Trypsin and trypsin-like proteases in the brain: proteolysis and cellular functions.

Several serine proteases including thrombin, tissue-type plasminogen activator and urokinase-type plasminogen activator have been well characterized in the brain. In this article, we review the brain-related trypsin and trypsin-like serine proteases. Accumulating evidence demonstrates that trypsin and trypsin-like serine proteases play very important roles in neural development, plasticity, neurodegeneration and neuroregeneration in the brain. Neuropsin is able to hydrolyze the extracellular matrix components by its active site serine, and regulates learning and memory in normal brain. The mutant neurotrypsin contributes to mental retardation in children. Neurosin seems to be involved in the pathogenesis of neurodegenerative disorders, like Alzheimer's disease, Parkinson's disease or multiple sclerosis. Although mesotrypsin/trypsin IV is also implicated in neurodegeneration, its functional significance still remains largely unknown. Particularly, mesotrypsin/trypsin IV, P22 and neurosin exert their physiological and pathological functions through activation of certain protease-activated receptors (PARs). In the brain, the presence of serpins controls the activity of serine proteases. Therefore, understanding the interaction among brain trypsin, serpins and PARs will provide invaluable tools for regulating normal brain functions and for the clinical treatment of neural disorders.