Regional Control Of Hairless Versus Hair-bearing Skin By Dkk2

In mammals most body areas are covered in hair or fur of multiple length,density and thickness,but some regions are completely hairless,like the palms of the hands,the soles of the feet and the ventral wrist in human and the plantar epidermis in mice.Canonical Wnt signaling pathway has been reported to be involved in the morphogenesis,regenerative growth of skin and hair follicles,Dickkopf(Dkk)plays a critical role as Wnt inhibitor.Dkk is a small gene family encodes secreted proteins that regulate Wnt/?-catenin signaling pathway.The hallmark function of Dkk1,Dkk2,Dkk4 is antagonizing canonical Wnt/?-catenin signaling pathway.There have been several studies showed the function and expression of Dkk in skin development,but the molecular mechanism by which Dkk2 and Wnt signaling pathway interact still remains unclear.In this study,we generated Dkk2-null mice to reveal its function in hair follicle formation and hair regeneration.After Dkk2 was knockout,we detected the hair growth cycle of mice by shaving the pelage?H&E staining and Ki67 staining and found that Dkk2-null mice were undergoing normal hair growth cycle,indicating knockout of Dkk2 had no effect to the hair growth cycle of the mice.In the meantime,however,we found that Dkk2-null mice established ectopic hair follicles in plantar.We collected mouse paws at different time points and checked grossly by serial section H&E staining?dermal papilla ALP staining and SEM and found that the ectopic hair follicles were initiated at E18.5 and could develop into normal hair structure and external hair shaft.These results indicated that knockout of Dkk2 caused ectopic hair formation in plantar.To look into the mechanism of the ectopic hair formation,we firstly checked the Dkk gene family expression in plantar by in situ hybridization and Real-time PCR.The results showed that Dkk2 was expressed in the dermis of plantar,no expression was detected in the heel?ventral and footpad;the expression of Dkk1?Dkk3?Dkk4 in plantar and back skin of Dkk2-null mice had no significant change compared with control mice,indicating that Dkk2 could inhibit hair follicle development during embryogenesis,Dkk gene family had no function redundency with each other.At last,we detected the expression of nuclear ?-catenin and LEF1 in plantar skin by immunofluorescence staining and found that both the expressions of these proteins were up-regulated in plantar skin,especially in the ectopic placodes,indicating the activation of canonical Wnt/?-catenin signaling pathway in this area;we then generated TCF/LEF1 GFP Dkk2-null mice and Axin2 Lac Z Dkk2-null mice and found that the expression patterns of GFP and X-gal were consistent with nuclear ?-catenin and LEF1,indicating that Dkk2 knockout could activate the Wnt/?-catenin signaling pathway in plantar and lead to ectopic hair formation.In sum,in this study,we report the phenotype of ectopic hair formation in Dkk2-null mice for the first time and confirmed that Dkk2 normally functioned as an inhibitor of Wnt/?-catenin signaling pathway in plantar to prevent hair follicle formation in this area.These results help to understand how hair follicles are established in different regions of the skin,and are likely to be applicable to understand the regulation of hair follicle formation in human.