Immunocytochemical localization of CD44 in the mouse retina.

The transmembrane glycoprotein CD44 is a cell adhesion molecule which has recently been localized in a variety of cell types. It mediates cell attachment to extracellular matrix components and also binds to the actin cytoskeleton within the cell. In this study, the presence and distribution of CD44 in the mouse retina was investigated in order to determine whether this molecule might be important for retinal cell adhesion. With immunoperoxidase techniques, positive labeling for CD44 was found at the level of the outer limiting membrane and in the region just sclerad to it. However, from these light microscope results it was not clear if the label was in the photoreceptor inner segments, the Müller cell apical microvilli, or both. In order to answer this question, cryoultramicrotomy and immunogold labeling were used to demonstrate that CD44 is specifically localized to the Müller cell microvilli which appose the interphotoreceptor matrix. Western blotting of a retina homogenate showed that the anti-CD44 antibody is specific for a protein of approximately 90 kDa which is the correct molecular weight for the most abundant form of CD44. These results, along with the known binding characteristics of CD44 for the extracellular matrix, suggest that CD44 could play a role in mediating the attachment of the neural retina to components of the interphotoreceptor matrix.