Research / Scientific departments / Cell Biology and Infection / Units and groups / Biology of Cell Interactions / Research interests
1. ENDOCYTOSIS OF CYTOKINE RECEPTORS
(N SAUVONNET, C BASQUIN, V MALARDE)
Binding of ligands to membrane receptors at the cell surface may induce their internalization by endocytosis. Once internalized, receptors and ligands are sorted in intracellular membrane compartments and can be degraded or recycled back to the cell surface.
Receptor-mediated endocytosis allows cells to communicate with their environment via membrane receptors that bind macromolecules in the extracellular milieu. It is an essential process for cells since it controls many functions including nutriment uptake, growth factor, hormone responses and antigen presentation. It represents also a way of entry of some pathogens into cells.
Receptor-mediated endocytosis through clathrin-coated pits and vesicles has been by far the most thoroughly investigated, however clathrin-independent route exists. Indeed, we identified interleukin 2 (IL-2) as the first physiological ligand endocytosed by a clathrin-independent mechanism. IL-2 receptors are essential for the immune response and endocytosis is a way to control the mitogenic signal induced by the cytokine.
The aim of the team is to characterize this unconventional endocytic pathway. So far we identified around 10 proteins involved in the process. Some are also involved in the fission of clathrin-coated vesicle such as dynamin, actin, N-WASP and cortactin. Others, like phosphatidylinositol 3-kinase (PI3-kinase), Vav2, Rac1 and the kinases Pak, are specific to this route. Our working model proposes that these specific actors are organised in a cascade promoting actin polymerization leading to the fission of the vesicle (Figure 1).
We are currently investigating the factors initiating the membrane curvature necessary to form the vesicle.
In addition, we study the dynamics of this endocytic process using Total Internal Reflection Fluorescent (TIRF) microscopy, restricting the observation at the plasma membrane level, to define the characteristics of the clathrin-caveolin-independent endocytosis (Movies 1 and 2).
Beside cytokine receptors endocytosis our team is also interested by intracellular trafficking, that can be affected by pathogenic bacteria or viruses.
Figure 1. Working model for clathrin-independent IL-2R endocytosis. IL-2R in association with PI 3-kinase leads to the induction of Vav2 and the activation of Rac1. Activated Rac1 in turn induces the kinase Pak1, the phosphorylation of cortactin and the recruitment of N-WASP thereby promoting actin polymerization during endocytosis.
Movie 1: Dynamics of IL-2R endocytosis using TIRF.
TIRF microscopy in real time observing IL-2R at the plasma membrane in control cells (1Hz).
Movie 2: Perturbation of actin cytoskeleton inhibits the dynamics of IL-2R endocytosis.
TIRF microscopy in real time observing IL-2R at the plasma membrane in cells treated with the acin-perturbing drug jasplakinolide (1Hz).
2. HOST-PATHOGEN INTERACTIONS : THE MODEL OF CHLAMYDIA
(A SUBTIL, F DE LUCIA, L GEHRE, M LAVERRIERRE, S PERRINET, F VROMMAN)
Schematic view of the infections cycle
Hela cells infected for 24 hours with Chlamydia trachomatis
Chlamydiae are bacteria that proliferate only within eukaryotic host cells. The two species pathogenic to humans, Chlamydia trachomatis and Chlamydia pneumoniae, cause a number of diseases, including trachoma, pelvic inflammatory disease, or pneumonia. Primary infections are often minor or asymptomatic; the sequelae, blindness, sterility or ectopic pregnancy appear long after infection. Throughout their cycle in the host cell, Chlamydia remain in a membrane-bound compartment called an inclusion. At the end of the cycle, the host cell is lysed and infectious forms are disseminated. We investigate the interactions between the bacteria and the cells during infection. We focus mainly on the functional study of proteins secreted by the bacteria into the host cytoplasm by a process called type III secretion. Some of these proteins are translocated into the nucleus of the host and we are particularly interested in the effect of infection at the nuclear level.