Supernumerary Centrosomes and Cell Proliferation

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PROJECT SUMMARY The centriole is a conserved organelle of metazoans that is found at the core of microtubule organizing centers, centrosomes, and at the base of cilia and flagellae. Centriole dysfunction leads to a wide range of diseases including the developmental defects of ciliopathies; defective brain development in microcephaly; and in cancer where supernumerary or defective centrosomes are associated with poor prognosis. Centriole duplication is promoted by Plk4, which phosphorylates cartwheel proteins to mediate their assembly. We and others have found that induction of Plk4-mediated centriole amplification results in hyperplasia of several tissues and increases the susceptibility to tumorigenesis in the mouse in the absence of the p53 tumor suppressor. Both the loss and acquisition of extra centrosomes normally block cell proliferation. However, the pathway that responds to loss of centrosomes differs from several pathways that respond to supernumerary centrosomes. Therefore, to identify proteins that signal or respond to the presence of extra centrosomes, we carried out a genome-wide screen for genes that when deleted or knocked- down permit the proliferation of cells that have elevated levels of Plk4. This has identified new pathways whereby cells respond to supernumerary centrosomes: a previously unknown involvement of specific Rac-mediated signaling that regulates centriole duplication; proteins that regulate the elongation, disengagement and separation of centrioles; and negative regulators of ciliogenesis. Here we follow three approaches to understand how centriole number is regulated in proliferating cells. First, we will determine how the Arh15gap GTPase Activating Protein acts upon the Pak1/2 protein kinases to regulate Plk4 levels or activity and upon Arp2/3 to regulate spindle associated actin to permit an increase in centriole- and cell-cycle arrest. Second, we examine how known components of the centriole affect the execution of the centriole and cell cycles at unexpected points. We will determine how the Usp33 deubiquitinase regulates the centriole capping protein complex to protect the nascent procentriole and how proteins required in the centriole cycle for both centriole disengagement and centrosome separation contribute to the block to cell proliferation in the presence of supernumerary centrosomes. Finally, we will explore how negative regulators of ciliogenesis block primary cilium formation in the presence of extra centrioles in such a way as to arrest cell proliferation. We anticipate that this research will advance our understanding of the multiple ways in which cells respond to supernumerary centrosomes. It will identify pathways that can be targeted for restoring centriole numbers or cell cycle regulation in tumor cells or for targeting such cells for apoptosis. This will find translational application in developing multiple drug strategies for cancer treatment.