Wnt Signaling Inhibitors Regulate the Transcriptional Response to Morphogenetic Shh-Gli Signaling in the Neural Tube

Abstract

Shh-Gli signaling controls cell fates in the developing ventral neural tube by regulating the patterned expression of transcription factors in neural progenitors. However, the molecular mechanisms that limit target gene responses to specific domains are unclear. Here, we show that Wnt pathway inhibitors regulate the threshold response of a ventral Shh target gene, Nkx2.2, to establish its restricted expression in the ventral spinal cord. Identification and characterization of an Nkx2.2 enhancer reveals that expression is directly regulated by positive Shh-Gli signaling and negative Tcf repressor activity. Our data indicate that the dorsal limit of Nkx2.2 is controlled by Tcf4-mediated transcriptional repression, and not by a direct requirement for high-level Shh-Gli signaling, arguing against a simple model based on differential Gli factor affinities in target genes. These results identify a transcriptional mechanism that integrates graded Shh and Wnt signaling to define progenitor gene expression domains and cell fates in the neural tube.

A direct fate exclusion mechanism by Sonic hedgehog-regulated transcriptional repressors

“…To examine the direct regulatory actions of the Shh-initiated transcriptional network (Lei et al, 2006;Oosterveen et al, 2012;Peterson et al, 2012), we performed ChIP-seq for Nkx2.2, Nkx6.1 and Olig2 on neural progenitors derived in vitro from mouse embryonic stem cells (mESCs); a model system that recapitulates in vivo patterning processes (Peterson et al, 2012;Wichterle et al, 2002) (supplementary material Table S1). The binding events were reproducibly detected in biological replicates (supplementary material Fig.…”

“…Detailed analyses showed that a number of known neural fate determinants as well as components of the Hedgehog pathway were co-targeted ( Fig. 1E,F (Lei et al, 2006;Oosterveen et al, 2012;Peterson et al, 2012;Wang et al, 2011), intersectional analysis showed a limited overlap with Gli1-and Gli3-bound regions overall ( Fig. 1G; supplementary material Fig.…”

“…Exceptions were putative cis-regulatory regions around ventral neural progenitor subtype specifiers, including Nkx2.2, Olig2 and Nkx6.1 (supplementary material Fig. S2) (Lei et al, 2006;Oosterveen et al, 2012;Peterson et al, 2012;Wang et al, 2011). At the target gene-level, Gli factors showed extensive overlap, particularly with genes targeted by all three repressors ( Fig.…”

“…Several lines of evidence, including the direct binding of Gli transcription factors to their cis-regulatory modules, have identified Nkx2.2, Nkx6.1 and Olig2 as direct transcriptional targets of the ventral neural patterning activity of Shh (Lei et al, 2006;Oosterveen et al, 2012;Peterson et al, 2012;Wang et al, 2011). Each of these factors has been shown to function as a transcriptional repressor in neural patterning: Olig2 is required for the specification of somatic motor neuron progenitors, Nkx2.2 for the specification of V3 interneuron progenitors, whereas Nkx6.1 expression overlaps V2 and V3 interneurons and somatic motor neuron progenitors and is essential for the normal specification of both populations (Briscoe et al, , 1999Lu et al, 2002;Mizuguchi et al, 2001;Muhr et al, 2001;Novitch et al, 2001;Sander et al, 2000;Vallstedt et al, 2001;Zhou and Anderson, 2002;Zhou et al, 2001) (Fig.…”

Wnt signaling interacts with Shh to regulate taste papilla development

“…Experiments in Drosophila have shown that Wnt and Shh interactions occur in the embryonic epidermis and in the larval leg imaginal disk (31,32). In the developing vertebrate spinal cord, interactions have been reported between Shh and Wnt signaling pathways in regulating patterning and cell specifications (32,33). In mammalian systems, Lef1 null mice had been observed to have reduced expression of Shh in the developing tooth germ (34), and ␤-catenin mutant mice showed reduced Shh expression in hair follicles (35).…”