Facteurs bHLH proneuraux et fiabilité des programmes de spécification neuronaux chez C. elegans

Au cours du développement du système nerveux, une grande diversité de types neuronaux est produite. Ce processus doit être étroitement régulé afin que le bon ensemble de types neuronaux soit généré. Durant ma thèse, j’ai analysé les mécanismes qui assurent la fiabilité des programmes de spécification neuronaux en utilisant C. elegans comme organisme modèle. Les facteurs de transcription bHLH proneuraux jouent un rôle clé dans les étapes précoces de la spécification des neurones chez de nombreux animaux. En analysant le développement d’une classe spécifique d’interneurones cholinergiques (AIY), j’ai observé que quatre facteurs bHLH différents sont impliqués dans leur spécification: NGN-1/Neurogénine, HLH-3/Achaete-Scute, HLH-16/Beta3 et HLH-2/E. Cela pose la question de savoir pourquoi autant de facteurs bHLH sont requis pour un unique événement de spécification. En utilisant une approche d’imagerie quantitative, j’ai établi qu’une coopération entre les différents bHLH proneuraux est requise afin d’activer un niveau d’expression correct des facteurs de transcription sélecteurs terminaux qui, par la suite, initient et maintiennent l’expression d’une large batterie de gènes de différenciation terminaux responsables de la fonction du neurone AIY. De manière surprenante, les différents bHLH proneuraux ont un effet antagoniste sur une autre cible, le facteur proapoptotique EGL-1 de la famille BH3-only, qui est normalement exprimé à faible niveau dans le neurone AIY et qui interfère avec sa spécification. Je propose que l’emploi de multiples facteurs bHLH proneuraux permet une spécification neuronale robuste tout en empêchant l’activation incontrôlée de gènes délétères.During nervous system development, a high diversity of neuronal cell types is produced. This process has to be tightly regulated to generate the correct set of neuron types. During my PhD, I analyzed the mechanisms that ensure the robustness of neuronal specification programs, using C. elegans as a model organism. Proneural bHLH transcription factors play a key role in the early steps of neuronal specification in many animals. By anaIyzing the development of a specific class of cholinergic interneurons (AIY), I observed that four different bHLH factors are involved in their specification : NGN-1/Neurogenin, HLH-3/Achaete-Scute, HLH-16/Beta3 and HLH-2/E. This raises the question of why so many bHLH factors are required for a single neuronal specification event. Using quantitative imaging, I established that a cooperation between the different proneural bHLH factors is required to activate the correct level of expression of terminal selector transcription factors that subsequently initiate and maintain the expression of a large battery of terminal differentiation genes responsible for the function of the AIY neuron. Surprisingly, the different proneural bHLH have an antagonistic effect on another target, the proapoptotic BH3-only factor EGL-1, normally expressed only at very low levels in the AIY neuron and detrimental for its specification. I propose that the use of multiple proneural bHLH factors allows robust neuronal specification while, at the same time, preventing spurious activation of deleterious genes

Multiple neural bHLHs ensure the precision of a neuronal specification event in C. elegans

International audienceNeural bHLH transcription factors play a key role in the early steps of neuronal specification in many animals. We have previously observed that the Achaete-Scute HLH-3, the Olig HLH-16 and their binding partner the E protein HLH-2 activate the terminal differentiation program of a specific class of cholinergic neurons, AIY, in C. elegans. Here we identify a role for a fourth bHLH, the Neurogenin NGN-1, in this process, raising the question of why so many neural bHLHs are required for a single neuronal specification event. Using quantitative imaging we show that the combined action of different bHLHs is needed to activate the correct level of expression of the terminal selector transcription factors TTX-3 and CEH-10 that subsequently initiate and maintain the expression of a large battery of terminal differentiation genes. Surprisingly, the different bHLHs have an antagonistic effect on another target, the proapoptotic BH3-only factor EGL-1, normally not expressed in AIY and otherwise detrimental for its specification. We propose that the use of multiple neural bHLHs allows robust neuronal specification while, at the same time, preventing spurious activation of deleterious genes

Quest for Bioactive Compounds in Our Diet with Anti-Ageing and Anti-Aggregation Properties

Ageing is a complex process affected by both genetic and environmental factors, characterized by a gradual failure of functionality, reduced stress response and resistance, leading to enhanced probability for age-related diseases and mortality. During the last decades, natural compounds have attracted the attention of researchers in the quest of bioactive phytochemicals with anti-ageing properties. For a few of these compounds an extra advantage appears; many of them have been shown to decelerate the progression of age-related diseases with emphasis on aggregation-related diseases. Using the nematode Caenorhabditis elegans along with the replicative senescence model of human primary fibroblasts, we have identified compounds that are part of our diet with anti-oxidation, anti-ageing and anti-aggregation activities. Some of the identified compounds promote their anti-ageing activity through activation of the proteasome, others through the activation of Nrf2 transcription factor, while others through inhibition of glucose transporters (GLUTs). Our work identifies new bioactive compounds with anti-ageing and/or anti-aggregation properties or reveals additional beneficial properties on already known bioactive compounds

PRC1 chromatin factors strengthen the consistency of neuronal cell fate specification and maintenance in C. elegans

International audienceIn the nervous system, the specific identity of a neuron is established and maintained by terminal selector transcription factors that directly activate large batteries of terminal differentiation genes and positively regulate their own expression via feedback loops. However, how this is achieved in a reliable manner despite noise in gene expression, genetic variability or environmental perturbations remains poorly understood. We addressed this question using the AIY cholinergic interneurons of C. elegans, whose specification and differentiation network is well characterized. Via a genetic screen, we found that a loss of function of PRC1 chromatin factors induces a stochastic loss of AIY differentiated state in a small proportion of the population. PRC1 factors act directly in the AIY neuron and independently of PRC2 factors. By quantifying mRNA and protein levels of terminal selector transcription factors in single neurons, using smFISH and CRISPR tagging, we observed that, in PRC1 mutants, terminal selector expression is still initiated during embryonic development but the level is reduced, and expression is subsequently lost in a stochastic manner during maintenance phase in part of the population. We also observed variability in the level of expression of terminal selectors in wild type animals and, using correlation analysis, established that this noise comes from both intrinsic and extrinsic sources. Finally, we found that PRC1 factors increase the resistance of AIY neuron fate to environmental stress, and also secure the terminal differentiation of other neuron types. We propose that PRC1 factors contribute to the consistency of neuronal cell fate specification and maintenance by protecting neurons against noise and perturbations in their differentiation program