Functional characteristics of skate connexin35, a member of the gamma subfamily of connexins expressed in the vertebrate retina

Increased Hemichannel Activity Displayed by a Connexin43 Mutation Causing a Familial Connexinopathy Exhibiting Hypotrichosis with Follicular Keratosis and Hyperostosis

Mutations in the GJA1 gene that encodes connexin43 (Cx43) cause several rare genetic disorders, including diseases affecting the epidermis. Here, we examined the in vitro functional consequences of a Cx43 mutation, Cx43-G38E, linked to a novel human phenotype of hypotrichosis, follicular keratosis and hyperostosis. We found that Cx43-G38E was efficiently translated in Xenopus oocytes and localized to gap junction plaques in transfected HeLa cells. Cx43-G38E formed functional gap junction channels with the same efficiency as wild-type Cx43 in Xenopus oocytes, although voltage gating of the gap junction channels was altered. Notably, Cx43-G38E significantly increased membrane current flow through the formation of active hemichannels when compared to wild-type Cx43. These data demonstrate the association of increased hemichannel activity to a connexin mutation linked to a skeletal-cutaneous phenotype, suggesting that augmented hemichannel activity could play a role in skin and skeletal disorders caused by human Cx43 mutations.

Signaling Between TRPV1/TRPV4 and Intracellular Hydrostatic Pressure in the Mouse Lens

Purpose

The lens uses feedback to maintain zero pressure in its surface cells. Positive pressures are detected by transient receptor potential vanilloid (TRPV4), which initiates a cascade that reduces surface cell osmolarity. The first step is opening of gap junction hemichannels. One purpose of the current study was to identify the connexin(s) in the hemichannels. Negative pressures are detected by TRPV1, which initiates a cascade that increases surface osmolarity. The increase in osmolarity was initially reported to be through inhibition of Na/K ATPase activity, but a recent study reported it was through stimulation of Na/K/2Cl (NKCC) cotransport. A second purpose of this study was to reconcile these two reports.

Methods

Intracellular hydrostatic pressures were measured using a microelectrode/manometer system. Lenses from TRPV1 or Cx50 null mice were studied. Specific inhibitors of Cx50 gap junction channels, NKCC, and Akt were used.

Results

Either knockout of Cx50 or blockade of Cx50 channels completely eliminated the response to positive surface pressures. Knockout of Cx50 also caused a positive drift in surface pressure. The short-term (∼20-minute) response to negative surface pressures was eliminated by blockade of NKCC, but a long-term (∼4-hour) response restored pressure to zero. Both short- and long-term responses were eliminated by knockout of TRPV1 or inhibition of Akt.

Conclusions

Hemichannels made from Cx50 are required for the response to positive surface pressures. Negative surface pressures first activate NKCC, but a backup system is inhibition of Na/K ATPase activity. Both responses are initiated by TRPV1 and go through PI3K/Akt before branching.

S-nitrosylation of connexin43 hemichannels elicits cardiac stress-induced arrhythmias in Duchenne muscular dystrophy mice

Patients with Duchenne muscular dystrophy (DMD) commonly present with severe ventricular arrhythmias that contribute to heart failure. Arrhythmias and lethality are also consistently observed in adult Dmdmdx mice, a mouse model of DMD, after acute β-adrenergic stimulation. These pathological features were previously linked to aberrant expression and remodeling of the cardiac gap junction protein connexin43 (Cx43). Here, we report that remodeled Cx43 protein forms Cx43 hemichannels in the lateral membrane of Dmdmdx cardiomyocytes and that the β-adrenergic agonist isoproterenol (Iso) aberrantly activates these hemichannels. Block of Cx43 hemichannels or a reduction in Cx43 levels (using Dmdmdx Cx43+/- mice) prevents the abnormal increase in membrane permeability, plasma membrane depolarization, and Iso-evoked electrical activity in these cells. Additionally, Iso treatment promotes nitric oxide (NO) production and S-nitrosylation of Cx43 hemichannels in Dmdmdx heart. Importantly, inhibition of NO production prevents arrhythmias evoked by Iso. We found that NO directly activates Cx43 hemichannels by S-nitrosylation of cysteine at position 271. Our results demonstrate that opening of remodeled and S-nitrosylated Cx43 hemichannels plays a key role in the development of arrhythmias in DMD mice and that these channels may serve as therapeutic targets to prevent fatal arrhythmias in patients with DMD .

Connexin43 mutations linked to skin disease have augmented hemichannel activity

Mutations in the gene (GJA1) encoding connexin43 (Cx43) are responsible for several rare genetic disorders, including non-syndromic skin-limited diseases. Here we used two different functional expression systems to characterize three Cx43 mutations linked to palmoplantar keratoderma and congenital alopecia-1, erythrokeratodermia variabilis et progressiva, or inflammatory linear verrucous epidermal nevus. In HeLa cells and Xenopus oocytes, we show that Cx43-G8V, Cx43-A44V and Cx43-E227D all formed functional gap junction channels with the same efficiency as wild-type Cx43, with normal voltage gating and a unitary conductance of ~110 pS. In HeLa cells, all three mutations also localized to regions of cell-cell contact and displayed a punctate staining pattern. In addition, we show that Cx43-G8V, Cx43-A44V and Cx43-E227D significantly increase membrane current flow through formation of active hemichannels, a novel activity that was not displayed by wild-type Cx43. The increased membrane current was inhibited by either 2 mM calcium, or 5 µM gadolinium, mediated by hemichannels with a unitary conductance of ~250 pS, and was not due to elevated mutant protein expression. The three Cx43 mutations all showed the same gain of function activity, suggesting that augmented hemichannel activity could play a role in skin-limited diseases caused by human Cx43 mutations.

Cancer Connectors: Connexins, Gap Junctions, and Communication

Despite concerted clinical and research efforts, cancer is a leading cause of death worldwide. Surgery, radiation, and chemotherapy have remained the most common standard-of-care strategies against cancer for decades. However, the side effects of these therapies demonstrate the need to investigate adjuvant novel treatment modalities that minimize the harm caused to healthy cells and tissues. Normal and cancerous cells require communication amongst themselves and with their surroundings to proliferate and drive tumor growth. It is vital to understand how intercellular and external communication impacts tumor cell malignancy. To survive and grow, tumor cells, and their normal counterparts utilize cell junction molecules including gap junctions (GJs), tight junctions, and adherens junctions to provide contact points between neighboring cells and the extracellular matrix. GJs are specialized structures composed of a family of connexin proteins that allow the free diffusion of small molecules and ions directly from the cytoplasm of adjacent cells, without encountering the extracellular milieu, which enables rapid, and coordinated cellular responses to internal and external stimuli. Importantly, connexins perform three main cellular functions. They enable direct gap junction intercellular communication (GJIC) between cells, form hemichannels to allow cell communication with the extracellular environment, and serve as a site for protein-protein interactions to regulate signaling pathways. Connexins themselves have been found to promote tumor cell growth and invasiveness, contributing to the overall tumorigenicity and have emerged as attractive anti-tumor targets due to their functional diversity. However, connexins can also serve as tumor suppressors, and therefore, a complete understanding of the roles of the connexins and GJs in physiological and pathophysiological conditions is needed before connexin targeting strategies are applied. Here, we discuss how the three aspects of connexin function, namely GJIC, hemichannel formation, and connexin-protein interactions, function in normal cells, and contribute to tumor cell growth, proliferation, and death. Finally, we discuss the current state of anti-connexin therapies and speculate which role may be most amenable for the development of targeting strategies.

Synaptic Functions of Hemichannels and Pannexons: A Double-Edged Sword

The classical view of synapses as the functional contact between presynaptic and postsynaptic neurons has been challenged in recent years by the emerging regulatory role of glial cells. Astrocytes, traditionally considered merely supportive elements are now recognized as active modulators of synaptic transmission and plasticity at the now so-called "tripartite synapse." In addition, an increasing body of evidence indicates that beyond immune functions microglia also participate in various processes aimed to shape synaptic plasticity. Release of neuroactive compounds of glial origin, -process known as gliotransmission-, constitute a widespread mechanism through which glial cells can either potentiate or reduce the synaptic strength. The prevailing vision states that gliotransmission depends on an intracellular Ca²⁺/exocytotic-mediated release; notwithstanding, growing evidence is pointing at hemichannels (connexons) and pannexin channels (pannexons) as alternative non-vesicular routes for gliotransmitters efflux. In concurrence with this novel concept, both hemichannels and pannexons are known to mediate the transfer of ions and signaling molecules -such as ATP and glutamate- between the cytoplasm and the extracellular milieu. Importantly, recent reports show that glial hemichannels and pannexons are capable to perceive synaptic activity and to respond to it through changes in their functional state. In this article, we will review the current information supporting the "double edge sword" role of hemichannels and pannexons in the function of central and peripheral synapses. At one end, available data support the idea that these channels are chief components of a feedback control mechanism through which gliotransmitters adjust the synaptic gain in either resting or stimulated conditions. At the other end, we will discuss how the excitotoxic release of gliotransmitters and [Ca²⁺]_i overload linked to the opening of hemichannels/pannexons might impact cell function and survival in the nervous system.

Gap Junction Blockers: An Overview of their Effects on Induced Seizures in Animal Models

BACKGROUND

Gap junctions are clusters of intercellular channels allowing the bidirectional pass of ions directly into the cytoplasm of adjacent cells. Electrical coupling mediated by gap junctions plays a role in the generation of highly synchronized electrical activity. The hypersynchronous neuronal activity is a distinctive characteristic of convulsive events. Therefore, it has been postulated that enhanced gap junctional communication is an underlying mechanism involved in the generation and maintenance of seizures. There are some chemical compounds characterized as gap junction blockers because of their ability to disrupt the gap junctional intercellular communication.

OBJECTIVE

Hence, the aim of this review is to analyze the available data concerning the effects of gap junction blockers specifically in seizure models.

RESULTS

Carbenoxolone, quinine, mefloquine, quinidine, anandamide, oleamide, heptanol, octanol, meclofenamic acid, niflumic acid, flufenamic acid, glycyrrhetinic acid and retinoic acid have all been evaluated on animal seizure models. In vitro, these compounds share anticonvulsant effects typically characterized by the reduction of both amplitude and frequency of the epileptiform activity induced in brain slices. In vivo, gap junction blockers modify the behavioral parameters related to seizures induced by 4-aminopyridine, pentylenetetrazole, pilocarpine, penicillin and maximal electroshock.

CONCLUSION

Although more studies are still required, these molecules could be a promising avenue in the search for new pharmaceutical alternatives for the treatment of epilepsy.

Connexin26 Mutations Causing Palmoplantar Keratoderma and Deafness Interact with Connexin43, Modifying Gap Junction and Hemichannel Properties

Mutations in GJB2 (Cx26) cause either deafness, or deafness associated with skin diseases. That different disorders can be caused by distinct mutations within the same gene suggests that unique channel activities are influenced by each class of mutation. We have examined the functional characteristics of two human mutations, Cx26-H73R and Cx26-S183F, causing palmoplantar keratoderma (PPK) and deafness. Both failed to form gap junction channels or hemichannels when expressed alone. Co-expression of the mutants with wild-type Cx43 showed a trans-dominant inhibition of Cx43 gap junction channels, without reductions in Cx43 protein synthesis. In addition, the presence of mutant Cx26 shifted Cx43 channel gating and kinetics towards a more Cx26-like behavior. Co-immunoprecipitation showed Cx43 being pulled down more efficiently with mutant Cx26, than wild-type, confirming the enhanced formation of heteromeric connexons. Finally, the formation of heteromeric connexons resulted in significantly increased Cx43 hemichannel activity in the presence of Cx26 mutants. These findings suggest a common mechanism whereby Cx26 mutations causing PPK and deafness trans-dominantly influence multiple functions of wild-type Cx43. They also implicate a role for aberrant hemichannel activity in the pathogenesis of PPK, and further highlight an emerging role for Cx43 in genetic skin diseases.

Protein kinase C-dependent regulation of connexin43 gap junctions and hemichannels

Connexin43 (Cx43) generates intercellular gap junction channels involved in, among others, cardiac and brain function. Gap junctions are formed by the docking of two hemichannels from neighbouring cells. Undocked Cx43 hemichannels can upon different stimuli open towards the extracellular matrix and allow transport of molecules such as fluorescent dyes and ATP. A range of phosphorylated amino acids have been detected in the C-terminus of Cx43 and their physiological role has been intensively studied both in the gap junctional form of Cx43 and in its hemichannel configuration. We present the current knowledge of protein kinase C (PKC)-dependent regulation of Cx43 and discuss the divergent results.

Connexin channels in congenital skin disorders

Gap junctions and hemichannels comprised of connexins influence epidermal proliferation and differentiation. Significant advances in our understanding of the functional role of connexins in the skin have been made by studying the diseases caused by connexin mutations. Eleven clinically defined cutaneous disorders with an overlapping spectrum of phenotypes are caused by mutations in five different connexin genes, highlighting that disease presentation must be deciphered with an understanding of how connexin functions are affected. Increasing evidence suggests that the skin diseases produced by connexin mutations result from dominant gains of function. In palmoplantar keratoderma with deafness, the connexin 26 mutations transdominantly alter the function of wild-type connexin 43 and create leaky heteromeric hemichannels. In keratitis-ichthyosis-deafness syndrome, different connexin 26 mutations can either form dominant hemichannels with altered calcium regulation or increased calcium permeability, leading to clinical subtypes of this syndrome. It is only with detailed understanding of these subtle functional differences that we can hope to create successful pathophysiology driven therapies for the connexin skin disorders.

Keratitis-ichthyosis-deafness syndrome-associated Cx26 mutants produce nonfunctional gap junctions but hyperactive hemichannels when co-expressed with wild type Cx43

Mutations in Cx26 gene are found in most cases of human genetic deafness. Some mutations produce syndromic deafness associated with skin disorders, like the Keratitis-Ichthyosis-Deafness syndrome (KID). Because in the human skin connexin 26 (Cx26) is co-expressed with other connexins, like Cx43 and Cx30, and as the KID syndrome is inherited as autosomal dominant condition, it is possible that KID mutations change the way Cx26 interacts with other co-expressed connexins. Indeed, some Cx26 syndromic mutations showed gap junction dominant negative effect when co-expressed with wild-type connexins, including Cx26 and Cx43. The nature of these interactions and the consequences on hemichannels and gap junction channel (GJC) functions remain unknown. In this study, we demonstrate that syndromic mutations, at the N terminus segment of Cx26, change connexin oligomerization compatibility, allowing aberrant interactions with Cx43. Strikingly, heteromeric oligomer formed by Cx43/Cx26 (syndromic mutants) shows exacerbated hemichannel activity but nonfunctional GJCs; this also occurs for those Cx26 KID mutants that do not show functional homomeric hemichannels. Heterologous expression of these hyperactive heteromeric hemichannels increases cell membrane permeability, favoring ATP release and Ca(2+) overload. The functional paradox produced by oligomerization of Cx43 and Cx26 KID mutants could underlie the severe syndromic phenotype in human skin.

Distinct permeation profiles of the connexin 30 and 43 hemichannels

Connexin 43 (Cx43) hemichannels may form open channels in the plasma membrane when exposed to specific stimuli, e.g. reduced extracellular concentration of divalent cations, and allow passage of fluorescent molecules and presumably a range of smaller physiologically relevant molecules. However, the permeability profile of Cx43 hemichannels remains unresolved. Exposure of Cx43-expressing Xenopus laevis oocytes to divalent cation free solution induced a gadolinium-sensitive uptake of the fluorescent dye ethidium. In spite thereof, a range of biological molecules smaller than ethidium, such as glutamate, lactate, and glucose, did not permeate the pore whereas ATP did. In contrast, permeability of glutamate, glucose and ATP was observed in oocytes expressing Cx30. Exposure to divalent cation free solutions induced a robust membrane conductance in Cx30-expressing oocytes but none in Cx43-expressing oocytes. C-terminally truncated Cx43 (M257) displayed increased dye uptake and, unlike wild type Cx43 channels, conducted current. Neither Cx30 nor Cx43 acted as water channels in their hemichannel configuration. Our results demonstrate that connexin hemichannels have isoform-specific permeability profiles and that dye uptake cannot be equaled to permeability of smaller physiologically relevant molecules in given settings.

Gap junctions

Gap junctions are essential to the function of multicellular animals, which require a high degree of coordination between cells. In vertebrates, gap junctions comprise connexins and currently 21 connexins are known in humans. The functions of gap junctions are highly diverse and include exchange of metabolites and electrical signals between cells, as well as functions, which are apparently unrelated to intercellular communication. Given the diversity of gap junction physiology, regulation of gap junction activity is complex. The structure of the various connexins is known to some extent; and structural rearrangements and intramolecular interactions are important for regulation of channel function. Intercellular coupling is further regulated by the number and activity of channels present in gap junctional plaques. The number of connexins in cell-cell channels is regulated by controlling transcription, translation, trafficking, and degradation; and all of these processes are under strict control. Once in the membrane, channel activity is determined by the conductive properties of the connexin involved, which can be regulated by voltage and chemical gating, as well as a large number of posttranslational modifications. The aim of the present article is to review our current knowledge on the structure, regulation, function, and pharmacology of gap junctions. This will be supported by examples of how different connexins and their regulation act in concert to achieve appropriate physiological control, and how disturbances of connexin function can lead to disease.

Regulation of connexin hemichannel activity by membrane potential and the extracellular calcium in health and disease

Connexins are thought to solely mediate cell-to-cell communication by forming gap junction channels composed of two membrane-spanning hemichannels positioned end-to-end. However, many if not all connexin isoforms also form functional hemichannels (i.e., the precursors of complete channels) that mediate the rapid exchange of ions, second messengers and metabolites between the cell interior and the interstitial space. Electrical and molecular signaling via connexin hemichannels is now widely recognized to be important in many physiological scenarios and pathological conditions. Indeed, mutations in connexins that alter hemichannel function have been implicated in several diseases. Here, we present a comprehensive overview of how hemichannel activity is tightly regulated by membrane potential and the external calcium concentration. In addition, we discuss the genetic mutations known to alter hemichannel function and their deleterious effects, of which a better understanding is necessary to develop novel therapeutic approaches for diseases caused by hemichannel dysfunction. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Structural and functional similarities of calcium homeostasis modulator 1 (CALHM1) ion channel with connexins, pannexins, and innexins

CALHM1 (calcium homeostasis modulator 1) forms a plasma membrane ion channel that mediates neuronal excitability in response to changes in extracellular Ca(2+) concentration. Six human CALHM homologs exist with no homology to other proteins, although CALHM1 is conserved across >20 species. Here we demonstrate that CALHM1 shares functional and quaternary and secondary structural similarities with connexins and evolutionarily distinct innexins and their vertebrate pannexin homologs. A CALHM1 channel is a hexamer, comprised of six monomers, each of which possesses four transmembrane domains, cytoplasmic amino and carboxyl termini, an amino-terminal helix, and conserved extracellular cysteines. The estimated pore diameter of the CALHM1 channel is ∼14 Å, enabling permeation of large charged molecules. Thus, CALHMs, connexins, and pannexins and innexins are structurally related protein families with shared and distinct functional properties.

Intercellular Ca(2+) waves: mechanisms and function

Intercellular calcium (Ca(2+)) waves (ICWs) represent the propagation of increases in intracellular Ca(2+) through a syncytium of cells and appear to be a fundamental mechanism for coordinating multicellular responses. ICWs occur in a wide diversity of cells and have been extensively studied in vitro. More recent studies focus on ICWs in vivo. ICWs are triggered by a variety of stimuli and involve the release of Ca(2+) from internal stores. The propagation of ICWs predominately involves cell communication with internal messengers moving via gap junctions or extracellular messengers mediating paracrine signaling. ICWs appear to be important in both normal physiology as well as pathophysiological processes in a variety of organs and tissues including brain, liver, retina, cochlea, and vascular tissue. We review here the mechanisms of initiation and propagation of ICWs, the key intra- and extracellular messengers (inositol 1,4,5-trisphosphate and ATP) mediating ICWs, and the proposed physiological functions of ICWs.

Pannexin protein expression in the rat middle cerebral artery

BACKGROUND

Connexin proteins are well known to participate in cell-to-cell communication within the cerebral vasculature. Pannexins are a recently discovered family of proteins that could potentially be involved in cell-to-cell communication. Herein, we sought to determine whether pannexins are expressed in rat middle cerebral artery (MCA).

METHODS

A combination of RT-PCR, immunoblotting and immunohistochemistry techniques was used to characterize the expression pattern of pannexins in rat MCA. A fluorescent dye uptake approach in cultured smooth muscle cells was used to determine whether these cells have functional hemichannels.

RESULTS

We report for the first time that pannexins are expressed in the cerebral vasculature. We reveal that pannexin 1 is expressed in smooth muscle but not in endothelium and pannexin 2 is expressed in both endothelium and smooth muscle. Fluorescent dye entered cultured smooth muscle cells in the absence of extracellular calcium or when the cells were depolarized, which was prevented by the putative hemichannel blocker carbenoxolone.

CONCLUSIONS

The identification of pannexins in rat MCA indicates that pannexin expression is not restricted to neuronal cells. Dye uptake in cultured smooth muscle cells exhibited properties similar to those of connexin and pannexin hemichannels, which may represent another form of cell-to-cell communication within the vasculature.

Mechanism of inhibition of connexin channels by the quinine derivative N-benzylquininium

The anti-malarial drug quinine and its quaternary derivative N-benzylquininium (BQ(+)) have been shown to inhibit gap junction (GJ) channels with specificity for Cx50 over its closely related homologue Cx46. Here, we examined the mechanism of BQ(+) action using undocked Cx46 and Cx50 hemichannels, which are more amenable to analyses at the single-channel level. We found that BQ(+) (300 µM-1 mM) robustly inhibited Cx50, but not Cx46, hemichannel currents, indicating that the Cx selectivity of BQ(+) is preserved in both hemichannel and GJ channel configurations. BQ(+) reduced Cx50 hemichannel open probability (P(o)) without appreciably altering unitary conductance of the fully open state and was effective when added from either extracellular or cytoplasmic sides. The reductions in P(o) were dependent on BQ(+) concentration with a Hill coefficient of 1.8, suggesting binding of at least two BQ(+) molecules. Inhibition by BQ(+) was voltage dependent, promoted by hyperpolarization from the extracellular side and conversely by depolarization from the cytoplasmic side. These results are consistent with binding of BQ(+) in the pore. Substitution of the N-terminal (NT) domain of Cx46 into Cx50 significantly impaired inhibition by BQ(+). The NT domain contributes to the formation of the wide cytoplasmic vestibule of the pore and, thus, may contribute to the binding of BQ(+). Single-channel analyses showed that BQ(+) induced transitions that did not resemble pore block, but rather transitions indistinguishable from the intrinsic gating events ascribed to loop gating, one of two mechanisms that gate Cx channels. Moreover, BQ(+) decreased mean open time and increased mean closed time, indicating that inhibition consists of an increase in hemichannel closing rate as well as a stabilization of the closed state. Collectively, these data suggest a mechanism of action for BQ(+) that involves modulation loop gating rather than channel block as a result of binding in the NT domain.

Pathological hemichannels associated with human Cx26 mutations causing Keratitis-Ichthyosis-Deafness syndrome

Connexin (Cx) proteins form intercellular gap junction channels by first assembling into single membrane hemichannels that then dock to connect the cytoplasm of two adjacent cells. Gap junctions are highly specialized structures that allow the direct passage of small molecules between cells to maintain tissue homeostasis. Functional activity of nonjunctional hemichannels has now been shown in several experimental systems. Hemichannels may constitute an important diffusional exchange pathway with the extracellular space, but the extent of their normal physiological role is currently unknown. Aberrant hemichannel activity has been linked to mutations of connexin proteins involved in genetic diseases. Here, we review a proposed role for hemichannels in the pathogenesis of Keratitis-Ichthyosis-Deafness (KID) syndrome associated with connexin26 (Cx26) mutations. Continued functional evaluation of mutated hemichannels linked to human hereditary disorders may provide additional insights into the mechanisms governing their regulation in normal physiology and dysregulation in disease. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Nature of plasmalemmal functional "hemichannels"

The molecular identity of the protein forming "hemichannels" at non-junctional membranes is disputed. The family of gap junction proteins, innexins, connexins, and pannexins share several common features, including permeability characteristics and sensitivity to blocking agents. Such overlap in properties renders the identification of which of these protein species actually establishes the non-junctional membrane conductance and permeability quite complicated, especially because in vertebrates pannexins and connexins have largely overlapping distributions in tissues. Recently, attempts to establish criteria to identify events that are "hemichannel" mediated and those to allow the distinction between connexin- from pannexin-mediated events have been proposed. Here, I present an update on that topic and discuss the most recent findings related to the nature of functional "hemichannels" focusing on connexin43 and pannexin1. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

The potency of the fs260 connexin43 mutant to impair keratinocyte differentiation is distinct from other disease-linked connexin43 mutants

Although there are currently 62 mutants of Cx43 (connexin43) that can cause ODDD (oculodentodigital dysplasia), only two mutants have also been reported to cause palmar plantar hyperkeratosis. To determine how mutants of Cx43 can lead to this skin disease, REKs (rat epidermal keratinocytes) were engineered to express an ODDD-associated Cx43 mutant always linked to skin disease (fs260), an ODDD-linked Cx43 mutant which has been reported to sometimes cause skin disease (fs230), Cx43 mutants which cause ODDD only (G21R, G138R), a mouse Cx43 mutant linked to ODDD (G60S), a non-disease-linked truncated Cx43 mutant that is trapped in the endoplasmic reticulum (Δ244*) or full-length Cx43. When grown in organotypic cultures, of all the mutants investigated, only the fs260-expressing REKs consistently developed a thinner stratum corneum and expressed lower levels of Cx43, Cx26 and loricrin in comparison with REKs overexpressing wild-type Cx43. REKs expressing the fs260 mutant also developed a larger organotypic vital layer after acetone-induced injury and exhibited characteristics of parakeratosis. Collectively, our results suggest that the increased skin disease burden exhibited in ODDD patients harbouring the fs260 mutant is probably due to multiple additive effects cause by the mutant during epidermal differentiation.

Intramolecular loop/tail interactions are essential for connexin 43-hemichannel activity

Connexin-assembled gap junctions (GJs) and hemichannels coordinate intercellular signaling processes. Although the regulation of connexins in GJs has been well characterized, the molecular determinants controlling connexin-hemichannel activity are unresolved. Here we investigated the regulation of Cx43-hemichannel activity by actomyosin contractility and intracellular [Ca(2+)] ([Ca(2+)](i)) using plasma membrane-permeable TAT peptides (100 μM) designed to interfere with interactions between the cytoplasmic loop (CL) and carboxy-terminal (CT) in primary bovine corneal endothelial cells and HeLa, C6 glioma, and Xenopus oocytes ectopically expressing Cx43. Peptides corresponding to the last 10 CT aa (TAT-Cx43CT) prevented the inhibition of Cx43-hemichannel activity by contractility/high [Ca(2+)](i), whereas a reverse peptide (TAT-Cx43CTrev) did not. These effects were independent of zonula occludens-1, a cytoskeletal-associated Cx43-binding protein. In contrast, peptides corresponding to CL (TAT-L2) inhibited Cx43-hemichannel responses, whereas a mutant peptide (TAT-L2(H126K/I130N)) did not inhibit. In these assays, TAT-Cx43CT acted as a scaffold for TAT-L2 and vice versa, a finding supported by surface plasmon resonance measurements. Loop/tail interactions appeared essential for Cx43-hemichannel activity, because TAT-Cx43CT restored the activity of nonfunctional hemichannels, consisting of either Cx43 lacking the C-terminal tail (Cx43(M239)) or intact Cx43 ectopically expressed in Xenopus oocytes. We conclude that intramolecular loop/tail interactions control Cx43-hemichannel activity, laying the basis for developing hemichannel-specific blockers.

Mefloquine blockade of Pannexin1 currents: resolution of a conflict

The authors' laboratory has reported potent block of Pannexin1 (Panx1) currents by the antimalarial quinine derivative mefloquine. However, other laboratories have found little or no mefloquine sensitivity of Panx1 currents or processes attributable to these channels. In order to resolve this issue, the authors have performed extensive dose-response studies on Panx1-transfected neuroblastoma (Neuro2A) and rat insulinoma (Rin) cells, comparing mefloquine obtained from three suppliers and also comparing the sensitivity to diastereomers. Results indicate a 20-fold difference in sensitivity to the (-)-threo-(11R/2R) diastereomer compared to the erythro enatiomers and much lower potency of (+/-)-erythro-(R*/S*)-mefloquine obtained from one of the commercial sources. This markedly lower efficacy presumably accounts for the disparity in results from different laboratories who have applied it in Panx1 studies.

Regulation of neuronal connexin-36 channels by pH

Neurotransmission through electrical synapses plays an important role in the spike synchrony among neurons and oscillation of neuronal networks. Indeed, electrical transmission has been implicated in the hypersynchronous electrical activity of epilepsy. We have investigated the influence of intracellular pH on the strength of electrical coupling mediated by connexin36 (Cx36), the principal gap junction protein in the electrical synapses of vertebrates. In striking contrast to other connexin isoforms, the activity of Cx36 channels decreases following alkalosis rather than acidosis when it is expressed in Xenopus oocytes and N2A cells. This uncoupling of Cx36 channels upon alkalinization occurred in the vertebrate orthologues analyzed (human, mouse, chicken, perch, and skate). While intracellular acidification caused a mild or moderate increase in the junctional conductance of virtually all these channels, the coupling of the skate Cx35 channel was partially blocked by acidosis. The mutational analysis suggests that the Cx36 channels may contain two gating mechanisms operating with opposing sensitivity to pH. One gate, the dominant mechanism, closes for alkalosis and it probably involves an interaction between the C- and N-terminal domains, while a secondary acid sensing gate only causes minor, albeit saturating, changes in coupling following acidosis and alkalosis. Thus, we conclude that neuronal Cx36 channels undergo unique regulation by pH(i) since their activity is inhibited by alkalosis rather than acidosis. These data provide a novel basis to define the relevance and consequences of the pH-dependent modulation of Cx36 synapses under physiological and pathological conditions.

CNQX and AMPA inhibit electrical synaptic transmission: a potential interaction between electrical and glutamatergic synapses

Electrical synapses play an important role in signaling between neurons and the synaptic connections between many neurons possess both electrical and chemical components. Although modulation of electrical synapses is frequently observed, the cellular processes that mediate such changes have not been studied as thoroughly as plasticity in chemical synapses. In the leech (Hirudo sp), the competitive AMPA receptor antagonist CNQX inhibited transmission at the rectifying electrical synapse of a mixed glutamatergic/electrical synaptic connection. This CNQX-mediated inhibition of the electrical synapse was blocked by concanavalin A (Con A) and dynamin inhibitory peptide (DIP), both of which are known to inhibit endocytosis of neurotransmitter receptors. CNQX-mediated inhibition was also blocked by pep2-SVKI (SVKI), a synthetic peptide that prevents internalization of AMPA-type glutamate receptor. AMPA itself also inhibited electrical synaptic transmission and this AMPA-mediated inhibition was partially blocked by Con A, DIP and SVKI. Low frequency stimulation induced long-term depression (LTD) in both the electrical and glutamatergic components of these synapses and this LTD was blocked by SVKI. GYKI 52466, a selective non-competitive antagonist of AMPA receptors, did not affect the electrical EPSP, although it did block the glutamatergic component of these synapses. CNQX did not affect non-rectifying electrical synapses in two different pairs of neurons. These results suggest an interaction between AMPA-type glutamate receptors and the gap junction proteins that mediate electrical synaptic transmission. This putative interaction between glutamate receptors and gap junction proteins represents a novel mechanism for regulating the strength of synaptic transmission.

Possible involvement of different connexin43 domains in plasma membrane permeabilization induced by ischemia-reperfusion

In vitro and in vivo studies support the involvement of connexin 43-based cell-cell channels and hemichannels in cell death propagation induced by ischemia-reperfusion. In this context, open connexin hemichannels in the plasma membrane have been proposed to act as accelerators of cell death. Progress on the mechanisms underlying the cell permeabilization induced by ischemia-reperfusion reveals the involvement of several factors leading to an augmented open probability and increased number of hemichannels on the cell surface. While open probability can be increased by a reduction in extracellular concentration of divalent cations and changes in covalent modifications of connexin 43 (oxidation and phosphorylation), increase in number of hemichannels requires an elevation of the intracellular free Ca(2+) concentration. Reversal of connexin 43 redox changes and membrane permeabilization can be induced by intracellular, but not extracellular, reducing agents, suggesting a cytoplasmic localization of the redox sensor(s). In agreement, hemichannels formed by connexin 45, which lacks cytoplasmic cysteines, or by connexin 43 with its C-terminal domain truncated to remove its cysteines are insensitive to reducing agents. Although further studies are required for a precise localization of the redox sensor of connexin 43 hemichannels, modulation of the redox potential is proposed as a target for the design of pharmacological tools to reduce cell death induced by ischemia-reperfusion in connexin 43-expressing cells.

Molecular basis of voltage dependence of connexin channels: An integrative appraisal

The importance of electrical and molecular signaling through connexin (Cx) channels is now widely recognized. The transfer of ions and other small molecules between adjacent cells is regulated by multiple stimuli, including voltage. Indeed, Cx channels typically exhibit complex voltage sensitivity. Most channels are sensitive to the voltage difference between the cell interiors (or transjunctional voltage, V(j)), while other channels are also sensitive to absolute inside-outside voltage (i.e., the membrane potential, V(m)). The first part of this review is focused on the description of the distinct forms of voltage sensitivity and the gating mechanisms that regulate hemichannel activity, both individually and as components of homotypic and heterotypic gap junctions. We then provide an up to date and precise picture of the molecular and structural aspects of how V(j) and V(m) are sensed, and how they, therefore, control channel opening and closing. Mutagenic strategies coupled with structural, biochemical and electrophysical studies are providing significant insights into how distinct forms of voltage dependence are brought about. The emerging picture indicates that Cx channels can undergo transitions between multiple conductance states driven by distinct voltage-gating mechanisms. Each hemichannel may contain a set of two V(j) gates, one fast and one slow, which mediate the transitions between the main open state to the residual state and to the fully closed state, respectively. Eventually, a V(m) gate regulates channel transitions between the open and closed states. Clusters of charged residues within separate domains of the Cx molecule have been identified as integral parts of the V(j) and V(m) sensors. The charges at the first positions of the amino terminal cytoplasmic domain determine the magnitude and polarity of the sensitivity to fast V(j)-gating, as well as contributing to the V(j)-rectifying properties of ion permeation. Additionally, important advances have been made in identifying the conformational rearrangements responsible for fast V(j)-gating transitions to the residual state in the Cx43 channel. These changes involve an intramolecular particle-receptor interaction between the carboxy terminal domain and the cytoplasmic loop.

Species specificity of mammalian connexin-26 to form open voltage-gated hemichannels

Mutations of connexin-26 (Cx26) cause nonsyndromic hearing loss and other syndromes affecting ectoderm-derived tissues. While the exact mechanisms underlying these diseases remain elusive, Cx's are generally considered to mediate cell-to-cell communication by forming gap junction channels. We show here that unlike rat Cx26, human and sheep Cx26 form voltage-gated hemichannels when expressed in oocytes and Neuro2A cells. A single evolutionary amino acidic change at position 159 of the rodent protein, the replacement of aspartic acid with asparagine in the human and sheep proteins, accounts for this species specificity. At the resting potential and in normal millimolar extracellular calcium, open human Cx26 hemichannels can be detected both electrophysiologically and by dye uptake, although they did not affect cell viability. These hemichannels opened at approximately -50 mV and their activation increased by depolarization until they inactivate at positive membrane potentials. Single-channel analysis revealed that activation and inactivation involved two distinct voltage gating mechanisms and that the fully open hemichannel displays a conductance twice that of the intercellular channel. The existence of a hemichannel that opens under physiological control of the membrane potential may have important implications for the normal and pathological activity of Cx26 in humans, particularly with respect to hearing and the epidermis.

Functional connexin "hemichannels": a critical appraisal

"Hemichannels" are defined as the halves of gap junction channels (also termed connexons) that are contributed by one cell; "hemichannels" are considered to be functional if they are open in nonjunctional membranes in the absence of pairing with partners from adjacent cells. Several recent reviews have summarized the blossoming literature regarding functional "hemichannels", in some cases encyclopedically. However, most of these previous reviews have been written with the assumption that all data reporting "hemichannel" involvement really have studied phenomena in which connexons actually form the permeability or conductance pathway. In this review, we have taken a slightly different approach. We review the concept of "hemichannels", summarize properties that might be expected of half gap junctions and evaluate the extent to which the properties of presumptive "hemichannels" match expectations. Then we consider functions attributed to hemichannels, provide an overview of other channel types that might fulfill similar roles and provide sets of criteria that might be applied to verify involvement of connexin hemichannels in cell and tissue function. One firm conclusion is reached. The study of hemichannels is technically challenging and fraught with opportunities for misinterpretation, so that future studies must apply rigorous standards for detection of hemichannel expression and function. At the same time there are reasons to expect surprises, including the possibility that some time honored techniques for studying gap junctions may prove unsuitable for detecting hemichannels. We advise hemichannel researchers to proceed with caution and an open mind.

Structure and function of gap junctions in the developing brain

Gap-junction-dependent neuronal communication is widespread in the developing brain, and the prevalence of gap-junctional coupling is well correlated with specific developmental events. We summarize here our current knowledge of the contribution of gap junctions to brain development and propose that they carry out this role by taking advantage of the full complement of their functional properties. Thus, hemichannel activation may represent a key step in the initiation of Ca(2+) waves that coordinate cell cycle events during early prenatal neurogenesis, whereas both hemichannels and/or gap junctions may control the division and migration of cohorts of precursor cells during late prenatal neurogenesis. Finally, the recent discovery that pannexins, a novel group of proteins prominently expressed in the brain, are able to form both hemichannels and gap-junction channels suggests that we need to seek more than just connexins with respect to these junctions.

Electrical synapses--gap junctions in the brain

In the nervous system, interneuronal communication can occur via indirect or direct transmission. The mode of indirect communication involves chemical synapses, in which transmitters are released into the extracellular space to subsequently bind to the postsynaptic cell membrane. Direct communication is mediated by electrical synapses, and will be the focus of this review. The most prevalent group of electrical synapses are neuronal gap junctions (both terms are used interchangeably in this article), which directly connect the intracellular space of two cells by gap junction channels. The structural components of gap junction channels in the nervous system are connexin proteins, and, as recently identified, pannexin proteins. Connexin gap junction channels enable the intercellular, bidirectional transport of ions, metabolites, second messengers and other molecules smaller than 1 kD. More than 20 connexin genes have been found in the mouse and human genome. With the cloning of connexin36 (Cx36), a connexin protein with predominantly neuronal expression, the biochemical correlate of electrotonic transmission between neurons was identified. We outline the distribution of Cx36 as well as two other neuronal connexins (Cx57 and Cx45) in the nervous system, describing their spatial and temporal expression patterns. One focus in this review was the retina, as it shows many and diverse electrical synapses whose connexin components have been identified in fish and mammals. In view of the function of neuronal gap junctions, the network of inhibitory interneurons will be reviewed in detail, focussing on the hippocampus. Although in vivo data on pannexin proteins are still restricted to information on mRNA expression, electrophysiological data and the expression pattern in the nervous system have been included.

Calcium-dependent binding of calmodulin to neuronal gap junction proteins

We examined the interactions of calmodulin with neuronal gap junction proteins connexin35 (Cx35) from perch, its mouse homologue Cx36, and the related perch Cx34.7 using surface plasmon resonance. Calmodulin bound to the C-terminal domains of all three connexins with rapid kinetics in a concentration- and Ca2+-dependent manner. Dissociation was also very rapid. K(d)'s for calmodulin binding at a high-affinity site ranged from 11 to 72 nM, and K(1/2)'s for Ca2+ were between 3 and 5 microM. No binding to the intracellular loops was observed. Binding competition experiments with synthetic peptides mapped the calmodulin binding site to a 10-30 amino acid segment at the beginning of the C-terminal domain of Cx36. The micromolar K(1/2)'s and rapid on and off rates suggest that this interaction may change dynamically in neurons, and may occur transiently when Ca2+ is elevated to a level that would occur in the near vicinity of an activated synapse.

Physiological properties of rod photoreceptor electrical coupling in the tiger salamander retina

Using dual whole-cell voltage and current clamp recording techniques, we investigated the gap junctional conductance and the coupling coefficient between neighbouring rods in live salamander retinal slices. The application of sinusoidal stimuli over a wide range of temporal frequencies allowed us to characterize the band-pass filtering properties of the rod network. We found that the electrical coupling of all neighbouring rods exhibited reciprocal and symmetrical conductivities. On average, the junctional conductance between paired rods was 500 pS and the coupling coefficient (the ratio of voltage responses of the follower cell to those of the driver cell), or K-value, was 0.07. Our experimental results also demonstrated that the rod network behaved like a band-pass filter with a peak frequency of about 2-5 Hz. However, the gap junctions between adjacent rods exhibited linearity and voltage independency within the physiological range of rods. These gap junctions did not contribute to the filtering mechanisms of the rod network. Combined with the computational modelling, our data suggest that the filtering of higher frequency rod signals by the network is largely mediated by the passive resistive and capacitive (RC) properties of rod plasma membranes. Furthermore, we found several attributes of rod electrical coupling resembling the physiological properties of gene-encoded Cx35/36 gap junctions examined in other in vitro studies. This indicates that the previously found Cx35/36 expression in the salamander rod network may be functionally involved in rod-rod electrical coupling.

Role of connexin-based gap junction channels and hemichannels in ischemia-induced cell death in nervous tissue

Gap junction channels and hemichannels formed of connexin subunits are found in most cell types in vertebrates. Gap junctions connect cells via channels not open to the extracellular space and permit the passage of ions and molecules of approximately 1 kDa. Single connexin hemichannels, which are connexin hexamers, are present in the surface membrane before docking with a hemichannel in an apposed membrane. Because of their high conductance and permeability in cell-cell channels, it had been thought that connexin hemichannels remained closed until docking to form a cell-cell channel. Now it is clear that at least some hemichannels can open to allow passage of molecules between the cytoplasm and extracellular space. Here we review evidence that gap junction channels may allow intercellular diffusion of necrotic or apoptotic signals, but may also allow diffusion of ions and substances from healthy to injured cells, thereby contributing to cell survival. Moreover, opening of gap junction hemichannels may exacerbate cell injury or mediate paracrine or autocrine signaling. In addition to the cell specific features of an ischemic insult, propagation of cell damage and death within affected tissues may be affected by expression and regulation of gap junction channels and hemichannels formed by connexins.

Connexin-based gap junction hemichannels: gating mechanisms

Connexins (Cxs) form hemichannels and gap junction channels. Each gap junction channel is composed of two hemichannels, also termed connexons, one from each of the coupled cells. Hemichannels are hexamers assembled in the ER, the Golgi, or a post Golgi compartment. They are transported to the cell surface in vesicles and inserted by vesicle fusion, and then dock with a hemichannel in an apposed membrane to form a cell-cell channel. It was thought that hemichannels should remain closed until docking with another hemichannel because of the leak they would provide if their permeability and conductance were like those of their corresponding cell-cell channels. Now it is clear that hemichannels formed by a number of different connexins can open in at least some cells with a finite if low probability, and that their opening can be modulated under various physiological and pathological conditions. Hemichannels open in different kinds of cells in culture with conductance and permeability properties predictable from those of the corresponding gap junction channels. Cx43 hemichannels are preferentially closed in cultured cells under resting conditions, but their open probability can be increased by the application of positive voltages and by changes in protein phosphorylation and/or redox state. In addition, increased activity can result from the recruitment of hemichannels to the plasma membrane as seen in metabolically inhibited astrocytes. Mutations of connexins that increase hemichannel open probability may explain cellular degeneration in several hereditary diseases. Taken together, the data indicate that hemichannels are gated by multiple mechanisms that independently or cooperatively affect their open probability under physiological as well as pathological conditions.

Cloning and functional expression of invertebrate connexins from Halocynthia pyriformis

Unlike many other ion channels, unrelated gene families encode gap junctions in different animal phyla. Connexin and pannexin genes are found in deuterostomes, while protostomal species use innexin genes. Connexins are often described as vertebrate genes, despite the existence of invertebrate deuterostomes. We have cloned connexin sequences from an invertebrate chordate, Halocynthia pyriformis. Invertebrate connexins shared 25-40% sequence identity with human connexins, had extracellular domains containing six invariant cysteine residues, coding regions that were interrupted by introns, and formed functional channels in vitro. These data show that gap junction channels based on connexins are present in animals that predate vertebrate evolution.

Pharmacological properties of homomeric and heteromeric pannexin hemichannels expressed in Xenopus oocytes

Several new findings have emphasized the role of neuron-specific gap junction proteins (connexins) and electrical synapses in processing sensory information and in synchronizing the activity of neuronal networks. We have recently shown that pannexins constitute an additional family of proteins that can form gap junction channels in a heterologous expression system and are also widely expressed in distinct neuronal populations in the brain, where they may represent a novel class of electrical synapses. In this study, we have exploited the hemichannel-forming properties of pannexins to investigate their sensitivity to well-known connexin blockers. By combining biochemical and electrophysiological approaches, we report here further evidence for the interaction of pannexin1 (Px1) with Px2 and demonstrate that the pharmacological sensitivity of heteromeric Px1/Px2 is similar to that of homomeric Px1 channels. In contrast to most connexins, both Px1 and Px1/Px2 hemichannels were not gated by external Ca2+. In addition, they exhibited a remarkable sensitivity to blockade by carbenoxolone (with an IC50 of approximately 5 microm), whereas flufenamic acid exerted only a modest inhibitory effect. The opposite was true in the case of connexin46 (Cx46), thus indicating that gap junction blockers are able to selectively modulate pannexin and connexin channels.

Fine-structural analysis and connexin expression in the retina of a transgenic model of Huntington's disease

Recent studies indicate that the visual system appears more frequently affected in polyglutamine diseases than expected previously. Here, we investigated retinal degenerations in adult transgenic R6/2 mice, a model for Huntington's disease (HD). Light microscopical analysis revealed retinal dystrophy all over the retina, with central areas showing major effects. Electron microscopical analysis showed strong degenerations of outer and inner photoreceptor segments, shrinkage of photoreceptor cell somata, and signs of degeneration in photoreceptor terminals in the outer plexiform layer. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling showed hints of apoptosis. Mutant huntingtin and ubiquitin were expressed in all classes of retinal neurons, the pigment epithelium, and to a minor extent in neuropil structures. For investigating possible links to functional impairments in the rod-cone pathway, expression levels of three connexins (Cx) were compared in R6/2 and wildtype mice retinae. In R6/2 mice, expression of Cx36, the major neuronal connexin in the retina, was slightly reduced in the outer plexiform layer, indicating affected photoreceptor terminals as detected at the electron microscopical level. In contrast, Cx45, a putative neuronal connexin in the retina, was remarkably reduced in the inner plexiform layer of R6/2 mice. This result corresponded to fainter signals of Cx45 mRNA as documented by in situ hybridization and to a lower level of mCx45 cDNA as obtained by polymerase chain reaction after reverse transcription, suggesting functional deficits in spatial processing of Cx45-mediated gap junction coupling due to transgene-induced retinal degenerations. Thus, it is important to clarify the meaning of visual involvement in HD.

Correlative studies of gating in Cx46 and Cx50 hemichannels and gap junction channels

Transjunctional voltage (V(j)) gating of gap junction (GJ) channels formed of connexins has been proposed to occur by gating of the component hemichannels. We took advantage of the ability of Cx46 and Cx50 to function as unapposed hemichannels to identify gating properties intrinsic to hemichannels and how they contribute to gating of GJ channels. We show that Cx46 and Cx50 hemichannels contain two distinct gating mechanisms that generate reductions in conductance for both membrane polarities. At positive voltages, gating is similar in Cx46 and Cx50 hemichannels, primarily showing increased transitioning to long-lived substates. At negative voltages, Cx46 currents deactivate completely and the underlying single hemichannels exhibit transitions to a fully closed state. In contrast, Cx50 currents do not deactivate completely at negative voltages and the underlying single hemichannels predominantly exhibit transitions to various substates. Transitions to a fully closed state occur, but are infrequent. In the respective GJ channels, both forms of gating contribute to the reduction in conductance by V(j). However, examination of gating of mutant hemichannels and GJ channels in which the Asp at position 3 was replaced with Asn (D3N) showed that the positive hemichannel gate predominantly closes Cx50 GJs, whereas the negative hemichannel gate predominantly closes Cx46 GJs in response to V(j). We also report, for the first time, single Cx50 hemichannels in oocytes to be inwardly rectifying, high conductance channels (gamma = 470 pS). The antimalarial drug mefloquine, which selectively blocks Cx50 and not Cx46 GJs, shows the same selectivity in Cx50 and Cx46 hemichannels indicating that the actions of such uncoupling agents, like voltage gating, are intrinsic hemichannel properties.

Dendrodendritic electrical synapses between mammalian retinal ganglion cells

Electrical synapses between alpha-type ganglion cells were detected using combined techniques of dual patch-clamp recordings, intracellular labeling, electron microscopy, and channel subunit connexin immunocytochemistry in the albino rat retina. After intracellular injection of Neurobiotin into alpha-cells of inner (ON-center) and outer (OFF-center) ramifying types, measurement of tracer coupling resulted in a preferentially homologous occurrence among cells of the same morphological type (n = 19 of 24). In high-voltage as well as conventional electron microscopic analysis, direct dendrodendritic gap junctions (average size, 0.86 mum long) were present in contact sites between tracer-coupled alpha-cells. In simultaneous dual whole-cell recordings from pairs of neighboring alpha-cells, these cells generated TTX-sensitive sustained spiking against extrinsic current injection, and bidirectional electrical synapses (maximum coupling coefficient, 0.32) with symmetrical junction conductance (average, 1.35 nS) were observed in pairs with cells of the same morphological type. Precise temporal synchronization of spike activity (average time delay, 2.7 msec) was detected when depolarizing currents were simultaneously injected into the pairs. To address whether physiologically identified electrical synapses constitute gap junctional connectivity between cell pairs, identified neuronal connexin36 immunoreactivity was undertaken in Lucifer yellow-labeled cell pairs after patch-clamp recordings. All alpha-cells expressed connexin36, and confocal laser-scanning imaging demonstrated that connexin36 is primarily located at dendritic crossings between electrically coupled cells (seven sites in a pair, on average). These results give conclusive evidence for electrical synapses via dendrodendritic gap junctions involving connexin36 in alpha retinal ganglion cells of the same physiological type.

Biophysical characterization of zebrafish connexin35 hemichannels

A subset of connexins can form unopposed hemichannels in expression systems, providing an opportunity for comparison of hemichannel gating properties with those of intact gap junction channels. Zebrafish connexin35 (Cx35) is a member of the Cx35/Cx36 subgroup of connexins highly expressed in the retina and brain. In the present study, we have shown that Cx35 expression in Xenopus oocytes and N2A cells produced large outward whole cell currents on cell depolarization. Using whole cell, cell-attached, and excised patch configurations, we obtained multichannel and single-channel current recordings attributable to the Cx35 hemichannels (I(hc)) that were activated and increased by stepwise depolarization of membrane potential (V(m)) and deactivated by hyperpolarization. The currents were not detected in untransfected N2A cells or in control oocytes injected with antisense Cx38. However, water-injected oocytes that were not treated with antisense showed activities attributable to Cx38 hemichannels that were easily distinguishable from Cx35 hemichannels by a significantly larger unitary conductance (gamma(hc): 250-320 pS). The gamma(hc) of Cx35 hemichannels exhibited a pronounced V(m) dependence; i.e., gamma(hc) increased/decreased with relative hyperpolarization/depolarization (gamma(hc) was 72 pS at V(m) = -100 mV and 35 pS at V(m) = 100 mV). Extrapolation to V(m) = 0 mV predicted a gamma(hc) of 48 pS, suggesting a unitary conductance of intact Cx35 gap junction channels of approximately 24 pS. Channel gating was also V(m) dependent: open time declined with negative V(m) and increased with positive V(m). The ability to break down the complex gating of intact intercellular channels into component hemichannels in vitro will help to evaluate putative physiological roles for hemichannels in vivo.

Electrical synapses: a dynamic signaling system that shapes the activity of neuronal networks

Gap junctions consist of intercellular channels dedicated to providing a direct pathway for ionic and biochemical communication between contacting cells. After an initial burst of publications describing electrical coupling in the brain, gap junctions progressively became less fashionable among neurobiologists, as the consensus was that this form of synaptic transmission would play a minimal role in shaping neuronal activity in higher vertebrates. Several new findings over the last decade (e.g. the implication of connexins in genetic diseases of the nervous system, in processing sensory information and in synchronizing the activity of neuronal networks) have brought gap junctions back into the spotlight. The appearance of gap junctional coupling in the nervous system is developmentally regulated, restricted to distinct cell types and persists after the establishment of chemical synapses, thus suggesting that this form of cell-cell signaling may be functionally interrelated with, rather than alternative to chemical transmission. This review focuses on gap junctions between neurons and summarizes the available data, derived from molecular, biological, electrophysiological, and genetic approaches, that are contributing to a new appreciation of their role in brain function.

Mechanism of regulation of the gap junction protein connexin 43 by protein kinase C-mediated phosphorylation

Phosphorylation of the gap junction protein connexin 43 (Cx43) by protein kinase C (PKC) decreases dye coupling in many cell types. We report an investigation of the regulation by PKC of Cx43 gap junctional hemichannels (GJH) expressed in Xenopus laevis oocytes. The activity of GJH was assessed from the uptake of hydrophilic fluorescent probes. PKC inhibitors increased probe uptake in isolated oocytes expressing recombinant Cx43, indicating that the regulatory effect occurs at the hemichannel level. We identified by mutational analysis the carboxy-terminal (CT) domain sequences involved in this response. We found that 1) Ser368 is responsible for the regulation of Cx43 GJH solute permeability by PKC-mediated phosphorylation, 2) CT domain residues 253-270 and 288-359 are not necessary for the effect of PKC, and 3) the prolinerich CT region is not involved in the effect of phosphorylation by PKC. Our results demonstrate that Ser368 (but not Ser372) is involved in the regulation of Cx43 solute permeability by PKC-mediated phosphorylation, and we conclude that different molecular mechanisms underlie the regulation of Cx43 by intracellular pH and PKC-mediated phosphorylation.

Connexin35/36 gap junction proteins are expressed in photoreceptors of the tiger salamander retina

Photoreceptors in the vertebrate retina are electrically coupled with one another. Such coupling plays important roles in visual information processing. Physiological properties of rod-rod and rod-cone coupling have been best studied in the salamander retina, yet the cellular and molecular basis of these electrical synapses has not been established. Recently, connexin35/36 (Cx35/36) gap junction proteins were found to be highly expressed in brain and retina, suggesting that it may mediate photoreceptor coupling. To test this idea, we examined the cellular distribution of Cx35/36 in the salamander retina. Western blot analysis showed the expression of Cx35/36 proteins, and confocal microscopy revealed characteristic punctate Cx35/36 immunoreactivity in both synaptic layers. In addition, Cx35/36-positive plaques were detected in the outer nuclear layer (ONL) between neighboring rods, and these plaques outlined the mosaic of the rod network at a level distal to the external limiting membrane. Moreover, although Cx35/36 plaques were detected between some cones and their adjacent rods, the number and size of these plaques was smaller, and their staining intensity was diminished compared with the plaques between adjacent rods. Furthermore, Lucifer yellow injection together with confocal microscopy revealed that Cx35/36-puncta were colocalized with finlike structures of rod cell membrane, with the ultrastructure of gap junctions between paired rod fins having been found by electron microscopy. Therefore, our findings demonstrate that Cx35/36 expression in photoreceptors is primarily located between rods and to a lesser extent between rods and cones, suggesting that Cx35/36 may participate in electrical coupling between rods and between rods and cones in the salamander retina.

Molecular Cloning and Functional Expression of zfCx52.6

Gap junction-mediated electrical coupling contributes to synchronous oscillatory activities of neurons, and considerable progress has been made in defining the molecular composition of gap junction channels. In particular, cloning and functional expression of gap junction proteins (connexins (Cx)) from zebrafish retina have shown that this part of the brain possesses a high degree of connexin diversity that may account for differential functional properties of electrical synapses. Here, we report the cloning and functional characterization of a new connexin, designated zebrafish Cx52.6 (zfCx52.6). This connexin shows little similarity to known connexins from fish and higher vertebrates. By combining in situ hybridization with Laser Capture Microdissection and RT-PCR, we found that this novel fish connexin is expressed in horizontal cells in the inner nuclear layer of the retina. Functional expression of zfCx52.6 in neuroblastoma cells and Xenopus oocytes led to functional gap junctional channels and, in single oocytes, induced large non-junctional membrane currents indicative of the formation of hemichannels, which were inhibited in reversible fashion by raising extracellular Ca(2+) concentrations.

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

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

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Pannexins, a family of gap junction proteins expressed in brain

Database search has led to the identification of a family of proteins, the pannexins, which share some structural features with the gap junction forming proteins of invertebrates and vertebrates. The function of these proteins has remained unclear so far. To test the possibility that pannexins underlie electrical communication in the brain, we have investigated their tissue distribution and functional properties. Here, we show that two of these genes, pannexin 1 (Px1) and Px2, are abundantly expressed in the CNS. In many neuronal cell populations, including hippocampus, olfactory bulb, cortex and cerebellum, there is coexpression of both pannexins, whereas in other brain regions, e.g., white matter, only Px1-positive cells were found. On expression in Xenopus oocytes, Px1, but not Px2 forms functional hemichannels. Coinjection of both pannexin RNAs results in hemichannels with functional properties that are different from those formed by Px1 only. In paired oocytes, Px1, alone and in combination with Px2, induces the formation of intercellular channels. The functional characteristics of homomeric Px1 versus heteromeric Px1/Px2 channels and the different expression patterns of Px1 and Px2 in the brain indicate that pannexins form cell type-specific gap junctions with distinct properties that may subserve different functions.

Zebrafish Cx35: cloning and characterization of a gap junction gene highly expressed in the retina

The vertebrate connexin gene family encodes protein subunits of gap junction channels, which provide a route for direct intercellular communication. Consequently, gap junctions play a vital role in many developmental and homeostatic processes. Aberrant functioning of gap junctions is implicated in many human diseases. Zebrafish are an ideal vertebrate model to study development of the visual system as they produce transparent embryos that develop rapidly, thereby facilitating morphological and behavioral testing. In this study, zebrafish connexin35 has been cloned from a P1 artificial chromosome (PAC) library. Sequence analysis shows a high degree of similarity to the Cx35/36 orthologous group, which are expressed primarily in nervous tissue, including the retina. The gene encodes a 304-amino acid protein with a predicted molecular weight of approximately 35 kDa. Injection of zebrafish Cx35 RNA into paired Xenopus oocytes elicited intercellular electrical coupling with weak voltage sensitivity. In development, Cx35 is first detectable by Northern analysis and RT-PCR, at 2 days post-fertilization (2 dpf), and in the adult it is expressed in the brain and retina. Immunohistochemical analysis revealed that the Cx35 protein is expressed in two sublaminae of the inner plexiform layer of the adult retina. A similar pattern was seen in the 4 and 5 dpf retina, but no labeling was detected in the retina of earlier embryos.