Lipid droplets (LDs) are ubiquitous organelles that store metabolic energy in the form of neutral lipids (typically triacylglycerols and steryl esters). Beyond being inert energy storage compartments, LDs are dynamic organelles that participate in numerous essential metabolic functions. Cells generate LDs de novo from distinct sub-regions at the endoplasmic reticulum (ER), but what determines sites of LD formation remains a key unanswered question. Here, we review the factors that determine LD formation at the ER, and discuss how they work together to spatially and temporally coordinate LD biogenesis. These factors include lipid synthesis enzymes, assembly proteins, and membrane structural requirements. LDs also make contact with other organelles, and these inter-organelle contacts contribute to defining sites of LD production. Finally, we highlight emerging non-canonical roles for LDs in maintaining cellular homeostasis during stress.

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Fatty acids (FAs) are central cellular metabolites that contribute to lipid synthesis, and can be stored or harvested for metabolic energy. Dysregulation in FA processing and storage causes toxic FA accumulation or altered membrane compositions and contributes to metabolic and neurological disorders. Saturated lipids are particularly detrimental to cells, but how lipid saturation levels are maintained remains poorly understood. Here, we identify the cerebellar ataxia SCAR20-associated protein Snx14, an endoplasmic reticulum (ER)-lipid droplet (LD) tethering protein, as a novel factor required to maintain the lipid saturation balance of cellular membranes. We show that Snx14-deficient cells and SCAR20 disease patient-derived cells are hypersensitive to saturated FA (SFA)-mediated lipotoxic cell death that compromises ER integrity. Using APEX2-based proximity labeling, we reveal the protein composition of Snx14-associated ER-LD contacts and define a functional interaction between Snx14 and Δ-9 FA desaturase SCD1. We show that SCD1 is upregulated in SNX14KO cells, and SNX14KO-associated SFA hypersensitivity can be rescued by ectopic SCD1 overexpression. The hydrophobic PXA domain of Snx14 and its interaction with SCD1 are required for Snx14-mediated SFA protection function. Lipidomic profiling reveals that SNX14KO cells exhibit increased membrane saturation, and mimics the lipid profile of SCD1-inhibited cells. Altogether these mechanistic insights reveal a functional interaction between Snx14 and SCD1 in the ER network to maintain FA homeostasis and membrane saturation, defects in which may underlie the neuropathology of SCAR20.Significance StatementSCAR20 disease is an autosomal recessive spinocerebellar ataxia primarily affecting children, and results from loss-of-function mutations in the SNX14 gene. Snx14 is an endoplasmic reticulum (ER)-localized protein that localizes to ER-lipid droplet (LD) contacts and promotes LD biogenesis, but why Snx14 loss causes SCAR20 is unclear. Here, we demonstrate that Snx14-deficient cells and SCAR20 patient-derived fibroblasts have defective ER homeostasis and altered lipid saturation profiles. We reveal a functional interaction between Snx14 and fatty acid desaturase SCD1. Lipidomics shows Snx14-deficient cells contain elevated saturated lipids, closely mirroring SCD1-defective cells. Furthermore, Snx14 and SCD1 interact in the ER, and SCD1 over-expression rescues Snx14 loss. We propose that Snx14 maintains cellular lipid homeostasis, the loss of which underlies the cellular basis for SCAR20 disease.

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Isogenic cells can manifest distinct cellular fates for a single stress, however the nongenetic mechanisms driving such fates remain poorly understood. Here, we implement a robust multi-channel live-cell imaging approach to uncover noncanonical factors governing cell fate. We show that in response to acute glucose removal (AGR), budding yeast undergo distinct fates becoming either quiescent or senescent. Senescent cells fail to resume mitotic cycles following glucose replenishment but remain responsive to nutrient stimuli. Whereas quiescent cells manifest starvation-induced adaptation, senescent cells display perturbed endomembrane trafficking and defective nucleus-vacuole junction (NVJ) expansion. Surprisingly, we also show senescence occurs in the absence of lipid droplets. Importantly, we identify the nutrient-sensing linked kinase Rim15 as a key biomarker that predicts cell fates before AGR stress. We propose that isogenic yeast challenged with acute nutrient shortage contain determinants that influence their post-stress fate, and demonstrate that specific nutrient signaling, stress-response, endomembrane trafficking, and inter-organelle tether biomarkers are early indicators for long-term fate outcomes.