Associated Principal Investigator, Research Group Leader MPI for Biology of Aging
Cell growth is a crucial and tightly regulated process. Cells take up nutrients (i.e. amino acids, sugars, lipids) from their environment, and use them to synthesize various macromolecules, which they incorporate to increase their mass and grow. As growth is very energy consuming, cells have developed mechanisms to sense environmental conditions and to adjust their metabolism accordingly, so that they only grow when conditions are optimal. These mechanisms are of great importance, as dysregulation of growth can lead to life-threatening disorders, such as cancer and other age-related diseases. Our work focuses on the intricate molecular and cellular mechanisms of nutrient sensing and growth control, mainly via the regulation of the TSC/mTOR signaling hub. Given the central role of mTOR in the aging process, and that dysregulation of the nutrient sensing machinery is a hallmark of aging, our research investigates fundamental aspects of aging and age-related diseases.
Our research: The mTOR kinase, as part of the mTOR complexes 1 (mTORC1) and 2 (mTORC2), is a master growth regulator. It functions as a sensor and a molecular rheostat that links information from the cellular milieu to the growth properties of the cells. A large number of inputs converge on mTORC1 to regulate growth. Nutrients, energy, and growth factors activate this complex, whereas various stresses strongly inhibit its activity. Besides cell growth, mTOR activity affects the majority of cellular functions and can therefore influence organismal health, lifespan and aging (Fig. 1). Importantly, mutations on upstream pathway components, such as the inhibitory Tuberous Sclerosis Complex (TSC) proteins, can lead to mTOR hyperactivation, and, thus, are clinically relevant. In line with this, compounds targeting mTOR or upstream regulatory components are currently being used against various diseases or as anti-aging drugs. Our research aims to elucidate existing and novel molecular mechanisms of cell growth control, mainly via regulation of the TSC and mTOR complexes, and to identify and functionally characterize novel components/regulators of these complexes, focusing on their putative implementation as new targets for drug development.
Our successes: The availability of nutrients, such as amino acids, is a prerequisite for cell growth, and therefore a robust regulator of mTORC1 activity. Dr Demetriades and colleagues have previously revealed the cellular and molecular mechanisms by which mTORC1 is inactivated in response to amino acid starvation (Fig. 2). 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 (Fig. 3). This work placed TSC in the amino acid sensing pathway and showed 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 the lysosomal recruitment of TSC, thereby inhibiting mTORC1 (Fig. 4) [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 Plescher 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 addition to the core funding from the MPG, the work in the Demetriades lab is also supported by a number of additional grants, fellowships and awards, including an ERC Starting grant.
Our goals: We study the intricate molecular and cellular mechanisms of nutrient sensing and cell growth control, mainly via the regulation of the master cellular nutrient sensor and growth coordinator: the mTOR kinase. Dr. Demetriades has previously elucidated the mechanistic details of mTOR inactivation in response to nutrient starvation and has revealed how information from multiple, diverse, cellular stresses is integrated to control cellular physiology. The vision of the Demetriades group at the MPI-AGE is to understand:
Our methods/techniques: We apply high-throughput omics (functional genomic screens, proteomics, metabolomics) to identify novel regulators of key cellular processes; and combine them with state-of-the-art molecular biology, biochemistry, cell biology and high-resolution microscopy techniques to understand the very mechanistic details of their function and to reveal new principles in nutrient sensing and cell growth research. We use cell lines of human, mouse and Drosophila origin, as well as mouse models, to investigate universal and evolutionarily conserved cellular processes, and to understand how cells function in health and what goes wrong in disease and aging.
Figure 1: mTORC1 is the master regulator of virtually all cellular and organismal processes in response to multiple intra- and extra-cellular stimuli. It functions as a cellular sensor, facilitating cellular adaptation to a very dynamic cellular environment.
Figure 2: The core lysosomal amino acid sensing machinery. In optimal growth conditions, the TSC complex localizes away from the lysosomal surface and mTORC1 is active promoting cell growth. Amino acid starvation (or other stresses) drives the Rag-dependent lysosomal relocalization of the TSC, where it inhibits Rheb to completely delocalize and inactivate mTORC1.
Figure 3: In cells lacking components of the TSC, mTOR demonstrates aberrant, constitutive lysosomal localization - and thus remains hyperactive - even under conditions of nutrient (amino acid) deprivation. Confocal microscopy images showing that mTOR (red) co-localizes with lysosomes (marked by LAMP2, blue) in TSC2-null cells (see upper right cell). Exogenous TSC2 expression (see lower left cell, marked by EGFP) restores mTOR localization, which becomes diffusely cytoplasmic.
Figure 4: TSC2 relocalizes to the lysosomal surface in response to cellular stress. Each individual stress stimulus, when applied singly to cells, is sufficient to drive lysosomal accumulation of TSC, thereby inhibiting mTORC1.