General and conditional replacement of connexin43-coding DNA by a lacZ reporter gene for cell-autonomous analysis of expression

A fatal alliance: Glial connexins, myelin pathology and mental disorders

Mature oligodendrocytes are myelin forming glial cells which are responsible for myelination of neuronal axons in the white matter of the central nervous system. Myelin pathology is a major feature of severe neurological disorders. Oligodendrocyte-specific gene mutations and/or white matter alterations have also been addressed in a variety of mental disorders. Breakdown of myelin integrity and demyelination is associated with severe symptoms, including impairments in motor coordination, breathing, dysarthria, perception (vision and hearing), and cognition. Furthermore, there is evidence indicating that myelin sheath defects and white matter pathology contributes to the affective and cognitive symptoms of patients with mental disorders. Oligodendrocytes express the connexins GJC2; mCx47 [human (GJC2) and mouse (mCx47) connexin gene nomenclature according to Söhl and Willecke (2003)], GJB1; mCx32, and GJD1; mCx29 in both white and gray matter. Preclinical findings indicate that alterations in connexin expression in oligodendrocytes and astrocytes can induce myelin defects. GJC2; mCx47 is expressed at early embryonic stages in oligodendrocyte precursors cells which precedes central nervous system myelination. In adult humans and animals GJC2, respectively mCx47 expression is essential for oligodendrocyte function and ensures adequate myelination as well as myelin maintenance in the central nervous system. In the past decade, evidence has accumulated suggesting that mental disorders can be accompanied by changes in connexin expression, myelin sheath defects and corresponding white matter alterations. This dual pathology could compromise inter-neuronal information transfer, processing and communication and eventually contribute to behavioral, sensory-motor, affective and cognitive symptoms in patients with mental disorders. The induction of myelin repair and remyelination in the central nervous system of patients with mental disorders could help to restore normal neuronal information propagation and ameliorate behavioral and cognitive symptoms in individuals with mental disorders.

Cx43 in Neural Progenitors Promotes Glioma Invasion in a 3D Culture System

The environment that envelops the cancer cells intimately affects the malignancy of human cancers. In the case of glioma, an aggressive adult brain cancer, its high rate of recurrence after total resection is responsible for a poor prognosis. Connexin43 (Cx43) is a gap junction protein with a prominent presence in glioma-associated normal brain cells, specifically in the reactive astrocytes. We previously demonstrated that elimination of Cx43 in these astrocytes reduces glioma invasion in a syngeneic mouse model. To further our investigation in human glioma cells, we developed a scaffold-free 3D platform that takes into account both the tumor and its interaction with the surrounding tissue. Using cell-tracking dyes and 3D laser scanning confocal microscopy, we now report that the elimination of Cx43 protein in neural progenitor spheroids reduced the invasiveness of human brain tumor-initiating cells, confirming our earlier observation in an intact mouse brain. By investigating the glioma invasion in a defined multicellular system with a tumor boundary that mimics the intact brain environment, our findings strengthen Cx43 as a candidate target for glioma control.

Astroglial Connexin 43 Deficiency Protects against LPS-Induced Neuroinflammation: A TSPO Brain µPET Study with [18F]FEPPA

Astroglial connexin 43 (Cx43) has been recognized as a crucial immunoregulating factor in the brain. Its inactivation leads to a continuous immune recruitment, cytokine expression modification and a specific humoral autoimmune response against the astrocytic extracellular matrix but without brain lesions or cell lysis. To assess the impact of Cx43 deletion on the brain’s inflammatory response, TSPO expression was studied by positron emission tomography (PET) imaging with a specific radioligand, [¹⁸F]FEPPA, in basal conditions or upon Lipopolysaccharides (LPS)-induced inflammatory challenge. Astroglial Cx43-deleted mice underwent [¹⁸F]FEPPA PET/CT dynamic imaging with or without LPS injection (5 mg/kg) 24 h before imaging. Quantification and pharmacokinetic data modelling with a 2TCM-1K compartment model were performed. After collecting the mice brains, TSPO expression was quantified and localized by Western blot and FISH analysis. We found that astroglial Cx43 deficiency does not significantly alter TSPO expression in the basal state as observed with [¹⁸F]FEPPA PET imaging, FISH and Western blot analysis. However, deletion of astrocyte Cx43 abolishes the LPS-induced TSPO increase. Autoimmune encephalopathy observed in astroglial Cx43-deleted mice does not involve TSPO overexpression. Consistent with previous studies showing a unique inflammatory status in the absence of astrocyte Cx43, we show that a deficient expression of astrocytic Cx43 protects the animals from LPS-induced neuroinflammation as addressed by TSPO expression.

Connexin 30.2 is expressed in exocrine vascular endothelial and ductal epithelial cells throughout pancreatic postnatal development

Previously we have demonstrated that the GJ protein connexin 30.2 (Cx30.2) is expressed in pancreatic beta cells and endothelial cells (ECs) of the islet. In the present study, we address whether Cx30.2 is expressed in the exocrine pancreas, including its vascular system. For this, adult mouse pancreatic sections were double labeled with specific antibodies against Cx30.2 and CD31, an endothelial cell marker, or with anti-α-actin smooth muscle, a smooth muscle cell (SMC) marker or anti-mucin-1, a marker of epithelial ductal cells, using immunofluorescence (IF) studies. Cx30.2-IF hot spots were found at junctional membranes of exocrine ECs and SMCs of blood vessels. Furthermore, Cx30.2 was localized in mucin-1 positive cells or epithelial ductal cells. Using immunohistochemistry (IHC) studies, it was found that in vessels and ducts of different diameters, Cx30.2 was also expressed in these cell types. In addition, it was found that Cx30.2 is already expressed in these cell types in pancreatic sections of 3, 14 and 21 days postpartum. Moreover, this cell specific pattern of expression was also found in the adult rat, hamster and guinea pig pancreas. Expression of Cx30.2 mRNA and protein in the pancreas of all these species was confirmed by RT-PCR and Western blot studies. Overall, our results suggest that intercellular coupling mediated by Cx30.2 intercellular channels may synchronize the functional activity of ECs and SMCs of vascular cells, as well as of epithelial ductal cells after birth.

Connexin 43 Controls the Astrocyte Immunoregulatory Phenotype

Astrocytes are the most abundant glial cells of the central nervous system and have recently been recognized as crucial in the regulation of brain immunity. In most neuropathological conditions, astrocytes are prone to a radical phenotypical change called reactivity, which plays a key role in astrocyte contribution to neuroinflammation. However, how astrocytes regulate brain immunity in healthy conditions is an understudied question. One of the astroglial molecule involved in these regulations might be Connexin 43 (Cx43), a gap junction protein highly enriched in astrocyte perivascular endfeet-terminated processes forming the glia limitans. Indeed, Cx43 deletion in astrocytes (Cx43KO) promotes a continuous immune recruitment and an autoimmune response against an astrocyte protein, without inducing any brain lesion. To investigate the molecular basis of this unique immune response, we characterized the polysomal transcriptome of hippocampal astrocytes deleted for Cx43. Our results demonstrate that, in the absence of Cx43, astrocytes adopt an atypical reactive status with no change in most canonical astrogliosis markers, but with an upregulation of molecules promoting immune recruitment, complement activation as well as anti-inflammatory processes. Intriguingly, while several of these upregulated transcriptional events suggested an activation of the γ-interferon pathway, no increase in this cytokine or activation of related signaling pathways were found in Cx43KO. Finally, deletion of astroglial Cx43 was associated with the upregulation of several angiogenic factors, consistent with an increase in microvascular density in Cx43KO brains. Collectively, these results strongly suggest that Cx43 controls immunoregulatory and angiogenic properties of astrocytes.

Connexins and microRNAs: Interlinked players in regulating islet function?

Pancreatic β-cells are connected to neighboring endocrine cells through the adherin proteins and gap junctions. Connexin 36 (Cx36) is one of the most well-studied and abundantly expressed gap-junction proteins within rodent islets, which is important in coordinated insulin secretion. The expression of connexins is regulated at various levels and by several mechanisms; one of which is via microRNAs. In past 2 decades, microRNAs (miRNAs) have emerged as key molecules in developmental, physiologic and pathological processes. However, very few studies have demonstrated miRNA-mediated regulation of connexins. Even though there are no reports yet on miRNAs and Cx36; we envisage that considering the important role of connexins and microRNAs in insulin secretion, there would be common pathways interlinking these biomolecules. Here, we discuss the current literature on connexins and miRNAs specifically with reference to islet function.

Immunoregulation at the gliovascular unit in the healthy brain: A focus on Connexin 43

In the brain, immune cell infiltration is normally kept at a very low level and a unique microenvironment strictly restricts immune reactions and inflammation. Even in such quiescent environment, a constant immune surveillance is at work allowing the brain to rapidly react to threats. To date, knowledge about the factors regulating the brain-immune system interrelationship in healthy conditions remains elusive. Interestingly, astrocytes, the most abundant glial cells in the brain, may participate in many aspects of this unique homeostasis, in particular due to their close interaction with the brain vascular system and expression of a specific molecular repertoire. Indeed, astrocytes maintain the blood-brain barrier (BBB) integrity, interact with immune cells, and participate in the regulation of intracerebral liquid movements. We recently showed that Connexin 43 (Cx43), a gap junction protein highly expressed by astrocytes at the BBB interface, is an immunoregulating factor. The absence of astroglial Cx43 leads to a transient endothelial activation, a continuous immune recruitment as well as the development of a specific humoral autoimmune response against the von Willebrand factor A domain-containing protein 5a, an extracellular matrix protein expressed by astrocytes. In this review, we propose to gather current knowledge on how astrocytes may influence the immune system in the healthy brain, focusing on their roles at the gliovascular interface. We will also consider pathological situations involving astrocyte-specific autoimmunities. Finally, we will discuss the specific role of astroglial Cx43 and the physiological consequences of immune regulations taking place on inflammation, cognition and behavior in the absence of Cx43.

Dexamethasone-induced muscular atrophy is mediated by functional expression of connexin-based hemichannels

Long-term treatment with high glucocorticoid doses induces skeletal muscle atrophy. However, the molecular mechanism of such atrophy remains unclear. We evaluated the possible involvement of connexin-based hemichannels (Cx HCs) in muscle atrophy induced by dexamethasone (DEX), a synthetic glucocorticoid, on control (Cx43(fl/fl)Cx45(fl/fl)) and Cx43/Cx45 expression-deficient (Cx43(fl/fl)Cx45(fl/fl):Myo-Cre) skeletal myofibers. Myofibers of Cx43(fl/fl)Cx45(fl/fl) mice treated with DEX (5h) expressed several proteins that form non-selective membrane channels (Cx39, Cx43, Cx45, Panx1, P2X7 receptor and TRPV2). After 5h DEX treatment in vivo, myofibers of Cx43(fl/fl)Cx45(fl/fl) mice showed Evans blue uptake, which was absent in myofibers of Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice. Similar results were obtained in vitro using ethidium as an HC permeability probe, and DEX-induced dye uptake in control myofibers was blocked by P2X7 receptor inhibitors. DEX also induced a significant increase in basal intracellular Ca(2+) signal and a reduction in resting membrane potential in Cx43(fl/fl)Cx45(fl/fl) myofibers, changes that were not elicited by myofibers deficient in Cx43/Cx45 expression. Moreover, treatment with DEX induced NFκB activation and increased mRNA levels of TNF-α in control but not in Cx43/Cx45 expression-deficient myofibers. Finally, a prolonged DEX treatment (7days) increased atrogin-1 and Murf-1 and reduced the cross sectional area of Cx43(fl/fl)Cx45(fl/fl) myofibers, but these parameters remained unaffected in Cx43(fl/fl)Cx45(fl/fl):Myo-Cre myofibers. Therefore, DEX-induced expression of Cx43 and Cx45 plays a critical role in early sarcolemma changes that lead to atrophy. Consequently, this side effect of chronic glucocorticoid treatment might be avoided by co-administration with a Cx HC blocker.

Attenuated Levels of Hippocampal Connexin 43 and its Phosphorylation Correlate with Antidepressant- and Anxiolytic-Like Activities in Mice

Clinical and preclinical studies have implicated glial anomalies in major depression. Conversely, evidence suggests that the activity of antidepressant drugs is based, at least in part, on their ability to stimulate density and/or activity of astrocytes, a major glial cell population. Despite this recent evidence, little is known about the mechanism(s) by which astrocytes regulate emotionality. Glial cells communicate with each other through gap junction channels (GJCs), while they can also directly interact with neurons by releasing gliotransmitters in the extracellular compartment via an hemichannels (HCs)-dependent process. Both GJCs and HCs are formed by two main protein subunits: connexins (Cx) 30 and 43 (Cx30 and Cx43). Here we investigate the role of hippocampal Cx43 in the regulation of depression-like symptoms using genetic and pharmacological approaches. The first aim of this study was to evaluate the impact of the constitutive knock-down of Cx43 on a set of behaviors known to be affected in depression. Conversely, the expression of Cx43 was assessed in the hippocampus of mice subjected to prolonged corticosterone (CORT) exposure, given either alone or in combination with an antidepressant drug, the selective serotonin reuptake inhibitor fluoxetine. Our results indicate that the constitutive deficiency of Cx43 resulted in the expression of some characteristic hallmarks of antidepressant-/anxiolytic-like behavioral activities along with an improvement of cognitive performances. Moreover, in a new cohort of wild-type mice, we showed that CORT exposure elicited anxiety and depression-like abnormalities that were reversed by chronic administration of fluoxetine. Remarkably, CORT also increased hippocampal amounts of phosphorylated form of Cx43 whereas fluoxetine treatment normalized this parameter. From these results, we envision that antidepressant drugs may exert their therapeutic activity by decreasing the expression and/or activity of Cx43 resulting from a lower level of phosphorylation in the hippocampus.

Immune quiescence of the brain is set by astroglial connexin 43

In the normal brain, immune cell trafficking and immune responses are strictly controlled and limited. This unique homeostatic equilibrium, also called brain immune quiescence, is crucial to maintaining proper brain functions and is altered in various pathological processes, from chronic immunopathological disorders to cognitive and psychiatric impairments. To date, the precise nature of factors regulating the brain/immune system interrelationship is poorly understood. In the present study, we demonstrate that one of these regulating factors is Connexin 43 (Cx43), a gap junction protein highly expressed by astrocytes at the blood-brain barrier (BBB) interface. We show that, by setting the activated state of cerebral endothelium, astroglial Cx43 controls immune recruitment as well as antigen presentation mechanisms in the mouse brain. Consequently, in the absence of astroglial Cx43, recruited immune cells elaborate a specific humoral autoimmune response against the von Willebrand factor A domain-containing protein 5a, an extracellular matrix protein of the brain. Altogether, our results demonstrate that Cx43 is a new astroglial factor promoting the immune quiescence of the brain.

Role of gap junction protein connexin43 in astrogliosis induced by brain injury

Astrogliosis is a process that involves morphological and biochemical changes associated with astrocyte activation in response to cell damage in the brain. The upregulation of intermediate filament proteins including glial fibrillary acidic protein (GFAP), nestin and vimentin are often used as indicators for astrogliosis. Although connexin43 (Cx43), a channel protein widely expressed in adult astrocytes, exhibits enhanced immunoreactivity in the peri-lesion region, its role in astrogliosis is still unclear. Here, we correlated the temporal and spatial expression of Cx43 to the activation of astrocytes and microglia in response to an acute needle stab wound in vivo. We found large numbers of microglia devoid of Cx43 in the needle wound at 3 days post injury (dpi) while reactive astrocytes expressing Cx43 were present in the peripheral zone surrounding the injury site. A redistribution of Cx43 to the needle site, corresponding to the increased presence of GFAP-positive reactive astrocytes in the region, was only apparent from 6 dpi and sustained until at least 15 dpi. Interestingly, the extent of microglial activation and subsequent astrogliosis in the brain of Cx43 knockout mice was significantly larger than those of wild type, suggesting that Cx43 expression limits the degree of microgliosis. Although Cx43 is not essential for astrogliosis and microglial activation induced by a needle injury, our results demonstrate that Cx43 is a useful marker for injury induced astrogliosis due to its enhanced expression specifically within a small region of the lesion for an extended period. As a channel protein, Cx43 is a potential in vivo diagnostic tool of asymptomatic brain injury.

Bisphosphonates improve trabecular bone mass and normalize cortical thickness in ovariectomized, osteoblast connexin43 deficient mice

The gap junction protein, connexin43 (Cx43) controls both bone formation and osteoclastogenesis via osteoblasts and/or osteocytes. Cx43 has also been proposed to mediate an anti-apoptotic effect of bisphosphonates, potent inhibitors of bone resorption. We studied whether bisphosphonates are effective in protecting mice with a conditional Cx43 gene deletion in osteoblasts and osteocytes (cKO) from the consequences of ovariectomy on bone mass and strength. Ovariectomy resulted in rapid loss of trabecular bone followed by a slight recovery in wild type (WT) mice, and a similar degree of trabecular bone loss, albeit slightly delayed, occurred in cKO mice. Treatment with either risedronate (20 μg/kg) or alendronate (40 μg/kg) prevented ovariectomy-induced bone loss in both genotypes. In basal conditions, bones of cKO mice have larger marrow area, higher endocortical osteoclast number, and lower cortical thickness and strength relative to WT. Ovariectomy increased endocortical osteoclast number in WT but not in cKO mice. Both bisphosphonates prevented these increases in WT mice, and normalized endocortical osteoclast number, cortical thickness and bone strength in cKO mice. Thus, lack of osteoblast/osteocyte Cx43 does not alter bisphosphonate action on bone mass and strength in estrogen deficiency. These results support the notion that one of the main functions of Cx43 in cortical bone is to restrain osteoblast and/or osteocytes from inducing osteoclastogenesis at the endocortical surface.

Deletion of astroglial connexins weakens the blood-brain barrier

Astrocytes, the most prominent glial cell type in the brain, send specialized processes named endfeet, which enwrap blood vessels and express a large molecular repertoire dedicated to the physiology of the vascular system. One of the most striking properties of astrocyte endfeet is their enrichment in gap junction protein connexins 43 and 30 (Cx43 and Cx30) allowing for direct intercellular trafficking of ions and small signaling molecules through perivascular astroglial networks. The contribution of astroglial connexins to the physiology of the brain vascular system has never been addressed. Here, we show that Cx43 and Cx30 expression at the level of perivascular endfeet starts from postnatal days 2 and 12 and is fully mature at postnatal days 15 and 20, respectively, indicating that astroglial perivascular connectivity occurs and develops during postnatal blood-brain barrier (BBB) maturation. We demonstrate that mice lacking Cx30 and Cx43 in GFAP (glial fibrillary acidic protein)-positive cells display astrocyte endfeet edema and a partial loss of the astroglial water channel aquaporin-4 and β-dystroglycan, a transmembrane receptor anchoring astrocyte endfeet to the perivascular basal lamina. Furthermore, the absence of astroglial connexins weakens the BBB, which opens upon increased hydrostatic vascular pressure and shear stress. These results demonstrate that astroglial connexins are necessary to maintain BBB integrity.

Hypoxia in high glucose followed by reoxygenation in normal glucose reduces the viability of cortical astrocytes through increased permeability of connexin 43 hemichannels

Brain ischemia causes more extensive injury in hyperglycemic than normoglycemic subjects, and the increased damage is to astroglia as well as neurons. In the present work, we found that in cortical astrocytes from rat or mouse, reoxygenation after hypoxia in a medium mimicking interstitial fluid during ischemia increases hemichannel activity and decreases cell-cell communication via gap junctions as indicated by dye uptake and dye coupling, respectively. These effects were potentiated by high glucose during the hypoxia in a concentration-dependent manner (and by zero glucose) and were not observed in connexin 43(-/-) astrocytes. The responses were transient and persistent after short and long periods of hypoxia, respectively. The persistent responses were associated with a progressive reduction in cell viability that was prevented by La(3+) or peptides that block connexin 43 (Cx43) hemichannels or by inhibition of p38 MAP kinase prior to hypoxia-reoxygenation but not by treatments that block pannexin hemichannels. Block of Cx43 hemichannels did not affect the reduction in gap junction mediated dye coupling observed during reoxygenation. Cx43 hemichannels may be a novel therapeutic target to reduce cell death following stroke, particularly in hyperglycemic conditions.

Cx43 CT domain influences infarct size and susceptibility to ventricular tachyarrhythmias in acute myocardial infarction

AIMS

Hearts of mice expressing K258stop in place of connexin43 (Cx43) protein were subjected to acute myocardial infarction in order to assess the importance of Cx43 regulation on infarct size and arrhythmia susceptibility. This mutation K258stop prevents chemical regulation of Cx43 channels, including by low intracellular pH.

METHODS AND RESULTS

Langendorff-perfused hearts of mice harbouring one Cx43 knockout (KO) allele and one K258stop or Cx43 allele (K258stop/KO; Cx43/KO as control) were subjected to 1 h of ischaemia and 4 h of reperfusion by reversibly occluding the left anterior descending (LAD) coronary artery. Inducibility of ventricular tachyarrhythmias (VTs) was tested by applying an endocardial burst-pacing protocol during LAD occlusion. Separately, time course and the extent of acidification-induced closure of gap junction channels were tested by dual-voltage clamp. Infarct volume (as per cent of area at risk) was significantly larger in K258stop/KO hearts compared with Cx43/KO controls (42.2 +/- 3 vs. 30.4 +/- 1.7%, P = 0.004, n = 8 each). During LAD occlusion, K258stop/KO hearts had a higher incidence of pacing-induced VT and a higher frequency of occurrence of spontaneous premature ventricular beats. The occurrence of ventricular arrhythmias was also significantly larger in the K258stop/KO hearts during reperfusion. In separate experiments, we demonstrated reduced sensitivity to acidification-induced uncoupling in cell pairs obtained from K258stop/KO hearts.

CONCLUSION

Loss of the regulatory domain of Cx43 leads to an increase in infarct size and increased susceptibility to arrhythmias following acute coronary occlusion.

A new conditional mouse mutant reveals specific expression and functions of connexin36 in neurons and pancreatic beta-cells

Connexin36 (Cx36) is the main connexin isoform expressed in neurons of the central nervous system (CNS) and in pancreatic beta-cells, i.e. two types of excitable cells that share - in spite of their different origins - a number of common features. Previous studies on Cx36 deficient mice have documented that loss of Cx36 resulted in phenotypic abnormalities in both the CNS and the pancreas which, however, could not be attributed to specific cell types due to the general deletion nature of the animal model used. Attempts to address this limitation using cell type specific deletions generated by the Cre/loxP strategy have so far been complicated by the lack of Cx36 expression from the floxed allele. We have now generated a conditional Cx36 deficient mouse mutant in which the coding region of Cx36 is flanked by loxP sites, followed by a cyan fluorescent protein (CFP) reporter gene. Here we show that Cx36 was still expressed from the floxed allele in neurons and pancreatic beta-cells. In these cells, a 30-60% decrease of this protein, relative to the expression level of the wildtype allele, did not significantly perturb cell coupling. The deletion of Cx36 by ubiquitously and cell type specifically expressed Cre recombinases revealed that CFP functions as a reliable reporter for Cx36 expression in brain neurons and to some extent in retina neurons, but not in pancreas. Loss of Cx36 by Cre-mediated recombination was documented at transcript and protein levels. Cell type specific deletion of Cx36 in the endocrine pancreas revealed major alterations in the basal as well as the glucose-induced insulin secretion, hence specifically attributing to pancreatic Cx36 an important regulatory role in the control of beta-cell function. Cell type specific deletion of Cx36 in the CNS by suitable Cre recombinases should also help to elucidate the functional role of Cx36 in different neuronal subtypes.

Adopting the good reFLEXes when generating conditional alterations in the mouse genome

Major advances have been made in the use of the Cre/loxP system for conditional gene targeting in the mouse. By combining the ability of Cre recombinase to invert or excise a DNA fragment, depending upon the orientation of the flanking loxP sites, and the use of wild-type loxP and variant lox511 sites, we devised an efficient and reliable Cre-mediated genetic switch, called FLEX, through which expression of a given gene can be turned off, while expression of another one can be simultaneously turned on. We discuss how this innovative, flexible and powerful approach, which virtually adapts to any kind of site-specific recombinase (e.g., Cre and Flp recombinases), can be used to easily generate, even at high throughput and genome wide scale, many genetic modifications in a conditional manner, including those which were considered as difficult or impossible to achieve.

Ablation of connexin43 in smooth muscle cells of the mouse intestine: functional insights into physiology and morphology

Connexin43 (Cx43) gap-junction channels are highly abundant in intestinal smooth muscle but their functional impact has not been studied so far. Here, we have aimed to elucidate the functional role of Cx43 in the tunica muscularis of the mouse intestine in vivo. Transgenic mice with conditional deletion of Cx43 in smooth muscle cells (SMC) were generated. Histological investigations by immunofluorescence analyses and organ-bath recordings to assess the contractility of intestinal tissue strips were carried out. Measurements of gastrointestinal transit and of the visceromotor response by utilizing a standardized colorectal distension model to quantify alterations of visceral sensory function were also performed in SMC-specific Cx43 null mice and control littermates. Histologically, we found thickening of the tunica muscularis and a 13-fold increase of neutrophil infiltration of the gastrointestinal wall of SMC-specific Cx43 null mice. These animals also exhibited a decrease of 29% in gastrointestinal transit time. In contrast, the visceromotor response to a standardized colorectal distension was elevated, as was the contractility in SMC-specific Cx43 null mice, compared with controls. Thus, SMC-specific ablation of Cx43 in mice leads to morphological and functional alterations of the intestinal tunica muscularis, to gastrointestinal motor dysfunction and to altered visceral sensory function.

Temporal regulation of connexin phosphorylation in embryonic and adult tissues

Gap junctions, composed of proteins from the connexin family, allow for intercellular communication between cells in tissues and are important in development, tissue/cellular homeostasis, and carcinogenesis. Genome databases indicate that there are at least 20 connexins in the mouse and human. Connexin phosphorylation has been implicated in connexin assembly into gap junctions, gap junction turnover, and cell signaling events that occur in response to tumor promoters and oncogenes. Connexin43 (Cx43), the most widely expressed and abundant gap junction protein, can be phosphorylated at several different serine and tyrosine residues. Here, we focus on the dynamic regulation of Cx43 phosphorylation in tissue and how these regulatory events are affected during development, wound healing, and carcinogenesis. The activation of several kinases, including protein kinase A, protein kinase C, p34cdc2/cyclin B kinase, casein kinase 1, mitogen-activated protein kinase, and pp60src kinase, can lead to the phosphorylation of different residues in the C-terminal region of Cx43. The use of antibodies specific for phosphorylation at defined residues has allowed the examination of specific phosphorylation events both in tissue culture and in vivo. These new antibody tools and those under development will allow us to correlate specific phosphorylation events with changes in connexin function.

Spontaneous ectopic recombination in cell-type-specific Cre mice removes loxP-flanked marker cassettes in vivo

Conditional gene targeting using the Cre/loxP technology generally includes integration of a selection marker cassette flanked by loxP recognition sites (floxed) in the target gene locus. Subsequent marker removal avoids possible impairment of gene expression or mosaicism due to partial and total deletions after Cre-mediated recombination in vivo. The use of deleter Cre mice for in vivo marker removal in floxed connexin43 mice revealed considerable mosaicism, but no selective marker removal. In addition, we noted that several Cre transgenic lines displayed spontaneous ectopic activity, reminiscent of deleter Cre mice, and required the confirmation of cell type-specific deletion in every individual mouse. When we used myosin heavy chain promoter Cre (alphaMyHC-Cre) mice for cardiomyocyte specific deletion, we observed, in addition to cardiomyocyte-restricted or complete excision, selective marker removal in a subgroup of mice as well. Thus, selective marker removal can be achieved as a byproduct of cell-type restricted deletion.

Development of mice with osteoblast-specific connexin43 gene deletion

Genetic deficiency of Cx43 in vivo causes skeletal developmental defects, osteoblast dysfunction and perinatal lethality. To determine the role of Cx43 in the adult skeleton, we developed two models of osteoblast-specific Cx43 gene deletion using Cre mediated replacement of a "floxed" Cx43 allele with a LacZ reporter gene. Cre recombinase expression in osteoblasts was driven by either the osteocalcin OG2 promoter or the 2.3 kb fragment of the Colalpha1(I) promoter. Homozygous Cx43(fl/fl) mice, in which the Cx43 coding region is flanked by two loxP sites, were crossed with Cre expressing mice in a heterozygous Cx43-null background [Cx43(+/-); Colalpha1(I)-Cre or Cx43(+/-); OG2-Cre]. Cx43 gene ablation was demonstrated in tissues by selective X-gal staining of cells lining the endosteal surface, and in cultured osteoblastic cells from calvaria using different approaches. Although no LacZ expression was observed in proliferating calvaria cells, before osteoblast differentiation begins, post-proliferative cells isolated from conditional knockout mice [Cx43(fl/-); Colalpha1(I)-Cre or Cx43(fl/-); OG2-Cre] developed strong LacZ expression as they differentiated, in parallel to a progressive disappearance of Cx43 mRNA and protein abundance relative to controls. Selective Cre mediated Cx43 gene inactivation in bone forming cells will be useful to determine the role of Cx43 in adult skeletal homeostasis and overcome the perinatal lethality of the conventional null model.

Defective epidermal barrier in neonatal mice lacking the C-terminal region of connexin43

More than 97% of mice in which the C-terminal region of connexin43 (Cx43) was removed (designated as Cx43K258stop) die shortly after birth due to a defect of the epidermal barrier. The abnormal expression of Cx43K258stop protein in the uppermost layers of the epidermis seems to perturb terminal differentiation of keratinocytes. In contrast to Cx43-deficient mice, neonatal Cx43K258stop hearts show no lethal obstruction of the right ventricular outflow tract, but signs of dilatation. Electrocardiographies of neonatal hearts reveal repolarization abnormalities in 20% of homozygous Cx43K258stop animals. The very rare adult Cx43K258stop mice show a compensation of the epidermal barrier defect but persisting impairment of cardiac function in echocardiography. Female Cx43K258stop mice are infertile due to impaired folliculogenesis. Our results indicate that the C-terminally truncated Cx43K258stop mice lack essential functions of Cx43, although the truncated Cx43 protein can form open gap junctional channels.

Electrical synapses: a dynamic signaling system that shapes the activity of neuronal networks

Gap junctions consist of intercellular channels dedicated to providing a direct pathway for ionic and biochemical communication between contacting cells. After an initial burst of publications describing electrical coupling in the brain, gap junctions progressively became less fashionable among neurobiologists, as the consensus was that this form of synaptic transmission would play a minimal role in shaping neuronal activity in higher vertebrates. Several new findings over the last decade (e.g. the implication of connexins in genetic diseases of the nervous system, in processing sensory information and in synchronizing the activity of neuronal networks) have brought gap junctions back into the spotlight. The appearance of gap junctional coupling in the nervous system is developmentally regulated, restricted to distinct cell types and persists after the establishment of chemical synapses, thus suggesting that this form of cell-cell signaling may be functionally interrelated with, rather than alternative to chemical transmission. This review focuses on gap junctions between neurons and summarizes the available data, derived from molecular, biological, electrophysiological, and genetic approaches, that are contributing to a new appreciation of their role in brain function.

Replacement by a lacZ reporter gene assigns mouse connexin36, 45 and 43 to distinct cell types in pancreatic islets

Transcripts of three connexin isoforms (Cx36, Cx43 and Cx45) have been reported in rodent pancreatic islets, but the precise distribution of the cognate proteins is still unknown. We determined expression of Cx36 in a cell-autonomous manner using mice with a targeted replacement of the Cx36 coding region by a lacZ reporter gene. For cell-autonomous monitoring of Cx43 expression, we used the Cre/loxP system: Mice carrying the Cx43 coding region flanked by loxP sites (floxed) also carried an embedded lacZ gene that is activated after Cre-mediated recombination in cells with transcriptional activity of the Cx43 gene. Deletion of the Cx43 coding region in beta-cells did not result in the activation of the embedded lacZ reporter gene. Instead, Cx43 expression was found in endothelial cells of the islets of Langerhans in mice with endothelium-specific deletion. Ubiquitous deletion of Cx43 led to a similar endothelial lacZ expression, but again, activity of the reporter gene was not detected in beta-cells. Mice with targeted replacement of the Cx45 coding region by lacZ showed a vascular expression similar to Cx43. The data show that native insulin-producing cells express a connexin isoform (Cx36) which differs from those (Cx43 and Cx45) expressed by vascular islet cells.

Reduced expression of P2Y1 receptors in connexin43-null mice alters calcium signaling and migration of neural progenitor cells

Glial calcium signals play important roles during CNS development. Calcium transients induced by ATP, acting on purinergic receptors, stimulate DNA synthesis, increase astrocytic and neural stem cell proliferation, and are prominent during the differentiation of radial glia. We have shown previously that expression of P2Y receptors in astrocytes is altered when connexin43 (Cx43) is downregulated. To evaluate the consequences of Cx43 deletion on calcium signaling during neural progenitor development, studies were performed on neurospheres derived from embryonic striatum. After adhesion, cells migrating from wild-type (WT) and Cx43-null neurospheres displayed spontaneous calcium oscillations. Such activity was blunted by apyrase, 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate (MRS-2179), and suramin, suggesting that ATP released by neural cells acts on purinergic receptors to induce calcium oscillations. The amplitudes of Ca2+ transients induced by P2Y but not P2X receptor agonists were larger in WT than in Cx43-null progenitors, suggesting that these two cell populations express different P2 receptors. Suramin, a nonselective P2 receptor antagonist, and MRS-2179, a P2Y1 receptor-selective antagonist, reduced the proliferation rate and the migration of WT progenitor cells to levels similar to those of Cx43-null cells. Conversely, exogenous expression of P2Y1 receptors in Cx43-null cells restored their migration pattern to levels seen in WT progenitors. However, treatment with P2 receptor antagonists did not alter the ratio of nestin to GFAP expression in WT neural progenitors. These data show that altered autocrine-paracrine communication attributable to reduced levels of P2Y1 receptors in neural progenitor cells lacking Cx43 affects proliferation and migration but not cell differentiation during early CNS development.

Astrocyte cultures from conditional connexin43-deficient mice

Connexin43 (Cx43) mainly provides the molecular basis for astrocytic gap junctions. Interastrocytic coupling is thought to mediate extracellular ion homeostasis, long-range signaling, and neuroprotection in the brain. Cx43 has been implicated in astrocytic growth control and is also expressed in other cell types in the brain, such as leptomeningeal and vascular cells. Cx43 function has been studied in astrocyte cultures of Cx43-deficient mice, which lack Cx43 in all cell types. We have generated conditionally deficient mice with an astrocyte-directed inactivation of Cx43, which leaves expression in other cell types unaffected. Other connexins have been detected in astrocytes. For the study of astrocytes lacking Cx45 and Cx26 in vitro, which deficiencies are embryonic lethal, conditionally deficient astrocyte cultures are essential. In the present study, we describe the developmental kinetics of Cx43 inactivation and loss of intercellular communication in astrocyte cultures derived from conditional Cx43-deficient mice. Conditional ablation of Cx43 is efficient, reaches a plateau at 4 weeks in culture, but retains Cx43 expression in contaminating nonastrocytic cells. Our findings indicate that conditional knockout astrocytes are a promising tool for the study of embryonic lethal genes in astrocyte cultures.

Connexin43 is not expressed in principal cells of mouse cortex and hippocampus

Previous immunofluorescence analyses in mice and rats showed a mainly astrocytic expression of the gap junction protein connexin43 (Cx43) in brain. However, in situ hybridization of murine brain sections suggested strong expression of Cx43 mRNA in hippocampal and cortical pyramidal neurons and Purkinje cells. These findings contrast with recent immunoelectron microscopic studies that excluded prominent Cx43 protein expression in neurons. Both contrasting results could be explained by post-transcriptional control mechanisms. Here we demonstrate by conditional replacement of the Cx43 coding region by a lacZ reporter gene, mimicking transcriptional activity of the Cx43 gene, that Cx43 is not expressed in principal cells of murine brain. This histochemical approach used is not prone to cross-reactivity of mRNA probes or antibodies. Furthermore, we show that in situ hybridization signals, suggested to be specific for Cx43 in mouse neurons, are retained even when the Cx43 coding DNA in neurons is removed by cre-mediated deletion. Our results confirm the previous findings of a mainly astrocytic expression of Cx43 in adult mouse brain and underscore the importance of connexin-deficient mice as controls for in situ hybridization studies. We found no evidence for post-transcriptional control of the Cx43 gene in principal neurons. Thus, the synchronized activity of neuronal networks cannot depend on Cx43 containing gap junctions in these cells.