Physiological roles of permeable junctions: some possibilities.

Abstract

It has been nearly two decades since penneable intercellular junctions were first detected between electrically nonexcitable cells (27, 33). Considerable prog­ ress has been made in our understanding of the structure, permeability, dis­ tribution, and biochemistry of the junctions (see 9, 24, 54, 71 in this volume). Much less progress has been made, however, in our understanding of the biological roles played by the junctions, despite the common use of the term communication. In fact most of the original functional specula­ tions, e.g. involvement in exchange of nutrients (27), maintenance of tissue homeostasis (73), regulation of cell growth and differentiation (25, 50), and transmission of intracellular molecular signals (61), are still frequently cited in the literature with rather little in the way of experimental support. A major problem in trying to design experiments to test these various ideas has been the lack of a specific and reversible method for interrupting junctional permeabil­ ity. Consequently, it has been necessary to rely on more indirect approaches, for example correlating patterns of junctional distribution and changes in junctional permeability with specific physiological and pathological events. In this review we discuss a few of the recently studied systems in which these indirect approaches have been quite useful. Rather than be exhaustive, we have selected cell systems that reflect general concepts regarding junctional com­ munication, that illustrate the range of strategies currently available to study the physiology and pathology of the junctions, and that have particular promise for