The proteolytic processing of the amyloid precursor protein gene family members APLP-1 and APLP-2 involves alpha-, beta-, gamma-, and epsilon-like cleavages: modulation of APLP-1 processing by n-glycosylation

APPepsilon, the epsilon-secretase-derived N-terminal product of the beta-amyloid precursor protein, behaves as a type I protein and undergoes alpha-, beta-, and gamma-secretase cleavages

beta-Amyloid peptide accumulates in the brain of patients affected by sporadic or familial forms of Alzheimer's disease. It derives from the proteolytic attacks of the beta-amyloid precursor protein (betaAPP) by beta- and gamma-secretase activities. An additional epsilon cleavage taking place a few residues C-terminal to the gamma-site has been reported, leading to the formation of an intracellular fragment referred to as APP intracellular domain C50. This epsilon cleavage received particular attention because it resembles the S3 Notch cleavage generating Notch intracellular domain. Indeed, APP intracellular domain, like its Notch counterpart, appears to mediate important physiological functions. gamma and epsilon cleavages on betaAPP appear spatio-temporally linked but pharmacologically distinct and discriminable by mutagenesis approaches. As these cleavages could be seen as either deleterious (gamma-site) or beneficial (epsilon-site), it appears of most interest to set up models aimed at studying these activities separately, particularly to design specific and bioavailable inhibitors. On the other hand, it is important to respect the topology of the substrates in order to examine physiologically relevant cleavages. Here we describe the obtention of cells overexpressing APPepsilon, the epsilon-secretase-derived N-terminal fragment of betaAPP. Interestingly, this N-terminal fragment of betaAPP was shown by biochemical and immunohistochemical approaches to behave as a genuine membrane-bound protein. APPepsilon undergoes constitutive and protein kinase C-regulated alpha-secretase cleavages. Furthermore, APPepsilon is targeted by the beta-secretase beta-site APP-cleaving enzyme and is subsequently cleaved by gamma-secretase. The resulting beta-amyloid peptide production is fully prevented by various gamma-secretase inhibitors. Altogether, our study shows that APPepsilon is a relevant betaAPP derivative to study gamma-secretase activities and to design specific inhibitors without facing any rate-limiting effect of epsilon-secretase-derived cleavage.

Energy inhibition elevates beta-secretase levels and activity and is potentially amyloidogenic in APP transgenic mice: possible early events in Alzheimer's disease pathogenesis

Beta-secretase [beta-site amyloid precursor protein-cleaving enzyme 1 (BACE1)] is the key rate-limiting enzyme for the production of the beta-amyloid (Abeta) peptide involved in the pathogenesis of Alzheimer's disease (AD). BACE1 levels and activity are increased in AD brain and are likely to drive Abeta overproduction, but the cause of BACE1 elevation in AD is unknown. Interestingly, cerebral glucose metabolism and blood flow are both reduced in preclinical AD, suggesting that impaired energy production may be an early pathologic event in AD. To determine whether reduced energy metabolism would cause BACE1 elevation, we used pharmacological agents (insulin, 2-deoxyglucose, 3-nitropropionic acid, and kainic acid) to induce acute energy inhibition in C57/B6 wild-type and amyloid precursor protein (APP) transgenic (Tg2576) mice. Four hours after treatment, we observed that reduced energy production caused a approximately 150% increase of cerebral BACE1 levels compared with control. Although this was a modest increase, the effect was long-lasting, because levels of the BACE1 enzyme remained elevated for at least 7 d after a single dose of energy inhibitor. In Tg2576 mice, levels of the BACE1-cleaved APP ectodomain APPsbeta were also elevated and paralleled the BACE1 increase in both relative amount and duration. Importantly, cerebral Abeta40 levels in Tg2576 were increased to approximately 200% of control at 7 d after injection, demonstrating that energy inhibition was potentially amyloidogenic. These results support the hypothesis that impaired energy production in the brain may drive AD pathogenesis by elevating BACE1 levels and activity, which, in turn, lead to Abeta overproduction. This process may represent one of the earliest pathogenic events in AD.

Development of beta-amyloid-induced neurodegeneration in Alzheimer's disease and novel neuroprotective strategies

Alzheimer's disease (AD) is a form of dementia in which people develop rapid neurodegeneration, complete loss of cognitive abilities, and are likely to die prematurely. At present, no treatment for AD is known. One of the hallmarks in the development of AD is the aggregation of amyloid protein fragments in the brain, and much evidence points towards beta-amyloid fragments being one of the main causes of the neurodegenerative processes. This review summarises the present concepts and theories on how AD develops, and lists the evidence that supports them. A cascade of biochemical events is initiated that ultimately leads to neuronal death involving an imbalance of intracellular calcium homeostasis via activation of calcium channels, intracellular calcium stores, and subsequent production of free radicals by calcium-sensitive enzymes. Secondary processes include inflammatory responses that produce more free radicals and the induction of apoptosis. Recently, several new strategies have been proposed to try to ameliorate the neurodegenerative developments associated with AD. These include the activation of neuronal growth factor receptors and insulin-like receptors, both of which have neuroprotective properties. Furthermore, the role of cholesterol and potential protective properties of cholesterol-lowering drugs are under intense investigation. Other promising strategies include the inhibition of beta- and gamma-secretases which produce beta-amyloid, activation of proteases that degrade beta-amyloid, glutamate receptor selective drugs, antioxidants, and metal chelating agents, all of which prevent formation of plaques. Novel drugs that act at different levels of the neurodegenerative processes show great promise to reduce neurodegeneration. They could help to prolong the time of unimpaired cognitive abilities of people who develop AD, allowing them to lead an independent life.

Shedding of the amyloid precursor protein-like protein APLP2 by disintegrin-metalloproteinases

Cleavage of the amyloid precursor protein (APP) within the amyloid-beta (Abeta) sequence by the alpha-secretase prevents the formation of toxic Abeta peptides. It has been shown that the disintegrin-metalloproteinases ADAM10 and TACE (ADAM17) act as alpha-secretases and stimulate the generation of a soluble neuroprotective fragment of APP, APPsalpha. Here we demonstrate that the related APP-like protein 2 (APLP2), which has been shown to be essential for development and survival of mice, is also a substrate for both proteinases. Overexpression of either ADAM10 or TACE in HEK293 cells increased the release of neurotrophic soluble APLP2 severalfold. The strongest inhibition of APLP2 shedding in neuroblastoma cells was observed with an ADAM10-preferring inhibitor. Transgenic mice with neuron-specific overexpression of ADAM10 showed significantly increased levels of soluble APLP2 and its C-terminal fragments. To elucidate a possible regulatory mechanism of APLP2 shedding in the neuronal context, we examined retinoic acid-induced differentiation of neuroblastoma cells. Retinoic acid treatment of two neuroblastoma cell lines upregulated the expression of both APLP2 and ADAM10, thus leading to an increased release of soluble APLP2.

Amyloid precursor-like protein 1 influences endocytosis and proteolytic processing of the amyloid precursor protein

Ectodomain shedding of the amyloid precursor protein (APP) is a key regulatory step in the generation of the Alzheimer disease amyloid beta peptide (Abeta). The molecular mechanisms underlying the control of APP shedding remain little understood but are in part dependent on the low density lipoprotein receptor-related protein (LRP), which is involved in APP endocytosis. Here, we show that the APP homolog APLP1 (amyloid precursor-like protein 1) influences APP shedding. In human embryonic kidney 293 cells expression of APLP1 strongly activated APP shedding by alpha-secretase and slightly reduced beta-secretase cleavage. As revealed by domain deletion analysis, the increase in APP shedding required the NPTY amino acid motif within the cytoplasmic domain of APLP1. This motif is conserved in APP and is essential for the endocytosis of APP and APLP1. Unrelated membrane proteins containing similar endocytic motifs did not affect APP shedding, showing that the increase in APP shedding was specific to APLP1. In LRP-deficient cells APLP1 no longer induced APP shedding, suggesting that in wild-type cells APLP1 interferes with the LRP-dependent endocytosis of APP and there by increases APP alpha-cleavage. In fact, an antibody uptake assay revealed that expression of APLP1 reduced the rate of APP endocytosis. In summary, our study provides a novel mechanism for APP shedding, in which APLP1 affects the endocytosis of APP and makes more APP available for alpha-secretase cleavage.

siRNA targeted against amyloid precursor protein impairs synaptic activity in vivo

The amyloid precursor protein (APP) plays a central role in Alzheimer's disease (AD) pathogenesis through its cleavage leading to the accumulation of the peptide betaA4. Diffusible oligomeric assemblies of amyloid beta peptide are thought to induce synaptic dysfunction, an early change in AD. We tested the hypothesis that a reduction in presynaptic APP could itself lead to a decrease in synaptic efficacy in vivo. Twenty-four hours after intraocular injection, siRNA targeted against APP accumulated in retinal cells and the APP in retinal terminals in the superior colliculus was significantly reduced. Surprisingly, the amyloid precursor-like protein 2 (APLP2) was reduced as well. Functional imaging experiments in rats during visual stimulation showed that knockdown of presynaptic APP/APLP2 significantly reduced the stimulation-induced glucose utilization in the superior colliculus. Our results suggest that perturbations in the amount of APP/APLP2 axonally transported to, and/or in their turnover in the nerve terminal alter synaptic function and could be a pathogenic mechanism in AD.

Stabilization of ubiquitous mitochondrial creatine kinase preprotein by APP family proteins

Amyloid precursor protein (APP) is involved in the pathogenesis of Alzheimer's disease (AD). However, the physiological role of APP and its family members is still unclear. To gain insights into APP function, we used a proteomic approach to identify APP interacting proteins. We report here for the first time a direct interaction between the C-terminal region of APP family proteins and ubiquitous mitochondrial creatine kinase (uMtCK). This interaction was confirmed in vitro as well as in cultured cells and in brain. Interestingly, expression of full-length and C-terminal domain of APP family proteins stabilized uMtCK preprotein in cultured cells. Our data suggest that APP may regulate cellular energy levels and mitochondrial function via a direct interaction and stabilization of uMtCK.

Increased processing of APLP2 and APP with concomitant formation of APP intracellular domains in BDNF and retinoic acid-differentiated human neuroblastoma cells

The amyloid precursor protein (APP) belongs to a conserved gene family, also including the amyloid precursor-like proteins, APLP1 and APLP2. We have previously shown that all members of the APP protein family are up-regulated upon retinoic acid (RA)-induced neuronal differentiation of SH-SY5Y neuroblastoma cells. Here, we demonstrate that RA also affects the processing of APLP2 and APP, as shown by increased shedding of both sAPLP2 and sAPPalpha, as well as elevated levels of the APP intracellular domains (AICDs). Brain-derived neurotrophic factor (BDNF) has been reported to induce APP promoter activity and RA induces expression of the tyrosine kinase receptor B (TrkB) in neuroblastoma cells. We show that the increase in shedding of both APLP2 and APP in response to RA is not mediated through the TrkB receptor. However, BDNF concomitant with RA increased the expression of APP even further. In addition, the secretion of sAPLP2 and sAPPalpha as well as the levels of AICDs were increased in response to BDNF. In contrast, the levels of membrane-bound APP C-terminal fragment C99 significantly decreased. Our results suggest that RA and BDNF shifts APP processing towards the alpha-secretase pathway. In addition, we show that RA and BDNF regulate N-linked glycosylation of APLP1.

The amyloid-β precursor-like protein APLP2 and its relative APP are differentially regulated during neuroendocrine cell activation

The amyloid-beta precursor-like protein APLP2 is structurally and functionally related to the amyloid-beta precursor protein APP, the protein generally accepted to be involved in Alzheimer's disease. Since we previously observed that the levels of APP mRNA and protein were up-regulated threefold in the active intermediate pituitary melanotrope cells of black-adapted Xenopus laevis, we now decided to study the regulation of APLP2 in these physiologically inducible neuroendocrine cells. Interestingly, both the mRNA and protein levels of Xenopus APLP2 were similar in the melanotrope cells of black and white frogs. Newly synthesized APLP2 became glycosylated and sulfated, chondroitin sulfate glycosaminoglycan chains were added, and eventually the protein was proteolytically cleaved. Unlike for APP, no phosphorylated APLP2 was observed. Our results show that, although APP and APLP2 are thought to be functionally related, their responses to neuroendocrine cell activation differ, suggesting distinct roles for these proteins.

Altered APP Processing in PDAPP (Val717 → Phe) Transgenic Mice Yields Extended-Length Aβ Peptides†

Central to the pathology of Alzheimer's disease (AD) is the profuse accumulation of amyloid-beta (Abeta) peptides in the brain of affected individuals, and several amyloid precursor protein (APP) transgenic (Tg) mice models have been created to mimic Abeta deposition. Among these, the PDAPP Tg mice carrying the familial AD APP 717 Val --> Phe mutation have been widely used to test potential AD therapeutic interventions including active and passive anti-Abeta immunizations. The structure and biochemistry of the PDAPP Tg mice Abeta-related peptides were investigated using acid and detergent lysis of brain tissue, ultracentrifugation, FPLC, HPLC, enzymatic and chemical cleavage of peptides, Western blot, immunoprecipitation, and MALDI-TOF and SELDI-TOF mass spectrometry. Our experiments reveal that PDAPP mice produce a variety of C-terminally elongated Abeta peptides in addition to Abeta n-40 and Abeta n-42, as well as N-terminally truncated peptides, suggesting anomalous proteolysis of both APP and Abeta. Important alterations in the overall APP degradation also occur in this model, resulting in a striking comparative lack of CT83 and CT99 fragments, which may be inherent to the strain of mice, a generalized gamma-secretase failure, or the ultimate manifestation of the overwhelming amount of expressed human transgene; these alterations are not observed in other strains of APP Tg mice or in sporadic AD. Understanding at the molecular level the nature of these important animal models will permit a better understanding of therapeutic interventions directed to prevent, delay, or reverse the ravages of sporadic AD.

Homo- and heterodimerization of APP family members promotes intercellular adhesion

The amyloid precursor protein (APP) plays a central role in Alzheimer's disease, but its physiological function and that of its mammalian paralogs, the amyloid precursor-like proteins 1 and 2 (APLPs), is still poorly understood. APP has been proposed to form dimers, a process that could promote cell adhesion via trans-dimerization. We investigated the dimerization and cell adhesion properties of APP/APLPs and provide evidence that all three paralogs are capable of forming homo- and heterocomplexes. Moreover, we show that trans-interaction of APP family proteins promotes cell-cell adhesion in a homo- and heterotypic fashion and that endogenous APLP2 is required for cell-cell adhesion in mouse embryonic fibroblasts. We further demonstrate interaction of all the three APP family members in mouse brain, genetic interdependence, and molecular interaction of APP and APLPs in synaptically enriched membrane compartments. Together, our results provide evidence that homo- and heterocomplexes of APP/APLPs promote trans-cellular adhesion in vivo.

Genetic and molecular aspects of Alzheimer's disease shed light on new mechanisms of transcriptional regulation

Rapid advances made in biological research aimed at understanding the molecular basis of the pathogenesis of Alzheimer's disease have led to the characterization of a novel catalytic activity termed gamma-secretase. First described for its beta-amyloid-producing function, gamma-secretase is now actively studied for its role in a novel signal transduction paradigm, which implicates cell-surface receptor proteolysis and direct surface-to-nucleus signal transduction. gamma-Secretase targets numerous type I protein receptors involved in diverse functions ranging from normal development to neurodegeneration. In this Review we discuss how the study of the genetic and molecular aspects of Alzheimer's disease has revealed a dual role of gamma-secretase in transcriptional regulation and in the pathogenesis of familial Alzheimer's disease.

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.

Phosphorylation of amyloid precursor carboxy-terminal fragments enhances their processing by a gamma-secretase-dependent mechanism

In Alzheimer's disease, the complex catabolism of amyloid precursor protein (APP) leads to the production of amyloid-beta (Abeta) peptide, the major component of amyloid deposits. APP is cleaved by beta- and alpha-secretases to generate APP carboxy-terminal fragments (CTFs). Abeta peptide and amyloid intracellular domain are resulting from the cleavage of APP-CTFs by the gamma-secretase. In the present study, we hypothesize that post-translational modification of APP-CTFs could modulate their processing by the gamma-secretase. Inhibition of the gamma-secretase was shown to increase the total amount of APP-CTFs. Moreover, we showed that this increase was more marked among the phosphorylated variants and directly related to the activity of the gamma-secretase, as shown by kinetics analyses. Phosphorylated CTFs were shown to associate to presenilin 1, a major protein of the gamma-secretase complex. The phosphorylation of CTFs at the threonine 668 resulting of the c-Jun N-terminal kinase activation was shown to enhance their degradation by the gamma-secretase. Altogether, our results demonstrated that phosphorylated CTFs can be the substrates of the gamma-secretase and that an increase in the phosphorylation of APP-CTFs facilitates their processing by gamma-secretase.

beta Subunits of voltage-gated sodium channels are novel substrates of beta-site amyloid precursor protein-cleaving enzyme (BACE1) and gamma-secretase

Sequential processing of amyloid precursor protein (APP) by membrane-bound proteases, BACE1 and gamma-secretase, plays a crucial role in the pathogenesis of Alzheimer disease. Much has been discovered on the properties of these proteases; however, regulatory mechanisms of enzyme-substrate interaction in neurons and their involvement in pathological changes are still not fully understood. It is mainly because of the membrane-associated cleavage of these proteases and the lack of information on new substrates processed in a similar way to APP. Here, using RNA interference-mediated BACE1 knockdown, mouse embryonic fibroblasts that are deficient in either BACE1 or presenilins, and BACE1-deficient mouse brain, we show clear evidence that beta subunits of voltage-gated sodium channels are sequentially processed by BACE1 and gamma-secretase. These results may provide new insights into the underlying pathology of Alzheimer disease.

Got RIP? Presenilin-dependent intramembrane proteolysis in growth factor receptor signaling

A number of cell surface growth factor receptors are subject to presenilin-dependent regulated intramembrane proteolysis (PS-RIP) after ligand binding and/or ectodomain cleavage. PS-RIP is mediated by a highly conserved multi-component membrane-bound protease, termed gamma-secretase, responsible for generating Alzheimer's disease (AD)-associated Abeta peptide from its membrane-bound beta-amyloid precursor protein (APP), as well as for cleaving a number of other type-I membrane receptors. PS-RIP is a conserved cellular process by which cells transmit signals from one compartment to another, including the liberation of membrane-bound transcription factors. Recent studies indicate that PS-RIP also mediates the proteolytic inactivation of heteromeric receptor complexes by removing the transmembrane domains required for receptor-receptor interaction. Thus, PS-RIP appears to regulate diverse cellular pathways either by generating soluble effectors from membrane-bound precursors, or by removing the transmembrane domain of a membrane-tethered signaling component.

Cortical dysplasia resembling human type 2 lissencephaly in mice lacking all three APP family members

The Alzheimer's disease beta-amyloid precursor protein (APP) is a member of a larger gene family that includes the amyloid precursor-like proteins, termed APLP1 and APLP2. We previously documented that APLP2-/-APLP1-/- and APLP2-/-APP-/- mice die postnatally, while APLP1-/-APP-/- mice and single mutants were viable. We now report that mice lacking all three APP/APLP family members survive through embryonic development, and die shortly after birth. In contrast to double-mutant animals with perinatal lethality, 81% of triple mutants showed cranial abnormalities. In 68% of triple mutants, we observed cortical dysplasias characterized by focal ectopic neuroblasts that had migrated through the basal lamina and pial membrane, a phenotype that resembles human type II lissencephaly. Moreover, at E18.5 triple mutants showed a partial loss of cortical Cajal Retzius (CR) cells, suggesting that APP/APLPs play a crucial role in the survival of CR cells and neuronal adhesion. Collectively, our data reveal an essential role for APP family members in normal brain development and early postnatal survival.