Cdx1 refines positional identity of the vertebrate hindbrain by directly repressingMafbexpression

Abstract

SUMMARYAn interplay of transcription factors interprets signalling pathways to define anteroposterior positions along the vertebrate axis. In the hindbrain, these transcription factors prompt the position-appropriate appearance of seven to eight segmental structures, known as rhombomeres (r1-r8). The evolutionarily conserved Cdx caudal-type homeodomain transcription factors help specify the vertebrate trunk and tail but have not been shown to directly regulate hindbrain patterning genes. Mafb (Kreisler, Krml1, valentino), a basic domain leucine zipper transcription factor, is required for development of r5 and r6 and is the first gene to show restricted expression within these two segments. The homeodomain protein vHnf1 (Hnf1b) directly activates Mafb expression. vHnf1 and Mafb share an anterior expression limit at the r4/r5 boundary but vHnf1 expression extends beyond the posterior limit of Mafb and, therefore, cannot establish the posterior Mafb expression boundary. Upon identifying regulatory sequences responsible for posterior Mafb repression, we have used in situ hybridization, immunofluorescence and chromatin immunoprecipitation (ChIP) analyses to determine that Cdx1 directly inhibits early Mafb expression in the neural tube posterior of the r6/r7 boundary, which is the anteriormost boundary of Cdx1 expression in the hindbrain. Cdx1 dependent repression of Mafb is transient. After the 10-somite stage, another mechanism acts to restrict Mafb expression in its normal r5 and r6 domain, even in the absence of Cdx1. Our findings identify Mafb as one of the earliest direct targets of Cdx1 and show that Cdx1 plays a direct role in early hindbrain patterning. Thus, just as Cdx2 and Cdx4 govern the trunk-to-tail transition, Cdx1 may regulate the hindbrain-to-spinal cord transition.

Caudal-related Homeobox (Cdx) Protein-dependent Integration of Canonical Wnt Signaling on Paired-box 3 (Pax3) Neural Crest Enhancer

“…Therefore, the transient reduction in Pax3 mRNA levels observed in the dorsal NT of Cdx1 Ϫ/Ϫ embryos most likely reflects the fact that Cdx1 is expressed slightly earlier and more anteriorly than other Cdx members. In this regard, it is noteworthy that a similar transient effect has been recently reported for expression of the Cdx neural target Mafb in Cdx1-null embryos (62). Taken together, this suggests that all Cdx proteins might be involved in the control of Pax3 expression.…”

“…Until recently, Cdx proteins were mostly known for their evolutionarily conserved role in the control of neural AP patterning via Hox-dependent mechanisms (37,64,67,97). Cdx proteins are now also known to regulate neural AP patterning via Hox-independent mechanisms in different species (62,63). More recently, Cdx1-Cdx2 double knock-out mice were generated and revealed a novel redundant role for Cdx members in the control of NT closure (54).…”

“…As in the mesoderm and endoderm, Cdx proteins control neurectoderm AP patterning via Hox-dependent and -independent mechanisms (37,43,(62)(63)(64). Multiple gainor loss-of-function mouse models have also demonstrated that Cdx proteins are important for proper formation of the NT and NC-derived spinal ganglia, but whether they do so in a tissueautonomous or -non-autonomous (via the mesoderm) manner remains an open question (53,54,(65)(66)(67).…”

Retinoic acid regulates size, pattern and alignment of tissues at the head-trunk transition

“…The hindbrain and spinal cord are specified from a common progenitor tissue, the caudal neural plate (Schoenwolf, 1992;Muhr et al, 1997Muhr et al, , 1999Brown and Storey, 2000), through the binary activity of Cdx transcription factors (Skromne et al, 2007). Transcription of cdx genes in the caudal neural plate is initially broad, but is quickly turned off rostrally to allow hindbrain segmentation and patterning, while it is maintained in the 'on' state caudally to specify spinal cord (Skromne et al, 2007;Sturgeon et al, 2011). These events take place from mid-gastrulation to early segmentation (Muhr et al, 1997(Muhr et al, , 1999Woo and Fraser, 1998;Nordström et al, 2006;Skromne et al, 2007;Sturgeon et al, 2011) and are crucial for proper cell fate allocation.…”

“…Transcription of cdx genes in the caudal neural plate is initially broad, but is quickly turned off rostrally to allow hindbrain segmentation and patterning, while it is maintained in the 'on' state caudally to specify spinal cord (Skromne et al, 2007;Sturgeon et al, 2011). These events take place from mid-gastrulation to early segmentation (Muhr et al, 1997(Muhr et al, , 1999Woo and Fraser, 1998;Nordström et al, 2006;Skromne et al, 2007;Sturgeon et al, 2011) and are crucial for proper cell fate allocation. In mouse and zebrafish, changes in the expression domain of cdx changes the size of the hindbrain and spinal cord territories in a reciprocal manner; a rostral expansion of cdx transcriptional domain reduces the hindbrain and expands the spinal cord, whereas a caudal reduction in cdx transcriptional domain expands the hindbrain and reduces the spinal cord (Charite et al, 1998;Shimizu et al, 2006;Skromne et al, 2007;van Rooijen et al, 2012).…”