Understanding the molecular and cellular mechanisms regulating the development, patterning and postnatal renewal of the skin and ectodermal appendage organs such as hair follicles, teeth, and taste papillae, and identifying stem and progenitor cell populations in these organs, is critical for developing new therapies to accelerate wound healing, treat hair loss diseases, repair or replace diseased teeth, and ameliorate taste dysfunction.

Research in the Millar lab focuses on cell-cell signaling and epigenetic mechanisms that underlie these processes. In published research, we identified Wnt/beta-catenin signaling as a key pathway required for initiating the formation of ectodermal appendages from multipotent cells in mammalian embryos, and in controlling development and patterning of haired versus hairy skin. By analyzing genetic mouse models and tissues from human patients carrying mutations in the WNT10A gene, we showed that Wnt signaling plays key roles in regulating the functions of a wide variety of adult epithelial stem cells, as well as controlling specialized differentiation programs in palmoplantar skin. We have also identified critical functions for epigenetic regulators including micro-RNAs and chromatin modifiers in skin development and regeneration.

Our current research projects include:

1: Investigating mechanisms that cause ectodermal dysplasia in patients with mutations in the WNT10A gene and testing potential therapeutic strategies.

We identified WNT10A as a key ligand controlling epithelial stem cell proliferation and region-specific differentiation, providing a molecular basis for understanding ectodermal dysplasia and regenerative defects in WNT10A mutant patients. We are further exploring the mechanisms underlying palmoplantar keratoderma, the most clinically relevant skin defect in WNT10A patients, and testing potential therapeutic approaches using pre-clinical models and patient-derived and gene-edited induced pluripotent stem cells (iPS).

2: Determining the mechanisms that underlie the formation and maintenance of hairy versus hairless skin and regulate hair patterning.

Different regions of mammalian skin vary in their functions, regenerative properties, and responses to injury and disease. Delineating the mechanisms that establish and maintain skin heterogeneity has potential to reveal improved therapeutic strategies. Positional information resides in the skin dermis, but the responsible molecular signals are poorly defined. We are using regional variation of hair follicle development and regeneration as a model system to address this question. We are particularly focusing on endogenous Wnt inhibitors as mediators of regional variation and hair patterning in the skin; our initial studies have identified the Dickkopf2 (DKK2) inhibitor as playing a critical role in permitting development of hairless plantar skin.

3: Defining the functions of histone deacetylase chromatin modifiers in skin development, stem cells and cancer.

Embryonic development and postnatal renewal of the epidermis and hair follicles require coordinated changes in gene expression that are often dysregulated in tumorigenesis. Histone deacetylases (HDACs) provide global control of gene expression programs by repressive modification of chromatin, and form several classes, of which Class 1 members display the highest histone deacetylase activity. Of these, HDACs 1 and 2 associate with Mi2b, MTA-2 and MDB3 in the NuRD complex and with Sin3, SAP18 and SAP30 in the Sin3 complex, while HDAC3 complexes with N-CoR or SMRT. HDAC inhibitors are promising therapeutic agents for multiple types of tumor. However, while the functions and targets of individual HDACs vary widely, most HDAC inhibitors broadly block Class 1 HDACs, and their precise mechanisms of action are poorly understood. We are using genetic, biochemical, and global genomic approaches to delineate and compare the functions of HDAC1, HDAC2 and HDAC3 in the development and regeneration of skin epithelia, to identify transcription factors that target individual HDACs to specific sets of promoters in skin epithelial cells, and to determine the consequences of deletion of Hdac1, 2 or 3 for the initiation and progression of skin tumors. Our published studies showed that HDAC1 and HDAC2 function redundantly to permit self-renewal and survival of embryonic epidermal stem cells. By contrast, we find that HDAC3 is necessary for orderly stepwise differentiation of the epidermis. Data from these experiments will be critical for evaluating the potential risks and benefits of targeting individual HDACs in the epidermis and for developing more specific and effective therapeutics.

4: Identifying pioneer transcription factors that control development and stem cell activity in skin and oral epithelia.

Regenerative processes in skin and oral epithelia require cross-talk of multiple cell signaling and epigenetic mechanisms; failure of this communication leads to a range of diseases from skin cancers to hair loss conditions. Delineating how these inputs are coordinated to impact gene expression at the level of chromatin has potential to identify novel therapies for skin dysfunction. Pioneer transcription factors initiate gene expression by binding nucleosome-enriched silent chromatin and recruiting chromatin modifiers to provide access for transcription machinery. Key regulatory genes in hair follicle stem cells are associated with “super enhancers” containing binding sites for multiple transcription factors, suggesting that transcription factors activated in response to diverse signals collaborate with each other and with chromatin modifiers to coordinate cell-type specific transcriptional output. We are using genetic mouse models to investigate the roles of several families of putative pioneer transcription factors in controlling skin and oral stem cell proliferation and differentiation.