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Regulation of Leucine‐induced mTORC1 Activation in Skeletal Muscle of Neonatal Pigs

Leucine acts as anabolic agent that stimulates protein synthesis in skeletal muscle via activation of mammalian/mechanistic target of rapamycin complex 1 (mTORC1). Studies using cell cultures have proposed several models of the leucine‐induced activation of mTORC1 that involve Sestrin2, leucyl‐tRNA synthetase (LRS), and SH3 domain binding protein 4 (SH3BP4). In the first model, Sestrin2 (a GATOR2‐interacting protein) is a leucine sensor upstream of mTORC1. Lack of leucine causes Sestrin2 to interact with GATOR2, leading to inhibition of mTORC1. Under leucine‐rich condition, the binding of leucine to Sestrin2 disturbs the Sestrin2‐GATOR2 complex resulting in the activation of mTORC1. In a second proposed model, LRS acts as a leucine sensor that positively regulates mTORC1. In this model, there are two proposed pathways involving RagD and Vps34. In the first pathway, leucine binding to LRS stimulates LRS‐RagD interaction, which then promotes activation of mTORC1. In the second pathway, Vps34 mediates amino acid signaling to mTORC1. Leucine‐induced LRS‐Vps34 interaction is necessary for Vps34 activation towards Phospholipase D 1 (PLD1). PLD1 is essential for mTORC1 activation. In the third model, the binding of SH3BP4 to RagB acts as a potent inhibitor of amino acid‐induced activation of mTORC1. Leucine obstructs the interaction of SH3BP4 with RagB, allowing RagB to become fully active and participate in mTORC1 activation.