Restrictions in gap junctional communication in the Drosophila larval epidermis

Gap junction gene and protein families: Connexins, innexins, and pannexins

Gap junction channels facilitate the intercellular exchange of ions and small molecules. While this process is critical to all multicellular organisms, the proteins that form gap junction channels are not conserved. Vertebrate gap junctions are formed by connexins, while invertebrate gap junctions are formed by innexins. Interestingly, vertebrates and lower chordates contain innexin homologs, the pannexins, which also form channels, but rarely (if ever) make intercellular channels. While the connexin and the innexin/pannexin polypeptides do not share significant sequence similarity, all three of these protein families share a similar membrane topology and some similarities in quaternary structure. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Larval defense against attack from parasitoid wasps requires nociceptive neurons

Parasitoid wasps are a fierce predator of Drosophila larvae. Female Leptopilina boulardi (LB) wasps use a sharp ovipositor to inject eggs into the bodies of Drosophila melanogaster larvae. The wasp then eats the Drosophila larva alive from the inside, and an adult wasp ecloses from the Drosophila pupal case instead of a fly. However, the Drosophila larvae are not defenseless as they may resist the attack of the wasps through somatosensory-triggered behavioral responses. Here we describe the full range of behaviors performed by the larval prey in immediate response to attacks by the wasps. Our results suggest that Drosophila larvae primarily sense the wasps using their mechanosensory systems. The range of behavioral responses included both “gentle touch” like responses as well as nociceptive responses. We found that the precise larval response depended on both the somatotopic location of the attack, and whether or not the larval cuticle was successfully penetrated during the course of the attack. Interestingly, nociceptive responses are more likely to be triggered by attacks in which the cuticle had been successfully penetrated by the wasp. Finally, we found that the class IV neurons, which are necessary for mechanical nociception, were also necessary for a nociceptive response to wasp attacks. Thus, the class IV neurons allow for a nociceptive behavioral response to a naturally occurring predator of Drosophila.

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.

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.

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.

Effects of hydrocortisone on the formation of gap junctions and the abnormal growth of cilia within the rat anterior pituitary gland: possible role of gap junctions on the regulation of cell development

We investigated the effects of hydrocortisone on the formation of gap junctions in and the growth of cilia on folliculo-stellate cells. The male rats of experimental groups were given daily intraperitoneal injections of 5 mg/kg of hydrocortisone from Day 20 to 60. Five rats were killed at ages 10, 20, 30 and 40 days after initiation of injections, and the pituitary gland was removed from each rat. Then, the specimens were prepared for observation by transmission electron microscopy. A delay in the formation of gap junctions between folliculo-stellate cells was observed in hydrocortisone treated rats compared with control rats on Day 30, 40 and 50. Another finding in the present study was the increase of ciliated follicles on Day 40 and 50 in the hydrocortisone treated groups, simultaneous with the delay in gap junction formation. The results suggest that hydrocortisone has a suppressive effect on the gap junction formation between folliculo-stellate cells, and loss of intercellular communication by way of gap junctions may lead to alteration of morphological development of the cell.

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.

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.

Gap junction-mediated cell-cell communication modulates mouse neural crest migration

Previous studies showed that conotruncal heart malformations can arise with the increase or decrease in alpha1 connexin function in neural crest cells. To elucidate the possible basis for the quantitative requirement for alpha1 connexin gap junctions in cardiac development, a neural crest outgrowth culture system was used to examine migration of neural crest cells derived from CMV43 transgenic embryos overexpressing alpha1 connexins, and from alpha1 connexin knockout (KO) mice and FC transgenic mice expressing a dominant-negative alpha1 connexin fusion protein. These studies showed that the migration rate of cardiac neural crest was increased in the CMV43 embryos, but decreased in the FC transgenic and alpha1 connexin KO embryos. Migration changes occurred in step with connexin gene or transgene dosage in the homozygous vs. hemizygous alpha1 connexin KO and CMV43 embryos, respectively. Dye coupling analysis in neural crest cells in the outgrowth cultures and also in the living embryos showed an elevation of gap junction communication in the CMV43 transgenic mice, while a reduction was observed in the FC transgenic and alpha1 connexin KO mice. Further analysis using oleamide to downregulate gap junction communication in nontransgenic outgrowth cultures showed that this independent method of reducing gap junction communication in cardiac crest cells also resulted in a reduction in the rate of crest migration. To determine the possible relevance of these findings to neural crest migration in vivo, a lacZ transgene was used to visualize the distribution of cardiac neural crest cells in the outflow tract. These studies showed more lacZ-positive cells in the outflow septum in the CMV43 transgenic mice, while a reduction was observed in the alpha1 connexin KO mice. Surprisingly, this was accompanied by cell proliferation changes, not in the cardiac neural crest cells, but in the myocardium- an elevation in the CMV43 mice vs. a reduction in the alpha1 connexin KO mice. The latter observation suggests that cardiac neural crest cells may have a role in modulating growth and development of non-neural crest- derived tissues. Overall, these findings suggest that gap junction communication mediated by alpha1 connexins plays an important role in cardiac neural crest migration. Furthermore, they indicate that cardiac neural crest perturbation is the likely underlying cause for heart defects in mice with the gain or loss of alpha1 connexin function.

The role of gap junction membrane channels in development

In most developmental systems, gap junction-mediated cell-cell communication (GJC) can be detected from very early stages of embryogenesis. This usually results in the entire embryo becoming linked as a syncytium. However, as development progresses, GJC becomes restricted at discrete boundaries, leading to the subdivision of the embryo into communication compartment domains. Analysis of gap junction gene expression suggests that this functional subdivision of GJC may be mediated by the differential expression of the connexin gene family. The temporal-spatial pattern of connexin gene expression during mouse embryogenesis is highly suggestive of a role for gap junctions in inductive interactions, being regionally restricted in distinct developmentally significant domains. Using reverse genetic approaches to manipulate connexin gene function, direct evidence has been obtained for the connexin 43 (Cx43) gap junction gene playing a role in mammalian development. The challenges in the future are the identification of the target cell populations and the cell signaling processes in which Cx43-mediated cell-cell interactions are critically required in mammalian development. Our preliminary observations suggest that neural crest cells may be one such cell population.

Negative growth control of mitotically active imaginal cells (histoblasts) of the abdominal epidermis during metamorphosis of the housefly

On the ventral side of each pupal abdominal segment of the housefly, there is a pair of histoblast nests, each containing about 600 diploid cells. These cells, during adult development, divide, replace intervening polytene larval epidermal cells (LEC), and form both the median sternite and the surrounding pleura of the adult segment. Since the histoblast nests and the LEC form a contiguous layer, we examined the role of these two types of cells in regulating the mitotic potential of the histoblasts during development of the median sternite. Two experimental approaches were used: deletion of one of the nests by thermocautery; and by disturbance of the continuity of the monolayered epidermis by thermocautery of, or topical application of heptanol on, the midventral LEC. Ablation of one of the contralateral nests resulted in a mirror image duplication of the hemisternite and pleura by the surviving nest. Disturbance of the continuity of the LEC produced mirror image duplication of the hemisternal pattern by each of the contralateral nests. From these results, we propose that the contralateral ventral nests mutually downregulate their mitotic potential by secreting regulatory factor(s) to produce the normal median sternite pattern and surrounding pleura. We also suggest that these chemicals act in a paracrine fashion, possibly through gap junctions in the LEC. © 1994 Wiley-Liss, Inc.

Voltage-clamp analysis of gap junctions between embryonic muscles in Drosophila

1. Intercellular communication between embryonic muscle fibres was examined in Drosophila melanogaster. 2. Injection of fluorescent dye revealed extensive coupling between muscle fibres which form a uniform communicating arrangement of cells without restriction at the segmental borders. 3. Dye transfer was blocked by octanol and membrane depolarization suggesting that it is mediated by gap junctions. 4. Double voltage-clamp experiments from cell pairs in situ showed that the ionic coupling is sensitive to the voltage difference between the cytoplasm and the extracellular space (transmembrane voltage, Vi-o) as well as between the cells (transjunctional voltage, Vj). 5. In steady-state conditions, the gap conductance (gj) was maximal for hyperpolarized Vi-o and decreased progressively to near zero as Vi-o became more positive than -50 mV. 6. Gap conductance decreased from a maximal value as Vj increased either in the positive or negative direction (by depolarizing or hyperpolarizing, respectively, one of the cells from a holding potential of -60 mV). In both cases, gj asymptotically approached a non-zero residual value which was different for negative and positive Vj (about 20% of the maximal conductance for negative transmembrane potentials and 10% for positive values). 7. Application of octanol (1 mM) resulted in an almost complete and reversible block of gj.

Gap junction communication and cell adhesion in development

During the past decade growing evidence has suggested that cell-cell communication via gap junctions is crucial for early developmental processes (Warneret al., 1984; Guthrie & Gilula, 1989; Serraset al., 1989). It has been shown that embryos of mice (Kalimi & Lo, 1988), teleosts (Kimmelet al., 1984), insects (Warner & Lawrence, 1982; Ruangvoravat & Lo, 1992) and molluscs (Serraset al., 1989) become regionally organized into restricted domains of junctionally connected cells that share developmental potential. In the mouse gastrula, for example, dye-coupling experiments have demonstrated that cells within a developmental compartment have a high degree of coupling whereas cells across compartmental boundaries have reduced coupling (Kalimi & Lo, 1988). Classic experiments (Townes & Holtfretter, 1955; Steinberg, 1963) demonstrated that as cells begin to differentiate along common pathways, they develop selective adhesion properties and the ability to sort themselves from unlike neighbours (for review see Edelman 1988; Edelmanet al., 1990). More recently a multitude of specific cell adhesion molecules (CAMs) have been identified that mediate these processes and trigger the cytoplasmic events that drive further differentiation (Edelmanet al., 1990; Albelda, 1991; Geiger & Ayalon, 1992).

Analog of vertebrate anionic sites in blood-brain interface of larval Drosophila

The blood-brain barrier ensures brain function in vertebrates and in some invertebrates by maintaining ionic integrity of the extraneuronal bathing fluid. Recent studies have demonstrated that anionic sites on the luminal surface of vascular endothelial cells collaborate with tight junctions to effect this barrier in vertebrates. We characterize these two analogous barrier factors for the first time on Drosophila larva by an electron-dense tracer and cationic gold labeling. Ionic lanthanum entered into but not through the extracellular channels between perineurial cells. Tracer is ultimately excluded from neurons in the ventral ganglion mainly by an extensive series of (pleated sheet) septate junctions between perineurial cells. Continuous junctions, a variant of the septate junction, were not as efficient as the pleated sheet variety in blocking tracer. An anionic domain now is demonstrated in Drosophila central nervous system through the use of cationic colloidal gold in LR White embedment. Anionic domains are specifically stationed in the neural lamella and not noted in the other cell levels of the blood-brain interface. It is proposed that in the central nervous system of the Drosophila larva the array of septate junctions between perineurial cells is the physical barrier, while the anionic domains in neural lamella are a "charge-selective barrier" for cations. All of these results are discussed relative to analogous characteristics of the vertebrate blood-brain barrier.

Cell-cell communication correlates with pattern formation in molting Manduca midgut epithelium

The midgut epithelium of larval Manduca sexta is constructed of single goblet cells surrounded by a one-cell-thick reticulum of columnar cells. This pattern is expanded at each molt by the addition of new cells. Between molts, these epithelial cells are not dye coupled, even though gap junctions are present. Proliferating stem cells are dye coupled in small groups early in the molt. Then, at mid-molt, the whole epithelium temporarily becomes dye coupled. This is when the new (expanded) pattern is being established. Later, at the end of the molt, the epithelium returns to the non-coupled state. These results suggest that cell communication via gap junctions may play a role in cell patterning.

Connexin 43 expression in the mouse embryo: localization of transcripts within developmentally significant domains

The expression of the gap junction gene, Cx43, during mouse embryogenesis was characterized by an in situ hybridization analysis of mouse embryos from gestation days 4.5 to 12.5. This analysis revealed that Cx43 transcripts are differentially expressed as a function of development beginning at the blastocyst stage. In many regions of the embryo, Cx43 transcripts were found in discrete spatially restricted domains. This was observed in conjunction with the development of the brain, neural tube, prevertebra, limb, and various aspects of organogenesis. In some cases, the differential localization of Cx43 transcripts is associated with developmental processes mediated by inductive interactions, such as that of the eye, otic vesicle, kidney, and the branchial arches. In addition, in the 10.5 day embryo, Cx43 transcripts appear to be distributed as a gradient in regions spanning the midbrain/hindbrain junction, in the telencephalon, and in the limb mesenchyme. Surprisingly, our results also suggest that neural crest and sclerotomal cells, i.e., cells that are presumably migratory, express high levels of Cx43 transcripts. Overall, these results suggest that gap junctions encoded by Cx43 may play a role in various aspects of mouse development, possibly including relaying second messengers emanating from signal transduction pathways that mediate inductive interactions.