The regulatory role of the C-terminal domain of connexin43

The C-terminal (CT) domain of connexin43 (Cx43) is thought to be important in the control of gap junction function via: a.) CT phosphorylation-dependent control of gap junction assembly and gating, b.) interactions of CT with key regulatory binding partners. To more closely examine CT-dependent regulation, we have expressed a hemagglutinin-Cx43CT (amino acids 235-382) fusion protein in Normal Rat Kidney (NRK) cells under a tetracycline-responsive inducible promoter. Western blot analysis shows that Cx43CT expression is markedly induced by at least 48 h oftreatment with the tetracycline analogue, doxycycline. Furthermore, Cx43CT is modified within the cell, as several treatments/conditions that increase endogenous Cx43 phosphorylation induced a mobility shift in Cx43CT. Treatment with kinase activators, including epidermal growth factor (EGF) and the tumor promoting phorbol ester 12-O-tetradecanylphorbol-13-acetate (TPA), caused a shift in the mobility of the Cx43CT in a manner consistent with the mobility shift observed upon increased phosphorylation of endogenous Cx43. Similarly, Cx43CT in mitotic cells is extensively shifted, consistent with reports which show that Cx43 is phosphorylated to a unique phosphoisoform in mitotic cells. These results indicate that the Cx43CT can interact with at least some of the kinases that phosphorylate endogenous Cx43 in cells and possibly modulate the effects of kinase activation on gap junctional communication.

Cellular basis and clinical implications of biological markers in salivary tissues: their topological distribution in murine submandibular gland

Cell proliferation and apoptosis as well as cell-cell adhesion and communication are essential processes that assure cell survival, renewal and coordination. Since junctional proteins have a tumor suppressor activity, their immunohistochemical characterization has diagnostic and prognostic value. The purpose of this report is to review the role played by junctional and proliferation-related proteins in the salivary glands and to illustrate their immunohistochemical localisation in normal murine submandibular gland. Normal salivary gland tissue was obtained from normal adult male BALB/c mice. After immediate fixation in formalin and ethanol, the samples were immunohistochemically stained for E-cadherin (HECD-1), Bcl-2, Ki67 (MIB-1), connexin26 and connexin 32, beta-catenin and gamma-catenin. Their topological distribution and reactivity were evaluated by light microscopy. The nuclei of submandibular acinar cells exhibited low to moderate staining for Ki67, but no reaction was observed in ductal cells. Murine Bcl-2 was light to moderately expressed in the latero-basal domain of cells of submandibular acini but was only lightly expressed in striated and eosinophilic ducts. The lateral domain of acinar cells were heavily stained with anti-E-cadherin, while only low levels were expressed at the cellular surface of ducts. beta-Catenin was consistently and evenly distributed along the latero-apical boundaries of eosinophilic secretory duct cells as well as on the lateral domain of acinar cells. On the contrary, gamma-catenin was generally expressed at lower levels than beta-catenin, was not expressed in ductal cells and was only lightly stained on the lateral membranes of acinar cells. No expression of connexin 32 was observed in ducts but it was significantly expressed in a spotted pattern along the plasma membrane of acinic cells. Connexin 26 showed similar localization to that of connexin 32 but the staining was much more intense. Since these proteins have been reported to play key roles in maintaining homeostasis via control of cell growth, differentiation and death, their analysis in normal salivary tissue will hopefully contribute to the study of salivary tumorigenesis.

Ser364 of connexin43 and the upregulation of gap junction assembly by cAMP

The assembly of gap junctions (GJs) is a process coordinated by growth factors, kinases, and other signaling molecules. GJ assembly can be enhanced via the elevation of cAMP and subsequent stimulation of connexon trafficking to the plasma membrane. To study the positive regulation of GJ assembly, fibroblasts derived from connexin (Cx)43 knockout (KO) and wild-type (WT) mice were transfected with WT Cx43 (WTCx43) or mutant Cx43. GJ assembly between untransfected WT fibroblasts or stably transfected WTCx43/KO fibroblasts was increased two- to fivefold by 8Br-cAMP, and this increase could be blocked by inhibition of cAMP-dependent protein kinase (PKA) or truncation of the Cx43 COOH terminus (CT). Although serine 364 (S364) of the Cx43 CT was determined to be a major site of phosphorylation, the molar ratio of Cx43 phosphorylation was not increased by 8Br-cAMP. Importantly, GJ assembly between either S364ECx43/KO or S364ECx43/WT fibroblasts was stimulated by 8Br-cAMP, but that between S364ACx43/KO or S364PCx43/KO fibroblasts was not stimulated, indicating that phosphorylation or a negative charge at S364 is required for enhancement of GJ assembly by cAMP. Furthermore, GJ assembly between S364ACx43/WT fibroblasts could be stimulated by 8Br-cAMP, but could not be between S364PCx43/WT fibroblasts. Thus, S364PCx43 interferes with enhanced GJ assembly when coexpressed with WTCx43.

Gap junction assembly: PTX-sensitive G proteins regulate the distribution of connexin43 within cells

Cells expressing connexin43 are able to upregulate gap junction (GJ) communication by enhancing the assembly of new GJs, apparently through increased connexin trafficking. Because G proteins are known to regulate different aspects of protein trafficking, we examined the effects of pertussis toxin (PTX; a specific inhibitor of certain G proteins) on GJ assembly. Dissociated Novikoff hepatoma cells were reaggregated for 60 min to form nascent junctions. PTX inhibited GJ assembly, as indicated by a reduction in dye transfer. Electron microscopy also revealed a 60% decrease in the number of GJ channels per cell interface. Importantly, PTX blocked the twofold enhancement in GJ assembly found in the presence of low-density lipoprotein. Two G(i alpha) proteins (G(i alpha 2) and G(i alpha 3)), which have been implicated in the control of membrane trafficking, reacted with PTX in ADP-ribosylation studies. PTX and/or the trafficking inhibitors, brefeldin A and monensin, inhibited GJ assembly to comparable degrees. In addition, assays for GJ hemichannels demonstrated reduced plasma membrane levels of connexin43 following PTX treatment. These results suggest that PTX-sensitive G proteins regulate connexin43 trafficking, and, as a result of inhibition with PTX, the number of plasma membrane hemichannels available for GJ assembly is reduced.

Regulation of gap junctions by phosphorylation of connexins

Gap junctions are a unique type of intercellular junction found in most animal cell types. Gap junctions permit the intercellular passage of small molecules and have been implicated in diverse biological processes, such as development, cellular metabolism, and cellular growth control. In vertebrates, gap junctions are composed of proteins from the "connexin" gene family. The majority of connexins are modified posttranslationally by phosphorylation, primarily on serine amino acids; however, phosphotyrosine has also been detected in connexin from cells coexpressing nonreceptor tyrosine protein kinases. Connexins are targeted by numerous protein kinases, of which some have been identified: protein kinase C, mitogen-activated protein kinase, and the v-Src tyrosine protein kinase. Phosphorylation has been implicated in the regulation of a broad variety of connexin processes, such as the trafficking, assembly/disassembly, degradation, as well as the gating of gap junction channels. This review examines the consequences of connexin phosphorylation for the regulation of gap junctional communication.

Cyclic AMP and LDL trigger a rapid enhancement in gap junction assembly through a stimulation of connexin trafficking

Given the rapid turnover of connexin proteins, gap junction (GJ) assembly represents an important means of regulating the extent of GJ communication between cells. This report describes an increase in the level of GJ assembly within one hour following treatment with cAMP-elevating reagents or low density lipoprotein (LDL). Dye transfer methods and freeze-fracture with electron microscopy were used to assay junctional permeability and structure, respectively, subsequent to the dissociation, recovery and reaggregation of Novikoff hepatoma cells. Reaggregating cells in the presence of agents that increase cAMP levels (8-Br-cAMP, forskolin and IBMX) enhanced both dye transfer rates between cells and the extent of GJ formation 2- to 3-fold. These data and studies with the protein kinase A inhibitor, H-89, indicate that cAMP signaling plays a key role in enhanced assembly. The response to LDL parallels that to cAMP and relies on the activity of both adenylyl cyclase and protein kinase A. Immunoblot analysis revealed no change in the level of connexin43 (Cx43) or its phosphorylation states over a period of 2.5 hours. However, three agents (brefeldin A, monensin and nocodazole), that inhibit intracellular membrane trafficking by different mechanisms, all blocked the enhanced assembly of GJs when triggered by either elevated cAMP or exposure to LDL. Related studies, which employed trafficking inhibitors at different stages in GJ assembly, suggested that Cx43 trafficking during enhanced assembly is regulated, in part, by cell contact. Intracellular sources of Cx43 were characterized by colabeling for several markers of cytoplasmic membrane systems. We conclude that an increase in GJ assembly: (i) occurs rapidly in the presence of elevated cAMP or LDL, (ii) does not require an increase in Cx43 levels or major changes in Cx43 phosphorylation and (iii) is dependent upon the trafficking of Cx43 from intracellular storage sites.

Phosphorylation of connexin43 on serine368 by protein kinase C regulates gap junctional communication

Phorbol esters (e.g., TPA) activate protein kinase C (PKC), increase connexin43 (Cx43) phosphorylation, and decrease cell-cell communication via gap junctions in many cell types. We asked whether PKC directly phosphorylates and regulates Cx43. Rat epithelial T51B cells metabolically labeled with (32)P(i) yielded two-dimensional phosphotryptic maps of Cx43 with several phosphopeptides that increased in intensity upon TPA treatment. One of these peptides comigrated with the major phosphopeptide observed after PKC phosphorylation of immunoaffinity-purified Cx43. Purification of this comigrating peptide and subsequent sequencing indicated that the phosphorylated serine was residue 368. To pursue the functional importance of phosphorylation at this site, fibroblasts from Cx43(-/-) mice were transfected with either wild-type (Cx43wt) or mutant Cx43 (Cx43-S368A). Intercellular dye transfer studies revealed different responses to TPA and were followed by single channel analyses. TPA stimulation of T51B cells or Cx43wt-transfected fibroblasts caused a large increase in the relative frequency of approximately 50-pS channel events and a concomitant loss of approximately 100-pS channel events. This change to approximately 50-pS events was absent when cells transfected with Cx43-S368A were treated with TPA. These data strongly suggest that PKC directly phosphorylates Cx43 on S368 in vivo, which results in a change in single channel behavior that contributes to a decrease in intercellular communication.

Clustering of connexin 43-enhanced green fluorescent protein gap junction channels and functional coupling in living cells

Communication-incompetent cell lines were transfected with connexin (Cx) 43 fused with enhanced green fluorescent protein (EGFP) to examine the relation between Cx distribution determined by fluorescence microscopy and electrical coupling measured at single-channel resolution in living cell pairs. Cx43-EGFP channel properties were like those of wild-type Cx43 except for reduced sensitivity to transjunctional voltage. Cx43-EGFP clustered into plaques at locations of cell-cell contact. Coupling was always absent in the absence of plaques and even in the presence of small plaques. Plaques exceeding several hundred channels always conferred coupling, but only a small fraction of channels were functional. These data indicate that clustering may be a requirement for opening of gap junction channels.

Analysis of connexin phosphorylation sites

Most connexins, the proteins that form gap junction channels, are phosphoproteins. Connexin phosphorylation has been thought to regulate gap junctional protein trafficking, gap junction assembly, channel gating, and turnover. Connexin phosphorylation has been investigated in a variety of ways. Some connexins show mobility shifts in sodium dodecyl sulfate-polyacrylamide gel electrophoresis on phosphorylation. Kinase modulators can change the level of connexin phosphorylation and affect gap junctional communication levels. Metabolic labeling of cultured cells has allowed both phosphoamino acid identification and generation of phosphotryptic peptide maps. However, identification of the location of phosphorylated residues within the connexin sequence has required either targeted peptide synthesis, in vitro phosphorylation of known sites, and two-dimensional comigration studies or liquid chromatographic separation and N-terminal sequencing of peptides. In addition to these conventional methods, we discuss new applications of mass spectrometry to the identification of phosphorylated peptides and the specific residues phosphorylated within the connexin-derived peptide.

Trafficking, assembly, and function of a connexin43-green fluorescent protein chimera in live mammalian cells

To examine the trafficking, assembly, and turnover of connexin43 (Cx43) in living cells, we used an enhanced red-shifted mutant of green fluorescent protein (GFP) to construct a Cx43-GFP chimera. When cDNA encoding Cx43-GFP was transfected into communication-competent normal rat kidney cells, Cx43-negative Madin-Darby canine kidney (MDCK) cells, or communication-deficient Neuro2A or HeLa cells, the fusion protein of predicted length was expressed, transported, and assembled into gap junctions that exhibited the classical pentalaminar profile. Dye transfer studies showed that Cx43-GFP formed functional gap junction channels when transfected into otherwise communication-deficient HeLa or Neuro2A cells. Live imaging of Cx43-GFP in MDCK cells revealed that many gap junction plaques remained relatively immobile, whereas others coalesced laterally within the plasma membrane. Time-lapse imaging of live MDCK cells also revealed that Cx43-GFP was transported via highly mobile transport intermediates that could be divided into two size classes of <0.5 microm and 0.5-1.5 microm. In some cases, the larger intracellular Cx43-GFP transport intermediates were observed to form from the internalization of gap junctions, whereas the smaller transport intermediates may represent other routes of trafficking to or from the plasma membrane. The localization of Cx43-GFP in two transport compartments suggests that the dynamic formation and turnover of connexins may involve at least two distinct pathways.

Cellular interaction of integrin alpha3beta1 with laminin 5 promotes gap junctional communication

Wounding of skin activates epidermal cell migration over exposed dermal collagen and fibronectin and over laminin 5 secreted into the provisional basement membrane. Gap junctional intercellular communication (GJIC) has been proposed to integrate the individual motile cells into a synchronized colony. We found that outgrowths of human keratinocytes in wounds or epibole cultures display parallel changes in the expression of laminin 5, integrin alpha3beta1, E-cadherin, and the gap junctional protein connexin 43. Adhesion of keratinocytes on laminin 5, collagen, and fibronectin was found to differentially regulate GJIC. When keratinocytes were adhered on laminin 5, both structural (assembly of connexin 43 in gap junctions) and functional (dye transfer) assays showed a two- to threefold increase compared with collagen and five- to eightfold over fibronectin. Based on studies with immobilized integrin antibody and integrin-transfected Chinese hamster ovary cells, the interaction of integrin alpha3beta1 with laminin 5 was sufficient to promote GJIC. Mapping of intermediate steps in the pathway linking alpha3beta1-laminin 5 interactions to GJIC indicated that protein trafficking and Rho signaling were both required. We suggest that adhesion of epithelial cells to laminin 5 in the basement membrane via alpha3beta1 promotes GJIC that integrates individual cells into synchronized epiboles.

Formation of a distinct connexin43 phosphoisoform in mitotic cells is dependent upon p34cdc2 kinase

The gap junction protein connexin43 is a phosphoprotein that typically migrates as three bands (nonphosphorylated, P1 and P2) during polyacrylamide gel electrophoresis. The electrophoretic mobility of connexin43 from mitotic cells was distinctly reduced to a form (P3) that migrated slower than P2 from Rat1 cells prepared by shakeoff of nocodazole-treated and untreated cultures. Mitotic FT210 cells, which contain a temperature-sensitive mutation in the p34(cdc2) kinase, showed abundant levels of the P3 connexin43 when maintained at the permissive temperature where p34(cdc2) is active. In contrast, nocodozole-treated FT210 cells grown at the nonpermissive temperature did not contain P3 connexin43. These results indicated that generation of the P3 connexin43 was dependent upon active p34(cdc2)/cyclin B kinase. Although the p34(cdc2)kinase phosphorylated connexin43 in vitro on peptides containing serine 255, the major phosphotryptic peptides in P3 connexin43 from mitotic cells appeared to be the consequence of another protein kinase(s), which may be activated by the p34(cdc2)/cyclin B kinase. The P3 connexin43 exhibited a marked redistribution from cell-cell plasma membrane interfaces to multiple, distinctly stained cytoplasmic structures. These events may be part of the dramatic structural changes observed in mitotic cells undergoing cell rounding and cytokinesis. Results of initial studies using inhibitors of protein degradative and synthetic pathways suggested the likelihood that protein degradation and synthesis participate in the disappearance of the P3 connexin43 and restoration of the pattern of connexin43 isoforms observed in nonmitotic cells.

Direct isolation and analysis of endogenous transjunctional ADP from Cx43 transfected C6 glioma cells

Gap junctional communication has been implicated in numerous cellular processes. However, the repertoire of specific transjunctional substances which mediate these processes remains relatively unexplored. A few selected secondary messengers have been identified, at least indirectly (e.g., cAMP and IP3) and phenotypic complementation experiments have indicated that gap junctions enable communicating cells to distribute nucleotide pools as a shared resource. The latter would include high energy compounds such as ADP and ATP, allowing cells to share energy resources. We have utilized a nonbiased process to directly capture, identify, and quantify transjunctional compounds from C6 glioma cells, the transformed phenotype of which has been ameliorated by transfection with connexin43 (Cx43). This technique involves the direct isolation, identification, and quantitation of radioactive transjunctional molecules that travel from metabolically labeled "donor" cells to "receiver" cells. This report demonstrates that ADP and/or ATP represents over 6% of the transjunctional material derived from glucose in Cx43-transfected C6 glioma cells. Furthermore, equilibration of these high energy metabolites among first order neighbors is shown to occur in less than 20 min of communication.

The gap-junction protein connexin 56 is phosphorylated in the intracellular loop and the carboxy-terminal region

The lens gap-junction protein, connexin 56, is modified by phosphorylation. Two-dimensional mapping of tryptic phosphopeptides of 32P-labeled connexin 56 from primary chicken-lens cultures showed that treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) induced an increase in phosphorylation of connexin 56 at specific constitutively phosphorylated sites. Treatment with 8-Br-cAMP or forskolin did not induce substantial changes in connexin 56 phosphorylation. Two phosphorylation sites within connexin 56, S493 and S118, were identified after HPLC purification and peptide sequencing of tryptic phosphopeptides from bacterially expressed connexin 56 fusion proteins phosphorylated by protein kinase C or protein kinase A in vitro. Comparisons of the two-dimensional maps of tryptic phosphopeptides from in vitro phosphorylated connexin 56 fusion proteins and in vivo phosphorylated connexin 56 showed that S493 and S118 were constitutively phosphorylated in lentoid-containing cultures, and that treatment with TPA induced an increase in phosphorylation of the peptides containing S118. It is suggested that phosphorylation of connexin 56 at S118 is involved in the TPA-induced decrease in intercellular communication and acceleration of connexin 56 degradation.

Properties of chicken lens MIP channels reconstituted into planar lipid bilayers

Membrane fractions highly enriched in chicken lens MIP (MIP28) were found to form ion channels when incorporated into planar lipid bilayers. The channels displayed prominent unitary conductances of about 60 and 290 pS in symmetric 150 mm KCl solution and were slightly anion selective. For both depolarizing and hyperpolarizing voltages, voltage sensitivity of the MIP28-induced conductance could be fit by a Boltzmann relation, symmetric around zero mV, with V0 = 18.5 mV, n = 4.5 and gmin/gmax = 0.17. Channel properties were not appreciably altered by pH in the range of 5.8 to 7, although channel incorporation was observed to occur more frequently at lower pH values. Calcium, at millimolar concentrations, decreased the channel mean open time. Partial proteolysis of MIP28 to yield MIP21 did not appreciably affect single-channel conductance or voltage sensitivity of the reconstituted channels. MIP28 was not phosphorylated by cAMP dependent protein kinase (PKA). Although unitary conductance and selectivity of the chicken MIP channel are similar to those reported for the bovine MIP (MIP26), the voltage sensitivity of MIP28 was higher than that of the bovine homologue, and voltage sensitivity of MIP28 was not modulated by treatments previously shown to affect MIP26 voltage gating (partial proteolysis and protein phosphorylation by PKA: (Ehring et al., 1990). The existence of such strikingly different functional properties in highly homologous channel isoforms may provide a useful system for exploration of the structure-function relations of MIP channels.

Lipid differentiation in MP26 junction enriched membranes of bovine lens fiber cells

The present study was undertaken to address the question whether lipid differentiation occurs in junctional domains which could imply a functional requirement for specific lipids in junctional structures. Junction enriched membranes were isolated from bovine lens fiber cells using Tris and urea treatment, and the presence of junctional structures was ascertained by electron microscopy. Enrichment in major intrinsic protein (MIP, MP26) was monitored by SDS polyacrylamide gel electrophoresis. Junctional lipids were extracted by a modified Folch procedure, to quantitatively recover cholesterol, and lipid classes were analyzed. While 99.5% of total lens protein was solubilized in the course of junction isolation, 43.9% of cell phospholipids (PL) and 64.1% of cell cholesterol (Chol) were conserved. Cholesterol was by far the predominant lipid in the junction enriched lens fiber cell membranes (833 nmol/mg protein) and was more abundant than all phospholipids combined (682 nmol/mg protein). In isolating the junctional membranes, cholesterol levels increased 144-fold, and average phospholipid levels increased 99-fold, which resulted in an increase in Chol/PL ratio from 0.84 to 1.22. Different phospholipids showed substantially different degrees of enrichment with highest enrichments seen for the phosphatidylethanolamine fraction (152-fold) and sphingomyelin (101-fold). Thus, the phospholipids of the junction enriched membranes consisted mainly of ethanolamine glycerophospholipids (37.3%) and sphingomyelin (28.6%), with lesser amounts of choline glycerophospholipids (23.5%) and phosphatidylserine (9.2%) present. Our data suggest that the MP26 junction enriched membranes of bovine lens fiber cells contain differentiated lipid domains, and that cholesterol, ethanolamine glycerophospholipids and sphingomyelin are the prevalent boundary lipids of the major intrinsic protein in these domains.

Properties and regulation of gap junctional hemichannels in the plasma membranes of cultured cells

During the assembly of gap junctions, a hemichannel in the plasma membrane of one cell is thought to align and dock with another in an apposed membrane to form a cell-to-cell channel. We report here on the existence and properties of nonjunctional, plasma membrane connexin43 (Cx43) hemichannels. The opening of the hemichannels was demonstrated by the cellular uptake of 5(6)-carboxyfluorescein from the culture medium when extracellular calcium levels were reduced. Dye uptake exhibited properties similar to those of gap junction channels. For example, using different dyes, the levels of uptake were correlated with molecular size: 5(6)-carboxyfluorescein (approximately 32%), 7-hydroxycoumarin-3-carboxylic acid (approximately 24%), fura-2 (approximately 11%), and fluorescein-dextran (approximately 0.4%). Octanol and heptanol also reduced dye uptake by approximately 50%. Detailed analysis of one clone of Novikoff cells transfected with a Cx43 antisense expression vector revealed a reduction in dye uptake levels according to uptake assays and a corresponding decrease in intercellular dye transfer rates in microinjection experiments. In addition, a more limited decrease in membrane resistance upon reduction of extracellular calcium was detected in electrophysiological studies of antisense transfectants, in contrast to control cells. Studies of dye uptake in HeLa cells also demonstrated a large increase following transfection with Cx43. Together these observations indicate that Cx43 is responsible for the hemichannel function in these cultured cells. Similar dye uptake results were obtained with normal rat kidney (NRK) cells, which express Cx43. Dye uptake can be dramatically inhibited by 12-O-tetradeconylphorbol-13-acetate-activated protein kinase C in these cell systems and by a temperature-sensitive tyrosine protein kinase, pp60v-src in LA25-NRK cells. We conclude that Cx43 hemichannels are found in the plasma membrane, where they are regulated by multiple signaling pathways, and likely represent an important stage in gap junction assembly.

Regulation of connexin43 function by activated tyrosine protein kinases

Gap junctions are specialized membrane structures that are involved in the normal functioning of numerous mammalian tissues and implicated in several human disease processes. This mini-review focuses on the regulation of gap junctions through phosphorylation of connexin43 induced by the v-Src or epidermal growth factor receptor tyrosine kinases. These tyrosine kinases markedly disrupt gap junctional communication in mammalian cells. here, we describe work correlating the alteration of connexin43 function with the ability of the v-Src tyrosine kinase to phosphorylate connexin43 directly on two distinct tyrosine sites in mammalian cells (Y247 and Y265). We also present evidence that proline-rich regions and phosphotyrosine sites of connexin43 may mediate interactions with the SH3 and SH2 domains of v-Src. In contrast to v-Src, the activated epidermal growth factor receptor acts indirectly through activated MAP kinase which may stimulate phosphorylation of connexin43 exclusively on serine. This phosphorylation event is complex because MAP kinase phosphorylates three serine sites in connexin43 (S255, S279, and S282). These findings suggest novel interactions between connexin43, the v-Src tyrosine kinase, and activated MAP kinase that set the stage for future investigations into the regulation of gap junctions by protein phosphorylation.

Characterization of the mitogen-activated protein kinase phosphorylation sites on the connexin-43 gap junction protein

We have previously demonstrated that epidermal growth factor induced a rapid, transient decrease in gap junctional communication and increase in serine phosphorylation on the connexin-43 gap junction protein in T51B rat liver epithelial cells. The kinase(s) responsible for phosphorylation and specific serine targets in connexin-43 have not been identified. There are three consensus mitogen-activated protein (MAP) kinase serine phosphorylation sequences in the carboxyl-terminal tail of connexin-43 and purified MAP kinase phosphorylated connexin-43 in vitro on tryptic peptides that comigrated with a subset of peptides from connexin-43 phosphorylated in vivo in cells treated with epidermal growth factor. These data suggested that MAP kinase may phosphorylate connexin-43 directly in vivo. We have utilized a glutathione S-transferase fusion protein containing the cytoplasmic tail of connexin-43 to characterize MAP kinase phosphorylation. Site-directed mutagenesis, phosphotryptic peptide analysis, and peptide sequencing have confirmed that MAP kinase can phosphorylate connexin-43 at Ser255, Ser279, and Ser282, which correspond to the consensus sites recognized earlier. Characterization of MAP kinase-mediated phosphorylation of connexin-43 has defined potential targets for phosphorylation in vivo following activation of the epidermal growth factor receptor and has provided the basis for studies of the effects of phosphorylation, at specific molecular sites, on the regulation of gap junctional communication.

Cyclic AMP modifies the cellular distribution of connexin43 and induces a persistent increase in the junctional permeability of mouse mammary tumor cells

Direct communication between cells via gap junctions is thought to be an important component of homeostasis and coordinated cellular responses to external signals. We investigated how the second messenger cAMP exerts its effects on junctional communication in a mouse mammary tumor cell line, MMT22. Junctional permeance was quantitatively assessed using dye microinjection and video microscopy. An increase of permeance was found after exposure to 8-bromo-cAMP, being detectable after 30 minutes of treatment and attaining a fourfold higher level of permeance by 24 hours. This elevated level was maintained with continuous exposure to 8-bromo-cAMP for seven days. The permeability change was accompanied by an increase in gap junctions as shown by freeze-fracture electron microscopy and by confocal microscopy using antibodies directed against the gap junction protein, connexin43. The amount of detergent-insoluble connexin43 also increased with 8-bromo-cAMP treatment, and most of the increase could be attributed to an increase of slower migrating (i.e. phosphorylated) species of connexin43. However, connexin43 mRNA and the total cellular content of connexin43 did not change over this period of exposure to 8-bromo-cAMP, as shown by densitometric analyses of northern and western blots. We conclude that 8-bromo-cAMP affects the distribution of connexin43 such that a greater proportion of the protein is utilized for channel formation. Since these changes were relatively slow to develop and persisted with prolonged exposure to 8-bromo-cAMP, it is possible that the junctional permeability of these mammary tumor cells is linked to the ‘basal’ level of cAMP, i.e. levels maintained by the cells in accordance with a particular cell state.

Expression of multiple connexins in cultured neonatal rat ventricular myocytes

Three gap junction proteins have been identified in mammalian cardiac myocytes: connexin43 (Cx43), connexin45 (Cx45), and connexin40 (Cx40). These proteins form channels with different electrophysiological properties and have different distributions in cardiac tissues with disparate conduction properties. We characterized the expression, phosphorylation, turnover, and subcellular distribution of these connexins in primary cultures of neonatal rat ventricular myocytes. Cx43, Cx45, and Cx40 mRNA were specifically detected in RNA blots. Immunofluorescent staining with antibodies specific for Cx43 and Cx45 revealed punctate labeling at appositional membranes, but no immunoreactive Cx40 was detected. Double-label immunofluorescence confocal microscopy of cultured myocytes revealed colocalization of Cx43 and Cx45. Cx43 and Cx45 were both identified by immunoprecipitation from [35S]methionine-labeled cultures, but anti-Cx40 antibodies did not precipitate any radiolabeled protein. Phosphorylated forms of both Cx45 and Cx43 were immunoprecipitated from cultures metabolically labeled with [32P]orthophosphate. Phosphoamino acid analysis demonstrated that Cx45 was modified on serine residues, and Cx43 was phosphorylated on serine and threonine residues. Pulse-chase labeling experiments demonstrated that the half-lives of Cx43 and Cx45 were 1.9 and 2.9 hours, respectively. Thus, both Cx43 and Cx45 turn over relatively rapidly, suggesting that myocardial gap junctions have the potential for dynamic remodeling. The results implicate multiple mechanisms of gap junction regulation that may differ for different connexins.

Analyzing phorbol ester effects on gap junctional communication: a dramatic inhibition of assembly

The effect of 12-O-tetradeconylphorbol-13-acetate (TPA) on gap junction assembly between Novikoff hepatoma cells was examined. Cells were dissociated with EDTA to single cells and then reaggregated to form new junctions. When TPA (25 nM) was added to the cells at the onset of the 60-min reaggregation, dye transfer was detected at only 0.6% of the cell-cell interfaces compared to 72% for the untreated control and 74% for 4-alpha TPA, an inactive isomer of TPA. Freeze-fracture electron microscopy of reaggregated control cells showed interfaces containing an average of more than 600 aggregated intramembranous gap junction particles, while TPA-treated cells had no gap junctions. However, Lucifer yellow dye transfer between nondissociated cells via gap junctions was unaffected by 60 min of TPA treatment. Therefore, TPA dramatically inhibited gap junction assembly but did not alter channel gating nor enhance disassembly of preexisting gap junction structures. Short term TPA treatment (< 30 min) increased phosphorylation of the gap junction protein molecular weight of 43,000 (Cx43), but did not change the cellular level of Cx43. Cell surface biotinylation experiments suggested that TPA did not substantially reduce the plasma membrane concentration of Cx43. Therefore, the simple presence of Cx43 in the plasma membrane is not sufficient for gap junction assembly, and protein kinase C probably exerts an effect on assembly of gap junctions at the plasma membrane level.

An activator of protein kinase C inhibits gap junction communication between cultured bovine lens cells

Currently little is known about the regulation of gap junction communication in the lens. We report here on the effects of the protein kinase C activator, 12-O-tetradecanoylphorbol-13-acetate (TPA), on cultured bovine lens cells which appeared to be epithelial in nature. Dramatically reduced intercellular transfer of the fluorescent dye Lucifer yellow was observed when the cultured lens cells were treated with octanol, a known inhibitor of gap junction communication. TPA (4 beta isomer) was also shown to reduce intercellular permeability within these cultures. In contrast, an inactive form of TPA, 4 alpha-TPA, did not decrease dye transfer. Permeability was evaluated in terms of both the number of cells receiving dye and the rate of decrease in fluorescence intensity in the injected cell. The maximum decreases in dye transfer occurred at 2 h of TPA treatment and dye transfer gradually increased to control levels over a time course of many hours. Incubation of cultures with 32Pi and immunoprecipitation using antibodies to the N- and C-terminal regions of connexin43 demonstrated a gap junction phosphoprotein of 43,000 Da. Phosphorylation of connexin43 increased during the first 2 h of TPA treatment. These results suggest that protein kinase C has a direct or indirect effect on gap junction communication in cultured lens cells.

In vitro assembly of gap junctions

Gap junction structures were assembled in vitro from octyl-beta-D-glucopyranoside-solubilized components of lens fiber cell membranes. Individual pore structures (connexons), short double-membrane structures, and other amorphous material were evident in the solubilized mixture. Following the removal of the detergent by dialysis, these connexons associated to form single- and double-layered, two-dimensional hexagonal arrays (unit cell size a = b = 8.5 nm). The formation of larger arrays was dependent on the lipid-to-protein ratio and the presence of Mg2+ ions. Crystallographic analysis of electron micrographs revealed that lens junctional connexons consisted of six subunits surrounding a stain-filled channel. Upon further detergent treatment, in vitro assembled gap junctions were insoluble and formed three-dimensional stacks while other components were solubilized. SDS-PAGE and mass data from scanning transmission electron microscopy strongly suggest that a 38-kDa polypeptide, which is a processed form of the lens specific gap junction protein MP70, is a major component of the arrays. The in vitro assembly of gap junctions opens new avenues for the structural analysis of gap junctions and for the study of the intermolecular interactions of connexons during junctional assembly.

Enhanced gap junction formation with LDL and apolipoprotein B

Gap junctions are plasma membrane specializations involved in direct cell-cell communication. Intercellular communication is dependent upon the assembly of gap junction structures and would be influenced by agents which alter the assembly process. We investigated the effects of low density lipoprotein (LDL) on gap junction assembly between cultured Novikoff cells using quantitative dye transfer and freeze-fracture electron microscopic methods. We observed a concentration-dependent increase in dye transfer (maximum effect at 2.5 micrograms/ml) and a sixfold increase in the number of aggregated gap junction particles per cell. Immunoblots of Novikoff cells probed with anti-connexin43 antibody revealed no detectable increase in gap junction protein (connexin) levels. The influence of the different components of LDL on junction formation was also examined. First, we treated cells with cholesterol (0-150 microM) in serum-free BSA media and observed a decrease in junction assembly. Second, we added apolipoprotein-B (apo-B) in phosphatidyl choline vesicles to the cells and observed a concentration-dependent increase in dye transfer (maximum effect at 2.5 micrograms protein/ml) and a fivefold increase in the number of aggregated gap junction particles per cell. The addition of phosphatidyl choline vesicles without apo-B had no effect on gap junction formation. Thus, we demonstrated that gap junction assembly can be modulated by LDL and apo-B treatments.

Amino acid sequence of in vivo phosphorylation sites in the main intrinsic protein (MIP) of lens membranes

The main intrinsic membrane protein of the lens fiber cell, MIP, has been previously shown to be phosphorylated in preparations of lens fragments. Phosphorylation occurred on serine residues near the cytoplasmic C-terminus of the molecule. Since MIP is thought to function as a channel protein in lens plasma membranes, possibly as a cell-to-cell channel protein, phosphorylation could regulate the assembly or gating of these channels. We sought to identify the specific serines which are phosphorylated in order to help identify the kinases involved in regulating MIP function. To this end we purified a peptide fragment from native membranes that had not been subjected to any exogenous kinases or kinase activators. Any phosphorylation detected in these fragments must be due to cellular phosphorylation and thus is termed in vivo phosphorylation. Purified membranes were also phosphorylated with cAMP-dependent protein kinase to determine the mobility of phosphorylated and unphosphorylated MIP-derived peptides on different HPLC columns and to determine possible cAMP-dependent protein kinase phosphorylation sites. Lens membranes, which contain 50-60% of the protein as MIP, were digested with lysylendopeptidase C. Peptides were released from the C-terminal region of MIP and a major product of 21-22 kDa remained membrane-associated. Separation of the lysylendopeptidase-C-released peptides on C8 reversed-phase HPLC demonstrated that one of these fragments, corresponding to residues 239-259 in MIP, was partially phosphorylated. The phosphorylated and nonphosphorylated forms of this peptide were separated on QAE HPLC. In vivo phosphorylation sites were found at residues 243 and 245 through phosphoserine modification via ethanethiol and sequence analysis. Phosphorylation was never detected on serine 240. The phosphorylation level of serine 243 could be increased by incubation of membranes with cAMP-dependent protein kinase under standard assay conditions. Other kinases that phosphorylate serines found near acidic amino acids must be responsible for the in vivo phosphorylation demonstrated at serine 245.

Structural organization of the lens fiber cell plasma membrane protein MP18

The 18,000-dalton bovine lens fiber cell intrinsic membrane protein MP18 was phosphorylated on a serine residue by both cAMP-dependent protein kinase and protein kinase C. In addition, this protein bound calmodulin and was recognized by a monoclonal antibody (2D10). These different regions were localized using enzymatic and chemical fragmentation of electrophoretically purified MP18 that had been phosphorylated with either cAMP-dependent protein kinase or protein kinase C. Partial digestion of 32P-labeled MP18 with protease V8 resulted in a Mr = 17,000 peptide that bound calmodulin, but neither contained 32P or was recognized by the monoclonal antibody 2D10. Furthermore, the 17-kDa peptide had the same N-terminal amino acid sequence as MP18. Thus, the monoclonal antibody 2D10 recognition site and the protein kinase phosphorylation site(s) are close together and confined to a small region in the C terminus of MP18. This conclusion was confirmed in experiments where MP18 was fragmented with trypsin, endoproteinase Lys-C, or CNBr. The location of the phosphorylation site was confirmed by sequencing the small 32P-labeled, C-terminal peptide that resulted from protease V8 digestion of 32P-labeled MP18. This peptide contained a consensus sequence for cAMP-dependent protein kinase.

Phosphorylation of MP26, a lens junction protein, is enhanced by activators of protein kinase C

MP26, a protein thought to form gap junctional channels in the lens, and other lens proteins were phosphorylated under conditions that activate protein kinase C. Phosphorylation was detected both in lens fiber cell fragments in an "in vivo" labeling procedure with 32P-phosphate and in cell homogenates with 32P-ATP. In these experiments, both calcium and 12-O-tetradecanoylphorbol 13-acetate (TPA) were necessary for maximal phosphorylation of MP26. Calcium stimulated the phosphorylation of MP26 approximately fourfold and TPA with calcium led to a sevenfold increase. If TPA was present, 1 microM calcium was sufficient for maximal labeling. Phosphoamino acid analysis demonstrated approximately 85% phosphoserine, 15% phosphothreonine, and no phosphotyrosine when MP26 was phosphorylated in lens homogenates in the presence of TPA and calcium and then electrophoretically purified. Phosphorylation occurred near the cytoplasmic, C-terminal of MP26. The possible involvement of other kinases was also examined. The Walsh inhibitor, which affects cAMP-dependent protein kinases, had no influence on the TPA-mediated increase in phosphorylation. In studies with isolated membranes and added kinases, MP26 was also found to not be a substrate for calcium/calmodulin-dependent protein kinase II. Thus, protein kinase C may have phosphorylated MP26 in a direct manner.

Phosphorylation of troponin I by protein kinase C: mechanism of inhibition by calmodulin and troponin C

The mechanism by which calmodulin and troponin C influence phosphorylation of troponin I (TnI) by protein kinase C was investigated. The phosphorylation of TnI by protein kinase C requires the presence of acidic phospholipid, calcium and diacylglycerol. Light scattering intensity and fluorescence intensity experiments showed that TnI associated with the phospholipid membranes and caused extensive aggregation. In the presence of Ca2+, TnI-phospholipid interactions were prevented by approximately stoichiometric amounts of either troponin C or calmodulin. Troponin C was shown to completely inhibit phosphorylation of TnI by either protein kinase C or by phosphorylase b kinase. In contrast, calmodulin completely inhibited phosphorylation of TnI by protein kinase C, but had only little effect on TnI phosphorylation by phosphorylase b kinase. Inhibition by calmodulin did not appear to be due to interaction with PKC, since calmodulin mildly increased protein kinase C phosphorylation of histone III-S. The ratio of phosphoserine to phosphothreonine in protein kinase C-phosphorylated TnI remained approximately constant for reactions inhibited by up to 90% by calmodulin. TnI interactions with phospholipid and phosphorylation of TnI by PKC were also prevented by high salt concentrations. However, salt concentrations adequate to inhibit phosphorylation were sufficient to dissociate only TnI, but not protein kinase C from the membrane. These results suggest that the binding of TnI to phospholipid is required for phosphorylation by protein kinase C and that prevention of this binding by any means completely inhibited phosphorylation of TnI by protein kinase C.

Phosphorylation of lens intrinsic membrane proteins by protein kinase C

Two intrinsic proteins of bovine lens membranes with apparent relative molecular masses (Mr, app) of 26,000 and 18,000 were phosphorylated in intact membranes by protein kinase C prepared from either bovine brain or lens. The kinase preparations exhibited histone H1 phosphorylation dependent on calcium and phospholipid but not on cAMP. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis of the lens membranes showed a major band at Mr, app = 26,000 (identified as MP26, the main intrinsic protein of lens fiber cells), an intermediate band at Mr, app = 18,000 and several minor bands. Autoradiography of complete assay mixture containing protein kinase C, calcium, magnesium and [gamma-32P]ATP showed major bands at Mr, app = 18,000 and 26,000. Several lines of evidence indicated that the label at Mr, app = 26,000 was associated with MP26, a protein which has been found in lens junctions and which may form cell-cell channels. Treatment of the phosphorylated membranes with chymotrypsin and V8 protease cleaved the major band at Mr, app = 26,000 to fragments of Mr, app .= 22,000 and 24,000. Label was not detected in the resulting Mr, app = 22,000 peptide, but the Mr, app = 24,000 peptide was found to be labeled. Phosphoamino acid analysis of MP26 indicated that approximately 75% of the label was on phosphoserine and 25% was on phosphothreonine. No label was found on phosphotyrosine. These results differ from those reported for cAMP-dependent phosphorylation of lens proteins. Phosphorylation by protein kinase C may account for some of the labeling of MP26 detected in vivo.

A lens intercellular junction protein, MP26, is a phosphoprotein

The major protein present in the plasma membrane of the bovine lens fiber cell (MP26), thought to be a component of intercellular junctions, was phosphorylated in an in vivo labeling procedure. After fragments of decapsulated fetal bovine lenses were incubated with [32P]orthophosphate, membranes were isolated and analyzed by SDS PAGE and autoradiography. A number of lens membrane proteins were routinely phosphorylated under these conditions. These proteins included species at Mr 17,000 and 26,000 as well as a series at both 34,000 and 55,000. The label at Mr 26,000 appeared to be associated with MP26, since (a) boiling the membrane sample in SDS led to both an aggregation of MP26 and a loss of label at Mr 26,000, (b) the label at 26,000 was resistant to both urea and nonionic detergents, and (c) two-dimensional gels showed that a phosphorylated Mr 24,000 fragment was derived from MP26 with V8 protease. Studies with proteases also provided for a localization of most label within approximately 20 to 40 residues from the COOH-terminus of MP26. Published work indicates that the phosphorylated portion of MP26 resides on the cytoplasmic side of the membrane, and that this region of MP26 contains a number of serine residues. The same region of MP26 was labeled when isolated lens membranes were reacted with a cAMP-dependent protein kinase prepared from the bovine lens. After the in vivo labeling of lens fragments, phosphoamino acid analysis of MP26 demonstrated primarily labeled serines, with 5-10% threonines and no tyrosines. Treatments that lowered the intracellular calcium levels in the in vivo system led to a selective reduction of MP26 phosphorylation. In addition, forskolin and cAMP stimulated the phosphorylation of MP26 and other proteins in concentrated lens homogenates. These findings are of interest because MP26 appears to serve as a protein of cell-to-cell channels in the lens, perhaps as a lens gap junction protein.

Electron microscopy and hydrodynamic properties of blood clotting factor V and activation fragments of factor V with phospholipid vesicles

The electron microscopic and hydrodynamic properties of factor V and factor Va-vesicle complexes were determined. Images of negatively stained factor V bound to vesicles showed the protein as a relatively large globular domain (9.5 nm diameter) connected to the membrane through a narrow protein region 0.5-3 nm in length. This connecting region was not always visible and was measured as the distance between the globular region and the apparent vesicle edge. Factor V protein alone usually appeared as two connected globular regions of 10.2 and 6.5 nm diameter. The two-domain protein structure appeared consistent with both the image of factor V alone and bound to the membrane. Factor V had no biological activity in a phospholipid-free prothrombinase assay system used. The proteolytically activated form of factor V generated by digestion with thrombin (factor Va) was at least 30,000 times more active. The electron microscopic images of factor Va-vesicle complexes showed a smaller protein that was more closely associated with the vesicle surface than was factor V. The light chain (Mr about 80,000) component of factor Va also bound to the surface of the vesicles and appeared to be largely external to the membrane. Protein-induced hydrodynamic radius changes for the factor V-vesicle and factor Va-vesicle complexes were 12.8 and 6.3 nm, respectively. The images observed in the electron microscope were used to calculate protein-induced radius changes. Comparison of these values with the experimentally determined hydrodynamic radius changes showed approximate agreement for factor Va-membrane complexes. However, the images of factor V-vesicle complexes suggested smaller hydrodynamic radius changes than were actually observed.

Stopped-flow studies of myelin basic protein association with phospholipid vesicles and subsequent vesicle aggregation

When mixed with vesicles containing acidic phospholipids, myelin basic protein causes vesicle aggregation. The kinetics of this vesicle cross-linking by myelin basic protein was investigated by using stopped-flow light scattering. The process was highly cooperative, requiring about 20 protein molecules per vesicle to produce a measurable aggregation rate and about 35 protein molecules per vesicle to produce the maximum rate. The maximum aggregation rate constant approached the theoretical vesicle-vesicle collisional rate constant. Vesicle aggregation was second order in vesicle concentration and was much slower than protein-vesicle interaction. The highest myelin basic protein concentration used here did not inhibit vesicle aggregation, indicating that vesicle cross-linking occurred through protein-protein interactions. In contrast, poly(L-lysine)-induced vesicle aggregation was easily inhibited by increasing peptide concentrations, indicating that it did cross-link vesicles as a peptide monomer. The myelin basic protein:vesicle stoichiometry required for aggregation and the low affinity for protein dimerization suggested that multiple protein cross-links were needed to form a stable aggregate. Stopped-flow fluorescence was used to estimate the kinetics of myelin basic protein-vesicle binding. The half-times obtained suggested a rate constant that approached the theoretical protein-vesicle collisional rate constant.

Myelin basic protein-enhanced fusion of membranes

Myelin basic protein caused rapid aggregation of vesicles containing acidic phospholipids. Aggregation could be reversed by trypsin digestion of the myelin basic protein. Aggregated vesicles containing gel phase phospholipids or vesicles containing greater than 15 mol% lysolecithin underwent fusion. The extent of fusion was measured by irreversible changes in the light-scattering intensities or diffusion coefficients of the vesicles. Fusion was also measured by the fluorescence quenching which occurred when vesicles containing a covalently bound fluorophore. N-4-nitrobenzo-2-oxa-1,3-diazole, were fused with vesicles containing the covalently bound spin label, 4,4-dimethyl-oxazolidine-N-oxyl. The kinetics of fusion were first order in phospholipid and had half-times of 0.5-5 min depending on lysolecithin composition. This protein-enhanced membrane fusion may provide a valuable model system for studying some types of biological membrane fusions.