Saez, Juan C., Viviana M. Berthoud, Maria C. Branes, Agustin D. Martinez, and Eric C. Beyer. Plasma Membrane Channels Formed by Connexins: Their Regulation and Functions. Physiol Rev 83: 1359-1400,...

Plasma Membrane Channels Formed by Connexins: Their Regulation and Functions

The blood-brain barrier (BBB) prevents neurotoxic plasma components, blood cells, and pathogens from entering the brain. At the same time, the BBB regulates transport of molecules into and out of t...

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Blood-Brain Barrier: From Physiology to Disease and Back

Gap junctions (Gj) form conduits between adjacent cells that are composed of connexin (Cx) protein subunits and allow direct intercellular communication. To date, the connexin gene family comprises 20 members in the mouse and 21 members in the human genome, 19 of which can be grouped as sequence-orthologous pairs. The structure of connexin genes is relatively simple. An untranslated exon 1 is separated by an intron of different length from exon 2, containing the uninterrupted coding region and the 3'-untranslated region (3'-UTR). However, in some connexin genes, the untranslated regions and the reading frame are spliced. Among the known "cardiovascular" connexins, Cx37 and Cx40 were demonstrated to be functionally expressed in mouse and human endothelial cells and Cx40, Cx43 as well as Cx45 in cardiomyocytes of both species. Functional properties, like permeabilities, charge selectivity and unitary conductivity were investigated after directed expression of these connexins in cultured cell lines or paired Xenopus oocytes. Targeted deletion of their coding sequence in the mouse genome allowed study of the biological relevance of Cx37, Cx40, Cx43 and Cx45 with regard to cardiovascular morphology and function. After ablation of Cx37 or Cx40, mice were viable and could be used to study defects in the adult cardiovascular system but loss of Cx43 or Cx45 led to neonatal or embryonic lethality, respectively. Conditional and cell-type specific deletion of both connexins in the heart or blood vessels can help to overcome this obstacle. As yet only little is known about mutations in the human genes for Cx37, Cx40, Cx43 and Cx45. Thus, a profound comparison between human and mouse phenotypes is not yet possible.

Gap junctions and the connexin protein family

The effects of connexin phosphorylation on gap junctional communication.

Regulation of gap junctions by phosphorylation of connexins.

Characterization of the gap junction protein connexin37 in murine endothelium, respiratory epithelium, and after transfection in human HeLa cells.

Phosphoamino acid analysis of mouse connexin45 (Cx45) expressed in human HeLa cells revealed that phosphorylation occurred mainly at serine residues, but also on tyrosine and threonine residues. To characterize the role of Cx45 phosphorylation, different serine residues of the serine-rich carboxy terminal region were deleted or exchanged for other amino acids residues. Human HeLa cells deficient in gap junctional intercellular communication were stably transfected with appropriate constructs and analyzed for expression, localization, phosphorylation, formation of functional gap junction channels and degradation of mutant Cx45. After exchange or deletion of nine carboxy terminal serine residues, phosphorylation was decreased by 90%, indicating that these serine residues represented main phosphorylation sites of mouse Cx45. The various serine residues of this region contributed differently to the phosphorylation of Cx45 suggesting a cooperative mechanism for phosphorylation. Substitution of different serine residues for other amino acids did not interfere with correct intracellular trafficking and assembly of functional gap junction channels, as shown by localization of mutant Cx45 at the plasma membrane and by dye transfer to neighboring cells. Truncated Cx45 was also weakly phosphorylated but was trapped in perinuclear locations. Dye transfer of these transfectants was similar as in nontransfected HeLa cells. The half-life of mouse Cx45 protein in HeLa cells was determined as 4.2 hr. Pulse-chase experiments with the different transfectants revealed an increased turnover of Cx45, when one or both of the serine residues at positions 381 and 382 or 384 and 385 were exchanged for other amino acids. The half-life of these mutants was diminished by 50% compared to wild type Cx45.

Phosphorylated carboxy terminal serine residues stabilize the mouse gap junction protein connexin45 against degradation.

Tyrosine and serine phosphorylation of the neural cell adhesion molecule L1 is implicated in its oligomannosidic glycan dependent association with NCAM and neurite outgrowth

The neural cell adhesion molecule L1 is highly homologous in its extracellular domain between species and completely identical in its cytoplasmic domain. We report here that tyrosine residues of L1 are phosphorylated in addition to serine residues, as determined by monoclonal phosphotyrosine antibodies and phosphoamino acid analysis. This result supports the suggestion that the cytoplasmic domain of L1 might be involved in signal transduction.

Birgit Hertlein

Papers

Characterization of the gap junction protein connexin37 in murine endothelium, respiratory epithelium, and after transfection in human HeLa cells.

Phosphorylated carboxy terminal serine residues stabilize the mouse gap junction protein connexin45 against degradation.

Tyrosine and serine phosphorylation of the neural cell adhesion molecule L1 is implicated in its oligomannosidic glycan dependent association with NCAM and neurite outgrowth

The neural adhesion molecule L1 is phosphorylated on tyrosine and serine residues

Immunochemical characterization of connexin31, −37, −40, −43, and −45 in cultured primary cells, transfected cell lines and murine tissues