Caveolae/lipid rafts in fibroblast-like synoviocytes: ectopeptidase-rich membrane microdomains

Membrane peptidases play important roles in cell activation, proliferation and communication. Human fibroblast-like synoviocytes express considerable amounts of aminopeptidase N/CD13, dipeptidyl peptidase IV/CD26, and neprily sin/CD10, transmembrane proteins previously proposed to be involved in the regulation of intra-articular levels of neuropeptides and chemotactic media tors as well as in adhesion and cell-cell interactions. Here, we report the se peptidases in synoviocytes to be localized predominantly in glycolipid- and cholesterol-rich membrane microdomains known as 'rafts'. At the ultrast ructural level, aminopeptidase N/CD13 and dipeptidyl peptidase IV/CD26 were found in caveolae, in particular in intracellular yet surface-connected ve sicle-like structures and 'rosettes' made up of several caveolae. In additi on, clusters of peptidases were seen at the cell surface in flat patches ra nging in size from about 60 to 160 nm. Cholesterol depletion of synoviocyte s by methyl-beta- cyclodextrin disrupted > 90% of the caveolae and reduced the raft localization of aminopeptidase N/CD13 without affecting Ala-p-nitr oanilide-cleaving activity of confluent cell cultures. In co-culture experi ments with T-lymphocytes, cholesterol depletion of synoviocytes greatly red uced their capability to induce an early lymphocytic expression of aminopep tidase N/CD13. We propose caveolae/rafts to be peptidase-rich 'hot-spot' re gions of the synoviocyte plasma membrane required for functional cell-cell interactions with lymphocytes. The peptidases may act in concert with other types of proteins such as receptors and signal transducers localized in th ese specialized membrane domains.