Ectodysplasin regulates the lymphotoxin-β pathway for hair differentiation

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Abstract
Mutations in theEDAgene cause anhidrotic/hypohidrotic ectodermal dysplasia, a disorder characterized by defective formation of hair, sweat glands, and teeth in humans and in a mouse model, “Tabby” (Ta). The gene encodes ectodysplasin, a TNF ligand family member that activates the NF-κB-signaling pathway, but downstream targets and the mechanism of skin appendage formation have been only partially analyzed. Comparative transcription profiling of embryonic skin during hair follicle development in WT and Ta mice identified critical anhidrotic/hypohidrotic ectodermal dysplasia (EDA) effectors in four pathways, three already implicated in follicle formation. They includedShhand its effectors, as well as antagonists for the Wnt (Dkk4) and BMP (Sostdc1) pathways. The fourth pathway was unexpected, a variant NF-κB-signaling cascade based on lymphotoxin-β (LTβ)/RelB. Previously known to participate only in lymphoid organogenesis, LTβ was enriched in developing hair follicles of WT but not in Ta mice. Furthermore, in mice lacking LTβ, all three types of mouse hair were still formed, but all were structurally abnormal. Guard hairs became wavy and irregular, zigzag/auchen hairs lost their kinks, and in a phenocopy of features of Ta animals, the awl hairs doubled in number and were characteristically distorted and pinched. LTβ-null mice that received WT bone marrow transplants maintained mutant hair phenotypes, consistent with autonomous LTβ action in skin independent of its expression in lymphoid cells. Thus, as an EDA target, LTβ regulates the form of hair in developing hair follicles; and when EDA is defective, failure of LTβ activation can account for part of the Ta phenotype.