Abstract
Alzheimer's disease is characterized by the extracellular deposition of beta-amyloid peptide (Abeta) in cerebral plaques and evidence is accumulating that amyloid is neurotoxic. Abeta is derived from the beta-amyloid precursor protein (APP). Proteolytic processing of APP by the enzyme, beta-secretase, produces the N terminus of Abeta, and releases a secreted ectodomain of APP (beta-s-APP). To develop animal models for measuring beta-secretase activity in specific brain cells in vivo, we have targeted the expression of the full-length human APP to either neurons or astrocytes in transgenic mice using the neuron- specific enolase (NSE) promoter or a modified glial fibrillary acidic protein (GFAP) gene, respectively. The APP cDNAs expressed were mutated (KM to NL at 670/671) to encode amino acid substitutions that enhance amyloidogenic processing in vitro. Western analyses revealed abundant production of beta-s-APP in the brains of NSE-APP mice and enzyme-linked immunosorbent assay analyses showed production of Abeta in fetal primary mixed brain cultures and brain homogenates from these transgenic animals. Because the NSE promoter drives expression primarily in neurons, this provides in vivo evidence that the beta-secretase cleavage necessary for generation of beta-s-APP and Abeta is efficiently performed in neurons. In contrast, only little beta-s-APP was detected in brain homogenates of GFAP-APP mice, indicating that astrocytes show very little beta-secretase activity in vivo. This provides strong in vivo evidence that the major source of Abeta in brain is from neurons and not from astrocytes.
Highlights
The extracellular deposition of -amyloid peptide (A)1 in senile plaques is an early and invariant feature of Alzheimer’s disease (AD). This 39 – 43-amino acid peptide is the major component of plaques and is proteolytically processed from the -amyloid precursor protein (APP) [1, 2]
Mutations in APP are responsible for some forms of familial AD, supporting the hypothesis that APP and A are central to the disease process [6]
We report that -secretase processing of APP was highly effective when this molecule was expressed in neurons and relatively ineffective when it was expressed in astrocytes of transgenic mice
Summary
The extracellular deposition of -amyloid peptide (A) in senile plaques is an early and invariant feature of Alzheimer’s disease (AD). With mutations at the 717 position of APP, have been shown to produce increased amounts of the more amyloidogenic 42-amino acid form of A from APP [9] These findings suggest that factors governing the metabolic processing of APP play a direct pathogenic role in Alzheimer’s disease. The majority of APP is cleaved in the middle of the A region, releasing a secreted ectodomain containing the first 16 amino acids of A (␣-s-APP) This processing, mediated by an unidentified enzymatic activity termed “␣-secretase” precludes A formation [10]. To obtain in vivo models for studying -secretase processing of APP in the brain, we have generated transgenic mice expressing the 751-amino acid form of human APP in either neurons or astrocytes. In addition to helping identify the main cellular source of A in the brain, these and related models should prove valuable for the screening for drugs that inhibit -secretase activity
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