Connexin45 directly binds to ZO-1 and localizes to the tight junction region in epithelial MDCK cells

High-resolution proteomic mapping in the vertebrate central nervous system: close proximity of connexin35 to NMDA glutamate receptor clusters and co-localization of connexin36 with immunoreactivity for zonula occludens protein-1 (ZO-1)

Combined confocal microscopy and freeze-fracture replica immunogold labeling (FRIL) were used to examine the connexin identity at electrical synapses in goldfish brain and rat retina, and to test for "co-localization" vs. "close proximity" of connexins to other functionally interacting proteins in synapses of goldfish and mouse brain and rat retina. In goldfish brain, confocal microscopy revealed immunofluorescence for connexin35 (Cx35) and NMDA-R1 (NR1) glutamate receptor protein in Mauthner Cell/Club Ending synapses. By FRIL double labeling, NR1 glutamate receptors were found in clusters of intramembrane particles in the postsynaptic membrane extraplasmic leaflets, and these distinctive postsynaptic densities were in close proximity (0.1-0.3 microm) to neuronal gap junctions labeled for Cx35, which is the fish ortholog of connexin36 (Cx36) found at neuronal gap junctions in mammals. Immunogold labeling for Cx36 in adult rat retina revealed abundant gap junctions, including several previously unrecognized morphological types. As in goldfish hindbrain, immunogold double labeling revealed NR1-containing postsynaptic densities localized near Cx36-labeled gap junction in rat inferior olive. Confocal immunofluorescence microscopy revealed widespread co-localization of Cx36 and ZO-1, particularly in the reticular thalamic nucleus and amygdala of mouse brain. By FRIL, ZO-1 immunoreactivity was co-localized with Cx36 at individual gap junction plaques in rat retinal neurons. As cytoplasmic accessory proteins, ZO-1 and possibly related members of the membrane-associated guanylate kinase (MAGUK) family represent scaffolding proteins that may bind to and regulate the activity of many neuronal gap junctions. These data document the power of combining immunofluorescence confocal microscopy with FRIL ultrastructural imaging and immunogold labeling to determine the relative proximities of proteins that are involved in short- vs. intermediate-range molecular interactions in the complex membrane appositions at synapses between neurons.

New roles for astrocytes: gap junction hemichannels have something to communicate

Gap junctions are clusters of aqueous channels that connect the cytoplasm of adjoining cells. Each cell contributes a hemichannel, or connexon, to each cell-cell channel. The cell-cell channels are permeable to relatively large molecules, and it was thought that opening of hemichannels to the extracellular space would kill cells through loss of metabolites, collapse of ionic gradients and influx of Ca(2+). Recent findings indicate that specific non-junctional hemichannels do open under both physiological and pathological conditions, and that opening is functional or deleterious depending on the situation. Most of these studies utilized cells in tissue culture that expressed a specific gap junction protein, connexin 43. Several such examples are reviewed here, with a particular focus on astrocytes.

A novel claudin 16 mutation associated with childhood hypercalciuria abolishes binding to ZO-1 and results in lysosomal mistargeting

Mutations in the gene coding for the renal tight junction protein claudin 16 cause familial hypomagnesemia with hypercalciuria and nephrocalcinosis, an autosomal recessive disorder of renal Ca(2+) and Mg(2+) handling that progressively leads to chronic renal failure, with nephrolithiasis having been reported in heterozygous carriers. Screening a cohort of 11 families with idiopathic hypercalciuria identified a novel homozygous mutation in the claudin 16 gene in two families. In contrast to classical symptoms of familial hypomagnesemia with hypercalciuria and nephrocalcinosis, the patients displayed serious but self-limiting childhood hypercalciuria with preserved glomerular filtration rate. The mutation results in inactivation of a PDZ-domain binding motif, thereby disabling the association of the tight junction scaffolding protein ZO-1 with claudin 16. In contrast to wild-type claudin 16, the mutant no longer localizes to tight junctions in kidney epithelial cells but instead accumulates in lysosomes. Thus, mutations at different intragenic sites in the claudin 16 gene may lead to particular clinical phenotypes with a distinct prognosis. Mutations in claudin 16 that affect interaction with ZO-1 lead to lysosomal mistargeting, providing-for the first time, to our knowledge-insight into the molecular mechanism of a disease-associated mutation in the claudin 16 gene.

Lens connexins alpha3Cx46 and alpha8Cx50 interact with zonula occludens protein-1 (ZO-1)

Connexin alpha1Cx43 has previously been shown to bind to the PDZ domain-containing protein ZO-1. The similarity of the carboxyl termini of this connexin and the lens fiber connexins alpha3Cx46 and alpha8Cx50 suggested that these connexins may also interact with ZO-1. ZO-1 was shown to be highly expressed in mouse lenses. Colocalization of ZO-1 with alpha3Cx46 and alpha8Cx50 connexins in fiber cells was demonstrated by immunofluorescence and by fracture-labeling electron microscopy but showed regional variations throughout the lens. ZO-1 was found to coimmunoprecipitate with alpha3Cx46 and alpha8Cx50, and pull-down experiments showed that the second PDZ domain of ZO-1 was involved in this interaction. Transiently expressed alpha3Cx46 and alpha8Cx50 connexins lacking the COOH-terminal residues did not bind to the second PDZ domain but still formed structures resembling gap junctions by immunofluorescence. These results indicate that ZO-1 interacts with lens fiber connexins alpha3Cx46 and alpha8Cx50 in a manner similar to that previously described for alpha1Cx43. The spatial variation in the interaction of ZO-1 with lens gap junctions is intriguing and is suggestive of multiple dynamic roles for this association.

Expression of connexins in the normal and obstructed developing kidney

Connections between cells are achieved by proteins called connexins that comprise the gap junction. Connexins play a major role in organ development. Our reverse transcription-polymerase chain reaction (RT-PCR) studies demonstrate that Cx30, Cx36, Cx37, Cx40, Cx45, Cx46, and Cx50 are expressed in the kidney. Quantitative RT-PCR indicates that Cx37, Cx45, and Cx46 are preferentially expressed during early renal development. We also explored the expression of connexins in neonatal unilateral ureteral obstruction (UUO). After 12 days of neonatal UUO, the renal mRNA expression of Cx30, Cx37, and Cx40 was significantly elevated. In contrast, there was no change in connexin renal mRNA levels in adult UUO. We conclude that multiple connexins are expressed in the rat kidney and several are aberrantly expressed in neonatal UUO.

Tight junction proteins

A fundamental function of epithelia and endothelia is to separate different compartments within the organism and to regulate the exchange of substances between them. The tight junction (TJ) constitutes the barrier both to the passage of ions and molecules through the paracellular pathway and to the movement of proteins and lipids between the apical and the basolateral domains of the plasma membrane. In recent years more than 40 different proteins have been discovered to be located at the TJs of epithelia, endothelia and myelinated cells. This unprecedented expansion of information has changed our view of TJs from merely a paracellular barrier to a complex structure involved in signaling cascades that control cell growth and differentiation. Both cortical and transmembrane proteins integrate TJs. Among the former are scaffolding proteins containing PDZ domains, tumor suppressors, transcription factors and proteins involved in vesicle transport. To date two components of the TJ filaments have been identified: occludin and claudin. The latter is a protein family with more than 20 members. Both occludin and claudins are integral proteins capable of interacting adhesively with complementary molecules on adjacent cells and of co-polymerizing laterally. These advancements in the knowledge of the molecular structure of TJ support previous physiological models that exhibited TJ as dynamic structures that present distinct permeability and morphological characteristics in different tissues and in response to changing natural, pathological or experimental conditions.

Sharing signals: connecting lung epithelial cells with gap junction channels

Gap junction channels enable the direct flow of signaling molecules and metabolites between cells. Alveolar epithelial cells show great variability in the expression of gap junction proteins (connexins) as a function of cell phenotype and cell state. Differential connexin expression and control by alveolar epithelial cells have the potential to enable these cells to regulate the extent of intercellular coupling in response to cell stress and to regulate surfactant secretion. However, defining the precise signals transmitted through gap junction channels and the cross talk between gap junctions and other signaling pathways has proven difficult. Insights from what is known about roles for gap junctions in other systems in the context of the connexin expression pattern by lung cells can be used to predict potential roles for gap junctional communication between alveolar epithelial cells.

Molecular cloning, functional expression, and tissue distribution of a novel human gap junction-forming protein, connexin-31.9. Interaction with zona occludens protein-1

A novel human connexin gene (GJA11) was cloned from a genomic library. The open reading frame encoded a hypothetical protein of 294 amino acid residues with a predicted molecular mass of 31,933, hence referred to as connexin-31.9 (Cx31.9) or alpha 11 connexin. A clone in GenBank containing the Cx31.9 gene localized to chromosome 17q21.2. Northern analysis of Cx31.9 showed a major 4.4-kilobase transcript, which was expressed at varying levels in all tissues analyzed. Two monoclonal antibodies generated against different domains of Cx31.9 recognized a 30-33-kDa protein from cells overexpressing Cx31.9. Immunofluorescence of overexpressing cells indicated the presence of Cx31.9 between adjacent cells, consistent with its localization to gap junctions. Double voltage clamp analyses of Cx31.9-overexpressing cells, and of paired Xenopus oocytes injected with Cx31.9 cRNA, demonstrated junctional currents indicative of gap junction channel formation. In contrast to previously characterized connexins, Cx31.9 showed no voltage-dependent gating within a physiologically relevant range. Cx31.9 was detected in human tissues by immunoblot analysis, and immunofluorescence localized Cx31.9 expression to vascular smooth muscle cells. Furthermore, it was demonstrated that Cx31.9 interacted with ZO-1. Thus, Cx31.9 represents a novel connexin gene that in vivo generates a protein with unique voltage gating properties.

Lighting up gap junction channels in a flash

Gap junction intercellular communication channels permit the exchange of small regulatory molecules and ions between neighbouring cells and coordinate cellular activity in diverse tissue and organ systems. These channels have short half-lives and complex assembly and degradation pathways. Much of the recent work elucidating gap junction biogenesis has featured the use of connexins (Cx), the constituent proteins of gap junctions, tagged with reporter proteins such as Green Fluorescent Protein (GFP) and has illuminated the dynamics of channel assembly in live cells by high-resolution time-lapse microscopy. With some studies, however, there are potential short-comings associated with the GFP chimeric protein technologies. A recent report by Gaietta et al., has highlighted the use of recombinant proteins with tetracysteine tags attached to the carboxyl terminus of Cx43, which differentially labels 'old' and 'new' connexins thus opening up new avenues for studying temporal and spatial localisation of proteins and in situ trafficking events.

Formation of the gap junction nexus: binding partners for connexins

Gap junctions are the morphological correlates of direct cell-cell communication and are formed of hexameric assemblies of gap junction proteins (connexins) into hemichannels (or connexons) provided by each coupled cell. Gap junction channels formed by each of the connexin subtypes (of which there are as many as 20) display different properties, which have been attributed to differences in amino acid sequences of gating domains of the connexins. Recent studies additionally indicate that connexin proteins interact with other cellular components to form a protein complex termed the Nexus. This review summarizes current knowledge regarding the protein-protein interactions involving of connexin proteins and proposes hypothesized functions for these interactions.

Keratin subunit expression in human cultured melanocytes and mouse neural crest cells without formation of filamentous structures

The synthesis of keratin is considered to occur in epithelial and epidermal cells. Previous studies have not reported on keratin synthesis within melanocytes that derive from neural crest cells. Epithelial and neural crest cells originally develop from ectodermal tissue. We previously reported that the expression of keratin is a universal phenomenon seen in cultured melanoma cell lines, as demonstrated by two-dimensional polyacrylamide gel electrophoresis, western blot, and electron microscopy analyses. To further investigate the specificity of keratin function in melanocytic cells, we first examined the presence of keratin proteins in cultured human melanocytes, and unexpectedly found keratin subunits in melanocytes by the above-mentioned procedures. The keratin (K) subunits were composed of K1, K5, K8, K10, K14, K16, and K18, together with vimentin. Neural crest cells, which contain immature embryonic melanocytes developing from ectoderm, already expressed keratins; however, under electron microscopy, the expressed keratin did not form filamentous structures. Although the ATP synthase alpha-chain, which is expressed universally in cultured epidermal tumor cell lines, was also expressed in cultured melanocytes and neural crest cells, a novel malignant melanoma-related protein (MMRP) was absent in melanocytes and neural crest cells. We concluded that keratin subunits are present in both cells, but do not construct keratin filaments.