The β-amyloid (Aβ) peptide is the major constituent of amyloid plaques in Alzheimer's disease (AD) brain and is likely to play a central role in the pathogenesis of this devastating neurodegenerative disorder. The β-secretase, β-site amyloid precursor protein cleaving enzyme (BACE1; also called Asp2, memapsin 2), is the enzyme responsible for initiating Aβ generation. Thus, BACE is a prime drug target for the therapeutic inhibition of Aβ production in AD. Since its discovery 10 years ago, much has been learned about BACE. This review summarizes BACE properties, describes BACE translation dysregulation in AD, and discusses BACE physiological functions in sodium current, synaptic transmission, myelination, and schizophrenia. The therapeutic potential of BACE will also be considered. This is a summary of topics covered at a symposium held at the 39th annual meeting of the Society for Neuroscience and is not meant to be a comprehensive review of BACE.
The β-secretase, β-site amyloid precursor protein cleaving enzyme 1 (BACE1), is a prime therapeutic target for lowering cerebral β-amyloid (Aβ) levels in Alzheimer's disease (AD). Clinical development of BACE1 inhibitors is being intensely pursued. However, little is known about the physiological functions of BACE1, and the possibility exists that BACE1 inhibition may cause mechanism-based side effects. Indeed, BACE1-/- mice exhibit a complex neurological phenotype. Interestingly, BACE1 co-localizes with presynaptic neuronal markers, indicating a role in axons and/or terminals. Moreover, recent studies suggest axon guidance molecules are potential BACE1 substrates. Here, we used a genetic approach to investigate the function of BACE1 in axon guidance of olfactory sensory neurons (OSNs), a well-studied model of axon targeting in vivo.
We bred BACE1-/- mice with gene-targeted mice in which GFP is expressed from the loci of two odorant-receptors (ORs), MOR23 and M72, and olfactory marker protein (OMP) to produce offspring that were heterozygous for MOR23-GFP, M72-GFP, or OMP-GFP and were either BACE1+/+ or BACE1-/-. BACE1-/- mice had olfactory bulbs (OBs) that were smaller and weighed less than OBs of BACE1+/+ mice. In wild-type mice, BACE1 was present in OSN axon terminals in OB glomeruli. In whole-mount preparations and tissue sections, many OB glomeruli from OMP-GFP; BACE1-/- mice were malformed compared to wild-type glomeruli. MOR23-GFP; BACE1-/- mice had an irregular MOR23 glomerulus that was innervated by randomly oriented, poorly fasciculated OSN axons compared to BACE1+/+ mice. Most importantly, M72-GFP; BACE1-/- mice exhibited M72 OSN axons that were mis-targeted to ectopic glomeruli, indicating impaired axon guidance in BACE1-/- mice.
Our results demonstrate that BACE1 is required for the accurate targeting of OSN axons and the proper formation of glomeruli in the OB, suggesting a role for BACE1 in axon guidance. OSNs continually undergo regeneration and hence require ongoing axon guidance. Neurogenesis and the regeneration of neurons and axons occur in other adult populations of peripheral and central neurons that also require axon guidance throughout life. Therefore, BACE1 inhibitors under development for the treatment of AD may potentially cause axon targeting defects in these neuronal populations as well.
Aβ; BACE1; β-amyloid; β-secretase; Alzheimer's disease; axon guidance; axon targeting; glomerulus; odorant receptor; olfactory sensory neuron; olfactory bulb; olfactory system
Accumulation of amyloid β-peptide (Aβ) in the plaques is one of the major pathological features in Alzheimer's disease (AD). Sequential cleavage of amyloid precursor protein (APP) by β-site APP cleaving enzyme 1 (BACE-1) and γ-secretase results in the formation of Aβ peptides. Preventing Aβ formation is believed to attenuate AD progression and BACE-1 and γ-secretase are thus attractive targets for AD drug development.
Combining BACE-1 and γ-secretase inhibition on Aβ secretion from human neuroblastoma SH-SY5Y cells was evaluated in this study. Secreted Aβ40 and Aβ42 levels were measured from SH-SY5Y cells stably transfected with APPwt or APPswe genes. A selective BACE inhibitor and the γ-secretase inhibitor LY450139 (semagacestat) were used to inhibit respective secretase.
LY450139 increased Aβ40 and Aβ42 secretion from SH-SY5Y APPwt cells at low concentrations (by 60% at 3 nM) followed by subsequent inhibition at higher concentrations (IC50 90 nM). Washout studies showed that the Aβ increase evoked by 3 nM LY450139 was not due to enhanced cleavage following substrate accumulation but rather to activation of Aβ formation. By contrast, LY450139 inhibited Aβ formation from SH-SY5Y APPswe in a monophasic manner (IC50 18 nM). The BACE inhibitor per se inhibited Aβ secretion from both SH-SY5Y APPwt and SH-SY5Y APPswe cells with IC50s ranging between 7 - 18 nM and also prevented the increased Aβ secretion evoked by 3 nM LY450139. Combining the BACE inhibitor with higher inhibitory concentrations of LY450139 failed to demonstrate any clear additive or synergistic effects.
BACE-1 inhibition attenuates the Aβ increase evoked by LY450139 while not providing any obvious synergistic effects on LY450139-mediated inhibition.
The most popular current hypothesis is that Alzheimer’s disease (AD) is caused by aggregates of the amyloid peptide (Aβ), which is generated by cleavage of the Aβ protein precursor (APP) by β-secretase (BACE-1) followed by γ-secretase. BACE-1 cleavage is limiting for the production of Aβ, making it a particularly good drug target for the generation of inhibitors that lower Aβ. A landmark discovery in AD was the identification of BACE-1 (a.k.a. Memapsin-2) as a novel class of type I transmembrane aspartic protease. Although BACE-2, a homologue of BACE-1, was quickly identified, follow up studies using knockout mice demonstrated that BACE-1 was necessary and sufficient for most neuronal Aβ generation. Despite the importance of BACE-1 as a drug target, development has been slow due to the incomplete understanding of its function and regulation and the difficulties in developing a brain penetrant drug that can specifically block its large catalytic pocket. This review summarizes the biological properties of BACE-1 and attempts to use phylogenetic perspectives to understand its function. The article also addresses the challenges in discovering a selective drug-like molecule targeting novel mechanisms of BACE-1 regulation.
BACE-1; secretase; Memapsin; Alzheimer; Amyloid; aspartyl protease
Since BACE1 was reported as the β-secretase in Alzheimer's disease (AD) over ten years ago, encouraging progress has been made toward understanding the cellular functions of BACE1. Genetic studies have further confirmed that BACE1 is essential for processing amyloid precursor protein (APP) at the β-secretase site. Only after this cleavage can the membrane-bound APP C-terminal fragment be subsequently cleaved by γ-secretase to release so-called AD-causing Aβ peptides. Hence, in the past decade, a wide variety of BACE1 inhibitors have been developed for AD therapy. This review will summarize the major historical events during the evolution of BACE1 inhibitors designed through different strategies of drug discovery. Although BACE1 inhibitors are expected to be safe in general, careful titration of drug dosage to avoid undesirable side effects in BACE1-directed AD therapy is also emphasized.
Alzheimer's disease; amyloid plaques; BACE1; aspartyl protease; drug discovery
β- site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is an aspartyl protease best known for its role in generating the amyloid β peptides that are present in plaques of Alzheimer's Disease. BACE1 has been an attractive target for drug development. In cultured embryonic neurons BACE1-cleaved N-terminal APP is further processed to generate a fragment that can trigger axonal degeneration, suggesting a vital role for BACE1 in axonal health. In addition, BACE1 cleaves neuregulin 1 type III, a protein critical for myelination of peripheral axons by Schwann cells during development. Here, we asked if axonal degeneration or axonal regeneration in adult nerves might be affected by inhibition or elimination of BACE1. We report that BACE1 knockout and wild-type nerves degenerated at a similar rate after axotomy and to a similar extent in the experimental neuropathies produced by administration of paclitaxel and acrylamide. These data indicate N-APP is not the sole culprit in axonal degeneration in adult nerves. Unexpectedly, however, we observed that BACE1 knockout mice had markedly enhanced clearance of axonal and myelin debris from degenerated fibers, accelerated axonal regeneration, and earlier reinnervation of neuromuscular junctions, compared to littermate controls. These observations were reproduced in part by pharmacological inhibition of BACE1. These data suggest BACE1 inhibition as a therapeutic approach to accelerate regeneration and recovery after peripheral nerve damage.
β-Amyloid protein (Aβ), the major component of neuritic plaques in Alzheimer’s disease (AD), is derived from proteolytic cleavages of the amyloid precursor protein (APP) by β-secretase (BACE1) and the γ-secretase complex. BACE1 is the rate-limiting enzyme for Aβ production and an increase in BACE1 level/activity contributes to the pathogenesis of sporadic AD. In addition to cleaving APP for Aβ generation, BACE1 plays multiple physiological roles including the regulation of synaptic functions. Here, we found that overexpression of BACE1 reduces CREB phosphorylation, PKA activity and cAMP levels, whereas down-regulation of BACE1 has the opposite effect. We showed that BACE1’s effect is independent of its activity for Aβ production, which is corroborated by the observation that BACE1 transgenic mice have impaired learning/memory in the absence of neurotoxic human Aβ. Furthermore, we demonstrated that BACE1 interacts via its transmembrane domain with adenylate cyclase, resulting in reduction of cellular cAMP levels and thus PKA inactivation and reduced CREB phosphorylation. Our study suggests that besides its function as the β-secretase to produce Aβ, BACE1 may contribute to the memory and cognitive deficits typical of AD by regulating the cAMP/PKA/CREB pathway, which is important for memory functions.
Alzheimer’s disease (AD) is an intractable, neurodegenerative disease that appears to be brought about by both genetic and non-genetic factors. The neuropathology associated with AD is complex, although amyloid plaques composed of the β-amyloid peptide (Aβ) are hallmark neuropathological lesions of AD brain. Indeed, Aβ plays an early and central role in this disease. β-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is the initiating enzyme in Aβ genesis and BACE1 levels are elevated under a variety of conditions. Given the strong correlation between Aβ and AD, and the elevation of BACE1 in this disease, this enzyme is a prime drug target for inhibiting Aβ production in AD. However, nine years on from the initial identification of BACE1, and despite intense research, a number of key questions regarding BACE1 remain unanswered. Indeed, drug discovery and development for AD continues to be challenging. While current AD therapies temporarily slow cognitive decline, treatments that address the underlying pathologic mechanisms of AD are completely lacking. Here we review the basic biology of BACE1. We pay special attention to recent research that has provided some answers to questions such as those involving the identification of novel BACE1 substrates, the potential causes of BACE1 elevation and the putative function of BACE1 in health and disease. Our increasing understanding of BACE1 biology should aid the development of compounds that interfere with BACE1 expression and activity and may lead to the generation of novel therapeutics for AD.
Alzheimer’s; BACE1; β-secretase; Aβ; vascular disease; regulation; stress.
Beta-site amyloid precursor protein cleaving enzyme (BACE-1) is a single-membrane protein belongs to the aspartyl protease class of catabolic enzymes. This enzyme involved in the processing of the amyloid precursor protein (APP). The cleavage of APP by BACE-1 is the rate-limiting step in the amyloid cascade leading to the production of two peptide fragments Aβ40 and Aβ42. Among two peptide fragments Aβ42 is the primary species thought to be responsible for the neurotoxicity and amyloid plaque formation that lead to memory and cognitive defects in Alzheimer’s disease (AD). AD is a ravaging neurodegenerative disorder for which no disease-modifying treatment is currently available. Inhibition of BACE-1 is expected to stop amyloid plaque formation and emerged as an interesting and attractive therapeutic target for AD.
Ligand-based computational approach was used to identify the molecular chemical features required for the inhibition of BACE-1 enzyme. A training set of 20 compounds with known experimental activity was used to generate pharmacophore hypotheses using 3D QSAR Pharmacophore Generation module available in Discovery studio. The hypothesis was validated by four different methods and the best hypothesis was utilized in database screening of four chemical databases like Maybridge, Chembridge, NCI and Asinex. The retrieved hit compounds were subjected to molecular docking study using GOLD 4.1 program.
Among ten generated pharmacophore hypotheses, Hypo 1 was chosen as best pharmacophore hypothesis. Hypo 1 consists of one hydrogen bond donor, one positive ionizable, one ring aromatic and two hydrophobic features with high correlation coefficient of 0.977, highest cost difference of 121.98 bits and lowest RMSD value of 0.804. Hypo 1 was validated using Fischer randomization method, test set with a correlation coefficient of 0.917, leave-one-out method and decoy set with a goodness of hit score of 0.76. The validated Hypo 1 was used as a 3D query in database screening and retrieved 773 compounds with the estimated activity value <100 nM. These hits were docked into the active site of BACE-1 and further refined based on molecular interactions with the essential amino acids and good GOLD fitness score.
The best pharmacophore hypothesis, Hypo 1, with high predictive ability contains chemical features required for the effective inhibition of BACE-1. Using Hypo 1, we have identified two compounds with diverse chemical scaffolds as potential virtual leads which, as such or upon further optimization, can be used in the designing of new BACE-1 inhibitors.
The β-secretase enzyme BACE1 (β-site amyloid precursor protein-cleaving enzyme 1), which initiates amyloid-β (Aβ) production, is an excellent therapeutic target for Alzheimer's disease (AD). However, recent evidence raises concern that BACE1-inhibiting approaches may encounter dramatic declines in their abilities to ameliorate AD-like pathology and memory deficits during disease progression. Here, we used BACE1 haploinsufficiency as a therapeutic relevant model to evaluate the efficacy of partial inhibition of this enzyme. Specifically, we crossed BACE1+/− mice with 5XFAD transgenic mice and investigated the mechanisms by which Aβ accumulation and related memory impairments become less sensitive to rescue by BACE1+/− reduction. Haploinsufficiency lowered BACE1 expression by ∼50% in 5XFAD mice regardless of age in concordance with reduction in gene copy number. However, profound Aβ plaque pathology and memory deficits concomitant with BACE1 equivalent to wild-type control levels remained in BACE1+/−·5XFAD mice with advanced age (15–18 months old). Therefore, BACE1 haploinsufficiency is not sufficient to block the elevation of BACE1 expression (approximately twofold), which is also reported to occur during human AD progression, in 5XFAD mice. Our investigation revealed that PERK (PKR-endoplasmic reticulum-related kinase)-dependent activation of eIF2α (eukaryotic translation initiation factor-2α) accounts for the persistent BACE1 upregulation in BACE1+/−·5XFAD mouse brains at 15–18 months of age. Moreover, BACE1 haploinsufficiency was also no longer able to prevent reduction in the expression of neprilysin, a crucial Aβ-degrading enzyme, in 5XFAD mice with advanced age. These findings demonstrate that partial BACE1 suppression cannot attenuate deleterious BACE1-elevating or neprilysin-reducing mechanisms, limiting its capabilities to reduce cerebral Aβ accumulation and rescue memory defects during the course of AD development.
amyloid; BACE1; elF2α; learning and memory; neprilysin; PERK
Beta-site amyloid precursor protein-cleaving enzyme-1 (BACE1) initiates generation of amyloid beta (Abeta), a pathological hallmark of Alzheimer’s disease. We investigated the impact of BACE1 protein level on endogenous Abeta. Endogenous Abeta and BACE1 protein levels were concurrently and significantly reduced during early life. However, Abeta levels were similar between BACE1 transgenic and wildtype mice. This suggests that BACE1 protein level has a minimal effect on the level of endogenous Abeta. Consequently, other factors must be involved in modulation of Abeta production in adult and ageing brain and investigation of such factors may yield therapeutic targets. Further, these results suggest that substantial inhibition of BACE1 in brain may be required for clinical benefit in Alzheimer’s disease.
Alzheimer’s disease; amyloid beta; beta-site amyloid precursor protein (APP); cleaving enzyme-1 (BACE1); non-transgenic (wild-type) mice
Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1)–the neuronal β-secretase responsible for producing β-amyloid (Aβ) peptides–emerged as one of the key therapeutic targets of Alzheimer's disease (AD). Although complete ablation of the BACE1 gene prevents Aβ formation, we reported that BACE1 knockout mice display severe presynaptic deficits at mossy fiber (MF) to CA3 synapses in the hippocampus, a major locus of BACE1 expression. We also found that the deficits are likely due to abnormal presynaptic Ca2+ regulation. Cholinergic system has been implicated in AD, in some cases involving Ca2+-permeable α7-nicotinic acetylcholine receptors (nAChRs). Here we report that brief application of nicotine, via α7-nAChRs, can restore mossy fiber LTP (mfLTP) in BACE1 knockouts. Our data suggest that activating α7-nAChRs can recover the presynaptic deficits in BACE1 knockouts.
long-term potentiation; presynaptic; paired-pulse facilitation; beta-secretase; Alzheimer's disease; α7-nAchR
Sequential cleavages of the β-amyloid precursor protein cleaving enzyme 1 (BACE1) by β-secretase and γ-secretase generate the amyloid β-peptides, believed to be responsible of synaptic dysfunction and neuronal cell death in Alzheimer's disease (AD). Levels of BACE1 are increased in vulnerable regions of the AD brain, but the underlying mechanism is unknown. Here we show that oxidative stress (OS) stimulates BACE1 expression by a mechanism requiring γ-secretase activity involving the c-jun N-terminal kinase (JNK)/c-jun pathway. BACE1 levels are increased in response to OS in normal cells, but not in cells lacking presenilins or amyloid precursor protein. Moreover, BACE1 is induced in association with OS in the brains of mice subjected to cerebral ischaemia/reperfusion. The OS-induced BACE1 expression correlates with an activation of JNK and c-jun, but is absent in cultured cells or mice lacking JNK. Our findings suggest a mechanism by which OS induces BACE1 transcription, thereby promoting production of pathological levels of amyloid β in AD.
Alzheimer's Disease; BACE1; JNK pathway; oxidative stress; γ-secretase
Beta-amyloid precursor protein cleavage enzyme 1 (BACE1) has been identified as a major neuronal β-secretase critical for the formation of β-amyloid (Aβ) peptide, which is thought responsible for the pathology of Alzheimer’s disease (AD). Therefore, BACE1 is one of the key therapeutic targets that can prevent the progression of AD. Previous studies showed that knocking out the BACE1 gene prevents Aβ formation, but results in behavioral deficits and specific synaptic dysfunctions at Schaffer collateral to CA1 synapses. However, BACE1 protein is most highly expressed at the mossy fiber projections in CA3. Here we report that BACE1 knockout mice display reduced presynaptic function, as measured by an increase in paired-pulse facilitation ratio. More dramatically, mossy fiber LTP, which is normally expressed via an increase in presynaptic release, was eliminated in the knockouts. While LTD was slightly larger in the BACE1 knockouts, it could not be reversed. The specific deficit in mossy fiber LTP was upstream of cAMP signaling, and could be “rescued” by transiently elevating extracellular Ca2+ concentration. These results suggest that BACE1 may play a critical role in regulating presynaptic function, especially activity-dependent strengthening of presynaptic release, at mossy fiber synapses.
long-term potentiation; long-term depression; presynaptic; paired-pulse facilitation; beta-secretase; Alzheimer’s disease
Background. Alzheimer's disease (AD) is a devastating neurological disorder and the main cause of dementia in the elderly population worldwide. Adult neurogenesis appears to be upregulated very early in AD pathogenesis in response to some specific aggregates of beta-amyloid (Aβ) peptides, exhausting the neuronal stem cell pools in the brain. Previously, we characterized a conserved nonprotein-coding antisense transcript for β-secretase-1 (BACE1), a critical enzyme in AD pathophysiology. We showed that the BACE1-antisense transcript (BACE1-AS) is markedly upregulated in brain samples from AD patients and promotes the stability of the (sense) BACE1 transcript. In the current paper, we examine the relationship between BACE1, BACE1-AS, adult neurogenesis markers, and amyloid plaque formation in amyloid precursor protein (APP) transgenic mice (Tg-19959) of various ages. Results. Consistent with previous publications in other APP overexpressing mouse models, we found adult neurogenesis markers to be noticeably upregulated in Tg-19959 mice very early in the development of the disease. Knockdown of either one of BACE1 or BACE1-AS transcripts by continuous infusion of locked nucleic acid- (LNA-) modified siRNAs into the third ventricle over the period of two weeks caused concordant downregulation of both transcripts in Tg-19959 mice. Downregulation of BACE1 mRNA was followed by reduction of BACE1 protein and insoluble Aβ. Modulation of BACE1 and BACE1-AS transcripts also altered oligomeric Aβ aggregation pattern, which was in turn associated with an increase in neurogenesis markers at the RNA and protein level. Conclusion. We found alterations in the RNA and protein concentrations of several adult neurogenesis markers, as well as non-protein-coding BACE1-AS transcripts, in parallel with the course of β-amyloid synthesis and aggregation in the brain of Tg15999 mice. In addition, by knocking down BACE1 or BACE1-AS (thereby reducing Aβ production and plaque deposition), we were able to modulate expression of these neurogenesis markers. Our findings suggest a distortion of adult neurogenesis that is associated with Aβ production very early in amyloid pathogenesis. We believe that these alterations, at the molecular level, could prove useful as novel therapeutic targets and/or as early biomarkers of AD.
Individuals with Down Syndrome (DS), or trisomy 21, develop Alzheimer's disease (AD) pathology by approximately 40 years of age. Chromosome 21 harbors several genes implicated in AD, including the amyloid precursor protein and one homologue of the β-site APP cleaving enzyme, BACE2. Processing of the amyloid precursor protein by β-secretase (BACE) is the rate-limiting step in the production of the pathogenic Aβ peptide. Increased amounts of APP in the DS brain result in increased amounts of Aβ and extracellular plaque formation beginning early in life. BACE dysregulation potentially represents an overlapping biological mechanism with sporadic AD and a common therapeutic target. As the lifespan for those with DS continues to increase, age-related concerns such as obesity, depression, and AD are of growing concern. The ability to prevent or delay the progression of neurodegenerative diseases will promote healthy aging and improve quality of life for those with DS.
β-Site amyloid precursor protein (APP) cleaving enzyme-1 (BACE1) is the β-secretase that initiates Aβ production in Alzheimer’s disease (AD). BACE1 levels are increased in AD, which could contribute to pathogenesis, yet the mechanism of BACE1 elevation is unclear. Furthermore, the normal function of BACE1 is poorly understood. We localized BACE1 in the brain at both the light and electron microscopic levels to gain insight into normal and pathophysiologic roles of BACE1 in health and AD, respectively. Our findings provide the first ultrastructural evidence that BACE1 localizes to vesicles (likely endosomes) in normal hippocampal mossy fiber terminals of both non-transgenic and APP transgenic (5XFAD) mouse brains. In some instances, BACE1-positive vesicles were located near active zones, implying a function for BACE1 at the synapse. In addition, BACE1 accumulated in swollen dystrophic autophagosome-poor presynaptic terminals surrounding amyloid plaques in 5XFAD cortex and hippocampus. Importantly, accumulations of BACE1 and APP co-localized in presynaptic dystrophies, implying increased BACE1 processing of APP in peri-plaque regions. In primary cortical neuron cultures, treatment with the lysosomal protease inhibitor leupeptin caused BACE1 levels to increase; however, exposure of neurons to the autophagy inducer trehalose did not reduce BACE1 levels. This suggests that BACE1 is degraded by lysosomes but not by autophagy. Our results imply that BACE1 elevation in AD could be linked to decreased lysosomal degradation of BACE1 within dystrophic presynaptic terminals. Elevated BACE1 and APP levels in plaque-associated presynaptic dystrophies could increase local peri-plaque Aβ generation and accelerate amyloid plaque growth in AD.
Electronic supplementary material
The online version of this article (doi:10.1007/s00401-013-1152-3) contains supplementary material, which is available to authorized users.
Alzheimer’s disease; BACE1; β-Secretase; Aβ; Amyloid plaque; Dystrophic neurite; Lysosome; Autophagy
The β-site amyloid precursor protein cleaving enzyme 1 (BACE1) is necessary to generate the Aβ peptide, which is implicated in Alzheimer's disease pathology. Studies show that the expression of BACE1 and its protease activity are tightly regulated, but the physiological function of BACE1 remains poorly understood. Recently, numerous axon guidance proteins were identified as potential substrates of BACE1. Here, we examined the consequences of loss of BACE1 function in a well-defined in vivo model system of axon guidance, mouse olfactory sensory neurons (OSNs). The BACE1 protein resides predominantly in proximal segment and the termini of OSN axons, and the expression of BACE1 inversely correlates with odor-evoked neural activity. The precision of targeting of OSN axons is disturbed in both BACE1 null and, surprisingly, in BACE1 heterozygous mice. We propose that BACE1 cleavage of axon guidance proteins is essential to maintain the connectivity of OSNs in vivo.
Amyloid precursor protein cleaving enzyme 1 (BACE1), an aspartyl protease, initiates processing of the amyloid precursor protein (APP) into β-amyloid (Aβ); the peptide likely contributes to development of Alzheimer’s disease (AD). BACE1 is an attractive therapeutic target for AD treatment, but it exhibits other physiological activities and has many other substrates besides APP. Thus, inhibition of BACE1 function may cause adverse side effects. Here, we present a peptide, S1, isolated from a peptide library that selectively inhibits BACE1 hydrolytic activity by binding to the β-proteolytic site on APP and Aβ N-terminal. The S1 peptide significantly reduced Aβ levels in vitro and in vivo and inhibited Aβ cytotoxicity in SH-SY5Y cells. When applied to APPswe/PS1dE9 double transgenic mice by intracerebroventricular injection, S1 significantly improved the spatial memory as determined by the Morris Water Maze, and also attenuated their Aβ burden. These results indicate that the dual-functional peptide S1 may have therapeutic potential for AD by both reducing Aβ generation and inhibiting Aβ cytotoxicity.
β-Site β-amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1) is the β-secretase in vivo for processing APP to generate amyloid β protein (Aβ). Aβ deposition in the brain is the hallmark of Alzheimer's disease (AD) neuropathology. Inhibition of BACE1 activity has major pharmaceutical potential for AD treatment. The expression of the BACE1 gene is relatively low in vivo. The control of BACE1 expression has not been well defined. There are six upstream AUGs (uAUGs) in the 5′ leader sequence of the human BACE1 mRNA. We investigated the role of the promoter and the uATGs in the 5′ untranslated region (UTR) of the human BACE1 gene in BACE1 gene transcription and translation initiation. Our results show that the first and second uATGs are the integral part of the core minimal promoter of the human BACE1 gene, while the third uAUG is skipped over by ribosomal scanning. The fourth uAUG can function as a translation initiation codon, and deletion or mutation of this uAUG increases downstream gene expression. The fourth uAUG of the BACE1 5′UTR is responsible for inhibiting the expression of BACE1. Translation initiation by the BACE1 uAUGs and physiological AUG requires intact eIF4G. Our results demonstrate that during human BACE1 gene expression, ribosomes skipped some uAUGs by leaky scanning and translated an upstream open reading frame, initiated efficiently at the fourth uAUG, and subsequently reinitiated BACE1 translation at the physiological AUG site. Such leaky scanning and reinitiation resulted in weak expression of BACE1 under normal conditions. Alterations of the leaky scanning and reinitiation in BACE1 gene expression could play an important role in AD pathogenesis.
Cleavage of amyloid precursor protein (APP) by the Alzheimer's β-secretase (BACE1) is a key step in generating amyloid β-peptide, the main component of amyloid plaques. Here we report evidence that heparan sulfate (HS) interacts with β-site APP-cleaving enzyme (BACE) 1 and regulates its cleavage of APP. We show that HS and heparin interact directly with BACE1 and inhibit in vitro processing of peptide and APP substrates. Inhibitory activity is dependent on saccharide size and specific structural characteristics, and the mechanism of action involves blocking access of substrate to the active site. In cellular assays, HS specifically inhibits BACE1 cleavage of APP but not alternative cleavage by α-secretase. Endogenous HS immunoprecipitates with BACE1 and colocalizes with BACE1 in the Golgi complex and at the cell surface, two of its putative sites of action. Furthermore, inhibition of cellular HS synthesis results in enhanced BACE1 activity. Our findings identify HS as a natural regulator of BACE1 and suggest a novel mechanism for control of APP processing.
Alzheimer's disease; amyloid processing; proteoglycan; protease; neurodegeneration
The β-site APP cleaving enzyme 1 (BACE1) is a transmembrane aspartyl protease involved in Alzheimer’s disease (AD) pathogenesis and in myelination. BACE1 initiates the generation of the pathogenic amyloid β-peptide, which makes BACE1 a major drug target for AD. BACE1 also cleaves and activates neuregulin 1, thereby contributing to postnatal myelination, in particular in the peripheral nervous system. Additional proteins are also cleaved by BACE1, but less is known about the physiological consequences of their cleavage. Recently, new phenotypes were described in BACE1-deficient mice. Although it remains unclear through which BACE1 substrates they are mediated, the phenotypes suggest a versatile role of this protease for diverse physiological processes. This review summarizes the enzymatic and cellular properties of BACE1 as well as its regulation by lipids, by transcriptional, and by translational mechanisms. The main focus will be on the recent progress in understanding BACE1 function and its implication for potential mechanism-based side effects upon therapeutic inhibition.
neurodegeneration; amyloid-beta; regulated intramembrane proteolysis; beta-secretase; aspartic protease
Beta-site APP-cleaving enzyme (BACE) is required for production of the Alzheimer's disease (AD)-associated Aβ protein. BACE levels are elevated in AD brain, and increasing evidence reveals BACE as a stress-related protease that is upregulated following cerebral ischemia. However, the molecular mechanism responsible is unknown. We show that increases in BACE and β-secretase activity are due to post-translational stabilization following caspase activation. We also found that during cerebral ischemia, levels of GGA3, an adaptor protein involved in BACE trafficking, are reduced, while BACE levels are increased. RNAi silencing of GGA3 also elevated levels of BACE and Aβ. Finally, in AD brain samples, GGA3 protein levels were significantly decreased and inversely correlated with increased levels of BACE. In summary, we have elucidated a novel GGA3-dependent mechanism regulating BACE levels and β-secretase activity. This mechanism may explain increased cerebral levels of BACE and Aβ following cerebral ischemia and in AD.
β-Site APP-cleaving enzyme 1 (BACE1) initiates amyloid-β (Aβ) generation and thus represents a prime therapeutic target in treating Alzheimer's disease (AD). Notably, increasing evidence indicates that BACE1 levels become elevated in AD brains as disease progresses; however, it remains unclear how the BACE1 upregulation may affect efficacies of therapeutic interventions including BACE1-inhibiting approaches. Here, we crossed heterozygous BACE1 knockout mice with AD transgenic mice (5XFAD model) and compared the abilities of partial BACE1 reduction to rescue AD-like phenotypes at earlier (6-month-old) and advanced (15–18-month-old) stages of disease, which expressed normal (∼100%) and elevated (∼200%) levels of BACE1, respectively. BACE1+/− deletion rescued memory deficits as tested by the spontaneous alternation Y-maze task in 5XFAD mice at the earlier stage and prevented their septohippocampal cholinergic deficits associated with significant neuronal loss. Importantly, BACE1+/− deletion was no longer able to rescue memory deficits or cholinergic neurodegeneration in 5XFAD mice at the advanced stage. Moreover, BACE1+/− deletion significantly reduced levels of Aβ42 and the β-secretase-cleaved C-terminal fragment (C99) in 6-month-old 5XFAD mouse brains, while these neurotoxic β-cleavage products dramatically elevated with age and were not affected by BACE1+/− deletion in 15–18-month-old 5XFAD brains. Interestingly, although BACE1+/− deletion lowered BACE1 expression by ∼50% in 5XFAD mice irrespective of age in concordance with the reduction in gene copy number, BACE1 equivalent to wild-type controls remained in BACE1+/−·5XFAD mice at the advanced age. In accord, phosphorylation of the translation initiation factor eIF2α, an important mediator of BACE1 elevation, was dramatically increased (∼9-fold) in 15–18-month-old 5XFAD mice and remained highly upregulated (∼6-fold) in age-matched BACE1+/−·5XFAD mice. Together, our results indicate that partial reduction of BACE1 is not sufficient to block the phospho-eIF2α-dependent BACE1 elevation during the progression of AD, thus limiting its abilities to reduce cerebral Aβ/C99 levels and rescue memory deficits and cholinergic neurodegeneration.
β-Site amyloid precursor protein cleaving enzyme 1 (BACE1) initiates amyloid-β (Aβ) generation that is central to the pathophysiology of Alzheimer’s disease (AD). Therefore, lowering Aβ levels by BACE1 manipulations represents a key therapeutic strategy, but it remains unclear whether partial inhibition of BACE1, as expected for AD treatments, can improve memory deficits. In this study, we used heterozygous BACE1 gene knockout (BACE1+/−) mice to evaluate the effects of partial BACE1 suppression on different types of synaptic and cognitive dysfunctions in Alzheimer’s transgenic mice (5XFAD model). We found that ~50% BACE1 reductions rescued deficits of 5XFAD mice not only in hippocampus-dependent memories as tested by contextual fear conditioning and spontaneous alternation Y-maze paradigms but also in cortex-dependent remote memory stabilization during 30 days after contextual conditioning. Furthermore, 5XFAD-associated impairments in long-term potentiation (a synaptic model of learning and memory) and declines in synaptic plasticity/learning-related BDNF-TrkB signaling pathways were prevented in BACE1+/−·5XFAD mice. Finally, these improvements were related with reduced levels of β-secretase-cleaved C-terminal fragment (C99), Aβ peptides and plaque burden in relevant brain regions of BACE1+/−·5XFAD mice. Therefore, our findings provide compelling evidence for beneficial effects of partially BACE1-inhibiting approaches on multiple forms of functional defects associated with AD.
β-secretase; knockout; amyloid-β; fear conditioning; long-term potentiation; brain-derived neurotrophic factor