Activators and inhibitors of the plasminogen system in Alzheimer's disease

Accumulation and deposition of A beta is one of the main neuropathological hallmarks of Alzheimers disease (AD) and impaired A beta degradation may be one mechanism of accumulation. Plasmin is the key protease of the plasminogen system and can cleave A beta. Plasmin is activated from plasminogen by tissue plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). The activators are regulated by inhibitors which include plasminogen activator inhibitor-1 (PAI-1) and neuroserpin. Plasmin is also regulated by inhibitors including a2-antiplasmin and a2-macroglobulin. Here, we investigate the mRNA levels of the activators and inhibitors of the plasminogen system and the protein levels of tPA, neuroserpin and a2-antiplasmin in post-mortem AD and control brain tissue. Distribution of the activators and inhibitors in human brain sections was assessed by immunoperoxidase staining. mRNA measurements were made in 20 AD and 20 control brains by real-time PCR. In an expanded cohort of 38 AD and 38 control brains tPA, neuroserpin and a2-antiplasmin protein levels were measured by ELISA. The activators and inhibitors were present mainly in neurons and a2-antiplasmin was also associated with A beta plaques in AD brain tissue. tPA, uPA, PAI-1 and a2-antiplasmin mRNA were all significantly increased in AD compared to controls, as were tPA and a2-antiplasmin protein, whereas neuroserpin mRNA and protein were significantly reduced. a2-macroglobulin mRNA was not significantly altered in AD. The increases in tPA, uPA, PAI-1 and a2-antiplasmin may counteract each other so that plasmin activity is not significantly altered in AD, but increased tPA may also affect synaptic plasticity, excitotoxic neuronal death and apoptosis.