During cell division, it is important to ensure that the genomic information is faithfully transmitted to daughter cells. To achieve correct segregation ofchromosomes, specialized apparatus are built and complexly regulated duringeukaryotic cell division. The kinetochore is one of these apparatus. The kinetochoreforms a large structure on centromere DNA and plays important roles in mitoticprogression by microtubule attachment. The kinetochore of higher eukaryotes has atrilaminar structure that consists of the inner plate, outer plate and midzone. Theinner plate functions as the foundation of kinetochore formation. While a largenumber of proteins that localize to the inner kinetochore have been identifiedrecently, precise functions of each kinetochore protein are poorly understood andlittle is known about their functional role especially in organismal context.The CENP-O complex that consists of CENP-50/U, O, P, Q, and R constitutivelylocalizes into the kinetochore and is required for recovery from spindle damage inchicken DT40 cells. While DT40 cells with KO of most proteins localized in innerkinetochore die, DT40 cells lacking CENP-O complex proteins are still viable. Therefore, we focus on functional role of the CENP-O complex proteins inorganismal context. Previous study showed that mouse ES cells and mice lacking CENP-50/U diewithin several days after depletion of the protein and during early development,respectively, while chicken DT40 cells with KO of CENP-50/U are viable. Thisobservation suggests that the CENP-O complex plays an essential role in the earlydevelopment of mice. Notably, knockout phenotype of CENP-R in DT40 cells isdifferent from that of other CENP-O complex proteins in some aspects andkinetochore localization of CENP-R occurs downstream of other complex proteins.CENP-R may play a different role from that of other CENP-O complex proteins. It isimportant to examine function of CENP-O complex including CENP-R inorganismal context. In this study, ES cell lines and mice lacking CENP-R were created to elucidate the function of CENP-O complex proteins in mice, and their phenotypes were compared with that of CENP-50/U. Although mice with knock out of CENP-50/U died during early embryogenesis, CENP-R null mice were viable and normally produced offspring, suggesting that CENP-R is not essential in mice and has different role from that of other CENP-Ocomplex proteins. To elucidate that the lethality of CENP-50/U deficient mice is caused at a cellular level, ES cells lacking CENP-50/U or R were generated. Although ES cells with KOof CENP-50 /U were not viable and died within several days, ES cells lackingCENP-R were normally propagated. In addition, mitotic index and the number ofanaphase with lagging chromosomes were increased in ES cells lacking CENP-50/U.These results suggest that the lethality of mice lacking CENP-50/U is caused at acellular level, and each cell in mice lacking CENP-50/U dies due to some mitoticdefects. I also confirmed that the kinetochore structure and the dependencies ofkinetochore localization of CENP-O complex proteins are conserved betweenchicken DT40 cells and mice ES cells. Chicken DT40 cells are derived from chicken B cells, and DT40 cells lackingCENP-50/U are viable. Therefore, I expected that the viabilities of mice cells lackingCENP-50/U are different among cell types. To examine this, I generated inducibleknockout mice, in which CENP-50/U is removed by ERT2-conjugated Crerecombinase. I isolated mouse embryonic fibroblast cells and B-cells from adultinducible knockout mice of CENP-50/U. As a result, both mouse B cells and mouseembryonic fibroblast cells were viable and normally propagated even afterCENP-50/U was removed from kinetochore by OHT addition although ES cellslacking CENP-50/U are lethal. I concluded that the importance of CENP-50/U inmitosis is different among cell types in mice. In living organism, there are various cell divisions such as meiosis, segmentation of fertilized egg or asymmetric cell division in which machinery of chromosome segregation, growth rate, mitotic regulation are different in detail. In addition, the frequency of chromosome missegregation is different among cell types.It is possible that the kinetochore has a function in strict regulation of theseparticular cell divisions, however, little is known about kinetochore functions andvariety in organismal context. In this study, I showed that the importance ofCENP-50/U differs among cell types. From this result, I suggested that CENP-50/Umay be required for these strict regulations of mitosis in mice
