Faculty Bibliography

The characterization of mesenchymal progenitors is central to understanding development, postnatal pathology and evolutionary adaptability. The precise identity of the mesenchymal precursors that generate the coronal suture, an important structural boundary in mammalian skull development, remains unclear. We show in mouse that coronal suture progenitors originate from hedgehog-responsive cephalic paraxial mesoderm (Mes) cells, which migrate rapidly to a supraorbital domain and establish a unidirectional lineage boundary with neural crest (NeuC) mesenchyme. Lineage tracing reveals clonal and stereotypical expansion of supraorbital mesenchymal cells to form the coronal suture between E11.0 and E13.5. We identify engrailed 1 (En1) as a necessary regulator of cell movement and NeuC/Mes lineage boundary positioning during coronal suture formation. In addition, we provide genetic evidence that En1 functions upstream of fibroblast growth factor receptor 2 (Fgfr2) in regulating early calvarial osteogenic differentiation, and postulate that it plays an additional role in precluding premature osteogenic conversion of the sutural mesenchyme.

In adult skin, stem cells in the hair follicle bulge cyclically regenerate the follicle, whereas a distinct stem cell population maintains the epidermis. The degree to which all bulge cells have equal regenerative potential is not known. We found that Sonic hedgehog (Shh) from neurons signals to a population of cells in the telogen bulge marked by the Hedgehog response gene Gli1. Gli1-expressing bulge cells function as multipotent stem cells in their native environment and repeatedly regenerate the anagen follicle. Shh-responding perineural bulge cells incorporate into healing skin wounds where, notably, they can change their lineage into epidermal stem cells. The perineural niche (including Shh) is dispensable for follicle contributions to acute wound healing and skin homeostasis, but is necessary to maintain bulge cells capable of becoming epidermal stem cells. Thus, nerves cultivate a microenvironment where Shh creates a molecularly and phenotypically distinct population of hair follicle stem cells

Wound healing is a complex process involving multiple cellular events, including cell proliferation, migration, and tissue remodeling. A disintegrin and metalloprotease 12 (ADAM12) is a membrane-anchored metalloprotease, which has been implicated in activation-inactivation of growth factors that play an important role in wound healing, including heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF) and insulin growth factor (IGF) binding proteins. Here, we report that expression of ADAM12 is fivefold upregulated in the nonhealing edge of chronic ulcers compared to healthy skin, based on microarrays of biopsies taken from five patients and from healthy controls (p = 0.013). The increase in ADAM12 expression in chronic ulcers was confirmed by quantitative real-time polymerase chain reaction (RT-PCR). Moreover, immunohistochemical analysis demonstrated a pronounced increase in the membranous and intracellular signal for ADAM12 in the epidermis of chronic wounds compared to healthy skin. These findings, coupled with our previous observations that lack of keratinocyte migration contributes to the pathogenesis of chronic ulcers, prompted us to evaluate how the absence of ADAM12 affects the migration of mouse keratinocytes. Skin explants from newborn ADAM12(-/-) or wild-type (WT) mice were used to quantify keratinocyte migration out of the explants over a period of 7 days. We found a statistically significant increase in the migration of ADAM12(-/-) keratinocytes compared to WT control (p = 0.0014) samples. Taken together, the upregulation of ADAM12 in chronic wounds and the increased migration of keratinocytes in the absence of ADAM12 suggest that ADAM12 is an important mediator of wound healing. We hypothesize that increased expression of ADAM12 in chronic wounds impairs wound healing through the inhibition of keratinocyte migration and that topical ADAM12 inhibitors may therefore prove useful for the treatment of chronic wounds

The membranous bones of the mammalian skull vault arise from discrete condensations of neural crest- and mesodermally-derived cells. Recently, a number of homeodomain transcription factors have been identified as critical regulators of this process. Here, we show that the homeoprotein engrailed 1 (EN1) is expressed during embryonic and perinatal craniofacial bone development, where it localizes to the skeletogenic mesenchyme, and, subsequently, to calvarial osteoblasts and osteoprogenitors. Mice lacking En1 exhibit generalized calvarial bone hypoplasia and persistent widening of the sutural joints. A reduction in calvarial membranous bone deposition and mineralization (osteopenia) is coupled to enhanced osteolytic resorption in En1 mutants. Consistent with these observations, expression of established osteoblast differentiation markers reveals that En1 function is required for both early and late phases of calvarial osteogenesis. Further analysis shows that EN1 regulates FGF signaling in calvarial osteoblasts. Moreover, EN1 indirectly influences calvarial osteoclast recruitment and bone resorption by regulating the expression of receptor activator of NFkappaB ligand (RANKL) in osteoblasts. Thus, during intramembranous bone formation, EN1 acts both cell autonomously and non-cell autonomously. In summary, this study identifies EN1 as a novel modulator of calvarial osteoblast differentiation and proliferation, processes that must be exquisitely balanced to ensure proper skull vault formation