Mitochondria play a central role in ageing. They are considered to be both a target of the ageing process, as well as a contributor to it1. Alterations in mitochondrial structure and function are evident during ageing in most eukaryotes2, but how this occurs is poorly understood. Here, we identify a functional link between the lysosome-like vacuole and mitochondria in S. cerevisiae, and show that mitochondrial dysfunction in replicatively-aged yeast arises from altered vacuolar pH. We found that vacuolar acidity declines during the early asymmetric divisions of a mother cell, and show that preventing this decline suppresses mitochondrial dysfunction and extends lifespan. Surprisingly, changes in vacuolar pH do not limit mitochondrial function by disrupting vacuolar protein degradation, but rather, by reducing pH-dependent amino acid storage in the vacuolar lumen. We also found that calorie restriction promotes lifespan extension at least in part by increasing vacuolar acidity via conserved nutrient sensing pathways3. Interestingly, although vacuolar acidity is reduced in aged mother cells, acidic vacuoles are regenerated in newborn daughters, coinciding with daughter cells having a renewed lifespan potential4. Overall, our results identify vacuolar pH as a critical regulator of ageing and mitochondrial function, and outline a potentially conserved mechanism by which calorie restriction delays the ageing process. Because functions of the vacuole are highly conserved throughout evolution5, we hypothesize that lysosomal pH may modulate mitochondrial function and lifespan in other eukaryotic cells.

Cholesterol and fatty acid synthesis in mammals are controlled by SREBPs, a family of membrane bound transcription factors. Our studies identified homologs of SREBP, its binding partner SCAP, and the ER retention protein Insig in Schizosaccharomyces pombe, named sre1+, scp1+, and ins1+. Like SREBP, Sre1 is cleaved and activated in response to sterol depletion in a Scp1-dependent manner. Microarray analysis revealed that Sre1 activates sterol biosynthetic enzymes as in mammals, and, surprisingly, Sre1 also stimulates transcription of genes required for adaptation to hypoxia. Furthermore, Sre1 rapidly activates these target genes in response to low oxygen and is itself required for anaerobic growth. Based on these findings, we propose and test a model in which Sre1 and Scp1 monitor oxygen-dependent sterol synthesis as an indirect measure of oxygen supply and mediate a hypoxic response in fission yeast.

Cholesterol is an essential component of mammalian cell membranes and is required for proper membrane permeability, fluidity, organelle identity, and protein function. Cells maintain sterol homeostasis by multiple feedback controls that act through transcriptional and posttranscriptional mechanisms. The membrane-bound transcription factor sterol regulatory element binding protein (SREBP) is the principal regulator of both sterol synthesis and uptake. In mammalian cells, the ER membrane protein Insig has emerged as a key component of homeostatic regulation by controlling both the activity of SREBP and the sterol-dependent degradation of the biosynthetic enzyme HMG-CoA reductase. In this review, we focus on recent advances in our understanding of the molecular mechanisms of the regulation of sterol synthesis. A comparative analysis of SREBP and HMG-CoA reductase regulation in mammals, yeast, and flies points toward an equilibrium model for how lipid signals regulate the activity of sterol-sensing proteins and their downstream effectors.