Mitochondrial stress by SAH and homocysteine 1 Converging evidence of mitochondrial dysfunction in a yeast model of homocysteine metabolism imbalance

An elevated level of homocysteine, a thiol amino acid, is associated with various complex disorders. The cellular effects of homocysteine and its precursors Sadenosyl homocysteine (SAH) and Sadenosyl methionine (SAM) are however poorly understood. We used Saccharomyces cerevisiae as a model to understand the basis of pathogenicity induced by homocysteine and its precursors. Both homocysteine and SAH but not SAM inhibited the growth of str4Δ strain (which lacks the enzyme that converts homocysteine to cystathionine mimicking vascular cells). Addition of SAM abrogated the inhibitory effect of SAH but not that of homocysteine indicating that increase in SAM/SAH ratio is sufficient to overcome SAH mediated growth defect but not that of homocysteine. Also, the transcriptomic profile of SAH and homocysteine showed gross dissimilarity based on gene enrichment analysis. Further, compared to homocysteine, SAH treatment caused a higher level of oxidative stress in the cells. However, unlike previously reported response in wild type (Kumar, A. et al. Biochem J 2006), str4Δ strain did not exhibit endoplasmic reticulum stress response. This suggests that homocysteine induces varied response depending on the flux of homocysteine metabolism. We also observed altered expression of mitochondrial genes, defective membrane potential and fragmentation of mitochondrial network together with the increased expression of fission genes indicating that the imbalance in homocysteine metabolism has major affect on mitochondrial functions. Further, treatment of cells with homocysteine or SAH resulted in apoptosis as revealed by annexin V staining and TUNEL assay. Cumulatively, our results suggest that elevated levels of homocysteine leads to mitochondrial dysfunction which could potentially initiate pro-apoptotic pathways and this could be one of the mechanisms underlying homocysteine induced pathogenecity.