Ras signaling through the RalGEF-Ral pathway in Caenorhabditis elegans

The classical Ras effector pathway involves activation of the Raf-MEK-ERK mitogen-activated protein kinase cascade. Recent studies show that a second Ras effector cascade, Ral guanine nucleotide exchange factor (RalGEF) activation of the Ras-like (Ral) small GTPases, also promotes tumorigenic, invasive and metastatic cancer cell growth. How RalGEF-Ral downstream effector signaling facilitates Ras activity in cancer cells remains poorly defined. Studies of the C. elegans Ras ortholog have provided critical clues for delineating Ras signaling in mammalian cells. Components of Raf and RalGEF effector pathways are conserved in C. elegans, but only Raf is known to promote Ras function in vulval fate induction. Vulval precursor cell fates are patterned through activation of the epidermal growth factor (EGF)-EGF receptor (EGFR)-Ras-Raf-MEK-ERK cascade to specify the 1° fate, followed by Notch signaling to specify the 2° fate. Recently, the Raf pro-1° signal was also shown to be transiently active in presumptive 2° cells with unknown consequences. My studies have focused on determining the role of C. elegans RalGEF and Ral in Ras-dependent vulval patterning, with the long-term goal of understanding the in vivo function of Ral in mammals. We found that Ras signaling through RalGEF-Ral antagonizes pro-1° Ras-Raf signaling in parallel with or downstream of the Ras-Raf signal. We showed that Ral regulates the balance of 1° and 2° fates, and contributes to EGF and Notch pro-2° activities. Ral expression was also found to be restricted to presumptive 2°s following initial induction. These results suggest that while Ras signals through Raf in 1° cells, in 2° cells Ras effector usage is switched to RalGEF-Ral. Thus, by such effector switching in presumptive 2° cells, the EGF signal transduced by Ras promotes a 2° fate instead of a 1° fate. From mammalian studies, we know that differential Ras effector usage exists, and can impact the efficacy of pharmacological inhibitors of Ras effector signaling currently under clinical trial evaluation. Our study provides insight into how cells spaced across gradients discriminate signal strength, and suggests that effector switching provides a mechanism by which the relative signal strength of two effector signals leads to distinct cellular outcomes.