Wnt/ß-catenin signaling in dermal condensates is required for hair follicle formation.

Broad dermal Wnt signaling is required for patterned induction of hair follicle placodes and subsequent Wnt signaling in placode stem cells is essential for induction of dermal condensates, cell clusters of precursors for the hair follicle dermal papilla (DP). Progression of hair follicle formation then requires coordinated signal exchange between dermal condensates and placode stem cells. However, it remains unknown whether continued Wnt signaling in DP precursor cells plays a role in this process, largely due to the long-standing inability to specifically target dermal condensates for gene ablation. Here we use the Tbx18(Cre) knockin mouse line to ablate the Wnt-responsive transcription factor ß-catenin specifically in these cells at E14.5 during the first wave of guard hair follicle formation. In the absence of ß-catenin, canonical Wnt signaling is effectively abolished in these cells. Sox2(+) dermal condensates initiate normally; however by E16.5 guard hair follicle numbers are strongly reduced and by E18.5 most whiskers and guard hair follicles are absent, suggesting that active Wnt signaling in dermal condensates is important for hair follicle formation to proceed after induction. To explore the molecular mechanisms by which Wnt signaling in dermal condensates regulates hair follicle formation, we analyze genome-wide the gene expression changes in embryonic ß-catenin null DP precursor cells. We find altered expression of several signaling pathway genes, including Fgfs and Activin, both previously implicated in hair follicle formation. In summary, these data reveal a functional role of Wnt signaling in DP precursors for embryonic hair follicle formation and identify Fgf and Activin signaling as potential effectors of Wnt signaling-regulated events.

PDGF signalling in the dermis and in dermal condensates is dispensable for hair follicle induction and formation.

Embryonic hair follicle (HF) induction and formation is dependent on signalling crosstalk between the dermis and specialized dermal condensates on the mesenchymal side and epidermal cells and incipient placodes on the epithelial side, but the precise nature and succession of signals remain unclear. Platelet-derived growth factor (PDGF) signalling is involved in the development of several organs and the maintenance of adult tissues, including HF regeneration in the hair cycle. As both PDGF receptors, PDGFRα and PDGFRß, are expressed in embryonic dermis and dermal condensates, we explored in this study the role of PDGF signalling in HF induction and formation in the developing skin mesenchyme. We conditionally ablated both PDGF receptors with Tbx18(Cre) in early dermal condensates before follicle formation, and with Prx1-Cre broadly in the ventral dermis prior to HF induction. In both PDGFR double mutants, HF induction and formation ensued normally, and the pattern of HF formation and HF numbers were unaffected. These data demonstrate that mesenchymal PDGF signalling, either in the specialized niche or broadly in the dermis, is dispensable for HF induction and formation.

Cxcr4 is transiently expressed in both epithelial and mesenchymal compartments of nascent hair follicles but is not required for follicle formation.

Hair follicle (HF) morphogenesis relies on the coordinated exchange of signals between mesenchymal and epithelial compartments of embryonic skin. Chemokine receptor Cxcr4 expression was recently identified in dermal condensates (DCs) of nascent HFs, but its role in promoting HF morphogenesis remains unknown. Our analyses confirmed Cxcr4 expression in condensate cells, and additionally revealed transient Cxcr4 expression in incipient epithelial hair placodes. Placodal Cxcr4 appeared prior to detection in DCs, representing a switch of expression between epithelial and mesenchymal compartments. To explore the functional role of this receptor in both compartments for early HF formation, we conditionally ablated Cxcr4 with condensate-targeting Tbx18(cre) knock-in and epidermis-targeting Krt14-cre transgenic mice. Conditional knockouts for both crosses were viable throughout embryogenesis and into adulthood. Morphological and biochemical marker analyses revealed comparable numbers of HFs forming in knockout embryos compared to wild-type littermate controls in both cases, suggesting that neither dermal nor epithelial Cxcr4 expression is required for early HF morphogenesis. We conclude that Cxcr4 expression and chemokine signaling through this receptor in embryonic mouse skin is dispensable for HF formation.

In vitro quantification of pigment production and transfer in 2D co-cultures and 3D skin organotypic.

Here, we present a protocol to set up and study 2D keratinocyte-melanocyte co-cultures and 3D full-thickness human skin equivalents. We describe steps for culturing of keratinocyte and melanocyte lines and the establishment of both 2D and 3D co-cultures. The cultures are utilized to measure melanin content and investigate mechanisms driving melanin production and transfer, through flow cytometry and immunohistochemistry. Culture conditions are highly amendable to different conditions, and analysis is simple and objective-thus allowing for medium to high throughput. For complete details on the use and execution of this protocol, please refer to Ng et al. (2022).1.

Differentiation potential of human postnatal mesenchymal stem cells, mesoangioblasts, and multipotent adult progenitor cells reflected in their transcriptome and partially influenced by the culture conditions.

Several adherent postnatal stem cells have been described with different phenotypic and functional properties. As many of these cells are being considered for clinical therapies, it is of great importance that the identity and potency of these products is validated. We compared the phenotype and functional characteristics of human mesenchymal stem cells (hMSCs), human mesoangioblasts (hMab), and human multipotent adult progenitor cells (hMAPCs) using uniform standardized methods. Human MAPCs could be expanded significantly longer in culture. Differences in cell surface marker expression were found among the three cell populations with CD140b being a distinctive marker among the three cell types. Differentiation capacity towards adipocytes, osteoblasts, chondrocytes, and smooth muscle cells in vitro, using established protocols, was similar among the three cell types. However, only hMab differentiated to skeletal myocytes, while only hMAPCs differentiated to endothelium in vitro and in vivo. A comparative transcriptome analysis confirmed that the three cell populations are distinct and revealed gene signatures that correlated with their specific functional properties. Furthermore, we assessed whether the phenotypic, functional, and transcriptome features were mediated by the culture conditions. Human MSCs and hMab cultured under MAPC conditions became capable of generating endothelial-like cells, whereas hMab lost some of their ability to generate myotubes. By contrast, hMAPCs cultured under MSC conditions lost their endothelial differentiation capacity, whereas this was retained when cultured under Mab conditions, however, myogenic capacity was not gained under Mab conditions. These studies demonstrate that hMSCs, hMab, and hMAPCs have different properties that are partially mediated by the culture conditions.

Sox2 in the dermal papilla regulates hair follicle pigmentation.

Within the hair follicle (HF) niche, dermal papilla (DP) cells are well known for the hair induction capacity; however, DP cell signaling also regulates HF pigmentation. Here we describe how Sox2 in the DP is a key regulator of melanocyte signaling. To study the largely unknown regulatory role the DP has on hair pigmentation, we characterize leptin receptor (Lepr) expression in the skin and as a genetic tool to target the DP. Sox2 ablation in the DP results in a phenotypic switch from eumelanin to pheomelanin. Mechanistically, we describe a temporal upregulation of Agouti and downregulation of Corin, directly by Sox2 in the DP. We also show that bone morphogenic protein (BMP) signaling regulation by Sox2 is responsible for downregulating MC1R, Dct, and Tyr in melanocytes of Sox2 cKO mice. Thus, we demonstrate that Sox2 in the DP regulates not only the choice of hair pigment but also the overall HF pigment production.

Oct4 and klf4 reprogram dermal papilla cells into induced pluripotent stem cells.

Direct reprogramming of somatic cells into induced pluripotent stem (iPS) cells by only four transcription factors (Oct4, Sox2, Klf4, and c-Myc) has great potential for tissue-specific regenerative therapies, eliminating the ethical issues surrounding the use of embryonic stem cells and the rejection problems of using non-autologous cells. The reprogramming efficiency generally is very low, however, and the problems surrounding the introduction of viral genetic material are only partially investigated. Recent efforts to reduce the number of virally expressed transcription factors succeeded at reprogramming neural stem cells into iPS cells by overexpressing Oct4 alone. However, the relative inaccessibility and difficulty of obtaining neural cells in humans remains to be resolved. Here we report that dermal papilla (DP) cells, which are specialized skin fibroblasts thought to instruct hair follicle stem cells, endogenously express high levels of Sox2 and c-Myc, and that these cells can be reprogrammed into iPS cells with only Oct4 and Klf4. Moreover, we show that DP cells are reprogrammed more efficiently than skin and embryonic fibroblasts. iPS cells derived from DP cells expressed pluripotency genes and differentiated into cells from all germ layers in vitro and widely contributed to chimeric mice in vivo, including the germline. Our work establishes DP cells as an easily accessible source to generate iPS cells with efficiency and with less genetic material. This opens up the possibility of streamlined generation of skin-derived, patient-specific pluripotent stem cells and of ultimately replacing the remaining two factors with small molecules for safe generation of transplantable cells.

Multipotent adult progenitor cell isolation and proliferation in cytokine and serum-free medium conditioned by rat B104 cells.

This report describes the isolation of rodent multipotent adult progenitor cells (MAPCs) and proliferation of these cells in both standard medium and medium without exogenous serum or growth factors conditioned by the rat cell line B104. MAPCs have exacting requirements for their proliferation in vitro but once established proliferate rapidly at low seeding density, requiring almost daily passage and media exchange. Previously published methods for growth of MAPCs in vitro all used media supplemented with serum and growth factors, which adds considerable expense.

Bone-marrow-derived cells and heart repair.

PURPOSE OF REVIEW: Cardiovascular diseases are the leading cause of morbidity/mortality in the world. Despite significant progress in cardiovascular medicine, mortality rates have remained steady during the past decade. Therefore, investigators are evaluating novel methods to repair/regenerate the damaged heart. RECENT FINDINGS: Within the past decade, stem cell therapy strategies have been evaluated for the treatment of cardiovascular diseases. Among all postnatal stem cell compartments, bone marrow has been most extensively studied. Recent studies using bone marrow stem cells have suggested that this approach may have beneficial effects on cardiac function, probably by enhancing new cell formation from endogenous cardiac and endothelial stem/progenitor cells. SUMMARY: The present review critically assesses the possible beneficial effects of bone-marrow-derived cell therapy in cardiac ischemia, demonstrating benefits through as yet incompletely understood mechanisms.

Multipotent Adult Progenitor Cells (MAPC) contribute to hepatocarcinoma neovasculature.

The use of stem cells as a vehicle of therapeutic genes is an attractive approach for the development of new antitumoral strategies based on gene therapy. The aim of our study was to assess the potential of bone marrow-derived Multipotent Adult Progenitor Cells (rMAPCs) to differentiate in vitro and in vivo into endothelial cells and to be recruited to areas of tumor vasculogenesis. In vitro, rMAPCs obtained from Buffalo rats differentiated into cells expressing endothelial markers and demonstrated functional endothelial capacity. Intravenous injection of undifferentiated rMAPC transduced with a lentivirus expressing GFP in an orthotopic rat model of hepatocellular carcinoma, resulted in tumor recruitment of the injected cells and in vivo differentiation into endothelial cells in the tumor area with contribution to vasculogenesis. In summary, our results suggest that rMAPCs can be efficiently recruited by vascularized tumors and differentiate to endothelium and thus may represent a useful vehicle for delivery of therapeutic genes to sites of active tumor neovascularization.

Loss of lamin B1 is a biomarker to quantify cellular senescence in photoaged skin.

Skin ageing is an inevitable consequence of life and accelerated by exposure to ultraviolet (UV) rays. Senescence is an irreversible growth arrest and senescent cells accumulate in ageing tissues, at sites of age-related pathologies and in pre-neoplastic lesions. Conventionally, senescent cells have been detected by senescence associated-ß-galactosidase (SA-ß-gal) staining, a procedure that requires enzymatic activity, which is lost in fixed tissue samples. We previously demonstrated that loss of lamin B1 is a novel marker to identify senescent cells. Here, we demonstrate that loss of lamin B1 facilitates the detection and quantification of senescent cells upon UV-exposure in vitro and upon chronic UV-exposure and skin regeneration in vivo. Taken together, this marker enables the study of environmental conditions on tissue ageing and regeneration in vivo, serves as a diagnostic tool to distinguish senescent from proliferating cells in pre-neoplastic lesions, and facilitates investigating the role of senescent cells in various age-related pathologies.

Signaling Networks among Stem Cell Precursors, Transit-Amplifying Progenitors, and their Niche in Developing Hair Follicles.

The hair follicle (HF) is a complex miniorgan that serves as an ideal model system to study stem cell (SC) interactions with the niche during growth and regeneration. Dermal papilla (DP) cells are required for SC activation during the adult hair cycle, but signal exchange between niche and SC precursors/transit-amplifying cell (TAC) progenitors that regulates HF morphogenetic growth is largely unknown. Here we use six transgenic reporters to isolate 14 major skin and HF cell populations. With next-generation RNA sequencing, we characterize their transcriptomes and define unique molecular signatures. SC precursors, TACs, and the DP niche express a plethora of ligands and receptors. Signaling interaction network analysis reveals a bird's-eye view of pathways implicated in epithelial-mesenchymal interactions. Using a systematic tissue-wide approach, this work provides a comprehensive platform, linked to an interactive online database, to identify and further explore the SC/TAC/niche crosstalk regulating HF growth.

Zfp281 Coordinates Opposing Functions of Tet1 and Tet2 in Pluripotent States.

Pluripotency is increasingly recognized as a spectrum of cell states defined by their growth conditions. Although naive and primed pluripotency states have been characterized molecularly, our understanding of events regulating state acquisition is wanting. Here, we performed comparative RNA sequencing of mouse embryonic stem cells (ESCs) and defined a pluripotent cell fate (PCF) gene signature associated with acquisition of naive and primed pluripotency. We identify Zfp281 as a key transcriptional regulator for primed pluripotency that also functions as a barrier toward achieving naive pluripotency in both mouse and human ESCs. Mechanistically, Zfp281 interacts with Tet1, but not Tet2, and its direct transcriptional target, miR-302/367, to negatively regulate Tet2 expression to establish and maintain primed pluripotency. Conversely, ectopic Tet2 alone, but not Tet1, efficiently reprograms primed cells toward naive pluripotency. Our study reveals a molecular circuitry in which opposing functions of Tet1 and Tet2 control acquisition of alternative pluripotent states.

An Integrated Transcriptome Atlas of Embryonic Hair Follicle Progenitors, Their Niche, and the Developing Skin.

Defining the unique molecular features of progenitors and their niche requires a genome-wide, whole-tissue approach with cellular resolution. Here, we co-isolate embryonic hair follicle (HF) placode and dermal condensate cells, precursors of adult HF stem cells and the dermal papilla/sheath niche, along with lineage-related keratinocytes and fibroblasts, Schwann cells, melanocytes, and a population inclusive of all remaining skin cells. With next-generation RNA sequencing, we define gene expression patterns in the context of the entire embryonic skin, and through transcriptome cross-comparisons, we uncover hundreds of enriched genes in cell-type-specific signatures. Axon guidance signaling and many other pathway genes are enriched in multiple signatures, implicating these factors in driving the large-scale cellular rearrangements necessary for HF formation. Finally, we share all data in an interactive, searchable companion website. Our study provides an overarching view of signaling within the entire embryonic skin and captures a molecular snapshot of HF progenitors and their niche.

Enpp2/Autotaxin in dermal papilla precursors is dispensable for hair follicle morphogenesis.

Systematic ablation of previously identified dermal papilla (DP) signature genes in embryonic DP precursors will reveal their functional roles during hair follicle morphogenesis. In this study, we validate Enpp2/Autotaxin as one of the highest expressed signature genes in postnatal DP, and demonstrate specific expression of this lysophosphatidic acid (LPA)-generating enzyme in embryonic dermal condensates. We further identify dermal and epidermal expression of several LPA receptors, suggesting that LPA signaling could contribute to follicle morphogenesis in both mesenchymal and epithelial compartments. We then use the recently characterized Cre-expressing Tbx18 knock-in line to conditionally ablate Enpp2 in embryonic DP precursors. Despite efficient gene knockout in embryonic day 14.5 (E14.5) dermal condensates, morphogenesis proceeds regularly with normal numbers, lengths, and sizes of all hair follicle types, suggesting that Enpp2 is not required for hair follicle formation. To interrogate DP signature gene expression, we finally isolate control and Enpp2-null DP precursors and identify the expression and upregulation of LIPH, an alternative LPA-producing enzyme, suggesting that this gene could functionally compensate for the absence of Enpp2. We conclude that future coablation of both LPA-producing enzymes or of several LPA receptors may reveal the functional role of LPA signaling during hair follicle morphogenesis.

Tbx18 targets dermal condensates for labeling, isolation, and gene ablation during embryonic hair follicle formation.

How cell fate decisions of stem and progenitor cells are regulated by their microenvironment or niche is a central question in stem cell and regenerative biology. Although functional analysis of hair follicle epithelial stem cells by gene targeting is well established, the molecular and genetic characterization of the dermal counterpart during embryonic morphogenesis has been lacking because of the absence of cell type-specific drivers. Here, we report that T-box transcription factor Tbx18 specifically marks dermal papilla (DP) precursor cells during embryonic hair follicle morphogenesis. With Tbx18(LacZ), Tbx18(H2BGFP), and Tbx18(Cre) knock-in mouse models, we demonstrate LacZ and H2BGFP (nuclear green fluorescent protein) expression and Cre activity in dermal condensates of nascent first-wave hair follicles at E14.5. As Tbx18 expression becomes more widespread throughout the dermis at later developmental stages, we use tamoxifen-inducible Cre-expressing mice, Tbx18(MerCreMer), to exclusively target DP precursor cells and their progeny. Finally, we ablate Tbx18 in full knockout mice, but find no perturbations in hair follicle formation, suggesting that Tbx18 is dispensable for normal DP function. In summary, our study establishes Tbx18 as a genetic driver to target for the first time embryonic DP precursors for labeling, isolation, and gene ablation that will greatly enhance investigations into their molecular functions during hair follicle morphogenesis.

Sox2 in the dermal papilla niche controls hair growth by fine-tuning BMP signaling in differentiating hair shaft progenitors.

How dermal papilla (DP) niche cells regulate hair follicle progenitors to control hair growth remains unclear. Using Tbx18(Cre) to target embryonic DP precursors, we ablate the transcription factor Sox2 early and efficiently, resulting in diminished hair shaft outgrowth. We find that DP niche expression of Sox2 controls the migration speed of differentiating hair shaft progenitors. Transcriptional profiling of Sox2 null DPs reveals increased Bmp6 and decreased BMP inhibitor Sostdc1, a direct Sox2 transcriptional target. Subsequently, we identify upregulated BMP signaling in knockout hair shaft progenitors and demonstrate that Bmp6 inhibits cell migration, an effect that can be attenuated by Sostdc1. A shorter and Sox2-negative hair type lacks Sostdc1 in the DP and shows reduced migration and increased BMP activity of hair shaft progenitors. Collectively, our data identify Sox2 as a key regulator of hair growth that controls progenitor migration by fine-tuning BMP-mediated mesenchymal-epithelial crosstalk.

Plasticity and cardiovascular applications of multipotent adult progenitor cells.

Cardiovascular disease is the leading cause of death worldwide, which has encouraged the search for new therapies that enable the treatment of patients in palliative and curative ways. In the past decade, the potential benefit of transplantation of cells that are able to substitute for the injured tissue has been studied with several cell populations, such as stem cells. Some of these cell populations, such as myoblasts and bone marrow cells, are already being used in clinical trials. The laboratory of CM Verfaillie has studied primitive progenitors, termed multipotent adult progenitor cells, which can be isolated from adult bone marrow. These cells can differentiate in vitro at the single-cell level into functional cells that belong to the three germ layers and contribute to most, if not all, somatic cell types after blastocyst injection. This remarkably broad differentiation potential makes this particular cell population a candidate for transplantation in tissues in need of regeneration. Here, we focus on the regenerative capacity of multipotent adult progenitor cells in several ischemic mouse models, such as acute and chronic myocardial infarction and limb ischemia.

In vitro and in vivo arterial differentiation of human multipotent adult progenitor cells.

Many stem cell types have been shown to differentiate into endothelial cells (ECs); however, their specification to arterial or venous endothelium remains unexplored. We tested whether a specific arterial or venous EC fate could be induced in human multipotent adult progenitor cells (hMAPCs) and AC133(+) cells (hAC133(+)). In vitro, in the presence of VEGF(165), hAC133(+) cells only adopted a venous and microvascular EC phenotype, while hMAPCs differentiated into both arterial and venous ECs, possibly because hMAPCs expressed significantly more sonic hedgehog (Shh) and its receptors as well as Notch 1 and 3 receptors and some of their ligands. Accordingly, blocking either of those pathways attenuated in vitro arterial EC differentiation from hMAPCs. Complementarily, stimulating these pathways by addition of Delta-like 4 (Dll-4), a Notch ligand, and Shh to VEGF(165) further boosted arterial differentiation in hMAPCs both in vitro and in an in vivo Matrigel model. These results represent the first demonstration of adult stem cells with the potential to be differentiated into different types of ECs in vitro and in vivo and provide a useful human model to study arteriovenous specification.