Editor | Enzyme Beauty
The lysosome is an organelle responsible for the degradation and recycling of materials within the cell, playing a key role in organism development, metabolic balance, and the maintenance of cellular homeostasis. Centered around the lysosome, the cell has structured a system that integrates responses to external stress, amino acid metabolism, and the transcriptional regulation of related genes; mTORC1 is a key regulatory component of this system【1】. mTORC1 is a protein kinase complex; it is recruited to the lysosomal membrane through the activated Ragulator complex and activates downstream pathways. mTORC1 can phosphorylate downstream substrates including S6 kinase（S6K）, translation initiation factor（4E-BP1）, and ULK1 protein responsible for autophagy initiation, thereby regulating cell size, protein translation, and autophagic flux（Figure 1）. The levels of intracellular amino acids and key metabolic substrates: leucine, arginine, and SAM can activate Ragulator, regulating Rag GTPase and mTORC1 activity. Meanwhile, lysosomal v-ATPase and amino acid transporter SLC39A8 are also involved in regulating mTORC1 activity（Figure 1）. Therefore, mTORC1 and lysosome-associated proteins have long been therapeutic targets for neurodegenerative diseases and cancer.
Figure 1: Regulation of mTORC1 Activity is Key to Nutrient Sensing Pathways
Recently, Nature Cell Biology published the latest research results from Stanford University’sTobias Meyer research group:The lysosomal GPCR-like protein GPR137B regulates Rag and mTORC1 localization and activity . This paper elucidates a novel regulatory mechanism for the mTORC1 pathway, where they identified a lysosomal membrane protein GPR137B through an RNAi library screen. GPR137B can directly bind to Rag GTPase A-Rag A, enhancing its localization and dynamic activity at the lysosome, thereby regulating mTORC1.
In this study, the authors identified the GPR137B gene by screening an siRNA library, and reducing its expression with siRNA could inhibit mTORC1 localization and activity at the lysosome（Figure 2）. Further experiments showed that in MEF cells lacking Rag A/B, overexpression of GPR137B still could not restore mTORC1 localization at the lysosome; in cells with continuously activated Rag A protein, reducing GPR137B expression had little effect on mTORC1 localization at the lysosome. Evidence from both directions suggests that GPR137B protein may regulate mTORC1 activation through Rag GTPase.
Figure 2: Reduced GPR37B Expression Affects mTORC1 Localization at the Lysosome
Next, the authors further analyzed the action mode of GPR137B. In cell lines with stable expression of GPR137B, co-immunoprecipitation was used to detect interacting proteins, revealing that GPR137B interacts with Rag A. It was also found that in the absence of amino acids, overexpression of GPR137B could still activate Rag; while knockdown of GPR137B could affect Rag’s activation of mTORC1, further suggesting that this protein has a direct regulatory effect on Rag A.
Activation of RagGTPase A correspondingly activates mTORC1 localization and activation at the lysosome. Additionally, the authors discovered that as mTORC1 localization and activity at the lysosome increased, Rag A/C would dissociate from the lysosomal membrane, indicating a rapid activation-dissociation dynamic cycle process for Rag A/C. Therefore, in cells expressing continuously activated Rag A CA, the signal of lysosomal localized Rag C rapidly recovered after FRAP（Figure 3）. In cells expressing GPR137B, the signal of Rag C was also very rapid（Figure 3）, proving that GPR137B not only activates Rag’s activity and lysosomal localization but also promotes the dynamic cycling process of Rag.
Figure 3: GPR137B Increases Dynamic Cycling of Rag A/C at the Lysosome
The authors identified a new lysosomal transmembrane protein GPR137B; through meticulous experiments, they revealed a novel mode of regulation for Rag activity and its lysosomal localization. The discovery of GPR137B provides new therapeutic targets for lysosome-related diseases.
Original Link:
https://doi.org/10.1038/s41556-019-0321-6
Editor: Xiao Xianzi
References
1. Saxton, R. A. & Sabatini, D. M. mTOR signaling in growth, metabolism, and disease. Cell 169, 361–371(2017).
