins play a role in GB progression and lethality.

Glioblastoma (GB) is the most frequent and aggressive type of brain tumor. It is originated from glial cells that expand and proliferate very fast in the brain. GB cells infiltrate and establish cell to cell communication with healthy neurons. Currently there is no effective treatment for GB and these tumors result incurable with an average survival of 16 months after diagnosis. Here we used a Drosophila melanogaster model to search for genetic suppressors of GB progression. The results show that genes involved in the formation of synapses are required for glial cell number increase, expansion of tumoral volume and premature death. Among these synaptic genes we found that post-synaptic genes that contribute to Neuron-GB interaction which validate previous findings in human GB. Moreover, we found electro dense structures between GB cells that are compatible with synapses and that expression of pre-synaptic genes, including brp, Lip-α and syt 1, is required for GB progression and aggressiveness. These results suggest a contribution of synapses between GB cells to disease progression, named as intratumoral synapses.

Molecular and morphological approach to study the innexin gap junctions in Rhynchosciara americana

Gap junctions mediate communication between adjacent cells and are fundamental to the development and homeostasis in multicellular organisms. In invertebrates, gap junctions are formed by transmembrane proteins called innexins. Gap junctions allow the passage of small molecules through an intercellular channel, between a cell and another adjacent cell. The dipteran Rhynchosciara americana has contributed to studying the biology of invertebrates and the study of the interaction and regulation of genes during biological development. Therefore, this paper aimed to study the R. americana innexin-2 by molecular characterization, analysis of the expression profile and cellular localization. The molecular characterization results confirm that the message is from a gap junction protein and analysis of the expression and cellular localization profile shows that innexin-2 can participate in many physiological processes during the development of R. americana.

Evolutionary dynamics of recent selection on cognitive abilities

Cognitive abilities can vary dramatically among species. The relative importance of social and ecological challenges in shaping cognitive evolution has been the subject of a long-running and recently renewed debate, but little work has sought to understand the selective dynamics underlying the evolution of cognitive abilities. Here, we investigate recent selection related to cognition in the paper wasp Polistes fuscatus-a wasp that has uniquely evolved visual individual recognition abilities. We generate high quality de novo genome assemblies and population genomic resources for multiple species of paper wasps and use a population genomic framework to interrogate the probable mode and tempo of cognitive evolution. Recent, strong, hard selective sweeps in P. fuscatus contain loci annotated with functions in long-term memory formation, mushroom body development, and visual processing, traits which have recently evolved in association with individual recognition. The homologous pathways are not under selection in closely related wasps that lack individual recognition. Indeed, the prevalence of candidate cognition loci within the strongest selective sweeps suggests that the evolution of cognitive abilities has been among the strongest selection pressures in P. fuscatus' recent evolutionary history. Detailed analyses of selective sweeps containing candidate cognition loci reveal multiple cases of hard selective sweeps within the last few thousand years on de novo mutations, mainly in noncoding regions. These data provide unprecedented insight into some of the processes by which cognition evolves.

The pleiotropic effects of Innexin genes expressed in Drosophila glia encompass wing chemosensory sensilla

The neuroanatomy of Drosophila wing chemosensilla and the analysis of their sensory organ precursor cell lineage have demonstrated that they are surprisingly related to taste perception. The microarchitecture of wing bristles limits the use of electrophysiology methods to investigate wing chemosensory mechanisms. However, by monitoring the fluorescence of the complex calcium/GCaMP, calcium flux triggered upon tastant stimulation was observed within sensilla aligned along the wing anterior nerve. This string of fluorescent puncta was impaired in wings of Innexin 2 (Inx2) mutant flies; although it is unclear whether the Innexin proteins act at the level of the wing imaginal disc, adult wing and/or at both levels. Glial cells known to shelter Innexin(s) expression have no documented role in adult chemosensory sensilla. Our data suggest that Innexin(s) are likely required for the maturation of functional wing chemosensilla in adulthood. The unexpected presence of most Innexin transcripts in adult wing RNAseq data set argues for the expression of Innexin proteins in the larval imaginal wing disc that are continued in wing chemosensilla at adulthood. OPEN PRACTICES: This article has earned an Open Data badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available as supporting materials and includes the electronic lab notebook. Learn more about the Open Practices badges from the Center for Open Science: https://osf.io/tvyxz/wiki.

Shaking-B misexpression increases the formation of gap junctions but not chemical synapses between auditory sensory neurons and the giant fiber of Drosophila melanogaster

The synapse between auditory Johnston's Organ neurons (JONs) and the giant fiber (GF) of Drosophila is structurally mixed, being composed of cholinergic chemical synapses and Neurobiotin- (NB) permeable gap junctions, which consist of the innexin Shaking-B (ShakB). Previous observations showed that misexpression of one ShakB isoform, ShakB(N+16), in a subset of JONs that do not normally form gap junctions results in their de novo dye coupling to the GF. Misexpression of the transcription factor Engrailed (En) in these neurons also has this effect, and in addition causes the formation of new chemical synapses. These results, along with earlier studies suggesting that gap junctions are required for the development of some chemical synapses, led to the hypothesis that ShakB would, like En, have an instructive effect on the distribution of mixed chemical/electrical contacts. To test this, we first confirmed quantitatively that ShakB(N+16) misexpression increased the dye-coupling of JONs with the GF, indicating the formation of ectopic gap junctions. Conversely, expression of the 'incorrect' isoform, ShakB(N), abolished dye coupling. Immunocytochemistry of the ShakB protein showed that ShakB(N+16) increased gap junctional plaques in JON axons but ShakB(N) did not. To test our hypothesis, fluorescently-labeled presynaptic active zone protein (Brp) was expressed in JONs and the changes in its distribution on the GF dendrites was assayed with confocal microscopy in animals with misexpression of ShakB(N+16), ShakB(N) or, as a positive control, En. Using different methods of image analysis, we confirmed our previous result that En misexpression increased the chemical synapses with the GF and the amount of GF medial dendrite branching. However, contrary to our hypothesis, misexpression of ShakB did not increase these parameters. Immunostaining showed no association between presynaptic active zones and the new ShakB plaques, further evidence against the hypothesis. We conclude that both subsets of JON form chemical synapses onto the GF dendrites but only one population forms gap junctions, comprised of ShakB(N+16). Misexpression of this isoform in all JONs does not instruct the formation of new mixed chemical/electrical synapses, but results in the insertion of new gap junctions, presumably at the sites of existing chemical synaptic contacts with the GF.

The interplay between electrical and chemical synaptogenesis

Neurons communicate with each other via electrical or chemical synaptic connections. The pattern and strength of connections between neurons are critical for generating appropriate output. What mechanisms govern the formation of electrical and/or chemical synapses between two neurons? Recent studies indicate that common molecular players could regulate the formation of both of these classes of synapses. In addition, electrical and chemical synapses can mutually coregulate each other’s formation. Electrical activity, generated spontaneously by the nervous system or initiated from sensory experience, plays an important role in this process, leading to the selection of appropriate connections and the elimination of inappropriate ones. In this review, we discuss recent studies that shed light on the formation and developmental interactions of chemical and electrical synapses.

Electrical synapses mediate synergism between pheromone and food odors in Drosophila melanogaster

In Drosophila melanogaster, the sex pheromone produced by males, cis-vaccenyl acetate (cVA), evokes a stereotypic gender-specific behavior in both males and females. As Drosophila adults feed, mate, and oviposit on food, they perceive the pheromone as a blend against a background of food odors. Previous studies have reported that food odors enhance flies' behavioral response to cVA, specifically in virgin females. However, how and where the different olfactory inputs interact has so far remained unknown. In this study, we elucidated the neuronal mechanism underlying the response at an anatomical, functional, and behavioral level. Our data show that in virgin females cVA and the complex food odor vinegar evoke a synergistic response in the cVA-responsive glomerulus DA1. This synergism, however, does not appear at the input level of the glomerulus, but is restricted to the projection neuron level only. Notably, it is abolished by a mutation in gap junctions in projection neurons and is found to be mediated by electrical synapses between excitatory local interneurons and projection neurons. As a behavioral consequence, we demonstrate that virgin females in the presence of vinegar become receptive more rapidly to courting males, while male courtship is not affected. Altogether, our results suggest that lateral excitation via gap junctions modulates odor tuning in the antennal lobe and drives synergistic interactions between two ecologically relevant odors, representing food and sex.

Shaking B Mediates Synaptic Coupling between Auditory Sensory Neurons and the Giant Fiber of Drosophila melanogaster

The Johnston’s Organ neurons (JONs) form chemical and electrical synapses onto the giant fiber neuron (GF), as part of the neuronal circuit that mediates the GF escape response in Drosophila melanogaster. The purpose of this study was to identify which of the 8 Drosophila innexins (invertebrate gap junction proteins) mediates the electrical connection at this synapse. The GF is known to express Shaking B (ShakB), specifically the ShakB(N+16) isoform only, at its output synapses in the thorax. The shakB² mutation disrupts these GF outputs and also abolishes JON-GF synaptic transmission. However, the identity of the innexin that forms the presynaptic hemichannels in the JONs remains unknown. We used electrophysiology, immunocytochemistry and dye injection, along with presynaptically-driven RNA interference, to investigate this question. The amplitude of the compound action potential recorded in response to sound from the base of the antenna (sound-evoked potential, or SEP) was reduced by RNAi of the innexins Ogre, Inx3, Inx6 and, to a lesser extent Inx2, suggesting that they could be required in JONs for proper development, excitability, or synchronization of action potentials. The strength of the JON-GF connection itself was reduced to background levels only by RNAi of shakB, not of the other seven innexins. ShakB knockdown prevented Neurobiotin coupling between GF and JONs and removed the plaques of ShakB protein immunoreactivity that are present at the region of contact. Specific shakB RNAi lines that are predicted to target the ShakB(L) or ShakB(N) isoforms alone did not reduce the synaptic strength, implying that it is ShakB(N+16) that is required in the presynaptic neurons. Overexpression of ShakB(N+16) in JONs caused the formation of ectopic dye coupling, whereas ShakB(N) prevented it altogether, supporting this conclusion and also suggesting that gap junction proteins may have an instructive role in synaptic target choice.

The molecular and immunochemical expression of innexins in the yellow fever mosquito, Aedes aegypti: insights into putative life stage- and tissue-specific functions of gap junctions

Gap junctions (GJ) mediate direct intercellular communication by forming channels through which certain small molecules and/or ions can pass. Connexins, the proteins that form vertebrate GJ, are well studied and known to contribute to neuronal, muscular and epithelial physiology. Innexins, the GJ proteins of insects, have only recently received much investigative attention and many of their physiological roles remain to be determined. Here we characterize the molecular expression of six innexin (Inx) genes in the yellow fever mosquito Aedes aegypti (AeInx1, AeInx2, AeInx3, AeInx4, AeInx7, and AeInx8) and the immunochemical expression of one innexin protein, AeInx3, in the alimentary canal. We detected the expression of no less than four innexin genes in each mosquito life stage (larva, pupa, adult) and tissue/body region from adult males and females (midgut, Malpighian tubules, hindgut, head, carcass, gonads), suggesting a remarkable potential molecular diversity of GJ in mosquitoes. Moreover, the expression patterns of some innexins were life stage and/or tissue specific, suggestive of potential functional specializations. Cloning of the four full-length cDNAs expressed in the Malpighian tubules of adult females (AeInx1, AeInx2, AeInx3, and AeInx7) revealed evidence for 1) alternative splicing of AeInx1 and AeInx3 transcripts, and 2) putative N-glycosylation of AeInx3 and AeInx7. Finally, immunohistochemistry of AeInx3 in the alimentary canal of larval and adult female mosquitoes confirmed localization of this innexin to the intercellular regions of Malpighian tubule and hindgut epithelial cells, suggesting that it is an important component of GJ in these tissues.

Hemichannel composition and electrical synaptic transmission: molecular diversity and its implications for electrical rectification

Unapposed hemichannels (HCs) formed by hexamers of gap junction proteins are now known to be involved in various cellular processes under both physiological and pathological conditions. On the other hand, less is known regarding how differences in the molecular composition of HCs impact electrical synaptic transmission between neurons when they form intercellular heterotypic gap junctions (GJs). Here we review data indicating that molecular differences between apposed HCs at electrical synapses are generally associated with rectification of electrical transmission. Furthermore, this association has been observed at both innexin and connexin (Cx) based electrical synapses. We discuss the possible molecular mechanisms underlying electrical rectification, as well as the potential contribution of intracellular soluble factors to this phenomenon. We conclude that asymmetries in molecular composition and sensitivity to cellular factors of each contributing hemichannel can profoundly influence the transmission of electrical signals, endowing electrical synapses with more complex functional properties.

Drosophila Ebony: a novel type of nonribosomal peptide synthetase related enzyme with unusually fast peptide bond formation kinetics

Drosophila Ebony is a β-alanyl biogenic amine synthetase with proven function in cuticle and in glia of the nervous system. It is closely related to nonribosomal peptide synthetases (NRPSs), which typically consist of at least an adenylation, a peptidyl carrier protein and a peptide bond forming condensation domain. Besides its role in cuticle formation, Ebony is in most glia of the brain thought to convert biogenic amines to β-alanyl conjugates. If the metabolization of the neurotransmitter histamine to β-alanyl histamine requires a fast reaction in visual signal transduction, Ebony must be able to fulfill this requirement. Since NRPSs are in general slowly acting multi-modular protein machineries, the enigma of how Ebony quickly facilitates this inactivation remains a key question for understanding its role in vision. To quantitatively analyze the reaction kinetics, we used phosphopantetheinylated holo-Ebony prepared from Baculovirus infected Sf9 cells. Kinetic parameters for the loading reaction, e.g. the formation of β-alanyl-Ebony thioester, complied with those of slow NRPSs. In contrast, single-turnover analysis of the last reaction step, peptide bond formation between pre-activated β-alanyl Ebony thioester and histamine, revealed a very rapid conjugation reaction. This biphasic nature of activity identifies Ebony as a novel type of NRPS related molecule that combines a slow amino acid activation phase with a very fast product formation step.

Electrical synapses and their functional interactions with chemical synapses

Brain function relies on the ability of neurons to communicate with each other. Interneuronal communication primarily takes place at synapses, where information from one neuron is rapidly conveyed to a second neuron. There are two main modalities of synaptic transmission: chemical and electrical. Far from functioning independently and serving unrelated functions, mounting evidence indicates that these two modalities of synaptic transmission closely interact, both during development and in the adult brain. Rather than conceiving synaptic transmission as either chemical or electrical, this article emphasizes the notion that synaptic transmission is both chemical and electrical, and that interactions between these two forms of interneuronal communication might be required for normal brain development and function.

Innexins Ogre and Inx2 are required in glial cells for normal postembryonic development of the Drosophila central nervous system

Innexins are one of two gene families that have evolved to permit neighbouring cells in multicellular systems to communicate directly. Innexins are found in prechordates and persist in small numbers in chordates as divergent sequences termed pannexins. Connexins are functionally analogous proteins exclusive to chordates. Members of these two families of proteins form intercellular channels, assemblies of which constitute gap junctions. Each intercellular channel is a composite of two hemichannels, one from each of two apposed cells. Hemichannels dock in the extracellular space to form a complete channel with a central aqueous pore that regulates the cell-cell exchange of ions and small signalling molecules. Hemichannels can also act independently by releasing paracrine signalling molecules. optic ganglion reduced (ogre) is a member of the Drosophila innexin family, originally identified as a gene essential for postembryonic neurogenesis. Here we demonstrate, by heterologous expression in paired Xenopus oocytes, that Ogre alone does not form homotypic gap-junction channels; however, co-expression of Ogre with Innexin2 (Inx2) induces formation of functional channels with properties distinct from Inx2 homotypic channels. In the Drosophila larval central nervous system, we find that Inx2 partially colocalises with Ogre in proliferative neuroepithelia and in glial cells. Downregulation of either ogre or inx2 selectively in glia, by targeted expression of RNA interference transgenes, leads to a significant reduction in the size of the larval nervous system and behavioural defects in surviving adults. We conclude that these innexins are crucially required in glial cells for normal postembryonic development of the central nervous system.

Distinct roles of TRP channels in auditory transduction and amplification in Drosophila

Auditory receptor cells rely on mechanically gated channels to transform sound stimuli into neural activity. Several TRP channels have been implicated in Drosophila auditory transduction, but mechanistic studies have been hampered by the inability to record subthreshold signals from receptor neurons. Here, we develop a non-invasive method for measuring these signals by recording from a central neuron that is electrically coupled to a genetically defined population of auditory receptor cells. We find that the TRPN family member NompC, which is necessary for the active amplification of sound-evoked motion by the auditory organ, is not required for transduction in auditory receptor cells. Instead, NompC sensitizes the transduction complex to movement and precisely regulates the static forces on the complex. In contrast, the TRPV channels Nanchung and Inactive are required for responses to sound, suggesting they are components of the transduction complex. Thus, transduction and active amplification are genetically separable processes in Drosophila hearing.

Preferential electrical coupling regulates neocortical lineage-dependent microcircuit assembly

Radial glial cells are the primary neural progenitor cells in the developing neocortex. Consecutive asymmetric divisions of individual radial glial progenitor cells produce a number of sister excitatory neurons that migrate along the elongated radial glial fibre, resulting in the formation of ontogenetic columns. Moreover, sister excitatory neurons in ontogenetic columns preferentially develop specific chemical synapses with each other rather than with nearby non-siblings. Although these findings provide crucial insight into the emergence of functional columns in the neocortex, little is known about the basis of this lineage-dependent assembly of excitatory neuron microcircuits at single-cell resolution. Here we show that transient electrical coupling between radially aligned sister excitatory neurons regulates the subsequent formation of specific chemical synapses in the neocortex. Multiple-electrode whole-cell recordings showed that sister excitatory neurons preferentially form strong electrical coupling with each other rather than with adjacent non-sister excitatory neurons during early postnatal stages. This preferential coupling allows selective electrical communication between sister excitatory neurons, promoting their action potential generation and synchronous firing. Interestingly, although this electrical communication largely disappears before the appearance of chemical synapses, blockade of the electrical communication impairs the subsequent formation of specific chemical synapses between sister excitatory neurons in ontogenetic columns. These results suggest a strong link between lineage-dependent transient electrical coupling and the assembly of precise excitatory neuron microcircuits in the neocortex.

Fly neurons in culture: a model for neural development and pathology

Primary neural cultures from the fruit fly, Drosophila melanogaster, enable a high-resolution glance into cellular processes and neuronal interaction. The development of the culture, along with its vitality and functionality, can be continuously monitored, and the abundance of available tools for D. melanogaster research can greatly assist in characterizing different aspects of the culture. The fly primary neural culture preparation thus offers a promising platform for studying a variety of processes relating to nervous system development, activity and pathology. Our data reveal that neural cultures derived from the CNS of third-instar D. melanogaster larvae undergo an organization process that is specific and consistent throughout different cultures, and culminates in the creation of an elaborate neural network. We demonstrate that this process is accompanied by detectable changes in the protein expression profile of the culture, indicating the involvement of multi-protein processes specific to each stage of the network's development. As a further proof of concept, we demonstrate differential expression of a particular protein family, the gap-junction constructing innexin protein family, throughout the network's life.

Electrical coupling between olfactory glomeruli

In the Drosophila antennal lobe, excitation can spread between glomerular processing channels. In this study, we investigated the mechanism of lateral excitation. Dual recordings from excitatory local neurons (eLNs) and projection neurons (PNs) showed that eLN-to-PN synapses transmit both hyperpolarization and depolarization, are not diminished by blocking chemical neurotransmission, and are abolished by a gap-junction mutation. This mutation eliminates odor-evoked lateral excitation in PNs and diminishes some PN odor responses. This implies that lateral excitation is mediated by electrical synapses from eLNs onto PNs. In addition, eLNs form synapses onto inhibitory LNs. Eliminating these synapses boosts some PN odor responses and reduces the disinhibitory effect of GABA receptor antagonists on PNs. Thus, eLNs have two opposing effects on PNs, driving both direct excitation and indirect inhibition. We propose that when stimuli are weak, lateral excitation promotes sensitivity, whereas when stimuli are strong, lateral excitation helps recruit inhibitory gain control.

High resolution map of Caenorhabditis elegans gap junction proteins

The innexin family of gap junction proteins has 25 members in Caenorhabditis elegans. Here, we describe the first high-resolution expression map of all members through analysis of live worms transformed with green fluorescent protein under the control of entire promoter regions. Our analyses show that innexins have dynamic expression patterns throughout development and are found in virtually all cell types and tissues. Complex tissues, such as the pharynx, intestine, gonad, as well as scaffolding tissues and guidepost cells express a variety of innexins in overlapping or complementary patterns, suggesting they may form heteromeric and heterotypic channels. Innexin expression occurs in several types of cells that are not known to form gap junctions as well as in a pair of migrating cells, suggesting they may have hemichannel function. Therefore, innexins likely play roles in almost all body functions, including embryonic development, cell fate determination, oogenesis, egg laying, pharyngeal pumping, excretion, and locomotion.

Experience-dependent maturation of the glomerular microcircuit

Spontaneous and patterned activity, largely attributed to chemical transmission, shape the development of virtually all neural circuits. However, electrical transmission also has an important role in coordinated activity in the brain. In the olfactory bulb, gap junctions between apical dendrites of mitral cells increase excitability and synchronize firing within each glomerulus. We report here that the development of the glomerular microcircuit requires both sensory experience and connexin (Cx)36-mediated gap junctions. Coupling coefficients, which measure electrical coupling between mitral cell dendrites, were high in young mice, but decreased after postnatal day (P)10 because of a maturational increase in membrane conductance. Sensory deprivation, induced by unilateral naris occlusion at birth, slowed the morphological development of mitral cells and arrested the maturational changes in membrane conductance and coupling coefficients. As the coupling coefficients decreased in normal mice, a glutamate-mediated excitatory postsynaptic current (EPSC) between mitral cells emerged by P30. Although mitral-mitral EPSCs were generally unidirectional, they were not present in young adult Cx36(-/-) mice, suggesting that gap junctions are required for the development and/or function of the mature circuit. The experience-dependent transition from electrical transmission to combined chemical and electrical transmission provides a previously unappreciated mechanism that may tune the response properties of the glomerular microcircuit.

DE-Cadherin, a Core Component of the Adherens Junction Complex Modifies Subcellular Localization of theDrosophilaGap Junction Protein Innexin2

The Drosophila innexin multigene family of gap junction encoding proteins consists of eight family members whose function in epithelial morphogenesis is mostly unknown. We have recently shown that innexin2 plays a crucial role in the organization of embryonic epithelia. Innexin2 protein accumulates in the epidermis in the apico-lateral membrane domain and colocalizes with core proteins of adherens junctions, such as DE-cadherin and Armadillo, the ss -catenin homolog. Innexin2 localization is altered in both armadillo and DE-cadherin mutants Biochemical interaction studies point to a direct interaction of DE-cadherin and Armadillo with innexin2 suggesting a close link between gap junction and adherens junction biogenesis. We have used the Drosophila Schneider cell tissue culture system to further study the interaction of innexin2 with DE-cadherin. Our results provide evidence that DE-cadherin may be a key component to control trafficking, and localization of Innexin2 to the plasma membrane.

Two gap junction channel (innexin) genes of the Bombyx mori and their expression

Gap junctions are clusters of intercellular channels that are associated with embryonic development and neural signaling. Innexins, invertebrate gap junction proteins, have been identified in Drosophila and Caenorhabditis. Here, we report the isolation and characterization of two novel members of the insect innexin family, Bm inx2 and Bm inx4, from embryos of the silkworm, Bombyx mori, during the germ-band formation stage. Bm inx2 is a single copy gene with one exon, while Bm inx4 is a single copy gene with four exons and three introns. The predicted proteins show structural similarities with other innexin family members, including four transmembrane (TM) domains, two extracellular loops (ELs), one cytoplasmic loop (CL), and typical conserved amino acids. Bm inx2 is phylogenetically orthologous to the other insect inx2 genes, but Bm inx4 is not orthologous to any known innexin including Dm inx4. Interestingly, Northern blotting and in situ hybridization showed that Bm inx2 was variously expressed across all developmental stages and in various tissues, with high expression seen in the nervous system at the time of embryogenesis. In contrast, Bm inx4 was transiently expressed at the germ-band formation stage of embryogenesis, and was specifically expressed in the ovary and testis during the larval and pupal stages. The isolation and characterization of these novel genes should form the basis for further study of the functional events that occur during development and neuronal communication in B. mori.

The chemical component of the mixed GF-TTMn synapse in Drosophila melanogaster uses acetylcholine as its neurotransmitter

The largest central synapse in adult Drosophila is a mixed electro-chemical synapse whose gap junctions require the product of the shaking-B (shak-B) gene. Shak-B² mutant flies lack gap junctions at this synapse, which is between the giant fibre (GF) and the tergotrochanteral motor neuron (TTMn), but it still exhibits a long latency response upon GF stimulation. We have targeted the expression of the light chain of tetanus toxin to the GF, to block chemical transmission, in shak-B² flies. The long latency response in the tergotrochanteral muscle (TTM) was abolished indicating that the chemical component of the synapse mediates this response. Attenuation of GAL4-mediated labelling by a cha-GAL80 transgene, reveals the GF to be cholinergic. We have used a temperature-sensitive allele of the choline acetyltransferase gene (cha^ts2) to block cholinergic synapses in adult flies and this also abolished the long latency response in shak-B² flies. Taken together the data provide evidence that both components of this mixed synapse are functional and that the chemical neurotransmitter between the GF and the TTMn is acetylcholine. Our findings show that the two components of this synapse can be separated to allow further studies into the mechanisms by which mixed synapses are built and function.

The connexin turnover, an important modulating factor of the level of cell-to-cell junctional communication: comparison with other integral membrane proteins

The constituent proteins of gap junctions, called "connexins" (Cxs) in chordates, are generally renewed several times a day, in approximately the same rate range as many other integral plasma membrane proteins and the proteins of other channels, other intercellular junctions or different membrane receptors. This permanent renewal turns on a fine-tuned balance among various processes, such as gene transcription, mRNA stability and processing, protein synthesis and oligomerization, posttranslational modifications, transport to the plasma membrane, anchoring to the cytoskeleton, connexon aggregation and docking, regulation of endocytosis and controlled degradations of the proteins. Subtle changes at one or some of these steps would represent an exquisite level of regulation that extends beyond the rapid channel opening and closure events associated with channel gating; membrane channels and receptors are constantly able to answer to physiological requirements to either up- or downregulate their activity. The Cx turnover rate thereby appears to be a key component in the regulation of any protein, particularly of gap junctional proteins. However, the physiological stimuli that control the assembly of Cxs into gap junctions and their degradation remain poorly understood.

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.

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.

Transient electrical coupling regulates formation of neuronal networks

Electrical synapses are abundant before and during developmental windows of intense chemical synapse formation, and might therefore contribute to the establishment of neuronal networks. Transient electrical coupling develops and is then eliminated between regenerating Helisoma motoneurons 110 and 19 during a period of 48-72 h in vivo and in vitro following nerve injury. An inverse relationship exists between electrical coupling and chemical synaptic transmission at these synapses, such that the decline in electrical coupling is coincident with the emergence of cholinergic synaptic transmission. In this study, we have generated two- and three-cell neuronal networks to test whether predicted synaptogenic capabilities were affected by previous synaptic interactions. Electrophysiological analyses demonstrated that synapses formed in three-cell neuronal networks were not those predicted based on synaptogenic outcomes in two-cell networks. Thus, new electrical and chemical synapse formation within a neuronal network is dependent on existing connectivity of that network. In addition, new contacts formed with established networks have little impact on these existing connections. These results suggest that network-dependent mechanisms, particularly those mediated by gap junctional coupling, regulate synapse formation within simple neural networks.

Molecular characterization and embryonic expression of innexins in the leech Hirudo medicinalis

Gap junctions are direct intercellular channels that permit the passage of ions and small signaling molecules. The temporal and spatial regulation of gap junctional communication is, thus, one mechanism by which cell interactions, and hence cell properties and cell fate, may be regulated during development. The nervous system of the leech, Hirudo medicinalis, is a particularly advantageous system in which to study developmental mechanisms involving gap junctions because interactions between identified cells may be studied in vivo in both the embryo and the adult. As in most invertebrates, gap junctions in the leech are composed of innexin proteins, which are distantly related to the vertebrate pannexins and are encoded by a multi-gene family. We have cloned ten novel leech innexins and describe the expression of these, plus two other previously reported members of this gene family, in the leech embryo between embryonic days 6 and 12, a period during which the main features of the central nervous system are established. Four innexins are expressed in neurons and two in glia, while several innexins are expressed in the excretory, circulatory, and reproductive organs. Of particular interest is Hm-inx6, whose expression appears to be restricted to the characterized S cell and two other neurons putatively identified as presynaptic to this cell. Two other innexins also show highly restricted expressions in neurons and may be developmentally regulated.

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.

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.

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.

Electrical synapse formation disrupts calcium-dependent exocytosis, but not vesicle mobilization

Electrical coupling exists prior to the onset of chemical connectivity at many developing and regenerating synapses. At cholinergic synapses in vitro, trophic factors facilitated the formation of electrical synapses and interfered with functional neurotransmitter release in response to photolytic elevations of intracellular calcium. In contrast, neurons lacking trophic factor induction and electrical coupling possessed flash-evoked transmitter release. Changes in cytosolic calcium and postsynaptic responsiveness to acetylcholine were not affected by electrical coupling. These data indicate that transient electrical synapse formation delayed chemical synaptic transmission by imposing a functional block between the accumulation of presynaptic calcium and synchronized, vesicular release. Despite the inability to release neurotransmitter, neurons that had possessed strong electrical coupling recruited secretory vesicles to sites of synaptic contact. These results suggest that the mechanism by which neurotransmission is disrupted during electrical synapse formation is downstream of both calcium influx and synaptic vesicle mobilization. Therefore, electrical synaptogenesis may inhibit synaptic vesicles from acquiring a readily releasable state. We hypothesize that gap junctions might negatively interact with exocytotic processes, thereby diminishing chemical neurotransmission.

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.

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.

Nonredundant gap junction functions

The need for molecular heterogeneity of gap junction channel proteins in vivo has been enigmatic. Recently, functional replacement of one channel gene with another in mice and flies has revealed that cellular health depends not simply on gap junction communication but also requires the correct type of intercellular channel subunit.