THE AXONALLY SECRETED SERINE PROTEINASE-INHIBITOR, NEUROSERPIN, INHIBITS PLASMINOGEN ACTIVATORS AND PLASMIN BUT NOT THROMBIN

Neuroserpin is an axonally secreted serine proteinase inhibitor that i s expressed in neurons during embryogenesis and in the adult nervous s ystem. To identify target proteinases, we used a eucaryotic expression system based on the mouse myeloma cell line J558L and vectors includi ng a promoter from an Ig-kappa-variable region, an Ig-kappa enhancer, and the exon encoding the Ig-kappa constant region (C kappa) and produ ced recombinant neuroserpin as a wild-type protein or as a fusion prot ein with C kappa. We investigated the capability of recombinant neuros erpin to form SDS-stable complexes with, and to reduce the amidolytic activity of, a variety of serine proteinases in vitro. Consistent with its primary structure at the reactive site, neuroserpin exhibited inh ibitory activity against trypsin-like proteinases. Although neuroserpi n bound and inactivated plasminogen activators and plasmin, no interac tion was observed with thrombin. A reactive site mutant of neuroserpin neither formed complexes with nor inhibited the amidolytic activity o f any of the tested proteinases. Kinetic analysis of the inhibitory ac tivity revealed neuroserpin to be a slow binding inhibitor of plasmino gen activators and plasmin. Thus, we postulate that neuroserpin could represent a regulatory element of extracellular proteolytic events in the nervous system mediated by plasminogen activators or plasmin.