Master of Science / Department of Biochemistry / Qize Wei / Proper regulation of mitosis and cytokinesis is fundamentally important for all living organisms. During anaphase, antiparallel microtubules are bundled between the separating chromosomes, forming the central spindle (also called the spindle midzone), and the myosin contractile ring is assembled at the equatorial cortex. Regulators of central spindle formation and myosin contractile ring assembly are mostly restricted to the interdigitated microtubules of central spindles and they can be collectively called midzone components. It is thought that characteristic microtubule configurations during mitosis and cytokinesis are dictated by the coordinated action of microtubule-stabilizing and -destabilizing factors. Although extensive investigations have focused on understanding the roles of microtubule-bundling/stabilizing factors in controlling central spindle formation, efforts have been lacking in aiming to understand how microtubule-destabilizing factors regulate the assembly of central spindles. This dissertation describes the role of a novel microtubule-destabilizing factor termed CCDC69 (coiled-coil domain-containing protein 69) in controlling the assembly of central spindles and the recruitment of midzone components. Endogenous CCDC69 was localized to the nucleus during interphase and to the central spindle during anaphase. Exogenous expression of CCDC69 in HeLa cells destabilized microtubules and disrupted the formation of bipolar mitotic spindles. RNA interference (RNAi)-mediated knockdown of CCDC69 led to the formation of aberrant central spindles and interfered with the localization of midzone components such as aurora B kinase, protein regulator of cytokinesis 1 (PRC1), MgcRacGAP/HsCYK-4, and pololike kinase 1 (Plk1) at the central spindle. CCDC69 knockdown also decreased equatorial RhoA staining, indicating that CCDC69 deficiency can impair equatorial RhoA activation and ultimately lead to cytokinesis defects. Four coiled-coil domains were found in CCDC69 and the C terminal coiled-coil domain was required for interaction with aurora B. Disruption of aurora B function in HeLa cells by treatment with a small chemical inhibitor led to the mislocalization of CCDC69 at the central spindle. Further, vitro kinase assay showed that Plk1 could phosphorylate CCDC69. Taken together, we propose that CCDC69 acts as a scaffold and a microtubule-destabilizing factor to control the recruitment of midzone components and the assembly of central spindles.

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La polykystose rénale autosomique dominante (PKRAD) est une maladie génétique rénale qui se manifeste par le développement de kystes au rein. Elle résulte de mutations dans le gène PKD1/polycystine-1 (PC-1), contenant un motif coiled-coil et dans le gène PKD2/PC-2 dont les fonctions restent à élucider. À partir d’un chromosome artificiel bactérien, le gène Pkd1 murin (BAC-Pkd1) a été modifié afin de générer 4 lignées de souris transgéniques délété du motif coiled-coil Pkd1Δcoiled-coil et 3 lignées contrôles Pkd1TAG possédant 1-35 copies du transgène. Ces 2 transgènes ont un profil d’expression identique à l’endogène et le niveau dépend du nombre de copies. Alors que les souris Pkd1TAG développent la PKRAD de sévérité proportionnelle au niveau d’expression, les lignées Pkd1Δcoiled-coil en sont épargnées. Ces résultats démontrent l’importance du motif coiled-coil dans la maladie. Les souris Pkd1Δcoiled-coil croisées par Pkd1-/- (létale à la naissance), Pkd1-/- ; Pkd1Δcoiled-coil survivent après la naissance et permettent d’analyser in vivo les interactions, la signalisation et le rôle physiologique du motif coiled-coil. Ces souris Pkd1-/- ; Pkd1Δcoiled-coil avec une copie du transgène présentent des kystes rénaux et meurent ~2 semaines alors que celles à hautes copies n’ont aucun phénotype comme les Pkd1TAG. Les résultats génétiques et biochimiques démontrent que Pc-1Δcoiled-coil est hypomorphe. Bien que Pc-1Δcoiled-coil subit son clivage autoprotéolytique, l’analyse du transport intracellulaire de Pc1Δcoiled-coil montre un délai de maturation. Alors qu’in vitro le trafic Pc-1/Pc-2 dépend du motif coiled-coil pour leur interaction, in vivo une interaction Pc-1Δcoiled-coil/Pc-2 est détectée, suggérant un site d’interaction distinct. Nos études in vivo démontrent que le motif coiled-coil de Pc-1 joue un rôle clé dans sa maturation et l’existence d’un nouveau partenaire de Pc-1 pour son transport intracellulaire. / Autosomal dominant polycystic kidney disease (ADPKD) is the most prevalent genetic disorder that is characterize by the formation of bilateral renal cysts and leads to kidney failure. It results from mutations in the PKD1/polycystin-1(PC-1) gene, containing a coiled-coil motif, and in the PKD2/PC-2 gene, the functions of which remain to be elucidated. From a bacterial artificial chromosome, the murine Pkd1 gene (Pkd1-BAC) was modified in order to obtain 4 transgenic mouse lines deleted from it coiled-coil motif (Pkd1Δcoiled-coil) and 3 Pkd1TAG control lines possessing 1-35 copies of the transgene. These two transgenes have an expression profile identical to the endogenous gene and their expression depends on the number of copies. While Pkd1TAG mice develop ADPKD with a severity proportional to the level of expression, the Pkd1Δcoiled-coil lines are spared. These results demonstrate the importance of the coiled-coil motif in the disease. Pkd1Δcoiled-coil mice crossed by Pkd1-/- (lethal by birth), Pkd1-/-; Pkd1Δcoiled-coil survive after birth and allow the interactions, signaling and physiological role of the coiled-coil motif to be analyzed in vivo. These Pkd1-/-; Pkd1Δcoiled-coil mice with one copy of the transgene show kidney cysts and die ~ 2-weeks while high-copy ones have no phenotype as opposed to Pkd1-/-; Pkd1TAG who eventually develop cyst after 1 year. The genetic and biochemical results demonstrate that Pc-1Δcoiled-coil is hypomorphic. Although Pc-1Δcoiled-coil undergoes its autoproteolytic cleavage, analysis of the intracellular transport of Pc-1Δcoiled-coil shows a delay in maturation. While Pc-1 and Pc-2 appear to interact in vitro through their coiled-coil motif and co-transport, a Pc-1Δcoiled-coil -Pc-2 interaction is conserved in the kidney, suggesting a distinct mechanism of interactions in vivo. Thus, our results show that the coiled-coil motif of Pc-1 in vivo is involved in the maturation independently of Pc-2, highlighting the existence of a critical partner in intracellular transport.

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