Spatiotemporal antagonism in mesenchymal-epithelial signaling in sweat versus hair fate decision

INTRODUCTION Across the vertebrate kingdom, epithelial appendages—including mammary, sweat, and salivary glands; hair follicles (HFs); teeth; scales; and feathers—begin to form during embryogenesis when WNT signaling instructs progenitors within the epithelial sheet to organize into morphologically similar placodes. Most animals restrict these epidermal appendages to distinct body regions. This paradigm shifted late in mammalian evolution; the dual presence of HFs and eccrine sweat glands (SwGs) in the skin is a recent acquisition of primates. Classical tissue recombination experiments from the 1960s revealed that mesenchyme directs the divergent downstream events that determine appendage specification and patterning. Relatively little is known about the specific spatiotemporal cross-talk and molecular mechanisms that underlie epithelial fate specification in response to mesenchymal signals. Elucidating the epithelial-mesenchymal cross-talk involved in regional skin appendage specification is integral to understanding how humans have adjusted these mechanisms to endow them with a greater capacity than that of their hairy cousins to live in diverse environments. RATIONALE The acquisition of SwGs and their importance in thermoregulation and water balance are underscored by human patients who suffer from the life-threatening condition of lacking SwGs, either from loss in severe burns or from genetic disorders. Conversely, a gain-of-function variant that elevates SwG numbers has been expanding among the Southeast Asian population, where excessive SwGs are desirable. By elucidating the underlying mechanisms that distinguish humans from other mammals in their ability to make both glands and follicles over their body skin, our findings could pave the way for future therapeutic advances in skin regeneration with dual appendages. RESULTS Using mouse as a model, we explored the differences between the back skin mesenchyme, which is only competent to specify HFs, and the foot skin mesenchyme, which can only make SwGs. Using genome-wide analyses and functional studies, we discovered that just after the formation of morphologically similar epidermal buds, appendage choice is determined through regional skin differences in mesenchymal expression of bone morphogenetic proteins (BMPs). Probing into mechanisms, we showed that when BMPs are elevated in foot skin mesenchyme, BMP signaling is activated in both dermis and epidermis. This triggers a cascade of downstream signaling events. WNT signaling is elevated in the dermis and reduced in the epidermis. Mesenchymal fibroblast growth factors (FGFs) appear and affect the overlying epithelium. These converging pathways lead to suppression of sonic hedgehog (SHH) in the epithelium. This BMP:SHH antagonism within the epithelial bud specifies SwG fate and prevents HF fate. Moreover, by manipulating gene expression in vivo at specific developmental stages, we demonstrated that this signaling circuitry acts only within a narrow window of time during mouse embryogenesis. Thus, when SHH is ectopically expressed in foot skin epithelium during the permissive phase, HF-specific gene expression is up-regulated in the epithelial bud, whereas SHH signaling in the mesenchyme stimulates expression of BMP antagonists, further suppressing local BMP signaling and blocking SwG fate. In human skins, this antagonistic interplay of BMP:SHH signaling occurs temporally, in addition to spatially. The first bud waves are specified as HFs, and then a tipping in the balance of BMP:SHH signaling results in the last waves of buds becoming SwGs. CONCLUSION Our findings revealed a differential impact of BMP signaling on appendage fate specification that has ancient roots and occurs repeatedly throughout vertebrate evolution. In the evolutionary developmental biology view of BMP signaling and fate specification of integument, chicken scales and mammalian SwGs require BMP signaling to specify their fate, whereas feathers and HFs must suppress it. Our discovery of BMP-SHH antagonism in bud fate choice uncovered additional evolutionary parallels, this time between two even more distantly related epidermal appendages, the mammalian SwG and the fly wing. Our studies provide new insights into how elevated mesenchymal BMP signaling triggers a self-perpetuating molecular cascade that culminates in silencing of SHH signaling to suppress one appendage fate and specify another. In most mammals, the BMP:SHH antagonism is regulated spatially. Humans, however, have evolved to regulate it temporally, endowing them with greater ability to run marathons and survive in extreme climates.