Limited survival and impaired hepatic fasting metabolism in mice with constitutive Rag GTPase signaling

Abstract

The mechanistic target of rapamycin complex 1 (mTORC1) integrates cellular nutrient signaling and hormonal cues to control metabolism. We have previously shown that constitutive nutrient signaling to mTORC1 by means of genetic activation of RagA (expression of GTP-locked RagA, or RagAGTP) in mice resulted in a fatal energetic crisis at birth. Herein, we rescue neonatal lethality in RagAGTP mice and find morphometric and metabolic alterations that span glucose, lipid, ketone, bile acid and amino acid homeostasis in adults, and a median lifespan of nine months. Proteomic and metabolomic analyses of livers from RagAGTP mice reveal a failed metabolic adaptation to fasting due to a global impairment in PPARα transcriptional program. These metabolic defects are partially recapitulated by restricting activation of RagA to hepatocytes, and revert by pharmacological inhibition of mTORC1. Constitutive hepatic nutrient signaling does not cause hepatocellular damage and carcinomas, unlike genetic activation of growth factor signaling upstream of mTORC1. In summary, RagA signaling dictates dynamic responses to feeding-fasting cycles to tune metabolism so as to match the nutritional state.

Phosphorylation of FAM134C by CK2 controls starvation-induced ER-phagy

“…Next, we infected mice with a liver-specific Tsc1 KO (Li- Tsc1 KO ) and with a guanosine triphosphate (GTP)–bound constitutively active form of RagA (RagA GTP ) ; both models showed constitutive mTORC1 signaling even under fasting conditions (fig. S9, A and B) ( 39 ). Li- Tsc1 KO and RagA GTP hepatocytes showed a significantly reduced number of red-only puncta compared to WT, suggesting defective starvation-induced ER-phagy ( Fig.…”

The mTOR–Autophagy Axis and the Control of Metabolism

“…It is noteworthy that in these mice, glycogen mobilization during fasting is not impaired, and neither is it in RagA GTP neonates ( Efeyan et al, 2013 ) nor in adult Atg7 -KO mice ( Karsli-Uzunbas et al, 2014 ), in spite of the relevance of autophagic degradation of glycogen (glycophagy) during fasting ( Schiaffino and Hanzlíková, 1972 ; Jiang et al, 2010 ). Surprisingly, mice with constitutive hepatic mTORC1 activity by means of genetic deletion of Tsc1 ( Sengupta et al, 2010 ; Yecies et al, 2011 ; Liko et al, 2020 ), or liver-specific activation of RagA ( de la Calle Arregui et al, 2021 ), do not phenocopy a deficiency in autophagy and instead have an impaired transcriptional adaptation to fasting that limits ketogenesis. Thus, in adult mice undergoing fasting, mTORC1 and autophagy control complementary adaptations to nutrient limitations (see Table 1 ).…”

Inhibition of Rag GTPase signaling in mice suppresses B cell responses and lymphomagenesis with minimal detrimental trade-offs

“…Acute genetic elimination of the heterodimeric Rag GTPase complex selectively suppresses B cell maturation without mirrored negative effects on T cell maturation ( Do et al, 2020 ; Efeyan et al, 2014 ; Kalaitzidis et al, 2017 ; Kim et al, 2014 ). Conversely, genetic activation of RagA, conferring complete insensitivity to nutrient deprivation ( de la Calle Arregui et al, 2021 , Efeyan et al, 2013 ), results in compromised B cell activation upon immunization ( Ersching et al, 2017 ). Milder activation of Rag GTPases in mice by heterozygous expression of mutant variants of RagC found in human B cell lymphomas results in accelerated lymphomagenesis and hyperactive B cells ( Ortega-Molina et al, 2019 ).…”