The gap junction cellular internet: connexin hemichannels enter the signalling limelight

Function of connexins in the renal circulation

Connexins form intercellular channels that span two plasma membranes and directly couple the cytoplasm of adjacent cells. This morphological contact enables the exchange of ions, second messengers, and metabolites, which act to regulate several biological functions. This review focuses on the significance of connexins in the renal circulation. Cells of the renal vasculature are coupled and express connexins in a vessel and cell-specific pattern. This finding indicates that renal connexins likely play an important role in renal autoregulatory mechanisms (Bayliss effect, tubuloglomerular feedback) and in the control of vasomotor responses. The described coupling of endothelial and vascular smooth muscle cells in the afferent arterioles may also contribute to the communication of neighboring nephrons, called 'nephron coupling.' Furthermore, deletion of the Cx40 and Cx43 genes results in an altered functional behavior of the renin-producing cells, suggesting involvement of these connexin isoforms in the regulation of renin secretion and synthesis. In addition, this review discusses the role of renal connexin expression in the pathogenesis of hypertension or diabetes.

The conditional connexin43G138R mouse mutant represents a new model of hereditary oculodentodigital dysplasia in humans

Oculodentodigital dysplasia (ODDD) is a dominant negatively inherited disorder with variable but characteristic anomalies of the fingers and toes, eyes, face and teeth, which are caused by mutations in the connexin 43 (Cx43) gene. All mutations analyzed so far have a negative influence on the conductance through gap junctional channels and hemichannels, as well as trafficking of Cx43 protein in transfected cells. In this study, we inserted the human Cx43G138R point mutation into the mouse Cx43 gene and generated mice conditionally expressing this mutation. All ODDD phenotypic manifestations observed in humans, including syndactyly and enamel hypoplasia as well as craniofacial, bone and heart anomalies, were also observed with significant penetrance in Cx43G138R mice. When this mutation was specifically expressed in cardiomyocytes, characteristic alterations in the electrocardiogram and spontaneous arrhythmias were recorded. In vitro studies with Cx43G138R-expressing cells revealed loss of the Cx43 P2 phosphorylation state, which was also absent in the mutated hearts. This loss has previously been associated with gap junctional dysfunction and increased cellular ATP release. The Cx43G138R mutated mice show significantly increased arrhythmogeneity ex vivo in Langendorff experiments with explanted hearts and in vivo in particular under hypoxic conditions. Our results suggest that the increased activity of ATP-releasing channels in Cx43G138R mutated cardiomyocytes may further reduce the already decreased gap junctional communication and thus aggravate arrhythmogenesis in the mouse mutant.

Gap junctions: basic structure and function

Gap junctions allow the exchange of ions, second messengers, and small metabolites between adjacent cells and are formed by two unrelated protein families, the pannexins and connexins. Mutations in connexin genes cause a variety of genetic disorders, implicating a critical role in tissue homeostasis. Association of congenital skin disorders to mutations in different connexins has underscored the importance of gap junctional communication in the skin and its appendages. Here, we discuss the basic structure of gap junction channels and the function of connexin genes that have been associated with human disorders to explore the physiology of intercellular communication in skin.

Pannexin 1 and pannexin 3 are glycoproteins that exhibit many distinct characteristics from the connexin family of gap junction proteins

Pannexins are mammalian orthologs of the invertebrate gap junction proteins innexins and thus have been proposed to play a role in gap junctional intercellular communication. Localization of exogenously expressed pannexin 1 (Panx1) and pannexin 3 (Panx3), together with pharmacological studies, revealed a cell surface distribution profile and life cycle dynamics that were distinct from connexin 43 (Cx43, encoded by Gja1). Furthermore, N-glycosidase treatment showed that both Panx1 (approximately 41-48 kD species) and Panx3 (approximately 43 kD) were glycosylated, whereas N-linked glycosylation-defective mutants exhibited a decreased ability to be transported to the cell surface. Tissue surveys revealed the expression of Panx1 in several murine tissues--including in cartilage, skin, spleen and brain--whereas Panx3 expression was prevalent in skin and cartilage with a second higher-molecular-weight species present in a broad range of tissues. Tissue-specific localization patterns of Panx1 and Panx3 ranging from distinct cell surface clusters to intracellular profiles were revealed by immunostaining of skin and spleen sections. Finally, functional assays in cultured cells transiently expressing Panx1 and Panx3 were incapable of forming intercellular channels, but assembled into functional cell surface channels. Collectively, these studies show that Panx1 and Panx3 have many characteristics that are distinct from Cx43 and that these proteins probably play an important biological role as single membrane channels.

Renin Release

The aspartyl-protease renin is the key regulator of the renin-angiotensin-aldosterone system, which is critically involved in salt, volume, and blood pressure homeostasis of the body. Renin is mainly produced and released into circulation by the so-called juxtaglomerular epithelioid cells, located in the walls of renal afferent arterioles at the entrance of the glomerular capillary network. It has been known for a long time that renin synthesis and secretion are stimulated by the sympathetic nerves and the prostaglandins and are inhibited in negative feedback loops by angiotensin II, high blood pressure, salt, and volume overload. In contrast, the events controlling the function of renin-secreting cells at the organ and cellular level are markedly less clear and remain mysterious in certain aspects. The unravelling of these mysteries has led to new and interesting insights into the process of renin release.

Purification and initiation of structural characterization of human peripheral myelin protein 22, an integral membrane protein linked to peripheral neuropathies

Gene duplications, deletions, and point mutations in peripheral myelin protein 22 (PMP22) are linked to several inherited peripheral neuropathies. However, the structural and biochemical properties of this very hydrophobic putative tetraspan integral membrane protein have received little attention, in part because of difficulties in obtaining milligram quantities of wild type and disease-linked mutant forms of the protein. In this study a fusion protein was constructed consisting of a fragment of lambda repressor, a decahistidine tag, an intervening TEV protease cleavage site, a Strep tag, and the human PMP22 sequence. This fusion protein was expressed in Escherichia coli at a level of 10-20 mg/L of protein. Following TEV cleavage of the fusion partner, PMP22 was purified and its structural properties were examined in several different types of detergent micelles using cross-linking, near and far-UV circular dichroism, and nuclear magnetic resonance (NMR) spectroscopy. PMP22 is highly helical and, in certain detergents, shows evidence of stable tertiary structure. The protein exhibits a strong tendency to dimerize. The 1H-15N TROSY NMR spectrum is well dispersed and contains signals from all regions of the protein. It appears that detergent-solubilized PMP22 is amenable to detailed structural characterization via crystallography or NMR. This work sets the stage for more detailed studies of the structure, folding, and misfolding of wild type and disease-linked mutants in order to unravel the molecular defects underlying peripheral neuropathies.

Bench to bedside and back again: molecular mechanisms of alpha-catenin function and roles in tumorigenesis

The cadherin/catenin complex, comprised of E-cadherin, beta-catenin and alpha-catenin, is essential for initiating cell-cell adhesion, establishing cellular polarity and maintaining tissue organization. Disruption or loss of the cadherin/catenin complex is common in cancer. As the primary cell-cell adhesion protein in epithelial cells, E-cadherin has long been studied in cancer progression. Similarly, additional roles for beta-catenin in the Wnt signaling pathway has led to many studies of the role of beta-catenin in cancer. Alpha-catenin, in contrast, has received less attention. However, recent data demonstrate novel functions for alpha-catenin in regulating the actin cytoskeleton and cell-cell adhesion, which when perturbed could contribute to cancer progression. In this review, we use cancer data to evaluate molecular models of alpha-catenin function, from the canonical role of alpha-catenin in cell-cell adhesion to non-canonical roles identified following conditional alpha-catenin deletion. This analysis identifies alpha-catenin as a prognostic factor in cancer progression.

Modulation of membrane channel currents by gap junction protein mimetic peptides: size matters

Connexin mimetic peptides are widely used to assess the contribution of nonjunctional connexin channels in several processes, including ATP release. These peptides are derived from various connexin sequences and have been shown to attenuate processes downstream of the putative channel activity. Yet so far, no documentation of effects of peptides on connexin channels has been presented. We tested several connexin and pannexin mimetic peptides and observed attenuation of channel currents that is not compatible with sequence specific actions of the peptides. Connexin mimetic peptides inhibited pannexin channel currents but not the currents of the channel formed by connexins from which the sequence was derived. Pannexin mimetic peptides did inhibit pannexin channel currents but also the channels formed by connexin 46. The same pattern of effects was observed for dye transfer, except that the inhibition levels were more pronounced than for the currents. The channel inhibition by peptides shares commonalities with channel effects of polyethylene glycol (PEG), suggesting a steric block as a mechanism. PEG accessibility is in the size range expected for the pore of innexin gap junction channels, consistent with a functional relatedness of innexin and pannexin channels.

Possible involvement of different connexin43 domains in plasma membrane permeabilization induced by ischemia-reperfusion

In vitro and in vivo studies support the involvement of connexin 43-based cell-cell channels and hemichannels in cell death propagation induced by ischemia-reperfusion. In this context, open connexin hemichannels in the plasma membrane have been proposed to act as accelerators of cell death. Progress on the mechanisms underlying the cell permeabilization induced by ischemia-reperfusion reveals the involvement of several factors leading to an augmented open probability and increased number of hemichannels on the cell surface. While open probability can be increased by a reduction in extracellular concentration of divalent cations and changes in covalent modifications of connexin 43 (oxidation and phosphorylation), increase in number of hemichannels requires an elevation of the intracellular free Ca(2+) concentration. Reversal of connexin 43 redox changes and membrane permeabilization can be induced by intracellular, but not extracellular, reducing agents, suggesting a cytoplasmic localization of the redox sensor(s). In agreement, hemichannels formed by connexin 45, which lacks cytoplasmic cysteines, or by connexin 43 with its C-terminal domain truncated to remove its cysteines are insensitive to reducing agents. Although further studies are required for a precise localization of the redox sensor of connexin 43 hemichannels, modulation of the redox potential is proposed as a target for the design of pharmacological tools to reduce cell death induced by ischemia-reperfusion in connexin 43-expressing cells.

Transcriptional modulation of genes encoding structural characteristics of differentiating enterocytes during development of a polarized epithelium in vitro

Although there is considerable evidence implicating posttranslational mechanisms in the development of epithelial cell polarity, little is known about the patterns of gene expression and transcriptional regulation during this process. We characterized the temporal program of gene expression during cell-cell adhesion-initiated polarization of human Caco-2 cells in tissue culture, which develop structural and functional polarity similar to that of enterocytes in vivo. A distinctive switch in gene expression patterns occurred upon formation of cell-cell contacts between neighboring cells. Expression of genes involved in cell proliferation was down-regulated concomitant with induction of genes necessary for functional specialization of polarized epithelial cells. Transcriptional up-regulation of these latter genes correlated with formation of important structural and functional features in enterocyte differentiation and establishment of structural and functional cell polarity; components of the apical microvilli were induced as the brush border formed during polarization; as barrier function was established, expression of tight junction transmembrane proteins peaked; transcripts encoding components of the apical, but not the basal-lateral trafficking machinery were increased during polarization. Coordinated expression of genes encoding components of functional cell structures were often observed indicating temporal control of expression and assembly of multiprotein complexes.

Some oculodentodigital dysplasia-associated Cx43 mutations cause increased hemichannel activity in addition to deficient gap junction channels

Oculodentodigital dysplasia (ODDD) is a dominantly inherited human disorder associated with different symptoms like craniofacial anomalies, syndactyly and heart dysfunction. ODDD is caused by mutations in the GJA1 gene encoding the gap junction protein connexin43 (Cx43). Here, we have characterized four Cx43 mutations (I31M, G138R, G143S and H194P) after stable expression in HeLa cells. In patients, the I31M and G138R mutations showed all phenotypic characteristics of ODDD, whereas G143S did not result in facial abnormalities and H194P mutated patients exhibited no syndactylies. In transfected HeLa cells, these mutations led to lack of the P2 phosphorylation state of the Cx43 protein, complete inhibition of gap junctional coupling measured by neurobiotin transfer and increased hemichannel activity. In addition, altered trafficking and delayed degradation were found in these mutants by immunofluorescence and pulse-chase analyses. In G138R and G143S mutants, the increased hemichannel activity correlated with an increased half-time of the Cx43 protein. However, the I31M mutated protein showed no extended half-time. Thus, the increased hemichannel activity may be directly caused by an altered conformation of the mutated channel forming protein. We hypothesize that increased hemichannel activity may aggravate the phenotypic abnormalities in ODDD patients who are deficient in Cx43 gap junction channels.

Mechanotransduction of keratinocytes in culture and in the epidermis

The epidermis, like many other tissues, reacts to mechanical stress by increasing cell proliferation. Mechanically stressed skin regions often develop thicker skin and hyperkeratosis. Interestingly, a large number of skin diseases are accompanied by epidermal proliferation and hyperkeratosis even under normal mechanical stress conditions. Although, some of the molecular pathways of mechanical signaling involving integrins, the epidermal growth factor receptor and mitogen-activated protein kinases are known it is still unclear, how mechanical force is sensed and transformed into the molecular signals that induce cell proliferation. This review focuses on the molecules and pathways known to play a role in mechanotransduction in epidermal keratinocytes and discusses the pathways identified in other well-studied cell types.

Inactivation of VEGF in mammary gland epithelium severely compromises mammary gland development and function

To investigate the role of the angiogenic cytokine vascular endothelial growth factor (VEGF) during pregnancy and lactation, we used mice in which VEGF had been inactivated in mammary gland epithelial cells. Pups born to mutant mothers failed to thrive, displaying little milk in their stomachs. However, when they were transferred to control mothers they developed normally. Investigation of the mammary gland morphology revealed that lobulo-alveolar expansion into the fat pad was not complete in lactating mutant glands, and an accumulation of fat globules was evident in their secretory epithelium. In contrast to control glands, lactating mutant glands failed to up-regulate mRNAs for genes involved in milk secretion. Blood vessel density was comparable in pregnant mice of both groups but was only half that of controls in lactating mutant mice. FITC-labeled albumin injected intravenously (i.v.) into lactating mice extravasated rapidly and accumulated in the mammary gland epithelial cells in control animals, but was almost completely retained within the vessels in the mutants. Injection of recombinant VEGF i.v. reversed this effect. These findings demonstrate that mammary epithelium-derived VEGF is partially dispensable for angiogenesis during pregnancy and lactation, and by regulating vascular function during lactation, this factor is crucial to mammary gland differentiation and milk production.

Requirement for ERK MAP kinase in mouse preimplantation development

Preimplantation development is a crucial step for successful implantation and pregnancy. Although both compaction and blastocyst formation have been extensively studied, mechanisms regulating the early cell division stages before compaction have remained unclear. Here, we show that extracellular signal regulated kinase (ERK) mitogen-activated protein (MAP) kinase function is required for early embryonic cell division before compaction. Our analysis demonstrates that inhibition of ERK activation in late two-cell-stage embryos leads to a reversible arrest in the G2 phase at the four-cell stage. The G2-arrested four-cell-stage embryos showed weakened cell-cell adhesion as compared with control embryos. Remarkably, microarray analyses showed that most of the programmed changes of upregulated and downregulated gene expression during the four- to eight-cell stages proceeded normally in four-cell-stage-arrested embryos that were subsequently released to resume development; however, the expression profiles of a proportion of genes in these embryos closely paralleled the stages of embryonic rather than normal development. These parallel genes included the genes encoding intercellular adhesion molecules, whose expression appeared to be positively regulated by the ERK pathway. We also show that, whereas ERK inactivation in eight-cell-stage embryos did not lead to cell division arrest, it did cause this arrest when cadherin-mediated cell-cell adhesion was disrupted. These results demonstrate an essential role of ERK function in two-cell to eight-cell-stage embryos, and suggest a loose parallelism between the gene expression programs and the developmental stages before compaction.

CD4+ T lymphocyte subsets express connexin 43 and establish gap junction channel communication with macrophages in vitro

Gap junction channels constructed of connexins (Cxs) are expressed by peripheral and secondary lymphoid organ-derived lymphocytes. These channels in the plasma membrane play key roles in a range of lymphocyte functions exemplified by the synthesis and secretion of Igs and cytokines and during transmigration across the endothelium. Most recently, their involvement in antigen cross-presentation has also been established. We report here for the first time the expression of mRNA and protein encoding Cx43 in mouse-derived CD4+ Th0, Th1, and Th2 lymphocyte subpopulations and demonstrate the establishment gap junction channel formation with primary macrophages in vitro. We show that this mode of direct communication is particularly favored in Th1-macrophage interactions and that LPS inhibits lymphocyte-macrophage cross-talk independently of the subset of lymphocyte involved. Our work suggests that gap junction-mediated communication can be modulated in the absence of specific antigenic stimulation. Therefore, a further mechanism featuring gap junction-mediated communication may be implicated in immune regulation.

Endothelial gap junctions are down-regulated by arsenic trioxide

We investigated the effect of As(2)O(3), an anti-cancer drug, on endothelial gap junctions. Human aortic endothelial cells (HAEC) were treated with As(2)O(3) at 1, 10, 100, and 1000 ng/ml and the cells were examined to evaluate the expression of connexin43 (Cx43) and to assess gap-junction communication. Endothelial nitric oxide synthase (eNOS) and nitric oxide (NO) were measured to assess for endothelial dysfunction. Male Sprague-Dawley rats were given intravenous As(2)O(3) (200 mug/kg/day) or saline for 4 weeks, after which aortic endothelial gap junctions, eNOS, and circulating NO levels were evaluated. We found that HAEC Cx43 transcripts and gap junctions were reduced and gap-junction communication was attenuated by As(2)O(3). This decrease of Cx43 gap junctions was prevented by the addition of protease inhibitors. At a dose of 100 ng/ml of As(2)O(3), eNOS was reduced at 48 h, but NO was markedly reduced by 1 h. In animals treated with As(2)O(3), endothelial gap junctions comprising Cx37, Cx40, or Cx43 were all reduced in amount, while eNOS and circulating NO levels remained unchanged. In both in vitro and in vivo rat experiments, endothelial gap junctions were consistently reduced by As(2)O(3), unlike the response of eNOS and NO, which were decreased in cells but not in the rat aortic endothelium. The reduction in Cx43 involved both down-regulation at the transcriptional level and increased degradation. These findings indicate that gap-junction communication in the vascular endothelium is inhibited by treatment with As(2)O(3).

Opening of connexin 43 hemichannels is increased by lowering intracellular redox potential

Nonjunctional membrane in many cells contains connexin gap junction hemichannels (or connexons) that can open to allow permeation of small molecules. Opening of Cx43 hemichannels is infrequent in normal extracellular Ca(2+) and enhanced by low Ca(2+), positive membrane potentials, and dephosphorylation of critical residues. Here we report that lowering intracellular redox potential increases Cx43 hemichannel open probability under otherwise normal conditions. We studied dye uptake and single-channel activity in HeLa cells transfected with wild-type Cx43, Cx43 with enhanced GFP attached to its C terminus (Cx43-EGFP), and Cx43 with enhanced GFP attached to its N terminus (EGFP-Cx43). Dithiothreitol [(DTT) 10 mM], a membrane permeant-reducing agent, increased the rate of dye uptake by cells expressing Cx43 and Cx43-EGFP, but not by parental cells or cells expressing EGFP-Cx43. Induced dye uptake was blocked by La(3+), by a peptide gap junction and hemichannel blocker (gap 26), and by flufenamic acid. DTT increased Cx43-EGFP hemichannel opening at positive voltages. Bath application of reduced glutathione, a membrane impermeant-reducing agent, did not increase dye uptake, but glutathione in the recording pipette increased hemichannel opening at positive voltages, suggesting that it acted intracellularly. DTT caused little change in levels of surface Cx43 or Cx43-EGFP, or in intracellular pH. These findings suggest that lowering intracellular redox potential increases the opening of Cx43 and Cx43-EGFP hemichannels, possibly by action on cytoplasmic cysteine residues in the connexin C terminus.

Cellular interactions of Plasmodium liver stage with its host mammalian cell

The Plasmodium liver forms are bridgehead stages between the mosquito sporozoite stages and mammalian blood stages that instigate the malaria disease. In hepatocytes, Plasmodium achieves one of the fastest growth rates among eukaryotic cells. However, nothing is known about host hepatic cell interactions, e.g. nutrient scavenging and/or subversion of cellular functions necessary for Plasmodium development and replication. Plasmodium usually invades hepatocytes by establishing a parasitophorous vacuole wherein it undergoes multiple nuclear division cycles. We show that Plasmodium preferentially develops in the host juxtanuclear region. By comparison with the parasitophorous vacuole of other apicomplexan parasites which associate with diverse host organelles, the Plasmodium parasitophorous vacuole only forms an association with the host endoplasmic reticulum. Intrahepatic Plasmodium actively modifies the permeability of its vacuole to allow the transfer of a large variety of molecules from the host cytosol to the vacuolar space through open channels. In contrast with malaria blood stages, the pores within the parasitophorous vacuole membrane of the liver stage display a smaller size as they restrict the passage of solutes to less than 855Da. These pores are stably maintained during parasite karyokinesis until complete cellularisation. Host-derived cholesterol accumulated at the parasitophorous vacuole membrane may modulate the channel activity. These observations define the parasitophorous vacuole of the Plasmodium liver stage as a dynamic and highly permeable compartment that can ensure the sustained supply of host molecules to support parasite growth in the nutrient-rich environment of liver cells.

Overexpression of biglycan in the heart of transgenic mice: an antibody microarray study

Biglycan, a small leucine rich proteoglycan, is expressed in almost every tissue of the body, mainly in the extracellular matrix of connective tissues. Although there is an increasing amount of data on the biological role of biglycan protein, its function is still poorly understood. We aimed to gather more information about the biological function of biglycan protein in the cardiac tissues, and its role in signal transduction. Therefore, we generated transgenic mice overexpressing the human biglycan protein and analyzed the cardiac protein profile of transgenic offsprings using quantitative real-time (QRT)-PCR and proteomics. QRT-PCR results showed that most members of extracellular matrix were downregulated whereas cadherins, TGF-beta1, and TGF-beta2 were upregulated. Antibody microarrayer experiment revealed that pyk2, RAF-1, Mcl-1, syntrophin, calmodulin, isoforms of NOS protein family (eNOS, nNOS, and iNOS), and synaptotagmin proteins were unambiguously upregulated in the heart of biglycan transgenic mice. In this study we show that biglycan directly or indirectly activates proteins involved in cardiac remodeling (TGF-beta, pyk2), signal transduction (RAF-1, Mcl-1, syntrophin, calmodulin, nNOS p38MAPK and MAP kinases), cardioprotection (NOS family, TGF-beta) and Ca++ signaling (connexin, calmodulin, synaptotagmin). On the basis of the results presented here, we conclude that biglycan is a multifunctional extracellular protein that has a pivotal role in pathological remodeling of cardiac tissue and mediates cardioprotection.

Gap-junction blocker carbenoxolone differentially enhances NMDA-induced cell death in hippocampal neurons and astrocytes in co-culture

The beneficial or detrimental role of gap junction communication in the pathophysiology of brain injury is still controversial. We used co-cultures of hippocampal astrocytes and neurons, where we identified homocellular astrocyte-astrocyte and heterocellular astrocyte-neuron coupling by fluorescence recovery after photobleaching, which was decreased by the gap junction blocker carbenoxolone (CBX). In these cultures, we determined the cell type-specific effects of CBX on the excitotoxic damage caused by N-methyl-D-aspartate (NMDA). We determined in both astrocytes and neurons the influence of CBX, alone or together with NMDA challenge, on cytotoxicity using propidium iodide labeling. CBX alone was not cytotoxic, but CBX treatment differentially accelerated the NMDA-induced cell death in both astrocytes and neurons. In addition, we measured mitochondrial potential using rhodamine 123, membrane potential using the oxonol dye bis(1,3-diethylthiobarbituric acid)trimethine oxonol, cytosolic Ca(2+) level using fura-2, and formation of reactive oxygen species (ROS) using dihydroethidium. CBX alone induced neither an intracellular Ca(2+) rise nor a membrane depolarization. However, CBX elicited a mitochondrial depolarization in both astrocytes and neurons and increased the ROS formation in neurons. In contrast, NMDA caused a membrane depolarization in neurons, coinciding with intracellular Ca(2+) rise, but neither mitochondrial depolarization nor ROS production seem to be involved in NMDA-mediated cytotoxicity. Pre-treatment with CBX accelerated the NMDA-induced membrane depolarization and prevented the repolarization of neurons after the NMDA challenge. We hypothesize that these effects are possibly mediated via blockage of gap junctions, and might be involved in the mechanism of CBX-induced acceleration of excitotoxic cell death, whereas the CBX-induced mitochondrial depolarization and ROS formation are not responsible for the increase in cytotoxicity. We conclude that both in astrocytes and neurons gap junctions provide protection against NMDA-induced cytotoxicity.

Cell adhesion molecules in chemically-induced renal injury

Cell adhesion molecules are integral cell-membrane proteins that maintain cell-cell and cell-substrate adhesion and in some cases act as regulators of intracellular signaling cascades. In the kidney, cell adhesion molecules, such as the cadherins, the catenins, the zonula occludens protein-1 (ZO-1), occludin and the claudins are essential for maintaining the epithelial polarity and barrier integrity that are necessary for the normal absorption/excretion of fluid and solutes. A growing volume of evidence indicates that these cell adhesion molecules are important early targets for a variety of nephrotoxic substances including metals, drugs, and venom components. In addition, it is now widely appreciated that molecules, such as intracellular adhesion molecule-1 (ICAM-1), integrins, and selectins play important roles in the recruitment of leukocytes and inflammatory responses that are associated with nephrotoxic injury. This review summarizes the results of recent in vitro and in vivo studies indicating that these cell adhesion molecules may be primary molecular targets in many types of chemically-induced renal injury. Some of the specific agents that are discussed include cadmium (Cd), mercury (Hg), bismuth (Bi), cisplatin, aminoglycoside antibiotics, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), and various venom toxins. This review also includes a discussion of the various mechanisms, by which these substances can affect cell adhesion molecules in the kidney.

Connexin channel permeability to cytoplasmic molecules

Connexin channels are known to be permeable to a variety of cytoplasmic molecules. The first observation of second messenger junctional permeability, made approximately 30 years ago, sparked broad interest in gap junction channels as mediators of intercellular molecular signaling. Since then, much has been learned about the diversity of connexin channels with regard to isoform diversity, tissue and developmental distribution, modes of channel regulation, assembly, expression, biochemical modification and permeability, all of which appear to be dynamically regulated. This information has expanded the potential roles of connexin channels in development, physiology and disease, and made their elucidation much more complex--30 years ago such an orchestra of junctional dynamics was unanticipated. Only recently, however, have investigators been able to directly address, in this more complex framework, the key issue: what specific biological molecules, second messengers and others, are able to permeate the various types of connexin channels, and how well? An important related issue, given the ever-growing list of connexin-related pathologies, is how these permeabilities are altered by disease-causing connexin mutations. Together, many studies show that a variety of cytoplasmic molecules can permeate the different types of connexin channels. A few studies reveal differences in permeation by different molecules through a particular type of connexin channel, and differences in permeation by a particular molecule through different types of connexin channels. This article describes and evaluates the various methods used to obtain these data, presents an annotated compilation of the results, and discusses the findings in the context of what can be inferred about mechanism of selectivity and potential relevance to signaling. The data strongly suggest that highly specific interactions take place between connexin pores and specific biological molecular permeants, and that those interactions determine which cytoplasmic molecules can permeate and how well. At this time, the nature of those interactions is unclear. One hopes that with more detailed permeability and structural information, the specific molecular mechanisms of the selectivity can be elucidated.

The modulatory effects of connexin 43 on cell death/survival beyond cell coupling

Connexins form a diverse and ubiquitous family of integral membrane proteins. Characteristically, connexins are assembled into intercellular channels that aggregate into discrete cell-cell contact areas termed gap junctions (GJ), allowing intercellular chemical communication, and are essential for propagation of electrical impulses in excitable tissues, including, prominently, myocardium, where connexin 43 (Cx43) is the most important isoform. Previous studies have shown that GJ-mediated communication has an important role in the cellular response to stress or ischemia. However, recent evidence suggests that connexins, and in particular Cx43, may have additional effects that may be important in cell death and survival by mechanisms independent of cell to cell communication. Connexin hemichannels, located at the plasma membrane, may be important in paracrine signaling that could influence intracellular calcium and cell survival by releasing intracellular mediators as ATP, NAD(+), or glutamate. In addition, recent studies have shown the presence of connexins in cell structures other than the plasma membrane, including the cell nucleus, where it has been suggested that Cx43 influences cell growth and differentiation. In addition, translocation of Cx43 to mitochondria appears to be important for certain forms of cardioprotection. These findings open a new field of research of previously unsuspected roles of Cx43 intracellular signaling.

Calcium Dynamics: Spatio‐Temporal Organization from the Subcellular to the Organ Level

Many essential physiological processes are controlled by calcium. To ensure reliability and specificity, calcium signals are highly organized in time and space in the form of oscillations and waves. Interesting findings have been obtained at various scales, ranging from the stochastic opening of a single calcium channel to the intercellular calcium wave spreading through an entire organ. A detailed understanding of calcium dynamics thus requires a link between observations at different scales. It appears that some regulations such as calcium-induced calcium release or PLC activation by calcium, as well as the weak diffusibility of calcium ions play a role at all levels of organization in most cell types. To comprehend how calcium waves spread from one cell to another, specific gap-junctional coupling and paracrine signaling must also be taken into account. On the basis of a pluridisciplinar approach ranging from physics to physiology, a unified description of calcium dynamics is emerging, which could help understanding how such a small ion can mediate so many vital functions in living systems.

Functional connexin "hemichannels": a critical appraisal

"Hemichannels" are defined as the halves of gap junction channels (also termed connexons) that are contributed by one cell; "hemichannels" are considered to be functional if they are open in nonjunctional membranes in the absence of pairing with partners from adjacent cells. Several recent reviews have summarized the blossoming literature regarding functional "hemichannels", in some cases encyclopedically. However, most of these previous reviews have been written with the assumption that all data reporting "hemichannel" involvement really have studied phenomena in which connexons actually form the permeability or conductance pathway. In this review, we have taken a slightly different approach. We review the concept of "hemichannels", summarize properties that might be expected of half gap junctions and evaluate the extent to which the properties of presumptive "hemichannels" match expectations. Then we consider functions attributed to hemichannels, provide an overview of other channel types that might fulfill similar roles and provide sets of criteria that might be applied to verify involvement of connexin hemichannels in cell and tissue function. One firm conclusion is reached. The study of hemichannels is technically challenging and fraught with opportunities for misinterpretation, so that future studies must apply rigorous standards for detection of hemichannel expression and function. At the same time there are reasons to expect surprises, including the possibility that some time honored techniques for studying gap junctions may prove unsuitable for detecting hemichannels. We advise hemichannel researchers to proceed with caution and an open mind.

Connexin hemichannels and gap junction channels are differentially influenced by lipopolysaccharide and basic fibroblast growth factor

Gap junction (GJ) channels are formed by two hemichannels (connexons), each contributed by the cells taking part in this direct cell-cell communication conduit. Hemichannels that do not interact with their counterparts on neighboring cells feature as a release pathway for small paracrine messengers such as nucleotides, glutamate, and prostaglandins. Connexins are phosphorylated by various kinases, and we compared the effect of various kinase-activating stimuli on GJ channels and hemichannels. Using peptides identical to a short connexin (Cx) amino acid sequence to specifically block hemichannels, we found that protein kinase C, Src, and lysophosphatidic acid (LPA) inhibited GJs and hemichannel-mediated ATP release in Cx43-expressing C6 glioma cells (C6-Cx43). Lipopolysaccharide (LPS) and basic fibroblast growth factor (bFGF) inhibited GJs, but they stimulated ATP release via hemichannels in C6-Cx43. LPS and bFGF inhibited hemichannel-mediated ATP release in HeLa-Cx43 cells, but they stimulated it in HeLa-Cx43 with a truncated carboxy-terminal (CT) domain or in HeLa-Cx26, which has a very short CT. Hemichannel potentiation by LPS was inhibited by blockers of the arachidonic acid metabolism, and arachidonic acid had a potentiating effect like LPS and bFGF. We conclude that GJ channels and hemichannels display similar or oppositely directed responses to modulatory influences, depending on the balance between kinase activity and the activity of the arachidonic acid pathway. Distinctive hemichannel responses to pathological stimulation with LPS or bFGF may serve to optimize the cell response, directed at strictly controlling cellular ATP release, switching from direct GJ communication to indirect paracrine signaling, or maximizing cell-protective strategies.