Lack of isocitrate lyase in Chlamydomonas leads to changes in carbon metabolism and in the response to oxidative stress under mixotrophic growth

Abstract

SUMMARYIsocitrate lyase is a key enzyme of the glyoxylate cycle. This cycle plays an essential role in cell growth on acetate, and is important for gluconeogenesis as it bypasses the two oxidative steps of the tricarboxylic acid (TCA) cycle in which CO 2 is evolved. In this paper, a null icl mutant of the green microalga Chlamydomonas reinhardtii is described. Our data show that isocitrate lyase is required for growth in darkness on acetate (heterotrophic conditions), as well as for efficient growth in the light when acetate is supplied (mixotrophic conditions). Under these latter conditions, reduced acetate assimilation and concomitant reduced respiration occur, and biomass composition analysis reveals an increase in total fatty acid content, including neutral lipids and free fatty acids. Quantitative proteomic analysis by 14 N/ 15 N labelling was performed, and more than 1600 proteins were identified. These analyses reveal a strong decrease in the amounts of enzymes of the glyoxylate cycle and gluconeogenesis in parallel with a shift of the TCA cycle towards amino acid synthesis, accompanied by an increase in free amino acids. The decrease of the glyoxylate cycle and gluconeogenesis, as well as the decrease in enzymes involved in b-oxidation of fatty acids in the icl mutant are probably major factors that contribute to remodelling of lipids in the icl mutant. These modifications are probably responsible for the elevation of the response to oxidative stress, with significantly augmented levels and activities of superoxide dismutase and ascorbate peroxidase, and increased resistance to paraquat.

The Path to Triacylglyceride Obesity in the sta6 Strain of Chlamydomonas reinhardtii

“…Support for this proposal comes from the recent study of an ICL1 deletion mutant (61), which is devoid of a glyoxylate cycle and displays many anomalies in central carbon metabolism.…”

“…Hence sensitive genes displaying a strong acetate boost prove to also display a strong transcriptional sensitivity to the presence/absence of exogenous acetate. Notably, 7 of these 13 proteins, listed in the previous paragraph, are also expressed aberrantly in the ICL1 mutant (61).…”

“…5 and Table 7). Several of the genes in this cohort are also anomalously expressed in a mutant strain that has a deletion of the isocitrate lyase gene (ICL1) and hence is blocked in its glyoxylate cycle (61). We suggest that the transcription patterns of this "sensitive" gene set are indicative of the biochemical pathways being pursued by cells under a given set of environmental/genetic conditions and that they may be less informative in indicating the defining differences between starch-forming and starch-null cells.…”

Synergism between Inositol Polyphosphates and TOR Kinase Signaling in Nutrient Sensing, Growth Control, and Lipid Metabolism in Chlamydomonas

“…Two recent studies implicated the TCA pathway in carbon utilization and lipid accumulation. A Chlamydomonas mutant missing isocitrate lyase, a key enzyme in the glyoxylate shunt within the TCA cycle, was found to have acetate-specific growth defects and to overaccumulate lipids (Plancke et al, 2014). A metabolic modeling study also found that changes in TCA flux were required to maintain metabolic balance when switching from phototrophic to mixotrophic growth (Chapman et al, 2015).…”

Impact of thiamine metabolites and spent medium from Chlorella sorokiniana on metabolism in the green algae Auxenochlorella prototheciodes

“…The glyoxylate cycle plays an essential role in acetate metabolism and also allows for bypass of OGDH. Plancke et al created a mutant of the green algae Chlamydomonas that lacked an isocitrate lyase gene [31]. They found that this mutant strain lost its ability to grow on acetate.…”