Branched-Chain Amino Acid Catabolism Impacts Triacylglycerol Homeostasis in Chlamydomonas reinhardtii

Abstract

Nitrogen (N) starvation-induced triacylglycerol (TAG) synthesis, and its complex relationship with starch metabolism in algal cells, has been intensively studied; however, few studies have examined the interaction between amino acid metabolism and TAG biosynthesis. Here, via a forward genetic screen for TAG homeostasis, we isolated a Chlamydomonas (Chlamydomonas reinhardtii) mutant (bkdE1a) that is deficient in the E1a subunit of the branched-chain ketoacid dehydrogenase (BCKDH) complex. Metabolomics analysis revealed a defect in the catabolism of branched-chain amino acids in bkdE1a. Furthermore, this mutant accumulated 30% less TAG than the parental strain during N starvation and was compromised in TAG remobilization upon N resupply. Intriguingly, the rate of mitochondrial respiration was 20% to 35% lower in bkdE1a compared with the parental strains. Three additional knockout mutants of the other components of the BCKDH complex exhibited phenotypes similar to that of bkdE1a. Transcriptional responses of BCKDH to different N status were consistent with its role in TAG homeostasis. Collectively, these results indicate that branched-chain amino acid catabolism contributes to TAG metabolism by providing carbon precursors and ATP, thus highlighting the complex interplay between distinct subcellular metabolisms for oil storage in green microalgae.

The Effects of Biostimulants, Biofertilizers and Water-Stress on Nutritional Value and Chemical Composition of Two Spinach Genotypes (Spinacia oleracea L.)

“…Moreover, the application of humic-like substances was shown to increase fatty acids content in microalgae, without however any postulations regarding the mechanisms of action being reported [62]. Amino acids application may also have an effect on fatty acids content, since specific amino acids synthesized in plants such as Leucine, Isoleucine, and Valine contain an a branched aliphatic chain and their degradation produces acetyl-CoA which is a substrate for fatty acids biosynthesis [63].…”

Subcellular Energetics and Carbon Storage in Chlamydomonas

“…Another example of the possible occurrence of an ATP import from mitochondria to chloroplast is the characterization of the bckdh mutants deficient in the E1α subunit of the branched-chain ketoacid dehydrogenase (BCKDH), a key enzyme in the catabolism of branched chain amino acids (BCAA) in the mitochondria [70]. The bckdh mutants made 30% less oil with a reduced rate of mitochondrial respiration (20–30% less).…”

“…The bckdh mutants made 30% less oil with a reduced rate of mitochondrial respiration (20–30% less). The reduction in total lipid (including oil content) in the bckdh mutants is considered due to a shortage in the supply of acetyl-CoA as well as ATP (which are both products of BCAA catabolism in mitochondria) to de novo fatty acid synthesis in the chloroplast [70].…”

“…In addition to above key steps of de novo fatty acid synthesis, precursor supply i.e., acetyl-CoA has been increasingly shown to play a critical role in determining cellular oil content [5,70,89]. Under photoautotrophic conditions, pyruvate is usually considered the “major” source of plastidial acetyl-CoA, a reaction catalyzed by pyruvate dehydrogenase (PDH).…”

Potassium channel KCN11 is required for maintaining cellular osmolarity during nitrogen starvation to control proper cell physiology and TAG accumulation in Chlamydomonas reinhardtii

“…These studies documented the changes in significant physiological reprogramming, such as ribosome abundance, starch accumulation, and organization of the thylakoid membrane [5][6][7][8][9]. Interest in these pathways has renewed recently because of the use of C. reinhardtii as a reference organism for understanding triacylglycerol (TAG) accumulation pathways induced by N starvation [10][11][12][13][14][15][16].…”