Target of rapamycin (TOR) plays a critical role in triacylglycerol accumulation in microalgae

Abstract

Most microalgae produce triacylglycerol (TAG) under stress conditions such as nitrogen depletion, but the underlying molecular mechanism remains unclear. In this study, we focused on the role of target of rapamycin (TOR) in TAG accumulation. TOR is a serine/threonine protein kinase that is highly conserved and plays pivotal roles in nitrogen and other signaling pathways in eukaryotes. We previously constructed a rapamycin-susceptible Cyanidioschyzon merolae, a unicellular red alga, by expressing yeast FKBP12 protein to evaluate the results of TOR inhibition (Imamura et al. in Biochem Biophys Res Commun 439:264-269, 2013). By using this strain, we here report that rapamycin-induced TOR inhibition results in accumulation of cytoplasmic lipid droplets containing TAG. Transcripts for TAG synthesis-related genes, such as glycerol-3-phosphate acyltransferase and acyl-CoA:diacylglycerol acyltransferase (DGAT), were increased by rapamycin treatment. We also found that fatty acid synthase-dependent de novo fatty acid synthesis was required for the accumulation of lipid droplets. Induction of TAG and up-regulation of DGAT gene expression by rapamycin were similarly observed in the unicellular green alga, Chlamydomonas reinhardtii. These results suggest the general involvement of TOR signaling in TAG accumulation in divergent microalgae.

TOR (target of rapamycin) is a key regulator of triacylglycerol accumulation in microalgae

“…7 Based on these findings, it was concluded that the TOR-signaling pathway negatively regulates TAG accumulation in microalgae. 8 Mammalian TOR (mTOR) inhibitors are generally used as immunosuppressors as well as antineoplastic drugs in various cancer treatments. 9 Therefore, development of new chemicals that specifically inhibit mTOR activity has been an intensive research area for future medical applications.…”

Analysis of triacylglycerol accumulation under nitrogen deprivation in the red alga Cyanidioschyzon merolae

“…This is consistent with the reports on other microalgae including Cy. merolae, which accumulate nonpolar lipids under nutrient deprivation (Chisti, 2007;Hu et al, 2008;Imamura et al, 2015;Iwai et al, 2014;Moellering & Benning, 2010;Msanne et al, 2012;Sakurai et al, 2014;Siaut et al, 2011;Tornabene et al, 1983). Compared with other microalgae, Cy.…”

“…We still need to know which of the three DGATs in Cy. merolae (CME100C, CMJ162C and CMQ199C) (Imamura et al, 2015;Sumiya et al, 2015) determines the specificity of acylation.…”

“…A nuclear genetic engineering (Ohnuma et al, 2008) and synchronous culture system (Itoh et al, 1996) have been established. These genetic technologies are expected to improve growth performance and to produce TAG as shown by previous studies (Imamura et al, 2015;Sumiya et al, 2015). The fatty acid composition of lipid classes is very simple, consisting of mainly 16 : 0, 18 : 0, 18 : 1(9) and 18 : 2(9,12) fatty acids (Sato & Moriyama, 2007).…”

“…merolae has unique properties as a producer for the source of bioenergy. Imamura et al (2015) showed that nitrogen deficiency increased the content of oil in Cy. merolae, but detailed characterization of TAG has not been attempted.…”