Contact inhibition and the regulation of endocytosis in the corneal endothelium: Correlation with a restricted surface receptor lateral mobility and the appearance of a fibronectin meshwork

Abstract

The correlation between adsorptive endocytosis, surface receptor lateral mobility, and the production of fibronectin was analyzed using corneal endothelial cells maintained in tissue culture as a model. These properties were studied as a function of the cell density and organization using sparse, subconfluent, and confluent cultures. Sparse, actively growing cultures bind, internalize, and degrade low density lipoprotein (LDL), whereas confluent endothelial cell cultures, although still capable of binding LDL, no longer internalize it and hence show no degradation of the lipoprotein. Disruption of the cell monolayer by means of wounding was associated with a gain of LDL uptake activity only in cells located at the edges of the wound or migrating into the wound. Cap formation induced by Concanavalin A was observed in sparse, but not in confluent cultures, indicating a lower degree of receptor lateral mobility concomitant with the formation of a confluent cell monolayer. Lactoper-oxidase catalyzed iodination of sparse and confluent cultures revealed an accumulation of fibronectin primarily in the pericellular matrix produced by confluent cultures. In contrast, no extracellular matrix and almost no fibronectin associated with the cell surface were detected in actively growing cells which were not yet in contact. The formation of a confluent cell monolayer is therefore associated with the inhibition of LDL uptake, possibly via membrane changes that lead to a restriction of surface receptor lateral mobility. This might be brought about by a direct or indirect interaction of the extracellular meshwork of fibronectin with various surface components and subjacent cytoskeletal elements. It is suggested that the organization of cells located as a single layer at the interface between fluid and tissue can dictate, both in vitro and in vivo, metabolic functions such as adsorptive endocytosis and thus be a prominent factor in determining their physiological role as a barrier.