Molecular bases of neuroserpin function and pathology

Serpins build a large and evolutionary widespread protein superfamily, hosting members that are mainly Ser-protease inhibitors. Typically, serpins display a conserved core domain composed of three main β-sheets and 9-10 α-helices, for a total of approximately 350 amino acids. Neuroserpin (NS) is mostly expressed in neurons and in the central and peripheral nervous systems, where it targets tissue-type plasminogen activator. NS activity is relevant for axogenesis, synaptogenesis and synaptic plasticity. Five (single amino acid) NS mutations are associated with severe neurodegenerative disease in man, leading to early onset dementia, epilepsy and neuronal death. The functional aspects of NS protease inhibition are linked to the presence of a long exposed loop (reactive center loop, RCL) that acts as bait for the incoming partner protease. Large NS conformational changes, associated with the cleavage of the RCL, trap the protease in an acyl-enzyme complex. Contrary to other serpins, this complex has a half-life of approximately 10 min. Conformational flexibility is held to be at the bases of NS polymerization leading to Collins bodies intracellular deposition and neuronal damage in the pathological NS variants. Two main general mechanisms of serpin polymerization are currently discussed. Both models require the swapping of the RCL among neighboring serpin molecules. Specific differences in the size of swapped regions, as well as differences in the folding stage at which polymerization can occur, distinguish the two models. The results provided by recent crystallographic and biophysical studies allow rationalization of the functional and pathological roles played by NS based on the analysis of four three-dimensional structures.