Expression and modulation of connexin 30.2, a novel gap junction protein in the mouse retina

Preservation of Intrinsically Photosensitive Retinal Ganglion Cells (ipRGCs) in Late Adult Mice: Implications as a Potential Biomarker for Early Onset Ocular Degenerative Diseases

Purpose

Intrinsically photosensitive retinal ganglion cells (ipRGCs) play a crucial role in non-image-forming visual functions. Given their significant loss observed in various ocular degenerative diseases at early stages, this study aimed to assess changes in both the morphology and associated behavioral functions of ipRGCs in mice between 6 (mature) and 12 (late adult) months old. The findings contribute to understanding the preservation of ipRGCs in late adults and their potential as a biomarker for early ocular degenerative diseases.

Methods

Female and male C57BL/6J mice were used to assess the behavioral consequences of aging to mature and old adults, including pupillary light reflex, light aversion, visual acuity, and contrast sensitivity. Immunohistochemistry on retinal wholemounts from these mice was then conducted to evaluate ipRGC dendritic morphology in the ganglion cell layer (GCL) and inner nuclear layer (INL).

Results

Morphological analysis showed that ipRGC dendritic field complexity was remarkably stable through 12 months old of age. Similarly, the pupillary light reflex, visual acuity, and contrast sensitivity were stable in mature and old adults. Although alterations were observed in ipRGC-independent light aversion distinct from the pupillary light reflex, aged wild-type mice continuously showed enhanced light aversion with dilation. No effect of sex was observed in any tests.

Conclusions

The preservation of both ipRGC morphology and function highlights the potential of ipRGC-mediated function as a valuable biomarker for ocular diseases characterized by early ipRGC loss. The consistent stability of ipRGCs in mature and old adult mice suggests that detected changes in ipRGC-mediated functions could serve as early indicators or diagnostic tools for early-onset conditions such as Alzheimer's disease, Parkinson's disease, and diabetes, where ipRGC loss has been documented.

Extrinsic and Intrinsic Factors Determine Expression Levels of Gap Junction-Forming Connexins in the Mammalian Retina

Gap junctions (GJs) are not static bridges; instead, GJs as well as the molecular building block connexin (Cx) proteins undergo major expression changes in the degenerating retinal tissue. Various progressive diseases, including retinitis pigmentosa, glaucoma, age-related retinal degeneration, etc., affect neurons of the retina and thus their neuronal connections endure irreversible changes as well. Although Cx expression changes might be the hallmarks of tissue deterioration, GJs are not static bridges and as such they undergo adaptive changes even in healthy tissue to respond to the ever-changing environment. It is, therefore, imperative to determine these latter adaptive changes in GJ functionality as well as in their morphology and Cx makeup to identify and distinguish them from alterations following tissue deterioration. In this review, we summarize GJ alterations that take place in healthy retinal tissue and occur on three different time scales: throughout the entire lifespan, during daily changes and as a result of quick changes of light adaptation.

Analytical methods for assessing retinal cell coupling using cut-loading

Electrical coupling between retinal neurons contributes to the functional complexity of visual circuits. “Cut-loading” methods allow simultaneous assessment of cell-coupling between multiple retinal cell-types, but existing analysis methods impede direct comparison with gold standard direct dye injection techniques. In the current study, we both improved an existing method and developed two new approaches to address observed limitations. Each method of analysis was applied to cut-loaded dark-adapted Guinea pig retinae (n = 29) to assess coupling strength in the axonless horizontal cell type (‘a-type’, aHCs). Method 1 was an improved version of the standard protocol and described the distance of dye-diffusion (space constant). Method 2 adjusted for the geometric path of dye-transfer through cut-loaded cells and extracted the rate of dye-transfer across gap-junctions in terms of the coupling coefficient (k_j). Method 3 measured the diffusion coefficient (De) perpendicular to the cut-axis. Dye transfer was measured after one of five diffusion times (1–20 mins), or with a coupling inhibitor, meclofenamic acid (MFA) (50–500μM after 20 mins diffusion). The standard protocol fits an exponential decay function to the fluorescence profile of a specified retina layer but includes non-specific background fluorescence. This was improved by measuring the fluorescence of individual cell soma and excluding from the fit non-horizontal cells located at the cut-edge (p<0.001) (Method 1). The space constant (Method 1) increased with diffusion time (p<0.01), whereas Methods 2 (p = 0.54) and 3 (p = 0.63) produced consistent results across all diffusion times. Adjusting distance by the mean cell-cell spacing within each tissue reduced the incidence of outliers across all three methods. Method 1 was less sensitive to detecting changes induced by MFA than Methods 2 (p<0.01) and 3 (p<0.01). Although the standard protocol was easily improved (Method 1), Methods 2 and 3 proved more sensitive and generalisable; allowing for detailed assessment of the tracer kinetics between different populations of gap-junction linked cell networks and direct comparison to dye-injection techniques.

The role of gap junctions in cell death and neuromodulation in the retina

Vision altering diseases, such as glaucoma, diabetic retinopathy, age-related macular degeneration, myopia, retinal vascular disease, traumatic brain injuries and others cripple many lives and are projected to continue to cause anguish in the foreseeable future. Gap junctions serve as an emerging target for neuromodulation and possible regeneration as they directly connect healthy and/or diseased cells, thereby playing a crucial role in pathophysiology. Since they are permeable for macromolecules, able to cross the cellular barriers, they show duality in illness as a cause and as a therapeutic target. In this review, we take recent advancements in gap junction neuromodulation (pharmacological blockade, gene therapy, electrical and light stimulation) into account, to show the gap junction's role in neuronal cell death and the possible routes of rescuing neuronal and glial cells in the retina succeeding illness or injury.

PTEN Expression Regulates Gap Junction Connectivity in the Retina

Manipulation of the phosphatase and tensin homolog (PTEN) pathway has been suggested as a therapeutic approach to treat or prevent vision loss due to retinal disease. In this study, we investigated the effects of deleting one copy of Pten in a well-characterized class of retinal ganglion cells called α-ganglion cells in the mouse retina. In Pten +/- retinas, α-ganglion cells did not exhibit major changes in their dendritic structure, although most cells developed a few, unusual loop-forming dendrites. By contrast, α-ganglion cells exhibited a significant decrease in heterologous and homologous gap junction mediated cell coupling with other retinal ganglion and amacrine cells. Additionally, the majority of OFF α-ganglion cells (12/18 cells) formed novel coupling to displaced amacrine cells. The number of connexin36 puncta, the predominant connexin that mediates gap junction communication at electrical synapses, was decreased by at least 50% on OFF α-ganglion cells. Reduced and incorrect gap junction connectivity of α-ganglion cells will affect their functional properties and alter visual image processing in the retina. The anomalous connectivity of retinal ganglion cells would potentially limit future therapeutic approaches involving manipulation of the Pten pathway for treating ganglion cell degeneration in diseases like glaucoma, traumatic brain injury, Parkinson's, and Alzheimer's diseases.

An Update on Connexin Gap Junction and Hemichannels in Diabetic Retinopathy

Diabetic retinopathy (DR) is one of the main causes of vision loss in the working age population. It is characterized by a progressive deterioration of the retinal microvasculature, caused by long-term metabolic alterations inherent to diabetes, leading to a progressive loss of retinal integrity and function. The mammalian retina presents an orderly layered structure that executes initial but complex visual processing and analysis. Gap junction channels (GJC) forming electrical synapses are present in each retinal layer and contribute to the communication between different cell types. In addition, connexin hemichannels (HCs) have emerged as relevant players that influence diverse physiological and pathological processes in the retina. This article highlights the impact of diabetic conditions on GJC and HCs physiology and their involvement in DR pathogenesis. Microvascular damage and concomitant loss of endothelial cells and pericytes are related to alterations in gap junction intercellular communication (GJIC) and decreased connexin 43 (Cx43) expression. On the other hand, it has been shown that the expression and activity of HCs are upregulated in DR, becoming a key element in the establishment of proinflammatory conditions that emerge during hyperglycemia. Hence, novel connexin HCs blockers or drugs to enhance GJIC are promising tools for the development of pharmacological interventions for diabetic retinopathy, and initial in vitro and in vivo studies have shown favorable results in this regard.

Amacrine Cells Forming Gap Junctions With Intrinsically Photosensitive Retinal Ganglion Cells: ipRGC Types, Neuromodulator Contents, and Connexin Isoform

Purpose

Intrinsically photosensitive retinal ganglion cells (ipRGCs) signal not only centrally to non-image-forming visual centers of the brain but also intraretinally to amacrine interneurons through gap junction electrical coupling, potentially modulating image-forming retinal processing. We aimed to determine (1) which ipRGC types couple with amacrine cells, (2) the neuromodulator contents of ipRGC-coupled amacrine cells, and (3) whether connexin36 (Cx36) contributes to ipRGC-amacrine coupling.

Methods

Gap junction-permeable Neurobiotin tracer was injected into green fluorescent protein (GFP)-labeled ipRGCs in Opn4Cre/+; Z/EG mice to stain coupled amacrine cells, and immunohistochemistry was performed to reveal the neuromodulator contents of the Neurobiotin-stained amacrine cells. We also created Opn4Cre/+; Cx36flox/flox; Z/EG mice to knock out Cx36 in GFP-labeled ipRGCs and looked for changes in the number of ipRGC-coupled amacrine cells.

Results

Seventy-three percent of ipRGCs, including all six types (M1-M6), were tracer-coupled with amacrine somas 5.7 to 16.5 µm in diameter but not with ganglion cells. Ninety-two percent of the ipRGC-coupled somas were in the ganglion cell layer and the rest in the inner nuclear layer. Some ipRGC-coupled amacrine cells were found to accumulate serotonin or to contain nitric oxide synthase or neuropeptide Y. Knocking out Cx36 in M2 and M4 dramatically reduced the number of coupled somas.

Conclusions

Heterologous gap junction coupling with amacrine cells is widespread across mouse ipRGC types. ipRGC-coupled amacrine cells probably comprise multiple morphologic types and use multiple neuromodulators, suggesting that gap junctional ipRGC-to-amacrine signaling likely exerts diverse modulatory effects on retinal physiology. ipRGC-amacrine coupling is mediated partly, but not solely, by Cx36.

Retinal ganglion cell interactions shape the developing mammalian visual system

Retinal ganglion cells (RGCs) serve as a crucial communication channel from the retina to the brain. In the adult, these cells receive input from defined sets of presynaptic partners and communicate with postsynaptic brain regions to convey features of the visual scene. However, in the developing visual system, RGC interactions extend beyond their synaptic partners such that they guide development before the onset of vision. In this Review, we summarize our current understanding of how interactions between RGCs and their environment influence cellular targeting, migration and circuit maturation during visual system development. We describe the roles of RGC subclasses in shaping unique developmental responses within the retina and at central targets. Finally, we highlight the utility of RNA sequencing and genetic tools in uncovering RGC type-specific roles during the development of the visual system.

Cx30.2 deletion causes imbalances in testicular Cx43, Cx46, and Cx50 and insulin receptors. Reciprocally, diabetes/obesity alters Cx30.2 in mouse testis

Cx30.2 protein content and localization were assessed during development. An account of Cx30.2, Cx43, Cx46, and Cx50, and insulin receptor (IR) responses to Cx30.2, Cx46, or Cx50 deficiency in mouse interstitial tissue (ITf)- and seminiferous tubule-enriched fractions (STf) is given. The impact of high glucose/insulin on Cx30.2 was investigated in spontaneously diabetic and obese db/db and ob/ob mouse testis and anterior pituitary (AP). Cx30.2 labeled contacts in vascular endothelial and Leydig cells and Sertoli cell junctions in stage V-VII. Cx30.2 expression is regulated differently in the interstitium and tubules. Cx30.2 at 30-kDa levels peaked by 28 days in ITf and by 14 days in STf. In STf, deleting Cx30.2 decreased Cx43 and Cx50, whereas deleting Cx50 downregulated Cx30.2. The opposite occurred in ITf. In STf, deleting Cx30.2 upregulated Cx46 except the full-length reciprocally, deleting Cx46 upregulated Cx30.2. In ITf, Cx30.2 deficiency upregulated full-length and phosphorylated Cx46, whereas deleting Cx46 downregulated 48- to 50-kDa Cx30.2. The db/db and ob/ob mouse ITf, STf, and AP showed imbalanced Cx30.2 levels. IRα levels at 135 kDa declined in Cx30.2-/- and Cx50-/- mouse ITf and Cx46-/- and Cx50-/- STf. IRβ at 98 to 110 kDa dropped in Cx30.2-/- and Cx46-/- mice STf suggesting that Cx30.2 deficiency decreases active IR sites. The results show the connexins interdependence and interaction and that altering a single connexin changes the remaining connexins expression, which can modify gap junction-mediated glucose exchanges in contacting cells. Data suggest that glucose/insulin influences Cx30.2 turnover in testis and AP and, reciprocally, that connexins modulate testis glucose uptake and response to insulin.

Electrical Coupling of Heterotypic Ganglion Cells in the Mammalian Retina

Electrical coupling has been reported to occur only between homotypic retinal ganglion cells, in line with the concept of parallel processing in the early visual system. Here, however, we show reciprocal correlated firing between heterotypic ganglion cells in multielectrode array recordings during light stimulation in retinas of adult guinea pigs of either sex. Heterotypic coupling was further confirmed via tracer spread after intracellular injections of single cells with neurobiotin. Both electrically coupled cell types were sustained ON center ganglion cells but showed distinct light response properties and receptive field sizes. We identified one of the involved cell types as sustained ON α-ganglion cells. The presence of electrical coupling between heterotypic ganglion cells introduces a network motif in which the signals of distinct ganglion cell types are partially mixed at the output stage of the retina.SIGNIFICANCE STATEMENT The visual information is split into parallel pathways, before it is sent to the brain via the output neurons of the retina, the ganglion cells. Ganglion cells can form electrical synapses between dendrites of neighboring cells in support of lateral information exchange. To date, ganglion-to-ganglion cell coupling is thought to occur only between cells of the same type. Here, however, we show that electrical coupling between different types of ganglion cells exists in the mammalian retina. We provide functional and anatomical evidence that two different types of ganglion cells share information via electrical coupling. This new network motif extends the impact of the heavily studied coding benefits of homotypic coupling to heterotypic coupling across parallel neuronal pathways.

Gap junction mediated signaling between satellite glia and neurons in trigeminal ganglia

Peripheral sensory ganglia contain the somata of neurons mediating mechanical, thermal, and painful sensations from somatic, visceral, and oro-facial organs. Each neuronal cell body is closely surrounded by satellite glial cells (SGCs) that have properties and functions similar to those of central astrocytes, including expression of gap junction proteins and functional dye coupling. As shown in other pain models, after systemic pain induction by intra-peritoneal injection of lipopolysaccharide, dye coupling among SGCs in intact trigeminal ganglion was enhanced. Moreover, neuron-neuron and neuron-SGC coupling was also detected. To verify the presence of gap junction-mediated coupling between SGCs and sensory neurons, we performed dual whole cell patch clamp recordings from both freshly isolated and short term cultured cell pairs dissociated from mouse trigeminal ganglia. Bidirectional gap junction mediated electrical responses were frequently recorded between SGCs, between neurons and between neurons and SGCs. Polarization of SGC altered neuronal excitability, providing evidence that gap junction-mediated interactions between neurons and glia within sensory ganglia may contribute to integration of peripheral sensory responses, and to the modulation and coordinaton of neuronal activity.

Strategic Positioning of Connexin36 Gap Junctions Across Human Retinal Ganglion Cell Dendritic Arbors

Connexin36 (Cx36) subunits form gap junctions (GJ) between neurons throughout the central nervous system. Such GJs of the mammalian retina serve the transmission, averaging and correlation of signals prior to conveying visual information to the brain. Retinal GJs have been exhaustively studied in various animal species, however, there is still a perplexing paucity of information regarding the presence and function of human retinal GJs. Particularly little is known about GJ formation of human retinal ganglion cells (hRGCs) due to the limited number of suitable experimental approaches. Compared to the neuronal coupling studies in animal models, where GJ permeable tracer injection is the gold standard method, the post-mortem nature of scarcely available human retinal samples leaves immunohistochemistry as a sole approach to obtain information on hRGC GJs. In this study Lucifer Yellow (LY) dye injections and Cx36 immunohistochemistry were performed in fixed short-post-mortem samples to stain hRGCs with complete dendritic arbors and locate dendritic Cx36 GJs. Subsequent neuronal reconstructions and morphometric analyses revealed that Cx36 plaques had a clear tendency to form clusters and particularly favored terminal dendritic segments.

Plasticity of Retinal Gap Junctions: Roles in Synaptic Physiology and Disease

Electrical synaptic transmission via gap junctions underlies direct and rapid neuronal communication in the central nervous system. The diversity of functional roles played by electrical synapses is perhaps best exemplified in the vertebrate retina, in which gap junctions are expressed by each of the five major neuronal types. These junctions are highly plastic; they are dynamically regulated by ambient illumination and circadian rhythms acting through light-activated neuromodulators. The networks formed by electrically coupled neurons provide plastic, reconfigurable circuits positioned to play key and diverse roles in the transmission and processing of visual information at every retinal level. Recent work indicates gap junctions also play a role in the progressive cell death and aberrant activity seen in various pathological conditions of the retina. Gap junctions thus form potential targets for novel neuroprotective therapies in the treatment of neurodegenerative retinal diseases such as glaucoma and ischemic retinopathies.

Connexin 30.2 is expressed in exocrine vascular endothelial and ductal epithelial cells throughout pancreatic postnatal development

Previously we have demonstrated that the GJ protein connexin 30.2 (Cx30.2) is expressed in pancreatic beta cells and endothelial cells (ECs) of the islet. In the present study, we address whether Cx30.2 is expressed in the exocrine pancreas, including its vascular system. For this, adult mouse pancreatic sections were double labeled with specific antibodies against Cx30.2 and CD31, an endothelial cell marker, or with anti-α-actin smooth muscle, a smooth muscle cell (SMC) marker or anti-mucin-1, a marker of epithelial ductal cells, using immunofluorescence (IF) studies. Cx30.2-IF hot spots were found at junctional membranes of exocrine ECs and SMCs of blood vessels. Furthermore, Cx30.2 was localized in mucin-1 positive cells or epithelial ductal cells. Using immunohistochemistry (IHC) studies, it was found that in vessels and ducts of different diameters, Cx30.2 was also expressed in these cell types. In addition, it was found that Cx30.2 is already expressed in these cell types in pancreatic sections of 3, 14 and 21 days postpartum. Moreover, this cell specific pattern of expression was also found in the adult rat, hamster and guinea pig pancreas. Expression of Cx30.2 mRNA and protein in the pancreas of all these species was confirmed by RT-PCR and Western blot studies. Overall, our results suggest that intercellular coupling mediated by Cx30.2 intercellular channels may synchronize the functional activity of ECs and SMCs of vascular cells, as well as of epithelial ductal cells after birth.

Electrical synapses in mammalian CNS: Past eras, present focus and future directions

Gap junctions provide the basis for electrical synapses between neurons. Early studies in well-defined circuits in lower vertebrates laid the foundation for understanding various properties conferred by electrical synaptic transmission. Knowledge surrounding electrical synapses in mammalian systems unfolded first with evidence indicating the presence of gap junctions between neurons in various brain regions, but with little appreciation of their functional roles. Beginning at about the turn of this century, new approaches were applied to scrutinize electrical synapses, revealing the prevalence of neuronal gap junctions, the connexin protein composition of many of those junctions, and the myriad diverse neural systems in which they occur in the mammalian CNS. Subsequent progress indicated that electrical synapses constitute key elements in synaptic circuitry, govern the collective activity of ensembles of electrically coupled neurons, and in part orchestrate the synchronized neuronal network activity and rhythmic oscillations that underlie fundamental integrative processes. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Design principles of electrical synaptic plasticity

Essentially all animals with nervous systems utilize electrical synapses as a core element of communication. Electrical synapses, formed by gap junctions between neurons, provide rapid, bidirectional communication that accomplishes tasks distinct from and complementary to chemical synapses. These include coordination of neuron activity, suppression of voltage noise, establishment of electrical pathways that define circuits, and modulation of high order network behavior. In keeping with the omnipresent demand to alter neural network function in order to respond to environmental cues and perform tasks, electrical synapses exhibit extensive plasticity. In some networks, this plasticity can have dramatic effects that completely remodel circuits or remove the influence of certain cell types from networks. Electrical synaptic plasticity occurs on three distinct time scales, ranging from milliseconds to days, with different mechanisms accounting for each. This essay highlights principles that dictate the properties of electrical coupling within networks and the plasticity of the electrical synapses, drawing examples extensively from retinal networks.

Bipolar cell gap junctions serve major signaling pathways in the human retina

Connexin36 (Cx36) constituent gap junctions (GJ) throughout the brain connect neurons into functional syncytia. In the retina they underlie the transmission, averaging and correlation of signals prior conveying visual information to the brain. This is the first study that describes retinal bipolar cell (BC) GJs in the human inner retina, whose function is enigmatic even in the examined animal models. Furthermore, a number of unique features (e.g. fovea, trichromacy, midget system) necessitate a reexamination of the animal model results in the human retina. Well-preserved postmortem human samples of this study are allowed to identify Cx36 expressing BCs neurochemically. Results reveal that both rod and cone pathway interneurons display strong Cx36 expression. Rod BC inputs to AII amacrine cells (AC) appear in juxtaposition to AII GJs, thus suggesting a strategic AII cell targeting by rod BCs. Cone BCs serving midget, parasol or koniocellular signaling pathways display a wealth of Cx36 expression to form homologously coupled arrays. In addition, they also establish heterologous GJ contacts to serve an exchange of information between parallel signaling streams. Interestingly, a prominent Cx36 expression was exhibited by midget system BCs that appear to maintain intimate contacts with bistratified BCs serving other pathways. These findings suggest that BC GJs in parallel signaling streams serve both an intra- and inter-pathway exchange of signals in the human retina.

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.

Connexin channel and its role in diabetic retinopathy

Diabetic retinopathy is the leading cause of blindness in the working age population. Unfortunately, there is no cure for this devastating ocular complication. The early stage of diabetic retinopathy is characterized by the loss of various cell types in the retina, namely endothelial cells and pericytes. As the disease progresses, vascular leakage, a clinical hallmark of diabetic retinopathy, becomes evident and may eventually lead to diabetic macular edema, the most common cause of vision loss in diabetic retinopathy. Substantial evidence indicates that the disruption of connexin-mediated cellular communication plays a critical role in the pathogenesis of diabetic retinopathy. Yet, it is unclear how altered communication via connexin channel mediated cell-to-cell and cell-to-extracellular microenvironment is linked to the development of diabetic retinopathy. Recent observations suggest the possibility that connexin hemichannels may play a role in the pathogenesis of diabetic retinopathy by allowing communication between cells and the microenvironment. Interestingly, recent studies suggest that connexin channels may be involved in regulating retinal vascular permeability. These cellular events are coordinated at least in part via connexin-mediated intercellular communication and the maintenance of retinal vascular homeostasis. This review highlights the effect of high glucose and diabetic condition on connexin channels and their impact on the development of diabetic retinopathy.

Multiple cell types form the VIP amacrine cell population

Amacrine cells are a heterogeneous group of interneurons that form microcircuits with bipolar, amacrine and ganglion cells to process visual information in the inner retina. This study has characterized the morphology, neurochemistry and major cell types of a VIP-ires-Cre amacrine cell population. VIP-tdTomato and -Confetti (Brainbow2.1) mouse lines were generated by crossing a VIP-ires-Cre line with either a Cre-dependent tdTomato or Brainbow2.1 reporter line. Retinal sections and whole-mounts were evaluated by quantitative, immunohistochemical, and intracellular labeling approaches. The majority of tdTomato and Confetti fluorescent cell bodies were in the inner nuclear layer (INL) and a few cell bodies were in the ganglion cell layer (GCL). Fluorescent processes ramified in strata 1, 3, 4, and 5 of the inner plexiform layer (IPL). All tdTomato fluorescent cells expressed syntaxin 1A and GABA-immunoreactivity indicating they were amacrine cells. The average VIP-tdTomato fluorescent cell density in the INL and GCL was 535 and 24 cells/mm² , respectively. TdTomato fluorescent cells in the INL and GCL contained VIP-immunoreactivity. The VIP-ires-Cre amacrine cell types were identified in VIP-Brainbow2.1 retinas or by intracellular labeling in VIP-tdTomato retinas. VIP-1 amacrine cells are bistratified, wide-field cells that ramify in strata 1, 4, and 5, VIP-2A and 2B amacrine cells are medium-field cells that mainly ramify in strata 3 and 4, and VIP-3 displaced amacrine cells are medium-field cells that ramify in strata 4 and 5 of the IPL. VIP-ires-Cre amacrine cells form a neuropeptide-expressing cell population with multiple cell types, which are likely to have distinct roles in visual processing.

Prox1 Is a Marker for AII Amacrine Cells in the Mouse Retina

The transcription factor Prox1 is expressed in multiple cells in the retina during eye development. This study has focused on neuronal Prox1 expression in the inner nuclear layer (INL) of the adult mouse retina. Prox1 immunostaining was evaluated in vertical retinal sections and whole mount preparations using a specific antibody directed to the C-terminus of Prox1. Strong immunostaining was observed in numerous amacrine cell bodies and in all horizontal cell bodies in the proximal and distal INL, respectively. Some bipolar cells were also weakly immunostained. Prox1-immunoreactive amacrine cells expressed glycine, and they formed 35 ± 3% of all glycinergic amacrine cells. Intracellular Neurobiotin injections into AII amacrine cells showed that all gap junction-coupled AII amacrine cells express Prox1, and no other Prox1-immunostained amacrine cells were in the immediate area surrounding the injected AII amacrine cell. Prox1-immunoreactive amacrine cell bodies were distributed across the retina, with their highest density (3887 ± 160 cells/mm²) in the central retina, 0.5 mm from the optic nerve head, and their lowest density (3133 ± 350 cells/mm²) in the mid-peripheral retina, 2 mm from the optic nerve head. Prox1-immunoreactive amacrine cell bodies comprised ~9.8% of the total amacrine cell population, and they formed a non-random mosaic with a regularity index (RI) of 3.4, similar to AII amacrine cells in the retinas of other mammals. Together, these findings indicate that AII amacrine cells are the predominant and likely only amacrine cell type strongly expressing Prox1 in the adult mouse retina, and establish Prox1 as a marker of AII amacrine cells.

Characterization of connexin36 gap junctions in the human outer retina

Retinal connexins (Cx) form gap junctions (GJ) in key circuits that transmit average or synchronize signals. Expression of Cx36, -45, -50 and -57 have been described in many species but there is still a disconcerting paucity of information regarding the Cx makeup of human retinal GJs. We used well-preserved human postmortem samples to characterize Cx36 GJ constituent circuits of the outer plexiform layer (OPL). Based on their location, morphometric characteristics and co-localizations with outer retinal neuronal markers, we distinguished four populations of Cx36 plaques in the human OPL. Three of these were comprised of loosely scattered Cx36 plaques; the distalmost population 1 formed cone-to-rod GJs, population 2 in the mid-OPL formed cone-to-cone GJs, whereas the proximalmost population 4 likely connected bipolar cell dendrites. The fourth population (population 3) of Cx36 plaques conglomerated beneath cone pedicles and connected dendritic tips of bipolar cells that shared a common presynaptic cone. Overall, we show that the human outer retina displays a diverse cohort of Cx36 GJ that follows the general mammalian scheme and display a great functional diversity.

Connexin30.2: In Vitro Interaction with Connexin36 in HeLa Cells and Expression in AII Amacrine Cells and Intrinsically Photosensitive Ganglion Cells in the Mouse Retina

Electrical coupling via gap junctions is an abundant phenomenon in the mammalian retina and occurs in all major cell types. Gap junction channels are assembled from different connexin subunits, and the connexin composition of the channel confers specific properties to the electrical synapse. In the mouse retina, gap junctions were demonstrated between intrinsically photosensitive ganglion cells and displaced amacrine cells but the underlying connexin remained undetermined. In the primary rod pathway, gap junctions play a crucial role, coupling AII amacrine cells among each other and to ON cone bipolar cells. Although it has long been known that connexin36 and connexin45 are necessary for the proper functioning of this most sensitive rod pathway, differences between homocellular AII/AII gap junctions and AII/ON bipolar cell gap junctions suggested the presence of an additional connexin in AII amacrine cells. Here, we used a connexin30.2-lacZ mouse line to study the expression of connexin30.2 in the retina. We show that connexin30.2 is expressed in intrinsically photosensitive ganglion cells and AII amacrine cells. Moreover, we tested whether connexin30.2 and connexin36-both expressed in AII amacrine cells-are able to interact with each other and are deposited in the same gap junctional plaques. Using newly generated anti-connexin30.2 antibodies, we show in HeLa cells that both connexins are indeed able to interact and may form heteromeric channels: both connexins were co-immunoprecipitated from transiently transfected HeLa cells and connexin30.2 gap junction plaques became significantly larger when co-expressed with connexin36. These data suggest that connexin36 is able to form heteromeric gap junctions with another connexin. We hypothesize that co-expression of connexin30.2 and connexin36 may endow AII amacrine cells with the means to differentially regulate its electrical coupling to different synaptic partners.

Electrical synapses and the development of inhibitory circuits in the thalamus

KEY POINTS

The thalamus is a structure critical for information processing and transfer to the cortex. Thalamic reticular neurons are inhibitory cells interconnected by electrical synapses, most of which require the gap junction protein connexin36 (Cx36). We investigated whether electrical synapses play a role in the maturation of thalamic networks by studying neurons in mice with and without Cx36. When Cx36 was deleted, inhibitory synapses were more numerous, although both divergent inhibitory connectivity and dendritic complexity were reduced. Surprisingly, we observed non-Cx36-dependent electrical synapses with unusual biophysical properties interconnecting some reticular neurons in mice lacking Cx36. The results of the present study suggest an important role for Cx36-dependent electrical synapses in the development of thalamic circuits.

ABSTRACT

Neurons within the mature thalamic reticular nucleus (TRN) powerfully inhibit ventrobasal (VB) thalamic relay neurons via GABAergic synapses. TRN neurons are also coupled to one another by electrical synapses that depend strongly on the gap junction protein connexin36 (Cx36). Electrical synapses in the TRN precede the postnatal development of TRN-to-VB inhibition. We investigated how the deletion of Cx36 affects the maturation of TRN and VB neurons, electrical coupling and GABAergic synapses by studying wild-type (WT) and Cx36 knockout (KO) mice. The incidence and strength of electrical coupling in TRN was sharply reduced, but not abolished, in KO mice. Surprisingly, electrical synapses between Cx36-KO neurons had faster voltage-dependent decay kinetics and conductance asymmetry (rectification) than did electrical synapses between WT neurons. The properties of TRN-mediated inhibition in VB also depended on the Cx36 genotype. Deletion of Cx36 increased the frequency and shifted the amplitude distributions of miniature IPSCs, whereas the paired-pulse ratio of evoked IPSCs was unaffected, suggesting that the absence of Cx36 led to an increase in GABAergic synaptic contacts. VB neurons from Cx36-KO mice also tended to have simpler dendritic trees and fewer divergent inputs from the TRN compared to WT cells. The findings obtained in the present study suggest that proper development of thalamic inhibitory circuitry, neuronal morphology, TRN cell function and electrical coupling requires Cx36. In the absence of Cx36, some TRN neurons express asymmetric electrical coupling mediated by other unidentified connexin subtypes.

Heterogeneous transgene expression in the retinas of the TH-RFP, TH-Cre, TH-BAC-Cre and DAT-Cre mouse lines

Transgenic mouse lines are essential tools for understanding the connectivity, physiology and function of neuronal circuits, including those in the retina. This report compares transgene expression in the retina of a tyrosine hydroxylase (TH)-red fluorescent protein (RFP) mouse line with three catecholamine-related Cre recombinase mouse lines [TH-bacterial artificial chromosome (BAC)-, TH-, and dopamine transporter (DAT)-Cre] that were crossed with a ROSA26-tdTomato reporter line. Retinas were evaluated and immunostained with commonly used antibodies including those directed to TH, GABA and glycine to characterize the RFP or tdTomato fluorescent-labeled amacrine cells, and an antibody directed to RNA-binding protein with multiple splicing to identify ganglion cells. In TH-RFP retinas, types 1 and 2 dopamine (DA) amacrine cells were identified by their characteristic cellular morphology and type 1 DA cells by their expression of TH immunoreactivity. In the TH-BAC-, TH-, and DAT-tdTomato retinas, less than 1%, ∼ 6%, and 0%, respectively, of the fluorescent cells were the expected type 1 DA amacrine cells. Instead, in the TH-BAC-tdTomato retinas, fluorescently labeled AII amacrine cells were predominant, with some medium diameter ganglion cells. In TH-tdTomato retinas, fluorescence was in multiple neurochemical amacrine cell types, including four types of polyaxonal amacrine cells. In DAT-tdTomato retinas, fluorescence was in GABA immunoreactive amacrine cells, including two types of bistratified and two types of monostratified amacrine cells. Although each of the Cre lines was generated with the intent to specifically label DA cells, our findings show a cellular diversity in Cre expression in the adult retina and indicate the importance of careful characterization of transgene labeling patterns. These mouse lines with their distinctive cellular labeling patterns will be useful tools for future studies of retinal function and visual processing.

Expression and Localization of Connexins in the Outer Retina of the Mouse

The identification of the proteins that make up the gap junction channels between rods and cones is of crucial importance to understand the functional role of photoreceptor coupling within the retinal network. In vertebrates, connexin proteins constitute the structural components of gap junction channels. Connexin36 is known to be expressed in cones whereas extensive investigations have failed to identify the corresponding connexin expressed in rods. Using immunoelectron microscopy, we demonstrate that connexin36 (Cx36) is present in gap junctions of cone but not rod photoreceptors in the mouse retina. To identify the rod connexin, we used nested reverse transcriptase polymerase chain reaction and tested retina and photoreceptor samples for messenger RNA (mRNA) expression of all known connexin genes. In addition to connexin36, we detected transcripts for connexin32, connexin43, connexin45, connexin50, and connexin57 in photoreceptor samples. Immunohistochemistry showed that connexin43, connexin45, connexin50, and connexin57 proteins are expressed in the outer plexiform layer. However, none of these connexins was detected at gap junctions between rods and cones as a counterpart of connexin36. Therefore, the sought-after rod protein must be either an unknown connexin sequence, a connexin36 splice product not detected by our antibodies, or a protein from a further gap junction protein family.

Morphology and connectivity of the small bistratified A8 amacrine cell in the mouse retina

Amacrine cells comprise ∼ 30 morphological types in the mammalian retina. The synaptic connectivity and function of a few γ-aminobutyric acid (GABA)ergic wide-field amacrine cells have recently been studied; however, with the exception of the rod pathway-specific AII amacrine cell, the connectivity of glycinergic small-field amacrine cells has not been investigated in the mouse retina. Here, we studied the morphology and connectivity pattern of the small-field A8 amacrine cell. A8 cells in mouse retina are bistratified with lobular processes in the ON sublamina and arboreal dendrites in the OFF sublamina of the inner plexiform layer. The distinct bistratified morphology was first visible at postnatal day 8, reaching the adult shape at P13, around eye opening. The connectivity of A8 cells to bipolar cells and ganglion cells was studied by double and triple immunolabeling experiments by using various cell markers combined with synaptic markers. Our data suggest that A8 amacrine cells receive glutamatergic input from both OFF and ON cone bipolar cells. Furthermore, A8 cells are coupled to ON cone bipolar cells by gap junctions, and provide inhibitory input via glycine receptor (GlyR) subunit α1 to OFF cone bipolar cells and to ON A-type ganglion cells. Measurements of spontaneous glycinergic postsynaptic currents and GlyR immunolabeling revealed that A8 cells express GlyRs containing the α2 subunit. The results show that the bistratified A8 cell makes very similar synaptic contacts with cone bipolar cells as the rod pathway-specific AII amacrine cell. However, unlike AII cells, A8 amacrine cells provide glycinergic input to ON A-type ganglion cells.

The RNA binding protein RBPMS is a selective marker of ganglion cells in the mammalian retina

There are few neurochemical markers that reliably identify retinal ganglion cells (RGCs), which are a heterogeneous population of cells that integrate and transmit the visual signal from the retina to the central visual nuclei. We have developed and characterized a new set of affinity-purified guinea pig and rabbit antibodies against RNA-binding protein with multiple splicing (RBPMS). On western blots these antibodies recognize a single band at 〜24 kDa, corresponding to RBPMS, and they strongly label RGC and displaced RGC (dRGC) somata in mouse, rat, guinea pig, rabbit, and monkey retina. RBPMS-immunoreactive cells and RGCs identified by other techniques have a similar range of somal diameters and areas. The density of RBPMS cells in mouse and rat retina is comparable to earlier semiquantitative estimates of RGCs. RBPMS is mainly expressed in medium and large DAPI-, DRAQ5-, NeuroTrace- and NeuN-stained cells in the ganglion cell layer (GCL), and RBPMS is not expressed in syntaxin (HPC-1)-immunoreactive cells in the inner nuclear layer (INL) and GCL, consistent with their identity as RGCs, and not displaced amacrine cells. In mouse and rat retina, most RBPMS cells are lost following optic nerve crush or transection at 3 weeks, and all Brn3a-, SMI-32-, and melanopsin-immunoreactive RGCs also express RBPMS immunoreactivity. RBPMS immunoreactivity is localized to cyan fluorescent protein (CFP)-fluorescent RGCs in the B6.Cg-Tg(Thy1-CFP)23Jrs/J mouse line. These findings show that antibodies against RBPMS are robust reagents that exclusively identify RGCs and dRGCs in multiple mammalian species, and they will be especially useful for quantification of RGCs.

Melanopsin ganglion cells are the most resistant retinal ganglion cell type to axonal injury in the rat retina

We report that the most common retinal ganglion cell type that remains after optic nerve transection is the M1 melanopsin ganglion cell. M1 ganglion cells are members of the intrinsically photosensitive retinal ganglion cell population that mediates non-image-forming vision, comprising ∼2.5% of all ganglion cells in the rat retina. In the present study, M1 ganglion cells comprised 1.7±1%, 28±14%, 55±13% and 82±8% of the surviving ganglion cells 7, 14, 21 and 60 days after optic nerve transection, respectively. Average M1 ganglion cell somal diameter and overall morphological appearance remained unchanged in non-injured and injured retinas, suggesting a lack of injury-induced degeneration. Average M1 dendritic field size increased at 7 and 60 days following optic nerve transection, while average dendritic field size remained similar in non-injured retinas and in retinas at 14 and 21 days after optic nerve transection. These findings demonstrate that M1 ganglion cells are more resistant to injury than other ganglion cell types following optic nerve injury, and provide an opportunity to develop pharmacological or genetic therapeutic approaches to mitigate ganglion cell death and save vision following optic nerve injury.

Dynamic tuning of electrical and chemical synaptic transmission in a network of motion coding retinal neurons

Recently, we demonstrated that gap junction coupling in the population of superior coding ON-OFF directionally selective ganglion cells (DSGCs) genetically labeled in the Hb9::eGFP mouse retina allows the passage of lateral anticipatory signals that help track moving stimuli. Here, we examine the properties of gap junctions in the DSGC network, and address how interactions between electrical and chemical synapses and intrinsic membrane properties contribute to the dynamic tuning of lateral anticipatory signals. When DSGC subtypes coding all four cardinal directions were individually loaded with the gap junction-permeable tracer Neurobiotin, only superior coding DSGCs exhibited homologous coupling. Consistent with these anatomical findings, gap junction-dependent feedback spikelets were only observed in Hb9(+) DSGCs. Recordings from pairs of neighboring Hb9(+) DSGCs revealed that coupling was reciprocal, non-inactivating, and relatively weak, and provided a substrate for an extensive subthreshold excitatory receptive field around each cell. This subthreshold activity appeared to boost coincident light-driven chemical synaptic responses. However, during responses to moving stimuli, gap junction-mediated boosting appeared to be dynamically modulated such that upstream DSGCs primed downstream cells, but not vice versa, giving rise to highly skewed responses in individual cells. We show that the asymmetry in priming arises from a combination of spatially offset GABAergic inhibition and activity-dependent changes in intrinsic membrane properties of DSGCs. Thus, dynamic interactions between electrical and chemical synapses and intrinsic membrane properties allow the network of DSGCs to propagate anticipatory responses most effectively along their preferred direction without leading to runaway excitation.

Survey on amacrine cells coupling to retrograde-identified ganglion cells in the mouse retina

PURPOSE

Retinal amacrine cells (ACs) may make inhibitory chemical synapses and potentially excitatory gap junctions on ganglion cells (GCs). The total number and subtypes of ACs coupled to the entire GC population were investigated in wild-type and three lines of transgenic mice.

METHODS

GCs and GC-coupled ACs were identified by the previously established LY-NB (Lucifer yellow-Neurobiotin) retrograde double-labeling technique, in conjunction with specific antibodies and confocal microscopy.

RESULTS

GC-coupled ACs (NB-positive and LY-negative) comprised nearly 11% of displaced ACs and 4% of conventional ACs in wild-type mice, and were 9% and 4% of displaced ACs in Cx45(-/-) and Cx36/45(-/-) mice, respectively. Their somas were small in Cx36/45(-/-) mice, but variable in other strains. They were mostly γ-aminobutyric acid (GABA)-immunoreactive (IR) and located in the GC layer. They comprised only a small portion in the AC subpopulations, including GABA-IR, glycine-IR, calretinin-IR, 5-HT-accumulating, and ON-type choline acetyltransferase (ChAT) ACs in wild-type and ChAT transgenic mice (ChAT- tdTomato). In the distal 80% of the inner plexiform layer (IPL), dense GC dendrites coexisted with rich glycine-IR and GABA-IR. In the inner 20% of the IPL, sparse GC dendrites presented with a major GABA band and sparse glycine-IR.

CONCLUSIONS

Various subtypes of ACs may couple to GCs. ACs of the same immunoreactivity may either couple or not couple to GCs. Cx36 and Cx45 dominate GC-AC coupling except for small ACs. The overall potency of GC-AC coupling is moderate, especially in the proximal 20% of the IPL, where inhibitory chemical signals are dominated by GABA ACs.

Changing coupling pattern of The ON-OFF direction-selective ganglion cells in early postnatal mouse retina

In the adult rabbit and mouse retina, about 30% of the ON-OFF direction selective ganglion cells (DSGCs) are coupled via gap junctions. In early postnatal rabbit retinas, a greater proportion of morphological ON-OFF DSGCs shows coupling with a larger number of nearby somas. It is not clear whether the coupled ON-OFF DSGCs belong to the same subtype, or how coupling patterns change during development. In this study, we showed that in adult mouse retinas, all coupled ON-OFF DSGCs exhibited preferred directions (PDs) to superior, and this pattern emerged at postnatal day 15 (P15). At P13, the ON-OFF DSGCs with PDs to posterior were also coupled. Every ON-OFF DSGC in every subtype injected at P12 exhibited coupling. Therefore, a rapid decoupling process takes place in DSGCs around eye opening. Light deprivation delayed but did not halt the decoupling process. By using a transgenic mouse line in which green fluorescent protein (GFP) is selectively expressed in DSGCs with PDs to posterior and by performing in situ hybridization of cadherin-6, a marker for the DSGCs with PDs to superior and inferior, we showed that heterologous coupling existed between DSGCs with PDs to anterior and posterior till P12, but this heterologous coupling never spread to DSGCs positive for cadherin-6.

High glucose-induced downregulation of connexin 30.2 promotes retinal vascular lesions: implications for diabetic retinopathy

PURPOSE

To investigate whether high glucose (HG) alters expression of connexin 30.2 (Cx30.2) and influences gap junction intercellular communication (GJIC) in retinal endothelial cells and promotes vascular lesions characteristic of diabetic retinopathy (DR).

METHODS

Western blot analysis and immunostaining were performed to determine Cx30.2 protein expression and localization in rat retinal endothelial cells (RRECs) grown in normal (N; 5 mM) or HG (30 mM) medium for 7 days. Concurrently, GJIC was assessed in cells grown in N or HG medium and in cells transfected with Cx30.2 siRNA. Similarly, retinal Cx30.2 expression was assessed in nondiabetic and diabetic rats. Additionally, the effect of reduced Cx30.2 on development of acellular capillaries (ACs) and pericyte loss (PL) was studied in retinas of Cx30.2 knockout mice.

RESULTS

Cx30.2 was identified in RRECs in vitro and in vascular cells of retinal capillaries. RRECs grown in HG exhibited significantly reduced Cx30.2 protein levels consistent with decreased Cx30.2 immunostaining compared with those grown in N medium. Cells grown in HG and cells transfected with Cx30.2 siRNA exhibited significantly diminished dye transfer compared with N or nontransfected cells. Importantly, Cx30.2 protein level and immunostaining were decreased in diabetic retinas compared with nondiabetic retinas. Retinal capillaries of Cx30.2 knockout mice exhibited increased numbers of ACs and PL compared with those of wild-type mice.

CONCLUSIONS

These results indicate that HG- or diabetes-induced downregulation of Cx30.2 expression and decrease in GJIC activity play a critical role in the development of retinal vascular lesions in early DR.

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.

Analyzing the effects of gap junction blockade on neural synchrony via a motoneuron network computational model

In specific regions of the central nervous system (CNS), gap junctions have been shown to participate in neuronal synchrony. Amongst the CNS regions identified, some populations of brainstem motoneurons are known to be coupled by gap junctions. The application of various gap junction blockers to these motoneuron populations, however, has led to mixed results regarding their synchronous firing behavior, with some studies reporting a decrease in synchrony while others surprisingly find an increase in synchrony. To address this discrepancy, we employ a neuronal network model of Hodgkin-Huxley-style motoneurons connected by gap junctions. Using this model, we implement a series of simulations and rigorously analyze their outcome, including the calculation of a measure of neuronal synchrony. Our simulations demonstrate that under specific conditions, uncoupling of gap junctions is capable of producing either a decrease or an increase in neuronal synchrony. Subsequently, these simulations provide mechanistic insight into these different outcomes.

The role of gap junction channels during physiologic and pathologic conditions of the human central nervous system

Gap junctions (GJs) are expressed in most cell types of the nervous system, including neuronal stem cells, neurons, astrocytes, oligodendrocytes, cells of the blood brain barrier (endothelial cells and astrocytes) and under inflammatory conditions in microglia/macrophages. GJs connect cells by the docking of two hemichannels, one from each cell with each hemichannel being formed by 6 proteins named connexins (Cx). Unapposed hemichannels (uHC) also can be open on the surface of the cells allowing the release of different intracellular factors to the extracellular space. GJs provide a mechanism of cell-to-cell communication between adjacent cells that enables the direct exchange of intracellular messengers, such as calcium, nucleotides, IP(3), and diverse metabolites, as well as electrical signals that ultimately coordinate tissue homeostasis, proliferation, differentiation, metabolism, cell survival and death. Despite their essential functions in physiological conditions, relatively little is known about the role of GJs and uHC in human diseases, especially within the nervous system. The focus of this review is to summarize recent findings related to the role of GJs and uHC in physiologic and pathologic conditions of the central nervous system.

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.

Functional redundancy and compensation among members of gap junction protein families?

Gap junctions are intercellular conduits for small molecules made up by protein subunits called connexins. A large number of connexin genes were found in mouse and man, and most cell types express several connexins, lending support to the view that redundancy and compensation among family members exist. This review gives an overview of the current knowledge on redundancy and functional compensation - or lack thereof. It takes into account the different properties of connexin subunits which comprise gap junctional intercellular channels, but also the compatibility of connexins in gap junctions. Most insight has been gained by the investigation of mice deficient for one or more connexins and transgenic mice with functional replacement of one connexin gene by another. Most single deficient mice show phenotypical alterations limited to critical developmental time points or to specific organs and tissues, while mice doubly deficient for connexins expressed in the same cell type usually show more severe phenotypical alterations. Replacement of a connexin by another connexin in some cases gave rise to rescue of phenotypical alterations of connexin deficiencies, which were restricted to specific tissues. In many tissues, connexin substitution did not restore phenotypical alterations of connexin deficiencies, indicating that connexins are specialized in function. In some cases, fatal consequences arose from the replacement. The current consensus gained from such studies is that redundancy and compensation among connexins exists at least to a limited extent. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Tracer coupling of intrinsically photosensitive retinal ganglion cells to amacrine cells in the mouse retina

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are a subtype of ganglion cell in the mammalian retina that expresses the photopigment melanopsin and drives non-image-forming visual functions. Three morphological subtypes of ipRGCs (M1, M2, and M3) have been described based on their dendritic stratifications in the inner plexiform layer (IPL), but the question of their potential interactions via electrical coupling remains unsettled. In this study, we have addressed this question in the mouse retina by, injecting the tracer Neurobiotin into ipRGCs that had been genetically labelled with the fluorescent protein, tdTomato. We confirmed the presence of the M1-M3 subtypes of ipRGCs based on their distinct dendritic stratifications. All three subtypes were tracer coupled to putative amacrine cells situated within the ganglion cell layer (GCL) but not the inner nuclear layer (INL). The cells tracer coupled to the M1 and M2 cells were shown to be widefield GABA-immunoreactive amacrine cells. We found no evidence of homologous tracer coupling of ipRGCs or heterologous coupling to other types of ganglion cells.

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.