Abnormal accumulation of lipid droplets in neurons induces the conversion of alpha-Synuclein to proteolytic resistant forms in a Drosophila model of Parkinson’s disease

Abstract

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by alpha-synuclein (αSyn) aggregation and associated with abnormalities in lipid metabolism. The accumulation of lipids in cytoplasmic organelles called lipid droplets (LDs) was observed in cellular models of PD. To investigate the pathophysiological consequences of interactions between αSyn and proteins that regulate the homeostasis of LDs, we used a transgenic Drosophila model of PD, in which human αSyn is specifically expressed in photoreceptor neurons. We first found that overexpression of the LD-coating proteins Perilipin 1 or 2 (dPlin1/2), which limit the access of lipases to LDs, markedly increased triacylglyclerol (TG) loaded LDs in neurons. However, dPlin-induced-LDs in neurons are independent of lipid anabolic (diacylglycerol acyltransferase 1/midway, fatty acid transport protein/dFatp) and catabolic (brummer TG lipase) enzymes, indicating that alternative mechanisms regulate neuronal LD homeostasis. Interestingly, the accumulation of LDs induced by various LD proteins (dPlin1, dPlin2, CG7900 or KlarsichtLD-BD) was synergistically amplified by the co-expression of αSyn, which localized to LDs in both Drosophila photoreceptor neurons and in human neuroblastoma cells. Finally, the accumulation of LDs increased the resistance of αSyn to proteolytic digestion, a characteristic of αSyn aggregation in human neurons. We propose that αSyn cooperates with LD proteins to inhibit lipolysis and that binding of αSyn to LDs contributes to the pathogenic misfolding and aggregation of αSyn in neurons.

Functions of Stress-Induced Lipid Droplets in the Nervous System

“…In the adult retina, part of the PNS, several different genetic models of neurodegeneration lead to an increase in LDs in glial-like retinal pigment cells (RPCs) ( Liu et al, 2015 ; Liu et al, 2017 ; Cabirol-Pol et al, 2018 ; Van Den Brink et al, 2018 ; Yeshaw et al, 2019 ; Girard et al, 2020 ; Muliyil et al, 2020 ). Sparse LDs have also been reported in Drosophila CNS and photoreceptor neurons and, in the latter, it is known that they increase in abundance in several neurodegeneration models ( Van Den Brink et al, 2018 ; Wat et al, 2020 ; Girard et al, 2021 ). In both the mammalian and Drosophil a nervous systems, a growing body of evidence indicates that a common feature of many of the stresses and pathologies that induce glial LDs is redox imbalance, which is associated with high levels of reactive oxygen species (ROS) ( Bailey et al, 2015 ; Liu et al, 2015 ; Liu et al, 2017 ; Ioannou et al, 2019 ; Cheng et al, 2020 ; Muliyil et al, 2020 ).…”

“…In the yeast S. cerevisiae , α-synuclein inhibits growth and this is rescued by inhibition of Pah1, a Lipin phosphatidate phosphatase, suggesting that diacylglycerol synthesis is harmful, contributing to toxicity ( Soste et al, 2019 ). A closer functional link to TAG metabolism is suggested by a Drosophila model of Parkinson’s disease where human α-synuclein is expressed in photoreceptor neurons ( Girard et al, 2021 ). Coexpression of the perilipin Lsd2 induces LDs in photoreceptors, which recruit α-synuclein to their surface and increases the proportion of protease-resistant α-synuclein, a characteristic associated with α-synuclein aggregation and neurodegeneration ( Cremades et al, 2012 ; Suzuki et al, 2015 ; Girard et al, 2021 ).…”

“…In C. elegans , it has also been shown that mutations in an Abhydrolase Domain-Containing Protein 5 (ABHD5)/Comparative Gene Identification-58 (CGI-58) orthologue, a known co-activator of ATGL, or overexpression of DGAT1/2 orthologues leads to ectopic LDs in neurons ( Yang et al, 2020a ). Similarly, some perilipins protect LDs from lipolysis, and overexpression of either the Lsd-1 or Lsd-2 perilipins in Drosophila photoreceptors results in a large increase in the usually sparse LDs in these neurons ( Girard et al, 2021 ). Collectively, these findings provide evidence that neurons do not usually accumulate LDs in vivo, because they actively turnover TAGs, favouring lipolysis over biosynthesis Figure 2 .…”

Lipotoxicity Downstream of α-Synuclein Imbalance: A Relevant Pathomechanism in Synucleinopathies?

“…An unbiased proteomic analysis of αS expression in this system revealed significant enrichment of KEGG pathways that included FA biosynthesis (upregulated) and FA metabolism (downregulated) [ 39 ]. In a transgenic Drosophila melanogaster model of PD, in which human αS is specifically expressed in photoreceptor neurons, the accumulation of LDs induced by various LD proteins (perilipin1, perilipin2 or CG7900) was synergistically amplified by the co-expression of αS [ 40 ]. αS localized to LDs in both Drosophila photoreceptor neurons and in human neuroblastoma cells [ 40 ].…”

“…In a transgenic Drosophila melanogaster model of PD, in which human αS is specifically expressed in photoreceptor neurons, the accumulation of LDs induced by various LD proteins (perilipin1, perilipin2 or CG7900) was synergistically amplified by the co-expression of αS [ 40 ]. αS localized to LDs in both Drosophila photoreceptor neurons and in human neuroblastoma cells [ 40 ]. As far as vertebrate animal models are concerned (which naturally possess αS), observations from cultured mouse/rat neurons as well as mouse models have been reported.…”