The Ral Small GTPase In C. elegans development
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abstract
Ras is the most frequently mutated oncogene in human cancers and the Raf-MEK-ERK protein kinase cascade is the best characterized downstream effector function of Ras. However, it is now clear that Ras function cannot be ascribed simply to one effector pathway, and that Ras utilizes a spectrum of functionally diverse effectors, with at least five implicated in oncogenesis. What remains unclear is how effector utilization is regulated. Recent studies have implicated a critical role for guanine nucleotide exchange factors for the Ral small GTPases (RalGEFs) as effectors of Ras in cancer. Yet we know little about the interplay between RalGEF-Ral and other effector pathways, and we know even less about the normal functions of RalGEF-Ral activity during development. Genetic analyses of the Ras homolog in C. elegans, LET-60, have elucidated the role of the Raf-MEK-ERK cascade in development and provided critical clues for understanding mechanisms of Ras signaling in mammalian cells. Since essentially all known components of RalGEF-Ral signaling are conserved in C. elegans, this prompted our interest in elucidating a role for the evolutionarily conserved C. elegans RGL-1 (RalGEF) and RAL-1 (Ral) in LET-60-regulated development. Our preliminary results indicate that, unexpectedly, RGL-1-RAL-1 signaling antagonized rather than cooperated with the Raf-MEK-ERK signal in inducing 10 cell fate in the vulva, and does so in parallel or downstream to the ERK substrate, the LIN- 31/HNF transcription factor. Additionally, our results suggest that RAL-1 is expressed only in 20 cells neighboring 10 vulval cells, suggesting that LET-60/Ras switches from the use of one effector (LIN-45/Raf) in 10 cells to another (RGL-1/RalGEF) in 20 cells, and that RAL-1 controls the balance of Ras-dependent cell fates. These observations provide the rationale for our analysis of the role of RGL-1-RAL-1 signaling in cell fate induction, a model for RalGEF-Ral oncogenic functions, and to our knowledge the first developmental model to study the potentially critical regulatory mechanism of differential effector usage by Ras. We propose studies that will apply a multi-faceted approach that utilizes genetic, biochemical and cell biological methods to elucidate the mechanisms and roles by which LET-60 dynamically regulates these two distinct effector- signaling networks. We propose aims to (1) determine the signaling properties of the RGL-1-RAL-1 module relative to the Ras-Raf-MEK-ERK pathway, (2) determine the developmental mechanisms by which RGL-1- RAL-1 controls equivalence group fate decisions, and (3) determine the genetic and molecular mechanism of RGL-1-RAL-1 function in 20 cells. Our studies will address outstanding questions of developmental patterning in response to a morphogen gradient, balance between Ras and Notch signaling, and differential use of effectors in development and cancer, all of which lie at a critical intersection between developmental biology and cancer. PUBLIC HEALTH RELEVANCE: Narrative Ras is the most commonly mutated oncogene in cancer,and an important Ras effector, RalGEF, functions to activate the Ral small GTPase to promote oncogenesis and metastasis. However, little is known about how RalGEF-Ral signaling promotes tumorigenesis, or even what functions are performed by RalGEF-Ral in an animal. Our studies will address the in vivo function of RalGEF-Ral and its interactions with Ras in one of the best known cell fate induction systems. These studies will provide insights into the previously uncharacterized developmental and molecular mechanisms of RalGEF-Ral signaling, a potential pharmaceutical target.
