Abstract
The six C. elegans vulval precursor cells (VPCs) are induced to form the 3°-3°-2°-1°-2°-3° pattern of cell fates with high fidelity. In response to EGF signal, the LET-60/Ras-LIN-45/Raf-MEK-2/MEK-MPK-1/ERK canonical MAP kinase cascade is necessary to induce 1° fate and synthesis of DSL ligands for the lateral Notch signal. In turn, LIN-12/Notch receptor is necessary to induce neighboring cells to become 2°. We previously showed that, in response to graded EGF signal, the modulatory LET-60/Ras-RGL-1/RalGEF-RAL-1/Ral signal promotes 2° fate in support of LIN-12. In this study, we identify two key differences between RGL-1 and RAL-1. First, deletion of RGL-1 confers no overt developmental defects, while previous studies showed RAL-1 to be essential for viability and fertility. From this observation, we hypothesize that the essential functions of RAL-1 are independent of upstream activation. Second, RGL-1 plays opposing and genetically separable roles in VPC fate patterning. RGL-1 promotes 2° fate via canonical GEF-dependent activation of RAL-1. Conversely, RGL-1 promotes 1° fate via a non-canonical GEF-independent activity. Our genetic epistasis experiments are consistent with RGL-1 functioning in the modulatory 1°-promoting AGE-1/PI3-Kinase-PDK-1-AKT-1 cascade. Additionally, animals lacking RGL-1 experience 15-fold higher rates of VPC patterning errors compared to the wild type. Yet VPC patterning in RGL-1 deletion mutants is not more sensitive to environmental perturbations. We propose that RGL-1 functions to orchestrate opposing 1°- and 2°-promoting modulatory cascades to decrease developmental stochasticity. We speculate that such switches are broadly conserved but mostly masked by paralog redundancy or essential functions.
Highlights
Developmental patterning of the C. elegans vulva precursor cell (VPC) fates is a textbook system for analysis of cell-cell signaling
Using GEF-specific mutations and genetic bypass experiments, we show that the opposing functions of RGL-1 in VPC fate patterning are genetically separable
Deletion of RAL-1 but not RGL-1 confers a net effect on 1 ̊ vs. 2 ̊ VPC induction
Summary
Developmental patterning of the C. elegans vulva precursor cell (VPC) fates is a textbook system for analysis of cell-cell signaling. The vulva develops from six roughly equipotent VPCs– P3.p through P8.p –that are induced to assume a 3 ̊-3 ̊-2 ̊-1 ̊-2 ̊-3 ̊ pattern of cells fates. Redundant DSL ligands produced by presumptive 1 ̊ cells induce neighboring VPCs via LIN-12/Notch to become 2 ̊ [2, 3]; LIN-12 is necessary and sufficient for 2 ̊ fate [4] and controls 2 ̊-specific transcriptional targets (Fig 1B; [5,6,7]). A graded signal was found to contribute to 2 ̊ fate induction [8,9,10], but a mechanism by which this signal is mediated was lacking
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