1. Research on the role of GCN1 --- Amino acid sensing and the regulation of cellular function
Amino acids exert many biological functions, serving as allosteric regulators and neurotransmitters, as constituents in proteins and as nutrients. mTOR is known as an important regulator of cellular homeostasis as a sensor of amino acid sufficiency. However, the research about the sensor of amino acid insufficiency (i.e. GCN2 pathway) has been less explored in mammals.
Protein translation is a highly energy demanding process. Therefore, it is important for the cells to repress overall protein translation and produce limited proteins which is necessary for cell survival during various stresses.
Upon exposure to stresses such as amino acid starvation (AAS), phosphorylation of translational initiation factor eIF2a represses general translation. At the same time, it increases the selective translation of cytoprotective proteins, such as ATF4, that transcriptionally activate gene expression involved in a stress response to promote cell survival. Among four eIF2? kinases, GCN2 responds to AAS and phosphorylates eIF2?. In yeast, Gcn1 interacts with both GCN2 and ribosome, and is required for Gcn2 activation by AAS. Upon AAS, uncharged tRNAs are increased and Gcn1 transfers the uncharged tRNA to Gcn2 at the ribosome.
Amino acids exert many biological functions, serving as allosteric regulators and neurotransmitters, as constituents in proteins and as nutrients. mTOR is known as an important regulator of cellular homeostasis as a sensor of amino acid sufficiency. However, the research about the sensor of amino acid insufficiency (i.e. GCN2 pathway) has been less explored in mammals.
Protein translation is a highly energy demanding process. Therefore, it is important for the cells to repress overall protein translation and produce limited proteins which is necessary for cell survival during various stresses.
Upon exposure to stresses such as amino acid starvation (AAS), phosphorylation of translational initiation factor eIF2a represses general translation. At the same time, it increases the selective translation of cytoprotective proteins, such as ATF4, that transcriptionally activate gene expression involved in a stress response to promote cell survival. Among four eIF2? kinases, GCN2 responds to AAS and phosphorylates eIF2?. In yeast, Gcn1 interacts with both GCN2 and ribosome, and is required for Gcn2 activation by AAS. Upon AAS, uncharged tRNAs are increased and Gcn1 transfers the uncharged tRNA to Gcn2 at the ribosome.
We explored the function of GCN1 using two types of mutant mouse lines: Gcn1 knockout mice and Gcn1?RWDBD mice which lack GCN2 binding domain (1). Both mutant mice showed growth retardation, which was not observed in the Gcn2 KO mice. Gcn1 KO mice died at the intermediate stage of embryonic development because of severe growth retardation. Gcn1??RWDBD embryos showed mild growth retardation and malformation, and died soon after birth, most likely due to respiratory failure. Collectively, it was revealed that GCN1 contributes to normal embryogenesis in a GCN2-independent manner. We further generated mice embryonic fibroblasts from Gcn1?RWDBD embryos, and showed that GCN1 is necessary for response to AAS. Interestingly, GCN1 regulates not only the eIF2?-mediated stress response but also cell cycle and cell proliferation in a GCN2-independent manner. Taking these findings together, we propose that GCN1 integrates cellular energetic status including amino acid availability to enhance cell viability.
Kim et al. recently demonstrated that GCN1 suppresses a set of inflammatory cytokine expression in a GCN2-independent manner in response to the increased uncharged tRNA by glutamyl-prolyl-tRNA synthetase (EPRS) inhibitor halofuginone mainly in the human fibroblast-like synoviocytes (FLS) (2). Thus, the above-mentioned two researches demonstrated that GCN1 also acts in a GCN2-independent manner in mice and human. Indeed, recent studies revealed that GCN1 regulates apoptosis (3) and immunity (4) in a GCN2-independent manner, in C. elegans and Arabidopsis, respectively.
Our lab thus aims to clarify the role of GCN1 in physiology and diseases in mammals.
Kim et al. recently demonstrated that GCN1 suppresses a set of inflammatory cytokine expression in a GCN2-independent manner in response to the increased uncharged tRNA by glutamyl-prolyl-tRNA synthetase (EPRS) inhibitor halofuginone mainly in the human fibroblast-like synoviocytes (FLS) (2). Thus, the above-mentioned two researches demonstrated that GCN1 also acts in a GCN2-independent manner in mice and human. Indeed, recent studies revealed that GCN1 regulates apoptosis (3) and immunity (4) in a GCN2-independent manner, in C. elegans and Arabidopsis, respectively.
Our lab thus aims to clarify the role of GCN1 in physiology and diseases in mammals.
References
1. Ribosome binding protein GCN1 regulates the cell cycle and cell proliferation and is essential for the embryonic development of mice. Yamazaki H, Kasai S, Mimura J, Ye P, Inose-Maruyama A, Tanji K, Wakabayashi K, Mizuno S, Sugiyama F, Takahashi S, Sato T, Ozaki T, Cavener DR, Yamamoto M, Itoh K. PLoS Genet. 2020 Apr 23;16(4):e1008693.
2. Aminoacyl-tRNA synthetase inhibition activates a pathway that branches from the canonical amino acid response in mammalian cells. Kim Y, Sundrud MS, Zhou C, Edenius M, Zocco D, Powers K, Zhang M, Mazitschek R, Rao A, Yeo CY, Noss EH, Brenner MB, Whitman M, Keller TL. Proc Natl Acad Sci U S A. 2020 Apr 21;117(16):8900-8911.
3. Arabidopsis ILITHYIA protein is necessary for proper chloroplast biogenesis and root development independent of eIF2? phosphorylation. Faus I, Niñoles R, Kesari V, Llabata P, Tam E, Nebauer SG, Santiago J, Hauser MT, Gadea J. J Plant Physiol. 2018 May - Jun;224-225:173-182.
4. The translational regulators GCN-1 and ABCF-3 act together to promote apoptosis in C. elegans. Hirose T, Horvitz HR. PLoS Genet. 2014 Aug 7;10(8):e1004512.
1. Ribosome binding protein GCN1 regulates the cell cycle and cell proliferation and is essential for the embryonic development of mice. Yamazaki H, Kasai S, Mimura J, Ye P, Inose-Maruyama A, Tanji K, Wakabayashi K, Mizuno S, Sugiyama F, Takahashi S, Sato T, Ozaki T, Cavener DR, Yamamoto M, Itoh K. PLoS Genet. 2020 Apr 23;16(4):e1008693.
2. Aminoacyl-tRNA synthetase inhibition activates a pathway that branches from the canonical amino acid response in mammalian cells. Kim Y, Sundrud MS, Zhou C, Edenius M, Zocco D, Powers K, Zhang M, Mazitschek R, Rao A, Yeo CY, Noss EH, Brenner MB, Whitman M, Keller TL. Proc Natl Acad Sci U S A. 2020 Apr 21;117(16):8900-8911.
3. Arabidopsis ILITHYIA protein is necessary for proper chloroplast biogenesis and root development independent of eIF2? phosphorylation. Faus I, Niñoles R, Kesari V, Llabata P, Tam E, Nebauer SG, Santiago J, Hauser MT, Gadea J. J Plant Physiol. 2018 May - Jun;224-225:173-182.
4. The translational regulators GCN-1 and ABCF-3 act together to promote apoptosis in C. elegans. Hirose T, Horvitz HR. PLoS Genet. 2014 Aug 7;10(8):e1004512.