Control Of Motile Multiciliogenesis In Tetrapods – COMMIT

Ciliogenesis has emerged as a key biological phenomenon in many developmental processes, such as the establishment of organ laterality and Shh signalling. Deficient ciliogenesis is the cause of numerous disorders collectively named ciliopathies. Distinct types of cilia can be detected, characterized by different sizes and biomechanical properties. They can range from immotile primary cilia, which can function as sensing and signalling platforms, to motile cilia, which can generate mechanical flows of the surrounding fluid. They can be expressed from one to several hundreds of copies in each specialized cell. Motile multiciliated cells (MCCs) lining the surface of some vertebrate epithelia are essential for physiological processes like airway cleansing, ovum implantation and cerebrospinal fluid circulation. In several chronic airway diseases, regeneration of a functional mucociliary epithelium (MCE) is important for proper healing. From this prospect, understanding the mechanisms governing the biosynthesis of multiciliated cells is a fundamental issue with high biomedical relevance. So far, the key mechanisms of multiciliogenesis have remained largely elusive.

The proposed COMMIT is aimed at identifying cellular and molecular cues controlling motile multiciliogenesis, conserved from Xenopus to human. The project stems from an ongoing collaboration between two groups (Laurent Kodjabachian, IBDML, Marseille, France, P1, coordinator; Pascal Barbry, IPMC, Sophia-Antipolis, France, P2) that has already identified conserved molecular components that control multiciliogenesis in these two models. This collaboration has established the first functional link existing between microRNAs (miRNAs) and multiciliogenesis, opening a novel and unique opportunity to further delve into this important question of cell biology. Members of the miR-449 family are evolutionary conserved regulators of vertebrate multiciliogenesis. In both human airway epithelium and Xenopus laevis embryonic epidermis, miR-449 specifically accumulate in MCCs at an early stage of differentiation. MiR-449 knockdown dramatically reduces centriole multiplication and multiciliogenesis in both model systems. Multiple lines of evidence indicate that a key mechanism of action of miR-449 in multiciliogenesis comes from a capacity to directly repress the Delta/Notch pathway (Marcet et al., revised manuscript submitted to Nature Cell Biology). Incidentally, this study is a nice illustration that “nothing in biology makes sense except in the light of evolution” (Dobzhansky, T. 1973. The American Biology Teacher, 35:125-129.) and establishes the power of a combination of functional and molecular studies in distant tetrapods.

However, this work also revealed the complexity underlying the genetic interaction between miR-449 and the Notch pathway and the existence of additional cues impacting on the process of multiciliogenesis. There is therefore a critical need for a better and more global understanding of the relevant cellular interactions during construction of the MCE in both human airways and frog epidermis. This is the ambition of COMMIT.

To reach this objective, we shall combine our expertise in genome-wide transcriptional analysis, molecular embryology, functional genomics and cell biology. We will focus on the role of the Notch, BMP and Wnt signalling pathways in controlling transcriptional programmes in the human airway and frog epidermis. Our final goal will be to define a minimal signature common to human and frog MCE, to initiate the construction of a GRN (Gene Regulatory Network) around key conserved nodes.

COMMIT is expected to uncover both fundamental principles of signalling pathway deployment and integration and to provide novel targets for diagnosis and therapeutics to the growing field of ciliopathies.