The tissue plasminogen activator-plasminogen proteolytic cascade accelerates amyloid-beta (Abeta) degradation and inhibits Abeta-induced neurodegeneration

Abstract

Accumulation of the amyloid-beta (Abeta) peptide depends on both its generation and clearance. To better define clearance pathways, we have evaluated the role of the tissue plasminogen activator (tPA)-plasmin system in Abeta degradation in vivo. In two different mouse models of Alzheimer's disease, chronically elevated Abeta peptide in the brain correlates with the upregulation of plasminogen activator inhibitor-1 (PAI-1) and inhibition of the tPA-plasmin system. In addition, Abeta injected into the hippocampus of mice lacking either tPA or plasminogen persists, inducing PAI-1 expression and causing activation of microglial cells and neuronal damage. Conversely, Abeta injected into wild-type mice is rapidly cleared and does not cause neuronal degeneration. Thus, the tPA-plasmin proteolytic cascade aids in the clearance of Abeta, and reduced activity of this system may contribute to the progression of Alzheimer's disease.

Figure 1.

tPA activity is decreased and PAI-1 expression increased in Tg2576 mice. Tg2576 and age-matched wild-type (WT) mice brain sections (n = 5-7) were subjected to in situ zymography to measure tPA activity. Dark, lytic zones are smaller in the hippocampus, amygdala, and hypothalamus of 12-week-old (B) and 12-month-old (D) Tg2576 mice as compared with wild-type mice (A, C). Twelve-week-old Tg2576 mice show increased PAI-1 immunoreactivity (F), whereas age-matched littermate controls do not (E). At 12 months, both Tg2576 (H) and littermate nontransgenic “wild-type” (G) show similar levels of PAI-1 expression in the hippocampus. CA2 and CA3 indicate respective hippocampal subregions.

Figure 2.

Quantification of tPA activity in the hippocampus and amygdala of TgCRND8 mice (hatched bars), Tg2576 mice (open bars), and their age-matched littermate nontransgenic controls (WT, solid bars). The two sets of controls were equivalent and averaged for this figure. See Results for detailed statistical analysis.

Figure 3.

Mice deficient for tPA or plasminogen show PAI-1 induction and persistent Aβ deposition after Aβ injection. tPA-/-, plg-/-, and C57BL/6 (WT) mice (n = 5-7) were injected with Aβ₄₀ into the CA1 region of the hippocampus. PAI-1 expression (bright green fluorescence, highlighted with arrows) was upregulated in the presence of Aβ in the tPA-/- (B) and plg-/- (C) mice but not in wild-type mice (A). Immunohistochemical analysis shows the presence of Aβ in tPA-/- (E) and plg-/- (F) but not in wild-type (D) mice 3 d after injection. The Aβ₄₀ deposited in tPA-/- and plg-/- mice causes microglial activation as assessed by F4/80 staining (H and I, respectively; compare with G) and neuronal degeneration as assessed by Fluoro-Jade B staining (K and L, respectively; compare with J).

Figure 4.

Quantification of Aβ immunoreactivity (A), microglial activation (B), and neurodegeneration (C) in wild-type (open bars), tPA-/- (solid bars), and plg-/- mice (hatched bars). See Results for detailed statistical analysis.