A Study in SCAR20 Neurologic Disorder Reveals Defective Cellular Lipid Homeostasis

Fatty acids (FAs) are important cellular metabolites that are utilized by the cells to perform important functions such as the generation of ATP, membrane biosynthesis, and cell signaling. Dysregulation in FA processing and storage causes toxic FA accumulation which alters membrane compositions and contributes to metabolic and neurological disorders. Excess lipids are stored as lipid droplets (LDs) which sequester toxic FAs and serve as metabolic buffers to maintain lipid and energy homeostasis. LDs emerge from the endoplasmic reticulum (ER) but how their formation is regulated is not completely understood. Recently, we identified sorting nexin family protein Snx14, implicated in cerebellar ataxia disease SCAR20, as a novel factor that enriches at ER-LD contacts following exogenous FA treatment independently of Seipin and promotes FA-induced LD growth. Loss of Snx14 perturbs LD morphology whereas Snx14 overexpression extends ER-LD contacts and promotes LD biogenesis. Proximity-based APEX2 labeling revealed the enrichment of Snx14 at ER-LD contacts during LD biogenesis. Capitalizing on this APEX technology, we also utilize Snx14-APEX2 localization to dissect the protein composition of ER-LD contact sites. We identify proteins involved in fatty acid activation, desaturation, and triacylglycerol synthesis as being enriched at ER-LD contacts, indicating these contact sites serve as lipogenic sub-domains of the ER network. Furthermore, we identify the major delta-9 FA desaturase SCD1 as a key interacting partner of Snx14. Consistent with this, Snx14-deficient cells are hypersensitive to saturated fatty acid (SFA)-mediated lipotoxic cell death that compromises ER integrity. We show that SCD1 is upregulated in SNX14-KO cells, and Snx14-associated SFA hypersensitivity can be rescued by ectopic SCD1 overexpression. The lipid-associated PXA domain of Snx14 and its interaction with SCD1 are required for Snx14-mediated SFA protection function. Snx14 loss mimics SCD1 inhibition and causes accumulation of free FAs and increased membrane lipid saturation. Altogether these mechanistic insights reveal a role for ER-LD contacts as lipogenic ER sub-domains, and Snx14 as an ER-LD tether with a key role in maintaining cellular FA homeostasis through a functional interaction with SCD1, defects of which may underlie the neuropathology of SCAR20.