Generation of heterozygous and homozygous NF1 lines from human-induced pluripotent stem cells using CRISPR/Cas9 to investigate bone defects associated with neurofibromatosis type 1.

Neurofibromatosis type 1 (NF1) is one of the most common genetic disorders caused by heterozygous germline NF1 mutations. NF1 affects many systems, including the skeletal system. To date, no curative therapies are available for skeletal manifestations such as scoliosis and tibial dysplasia, mainly due to the lack of knowledge about the mechanisms that underlie this process. By using CRISPR/Cas9-mediated gene editing in human-induced pluripotent stem cells (hiPSCs) to minimize the variability due to genetic background and epigenetic factors, we generated isogenic heterozygous and homozygous NF1-deficient hiPSC lines to investigate the consequences of neurofibromin inactivation on osteoblastic differentiation. Here, we demonstrate that loss of one or both copies of NF1 does not alter the potential of isogenic hiPSCs to differentiate into mesenchymal stem cells (hiPSC-MSCs). However, NF1 (+/-) and NF1 (-/-) hiPSC-MSCs show a defect in osteogenic differentiation and mineralization. In addition, we show that a mono-allelic deletion in NF1 in an isogenic context is sufficient to impair cell differentiation into osteoblasts. Overall, this study highlights the relevance of generating isogenic lines, which may help in genotype-phenotype correlation and provide a human cellular model to understand the molecular mechanisms underlying NF1 and, thus, discover new therapeutic strategies.

Generating Functional and Highly Proliferative Melanocytes Derived from Human Pluripotent Stem Cells: A Promising Tool for Biotherapeutic Approaches to Treat Skin Pigmentation Disorders.

Melanocytes are essential for skin homeostasis and protection, and their loss or misfunction leads to a wide spectrum of diseases. Cell therapy utilizing autologous melanocytes has been used for years as an adjunct treatment for hypopigmentary disorders such as vitiligo. However, these approaches are hindered by the poor proliferative capacity of melanocytes obtained from skin biopsies. Recent advances in the field of human pluripotent stem cells have fueled the prospect of generating melanocytes. Here, we have developed a well-characterized method to produce a pure and homogenous population of functional and proliferative melanocytes. The genetic stability and potential transformation of melanocytes from pluripotent stem cells have been evaluated over time during the in vitro culture process. Thanks to transcriptomic analysis, the molecular signatures all along the differentiation protocol have been characterized, providing a solid basis for standardizing the protocol. Altogether, our results promise meaningful, broadly applicable, and longer-lasting advances for pigmentation disorders and open perspectives for innovative biotherapies for pigment disorders.

Human-Induced Pluripotent Stem Cell‒Derived Keratinocytes, a Useful Model to Identify and Explore the Pathological Phenotype of Epidermolysis Bullosa Simplex.

Epidermolysis bullosa simplex (EBS), an autosomal dominant skin disorder, is characterized by skin fragility. Genetically, the majority of cases are related to missense sequence variations in two keratin genes K5 or K14, leading to cytolysis of basal keratinocytes (KCs) and intraepidermal blistering. Progress toward the identification of treatments has been hampered by an incomplete understanding of the mechanisms underlying this disease and availability of relevant and reliable in vitro models recapitulating the physiopathological mechanisms. Recent advances in stem cell field have fueled the prospect that these limitations could be overcome, thanks to the availability of disease-specific human induced pluripotent stem cells (hiPSCs). In this study, we generated hiPSC-derived KCs from patients carrying keratin gene K5-dominant sequence variations and compared them with nonaffected hiPSC-derived KCs as well as their primary counterparts. Our results showed that EBS hiPSC-derived KCs displayed proliferative defects, increased capacity to migrate, alteration of extracellular signal‒regulated kinase signaling pathway, and cytoplasmic keratin filament aggregates as observed in primary EBS KCs. Of interest, EBS hiPSC-derived KCs exhibited downregulation of hemidesmosomal proteins, revealing the different effects of keratin gene K5 sequence variations on keratin cytoskeletal organization. With a combination of culture miniaturization and treatment with the chaperone molecule 4-phenylbutyric acid, our results showed that hiPSC-derived KCs represent a suitable model for identifying novel therapies for EBS.

Clinical Grade Human Pluripotent Stem Cell-Derived Engineered Skin Substitutes Promote Keratinocytes Wound Closure In Vitro.

Chronic wounds, such as leg ulcers associated with sickle cell disease, occur as a consequence of a prolonged inflammatory phase during the healing process. They are extremely hard to heal and persist as a significant health care problem due to the absence of effective treatment and the uprising number of patients. Indeed, there is a critical need to develop novel cell- and tissue-based therapies to treat these chronic wounds. Development in skin engineering leads to a small catalogue of available substitutes manufactured in Good Manufacturing Practices compliant (GMPc) conditions. Those substitutes are produced using primary cells that could limit their use due to restricted sourcing. Here, we propose GMPc protocols to produce functional populations of keratinocytes and fibroblasts derived from pluripotent stem cells to reconstruct the associated dermo-epidermal substitute with plasma-based fibrin matrix. In addition, this manufactured composite skin is biologically active and enhances in vitro wounding of keratinocytes. The proposed composite skin opens new perspectives for skin replacement using allogeneic substitute.

The Future of Regenerative Medicine: Cell Therapy Using Pluripotent Stem Cells and Acellular Therapies Based on Extracellular Vesicles.

The rapid progress in the field of stem cell research has laid strong foundations for their use in regenerative medicine applications of injured or diseased tissues. Growing evidences indicate that some observed therapeutic outcomes of stem cell-based therapy are due to paracrine effects rather than long-term engraftment and survival of transplanted cells. Given their ability to cross biological barriers and mediate intercellular information transfer of bioactive molecules, extracellular vesicles are being explored as potential cell-free therapeutic agents. In this review, we first discuss the state of the art of regenerative medicine and its current limitations and challenges, with particular attention on pluripotent stem cell-derived products to repair organs like the eye, heart, skeletal muscle and skin. We then focus on emerging beneficial roles of extracellular vesicles to alleviate these pathological conditions and address hurdles and operational issues of this acellular strategy. Finally, we discuss future directions and examine how careful integration of different approaches presented in this review could help to potentiate therapeutic results in preclinical models and their good manufacturing practice (GMP) implementation for future clinical trials.

Pathological modelling of pigmentation disorders associated with Hutchinson-Gilford Progeria Syndrome (HGPS) revealed an impaired melanogenesis pathway in iPS-derived melanocytes.

Hutchinson-Gilford Progeria Syndrome (HGPS) is a rare genetic disorder that leads to premature aging. In this study, we used induced pluripotent stem cells to investigate the hypopigmentation phenotypes observed in patients with progeria. Accordingly, two iPS cell lines were derived from cells from HGPS patients and differentiated into melanocytes. Measurements of melanin content revealed a lower synthesis of melanin in HGPS melanocytes as compared to non-pathologic cells. Analysis of the melanosome maturation process by electron microscopy revealed a lower percentage of mature, fully pigmented melanosomes. Finally, a functional rescue experiment revealed the direct role of progerin in the regulation of melanogenesis. Overall, these results report a new dysregulated pathway in HGPS and open up novel perspectives in the study of pigmentation phenotypes that are associated with normal and pathological aging.

Differentiation of nonhuman primate pluripotent stem cells into functional keratinocytes.

BACKGROUND: Epidermal grafting using cells derived from pluripotent stem cells will change the face of this side of regenerative cutaneous medicine. To date, the safety of the graft would be the major unmet deal in order to implement long-term skin grafting. In this context, experiments on large animals appear unavoidable to assess this question and possible rejection. Cellular tools for large animal models should be constructed. METHODS: In this study, we generated monkey pluripotent stem cell-derived keratinocytes and evaluated their capacities to reconstruct an epidermis, in vitro as well as in vivo. RESULTS: Monkey pluripotent stem cells were differentiated efficiently into keratinocytes able to reconstruct fully epidermis presenting a low level of major histocompatibility complex class-I antigens, opening the way for autologous or allogeneic epidermal long-term grafting. CONCLUSIONS: Functional keratinocytes generated from nonhuman primate embryonic stem cells and induced pluripotent stem cells reproduce an in-vitro and in-vivo stratified epidermis. These monkey skin grafts will be considered to model autologous or allogeneic epidermal grafting using either embryonic stem cells or induced pluripotent stem cells. This graft model will allow us to further investigate the safety, efficacy and immunogenicity of nonhuman primate PSC-derived epidermis in the perspective of human skin cell therapy.

In vitro modeling of hyperpigmentation associated to neurofibromatosis type 1 using melanocytes derived from human embryonic stem cells.

"Café-au-lait" macules (CALMs) and overall skin hyperpigmentation are early hallmarks of neurofibromatosis type 1 (NF1). One of the most frequent monogenic diseases, NF1 has subsequently been characterized with numerous benign Schwann cell-derived tumors. It is well established that neurofibromin, the NF1 gene product, is an antioncogene that down-regulates the RAS oncogene. In contrast, the molecular mechanisms associated with alteration of skin pigmentation have remained elusive. We have reassessed this issue by differentiating human embryonic stem cells into melanocytes. In the present study, we demonstrate that NF1 melanocytes reproduce the hyperpigmentation phenotype in vitro, and further characterize the link between loss of heterozygosity and the typical CALMs that appear over the general hyperpigmentation. Molecular mechanisms associated with these pathological phenotypes correlate with an increased activity of cAMP-mediated PKA and ERK1/2 signaling pathways, leading to overexpression of the transcription factor MITF and of the melanogenic enzymes tyrosinase and dopachrome tautomerase, all major players in melanogenesis. Finally, the hyperpigmentation phenotype can be rescued using specific inhibitors of these signaling pathways. These results open avenues for deciphering the pathological mechanisms involved in pigmentation diseases, and provide a robust assay for the development of new strategies for treating these diseases.

Antagonistic effect of the inflammasome on thymic stromal lymphopoietin expression in the skin.

BACKGROUND: Innate immune sensors control key cytokines that regulate T-cell priming and T-cell fate. This is particularly evident in allergic reactions, which represent ideal systems to study the interplay of innate and adaptive immunity. In patients with contact dermatitis, inflammasome-mediated IL-1 activation is responsible for a TH1 immune response. Surprisingly, the IL-1 signaling pathway was also proposed to control the activation of thymic stromal lymphopoietin (TSLP), a cytokine implicated in development of the T(H)2 response in patients with atopic dermatitis (AD) and asthma. OBJECTIVES: We sought to assess the effect of the inflammasome on TSLP expression levels and the development of AD. METHODS: We studied the effect of the inflammasome activator 2,4-dinitrofluorobenzene, and IL-1ß on TSLP mRNA expression levels in mouse and human cell lines (in vitro assays), as well as in live mice and on human skin transplants. We also assessed the effect of 2,4-dinitrofluorobenzene on TSLP and the TH2 response in mice in which the inflammasome and IL-1 signaling pathways were blocked, either genetically or pharmacologically, in 2 models of AD. RESULTS: We provide in vitro and in vivo evidence that inflammasome activation has an inhibitory role on TSLP mRNA expression and T(H)2 cell fate in the skin. We also show that solvents influence the activation of TSLP and IL-1ß and direct the T-cell fate to a given hapten. CONCLUSION: Our observations strongly suggest that the TH1 versus TH2 cell fate decision is regulated at multiple levels and starts with innate immune events occurring within peripheral epithelial tissues.

A defective Krab-domain zinc-finger transcription factor contributes to altered myogenesis in myotonic dystrophy type 1.

Myotonic dystrophy type 1 (DM1) is an RNA-mediated disorder caused by a non-coding CTG repeat expansion that, in particular, provokes functional alteration of CUG-binding proteins. As a consequence, several genes with misregulated alternative splicing have been linked to clinical symptoms. In our search for additional molecular mechanisms that would trigger functional defects in DM1, we took advantage of mutant gene-carrying human embryonic stem cell lines to identify differentially expressed genes. Among the different genes found to be misregulated by DM1 mutation, one strongly downregulated gene encodes a transcription factor, ZNF37A. In this paper, we show that this defect in expression, which derives from a loss of RNA stability, is controlled by the RNA-binding protein, CUGBP1, and is associated with impaired myogenesis-a functional defect reminiscent of that observed in DM1. Loss of the ZNF37A protein results in changes in the expression of the subunit α1 of the receptor for the interleukin 13. This suggests that the pathological molecular mechanisms linking ZNF37A and myogenesis may involve the signaling pathway that is known to promote myoblast recruitment during development and regeneration.

mTOR-dependent proliferation defect in human ES-derived neural stem cells affected by myotonic dystrophy type 1.

Patients with myotonic dystrophy type 1 exhibit a diversity of symptoms that affect many different organs. Among these are cognitive dysfunctions, the origin of which has remained elusive, partly because of the difficulty in accessing neural cells. Here, we have taken advantage of pluripotent stem cell lines derived from embryos identified during a pre-implantation genetic diagnosis for mutant-gene carriers, to produce early neuronal cells. Functional characterization of these cells revealed reduced proliferative capacity and increased autophagy linked to mTOR signaling pathway alterations. Interestingly, loss of function of MBNL1, an RNA-binding protein whose function is defective in DM1 patients, resulted in alteration of mTOR signaling, whereas gain-of-function experiments rescued the phenotype. Collectively, these results provide a mechanism by which DM1 mutation might affect a major signaling pathway and highlight the pertinence of using pluripotent stem cells to study neuronal defects.

Functional melanocytes derived from human pluripotent stem cells engraft into pluristratified epidermis.

Melanocytes are essential for skin homeostasis and protection, and their defects in humans lead to a wide array of diseases that are potentially extremely severe. To date, the analysis of molecular mechanisms and the function of human melanocytes have been limited because of the difficulties in accessing large numbers of cells with the specific phenotypes. This issue can now be addressed via a differentiation protocol that allows melanocytes to be obtained from pluripotent stem cell lines, either induced or of embryonic origin, based on the use of moderate concentrations of a single cytokine, bone morphogenic protein 4. Human melanocytes derived from pluripotent stem cells exhibit all the characteristic features of their adult counterparts. This includes the enzymatic machinery required for the production and functional delivery of melanin to keratinocytes. Melanocytes also integrate appropriately into organotypic epidermis reconstructed in vitro. The availability of human cells committed to the melanocytic lineage in vitro will enable the investigation of those mechanisms that guide the developmental processes and will facilitate analysis of the molecular mechanisms responsible for genetic diseases. Access to an unlimited resource may also prove a vital tool for the treatment of hypopigmentation disorders when donors with matching haplotypes become available in clinically relevant banks of pluripotent stem cell lines.

miR-203 modulates epithelial differentiation of human embryonic stem cells towards epidermal stratification.

The molecular mechanisms controlling the differentiation of human basal keratinocyte stem cells towards the epidermis are well characterized, whereas the earliest process leading to the specification of embryonic stem cells into keratinocytes is still not well understood. MicroRNAs are regulators of many cellular events, but evidence for microRNA acting on the differentiation of human embryonic stem cells into a specific lineage has been elusive. By using our recent protocol for obtaining functional keratinocytes from hESC, we attempted to analyze the role of microRNAs in the early stages of epidermal differentiation. Thus, we identified a set of 5 microRNAs, namely miR-200a, miR-200b, miR-203, miR-205 and miR-429, that are specifically overexpressed during the early stages of the differentiation process. Interestingly, our functional analyses revealed an instrumental role of miR-203, which had been previously shown to play a key role during the formation of the pluristratified epidermis by basal keratinocyte stem cells, in the early keratinocyte commitment. These results highlight the determinant and unique role of miR-203 during the entire process of epidermal development by extending its spectrum of action from the early commitment of embryonic stem cells to ultimate differentiation of the organ.

Concise review: Epidermal grafting: the case for pluripotent stem cells.

Although cell therapy has been clinically implemented for several decades, its use is hampered by the difficulty in supplying the amount of epidermal substitute needed to extend the application to all patients who may benefit from it. How human pluripotent stem cells may help meet this challenge is the topic of this review. After reporting on the main current applications and needs of skin grafting, we explore the potential of pluripotent stem cells--either of embryonic origin or produced by genetic reprogramming--to provide the needed clinical-grade keratinocytes, fulfilling industrial scale production, and quality standards. Immunogenicity is clearly an issue, although one may expect cells displaying characteristics of fetal or embryonic skin to have a much better tolerance than adult keratinocytes. The open possibility of a bank of pluripotent stem cell lines selected on the basis of interesting haplotypes may eventually provide a definitive answer. Actually, making the case for pluripotent stem cells in skin grafting goes well beyond that specific cell type. Most cell phenotypes that normally participate to the formation of dermis and epidermis can either already be obtained through in vitro differentiation from pluripotent stem cells or would likely migrate from the host into a graft. However, differentiation protocols for specialized glands and hair follicles remain to be designed. A future can be foreseen when reconstructive medicine will make use of composite grafts integrating several different cell types and biomaterials.

CD98hc (SLC3A2) is a key regulator of keratinocyte adhesion.

BACKGROUND: Adhesion of keratinocytes is crucial for maintaining the integrity of the skin, as demonstrated by the number of dermatological disorders of genetic origin that are associated with a defect of basal keratinocyte adhesion. Integrins are the main component of the molecular networks involved in this phenomenon, but there are many others. In a recent description of proteins associated to caveolae at the plasma membrane of human basal epidermal cells, we demonstrated that CD98hc is localized with ß1 integrin. OBJECTIVES: We investigated the CD98hc proteins interactions and the role of CD98hc in keratinocyte adhesion. METHODS: CD98hc protein interaction was identified following co-immunoprecipitation and proteomic analysis using LTQ-FT mass spectrometer. Extinction of CD98hc gene expression using specific short hairpin RNA or over-expression of CD98hc lacking the ß1 integrin binding site was used to evaluate the role of this protein in keratinocyte fate. RESULTS: We show that CD98hc forms molecular complexes with ß1 and ß4 integrins in primary human keratinocytes and, using immunofluorescence, that these complexes are localized at the plasma membrane, in keeping with a role in adhesion. We confirmed that this protein is a key player of keratinocyte adhesion because in absence of interaction between CD98hc and integrins, ß1 integrin failed to translocate from the cytoplasm to the plasma membrane and keratinocytes expressed epidermal differentiation markers. CONCLUSIONS: All these data strongly suggested that CD98hc is involved in integrin trafficking and by consequence, in keratinocyte adhesion and differentiation.

Human embryonic stem-cell derivatives for full reconstruction of the pluristratified epidermis: a preclinical study.

BACKGROUND: Cell therapy for large burns is dependent upon autologous epidermis reconstructed in vitro. However, the effectiveness of current procedures is limited by the delay needed to culture the patient's own keratinocytes. To assess whether the keratinocyte progeny of human embryonic stem cells (hESCs) could be used to form a temporary skin substitute for use in patients awaiting autologous grafts, we investigated the cells' capability of constructing a pluristratified epidermis. METHODS: hESCs from lines H9 and SA01 were seeded at least in triplicate on fibroblast feeder cells for 40 days in a medium supplemented with bone morphogenetic protein 4 and ascorbic acid. Molecular characterisation of cell differentiation was done throughout the process by quantitative PCR, fluorescence-activated cell sorting, and immunocytochemical techniques. Keratinocyte molecular differentiation and functional capacity to construct a human epidermis were assessed in vitro and in vivo. FINDINGS: From hESCs, we generated a homogeneous population of cells that showed phenotypic characteristics of basal keratinocytes. Expression levels of genes encoding keratin 14, keratin 5, integrin alpha6, integrin beta4, collagen VII, and laminin 5 in these cells were similar to those in basal keratinocytes. After seeding on an artificial matrix, keratinocytes derived from hESCs (K-hESCs) formed a pluristratified epidermis. Keratin-14 immunostaining was seen in the basal compartment, with keratin 10 present in layers overlying the basal layer. Involucrin and filaggrin, late markers of epidermal differentiation, were detected in the uppermost layers only. 12 weeks after grafting onto five immunodeficient mice, epidermis derived from K-hESCs had a structure consistent with that of mature human skin. Human involucrin was appropriately located in spinous and granular layers and few Ki67-positive cells were detected in the basal layer. INTERPRETATION: hESCs can be differentiated into basal keratinocytes that are fully functional--ie, able to construct a pluristratified epidermis. This resource could be developed to provide temporary skin substitutes for patients awaiting autologous grafts. FUNDING: Institut National de la Santé et de la Recherche Médicale, University Evry Val d'Essonne, Association Française contre les Myopathies, Fondation René Touraine, and Genopole.