Hypoglycemia-Exacerbated Mitochondrial Connexin 43 Accumulation Aggravates Cardiac Dysfunction in Diabetic Cardiomyopathy

Phosphoproteomics reveals a novel mechanism underlying the proarrhythmic effects of nilotinib, vandetanib, and mobocertinib

The use of tyrosine kinase inhibitors (TKIs) has resulted in significant occurrence of arrhythmias. However, the precise mechanism of the proarrhythmic effect is not fully understood. In this study, we found that nilotinib (NIL), vandetanib (VAN), and mobocertinib (MOB) induced the development of "cellrhythmia" (arrhythmia-like events) in a concentration-dependent manner in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). Continuous administration of NIL, VAN, or MOB in animals significantly prolonged the action potential durations (APD) and increased susceptibility to arrhythmias. Using phosphoproteomic analysis, we identified proteins with altered phosphorylation levels after treatment with 3 μM NIL, VAN, and MOB for 1.5 h. Using these identified proteins as substrates, we performed kinase-substrate enrichment analysis to identify the kinases driving the changes in phosphorylation levels of these proteins. MAPK and WNK were both inhibited by NIL, VAN, and MOB. A selective inhibitor of WNK1, WNK-IN-11, induced concentration- and time-dependent cellrhythmias and prolonged field potential duration (FPD) in hiPSC-CMs in vitro; furthermore, administration in guinea pigs confirmed that WNK-IN-11 prolonged ventricular repolarization and increased susceptibility to arrhythmias. Fingding indicated that WNK1 inhibition had an in vivo and in vitro arrhythmogenic phenotype similar to TKIs. Additionally,three of TKIs reduced hERG and KCNQ1 expression at protein level, not at transcription level. Similarly, the knockdown of WNK1 decreased hERG and KCNQ1 protein expression in hiPSC-CMs. Collectively, our data suggest that the proarrhythmic effects of NIL, VAN, and MOB occur through a kinase inhibition mechanism. NIL, VAN, and MOB inhibit WNK1 kinase, leading to a decrease in hERG and KCNQ1 protein expression, thereby prolonging action potential repolarization and consequently cause arrhythmias.

Gap Junctions or Hemichannel-Dependent and Independent Roles of Connexins in Fibrosis, Epithelial-Mesenchymal Transitions, and Wound Healing

Fibrosis initially appears as a normal response to damage, where activated fibroblasts produce large amounts of the extracellular matrix (ECM) during the wound healing process to assist in the repair of injured tissue. However, the excessive accumulation of the ECM, unresolved by remodeling mechanisms, leads to organ dysfunction. Connexins, a family of transmembrane channel proteins, are widely recognized for their major roles in fibrosis, the epithelial-mesenchymal transition (EMT), and wound healing. Efforts have been made in recent years to identify novel mediators and targets for this regulation. Connexins form gap junctions and hemichannels, mediating communications between neighboring cells and inside and outside of cells, respectively. Recent evidence suggests that connexins, beyond forming channels, possess channel-independent functions in fibrosis, the EMT, and wound healing. One crucial channel-independent function is their role as the primary functional component for cell adhesion. Other channel-independent functions of connexins involve their roles in mitochondria and exosomes. This review summarizes the latest advances in the channel-dependent and independent roles of connexins in fibrosis, the EMT, and wound healing, with a particular focus on eye diseases, emphasizing their potential as novel, promising therapeutic targets.

Mitochondrial Connexins and Mitochondrial Contact Sites with Gap Junction Structure

Mitochondria contain connexins, a family of proteins that is known to form gap junction channels. Connexins are synthesized in the endoplasmic reticulum and oligomerized in the Golgi to form hemichannels. Hemichannels from adjacent cells dock with one another to form gap junction channels that aggregate into plaques and allow cell-cell communication. Cell-cell communication was once thought to be the only function of connexins and their gap junction channels. In the mitochondria, however, connexins have been identified as monomers and assembled into hemichannels, thus questioning their role solely as cell-cell communication channels. Accordingly, mitochondrial connexins have been suggested to play critical roles in the regulation of mitochondrial functions, including potassium fluxes and respiration. However, while much is known about plasma membrane gap junction channel connexins, the presence and function of mitochondrial connexins remain poorly understood. In this review, the presence and role of mitochondrial connexins and mitochondrial/connexin-containing structure contact sites will be discussed. An understanding of the significance of mitochondrial connexins and their connexin contact sites is essential to our knowledge of connexins' functions in normal and pathological conditions, and this information may aid in the development of therapeutic interventions in diseases linked to mitochondria.

Regulation of Cx43 and its role in trichloroethylene-induced cardiac toxicity in H9C2 rat cardiomyocytes

Trichloroethylene (TCE), a widespread environmental contaminant, has been linked to congenital heart defects. Abnormal regulation of Connexin 43 is closely associated with various cardiac diseases. However, it is yet to be established how Cx43 responds to environmental pollutants. Here, we aim to explore the role of Cx43 in TCE-induced cardiac toxicity using H9C2 cardiomyocytes. EdU incorporation assay and cell cycle analysis revealed that increased number of TCE-treated cells entered into the S stage, indicating that TCE exposure provoked cell proliferation. Additionally, compromised mitochondrial function was observed in TCE-treated cells, and inhibition of mitochondrial permeability transition pore (mPTP) with Cyclosporin A or eliminating mitochondrial ROS by MitoQ alleviated the TCE-induced cardiac toxicity. Importantly, TCE exposure increased the protein expression levels of Cx43 and stimulated the recruitment of Cx43 to the mitochondria. TCE exposure disrupted canonical Wnt signal pathway, resulting in downregulation of antioxidant genes and β-catenin. The adverse effects of TCE on Wnt signal pathway activation, mitochondrial function and cell proliferation were efficiently counteracted by either Cx43 knockdown or pharmaceutical activator of Wnt signaling, CHIR-99021. Taken together, our results for the first time revealed that dysregulation of Cx43 mediates TCE-induced heart defects via mitochondrial dysfunction and Wnt signaling inhibition, suggesting that Cx43 can be a potential molecular marker or therapeutic target for cardiac diseases caused by environmental pollutants.

Connexins and Glucose Metabolism in Cancer

Connexins are a family of transmembrane proteins that regulate diverse cellular functions. Originally characterized for their ability to mediate direct intercellular communication through the formation of highly regulated membrane channels, their functions have been extended to the exchange of molecules with the extracellular environment, and the ability to modulate numerous channel-independent effects on processes such as motility and survival. Notably, connexins have been implicated in cancer biology for their context-dependent roles that can both promote or suppress cancer cell function. Moreover, connexins are able to mediate many aspects of cellular metabolism including the intercellular coupling of nutrients and signaling molecules. During cancer progression, changes to substrate utilization occur to support energy production and biomass accumulation. This results in metabolic plasticity that promotes cell survival and proliferation, and can impact therapeutic resistance. Significant progress has been made in our understanding of connexin and cancer biology, however, delineating the roles these multi-faceted proteins play in metabolic adaptation of cancer cells is just beginning. Glucose represents a major carbon substrate for energy production, nucleotide synthesis, carbohydrate modifications and generation of biosynthetic intermediates. While cancer cells often exhibit a dependence on glycolytic metabolism for survival, cellular reprogramming of metabolic pathways is common when blood perfusion is limited in growing tumors. These metabolic changes drive aggressive phenotypes through the acquisition of functional traits. Connections between glucose metabolism and connexin function in cancer cells and the surrounding stroma are now apparent, however much remains to be discovered regarding these relationships. This review discusses the existing evidence in this area and highlights directions for continued investigation.

Connexins and angiogenesis: Functional aspects, pathogenesis, and emerging therapies (Review)

Connexins (Cxs) play key roles in cellular communication. By facilitating metabolite exchange or interfering with distinct signaling pathways, Cxs affect cell homeostasis, proliferation, and differentiation. Variations in the activity and expression of Cxs have been linked to numerous clinical conditions including carcinomas, cardiac disorders, and wound healing. Recent discoveries on the association between Cxs and angiogenesis have sparked interest in Cx-mediated angiogenesis due to its essential functions in tissue formation, wound repair, tumor growth, and metastasis. It is now widely recognized that understanding the association between Cxs and angiogenesis may aid in the development of new targeted therapies for angiogenic diseases. The aim of the present review was to provide a comprehensive overview of Cxs and Cx-mediated angiogenesis, with a focus on therapeutic implications.

Connexin 43 in Mitochondria: What Do We Really Know About Its Function?

Connexins are known for their ability to mediate cell-cell communication via gap junctions and also form hemichannels that pass ions and molecules over the plasma membrane when open. Connexins have also been detected within mitochondria, with mitochondrial connexin 43 (Cx43) being the best studied to date. In this review, we discuss evidence for Cx43 presence in mitochondria of cell lines, primary cells and organs and summarize data on its localization, import and phosphorylation status. We further highlight the influence of Cx43 on mitochondrial function in terms of respiration, opening of the mitochondrial permeability transition pore and formation of reactive oxygen species, and also address the presence of a truncated form of Cx43 termed Gja1-20k. Finally, the role of mitochondrial Cx43 in pathological conditions, particularly in the heart, is discussed.