Abstract
Autophagy is a highly conserved catabolic process in eukaryotic cells by which waste cellular components are recycled to maintain growth in both favorable and stress conditions. Autophagy has been linked to lipid metabolism in microalgae; however, the mechanism underlying this interaction remains unclear. In this study, transgenic Chlamydomonas reinhardtii cells that stably express the red fluorescent protein (mCherry) tagged-ATG8 as an autophagy marker were established. By using this tool, we were able to follow the autophagy process in live microalgal cells under various conditions. Live-cell and transmission electron microscopy (TEM) imaging revealed physical contacts between lipid droplets and autophagic structures during the early stage of nitrogen starvation, while fusion of these two organelles was observed in prolonged nutritional deficiency, suggesting that an autophagy-related pathway might be involved in lipid droplet turnover in this alga. Our results thus shed light on the interplay between autophagy and lipid metabolism in C. reinhardtii, and this autophagy marker would be a valuable asset for further investigations on autophagic processes in microalgae.
Highlights
Microalgae are plant-like organisms that inhabit freshwater, inter-tidal, marine, and extreme environments
These results suggested that autophagy might be involved in the biogenesis of lipid droplets in this alga
The mCherry-autophagy-related protein 8 (ATG8) fusion construct was expressed in C. reinhardtii using pChlamiRNA3int expression system
Summary
Microalgae are plant-like organisms that inhabit freshwater, inter-tidal, marine, and extreme environments. Two studies demonstrated that inhibition of autophagic process by treatment with autophagy inhibitors including concanamycin A, bafilomycin A1, and wortmannin reduced the number of lipid droplets accumulated in C. reinhardtii cells under nutrient starvation [15,16]. These results suggested that autophagy might be involved in the biogenesis of lipid droplets in this alga. By using live-cell imaging, we observed physical interactions between mCherry-labeled autophagosomes and lipid droplets in this green alga under nitrogen starvation To our knowledge, this provides the first visual evidence for lipid droplet–autophagosome interaction in microalgae
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