Alzheimer's disease is characterized by the extracellular deposition of ␤-amyloid peptide (A␤) in cerebral plaques and evidence is accumulating that amyloid is neurotoxic. A␤ is derived from the ␤-amyloid precursor protein (APP). Proteolytic processing of APP by the enzyme, ␤-secretase, produces the N terminus of A␤, and releases a secreted ectodomain of APP (␤-s-APP). To develop animal models for measuring ␤-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 neuronspecific 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 ␤-s-APP in the brains of NSE-APP mice and enzyme-linked immunosorbent assay analyses showed production of A␤ 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 ␤-secretase cleavage necessary for generation of ␤-s-APP and A␤ is efficiently performed in neurons. In contrast, only little ␤-s-APP was detected in brain homogenates of GFAP-APP mice, indicating that astrocytes show very little ␤-secretase activity in vivo. This provides strong in vivo evidence that the major source of A␤ in brain is from neurons and not from astrocytes.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). APP is expressed in all tissues, and the relative amount of A␤ processed from APP varies in different cell types in culture (3-5). The cellular source of A␤ deposited into plaques in the brain is unknown. 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). Missense mutations immediately Nterminal to the A␤ region of APP 2 lead to a 5-10-fold enhancement of A␤ produced from APP in vitro, strongly supporting the role of A␤ in the development of AD in this family (7,8). Other families, 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). In an alternative pathway, cleavage between Met 671 and Asp 672 by a likewise unid...

Free-living amoebae are rare causes of morbidity and mortality in humans and animals around the globe. Because the route of exposure and clinical progression of disease caused by different species of amoebae may vary in people and animals, determining the species of amoeba present is important. We describe here a fatal infection by the free-living amoeba Balamuthia mandrillaris in a Siberian tiger ( Panthera tigris altaica). The 17-y-old patient had a rapid clinical decline after a peracute onset of severe lethargy, dull mentation, and anorexia. Autopsy did not identify a cause of death. Histology revealed inflammation associated with amoebic trophozoites in the brain, lungs, and iris of one eye. These amoebae were confirmed to be B. mandrillaris based on a PCR assay and sequencing. Although there are subtle morphologic differences between cyst stages of Acanthamoeba spp., B. mandrillaris, and Naegleria fowleri when present and identified on routine staining, other modalities, including PCR, immunofluorescence, electron microscopy, and immunohistochemistry, are typically utilized to confirm the pathogen involved in these cases. We review the reports of balamuthosis in animals.

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