Regulation of LIM homeodomain 2 (Lhx2) in the distal limb bud during patterning and development

Abstract

During limb morphogenesis, fibroblast growth factors (Fgfs) secreted from the apical ectodermal ridge (AER) direct proximodistal outgrowth of the limb, while sonic hedgehog (Shh) released from the zone of polarizing activity modulate anteroposterior expansion of the limb. Fgfs and Shh maintain each other’s expression through an autoregulatory loop. The transcription factor LIM homeodomain 2 (Lhx2) is expressed in the distal sub‐AER mesoderm and is important for Shh expression. We have identified a chicken enhancer upstream of the LHX2 promoter that is active in the distal sub‐AER LHX2 expression domain that contains putative transcription factor binding sites for known downstream targets of FGF signaling that are critical for activity. Therefore, we hypothesized that LASARM1 interacts with the LHX2 promoter and that FGF works through LASARM1 to regulate LHX2 expression. To determine this, we performed chromosome conformation capture (3C) on cells isolated from distal chicken limb buds and compared them to non‐LHX2 expressing tail cells. Promoter and enhancer specific primers were used to isolate ligated promoter‐enhancer fragments by end‐point PCR. Amplified sequences were then sequence validated. A 3C ligation product library was used as a control. We also evaluated the role of FGF to regulate LASARM1 activity in subAER mesoderm by analyzing LASARM1 activity following inhibition of FGF signaling by an FGF‐receptor inhibitor. End‐point PCR revealed promoter‐enhancer ligated amplicons in distal limb bud cells, but not in tail cells. DNA sequencing of enhancer‐ligated fragments confirmed LHX2‐LASARM1 fusion fragments for distal limb bud cells. Interestingly, disruption of FGF signaling via a specific FGF receptor inhibitor did not alter LASARM1 activity, although LHX2 expression was markedly reduced. These findings suggest that LASARM1 interacts with the LHX2 promoter during limb patterning, but additional enhancers are required to mediate the FGF‐mediated up‐regulation of LHX2. Evaluation of additional LHX2 enhancers is underway. Investigations regarding the relationship between FGF and LHX2 will provide important insights into the mechanisms underlying LHX2 regulation and the FGF‐to‐SHH reciprocal loop.