Inhibition of amyloid beta-peptide production by blockage of beta-secretase cleavage site of amyloid precursor protein

Amyloid beta-peptide (Abeta) is implicated as the major causative agent in Alzheimer's disease (AD). Abeta is produced by the processing of the amyloid precursor protein (APP) by BACE1 (beta-secretase) and gamma-secretase. Many inhibitors have been developed for the secretases. However, the inhibitors will interfere with the processing of not only APP but also of other secretase substrates. In this study, we describe the development of inhibitors that prevent production of Abeta by specific binding to the beta-cleavage site of APP. We used the hydropathic complementarity (HC) approach for the design of short peptide inhibitors. Some of the HC peptides were bound to the substrate peptide (Sub W) corresponding to the beta-cleavage site of APP and blocked its cleavage by recombinant human BACE1 (rhBACE1) in vitro. In addition, HC peptides specifically inhibited the cleavage of Sub W, and not affecting other BACE1 substrates. Chemical modification allowed an HC peptide (CIQIHF) to inhibit the processing of APP as well as the production of Abeta in the treated cells. Such novel APP-specific inhibitors will provide opportunity for the development of drugs that can be used for the prevention and treatment of AD with minimal side effects.     

Inhibition of glycosphingolipid biosynthesis reduces secretion of the beta-amyloid precursor protein and amyloid beta-peptide

Alzheimer disease is associated with extracellular deposits of amyloid beta-peptides in the brain. Amyloid beta-peptides are generated by proteolytic processing of the beta-amyloid precursor protein by beta- and gamma-secretases. The cleavage by secretases occurs predominantly in post-Golgi secretory and endocytic compartments and is influenced by cholesterol, indicating a role of the membrane lipid composition in proteolytic processing of the beta-amyloid precursor protein. To analyze the role of glycosphingolipids in these processes we inhibited glycosyl ceramide synthase, which catalyzes the first step in glycosphingolipid biosynthesis. The depletion of glycosphingolipids markedly reduced the secretion of endogenous beta-amyloid precursor protein in different cell types, including human neuroblastoma SH-SY5Y cells. Importantly, secretion of amyloid beta-peptides was also strongly decreased by inhibition of glycosphingolipid biosynthesis. Conversely, the addition of exogenous brain gangliosides to cultured cells reversed these effects. Biochemical and cell biological experiments demonstrate that the pharmacological reduction of cellular glycosphingolipid levels inhibited maturation and cell surface transport of the beta-amyloid precursor protein. In the glycosphingolipid-deficient cell line GM95, cellular levels and maturation of beta-amyloid precursor protein were also significantly reduced as compared with normal B16 cells. Together, these data demonstrate that glycosphingolipids are implicated in the regulation of the subcellular transport of the beta-amyloid precursor protein in the secretory pathway and its proteolytic processing. Thus, enzymes involved in glycosphingolipid metabolism might represent targets to inhibit the production of amyloid beta-peptides.     

Ceramide stabilizes beta-site amyloid precursor protein-cleaving enzyme 1 and promotes amyloid beta-peptide biogenesis

The lipid second messenger ceramide regulates several biochemical events that occur during aging. In addition, its level is highly elevated in the amyloid-burdened brains of Alzheimer's disease patients. Here, we analyzed the impact of aberrant ceramide levels on amyloid beta-peptide (Abeta) generation by using a cell-permeable analog of ceramide, C6-ceramide, and several biochemical inhibitors of the sphingomyelin/glycosphingolipid biosynthetic pathway. We found that C6-ceramide increased the biogenesis of Abeta by affecting beta-but not gamma-cleavage of the amyloid precursor protein. Similarly to C6-ceramide, increased levels of endogenous ceramide induced by neutral sphingomyelinase treatment also promoted the biogenesis of Abeta. Conversely, fumonisin B1, which inhibits the biosynthesis of endogenous ceramide, reduced Abeta production. Exogenous C6-ceramide restored both intracellular ceramide levels and Abeta generation in fumonisin B1-treated cells. These events were specific for amyloid precursor protein and were not associated with apoptotic cell death. Pulse-chase and time-course degradation experiments showed that ceramide post-translationally stabilizes the beta-secretase BACE1. Taken together, these data indicate that the lipid second messenger ceramide, which is elevated in the brains of Alzheimer's disease patients, increases the half-life of BACE1 and thereby promotes Abeta biogenesis.     

Axonal amyloid precursor protein and its fragments undergo somatodendritic endocytosis and processing

Deposition of potentially neurotoxic Aβ fragments derived from amyloid precursor protein (APP) at synapses may be a key contributor to Alzheimer''s disease. However, the location(s) of proteolytic processing and subsequent secretion of APP fragments from highly compartmentalized, euploid neurons that express APP and processing enzymes at normal levels is not well understood. To probe the behavior of endogenous APP, particularly in human neurons, we developed a system using neurons differentiated from human embryonic stem cells, cultured in microfluidic devices, to enable direct biochemical measurements from axons. Using human or mouse neurons in these devices, we measured levels of Aβ, sAPPα, and sAPPβ secreted solely from axons. We found that a majority of the fragments secreted from axons were processed in the soma, and many were dependent on somatic endocytosis for axonal secretion. We also observed that APP and the β-site APP cleaving enzyme were, for the most part, not dependent on endocytosis for axonal entry. These data establish that axonal entry and secretion of APP and its proteolytic processing products traverse different pathways in the somatodendritic compartment before axonal entry.     

The intracellular domain of amyloid precursor protein interacts with FKBP12

To elucidate the roles of the APP intracellular domain (AICD) in the development of Alzheimer's disease, a yeast two-hybrid system was used to screen for AICD-interacting proteins. Our result revealed that FKBP12, an immunophilin with a peptidyl-prolyl cis-trans isomerase (PPIase) activity, may interact with AICD. This interaction was confirmed by coimmunoprecipitation studies. FKBP12 has been shown to be expressed at a higher level in areas of pathology of patients with neurodegenerative diseases. In addition, Pin1, a member of another PPIase family, has been suggested to be involved in the amyloidogenic APP processing and Abeta production. The interaction between FKBP12 and AICD might hint at a possible role FKBP12 plays, probably in a fashion similar to Pin1, in the amyloidogenesis of APP. We also found that the interaction was interfered, in a dose-dependent manner, by FK506, whose neuroprotective effect has been suggested to be correlated with its PPIase inhibitory activity.     

Equimolar production of amyloid beta-protein and amyloid precursor protein intracellular domain from beta-carboxyl-terminal fragment by gamma-secretase

We showed previously that cells expressing wild-type (WT) beta-amyloid precursor protein (APP) or coexpressing WTAPP and WT presenilin (PS) 1/2 produced APP intracellular domains (AICD) 49-99 and 50-99, with the latter predominating. On the other hand, the cells expressing mutant (MT) APP or coexpressing WTAPP and MTPS1/2 produced a greater proportion of AICD-(49-99) than AICD-(50-99). In addition, the expression of amyloid beta-protein (Abeta) 49 in cells resulted in predominant production of Abeta40 and that of Abeta48 leads to preferential production of Abeta42. These observations suggest that epsilon-cleavage and gamma-cleavage are interrelated. To determine the stoichiometry between Abeta and AICD, we have established a 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonic acid-solubilized gamma-secretase assay system that exhibits high specific activity. By using this assay system, we have shown that equal amounts of Abeta and AICD are produced from beta-carboxyl-terminal fragment (C99) by gamma-secretase, irrespective of WT or MTAPP and PS1/2. Although various Abeta species, including Abeta40, Abeta42, Abeta43, Abeta45, Abeta48, and Abeta49, are generated, only two species of AICD, AICD-(49-99) and AICD-(50-99), are detected. We also have found that M233T MTPS1 produced only one species of AICD, AICD-(49-99), and only one for its counterpart, Abeta48, in contrast to WT and other MTPS1s. These strongly suggest that epsilon-cleavage is the primary event, and the produced Abeta48 and Abeta49 rapidly undergo gamma-cleavage, resulting in generation of various Abeta species.     

Metalloprotease meprin beta generates nontoxic N-terminal amyloid precursor protein fragments in vivo

Identification of physiologically relevant substrates is still the most challenging part in protease research for understanding the biological activity of these enzymes. The zinc-dependent metalloprotease meprin β is known to be expressed in many tissues with functions in health and disease. Here, we demonstrate unique interactions between meprin β and the amyloid precursor protein (APP). Although APP is intensively studied as a ubiquitously expressed cell surface protein, which is involved in Alzheimer disease, its precise physiological role and relevance remain elusive. Based on a novel proteomics technique termed terminal amine isotopic labeling of substrates (TAILS), APP was identified as a substrate for meprin β. Processing of APP by meprin β was subsequently validated using in vitro and in vivo approaches. N-terminal APP fragments of about 11 and 20 kDa were found in human and mouse brain lysates but not in meprin β(-/-) mouse brain lysates. Although these APP fragments were in the range of those responsible for caspase-induced neurodegeneration, we did not detect cytotoxicity to primary neurons treated by these fragments. Our data demonstrate that meprin β is a physiologically relevant enzyme in APP processing.     

Lead-induced morphological changes and amyloid precursor protein accumulation in adult rat hippocampus

Lead is an important environmental pollutant that exerts potent toxic effects on many organs. The toxic effects of lead are less well known for adult brain than for children. We investigated the morphological changes and amyloid precursor protein (APP) accumulation in the adult rat hippocampus following exposure to lead. Forty rats were divided into two groups of 20. One group was exposed to 580 parts per million (ppm) lead acetate and other group to an identical concentration of sodium acetate as a control group. After exposure to lead for 3 months, the hippocampus was examined by electron microscopy and APP levels in the hippocampus were detected using immunohistochemistry. Lead levels in the blood of rats exposed to lead were significantly higher than in the controls. The morphological changes in the hippocampus included mitochondrial degeneration, apoptosis and abnormal synapses in the rats exposed to lead. APP in hippocampus was increased significantly in the group exposed to lead compared to controls. We determined that lead exposure causes accumulation of APP and morphological changes in the adult rat hippocampus.     

Truncated carboxyl-terminal fragments of beta-amyloid precursor protein are processed to amyloid beta-proteins 40 and 42

We previously showed that beta-amyloid precursor protein (APP) is cleaved not only in the middle of the membrane (gamma-cleavage) but also at novel cleavage sites close to the membrane/cytoplasmic boundary (epsilon-cleavage), releasing APP intracellular domains (AICDs) 49-99 and 50-99. To learn more about the relationship between gamma- and epsilon-cleavage, C-terminally truncated carboxyl-terminal fragments (CTFs) of APP, especially CTFs1-48 and 1-49 (the postulated products that are generated by epsilon-cleavage), were transiently expressed in CHO cells. Most importantly, the cells expressing CTF1-49 secreted predominantly amyloid beta-protein (Abeta) 40, while those expressing CTF1-48 secreted preferentially Abeta42. This supports our assumption that epsilon-cleavage precedes Alphabeta production and that preceding epsilon-cleavage determines the preference for the final Abeta species. The gamma-secretase inhibitors, L-685,458 and DAPT, suppressed Abeta production from CTF1-49. Regarding Abeta production from CTF1-48, L-685,458 suppressed it, but DAPT failed to do so. A dominant negative mutant of presenilin 1 suppressed the production of Abeta40 and 42 from both CTFs1-48 and 1-49. These data should shed significant light into the mechanism of Abeta production.     

Calcium-mediated transient phosphorylation of tau and amyloid precursor protein followed by intraneuronal amyloid-beta accumulation

Intraneuronal accumulation of hyperphosphorylated protein tau in paired helical filaments together with amyloid-beta peptide (Abeta) deposits confirm the clinical diagnosis of Alzheimer disease. A common cellular mechanism leading to the production of these potent toxins remains elusive. Here we show that, in cultured neurons, membrane depolarization induced a calcium-mediated transient phosphorylation of both microtubule-associated protein tau and amyloid precursor protein (APP), followed by a dephosphorylation of these proteins. Phosphorylation was mediated by glycogen synthase kinase 3 and cyclin-dependent kinase 5 protein kinases, while calcineurin was responsible for dephosphorylation. Following the transient phosphorylation of APP, intraneuronal Abeta accumulated and induced neurotoxicity. Phosphorylation of APP on Thr-668 was indispensable for intraneuronal accumulation of Abeta. Our data demonstrate that an increase in cytosolic calcium concentration induces modifications of neuronal metabolism of APP and tau, similar to those found in Alzheimer disease.     

Membrane microdomain switching: a regulatory mechanism of amyloid precursor protein processing

Neuronal activity has an impact on β cleavage of amyloid precursor protein (APP) by BACE1 to generate amyloid-β peptide (Aβ). However, the molecular mechanisms underlying this effect remain to be elucidated. Cholesterol dependency of β cleavage prompted us to analyze immunoisolated APP-containing detergent-resistant membranes from rodent brains. We found syntaxin 1 as a key molecule for activity-dependent regulation of APP processing in cholesterol-dependent microdomains. In living cells, APP associates with syntaxin 1–containing microdomains through X11–Munc18, which inhibits the APP–BACE1 interaction and β cleavage via microdomain segregation. Phosphorylation of Munc18 by cdk5 causes a shift of APP to BACE1-containing microdomains. Neuronal hyperactivity, implicated in Aβ overproduction, promotes the switching of APP microdomain association as well as β cleavage in a partially cdk5-dependent manner. We propose that microdomain switching is a mechanism of cholesterol- and activity-dependent regulation of APP processing in neurons.     

Release of amino-terminal fragments from amyloid precursor protein reporter and mutated derivatives in cultured cells.

Abnormal proteolytic processing of amyloid precursor protein (APP) is thought to be central to the formation and deposition of beta amyloid peptide in Alzheimer's disease. A putative "secretase" activity normally releases an amino-terminal APP fragment by cleaving APP at residues within the beta amyloid peptide thereby precluding amyloidogenesis. In order to better understand the requirements for APP cleavage by secretase, we have expressed a modified cDNA construct representing the 751-amino acid isoform of APP (APP-REP) and mutated APP-REP proteins in cultured cells. Here, we show that: (a) APP-REP is predominantly associated with membranes; (b) intracellular turnover and processing of APP-REP is similar to that reported for the intact APP protein; (c) secretion appears unaltered by introduction of the glutamate to glutamine mutation found in the APP gene of patients suffering from hereditary cerebral hemorrhage with amyloidosis of Dutch origin; (d) a mutation in which the 18 juxtamembranous amino acids encompassing the secretase site are deleted also allows release of an amino-terminal fragment into the conditioned medium; and (e) kinetics of cleavage of APP-REP and its mutated derivatives are similar. These results indicate that the secretory cleavage of the extracellular amino-terminal fragments of APP-REP can occur in the presence of different novel juxtamembranous amino acid sequences.     

Retention of the Alzheimer's amyloid precursor fragment C99 in the endoplasmic reticulum prevents formation of amyloid beta-peptide

gamma-Secretase is a membrane-associated endoprotease that catalyzes the final step in the processing of Alzheimer's beta-amyloid precursor protein (APP), resulting in the release of amyloid beta-peptide (Abeta). The molecular identity of gamma-secretase remains in question, although recent studies have implicated the presenilins, which are membrane-spanning proteins localized predominantly in the endoplasmic reticulum (ER). Based on these observations, we have tested the hypothesis that gamma-secretase cleavage of the membrane-anchored C-terminal stump of APP (i.e. C99) occurs in the ER compartment. When recombinant C99 was expressed in 293 cells, it was localized mainly in the Golgi apparatus and gave rise to abundant amounts of Abeta. Co-expression of C99 with mutant forms of presenilin-1 (PS1) found in familial Alzheimer's disease resulted in a characteristic elevation of the Abeta(42)/Abeta(40) ratio, indicating that the N-terminal exodomain of APP is not required for mutant PS1 to influence the site of gamma-secretase cleavage. Biogenesis of both Abeta(40) and Abeta(42) was almost completely eliminated when C99 was prevented from leaving the ER by addition of a di-lysine retention motif (KKQN) or by co-expression with a dominant-negative mutant of the Rab1B GTPase. These findings indicate that the ER is not a major intracellular site for gamma-secretase cleavage of C99. Thus, by inference, PS1 localized in this compartment does not appear to be active as gamma-secretase. The results suggest that presenilins may acquire the characteristics of gamma-secretase after leaving the ER, possibly by assembling with other proteins in peripheral membranes.