Xavier Meckler

Alzheimer Disease Aβ Production in the Absence of S-Palmitoylation-dependent Targeting of BACE1 to Lipid Rafts

Alzheimer disease β-amyloid (Aβ) peptides are generated via sequential proteolysis of amyloid precursor protein (APP) by BACE1 and γ-secretase. A subset of BACE1 localizes to cholesterol-rich membrane microdomains, termed lipid rafts. BACE1 processing in raft microdomains of cultured cells and neurons was characterized in previous studies by disrupting the integrity of lipid rafts by cholesterol depletion. These studies found either inhibition or elevation of Aβ production depending on the extent of cholesterol depletion, generating controversy. The intricate interplay between cholesterol levels, APP trafficking, and BACE1 processing is not clearly understood because cholesterol depletion has pleiotropic effects on Golgi morphology, vesicular trafficking, and membrane bulk fluidity. In this study, we used an alternate strategy to explore the function of BACE1 in membrane microdomains without altering the cellular cholesterol level. We demonstrate that BACE1 undergoes S-palmitoylation at four Cys residues at the junction of transmembrane and cytosolic domains, and Ala substitution at these four residues is sufficient to displace BACE1 from lipid rafts. Analysis of wild type and mutant BACE1 expressed in BACE1 null fibroblasts and neuroblastoma cells revealed that S-palmitoylation neither contributes to protein stability nor subcellular localization of BACE1. Surprisingly, non-raft localization of palmitoylation-deficient BACE1 did not have discernible influence on BACE1 processing of APP or secretion of Aβ. These results indicate that post-translational S-palmitoylation of BACE1 is not required for APP processing, and that BACE1 can efficiently cleave APP in both raft and non-raft microdomains.

Mechanisms of Disease: new therapeutic strategies for Alzheimer's disease—targeting APP processing in lipid rafts

Alzheimers disease (AD) is the most common cause of age-related dementia. Pathologically, AD is characterized by the deposition in the brain of amyloid-β peptides derived from proteolysis of amyloid precursor protein (APP) by β-site APP cleaving enzyme 1 (BACE1) and γ-secretase. A growing body of evidence implicates cholesterol and cholesterol-rich membrane microdomains in amyloidogenic processing of APP. Here, we review recent findings regarding the association of BACE1, γ-secretase and APP in lipid rafts, and discuss potential therapeutic strategies for AD that are based on knowledge gleaned from the membrane environment that fosters APP processing.

S-Palmitoylation of γ-Secretase Subunits Nicastrin and APH-1

Proteolytic processing of amyloid precursor protein (APP) by β- and γ-secretases generates β-amyloid (Aβ) peptides, which accumulate in the brains of individuals affected by Alzheimer disease. Detergent-resistant membrane microdomains (DRM) rich in cholesterol and sphingolipid, termed lipid rafts, have been implicated in Aβ production. Previously, we and others reported that the four integral subunits of the γ-secretase associate with DRM. In this study we investigated the mechanisms underlying DRM association of γ-secretase subunits. We report that in cultured cells and in brain the γ-secretase subunits nicastrin and APH-1 undergo S-palmitoylation, the post-translational covalent attachment of the long chain fatty acid palmitate common in lipid raft-associated proteins. By mutagenesis we show that nicastrin is S-palmitoylated at Cys689, and APH-1 is S-palmitoylated at Cys182 and Cys245. S-Palmitoylation-defective nicastrin and APH-1 form stable γ-secretase complexes when expressed in knock-out fibroblasts lacking wild type subunits, suggesting that S-palmitoylation is not essential for γ-secretase assembly. Nevertheless, fractionation studies show that S-palmitoylation contributes to DRM association of nicastrin and APH-1. Moreover, pulse-chase analyses reveal that S-palmitoylation is important for nascent polypeptide stability of both proteins. Co-expression of S-palmitoylation-deficient nicastrin and APH-1 in cultured cells neither affects Aβ40, Aβ42, and AICD production, nor intramembrane processing of Notch and N-cadherin. Our findings suggest that S-palmitoylation plays a role in stability and raft localization of nicastrin and APH-1, but does not directly modulate γ-secretase processing of APP and other substrates.

A Function for EHD Family Proteins in Unidirectional Retrograde Dendritic Transport of BACE1 and Alzheimer’s Disease Aβ Production

Abnormal accumulation of β-secretase (BACE1) in dystrophic neurites and presynaptic β-amyloid (Aβ) production contribute to Alzheimers disease pathogenesis. Little, however, is known about BACE1 sorting and dynamic transport in neurons. We investigated BACE1 trafficking in hippocampal neurons using live-cell imaging and selective labeling. We report that transport vesicles containing internalized BACE1 in dendrites undergo exclusive retrograde transport toward the soma, whereas they undergo bidirectional transport in axons. Unidirectional dendritic transport requires Eps15-homology-domain-containing (EHD) 1 and 3 protein function. Furthermore, loss of EHD function compromises dynamic axonal transport and overall BACE1 levels in axons. EHD1/3 colocalize with BACE1 and APP β-C-terminal fragments in hippocampal mossy fiber terminals, and their depletion in neurons significantly attenuates Aβ levels. These results demonstrate unidirectional endocytic transport of a dendritic cargo and reveal a role for EHD proteins in neuronal BACE1 transcytosis and Aβ production, processes that are highly relevant for Alzheimers disease.

Evidence That CD147 Modulation of β-Amyloid (Aβ) Levels Is Mediated by Extracellular Degradation of Secreted Aβ

Cerebral deposition of β-amyloid (Aβ) peptides is a pathological hallmark of Alzheimer disease. Intramembranous proteolysis of amyloid precursor protein by a multiprotein γ-secretase complex generates Aβ. Previously, it was reported that CD147, a glycoprotein that stimulates production of matrix metalloproteinases (MMPs), is a subunit of γ-secretase and that the levels of secreted Aβ inversely correlate with CD147 expression. Here, we show that the levels and localization of CD147 in fibroblasts, as well as postnatal expression and distribution in brain, are distinct from those of integral γ-secretase subunits. Notably, we show that although depletion of CD147 increased extracellular Aβ levels in intact cells, membranes isolated from CD147-depleted cells failed to elevate Aβ production in an in vitro γ-secretase assay. Consistent with an extracellular source that modulates Aβ metabolism, synthetic Aβ was degraded more rapidly in the conditioned medium of cells overexpressing CD147. Moreover, modulation of CD147 expression had no effect on ϵ-site cleavage of amyloid precursor protein and Notch1 receptor. Collectively, our results demonstrate that CD147 modulates Aβ levels not by regulating γ-secretase activity, but by stimulating extracellular degradation of Aβ. In view of the known function of CD147 in MMP production, we postulate that CD147 expression influences Aβ levels by an indirect mechanism involving MMPs that can degrade extracellular Aβ.

Reduced Alzheimer's Disease β-Amyloid Deposition in Transgenic Mice Expressing S-Palmitoylation-Deficient APH1aL and Nicastrin

Sequential cleavage of amyloid precursor protein by β- and γ-secretases generates β-amyloid peptides (Aβ), which accumulate in the brains of patients with Alzheimers disease. We recently identified S-palmitoylation of two γ-secretase subunits, APH1 and nicastrin. S-Palmitoylation is an essential posttranslational modification for the proper trafficking and function of many neuronal proteins. In cultured cell lines, lack of S-palmitoylation causes instability of nascent APH1 and nicastrin but does not affect γ-secretase processing of amyloid precursor protein. To determine the importance of γ-secretase S-palmitoylation for Aβ deposition in the brain, we generated transgenic mice coexpressing human wild-type or S-palmitoylation-deficient APH1aL and nicastrin in neurons in the forebrain. We found that lack of S-palmitoylation did not impair the ability of APH1aL and nicastrin to form enzymatically active protein complexes with endogenous presenilin 1 and PEN2 or affect the localization of γ-secretase subunits in dendrites and axons of cortical neurons. When we crossed these mice with 85Dbo transgenic mice, which coexpress familial Alzheimers disease-causing amyloid precursor protein and presenilin 1 variants, we found that coexpression of wild-type or mutant APH1aL and nicastrin led to marked stabilization of transgenic presenilin 1 in the brains of double-transgenic mice. Interestingly, we observed a moderate, but significant, reduction in amyloid deposits in the forebrain of mice expressing S-palmitoylation-deficient γ-secretase subunits compared with mice overexpressing wild-type subunits, as well as a reduction in the levels of insoluble Aβ40–42. These results indicate that γ-secretase S-palmitoylation modulates Aβ deposition in the brain.

Mutation Analysis of the Presenilin 1 N-terminal Domain Reveals a Broad Spectrum of γ-Secretase Activity toward Amyloid Precursor Protein and Other Substrates

The γ-secretase protein complex executes the intramembrane proteolysis of amyloid precursor protein (APP), which releases Alzheimer disease β-amyloid peptide. In addition to APP, γ-secretase also cleaves several other type I membrane protein substrates including Notch1 and N-cadherin. γ-Secretase is made of four integral transmembrane protein subunits: presenilin (PS), nicastrin, APH1, and PEN2. Multiple lines of evidence indicate that a heteromer of PS-derived N- and C-terminal fragments functions as the catalytic subunit of γ-secretase. Only limited information is available on the domains within each subunit involved in the recognition and recruitment of diverse substrates and the transfer of substrates to the catalytic site. Here, we performed mutagenesis of two domains of PS1, namely the first luminal loop domain (LL1) and the second transmembrane domain (TM2), and analyzed PS1 endoproteolysis as well as the catalytic activities of PS1 toward APP, Notch, and N-cadherin. Our results show that distinct residues within LL1 and TM2 domains as well as the length of the LL1 domain are critical for PS1 endoproteolysis, but not for PS1 complex formation with nicastrin, APH1, and PEN2. Furthermore, our experimental PS1 mutants formed γ-secretase complexes with distinct catalytic properties toward the three substrates examined in this study; however, the mutations did not affect PS1 interaction with the substrates. We conclude that the N-terminal LL1 and TM2 domains are critical for PS1 endoproteolysis and the coordination between the putative substrate-docking site and the catalytic core of the γ-secretase.

Regulation of dynamic BACE1 trafficking in neurons

Proteolytic processing of amyloid precursor protein (APP) by β-site APP cleaving enzyme 1 (BACE1) and γ-secretase generates Aβ peptides. APP is constitutively trafficked through the secretory and endocytic pathways in cultured cells and neurons. The identities of cellular organelles and sorting pathways involved in amyloidogenic processing of APP have been extensively investigated. Although a consensus has not yet emerged, there is a general agreement from biochemical and genetic studies on the importance of endocytic APP trafficking for Aβ production. In neurons, APP is trafficked anterogradely along peripheral and central axons, and proteolytically processed during transit. Recent in vivo studies estimated that ~70% of Aβ released in the brain requires ongoing endocytosis, and synaptic activity regulates the vast majority of this endocytosis-dependent Aβ. BACE1 cleavage is thought to be the rate-limiting step in amyloidogenic processing of APP. Little, however, is known about endocytic BACE1 sorting and dynamic transport in neurons. We investigated BACE1 trafficking in cultured hippocampal neurons using live-cell imaging and selective labeling. This approach revealed dynamic neuronal transport of internalized BACE1 in dendrites and axons. BACE1 was transported in vesicles that were positive for markers of recycling endosomes. Dominant-negative expression and siRNA knock-down of proteins involved in endocytic transit revealed that BACE1 is dynamically transported in recycling endosomes and this process significantly contributes to amyloidogenic APP processing. Our results suggest that BACE1 trafficking in neuronal recycling endosomes is likely relevant for presynaptic Aβ production and contributes to Alzheimer’s disease pathogenesis.