Ral Signals through a MAP4 Kinase-p38 MAP Kinase Cascade in C. elegans Cell Fate Patterning

Hanna Shin,Rebecca E.W Kaplan,Tam Duong,Razan Fakieh,David J Reiner

doi:10.1016/j.celrep.2018.08.011

Hanna Shin, Rebecca E.W Kaplan + Show 3 more

Open Access

https://doi.org/10.1016/j.celrep.2018.08.011

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Abstract

C.elegans vulval precursor cell (VPC) fates are patterned by an epidermal growth factor (EGF) gradient. High-dose EGF induces 1° VPC fate, and lower dose EGF contributes to 2° fate in support ofLIN-12/Notch. We previously showed that the EGF 2°-promoting signal is mediated by LET-60/Ras switching effectors, from the canonical Raf-MEK-ERK mitogen-activated protein (MAP) kinase cascade that promotes 1° fate to the non-canonical RalGEF-Ral that promotes 2° fate. Of oncogenic Ras effectors, RalGEF-Ral is by far the least well understood. We use genetic analysis to identify an effector cascade downstream of C.elegans RAL-1/Ral, starting with an established Ral binding partner, Exo84 of the exocyst complex. Additionally, RAL-1 signals through GCK-2, a citron-N-terminal-homology-domain-containing MAP4 kinase, and PMK-1/p38 MAP kinase cascade to promote 2° fate. Our study delineates a Ral-dependent developmental signaling cascade invivo, thus providing the mechanism by which lower EGF dose is transduced.

Highlights

We found that LET-60/Ras uses the non-canonical RGL-1/RalGEF-RAL-1/Ral effector to promote 2 fate in support of LIN-12/Notch (Zand et al, 2011)
Criteria for a RAL-1-Dependent 2-Promoting Signal Because LIN-12/Notch, but not the LET-60/Ras-RGL-1/RalGEFRAL-1/Ral signal, is necessary for 2 fate induction, we used a combination of parallelism and epistasis to test the genetic relationships among members of the vulval precursor cell (VPC) fate patterning network
In the lin-12(n379d) background at 15C, n765ts strongly increased ectopic 2 induction. We showed that this 2-promoting activity depends on LIN-3/epidermal growth factor (EGF), LET-60/Ras, RGL-1/RalGEF, and RAL-1: RNAi depletion of let-60, rgl-1, and ral-1 blocked the increased ectopic 2s conferred by lin-12(n379d); lin-15(n765ts) at 15C

Summary

Introduction

Strategies to inhibit oncogenic Ras have failed, so Ras is considered to be mostly ‘‘undruggable’’ (Papke and Der, 2017). Attention has shifted to oncogenic Ras effectors to identify therapeutic targets. Canonical oncogenic Ras effectors, the Raf-MEK-ERK and phosphatidylinositol 3-kinase (PI3K)-PDK-Akt cascades, are among the best studied and most targeted signaling cascades (Ryan et al, 2015; Wong et al, 2010). Even potent smallmolecule inhibitors, like the BRAF inhibitor vemurafenib, are subject to multiple bypass mechanisms that permit initially responsive tumors to relapse (Sun et al, 2014). Successful treatment will likely require multi-pronged regimens to simultaneously inhibit multiple Ras effectors

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