Alpha-synuclein (αSyn) misfolding is associated with several devastating neurodegenerative disorders, including Parkinson's disease (PD). In yeast cells and in neurons αSyn accumulation is cytotoxic, but little is known about its normal function or pathobiology. The earliest defect following αSyn expression in yeast was a block in endoplasmic reticulum (ER)-to-Golgi vesicular trafficking. In a genomewide screen, the largest class of toxicity modifiers were proteins functioning at this same step, including the Rab guanosine triphosphatase Ypt1p, which associated with cytoplasmic αSyn inclusions. Elevated expression of Rab1, the mammalian YPT1 homolog, protected against αSyn-induced dopaminergic neuron loss in animal models of PD. Thus, synucleinopathies may result from disruptions in basic cellular functions that interface with the unique biology of particular neurons to make them especially vulnerable.Parkinson's disease (PD) is the second most common neurodegenerative disorder (1,2). Accruing evidence points to a causative role for the presynaptic protein alpha-synuclein (αSyn) in PD pathogenesis. αSyn is a major constituent of Lewy Bodies-cellular inclusions that are the hallmark pathological feature of PD and other neurodegenerative disorders collectively

The amyloid ␤ (A␤) peptide that accumulates in Alzheimer's disease brain is derived from the proteolytic processing of the amyloid precursor protein by ␤-and ␥-secretase activities. The ␤-secretase enzyme ␤-site amyloid precursor protein-cleaving enzyme (BACE) generates the N terminus of A␤ by cleavage at either Asp 1 (␤-site) or Glu 11 (␤-site), ultimately leading to the production of full-length A␤1-40/42 or truncated A␤11-40/42. The functional significance of this variable cleavage site specificity as well as the relative pathological impact of full-length versus N-terminally truncated A␤ remains largely unknown. In our analysis of BACE reactivity in cell culture, we found that the preference of the protease for either ␤-or ␤-cleavage was strongly dependent on intracellular localization. Within the endoplasmic reticulum, ␤-site proteolysis predominated, whereas in the trans-Golgi network, ␤-cleavage was favored. Furthermore, the contrasting cleavage site specificities of BACE were not simply due to differences in organelle pH or the oligosaccharide composition of the glycoproteins involved. Examination of post-mortem brain specimens revealed significant levels of A␤11-40/42 within insoluble amyloid pools. Taken together, these data support an important role for ␤-cleavage in the process of cerebral amyloid deposition and localize the processing event to the trans-Golgi network.Senile plaques, lesions composed largely of aggregated amyloid ␤ (A␤) 1 protein, are a pathologic hallmark of Alzheimer's disease (AD) (1, 2). A␤ is derived from proteolytic processing of the type 1 membrane glycoprotein APP (3, 4), and its deposition most likely represents a crucial causative event in AD pathogenesis (5). The membrane-anchored aspartyl protease BACE acts on APP first at its ␤-cleavage site (6 -10), generating a membrane-bound C-terminal stub (C99) whose subsequent proteolysis by a second enzyme, ␥-secretase, yields A␤. In an alternative cellular pathway precluding A␤ production, APP is initially cleaved by ␣-secretase activity, ultimately leading to the release of a shorter peptide known as p3 (Fig. 1A) (11).Full-length A␤ encompasses a well-defined 40-or 42-amino acid residue stretch within the APP backbone (A␤1-40 and A␤1-42). However, in cerebral amyloid deposits, numerous N-terminally truncated variants of A␤40 and A␤42 (NtA␤), frequently harboring additional structural modifications, have been isolated (2,12,13). Whereas the functional significance of this N-terminal heterogeneity remains unclear, a variety of NtA␤ species aggregate more quickly in vitro than their fulllength counterparts (14). Whereas most types of NtA␤ are assumed to arise from the proteolysis of full-length peptides after their release from cells in the central nervous system, two such variants, A␤11-40 and A␤11-42, are generated directly from APP by BACE proteolysis at an alternative site, termed ␤Ј, between Tyr 10 and Glu 11 of A␤ (8,15,16). This event initially produces a shorter C-terminal stub (C89), which then acts as a substrate for ␥-se...

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Introducing mutations within the amyloid precursor protein (APP) that affect β- and γ-secretase cleavages results in amyloid plaque formation in vivo. However, the relationship between β-amyloid deposition and the subcellular site of Aβ production is unknown. To determine the effect of increasing β-secretase (BACE) activity on Aβ deposition, we generated transgenic mice overexpressing human BACE. Although modest overexpression enhanced amyloid deposition, high BACE overexpression inhibited amyloid formation despite increased β-cleavage of APP. However, high BACE expression shifted the subcellular location of APP cleavage to the neuronal perikarya early in the secretory pathway. These results suggest that the production, clearance, and aggregation of Aβ peptides are highly dependent on the specific neuronal subcellular domain wherein Aβ is generated and highlight the importance of perikaryal versus axonal APP proteolysis in the development of Aβ amyloid pathology in Alzheimer's disease.