Connexin 43 connexon to gap junction transition is regulated by zonula occludens-1

Altered connexin 43 expression underlies age-dependent decrease of regulatory T cell suppressor function in nonobese diabetic mice

Type 1 diabetes is one of the most extensively studied autoimmune diseases, but the cellular and molecular mechanisms leading to T cell-mediated destruction of insulin-producing β cells are still not well understood. In this study, we show that regulatory T cells (T(regs)) in NOD mice undergo age-dependent loss of suppressor functions exacerbated by the decreased ability of activated effector T cells to upregulate Foxp3 and generate T(regs) in the peripheral organs. This age-dependent loss is associated with reduced intercellular communication mediated by gap junctions, which is caused by impaired upregulation and decreased expression of connexin 43. Regulatory functions can be corrected, even in T cells isolated from aged, diabetic mice, by a synergistic activity of retinoic acid, TGF-β, and IL-2, which enhance connexin 43 and Foxp3 expression in T(regs) and restore the ability of conventional CD4(+) T cells to upregulate Foxp3 and generate peripherally derived T(regs). Moreover, we demonstrate that suppression mediated by T(regs) from diabetic mice is enhanced by a novel reagent, which facilitates gap junction aggregation. In summary, our report identifies gap junction-mediated intercellular communication as an important component of the T(reg) suppression mechanism compromised in NOD mice and suggests how T(reg) mediated immune regulation can be improved.

Targeting connexin 43 with α-connexin carboxyl-terminal (ACT1) peptide enhances the activity of the targeted inhibitors, tamoxifen and lapatinib, in breast cancer: clinical implication for ACT1

BACKGROUND

Treatment failure is a critical issue in breast cancer and identifying useful interventions that optimize current cancer therapies remains a critical unmet need. Expression and functional studies have identified connexins (Cxs), a family of gap junction proteins, as potential tumor suppressors. Studies suggest that Cx43 has a role in breast cancer cell proliferation, differentiation, and migration. Although pan-gap junction drugs are available, the lack of specificity of these agents increases the opportunity for off target effects. Consequently, a therapeutic agent that specifically modulates Cx43 would be beneficial and has not been tested in breast cancer. In this study, we now test an agent that specifically targets Cx43, called ACT1, in breast cancer.

METHODS

We evaluated whether direct modulation of Cx43 using a Cx43-directed therapeutic peptide, called ACT1, enhances Cx43 gap junctional activity in breast cancer cells, impairs breast cancer cell proliferation or survival, and enhances the activity of the targeted inhibitors tamoxifen and lapatinib.

RESULTS

Our results show that therapeutic modulation of Cx43 by ACT1 maintains Cx43 at gap junction sites between cell-cell membrane borders of breast cancer cells and augments gap junction activity in functional assays. The increase in Cx43 gap junctional activity achieved by ACT1 treatment impairs proliferation or survival of breast cancer cells but ACT1 has no effect on non-transformed MCF10A cells. Furthermore, treating ER+ breast cancer cells with a combination of ACT1 and tamoxifen or HER2+ breast cancer cells with ACT1 and lapatinib augments the activity of these targeted inhibitors.

CONCLUSIONS

Based on our findings, we conclude that modulation of Cx43 activity in breast cancer can be effectively achieved with the agent ACT1 to sustain Cx43-mediated gap junctional activity resulting in impaired malignant progression and enhanced activity of lapatinib and tamoxifen, implicating ACT1 as part of a combination regimen in breast cancer.

Inhibition of connexin 43 hemichannel-mediated ATP release attenuates early inflammation during the foreign body response

BACKGROUND

In the last 50 years, the use of medical implants has increased dramatically. Failure of implanted devices and biomaterials is a significant source of morbidity and increasing healthcare expenditures. An important cause of implant failure is the host inflammatory response. Recent evidence implicates extracellular ATP as an important inflammatory signaling molecule. A major pathway for release of cytoplasmic ATP into the extracellular space is through connexin hemichannels, which are the unpaired constituents of gap junction intercellular channels. Blockade of hemichannels of the connexin 43 (Cx43) isoform has been shown to reduce inflammation and improve healing. We have developed a Cx43 mimetic peptide (JM2) that targets the microtubule-binding domain of Cx43. The following report investigates the role of the Cx43 microtubule-binding domain in extracellular ATP release by Cx43 hemichannels and how this impacts early inflammatory events of the foreign body reaction.

METHODS

In vitro Cx43 hemichannel-mediated ATP release by cultured human microvascular endothelial cells subjected to hypocalcemic and normocalcemic conditions was measured after application of JM2 and the known hemichannel blocker, flufenamic acid. A submuscular silicone implant model was used to investigate in vivo ATP signaling during the early foreign body response. Implants were coated with control pluronic vehicle or pluronic carrying JM2, ATP, JM2+ATP, or known hemichannel blockers and harvested at 24 h for analysis.

RESULTS

JM2 significantly inhibited connexin hemichannel-mediated ATP release from cultured endothelial cells. Importantly, the early inflammatory response to submuscular silicone implants was inhibited by JM2. The reduction in inflammation by JM2 was reversed by the addition of exogenous ATP to the pluronic vehicle.

CONCLUSIONS

These data indicate that ATP released through Cx43 hemichannels into the vasculature is an important signal driving the early inflammatory response to implanted devices. A vital aspect of this work is that it demonstrates that targeted molecular therapeutics, such as JM2, provide the capacity to regulate inflammation in a clinically relevant system.

The role of connexin43-Src interaction in astrocytomas: A molecular puzzle

Connexin43 (Cx43) as a building block of gap junction channels and hemichannels exerts important functions in astrocytes. When these cells acquire a malignant phenotype Cx43 protein but not mRNA levels are downregulated, being negligible in high-grade astrocytoma or glioblastoma multiforme, the most common and deadliest of malignant primary brain tumors in adults. Some microRNAs associated to glioma target Cx43 and could explain the lack of correlation between mRNA and protein levels of Cx43 found in some high-grade astrocytomas. More importantly, these microRNAs could be a promising therapeutic target. A great number of studies have confirmed the relationship between cancer and connexins that was proposed by Loewenstein more than 40years ago, but these studies have also revealed that this is a very complex relationship. Indeed, restoring Cx43 to glioma cells reduces their rate of proliferation and their tumorigenicity but this tumor suppressor effect could be counterbalanced by its effects on invasiveness, adhesion and migration. The mechanisms underlying these effects suggest the participation of a great variety of proteins that bind to different regions of Cx43. The present review focuses on an intrinsically disordered region of the C-terminal domain of Cx43 in which converges the interaction of several proteins, including the proto-oncogene Src. We summarize data that indicate that Cx43-Src interaction inhibits the oncogenic activity of Src and promotes a conformational change in the structure of Cx43 that allosterically modifies the binding to other important signaling proteins. As a consequence, crucial cell functions, such as proliferation or migration, could be strongly affected. We propose that the knowledge of the structural basis of the antitumorigenic effect of Cx43 on astrocytomas could help to design new therapies against this incurable disease.

The carboxyl tail of connexin32 regulates gap junction assembly in human prostate and pancreatic cancer cells

Connexins, the constituent proteins of gap junctions, are transmembrane proteins. A connexin (Cx) traverses the membrane four times and has one intracellular and two extracellular loops with the amino and carboxyl termini facing the cytoplasm. The transmembrane and the extracellular loop domains are highly conserved among different Cxs, whereas the carboxyl termini, often called the cytoplasmic tails, are highly divergent. We have explored the role of the cytoplasmic tail of Cx32, a Cx expressed in polarized and differentiated cells, in regulating gap junction assembly. Our results demonstrate that compared with the full-length Cx32, the cytoplasmic tail-deleted Cx32 is assembled into small gap junctions in human pancreatic and prostatic cancer cells. Our results further document that the expression of the full-length Cx32 in cells, which express the tail-deleted Cx32, increases the size of gap junctions, whereas the expression of the tail-deleted Cx32 in cells, which express the full-length Cx32, has the opposite effect. Moreover, we show that the tail is required for the clustering of cell-cell channels and that in cells expressing the tail-deleted Cx32, the expression of cell surface-targeted cytoplasmic tail alone is sufficient to enhance the size of gap junctions. Our live-cell imaging data further demonstrate that gap junctions formed of the tail-deleted Cx32 are highly mobile compared with those formed of full-length Cx32. Our results suggest that the cytoplasmic tail of Cx32 is not required to initiate the assembly of gap junctions but for their subsequent growth and stability. Our findings suggest that the cytoplasmic tail of Cx32 may be involved in regulating the permeability of gap junctions by regulating their size.

Connexins and skin disease: insights into the role of beta connexins in skin homeostasis

Cell-to-cell communication triggered by connexin channels plays a central role in maintaining epidermal homeostasis. Here, we discuss the role of the beta connexin subgroup, where site-specific mutations in at least 4 of these proteins lead to distinctive non-inflammatory and inflammatory hyperproliferative epidermal disorders. Recent advances in the molecular pathways evoked and correlation with clinical outcome are discussed. The latest data provide increasing evidence that connexins in the epidermis are sensors to environmental stress and that targeting aberrant hemichannel activity holds significant therapeutic potential for inflammatory skin disorders.

Sodium channels in the Cx43 gap junction perinexus may constitute a cardiac ephapse: an experimental and modeling study

It has long been held that electrical excitation spreads from cell-to-cell in the heart via low resistance gap junctions (GJ). However, it has also been proposed that myocytes could interact by non-GJ-mediated “ephaptic” mechanisms, facilitating propagation of action potentials in tandem with direct GJ-mediated coupling. We sought evidence that such mechanisms contribute to cardiac conduction. Using super-resolution microscopy, we demonstrate that Na_v1.5 is localized within 200 nm of the GJ plaque (a region termed the perinexus). Electron microscopy revealed close apposition of adjacent cell membranes within perinexi suggesting that perinexal sodium channels could function as an ephapse, enabling ephaptic cell-to-cell transfer of electrical excitation. Acute interstitial edema (AIE) increased intermembrane distance at the perinexus and was associated with preferential transverse conduction slowing and increased spontaneous arrhythmia incidence. Inhibiting sodium channels with 0.5 μM flecainide uniformly slowed conduction, but sodium channel inhibition during AIE slowed conduction anisotropically and increased arrhythmia incidence more than AIE alone. Sodium channel inhibition during GJ uncoupling with 25 μM carbenoxolone slowed conduction anisotropically and was also highly proarrhythmic. A computational model of discretized extracellular microdomains (including ephaptic coupling) revealed that conduction trends associated with altered perinexal width, sodium channel conductance, and GJ coupling can be predicted when sodium channel density in the intercalated disk is relatively high. We provide evidence that cardiac conduction depends on a mathematically predicted ephaptic mode of coupling as well as GJ coupling. These data suggest opportunities for novel anti-arrhythmic therapies targeting noncanonical conduction pathways in the heart.

Cardiomyocyte protein trafficking: Relevance to heart disease and opportunities for therapeutic intervention

Cardiomyocytes, the individual contractile units of heart muscle, are long-lived and robust. Given the longevity of these cells, it can be easy to overlook their dynamic intracellular environment that contain rapid protein movements and frequent protein turnover. Critical gene transcription and protein translation occur continuously, as well as trafficking and localization of proteins to specific functional zones of cell membrane. As heart failure becomes an increasingly important clinical entity, growing numbers of investigative teams are examining the cell biology of healthy and diseased cardiomyocytes. In this review, we introduce the major architectural structures and types of protein movements within cardiac cells, and then review recent studies that explore the regulation of such movements. We conclude by introducing current translational directions of the basic studies with a focus on novel areas of therapeutic development.

TMEM43 mutation p.S358L alters intercalated disc protein expression and reduces conduction velocity in arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a myocardial disease characterized by fibro-fatty replacement of myocardium in the right ventricular free wall and frequently results in life-threatening ventricular arrhythmias and sudden cardiac death. A heterozygous missense mutation in the transmembrane protein 43 (TMEM43) gene, p.S358L, has been genetically identified to cause autosomal dominant ARVC type 5 in a founder population from the island of Newfoundland, Canada. Little is known about the function of the TMEM43 protein or how it leads to the pathogenesis of ARVC. We sought to determine the distribution of TMEM43 and the effect of the p.S358L mutation on the expression and distribution of various intercalated (IC) disc proteins as well as functional effects on IC disc gap junction dye transfer and conduction velocity in cell culture. Through Western blot analysis, transmission electron microscopy (TEM), immunofluorescence (IF), and electrophysiological analysis, our results showed that the stable expression of p.S358L mutation in the HL-1 cardiac cell line resulted in decreased Zonula Occludens (ZO-1) expression and the loss of ZO-1 localization to cell-cell junctions. Junctional Plakoglobin (JUP) and α-catenin proteins were redistributed to the cytoplasm with decreased localization to cell-cell junctions. Connexin-43 (Cx43) phosphorylation was altered, and there was reduced gap junction dye transfer and conduction velocity in mutant TMEM43-transfected cells. These observations suggest that expression of the p.S358L mutant of TMEM43 found in ARVC type 5 may affect localization of proteins involved in conduction, alter gap junction function and reduce conduction velocity in cardiac tissue.

Benzalkonium chloride suppresses rabbit corneal endothelium intercellular gap junction communication

Purpose

Gap junction intercellular communication (GJIC) plays a critical role in the maintenance of corneal endothelium homeostasis. We determined if benzalkonium chloride (BAK) alters GJIC activity in the rabbit corneal endothelium since it is commonly used as a drug preservative in ocular eyedrop preparations even though it can have cytotoxic effects.

Methods

Thirty-six adult New Zealand albino rabbits were randomly divided into three groups. BAK at 0.01%, 0.05%, and 0.1% was applied twice daily to one eye of each of the rabbits in one of the three groups for seven days. The contralateral untreated eyes were used as controls. Corneal endothelial morphological features were observed by in vivo confocal microscopy (IVCM). Immunofluorescent staining resolved changes in gap junction integrity and localization. Western blot analysis and RT-PCR evaluated changes in levels of connexin43 (Cx43) and tight junction zonula occludens-1 (ZO-1) gene and protein expression, respectively. Cx43 and ZO-1 physical interaction was detected by immunoprecipitation (IP). Primary rabbit corneal endothelial cells were cultured in Dulbecco's Modified Eagle Medium (DMEM) containing BAK for 24 hours. The scrape-loading dye transfer technique (SLDT) was used to assess GJIC activity.

Results

Topical administration of BAK (0.05%, 0.1%) dose dependently disrupted corneal endothelial cell morphology, altered Cx43 and ZO-1 distribution and reduced Cx43 expression. BAK also markedly induced increases in Cx43 phosphorylation status concomitant with decreases in the Cx43-ZO-1 protein-protein interaction. These changes were associated with marked declines in GJIC activity.

Conclusions

The dose dependent declines in rabbit corneal endothelial GJIC activity induced by BAK are associated with less Cx43-ZO-1 interaction possibly arising from increases in Cx43 phosphorylation and declines in its protein expression. These novel changes provide additional evidence that BAK containing eyedrop preparations should be used with caution to avoid declines in corneal transparency resulting from losses in GJIC activity and endothelial function.

Trafficking highways to the intercalated disc: new insights unlocking the specificity of connexin 43 localization

With each heartbeat, billions of cardiomyocytes work in concert to propagate the electrical excitation needed to effectively circulate blood. Regulated expression and timely delivery of connexin proteins to form gap junctions at the specialized cell-cell contact region, known as the intercalated disc, is essential to ventricular cardiomyocyte coupling. We focus this review on several regulatory mechanisms that have been recently found to govern the lifecycle of connexin 43 (Cx43), the short-lived and most abundantly expressed connexin in cardiac ventricular muscle. The Cx43 lifecycle begins with gene expression, followed by oligomerization into hexameric channels, and then cytoskeletal-based transport toward the disc region. Once delivered, hemichannels interact with resident disc proteins and are organized to effect intercellular coupling. We highlight recent studies exploring regulation of Cx43 localization to the intercalated disc, with emphasis on alternatively translated Cx43 isoforms and cytoskeletal transport machinery that together regulate Cx43 gap junction coupling between cardiomyocytes.

Cx43-hemichannel function and regulation in physiology and pathophysiology: insights from the bovine corneal endothelial cell system and beyond

Intercellular communication in primary bovine corneal endothelial cells (BCECs) is mainly driven by the release of extracellular ATP through Cx43 hemichannels. Studying the characteristics of Ca²⁺-wave propagation in BCECs, an important form of intercellular communication, in response to physiological signaling events has led to the discovery of important insights in the functional properties and regulation of native Cx43 hemichannels. Together with ectopic expression models for Cx43 hemichannels and truncated/mutated Cx43 versions, it became very clear that loop/tail interactions play a key role in controlling the activity of Cx43 hemichannels. Interestingly, the negative regulation of Cx43 hemichannels by enhanced actin/myosin contractility seems to impinge upon loss of these loop/tail interactions essential for opening Cx43 hemichannels. Finally, these molecular insights have spurred the development of novel peptide tools that can selectively inhibit Cx43 hemichannels, but neither Cx43 gap junctions nor hemichannels formed by other Cx isoforms. These tools now set the stage to hunt for novel physiological functions for Cx43 hemichannels in primary cells and tissues and to tackle disease conditions associated with excessive, pathological Cx43-hemichannel openings.

Super-resolution imaging reveals that loss of the C-terminus of connexin43 limits microtubule plus-end capture and NaV1.5 localization at the intercalated disc

AIMS

It is well known that connexin43 (Cx43) forms gap junctions. We recently showed that Cx43 is also part of a protein-interacting network that regulates excitability. Cardiac-specific truncation of Cx43 C-terminus (mutant 'Cx43D378stop') led to lethal arrhythmias. Cx43D378stop localized to the intercalated disc (ID); cell-cell coupling was normal, but there was significant sodium current (INa) loss. We proposed that the microtubule plus-end is at the crux of the Cx43-INa relation. Yet, specific localization of relevant molecular players was prevented due to the resolution limit of fluorescence microscopy. Here, we use nanoscale imaging to establish: (i) the morphology of clusters formed by the microtubule plus-end tracking protein 'end-binding 1' (EB1), (ii) their position, and that of sodium channel alpha-subunit NaV1.5, relative to N-cadherin-rich sites, and (iii) the role of Cx43 C-terminus on the above-mentioned parameters and on the location-specific function of INa.

METHODS AND RESULTS

Super-resolution fluorescence localization microscopy in murine adult cardiomyocytes revealed EB1 and NaV1.5 as distinct clusters preferentially localized to N-cadherin-rich sites. Extent of co-localization decreased in Cx43D378stop cells. Macropatch and scanning patch clamp showed reduced INa exclusively at cell end, without changes in unitary conductance. Experiments in Cx43-modified HL1 cells confirmed the relation between Cx43, INa, and microtubules.

CONCLUSIONS

NaV1.5 and EB1 localization at the cell end is Cx43-dependent. Cx43 is part of a molecular complex that determines capture of the microtubule plus-end at the ID, facilitating cargo delivery. These observations link excitability and electrical coupling through a common molecular mechanism.

The effect of a connexin43-based Peptide on the healing of chronic venous leg ulcers: a multicenter, randomized trial

The gap junction protein, connexin43 (Cx43), has critical roles in the inflammatory, edematous, and fibrotic processes following dermal injury and during wound healing, and is abnormally upregulated at the epidermal wound margins of venous leg ulcers (VLUs). Targeting Cx43 with ACT1, a peptide mimetic of the carboxyl-terminus of Cx43, accelerates fibroblast migration and proliferation, and wound reepithelialization. In a prospective, multicenter clinical trial conducted in India, adults with chronic VLUs were randomized to treatment with an ACT1 gel formulation plus conventional standard-of-care (SOC) protocols, involving maintaining wound moisture and four-layer compression bandage therapy, or SOC protocols alone. The primary end point was mean percent ulcer reepithelialization from baseline to 12 weeks. A significantly greater reduction in mean percent ulcer area from baseline to 12 weeks was associated with the incorporation of ACT1 therapy (79% (SD 50.4)) as compared with compression bandage therapy alone (36% (SD 179.8); P=0.02). Evaluation of secondary efficacy end points indicated a reduced median time to 50 and 100% ulcer reepithelialization for ACT1-treated ulcers. Incorporation of ACT1 in SOC protocols may represent a well-tolerated, highly effective therapeutic strategy that expedites chronic venous ulcer healing by treating the underlying ulcer pathophysiology through Cx43-mediated pathways.

Reactionary Dentinogenesis and Neuroimmune Response in Dental Caries

Reactionary dentin formation is an adaptive secretory response mediated by odontoblasts to moderate dentin injury. The implications of this process for neuroimmune interactions operating to contain pathogens have not been fully appreciated. The purpose of the present study was to describe the relationship between reactionary dentinogenesis, the neurogenic changes of dental pulp innervation, and dendritic cell recruitment to caries progression, using a comparative immunohistochemical approach in human teeth from young adult individuals. Reactionary dentin formation during dentin caries progression is associated with changes in the integrity of junctional complexes within the odontoblast layer. Diminished coexpression of Cx43 and zonula occludens 1 implies a reduced level of intercellular connectivity between odontoblasts. Dentin caries also causes overexpression of growth-associated protein 43, a modulator of neural plasticity that promotes extensive sprouting of nerve endings into the reactionary dentin matrix. At the same time, an elevated number of HLA-DR-positive dendritic cells infiltrate the odontoblast layer and subsequently invade reactionary dentin formed underneath the early caries-affected regions. Simultaneous odontoblast layer remodeling, nerve fiber sprouting, and activation of dendritic cells during caries progression suggest a coordinated neuroimmune response to fight caries pathogen invasion and to promote dentin-pulp healing. We propose that reactionary dentin formation hinders pathogen invasion and supports defensive neuroimmune interactions against infection. The eventual understanding of this complex scenario may contribute to the development of novel approaches to dental caries treatment.

A 14-3-3 mode-1 binding motif initiates gap junction internalization during acute cardiac ischemia

Altered phosphorylation and trafficking of connexin 43 (Cx43) during acute ischemia contributes to arrhythmogenic gap junction remodeling, yet the critical sequence and accessory proteins necessary for Cx43 internalization remain unresolved. 14-3-3 proteins can regulate protein trafficking, and a 14-3-3 mode-1 binding motif is activated upon phosphorylation of Ser373 of the Cx43 C-terminus. We hypothesized that Cx43(Ser373) phosphorylation is important to pathological gap junction remodeling. Immunofluorescence in human heart reveals the enrichment of 14-3-3 proteins at intercalated discs, suggesting interaction with gap junctions. Knockdown of 14-3-3τ in cell lines increases gap junction plaque size at cell-cell borders. Cx43(S373A) mutation prevents Cx43/14-3-3 complexing and stabilizes Cx43 at the cell surface, indicating avoidance of degradation. Using Langendorff-perfused mouse hearts, we detect phosphorylation of newly internalized Cx43 at Ser373 and Ser368 within 30 min of no-flow ischemia. Phosphorylation of Cx43 at Ser368 by protein kinase C and Ser255 by mitogen-activated protein kinase has previously been implicated in Cx43 internalization. The Cx43(S373A) mutant is resistant to phosphorylation at both these residues and does not undergo ubiquitination, revealing Ser373 phosphorylation as an upstream gatekeeper of a posttranslational modification cascade necessary for Cx43 internalization. Cx43(Ser373) phosphorylation is a potent target for therapeutic interventions to preserve gap junction coupling in the stressed myocardium.

Atrial fibrillation-linked GJA5/connexin40 mutants impaired gap junctions via different mechanisms

The gap junctions (GJs) formed by Cx40 and Cx43 provide a low resistance passage allowing for rapid propagation of action potentials. Sporadic somatic mutations in GJA5 (encoding Cx40) have been identified in lone atrial fibrillation (AF) patients. More recently germline autosomal dominantly inherited mutations in GJA5 have been found in early onset lone AF patients in several families over generations. Characterizations of these AF-linked Cx40 mutants in model cells and in patient tissues revealed that some of the mutants reduced GJ channel function due to an impaired trafficking or channel formation. While others showed a gain-of-function in hemichannels. These functional alterations in GJs or hemichannel may play an important role in the pathogenesis of AF in the mutant carriers.

Intercellular electrical communication in the heart: a new, active role for the intercalated disk

Cardiac conduction is the propagation of electrical excitation through the heart and is responsible for triggering individual myocytes to contract in synchrony. Canonically, this process has been thought to occur electrotonically, by means of direct flow of ions from cell to cell. The intercalated disk (ID), the site of contact between adjacent myocytes, has been viewed as a structure composed of mechanical junctions that stabilize the apposition of cell membranes and gap junctions which constitute low resistance pathways between cells. However, emerging evidence suggests a more active role for structures within the ID in mediating intercellular electrical communication by means of non-canonical ephaptic mechanisms. This review will discuss the role of the ID in the context of the canonical, electrotonic view of conduction and highlight new, emerging possibilities of its playing a more active role in ephaptic coupling between cardiac myocytes.

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.

Electrical synapses and their functional interactions with chemical synapses

Brain function relies on the ability of neurons to communicate with each other. Interneuronal communication primarily takes place at synapses, where information from one neuron is rapidly conveyed to a second neuron. There are two main modalities of synaptic transmission: chemical and electrical. Far from functioning independently and serving unrelated functions, mounting evidence indicates that these two modalities of synaptic transmission closely interact, both during development and in the adult brain. Rather than conceiving synaptic transmission as either chemical or electrical, this article emphasizes the notion that synaptic transmission is both chemical and electrical, and that interactions between these two forms of interneuronal communication might be required for normal brain development and function.

c-Src kinase inhibition reduces arrhythmia inducibility and connexin43 dysregulation after myocardial infarction

OBJECTIVES

The aim of this study was to evaluate the role of tyrosine kinase cellular-Src (c-Src) inhibition on connexin43 (Cx43) regulation in a mouse model of myocardial infarction (MI).

BACKGROUND

MI is associated with decreased expression of Cx43, the principal gap junction protein responsible for propagating current in ventricles. Activated c-Src has been linked to Cx43 dysregulation.

METHODS

MI was induced in 12-week-old mice by coronary artery occlusion. MI mice were treated with c-Src inhibitors (PP1 or AZD0530), PP3 (an inactive analogue of PP1), or saline. Treated hearts were compared to sham mice by echocardiography, optical mapping, telemetry electrocardiographic monitoring, and inducibility studies. Tissues were collected for immunoblotting, quantitative polymerase chain reaction, and immunohistochemistry.

RESULTS

Active c-Src was elevated in PP3-treated MI mice compared to sham at the scar border (280%, p = 0.003) and distal ventricle (346%, p = 0.013). PP1 treatment restored active c-Src to sham levels at the scar border (86%, p = 0.95) and distal ventricle (94%, p = 1.0). PP1 raised Cx43 expression by 69% in the scar border (p = 0.048) and by 73% in the distal ventricle (p = 0.043) compared with PP3 mice. PP1-treated mice had restored conduction velocity at the scar border (PP3: 32 cm/s, PP1: 41 cm/s, p < 0.05) and lower arrhythmic inducibility (PP3: 71%, PP1: 35%, p < 0.05) than PP3 mice. PP1 did not change infarct size, electrocardiographic pattern, or cardiac function. AZD0530 treatment demonstrated restoration of Cx43 comparable to PP1.

CONCLUSIONS

c-Src inhibition improved Cx43 levels and conduction velocity and lowered arrhythmia inducibility after MI, suggesting a new approach for arrhythmia reduction following MI.

Cardiac to cancer: connecting connexins to clinical opportunity

Gap junctions and their connexin components are indispensable in mediating the cellular coordination required for tissue and organ homeostasis. The critical nature of their existence mandates a connection to disease while at the same time offering therapeutic potential. Therapeutic intervention may be offered through the pharmacological and molecular disruption of the pathways involved in connexin biosynthesis, gap junction assembly, stabilization, or degradation. Chemical inhibitors aimed at closing connexin channels, peptide mimetics corresponding to short connexin sequences, and gene therapy approaches have been incredibly useful molecular tools in deciphering the complexities associated with connexin biology. Recently, therapeutic potential in targeting connexins has evolved from basic research in cell-based models to clinical opportunity in the form of human trials. Clinical promise is particularly evident with regards to targeting connexin43 in the context of wound healing. The following review is aimed at highlighting novel advances where the pharmacological manipulation of connexin biology has proven beneficial in animals or humans.

Connexins: sensors of epidermal integrity that are therapeutic targets

Gap junction proteins (connexins) are differentially expressed throughout the multiple layers of the epidermis. A variety of skin conditions arise with aberrant connexin expression or function and suggest that maintaining the epidermal gap junction network has many important roles in preserving epidermal integrity and homeostasis. Mutations in a number of connexins lead to epidermal dysplasias giving rise to a range of dermatological disorders of differing severity. 'Gain of function' mutations reveal connexin-mediated roles in calcium signalling within the epidermis. Connexins are involved in epidermal innate immunity, inflammation control and in wound repair. The therapeutic potential of targeting connexins to improve wound healing responses is now clear. This review discusses the role of connexins in epidermal integrity, and examines the emerging evidence that connexins act as epidermal sensors to a variety of mechanical, temperature, pathogen-induced and chemical stimuli. Connexins thus act as an integral component of the skin's protective barrier.

Gap junction modulation and its implications for heart function

Gap junction communication (GJC) mediated by connexins is critical for heart function. To gain insight into the causal relationship of molecular mechanisms of disease pathology, it is important to understand which mechanisms contribute to impairment of gap junctional communication. Here, we present an update on the known modulators of connexins, including various interaction partners, kinases, and signaling cascades. This gap junction network (GJN) can serve as a blueprint for data mining approaches exploring the growing number of publicly available data sets from experimental and clinical studies.

Mix and match: investigating heteromeric and heterotypic gap junction channels in model systems and native tissues

This review is based in part on a roundtable discussion session: "Physiological roles for heterotypic/heteromeric channels" at the 2013 International Gap Junction Conference (IGJC 2013) in Charleston, South Carolina. It is well recognized that multiple connexins can specifically co-assemble to form mixed gap junction channels with unique properties as a means to regulate intercellular communication. Compatibility determinants for both heteromeric and heterotypic gap junction channel formation have been identified and associated with specific connexin amino acid motifs. Hetero-oligomerization is also a regulated process; differences in connexin quality control and monomer stability are likely to play integral roles to control interactions between compatible connexins. Gap junctions in oligodendrocyte:astrocyte communication and in the cardiovascular system have emerged as key systems where heterotypic and heteromeric channels have unique physiologic roles. There are several methodologies to study heteromeric and heterotypic channels that are best applied to either heterologous expression systems, native tissues or both. There remains a need to use and develop different experimental approaches in order to understand the prevalence and roles for mixed gap junction channels in human physiology.

Arrhythmogenic cardiomyopathy and Brugada syndrome: diseases of the connexome

This review summarizes data in support of the notion that the cardiac intercalated disc is the host of a protein interacting network, called "the connexome", where molecules classically defined as belonging to one particular structure (e.g., desmosomes, gap junctions, sodium channel complex) actually interact with others, and together, control excitability, electrical coupling and intercellular adhesion in the heart. The concept of the connexome is then translated into the understanding of the mechanisms leading to two inherited arrhythmia diseases: arrhythmogenic cardiomyopathy, and Brugada syndrome. The cross-over points in these two diseases are addressed to then suggest that, though separate identifiable clinical entities, they represent "bookends" of a spectrum of manifestations that vary depending on the effect that a particular mutation has on the connexome as a whole.

Old cogs, new tricks: a scaffolding role for connexin43 and a junctional role for sodium channels?

Cardiac conduction is the process by which electrical excitation is communicated from cell to cell within the heart, triggering synchronous contraction of the myocardium. The role of conduction defects in precipitating life-threatening arrhythmias in various disease states has spurred scientific interest in the phenomenon. While the understanding of conduction has evolved greatly over the last century, the process has largely been thought to occur via movement of charge between cells via gap junctions. However, it has long been hypothesized that electrical coupling between cardiac myocytes could also occur ephaptically, without direct transfer of ions between cells. This review will focus on recent insights into cardiac myocyte intercalated disk ultrastructure and their implications for conduction research, particularly the ephaptic coupling hypothesis.

Impact of the controlled release of a connexin 43 peptide on corneal wound closure in an STZ model of type I diabetes

The alpha-carboxy terminus 1 (αCT1) peptide is a synthetically produced mimetic modified from the DDLEI C-terminus sequence of connexin 43 (Cx43). Previous research using various wound healing models have found promising therapeutic effects when applying the drug, resulting in increased wound healing rates and reduced scarring. Previous data suggested a rapid metabolism rate in vitro, creating an interest in long term release. Using a streptozotocin (STZ) type I diabetic rat model with a surgically induced corneal injury, we delivered αCT1 both directly, in a pluronic gel solution, and in a sustained system, using polymeric alginate-poly-l-ornithine (A-PLO) microcapsules (MC). Fluorescent staining of wound area over a 5 day period indicated a significant increase in wound closure rates for both αCT1 and αCT1 MC treated groups, withαCT1 MC groups showing the most rapid wound closure overall. Analysis of inflammatory reaction to the treatment groups indicated significantly lower levels of both Interferon Inducible T-Cell Alpha Chemoattractant (ITAC) and Tumor Necrosis Factor Alpha (TNFα) markers using confocal quantification and ELISA assays. Additional analysis examining genes selected from the EMT pathway using RT-PCR and Western blotting suggested αCT1 modification of Transforming Growth Factor Beta 2 (TGFβ2), Keratin 8 (Krt8), Estrogen Receptor 1 (Esr1), and Glucose Transporter 4 (Glut4) over a 14 day period. Combined, this data indicated a possible suppression of the inflammatory response by αCT1, leading to increased wound healing rates.

Purinergic signaling in early inflammatory events of the foreign body response: modulating extracellular ATP as an enabling technology for engineered implants and tissues

Purinergic signaling is a ubiquitous and vital aspect of mammalian biology in which purines--mainly adenosine triphosphate (ATP)--are released from cells through loss of membrane integrity (cell death), exocytosis, or transport/diffusion across membrane channels, and exert paracrine or autocrine signaling effects through three subclasses of well-characterized receptors: the P1 adenosine receptors, the P2X ionotropic nucleotide receptors, and the P2Y metabotropic receptors. ATP and its metabolites are released by damaged and stressed cells in injured tissues. The early events of wound healing, hemostasis, and inflammation are highly regulated by these signals through activation of purinergic receptors on platelets and neutrophils. Recent data have demonstrated that ATP signaling is of particular importance to targeting leukocytes to sites of injury. This is particularly relevant to the subject of implanted medical devices, engineered tissues, and grafts as all these technologies elicit a wound healing response with varying degrees of encapsulation, rejection, extrusion, or destruction of the tissue or device. Here, we review the biology of purinergic signaling and focus on ATP release and response mechanisms that pertain to the early inflammatory phase of wound healing. Finally, therapeutic options are explored, including a new class of peptidomimetic drugs based on the ATP-conductive channel connexin43.

Vinculin directly binds zonula occludens-1 and is essential for stabilizing connexin-43-containing gap junctions in cardiac myocytes

Vinculin (Vcl) links actin filaments to integrin- and cadherin-based cellular junctions. Zonula occludens-1 (ZO-1, also known as TJP1) binds connexin-43 (Cx43, also known as GJA1), cadherin and actin. Vcl and ZO-1 anchor the actin cytoskeleton to the sarcolemma. Given that loss of Vcl from cardiomyocytes causes maldistribution of Cx43 and predisposes cardiomyocyte-specific Vcl-knockout mice with preserved heart function to arrhythmia and sudden death, we hypothesized that Vcl and ZO-1 interact and that loss of this interaction destabilizes gap junctions. We found that Vcl, Cx43 and ZO-1 colocalized at the intercalated disc. Loss of cardiomyocyte Vcl caused parallel loss of ZO-1 from intercalated dics. Vcl co-immunoprecipitated Cx43 and ZO-1, and directly bound ZO-1 in yeast two-hybrid studies. Excision of the Vcl gene in neonatal mouse cardiomyocytes caused a reduction in the amount of Vcl mRNA transcript and protein expression leading to (1) decreased protein expression of Cx43, ZO-1, talin, and β1D-integrin, (2) reduced PI3K activation, (3) increased activation of Akt, Erk1 and Erk2, and (4) cardiomyocyte necrosis. In summary, this is the first study showing a direct interaction between Vcl and ZO-1 and illustrates how Vcl plays a crucial role in stabilizing gap junctions and myocyte integrity.

Proteins and mechanisms regulating gap-junction assembly, internalization, and degradation

Gap junctions (GJs) are the only known cellular structures that allow a direct cell-to-cell transfer of signaling molecules by forming densely packed arrays or "plaques" of hydrophilic channels that bridge the apposing membranes of neighboring cells. The crucial role of GJ-mediated intercellular communication (GJIC) for all aspects of multicellular life, including coordination of development, tissue function, and cell homeostasis, has been well documented. Assembly and degradation of these membrane channels is a complex process that includes biosynthesis of the connexin (Cx) subunit proteins (innexins in invertebrates) on endoplasmic reticulum (ER) membranes, oligomerization of compatible subunits into hexameric hemichannels (connexons), delivery of the connexons to the plasma membrane (PM), head-on docking of compatible connexons in the extracellular space at distinct locations, arrangement of channels into dynamic spatially and temporally organized GJ channel plaques, as well as internalization of GJs into the cytoplasm followed by their degradation. Clearly, precise modulation of GJIC, biosynthesis, and degradation are crucial for accurate function, and much research currently addresses how these fundamental processes are regulated. Here, we review posttranslational protein modifications (e.g., phosphorylation and ubiquitination) and the binding of protein partners (e.g., the scaffolding protein ZO-1) known to regulate GJ biosynthesis, internalization, and degradation. We also look closely at the atomic resolution structure of a GJ channel, since the structure harbors vital cues relevant to GJ biosynthesis and turnover.

Injury-triggered Akt phosphorylation of Cx43: a ZO-1-driven molecular switch that regulates gap junction size

The proteins that form vertebrate gap junctions, the connexins, are highly regulated and have short (<2 hour) half-lives. Phosphorylation of connexin43 (Cx43) affects gap junction assembly, channel gating and turnover. After finding dramatic effects on gap junctions with Akt inhibitors, we created an antibody specific for Cx43 phosphorylated on S373, a potential Akt substrate. We found S373 phosphorylation in cells and skin or heart almost exclusively in larger gap-junctional structures that increased dramatically after wounding or hypoxia. We were able to mechanistically show that Akt-dependent phosphorylation of S373 increases gap junction size and communication by completely eliminating the interaction between Cx43 and ZO-1. Thus, phosphorylation on S373 acts as a molecular 'switch' to rapidly increase gap-junctional communication, potentially leading to initiation of activation and migration of keratinocytes or ischemic injury response in the skin and the heart, respectively.

Super-resolution fluorescence microscopy of the cardiac connexome reveals plakophilin-2 inside the connexin43 plaque

AIMS

Cell function requires formation of molecular clusters localized to discrete subdomains. The composition of these interactomes, and their spatial organization, cannot be discerned by conventional microscopy given the resolution constraints imposed by the diffraction limit of light (∼200-300 nm). Our aims were (i) Implement single-molecule imaging and analysis tools to resolve the nano-scale architecture of cardiac myocytes. (ii) Using these tools, to map two molecules classically defined as components 'of the desmosome' and 'of the gap junction', and defined their spatial organization.

METHODS AND RESULTS

We built a set-up on a conventional inverted microscope using commercially available optics. Laser illumination, reducing, and oxygen scavenging conditions were used to manipulate the blinking behaviour of individual fluorescent reporters. Movies of blinking fluorophores were reconstructed to generate subdiffraction images at ∼20 nm resolution. With this method, we characterized clusters of connexin43 (Cx43) and of 'the desmosomal protein' plakophilin-2 (PKP2). In about half of Cx43 clusters, we observed overlay of Cx43 and PKP2 at the Cx43 plaque edge. SiRNA-mediated loss of Ankyrin-G expression yielded larger Cx43 clusters, of less regular shape, and larger Cx43-PKP2 subdomains. The Cx43-PKP2 subdomain was validated by a proximity ligation assay (PLA) and by Monte-Carlo simulations indicating an attraction between PKP2 and Cx43.

CONCLUSIONS

(i) Super-resolution fluorescence microscopy, complemented with Monte-Carlo simulations and PLAs, allows the study of the nanoscale organization of an interactome in cardiomyocytes. (ii) PKP2 and Cx43 share a common hub that permits direct physical interaction. Its relevance to excitability, electrical coupling, and arrhythmogenic right ventricular cardiomyopathy, is discussed.

Two tyrosine-based sorting signals in the Cx43 C-terminus cooperate to mediate gap junction endocytosis

Three tyrosine-based sorting signals in the gap junction protein connexin 43 were identified, two of which function cooperatively as adaptor protein complex-2 binding sites. The analyses provide a molecular model for clathrin to efficiently internalize large plasma membrane structures and suggest a mechanism for regulating constitutive versus acute gap junction internalization.

Gap junction (GJ) channels that electrically and chemically couple neighboring cells are formed when two hemichannels (connexons) of apposed cells dock head-on in the extracellular space. Remarkably, docked connexons are inseparable under physiological conditions, and we and others have shown that GJs are internalized in whole, utilizing the endocytic clathrin machinery. Endocytosis generates double-membrane vesicles (annular GJs or connexosomes) in the cytoplasm of one of the apposed cells that are degraded by autophagosomal and, potentially, endo/lysosomal pathways. In this study, we investigated the structural motifs that mediate Cx43 GJ endocytosis. We identified three canonical tyrosine-based sorting signals of the type “YXXΦ” in the Cx43 C-terminus, two of which function cooperatively as AP-2 binding sites. We generated a set of green fluorescent protein–tagged and untagged Cx43 mutants that targeted these two sites either individually or together. Mutating both sites completely abolished Cx43-AP-2/Dab2/clathrin interaction and resulted in increased GJ plaque size, longer Cx43 protein half-lives, and impaired GJ internalization. Interestingly, Dab2, an accessory clathrin adaptor found earlier to be important for GJ endocytosis, interacts indirectly with Cx43 via AP-2, permitting the recruitment of up to four clathrin complexes per Cx43 protein. Our analyses provide a mechanistic model for clathrin's efficient internalization of large plasma membrane structures, such as GJs.

A synthetic connexin 43 mimetic peptide augments corneal wound healing

The ability to safely and quickly close wounds and lacerations is an area of need in regenerative medicine, with implications toward healing a wide range of tissues and wounds. Using an in vivo corneal injury model, our study applied a newly developed peptide capable of promotion of wound healing and epithelial regeneration. The alpha-carboxy terminus 1 (αCT1) peptide is a 25 amino acid peptide from the C-terminus of connexin 43 (Cx43), modified to promote cellular uptake. Previous studies applying αCT1 to excisional skin wounds in porcine models produced tissues having an overall reduced level of scar tissue and decreased healing time. Rapid metabolism of αCT1 in previous work led to the investigation of extended release on wound healing rate used in this study. Here we delivered αCT1 both directly, in a concentrated pluronic solution, and in a sustained system, using polymeric alginate-poly-l-ornithine (A-PLO) microcapsules. Cell toxicity analysis showed minimal cell-loss with microcapsule treatment. Measurement of wound healing using histology and fluorescence microscopy indicated significant reduction in healing time of αCT1 microcapsule treated rat corneas compared with controls (88% vs. 38%). RT-PCR analysis showed an initial up regulation followed by down regulation of the gene keratin-19 (Krt19). Zonula occludens 1 (ZO-1) showed an opposite down regulation followed by an up regulation whereas Cx43 showed a biphasic response. Inflammatory indexes demonstrated a reduction in the inflammation of corneas treated with αCT1 microcapsules when compared with pluronic gel vehicle. These results suggest αCT1, when applied in a sustained release system, acts as a beneficial wound healing treatment.

Endothelial calcium dynamics, connexin channels and blood-brain barrier function

Situated between the circulation and the brain, the blood-brain barrier (BBB) protects the brain from circulating toxins while securing a specialized environment for neuro-glial signaling. BBB capillary endothelial cells exhibit low transcytotic activity and a tight, junctional network that, aided by the cytoskeleton, restricts paracellular permeability. The latter is subject of extensive research as it relates to neuropathology, edema and inflammation. A key determinant in regulating paracellular permeability is the endothelial cytoplasmic Ca(2+) concentration ([Ca(2+)]i) that affects junctional and cytoskeletal proteins. Ca(2+) signals are not one-time events restricted to a single cell but often appear as oscillatory [Ca(2+)]i changes that may propagate between cells as intercellular Ca(2+) waves. The effect of Ca(2+) oscillations/waves on BBB function is largely unknown and we here review current evidence on how [Ca(2+)]i dynamics influence BBB permeability.

The intracellular fate of zonula occludens 2 is regulated by the phosphorylation of SR repeats and the phosphorylation/O-GlcNAcylation of S257

ZO-2 nuclear import and accumulation in speckles is regulated by phosphorylation of its SR repeats by SRPK1 in a process initiated by EGF activation of AKT. ZO-2 nuclear exportation is favored by O-GlcNAc of S257 at the nucleus, whereas maturation of tight junctions is accompanied by ZO-2 phosphorylation at S257 by PKCζ.

Zona occludens 2 (ZO-2) has a dual localization. In confluent epithelia, ZO-2 is present at tight junctions (TJs), whereas in sparse proliferating cells it is also found at the nucleus. Previously we demonstrated that in sparse cultures, newly synthesized ZO-2 travels to the nucleus before reaching the plasma membrane. Now we find that in confluent cultures newly synthesized ZO-2 goes directly to the plasma membrane. Epidermal growth factor induces through AKT activation the phosphorylation of the kinase for SR repeats, serine arginine protein kinase 1, which in turn phosphorylates ZO-2, which contains 16 SR repeats. This phosphorylation induces ZO-2 entry into the nucleus and accumulation in speckles. ZO-2 departure from the nucleus requires intact S257, and stabilizing the β-O-linked N-acetylglucosylation (O-GlcNAc) of S257 with O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate, an inhibitor of O-GlcNAcase, triggers nuclear exportation and proteosomal degradation of ZO-2. At the plasma membrane ZO-2 is not O-GlcNAc, and instead, as TJs mature, it becomes phosphorylated at S257 by protein kinase Cζ. This late phosphorylation of S257 is required for the correct cytoarchitecture to develop, as cells transfected with ZO-2 mutant S257A or S257E form aberrant cysts with multiple lumens. These results reveal novel posttranslational modifications of ZO-2 that regulate the intracellular fate of this protein.

Peptides and peptide-derived molecules targeting the intracellular domains of Cx43: gap junctions versus hemichannels

About a decade ago, the molecular determinants controlling the opening and closing of Cx43 gap junction channels have been identified. Advanced biophysical approaches revealed a critical role for structural rearrangements in the cytoplasmic loop and dimerization of the C-terminal tail, resulting in binding of the C-terminal tail to the cytoplasmic loop and Cx43 gap junction channel closure during cellular acidosis. This has spurred the development of Cx43-mimetic peptides and peptidomimetics that interfere with these loop/tail interactions, thereby preventing the closure of Cx43 gap junctions, e.g. in the heart upon ischemia. Recently, we found that loop/tail interactions control Cx43-hemichannel activity but with an opposite effect. Binding of the C-terminal tail to the cytoplasmic loop is a requisite for the opening of Cx43 hemichannels in response to different stimuli, like decreased extracellular [Ca2+], increased intracellular [Ca2+], positive membrane potentials or ischemia. Strikingly, peptides that favor the open state of Cx43 gap junctions like the L2 peptide inhibit Cx43-hemichannel opening. These tools now provide unprecedented opportunities to selectively inhibit Cx43 hemichannels while maintaining Cx43 gap junction communication, impossible to achieve with siRNA or knockdown approaches both affecting gap junctions and hemichannels. These tools not only are very helpful to unravel the role of Cx43 hemichannels in complex biological systems, but also hold therapeutic potential to counteract excessive Cx43-hemichannel activity like in ischemia/reperfusion in the brain and the heart or to prevent Cx43 hemichannel-mediated gliotransmitter release in the basal amygdala during memory consolidation in response to emotional events. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Cx43, ZO-1, alpha-catenin and beta-catenin in cataractous lens epithelial cells

Specimens of the anterior lens capsule with an attached monolayer of lens epithelial cells (LECs) were obtained from patients (n=52) undergoing cataract surgery. Specimens were divided into three groups based on the type of cataract: nuclear cataract, cortical cataract and posterior subcapsular cataract (PSC). Clear lenses (n=11) obtained from donor eyes were used as controls. Expression was studied by immunofluorescence, real-time PCR and Western blot. Statistical analysis was done using the student's t-test. Immunofluorescence results showed punctate localization of Cx43 at the cell boundaries in controls, nuclear cataract and PSC groups. In the cortical cataract group, cytoplasmic pools of Cx43 without any localization at the cell boundaries were observed. Real-time PCR results showed significant up-regulation of Cx43 in nuclear and cortical cataract groups. Western blot results revealed significant increase in protein levels of Cx43 and significant decrease of ZO-1 in all three cataract groups. Protein levels of alpha-catenin were decreased significantly in nuclear and cortical cataract group. There was no significant change in expression of beta-catenin in the cataractous groups. Our findings suggest that ZO-1 and alpha-catenin are important for gap junctions containing Cx43 in the LECs. Alterations in cell junction proteins may play a role during formation of different types of cataract.

AKAP9 is essential for spermatogenesis and sertoli cell maturation in mice

Mammalian male fertility relies on complex inter- and intracellular signaling during spermatogenesis. Here we describe three alleles of the widely expressed A-kinase anchoring protein 9 (Akap9) gene, all of which cause gametogenic failure and infertility in the absence of marked somatic phenotypes. Akap9 disruption does not affect spindle nucleation or progression of prophase I of meiosis but does inhibit maturation of Sertoli cells, which continue to express the immaturity markers anti-Mullerian hormone and thyroid hormone receptor alpha in adults and fail to express the maturation marker p27(Kip1). Furthermore, gap and tight junctions essential for blood-testis barrier (BTB) organization are disrupted. Connexin43 (Cx43) and zona occludens-1 are improperly localized in Akap9 mutant testes, and Cx43 fails to compartmentalize germ cells near the BTB. These results identify and support a novel reproductive tissue-specific role for Akap9 in the coordinated regulation of Sertoli cells in the testis.

Deletion of the last five C-terminal amino acid residues of connexin43 leads to lethal ventricular arrhythmias in mice without affecting coupling via gap junction channels

The cardiac intercalated disc harbors mechanical and electrical junctions as well as ion channel complexes mediating propagation of electrical impulses. Cardiac connexin43 (Cx43) co-localizes and interacts with several of the proteins located at intercalated discs in the ventricular myocardium. We have generated conditional Cx43D378stop mice lacking the last five C-terminal amino acid residues, representing a binding motif for zonula occludens protein-1 (ZO-1), and investigated the functional consequences of this mutation on cardiac physiology and morphology. Newborn and adult homozygous Cx43D378stop mice displayed markedly impaired and heterogeneous cardiac electrical activation properties and died from severe ventricular arrhythmias. Cx43 and ZO-1 were co-localized at intercalated discs in Cx43D378stop hearts, and the Cx43D378stop gap junction channels showed normal coupling properties. Patch clamp analyses of isolated adult Cx43D378stop cardiomyocytes revealed a significant decrease in sodium and potassium current densities. Furthermore, we also observed a significant loss of Nav1.5 protein from intercalated discs in Cx43D378stop hearts. The phenotypic lethality of the Cx43D378stop mutation was very similar to the one previously reported for adult Cx43 deficient (Cx43KO) mice. Yet, in contrast to Cx43KO mice, the Cx43 gap junction channel was still functional in the Cx43D378stop mutant. We conclude that the lethality of Cx43D378stop mice is independent of the loss of gap junctional intercellular communication, but most likely results from impaired cardiac sodium and potassium currents. The Cx43D378stop mice reveal for the first time that Cx43 dependent arrhythmias can develop by mechanisms other than impairment of gap junction channel function.

Multilayered regulation of cardiac ion channels

Essential to beat-to-beat heart function is the ability for cardiomyocytes to propagate electrical excitation and generate contractile force. Both excitation and contractility depend on specific ventricular ion channels, which include the L-type calcium channel (LTCC) and the connexin 43 (Cx43) gap junction. Each of these two channels is localized to a distinct subdomain of the cardiomyocyte plasma membrane. In this review, we focus on regulatory mechanisms that govern the lifecycles of LTCC and Cx43, from their biogenesis in the nucleus to directed delivery to T-tubules and intercalated discs, respectively. We discuss recent findings on how alternative promoter usage, tissue-specific transcription, and alternative splicing determine precise ion channel expression levels within a cardiomyocyte. Moreover, recent work on microtubule and actin-dependent trafficking for Cx43 and LTCC are introduced. Lastly, we discuss how human cardiac disease phenotypes can be attributed to defects in distinct mechanisms of channel regulation at the level of gene expression and channel trafficking. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

The perinexus: sign-post on the path to a new model of cardiac conduction?

The perinexus is a recently identified microdomain surrounding the cardiac gap junction that contains elevated levels of connexin43 and the sodium channel protein, Nav1.5. Ongoing work has established a role for the perinexus in regulating gap junction aggregation. However, recent studies have raised the possibility of a perinexal contribution at the gap junction cleft to intercellular propagation of action potential via non-electrotonic mechanisms. The latter possibility could modify the current theoretical understanding of cardiac conduction, help explain paradoxical experimental findings, and open up entirely new avenues for antiarrhythmic therapy. We review recent structural insights into the perinexus and its potential novel functional role in cardiac-excitation spread, highlighting presently unanswered questions, the evidence for ephaptic conduction in the heart and how structural insights may help complete this picture.

A Connexin43 Mimetic Peptide Promotes Regenerative Healing and Improves Mechanical Properties in Skin and Heart

SIGNIFICANCE

Evidence is building that the gap junction protein connexin43 (Cx43) is an important molecule in regenerative healing of skin and heart. Excess scarring from skin wound healing is a continuing clinical problem. Humans generally lack the ability to regenerate tissue following injury, and some degree of fibrotic repair occurs. In the skin, this results in unsightly scars with inferior mechanical properties. In the heart, scarring causes disruption in the contractility of cardiac muscle and increases the risk of deadly arrhythmia. Therapies that tip the balance of wound healing away from scar tissue and toward regeneration would thus represent a significant medical advance.

RECENT ADVANCES

A cell-permeant peptide, αCT1 (alpha connexin carboxyl-terminal peptide), based on the carboxyl-terminus of connexin43, has been shown to elicit changes in gap junction organization and intracellular communication. In the skin, αCT1 applied at acute time points results in decreased inflammatory response, reduced area of scar progenitor tissue, and restoration of more normal dermal structure and mechanical strength. αCT1 application to infarcted hearts improved cardiac contractility, reduced the propensity for arrhythmia, and increased conduction velocity through the injured heart.

CRITICAL ISSUES

Application of therapies like αCT1 could reduce cutaneous scarring and improve mechanical properties of healed skin and the contractile function and electrical stability of the heart following injury or surgery.

FUTURE DIRECTIONS

αCT1 is a potential therapy for cutaneous wounds that could lead to reduced scarring and improvements in the mechanical properties of healed skin. For injured myocardial tissues, this Cx43 mimetic peptide may also provide a therapeutic approach for targeting pathological fibrosis and reducing the likelihood of sudden death from cardiac arrhythmias.

Characterization of polymeric microcapsules containing a low molecular weight peptide for controlled release

A need exists to prolong the release of rapidly metabolized peptides of a low molecular weight, while delivering this peptide without environmental interference. Previous studies have used bovine serum albumin (BSA) as a model peptide to study release characteristics from alginate microcapsules. BSA is 66 kDa in size, while the peptide of interest here, connexin-43 carboxyl-terminus mimetic peptide (αCT1), is only 3.4 kDa. Such a change in size results in a much different set of release parameters. Our overall goal is a sustained release over a 24+ h period. Prolonged application of the peptide to a wound site to investigate therapeutic effects is ideal. As a result, a diffusion method using alginate microcapsules, along with the addition of poly-l-lysine and poly-l-ornithine, has been explored. We first aimed to establish and characterize our parameters through a set of parametric tests. Variations in polymer coating, change in pH, and changes in loading ratio have previously been shown to effect release using model compounds. Here we test specific changes in these parameters to show effects on the release of αCT1. Additionally, the microcapsules were attached to several biomaterials and surgical implants by ultraviolet cross-linking to study the effectiveness of attachment and delivery. Analysis and measurements using phase contrast microscopy, scanning electron microscopy, and atomic force microscopy were used to characterize changes in microcapsule morphology.