PHluorin-BACE1-mCherry Acts as a Reporter for the Intracellular Distribution of Active BACE1 In Vitro and In Vivo.
ABSTRACT: β-site APP-cleaving enzyme 1 (BACE1) initiates amyloid precursor protein (APP) cleavage and β-amyloid (Aβ) production, a critical step in the pathogenesis of Alzheimer's disease (AD). It is thus of considerable interest to investigate how BACE1 activity is regulated. BACE1 has its maximal activity at acidic pH and GFP variant-pHluorin-displays pH dependence. In light of these observations, we generated three tandem fluorescence-tagged BACE1 fusion proteins, named pHluorin-BACE1-mCherry, BACE1-mCherry-pHluorin and BACE1-mCherry-EGFP. Comparing the fluorescence characteristics of these proteins in response to intracellular pH changes induced by chloroquine or bafilomycin A1, we found that pHluorin-BACE1-mCherry is a better pH sensor for BACE1 because its fluorescence intensity responds to pH changes more dramatically and more quickly. Additionally, we found that (pro)renin receptor (PRR), a subunit of the v-ATPase complex, which is critical for maintaining vesicular pH, regulates pHluorin's fluorescence and BACE1 activity in pHluorin-BACE1-mCherry expressing cells. Finally, we found that the expression of Swedish mutant APP (APPswe) suppresses pHluorin fluorescence in pHluorin-BACE1-mCherry expressing cells in culture and in vivo, implicating APPswe not only as a substrate but also as an activator of BACE1. Taken together, these results suggest that the pHluorin-BACE1-mCherry fusion protein may serve as a useful tool for visualizing active/inactive BACE1 in culture and in vivo.
Project description:BACE1 initiates amyloid-? (A?) generation and the resultant cerebral amyloidosis, as a characteristic of Alzheimer's disease (AD). Thus, inhibition of BACE1 has been the focus of a large body of research. The most recent clinical trials highlight the difficulty involved in this type of anti-AD therapy as evidenced by side effects likely due to the ubiquitous nature of BACE1, which cleaves multiple substrates. The human Swedish mutant form of amyloid protein precursor (APPswe) has been shown to possess a higher affinity for BACE1 compared to wild-type APP (APPwt). We pursued a new approach wherein harnessing this greater affinity to modulate BACE1 APP processing activity. We found that one peptide derived from APPswe, containing the ?-cleavage site, strongly inhibits BACE1 activity and thereby reduces A? production. This peptide, termed APPswe BACE1 binding site peptide (APPsweBBP), was further conjugated to the fusion domain of the HIV-1 Tat protein (TAT) at the C-terminus to facilitate its biomembrane-penetrating activity. APPwt and APPswe over-expressing CHO cells treated with this TAT-conjugated peptide resulted in a marked reduction of A? and a significant increase of soluble APP?. Intraperitoneal administration of this peptide to 5XFAD mice markedly reduced ?-amyloid deposits as well as improved hippocampal-dependent learning and memory.
Project description:A transmembrane aspartyl protease termed beta-site APP cleavage enzyme 1 (BACE1) that cleaves the amyloid-beta precursor protein (APP), which is abundant in neurons, is required for the generation of amyloid-beta (Abeta) peptides implicated in the pathogenesis of Alzheimer's disease (AD). We now demonstrate that BACE1, enriched in neurons of the CNS, is a major determinant that predisposes the brain to Abeta amyloidogenesis. The physiologically high levels of BACE1 activity coupled with low levels of BACE2 and alpha-secretase anti-amyloidogenic activities in neurons is a major contributor to the accumulation of Abeta in the CNS, whereas other organs are spared. Significantly, deletion of BACE1 in APPswe;PS1DeltaE9 mice prevents both Abeta deposition and age-associated cognitive abnormalities that occur in this model of Abeta amyloidosis. Moreover, Abeta deposits are sensitive to BACE1 dosage and can be efficiently cleared from the CNS when BACE1 is silenced. However, BACE1 null mice manifest alterations in hippocampal synaptic plasticity as well as in performance on tests of cognition and emotion. Importantly, memory deficits but not emotional alterations in BACE1(-/-) mice are prevented by coexpressing APPswe;PS1DeltaE9 transgenes, indicating that other potential substrates of BACE1 may affect neural circuits related to emotion. Our results establish BACE1 and APP processing pathways as critical for cognitive, emotional, and synaptic functions, and future studies should be alert to potential mechanism-based side effects that may occur with BACE1 inhibitors designed to ameliorate Abeta amyloidosis in AD.
Project description:Alzheimer's disease (AD) is characterized by amyloid plaques and neurofibrillary tangles. Substantial evidence for AD pathogenesis suggests that ?-site APP cleaving enzyme 1 (BACE1) and ?-secretase enzyme initiate the amyloidogenic pathway and produces toxic A? peptides that prone to aggregate in the brain. Therefore, the inhibition of BACE1 expression and function is an attractive strategy for AD therapy. In the present work, we made the first finding that activating ?-opioid receptors (DOR) with a specific DOR agonist significantly attenuated BACE1 expression and activity in the highly differentiated PC12 cells with mimicked AD injury, while the application of DOR inhibitor naltrindole reversed the UFP-512 effects, and even caused a major increase in BACE1 expression and activity as well as A?42 production in physiological conditions. Knocking-down DOR also enhanced BACE1 protein expression and its activity for APP processing, associating with a significant increase in A?42 production. In sharp contrast, activation of ?-opioid receptor (MOR) with DAMGO greatly promoted BACE1 expression and activity with an acceleration of APP cleavage, thus contributing to increased A?42 production. DADLE, a less selective DOR agonist that may bind to MOR, had no stable inhibitory effect on BACE1. Similar results were also found in APP mutant (APPswe) SH-SY5Y cell line, providing further validation of the DOR action on BACE1 regulation. Our novel data demonstrated entirely different roles of DOR and MOR in the regulation of BACE1 expression and activity with DOR being neuroprotective against AD injury. These findings provided a novel clue for new strategies of AD therapy via targeting endogenous opioid receptors.
Project description:The cleavage of amyloid precursor protein (APP) by ?-site APP cleaving enzyme 1 (BACE1) is the rate-limiting step in beta amyloid generation during Alzheimer's disease (AD) pathogenesis. In AD brains, BACE1 is abnormally accumulated in endocytic compartments, where the acidic pH is optimal for its activity. However, mechanisms regulating the endosome-to-trans-Golgi network (TGN) retrieval of BACE1 remain unclear. Here, we show that partitioning defective 3 (Par3) facilitates BACE1 retrograde trafficking from endosomes to the TGN. Par3 functions through aPKC-mediated phosphorylation of BACE1 on Ser498, which in turn promotes the interaction between BACE1 and phosphofurin acidic cluster sorting protein 1 and facilitates the retrograde trafficking of BACE1 to the TGN. In human AD brains, there is a significant decrease in Ser498 phosphorylation of BACE1 suggesting that defective phosphorylation-dependent retrograde transport of BACE1 is important in AD pathogenesis. Together, our studies provide mechanistic insight into a novel role for Par3 and aPKC in regulating the retrograde endosome-to-TGN trafficking of BACE1 and shed light on the mechanisms of AD pathogenesis.
Project description:?-amyloid (A?) plays an essential role in the pathogenesis of Alzheimer's disease (AD). Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) is indispensable for A? production, and knockout of BACE1 has no overt phenotypes in mouse. Thus, fine modulation of BACE1 may be a safe and effective treatment for AD patients. However, the large active site of BACE1 makes it challenging to target BACE1 with classical small-molecule inhibitors. DNA aptamer can have high affinity and specificity against diverse targets, and it provides an alternative strategy to target BACE1. In this study, we used a novel cell-systematic evolution of ligands by exponential enrichment (SELEX) strategy to select specific DNA aptamers optimized to target BACE1 under physiological status. After 17 rounds of selection, we identified two DNA aptamers against BACE1: BI1 and BI2. The identified aptamers interacted with BACE1 in pull-down assay, inhibited BACE1 activity in in vitro fluorescence resonance energy transfer (FRET) assay and HEK293-APP stable cell line, reduced A? in the culture medium of HEK293-amyloid protein precursor (APP) stable cell line and APP-PS1 primary cultured neurons, and rescued A?-induced neuronal deficiency in APP-PS1 primary cultured neurons. In contrast, the identified aptamers had no effect on ?- or ?-secretase. In addition, cholesteryl tetraetylene glycol (TEG) modification further improved the potency of the identified aptamers. Our study suggests that it is feasible and effective to target BACE1 with DNA aptamers, and the therapeutic potential of the identified aptamers deserves further investigation.
Project description:The BACE1 gene encodes the beta-site APP-cleaving enzyme 1 and has been associated with Alzheimer's disease (AD). BACE1 is the most important ?-secretase responsible for the generation of Alzheimer-associated amyloid ?-proteins (A?) and may play a role in the amyloidogenic process in AD. We hypothesized that BACE1 gene variants might influence BACE1 activity or other markers for APP metabolism in the cerebrospinal fluid (CSF) and thereby contribute to the development of AD. We genotyped a Swedish sample of 269 AD patients for the rs638405 single nucleotide polymorphism (SNP) in the BACE1 gene and correlated genotype data to a broad range of amyloid-related biomarkers in CSF, including BACE1 activity, levels of A?40, A?42, ?- and ?-cleaved soluble APP (?-sAPP and ?-sAPP), as well as markers for Alzheimer-type axonal degeneration, i.e., total-tau and phospho-tau181. Gene variants of BACE1 were neither associated with amyloid-related biomarkers, nor with markers for axonal degeneration in AD.
Project description:Neurodegenerative plaques characteristic of Alzheimer's disease (AD) are composed of amyloid beta (A?) peptide, which is proteolyzed from amyloid precursor protein (APP) by ?-secretase (beta-site APP cleaving enzyme [BACE1]) and ?-secretase. Although ?-secretase has essential functions across metazoans, no essential roles have been identified for BACE1 or A?. Because their only known function results in a disease phenotype, we sought to understand these components from an evolutionary perspective. We show that APP-like proteins are found throughout most animal taxa, but sequences homologous to A? are not found outside gnathostomes and the ? cut site is only conserved within sarcopterygians. BACE1 enzymes, however, extend through basal chordates and as far as cnidaria. We then sought to determine whether BACE1 from a species that never evolved A? could proteolyze APP substrates that include A?. We demonstrate that BACE1 from a basal chordate is a functional ortholog that can liberate A? from full-length human APP, indicating BACE1 activity evolved at least 360 My before A?.
Project description:The convergence between amyloid precursor protein (APP) and its ?-secretase ?-site APP cleaving enzyme 1 (BACE1) is a prerequisite for the generation of ?-amyloid peptide, a key pathogenic agent for Alzheimer's disease. Yet the underlying molecular mechanisms regulating their convergence remain unclear. Here, we show that the polarity protein partitioning-defective 3 (Par3) regulates the polarized convergence between APP and BACE1 in hippocampal neurons. Par3 forms a complex with BACE1 through its first PDZ domain, which is important for regulating BACE1 endosome-to-TGN trafficking. In the absence of Par3, there is an increase in the convergence between internalized APP and BACE1. In hippocampal neurons, loss of Par3 leads to increased APP and BACE1 convergence in axons but not in dendrites. This polarized convergence mainly occurs in retrograde or stalled axonal late endocytic organelles and is likely due to compartment-specific regulation of APP trafficking by Par3. Together, our data show a novel function for Par3 in regulating polarized convergence between APP and BACE1 in hippocampal neurons.
Project description:Mutations in amyloid ? precursor protein (APP) gene alter APP processing, either causing familial Alzheimer's disease (AD) or protecting against dementia. Under normal conditions, ?-site APP cleaving enzyme 1 (BACE1) cleaves APP at minor Asp1 site to generate C99 for amyloid ? protein (A?) production, and predominantly at major Glu11 site to generate C89, resulting in truncated A? production. We discovered that A673V mutation, the only recessive AD-associated APP mutation, shifted the preferential ?-cleavage site of BACE1 in APP from the Glu11 site to the Asp1 site both in male and female transgenic mice in vivo and in cell lines and primary neuronal culture derived from timed pregnant rats in vitro, resulting in a much higher C99 level and C99/C89 ratio. All other mutations at this site, including the protective Icelandic A673T mutation, reduced C99 generation, and decreased the C99/C89 ratio. Furthermore, A673V mutation caused stronger dimerization between mutant and wild-type APP, enhanced the lysosomal degradation of the mutant APP, and inhibited ?-secretase cleavage of the mutant C99 to generate A?, leading to recessively inherited AD. The results demonstrate that APP673 regulates APP processing and the BACE1 cleavage site selection is critical for amyloidogenesis in AD pathogenesis, and implicate a pharmaceutical potential for targeting the APP673 site for AD drug development.SIGNIFICANCE STATEMENT ?-site APP cleaving enzyme 1 (BACE1) is essential for amyloid ? protein production. We discovered that A673V mutation shifted the BACE1 cleavage site from the Glu11 to the Asp1 site, resulting in much higher C99 level and C99/C89 ratio. All other mutations at this site of amyloid ? precursor protein (APP) reduced C99 generation and decreased the C99/C89 ratio. Furthermore, A673V mutation resulted in stronger dimerization between mutant and wild-type APP, enhanced the lysosomal degradation of the mutant APP, and inhibited ?-secretase cleavage of the mutant C99 to generate amyloid ? protein, leading to recessively inherited Alzheimer's disease (AD). The results demonstrate that APP673 regulates APP processing, and the BACE1 cleavage site selection is critical for amyloidogenesis in AD pathogenesis, and implicate a pharmaceutical potential for targeting the APP673 site for AD drug development.
Project description:A molecular imaging probe to fluorescently image the ?-site of the amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) and cathepsin D (CatD) enzymes associated with Alzheimer's disease (AD) was designed and synthesized. This imaging probe was built upon iron oxide nanoparticles (cross-linked dextran iron oxide nanoparticles, or CLIO). Peptide substrates containing a terminal near-infrared fluorochrome (fluorophore emitting at 775 nm for CatD or fluorophore emitting at 669 nm for BACE1) were conjugated to the CLIO nanoparticles. The CatD substrate contained a phenylalanine-phenylalanine cleavage site more specific to CatD than BACE1. The BACE1 substrate contained the sequence surrounding the leucine-asparagine cleavage site of the BACE1 found in the Swedish mutation of APP, which is more specific to BACE1 than CatD. These fluorescently-labeled peptide substrates were then conjugated to the nanoparticle. The nanoparticle probes were purified by gel filtration, and their fluorescence intensities were determined using a fluorescence plate reader. The CatD peptide substrate demonstrated a 15.5-fold increase in fluorescence when incubated with purified CatD enzyme, and the BACE1 substrate exhibited a 31.5-fold increase in fluorescence when incubated with purified BACE1 enzyme. Probe specificity was also demonstrated in the human H4 neuroglioma cells and the H4 cells stably transfected with BACE1 in which the probe monitored enzymatic cleavage. In the H4 and H4-BACE1 cells, BACE1 and active CatD activity increased, an occurrence that was reflected in enzyme expression levels as determined by immunoblotting. These results demonstrate the applicability of this probe for detecting potential Alzheimer's enzyme biomarkers.