Connexin43: emerging role in erectile function

Salidroside Attenuates Hypoxia-Induced Expression of Connexin 43 in Corpus Cavernosum Smooth Muscle Cells

<b><i>Introduction:</i></b> Connexin 43 (Cx43) is the major component of gap junction in corpus cavernosum smooth muscle, which allows rapid intercellular communication. Cx43 coordinates corpus cavernosum smooth muscle cells and ensures erectile function. The role of hypoxia in Cx43 dysfunction resulting in erectile dysfunction has not been well studied, and salidroside has shown cell protective effects under hypoxia. <b><i>Objective:</i></b> We aimed to investigate the protective role of salidroside and the underlying mechanisms in hypoxia-induced dysfunction of Cx43. <b><i>Methods:</i></b> Corpus cavernosum smooth muscle cells prepared from young male Sprague-Dawley rats were pretreated with or without salidroside and exposed to hypoxic condition for 48 h. The cell viability, expression of hypoxia-inducible factor-1α (HIF-1α) and Cx43, and Ca²⁺ signals were investigated. <b><i>Results:</i></b> Pretreatment with salidroside attenuated loss of hypoxia-induced cell viability markedly and could downregulate the HIF-1α protein expression under hypoxia. Moreover, the expression of Cx43 was significantly increased by hypoxia but was decreased with salidroside pretreatment. The salidroside pretreated group exhibited enhanced release of intracellular Ca²⁺ in corpus cavernosum smooth muscle cells compared with the hypoxia group after stimulation. <b><i>Conclusion:</i></b> Salidroside has a protective effect against hypoxia-induced damage to corpus cavernosum smooth muscle cells.

Effect of platelet-derived growth factor-BB on gap junction and connexin43 in rat penile corpus cavernosum smooth muscle cells

We explored whether platelet-derived growth factor (PDGF)-BB regulates corpus cavernosum smooth muscle cell gap junctions and can ameliorate erectile dysfunction and how it modulates connexin43 (CX43) after bilateral cavernous neurectomy. Primary cultured rat corpus cavernosum smooth muscle cells were treated with PDGF-BB with or without a PDGFR inhibitor, Akt siRNA or the depletion or promotion of β-catenin. PDGF-BB improved CCSMCs gap junction coupling and increased CX43 and PDGFRβ expression; inhibition of PDGFR activity down-regulated CX43 and decreased Akt and nuclear β-catenin. Knockdown or promotion of β-catenin down-regulated and up-regulated CX43 expression respectively. Moreover, β-catenin activation induced CX43 nuclear accumulation, which impeded CX43 down-regulation induced by PDGFR inhibition, suggesting that CX43 expression is positively correlated with nuclear β-catenin expression. Furthermore, CX43 promoter luciferase and chromatin immunoprecipitation assays indicated that β-catenin regulates CX43 transcription by directly interacting with its promoter. Male rats underwent bilateral cavernous neurectomy. After 12 weeks, they were injected with PDGF-BB, CX43 and PDGFRβ expression was significantly lower than in the control group, which was reversed by PDGF-BB injection. These results suggested that PDGF-BB contributed to the improvement of gap junction intracellular communication among corpus cavernosum smooth muscle cells, increased CX43 through PDGFRβ/Akt/nuclear β-catenin signalling, and ameliorated cavernous nerve injury-induced erectile dysfunction.

PDGF-mediated PI3K/AKT/β-catenin signaling regulates gap junctions in corpus cavernosum smooth muscle cells

Erectile dysfunction (ED) is the most common sexual disorder that men report to healthcare providers. Gap junctions (GJs) are thought to be responsible for synchronous shrinkage of corpus cavernosum smooth muscle cells (CCSMCs), and play thus an important role in the maintenance of an erection. Hypoxia has been suggested as a pathological mechanism underlying ED. Here we demonstrate that hypoxia increased the expression of platelet-derived growth factor (PDGF) and the main GJ component connexin (Cx)43 in CCSMCs. Inhibiting PDGF receptor (PDGFR) activity decreased Cx43 expression. Treatment with different concentrations of PDGF increased the levels of phosphorylated protein kinase B (AKT), β-catenin, and Cx43, whereas inhibition of PDGFR or activation of phosphatidylinositol 3 kinase (PI3K)/AKT signaling altered β-catenin and Cx43 expression. Meanwhile, silencing β-catenin resulted in the downregulation of Cx43. These results demonstrate that PDGF secretion by CCSMCs and vascular endothelial cells is enhanced under hypoxic conditions, leading to increased Cx43 expression through PI3K/AKT/β-catenin signaling and ultimately affecting GJ function in ED. Thus, targeting this pathway is a potential therapeutic strategy for the treatment of ED.

Physiological roles of connexins and pannexins in reproductive organs

Reproductive organs are complex and well-structured tissues essential to perpetuate the species. In mammals, the male and female reproductive organs vary on their organization, morphology and function. Connectivity between cells in such tissues plays pivotal roles in organogenesis and tissue functions through the regulation of cellular proliferation, migration, differentiation and apoptosis. Connexins and pannexins can be seen as major regulators of these physiological processes. In the present review, we assembled several lines of evidence demonstrating that these two families of proteins are essential for male and female reproduction.

Inhibition of histone deacetylase (HDAC) by 4-phenylbutyrate results in increased junctional conductance between rat corpora smooth muscle cells

4-phenylbutyrate (4-PB) has been shown to increase the protein content in a number of cells types. One such protein is Connexin43 (Cx43). We show here that 4-phenylbutyrate exposure results in significantly elevated cell to cell coupling, as determined by dual whole cell patch clamp. Incubation with 5 mM 4PB for 24 h or more nearly doubles junctional conductance. Interestingly, mRNA levels for Cx43 declined with exposure to 4-PB while western blot analysis revealed not significant change in protein levels. These data are most consistent with stabilization of the existing Cx43 pool or alterations in the number of functional channels within an existing pool of active and silent channels. These data represent a baseline for testing the efficacy of increased connexin mediated coupling in a variety of multicellular functions including erectile function.

The effects of the histone deacetylase inhibitor 4-phenylbutyrate on gap junction conductance and permeability

Longitudinal resistance is a key factor in determining cardiac action potential propagation. Action potential conduction velocity has been shown to be proportional to the square root of longitudinal resistance. A major determinant of longitudinal resistance in myocardium is the gap junction channel, comprised connexin proteins. Within the ventricular myocardium connexin43 (Cx43) is the dominantly expressed connexin. Reduced numbers of gap junction channels will result in an increase in longitudinal resistance creating the possibility of slowed conduction velocity while increased numbers of channels would potentially result in an increase in conduction velocity. We sought to determine if inhibition of histone deacetylase (HDAC) by 4-phenylbutyrate (4-PB), a known inhibitor of HDAC resulted in an increase in junctional conductance and permeability, which is not the result of changes in single channel unitary conductance. These experiments were performed using HEK-293 cells and HeLa cells stably transfected with Cx43. Following treatment with increasing concentrations of 4-PB up-regulation of Cx43 was observed via Western blot analysis. Junctional (g_j) conductance and unitary single channel conductance were measured via whole-cell patch clamp. In addition intercellular transfer of lucifer yellow (LY) was determined by fluorescence microscopy. The data in this study indicate that 4-PB is able to enhance functional Cx43 gap junction coupling as indicated by LY dye transfer and multichannel and single channel data along with Western blot analysis. As a corollary, pharmacological agents such as 4-PB have the potential, by increasing intercellular coupling, to reduce the effect of ischemia. It remains to be seen whether drugs like 4-PB will be effective in preventing cardiac maladies.

Connexin 43 a check-point component of cell proliferation implicated in a wide range of human testis diseases

Gap junction channels link cytoplasms of adjacent cells. Connexins, their constitutive proteins, are essential in cell homeostasis and are implicated in numerous physiological processes. Spermatogenesis is a sophisticated model of germ cell proliferation, differentiation, survival, and apoptosis, in which a connexin isotype, connexin 43, plays a crucial role as evidenced by genomic approaches based on gene deletion. The balance between cell proliferation/differentiation/apoptosis is a prerequisite for maintaining levels of spermatozoa essential for fertility and for limiting anarchic cell proliferation, a major risk of testis tumor. The present review highlights the emerging role of connexins in testis pathogenesis, focusing specifically on two intimately interconnected human testicular diseases (azoospermia with impaired spermatogenesis and testicular germ cell tumors), whose incidence increased during the last decades. This work proposes connexin 43 as a potential cancer diagnostic and prognostic marker, as well as a promising therapeutic target for testicular diseases.

Density and distribution of connexin 43 in corpus cavernosum tissue from diabetic and hypogonadal patients with erectile dysfunction

Altered expression of connexin 43 (Cx43) has been postulated to be involved in the development and progression of various diseases including erectile dysfunction (ED). The aim of this study was to determine whether distribution and density of the gap junction protein Cx43 are altered in human corpus cavernosum (HCC) tissue samples derived from diabetic or hypogonadal patients with ED compared to those from normal subjects.

HCC tissue sections derived from normal, diabetic and hypogonadal subjects were fixed in 4% formaldehyde, embedded in paraffin and immunostained with a monoclonal mouse anti-rat Cx43 antibody. Cx43 density was expressed as the cumulative number of gap junction plaques per unit area of tissue corrected for number of 4′,6-diamidino-2-phenylindole, dihydrochloride-labeled smooth muscle cells (dots per unit area corrected for number of cells).

The distribution of Cx43 plaques in smooth muscle was not affected in tissues derived from diabetic or hypogonadal subjects with ED compared with those from normal subjects. However, the number of Cx43 plaques was significantly reduced in HCC tissues derived from diabetic or hypogonadal subjects (73±8% and 68±11% of normal, respectively), indicating reduced Cx43 gap junctions in diabetic and hypogonadal subjects with ED.

Cx43 density in the HCC was diminished in tissue samples derived from diabetic or hypogonadal patients with ED compared to tissue samples from normal non-diabetic subjects. This marked decrease in Cx43 gap junction channels may contribute to attenuated gap junction function and to diminished erectile physiology.

Connexins and diabetes

Cell-to-cell interactions via gap junctional communication and connexon hemichannels are involved in the pathogenesis of diabetes. Gap junctions are highly specialized transmembrane structures that are formed by connexon hemichannels, which are further assembled from proteins called “connexins.” In this paper, we discuss current knowledge about connexins in diabetes. We also discuss mechanisms of connexin influence and the role of individual connexins in various tissues and how these are affected in diabetes. Connexins may be a future target by both genetic and pharmacological approaches to develop treatments for the treatment of diabetes and its complications.

The effects of 2-aminoethoxydiphenyl borate and diphenylboronic anhydride on gap junctions composed of Connexin43 in TM₄ sertoli cells

2-Aminoethoxydiphenyl borate (2-APB) has recently been demonstrated to inhibit gap junction (GJ) channels, whereas the underlying mechanisms are still unknown. Using mouse TM₄ Sertoli cell which expresses connexin43 (Cx43), we explored the effects of 2-APB and its analogues on dye-coupling through junctional channels formed by Cx43 and on expression of Cx43. Exposure of the cells to 2-APB (1-50 µM) and one of its analogues diphenylboronic anhydride (DPBA) (1-30 µM) for 4 h leads to a significant decrease in dye coupling of GJ in a concentration-dependent manner. The inhibitory effects of 2-APB and DPBA are reversible since decreased GJ coupling resumes after the two compounds are washed out. The disfunction of GJ induced by 2-APB and DPBA is associated with a decrease in total amount of Cx43 protein and number of GJs on the cell membrane. 2-APB and DPBA do not alter Cx43 phosphorylation state and the level of Cx43 mRNA expression. The loss of Cx43 protein is prevented by either lysosomal or proteasomal inhibitor, suggesting that the decrease in Cx43 results from a 2-APB or DPBA-enhanced degradation of Cx43. The present results indicate that 2-APB and DPBA inhibit GJ communication through decreasing Cx43 expression in TM₄ cells.