Faculty Bibliography

Melanocyte stem cells (McSCs) reside in the hair follicle bulge/ secondary hair germ niche where they are essential for hair pigmentation and have the potential to also regulate epidermal pigmentation. A better understanding of the molecular mechanisms that govern these stem cells holds broad implications in pigmentation disorders including hair graying, vitiligo and melanoma. We show that Wnt signaling is temporarily activated in McSCs at the onset of hair follicle regeneration and is necessary for their differentiation. Nonetheless, lineage tracing of Wnt-active differentiated McSCs demonstrate that McSCs can revert back to undifferentiated McSCs following withdrawal of Wnt signal activation. This suggests that McSC differentiation driven by Wnt signaling can be reversible, and temporal Wnt activation in McSCs does not deprive their self-renewing capacity. This ability of McSCs to oscillate between the differentiated and undifferentiated/stem cell states is not prevented when they differentiate into mature melanocytes after UVB irradiation. In aged mice, this process is compromised due to the failure to cease Wnt signaling, leading to ectopic McSC differentiation and a failure to return to their undifferentiated state, ultimately resulting in their loss. Upon induction of melanoma forming mutations, McSCs exhibit the potential to form melanoma. The tumorigenic potential of McSCs is regulated by Wnt signaling. Our results show the critical function of Wnt signaling in governing behavior of McSCs in adult skin during normal tissue homeostasis and melanoma

Adenosine A2a receptor (A2aR) stimulation promotes the synthesis of collagens I and III, and we have recently demonstrated that there is crosstalk between the A2aR and WNT/beta-catenin signaling pathway. In in vitro studies, A2aR signaling for collagen III expression was mediated by WNT/beta-catenin signaling in human dermal fibroblasts; we further verified whether the crosstalk between A2aR and Wnt/beta-catenin signaling was involved in diffuse dermal fibrosis in vivo. Wnt-signaling reporter mice (Tcf/Lef:H2B-GFP) were challenged with bleomycin and treated with the selective A2aR antagonist istradefylline (KW6002) or vehicle. Dermal fibrosis was quantitated and nuclear translocation of beta-catenin in fibroblasts was assessed by double-staining for Green fluorescent protein or dephosphorylated beta-catenin or beta-catenin phosphorylated at Ser552, and vimentin. KW6002 significantly reduced skin thickness, skinfold thickness, breaking tension, dermal hydroxyproline content, myofibroblast accumulation, and collagen alignment in bleomycin-induced dermal fibrosis. Also, there was increased expression of Tcf/Lef:H2B-GFP reporter in bleomycin-induced dermal fibrosis, an effect that was diminished by treatment with KW6002. Moreover, KW6002 significantly inhibited nuclear translocation of Tcf/Lef:H2B-GFP reporter, as well as dephosphorylated beta-catenin and beta-catenin phosphorylated at Ser552. Our work supports the hypothesis that pharmacologic blockade of A2aR inhibits the WNT/beta-catenin signaling pathway, contributing to its capacity to inhibit bleomycin-induced dermal fibrosis.

Although regeneration via the reprogramming of one cell lineage to another occurs in fish and amphibians, it is not observed in mammals. We discovered in mouse that during wound healing adipocytes regenerate from myofibroblasts, a cell type thought to be differentiated and non-adipogenic. Myofibroblast reprogramming required neogenic hair follicles, which triggered BMP signaling and then activation of adipocyte transcription factors expressed during development. Overexpression of the BMP antagonist, noggin, in hair follicles or deletion of the BMP receptor in myofibroblasts prevented adipocyte formation. Adipocytes formed from human keloid fibroblasts when treated with either BMP or when placed with human hair follicles in vitro. Thus, we identify the myofibroblast as a plastic cell type that may be manipulated to treat scars in humans.

Melanocyte stem cells (McSCs) reside in the hair follicle bulge/ secondary hair germ niche where they are essential for hair pigmentation and have the potential to also regulate epidermal pigmentation. A better understanding of the molecular mechanisms that govern these stem cells holds broad implications in pigmentation disorders including hair graying, vitiligo and melanoma. We show that Wnt signaling is temporarily activated in McSCs at the onset of hair follicle regeneration and is necessary and sufficient for their differentiation. Nonetheless, lineage tracing of Wnt-active differentiated McSCs demonstrates that McSCs can revert back to undifferentiated McSCs following withdrawal of Wnt signal activation. This suggests that McSC differentiation driven by Wnt signaling can be reversible, and temporal Wnt activation in McSCs does not deprive their self-renewing capacity. This ability of McSCs to oscillate between the differentiated and undifferentiated/stem cell states is not prevented when they differentiate into mature melanocytes after UVB irradiation. In aged mice, this process is compromised due to the failure to cease Wnt signaling, leading to ectopic McSC differentiation and a failure to return to their undifferentiated state, ultimately resulting in their loss. Our results show the critical regulation of Wnt signaling in governing maintenance as well as differentiation of McSCs in adult skin

Clinically, many nail disorders accompany bone deformities, but whether the two defects are causally related is under debate. To investigate the potential interactions between the two tissue types, we analyzed epithelial-specific beta-catenin-deficient mice, in which nail differentiation is abrogated. These mice showed regression of not only the nail plate but also of the underlying digit bone. Characterization of these bone defects revealed active bone resorption, which is suppressed by Wnt activation in osteoblast and osteoclast precursors. Furthermore, we found that Wntless expression, essential for Wnt ligand secretion, was lacking in the beta-catenin-deficient nail epithelium and that genetic deletion of Wntless (Wls) in the nail epithelium led to the lack of Wnt activation in osteoblast and osteoclast precursors and subsequently led to defective regression of the underlying digit bone. Together, these data show that epithelial Wnt ligands can ultimately regulate Wnt signaling in osteoblast and osteoclast precursors, known to regulate bone homeostasis. These results reveal a critical role for the nail epithelium on the digit bone during homeostatic regeneration and show that Wnt/beta-catenin signaling is critical for this interaction.

Delineating the crosstalk between distinct signaling pathways is key to understanding the diverse and dynamic responses of adult stem cells during tissue regeneration. Here, we demonstrate that the Edn/EdnrB signaling pathway can interact with other signaling pathways to elicit distinct stem cell functions during tissue regeneration. EdnrB signaling promotes proliferation and differentiation of melanocyte stem cells (McSCs), dramatically enhancing the regeneration of hair and epidermal melanocytes. This effect is dependent upon active Wnt signaling that is initiated by Wnt ligand secretion from the hair follicle epithelial niche. Further, this Wnt-dependent EdnrB signaling can rescue the defects in melanocyte regeneration caused by Mc1R loss. This suggests that targeting Edn/EdnrB signaling in McSCs can be a therapeutic approach to promote photoprotective-melanocyte regeneration, which may be useful for those with increased risk of skin cancers due to Mc1R variants.

Hair thinning and loss are prominent aging phenotypes but have an unknown mechanism. We show that hair follicle stem cell (HFSC) aging causes the stepwise miniaturization of hair follicles and eventual hair loss in wild-type mice and in humans. In vivo fate analysis of HFSCs revealed that the DNA damage response in HFSCs causes proteolysis of type XVII collagen (COL17A1/BP180), a critical molecule for HFSC maintenance, to trigger HFSC aging, characterized by the loss of stemness signatures and by epidermal commitment. Aged HFSCs are cyclically eliminated from the skin through terminal epidermal differentiation, thereby causing hair follicle miniaturization. The aging process can be recapitulated by Col17a1 deficiency and prevented by the forced maintenance of COL17A1 in HFSCs, demonstrating that COL17A1 in HFSCs orchestrates the stem cell-centric aging program of the epithelial mini-organ.

The skin is a complex organ consisting of the epidermis, dermis, and skin appendages, including the hair follicle and sebaceous gland. Wound healing in adult mammals results in scar formation without any skin appendages. Studies have reported remarkable examples of scarless healing in fetal skin and appendage regeneration in adult skin following the infliction of large wounds. The models used in these studies have offered a new platform for investigations of the cellular and molecular mechanisms underlying wound healing and skin regeneration in mammals. In this article, we will focus on the contribution of skin appendages to wound healing and, conversely, skin appendage regeneration following injuries.