Binary cell fate specification duringC. elegansembryogenesis driven by reiterated reciprocal asymmetry of TCF POP-1 and its coactivatorβ-catenin SYS-1

Abstract

C. elegans embryos exhibit an invariant lineage comprised primarily of a stepwise binary diversification of anterior-posterior (A-P) blastomere identities. This binary cell fate specification requires input from both the Wnt and MAP kinase signaling pathways. The nuclear level of the TCF protein POP-1 is lowered in all posterior cells. We show here that the ␤-catenin SYS-1 also exhibits reiterated asymmetry throughout multiple A-P divisions and that this asymmetry is reciprocal to that of POP-1. Furthermore, we show that SYS-1 functions as a coactivator for POP-1, and that the SYS-1-to-POP-1 ratio appears critical for both the anterior and posterior cell fates. A high ratio drives posterior cell fates, whereas a low ratio drives anterior cell fates. We show that the SYS-1 and POP-1 asymmetries are regulated independently, each by a subset of genes in the Wnt/MAP kinase pathways. We propose that two genetic pathways, one increasing SYS-1 and the other decreasing POP-1 levels, robustly elevate the SYS-1-to-POP-1 ratio in the posterior cell, thereby driving A-P differential cell fates.KEY WORDS: C. elegans, TCF/POP-1, ␤-catenin/SYS-1, Cell fate specification Development 134, 2685Development 134, -2695Development 134, (2007 POP-1 (Kidd et al., 2005). Recently, SYS-1 has been implicated in endoderm precursor specification (Phillips et al., 2007). Whereas animals homozygous for a reduction-of-function mutation are sterile, sys-1(RNAi) resulted in a very low penetrance gutless phenotype.We show here that SYS-1 is a limiting coactivator for POP-1 in the activation of Wnt/MAPK-responsive genes in the E blastomere. SYS-1 exhibits a reiterated asymmetry that is reciprocal to the reiterated asymmetry of nuclear POP-1 through all A-P divisions examined. We show that the SYS-1-to-POP-1 ratio appears critical for both anterior and posterior cell fates at multiple divisions: a high ratio drives the posterior cell fate, whereas a low ratio drives the anterior cell fate. SYS-1 and POP-1 levels are regulated in opposite directions by two pathways known to regulate endoderm specification: SYS-1 levels are increased primarily by the MOM-2/MOM-5/APR-1 pathway, whereas nuclear POP-1 levels are decreased primarily by the MOM-4/LIT-1/WRM-1 pathway. Together, these two pathways efficiently increase the SYS-1-to-POP-1 ratio in the posterior cell, promoting asymmetric cell fates. MATERIALS AND METHODS StrainsN2 was used as the wild-type strain. Genetic markers:LGI, pop-1(zu189), dpy-5(e61), mom-5(or57), mom-4(ne19), sys-1(q544), fog-3(q520), teIs3 (P med-1 gfp::pop-1), hT1(I:V), szT1(I:X);LGII, rol-1(e91), mom-3(or78), mnC1; LGIII, unc-119(ed3), lit-1(t1512), lit-1(t1534), unc32(e189);LGIV, teIs46(P end-1 gfp::H2B); LGV, mom-2(or42), DnT1(IV;V), teIs18 (P sdz-23 gfp::H2B); LGX,. TX796 [teEx321(P med-1 gfp::sys-1)]. gfp::H2B)(V)]. TX691 [teIs46(P end-1 gfp::H2B)(IV)] (Shetty et al., 2005). TX932 [sys-1(q544)/fog-3(q470)(I); teIs3(P med-1 gfp::pop-1)(V)] (Miskowski et al., 2001). TX964 [teIs98(P pie-1 gfp::sys-1)]. JM139 [P pho-1 ...

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

“…4B, right) (Calvo et al 2001;Zhao et al 2002;Maduro et al 2005;Shetty et al 2005;Arata et al 2006;Lam et al 2006;Huang et al 2007;Owraghi et al 2010). Molecular genetic manipulations that artificially alter the ratio of TCF to b-catenin between daughter cells lead to changes in gene expression or cell fate that are consistent with this model of Wnt pathway activation, switching the transcriptional behavior of TCF in the nucleus (Kidd et al 2005;Lam et al 2006;Huang et al 2007;Bertrand and Hobert 2009).…”

“…Surprisingly, only one of the C. elegans Wnts, MOM-2, appears to play a major role during embryogenesis: 70% of mom-2 zygotic mutant animals die during embryogenesis, whereas ,5% of lin-44; cwn-1; cwn-2 or lin-44; cwn-1; egl-20 mutants display embryonic lethality (Thorpe et al 1997;Gleason et al 2006). This result is surprising given the apparent reiterated use of Wnt signaling in a large number of asymmetric cell divisions during embryonic development (Park and Priess 2003;Huang et al 2007). However, both cwn-1 and cwn-2 mutations enhance the mom-2 lethal phenotype, and the cwn-1; cwn-2; mom-2 triple mutant shows 100% lethality, indicating that there is some functional redundancy of Wnts in embryonic development (Gleason et al 2006).…”

“…The subsequent phosphorylation of POP-1 marks it for nuclear export by PAR-5/14-3-3 and CRM-1/ Exportin (Rocheleau et al 1997;Meneghini et al 1999;Shin et al 1999;Lo et al 2004), leading to a lowering of POP-1/TCF levels in the nucleus of the signaled cell (Lin et al 1998;Meneghini et al 1999;Rocheleau et al 1999;Herman 2001;Maduro et al 2002;Park and Priess 2003;Siegfried et al 2004;Huang et al 2007;Phillips et al 2007). Although the mechanism is unclear, in this daughter cell there is also a Wnt-induced increase in the levels of the b-catenin SYS-1 in the nucleus (Huang et al 2007;Phillips et al 2007). The end result of these Wnt-induced events is an asymmetric distribution of TCF and b-catenin between sister cell nuclei (Fig.…”

“…Forward and reverse genetic screens identified components of Wnt, MAPK, and Src signaling pathways as being required for endoderm formation (Lin et al 1995;Kaletta et al 1997;Rocheleau et al 1997Rocheleau et al , 1999Meneghini et al 1999;Peters et al 1999;Schlesinger et al 1999;Shin et al 1999;Bei et al 2002;Thorpe and Moon 2004;Walston et al 2004;Huang et al 2007;Phillips et al 2007). Wnt pathway components functioning in endoderm induction include MOM-1/Porcupine, MOM-2/Wnt, MIG-14/ Wntless (a.k.a.…”

Wnt signaling through T-cell factor phosphorylation

“…This POP-1 asymmetry requires LIT-1, a protein kinase that regulates asymmetric cell divisions [50] and promotes the nuclear export of POP-1 [49,51,52]. Paradoxically, the small amount of POP-1 that remains in the E-cell nucleus is required, together with SYS-1, a distant member of the β-catenin family [53,54], for Wnt-dependent activation of endoderm-specific end-1 and end-3 target genes [44,45,47,[55][56][57]. POP-1 also functions as a transcriptional activator in T neuroblasts and somatic gonadal precursors, in which POP-1 and SYS-1 directly activate the ceh-22/tinman gene [47,53,54,58].…”