Identification of connexin 50 and 57 mRNA in A-type horizontal cells of the rabbit retina

Connexin-36 distribution and layer-specific topography in the cat retina

Connexin-36 (Cx36) is the major constituent of mammalian retinal gap junctions positioned in key signal pathways. Here, we examined the laminar and large-scale topographical distribution of Cx36 punctate immunolabels in the retina of the cat, a classical model of the mammalian visual system. Calretinin-immunoreactive (CaR-IR) cell populations served to outline the nuclear and plexiform layers and to stain specific neuronal populations. CaR-IR cells included horizontal cells in the outer retina, numerous amacrine cells, and scattered cells in the ganglion cell layer. Cx36-IR plaques were found among horizontal cell dendrites albeit without systematic colocalization of the two labels. Diffuse Cx36 immunoreactivity was found in the cytoplasm of AII amacrine cells, but no colocalization of Cx36 plaques was observed with either the perikarya or the long varicose dendrites of the CaR-IR non-AII amacrine cells. Cx36 puncta were seen throughout the entire inner plexiform layer showing their highest density in the ON sublamina. The densities of AII amacrine cell bodies and Cx36 plaques in the ON sublamina were strongly correlated across a wide range of eccentricities suggesting their anatomical association. However, the high number of plaques per AII cell suggests that a considerable fraction of Cx36 gap junctions in the ON sublamina is formed by other cell types than AII amacrine cells drawing attention to extensive but less studied electrically coupled networks.

Cell-cell communication in diabetic retinopathy

In diabetic retinopathy, high glucose (HG)-mediated breakdown in cell-cell communication promotes disruption of retinal homeostasis. Several studies indicate that HG condition alters expression of connexin genes and subsequent gap junction intercellular communication (GJIC) in retinal vascular cells and non-vascular cells. A serious consequence of disrupted cell-cell communication is apoptosis and breakdown of the blood-retinal barrier (BRB). More recently, studies suggest adverse effects from HG on retinal Müller cells. This article focuses on HG-mediated changes in connexin expression and GJIC and their subsequent effects on the breakdown of retinal homeostasis, cell death, compromised vascular permeability, and interactions between endothelial cells, pericytes and retinal Müller cells in the pathogenesis of diabetic retinopathy. Additionally, options for rectifying disrupted homeostasis under HG condition associated with diabetic retinopathy are reviewed.

Gap junctional coupling in the vertebrate retina: variations on one theme?

Gap junctions connect cells in the bodies of all multicellular organisms, forming either homologous or heterologous (i.e. established between identical or different cell types, respectively) cell-to-cell contacts by utilizing identical (homotypic) or different (heterotypic) connexin protein subunits. Gap junctions in the nervous system serve electrical signaling between neurons, thus they are also called electrical synapses. Such electrical synapses are particularly abundant in the vertebrate retina where they are specialized to form links between neurons as well as glial cells. In this article, we summarize recent findings on retinal cell-to-cell coupling in different vertebrates and identify general features in the light of the evergrowing body of data. In particular, we describe and discuss tracer coupling patterns, connexin proteins, junctional conductances and modulatory processes. This multispecies comparison serves to point out that most features are remarkably conserved across the vertebrate classes, including (i) the cell types connected via electrical synapses; (ii) the connexin makeup and the conductance of each cell-to-cell contact; (iii) the probable function of each gap junction in retinal circuitry; (iv) the fact that gap junctions underlie both electrical and/or tracer coupling between glial cells. These pan-vertebrate features thus demonstrate that retinal gap junctions have changed little during the over 500 million years of vertebrate evolution. Therefore, the fundamental architecture of electrically coupled retinal circuits seems as old as the retina itself, indicating that gap junctions deeply incorporated in retinal wiring from the very beginning of the eye formation of vertebrates. In addition to hard wiring provided by fast synaptic transmitter-releasing neurons and soft wiring contributed by peptidergic, aminergic and purinergic systems, electrical coupling may serve as the 'skeleton' of lateral processing, enabling important functions such as signal averaging and synchronization.

Gap junction-mediated electrical transmission: regulatory mechanisms and plasticity

The term synapse applies to cellular specializations that articulate the processing of information within neural circuits by providing a mechanism for the transfer of information between two different neurons. There are two main modalities of synaptic transmission: chemical and electrical. While most efforts have been dedicated to the understanding of the properties and modifiability of chemical transmission, less is still known regarding the plastic properties of electrical synapses, whose structural correlate is the gap junction. A wealth of data indicates that, rather than passive intercellular channels, electrical synapses are more dynamic and modifiable than was generally perceived. This article will discuss the factors determining the strength of electrical transmission and review current evidence demonstrating its dynamic properties. Like their chemical counterparts, electrical synapses can also be plastic and modifiable. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

The role of neuronal connexins 36 and 45 in shaping spontaneous firing patterns in the developing retina

Gap junction coupling synchronizes activity among neurons in adult neural circuits, but its role in coordinating activity during development is less known. The developing retina exhibits retinal waves--spontaneous depolarizations that propagate among retinal interneurons and drive retinal ganglion cells (RGCs) to fire correlated bursts of action potentials. During development, two connexin isoforms, connexin 36 (Cx36) and Cx45, are expressed in bipolar cells and RGCs, and therefore provide a potential substrate for coordinating network activity. To determine whether gap junctions contribute to retinal waves, we compared spontaneous activity patterns using calcium imaging, whole-cell recording, and multielectrode array recording in control, single-knock-out (ko) mice lacking Cx45 and double-knock-out (dko) mice lacking both isoforms. Wave frequency, propagation speed, and bias in propagation direction were similar in control, Cx36ko, Cx45ko, and Cx36/45dko retinas. However, the spontaneous firing rate of individual retinal ganglion cells was elevated in Cx45ko retinas, similar to Cx36ko retinas (Hansen et al., 2005; Torborg and Feller, 2005), a phenotype that was more pronounced in Cx36/45dko retinas. As a result, spatial correlations, as assayed by nearest-neighbor correlation and functional connectivity maps, were significantly altered. In addition, Cx36/45dko mice had reduced eye-specific segregation of retinogeniculate afferents. Together, these findings suggest that although Cx36 and Cx45 do not play a role in gross spatial and temporal propagation properties of retinal waves, they strongly modulate the firing pattern of individual RGCs, ensuring strongly correlated firing between nearby RGCs and normal patterning of retinogeniculate projections.

Expression of connexin genes in the human retina

BACKGROUND

Gap junction channels allow direct metabolically and electrical coupling between adjacent cells in various mammalian tissues. Each channel is composed of 12 protein subunits, termed connexins (Cx). In the mouse retina, Cx43 could be localized mostly between astroglial cells whereas expression of Cx36, Cx45 and Cx57 genes has been detected in different neuronal subtypes. In the human retina, however, the expression pattern of connexin genes is largely unknown.

METHODS

Northern blot hybridizations, RT-PCR as well as immunofluorescence analyses helped to explore at least partially the expression pattern of the following human connexin genes GJD2 (hCx36), GJC1 (hCx45), GJA9 (hCx59) and GJA10 (hCx62) in the human retina.

RESULTS

Here we report that Northern blot hybridization signals of the orthologuous hCx36 and hCx45 were found in human retinal RNA. Immunofluorescence signals for both connexins could be located in both inner and outer plexiform layer (IPL, OPL). Expression of a third connexin gene denoted as GJA10 (Cx62) was also detected after Northern blot hybridization in the human retina. Interestingly, its gene structure is similar to that of Gja10 (mCx57) being expressed in mouse horizontal cells. RT-PCR analysis suggested that an additional exon of about 25 kb further downstream, coding for 12 amino acid residues, is spliced to the nearly complete reading frame on exon2 of GJA10 (Cx62). Cx59 mRNA, however, with high sequence identity to zebrafish Cx55.5 was only weakly detectable by RT-PCR in cDNA of human retina.

CONCLUSION

In contrast to the neuron-expressed connexin genes Gjd2 coding for mCx36, Gjc1 coding for mCx45 and Gja10 coding for mCx57 in the mouse, a subset of 4 connexin genes, including the unique GJA9 (Cx59) and GJA10 (Cx62), could be detected at least as transcript isoforms in the human retina. First immunofluorescence analyses revealed a staining pattern of hCx36 and hCx45 expression both in the IPL and OPL, partially reminiscent to that in the mouse, although additional post-mortem material is needed to further explore their sublamina-specific distribution. Appropriate antibodies against Cx59 and Cx62 protein will clarify expression of these proteins in future studies.

Differential expression of connexins during histogenesis of the chick retina

Gap junction (GJ) channels couple adjacent cells, allowing transfer of second messengers, ions, and molecules up to 1 kDa. These channels are composed by a multigene family of integral membrane proteins called connexins (Cx). In the retina, besides being essential circuit element in the visual processing, GJ channels also play important roles during its development. Herein, we analyzed Cx43, Cx45, Cx50, and Cx56 expression during chick retinal histogenesis. Cx exhibited distinct expression profiles during retinal development, except for Cx56, whose expression was not detected. Cx43 immunolabeling was observed at early development, in the transition of ventricular zone and pigmented epithelium. Later, Cx43 was seen in the outer plexiform and ganglion cell layers, and afterwards also in the inner plexiform layer. We observed remarkable changes in the phosphorylation status of this protein, which indicated modifications in functional properties of this Cx during retinal histogenesis. By contrast, Cx45 showed stable gene expression levels throughout development and ubiquitous immunoreactivity in progenitor cells. From later embryonic development, Cx45 was mainly observed in the inner retina, and it was expressed by glial cells and neurons. In turn, Cx50 was virtually absent in the chick retina at initial embryonic phases. Combination of PCR, immunohistochemistry and Western blot indicated that this Cx was present in differentiated cells, arising in parallel with the formation of the visual circuitry. Characterization of Cx expression in the developing chick retina indicated particular roles for these proteins and revealed similarities and differences when compared to other species.

Expression pattern of connexins in the corneal and limbal epithelium of a primate

PURPOSE

To detect the expression pattern of connexins in epithelial cells of the central cornea and limbus of the macaque.

METHODS

Total RNA was extracted from the central corneal and limbal epithelia of Macaca fascicularis and processed by reverse transcriptase-polymerase chain reaction with isoform primers to detect the expression of 16 connexin (Cx). Immunofluorescent staining of frozen sections of corneal tissue confirmed and localized connexin proteins expression.

RESULTS

Transcripts encoding 10 Cx isoforms (Cx26, Cx30, Cx30.3, Cx31, Cx31.1, Cx32, Cx43, Cx45, Cx50, and Cx58) were detected by reverse transcriptase-polymerase chain reaction in both central and peripheral corneal epithelium. Six (Cx26, Cx31, Cx32, Cx43, Cx45, and Cx58) were confirmed by laser scanning confocal microscopy. Cx26 was detected throughout the central corneal epithelium and in the mid and superficial layers of the limbal epithelium. Cx43 and Cx45 were localized to the basal and suprabasal epithelial cells. Cx58 was expressed in the superficial epithelium throughout the cornea, whereas Cx31 and Cx32 were mainly expressed in the central corneal epithelium and weakly in the limbal area.

CONCLUSIONS

The complex distribution pattern of the connexins suggests that selected isoforms play important roles in maintaining corneal homeostasis.

Loss of protein kinase Cgamma in knockout mice and increased retinal sensitivity to hyperbaric oxygen

OBJECTIVE

To determine if loss of protein kinase Cgamma (PKCgamma) results in increased structural damage to the retina by hyperbaric oxygen (HBO), a treatment used for several ocular disorders.

METHODS

Six-week-old mice were exposed in vivo to 100% HBO 3 times a week for 8 weeks. Eyes were dissected, fixed, embedded in Epon, sectioned, stained with toluidine blue O, and examined by light microscopy.

RESULTS

The thicknesses of the inner nuclear and ganglion cell layers were increased. Destruction of the outer plexiform layer was observed in the retinas of the PKCgamma-knockout mice relative to control mice. Exposure to HBO caused significant degradation of the retina in knockout mice compared with control mice. Damage to the outer segments of the photoreceptor layer and ganglion cell layer was apparent in central retinas of HBO-treated knockout mice.

CONCLUSIONS

Protein kinase Cgamma-knockout mice had increased retinal sensitivity to HBO. Results demonstrate that PKCgamma protects retinas from HBO damage.

CLINICAL RELEVANCE

Care should be taken in treating patients with HBO, particularly if they have a genetic disease, such as spinocerebellar ataxia type 14, a condition in which the PKCgamma is mutated and nonfunctional.

Spatial relationships of connexin36, connexin57 and zonula occludens-1 in the outer plexiform layer of mouse retina

Horizontal cells form gap junctions with each other in mammalian retina, and lacZ reporter analyses have recently indicated that these cells express the Cx57 gene, which codes for the corresponding gap junctional protein. Using anti-connexin57 antibodies, we detected connexin57 protein in immunoblots of mouse retina, and found punctate immunolabeling of this connexin co-distributed with calbindin-positive horizontal cells in the retinal outer plexiform layer. Double immunofluorescence labeling was conducted to determine the spatial relationships of connexin36, connexin57, the gap junction-associated protein zonula occludens-1 and the photoreceptor ribbon synapse-associated protein bassoon in the outer plexiform layer. Connexin36 was substantially co-localized with zonula occludens-1 in the outer plexiform layer, and both of these proteins were frequently located in close spatial proximity to bassoon-positive ribbon synapses. Connexin57 was often found adjacent to, but not overlapping with, connexin36-positive and zonula occludens-1-positive puncta, and was also located adjacent to bassoon-positive ribbon synapses at rod spherules, and intermingled with such synapses at cone pedicles. These results suggest zonula occludens-1 interaction with connexin36 but not with Cx57 in the outer plexiform layer, and an absence of connexin57/connexin36 heterotypic gap junctional coupling in mouse retina. Further, an arrangement of synaptic contacts within rod spherules is suggested whereby gap junctions between horizontal cell terminals containing connexin57 occur in very close proximity to ribbon synapses formed by rod photoreceptors, as well as in close proximity to Cx36-containing gap junctions between rods and cones.

Retinal horizontal cell-specific promoter activity and protein expression of zebrafish connexin 52.6 and connexin 55.5

Connexins in retinal horizontal cells (HC) function in the processing of visual information. For example, gap junction-forming connexins may contribute to the spatial integration of visual stimuli. Additionally, connexin hemichannels have been hypothesized to participate in the feedback pathway from HCs to cones. To verify the identities of the zebrafish HC connexins, we performed promoter expression and immunohistochemical studies of connexin 52.6 (Cx52.6) and Cx55.5. Zebrafish embryos were microinjected with Cx52.6 or Cx55.5 promoter sequences and a green fluorescent protein reporter construct. Light and electron microscopic (EM) analysis showed green fluorescent protein expression exclusively in retinal HCs. Immunohistochemistry confirmed that HCs express Cx52.6 and Cx55.5 proteins. Light microscopy revealed Cx52.6 and Cx55.5 in the retinal inner nuclear and outer plexiform layers. Double labeling for Cx55.5 or Cx52.6 and cell-specific markers (tyrosine hydroxylase, protein kinase C-alpha, or GluR2) demonstrated that these connexins do not localize to interplexiform or ON bipolar cells, but most likely are present in HCs. Preembedding immuno-EM confirmed the HC-specific expression of Cx52.6 and Cx55.5 and illustrated the presence of these two connexins in gap junctions between HCs. The EM data also revealed robust labeling for Cx55.5 in hemichannels on HC dendrites in photoreceptor synaptic terminals. Voltage-clamp experiments in cultured cells demonstrated that Cx55.5-containing hemichannels can open at physiological membrane potentials. These results offer the first in vivo demonstration of the HC-specific activities of the Cx52.6 and Cx55.5 promoters. Furthermore, these data provide the first proof at the protein level for retinal HC-specific connexins in the zebrafish.

Coupling between A-type horizontal cells is mediated by connexin 50 gap junctions in the rabbit retina

There are many examples of neuronal coupling via gap junctions in the retina. Of these, perhaps the best known is the extensive coupling between horizontal cells. In the rabbit retina, there are two types of horizontal cells, A-type and B-type, both of which are independently coupled. Connexin 50 (Cx50) cDNA, encoding a 440 aa protein, was successfully isolated from rabbit retina RNA. Cx50 was also obtained from isolated A-type horizontal cells (A-type HCs) by single-cell RT-PCR. A-type HCs were visualized by intracellular dye injection or with an antibody against calbindin. Confocal analysis revealed all Cx50 labeling occurred on the A-type HC matrix, typically at dendritic intersections. The Cx50 plaques varied in size, from punctate signals in which fine dendrites cofasciculated, to giant plaques, >50 microm(2), in which large dendrites crossed. The numerous Cx50 plaques between A-type HCs may adequately account for the remarkable coupling observed in this network. We could not detect Cx50 staining on the tips of horizontal cell dendrites within the cone pedicle invagination. This distribution does not support a role for Cx50 in hemichannel-mediated feedback. In addition, the absence of Cx50 in B-type HCs suggests the presence of a different connexin for this cell type. In summary, these results suggest that gap junctions in the A-type horizontal cell matrix are composed from Cx50. Multiple neuronal connexins are expressed in the mammalian retina and different cell types express specific connexins.

Association of connexin36 and zonula occludens-1 with zonula occludens-2 and the transcription factor zonula occludens-1-associated nucleic acid-binding protein at neuronal gap junctions in rodent retina

Most gap junctions between neurons in mammalian retina contain abundant connexin36, often in association with the scaffolding protein zonula occludens-1. We now investigate co-association of connexin36, zonula occludens-1, zonula occludens-2 and Y-box transcription factor 3 (zonula occludens-1-associated nucleic acid-binding protein) in mouse and rat retina. By immunoblotting, zonula occludens-1-associated nucleic acid-binding protein and zonula occludens-2 were both detected in retina, and zonula occludens-2 in retina was found to co-immunoprecipitate with connexin36. By immunofluorescence, the four proteins appeared as puncta distributed in the plexiform layers. In the inner plexiform layer, most connexin36-puncta were co-localized with zonula occludens-1, and many were co-localized with zonula occludens-1-associated nucleic acid-binding protein. Moreover, zonula occludens-1-associated nucleic acid-binding protein was often co-localized with zonula occludens-1. Nearly all zonula occludens-2-puncta were positive for connexin36, zonula occludens-1 and zonula occludens-1-associated nucleic acid-binding protein. In the outer plexiform layer, connexin36 was also often co-localized with zonula occludens-1-associated nucleic acid-binding protein. In connexin36 knockout mice, labeling of zonula occludens-1 was slightly reduced in the inner plexiform layer, zonula occludens-1-associated nucleic acid-binding protein was decreased in the outer plexiform layer, and both zonula occludens-1-associated nucleic acid-binding protein and zonula occludens-2 were markedly decreased in the inner sublamina of the inner plexiform layer, whereas zonula occludens-1, zonula occludens-2 and zonula occludens-1-associated nucleic acid-binding protein puncta persisted and remained co-localized in the outer sublamina of the inner plexiform layer. By freeze-fracture replica immunogold labeling, connexin36 was found to be co-localized with zonula occludens-2 within individual neuronal gap junctions. In addition, zonula occludens-1-associated nucleic acid-binding protein was abundant in a portion of ultrastructurally-defined gap junctions throughout the inner plexiform layer, and some of these junctions contained both connexin36 and zonula occludens-1-associated nucleic acid-binding protein. These distinct patterns of connexin36 association with zonula occludens-1, zonula occludens-2 and zonula occludens-1-associated nucleic acid-binding protein in different sublaminae of retina, and differential responses of these proteins to connexin36 gene deletion suggest differential regulatory and scaffolding roles of these gap junction accessory proteins. Further, the persistence of a subpopulation of zonula occludens-1/zonula occludens-2/zonula occludens-1-associated nucleic acid-binding protein co-localized puncta in the outer part of the inner plexiform layer of connexin36 knockout mice suggests close association of these proteins with other structures in retina, possibly including gap junctions composed of an as-yet-unidentified connexin.