Altered pattern of vascular connexin expression in atherosclerotic plaques

Omega-3 fatty acids and atorvastatin affect connexin 43 expression in the aorta of hereditary hypertriglyceridemic rats

Statins and omega-3 polyunsaturated fatty acids (n-3 PUFA) reduce cardiovascular disease incidence during hypertriglyceridemia (HTG). To elucidate possible cardioprotective mechanisms, we focused on gap junction protein connexin 43 (Cx43). Its expression is disturbed during atherogenesis, but little information is available on its expression during HTG. Experiments were performed on adult male hereditary HTG (hHTG) rats treated with n-3 PUFA (30 mg/day) and atorvastatin (0.5 mg/100 g body weight per day) for 2 months. Cx43 expression and distribution in the aorta were investigated by using Western blotting and immunolabeling, followed by quantitative analysis. Transmission electronmicroscopy was used to study ultrastructure of endothelial contact sites. In contrast to age-matched Wistar, Cx43 expression in aorta of hHTG rats was significantly higher (p < 0.05), and prominent Cx43 immunospots were seen in tunica media and less in endothelium of hHTG rats. Changes in Cx43 expression were accompanied by local qualitative subcellular alterations of interendothelial connections. Treatment of hHTG rats with n-3 PUFA and atorvastatin markedly lowered Cx43 expression in aorta and modified connexin distribution in endothelium and media (p < 0.05 vs. untreated hHTG). The protective effect of treatment of HTG was observed on the structural integrity of the endothelium and was readily visible at the molecular level. Results indicate the involvement of altered Cx43 expression in vascular pathophysiology during HTG and during HTG treatment.

Asymmetric dimethylarginine damages connexin43-mediated endothelial gap junction intercellular communication

Asymmetric dimethylarginine (ADMA), a major endogenous inhibitor of nitric oxide synthase, is recently defined as a novel atherogenic factor. Communication via gap junction (GJIC) is involved in the regulation of a variety of endothelial activities, such as cell differentiation and senescence. The aim of this study is to explore the effects of ADMA on connexin43 (Cx43) mediated endothelial GJIC. Lysophosphatidylcholine (LPC) caused the downregulation of Cx43 expression and GJIC dysfunction in cultured human umbilical vein endothelial cells (HUVECs), which were significantly ameliorated by decreasing ADMA accumulation. Furthermore, we found that ADMA (10 µmol·L–1, 24 h) markedly downregulated Cx43 expression and damaged GJIC function in HUVECs. ADMA also increased production of intracellular reactive oxygen species (ROS) and induced phosphorylation of p38 MAPK. Furthermore, the inhibitory effect of ADMA on Cx43-mediated GJIC could be attenuated by NADPH oxidase inhibitor diphenyleneiodonium and apocynin as well as p38 MAPK inhibitor SB203580, respectively. In conclusion, our present results suggest that ADMA inhibits endothelial GJIC function via downregulating Cx43 expression, which suggesting a novel mechanism linking between elevated ADMA level and progression of atherosclerosis.

Blockade of Connexin 43 Hemichannels Reduces Neointima Formation After Vascular Injury by Inhibiting Proliferation and Phenotypic Modulation of Smooth Muscle Cells

Connexins 43 (Cx43) plays a key role in neointimal formation after vascular injury, but the mechanism still needs to be further explored. We hypothesized that the gap junction-dependent function of Cx43 to mediate intercellular communication has a crucial role in the development and progression of vascular diseases. The effect of intercellular communication mediated by Cx43 hemichannels on neointimal formation after vascular injury was investigated. Cx43 was overexpressed or knockdown in rat vascular smooth muscle cell (SMC) by transfection pcDNA-Cx43 plasmid or small interfering RNA (siRNA) against Cx43 (siCx43). SMC proliferation and marker genes expression after Cx43 alteration and blockade of the Cx43 hemichannel were analyzed by 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide assay and RT-PCR. The effect of carbenoxolone on neointimal formation was investigated in carotid artery injured rat model. We demonstrated that overexpression of Cx43 promoted SMC proliferation, meanwhile, mRNA expression level of smooth muscle α-actin and calponin, which were important markers of SMC in a contractile state, were down-regulated in smooth muscle. Knockdown of Cx43 inhibited SMC proliferation but increased SMC marker genes expression level. Carbenoxolone (50 μM) improved SMC contractile differentiation and inhibited its proliferation. Our data showed that carbenoxolone reduced neointimal formation after carotid artery injury. In summary, blockade of intercellular communication via Cx43 hemichannels reduces neointimal formation after vascular injury by inhibiting proliferation and phenotypic modulation of SMCs.

Molecular role of Cx37 in advanced atherosclerosis: a micro-array study

Recently, we showed that connexin37 (Cx37) protects against early atherosclerotic lesion development by regulating monocyte adhesion. The expression of this gap junction protein is altered in mouse and human atherosclerotic lesions; it is increased in macrophages newly recruited to the lesions and disappears from the endothelium of advanced plaques. To obtain more insight into the molecular role of Cx37 in advanced atherosclerosis, we used micro-array analysis for gene expression profiling in aortas of ApoE(-/-) and Cx37(-/-)ApoE(-/-) mice before and after 18 weeks of cholesterol-rich diet. Out of >15,000 genes, 106 genes were significantly differentially expressed in young mice before diet (P-value of <0.05, fold change of >0.7 or <-0.7, and intensity value >2.2 times background). Ingenuity pathway analysis (IPA) revealed differences in genes involved in cell-to-cell signaling and interaction, cellular compromise and nutritional disease. In addition, we identified 100 genes that were significantly perturbed after the cholesterol-rich diet. Similar to the analysis on 10-week-old mice, IPA revealed differences in genes involved in cell-to-cell signaling and interaction as well as to immuno-inflammatory disease. Furthermore, we found important changes in genes involved in vascular calcification and matrix degradation, some of which were confirmed at protein level by (immuno-)histochemistry. In conclusion, we suggest that Cx37 deficiency alters the global differential gene expression profiles in young mice towards a pro-inflammatory phenotype, which are then further influenced in advanced atherosclerosis. The results provide new insights into the significance of Cx37 in plaque calcification.

Heterogeneous expression of endothelial connexin (Cx) 37, Cx40, and Cx43 in rat large veins

Gap junctions are clusters of transmembrane protein channels for intercellular communication and are composed of connexin (Cx). The vascular endothelial cells express Cx37, Cx40, and Cx43. We herein examined the spatial distribution of the endothelial connexins Cx37, Cx40, and Cx43 in rat large veins including the cranial vena cava, thoracic section of the caudal vena cava, and abdominal section of the caudal vena cava. We also examined the mean size of the endothelial cells and quantified the protein expression levels of the endothelial connexins. We found that the large veins heterogeneously expressed Cx37, Cx40, and Cx43 as follows: Cx40 > Cx37 > > Cx43 in the cranial vena cava, Cx37 > Cx43 > > Cx40 in the thoracic section of the caudal vena cava, and Cx40 > Cx43 > > Cx37 in the abdominal section of the caudal vena cava. Double immunostaining of two of the endothelial connexins revealed that the gap-junction plaques were composed of various combinations of endothelial connexins. The mean size of the endothelial cells was large, moderate, or small in the cranial vena cava, the abdominal section of the caudal vena cava, or the thoracic section of the caudal vena cava, respectively. The heterogeneity of the endothelial cells of the rat large veins in terms of the connexin expression suggests that the endothelial cells are differently coupled in the large veins. The present data are useful for investigating, for example, disease-related alterations in expression of endothelial connexins in large veins.

Oxidized phospholipid species promote in vivo differential cx43 phosphorylation and vascular smooth muscle cell proliferation

Regulation of both the expression and function of connexins in the vascular wall is important during atherosclerosis. Progression of the disease state is marked by vascular smooth muscle cell (VSMC) proliferation, which coincides with the reduced expression levels of connexin 43 (Cx43). However, nothing is currently known about the factors that regulate post-translational modifications of Cx43 in atherogenesis, which could be of particular importance, due to the association between site-specific Cx43 phosphorylation and cellular proliferation. We compared the effects of direct carotid applications of two oxidized phospholipid derivatives, 1-palmitoyl-2-oxovaleroyl-sn-glycero-3-phosphorylcholine (POVPC) and 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphorylcholine (PGPC), on Cx43 expression and phosphorylation, and on cell proliferation. Since both POVPC and PGPC have been shown to act through different intracellular pathways, we hypothesized that each oxidized phospholipid species could induce differential Cx43 phosphorylation events in the cytoplasmically located carboxyl-terminal region of the protein, which could potentially enhance cell proliferation. Application of POVPC caused a reduction in VSMC Cx43 levels, enhanced its phosphorylation at serine (pS) 279/282, and increased VSMC proliferation both in vivo and in vitro. Treatment with PGPC enhanced VSMC pS368 levels with no associated change in proliferation. These oxidized phospholipid-induced Cx43 post-translational changes in VSMCs were consistent with those identified in ApoE(-/-) mice. Taken together, these results demonstrate that post-translational phosphorylation of Cx43 could be a key factor in the pathogenesis of atherosclerosis.

A model system to study Connexin 43 in the immune system

Connexin 43 (Cx43) is the predominant gap junction protein expressed in immune cells. Previous manuscripts have stated that gap junctions may play a role in antigen cross-presentation, dendritic cell maturation, T cell development, and regulatory T cell function. Many of these previous studies were performed in vitro. In vivo studies were not directly possible in adult mice because Cx43-/- mice die shortly after birth due to a cardiac malformation. To overcome these drawbacks, we have developed a mouse model that deletes Cx43 in the immune system while maintaining normal cardiac function. In our model, irradiated CD45.1+ wild-type mice were reconstituted with Cx43WT, Cx43+/-, or Cx43-/- hematopoietic fetal liver cells that were derived from CD45.2+ mice. The presence of CD45.2 allowed us to identify and track the donor cells following reconstitution. We determined that Cx43+/- and Cx43-/- hematopoietic cells were able to reconstitute irradiated mice as well as Cx43WT cells. Reconstitution was nearly 100% in the thymus and over 90% in the spleen. There appeared to be no difference in thymocyte development or in the ability of lymphocytes to transmigrate to peripheral lymphoid organs. However in response to inflammation, Cx43+/- radiation chimeras had increased peritoneal infiltration compared to Cx43WT and Cx43-/- groups. IgG responses were normal in all groups but the Cx43-/- reconstituted mice had an elevated IgM response. Our data suggests that Cx43 may not be involved in the normal development of the immune system but may regulate certain effector functions in vivo.

Shear Stress and Cyclic Circumferential Stretch, But Not Pressure, Alter Connexin43 Expression in Endothelial Cells

Hemodynamic forces play a critical role in atherogenesis, as evidenced by the focal pattern of development of atherosclerotic lesions. Whereas disturbed flow in the branches and curved regions of large arteries is proatherogenic, laminar flow in the straight parts of vessels is atheroprotective. In addition, hypertension and age-related changes in arterial stiffness are important risk factors of the disease. Hemodynamic forces induce various changes in the structure and function of vascular endothelium, many of which reflect alterations in gene expression. Endothelial cells are linked by gap junctions, which facilitate the propagation of electrical and chemical signals along the vascular wall. Using an in vitro perfusion system, we investigated the effects of pulsed unidirectional and oscillatory flows in combination with different levels of hydrostatic pressure and circumferential stretch on the expression of Cx43 in endothelial cells. Our results show that shear stress and circumferential stretch, but not pressure, modulate the expression of Cx43. In view of the distribution of this protein along the vascular tree, our findings provide new insights into the role of mechanical forces on gap junctional communication in regions prone to the development of atherosclerosis.

Connexin32 is expressed in vascular endothelial cells and participates in gap-junction intercellular communication

Endothelial cells (ECs) play many roles in vascular biology, including control of blood pressure, blood clotting, atherosclerosis, angiogenesis, and inflammation. Gap junctions (GJs) are channel-like assemblies of connexin (Cx) family proteins that connect neighboring cells and modulate and synchronize their intracellular environments by the transfer of intracellular mediators. It has been reported that vascular ECs express Cx37, Cx40, and Cx43, but not Cx32. Here, we showed that Cx32 mRNA and protein are expressed in various cultured human ECs. We confirmed Cx32 expression in blood vessel ECs using wild-type and Cx32 knock-out mice. We observed that dye transfer between cultured ECs through gap junctions is suppressed by an anti-Cx32 monoclonal antibody. These findings suggest that vascular ECs express Cx32, which participates in endothelial gap-junction intercellular communication.

Functional differences between human Cx37 polymorphic hemichannels

A polymorphism in the human Cx37 gene (C1019T), resulting in a non-conservative amino acid change in the regulatory C-terminus of the Cx37 protein (P319S), has been proposed as a prognostic marker for atherosclerosis. We have recently demonstrated that Cx37 hemichannels control the initiation of atherosclerotic plaque development by regulating ATP-dependent monocyte adhesion in atherosclerosis-susceptible apolipoprotein E-deficient mice. In this study, we have measured the electrical properties of Cx37 hemichannels (HCs) and gap junction channels (GJCs) with voltage-clamp methods. To this end, we have transfected hCx37-P319, hCx37-S319 or empty pIRES-eGFP vector cDNA into communication-deficient HeLa cells. In clones expressing similar levels of Cx37, exposure of single cells to low-Ca(2+) solution induced a voltage-sensitive HC current. The analysis yielded a bell-shaped function g(hc)=f(V(m)) (g(hc): normalized conductance at steady state; V(m): membrane potential) with a maximum around V(m)=-30 mV. The peak g(hc) of Cx37-P319 was 3-fold larger than that of Cx37-S319 HCs. Experiments on cell pairs revealed that Cx37-P319 GJCs exhibited a 1.5-fold larger unitary conductance than Cx37-S319 GJCs. Hence, the larger peak g(hc) of the former may reflect a larger conductance of their HCs. Using the same clones, we found that Cx37-P319 cells released more ATP and were less adhesive than Cx37-S319 cells. The reduction in adhesiveness of Cx37-expressing cells was prevented by extracellular apyrase. We conclude that the differences in biophysical properties between polymorphic HCs may be responsible for inequality in ATP release between Cx37-P319 and Cx37-S319 cells, which results in differential cell adhesion.

Intercellular Communication in Atherosclerosis

Cell-to-cell communication is a process necessary for physiological tissue homeostasis and appears often altered during disease. Gap junction channels, formed by connexins, allow the direct intercellular communication between adjacent cells. After a brief review of the pathophysiology of atherosclerosis, we will discuss the role of connexins throughout the different stages of the disease.

Connexins in vascular physiology and pathology

Cellular interaction in blood vessels is maintained by multiple communication pathways, including gap junctions. They consist of intercellular channels ensuring direct interaction between endothelial and smooth muscle cells and the synchronization of their behavior along the vascular wall. Gap-junction channels arise from the docking of two hemichannels or connexons, formed by the assembly of six connexins, and achieve direct cellular communication by allowing the transport of small metabolites, second messengers, and ions between two adjacent cells. Physiologic variations in connexin expression are observed along the vascular tree, with most common connexins being Cx37, Cx40, and Cx43. Changes in the level of expression of connexins have been correlated to the development of vascular disease, such as hypertension, atherosclerosis, or restenosis. Recent studies on connexin-deficient mice highlighted key roles of these communication pathways in the development of these pathologies and confirmed the need for targeted pharmacologic approaches for their prevention and treatment. The aim of this issue is to review the current knowledge on the implication of gap junctions in vascular function and most common cardiovascular diseases.

Gap junctions in the control of vascular function

Direct intercellular communication via gap junctions is critical in the control and coordination of vascular function. In the cardiovascular system, gap junctions are made up of one or more of four connexin proteins: Cx37, Cx40, Cx43, and Cx45. The expression of more than one gap-junction protein in the vasculature is not redundant. Rather, vascular connexins work in concert, first during the development of the cardiovascular system, and then in integrating smooth muscle and endothelial cell function, and in coordinating cell function along the length of the vessel wall. In addition, connexin-based channels have emerged as an important signaling pathway in the astrocyte-mediated neurovascular coupling. Direct electrical communication between endothelial cells and vascular smooth muscle cells via gap junctions is thought to play a relevant role in the control of vasomotor tone, providing the signaling pathway known as endothelium-derived hyperpolarizing factor (EDHF). Consistent with the importance of gap junctions in the regulation of vasomotor tone and arterial blood pressure, the expression of connexins is altered in diseases associated with vascular complications. In this review, we discuss the participation of connexin-based channels in the control of vascular function in physiologic and pathologic conditions, with a special emphasis on hypertension and diabetes.

Biological and biophysical properties of vascular connexin channels

Intercellular channels formed by connexin proteins play a pivotal role in the direct movement of ions and larger cytoplasmic solutes between vascular endothelial cells, between vascular smooth muscle cells, and between endothelial and smooth muscle cells. Multiple genetic and epigenetic factors modulate connexin expression levels and/or channel function, including cell-type-independent and cell-type-specific transcription factors, posttranslational modifications, and localized membrane targeting. Additionally, differences in protein-protein interactions, including those between connexins, significantly contribute to both vascular homeostasis and disease progression. The biophysical properties of the connexin channels identified in the vasculature, those formed by Cx37, Cx40, Cx43 and/or Cx45 proteins, are discussed in this chapter in the physiological and pathophysiological context of vessel function.

Altered gene expression in highly purified enterocytes from patients with active coeliac disease

Background

Coeliac disease is a multifactorial inflammatory disorder of the intestine caused by ingestion of gluten in genetically susceptible individuals. Genes within the HLA-DQ locus are considered to contribute some 40% of the genetic influence on this disease. However, information on other disease causing genes is sparse. Since enterocytes are considered to play a central role in coeliac pathology, the aim of this study was to examine gene expression in a highly purified isolate of these cells taken from patients with active disease. Epithelial cells were isolated from duodenal biopsies taken from five coeliac patients with active disease and five non-coeliac control subjects. Contaminating T cells were removed by magnetic sorting. The gene expression profile of the cells was examined using microarray analysis. Validation of significantly altered genes was performed by real-time RT-PCR and immunohistochemistry.

Results

Enterocyte suspensions of high purity (98–99%) were isolated from intestinal biopsies. Of the 3,800 genes investigated, 102 genes were found to have significantly altered expression between coeliac disease patients and controls (p < 0.05). Analysis of these altered genes revealed a number of biological processes that are potentially modified in active coeliac disease. These processes include events likely to contibute to coeliac pathology, such as altered cell proliferation, differentiation, survival, structure and transport.

Conclusion

This study provides a profile of the molecular changes that occur in the intestinal epithelium of coeliac patients with active disease. Novel candidate genes were revealed which highlight the contribution of the epithelial cell to the pathogenesis of coeliac disease.

Functional role of gap junctions in cytokine-induced leukocyte adhesion to endothelium in vivo

To assess the hypothesis that gap junctions (GJs) participate on leukocyte-endothelium interactions in the inflammatory response, we compared leukocyte adhesion and transmigration elicited by cytokine stimulation in the presence or absence of GJ blockers in the hamster cheek pouch and also in the cremaster muscle of wild-type (WT) and endothelium-specific connexin 43 (Cx43) null mice (Cx43e(-/-)). In the cheek pouch, topical tumor necrosis factor-alpha (TNF-alpha; 150 ng/ml, 15 min) caused a sustained increment in the number of leukocytes adhered to venular endothelium (LAV) and located at perivenular regions (LPV). Superfusion with the GJ blockers 18-alpha-glycyrrhetinic acid (AGA; 75 microM) or 18-beta-glycyrrhetinic acid (50 microM) abolished the TNF-alpha-induced increase in LAV and LPV; carbenoxolone (75 microM) or oleamide (100 microM) reduced LAV by 50 and 75%, respectively, and LPV to a lesser extent. None of these GJ blockers modified venular diameter, blood flow, or leukocyte rolling. In contrast, glycyrrhizin (75 microM), a non-GJ blocker analog of AGA, was devoid of effect. Interestingly, when AGA was removed 90 min after TNF-alpha stimulation, LAV started to rise at a similar rate as in control. Conversely, application of AGA 90 min after TNF-alpha reduced the number of previously adhered cells. In WT mice, intrascrotal injection of TNF-alpha (0.5 microg/0.3 ml) increased LAV (fourfold) and LPV (threefold) compared with saline-injected controls. In contrast to the observations in WT animals, TNF-alpha stimulation did not increase LAV or LPV in Cx43e(-/-) mice. These results demonstrate an important role for GJ communication in leukocyte adhesion and transmigration during acute inflammation in vivo and further suggest that endothelial Cx43 is key in these processes.

Involvement of connexin 43 in angiotensin II-induced migration and proliferation of saphenous vein smooth muscle cells via the MAPK-AP-1 signaling pathway

Proliferation and migration of vascular smooth muscle cells (VSMCs) lead to intimal thickening and influence the long-term patency of venous graft post coronary arterial bypass graft. There is increasing evidence that connexins are involved in the development of intimal hyperplasia and restenosis. We assessed connexin 43 (Cx43) expression and its role in angiotensin II-induced proliferation and migration of smooth muscle cells and the signal pathways involved in human saphenous vein bypass conduits. Angiotensin II significantly increased gap junctional intercellular communication and induced the expression of Cx43 in human saphenous vein SMCs in a dose- and time-dependent manner through angiotensin II type 1 receptor. The effect of angiotensin II was blocked by siRNA of ERK 1/2, p38 MAPK and JNK, respectively. Overexpression of Cx43 markedly increased the proliferation of saphenous vein SMCs. However, siRNA for Cx43 inhibited angiotensin II-induced proliferation, cyclin E expression and migration of human saphenous vein SMCs. In dual-luciferase reporter assay, angiotensin II markedly activated AP-1 transcription factor, which was significantly attenuated by a dominant-negative AP-1 (A-Fos) with subsequent inhibition of angiotensin II-induced transcriptional expression of Cx43. These data demonstrate the role of Cx43 in the proliferation and migration of human saphenous vein SMCs and angiotensin II-induced Cx43 expression via mitogen-activated protein kinases (MAPK)-AP-1 signaling pathway.

Effects of disturbed flow on endothelial cells

Vascular endothelial cells (ECs) play significant roles in regulating circulatory functions. The shear stress resulting from blood flow modulates EC functions by activating mechano-sensors, signaling pathways, and gene and protein expressions. Shear stress with a clear direction resulting form pulsatile or steady flow causes only transient activation of pro-inflammatory and proliferative pathways, which become down-regulated when such directed shearing is sustained. In contrast, shear flow without a definitive direction (e.g., disturbed flow in regions of complex geometry) causes sustained molecular signaling of pro-inflammatory and proliferative pathways. The EC responses to shear flows with a clear direction involve the remodeling of EC structure to maintain vascular homeostasis and are athero-protective. Such regulatory mechanism does not operate effectively when the flow pattern is disturbed. Therefore, the branch points and other regions of the arterial tree with a complex geometry are prone to atherogenesis, whereas the straight part of the arterial tree is generally spared. Understanding of the EC responses to different flow patters helps to elucidate the mechanism of the region-specific localization of atherosclerosis in the arterial system.

Associations between connexin37 gene polymorphism and markers of subclinical atherosclerosis: the Cardiovascular Risk in Young Finns study

OBJECTIVE

Connexin37 (cx37) C1019T polymorphism has been shown to associate with coronary artery disease in different populations. We investigated whether this polymorphism associates with carotid artery intima-media thickness (IMT), carotid artery compliance (CAC) and brachial artery flow mediated dilatation (FMD) - i.e., early ultrasound markers of subclinical atherosclerosis - in a clinically healthy population of young Finnish adults.

METHODS AND RESULTS

1440 individuals from the Cardiovascular Risk in Young Finns study were genotyped and studied using cardiovascular risk factor and ultrasound data obtained in 2001. In linear regression models, no significant association between the cx37 polymorphism and carotid IMT, CAC or brachial artery FMD (ANOVA, p=0.159, 0.151 and 0.547), respectively, was found in the whole population or in women and men separately.

CONCLUSIONS

The connexin37 C1019T polymorphism is not related with markers of subclinical atherosclerosis in young adults.

The stage-specific function of gap junctions during tumourigenesis

Tumour development is a process resulting from the disturbance of various cellular functions including cell proliferation, adhesion and motility. While the role of these cell parameters in tumour promotion and progression has been widely recognized, the mechanisms that influence gap junctional coupling during tumorigenesis remain elusive. Neoplastic cells usually display decreased levels of connexin expression and/or gap junctional coupling. Thus, impaired intercellular communication via gap junctions may facilitate the release of a potentially neoplastic cell from the controlling regime of the surrounding tissue, leading to tumour promotion. However, recent data indicates that metastatic tumour cell lines are often characterized by relatively high levels of connexin expression and gap junctional coupling. This review outlines current knowledge on the role of connexins in tumorigenesis and the possible mechanisms of the interference of gap junctional coupling with the processes of tumour invasion and metastasis.

Angiotensin II and IGF-1 regulate connexin43 expression via ERK and p38 signaling pathways in vascular smooth muscle cells of coronary artery bypass conduits

BACKGROUND

Changes in connexin expression have been found in vascular smooth muscle cells (VSMCs) during the progression of atherosclerotic lesion and intimal hyperplasia. It is our hypothesis that increased connexin43 expression following stimulation of VSMCs with Ang II and IGF-1 contributes to more proliferation in saphenous vein (SV) than in the internal mammary artery (IMA).

MATERIALS AND METHOD

Using immunohistochemistry, Western blot, and reverse-transcription polymerase chain reaction, we assessed the effect of Ang II and IGF-1 stimulation on connexin43 expression and the signaling pathways involved in VSMCs of SV and IMA.

RESULTS

Immunostaining demonstrated strong expression of connexin43 in SV compared with IMA after stimulation with Ang II and IGF-1. Ang II up-regulated the expression of connexin43 in VSMCs of SV in a dose- and time-dependent manner. This was inhibited by p38 and ERK MAP kinase inhibitors, SB203580 and PD98059, respectively. In the VSMCs of IMA, the connexin43 expression was markedly low and maintained at a reduced level even after 3 h stimulation. IGF-1 dose-dependently induced mRNA expression of connexin43 in the VSMCs of SV, which was blocked by PD98059. However, in VSMCs of IMA there was no significant effect of IGF-1 on the connexin43 mRNA expression.

CONCLUSION

These data suggest that connexin43 expression can be influenced by Ang II and IGF-1 through ERK and p38 pathways and may contribute to the pathogenesis of vein graft disease following coronary artery bypass grafting.

Endothelial connexin 37, connexin 40, and connexin 43 respond uniquely to substrate and shear stress

Endothelial connexins have been linked to atherosclerosis and hypertension; however, little is know about their sensitivity to stimuli and individual functions. This study investigates the responses of endothelial connexin 37, connexin 40, and connexin 43 (Cx37, Cx40, and Cx43) to shear stress and substrate. Human endothelial cells were seeded on adsorbed collagen or a collagen gel containing smooth muscle cells and exposed to static or laminar shear stress. Connexin mRNA, protein, and gap junction communication were examined. Endothelial monolayers were treated with connexin-specific short interfering RNA (siRNA) and evaluated for communication, proliferation, and morphology under static and shear stress. Results show differential responses of Cx37, Cx40, and Cx43 to substrate and shear stress with reduced communication after shear exposure. RNA interference of individual connexins resulted in expression change of nontarget connexins, which suggests linked expression. Gap junction communication under static conditions is reduced following Cx43 siRNA treatment. Endothelial cells are more elongated with RNA interference (RNAi) targeting Cx40. In conclusion, endothelial connexins demonstrated novel sensitivity to mechanical environment and substrate. Individual isotypes show differential responses and RNAi knockdown provides new insight into connexin function and potential roles in the vasculature.

Gap junction-mediated intercellular communication in the immune system

Immune cells are usually considered non-attached blood cells, which would exclude the formation of gap junctions. This is a misconception since many immune cells express connexin 43 (Cx43) and other connexins and are often residing in tissue. The role of gap junctions is largely ignored by immunologists as is the immune system in the field of gap junction research. Here, the current knowledge of the distribution of connexins and the function of gap junctions in the immune system is discussed. Gap junctions appear to play many roles in antibody productions and specific immune responses and may be important in sensing danger in tissue by the immune system. Gap junctions not only transfer electrical and metabolical but also immunological information in the form of peptides for a process called cross-presentation. This is essential for proper immune responses to viruses and possibly tumours. Until now only 40 research papers on gap junctions in the immune system appeared and this will almost certainly expand with the increased mutual interest between the fields of immunology and gap junction research.

Connexin37: a potential modifier gene of inflammatory disease

There is an increasing appreciation of the importance of gap junction proteins (connexins) in modulating the severity of inflammatory diseases. Multiple epidemiological gene association studies have detected a link between a single nucleotide polymorphism in the human connexin37 (Cx37) gene and coronary artery disease or myocardial infarction in various populations. This C1019T polymorphism causes a proline-to-serine substitution (P319S) in the regulatory C terminal tail of Cx37, a protein that is expressed in the vascular endothelium as well as in monocytes and macrophages. Indeed, these three cell types are key players in atherogenesis. In the early phases of atherosclerosis, blood monocytes are recruited to the sites of injury in response to chemotactic factors. Monocytes adhere to the dysfunctional endothelium and transmigrate across endothelial cells to penetrate the arterial intima. In the intima, monocytes proliferate, mature, and accumulate lipids to progress into macrophage foam cells. This review focuses on Cx37 and its impact on the cellular and molecular events underlying tissue function, with particular emphasis of the contribution of the C1019T polymorphism in atherosclerosis. We will also discuss evidence for a potential mechanism by which allelic variants of Cx37 are differentially predictive of increased risk for inflammatory diseases.

Connexins: new genes in atherosclerosis

Atherosclerosis, the main cause of death and disability in adult populations of industrialized societies, is a multifactorial progressive process involving a variety of pathogenic mechanisms. Our current view on the pathogenesis of the disease implies complex patterns of interactions between a dysfunctional endothelium, leukocytes, and activated smooth muscle cells in which cytokines and growth factors are known to play a crucial role. Apart from paracrine cell-to-cell signalling, a role for gap junction-mediated intercellular communication in the development of the disease has been recently suggested. Gap junction channels result from the docking of two hemichannels or connexons, formed by the hexameric assembly of connexins, and directly connect the cytoplasm of adjacent cells. In this review, we summarize existing evidence implicating connexins in atherosclerosis. Indeed, the expression pattern of vascular connexins is altered during atherosclerotic plaque formation. In addition, changes in connexin expression or gap junctional communication have been observed in vascular cells in vitro by disturbances in blood flow, cholesterol, inflammatory cytokines, and growth factors. Furthermore, genetically modifying connexin expression affects the course of the atherosclerotic process in mouse models of the disease. Finally, the involvement of connexins in treatment of atherosclerotic disease will be discussed.

Connexin37 protects against atherosclerosis by regulating monocyte adhesion

A genetic polymorphism in the human gene encoding connexin37 (CX37, encoded by GJA4, also known as CX37) has been reported as a potential prognostic marker for atherosclerosis. The expression of this gap-junction protein is altered in mouse and human atherosclerotic lesions: it disappears from the endothelium of advanced plaques but is detected in macrophages recruited to the lesions. The role of CX37 in atherogenesis, however, remains unknown. Here we have investigated the effect of deleting the mouse connexin37 (Cx37) gene (Gja4, also known as Cx37) on atherosclerosis in apolipoprotein E-deficient (Apoe(-/-)) mice, an animal model of this disease. We find that Gja4(-/-)Apoe(-/-) mice develop more aortic lesions than Gja4(+/+)Apoe(-/-) mice that express Cx37. Using in vivo adoptive transfer, we show that monocyte and macrophage recruitment is enhanced by eliminating expression of Cx37 in these leukocytes but not by eliminating its expression in the endothelium. We further show that Cx37 hemichannel activity in primary monocytes, macrophages and a macrophage cell line (H36.12j) inhibits leukocyte adhesion. This antiadhesive effect is mediated by release of ATP into the extracellular space. Thus, Cx37 hemichannels may control initiation of the development of atherosclerotic plaques by regulating monocyte adhesion. H36.12j macrophages expressing either of the two CX37 proteins encoded by a polymorphism in the human GJA4 gene show differential ATP-dependent adhesion. These results provide a potential mechanism by which a polymorphism in CX37 protects against atherosclerosis.

Effect of gap junction uncoupler heptanol on resistance arteries reactivity in experimental models of diabetes, hyperlipemia and hyperlipemia-diabetes

The understanding of the involvement of the gap junctions (GJ) in the vascular reactivity is an ongoing effort. In this study we questioned on impact of pathologies such as diabetes, hyperlipemia, and simultaneous hyperlipemia-diabetes on GJ involvement in the contractile/relaxant response of the mesenteric resistance arteries. To this purpose, four groups of Golden Syrian hamsters were used: (i) diabetics (D), injected by streptozotocin, (ii) hyperlipemics (H), fed the standard chow of the species supplemented with 3% cholesterol and 15% butter, (iii) simultaneously hyperlipemic-diabetics (HD), and (iv) controls (C), age-matched normal healthy animals. At 24 weeks after the beginning of the experiment, the vascular reactivity of the resistance arteries was measured by the myograph technique in the presence/absence of 1 mM Heptanol (Hep) and of vasoconstrictors and vasodilators. The results showed that: (i) in pathological conditions 1 mM Hep significantly impaired the constrictor response of the hamster resistance arteries to both 10(-5) M NA (noradrenaline, agonist of alpha(1)-adrenoceptors) and 64.1 mM K+ (potassium ion, the major intracellular cation). The impairment occur in the group range: HD < H < D < C being the highest at the simultaneous insult of hyperlipemia and diabetes; (ii) independently of the pathological condition, 1 mM Hep abolishes both endothelium-dependent and independent relaxation of the hamster resistance arteries. At 1 mM Hep we noticed a reversible effect on endothelium-dependent relaxation that may be partially restored (in normal) in the presence of L-arginine. It is hoped that these results may contribute to understanding of the involvement of GJ in vascular pathology/dysfunction.

Connexin40 and connexin43 in mouse aortic endothelium: evidence for coordinated regulation

In the vessel wall, endothelial cells are metabolically and electrically coupled to each other and to the adjacent smooth muscle cells by gap junctions composed of connexins. Gap junctions may be formed from combinations of several different connexin proteins, and deletion of one connexin can lead to modification of the expression of another. To reveal a possible interaction between connexin40 (Cx40) and connexin43 (Cx43) in endothelium, we studied their distribution in vessels from C57Bl/6 and Cx40 knockout mice (Cx40-/-) using immunoblots and immunocytochemistry on aortic cross sections and en face whole mounts. En face preparations from C57Bl/6 mice revealed two distinct pools of Cx43, one pericellular and the other intracellular. Cx40 was largely restricted to the periphery of the cells, and in Cx40-/- mice it was, as expected, undetectable. In the Cx40-/- mice, total Cx43 protein was also modestly reduced (immunoblots), but there was a major redistribution of the protein within the cell. The pericellular component of Cx43 was rendered virtually undetectable, and the intracellular compartments were normal or even slightly elevated. Smooth muscle Cx43 was also reduced in the Cx40-/- animals. These findings indicate that the cellular distribution of Cx43 is dependent on the presence of Cx40, and in view of the profound effects on the pericellular pool of the Cx43, the findings suggest that interactions between Cx40 and Cx43 regulate communication between endothelial cells and perhaps between smooth muscle and endothelial cells as well.

Mechanisms Underlying Nano-Sized Air-Pollution-Mediated Progression of Atherosclerosis Carbon Black Causes Cytotoxic Injury/Inflammation and Inhibits Cell Growth in Vascular Endothelial Cells

BACKGROUND

Epidemiological studies indicate a significant link between exposure to environmental air pollution and mortality and morbidity from ischemic heart disease. Because nanoparticles can translocate into blood circulation, the present study aimed to clarify their direct effects on human vascular endothelial cells (ECs).

METHODS AND RESULTS

Human umbilical vein ECs (HUVECs) were treated with carbon black (CB), a component of diesel exhaust particles, for 24 h. CB induced cytotoxic morphological changes such as cytosolic vacuole formation, cell disorientation and decreased density. Lactate dehydrogenase assay revealed that CB induced cytotoxic injury in both the cells and plasma membranes. Proliferation assay showed that CB inhibited cell growth. Monocyte chemoattractant protein-1 but not vascular cell adhesion molecule-1 was induced by CB. CB reduced the expressions of connexin37 and endothelial nitric oxide (NO) synthase. Microarray analysis revealed the induction of pro-inflammatory molecules by CB.

CONCLUSIONS

The present results demonstrate for the first time that CB directly affects the endothelium, causing cytotoxic injury, inflammatory responses, and inhibition of cell growth. As EC injury/inflammation and membrane disintegration are related to the initiation of atherosclerosis, and NO is anti-atherogenic and anti-thrombogenic, the direct effects of nanoparticles on ECs may represent one mechanism behind environmental air pollution-mediated atherosclerosis and ischemic heart disease.

Flow regulates intercellular communication in HAEC by assembling functional Cx40 and Cx37 gap junctional channels

Direct cell-to-cell transfer of ions and small signaling molecules via gap junctions plays a key role in vessel wall homeostasis. Vascular endothelial gap junctional channels are formed by the connexin (Cx) proteins Cx37, Cx40, and Cx43. The mechanisms regulating connexin expression and assembly into functional channels have not been fully identified. We investigated the dynamic regulation of endothelial gap junctional intercellular communication (GJIC) by fluid flow and the participation of each vascular connexin in functional human endothelial gap junctions in vitro. Human aortic endothelial cells (HAEC) were exposed for 5, 16, and 24 h to physiological flows in a parallel-plate flow chamber. Connexin protein expression and localization were evaluated by immunocytochemistry, and functional GJIC was evaluated by dye injection. Connexin-mimetic peptide inhibitors were used to assess the specific connexin composition of functional channels. HAEC monolayers in culture exhibited baseline functional communication at a striking low level despite abundant expression of Cx43 and Cx40 localized at cell-to-cell appositions. Upon exposure to flow, GJIC by dye spread demonstrated a significant time-dependent increase from baseline levels, reaching 7.5-fold in 24 h. Inhibition studies revealed that this response was mediated primarily by Cx40, with lesser contributions of the other two vascular connexins assembled into functional homotypic and/or heterotypic channels. This is the first study to demonstrate that flow simultaneously and differentially regulates expression of the Cx37, Cx40, and Cx43 proteins and their involvement in the augmentation of intercellular communication by dye transfer in human endothelial cells in vitro.

Prospects for Personalized Cardiovascular Medicine

Sequencing of the human genome has ushered in prospects for individualizing cardiovascular health care. There is growing evidence that the practice of cardiovascular medicine might soon have a new toolbox to predict and treat disease more effectively. The Human Genome Project has spawned several important "omic" technologies that allow "whole genome" interrogation of sequence variation (re-sequencing, genotyping, comparative genome hybridization), transcription (expression profiling, tissue arrays), proteins (gas or liquid chromatography and tandem mass spectroscopy [MS]), and metabolites (MS or nuclear magnetic resonance profiling); deoxyribonucleic acid, ribonucleic acid, protein, and metabolic approaches all provide more exacting detail of cardiovascular disease mechanisms and, in some cases, are redefining its taxonomy. Pharmacogenomic approaches are emerging across broad classes of cardiovascular therapeutics to assist practitioners in making more precise decisions about which drugs to give to which patients to optimize the benefit-to-risk ratio. Molecular imaging is developing chemical and biological probes that can sense molecular pathway mechanisms that will allow us to monitor health and disease. Together, these tools will enable a paradigm shift from genetic medicine--on the basis of the study of individual inherited characteristics, most often single genes--to genomic medicine, which by its nature is comprehensive and focuses on the functions and interactions of multiple genes and gene products, among themselves and with their environment. The information gained from such analyses, in combination with clinical data, is now allowing us to assess individual risks and guide clinical management and decision-making, all of which form the basis for cardiovascular genomic medicine.

Pharmacology of gap junctions. New pharmacological targets for treatment of arrhythmia, seizure and cancer?

Intercellular communication in many organs is maintained via intercellular gap junction channels composed of connexins, a large protein family with a number of isoforms. This gap junction intercellular communication (GJIC) allows the propagation of action potentials (e.g., in brain, heart), and the transfer of small molecules which may regulate cell growth, differentiation and function. The latter has been shown to be involved in cancer growth: reduced GJIC often is associated with increased tumor growth or with de-differentiation processes. Disturbances of GJIC in the heart can cause arrhythmia, while in brain electrical activity during seizures seems to be propagated via gap junction channels. Many diseases or pathophysiological conditions seem to be associated with alterations of gap junction protein expression. Thus, depending on the target disease opening or closure of gap junctions may be of interest, or alteration of connexin expression. GJIC can be affected acutely by changing gap junction conductance or--more chronic--by altering connexin expression and membrane localisation. This review gives an overview on drugs affecting GJIC.

Intact endothelium-dependent dilation and conducted responses in resistance vessels of hypercholesterolemic mice in vivo

Atherosclerosis and hyperlipidemia impair endothelium-dependent nitric oxide (NO)-mediated dilations in conducting arteries. In addition to NO, the endothelium releases an endothelium-derived hyperpolarizing factor (EDHF) in response to acetylcholine (ACh), which is particularly important in microvessels and initiates a dilation that conducts along the vessel through gap junctional communication. The expression of connexins is, however, altered by hypercholesterolemia. Therefore, we studied endothelium-dependent dilations and their conduction in murine hypercholesterolemic models. Dilations were assessed by intravital microscopy in arterioles with a diameter of approximately 35 microm in ApoE and LDL receptor (LDLR(-/-))-deficient mice after superfusion or locally confined application of ACh. ACh induced comparable concentration-dependent dilations in wild-type, LDLR(-/-), and ApoE(-/-) mice fed a normal or high-cholesterol diet, however EC(50) was slightly higher in ApoE(-/-) mice. Furthermore, the NO donor sodium-nitroprusside dilated arterioles to a similar extent (approximately 60%). Locally initiated ACh dilations (approximately 68%) conducted up to a distance of 1,100 microm without significant attenuation even under severe hypercholesterolemic conditions. Since ACh dilation in the arterioles of mice is mainly mediated via EDHF, we conclude that hypercholesterolemia does not alter EDHF release and efficacy. This conclusion is confirmed by an intact conducted response since EDHF is a prerequisite for this response. The intact conduction also suggests that gap-junctional communication is functionally preserved in these models.

Association between C1019T polymorphism of connexin37 and acute myocardial infarction: a study in patients from Sicily

During atherogenesis, a critical role is played by intercellular communication via gap junctions, cell membrane channels linking the cytoplasmic compartments of adjacent cells. The component protein subunits of these channels, called connexin (Cx), belong to a multigene family. Cx37 is involved in growth, regeneration after injury and ageing of the endothelial cells, suggesting its role in atherosclerosis. The C1019 single nucleotide polymorphism (SNP) of Cx37 gene was associated with thickening of the carotid intima in Swedish men and was also associated with coronary artery disease in a Taiwanese population. On the other hand, in two more recent studies performed in male Japanese population, T1019 Cx37 SNP has shown to be a risk factor for acute myocardial infarction (AMI). In the light of these discrepant results, we have studied the frequency of this SNP in a very homogeneous cohort of young male people affected by AMI. We analysed 97 male Sicilian patients (mean age 40, age range 20-46) and 196 healthy male controls (mean age 39, age range 20-55) for C1019T of the Cx37. The 1019T SNP was significantly increased in the patients compared to the controls (43.8% vs. 34.4%; p=0.03 by chi2 test with Yates' correction; odds ratio (OR) 1.5, (1.0-2.1) 95% confidence interval (CI)). The present case control study performed in a homogeneous Caucasoid population confirms the Japanese results that T SNP of Cx37 gene is involved in AMI phenotype, demonstrating the consistency of the association across past studies and across different populations. The differences between patients and controls are significant but relatively small with an odd ratio risk of 1.5. However, as AMI is a multifactorial disease, any single mutation will only provide a small or modest contribution to risk, also depending on interaction with other genes and/or a particular environment.

Gap junctional communication in tissue inflammation and repair

Local injury induces a complex orchestrated response to stimulate healing of injured tissues, cellular regeneration and phagocytosis. Practically, inflammation is defined as a defense process whereby fluid and white blood cells accumulate at a site of injury. The balance of cytokines, chemokines, and growth factors is likely to play a key role in regulating important cell functions such as migration, proliferation, and matrix synthesis during the process of inflammation. Hence, the initiation, maintenance, and resolution of innate responses depend upon cellular communication. A process similar to tissue repair and subsequent scarring is found in a variety of fibrotic diseases. This may occur in a single organ such as liver, kidneys, pancreas, lung, skin, and heart, but fibrosis may also have a more generalized distribution such as in atherosclerosis. The purpose of this review is to summarize recent advances on the contribution of gap junction-mediated intercellular communication in the modulation of the inflammatory response and tissue repair.

Increased connexin43 expression in human saphenous veins in culture is associated with intimal hyperplasia

OBJECTIVE

Intimal hyperplasia is a vascular remodelling process that occurs after a vascular injury. The mechanisms involved in intimal hyperplasia are proliferation, dedifferentiation, and migration of medial smooth muscle cells towards the subintimal space. We postulated that gap junctions, which coordinate physiologic processes such as cell growth and differentiation, might participate in the development of intimal hyperplasia. Connexin43 (Cx43) expression levels may be altered in intimal hyperplasia, and we therefore evaluated the regulated expression of Cx43 in human saphenous veins in culture in the presence or not of fluvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity.

METHODS

Segments of harvested human saphenous veins, obtained at the time of bypass graft, were opened longitudinally with the luminal surface uppermost and maintained in culture for 14 days. Vein fragments were then processed for histologic examination, neointimal thickness measurements, immunocytochemistry, RNA, and proteins analysis.

RESULTS

Of the four connexins (Cx37, 40, 43, and 45), we focused on Cx43 and Cx40, which we found by real-time polymerase chain reaction to be expressed in the saphenous vein because they are the predominant connexins expressed by smooth muscle cells and endothelial cells. After 14 days of culture, histomorphometric analysis showed a significant increase in the intimal thickness as observed during the process of intimal hyperplasia. A time-course analysis revealed a progressive upregulation of Cx43 to reach a maximal increase of sixfold to eightfold at both transcript and protein levels after 14 days in culture. In contrast, the expression of Cx40, abundantly expressed in the endothelial cells, was not altered. Immunofluorescence showed a large increase in Cx43 within smooth muscle cell membranes of the media layer. The development of intimal hyperplasia in vitro was decreased in presence of fluvastatin and was associated with reduced Cx43 expression.

CONCLUSIONS

These data show that Cx43 is increased in vitro during the process of intimal hyperplasia and that fluvastatin could prevent this induction, supporting a critical role for Cx43-mediated gap-junctional communication in the human vein during the development of intimal hyperplasia.

CLINICAL RELEVANCE

Stenosis due to intimal hyperplasia is the most common cause of failure of venous bypass grafts. To better understand the development of intimal hyperplasia, we used an ex vivo organ culture model to study saphenous veins harvested from patients undergoing a lower limb bypass surgery. In this model, the morphologic and functional integrity of the vessel wall is maintained and significant intimal hyperplasia development occurs after 14 days in culture. We have postulated that gap junctions, which coordinate physiologic processes such as cell growth and differentiation, may participate in the development of intimal hyperplasia. Indeed, intimal hyperplasia consists of proliferation and migration of smooth muscle cells into the subendothelial space. Intercellular communication is responsible for the direct transfer of ions and small molecules from one cell to the other through gap-junction channels found at cell-cell appositions. No study to date has evaluated whether gap junctional communication is involved in the process of intimal hyperplasia in humans. This assertion was investigated by using the aforementioned organ culture model of intimal hyperplasia in human saphenous veins, and our data support a critical role for Cx43-mediated gap junctional communication in human vein during the development of intimal hyperplasia.

Chlamydia pneumoniae (C. pneumoniae) infection upregulates atherosclerosis-related gene expression in human umbilical vein endothelial cells (HUVECs)

There is accumulating evidence that supports a role of infection in atherosclerosis, with possible mechanism by injuring to the endothelium and inducing an autoimmune response to heat shock proteins (HSPs). In this study, a cDNA array, containing 588 human cardiovascular genes, was utilized to analyze the gene expression profile of Chlamydia pneumoniae (C. pneumoniae) infected human umbilical vein endothelial cells (HUVECs). After 48h of C. pneumoniae infection, the HUVECs were harvested and subjected to immunofluorescent staining, electron microscopy, cDNA array hybridization, RT-PCR, and immunoblotting. This study found a panel of human host genes that were upregulated by C. pneumoniae. The majority of these genes were related to complex lipid metabolism, adhesion receptors, hormones, hormone receptors, and a metalloproteinase that may contribute to atherosclerosis in vivo. Representatives of upregulated gene products, i.e., heat shock protein 60 (HSP60), macrophage scavenger receptor, cytochrome P450, and VEGF165R were immunofluorescently detected in HUVECs, with their greater expression induced by C. pneumoniae infection. These findings supported the opinion that C. pneumoniae might contribute to atherosclerotic development in vivo.

Nitric oxide specifically reduces the permeability of Cx37-containing gap junctions to small molecules

Gap junction intercellular communication (GJIC) plays a significant role in the vascular system. Regulation of GJIC is a dynamic process, with alterations in connexin (Cx) protein expression and post-translational modification as contributing mechanisms. We hypothesized that the endothelial autacoid nitric oxide (NO) would reduce dye coupling in human umbilical vein endothelial cells (HUVECs). In our subsequent experiments, we sought to isolate the specific Cx isoform(s) targeted by NO or NO-activated signaling pathways. Since HUVEC cells variably express three Cx (Cx37, Cx40, and Cx43), this latter aim required the use of transfected HeLa cells (HeLaCx37, HeLaCx43), which do not express Cx proteins in their wild type form. Dye coupling was measured by injecting fluorescent dye (e.g., Alexa Fluor 488) into a single cell and determining the number of stained adjacent cells. Application of the NO donor SNAP (2 microM, 20 min) reduced dye coupling in HUVEC by 30%. In HeLa cells, SNAP did not reduce dye transfer of cells expressing Cx43, but decreased the dye transfer from Cx37-expressing cells to Cx43-expressing cells by 76%. The effect of SNAP on dye coupling was not mediated via cGMP. In contrast to its effect on dye coupling, SNAP had no effect on electrical coupling, measured by a double patch clamp in whole cell mode. Our results demonstrate that NO inhibits the intercellular transfer of small molecules by a specific influence on Cx37, suggesting a potential role of NO in controlling certain aspects of vascular GJIC.

Shear stress-induced upregulation of connexin 43 expression in endothelial cells on upstream surfaces of rat cardiac valves

Endothelial expression of the gap junction proteins, connexin (Cx) 37, Cx40, and Cx43, varies within the vascular network. While previous studies suggest that shear stress may upregulate Cx43, it is not well understood if shear stress affects the expression of all endothelial connexins and to what extent. Endothelial cells on the upstream and downstream surfaces of cardiac valves are subjected to considerably different intensities of shear stress. We therefore reasoned that we could determine the extent hemodynamic forces affect the expression of Cx37, Cx40, and Cx43 by comparing their immunohistochemical distribution on the upstream and downstream surfaces of rat cardiac valves. We found 70- to 200-fold greater expression of Cx43 in the endothelial cells on the upstream than on the downstream surfaces. However, Cx37 was expressed almost equally in the endothelial cells on upstream and downstream surfaces, and Cx40, a major connexin in most vascular endothelial cells, was not detected on either surface. In addition to the heterogeneity in Cx43 expression, endothelial cells on the upstream surface were 35% to 65% smaller than those on the corresponding downstream surface. These results suggest that shear stress may affect endothelial cell size and Cx43 expression but not Cx37 expression.

Distinct endothelial phenotypes evoked by arterial waveforms derived from atherosclerosis-susceptible and -resistant regions of human vasculature

Atherosclerotic lesion localization to regions of disturbed flow within certain arterial geometries, in humans and experimental animals, suggests an important role for local hemodynamic forces in atherogenesis. To explore how endothelial cells (EC) acquire functional/dysfunctional phenotypes in response to vascular region-specific flow patterns, we have used an in vitro dynamic flow system to accurately reproduce arterial shear stress waveforms on cultured human EC and have examined the effects on EC gene expression by using a high-throughput transcriptional profiling approach. The flow patterns in the carotid artery bifurcations of several normal human subjects were characterized by using 3D flow analysis based on actual vascular geometries and blood flow profiles. Two prototypic arterial waveforms, "athero-prone" and "athero-protective," were defined as representative of the wall shear stresses in two distinct regions of the carotid artery (carotid sinus and distal internal carotid artery) that are typically "susceptible" or "resistant," respectively, to atherosclerotic lesion development. These two waveforms were applied to cultured EC, and cDNA microarrays were used to analyze the differential patterns of EC gene expression. In addition, the differential effects of athero-prone vs. athero-protective waveforms were further characterized on several parameters of EC structure and function, including actin cytoskeletal organization, expression and localization of junctional proteins, activation of the NF-kappaB transcriptional pathway, and expression of proinflammatory cytokines and adhesion molecules. These global gene expression patterns and functional data reveal a distinct phenotypic modulation in response to the wall shear stresses present in atherosclerosis-susceptible vs. atherosclerosis-resistant human arterial geometries.

Adenoviral delivery of human connexin37 induces endothelial cell death through apoptosis

Gap junction channels formed of connexins directly link the cytoplasm of adjacent cells and have been implicated in intercellular signaling that may regulate the functions of vascular cells. To facilitate connexin manipulation and analysis of their roles in adult endothelial cells, we developed adenoviruses containing the vascular connexins (Cx37, Cx40, and Cx43). We infected cultured human umbilical vein endothelial cells with control or connexin adenoviruses. Connexin expression was verified by immunoblotting and immunofluorescence. Infection with the Cx37 adenovirus (but not control or other connexin adenoviruses) led to a dose-dependent death of the endothelial cells that was partially antagonized by the gap junction blocker alpha-glycyrrhetinic acid and altered the intercellular transfer of Lucifer yellow and neurobiotin. Cell morphology, Annexin V and TUNEL staining, and caspase 3 assays all implicated apoptosis in the cell death. These data suggest that connexin-specific alterations of intercellular communication may modulate endothelial cell growth and death.

Gap junctions and connexin-mediated communication in the immune system

Gap junctions and connexins are present in the immune system. In haematopoiesis, connexin 43, the most widely distributed gap junction protein, appears to be a key player in the development of progenitor cells and their communication with stromal cells. Connexin 43 is expressed by macrophages, neutrophils and mast cells. Lymphocytes also express connexin 43, and inhibition of gap junction channels in these cells by using highly specific connexin mimetic reagents has profound effects on immunoglobulin secretion and synthesis of cytokines. Lymphocytes and leukocytes also communicate directly in vitro with endothelial cells via gap junctions. Connexins are implicated in inflammatory reactions in a range of tissues. Their involvement in atherosclerotic plaque formation in the vascular system is also a current growth point in research, and could lead to the development of therapeutic interventions.

Upregulation of connexin43 gap junctions between neointimal smooth muscle cells

Increased expression of connexin43 gap junctions in smooth muscle cells (SMC) is implicated in the response to primary arterial injury and in the early stages of human coronary atherosclerosis, but the relevance of these findings to restenosis is unknown. Here we investigated the expression of connexin43 gap junctions in restenotic aortas of cholesterol-fed double injured rabbits. Immunofluorescence confocal microscopy was used to evaluate temporal and spatial expression patterns and to characterize the major expressing cell type. Parallel studies were conducted by electron microscopy, in situ hybridization and Northern blot analysis. Connexin43 gap junctions- and connexin43 mRNA-expressing cells were abundant in the media of non-injured control aorta. Following primary injury and 6 weeks cholesterol diet, connexin43 gap junctions were found distributed throughout the primary intimal layer; although medial expression was reduced, the overall mRNA expression level remained similar to that of non-injured controls. After secondary injury, no major change in distribution pattern of connexin43 gap junctions occurred up to day 10, when marked neointimal labeling was observed. This overall pattern persisted, though with some diminution, at later stages. On the mRNA level total connexin43 mRNA expression declined to about 40% of control values within 4 days after secondary injury (P < 0.05), but subsequently increased four-fold, attaining levels double that of non-injured controls in the 10-day group (P < 0.005 versus control and 4 days). At later stages mRNA expression levels returned to values similar to those of non-injured controls. At all stages, connexin43 gap junctions were localized to the SMC, not to macrophages. We conclude that the enhanced gap junction formation may contribute to the coordination of the response of SMC after secondary injury, particularly in the early phase of restenosis.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Role of connexin37 and connexin40 in vascular development

Mice lacking both connexin37 (Cx37) and connexin40 (Cx40), gap junction proteins expressed in vascular endothelium, die perinatally with pronounced vascular abnormalities. Early vasculogenesis proceeds normally, but by E18.5 Cx37(-/-)Cx40(-/-) animals display vessel dilatation and congestion as well as localized hemorrhages in skin, testis, intestines, and lungs. Abnormal vascular channels are present in the testis, often forming cavernous hemangioma-like defects. Unusually large, distended vessels are also present in the submucosa and lamina propria of the intestine. Ablation of Cx40 has a greater effect on endothelial dye-transfer than ablation of Cx37, and the effect of Cx40 ablation is age-dependent. Only in embryonic aortas lacking both Cx37 and Cx40 is there a complete loss of endothelial coupling. Surprisingly, elimination of Cx40 results in a large drop in aortic endothelial Cx37 on western blots, and deletion of Cx37 also reduces endothelial Cx40 levels. In contrast, in the medial layer, both Cx37 and Cx43 increase when Cx40 is ablated. These studies indicate that Cx37 and Cx40 are collectively critical for endothelial communication and provide evidence of an important role for gap junctions in vascular development. In addition, Cx37 and Cx40 appear to be mutually dependent on each other for normal expression in vascular endothelium.