Understanding cilia and flagella diversity – DIVERCIL

The overall goal of this proposal is to understand how cilia and flagella are assembled and how cilia diversity is generated. These highly organized organelles play fundamental biological functions in a wide range of organisms. For example, motile cilia and flagella are required for fluid flow and cell motility in various tissues. Cilia are also involved in cell signaling during development. As a result, in humans, cilia dysfunction leads to a wide range of pathologies, called ciliopathies.
The first critical step required for cilia or flagella assembly is the conversion of the centriole into the basal body. This step involves the docking of the centriole to cytoplasmic vesicles and the formation of the Transition Zone (TZ), a complex structure of the ciliary base. The TZ plays an essential role by bridging the basal body to the plasma membrane and gating proteins in and out of the cilium. Numerous components of the TZ have been identified, among which many are mutated in human ciliopathies. However, the TZ shows structural and molecular variations between cell types and species, which still need to be understood. As well, many unsolved questions remain regarding the molecular mechanisms supporting the existence of different types of cilia and different modes of assembly between organisms or cell types.
In humans, cilia diversity is emphasized by comparing the architecture of the highly organized flagellum of sperm cells to that of primary cilium found on many cell types during development. Specific features are associated with mammalian sperm flagella, such as the annulus, but the mechanisms that control the assembly of the mammalian sperm flagella and its specific attributes are largely unknown. Ancient observations proposed that the assembly of sperm flagella in Drosophila and humans involves analogous elements, the ring centriole and the annulus respectively, which are found to be required for flagella formation or integrity during spermiogenesis. The composition of the ring centriole and of the annulus is poorly understood, but recent work demonstrates that proteins required to build the TZ of cilia in all organisms are involved in their formation. Therefore, our working hypothesis is that the ring centriole and the annulus are evolutionary derived TZ structures and our objectives are to determine how these structures are built and related to the ciliary TZ and how variations in TZ composition may account for cilia diversity.
In this context, our project aims at: 1) Characterizing proteins required for TZ and/or ring centriole assembly in mouse and Drosophila, by identifying the proteins associated with these compartments using the innovative APEX proximity-dependent labelling strategy; 2) Establishing the first thorough description of the proteome of mammalian sperm annulus; 3) Identifying novel genes required for TZ assembly by a genetic screen in Drosophila; 4) Investigating the implication of the above identified proteins in the process of cilia and flagella assembly in Drosophila and mouse.
This project relies on original preliminary observations and bridges two teams with perfect complementary expertise in cilia and male reproductive biology and two distant model organisms, Drosophila and mouse. It focuses on a key feature involved in cilia formation: the Transition Zone (TZ), and explores a new avenue which questions how the TZ has evolved and may contribute to the assembly of the sperm flagella. Altogether, this project will unravel novel components required to build the TZ and bring novel insights into the mechanisms that contribute to generate cilia diversity in animals. Last, it will have important spin-offs in medical research by bringing new understandings of human ciliopathies, in particular of male infertility.