Loss of connexin40 is associated with decreased endothelium-dependent relaxations and eNOS levels in the mouse aorta

The Potential Role of Connexins in the Pathogenesis of Atherosclerosis

Connexins (Cx) are members of a protein family which enable extracellular and intercellular communication through hemichannels and gap junctions (GJ), respectively. Cx take part in transporting important cell-cell messengers such as 3',5'-cyclic adenosine monophosphate (cAMP), adenosine triphosphate (ATP), and inositol 1,4,5-trisphosphate (IP3), among others. Therefore, they play a significant role in regulating cell homeostasis, proliferation, and differentiation. Alterations in Cx distribution, degradation, and post-translational modifications have been correlated with cancers, as well as cardiovascular and neurological diseases. Depending on the isoform, Cx have been shown either to promote or suppress the development of atherosclerosis, a progressive inflammatory disease affecting large and medium-sized arteries. Cx might contribute to the progression of the disease by enhancing endothelial dysfunction, monocyte recruitment, vascular smooth muscle cell (VSMC) activation, or by inhibiting VSMC autophagy. Inhibition or modulation of the expression of specific isoforms could suppress atherosclerotic plaque formation and diminish pro-inflammatory conditions. A better understanding of the complexity of atherosclerosis pathophysiology linked with Cx could result in developing novel therapeutic strategies. This review aims to present the role of Cx in the pathogenesis of atherosclerosis and discusses whether they can become novel therapeutic targets.

Endothelial Connexins in Developmental and Pathological Angiogenesis

Connexins (Cxs) constitute a large family of transmembrane proteins that form gap junction channels, which enable the direct transfer of small signaling molecules from cell to cell. In blood vessels, Cx channels allow the endothelial cells (ECs) to respond to external and internal cues as a whole and, thus, contribute to the maintenance of vascular homeostasis. While the role of Cxs has been extensively studied in large arteries, a growing body of evidence suggests that they also play a role in the formation of microvascular networks. Since the formation of new blood vessels requires the coordinated response of ECs to external stimuli, endothelial Cxs may play an important role there. Recent studies in developmental and pathologic models reveal that EC Cxs regulate physiological and pathological angiogenesis through canonical and noncanonical functions, making these proteins potential therapeutic targets for the development of new strategies aimed at a better control of angiogenesis.

Targeting Endothelial Connexin37 Reduces Angiogenesis and Decreases Tumor Growth

Connexin37 (Cx37) and Cx40 form intercellular channels between endothelial cells (EC), which contribute to the regulation of the functions of vessels. We previously documented the participation of both Cx in developmental angiogenesis and have further shown that loss of Cx40 decreases the growth of different tumors. Here, we report that loss of Cx37 reduces (1) the in vitro proliferation of primary human EC; (2) the vascularization of subcutaneously implanted matrigel plugs in Cx37−/− mice or in WT using matrigel plugs supplemented with a peptide targeting Cx37 channels; (3) tumor angiogenesis; and (4) the growth of TC-1 and B16 tumors, resulting in a longer mice survival. We further document that Cx37 and Cx40 function in a collaborative manner to promote tumor growth, inasmuch as the injection of a peptide targeting Cx40 into Cx37−/− mice decreased the growth of TC-1 tumors to a larger extent than after loss of Cx37. This loss did not alter vessel perfusion, mural cells coverage and tumor hypoxia compared to tumors grown in WT mice. The data show that Cx37 is relevant for the control of EC proliferation and growth in different tumor models, suggesting that it may be a target, alone or in combination with Cx40, in the development of anti-tumoral treatments.

Endothelium-Derived Hyperpolarizing Factor and Myoendothelial Coupling: The in vivo Perspective

The endothelium controls vascular tone adopting blood flow to tissue needs. It releases chemical mediators [e.g., nitric oxide (NO), prostaglandins (PG)] and exerts appreciable dilation through smooth muscle hyperpolarization, thus termed endothelium-dependent hyperpolarization (EDH). Initially, EDH was attributed to release of a factor, but later it was suggested that smooth muscle hyperpolarization might be derived from radial spread of an initial endothelial hyperpolarization through heterocellular channels coupling these vascular cells. The channels are indeed present and formed by connexins that enrich in gap junctions (GJ). In vitro data suggest that myoendothelial coupling underlies EDH-type dilations as evidenced by blocking experiments as well as simultaneous, merely identical membrane potential changes in endothelial and smooth muscle cells (SMCs), which is indicative of coupling through ohmic resistors. However, connexin-deficient animals do not display any attenuation of EDH-type dilations in vivo, and endothelial and SMCs exhibit distinct and barely superimposable membrane potential changes exerted by different means in vivo. Even if studied in the exact same artery EDH-type dilation exhibits distinct features in vitro and in vivo: in isometrically mounted vessels, it is rather weak and depends on myoendothelial coupling through connexin40 (Cx40), whereas in vivo as well as in vitro under isobaric conditions it is powerful and independent of myoendothelial coupling through Cx40. It is concluded that EDH-type dilations are distinct and a significant dependence on myoendothelial coupling in vitro does not reflect the situation under physiologic conditions in vivo. Myoendothelial coupling may act as a backup mechanism that is uncovered in the absence of the powerful EDH-type response and possibly reflects a situation in a pathophysiologic environment.

The role of connexin proteins and their channels in radiation-induced atherosclerosis

Radiotherapy is an effective treatment for breast cancer and other thoracic tumors. However, while high-energy radiotherapy treatment successfully kills cancer cells, radiation exposure of the heart and large arteries cannot always be avoided, resulting in secondary cardiovascular disease in cancer survivors. Radiation-induced changes in the cardiac vasculature may thereby lead to coronary artery atherosclerosis, which is a major cardiovascular complication nowadays in thoracic radiotherapy-treated patients. The underlying biological and molecular mechanisms of radiation-induced atherosclerosis are complex and still not fully understood, resulting in potentially improper radiation protection. Ionizing radiation (IR) exposure may damage the vascular endothelium by inducing DNA damage, oxidative stress, premature cellular senescence, cell death and inflammation, which act to promote the atherosclerotic process. Intercellular communication mediated by connexin (Cx)-based gap junctions and hemichannels may modulate IR-induced responses and thereby the atherosclerotic process. However, the role of endothelial Cxs and their channels in atherosclerotic development after IR exposure is still poorly defined. A better understanding of the underlying biological pathways involved in secondary cardiovascular toxicity after radiotherapy would facilitate the development of effective strategies that prevent or mitigate these adverse effects. Here, we review the possible roles of intercellular Cx driven signaling and communication in radiation-induced atherosclerosis.

Mouse models of atherosclerosis and their suitability for the study of myocardial infarction

Atherosclerotic plaques impair vascular function and can lead to arterial obstruction and tissue ischaemia. Rupture of an atherosclerotic plaque within a coronary artery can result in an acute myocardial infarction, which is responsible for significant morbidity and mortality worldwide. Prompt reperfusion can salvage some of the ischaemic territory, but ischaemia and reperfusion (IR) still causes substantial injury and is, therefore, a therapeutic target for further infarct limitation. Numerous cardioprotective strategies have been identified that can limit IR injury in animal models, but none have yet been translated effectively to patients. This disconnect prompts an urgent re-examination of the experimental models used to study IR. Since coronary atherosclerosis is the most prevalent morbidity in this patient population, and impairs coronary vessel function, it is potentially a major confounder in cardioprotective studies. Surprisingly, most studies suggest that atherosclerosis does not have a major impact on cardioprotection in mouse models. However, a major limitation of atherosclerotic animal models is that the plaques usually manifest in the aorta and proximal great vessels, and rarely in the coronary vessels. In this review, we examine the commonly used mouse models of atherosclerosis and their effect on coronary artery function and infarct size. We conclude that none of the commonly used strains of mice are ideal for this purpose; however, more recently developed mouse models of atherosclerosis fulfil the requirement for coronary artery lesions, plaque rupture and lipoprotein patterns resembling the human profile, and may enable the identification of therapeutic interventions more applicable in the clinical setting.

Modulating the Bioactivity of Nitric Oxide as a Therapeutic Strategy in Cardiac Surgery

Cardiac surgery, including cardioplegic arrest and extracorporeal circulation, causes endothelial dysfunction, which can lead to no-reflow phenomenon and reduction of myocardial pump function. Nitric oxide (NO) deficiency is involved in this pathologic process, thereby providing a fundamental basis for the use of NO replacement therapy. Presently used drugs and additives to cardioplegic and heart preservation solutions are not able to reliably protect endothelial cells and cardiomyocytes from ischemia-reperfusion injury. This review discusses promising NO-releasing compounds of various chemical classes for cardioplegia and reperfusion, which effectively maintain NO homeostasis under experimental conditions, and presents the mechanisms of their action on the cardiovascular system. Incomplete preclinical studies and a lack of toxicity assessment, however, hinder translation of these drug candidates into the clinic. Perspectives for modulation of endothelial function using NO-mediated mechanisms are discussed. They are based on the cardioprotective potential of targeting vascular gap junctions and endothelial ion channels, intracoronary administration of progenitor cells, and endothelial-specific microRNAs. Some of these strategies may provide important therapeutic benefits for human cardiovascular interventions.

Connexins: Key Players in the Control of Vascular Plasticity and Function

Of the 21 members of the connexin family, 4 (Cx37, Cx40, Cx43, and Cx45) are expressed in the endothelium and/or smooth muscle of intact blood vessels to a variable and dynamically regulated degree. Full-length connexins oligomerize and form channel structures connecting the cytosol of adjacent cells (gap junctions) or the cytosol with the extracellular space (hemichannels). The different connexins vary mainly with regard to length and sequence of their cytosolic COOH-terminal tails. These COOH-terminal parts, which in the case of Cx43 are also translated as independent short isoforms, are involved in various cellular signaling cascades and regulate cell functions. This review focuses on channel-dependent and -independent effects of connexins in vascular cells. Channels play an essential role in coordinating and synchronizing endothelial and smooth muscle activity and in their interplay, in the control of vasomotor actions of blood vessels including endothelial cell reactivity to agonist stimulation, nitric oxide-dependent dilation, and endothelial-derived hyperpolarizing factor-type responses. Further channel-dependent and -independent roles of connexins in blood vessel function range from basic processes of vascular remodeling and angiogenesis to vascular permeability and interactions with leukocytes with the vessel wall. Together, these connexin functions constitute an often underestimated basis for the enormous plasticity of vascular morphology and function enabling the required dynamic adaptation of the vascular system to varying tissue demands.

Role of ROS/RNS in Preeclampsia: Are Connexins the Missing Piece?

Preeclampsia is a pregnancy complication that appears after 20 weeks of gestation and is characterized by hypertension and proteinuria, affecting both mother and offspring. The cellular and molecular mechanisms that cause the development of preeclampsia are poorly understood. An important feature of preeclampsia is an increase in oxygen and nitrogen derived free radicals (reactive oxygen species/reactive nitrogen species (ROS/RNS), which seem to be central players setting the development and progression of preeclampsia. Cell-to-cell communication may be disrupted as well. Connexins (Cxs), a family of transmembrane proteins that form hemichannels and gap junction channels (GJCs), are essential in paracrine and autocrine cell communication, allowing the movement of signaling molecules between cells as well as between the cytoplasm and the extracellular media. GJCs and hemichannels are fundamental for communication between endothelial and smooth muscle cells and, therefore, in the control of vascular contraction and relaxation. In systemic vasculature, the activity of GJCs and hemichannels is modulated by ROS and RNS. Cxs participate in the development of the placenta and are expressed in placental vasculature. However, it is unknown whether Cxs are modulated by ROS/RNS in the placenta, or whether this potential modulation contributes to the pathogenesis of preeclampsia. Our review addresses the possible role of Cxs in preeclampsia, and the plausible modulation of Cxs-formed channels by ROS and RNS. We suggest these factors may contribute to the development of preeclampsia.

Targeting connexin37 alters angiogenesis and arteriovenous differentiation in the developing mouse retina

Connexin37 (Cx37) forms intercellular channels between endothelial cells (EC), and contributes to coordinate the motor tone of vessels. We investigated the contribution of this protein during physiological angiogenesis. We show that, compared to WT littermates, mice lacking Cx37 (Cx37-/- ) featured (i) a decreased extension of the superficial vascular plexus during the first 4 days after birth; (ii) an increased vascular density at the angiogenic front at P6, due to an increase in the proliferative rate of EC and in the sprouting of the venous compartment, as well as to a somewhat displaced position of tip cells; (iii) a decreased coverage of newly formed arteries and veins by mural cells; (iv) altered ERK-dependent endothelial cells proliferation through the EphB4 signaling pathway, which is involved in the specification of veins and arteries. In vitro studies documented that, in the absence of Cx37, human venous EC (HUVEC) released less platelet-derived growth factor (PDGF) and more Angiopoietin-2, two molecules involved in the recruitment of mural cells. Treatment of mice with DAPT, an inhibitor of the Notch pathway, decreased the expression of Cx37, and partially mimicked in WT retinas, the alterations observed in Cx37-/- mice. Thus, Cx37 contributes to (i) the early angiogenesis of retina, by interacting with the Notch pathway; (ii) the growth and maturation of neo-vessels, by modulating tip, stalk, and mural cells; (iii) the regulation of arteriovenous specification, thus, representing a novel target for treatments of retina diseases.

Single and fractionated ionizing radiation induce alterations in endothelial connexin expression and channel function

Radiotherapy is an effective treatment for most tumor types. However, emerging evidence indicates an increased risk for atherosclerosis after ionizing radiation exposure, initiated by endothelial cell dysfunction. Interestingly, endothelial cells express connexin (Cx) proteins that are reported to exert proatherogenic as well as atheroprotective effects. Furthermore, Cxs form channels, gap junctions and hemichannels, that are involved in bystander signaling that leads to indirect radiation effects in non-exposed cells. We here aimed to investigate the consequences of endothelial cell irradiation on Cx expression and channel function. Telomerase immortalized human Coronary Artery/Microvascular Endothelial cells were exposed to single and fractionated X-rays. Several biological endpoints were investigated at different time points after exposure: Cx gene and protein expression, gap junctional dye coupling and hemichannel function. We demonstrate that single and fractionated irradiation induce upregulation of proatherogenic Cx43 and downregulation of atheroprotective Cx40 gene and protein levels in a dose-dependent manner. Single and fractionated irradiation furthermore increased gap junctional communication and induced hemichannel opening. Our findings indicate alterations in Cx expression that are typically observed in endothelial cells covering atherosclerotic plaques. The observed radiation-induced increase in Cx channel function may promote bystander signaling thereby exacerbating endothelial cell damage and atherogenesis.

Connexin37-Dependent Mechanisms Selectively Contribute to Modulate Angiotensin II -Mediated Hypertension

Background Gap junction channels made of Connexin37 (Cx37) are expressed by aortic endothelial and smooth muscle cells of hypertensive mice, as well as by the renin-secreting cells of kidneys. Methods and Results To decipher whether Cx37 has any role in hypertension, angiotensin II (Ang II ) was infused in normotensive wild-type and Cx37-deficient mice (Cx37-/-). After 2 to 4 weeks, the resulting increase in blood pressure was lower in Cx37-/- than in wild-type mice, suggesting an alteration in the Ang II response. To investigate this possibility, mice were submitted to a 2-kidney, 1-clip procedure, a renin-dependent model of hypertension. Two weeks after this clipping, Cx37-/- mice were less hypertensive than wild-type mice and, 2 weeks later, their blood pressure had returned to control values, in spite of abnormally high plasma renin levels. In contrast, Cx37-/- and wild-type mice that received N-nitro-l-arginine-methyl-ester, a renin-independent model of hypertension, featured a similar and sustained increase in blood pressure. The data indicate that loss of Cx37 selectively altered the Ang II -dependent pathways. Consistent with this conclusion, aortas of Cx37-/- mice featured an increased basal expression of the Ang II type 2 receptors ( AT 2R), and increased transcripts levels of downstream signaling proteins, such as Cnksr1 and Ptpn6 ( SHP -1). Accordingly, the response of Cx37-/- mice aortas to an ex vivo Ang II exposure was altered, since phosphorylation levels of several proteins of the Ang II pathway ( MLC 2, ERK , and AKT ) remained unchanged. Conclusions These findings provide evidence that Cx37 selectively influences Ang II signaling, mostly via a modulation of the expression of the Ang II type 2 receptor.

Ablation of both Cx40 and Panx1 results in similar cardiovascular phenotypes exhibited in Cx40 knockout mice

Connexins (Cxs) and pannexins (Panxs) are highly regulated large-pore channel-forming proteins that participate in cellular communication via small molecular exchange with the extracellular microenvironment, or in the case of connexins, directly between cells. Given the putative functional overlap between single membrane-spanning connexin hemichannels and Panx channels, and cardiovascular system prevalence, we generated the first Cx40-/-Panx1-/- mouse with the anticipation that this genetic modification would lead to a severe cardiovascular phenotype. Mice null for both Cx40 and Panx1 produced litter sizes and adult growth progression similar to wild-type (WT), Cx40-/- and Panx1-/- mice. Akin to Cx40-/- mice, Cx40-/-Panx1-/- mice exhibited cardiac hypertrophy and elevated systolic, diastolic, and mean arterial blood pressure compared with WT and Panx1-/- mice; however assessment of left ventricular ejection fraction and fractional shortening revealed no evidence of cardiac dysfunction between groups. Furthermore, Cx40-/-, Panx1-/-, and Cx40-/-Panx1-/- mice demonstrated impaired endothelial-mediated vasodilation of aortic segments to increasing concentrations of methacholine (MCh) compared with WT, highlighting roles for both Cx40 and Panx1 in vascular endothelial cell (EC) function. Surprisingly, elevated kidney renin mRNA expression, plasma renin activity, and extraglomerular renin-producing cell populations found in Cx40-/- mice was further exaggerated in double knockout mice. Thus, while gestation and gross development were conserved in Cx40-/-Panx1-/- mice, they exhibit cardiac hypertrophy, hypertension, and impaired endothelial-mediated vasodilation that phenocopies Cx40-/- mice. Nevertheless, the augmented renin homeostasis observed in the double knockout mice suggests that both Cx40 and Panx1 may play an integrative role.

Connexins in the control of vasomotor function

Vascular endothelial cells, as well as smooth muscle cells, show heterogeneity with regard to their receptor expression and reactivity. For the vascular wall to act as a functional unit, the various cells' responses require integration. Such an integration is not only required for a homogeneous response of the vascular wall, but also for the vasomotor behaviour of consecutive segments of the microvascular arteriolar tree. As flow resistances of individual sections are connected in series, sections require synchronization and coordination to allow effective changes of conductivity and blood flow. A prerequisite for the local coordination of individual vascular cells and different sections of an arteriolar tree is intercellular communication. Connexins are involved in a dual manner in this coordination. (i) By forming gap junctions between cells, they allow an intercellular exchange of signalling molecules and electrical currents. In particular, the spread of electrical currents allows for coordination of cell responses over longer distances. (ii) Connexins are able to interact with other proteins to form signalling complexes. In this way, they can modulate and integrate individual cells' responses also in a channel-independent manner. This review outlines mechanisms allowing the vascular connexins to exert their coordinating function and to regulate the vasomotor reactions of blood vessels both locally, and in vascular networks. Wherever possible, we focus on the vasomotor behaviour of small vessels and arterioles which are the main vessels determining vascular resistance, blood pressure and local blood flow.

Enhanced Function of Induced Pluripotent Stem Cell-Derived Endothelial Cells Through ESM1 Signaling

The mortality rate for (cardio)‐vascular disease is one of the highest in the world, so a healthy functional endothelium is of outmost importance against vascular disease. In this study, human induced pluripotent stem (iPS) cells were reprogrammed from 1 ml blood of healthy donors and subsequently differentiated into endothelial cells (iPS‐ECs) with typical EC characteristics. This research combined iPS cell technologies and next‐generation sequencing to acquire an insight into the transcriptional regulation of iPS‐ECs. We identified endothelial cell‐specific molecule 1 (ESM1) as one of the highest expressed genes during EC differentiation, playing a key role in EC enrichment and function by regulating connexin 40 (CX40) and eNOS. Importantly, ESM1 enhanced the iPS‐ECs potential to improve angiogenesis and neovascularisation in in vivo models of angiogenesis and hind limb ischemia. These findings demonstrated for the first time that enriched functional ECs are derived through cell reprogramming and ESM1 signaling, opening the horizon for drug screening and cell‐based therapies for vascular diseases. Therefore, this study showcases a new approach for enriching and enhancing the function of induced pluripotent stem (iPS) cell‐derived ECs from a very small amount of blood through ESM1 signaling, which greatly enhances their functionality and increases their therapeutic potential. Stem Cells2019;37:226–239

Connexin 43 Plays a Role in Pulmonary Vascular Reactivity in Mice

Pulmonary arterial hypertension (PAH) is a chronic condition characterized by vascular remodeling and increased vaso-reactivity. PAH is more common in females than in males (~3:1). Connexin (Cx)43 has been shown to be involved in cellular communication within the pulmonary vasculature. Therefore, we investigated the role of Cx43 in pulmonary vascular reactivity using Cx43 heterozygous (Cx43+/−) mice and 37,43Gap27, which is a pharmacological inhibitor of Cx37 and Cx43. Contraction and relaxation responses were studied in intra-lobar pulmonary arteries (IPAs) derived from normoxic mice and hypoxic mice using wire myography. IPAs from male Cx43+/− mice displayed a small but significant increase in the contractile response to endothelin-1 (but not 5-hydroxytryptamine) under both normoxic and hypoxic conditions. There was no difference in the contractile response to endothelin-1 (ET-1) or 5-hydroxytryptamine (5-HT) in IPAs derived from female Cx43+/−mice compared to wildtype mice. Relaxation responses to methacholine (MCh) were attenuated in IPAs from male and female Cx43+/− mice or by pre-incubation of IPAs with 37,43Gap27. Nω-Nitro-L-arginine methyl ester (l-NAME) fully inhibited MCh-induced relaxation. In conclusion, Cx43 is involved in nitric oxide (NO)-induced pulmonary vascular relaxation and plays a gender-specific and agonist-specific role in pulmonary vascular contractility. Therefore, reduced Cx43 signaling may contribute to pulmonary vascular dysfunction.

Polymorphism of CONNEXIN37 gene is a risk factor for ischemic stroke in Han Chinese population

BACKGROUND

Stroke has a high fatality and disability rate, and is one of the main burdens to human health. It is thus very important to identify biomarkers for the development of effective approaches for the prevention and treatment of stroke. Connexin37 is an anti-inflammatory cytokine and is involved in chronic inflammation and atherosclerosis. Recent studies have found that CONNEXIN37 gene variations are associated with atherosclerosis diseases, such as coronary heart disease and stroke, but its association with stroke in distinct human populations remains to be determined. We report here the analysis of the association of the single nucleotide polymorphisms (SNPs) of CONNEXIN37 with ischemic stroke in Han Chinese population.

METHODS

Two SNPs of CONNEXIN37 gene were analyzed in 385 ischemic stroke patients and 362 hypertension control patients using ligase detection reaction (LDR) method.

RESULTS

Logistic regression analysis demonstrated that, AG and GG genotypes of SNP rs1764390 and CC genotype of rs1764391 of CONNEXIN37 were associated with an increased risk of ischemic stroke, and that G allele of rs1764390 is a risk factor for ischemic stroke. Further, we found that SNP rs1764390 and SNP rs1764391 in CONNEXIN37 were associated with ischemic stroke under additive/dominant model, and recessive/dominant model, respectively.

CONCLUSION

Our results indicate that CONNEXIN37 gene polymorphism is an ischemic stroke risk factor in Northern Han Chinese.

Role of Non-Myocyte Gap Junctions and Connexin Hemichannels in Cardiovascular Health and Disease: Novel Therapeutic Targets?

The heart is a complex organ composed of multiple cell types, including cardiomyocytes and different non-myocyte populations, all working closely together to determine the hearts properties and maintain normal cardiac function. Connexins are abundantly expressed proteins that form plasma membrane hemichannels and gap junctions between cells. Gap junctions are intracellular channels that allow for communication between cells, and in the heart they play a crucial role in cardiac conduction by coupling adjacent cardiomyocytes. Connexins are expressed in both cardiomyocytes and non-myocytes, including cardiac fibroblasts, endothelial cells, and macrophages. Non-myocytes are the largest population of cells in the heart, and therefore it is important to consider what roles connexins, hemichannels, and gap junctions play in these cell types. The aim of this review is to provide insight into connexin-based signalling in non-myocytes during health and disease, and highlight how targeting these proteins could lead to the development of novel therapies. We conclude that connexins in non-myocytes contribute to arrhythmias and adverse ventricular remodelling following myocardial infarction, and are associated with the initiation and development of atherosclerosis. Therefore, therapeutic interventions targeting these connexins represent an exciting new research avenue with great potential.

Communication Is Key: Mechanisms of Intercellular Signaling in Vasodilation

Thirty years ago, Robert F. Furchgott concluded that nitric oxide, a compound traditionally known to be a toxic component of fuel exhaust, is in fact released from the endothelium, and in a paracrine fashion, induces relaxation of underlying vascular smooth muscle resulting in vasodilation. This discovery has helped pave the way for a more thorough understanding of vascular intercellular and intracellular communication that supports the process of regulating regional perfusion to match the local tissue oxygen demand. Vasoregulation is controlled not only by endothelial release of a diverse class of vasoactive compounds such as nitric oxide, arachidonic acid metabolites, and reactive oxygen species, but also by physical forces on the vascular wall and through electrotonic conduction through gap junctions. Although the endothelium is a critical source of vasoactive compounds, paracrine mediators can also be released from surrounding parenchyma such as perivascular fat, myocardium, and cells in the arterial adventitia to exert either local or remote vasomotor effects. The focus of this review will highlight the various means by which intercellular communication contributes to mechanisms of vasodilation. Paracrine signaling and parenchymal influences will be reviewed as well as regional vessel communication through gap junctions, connexons, and myoendothelial feedback. More recent modes of communication such as vesicular and microRNA signaling will also be discussed.

The Role of Gap Junction-Mediated Endothelial Cell-Cell Interaction in the Crosstalk between Inflammation and Blood Coagulation

Endothelial cells (ECs) play a pivotal role in the crosstalk between blood coagulation and inflammation. Endothelial cellular dysfunction underlies the development of vascular inflammatory diseases. Recent studies have revealed that aberrant gap junctions (GJs) and connexin (Cx) hemichannels participate in the progression of cardiovascular diseases such as cardiac infarction, hypertension and atherosclerosis. ECs can communicate with adjacent ECs, vascular smooth muscle cells, leukocytes and platelets via GJs and Cx channels. ECs dynamically regulate the expression of numerous Cxs, as well as GJ functionality, in the context of inflammation. Alterations to either result in various side effects across a wide range of vascular functions. Here, we review the roles of endothelial GJs and Cx channels in vascular inflammation, blood coagulation and leukocyte adhesion. In addition, we discuss the relevant molecular mechanisms that endothelial GJs and Cx channels regulate, both the endothelial functions and mechanical properties of ECs. A better understanding of these processes promises the possibility of pharmacological treatments for vascular pathogenesis.

Mitochondrial connexin40 regulates mitochondrial calcium uptake in coronary endothelial cells

Connexins (Cxs) are a group of integral membrane proteins that can form gap junctions between adjacent cells. Recently, it was reported that Cx43 is expressed not only in the plasma membrane but also in the inner mitochondrial membrane and that it regulates mitochondrial functions. Cx40 is predominantly expressed in vascular endothelial cells (ECs) and plays an important role in the electrical propagation between ECs and endothelial/smooth muscle cells. However, it is unknown whether Cx40 is expressed in the mitochondria and what the role of mitochondrial Cx40 is in endothelial functions. We observed in coronary ECs that Cx40 protein was expressed in the mitochondria, as determined by Western blot and immunofluorescence studies. We found that mouse coronary ECs (MCECs) isolated from Cx40 knockout (Cx40 KO) mice exhibited significantly lower resting mitochondrial calcium concentration ([Ca2+]mito) than MCECs from wild-type (WT) mice. After increase in cytosolic Ca2+ concentration ([Ca2+]cyto) with cyclopiazonic acid, calcium uptake into the mitochondria was significantly attenuated in MCECs from Cx40 KO mice compared with WT MCECs. There was no difference in resting [Ca2+]cyto and store-operated calcium entry in MCECs from WT and Cx40 KO mice. We also detected a significant decrease in the concentration of mitochondrial reactive oxygen species (ROS) in Cx40 KO MCECs. Cx40 overexpression in ECs significantly increased resting [Ca2+]mito level and calcium uptake by mitochondria in response to increased [Ca2+]cyto and augmented mitochondrial ROS production. These data suggest that mitochondrial Cx40 contributes to the regulation of mitochondrial calcium homeostasis.

Omega-3 fatty acids reduce lipopolysaccharide-induced abnormalities in expression of connexin-40 in aorta of hereditary hypertriglyceridemic rats

Omega-3 fatty acids (omega3FA) are known to reduce hypertriglyceridemia- and inflammation-induced vascular wall diseases. However, mechanisms of their effects are not completely clear. We examined, whether 10-day omega3FA diet can reduce bacterial lipopolysaccharide-induced changes in expression of gap junction protein connexin40 (Cx40) in the aorta of hereditary hypertriglyceridemic (hHTG) rats. After administration of a single dose of lipopolysaccharide (LPS, 1 mg/kg, i.p.) to adult hHTG rats, animals were fed with omega3FA diet (30 mg/kg/day) for 10 days. LPS decreased Cx40 expression that was associated with reduced acetylcholine-induced relaxation of aorta. Omega3FA administration to LPS rats had partial anti-inflammatory effects, associated with increased Cx40 expression and improved endothelium dependent relaxation of the aorta. Our results suggest that 10-day omega3FA diet could protect endothelium-dependent relaxation of the aorta of hHTG rats against LPS-induced damage through the modulation of endothelial Cx40 expression.

Induction of Genes Expressed in Endothelial Cells of the Corpus Callosum in the Chronic Cerebral Hypoperfusion Rat Model

BACKGROUND

Cerebrovascular white matter lesions (WMLs) are associated with cognitive impairment in patients with subcortical vascular dementia. We performed a comprehensive gene expression analysis to elucidate genes associated with WML development in a chronic cerebral hypoperfusion rat model.

METHODS

Brains of rats with bilateral carotid ligation (2VO, n = 10) and sham-operated rats (n = 5-10/group) were removed on days 1, 7, or 28 after surgery. Total RNA isolated from the corpus callosum was evaluated by microarray analysis and quantitative reverse transcription-polymerase chain reaction.

RESULTS

On days 7 and 28, WMLs exhibited histologic changes. On day 7, 16 genes were differentially expressed between groups. mRNA levels of Ptprb, Kcnj8, Crispld2, Bcl6b, and Gja5 were differentially expressed in 2VO rats on day 7, but then returned to normal, whereas mRNA levels of Vwf and Trappc6a were upregulated after day 7. Immunohistochemistry showed that GJA5 and vWF were detected in endothelial cells, KCNJ8 in endothelial cells and astrocytes, CRISPLD2 in neurons and astrocytes, and TRAPPC6A in neurons.

CONCLUSION

Our findings indicate novel genes that may be associated with WML development in the chronic cerebral hypoperfusion rat model, and suggest an important role of neurovascular dysfunction in the pathophysiology.

Two functionally distinct pools of eNOS in endothelium are facilitated by myoendothelial junction lipid composition

In resistance arteries, endothelial cells (EC) make contact with smooth muscle cells (SMC), forming myoendothelial junctions (MEJ). Endothelial nitric oxide synthase (eNOS) is present in the luminal side of the EC (apical EC) and the basal side of the EC (MEJ). To test if these eNOS pools acted in sync or separately, we co-cultured ECs and SMCs, then stimulated SMCs with phenylephrine (PE). Adrenergic activation causes inositol [1,4,5] triphosphate (IP3) to move from SMC to EC through gap junctions at the MEJ. PE increases MEJ eNOS phosphorylation (eNOS-P) at S1177, but not in EC. Conversely, we used bradykinin (BK) to increase EC calcium; this increased EC eNOS-P but did not affect MEJ eNOS-P. Inhibiting gap junctions abrogated the MEJ eNOS-P after PE, but had no effect on BK eNOS-P. Differential lipid composition between apical EC and MEJ may account for the compartmentalized eNOS-P response. Indeed, DAG and phosphatidylserine are both enriched in MEJ. These lipids are cofactors for PKC activity, which was significantly increased at the MEJ after PE. Because PKC activity also relies on endoplasmic reticulum (ER) calcium release, we used thapsigargin and xestospongin C, BAPTA, and PKC inhibitors, which caused significant decreases in MEJ eNOS-P after PE. Functionally, BK inhibited leukocyte adhesion and PE caused an increase in SMC cGMP. We hypothesize that local lipid composition of the MEJ primes PKC and eNOS-P for stimulation by PE, allowing for compartmentalized function of eNOS in the blood vessel wall.

Targeting endothelial connexin40 inhibits tumor growth by reducing angiogenesis and improving vessel perfusion

Endothelial connexin40 (Cx40) contributes to regulate the structure and function of vessels. We have examined whether the protein also modulates the altered growth of vessels in tumor models established in control mice (WT), mice lacking Cx40 (Cx40-/-), and mice expressing the protein solely in endothelial cells (Tie2-Cx40). Tumoral angiogenesis and growth were reduced, whereas vessel perfusion, smooth muscle cell (SMC) coverage and animal survival were increased in Cx40-/- but not Tie2-Cx40 mice, revealing a critical involvement of endothelial Cx40 in transformed tissues independently of the hypertensive status of Cx40-/- mice. As a result, Cx40-/- mice bearing tumors survived significantly longer than corresponding controls, including after a cytotoxic administration. Comparable observations were made in WT mice injected with a peptide targeting Cx40, supporting the Cx40 involvement. This involvement was further confirmed in the absence of Cx40 or by peptide-inhibition of this connexin in aorta-sprouting, matrigel plug and SMC migration assays, and associated with a decreased expression of the phosphorylated form of endothelial nitric oxide synthase. The data identify Cx40 as a potential novel target in cancer treatment.

Store-operated Ca2+ Entry Mediated by Orai1 and TRPC1 Participates to Insulin Secretion in Rat β-Cells

Store-operated Ca(2+) channels (SOCs) are voltage-independent Ca(2+) channels activated upon depletion of the endoplasmic reticulum Ca(2+) stores. Early studies suggest the contribution of such channels to Ca(2+) homeostasis in insulin-secreting pancreatic β-cells. However, their composition and contribution to glucose-stimulated insulin secretion (GSIS) remains unclear. In this study, endoplasmic reticulum Ca(2+) depletion triggered by acetylcholine (ACh) or thapsigargin stimulated the formation of a ternary complex composed of Orai1, TRPC1, and STIM1, the key proteins involved in the formation of SOCs. Ca(2+) imaging further revealed that Orai1 and TRPC1 are required to form functional SOCs and that these channels are activated by STIM1 in response to thapsigargin or ACh. Pharmacological SOCs inhibition or dominant negative blockade of Orai1 or TRPC1 using the specific pore mutants Orai1-E106D and TRPC1-F562A impaired GSIS in rat β-cells and fully blocked the potentiating effect of ACh on secretion. In contrast, pharmacological or dominant negative blockade of TRPC3 had no effect on extracellular Ca(2+) entry and GSIS. Finally, we observed that prolonged exposure to supraphysiological glucose concentration impaired SOCs function without altering the expression levels of STIM1, Orai1, and TRPC1. We conclude that Orai1 and TRPC1, which form SOCs regulated by STIM1, play a key role in the effect of ACh on GSIS, a process that may be impaired in type 2 diabetes.

Connexin43 Inhibition Prevents Human Vein Grafts Intimal Hyperplasia

Venous bypass grafts often fail following arterial implantation due to excessive smooth muscle cells (VSMC) proliferation and consequent intimal hyperplasia (IH). Intercellular communication mediated by Connexins (Cx) regulates differentiation, growth and proliferation in various cell types. Microarray analysis of vein grafts in a model of bilateral rabbit jugular vein graft revealed Cx43 as an early upregulated gene. Additional experiments conducted using an ex-vivo human saphenous veins perfusion system (EVPS) confirmed that Cx43 was rapidly increased in human veins subjected ex-vivo to arterial hemodynamics. Cx43 knock-down by RNA interference, or adenoviral-mediated overexpression, respectively inhibited or stimulated the proliferation of primary human VSMC in vitro. Furthermore, Cx blockade with carbenoxolone or the specific Cx43 inhibitory peptide ⁴³gap26 prevented the burst in myointimal proliferation and IH formation in human saphenous veins. Our data demonstrated that Cx43 controls proliferation and the formation of IH after arterial engraftment.

Role of connexins and pannexins in cardiovascular physiology

Connexins and pannexins form connexons, pannexons and membrane channels, which are critically involved in many aspects of cardiovascular physiology. For that reason, a vast number of studies have addressed the role of connexins and pannexins in the arterial and venous systems as well as in the heart. Moreover, a role for connexins in lymphatics has recently also been suggested. This review provides an overview of the current knowledge regarding the involvement of connexins and pannexins in cardiovascular physiology.

Endothelial Connexin37 and Connexin40 participate in basal but not agonist-induced NO release

BACKGROUND

Connexin37 (Cx37) and Cx40 are crucial for endothelial cell-cell communication and homeostasis. Both connexins interact with endothelial nitric oxide synthase (eNOS). The exact contribution of these interactions to the regulation of vascular tone is unknown.

RESULTS

Cx37 and Cx40 were expressed in close proximity to eNOS at cell-cell interfaces of mouse aortic endothelial cells. Absence of Cx37 did not affect expression of Cx40 and a 50 % reduction of Cx40 in Cx40(+/-) aortas did not affect the expression of Cx37. However, absence of Cx40 was associated with reduced expression of Cx37. Basal NO release and the sensitivity for ACh were decreased in Cx37(-/-) and Cx40(-/-) aortas but not in Cx40(+/-) aortas. Moreover, ACh-induced release of constricting cyclooxygenase products was present in WT, Cx40(-/-) and Cx40(+/-) aortas but not in Cx37(-/-) aortas. Finally, agonist-induced NO-dependent relaxations and the sensitivity for exogenous NO were not affected by genotype.

CONCLUSIONS

Cx37 is more markedly involved in basal NO release, release of cyclooxygenase products and the regulation of the sensitivity for ACh as compared to Cx40.

Compartmentalized nitric oxide signaling in the resistance vasculature

Nitric oxide (NO) was first described as a bioactive molecule through its ability to stimulate soluble guanylate cyclase, but the revelation that NO was the endothelium derived relaxation factor drove the field to its modern state. The wealth of research conducted over the past 30 years has provided us with a picture of how diverse NO signaling can be within the vascular wall, going beyond simple vasodilation to include such roles as signaling through protein S-nitrosation. This expanded view of NO's actions requires highly regulated and compartmentalized production. Importantly, resistance arteries house multiple proteins involved in the production and transduction of NO allowing for efficient movement of the molecule to regulate vascular tone and reactivity. In this review, we focus on the many mechanisms regulating NO production and signaling action in the vascular wall, with a focus on the control of endothelial nitric oxide synthase (eNOS), the enzyme responsible for synthesizing most of the NO within these confines. We also explore how cross talk between the endothelium and smooth muscle in the microcirculation can modulate NO signaling, illustrating that this one small molecule has the capability to produce a plethora of responses.

Interplay between connexin40 and nitric oxide signaling during hypertension

Connexins (Cxs) and endothelial nitric oxide synthase (eNOS) contribute to the adaptation of endothelial and smooth muscle cells to hemodynamic changes. To decipher the in vivo interplay between these proteins, we studied Cx40-null mice, a model of renin-dependent hypertension which displays an altered endothelium-dependent relaxation of the aorta because of reduced eNOS levels. These mice, which were either untreated or subjected to the 1-kidney, 1-clip (1K1C) procedure, a model of volume-dependent hypertension, were compared with control mice submitted to either the 1K1C or the 2-kidney, 1-clip (2K1C) procedure, a model of renin-dependent hypertension. All operated mice became hypertensive and featured hypertrophy and altered Cx expression of the aorta. The combination of volume- and renin-dependent hypertension in Cx40-/- 1K1C mice raised blood pressure and cardiac weight index. Under these conditions, all aortas showed increased levels of Cx40 in endothelial cells and of both Cx37 and Cx45 in smooth muscle cells. In the wild-type 1K1C mice, the interactions between Cx40 and Cx37 with eNOS were enhanced, resulting in increased NO release. The Cx40-eNOS interaction could not be observed in mice lacking Cx40, which also featured decreased levels of eNOS. In these animals, the volume overload caused by the 1K1C procedure resulted in increased phosphorylation of eNOS and in a higher NO release. The findings provide evidence that Cx40 and Cx37 play an in vivo role in the regulation of eNOS.

Mutations in cardiovascular connexin genes

Connexins (Cxs) form a family of transmembrane proteins comprising 21 members in humans. Cxs differ in their expression patterns, biophysical properties and ability to combine into homomeric or heteromeric gap junction channels between neighbouring cells. The permeation of ions and small metabolites through gap junction channels or hemichannels confers a crucial role to these proteins in intercellular communication and in maintaining tissue homeostasis. Among others, Cx37, Cx40, Cx43, Cx45 and Cx47 are found in heart, blood and lymphatic vessels. Mutations or polymorphisms in the genes coding for these Cxs have not only been implicated in cardiovascular pathologies but also in a variety of other disorders. While mutations in Cx43 are mostly linked to oculodentodigital dysplasia, Cx47 mutations are associated with Pelizaeus-Merzbacher-like disease and lymphoedema. Cx40 mutations are principally linked to atrial fibrillation. Mutations in Cx37 have not yet been described, but polymorphisms in the Cx37 gene have been implicated in the development of arterial disease. This review addresses current knowledge on gene mutations in cardiovascular Cxs systematically and links them to alterations in channel properties and disease.

Endothelial arginine resynthesis contributes to the maintenance of vasomotor function in male diabetic mice

Aim

Argininosuccinate synthetase (ASS) is essential for recycling L-citrulline, the by-product of NO synthase (NOS), to the NOS substrate L-arginine. Here, we assessed whether disturbed arginine resynthesis modulates endothelium-dependent vasodilatation in normal and diabetic male mice.

Methods and Results

Endothelium-selective Ass-deficient mice (Ass^fl/fl/Tie2Cre^tg/− = Ass-KO^Tie2) were generated by crossing Ass^fl/fl mice ( = control) with Tie2Cre mice. Gene ablation in endothelial cells was confirmed by immunohistochemistry. Blood pressure (MAP) was recorded in 34-week-old male mice. Vasomotor responses were studied in isolated saphenous arteries of 12- and 34-week-old Ass-KO^Tie2 and control animals. At the age of 10 weeks, diabetes was induced in control and Ass-KO^Tie2 mice by streptozotocin injections. Vasomotor responses of diabetic animals were studied 10 weeks later. MAP was similar in control and Ass-KO^Tie2 mice. Depletion of circulating L-arginine by arginase 1 infusion or inhibition of NOS activity with L-NAME resulted in an increased MAP (10 and 30 mmHg, respectively) in control and Ass-KO^Tie2 mice. Optimal arterial diameter, contractile responses to phenylephrine, and relaxing responses to acetylcholine and sodium nitroprusside were similar in healthy control and Ass-KO^Tie2 mice. However, in diabetic Ass-KO^Tie2 mice, relaxation responses to acetylcholine and endothelium-derived NO (EDNO) were significantly reduced when compared to diabetic control mice.

Conclusions

Absence of endothelial citrulline recycling to arginine did not affect blood pressure and systemic arterial vasomotor responses in healthy mice. EDNO-mediated vasodilatation was significantly more impaired in diabetic Ass-KO^Tie2 than in control mice demonstrating that endothelial arginine recycling becomes a limiting endothelial function in diabetes.

Lipopolysaccharide exposure during pregnancy leads to aortic dysfunction in offspring rats

BACKGROUND

Prenatal exposure to Lipopolysaccharide (LPS) produces hypertension in adult offspring rats. The present study was to explore the effects of prenatal inflammation on morphological and functional changes in the aorta from offspring rats and to further assess its susceptibility to cardiovascular diseases.

METHODS AND RESULTS

Pregnant rats were treated intraperitoneally on gestation Days 8, 10 and 12 with saline, LPS (0.79 mg/kg), or pyrrolidine dithiocarbamate (PDTC, 100 mg/kg)+LPS, respectively. Aortic ring reactivity and histopathological alteration were analyzed in offspring at the age of 12 weeks. The detections of connexin (Cx) 37, Cx40, Cx43, and Cx45, including immunofluorescent patterns, protein levels and mRNA expression in the aorta, were performed as well. Furthermore, the expressions of Nuclear factor (NF)-κB (p65), IκBα, phospho-IκBα and IκBβ were determined. The results showed that prenatal LPS exposure leads to morphological abnormalities and impaired aortic reactivity in offspring. Prenatal LPS exposure also decreased the protein and mRNA expression of Cx37 in the aorta from offspring rats. NF-κB and phospho-IκBα levels were both increased, IκBα level, however, was decreased in the aorta of offspring from the maternal LPS exposure compared to the controls. Simultaneously, PDTC treatment markedly reversed the action of LPS.

CONCLUSIONS

Decreased expression of Cx37 contributed to the aortic dysfunction of prenatal LPS exposure offspring, which should be associated with NF-κB activation.

Restoration of connexin 40 (Cx40) in Renin-producing cells reduces the hypertension of Cx40 null mice

Connexin 40 (Cx40) is expressed by the renin-producing cells (RSCs) of the kidneys and the endothelial cells of blood vessels. Cx40 null mice (Cx40(-/-)) feature a much increased renin synthesis and secretion, which results in chronic hypertension, and also display an altered endothelium-dependent relaxation of the aorta because of reduced eNOS levels and nitric oxide production. To discriminate the effect of Cx40 in renin secretion and vascular signaling, we targeted Cx40 to either the RSCs or the endothelial cells of Cx40 null mice. When compared with Cx40(-/-) controls, the animals expressing Cx40 in RSCs were less hypertensive and featured reduced renin levels, still numerous RSCs outside the wall of the afferent arterioles. In contrast, mice expressing Cx40 in the endothelial cells were as hypertensive as Cx40(-/-) mice, in spite of control levels of Cx37 and eNOS. Our data show that blood pressure is improved by restoration of Cx40 expression in RSCs but not in endothelial cells, stressing the prominent role of renin in the mouse hypertension linked to loss of Cx40.

Multiple roles of connexins in atherosclerosis- and restenosis-induced vascular remodelling

Endothelial dysfunction is the initial step in atherosclerotic plaque development in large- and medium-sized arteries. This progressive disease, which starts during childhood, is characterized by the accumulation of lipids, macrophages, neutrophils, T lymphocytes and smooth muscle cells in the intima of the vessels. Erosion and rupture of the atherosclerotic plaque may induce myocardial infarction and cerebrovascular accidents, which are responsible for a large percentage of sudden deaths. The most common treatment for atherosclerosis is angioplasty and stent implantation, but these surgical interventions favour a vascular reaction called restenosis and the associated de-endothelialization increases the risk of thrombosis. This review provides an overview of the role of connexins, a large family of transmembrane proteins, in vascular remodelling associated with atherosclerosis and restenosis. The connexins expressed in the vascular wall are Cx37, Cx40, Cx43 and Cx45; their expressions vary with vascular territory and species. Connexins form hemichannels or gap junction channels, allowing the exchange of ions and small metabolites between the cytosol and extracellular space or between neighbouring cells, respectively. Connexins have important roles in vascular physiology; they support radial and longitudinal cell-to-cell communication in the vascular wall, and significant changes in their expression patterns have been described during atherosclerosis and restenosis.

Ca²⁺-dependent nitric oxide release in the injured endothelium of excised rat aorta: a promising mechanism applying in vascular prosthetic devices in aging patients

Background

Nitric oxide is key to endothelial regeneration, but it is still unknown whether endothelial cell (EC) loss results in an increase in NO levels at the wound edge. We have already shown that endothelial damage induces a long-lasting Ca²⁺ entry into surviving cells though connexin hemichannels (CxHcs) uncoupled from their counterparts on ruptured cells. The physiological outcome of injury-induced Ca²⁺ inflow is, however, unknown.

Methods

In this study, we sought to determine whether and how endothelial scraping induces NO production (NOP) in the endothelium of excised rat aorta by exploiting the NO-sensitive fluorochrome, DAF-FM diacetate and the Ca²⁺-sensitive fluorescent dye, Fura-2/AM.

Results

We demonstrated that injury-induced NOP at the lesion site is prevented in presence of the endothelial NO synthase inhibitor, L-NAME, and in absence of extracellular Ca²⁺. Unlike ATP-dependent NO liberation, the NO response to injury is insensitive to BTP-2, which selectively blocks store-operated Ca²⁺ inflow. However, injury-induced NOP is significantly reduced by classic gap junction blockers, and by connexin mimetic peptides specifically targeting Cx37Hcs, Cx40HCs, and Cx43Hcs. Moreover, disruption of caveolar integrity prevents injury-elicited NO signaling, but not the accompanying Ca²⁺ response.

Conclusions

The data presented provide the first evidence that endothelial scraping stimulates NO synthesis at the wound edge, which might both exert an immediate anti-thrombotic and anti-inflammatory action and promote the subsequent re-endothelialization.

Atorvastatin-loaded hydrogel affects the smooth muscle cells of human veins

Intimal hyperplasia (IH) is the major cause of stenosis of vein grafts. Drugs such as statins prevent stenosis, but their systemic administration has limited effects. We developed a hyaluronic acid hydrogel matrix, which ensures a controlled release of atorvastatin (ATV) at the site of injury. The release kinetics demonstrated that 100% of ATV was released over 10 hours, independent of the loading concentration of the hydrogel. We investigated the effects of such a delivery on primary vascular smooth muscle cells isolated from human veins. ATV decreased the proliferation, migration, and passage of human smooth muscle cells (HSMCs) across a matrix barrier in a similar dose-dependent (5-10 µM) and time-dependent manner (24-72 hours), whether the drug was directly added to the culture medium or released from the hydrogel. Expression analysis of genes known to be involved in the development of IH demonstrated that the transcripts of both the gap junction protein connexin43 (Cx43) and plasminogen activator inhibitor-1 (PAI-1) were decreased after a 24-48-hour exposure to the hydrogel loaded with ATV, whereas the transcripts of the heme oxygenase (HO-1) and the inhibitor of tissue plasminogen activator were increased. At the protein level, Cx43, PAI-1, and metalloproteinase-9 expression were decreased, whereas HO-1 was upregulated in the presence of ATV. The data demonstrate that ATV released from a hydrogel has effects on HSMCs similar to the drug being freely dissolved in the environment.

Characterization of vasomotor responses in different vascular territories of C57BL/6J mice

The mouse is a quickly reproducing, inexpensive animal and often used for transgenic approaches. Due to its small size, only the aorta is frequently taken to assess vascular function. However, atherosclerosis is a generalized disease and becomes symptomatic when the perfusion of specific organs is impaired. We have therefore compared the thoracic and abdominal aorta with carotid, femoral, mesenteric, renal and coronary arteries to see whether aortic vasomotion can indeed serve as a surrogate for other, organ-specific vascular territories. Arterial segments of male C57BL/6J mice were dissected and mounted on a myograph for isometric force measurement. Vasoconstriction was determined in response to depolarization by potassium chloride (KCl), which was not different with or without an α-adrenoceptor antagonist. Vascular responses were determined in response to receptor activation by the neurotransmitter norepinephrine (±inhibition of nitric oxide synthase; ±α- and β-adrenoceptor antagonists) and the platelet-derived mediator serotonin (±inhibition of nitric oxide synthesis; ±5-hydroxytryptamine receptor antagonist). Endothelium-dependent and -independent vasodilation was determined in response to carbachol and nitroprusside after norepinephrine-induced pre-constriction (±β-adrenoceptor antagonist). Vasoconstriction in response to KCl, norepinephrine and serotonin differed in magnitude between thoracic and abdominal aorta and between aorta and the other arterial segments. Endothelium-dependent and -independent vasodilation differed also in magnitude between the arterial segments. Thus, the murine aorta is not a general surrogate to assess vascular function of organ-specific vascular territories.

Regulation of cardiovascular connexins by mechanical forces and junctions

Connexins form a family of transmembrane proteins that consists of 20 members in humans and 21 members in mice. Six connexins assemble into a connexon that can function as a hemichannel or connexon that can dock to a connexon expressed by a neighbouring cell, thereby forming a gap junction channel. Such intercellular channels synchronize responses in multicellular organisms through direct exchange of ions, small metabolites, and other second messenger molecules between the cytoplasms of adjacent cells. Multiple connexins are expressed in the cardiovascular system. These connexins not only experience the different biomechanical forces within this system, but may also act as effector proteins in co-ordinating responses within groups of cells towards these forces. This review discusses recent insights regarding regulation of cardiovascular connexins by mechanical forces and junctions. It specifically addresses effects of (i) shear stress on endothelial connexins, (ii) hypertension on vascular connexins, and (iii) changes in afterload and the composition of myocardial mechanical junctions on cardiac connexins.

Role of hemodynamic forces in the ex vivo arterialization of human saphenous veins

BACKGROUND

Human saphenous vein grafts are one of the salvage bypass conduits when endovascular procedures are not feasible or fail. Understanding the remodeling process that venous grafts undergo during exposure to arterial conditions is crucial to improve their patency, which is often compromised by intimal hyperplasia. The precise role of hemodynamic forces such as shear stress and arterial pressure in this remodeling is not fully characterized. The aim of this study was to determine the involvement of arterial shear stress and pressure on vein wall remodeling and to unravel the underlying molecular mechanisms.

METHODS

An ex vivo vein support system was modified for chronic (up to 1 week), pulsatile perfusion of human saphenous veins under controlled conditions that permitted the separate control of arterial shear stress and different arterial pressure (7 mm Hg or 70 mm Hg).

RESULTS

Veins perfused for 7 days under high pressure (70 mm Hg) underwent significant development of a neointima compared with veins exposed to low pressure (7 mm Hg). These structural changes were associated with altered expression of several molecular markers. Exposure to an arterial shear stress under low pressure increased the expression of matrix metalloproteinase (MMP)-2 and MMP-9 and tissue inhibitor of metalloproteinase (TIMP)-1 at the transcript, protein, and activity levels. This increase was enhanced by high pressure, which also increased TIMP-2 protein expression despite decreased levels of the cognate transcript. In contrast, the expression of plasminogen activator inhibitor-1 increased with shear stress but was not modified by pressure. Levels of the venous marker Eph-B4 were decreased under arterial shear stress, and levels of the arterial marker Ephrin-B2 were downregulated under high-pressure conditions.

CONCLUSIONS

This model is a valuable tool to identify the role of hemodynamic forces and to decipher the molecular mechanisms leading to failure of human saphenous vein grafts. Under ex vivo conditions, arterial perfusion is sufficient to activate the remodeling of human veins, a change that is associated with the loss of specific vein markers. Elevation of pressure generates intimal hyperplasia, even though veins do not acquire arterial markers.

CLINICAL RELEVANCE

The pathological remodeling of the venous wall, which leads to stenosis and ultimately graft failure, is the main limiting factor of human saphenous vein graft bypass. This remodeling is due to the hemodynamic adaptation of the vein to the arterial environment and cannot be prevented by conventional therapy. To develop a more targeted therapy, a better understanding of the molecular mechanisms involved in intimal hyperplasia is essential, which requires the development of ex vivo models of chronic perfusion of human veins.

Defective Cx40 maintains Cx37 expression but intact Cx40 is crucial for conducted dilations irrespective of hypertension

The gap junction channel protein connexin40 (Cx40) is crucial in vascular and renal physiology, because Cx40-deficient mice exhibit impaired conduction of endothelium-dependent dilations and pronounced hypertension. The latter precludes mechanistic insights into the role of endothelial Cx40, because long-lasting hypertension itself may affect conduction and Cx expression. We aimed to identify endothelial Cx40 functions, their dependency on the conductive capability, and to separate these from hypertension-related alterations. We assessed conduction and Cx expression in mice with cell type-specific deletion of Cx40 and in mice expressing a defective Cx40 (Cx40A96S) identified in humans, which forms nonconducting gap junction channels. Confined arteriolar stimulation with acetylcholine or bradykinin elicited local dilations that conducted upstream without attenuation of the amplitude for distances up to 1.2-mm in controls with a floxed Cx40 gene (Cx40(fl/fl)). Conducted responses in hypertensive animals devoid of Cx40 in renin-producing cells were unaltered but remote dilations were reduced in normotensive animals deficient for Cx40 in endothelial cells (Cx40(fl/fl):Tie2-Cre). Surprisingly, Cx37 expression was undetectable by immunostaining in arteriolar endothelium only in Cx40(fl/fl):Tie2-Cre; however, transcriptional activity of Cx37 in the cremaster was comparable with Cx40(fl/fl) controls. Cx40A96S mice were hypertensive with preserved expression of Cx40 and Cx37. Nevertheless, conducted responses were blunted. We conclude that endothelial Cx40 is necessary to support conducted dilations initiated by endothelial agonists and to locate Cx37 into the plasma membrane. These functions are unaltered by long-lasting hypertension. In the presence of a nonconducting Cx40, Cx37 is present but cannot support the conduction highlighting the importance of endothelial Cx40.

Cell-cell communication in the kidney microcirculation

In the renal vasculature of humans, rats, and mice, at least four isoforms of Cx, Cxs 37, 40, 43, and 45 are expressed. In the ECs, Cx40 is the predominantly expressed Cx, whereas Cx45 is suggested to be expressed in the VSMCs. The preglomerular vasculature has a higher expression of Cxs than the postglomerular vasculature. Cxs form gap junctions between neighboring cells, and as in other organ systems, the major function of Cxs in the kidney appears to be mediation of intercellular communication. Cxs may also form hemichannels that allow cellular secretion of signaling molecules like ATP, and thereby mediate paracrine signaling. Renal Cxs facilitate vascular conduction, juxtaglomerlar apparatus calcium signaling, and enable ECs and VSMCs to communicate. Thus, current research suggests multiple roles for Cxs in important regulatory mechanisms within the kidney, including the renin-angiotensin system, TGF, and salt and water homeostasis. Interestingly, changes in the activity of the renin-angiotensin system or changes in blood pressure seem to affect the expression of the renal vascular Cxs. At the systemic level, renal Cxs may be involved in blood pressure regulation, and possibly in the pathogenesis of hypertension and diabetes.

Hypoxic pulmonary vasoconstriction requires connexin 40-mediated endothelial signal conduction

Hypoxic pulmonary vasoconstriction (HPV) is a physiological mechanism by which pulmonary arteries constrict in hypoxic lung areas in order to redirect blood flow to areas with greater oxygen supply. Both oxygen sensing and the contractile response are thought to be intrinsic to pulmonary arterial smooth muscle cells. Here we speculated that the ideal site for oxygen sensing might instead be at the alveolocapillary level, with subsequent retrograde propagation to upstream arterioles via connexin 40 (Cx40) endothelial gap junctions. HPV was largely attenuated by Cx40-specific and nonspecific gap junction uncouplers in the lungs of wild-type mice and in lungs from mice lacking Cx40 (Cx40-/-). In vivo, hypoxemia was more severe in Cx40-/- mice than in wild-type mice. Real-time fluorescence imaging revealed that hypoxia caused endothelial membrane depolarization in alveolar capillaries that propagated to upstream arterioles in wild-type, but not Cx40-/-, mice. Transformation of endothelial depolarization into vasoconstriction involved endothelial voltage-dependent α1G subtype Ca2+ channels, cytosolic phospholipase A2, and epoxyeicosatrienoic acids. Based on these data, we propose that HPV originates at the alveolocapillary level, from which the hypoxic signal is propagated as endothelial membrane depolarization to upstream arterioles in a Cx40-dependent manner.

Vascular Hypoxic Preconditioning Relies on TRPV4-Dependent Calcium Influx and Proper Intercellular Gap Junctions Communication

Objective—

We investigated the impact of hypoxia-reoxygenation on endothelial relaxation and aimed to clarify the role of transient receptor potential cation channels V4 (TRPV4) and gap junctions in the protective effect associated with hypoxic preconditioning on the vascular function.

Methods and Results—

By mimicking ischemia-reperfusion in C57BL/6 male mice in vivo, we documented a reduced NO-mediated relaxation and an increased endothelium-derived hyperpolarization (EDH[F])-mediated relaxation. Hypoxic preconditioning, however, restored NO relaxation and further improved the EDH(F) response. We also examined specifically 2 major effectors of the EDH(F) pathway, transient receptor potential cation channels V4 and connexins. We found that in endothelial cells, expression and activity of transient receptor potential cation channels V4 were increased by hypoxic stimuli independently of preconditioning which was interestingly associated with an increase of structural caveolar component caveolin-1 at membrane locations. Gap junctions, however, seemed to directly support EDH(F)-driven preconditioning as connexin 40 and connexin 43 expression increased and as in vivo carbenoxolone treatment completely inhibited the EDH(F) pathway and significantly reduced the protection afforded by preconditioning for the concomitant NO-mediated relaxation.

Conclusion—

Our work provides evidence on how transient receptor potential cation channels V4 and connexins might participate in preserving vasorelaxation under hypoxia and restoring the NO-mediated pathway in hypoxic preconditioning conditions pointing out caveolae as a common signaling location.

Renal connexins and blood pressure

The kidneys are centrally involved in the regulation of blood pressure. Kidney function requires the coordinated actions of a number of different vascular and tubular cell types in the renal vasculature and in the renal tubular system. The intrarenal coordination of these actions is not well understood. Since gap junctions have been identified in the kidneys, possible pathways involved in this context could be direct intercellular communication via gap junctions or via connexin hemichannels. In this context nine different connexins have been found to be expressed in the kidney, either localized to the vasculature or to the tubular system. Evidence is arising that malfunctions of certain connexins have an impact on the capability of the kidney to maintain blood pressure homeostasis. Findings reported in this context will be outlined and discussed in this review. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.