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The humoral immune response requires that B cells undergo a sudden anabolic shift and high cellular nutrient levels, which are required to sustain the subsequent proliferative burst. Follicular lymphoma (FL) originates from B cells that have participated in the humoral response, and 15% of FL samples harbour point-activating mutations in RRAGC, an essential activator of mTORC1 downstream of the sensing of cellular nutrients. The impact of recurrent RRAGC mutations in B cell function and lymphoma is unexplored. RRAGC mutations, targeted to the endogenous locus in mice, confer a partial insensitivity to nutrient deprivation, but strongly exacerbate B cell responses and accelerate lymphomagenesis, while creating a selective vulnerability to pharmacological inhibition of mTORC1. This moderate increase in nutrient signalling synergizes with paracrine cues from the supportive T cell microenvironment that activate B cells via the PI3K–Akt–mTORC1 axis. Hence, Rragc mutations sustain induced germinal centres and murine and human FL in the presence of decreased T cell help. Our results support a model in which activating mutations in the nutrient signalling pathway foster lymphomagenesis by corrupting a nutrient-dependent control over paracrine signals from the T cell microenvironment.

During antibody affinity maturation, germinal center (GC) B cells cycle between affinity-driven selection in the light zone (LZ) and proliferation and somatic hypermutation in the dark zone (DZ). Although selection of GC B cells is triggered by antigen-dependent signals delivered in the LZ, DZ proliferation occurs in the absence of such signals. We show that positive selection triggered by T cell help activates the mechanistic target of rapamycin complex 1 (mTORC1), which promotes the anabolic program that supports DZ proliferation. Blocking mTORC1 prior to growth prevented clonal expansion, whereas blockade after cells reached peak size had little to no effect. Conversely, constitutively active mTORC1 led to DZ enrichment but loss of competitiveness and impaired affinity maturation. Thus, mTORC1 activation is required for fueling B cells prior to DZ proliferation rather than for allowing cell-cycle progression itself and must be regulated dynamically during cyclic re-entry to ensure efficient affinity-based selection. During germinal center selection, signals from Tfh cells in the light zone dictate the extent of B cell proliferation in the dark zone. Ersching et al. (2017) show that Tfh help induces mTORC1 activation in light zone B cells, leading to cell growth that sustains the subsequent dark zone proliferative burst.

The mechanistic target of rapamycin complex 1 (mTORC1) integrates cues from growth factors and nutrients to control metabolism. In contrast to the growth factor input, genetic disruption of nutrient-dependent activation of mTORC1 in mammals remains unexplored. We engineered mice lacking RagA and RagB genes, which encode the GTPases responsible for mTORC1 activation by nutrients. RagB has limited expression, and its loss shows no effects on mammalian physiology. RagA deficiency leads to E10.5 embryonic death, loss of mTORC1 activity, and severe growth defects. Primary cells derived from these mice exhibit no regulation of mTORC1 by nutrients and maintain high sensitivity to growth factors. Deletion of RagA in adult mice is lethal. Upon RagA loss, a myeloid population expands in peripheral tissues. RagA-specific deletion in liver increases cellular responses to growth factors. These results show the essentiality of nutrient sensing for mTORC1 activity in mice and its suppression of PI3K/Akt signaling., United States. National Institutes of Health (R01 CA129105), United States. National Institutes of Health (R01 CA103866), United States. National Institutes of Health (R01 AI047389), United States. National Institutes of Health (R21 AG042876), American Federation for Aging Research, Starr Foundation, David H. Koch Institute for Integrative Cancer Research at MIT. Frontier Research Program, Ellison Medical Foundation, United States. National Institutes of Health (AG041765), National Cancer Institute (U.S.) (F31CA167872)

The ability to sense and respond to fluctuations in environmental nutrient levels is a requisite for life. Nutrient scarcity is a selective pressure that has shaped the evolution of most cellular processes. Different pathways that detect intracellular and extracellular levels of sugars, amino acids, lipids and surrogate metabolites are integrated and coordinated at the organismal level through hormonal signals. During food abundance, nutrient-sensing pathways engage anabolism and storage, whereas scarcity triggers homeostatic mechanisms, such as the mobilization of internal stores through autophagy. Nutrient-sensing pathways are commonly deregulated in human metabolic diseases., National Institutes of Health (U.S.) (Grant R01 CA129105), National Institutes of Health (U.S.) (Grant R01 CA103866), National Institutes of Health (U.S.) (Grant R01 AI047389), National Institutes of Health (U.S.) (Grant R21 AG042876), American Federation for Aging Research, Starr Foundation, David H. Koch Institute for Integrative Cancer Research at MIT (Frontier Research Program), Ellison Medical Foundation, Charles A. King Trust, American Cancer Society (Ellison Medical Foundation Postdoctoral Fellowship PF-13-356-01-TBE)

The mTOR complex 1 (mTORC1) protein kinase is a master growth regulator that is stimulated by amino acids. Amino acids activate the Rag guanosine triphosphatases (GTPases), which promote the translocation of mTORC1 to the lysosomal surface, the site of mTORC1 activation. We found that the vacuolar H+–adenosine triphosphatase ATPase (v-ATPase) is necessary for amino acids to activate mTORC1. The v-ATPase engages in extensive amino acid–sensitive interactions with the Ragulator, a scaffolding complex that anchors the Rag GTPases to the lysosome. In a cell-free system, ATP hydrolysis by the v-ATPase was necessary for amino acids to regulate the v-ATPase-Ragulator interaction and promote mTORC1 translocation. Results obtained in vitro and in human cells suggest that amino acid signaling begins within the lysosomal lumen. These results identify the v-ATPase as a component of the mTOR pathway and delineate a lysosome-associated machinery for amino acid sensing., Damon Runyon Cancer Research Foundation, Millennium Pharmaceuticals, Inc., American Lebanese Syrian Associated Charities, Howard Hughes Medical Institute

The mechanistic target of rapamycin complex 1 (mTORC1) pathway regulates organismal growth in response to many environmental cues, including nutrients and growth factors. Cell-based studies showed that mTORC1 senses amino acids through the RagA–D family of GTPases (also known as RRAGA, B, C and D), but their importance in mammalian physiology is unknown. Here we generate knock-in mice that express a constitutively active form of RagA (RagA[superscript GTP]) from its endogenous promoter. RagA[superscript GTP/GTP] mice develop normally, but fail to survive postnatal day 1. When delivered by Caesarean section, fasted RagA[superscript GTP/GTP] neonates die almost twice as rapidly as wild-type littermates. Within an hour of birth, wild-type neonates strongly inhibit mTORC1, which coincides with profound hypoglycaemia and a decrease in plasma amino-acid concentrations. In contrast, mTORC1 inhibition does not occur in RagA[superscript GTP/GTP] neonates, despite identical reductions in blood nutrient amounts. With prolonged fasting, wild-type neonates recover their plasma glucose concentrations, but RagA[superscript GTP/GTP] mice remain hypoglycaemic until death, despite using glycogen at a faster rate. The glucose homeostasis defect correlates with the inability of fasted RagA[superscript GTP/GTP] neonates to trigger autophagy and produce amino acids for de novo glucose production. Because profound hypoglycaemia does not inhibit mTORC1 in RagA[superscript GTP/GTP] neonates, we considered the possibility that the Rag pathway signals glucose as well as amino-acid sufficiency to mTORC1. Indeed, mTORC1 is resistant to glucose deprivation in RagA[superscript GTP/GTP] fibroblasts, and glucose, like amino acids, controls its recruitment to the lysosomal surface, the site of mTORC1 activation. Thus, the Rag GTPases signal glucose and amino-acid concentrations to mTORC1, and have an unexpectedly key role in neonates in autophagy induction and thus nutrient homeostasis and viability, National Institutes of Health (U.S.) (Grant R01 CA129105), National Institutes of Health (U.S.) (Grant R01 CA103866), National Institutes of Health (U.S.) (Grant R37 AI047389), American Federation for Aging Research, Starr Cancer Consortium, David H. Koch Institute for Integrative Cancer Research at MIT (Frontier Research Program), Ellison Medical Foundation

Growth factors and nutrients regulate the mTORC1 [mammalian (or mechanistic) target of rapamycin complex 1] by different mechanisms. The players that link growth factors and mTORC1 activation have been known for several years and mouse models have validated its relevance for human physiology and disease. In contrast with the picture for growth factor signalling, the means by which nutrient availability leads to mTORC1 activation have remained elusive until recently, with the discovery of the Rag GTPases upstream of mTORC1. The Rag GTPases recruit mTORC1 to the outer lysosomal surface, where growth factor signalling and nutrient signalling converge on mTORC1 activation. A mouse model of constitutive RagA activity has revealed qualitative differences between growth-factor- and nutrient-dependent regulation of mTORC1. Regulation of mTORC1 activity by the Rag GTPases in vivo is key for enduring early neonatal starvation, showing its importance for mammalian physiology., National Institutes of Health (U.S.) (Grant R01 CA129105), National Institutes of Health (U.S.) (Grant R01 CA103866), National Institutes of Health (U.S.) (Grant R01 AI047389), National Institutes of Health (U.S.) (Grant R21 AG042876)

In all eukaryotes, the target of rapamycin (TOR) signalling pathway couples energy and nutrient abundance to the execution of cell growth and division, owing to the ability of TOR protein kinase to simultaneously sense energy, nutrients and stress and, in metazoans, growth factors. Mammalian TOR complex 1 (mTORC1) and mTORC2 exert their actions by regulating other important kinases, such as S6 kinase (S6K) and Akt. In the past few years, a significant advance in our understanding of the regulation and functions of mTOR has revealed the crucial involvement of this signalling pathway in the onset and progression of diabetes, cancer and ageing., National Institutes of Health (U.S.), Howard Hughes Medical Institute, Whitehead Institute for Biomedical Research, Jane Coffin Childs Memorial Fund for Medical Research (Postdoctoral Fellowship), Human Frontier Science Program (Strasbourg, France)

The mammalian target of rapamycin (mTOR) is a master regulator of cell growth and division that responds to a variety of stimuli, including nutrient, energy, and growth factors. In the last years, a significant number of pieces have been added to the puzzle of how mTOR coordinates and executes its functions. Extensive research on mTOR has also uncovered a complex network of regulatory loops that impact the therapeutic approaches aimed at targeting mTOR., Howard Hughes Medical Institute, National Institutes of Health (U.S.), Human Frontier Science Program (Strasbourg, France)

Oncogene-induced senescence is a cellular response that may be crucial for protection against cancer development1,2, but its investigation has so far been restricted to cultured cells that have been manipulated to overexpress an oncogene. Here we analyse tumours initiated by an endogenous oncogene, ras, and show that senescent cells exist in premalignant tumours but not in malignant ones. Senescence is therefore a defining feature of premalignant tumours that could prove valuable in the diagnosis and prognosis of cancer.