Context-specific function of the LIM homeobox 1 transcription factor in head formation of the mouse embryo

Abstract

Lhx1 encodes a LIM homeobox transcription factor that is expressed in the primitive streak, mesoderm and anterior mesendoderm of the mouse embryo. Using a conditional Lhx1 flox mutation and three different Cre deleters, we demonstrated that LHX1 is required in the anterior mesendoderm, but not in the mesoderm, for formation of the head. LHX1 enables the morphogenetic movement of cells that accompanies the formation of the anterior mesendoderm, in part through regulation of Pcdh7 expression. LHX1 also regulates, in the anterior mesendoderm, the transcription of genes encoding negative regulators of WNT signalling, such as Dkk1, Hesx1, Cer1 and Gsc. Embryos carrying mutations in Pcdh7, generated using CRISPRCas9 technology, and embryos without Lhx1 function specifically in the anterior mesendoderm displayed head defects that partially phenocopied the truncation defects of Lhx1-null mutants. Therefore, disruption of Lhx1-dependent movement of the anterior mesendoderm cells and failure to modulate WNT signalling both resulted in the truncation of head structures. Compound mutants of Lhx1, Dkk1 and Ctnnb1 show an enhanced head truncation phenotype, pointing to a functional link between LHX1 transcriptional activity and the regulation of WNT signalling. Collectively, these results provide comprehensive insight into the context-specific function of LHX1 in head formation: LHX1 enables the formation of the anterior mesendoderm that is instrumental for mediating the inductive interaction with the anterior neuroectoderm and LHX1 also regulates the expression of factors in the signalling cascade that modulate the level of WNT activity.

Lhx1 functions together with Otx2, Foxa2, and Ldb1 to govern anterior mesendoderm, node, and midline development

“…The present results confirm and extend earlier work suggesting that Lhx1 modulates the strength of Wnt signaling in the midline to ensure correct anterior neuroectoderm patterning (Fossat et al 2015). Our transcriptional profiling experiments demonstrated that expression of multiple Wnt signaling components depends on Lhx1 activity.…”

“…As for the Eomes + cell population (Costello et al 2011), Lhx1 + progenitors exclusively colonize the head and cardiac mesoderm and the entire gut endoderm as well as APS derivatives, including the AME, node, and notochord. Consistent with previous studies (Fossat et al 2015), we found here that conditional deletion from the epiblast does not perturb early mesoderm induction. However, high-resolution imaging revealed striking defects in anterior DE (ADE) emergence and dispersal of the VE population.…”

“…Foxa2 is critical for polarization of DE cells and epithelization of the midline structures (Burtscher and Lickert 2009). Conditional Lhx1 ablation selectively within the Foxa2 expression domain disrupts head development (Fossat et al 2015). Our ChIP-seq experiments identified Lhx1-binding sites present at key enhancer regions controlling expression of Otx2 and Foxa2.…”

“…Partial loss (∼70%) of Lhx1 from the epiblast causes abnormalities in anterior patterning associated with Wnt signaling defects (Tanaka et al 2010;Fossat et al 2015). To completely eliminate Lhx1 function in the epiblast, we exploited the Sox2Cre deleter strain (Hayashi et al 2002) together with a novel Lhx1 conditional allele generated using a EUCOMM (European Conditional Mouse Mutagenesis Program) resource targeting vector (Supplemental Fig.…”

“…Loss-of-function Lhx1 mutant embryos display a severe block to gastrulation, characterized by constriction of the VE at the extraembryonic ectoderm/epiblast boundary that acutely disrupts morphogenetic cell movements (Shawlot and Behringer 1995). Decreasing Lhx1 expression in the epiblast causes defective AME formation and consequently leads to anterior truncations (Shawlot et al 1999;Fossat et al 2015). …”