Structural and Functional Characterization of a Caenorhabditis elegans Genetic Interaction Network within Pathways.

A genetic interaction (GI) is defined when the mutation of one gene modifies the phenotypic expression associated with the mutation of a second gene. Genome-wide efforts to map GIs in yeast revealed structural and functional properties of a GI network. This provided insights into the mechanisms underlying the robustness of yeast to genetic and environmental insults, and also into the link existing between genotype and phenotype. While a significant conservation of GIs and GI network structure has been reported between distant yeast species, such a conservation is not clear between unicellular and multicellular organisms. Structural and functional characterization of a GI network in these latter organisms is consequently of high interest. In this study, we present an in-depth characterization of ~1.5K GIs in the nematode Caenorhabditis elegans. We identify and characterize six distinct classes of GIs by examining a wide-range of structural and functional properties of genes and network, including co-expression, phenotypical manifestations, relationship with protein-protein interaction dense subnetworks (PDS) and pathways, molecular and biological functions, gene essentiality and pleiotropy. Our study shows that GI classes link genes within pathways and display distinctive properties, specifically towards PDS. It suggests a model in which pathways are composed of PDS-centric and PDS-independent GIs coordinating molecular machines through two specific classes of GIs involving pleiotropic and non-pleiotropic connectors. Our study provides the first in-depth characterization of a GI network within pathways of a multicellular organism. It also suggests a model to understand better how GIs control system robustness and evolution

Études comparatives de la nociception chez Caenorhabditis elegans souche sauvage (N2) et mutants (egl-3 et egl-21)

L’objectif de cette étude était de vérifier l’hypothèse selon laquelle, les mutants C. elegans (egl-3, egl-21, flp-18 et flp-21) auraient une diminution significative dans la production de neuropeptides matures bioactifs et présenteraient donc une diminution significative de la sensibilité aux stimuli nociceptifs. La nociception est une fonction défensive, une sorte d'alarme qui permet l’intégration de stimuli potentiellement dangereux pour l’organisme. L’intégration du stimulus au niveau du système nerveux se fait via l’activation de nocicepteurs. Caenorhabditis elegans (C. elegans) est un petit nématode majoritairement hermaphrodite, transparent, non parasite et non segmenté qui se nourrit de bactéries du genre Escherichia coli (E. coli). Il a un cycle de développement d’environ 3 jours à 22°C et une durée de vie d’environ 3 semaines. C. elegans est un organisme modèle largement utilisé pour examiner la réponse nocifensive aux stimuli nocifs. Une analyse complète de la séquence du génome révèle plusieurs gènes codant pour des pro-neuropeptides menant à une série de neuropeptides bioactifs. Les neuropeptides de C. elegans sont impliqués dans la modulation de tous les comportements, y compris la locomotion, la mécanosensation, la thermosensation et la chimiosensation. Nous avons soumis nos nématodes à des stimuli thermiques (30°C - 35°C) afin d’observer le comportement des souches sauvages et mutantes. Les résultats de nos études ont révélé une altération du comportement d'évitement thermique chez les mutants. Ce comportement d'évitement thermique des C. elegans mutants egl-3 et egl-21 était significativement entravé par rapport à la souche sauvage (N2). De plus, les C. elegans mutants flp-18 et flp-21 présentaient un phénotype similaire à egl-3 et egl-21. EGL-3 pro-protéine convertase et EGL-21 carboxypeptidase E sont des enzymes essentielles pour la maturation des pro-neuropeptides en neuropeptides actifs chez C. elegans. Les analyses d’abondance relative avec des homogénats de C. elegans mutants egl-3 et egl-21 ont démontré que la protéolyse de ProFLP-18 et ProFLP-21 était gravement entravée, conduisant à un manque de neuropeptides bioactifs matures. Pour déclencher une réponse nocifensive de C. elegans au stimuli thermique, les voies de signalisation des neuropeptides FLP-18 ou FLP-21 / NPR-1 sont nécessaires.This study’s objective was to test the hypothesis that C. elegans mutants (egl-3, egl-21, flp-18 and flp-21) will have a significant decrease in mature bioactive neuropeptides production and that they will have a significant decrease in sensitivity to stimuli. Nociception is a defensive function, an alarm that allows integration of potentially harmful stimuli. The integration of the stimulus into the nervous system is done by activation of nociceptors. Caenorhabditis elegans (C. elegans) is a small, non-parasitic, non segmented nematode. It is easily grown in petri dishes where it is fed with bacteria, specifically Escherichia coli (E. coli). C. elegans is a transparent worm, mostly hermaphrodite, with a development cycle and a lifespan of about three days at 22° C and three weeks respectively. C. elegans is a model organism widely used to examine the harmful response to stimulation. A complete analysis of the genome sequence reveals several pro-neuropeptide genes, encoding a series of bioactive neuropeptides. The neuropeptides of C. elegans are involved in the modulation of all behaviors, including locomotion, mechanosensation, thermosensation and chemiosensation. We exposed our nematodes on thermal stimuli (30°C - 35°C) to observe the behavior of wild strains and mutants. Our results revealed that only mutants had impaired thermal avoidance behavior. This thermal avoidance behavior of C. elegans mutants egl-3 and egl-21 was significantly impaired compared to the wild-type strain (N2). In addition, C. elegans mutants flp-18 and flp-21 exhibited a phenotype similar to egl-3 and egl-21. EGL-3 pro-protein convertase and EGL-21 carboxypeptidase E are essential enzymes for the maturation of pro-neuropeptides to the mature bioactive neuropeptides in C. elegans. Relative abundance analyzes with egl-3 and egl-21 mutant C. elegans homogenates demonstrate that proteolysis of ProFLP-18 and ProFLP-21 are severely impeded, leading to a lack of mature bioactive neuropeptides. FLP-18 or FLP-21/NPR-1 neuropeptide signaling pathways are needed to trigger nocifensive response of C. elegans to thermal stimuli

Role of phytoplasma effector proteins in plant development and plant-insect interactions

Phytoplasmas are insect-transmitted plant pathogenic bacteria that dramatically alter plant development. Phytoplasma virulence protein (effector) SAP54 mediates degradation of host MADS-box transcription factors (MTFs) via 26S proteasome shuttle protein RAD23 to abolish normal flower development and produce leaf-like flowers (phyllody). Phyllodies are common symptoms in phytoplasma-infected plants worldwide. Why do phytoplasmas degrade MTFs and induce phyllody? Are changes in host plant morphology adaptive and benefit phytoplasma spread? Because phytoplasmas rely on their insect (leafhopper) vectors for transmission from plant to plant, I hypothesized that the vegetative tissues of the leaf-like flowers render plants more attractive to the insect vectors that will aid phytoplasma dispersal in nature. I discovered that the induction of phyllody is genetically linked with enhanced insect egg-laying preference on the infected plants that exhibit the leaflike flower phenotype. However, SAP54 enhances insect colonisation of plants independently from floral transition and the changes in plant morphology. Interestingly, male leafhoppers are required for the preference of females to lay eggs on SAP54 plants. Moreover, SAP54 suppresses insect induced plant responses in sex-specific manner by selectively downregulating male-induced defence and secondary metabolism pathways. Furthermore, I identified four MTFs that are expressed in plant leaves and play important roles in egg-laying preferences by leafhoppers and demonstrate sex-specific regulation by SAP54. Taken together, phytoplasma effector SAP54 enhances insect vector colonisation of plants by suppression of insect-induced plant responses independent of developmental changes. This is likely to occur by targeting MTFs – a conserved regulators of both plant development as well as plant defence against herbivorous insects. In addition to developmental changes, degradation of MTFs by SAP54 may result in modulation of male-induced plant responses to attract female insects for egg-laying and aid phytoplasma spread in nature