A tumor suppressor Retinoblastoma1 is essential for embryonic development in the sea urchin

Abstract

Background Embryonic cells and cancer cells share various cellular characteristics important for their functions. It has been thus proposed that similar mechanisms of regulation may be present in these otherwise disparate cell types. Results To explore how regulative embryonic cells are fundamentally different from cancerous cells, we report here that a fine balance of a tumor suppressor protein Retinoblastoma1 (Rb1) and a germline factor Vasa are important for proper cell proliferation and differentiation of the somatic cells during embryogenesis of the sea urchin. Rb1 knockdown blocked embryonic development and induced Vasa accumulation in the entire embryo, while its overexpression resulted in a smaller‐sized embryo with differentiated body structures. These results suggest that a titrated level of Rb1 protein may be essential for a proper balance of cell proliferation and differentiation during development. Vasa knockdown or overexpression, on the other hand, reduced or increased Rb1 protein expression, respectively. Conclusions Taken together, it appears that Vasa protein positively regulates Rb1 protein while Rb1 protein negatively regulates Vasa protein, balancing the act of these two antagonistic molecules in somatic cells. This mechanism may provide a fine control of cell proliferation and differentiation, which is essential for regulative embryonic development.

Functional contribution of DCLKs in sea urchin development

“…We found, in a tumor‐suppressive condition, DCLK upregulation after damage was reduced, suggesting Vasa may be important for DCLK protein expression as initially predicted (Figure 5A‐B, arrows). In support of this result, the Rb1‐MO embryos in which Vasa expression is increased 3 also showed upregulation of DCLKs compared the control group even without damage (Figure 5C,D, arrows). This suggests again that SpDCLKs may be regulated by the balancing act of Vasa and Rb1.…”

“…To test our initial question of if/how DCLK contributes to different biological phenotypes depending on the cellular environment, we performed the same damage experiment using the embryos injected with Rb1‐MO and/or Vasa‐MO to induce a tumor‐prone or tumor‐suppressive condition, respectively 3 . Vasa‐MO delays cell cycling of the embryo 18 and Rb1‐MO blocks cell differentiation of the embryo, 3 and both of which cause significant developmental defects such as a general delay in development and a failure in gastrulation and embryonic body growth. For this experiment, we used day 2 embryos and tested for 4‐hour recovery.…”

“…To test this possibility further, the embryo was also knocked down both with Vasa and Rb1 to disturb a balance of these two antagonistic molecules. Vasa and Rb1 regulate their expression levels to each, 3 therefore, double knockdown of these two molecules were used as a way to remove both sides of the balancing factors for plasticity regulation. This double‐knockdown caused a significant developmental abnormality with no clear blastocoel formation especially after damage, supporting a contention further that Vasa and Rb1 are essential in embryogenesis of the sea urchin (Figure 5E, arrow).…”

“…We previously reported that a delicate balance of Retinoblastoma1 (Rb1, tumor suppressor) and Vasa (pluripotency facilitator) may be important for controlling embryonic plasticity of the sea urchin embryo 3 . Rb1 knockdown induces excess Vasa protein accumulation in the entire embryo, while Rb1 overexpression reduces Vasa protein level.…”

“…These observations suggest that a fine balance of these antagonistic factors may be essential for proper cell proliferation and differentiation of somatic cells during embryogenesis. Since both Rb1 and Vasa are considered to regulate a number of downstream targets in this embryo, 3,4 we hypothesize these factors might provide the cellular environment rather than directly control pluripotency or tumorigenesis. If this hypothesis is correct, the same downstream factor(s) may drive both embryonic and tumorigenic pluripotency in a context dependent manner.…”