Minerva Fast Track Group Fernandes

Research

Our lab investigates how mTORC1 (mechanistic Target Of Rapamycin complex 1) localization and function shape anabolic and catabolic responses in skeletal muscle cells, and how these change with age. We aim to elucidate how mTORC1 signaling controls the muscle’s capacity to grow and regenerate throughout ageing. Specifically, we seek to answer the following questions:

How is mTORC1 signaling coordinated across different skeletal muscle cell types?
Which factors influence mTORC1 activity in skeletal muscle cells?
How does mTORC1 regulation change with age, and what are the implications for maintenance of muscle mass?

Skeletal muscle is one of the main tissues of our bodies. It comprises 40-50% of our body mass and has key roles in movement, posture and body metabolism. In addition, due to its high protein content, skeletal muscle serves as a critical protein reservoir for the entire organism. Its cellular composition is unique, with multinucleated muscle fibers and resident adult stem cells. This structural organization is key for enabling contraction and voluntary movement, which defines the tissue’s main function. Moreover, owing to the presence of stem cells, the skeletal muscle retains a substantial regenerative capacity.

Each of the muscle cell types has unique intrinsic metabolic needs to sustain activities essential to their function. In particular, these cells heavily rely on anabolic and catabolic processes that determine their growth and the maintenance of their state. Importantly, dysfunction of such programs is thought to strongly contribute to loss of muscle mass with age, a common feature that occurs independently of disease, and that can itself become pathological in severe cases (sarcopenia).

The image illustrates the dual role of mTORC1, promoting protein synthesis in anabolic processes while regulating lysosomal biogenesis during catabolic activities. — **Figure 1.** **mTORC1 regulation of metabolism**. mTORC1 promotes anabolism via the activation of biosynthetic pathways, such as protein synthesis. Conversely, it inhibits catabolic processes, like lysosomal biogenesis. Created with BioRender.com

**Figure 1.** **mTORC1 regulation of metabolism**. mTORC1 promotes anabolism via the activation of biosynthetic pathways, such as protein synthesis. Conversely, it inhibits catabolic processes, like lysosomal biogenesis. Created with BioRender.com

The balance between anabolism and catabolism is fundamental for cellular homeostasis. Cellular growth, a key outcome of anabolism, is characterized by the accumulation of cell mass, achieved by the activation of biosynthetic pathways. Conversely, catabolism, or the breakdown of cellular components, is often linked to the cell’s main recycling centers, the lysosomes. Degradation of macromolecules and damaged organelles inside lysosomes, for instance, supplies cells with molecular precursors necessary for cell survival.

Central to the regulation of anabolic and catabolic processes is mTORC1, a signaling hub that integrates information about the cellular environment to control metabolism. mTORC1 activates core anabolic processes, for example, the protein synthesis machinery (Figure 1). Simultaneously, because growing cells conserve energy by minimizing catabolic processes, mTORC1 inhibits key catabolic factors, such as those involved in lysosome biogenesis (Figure 1).

Diagram of mTORC1 signaling involving amino acids for anabolism and catabolism. — **Figure 2.** **Spatial and functional regulation of mTORC1.** mTORC1 is found both at lysosomal and non-lysosomal locations, with different mTORC1 pools responding to nutrients, such as amino acids (AAs), coming from distinct sources to control diverse downstream processes. Created with BioRender.com

**Figure 2.** **Spatial and functional regulation of mTORC1.** mTORC1 is found both at lysosomal and non-lysosomal locations, with different mTORC1 pools responding to nutrients, such as amino acids (AAs), coming from distinct sources to control diverse downstream processes. Created with BioRender.com

Our previous work has shown that mTORC1 is not simply in a binary ‘on’ or ‘off’ state, but can selectively regulate specific processes. Importantly, selective mTORC1 activity is achieved by its localization within cells, which is dictated by the nutrient sources that cells rely on. Cytoplasmic mTORC1, primarily activated by exogenous nutrients, promotes protein synthesis. In contrast, lysosome-localized mTORC1, responsive to nutrients generated there, regulates lysosome biogenesis (Figure 2).

To answer our questions, we combine whole-cell and organelle isolation coupled with high-throughput omics approaches (proteomics, metabolomics, proximome analyses) and state-of-the-art molecular biology, biochemistry, cell biology and high-resolution microscopy techniques. We make use of mouse models, skeletal muscle cell lines and iPSC (induced pluripotent stem cell)-derived skeletal muscle cells, to identify evolutionarily conserved processes and understand which molecular mechanisms can be targeted to promote a longer health span.