tcl-2 encodes a novel protein that acts synergistically with Wnt signaling pathways in C. elegans

Mutations in tcl-2 cause defects in the specification of the fates of the descendants of the TL and TR blast cells, whose polarity is regulated by lin-44/Wnt and lin-17/frizzled, during Caenorhabditis elegans development. In wild-type animals, POP-1/TCF/LEF, is asymmetrically distributed to the T cell daughters, resulting in a higher level of POP-1 in the nucleus of the anterior daughter. The POP-1 asymmetric distribution is controlled by lin-44 and lin-17. However, in tcl-2 mutants, POP-1 is equally distributed to T cell daughters as is observed in lin-17 mutants, indicating that, like lin-17, tcl-2 functions upstream of pop-1. In addition, tcl-2 mutations cause defects in the development of the gonad and the specification of fate of the posterior daughter of the P12 cell, both of which are controlled by the Wnt pathway. Double mutant analyses indicate that tcl-2 can act synergistically with the Wnt pathway to control gonad development as well as P12 descendant cell fate specification. tcl-2 encodes a novel protein. A functional tcl-2::gfp construct was weakly expressed in the nuclei of the T cell and its descendants. Our results suggest that tcl-2 functions with Wnt pathways to control T cell fate specification, gonad development, and P12 cell fate specification.     

A novel noncanonical Wnt pathway is involved in the regulation of the asymmetric B cell division in C. elegans

The polarities of several cells that divide asymmetrically during Caenorhabditis elegans development are controlled by Wnt signaling. LIN-44/Wnt and LIN-17/Fz control the polarities of cells in the tail of developing C. elegans larvae, including the male-specific blast cell, B, that divides asymmetrically to generate a larger anterior daughter and a smaller posterior daughter. We determined that WRM-1 and the major canonical Wnt pathway components: BAR-1, SGG-1/GSK-3 and PRY-1/Axin were not involved in the control of B cell polarity. However, POP-1/Tcf is involved and is asymmetrically distributed to the B daughter nuclei, as it is in many cell divisions during C. elegans development. Aspects of the B cell division are reminiscent of the divisions controlled by the planar cell polarity (PCP) pathway that has been described in both Drosophila and vertebrate systems. We identified C. elegans homologs of Wnt/PCP signaling components and have determined that many of them appear to be involved in the regulation of B cell polarity. Specifically, MIG-5/Dsh, RHO-1/RhoA and LET-502/ROCK appear to play major roles, while other PCP components appear to play minor roles. We conclude that a noncanonical Wnt pathway, which is different from other Wnt pathways in C. elegans, regulates B cell polarity.     

Identification of genes that regulate a left-right asymmetric neuronal migration in Caenorhabditis elegans

In C. elegans, cells of the QL and QR neuroblast lineages migrate with left-right asymmetry; QL and its descendants migrate posteriorly whereas QR and its descendants migrate anteriorly. One key step in generating this asymmetry is the expression of the Hox gene mab-5 in the QL descendants but not in the QR descendants. This asymmetry appears to be coupled to the asymmetric polarizations and movements of QL and QR as they migrate and relies on an asymmetric response to an EGL-20/Wnt signal. To identify genes involved in these complex layers of regulation and to isolate targets of mab-5 that direct posterior migrations, we screened visually for mutants with cell migration defects in the QL and QR lineages. Here, we describe a set of new mutants (qid-5, qid-6, qid-7, and qid-8) that primarily disrupt the migrations of the QL descendants. Most of these mutants were defective in mab-5 expression in the QL lineage and might identify genes that interact directly or indirectly with the EGL-20/Wnt signaling pathway.     

Control of cell polarity by noncanonical Wnt signaling in C. elegans

The three Caenorhabditis elegans beta-catenin each function in distinct processes: BAR-1 in canonical Wnt signaling that controls cell fates and cell migrations, HMP-2 in cell adhesion and WRM-1 in Wnt signaling pathways that function in conjunction with a mitogen-activated kinase (MAPK) pathway to control the orientations, or cell polarities, of cells that undergo asymmetric cell divisions. In addition, WRM-1 does not interact with the canonical beta-catenin binding site in POP-1/Tcf. Thus, Wnt signaling through WRM-1 is noncanonical and, except for one division that might not include any of the three C. elegans beta-catenin, controls cell polarity in C. elegans.     

Wnt signaling and a Hox protein cooperatively regulate psa-3/Meis to determine daughter cell fate after asymmetric cell division in C. elegans

Asymmetric cell division is a mechanism for achieving cellular diversity. In C. elegans, many asymmetric cell divisions are controlled by the Wnt-MAPK pathway through POP-1/TCF. It is poorly understood, however, how POP-1 determines the specific fates of daughter cells. We found that nob-1/Hox, ceh-20/Pbx, and a Meis-related gene, psa-3, are required for asymmetric division of the T hypodermal cell. psa-3 expression was asymmetric between the T cell daughters, and it was regulated by POP-1 through a POP-1 binding site in the psa-3 gene. psa-3 expression was also regulated by NOB-1 and CEH-20 through a NOB-1 binding sequence in a psa-3 intron. PSA-3 can bind CEH-20 and function after the T cell division to promote the proper fate of the daughter cell. These results indicate that cooperation between Wnt signaling and a Hox protein functions to determine the specific fate of a daughter cell.     

A Wnt signaling system that specifies two patterns of cell migration in C. elegans

In C. elegans, a bilateral pair of neuroblasts, QL and QR, give rise to cells that migrate in opposite directions along the anteroposterior (A/P) body axis. QL and its descendants migrate posteriorly whereas QR and its descendants migrate anteriorly. We find that a Wnt family member, EGL-20, acts in a dose-dependent manner to specify these opposite migratory behaviors. High levels of EGL-20 promote posterior migration by activating a canonical Wnt signal transduction pathway, whereas low levels promote anterior migration by activating a separate, undefined pathway. We find that the two Q cells respond differently to EGL-20 because they have different response thresholds. Thus, in this system two distinct dose-dependent responses are specified not by graded levels of the Wnt signal, but instead by left-right asymmetrical differences in the cellular responsiveness to Wnt signaling.     

DSH-2 regulates asymmetric cell division in the early C. elegans somatic gonad

Like other organs, the C. elegans gonad develops from a simple primordium that must undergo axial patterning to generate correct adult morphology. Proximal/distal (PD) polarity in the C. elegans gonad is established early during gonadogenesis by the somatic gonad precursor cells, Z1 and Z4. Z1 and Z4 each divide asymmetrically to generate one daughter with a proximal fate and one with a distal fate. PD polarity of the Z1/Z4 lineages requires the activity of a Wnt pathway that activates the TCF/LEF homolog pop-1. How the gonadal pathway controlling pop-1 is regulated by upstream factors has been unclear, as neither Wnt nor Dishevelled (Dsh) proteins have been shown to be required. Here we show that the C. elegansdsh homolog dsh-2 controls gonadal polarity. As in pop-1 mutants, dsh-2 hermaphrodites have Z1 and Z4 lineage defects indicative of defective PD polarity and are missing gonadal arms. Males have an elongated but disorganized gonad, also with lineage defects. DSH-2 protein is expressed in the Z1/Z4 gonadal precursor cells. Asymmetric distribution of nuclear GFP::POP-1 in Z1 and Z4 daughter cells is reversed in dsh-2 mutants, with higher levels in distal than proximal daughters. dsh-2 and the frizzled receptor homolog lin-17 have a strong genetic interaction, suggesting that they act in a common pathway. We suggest that DSH-2 functions as an upstream regulator of POP-1 in the somatic gonad to control asymmetric cell division, thereby establishing proximal-distal polarity of the developing organ.     

The thrombospondin repeat containing protein MIG-21 controls a left-right asymmetric Wnt signaling response in migrating C. elegans neuroblasts

Wnt proteins are secreted signaling molecules that play a central role in development and adult tissue homeostasis. Although several Wnt signal transduction mechanisms have been described in detail, it is still largely unknown how cells are specified to adopt such different Wnt signaling responses. Here, we have used the stereotypic migration of the C. elegans Q neuroblasts as a model to study how two initially equivalent cells are instructed to activate either β-catenin dependent or independent Wnt signaling pathways to control the migration of their descendants along the anteroposterior axis. We find that the specification of this difference in Wnt signaling response is dependent on the thrombospondin repeat containing protein MIG-21, which acts together with the netrin receptor UNC-40/DCC to control an initial left-right asymmetric polarization of the Q neuroblasts. Furthermore, we show that the direction of this polarization determines the threshold for Wnt/β-catenin signaling, with posterior polarization sensitizing for activation of this pathway. We conclude that MIG-21 and UNC-40 control the asymmetry in Wnt signaling response by restricting posterior polarization to one of the two Q neuroblasts.     

WRM-1 activates the LIT-1 protein kinase to transduce anterior/posterior polarity signals in C. elegans.

During C. elegans development, Wnt/WG signaling is required for differences in cell fate between sister cells born from anterior/posterior divisions. A beta-catenin-related gene, wrm-1, and the lit-1 gene are effectors of this signaling pathway and appear to downregulate the activity of POP-1, a TCF/LEF-related protein, in posterior daughter cells. We show here that lit-1 encodes a serine/threonine protein kinase homolog related to the Drosophila tissue polarity protein Nemo. We demonstrate that the WRM-1 protein binds to LIT-1 in vivo and that WRM-1 can activate the LIT-1 protein kinase when coexpressed in vertebrate tissue culture cells. This activation leads to phosphorylation of POP-1 and to apparent changes in its subcellular localization. Our findings provide evidence for novel regulatory avenues for an evolutionarily conserved Wnt/WG signaling pathway.     

Patterning of the C. elegans 1 degrees vulval lineage by RAS and Wnt pathways

In C. elegans, the descendants of the 1 degrees vulval precursor cell (VPC) establish a fixed spatial pattern of two different cell fates: E-F-F-E. The two inner granddaughters attach to the somatic gonadal anchor cell (AC) and generate four vulF cells, while the two outer granddaughters produce four vulE progeny. zmp-1::GFP, a molecular marker that distinguishes these two fates, is expressed in vulE cells, but not vulF cells. We demonstrate that a short-range AC signal is required to ensure that the pattern of vulE and vulF fates is properly established. In addition, signaling between the inner and outer 1 degrees VPC descendants, as well as intrinsic polarity of the 1 degrees VPC daughters, is involved in the asymmetric divisions of the 1 degrees VPC daughters and the proper orientation of the outcome. Finally, we provide evidence that RAS signaling is used during this new AC signaling event, while the Wnt receptor LIN-17 appears to mediate signaling between the inner and outer 1 degrees VPC descendants.