The involvement of the tissue plasminogen activator system in Alzheimer's disease

Amyloid-beta plaques are a pathological hallmark of Alzheimer's disease. Several proteases are known to cleave/remove amyloid-beta, including plasmin, the product of tissue plasminogen activator cleavage of the pro-enzyme plasminogen. Although plasmin levels are lower in Alzheimer brain, there has been little analysis of the plasminogen activator/plasmin system in the brains of Alzheimer patients. In this study, zymography, immunocapture and ELISA assays were utilized to show that tissue plasminogen activator activity in frontal cortex tissue of Alzheimer patients is dramatically reduced compared to age-matched controls, while tissue plasminogen activator and plasminogen protein levels are unchanged; suggesting that plasminogen activator activity is inhibited in the Alzheimer brain. Analysis of endogenous plasminogen activator inhibitors shows that neuroserpin levels are significantly elevated in brains of Alzheimer patients. Furthermore, elevated amounts of tissue plasminogen activator-neuroserpin complexes are seen in the Alzheimer brain, and immunohistochemical studies demonstrate that both tissue plasminogen activator and neuroserpin are associated with amyloid-beta plaques in Alzheimer brain tissue. Thus, neuroserpin inhibition of tissue plasminogen activator activity leads to reduced plasmin and may be responsible for reduced clearance of amyloid-beta in the Alzheimer disease brain. Furthermore, decreased tPA activity in the Alzheimer brain may directly influence synaptic activity and impair cognitive function. These findings in human AD patients are further supported by experiments done in mice. Injection of the Abeta42 peptide into the frontal cortex of wild type and mice that are deficient for neuroserpin, tPA, or PAI-1 demonstrates that NS-/- mice are able to clear the peptide faster compared to wild type mice and that tPA-/- mice take longer to degrade the peptide. A novel animal model was created by crossing an AD transgenic mouse to NS-/- mice. In Morris water maze studies, these AD+NS-/- mice show dramatic improvement in the spatial orientation deficit characteristic of the AD transgenic mouse, as well as less of the Abeta 40 and Abeta42 as demonstrated by ELISA analysis. The AD+NS-/- mice also show smaller and fewer plaques compared to AD transgenic mice.