Le Complexe Gamma-secrétase et la Mort Cellulaire par Apoptose : Implication dans la Maladie d'Alzheimer

Alzheimer's disease is characterized at the histopathological level by neurofibrillary tangles, senile plaques and massive neuronal loss. Senile plaques are composed by the aggregation of amyloid-beta peptide., which is produced after cleavage of a transmembrane protein, betaAPP, by two enzymatic activities, named beta- and gamma-secretases. The gamma-secretase activity is borne by a high molecular weight complex composed of at least four proteins, a Presenilin, Nicastrin (NCT), Pen-2 and Aph-1. The lack of one of the four members induces an inhibition of the gamma-secretase activity. This PhD focused on degradation processes involved in Aph-1 and Pen-2 catabolism and on the role that could play these proteins, as NCT on regulation of neuronal apoptosis. We have shown that Aph-1, Pen-2 and NCT decrease neuronal sensitivity to apoptosis by lowering caspase-3 activity. These processes are under control of p53 that expression is regulated by Aph-1, Pen-2 and NCT. Cell signalling induced by the three proteins are although different. Aph-1 and Pen-2 antiapoptotic functions require integrity of the gamma-secretase complex, but not its activity, while NCT antiapoptotic function is independent of the complex. We examine also Pen-2 and presenilins transcription. We show here that the link between p53 and Pen-2 is not unidirectional. p53 transcription is regulated by an intracellular fragment, AICD, produced after gamma-secretase cleavage of betaAPP. We demonstrate that AICD and p53 increase Pen-2 transcription. Otherwise, we show Pen-2 is involved in transcriptional control of presenilins, demonstrating that members of the gamma-secretase complex could regulate each other's.La maladie d'Alzheimer se caractérise par des dégénérescences neurofibrillaires, des plaques séniles et une mort cellulaire neuronale massive. Les plaques séniles sont constituées par l'agrégation du peptide amyloïde, formé suite au clivage de la betaAPP, par deux activités enzymatiques, nommées beta- et gamma-secrétases. L'activité gamma-secrétase est portée par un complexe protéique composé au minimum par une préséniline (PS), la nicastrine (NCT), Aph-1 et Pen-2. L'absence d'une seule de ces quatre protéines induit l'inhibition de l'activité gamma-secrétase. Nous avons examiné les processus de dégradation des protéines Aph-1 et Pen-2, ainsi que leur rôle dans la régulation de la mort neuronale. Nous montrons que ces deux protéines, ainsi que la NCT, réduisent la sensibilité des neurones à l'apoptose, en inhibant l'activité de la caspase-3. Ces phénomènes sont contrôlés par l'oncogène p53, dont l'expression est régulée par la NCT, Aph-1 et Pen-2. Cependant, les membres du complexe gamma-secrétase n'engagent pas la même voie de signalisation. La fonction protectrice d'Aph-1 et de Pen-2 requiert l'intégrité du complexe, mais pas l'activité gamma-secrétase, tandis que celle de la NCT est totalement indépendante du complexe. Nous avons examiné la régulation transcriptionnelle de Pen-2 et des présénilines. Nous montrons que le lien existant entre p53 et Pen-2 n'est pas unidirectionnel. La transcription de p53 est régulée par le fragment AICD issu du clivage de la betaAPP. Nos résultats montrent qu'AICD et p53 potentialisent la transcription de Pen-2. De plus, Pen-2 intervient également dans le contrôle transcriptionnel des présénilines, soulignant qu'une modulation de l'expression d'un membre du complexe peut influencer l'expression des autres membres

Le complexe gamma-secrétase et la mort cellulaire par apoptose (implication dans la maladie d'Alzheimer)

La maladie d'Alzheimer se caractérise par des dégénérescences neurofibrillaires, des plaques séniles et une mort cellulaire neuronale massive. Les plaques séniles sont constituées par l'agrégation du peptide amyloïde, formé suite au clivage de la bAPP, par deux activités enzymatiques, nommées b- et g-secrétases. L'activité g-secrétase est portée par un complexe protéique composé au minimum par une préséniline (PS), la nicastrine (NCT), Aph-1 et Pen-2. L'absence de l'une d'entre elles induit l'inhibition de l'activité g-secrétase. Nous avons examiné les processus de dégradation des protéines Aph-1 et Pen-2, ainsi que leur rôle dans la régulation de la mort neuronale. Nous montrons que ces deux protéines, ainsi que la NCT, réduisent la sensibilité des neurones à l'apoptose, en inhibant l'activité de la caspase-3. Ces phénomènes sont contrôlés par p53, dont l'expression est régulée par la NCT, Aph-1 et Pen-2. Cependant, ces protéines n'engagent pas la même voie de signalisation. La fonction protectrice d'Aph-1 et de Pen-2 requiert l'intégrité du complexe g-secrétase, mais pas son activité, tandis que celle de la NCT est indépendante du complexe. Nous avons examiné la régulation transcriptionnelle de Pen-2 et des PS. Nous montrons que le lien existant entre p53 et Pen-2 n'est pas unidirectionnel. La transcription de p53 est régulée par le fragment AICD issu du clivage de la bAPP. Nous montrons qu'AICD et p53 potentialisent la transcription de Pen-2. De plus, Pen-2 intervient également dans le contrôle transcriptionnel des présénilines, soulignant qu'une modulation de l'expression d'un membre du complexe peut influencer l'expression des autres membres.NICE-BU Sciences (060882101) / SudocSudocFranceF

γ-Secretase-mediated regulation of neprilysin: influence of cell density and aging and modulation by imatinib.

International audienceProteolytic degradation has emerged as a key pathway involved in controlling levels of the Alzheimer's 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

p53-dependent control of cell death by nicastrin: lack of requirement for presenilin-dependent gamma-secretase complex.

International audienceNicastrin (NCT) is a component of the presenilin (PS)-dependent gamma-secretase complexes that liberate amyloid beta-peptides from the beta-Amyloid Precursor Protein. Several lines of evidence indicate that the members of these complexes could also contribute to the control of cell death. Here we show that over-expression of NCT increases the viability of human embryonic kidney (HEK293) cells and decreases staurosporine (STS)- and thapsigargin (TPS)-induced caspase-3 activation in various cell lines from human and neuronal origins by Akt-dependent pathway. NCT lowers p53 expression, transcriptional activity and promoter transactivation and reduces p53 phosphorylation. NCT-associated protection against STS-stimulated cell death was completely abolished by p53 deficiency. Conversely, the depletion of NCT drastically enhances STS-induced caspase-3 activation and p53 pathway and favored p53 nuclear translocation. We examined whether NCT protective function depends on PS-dependent gamma-secretase activity. First, a 29-amino acid deletion known to reduce NCT-dependent amyloid beta-peptide production did not affect NCT-associated protective phenotype. Second, NCT still reduces STS-induced caspase-3 activation in fibroblasts lacking PS1 and PS2. Third, the gamma-secretase inhibitor DFK167 did not affect NCT-mediated reduction of p53 activity. Altogether, our study indicates that NCT controls cell death via phosphoinositide 3-kinase/Akt and p53-dependent pathways and that this function remains independent of the activity and molecular integrity of the gamma-secretase complexes

β-Amyloid Precursor Protein Intracellular Domain Controls Mitochondrial Function by Modulating Phosphatase and Tensin Homolog–Induced Kinase 1 Transcription in Cells and in Alzheimer Mice Models

International audienceBACKGROUND: Mitophagy and mitochondrial dynamics alterations are two major hallmarks of neurodegenerative diseases. Dysfunctional mitochondria accumulate in Alzheimer's disease-affected brains by yet unexplained mechanisms. METHODS: We combined cell biology, molecular biology, and pharmacological approaches to unravel a novel molecular pathway by which presenilins control phosphatase and tensin homolog-induced kinase 1 (Pink-1) expression and transcription. In vivo approaches were carried out on various transgenic and knockout animals as well as in adeno-associated virus-infected mice. Functional readout and mitochondrial physiology (mitochondrial potential) were assessed by combined procedures including flow cytometry, live imaging analysis, and immunohistochemistry. RESULTS: We show that presenilins 1 and 2 trigger opposite effects on promoter transactivation, messenger RNA, and protein expression of Pink-1. This control is linked to g-secretase activity and b-amyloid precursor protein but is independent of phosphatase and tensin homolog. We show that amyloid precursor protein intracellular domain (AICD) accounts for presenilin-dependent phenotype and upregulates Pink-1 transactivation in cells as well as in vivo in a Forkhead box O3a-dependent manner. Interestingly, the modulation of g-secretase activity or AICD expression affects Pink-1-related control of mitophagy and mitochondrial dynamics. Finally, we show that parkin acts upstream of presenilins to control Pink-1 promoter transactivation and protein expression. CONCLUSIONS: Overall, we delineate a molecular cascade presenilins-AICD-Forkhead box O3a linking parkin to Pink-1. Our study demonstrates AICD-mediated Pink-1-dependent control of mitochondrial physiology by presenilins. Furthermore, it unravels a parkin-Pink-1 feedback loop controlling mitochondrial physiology that could be disrupted in neurodegenerative conditions

Catabolism of endogenous and overexpressed APH1a and PEN2: evidence for artifactual involvement of the proteasome in the degradation of overexpressed proteins

PS (presenilin)-dependent γ-secretase occurs as a high-molecular-mass complex composed of either PS1 or PS2 associated with Nct (nicastrin), PEN2 (presenilin enhancer 2 homologue) and APH1 (anterior pharynx defective 1 homologue). Numerous reports have documented the very complicated physical and functional cross-talk between these proteins that ultimately governs the biological activity of the γ-secretase, but very few studies examined the fate of the components of the complex. We show that, in both HEK-293 cells and the TSM1 neuronal cell line, the immunoreactivities of overexpressed myc-tagged-APH1a and -PEN2 were enhanced by the proteasome inhibitors ZIE and lactacystin, whereas a broad range of protease inhibitors had no effect. By contrast, proteasome inhibitors were totally unable to affect the cellular expression of endogenous APH1aL and PEN2 in HEK-293 cells, TSM1 and primary cultured cortical neurons. To explain this apparent discrepancy, we examined the degradation of myc-tagged-APH1a and -PEN2, in vitro, by cell extracts containing endogenous proteasome and by purified 20S proteasome. Strikingly, myc-tagged-APH1a and -PEN2 resist proteolysis by endogenous proteasome and purified 20S proteasome. We also show that endogenous PEN2 expression was drastically higher in wild-type than in PS- and Nct-deficient fibroblasts and was enhanced by proteasome inhibitors only in the two deficient cell systems. However, here again, purified 20S proteasome appeared unable to cleave endogenous PEN2 present in PS-deficient fibroblasts. The levels of endogenous APH1aL-like immunoreactivity were not modified by proteasome inhibitors and were unaffected by PS deficiency. Altogether, our results indicate that endogenous PEN2 and APH1aL do not undergo proteasomal degradation under physiological conditions in HEK-293 cells, TSM1 cells and fibroblasts and that the clearance of PEN2 in PS- and Nct-deficient fibroblasts is not mediated by 20S proteasome. Whether the 26S proteasome participates to PEN2 proteolysis in deficient fibroblasts remains to be established