Author:

Keywords:

Yeast ageing, Sch9

Abstract:

The interest in ageing research not only stems from an intrinsic curiosity towards the matter, but also from the fact that the incidence of several diseases increase exponentially with age. The examples of such age-related disorders are ample and include multiple cancers and neurodegenerative disorders including Alzheimer’s and Parkinson’s disease. As such, a lot of research has been performed to unravel the mysteries of ageing. The budding yeast Saccharomyces cerevisiae has proven to be an extremely valuable tool to study the molecular mechanisms by which genetic, cellular and environmental factors regulate lifespan, senescence and human pathologies. Indeed, it has become apparent that many aspects involved in the control of lifespan appear to be well conserved among species. Of these, the lifespan-extending effects of calorie restriction and downregulation of nutrient signaling through the evolutionary conserved TORC1-Sch9 and Ras/cAMP/PKA pathways are prime examples. To help coordinate cellular responses to changing nutrient levels, yeast depends on Sch9, a protein kinase with a central role in the nutrient-induced signaling network that controls a plethora of phenotypes including cell growth, cell cycle progression, stress resistance and longevity. Although a lot of research has been performed to elucidate the molecular mechanisms by which Sch9 regulates these phenotypes, the direct physiological targets of Sch9 remain largely unknown. Therefore, this PhD project aimed at identifying novel downstream effectors and/or direct physiological targets of Sch9, with a special focus on those modulating yeast ageing. To this end, we investigate the role of Sch9 in sphingolipid metabolism and vacuolar functioning, two processes essential for yeast and mammalian cells to ensure proper growth and survival. The results described in this PhD manuscript reveal that the protein kinase Sch9 is not just a downstream effector, but also a key regulator of sphingolipid metabolism. We show that the TORC1-Sch9 signaling pathway controls biosynthetic ceramide production by repressing expression of the ceramidase genes. In addition, we establish that Sch9 also modulates hydrolytic ceramide production by controlling the activity of inositol phospholipase C, Isc1. As such, Sch9 comprises a major gatekeeper of sphingolipid homeostasis, optimizing ceramidase activity by integrating information on internal sphingolipid levels through Pkh1/2, as well as information on nutritional status via TORC1, thereby adjusting sphingolipid metabolite levels to cellular needs. As the dynamic balance between these intermediary sphingolipid signaling metabolites, the so-called sphingolipid rheostat, has important functions in regulating cell survival, our data also provide additional molecular links that help explain the extended life span of the sch9∆ strain. In addition to being a regulator of sphingolipid metabolism, we also show in this work that Sch9 is an important modulator of the vacuolar proton pump (V-ATPase) in response to glucose availability, and that this functional interaction has important implications for yeast lifespan and pH homeostasis. Indeed, Sch9 not only genetically, but also physically interacts with the V-ATPase to control its glucose-dependent assembly state downstream of TORC1. Moreover, we reveal a dual role for Sch9 in regulating ageing as its function in lifespan determination switches from pro-ageing to pro-survival upon impairment of V-ATPase function. Analogous to the situation in mammals, Sch9 thus functions in a feedback loop that allows fine-tuning of V-ATPase and TORC1 activity in response to nutrient availability to properly regulate cell growth and lifespan. Taken together, the results in this PhD manuscript show that Sch9 modulates ageing by impacting on mitochondrial and vacuolar integrity via its role in sphingolipid metabolism and V-ATPase functioning, respectively. Furthermore, our results, in combination with the work of others, suggest that Sch9 functions in homeostatic feedback loops to adjust growth to cellular needs by integrating information from lipid metabolism, the nutritional status, pH, V-ATPase activity and protein synthesis. This notion is fundamentally different from the current view of TORC1 and Sch9 being merely transducers of extracellular cues.