Chemical genetic analysis of zinc-neutral lipid interaction in yeast

Recently, the beneficial effects of zinc supplementation on pathological conditions associated with the accumulation of triglycerides (TAG) and cholesterol, also accompanied by reduced levels of zinc, suggest an interaction between neutral lipid metabolism and zinc homeostasis. The molecular basis of the zinc-neutral lipid interaction, however, is not understood. Exploiting the simplicity of the neutral lipid metabolism in \emph{Saccharomyces cervisiae}, my thesis aims to determine the underlying mechanisms driving this zinc-neutral lipid interaction.

In chapter 2, I elucidated that the TAG content is correlated with intracellular zinc levels in yeast, demonstrated by reduced LD/TAG content in cells exposed to an increasing range of zinc concentrations. Using TAG lipase mutants, it is revealed that LD/TAG depletion occurs independent of TAG mobilization and is triggered by the repression of TAG synthesis instead. Supplementation with FA restored diminished TAG levels, implying that the repression of TAG synthesis arises from the unavailability of FA, the primary precursor for TAG synthesis. Further investigations pointed out the disruption of Acc1 activity as causal for zinc-mediated repression of FA synthesis, leading to a concomitant reduction in cellular TAG levels.

In chapter 3, I elucidated the genetic requirement for impaired zinc homeostasis in the presence and absence of TAG via analysing the growth of gene deletions in DMA and TAG-deficient libraries. The protein-protein interaction network of zinc sensitivity exacerbators or suppressors revealed high enrichment for different pathways in the presence and absence of TAG. High enrichment for proteins related to carbon metabolism and energy generating processes in the absence of TAG, suggested a change in the metabolism of the primary energy generating compound, glucose, which necessitates the formation of TAG as an alternative carbon source in impaired zinc homeostasis.

In chapter 4, I investigated the link between ergosterol metabolism and zinc homeostasis, suggested by bioinformatic predictions in chapter 3 and also experimental findings in chapter 2, whereby zinc supplementation resulted in the modulation of LDs containing the storage form of ergosterol, steryl ester (SE). Notably, impaired zinc homeostasis resulted in the accumulation of SE, as well as several ergosterol intermediates. Even though the synthesis of ergosterol and steryl ester is interconnected via the regulation of ERG genes involved in the last module of ergosterol synthesis, zinc-mediated modulation of ergosterol occurs independent of SE levels. Overall, this thesis provided mechanistic insight into the interaction between zinc homeostasis and neutral lipid metabolism in yeast. The bioinformatic predictions in tandem with experimental analysis argue in favor of metabolic remodeling in impaired zinc homeostasis triggered by glucose depletion, promoting the utilization of alternative carbon sources, mainly neutral lipids, while inhibiting the synthesis of energy-demanding products such as FA.