13Maternal β-catenin activity is essential and critical for dorsal induction and its 14 dorsal activation has been thoroughly studied. However, how the maternal β-15 catenin activity is suppressed in the non-dorsal cells remains poorly understood. 16 Nanog is known to play a central role for maintenance of the pluripotency and 17 maternal-to-zygotic transition. Here we reveal a novel role of Nanog as a strong 18 repressor of maternal Wnt/β-catenin signaling to safeguard the embryo against 19 hyper-activation of maternal β-catenin activity and hyper-dorsalization. 20 Knockdown of nanog at different levels led to either posteriorization or 21 dorsalization, mimicking zygotic or maternal activation of Wnt/β-catenin 22 activities, and the maternal-zygotic mutant of nanog (MZnanog) showed strong 23 activation of maternal β-catenin and hyper-dorsalization. Although a 24 constitutive-activator-type Nanog (Vp16-Nanog, lacking the N-terminal) 25 perfectly rescued the defects of maternal to zygotic transition in MZnanog, it did 26 not rescue the phenotypes resulting from β-catenin activation. Mechanistically, 27 the N-terminal of Nanog directly interacts with TCF and interferes with the 28 binding of β-catenin to TCF, thereby attenuating the transcriptional activity of β-29 catenin. Therefore, our study establishes a novel role for Nanog in repressing 30 maternal β-catenin activity and demonstrates a transcriptional switch between 31 β-catenin/TCF and Nanog/TCF complexes, which safeguards the embryo from 32 global activation of maternal β-catenin activity. 34The Wnt/β-catenin signaling pathway, known as the canonical Wnt 35 signaling pathway, is highly conserved during evolution. It plays crucial roles in 36 organogenesis, stem cell renewal and differentiation, homeostasis, 37 reproduction, carcinogenesis and embryonic development [1-5]. Decades of 38 studies have shown that the central scheme of the Wnt/β-catenin pathway is to 39 stabilize the transcription coactivator β-catenin and protect it from 40 phosphorylation-dependent degradation [6, 7]. The Wnt/β-catenin pathway is 41 well-known for its ON/OFF regulation model. In the absence of Wnt ligand, 42 cytoplasmic β-catenin forms a complex with Axin, APC, GSK3 and CK1, and is 43 phosphorylated by CK1 and GSK3, recognized by the E3 ubiquitin ligase 44 subunit β-Trcp, and processed through ubiquitination and proteasomal 45 degradation. At this point, the nuclear TCFs is believe to physically interact with 46 the co-repressors (the Groucho/TLE family members) and act as transcriptional 47 repressors [8-11]. In the present of Wnt ligand, a receptor complex forms 48 between Frizzled and LRP5/6, and Frizzled is recruited by Dvl which leads to 49 LRP5/6 phosphorylation and Axin recruitment, which in turn disrupts Axin-50 mediated phosphorylation/degradation of β-catenin, allowing β-catenin to 51 accumulate in the nucleus where it serves as a coactivator for TCF to activate52Wnt-responsive genes [12, 13]. Thus, the activity of Wnt/β-catenin pathway is 53 con...

Maternal β-catenin activity is essential and critical for dorsal induction and its dorsal activation has been thoroughly studied. However, how the maternal β-catenin activity is suppressed in the nondorsal cells remains poorly understood. Nanog is known to play a central role for maintenance of the pluripotency and maternal-zygotic transition (MZT). Here, we reveal a novel role of Nanog as a strong repressor of maternal β-catenin signaling to safeguard the embryo against hyperactivation of maternal β-catenin activity and hyperdorsalization. In zebrafish, knockdown of nanog at different levels led to either posteriorization or dorsalization, mimicking zygotic or maternal activation of Wnt/β-catenin activities, and the maternal zygotic mutant of nanog (MZnanog) showed strong activation of maternal β-catenin activity and hyperdorsalization. Although a constitutive activator-type Nanog (Vp16-Nanog, lacking the N terminal) perfectly rescued the MZT defects of MZnanog, it did not rescue the phenotypes resulting from β-catenin signaling activation. Mechanistically, the N terminal of Nanog directly interacts with T-cell factor (TCF) and interferes with the binding of βcatenin to TCF, thereby attenuating the transcriptional activity of β-catenin. Therefore, our study establishes a novel role for Nanog in repressing maternal β-catenin activity and demonstrates a transcriptional switch between β-catenin/TCF and Nanog/TCF complexes, which safeguards the embryo from global activation of maternal β-catenin activity.