Intracellular trafficking of the β-secretase and processing of amyloid precursor protein

The main component of the amyloid plaques found in the brains of those with Alzheimer's disease (AD) is a polymerized form of the β-amyloid peptide (Aβ) and is considered to play a central role in the pathogenesis of this neurodegenerative disorder. Aβ is derived from the proteolytic processing of the amyloid precursor protein (APP). Beta site APP-cleaving enzyme, BACE1 (also known as β-secretase) is a membrane-bound aspartyl protease responsible for the initial step in the generation of Aβ peptide and is thus a prime target for therapeutic intervention. Substantive evidence now indicates that the processing of APP by BACE1 is regulated by the intracellular sorting of the enzyme and, moreover, perturbations in these intracellular trafficking pathways have been linked to late-onset AD. In this review, we highlight the recent advances in the understanding of the regulation of the intracellular sorting of BACE1 and APP and illustrate why the trafficking of these cargos represent a key issue for understanding the membrane-mediated events associated with the generation of the neurotoxic Aβ products in AD.     

MEK inhibitors suppress β-amyloid production by altering the level of a β-C-terminal fragment of amyloid precursor protein in neuronal cells

β-Amyloid peptide (Aβ) is generated via sequential proteolysis of amyloid precursor protein (APP) by β- and γ-secretases. Cell-based screening experiments disclosed that the MEK (MAP kinase kinase) inhibitors, U0126 and PD184352, suppress Aβ secretion from human neuronal SH-SY5Y cells expressing Swedish mutant APP. These inhibitors did not affect the cellular levels of APP but significantly reduced those of the APP β-C-terminal fragment (β-CTF). Additionally, β-CTF levels were markedly reduced by these inhibitors in cells expressing the fragment in a γ-secretase-independent and proteasome-dependent manner. Our results suggest that MEK inhibitors reduce Aβ generation via secretase-independent alteration of β-CTF levels.     

Adaptor protein 2-mediated endocytosis of the β-secretase BACE1 is dispensable for amyloid precursor protein processing

The β-site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1) is a transmembrane aspartyl protease that catalyzes the proteolytic processing of APP and other plasma membrane protein precursors. BACE1 cycles between the trans-Golgi network (TGN), the plasma membrane, and endosomes by virtue of signals contained within its cytosolic C-terminal domain. One of these signals is the DXXLL-motif sequence DISLL, which controls transport between the TGN and endosomes via interaction with GGA proteins. Here we show that the DISLL sequence is embedded within a longer [DE]XXXL[LI]-motif sequence, DDISLL, which mediates internalization from the plasma membrane by interaction with the clathrin-associated, heterotetrameric adaptor protein 2 (AP-2) complex. Mutation of this signal or knockdown of either AP-2 or clathrin decreases endosomal localization and increases plasma membrane localization of BACE1. Remarkably, internalization-defective BACE1 is able to cleave an APP mutant that itself cannot be delivered to endosomes. The drug brefeldin A reversibly prevents BACE1-catalyzed APP cleavage, ruling out that this reaction occurs in the endoplasmic reticulum (ER) or ER-Golgi intermediate compartment. Taken together, these observations support the notion that BACE1 is capable of cleaving APP in late compartments of the secretory pathway.     

Evidence for lysosomal processing of amyloid β-protein precursor in cultured cells

Amyloid -protein precursor (ABPP) of Alzheimer's disease (AD) represents a family of proteins which includes the parent protein which generates a small (4 kD) fragment that self-assembles to form amyloid fibrils in AD. Thus, the normal and abnormal proteolysis of ABPP may be directly relevant to AD pathogenesis. We have examined the accumulation of ABPP in cultured rodent and human neuronal cell lines in the presence and absence of a battery of protease inhibitors using immunohistochemistry and Western blot analysis. Here we present evidence for a lysosomal pathway for the turnover of ABPP and discuss the relevance of these results to plaque pathology and abnormal ABPP immunostaining in AD.Special issue dedicated to Dr. Paola S. Timiras     

Hydrogen sulfide reduces mRNA and protein levels of β-site amyloid precursor protein cleaving enzyme 1 in PC12 cells

Hydrogen sulfide (H(2)S) is now identified as a new neuromodulator. Increasing evidence suggest that H(2)S may play an important role in the progression of Alzheimer's disease (AD). The aim of the present study is to investigate the effects of H(2)S on beta-site amyloid precursor protein cleaving enzyme 1 (BACE-1) expression and amyloid beta (Aβ) secretion in PC12 cells. The levels of BACE-1 mRNA were measured by quantitative polymerase chain reaction analysis. BACE-1 protein levels were assessed by Western blot. Cellular culture medium levels of Aβ1-42 were analyzed by ELISA. We found that sodium hydrosulfide (NaHS), a H(2)S donor, decreased BACE-1 mRNA and protein levels and Aβ1-42 release. Furthermore, NaHS promoted the phosphorylation of Akt and ERK but not JNK or p38 MAPK. However, the effects of NaHS on BACE-1 expression and Aβ1-42 secretion were abolished by inhibitors of phosphatidylinositol 3-kinase (PI3-K), but not of mitogen-activated protein kinase kinases (MEK). Our data indicate that H(2)S reduces BACE-1 expression in PC12 cells via activation of PI3-K/Akt signaling pathways. H(2)S releasing drugs may have therapeutic potential in AD patients.     

Decreased levels of soluble amyloid β-protein precursor and β-amyloid protein in cerebrospinal fluid of patients with systemic lupus erythematosus

Symptoms originating from the central nervous system (CNS) frequently occur in patients with systemic lupus erythematosus (SLE). These symptoms are extremely diverse, including a state of dementia. The aim of this study was to examine the cerebrospinal fluid (CSF) content of soluble molecules indicating axonal degeneration and amyloidogenesis. One hundred and fourteen patients with SLE and age-matched controls were evaluated clinically, with magnetic resonance imaging of the brain and CSF analyses. Levels of tau, amyloid precursor protein (APP), beta-amyloid protein (Abeta42), and transforming growth factor beta (TGF-beta) were all determined using sandwich ELISAs.APP and Abeta42 levels were significantly decreased in SLE patients irrespective of their CNS involvement, as compared with healthy controls. Patients with neuropsychiatric SLE who underwent a second lumbar puncture following successful cyclophosphamide treatment showed further decreases of Abeta42. CSF-tau levels were significantly increased in SLE patients showing magnetic resonance imaging-verified brain pathology as compared with SLE patients without such engagement. Importantly, tau levels displayed significant correlation to Abeta42 levels in the CSF. Finally, TGF-beta levels were significantly increased in patients with neuropsychiatric SLE as compared with those without. Low intrathecal levels of Abeta42 found in SLE patients seem to be a direct consequence of a diminished production of APP, probably mediated by heavy anti-inflammatory/immuno-suppressive therapy. Furthermore, our findings suggest that CSF tau can be used as a biochemical marker for neuronal degeneration in SLE. Finally, the increased TGF-beta levels observed may support a notion of an ongoing anti-inflammatory response counteracting tissue injury caused by CNS lupus.     

Ultrastructural localization of amyloid β/A4 protein precursor in the normal rat brain

The ultrastructural localization of amyloidβ/ A4 protein precursor (APP) was studied immunohistochemically in normal rat brains using antibodies against different portions of APP. In cerebral cortical neurons and Purkinje cells, APP reaction products were located in the cytoplasm and on cell surface membranes. Some Golgi apparatuses and rough endoplasmic reticulum also showed APP immunoreactivity on their membranes and some vesicles near the trans face of the Golgi apparatuses were stained. In the neuropil of the cerebral cortex and the cerebellar molecular layer, many cell processes, which surrounded synapses and were considered to be astrocytic, were APP-positive. Foot processes around capillaries and subpial astrocytic processes were also immuno-positive. At the ultrastructural level, APP-positive astrocytic processes were identified.     

ADAM10 is the physiologically relevant, constitutive ��-secretase of the amyloid precursor protein in primary neurons

The amyloid precursor protein (APP) undergoes constitutive shedding by a protease activity called α‐secretase. This is considered an important mechanism preventing the generation of the Alzheimer's disease amyloid‐β peptide (Aβ). α‐Secretase appears to be a metalloprotease of the ADAM family, but its identity remains to be established. Using a novel α‐secretase‐cleavage site‐specific antibody, we found that RNAi‐mediated knockdown of ADAM10, but surprisingly not of ADAM9 or 17, completely suppressed APP α‐secretase cleavage in different cell lines and in primary murine neurons. Other proteases were not able to compensate for this loss of α‐cleavage. This finding was further confirmed by mass‐spectrometric detection of APP‐cleavage fragments. Surprisingly, in different cell lines, the reduction of α‐secretase cleavage was not paralleled by a corresponding increase in the Aβ‐generating β‐secretase cleavage, revealing that both proteases do not always compete for APP as a substrate. Instead, our data suggest a novel pathway for APP processing, in which ADAM10 can partially compete with γ‐secretase for the cleavage of a C‐terminal APP fragment generated by β‐secretase. We conclude that ADAM10 is the physiologically relevant, constitutive α‐secretase of APP.     

Lysine 624 of the amyloid precursor protein (APP) is a critical determinant of amyloid β peptide length: support for a sequential model of γ-secretase intramembrane proteolysis and regulation by the amyloid β precursor protein (APP) juxtamembrane region

γ-Secretase is a multiprotein intramembrane cleaving aspartyl protease (I-CLiP) that catalyzes the final cleavage of the amyloid β precursor protein (APP) to release the amyloid β peptide (Aβ). Aβ is the primary component of senile plaques in Alzheimer's disease (AD), and its mechanism of production has been studied intensely. γ-Secretase executes multiple cleavages within the transmembrane domain of APP, with cleavages producing Aβ and the APP intracellular domain (AICD), referred to as γ and ε, respectively. The heterogeneous nature of the γ cleavage that produces various Aβ peptides is highly relevant to AD, as increased production of Aβ 1-42 is genetically and biochemically linked to the development of AD. We have identified an amino acid in the juxtamembrane region of APP, lysine 624, on the basis of APP695 numbering (position 28 relative to Aβ) that plays a critical role in determining the final length of Aβ peptides released by γ-secretase. Mutation of this lysine to alanine (K28A) shifts the primary site of γ-secretase cleavage from 1-40 to 1-33 without significant changes to ε cleavage. These results further support a model where ε cleavage occurs first, followed by sequential proteolysis of the remaining transmembrane fragment, but extend these observations by demonstrating that charged residues at the luminal boundary of the APP transmembrane domain limit processivity of γ-secretase.     

Substrate sequence influences γ-secretase modulator activity, role of the transmembrane domain of the amyloid precursor protein

A subset of non-steroidal anti-inflammatory drugs modulates the γ cleavage site in the amyloid precursor protein (APP) to selectively reduce production of Aβ42. It is unclear precisely how these γ-secretase modulators (GSMs) act to preferentially spare Aβ40 production as well as Notch processing and signaling. In an effort to determine the substrate requirements in NSAID/GSM activity, we determined the effects of sulindac sulfide and flurbiprofen on γ-cleavage of artificial constructs containing several γ-secretase substrates. Using FLAG-tagged constructs that expressed extracellularly truncated APP, Notch-1, or CD44, we found that these substrates have different sensitivities to sulindac sulfide. γ-Secretase cleavage of APP was altered by sulindac sulfide, but CD44 and Notch-1 were either insensitive or only minimally altered by this compound. Using chimeric APP constructs, we observed that the transmembrane domain (TMD) of APP played a pivotal role in determining drug sensitivity. Substituting the APP TMD with that of APLP2 retained the sensitivity to γ-cleavage modulation, but replacing TMDs from Notch-1 or ErbB4 rendered the resultant molecules insensitive to drug treatment. Specifically, the GXXXG motif within APP appeared to be critical to GSM activity. Consequently, the modulatory effects on γ-cleavage appears to be substrate-dependent. We hypothesize that the substrate present in the γ-secretase complex influences the conformation of the complex so that the binding site of GSMs is either stabilized or less favorable to influence the cleavage of the respective substrates.     

ADAM9 inhibition increases membrane activity of ADAM10 and controls α-secretase processing of amyloid precursor protein

Prodomains of A disintegrin and metalloproteinase (ADAM) metallopeptidases can act as highly specific intra- and intermolecular inhibitors of ADAM catalytic activity. The mouse ADAM9 prodomain (proA9; amino acids 24-204), expressed and characterized from Escherichia coli, is a competitive inhibitor of human ADAM9 catalytic/disintegrin domain with an overall inhibition constant of 280 ± 34 nM and high specificity toward ADAM9. In SY5Y neuroblastoma cells overexpressing amyloid precursor protein, proA9 treatment reduces the amount of endogenous ADAM10 enzyme in the medium while increasing membrane-bound ADAM10, as shown both by Western and activity assays with selective fluorescent peptide substrates using proteolytic activity matrix analysis. An increase in membrane-bound ADAM10 generates higher levels of soluble amyloid precursor protein α in the medium, whereas soluble amyloid precursor protein β levels are decreased, demonstrating that inhibition of ADAM9 increases α-secretase activity on the cell membrane. Quantification of physiological ADAM10 substrates by a proteomic approach revealed that substrates, such as epidermal growth factor (EGF), HER2, osteoactivin, and CD40-ligand, are increased in the medium of BT474 breast tumor cells that were incubated with proA9, demonstrating that the regulation of ADAM10 by ADAM9 applies for many ADAM10 substrates. Taken together, our results demonstrate that ADAM10 activity is regulated by inhibition of ADAM9, and this regulation may be used to control shedding of amyloid precursor protein by enhancing α-secretase activity, a key regulatory step in the etiology of Alzheimer disease.     

Evolutionary origin of a Kunitz-type trypsin inhibitor domain inserted in the amyloid β precursor protein of Alzheimer's disease

Summary The Kunitz-type protease inhibitor is one of the serine protease inhibitors. It is found in blood, saliva, and all tissues in mammals. Recently, a Kunitz-type sequence was found in the protein sequence of the amyloid precursor protein (APP). It is known that APP accumulates in the neuritic plaques and cerebrovascular deposits of patients with Alzheimer's disease. Collagen type VI in chicken also has an insertion of a Kunitz-type sequence. To elucidate the evolutionary origin of these insertion sequences, we constructed a phylogenetic tree by use of all the available sequences of Kunitz-type inhibitors. The tree shows that the ancestral gene of the Kunitz-type inhibitor appeared about 500 million years ago. Thereafter, this gene duplicated itself many times, and some of the duplicates were inserted into other protein-coding genes. During this process, the Kunitz-type sequence in the present APP gene diverged from its ancestral gene about 270 million years ago and was inserted into the gene soon after duplication. Although the function of the insertion sequences is unknown, our molecular evolutionary analysis shows that these insertion sequences in APP have an evolutionarily close relationship with the inter--trypsin inhibitor or trypstatin, which inhibits the activity of tryptase, a novel membrane-bound serine protease in human T4⁺ lymphocytes.Offprint requests to: T. Gojobori     

Inhibition of dynamin-dependent endocytosis increases shedding of the amyloid precursor protein ectodomain and reduces generation of amyloid β protein

Background  

The amyloid precursor protein (APP) is transported via the secretory pathway to the cell surface, where it may be cleaved within its ectodomain by α-secretase, or internalized within clathrin-coated vesicles. An alternative proteolytic pathway occurs within the endocytic compartment, where the sequential action of β- and γ-secretases generates the amyloid β protein (Aβ). In this study, we investigated the effects of modulators of endocytosis on APP processing.     

Secreted amyloid precursor protein β and secreted amyloid precursor protein α induce axon outgrowth in vitro through Egr1 signaling pathway

Background

sAPPα released after α secretase cleavage of Amyloid Precursor Protein (APP) has several functions including the stimulation of neurite outgrowth although detailed morphometric analysis has not been done. Two domains involved in this function have been described and are present in sAPPβ released at the first step of amyloid peptide cleavage, raising the possibility that sAPPβ could also stimulate neurite outgrowth. We investigated the morphological effects of sAPPα and sAPPβ on primary neurons and identified a key signaling event required for the changes observed.

Methodology/Principal Findings

Final concentrations of 50 to 150 nM bacterial recombinant sAPPα or sAPPβ added to primary neuronal cultures after 1 day in vitro decreased cell adhesion 24 hours later and primary dendrite length 96 hours later. 150 nM sAPPα and sAPPβ induced a similar increase of axon outgrowth, although this increase was already significant at 100 nM sAPPα. These morphological changes induced by sAPPs were also observed when added to differentiated neurons at 5 days in vitro. Real time PCR and immunocytochemistry showed that sAPPα and sAPPβ stimulated Egr1 expression downstream of MAPK/ERK activation. Furthermore, in primary neurons from Egr1 −/− mice, sAPPs affected dendritic length but did not induce any increase of axon length.

Conclusion/Significance

sAPPα and sAPPβ decrease cell adhesion and increase axon elongation. These morphological changes are similar to what has been observed in response to heparan sulfate. The sAPPα/sAPPβ stimulated increase in axon growth requires Egr1 signaling. These data suggest that sAPPβ is not deleterious per se. Since sAPPβ and sAPPα are present in the embryonic brain, these two APP metabolites might play a role in axon outgrowth during development and in response to brain damage.     

Genetic cathepsin B deficiency reduces β-amyloid in transgenic mice expressing human wild-type amyloid precursor protein

Neurotoxic β-amyloid (Aβ) peptides participate in Alzheimer’s disease (AD); therefore, reduction of Aβ generated from APP may provide a therapeutic approach for AD. Gene knockout studies in transgenic mice producing human Aβ may identify targets for reducing Aβ. This study shows that knockout of the cathepsin B gene in mice expressing human wild-type APP (hAPPwt) results in substantial decreases in brain Aβ40 and Aβ42 by 67% and decreases in levels of the C-terminal β-secretase fragment (CTFβ) derived from APP. In contrast, knockout of cathepsin B in mice expressing hAPP with the rare Swedish (Swe) and Indiana (Ind) mutations had no effect on Aβ. The difference in reduction of Aβ in hAPPwt mice, but not in hAPPSwe/Ind mice, shows that the transgenic model can affect cathepsin B gene knockout results. Since most AD patients express hAPPwt, these data validate cathepsin B as a target for development of inhibitors to lower Aβ in AD.     

Loss of cleavage at β'-site contributes to apparent increase in β-amyloid peptide (Aβ) secretion by β-secretase (BACE1)-glycosylphosphatidylinositol (GPI) processing of amyloid precursor protein

Several lines of evidence implicate lipid raft microdomains in Alzheimer disease-associated β-amyloid peptide (Aβ) production. Notably, targeting β-secretase (β-site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1)) exclusively to lipid rafts by the addition of a glycosylphosphatidylinositol (GPI) anchor to its ectodomain has been reported to elevate Aβ secretion. Paradoxically, Aβ secretion is not reduced by the expression of non-raft resident S-palmitoylation-deficient BACE1 (BACE1-4C/A (C474A/C478A/C482A/C485A)). We addressed this apparent discrepancy in raft microdomain-associated BACE1 processing of APP in this study. As previously reported, we found that expression of BACE1-GPI elevated Aβ secretion as compared with wild-type BACE1 (WTBACE1) or BACE1-4C/A. However, this increase occurred without any difference in the levels of APP ectodomain released following BACE1 cleavage (soluble APPβ), arguing against an overall increase in BACE1 processing of APP per se. Further analysis revealed that WTBACE1 cleaves APP at β- and β'-sites, generating +1 and +11 β-C-terminal fragments and secreting intact as well as N-terminally truncated Aβ. In contrast, three different BACE1-GPI chimeras preferentially cleaved APP at the β-site, mainly generating +1 β-C-terminal fragment and secreting intact Aβ. As a consequence, cells expressing BACE1-GPI secreted relatively higher levels of intact Aβ without an increase in BACE1 processing of APP. Markedly reduced cleavage at β'-site exhibited by BACE1-GPI was cell type-independent and insensitive to subcellular localization of APP or the pathogenic KM/NL mutant. We conclude that the apparent elevation in Aβ secretion by BACE1-GPI is mainly attributed to preferential cleavage at the β-site and failure to detect +11 Aβ species secreted by cells expressing WTBACE1.