Evidence for Two Physiologically Distinct Gap Junctions Expressed by the Chick Lens Epithelial Cell

Lens epithelial cells communicate with two different cell types. They communicate with other epithelial cells via gap junctions on their lateral membranes, and with fiber cells via junctions on their apices. We tested independently these two routes of cellcell communica t ion to determine if t reatment with a 90% CO2-equilibrated medium caused a decrease in junctional permeability; the transfer of fluorescent dye was used as the assay. We found that the high-CO2 treatment blocked intraepithelial dye transfer but not fiber-to-epithelium dye transfer. The lens epithelial cell thus forms at least two physiologically distinct classes of gap junctions. AP junctions are probably the intercellular junctions responsible for low-resistance pathways between cells (2, 12, 32). They are structures that can be found in numerous tissues and organs and in nearly all animal species. Gap junctions from diverse sources might be a single class of intercellular junction, composed of a single protein species or a set of proteins that have been conserved throughout the evolution of the different phyla. However, a number of lines of evidence indicate that there may be classes of gap junctions that are distinct from one another. Gap junctions from different sources are sometimes anatomically different from each other, particularly as viewed by freeze-fracture electron microscopy (23). Biochemical comparisons of junctions isolated from different organs have demonstrated nonhomology between the component peptides (14, 17, 26). Functional differences between junction types are evidenced by different sieving properties of junctions that join cells in culture, depending on the cell type used (10); and a gap junction that connects two cell lines that normally make junctions with different permeability characteristics can be asymmetrically permeable (9). Where experimental systems have been developed to perfuse the inner surfaces of gap junctions with test solutions, junctional permeability is found to be sensitive to hydrogen ions in one case (35) and insensitive in another (19). In addition, a differentiating cell may express anatomically and physiologically distinct gap junctions at different developmental stages (33). These observations can be taken to suggest that gap junctions are more than a single class of intercellular junction. However, in each case the junctions compared faced the cytoplasms of different cell types, were isolated via different methods, or were subjected to different experimental conditions. Some of the evidence strongly indicates that gap junctions can be nonidentical, particularly the biochemical evidence ( 14, 17, 26) and the data gathered on perfused junctions (19, 35). But the most effective means of comparing gap junctions is to do so in a system in which a single cell type expresses more than one junction type. A cell at the boundary of two groups of cells that are unlike and functionally coupled to each other must make gap junctions to communicate with its own cell type (homotypic junctions) and the other cell type (heterotypic junctions). These two routes of communication are not necessarily mediated by different classes of gap junctions. There are examples of electrical coupling between embryonic tissues (30, 34), between developmental compartments (5, 38), and between unlike cells in a single tissue (28, 31). In several cases gap junctions connecting two different cell types in a single tissue have been observed anatomically ( 1, 8, 15, 18, 20, 25, 27). In culture, some cell lines will form heterotypic junctions whereas others will not (6, 7, 11). Freeze-fracture observations have demonstrated anatomical differences between homotypic and heterotypic junctions in chick lens (13) and goldfish saccular macula (15). There is also a report of morphologically distinct homotypic junctions between rat intestinal epithelial cells (36). An example of a functional difference between two gap junctions comes from insect epidermis, where fluorescent dye does not pass the intersegmental border even though the segments are electrically coupled (39). There is an example of a difference in regulatory properties from a culture system in which rat neonatal ventricular cells are coupled to each other and to fibroblasts. The junctional permeability of the homotypic junctions is not sensitive to the uncoupling properties of the calcium ionophore A23187, whereas the heterotypic junctions are sensitive to the treatment (4). In this paper we report the first evidence for a difference in regulatory properties between two naturally occurring gap junctions from a single cell type. Embryonic lens epithelial cells of the chick communicate with other epithelial cells via gap junctions on their lateral membranes (homotypic junctions), and they also communicate with lens fiber cells via The Rockefeller University Press, 0021-9525/86/01/0194/06 $1.00 194 The Journal of Cell Biology, Volume 102, January 1986 194-199 on Jne 8, 2017 D ow nladed fom Published January 1, 1986