PI of the "Centrosome, cilia and cancer" group in the "Molecular Oncology" team (Head: Daniel Birnbaum, MD, PhD)
Our team is dedicated to the study of centrosomes and cilia, two cellular entities that play many roles in cell and tissue biology. The centrosome is long-known as a major microtubule organizing center, but it also plays roles in the control of the cell cycle. In addition the older centriole of the centrosome generates highly organised microtubules forming cilia at the surface of cells. There are two kinds of cilia : motile and non motile. Primary cilia are non motile cilia at the surface of many resting and/or terminally differentiated cells, which act in chemo and mechanosensation, and in the activation and regulation of signalling pathways crucial during development. Motile cilia generate fluid movement along specialized epithelia. They can also have sensory functions. An increase in centrosome number is linked to genetic instability and tumor development, and mutation in many genes coding for centrosome and cilia proteins are causes of developmental diseases, like ciliopathies and microcephalies. We are interested in two related proteins identified in the laboratory, which are located at the centrosome and at the basal body, the modified centriole that anchors the cilium to the cell surface. To study the function of the first protein, FOP, we have generated mutant DT40 cells in which FOP can be eliminated in an inducible manner. In this model system, loss of FOP induces cell death by apoptosis at the G1/S cell cycle transition. We have also shown that FOP depletion by siRNA in human epithelial RPE1 cells causes cell cycle arrest at G1/S. Ablation of FOP may affect the signaling properties of the centrosome. This could work in two different ways : A stress signal could be induced when FOP absence is detected, and/or a cycling signal coming from the centrosome may be affected by FOP absence. The existence of a centrosome integrity checkpoint as been postulated in RPE1 cells, which could be activated upon FOP depletion in these cells. Importantly, our study in DT40 indicates that activation of such a checkpoint may not only result in a cell cycle arrest, but also in cell apoptosis, depending on the cellular context. We are currently analysing FOP mutant mice to shed light on the biological and physiological functions of FOP. The second protein, FOR20, is highly conserved in evolution, and only found in ciliated eukaryotes, from protists to mammals. Our work has shown that FOR20 not only localizes at the centrosomes and basal bodies, but also at pericentriolar satellites. Satellites are small (diameter 50 nm) electron dense granular structures with poorly characterized function. They have minus-end directed motility along microtubules and they may have a role as carriers to bring cargoes to the centrosomes and basal bodies. FOR20 depletion disperses satellites from their pericentriolar location and strongly affects the capacity of quiescent RPE1 cells to form a primary cilium. Future efforts aim at understanding the mechanisms by which FOR20 contributes to the biogenesis of primary cilia and motile cilia on multiciliated cells. To address this question we carried out two-hybrid screens and we are now characterizing FOR20-associated proteins that we have identified.