Integration of EGFR and LIN-12/Notch Signaling by LIN-1/Elk1, the Cdk8 Kinase Module, and SUR-2/Med23 in Vulval Precursor Cell Fate Patterning in Caenorhabditis elegans

Abstract

Six initially equivalent, multipotential Vulval Precursor Cells (VPCs) in adopt distinct cell fates in a precise spatial pattern, with each fate associated with transcription of different target genes. The pattern is centered on a cell that adopts the "1°" fate through Epidermal Growth Factor Receptor (EGFR) activity, and produces a lateral signal composed of ligands that activate LIN-12/Notch in the two flanking VPCs to cause them to adopt "2°" fate. Here, we investigate orthologs of a transcription complex that acts in mammalian EGFR signaling-/Elk1, Med23, and the Cdk8 Kinase module (CKM)-previously implicated in aspects of 1° fate in and show they act in different combinations for different processes for 2° fate. When EGFR is inactive, the CKM, but not SUR-2, helps to set a threshold for LIN-12/Notch activity in all VPCs. When EGFR is active, all three factors act to resist LIN-12/Notch, as revealed by the reduced ability of ectopically-activated LIN-12/Notch to activate target gene reporters. We show that overcoming this resistance in the 1° VPC leads to repression of lateral signal gene reporters, suggesting that resistance to LIN-12/Notch helps ensure that P6.p becomes a robust source of the lateral signal. In addition, we show that Med23 and/Elk1, and not the CKM, are required to promote endocytic downregulation of LIN-12-GFP in the 1° VPC. Finally, our analysis using cell fate reporters reveals that both EGFR and LIN-12/Notch signal transduction pathways are active in all VPCs in /Elk1 mutants, and that/Elk1 is important for integrating EGFR and /Notch signaling inputs in the VPCs so that the proper gene complement is transcribed.

Outstanding questions in developmental ERK signaling

“…Although ERK-mediated phosphorylation of LIN-1 converts it to an activator of 1°fate genes (Leight et al, 2015), for other target genes, LIN-1 may act solely as a repressor. For example, transcription of lateral Notch ligand genes (see below) does not require LIN-1 activation (Underwood et al, 2017). Furthermore, dissociation of the LIN-1/LIN-31 complex exposes a phosphorylation site in the transactivation domain of LIN-31 such that it also becomes an activator when phosphorylated by ERK ( Fig.…”

Hydrogen peroxide as regeneration-initiation signal that activates pERK to trigger planarian regeneration

“…In many organisms, the epidermal growth factor receptors (EGFR), one of families of the receptor tyrosine kinases (RTK), are known to regulate several biological processes by activating key downstream pathways including the MAPK pathway [26][27][28][29][43][44][45] . In planarians, Smed-egfr-3 is required for proper regeneration as well as for ERK activation (Fig.4) 24,46 .…”

“…Literature has already described examples of the activation of EGFR signaling by ROS and the potential of EGFR in mediating MAPK signaling including ERK activation [24][25][26][27][28][29][30][31] . Additionally, the egr (early growth response) family of transcription factors is suggested to be a downstream target of EGFR signaling in planarian regeneration as well as in other models [32][33][34][35] .…”

“…Many studies already indicated an interplay between ROS and mitogenactivated protein kinases (MAPK) signaling pathways [18][19][20][21][22] 24,25 and other models [26][27][28][29] . Also, the activation of the EGFR pathway by ROS has been described in different models 30,31 .…”

A broad mutational target explains an evolutionary trend

“…In C. elegans, the CDK-8 module acts in a highly pleiotropic fashion yet a P3.p division frequency phenotype was not previously reported. In the ventral epidermis, the CDK-8 module was shown to act at many other steps, contributing in the L1 stage to the fusion to hyp7 of anterior and posterior Pn.p cells (such as P2.p and P9.p) (Yoda et al, 2005), to the block of division of all VPCs in the L2 stage (Clayton et al, 2008) and to the level of induction of 2° and 1° VPC fates via cell-autonomous repression of EGF and Notch signalling in the L3 stage; these activities being mostly revealed in a sensitized genetic background (Moghal et al, 2003, Grants et al, 2016, Underwood et al, 2017. We found that mutation in three other genes encoding components of the CDK-8 module also increased P3.p division frequency in an otherwise wild-type genetic background ( Fig.…”