The power of yeast to model diseases of the powerhouse of the cell

Abstract

Mitochondria participate in a variety of cellular functions. As such, mitochondrial diseases exhibit numerous clinical phenotypes. Because mitochondrial functions are highly conserved between humans and Saccharomyces cerevisiae, yeast are an excellent model to study mitochondrial disease, providing insight into both physiological and pathophysiological processes.

Figure 1

Summary of OXPHOS in yeast and the multitude of patient phenotypes associated with pathogenic lesions that impact each individual complex. Complex II (brown), also referred to as SDH, is a physical link between the TCA cycle and the electron transport chain. Electrons derived from the oxidation of succinate to fumarate by Complex II ultimately reduce CoQ (QH₂). QH₂ engages one of two CoQ binding sites within Complex III (pink) where its two electrons reach a fork in the road. One electron ultimately reduces cytochrome c while the other electron is delivered to oxidized CoQ (Q) in the 2^nd CoQ binding site within Complex III producing a reactive semiquinone. The semiquinone is fully reduced to QH₂ upon the release of a second wave of electrons from another QH₂ completing the Q cycle. The release of electrons by QH₂ is coupled to the discharge of H⁺ into the IMS. Reduced cytochrome c donates its single electron to Complex IV (grey) which couples the reduction of O₂ to H₂O with the vectorial transport of H⁺ into the IMS. Complexes III and IV assemble in supercomplexes that increase the efficiency of the electron transport chain by substrate channeling. The proton-motive force established by Complex III and IV is harnessed by the Complex V (aka the ATP synthase) to produce ATP. The numerous phenotypes observed in patients with mutations associated with each individual OXPHOS complex is indicated.

Figure 2

Two non-mutually exclusive models of how mutations in SDH promote tumorigenesis. (A) SDH is a nexus between the TCA cycle and the electron transport chain. In the TCA cycle, it oxidizes succinate to fumarate. The two released hydrogens in turn reduce the FAD cofactor associated with SDHA/Sdh1p. Electrons are transferred from FADH₂ to a series of three Fe/S clusters in SDHB/Sdh2p and ultimately to oxidized CoQ (Q) generating reduced CoQ (QH₂) which donates its electrons to Complex III. In the cytosol, HIF-1 alpha is rapidly degraded subsequent to its prolyl hydroxylation (degradation indicated by faded coloring). (B) Mutations in SDH result in an accumulation of succinate which is transported into the cytosol by the dicarboxylate carrier in exchange for P_i. If the pathogenic mutation occurs in a subunit other than SDHA/Sdh1p, the flow of electrons is uncoupled from the reduction of CoQ and ROS are generated. ROS and succinate can both inhibit HIF-1 alpha prolyl hydroxylases leading to HIF-1 alpha stabilization. HIF-1 alpha induces the expression of genes involved in angiogenesis, proliferation, cell survival, and glycolysis; pathways that would benefit a tumor.

Figure 3

CL structure and schematic showing CL assembly and remodeling pathways in mitochondria. (A) Structure of CL. The position of the four attached acyl chains is indicated (R₁ thru R₄). (B) CL biosynthetic (boxed in yellow) and remodeling (boxed in blue) pathways. Pgs1p catalyzes the first and committed step in CL biosynthesis producing phosphatidylglycerophosphate from CDP-DAG and G3P (385). The recently identified phosphatidylglycerophosphate phosphatase, Gep4 (386), generates phosphatidylglycerol, the precursor of “immature” CL. Finally, “immature” CL is produced by Crd1p from phosphatidylglycerol and CDP-DAG. The acyl chains of “immature” CL are remodeled by the sequential action of the CL deacylase, Cld1p, and MLCL transacylase, Taz1p. The difference in acyl chain composition between “immature” and “mature” CL is illustrated on the right. CDP-DAG, cytidine diphosphatediphosphate-diacylglycerol; CMP, cytodine monophosphate; G3P, glycerol-3-phosphate; FA, fatty acid; PC, phosphatidylcholine; lyso-PC, lyso- phosphatidylcholine.

Figure 4

The power of yeast as a model for human mitochondrial disease. When the weaknesses of the yeast model (unicellular, absence of respiratory complex I, and homoplasmic mtDNA) are directly compared to its strengths (high degree of conservation of basic mitochondrial processes, easy genetics, robust biochemistry, ability to survive in the absence of a functional OXPHOS system, homoplasmic mtDNA, and capacity for large scale genetic, pharmacologic, and suppressor screens) with respect to modeling human mitochondrial disease, the value of yeast is clearly evident.