| Details: |
Fatty acids (FAs) are important cellular metabolites which are utilized by the cells for the generation of ATP, membrane biosynthesis, and cell signaling. An imbalance in the processing of these FAs causes them to accumulate resulting in lipotoxicity – a primary cause behind several metabolic disorders. Hence cells require to spatially organize FAs for efficient metabolism. Excess lipids are stored for future use in the form of lipid droplets (LDs) which emerge from the endoplasmic reticulum (ER). In yeast, the LDs spatially cluster at the Nuclear-ER Vacuole Junction (NVJ) under nutrient depletion. However, how the budding of LDs from the ER is regulated is not completely understood. Recently, we showed that in mammals sorting nexin protein Snx14, implicated in cerebellar ataxia disease SCAR20, is a novel marker for ER-LD contacts where it promotes oleate-induced LD growth (Datta, et al, JCB, 2019). We used proximity based APEX2 technology and structure-function analysis to show that Snx14 tethers LDs to ER to maintain LD homeostasis. Loss of Snx14 perturbs LD morphology whereas Snx14 overexpression extends ER-LD contacts and promotes LD biogenesis.
To further understand the implications of Snx14 in regulating ER-LD contacts, we conducted a proteomic screen to determine the proteome at ER-LD contacts using the proximity based APEX2 method. Proximity labeling of Snx14 revealed known LD proteins as well as FA desaturase SCD1 (stearoyl-CoA desaturase-1).The enzyme SCD1 catalyses the conversion of saturated fatty acids (SFAs) to monounsaturated FAs. SFAs such as palmitate are lipotoxic to the cells and are often associated with heart and brain diseases. Interestingly, we found that SNX14-KO cells are hyper-sensitive to palmitate-induced lipotoxicity, similar to SCD1-inhibited conditions. Lipid analysis indicates defects in palmitate processing in SNX14-KO cells. We showed that Snx14 overexpression induces clustering of the SCD1 enzyme at ER-LD contact sites. We also found that SCD1 over-expression can rescue SNX14-KO cells. We hypothesize that Snx14 affects SCD1 activity via recruiting SCD1 to ER-LD contact sites where it engages SFAs in order to process them prior to their incorporation into mature lipids. Current and future studies aim to dissect mechanistically the interplay of Snx14 and SCD1 in maintaining lipid homeostasis.
|