Krox20, a novel candidate for the regulatory hierarchy that controls hair shaft bending.

Murine hair follicle morphogenesis gives rise to four distinct pelage follicle types that produce hair shafts differing in length, the number of medulla columns, and the presence and number of bends. Recently, Igfbp5 was identified as the first molecular marker that distinguishes among different hair follicle types and shown to mark zigzag hairs. Further, it was demonstrated that Igfbp5 expression is modulated by FGF signalling. Here, we identify Krox20 as a molecular marker whose expression in the proximal follicle appears to be restricted to zigzag hair follicles. Gene transcription occurs in precursors and early differentiating cells of the medulla. Spatial and temporal expression of Krox20 and Igfbp5 seem to be tightly co-regulated in wildtype follicles. This correlation also holds in transgenic mice in which Igfbp5 expression is disturbed. Inspection of the Igfbp5 promoter reveals several putative binding sites for KROX20. In transfection studies, KROX20 strongly stimulates transcription from a 1.5kb Igfbp5 promoter fragment which is significantly reduced by site-directed mutagenesis of putative KROX20 binding sites, indicating a potential role of KROX20 in activating Igfbp5 expression. Our data suggest Krox20 as a nodal point of FGF and IGF signalling pathways controlling Igfbp5 expression which is associated with hair shaft differentiation and may generate the periodicity of the zigzag hair.