WNT-responsive SUMOylation of ZIC5 promotes murine neural crest cell development, having multiple effects on transcription.

Zinc finger of the cerebellum (Zic) proteins act as classic transcription factors to promote transcription of the Foxd3 gene during neural crest cell specification. Additionally, they can act as co-factors that bind proteins from the T-cell factor/lymphoid enhancing factor (TCF/LEF) family (TCFs) to repress WNT-ß-catenin-dependent transcription without contacting DNA. Here, we show that ZIC activity at the neural plate border is influenced by WNT-dependent SUMOylation. In the presence of high canonical WNT activity, a lysine residue within the highly conserved zinc finger N-terminally conserved (ZF-NC) domain of ZIC5 is SUMOylated, which reduces formation of the ZIC-TCF co-repressor complex and shifts the balance towards transcription factor function. The modification is crucial in vivo, as a ZIC5 SUMO-incompetent mouse strain exhibits neural crest specification defects. This work reveals the function of the ZF-NC domain within ZIC, provides in vivo validation of target protein SUMOylation and demonstrates that WNT-ß-catenin signalling directs transcription at non-TCF DNA-binding sites. Furthermore, it can explain how WNT signals convert a broad region of Zic ectodermal expression into a restricted region of neural crest cell specification.

SUMOylation Potentiates ZIC Protein Activity to Influence Murine Neural Crest Cell Specification.

The mechanisms of neural crest cell induction and specification are highly conserved among vertebrate model organisms, but how similar these mechanisms are in mammalian neural crest cell formation remains open to question. The zinc finger of the cerebellum 1 (ZIC1) transcription factor is considered a core component of the vertebrate gene regulatory network that specifies neural crest fate at the neural plate border. In mouse embryos, however, Zic1 mutation does not cause neural crest defects. Instead, we and others have shown that murine Zic2 and Zic5 mutate to give a neural crest phenotype. Here, we extend this knowledge by demonstrating that murine Zic3 is also required for, and co-operates with, Zic2 and Zic5 during mammalian neural crest specification. At the murine neural plate border (a region of high canonical WNT activity) ZIC2, ZIC3, and ZIC5 function as transcription factors to jointly activate the Foxd3 specifier gene. This function is promoted by SUMOylation of the ZIC proteins at a conserved lysine immediately N-terminal of the ZIC zinc finger domain. In contrast, in the lateral regions of the neurectoderm (a region of low canonical WNT activity) basal ZIC proteins act as co-repressors of WNT/TCF-mediated transcription. Our work provides a mechanism by which mammalian neural crest specification is restricted to the neural plate border. Furthermore, given that WNT signaling and SUMOylation are also features of non-mammalian neural crest specification, it suggests that mammalian neural crest induction shares broad conservation, but altered molecular detail, with chicken, zebrafish, and Xenopus neural crest induction.