Tetraspanins play a crucial role in many physiological functions such as sperm-egg fusion and immunity, but also in major pathologies including cancer, malaria, hepatitis C and AIDS. They function as molecular organizers of discrete microdomains, to which they connect other important molecules. The goal of the "tetraspanin" team is to determine how tetraspanins function at the molecular level, with emphasis in cancer and malaria.
Cells communicate with their environment through membrane receptors which recognize molecules and particles in the external milieu and transmit an intracellular signal that can lead to gene activation or alternatively to activation of other membrane receptors and cytoplasmic targets. It has recently become evidenced that ligand binding and signal transduction can be regulated at the level of the plasma membrane through receptor compartmentalization in discrete membrane structures called microdomains. One type of microdomain, the so-called rafts or GPI molecules-enriched microdomains has been extensively studied, and has been shown to play a key role in trafficking in polarized cells, in signal transduction and in intracellular pathogen entry and/or release (Simons et toomre, Nat. Rev. Mol. Cell Biol., 2000, 1: 31-39).
Other membrane microdomains are much less studied. Over the years it has been evidenced by a few groups worldwide, including ours, that tetraspanins assemble a new kind of microdomains collectively referred as to the "tetraspanin web" or tetraspanin-enriched microdomains (Berditchevski, J Cell Sci., 2001, 114: 4143; Boucheix et Rubinstein, Cell Mol. Life Sci., 2001, 58: 1189; Hemler, Annu. Rev. Cell Dev. Biol., 2003, 19: 397; Yanez-Mo et al., Microcirculation., 2001, 8: 153). These microdomains are highly organized, with a particular tetraspanin targeting specific non-tetraspanin partner molecules (to which they associate directly) to these domains. We have demonstrated that they are built with the help of lipids, cholesterol participating to the maintenance of these structures, as well as lipid-modifications (palmitoylation) of tetraspanins (Charrin et al., FEBS Lett., 2002, 516: 139; Charrin et al., Eur. J. Immunol., 2003, 33: 2479). We have recently published the first proteomic characterization of these structures: They contain 4 major groups of proteins, including integrins, proteins with Ig domains, ectoenzymes and heterotrimeric G proteins (Andre et al., Proteomics., 2006, 6: 1437; Le Naour et al., Mol. Cell Proteomics., 2006, 5: 845).
Fig. 1 A model for the tetraspanin web:
Biochemical studies have demonstrated specific interactions of certain tetraspanins (here in green) with a few numbers of associated molecules (their molecular partners). For example, CD9 and CD81 have two common partners designed as CD9P-1 or EWI-2. Discrete microdomains arise as tetraspanins associate with each others. Thus each tetraspanin is likely to induce the association of its molecular partners with the microdomains.
Tetraspanins are four transmembrane domains proteins with discrete structural features. There are 33 tetraspanins in mammals, and so far all cell types studied express several of these molecules, often at a high level. Tetraspanins have been implicated in various biological processes such as cell adhesion, migration, cell fusion, co-stimulation, signal transduction, and differentiation. Altogether, these data suggest that tetraspanins play a role in basal functions of the cell.
The importance of these molecules in various physiological and pathological conditions has been unravelled during the recent years. Genetic approaches such as homologous recombination or the identification of diseases linked to mutations in tetraspanins highlighted the crucial role of these molecules in important physiological functions. We have reported a role of CD9 and CD81 in gamete fusion (Le Naour et al., Science, 2000, 287: 319; Rubinstein et al., Dev. Biol., 2006, 290: 351)), and other have shown the role of CD81 in the immune response (Maecker et Levy, J. Exp. Med., 1997, 185: 1505). Very recently, knock out of CD151 has revealed the role of this tetraspanin in platelet function and in wound healing (Cowin et al., J. Invest Dermatol., 2006, 126: 680; Lau et al., Blood, 2004, 104: 2368). Patients with mutations in CD151 suffer from skin defects and glomerulopathies (Karamatic, V et al., Blood, 2004, 104: 2217), probably as a consequence of a defect in basement membrane maintenance. Patients with mutations in TM4SF2 suffer from mental retardation (Zemni et al., Nat. Genet., 2000, 24: 167).
Fig.2: the structure of a typical tetraspanin, according to the modelling of Michel Seigneuret.
Several studies have shown that some of these molecules can regulate the development of metastasis and some viral infections. For example, CD81 is an obligatory receptor for the hepatitis C virus (Bartosch et al., J. Exp. Med., 2003, 197: 633), a major cause of hepatocellular carcinoma. Additionally, tetraspanins microdomain have recently emerged as gates for HIV budding. In collaboration with the team of Dominique Mazier at Inserm U511, we recently shown the essential role of CD81 in the initiation of the hepatic phase of the infection by Plasmodium .yoelii and P. falciparum, the parasites responsible for malaria in mice and human (Silvie et al., Nat. Med., 2003, 9: 93). We could demonstrate that these parasites require not only CD81 to infect the cells, but also the integrity of tetraspanin-enriched microdomains (Silvie et al., J. Cell Sci., 2006, 119: 1992).
Despite great advances in the recent years showing the functional importance of tetraspanins, and the organization by these molecules of membrane microdomains, their mechanisms of action remain obscure. Is the localization of tetraspanins in microdomains a key step of their function? Is the function of tetraspanins to regulate the function of partner proteins? What kind of molecular activity is regulated? These are the kind of questions we address. We develop models that are pertinent to understand the role in tetraspanins in cancer and we continue our close collaboration with Inserm U511 to investigate the mechanisms by which CD81 supports infection by the malaria parasite.
Charrin S, Le Naour F., Silvie O, Milhiet PE, Boucheix C and Rubinstein E. Lateral organization of membrane proteins: tetraspanins spin their web. Biochem J. 2009 May 13;420(2):133-54. PMID: 19426143
Boucheix C, Rubinstein E. Tetraspanins. Cell Mol Life Sci. 2001 Aug;58(9):1189-205. PMID: 11577978 (accès direct texte intégral)
Hemler ME. Tetraspanin proteins mediate cellular penetration, invasion, and fusion events and define a novel type of membrane microdomain. Annu Rev Cell Dev Biol. 2003;19:397-422. PMID: 14570575
Levy S, Shoham T. The tetraspanin web modulates immune-signalling complexes. Nat Rev Immunol. 2005 Feb;5(2):136-48. PMID: 15688041
Hemler ME. Tetraspanin functions and associated microdomains. Nat Rev Mol Cell Biol. 2005 Oct;6(10):801-11. PMID: 16314869