We are interested in retroviral envelope glycoproteins (Env) that use nutrient transporters from the solute carrier (SLC) family as receptors. We exploit our discoveries on Env-SLC interactions to explore metabolic regulations, identify new genes and functions for exaptated env, and generate new soluble Env-derived receptor-binding domains (RBD) to investigate normal and pathological physiological processes and metabolic modulations.
1.The retroviruses in us: From cell fusion to metabolism; exaptation of SLC-binding Env and Env-derived RBD
Retrovirus co-evolution with vertebrates has accompanied speciation through endogeneization of retroviral sequences into the host germline cells. A striking example of exaptation of retroviral sequences is the use of an Env-related proteins by mammalians as cell membrane fusiogenic agents that lead to syncytiotrophoblast and placenta formation (Env-triggered syncytium formation in a link to a time-lapse video).
Among the retroviruses of vertebrates, the gammaretroviruses are the most widely spread in mammals, whether as exogenous infectious agents or as bona fide host genes (Endogenous RetroViruses, ERV). All Env from gammaretroviruses and deltaretroviruses, and some members of the alpha- and beta-retroviral generae, for which a fusion or entry receptor has been identified, have been shown to specifically bind SLC members through their aminoterminal RBD. We have used these properties to derive soluble tagged RBD ligands to monitor SLC cell surface expression in different contexts (Fig.1).
2.GLUT1, the entry receptor for the Human T-cell Leukemia Viruses (HTLV) and HRBD, a new metabolic marker of GLUT1-dependent activities
In 2000, 20 years after its identification in the early '80s as the first member of the deltaretrovirus genus, we have discovered that the Human T-cell Leukemia Virus (HTLV), and its simian STLV counterparts, have an Env structurally related to that of gammaretroviruses, although remotely related to the latter viruses (Kim, JBC 2000). This discovery led us to the identification of GLUT1 as the HTLV receptor and the precise characterization of the key residues in GLUT1 that are responsible for Env binding and HTLV entry (Manel, Cell 2003; Manel, J Biol Chem 2005) (Fig.2).
3."RBD shuffle"; a datamining tool to identify env-related genes encoding RBD; "Transportome", an RBD-based metabolic profiling; and SLCeome, a human SLC expression library
We have developed a new datamining tool, "RBD shuffle", that allows us to identify non-annotated env-related RBD in human and other species genomes. We hypothesized that positive selection has been exerted on RBD based on their potential to produced soluble SLC ligands. Such a property would lead to exaptation of RBD as modulators of nutrient transport and metabolic regulation.
In this context, we use "Transportome" metabolic profiling with our expanding panel of RBD to track a series of SLC known as, or that we unveiled to be, key players in hematopoiesis. This project is developed notably in collaboration with Naomi Taylor's group (see Montel-Hagen, Cell 2008; Oburoglu, Cell Stem Cell, 2014) and Mohandas Narla's laboratory (New York Blood Center).
We have set up "Transpotome" to help monitoring oncogenic processes (Petit et al. 2013) and we are presently exploiting this tool in ongoing collaborations with several teams at the Montpellier Cancer Institute and IRCM teams.
The tracking of specific SLC with different RBD has allowed us to unveil metabolic reprogrammation of neutrophils during inflammation (Laval et al. J Immunol 2013); to characterize the role of XPR1 in a rare idiopathic neuropathological disease - see below (Legati/Giovannini et al. 2015; Anheim/López-Sánchez et al. 2016); or to detect rapidly statin-driven myotoxicity in collaboration with Istem (Peric et al. 2015).
"Transportome", an RBD-based SLC monitoring is a concept at the origin of Metafora-biosystems, a start-up co-founded by Vincent Petit, a former member of our group.
Further developments with RBD include their use as tools for specific tumor imaging and detection (Fig.3), antitumoral activity, and metabolic regulators, notably in collaboration with several teams at IGMM and IRCM.
Also, thanks to Marc Vidal's human ORFeome generous endeavor (hORFeome Database), we are mounting an expression library in mammalian cells of all identified human SLC, in a semi-automated screen for ligands and SLC modulators (SLCeome). While expanding our RBD collection, using "Transportome" and "SLCeome" screening, we identified the long-sought receptors for the bovine leukemia virus (BLV) and porcine endogenous retrovirus type B (PERVB); two retroviruses characterized by their costly massive economical impacts and their ability to infect human cells.
4.XPR1, a mouse retrovirus receptor, is the first and only know inorganic phosphate exporter identified in metazoans and a new actor in pathological calcification
Although numerous animal retroviral Env can drive human cell infection, identification of their receptors and their functions have been slow and fastidious.
Thus, xenotropic and polytropic murine leukemia retroviruses (X- and P-MLV), which have not shown to be human infectious agents (see Courgnaud et al. PNAS 2010; Jeziorski et al. 2016) despite their ability to infect human cells, were among the first mammalian retroviruses identified in the '70s; XPR1, their common receptor, has been discovered only several decades later, in 1998, and we recently elucidated XPR1 function as the first metazoan exporter of inorganic phosphate; a function that we have shown to be conserved from invertebrates to humans (Giovannini et al. 2013) (Fig.4a). Moreover, we showed that XRBD, the receptor-binding Env-derived soluble ligand, inhibited XPR1-mediated phosphate transport (Giovannini 2013) (Fig.4b).
More recently, we have identified XPR1 as one of the genes responsible for a rare neurodegenerative disease, the primary familial brain calcification, formerly known as Fahr's disease (Legati/Giovannini et al. 2015). Furthermore, we have identified several sets of mutations in the SPX cytoplasmic domain of XPR1 as responsible for this disease (Anheim/López-Sánchez et al. 2016). We are currently using these mutants and different XPR1 expression molecules to develop our understanding at the molecular and physiological levels of phosphate bidirectional transport and its link to normal and pathological calcification. We are also currently identifying and characterizing XPR1 cellular partners that are likely to play a role in XPR1-dependent phosphate and calcium homeostasis.