BMP signaling in dermal papilla cells is required for their hair follicle-inductive properties

A two-step mechanism for stem cell activation during hair regeneration

Hair follicles (HFs) undergo cyclic bouts of degeneration, rest, and regeneration. During rest (telogen), the hair germ (HG) appears as a small cell cluster between the slow-cycling bulge and dermal papilla (DP). Here we show that HG cells are derived from bulge stem cells (SCs) but become responsive quicker to DP-promoting signals. In vitro, HG cells also proliferate sooner but display shorter-lived potential than bulge cells. Molecularly, they more closely resemble activated bulge rather than transit-amplifying (matrix) cells. Transcriptional profiling reveals precocious activity of both HG and DP in late telogen, accompanied by Wnt signaling in HG and elevated FGFs and BMP inhibitors in DP. FGFs and BMP inhibitors participate with Wnts in exerting selective and potent stimuli to the HG both in vivo and in vitro. Our findings suggest a model where HG cells fuel initial steps in hair regeneration, while the bulge is the engine maintaining the process.

TGF-beta is specifically expressed in human dermal papilla cells and modulates hair folliculogenesis

Dermal papilla cells (DPCs) in the mammalian hair follicle have been shown to develop hair follicles through epithelial-mesenchymal interactions. A cell therapy to regenerate human hair is theoretically possible by expanding autologous human DPCs (hDPCs) and transplanting them into bald skin, though much remains to be overcome before clinical success. In this study, we compared gene signatures of hDPCs at different passages and human dermal fibroblasts, and found transforming growth factor (TGF)-beta(2) to be highly expressed in cultured hDPCs. Keratinocyte conditioned medium, which is known to help preserve the hair-inducing capacity of hDPCs, up-regulated TGF-beta(2) expression of hDPCs and also enhanced their alkaline phosphatase (ALP) activity, a known index for hair-inductive capacity. Through screening of components secreted from keratinocytes, the vitamin D(3) analogue was found to promote TGF-beta(2) expression and ALP activity of hDPCs. In animal hair folliculogenesis models using rat epidermis and expanded hDPCs, inhibition of TGF-beta(2) signalling at the ligand or receptor level significantly impaired hair folliculogenesis and maturation. These results suggest an important role for TGF-beta(2) in hair follicle morphogenesis and provide insights into the establishment of future cell therapies for hair regrowth by transplanting expanded DPCs.

Epidermal homeostasis: a balancing act of stem cells in the skin

The skin epidermis and its array of appendages undergo ongoing renewal by a process called homeostasis. Stem cells in the epidermis have a crucial role in maintaining tissue homeostasis by providing new cells to replace those that are constantly lost during tissue turnover or following injury. Different resident skin stem cell pools contribute to the maintenance and repair of the various epidermal tissues of the skin, including interfollicular epidermis, hair follicles and sebaceous glands. Interestingly, the basic mechanisms and signalling pathways that orchestrate epithelial morphogenesis in the skin are reused during adult life to regulate skin homeostasis.

Building epithelial tissues from skin stem cells

The skin epidermis and its appendages provide a protective barrier that guards against loss of fluids, physical trauma, and invasion by harmful microbes. To perform these functions while confronting the harsh environs of the outside world, our body surface undergoes constant rejuvenation through homeostasis. In addition, it must be primed to repair wounds in response to injury. The adult skin maintains epidermal homeostasis, hair regeneration, and wound repair through the use of its stem cells. What are the properties of skin stem cells, when do they become established during embryogenesis, and how are they able to build tissues with such remarkably distinct architectures? How do stem cells maintain tissue homeostasis and repair wounds and how do they regulate the delicate balance between proliferation and differentiation? What is the relationship between skin cancer and mutations that perturbs the regulation of stem cells? In the past 5 years, the field of skin stem cells has bloomed as we and others have been able to purify and dissect the molecular properties of these tiny reservoirs of goliath potential. We report here progress on these fronts, with emphasis on our laboratory's contributions to the fascinating world of skin stem cells.

Conversion of the nipple to hair-bearing epithelia by lowering bone morphogenetic protein pathway activity at the dermal-epidermal interface

Epithelial appendages, such as mammary glands and hair, arise as a result of epithelial-mesenchymal interactions. Bone morphogenetic proteins (BMPs) are important for hair follicle morphogenesis and cycling and are known to regulate a wide variety of developmental processes. For example, overexpression of BMPs inhibits hair follicle formation. We hypothesized that the down-regulation of the BMP signaling pathway in the basal epidermis expands regions that are competent to form hair follicles and could alter the fate of the epithelium in the mouse nipple to a hair-covered epidermal phenotype. To test our hypothesis, we used a transgenic mouse model in which keratin 14 (KRT14) promoter-mediated overexpression of Noggin, a BMP antagonist, modulates BMP activity. We observed the conversion of nipple epithelium into pilosebaceous units. During normal mammary gland organogenesis, BMPs are likely used by the nipple epithelium to suppress keratinocyte differentiation, thus preventing the formation of pilosebaceous units. In this report, we characterize the morphology and processes that influence the development of hairs within the nipple of the KRT14-Noggin mouse. We demonstrate that Noggin acts, in part, by reducing the BMP signal in the epithelium. Reduction of the BMP signal in turn leads to a reduction in the levels of parathyroid hormone-related protein. We propose that during evolution of the nipple, the BMP pathway was co-opted to suppress hair follicle formation and create a more functional milk delivery apparatus.

An in vivo reporter of BMP signaling in organogenesis reveals targets in the developing kidney

Background

Bone morphogenetic proteins (BMPs) regulate essential processes during organogenesis, and a functional understanding of these secreted proteins depends on identification of their target cells. In this study, we generate a transgenic reporter for organogenesis studies that we use to define BMP pathway activation in the developing kidney.

Results

Mouse strains reporting on BMP pathway activation were generated by transgenically expressing β-galactosidase under the control of BMP responsive elements from Id1. Reporter expression corresponds well with immunoassays for pathway activation in all organs studied, validating the model. Using these reporters we have generated a detailed map of cellular targets of BMP signaling in the developing kidney. We find that SMAD dependent BMP signaling is active in collecting duct trunks, but not tips. Furthermore, glomerular endothelial cells, and proximal nephron tubules from the renal vesicle stage onward show pathway activation. Surprisingly, little activation is detected in the nephrogenic zone of the kidney, and in organ culture BMP treatment fails to activate SMAD dependent BMP signaling in nephron progenitor cells. In contrast, signaling is efficiently induced in collecting duct tips.

Conclusion

Transgenic reporters driven by control elements from BMP responsive genes such as Id1 offer significant advantages in sensitivity and consistency over immunostaining for studies of BMP pathway activation. They also provide opportunities for analysis of BMP signaling in organ and primary cell cultures subjected to experimental manipulation. Using such a reporter, we made the surprising finding that SMAD dependent BMP signaling is inactive in nephron progenitors, and that these cells are refractory to activation by applied growth factors. Furthermore, we find that the BMP pathway is not normally active in collecting duct tips, but that it can be ectopically activated by BMP treatment, offering a possible explanation for the inhibitory effects of BMP treatment on collecting duct growth and branching.

A microRNA signature for a BMP2-induced osteoblast lineage commitment program

Bone morphogenetic proteins (BMPs) are potent morphogens that activate transcriptional programs for lineage determination. How BMP induction of a phenotype is coordinated with microRNAs (miRNAs) that inhibit biological pathways to control cell differentiation, remains unknown. Here, we show by profiling miRNAs during BMP2 induced osteogenesis of C2C12 mesenchymal cells, that 22 of 25 miRNAs which significantly changed in response to BMP2 are down-regulated. These miRNAs are each predicted to target components of multiple osteogenic pathways. We characterize two representative miRNAs and show that miR-133 directly targets Runx2, an early BMP response gene essential for bone formation, and miR-135 targets Smad5, a key transducer of the BMP2 osteogenic signal, controlled through their 3'UTR sequences. Both miRNAs functionally inhibit differentiation of osteoprogenitors by attenuating Runx2 and Smad5 pathways that synergistically contribute to bone formation. Although miR-133 is known to promote MEF-2-dependent myogenesis, we have identified a second complementary function to inhibit Runx2-mediated osteogenesis. Our key finding is that BMP2 controls bone cell determination by inducing miRNAs that target muscle genes but mainly by down-regulating multiple miRNAs that constitute an osteogenic program, thereby releasing from inhibition pathway components required for cell lineage commitment. Thus, our studies establish a mechanism for BMP morphogens to selectively induce a tissue-specific phenotype and suppress alternative lineages.

Skin and hair: models for exploring organ regeneration

Skin is an excellent model to study the basic biology of organ regeneration and translational approaches to regenerative medicine. Because of the accessibility of the skin, a long history of regenerative approaches already exists. Identifying the commonalities between skin regeneration and the regeneration of other organs could provide major breakthroughs in regenerative medicine. The hair follicle represents a miniature organ with readily accessible stem cells, multiple cell lineages, and signaling centers. During the normal lifespan of a human, this miniature organ regenerates itself more than 10 times. The cells responsible for this remarkable process are called bulge stem cells. A plethora of molecular and genetic tools have been developed to follow their fate and to explore their ontogeny. Major advances have been made toward understanding the normal cell fate of bulge stem cells and their developmental plasticity. Recent studies suggest the epidermis and hair may have an untapped potential to form other organs. Understanding the mechanisms that regulate adult stem-cell proliferation is a major goal for regenerative medicine. In the hair follicle, pharmacologic agents, recombinant proteins, and artificial cell-permeable proteins have been developed to manipulate the proliferation of the quiescent bulge stem cells. These advances illustrate a potential roadmap for regenerative medicine using molecular tools developed for skin biology to promote organ regeneration by manipulating adult stem cells in situ.

Dlx3 is a crucial regulator of hair follicle differentiation and cycling

Dlx homeobox transcription factors regulate epidermal, neural and osteogenic cellular differentiation. Here, we demonstrate the central role of Dlx3 as a crucial transcriptional regulator of hair formation and regeneration. The selective ablation of Dlx3 in the epidermis results in complete alopecia owing to failure of the hair shaft and inner root sheath to form, which is caused by the abnormal differentiation of the cortex. Significantly, we elucidate the regulatory cascade that positions Dlx3 downstream of Wnt signaling and as an upstream regulator of other transcription factors that regulate hair follicle differentiation, such as Hoxc13 and Gata3. Colocalization of phospho-Smad1/5/8 and Dlx3 is consistent with a regulatory role for BMP signaling to Dlx3 during hair morphogenesis. Importantly, mutant catagen follicles undergo delayed regression and display persistent proliferation. Moreover, ablation of Dlx3 expression in the telogen bulge stem cells is associated with a loss of BMP signaling, precluding re-initiation of the hair follicle growth cycle. Taken together with hair follicle abnormalities in humans with Tricho-Dento-Osseous (TDO) syndrome, an autosomal dominant ectodermal dysplasia linked to mutations in the DLX3 gene, our results establish that Dlx3 is essential for hair morphogenesis, differentiation and cycling programs.