Neuronal connexin36 association with zonula occludens-1 protein (ZO-1) in mouse brain and interaction with the first PDZ domain of ZO-1

Dynamic electrical synapses rewire brain networks for persistent oscillations and epileptogenesis

One of the very fundamental attributes for telencephalic neural computation in mammals involves network activities oscillating beyond the initial trigger. The continuing and automated processing of transient inputs shall constitute the basis of cognition and intelligence but may lead to neuropsychiatric disorders such as epileptic seizures if carried so far as to engross part of or the whole telencephalic system. From a conventional view of the basic design of the telencephalic local circuitry, the GABAergic interneurons (INs) and glutamatergic pyramidal neurons (PNs) make negative feedback loops which would regulate the neural activities back to the original state. The drive for the most intriguing self-perpetuating telencephalic activities, then, has not been posed and characterized. We found activity-dependent deployment and delineated functional consequences of the electrical synapses directly linking INs and PNs in the amygdala, a prototypical telencephalic circuitry. These electrical synapses endow INs dual (a faster excitatory and a slower inhibitory) actions on PNs, providing a network-intrinsic excitatory drive that fuels the IN-PN interconnected circuitries and enables persistent oscillations with preservation of GABAergic negative feedback. Moreover, the entities of electrical synapses between INs and PNs are engaged in and disengaged from functioning in a highly dynamic way according to neural activities, which then determine the spatiotemporal scale of recruited oscillating networks. This study uncovers a special wide-range and context-dependent plasticity for wiring/rewiring of brain networks. Epileptogenesis or a wide spectrum of clinical disorders may ensue, however, from different scales of pathological extension of this unique form of telencephalic plasticity.

Intralumenal docking of connexin 36 channels in the ER isolates mistrafficked protein

The intracellular domains of connexins are essential for the assembly of gap junctions. For connexin 36 (Cx36), the major neuronal connexin, it has been shown that a dysfunctional PDZ-binding motif interferes with electrical synapse formation. However, it is still unknown how this motif coordinates the transport of Cx36. In the present study, we characterize a phenotype of Cx36 mutants that lack a functional PDZ-binding motif using HEK293T cells as an expression system. We provide evidence that an intact PDZ-binding motif is critical for proper endoplasmic reticulum (ER) export of Cx36. Removing the PDZ-binding motif of Cx36 results in ER retention and the formation of multimembrane vesicles containing gap junction-like connexin aggregates. Using a combination of site-directed mutagenesis and electron micrographs, we reveal that these vesicles consist of Cx36 channels that docked prematurely in the ER. Our data suggest a model in which ER-retained Cx36 channels reshape the ER membrane into concentric whorls that are released into the cytoplasm.

Electrical synapse structure requires distinct isoforms of a postsynaptic scaffold

Electrical synapses are neuronal gap junction (GJ) channels associated with a macromolecular complex called the electrical synapse density (ESD), which regulates development and dynamically modifies electrical transmission. However, the proteomic makeup and molecular mechanisms utilized by the ESD that direct electrical synapse formation are not well understood. Using the Mauthner cell of zebrafish as a model, we previously found that the intracellular scaffolding protein ZO1b is a member of the ESD, localizing postsynaptically, where it is required for GJ channel localization, electrical communication, neural network function, and behavior. Here, we show that the complexity of the ESD is further diversified by the genomic structure of the ZO1b gene locus. The ZO1b gene is alternatively initiated at three transcriptional start sites resulting in isoforms with unique N-termini that we call ZO1b-Alpha, -Beta, and -Gamma. We demonstrate that ZO1b-Beta and ZO1b-Gamma are broadly expressed throughout the nervous system and localize to electrical synapses. By contrast, ZO1b-Alpha is expressed mainly non-neuronally and is not found at synapses. We generate mutants in all individual isoforms, as well as double mutant combinations in cis on individual chromosomes, and find that ZO1b-Beta is necessary and sufficient for robust GJ channel localization. ZO1b-Gamma, despite its localization to the synapse, plays an auxiliary role in channel localization. This study expands the notion of molecular complexity at the ESD, revealing that an individual genomic locus can contribute distinct isoforms to the macromolecular complex at electrical synapses. Further, independent scaffold isoforms have differential contributions to developmental assembly of the interneuronal GJ channels. We propose that ESD molecular complexity arises both from the diversity of unique genes and from distinct isoforms encoded by single genes. Overall, ESD proteomic diversity is expected to have critical impacts on the development, structure, function, and plasticity of electrical transmission.

Loss of Ephaptic Contacts in the Murine Thalamus during Osmotic Demyelination Syndrome

BACKGROUND AND AIM

A murine model mimicking osmotic demyelination syndrome (ODS) revealed with histology in the relay posterolateral (VPL) and ventral posteromedial (VPM) thalamic nuclei adjoined nerve cell bodies in chronic hyponatremia, amongst the damaged 12 h and 48 h after reinstatement of osmolality. This report aims to verify and complement with ultrastructure other neurophysiology, immunohistochemistry, and molecular biochemistry data to assess the connexin-36 protein, as part of those hinted close contacts.This ODS investigation included four groups of mice: Sham (NN; n = 13), hyponatremic (HN; n = 11), those sacrificed 12 h after a fast restoration of normal natremia (ODS12h; n = 6) and mice sacrificed 48 h afterward, or ODS48 h (n = 9). Out of these, thalamic zones samples included NN (n = 2), HN (n = 2), ODS12h (n = 3) and ODS48h (n = 3).

RESULTS

Ultrastructure illustrated junctions between nerve cell bodies that were immunolabeled with connexin36 (Cx36) with light microscopy and Western blots. These cell's junctions were reminiscent of low resistance junctions characterized in other regions of the CNS with electrophysiology. Contiguous neurons showed neurolemma contacts in intact and damaged tissues according to their location in the ODS zones, at 12 h and 48 h post correction along with other demyelinating alterations. Neurons and ephaptic contact measurements indicated the highest alterations, including nerve cell necrosis in the ODS epicenter and damages decreased toward the outskirts of the demyelinated zone.

CONCLUSION

Ephapses contained C × 36between intact or ODS injured neurons in the thalamus appeared to be resilient beyond the core degraded tissue injuries. These could maintain intercellular ionic and metabolite exchanges between these lesser injured regions and, thus, would partake to some brain plasticity repairs.

The IgSF Cell Adhesion Protein CLMP and Congenital Short Bowel Syndrome (CSBS)

The immunoglobulin-like cell adhesion molecule CLMP is a member of the CAR family of cell adhesion proteins and is implicated in human congenital short-bowel syndrome (CSBS). CSBS is a rare but very severe disease for which no cure is currently available. In this review, we compare data from human CSBS patients and a mouse knockout model. These data indicate that CSBS is characterized by a defect in intestinal elongation during embryonic development and impaired peristalsis. The latter is driven by uncoordinated calcium signaling via gap junctions, which is linked to a reduction in connexin43 and 45 levels in the circumferential smooth muscle layer of the intestine. Furthermore, we discuss how mutations in the CLMP gene affect other organs and tissues, including the ureter. Here, the absence of CLMP produces a severe bilateral hydronephrosis—also caused by a reduced level of connexin43 and associated uncoordinated calcium signaling via gap junctions.

Activation of the unfolded protein response by Connexin47 mutations associated with Pelizaeus-Merzbacher-like disease

Pelizaeus-Merzbacher-like disease type 1 (PMLD1) is a hypomyelinating disorder arising in patients with mutations in GJC2, encoding Connexin47 (Cx47). PMLD1 causes nystagmus, cerebellar ataxia, spasticity and changes in CNS white matter detected by MRI. At least one mutation (p.I33M) yields a much milder phenotype, spastic paraplegia type 44 (SPG44). Cx47 contributes to gap junction communication channels between oligodendrocytes (OLs), the myelinating cells in the central nervous system (CNS), and between OLs and astrocytes. Prior studies in cell lines have shown that PMLD1 mutants such as p.P87S display defective protein trafficking, intracellular retention in the ER and loss-of-function. Here we show that when expressed in primary OLs, three PMLD1 associated mutants (p.P87S, p.Y269D and p.M283T) show ER retention of Cx47 and evidence of activation of the cellular stress (unfolded protein response, UPR) and apoptotic pathways. On the other hand, the milder SPG44 associated mutation p.I33M shows a wild-type-like subcellular distribution and no activation of the UPR or apoptotic pathways. These studies provide new insight into a potential element of toxic gain of function underlying the mechanism of PMLD1 that should help guide future therapeutic approaches.

Intrinsic Sources and Functional Impacts of Asymmetry at Electrical Synapses

Electrical synapses couple inhibitory neurons across the brain, underlying a variety of functions that are modifiable by activity. Despite recent advances, many functions and contributions of electrical synapses within neural circuitry remain underappreciated. Among these are the sources and impacts of electrical synapse asymmetry. Using multi-compartmental models of neurons coupled through dendritic electrical synapses, we investigated intrinsic factors that contribute to effective synaptic asymmetry and that result in modulation of spike timing and synchrony between coupled cells. We show that electrical synapse location along a dendrite, input resistance, internal dendritic resistance, or directional conduction of the electrical synapse itself each alter asymmetry as measured by coupling between cell somas. Conversely, we note that asymmetrical gap junction (GJ) conductance can be masked by each of these properties. Furthermore, we show that asymmetry modulates spike timing and latency of coupled cells by up to tens of milliseconds, depending on direction of conduction or dendritic location of the electrical synapse. Coordination of rhythmic activity between two cells also depends on asymmetry. These simulations illustrate that causes of asymmetry are diverse, may not be apparent in somatic measurements of electrical coupling, influence dendritic processing, and produce a variety of outcomes on spiking and synchrony of coupled cells. Our findings highlight aspects of electrical synapses that should always be included in experimental demonstrations of coupling, and when assembling simulated networks containing electrical synapses.

Convergent NMDA receptor-Pannexin1 signaling pathways regulate the interaction of CaMKII with Connexin-36

Ca2+/calmodulin-dependent protein kinase II (CaMKII) binding and phosphorylation of mammalian connexin-36 (Cx36) potentiate electrical coupling. To explain the molecular mechanism of how Cx36 modifies plasticity at gap junctions, we investigated the roles of ionotropic N-methyl-D-aspartate receptors and pannexin1 (Panx1) channels in regulating Cx36 binding to CaMKII. Pharmacological interference and site-directed mutagenesis of protein interaction sites shows that NMDA receptor activation opens Cx36 channels, causing the Cx36- CaMKII binding complex to adopt a compact conformation. Ectopic Panx1 expression in a Panx1 knock-down cell line is required to restore CaMKII mediated opening of Cx36. Furthermore, blocking of Src-family kinase activation of Panx1 is sufficient to prevent the opening of Cx36 channels. Our research demonstrates that the efficacy of Cx36 channels requires convergent calcium-dependent signaling processes in which activation of ionotropic N-methyl-D-aspartate receptor, Src-family kinase, and Pannexin1 open Cx36. Our results add to the best of our knowledge a new twist to mounting evidence for molecular communication between these core components of electrical and chemical synapses.

The Roles of Calmodulin and CaMKII in Cx36 Plasticity

Anatomical and electrophysiological evidence that gap junctions and electrical coupling occur between neurons was initially confined to invertebrates and nonmammals and was thought to be a primitive form of synaptic transmission. More recent studies revealed that electrical communication is common in the mammalian central nervous system (CNS), often coexisting with chemical synaptic transmission. The subsequent progress indicated that electrical synapses formed by the gap junction protein connexin-36 (Cx36) and its paralogs in nonmammals constitute vital elements in mammalian and fish synaptic circuitry. They govern the collective activity of ensembles of coupled neurons, and Cx36 gap junctions endow them with enormous adaptive plasticity, like that seen at chemical synapses. Moreover, they orchestrate the synchronized neuronal network activity and rhythmic oscillations that underlie the fundamental integrative processes, such as memory and learning. Here, we review the available mechanistic evidence and models that argue for the essential roles of calcium, calmodulin, and the Ca2+/calmodulin-dependent protein kinase II in integrating calcium signals to modulate the strength of electrical synapses through interactions with the gap junction protein Cx36.

ZO-1 Regulates Intercalated Disc Composition and Atrioventricular Node Conduction

RATIONALE

ZO-1 (Zona occludens 1), encoded by the tight junction protein 1 (TJP1) gene, is a regulator of paracellular permeability in epithelia and endothelia. ZO-1 interacts with the actin cytoskeleton, gap, and adherens junction proteins and localizes to intercalated discs in cardiomyocytes. However, the contribution of ZO-1 to cardiac physiology remains poorly defined.

OBJECTIVE

We aim to determine the role of ZO-1 in cardiac function.

METHODS AND RESULTS

Inducible cardiomyocyte-specific Tjp1 deletion mice (Tjp1fl/fl; Myh6Cre/Esr1*) were generated by crossing the Tjp1 floxed mice and Myh6Cre/Esr1* transgenic mice. Tamoxifen-induced loss of ZO-1 led to atrioventricular (AV) block without changes in heart rate, as measured by ECG and ex vivo optical mapping. Mice with tamoxifen-induced conduction system-specific deletion of Tjp1 (Tjp1fl/fl; Hcn4CreERt2) developed AV block while tamoxifen-induced conduction system deletion of Tjp1 distal to the AV node (Tjp1fl/fl; Kcne1CreERt2) did not demonstrate conduction defects. Western blot and immunostaining analyses of AV nodes showed that ZO-1 loss decreased Cx (connexin) 40 expression and intercalated disc localization. Consistent with the mouse model study, immunohistochemical staining showed that ZO-1 is abundantly expressed in the human AV node and colocalizes with Cx40. Ventricular conduction was not altered despite decreased localization of ZO-1 and Cx43 at the ventricular intercalated disc and modestly decreased left ventricular ejection fraction, suggesting ZO-1 is differentially required for AV node and ventricular conduction.

CONCLUSIONS

ZO-1 is a key protein responsible for maintaining appropriate AV node conduction through maintaining gap junction protein localization.

Understanding the Molecular and Cell Biological Mechanisms of Electrical Synapse Formation

In this review article, we will describe the recent advances made towards understanding the molecular and cell biological mechanisms of electrical synapse formation. New evidence indicates that electrical synapses, which are gap junctions between neurons, can have complex molecular compositions including protein asymmetries across joined cells, diverse morphological arrangements, and overlooked similarities with other junctions, all of which indicate new potential roles in neurodevelopmental disease. Aquatic organisms, and in particular the vertebrate zebrafish, have proven to be excellent models for elucidating the molecular mechanisms of electrical synapse formation. Zebrafish will serve as our main exemplar throughout this review and will be compared with other model organisms. We highlight the known cell biological processes that build neuronal gap junctions and compare these with the assemblies of adherens junctions, tight junctions, non-neuronal gap junctions, and chemical synapses to explore the unknown frontiers remaining in our understanding of the critical and ubiquitous electrical synapse.

Regulation of gap junction intercellular communication by connexin ubiquitination: physiological and pathophysiological implications

Gap junctions consist of arrays of intercellular channels that enable adjacent cells to communicate both electrically and metabolically. Gap junctions have a wide diversity of physiological functions, playing critical roles in both excitable and non-excitable tissues. Gap junction channels are formed by integral membrane proteins called connexins. Inherited or acquired alterations in connexins are associated with numerous diseases, including heart failure, neuropathologies, deafness, skin disorders, cataracts and cancer. Gap junctions are highly dynamic structures and by modulating the turnover rate of connexins, cells can rapidly alter the number of gap junction channels at the plasma membrane in response to extracellular or intracellular cues. Increasing evidence suggests that ubiquitination has important roles in the regulation of endoplasmic reticulum-associated degradation of connexins as well as in the modulation of gap junction endocytosis and post-endocytic sorting of connexins to lysosomes. In recent years, researchers have also started to provide insights into the physiological roles of connexin ubiquitination in specific tissue types. This review provides an overview of the advances made in understanding the roles of connexin ubiquitination in the regulation of gap junction intercellular communication and discusses the emerging physiological and pathophysiological implications of these processes.

Exendin-4 overcomes cytokine-induced decreases in gap junction coupling via protein kinase A and Epac2 in mouse and human islets

KEY POINTS

The pancreatic islets of Langerhans maintain glucose homeostasis through insulin secretion, where insulin secretion dynamics are regulated by intracellular Ca2+ signalling and electrical coupling of the insulin producing β-cells in the islet. We have previously shown that cytokines decrease β-cell coupling and that compounds which increase cAMP can increase coupling. In both mouse and human islets exendin-4, which increases cAMP, protected against cytokine-induced decreases in coupling and in mouse islets preserved glucose-stimulated calcium signalling by increasing connexin36 gap junction levels on the plasma membrane. Our data indicate that protein kinase A regulates β-cell coupling through a fast mechanism, such as channel gating or membrane organization, while Epac2 regulates slower mechanisms of regulation, such as gap junction turnover. Increases in β-cell coupling with exendin-4 may protect against cytokine-mediated β-cell death as well as preserve insulin secretion dynamics during the development of diabetes.

ABSTRACT

The pancreatic islets of Langerhans maintain glucose homeostasis. Insulin secretion from islet β-cells is driven by glucose metabolism, depolarization of the cell membrane and an influx of calcium, which initiates the release of insulin. Gap junctions composed of connexin36 (Cx36) electrically couple β-cells, regulating calcium signalling and insulin secretion dynamics. Cx36 coupling is decreased in pre-diabetic mice, suggesting a role for altered coupling in diabetes. Our previous work has shown that pro-inflammatory cytokines decrease Cx36 coupling and that compounds which increase cAMP can increase Cx36 coupling. The goal of this study was to determine if exendin-4, which increases cAMP, can protect against cytokine-induced decreases in Cx36 coupling and altered islet function. In both mouse and human islets, exendin-4 protected against cytokine-induced decreases in coupling and preserved glucose-stimulated calcium signalling. Exendin-4 also protected against protein kinase Cδ-mediated decreases in Cx36 coupling. Exendin-4 preserved coupling in mouse islets by preserving Cx36 levels on the plasma membrane. Exendin-4 regulated Cx36 coupling via both protein kinase A (PKA)- and Epac2-mediated mechanisms in cytokine-treated islets. In mouse islets, modulating Epac2 had a greater impact in mediating Cx36 coupling, while in human islets modulating PKA had a greater impact on Cx36 coupling. Our data indicate that PKA regulates Cx36 coupling through a fast mechanism, such as channel gating, while Epac2 regulates slower mechanisms of regulation, such as Cx36 turnover in the membrane. Increases in Cx36 coupling with exendin-4 may protect against cytokine-mediated β-cell dysfunction to insulin secretion dynamics during the development of diabetes.

Downregulation of Neuronal and Dendritic Connexin36-Made Electrical Synapses Without Glutamatergic Axon Terminals in Spinal Anterior Horn Cells From the Early Stage of Amyotrophic Lateral Sclerosis

Connexin36 (Cx36) forms gap junctions between neurons, which are called electrical synapses, enabling adjacent neurons to communicate directly. The participation of chemical synapses in neurodegeneration in amyotrophic lateral sclerosis (ALS) has long been indicated, but it remains unclear whether electrical synapses are involved in the pathogenesis of ALS. We performed extensive immunopathological analyses using mutant superoxide dismutase 1 (SOD1^G93A) transgenic mice and their littermates to investigate whether Cx36-made electrical synapses are affected in motor neuron diseases. We found that in the lamina IX of the lumbar spinal cord from wild type mice, about half of the Cx36 puncta existed independently of chemical synapse markers, while the rest coexisted with chemical synapse markers, such as vesicular glutamate transporter 1 (VGLUT1), which is a glutamatergic axon terminal marker, and/or glutamate decarboxylase 65 (GAD65), which is a GABAergic axon terminal marker. Cx36 single or Cx36/GAD65 double positive puncta, but not VGLUT1-containing puncta, were preferentially decreased on neuronal and dendritic surfaces of the anterior horn cells in the early stage of SOD1^G93A ALS mice. Moreover, in five human autopsied sporadic ALS cases with bulbar or upper limb onset, Cx36 immunoreactivity was diminished in the proximal dendrites and neuropils of well-preserved large motor neurons in the lumbar anterior horns. These findings suggest that downregulation of neuronal and dendritic Cx36 in the spinal anterior horns commonly occurs from the early stage of hereditary and sporadic ALS. Cx36-made electrical synapses without glutamatergic signaling appear to be more vulnerable than other chemical synapses and electrical synapses with glutamatergic signaling in the early stage of motor neuron degeneration, suggesting involvement of Cx36-made electrical synapses in the pathogenesis of human ALS.

E3 ubiquitin ligases LNX1 and LNX2 localize at neuronal gap junctions formed by connexin36 in rodent brain and molecularly interact with connexin36

Electrical synapses in the mammalian central nervous system (CNS) are increasingly recognized as highly complex structures for mediation of neuronal communication, both with respect to their capacity for dynamic short- and long-term modification in efficacy of synaptic transmission and their multimolecular regulatory and structural components. These two characteristics are inextricably linked, such that understanding of mechanisms that contribute to electrical synaptic plasticity requires knowledge of the molecular composition of electrical synapses and the functions of proteins associated with these synapses. Here, we provide evidence that the key component of gap junctions that form the majority of electrical synapses in the mammalian CNS, namely connexin36 (Cx36), directly interacts with the related E3 ubiquitin ligase proteins Ligand of NUMB protein X1 (LNX1) and Ligand of NUMB protein X2 (LNX2). This is based on immunofluorescence colocalization of LNX1 and LNX2 with Cx36-containing gap junctions in adult mouse brain versus lack of such coassociation in LNX null mice, coimmunoprecipitation of LNX proteins with Cx36, and pull-down of Cx36 with the second PDZ domain of LNX1 and LNX2. Furthermore, cotransfection of cultured cells with Cx36 and E3 ubiquitin ligase-competent LNX1 and LNX2 isoforms led to loss of Cx36-containing gap junctions between cells, whereas these junctions persisted following transfection with isoforms of these proteins that lack ligase activity. Our results suggest that a LNX protein mediates ubiquitination of Cx36 at neuronal gap junctions, with consequent Cx36 internalization, and may thereby contribute to intracellular mechanisms that govern the recently identified modifiability of synaptic transmission at electrical synapses.

Alterations at Arg76 of human connexin 46, a residue associated with cataract formation, cause loss of gap junction formation but preserve hemichannel function

The connexins are members of a family of integral membrane proteins that form gap junction channels between apposed cells and/or hemichannels across the plasma membranes. The importance of the arginine at position 76 (Arg76) in the structure and/or function of connexin 46 (Cx46) is highlighted by its conservation across the entire connexin family and the occurrence of pathogenic mutations at this (or the corresponding homologous) residue in a number of human diseases. Two mutations at Arg76 in Cx46 are associated with cataracts in humans, highlighting the importance of this residue. We examined the expression levels and macroscopic and single-channel properties of human Cx46 and compared them with those for two pathogenic mutants, namely R76H and R76G. To gain further insight into the role of charge at this position, we generated two additional nonnaturally occurring mutants, R76K (charge conserving) and R76E (charge inverting). We found that, when expressed exogenously in Neuro2a cells, all four mutants formed membrane hemichannels, inducing membrane permeability at levels comparable to those recorded in cells expressing the wild-type Cx46. In contrast, the number of gap-junction plaques and the magnitude of junctional coupling were reduced by all four mutations. To gain further insight into the role of Arg76 in the function of Cx46, we performed homology modeling of Cx46 and in silico mutagenesis of Arg76 to Gly, His, or Glu. Our studies suggest that the loss of interprotomeric interactions has a significant effect on the extracellular domain conformation and dynamics, thus affecting the hemichannel docking required for formation of cell-cell channels.

A Cell Junctional Protein Network Associated with Connexin-26

GJB2 mutations are the leading cause of non-syndromic inherited hearing loss. GJB2 encodes connexin-26 (CX26), which is a connexin (CX) family protein expressed in cochlea, skin, liver, and brain, displaying short cytoplasmic N-termini and C-termini. We searched for CX26 C-terminus binding partners by affinity capture and identified 12 unique proteins associated with cell junctions or cytoskeleton (CGN, DAAM1, FLNB, GAPDH, HOMER2, MAP7, MAPRE2 (EB2), JUP, PTK2B, RAI14, TJP1, and VCL) by using mass spectrometry. We show that, similar to other CX family members, CX26 co-fractionates with TJP1, VCL, and EB2 (EB1 paralogue) as well as the membrane-associated protein ASS1. The adaptor protein CGN (cingulin) co-immuno-precipitates with CX26, ASS1, and TJP1. In addition, CGN co-immunoprecipitation with CX30, CX31, and CX43 indicates that CX association is independent on the CX C-terminus length or sequence. CX26, CGN, FLNB, and DAMM1 were shown to distribute to the organ of Corti and hepatocyte plasma membrane. In the mouse liver, CX26 and TJP1 co-localized at the plasma membrane. In conclusion, CX26 associates with components of other membrane junctions that integrate with the cytoskeleton.

Structural and Intermolecular Associations Between Connexin36 and Protein Components of the Adherens Junction-Neuronal Gap Junction Complex

Intimate structural and functional relationships between gap junctions and adherens junctions have been demonstrated in peripheral tissues, but have not been thoroughly examined in the central nervous system, where adherens junctions are often found in close proximity to neuronal gap junctions. Here, we used immunofluorescence approaches to document the localization of various protein components of adherens junctions in relation to those that we have previously reported to occur at electrical synapses formed by neuronal gap junctions composed of connexin36 (Cx36). The adherens junction constituents N-cadherin and nectin-1 were frequently found to localize near or overlap with Cx36-containing gap junctions in several brain regions examined. This was also true of the adherens junction-associated proteins α-catenin and β-catenin, as well as the proteins zonula occludens-1 and AF6 (aka, afadin) that were reported constituents of both adherens junctions and gap junctions. The deployment of the protein constituents of these junctions was especially striking at somatic contacts between primary afferent neurons in the mesencephalic trigeminal nucleus (MesV), where the structural components of adherens junctions appeared to be maintained in connexin36 null mice. These results support emerging views concerning the multi-molecular composition of electrical synapses and raise possibilities for various structural and functional protein-protein interactions at what now can be considered the adherens junction-neuronal gap junction complex. Further, the results point to intracellular signaling pathways that could potentially contribute to the assembly, maintenance and turnover of this complex, as well as to the dynamic nature of neuronal communication at electrical synapses.

The interplay between electrical and chemical synaptogenesis

Neurons communicate with each other via electrical or chemical synaptic connections. The pattern and strength of connections between neurons are critical for generating appropriate output. What mechanisms govern the formation of electrical and/or chemical synapses between two neurons? Recent studies indicate that common molecular players could regulate the formation of both of these classes of synapses. In addition, electrical and chemical synapses can mutually coregulate each other’s formation. Electrical activity, generated spontaneously by the nervous system or initiated from sensory experience, plays an important role in this process, leading to the selection of appropriate connections and the elimination of inappropriate ones. In this review, we discuss recent studies that shed light on the formation and developmental interactions of chemical and electrical synapses.

Connexin36 Expression in Primary Afferent Neurons in Relation to the Axon Reflex and Modality Coding of Somatic Sensation

Electrical coupling mediated by connexin36-containing gap junctions that form electrical synapses is known to be prevalent in the central nervous system, but such coupling was long ago reported also to occur between cutaneous sensory fibers. Here, we provide evidence supporting the capability of primary afferent fibers to engage in electrical coupling. In transgenic mice with enhanced green fluorescent protein (eGFP) serving as a reporter for connexin36 expression, immunofluorescence labeling of eGFP was found in subpopulations of neurons in lumbar dorsal root and trigeminal sensory ganglia, and in fibers within peripheral nerves and tissues. Immunolabeling of connexin36 was robust in the sciatic nerve, weaker in sensory ganglia than in peripheral nerve, and absent in these tissues from Cx36 null mice. Connexin36 mRNA was detected in ganglia from wild-type mice, but not in those from Cx36 null mice. Labeling of eGFP was localized within a subpopulation of ganglion cells containing substance P and calcitonin gene-releasing peptide, and in peripheral fibers containing these peptides. Expression of eGFP was also found in various proportions of sensory ganglion neurons containing transient receptor potential (TRP) channels, including TRPV1 and TRPM8. Ganglion cells labeled for isolectin B4 and tyrosine hydroxylase displayed very little co-localization with eGFP. Our results suggest that previously observed electrical coupling between peripheral sensory fibers occurs via electrical synapses formed by Cx36-containing gap junctions, and that some degree of selectivity in the extent of electrical coupling may occur between fibers belonging to subpopulations of sensory neurons identified according to their sensory modality responsiveness.

Function of Connexins in the Interaction between Glial and Vascular Cells in the Central Nervous System and Related Neurological Diseases

Neuronal signaling together with synapse activity in the central nervous system requires a precisely regulated microenvironment. Recently, the blood-brain barrier is considered as a “neuro-glia-vascular unit,” a structural and functional compound composed of capillary endothelial cells, glial cells, pericytes, and neurons, which plays a pivotal role in maintaining the balance of the microenvironment in and out of the brain. Tight junctions and adherens junctions, which function as barriers of the blood-brain barrier, are two well-known kinds in the endothelial cell junctions. In this review, we focus on the less-concerned contribution of gap junction proteins, connexins in blood-brain barrier integrity under physio-/pathology conditions. In the neuro-glia-vascular unit, connexins are expressed in the capillary endothelial cells and prominent in astrocyte endfeet around and associated with maturation and function of the blood-brain barrier through a unique signaling pathway and an interaction with tight junction proteins. Connexin hemichannels and connexin gap junction channels contribute to the physiological or pathological progress of the blood-brain barrier; in addition, the interaction with other cell-cell-adhesive proteins is also associated with the maintenance of the blood-brain barrier. Lastly, we explore the connexins and connexin channels involved in the blood-brain barrier in neurological diseases and any programme for drug discovery or delivery to target or avoid the blood-brain barrier.

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.

Asymmetry of an Intracellular Scaffold at Vertebrate Electrical Synapses

Neuronal synaptic connections are either chemical or electrical, and these two types of synapses work together to dynamically define neural circuit function [1]. Although we know a great deal about the molecules that support chemical synapse formation and function, we know little about the macromolecular complexes that regulate electrical synapses. Electrical synapses are created by gap junction (GJ) channels that provide direct ionic communication between neurons [2]. Although they are often molecularly and functionally symmetric, recent work has found that pre- and postsynaptic neurons can contribute different GJ-forming proteins, creating molecularly asymmetric channels that are correlated with functional asymmetry at the synapse [3, 4]. Associated with the GJs are structures observed by electron microscopy termed the electrical synapse density (ESD) [5]. The ESD has been suggested to be critical for the formation and function of the electrical synapse, yet the biochemical makeup of these structures is poorly understood. Here we find that electrical synapse formation in vivo requires an intracellular scaffold called Tight Junction Protein 1b (Tjp1b). Tjp1b is localized to the electrical synapse, where it is required for the stabilization of the GJs and for electrical synapse function. Strikingly, we find that Tjp1b protein localizes and functions asymmetrically, exclusively on the postsynaptic side of the synapse. Our findings support a novel model of electrical synapse molecular asymmetry at the level of an intracellular scaffold that is required for building the electrical synapse. We propose that such ESD asymmetries could be used by all nervous systems to support molecular and functional asymmetries at electrical synapses.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.

Cx36, Cx43 and Cx45 in mouse and rat cerebellar cortex: species-specific expression, compensation in Cx36 null mice and co-localization in neurons vs. glia

Electrical synapses formed by connexin36 (Cx36)-containing gap junctions between interneurons in the cerebellar cortex have been well characterized, including those formed between basket cells and between Golgi cells, and there is gene reporter-based evidence for the expression of connexin45 (Cx45) in the cerebellar molecular layer. Here, we used immunofluorescence approaches to further investigate expression patterns of Cx36 and Cx45 in this layer and to examine localization relationships of these connexins with each other and with glial connexin43 (Cx43). In mice, strain differences were found, such that punctate labelling for Cx36 was differentially distributed in the molecular layer of C57BL/6 vs. CD1 mice. In mice with EGFP reporter representing Cx36 expression, Cx36-puncta were localized to processes of stellate cells and other cerebellar interneurons. Punctate labelling of Cx45 was faint in the molecular layer of wild-type mice and was increased in intensity in mice with Cx36 gene ablation. The vast majority of Cx36-puncta co-localized with Cx45-puncta, which in turn was associated with the scaffolding protein zonula occludens-1. In rats, Cx45-puncta were also co-localized with Cx36-puncta and additionally occurred along Bergmann glial processes adjacent to Cx43-puncta. The results indicate strain and species differences in Cx36 as well as Cx45 expression, possible compensatory processes after loss of Cx36 expression and localization of Cx45 to both neuronal and Bergmann glial gap junctions. Further, expression of both Cx43 and Cx45 in Bergmann glia of rat may contribute to the complex properties of junctional coupling between these cells and perhaps to their reported coupling with Purkinje cells.

Connexin36 localization to pinealocytes in the pineal gland of mouse and rat

Several cell types in the pineal gland are known to establish intercellular gap junctions, but the connexin constituents of those junctions have not been fully characterized. Specifically, the expression of connexin36 (Cx36) protein and mRNA has been examined in the pineal, but the identity of cells that produce Cx36 and that form Cx36-containing gap junctions has not been determined. We used immunofluorescence and freeze fracture replica immunogold labelling (FRIL) of Cx36 to investigate the cellular and subcellular localization of Cx36 in the pineal gland of adult mouse and rat. Immunofluorescence labelling of Cx36 was visualized exclusively as puncta or short immunopositive strands that were distributed throughout the pineal, and which were absent in pineal sections from Cx36 null mice. By double immunofluorescence labelling, Cx36 was localized to tryptophan hydroxylase-positive and 5-hydroxytryptamine-positive pinealocyte cell bodies and their large initial processes, including at intersections of those processes and at sites displaying a confluence of processes. Labelling for the cell junction marker zonula occludens-1 (ZO-1) either overlapped or was closely associated with labelling for Cx36. Pinealocytes thus form Cx36-containing gap junctions that also incorporate the scaffolding protein ZO-1. FRIL revealed labelling of Cx36 at ultrastructurally defined gap junctions between pinealocytes, most of which was at gap junctions having reticular, ribbon or string configurations. The results suggest that the endocrine functions of pinealocytes and their secretion of melatonin is supported by their intercellular communication via Cx36-containing gap junctions, which may now be tested by the use of Cx36 null mice.

Zonula occludens-2 regulates Rho proteins activity and the development of epithelial cytoarchitecture and barrier function

Silencing Zonula occludens 2 (ZO-2), a tight junctions (TJ) scaffold protein, in epithelial cells (MDCK ZO-2 KD) triggers: 1) Decreased cell to substratum attachment, accompanied by reduced expression of claudin-7 and integrin β1, and increased vinculin recruitment to focal adhesions and stress fibers formation; 2) Lowered cell-cell aggregation and appearance of wider intercellular spaces; 3) Increased RhoA/ROCK activity, mediated by GEF-HI recruitment to cell borders by cingulin; 4) Increased Cdc42 activity, mitotic spindle disorientation and the appearance of cysts with multiple lumens; 5) Increased Rac and cofilin activity, multiple lamellipodia formation and random cell migration but increased wound closure; 6) Diminished cingulin phosphorylation and disappearance of planar network of microtubules at the TJ region; and 7) Increased transepithelial electrical resistance at steady state, coupled to an increased expression of ZO-1 and claudin-4 and a decreased expression of claudin-2 and paracingulin. Hence, ZO-2 is a crucial regulator of Rho proteins activity and the development of epithelial cytoarchitecture and barrier function.

Immunofluorescence reveals unusual patterns of labelling for connexin43 localized to calbindin-D28K-positive interstitial cells in the pineal gland

Gap junctions between cells in the pineal gland have been described ultrastructurally, but their connexin constituents have not been fully characterized. We used immunofluorescence in combination with markers of pineal cells to document the cellular localization of connexin43 (Cx43). Immunofluorescence labelling of Cx43 with several different antibodies was widely distributed throughout the pineal, whereas another connexin examined, connexin26, was not found in pineal but only in surrounding leptomeninges. Labelling apparently associated with plasma membranes was visualized either as fine Cx43-puncta (1-2 μm) or as unusually large pools of Cx43 ranging up to 4-7 μm in diameter or length. These puncta and pools were highly concentrated in perivascular spaces, where they were associated with numerous cells devoid of labelling for markers of pinealocytes (e.g. tryptophan hydroxylase and serotonin), and where they were minimally associated with blood vessels and lacked association with resident macrophages. Astrocytes labelled for glial fibrillary acidic protein were largely restricted to the anterior pole of the pineal gland, where they displayed only fine and sparse Cx43-puncta along their processes. Labelling for Cx43 was localized largely though not exclusively to the somata and long processes of a subpopulation of perivascular interstitial cells that were immunopositive for calbindin-D28K. These cells were often located among dense bundles or termination areas of sympathetic fibres labelled for tyrosine hydroxylase or serotonin. The results indicate that interstitial cells form abundant gap junctions composed of Cx43, and suggest that gap junction-mediated intracellular communication by these cells supports the activities of pinealocytes.

Two-color fluorescent analysis of connexin 36 turnover: relationship to functional plasticity

Gap junctions formed of connexin 36 (Cx36, also known as Gjd2) show tremendous functional plasticity on several time scales. Changes in connexin phosphorylation modify coupling in minutes through an order of magnitude, but recent studies also imply involvement of connexin turnover in regulating cell-cell communication. We utilized Cx36 with an internal HaloTag to study Cx36 turnover and trafficking in cultured cells. Irreversible, covalent pulse-chase labeling with fluorescent HaloTag ligands allowed clear discrimination of newly formed and pre-existing Cx36. Cx36 in junctional plaques turned over with a half-life of 3.1 h, and the turnover rate was unchanged by manipulations of protein kinase A (PKA) activity. In contrast, changes in PKA activity altered coupling within 20 min. New Cx36 in cargo vesicles was added directly to existing gap junctions and newly made Cx36 was not confined to points of addition, but diffused throughout existing gap junctions. Existing connexins also diffused into photobleached areas with a half-time of less than 2 s. In conclusion, studies of Cx36-HaloTag revealed novel features of connexin trafficking and demonstrated that phosphorylation-based changes in coupling occur on a different time scale than turnover.

Connexin36 expression in major centers of the auditory system in the CNS of mouse and rat: Evidence for neurons forming purely electrical synapses and morphologically mixed synapses

Electrical synapses formed by gap junctions composed of connexin36 (Cx36) are widely distributed in the mammalian central nervous system (CNS). Here, we used immunofluorescence methods to document the expression of Cx36 in the cochlear nucleus and in various structures of the auditory pathway of rat and mouse. Labeling of Cx36 visualized exclusively as Cx36-puncta was densely distributed primarily on the somata and initial dendrites of neuronal populations in the ventral cochlear nucleus, and was abundant in superficial layers of the dorsal cochlear nucleus. Other auditory centers displaying Cx36-puncta included the medial nucleus of the trapezoid body (MNTB), regions surrounding the lateral superior olivary nucleus, the dorsal nucleus of the medial lemniscus, the nucleus sagulum, all subnuclei of the inferior colliculus, and the auditory cerebral cortex. In EGFP-Cx36 transgenic mice, EGFP reporter was detected in neurons located in each of auditory centers that harbored Cx36-puncta. In the ventral cochlear nuclei and the MNTB, many neuronal somata were heavily innervated by nerve terminals containing vesicular glutamate transporter-1 (vglut1) and Cx36 was frequently localized at these terminals. Cochlear ablation caused a near total depletion of vglut1-positive terminals in the ventral cochlear nuclei, with a commensurate loss of labeling for Cx36 around most neuronal somata, but preserved Cx36-puncta at somatic neuronal appositions. The results suggest that electrical synapses formed by Cx36-containing gap junctions occur in most of the widely distributed centers of the auditory system. Further, it appears that morphologically mixed chemical/electrical synapses formed by nerve terminals are abundant in the ventral cochlear nucleus, including those at endbulbs of Held formed by cochlear primary afferent fibers, and those at calyx of Held synapses on MNTB neurons.

Type 2 wide-field amacrine cells in TH::GFP mice show a homogenous synapse distribution and contact small ganglion cells

In vertebrate retinas, wide-field amacrine cells represent a diverse class of interneurons, important for the extraction of selective features, like motion or objects, from the visual scene. Most types of wide-field amacrine cells lack dedicated output processes, whereas some types spatially segregate outputs from inputs. In the tyrosine hydroxylase (TH)::green fluorescent protein (GFP) mouse line, two types of GFP-expressing wide-field amacrine cells have been described: dopaminergic type 1 and γ-aminobutyric acid-ergic type 2 cells (TH2). TH2 cells possess short and long radial processes stratifying in the middle of the inner plexiform layer, where they collect excitatory and inhibitory inputs from bipolar cells and other amacrine cells, respectively. Although it was shown that these inputs lead to ON-OFF light responses, their spatial distribution along TH2 cell processes is unknown. Also, the postsynaptic targets of TH2 cells have not been identified so far. Here, we analysed the synapse distribution of these cells in TH::GFP mice and show that they form a weakly coupled network. Electrical synapses (made of connexin36) and chemical (excitatory and inhibitory) synapses are uniformly distributed along TH2 dendrites, independent of dendrite length or distance from soma. Moreover, we reveal that TH2 cells contact at least two types of small ganglion cells; one of them is the W3 cell, a ganglion cell sensitive to object motion. Contacts were often associated with markers of inhibitory synapses. Thus, TH2 wide-field amacrine cells likely provide postsynaptic inhibition to W3 ganglion cells and may contribute to object-motion detection in the mouse retina.

Heterotypic gap junctions at glutamatergic mixed synapses are abundant in goldfish brain

Gap junctions provide for direct intercellular electrical and metabolic coupling. The abundance of gap junctions at "large myelinated club ending (LMCE)" synapses on Mauthner cells (M-cells) of the teleost brain provided a convenient model to correlate anatomical and physiological properties of electrical synapses. There, presynaptic action potentials were found to evoke short-latency electrical "pre-potentials" immediately preceding their accompanying glutamate-induced depolarizations, making these the first unambiguously identified "mixed" (i.e., chemical plus electrical) synapses in the vertebrate CNS. We recently showed that gap junctions at these synapses exhibit asymmetric electrical resistance (i.e., electrical rectification), which we correlated with total molecular asymmetry of connexin composition in their apposing gap junction hemiplaques, with connexin35 (Cx35) restricted to axon terminal hemiplaques and connexin34.7 (Cx34.7) restricted to apposing M-cell plasma membranes. We now show that similarly heterotypic neuronal gap junctions are abundant throughout goldfish brain, with labeling exclusively for Cx35 in presynaptic hemiplaques and exclusively for Cx34.7 in postsynaptic hemiplaques. Moreover, the vast majority of these asymmetric gap junctions occur at glutamatergic axon terminals. The widespread distribution of heterotypic gap junctions at glutamatergic mixed synapses throughout goldfish brain and spinal cord implies that pre- vs. postsynaptic asymmetry at electrical synapses evolved early in the chordate lineage. We propose that the advantages of the molecular and functional asymmetry of connexins at electrical synapses that are so prominently expressed in the teleost CNS are unlikely to have been abandoned in higher vertebrates. However, to create asymmetric coupling in mammals, where most gap junctions are composed of connexin36 (Cx36) on both sides, would require some other mechanism, such as differential phosphorylation of connexins on opposite sides of the same gap junction or on asymmetric differences in the complement of their scaffolding and regulatory proteins.

Hemichannel composition and electrical synaptic transmission: molecular diversity and its implications for electrical rectification

Unapposed hemichannels (HCs) formed by hexamers of gap junction proteins are now known to be involved in various cellular processes under both physiological and pathological conditions. On the other hand, less is known regarding how differences in the molecular composition of HCs impact electrical synaptic transmission between neurons when they form intercellular heterotypic gap junctions (GJs). Here we review data indicating that molecular differences between apposed HCs at electrical synapses are generally associated with rectification of electrical transmission. Furthermore, this association has been observed at both innexin and connexin (Cx) based electrical synapses. We discuss the possible molecular mechanisms underlying electrical rectification, as well as the potential contribution of intracellular soluble factors to this phenomenon. We conclude that asymmetries in molecular composition and sensitivity to cellular factors of each contributing hemichannel can profoundly influence the transmission of electrical signals, endowing electrical synapses with more complex functional properties.

Connexin36 identified at morphologically mixed chemical/electrical synapses on trigeminal motoneurons and at primary afferent terminals on spinal cord neurons in adult mouse and rat

Morphologically mixed chemical/electrical synapses at axon terminals, with the electrical component formed by gap junctions, is common in the CNS of lower vertebrates. In mammalian CNS, evidence for morphologically mixed synapses has been obtained in only a few locations. Here, we used immunofluorescence approaches to examine the localization of the neuronally expressed gap junction forming protein connexin36 (Cx36) in relation to the axon terminal marker vesicular glutamate transporter-1 (vglut1) in the spinal cord and the trigeminal motor nucleus (Mo5) of rat and mouse. In adult rodents, immunolabeling for Cx36 appeared exclusively as Cx36-puncta, and was widely distributed at all rostro-caudal levels in most spinal cord laminae and in the Mo5. A high proportion of Cx36-puncta was co-localized with vglut1, forming morphologically mixed synapses on motoneurons, in intermediate spinal cord lamina, and in regions of medial lamina VII, where vglut1-containing terminals associated with Cx36 converged on neurons adjacent to the central canal. Unilateral transection of lumbar dorsal roots reduced immunolabeling of both vglut1 and Cx36 in intermediate laminae and lamina IX. Further, vglut1-terminals displaying Cx36-puncta were contacted by terminals labeled for glutamic acid decarboxylase65, which is known to be contained in presynaptic terminals on large-diameter primary afferents. Developmentally, mixed synapses begin to emerge in the spinal cord only after the second to third postnatal week and thereafter increase to adult levels. Our findings demonstrate that axon terminals of primary afferent origin form morphologically mixed synapses containing Cx36 in broadly distributed areas of adult rodent spinal cord and Mo5.

AII amacrine cells discriminate between heterocellular and homocellular locations when assembling connexin36-containing gap junctions

Electrical synapses (gap junctions) rapidly transmit signals between neurons and are composed of connexins. In neurons, connexin36 (Cx36) is the most abundant isoform; however, the mechanisms underlying formation of Cx36-containing electrical synapses are unknown. We focus on homocellular and heterocellular gap junctions formed by an AII amacrine cell, a key interneuron found in all mammalian retinas. In mice lacking native Cx36 but expressing a variant tagged with enhanced green fluorescent protein at the C-terminus (KO-Cx36-EGFP), heterocellular gap junctions formed between AII cells and ON cone bipolar cells are fully functional, whereas homocellular gap junctions between two AII cells are not formed. A tracer injected into an AII amacrine cell spreads into ON cone bipolar cells but is excluded from other AII cells. Reconstruction of Cx36-EGFP clusters on an AII cell in the KO-Cx36-EGFP genotype confirmed that the number, but not average size, of the clusters is reduced - as expected for AII cells lacking a subset of electrical synapses. Our studies indicate that some neurons exhibit at least two discriminatory mechanisms for assembling Cx36. We suggest that employing different gap-junction-forming mechanisms could provide the means for a cell to regulate its gap junctions in a target-cell-specific manner, even if these junctions contain the same connexin.

Electrical synapses and their functional interactions with chemical synapses

Brain function relies on the ability of neurons to communicate with each other. Interneuronal communication primarily takes place at synapses, where information from one neuron is rapidly conveyed to a second neuron. There are two main modalities of synaptic transmission: chemical and electrical. Far from functioning independently and serving unrelated functions, mounting evidence indicates that these two modalities of synaptic transmission closely interact, both during development and in the adult brain. Rather than conceiving synaptic transmission as either chemical or electrical, this article emphasizes the notion that synaptic transmission is both chemical and electrical, and that interactions between these two forms of interneuronal communication might be required for normal brain development and function.

Cardiac to cancer: connecting connexins to clinical opportunity

Gap junctions and their connexin components are indispensable in mediating the cellular coordination required for tissue and organ homeostasis. The critical nature of their existence mandates a connection to disease while at the same time offering therapeutic potential. Therapeutic intervention may be offered through the pharmacological and molecular disruption of the pathways involved in connexin biosynthesis, gap junction assembly, stabilization, or degradation. Chemical inhibitors aimed at closing connexin channels, peptide mimetics corresponding to short connexin sequences, and gene therapy approaches have been incredibly useful molecular tools in deciphering the complexities associated with connexin biology. Recently, therapeutic potential in targeting connexins has evolved from basic research in cell-based models to clinical opportunity in the form of human trials. Clinical promise is particularly evident with regards to targeting connexin43 in the context of wound healing. The following review is aimed at highlighting novel advances where the pharmacological manipulation of connexin biology has proven beneficial in animals or humans.

Connexin36 in gap junctions forming electrical synapses between motoneurons in sexually dimorphic motor nuclei in spinal cord of rat and mouse

Pools of motoneurons in the lumbar spinal cord innervate the sexually dimorphic perineal musculature, and are themselves sexually dimorphic, showing differences in number and size between male and female rodents. In two of these pools, the dorsomedial nucleus (DMN) and the dorsolateral nucleus (DLN), dimorphic motoneurons are intermixed with non-dimorphic neurons innervating anal and external urethral sphincter muscles. As motoneurons in these nuclei are reportedly linked by gap junctions, we examined immunofluorescence labeling for the gap junction-forming protein connexin36 (Cx36) in male and female mice and rats. Fluorescent Cx36-labeled puncta occurred in distinctly greater amounts in the DMN and DLN of male rodents than in other spinal cord regions. These puncta were localized to motoneuron somata, proximal dendrites, and neuronal appositions, and were distributed either as isolated or large patches of puncta. In both rats and mice, Cx36-labeled puncta were associated with nearly all (> 94%) DMN and DLN motoneurons. The density of Cx36-labeled puncta increased dramatically from postnatal days 9 to 15, unlike the developmental decreases in these puncta observed in other central nervous system regions. In females, Cx36 labeling of puncta in the DLN was similar to that in males, but was sparse in the DMN. In enhanced green fluorescent protein (EGFP)-Cx36 transgenic mice, motoneurons in the DMN and DLN were intensely labeled for the EGFP reporter in males, but less so in females. The results indicate the presence of Cx36-containing gap junctions in the sexually dimorphic DMN and DLN of both male and female rodents, suggesting coupling of not only sexually dimorphic but also non-dimorphic motoneurons in these nuclei.

New insights into the role of connexins in pancreatic islet function and diabetes

Multi-cellular systems require complex signaling mechanisms for proper tissue function, to mediate signaling between cells in close proximity and at distances. This holds true for the islets of Langerhans, which are multicellular micro-organs located in the pancreas responsible for glycemic control, through secretion of insulin and other hormones. Coupling of electrical and metabolic signaling between islet β-cells is required for proper insulin secretion and effective glycemic control. β-cell specific coupling is established through gap junctions composed of connexin36, which results in coordinated insulin release across the islet. Islet connexins have been implicated in both Type-1 and Type-2 diabetes; however a clear link remains to be determined. The goal of this review is to discuss recent discoveries regarding the role of connexins in regulating insulin secretion, the regulation of connexins within the islet, and recent studies which support a role for connexins in diabetes. Further studies which investigate the regulation of connexins in the islet and their role in diabetes may lead to novel diabetes therapies which regulate islet function and β-cell survival through modulation of gap junction coupling.

Proteins and mechanisms regulating gap-junction assembly, internalization, and degradation

Gap junctions (GJs) are the only known cellular structures that allow a direct cell-to-cell transfer of signaling molecules by forming densely packed arrays or "plaques" of hydrophilic channels that bridge the apposing membranes of neighboring cells. The crucial role of GJ-mediated intercellular communication (GJIC) for all aspects of multicellular life, including coordination of development, tissue function, and cell homeostasis, has been well documented. Assembly and degradation of these membrane channels is a complex process that includes biosynthesis of the connexin (Cx) subunit proteins (innexins in invertebrates) on endoplasmic reticulum (ER) membranes, oligomerization of compatible subunits into hexameric hemichannels (connexons), delivery of the connexons to the plasma membrane (PM), head-on docking of compatible connexons in the extracellular space at distinct locations, arrangement of channels into dynamic spatially and temporally organized GJ channel plaques, as well as internalization of GJs into the cytoplasm followed by their degradation. Clearly, precise modulation of GJIC, biosynthesis, and degradation are crucial for accurate function, and much research currently addresses how these fundamental processes are regulated. Here, we review posttranslational protein modifications (e.g., phosphorylation and ubiquitination) and the binding of protein partners (e.g., the scaffolding protein ZO-1) known to regulate GJ biosynthesis, internalization, and degradation. We also look closely at the atomic resolution structure of a GJ channel, since the structure harbors vital cues relevant to GJ biosynthesis and turnover.

Re-evaluation of connexins associated with motoneurons in rodent spinal cord, sexually dimorphic motor nuclei and trigeminal motor nucleus

Electrical synapses formed by neuronal gap junctions composed of connexin36 (Cx36) are a common feature in mammalian brain circuitry, but less is known about their deployment in spinal cord. It has been reported based on connexin mRNA and/or protein detection that developing and/or mature motoneurons express a variety of connexins, including Cx26, Cx32, Cx36 and Cx43 in trigeminal motoneurons, Cx36, Cx37, Cx40, Cx43 and Cx45 in spinal motoneurons, and Cx32 in sexually dimorphic motoneurons. We re-examined the localization of these connexins during postnatal development and in adult rat and mouse using immunofluorescence labeling for each connexin. We found Cx26 in association only with leptomeninges in the trigeminal motor nucleus (Mo5), Cx32 only with oligodendrocytes and myelinated fibers among motoneurons in this nucleus and in the spinal cord, and Cx37, Cx40 and Cx45 only with blood vessels in the ventral horn of spinal cord, including those among motoneurons. By freeze-fracture replica immunolabeling, > 100 astrocyte gap junctions but no neuronal gap junctions were found based on immunogold labeling for Cx43, whereas 16 neuronal gap junctions at postnatal day (P)4, P7 and P18 were detected based on Cx36 labeling. Punctate labeling for Cx36 was localized to the somatic and dendritic surfaces of peripherin-positive motoneurons in the Mo5, motoneurons throughout the spinal cord, and sexually dimorphic motoneurons at lower lumbar levels. In studies of electrical synapses and electrical transmission between developing and between adult motoneurons, our results serve to focus attention on mediation of this transmission by gap junctions composed of Cx36.

Morphologically mixed chemical-electrical synapses formed by primary afferents in rodent vestibular nuclei as revealed by immunofluorescence detection of connexin36 and vesicular glutamate transporter-1

Axon terminals forming mixed chemical/electrical synapses in the lateral vestibular nucleus of rat were described over 40 years ago. Because gap junctions formed by connexins are the morphological correlate of electrical synapses, and with demonstrations of widespread expression of the gap junction protein connexin36 (Cx36) in neurons, we investigated the distribution and cellular localization of electrical synapses in the adult and developing rodent vestibular nuclear complex, using immunofluorescence detection of Cx36 as a marker for these synapses. In addition, we examined Cx36 localization in relation to that of the nerve terminal marker vesicular glutamate transporter-1 (vglut-1). An abundance of immunolabeling for Cx36 in the form of Cx36-puncta was found in each of the four major vestibular nuclei of adult rat and mouse. Immunolabeling was associated with somata and initial dendrites of medium and large neurons, and was absent in vestibular nuclei of Cx36 knockout mice. Cx36-puncta were seen either dispersed or aggregated into clusters on the surface of neurons, and were never found to occur intracellularly. Nearly all Cx36-puncta were localized to large nerve terminals immunolabeled for vglut-1. These terminals and their associated Cx36-puncta were substantially depleted after labyrinthectomy. Developmentally, labeling for Cx36 was already present in the vestibular nuclei at postnatal day 5, where it was only partially co-localized with vglut-1, and did not become fully associated with vglut-1-positive terminals until postnatal day 20-25. The results show that vglut-1-positive primary afferent nerve terminals form mixed synapses throughout the vestibular nuclear complex, that the gap junction component of these synapses contains Cx36, that multiple Cx36-containing gap junctions are associated with individual vglut-1 terminals and that the development of these mixed synapses is protracted over several postnatal weeks.

Molecular and functional asymmetry at a vertebrate electrical synapse

Electrical synapses are abundant in the vertebrate brain, but their functional and molecular complexities are still poorly understood. We report here that electrical synapses between auditory afferents and goldfish Mauthner cells are constructed by apposition of hemichannels formed by two homologs of mammalian connexin 36 (Cx36) and that, while Cx35 is restricted to presynaptic hemiplaques, Cx34.7 is restricted to postsynaptic hemiplaques, forming heterotypic junctions. This molecular asymmetry is associated with rectification of electrical transmission that may act to promote cooperativity between auditory afferents. Our data suggest that, in similarity to pre- and postsynaptic sites at chemical synapses, one side in electrical synapses should not necessarily be considered the mirror image of the other. While asymmetry based on the presence of two Cx36 homologs is restricted to teleost fish, it might also be based on differences in posttranslational modifications of individual connexins or in the complement of gap junction-associated proteins.

Polarity proteins as regulators of cell junction complexes: implications for breast cancer

The epithelium of multicellular organisms possesses a well-defined architecture, referred to as polarity that coordinates the regulation of essential cell features. Polarity proteins are intimately linked to the protein complexes that make the tight, adherens and gap junctions; they contribute to the proper localization and assembly of these cell-cell junctions within cells and consequently to functional tissue organization. The establishment of cell-cell junctions and polarity are both implicated in the regulation of epithelial modifications in normal and cancer situations. Uncovering the mechanisms through which cell-cell junctions and epithelial polarization are established and how their interaction with the microenvironment directs cell and tissue organization has opened new venues for the development of cancer therapies. In this review, we focus on the breast epithelium to highlight how polarity and cell-cell junction proteins interact together in normal and cancerous contexts to regulate major cellular mechanisms such as migration. The impact of these proteins on epigenetic mechanisms responsible for resetting cells toward oncogenesis is discussed in light of increasing evidence that tissue polarity modulates chromatin function. Finally, we give an overview of recent breast cancer therapies that target proteins involved in cell-cell junctions.

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.