The incidence of Alzheimer disease (AD) and vascular dementia is greatly increased following cerebral ischemia and stroke in which hypoxic conditions occur in affected brain areas. β-Amyloid peptide (Aβ), which is derived from the β-amyloid precursor protein (APP) by sequential proteolytic cleavages from β-secretase (BACE1) and presenilin-1 (PS1)/γ-secretase, is widely believed to trigger a cascade of pathological events culminating in AD and vascular dementia. However, a direct molecular link between hypoxic insults and APP processing has yet to be established. Here, we demonstrate that acute hypoxia increases the expression and the enzymatic activity of BACE1 by up-regulating the level of BACE1 mRNA, resulting in increases in the APP C-terminal fragment-β (βCTF) and Aβ. Hypoxia has no effect on the level of PS1, APP, and tumor necrosis factor-α-converting enzyme (TACE, an enzyme known to cleave APP at the α-secretase cleavage site). Sequence analysis, mutagenesis, and gel shift studies revealed binding of HIF-1 to the BACE1 promoter. Overexpression of HIF-1α increases BACE1 mRNA and protein level, whereas down-regulation of HIF-1α reduced the level of BACE1. Hypoxic treatment fails to further potentiate the stimulatory effect of HIF-1α overexpression on BACE1 expression, suggesting that hypoxic induction of BACE1 expression is primarily mediated by HIF-1α. Finally, we observed significant reduction in BACE1 protein levels in the hippocampus and the cortex of HIF-1α conditional knock-out mice. Our results demonstrate an important role for hypoxia/HIF-1α in modulating the amyloidogenic processing of APP and provide a molecular mechanism for increased incidence of AD following cerebral ischemic and stroke injuries.

Presenilins (PS, PS1/PS2) are necessary for the proteolytic activity of γ-secretase, which cleaves multiple type I transmembrane proteins including Alzheimer's β-amyloid precursor protein (APP), Notch, ErbB4, etc. Cleavage by PS/γ-secretase releases the intracellular domain (ICD) of its substrates. Notch ICD translocates into the nucleus to regulate expression of genes important for development. However, the patho/physiological role of other ICDs, especially APP ICD (AICD), in regulating gene expression remains controversial because evidence supporting this functionality stems mainly from studies performed under supraphysiological conditions. EGF receptor (EGFR) is up-regulated in a wide variety of tumors and hence is a target for cancer therapeutics. Abnormal expression/activation of EGFR contributes to keratinocytic carcinomas, and mice with reduced PS dosages have been shown to develop skin tumors. Here we demonstrate that the levels of PS and EGFR in the skin tumors of PS1 ^+/− / PS2 ^−/− mice and the brains of PS1/2 conditional double knockout mice are inversely correlated. Deficiency in PS/γ-secretase activity or APP expression results in a significant increase of EGFR in fibroblasts. Importantly, we show that AICD mediates transcriptional regulation of EGFR . Furthermore, we provide in vivo evidence demonstrating direct binding of endogenous AICD to the EGFR promoter. Our results indicate an important role of PS/γ-secretase-generated APP metabolite AICD in gene transcription and in EGFR-mediated tumorigenesis.

Mutations in PSEN1 and PSEN2 genes account for the majority of cases of early-onset familial Alzheimer disease. Since the first prediction of a genetic link between PSEN1 and PSEN2 with Alzheimer's disease, many research groups from both academia and pharmaceutical industry have sought to unravel how pathogenic mutations in PSEN cause presenile dementia. PSEN genes encode polytopic membrane proteins termed presenilins (PS1 and PS2), which function as the catalytic subunit of γ-secretase, an intramembrane protease that has a wide spectrum of type I membrane protein substrates. Sequential cleavage of amyloid precursor protein by BACE and γ-secretase releases highly fibrillogenic β-amyloid peptides, which accumulate in the brains of aged individuals and patients with Alzheimer's disease. Familial Alzheimer's disease-associated presenilin variants are thought to exert their pathogenic function by selectively elevating the levels of highly amyloidogenic Aβ42 peptides. In addition to Alzheimer's disease, several recent studies have linked PSEN1 to familiar frontotemporal dementia. Here, we review the biology of PS1, its role in γ-secretase activity, and discuss recent developments in the cell biology of PS1 with respect to Alzheimer's disease pathogenesis.

Background: The phosphatase PTEN governs the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway which is arguably the most important pro-survival pathway in neurons. Recently, PTEN has also been implicated in multiple important CNS functions such as neuronal differentiation, plasticity, injury and drug addiction. It has been reported that loss of PTEN protein, accompanied by Akt activation, occurs under excitotoxic conditions (stroke) as well as in Alzheimer's (AD) brains. However the molecular signals and mechanism underlying PTEN loss are unknown. Results: In this study, we investigated redox regulation of PTEN, namely S-nitrosylation, a covalent modification of cysteine residues by nitric oxide (NO), and H₂O₂-mediated oxidation. We found that S-nitrosylation of PTEN was markedly elevated in brains in the early stages of AD (MCI). Surprisingly, there was no increase in the H₂O₂-mediated oxidation of PTEN, a modification common in cancer cell types, in the MCI/AD brains as compared to normal aged control. Using several cultured neuronal models, we further demonstrate that S-nitrosylation, in conjunction with NO-mediated enhanced ubiquitination, regulates both the lipid phosphatase activity and protein stability of PTEN. S-nitrosylation and oxidation occur on overlapping and distinct Cys residues of PTEN. The NO signal induces PTEN protein degradation via the ubiquitin-proteasome system (UPS) through NEDD4-1-mediated ubiquitination. Conclusion: This study demonstrates for the first time that NO-mediated redox regulation is the mechanism of PTEN protein degradation, which is distinguished from the H₂O₂-mediated PTEN oxidation, known to only inactivate the enzyme. This novel regulatory mechanism likely accounts for the PTEN loss observed in neurodegeneration such as in AD, in which NO plays a critical pathophysiological role.

Presenilins (PS1/PS2) regulate proteolysis of β-amyloid precursor protein (βAPP) and affect its intracellular trafficking. Here, we demonstrate that a PS1-interacting protein, phospholipase D1 (PLD1), affects intracellular trafficking of βAPP. Overexpression of PLD1 in PS1wt cells promotes generation of βAPP-containing vesicles from the trans-Golgi network. Conversely, inhibition of PLD1 activity by 1-butanol decreases βAPP trafficking in both wt and PS1-deficient cells. The subcellular localization of PLD1 is altered, and PLD enzymatic activity is reduced in cells expressing familial Alzheimer’s disease (FAD) PS1 mutations compared with PS1wt cells. Overexpression of wt, but not catalytically inactive, PLD1 increases budding of βAPP-containing vesicles from the trans-Golgi network in FAD mutant cells. Surface delivery of βAPP is also increased by PLD1 in these cells. The impaired neurite outgrowth capacity in FAD mutant neurons was corrected by introducing PLD1 into these cells. The results indicate that PLD1 may represent a therapeutic target for rescuing compromised neuronal function in AD.

γ-Secretase, which is responsible for the intramembranous cleavage of Alzheimer β-amyloid precursor protein and the signaling receptor Notch, is a multiprotein complex consisting of at least four components: presenilin (PS); nicastrin (Nct); APH-1 (anterior pharynx-defective-1); and presenilin enhancer-2 (PEN-2). Presenilin 1 (PS1) is known to be essential for the stability, interaction, and trafficking of the other PS1/γ-secretase components. However, the precise functions of the other components remain elusive. Here, we investigated the functions of Nct within the PS1/γ-secretase complex. We demonstrated that the loss of Nct expression in the embryonic fibroblast cells (Nct KO cells) results in dramatically decreased levels of APH-1, PEN-2, and PS1 fragments accompanied by a significant accumulation of full-length PS1. In the absence of Nct, PEN-2 and full-length PS1 are subjected to proteasome-mediated degradation, whereas the degradation of APH-1 is mediated by both proteasomal and lysosomal pathways. Unlike the case of wild type cells in which the γ-secretase complex mainly locates in the trans-Golgi network, the majority of residual PEN-2, APH-1, and the uncleaved full-length PS1 in Nct KO cells reside in the endoplasmic reticulum, which remain associated with each other in the absence of Nct. Interestingly, significant amounts of full-length PS1 and PEN-2, but not APH-1, are detected on the plasma membrane in Nct KO cells, suggesting the Nct-independent cell surface delivery of the PEN-2·PS1. Finally, the diminished PEN-2 protein level in Nct-deficient cells can be partially restored by overexpression of exogenous PS1, APH-1, or PEN-2 individually or collectively, indicating a dispensable role for Nct in controlling PEN-2 level. Taken together, our study demonstrates a critical role of Nct in the stability and proper intracellular trafficking of other components of the PS1/ γ-secretase complex but not in maintaining the association of PEN-2, APH-1, and full-length PS1.