Abstract
Accumulation and deposition of Aβ is one of the main neuropathological hallmarks of Alzheimer’s disease (AD) and impaired Aβ degradation may be one mechanism of accumulation. Plasmin is the key protease of the plasminogen system and can cleave Aβ. 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 α2-antiplasmin and α2-macroglobulin. Here, we investigate the mRNA levels of the activators and inhibitors of the plasminogen system and the protein levels of tPA, neuroserpin and α2-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 α2-antiplasmin protein levels were measured by ELISA. The activators and inhibitors were present mainly in neurons and α2-antiplasmin was also associated with Aβ plaques in AD brain tissue. tPA, uPA, PAI-1 and α2-antiplasmin mRNA were all significantly increased in AD compared to controls, as were tPA and α2-antiplasmin protein, whereas neuroserpin mRNA and protein were significantly reduced. α2-macroglobulin mRNA was not significantly altered in AD. The increases in tPA, uPA, PAI-1 and α2-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.
Highlights
The abnormal accumulation and deposition of amyloid  (A) peptide is one of the key neuropathological hallmarks of Alzheimer’s disease (AD) and is thought to initiate a series of processes that cause synaptic dysfunction and neuronal death [1,2]
None of the proteins was associated with neurofibrillary tangles and, with the exception of co-localization of ␣2-antiplasmin with A plaques in AD, there were no obvious differences in the distribution of any of the proteins in AD and control brains
We have shown that there are marked changes in the expression of the activators and inhibitors of the plasminogen system in AD: significant increases in tissue plasminogen activator (tPA), urokinase-type plasminogen activator (uPA), plasminogen activator inhibitor-1 (PAI-1) and ␣2-antiplasmin mRNA and increased tPA and ␣2-antiplasmin protein, and significant reduction in neuroserpin mRNA and protein
Summary
The abnormal accumulation and deposition of amyloid  (A) peptide is one of the key neuropathological hallmarks of Alzheimer’s disease (AD) and is thought to initiate a series of processes that cause synaptic dysfunction and neuronal death [1,2]. In recent years, reduced activity of enzymes capable of degrading A has been suggested as a potential contributor to AD pathogenesis [3,4,5]. These enzymes include angiotensin-converting enzyme [6,7], neprilysin [8], endothelin-converting enzymes [9], insulin-degrading enzyme [10] and plasmin [11]. A activates the plasminogen activators both in vitro and in vivo [11,17,18,19] This has the potential to be a protective mechanism to limit the accumulation of A. Plasmin protects cultured neurons from A-induced cell death [11,21,22] and enhances clearance of A in vivo [23]
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