A Sweet Solution: Glycolysis-Dependent Treg Cell Migration
Authors: Ronen Alon
Journal: Immunity (2017)

mTORC2 is a metabolic regulatory complex activated by PI3K. In this issue of Immunity, Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar demonstrate a specialized role for this complex in the migration of regulatory T (Treg) cells to sites of inflammation rather than their differentiation and survival. mTORC2 is a metabolic regulatory complex activated by PI3K. In this issue of Immunity, Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar demonstrate a specialized role for this complex in the migration of regulatory T (Treg) cells to sites of inflammation rather than their differentiation and survival. Upon activation, the energy needs of naive T cells increase substantially and both glucose and particular amino acids like glutamine are used to meet this demand (Newton et al., 2016Newton R. Priyadharshini B. Turka L.A. Nat. Immunol. 2016; 17: 618-625Crossref PubMed Scopus (210) Google Scholar). Early studies have focused on these metabolic switches in naive and effector T cells (Teff cells). Yet recently, increasing attention has been focused on different types of regulatory T cells (Treg cells), with the recognition that these cells have their own signaling and metabolic signatures. Recent studies suggest that Teff and Treg cells use different metabolic programs for survival and differentiation: while T helper 1 (Th1) and T helper 17 (Th17) effector cells are highly glycolytic and express elevated levels of the main glucose transporter Glut1 (Macintyre et al., 2014Macintyre A.N. Gerriets V.A. Nichols A.G. Michalek R.D. Rudolph M.C. Deoliveira D. Anderson S.M. Abel E.D. Chen B.J. Hale L.P. Rathmell J.C. Cell Metab. 2014; 20: 61-72Abstract Full Text Full Text PDF PubMed Scopus (675) Google Scholar), Treg cells express low Glut1 and rely on the more energetically favorable oxidative phosphorylation process using fatty acid β-oxidation as their main fuel (Michalek et al., 2011Michalek R.D. Gerriets V.A. Jacobs S.R. Macintyre A.N. MacIver N.J. Mason E.F. Sullivan S.A. Nichols A.G. Rathmell J.C. J. Immunol. 2011; 186: 3299-3303Crossref PubMed Scopus (1322) Google Scholar). Glycolytic metabolic pathways are tightly regulated by the mechanistic target of rapamycin complex (mTORC1) and its downstream hypoxia-inducible transcription factor HIF-1α (Newton et al., 2016Newton R. Priyadharshini B. Turka L.A. Nat. Immunol. 2016; 17: 618-625Crossref PubMed Scopus (210) Google Scholar). HIF-1α also destabilizes the master Treg cell transcription factor (TF) FoxP3. Nevertheless, recent data indicate that this metabolic dichotomy between Teff and Treg cells is oversimplified. For instance, thymus-derived Treg (tTreg) cells with constitutive expression of the transcription factor FoxP3 in fact resemble effector T cells in that they rely on glycolytic pathways for proliferation. In contrast, peripherally induced Treg (pTreg) cells, regulatory subsets that do not express FoxP3 in a constitutive manner, preferentially use oxidative phosphorylation for their proliferation and functional fitness (Zeng et al., 2013Zeng H. Yang K. Cloer C. Neale G. Vogel P. Chi H. Nature. 2013; 499: 485-490Crossref PubMed Scopus (549) Google Scholar). Nevertheless, these Treg cells can undergo a temporal context-dependent metabolic switch to glycolysis under specific proliferative settings. For instance, some Toll-like receptor (TLR) signals can promote Treg cell proliferation via mTORC1-dependent glycolysis (Newton et al., 2016Newton R. Priyadharshini B. Turka L.A. Nat. Immunol. 2016; 17: 618-625Crossref PubMed Scopus (210) Google Scholar). These and other data collectively suggest that the metabolic programs used by various types of Treg cells for survival and function are far more flexible than what was originally anticipated (Newton et al., 2016Newton R. Priyadharshini B. Turka L.A. Nat. Immunol. 2016; 17: 618-625Crossref PubMed Scopus (210) Google Scholar). A new example for this flexibility, a switch from oxidative phosphorylation into glycolytic metabolism, is demonstrated by Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar in this issue of Immunity. In this study various types of Treg cells were shown to undergo this switch in response to signals triggered via their CD28 costimulatory molecule and LFA-1 integrin and reversed by CTLA-4 occupancy. This study provides new evidence that Treg cells can use glycolytic machineries to migrate into inflamed peripheral tissues. On the other hand, and in contrast to T effector cells, these very Treg cells continued to use oxidative phosphorylation to proliferate and maintain their suppressive functions. A key player in this glycolytic promigratory program of Treg cells is the hexokinase isoenzyme glucokinase (GCK) (Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar). Using both biochemical and genetic tools, Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar revealed that the mechanistic TOCR2 complex (mTORC2) transcriptionally upregulated this glycolysis-associated enzyme (Figure 1, I). This mTOR assembly regulates cellular metabolism, apoptosis, autophagy, as well as cytoskeletal remodeling in many species (Cybulski and Hall, 2009Cybulski N. Hall M.N. Trends Biochem. Sci. 2009; 34: 620-627Abstract Full Text Full Text PDF PubMed Scopus (217) Google Scholar). mTORC2 shares with mTORC1 the evolutionary conserved core serine kinase mTOR, which is directly activated by the PI3K lipid product PIP3. The mTORC1 and mTORC2 multi-protein complexes comprise of distinct components and while mTORC1 activity is rapamycin sensitive, mTORC2 is tightly regulated by a rapamycin-insensitive scaffold protein termed Rictor. The functions of other components of this complex, such as the scaffold protein mSin1, as well as regulatory components such as mLST8, Protor1/2, and Deptor, remain largely obscure. GCK is also a main target of mTORC2 in the liver (Hagiwara et al., 2012Hagiwara A. Cornu M. Cybulski N. Polak P. Betz C. Trapani F. Terracciano L. Heim M.H. Rüegg M.A. Hall M.N. Cell Metab. 2012; 15: 725-738Abstract Full Text Full Text PDF PubMed Scopus (391) Google Scholar). Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar found that Treg cell mTORC2 was involved not only in the upregulation of GCK but also in the downregulation of its inhibitor, the GCK regulatory protein, GCKR, a protein that normally sequesters cytosolic GCK and thereby attenuates glycolysis (Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar). Along these findings, Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar have presented nice supportive evidence from human homozygous carriers of a loss-of-function GCKR. The Treg cells of these patients displayed increased GCK activity and enhanced motility ex vivo. These patients also possessed reduced numbers of circulating Treg cells with normal T effector cells eluding to enhanced emigration of GCKR mutated Treg cells from the circulation to the periphery under homeostatic conditions (Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar). A second means by which mTOCR2 may promote Treg cell migration is a direct participation in actin remodeling events of these T cells since this complex participates in actin cytoskeletal rearrangements mediated by RhoGTPases (Cybulski and Hall, 2009Cybulski N. Hall M.N. Trends Biochem. Sci. 2009; 34: 620-627Abstract Full Text Full Text PDF PubMed Scopus (217) Google Scholar). It seems that both mTORC2 and GCK were not required for strict chemokine sensing by Treg cells because Treg cell migration through permissive barriers such as large pore transwell filters was insensitive to mTORC2 or to GCK silencing. Nevertheless, Treg cell migration through small confinements depended on GCK activity eluding to an involvement of this enzyme in T cell nucleus squeezing. This step is tightly regulated by actomyosin-generated forces both at the back and the front of chemokine-attracted lymphocytes (Barzilai et al., 2017Barzilai S. Yadav S.K. Morrell S. Roncato F. Klein E. Stoler-Barak L. Golani O. Feigelson S.W. Zemel A. Nourshargh S. Alon R. Cell Rep. 2017; 18: 685-699Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar). Thus, enrichment of GCK activity behind the Treg cell nucleus would be consistent with preferential use of their glycolytic machineries to generate actomyosin-driven forces on the Treg cell nucleus (Figure 1, II). This possibility would also be consistent with earlier findings of mitochondrial enrichment at the uropod of migrating leukocytes (Campello et al., 2006Campello S. Lacalle R.A. Bettella M. Mañes S. Scorrano L. Viola A. J. Exp. Med. 2006; 203: 2879-2886Crossref PubMed Scopus (255) Google Scholar). Treg cell GCK could act as a glycolytic ATP feeder for the ATP-hydrolyzing sodium pump Na,K-ATPase (Jung et al., 2002Jung J. Yoon T. Choi E.C. Lee K. J. Biol. Chem. 2002; 277: 48931-48937Crossref PubMed Scopus (43) Google Scholar) as inferred from the high levels of F-actin associated with GCK and Na,K-ATPase in the motile Treg cells reported by Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar. Notably, mTORC2 directly acted on the serine threonine serum- and glucocorticoid- regulated kinase (SGK1). This enzyme is expressed in Treg cells and may modulate the activity of Na+/K+-ATPase, as reported in other systems (Figure 1, II). mTORC2 also directly activates PKC-α, a regulator of actin turnover, myosin activity, and microtubule stability. Co-localization experiments will be required to find out whether the mTORC2 complex is enriched with any of these enzyme in specific compartments of motile Treg cells. LFA-1 outside-in signaling has been shown to enhance TCR-triggered AP-1 transcriptional programs (Perez et al., 2003Perez O.D. Mitchell D. Jager G.C. South S. Murriel C. McBride J. Herzenberg L.A. Kinoshita S. Nolan G.P. Nat. Immunol. 2003; 4: 1083-1092Crossref PubMed Scopus (172) Google Scholar) but so far the contribution of LFA-1 outside-in signals and any of its orchestrators (e.g., cytohesin-1) to Treg cell migration has not been suggested. The precise physiological scenarios used by Treg cell-expressed LFA-1 and Treg cell-expressed CD28 for optimized migration in vivo are also unclear because the Treg cells used in Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar were primed and differentiated in vitro. The in vivo surface densities of ICAM-1 and of other LFA-1 ligands on various APCs and on stromal cells (e.g., endothelial cells) as well as the content and surface densities of CD28-stimulating B7 family members (CD80 and CD86) on these various cells are likely lower than those used in Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar even under highly inflammatory settings. It is therefore possible that in vivo, in addition to ICAMs and CD28 ligands, Treg cells make use of additional adhesive, co-stimulatory, and chemokinetic signals to trigger the specific TORC2 activities necessary for their migration to different sites of inflammation. Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar elude to the possibility that pTreg cells trigger these programs both in the lymphoid organs they originate at and at the peripheral inflamed sites to which they migrate. While CD28 ligands would be restricted to subsets of properly activated APCs, non-professional APCs with high ICAM-1 expression generally lack B7 family CD28 ligands, and therefore might use alternative CD28-independent PI3K activating molecules (e.g., CD2 ligands) to drive promigratory mTORC2-GCK programs in the Treg cells with which they engage. Taken together, although largely based on in vitro generated Treg cells possibly resembling pTreg cells, Kishore et al., 2017Kishore M. Cheung K. Fu H. Bonacina F. Wang G. Coe D. Ward E.J. Colamatteo A. Jangani M. Baragetti A. et al.Immunity. 2017; 47 (this issue): 875-889Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar opens up additional avenues of research in the field of pTreg and tTreg cell migration and function at distinct peripheral sites of inflammation. Future studies should introduce new genetic systems to temporally interfere with mTORC2 and GCK expression in various types of in vivo generated Treg cells. It would be particularly interesting to assess what subsets of Treg cells navigating inside specific inflammatory and tumor microenvironments make use of the mTORC2 complex and specific GCK isoforms. It would also be clinically important to dissect how mTOR1 and mTORC2 might be selectively manipulated to achieve a therapeutic balance between Treg and Teff cell migration and function at these distinct microenvironments. Regulatory T Cell Migration Is Dependent on Glucokinase-Mediated GlycolysisKishore et al.ImmunityNovember 21, 2017In BriefRegulatory T cell localization to inflammatory sites is key to their homeostatic function. Kishore and colleagues demonstrate that Treg cell migration requires the activation of glycolysis by the enzyme glucokinase induced via a Treg cell-selective PI3K-mTORC2 pathway. Full-Text PDF Open Access