A spectrophotometric assay of intercellular communication.

Cell signalling and communication mechanisms are central to an understandmg of cell and tissue differentiation and many pathological processes. Intercellular communication in metazoans is a hnction of cell-cell conduits known as gap junctions [ I ] . There are three main classes of cell-cell junction occluding junctions, anchoring junctions and comunicating junctions. Gap junctions belong to the last of these classes and were first morphologically d e s c n i in 1967. They consist of two aligned hemichannels, one donated by each adjacent cell and are assembled from six connexin protein units [2] CoMKxins demonstrate extracdlular, transmembrane and intracdhtlar domaim with the latter being the most variable and subject to modification by phosphorylation [3]. There are four main classes of connexh designated on the basis of molecukr mass, Cx26,32,43 and 46, coded by a scrperfarniy of g e m [4). Gap junctions permit the exchange of low molecular weight mokuIes and ions between cells, < 1kDa. Loss of communication between tumour cells or between tumour cells and surrounding normal cells is a feature of many neoplasms and this model predicts that gap junctions represent pathways for the dissemination ofcritical growth regulatory ndecuks throughout a tissues c o m n i d n g compartments [ 1.51. In the multistage model ofcarcinogenesis, loss of communication with an initiated cell releases it fkom the predominantly inhibitory growth control of surrounding normal cells and permits the clonal expansion which charactpromotion phase [6, 71. Restoration of c a m r m d i o n in tumour tissue by tramfeetion and expression of Cx genes or the treatment of an upregulating compound has been reported to regress tumour development [8, 9, 10, 1 I]. Connexin genes are consequently viewed as