The role of B-Myb and its target genes in ES cell cycle progression

Abstract

Embryonic stem cells and induced pluripotent stem cells represent a unique cell type that features the capacity of unlimited self-renewal and the ability to differentiate into all derivatives of the three primary germ layers. ES and iPS cells are therefore a promising source for regenerative therapies such as tissue engineering or cell replacement therapy. Nevertheless, before widespread stem cell applications in regenerative medicine can be realized, a detailed understanding of the regulatory pathways of pluripotency and self-renewal is necessary. <br /> In this connection, the transcription factor B-Myb plays a decisive role in the regulatory network of ES cells and is involved in maintaining pluripotency, normal cell cycle progression and chromosomal integrity. Since B-Myb regulates gene expression of many proteins that are either associated with the cell cycle or implicated in differentiation, the question has been raised whether individual target genes act as major contributors of B-Myb or if the interaction of several proteins fulfills this task. <br /> Taking previous data in somatic cells into account, the target genes Aurora Kinase A and Cyclin B1 were chosen to detect whether they serve as major contributors for B-Myb in embryonic stem cells. In the present study, miRNAs for knockdown of Aurora Kinase A and Cyclin B1 were used for cell cycle and spindle assembly analysis in order to test whether the phenotype described for B-Myb knockdown could be reproduced by one of those target genes. Contrary to preliminary experiments in somatic cells, incomplete knockdown of neither Aurora Kinase A nor Cyclin B1 in ES stem cells led to abnormalities or defects in cell cycle or mitotic progression. Partial knockdown of both proteins did not reproduce the phenotype caused by B-Myb knockdown. <br /> The effects of B-Myb in embryonic stem cells must involve multiple factors or another critical factor that has to be identified. <br /> Other B-Myb target genes than Aurora Kinase A or Cyclin B1 can be tested in an analo-gous manner or by using the Ainv15 cells. Furthermore, dysregulation of several target genes could cause the aneuploidy observed with loss of B-Myb. Thus, it is still unclear, how B-Myb regulates cell cycle progression precisely in embryonic stem cells.