Regulation Of Aβ Production Research Articles

GGA Proteins Mediate the Recycling Pathway of Memapsin 2 (BACE)

Memapsin 2 (BACE, beta-secretase) is a membrane-associated aspartic protease that initiates the hydrolysis of beta-amyloid precursor protein (APP) leading to the production of amyloid-beta (A beta) and the progression of Alzheimer disease. Both memapsin 2 and APP are transported from the cell surface to endosomes where APP is cleaved by memapsin 2. We described previously that the cytosolic domain of memapsin 2 contains an acid cluster-dileucine motif (ACDL) that binds the VHS (Vps-27, Hrs, and STAM) domain of Golgi-localized gamma-ear-containing ARF-binding (GGA) proteins (He, X., Zhu, G., Koelsch, G., Rodgers, K. K., Zhang, X. C., and Tang, J. (2003) Biochemistry 42, 12174-12180). Here we report that GGA proteins colocalize in the trans-Golgi network and endosomes with memapsin 2 and a memapsin 2 chimera containing a cytosolic domain of a mannose-6-phosphate receptor. Depleting cellular GGA proteins with RNA interference or mutation of serine 498 to stop the phosphorylation of ACDL resulted in the accumulation of memapsin 2 in early endosomes. A similar change of memapsin 2 localization also was observed when a retromer subunit, VPS26, was depleted. These observations suggest that GGA proteins function with the phosphorylated ACDL in the memapsin 2-recycling pathway from endosomes to trans-Golgi on the way back to the cell surface.

Sp1 and Smad transcription factors co-operate to mediate TGF-beta-dependent activation of amyloid-beta precursor protein gene transcription.

Abnormal deposition of Abeta (amyloid-beta peptide) is one of the hallmarks of AD (Alzheimer's disease). This peptide results from the processing and cleavage of its precursor protein, APP (amyloid-beta precursor protein). We have demonstrated previously that TGF-beta (transforming growth factor-beta), which is overexpressed in AD patients, is capable of enhancing the synthesis of APP by astrocytes by a transcriptional mechanism leading to the accumulation of Abeta. In the present study, we aimed at further characterization of the molecular mechanisms sustaining this TGF-beta-dependent transcriptional activity. We report the following findings: first, TGF-beta is capable of inducing the transcriptional activity of a reporter gene construct corresponding to the +54/+74 region of the APP promoter, named APP(TRE) (APP TGF-beta-responsive element); secondly, although this effect is mediated by a transduction pathway involving Smad3 (signalling mother against decapentaplegic peptide 3) and Smad4, Smad2 or other Smads failed to induce the activity of APP(TRE). We also observed that the APP(TRE) sequence not only responds to the Smad3 transcription factor, but also the Sp1 (signal protein 1) transcription factor co-operates with Smads to potentiate the TGF-beta-dependent activation of APP. TGF-beta signalling induces the formation of nuclear complexes composed of Sp1, Smad3 and Smad4. Overall, the present study gives new insights for a better understanding of the fine molecular mechanisms occurring at the transcriptional level and regulating TGF-beta-dependent transcription. In the context of AD, our results provide additional evidence for a key role for TGF-beta in the regulation of Abeta production.

High throughput screens for the identification of compounds that alter the accumulation of the Alzheimer's amyloid β peptide (Aβ)

Evidence gathered over the last two decades suggests that β amyloid (Aβ), the predominant proteinaceous component of senile plaques, plays an early and critical role in the etiology and pathogenesis of Alzheimer's disease (AD). Thus, it is reasonable to hypothesize that compounds capable of reducing the accumulation of Aβ may be of value therapeutically. Additionally, compounds that influence Aβ accumulation may be useful as tools to further dissect the cellular pathways that regulate Aβ production and accumulation. To screen for compounds that affect Aβ levels, we have established high throughput, cell-based assays capable of the sensitive and selective detection of Aβ40 in parallel with the more amyloidogenic form of the peptide, Aβ42. To validate the approach, we examined the effects of several compounds previously identified to influence Aβ accumulation. Analysis of peptide accumulation following treatment with these compounds showed results similar to those previously published. Currently, we are using this assay to screen drugs that have already received FDA approval for the treatment of other diseases and over-the-counter natural product extracts. If compounds such as these can be identified that lower Aβ in the brain, they may represent one of the fastest and most cost effective methods to therapy.

PDZ Domain-dependent Suppression of NF-κB/p65-induced Aβ42 Production by a Neuron-specific X11-like Protein

It is widely believed that one of the causes of Alzheimer's disease (AD) is the generation and secretion of beta-amyloid (Abeta) from amyloid precursor protein in the brain. Here we report that a transcription factor, NF-kappaB/p65, induces increased secretion of amyloidogenic Abeta42 but not Abeta40. The kappaB motif-dependent production of Abeta42 was suppressed by binding of NF-kappaB/p65 to the PDZ domain of the X11-like protein (X11L), which a human homologue protein of LIN-10. The results suggest that the PDZ domain of X11L can control the ability of NF-kappaB/p65 to induce expression of protein(s) involved in Abeta42 production. The amino acids 161-163 in Rel homology domain (RHD) of NF-kappaB/p65 is important in interaction of NF-kappaB/p65 with X11L. Another subunit NF-kappaB/p50 and heterodimers of p65 and p50 do not bind to X11L. Our finding indicates NF-kappaB and X11L may, in novel way, regulate Abeta production in neuronal cells. Targeting X11L by specific therapy may provide the possibility to control the progression of AD.

Alternative, Non-secretase Processing of Alzheimer's β-Amyloid Precursor Protein during Apoptosis by Caspase-6 and -8

Alzheimer's disease (AD) is a progressive neurodegenerative disorder. Although the pathogenesis of AD is unknown, it is widely accepted that AD is caused by extracellular accumulation of a neurotoxic peptide, known as Abeta. Mutations in the beta-amyloid precursor protein (APP), from which Abeta arises by proteolysis, are associated with some forms of familial AD (FAD) and result in increased Abeta production. Two other FAD genes, presenilin-1 and -2, have also been shown to regulate Abeta production; however, studies examining the biological role of these FAD genes suggest an alternative theory for the pathogenesis of AD. In fact, all three genes have been shown to regulate programmed cell death, hinting at the possibility that dysregulation of apoptosis plays a primary role in causing neuronal loss in AD. In an attempt to reconcile these two hypotheses, we investigated APP processing during apoptosis and found that APP is processed by the cell death proteases caspase-6 and -8. APP is cleaved by caspases in the intracellular portion of the protein, in a site distinct from those processed by secretases. Moreover, it represents a general effect of apoptosis, because it occurs during cell death induced by several stimuli both in T cells and in neuronal cells.