EdnrB Governs Regenerative Response of Melanocyte Stem Cells by Crosstalk with Wnt Signaling

News from the endothelin-3/EDNRB signaling pathway: Role during enteric nervous system development and involvement in neural crest-associated disorders

The endothelin system is a vertebrate-specific innovation with important roles in regulating the cardiovascular system and renal and pulmonary processes, as well as the development of the vertebrate-specific neural crest cell population and its derivatives. This system is comprised of three structurally similar 21-amino acid peptides that bind and activate two G-protein coupled receptors. In 1994, knockouts of the Edn3 and Ednrb genes revealed their crucial function during development of the enteric nervous system and melanocytes, two neural-crest derivatives. Since then, human and mouse genetics, combined with cellular and developmental studies, have helped to unravel the role of this signaling pathway during development and adulthood. In this review, we will summarize the known functions of the EDN3/EDNRB pathway during neural crest development, with a specific focus on recent scientific advances, and the enteric nervous system in normal and pathological conditions.

A direct link between MITF, innate immunity, and hair graying

Melanocyte stem cells (McSCs) and mouse models of hair graying serve as useful systems to uncover mechanisms involved in stem cell self-renewal and the maintenance of regenerating tissues. Interested in assessing genetic variants that influence McSC maintenance, we found previously that heterozygosity for the melanogenesis associated transcription factor, Mitf, exacerbates McSC differentiation and hair graying in mice that are predisposed for this phenotype. Based on transcriptome and molecular analyses of Mitf^mi-vga9/+ mice, we report a novel role for MITF in the regulation of systemic innate immune gene expression. We also demonstrate that the viral mimic poly(I:C) is sufficient to expose genetic susceptibility to hair graying. These observations point to a critical suppressor of innate immunity, the consequences of innate immune dysregulation on pigmentation, both of which may have implications in the autoimmune, depigmenting disease, vitiligo.

Hair pigmentation over the course of a lifetime depends on melanocyte stem cells that reside in the hair follicle. As old hairs fall out and new hairs grow in, melanocyte stem cells serve as a reservoir for the melanocytes that produce the pigment that gives hair its visible color. The loss of these stem cells leads to the growth of nonpigmented, or gray, hairs. Evaluating mouse models of hair graying can reveal key aspects of melanocyte stem cell biology. Using this approach, we discovered a novel role for the melanogenesis associated transcription factor, MITF, in repressing the expression of innate immune genes within cells of the melanocyte lineage. The importance of this repression is revealed in animals that have a predisposition for hair graying. In these animals, artificial elevation of the innate immune response, either through a genetic mechanism or via exposure to viral mimic, results in significant melanocyte and melanocyte stem cell loss and leads to the production of an increased number of gray hairs. These observations highlight the negative effects of innate immune activation on melanocyte and melanocyte stem cell physiology and suggest a connection between viral infection and hair graying.

Regulation of melanocyte stem cell behavior by the niche microenvironment

Somatic stem cells are regulated by their niches to maintain tissue homeostasis and repair throughout the lifetime of an organism. An excellent example to study stem cell/niche interactions is provided by the regeneration of melanocytes during the hair cycle and in response to various types of injury. These processes are regulated by neighboring stem cells and multiple signaling pathways, including WNT/β-catenin, KITL/KIT, EDNs/EDNRB, TGF-β/TGF-βR, α-MSH/MC1R, and Notch signaling. In this review, we highlight recent studies that have advanced our understanding of the molecular crosstalk between melanocyte stem cells and their neighboring cells, which collectively form the niche microenvironment, and we focus on the question of how McSCs/niche interactions shape the responses to genotoxic damages and mechanical injury.

Dissecting Wnt Signaling for Melanocyte Regulation during Wound Healing

Abnormal pigmentation is commonly seen in the wound scar. Despite advancements in the research of wound healing, little is known about the repopulation of melanocytes in the healed skin. Previous studies have shown the capacity of melanocyte stem cells in the hair follicle to contribute skin epidermal melanocytes after injury in mice and humans. Here, we focused on the Wnt pathway, known to be a vital regulator of melanocyte stem cells in efforts to better understand the regulation of follicle-derived epidermal melanocytes during wound healing. We showed that transgenic expression of Wnt inhibitor Dkk1 in melanocytes reduced epidermal melanocytes in the wound scar. Conversely, forced activation of Wnt signaling by genetically stabilizing β-catenin in melanocytes increases epidermal melanocytes. Furthermore, we show that deletion of Wntless (Wls), a gene required for Wnt ligand secretion, within epithelial cells results in failure in activating Wnt signaling in adjacent epidermal melanocytes. These results show the essential function of extrinsic Wnt ligands in initiating Wnt signaling in follicle-derived epidermal melanocytes during wound healing. Collectively, our results suggest the potential for Wnt signal regulation to promote melanocyte regeneration and provide a potential molecular window to promote proper melanocyte regeneration after wounding and in conditions such as vitiligo.

Restoration of skin pigmentation after deep partial or full-thickness burn injury

Significant skin pigmentation changes occur when patients suffer deep burn injuries. These pigmentation disorders may cause not only cosmetic and psychological issues, but more importantly it increases the risk of skin cancer or photoaging. Severe burns significantly effect on the process of repigmentation as the pigmentation is tightly regulated by cell proliferation and differentiation of melanocytes and melanocyte stem cells which are housing in the epidermis and hair follicles of the skin. In the present review, we discuss the possible mechanisms to replenish the melanocytes from the healthy epidermis and hair follicles surrounding burn wounds. The molecular mechanisms of skin repigmentation following healing of burn injuries includes the differentiation of melanoblasts into melanocytes, the distribution and responses of melanocytes and melanocyte stem cells after burn injury, and the regulation of melanin production. We also reviewed advanced therapeutic strategies to treat pigmentation disorders, such as convectional surgery, laser, UV treatment and emerging concepts in skin tissue-engineering.

Keratinocyte Sonic Hedgehog Upregulation Drives the Development of Giant Congenital Nevi via Paracrine Endothelin-1 Secretion

Giant congenital nevi are associated with clinical complications such as neurocutaneous melanosis and melanoma. Virtually nothing is known about why some individuals develop these lesions. We previously identified the sonic hedgehog (Shh) pathway regulator Cdon as a candidate nevus modifier gene. Here we validate this by studying Cdon knockout mice, and go on to establishing the mechanism by which Shh exacerbates nevogenesis. Cdon knockout mice develop blue nevi without the need for somatic melanocyte oncogenic mutation. In a mouse model carrying melanocyte NRAS^Q61K, we found that strain backgrounds that carry genetic variants that cause increased keratinocyte Shh pathway activity, as measured by Gli1 and Gli2 expression, develop giant congenital nevi. Shh components are also active adjacent to human congenital nevi. Mechanistically, this exacerbation of nevogenesis is driven via the release of the melanocyte mitogen endothelin-1 from keratinocytes. We then suppressed nevus development in mice using Shh and endothelin antagonists. Our work suggests an aspect of nevus development whereby keratinocyte cytokines such as endothelin-1 can exacerbate nevogenesis, and provides potential therapeutic approaches for giant congenital nevi. Furthermore, it highlights the notion that germline genetic variation, in addition to somatic melanocyte mutation, can strongly influence the histopathological features of melanocytic nevi.

Epilation induces hair and skin pigmentation through an EDN3/EDNRB-dependent regenerative response of melanocyte stem cells

In response to various types of injury, melanocyte stem cells (McSCs) located in the bulge of hair follicles can regenerate mature melanocytes for hair and skin pigmentation. How McSCs respond to injury, however, remains largely unknown. Here we show that after epilation of mice, McSCs regenerate follicular and epidermal melanocytes, resulting in skin and hair hyperpigmentation. We further show that epilation leads to endogenous EDN3 upregulation in the dermal papilla, the secondary hair germ cells, and the epidermis. Genetic and pharmacological disruption of the EDN3 receptor EDNRB in vivo significantly blocks the effect of epilation on follicular and epidermal melanocyte regeneration as well as skin and hair hyperpigmentation. Taken together, these results indicate that epilation induces McSCs activation through EDN3/EDNRB signaling and in turn leads to skin and hair hyperpigmentation. The findings suggest that EDN/EDNRB signaling may serve as a potential therapeutic target to promote repigmentation in hypopigmentation disorders.