DECREASED RECEPTOR-MEDIATED CALCIUM RESPONSE IN PRION-INFECTED CELLS CORRELATES WITH DECREASED MEMBRANE FLUIDITY AND IP3 RELEASE

The most characteristic neuropathologic features of prion diseases are accumulation of PrPSc in the brain and vacuolation of neurons, Neuron al vacuolation suggests plasma membrane dysfunction. In an earlier stu dy, we found that bradykinin (Bk)-stimulated Ca2+ responses in scrapie -infected ScN(2)a cells were reduced by 30 to 50% compared with uninfe cted N(2)a cells, In this study, we investigated the cause. The IP3 se cond-messenger response to Bk stimulation was reduced by 90%, indicati ng that a defect occurs in the plasma membrane, Receptor-binding assay s showed a 3- to 4-fold increase in Bk receptor numbers on ScN(2)a cel ls; however, their binding affinity was reduced 5- to 13-fold, which m ay account for the decreased IP3 and Ca2+ responses, These results arg ue that scrapie causes a more fundamental change in the properties of the plasma membrane, We verified this by fluorescence recovery after p hotobleaching (FRAP) analysis with a lipid probe that measures lateral membrane fluidity. A 7-fold reduction of fluidity was found. These re sults support the hypothesis that the conversion of PrPC to PrPSc or t he accumulation of PrPSc in scrapie-infected cells alters the composit ion of their plasma membranes that secondarily causes the abnormal rec eptor-mediated function.