The mammalian hairless (hr) gene plays a critical role in the maintenance of hair growth. Although the hr gene has been identified, the biochemical function of its encoded protein (Hr) has remained obscure. Here, we show that Hr functions as a transcriptional corepressor for thyroid hormone receptors (TRs). We find that two independent regions of Hr mediate TR binding and that interaction requires a cluster of hydrophobic residues similar to the binding motifs proposed for nuclear receptor corepressors (N-CoR and SMRT). Similarly, we show that Hr binds to the same region of TR as known corepressors. We show that Hr interacts with histone deacetylases (HDACs) and is localized to matrix-associated deacetylase (MAD) bodies, indicating that the mechanism of Hr-mediated repression is likely through associated HDAC activity. Thus, Hr is a component of the corepressor machinery, and despite its lack of sequence identity with previously described corepressors, its mode of action is remarkably conserved. On the basis of its thyroid hormone-inducible and tissue-and developmental-specific expression, Hr likely defines a new class of nuclear receptor corepressors that serve a more specialized role than ubiquitous corepressors. The discovery that Hr is a corepressor provides a molecular basis for specific hair loss syndromes in both humans and mice.

Although mutations in the mammalian hairless (Hr) gene result in congenital hair loss disorders in both mice and humans, the precise role of Hr in skin biology remains unknown. We have shown that the protein encoded by Hr (HR) functions as a nuclear receptor co-repressor. To address the role of HR in vivo, we generated a loss-of-function (Hr-/-) mouse model. The Hr-/- phenotype includes both hair loss and severe wrinkling of the skin. Wrinkling is correlated with increased cell proliferation in the epidermis and the presence of dermal cysts. In addition,a normally undifferentiated region, the infundibulum, is transformed into a morphologically distinct structure (utricle) that maintains epidermal function. Analysis of gene expression revealed upregulation of keratinocyte terminal differentiation markers and a novel caspase in Hr-/- skin, substantiating HR action as a co-repressor in vivo. Differences in gene expression occur prior to morphological changes in vivo, as well as in cultured keratinocytes, indicating that aberrant transcriptional regulation contributes to the Hr-/-phenotype. The properties of the cell types present in Hr-/- skin suggest that the normal balance of cell proliferation and differentiation is disrupted, supporting a model in which HR regulates the timing of epithelial cell differentiation in both the epidermis and hair follicle.

Thyroid hormone (TH) influences multiple aspects of neural development, presumably by controlling the transcriptional activity of TH receptors to modulate gene expression. The mammalian hairless (hr) gene is likely an important component of TH action as 1) hr expression is directly regulated by TH in brain, and 2) the protein encoded by hr (Hr) acts as a corepressor, facilitating transcriptional repression by unliganded TH receptors. Here we examine the properties of endogenous Hr in developing rat brain. Using coimmunoprecipitation, we show that Hr interacts with TH receptor and histone deacetylases (HDACs) in brain extracts. We find that inhibition of HDAC activity impairs Hr-mediated transcriptional repression, indicating that Hr-HDAC interaction is functionally significant. To identify potential sites of Hr action in developing brain, we assessed hr transcript and protein expression. We show that hr is broadly expressed in brain and overlaps with the expression of multiple HDACs in multiple regions including cortex, hippocampus, and cerebellum. Additionally, Hr expression is TH sensitive and developmentally regulated. The striking correlation of Hr expression with brain regions, cell types, and developmental stages influenced by TH, together with its function as a corepressor, suggests Hr is a key mediator of TH action in developing brain.