Hairless and Wnt signaling: allies in epithelial stem cell differentiation.

Nuclear receptors and Wnt signaling are both important regulators of developmental and physiological processes. Recent work linking these pathways in epithelial stem cell differentiation has come from studies analyzing the in vivo function of the nuclear receptor corepressor, Hairless (HR). The HR protein has long been suspected to regulate a stem cell-mediated process, hair cycling, as mutations in the Hr gene cause hair loss in both mice and men. The discovery that the HR protein is a nuclear receptor corepressor indicated that HR function in hair cycling is by regulating gene expression. A recent study revealed that HR represses expression of Wise, an inhibitor of Wnt signaling, leading to a model in which HR controls the timing of Wnt signaling required for hair cycling. Here we review these data, and provide new data showing that HR corepressor activity is essential for its in vivo function, and identify an additional putative Wnt inhibitor regulated by HR. This work complements previous studies demonstrating the role of Wnt signaling in epithelial stem cell differentiation.