Using yeast as a model organism to study human disease is not a new concept. Yeast are useful for this purpose for many reasons as discussed in a previous post, but potentially of most importance is that yeast cell biology is similar to that of mammalian cells, and yeast have homologues of many heritable genes causing human disease. Using yeast to understand ancient cell biology and how mutations in conserved genes results in pathologies started years ago by Leland Hartwell, who used yeast to characterize cell cycle regulation. Through yeast genetics, he identified mutations that cause cell cycle defects. These mutant strains have been used to understand cell cycle regulation and to discover novel cancer therapeutics, eg., palbociclib. Another example in the field of cancer research is based on work out of the laboratory of Rodney Rothstein, demonstrating that overexpression of cancer causing genes in yeast deletion strains reveals genes that contribute to cancer-promoting pathways. This line of inquiry is leading to the identification of novel targets for cancer therapeutics.
One recent example in the media is the use of yeast to understand the pathologies behind Parkinson’s disease. Daniel Tardiff and his colleagues in the laboratory of Susan Lindquist at MIT’s Whitehead Institute, expressed human alpha-synuclein in yeast and found that the protein behaves similarly in yeast cells as it does in humans cells, ultimately leading to cellular toxicity. Yeast expressing alpha-synulciein can be used in screens to find genes and/or compounds that enhance or suppress the toxic phenotype. Through a phenotypic screen, a small molecule was identified that rescued the toxic phenotype in yeast as well as in Parkinson’s patient neuronal cells. Susan Lindquist and others started Yumanity Therapeutics to discover therapeutics for diseases caused by misfolded proteins such as Alzheimer’s and ALS using a platform based on yeast phenotypic screens.
The aforementioned are great examples of how yeast can be used as a model to study human disease and discover novel therapeutics. But to better understand the potential of yeast cells, I am going to go to focus on a recent publication on the use of yeast to find therapeutic targets for ATP synthase disorders. ATP synthase disorders are a group of mitochondrial disorders. ATP synthase synthesizes the fuel required for cellular function, ATP. The inability to properly synthesize ATP leads to the early onset of diseases such as NARP (neuropathy, ataxia and retinosa pigmentosa) and Leigh syndrome.
Aiyer and her colleagues used a yeast strain lacking Fmc1 to study ATP synthase disorders. Fmc1 aids in the assembly of ATP synthase at high temperatures. If ATP synthase does not assemble properly, it leads to defects in the formation of the respiratory chain. Yeast lacking Fmc1 (fmc1Δ) do not grow well on non-fermentable media and can therefore be used in a screen to find compounds that rescue the ATP synthase deficiency.
For the chemical screen, fmc1Δ yeast were spread on an agar plate. Then, small disks of filter paper with droplets of DMSO-dissolved compounds were placed on top of the yeast. If there is a compound that rescues the ATP synthase deficiency, researchers will observe yeast growth around the disk.
Out of this screen, researchers identified sodium pyrithione, an antimicrobial used in many products, as a hit. In the above image, there is a ring of yeast growth around the 3filters containing NaPT but not the DMSO control. This compound also enhanced the survival of cytoplasmic hybrid cells derived from NARP patients.
Sodium pyrithione (NaPT):
Researchers then treated a yeast gene deletion collection with NaPT to determine its mechanism of action and found that strains with mitochondrial protein sorting gene deletions were the most sensitive to NaPT: TIM17 and TIM23.
Mitochondrial protein sorting
The TIM17 and TIM23 deletion strains are specifically sensitive to NaPT. TIM23 plays a role in transporting subunits of ATP synthase and respiratory chain complexes. Through further biochemical studies it was determined that NaPT is affecting mitochondrial membrane protein sorting. fmc1Δ yeast treated with NaPT display an inhibition of protein targeting to the matrix and an enhancement of protein targeting to the inner membrane, and the inner membrane is where ATP synthase functions as shown in the image below:
This phenomenon was recapitulated genetically. Tim21 is a regulatory subunit of the TIM23 complex and regulates the translocation TIM23 from the mitochondrial matrix to the inner membrane. It was shown that over-expression of Tim21 in fmc1Δ cells rescues the respiratory growth defect. The expression of TIMM21 in NARP patient cells lead to an increase in cell survival.
Interestingly, Tim21 also promotes the formation of oxidative phosphorylation complexes in yeast and humans, and expression of Tim21 in yeast reduces aggregates of unassembled ATP synthase subunits in the mitochondrial matrix. Therefore, targeting mitochondrial protein import and oxidative phosphorylation complex assembly may prove to be beneficial in reversing the affects of mitochondrial disease. The mitochondrial sorting pathway warrants further investigation as a target for these mitochondrial disorders, and investigators are looking into how NaPT or related compounds might be further developed into therapeutics. We look forward to hearing about the progress.
This is an excellent example of how yeast can be used as a disease model in an unbiased phenotypic screen. This is similar to the type of work we are doing at Perlstein Lab on other Mendelian diseases affecting ancient cell biology. We too believe in the power of the phenotypic screens in yeast (and other model org) to identify precision orphan drug candidates.