Cx46 hemichannel modulation by nitric oxide: Role of the fourth transmembrane helix cysteine and its possible involvement in cataract formation

Connexins in Cancer, the Possible Role of Connexin46 as a Cancer Stem Cell-Determining Protein

Cancer is a widespread and incurable disease caused by genetic mutations, leading to uncontrolled cell proliferation and metastasis. Connexins (Cx) are transmembrane proteins that facilitate intercellular communication via hemichannels and gap junction channels. Among them, Cx46 is found mostly in the eye lens. However, in pathological conditions, Cx46 has been observed in various types of cancers, such as glioblastoma, melanoma, and breast cancer. It has been demonstrated that elevated Cx46 levels in breast cancer contribute to cellular resistance to hypoxia, and it is an enhancer of cancer aggressiveness supporting a pro-tumoral role. Accordingly, Cx46 is associated with an increase in cancer stem cell phenotype. These cells display radio- and chemoresistance, high proliferative abilities, self-renewal, and differentiation capacities. This review aims to consolidate the knowledge of the relationship between Cx46, its role in forming hemichannels and gap junctions, and its connection with cancer and cancer stem cells.

Extracellular Cysteines Are Critical to Form Functional Cx46 Hemichannels

Connexin (Cxs) hemichannels participate in several physiological and pathological processes, but the molecular mechanisms that control their gating remain elusive. We aimed at determining the role of extracellular cysteines (Cys) in the gating and function of Cx46 hemichannels. We studied Cx46 and mutated all of its extracellular Cys to alanine (Ala) (one at a time) and studied the effects of the Cys mutations on Cx46 expression, localization, and hemichannel activity. Wild-type Cx46 and Cys mutants were expressed at comparable levels, with similar cellular localization. However, functional experiments showed that hemichannels formed by the Cys mutants did not open either in response to membrane depolarization or removal of extracellular divalent cations. Molecular-dynamics simulations showed that Cys mutants may show a possible alteration in the electrostatic potential of the hemichannel pore and an altered disposition of important residues that could contribute to the selectivity and voltage dependency in the hemichannels. Replacement of extracellular Cys resulted in "permanently closed hemichannels", which is congruent with the inhibition of the Cx46 hemichannel by lipid peroxides, through the oxidation of extracellular Cys. These results point to the modification of extracellular Cys as potential targets for the treatment of Cx46-hemichannel associated pathologies, such as cataracts and cancer, and may shed light into the gating mechanisms of other Cx hemichannels.

Role and Posttranslational Regulation of Cx46 Hemichannels and Gap Junction Channels in the Eye Lens

Connexins are a family of proteins that can form two distinct types of channels: hemichannels and gap junction channels. Hemichannels are composed of six connexin subunits and when open allow for exchanges between the cytoplasm and the extracellular milieu. Gap junction channels are formed by head-to-head docking of two hemichannels in series, each one from one of two adjacent cells. These channels allow for exchanges between the cytoplasms of contacting cells. The lens is a transparent structure located in the eye that focuses light on the retina. The transparency of the lens depends on its lack of blood irrigation and the absence of organelles in its cells. To survive such complex metabolic scenario, lens cells express Cx43, Cx46 and Cx50, three connexins isoforms that form hemichannels and gap junction channels that allow for metabolic cooperation between lens cells. This review focuses on the roles of Cx46 hemichannels and gap junction channels in the lens under physiological conditions and in the formation of cataracts, with emphasis on the modulation by posttranslational modifications.

Mutations of CX46/CX50 and Cataract Development

Cataract is a common disease in the aging population. Gap junction has been considered a central component in maintaining homeostasis for preventing cataract formation. Gap junction channels consist of connexin proteins with more than 20 members. Three genes including GJA1, GJA3, and GJA8, that encode protein Cx43 (connexin43), Cx46 (connexin46), and Cx50 (connexin50), respectively, have been identified in human and rodent lens. Cx46 together with Cx50 have been detected in lens fiber cells with high expression, whereas Cx43 is mainly expressed in lens epithelial cells. Disrupted expression of the two connexin proteins Cx46 and Cx50 is directly related to the development of severe cataract in human and mice. In this review article, we describe the main role of Cx46 and Cx50 connexin proteins in the lens and the relationship between mutations of Cx46 or Cx50 and hereditary cataracts. Furthermore, the latest progress in the fundamental research of lens connexin and the mechanism of cataract formation caused by lens connexin dysfunction are summarized. Overall, targeting connexin could be a novel approach for the treatment of cataract.

Over-activated hemichannels: A possible therapeutic target for human diseases

In our body, all the cells are constantly sharing chemical and electrical information with other cells. This intercellular communication allows them to respond in a concerted way to changes in the extracellular milieu. Connexins are transmembrane proteins that have the particularity of forming two types of channels; hemichannels and gap junction channels. Under normal conditions, hemichannels allow the controlled release of signaling molecules to the extracellular milieu. However, under certain pathological conditions, over-activated hemichannels can induce and/or exacerbate symptoms. In the last decade, great efforts have been put into developing new tools that can modulate these over-activated hemichannels. Small molecules, antibodies and mimetic peptides have shown a potential for the treatment of human diseases. In this review, we summarize recent findings in the field of hemichannel modulation via specific tools, and how these tools could improve patient outcome in certain pathological conditions.

The mutual interplay of redox signaling and connexins

Connexins (Cxs) are ubiquitous transmembrane proteins that possess both channel function (e.g., formations of gap junction and hemichannel) and non-channel properties (e.g., gene transcription and protein-protein interaction). Several factors have been identified to play a role in the regulation of Cxs, which include those acting intracellularly, as redox potential, pH, intramolecular interactions, and post-translational modifications (e.g., phosphorylation, S-nitrosylation) as well as those acting extracellularly, such as Ca²⁺ and Mg²⁺. The relationship between redox signaling and Cxs attracts considerable attention in recent years. There is ample evidence showing that redox signaling molecules (e.g., hydrogen peroxide (H₂O₂), nitric oxide (NO)) affect Cxs-based channel function while the opening of Cx channels also triggers the transfer of various redox-related metabolites (e.g., reactive oxygen species, glutathione, nicotinamide adenine dinucleotide, and NO). On the basis of these evidences, we propose the existence of redox-Cxs crosstalk. In this review, we briefly discuss the interaction between redox signaling and Cxs and the implications of the intersection in disease pathology and future therapeutic interventions.

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.

4-Hydroxynonenal induces Cx46 hemichannel inhibition through its carbonylation

Hemichannels formed by connexins mediate the exchange of ions and signaling molecules between the cytoplasm and the extracellular milieu. Under physiological conditions hemichannels have a low open probability, but in certain pathologies their open probability increases, which can result in cell damage. Pathological conditions are characterized by the production of a number of proinflammatory molecules, including 4-hydroxynonenal (4-HNE), one of the most common lipid peroxides produced in response to inflammation and oxidative stress. The aim of this work was to evaluate whether 4-HNE modulates the activity of Cx46 hemichannels. We found that 4-HNE (100 μM) reduced the rate of 4',6-diamino-2-fenilindol (DAPI) uptake through hemichannels formed by recombinant human Cx46 fused to green fluorescent protein, an inhibition that was reversed partially by 10 mM dithiothreitol. Immunoblot analysis showed that the recombinant Cx46 expressed in HeLa cells becomes carbonylated after exposure to 4-HNE, and that 10 mM dithiothreitol reduced its carbonylation. We also found that Cx46 was carbonylated by 4-HNE in the lens of a selenite-induced cataract animal model. The exposure to 100 μM 4-HNE decreased hemichannel currents formed by recombinant rat Cx46 in Xenopus laevis oocytes. This inhibition also occurred in a mutant expressing only the extracellular loop cysteines, suggesting that other Cys are not responsible for the hemichannel inhibition by carbonylation. This work demonstrates for the first time that Cx46 is post-translationally modified by a lipid peroxide and that this modification reduces Cx46 hemichannel activity.

Role of ROS/RNS in Preeclampsia: Are Connexins the Missing Piece?

Preeclampsia is a pregnancy complication that appears after 20 weeks of gestation and is characterized by hypertension and proteinuria, affecting both mother and offspring. The cellular and molecular mechanisms that cause the development of preeclampsia are poorly understood. An important feature of preeclampsia is an increase in oxygen and nitrogen derived free radicals (reactive oxygen species/reactive nitrogen species (ROS/RNS), which seem to be central players setting the development and progression of preeclampsia. Cell-to-cell communication may be disrupted as well. Connexins (Cxs), a family of transmembrane proteins that form hemichannels and gap junction channels (GJCs), are essential in paracrine and autocrine cell communication, allowing the movement of signaling molecules between cells as well as between the cytoplasm and the extracellular media. GJCs and hemichannels are fundamental for communication between endothelial and smooth muscle cells and, therefore, in the control of vascular contraction and relaxation. In systemic vasculature, the activity of GJCs and hemichannels is modulated by ROS and RNS. Cxs participate in the development of the placenta and are expressed in placental vasculature. However, it is unknown whether Cxs are modulated by ROS/RNS in the placenta, or whether this potential modulation contributes to the pathogenesis of preeclampsia. Our review addresses the possible role of Cxs in preeclampsia, and the plausible modulation of Cxs-formed channels by ROS and RNS. We suggest these factors may contribute to the development of preeclampsia.

A Simple Assay to Evaluate the Function of Human Connexin Hemichannels Expressed in Escherichia coli that Can Be Used for Drug Discovery and Mutant Analysis

Abnormally increased activity of connexin hemichannels contributes to cell damage in many disorders, including deafness, stroke, and cardiac infarct, and therefore hemichannels constitute a potentially important therapeutic target. Unfortunately, the available hemichannel inhibitors are not specific and most are toxic. The absence of a simple and cost-effective screening assay has made the discovery of hemichannel inhibitors difficult. Here, we present an optimized assay where human connexins are expressed in genetically modified Escherichia coli cells deficient in potassium uptake (LB2003 cells). These cells cannot grow in low-potassium medium, and hemichannel function is assayed by the reversion of the no-growth phenotype. Since functional hemichannels are permeable to potassium, they allow for its uptake and cell growth. The simple reading of bacterial growth in low-potassium medium distinguishes functional hemichannels (growth) from those inhibited (no growth). This assay is simple, robust, inexpensive, and reliable, and is easily scaled to high-throughput multiwell platforms. © 2019 by John Wiley & Sons, Inc. Basic Protocol 1: Preparation of competent LB2003 cells resistant to kanamycin Basic Protocol 2: Growth complementation assay Support Protocol: Evaluation of cytotoxic effects of potential connexin hemichannel inhibitors.