Our lab aims to understand how cells control their growth in health and disease. The mechanistic Target of Rapamycin (mTOR) - as part of the mTOR complexes 1 (mTORC1) and 2 (mTORC2) - is a master growth regulator. Importantly, mTORC1 does not function as an "ON-OFF switch", but rather as a finely tuned rheostat; quantitative fluctuations of mTORC1 activity are critical for the determination of the cellular metabolic and thus growth program. The immediate upstream activator of mTORC1 is the small GTPase Rheb, whose activity is in turn regulated by the inhibitory Tuberous Sclerosis Complex (TSC). Hyperactivation of mTORC1 - caused mainly by mutations on its upstream regulators, such as the TSC - is of clinical relevance and a common feature of most cancer types. Moreover, perturbations of mTOR activity are interrelated to normal ageing, lifespan, and healthspan; the immune system decline, muscular atrophy, as well as the onset and progression of multiple age-related, metabolic (diabetes) and neurological disorders (cognitive decline, Alzheimer's disease), are at least in part caused by dysregulated mTOR function. Consequently, a number of pharmacological inhibitors that target components of the mTOR network are currently applied to the clinic for the treatment of mTOR-related diseases.
Underscoring the importance of mTORC1 for the maintenance of cellular homeostasis, a large number of inputs converge on this complex to regulate growth. Nutrients, energy and growth factors activate mTORC1, whereas various stresses strongly inhibit its activity. Notably, virtually all upstream stimuli that affect mTORC1 activity signal via the TSC. Despite the significance of this complex in mTORC1 regulation, the cellular mechanisms by which upstream signals are integrated to control its function have been largely unknown until recently.
The availability of nutrients, such as amino acids, is a prerequisite for cell growth, and therefore a robust regulator of mTORC1 activity. Our previous work revealed the cellular and molecular mechanisms by which mTORC1 is inactivated in response to amino acid starvation. When cells lack amino acids, the TSC is rapidly recruited to the lysosomal surface to act on its target Rheb and thereby influence mTORC1 localization and activity. This work places TSC in the amino acid sensing pathway and shows that amino acid starvation inactivates mTORC1 via changes in the subcellular localization of TSC [see Demetriades et al., 2014]. In a follow-up study, we revealed that the lysosomal relocalization of TSC is a universal response to cellular stress; each individual stress stimulus, when applied singly to cells, is sufficient to drive lysosomal recruitment of TSC, thereby inhibiting mTORC1 [see Demetriades et al., 2016]. Hence the Boolean Operator for the lysosomal relocalization of TSC in response to multiple stimuli is the "OR" operator. This way, cells ensure that, under unfavorable conditions, mTORC1 will become inactive to cease growth, thus preventing a metabolic catastrophe and ultimately cellular death. One important but poorly understood stress stimulus is hyperosmotic stress. Consequently, we also focused on the signaling events by which osmostress inactivates mTORC1 and put together the complete signaling pathway, which involves multiple kinases that impinge on TSC2 and regulate its localization [see Demetriades et al., 2015]. Importantly, these projects revealed the qualitative and quantitative aspects of how multiple upstream stimuli are mechanistically integrated to regulate cell growth in a spatiotemporal manner.
In an independent approach, we previously aimed to identify novel mediators of amino acid signaling to mTORC1. Along with other people in the lab of Aurelio Teleman at DKFZ, Heidelberg, we performed genome-wide RNAi screens, and developed quantitative assays to measure mTORC1 activity in the lysates of knockdown cells for each gene. This effort revealed the role of the translation initiation factor eIF4A as a conserved inhibitor of mTOR in response to amino acid starvation, independently of its role in general translation. This novel function of eIF4A in mTORC1 regulation was verified by multiple independent assays, and was shown to involve TSC2. Importantly, this work revealed a feedback loop whereby specific components of the translation machinery sense amino acid availability to regulate mTORC1 activity [see Tsokanos et al, 2016].
Our current research follows up on our previous findings and is expected to shed light on the molecular mechanisms of cellular growth control and the regulation of the TSC/mTOR axis; therefore, it covers important aspects of both basic and translational research. Ultimately, novel targets for drug development will be added to the arsenal against mTOR-related diseases.