Yuko Oda

Postnatal cell fate is postulated to be primarily determined by the local tissue microenvironment. Here, we find that Mediator 1 (Med1) dependent epigenetic mechanisms dictate tissue-specific lineage commitment and progression of dental epithelia. Deletion of Med1, a key component of the Mediator complex linking enhancer activities to gene transcription, provokes a tissue extrinsic lineage shift, causing hair generation in incisors. Med1 deficiency gives rise to unusual hair growth via primitive cellular aggregates. Mechanistically, we find that MED1 establishes super-enhancers that control enamel lineage transcription factors in dental stem cells and their progenies. However, Med1 deficiency reshapes the enhancer landscape and causes a switch from the dental transcriptional program towards hair and epidermis on incisors in vivo, and in dental epithelial stem cells in vitro. Med1 loss also provokes an increase in the number and size of enhancers. Interestingly, control dental epithelia already exhibit enhancers for hair and epidermal key transcription factors; these transform into super-enhancers upon Med1 loss suggesting that these epigenetic mechanisms cause the shift towards epidermal and hair lineages. Thus, we propose a role for Med1 in safeguarding lineage specific enhancers, highlight the central role of enhancer accessibility in lineage reprogramming and provide insights into ectodermal regeneration.

Postnatal cell fate has been postulated to be primarily determined by the local tissue microenvironment. Here, we found that Mediator 1 ( Med1 ) dependent epigenetic mechanisms dictate tissue-specific lineage commitment and progression of dental epithelia. Deletion of Med1 , a key component of the Mediator complex linking enhancer activities to gene transcription, provokes a tissue extrinsic lineage shift, causing hair generation in the dental environment. Med1 deficiency gives rise to unusual hair growth via primitive cellular aggregates on incisors. Mechanistically, we found that Med1 establishes super-enhancers that control enamel lineage transcription factors in dental stem cells and their progenies. However, Med1 deficiency reshapes the enhancer landscapes and causes a switch from the dental epithelial transcriptional program towards hair and epidermis on incisors in vivo , and in dental epithelial stem cells in vitro. Med1 loss also provokes an increase in the number and size of enhancers. Interestingly, control dental epithelia already exhibit enhancers for hair and epidermal key transcription factors; these expand in size and transform into active super-enhancers upon Med1 loss suggesting that these epigenetic mechanisms cause the transcriptomic and phenotypic shift towards epidermal and hair lineages. Thus, we propose a role for Med1 in safeguarding lineage specific enhancers, highlight the central role of enhancer accessibility and usage in lineage reprogramming and provide new insights into ectodermal regeneration.

Dental enamel is hardest tissue in the body and is produced by dental epithelial cells residing in the tooth. Their cell fates are tightly controlled by transcriptional programs that are facilitated by fate determining transcription factors and chromatin regulators. Understanding the transcriptional program controlling dental cell fate is critical for our efforts to build and repair teeth. In this review, we describe the current understanding of these regulators essential for regeneration of dental epithelial stem cells and progeny, which are identified through transgenic mouse models. We first describe the development and morphogenesis of mouse dental epithelium in which different subpopulations of epithelia such as ameloblasts contribute to enamel formation. Then, we describe the function of critical factors in stem cells or progeny to drive enamel lineages. We also show that gene mutations of these factors are associated with dental anomalies in craniofacial diseases in humans. We also describe the function of the master regulators to govern dental lineages, in which the genetic removal of each factor switches dental cell fate to that generating hair. The distinct and related mechanisms responsible for the lineage plasticity are discussed. This knowledge will lead us to develop a potential tool for bioengineering new teeth.

Epidermal lineages and injury induced regeneration are controlled by transcriptional programs coordinating cellular signaling and epigenetic regulators, but the mechanism remains unclear. Previous studies showed that conditional deletion of the transcriptional coactivator Mediator 1 (Med1) changes epidermal lineages and accelerates wound re-epithelialization. Here, we studied a molecular mechanism by which Med1 facilitates these processes, in particular, by focusing on TGFβ signaling through genome wide transcriptome analysis. The expression of the TGF ligands (Tgfβ1/β2) and their downstream target genes is decreased in both normal and wounded Med1 null skin. Med1 silencing in cultured keratinocytes likewise reduces the expression of the ligands (TGFβ1/β2) and diminishes activity of TGFβ signaling as shown by decreased p-Smad2/3. Silencing Med1 increases keratinocyte proliferation and migration in vitro. Epigenetic studies using chromatin immuno-precipitation and next generation DNA sequencing reveals that Med1 regulates transcription of TGFβ components by forming large clusters of enhancers called super-enhancers at the regulatory regions of the TGFβ ligand and SMAD3 genes. These results demonstrate that Med1 is required for the maintenance of the TGFβ signaling pathway. Finally, we show that pharmacological inhibition of TGFβ signaling enhances epidermal lineages and accelerates wound re-epithelialization in skin similar to that seen in the Med1 null mice, providing new insights into epidermal regeneration.

Epidermal stem cells (SCs) residing in the skin play an essential role for epidermal regeneration during cutaneous wound healing. Upon injury, distinct epidermal SCs residing in the interfollicular epidermis and/or hair follicles are activated to proliferate. Subsequently, SCs and progeny migrate, differentiate and restore the epidermis. We review a role of the vitamin D signaling through its receptor of vitamin D receptor (Vdr) in these processes.Vdrconditional knockout (cKO) mouse skin experiences a delay in wound re-epithelialization under low dietary calcium conditions, stimulating our efforts to examine a cooperative role ofVdrwith calcium signaling through the calcium sensing receptor in the epidermis. We review the role of vitamin D and calcium signaling in different processes essential for injury induced epidermal regeneration during cutaneous wound repair. First, we discuss their roles in self-renewal of epidermal SCs through beta-catenin signaling. Then, we describe epidermal remodeling, in which SCs and progeny migrate and differentiate to restore the epidermis, events controlled by the E-cadherin mediated adherens junction signaling. Finally, we discuss the potential mechanisms for vitamin D and calcium signaling to regulate injury induced epidermal regeneration mutually and interdependently.

When the skin is injured, keratinocytes proliferate, migrate, and differentiate to regenerate the epidermis. We recently showed that ablation of the vitamin D receptor (Vdr) in keratinocytes delays wound re-epithelialization in mice also fed a low-calcium diet, implicating a cooperative role of Vdr and calcium signaling in this process. In this study, we examined the role of vitamin D and calcium signaling in wound healing by deleting their receptors, Vdr and the calcium-sensing receptor (Casr). Gene expression profiling of neonatal epidermis lacking both Vdr and Casr [Vdr and Casr double knockout (DKO)] specifically in keratinocytes revealed that DKO affects a number of pathways relevant to wound healing, including Vdr, β-catenin, and adherens junction (AJ) signaling. In adult skin, DKO caused a significant delay in wound closure and re-epithelialization, whereas myofibroblast numbers and matrix deposition were unaffected. The injury-induced proliferation of epidermal keratinocytes was blunted in both epidermis and hair follicles, and expression of β-catenin target genes was reduced in the DKO. Expression of E-cadherin and desmoglein 1 was reduced in the shortened leading edges of the epithelial tongues re-epithelializing the wounds, consistent with the decreased migration rate of DKO keratinocytes in vitro. These results demonstrate that Vdr and Casr are required for β-catenin–regulated cell proliferation and AJ formation essential for re-epithelialization after wounding. We conclude that vitamin D and calcium signaling in keratinocytes are required for a normal regenerative response of the skin to wounding.