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Cellular dynamics and molecular regulation of microglia development: insights from the zebrafish model

par Ferrero, Giuliano
Président du jury Remmelink, Myriam
Promoteur Wittamer, Valérie
Publication Non publié, 2022-01-26

Thèse de doctorat

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Résumé :

Microglia, the resident macrophage population of the central nervous system (CNS), contribute to the development and homeostasis of the brain and spinal cord, and to the maintenance of parenchymal integrity in the event of injury or pathogenic aggression. Recent studies have also demonstrated the involvement of these cells in a wide range of severe pathological conditions, for which the replacement of "defective" microglia by in vitro reprogrammed cells is now considered a promising therapeutic avenue. Consequently, in recent years, the mechanisms controlling the origin and development of microglia have been the subject of intense research. Although numerous fate mapping studies in mice have revealed that microglia originate from embryonic hematopoietic progenitors with self-renewal properties within the CNS parenchyma, the precise nature of these progenitors is, however, still debated.In this work, we set out to answer this important question by using the zebrafish, a model of choice for the study of vertebrate developmental hematopoiesis, to better understand the dynamics of microglia development. Because the early steps of microglia ontogeny take place early during embryogenesis, the optical transparency of the zebrafish embryo is an unprecedented advantage for studying these processes non-invasively. By combining the use of hematopoietic mutants with novel cell fate tracking approaches, we identified primitive macrophages as the unique precursors of embryonic microglia in the zebrafish embryo. Surprisingly, our results also demonstrate that this initial wave of microglial cells is subsequently replaced at the larval stage by a distinct population that maintains through adulthood, and originates from embryonic hematopoietic stem cells.These observations led us to investigate the signaling pathways that control the development of each of the two microglial waves in vivo. We focused on the role of Colony-stimulating factor 1 receptor (CSF1-1R), a fundamental regulator of macrophage development in vertebrates, of which zebrafish harbor two paralogous copies: csf1ra and csf1rb. This work, based on the use of mutant lines for each paralog, revealed distinct roles for csf1ra and csf1rb during myelopoiesis, identifying csf1rb as an essential regulator of adult microglia development. The importance of the Csf1r pathway in macrophage biology was also assessed in real-time imaging analyses performed in embryos mutant for both csf1r paralogs (csf1rDM), leading to a better understanding of the role of csf1r in the temporal dynamics of myelopoiesis.Finally, by performing studies on fish mutant for the irf8 (interferon regulatory factor 8) gene, we demonstrated that the key role of this transcription factor during myelopoiesis is conserved in zebrafish. Unexpectedly, phenotypic analysis of these animals revealed the presence of a macrophage-like population of non-hematopoietic ancestry, previously identified as metaphocytes, and thus developing independently of Irf8 signaling.In conclusion, the results of this work establish zebrafish as a valuable model to study vertebrate microglia development and pave the way to further investigating the molecular pathways regulating each wave of microglia ontogeny. Understanding how microglia differentiates from different hematopoietic progenitors should ultimately help replicating the process in vitro for therapeutic applications.