Virtual cloning, functional expression, and gating analysis of human connexin31.9

Gap Junction Channels of Innexins and Connexins: Relations and Computational Perspectives

Gap junction (GJ) channels in invertebrates have been used to understand cell-to-cell communication in vertebrates. GJs are a common form of intercellular communication channels which connect the cytoplasm of adjacent cells. Dysregulation and structural alteration of the gap junction-mediated communication have been proven to be associated with a myriad of symptoms and tissue-specific pathologies. Animal models relying on the invertebrate nervous system have exposed a relationship between GJs and the formation of electrical synapses during embryogenesis and adulthood. The modulation of GJs as a therapeutic and clinical tool may eventually provide an alternative for treating tissue formation-related diseases and cell propagation. This review concerns the similarities between Hirudo medicinalis innexins and human connexins from nucleotide and protein sequence level perspectives. It also sets forth evidence of computational techniques applied to the study of proteins, sequences, and molecular dynamics. Furthermore, we propose machine learning techniques as a method that could be used to study protein structure, gap junction inhibition, metabolism, and drug development.

Administration of Lactobacillus paracasei ameliorates type 2 diabetes in mice

Probiotics have been proposed as an option for the prevention of type 2 diabetes mellitus (T2DM). The objective of this study was to evaluate the hypoglycemic effects of Lactobacillus paracasei on diabetic mice and explore the possible underlying molecular mechanism. The α-glucosidase inhibitory activities of eight L. paracasei strains were assessed in vitro. L. paracasei TD062 with high α-glucosidase inhibitory activity (31.9%) showed an excellent antidiabetic ability and it could survive in simulated gastrointestinal juices. To investigate the beneficial effects of L. paracasei TD062, diabetic mice were treated with the strain at 109, 108 and 107 CFU ml-1. The results indicated that the administration of L. paracasei TD062 could regulate the levels of fasting blood glucose (FBG), postprandial blood glucose (PBG), glucose tolerance, hepatic glycogen and lipid metabolism. In addition, the antioxidant capacity was also improved by oral administration of L. paracasei TD062. And the hypoglycemic effects exhibited dose dependence to some extent. Furthermore, it was revealed that L. paracasei TD062 had a positive effect on the expression levels of genes related to glucose metabolism and the PI3K/Akt pathway. These results demonstrated that L. paracasei TD062 played an important role in preventing the development of T2DM and might be applied as a new type of hypoglycemic agent in functional foods.

Connexin 43 a check-point component of cell proliferation implicated in a wide range of human testis diseases

Gap junction channels link cytoplasms of adjacent cells. Connexins, their constitutive proteins, are essential in cell homeostasis and are implicated in numerous physiological processes. Spermatogenesis is a sophisticated model of germ cell proliferation, differentiation, survival, and apoptosis, in which a connexin isotype, connexin 43, plays a crucial role as evidenced by genomic approaches based on gene deletion. The balance between cell proliferation/differentiation/apoptosis is a prerequisite for maintaining levels of spermatozoa essential for fertility and for limiting anarchic cell proliferation, a major risk of testis tumor. The present review highlights the emerging role of connexins in testis pathogenesis, focusing specifically on two intimately interconnected human testicular diseases (azoospermia with impaired spermatogenesis and testicular germ cell tumors), whose incidence increased during the last decades. This work proposes connexin 43 as a potential cancer diagnostic and prognostic marker, as well as a promising therapeutic target for testicular diseases.

Aspartic acid residue D3 critically determines Cx50 gap junction channel transjunctional voltage-dependent gating and unitary conductance

Previous studies have suggested that the aspartic acid residue (D) at the third position is critical in determining the voltage polarity of fast V(j)-gating of Cx50 channels. To test whether another negatively charged residue (a glutamic acid residue, E) could fulfill the role of the D3 residue, we generated the mutant Cx50D3E. V(j)-dependent gating properties of this mutant channel were characterized by double-patch-clamp recordings in N2A cells. Macroscopically, the D3E substitution reduced the residual conductance (G(min)) to near zero and outwardly shifted the half-inactivation voltage (V(0)), which is a result of both a reduced aggregate gating charge (z) and a reduced free-energy difference between the open and closed states. Single Cx50D3E gap junction channels showed reduced unitary conductance (γ(j)) of the main open state, reduced open dwell time at ±40 mV, and absence of a long-lived substate. In contrast, a G8E substitution tested to compare the effects of the E residue at the third and eighth positions did not modify the V(j)-dependent gating profile or γ(j). In summary, this study is the first that we know of to suggest that the D3 residue plays an essential role, in addition to serving as a negative-charge provider, as a critical determinant of the V(j)-dependent gating sensitivity, open-closed stability, and unitary conductance of Cx50 gap junction channels.

Mechanism of activation of PKB/Akt by the protein phosphatase inhibitor Calyculin A

The protein phosphatase inhibitor calyculin A activates PKB/Akt to ~50% of the activity induced by insulin-like growth factor 1 (IGF1) in HeLa cells promoting an evident increased phosphorylation of Ser473 despite the apparent lack of Thr308 phosphorylation of PKB. Nevertheless, calyculin A-induced activation of PKB seems to be dependent on basal levels of Thr308 phosphorylation, since a PDK1-dependent mechanism is required for calyculin A-dependent PKB activation by using embryonic stem cells derived from PDK1 wild-type and knockout mice. Data shown suggest that calyculin A-induced phosphorylation of Ser473 was largely blocked by LY294002 and SB-203580 inhibitors, indicating that both PI3-kinase/TORC2-dependent and SAPK2/p38-dependent protein kinases contributed to phosphorylation of Ser473 in calyculin A-treated cells. Additionally, our results suggest that calyculin A blocks the IGF1-dependent Thr308 phosphorylation and activation of PKB, likely due to an enhanced Ser612 phosphorylation of insulin receptor substrate 1 (IRS1), which can be inhibitory to its activation of PI3-kinase, a requirement for PDK1-induced Thr308 phosphorylation and IGF1-dependent activation of PKB. Our data suggest that PKB activity is most dependent on the level of Ser473 phosphorylation rather than Thr308, but basal levels of Thr308 phosphorylation are a requirement. Additionally, we suggest here that calyculin A regulates the IGF1-dependent PKB activation by controlling the PI3-kinase-associated IRS1 Ser/Thr phosphorylation levels.

Physiological and physiopathological aspects of connexins and communicating gap junctions in spermatogenesis

Spermatogenesis is a highly regulated process of germ cell proliferation and differentiation, starting from spermatogonia to spermatocytes and giving rise to spermatids, the future spermatozoa. In addition to endocrine regulation, testicular cell-cell interactions are essential for spermatogenesis. This precise control is mediated through paracrine/autocrine pathways, direct intercellular contacts and through intercellular communication channels, consisting of gap junctions and their constitutive proteins, the connexins. Gap junctions are localized between adjacent Leydig cells, between Sertoli cells and between Sertoli cells and specific germ cells. This review focuses on the distribution of connexins within the seminiferous epithelium, their participation in gap junction channel formation, the control of their expression and the physiological relevance of these junctions in both the Sertoli-Sertoli cell functional synchronization and the Sertoli-germ cell dialogue. In this review, we also discuss the potential implication of disrupted connexin in testis cancer, since impaired expression of connexin has been described as a typical feature of tumoral proliferation.

Molecular remodeling of ion channels, exchangers and pumps in atrial and ventricular myocytes in ischemic cardiomyopathy

Existing molecular knowledge base of cardiovascular diseases is rudimentary because of lack of specific attribution to cell type and function. The aim of this study was to investigate cell-specific molecular remodeling in human atrial and ventricular myocytes associated with ischemic cardiomyopathy. Our strategy combines two technological innovations, laser-capture microdissection of identified cardiac cells in selected anatomical regions of the heart and splice microarray of a narrow catalog of the functionally most important genes regulating ion homeostasis. We focused on expression of a principal family of genes coding for ion channels, exchangers and pumps (CE&P genes) that are involved in electrical, mechanical and signaling functions of the heart and constitute the most utilized drug targets. We found that (1) CE&P genes remodel in a cell-specific manner: ischemic cardiomyopathy affected 63 CE&P genes in ventricular myocytes and 12 essentially different genes in atrial myocytes. (2) Only few of the identified CE&P genes were previously linked to human cardiac disfunctions. (3) The ischemia-affected CE&P genes include nuclear chloride channels, adrenoceptors, cyclic nucleotide-gated channels, auxiliary subunits of Na(+), K(+) and Ca(2+) channels, and cell-surface CE&Ps. (4) In both atrial and ventricular myocytes ischemic cardiomyopathy reduced expression of CACNG7 and induced overexpression of FXYD1, the gene crucial for Na(+) and K(+) homeostasis. Thus, our cell-specific molecular profiling defined new landmarks for correct molecular modeling of ischemic cardiomyopathy and development of underlying targeted therapies.

An extensive survey of tyrosine phosphorylation revealing new sites in human mammary epithelial cells

Protein tyrosine phosphorylation represents a central regulatory mechanism in cell signaling. Here, we present an extensive survey of tyrosine phosphorylation sites in a normal-derived human mammary epithelial cell (HMEC) line by applying antiphosphotyrosine peptide immunoaffinity purification coupled with high sensitivity capillary liquid chromatography tandem mass spectrometry. A total of 481 tyrosine phosphorylation sites (covered by 716 unique peptides) from 285 proteins were confidently identified in HMEC following the analysis of both the basal condition and acute stimulation with epidermal growth factor (EGF). The estimated false discovery rate was 1.0% as determined by searching against a scrambled database. Comparison of these data with existing literature showed significant agreement for previously reported sites. However, we observed 281 sites that were not previously reported for HMEC cultures and 29 of which have not been reported for any human cell or tissue system. The analysis showed that a majority of highly phosphorylated proteins were relatively low-abundance. Large differences in phosphorylation stoichiometry for sites within the same protein were also observed, raising the possibility of more important functional roles for such highly phosphorylated pTyr sites. By mapping to major signaling networks, such as the EGF receptor and insulin growth factor-1 receptor signaling pathways, many known proteins involved in these pathways were revealed to be tyrosine phosphorylated, which provides interesting targets for future hypothesis-driven and targeted quantitative studies involving tyrosine phosphorylation in HMEC or other human systems.

An Update on Connexin Genes and their Nomenclature in Mouse and Man

Gap junctions, composed of connexin protein subunits, allow direct communication through conduits between neighboring cells. Twenty and twenty-one members of the connexin gene family are likely to be expressed in the mouse and human genome, respectively, 19 of which can be grouped into sequence-orthologous pairs. Their gene structure appears to be relatively simple. In most cases, an untranslated exon1 is separated by an intron of different lengh from exon2 that includes the uninterrupted coding region and the 3'-untranslated region. However, there are several exceptions to this scheme, since some mouse connexin genes contain different 5'-untranslated regions spliced either in an alternative and/or consecutive manner. Additionally, in at least 3 mouse and human connexin genes (mCx36, mCx39, mCx57 and hCx31.3, hCx36, as well as hCx40.1) the reading frame is spliced together from 2 different exons. So far, there are two nomenclatures to classify the known connexin genes: The "Gja/Gjb" nomenclature, as it is currently adopted by the NCBI data base, contains some inconsistencies compared to the "Cx" nomenclature. Here we suggest some minor corrections to co-ordinate the "Gja/Gjb" nomenclature with the "Cx" nomenclature. Furthermore, this short review contains an update on phenotypic correlations between connexin deficient mice and patients bearing mutations in their orthologous connexin genes.

Targeted disruption of ROCK1 causes insulin resistance in vivo

Insulin signaling is essential for normal glucose homeostasis. Rho-kinase (ROCK) isoforms have been shown to participate in insulin signaling and glucose metabolism in cultured cell lines. To investigate the physiological role of ROCK1 in the regulation of whole body glucose homeostasis and insulin sensitivity in vivo, we studied mice with global disruption of ROCK1. Here we show that, at 16-18 weeks of age, ROCK1-deficient mice exhibited insulin resistance, as revealed by the failure of blood glucose levels to decrease after insulin injection. However, glucose tolerance was normal in the absence of ROCK1. These effects were independent of changes in adiposity. Interestingly, ROCK1 gene ablation caused a significant increase in glucose-induced insulin secretion, leading to hyperinsulinemia. To determine the mechanism(s) by which deletion of ROCK1 causes insulin resistance, we measured the ability of insulin to activate phosphatidylinositol 3-kinase and multiple distal pathways in skeletal muscle. Insulin-stimulated phosphatidylinositol 3-kinase activity associated with IRS-1 or phospho-tyrosine was also reduced approximately 40% without any alteration in tyrosine phosphorylation of insulin receptor in skeletal muscle. Concurrently, serine phosphorylation of IRS-1 at serine 632/635, which is phosphorylated by ROCK in vitro, was also impaired in these mice. Insulin-induced phosphorylation of Akt, AS160, S6K, and S6 was also decreased in skeletal muscle. These data suggest that ROCK1 deficiency causes systemic insulin resistance by impairing insulin signaling in skeletal muscle. Thus, our results identify ROCK1 as a novel regulator of glucose homeostasis and insulin sensitivity in vivo, which could lead to new treatment approaches for obesity and type 2 diabetes.

mTORC1 activation regulates beta-cell mass and proliferation by modulation of cyclin D2 synthesis and stability

Growth factors, insulin signaling, and nutrients are important regulators of beta-cell mass and function. The events linking these signals to the regulation of beta-cell mass are not completely understood. The mTOR pathway integrates signals from growth factors and nutrients. Here, we evaluated the role of the mTOR/raptor (mTORC1) signaling in proliferative conditions induced by controlled activation of Akt signaling. These experiments show that the mTORC1 is a major regulator of beta-cell cycle progression by modulation of cyclin D2, D3, and Cdk4 activity. The regulation of cell cycle progression by mTORC1 signaling resulted from modulation of the synthesis and stability of cyclin D2, a critical regulator of beta-cell cycle, proliferation, and mass. These studies provide novel insights into the regulation of cell cycle by the mTORC1, provide a mechanism for the antiproliferative effects of rapamycin, and imply that the use of rapamycin could negatively impact the success of islet transplantation and the adaptation of beta-cells to insulin resistance.

Identification of MUP1 as a regulator for glucose and lipid metabolism in mice

Major urinary protein (MUP) 1 is a lipocalin family member abundantly secreted into the circulation by the liver. MUP1 binds to lipophilic pheromones and is excreted in urine. Urinary MUP1/pheromone complexes mediate chemical communication in rodents. However, it is unclear whether circulatory MUP1 has additional physiological functions. Here we show that MUP1 regulates glucose and lipid metabolism. MUP1 expression was markedly reduced in both genetic and dietary fat-induced obesity and diabetes. Mice were infected with MUP1 adenoviruses via tail vein injection, and recombinant MUP1 was overexpressed in the liver and secreted into the bloodstream. Recombinant MUP1 markedly attenuated hyperglycemia and glucose intolerance in genetic (db/db) and dietary fat-induced type 2 diabetic mice as well as in streptozotocin-induced type 1 diabetic mice. MUP1 inhibited the expression of both gluconeogenic genes and lipogenic genes in the liver. Moreover, recombinant MUP1 directly decreased glucose production in primary hepatocyte cultures by inhibiting the expression of gluconeogenic genes. These data suggest that MUP1 regulates systemic glucose and/or lipid metabolism through the paracrine/autocrine regulation of the hepatic gluconeogenic and/or lipogenic programs, respectively.

Growth hormone inhibition of glucose uptake in adipocytes occurs without affecting GLUT4 translocation through an insulin receptor substrate-2-phosphatidylinositol 3-kinase-dependent pathway

Growth hormone (GH) pretreatment of 3T3-L1 adipocytes resulted in a concentration- and time-dependent inhibition of insulin-stimulated glucose uptake. Surprisingly, this occurred without significant effect on insulin-stimulated glucose transporter (GLUT) 4 translocation or fusion with the plasma membrane. In parallel, the inhibitory actions of chronic GH pretreatment also impaired insulin-dependent activation of phosphatidylinositol (PI) 3-kinase bound to insulin receptor substrate (IRS)-2 but not to IRS-1. In addition, insulin-stimulated Akt phosphorylation was inhibited by GH pretreatment. In contrast, overexpression of IRS-2 or expression of a constitutively active Akt mutant prevented the GH-induced insulin resistance of glucose uptake. Moreover, small interfering RNA-mediated IRS-2 knockdown also inhibited insulin-stimulated Akt activation and glucose uptake without affecting GLUT4 translocation and plasma membrane fusion. Together, these data support a model in which chronic GH stimulation inhibits insulin-dependent activation of phosphatidylinositol 3-kinase through a specific interaction of phosphatidylinositol 3-kinase bound to IRS-2. This inhibition leads to suppression of Akt activation coupled to glucose transport activity but not translocation or plasma membrane fusion of GLUT4.

Cx30.2 can form heteromeric gap junction channels with other cardiac connexins

Since most cells in the heart co-express multiple connexins, we studied the possible heteromeric interactions between connexin30.2 and connexin40, connexin43 or connexin45 in transfected cells. Double-label immunofluorescence microscopy showed that connexin30.2 extensively co-localized with each co-expressed connexin at appositional membranes. When Triton X-100 solubilized connexons were affinity purified from co-expressing cells, connexin30.2 was isolated together with connexin40, connexin43, or connexin45. Co-expression of connexin30.2 with connexin40, connexin43, or connexin45 did not significantly reduce total junctional conductance. Gap junction channels in cells co-expressing connexin30.2 with connexin43 or connexin45 exhibited voltage-dependent gating intermediate between that of either connexin alone. In contrast, connexin30.2 dominated the voltage-dependence when co-expressed with connexin40. Our data suggest that connexin30.2 can form heteromers with the other cardiac connexins and that mixed channel formation will influence the gating properties of gap junctions in cardiac regions that co-express these connexins.

Phosphorylation of Ser357 of rat insulin receptor substrate-1 mediates adverse effects of protein kinase C-delta on insulin action in skeletal muscle cells

The activation of the protein kinase C (PKC) family of serine/threonine kinases contributes to the modulation of insulin signaling, and the PKC-dependent phosphorylation of insulin receptor substrate (IRS)-1 has been implicated in the development of insulin resistance. Here we demonstrate Ser(357) of rat IRS-1 as a novel PKC-delta-dependent phosphorylation site in skeletal muscle cells upon stimulation with insulin and phorbol ester using Ser(P)(357) antibodies and active and kinase dead mutants of PKC-delta. Phosphorylation of this site was simulated using IRS-1 Glu(357) and shown to reduce insulin-induced tyrosine phosphorylation of IRS-1, to decrease activation of Akt, and to subsequently diminish phosphorylation of glycogen synthase kinase-3. When the phosphorylation was prevented by mutation of Ser(357) to alanine, these effects of insulin were enhanced. When the adjacent Ser(358), present in mouse and rat IRS-1, was mutated to alanine, which is homologous to the human sequence, the insulin-induced phosphorylation of glycogen synthase kinase-3 or tyrosine phosphorylation of IRS-1 was not increased. Moreover, both active PKC-delta and phosphorylation of Ser(357) were shown to be necessary for the attenuation of insulin-stimulated Akt phosphorylation. The phosphorylation of Ser(357) could lead to increased association of PKC-delta to IRS-1 upon insulin stimulation, which was demonstrated with IRS-1 Glu(357). Together, these data suggest that phosphorylation of Ser(357) mediates at least in part the adverse effects of PKC-delta activation on insulin action.

53BP2S, interacting with insulin receptor substrates, modulates insulin signaling

It is well known that insulin receptor substrates (IRS) act as a mediator for signal transduction of insulin, insulin-like growth factors, and several cytokines. To identify proteins that interact with IRS and modulate IRS-mediated signals, we performed yeast two-hybrid screening with IRS-1 as bait. Out of 109 cDNA-positive clones identified from a human placental cDNA library, two clones encoded 53BP2, p53-binding protein 2 (53BP2S), a short form splicing variant of the apoptosis-stimulating protein of p53 that possesses Src homology region 3 domain, and ankyrin repeats domain, and had been reported to interact with p53, Bcl-2, and NF-kappaB. Interaction of 53BP2S with IRS-1 was confirmed by glutathione S-transferase pull-down and co-immunoprecipitation assays in COS-7 cells and 3T3-L1 adipocytes. The Src homology region 3 domain and ankyrin repeats domain of 53BP2S were responsible for its interaction with IRS-1, whereas the phosphotyrosine binding domain and a central domain (amino acid residues 750-861) of IRS-1 were required for its interaction with 53BP2S. In CHO-C400 cells, expression of 53BP2S reduced insulin-stimulated IRS-1 tyrosine phosphorylation with a concomitant enhancement of IRS-2 tyrosine phosphorylation. In addition, the amount of the phosphatidylinositol 3-kinase regulatory p85 subunit associated with tyrosine-phosphorylated proteins, and activation of Akt was enhanced by 53BP2S expression. Although 53BP2S also enhanced Akt activation in 3T3-L1 adipocytes, insulin-induced glucose transporter 4 translocation was markedly inhibited in accordance with reduction of insulin-induced AS160 phosphorylation. Together these data demonstrate that 53BP2S interacts and modulates the insulin signals mediated by IRSs.

Permeability of homotypic and heterotypic gap junction channels formed of cardiac connexins mCx30.2, Cx40, Cx43, and Cx45

We examined the permeabilities of homotypic and heterotypic gap junction (GJ) channels formed of rodent connexins (Cx) 30.2, 40, 43, and 45, which are expressed in the heart and other tissues, using fluorescent dyes differing in net charge and molecular mass. Combining fluorescent imaging and electrophysiological recordings in the same cell pairs, we evaluated the single-channel permeability (P(gamma)). All homotypic channels were permeable to the anionic monovalent dye Alexa Fluor-350 (AF(350)), but mCx30.2 channels exhibited a significantly lower P(gamma) than the others. The anionic divalent dye Lucifer yellow (LY) remained permeant in Cx40, Cx43, and Cx45 channels, but transfer through mCx30.2 channels was not detected. Heterotypic channels generally exhibited P(gamma) values that were intermediate to the corresponding homotypic channels. P(gamma) values of mCx30.2/Cx40, mCx30.2/Cx43, or mCx30.2/Cx45 heterotypic channels for AF(350) were similar and approximately twofold higher than P(gamma) values of mCx30.2 homotypic channels. Permeabilities for cationic dyes were assessed only qualitatively because of their binding to nucleic acids. All homotypic and heterotypic channel configurations were permeable to ethidium bromide and 4,6-diamidino-2-phenylindole. Permeability for propidium iodide was limited only for GJ channels that contain at least one mCx30.2 hemichannel. In summary, we have demonstrated that Cx40, Cx43, and Cx45 are permeant to all examined cationic and anionic dyes, whereas mCx30.2 demonstrates permeation restrictions for molecules with molecular mass over approximately 400 Da. The ratio of single-channel conductance to permeability for AF(350) was approximately 40- to 170-fold higher for mCx30.2 than for Cx40, Cx43, and Cx45, suggesting that mCx30.2 GJs are notably more adapted to perform electrical rather than metabolic cell-cell communication.

Molecular basis of voltage dependence of connexin channels: An integrative appraisal

The importance of electrical and molecular signaling through connexin (Cx) channels is now widely recognized. The transfer of ions and other small molecules between adjacent cells is regulated by multiple stimuli, including voltage. Indeed, Cx channels typically exhibit complex voltage sensitivity. Most channels are sensitive to the voltage difference between the cell interiors (or transjunctional voltage, V(j)), while other channels are also sensitive to absolute inside-outside voltage (i.e., the membrane potential, V(m)). The first part of this review is focused on the description of the distinct forms of voltage sensitivity and the gating mechanisms that regulate hemichannel activity, both individually and as components of homotypic and heterotypic gap junctions. We then provide an up to date and precise picture of the molecular and structural aspects of how V(j) and V(m) are sensed, and how they, therefore, control channel opening and closing. Mutagenic strategies coupled with structural, biochemical and electrophysical studies are providing significant insights into how distinct forms of voltage dependence are brought about. The emerging picture indicates that Cx channels can undergo transitions between multiple conductance states driven by distinct voltage-gating mechanisms. Each hemichannel may contain a set of two V(j) gates, one fast and one slow, which mediate the transitions between the main open state to the residual state and to the fully closed state, respectively. Eventually, a V(m) gate regulates channel transitions between the open and closed states. Clusters of charged residues within separate domains of the Cx molecule have been identified as integral parts of the V(j) and V(m) sensors. The charges at the first positions of the amino terminal cytoplasmic domain determine the magnitude and polarity of the sensitivity to fast V(j)-gating, as well as contributing to the V(j)-rectifying properties of ion permeation. Additionally, important advances have been made in identifying the conformational rearrangements responsible for fast V(j)-gating transitions to the residual state in the Cx43 channel. These changes involve an intramolecular particle-receptor interaction between the carboxy terminal domain and the cytoplasmic loop.

Treating insulin resistance: future prospects

Insulin resistance typically reflects multiple defects of insulin receptor and post-receptor signalling that impair a diverse range of metabolic and vascular actions. Many potential intervention targets and compounds with therapeutic activity have been described. Proof of principle for a non-peptide insulin mimetic has been demonstrated by specific activation of the intracellular B-subunit of the insulin receptor. Potentiation of insulin action has been achieved with agents that enhance phosphorylation and prolong the tyrosine kinase activity of the insulin receptor and its protein substrates after activation by insulin. These include inhibitors of phosphatases and serine kinases that normally prevent or terminate tyrosine kinase signalling. Additional approaches involve increasing the activity of phosphatidylinositol 3-kinase and other downstream components of the insulin signalling pathways. Experimental interventions to remove signalling defects caused by cytokines, certain adipocyte hormones, excess fatty acids, glucotoxicity and negative feedback by distal signalling steps have also indicated therapeutic possibilities. Several hormones, metabolic enzymes, minerals, co-factors and transcription co-activators have shown insulin-sensitising potential. Since insulin resistance affects many metabolic and cardiovascular diseases, it provides an opportunity for simultaneous therapeutic attack on a broad front.

Connexin-mediated cardiac impulse propagation: connexin 30.2 slows atrioventricular conduction in mouse heart

In mouse heart, four connexins (Cxs), Cx30.2, Cx40, Cx43, and Cx45, form gap junction (GJ) channels for electric and metabolic cell-to-cell signaling. Extent and pattern of Cx isoform expression together with cytoarchitecture and excitability of cells determine the velocity of excitation spread in different regions of the heart. In the SA node, cell-cell coupling is mediated by Cx30.2 and Cx45, which form low-conductance (approximately 9 and 32 pS, respectively) GJ channels. In contrast, the working cardiomyocytes of atria and ventricles express mainly Cx40 and Cx43, which form GJ channels of high conductance (approximately 180 and 115 pS, respectively) that facilitate the fast conduction necessary for efficient mechanical contraction. In the AV node, cell-cell coupling is mediated by abundantly expressed Cx30.2 and Cx45 and Cx40, which is expressed to a lesser extent. Cx30.2 and Cx45 may determine higher intercellular resistance and slower conduction in the SA- and AV-nodal regions than in the ventricular conduction system or the atrial and ventricular working myocardium. Cx30.2 and its putative human ortholog, Cx31.9, under physiologic conditions form unapposed hemichannels in nonjunctional plasma membrane; these hemichannels have a conductance of approximately 20 pS and are permeable to cationic dyes up to approximately 400 Da in molecular mass. Genetic ablation of Cxs confirmed that Cx40 and Cx43 are important in determining the high conduction velocities in atria and ventricles, whereas the deletion of the Cx30.2 complementary DNA led to accelerated conduction in the AV node and reduced the Wenckebach period. We suggest that these effects are caused by (1) a dominant-negative effect of Cx30.2 on junctional conductance via formation of low-conductance homotypic and heterotypic GJ channels, and (2) open Cx30.2 hemichannels in non-junctional membranes, which shorten the space constant and depolarize the excitable membrane.

Dephosphorylation by Default, a Potential Mechanism for Regulation of Insulin Receptor Substrate-1/2, Akt, and ERK1/2

Protein phosphorylation is an important mechanism that controls many cellular activities. Phosphorylation of a given protein is precisely controlled by two opposing biochemical reactions catalyzed by protein kinases and protein phosphatases. How these two opposing processes are coordinated to achieve regulation of protein phosphorylation is unresolved. We have developed a novel experimental approach to directly study protein dephosphorylation in cells. We determined the kinetics of dephosphorylation of insulin receptor substrate-1/2, Akt, and ERK1/2, phosphoproteins involved in insulin receptor signaling. We found that insulin-induced ERK1/2 and Akt kinase activities were completely abolished 10 min after inhibition of the corresponding upstream kinases with PD98059 and LY294002, respectively. In parallel experiments, insulin-induced phosphorylation of Akt, ERK1/2, and insulin receptor substrate-1/2 was decreased and followed similar kinetics. Our findings suggest that these proteins are dephosphorylated by a default mechanism, presumably via constitutively active phosphatases. However, dephosphorylation of these proteins is overcome by activation of protein kinases following stimulation of the insulin receptor. We propose that, during acute insulin stimulation, the kinetics of protein phosphorylation is determined by the interplay between upstream kinase activity and dephosphorylation by default.

Altered intercellular communication in lung fibroblast cultures from patients with idiopathic pulmonary fibrosis

Rationale

Gap junctions are membrane channels formed by an array of connexins which links adjacent cells realizing an electro- metabolic synapse. Connexin-mediated communication is crucial in the regulation of cell growth, differentiation, and development. The activation and proliferation of phenotypically altered fibroblasts are central events in the pathogenesis of idiopathic pulmonary fibrosis. We sought to evaluate the role of connexin-43, the most abundant gap-junction subunit in the human lung, in the pathogenesis of this condition.

Methods

We investigated the transcription and protein expression of connexin-43 and the gap-junctional intercellular communication (GJIC) in 5 primary lung fibroblast lines derived from normal subjects (NF) and from 3 histologically proven IPF patients (FF).

Results

Here we show that connexin-43 mRNA was significantly reduced in FF as demonstrated by standard and quantitative RT-PCR. GJIC was functionally evaluated by means of flow-cytometry. In order to demonstrate that dye spreading was taking place through gap junctions, we used carbenoxolone as a pharmacological gap-junction blocker. Carbenoxolone specifically blocked GJIC in our system in a concentration dependent manner. FF showed a significantly reduced homologous GJIC compared to NF. Similarly, GJIC was significantly impaired in FF when a heterologous NF line was used as dye donor, suggesting a complete defect in GJIC of FF.

Conclusion

These results suggest a novel alteration in primary lung fibroblasts from IPF patients. The reduced Cx43 expression and the associated alteration in cell-to-cell communication may justify some of the known pathological characteristic of this devastating disease that still represents a challenge to the medical practice.

Insulin-stimulated Interaction between Insulin Receptor Substrate 1 and p85α and Activation of Protein Kinase B/Akt Require Rab5

Binding of insulin to the insulin receptor initiates a cascade of protein phosphorylation and effector recruitment events leading to the activation of multiple distinct signaling pathways. Previous studies suggested that the diversity and specificity of insulin signal transduction are accomplished by both subcellular localization of receptor and the selective activation of downstream signaling molecules. The small GTPase Rab5 is a key regulator of endocytosis. Three Rab5 isoforms (Rab5a, -5b, and -5c) have been identified. Here we exploited the RNA interference technique to specifically knock down individual Rab5 isoforms to determine the cellular function of Rab5 in distinct insulin signaling pathways. Small interference RNA against a single Rab5 isoform had no effect on protein kinase B (PKB)/Akt or MAPK activation by insulin in NIH3T3 cells overexpressing human insulin receptor. However, simultaneous knockdown of all three Rab5 isoforms dramatically attenuated PKB/Akt activation by insulin without affecting MAPK activation. This inhibition of PKB/Akt activation was because of the impaired interaction between insulin receptor substrate 1 and the p85alpha subunit of phosphatidylinositol 3-kinase. These results indicate a requirement of Rab5 in presenting p85 to insulin receptor substrate 1. Additional evidence supporting a role for Rab5 was suggested by studies with GAPex-5, a vps9 domain containing exchange factor. Down-regulation of GAPex-5 impaired insulin-stimulated PKB/Akt activation. Collectively, this study indicates the involvement of Rab5 in insulin signaling.

Properties of mouse connexin 30.2 and human connexin 31.9 hemichannels: implications for atrioventricular conduction in the heart

Four connexins (Cxs), mouse (m)Cx30.2, Cx40, Cx43, and Cx45, determine cell-cell electrical signaling in mouse heart, and Cx43 and Cx45 are known to form unapposed hemichannels. Here we show that mCx30.2, which is most abundantly expressed in sinoatrial and atrioventricular nodal regions of the heart, and its putative human ortholog, human (h)Cx31.9, also form functional hemichannels, which, like mCx30.2 cell-cell channels, are permeable to cationic dyes up to approximately 400 Da in size. DAPI uptake by HeLa cells expressing mCx30.2 was >10-fold faster than that by HeLa parental cells. In Ca(2+)-free medium, uptake of DAPI by HeLaCx30.2-EGFP cells was increased approximately 2-fold, but uptake by parental cells was not affected. Conversely, acidification by application of CO(2) reduced DAPI uptake by HeLaCx30.2-EGFP cells but had little effect on uptake by parental cells. Cells expressing mCx30.2 exhibited higher rates of DAPI uptake than did cells expressing any of the other cardiac Cxs. Cardiomyocytes of 2-day-old rats transfected with hCx31.9-EGFP took up DAPI and ethidium bromide 5-10 times faster than wild-type cardiomyocytes. Mefloquine, a close derivative of quinine and quinidine that exhibits antimalarial and antiarrhythmic properties, reduced conductance of cell-cell junctions and dye uptake through mCx30.2 hemichannels with approximately the same affinity (IC(50) = approximately 10 microM) and increased dependence of junctional conductance on transjunctional voltage. Unitary conductance of mCx30.2 hemichannels was approximately 20 pS, about twice the cell-cell channel conductance. Hemichannels formed of mCx30.2 and hCx31.9 may slow propagation of excitation in the sinoatrial and atrioventricular nodes by shortening the space constant and depolarizing the excitable membrane.

Specific regulation of IRS-2 expression by glucose in rat primary pancreatic islet beta-cells

Insulin receptor substrate 2 (IRS-2) plays a critical role in pancreatic beta-cells. Increased IRS-2 expression promotes beta-cell growth and survival, whereas decreased IRS-2 levels lead to apoptosis. It was found that IRS-2 turnover in rat islet beta-cells was rapid, with mRNA and protein half-lives of approximately 90 min and approximately 2 h, respectively. However, this was countered by specific glucose-regulated IRS-2 expression mediated at the transcriptional level. Glucose (> or = 6 mM) increased IRS-2 mRNA and protein levels in a dose-dependent manner, reaching a maximum 4-fold increase in IRS-2 mRNA and a 5-6-fold increase in IRS-2 protein levels at > or = 12 mM glucose (p < or = 0.01). Glucose (15 mM) regulation of islet beta-cell IRS-2 gene expression was rapid, with a significant increase in IRS-2 mRNA levels within 2 h that reached a maximum 4-fold increase by 4 h. IRS-2 protein expression in beta-cells followed that of IRS-2 mRNA. Glucose metabolism was necessary for increased IRS-2 expression in beta-cells. Moreover, inhibition of a glucose-induced rise in islet beta-cell cytosolic [Ca2+]i prevented an increase in IRS-2 expression, indicating this was Ca2+-dependent. The glucose-induced rise in IRS-2 levels correlated with increased IRS-2 tyrosine phosphorylation and downstream activation of protein kinase B. These data indicate that fluctuations of glucose in the normal physiological range (5-15 mM) promote beta-cell survival via regulation of IRS-2 expression and a subsequent parallel protein kinase B activation. Given that the onset of type-2 diabetes is marked by loss of beta-cells, these data further the idea that controlled IRS-2 expression in beta-cells could be a therapeutic means to promote beta-cell survival and delay the onset of the disease.

Connexin30.2 containing gap junction channels decelerate impulse propagation through the atrioventricular node

In the mammalian heart, gap junction channels between electrically coupled cardiomyocytes are necessary for impulse propagation and coordinated contraction of atria and ventricles. Recently, mouse connexin30.2 (Cx30.2) was shown to be expressed in the cardiac conduction system, predominantly in sinoatrial and atrioventricular (AV) nodes. The corresponding gap junctional channels expressed in HeLa cells exhibit the lowest unitary conductance (9 pS) of all connexin channels. Here we report that Cx30.2 slows down the propagation of excitation through the AV node. Mice expressing a LacZ reporter gene instead of the Cx30.2 coding region (Cx30.2(LacZ/LacZ)) exhibit a PQ interval that is approximately 25% shorter than in WT littermates. By recording atrial, His, and ventricular signals with intracardiac electrodes, we show that this decrease is attributed to significantly accelerated conduction above the His bundle (atrial-His interval: 27.9 +/- 5.1 ms in Cx30.2(LacZ/LacZ) versus 37.1 +/- 4.1 ms in Cx30.2(+/+) mice), whereas HV conduction is unaltered. Atrial stimulation revealed an elevated AV-nodal conduction capacity and faster ventricular response rates during induced episodes of atrial fibrillation in Cx30.2(LacZ/LacZ) mice. Our results show that Cx30.2 contributes to the slowdown of impulse propagation in the AV node and additionally limits the maximum number of beats conducted from atria to ventricles. Thus, it is likely to be involved in coordination of atrial and ventricular contraction and to fulfill a protective role toward pathophysiological states such as atrial tachyarrhythmias (e.g., atrial fibrillation) by preventing rapid conduction to the ventricles potentially associated with hemodynamic deterioration.

Phylogenetic analysis of three complete gap junction gene families reveals lineage-specific duplications and highly supported gene classes

Gap junctions, composed of connexin proteins in chordates, are the most ubiquitous form of intercellular communication. Complete connexin gene families have been identified from human (20) and mouse (19), revealing significant diversity in gap junction channels. We searched current databases and identified 37 putative zebrafish connexin genes, almost twice the number found in mammals. Phylogenetic comparison of entire connexin gene families from human, mouse, and zebrafish revealed 23 zebrafish relatives of 16 mammalian connexins, and 14 connexins apparently unique to zebrafish. We found evidence for duplication events in all genomes, as well as evidence for recent tandem duplication events in the zebrafish, indicating that the complexity of the connexin family is growing. The identification of a third complete connexin gene family provides novel insight into the evolution of connexins, and sheds light into the phenotypic evolution of intercellular communication via gap junctions.

Direct cross-talk of interleukin-6 and insulin signal transduction via insulin receptor substrate-1 in skeletal muscle cells

The exercise-induced interleukin (IL)-6 production and secretion within skeletal muscle fibers has raised the question of a putative tissue-specific function of IL-6 in the energy metabolism of the muscle during and after the exercise. In the present study, we followed the hypothesis that IL-6 signaling may directly interact with insulin receptor substrate (IRS)-1, a keystone in the insulin signaling cascade. We showed that IL-6 induces a rapid recruitment of IRS-1 to the IL-6 receptor complex in cultured skeletal muscle cells. Moreover, IL-6 induced a rapid and transient phosphorylation of Ser-318 of IRS-1 in muscle cells and in muscle tissue, but not in the liver of IL-6-treated mice, probably via the IL-6-induced co-recruitment of protein kinase C-delta. This Ser-318 phosphorylation improved insulin-stimulated Akt phosphorylation and glucose uptake in myotubes since transfection with an IRS-1/Glu-318 mutant simulating a permanent phospho-Ser-318 modification increased Akt phosphorylation and glucose uptake. Noteworthily, two inhibitory mechanisms of IL-6 on insulin action, phosphorylation of the inhibitory Ser-307 residue of IRS-1 and induction of SOCS-3 expression, were only found in liver but not in muscle of IL-6-treated mice. Thus, the data provided evidence for a possible molecular mechanism of the physiological metabolic effects of IL-6 in skeletal muscle, thereby exerting short term beneficial effects on insulin action.

Expression of gap junctional connexins 26, 32 and 43 in bovine placentomes during pregnancy

Gap junctional connexins (Cx) are induced in the endometrium during implantation in rodents, the human receptive window, and in the decidua Cx26 and Cx43 expression increases in response to trophoblast invasion. In contrast, this gap junctional response and decidualization is absent in non-invasive epitheliochorial placentae of pigs and horses. Bovine (syn)epitheliochorial placentation represents an intermediate type of trophoblast invasion, since it is characterized by the continuous migration and fusion of trophoblast giant cells (TGC) with uterine epithelial cells. Therefore the objective of the present study was to investigate the expression of Cx26, Cx32, and Cx43 in placental tissues during bovine pregnancy, to determine if Cx expression patterns correlate with the depth of trophoblast invasion. Cx26, Cx32, and Cx43 proteins were detected by immunohistochemistry and corresponding specific mRNAs were shown by RT-PCR and localized in tissue sections by in situ hybridization. Cx26 protein was detected at the feto-maternal contact interface and as cytoplasmic staining in TGC. Cx26 mRNA was located in maternal epithelium and in TGC. Cx32 protein expression was observed in the maternal epithelium exclusively on the tips of maternal septa, whereas Cx32 mRNA was detected in all maternal epithelial cells and single TGC. Cx43 protein and mRNA were coexpressed in TGC. Cx43 protein was present in maternal septal stroma and to a lesser extent in chorionic villous mesenchyme, while Cx43 mRNA was associated with the vasculature. In the course of gestation, expression of Cx26, Cx32, and Cx43 did not change. In conclusion, the intermediate invasive status of bovine trophoblast is supported by the fact that TGC coexpress Cx26, Cx32, and Cx43, which may be important for trophoblast migration (invasion), and fusion with maternal epithelial cells. Cx32 could be involved in the control of invasion.

The detection of hamster connexins: a comparison of expression profiles with wild-type mouse and the cancer-prone Min mouse

The open reading frames of 17 connexins from Syrian hamster (using tissues) and 16 connexins from the Chinese hamster cell line V79, were fully (Cx30, Cx31, Cx37, Cx43 and Cx45) or partially sequenced. We have also detected, and partially sequenced, seven rat connexins that previously were unavailable. The expression of connexin genes was examined in some hamster organs and cultured hamster cells, and compared with wild-type mouse and the cancer-prone Min mouse. Although the expression patterns were similar for most organs and connexins in hamster and mouse, there were also some prominent differences (Cx29 and 30.3 in testis; Cx31.1 and 32 in eye; Cx46 in brain, kidney and testis; Cx47 in kidney). This suggests that some connexins have species-specific expression profiles. In contrast, there were minimal differences in expression profiles between wild type and Min mice. Species-specific expression profiles should be considered in attempts to make animal models of human connexin-associated diseases.

Gap junctional communication in the male reproductive system

Male fertility is a highly controlled process that allows proliferation, meiosis and differentiation of male germ cells in the testis, final maturation in the epididymis and also requires functional male accessory glands: seminal vesicles, prostate and corpus cavernosum. In addition to classical endocrine and paracrine controls, mainly by gonadotropins LH and FSH and steroids, there is now strong evidence that all these processes are dependent upon the presence of homocellular or heterocellular junctions, including gap junctions and their specific connexins (Cxs), between the different cell types that structure the male reproductive tract. The present review is focused on the identification of Cxs, their distribution in the testis and in different structures of the male genital tract (epididymis, seminal vesicle, prostate, corpus cavernosum), their crucial role in the control of spermatogenesis and their implication in the function of the male accessory glands, including functional smooth muscle tone. Their potential dysfunctions in some testis (spermatogenic arrest, seminoma) and prostate (benign hyperplasia, adenocarcinoma) diseases and in the physiopathology of the human erectile function are also discussed.

Cloning and functional characterization of a novel connexin expressed in somites of Xenopus laevis

Connexin-containing gap junctions play an essential role in vertebrate development. More than 20 connexin isoforms have been identified in mammals. However, the number identified in Xenopus trails with only six isoforms described. Here, identification of a new connexin isoform from Xenopus laevis is described. Connexin40.4 was found by screening expressed sequence tag databases and carrying out polymerase chain reaction on genomic DNA. This new connexin has limited amino acid identity with mammalian (<50%) connexins, but conservation is higher (approximately 62%) with fish. During Xenopus laevis development, connexin40.4 was first expressed after the mid-blastula transition. There was prominent expression in the presomitic paraxial mesoderm and later in the developing somites. In adult frogs, expression was detected in kidney and stomach as well as in brain, heart, and skeletal muscle. Ectopic expression of connexin40.4 in HEK293 cells, resulted in formation of gap junction like structures at the cell interfaces. Similar ectopic expression in neural N2A cells resulted in functional electrical coupling, displaying mild, asymmetric voltage dependence. We thus cloned a novel connexin from Xenopus laevis, strongly expressed in developing somites, with no apparent orthologue in mammals.

Protein kinase C-alpha regulates insulin action and degradation by interacting with insulin receptor substrate-1 and 14-3-3 epsilon

Protein kinase C (PKC)-alpha exerts a regulatory function on insulin action. We showed by overlay blot that PKCalpha directly binds a 180-kDa protein, corresponding to IRS-1, and a 30-kDa molecular species, identified as 14-3-3epsilon. In intact NIH-3T3 cells overexpressing insulin receptors (3T3-hIR), insulin selectively increased PKCalpha co-precipitation with IRS-1, but not with IRS-2, and with 14-3-3epsilon, but not with other 14-3-3 isoforms. Overexpression of 14-3-3epsilon in 3T3-hIR cells significantly reduced IRS-1-bound PKCalpha activity, without altering IRS-1/PKCalpha co-precipitation. 14-3-3epsilon overexpression also increased insulin-stimulated insulin receptor and IRS-1 tyrosine phosphorylation, followed by increased activation of Raf1, ERK1/2, and Akt/protein kinase B. Insulin-induced glycogen synthase activity and thymidine incorporation were also augmented. Consistently, selective depletion of 14-3-3epsilon by antisense oligonucleotides caused a 3-fold increase of IRS-1-bound PKCalpha activity and a similarly sized reduction of insulin receptor and IRS-1 tyrosine phosphorylation and signaling. In turn, selective inhibition of PKCalpha expression by antisense oligonucleotides reverted the negative effect of 14-3-3epsilon depletion on insulin signaling. Moreover, PKCalpha inhibition was accompanied by a >2-fold decrease of insulin degradation. Similar results were also obtained by overexpressing 14-3-3epsilon. Thus, in NIH-3T3 cells, insulin induces the formation of multimolecular complexes, including IRS-1, PKCalpha, and 14-3-3epsilon. The presence of 14-3-3epsilon in the complex is not necessary for IRS-1/PKCalpha interaction but modulates PKCalpha activity, thereby regulating insulin signaling and degradation.

Phosphatase and tensin homolog regulation of islet growth and glucose homeostasis

The Irs2 branch of the insulin/insulin-like growth factor signaling cascade activates the phosphatidylinositol 3-kinase --> Akt --> Foxo1 cascade in many tissues, including hepatocytes and pancreatic beta-cells. The 3'-lipid phosphatase Pten ordinarily attenuates this cascade; however, its influence on beta-cell growth or function is unknown. To determine whether decreased Pten expression could restore beta-cell function and prevent diabetes in Irs2(-/-) mice, we generated wild type or Irs2 knock-out mice that were haploinsufficient for Pten (Irs2(-/-)::Pten(+/-)). Irs2(-/-) mice develop diabetes by 3 months of age as beta-cell mass declined progressively until insulin production was lost. Pten insufficiency increased peripheral insulin sensitivity in wild type and Irs2(-/-) mice and increased Akt and Foxo1 phosphorylation in the islets. Glucose tolerance improved in the Pten(+/-) mice, although beta-cell mass and circulating insulin levels decreased. Compared with Irs2(-/-) mice, the Irs2(-/-)::Pten(+/-) mice displayed nearly normal glucose tolerance and survived without diabetes, because normal but small islets produced sufficient insulin until the mice died of lymphoproliferative disease at 12 months age. Thus, steps to enhance phosphatidylinositol 3-kinase signaling can promote beta-cell growth, function, and survival without the Irs2 branch of the insulin/insulin-like growth factor signaling cascade.

Role of connexin-based gap junction channels in testis

Spermatogenesis is a highly controlled process that allows proliferation and differentiation of male germ cells. This is under classical endocrine and paracrine controls. There is also evidence that gap junctions between Leydig cells, between Sertoli cells and between Sertoli and germ cells participate in the local regulation of spermatogenesis. Recent studies reveal that connexin 43 (Cx43), the predominant gap junction protein in the testis, is essential for the initiation and maintenance of spermatogenesis. In this review, we focus on the identification, distribution and control of connexins in the mammalian testis. The implication of connexin-based gap junctions in testicular physiology and in pathological disorders of spermatogenesis (spermatogenic arrest and testis cancer) is also discussed.

The phosphorylation of Ser318 of insulin receptor substrate 1 is not per se inhibitory in skeletal muscle cells but is necessary to trigger the attenuation of the insulin-stimulated signal

The Ser/Thr phosphorylation of insulin receptor substrate 1 (IRS) is one key mechanism to stimulate and/or attenuate insulin signal transduction. Using a phospho-specific polyclonal antibody directed against phosphorylated Ser(318) of IRS-1, we found a rapid and transient insulin-stimulated phosphorylation of Ser(318) in human and rodent skeletal muscle cell models and in muscle tissue of insulin-treated mice. None of the investigated insulin resistance-associated factors (e.g. high glucose, tumor necrosis factor-alpha, adrenaline) stimulated the phosphorylation of Ser(318). Studying the function of this phosphorylation, we found that replacing Ser(318) by alanine completely prevented both the attenuation of insulin-stimulated Akt/protein kinase B Ser(473) phosphorylation and glucose uptake after 60 min of insulin stimulation. Unexpectedly, after acute insulin stimulation, we observed that phosphorylation of Ser(318) is not inhibitory but rather enhances insulin signal transduction because introduction of Ala(318) led to a reduction of the insulin-stimulated Akt/protein kinase B phosphorylation. Furthermore, replacing Ser(318) by glutamate, i.e. mimicking phosphorylation, improved glucose uptake after acute insulin stimulation. These data suggest that phosphorylation of Ser(318) is not per se inhibitory but is necessary to trigger the attenuation of the insulin-stimulated signal in skeletal muscle cells. Investigating the molecular mechanism of insulin-stimulated Ser(318) phosphorylation, we found that phosphatidylinositol 3-kinase-mediated activation of atypical protein kinase C-zeta and recruitment of protein kinase C-zeta to IRS-1 was responsible for this phosphorylation. We conclude that Ser(318) phosphorylation of IRS-1 is an early physiological event in insulin-stimulated signal transduction, which attenuates the continuing action of insulin.

PTEN, but Not SHIP2, Suppresses Insulin Signaling through the Phosphatidylinositol 3-Kinase/Akt Pathway in 3T3-L1 Adipocytes

Glucose homeostasis is controlled by insulin in part through the stimulation of glucose transport in muscle and fat cells. This insulin signaling pathway requires phosphatidylinositol (PI) 3-kinase-mediated 3'-polyphosphoinositide generation and activation of Akt/protein kinase B. Previous experiments using dominant negative constructs and gene ablation in mice suggested that two phosphoinositide phosphatases, SH2 domain-containing inositol 5'-phosphatase 2 (SHIP2) and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) negatively regulate this insulin signaling pathway. Here we directly tested this hypothesis by selectively inhibiting the expression of SHIP2 or PTEN in intact cultured 3T3-L1 adipocytes through the use of short interfering RNA (siRNA). Attenuation of PTEN expression by RNAi markedly enhanced insulin-stimulated Akt and glycogen synthase kinase 3alpha (GSK-3alpha) phosphorylation, as well as deoxyglucose transport in 3T3-L1 adipocytes. In contrast, depletion of SHIP2 protein by about 90% surprisingly failed to modulate these insulin-regulated events under identical assay conditions. In control studies, no diminution of insulin signaling to the mitogen-activated protein kinases Erk1 and Erk2 was observed when either PTEN or SHIP2 were depleted. Taken together, these results demonstrate that endogenous PTEN functions as a suppressor of insulin signaling to glucose transport through the PI 3-kinase pathway in cultured 3T3-L1 adipocytes.

Functional properties of mouse connexin30.2 expressed in the conduction system of the heart

Gap junction channels composed of connexin (Cx) 40, Cx43, and Cx45 proteins are known to be necessary for impulse propagation through the heart. Here, we report mouse connexin30.2 (mCx30.2) to be a new cardiac connexin that is expressed mainly in the conduction system of the heart. Antibodies raised to the cytoplasmic loop or the C-terminal regions of mCx30.2 recognized this protein in mouse heart as well as in HeLa cells transfected with wild-type mCx30.2 or mCx30.2 fused with enhanced green fluorescent protein (mCx30.2-EGFP). Immunofluorescence analyses of adult hearts yielded positive signals within the sinoatrial node, atrioventricular node, and A-V bundle of the cardiac conduction system. Dye transfer studies demonstrated that mCx30.2 and mCx30.2-EGFP channels discriminate poorly on the basis of charge, but do not allow permeation of tracers >400 Da. Both mCx30.2 and mCx30.2-EGFP gap junctional channels exhibited weak sensitivity to transjunctional voltage (Vj) and a single channel conductance of approximately 9 pS, which is the lowest among all members of the connexin family measured in HeLa cell transfectants. HeLa mCx30.2-EGFP transfectants when paired with cells expressing Cx40, Cx43, or Cx45 formed functional heterotypic gap junction channels that exhibited low unitary conductances (15 to 18 pS), rectifying open channel I-V relations and asymmetric Vj dependence. The electrical properties of homo- and hetero-typic junctions involving mCx30.2 may contribute to slow propagation velocity in nodal tissues and directional asymmetry of excitation spread in the AV nodal region.

Expression of the rat connexin 39 (rCx39) gene in myoblasts and myotubes in developing and regenerating skeletal muscles: an in situ hybridization study

We report a detailed analysis of the expression pattern of the recently identified rat connexin gene, named rat connexin 39 (rCx39), both during embryonic development and in adult life. Qualitative and quantitative reverse transcription/polymerase chain reaction analysis showed intense expression of rCx39 restricted to differentiating skeletal muscles, with a peak of expression detected at 18 days of embryonic life, followed by a rapid decline to undetectable levels within the first week of postnatal life. A combination of the in situ hybridization technique for the detection of rCx39 mRNA and immunohistochemistry for myogenin, a myoblast-specific marker, allowed us to establish that the mRNA for this connexin was expressed in myogenin-positive myoblasts and early myotubes but disappeared in mature myotubes. Moreover, in adult animals, rCx39 mRNA was expressed in myogenic cells involved in skeletal myofiber regeneration following a crush injury. This is the first case of a connexin being mainly expressed in the myogenic cell lineage. The information presented should pave the way to novel molecular approaches in studies on the role of connexin-based gap-junctional communication in skeletal muscle differentiation and regeneration.

Differential contribution of insulin receptor substrates 1 versus 2 to insulin signaling and glucose uptake in l6 myotubes

Insulin receptor substrates-1 and 2 (IRS-1 and IRS-2) are pivotal in relaying insulin signaling in insulin-responsive tissues such as muscle. However, the precise contribution of IRS-1 vis-a-vis IRS-2 in insulin-mediated metabolic and mitogenic responses has not been compared directly in differentiated muscle cells. This study aimed to determine the relative contribution of IRS-1 versus IRS-2 in these responses, using small interfering RNA (siRNA)-mediated specific gene silencing. In L6 myotubes, transfection of siRNA targeted specifically against IRS-1 (siIRS-1) or IRS-2 (siIRS-2) reduced the cognate protein expression by 70-75%. Insulin-induced ERK phosphorylation was much more sensitive to IRS-2 than IRS-1 ablation, whereas p38MAPK phosphorylation was reduced by 43 or 62% in myotubes treated with siIRS-1 or siIRS-2, respectively. Insulin-induced Akt1 and Akt2 phosphorylation was reduced in myotubes treated with siIRS-1, but only Akt2 phosphorylation was reduced in myotubes treated with siIRS-2. In contrast, siIRS-1 treatment caused a marked reduction in insulin-induced actin remodeling, glucose uptake, and GLUT4 translocation, and siIRS-2 was without effect on these responses. Notably, combined siIRS-1 and siIRS-2, although reducing each IRS by around 75%, caused no further drop in glucose uptake than that achieved with siIRS-1 alone, but abolished p38MAPK phosphorylation. We conclude that insulin-stimulated Akt1 phosphorylation, actin remodeling, GLUT4 translocation, and glucose uptake are regulated mainly by IRS-1, whereas IRS-2 contributes selectively to ERK signaling, and Akt2 and p38MAPK lie downstream of both IRS in muscle cells.

Cloning and functional expression of invertebrate connexins from Halocynthia pyriformis

Unlike many other ion channels, unrelated gene families encode gap junctions in different animal phyla. Connexin and pannexin genes are found in deuterostomes, while protostomal species use innexin genes. Connexins are often described as vertebrate genes, despite the existence of invertebrate deuterostomes. We have cloned connexin sequences from an invertebrate chordate, Halocynthia pyriformis. Invertebrate connexins shared 25-40% sequence identity with human connexins, had extracellular domains containing six invariant cysteine residues, coding regions that were interrupted by introns, and formed functional channels in vitro. These data show that gap junction channels based on connexins are present in animals that predate vertebrate evolution.

Hepatocyte CYP2E1 overexpression and steatohepatitis lead to impaired hepatic insulin signaling

Insulin resistance and increased cytochrome P450 2E1 (CYP2E1) expression are both associated with and mechanistically implicated in the development of nonalcoholic fatty liver disease. Although currently viewed as distinct factors, insulin resistance and CYP2E1 expression may be interrelated through the ability of CYP2E1-induced oxidant stress to impair hepatic insulin signaling. To test this possibility, the effects of in vitro and in vivo CYP2E1 overexpression on hepatocyte insulin signaling were examined. CYP2E1 overexpression in a hepatocyte cell line decreased tyrosine phosphorylation of insulin receptor substrate (IRS)-1 and IRS-2 in response to insulin. CYP2E1 overexpression was also associated with increased inhibitory serine 307 and 636/639 IRS-1 phosphorylation. In parallel, the effects of insulin on Akt activation, glycogen synthase kinase 3, and FoxO1a phosphorylation, and glucose secretion were all significantly decreased in CYP2E1 overexpressing cells. This inhibition of insulin signaling by CYP2E1 overexpression was partially c-Jun N-terminal kinase dependent. In the methionine- and choline-deficient diet mouse model of steatohepatitis with CYP2E1 overexpression, insulin-induced IRS-1, IRS-2, and Akt phosphorylation were similarly decreased. These findings indicate that increased hepatocyte CYP2E1 expression and the presence of steatohepatitis result in the down-regulation of insulin signaling, potentially contributing to the insulin resistance associated with nonalcoholic fatty liver disease.

Serine 332 phosphorylation of insulin receptor substrate-1 by glycogen synthase kinase-3 attenuates insulin signaling

The ability of glycogen synthase kinase-3 (GSK-3) to phosphorylate insulin receptor substrate-1 (IRS-1) is a potential inhibitory mechanism for insulin resistance in type 2 diabetes. However, the serine site(s) phosphorylated by GSK-3 within IRS-1 had not been yet identified. Using an N-terminal deleted IRS-1 mutant and two IRS-1 fragments, PTB-1 1-320 and PTB-2 1-350, we localized GSK-3 phosphorylation site(s) within amino acid sequence 320-350. Mutations of serine 332 or 336, which lie in the GSK-3 consensus motif (SXXXS) within PTB-2 or IRS-1, to alanine abolished their phosphorylation by GSK-3. This suggested that Ser332 is a GSK-3 phosphorylation site and that Ser336 serves as the "priming" site typically required for GSK-3 action. Indeed, dephosphorylation of IRS-1 prevented GSK-3 phosphorylation. Furthermore, the phosphorylated peptide derived from the IRS-1 sequence was readily phosphorylated by GSK-3, in contrast to the nonphosphorylated peptide, which was not phosphorylated by the enzyme. When IRS-1 mutants S332A(IRS-1), S336A(IRS-1), or S332A/336A(IRS-1) were expressed in Chinese hamster ovary cells overexpressing insulin receptors, their insulin-induced tyrosine phosphorylation levels increased compared with that of wild-type (WT) IRS-1. This effect was stronger in the double mutant S332A/336A(IRS-1) and led to enhanced insulin-mediated activation of protein kinase B. Finally, immunoblot analysis with polyclonal antibody directed against IRS-1 phosphorylated at Ser332 confirmed IRS-1 phosphorylation in cultured cells. Moreover, treatment with the GSK-3 inhibitor lithium reduced Ser332 phosphorylation, whereas overexpression of GSK-3 enhanced this phosphorylation. In summary, our studies identify Ser332 as the GSK-3 phosphorylation target in IRS-1, indicating its physiological relevance and demonstrating its novel inhibitory role in insulin signaling.

Insulin receptor substrate-2 proteasomal degradation mediated by a mammalian target of rapamycin (mTOR)-induced negative feedback down-regulates protein kinase B-mediated signaling pathway in beta-cells

Regulation of insulin receptor substrate (IRS)-2 expression is critical to beta-cell survival, but the mechanisms that control this are complex and undefined. Here in pancreatic beta-cells (INS-1), chronic exposure (>8 h) to 15 mm glucose and/or 5 nm IGF-1, increased Ser/Thr phosphorylation of IRS-2, which correlated with decreased IRS-2 levels. This glucose/IGF-1-induced decrease in IRS-2 levels was prevented by the proteasomal inhibitor, lactacystin. In addition, the glucose/IGF-1-induced increase in Ser/Thr phosphorylation of IRS-2 and the subsequent decrease in INS-1 cell IRS-2 protein levels was thwarted by the mammalian target of rapamycin(mTOR) inhibitor, rapamycin. Moreover, adenoviral-mediated expression of constitutively active mTOR (mTORDelta) further increased glucose/IGF-1-induced Ser/Thr phosphorylation of IRS-2 and decreased IRS-2 protein levels, whereas adenoviral-mediated expression of "kinase-dead" mTOR (mTOR-KD) conversely reduced Ser/Thr phosphorylation of IRS-2 and maintained IRS-2 protein levels. In adenoviral-infected beta-cells expressing mTORDelta, the decrease in IRS-2 protein levels was also prevented by rapamycin or lactacystin, further indicating a proteasomal mediated degradation of IRS-2 mediated via mTOR-induced Ser/Thr phosphorylation of IRS-2. Finally, we found that chronic activation of mTOR leading to decreased levels of IRS-2 in INS-1 cells led to a significant decrease in PKB activation and consequently increased beta-cell apoptosis. Thus, chronic activation of mTOR by glucose (and/or IGF-1) in beta-cells leads to increased Ser/Thr phosphorylation of IRS-2 that targets it for proteasomal degradation, resulting in decreased IRS-2 expression and increased beta-cell apoptosis. This may be a contributing mechanism as to how beta-cell mass is decreased by chronic hyperglycemia in the pathogenesis of type-2 diabetes.

In vitro phosphorylation of insulin receptor substrate 1 by protein kinase C-zeta: functional analysis and identification of novel phosphorylation sites

Protein kinase C-zeta (PKC-zeta) participates both in downstream insulin signaling and in the negative feedback control of insulin action. Here we used an in vitro approach to identify PKC-zeta phosphorylation sites within insulin receptor substrate 1 (IRS-1) and to characterize the functional implications. A recombinant IRS-1 fragment (rIRS-1(449)(-)(664)) containing major tyrosine motifs for interaction with phosphatidylinositol (PI) 3-kinase strongly associated to the p85alpha subunit of PI 3-kinase after Tyr phosphorylation by the insulin receptor. Phosphorylation of rIRS-1(449)(-)(664) by PKC-zeta induced a prominent inhibition of this process with a mixture of classical PKC isoforms being less effective. Both PKC-zeta and the classical isoforms phosphorylated rIRS-1(449)(-)(664) on Ser(612). However, modification of this residue did not reduce the affinity of p85alpha binding to pTyr-containing peptides (amino acids 605-615 of rat IRS-1), as determined by surface plasmon resonance. rIRS-1(449)(-)(664) was then phosphorylated by PKC-zeta using [(32)P]ATP and subjected to tryptic phosphopeptide mapping based on two-dimensional HPLC coupled to mass spectrometry. Ser(498) and Ser(570) were identified as novel phosphoserine sites targeted by PKC-zeta. Both sites were additionally confirmed by phosphopeptide mapping of the corresponding Ser --> Ala mutants of rIRS-1(449)(-)(664). Ser(570) was specifically targeted by PKC-zeta, as shown by immunoblotting with a phosphospecific antiserum against Ser(570) of IRS-1. Binding of p85alpha to the S570A mutant was less susceptible to inhibition by PKC-zeta, when compared to the S612A mutant. In conclusion, our in vitro data demonstrate a strong inhibitory action of PKC-zeta at the level of IRS-1/PI 3-kinase interaction involving multiple serine phosphorylation sites. Whereas Ser(612) appears not to participate in the negative control of insulin signaling, Ser(570) may at least partly contribute to this process.

Protein phosphatase 4 interacts with and down-regulates insulin receptor substrate 4 following tumor necrosis factor-alpha stimulation

Protein phosphatase 4 (PP4; also named PPX or PPP4) is a PP2A-related protein serine/threonine phosphatase with important roles in a variety of cellular processes such as microtubule growth/organization, apoptosis, tumor necrosis factor (TNF)-alpha signaling, and activation of c-Jun N-terminal kinase and NF-kappaB. To further investigate the cellular functions of PP4, we isolated and identified PP4-interacting proteins using a proteomic approach. We found that insulin receptor substrate 4 (IRS-4) interacted with PP4 and that this interaction was enhanced following TNF-alpha stimulation. We also found that PP4, but not PP2A, down-regulated IRS-4 in a phosphatase activity-dependent manner. Pulse-chase analysis revealed that PP4 decreased the half-life of IRS-4 from 4 to 1 h. Moreover, we found that TNF-alpha stimulated a PP4-dependent degradation of IRS-4, as indicated by the blockage of the degradation by a potent PP4 inhibitor (okadaic acid) and a phosphatase-dead PP4 mutant (PP4-RL). Taken together, our studies indicate that IRS-4 is subject to regulation by TNF-alpha and that PP4 mediates TNF-alpha-induced degradation of IRS-4.

SH2-B promotes insulin receptor substrate 1 (IRS1)- and IRS2-mediated activation of the phosphatidylinositol 3-kinase pathway in response to leptin

Leptin regulates energy homeostasis primarily by binding and activating its long form receptor (LRb). Deficiency of either leptin or LRb causes morbid obesity. Leptin stimulates LRb-associated JAK2, thus initiating multiple pathways including the Stat3 and phosphatidylinositol (PI) 3-kinase pathways that mediate leptin biological actions. Here we report that SH2-B, a JAK2-interacting protein, promotes activation of the PI 3-kinase pathway by recruiting insulin receptor substrate 1 (IRS1) and IRS2 in response to leptin. SH2-B directly bound, via its PH and SH2 domain, to both IRS1 and IRS2 both in vitro and in intact cells and mediated formation of a JAK2/SH2-B/IRS1 or IRS2 tertiary complex. Consequently, SH2-B dramatically enhanced leptin-stimulated tyrosine phosphorylation of IRS1 and IRS2 in HEK293 cells stably expressing LRb, thus promoting association of IRS1 and IRS2 with the p85 regulatory subunit of PI 3-kinase and phosphorylation and activation of Akt. SH2-B mutants with lower affinity for IRS1 and IRS2 exhibited reduced ability to promote association of JAK2 with IRS1, tyrosine phosphorylation of IRS1, and association of IRS1 with p85 in response to leptin. Moreover, deletion of the SH2-B gene impaired leptin-stimulated tyrosine phosphorylation of endogenous IRS1 in mouse embryonic fibroblasts (MEF), which was reversed by reintroduction of SH2-B. Similarly, SH2-B promoted growth hormone-stimulated tyrosine phosphorylation of IRS1 in both HEK293 and MEF cells. Our data suggest that SH2-B is a novel mediator of the PI 3-kinase pathway in response to leptin or other hormones and cytokines that activate JAK2.

Protein Kinase C-ζ-induced Phosphorylation of Ser318 in Insulin Receptor Substrate-1 (IRS-1) Attenuates the Interaction with the Insulin Receptor and the Tyrosine Phosphorylation of IRS-1

Insulin receptor substrate-1 (IRS-1) was recently identified as a novel upstream substrate for the insulin-activated protein kinase C (PKC)-zeta. This interaction down-regulates insulin signal transduction under hyper-insulinemic conditions. To clarify the molecular mechanism of this feedback loop, we sought to identify the PKC-zeta phosphorylation sites of IRS-1 and to investigate their biological significance. Upon incubation of recombinant IRS-1 fragments with PKC-zeta, we identified Ser(318) of rat IRS-1 (Ser(323) in human IRS-1) as the major in vitro phosphorylation site (confirmed by mutation of Ser(318) to alanine). To monitor phosphorylation of Ser(318) in cellular extracts, we prepared a polyclonal phosphosite-specific antibody. The biological significance was studied in baby hamster kidney cells stably expressing the insulin receptor (BHK(IR)). Using the phospho-Ser(318)-specific antibody we observed that insulin stimulates phosphorylation of Ser(318) in IRS-1, which is mediated, at least partially, by PKC-zeta. Moreover, we found that the previously described insulin-stimulated, PKC-zeta-mediated inhibition of the interaction of IRS-1 with the insulin receptor and the reduced tyrosine phosphorylation of IRS-1 was abrogated by mutation of IRS-1 Ser(318) to alanine. These results, generated in BHK(IR) cells, suggest that phosphorylation of Ser(318) by PKC-zeta might contribute to the inhibitory effect of prolonged hyperinsulinemia on IRS-1 function.