Abstract :

Autophagy is a conserved degradative pathway used as a host defense mechanism against intracellular pathogens. However, several viruses can evade or subvert autophagy to insure their own replication. Nevertheless, the molecular details of viral interaction with autophagy remain largely unknown. We have determined the ability of 83 proteins of several families of RNA viruses (Paramyxoviridae, Flaviviridae, Orthomyxoviridae, Retroviridae and Togaviridae), to interact with 44 human autophagy-associated proteins using yeast two-hybrid and bioinformatic analysis. We found that the autophagy network is highly targeted by RNA viruses. Although central to autophagy, targeted proteins have also a high number of connections with proteins of other cellular functions. Interestingly, immunity-associated GTPase family M (IRGM), the most targeted protein, was found to interact with the autophagy-associated proteins ATG5, ATG10, MAP1CL3C and SH3GLB1. Strikingly, reduction of IRGM expression using small interfering RNA impairs both Measles virus (MeV), Hepatitis C virus (HCV) and human immunodeficiency virus-1 (HIV-1)-induced autophagy and viral particle production. Moreover we found that the expression of IRGM-interacting MeV-C, HCV-NS3 or HIV-NEF proteins per se is sufficient to induce autophagy, through an IRGM dependent pathway. Our work reveals an unexpected role of IRGM in virus-induced autophagy and suggests that several different families of RNA viruses may use common strategies to manipulate autophagy to improve viral infectivity.                                     

DEC        2011     

DOI :   10.1371/journal.ppat.1002422         

Pubmed ID : 22174682

Abstract :

Insertions or deletions (indels) of amino acids residues have been recognized as an important source of genetic and structural divergence between paralogous Bcl-2 family members. However, these signature sequences have not so far been extensively investigated amongst orthologous Bcl-2 family proteins. Bcl2l10 is an antiapoptotic member of the Bcl-2 family that has evolved rapidly throughout the vertebrate lineage and which shows conserved abundant expression in eggs and oocytes. In this paper, we have unraveled two major sites of divergence between human Bcl2l10 and its vertebrate homologs. The first one provides length variation at the N-terminus (before the BH4 domain) and the second one is located between the predicted alpha 5-alpha 6 pore-forming helices, providing an unprecedented case in the superfamily of helix-bundled pore-forming proteins. These two particular indels were studied phylogenetically and through biochemical and cell biological techniques, including truncation and site-directed mutagenesis. While deletion of the N-terminal extension had no significant functional impact in HeLa cells, our results suggest that the human Bcl2l10 protein evolved a calcium-binding motif in its alpha 5-alpha 6 interhelical region by acquiring critical negatively charged residues. Considering the reliance of female eggs on calcium-dependent proteins and calcium-regulated processes and the exceptional longevity of oocytes in the primate lineage, we propose that this microstructural variation may be an adaptive feature associated with high maternal expression of this Bcl-2 family member.                   

DEC        2011     

DOI :  10.1093/molbev/msr152             

Pubmed ID : 21705382

Abstract :

During development, natural killer (NK) cells exit the BM to reach the blood. CXCR4 retains NK cells in the BM, whereas the sphingosine-1 phosphate receptor 5 (S1P5) promotes their exit from this organ. However, how the action of these receptors is coordinated to preserve NK-cell development in the BM parenchyma while providing mature NK cells at the periphery is unclear. The role of CXCR4 and S1P5 in NK-cell recirculation at the periphery is also unknown. In the present study, we show that, during NK-cell differentiation, CXCR4 expression decreases whereas S1P5 expression increases, thus favoring the exit of mature NK cells via BM sinusoids. Using S1P5(-/-) mice and a new knockin mouse model in which CXCR4 cannot be desensitized (a mouse model of warts, hypogammaglobulinemia, infections, and myelokathexis [WHIM] syndrome), we demonstrate that NK-cell exit from the BM requires both CXCR4 desensitization and S1P5 engagement. These 2 signals occur independently of each other: CXCR4 desensitization is not induced by S1P5 engagement and vice versa. Once in the blood, the S1P concentration increases and S1P5 responsiveness decreases. This responsiveness is recovered in the lymph nodes to allow NK-cell exit via lymphatics in a CXCR4-independent manner. Therefore, coordinated changes in CXCR4 and S1P5 responsiveness govern NK-cell trafficking. (Blood. 2011;118(18):4863-4871)             

Nov-03  2011     

DOI : 10.1182/blood-2011-06-362574               

Pubmed ID : 21911833

Abstract :

RIG-I is a key innate immune pattern-recognition receptor that triggers interferon expression upon detection of intracellular 5'triphosphate double-stranded RNA (5'ppp-dsRNA) of viral origin. RIG-I comprises N-terminal caspase activation and recruitment domains (CARDs), a DECH helicase, and a C-terminal domain (CTD). We present crystal structures of the ligand-free, autorepressed, and RNA-bound, activated states of RIG-I. Inactive RIG-I has an open conformation with the CARDs sequestered by a helical domain inserted between the two helicase moieties. ATP and dsRNA binding induce a major rearrangement to a closed conformation in which the helicase and CTD bind the blunt end 5'ppp-dsRNA with perfect complementarity but incompatibly with continued CARD binding. We propose that after initial binding of 5'ppp-dsRNA to the flexibly linked CTD, co-operative tight binding of ATP and RNA to the helicase domain liberates the CARDs for downstream signaling. These findings significantly advance our molecular understanding of the activation of innate immune signaling helicases.      

Oct-14   2011     

DOI : 10.1016/j.cell.2011.09.039                

Pubmed ID : 22000019

Abstract :

Alteration of early haematopoietic development is thought to be responsible for the onset of immature leukemias and lymphomas. We have previously demonstrated that Tax(HTLV-1) interferes with beta-selection, an important checkpoint of early thymopoiesis, indicating that human T-cell leukemia virus type 1 (HTLV-1) infection has the potential to perturb thymic human alpha beta T-cell development. To verify that inference and to clarify the impact of HTLV-1 infection on human T-cell development, we investigated the in vivo effects of HTLV-1 infection in a "Human Immune System" (HIS) Rag2(-/-)gamma(-/-)(c) mouse model. These mice were infected with HTLV-1, at a time when the three main subpopulations of human thymocytes have been detected. In all but two inoculated mice, the HTLV-1 provirus was found integrated in thymocytes; the proviral load increased with the length of the infection period. In the HTLV-1-infected mice we observed alterations in human T-cell development, the extent of which correlated with the proviral load. Thus, in the thymus of HTLV-1-infected HIS Rag2(-/-)gamma(-/-)(c) mice, mature single-positive (SP) CD4(+) and CD8(+) cells were most numerous, at the expense of immature and double-positive (DP) thymocytes. These SP cells also accumulated in the spleen. Human lymphocytes from thymus and spleen were activated, as shown by the expression of CD25: this activation was correlated with the presence of tax mRNA and with increased expression of NF-kB dependent genes such as bfl-1, an anti-apoptotic gene, in thymocytes. Finally, hepatosplenomegaly, lymphadenopathy and lymphoma/thymoma, in which Tax was detected, were observed in HTLV-1-infected mice, several months after HTLV-1 infection. These results demonstrate the potential of the HIS Rag2(-/-)gamma(-/-)(c) animal model to elucidate the initial steps of the leukemogenic process induced by HTLV-1.          

SEP        2011     

DOI : 10.1371/journal.ppat.1002231   

Pubmed ID : 21909275

Abstract :

Podosomes are dynamic, actin-containing adhesion structures that collectively self-organize as rings. In this study, we first show by observing osteoclasts plated on bead-seeded soft substrates that podosome assemblies, such as rings, are involved in tension forces. During the expansion of a podosome ring, substrate displacement is oriented outward, suggesting that podosomal structures push the substrate away. To further elucidate the function of forces generated by podosomes, we analyze osteoclast migration. Determining the centers of mass of the whole cell (G) and of actin (P), we demonstrate that osteoclasts migrate by "jumps" and that the trajectories of G and P are strongly correlated. The velocity of the center of mass as a function of time reveals that osteoclasts rapidly catch up with podosomal structures in a periodic pattern. We conclude that actin dynamics inside the cell are not only correlated with cell migration, but drive it.       

Sept-01 2011     

DOI : 10.1091/mbc.E11-01-0086          

Pubmed ID :  21737683