Connexin45 gap junction channels in rat cerebral vascular smooth muscle cells

Connexin and Pannexin Large-Pore Channels in Microcirculation and Neurovascular Coupling Function

Microcirculation homeostasis depends on several channels permeable to ions and/or small molecules that facilitate the regulation of the vasomotor tone, hyperpermeability, the blood–brain barrier, and the neurovascular coupling function. Connexin (Cxs) and Pannexin (Panxs) large-pore channel proteins are implicated in several aspects of vascular physiology. The permeation of ions (i.e., Ca²⁺) and key metabolites (ATP, prostaglandins, D-serine, etc.) through Cxs (i.e., gap junction channels or hemichannels) and Panxs proteins plays a vital role in intercellular communication and maintaining vascular homeostasis. Therefore, dysregulation or genetic pathologies associated with these channels promote deleterious tissue consequences. This review provides an overview of current knowledge concerning the physiological role of these large-pore molecule channels in microcirculation (arterioles, capillaries, venules) and in the neurovascular coupling function.

New Insights into Pulmonary Hypertension: A Role for Connexin-Mediated Signalling

Pulmonary hypertension is a serious clinical condition characterised by increased pulmonary arterial pressure. This can lead to right ventricular failure which can be fatal. Connexins are gap junction-forming membrane proteins which serve to exchange small molecules of less than 1 kD between cells. Connexins can also form hemi-channels connecting the intracellular and extracellular environments. Hemi-channels can mediate adenosine triphosphate release and are involved in autocrine and paracrine signalling. Recently, our group and others have identified evidence that connexin-mediated signalling may be involved in the pathogenesis of pulmonary hypertension. In this review, we discuss the evidence that dysregulated connexin-mediated signalling is associated with pulmonary hypertension.

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.

Intercellular Conduction Optimizes Arterial Network Function and Conserves Blood Flow Homeostasis During Cerebrovascular Challenges

OBJECTIVE

Cerebral arterial networks match blood flow delivery with neural activity. Neurovascular response begins with a stimulus and a focal change in vessel diameter, which by themselves is inconsequential to blood flow magnitude, until they spread and alter the contractile status of neighboring arterial segments. We sought to define the mechanisms underlying integrated vascular behavior and considered the role of intercellular electrical signaling in this phenomenon. Approach and Results: Electron microscopic and histochemical analysis revealed the structural coupling of cerebrovascular cells and the expression of gap junctional subunits at the cell interfaces, enabling intercellular signaling among vascular cells. Indeed, robust vasomotor conduction was detected in human and mice cerebral arteries after focal vessel stimulation: a response attributed to endothelial gap junctional communication, as its genetic alteration attenuated this behavior. Conducted responses were observed to ascend from the penetrating arterioles, influencing the contractile status of cortical surface vessels, in a simulated model of cerebral arterial network. Ascending responses recognized in vivo after whisker stimulation were significantly attenuated in mice with altered endothelial gap junctional signaling confirming that gap junctional communication drives integrated vessel responses. The diminishment in vascular communication also impaired the critical ability of the cerebral vasculature to maintain blood flow homeostasis and hence tissue viability after stroke.

CONCLUSIONS

Our findings highlight the integral role of intercellular electrical signaling in transcribing focal stimuli into coordinated changes in cerebrovascular contractile activity and expose, a hitherto unknown mechanism for flow regulation after stroke.

Polarized hemichannel opening of pannexin 1/connexin 43 contributes to dysregulation of transport function in blood-brain barrier endothelial cells

Dysregulation of blood-brain barrier (BBB) transport exacerbates brain damage in acute ischemic stroke. Here, we aimed to investigate the mechanism of this BBB transport dysregulation by studying the localization and function of pannexin (Px) and connexin (Cx) hemichannels in blood-brain barrier endothelial cells of rat (TR-BBB13 cells) and human (hCMEC/D3 cells) under acute ischemic stroke-mimicking oxygen/glucose deprivation (OGD) and extracellular Ca²⁺ ([Ca²⁺]_e)-free conditions. TR-BBB13 cells showed increased uptake of hemichannel-permeable sulforhodamine 101, and this increase was markedly inhibited by carbenoxolone, a hemichannel inhibitor. Transcripts of Px1 and Cx43 were detected in TR-BBB13 cells and freshly isolated brain microvascular endothelial cells. The basal compartment-to-cell uptake of hemichannel-permeable propidium iodide was selectively enhanced in hCMEC/D3 cells under [Ca²⁺]_e-free conditions in the basal Transwell chamber. Immunohistochemical analysis revealed the predominant localization of Cx43 on the lateral membranes of hCMEC/D3 cells. [³H]Taurine uptake by hCMEC/D3 cells was significantly reduced in the absence of [Ca²⁺]_e. Functional knock-down of Px1 and Cx43 with mimetic peptides significantly inhibited the increase of ATP release from hCMEC/D3 cells under [Ca²⁺]_e-free conditions. These results suggest that polarized Px1/Cx43 hemichannel opening in brain capillary endothelial cells under acute ischemic stroke-mimicking conditions contributes to dysregulation of BBB transport function, resulting in release of intracellular taurine and ATP.

Smooth muscle gap-junctions allow propagation of intercellular Ca2+ waves and vasoconstriction due to Ca2+ based action potentials in rat mesenteric resistance arteries

The role of vascular gap junctions in the conduction of intercellular Ca2+ and vasoconstriction along small resistance arteries is not entirely understood. Some depolarizing agents trigger conducted vasoconstriction while others only evoke a local depolarization. Here we use a novel technique to investigate the temporal and spatial relationship between intercellular Ca2+ signals generated by smooth muscle action potentials (APs) and vasoconstriction in mesenteric resistance arteries (MA). Pulses of exogenous KCl to depolarize the downstream end (T1) of a 3 mm long artery increased intracellular Ca2+ associated with vasoconstriction. The spatial spread and amplitude of both depended on the duration of the pulse, with only a restricted non-conducting vasoconstriction to a 1 s pulse. While blocking smooth muscle cell (SMC) K+ channels with TEA and activating L-type voltage-gated Ca2+ channels (VGCCs) with BayK 8644 spread was dramatically facilitated, so the 1 s pulse evoked intercellular Ca2+ waves and vasoconstriction that spread along an entire artery segment 3000 μm long. Ca2+ waves spread as nifedipine-sensitive Ca2+ spikes due to SMC action potentials, and evoked vasoconstriction. Both intercellular Ca2+ and vasoconstriction spread at circa 3 mm s-1 and were independent of the endothelium. The spread but not the generation of Ca2+ spikes was reversibly blocked by the gap junction inhibitor 18β-GA. Thus, smooth muscle gap junctions enable depolarization to spread along resistance arteries, and once regenerative Ca2+-based APs occur, spread along the entire length of an artery followed by widespread vasoconstriction.

The Conducted Vasomotor Response: Function, Biophysical Basis, and Pharmacological Control

Arterial tone is coordinated among vessel segments to optimize nutrient transport and organ function. Coordinated vasomotor activity is remarkable to observe and depends on stimuli, sparsely generated in tissue, eliciting electrical responses that conduct lengthwise among electrically coupled vascular cells. The conducted response is the focus of this topical review, and in this regard, the authors highlight literature that advances an appreciation of functional significance, cellular mechanisms, and biophysical principles. Of particular note, this review stresses that conduction is enabled by a defined pattern of charge movement along the arterial wall as set by three key parameters (tissue structure, gap junctional resistivity, and ion channel activity). The impact of disease on conduction is carefully discussed, as are potential strategies to restore this key biological response and, along with it, the match of blood flow delivery with tissue energetic demand.

KIR channels tune electrical communication in cerebral arteries

The conducted vasomotor response reflects electrical communication in the arterial wall and the distance signals spread is regulated by three factors including resident ion channels. This study defined the role of inward-rectifying K⁺ channels (K_IR) in governing electrical communication along hamster cerebral arteries. Focal KCl application induced a vasoconstriction that conducted robustly, indicative of electrical communication among cells. Inhibiting dominant K⁺ conductances had no attenuating effect, the exception being Ba²⁺ blockade of K_IR. Electrophysiology and Q-PCR analysis of smooth muscle cells revealed a Ba²⁺-sensitive K_IR current comprised of K_IR2.1/2.2 subunits. This current was surprisingly small and when incorporated into a model, failed to account for the observed changes in conduction. We theorized a second population of K_IR channels exist and consistent with this idea, a robust Ba²⁺-sensitive K_IR2.1/2.2 current was observed in endothelial cells. When both K_IR currents were incorporated into, and then inhibited in our model, conduction decay was substantive, aligning with experiments. Enhanced decay was ascribed to the rightward shift in membrane potential and the increased feedback arising from voltage-dependent-K⁺ channels. In summary, this study shows that two K_IR populations work collaboratively to govern electrical communication and the spread of vasomotor responses along cerebral arteries.

Enhanced gap junctional channel activity between vascular smooth muscle cells in cerebral artery of spontaneously hypertensive rats

The aim of the present study is to investigate the effects of hypertension on the gap junctions between vascular smooth muscle cells (VSMCs) in the cerebral arteries (CAs) of spontaneously hypertensive rats (SHRs). The functions of gap junctions in the CAs of VSMCs in SHRs and control normotensive Wistar-Kyoto (WKY) rats were studied using whole-cell patch clamp recordings and pressure myography, and the expression levels of connexins were analyzed using reverse transcription-quantitative polymerase chain reaction and Western blot analyses. Whole-cell patch clamp measurements revealed that the membrane capacitance and conductance of in situ VSMCs in the CAs were significantly greater in SHRs than in WKY rats, suggesting that gap junction coupling is enhanced between VSMCs in the CAs of SHRs. Application of the endothelium-independent vasoconstrictors KCl or phenylephrine (PE) stimulated a greater vasoconstriction in the CAs of SHRs than in those of WKY rats. The EC₅₀ value of KCl was 24.9 mM (n = 14) and 36.9 mM (n=12) for SHRs and WKY rats, respectively. The EC₅₀ value of PE was 0.9 µM (n = 7) and 2.2 µM (n = 7) for SHRs and WKY rats, respectively. Gap junction inhibitors 18β-glycyrrhetinic acid (18β-GA), niflumic acid (NFA), and 2-aminoethoxydiphenyl borate (2-APB) attenuated KCl-induced vasoconstriction in SHRs and WKY rats. The mRNA and protein expression levels of the gap junction protein connexin 45 (Cx45) were significantly higher in the CAs of SHRs than in those of WKY rats. Phosphorylated Cx43 protein expression was significantly higher in the CAs of SHRs than in those of WKY rats, despite the total Cx43 mRNA and protein expression levels in the cerebral artery (CA) exhibiting no significant difference between SHRs and WKY rats. Increases in the expression of Cx45 and phosphorylation of Cx43 may promote gap junction communication among VSMCs in the CAs of SHRs, which may enhance the contractile response of the CA to vasoconstrictors.

Gap junction regulation of vascular tone: implications of modulatory intercellular communication during gestation

In the vasculature, gap junctions (GJ) play a multifaceted role by serving as direct conduits for cell-cell intercellular communication via the facilitated diffusion of signaling molecules. GJs are essential for the control of gene expression and coordinated vascular development in addition to vascular function. The coupling of endothelial cells to each other, as well as with vascular smooth muscle cells via GJs, plays a relevant role in the control of vasomotor tone, tissue perfusion and arterial blood pressure. The regulation of cell-signaling is paramount to cardiovascular adaptations of pregnancy. Pregnancy requires highly developed cell-to-cell coupling, which is affected partly through the formation of intercellular GJs by Cx43, a gap junction protein, within adjacent cell membranes to help facilitate the increase of uterine blood flow (UBF) in order to ensure adequate perfusion for nutrient and oxygen delivery to the placenta and thus the fetus. One mode of communication that plays a critical role in regulating Cx43 is the release of endothelial-derived vasodilators such as prostacyclin (PGI2) and nitric oxide (NO) and their respective signaling mechanisms involving second messengers (cAMP and cGMP, respectively) that are likely to be important in maintaining UBF. Therefore, the assertion we present in this review is that GJs play an integral if not a central role in maintaining UBF by controlling rises in vasodilators (PGI2 and NO) via cyclic nucleotides. In this review, we discuss: (1) GJ structure and regulation; (2) second messenger regulation of GJ phosphorylation and formation; (3) pregnancy-induced changes in cell-signaling; and (4) the role of uterine arterial endothelial GJs during gestation. These topics integrate the current knowledge of this scientific field with interpretations and hypotheses regarding the vascular effects that are mediated by GJs and their relationship with vasodilatory vascular adaptations required for modulating the dramatic physiological rises in uteroplacental perfusion and blood flow observed during normal pregnancy.

Emerging trend in second messenger communication and myoendothelial feedback

Over the past decade, second messenger communication has emerged as one of the intriguing topics in the field of vasomotor control. Of particular interest has been the idea of second messenger flux from smooth muscle to endothelium initiating a feedback response that attenuates constriction. Mechanistic details of the precise signaling cascade have until recently remained elusive. In this perspective, we introduce readers to how myoendothelial gap junctions could enable sufficient inositol trisphosphate flux to initiate endothelial Ca(2+) events that activate Ca(2+) sensitive K(+) channels. The resulting hyperpolarizing current would in turn spread back through the same myoendothelial gap junctions to moderate smooth muscle depolarization and constriction. In discussing this defined feedback mechanism, this brief manuscript will stress the importance of microdomains and of discrete cellular signaling.

Traumatic brain injury in vivo and in vitro contributes to cerebral vascular dysfunction through impaired gap junction communication between vascular smooth muscle cells

Gap junctions (GJs) contribute to cerebral vasodilation, vasoconstriction, and, perhaps, to vascular compensatory mechanisms, such as autoregulation. To explore the effects of traumatic brain injury (TBI) on vascular GJ communication, we assessed GJ coupling in A7r5 vascular smooth muscle (VSM) cells subjected to rapid stretch injury (RSI) in vitro and VSM in middle cerebral arteries (MCAs) harvested from rats subjected to fluid percussion TBI in vivo. Intercellular communication was evaluated by measuring fluorescence recovery after photobleaching (FRAP). In VSM cells in vitro, FRAP increased significantly (p<0.05 vs. sham RSI) after mild RSI, but decreased significantly (p<0.05 vs. sham RSI) after moderate or severe RSI. FRAP decreased significantly (p<0.05 vs. sham RSI) 30 min and 2 h, but increased significantly (p<0.05 vs. sham RSI) 24 h after RSI. In MCAs harvested from rats 30 min after moderate TBI in vivo, FRAP was reduced significantly (p<0.05), compared to MCAs from rats after sham TBI. In VSM cells in vitro, pretreatment with the peroxynitrite (ONOO(-)) scavenger, 5,10,15,20-tetrakis(4-sulfonatophenyl)prophyrinato iron[III], prevented RSI-induced reductions in FRAP. In isolated MCAs from rats treated with the ONOO(-) scavenger, penicillamine, GJ coupling was not impaired by fluid percussion TBI. In addition, penicillamine treatment improved vasodilatory responses to reduced intravascular pressure in MCAs harvested from rats subjected to moderate fluid percussion TBI. These results indicate that TBI reduced GJ coupling in VSM cells in vitro and in vivo through mechanisms related to generation of the potent oxidant, ONOO(-).

Role of connexins in infantile hemangiomas

The circulatory system is one of the first systems that develops during embryogenesis. Angiogenesis describes the formation of blood vessels as a part of the circulatory system and is essential for organ growth in embryogenesis as well as repair in adulthood. A dysregulation of vessel growth contributes to the pathogenesis of many disorders. Thus, an imbalance between pro- and antiangiogenic factors could be observed in infantile hemangioma (IH). IH is the most common benign tumor during infancy, which appears during the first month of life. These vascular tumors are characterized by rapid proliferation and subsequently slower involution. Most IHs regress spontaneously, but in some cases they cause disfigurement and systemic complications, which requires immediate treatment. Recently, a therapeutic effect of propranolol on IH has been demonstrated. Hence, this non-selective β-blocker became the first-line therapy for IH. Over the last years, our understanding of the underlying mechanisms of IH has been improved and possible mechanisms of action of propranolol in IH have postulated. Previous studies revealed that gap junction proteins, the connexins (Cx), might also play a role in the pathogenesis of IH. Therefore, affecting gap junctional intercellular communication is suggested as a novel therapeutic target of propranolol in IH. In this review we summarize the current knowledge of the molecular processes, leading to IH and provide new insights of how Cxs might be involved in the development of these vascular tumors.

Connexin 30 expression and frequency of connexin heterogeneity in astrocyte gap junction plaques increase with age in the rat retina

We investigated age-associated changes in retinal astrocyte connexins (Cx) by assaying Cx numbers, plaque sizes, protein expression levels and heterogeneity of gap junctions utilizing six-marker immunohistochemistry (IHC). We compared Wistar rat retinal wholemounts in animals aged 3 (young adult), 9 (middle-aged) and 22 months (aged). We determined that retinal astrocytes have gap junctions composed of Cx26, -30, -43 and -45. Cx30 was consistently elevated at 22 months compared to younger ages both when associated with parenchymal astrocytes and vascular-associated astrocytes. Not only was the absolute number of Cx30 plaques significantly higher (P<0.05) but the size of the plaques was significantly larger at 22 months compared to younger ages (p<0.05). With age, Cx26 increased significantly initially, but returned to basal levels; whereas Cx43 expression remained low and stable with age. Evidence that astrocytes alter connexin compositions of gap junctions was demonstrated by the significant increase in the number of Cx26/Cx45 gap junctions with age. We also found gap junctions comprised of 1, 2, 3 or 4 Cx proteins suggesting that retinal astrocytes use various connexin protein combinations in their gap junctions during development and aging. These data provides new insight into the dynamic and extensive Cx network utilized by retinal astrocytes for communication within both the parenchyma and vasculature for the maintenance of normal retinal physiology with age. This characterisation of the changes in astrocytic gap junctional communication with age in the CNS is crucial to the understanding of physiological aging and age-related neurodegenerative diseases.

Connexin45 regulates endothelial-induced mesenchymal cell differentiation toward a mural cell phenotype

OBJECTIVE

The focus of this study was to investigate the role of connexin (Cx) 45 in endothelial-induced mural cell differentiation.

METHODS AND RESULTS

We created mural cell precursors that stably express only Cx45 in Cx43-deficient mesenchymal cells (ReCx45), and used our in vitro model of blood vessel assembly to assess the capacity of this Cx to support endothelial-induced mural cell differentiation. Lucifer Yellow dye injection and dual whole-cell patch clamping revealed that functional gap junctions exhibiting properties of Cx45-containing channels formed among ReCx45 transfectants, and between ReCx45 and endothelial cells. Heterocellular Cx45-containing gap junction channels enabled transforming growth factor-β activation and promoted the upregulation of mural cell-specific proteins in the mesenchymal precursors.

CONCLUSIONS

These studies reveal a critical role for Cx45 in the regulation of endothelial-induced mural cell differentiation, which is consistent with the phenotype of Cx45-deficient embryos that exhibit dysregulated transforming growth factor-β and lack mural cell development.

The role of gap junction channels during physiologic and pathologic conditions of the human central nervous system

Gap junctions (GJs) are expressed in most cell types of the nervous system, including neuronal stem cells, neurons, astrocytes, oligodendrocytes, cells of the blood brain barrier (endothelial cells and astrocytes) and under inflammatory conditions in microglia/macrophages. GJs connect cells by the docking of two hemichannels, one from each cell with each hemichannel being formed by 6 proteins named connexins (Cx). Unapposed hemichannels (uHC) also can be open on the surface of the cells allowing the release of different intracellular factors to the extracellular space. GJs provide a mechanism of cell-to-cell communication between adjacent cells that enables the direct exchange of intracellular messengers, such as calcium, nucleotides, IP(3), and diverse metabolites, as well as electrical signals that ultimately coordinate tissue homeostasis, proliferation, differentiation, metabolism, cell survival and death. Despite their essential functions in physiological conditions, relatively little is known about the role of GJs and uHC in human diseases, especially within the nervous system. The focus of this review is to summarize recent findings related to the role of GJs and uHC in physiologic and pathologic conditions of the central nervous system.

Connexin45 is expressed in vascular smooth muscle but its function remains elusive

Connexins (Cx) form gap junctions and allow the coordination of cellular behaviour. In vessels, expression of Cx40, Cx37, and Cx43 is well established and specifically Cx40 serves important functions in endothelial cells. In contrast, expression and physiological functions of Cx45 is unclear although its expression has been suggested in vascular smooth muscle (VSM). Therefore, we studied expression and function of Cx45 in vessels using different mice models allowing to identify and delete Cx45. Smooth muscle cell (SMC)-specific deletion was achieved by the Cre/loxP system using Cre-recombinase driven by a Nestin promoter. Deletion of Cx45 leads concomitantly to the expression of enhanced green fluorescence protein (EGFP) in these mice. Conduction of vasomotor responses was studied in cremasteric arterioles using intravital microscopy and arterial pressure was measured telemetrically. Cx45 is transcriptionally expressed in VSM as detected by EGFP expression in SMC-specific Cx45-deficient mice (Cx45fl/fl:Nestin-Cre) but not in endothelial cells (Cx45fl/fl:TIE2-Cre). Moreover, EGFP was located at VSM cell borders in arterioles of transgenic mice carrying an EGFP-tagged Cx45. Expectedly, arteriolar conduction of dilations evoked by the endothelium-dependent agonist acetylcholine were not different between Cx45fl/fl:Nestin-Cre mice and controls carrying homozygously a floxed Cx45 gene (Cx45fl/fl). Surprisingly, the amplitude of locally initiated endothelium-independent constrictions (K⁺) and dilations (adenosine) declined similarly with distance in both genotypes indicating an intact VSM conduction pathway also in mice being deficient for Cx45 in VSM. Arterial pressure was not different between freely moving Cx45fl/fl and Cx45fl/fl:Nestin-Cre mice during day or night. We conclude that Cx45 is physiologically expressed in VSM, but not in EC in murine arterioles. However, Cx45 is dispensable for the conduction of vasomotor responses along these arterioles. Possibly, other Cx functionally replace the lack of Cx45 in VSM. The reported role of Cx45 in renin secretion does not seem to alter arterial pressure in freely moving mice.

Electrical transmission between mammalian neurons is supported by a small fraction of gap junction channels

Electrical synapses formed by gap junctions between neurons create networks of electrically coupled neurons in the mammalian brain, where these networks have been found to play important functional roles. In most cases, interneuronal gap junctions occur at remote dendro-dendritic contacts, making difficult accurate characterization of their physiological properties and correlation of these properties with their anatomical and morphological features of the gap junctions. In the mesencephalic trigeminal (MesV) nucleus where neurons are readily accessible for paired electrophysiological recordings in brain stem slices, our recent data indicate that electrical transmission between MesV neurons is mediated by connexin36 (Cx36)-containing gap junctions located at somato-somatic contacts. We here review evidence indicating that electrical transmission between these neurons is supported by a very small fraction of the gap junction channels present at cell-cell contacts. Acquisition of this evidence was enabled by the unprecedented experimental access of electrical synapses between MesV neurons, which allowed estimation of the average number of open channels mediating electrical coupling in relation to the average number of gap junction channels present at these contacts. Our results indicate that only a small proportion of channels (~0.1 %) appear to be conductive. On the basis of similarities with other preparations, we postulate that this phenomenon might constitute a general property of vertebrate electrical synapses, reflecting essential aspects of gap junction function and maintenance.

Modulation of metabolic communication through gap junction channels by transjunctional voltage; synergistic and antagonistic effects of gating and ionophoresis

Gap junction (GJ) channels assembled from connexin (Cx) proteins provide a structural basis for direct electrical and metabolic cell-cell communication. Here, we focus on gating and permeability properties of Cx43/Cx45 heterotypic GJs exhibiting asymmetries of both voltage-gating and transjunctional flux (J(j)) of fluorescent dyes depending on transjunctional voltage (V(j)). Relatively small differences in the resting potential of communicating cells can substantially reduce or enhance this flux at relative negativity or positivity on Cx45 side, respectively. Similarly, series of V(j) pulses resembling bursts of action potentials (APs) reduce J(j) when APs initiate in the cell expressing Cx43 and increase J(j) when APs initiate in the cell expressing Cx45. J(j) of charged fluorescent dyes is affected by ionophoresis and V(j)-gating and the asymmetry of J(j)-V(j) dependence in heterotypic GJs is enhanced or reduced when ionophoresis and V(j)-gating work in a synergistic or antagonistic manner, respectively. Modulation of cell-to-cell transfer of metabolites and signaling molecules by V(j) may occur in excitable as well as non-excitable tissues and may be more expressed in the border between normal and pathological regions where intercellular gradients of membrane potential and concentration of ions are substantially altered. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Cerebrovascular connexin expression: effects of traumatic brain injury

Traumatic brain injury (TBI) results in dysfunction of the cerebrovasculature. Gap junctions coordinate vasomotor responses and evidence suggests that they are involved in cerebrovascular dysfunction after TBI. Gap junctions are comprised of connexin proteins (Cxs), of which Cx37, Cx40, Cx43, and Cx45 are expressed in vascular tissue. This study tests the hypothesis that TBI alters Cx mRNA and protein expression in cerebral vascular smooth muscle and endothelial cells. Anesthetized (1.5% isoflurane) male Sprague-Dawley rats received sham or fluid-percussion TBI. Two, 6, and 24 h after, cerebral arteries were harvested, fresh-frozen for RNA isolation, or homogenized for Western blot analysis. Cerebral vascular endothelial and smooth muscle cells were selected from frozen sections using laser capture microdissection. RNA was quantified by ribonuclease protection assay. The mRNA for all four Cx genes showed greater expression in the smooth muscle layer compared to the endothelial layer. Smooth muscle Cx43 mRNA expression was reduced 2  h and endothelial Cx45 mRNA expression was reduced 24  h after injury. Western blot analysis revealed that Cx40 protein expression increased, while Cx45 protein expression decreased 24  h after injury. These studies revealed significant changes in the mRNA and protein expression of specific vascular Cxs after TBI. This is the first demonstration of cell type-related differential expression of Cx mRNA in cerebral arteries, and is a first step in evaluating the effects of TBI on gap junction communication in the cerebrovasculature.

CXCL12 secretion by bone marrow stromal cells is dependent on cell contact and mediated by connexin-43 and connexin-45 gap junctions

The chemokine CXCL12 is essential for the function of hematopoietic stem and progenitor cells. Here we report that secretion of functional CXCL12 from human bone marrow stromal cells (BMSCs) was a cell contact-dependent event mediated by connexin-43 (Cx43) and Cx45 gap junctions. Inhibition of connexin gap junctions impaired the secretion of CXCL12 and homing of leukocytes to mouse bone marrow. Purified human CD34(+) progenitor cells did not adhere to noncontacting BMSCs, which led to a much smaller pool of immature cells. Calcium conduction activated signaling by cAMP-protein kinase A (PKA) and induced CXCL12 secretion mediated by the GTPase RalA. Cx43 and Cx45 additionally controlled Cxcl12 transcription by regulating the nuclear localization of the transcription factor Sp1. We suggest that BMSCs form a dynamic syncytium via connexin gap junctions that regulates CXC12 secretion and the homeostasis of hematopoietic stem cells.

pH-dependent modulation of voltage gating in connexin45 homotypic and connexin45/connexin43 heterotypic gap junctions

Intracellular pH (pH(i)) can change during physiological and pathological conditions causing significant changes of electrical and metabolic cell-cell communication through gap junction (GJ) channels. In HeLa cells expressing wild-type connexin45 (Cx45) as well as Cx45 and Cx43 tagged with EGFP, we examined how pH(i) affects junctional conductance (g(j)) and g(j) dependence on transjunctional voltage (V(j)). To characterize V(j) gating, we fit the g(j)-V(j) relation using a stochastic four-state model containing one V(j)-sensitive gate in each apposed hemichannel (aHC); aHC open probability was a Boltzmann function of the fraction of V(j) across it. Using the model, we estimated gating parameters characterizing sensitivity to V(j) and number of functional channels. In homotypic Cx45 and heterotypic Cx45/Cx43-EGFP GJs, pH(i) changes from 7.2 to approximately 8.0 shifted g(j)-V(j) dependence of Cx45 aHCs along the V(j) axis resulting in increased probability of GJ channels being in the fully open state without change in the slope of g(j) dependence on V(j). In contrast, acidification shifted g(j)-V(j) dependence in the opposite direction, reducing open probability; acidification also reduced the number of functional channels. Correlation between the number of channels in Cx45-EGFP GJs and maximal g(j) achieved under alkaline conditions showed that only approximately 4% of channels were functional. The acid dissociation constant (pK(a)) of g(j)-pH(i) dependence of Cx45/Cx45 GJs was approximately 7. The pK(a) of heterotypic Cx45/Cx43-EGFP GJs was lower, approximately 6.7, between the pK(a)s of Cx45 and Cx43-EGFP (approximately 6.5) homotypic GJs. In summary, pH(i) significantly modulates junctional conductance of Cx45 by affecting both V(j) gating and number of functional channels.

Connexins and the kidney

Connexins (Cxs) are widely-expressed proteins that form gap junctions in most organs, including the kidney. In the renal vasculature, Cx37, Cx40, Cx43, and Cx45 are expressed, with predominant expression of Cx40 in the endothelial cells and Cx45 in the vascular smooth muscle cells. In the tubules, there is morphological evidence for the presence of gap junction plaques only in the proximal tubules. In the distal nephron, Cx30, Cx30.3, and Cx37 are expressed, but it is not known whether they form gap junctions connecting neighboring cells or whether they primarily act as hemichannels. As in other systems, the major function of Cxs in the kidney appears to be intercellular communication, although they may also form hemichannels that allow cellular secretion of large signaling molecules. Renal Cxs facilitate vascular conduction, juxtaglomerular apparatus calcium signaling, and tubular purinergic signaling. Accordingly, current evidence points to roles for these Cxs in several important regulatory mechanisms in the kidney, including the renin angiotensin system, tubuloglomerular feedback, and salt and water reabsorption. At the systemic level, renal Cxs may help regulate blood pressure and may be involved in hypertension and diabetes.

Heterotypic gap junction channels as voltage-sensitive valves for intercellular signaling

Gap junction (GJ) channels assembled from connexin (Cx) proteins provide a structural basis for direct electrical and metabolic cell-cell communication. By combining fluorescence imaging and dual whole-cell voltage clamp methods, we demonstrate that in response to transjunctional voltage (Vj) Cx43/Cx45 heterotypic GJs exhibit both Vj-gating and dye transfer asymmetries. The later is affected by ionophoresis of charged fluorescent dyes and voltage-dependent gating. We demonstrate that small differences in resting (holding) potentials of communicating cells can fully block (at relative negativity on Cx45 side) or enhance (at relative positivity on Cx45 side) dye transfer. Similarly, series of high frequency Vj pulses resembling bursts of action potentials (APs) can fully block or increase the transjunctional flux (Jj) of dye depending on whether pulses are generated in the cell expressing Cx43 or Cx45, respectively. Asymmetry of Jj-Vj dependence is enhanced or reduced when ionophoresis and Vj-gating act synergistically or antagonistically, whereas single channel permeability (Pgamma) remains unaffected. This modulation of intercellular signaling by Vj can play a crucial role in many aspects of intercellular communication in the adult, in embryonic development, and in tissue regeneration.

Current perspective on differential communication in small resistance arteries

Blood flow is controlled by an integrated network of resistance arteries that are coupled in series and parallel with one another. To dramatically alter tissue perfusion as required during periods of high metabolic demand, arterial networks must dilate in a coordinated manner. Gap junctions facilitate arterial coordination by enabling electrical stimuli to conduct among endothelial and (or) smooth muscle cells. The goal of this review was to provide an introduction to the field of vascular communication, the process of intercellular conduction, and the manner in which key properties influence charge flow. After a brief historical introduction, we establish the idea that electrical stimuli conduct differentially among neighbouring endothelial and smooth muscle cells. Highlighting recent studies that have synergistically combined computational and experimental approaches, this perspective explores how specific structural, electrical, and gap junctional properties enable electrical phenomenon to conduct differentially. To close, the concept of differential communication is functionally integrated into a mechanistic understanding of blood flow control.

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.

Limited intravascular coupling in the rodent brainstem and retina supports a role for glia in regional blood flow

Regional synaptic activity induces local increases in perfusion that are coupled to upstream vasodilation and improved blood flow. In the cerebral circulation, it has been proposed that astrocytes mediate the link between the initiating stimulus and local vasodilation through propagated intracellular calcium waves. In the systemic circulation the mechanism by which local vasodilation triggers upstream alterations in blood flow involves electrotonic propagation of hyperpolarization via endothelial gap junctions, although less is known concerning the cerebral circulation. The present study aimed to investigate the extent of coupling in microvessels of the rodent brainstem and retina and the subtypes of intracellular calcium stores that might mediate astrocytic signaling. Within the brainstem, connexins (Cxs) 37 and 40 were restricted to the endothelium of pial vessels and larger penetrating arterioles, whereas astrocytic Cxs30 and 43 were found closely associated with pre- and postsynaptic neurons and nearby microvessels. Within the rat retina, Cxs37 and 40 were expressed in large radiating arterioles, but were not found in smaller vessels on the retinal surface or in the deeper retinal layers. These Cxs were absent from all retinal vessels in mice. Astrocytes, expressing Cxs30 and 43 in the rat, but only Cx43 in the mouse, were found closely associated with superficial, but not deeper blood vessels. Inositol-trisphosphate receptors (IP(3)R) 1 and 2 were expressed within brainstem astrocytes, whereas IP(3)R1 and 3 were expressed within retinal astrocytes. Limited intravascular coupling and the proximity of astrocytic networks to blood vessels supports a role for glia in activity-dependent alterations in central blood flow.

Connexin45-containing neuronal gap junctions in rodent retina also contain connexin36 in both apposing hemiplaques, forming bihomotypic gap junctions, with scaffolding contributed by zonula occludens-1

Mammalian retinas contain abundant neuronal gap junctions, particularly in the inner plexiform layer (IPL), where the two principal neuronal connexin proteins are Cx36 and Cx45. Currently undetermined are coupling relationships between these connexins and whether both are expressed together or separately in a neuronal subtype-specific manner. Although Cx45-expressing neurons strongly couple with Cx36-expressing neurons, possibly via heterotypic gap junctions, Cx45 and Cx36 failed to form functional heterotypic channels in vitro. We now show that Cx36 and Cx45 coexpressed in HeLa cells were colocalized in immunofluorescent puncta between contacting cells, demonstrating targeting/scaffolding competence for both connexins in vitro. However, Cx36 and Cx45 expressed separately did not form immunofluorescent puncta containing both connexins, supporting lack of heterotypic coupling competence. In IPL, 87% of Cx45-immunofluorescent puncta were colocalized with Cx36, supporting either widespread heterotypic coupling or bihomotypic coupling. Ultrastructurally, Cx45 was detected in 9% of IPL gap junction hemiplaques, 90-100% of which also contained Cx36, demonstrating connexin coexpression and cotargeting in virtually all IPL neurons that express Cx45. Moreover, double replicas revealed both connexins in separate domains mirrored on both sides of matched hemiplaques. With previous evidence that Cx36 interacts with PDZ1 domain of zonula occludens-1 (ZO-1), we show that Cx45 interacts with PDZ2 domain of ZO-1, and that Cx36, Cx45, and ZO-1 coimmunoprecipitate, suggesting that ZO-1 provides for coscaffolding of Cx45 with Cx36. These data document that in Cx45-expressing neurons of IPL, Cx45 is almost always accompanied by Cx36, forming "bihomotypic" gap junctions, with Cx45 structurally coupling to Cx45 and Cx36 coupling to Cx36.

Gap junction inhibitors reduce endothelium-dependent contractions in the aorta of spontaneously hypertensive rats

Experiments were designed to determine the effect of gap junction inhibitors on endothelium-dependent contractions. Isolated aortic rings of spontaneously hypertensive rats (SHR) were suspended in vitro for isometric force recording. The nonselective gap junction inhibitor, carbenoxolone, reduced endothelium-dependent contractions to acetylcholine and the calcium ionophore A23187 [5-methylamino-2-(2S,3R,5R,8S,9S)-3,5,9-trimethyl-2-(1-oxo-(1H-pyrrol-2-yl)propan-2-yl)-1,7-dioxaspiro-(5,5)undecan-8-yl)methyl)benzooxazole-4-carboxylic acid]. There was no or modest effect of the gap peptides (40)Gap27, (37,43)Gap27, or (43)Gap26 when applied alone on endothelium-dependent contractions. However, the combined treatment with the three gap peptides significantly decreased endothelium-dependent contractions. The combined inhibition of the three connexins was not as effective as carbenoxolone, suggesting the involvement of other connexins in the process of endothelium-dependent contraction. The present study shows the involvement of gap junctions in endothelium-dependent contractions of the SHR aorta, presumably that of the combination of connexins 37, 40, and 43 rather than a single subtype of these proteins. Contractions of the vascular smooth muscle caused by 9,11-dideoxy-11alpha, 9alpha-epoxymethanoprostaglandin F(2alpha) (U46619) and prostacyclin, but not to those of endoperoxides and phenylephrine, were reduced only minimally by carbenoxolone. Thus, if gap junction signaling is involved in the contraction of the vascular smooth muscle to thromboxane-prostanoid receptor agonists, their contribution is small. This suggests that the reduction of endothelium-dependent contractions by carbenoxolone and the gap peptides cannot be attributed to the homocellular gap junctions between vascular smooth muscle, but is more likely to involve the homocellular gap junctions between endothelial cells and/or myoendothelial gap junctions.

Connexin45 is expressed in the juxtaglomerular apparatus and is involved in the regulation of renin secretion and blood pressure

Connexin (Cx) proteins are known to play a role in cell-to-cell communication via intercellular gap junction channels or transiently open hemichannels. Previous studies have identified several connexin isoforms in the juxtaglomerular apparatus (JGA), but the vascular connexin isoform Cx45 has not yet been studied in this region. The present work aimed to identify in detail the localization of Cx45 in the JGA and to suggest a functional role for Cx45 in the kidney using conditions where Cx45 expression or function was altered. Using mice that express lacZ coding DNA under the control of the Cx45 promoter, we observed beta-galactosidase staining in cortical vasculature and glomeruli, with specific localization to the JGA region. Renal vascular localization of Cx45 was further confirmed with the use of conditional Cx45-deficient (Cx45fl/fl:Nestin-Cre) mice, which express enhanced green fluorescence protein (EGFP) instead of Cx45 only in cells that, during development, expressed the intermediate filament nestin. EGFP fluorescence was found in the afferent and efferent arteriole smooth muscle cells, in the renin-producing juxtaglomerular cells, and in the extra- and intraglomerular mesangium. Cx45fl/fl:Nestin-Cre mice exhibited increased renin expression and activity, as well as higher systemic blood pressure. The propagation of mechanically induced calcium waves was slower in cultured vascular smooth muscle cells (VSMCs) from Cx45fl/fl:Nestin-Cre mice and in control VSMC treated with a Cx45 gap mimetic peptide that inhibits Cx45 gap junctional communication. VSMCs allowed the cell-to-cell passage of the gap junction permeable dye Lucifer yellow, and calcium wave propagation was not altered by addition of the ATP receptor blocker suramin, suggesting that Cx45 regulates calcium wave propagation via direct gap junction coupling. In conclusion, the localization of Cx45 to the JGA and functional data from Cx45fl/fl:Nestin-Cre mice suggest that Cx45 is involved in the propagation of JGA vascular signals and in the regulation of renin release and blood pressure.

Targeting connexin 43 prevents platelet-derived growth factor-BB-induced phenotypic change in porcine coronary artery smooth muscle cells

We previously reported that reducing the expression of the gap junction protein connexin (Cx)43 in mice restricts intimal thickening formation after acute vascular injury by limiting the inflammatory response and the proliferation and migration of smooth muscle cells (SMCs) toward the damaged site. SMC populations isolated from porcine coronary artery exhibit distinct phenotypes: spindle-shaped (S) and rhomboid (R). S-SMCs are predominant in the normal media, whereas R-SMCs are recovered in higher proportion from stent-induced intimal thickening, suggesting that they participate in the restenotic process. Here, we further investigate the relationship between connexin expression and SMC phenotypes using porcine coronary artery SMCs. Cx40 was highly expressed in normal media of porcine coronary artery in vivo, whereas Cx43 was barely detectable. In contrast, Cx40 was downregulated and Cx43 was markedly upregulated in stent-induced intimal thickening. In vitro, S-SMCs expressed Cx40 and Cx43. In R-SMCs, Cx43 expression was increased and Cx40 was absent. We confirmed that S-SMCs treated with platelet-derived growth factor-BB acquire an R phenotype. This was accompanied by an upregulation of Cx43 and a loss of Cx40. Importantly, platelet-derived growth factor-BB-induced S-to-R phenotypic change was prevented by a reduction of Cx43 expression with antisense, ie, S-SMCs retained their typical elongated appearance and the expression of alpha-smooth muscle actin, a well-known SMC differentiation marker, whereas the expression of S100A4, a typical marker of R-SMCs, was prevented. In conclusion, limiting Cx43 expression in S-SMCs prevents platelet-derived growth factor-BB-induced S-to-R modulation. This suggests that Cx43 may be an additional target for local delivery strategies aimed at reducing restenosis.

Localization of heterotypic gap junctions composed of connexin45 and connexin36 in the rod pathway of the mouse retina

The primary rod pathway in mammals contains gap junctions between AII amacrine cells and ON cone bipolar cells which relay the rod signal into the cone pathway under scotopic conditions. Two gap junctional proteins, connexin36 (Cx36) and connexin45 (Cx45), appear to play a pivotal role in this pathway because lack of either protein leads to an impairment of visual transmission under scotopic conditions. To investigate whether these connexins form heterotypic gap junctions between ON cone bipolar and AII amacrine cells, we used newly developed Cx45 antibodies and studied the cellular and subcellular distribution of this protein in the mouse retina. Specificity of the Cx45 antibodies was determined, among others, by Western blot and immunostaining of mouse heart, where Cx45 is abundantly expressed. In mouse retina, Cx45 immunosignals were detected in both plexiform layers and the ganglion cell layer. Double staining for Cx45 and Cx36 revealed a partial overlap in the punctate patterns in the ON sublamina of the inner plexiform layer of the retina. We quantified the distributions of these two connexins in the ON sublamina, and detected 30% of the Cx45 signals to be co-localized with or in close apposition to Cx36 signals. Combining immunostaining and intracellular dye injection revealed an overlap or tight association of Cx36 and Cx45 signals on the terminals of injected AII amacrine and two types of ON cone bipolar cells. Our results provide direct evidence for heterotypic gap junctions composed of Cx36 and Cx45 between AII amacrine and certain types of ON cone bipolar cells.

Inward rectifying potassium channels facilitate cell-to-cell communication in hamster retractor muscle feed arteries

This study examined whether inward rectifying K+(KIR) channels facilitate cell-to-cell communication along skeletal muscle resistance arteries. With the use of feed arteries from the hamster retractor muscle, experiments examined whether KIRchannels were functionally expressed and whether channel blockade attenuated the conduction of acetylcholine-induced vasodilation, an index of cell-to-cell communication. Consistent with KIRchannel expression, this study observed the following: 1) a sustained Ba2+-sensitive, K+-induced dilation in preconstricted arteries; 2) a Ba2+-sensitive inwardly rectifying K+current in arterial smooth muscle cells; and 3) KIR2.1 and KIR2.2 expression in the smooth muscle layer of these arteries. It was subsequently shown that the discrete application of acetylcholine elicits a vasodilation that conducts with limited decay along the feed artery wall. In the presence of 100 μM Ba2+, the local and conducted response to acetylcholine was attenuated, a finding consistent with a role for KIRin facilitating cell-to-cell communication. A computational model of vascular communication accurately predicted these observations. Control experiments revealed that in contrast to Ba2+, ATP-sensitive- and large-conductance Ca2+activated-K+channel inhibitors had no effect on the local or conducted vasodilatory response to acetylcholine. We conclude that smooth muscle KIRchannels play a key role in facilitating cell-to-cell communication along skeletal muscle resistance arteries. We attribute this facilitation to the intrinsic property of negative slope conductance, a biophysical feature common to KIR2.1- and 2.2-containing channels, which enables them to increase their activity as a cell hyperpolarizes.

Microarray analysis of blood microvessels from PDGF-B and PDGF-Rbeta mutant mice identifies novel markers for brain pericytes

Normal blood microvessels are lined by pericytes, which contribute to microvessel development and stability through mechanisms that are poorly understood. Pericyte deficiency has been implicated in the pathogenesis of microvascular abnormalities associated with diabetes and tumors. However, the unambiguous identification of pericytes is still a problem because of cellular heterogeneity and few available molecular markers. Here we describe an approach to identify pericyte markers based on transcription profiling of pericyte-deficient brain microvessels isolated from platelet-derived growth factor (PDGF-B)-/- and PDGF beta receptor (PDGFRbeta)-/- mouse mutants. The approach was validated by the identification of known pericyte markers among the most down-regulated genes in PDGF-B-/- and PDGFRbeta-/- microvessels. Of candidates for novel pericyte markers, we selected ATP-sensitive potassium-channel Kir6.1 (also known as Kcnj8) and sulfonylurea receptor 2, (SUR2, also known as Abcc9), both part of the same channel complex, as well as delta homologue 1 (DLK1) for in situ hybridization, which demonstrated their specific expression in brain pericytes of mouse embryos. We also show that Kir6.1 is highly expressed in pericytes in brain but undetectable in pericytes in skin and heart. The three new brain pericyte markers are signaling molecules implicated in ion transport and intercellular signaling, potentially opening new windows on pericyte function in brain microvessels.

Experimental diabetes alters connexin43 derived gap junction permeability in short-term cultures of rat corporeal vascular smooth muscle cells

PURPOSE

Intercellular communication through gap junctions was assessed in 8 to 10-week STZ diabetic rats to evaluate diabetes related effects on gap junctional conductance and permeability in short-term cultures of corporeal myocytes.

MATERIALS AND METHODS

Rats were made diabetic by a single intraperitoneal injection of STZ. Eight to 10 weeks later erectile function was evaluated in vivo and corporeal tissue was harvested to isolate corporeal myocytes. Dual whole cell patch clamp studies of intercellular communication through connexin43 (Cx43) derived gap junction channels were done in short-term, ie passages 0 to 2, cultured corporeal myocytes excised from STZ diabetic rats with documented erectile impairment as well as in myocytes from age matched control rats.

RESULTS

No differences in macroscopic junctional conductance, single channel conductance or open probability were detected between myocytes from age matched control and STZ diabetic rats, confirming the lack of diabetes related alterations in Cx43 gating or conductance. However, fluorescence dye transfer experiments revealed a marked 3-fold increase in Cx43 mediated junctional permeability in the absence of any detectable change in Cx43 protein expression.

CONCLUSIONS

These data suggest that an alteration in the selectivity filter of Cx43 in diabetic animals affects the permeability of specifically sized and charged solutes. To our knowledge these studies provide the first evidence of a diabetes related increase in intercellular permselectivity in corporeal myocytes and, thus, they may have important implications for diabetes related erectile dysfunction.

Endothelium-dependent smooth muscle hyperpolarization: do gap junctions provide a unifying hypothesis?

An endothelium-derived hyperpolarizing factor (EDHF) that is distinct from nitric oxide (NO) and prostanoids has been widely hypothesized to hyperpolarize and relax vascular smooth muscle following stimulation of the endothelium by agonists. Candidates as diverse as K(+) ions, eicosanoids, hydrogen peroxide and C-type natriuretic peptide have been implicated as the putative mediator, but none has emerged as a 'universal EDHF'. An alternative explanation for the EDHF phenomenon is that direct intercellular communication via gap junctions allows passive spread of agonist-induced endothelial hyperpolarization through the vessel wall. In some arteries, eicosanoids and K(+) ions may themselves initiate a conducted endothelial hyperpolarization, thus suggesting that electrotonic signalling may represent a general mechanism through which the endothelium participates in the regulation of vascular tone.

Vascular gap junctions and implications for hypertension

Four connexin (Cx) molecules, namely Cx37, Cx40, Cx43 and Cx45, are expressed in the gap junctions that exist within and between the cellular layers of arteries. Endothelial cells are well coupled by large gap junctions expressing Cx37, Cx40 and, to a lesser extent, Cx43, whose expression may be more subject to regulation by physical factors. Smooth muscle cells are more heterogeneously coupled by gap junctions that are small and rare. The identity of the Cx expressed in the media may vary among different arteries. Myoendothelial gap junctions are small and more common in resistance arteries with fewer layers of smooth muscle cells. Given the small size of these gap junctions and the rapid turnover rate of Cxs, homocellular coupling in the media and heterocellular coupling between the cell layers may be subject to more dynamic control than coupling in the endothelium. Vascular gap junctions have been implicated in a number of vasomotor responses that may regulate vascular tone and blood pressure. These include the mechanism of action of the vasodilator, endothelium-derived hyperpolarizing factor (EDHF), the myogenic constriction to intramural pressure increase, the spontaneous or agonist-induced vasomotion of arteries and arterioles and the spreading vasodilation and constriction observed in microcirculatory networks. Few data are available on Cx expression in the media of resistance arteries during hypertension. Changes in the expression of Cx43 described in the media of the aorta of hypertensive rats vary with the hypertensive model studied and are likely to represent adaptations to structural changes in the vascular wall. In contrast, in the endothelium of the caudal and mesenteric arteries of spontaneously hypertensive rats, expression of Cxs is significantly decreased compared with arteries from normotensive rats and this decrease is reversed by inhibitors of the renin-angiotensin system. During hypertension, the activity of EDHF is decreased in the mesenteric artery, but this occurs much later than the initial increase in blood pressure and the decrease in endothelial Cxs, suggesting that changes in EDHF may not be causally related to hypertension or to the changes in endothelial Cxs. Upregulation of the myogenic response and the incidence of vasomotion has been reported in hypertension. Little is currently known of the effects of hypertension on spreading vasomotor responses. Deletion of specific Cxs in genetically modified mice is complicated by neonatal lethality or coordinate regulation and compensatory changes in the remaining Cxs. Nevertheless, mice in which Cx40 has been deleted are hypertensive and spreading vasodilatory responses are significantly impaired. Determination of a role for specific Cxs in the control of blood pressure must await the development of animals in which Cx expression can be modulated in a more complex temporal and tissue-specific manner.

Hypertension attenuates cell-to-cell communication in hamster retractor muscle feed arteries

This study examined whether hypertension attenuated cell-to-cell communication in skeletal muscle resistance arteries. Briefly, arteries feeding the retractor muscle of normotensive and hypertensive hamsters were cannulated, pressurized, and superfused with a physiological saline solution. Cell-to-cell communication was functionally assessed by application of vasoactive stimuli (via micropipette) to a small portion of a feed artery while diameter at sites distal to the point of agent application was monitored. In keeping with past observations, discrete application of a smooth muscle depolarizing agent (phenylephrine or KCl) elicited a localized vasoconstriction that conducted poorly along feed arteries from normotensive hamsters. In contrast, acetylcholine, an agent known to hyperpolarize endothelial cells, elicited a vasodilation in normotensive feed arteries that conducted with little decay. Whereas smooth muscle depolarizing agents continued to elicit a localized response, conduction of endothelium-dependent vasodilation was attenuated in hypertensive hamsters. This decrease occurred in the absence of changes in vessel reactivity to intravascular pressure or to global application of phenylephrine, U-46619, or acetylcholine. We propose, on the basis of these physiological observations, quantitative mRNA measurements of connexins 37, 40, 43, and 45, and analysis of the literature, that an increase in endothelial-to-endothelial or smooth muscle-to-endothelial coupling resistance is likely responsible for hypertension-induced impairment in vascular communication. We hypothesize that this attenuation could contribute to the rise in total peripheral resistance characteristically observed in hypertension.

Unique distributions of the gap junction proteins connexin29, connexin32, and connexin47 in oligodendrocytes

Oligodendrocytes of adult rodents express three different connexins: connexin29 (Cx29), Cx32, and Cx47. In this study, we show that Cx29 is localized to the inner membrane of small myelin sheaths, whereas Cx32 is localized on the outer membrane of large myelin sheaths; Cx29 does not colocalize with Cx32 in gap junction plaques. All oligodendrocytes appear to express Cx47, which is largely restricted to their perikarya. Cx32 and Cx47 are colocalized in many gap junction plaques on oligodendrocyte somata, particularly in gray matter. Cx45 is detected in the cerebral vasculature, but not in oligodendrocytes or myelin sheaths. This diversity of connexins in oligodendrocytes (in different populations of cells and in different subcellular compartments) likely reflects functional differences between these connexins and perhaps the oligodendrocytes themselves.

Cellular expression of connexins in the rat brain: neuronal localization, effects of kainate-induced seizures and expression in apoptotic neuronal cells

The identification of connexins (Cxs) expressed in neuronal cells represents a crucial step for understanding the direct communication between neurons and between neuron and glia. In the present work, using a double-labelling method combining in situ hybridization for Cx mRNAs with immunohistochemical detection for neuronal markers, we provide evidence that, among cerebral connexins (Cx26, Cx32, Cx36, Cx37, Cx40, Cx43, Cx45 and Cx47), only Cx45 and Cx36 mRNAs are localized in neuronal cells in both developing and adult rat brain. In order to establish whether connexin expression is influenced in vivo by abnormal neuronal activity, we examined the short-term effects of kainate-induced seizures. The results revealed an unexpected expression of Cx26 and Cx45 mRNA in neuronal cells undergoing apoptotic cell death in the CA3-CA4, in the hilus of the hippocampus and in other brain regions involved in seizure-induced lesion. However, the expression of Cx26 and Cx45 mRNAs was not associated with detectable expression of corresponding proteins as evaluated by immunohistochemistry with specific antibodies. Moreover, in the same brain regions Cx32 and Cx43 were up-regulated in non-neruronal cells whereas the neuronal Cx36 was down-regulated. Taken together the present results provide novel information regarding the specific subpopulation of neurons expressing Cx45 and raise the question of the meaning of connexin mRNA expression in the neuronal apoptotic process.

Essential role of Gap junctions in NO- and prostanoid-independent relaxations evoked by acetylcholine in rabbit intracerebral arteries

BACKGROUND AND PURPOSE

Direct intercellular communication via gap junctions may play a central role in endothelium-dependent relaxations that are mediated by a conducted hyperpolarization and do not involve the synthesis of NO and prostanoids. In the present study, inhibitory peptides homologous to the Gap27 domain of the second extracellular loop of connexin37/connexin43 and connexin40, designated as 37,43Gap27 and 40Gap27, respectively, were used to evaluate the role of this mechanism in intracerebral arteries.

METHODS

Isolated rings of rabbit middle cerebral artery were constricted by histamine (10 micromol/L) in the presence of N(G)-nitro-L-arginine methyl ester (300 micromol/L) and indomethacin (10 micromol/L). Concentration-relaxation curves for acetylcholine were constructed in the presence and absence of 37,43Gap27 and 40Gap27. Specific antibodies were used to delineate the distribution of connexin37, connexin40, connexin43, and connexin45 within the arterial wall.

RESULTS

Individually, 37,43Gap27 and 40Gap27 minimally affected endothelium-dependent relaxations to acetylcholine at concentrations of 300 micro mol/L, whereas their combination (at 300 micromol/L each) inhibited the maximal response by approximately 70% and increased the EC50 value for relaxation by approximately 15-fold. In endothelium-denuded rings, this peptide combination did not attenuate responses to sodium nitroprusside, an exogenous source of NO. Gap junction plaques, whose incidence was highest in endothelium, were constructed from connexin40 and connexin43 in the media and connexin37, connexin40, and connexin43 in the endothelium.

CONCLUSIONS

The findings confirm that direct communication via gap junctions contributes to agonist-induced relaxations of intracerebral arteries. More than one connexin subtype appears to participate in such responses.

Inhibitors of gap junctions attenuate myogenic tone in cerebral arteries

The effects of two structurally distinct inhibitors of gap junction communication were studied by using three different forms of vasoconstriction in pressurized rat middle cerebral arteries. The sensitivity of myogenic tone (at 60 mmHg), vasopressin-induced tone (10 nM, at 20 mmHg), and depolarizing solution-induced tone (80 mM K(+), at 20 mmHg) to inhibition by heptanol (1.0 microM to 3.0 mM) or 18alpha-glycyrrhetinic acid (18alpha-GA, 1.0 to 50 microM) were determined. Pressure-induced myogenic tone was inhibited by heptanol (IC(50) = 0.75 +/- 0.09 mM) and 18alpha-GA ( approximately 30 microM). Vasopressin-induced vasoconstriction was also inhibited by heptanol (IC(50) = 0.4 +/- 0.3 mM) and 18alpha-GA (>1 microM). Depolarizing solution-induced vasoconstriction was less sensitive to inhibition by heptanol compared to vasopressin (P < 0.01) or pressure-induced constriction (P < 0.05). However, 18alpha-GA did not inhibit depolarization-induced constriction. Sharp microelectrode experiments on isolated arteries revealed stable membrane potentials, with no detectable effect of heptanol (1 mM) or 18alpha-GA (20-30 microM) on the average membrane potential at 20 mmHg. However, approximately 20% of impaled cells (5 of 28) exhibited uncharacteristic oscillations in membrane potential after pharmacological uncoupling. At 60 mmHg a approximately 7- to 9-mV hyperpolarization and corresponding vasodilation (approximately 50%) was observed, and the frequency of membrane potential oscillations doubled (9 of 23 cells). These data indicate that gap junctions play an important role in the maintenance and modulation of membrane potential and tone in cerebral resistance arteries.

Increase in Cx45 gap junction channels in cerebral smooth muscle cells from SHR

We recently reported the novel finding of expression and function of connexin45 (Cx45) in cerebrovascular smooth muscle cells. We examined the hypothesis that Cx45 is altered in hypertension. Immunoblots for Cx45 showed a significant increase in Cx45 in cerebral arteries from adult spontaneously hypertensive rats (SHR) compared with adult Wistar-Kyoto (WKY) rats, with no difference in aorta or femoral artery. Patch-clamp of cerebral smooth muscle cells pairs from SHR versus WKY showed a significantly steeper voltage dependence of deactivation and partial block of junctional currents by quinine and by a peptide that interferes with docking of Cx45, consistent with dominance of functional Cx45 channels in SHR. We examined potential roles of blood pressure versus angiotensin in elevated Cx45 in SHR by measuring Cx45 protein in 4 groups: (1) long-term administration in Wistar rats of the nitric oxide synthase inhibitor L-NAME; (2) long-term administration in SHR of the ACE inhibitor captopril; (3) long-term administration in Wistar rats of angiotensin; and (4) exposure of basilar artery segments in organ culture to angiotensin. Blood pressure was significantly elevated in groups 1 and 3 and was normal in group 2. In groups 1, 2, and 4, there was no significant change in Cx45 protein. In group 3, there was a modest but insignificant increase in Cx45 protein but no change in voltage dependence of deactivation of junctional currents. Overall, our data show increased Cx45 in SHR that is unlikely to be due to either elevated blood pressure or to angiotensin. Relative dominance of Cx45 over Cx43 in cerebral vessels may predispose SHR to ischemic stroke.

Connexin37 is the major connexin expressed in the media of caudal artery

OBJECTIVE

To determine the connexins (Cxs) involved in intercellular coupling within vascular muscle, the present study has quantified mRNA and protein expression for Cx37, Cx40, Cx43, and Cx45 in the caudal artery (CA) and thoracic aorta (ThA) of the rat.

METHODS AND RESULTS

Real-time polymerase chain reaction and immunohistochemistry identified Cx37 as the most abundantly expressed Cx in the CA, with fine punctate staining observed in the media. Conversely, mRNA for Cx43 was 40-fold greater in the ThA than in the CA, with punctate staining in the endothelium and media of the ThA but confined to the endothelium in the CA. Western blotting confirmed the differences in the relative amounts of Cx43 between the 2 vessels. For both arteries, Cx45 was expressed to a lesser degree in the media but not in the endothelium, whereas Cx40 was found only in the endothelium. Cx37, Cx40, and Cx43 were expressed in the endothelium of both vessels, although the density of Cx40 plaques was significantly greater in the CA.

CONCLUSIONS

The demonstration of Cx37 as the dominant Cx in the media of the CA highlights the potential heterogeneity in Cx involvement in vascular smooth muscle.

ADP-induced pial arteriolar dilation in ovariectomized rats involves gap junctional communication

It was previously shown that, despite the loss of nitric oxide (NO) dependence, ADP-induced pial arteriolar dilation was not attenuated in estrogen-depleted [i.e., ovariectomized (Ovx)] rats. Additional evidence suggested that the NO was replaced by an endothelium-dependent hyperpolarizing factor (EDHF)-like mechanism. To further characterize the nascent EDHF role in Ovx females, the current study was undertaken to test whether, in Ovx rats, ADP-induced pial arteriolar dilation retained its endothelial dependence and whether gap junctions are involved in that response. A closed cranial window and intravital microscopy system was used to monitor pial arteriolar diameter changes in anesthetized rats. The endothelial portion of the ADP-induced dilation was evaluated using light dye endothelial injury (L/D). The study was organized around three experimental approaches. First, the responses of pial arterioles to ADP before and after L/D exposure in intact and Ovx female rats were tested. L/D reduced the ADP response by 50-70% in both groups, thereby indicating that the endothelium dependence of ADP-induced vasodilation is not altered by chronic estrogen depletion. Second, the NO synthase inhibitor N(omega)-nitro-L-arginine (L-NNA) and the prostanoid synthesis inhibitor indomethacin (Indo) were coapplied. In intact females, L-NNA-Indo attenuated the response to ADP by 50%, with no further changes upon the addition of L/D. On the other hand, L-NNA-Indo did not affect ADP reactivity in Ovx rats, but subsequent L/D exposure reduced the ADP response by >50%. The NO-prostanoid-independent, but endothelium-dependent, nature of the response in Ovx females is a hallmark of EDHF participation. Third, gap junctional inhibition strategies were applied. A selective inhibitor of gap junctional function, Gap 27, did not affect ADP reactivity in intact females but reduced the the ADP response by 50% in Ovx females. A similar result was obtained following application of a connexin43 antisense oligonucleotide. These findings suggest that the nascent EDHF dependency of ADP-induced pial arteriolar dilation in Ovx females involves connexin43-related gap junctional communication.