Raphaelle Pardossi-Piquard

The neuronal sortilin-related receptor SORL1 is genetically associated with Alzheimer disease

The recycling of the amyloid precursor protein (APP) from the cell surface via the endocytic pathways plays a key role in the generation of amyloid β peptide (Aβ) in Alzheimer disease. We report here that inherited variants in the SORL1 neuronal sorting receptor are associated with late-onset Alzheimer disease. These variants, which occur in at least two different clusters of intronic sequences within the SORL1 gene (also known as LR11 or SORLA) may regulate tissue-specific expression of SORL1. We also show that SORL1 directs trafficking of APP into recycling pathways and that when SORL1 is underexpressed, APP is sorted into Aβ-generating compartments. These data suggest that inherited or acquired changes in SORL1 expression or function are mechanistically involved in causing Alzheimer disease.

Presenilin-Dependent Transcriptional Control of the Aβ-Degrading Enzyme Neprilysin by Intracellular Domains of βAPP and APLP

Amyloid beta-peptide (Abeta), which plays a central role in Alzheimers disease, is generated by presenilin-dependent gamma-secretase cleavage of beta-amyloid precursor protein (betaAPP). We report that the presenilins (PS1 and PS2) also regulate Abeta degradation. Presenilin-deficient cells fail to degrade Abeta and have drastic reductions in the transcription, expression, and activity of neprilysin, a key Abeta-degrading enzyme. Neprilysin activity and expression are also lowered by gamma-secretase inhibitors and by PS1/PS2 deficiency in mouse brain. Neprilysin activity is restored by transient expression of PS1 or PS2 and by expression of the amyloid intracellular domain (AICD), which is cogenerated with Abeta, during gamma-secretase cleavage of betaAPP. Neprilysin gene promoters are transactivated by AICDs from APP-like proteins (APP, APLP1, and APLP2), but not by Abeta or by the gamma-secretase cleavage products of Notch, N- or E- cadherins. The presenilin-dependent regulation of neprilysin, mediated by AICDs, provides a physiological means to modulate Abeta levels with varying levels of gamma-secretase activity.

TMP21 is a presenilin complex component that modulates gamma-secretase but not epsilon-secretase activity.

The presenilin proteins (PS1 and PS2) and their interacting partners nicastrin, aph-1 (refs 4, 5) and pen-2 (ref. 5) form a series of high-molecular-mass, membrane-bound protein complexes that are necessary for γ-secretase and ɛ-secretase cleavage of selected type 1 transmembrane proteins, including the amyloid precursor protein, Notch and cadherins. Modest cleavage activity can be generated by reconstituting these four proteins in yeast and Spodoptera frugiperda (sf9) cells. However, a critical but unanswered question about the biology of the presenilin complexes is how their activity is modulated in terms of substrate specificity and/or relative activities at the γ and ɛ sites. A corollary to this question is whether additional proteins in the presenilin complexes might subsume these putative regulatory functions. The hypothesis that additional proteins might exist in the presenilin complexes is supported by the fact that enzymatically active complexes have a mass that is much greater than predicted for a 1:1:1:1 stoichiometric complex (at least 650 kDa observed, compared with about 220 kDa predicted). To address these questions we undertook a search for presenilin-interacting proteins that differentially affected γ- and ɛ-site cleavage events. Here we report that TMP21, a member of the p24 cargo protein family, is a component of presenilin complexes and differentially regulates γ-secretase cleavage without affecting ɛ-secretase activity.

The γ /η-Secretase-Derived APP Intracellular Domain Fragments Regulate p53

Amyloid β-peptide (Aβ), which plays a central role in Alzheimer Disease, is generated by presenilin-dependent and presenilin-independent γ-secretase cleavages of β-amyloid precursor protein (βAPP). We report that the presenilins (PS1 and PS2) also regulate p53-associated cell death Thus, we established that PS deficiency, catalytically inactive PS mutants, γ-secretase inhibitors and βAPP or APLP2 depletion reduced the expression and activity of p53, and lowered the transactivation of its promoter and mRNA levels. p53 expression was also reduced in the brains or βAPP-deficient mice or in brains where both PS had been invalidated by double conditional knock out. AICDC59 and AICDC50, the γ- and η- secretase-derived C-terminal fragments of βAPP, respectively, trigger the activation of caspase-3, p53-dependent cell death, and increase p53 activity and mRNA. Finally, HEK293 cells expressing PS1 harboring familial AD (FAD) mutations or FAD-affected brains, all display enhanced p53 activity and p53 expression. Our studies demonstrate that AICDs control p53 at a transcriptional level, in vitro and in vivo and unravel a still unknown function for presenilins.

p53 Is Regulated by and Regulates Members of the γ-Secretase Complex

Amyloid β-peptides is the generic term for a set of hydrophobic peptides that accumulate in Alzheimer’s disease (AD)-affected brains. These amyloid-β peptide fragments are mainly generated by an enzymatic machinery referred to as γ-secretase complex that is built up by the association of four distinct proteins, namely presenilin 1 (PS1) or PS2, nicastrin, Aph-1 and Pen-2. AD is also characterized by exacerbated cell death that appears linked to the tumor suppressor p53. Interestingly, all members of the γ-secretase complex control p53-dependent cell death. On the other hand, p53 appears to be able to regulate directly or indirectly the expression and transcription of PS1, PS2 and Pen-2. This review will focus on the functional cross-talk between the members of the γ-secretase complex and p53 and will discuss the putative implication of this oncogene in AD pathology.

Neprilysin activity and expression are controlled by nicastrin

We recently demonstrated that the presenilin‐dependent γ‐secretase complex regulates the expression and activity of neprilysin, one of the main enzymes that degrade the amyloid β‐peptide (Aβ) which accumulates in Alzheimers disease. Here, we examined the influence of endogenous nicastrin (NCT), a member of the γ‐secretase complex, on neprilysin physiology. We show that nicastrin deficiency drastically lowers neprilysin expression, membrane‐bound activity and mRNA levels, but it did not modulate the expression of two other putative Aβ‐cleaving enzymes, endothelin‐converting enzyme and insulin‐degrading enzyme. Furthermore, we show that nicastrin restores neprilysin activity and expression in nicastrin‐deficient, but not presenilin‐deficient fibroblasts, indicating that the control of neprilysin necessitates the complete γ‐secretase complex harbouring its four reported components. Finally, we show that NCT expression peaked 24 h after NCT cDNA transfection of wild‐type and NCT–/– fibroblasts, while neprilysin expression drastically increased only after 36 h and was maximal at 48 h. This delayed effect on neprilysin expression correlates well with our demonstration of an indirect γ‐secretase‐dependent modulation of neprilysin at its transcriptional level.

APH1 polar transmembrane residues regulate the assembly and activity of presenilin complexes

Complexes involved in the γ/ϵ-secretase-regulated intramembranous proteolysis of substrates such as the amyloid-β precursor protein are composed primarily of presenilin (PS1 or PS2), nicastrin, anterior pharynx defective-1 (APH1), and PEN2. The presenilin aspartyl residues form the catalytic site, and similar potentially functional polar transmembrane residues in APH1 have been identified. Substitution of charged (E84A, R87A) or polar (Q83A) residues in TM3 had no effect on complex assembly or activity. In contrast, changes to either of two highly conserved histidines (H171A, H197A) located in TM5 and TM6 negatively affected PS1 cleavage and altered binding to other secretase components, resulting in decreased amyloid generating activity. Charge replacement with His-to-Lys substitutions rescued nicastrin maturation and PS1 endoproteolysis leading to assembly of the formation of structurally normal but proteolytically inactive γ-secretase complexes. Substitution with a negatively charged side chain (His-to-Asp) or altering the structural location of the histidines also disrupted γ-secretase binding and abolished functionality of APH1. These results suggest that the conserved transmembrane histidine residues contribute to APH1 function and can affect presenilin catalytic activity.

p53-dependent control of transactivation of the Pen2 promoter by presenilins

The senile plaques found in the brains of patients with Alzheimers disease are mainly due to the accumulation of amyloid β-peptides (Aβ) that are liberated by γ-secretase, a high molecular weight complex including presenilins, PEN-2, APH-1 and nicastrin. The depletion of each of these proteins disrupts the complex assembly into a functional protease. Here, we describe another level of regulation of this multimeric protease. The depletion of both presenilins drastically reduces Pen2 mRNA levels and its promoter transactivation. Furthermore, overexpression of presenilin-1 lowers Pen2 promoter transactivation, a phenotype abolished by a double mutation known to prevent presenilin-dependent γ-secretase activity. PEN-2 expression is decreased by depletion of β-amyloid precursor protein (APP) and increased by the APP intracellular domain (AICD). We show that AICD and APP complement for Pen2 mRNA levels in APP/APLP1-2 knockout fibroblasts. Interestingly, overexpression of presenilin-2 greatly increases Pen2 promoter transactivation. The opposite effect triggered by both presenilins was reminiscent of our previous study, which showed that these two proteins elicit antagonistic effects on p53. Therefore, we examined the contribution of p53 on Pen2 transcription. Pen2 promoter transactivation, and Pen2 mRNA and protein levels were drastically reduced in p53–/– fibroblasts. Furthermore, PEN-2 expression could be rescued by p53 complementation in p53- and APP-deficient cells. Interestingly, PEN-2 expression was also reduced in p53-deficient mouse brain. Overall, our study describes a p53-dependent regulation of PEN-2 expression by other members of the γ-secretase complex, namely presenilins.

γ-Secretase-Mediated Regulation of Neprilysin: Influence of Cell Density and Aging and Modulation by Imatinib

Proteolytic degradation has emerged as a key pathway involved in controlling levels of the Alzheimers disease (AD)-associated amyloid-β peptides (Aβ) in the brain. The ectopeptidase, neprilysin (NEP), has been reported as the major Aβ-degrading enzyme in mice and human brains. We have previously shown that NEP expression and activity are regulated by AICD, the intracellular domain of the amyloid-β protein precursor (AβPP) generated by γ-secretase. Thus, NEP transcription, expression, and enzymatic activity are dramatically reduced in fibroblasts devoid of AβPP (the precursor of AICD) or lacking both presenilin (PS) 1 and 2 (two parent proteins contributing to AICD formation). We demonstrate here that NEP expression and activity are influenced by a number of cell passages and density, and we confirm a drastic reduction of NEP expression and activity in AβPP and PS null fibroblasts examined at similar passages and cell densities. Furthermore, Imatinib (Gleevec), a known tyrosine kinase inhibitor was recently shown to elevate AICD in H4 human neuroglioma cells, and this was accompanied by concomitant increases of NEP protein, mRNA levels, and activity. However, the demonstration of a causal link between Imatinib and AICD levels was still lacking. We show here an Imatinib-dependent effect on NEP expression and activity in murine fibroblasts and establish that Imatinib-induced modulation of NEP was abolished by the depletion of AβPP or its homologues APLP1 and APLP2, thereby confirming that Imatinib-mediated control of NEP could indeed be accounted for its effect on AICD.

TMP21 Transmembrane Domain Regulates γ−Secretase Cleavage

TMP21 has been shown to be associated with the γ-secretase complex and can specifically regulate γ-cleavage without affecting ϵ-mediated proteolysis. To explore the basis of this activity, TMP21 modulation of γ-secretase activity was investigated independent of ϵ-cleavage using an amyloid-β precursor proteinϵ (APPϵ) construct which lacks the amyloid intracellular domain domain. The APPϵ construct behaves similarly to the full-length precursor protein with respect to α- and β-cleavages and is able to undergo normal γ-processing. Co-expression of APPϵ and TMP21 resulted in the accumulation of membrane-embedded higher molecular weight Aβ-positive fragments, consistent with an inhibition of γ-secretase cleavage. The APPϵ system was used to examine the functional domains of TMP21 through the investigation of a series of TMP21-p24a chimera proteins. It was found that chimeras containing the transmembrane domain bound to the γ-secretase complex and could decrease γ-secretase proteolytic processing. This was confirmed though investigation of a synthetic peptide corresponding to the TMP21 transmembrane helix. The isolated TMP21 TM peptide but not the homologous p24a domain was able to reduce Aβ production in a dose-dependent fashion. These observations suggest that the TMP21 transmembrane domain promotes its association with the presenilin complex that results in decreased γ-cleavage activity.