The Roles ntl, Wnt, and BMP Play on p21 Expression Levels in Developmen

p21 is an important CDK inhibitor (CKI) which is known to inhibit cell cycle progression by down-regulating CDK2 levels. For that reason, p21 is an excellent indicator to study cell cycle regulation. By the end of mitosis, cells with low levels of p21 proceed to S phase, while cells with high levels of p21 arrest in the G1/G0 phase of the cell cycle. In previous studies, the embryonic transcription factor Brachyury is shown to be crucial to posterior body development. However, it is unknown whether brachyury also plays a role in cell cycle regulation, which may contribute to the phenotype of brachyury mutants. In this study I show that p21 levels are positively correlated with the expression of ntl, the zebrafish homolog of mammalian brachyury. In ntl mutants, p21 expression is absent. Conversely, in HS: ntl mutants with increased expression of ntl, I observed significant increase in p21 levels. Previous studies in our lab showed the notochord cells are in a G1/G0 arrest state during convergence and extension. My results suggest that ntl may be responsible for inducing this G1/G0 arrest as p21 expression in the notochord of ntl mutants is lost, and may explain the poorly developed notochord observed in ntl mutants. The loss of p21 expression in the tailbud region of ntl mutants also suggests the potential role of stem cell regulation by ntl, as stem cells and progenitor cells reside in the tailbud. In addition, I also looked at p21 expression in embryos deficient in Wnt or BMP signaling pathways. Interestingly, two different BMP mutant lines used in our study showed discrepancy in p21 expression. This could be due to the complicated roles BMP pathway plays in different stages of development. Overall, our results show BMP increases p21 expression at 15 somite stage. Contrary to our expectations, the decreased p21 levels observed in Wnt dominant negative HS:TCF?C embryos suggest knocking out Wnt does not necessarily reduce CKI levels. These results may be clinically important, as the cancer microenvironment highly resembles the embryonic environment, and the Wnt signaling pathway is frequently targeted by pharmaceutical companies for new anticancer drug development.