Akt phosphorylates Connexin43 on Ser373, a "mode-1" binding site for 14-3-3

Connexin43 (Cx43) is a membrane-spanning protein that forms channels that bridge the gap between adjacent cells and this allows for the intercellular exchange of information. Cx43 is regulated by phosphorylation and by interacting proteins. "Mode-1" interaction with 14-3-3 requires phosphorylation of Ser373 on Cx43 (Park et al. 2006). Akt phosphorylates and targets a number of proteins to interactions with 14-3-3. Here we demonstrate that Akt phosphorylates Cx43 on Ser373 and Ser369; antibodies recognizing Akt-phosphorylated sites or phospho-Ser "mode-1" 14-3-3-binding sites recognize a protein from EGF-treated cells that migrates as Cx43, and GST-14-3-3 binds to Cx43 phosphorylated endogenously in EGF-treated cells. Confocal microscopy supports the co-localization of Cx43 with Akt and with 14-3-3 at the outer edges of gap junctional plaques. These data suggest that Akt could target Cx43 to an interaction with 14-3-3 that may play a role in the forward trafficking of Cx43 multimers and/or their incorporation into existing gap junctional plaques.

v-Src tyrosine phosphorylation of connexin43: regulation of gap junction communication and effects on cell transformation

The oncogenic tyrosine kinase, v-Src, phosphorylates connexin43 (Cx43) on Y247 and Y265 and inhibits Cx43 gap junctional communication (GJC), the process of intercellular exchange of ions and metabolites. To test the role of a negative charge on Cx43 induced by tyrosine phosphorylation, we expressed Cx43 with glutamic acid substitutions at Y247 or Y265. The Cx43Y247E or Cx43Y265E channels were functional in Cx43 knockout fibroblasts, indicating that introducing a negative charge on Cx43 was not likely the mechanism for v-Src disruption of GJC. Cells coexpressing v-Src and the triple serine to alanine mutant, Cx43S255/279/282A, confirmed that mitogen-activated protein (MAP) kinase phosphorylation of Cx43 was not required for v-Src-induced disruption of GJC and that tyrosine phosphorylation was sufficient. In addition, v-Src cells containing v-Src-resistant gap junctions, Cx43Y247/265F, displayed properties of cell migration, adhesion, and proliferation similar to Cx43wt/v-Src cells, suggesting that Cx43 tyrosine phosphorylation and disruption of GJC are not involved in these transformed cell properties.

Functional analysis of Nox4 reveals unique characteristics compared to other NADPH oxidases

Reactive oxygen species (ROS) are important signal transduction molecules in ligand-induced signaling, regulation of cell growth, differentiation, apoptosis and motility. Recently NADPH oxidases (Nox) homologous to Nox2 (gp91phox) of phagocyte cytochrome b558 have been identified, which are an enzymatic source for ROS generation in epithelial cells. This study was undertaken to delineate the requirements for ROS generation by Nox4. Nox4, in contrast to other Nox proteins, produces large amounts of hydrogen peroxide constitutively. Known cytosolic oxidase proteins or the GTPase Rac are not required for this activity. Nox4 associates with the protein p22phox on internal membranes, where ROS generation occurs. Knockdown and gene transfection studies confirmed that Nox4 requires p22phox for ROS generation. Mutational analysis revealed structural requirements affecting expression of the p22phox protein and Nox activity. Mechanistic insight into ROS regulation is significant for understanding fundamental cell biology and pathophysiological conditions.

Connexin43 PDZ2 binding domain mutants create functional gap junctions and exhibit altered phosphorylation

Connexin43 (Cx43) is the most abundantly expressed gap junction protein. The C-terminal tail of Cx43 is important for regulation of gap junctions via phosphorylation of specific tyrosine and serine residues and through interactions with cellular proteins. The C-terminus of Cx43 has been shown to interact with the PDZ2 domain of the tight and adherens junction associated zona occludens 1 (ZO-1) protein. Analysis of the PDZ2 binding domain of Cx43 indicated that positions -3 and -2, and the final hydrophobic amino acid at the C-terminus, are critical for ZO-1 binding. In addition, the C-termini of connexins 40 and 45, but not Cx32, interacted with ZO-1. To evaluate the functional significance of the Cx43-ZO-1 interaction, Cx43 wild type (Cx43wt) and mutants lacking either the C-terminal hydrophobic isoleucine (Cx43deltaI382) or the last five amino acids (Cx43delta378-382), required for ZO-1 binding in vitro, were introduced into a Cx43-deficient MDCK cell line. In vitro binding studies and coimmunoprecipitation assays indicated that these Cx43 mutants failed to interact with ZO-1. Confocal and deconvolution microscopy revealed that a fraction of Cx43wt colocalized with ZO-1 at the plasma membrane. A similar colocalization pattern was observed for the Cx43deltaI382 and Cx43 delta378-382 mutants, which were translocated to the plasma membrane and formed functional gap junction channels. The wt and mutant Cx43 appeared to have similar turnover rates. However, the P2 and P3 phosphoisoforms of the Cx43 mutants were significantly reduced compared to Cx43wt. These studies indicated that the interaction of Cx43 with ZO-1 may contribute to the regulation of Cx43 phosphorylation.

p21-activated kinase (Pak) regulates NADPH oxidase activation in human neutrophils

The phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase plays an instrumental role in host defense and contributes to microbicial killing by releasing highly reactive oxygen species. This multicomponent enzyme is composed of membrane and cytosolic components that assemble in the plasma membrane or phagolysosome. While the guanosine S'-triphosphatase (GTPase) Rac2 has been shown to be a critical regulator of NADPH oxidase activity and assembly, the role of its effector, p21-activated kinase (Pak), in oxidase function has not been well defined. Using HIV-1 Tat-mediated protein transduction of Pak inhibitory domain, we show here that Pak activity is indeed required for efficient superoxide generation in intact neutrophils. Furthermore, we show that Pak translocates to the plasma membrane upon N-formyl-methionyl-leucyl-phenylalanine (fMLF) stimulation and colocalizes with translocated p47(phox) and with p22phox, a subunit of flavocytochrome b558. Although activated Pak phosphorylated several essential serine residues in the C-terminus of p47phox, direct binding to p47phox was not observed. In contrast, active Pak bound directly to p22phox, suggesting flavocytochrome b was the oxidase-associated membrane target of this kinase and this association may facilitate further phosphorylation of p47phox in the assembling NADPH oxidase complex.

Novel rab GAP-like protein, CIP85, interacts with connexin43 and induces its degradation

Gap junctions play critical roles in tissue function and homeostasis. Connexin43 (Cx43) is a major gap junction protein expressed in the mammalian heart and other tissues and may be regulated by its interaction with other cellular proteins. Using the yeast two-hybrid screen, we identified a novel Cx43-interacting protein of 85-kDa, CIP85, which contains a single TBC, SH3, and RUN domain, in addition to a short coiled coil region. Homologues containing this unique combination of domains were found in human, D. melanogaster, and C. elegans. CIP85 mRNA is expressed ubiquitously in mouse and human tissues. In vitro interaction assays and in vivo co-immunoprecipitation experiments confirmed the interaction of endogenous CIP85 with Cx43. In vitro interaction experiments using CIP85 mutants with in-frame deletions of the TBC, SH3, and RUN domains indicated that the SH3 domain of CIP85 is involved in its interaction with Cx43. Conversely, analysis of Cx43 mutants with proline to alanine substitutions in the two proline-rich regions of Cx43 revealed that the P(253)LSP(256) motif is an important determinant of the ability of Cx43 to interact with CIP85. Laser-scanning confocal microscopy showed that CIP85 colocalized with Cx43 at the cell periphery, particularly in areas reminiscent of gap junction plaques. The functional importance of the interaction between CIP85 and Cx43 was suggested by the observation that CIP85 appears to induce the turnover of Cx43 through the lysosomal pathway.

Identification of connexin-interacting proteins: application of the yeast two-hybrid screen

Protein-protein interactions are recognized as one of the fundamental mechanisms for relaying the intra- and intercellular signals that are required for normal cellular activities affecting growth, development, and maintenance of homeostasis in tissues and organs. The yeast two-hybrid screen has become a valuable tool for identifying protein-protein interactions. The gap junction protein connexin 43 (Cx43) has been implicated in a number of biological processes including development and cellular growth control. To further advance our understanding of the ways in which Cx43 may influence these cellular activities, and to extend our knowledge of the regulation of Cx43 function and/or processing, we have employed the yeast two-hybrid screen technique to identify Cx43-interacting proteins. We present detailed methods for the yeast two-hybrid screen of a mouse embryonic cDNA library using the C terminus of Cx43 as "bait." We also describe additional methods to confirm the interactions between Cx43 and the identified proteins. These methods include in vitro binding assays, coimmunoprecipitation, and subcellular localization using immunofluorescence microscopy.

Immortalized connexin43 knockout cell lines display a subset of biological properties associated with the transformed phenotype

Immortalized cells from embryonic connexin43 knockout mice (Cx43-/-) and homozygous littermates (Cx43+/+) were cloned and characterized to determine whether the absence of Cx43 function would induce observable phenotypic changes. Cells of the Cx43+/+ clones expressed Cx43 and engaged in gap junctional communication with 10-12 neighboring cells. The Cx43-/- cells were devoid of Cx43 and communicated to less than 1 cell. Electrophysiological analysis indicated that the Cx43-/- cells communicated through Cx45 channels from 8-80-fold less than did the Cx43+/+ subclones, which seemed to communicate through Cx43 and Cx45 channels. The Cx43-/- clones grew at faster rates and to higher saturation densities, had a more spindly morphology, were more refractile, and adhered less well to the substratum than did the Cx43+/+ clones. Reintroducing the Cx43 gene into the Cx43-/- clones resulted in three subclones that communicated to 3-4 cells. Partial restoration of gap junctional communication in the three subclones was accompanied by reduced growth rates and saturation densities (2-fold compared to that of parental Cx43-/- clones) but no reversions in morphology or cell-substratum adhesion. The increased growth rates and saturation densities, altered morphology, and decreased cell adhesion displayed by the Cx43-/- clones reflect a subset of the properties of transformed cells. These studies advance the hypothesis that loss of Cx43 function during development may cause cells to acquire a preneoplastic condition.

A connexin 43 antisense vector reduces the ability of normal cells to inhibit the foci formation of transformed cells

Antisense gene constructs have been very useful in the functional analysis of genes and their products. In this report we used a connexin 43 (Cx43) antisense gene construct to study the role that heterologous gap-junctional intracellular communication (GJIC) plays in the ability of untransformed fibroblasts to suppress the foci-forming ability of src oncogene-transformed cells. Untransformed Rat-1 fibroblasts transfected with the Cx43 antisense DNA construct showed marked decreases in Cx43 RNA and protein, which were accompanied by a corresponding decrease in GJIC. These Cx43 antisense-transfected cells maintained normal cell morphology, growth rates, and saturation densities and did not grow in soft-agar suspension. However, in coculture experiments, the Cx43 antisense cells were less effective than vector-alone-transfected, sense-transfected, and untransfected cells at inhibiting foci formation of pp60v-src-transformed cells. These effects of junctionally competent, normal cells were associated with the existence of heterologous GJIC with the transformed cells and did not appear to result from the elaboration of a stable, diffusible inhibitory factor. Thus, gap-junction-mediated transfer of putative regulatory molecules may play a role in the ability of untransformed cells to suppress the expression of certain properties of transformed cells.