The Axin-like protein PRY-1 is a negative regulator of a canonical Wnt pathway in C. elegans

Abstract

Axin, APC, and the kinase GSK3␤ are part of a destruction complex that regulates the stability of the Wnt pathway effector ␤-catenin. In C. elegans, several Wnt-controlled developmental processes have been described, but an Axin ortholog has not been found in the genome sequence and SGG-1/GSK3␤, and the APC-related protein APR-1 have been shown to act in a positive, rather than negative fashion in Wnt signaling. We have shown previously that the EGL-20/Wnt-dependent expression of the homeobox gene mab-5 in the Q neuroblast lineage requires BAR-1/␤-catenin and POP-1/Tcf. Here, we have investigated how BAR-1 is regulated by the EGL-20 pathway. First, we have characterized a negative regulator of the EGL-20 pathway, pry-1. We show that pry-1 encodes an RGS and DIX domain-containing protein that is distantly related to Axin/Conductin. Our results demonstrate that despite its sequence divergence, PRY-1 is a functional Axin homolog. We show that PRY-1 interacts with BAR-1, SGG-1, and APR-1 and that overexpression of PRY-1 inhibits mab-5 expression. Furthermore, pry-1 rescues the zebrafish axin1 mutation masterblind, showing that it can functionally interact with vertebrate destruction complex components. Finally, we show that SGG-1, in addition to its positive regulatory role in early embryonic Wnt signaling, may function as a negative regulator of the EGL-20 pathway. We conclude that a highly divergent destruction complex consisting of PRY-1, SGG-1, and APR-1 regulates BAR-1/␤-catenin signaling in C. elegans.

-Catenin-Dependent Wnt Signaling in C. elegans: Teaching an Old Dog a New Trick

“…PRY-1 physically interacts with APR-1, GSK-3, and BAR-1 , presumably to phosphorylate BAR-1, which contains eight GSK-3 consensus phosphorylation sites (Eisenmann et al 1998). Deletion of these sites results in a hyperactive form of the protein that causes Wnt pathway gain-of-function phenotypes when overexpressed (Eisenmann et al 1998;Gleason et al 2002Gleason et al , 2006Korswagen et al 2002). Therefore, these data suggest that a Wnt/b-catenin-dependent pathway similar to that characterized in other species also functions in C. elegans during larval development.…”

“…The C. elegans WBC pathway uses all of the positively acting core components of a canonical Wnt pathway (Wnt, Fz, Dsh, b-catenin, and TCF) except for LRP, and the homologs of the b-catenin destruction complex components (PRY-1/Axin, GSK-3/GSK3b, APR-1/ APC, and KIN-19/CK1a) function as negative regulators of this pathway (Eisenmann 2005;Gleason et al 2006). For example, in pry-1/ Axin mutants, several Wnt-responsive Hox genes (lin-39, mab-1, and egl-5) are expressed outside their normal domains and mutant animals display phenotypes consistent with over-or ectopic activation of the Wnt pathway (Harris et al 1996;Eisenmann et al 1998;Maloof et al 1999;Gleason et al 2002Gleason et al , 2006Howard and Sundaram 2002;Korswagen et al 2002;Oosterveen et al 2007). PRY-1 physically interacts with APR-1, GSK-3, and BAR-1 , presumably to phosphorylate BAR-1, which contains eight GSK-3 consensus phosphorylation sites (Eisenmann et al 1998).…”

“…In three different larval developmental processes, WBC pathway activity is necessary to maintain Hox gene expression in Wnt-receiving cells: WBC activity maintains expression of the midbody Hox gene lin-39 during vulval precursor cell-fate specification (Eisenmann et al 1998;Penigault and Felix 2011b), and is believed to be required for expression of the posterior Hox gene egl-5 during P12 fate specification (Jiang and Sternberg 1998;Eisenmann and Kim 2000;Li et al 2009) and for expression of the mid/posterior Hox gene mab-5 in the daughters of the QL neuroblast to mediate their proper cell migration (Harris et al 1996;Maloof et al 1999;Eisenmann and Kim 2000;Herman 2001;Korswagen et al 2002). As an example of WBC pathway function in C. elegans, we consider this last process.…”

“…Both cells begin with an anterior migration; however, the three QL neuroblast descendants (QL.d) migrate to the posterior in response to the Wnt EGL-20 (Herman 2002;Korswagen et al 2002;Silhankova and Korswagen 2007), which is expressed in a posterior-to-anterior gradient (Coudreuse et al 2006). Although both the QL and QR cells are exposed to the ligand, they have different intrinsic sensitivities.…”

“…Genes encoding the core b-catenin destruction complex components CK1a (kin-19) (Peters et al 1999), GSK3b (gsk-3) (Schlesinger et al 1999), Axin ( pry-1 and axl-1) Korswagen et al 2002;Oosterveen et al 2007), and APC (apr-1) (Rocheleau et al 1997) are found in C. elegans. In the case of Axin and APC, the encoded proteins are fairly diverged from family members in other species, with several conserved domains absent.…”