Carlos Clavel

Hair follicle dermal papilla cells at a glance

Mammalian skin is a highly tractable tissue in which to explore epithelial–mesenchymal interactions during development and in postnatal life ([Blanpain and Fuchs, 2009][1]; [Muller-Rover et al., 2001][2]; [Schmidt-Ullrich and Paus, 2005][3]; [Watt and Jensen, 2009][4]). One population of

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. STEM CELLS 2011;29:871–882

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. STEM CELLS 2010;28:221–228

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.

Wnt/β-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 β-catenin specifically in these cells at E14.5 during the first wave of guard hair follicle formation. In the absence of β-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 β-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.

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. While 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 due to 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 Tbx18LacZ, Tbx18H2BGFP and Tbx18Cre knock-in mouse models we demonstrate LacZ/GFP expression and Cre activity in dermal condensates of nascent first-wave hair follicles at E14.5. Since Tbx18 expression becomes more widespread throughout the dermis at later developmental stages, we utilize tamoxifen-inducible Cre expressing mice, Tbx18MerCreMer, 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 embryonic DP precursors for labeling, isolation and gene ablation that will greatly enhance investigations into their molecular functions during hair follicle morphogenesis.

Bone-marrow-derived cells and heart repair.

Purpose of reviewCardiovascular 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 findingsWithin 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. SummaryThe 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.

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.

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 PDGFRβ, 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 Tbx18Cre 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.

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 utilize the recently characterized Cre-expressing Tbx18 knock-in line to conditionally ablate Enpp2 in embryonic DP precursors. Despite efficient gene knockout in 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 co-ablation of both LPA producing enzymes or of several LPA receptors may reveal the functional role of LPA signaling during hair follicle morphogenesis.