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, Neuronal vacuolation suggests plasma membrane dysfunction. In an earlier study, we found that bradykinin (Bk)-stimulated Ca2+ responses in scrapie-infected ScN(2)a cells were reduced by 30 to 50% compared with uninfected N(2)a cells, In this study, we investigated the cause. The IP3 second-messenger response to Bk stimulation was reduced by 90%, indicating that a defect occurs in the plasma membrane, Receptor-binding assays showed a 3- to 4-fold increase in Bk receptor numbers on ScN(2)a cells; however, their binding affinity was reduced 5- to 13-fold, which may account for the decreased IP3 and Ca2+ responses, These results argue that scrapie causes a more fundamental change in the properties of the plasma membrane, We verified this by fluorescence recovery after photobleaching (FRAP) analysis with a lipid probe that measures lateral membrane fluidity. A 7-fold reduction of fluidity was found. These results support the hypothesis that the conversion of PrPC to PrPSc or the accumulation of PrPSc in scrapie-infected cells alters the composition of their plasma membranes that secondarily causes the abnormal receptor-mediated function.