Protease Inhibitor Coinfusion with Amyloid β-Protein Results in Enhanced Deposition and Toxicity in Rat Brain

Abstract

Amyloid ␤-protein, A␤, is normally produced in brain and is cleared by unknown mechanisms. In Alzheimer's disease (AD), A␤ accumulates in plaque-like deposits and is implicated genetically in neurodegeneration. Here we investigate mechanisms for A␤ degradation and A␤ toxicity in vivo, focusing on the effects of A␤40, which is the peptide that accumulates in apolipoprotein E4-associated AD. Chronic intraventricular infusion of A␤40 into rat brain resulted in limited deposition and toxicity. Coinfusion of A␤40 with the cysteine protease inhibitor leupeptin resulted in increased extracellular and intracellular A␤ immunoreactivity. Analysis of gliosis and TUNEL in neuron layers of the frontal and entorhinal cortex suggested that leupeptin exacerbated A␤40 toxicity. This was supported further by the neuronal staining of cathepsin B in endosomes or lysosomes, colocalizing with intracellular A␤ immunoreactivity in pyknotic cells. Leupeptin plus A␤40 caused limited but significant neuronal phospho-tau immunostaining in the entorhinal cortex. Intriguingly, A␤40 plus leupeptin induced intracellular accumulation of the more toxic A␤, A␤42, in a small group of septal neurons. Leupeptin infusion previously has been reported to interfere with lysosomal proteolysis and to result in the accumulation of lipofuscin, dystrophic neurites, tau-and ubiquitinpositive inclusions, and structures resembling paired helical filaments. Coinfusion of A␤40 with the serine protease inhibitor aprotinin also increased diffuse extracellular deposition but reduced astrocytosis and TUNEL and was not associated with intracellular A␤ staining. Collectively, these data suggest that an age or Alzheimer's-related defect in lysosomal/endosomal function could promote A␤ deposition and DNA fragmentation in neurons and glia similar to that found in Alzheimer's disease.

Calpain Activation Promotes BACE1 Expression, Amyloid Precursor Protein Processing, and Amyloid Plaque Formation in a Transgenic Mouse Model of Alzheimer Disease

“…In the brains of AD patients and the AD mouse model, there are remarkable activated astrocytes around the area of A␤ plaques (46,47). In line with this notion, a number of GFAPpositive areas were observed around the plaques (Fig.…”

Retinoic Acid Attenuates β-Amyloid Deposition and Rescues Memory Deficits in an Alzheimer's Disease Transgenic Mouse Model

“…Given the central role of fibrillar A␤ in the activation of microglia and astrocytes seen in AD brain (Rozemuller et al, 2005) and in AD animal models (Frautschy et al, 1998;Apelt and Schliebs, 2001;Matsuoka et al, 2001), the significant decrease in activated microglia and astrocytes seen in the ATRA-treated APP/PS1 mice can be attributed to its inhibition of A␤ accumulation. However, ATRA appears to possess an inherent anti-inflammatory function independent of A␤ (Mehta et al, 1994;Datta et al, 2001).…”

“…In the brains of human AD patients and transgenic AD mouse models, infiltration of activated astrocytes and microglia are seen in the area of A␤ plaques (Itagaki et al, 1989;Frautschy et al, 1998;Stalder et al, 1999;Bornemann et al, 2001;Matsuoka et al, 2001), which are characteristic components of an inflammatory process that develops around injury in the brain (McGeer and McGeer, 1999). Based on previous in vitro studies showing that RA inhibited the neurotoxic effect of activated microglia by suppressing the production of inflammatory cytokines and cytotoxic molecules (Dheen et al, 2005), we compared astrocytic and microglial reactivity in APP/PS1 mice treated with ATRA or vehicle as a control.…”

Silymarin Attenuated the Amyloid β Plaque Burden and Improved Behavioral Abnormalities in an Alzheimer’s Disease Mouse Model

“…[34][35][36][37][38][39] We estimated neuroinflammation in the brain using an Iba1 antibody as a microglia marker (Fig. 4).…”