(A, B) X-gal stained E13.5 and E14.5 embryos of the following genotypes: control TOPGAL, TOPGAL;ΔN, and TOPGAL;dl/dl (A); and control cond-lacZ, cond-lacZ; ΔN, cond-lacZ;dl/dl and cond-lacZ;Ta/Ta (B). Arrows indicate hair placodes displaying Wnt reporter gene expression. Arrows in insets indicate hair placodes, eyelids and vibrissae. (C) X-gal stained E13.5 and E14.5 βcatlacZ, βcatlacZ;dl/dl and βcatlacZ;Ta/Ta embryos. (D) In situ hybridization for β-catenin mRNA using sagittal skin sections of control, dl/dl and ΔN embryos at the time points indicated. Arrows indicate developing placodes. (E) Dkk4 is an NF-κB target gene. Upper panel: The upstream promoter of Dkk4 contains a consensus NF-κB DNA binding site, located at -2303 - -2312 in mouse Dkk4 (vertical red line). Green and orange arrows indicate the positions of ChIP primers in the mouse Dkk4 promoter. Lower panel: ChIP using wild type E14.5 epidermal extracts, anti-p65 antibody (α-p65 ab) or IgG control, and Dkk4 or control Gapdh primers.

(A) TOPGAL and cond-lacZ Wnt reporter expression analyzed by X-Gal staining in control, ΔN, and dl/dl backgrounds at E15.5. (B) Whole mount X-gal staining of E15.5 β-catenin^+/lacZ (βcatlacZ), β-catenin^+/lacZ;dl/dl (βcatlacZ;dl/dl), and β-catenin^+/lacZ;Ta/Ta (βcatlacZ;Ta/Ta) embryos as indicated. Patches of β-catenin expression appear fused and form strings in downless and tabby mutant embryos (red arrows), as also observed in embryos of these genotypes at E13.5 and E14.5 (Figure 2C). (C) Whole mount in situ hybridization of control (c.) and ΔN embryos at the time points indicated using Wnt10b probe. Arrowheads indicate focal expression. (D) In situ hybridization for Wnt10b using sagittal sections of control and dl/dl skin at E14.0 and E15.5. (E) Whole mount in situ hybridization of E13.5 (upper panels) and E14.5 (lower panels) Ta/Ta skin explants using Wnt10b probe. Explants were treated for 24 hours with recombinant Fc-Eda-A1 (+ A1), Fc-Eda A2 (+A2), TNFα (+ TNFα), or were untreated (nt). (F) Upper panel: Conserved NF-κB binding sites in the human and murine Wnt10b promoters at the positions indicated. A verified NF-κB DNA binding site in the IκBα promoter is listed for comparison. Lower panels: ChIP using E14.5 dorsal skin extracts, primers that amplify a region encompassing the two proximal NF-κB consensus sequences in murine Wnt10b, and anti-p65 or IgG control antibodies. Primers amplifying a Gapdh promoter fragment were used as a negative control.

(A) Technovit sections of X-Gal stained cond-lacZ and TOPGAL Wnt reporter embryos at E12.5 – E15.5. Arrowheads indicate X-Gal staining. Dashed lines indicate the dermal-epidermal boundary. Numbers specify developmental stages of placodes. (B) Immunofluorescence detection of β-catenin (green). α-KRT14 antibody (red) was used to mark the epidermis. Yellow arrowheads indicate nuclear β-catenin staining in the dermis. A yellow bracket indicates localized dermal nuclear β-catenin at E13.5. Dashed lines indicate the dermal-epidermal boundary. (C) Technovit sections of E14.5 X-Gal stained cond-lacZ and cond-lacZ;KRT14-βcat−/− embryos. Arrowheads indicate X-Gal staining. Dashed lines indicate the dermal-epidermal boundary. (D) Whole mount X-Gal staining of Wnt reporter (TOPGAL, BAT-gal, cond-lacZ) and NF-κB reporter (κ-Gal) embryos at the time points indicated. Red arrows in insets indicate hair placodes and vibrissae, blood vessels, eyelids, and developing sweat glands in footpads.

(A, B) Whole mount in situ hybridization of E14.5 littermate control and KRT5-rtTA;tetO-Dkk1 embryos doxycycline treated from E0.5, using Edar and Wnt10b (A) and Eda (B) probes. (C) Whole mount X-gal staining of E14.5 and E16.5 κ-Gal control and KRT5-rtTA;tetO-Dkk1; κ-Gal embryos doxycycline treated from E0.5. Red arrows indicate primary hair placodes; yellow arrow indicates a secondary hair placode. (D, E) E14.5 KRT14-Cre;Ctnnb1^fl/fl (KRT14-βcat−/−) and control littermate embryos hybridized with the probes indicated. (F) Whole mount X-gal stained E14.5 KRT14-Cre;Ctnnb1^fl/fl; κGal (KRT14-βcat−/−; κ-Gal) and control littermate κ-Gal NF-κB reporter embryos. Red arrowheads indicate hair placodes.

(A–C) Epidermal Eda and Edar mRNA expression assayed by real time RT-PCR in E13.5–E15.5 littermate controls and embryos with forced epidermal Dkk1 expression (KRT5-rtTA;tetO-Dkk1 (A)), epidermal β-catenin deficiency (KRT14-βcat−/−, (B)), or carrying an epidermal β-catenin gain of function mutation (KRT14-Cre; β-cat^(ex3)fl, (C)). Results are mean ± SEM. Two or three embryos were analyzed for each genotype. (D) Edar mRNA expression assayed by real time RT-PCR in epidermis from doxycycline treated control, KRT14-Eda-A1 (Eda), KRT5-rtTA;tetO-Dkk1 (Dkk1) and KRT5-rtTA;tetO-Dkk1;KRT14-Eda-A1 (Dkk1;Eda) embryos at E14.5 and E15.5. Two embryos were analyzed for each genotype. (E) Upper panel: two conserved LEF/TCF binding sites in murine and human Edar promoters. Middle panel: vertical red bar indicates the position of the proximal LEF/TCF site in murine Edar; arrows indicate the locations of primers. Lower panels:- left: ChIP using wild-type E14.5 dorsal epidermal extracts, primers for Edar promoter, and anti-β-catenin antibody (α β-catenin) or IgG control; right: ChIP using dorsal epidermal extracts from doxycycline treated E14.5 control or KRT5-rtTA;tetO-Dkk1 (DKK1) embryos, anti-β-catenin antibody, and primers for Edar promoter (upper right) or Gapdh negative control (lower right).

(A) Hematoxylin/eosin (H&E) stained sections of skin from doxycycline treated KRT5-rtTA;tetO-Dkk1, KRT5-rtTA;tetO-Dkk1;KRT14-Eda-A1 and control littermate embryos. Yellow arrows indicate developing hair follicles in controls. (B, C) In situ hybridization of doxycycline treated E14.5 KRT5-rtTA;tetO-Dkk1;KRT14-Eda-A1 and control embryos for Edar (B) and Wnt10b (C). (D) H&E stained sections of KRT14-Cre;Ctnnb1^fl/fl;KRT14-Eda-A1 (KRT14-βcat−/−;KRT14-Eda-A1) and control littermate skin at E15.5 and E19.5. (E) In situ hybridization of E14.5 KRT14-βcat−/−;KRT14-Eda-A1 and control embryos for Wnt10b. (F) H&E stained E14.5 and P0 skin sections of the indicated genotypes. (G) Whole mount in situ hybridization of doxycycline treated E14.5 KRT14-LMP-Edar, KRT5-rtTA;tetO-Dkk1 and KRT5-rtTA;tetO-Dkk1;KRT14-LMP-Edar and control embryos, using Shh probe. (H) Treatment of skin explants from doxycycline treated E13.5 and E16.5 littermate control and KRT5-rtTA;tetO-Dkk1 embryos with or without Noggin for 24 hrs. Hair follicle induction was monitored by in situ hybridization for Shh (E13.5) or staining with alkaline phosphatase (AP) to reveal dermal condensates (E16.5).

Wnt activity lies both upstream and downstream of Edar signaling. At early stages of hair follicle development an irregular pre-pattern of Wnt activity is established in the epithelium and is required for patterned inhibition of Wnt signaling in interplacodal regions, possibly via the actions of secreted Wnt-induced inhibitors such as DKK4. Reinforcement of signaling in pre-placodes involves elevation of β-catenin and Wnt10b transcription. Expression of Eda and Edar requires Wnt signaling. Maintenance of Wnt signaling and elevated Wnt10a, Wnt10b, and Dkk4 expression at later stages requires Eda-A1/Edar/NF-κB activity. HF, hair follicle.