Mutant neuroserpin (S49P) that causes familial encephalopathy with neuroserpin inclusion bodies is a poor proteinase inhibitor and readily forms polymers in vitro

Familial encephalopathy with neuroserpin inclusion bodies (FENIB) is an autosomal dominant dementia that is characterized by intraneuronal inclusions of mutant neuroserpin. We report here the expression, purification, and characterization of wild-type neuroserpin and neuroserpin containing the S49P mutation that causes FENIB. Wild-type neuroserpin formed SDS-stable complexes with tPA with an association rate constant and K-i of 1.2 x 10(4) M-1 s(-1) and 5.8 nM, respectively. In contrast, S49P neuroserpin formed unstable complexes with an association rate constant and K-i of 0.3 x 10(4) M-1 s(-1) and 533.3 nM, respectively. Am assessment by circular dichroism showed that S49P neuroserpin had a lower melting temperature than wild-type protein (49.9 and 56.6 degreesC, respectively) and more readily formed loop-sheet polymers under physiological conditions. Neither the wildtype nor S49P neuroserpin accepted the P7-P2 alpha(1)-antitrypsin or P14-P3 antithrombin-reactive loop peptides that have been shown to block polymer formation in other members of the serpin superfamily. Taken together, these data demonstrate that S49P neuroserpin is a poor proteinase inhibitor and readily forms loop-sheet polymers. These findings provide strong support for the role of neuroserpin polymerization in the formation of the intraneuronal inclusions that are characteristic of FENIB.