The role of gap junctions in health and disease

Cx43 and the Actin Cytoskeleton: Novel Roles and Implications for Cell-Cell Junction-Based Barrier Function Regulation

Barrier function is a vital homeostatic mechanism employed by epithelial and endothelial tissue. Diseases across a wide range of tissue types involve dynamic changes in transcellular junctional complexes and the actin cytoskeleton in the regulation of substance exchange across tissue compartments. In this review, we focus on the contribution of the gap junction protein, Cx43, to the biophysical and biochemical regulation of barrier function. First, we introduce the structure and canonical channel-dependent functions of Cx43. Second, we define barrier function and examine the key molecular structures fundamental to its regulation. Third, we survey the literature on the channel-dependent roles of connexins in barrier function, with an emphasis on the role of Cx43 and the actin cytoskeleton. Lastly, we discuss findings on the channel-independent roles of Cx43 in its associations with the actin cytoskeleton and focal adhesion structures highlighted by PI3K signaling, in the potential modulation of cellular barriers. Mounting evidence of crosstalk between connexins, the cytoskeleton, focal adhesion complexes, and junctional structures has led to a growing appreciation of how barrier-modulating mechanisms may work together to effect solute and cellular flux across tissue boundaries. This new understanding could translate into improved therapeutic outcomes in the treatment of barrier-associated diseases.

Inhibition of gap junction composed of Cx43 prevents against acute kidney injury following liver transplantation

Postoperative acute kidney injury (AKI) is a severe complication after liver transplantation (LT). Its deterioration and magnification lead to the increase in mortality. Connexin43 (Cx43) mediates direct transmission of intracellular signals between neighboring cells, always considered to be the potent biological basis of organ damage deterioration and magnification. Thus, we explored the effects of Cx43 on AKI following LT and its related possible mechanism. In this study, alternations of Cx43 expression were observed in 82 patients, receiving the first-time orthotopic LT. We built autologous orthotopic liver transplantation (AOLT) models with Sprague–Dawley (SD) rats in vivo, and hypoxia-reoxygenation (H/R) or lipopolysaccharide (LPS) pretreatment models with kidney tubular epithelial cells (NRK-52E) in vitro, both of which were the most important independent risk factors of AKI following LT. Then, different methods were used to alter the function of Cx43 channels to determine its protective effects on AKI. The results indicated that patients with AKI suffering from longer time of tracheal intubation or intensive care unit stay, importantly, had significantly lower survival rate at postoperative 30 days and 3 years. In rat AOLT models, as Cx43 was inhibited with heptanol, postoperative AKI was attenuated significantly. In vitro experiments, downregulation of Cx43 with selective inhibitors, or siRNA protected against post-hypoxic NRK-52E cell injuries caused by H/R and/or LPS, while upregulation of Cx43 exacerbated the above-mentioned cell injuries. Of note, alternation of Cx43 function regulated the content of reactive oxygen species (ROS), which not only mediated oxidative stress and inflammation reactions effectively, but also regulated necroptosis. Therefore, we concluded that Cx43 inhibition protected against AKI following LT through attenuating ROS transmission between the neighboring cells. ROS alternation depressed oxidative stress and inflammation reaction, which ultimately reduced necroptosis. This might offer new insights for targeted intervention for organ protection in LT, or even in other major surgeries.

Connexin32 plays a crucial role in ROS-mediated endoplasmic reticulum stress apoptosis signaling pathway in ischemia reperfusion-induced acute kidney injury

BACKGROUND

Ischemia-reperfusion (I/R)-induced acute kidney injury (AKI) not only prolongs the length of hospital stay, but also seriously affects the patient's survival rate. Although our previous investigation has verified that reactive oxygen species (ROS) transferred through gap junction composed of connexin32 (Cx32) contributed to AKI, its underlying mechanisms were not fully understood and viable preventive or therapeutic regimens were still lacking. Among various mechanisms involved in organs I/R-induced injuries, endoplasmic reticulum stress (ERS)-related apoptosis is currently considered to be an important participant. Thus, in present study, we focused on the underlying mechanisms of I/R-induced AKI, and postulated that Cx32 mediated ROS/ERS/apoptosis signal pathway activation played an important part in I/R-induced AKI.

METHODS

We established renal I/R models with Cx32+/+ and Cx32-/- mice, which underwent double kidneys clamping and recanalization. ROS scavenger (N-acetylcysteine, NAC) and ERS inhibitors (4-phenyl butyric acid, 4-PBA, and tauroursodeoxycholic acid, TUDCA) were used to decrease the content of ROS and attenuate ERS activation, respectively.

RESULTS

Renal damage was progressively exacerbated in a time-dependent manner at the reperfusion stage, that was consistent with the alternation of ERS activation, including glucose regulated protein 78 (BiP/GRP78), X box-binding protein1, and C/EBP homologous protein expression. TUDCA or 4-PBA application attenuated I/R-induced ERS activation and protected against renal tubular epithelial cells apoptosis and renal damage. Cx32 deficiency decreased ROS generation and distribution between the neighboring cells, which attenuated I/R-induced ERS activation, and improved cell apoptosis and renal damage.

CONCLUSION

Cx32 mediated ROS/ERS/apoptosis signal pathway activation played an important part in I/R-induced AKI. Cx32 deficiency, ROS elimination, and ERS inhibition all could protect against I/R-induced AKI.

Increased expression of CX43 on stromal cells promotes leukemia apoptosis

Connexin 43 (Cx43) induced apoptosis has been reported in solid tumors, but the effect of Cx43 expressed by bone marrow stromal cells (BMSC) in leukemia has not been fully investigated. Manipulating Cx43 expression could be a potential therapeutic strategy for leukemia. Here, we investigate the effect of Cx43 expressed by BMSCs (human Umbilical Cord Stem Cells over-expressed CX43, Cx43-hUCSC) on leukemia cells. When co-cultured with Cx43-hUCSC, leukemia cells show significant lower growth rate with increasing apoptosis activity, and more leukemia cells enter S phase. Functional assays of fluorescence recovery after photo bleaching (FRAP) showed improved gap junctional intercellular communication (GJIC) on leukemia cells when co-cultured with Cx43-hUCSC (p < 0.01). In a mouse minimal disease model, the mean survival time and mortality rate were significantly improved in mice transplanted with Cx43-hUCSC. Our results indicate that Cx43 expressed by BMSC induces apoptosis on leukemia cells. Small molecules or other pharmaceutical approaches for modulating Cx43 expression in BMSCs could be used for delaying relapse of leukemia.

All-trans retinoic acid arrests cell cycle in leukemic bone marrow stromal cells by increasing intercellular communication through connexin 43-mediated gap junction

Background

Gap junctional intercellular communication (GJIC) is typically decreased in malignant tumors. Gap junction is not presented between hematopoietic cells but occurred in bone marrow stromal cells (BMSCs). Connexin 43 (Cx43) is the major gap junction (GJ) protein; our previous study revealed that Cx43 expression and GJIC were decreased in acute leukemic BMSCs. All-trans retinoic acid (ATRA) increases GJIC in a variety of cancer cells and has been used to treat acute promyelocytic leukemia, but the effects of ATRA on leukemic BMSCs is unknown. In this study, we evaluated the potential effects of ATRA on cell cycle, proliferation, and apoptosis of leukemic BMSCs. Effects of ATRA on Cx43 expression and GJIC were also examined.

Methods

Human BMSCs obtained from 25 patients with primary acute leukemia, and 10 normal healthy donors were cultured. Effects of ATRA on cell cycle, cell proliferation, and apoptosis were examined with or without co-treatment with amphotericin-B. Cx43 expression was examined at both the mRNA and protein expression levels. GJIC was examined by using a dye transfer assay and measuring the rate of fluorescence recovery after photobleaching (FRAP).

Results

ATRA arrested the cell cycle progression, inhibited cell growth, and increased apoptosis in leukemic BMSCs. Both Cx43 expression and GJIC function were increased by ATRA treatment. Most of the observed effects mediated by ATRA were abolished by amphotericin-B pretreatment.

Conclusions

ATRA arrests cell cycle progression in leukemic BMSCs, likely due to upregulating Cx43 expression and enhancing GJIC function.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-015-0212-7) contains supplementary material, which is available to authorized users.

Conserved glycine at position 45 of major cochlear connexins constitutes a vital component of the Ca²⁺ sensor for gating of gap junction hemichannels

Mutations in gap junction (GJ) family of proteins, especially in the connexin (Cx) 26, are responsible for causing severe congenital hearing loss in a significant portion of patients (30-50% in various ethnic groups). Substitution of glycine at the position 45 of Cx26 to glutamic acid (p.G45E mutation) causes the Keratitis-ichthyosis-deafness (KID) syndrome. Previous studies have suggested that this point mutation caused a gain-of-function defect. However, the molecular mechanism of KID syndrome remains unclear. Since glycine at this position is conserved in many Cxs expressed in the cochlea, we tested the hypothesis that glycine at position 45 is an important component of the sensor regulating the Ca(2+) gating of GJ hemichannels. Using reconstituted Cx30, 32 and 43 expressed in the HEK 293 cells, we compared the functions of wild type and p.G45E mutant Cxs. We found that G45E in Cx30 resulted in similar deleterious cellular effects as Cx26 did. Cell death occurred within 24h of transfection, which was rescued by increasing extracellular Ca(2+) concentration ([Ca(2+)]o). Dye loading assay showed that Cx30 G45E, similar to Cx26 G45E, had leaky hemichannels at physiological [Ca(2+)]o (1.2 mM). Higher [Ca(2+)]o reduced the dye loading in a dose-dependent manner. Whole cell membrane current recordings also indicated that G45E caused increased hemichannel activities. p.G45E mutations of Cx32 and 43 also resulted in leaky hemichannels compared to their respective wild types in lower [Ca(2+)]o. Our data in this study provided further support for the hypothesis that glycine at position 45 is a conserved Ca(2+) sensor for the gating of GJ hemichannels among multiple Cx subtypes expressed in the cochlea.

Effect of Cx43 gene-modified leukemic bone marrow stromal cells on the regulation of Jurkat cell line in vitro

We recently reported that Cx43 expression and gap junction intercellular communication (GJIC) between acute leukemic bone marrow stromal cells (BMSCs) were deficient, which could recovery after effective chemotherapy. However, the exact role of GJIC in the hematopoietic microenvironment in leukemic cell death and proliferation is not clear. We show here that following transfection with the Cx43 gene, GJIC function was increased between leukemic BMSCs. Furthermore, compared with leukemic cells alone, the proliferation and apoptosis of leukemic cells co-cultured with BMSCs were inhibited, the percentage of G0-phase cells was higher; and expression of p53 increased and bax decreased. However, after co-culturing leukemic cells with Cx43-modified BMSCs, the number of proliferative and spontaneously apoptotic Jurkat cells increased; the percentage of G0-phase cells decreased; the expression of p53 decreased; and bax increased. Compared with Jurkat cells co-cultured with BMSCs and Jurkat cells alone, the sensitivity of leukemic cells co-cultured with Cx43-modified BMSCs to chemotherapeutics increased. Our data suggests that GJIC between leukemia BMSCs is one of the impact factor to the proliferation, apoptosis and drug sensitivity of co-cultured leukemic cells. Up-regulating its function can inhibit the protective effects of leukemic BMSCS and enhance the efficacy of therapies in hematologic malignancies.

Pathological hemichannels associated with human Cx26 mutations causing Keratitis-Ichthyosis-Deafness syndrome

Connexin (Cx) proteins form intercellular gap junction channels by first assembling into single membrane hemichannels that then dock to connect the cytoplasm of two adjacent cells. Gap junctions are highly specialized structures that allow the direct passage of small molecules between cells to maintain tissue homeostasis. Functional activity of nonjunctional hemichannels has now been shown in several experimental systems. Hemichannels may constitute an important diffusional exchange pathway with the extracellular space, but the extent of their normal physiological role is currently unknown. Aberrant hemichannel activity has been linked to mutations of connexin proteins involved in genetic diseases. Here, we review a proposed role for hemichannels in the pathogenesis of Keratitis-Ichthyosis-Deafness (KID) syndrome associated with connexin26 (Cx26) mutations. Continued functional evaluation of mutated hemichannels linked to human hereditary disorders may provide additional insights into the mechanisms governing their regulation in normal physiology and dysregulation in disease. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Cytoplasmic accumulation of connexin32 expands cancer stem cell population in human HuH7 hepatoma cells by enhancing its self-renewal

Although the connexin32 (Cx32)-mediated gap junction is abolished in hepatocellular carcinoma (HCC), the expression of cytoplasmic Cx32 tends to increase in correspondence with the grade of malignancy. Establishing a Tet-off expression system in human nonmetastatic HuH7 HCC cells where cytoplasmic Cx32 was overexpressed by doxycycline (Dox) withdrawal, we previously demonstrated that overexpression of cytoplasmic Cx32 made HuH7 cells metastatic in mice. In our study, hypothesizing that the cytoplasmic Cx32-induced metastasis may involve expansion of the cancer stem cell (CSC) population, we examined whether cytoplasmic Cx32 controlled the size of the side population (SP) in HuH7 Tet-off Cx32 cells. Fluorescence-activated cell sorting revealed that SP was expanded in a Dox-free medium compared with a Dox-supplemented one. Although cytoplasmic Cx32 did not block maturation from SP to non-SP, purified SP reconstituted a larger SP fraction in the Dox-free medium than in the Dox-supplemented one. Furthermore, although SP from HuH7 Tet-off mock cells formed a similar number of CSC spheres of a similar size whether with or without Dox, SP from HuH7 Tet-off Cx32 cells developed a greater number of larger CSC spheres in the Dox-free medium than in the Dox-supplemented one. Taken together, these results suggest that accumulation of cytoplasmic Cx32 should enhance self-renewal of CSC to expand the CSC population in HCC.

Up-regulation of Cx43 expression and GJIC function in acute leukemia bone marrow stromal cells post-chemotherapy

Gap junction intercellular communication (GJIC) among bone marrow stromal cells (BMSCs) most frequently occurs through a channel composed of connexin43 (Cx43). Dysregulation of connexin expression is believed to have a role in carcinogenesis. In earlier work, we found that in acute leukemia BMSCs, expression of Cx43 and functioning GJIC declined. However, there has been no evaluation of whether GJIC in BMSCs in complete remission (CR) post-chemotherapy is different from GJIC pre-chemotherapy. We studied Cx43 expression and tested GJIC function in human bone marrow cultures under different physiological and pathological conditions. To assay Cx43 expression we used immunocytochemistry, laser scan confocal microscopy (LSCM), flow cytometry and RT-PCR. The results showed that the expression level of Cx43 and its mRNA in acute leukemia BMSCs post-chemotherapy was significantly higher and similar to normal levels than in primary acute leukemia BMSCs (p<0.01). Functional tests in cultures using dye transfer and fluorescence recovery after photobleaching (FRAP) assays showed that the function of GJIC in acute leukemia BMSCs was significantly improved following effective chemotherapy. Our findings suggest Cx43 and GJIC might be involved in the courses of occurrence, development and termination of acute leukemia, and effective chemotherapy could improve Cx43 expression and GJIC function that were dysfunctional prior to treatment.

Connexins: a myriad of functions extending beyond assembly of gap junction channels

Connexins constitute a large family of trans-membrane proteins that allow intercellular communication and the transfer of ions and small signaling molecules between cells. Recent studies have revealed complex translational and post-translational mechanisms that regulate connexin synthesis, maturation, membrane transport and degradation that in turn modulate gap junction intercellular communication. With the growing myriad of connexin interacting proteins, including cytoskeletal elements, junctional proteins, and enzymes, gap junctions are now perceived, not only as channels between neighboring cells, but as signaling complexes that regulate cell function and transformation. Connexins have also been shown to form functional hemichannels and have roles altogether independent of channel functions, where they exert their effects on proliferation and other aspects of life and death of the cell through mostly-undefined mechanisms. This review provides an updated overview of current knowledge of connexins and their interacting proteins, and it describes connexin modulation in disease and tumorigenesis.

Connexin mutations causing skin disease and deafness increase hemichannel activity and cell death when expressed in Xenopus oocytes

Mutations in the GJB2 gene-encoding connexin 26 (Cx26) have been linked to skin disorders and genetic deafness. However, the severity and type of the skin disorders caused by Cx26 mutations are heterogeneous. Here we explored the effect of Cx26 KID syndrome-associated mutations, G12R, S17F, and D50N on channel function. The Cx26 N14K mutation was also examined that is associated with deafness but has a skin disorder distinct from the KID syndrome mutations. The proteins were all expressed in Xenopus oocytes with levels equal to wild-type Cx26. The G12R, N14K, and D50N mutations resulted in larger hemichannel currents than the wild-type-expressing cells, but the S17F mutation resulted in a complete loss of hemichannel activity. Elevated hemichannel activity correlated with an increased cell death. This result could be reversed through the elevation of calcium (Ca2+) in the extracellular media. Functional gap junctions were only produced by paired N14K cells, which had a similar conductance level to wild type, even though they exhibited a complete loss of voltage sensitivity. This set of data confirms that aberrant hemichannel activity is a common feature of Cx26 mutations associated with KID syndrome, and this may contribute to a loss of cell viability and tissue integrity.

Activated macrophages inhibit enterocyte gap junctions via the release of nitric oxide

Enterocytes exist in close association with tissue macrophages, whose activation during inflammatory processes leads to the release of nitric oxide (NO). Repair from mucosal injury requires the migration of enterocytes into the mucosal defect, a process that requires connexin43 (Cx43)-mediated gap junction communication between adjacent enterocytes. Enterocyte migration is inhibited during inflammatory conditions including necrotizing enterocolitis, in part, through impaired gap junction communication. We now hypothesize that activated macrophages inhibit gap junctions of adjacent enterocytes and seek to determine whether NO release from macrophages was involved. Using a coculture system of enterocytes and macrophages, we now demonstrate that "activation" of macrophages with lipopolysaccharide and interferon reduces the phosphorylation of Cx43 in adjacent enterocytes, an event known to inhibit gap junction communication. The effects of macrophages on enterocyte gap junctions could be reversed by treatment of macrophages with the inducible nitric oxide synthase (iNOS) inhibitor l-Lysine omega-acetamidine hydrochloride (l-NIL) and by incubation with macrophages from iNOS(-/-) mice, implicating NO in the process. Activated macrophages also caused a NO-dependent redistribution of connexin43 in adjacent enterocytes from the cell surface to an intracellular location, further suggesting NO release may inhibit gap junction function. Treatment of enterocytes with the NO donor S-nitroso-N-acetylpenicillamine (SNAP) markedly inhibited gap junction communication as determined using single cell microinjection of the gap junction tracer Lucifer yellow. Strikingly, activated macrophages inhibited enterocyte migration into a scraped wound, which was reversed by l-NIL pretreatment. These results implicate enterocyte gap junctions as a target of the NO-mediated effects of macrophages during intestinal inflammation, particularly where enterocyte migration is impaired.

Aberrant hemichannel properties of Cx26 mutations causing skin disease and deafness

Mutations in the human GJB2 gene, which encodes connexin26 (Cx26), underlie various forms of hereditary deafness and skin disease. While it has proven difficult to discern the exact pathological mechanisms that cause these disorders, studies have shown that the loss or abnormal function of Cx26 protein has a profound effect on tissue homeostasis. Here, we used the Xenopus oocyte expression system to examine the functional characteristics of a Cx26 mutation (G45E) that results in keratitis-ichthyosis-deafness syndrome (KIDS) with a fatal outcome. Our data showed that oocytes were able to express both wild-type Cx26 and its G45E variant, each of which formed hemichannels and gap junction channels. However, Cx26-G45E hemichannels displayed significantly greater whole cell currents than wild-type Cx26, leading to cell lysis and death. This severe phenotype could be rescued in the presence of elevated Ca(2+) levels in the extracellular milieu. Cx26-G45E could also form intercellular channels with a similar efficiency as wild-type Cx26, however, with increased voltage sensitive gating. We also compared Cx26-G45E with a previously described Cx26 mutant, A40V, which has an overlapping human phenotype. We found that both dominant Cx26 mutants elicited similar functional consequences and that cells coexpressing mutant and wild-type connexins predominantly displayed mutant-like behavior. These data suggest that mutant hemichannels may act on cellular homeostasis in a manner that can be detrimental to the tissues in which they are expressed.