A lysosomal lipid transport pathway that enables cell survival under cholinelimitation
Lysosomes degrade macromolecules and recycle their nutrient content to support cell function and survival over a broad range of metabolic conditions. Yet, the machineries involved in lysosomal recycling of many essential nutrients remain to be discovered, with a notable example being choline, an essential metabolite liberated in large quantities within the lysosome via the degradation of choline-containing lipids. To identify critical lysosomal choline transport pathways, we engineered metabolic dependency on lysosome-derived choline in pancreatic cancer cells. We then exploited this dependency to perform an endolysosome-focused CRISPR-Cas9 negative selection screen for genes mediating lysosomal choline recycling. Our screen identified the orphan lysosomal transmembrane protein SPNS1, whose loss leads to neurodegeneration-like disease in animal models, as critical for cell survival under free choline limitation. We find that SPNS1 loss leads to massive accumulation of lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE) within the lysosome. Mechanistically, we revealed that SPNS1 is required for the efflux of LPC species from the lysosome to enable their reesterification into choline-containing phospholipids in the cytosol. Using cell-based lipid uptake assays, we determine that SPNS1 functions as a proton gradient-dependent transporter of LPC. Collectively, our work defines a novel lysosomal phospholipid salvage pathway that is required for cell survival under conditions of choline limitation, and more broadly, provides a robust platform to deorphan lysosomal gene functions.