Genetic interplays between Msx2 and Foxn1 are required for Notch1 expression and hair shaft differentiation
Description
We use compound genetic mutants to analyze two transcription factors with demonstrated functions in hair shaft differentiation, Msx2 and Foxn1. First, we show that maintenance of Notch1 expression in the hair follicle matrix requires both factors, indicating a redundant activity of these proteins. In their absence, matrix cell proliferation is compromised; medulla and inner root sheath (IRS) differentiation is affected as evidenced by reduced K17 and Gata3 expression, respectively. We also identify a 4.7 kb of Notch1 promoter sequence which specifically drives EYFP expression in hair follicles and is regulated by Foxn1. Moreover, Foxn1 directly binds to this promoter as evidenced by ChIP assay. Whereas these aspects of the phenotype can be attributed to loss of Notch1, Msx2/Foxn1 also contribute independently of Notch1 to cortex and cuticle differentiation - several cortical and cuticle keratins are no longer expressed in the double mutant follicles. Despite the presence of a normal number of hair follicles and lineage-restricted stem cells, the impact of these defects is the complete loss of all visible external hairs. Since we find that BMP4 can regulate Foxn1 expression independent of Msx2, we suggest that these two genes function in parallel pathways downstream of BMP signaling and upstream of Notch1. Our results reveal a global requirement for Msx2 and Foxn1 in the hair matrix where they maintain Notch1 expression and thus indirectly control IRS and medulla differentiation, while directly supporting many aspects of cortical differentiation. Except for the genetic interplays between Msx2 and Foxn1 in the hair follicles, both Msx2 and Foxn1 are required for proper differentiation of the keratogenous zone, proliferation of the distal matrix and organization of the nail bed in the nail unit. Loss of both Msx2 and Foxn1 results in reduced hair specific keratin expression in the keratogenous zone, hyperproliferation of the distal matrix and stratification of the nail bed