Hemichannel and junctional properties of connexin 50

Mechano-activated connexin hemichannels and glutathione transport protect lens fiber cells against oxidative insults

Long-lived lens fiber cells require a robust cellular protective function against oxidative insults to maintain their hemostasis and viability; however, the underlying mechanism is largely obscure. In this study, we unveiled a new mechanism that protects lens fiber cells against oxidative stress-induced cell death. We found that mechano-activated connexin (Cx) hemichannels (HCs) mediate the transport of glutathione (GSH) into chick embryonic fibroblasts (CEF) and primary lens fiber cells, resulting in a decrease in the accumulation of intracellular reactive oxygen species induced by both H2O2 and ultraviolet B, providing protection to lens fiber cells against cell apoptosis and necrosis. Furthermore, HCs formed by both homomeric Cx50 or Cx46 and heteromeric Cx50/Cx46 were mechanosensitive and could transport GSH into CEF cells. Notably, mechano-activated Cx50 HCs exhibited a greater capacity to transport GSH than Cx46 HCs. Consistently, the deficiency of Cx50 in single lens fiber cells led to a higher level of oxidative stress. Additionally, outer cortical short lens fiber cells expressing full length Cxs demonstrated greater resistance to oxidative injury compared to central core long lens fibers. Taken together, our results suggest that the activation of Cx HCs by interstitial fluid flow in cultured epithelial cells and isolated fiber cells shows that HCs can serve as a pathway for moving GSH across the cell membrane to offer protection against oxidative stress.

Calcium Regulation of Connexin Hemichannels

Connexin hemichannels (HCs) expressed at the plasma membrane of mammalian cells are of paramount importance for intercellular communication. In physiological conditions, HCs can form gap junction (GJ) channels, providing a direct diffusive path between neighbouring cells. In addition, unpaired HCs provide conduits for the exchange of solutes between the cytoplasm and the extracellular milieu, including messenger molecules involved in paracrine signalling. The synergistic action of membrane potential and Ca2+ ions controls the gating of the large and relatively unselective pore of connexin HCs. The four orders of magnitude difference in gating sensitivity to the extracellular ([Ca2+]e) and the cytosolic ([Ca2+]c) Ca2+ concentrations suggests that at least two different Ca2+ sensors may exist. While [Ca2+]e acts as a spatial modulator of the HC opening, which is most likely dependent on the cell layer, compartment, and organ, [Ca2+]c triggers HC opening and the release of extracellular bursts of messenger molecules. Such molecules include ATP, cAMP, glutamate, NAD+, glutathione, D-serine, and prostaglandins. Lost or abnormal HC regulation by Ca2+ has been associated with several diseases, including deafness, keratitis ichthyosis, palmoplantar keratoderma, Charcot-Marie-Tooth neuropathy, oculodentodigital dysplasia, and congenital cataracts. The fact that both an increased and a decreased Ca2+ sensitivity has been linked to pathological conditions suggests that Ca2+ in healthy cells finely tunes the normal HC function. Overall, further investigation is needed to clarify the structural and chemical modifications of connexin HCs during [Ca2+]e and [Ca2+]c variations. A molecular model that accounts for changes in both Ca2+ and the transmembrane voltage will undoubtedly enhance our interpretation of the experimental results and pave the way for developing therapeutic compounds targeting specific HC dysfunctions.

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.

Protein kinase A activation alleviates cataract formation via increased gap junction intercellular communication

Cataract is the leading cause of blindness worldwide. Here, we reported a potential, effective therapeutic mean for cataract prevention and treatment. Gap junction communication, an important mechanism in maintaining lens transparency, is increased by protein kinase A (PKA). We found that PKA activation reduced cataracts induced by oxidative stress, increased gap junctions/hemichannels in connexin (Cx) 50, Cx46 or Cx50 and Cx46 co-expressing cells, and decreased reactive oxygen species (ROS) levels. However, ROS reduction was shown in wild-type, Cx46 and Cx50 knockout, but not in Cx46/Cx50 double KO lens. In addition, PKA activation protects lens fiber cell death induced by oxidative stress via hemichannel-mediated glutathione transport. Connexin deletion increased lens opacity induced by oxidative stress associated with reduction of anti-oxidative stress gene expression. Together, our results suggest that PKA activation through increased connexin channels in lens fiber cell decreases ROS levels and cell death, leading to alleviated cataracts.

Hyposmotic stress causes ATP release in a discrete zone within the outer cortex of rat lens

PURPOSE

Purinergic signaling pathways activated by extracellular ATP have been implicated in the regulation of lens volume and transparency. In this study, we investigated the location of ATP release from whole rat lenses and the mechanism by which osmotic challenge alters such ATP release.

METHODS

Three-week-old rat lenses were cultured for 1 h in isotonic artificial aqueous humor (AAH) with no extracellular Ca²⁺, hypotonic AAH, or hypertonic AAH. The hypotonic AAH-treated lenses were also cultured in the absence or presence of connexin hemichannels and the pannexin channel blockers carbenoxolone, probenecid, and flufenamic acid. The ATP concentration in the AAH was determined using a Luciferin/luciferase bioluminescence assay. To visualize sites of ATP release induced by hemichannel and/or pannexin opening, the lenses were cultured in different AAH solutions, as described above, and incubated in the presence of Lucifer yellow (MW = 456 Da) and Texas red-dextran (MW = 10 kDa) for 1 h. Then the lenses were fixed, cryosectioned, and imaged using confocal microscopy to visualize areas of dye uptake from the extracellular space.

RESULTS

The incubation of the rat lenses in the AAH that lacked Ca²⁺ induced a significant increase in the extracellular ATP concentration. This was associated with an increased uptake of Lucifer yellow but not of Texas red-dextran in a discrete region of the outer cortex of the lens. Hypotonic stress caused a similar increase in ATP release and an increase in the uptake of Lucifer yellow in the outer cortex, which was significantly reduced by probenecid but not by carbenoxolone or flufenamic acid.

CONCLUSIONS

Our data suggest that in response to hypotonic stress, the intact rat lens is capable of releasing ATP. This seems to be mediated via the opening of pannexin channels in a specific zone of the outer cortex of the lens. Our results support the growing evidence that the lens actively regulates its volume and therefore, its optical properties, via puerinergic signaling pathways.

A novel mutation GJA8 NM_005267.5: c.124G > A, p.(E42K) causing congenital nuclear cataract

Background

To identify the genetic mutation of a four-generation autosomal dominant congenital cataract family in China.

Methods

Targeted region sequencing containing 778 genes associated with ocular diseases was performed to screen for the potential mutation, and Sanger sequencing was used to confirm the mutation. The homology model was constructed to identify the protein structural change, several online software were used to predict the mutation impact. CLUSTALW was used to perform multiple sequence alignment from different species.

Results

A novel heterozygous mutation, GJA8 NM_005267.5: c.124G > A, p.(E42K) was found, which cosegregated with congenital cataract phenotype in this family. Bioinformatics analysis of the mutation showed that the surface potential diagram of proteins changed. Several online programs predicted the mutation was ‘Pathogenic’, ‘Damaging’, ‘Disease causing’ or ‘Deleterious’.

Conclusions

A novel mutation NM_005267.5(GJA8):c.124G > A was identified in our study. Our finding can broaden the mutation spectrum of GJA8, enrich the phenotype-genotype correlation of congenital cataract and help to better understand the genetic background of congenital cataract.

Molecular mechanisms underlying enhanced hemichannel function of a cataract-associated Cx50 mutant

Connexin-50 (Cx50) is among the most frequently mutated genes associated with congenital cataracts. Although most of these disease-linked variants cause loss of function because of misfolding or aberrant trafficking, others directly alter channel properties. The mechanistic bases for such functional defects are mostly unknown. We investigated the functional and structural properties of a cataract-linked mutant, Cx50T39R (T39R), in the Xenopus oocyte system. T39R exhibited greatly enhanced hemichannel currents with altered voltage-gating properties compared to Cx50 and induced cell death. Coexpression of mutant T39R with wild-type Cx50 (to mimic the heterozygous state) resulted in hemichannel currents whose properties were indistinguishable from those induced by T39R alone, suggesting that the mutant had a dominant effect. Furthermore, when T39R was coexpressed with Cx46, it produced hemichannels with increased activity, particularly at negative potentials, which could potentially contribute to its pathogenicity in the lens. In contrast, coexpression of wild-type Cx50 with Cx46 was associated with a marked reduction in hemichannel activity, indicating that it may have a protective effect. All-atom molecular dynamics simulations indicate that the R39 substitution can form multiple electrostatic salt-bridge interactions between neighboring subunits that could stabilize the open-state conformation of the N-terminal (NT) domain while also neutralizing the voltage-sensing residue D3 as well as residue E42, which participates in loop gating. Together, these results suggest T39R acts as a dominant gain-of-function mutation that produces leaky hemichannels that may cause cytotoxicity in the lens and lead to development of cataracts.

Long-range interaction effects on coupled excitable nodes: traveling waves and chimera state

In this paper, analytical and numerical studies of the influence of the long-range interaction parameter on the excitability threshold in a ring of FitzHugh-Nagumo (FHN) system are investigated. The long-range interaction is introduced to the network to model regulation of the Gap junctions or hemichannels activity at the connexins level, which provides links between pre-synaptic and post-synaptic neurons. Results show that the long-range coupling enhances the range of the threshold parameter. We also investigate the long-range effects on the network dynamics, which induces enlargement of the oscillatory zone before the excitable regime. When considering bidirectional coupling, the long-range interaction induces traveling patterns such as traveling waves, while when considering unidirectional coupling, the long-range interaction induces multi-chimera states. We also studied the difference between the dynamics of coupled oscillators and coupled excitable neurons. We found that, for the coupled system, the oscillation period decreases with the increasing of the coupling parameter. For the same values of the coupling parameter, the oscillation period of the Oscillatory dynamics is greater than the oscillation period of the excitable dynamics. The analytical approximation shows good agreement with the numerical results.

Identification and functional analysis of a novel missense mutation in GJA8, p.Ala69Thr

BACKGROUND

To explore the molecular genetic cause of a four-generation autosomal dominant congenital cataract family in China.

METHODS

Targeted region sequencing was performed to screen for the potential mutation, and Sanger sequencing was used to confirm the mutation. The homology model was constructed to identify the protein structural change, PolyPhen-2 and Provean were used to predict the mutation impact. Functional and cellular analysis of the wild and mutant GJA8 were performed in DF-1 cells by western blotting, dye uptake assay, immunofluorescence, Annexin V-FITC staining.

RESULTS

A novel heterozygous mutation (c.205G > A; p.Ala69Thr) was identified within GJA8, which cosegregated with congenital cataract phenotype in this family. Bioinformatics analysis showed the mutation was located in a highly conserved region, and the mutation was predicted to be pathogenic. Function analysis indicated that the mutation inhibited GJA8 hemichannel activity, reduced cell tolerance to oxidative stress, changed the protein distribution pattern and inhibited the cell growth.

CONCLUSIONS

We have identified a novel missense mutation in GJA8 (c.205G > A, p.Ala69Thr) in a four-generation Chinese family and our results will further broaden the gene mutation spectrum of GJA8.

Connexin hemichannels mediate glutathione transport and protect lens fiber cells from oxidative stress

Elevated oxidized stress contributes to lens cataracts, and gap junctions play important roles in maintaining lens transparency. As well as forming gap junctions, connexin (Cx) proteins also form hemichannels. Here, we report a new mechanism whereby hemichannels mediate transport of reductant glutathione into lens fiber cells and protect cells against oxidative stress. We found that Cx50 (also known as GJA8) hemichannels opened in response to H2O2 in lens fiber cells but that transport through the channels was inhibited by two dominant-negative mutants in Cx50, Cx50P88S, which inhibits transport through both gap junctions and hemichannels, and Cx50H156N, which only inhibits transport through hemichannels and not gap junctions. Treatment with H2O2 increased the number of fiber cells undergoing apoptosis, and this increase was augmented with dominant-negative mutants that disrupted both hemichannels formed from Cx46 (also known as GJA3) and Cx50, while Cx50E48K, which only impairs gap junctions, did not have such an effect. Moreover, hemichannels mediate uptake of glutathione, and this uptake protected lens fiber cells against oxidative stress, while hemichannels with impaired transport had less protective benefit from glutathione. Taken together, these results show that oxidative stress activates connexin hemichannels in the lens fiber cells and that hemichannels likely protect lens cell against oxidative damage through transporting extracellular reductants.

Connexins in the Central Nervous System: Physiological Traits and Neuroprotective Targets

Cell-to-cell interaction and cell-to-extracellular environment communication are emerging as new therapeutic targets in neurodegenerative disorders. Dynamic expression of connexins leads to distinctive hemichannels and gap junctions, characterized by cell-specific conduction, exchange of stimuli or metabolites, and particular channel functions. Herein, we briefly reviewed classical physiological traits and functions of connexins, hemichannels, and gap junctions, in order to discuss the controversial role of these proteins and their mediated interactions during neuroprotection, with a particular focus on Cx43-based channels. We pointed out the contribution of connexins in neural cells populations during neurodegenerative processes to explore potential neuroprotective therapeutic applications.

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.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

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

On Biophysical Properties and Sensitivity to Gap Junction Blockers of Connexin 39 Hemichannels Expressed in HeLa Cells

Although connexins (Cxs) are broadly expressed by cells of mammalian organisms, Cx39 has a very restricted pattern of expression and the biophysical properties of Cx39-based channels [hemichannels (HCs) and gap junction channels (GJCs)] remain largely unknown. Here, we used HeLa cells transfected with Cx39 (HeLa-Cx39 cells) in which intercellular electrical coupling was not detected, indicating the absence of GJCs. However, functional HCs were found on the surface of cells exposed to conditions known to increase the open probability of other Cx HCs (e.g., extracellular divalent cationic-free solution (DCFS), extracellular alkaline pH, mechanical stimulus and depolarization to positive membrane potentials). Cx39 HCs were blocked by some traditional Cx HC blockers, but not by others or a pannexin1 channel blocker. HeLa-Cx39 cells showed similar resting membrane potentials (RMPs) to those of parental cells, and exposure to DCFS reduced RMPs in Cx39 transfectants, but not in parental cells. Under these conditions, unitary events of ~75 pS were frequent in HeLa-Cx39 cells and absent in parental cells. Real-time cellular uptake experiments of dyes with different physicochemical features, as well as the application of a machine-learning approach revealed that Cx39 HCs are preferentially permeable to molecules characterized by six categories of descriptors, namely: (1) electronegativity, (2) ionization potential, (3) polarizability, (4) size and geometry, (5) topological flexibility and (6) valence. However, Cx39 HCs opened by mechanical stimulation or alkaline pH were impermeable to Ca²⁺. Molecular modeling of Cx39-based channels suggest that a constriction present at the intracellular portion of the para helix region co-localizes with an electronegative patch, imposing an energetic and steric barrier, which in the case of GJCs may hinder channel function. Results reported here demonstrate that Cx39 form HCs and add to our understanding of the functional roles of Cx39 HCs under physiological and pathological conditions in cells that express them.

Focus on lens connexins

The lens is an avascular organ composed of an anterior epithelial cell layer and fiber cells that form the bulk of the organ. The lens expresses connexin43 (Cx43), connexin46 (Cx46) and connexin50 (Cx50). Epithelial Cx50 has critical roles in cell proliferation and differentiation, likely involving growth factor-dependent signaling pathways. Both Cx46 and Cx50 are crucial for lens transparency; mutations in their genes have been linked to congenital and age-related cataracts. Congenital cataract-associated connexin mutants can affect protein trafficking, stability and/or function, and the functional effects may differ between gap junction channels and hemichannels. Dominantly inherited cataracts may result from effects of the connexin mutant on its wild type isotype, the other co-expressed wild type connexin and/or its interaction with other cellular components.

Mechanism of gating by calcium in connexin hemichannels

Aberrant opening of nonjunctional connexin hemichannels at the plasma membrane is associated with many diseases, including ischemia and muscular dystrophy. Proper control of hemichannel opening is essential to maintain cell viability and is achieved by physiological levels of extracellular Ca²⁺, which drastically reduce hemichannel activity. Here we examined the role of conserved charged residues that form electrostatic networks near the extracellular entrance of the connexin pore, a region thought to be involved in gating rearrangements of hemichannels. Molecular dynamics simulations indicate discrete sites for Ca²⁺ interaction and consequent disruption of salt bridges in the open hemichannels. Experimentally, we found that disruption of these salt bridges by mutations facilitates hemichannel closing. Two negatively charged residues in these networks are putative Ca²⁺ binding sites, forming a Ca²⁺-gating ring near the extracellular entrance of the pore. Accessibility studies showed that this Ca²⁺-bound gating ring does not prevent access of ions or small molecules to positions deeper into the pore, indicating that the physical gate is below the Ca²⁺-gating ring. We conclude that intra- and intersubunit electrostatic networks at the extracellular entrance of the hemichannel pore play critical roles in hemichannel gating reactions and are tightly controlled by extracellular Ca²⁺ Our findings provide a general mechanism for Ca²⁺ gating among different connexin hemichannel isoforms.

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.

Connexin50 couples axon terminals of mouse horizontal cells by homotypic gap junctions

Horizontal cells in the mouse retina are of the axon-bearing B-type and contribute to the gain control of photoreceptors and to the center-surround organization of bipolar cells by providing feedback and feedforward signals to photoreceptors and bipolar cells, respectively. Horizontal cells form two independent networks, coupled by dendro-dendritic and axo-axonal gap junctions composed of connexin57 (Cx57). In Cx57-deficient mice, occasionally the residual tracer coupling of horizontal cell somata was observed. Also, negative feedback from horizontal cells to photoreceptors, potentially mediated by connexin hemichannels, appeared unaffected. These results point to the expression of a second connexin in mouse horizontal cells. We investigated the expression of Cx50, which was recently identified in axonless A-type horizontal cells of the rabbit retina. In the mouse retina, Cx50-immunoreactive puncta were predominantly localized on large axon terminals of horizontal cells. Electron microscopy did not reveal any Cx50-immunolabeling at the membrane of horizontal cell tips invaginating photoreceptor terminals, ruling out the involvement of Cx50 in negative feedback. Moreover, Cx50 colocalized only rarely with Cx57 on horizontal cell processes, indicating that both connexins form homotypic rather than heterotypic or heteromeric gap junctions. To check whether the expression of Cx50 is changed when Cx57 is lacking, we compared the Cx50 expression in wildtype and Cx57-deficient mice. However, Cx50 expression was unaffected in Cx57-deficient mice. In summary, our results indicate that horizontal cell axon terminals form two independent sets of homotypic gap junctions, a feature which might be important for light adaptation in the retina.

A novel GJA8 mutation (p.V44A) causing autosomal dominant congenital cataract

Purpose

To examine the mechanism by which a novel connexin 50 (Cx50) mutation, Cx50 V44A, in a Chinese family causes suture-sparing autosomal dominant congenital nuclear cataracts.

Methods

Family history and clinical data were recorded and direct gene sequencing was used to identify the disease-causing mutation. The Cx50 gene was cloned from a human lens cDNA library. Connexin protein distributions were assessed by fluorescence microscopy. Hemichannel functions were analyzed by dye uptake assay. Formation of functional channels was assessed by dye transfer experiments.

Results

Direct sequencing of the candidate GJA8 gene revealed a novel c.131T>C transition in exon 2, which cosegregated with the disease in the family and resulted in the substitution of a valine residue with alanine at codon 44 (p. V44A) in the extracellular loop 1 of the Cx50 protein. Both Cx50 and Cx50V44A formed functional gap junctions, as shown by the neurobiotin transfer assay. However, unlike wild-type Cx50, Cx50V44A was unable to form open hemichannels in dye uptake experiments.

Conclusion

This work identified a unique congenital cataract in the Chinese population, caused by the novel mutation Cx50V44A, and it showed that the V44A mutation specifically impairs the gating of the hemichannels but not the gap junction channels. The dysfunctional hemichannels resulted in the development of human congenital cataracts.

Temperature-sensitive gating of hCx26: high-resolution Raman spectroscopy sheds light on conformational changes

The temperature-sensitive gating of human Connexin 26 (hCx26) was analyzed with confocal Raman microscopy. High-resolution Raman spectra covering the spectral range between 400 and 1500 rel. cm(-1) with a spectral resolution of 1 cm(-1) were fully annotated, revealing notable differences between the spectrum recorded from solubilized hCx26 in Ca(2+)-buffered POPC at 10°C and any other set of protein conditions (temperature, Ca(2+) presence, POPC presence). Spectral components originating from specific amino acids show that the TM1/EL1 parahelix and probably the TM4 trans-membrane helix and the plug domain are involved in the gating process responsible for fully closing the hemichannel.

Altered inhibition of Cx26 hemichannels by pH and Zn2+ in the A40V mutation associated with keratitis-ichthyosis-deafness syndrome

Excessive opening of undocked Cx26 hemichannels in the plasma membrane is associated with disease pathogenesis in keratitis-ichthyosis-deafness (KID) syndrome. Thus far, excessive opening of KID mutant hemichannels has been attributed, almost solely, to aberrant inhibition by extracellular Ca(2+). This study presents two new possible contributing factors, pH and Zn(2+). Plasma pH levels and micromolar concentrations of Zn(2+) inhibit WT Cx26 hemichannels. However, A40V KID mutant hemichannels show substantially reduced inhibition by these factors. Using excised patches, acidification was shown to be effective from either side of the membrane, suggesting a protonation site accessible to H(+) flux through the pore. Sensitivity to pH was not dependent on extracellular aminosulfonate pH buffers. Single channel recordings showed that acidification did not affect unitary conductance or block the hemichannel but rather promoted gating to the closed state with transitions characteristic of the intrinsic loop gating mechanism. Examination of two nearby KID mutants in the E1 domain, G45E and D50N, showed no changes in modulation by pH or Zn(2+). N-bromo-succinimide, but not thiol-specific reagents, attenuated both pH and Zn(2+) responses. Individually mutating each of the five His residues in WT Cx26 did not reveal a key His residue that conferred sensitivity to pH or Zn(2+). From these data and the crystal structure of Cx26 that suggests that Ala-40 contributes to an intrasubunit hydrophobic core, the principal effect of the A40V mutation is probably a perturbation in structure that affects loop gating, thereby affecting multiple factors that act to close Cx26 hemichannels via this gating mechanism.

Motifs in the permeation pathway of connexin channels mediate voltage and Ca (2+) sensing

Connexin channels mediate electrical coupling, intercellular molecular signaling, and extracellular release of signaling molecules. Connexin proteins assemble intracellularly as hexamers to form plasma membrane hemichannels. The docking of two hemichannels in apposed cells forms a gap junction channel that allows direct electrical and selective cytoplasmic communication between adjacent cells. Hemichannels and junctional channels are gated by voltage, but extracellular Ca (2+) also gates unpaired plasma membrane hemichannels. Unlike other ion channels, connexin channels do not contain discrete voltage- or Ca (2+)-sensing modules linked to a separate pore-forming module. All studies to date indicate that voltage and Ca (2+) sensing are predominantly mediated by motifs that lie within or are exposed to the pore lumen. The sensors appear to be integral components of the gates, imposing an obligatory structural linkage between sensing and gating not commonly present in other ion channels, in which the sensors are semi-independent domains distinct from the pore. Because of this, the structural and electrostatic features that define connexin channel gating also define pore permeability properties, and vice versa; analysis/mutagenesis of gating and of permeability properties are linked. This offers unique challenges and opportunities for elucidating mechanisms of ligand and voltage-driven gating.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Connexin hemichannels in the lens

The normal function and survival of cells in the avascular lens is facilitated by intercellular communication through an extensive network of gap junctions formed predominantly by three connexins (Cx43, Cx46, and Cx50). In expression systems, these connexins can all induce hemichannel currents, but other lens proteins (e.g., pannexin1) can also induce similar currents. Hemichannel currents have been detected in isolated lens fiber cells. These hemichannels may make significant contributions to normal lens physiology and pathophysiology. Studies of some connexin mutants linked to congenital cataracts have implicated hemichannels with aberrant voltage-dependent gating or modulation by divalent cations in disease pathogenesis. Hemichannels may also contribute to age- and disease-related cataracts.

Insights on the mechanisms of Ca(2+) regulation of connexin26 hemichannels revealed by human pathogenic mutations (D50N/Y)

Because of the large size and modest selectivity of the connexin hemichannel aqueous pore, hemichannel opening must be highly regulated to maintain cell viability. At normal resting potentials, this regulation is achieved predominantly by the physiological extracellular Ca²⁺ concentration, which drastically reduces hemichannel activity. Here, we characterize the Ca²⁺ regulation of channels formed by wild-type human connexin26 (hCx26) and its human mutations, D50N/Y, that cause aberrant hemichannel opening and result in deafness and skin disorders. We found that in hCx26 wild-type channels, deactivation kinetics are accelerated as a function of Ca²⁺ concentration, indicating that Ca²⁺ facilitates transition to, and stabilizes, the closed state of the hemichannels. The D50N/Y mutant hemichannels show lower apparent affinities for Ca²⁺-induced closing than wild-type channels and have more rapid deactivation kinetics, which are Ca²⁺ insensitive. These results suggest that D50 plays a role in (a) stabilizing the open state in the absence of Ca²⁺, and (b) facilitating closing and stabilization of the closed state in the presence of Ca²⁺. To explore the role of a negatively charged residue at position 50 in regulation by Ca²⁺, this position was substituted with a cysteine residue, which was then modified with a negatively charged methanethiosulfonate reagent, sodium (2-sulfanoethyl) methanethiosulfonate (MTSES)⁻. D50C mutant hemichannels display properties similar to those of D50N/Y mutants. Recovery of the negative charge with chemical modification by MTSES⁻ restores the wild-type Ca²⁺ regulation of the channels. These results confirm the essential role of a negative charge at position 50 for Ca²⁺ regulation. Additionally, charge-swapping mutagenesis studies suggest involvement of a salt bridge interaction between D50 and K61 in the adjacent connexin subunit in stabilizing the open state in low extracellular Ca²⁺. Mutant cycle analysis supports a Ca²⁺-sensitive interaction between these two residues in the open state of the channel. We propose that disruption of this interaction by extracellular Ca²⁺ destabilizes the open state and facilitates hemichannel closing. Our data provide a mechanistic understanding of how mutations at position 50 that cause human diseases are linked to dysfunction of hemichannel gating by external Ca²⁺.

Roles and regulation of lens epithelial cell connexins

The avascular lens of the eye is covered anteriorly by an epithelium containing nucleated, metabolically active cells. This epithelium contains the first lens cells to encounter noxious external stimuli and cells that can develop compensatory or protective responses. Lens epithelial cells express the gap junction proteins, connexin43 (Cx43) and connexin50 (Cx50). Cx43 and Cx50 form gap junction channels and hemichannels with different properties. Although they may form heteromeric hemichannels, Cx43 and Cx50 probably do not form heterotypic channels in the lens. Cx50 channels make their greatest contribution to intercellular communication during the early postnatal period; subsequently, Cx43 becomes the predominant connexin supporting intercellular communication. Although epithelial Cx43 appears dispensable for lens development, Cx50 is critical for epithelial cell proliferation and differentiation. Cx43 and Cx50 hemichannels and gap junction channels are regulated by multiple different agents. Lens epithelial cell connexins contribute to both normal lens physiology and pathology.

Connexins form functional hemichannels in porcine ciliary epithelium

The expression of connexins in the ciliary epithelium is consistent with gap junctions between the pigmented (PE) and nonpigmented ciliary epithelium (NPE) that form when connexon hemichannels from adjacent cells pair to form a channel. Here we present evidence that suggests undocked connexons may form functional hemichannels that permit exchange of substances between NPE and the aqueous humor. Intact porcine eyes were perfused via the ciliary artery and propidium iodide (PI) (MW 668) was added to the aqueous humor compartment as a tracer. After calcium-free solution containing PI was introduced into the aqueous humor compartment for 30 min, fluorescence microscopy revealed PI in the NPE cell layer. PI entry into the NPE was inhibited by calcium and by the connexin antagonist 18α-glycyrrhetinic acid (18-AGA). Studies also were carried out with cultured porcine NPE. Under normal conditions, little PI entered the cultured cells but calcium-free medium stimulated PI accumulation and the entry was inhibited by 18-AGA. In cells loaded with calcein (MW 622), calcium-free solution stimulated calcein exit. 18-AGA partially suppressed calcein exit in calcium-free medium. Connexin 43 and connexin 50 proteins were detected by western blot analysis in both native and cultured NPE. In the intact eye, immunolocalization studies revealed connexin 50 at the basolateral, aqueous humor-facing, margin of the NPE. In contrast, connexin 43 was observed at the junction of the PE and NPE layer and on the basolateral membrane of PE. The results point to functional hemichannels at the NPE basolateral surface. It is feasible that hemichannels might contribute to the transfer of substances between the ciliary epithelium cytoplasm and aqueous humor.

Functional effects of Cx50 mutations associated with congenital cataracts

Mutations in connexin50 (Cx50) cause dominant cataracts in both humans and mice. The exact mechanisms by which mutations cause these variable phenotypes are poorly understood. We have examined the functional properties of gap junctions made by three Cx50 mutations, V44E, D47N, and V79L, expressed in mammalian cell lines. V44E trafficked to the plasma membrane properly and formed gap junctional plaques. However, the mutant did not form functional gap junctions when expressed alone, or with wild-type (WT) Cx46 and Cx50, indicating that V44E is a dominant negative inhibitor of WT connexin function. In contrast, D47N subunits did not localize to junctional plaques or form functional homotypic gap junctions; however, mixed expression of D47N and WT subunits of either Cx50 or Cx46 resulted in functional intercellular channels, with high levels of coupling. Single-channel studies indicated that D47N formed heteromeric channels with WT Cx46 with unique properties. Unlike either V44E or D47N, V79L formed functional homotypic intercellular channels. However, the mutation caused an alteration in voltage gating and a dramatic reduction in the single-channel open probability, resulting in much lower levels of conductance in cells expressing V79L alone, or together with WT connexin subunits. Thus, each mutation produced distinct changes in the properties of junctional coupling. V44E failed to form intercellular channels in any configuration, D47N formed only heteromeric channels with WT connexins, and V79L formed homotypic and heteromeric channels with altered properties. These results suggest that unique interactions between mutant and wild-type lens connexins might underlie the development of various cataract phenotypes in humans.

Cyclic nucleotide permeability through unopposed connexin hemichannels

Cyclic adenosine monophosphate (cAMP) is a well-known intracellular and intercellular second messenger. The membrane permeability of such molecules has potential importance for autocrine-like or paracrine-like delivery. Here experiments have been designed to demonstrate whether gap junction hemichannels, composed of connexins, are a possible entrance pathway for cyclic nucleotides into the interior of cells. HeLa cells stably expressing connexin43 (Cx43) and connexin26 (Cx26) were used to study the cyclic nucleotide permeability of gap junction hemichannels. For the detection of cAMP uptake, the cells were transfected using the cyclic nucleotide-modulated channel from sea urchin sperm (SpIH) as the cAMP sensor. SpIH derived currents (I_m) were recorded in whole-cell/perforated patch clamp configuration. Perfusion of the cells in an external K⁺ aspartate^- (KAsp) solution containing 500 μM cAMP and no extracellular Ca²⁺, yielded a five to sevenfold increase in the I_m current level. The SpIH current increase was associated with detectable hemichannel current activity. Depolarization of cells in Ca²⁺-free NaCl perfusate with 500 μM cAMP also induced a SpIH current increase. Elevating extracellular Ca²⁺ to mM levels inhibited hemichannel activity. Perfusion with a depolarizing KAsp solution containing 500 μM cAMP and 2 mM Ca²⁺ did not increase SpIH currents. The addition of the gap junction blocker carbenoxolone to the external solution inhibited cAMP uptake. Both cell depolarization and lowered extracellular Ca²⁺ increase the open probability of non-junctional hemichannels. Accordingly, the SpIH current augmentation was induced by the uptake of extracellular cAMP via open membrane hemichannels in Cx43 and Cx26 expressing cells. The data presented here show that hemichannels of Cx43 and Cx26 are permeable to cAMP, and further the data suggest that hemichannels are, in fact, a potential pathway for cAMP mediated cell-to-cell signaling.

Connexin channel modulators and their mechanisms of action

Gap junction channels and hemichannels formed by the connexin family of proteins play important roles in many aspects of tissue homeostasis in the brain and in other organs. In addition, connexin channels have been proposed as pharmacological targets in the treatment of a number of human disorders. In this review, we describe the connexin-subtype selectivity and specificity of pharmacological agents that are commonly used to modulate connexin channels. We also highlight recent progress made toward identifying new agents for connexin channels that act in a selective and specific manner. Finally, we discuss developing insights into possible mechanisms by which these agents modulate connexin channel function. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Connexin mutants and cataracts

The lens is a multicellular, but avascular tissue that must stay transparent to allow normal transmission of light and focusing of it on the retina. Damage to lens cells and/or proteins can cause cataracts, opacities that disrupt these processes. The normal survival of the lens is facilitated by an extensive network of gap junctions formed predominantly of connexin46 and connexin50. Mutations of the genes that encode these connexins (GJA3 and GJA8) have been identified and linked to inheritance of cataracts in human families and mouse lines. In vitro expression studies of several of these mutants have shown that they exhibit abnormalities that may lead to disease. Many of the mutants reduce or modify intercellular communication due to channel alterations (including loss of function or altered gating) or due to impaired cellular trafficking which reduces the number of gap junction channels within the plasma membrane. However, the abnormalities detected in studies of other mutants suggest that they cause cataracts through other mechanisms including gain of hemichannel function (leading to cell injury and death) and formation of cytoplasmic accumulations (that may act as light scattering particles). These observations and the anticipated results of ongoing studies should elucidate the mechanisms of cataract development due to mutations of lens connexins and abnormalities of other lens proteins. They may also contribute to our understanding of the mechanisms of disease due to connexin mutations in other tissues.

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'.

Identification and functional analysis of GJA8 mutation in a Chinese family with autosomal dominant perinuclear cataracts

Congenital cataract is a clinically and genetically heterogeneous group of eye disorders that causes visual impairment and childhood blindness. The purpose of this study was to identify the genetic defect associated with autosomal dominant congenital perinuclear cataract in a Chinese family. A detailed family history and clinical data of the family were recorded, and candidate gene sequencing was performed to screen for mutation-causing disease in our study. Direct sequencing revealed a c.601G>A (p.E201K) transversion in exon 2 of GJA8. This mutation co-segregated with all affected individuals in the family and was not found in unaffected family members or 100 unrelated controls. The function and mechanism of novel GJA8 point mutation E201K in Chinese patients were then investigated in this study. We found E201K aberrantly located in cytoplasm and prevented its location in the plasma membrane. Induction of E201K expression led to a decrease in cell growth and viability by MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. Our study provides important evidence that GJA8 is a disease-causing gene for congenital cataract and that mutation of GJA8 has a potential causative effect.

Voltage-dependent conformational changes in connexin channels

Channels formed by connexins display two distinct types of voltage-dependent gating, termed V(j)- or fast-gating and loop- or slow-gating. Recent studies, using metal bridge formation and chemical cross-linking have identified a region within the channel pore that contributes to the formation of the loop-gate permeability barrier. The conformational changes are remarkably large, reducing the channel pore diameter from 15 to 20Å to less than 4Å. Surprisingly, the largest conformational change occurs in the most stable region of the channel pore, the 3(10) or parahelix formed by amino acids in the 42-51 segment. The data provide a set of positional constraints that can be used to model the structure of the loop-gate closed state. Less is known about the conformation of the V(j)-gate closed state. There appear to be two different mechanisms; one in which conformational changes in channel structure are linked to a voltage sensor contained in the N-terminus of Cx26 and Cx32 and a second in which the C-terminus of Cx43 and Cx40 may act either as a gating particle to block the channel pore or alternatively to stabilize the closed state. The later mechanism utilizes the same domains as implicated in effecting pH gating of Cx43 channels. It is unclear if the two V(j)-gating mechanisms are related or if they represent different gating mechanisms that operate separately in different subsets of connexin channels. A model of the V(j)-closed state of Cx26 hemichannel that is based on the X-ray structure of Cx26 and electron crystallographic structures of a Cx26 mutation suggests that the permeability barrier for V(j)-gating is formed exclusively by the N-terminus, but recent information suggests that this conformation may not represent a voltage-closed state. Closed state models are considered from a thermodynamic perspective based on information from the 3.5Å Cx26 crystal structure and molecular dynamics (MD) simulations. The applications of computational and experimental methods to define the path of allosteric molecular transitions that link the open and closed states are discussed. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Paracrine signaling through plasma membrane hemichannels

Plasma membrane hemichannels composed of connexin (Cx) proteins are essential components of gap junction channels but accumulating evidence suggests functions of hemichannels beyond the communication provided by junctional channels. Hemichannels not incorporated into gap junctions, called unapposed hemichannels, can open in response to a variety of signals, electrical and chemical, thereby forming a conduit between the cell's interior and the extracellular milieu. Open hemichannels allow the bidirectional passage of ions and small metabolic or signaling molecules of below 1-2kDa molecular weight. In addition to connexins, hemichannels can also be formed by pannexin (Panx) proteins and current evidence suggests that Cx26, Cx32, Cx36, Cx43 and Panx1, form hemichannels that allow the diffusive release of paracrine messengers. In particular, the case is strong for ATP but substantial evidence is also available for other messengers like glutamate and prostaglandins or metabolic substances like NAD(+) or glutathione. While this field is clearly in expansion, evidence is still lacking at essential points of the paracrine signaling cascade that includes not only messenger release, but also downstream receptor signaling and consequent functional effects. The data available at this moment largely derives from in vitro experiments and still suffers from the difficulty of separating the functions of connexin-based hemichannels from gap junctions and from pannexin hemichannels. However, messengers like ATP or glutamate have universal roles in the body and further defining the contribution of hemichannels as a possible release pathway is expected to open novel avenues for better understanding their contribution to a variety of physiological and pathological processes. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Physiological and molecular characterization of connexin hemichannels in zebrafish retinal horizontal cells

Connexin channels mediate electrical synaptic transmission when assembled as cell-to-cell pores at gap junctions and can mediate transmembrane currents when expressed in plasma membranes as hemichannels. They are widely expressed in the vertebrate retina where in electrical synapses they are critical for transmission of visual signals. While the roles of connexins in electrical synapses are well-studied, the function and roles of connexin hemichannels in the nervous system are less well understood. Genetic deletion in zebrafish of connexin (Cx) 55.5 alters horizontal cell feedback to cones, spectral responses, and visual behavior. Here, we have characterized the properties of hemichannel currents in zebrafish retinal horizontal cells and examined the roles of two connexin isoforms, Cx55.5 and Cx52.6, that are coexpressed in these cells. We report that zebrafish horizontal cells express hemichannel currents that conduct inward current at physiological negative potentials and Ca(2+) levels. Manipulation of Cx55.5 and Cx52.6 gene expression in horizontal cells of adult zebrafish revealed that both Cx55.5 and Cx52.6 contribute to hemichannel currents; however, Cx55.5 expression is necessary for high-amplitude currents. Similarly, coexpression of Cx55.5 with Cx52.6 in oocytes increased hemichannel currents in a supra-additive manner. Taken together these results demonstrate that zebrafish horizontal cell hemichannel currents exhibit the functional characteristics necessary to contribute to synaptic feedback at the first visual synapse, that both Cx55.5 and Cx52.6 contribute to hemichannel currents, and that Cx55.5 may have an additional regulatory function enhancing the amplitude of hemichannel currents.

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.

Mechanism for modulation of gating of connexin26-containing channels by taurine

The mechanisms of action of endogenous modulatory ligands of connexin channels are largely unknown. Previous work showed that protonated aminosulfonates (AS), notably taurine, directly and reversibly inhibit homomeric and heteromeric channels that contain Cx26, a widely distributed connexin, but not homomeric Cx32 channels. The present study investigated the molecular mechanisms of connexin channel modulation by taurine, using hemichannels and junctional channels composed of Cx26 (homomeric) and Cx26/Cx32 (heteromeric). The addition of a 28-amino acid "tag" to the carboxyl-terminal domain (CT) of Cx26 (Cx26(T)) eliminated taurine sensitivity of homomeric and heteromeric hemichannels in cells and liposomes. Cleavage of all but four residues of the tag (Cx26(Tc)) resulted in taurine-induced pore narrowing in homomeric hemichannels, and restored taurine inhibition of heteromeric hemichannels (Cx26(Tc)/Cx32). Taurine actions on junctional channels were fully consistent with those on hemichannels. Taurine-induced inhibition of Cx26/Cx32(T) and nontagged Cx26 junctional channels was blocked by extracellular HEPES, a blocker of the taurine transporter, confirming that the taurine-sensitive site of Cx26 is cytoplasmic. Nuclear magnetic resonance of peptides corresponding to Cx26 cytoplasmic domains showed that taurine binds to the cytoplasmic loop (CL) and not the CT, and that the CT and CL directly interact. ELISA showed that taurine disrupts a pH-dependent interaction between the CT and the CT-proximal half of the CL. These studies reveal that AS disrupt a pH-driven cytoplasmic interdomain interaction in Cx26-containing channels, causing closure, and that the Cx26CT has a modulatory role in Cx26 function.

Molecular interaction and functional regulation of connexin50 gap junctions by calmodulin

Cx50 (connexin50), a member of the α-family of gap junction proteins expressed in the lens of the eye, has been shown to be essential for normal lens development. In the present study, we identified a CaMBD [CaM (calmodulin)-binding domain] (residues 141-166) in the intracellular loop of Cx50. Elevations in intracellular Ca2+ concentration effected a 95% decline in gj (junctional conductance) of Cx50 in N2a cells that is likely to be mediated by CaM, because inclusion of the CaM inhibitor calmidazolium prevented this Ca2+-dependent decrease in gj. The direct involvement of the Cx50 CaMBD in this Ca2+/CaM-dependent regulation was demonstrated further by the inclusion of a synthetic peptide encompassing the CaMBD in both whole-cell patch pipettes, which effectively prevented the intracellular Ca2+-dependent decline in gj. Biophysical studies using NMR and fluorescence spectroscopy reveal further that the peptide stoichiometrically binds to Ca2+/CaM with an affinity of ~5 nM. The binding of the peptide expanded the Ca2+-sensing range of CaM by increasing the Ca2+ affinity of the C-lobe of CaM, while decreasing the Ca2+ affinity of the N-lobe of CaM. Overall, these results demonstrate that the binding of Ca2+/CaM to the intracellular loop of Cx50 is critical for mediating the Ca2+-dependent inhibition of Cx50 gap junctions in the lens of the eye.

pH-dependent modulation of connexin-based gap junctional uncouplers

Gap junction (GJ) channels formed from connexin (Cx) proteins provide a direct pathway for electrical and metabolic cell–cell communication exhibiting high sensitivity to intracellular pH (pH(i)). We examined pH(i)-dependent modulation of junctional conductance (g(j)) of GJs formed of Cx26, mCx30.2, Cx36, Cx40, Cx43, Cx45, Cx46, Cx47 and Cx50 by reagents representing several distinct groups of uncouplers, such as long carbon chain alkanols (LCCAs), arachidonic acid, carbenoxolone, isoflurane, flufenamic acid and mefloquine. We demonstrate that alkalization by NH4Cl to pH ∼8 increased g(j) in cells expressing mCx30.2 and Cx45, yet did not affect g(j) of Cx26, Cx40, Cx46, Cx47 and Cx50 and decreased it in Cx43 and Cx36 GJs. Unexpectedly, cells expressing Cx45, but not other Cxs, exhibited full coupling recovery after alkalization with NH4Cl under the continuous presence of LCCAs, isoflurane and mefloquine. There was no coupling recovery by alkalization in the presence of arachidonic acid, carbenoxolone and flufenamic acid. In cells expressing Cx45, IC50 for octanol was 0.1, 0.25 and 2.68 mm at pH(i) values of 6.9, 7.2 and 8.1, respectively. Histidine modification of Cx45 protein by N-bromosuccinimide reduced the coupling-promoting effect of NH4Cl as well as the uncoupling effect of octanol. This suggests that LCCAs and some other uncouplers may act through the formation of hydrogen bonds with the as-of-yet unidentified histidine/s of the Cx45 GJ channel protein.

Different consequences of cataract-associated mutations at adjacent positions in the first extracellular boundary of connexin50

Gap junction channels, which are made of connexins, are critical for intercellular communication, a function that may be disrupted in a variety of diseases. We studied the consequences of two cataract-associated mutations at adjacent positions at the first extracellular boundary in human connexin50 (Cx50), W45S and G46V. Both of these mutants formed gap junctional plaques when they were expressed in HeLa cells, suggesting that they trafficked to the plasma membrane properly. However, their functional properties differed. Dual two-microelectrode voltage-clamp studies showed that W45S did not form functional intercellular channels in paired Xenopus oocytes or hemichannel currents in single oocytes. When W45S was coexpressed with wild-type Cx50, the mutant acted as a dominant negative inhibitor of wild-type function. In contrast, G46V formed both functional gap junctional channels and hemichannels. G46V exhibited greatly enhanced currents compared with wild-type Cx50 in the presence of physiological calcium concentrations. This increase in hemichannel activity persisted when G46V was coexpressed with wild-type lens connexins, consistent with a dominant gain of hemichannel function for G46V. These data suggest that although these two mutations are in adjacent amino acids, they have very different effects on connexin function and cause disease by different mechanisms: W45S inhibits gap junctional channel function; G46V reduces cell viability by forming open hemichannels.

A co-operative regulation of neuronal excitability by UNC-7 innexin and NCA/NALCN leak channel

Gap junctions mediate the electrical coupling and intercellular communication between neighboring cells. Some gap junction proteins, namely connexins and pannexins in vertebrates, and innexins in invertebrates, may also function as hemichannels. A conserved NCA/Dmα1U/NALCN family cation leak channel regulates the excitability and activity of vertebrate and invertebrate neurons. In the present study, we describe a genetic and functional interaction between the innexin UNC-7 and the cation leak channel NCA in Caenorhabditis elegans neurons. While the loss of the neuronal NCA channel function leads to a reduced evoked postsynaptic current at neuromuscular junctions, a simultaneous loss of the UNC-7 function restores the evoked response. The expression of UNC-7 in neurons reverts the effect of the unc-7 mutation; moreover, the expression of UNC-7 mutant proteins that are predicted to be unable to form gap junctions also reverts this effect, suggesting that UNC-7 innexin regulates neuronal activity, in part, through gap junction-independent functions. We propose that, in addition to gap junction-mediated functions, UNC-7 innexin may also form hemichannels to regulate C. elegans' neuronal activity cooperatively with the NCA family leak channels.

Properties of connexin 46 hemichannels in dissociated lens fiber cells

PURPOSE

To characterize the properties of connexin 46 hemichannels in differentiating fiber cells isolated from mouse lenses.

METHODS

Differentiating fiber cells were isolated from mouse lenses using collagenase. Cellular localization of connexin 50 (Cx50) and connexin 46 (Cx46) was assessed by immunofluorescence. Membrane currents were recorded using whole cell patch clamping. Dye uptake was measured using time-lapse imaging.

RESULTS

In freshly dissociated fiber cells isolated from knockout Cx50 (KOCx50) mouse lenses, removal of external divalent cations induced a macroscopic current composed of large conductance channels. This current was reduced at a holding potential of -60 mV, activated on depolarization, and had a reversal potential near 0 mV. These properties were very similar to those of Cx46 hemichannel currents recorded in single Xenopus oocytes. If the currents observed in divalent cation-free Ringer's solution were due to Cx46 hemichannel opening, then dye influx by gap junctional/hemichannel permeable dyes should be measurable in the fiber cells. To measure dye influx, the authors used the positively charged dyes, propidium iodide (PrI) and 4'-6-diamidino-2-phenylindole (DAPI). In the absence of external calcium, fiber cells took up both dyes. Furthermore, dye influx could be inhibited by hemichannel blockers. To confirm that this current was due to Cx46 hemichannels, the authors studied fiber cells isolated from the lenses of double knockout (Cx46(-/-); Cx50(-/-)) mice and demonstrated that both the calcium-sensitive conductance and dye influx were absent.

CONCLUSIONS

These results show that Cx46 can form functional hemichannels in the nonjunctional membrane of fiber cells.

Aquaporin 0 enhances gap junction coupling via its cell adhesion function and interaction with connexin 50

Both connexin 50 (Cx50) and aquaporin 0 (AQP0) have important roles in lens development and homeostasis, and their mutations are associated with human congenital cataracts. We have previously shown that Cx50 directly interacts with AQP0. Here, we demonstrate the importance of the Cx50 intracellular loop (IL) domain in mediating the interaction with AQP0 in the lens in vivo. AQP0 significantly increased (~20-30%) the intercellular coupling and conductance of Cx50 gap junctions. However, this increase was not observed when the IL domain was replaced with those from other lens connexins. The Cx50-AQP0 interaction had no effect on Cx50 hemichannel function. A fusion protein containing three extracellular loop domains of AQP0 efficiently blocked the cell-to-cell adhesion of AQP0 and attenuated the stimulatory effect of AQP0 on Cx50 gap junction conductance. These data suggest that the specific interaction between Cx50 and AQP0 enhances the coupling of Cx50 gap junctions, but not hemichannels, through the cell adhesion function of AQP0. This result establishes a physiological role of AQP0 in the functional regulation of gap junction channels.

Gap junctions or hemichannel-dependent and independent roles of connexins in cataractogenesis and lens development

In the last decade or so, increasing evidences suggest that the mutations of two connexin genes, GJA3 and GJA8, are directly linked to human congenital cataracts in North and Central America, Europe and Asia. GIA3 and GIA8 genes encode gap junction-forming proteins, connexin (Cx) 46 and Cx50, respectively. These two connexins are predominantly expressed in lens fiber cells. Majority of identified mutations are missense, and the mutated sites are scattered across various domains of connexin molecules. Genetic deletion of either of these two genes leads to the development of cataracts; however, the types of cataracts developed are distinctive. More interestingly, microphthalmia is only developed in Cx50, but not Cx46 deficient mice, suggesting the unique role of Cx50 in lens cell growth and development. Knockin studies with the replacement of Cx46 or Cx50 at their respective gene locus further demonstrate the unique properties of these two connexins. Furthermore, the function of Cx50 in epithelial-fiber differentiation appears to be independent of its conventional role in forming gap junction junction channels. Due to their specific functions in maintaining lens clarity and development, and their malfunctions resulting in lens cataractogenesis and developmental impairment, connexin molecules could be developed as potential drug targets for therapeutic intervention for treatment of cataracts and other eye disorders. Recent advances in basic research of lens connexins and the discoveries of clinical disorders as a result of lens connexin dysfunctions are summarized and discussed here.

A mutant connexin50 with enhanced hemichannel function leads to cell death

PURPOSE

To determine the consequences of expression of a novel connexin50 (CX50) mutant identified in a child with congenital total cataracts.

METHODS

The GJA8 gene was directly sequenced. Formation of functional channels was assessed by the two-microelectrode voltage-clamp

METHOD

Connexin protein levels and distribution were assessed by immunoblot analysis and immunofluorescence. The proportion of apoptotic cells was determined by flow cytometry.

RESULTS

Direct sequencing of the GJA8 gene identified a 137 G>T transition that resulted in the replacement of glycine by valine at position 46 of the coding region of CX50 (CX50G46V). Both CX50 and CX50G46V induced gap junctional currents in pairs of Xenopus oocytes. In single Xenopus oocytes, CX50G46V induced connexin hemichannel currents that were activated by removal of external calcium; their magnitudes were much higher than those in oocytes injected with similar amounts of CX50 cRNA. When expressed in HeLa cells under the control of an inducible promoter, both CX50 and CX50G46V formed gap junctional plaques. Induction of CX50G46V expression led to a decrease in the number of cells and an increase in the proportion of apoptotic cells. CX50G46V-induced cell death was prevented by high concentrations of extracellular calcium ions.

CONCLUSIONS

Unlike previously characterized CX50 mutants that exhibit impaired trafficking and/or lack of function, CX50G46V traffics properly to the plasma membrane and forms functional hemichannels and gap junction channels; however, it causes cell death even when expressed at minute levels. The biochemical results indirectly suggest a potential novel mechanism by which connexin mutants could lead to cataracts: cytotoxicity due to enhanced hemichannel function.

Connexin channels and phospholipids: association and modulation

BACKGROUND

For membrane proteins, lipids provide a structural framework and means to modulate function. Paired connexin hemichannels form the intercellular channels that compose gap junction plaques while unpaired hemichannels have regulated functions in non-junctional plasma membrane. The importance of interactions between connexin channels and phospholipids is poorly understood.

RESULTS

Endogenous phospholipids most tightly associated with purified connexin26 or connexin32 hemichannels or with junctional plaques in cell membranes, those likely to have structural and/or modulatory effects, were identified by tandem electrospray ionization-mass spectrometry using class-specific interpretative methods. Phospholipids were characterized by headgroup class, charge, glycerol-alkyl chain linkage and by acyl chain length and saturation. The results indicate that specific endogenous phospholipids are uniquely associated with either connexin26 or connexin32 channels, and some phospholipids are associated with both. Functional effects of the major phospholipid classes on connexin channel activity were assessed by molecular permeability of hemichannels reconstituted into liposomes. Changes to phospholipid composition(s) of the liposome membrane altered the activity of connexin channels in a manner reflecting changes to the surface charge/potential of the membrane and, secondarily, to cholesterol content. Together, the data show that connexin26 and connexin32 channels have a preference for tight association with unique anionic phospholipids, and that these, independent of headgroup, have a positive effect on the activity of both connexin26 and connexin32 channels. Additionally, the data suggest that the likely in vivo phospholipid modulators of connexin channel structure-function that are connexin isoform-specific are found in the cytoplasmic leaflet. A modulatory role for phospholipids that promote negative curvature is also inferred.

CONCLUSION

This study is the first to identify (endogenous) phospholipids that tightly associate with connexin channels. The finding that specific phospholipids are associated with different connexin isoforms suggests connexin-specific regulatory and/or structural interactions with lipid membranes. The results are interpreted in light of connexin channel function and cell biology, as informed by current knowledge of lipid-protein interactions and membrane biophysics. The intimate involvement of distinct phospholipids with different connexins contributes to channel structure and/or function, as well as plaque integrity, and to modulation of connexin channels by lipophilic agents.

The N terminus of connexin37 contains an alpha-helix that is required for channel function

The cytoplasmic N-terminal domain of connexins has been implicated in multiple aspects of gap junction function, including connexin trafficking/assembly and channel gating. A synthetic peptide corresponding to the first 23 amino acids of human connexin37 was prepared, and circular dichroism and nuclear magnetic resonance studies showed that this N-terminal peptide was predominantly alpha-helical between glycine 5 and glutamate 16. The importance of this structure for localization of the protein at appositional membranes and channel function was tested by expression of site-directed mutants of connexin37 in which amino acids leucine 10 and glutamine 15 were replaced with prolines or alanines. Wild type connexin37 and both substitution mutants localized to appositional membranes between transfected HeLa cells. The proline mutant did not allow intercellular transfer of microinjected neurobiotin; the alanine mutant allowed transfer, but less extensively than wild type connexin37. When expressed alone in Xenopus oocytes, wild type connexin37 produced hemichannel currents, but neither of the double substitution mutants produced detectable currents. The proline mutant (but not the alanine mutant) inhibited co-expressed wild type connexin37. Taken together, our data suggest that the alpha-helical structure of the connexin37 N terminus may be dispensable for protein localization, but it is required for channel and hemichannel function.