Andrew J. Holland - US grants

? DESCRIPTION (provided by applicant): The long-term goal of our research is to understand the molecular mechanisms that control centrosome biogenesis. Centrosomes are microtubule-nucleating organelles that play a central role in most microtubule- related functions, including cell motility, intracellular transport and chromosome segregation. In addition, the centrosome forms the basis of the basal body, which is required for the formation of cilia and flagella, and i therefore crucial for cells to both sense their environment and transduce signals. Centrosomes are present as a single copy at the beginning of the cycle and duplicate once during S phase to ensure only two copies are present to organize the poles of the mitotic spindle. Abnormalities in centrosome number are commonly observed in human cancer cells, where extra centrosomes lead to chromosome segregation errors that are thought to drive tumor formation. Understanding the mechanism by which cells achieve the once per cycle duplication of the centrosome is therefore an important fundamental question of considerable relevance to human health. Polo-like kinase 4 (Plk4) has emerged as the central, dose-dependent regulator of centrosome duplication. Suppressing Plk4 inhibits centrosome duplication, while Plk4 overexpression leads to the production of too many centrosomes. However, we understand little about how this kinase functions; and in particular, the critical Plk4 targets that control centrosome biogenesis remain to be identified. Our proposed research seeks to establish the mechanisms through which Plk4 orchestrates and coordinates centrosome biogenesis. Previous efforts to study Plk4 have been hampered because tools to specifically and rapidly manipulate Plk4 function have not been available. In this application we have overcome this limitation by developing two complementary methodologies that allow us to regulate Plk4 levels and activity in living cells. Using these tools we will establish the direct effect of altering Plk4 levels/actiity and distinguish between kinase-dependent and scaffolding functions of Plk4 in centrosome biogenesis. In Aim 1, we will study the effect of rapid loss/inhibition of Plk4 on cell growth and centrosome structure. In our preliminary data we have identified a highly conserved centrosome protein as a key Plk4 substrate required for centrosome duplication. In Aim 2, we propose to characterize how Plk4-mediated phosphorylation of this substrate contributes to centrosome assembly. These studies are relevant for understanding the normal regulation of centrosome assembly and for furthering ongoing efforts to target Plk4 in cancer therapy.

Project Summary The long-term goal of our research is to understand the molecular mechanisms that control centriole biogenesis and how errors in this process contribute to human disease. Centrioles are the structural core of centrosomes, organelles that nucleate microtubules to build mitotic/meiotic spindles and cilia. During a normal cell cycle, centrioles duplicate once to ensure their copy number is precisely maintained. The presence of supernumerary centrioles is a common feature of human tumors and can promote chromosome segregation errors that are sufficient to drive tumor development in mice. To maintain genome integrity, cells have evolved a protective centriole surveillance pathway to restrict the proliferation of cells with extra centrioles. The goal of our application is to unravel the molecular mechanism responsible for ?sensing? supernumerary centrioles and evaluate whether inactivation of this pathway facilitates tumor development in cells with extra centrioles. Centriole amplification triggers the activation of the PIDDosome, a trimeric protein complex that acts as an activation platform for Caspase-2. Once activated, Caspase-2 promotes the cleavage of MDM2 and subsequent stabilization of P53. However, there exists a gap in our understanding of how extra centrioles are sensed and how this information is relayed to the PIDDosome to trigger P53 activation. To address this knowledge gap, we developed a genome-wide screening approach to identify genes required to arrest the growth of non-transformed cells with extra centrioles. Our preliminary data show that distal appendages that form on mature centrioles are responsible for activating the PIDDosome following centriole amplification. In Aim 1 of this proposal we will use cell biological, genetic and biochemical approaches to mechanistically dissect how cells ?sense? supernumerary centrioles to trigger PIDDosome activation. In Aim 2, we will determine the impact of specifically inactivating the centriole surveillance pathway on the proliferation and oncogenic transformation of cells with extra centrioles in vivo. We are well suited to pursue these studies given our expertise in studying centriole biology; our development of a unique mouse model to study the impact of centriole amplification in vivo; and our collaborative relationship with the Regot and Loncarek laboratories, who are world-experts in high resolution live-cell imaging and correlative light/EM analysis of centriole ultrastructure. Understanding how normal cells detect centriole amplification addresses a fundamental question that will provide insight into how aneuploid tumor cells adapt to proliferate robustly with extra centrioles.