[Introduction to the structure and functions of junction communications or gap junctions]

Cell-to-cell communication through gap junctions (GJ) represents a direct route of exchange of informations between neighboring cells within tissues and organs. GJ are formed from the assembly of a large number of channels that differ from the other known channels because they connect the cytoplasm of adjacent cells. The GJ channel is built from two parts: the connexons. A connexon inserted into the plasma membrane of a cell interacts with another connexon belonging to an adjacent cell. Connexons are composed of proteins with four transmembrane domains that are named connexins (Cx). Six Cx form a connexon. Cx belong to a protein family with 13 known members at present. Each Cx is defined by its molecular mass in kDa (ex: Cx32, Cx43...). A given cell type expresses one or several Cx. The cell to cell transfer of molecules through GJ channels exhibit a size selectivity; only molecules with a molecular mass lower than 1000 Da such as ions and second messengers freely pass through GJ. Depending on the Cx they are made of, GJ seem to differ somewhat in their permeability properties. Cell-to-cell communication via GJ is a regulated process. GJ channels can be either open or closed. GJ mediated cell-to-cell communication or junctional coupling can be detected and quantified by visualization of the cell to cell transfer of a fluorescent probe (such as Lucifer Yellow...) previously introduced in a single cell by microinjection. The presence of GJ channels can also be identified by recording the passage of an electric current between contiguous cells. GJ are involved in numerous fundamental biological processes from the embryonic development to the homeostasis in adult tissues and organs. GJ coordinate cell activities and sometimes synchronize cell behaviour. This is the case for the propagation of the excitation wave in the cardiac muscle and smooth muscle. GJ mediate metabolic cooperation between cells; they represent a way of supply of nutrients for tissues that are weakly or not vascularized. GJ take part in the control of cell proliferation. The loss of GJ-mediated cell-to-cell communication is a common feature of transformed cells and the re-establishment of junctional coupling is associated with a decrease of tumoregenicity. Allowing the cell-to-cell transfer of second messengers, GJ participate (and sometimes control) the response of a cell population to signalling molecules. It is known for example that hormones influence the expression of Cx and thus the level of the junctional coupling and that communication via GJ has an effect on the type and extent of action of hormones.