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Vol. 17, Issue 4, 1676-1685, April 2006
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* Institut für Molekulare Physiologie und Entwicklungsbiologie der Universität Bonn, 53115 Bonn, Germany;
Kekulé-Institut für Organische Chemie und Biochemie der Universität Bonn, 53121 Bonn, Germany
Submitted November 17, 2005;
Revised January 6, 2006;
Accepted January 13, 2006
Monitoring Editor: Ben Margolis
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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; Wei et al., 2004). They form spatial microdomains in the plasma membrane at regions of cell adhesion and enable the integration of metabolic and signaling activities by allowing the direct exchange of ions and small molecules among neighboring cells. For the formation of a functional gap junction channel, two hexameric hemichannels, one contributed by each of the opposing cells, dock head to head in the extracellular space to form a double membrane-spanning intercellular channel (Segretain and Falk, 2004; Martin and Evans, 2004).
Three unrelated gene families have evolved to construct gap junction channels, the connexins and pannexins in deuterostomes, including all vertebrates, and the innexins in protostomes, including the invertebrates Drosophila and Caenorhabditis elegans (Phelan et al., 1998a; Söhl and Willecke, 2004; White et al., 2004; Bauer et al., 2005; Phelan, 2005). All three gene families encode four-pass membrane domains with two extracellular loops, cytoplasmic N and C termini, and a cytoplasmic loop domain. The connexin multigene family consists of 20 members in mice and 21 members in humans, and its contribution to intercellular communication has been extensively studied (Goodenough et al., 1996; Söhl and Willecke, 2004; Wei et al., 2004). Three pannexin genes with distinct expression patterns in the brain have been recently identified in mouse and humans (Panchin et al., 2000; Bruzzone et al., 2003; Baranova et al., 2004; Panchin, 2005). Genome sequencing projects identified eight innexin genes in the fruit fly Drosophila and 25 in the nematode C. elegans (Phelan and Starich, 2001). Connexin and pannexin genes are lacking in the fly and the nematode. Meanwhile, innexin genes have been cloned from several other invertebrate species including the cnidarian Hydra vulgaris (Alexopoulos et al., 2004), suggesting that innexins may encode gap junction proteins in all protostomes. Although innexins, connexins, and pannexins are structurally and functionally analogous, they show very little sequence similarity to each other.
For the eight innexin genes known in Drosophila, functions have been assigned via mutant and expression analysis to ogre and shakingB in the adult visual system (Watanabe and Kankel, 1992; Krishnan et al., 1993; Phelan et al., 1996, 1998b; Shimohigashi and Meinertzhagen, 1998; Zhang et al., 1999; Jacobs et al., 2000; Curtin et al., 2002) and to innexin4 (zero population growth) in germ cell differentiation (Tazuke et al., 2002; Gilboa et al., 2003). innexin2 (kropf) is required for the organization of epithelial cell layers in various organs, including the developing epidermis and the foregut (Bauer et al., 2002, 2004). No functional data are available to date on innexins 3, 5, 6, and 7. Mutations in connexin genes have been linked to a variety of human disorders, including peripheral neuropathy, nonsyndromal deafness, and cardiovascular anomalies (reviewed in Wei et al., 2004).
Both connexins and innexins are expressed in complex and overlapping expression patterns during development, and most cells and tissues express more than one connexin or innexin isoform. This enables cells to assemble homomeric (composed of identical subunits) or heteromeric (composed of different subunits) hemichannels resulting in the formation of homotypic (two hemichannels are identical) or heterotypic (two hemichannels differ in molecular composition) intercellular channels that may provide greater complexity in the regulation of gap junction communication (Cottrell and Burt, 2005). Although many examples for heteromeric/heterotypic connexin channels have been demonstrated in the heterologous Xenopus oocyte system, in tissue culture cells and also in a few cases in vivo, to what degree connexin hetero-oligomerization occurs in vivo and what physiological function it serves has still remained unclear. Similarly, expression of Shaking B-lethal and innexin2 in the heterologous Xenopus oocyte expression system and subsequent electrophysiological studies have demonstrated the potential for these two innexins to form homotypic channels, whereas innexin3 and innexin2 were shown to have the capacity to form heteromeric channels (Stebbings et al., 2000). However, it is not clear whether homomeric or heteromeric innexin channels occur in vivo, and if so, how they are assembled and what function they may serve during tissue and organ development.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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). Ectopic innexin (inx) expression studies were performed at 29°C using UASinx2, UASinx2-GFP, UASinx3-GFP, and UASwizinx3 (see below) as effector lines and paired-Gal4 and 69B-Gal4 as driver lines (Brand and Perrimon, 1993; Xiao et al., 1996; Bauer et al., 2004).
Antibody Generation
The peptide CPDDYRRDRQDRILKY containing the aa 157–171 of the inx3 protein sequence was synthesized, coupled to keyhole limpet hemocyanin (KLH), and used to generate an affinity-purified anti-inx3 rabbit polyclonal antibody. Synthesis and immunization was done in rabbits by Davids Biotechnology (Regensburg, Germany), using standard protocols.
Antibody Stainings and In Situ Hybridization
For in situ hybridization, the entire inx3 mRNA was subcloned into the EcoRI/XhoI sites of the pScript vector. Full-length digoxygenin RNA anti-sense probes were generated by in vitro transcription and labeled during run off transcription according to the manufacturer's instructions (Roche Diagnostics, Mannheim, Germany). In situ hybridization and antibody stainings were performed using standard protocols as described by Bauer et al. (2001). As primary antibodies, we used anti-arm (1:20; Developmental Studies Hybridoma Bank, University of Iowa, Iowa City, IA), anti-inx2 rabbit (1:75; Bauer et al., 2004), anti-inx2 chicken (1:50), anti-22C10 (1:10; Hybridoma Bank), anti-green fluorescent protein (GFP) (1:100; Roche Diagnostics), and anti-DE-cadherin (1:50; Santa Cruz Biotechnology, Santa Cruz, CA) and anti-inx3 (1:75). As secondary antibodies, we used Alexa 488 (1:100; MoBiTec, Göttingen, Germany); Alexa Fluor 546 (1:200; MoBiTec); Cy3, Cy2, and Cy5 (each 1:100; Dianova, Hamburg, Germany). Embryos stained with fluorescent antibodies were analyzed by laser scanning microscopy (Leica TSC2). Each fluorochrome was scanned individually to avoid cross-talk between channels. Images were subsequently combined using Adobe Photoshop 6.0 (Adobe Systems, Mountain View, CA).
Yeast Two-Hybrid Analysis and Plasmid Construction
The inx2 C terminus (CT) corresponding to the region from aa 289 (primer 5'-GGG ATC CCG AGA ATC GCT GTT GTG GCG GGT-3') to aa 367 (primer 5'-CGA GCT CGT TAG GCG TCG AAG GGC CGC-3') was amplified by PCR from LD11658 containing the entire inx2 cDNA. Subsequently, the amplification product was cut with BamHI and SacI and ligated in frame with the hSOS domain of the pSOS CytoTrap system vector (Stratagene, La Jolla, CA). It was used as bait, screening an embryonic library cloned in pMyr vectors and derived from the CytoTrap system (Stratagene). Screening was performed as described in the manual. Autoactivation of the inx3 target could be excluded after cotransformation of target and pSOS vector.
RNA Interference (RNAi) Experiments and pWIZ Constructs
A 620-base pair fragment (nucleotide 921-1540; GenBank accession no. AF172258
[GenBank]
) of inx3 was amplified and flanked with T7 binding motives by PCR (primer sequences: start, 5'-CTT TTA ATA CGA CTC ACT ATA GGG AGA GCT TGG CCA CCA TCT CCG GCG T-3'; stop, 5'-CTT TTA ATA CGA CTC ACT ATA GGG CTG GTA AAT GGT CCG TTA TTT AGG-3'). The fragment was used as template in the RiboMax Express Large Scale RNA Production system to produce complementary RNA of both strands. Injections were performed as described by Carthew (2001) with a final concentration of 0.5 µg/µl double-stranded RNA (dsRNA). Injections were performed with RNase-free needles into the posterior domain of the embryo at the syncytial blastoderm stage (injector Eppendorf TransferMan NK2). The volume ranged from 60 to 100 pl. Injection of buffer served as controls. For cuticle analyses embryos were incubated at 18°C under halocarbon oil for 48 h. The vitelline membrane was dissected, embryos were washed, and cuticles were prepared as described in Bauer et al. (2004). For in situ staining, devitellinized embryos were washed, fixed, and stained as described above.
To establish the stable transgenic UASwizinx3 knockdown lines, recombinant plasmids for the creation of stable RNAi-inducible fly strains were cloned as described by Lee and Carthew (2003). A 518-base pair DNA fragment of inx3 (bases 668–1186 of the cDNA) was amplified by PCR and cloned into the AvrII and NheI of pWIZ vector (gift from R. W. Carthew, Northwestern University, Evanston, IL). Recombinants with inverted repeats of both orientations were selected. Recombinant plasmids were injected into white– embryos, and transformant flies were generated by standard P element transformation (Spradling and Rubin, 1982) and crossed to Gal4 driver lines.
Cloning of Expression Vectors
Inx2 and 3 domains correspond to the following aa fragments: Inx2NT, aa 1–25; Inx2CL, aa 132–180; Inx2CT, aa 287–367; Inx3NT, aa 1–31; Inx3CL, aa 136–186; and Inx3CT, aa 293–395. The following primers were used to amplify inx domains toward cloning them into different expressions vectors (see below): BamInx2NTStart, 5'-CCG AGG ATC CAT ATG TTT GAT GTC TTT GGG TCC-3'; EcoInx2NTStop, 5'-GCG AAT TCC TAA TTG TTG TCG ATG CAC ACC TG-3'; BamInx2CLStart, 5'-CCG AGG ATC CAT AAG TCC TGG GAA GGC GGA-3'; EcoInx2CLStop, 5'-GCG AAT TCC TAT CGG AAG GCG TAG AAA TTG TG-3'; BamInx2CTStart, 5'-GGG ATC CCG AGA ATC GCT GTT GTG GCG GGT-3'; SacInx2CTStop, 5'-CGA GCT CGT TAG GCG TCG AAG GGC CGC-3'; EcoInx3NTStart, 5'-GGA ATT CCA TGG CGG TCT TTG GCA TGG TC-3'; XhoInx3NTStop, 5'-CCG CTC GAG CGG CGT GAT CCT GTA GTG GCA-3'; EcoInx3CSStart, 5'-GGA ATT CCA AGA ACA TGG AAG ACG GC-3'; XhoInx3CSStop, 5'-CCG CTC GAG CGG CGA GTA GCC GTT GTG G-3'; EcoInx3CTStart, 5'-GGA ATT CCT ATT CAC TGG TGG TTA TCA TG-3'; XhoInx3CTStop, 5'-CCG CTC GAG CGG TCA TGT CTC CGT CTC CTT-3'; and ApaInx3Stop, 5'-CGG GCC CGT GTC TCG GTC TCC TTG CCA CC-5'. The following primer was used to perform the insertion of a FLAG tag via PCR: 5'-CCG CCA CCG TTC CTC TGC CTC TGT CTA ATG TTC CTA CTG CTG CTA TTC TAG ACT GAG CTC GCC-3'. The following vectors were used to express tagged inx proteins: pMJGreen (kind gift from K. Willecke, University of Bonn, Bonn, Germany) for GFP-tagged inx2 and inx3; pGex5x-3 (GE Healthcare, Little Chalfont, Buckinghamshire, United Kingdom) for GST-tagged protein fragments; pQE-30Xa (QIAGEN, Valencia, CA) for His-tagged protein fragments; and pFast Bac (Invitrogen, Carlsbad, CA) for FLAG-tagged proteins. Proteins were expressed in Escherichia coli BL21 (Stratagene) cells or in case of InxCT FLAG-tagged protein in Sf21 cells.
Surface Plasmon Resonance Measurements
All SPR measurements were carried out with a BIAcore 3000 system (BIAcore, Uppsala, Sweden). Ligand proteins were coupled to final responses of 1.100 resonance units (RU) to 3.000 RU on CM5 sensor chips using the automated amino coupling procedure. For binding experiments and KD measurements, the chip was equilibrated to 1 x phosphate-buffered saline, pH 7.4, at 25°C. Injection of 120 µl of the analyte proteins glutathione S-transferase (GST), HisInx2CT, or GSTInx3CT was done at a constant flow rate of 30 µl/min. After 360 s of dissociation, the analyte was removed by injection of a short pulse 0.1% SDS in 5 mM NaOH as regeneration buffer. All binding curves were collected as double measurements and automatically subtracted from an underivatized reference surface. Obtained sensorgrams were analyzed with the BIAevaluation 3.1 software using the "steady-state affinity" fitting model.
Immunoprecipitation and Immunoblotting
Coimmunoprecipitation analyses of whole embryo lysates were performed using embryonic extracts of 0- to 16-h-old embryos and adult flies. The preparation of the extracts and the immunoprecipitation procedure was carried out as described in Bauer et al. (2004). Anti-inx3 and anti-inx2 antibodies were used at dilutions of 1:400 and anti-GFP antibody (Roche Diagnostics) at 1:1000. Secondary antibodies (anti-mouse and anti-rabbit; Dianova) were both used at a dilution of 1:2000 in 1% blocking solution (Roche Diagnostics).
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; Kistler et al., 1994). The TX100-solubilized extract was separated from TX100-insoluble material by high-speed centrifugation at 100,000 x g. Proteins from these fractions were concentrated in Amicon Ultra-15 centrifugal filter devices (Millipore, Billerica, MA). The sample was diluted with radioimmunoprecipitation assay buffer to a final concentration of 1 µg of protein/µl. After additional homogenization with a 26-gauge syringe and treatment for 1 min in an ultrasonic bath followed by centrifugation for 15 min at 11,000 x g. The supernatant was divided in equal aliquots. Coimmunoprecipitation analysis was performed as described above and in Bauer et al. (2004). |
RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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, 2002; Bauer et al., 2001, 2002). Both innexins are expressed rather ubiquitously during the blastoderm stage and in a segmentally reiterated pattern in the epidermis during later stages of development. Most prominent coexpression is found in the developing fore- and hindgut and in the epidermis (Bauer et al., 2002; Stebbings et al., 2002). Innexin2 protein was found to be localized to the membrane of epithelial cells in an apico-lateral position, and mutant analysis has shown that innexin2 is required for the organization of epithelial cell layers (Bauer et al., 2002, 2004). In contrast, functional studies on innexin3 are lacking because no innexin3 mutants have been isolated. As a first step to study the developmental role of innexin3, we generated an anti-innexin3 antibody. We used the peptide CPDDYRRDRQDRILKY, which contains the amino acids 157–171 of innexin3 within the cytoplasmic loop for immunization (Figure 1A). The antibody detects a 45-kDa protein corresponding to the predicted size of innexin3 (Figure 1C); its specificity was determined by testing various GST fusion proteins representing different innexin3 protein domains (Figure 1B), and by peptide competition experiments using protein extracts of wild-type embryos and adult flies (Figure 1C). On overexpression of an innexin3-GFP construct in every other segment of the epidermis using paired-Gal4 driver and UASinx3-GFP effector transgenic flies (see Materials and Methods), the antibody detects the stripe pattern (Figure 1D), further demonstrating the specificity of the antibody.
Anti-innexin3 antibody staining of wild-type embryos shows that innexin3 protein is distributed in a punctuate pattern mainly at the plasma membrane and to some extent also within the cytoplasm of the epithelial cells from early embryonic stages onward (Figure 1, E–H). A similar kind of protein distribution was found for other innexins (Bauer et al., 2001, Gilboa et al., 2003) and for vertebrate connexins, which are localized in gap junctions in the membranes of the cells and within intracellular stores in the endoplasmic reticulum–Golgi interfacial regions (Laird, 1996). At later stages of embryonic development, innexin3 is most prominently expressed in the epidermis, the developing fore- and hindgut, and in the central nervous system (CNS), as determined by using antibody double stainings with tissue-specific markers (Figure 1, F–H).
From blastoderm stage onwards until late stages of embryonic development, we find coexpression of innexins 2 and 3 in most tissues (Figure 1, I–K). Both proteins are colocalized in the membrane of epithelial cells, e.g., in the developing hindgut (Figure 1J) and in epidermal cells, from very early stages onward (Figure 1K). It is noteworthy, however, that we also find nonoverlapping signals of both innexins, which may indicate homomeric channels (Figure 1J).
Innexin3 Knockdown Causes Innexin2-like Mutant Phenotypes
We have previously shown that in maternal and zygotic null mutants of innexin2 (kropf mutants), epithelial morphogenesis is severely disrupted, resulting in cuticle holes and loss of cuticle formation in the most extreme cases (Bauer et al., 2002, 2004). During epidermal development, innexin2 was found to colocalize with armadillo and DE-cadherin, which are core proteins of adherens junctions (reviewed in Wheelock and Johnson, 2003) and evidence for direct molecular interactions between innexin2, DE-cadherin, and armadillo could be obtained in vitro and in vivo (Bauer et al., 2004). These data suggested that the positioning of gap junctions may depend on adherens junction proteins.
To further test the function of innexin3, we performed RNAi experiments to knock down innexin3 mRNA expression (Figure 2). We first injected 620-base pair dsRNA fragment into the posterior part of stage 3 embryos. In these embryos, innexin3 transcripts were strongly reduced, and we observed severe cuticle defects in the injected embryos, reflecting misdevelopment of the epidermis (our unpublished data). To investigate this in more detail, we generated a transgenic line carrying an UAS/Gal4 RNAi construct for innexin3 (UASwizinx3) in which part of its coding regions was cloned into a "face-to-face" orientation (Figure 2A). Using the UASwizinx3 effector in combination with the driver lines paired-Gal4, which knocks down innexin3 mRNA expression in every other segment (Figure 2D, compare with wild type in 2B), or 69B-Gal4, which mediates knockdown ubiquitously in the epidermis (Figure 2F), we find cuticle phenotypes similar to the dsRNA injection experiments: holes and irregular denticle belts or a complete loss of cuticle in the most extreme cases (Figure 2, E and G, compare with wild type in C). It is of note that kropf mutant embryos show similar defects during epidermis development (Bauer et al., 2004). In earlier developmental stages, the membrane localization of innexin2 is severely affected in 69B-Gal4 x UASwizinx3 knockdown embryos, and innexin2 protein accumulates to a considerable extent in the cytoplasm (Figure 2H, compare with wild type in inset of H). The distribution of the adherens junction core proteins DE-cadherin and armadillo (our unpublished data) is also changed in these embryos, from an apico-lateral localization in wild type to an accumulation in a more lateral membrane domain in the mutants (Figure 2, J and L, compare with wild type in I, K, and M); consistently, we observe a rounding up of the affected cells, characteristic for cell polarity defects (Tepass et al., 1996; Uemura et al., 1996). These data suggest an essential role for innexin3 in proper epithelial development of the epidermis, and they indicate that it controls innexin2 membrane localization, explaining the close similarity of the mutant defects of innexin3 knockdown and innexin2 mutant embryos. Conversely, we find innexin3 membrane localization affected in innexin2 mutants (Figure 2N, compare with wild type in I) and when innexin2 is misexpressed in stripes using an UASinx2 effector and a paired-Gal4 driver line, innexin3 is recruited into the ectopic expression domains of innexin2 (Figure 2O). In summary, the coexpression data, the similarities of the innexin2 and innexin3 mutant phenotypes and the observation that membrane localization of innexin2 is altered in innexin3 mutants and vice versa, strongly suggest the possibility that innexin2 and innexin3 may interact with each other to form heteromeric channels. To obtain further evidence for hetero-oligomerization of both innexins and to identify interacting protein domains, we carried out a series of molecular and biochemical experiments.
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Direct Interaction of Innexin2 with Innexin3
In a yeast two-hybrid interaction screen using the C-terminal cytoplasmic domain of innexin2 as a bait, we identified innexin3 as an interaction partner, providing the first molecular evidence that both innexins directly interact (Figure 3A). We screened a 0- to 24-h-old embryonic library derived from the CytoTrap system. After screening 
108 transformants and incubation at 37°C for 5 d, 34 potential positive clones were isolated of which 10 clones could be confirmed in a rescreen (Lehmann and Hoch, unpublished data). Among them was one clone containing the entire innexin3 cDNA. To confirm the yeast two-hybrid data, we performed coimmunoprecipitation analysis by using anti-innexin2 and anti-innexin3 antibodies in combination with embryonic extracts of wild-type embryos. As shown in Figure 3C, innexin3 can be precipitated specifically by using anti-innexin2 antibodies and vice versa (Figure 3D). When using extracts of transgenic embryos, which express a C-terminal-tagged innexin2-GFP fusion protein or when deleting the C-terminal domain, the interaction with innexin3 fails to occur (Figure 3E; our unpublished data). Consistent with the coimmunoprecipitation data, we find that in transgenic embryos expressing the innexin2-GFP fusion protein, innexin3 is not colocalized to innexin2-GFP (Figure 3G, compare with wild type in F). Consistently, overexpression of innexin2-GFP does not recruit innexin3 into the ectopic expression domains (Figure 3, I and J), and cells do not loose their polarity and do not round up (Figure 3H). These data further underline the significance of the C-terminal cytoplasmic domain of innexin2 for the interaction with innexin3.
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Innexin2 and Innexin3 Form Heterodimers In Vivo
To test whether innexin2 and innexin3 form heteromers in vivo, we performed biochemical fractionation experiments. We prepared embryonic extracts of 0- to 16-h-old wild-type embryos and used high-speed centrifugation in combination with coimmunoprecipitation to study intracellular innexin2/innexin3 heteromerization. We used a protocol that was previously applied to study the assembly of connexin proteins into oligomeric channels (Musil and Goodenough, 1993; Kistler et al., 1994). In this procedure, insoluble gap junction plaques located at the plasma membrane are separated from newly synthesized innexin monomers and oligomers located in the cytoplasmic fraction, by Triton X-100 treatment and centrifugation (Figure 5A). In coimmunoprecipitation experiments using the Triton X-100–soluble fraction, which includes all intracellular intermediate assembly products of gap junction channels, innexin2 is specifically coimmunoprecipitated by anti-innexin3 antibody and vice versa (Figure 5B). These data strongly suggest that innexins 2 and 3 form heteromers in vivo during the assembly of heteromeric channels.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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and Segretain and Falk, 2004), the molecular composition of innexin-containing gap junctions and the functional significance of innexin oligomerization in developmental processes is unknown (reviewed in Bauer et al., 2005 and Phelan, 2005). We have used a combination of in vitro approaches, including yeast two-hybrid analysis, coimmunoprecipitation, and surface plasmon resonance studies, and in vivo analysis, including the study of mutants, RNAi knockdown embryos, and transgenic animals to investigate the role of the Drosophila innexins 2 and 3 during embryonic development. Our results provide strong evidence that both innexins are essential for epidermis development and that they form heteromers via C termini-mediated interactions, suggesting a mechanism of how innexins may oligomerize into heteromeric gap junction channels.
Innexins2 and 3 Are Essential for Epithelial Tissue Morphogenesis
Our coimmunostainings indicate that innexins 2 and 3 are colocalized in the membrane of epidermal cells, and our genetic loss and gain-of-function experiments show that membrane localization and cellular distribution of both innexins is mutually dependent on each other (Figure 6): innexin2 is mislocalized to the cytoplasm upon RNAi knockdown of innexin3, and innexin3 is mislocalized to the cytoplasm in innexin2 mutants (Figure 2, H and N). Furthermore, innexin3 is recruited into ectopic expression domains defined by misexpression of innexin2. Consistent with the dependence of both innexins on each other, mutants or RNAi knockdown of either of the genes shows very similar cell polarity defects in the epidermis: on RNAi knockdown of innexin3 or in innexin2 mutants, we find large holes in the cuticle or even a complete loss of cuticle in the most extreme cases (Figure 2, E and G; Bauer et al., 2004), the proper membrane distribution of DE-cadherin is affected in the mutant embryos, and we observe a rounding up of the affected cells, characteristic for cell polarity defects (Figure 2L; Tepass et al., 1996; Uemura et al., 1996). These data suggest that heteromerization of both innexins is essential for proper epithelial development of the epidermis. It has been shown previously that innexin2 protein accumulates in the apico-lateral membrane domain and colocalizes with armadillo and DE-cadherin (Bauer et al., 2004). In mutants for both zygotic armadillo and DE-cadherin, the localization of innexin2 is altered and in innexin2 overexpression experiments, armadillo and DE-cadherin are organized into the ectopic innexin2 pattern. Further evidence for a more direct interaction between innexin2 and adherens junction proteins was provided by yeast two-hybrid analysis and coimmunoprecipitation experiments using embryonic extracts, which showed that innexin2 interacts via its cytoplasmic loop domain with the C terminus of DE-cadherin (Bauer et al., 2004). It is not clear whether innexin3 also directly binds to DE-cadherin or whether the heteromeric interactions of innexin3 with innexin2 explain the alterations of DE-cadherin localization in innexin3 knockdowns in which innexin2 is mislocalized. Also, we cannot exclude that the cell polarity defects observed in innexin knockdown embryos are an indirect consequence of defective cell-to-cell communication among epidermal cells. However, we currently favor the possibility that interactions between DE-cadherin and innexins may reflect common trafficking routes of adherens junction and gap junction proteins within cells that ensure the positioning of innexin-containing hemichannels in membrane domains close to adherens junctions in cells of the epidermis. This is supported by recent data in the Drosophila Schneider cell system showing that DE-cadherin controls trafficking and localization of innexin2 to the plasma membrane (Bauer et al., 2006). It was recently shown for the mammalian connexin43 
1 that it coassembles in a multiprotein complex containing N-cadherin and various N-cadherin–associated proteins, and it was suggested that the intracellular coassembly of connexins and cadherin is required for gap junction and adherens junction formation (Wei et al., 2005). In mammals as in Drosophila, there thus seems to be an intimate linkage between the assembly of connexin-containing gap junctions and adherens junctions. This is supported by a number of studies in mammals, which have shown that the formation of connexin-containing gap junctions is dependent on the assembly of adherens junctions (Meyer et al., 1992; Frenzel and Johnson, 1996; Hertig et al., 1996; Luo and Radice, 2003). Inhibition of cadherin function can disrupt gap junction formation and inhibit cell–cell coupling, suggesting that localization of cadherin to cell–cell contact sites may be a prerequisite for gap junction formation (Meyer et al., 1992). Conversely, inhibition of connexin43 can block adherens junction formation (Zuppinger et al., 2000).
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Innexins 2 and 3 Form Heteromers during Channel Assembly via C-Terminal Domain-mediated Interactions
Our yeast two-hybrid analysis, the coimmunoprecipitation and surface plasmon resonance studies, the biochemical fractionation experiments, and the in vivo analysis using transgenic embryos strongly suggest that the heteromeric interaction of both innexins occur during heteromeric channel assembly and are mediated via the cytoplasmic C-terminal domains of both innexins. Our data provide evidence that the heteromer of both proteins may be the smallest assembly unit during the formation of heteromeric innexin2/innexin3 channels. Coimmunoprecipitation analyses of full-length and progressively truncated versions of connexins 43, 32, and 26 have suggested the presence of isoform-specific intrinsic signals that regulate hetero-oligomerization of connexins (Falk et al., 1997; Falk, 2000). An assembly signal allowing connexin subunits to recognize each other is supposed to be located in the third transmembrane domain, whereas a selectivity signal regulating subunit compatibility is located in the N-terminal domain and the first transmembrane domain (for reviews, see Lagree et al., 2003 and Segretain and Falk, 2004). The corresponding regions are not conserved on the amino acid sequence level between connexins and innexins, and it is not clear whether such signals also exist for innexins. Rather, our data suggest that heteromerization of innexins 2 and 3 is mediated by C-terminal interactions. In normal development, the heteromeric interactions of both innexins may be required for proper vesicular transport and membrane insertion of mixed hemichannels, because we observe an accumulation of innexin3 in the cytoplasm of epithelial cells of kropf mutants and vice versa. Specific heteromerization signals may exist in the C termini of both proteins determining isoform compatibility. It is known for connexins that hemichannel assembly is not a random process but rather can be regulated in a cell-specific manner (Das Sarma et al., 2001). Furthermore, connexins do not contain conventional signal sequences for trafficking and assembly, thereby opening the option of multiple routes for trafficking and assembly in the same cell (Diez et al., 1999). It is interesting to note from our BIAcore experiments that the C terminus of innexin2 seems much more restricted in its interaction range than the C terminus of innexin3, which interacts also with its own cytoplasmic loop and its own N terminus, suggesting the potential to form homomeric innexin3 channels. This may allow the regulation of heterotypic innexin 2/innexin 3 channels via a "ball and chain" mechanism that was proposed previously for the chemical gating of connexin43 (Delmar et al., 2000) and requires the interaction of the carboxyl tail of connexin43 with its intracellular loop. Previous studies in the heterologous Xenopus expression system have shown that innexin2 can form homomeric channels in frog oocytes, whereas homomeric innexin3 channels were not functional (Stebbings et al., 2000). In our coimmunostainings, we find some individual, nonoverlapping signals for both innexin2 and innexin3 in epithelial cells, suggesting that homomeric channels for innexin2 and 3 also exist in vivo (Figure 1J). The function of these channels is, however, not known. In summary, our results demonstrate that innexin heteromerization is crucial for epithelial tissue morphogenesis and polarity in Drosophila epidermis development. The binding of both innexins via their C termini provides a mechanism for oligomerization of heteromeric channels. In view of the evolutionary conservation of innexins in many protostomal species, including grasshoppers, molluscs, flatworms, polychaete annelids, leeches, protochordates, and cnidarians (for reviews, see Alexopoulos et al., 2004 and Bauer et al., 2005), our findings suggest that innexins may have an evolutionary conserved role in tissue morphogenesis and polarity in animals.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Footnotes |
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Address correspondence to: Michael Hoch (m.hoch{at}uni-bonn.de).
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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