Spatial distribution of lipid droplets during starvation: Implications for lipophagy

Abstract

Survival during starvation depends largely on metabolic energy, which is stored in the form of neutral lipids in specialized organelles known as lipid droplets. The precursors for the synthesis of neutral lipids are also used for membrane biogenesis, which is required for cell growth and proliferation. Therefore cells must possess mechanisms to preferentially channel lipid precursors toward either membrane synthesis or lipid droplet storage, in response to nutrient status. How this partitioning is spatially regulated within the endoplasmic reticulum (ER) where lipid droplets co-localize, remains poorly understood. We have recently shown that at the onset of starvation lipid droplets concentrate at a perinuclear ER subdomain flanking the nucleus-vacuole junction (NVJ) and that this is crucial for maintaining proper nuclear shape and ER membrane organization. Here we show that disruption of the NVJ does not block the translocation and internalization of lipid droplets into the vacuole for their degradation, which takes place at later stages of starvation. We propose that alternative pathways of lipid droplet translocation from the ER to the vacuole may exist to enable stationary phase-induced lipophagy.

Keywords: lipid droplets; lipophagy; nuclear membrane; nucleus-vacuole junction.

Figure 1.

Deletion of NVJ1 does not disrupt lipophagy in budding yeast. (A) Wild-type (RS453), and the isogenic nvj1Δ (nvj1::HIS3MX6) and nvj1Δ atg15Δ (nvj1::HIS3MX6 atg15::TRP1) strains expressing a chromosomally integrated VPH1-mCherry fusion were grown in synthetic medium as previously described. Cultures were inoculated at OD₆₀₀ of 0.1 and imaged after 1 or 4 d of continuous growth at 30°C. LDs were labeled with BODIPY 493/503 as previously described. Cells were imaged using a Zeiss LSM880 confocal microscope and the ZEN2 software. Cells were visualized from the periphery by taking 10 optical sections, each 0.8 μm thick. A single mid-section is shown in all panels. At day 1, LDs concentrate at one side of the vacuole that is in contact with the nucleus. At day 4, LDs associate with vacuolar membrane domains that are devoid of Vph1-mCherry (see magnified inset for nvj1Δ cells). At this stage, and similar to the wild-type, many nvj1Δ and nvj1Δ atg15Δ cells display strong vacuolar BODIPY signal indicating the presence of internalized LDs. Bar, 5 μm. (B) Quantification of LD distribution shown in wild-type and nvj1Δ cells shown in A from 3 independent experiments. The schematic on the right depicts the LD distribution patterns quantified. Red, vacuole; green, LDs.