mTOR signaling in stem and progenitor cells

Abstract

The mammalian target of rapamycin (mTOR) senses nutrients and growth factors to coordinate cell growth, metabolism and autophagy. Extensive research has mapped the signaling pathways regulated by mTOR that are involved in human diseases, such as cancer, and in diabetes and ageing. Recently, however, new studies have demonstrated important roles for mTOR in promoting the differentiation of adult stem cells, driving the growth and proliferation of stem and progenitor cells, and dictating the differentiation program of multipotent stem cell populations. Here, we review these advances, providing an overview of mTOR signaling and its role in murine and human stem and progenitor cells.

Keywords: Amino acids; Metabolism; Signaling; Stem cell; mTOR.

Fig. 1.

Components of the mTORC1 and mTORC2 complexes. (Left) mTORC1 consists of the proteins mTOR, Raptor, mLST8, PRAS40 and DEPTOR. It regulates protein synthesis, lipid synthesis, autophagy, lysosome biogenesis and growth factor signaling by phosphorylating its substrates S6K, 4EBP1, lipin 1, ULK1, TFEB and Grb10. (Right) mTORC2 consists of mTOR, Rictor, mLST8, mSin1, DEPTOR and Protor1/2. It regulates cytoskeletal remodeling, cell growth and proliferation, ion transport, and cell survival through its downstream substrates PKC, AKT and SGK. mTORC1 is inhibited by acute rapamycin treatment (indicated by a solid inhibitory line), whereas mTORC2 is not inhibited by acute rapamycin treatment but is inhibited by prolonged rapamycin treatment (indicated by broken inhibitory line). Positive regulators in each complex are shown in green and negative regulators in red. 4EBP1, eIF4E-binding protein; AKT, RAC-α serine/threonine-protein kinase; DEPTOR, DEP-domain-containing mTOR-interacting protein; mLST8, mammalian lethal with Sec13 protein 8; mSin1, mammalian stress-activated MAPK-interacting protein 1; mTOR, mammalian target of rapamycin or mechanistic target of rapamycin; mTORC1, mTOR complex 1; mTORC2, mTOR complex 2; PKC, protein kinase C; PRAS40, proline-rich AKT substrate 40 kDa; Protor1/2, protein observed with Rictor 1 and 2; Raptor, regulatory-associated protein of mTOR; Rictor, rapamycin-insensitive companion of mTOR; S6K, ribosomal S6 kinase; SGK, serum/glucocorticoid-regulated kinase; TFEB, transcription factor EB; ULK1, Unc-51-like kinase 1.

Fig. 2.

Pathways that regulate mTORC1 and mTORC2 signaling. A schematic of a cell showing insulin and EGFR receptors at the plasma membrane, and components of the mTOR signaling pathways located near the lysosome. mTORC1 is controlled by growth factors via the PI3K-AKT and Ras-MAPK pathways. PI3K catalyzes the phosphorylation of PIP₂ to PIP₃, which in turn recruits PDK1 and AKT, facilitating the phosphorylation of AKT at T308 by PDK1. mTORC2 also phosphorylates AKT at S473. Activated AKT then promotes mTORC1 activity by inhibiting the GAP activity of the TSC, thus activating Rheb, a potent mTORC1 activator. The Ras-MAPK pathway regulates mTORC1 through the Ras-Raf-MEK-ERK signaling cascade. ERK or its downstream substrate RSK inhibits TSC by direct phosphorylation. In addition, mTORC1 is controlled by Wnt signaling (through GSK3), by TNFα (through IKKβ), by hypoxia (through REDD1) and by DNA damage (through p53). Energy stress also regulates mTORC1, acting via LKB1 and AMPK. Positive regulators of mTORC1 or mTORC2 are shown in green and negative regulators in red. AMPK, AMP-activated protein kinase; AKT, RAC-α serine/threonine-protein kinase; EGFR, epidermal growth factor receptor; ERK, extracellular signal-regulated kinase; GAP, GTPase activating protein; Grb2, growth factor receptor-bound protein 2; GSK3, glycogen synthase kinase 3; IKKβ, IκB kinase β; IRS, insulin receptor substrate; LKB1, liver kinase B1; MAPK, mitogen-activated protein kinase; MEK, MAPK/ERK kinase; mTOR, mammalian target of rapamycin or mechanistic target of rapamycin; mTORC1, mTOR complex 1; mTORC2, mTOR complex 2; p53, protein 53; PDK1, 3′-phosphoinositide-dependent kinase 1; PI3K, phosphoinositide 3 kinase; PIP₂, phosphatidylinositol (4,5) bisphosphate; PIP₃, phosphatidylinositol (3,4,5) trisphosphate; PTEN, phosphatase and tensin homolog; Raf, rapidly accelerated fibrosarcoma; Ras, rat sarcoma; REDD1, DNA damage response 1; Rheb, Ras homolog enriched in brain; RSK, ribosomal S6 kinase; SOS, son of sevenless homolog; TNFα, tumor necrosis factor α; TSC1/2, tuberous sclerosis complex 1/2; Wnt, wingless-type.

Fig. 3.

Amino acid signaling pathways upstream of mTORC1. (A) Model of how leucine and arginine activate mTORC1 through the Rag GTPases. Under amino acid-sufficient conditions, a heterodimer of GTP-bound RagA or RagB and GDP-bound RagC or RagD interacts with mTORC1 at the lysosome, where Rheb resides. Ragulator is a scaffold that anchors the Rag proteins to the lysosome and serves as a GEF for RagA/B. It consists of p18, p14, MP1, C7orf59 and HBXIP. A FLCN-FNIP complex functions as a GAP for RagC/D. The v-ATPase binds to the Ragulator and is required for amino acid signaling to mTORC1. The KICSTOR complex, which contains KPTN, ITFG2, C12orf66 and SZT2, anchors GATOR1 (which consists of DEPDC5, NPRL2 and NPRL3), a GAP for RagA/B, to the lysosome. GATOR2 (which consists of SEC13, SEH1L, WDR24, WDR59 and MIOS) inhibits GATOR1 through an unclear mechanism. Sestrin 2 and CASTOR1 bind to GATOR2 and prevent the inhibitory action of GATOR2 on GATOR1. Leucine and arginine bind to their sensors sestrin 2 and CASTOR1, respectively, blocking sestrin 2-GATOR2 and CASTOR1-GATOR2 interactions. (B) Model of how glutamine activates mTORC1 independently of the Rag GTPases. In this model, the v-ATPase is required, and the cycling of Arf1, between a GTP- and GDP-bound state, promotes glutamine-induced mTORC1 activation and its lysosomal localization. Positive regulators of mTORC1 are indicated in green and negative regulators are indicated in red. Arf1, adenosine diphosphate ribosylation factor 1; CASTOR1, cytosolic arginine sensor for mTORC1 subunit 1; DEPDC5, DEP domain containing 5; FLCN, folliculin; FNIP, folliculin interacting protein; GATOR1, GTPase-activating protein activity toward Rags; GEF, guanine nucleotide exchange factor; GAP, GTPase activating protein; HBXIP, hepatitis B virus X-interacting protein; ITFG2, integrin α FG-GAP repeat containing 2; KPTN, kaptin (actin binding protein); MIOS, meiosis regulator for oocyte development; MP1, MEK partner 1; mTORC1, mTOR complex 1; NPRL2, NPR2 like, GATOR1 complex subunit; NPRL3, NPR3 like, GATOR1 complex subunit; v-ATPase, vacuolar H⁺-ATPase; Rheb, Ras homolog enriched in brain; SEH1L, SEH1-like nucleoporin; SLC38A9, solute carrier family 38 member 9; SZT2, seizure threshold 2; TSC, tuberous sclerosis complex; WDR24, WD repeat-containing protein 24; WDR59, WD repeat-containing protein 59.

Fig. 4.

Regulation of mTOR signaling in stem and progenitor cells. (A) In neural stem cells (NSCs), mTORC1 activity is regulated by upstream regulators, such as LIN28, FBXL5 and ketamine. mTORC1 regulates NSC and neural progenitor cell (NPC) proliferation, differentiation and dendrite formation. (B) In hematopoietic stem cells (HSCs), mTORC1 activity is regulated by WIP1, ITPKB, TAL1 and Dlk1-Gtl2 microRNAs. mTORC1 activity promotes HSC and hematopoietic progenitor cell (HPC) proliferation, differentiation and long-lived HSC maintenance. (C) mTORC1 signaling is required to suppress BMP signaling and to promote the activation of hair follicle stem cells (HFSCs) during the transition from telogen to anagen during hair growth. (D) In mesenchymal stem cells (MSCs), mTORC1 and mTORC2 have distinct roles in cell fate decisions. mTORC1 facilitates adipocyte differentiation, while having conflicting roles in osteoblast differentiation. In contrast, mTORC2 inhibits PPARγ-dependent adipocyte differentiation and promotes RUNX2-dependent osteoblast differentiation. (E) Calorie restriction inhibits mTORC1 activity in intestinal niche cells, which stimulates cADPR production. cADPR promotes S6K1 phosphorylation, essentially enhancing mTORC1 activity in intestinal stem cells, which promotes self-renewal. (F) EGFR signaling activates mTORC1 in mammary stem cells (MaSCs), and is inhibited by Cbl ubiquitin ligases. mTORC1 activity promotes MaSC differentiation. (G) mTORC1 promotes germline stem cell (GSC) differentiation at the expense of self-renewal. mTORC1 is activated by ATRA in GSCs and inhibited by Plzf through expression of REDD1. (H) mTORC1 activity is regulated by CXCR2 and LIF signaling in pluripotent stem cell (PSCs). Inhibition of mTORC1 promotes mesendodermal differentiation in PSCs. Ac, acetylation; AKT, RAC-α serine/threonine-protein kinase; AMPAR, AMPA receptor; ATRA, all-trans retinoic acid; BDNF, brain-derived neurotrophic factor; BMP, bone morphogenetic protein; BST1, bone stromal antigen 1; cADPR, cyclic ADP ribose; Cbl, casitas B-lineage lymphoma; CXCR2, chemokine receptor 2; Dlk1-Gtl2, delta homolog 1-gene trap locus 2; EGFR, epidermal growth factor receptor; ERK, extracellular signal-regulated kinase; ERRα, estrogen-related receptor α; FBXL5, F box and leucine-rich repeat protein 5; GDH, glutamate dehydrogenase; GLS, glutaminase; IGF1R, IGF1 receptor; IGF2, insulin-like growth factor 2; IP₄, inositol (1,3,4,5)-tetrakisphosphate; ITPKB, inositol-trisphosphate 3 kinase B; LIF, leukemia inhibitory factor; LIN28, protein lin-28 homolog; mTOR, mammalian target of rapamycin or mechanistic target of rapamycin; mTORC1, mTOR complex 1; mTORC2, mTOR complex 2; P, phosphorylation; PI3K, phosphoinositide 3 kinase; Plzf, promyelocytic leukemia zinc finger; PPARγ, peroxisome-proliferator activated receptor gamma; Raptor, regulatory-associated protein of mTOR; REDD1, DNA damage response 1; ROS, reactive oxygen species; RUNX2, runt-related transcription factor 2; S6K1, ribosomal S6 kinase 1; SIRT1, sirtuin 1; SIRT4, sirtuin 4; TAL1, T-cell acute leukemia 1; TrKBR, tyrosine kinase B receptor; WIP1, wild-type p53-induced phosphatase 1.