Apolipoprotein E (APOE) is a 34kDa secreted protein that plays a crucial role in lipid metabolism. Human APOE has three common isoforms: APOE2, APOE3, and APOE4. Each isoform differs by a single amino acid substitution and is linked to various diseases, with Alzheimer's disease (AD) being the most well-known. In particular, the APOE4 allele is a genetic risk factor for late-onset AD, while APOE2 offers a protective effect.
In the central nervous system, APOE is mainly secreted by astrocytes and is part of a metabolic crosstalk between neurons and astrocytes. Astrocytic APOE acquires cholesterol via ATP-binding cassette transporter A1 (ABCA1) and nourishes neurons with cholesterol. On the other hand, the toxic fatty acids produced in neurons (as by-products of their activities) are loaded on APOE and transferred to astrocytes to be transiently stored in lipid droplets (LD) before degradation by ß-oxidation.
Our group recently demonstrated that oleic acid, loaded onto bovine serum albumin (which acts as a carrier), is taken up by astrocytes and metabolized into triacylglycerol (TAG), which is then stored in LD. These fatty astrocytes, which have accumulated excess TAG in lipid droplets (a phenomenon often observed in aging brains or under stress), secrete APOE associated with TAG rather than cholesterol. In this process, APOE4 was more efficient at loading TAG than the other isoforms. These findings suggest the existence of a second, previously unknown mechanism of APOE lipidation that comes into play when astrocytes are exposed to an excess of fat. My aims are to investigate this new pathway through which APOE can acquire an unphysiological (in the context of the brain) lipid, TAG, in fatty astrocytes and how it is affected by APOE polymorphism.
It is known that, in AD and normal aging, astrocytes undergo significant changes that affect their function, a process known as “astrocyte reactivity”. In Chapter 1, I investigate the extent to which our in vitro astrocytes with accumulated LD exhibit reactivity patterns similar to those of astrocytes from patients with different neurodegenerative diseases. In Chapters 2 and 3, I studied the mechanisms underlying the secretion of TAG-loaded APOE particles and how this process is affected by APOE polymorphism. I used pharmacological tools to perturb TAG metabolism in the human astrocytic cell line CCF-STTG1 and in iPSC-derived astrocytes. Finally, in Chapter 4, I developed protocols and conducted proof-of-concept experiments with Christoph Lamy (Unit of Anatomy, University of Geneva) and Aurélien Lathuilière (Department of Readaptation and Geriatrics, University of Geneva) to determine whether the new APOE lipidation pathway might also be involved in AD patients.
