Connexin channels and hemichannels are modulated differently by charge reversal at residues forming the intracellular pocket

BACKGROUND

Members of the β-subfamily of connexins contain an intracellular pocket surrounded by amino acid residues from the four transmembrane helices. The presence of this pocket has not previously been investigated in members of the α-, γ-, δ-, and ε-subfamilies. We studied connexin50 (Cx50) as a representative of the α-subfamily, because its structure has been determined and mutations of Cx50 are among the most common genetic causes of congenital cataracts.

METHODS

To investigate the presence and function of the intracellular pocket in Cx50 we used molecular dynamics simulation, site-directed mutagenesis, gap junction tracer intercellular transfer, and hemichannel activity detected by electrophysiology and by permeation of charged molecules.

RESULTS

Employing molecular dynamics, we determined the presence of the intracellular pocket in Cx50 hemichannels and identified the amino acids participating in its formation. We utilized site-directed mutagenesis to alter a salt-bridge interaction that supports the intracellular pocket and occurs between two residues highly conserved in the connexin family, R33 and E162. Substitution of opposite charges at either position decreased formation of gap junctional plaques and cell-cell communication and modestly reduced hemichannel currents. Simultaneous charge reversal at these positions produced plaque-forming non-functional gap junction channels with highly active hemichannels.

CONCLUSIONS

These results show that interactions within the intracellular pocket influence both gap junction channel and hemichannel functions. Disruption of these interactions may be responsible for diseases associated with mutations at these positions.

Autocrine activation of P2X7 receptors mediates catecholamine secretion in chromaffin cells

BACKGROUND AND PURPOSE

ATP is highly accumulated in secretory vesicles and secreted upon exocytosis from neurons and endocrine cells. In adrenal chromaffin granules, intraluminal ATP reaches concentrations over 100 mM. However, how these large amounts of ATP contribute to exocytosis has not been investigated.

EXPERIMENTAL APPROACH

Exocytotic events in bovine and mouse adrenal chromaffin cells were measured with single cell amperometry. Cytosolic Ca2+ measurements were carried out in Fluo-4 loaded cells. Submembrane Ca2+ was examined in PC12 cells transfected with a membrane-tethered Ca2+ indicator Lck-GCaMP3. ATP release was measured using the luciferin/luciferase assay. Knockdown of P2X7 receptors was induced with short interfering RNA (siRNA). Direct Ca2+ influx through this receptor was measured using a P2X7 receptor-GCamp6 construct.

KEY RESULTS

ATP induced exocytosis in chromaffin cells, whereas the ectonucleotidase apyrase reduced the release events induced by the nicotinic agonist dimethylphenylpiperazinium (DMPP), high KCl, or ionomycin. The purinergic agonist BzATP also promoted a secretory response that was dependent on extracellular Ca2+. A740003, a P2X7 receptor antagonist, abolished secretory responses of these secretagogues. Exocytosis was also diminished in chromaffin cells when P2X7 receptors were silenced using siRNAs and in cells of P2X7 receptor knockout mice. In PC12 cells, DMPP induced ATP release, triggering Ca2+ influx through P2X7 receptors. Furthermore, BzATP, DMPP, and KCl allowed the formation of submembrane Ca2+ microdomains inhibited by A740003.

CONCLUSION AND IMPLICATIONS

Autocrine activation of P2X7 receptors constitutes a crucial feedback system that amplifies the secretion of catecholamines in chromaffin cells by favouring submembrane Ca2+ microdomains.

Correlative light and X-ray tomography jointly unveil the critical role of connexin43 channels on inflammation-induced cellular ultrastructural alterations

Non-junctional connexin43 (Cx43) plasma membrane hemichannels have been implicated in several inflammatory diseases, particularly playing a role in ATP release that triggers activation of the inflammasome. Therapies targeting the blocking of the hemichannels to prevent the pathological release or uptake of ions and signalling molecules through its pores are of therapeutic interest. To date, there is no close-to-native, high-definition documentation of the impact of Cx43 hemichannel-mediated inflammation on cellular ultrastructure, neither is there a robust account of the ultrastructural changes that occur following treatment with selective Cx43 hemichannel blockers such as Xentry-Gap19 (XG19). A combination of same-sample correlative high-resolution three-dimensional fluorescence microscopy and soft X-ray tomography at cryogenic temperatures, enabled in the identification of novel 3D molecular interactions within the cellular milieu when comparing behaviour in healthy states and during the early onset or late stages under inflammatory conditions. Notably, our findings suggest that XG19 blockage of connexin hemichannels under pro-inflammatory conditions may be crucial in preventing the direct degradation of connexosomes by lysosomes, without affecting connexin protein translation and trafficking. We also delineated fine and gross cellular phenotypes, characteristic of inflammatory insult or road-to-recovery from inflammation, where XG19 could indirectly prevent and reverse inflammatory cytokine-induced mitochondrial swelling and cellular hypertrophy through its action on Cx43 hemichannels. Our findings suggest that XG19 might have prophylactic and therapeutic effects on the inflammatory response, in line with functional studies.

Unnexin is a protein subunit of a large-pore channel expressed by unicellular organisms

Cells of vertebrate and invertebrate organisms express proteins specialized in membrane channel-based cell-cell communication that are absent in unicellular organisms. We recently described the prediction of some members of the large-pore channel family in kinetoplastids, consisting of proteins called unnexins, which share several structural features with innexin and pannexin proteins. Here, we demonstrated that the unnexin1 protein (Unx1) is delivered to the cell membrane, displaying a topology consisting of four transmembrane domains with C and N termini on the cytoplasmic side and form large-pore channels that are permeable to small molecules. Low extracellular Ca2+/Mg2+ levels or extracellular alkalinization, but not mechanical stretching, increases channel activity. The Unx1 channel mediates the influx of Ca2+ and does not form intercellular dye coupling between HeLa Unx1 transfected cells. Unx1 channel function was further evidenced by its ability to mediate ionic currents when expressed in Xenopus oocytes. Downregulation of Unx1 mRNA with morpholine contains Trypanosoma cruzi invasion. Phylogenetic analysis revealed the presence of Unx1 homologs in other protozoan parasites, suggesting a conserved function for these channel parasites in other protists. Our data demonstrate that Unx1 forms large-pore membrane channels, which may serve as a diffusional pathway for ions and small molecules that are likely to be metabolic substrates or waste products, and signaling autocrine and paracrine molecules that could be involved in cell invasion. As morpholinos-induced downregulation of Unx1 reduces the infectivity of trypomastigotes, the Unx1 channels might be an attractive target for developing trypanocide drugs.

Pannexin-1 Modulates Inhibitory Transmission and Hippocampal Synaptic Plasticity

Pannexin-1 (Panx1) hemichannel is a non-selective transmembrane channel that may play important roles in intercellular signaling by allowing the permeation of ions and metabolites, such as ATP. Although recent evidence shows that the Panx1 hemichannel is involved in controlling excitatory synaptic transmission, the role of Panx1 in inhibitory transmission remains unknown. Here, we studied the contribution of Panx1 to the GABAergic synaptic efficacy onto CA1 pyramidal neurons (PyNs) by using patch-clamp recordings and pharmacological approaches in wild-type and Panx1 knock-out (Panx1-KO) mice. We reported that blockage of the Panx1 hemichannel with the mimetic peptide ¹⁰Panx1 increases the synaptic level of endocannabinoids (eCB) and the activation of cannabinoid receptors type 1 (CB1Rs), which results in a decrease in hippocampal GABAergic efficacy, shifting excitation/inhibition (E/I) balance toward excitation and facilitating the induction of long-term potentiation. Our finding provides important insight unveiling that Panx1 can strongly influence the overall neuronal excitability and play a key role in shaping synaptic changes affecting the amplitude and direction of plasticity, as well as learning and memory processes.

MATE1 expression in the cochlea and its potential involvement in cisplatin cellular uptake and ototoxicity

BACKGROUND

Cisplatin appears to enter the cochlear cells through the organic cation transporter 2 (OCT2). There is recent evidence that multidrug and toxin extrusion protein 1 (MATE1) is involved in cisplatin-induced nephrotoxicity. Its presence and role in the ear are unknown.

AIMS/OBJECTIVES

Evaluate the presence and localization of MATE1, and determine the localization of OCT2, in the cochlea. Evaluate cisplatin uptake with regard to MATE1 and OCT2 expression.

MATERIAL AND METHODS

Murine cochlear explants and paraffin-embedded cochleae were evaluated with immunohistochemistry for OCT2 and MATE1. Explant cultures were also treated with Texas Red cisplatin to determine their cellular uptake.

RESULTS

MATE1 is present in the cochlea. Most intense labeling of MATE1 and OCT2 was seen in the outer hair cells (OHCs) and pillar cells, respectively. Both transporters were observed in the spiral ganglion neurons and stria vascularis. Expression levels of OCT2 and MATE1 decreased following cisplatin exposure. Texas Red cisplatin staining was strong in OHCs and pillar cells.

CONCLUSIONS AND SIGNIFICANCE

To the best of our knowledge, this is the first study demonstrating the presence and localization of MATE1 in the cochlea. OCT2 labeling was seen in pillar cells. Consistently, OHCs and pillar cells uptake Texas Red cisplatin.

Expression of KID syndromic mutation Cx26S17F produces hyperactive hemichannels in supporting cells of the organ of Corti

Some mutations in gap junction protein Connexin 26 (Cx26) lead to syndromic deafness, where hearing impairment is associated with skin disease, like in Keratitis Ichthyosis Deafness (KID) syndrome. This condition has been linked to hyperactivity of connexin hemichannels but this has never been demonstrated in cochlear tissue. Moreover, some KID mutants, like Cx26S17F, form hyperactive HCs only when co-expressed with other wild-type connexins. In this work, we evaluated the functional consequences of expressing a KID syndromic mutation, Cx26S17F, in the transgenic mouse cochlea and whether co-expression of Cx26S17F and Cx30 leads to the formation of hyperactive HCs. Indeed, we found that cochlear explants from a constitutive knock-in Cx26S17F mouse or conditional in vitro cochlear expression of Cx26S17F produces hyperactive HCs in supporting cells of the organ of Corti. These conditions also produce loss of hair cells stereocilia. In supporting cells, we found high co-localization between Cx26S17F and Cx30. The functional properties of HCs formed in cells co-expressing Cx26S17F and Cx30 were also studied in oocytes and HeLa cells. Under the recording conditions used in this study Cx26S17F did not form functional HCs and GJCs, but cells co-expressing Cx26S17F and Cx30 present hyperactive HCs insensitive to HCs blockers, Ca²⁺ and La³⁺, resulting in more Ca²⁺ influx and cellular damage. Molecular dynamic analysis of putative heteromeric HC formed by Cx26S17F and Cx30 presents alterations in extracellular Ca²⁺ binding sites. These results support that in KID syndrome, hyperactive HCs are formed by the interaction between Cx26S17F and Cx30 in supporting cells probably causing damage to hair cells associated to deafness.

The Long-Term Pannexin 1 Ablation Produces Structural and Functional Modifications in Hippocampal Neurons

Enhanced activity and overexpression of Pannexin 1 (Panx1) channels contribute to neuronal pathologies such as epilepsy and Alzheimer's disease (AD). The Panx1 channel ablation alters the hippocampus's glutamatergic neurotransmission, synaptic plasticity, and memory flexibility. Nevertheless, Panx1-knockout (Panx1-KO) mice still retain the ability to learn, suggesting that compensatory mechanisms stabilize their neuronal activity. Here, we show that the absence of Panx1 in the adult brain promotes a series of structural and functional modifications in the Panx1-KO hippocampal synapses, preserving spontaneous activity. Compared to the wild-type (WT) condition, the adult hippocampal neurons of Panx1-KO mice exhibit enhanced excitability, a more complex dendritic branching, enhanced spine maturation, and an increased proportion of multiple synaptic contacts. These modifications seem to rely on the actin-cytoskeleton dynamics as an increase in the actin polymerization and an imbalance between the Rac1 and the RhoA GTPase activities were observed in Panx1-KO brain tissues. Our findings highlight a novel interaction between Panx1 channels, actin, and Rho GTPases, which appear to be relevant for synapse stability.

Gain-of-Function Dynamin-2 Mutations Linked to Centronuclear Myopathy Impair Ca2+-Induced Exocytosis in Human Myoblasts

Gain-of-function mutations of dynamin-2, a mechano-GTPase that remodels membrane and actin filaments, cause centronuclear myopathy (CNM), a congenital disease that mainly affects skeletal muscle tissue. Among these mutations, the variants p.A618T and p.S619L lead to a gain of function and cause a severe neonatal phenotype. By using total internal reflection fluorescence microscopy (TIRFM) in immortalized human myoblasts expressing the pH-sensitive fluorescent protein (pHluorin) fused to the insulin-responsive aminopeptidase IRAP as a reporter of the GLUT4 vesicle trafficking, we measured single pHluorin signals to investigate how p.A618T and p.S619L mutations influence exocytosis. We show here that both dynamin-2 mutations significantly reduced the number and durations of pHluorin signals induced by 10 μM ionomycin, indicating that in addition to impairing exocytosis, they also affect the fusion pore dynamics. These mutations also disrupt the formation of actin filaments, a process that reportedly favors exocytosis. This altered exocytosis might importantly disturb the plasmalemma expression of functional proteins such as the glucose transporter GLUT4 in skeletal muscle cells, impacting the physiology of the skeletal muscle tissue and contributing to the CNM disease.

Supporting Cells and Their Potential Roles in Cisplatin-Induced Ototoxicity

Cisplatin is a known ototoxic chemotherapy drug, causing irreversible hearing loss. Evidence has shown that cisplatin causes inner ear damage as a result of adduct formation, a proinflammatory environment and the generation of reactive oxygen species within the inner ear. The main cochlear targets for cisplatin are commonly known to be the outer hair cells, the stria vascularis and the spiral ganglion neurons. Further evidence has shown that certain transporters can mediate cisplatin influx into the inner ear cells including organic cation transporter 2 (OCT2) and the copper transporter Ctr1. However, the expression profiles for these transporters within inner ear cells are not consistent in the literature, and expression of OCT2 and Ctr1 has also been observed in supporting cells. Organ of Corti supporting cells are essential for hair cell activity and survival. Special interest has been devoted to gap junction expression by these cells as certain mutations have been linked to hearing loss. Interestingly, cisplatin appears to affect connexin expression in the inner ear. While investigations regarding cisplatin-induced hearing loss have been focused mainly on the known targets previously mentioned, the role of supporting cells for cisplatin-induced ototoxicity has been overlooked. In this mini review, we discuss the implications of supporting cells expressing OCT2 and Ctr1 as well as the potential role of gap junctions in cisplatin-induced cytotoxicity.

An Update on Connexin Gap Junction and Hemichannels in Diabetic Retinopathy

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

Restraint of Human Skin Fibroblast Motility, Migration, and Cell Surface Actin Dynamics, by Pannexin 1 and P2X7 Receptor Signaling

Wound healing is a dynamic process required to maintain skin integrity and which relies on the precise migration of different cell types. A key molecule that regulates this process is ATP. However, the mechanisms involved in extracellular ATP management are poorly understood, particularly in the human dermis. Here, we explore the role, in human fibroblast migration during wound healing, of Pannexin 1 channels and their relationship with purinergic signals and in vivo cell surface filamentous actin dynamics. Using siRNA against Panx isoforms and different Panx1 channel inhibitors, we demonstrate in cultured human dermal fibroblasts that the absence or inhibition of Panx1 channels accelerates cell migration, increases single-cell motility, and promotes actin redistribution. These changes occur through a mechanism that involves the release of ATP to the extracellular space through a Panx1-dependent mechanism and the activation of the purinergic receptor P2X7. Together, these findings point to a pivotal role of Panx1 channels in skin fibroblast migration and suggest that these channels could be a useful pharmacological target to promote damaged skin healing.

The interplay between α7 nicotinic acetylcholine receptors, pannexin-1 channels and P2X7 receptors elicit exocytosis in chromaffin cells

Pannexin-1 (Panx1) forms plasma membrane channels that allow the exchange of small molecules between the intracellular and extracellular compartments, and are involved in diverse physiological and pathological responses in the nervous system. However, the signaling mechanisms that induce their opening still remain elusive. Here, we propose a new mechanism for Panx1 channel activation through a functional crosstalk with the highly Ca²⁺ permeable α7 nicotinic acetylcholine receptor (nAChR). Consistent with this hypothesis, we found that activation of α7 nAChRs induces Panx1-mediated dye uptake and ATP release in the neuroblastoma cell line SH-SY5Y-α7. Using membrane permeant Ca²⁺ chelators, total internal reflection fluorescence microscopy in SH-SY5Y-α7 cells expressing a membrane-tethered GCAMP3, and Src kinase inhibitors, we further demonstrated that Panx1 channel opening depends on Ca²⁺ signals localized in submembrane areas, as well as on Src kinases. In turn, Panx1 channels amplify cytosolic Ca²⁺ signals induced by the activation of α7 nAChRs, by a mechanism that seems to involve ATP release and P2X7 receptor activation, as hydrolysis of extracellular ATP with apyrase or blockage of P2X7 receptors with oxidized ATP significantly reduces the α7 nAChR-Ca²⁺ signal. The physiological relevance of this crosstalk was also demonstrated in neuroendocrine chromaffin cells, wherein Panx1 channels and P2X7 receptors contribute to the exocytotic release of catecholamines triggered by α7 nAChRs, as measured by amperometry. Together these findings point to a functional coupling between α7 nAChRs, Panx1 channels and P2X7 receptors with physiological relevance in neurosecretion.

Contribution of Connexin Hemichannels to the Decreases in Cell Viability Induced by Linoleic Acid in the Human Lens Epithelial Cells (HLE-B3)

Connexin (Cx) proteins form gap junction channels (GJC) and hemichannels that a allow bidirectional flow of ions and metabolites between the cytoplasm and extracellular space, respectively. Under physiological conditions, hemichannels have a very low probability of opening, but in certain pathologies, hemichannels activity can increase and induce and/or accelerate cell death. Several mechanisms control hemichannels activity, including phosphorylation and oxidation (i.e., S-nitrosylation). Recently, the effect of polyunsaturated fatty acids (PUFAs) such as linoleic acid (LA), were found to modulate Cxs. It has been seen that LA increase cell death in bovine and human lens cells. The lens is a structure allocated in the eye that highly depends on Cx for the metabolic coupling between its cells, a condition necessary for its transparency. Therefore, we hypothesized that LA induces lens cells death by modulating hemichannel activity. In this work, we characterized the effect of LA on hemichannel activity and survival of HLE-B3 cells (a human lens epithelial cell line). We found that HLE-B3 cells expresses Cx43, Cx46, and Cx50 and can form functional hemichannels in their plasma membrane. The extracellular exposure to 10–50 μM of LA increases hemichannels activity (dye uptake) in a concentration-dependent manner, which was reduced by Cx-channel blockers, such as the Cx-mimetic peptide Gap27 and TATGap19, La³⁺, carbenoxolone (CBX) and the Akt kinase inhibitor. Additionally, LA increases intracellular calcium, which is attenuated in the presence of TATGap19, a specific Cx43-hemichannel inhibitor. Finally, the long exposure of HLE-B3 cells to LA 20 and 50 μM, reduced cell viability, which was prevented by CBX. Moreover, LA increased the proportion of apoptotic HLE-B3 cells, effect that was prevented by the Cx-mimetic peptide TAT-Gap19 but not by Akt inhibitor. Altogether, these findings strongly suggest a contribution of hemichannels opening in the cell death induced by LA in HLE-B3 cells. These cells can be an excellent tool to develop pharmacological studies in vitro.

Pannexin-1 Channels Are Essential for Mast Cell Degranulation Triggered During Type I Hypersensitivity Reactions

Mast cells (MCs) release pro-inflammatory mediators through a process called degranulation response. The latter may be induced by several conditions, including antigen recognition through immunoglobulin E (IgE) or "cross-linking," classically associated with Type I hypersensitivity reactions. Early in this reaction, Ca²⁺ influx and subsequent increase of intracellular free Ca²⁺ concentration are essential for MC degranulation. Several membrane channels that mediate Ca²⁺ influx have been proposed, but their role remains elusive. Here, we evaluated the possible contribution of pannexin-1 channels (Panx1 Chs), well-known as ATP-releasing channels, in the increase of intracellular Ca²⁺ triggered during cross-linking reaction of MCs. The contribution of Panx1 Chs in the degranulation response was evaluated in MCs from wild type (WT) and Panx1 knock out (Panx1^-/-) mice after anti-ovalbumin (OVA) IgE sensitization. Notably, the degranulation response (toluidine blue and histamine release) was absent in Panx1^-/- MCs. Moreover, WT MCs showed a rapid and transient increase in Ca²⁺ signal followed by a sustained increase after antigen stimulation. However, the sustained increase in Ca²⁺ signal triggered by OVA was absent in Panx1^-/- MCs. Furthermore, OVA stimulation increased the membrane permeability assessed by dye uptake, a prevented response by Panx1 Ch but not by connexin hemichannel blockers and without effect on Panx1^-/- MCs. Interestingly, the increase in membrane permeability of WT MCs was also prevented by suramin, a P2 purinergic inhibitor, suggesting that Panx1 Chs act as ATP-releasing channels impermeable to Ca²⁺. Accordingly, stimulation with exogenous ATP restored the degranulation response and sustained increase in Ca²⁺ signal of OVA stimulated Panx1^-/- MCs. Moreover, opening of Panx1 Chs in Panx1 transfected HeLa cells increased dye uptake and ATP release but did not promote Ca²⁺ influx, confirming that Panx1 Chs permeable to ATP are not permeable to Ca²⁺. These data strongly suggest that during antigen recognition, Panx1 Chs contribute to the sustained Ca²⁺ signal increase via release of ATP that activates P2 receptors, playing a critical role in the sequential events that leads to degranulation response during Type I hypersensitivity reactions.

RCAN1 Knockdown Reverts Defects in the Number of Calcium-Induced Exocytotic Events in a Cellular Model of Down Syndrome

In humans, Down Syndrome (DS) is a condition caused by partial or full trisomy of chromosome 21. Genes present in the DS critical region can result in excess gene dosage, which at least partially can account for DS phenotype. Although regulator of calcineurin 1 (RCAN1) belongs to this region and its ectopic overexpression in neurons impairs transmitter release, synaptic plasticity, learning and memory, the relative contribution of RCAN1 in a context of DS has yet to be clarified. In the present work, we utilized an in vitro model of DS, the CTb neuronal cell line derived from the brain cortex of a trisomy 16 (Ts16) fetal mouse, which reportedly exhibits acetylcholine release impairments compared to CNh cells (a neuronal cell line established from a normal littermate). We analyzed single exocytotic events by using total internal reflection fluorescence microscopy (TIRFM) and the vesicular acetylcholine transporter fused to the pH-sensitive green fluorescent protein (VAChT-pHluorin) as a reporter. Our analyses showed that, compared with control CNh cells, the trisomic CTb cells overexpress RCAN1, and they display a reduced number of Ca²⁺-induced exocytotic events. Remarkably, RCAN1 knockdown increases the extent of exocytosis at levels comparable to those of CNh cells. These results support a critical contribution of RCAN1 to the exocytosis process in the trisomic condition.

The syndromic deafness mutation G12R impairs fast and slow gating in Cx26 hemichannels

Mutations in connexin 26 hemichannels that cause syndromic deafness have a gain-of-function phenotype that is poorly understood. García et al. show that one such mutation impairs fast and slow gating in these hemichannels because of an interaction between the N terminus and intracellular loop.

Mutations in connexin 26 (Cx26) hemichannels can lead to syndromic deafness that affects the cochlea and skin. These mutations lead to gain-of-function hemichannel phenotypes by unknown molecular mechanisms. In this study, we investigate the biophysical properties of the syndromic mutant Cx26G12R (G12R). Unlike wild-type Cx26, G12R macroscopic hemichannel currents do not saturate upon depolarization, and deactivation is faster during hyperpolarization, suggesting that these channels have impaired fast and slow gating. Single G12R hemichannels show a large increase in open probability, and transitions to the subconductance state are rare and short-lived, demonstrating an inoperative fast gating mechanism. Molecular dynamics simulations indicate that G12R causes a displacement of the N terminus toward the cytoplasm, favoring an interaction between R12 in the N terminus and R99 in the intracellular loop. Disruption of this interaction recovers the fast and slow voltage-dependent gating mechanisms. These results suggest that the mechanisms of fast and slow gating in connexin hemichannels are coupled and provide a molecular mechanism for the gain-of-function phenotype displayed by the syndromic G12R mutation.

Lack of Pannexin 1 Alters Synaptic GluN2 Subunit Composition and Spatial Reversal Learning in Mice

Long-term potentiation (LTP) and long-term depression (LTD) are two forms of synaptic plasticity that have been considered as the cellular substrate of memory formation. Although LTP has received considerable more attention, recent evidences indicate that LTD plays also important roles in the acquisition and storage of novel information in the brain. Pannexin 1 (Panx1) is a membrane protein that forms non-selective channels which have been shown to modulate the induction of hippocampal synaptic plasticity. Animals lacking Panx1 or blockade of Pannexin 1 channels precludes the induction of LTD and facilitates LTP. To evaluate if the absence of Panx1 also affects the acquisition of rapidly changing information we trained Panx1 knockout (KO) mice and wild type (WT) littermates in a visual and hidden version of the Morris water maze (MWM). We found that KO mice find the hidden platform similarly although slightly quicker than WT animals, nonetheless, when the hidden platform was located in the opposite quadrant (OQ) to the previous learned location, KO mice spent significantly more time in the previous quadrant than in the new location indicating that the absence of Panx1 affects the reversion of a previously acquired spatial memory. Consistently, we observed changes in the content of synaptic proteins critical to LTD, such as GluN2 subunits of N-methyl-D-aspartate receptors (NMDARs), which changed their contribution to synaptic plasticity in conditions of Panx1 ablation. Our findings give further support to the role of Panx1 channels on the modulation of synaptic plasticity induction, learning and memory processes.

Calcium binding and voltage gating in Cx46 hemichannels

The opening of connexin (Cx) hemichannels in the membrane is tightly regulated by calcium (Ca2+) and membrane voltage. Electrophysiological and atomic force microscopy experiments indicate that Ca2+ stabilizes the hemichannel closed state. However, structural data show that Ca2+ binding induces an electrostatic seal preventing ion transport without significant structural rearrangements. In agreement with the closed-state stabilization hypothesis, we found that the apparent Ca2+ sensitivity is increased as the voltage is made more negative. Moreover, the voltage and Ca2+ dependence of the channel kinetics indicate that the voltage sensor movement and Ca2+ binding are allosterically coupled. An allosteric kinetic model in which the Ca2+ decreases the energy necessary to deactivate the voltage sensor reproduces the effects of Ca2+ and voltage in Cx46 hemichannels. In agreement with the model and suggesting a conformational change that narrows the pore, Ca2+ inhibits the water flux through Cx hemichannels. We conclude that Ca2+ and voltage act allosterically to stabilize the closed conformation of Cx46 hemichannels.

Connexinopathies: a structural and functional glimpse

Mutations in human connexin (Cx) genes have been related to diseases, which we termed connexinopathies. Such hereditary disorders include nonsyndromic or syndromic deafness (Cx26, Cx30), Charcot Marie Tooth disease (Cx32), occulodentodigital dysplasia and cardiopathies (Cx43), and cataracts (Cx46, Cx50). Despite the clinical phenotypes of connexinopathies have been well documented, their pathogenic molecular determinants remain elusive. The purpose of this work is to identify common/uncommon patterns in channels function among Cx mutations linked to human diseases. To this end, we compiled and discussed the effect of mutations associated to Cx26, Cx32, Cx43, and Cx50 over gap junction channels and hemichannels, highlighting the function of the structural channel domains in which mutations are located and their possible role affecting oligomerization, gating and perm/selectivity processes.

Charged Residues at the First Transmembrane Region Contribute to the Voltage Dependence of the Slow Gate of Connexins

Connexins (Cxs) are a family of membrane-spanning proteins that form gap junction channels and hemichannels. Connexin-based channels exhibit two distinct voltage-dependent gating mechanisms termed slow and fast gating. Residues located at the C terminus of the first transmembrane segment (TM-1) are important structural components of the slow gate. Here, we determined the role of the charged residues at the end of TM-1 in voltage sensing in Cx26, Cx46, and Cx50. Conductance/voltage curves obtained from tail currents together with kinetics analysis reveal that the fast and slow gates of Cx26 involves the movement of two and four charges across the electric field, respectively. Primary sequence alignment of different Cxs shows the presence of well conserved glutamate residues in the C terminus of TM-1; only Cx26 contains a lysine in that position (lysine 41). Neutralization of lysine 41 in Cx26 increases the voltage dependence of the slow gate. Swapping of lysine 41 with glutamate 42 maintains the voltage dependence. In Cx46, neutralization of negative charges or addition of a positive charge in the Cx26 equivalent region reduced the slow gate voltage dependence. In Cx50, the addition of a glutamate in the same region decreased the voltage dependence, and the neutralization of a negative charge increased it. These results indicate that the charges at the end of TM-1 are part of the slow gate voltage sensor in Cxs. The fact that Cx42, which has no charge in this region, still presents voltage-dependent slow gating suggests that charges still unidentified also contribute to the slow gate voltage sensitivity.

From Hyperactive Connexin26 Hemichannels to Impairments in Epidermal Calcium Gradient and Permeability Barrier in the Keratitis-Ichthyosis-Deafness Syndrome

The keratitis-ichthyosis-deafness (KID) syndrome is characterized by corneal, skin, and hearing abnormalities. KID has been linked to heterozygous dominant missense mutations in the GJB2 and GJB6 genes, encoding connexin26 and 30, respectively. In vitro evidence indicates that KID mutations lead to hyperactive (open) hemichannels, which in some cases is accompanied by abnormal function of gap junction channels. Transgenic mouse models expressing connexin26 KID mutations reproduce human phenotypes and present impaired epidermal calcium homeostasis and abnormal lipid composition of the stratum corneum affecting the water barrier. Here we have compiled relevant data regarding the KID syndrome and propose a mechanism for the epidermal aspects of the disease.

Diseases associated with leaky hemichannels

Hemichannels (HCs) and gap junction channels (GJCs) formed by protein subunits called connexins (Cxs) are major pathways for intercellular communication. While HCs connect the intracellular compartment with the extracellular milieu, GJCs allow the interchange of molecules between cytoplasm of two contacting cells. Under physiological conditions, HCs are mostly closed, but they can open under certain stimuli allowing the release of autocrine and paracrine molecules. Moreover, some pathological conditions, like ischemia or other inflammation conditions, significantly increase HCs activity. In addition, some mutations in Cx genes associated with human diseases, such as deafness or cataracts, lead to the formation of more active HCs or “leaky HCs.” In this article we will revise cellular and molecular mechanisms underlying the appearance of leaky HCs, and the consequences of their expression in different cellular systems and animal models, in seeking a common pattern or pathological mechanism of disease.

Conversion from clinically isolated syndrome to multiple sclerosis: A large multicentre study

Background and objective:

We explored which clinical and biochemical variables predict conversion from clinically isolated syndrome (CIS) to clinically definite multiple sclerosis (CDMS) in a large international cohort.

Methods:

Thirty-three centres provided serum samples from 1047 CIS cases with at least two years’ follow-up. Age, sex, clinical presentation, T2-hyperintense lesions, cerebrospinal fluid (CSF) oligoclonal bands (OCBs), CSF IgG index, CSF cell count, serum 25-hydroxyvitamin D3 (25-OH-D), cotinine and IgG titres against Epstein-Barr nuclear antigen 1 (EBNA-1) and cytomegalovirus were tested for association with risk of CDMS.

Results:

At median follow-up of 4.31 years, 623 CIS cases converted to CDMS. Predictors of conversion in multivariable analyses were OCB (HR = 2.18, 95% CI = 1.71–2.77, p < 0.001), number of T2 lesions (two to nine lesions vs 0/1 lesions: HR = 1.97, 95% CI = 1.52–2.55, p < 0.001; >9 lesions vs 0/1 lesions: HR = 2.74, 95% CI = 2.04–3.68, p < 0.001) and age at CIS (HR per year inversely increase = 0.98, 95% CI = 0.98–0.99, p < 0.001). Lower 25-OH-D levels were associated with CDMS in univariable analysis, but this was attenuated in the multivariable model. OCB positivity was associated with higher EBNA-1 IgG titres.

Conclusions:

We validated MRI lesion load, OCB and age at CIS as the strongest independent predictors of conversion to CDMS in this multicentre setting. A role for vitamin D is suggested but requires further investigation.

Habituation of Auditory Steady State Responses Evoked by Amplitude-Modulated Acoustic Signals in Rats

Generation of the auditory steady state responses (ASSR) is commonly explained by the linear combination of random background noise activity and the stationary response. Based on this model, the decrease of amplitude that occurs over the sequential averaging of epochs of the raw data has been exclusively linked to the cancelation of noise. Nevertheless, this behavior might also reflect the non-stationary response of the ASSR generators. We tested this hypothesis by characterizing the ASSR time course in rats with different auditory maturational stages. ASSR were evoked by 8-kHz tones of different supra-threshold intensities, modulated in amplitude at 115 Hz. Results show that the ASSR amplitude habituated to the sustained stimulation and that dishabituation occurred when deviant stimuli were presented. ASSR habituation increased as animals became adults, suggesting that the ability to filter acoustic stimuli with no-relevant temporal information increased with age. Results are discussed in terms of the current model of the ASSR generation and analysis procedures. They might have implications for audiometric tests designed to assess hearing in subjects who cannot provide reliable results in the psychophysical trials.

Pannexin 1 regulates bidirectional hippocampal synaptic plasticity in adult mice

The threshold for bidirectional modification of synaptic plasticity is known to be controlled by several factors, including the balance between protein phosphorylation and dephosphorylation, postsynaptic free Ca(2+) concentration and NMDA receptor (NMDAR) composition of GluN2 subunits. Pannexin 1 (Panx1), a member of the integral membrane protein family, has been shown to form non-selective channels and to regulate the induction of synaptic plasticity as well as hippocampal-dependent learning. Although Panx1 channels have been suggested to play a role in excitatory long-term potentiation (LTP), it remains unknown whether these channels also modulate long-term depression (LTD) or the balance between both types of synaptic plasticity. To study how Panx1 contributes to excitatory synaptic efficacy, we examined the age-dependent effects of eliminating or blocking Panx1 channels on excitatory synaptic plasticity within the CA1 region of the mouse hippocampus. By using different protocols to induce bidirectional synaptic plasticity, Panx1 channel blockade or lack of Panx1 were found to enhance LTP, whereas both conditions precluded the induction of LTD in adults, but not in young animals. These findings suggest that Panx1 channels restrain the sliding threshold for the induction of synaptic plasticity and underlying brain mechanisms of learning and memory.

Extracellular gentamicin reduces the activity of connexin hemichannels and interferes with purinergic Ca(2+) signaling in HeLa cells

Gap junction channels (GJCs) and hemichannels (HCs) are composed of protein subunits termed connexins (Cxs) and are permeable to ions and small molecules. In most organs, GJCs communicate the cytoplasm of adjacent cells, while HCs communicate the intra and extracellular compartments. In this way, both channel types coordinate physiological responses of cell communities. Cx mutations explain several genetic diseases, including about 50% of autosomal recessive non-syndromic hearing loss. However, the possible involvement of Cxs in the etiology of acquired hearing loss remains virtually unknown. Factors that induce post-lingual hearing loss are diverse, exposure to gentamicin an aminoglycoside antibiotic, being the most common. Gentamicin has been proposed to block GJCs, but its effect on HCs remains unknown. In this work, the effect of gentamicin on the functional state of HCs was studied and its effect on GJCs was reevaluated in HeLa cells stably transfected with Cxs. We focused on Cx26 because it is the main Cx expressed in the cochlea of mammals where it participates in purinergic signaling pathways. We found that gentamicin applied extracellularly reduces the activity of HCs, while dye transfer across GJCs was not affected. HCs were also blocked by streptomycin, another aminoglycoside antibiotic. Gentamicin also reduced the adenosine triphosphate release and the HC-dependent oscillations of cytosolic free-Ca²⁺ signal. Moreover, gentamicin drastically reduced the Cx26 HC-mediated membrane currents in Xenopus laevis oocytes. Therefore, the extracellular gentamicin-induced inhibition of Cx HCs may adversely affect autocrine and paracrine signaling, including the purinergic one, which might partially explain its ototoxic effects.

Pannexin 1 channels: new actors in the regulation of catecholamine release from adrenal chromaffin cells

Chromaffin cells of the adrenal gland medulla synthesize and store hormones and peptides, which are released into the blood circulation in response to stress. Among them, adrenaline is critical for the fight-or-flight response. This neurosecretory process is highly regulated and depends on cytosolic [Ca²⁺]. By forming channels at the plasma membrane, pannexin-1 (Panx1) is a protein involved in many physiological and pathological processes amplifying ATP release and/or Ca²⁺ signals. Here, we show that Panx1 is expressed in the adrenal gland where it plays a role by regulating the release of catecholamines. In fact, inhibitors of Panx1 channels, such as carbenoxolone (Cbx) and probenecid, reduced the secretory activity induced with the nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP, 50 μM) in whole adrenal glands. A similar inhibitory effect was observed in single chromaffin cells using Cbx or ¹⁰Panx1 peptide, another Panx1 channel inhibitors. Given that the secretory response depends on cytosolic [Ca²⁺] and Panx1 channels are permeable to Ca²⁺, we studied the possible implication of Panx1 channels in the Ca²⁺ signaling occurring during the secretory process. In support of this possibility, Panx1 channel inhibitors significantly reduced the Ca²⁺ signals evoked by DMPP in single chromaffin cells. However, the Ca²⁺ signals induced by caffeine in the absence of extracellular Ca²⁺ was not affected by Panx1 channel inhibitors, suggesting that this mechanism does not involve Ca²⁺ release from the endoplasmic reticulum. Conversely, Panx1 inhibitors significantly blocked the DMPP-induce dye uptake, supporting the idea that Panx1 forms functional channels at the plasma membrane. These findings indicate that Panx1 channels participate in the control the Ca²⁺ signal that triggers the secretory response of adrenal chromaffin cells. This mechanism could have physiological implications during the response to stress.

Role of connexin channels in the retinal light response of a diurnal rodent

Several studies have shown that connexin channels play an important role in retinal neural coding in nocturnal rodents. However, the contribution of these channels to signal processing in the retina of diurnal rodents remains unclear. To gain insight into this problem, we studied connexin expression and the contribution of connexin channels to the retinal light response in the diurnal rodent Octodon degus (degu) compared to rat, using in vivo ERG recording under scotopic and photopic light adaptation. Analysis of the degu genome showed that the common retinal connexins present a high degree of homology to orthologs expressed in other mammals, and expression of Cx36 and Cx43 was confirmed in degu retina. Cx36 localized mainly to the outer and inner plexiform layers (IPLs), while Cx43 was expressed mostly in cells of the retinal pigment epithelium. Under scotopic conditions, the b-wave response amplitude was strongly reduced by 18-β-glycyrrhetinic acid (β-GA) (−45.1% in degu, compared to −52.2% in rat), suggesting that connexins are modulating this response. Remarkably, under photopic adaptation, β-GA increased the ERG b-wave amplitude in degu (+107.2%) while reducing it in rat (−62.3%). Moreover, β-GA diminished the spontaneous action potential firing rate in ganglion cells (GCs) and increased the response latency of ON and OFF GCs. Our results support the notion that connexins exert a fine-tuning control of the retinal light response and have an important role in retinal neural coding.

SVCT2 vitamin C transporter expression in progenitor cells of the postnatal neurogenic niche

Known as a critical antioxidant, recent studies suggest that vitamin C plays an important role in stem cell generation, proliferation and differentiation. Vitamin C also enhances neural differentiation during cerebral development, a function that has not been studied in brain precursor cells. We observed that the rat neurogenic niche is structurally organized at day 15 of postnatal development, and proliferation and neural differentiation increase at day 21. In the human brain, a similar subventricular niche was observed at 1-month of postnatal development. Using immunohistochemistry, sodium-vitamin C cotransporter 2 (SVCT2) expression was detected in the subventricular zone (SVZ) and rostral migratory stream (RMS). Low co-distribution of SVCT2 and βIII-tubulin in neuroblasts or type-A cells was detected, and minimal co-localization of SVCT2 and GFAP in type-B or precursor cells was observed. Similar results were obtained in the human neurogenic niche. However, BrdU-positive cells also expressed SVCT2, suggesting a role of vitamin C in neural progenitor proliferation. Primary neurospheres prepared from rat brain and the P19 teratocarcinoma cell line, which forms neurospheres in vitro, were used to analyze the effect of vitamin C in neural stem cells. Both cell types expressed functional SVCT2 in vitro, and ascorbic acid (AA) induced their neural differentiation, increased βIII-tubulin and SVCT2 expression, and amplified vitamin C uptake.

Dynamin-2 regulates fusion pore expansion and quantal release through a mechanism that involves actin dynamics in neuroendocrine chromaffin cells

Over the past years, dynamin has been implicated in tuning the amount and nature of transmitter released during exocytosis. However, the mechanism involved remains poorly understood. Here, using bovine adrenal chromaffin cells, we investigated whether this mechanism rely on dynamin’s ability to remodel actin cytoskeleton. According to this idea, inhibition of dynamin GTPase activity suppressed the calcium-dependent de novo cortical actin and altered the cortical actin network. Similarly, expression of a small interfering RNA directed against dynamin-2, an isoform highly expressed in chromaffin cells, changed the cortical actin network pattern. Disruption of dynamin-2 function, as well as the pharmacological inhibition of actin polymerization with cytochalasine-D, slowed down fusion pore expansion and increased the quantal size of individual exocytotic events. The effects of cytochalasine-D and dynamin-2 disruption were not additive indicating that dynamin-2 and F-actin regulate the late steps of exocytosis by a common mechanism. Together our data support a model in which dynamin-2 directs actin polymerization at the exocytosis site where both, in concert, adjust the hormone quantal release to efficiently respond to physiological demands.

Critical role of the first transmembrane domain of Cx26 in regulating oligomerization and function

This study identifies a motif within the first transmembrane domain of Cx26, from amino acids Val-37 through Ala-40, that is critical for oligomerization and function. The impacts of deafness-associated mutations within this motif upon gap junction channel and hemichannel functions correlate with the severity of disease that they cause.

To identify motifs involved in oligomerization of the gap junction protein Cx26, we studied individual transmembrane (TM) domains and the full-length protein. Using the TOXCAT assay for interactions of isolated TM α-helices, we found that TM1, a Cx26 pore domain, had a strong propensity to homodimerize. We identified amino acids Val-37–Ala-40 (VVAA) as the TM1 motif required for homodimerization. Two deafness-associated Cx26 mutations localized in this region, Cx26V37I and Cx26A40G, differentially affected dimerization. TM1-V37I dimerized only weakly, whereas TM1-A40G did not dimerize. When the full-length mutants were expressed in HeLa cells, both Cx26V37I and Cx26A40G formed oligomers less efficiently than wild-type Cx26. A Cx26 cysteine substitution mutant, Cx26V37C formed dithiothreitol-sensitive dimers. Substitution mutants of Val-37 formed intercellular channels with reduced function, while mutants of Ala-40 did not form functional gap junction channels. Unlike wild-type Cx26, neither Cx26V37I nor Cx26A40G formed functional hemichannels in low extracellular calcium. Thus the VVAA motif of Cx26 is critical for TM1 dimerization, hexamer formation, and channel function. The differential effects of VVAA mutants on hemichannels and gap junction channels imply that inter-TM interactions can differ in unapposed and docked hemichannels. Moreover, Cx26 oligomerization appears dependent on transient TM1 dimerization as an intermediate step.

Modulation of gap junction channels and hemichannels by growth factors

Gap junction hemichannels and cell-cell channels have roles in coordinating numerous cellular processes, due to their permeability to extra and intracellular signaling molecules. Another mechanism of cellular coordination is provided by a vast array of growth factors that interact with relatively selective cell membrane receptors. These receptors can affect cellular transduction pathways, including alteration of intracellular concentration of free Ca(2+) and free radicals and activation of protein kinases or phosphatases. Connexin and pannexin based channels constitute recently described targets of growth factor signal transduction pathways, but little is known regarding the effects of growth factor signaling on pannexin based channels. The effects of growth factors on these two channel types seem to depend on the cell type, cell stage and connexin and pannexin isoform expressed. The functional state of hemichannels and gap junction channels are affected in opposite directions by FGF-1 via protein kinase-dependent mechanisms. These changes are largely explained by channels insertion in or withdrawal from the cell membrane, but changes in open probability might also occur due to changes in phosphorylation and redox state of channel subunits. The functional consequence of variation in cell-cell communication via these membrane channels is implicated in disease as well as normal cellular responses.

Gap-junction channels dysfunction in deafness and hearing loss

Gap-junction channels connect the cytoplasm of adjacent cells, allowing the diffusion of ions and small metabolites. They are formed at the appositional plasma membranes by a family of related proteins named connexins. Mutations in connexins 26, 31, 30, 32, and 43 have been associated with nonsyndromic or syndromic deafness. The majority of these mutations are inherited in an autosomal recessive manner, but a few of them have been associated with dominantly inherited hearing loss. Mutations in the connexin26 gene (GJB2) are the most common cause of genetic deafness. This review summarizes the most relevant and recent information about different mutations in connexin genes found in human patients, with emphasis on GJB2. The possible effects of the mutations on channel expression and function are discussed, in addition to their possible physiologic consequences for inner ear physiology. Finally, we propose that connexin channels (gap junctions and hemichannels) may be targets for age-related hearing loss induced by oxidative damage.

Connexin hemichannel composition determines the FGF-1-induced membrane permeability and free [Ca2+]i responses

Cell surface hemichannels (HCs) composed of different connexin (Cx) types are present in diverse cells and their possible role on FGF-1-induced cellular responses remains unknown. Here, we show that FGF-1 transiently (4-14 h, maximal at 7 h) increases the membrane permeability through HCs in HeLa cells expressing Cx43 or Cx45 under physiological extracellular Ca(2+)/Mg(2+) concentrations. The effect does not occur in HeLa cells expressing HCs constituted of Cx26 or Cx43 with its C-terminus truncated at aa 257, or in parental nontransfected HeLa cells. The increase in membrane permeability is associated with a rise in HC levels at the cell surface and a proportional increase in HC unitary events. The response requires an early intracellular free Ca(2+) concentration increase, activation of a p38 MAP kinase-dependent pathway, and a regulatory site of Cx subunit C-terminus. The FGF-1-induced rise in membrane permeability is also associated with a late increase in intracellular free Ca(2+) concentration, suggesting that responsive HCs allow Ca(2+) influx. The cell density of Cx26 and Cx43 HeLa transfectants cultured in serum-free medium was differentially affected by FGF-1. Thus, the FGF-1-induced cell permeabilization and derived consequences depend on the Cx composition of HCs.

Connexin43 with a cytoplasmic loop deletion inhibits the function of several connexins

Connexins (Cx) form gap junction channels mediating direct intercellular communication. To study the role of amino acids within the cytoplasmic loop, we produced a recombinant adenovirus containing Cx43 with a deletion of amino acids 130-136 (Cx43del(130-136)). Cx43del(130-136) expressed alone in HeLa cells localized within the cytoplasm and did not allow transfer of ions, neurobiotin or Lucifer yellow. When co-expressed with wild type Cx43, Cx43del(130-136) blocked electrical coupling and transfer of neurobiotin or Lucifer yellow. Cx43del(130-136) and Cx43 co-localized by immunofluorescence and were co-purified from Triton X-100-solubilized cell extracts. Intercellular transfer mediated by Cx37 and Cx45 (but not Cx26 or Cx40) was inhibited when co-expressed with Cx43del(130-136). Cx43del(130-136) co-localized with Cx37, Cx40, or Cx45, but not Cx26. These data suggest that Cx43del(130-136) produces connexin-specific inhibition of intercellular communication through formation of heteromeric connexons that are non-functional and/or retained in the cytoplasm.

Histamine reduces gap junctional communication of human tonsil high endothelial cells in culture

The regulation of gap junctional communication by histamine was studied in primary cultures of human tonsil high endothelial cells (HUTECs). We evaluated intercellular communication, levels, state of phosphorylation, and cellular distribution of gap junction protein subunits, mainly connexin (Cx)43. Histamine induced a time-dependent reduction in dye coupling (Lucifer yellow) associated with reduction in connexin43 localized at cell-cell appositions (immunofluorescence), without changes in levels and phosphorylation state of connexin43 (immunoblots). These effects were prevented with chlorpheniramine, an H1 receptor blocker; indomethacin, a cyclooxygenase blocker; or GF109203X, a protein kinase C inhibitor. Treatment with phorbol myristate acetate, a protein kinase C activator, and 4bromo (4Br)-A23187, a calcium ionophore, mimicked the histamine-induced effects on dye coupling. 8Bromo-cAMP doubled the dye coupling extent and prevented the histamine-induced reduction in incidence of dye coupling. After 24-h histamine treatment, known to desensitize H1 receptors, reapplication of histamine increased cell coupling in a way prevented by ranitidine, an H2 receptor blocker. Thus, activation of H1 and H2 receptors, which increase intracellular levels of free Ca2+ and cAMP, respectively, may affect gap junctional communication in opposite ways. Stabilization of actin filaments with phalloidine diminished but did not totally prevent histamine-induced cell shape changes and reduction in dye coupling. Hence, the histamine-induced reduction in gap junctional communication between HUTEC is mediated by cytoskeleton-dependent and -independent mechanisms and might contribute to modulate endothelial function in lymphoid tissue.

A carboxyl terminal domain of connexin43 is critical for gap junction plaque formation but not for homo- or hetero-oligomerization

We have initiated a series of experiments to analyze the biosynthesis and oligomerization of Cx43 in cells containing other connexins through the expression of site-directed mutants and chimeric connexin polypeptides. Here we report studies concerning a mutant of Cx43 (Cx43tr) that has been truncated after amino acid 251 to remove most of the Cx43 carboxy-terminal region. In stably transfected HeLa cells, full length Cx43 localized primarily to appositional membranes while much more Cx43tr was observed in the cytoplasm. Both Cx43 and Cx43tr showed similar oligomerization profiles based on centrifugation through sucrose gradients. HeLaCx43tr cells showed limited transfer of microinjected Lucifer Yellow but did show electrical coupling. Co-expression of Cx43tr with Cx43 or Cx45 led to Cx43tr localization at appositional membranes and co-localization with the other connexins. Moreover, cells co-expressing Cx43tr with Cx43 or Cx45 showed extensive intercellular dye coupling. Thus, Cx43tr was able to oligomerize and form functional channels when expressed alone or with a compatible connexin, but it only formed plaques when co-expressed. These results suggest that the carboxyl tail of Cx43 is not important for oligomerization, but they implicate critical residues in the formation of gap junction plaques.

ACh-induced endothelial NO synthase translocation, NO release and vasodilatation in the hamster microcirculation in vivo

Studies in cultured cells show that activation of endothelial nitric oxide (NO) synthase (eNOS) requires the dissociation of this enzyme from its inhibitory association with caveolin-1 (Cav-1), and perhaps its translocation from plasma membrane caveolae to other cellular compartments. We investigated the hypothesis that in vivo NO-dependent vasodilatation is associated with the translocation of eNOS from the cell membrane. To this end, we applied ACh topically (10-100 microM for 10 min) to the hamster cheek pouch microcirculation and measured NO production, blood flow and vessel diameter, and assessed subcellular eNOS distribution by Western blotting. Baseline NO production was 54.4 +/- 5.2 pmol min(-1) (n = 16). ACh increased NO release, caused arteriolar and venular dilatation and elevated microvascular flow. These responses were inhibited by N(G)-nitro-L-arginine (30 microM). The maximal increase in NO production induced by 10 microM and 100 microM ACh was 45 +/- 20 % and 111 +/- 33 %, respectively; the corresponding blood flow increases were 50 +/- 10 % and 130 +/- 24 %, respectively (n = 4-6). Both responses followed a similar time course, although increases in NO preceded flow changes. In non-stimulated tissues, eNOS was distributed mainly in the microsomal fraction. ACh-induced vasodilatation was associated with eNOS translocation to the cytosolic and Golgi-enriched fractions. After 1.5, 3.0 or 6.0 min of application, 10 microM ACh decreased the level of membrane-bound eNOS by -13 +/- 4 %, -60 +/- 4 % and -19 +/- 17 %, respectively; at the same time points, 100 microM ACh reduced microsomal eNOS content by -38 +/- 9 %, -61 +/- 16 % and -40 +/- 18 %, respectively (n = 4-5). In all cases, microsomal Cav-1 content did not change. The close ACh concentration dependence and the concomitance between eNOS subcellular redistribution and NO release support the concept that eNOS translocation from the plasma membrane is part of an activation mechanism that induces NO-dependent vasodilatation in vivo.

Identification of second messengers that induce expression of functional gap junctions in microglia cultured from newborn rats

The effect of several second messengers on the functional expression of gap junctions was investigated in primary cultures of newborn rat microglia. As previously reported, microglia cultured under resting conditions expressed low levels of the gap junction protein connexin 43, and exhibited little dye coupling. After treatment with 4bromo-A23187, a Ca(2+) ionophore, the incidence of dye coupling between microglia increased progressively over a 12-h period. Dye coupling was markedly reduced by gap junction blockers. Induction of dye coupling by 4bromo-A23187 was prevented by the addition of a synthetic peptide with the same sequence as a region of the extracellular loop 1 of connexin 43 (residues 53-66). The increase in dye coupling induced by 4bromo-A23187 was associated with increased connexin 43 mRNA and protein levels. Treatment of microglia with phorbol 12-myristate 13-acetate, an activator of protein kinase C, did not promote gap junctional communication in untreated microglia and reversed 4bromo-A23187-induced dye coupling. Thus, gap junctional communication between microglia can be regulated oppositely by calcium- and protein kinase C-dependent pathways. Activators of cGMP-dependent protein kinase (8bromo-cGMP) or protein kinase A (8bromo-cAMP) had no effect on untreated microglia or on 4bromo-A23187-induced dye coupling. Differential regulation of gap junctions by intracellular calcium concentration and protein kinase C activity may help to explain how various stimuli evoke differences in microglia responses, such as synthesis and secretion of cytokines and proteases.

In vivo assessment of microvascular nitric oxide production and its relation with blood flow

To assess the hypothesis that microvascular nitric oxide (NO) is critical to maintain blood flow and solute exchange, we quantified NO production in the hamster cheek pouch in vivo, correlating it with vascular dynamics. Hamsters (100-120 g) were anesthetized and prepared for measurement of microvessel diameters by intravital microscopy, of plasma flow by isotopic sodium clearance, and of NO production by chemiluminescence. Analysis of endothelial NO synthase (eNOS) location by immunocytochemistry and subcellular fractionation revealed that eNOS was present in arterioles and venules and was 67 +/- 7% membrane bound. Basal NO release was 60.1 +/- 5.1 pM/min (n = 35), and plasma flow was 2.95 +/- 0.27 microl/min (n = 29). Local NO synthase inhibition with 30 microM N(omega)-nitro-L-arginine reduced NO production to 8.6 +/- 2.6 pmol/min (-83 +/- 5%, n = 9) and plasma flow to 1.95 +/- 0.15 microl/min (-28 +/- 12%, n = 17) within 30-45 min, in parallel with constriction of arterioles (9-14%) and venules (19-25%). The effects of N(omega)-nitro-L-arginine (10-30 microM) were proportional to basal microvascular conductance (r = 0.7, P < 0.05) and fully prevented by 1 mM L-arginine. We conclude that in this tissue, NO production contributes to 35-50% of resting microvascular conductance and plasma-tissue exchange.

Gap junctions in cells of the immune system: structure, regulation and possible functional roles

Gap junction channels are sites of cytoplasmic communication between contacting cells. In vertebrates, they consist of protein subunits denoted connexins (Cxs) which are encoded by a gene family. According to their Cx composition, gap junction channels show different gating and permeability properties that define which ions and small molecules permeate them. Differences in Cx primary sequences suggest that channels composed of different Cxs are regulated differentially by intracellular pathways under specific physiological conditions. Functional roles of gap junction channels could be defined by the relative importance of permeant substances, resulting in coordination of electrical and/or metabolic cellular responses. Cells of the native and specific immune systems establish transient homo- and heterocellular contacts at various steps of the immune response. Morphological and functional studies reported during the last three decades have revealed that many intercellular contacts between cells in the immune response present gap junctions or "gap junction-like" structures. Partial characterization of the molecular composition of some of these plasma membrane structures and regulatory mechanisms that control them have been published recently. Studies designed to elucidate their physiological roles suggest that they might permit coordination of cellular events which favor the effective and timely response of the immune system.

Regulation of astrocyte gap junctions by hypoxia-reoxygenation

Confluent cultures of rat cortical astrocytes were subjected to 12-h hypoxia (<1% O(2)) followed by reoxygenation. Just after hypoxia, the cellular distribution, phosphorylation state and levels of connexin43 (Cx43), as well as the extent of dye coupling were as in control conditions. Nonetheless, 15-30 min after reoxygenation, dye coupling was transiently reduced by approximately 70%. The reduction in dye coupling occurred without changes in the state of phosphorylation or levels of Cx43. Nevertheless, it was correlated with a decrease in Cx43 reactivity found at membrane appositions and the appearance of intracellular Cx43-positive vesicle-like structures of variable size, suggesting internalization of gap junction channels. Reoxygenation-induced cellular uncoupling and redistribution of Cx43 were prevented by melatonin (500 microM), a potent-free radical scavenger, or indomethacin (50 microM), an inhibitor of the cyclooxygenase-dependent arachidonic acid metabolism. In astrocytes cultured under normoxia, the state of phosphorylation of Cx43 was not affected by antimycin A, a blocker of the mitochondrial oxidative metabolism, but phosphorylation was drastically reduced by iodoacetate, a blocker of anaerobic glycolysis. Thus, these results strongly suggest that reoxygenation-induced uncoupling is mediated by arachidonic acid byproducts that induce, at least, disorganization of Cx43 gap junction channels.