Mesenchymal-epidermal interactions in hair follicle cycling and regeneration

Abstract

The hair follicle is an important skin appendage that arises during development from complex reciprocal signalling between the epidermis and dermis. The multiple epidermal layers of the hair follicle are known to be under the control of the mesenchymal component, the dermal papilla (DP), which has been described as the key regulator of hair follicle induction and maintenance. The hair follicle has elicited interest as a model system for the investigation of tissue and organ regeneration due to its ability to regenerate completely throughout the life of a mammal. This characteristic is attributed to the reservoir of stem cells within the hair follicle, which is modulated by the DP.Although it is known that the DP is central to regulation of the hair follicle, mediators of its signalling are incompletely understood. Potential mediators of DP activity continue to emerge, two of which include the SoxF transcription factor, Sox18, and the transcription factor, STAT5. Sox18 has been implicated in hair type specification by the DP; however, its expression and function remains unclear. STAT5 has been shown in microarray analyses of the DP to be specifically expressed and upregulated, while studies involving STAT5 genetic knockouts display a hair phenotype. More recently, STAT5 has been implicated in autoimmune conditions affecting the hair follicle such as alopecia areata.To investigate further the role of STAT5 in the DP, I have hypothesised that STAT5 plays a key role in the DP's hair follicle inductive properties and regulation of cycling.STAT5 activation in the DP was characterised through immunofluorescence staining of mouse back skin at various stages of hair follicle development and cycling. Back skin was obtained from Topflash transgenic mice, a canonical Wnt reporter transgenic mouse driving luciferase expression, using β-catenin activity to monitor hair cycling via live bioluminescence imaging. While embryonic skin did not display any activation of STAT5, during postnatal hair follicle development (P0-P19), the majority of phospho-STAT5 positive DP were present within telogen phase, to a lesser extent within catagen and not at all during anagen. Adult skin showed phospho-STAT5 activation beginning in late catagen and persisting through to early anagen.Characterisation of SOCS2 by immunofluorescence, a downstream STAT5 transcriptional target, showed expression corresponding to the chronology of phospho-STAT5 activity.Hair regeneration assays were performed with DP cells using SKP culture with STAT5 activity modified through adenovirus constructs. SKPs derived from C57BL/6 neonates and STAT5 lox/lox neonates were transduced with STAT5A/B constitutively-active and crerecombinase constructs, respectively. These transduced SKPs were then used in patch assay experiments and DP formation was quantified using a green fluorescent protein ii (GFP) reporter. SKPs transduced with constitutively-active STAT5 were significantly better at forming hair follicle DP than control SKPs transduced with GFP alone. SKPs with...