NDR kinases are tumor suppressors implicated in a variety of cancers, including soft tissue sarcomas, B and T cell lymphomas, pancreatic, prostate and ovarian cancers. NDR mutations cause cell growth and polarized morphogenesis defects in a variety of cells, including neurons. Despite the importance of NDR kinases, the mechanisms by which they influence cellular morphogenesis and cancer progresion are poorly understood, thus limiting the development of diagnostics and therapeutics for NDR-mediated cancers. Our recent data support a novel role for the S. cerevisiae NDR1/2 orthologue Cbk1 in regulating mRNA polarity and translation, processes that play defining roles in cell polarity, asymmetric cell division, development, and morphogenesis. Specifically, we have shown that Cbk1 phosphorylates the ribonucleoprotein (RNP) Ssd1 and promotes the polar localization of a subset of Ssd1-mRNA complexes, suggesting that NDR kinases promote cell polarity by directing mRNA-RNP complexes to sites of localized protein expression. In contrast, Cbk1 inhibition or cellular stress redirect Ssd1-mRNA complexes to P bodies and stress granules, thereby repressing mRNA translation and causing cell morphology and lysis defects. Moreover, Ssd1 recruitment to the P bodies is abolished by Sit4/PP6 phosphatase deletion, suggesting that Sit4/PP6 is an NDR-opposing phosphatase. Finally, preliminary data suggest that human hnRNP A1 and A2/B1 serve as functional Ssd1 orthologues. Based on these data, we hypothesize that NDR kinases play a universal role in promoting cell polarity by enhancing mRNA-RNP interactions with mRNA transport proteins and concurrently inhibiting mRNA-RNP recruitment to translational repressing P bodies/stress granules. Because mRNA polarity and localized translation are critical for cell polarity, development and cell motility these data may help explain how NDR mutations give rise to morphogenesis defects in neurons and other cells. Moreover, the regulatory mechanisms for modulating active transport of mRNA-RNP complexes during polarized growth and in response to stress are unknown. The main objective of this proposal is to reveal how NDR kinases and a counteracting phosphatase pathway control cell growth and development via RNP regulation.
In Aim 1 we determine how Cbk1 promotes Ssd1- mediated asymmetric mRNA localization, in Aim 2 we will determine how Sit4 phosphatase regulates Ssd1 recruitment to P bodies and in Aim 3 we test the hypothesis that NDR kinases regulate cell polarity via hnRNP A1 and A2/B1. It is expected that these studies will provide mechanistic insight into how NDR and counteracting pathways regulate hnRNP-dependent mRNA localization. As such, these studies may reveal a key mechanism for NDR kinases in cell polarity, neuronal morphogenesis and tumor suppressor activity.
The proposed research is relevant to public health because it will provide new insights into NDR tumor suppressor function that will influence the future development of novel anti-cancer therapeutics. Specially, the proposed experiments will reveal how NDR tumor suppressors regulate cellular morphogenesis and cancer development by influencing when and where cell growth proteins are expressed. The project is relevant to NIH's mission because it will increase an understanding of the fundamental mechanisms of growth control and cancer development.