Triacylglycerol homeostasis: insights from yeast

Abstract

The endemic increase in lipid-associated disorders such as obesity and type 2 diabetes mellitus has placed triacylglycerol metabolism and its associated organelle, lipid droplets, in the spotlight of biomedical research. Key enzymes of triacylglycerol metabolism are structurally and functionally conserved between yeast and mammalian cells, and studies in yeast have contributed significantly to the understanding of their biological function(s). Based on these similarities, studies performed in yeast may provide further significant mechanistic insight into the molecular basis of triacylglycerol homeostasis and its important physiological roles in healthy and diseased cells.

FIGURE 1.

Enzymes involved in TAG homeostasis and their spatial organization. Gene names (italicized) and their functions are provided in supplemental Table 1 and in text. The area in light blue indicates the ER; the area in red indicates LD. The inset in gray indicates alternative pathways for the synthesis of potential TAG precursors; their specific contribution to TAG formation is unclear. Dashed lines indicate multiple enzymatic steps. FFA, free FA; LPA, lysophosphatidic acid; DAG-PP, DAG pyrophosphate; MAG, monoacylglycerol; Gro, glycerol; DHAP, dihydroxyacetone phosphate; PL, phospholipid; PI, phosphatidylinositol; PG, phosphatidylglycerol; GPI, glycosylphosphatidylinositol. Note the dual functionality of enzymes encoded by TGL3 and TGL5 genes as TAG lipases and TAG:phospholipid acyltransferases (18).

FIGURE 2.

Analyzing LD morphology as an indicator of TAG homeostasis using different staining and microscopic techniques. A–E, left panels, fluorescence; right panels, transmission (differential interference contrast). Scale bars = 10 μm. A, LD visualization in wild-type cells using a green fluorescent protein-tagged reporter construct (1). B, accumulation of LD in an snf1 deletion strain that displays hyperactive Acc1p (8) and TAG accumulation. Note that the size of LD does not increase in this mutant but rather their number (BODIPY 493/503 staining). C, morphologically altered LD in an fld1 mutant (42, 43) lacking the yeast ortholog of mammalian BSCL2, implicated in Berardinelli-Seip congenital lipodystrophy type 2 (BODIPY 493/503 staining). D, LD accumulation in an obese yeast mutant lacking the two major TAG lipases, Tgl3p and Tgl4p (Nile red staining) (1). E, a quadruple mutant lacking the four acyltransferases involved in TAG synthesis (35, 36) that also lacks LD and any detectable Nile red staining in the 550–570-nm emission range (see Ref. for experimental details). F, coherent anti-Stokes Raman scattering microscopy of a wild-type strain (left panel) and the quadruple mutant (right panel). Coherent anti-Stokes Raman scattering enables label-free detection of LD based on the spectroscopic properties of lipid molecules. A–C and E are courtesy of Heimo Wolinski (University of Graz); F is courtesy of Lu Fake and Huang Zhiwei (National University of Singapore).