Aberrant connexin26 hemichannels underlying keratitis-ichthyosis-deafness syndrome are potently inhibited by mefloquine

Connexin Hemichannel Inhibition and Human Genodermatoses

Pathogenic variants in genes encoding connexins that cause skin diseases, such as keratitis-ichthyosis-deafness (KID) syndrome and hidrotic ectodermal dysplasia (HED) or Clouston syndrome, display increased hemichannel activity. Mechanistic insights derived from biophysical studies of the variant connexins support the hypothesis that inhibition of the acquired hemichannel activity could alleviate epidermal pathology. Use of pharmacological blockers and engineered mAbs in mouse models of HED and KID confirm that hemichannel inhibition is a promising target for new therapeutic approaches to KID and HED. Insights from this work could apply to other connexin-based genetic skin diseases in which hemichannel activity is elevated.

Connexins in epidermal health and diseases: insights into their mutations, implications, and therapeutic solutions

The epidermis, the outermost layer of the skin, serves as a protective barrier against external factors. Epidermal differentiation, a tightly regulated process essential for epidermal homeostasis, epidermal barrier formation and skin integrity maintenance, is orchestrated by several players, including signaling molecules, calcium gradient and junctional complexes such as gap junctions (GJs). GJ proteins, known as connexins facilitate cell-to-cell communication between adjacent keratinocytes. Connexins can function as either hemichannels or GJs, depending on their interaction with other connexons from neighboring keratinocytes. These channels enable the transport of metabolites, cAMP, microRNAs, and ions, including Ca2+, across cell membranes. At least ten distinct connexins are expressed within the epidermis and mutations in at least five of them has been linked to various skin disorders. Connexin mutations may cause aberrant channel activity by altering their synthesis, their gating properties, their intracellular trafficking, and the assembly of hemichannels and GJ channels. In addition to mutations, connexin expression is dysregulated in other skin conditions including psoriasis, chronic wound and skin cancers, indicating the crucial role of connexins in skin homeostasis. Current treatment options for conditions with mutant or altered connexins are limited and primarily focus on symptom management. Several therapeutics, including non-peptide chemicals, antibodies, mimetic peptides and allele-specific small interfering RNAs are promising in treating connexin-related skin disorders. Since connexins play crucial roles in maintaining epidermal homeostasis as shown with linkage to a range of skin disorders and cancer, further investigations are warranted to decipher the molecular and cellular alterations within cells due to mutations or altered expression, leading to abnormal proliferation and differentiation. This would also help characterize the roles of each isoform in skin homeostasis, in addition to the development of innovative therapeutic interventions. This review highlights the critical functions of connexins in the epidermis and the association between connexins and skin disorders, and discusses potential therapeutic options.

Keratitis-ichthyosis-deafness syndrome with lethal p.Ala88Val variant and severe hypercalcemia

Keratitis-ichthyosis-deafness (KID) syndrome is a rare genetic disease caused by pathogenic variants in connexin 26 (gene GJB2), which is part of the transmembrane channels of the epithelia. Connexin 26 is expressed mainly in the cornea, the sensory epithelium of the inner ear, and in the skin keratinocytes, which are the three main target organs in KID syndrome. Approximately a dozen pathogenic variants have been described to date, including some lethal forms. Patients with lethal pathogenic variants present with severe symptoms from birth and die from sepsis during the first year of life. We present a premature female patient with KID syndrome carrying the lethal p.Ala88Val pathogenic variant in GJB2. In addition to the respiratory distress associated with this variant, our patient presented severe hypercalcemia of unexplained origin refractory to treatment. This abnormality has not been reported earlier in other patients with KID syndrome with the same variant.

Cellular mechanisms of connexin-based inherited diseases

The 21-member connexin gene family exhibits distinct tissue expression patterns that can cause a diverse array of over 30 inherited connexin-linked diseases ranging from deafness to skin defects and blindness. Intriguingly, germline mutations can cause disease in one tissue while other tissues that abundantly express the mutant connexin remain disease free, highlighting the importance of the cellular context of mutant expression. Modeling connexin pathologies in genetically modified mice and tissue-relevant cells has informed extensively on no less than a dozen gain- and loss-of-function mechanisms that underpin disease. This review focuses on how a deeper molecular understanding of the over 930 mutations in 11 connexin-encoding genes is foundational for creating a framework for therapeutic interventions.

Connexin hemichannel inhibition ameliorates epidermal pathology in a mouse model of keratitis ichthyosis deafness syndrome

Mutations in five different genes encoding connexin channels cause eleven clinically defined human skin diseases. Keratitis ichthyosis deafness (KID) syndrome is caused by point mutations in the GJB2 gene encoding Connexin 26 (Cx26) which result in aberrant activation of connexin hemichannels. KID syndrome has no cure and is associated with bilateral hearing loss, blinding keratitis, palmoplantar keratoderma, ichthyosiform erythroderma and a high incidence of childhood mortality. Here, we have tested whether a topically applied hemichhanel inhibitor (flufenamic acid, FFA) could ameliorate the skin pathology associated with KID syndrome in a transgenic mouse model expressing the lethal Cx26-G45E mutation. We found that FFA blocked the hemichannel activity of Cx26-G45E in vitro, and substantially reduced epidermal pathology in vivo, compared to untreated, or vehicle treated control animals. FFA did not reduce the expression of mutant connexin hemichannel protein, and cessation of FFA treatment allowed disease progression to continue. These results suggested that aberrant hemichannel activity is a major driver of skin disease in KID syndrome, and that the inhibition of mutant hemichannel activity could provide an attractive target to develop novel therapeutic interventions to treat this incurable disease.

Cellular targets of mefloquine

Mefloquine is a quinoline-based compound widely used as an antimalarial drug, particularly in chemoprophylaxis. Although decades of research have identified various aspects of mefloquine's anti-Plasmodium properties, toxic effects offset its robust use in humans. Mefloquine exerts harmful effects in several types of human cells by targeting many of the cellular lipids, proteins, and complexes, thereby blocking a number of downstream signaling cascades. In general, mefloquine modulates several cellular phenomena, such as alteration of membrane potential, induction of oxidative stress, imbalance of ion homeostasis, disruption of metabolism, failure of organelle function, etc., leading to cell cycle arrest and programmed cell death. This review aims to summarize the information on functional and mechanistic findings related to the cytotoxic effects of mefloquine.

Mechanisms Underlying Connexin Hemichannel Activation in Disease

Gap junctions and connexin hemichannels mediate intercellular and extracellular communication, respectively. While gap junctions are seen as the “good guys” by controlling homeostasis, connexin hemichannels are considered as the “bad guys”, as their activation is associated with the onset and dissemination of disease. Open connexin hemichannels indeed mediate the transport of messengers between the cytosol and extracellular environment and, by doing so, fuel inflammation and cell death in a plethora of diseases. The present mini-review discusses the mechanisms involved in the activation of connexin hemichannels during pathology.

Connexins and the Epithelial Tissue Barrier: A Focus on Connexin 26

Simple Summary

Tissues that face the external environment are known as ‘epithelial tissue’ and form barriers between different body compartments. This includes the outer layer of the skin, linings of the intestine and airways that project into the lumen connecting with the external environment, and the cornea of the eye. These tissues do not have a direct blood supply and are dependent on exchange of regulatory molecules between cells to ensure co-ordination of tissue events. Proteins known as connexins form channels linking cells directly and permit exchange of small regulatory signals. A range of environmental stimuli can dysregulate the level of connexin proteins and or protein function within the epithelia, leading to pathologies including non-healing wounds. Mutations in these proteins are linked with hearing loss, skin and eye disorders of differing severity. As such, connexins emerge as prime therapeutic targets with several agents currently in clinical trials. This review outlines the role of connexins in epithelial tissue and how their dysregulation contributes to pathological pathways.

Abstract

Epithelial tissue responds rapidly to environmental triggers and is constantly renewed. This tissue is also highly accessible for therapeutic targeting. This review highlights the role of connexin mediated communication in avascular epithelial tissue. These proteins form communication conduits with the extracellular space (hemichannels) and between neighboring cells (gap junctions). Regulated exchange of small metabolites less than 1kDa aide the co-ordination of cellular activities and in spatial communication compartments segregating tissue networks. Dysregulation of connexin expression and function has profound impact on physiological processes in epithelial tissue including wound healing. Connexin 26, one of the smallest connexins, is expressed in diverse epithelial tissue and mutations in this protein are associated with hearing loss, skin and eye conditions of differing severity. The functional consequences of dysregulated connexin activity is discussed and the development of connexin targeted therapeutic strategies highlighted.

Glial Connexins and Pannexins in the Healthy and Diseased Brain

Over the past several decades a large amount of data have established that glial cells, the main cell population in the brain, dynamically interact with neurons and thus impact their activity and survival. One typical feature of glia is their marked expression of several connexins, the membrane proteins forming intercellular gap junction channels and hemichannels. Pannexins, which have a tetraspan membrane topology as connexins, are also detected in glial cells. Here, we review the evidence that connexin and pannexin channels are actively involved in dynamic and metabolic neuroglial interactions in physiological as well as in pathological situations. These features of neuroglial interactions open the way to identify novel non-neuronal aspects that allow for a better understanding of behavior and information processing performed by neurons. This will also complement the "neurocentric" view by facilitating the development of glia-targeted therapeutic strategies in brain disease.

Allele-Specific Small Interfering RNA Corrects Aberrant Cellular Phenotype in Keratitis-Ichthyosis-Deafness Syndrome Keratinocytes

Keratitis-ichthyosis-deafness (KID) syndrome is a severe, untreatable condition characterized by ocular, auditory, and cutaneous abnormalities, with major complications of infection and skin cancer. Most cases of KID syndrome (86%) are caused by a heterozygous missense mutation (c.148G>A, p.D50N) in the GJB2 gene, encoding gap junction protein Cx26, which alters gating properties of Cx26 channels in a dominant manner. We hypothesized that a mutant allele-specific small interfering RNA could rescue the cellular phenotype in patient keratinocytes (KCs). A KID syndrome cell line (KID-KC) was established from primary patient KCs with a heterozygous p.D50N mutation. This cell line displayed impaired gap junction communication and hyperactive hemichannels, confirmed by dye transfer, patch clamp, and neurobiotin uptake assays. A human-murine chimeric skin graft model constructed with KID-KCs mimicked patient skin in vivo, further confirming the validity of these cells as a model. In vitro treatment with allele-specific small interfering RNA led to robust inhibition of the mutant GJB2 allele without altering expression of the wild-type allele. This corrected both gap junction and hemichannel activity. Notably, allele-specific small interfering RNA treatment caused only low-level off-target effects in KID-KCs, as detected by genome-wide RNA sequencing. Our data provide an important proof-of-concept and model system for the potential use of allele-specific small interfering RNA in treating KID syndrome and other dominant genetic conditions.

Cx26 keratitis ichthyosis deafness syndrome mutations trigger alternative splicing of Cx26 to prevent expression and cause toxicity in vitro

The Cx26 mRNA has not been reported to undergo alternative splicing. In expressing a series of human keratitis ichthyosis deafness (KID) syndrome mutations of Cx26 (A88V, N14K and A40V), we found the production of a truncated mRNA product. These mutations, although not creating a cryptic splice site, appeared to activate a pre-existing cryptic splice site. The alternative splicing of the mutant Cx26 mRNA could be prevented by mutating the predicted 3', 5' splice sites and the branch point. The presence of a C-terminal fluorescent protein tag (mCherry or Clover) was necessary for this alternative splicing to occur. Strangely, Cx26^A88V could cause the alternative splicing of co-expressed WT Cx26-suggesting a trans effect. The alternative splicing of Cx26^A88V caused cell death, and this could be prevented by the 3', 5' and branch point mutations. Expression of the KID syndrome mutants could be rescued by combining them with removal of the 5' splice site. We used this strategy to enable expression of Cx26^A40V-5' and demonstrate that this KID syndrome mutation removed CO₂ sensitivity from the Cx26 hemichannel. This is the fourth KID syndrome mutation found to abolish the CO₂-sensitivity of the Cx26 hemichannel, and suggests that the altered CO_-2-sensitivity could contribute to the pathology of this mutation. Future research on KID syndrome mutations should take care to avoid using a C-terminal tag to track cellular localization and expression or if this is unavoidable, combine this mutation with removal of the 5' splice site.

Inhibitors of connexin and pannexin channels as potential therapeutics

While gap junctions support the exchange of a number of molecules between neighboring cells, connexin hemichannels provide communication between the cytosol and the extracellular environment of an individual cell. The latter equally holds true for channels composed of pannexin proteins, which display an architecture reminiscent of connexin hemichannels. In physiological conditions, gap junctions are usually open, while connexin hemichannels and, to a lesser extent, pannexin channels are typically closed, yet they can be activated by a number of pathological triggers. Several agents are available to inhibit channels built up by connexin and pannexin proteins, including alcoholic substances, glycyrrhetinic acid, anesthetics and fatty acids. These compounds not always strictly distinguish between gap junctions, connexin hemichannels and pannexin channels, and may have effects on other targets as well. An exception lies with mimetic peptides, which reproduce specific amino acid sequences in connexin or pannexin primary protein structure. In this paper, a state-of-the-art overview is provided on inhibitors of cellular channels consisting of connexins and pannexins with specific focus on their mode-of-action and therapeutic potential.

Altered CO2 sensitivity of connexin26 mutant hemichannels in vitro

Connexin26 (Cx26) mutations underlie human pathologies ranging from hearing loss to keratitis ichthyosis deafness (KID) syndrome. Cx26 hemichannels are directly gated by CO₂ and contribute to the chemosensory regulation of breathing. The KID syndrome mutation A88V is insensitive to CO₂, and has a dominant negative action on the CO₂ sensitivity of Cx26^WT hemichannels, and reduces respiratory drive in humans. We have now examined the effect of further human mutations of Cx26 on its sensitivity to CO₂ : Mutated Cx26 subunits, carrying one of A88S, N14K, N14Y, M34T, or V84L, were transiently expressed in HeLa cells. The CO₂-dependence of hemichannel activity, and their ability to exert dominant negative actions on cells stably expressing Cx26^WT, was quantified by a dye-loading assay. The KID syndrome mutation, N14K, abolished the sensitivity of Cx26 to CO₂ Both N14Y and N14K exerted a powerful dominant negative action on the CO₂ sensitivity of Cx26^WT None of the other mutations (all recessive) had a dominant negative action. A88S shifted the affinity of Cx26 to slightly higher levels without reducing its ability to fully open to CO₂ M34T did not change the affinity of Cx26 for CO₂ but reduced its ability to open in response to CO₂ V84L had no effect on the CO₂-sensitivity of Cx26. Some pathological mutations of Cx26 can therefore alter the CO₂ sensitivity of Cx26 hemichannels. The loss of CO₂ sensitivity could contribute to pathology and consequent reduced respiratory drive could be an unrecognized comorbidity of these pathologies.

Design and Characterization of a Human Monoclonal Antibody that Modulates Mutant Connexin 26 Hemichannels Implicated in Deafness and Skin Disorders

Background: Mutations leading to changes in properties, regulation, or expression of connexin-made channels have been implicated in 28 distinct human hereditary diseases. Eight of these result from variants of connexin 26 (Cx26), a protein critically involved in cell-cell signaling in the inner ear and skin. Lack of non-toxic drugs with defined mechanisms of action poses a serious obstacle to therapeutic interventions for diseases caused by mutant connexins. In particular, molecules that specifically modulate connexin hemichannel function without affecting gap junction channels are considered of primary importance for the study of connexin hemichannel role in physiological as well as pathological conditions. Monoclonal antibodies developed in the last three decades have become the most important class of therapeutic biologicals. Recombinant methods permit rapid selection and improvement of monoclonal antibodies from libraries with large diversity.

Methods: By screening a combinatorial library of human single-chain fragment variable (scFv) antibodies expressed in phage, we identified a candidate that binds an extracellular epitope of Cx26. We characterized antibody action using a variety of biochemical and biophysical assays in HeLa cells, organotypic cultures of mouse cochlea and human keratinocyte-derived cells.

Results: We determined that the antibody is a remarkably efficient, non-toxic, and completely reversible inhibitor of hemichannels formed by connexin 26 and does not affect direct cell-cell communication via gap junction channels. Importantly, we also demonstrate that the antibody efficiently inhibits hyperative mutant Cx26 hemichannels implicated in autosomal dominant non-syndromic hearing impairment accompanied by keratitis and hystrix-like ichthyosis-deafness (KID/HID) syndrome. We solved the crystal structure of the antibody, identified residues that are critical for binding and used molecular dynamics to uncover its mechanism of action.

Conclusions: Although further studies will be necessary to validate the effect of the antibody in vivo, the methodology described here can be extended to select antibodies against hemichannels composed by other connexin isoforms and, consequently, to target other pathologies associated with hyperactive hemichannels. Our study highlights the potential of this approach and identifies connexins as therapeutic targets addressable by screening phage display libraries expressing human randomized antibodies.

Altered cellular localization and hemichannel activities of KID syndrome associated connexin26 I30N and D50Y mutations

Background

Gap junctions facilitate exchange of small molecules between adjacent cells, serving a crucial function for the maintenance of cellular homeostasis. Mutations in connexins, the basic unit of gap junctions, are associated with several human hereditary disorders. For example, mutations in connexin26 (Cx26) cause both non-syndromic deafness and syndromic deafness associated with skin abnormalities such as keratitis-ichthyosis-deafness (KID) syndrome. These mutations can alter the formation and function of gap junction channels through different mechanisms, and in turn interfere with various cellular processes leading to distinct disorders. The KID associated Cx26 mutations were mostly shown to result in elevated hemichannel activities. However, the effects of these aberrant hemichannels on cellular processes are recently being deciphered. Here, we assessed the effect of two Cx26 mutations associated with KID syndrome, Cx26I30N and D50Y, on protein biosynthesis and channel function in N2A and HeLa cells.

Results

Immunostaining experiments showed that Cx26I30N and D50Y failed to form gap junction plaques at cell-cell contact sites. Further, these mutations resulted in the retention of Cx26 protein in the Golgi apparatus. Examination of hemichannel function by fluorescent dye uptake assays revealed that cells with Cx26I30N and D50Y mutations had increased dye uptake compared to Cx26WT (wild-type) containing cells, indicating abnormal hemichannel activities. Cells with mutant proteins had elevated intracellular calcium levels compared to Cx26WT transfected cells, which were abolished by a hemichannel blocker, carbenoxolone (CBX), as measured by Fluo-3 AM loading and flow cytometry.

Conclusions

Here, we demonstrated that Cx26I30N and D50Y mutations resulted in the formation of aberrant hemichannels that might result in elevated intracellular calcium levels, a process which may contribute to the hyperproliferative epidermal phenotypes of KID syndrome.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-016-0081-0) contains supplementary material, which is available to authorized users.

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.

Complex Membrane Channel Blockade: A Unifying Hypothesis for the Prodromal and Acute Neuropsychiatric Sequelae Resulting from Exposure to the Antimalarial Drug Mefloquine

The alkaloid toxin quinine and its derivative compounds have been used for many centuries as effective medications for the prevention and treatment of malaria. More recently, synthetic derivatives, such as the quinoline derivative mefloquine (bis(trifluoromethyl)-(2-piperidyl)-4-quinolinemethanol), have been widely used to combat disease caused by chloroquine-resistant strains of the malaria parasite, Plasmodium falciparum. However, the parent compound quinine, as well as its more recent counterparts, suffers from an incidence of adverse neuropsychiatric side effects ranging from mild mood disturbances and anxiety to hallucinations, seizures, and psychosis. This review considers how the pharmacology, cellular neurobiology, and membrane channel kinetics of mefloquine could lead to the significant and sometimes life-threatening neurotoxicity associated with mefloquine exposure. A key role for mefloquine blockade of ATP-sensitive potassium channels and connexins in the substantia nigra is considered as a unifying hypothesis for the pathogenesis of severe neuropsychiatric events after mefloquine exposure in humans.

Erythrokeratodermia variabilis et progressiva allelic to oculo-dento-digital dysplasia

Erythrokeratodermia variabilis et progressiva (EKVP) is a genodermatosis with clinical and genetic heterogeneity, most often transmitted in an autosomal dominant manner, caused by mutations in GJB3 and GJB4 genes encoding connexins (Cx)31 and 30.3, respectively. In this issue, Boyden et al. (2015) report for the first time de novo dominant mutations in GJA1 encoding the ubiquitous Cx43 in patients with EKVP. These results expand the genetic heterogeneity of EKVP and the human disease phenotypes associated with GJA1 mutations. They disclose that EKVP is allelic to oculo-dento-digital dysplasia, a rare syndrome previously known to be caused by dominant GJA1 mutations.

Connexin hemichannels influence genetically determined inflammatory and hyperproliferative skin diseases

Connexin mutations underlie numerous human genetic diseases. Several connexin genes have been linked to skin diseases, and mechanistic studies have indicated that a gain of abnormal channel function may be responsible for pathology. The topical accessibility of the epidermal connexins, the existence of several mouse models of human skin disease, and the ongoing identification of pharmacological inhibitors targeting connexins provide an opportunity to test new therapeutic approaches.

The double life of connexin channels: single is a treat

Although several genetic diseases are caused by mutations in channels made by connexin family members, there has been little progress in the development and validation of therapeutic options. An in vitro study in this issue of JID suggests that an anti-malarial drug may be beneficial in keratitis-ichthyosis deafness, a severe conexin channel disease associated with potentially fatal recurrent infections.