Intersections between alpha-synuclein and cholesterol: an unsolved case

Abstract

A pathological sign of synucleinopathies, including Parkinson's disease, is the aggregation and fibrillation of α-synuclein (αsyn), a small presynaptic protein characterized by a high lipid binding affinity. How α-syn and cholesterol are interconnected in the context of these neurodegenerative disorders remains an open question. In this work, we investigated cholesterol homeostasis in a neuronal model of α-syn overexpression (WT α-syn cells). We found that both free cholesterol and cholesteryl ester levels were increased by α-syn overexpression. The raise in cholesteryl esters was associated with an increased acylCoA:cholesterol acyltransferase activity. While cholesteryl esters were part of the lipid droplet core in WT α-syn cells, free cholesterol was located in membrane compartments and lysosomes. Co-staining experiments revealed that cholesterol was accumulated in lysosomes in cells overexpressing α-syn, though the distribution of these organelles and the expression of lysosomal markers, LAMP-1 and Lysotracker, were not altered. In order to determine the mechanism for the increased cholesterol levels, the status of the transcription factor sterol regulatory element-binding protein (SREBP)-2 was evaluated. SREBP-2 nuclear translocation was induced by α-syn overexpression, in agreement with the in silico analysis carried out through the MyProteinNet server (Yeger-Lotem lab). The latter showed a relationship between α-syn and sterol regulatory element-binding gen (SREBF-2). Paradoxically, the activation of SREBP-2 was not accompanied by the expected upregulation of involved canonical downstream genes in cholesterol synthesis, such as 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) and 24-dehydrocholesterol reductase. Moreover, cholesterol content was not altered by the inhibition of HMGCR by mevastatin in WT α-syn cells, thus implying that cholesterolgenesis was not responsible for its increase. Our findings suggest that α-syn overexpression disrupts cholesterol trafficking resulting in the lysosomal sequestration and consequent ER transport impairment. Further studies should be performed to ascertain the mechanisms and functional implications of cholesterol uptake and intracellular trafficking alterations associated with synucleinopathies. Funding: ANPCyT, CONICET, and UNS.