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.

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.

Connexin Mutants Compromise the Lens Circulation and Cause Cataracts through Biomineralization

Gap junction-mediated intercellular communication facilitates the circulation of ions, small molecules, and metabolites in the avascular eye lens. Mutants of the lens fiber cell gap junction proteins, connexin46 (Cx46) and connexin50 (Cx50), cause cataracts in people and in mice. Studies in mouse models have begun to elucidate the mechanisms by which these mutants lead to cataracts. The expression of the dominant mutants causes severe decreases in connexin levels, reducing the gap junctional communication between lens fiber cells and compromising the lens circulation. The impairment of the lens circulation results in several changes, including the accumulation of Ca2+ in central lens regions, leading to the formation of precipitates that stain with Alizarin red. The cataract morphology and the distribution of Alizarin red-stained material are similar, suggesting that the cataracts result from biomineralization within the organ. In this review, we suggest that this may be a general process for the formation of cataracts of different etiologies.

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.

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.

The Connexin50D47A Mutant Causes Cataracts by Calcium Precipitation

Purpose

Mutations in connexin50 (Cx50) and connexin46 (Cx46) cause cataracts. Because the expression of Cx46fs380 leads to decreased gap junctional coupling and formation of calcium precipitates, we studied Cx50D47A lenses to test whether Cx50 mutants also cause cataracts due to calcium precipitation.

Methods

Connexin levels were determined by immunoblotting. Gap junctional coupling conductance was calculated from intracellular impedance studies of intact lenses. Intracellular hydrostatic pressure was measured using a microelectrode/manometer system. Intracellular free calcium ion concentrations ([Ca²⁺]_i) were measured using Fura-2 and fluorescence imaging. Calcium precipitation was assessed by Alizarin red staining and compared to the distribution of opacities in darkfield images.

Results

In Cx50D47A lenses, Cx50 levels were 11% (heterozygotes) and 1.2% (homozygotes), and Cx46 levels were 52% (heterozygotes) and 30% (homozygotes) when compared to wild-type at 2.5 months. Gap junctional coupling in differentiating fibers of Cx50D47A lenses was 49% (heterozygotes) and 29% (homozygotes), and in mature fibers, it was 24% (heterozygotes) and 4% (homozygotes) compared to wild-type lenses. Hydrostatic pressure was significantly increased in Cx50D47A lenses. [Ca²⁺]_i was significantly increased in Cx50D47A lenses. Alizarin red-stained calcium precipitates were present in homozygous Cx50D47A lenses with a similar distribution to the cataracts.

Conclusions

Cx50D47A expression altered the lens internal circulation by decreasing connexin levels and gap junctional coupling. Reduced water and ion outflow through gap junctions increased the gradients of intracellular hydrostatic pressure and concentrations of free calcium ions. In these lenses, calcium ions accumulated, precipitated, and formed cataracts. These results suggest that mutant lens fiber connexins lead to calcium precipitates, which may cause cataracts.

Chemical chaperone treatment improves levels and distributions of connexins in Cx50D47A mouse lenses

Mouse Cx50D47A and human Cx50D47N are non-functional connexin mutants that cause dominantly-inherited cataracts. In tissue culture expression experiments, they both exhibit impaired cellular trafficking and gap junction plaque formation. Lenses of mice expressing Cx50D47A have cataracts, reduced size, drastically decreased levels of connexin50, and less severely reduced levels of connexin46. The PERK-dependent pathway of the ER response to misfolded proteins is activated, and they have impaired differentiation with retained cellular organelles. Since treatments that enhance protein folding improve trafficking and plaque formation by Cx50D47N and other mutant connexins in vitro, and they are successful therapeutics for some other diseases caused by misfolded proteins, we tested the efficacy of the chemical chaperone, 4-phenylbutyrate (4-PBA) in cultured cells and mice expressing Cx50D47A. 4-PBA treatment increased the formation of Cx50D47A-containing plaques at appositional membranes of transiently transfected HeLa cells. Heterozygous Cx50D47A mice were treated with 4-PBA by addition to the drinking water and parenteral injection of pregnant mice (starting 10 days after pairing of males and females) and their pups. Lenses from 1-month-old mice were examined by darkfield illumination and immunofluorescence microscopy. Protein levels were determined by immunoblotting. Cataract size and density were not detectably different between the control and the 4-PBA-treated groups. Lens size was not increased following treatment. Levels of connexin46 and connexin50 were significantly increased in lenses of 4-PBA-treated mice compared with saline-treated animals. Immunofluorescence showed an increased abundance of connexin46 immunoreactivity and puncta. The ratio of phosphorylated to total EIF2α was not altered, and levels of organellar proteins were not significantly reduced, suggesting that the ER response to misfolded proteins and differentiation were not changed. Thus, treatment with 4-PBA improved critical pathological issues in these mice (low connexin and gap junction abundance), but the magnitude of this recovery (especially for Cx50) was inadequate to impact the reduced size or the opacification of Cx50D47A lenses.

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.

Do Connexin Mutants Cause Cataracts by Perturbing Glutathione Levels and Redox Metabolism in the Lens?

Cataracts of many different etiologies are associated with oxidation of lens components. The lens is protected by maintenance of a pool of reduced glutathione (GSH) and other antioxidants. Because gap junction channels made of the lens connexins, Cx46 and Cx50, are permeable to GSH, we tested whether mice expressing two different mutants, Cx46fs380 and Cx50D47A, cause cataracts by impairing lens glutathione metabolism and facilitating oxidative damage. Levels of GSH were not reduced in homogenates of whole mutant lenses. Oxidized glutathione (GSSG) and the GSSG/GSH ratio were increased in whole lenses of Cx50D47A, but not Cx46fs380 mice. The GSSG/GSH ratio was increased in the lens nucleus (but not cortex) of Cx46fs380 mice at 4.5 months of age, but it was not altered in younger animals. Carbonylated proteins were increased in Cx50D47A, but not Cx46fs380 lenses. Thus, both mouse lines have oxidizing lens environments, but oxidative modification is greater in Cx50D47A than in Cx46fs380 mice. The results suggest that GSH permeation through lens connexin channels is not a critical early event in cataract formation in these mice. Moreover, because oxidative damage was only detected in animals with significant cataracts, it cannot be an early event in their cataractogenesis.

Differential Regulation of Hemichannels and Gap Junction Channels by RhoA GTPase and Actin Cytoskeleton: A Comparative Analysis of Cx43 and Cx26

Connexins (Cxs) are transmembrane proteins that assemble into gap junction channels (GJCs) and hemichannels (HCs). Previous researches support the involvement of Rho GTPases and actin microfilaments in the trafficking of Cxs, formation of GJCs plaques, and regulation of channel activity. Nonetheless, it remains uncertain whether distinct types of Cxs HCs and GJCs respond differently to Rho GTPases or changes in actin polymerization/depolymerization dynamics. Our investigation revealed that inhibiting RhoA, a small GTPase that controls actin polymerization, or disrupting actin microfilaments with cytochalasin B (Cyto-B), resulted in reduced GJCs plaque size at appositional membranes and increased transport of HCs to non-appositional plasma membrane regions. Notably, these effects were consistent across different Cx types, since Cx26 and Cx43 exhibited similar responses, despite having distinct trafficking routes to the plasma membrane. Functional assessments showed that RhoA inhibition and actin depolymerization decreased the activity of Cx43 GJCs while significantly increasing HC activity. However, the functional status of GJCs and HCs composed of Cx26 remained unaffected. These results support the hypothesis that RhoA, through its control of the actin cytoskeleton, facilitates the transport of HCs to appositional cell membranes for GJCs formation while simultaneously limiting the positioning of free HCs at non-appositional cell membranes, independently of Cx type. This dynamic regulation promotes intercellular communications and reduces non-selective plasma membrane permeability through a Cx-type dependent mechanism, whereby the activity of Cx43 HCs and GJCs are differentially affected but Cx26 channels remain unchanged.

p62/Sequestosome 1 levels increase and phosphorylation is altered in Cx50D47A lenses, but deletion of p62/sequestosome 1 does not improve transparency

Purpose

p62/Sequestosome 1 (p62) is a stress-induced protein that is involved in several different intracellular pathways, including regulation of aspects of protein degradation. p62 levels are elevated in several types of cataracts. We investigated whether levels of p62 and its phosphorylation were altered in the lenses of Cx50D47A mice, which express a mutant of connexin50 (Cx50) that leads to cataracts and impaired lens differentiation. To evaluate the importance of p62 in the lens defects caused by a connexin50 mutant, we also examined the effect of deleting p62 in homozygous Cx50D47A mice.

Methods

Protein levels were determined with immunoblotting. Mouse lenses were examined with dark-field illumination microscopy. Intensities of the opacities and lens equatorial diameters were quantified using ImageJ. Nuclei and nuclear remnants were detected with fluorescence microscopy of lens sections stained with 4',6-diamino-2-phenylindole dihydrochloride (DAPI).

Results

Levels of total p62 were increased in the lenses of homozygous Cx50D47A mice compared to those of the wild-type animals. The ratio of p62 phosphorylated at threonine-269/serine-272 (T269/S272) to total p62 was significantly decreased, whereas the ratio of p62 phosphorylated at serine-349 (S349) to total p62 was significantly increased in lenses of homozygous Cx50D47A mice. However, deletion of p62 did not affect the sizes of the lenses or the severity of their cataracts in homozygous Cx50D47A mice. Deletion of p62 did not improve connexin50 or connexin46 levels. Moreover, deletion of p62 did not change the levels of crystallins, histone H3, the mitochondrial import receptor subunit TOM20 homolog, or the abundance of nuclei and nuclear fragments in the lenses of homozygous Cx50D47A mice. Homozygous deletion of p62 led to an 84% increase in the levels of ubiquilin 2, but did not significantly affect the levels of ubiquilin 1 or ubiquilin 4.

Conclusions

Although homozygous Cx50D47A lenses have increased levels of p62, a specific reduction in p62 phosphorylation at T269/S272, and a specific increase in p62 phosphorylation at S349, this protein is not a critical determinant of the severity of the abnormalities of these lenses (reduced growth or differentiation and cataracts). The lens may utilize redundant or compensatory systems (such as changes in levels of ubiquilin 2) to compensate for the lack of p62 in homozygous Cx50D47A lenses.

Levels and Modifications of Both Lens Fiber Cell Connexins Are Affected in Connexin Mutant Mice

In the lens, cell home ostasis and transparency are supported by intercellular communication facilitated by the channels formed of connexin46 (Cx46) and connexin50 (Cx50). Mutations of these connexins are linked to inherited cataracts. We studied the levels and the variations in electrophoretic mobilities of the immunoreactive Cx46 and Cx50 bands between 1 and 21 days after birth in the lenses of wild-type mice and homozygous animals from two different mouse models of connexin-linked cataracts (Cx46fs380 and Cx50D47A). In Cx50D47A mice, the expression of the mutant Cx50 reduced the normal phosphorylation of the co-expressed wild-type Cx46. In both models, levels of the mutant connexin and the co-expressed wild-type connexin decayed more rapidly than in wild-type mice but with different time courses. In the Cx46fs380 mice, modeling suggested that Cx50 degradation could be explained by the mixing of mutant Cx46 with wild-type Cx50. However, in Cx50D47A mice, similar modeling suggested that mixing alone could not explain the decrease in Cx46 levels. These data highlight the complex influences between two connexin proteins expressed in the same cell, some of which occur through direct mixing, while others occur indirectly, as in Cx50D47A mice, where the expression of the mutant connexin causes endoplasmic reticulum stress and impaired differentiation.