Autocellular coupling by gap junctions in cultured astrocytes: a new view on cellular autoregulation during process formation

Dynamics of gap junctions observed in living cells with connexin43-GFP chimeric protein

To study the aggregation of cell-to-cell channels into gap junctions at individual cell-cell contacts, we transfected cells with an expression vector for a chimeric protein composed of the cell-to-cell channel protein connexin43 and a green fluorescent protein. The chimeric channel protein was visualized in the fluorescence microscope and was found to form gap junctions at the cell-cell contacts just like wild-type connexin43. Cells expressing the chimeric protein had functional cell-to-cell channels. Using timelapse videomicroscopy on live cells we observed individual gap junctions over long periods and recorded the time course of aggregation of the chimeric channel protein into gap junctions at newly formed cell-cell contacts. We found that individual small gap junctions were very dynamic, moving about or becoming assembled and disassembled in the course of minutes. Larger gap junctions were more stable than small punctate ones. In control condition, stable new gap junctions were not formed during observation times of 30 min or longer. But at elevated levels of cyclic adenosine monophosphate, the chimeric channel protein began aggregating at new junctions 5-10 minutes after cell-cell contact and continued to concentrate there for at least one hour. Also already established junctions grew in size. The fluorescent chimeric channel protein will be an excellent tool to investigate the regulation of trafficking of connexin from and to the membrane and the mechanism of connexin channel aggregation into gap junctions.

Dynamics of astrocyte adhesion as analyzed by a combination of atomic force microscopy and immuno-cytochemistry: the involvement of actin filaments and connexin 43 in the early stage of adhesion

We observed the time-dependent morphological alteration of astrocytes during their adhesion by atomic force microscopy (AFM) and investigated the relationships between this morphological alteration and the localization of actin filaments and connexin 43 by immunocytochemistry. The fine processes observed as fine ridge-like structures by AFM were closely concerned with actin filaments by immunocytochemistry. During the adhesion of astrocytes, actin filaments appeared to be aligned regularly beyond the borders among different cells. Detectable connexin immunoreactivity was changed in the following regions: 1) the tips of fine cell processes and the cell margin when astrocytes started to adhere; 2) the border of cells when astrocytes tightly adhered; and 3) non-specific sites when astrocytes became a cluster. In the former two cases, the immunopositive spots for connexin were observed to colocalize with the tips of cell processes with actin filaments. These results strongly suggest that connexin associated with actin filaments at the tip of cell processes plays an important role in the early stage of the adhesion of astrocytes. These observations afford valuable clues for understanding the glial communication.

Visualization and functional blocking of gap junction hemichannels (connexons) with antibodies against external loop domains in astrocytes

Astrocytes constitute a metabolically and electrically coupled syncytium which is essential for the regulation of ionic homeostasis of the interstitial brain fluid and for coordinating responses to neuronal activities. The structural avenues that couple individual astrocytes are provided by gap junctions which consist of transmembraneous channels that bridge the extracellular space. Each gap junction channel is composed of two hemichannels (connexons) that dock to each other via their extracellular loops. Here we report on the characterization of external loop antibodies that visualize hemichannels in specific plasma membranes of vital astrocytes. Hemichannels were found to be concentrated on cytoplasmic processes and filopodia of subconfluent cells. Specificity of hemichannel-binding was evaluated by blockage of Lucifer Yellow (LY) dye-transfer and Ca2+ transmission, as well as LY uptake under calcium-deficient conditions. Our data indicate that hemichannels are accessible from the extracytoplasmic side either for direct visualization or functional manipulation. The availability of such a probe will allow in vivo experiments which require selective and/or temporal blocking of gap junctions in animal models.