Multipartite viruses: A decentralized mode of functioning. [O.24]

Multipartite viruses are characterized by a genome composed of two or more nucleic acid segments, each encapsidated indivually. A classical view in virology assumes that the viral replication cycle occurs within individual cells, where the whole viral genome information is replicated, and is then reiterated in successively infected cells during host invasion. In the context of multipartite viruses, this view implies that at least one copy of each of the genome segments must enter in each of the infected cells. The genome of the Faba bean necrotic stunt virus (FBNSV, Family Nanoviridae) is composed of 8 ssDNA circles of about 1000 bases, each encapsidated in an individual virus particle. We have previously shown that each of the eight segments reproducibly accumulates at a specific relative frequency, some representing around 30% of the total viral DNA within an infected plant and others not exceeding 2%. In this situation, it is difficult to conceive how FBNSV can actually transmit the whole genome information both from cell to cell and from host to host. If the segments enter cells indifferently, solely according to their frequency within the population, we could calculate that a successful infection of 95% of the susceptible cells would require the entry of nearly 200 particles per cell. This figure illustrates the enormous cost that FBNSV might bear at each cell-to-cell transmission step. Alternatively, this virus might infect individual cells with subgroups of genome segments, partial genome information being replicated at distinct location within a host. This may alleviate the cost at cell-to-cell passage but would imply a sort of unknown viral communication or complementation in between these subgroups of segments to maintain the integrity of the genome information. In any cases, the actual functioning of FBNSV is an enigma, because it is hard to conceive that a virus could force hundreds of particles in each newly colonized cells, or that the genome could function with separate subunits in distinct cells. We are currently developing tools to test the above alternatives. (Résumé d'auteur

Xanthomonas albilineans is able to move outside of the sugarcane xylem despite its reduced genome and the absence of a Hrp type III secretion system.

Xanthomonas albilineans, the causal agent of leaf scald disease of sugarcane, is a pathogen that experienced genome reduction during its speciation. Additionally, this xanthomonad is notably missing the Hrp type III secretion system and the xanthan gene cluster that are commonly found in pathogenic Xanthomonas species. X. albilineans was up to now considered as limited to the xylem of sugarcane. However, recently published studies suggested that X. albilineans was able to invade tissues other than the xylem of sugarcane leaves but the occurrence of X. albilineans outside the xylem has not been clearly proven. In this study, we used confocal microscopy and transmission electron microscopy to investigate the localization of this pathogen in diseased leaves and stalks of sugarcane. Three sugarcane cultivars with different levels of resistance to leaf scald were inoculated with the green fluorescent protein labelled X. albilineans strains XaFL07-1 (from Florida) and GPE PC73 (from Guadeloupe). Sections of sugarcane leaves and stalks were examined 8-60 days after inoculation in order to localize X. albilineans in the different plant tissues. Confocal microscopy observation of symptomatic leaves confirmed the presence of the pathogen in the protoxylem and the metaxylem, however, X. albilineans was also observed in the phloem, the parenchyma and the bulliform cells of the leaves. Similarly, the protoxylem and the metaxylem of infected sugarcane stalks were invaded by X. albilineans. Surprisingly, the pathogen was also observed in apparently intact storage cells of the stalk and in the intercellular spaces between these cells. Several of these observations made by confocal microscopy have been confirmed by transmission electron microscopy. X. albilineans can therefore no longer be considered as a xylem-limited pathogen. To our knowledge, this is the first description of a plant pathogenic bacterium invading apparently intact non-vascular plant tissue and multiplying in parenchyma cells. The mechanisms and virulence factors used by X. albilineans to enter and invade different tissues of sugarcane remain to be identified. (Résumé d'auteur

Understanding the evolutionary role of viral integration in banana genome: which similitude with retrotransposons?

The genome of banana (Musa sp.) harbours multiple integrations of several species of Banana streak virus (BSV), certainly resulting from illegitimate recombination between host and viral DNA. Surprisingly, this badnavirus does not require integration for its replication. Some integrations, only existing in the Musa balbisiana genome (denoted B), are infectious by releasing a functional viral genome following stresses such as in vitro culture and interspecific crosses. To date, four widespread species of BSV (Goldfinger -BSGFV, Imové - BSImV, Mysore - BSMysV and Obino l'Ewai - BSOLV) have been reported as integrated into the B genome with three of them as infectious (eBSGFV, eBSImV and eBSOLV). In order to study BSV expression from such viral integrants and to retrace their evolutionary story, a full genomic and genetic characterisation of BSV integrants (eBSV) was undergone including cytogenetic localization on chromosomes. Very low copies of integrations were recorded for each BSV species. The full characterisation of eBSGFV was recently performed in our lab (Gayral et al., 2008). eBSGFV results from a single event of integration corresponding to an allelic insertion of at least one full-length viral genome extensively rearranged with several viral regions duplicated. Although the four BSV species present important differences with each other, the organisation of eBSOLV and eBSImV looks like eBSGFV. Indeed, each of them is more or less extensively rearranged in PKW and is present as allelic insertions at the same locus. In contrary, the non infectious eBSMysV presents two independent insertions sites. The evolutionary history of each BSV species was studied by analysing their distribution, their insertion polymorphism and their structure evolution among representative banana species, in relation to the phylogeny of Musa genus. The early evolutionary stages of infectious eBSV for BSGFV and BSImV were investigated among selected banana genotypes representative of the diversity of 60 wild Musa species and genotypes. Both BSV species integrated recently in banana evolution, circa 640,000 years ago, and after speciation between Musa acuminata and Musa balbisiana, circa 4.5 MYA. These two species were subject to different selective pressures and showed distinct levels of rearrangement within their final structure. Unlike other pathosystems harboring viral integrants, there is no colonization of host genomes by duplication of the viral sequences once integrated. The strong diversity of eBSV in the Musa genome could be rather explained by independent integrations from each of the numerous BSV species. Interestingly, M. balbisiana diploid genotypes (BB) such as Pisang Klutuk Wulung (PKW), harbor infectious eBSVs in their genome but are nevertheless resistant to any multiplication of BSV. The mechanisms underlying such resistance are believed to be driven by epigenetic phenomena but no evidence has been obtained so far in banana plants. (Texte intégral

Stress-fiber mechanics and cell mechano-sensitivity

In recent years, research in cell biology has shown that mechanics is a key to cell response, differentiation, and disease. For instance, an increasing number of observations have shown that the ability of cells to contract, spread, and differentiate is highly dependent on the stiffness and architecture of the surrounding matrix. Although the origins of these intriguing behaviors are still poorly understood, it is now clear that cells fully make use of cross-talks between mechanics, chemistry, and transport processes to organize their structure, generate forces, and make appropriate decisions. To better understand the underlying mechanisms of mechano-transduction, this presentation will introduce a multiscale approach to the actin cytoskeleton of an adherent scale, spanning from the molecular to the cellular scale. At the cellular scale, the cytoskeleton is viewed as an active gel, which can acquire a specific structure and exert contractile forces in response to its mechanical environment. The way by which these forces arise and stabilize the cytoskeleton is explained in terms of a fine scale model of the interactions between the actin filaments and myosin motors found within each individual sarcomere of a stress fiber. At this scale, a cross-bridge model is used to explain the stabilization of active acto–myosin complex in the presence of so-called a catch-bond behavior between the two molecular units. The idea of a catch-bond response acto–myosin assembly was indeed discovered recently but never related to the mechano-sensitivity of stress-fibers. After further derivations, these concepts are summarized into a coupled system of differential equation whose solution is analyzed using numerical methods such as finite elements. Numerical simulations show that the model is able to capture the dependency of cell contraction on substrate stiffness, adhesion or the application of external force on the cell boundary. The very good agreement between model predictions and experimental observations not only confirms that catch bonds may play a significant role in the mechano-sensitivity of adherent cells, but also pinpoint the importance of the hierarchical structure of stress-fibers across the scales

Growth mechanics in degradable hydrogel scaffolds

Despite tremendous advances in the field of tissue engineering, a number of obstacles are still hindering its successful translation to the clinic. One of these challenges has been to design cell-laden scaffolds that can provide an appropriate environment for cells to successfully synthesize new tissue while providing a mechanical support that can resist physiological loads at the early stage of in situ implementation. A solution to this problem has been to balance tissue growth and scaffold degradation by creating new hydrogel systems that possess both hydrolytic and enzymatic degradation behaviors. Very little is known, however, about the complex behavior of these systems, emphasizing the need for a rigorous mathematical approach that can eventually assist and guide experimental advances. This presentation will introduce a mathematical and numerical formulation based on mixture theory, to describe the degradation, swelling, and transport of extracellular matrix (ECM) molecules released by cartilage cells (chondrocytes) within a hydrogel scaffold. The model particularly investigates the relative roles of hydrolytic and enzymatic degradations on ECM diffusion and their impacts on two important outcomes: the extent of ECM transport (and deposition) and the evolution of the scaffold’s mechanical integrity. Numerical results based on finite element show that if properly tuned, enzymatic degradation differs from hydrolytic degradation, in that it can create a degradation front that is a key to maintain scaffold stiffness while allowing ECM deposition. These results, therefore, suggest a hydrogel design that could enable successful in situ cartilage tissue engineering

Duration of adjuvant chemotherapy for stage III colon cancer

BACKGROUND Since 2004, a regimen of 6 months of treatment with oxaliplatin plus a fluoropyrimidine has been standard adjuvant therapy in patients with stage III colon cancer. However, since oxaliplatin is associated with cumulative neurotoxicity, a shorter duration of therapy could spare toxic effects and health expenditures. METHODS We performed a prospective, preplanned, pooled analysis of six randomized, phase 3 trials that were conducted concurrently to evaluate the noninferiority of adjuvant therapy with either FOLFOX (fluorouracil, leucovorin, and oxaliplatin) or CAPOX (capecitabine and oxaliplatin) administered for 3 months, as compared with 6 months. The primary end point was the rate of disease-free survival at 3 years. Noninferiority of 3 months versus 6 months of therapy could be claimed if the upper limit of the two-sided 95% confidence interval of the hazard ratio did not exceed 1.12. RESULTS After 3263 events of disease recurrence or death had been reported in 12,834 patients, the noninferiority of 3 months of treatment versus 6 months was not confirmed in the overall study population (hazard ratio, 1.07; 95% confidence interval [CI], 1.00 to 1.15). Noninferiority of the shorter regimen was seen for CAPOX (hazard ratio, 0.95; 95% CI, 0.85 to 1.06) but not for FOLFOX (hazard ratio, 1.16; 95% CI, 1.06 to 1.26). In an exploratory analysis of the combined regimens, among the patients with T1, T2, or T3 and N1 cancers, 3 months of therapy was noninferior to 6 months, with a 3-year rate of disease-free survival of 83.1% and 83.3%, respectively (hazard ratio, 1.01; 95% CI, 0.90 to 1.12). Among patients with cancers that were classified as T4, N2, or both, the disease-free survival rate for a 6-month duration of therapy was superior to that for a 3-month duration (64.4% vs. 62.7%) for the combined treatments (hazard ratio, 1.12; 95% CI, 1.03 to 1.23; P=0.01 for superiority). CONCLUSIONS Among patients with stage III colon cancer receiving adjuvant therapy with FOLFOX or CAPOX, noninferiority of 3 months of therapy, as compared with 6 months, was not confirmed in the overall population. However, in patients treated with CAPOX, 3 months of therapy was as effective as 6 months, particularly in the lower-risk subgroup. (Funded by the National Cancer Institute and others.

A TBLMI Framework for Harmonic Robust Control

The primary objective of this paper is to demonstrate that problems related to stability and robust control in the harmonic context can be effectively addressed by formulating them as semidefinite optimization problems, invoking the concept of infinite-dimensional Toeplitz Block LMIs (TBLMIs). One of the central challenges tackled in this study pertains to the efficient resolution of these infinite-dimensional TBLMIs. Exploiting the structured nature of such problems, we introduce a consistent truncation method that effectively reduces the problem to a finite-dimensional convex optimization problem. By consistent we mean that the solution to this finite-dimensional problem allows to closely approximate the infinite-dimensional solution with arbitrary precision. Furthermore, we establish a link between the harmonic framework and the time domain setting, emphasizing the advantages over Periodic Differential LMIs (PDLMIs). We illustrate that our proposed framework is not only theoretically sound but also practically applicable to solving H 2 and H$\infty$ harmonic control design problems. To enable this, we extend the definitions of H 2 and H$\infty$ norms into the harmonic space, leveraging the concepts of the harmonic transfer function and the average trace operator for Toeplitz Block operators. Throughout this paper, we support our theoretical contributions with a range of illustrative examples that demonstrate the effectiveness of our approach

La stratégie des vertus

Si les principes éthiques ont évolué au cours des siècles, ils orientent aujourd'hui comme hier les jugements de l'homme. La Vertu est un idéal qui n'a pas pris une ride. Mais la connaît-on vraiment ? En voici les effigies, préservées dans la pierre depuis le Moyen-Âge