N-terminome analyses underscore the prevalence of SPPL3-mediated intramembrane proteolysis among Golgi-resident enzymes and its role in Golgi enzyme secretion

Golgi membrane proteins such as glycosyltransferases and other glycan-modifying enzymes are key to glycosylation of proteins and lipids. Secretion of soluble Golgi enzymes that are released from their membrane anchor by endoprotease activity is a wide-spread yet largely unexplored phenomenon. The intramembrane protease SPPL3 can specifically cleave select Golgi enzymes, enabling their secretion and concomitantly altering global cellular glycosylation, yet the entire range of Golgi enzymes cleaved by SPPL3 under physiological conditions remains to be defined. Here, we established isogenic SPPL3-deficient HEK293 and HeLa cell lines and applied N-terminomics to identify substrates cleaved by SPPL3 and released into cell culture supernatants. With high confidence, our study identifies more than 20 substrates of SPPL3, including entirely novel substrates. Notably, our N-terminome analyses provide a comprehensive list of SPPL3 cleavage sites demonstrating that SPPL3-mediated shedding of Golgi enzymes occurs through intramembrane proteolysis. Through the use of chimeric glycosyltransferase constructs we show that transmembrane domains can determine cleavage by SPPL3. Using our cleavage site data, we surveyed public proteome data and found that SPPL3 cleavage products are present in human blood. We also generated HEK293 knock-in cells expressing the active site mutant D271A from the endogenous SPPL3 locus. Immunoblot analyses revealed that secretion of select novel substrates such as the key mucin-type O-glycosylation enzyme GALNT2 is dependent on endogenous SPPL3 protease activity. In sum, our study expands the spectrum of known physiological substrates of SPPL3 corroborating its significant role in Golgi enzyme turnover and secretion as well as in the regulation of global glycosylation pathways.

BACE1 partial deletion induces synaptic plasticity deficit in adult mice

BACE1 is the first enzyme involved in APP processing, thus it is a strong therapeutic target candidate for Alzheimer's disease. The observation of deleterious phenotypes in BACE1 Knock-out (KO) mouse models (germline and conditional) raised some concerns on the safety and tolerability of BACE1 inhibition. Here, we have employed a tamoxifen inducible BACE1 conditional Knock-out (cKO) mouse model to achieve a controlled partial depletion of BACE1 in adult mice. Biochemical and behavioural characterization was performed at two time points: 4-5 months (young mice) and 12-13 months (aged mice). A ~50% to ~70% BACE1 protein reduction in hippocampus and cortex, respectively, induced a significant reduction of BACE1 substrates processing and decrease of Aβx-40 levels at both ages. Hippocampal axonal guidance and peripheral nerve myelination were not affected. Aged mice displayed a CA1 long-term potentiation (LTP) deficit that was not associated with memory impairment. Our findings indicate that numerous phenotypes observed in germline BACE1 KO reflect a fundamental role of BACE1 during development while other phenotypes, observed in adult cKO, may be absent when partially rather than completely deleting BACE1. However, we demonstrated that partial depletion of BACE1 still induces CA1 LTP impairment, supporting a role of BACE1 in synaptic plasticity in adulthood.

Axonal organization defects in the hippocampus of adult conditional BACE1 knockout mice

β-Site APP (amyloid precursor protein) cleaving enzyme 1 (BACE1) is the β-secretase enzyme that initiates production of the toxic amyloid-β peptide that accumulates in the brains of patients with Alzheimer's disease (AD). Hence, BACE1 is a prime therapeutic target, and several BACE1 inhibitor drugs are currently being tested in clinical trials for AD. However, the safety of BACE1 inhibition is unclear. Germline BACE1 knockout mice have multiple neurological phenotypes, although these could arise from BACE1 deficiency during development. To address this question, we report that tamoxifen-inducible conditional BACE1 knockout mice in which the Bace1 gene was ablated in the adult largely lacked the phenotypes observed in germline BACE1 knockout mice. However, one BACE1-null phenotype was induced after Bace1 gene deletion in the adult mouse brain. This phenotype showed reduced length and disorganization of the hippocampal mossy fiber infrapyramidal bundle, the axonal pathway of dentate gyrus granule cells that is maintained by neurogenesis in the mouse brain. This defect in axonal organization correlated with reduced BACE1-mediated cleavage of the neural cell adhesion protein close homolog of L1 (CHL1), which has previously been associated with axon guidance. Although our results indicate that BACE1 inhibition in the adult mouse brain may avoid phenotypes associated with BACE1 deficiency during embryonic and postnatal development, they also suggest that BACE1 inhibitor drugs developed for treating AD may disrupt the organization of an axonal pathway in the hippocampus, an important structure for learning and memory.

Loss of Bace1 in mice does not alter the severity of caerulein induced pancreatitis

Context

Beta-site alpha-amyloid protein cleaving enzyme1 (BACE1) plays a key role in the pathogenesis of Alzheimer’s disease. Additional to its moderate expression in the brain, high levels of BACE1 mRNA were found in the pancreas. Murine Bace1 has been immunohistochemicaly detected at the apical pole of acinar cells within the exocrine pancreas of mice and Bace1 activity was observed in pancreatic juice. In vitro experiments revealed enteropeptidase as a putative substrate for Bace1 suggesting a role in acute pancreatitis.

Objective

The aim of this study was to address a protective mechanism of Bace1 in acute experimental pancreatitis in mice.

Methods

Acute experimental pancreatitis was induced by intraperitoneal injection of caerulein in homozygote Bace1^-/- mice and wild type mice. Serum and tissue analyses were carried out after 4 h, 8 h and 24 h. Measurement of plasma amylase and lipase was performed to confirm pancreatitis induction. In order to assess the severity of pancreatitis H&E stained pancreatic sections were examined regarding edema, inflammation and apoptosis. Immunohistochemical detection of myeloperoxidase (MPO) positive cells was carried out to further quantify the extent of inflammation. Expression of Bace2 within the pancreas was analyzed by immunohistochemistry and RT-qPCR.

Results

We demonstrate that total loss of Bace1 in mice leads to no alterations in the course of acute experimental caerulein-pancreatitis. Bace1^-/- mice develop a moderate pancreatitis that is comparable in histomorphological and serological features with those seen in wild type mice.

Discussion

We discuss the results in the context of the applied caerulein induced edematous pancreatitis model and possible compensatory mechanisms via Bace2 that might be responsible for the observed results.

APP is cleaved by Bace1 in pre-synaptic vesicles and establishes a pre-synaptic interactome, via its intracellular domain, with molecular complexes that regulate pre-synaptic vesicles functions

Amyloid Precursor Protein (APP) is a type I membrane protein that undergoes extensive processing by secretases, including BACE1. Although mutations in APP and genes that regulate processing of APP, such as PSENs and BRI2/ITM2B, cause dementias, the normal function of APP in synaptic transmission, synaptic plasticity and memory formation is poorly understood. To grasp the biochemical mechanisms underlying the function of APP in the central nervous system, it is important to first define the sub-cellular localization of APP in synapses and the synaptic interactome of APP. Using biochemical and electron microscopy approaches, we have found that APP is localized in pre-synaptic vesicles, where it is processed by Bace1. By means of a proteomic approach, we have characterized the synaptic interactome of the APP intracellular domain. We focused on this region of APP because in vivo data underline the central functional and pathological role of the intracellular domain of APP. Consistent with the expression of APP in pre-synaptic vesicles, the synaptic APP intracellular domain interactome is predominantly constituted by pre-synaptic, rather than post-synaptic, proteins. This pre-synaptic interactome of the APP intracellular domain includes proteins expressed on pre-synaptic vesicles such as the vesicular SNARE Vamp2/Vamp1 and the Ca2+ sensors Synaptotagmin-1/Synaptotagmin-2, and non-vesicular pre-synaptic proteins that regulate exocytosis, endocytosis and recycling of pre-synaptic vesicles, such as target-membrane-SNAREs (Syntaxin-1b, Syntaxin-1a, Snap25 and Snap47), Munc-18, Nsf, α/β/γ-Snaps and complexin. These data are consistent with a functional role for APP, via its carboxyl-terminal domain, in exocytosis, endocytosis and/or recycling of pre-synaptic vesicles.

Postsynaptic target specific synaptic dysfunctions in the CA3 area of BACE1 knockout mice

Beta-amyloid precursor protein cleaving enzyme 1 (BACE1), a major neuronal β-secretase critical for the formation of β-amyloid (Aβ) peptide, is considered one of the key therapeutic targets that can prevent the progression of Alzheimer's disease (AD). Although a complete ablation of BACE1 gene prevents Aβ formation, we previously reported that BACE1 knockouts (KOs) display presynaptic deficits, especially at the mossy fiber (MF) to CA3 synapses. Whether the defect is specific to certain inputs or postsynaptic targets in CA3 is unknown. To determine this, we performed whole-cell recording from pyramidal cells (PYR) and the stratum lucidum (SL) interneurons in the CA3, both of which receive excitatory MF terminals with high levels of BACE1 expression. BACE1 KOs displayed an enhancement of paired-pulse facilitation at the MF inputs to CA3 PYRs without changes at the MF inputs to SL interneurons, which suggests postsynaptic target specific regulation. The synaptic dysfunction in CA3 PYRs was not restricted to excitatory synapses, as seen by an increase in the paired-pulse ratio of evoked inhibitory postsynaptic currents from SL to CA3 PYRs. In addition to the changes in evoked synaptic transmission, BACE1 KOs displayed a reduction in the frequency of miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs) in CA3 PYRs without alteration in mEPSCs recorded from SL interneurons. This suggests that the impairment may be more global across diverse inputs to CA3 PYRs. Our results indicate that the synaptic dysfunctions seen in BACE1 KOs are specific to the postsynaptic target, the CA3 PYRs, independent of the input type.

Does folic acid supplementation rescue defects in ECE-1-deficient mouse embryos?

Endothelin (ET) signalling is essential for normal embryonic development. Disruption of this pathway leads to defects in the development of subsets of cranial and cephalic neural crest derivatives. Endothelin-converting enzyme 1 (ECE-1) is a ratelimiting step in the biosynthesis of ET-1. Recently, there has been considerable interest in the protective role of folic acid (FA) against congenital anomalies via increasing the expression of ET-1. We have tested whether FA supplementation can rescue craniofacial and cardiac defects observed in the ECE1-/- embryos. ECE1+/- mice were caged together to obtain litters containing embryos of all possible genotypes. The treatment group had the diet supplemented with 20 mg/kg of FA from the day of discovery of the vaginal plug. FA supplementation did not result in modified proportions of the genotypes, indicating no rescue of the embryonic mortality. There was also no effect on the litter size. Craniofacial and cardiac defects were likewise identical in the ECE1-/- embryos of both groups. There was a mild but significant reduction in the embryo size in wild-type and heterozygous FA-supplemented embryos, and there were haemorrhages in the wild-type supplemented embryos at ED14.5. Expression of ET receptor A detected by immunohistochemistry was up-regulated in the ECE1-/- embryos, but FA supplementation had no effects on the distribution of staining intensity. We conclude that FA is not able to rescue the phenotype in this model, suggesting an alternative pathway for its action. These results also caution against indiscriminate use of dietary supplements in attempts to prevent congenital anomalies.

APP knockout mice experience acute mortality as the result of ischemia

The incidence of Alzheimer’s disease increases in people who have had an ischemic episode. Furthermore, APP expression is increased following ischemic or hypoxic conditions, as is the production of the Aβ peptide. To address the question of why APP and Aβ are increased in hypoxic and ischemic conditions we induced an ischemic episode in APP knockout mice (APP−/−) and BACE1 knockout mice (BACE−/−). We find that both APP−/− and BACE−/− mice have a dramatically increased risk of mortality as a result of cerebral ischemia. Furthermore, APP knockout mice have reduced cerebral blood flow in response to hypoxia, while wild-type mice maintain or increase cerebral blood flow to the same conditions. The transcription factor, serum response factor (SRF), and calcium-binding molecule, calsequestrin, both involved in vascular regulation, are significantly altered in the brains of APP−/− mice compared to wild type controls. These results show that APP regulates cerebral blood flow in response to hypoxia, and that it, and its cleavage fragments, are crucial for rapid adaptation to ischemic conditions.

β-Site APP-cleaving enzyme 1 (BACE1) cleaves cerebellar Na+ channel β4-subunit and promotes Purkinje cell firing by slowing the decay of resurgent Na+ current

In cerebellar Purkinje cells, the β4-subunit of voltage-dependent Na(+) channels has been proposed to serve as an open-channel blocker giving rise to a "resurgent" Na(+) current (I (NaR)) upon membrane repolarization. Notably, the β4-subunit was recently identified as a novel substrate of the β-secretase, BACE1, a key enzyme of the amyloidogenic pathway in Alzheimer's disease. Here, we asked whether BACE1-mediated cleavage of β4-subunit has an impact on I (NaR) and, consequently, on the firing properties of Purkinje cells. In cerebellar tissue of BACE1-/- mice, mRNA levels of Na(+) channel α-subunits 1.1, 1.2, and 1.6 and of β-subunits 1-4 remained unchanged, but processing of β4 peptide was profoundly altered. Patch-clamp recordings from acutely isolated Purkinje cells of BACE1-/- and WT mice did not reveal any differences in steady-state properties and in current densities of transient, persistent, and resurgent Na(+) currents. However, I (NaR) was found to decay significantly faster in BACE1-deficient Purkinje cells than in WT cells. In modeling studies, the altered time course of I (NaR) decay could be replicated when we decreased the efficiency of open-channel block. In current-clamp recordings, BACE1-/- Purkinje cells displayed lower spontaneous firing rate than normal cells. Computer simulations supported the hypothesis that the accelerated decay kinetics of I (NaR) are responsible for the slower firing rate. Our study elucidates a novel function of BACE1 in the regulation of neuronal excitability that serves to tune the firing pattern of Purkinje cells and presumably other neurons endowed with I (NaR).

Partial reduction of BACE1 has dramatic effects on Alzheimer plaque and synaptic pathology in APP Transgenic Mice

The aspartyl protease beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) initiates processing of amyloid precursor protein (APP) into amyloid beta (Abeta) peptide, the major component of Alzheimer disease (AD) plaques. To determine the role that BACE1 plays in the development of Abeta-driven AD-like pathology, we have crossed PDAPP mice, a transgenic mouse model of AD overexpressing human mutated APP, onto mice with either a homozygous or heterozygous BACE1 gene knockout. Analysis of PDAPP/BACE(-/-) mice demonstrated that BACE1 is absolutely required for both Abeta generation and the development of age-associated plaque pathology. Furthermore, synaptic deficits, a neurodegenerative pathology characteristic of AD, were also reversed in the bigenic mice. To determine the extent of BACE1 reduction required to significantly inhibit pathology, PDAPP mice having a heterozygous BACE1 gene knock-out were evaluated for Abeta generation and for the development of pathology. Although the 50% reduction in BACE1 enzyme levels caused only a 12% decrease in Abeta levels in young mice, it nonetheless resulted in a dramatic reduction in Abeta plaques, neuritic burden, and synaptic deficits in older mice. Quantitative analyses indicate that brain Abeta levels in young APP transgenic mice are not the sole determinant for the changes in plaque pathology mediated by reduced BACE1. These observations demonstrate that partial reductions of BACE1 enzyme activity and concomitant Abeta levels lead to dramatic inhibition of Abeta-driven AD-like pathology, making BACE1 an excellent target for therapeutic intervention in AD.

Beta-site amyloid precursor protein cleaving enzyme 1 levels become elevated in neurons around amyloid plaques: implications for Alzheimer's disease pathogenesis

Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) (beta-secretase) initiates generation of beta-amyloid (Abeta), which plays an early role in Alzheimer's disease (AD). BACE1 levels are increased in postmortem AD brain, suggesting BACE1 elevation promotes Abeta production and AD. Alternatively, the BACE1 increase may be an epiphenomenon of late-stage AD. To distinguish between these possibilities, we analyzed BACE1 elevation using a highly specific BACE1 antibody, BACE-Cat1, made in BACE1-/- mice, which mount a robust anti-BACE1 immune response. Previous BACE1 immunohistochemical studies lack consistent results because typical BACE1 antibodies produce nonspecific background, but BACE-Cat1 immunolabels BACE1 only. BACE1 elevation was recapitulated in two amyloid precursor protein (APP) transgenic mouse lines. 5XFAD mice form amyloid plaques at young ages and exhibit neuron loss. In contrast, Tg2576 form plaques at a more advanced age and do not show cell death. These two mouse lines allow differentiation between early Abeta-induced events and late phenomena related to neuron death. BACE1 levels became elevated in parallel with amyloid burden in each APP transgenic, starting early in 5XFAD and late in Tg2576. The increase in BACE1 protein occurred without any change in BACE1 mRNA level, indicating a posttranscriptional mechanism. In APP transgenic and AD brains, high BACE1 levels were observed in an annulus around Abeta42-positive plaque cores and colocalized with neuronal proteins. These results demonstrate that amyloid plaques induce BACE1 in surrounding neurons at early stages of pathology before neuron death occurs. We conclude that BACE1 elevation is most likely triggered by the amyloid pathway and may drive a positive-feedback loop in AD.

Activation of protein kinase C modulates BACE1-mediated beta-secretase activity

beta-Site APP cleavage enzyme 1 (BACE1) is the beta-secretase responsible for generating amyloid-beta (A beta) peptides in Alzheimer's disease (AD). Previous studies suggest that activation of protein kinase C (PKC) modulates the beta-secretase-mediated cleavage of APP and reduces the production of A beta. The mechanism of PKC-mediated modulation of beta-secretase activity, however, remains elusive. We report here that activation of PKC modulated beta-secretase activity through either suppressing the accumulation or promoting the translocation of BACE1 protein in a cell type-dependent manner. We found that activation of PKC suppressed the accumulation of BACE1 protein in fibroblasts through an enhancement of intracellular protease activities. In neurons, activation of PKC did not alter the expression level of BACE1, but led to more BACE1 translocated to the cell surface, resulting in a decreased cleavage of APP at the beta1 site. Together, Our findings provide novel mechanisms of PKC-mediated modulation of beta-secretase activity, suggesting that alteration of the intracellular trafficking of BACE1 may serve as a useful therapeutic strategy to lower the production of A beta in AD.

BACE1 inhibition reduces endogenous Abeta and alters APP processing in wild-type mice

Accumulation of amyloid beta peptide (Abeta) in brain is a hallmark of Alzheimer's disease (AD). Inhibition of beta-site amyloid precursor protein (APP)-cleaving enzyme-1 (BACE1), the enzyme that initiates Abeta production, and other Abeta-lowering strategies are commonly tested in transgenic mice overexpressing mutant APP. However, sporadic AD cases, which represent the majority of AD patients, are free from the mutation and do not necessarily have overproduction of APP. In addition, the commonly used Swedish mutant APP alters APP cleavage. Therefore, testing Abeta-lowering strategies in transgenic mice may not be optimal. In this study, we investigated the impact of BACE1 inhibition in non-transgenic mice with physiologically relevant APP expression. Existing Abeta ELISAs are either relatively insensitive to mouse Abeta or not specific to full-length Abeta. A newly developed ELISA detected a significant reduction of full-length soluble Abeta 1-40 in mice with the BACE1 homozygous gene deletion or BACE1 inhibitor treatment, while the level of x-40 Abeta was moderately reduced due to detection of non-full-length Abeta and compensatory activation of alpha-secretase. These results confirmed the feasibility of Abeta reduction through BACE1 inhibition under physiological conditions. Studies using our new ELISA in non-transgenic mice provide more accurate evaluation of Abeta-reducing strategies than was previously feasible.

Lysosomal biogenesis and function is critical for necrotic cell death in Caenorhabditis elegans

Necrotic cell death is defined by distinctive morphological characteristics that are displayed by dying cells (Walker, N.I., B.V. Harmon, G.C. Gobe, and J.F. Kerr. 1988. Methods Achiev. Exp. Pathol. 13:18–54). The cellular events that transpire during necrosis to generate these necrotic traits are poorly understood. Recent studies in the nematode Caenorhabditis elegans show that cytoplasmic acidification develops during necrosis and is required for cell death (Syntichaki, P., C. Samara, and N. Tavernarakis. 2005. Curr. Biol. 15:1249–1254). However, the origin of cytoplasmic acidification remains elusive. We show that the alkalization of endosomal and lysosomal compartments ameliorates necrotic cell death triggered by diverse stimuli. In addition, mutations in genes that result in altered lysosomal biogenesis and function markedly affect neuronal necrosis. We used a genetically encoded fluorescent marker to follow lysosome fate during neurodegeneration in vivo. Strikingly, we found that lysosomes fuse and localize exclusively around a swollen nucleus. In the advanced stages of cell death, the nucleus condenses and migrates toward the periphery of the cell, whereas green fluorescent protein–labeled lysosomal membranes fade, indicating lysosomal rupture. Our findings demonstrate a prominent role for lysosomes in cellular destruction during necrotic cell death, which is likely conserved in metazoans.

Generation and characterization of mutant cell lines defective in gamma-secretase processing of Notch and amyloid precursor protein

Several type I integral membrane proteins, such as the Notch receptor or the amyloid precursor protein, are cleaved in their intramembrane domain by a gamma-secretase enzyme, which is carried within a multiprotein complex. These cleavages generate molecules that are involved in intracellular or extracellular signaling. At least four transmembrane proteins belong to the gamma-secretase complex: presenilin, nicastrin, Aph-1, and Pen-2. It is still unclear whether these proteins are the only components of the complex and whether a unique complex is involved in the different gamma-secretase cleavage events. We have set up a genetic screen based on the permanent acquisition or loss of an antibiotic resistance depending on the presence of an active gamma-secretase able to cleave a Notch-derived substrate. We selected clones deficient in gamma-secretase activity using this screen on mammalian cells after random mutagenesis. We further analyzed two of these clones and identified previously undescribed mutations in the nicastrin gene. The first mutation abolishes nicastrin production, and the second mutation, a point mutation in the ectodomain, abolishes nicastrin maturation. In both cases, gamma-secretase activity on Notch and APP is impaired.

beta-Secretase, APP and Abeta in Alzheimer's disease

Amyloid plaques, hallmark neuropathological lesions in Alzheimer's disease (AD) brain, are composed of the beta-amyloid peptide (Abeta). A large body of evidence suggests Abeta is central to the pathophysiology of AD and is likely to start this intractable neurodegenerative disorder. Mutations in three genes (amyloid precursor protein/APP, presenilin1, presenilin2) cause early on-set familial AD by increasing synthesis of the toxic 42 amino acid species of Abeta (Abeta42). Fibrillar Abeta in amyloid plaques appears to cause neurodegeneration, although recent studies suggest soluble Abeta oligomers may also be neurotoxic. Regardless, given the strong correlation between Abeta and AD, therapeutic strategies to lower cerebral Abeta levels should prove beneficial for the treatment of AD. Abeta is derived from APP via cleavage by two proteases, beta- and gamma-secretase. beta-secretase, recently identified as the novel aspartic protease BACEI, initiates the formation of Abeta. Consequently, BACE1 in principle is an excellent therapeutic target for strategies to reduce the production of Abeta in AD. However, the discovery of the homologue BACE2 raised the question of whether it too may be a beta-secretase. To settle this issue, our group and others have used gene targeting to generate BACE1 deficient (knockout) mice. These BACEI knockout mice have been instrumental in validating BACEI as the authentic beta-secretase in vivo. Here, I review the roles of BACE1, APP, and Abeta in AD and discuss the implications of therapeutic approaches that target BACE1 for the treatment of AD.

The role of Plasmodium falciparum food vacuole plasmepsins

Plasmepsins (PMs) are thought to have an important function in hemoglobin degradation in the malarial parasite Plasmodium falciparum and have generated interest as antimalarial drug targets. Four paralogous plasmepsins reside in the food vacuole of P. falciparum. Targeted gene disruption by double crossover homologous recombination has been employed to study food vacuole plasmepsin function in cultured parasites. Parasite clones with deletions in each of the individual PM I, PM II, and HAP genes as well as clones with a double PM IV/PM I disruption have been generated. All of these clones lack the corresponding PMs, are viable, and appear morphologically normal. PM II and PM IV/I disruptions have longer doubling times than the 3D7 parental line in rich RPMI medium. This appears to be because of a decreased level of productive progeny rather than an increased cell cycle time. In amino acid-limited medium, all four knockouts exhibit slower growth than the parental strain. Compared with 3D7, knock-out clone sensitivity to aspartic and cysteine protease inhibitors is changed minimally. These results suggest substantial functional redundancy and have important implications for the design of antimalarial drugs. The slow growth phenotype may explain why P. falciparum has maintained four plasmepsin genes with overlapping functions.

Signal peptide of eosinophil cationic protein is toxic to cells lacking signal peptide peptidase

Eosinophil cationic protein (ECP) is a toxin secreted by activated human eosinophils. The properties of mature ECP have been well studied but those of the signal peptide of ECP (ECPsp) are not clear. In this study, several chimeric proteins containing N-terminal fusion of ECPsp were generated, and introduced into Escherichia coli, Pichia pastoris, and human epidermoid carcinoma cell line A431 to study the function of ECPsp. We found that expression of ECPsp chimeric proteins inhibited the growth of E. coli and P. pastoris but not A431 cells. Primary sequence analysis and in vitro transcription/translation of ECPsp have revealed that it is a potential substrate for human signal peptide peptidase (hSPP), an intramembrane protease located in endoplasmic reticulum. In addition, knockdown of the hSPP mRNA expression in ECPsp-eGFP/A431 cells caused the growth inhibitory effect, whereas complementally expression of hSPP in P. pastoris system rescued the cell growth. Taken together, we have demonstrated that ECPsp is a toxic signal peptide, and expression of hSPP protects the cells from growth inhibition.

BACE (β-secretase) modulates the processing of APLP2 in vivo

BACE is an aspartyl protease that cleaves the amyloid precursor protein (APP) at the beta-secretase cleavage site and is involved in Alzheimer's disease. The aim of our study was to determine whether BACE affects the processing of the APP homolog APLP2. To this end, we developed BACE knockout mice with a targeted insertion of the gene for beta-galactosidase. BACE appeared to be exclusively expressed in neurons as determined by differential staining. BACE was expressed in specific areas in the cortex, hippocampus, cerebellum, pons, and spinal cord. APP processing was altered in the BACE knockouts with Abeta levels decreasing. The levels of APLP2 proteolytic products were decreased in BACE KO mice, but increased in BACE transgenic mice. Overexpression of BACE in cultured cells led to increased APLP2 processing. Our results strongly suggest that BACE is a neuronal protein that modulates the processing of both APP and APLP2.

BACE1 Deficiency Rescues Memory Deficits and Cholinergic Dysfunction in a Mouse Model of Alzheimer's Disease

beta-site APP cleaving enzyme 1 (BACE1) is the beta-secretase enzyme required for generating pathogenic beta-amyloid (Abeta) peptides in Alzheimer's disease (AD). BACE1 knockout mice lack Abeta and are phenotypically normal, suggesting that therapeutic inhibition of BACE1 may be free of mechanism-based side effects. However, direct evidence that BACE1 inhibition would improve cognition is lacking. Here we show that BACE1 null mice engineered to overexpress human APP (BACE1(-/-).Tg2576(+)) are rescued from Abeta-dependent hippocampal memory deficits. Moreover, impaired hippocampal cholinergic regulation of neuronal excitability found in the Tg2576 AD model is ameliorated in BACE1(-/-).Tg2576(+) bigenic mice. The behavioral and electrophysiological rescue of deficits in BACE1(-/-).Tg2576(+) mice is correlated with a dramatic reduction of cerebral Abeta40 and Abeta42 levels and occurs before amyloid deposition in Tg2576 mice. Our gene-based approach demonstrates that lower Abeta levels are beneficial for AD-associated memory impairments, validating BACE1 as a therapeutic target for AD.

BACE1 (β-secretase) transgenic and knockout mice: identification of neurochemical deficits and behavioral changes

BACE1 is a key enzyme in the generation of Abeta, the major component of senile plaques in the brains of Alzheimer's disease patients. We have generated transgenic mice expressing human BACE1 with the Cam Kinase II promoter driving neuronal-specific expression. The transgene contains the full-length coding sequence of human BACE1 preceding an internal ribosome entry site element followed by a LacZ reporter gene. These animals exhibit a bold, exploratory behavior and show elevated 5-hydroxytryptamine turnover. We have also generated a knockout mouse in which LacZ replaces the first exon of murine BACE1. Interestingly these animals show a contrasting behavior, being timid and less exploratory. Despite these clear differences both mouse lines are viable and fertile with no changes in morbidity. These results suggest an unexpected role for BACE1 in neurotransmission, perhaps through changes in amyloid precursor protein processing and Abeta levels.

BACE1 (beta-secretase) knockout mice do not acquire compensatory gene expression changes or develop neural lesions over time

The formation of Alzheimer's Abeta peptide is initiated when the amyloid precursor protein (APP) is cleaved by the enzyme beta-secretase (BACE1); inhibition of this cleavage has been proposed as a means of treating Alzheimer's disease. (AD) We have previously shown that young BACE1 knockout mice (BACE1 KO) do not generate Abeta but in other respects appear normal. Here we have extended this analysis to include both gene expression profiling and phenotypic assessment of older BACE1 KO animals to evaluate the impact of chronic Abeta deficiency. We did not detect global compensatory changes in neural gene expression in young BACE1 KO mice. In particular, expression of the beta-secretase homolog BACE2 was not upregulated. Furthermore, we found no structural alterations in any organ, including all central and peripheral neural tissues, of BACE1 KO mice up to 14 months of age. Aged BACE1 KO mice engineered to overexpress human APP (BACE1 KO/APPtg) did not develop amyloid plaques. These data provide evidence that neither beta-secretase nor Abeta plays a vital role in mouse physiology and that chronic beta-secretase inhibition could be a useful approach in treating AD.

Expression of a synthetic copy of the bovine chymosin gene in Aspergillus awamori from constitutive and pH-regulated promoters and secretion using two different pre-pro sequences

A copy of the bovine chymosin gene (chy) with a codon usage optimized for its expression in Aspergillus awamori was constructed starting from synthetic oligonucleotides. To study the ability of this filamentous fungus to secrete bovine prochymosin, two plasmids were constructed in which the transcriptional, translational, and secretory control regions of the A. nidulans gpdA gene and pepB genes were coupled to either preprochymosin or prochymosin genes. Secretion of a protein enzymatically and immunologically indistinguishable from bovine chymosin was achieved in A. awamori transformants with each of these constructions. In all cases, the primary translation product (40.5 kDa) was self-processed to a mature chymosin polypeptide having a molecular weight of 35.6 kDa. Immunological assays indicated that most of the chymosin was secreted to the extracellular medium. Hybridization analysis of genomic DNA from chymosin transformants showed chromosomal integration of prochymosin sequences and, in some transformants, multiple copies of the expression cassettes were observed. Expression from the gpdA promoter was constitutive, whereas expression from the pepB promoter was strongly influenced by pH. A very high expression from the pepB promoter was observed during the growth phase. The A. awamori pepB gene terminator was more favorable for chymosin production than the S. cerevisiae CYC1 terminator.

Beta-secretase as a target for the treatment of Alzheimer's disease

Finding inhibitors of the first step of the amyloid cascade, A beta (42) generation, is a major goal of Alzheimer's disease drug development. Two target protease activities, beta-and gamma-secretase, were detected more than 10 years ago but progress in this area has been slow because the enzymes were not identified. Using an expression cloning strategy we have identified a novel membrane bound aspartic protease, BACE1, as beta-secretase. The enzyme has been characterized in detail. The X-ray crystal structure, which is critical for rational inhibitor design, has been solved and shown to be similar to that of other pepsin family members. Our recent knockout studies show that BACE1 is critical for A beta generation, but the knockout mice show an otherwise normal phenotype, raising the possibility that therapeutic BACE1 inhibition could be accomplished without major mechanism based toxicity. Target-mediated toxicity of beta-secretase inhibition cannot be ruled out, however, as long as the major substrates of this enzyme are unknown. Although various peptidic beta-secretase inhibitors have been published, the key challenge now is the generation of more drug-like compounds that could be developed for therapeutic purposes. The focus of this review is progress in the beta-secretase field from the identification of the enzyme in 1999 to the most recent publications.

Specific aspartyl and calpain proteases are required for neurodegeneration in C. elegans

Necrotic cell death underlies the pathology of numerous human neurodegenerative conditions. In the nematode Caenorhabditis elegans, gain-of-function mutations in specific ion channel genes such as the degenerin genes deg-1 and mec-4, the acetylcholine receptor channel subunit gene deg-3 and the G(s) protein alpha-subunit gene gsa-1 evoke an analogous pattern of degenerative (necrotic-like) cell death in neurons that express the mutant proteins. An increase in concentrations of cytoplasmic calcium in dying cells, elicited either by extracellular calcium influx or by release of endoplasmic reticulum stores, is thought to comprise a major death-signalling event. But the biochemical mechanisms by which calcium triggers cellular demise remain largely unknown. Here we report that neuronal degeneration inflicted by various genetic lesions in C. elegans requires the activity of the calcium-regulated CLP-1 and TRA-3 calpain proteases and aspartyl proteases ASP-3 and ASP-4. Our findings show that two distinct classes of proteases are involved in necrotic cell death and suggest that perturbation of intracellular concentrations of calcium may initiate neuronal degeneration by deregulating proteolysis. Similar proteases may mediate necrotic cell death in humans.