Abstract. Calpain (a Ca2+-dependent protease) is present in many cell types. Because it is present in the cytosol, the potential exists that it may regulate critical intracellular events by inducing crucial proteolytic cleavages. However, the concentrations of Ca 2 § required to activate calpain are higher than those attained in the cytoplasm of most cells. Thus, the physiological importance of calpain and the mechanisms involved in its activation have remained elusive. In this study, we show that calpain rapidly moved to a peripheral location upon the addition of an agonist to suspensions of platelets, but it remained unactivated. We provide three lines of evidence that calpain was subsequently activated by a mechanism that required the binding of an adhesive ligand to the major platelet integrin, glycoprotein (GP) IIb-IIIa: calpain activation was prevented by RGDS, a tetrapeptide that inhibits the binding of adhesive ligand to GP Ub-l/Ia; it was also prevented by monoclonal antibodies that inhibit adhesive ligand binding to GP llb-IIIa; and its activation was markedly reduced in platelets from patients whose platelets have greatly reduced levels of functional GP llb-IIIa. Thus, in platelets, binding of the extracellular domain of GP l/b-IIIa to its adhesive ligand can initiate a transmembrane signal that activates intraceUular calpain. Because calpain is present in focal contacts of adherent cells, the interaction of integrins with adhesive ligands in the extracellular matrix may regulate activation of calpain in other cell types as well.

No abstract

Glial cells in higher invertebrate groups are usually recognized on the basis of their location and general morphological or functional criteria. In this study of the crustacean visual system, we have approached the analysis of the relations between glial cells and the extracellular matrix by classical histochemical methods for carbohydrates at the light and electron microscopic levels, carbonic anhydrase histochemistry and by the biochemical characterization of sulphated polysaccharides. Periodic acid-Schiff-positive glial cells and processes were observed in the retina, basement membrane below the retina and in the optic ganglia. Carbonic anhydrase was not detected in the retina but it was demonstrated in all optic ganglia. The biochemical analysis of the extracellular matrix confirmed the alcian blue reaction and showed that sulphated polysaccharides are not abundant in the optic neuropils. This article describes into more details the crustacean visual system glial cells classification, and the relation between them and the extracellular matrix. In addition, they show that glial cells are the main components of the retinal basement membrane.

No abstract