In vivo regulation of murine hair growth: insights from grafting defined cell populations onto nude mice

An Intrinsic Oscillation of Gene Networks Inside Hair Follicle Stem Cells: An Additional Layer That Can Modulate Hair Stem Cell Activities

This article explores and summarizes recent progress in and the characterization of main players in the regulation and cyclic regeneration of hair follicles. The review discusses current views and discoveries on the molecular mechanisms that allow hair follicle stem cells (hfSCs) to synergistically integrate homeostasis during quiescence and activation. Discussion elaborates on a model that shows how different populations of skin stem cells coalesce intrinsic and extrinsic mechanisms, resulting in the maintenance of stemness and hair regenerative potential during an organism’s lifespan. Primarily, we focus on the question of how the intrinsic oscillation of gene networks in hfSCs sense and respond to the surrounding niche environment. The review also investigates the existence of a cell-autonomous mechanism and the reciprocal interactions between molecular signaling axes in hfSCs and niche components, which demonstrates its critical driving force in either the activation of whole mini-organ regeneration or quiescent homeostasis maintenance. These exciting novel discoveries in skin stem cells and the surrounding niche components propose a model of the intrinsic stem cell oscillator which is potentially instructive for translational regenerative medicine. Further studies, deciphering of the distribution of molecular signals coupled with the nature of their oscillation within the stem cells and niche environments, may impact the speed and efficiency of various approaches that could stimulate the development of self-renewal and cell-based therapies for hair follicle stem cell regeneration.

The aging skin microenvironment dictates stem cell behavior

Aging manifests with architectural alteration and functional decline of multiple organs throughout an organism. In mammals, aged skin is accompanied by a marked reduction in hair cycling and appearance of bald patches, leading researchers to propose that hair follicle stem cells (HFSCs) are either lost, differentiate, or change to an epidermal fate during aging. Here, we employed single-cell RNA-sequencing to interrogate aging-related changes in the HFSCs. Surprisingly, although numbers declined, aging HFSCs were present, maintained their identity, and showed no overt signs of shifting to an epidermal fate. However, they did exhibit prevalent transcriptional changes particularly in extracellular matrix genes, and this was accompanied by profound structural perturbations in the aging SC niche. Moreover, marked age-related changes occurred in many nonepithelial cell types, including resident immune cells, sensory neurons, and arrector pili muscles. Each of these SC niche components has been shown to influence HF regeneration. When we performed skin injuries that are known to mobilize young HFSCs to exit their niche and regenerate HFs, we discovered that aged skin is defective at doing so. Interestingly, however, in transplantation assays in vivo, aged HFSCs regenerated HFs when supported with young dermis, while young HFSCs failed to regenerate HFs when combined with aged dermis. Together, our findings highlight the importance of SC:niche interactions and favor a model where youthfulness of the niche microenvironment plays a dominant role in dictating the properties of its SCs and tissue health and fitness.

Tissue engineering strategies for human hair follicle regeneration: How far from a hairy goal?

The demand for an efficient therapy for alopecia disease has fueled the hair research field in recent decades. However, despite significant improvements in the knowledge of key processes of hair follicle biology such as genesis and cycling, translation into hair follicle replacement therapies has not occurred. Great expectation has been recently put on hair follicle bioengineering, which is based on the development of fully functional hair follicles with cycling activity from an expanded population of hair‐inductive (trichogenic) cells. Most bioengineering approaches focus on in vitro reconstruction of folliculogenesis by manipulating key regulatory molecular/physical features of hair follicle growth/cycling in vivo. Despite their great potential, no cell‐based product is clinically available for hair regeneration therapy to date. This is mainly due to demanding issues that still hinder the functionality of cultured human hair cells. The present review comprehensively compares emergent strategies using different cell sources and tissue engineering approaches, aiming to successfully achieve a clinical cure for hair loss. The hurdles of these strategies are discussed, as well as the future directions to overcome the obstacles and fulfill the promise of a “hairy” feat.

Use of human intra-tissue stem/progenitor cells and induced pluripotent stem cells for hair follicle regeneration

Background

The hair follicle (HF) is a unique miniorgan, which self-renews for a lifetime. Stem cell populations of multiple lineages reside within human HF and enable its regeneration. In addition to resident HF stem/progenitor cells (HFSPCs), the cells with similar biological properties can be induced from human-induced pluripotent stem cells (hiPSCs). As approaches to regenerate HF by combining HF-derived cells have been established in rodents and a huge demand exists to treat hair loss diseases, attempts have been made to bioengineer human HF using HFSPCs or hiPSCs.

Main body of the abstract

The aim of this review is to comprehensively summarize the strategies to regenerate human HF using HFSPCs or hiPSCs. HF morphogenesis and regeneration are enabled by well-orchestrated epithelial-mesenchymal interactions (EMIs). In rodents, various combinations of keratinocytes with mesenchymal (dermal) cells with trichogenic capacity, which were transplanted into in vivo environment, have successfully generated HF structures. The regeneration efficiency was higher, when epithelial or dermal HFSPCs were adopted. The success in HF formation most likely depended on high receptivity to trichogenic dermal signals and/or potent hair inductive capacity of HFSPCs. In theory, the use of epithelial HFSPCs in the bulge area and dermal papilla cells, their precursor cells in the dermal sheath, or trichogenic neonatal dermal cells should elicit intense EMI sufficient for HF formation. However, technical hurdles, represented by the limitation in starting materials and the loss of intrinsic properties during in vitro expansion, hamper the stable reconstitution of human HFs with this approach. Several strategies, including the amelioration of culture condition or compartmentalization of cells to strengthen EMI, can be conceived to overcome this obstacle. Obviously, use of hiPSCs can resolve the shortage of the materials once reliable protocols to induce wanted HFSPC subsets have been developed, which is in progress. Taking advantage of their pluripotency, hiPSCs may facilitate previously unthinkable approaches to regenerate human HFs, for instance, via bioengineering of 3D integumentary organ system, which can also be applied for the treatment of other diseases.

Short conclusion

Further development of methodologies to reproduce bona fide EMI in HF formation is indispensable. However, human HFSPCs and hiPSCs hold promise as materials for human HF regeneration.

Self-organization process in newborn skin organoid formation inspires strategy to restore hair regeneration of adult cells

Organoids made from dissociated progenitor cells undergo tissue-like organization. This in vitro self-organization process is not identical to embryonic organ formation, but it achieves a similar phenotype in vivo. This implies genetic codes do not specify morphology directly; instead, complex tissue architectures may be achieved through several intermediate layers of cross talk between genetic information and biophysical processes. Here we use newborn and adult skin organoids for analyses. Dissociated cells from newborn mouse skin form hair primordia-bearing organoids that grow hairs robustly in vivo after transplantation to nude mice. Detailed time-lapse imaging of 3D cultures revealed unexpected morphological transitions between six distinct phases: dissociated cells, cell aggregates, polarized cysts, cyst coalescence, planar skin, and hair-bearing skin. Transcriptome profiling reveals the sequential expression of adhesion molecules, growth factors, Wnts, and matrix metalloproteinases (MMPs). Functional perturbations at different times discern their roles in regulating the switch from one phase to another. In contrast, adult cells form small aggregates, but then development stalls in vitro. Comparative transcriptome analyses suggest suppressing epidermal differentiation in adult cells is critical. These results inspire a strategy that can restore morphological transitions and rescue the hair-forming ability of adult organoids: (i) continuous PKC inhibition and (ii) timely supply of growth factors (IGF, VEGF), Wnts, and MMPs. This comprehensive study demonstrates that alternating molecular events and physical processes are in action during organoid morphogenesis and that the self-organizing processes can be restored via environmental reprogramming. This tissue-level phase transition could drive self-organization behavior in organoid morphogenies beyond the skin.

The patch assay reconstitutes mature hair follicles by culture-expanded human cells

AIM

We tested whether the a simple injection known as the patch assay could reconstitute mature hair follicles by culture-expanded human cells and explored whether the assay could reflect the trichogenicity of cultured cells.

MATERIALS & METHODS

Dissociated culture-expanded fetal or adult scalp dermal cells combined with foreskin keratinocytes were subcutaneously injected into the back skin of immunosuppressive mice to form the patch skin. The patches were collected and characterized and were analyzed for hair formation efficiency.

RESULTS

Using culture-expanded human fetal cells, the patch assay can efficiently reconstitute mature hair follicles and the efficiency of hair formation in the patch assay correlates with cell trichogenicity.

CONCLUSION

The patch assay has the potential for testing the trichogenicity of human cells.

Laboratory Models for the Study of Normal and Pathologic Wound Healing

Current knowledge of wound healing is based on studies using various in vitro and in vivo wound models. In vitro models allow for biological examination of specific cell types involved in wound healing. In vivo models generally provide the full spectrum of biological responses required for wound healing, including inflammation and angiogenesis, and provide cell-cell interactions not seen in vitro. In this review, the authors aim to delineate the most relevant wound healing models currently available and to discuss their strengths and limitations in their approximation of the human wound healing processes to aid scientists in choosing the most appropriate wound healing models for designing, testing, and validating their experiments.

Estrogen modulates mesenchyme-epidermis interactions in the adult nipple

Maintenance of specialized epidermis requires signals from the underlying mesenchyme; however, the specific pathways involved remain to be identified. By recombining cells from the ventral skin of the K14-PTHrP transgenic mice [which overexpress parathyroid hormone-related protein (PTHrP) in their developing epidermis and mammary glands] with those from wild type, we show that transgenic stroma is sufficient to reprogram wild-type keratinocytes into nipple-like epidermis. To identify candidate nipple-specific signaling factors, we compared gene expression signatures of sorted Pdgfrα-positive ventral K14-PTHrP and wild-type fibroblasts, identifying differentially expressed transcripts that are involved in WNT, HGF, TGFβ, IGF, BMP, FGF and estrogen signaling. Considering that some of the growth factor pathways are targets for estrogen regulation, we examined the upstream role of this hormone in maintaining the nipple. Ablation of estrogen signaling through ovariectomy produced nipples with abnormally thin epidermis, and we identified TGFβ as a negatively regulated target of estrogen signaling. Estrogen treatment represses Tgfβ1 at the transcript and protein levels in K14-PTHrP fibroblasts in vitro, while ovariectomy increases Tgfb1 levels in K14-PTHrP ventral skin. Moreover, ectopic delivery of Tgfβ1 protein into nipple connective tissue reduced epidermal proliferation. Taken together, these results show that specialized nipple epidermis is maintained by estrogen-induced repression of TGFβ signaling in the local fibroblasts.

Isolation of Mouse Hair Follicle Bulge Stem Cells and Their Functional Analysis in a Reconstitution Assay

The hair follicle (HF) is a dynamic structure readily accessible within the skin, and contains various pools of stem cells that have a broad regenerative potential during normal homeostasis and in response to injury. Recent discoveries demonstrating the multipotent capabilities of hair follicle stem cells and the easy access to skin tissue make the HF an attractive source for isolating stem cells and their subsequent application in tissue engineering and regenerative medicine. Here, we describe the isolation and purification of hair follicle bulge stem cells from mouse skin, and hair reconstitution assays that allows the functional analysis of multipotent stem cells.

A Two-Stepped Culture Method for Efficient Production of Trichogenic Keratinocytes

Successful hair follicle (HF) neogenesis in adult life depends on the existence of both capable dermal cells and competent epidermal keratinocytes that recapitulate embryonic organogenesis through epithelial-mesenchymal interaction. In tissue engineering, the maintenance of trichogenic potential of adult epidermal cells, while expanding them remains a challenging issue. We found that although HF outer root sheath keratinocytes could be expanded for more than 100 passages as clonogenic cells without losing the proliferative potential with a 3T3J2 fibroblast feeder layer, these keratinocytes were unable to form new HFs when combined with inductive HF dermal papilla (DP) cells. However, when these high-passage keratinocytes were cocultured with HF DP cells for 4 days in vitro, they regained the trichogenic ability to form new HFs after transplantation. We found that the short-term coculture with DP cells enhanced both Wnt/β-catenin signaling, a signaling cascade key to HF development, and upregulated the expression of HF-specific genes, including K6, K16, K17, and K75, in keratinocytes, indicating that these cells were poised toward a HF fate. Hence, efficient production of trichogenic keratinocytes can be obtained by a two-stepped procedure with initial cell expansion with a 3T3J2 fibroblast feeder followed by short-term coculture with DP cells.

A simple and rapid model for hair-follicle regeneration in the nude mouse

BACKGROUND

Methods for hair-follicle regeneration are important tools for investigating signalling and cytokines during hair-follicle morphogenesis and cycling. Several animal models for hair reconstitution have been established; however, these models have several shortcomings.

AIM

To develop a simple and rapid model for hair induction in nude mouse.

METHODS

We designed an improved flap model (IFM) for hair regeneration based on the existing flap assay. Histological sections and scanning electron microscopy were used to evaluate the regenerated hair. The fates of grafted cells were traced by fluorescence. The time required for hair induction was analysed and compared.

RESULTS

IFM produced a large number of normal hairs, and the time required for hair induction using IFM was 20.67 ± 0.67 days, compared with 29.33 ± 0.67 days for the traditional flap assay.

CONCLUSIONS

The time required for hair regeneration is considerably shortened with IFM. We speculate that this is due to increased blood supply at the transplantation sites.

Derivation of hair-inducing cell from human pluripotent stem cells

Dermal Papillae (DP) is a unique population of mesenchymal cells that was shown to regulate hair follicle formation and growth cycle. During development most DP cells are derived from mesoderm, however, functionally equivalent DP cells of cephalic hairs originate from Neural Crest (NC). Here we directed human embryonic stem cells (hESCs) to generate first NC cells and then hair-inducing DP-like cells in culture. We showed that hESC-derived DP-like cells (hESC-DPs) express markers typically found in adult human DP cells (e.g. p-75, nestin, versican, SMA, alkaline phosphatase) and are able to induce hair follicle formation when transplanted under the skin of immunodeficient NUDE mice. Engineered to express GFP, hESC-derived DP-like cells incorporate into DP of newly formed hair follicles and express appropriate markers. We demonstrated that BMP signaling is critical for hESC-DP derivation since BMP inhibitor dorsomorphin completely eliminated hair-inducing activity from hESC-DP cultures. DP cells were proposed as the cell-based treatment for hair loss diseases. Unfortunately human DP cells are not suitable for this purpose because they cannot be obtained in necessary amounts and rapidly loose their ability to induce hair follicle formation when cultured. In this context derivation of functional hESC-DP cells capable of inducing a robust hair growth for the first time shown here can become an important finding for the biomedical science.

Direct conversion of mouse and human fibroblasts to functional melanocytes by defined factors

Direct reprogramming provides a fundamentally new approach for the generation of patient-specific cells. Here, by screening a pool of candidate transcription factors, we identify that a combination of three factors, MITF, SOX10 and PAX3, directly converts mouse and human fibroblasts to functional melanocytes. Induced melanocytes (iMels) activate melanocyte-specific networks, express components of pigment production and delivery system, and produce melanosomes. Human iMels properly integrate into the dermal-epidermal junction, and produce and deliver melanin pigment to surrounding keratinocytes in a 3D organotypic skin reconstruct. Human iMels generate pigmented epidermis and hair follicles in skin reconstitution assays in vivo. The generation of iMels has important implications for studies of melanocyte lineage commitment, pigmentation disorders and cell replacement therapies.

Understanding fibroblast heterogeneity in the skin

Fibroblasts are found in most tissues, yet they remain poorly characterised. Different fibroblast subpopulations with distinct functions have been identified in the skin. This functional heterogeneity reflects the varied fibroblast lineages that arise from a common embryonic precursor. In addition to autocrine signals, fibroblasts are highly responsive to Wnt-regulated signals from the overlying epidermis, which can act both locally, via extracellular matrix (ECM) deposition, and via secreted factors that impact the behaviour of fibroblasts in different dermal locations. These findings may explain some of the changes that occur in connective tissue during wound healing and cancer progression.

Dermal papilla cells improve the wound healing process and generate hair bud-like structures in grafted skin substitutes using hair follicle stem cells

Tissue-engineered skin represents a useful strategy for the treatment of deep skin injuries and might contribute to the understanding of skin regeneration. The use of dermal papilla cells (DPCs) as a dermal component in a permanent composite skin with human hair follicle stem cells (HFSCs) was evaluated by studying the tissue-engineered skin architecture, stem cell persistence, hair regeneration, and graft-take in nude mice. A porcine acellular dermal matrix was seeded with HFSCs alone and with HFSCs plus human DPCs or dermal fibroblasts (DFs). In vitro, the presence of DPCs induced a more regular and multilayered stratified epidermis with more basal p63-positive cells and invaginations. The DPC-containing constructs more accurately mimicked the skin architecture by properly stratifying the differentiating HFSCs and developing a well-ordered epithelia that contributed to more closely recapitulate an artificial human skin. This acellular dermal matrix previously repopulated in vitro with HFSCs and DFs or DPCs as the dermal component was grafted in nude mice. The presence of DPCs in the composite substitute not only favored early neovascularization, good assimilation and remodeling after grafting but also contributed to the neovascular network maturation, which might reduce the inflammation process, resulting in a better healing process, with less scarring and wound contraction. Interestingly, only DPC-containing constructs showed embryonic hair bud-like structures with cells of human origin, presence of precursor epithelial cells, and expression of a hair differentiation marker. Although preliminary, these findings have demonstrated the importance of the presence of DPCs for proper skin repair.

Neonatal murine skin-derived cells transplanted using a mini-chamber model produce robust and normal hair

Hair follicle reconstitution models are useful tools for investigating signalling and cytokines during hair follicle morphogenesis and cycling. The chamber model is one of the most established methods available for the study of hair follicle reconstitution and appears to be the most reproducible. However, the chamber model has several deficiencies: infection of skin wounds and subsequent animal death commonly occur, a large number of cells are required and only one chamber can be transplanted onto each animal. We modified these deficiencies by using a mini-chamber method, which has the advantages of having a high graft take rate, requiring fewer cells and allowing several mini-chambers to be transplanted onto each animal. In our study, cultured dermal cells at different passages (0 to high) lost the ability to reconstruct hair follicles, but dermal cells cultured overnight (12 h) retained this ability. Using the assay, newborn mice dermal cells that were freshly isolated and cultured overnight (12 h), as well as cultured dermal papilla cells from mice vibrissa follicles, all reconstructed hair follicles. However, cultured dermal papilla cells from human scalp follicles could not reconstruct hair follicles. Copyright © 2013 John Wiley & Sons, Ltd.

Direct migration of follicular melanocyte stem cells to the epidermis after wounding or UVB irradiation is dependent on Mc1r signaling

During wound healing, stem cells provide functional mature cells to meet acute demands for tissue regeneration. Simultaneously, the tissue must maintain a pool of stem cells to sustain its future regeneration capability. However, how these requirements are balanced in response to injury is unknown. Here we demonstrate that after wounding or ultraviolet type B irradiation, melanocyte stem cells (McSCs) in the hair follicle exit the stem cell niche before their initial cell division, potentially depleting the pool of these cells. We also found that McSCs migrate to the epidermis in a melanocortin 1 receptor (Mc1r)-dependent manner and differentiate into functional epidermal melanocytes, providing a pigmented protective barrier against ultraviolet irradiation over the damaged skin. These findings provide an example in which stem cell differentiation due to injury takes precedence over stem cell maintenance and show the potential for developing therapies for skin pigmentation disorders by manipulating McSCs.

Therapeutic strategy for hair regeneration: hair cycle activation, niche environment modulation, wound-induced follicle neogenesis, and stem cell engineering

INTRODUCTION

There are major new advancements in the fields of stem cell biology, developmental biology, regenerative hair cycling, and tissue engineering. The time is ripe to integrate, translate, and apply these findings to tissue engineering and regenerative medicine. Readers will learn about new progress in cellular and molecular aspects of hair follicle development, regeneration, and potential therapeutic opportunities these advances may offer.

AREAS COVERED

Here, we use hair follicle formation to illustrate this progress and to identify targets for potential strategies in therapeutics. Hair regeneration is discussed in four different categories: i) Intra-follicle regeneration (or renewal) is the basic production of hair fibers from hair stem cells and dermal papillae in existing follicles. ii) Chimeric follicles via epithelial-mesenchymal recombination to identify stem cells and signaling centers. iii) Extra-follicular factors including local dermal and systemic factors can modulate the regenerative behavior of hair follicles, and may be relatively easy therapeutic targets. iv) Follicular neogenesis means the de novo formation of new follicles. In addition, scientists are working to engineer hair follicles, which require hair-forming competent epidermal cells and hair-inducing dermal cells.

EXPERT OPINION

Ideally self-organizing processes similar to those occurring during embryonic development should be elicited with some help from biomaterials.

Rapid genetic analysis of epithelial-mesenchymal signaling during hair regeneration

Hair follicle morphogenesis, a complex process requiring interaction between epithelia-derived keratinocytes and the underlying mesenchyme, is an attractive model system to study organ development and tissue-specific signaling. Although hair follicle development is genetically tractable, fast and reproducible analysis of factors essential for this process remains a challenge. Here we describe a procedure to generate targeted overexpression or shRNA-mediated knockdown of factors using lentivirus in a tissue-specific manner. Using a modified version of a hair regeneration model ^{5, 6, 11}, we can achieve robust gain- or loss-of-function analysis in primary mouse keratinocytes or dermal cells to facilitate study of epithelial-mesenchymal signaling pathways that lead to hair follicle morphogenesis. We describe how to isolate fresh primary mouse keratinocytes and dermal cells, which contain dermal papilla cells and their precursors, deliver lentivirus containing either shRNA or cDNA to one of the cell populations, and combine the cells to generate fully formed hair follicles on the backs of nude mice. This approach allows analysis of tissue-specific factors required to generate hair follicles within three weeks and provides a fast and convenient companion to existing genetic models.

Strategies to enhance epithelial-mesenchymal interactions for human hair follicle bioengineering

Hair follicle morphogenesis and regeneration depend on intensive but well-orchestrated interactions between epithelial and mesenchymal components. Accordingly, the enhancement of this crosstalk represents a promising approach to achieve successful bioengineering of human hair follicles. The present article summarizes the techniques, both currently available and potentially feasible, to promote epithelial-mesenchymal interactions (EMIs) necessary for human hair follicle regeneration. The strategies include the preparation of epithelial components with high receptivity to trichogenic dermal signals and/or mesenchymal cell populations with potent hair inductive capacity. In this regard, bulge epithelial stem cells, keratinocytes predisposed to hair follicle fate or keratinocyte precursor cells with plasticity may provide favorable epithelial cell populations. Dermal papilla cells sustaining intrinsic hair inductive capacity, putative dermal papilla precursor cells in the dermal sheath/neonatal dermis or trichogenic dermal cells derived from undifferentiated stem/progenitor cells are promising candidates as hair inductive dermal cells. The most established protocol for in vivo hair follicle reconstitution is co-grafting of epithelial and mesenchymal components into immunodeficient mice. In theory, combination of individually optimized cellular components of respective lineages should elicit most intensive EMIs to form hair follicles. Still, EMIs can be further ameliorated by the modulation of non-cell autonomous conditions, including cell compartmentalization to replicate the positional relationship in vivo and humanization of host environment by preparing human stromal bed. These approaches may not always synergistically intensify EMIs, however, step-by-step investigation probing optimal combinations should maximally enhance EMIs to achieve successful human hair follicle bioengineering.

Cellular populations isolated from newborn mouse skin including mesenchymal stem cells

We developed protocols for isolation and characterization of mesenchymal progenitors from murine dermis. Our protocols are part of a more general isolation procedure starting with neonatal murine skin, which has been described in detail by U. Lichti and coauthors (Nat Protoc 3(5):799-810, 2008). We list Lichti's procedures in an abbreviated form as part of this methods section. Our methods to isolate mesenchymal stem cells are presented as a continuous workflow of isolation and characterization, including flow cytometry, cell survival assays, colony formation assays, immunoblotting, immunostaining, multipotential differentiation assays, and in vivo engraftment. In most cases, the protocols are standard; in others, they were adapted to our particular purpose. We made special emphasis on the use of in vitro three-dimensional cultures to cue mesenchymal progenitors into epidermal cells.

Genetic manipulation of keratinocyte stem cells with lentiviral vectors

The epidermis of the skin and its appendages, such as the hair follicles, are formed and maintained by keratinocyte stem cells. Highly efficient and permanent genetic modifications are valuable tools to examine the multipotency and regenerative capacity of keratinocyte stem cells in skin and hair follicle development, homeostasis, and regeneration. Herein, we describe an ex vivo approach by which primary mouse keratinocytes can be permanently manipulated by lentiviral vectors at the genetic level. This protocol can be used to permanently express a gene-of-interest or selectively silence the expression of an endogenous gene, which can be used in preclinical development of gene-based therapies for skin and systemic disorders.

Isolation and differentiation of hair follicle-derived dermal precursors

Several different precursor populations participate in renewal and regeneration of the mammalian skin and hair follicle. Recently, we described the existence of multipotent dermal precursors that exhibit properties of stem cells, and reside in the mesenchymal compartment of the hair follicle. When isolated and grown in vitro, these cells give rise to self-renewing, multipotent, spherical colonies of cells called Skin-derived Precursors (or "SkPs"). Here we describe methods to isolate SkPs from rodent and human skin and provide assays to determine functional differentiation of their progeny.

Mesenchymal-epithelial interactions during hair follicle morphogenesis and cycling

Embryonic hair follicle induction and formation are regulated by mesenchymal-epithelial interactions between specialized dermal cells and epidermal stem cells that switch to a hair fate. Similarly, during postnatal hair growth, communication between mesenchymal dermal papilla cells and surrounding epithelial matrix cells coordinates hair shaft production. Adult hair follicle regeneration in the hair cycle again is thought to be controlled by activating signals originating from the mesenchymal compartment and acting on hair follicle stem cells. Although many signaling pathways are implicated in hair follicle formation and growth, the precise nature, timing, and intersection of these inductive and regulatory signals remains elusive. The goal of this review is to summarize our current understanding and to discuss recent new insights into mesenchymal-epithelial interactions during hair follicle morphogenesis and cycling.

Shh maintains dermal papilla identity and hair morphogenesis via a Noggin-Shh regulatory loop

During hair follicle morphogenesis, dermal papillae (DPs) function as mesenchymal signaling centers that cross-talk with overlying epithelium to regulate morphogenesis. While the DP regulates hair follicle formation, relatively little is known about the molecular basis of DP formation. The morphogen Sonic hedgehog (Shh) is known for regulating hair follicle epithelial growth, with excessive signaling resulting in basal cell carcinomas. Here, we investigate how dermal-specific Shh signaling contributes to DP formation and hair growth. Using a Cre-lox genetic model and RNAi in hair follicle reconstitution assays, we demonstrate that dermal Smoothened (Smo) loss of function results in the loss of the DP precursor, the dermal condensate, and a stage 2 hair follicle arrest phenotype reminiscent of Shh(-/-) skin. Surprisingly, dermal Smo does not regulate cell survival or epithelial proliferation. Rather, molecular screening and immunostaining studies reveal that dermal Shh signaling controls the expression of a subset of DP-specific signature genes. Using a hairpin/cDNA lentiviral system, we show that overexpression of the Shh-dependent gene Noggin, but not Sox2 or Sox18, can partially rescue the dermal Smo knockdown hair follicle phenotype by increasing the expression of epithelial Shh. Our findings suggest that dermal Shh signaling regulates specific DP signatures to maintain DP maturation while maintaining a reciprocal Shh-Noggin signaling loop to drive hair follicle morphogenesis.

1α,25-dihydroxyvitamin D3 modulates the hair-inductive capacity of dermal papilla cells: therapeutic potential for hair regeneration

Dermal papilla cells (DPCs) have the potential to induce differentiation of epithelial stem cells into hair, and Wnt signaling is deeply involved in the initiation process. The functional limitation of expanded adult DPCs has been a difficult challenge for cell-based hair regrowth therapy. We previously reported that 1α,25-dihydroxyvitamin D(3) (VD(3)) upregulates expression of transforming growth factor (TGF)-β2 and alkaline phosphatase (ALP) activity, both features of hair-inducing human DPCs (hDPCs). In this study, we further examined the effects and signaling pathways associated with VD(3) actions on DPCs. VD(3) suppressed hDPC proliferation in a dose-dependent, noncytotoxic manner. Among the Wnt-related genes investigated, Wnt10b expression was significantly upregulated by VD(3) in hDPCs. Wnt10b upregulation, as well as upregulation of ALPL (ALP, liver/bone/kidney) and TGF-β2, by VD(3) was specific in hDPCs and not detected in human dermal fibroblasts. Screening of paracrine or endocrine factors in the skin indicated that all-trans retinoic acid (atRA) upregulated Wnt10b gene expression, although synergistic upregulation (combined atRA and VD(3)) was not seen. RNA interference with vitamin D receptor (VDR) revealed that VD(3) upregulation of Wnt10b, ALPL, and TGF-β2 was mediated through the genomic VDR pathway. In a rat model of de novo hair regeneration by murine DPC transplantation, pretreatment with VD(3) significantly enhanced hair folliculogenesis. Specifically, a greater number of outgrowing hair shafts and higher maturation of regenerated follicles were observed. Together, these data suggest that VD(3) may promote functional differentiation of DPCs and be useful in preserving the hair follicle-inductive capacity of cultured DPCs for hair regeneration therapies.

Restoration of the intrinsic properties of human dermal papilla in vitro

The dermal papilla (DP) plays pivotal roles in hair follicle morphogenesis and cycling. However, characterization and/or propagation of human DPs have been unsatisfactory because of the lack of efficient isolation methods and the loss of innate characteristics in vitro. We hypothesized that culture conditions sustaining the intrinsic molecular signature of the human DP could facilitate expansion of functional DP cells. To test this, we first characterized the global gene expression profile of microdissected, non-cultured human DPs. We performed a 'two-step' microarray analysis to exclude the influence of unwanted contaminants in isolated DPs and successfully identified 118 human DP signature genes, including 38 genes listed in the mouse DP signature. The bioinformatics analysis of the DP gene list revealed that WNT, BMP and FGF signaling pathways were upregulated in intact DPs and addition of 6-bromoindirubin-3'-oxime, recombinant BMP2 and basic FGF to stimulate these respective signaling pathways resulted in maintained expression of in situ DP signature genes in primarily cultured human DP cells. More importantly, the exposure to these stimulants restored normally reduced DP biomarker expression in conventionally cultured DP cells. Cell growth was moderate in the newly developed culture medium. However, rapid DP cell expansion by conventional culture followed by the restoration by defined activators provided a sufficient number of DP cells that demonstrated characteristic DP activities in functional assays. The study reported here revealed previously unreported molecular mechanisms contributing to human DP properties and describes a useful technique for the investigation of human DP biology and hair follicle bioengineering.

Determinants of the direction of hair follicle growth after reconstruction with dissociated cells

BACKGROUND/OBJECTIVES

Several hair follicle reconstruction models based on the implantation of dissociated cells have been established. However, little has been said about the direction of growth, which is a very important issue for clinical applications. The aim of this study was to reveal the factors that determine the direction of hair follicle growth after reconstruction with dissociated cells.

METHODS

We designed an enclosed chamber assay in which a chamber was buried under nude mouse skin that was not exposed to the air, as in a traditional chamber assay. Inductive dermal cells and epithelial cells dissociated from neonatal C57 BL/6 mice were injected into the chamber and dermis for patch assays. The direction of newly formed hair follicles was analysed.

RESULTS

Newly formed hair follicles in the enclosed chamber were arranged randomly parallel to the skin on the surface of the panniculus carnosus.

CONCLUSIONS

These results indicate that developmental space provides the possibility for growth of reconstructed hair follicles and that the air-liquid interface determines the final direction of growth.

Clinically applicable transplantation procedure of dermal papilla cells for hair follicle regeneration

Dermal papilla cells (DPCs) interact with epithelial stem cells and induce hair folliculogenesis. Cell-based therapies using expanded DPCs for hair regeneration have been unsuccessful in humans. Two major challenges remain: first, expanded DPCs obtained from adult hair follicles have functional limitations; second, a clinically applicable method is needed for transplanting DPCs. This study aimed to identify an efficient, minimally invasive and economical DPC transplantation procedure for use in clinical settings. Five clinically applicable transplantation procedures were tested, termed the Pinhole, Laser, Slit, Non-vascularized sandwich (NVS) and Hemi-vascularized sandwich (HVS) methods. Labelled rat dermal papilla tissue was transplanted into rat sole skin, and hair follicle regeneration was evaluated histologically. Regenerated follicles and labelled DPCs were detected for all methods, although some follicles showed abnormal growth, i.e. a cystic or inverted appearance. The HVS method, pioneered here, resulted in significantly larger number of regenerated follicles that were more mature and regular than those observed using the other methods. Moreover, hair growth was detected after expanded adult-derived DPC transplantation using the HVS method. These results suggest that direct contact of epithelial and dermal components and better vascularization/oxygenation of the recipient site are critical for hair regeneration in cell-based therapies.

A simplified procedure to reconstitute hair-producing skin

One of the major objectives of tissue engineering is to reconstitute skin from stem cells. This requires multipotent skin stem cells and the ability to guide these cells to form a piece of skin with proper architecture and skin appendages. Based on previous progress, we develop a simplified procedure that can be useful for large-scale screening of factors that can modulate the hair formation ability of candidate cells. Newborn mouse cells are used. Dissociated epidermal and dermal cells in high-density suspension are allowed to reconstitute in vitro to generate its own matrix, or seeded into a scaffold-like matrix already used clinically. These cells self-organize and form a reconstituted skin with proper proportions and topological organization of different components. Large numbers of hair follicles form. The cellular and molecular events are characterized, showing a distinct but parallel morphogenetic process compared to those occurring in embryonic development. The formed hair follicles can cycle and regenerate and the reconstituted skin can heal after injury. The skins are in good condition 1 year after transplant. This procedure enables flexible size and shape of the reconstituted skin, so clinical applications can be envisioned for the future when large numbers of multipotential skin stem cells become available.

c-Met-induced epithelial carcinogenesis is initiated by the serine protease matriptase

The progression and negative outcome of a variety of human carcinomas are intimately associated with aberrant activity of the c-Met oncogene. The underlying cause of this dysregulation, however, remains a subject of discussion, as the majority of cancer patients do not present with activating mutations in c-Met receptor itself. In this study, we show that the oncogenic protease matriptase is ubiquitously co-expressed with the c-Met in human squamous cell carcinomas and amplifies migratory and proliferative responses of primary epithelial cells to the cognate ligand for c-Met, pro-hepatocyte growth factor/scatter factor (proHGF/SF), through c-Met and Gab1 signaling. Furthermore, the selective genetic ablation of c-Met from matriptase-expressing keratinocytes completely negates the oncogenic potential of matriptase. In addition, matriptase-dependent carcinoma formation could be blocked by the pharmacological inhibition of the Akt-mammalian target of Rapamycin (mTor) pathway. Our data identify matriptase as an initiator of c-Met-Akt-mTor-dependent signaling axis in tumors and reveal mTor activation as an essential component of matriptase/c-Met-induced carcinogenesis. The study provides a specific example of how epithelial transformation can be promoted by epigenetic acquisition of the capacity to convert a widely available paracrine growth factor precursor to its signaling competent state.

Tissue-engineered skin preserving the potential of epithelial cells to differentiate into hair after grafting

The aim of this study was to evaluate whether tissue-engineered skin produced in vitro was able to sustain growth of hair follicles in vitro and after grafting. Different tissues were designed. Dissociated newborn mouse keratinocytes or newborn mouse hair buds (HBs) were added onto dermal constructs consisting of a tissue-engineered cell-derived matrix elaborated from either newborn mouse or adult human fibroblasts cultured with ascorbic acid. After 7-21 days of maturation at the air-liquid interface, no hair was noticed in vitro. Epidermal differentiation was observed in all tissue-engineered skin. However, human fibroblast-derived tissue-engineered dermis (hD) promoted a thicker epidermis than mouse fibroblast-derived tissue-engineered dermis (mD). In association with mD, HBs developed epithelial cyst-like inclusions presenting outer root sheath-like attributes. In contrast, epidermoid cyst-like inclusions lined by a stratified squamous epithelium were present in tissues composed of HBs and hD. After grafting, pilo-sebaceous units formed and hair grew in skin elaborated from HBs cultured 10-26 days submerged in culture medium in association with mD. However, the number of normal hair follicles decreased with longer culture time. This hair-forming capacity after grafting was not observed in tissues composed of hD overlaid with HBs. These results demonstrate that epithelial stem cells can be kept in vitro in a permissive tissue-engineered dermal environment without losing their potential to induce hair growth after grafting.

Functionally distinct melanocyte populations revealed by reconstitution of hair follicles in mice

Hair follicle reconstitution analysis was used to test the contribution of melanocytes or their precursors to regenerated hair follicles. In this study, we first confirmed the process of chimeric hair follicle regeneration by both hair keratinocytes and follicular melanocytes. Then, as first suggested from the differential growth requirements of epidermal skin melanocytes and non-cutaneous or dermal melanocytes, we confirmed the inability of the latter to be involved as follicular melanocytes to regenerate hair follicles during the hair reconstitution assay. This clear functional discrimination between non-cutaneous or dermal melanocytes and epidermal melanocytes suggests the presence of two different melanocyte cell lineages, a finding that might be important in the pathogenesis of melanocyte-related diseases and melanomas.

Contact between dermal papilla cells and dermal sheath cells enhances the ability of DPCs to induce hair growth

We previously showed that cultured rat dermal papilla cells (DPCs) retain their hair-inducing capacity on afollicular epidermal cell (EPCs). Here, we examined the hair growth-inducing capacity of differently subcultured DPCs by transplanting them, along with rat EPCs, onto the backs of nude mice (graft chamber assay). DPCs at passage (p) 6 (DPCs(p6) or, more generally, low-passage DPCs) induced hair formation. However, DPCs(p>30) (high-passage DPCs) had no such activity and induced only subepidermal hair follicles (HFs) that were not encapsulated by the dermal sheath (DS). Thus, we examined the effect of DS cells (DSCs(p=1)) on the ability of DPCs(p=60) to induce hair growth by testing a mixture of these two cell types (cotransplant) in the graft chamber assay, in which DSCs(p=1) and DPCs(p=60) were labeled with enhanced green fluorescent protein (EGFP) and 1,1-dioctadecyl-3,3,3,3-tetramethylindocarbocyanine perchlorate (DiI), respectively. These cotransplants generated hairs as actively as did DPCs(p=6) transplants. Their HFs were encapsulated with EGFP(+)-DS and had DPs consisting largely of EGFP(+)-DPCs (47%) and DiI(+)-DPCs (43%), indicating a major contribution of DSC(p=1)-derived DPCs to HF induction. In addition, the results of in vitro coculture of DPCs(p=60) and DSCs(p=1) suggest that high-passage DPCs stimulate the expression of certain trichogenic genes in DSCs.

Compartmentalized organization: a common and required feature of stem cell niches?

A key question in the stem cell field is how to balance the slow cycling of stem cells with active organ growth. Recent studies of the hair follicle stem cell niche have shown that this can be achieved by organizing the stem cell niche into two compartments: one that engages in immediate, rapid new growth and one that contributes later to long-term growth that fuels hair regeneration. Based on these and other recent findings, we propose that several other adult stem cell niches, including those in the blood, intestine and brain, have a similar bi-compartmental organization and that stem cells might work cooperatively with their progeny to sustain tissue regeneration.

Review of hair follicle dermal cells

Hair follicle stem cells in the epithelial bulge are responsible for the continual regeneration of the hair follicle during cycling. The bulge cells reside in a niche composed of dermal cells. The dermal compartment of the hair follicle consists of the dermal papilla and dermal sheath. Interactions between hair follicle epithelial and dermal cells are necessary for hair follicle morphogenesis during development and in hair reconstitution assays. Dermal papilla and dermal sheath cells express specific markers and possess distinctive morphology and behavior in culture. These cells can induce hair follicle differentiation in epithelial cells and are required in hair reconstitution assays either in the form of intact tissue, dissociated freshly prepared cells or cultured cells. This review will focus on hair follicle dermal cells since most therapeutic efforts to date have concentrated on this aspect of the hair follicle, with the idea that enriching hair-inductive dermal cell populations and expanding their number by culture while maintaining their properties, will establish an efficient hair reconstitution assay that could eventually have therapeutic implications.

Dermal papilla cells induce keratinocyte tubulogenesis in culture

The ability of dermal papilla (DP) cells to induce hair growth was reported in many studies. However, early stages of hair follicle development and signals that govern this process are poorly understood. Therefore, an in vitro model may be a convenient system to study epithelial-mesenchymal interactions and early stages of epidermal morphogenesis, especially in humans. To investigate the role of DP cells in epidermal morphogenesis we modified the method of isolation of DP cells from hair follicle of human scalp and developed the three-dimensional model of epidermal morphogenesis. Isolated DP cells were able to differentiate in adipogenic and osteogenic directions and retained activity of alkaline phosphatase (AP) for seven passages in culture. DP cells were able to induce tubule-like structures in three-dimensional model in vitro and to reorganize collagen matrix. Prolonged cultivation of DP cells has been a big problem because of the loss of hair follicle-inducing ability and growth activity after several passages. To solve this problem we immortalized DP cells by the transfection of the human telomerase reverse transcriptase cDNA (hTERT). Immortalized DP-hTERT cells retained AP activity and demonstrated low ability to osteogenic differentiation. The conditioned medium collected from actively proliferated cells as well as DP-hTERT cells themselves were capable to induce tubulogenesis after prolonged keratinocyte cultivation.

High-throughput reconstitution of epithelial-mesenchymal interaction in folliculoid microtissues by biomaterial-facilitated self-assembly of dissociated heterotypic adult cells

The aim of this study was to develop a method for efficient production of folliculoid keratinocyte-dermal papilla (DP) microtissues to facilitate epithelial-mesenchymal interaction. The behavior of DP cells and adult keratinocytes from hairless skin on poly(ethylene-co-vinyl alcohol) (EVAL) surface was investigated. Keratinocytes, poorly adherent both to substrate and between homotypic cells, become suspended disperse cells after homotypic cell seeding. Seeded simultaneously, keratinocytes and DP cells are able to aggregate into spheroidal microtissues. Dynamical analysis shows that DP cells act as a carrier in the process due to the heterotypic intercellular adhesion. DP cells attach faster to EVAL and start to aggregate. Keratinocytes adhere to DP cells and are then carried by DP cells to form initial hybrid aggregates. Due to the high motility of DP cells, these hybrid aggregates move collectively as clusters and merge into larger spheroids which subsequently detach from the substratum and can be easily collected. Compared with random cell distribution in spheroids generated in hanging drops, these hybrid spheroids have a preferential compartmented core-shell structure: an aggregated DP cell core surrounded by a keratinocyte shell. In addition to ameliorated DP signature gene expression, keratinocytes show down-regulated epidermal terminal differentiation and enhanced follicular differentiation. Functionally, these microtissues are able to grow hairs in vivo. This work sheds light on the complex effects and dynamics of cell-cell and cell-substratum interaction in the patterning of heterotypic cells into tissue forms and is of potential to be applied to mass generation of other epithelial organ primordia in vitro.

Self-assembly of dermal papilla cells into inductive spheroidal microtissues on poly(ethylene-co-vinyl alcohol) membranes for hair follicle regeneration

Self-aggregation is key to hair follicle (HF) induction ability of dermal papilla (DP) cells and neogenesis of HF can be achieved by transplanting DP microtissues. However, there is currently lack of a suitable system that allows efficient production of DP microtissues and analysis of DP self-aggregation in vitro. We demonstrate that, at a higher seeding cell density, poly(ethylene-co-vinyl alcohol) (EVAL) membranes facilitate DP self-assembly into many compact spheroidal microtissues that are able to induce new HFs. This self-assembling process is associated with an enhanced cell movement and a declined cell-substrate adhesivity on EVAL. A compromised cell growth is also revealed on EVAL. On the contrary, a more adherent surface allows faster cell expansion but maintains DP cells in a flat morphology. Dynamically, cell migration, intercellular collision and intercellular adhesion contribute to DP microtissue formation on EVAL. Our results suggest that, for large-scale production of DP microtissues for HF regeneration, an adhesive surface is needed for quick cell expansion and a biomaterial with a lower adhesivity is required for self-aggregation. In addition, this system can be a model for investigation of DP self-aggregation in vitro.

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

Hair follicle (HF) formation is initiated when epithelial stem cells receive cues from specialized mesenchymal dermal papilla (DP) cells. In culture, DP cells lose their HF-inducing properties, but during hair growth in vivo, they reside within the HF bulb and instruct surrounding epithelial progenitors to orchestrate the complex hair differentiation program. To gain insights into the molecular program that maintains DP cell fate, we previously purified DP cells and four neighboring populations and defined their cell-type-specific molecular signatures. Here, we exploit this information to show that the bulb microenvironment is rich in bone morphogenetic proteins (BMPs) that act on DP cells to maintain key signature features in vitro and hair-inducing activity in vivo. By employing a novel in vitro/in vivo hybrid knockout assay, we ablate BMP receptor 1a in purified DP cells. When DPs cannot receive BMP signals, they lose signature characteristics in vitro and fail to generate HFs when engrafted with epithelial stem cells in vivo. These results reveal that BMP signaling, in addition to its key role in epithelial stem cell maintenance and progenitor cell differentiation, is essential for DP cell function, and suggest that it is a critical feature of the complex epithelial-mesenchymal cross-talk necessary to make hair.