Search

Metazoan stem cells repopulate tissues during adult life by dividing asymmetrically to generate another stem cell and a cell that terminally differentiates. Wnt signaling regulates the division pattern of stem cells in flies and vertebrates. While the short-lived nematode C. elegans has no adult somatic stem cells, the lateral epithelial seam cells divide in a stem cell-like manner in each larval stage, usually generating a posterior daughter that retains the seam cell fate and an anterior daughter that terminally differentiates. We show that while wild-type adult animals have 16 seam cells per side, animals with reduced function of the TCF homolog POP-1 have as many as 67 seam cells, and animals with reduced function of the β-catenins SYS-1 and WRM-1 have as few as three. Analysis of seam cell division patterns showed alterations in their stem cell-like divisions in the L2-L4 stages: reduced Wnt signaling caused both daughters to adopt non-seam fates, while activated Wnt signaling caused both daughters to adopt the seam fate. Therefore, our results indicate that Wnt signaling globally regulates the asymmetric, stem cell-like division of most or all somatic seam cells during C. elegans larval development, and that Wnt pathway regulation of stem cell-like behavior is conserved in nematodes., (Copyright © 2010 Elsevier Inc. All rights reserved.)

In Caenorhabditis elegans, vulval precursor cell (VPC) fate is specified by the action of RTK/Ras, Notch and Wnt signaling pathways. While the identity of signals for the Ras and Notch pathways is known, the source and identity of the Wnt ligand acting on the VPCs are unknown. Single mutations in any of the five Wnt genes (lin-44, cwn-1, cwn-2, egl-20 and mom-2) do not cause strong defects in VPC fate specification, suggesting that functionally redundant Wnts are required. Surprisingly, we found that all five Wnts influence VPC fate. The strongest defects we observed were in the lin-44; cwn-1; egl-20 triple mutant. Anterior VPCs were more strongly affected by loss of Wnt function than posterior VPCs, and expression from WntColon, two colonsGFP transcriptional reporters showed that the Wnts most strongly affecting VPC fate were expressed predominantly in the posterior, suggesting that some of the redundant Wnt ligands act over a distance to affect the VPCs. In addition to ligand redundancy, we found that at least three Wnt receptors, lin-17, mom-5 and mig-1, function in the VPCs. We also examined ligand and receptor function in another Wnt-mediated vulval process, the orientation of the P7.p lineage. Here, too, we found that four of five Wnt receptors can influence P7.p orientation, suggesting that a surprising amount of functional redundancy exists in Wnt signaling during C. elegans vulval induction.

Extracellular signaling pathways and transcriptional regulatory networks function during development to specify metazoan cell fates. During Caenorhabditis elegans vulval development, the specification of three vulval precursor cells (VPCs) requires the activity of Wnt, Notch, and Ras signaling pathways, and function of the Hox gene lin-39. LIN-39 protein levels are regulated in the VPCs by both Wnt and Ras signaling. In particular, activation of Ras signaling leads to an increase in LIN-39 protein in P6.p at the time of VPC fate specification. We wish to understand the regulation of lin-39 by these pathways. We first show that LIN-39 is a target for MAP kinase in vitro, suggesting that the Ras-dependent LIN-39 upregulation could be mediated post-translationally. To test this idea, we created transcriptional and translational lin-39::GFP fusions that include the entire lin-39 genomic region, allowing observation of lin-39 expression in live animals. The reporters express GFP in most, if not all, sites of expression previously observed by LIN-39 antibody staining. We used these constructs to show that at the time of vulval induction both lin-39::GFP reporters are upregulated in P6.p, indicating that the accumulation of high levels of LIN-39 protein detected previously corresponds to transcriptional upregulation of lin-39 expression. This transcriptional upregulation of lin-39 is dependent on Ras signaling. We tested the requirement for several transcription factors acting downstream of Ras signaling in the VPCs, and found that P6.p upregulation requires the transcription factors LIN-1 and LIN-25, but appears to be independent of LIN-31, SEM-4, EOR-1 and EOR-2.Finally, we found that when the Wnt pathway is over activated, expression from the transcriptional lin-39::GFP increases, suggesting that the Wnt pathway also regulates lin-39 at the transcriptional level.

During Caenorhabditis elegans vulval development, activation of receptor tyrosine kinase/Ras and Notch signaling pathways causes three vulval precursor cells (VPCs) to adopt induced cell fates. A Wnt signaling pathway also acts in cell fate specification by the VPCs, via regulation of the Hox gene lin-39. We show here that either mutation of pry-1 or expression of an activated BAR-1 beta-catenin protein causes an Overinduced phenotype, in which greater than three VPCs adopt induced cell fates. This indicates that pry-1, which encodes a C. elegans axin homolog, acts as a negative regulator of Wnt signaling in the VPCs. Loss of activity of the APC homolog apr-1 increases the penetrance of this Overinduced phenotype, suggesting that APR-1 may play a negative role in Wnt signaling in this process in C. elegans similar to APC proteins in other systems. The Overinduced phenotype is suppressed by reduction of function of the genes pop-1 TCF and lin-39 Hox. Surprisingly, the Overinduced phenotype caused by hyperactivated Wnt signaling is not dependent on signaling through the Ras pathway. These data suggest that hyperactivation of Wnt signaling is sufficient to cause VPCs to adopt induced fates and that a canonical Wnt pathway may play an important role during C. elegans vulval induction.