Down-regulation Of Gap Junctions Research Articles

Endothelial connexins are down-regulated by atherogenic factors

Connexins are single polypeptides that assemble to form paired connexon hexamers participating in gap-junctional intercellular communication. In addition, unpaired connexons at cell membrane also act as channels connecting cytosols and extracellular space. These channels' properties plus other unique functions of connexins give the molecules significant roles in endothelial cells, which mainly express connexin43 (Cx43), Cx40, and Cx37. In vitro studies have shown that expression of endothelial connexins are regulated by both physiological and pathological factors, a majority of which are involved in atherogenesis. In vascular disorders, endothelial connexins are differentially regulated. However, down-regulation of gap junctions is a common phenomenon. These findings suggest that reduced expression of endothelial gap junctions is a potential indicator of endothelial dysfunction and warrant investigators to explore the molecular mechanisms as well as therapeutic implications.

Losartan Preserves Integrity of Cardiac Gap Junctions and PGC-1 .ALPHA. Gene Expression and Prevents Cellular Apoptosis in Remote Area of Left Ventricular Myocardium Following Acute Myocardial Infarction

This study tests the hypothesis that peroxisome proliferator activated receptor-gamma coactivator 1alpha (PGC-1alpha) and the integrity of gap junctions (GJs) were suppressed and the number of apoptotic bodies was increased in remote viable areas of left ventricle following acute myocardial infarction (AMI), which can be reversed by losartan therapy. Open chest surgery was consecutively performed on 32 adult male Sprague-Dawley rats. These rats were classified into 4 groups (n = 8/each group): group I, AMI (by ligation of left coronary artery (LCA) without treatment); group II, AMI with losartan 20 mg/kg/day; group III, sham control (without LAD ligation); and group IV, sham control with losartan 20 mg/kg/day. Echocardiography was performed on day 1 prior to AMI and on day 14 just before the rats were to be sacrificed for cellular and molecular studies. The results showed that mRNA expression of PGC-1alpha, integrated area (microm(2)) of clustered connexin43 (Cx43) spots, and Cx43 GJs were substantially down-regulated and the number of apoptotic bodies was markedly increased in nontreated AMI rats compared with healthy control and losartan-treated AMI rats on day 14 following AMI (all values of P < 0.001). Additionally, day 14 left ventricular (LV) ejection fraction was significantly lower in nontreated AMI rats than in healthy control and losartan-treated AMI rats (all values of P < 0.0001). Down-regulation of GJs and PGC-1alpha gene expression and cellular death were frequently observed in remote viable areas of LV following AMI. Losartan therapy reversed the adverse effects of AMI and preserved LV function.

Downregulation of Gap Junctions in Cancer Cells

Gap junctions are intercellular plasma membrane domains enriched in channels that allow direct exchange of ions and small molecules between adjacent cells. Gap junction channels are composed of a family of transmembrane proteins called connexin. Connexins play important roles in the regulation of cell growth and differentiation. Cancer cells usually have downregulated levels of gap junctions, and several lines of evidence suggest that loss of gap junctional intercellular communication is an important step in carcinogenesis. In support of this hypothesis are studies showing that reexpression of connexins in cancer cells causes normalization of cell growth control and reduced tumor growth. To gain a more detailed understanding of the role of connexins as tumor suppressors, a clearer picture of the mechanisms involved in loss of gap junctions in cancer cells is needed. Furthermore, defining the mechanisms involved in downregulation of connexins in carcinogenesis will be an important step toward utilizing the potential of connexins as targets in cancer prevention and therapy. Various mechanisms are involved in the loss of gap junctions in cancer cells, ranging from loss of connexin gene transcription to aberrant trafficking of connexin proteins. This review will discuss our current knowledge on the molecular mechanisms involved in the downregulation of gap junctions in cancer cells.

Calmodulin transmitted through gap junctions stimulates endocytic incorporation of yolk precursors in insect oocytes

In ovarian follicles of Oncopeltus fasciatus, and of Xylocopa virginica, calmodulin (CaM) of epithelial cell origin is required by oocytes for endocytic uptake of yolk precursor molecules. Furthermore, this 17–19 kDa protein is normally transported to the oocytes via gap junctions. Downregulation of gap junctions by treatment with 1 mM octanol or separation of the epithelial cells from their oocytes terminated precursor uptake, and this activity could be rescued by microinjection of 60 μM CaM, but not by injections of incubation medium, nor solutions of other molecular species tested. That endogenous CaM is required was confirmed by incubating otherwise untreated follicles in physiological salt solution (PSS) containing either calmidazolium or W-7, both known antagonists of CaM. By radioimmunoprecipitation, we show that the epithelial cells surrounding an oocyte synthesized 15 times as much calmodulin as did the oocytes they encircled. Neither octanol-treated follicles nor denuded oocytes incubated in medium containing calmodulin were able to resume endocytosis, arguing against an extracellular route. However, fluorescently labeled calmodulin microinjected into oocytes is shown to have crossed through gap junctions, making epithelial cells distinctly fluorescent.

HSV-2 disrupts gap junctional intercellular communication between mammalian cells in vitro

Infection by herpes simplex virus-2 (HSV-2) disrupts both dye and electrical coupling in Vero (African green monkey kidney) cell cultures. Vero cells in vitro were iontophoretically injected with the fluorescent dye Lucifer yellow CH, the spread of which revealed that cells throughout the confluent sheet shared open gap junctions. However, 24 h after infection with the virus (but before cells became rounded), dye always remained only within the target cell. Intracellular electrophysiological measurements of ionic coupling revealed a 0.4 coupling coefficient for adjacent cells in uninfected control cultures. By 3 h following infection significant down-regulation of gap junctions had begun, preceding by many hours any signs of infection visible with the light microscope. Measurements between adjacent cells 3 h post-infection, a period when HSV-2 gene expression is known to be at a maximum, yielded an average coupling coefficient of 0.35. By 6 h post-infection (a period of known viral DNA replication) average coupling coefficient for adjacent cells was 0.25, while by 24 h post-infection the average fill still further to <0.08. A coupling coefficient of <0.08 suggests that infection by HSV-2 completely disabled the gap junctions.