PHD Defence

Published on 10/11/2022

Maxime VIGIER will defend his thesis on december 12, 2022, co-supervised by Catherine MALAPLATE (URAFPA, Université de Lorraine, Nancy, France) and Thierry OSTER (URAFPA, Université de Lorraine, Nancy, France).

Reviewers:

Mme Raphaëlle PARDOSSI-PIQUARD, Chargée de recherche, CNRS, Nice
M. Mustapha CHERKAOUI-MALKI, Professeur, Université de Bourgogne,

Examiners:

Mme Isabelle DENIS, Chargée de recherche, INRAE, Jouy-en-Josas,
M. Athanase VISVIKIS, Professeur, Université de Lorraine.

ABSTRACT

Alzheimer's disease (AD) is a complex and multifactorial pathology for which there is no current treatment. Several hypotheses have been proposed to explain the onset and progression of this disease, including the amyloid cascade, which predominates the field of research for the past 30 years. The amyloidogenic pathway requires the endocytosis of the APP protein in early endosomes where it undergoes two proteolytic cleavages, first by β-secretase to produce the C99 fragment, and then by γ-secretase to produce the Aβ peptide. One of the current hypotheses is that abnormalities of endocytosis and dysfunction of the endolysosomal system in neurons would constitute one of the early neuropathological mechanisms of AD, well before the neurotoxic cascade generated by Aβ and amyloid deposits. We advocate the hypothesis that changes in membrane organization, particularly during aging or due to lipid imbalances, may exacerbate or promote these dysfunctions. For this study, we used a human neuroblastoma model overexpressing the mutant protein APPswe. We first verified the presence of typical AD endolysosomal abnormalities (enlarged endosomes, blocked vesicular trafficking), to which we also associated low exosome production, chronic stress conditions that we correlated with neuronal death. Initially incriminating continuously produced Aβ in these cells, we sought to reduce its impact by inhibiting γ-secretase activity. This did not ameliorate the stress, but instead aggravated it, leading us to consider that it is the C99 fragment of APP, i.e. the substrate of Aβ production, that is the central amyloid product in the neurotoxic cascade seen in APP-overexpressing cells. The deleterious effects of C99 must occur before those of Aβ, explaining the known precocity of endolysosomal alterations. Accumulating as a result of γ-secretase inhibition, the C99 fragment interacts further with the early endosome-specific Rab5 protein. Maturation of the latter is thus prevented, blocking vesicular trafficking of the endolysosomal system. As the interactions between C99 and Rab5 occur at the membrane level of endosomes, we have modified the lipid composition of the bilayer and explored the consequences on these interactions. For this purpose, we treated SH-SY5Y-APPswe cells with docosahexaenoic acid (DHA, C22:6 n-3), the major polyunsaturated fatty acid in neuronal membranes and known for its neuroprotective properties against Aβ toxicity and AD. The expected beneficial effect on neuronal survival was indeed observed, in parallel with the unblocking of endolysosomal trafficking and exosomal production. All these changes were correlated with a dispersion between C99 and Rab5 in the membrane, suggesting that DHA treatment may initiate membrane remodeling. This remodeling may lead to protein relocalization, whereby endosomes may exchange Rab5 for Rab7 to evolve into late endosomes, thereby overcoming the initial blockage. To our knowledge, this is the first evidence that DHA can correct a phenotype directly related to AD, but its ability to remodel the neuronal membrane was previously demonstrated by our team to preserve the neurotrophic CNTF signaling in the brain of aged mice. We do not know what mechanistic principles might govern these beneficial effects, which are certainly non-specific, but we assume that by preserving the organization of the membranes of aged or chronically stressed neurons, they may prevent or restore some of the damage suffered, increase the chances of neuronal survival and thus slow AD development.

 

Keywords : Alzheimer's disease, neuronal membrane, endolysosomal system, membrane lipids, docosahexaenoic acid, APP and amyloid fragments