Induction and differentiation during vertebrate embryonic development

Our team is interested in understanding the molecular and cellular mechanisms that control the development in vertebrate embryos and the deregulation of specific developmental processes associated with the emergence of human diseases. We focus our research on two aspects: 1) the post-transcriptional regulation of muscle cell differentiation by RNA-binding proteins and 2) the role of Wnt/T-cell Factors (TCF) signaling in the emergence of neural stem cell niches.

Embryonic development is an extremely complex and highly regulated system. The research in our group is focused on the gene regulatory networks implicated in muscle and neural cell differentiation. We use Xenopus and zebrafish as models, which are most suitable for in vivo and in vitro analyses by large-scale cell and molecular biology, biochemistry, and experimental biology approaches. Our goal is to decipher common mechanisms underlying early development and human pathologies. The research interests include important aspects of transcriptional and post-transcriptional regulations of gene expression during early development.

  1. The post-transcriptional regulation of gene expression orchestrated by RNA-binding proteins plays essential roles in a variety of physiological and pathological processes. The evolutionarily conserved RBM24 (RNA-Binding Motif Protein 24) is highly expressed in muscles and head sensory organs of all vertebrate embryos. It displays dynamic subcellular localization and represents a novel regulator of cell differentiation. We have been endeavoring in understanding the mechanisms by which it functions in muscle cell differentiation and regeneration (Project leader: Raphaëlle Grifone).
  2. Wnt signaling is critically involved in development and disease. We have been interested in the identification and functional analysis of key factors that modulate the activation and/or repression of Wnt signaling in vertebrate embryos. More specifically, we are interested in TCF regulators that play a major role in the irreversible neural fate commitment, and in BarH-type homeodomain transcription factors (BARHL1/2), as BARHL2 functions as an irreversible inhibitor of Wnt/TCF-dependent transcription. We are also working on the non-lethal function of Caspase-3 that modulates canonical Wnt signaling in neural development (Project leader: Béatrice Durand)

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Cell differentiation is a fundamental process that generates all the cell types of our body from a fertilized egg. Its deregulation causes defective development and the occurrence of various pathological conditions. In this context, we have been engaged in understanding the molecular events during muscle and neural cell development in vertebrate embryos.

  1. RNA-binding proteins (RBPs) control RNA metabolism at multiple levels, and are critical for maintaining the homeostasis of protein synthesis during early development and in adult life. Due to their importance in the temporal and spatial control of gene expression, a growing number of human diseases are associated with RNAs and RBPs. Analyses of RBM24 loss of function reveal that it is critically required for cellular differentiation. Deficiency in its expression level could be the cause of congenital disorders, such as cardiomyopathy, myopathy or blindness, which affect the normal function of related tissues where crucial roles of this gene have been demonstrated in different animal models. Thus, our aim is to decipher the post-transcriptional mechanisms mediated by RBM24 in muscle development and regeneration. We will focus our work in the identification of its interacting partners and mRNA targets to understand how it regulates mRNA stability, polyadenylation and translation.
  2. During embryonic development stem/progenitor cells progressively lose their multipotence. The commitment/differentiation process is unidirectional, and prevented to go backwards by molecular locks. Understanding transcriptional and epigenetic events behind these locks in stem cells/progenitors is crucial; when corrupt they endow the emergence of tumors, and support their propagation. Wnt pathways play critical roles in a wide variety of biological process, including embryonic axis formation, cell proliferation, differentiation and migration, polarity establishment, and stem cell self-renewal. Aberrant Wnt signaling is implicated in various human diseases such as cancers. In embryonic blastomeres and rodent embryonic stem cells stabilization of TCF-mediated repression inhibits self-renewal and represses the pluripotency state. One of the cerebellar primordium - the rhombic lip (cRL) - generates the largest brain neuronal population: the granular neurons. The cRL is at the origin of Medulloblastoma (MB), the most common childhood malignant brain tumor. Another team project concerns the identification and functional analysis of key factors that modulate the activation and/or repression of Wnt signaling during Xenopus early development and specifically the role of TCF and their regulators, the BARHLs, in the emergence of cerebellar stem/progenitors cells. We also work on the non-lethal function of Caspase-3 that modulates canonical Wnt signaling, to specifically investigate its implication in the development of neural tissue.

Highlights

  1. Dishevelled (Dvl) protein mediates the activation of both canonical and non-canonical Wnt signaling, but the mechanism remains unclear. Moreover, its requirement for the activation of maternal Wnt/ß-catenin signaling in dorsal axis specification has been largely enigmatic. We have demonstrated that the carboxyl-terminus of Dvl plays an important role in regulating Dvl conformational changes in canonical versus non-canonical Wnt signaling, and that Dvl maternal function is dispensable for dorsal axis formation, but indispensable for cell polarity changes and posterior development (Elife 2016; Nat Commun 2017; J Biol Chem 2017; PloS Genet 2018).
  2. The homeodomain-containing transcription factor BarH-like homeobox-2 (Barhl2) is well conserved over the course of evolution. We have demonstrated that it acts as a transcriptional repressor in the Wnt pathway via its interaction with Tcf7l1, Groucho, and histone deacetylase 1 (Hdac1), which establishes regionalized and hereditary transmitted repressive chromatin structures (Development 2019).
  3. Caspases have non-apoptotic activities that are just as important as their function in apoptosis. In the neuroepithelium of the forebrain, we have shown that Caspase-3, one of the main executors of the apoptotic cascade, acts in parallel or downstream of Barhl2 to limit the activity of the Wnt/ß-catenin pathway and thus acts as a barrier to proliferation. This role of Caspase-3 does not depend on its function as an apoptosis effector (PNAS 2011).
  4. Transcriptional derepression and post-translational regulation of gene expression play important roles in tumorigenesis, but their implication in early development remains elusive. We demonstrated that XDSCR6, a Xenopus homolog of human Down syndrome critical region protein 6, regulates mesoderm and embryonic axis formation through derepression of polycomb group (PcG) proteins, including Ezh2. Moreover, we found that XDSCR6 and Ezh2 are important regulators of Stat3 transcriptional activity in mesoderm patterning during early development. They play an opposite role in Stat3 protein methylation (Development 2013; J Biol Chem 2020).
  5. ·The RNA-binding protein RBM24 displays highly conserved expression pattern in vertebrate embryos. We showed that it represents a direct target of MyoD, and is required for myogenic differentiation. Furthermore, we found that RBM24 protein is mainly expressed in cells entering into the differentiation program, and is localized in the cytoplasmic compartment of these cells. Most recently and importantly, we have provided the unprecedented finding that it interacts with the cytoplasmic polyadenylation complex to regulate poly(A) tail lengths of crystallin mRNAs during lens terminal differentiation (Mech Dev 2010, 2014; Dev Dyn 2018; PNAS 2020).

Future directions

Based on the results obtained during the last few years, we plan to concentrate our efforts in three directions:

  1. Transcriptional regulation of muscle differentiation is well understood, but post-transcriptional control of this process remains largely elusive. To understand the post-transcriptional mechanism implicating RBM24 in myogenic differentiation, we will perform large-scale analysis of Rbm24-regulated events during myogenesis, including the expression, polyadenylation, stability and translation efficiency of muscle-specific mRNAs. We will also identify novel RBM24-interacting partners and determine the functional interaction between RBM24 and the cytoplasmic polyadenylation complex in the regulation of muscle cell differentiation.
  2. In the nervous system, TCF transcription factors are involved in the irreversible commitment of stem/progenitor cells. We are investigating the role of transcription factors BARHLs in stem/progenitor cell biology with a specific interest in the cerebellum, which is at the origin of a malignant brain tumor in children: medulloblastoma.
  3. A non-lethal activity of Caspase-3 modulates canonical Wnt signaling during neural development. The molecular mechanisms underlying such functions are elusive. The objective is to study the molecular mechanisms regulating the non-lethal activity of Caspase-3 by the X-linked Inhibitor of Apoptosis Protein (XIAP), in the modulation of the Wnt-dependent TCF function in neuroepithelial proliferation and differentiation of the forebrain.

Collaborations

  1. Genome-editing and generation of zebrafish mutants by CRISPR/Cas9 approach to study post-transcriptional regulation of cell differentiation - School of Life Science, Shandong University, Qingdao, China.
  2. Bioinformatics platform - Curie Institute - U1021, Orsay, France.