Genetically matched human iPS cells reveal that propensity for cartilage and bone differentiation differs with clones, not cell type of origin

Variation of DNA methylation on the IRX1/2 genes is responsible for the neural differentiation propensity in human induced pluripotent stem cells

Introduction

Human induced pluripotent stem cells (hiPSCs) are useful tools for reproducing neural development in vitro. However, each hiPSC line has a different ability to differentiate into specific lineages, known as differentiation propensity, resulting in reduced reproducibility and increased time and funding requirements for research. To overcome this issue, we searched for predictive signatures of neural differentiation propensity of hiPSCs focusing on DNA methylation, which is the main modulator of cellular properties.

Methods

We obtained 32 hiPSC lines and their comprehensive DNA methylation data using the Infinium MethylationEPIC BeadChip. To assess the neural differentiation efficiency of these hiPSCs, we measured the percentage of neural stem cells on day 7 of induction. Using the DNA methylation data of undifferentiated hiPSCs and their measured differentiation efficiency into neural stem cells as the set of data, and HSIC Lasso, a machine learning-based nonlinear feature selection method, we attempted to identify neural differentiation-associated differentially methylated sites.

Results

Epigenome-wide unsupervised clustering cannot distinguish hiPSCs with varying differentiation efficiencies. In contrast, HSIC Lasso identified 62 CpG sites that could explain the neural differentiation efficiency of hiPSCs. Features selected by HSIC Lasso were particularly enriched within 3 Mbp of chromosome 5, harboring IRX1, IRX2, and C5orf38 genes. Within this region, DNA methylation rates were correlated with neural differentiation efficiency and were negatively correlated with gene expression of the IRX1/2 genes, particularly in female hiPSCs. In addition, forced expression of the IRX1/2 impaired the neural differentiation ability of hiPSCs in both sexes.

Conclusion

We for the first time showed that the DNA methylation state of the IRX1/2 genes of hiPSCs is a predictive biomarker of their potential for neural differentiation. The predictive markers for neural differentiation efficiency identified in this study may be useful for the selection of suitable undifferentiated hiPSCs prior to differentiation induction.

Plating human iPSC lines on micropatterned substrates reveals role for ITGB1 nsSNV in endoderm formation

Quantitative analysis of human induced pluripotent stem cell (iPSC) lines from healthy donors is a powerful tool for uncovering the relationship between genetic variants and cellular behavior. We previously identified rare, deleterious non-synonymous single nucleotide variants (nsSNVs) in cell adhesion genes that are associated with outlier iPSC phenotypes in the pluripotent state. Here, we generated micropatterned colonies of iPSCs to test whether nsSNVs influence patterning of radially ordered germ layers. Using a custom-built image analysis pipeline, we quantified the differentiation phenotypes of 13 iPSC lines that harbor nsSNVs in genes related to cell adhesion or germ layer development. All iPSC lines differentiated into the three germ layers; however, there was donor-specific variation in germ layer patterning. We identified one line that presented an outlier phenotype of expanded endodermal differentiation, which was associated with a nsSNV in ITGB1. Our study establishes a platform for investigating the impact of nsSNVs on differentiation.

From Mesenchymal Stromal Cells to Engineered Extracellular Vesicles: A New Therapeutic Paradigm

Mesenchymal stromal cells (MSCs) are multipotent cells obtained from many tissues including bone marrow, adipose tissue, umbilical cord, amniotic fluid, and placenta. MSCs are the leading cell source for stem cell therapy due to their regenerative and immunomodulatory properties, their low risk of tumorigenesis and lack of ethical constraints. However, clinical applications of MSCs remain limited. MSC therapeutic development continues to pose challenges in terms of preparation, purity, consistency, efficiency, reproducibility, processing time and scalability. Additionally, there are issues with their poor engraftment and survival in sites of disease or damage that limit their capacity to directly replace damaged cells. A key recent development in MSC research, however, is the now widely accepted view that MSCs primarily exert therapeutic effects via paracrine factor secretion. One of the major paracrine effectors are extracellular vesicles (EVs). EVs represent a potential cell-free alternative to stem cell therapy but are also rapidly emerging as a novel therapeutic platform in their own right, particularly in the form of engineered EVs (EEVs) tailored to target a broad range of clinical indications. However, the development of EVs and EEVs for therapeutic application still faces a number of hurdles, including the establishment of a consistent, scalable cell source, and the development of robust GMP-compliant upstream and downstream manufacturing processes. In this review we will highlight the clinical challenges of MSC therapeutic development and discuss how EVs and EEVs can overcome the challenges faced in the clinical application of MSCs.

Endochondral Bone Tissue Engineering Using Human Induced Pluripotent Stem Cells

There has been great interest in the use of induced pluripotent stem cells (iPSCs) in bone regenerative strategies for bone defects. In the present study, we investigated whether the implantation of chondrogenically differentiated iPSC-derived mesenchymal stem cells (iMSCs) could lead to the successful regeneration of bone defects in nude mice. Two clones of human iPSCs (201B7 and 454E2) were used. After the generation of iMSCs, chondrogenic differentiation was achieved using a three-dimensional pellet culture. Then, a 2-mm defect was created in the radius of nude mice and chondrogenically differentiated iMSC pellets were placed in the defect. Micro-computed tomography (μ-CT) imaging analysis was performed 8 weeks after transplantation to assess bone regeneration. Eleven out of 11 (100%) radii in the 201B7 cell-derived-pellet transplantation group and 7 out of 10 (70%) radii in the 454E2 cell-derived-pellet transplantation group showed bone union. On the other hand, only 2 out of 11 radii (18%) in the control group showed bone union. Therefore, the bone union rates in the experimental groups were significantly higher than that in the control group (p < 0.05). Histological analysis 2 weeks post-implantation in the experimental groups revealed hypertrophic chondrocytes within grafted iMSC pellets, and the formation of woven bone around them; this hypertrophic chondrocyte transitioning to the newly formed bone suggests that the cartilaginous template can trigger the process of endochondral bone ossification (ECO). Four weeks post-implantation, the cartilage template was reduced in size; newly formed woven bone predominated at the defect site. New vessels were surrounded by a matrix of woven bone and the hypertrophic chondrocytes transitioning to the newly formed bone indicated the progression of ECO. Eight weeks post-implantation, the pellets were completely resorbed and replaced by bone; complete bone union was overall observed. Dense mature bone developed with evidence of lamellar-like bone formation. Collectively, our results suggest that iMSC-based cartilage grafts recapitulating the morphogenetic process of ECO in the context of embryonic skeletogenesis are a novel and promising strategy for the repair of large bone defects.

Novel cell sources for bone regeneration

A plethora of both acute and chronic conditions, including traumatic, degenerative, malignant, or congenital disorders, commonly induce bone disorders often associated with severe persisting pain and limited mobility. Over 1 million surgical procedures involving bone excision, bone grafting, and fracture repair are performed each year in the U.S. alone, resulting in immense levels of public health challenges and corresponding financial burdens. Unfortunately, the innate self-healing capacity of bone is often inadequate for larger defects over a critical size. Moreover, as direct transplantation of committed osteoblasts is hindered by deficient cell availability, limited cell spreading, and poor survivability, an urgent need for novel cell sources for bone regeneration is concurrent. Thanks to the development in stem cell biology and cell reprogramming technology, many multipotent and pluripotent cells that manifest promising osteogenic potential are considered the regenerative remedy for bone defects. Considering these cells' investigation is still in its relative infancy, each of them offers their own particular challenges that must be conquered before the large-scale clinical application.

Gene Editing Correction of a Urea Cycle Defect in Organoid Stem Cell Derived Hepatocyte-like Cells

Urea cycle disorders are enzymopathies resulting from inherited deficiencies in any genes of the cycle. In severe cases, currently available therapies are marginally effective, with liver transplantation being the only definitive treatment. Donor liver availability can limit even this therapy. Identification of novel therapeutics for genetic-based liver diseases requires models that provide measurable hepatic functions and phenotypes. Advances in stem cell and genome editing technologies could provide models for the investigation of cell-based genetic diseases, as well as the platforms for drug discovery. This report demonstrates a practical, and widely applicable, approach that includes the successful reprogramming of somatic cells from a patient with a urea cycle defect, their genetic correction and differentiation into hepatic organoids, and the subsequent demonstration of genetic and phenotypic change in the edited cells consistent with the correction of the defect. While individually rare, there is a large number of other genetic-based liver diseases. The approach described here could be applied to a broad range and a large number of patients with these hepatic diseases where it could serve as an in vitro model, as well as identify successful strategies for corrective cell-based therapy.

Proliferative and chondrogenic potential of mesenchymal stromal cells from pluripotent and bone marrow cells

INTRODUCTION

Mesenchymal stromal cells (MSCs) can be derived from a wide range of fetal and adult sources including pluripotent stem cells (PSCs). The properties of PSC-derived MSCs need to be fully characterized, in order to evaluate the feasibility of their use in clinical applications. PSC-MSC proliferation and differentiation potential in comparison with bone marrow (BM)-MSCs is still under investigation. The objective of this study was to determine the proliferative and chondrogenic capabilities of both human induced pluripotent stem cell (hiPSC-) and embryonic stem cell (hESC-) derived MSCs, by comparing them with BM-MSCs.

METHODS

MSCs were derived from two hiPSC lines (hiPSC-MSCs), the well characterized Hues9 hESC line (hESC-MSCs) and BM from two healthy donors (BM-MSCs). Proliferation potential was investigated using appropriate culture conditions, with serial passaging, until cells entered into senescence. Differentiation potential to cartilage was examined after in vitro chondrogenic culture conditions.

RESULTS

BM-MSCs revealed a fold expansion of 1.18x10⁵ and 2.3x10⁵ while the two hiPSC-MSC lines and hESC-MSC showed 5.88x10¹⁰, 3.49x10⁸ and 2.88x10⁸, respectively. Under chondrogenic conditions, all MSC lines showed a degree of chondrogenesis. However, when we examined the formed chondrocyte micromasses by histological analysis of the cartilage morphology and immunohistochemistry for the chondrocyte specific markers Sox9 and Collagen II, we observed that PSC-derived MSC lines had formed pink rather than hyaline cartilage, in contrast to BM-MSCs.

CONCLUSION

In conclusion, MSCs derived from both hESCs and hiPSCs had superior proliferative capacity compared to BM-MSCs, but they were inefficient in their ability to form hyaline cartilage.

Origin of the Induced Pluripotent Stem Cells Affects Their Differentiation into Dopaminergic Neurons

Neuronal differentiation of human induced pluripotent stem (iPS) cells, both in 2D models and 3D systems in vitro, allows for the study of disease pathomechanisms and the development of novel therapies. To verify if the origin of donor cells used for reprogramming to iPS cells can influence the differentiation abilities of iPS cells, peripheral blood mononuclear cells (PBMC) and keratinocytes were reprogrammed to iPS cells using the Sendai viral vector and were subsequently checked for pluripotency markers and the ability to form teratomas in vivo. Then, iPS cells were differentiated into dopaminergic neurons in 2D and 3D cultures. Both PBMC and keratinocyte-derived iPS cells were similarly reprogrammed to iPS cells, but they displayed differences in gene expression profiles and in teratoma compositions in vivo. During 3D organoid formation, the origin of iPS cells affected the levels of FOXA2 and LMX1A only in the first stages of neural differentiation, whereas in the 2D model, differences were detected at the levels of both early and late neural markers FOXA2, LMX1A, NURR1, TUBB and TH. To conclude, the origin of iPS cells may significantly affect iPS differentiation abilities in teratomas, as well as exerting effects on 2D differentiation into dopaminergic neurons and the early stages of 3D midbrain organoid formation.

Induced Pluripotent Stem Cells in Dental and Nondental Tissue Regeneration: A Review of an Unexploited Potential

Cell-based therapies currently represent the state of art for tissue regenerative treatment approaches for various diseases and disorders. Induced pluripotent stem cells (iPSCs), reprogrammed from adult somatic cells, using vectors carrying definite transcription factors, have manifested a breakthrough in regenerative medicine, relying on their pluripotent nature and ease of generation in large amounts from various dental and nondental tissues. In addition to their potential applications in regenerative medicine and dentistry, iPSCs can also be used in disease modeling and drug testing for personalized medicine. The current review discusses various techniques for the production of iPSC-derived osteogenic and odontogenic progenitors, the therapeutic applications of iPSCs, and their regenerative potential in vivo and in vitro. Through the present review, we aim to explore the potential applications of iPSCs in dental and nondental tissue regeneration and to highlight different protocols used for the generation of different tissues and cell lines from iPSCs.

A lithium-containing biomaterial promotes chondrogenic differentiation of induced pluripotent stem cells with reducing hypertrophy

Background

Induced pluripotent stem cells (iPSCs) exhibit limitless pluripotent plasticity and proliferation capability to provide an abundant cell source for tissue regenerative medicine. Thus, inducing iPSCs toward a specific differentiation direction is an important scientific question. Traditionally, iPSCs have been induced to chondrocytes with the help of some small molecules within 21–36 days. To speed up the differentiation of iPSCs, we supposed to utilize bioactive ceramics to assist chondrogenic-induction process.

Methods

In this study, we applied ionic products (3.125~12.5 mg/mL) of the lithium-containing bioceramic (Li₂Ca₄Si₄O₁₃, L2C4S4) and individual Li⁺ (5.78~23.73 mg/L) in the direct chondrogenic differentiation of human iPSCs.

Results

Compared to pure chondrogenic medium and extracts of tricalcium phosphate (TCP), the extracts of L2C4S4 at a certain concentration range (3.125~12.5 mg/mL) significantly enhanced chondrogenic proteins Type II Collagen (COL II)/Aggrecan/ SRY-Box 9 (SOX9) synthesis and reduced hypertrophic protein type X collagen (COL X)/matrix metallopeptidase 13 (MMP13) production in iPSCs-derived chondrocytes within 14 days, suggesting that these newly generated chondrocytes exhibited favorable chondrocytes characteristics and maintained a low-hypertrophy state. Further studies demonstrated that the individual Li⁺ ions at the concentration range of 5.78~23.73 mg/L also accelerated the chondrogenic differentiation of iPSCs, indicating that Li⁺ ions played a pivotal role in chondrogenic differentiation process.

Conclusions

These findings indicated that lithium-containing bioceramic with bioactive specific ionic components may be used for a promising platform for inducing iPSCs toward chondrogenic differentiation and cartilage regeneration.

Aging: A cell source limiting factor in tissue engineering

Tissue engineering has yet to reach its ideal goal, i.e. creating profitable off-the-shelf tissues and organs, designing scaffolds and three-dimensional tissue architectures that can maintain the blood supply, proper biomaterial selection, and identifying the most efficient cell source for use in cell therapy and tissue engineering. These are still the major challenges in this field. Regarding the identification of the most appropriate cell source, aging as a factor that affects both somatic and stem cells and limits their function and applications is a preventable and, at least to some extents, a reversible phenomenon. Here, we reviewed different stem cell types, namely embryonic stem cells, adult stem cells, induced pluripotent stem cells, and genetically modified stem cells, as well as their sources, i.e. autologous, allogeneic, and xenogeneic sources. Afterward, we approached aging by discussing the functional decline of aged stem cells and different intrinsic and extrinsic factors that are involved in stem cell aging including replicative senescence and Hayflick limit, autophagy, epigenetic changes, miRNAs, mTOR and AMPK pathways, and the role of mitochondria in stem cell senescence. Finally, various interventions for rejuvenation and geroprotection of stem cells are discussed. These interventions can be applied in cell therapy and tissue engineering methods to conquer aging as a limiting factor, both in original cell source and in the in vitro proliferated cells.

The Role of Extracellular Matrix Expression, ERK1/2 Signaling and Cell Cohesiveness for Cartilage Yield from iPSCs

Current therapies involving chondrocytes or mesenchymal stromal cells (MSCs) remain inefficient in restoring cartilage properties upon injury. The induced pluripotent stem-cell (iPSC)-derived mesenchymal progenitor cells (iMPCs) have been put forward as a promising alternative cell source due to their high proliferation and differentiation potential. However, the observed cell loss during in vitro chondrogenesis is currently a bottleneck in establishing articular chondrocyte generation from iPSCs. In a search for candidate mechanisms underlying the low iPSC-derived cartilage tissue yield, global transcriptomes were compared between iMPCs and MSCs and the cell properties were analyzed via a condensation assay. The iMPCs had a more juvenile mesenchymal gene signature than MSCs with less myofibroblast-like characteristics, including significantly lower ECM- and integrin-ligand-related as well as lower α-smooth-muscle-actin expression. This correlated with less substrate and more cell-cell adhesion, impaired aggregate formation and consequently inferior cohesive tissue properties of the iMPC-pellets. Along lower expression of pro-survival ECM molecules, like decorin, collagen VI, lumican and laminin, the iMPC populations had significantly less active ERK1/2 compared to MSCs. Overall, this study proposes that this ECM and integrin-ligand shortage, together with insufficient pro-survival ERK1/2-activity, explains the loss of a non-aggregating iMPC sub-fraction during pellet formation and reduced survival of cells in early pellets. Enhancing ECM production and related signaling in iMPCs may be a promising new means to enrich the instructive microenvironment with pro-survival cues allowing to improve the final cartilage tissue yield from iPSCs.

Human iPSC-derived MSCs (iMSCs) from aged individuals acquire a rejuvenation signature

BACKGROUND

Primary mesenchymal stem cells (MSCs) are fraught with aging-related shortfalls. Human-induced pluripotent stem cell (iPSC)-derived MSCs (iMSCs) have been shown to be a useful clinically relevant source of MSCs that circumvent these aging-associated drawbacks. To date, the extent of the retention of aging-hallmarks in iMSCs differentiated from iPSCs derived from elderly donors remains unclear.

METHODS

Fetal femur-derived MSCs (fMSCs) and adult bone marrow MSCs (aMSCs) were isolated, corresponding iPSCs were generated, and iMSCs were differentiated from fMSC-iPSCs, from aMSC-iPSCs, and from human embryonic stem cells (ESCs) H1. In addition, typical MSC characterization such as cell surface marker expression, differentiation capacity, secretome profile, and trancriptome analysis were conducted for the three distinct iMSC preparations-fMSC-iMSCs, aMSC-iMSCs, and ESC-iMSCs. To verify these results, previously published data sets were used, and also, additional aMSCs and iMSCs were analyzed.

RESULTS

fMSCs and aMSCs both express the typical MSC cell surface markers and can be differentiated into osteogenic, adipogenic, and chondrogenic lineages in vitro. However, the transcriptome analysis revealed overlapping and distinct gene expression patterns and showed that fMSCs express more genes in common with ESCs than with aMSCs. fMSC-iMSCs, aMSC-iMSCs, and ESC-iMSCs met the criteria set out for MSCs. Dendrogram analyses confirmed that the transcriptomes of all iMSCs clustered together with the parental MSCs and separated from the MSC-iPSCs and ESCs. iMSCs irrespective of donor age and cell type acquired a rejuvenation-associated gene signature, specifically, the expression of INHBE, DNMT3B, POU5F1P1, CDKN1C, and GCNT2 which are also expressed in pluripotent stem cells (iPSCs and ESC) but not in the parental aMSCs. iMSCs expressed more genes in common with fMSCs than with aMSCs. Independent real-time PCR comparing aMSCs, fMSCs, and iMSCs confirmed the differential expression of the rejuvenation (COX7A, EZA2, and TMEM119) and aging (CXADR and IGSF3) signatures. Importantly, in terms of regenerative medicine, iMSCs acquired a secretome (e.g., angiogenin, DKK-1, IL-8, PDGF-AA, osteopontin, SERPINE1, and VEGF) similar to that of fMSCs and aMSCs, thus highlighting their ability to act via paracrine signaling.

CONCLUSIONS

iMSCs irrespective of donor age and cell source acquire a rejuvenation gene signature. The iMSC concept could allow circumventing the drawbacks associated with the use of adult MSCs und thus provide a promising tool for use in various clinical settings in the future.

The role of the reprogramming method and pluripotency state in gamete differentiation from patient-specific human pluripotent stem cells

The derivation of gametes from patient-specific pluripotent stem cells may provide new perspectives for genetic parenthood for patients currently facing sterility. We use current data to assess the gamete differentiation potential of patient-specific pluripotent stem cells and to determine which reprogramming strategy holds the greatest promise for future clinical applications. First, we compare the two best established somatic cell reprogramming strategies: the production of induced pluripotent stem cells (iPSC) and somatic cell nuclear transfer followed by embryonic stem cell derivation (SCNT-ESC). Recent reports have indicated that these stem cells, though displaying a similar pluripotency potential, show important differences at the epigenomic level, which may have repercussions on their applicability. By comparing data on the genetic and epigenetic stability of these cell types during derivation and in-vitro culture, we assess the reprogramming efficiency of both technologies and possible effects on the subsequent differentiation potential of these cells. Moreover, we discuss possible implications of mitochondrial heteroplasmy. We also address the ethical aspects of both cell types, as well as the safety considerations associated with clinical applications using these cells, e.g. the known genomic instability of human PSCs during long-term culture. Secondly, we discuss the role of the stem cell pluripotency state in germ cell differentiation. In mice, success in germ cell development from pluripotent stem cells could only be achieved when starting from a naive state of pluripotency. It remains to be investigated if the naive state is also crucial for germ cell differentiation in human cells and to what extent human naive pluripotency resembles the naive state in mouse.

Low Osteogenic Yield in Human Pluripotent Stem Cells Associates with Differential Neural Crest Promoter Methylation

Human pluripotent stem cell-derived osteoblasts possess great potential for use in bone disorder elucidation and repair; however, while the general ability of human pluripotent stem cells to differentiate into osteoblasts and lay down bone-specific matrix has been shown, previous studies lack the complete characterization of the process whereby such osteoblasts are derived as well as a comparison between the osteogenic efficiency of multiple cell lines. Here, we compared the osteogenic potential of two human induced pluripotent stem cell lines (RIV9 and RIV4) to human H9 embryonic stem cells. Generally capable of osteogenic differentiation, the overall osteogenic yield was lower in the RIV9 and RIV4 lines and correlated with differential expression of osteocalcin (OCN) in mature cultures and PAX7 and TWIST1 during early differentiation. In the undifferentiated cells, the promoters of the latter two genes were differentially methylated potentially explaining the variation in differentiation efficiency. Furthermore, the expression signatures of selected neural crest and mesodermal genes and proteins suggested that H9 cells preferentially gave rise to neural crest-derived osteoblasts, whereas the osteoblasts in the RIV9 cultures were generated both through a mesodermal and a neural crest route although each at a lower rate. These data suggest that epigenetic dissimilarities between multiple PSC lines may lead to differences in lineage derivation and mineralization. Since osteoblast progenitors from one origin inadequately repair a defect in the other, these data underscore the importance of screening human pluripotent stem cells lines for the identity of the osteoprogenitors they lay down. Stem Cells 2018;36:349-362.

An mTOR Signaling Modulator Suppressed Heterotopic Ossification of Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare and intractable disorder characterized by extraskeletal bone formation through endochondral ossification. FOP patients harbor gain-of-function mutations in ACVR1 (FOP-ACVR1), a type I receptor for bone morphogenetic proteins. Despite numerous studies, no drugs have been approved for FOP. Here, we developed a high-throughput screening (HTS) system focused on the constitutive activation of FOP-ACVR1 by utilizing a chondrogenic ATDC5 cell line that stably expresses FOP-ACVR1. After HTS of 5,000 small-molecule compounds, we identified two hit compounds that are effective at suppressing the enhanced chondrogenesis of FOP patient-derived induced pluripotent stem cells (FOP-iPSCs) and suppressed the heterotopic ossification (HO) of multiple model mice, including FOP-ACVR1 transgenic mice and HO model mice utilizing FOP-iPSCs. Furthermore, we revealed that one of the hit compounds is an mTOR signaling modulator that indirectly inhibits mTOR signaling. Our results demonstrate that these hit compounds could contribute to future drug repositioning and the mechanistic analysis of mTOR signaling.

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Established a screening system for fibrodysplasia ossificans progressiva (FOP)

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Identified two hit compounds that are effective in multiple FOP model mice

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An mTOR signaling modulator opens the door to a therapeutic strategy

Comparative study of periodontal differentiation propensity of induced pluripotent stem cells from different tissue origins

BACKGROUND

Despite being almost identical to embryonic stem cells, induced pluripotent stem cells (iPSCs) have been shown to possess a residual somatic memory that favors their differentiation propensity into donor tissue. To further confirm this assumption, we compare for the first time the periodontal differentiation tendency of human gingival fibroblast-derived iPSCs (G-iPSCs) and human neonatal skin fibroblast-derived iPSCs (S-iPSCs) to assess whether G-iPSCs could be more efficiently induced toward periodontal cells.

METHODS

We induced G- and S-iPSCs under the treatment of growth/differentiation factor-5 and connective tissue growth factor, respectively, for 14 days. Immunofluorescence staining and real-time polymerase chain reaction were used to compare their expression levels of related markers. Furthermore, a hydrogel carrier was developed to seed these periodontal progenitors for subcutaneous implantation in non-obese diabetic-severe combined immunodeficiency disease mice. Their differentiated periodontal phenotype maintenance was further assayed by HE observation, immunohistochemical staining and immunofluorescence co-localization with pre-labeled PKH67.

RESULTS

As expected, both iPSCs were inclined to differentiate back into their original lineage by expressing higher markers at both gene and protein levels in vitro. HE observation of G-iPSCs-seeded hydrogel constructs present more mineralized structure formation than S-iPSCs-seeded ones. Immunohistochemical staining and immunofluorescence analysis also showed stronger positive staining for periodontal related markers in G-iPSCs-seeded hydrogel constructs.

CONCLUSIONS

Our results preliminarily confirmed that both G- and S-iPSCs were inclined to differentiate back into their original tissue in vitro. Animal study further confirmed the phenotype maintenance of periodontal differentiated G-iPSCs, which highlighted their significant implications for therapeutic use in periodontal regeneration.

Genetic and epigenetic factors which modulate differentiation propensity in human pluripotent stem cells

BACKGROUND

Human pluripotent stem cell (hPSC) lines are known to have a bias in their differentiation. This gives individual cell lines a propensity to preferentially differentiate towards one germ layer or cell type over others. Chromosomal aberrations, mitochondrial mutations, genetic diversity and epigenetic variance are the main drivers of this phenomenon, and can lead to a wide range of phenotypes.

OBJECTIVE AND RATIONALE

Our aim is to provide a comprehensive overview of the different factors which influence differentiation propensity. Specifically, we sought to highlight known genetic variances and their mechanisms, in addition to more general observations from larger abnormalities. Furthermore, we wanted to provide an up-to-date list of a growing number of predictive indicators which are able to identify differentiation propensity before the initiation of differentiation. As differentiation propensity can lead to difficulties in both research as well as clinical translation, our thorough overview could be a useful tool.

SEARCH METHODS

Combinations of the following key words were applied as search criteria in the PubMed database: embryonic stem cells, induced pluripotent stem cells, differentiation propensity (also: potential, efficiency, capacity, bias, variability), epigenetics, chromosomal abnormalities, genetic aberrations, X chromosome inactivation, mitochondrial function, mitochondrial metabolism, genetic diversity, reprogramming, predictive marker, residual stem cell, clinic. Only studies in English were included, ranging from 2000 to 2017, with a majority ranging from 2010 to 1017. Further manuscripts were added from cross-references.

OUTCOMES

Differentiation propensity is affected by a wide variety of (epi)genetic factors. These factors clearly lead to a loss of differentiation capacity, preference towards certain cell types and oftentimes, phenotypes which begin to resemble cancer. Broad changes in (epi)genetics, such as aneuploidies or wide-ranging modifications to the epigenetic landscape tend to lead to extensive, less definite changes in differentiation capacity, whereas more specific abnormalities often have precise ramifications in which certain cell types become more preferential. Furthermore, there appears to be a greater, though often less considered, contribution to differentiation propensity by factors such as mitochondria and inherent genetic diversity. Varied differentiation capacity can also lead to potential consequences in the clinical translation of hPSC, including the occurrence of residual undifferentiated stem cells, and the transplantation of potentially transformed cells.

WIDER IMPLICATIONS

As hPSC continue to advance towards the clinic, our understanding of them progresses as well. As a result, the challenges faced become more numerous, but also more clear. If the transition to the clinic is to be achieved with a minimum number of potential setbacks, thorough evaluation of the cells will be an absolute necessity. Altered differentiation propensity represents at least one such hurdle, for which researchers and eventually clinicians will need to find solutions. Already, steps are being taken to tackle the issue, though further research will be required to evaluate any long-term risks it poses.

Activin-A enhances mTOR signaling to promote aberrant chondrogenesis in fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare and intractable disease characterized by extraskeletal bone formation through endochondral ossification. Patients with FOP harbor point mutations in ACVR1, a type I receptor for BMPs. Although mutated ACVR1 (FOP-ACVR1) has been shown to render hyperactivity in BMP signaling, we and others have uncovered a mechanism by which FOP-ACVR1 mistransduces BMP signaling in response to Activin-A, a molecule that normally transduces TGF-β signaling. Although Activin-A evokes enhanced chondrogenesis in vitro and heterotopic ossification (HO) in vivo, the underlying mechanisms have yet to be revealed. To this end, we developed a high-throughput screening (HTS) system using FOP patient-derived induced pluripotent stem cells (FOP-iPSCs) to identify pivotal pathways in enhanced chondrogenesis that are initiated by Activin-A. In a screen of 6,809 small-molecule compounds, we identified mTOR signaling as a critical pathway for the aberrant chondrogenesis of mesenchymal stromal cells derived from FOP-iPSCs (FOP-iMSCs). Two different HO mouse models, an FOP model mouse expressing FOP-ACVR1 and an FOP-iPSC-based HO model mouse, revealed critical roles for mTOR signaling in vivo. Moreover, we identified ENPP2, an enzyme that generates lysophosphatidic acid, as a linker of FOP-ACVR1 and mTOR signaling in chondrogenesis. These results uncovered the crucial role of the Activin-A/FOP-ACVR1/ENPP2/mTOR axis in FOP pathogenesis.

Rejuvenation of Cardiac Tissue Developed from Reprogrammed Aged Somatic Cells

Induced pluripotent stem cells (iPSCs) derived via somatic cell reprogramming have been reported to reset aged somatic cells to a more youthful state, characterized by elongated telomeres, a rearranged mitochondrial network, reduced oxidative stress, and restored pluripotency. However, it is still unclear whether the reprogrammed aged somatic cells can function normally as embryonic stem cells (ESCs) during development and be rejuvenated. In the current study, we applied the aggregation technique to investigate whether iPSCs derived from aged somatic cells could develop normally and be rejuvenated. iPSCs derived from bone marrow myeloid cells of 2-month-old (2 M) and 18-month-old (18 M) C57BL/6-Tg (CAG-EGFP)1Osb/J mice were aggregated with embryos derived from wild-type ICR mice to produce chimeras (referred to as 2 M CA and 18 M CA, respectively). Our observations focused on comparing the ability of the iPSCs derived from 18 M and 2 M bone marrow cells to develop rejuvenated cardiac tissue (the heart is the most vital organ during aging). The results showed an absence of p16 and p53 upregulation, telomere length shortening, and mitochondrial gene expression and deletion in 18 M CA, whereas slight changes in mitochondrial ultrastructure, cytochrome C oxidase activity, ATP production, and reactive oxygen species production were observed in CA cardiac tissues. The data implied that all of the aging characteristics observed in the newborn cardiac tissue of 18 M CA were comparable with those of 2 M CA newborn cardiac tissue. This study provides the first direct evidence of the aging-related characteristics of cardiac tissue developed from aged iPSCs, and our observations demonstrate that partial rejuvenation can be achieved by reprogramming aged somatic cells to a pluripotent state.

Induced pluripotent stem cells as a new getaway for bone tissue engineering: A systematic review

OBJECTIVES

Mesenchymal stem cells (MSCs) are frequently used for bone regeneration, however, they are limited in quantity. Moreover, their proliferation and differentiation capabilities reduce during cell culture expansion. Potential application of induced pluripotent stem cells (iPSCs) has been reported as a promising alternative source for bone regeneration. This study aimed to systematically review the available literature on osteogenic potential of iPSCs and to discuss methods applied to enhance their osteogenic potential.

METHODS AND MATERIALS

A thorough search of MEDLINE database was performed from January 2006 to September 2016, limited to English-language articles. All in vitro and in vivo studies on application of iPSCs in bone regeneration were included.

RESULTS

The current review is organized according to the PRISMA statement. Studies were categorized according to three different approaches used for osteo-induction of iPSCs. Data are summarized and reported according to the following variables: types of study, cell sources used for iPSC generation, applied reprogramming methods, applied osteo-induction methods and treatment groups.

CONCLUSION

According to the articles reviewed, osteo-induced iPSCs revealed osteogenic capability equal to or superior than MSCs; cell sources do not significantly affect osteogenic potential of iPSCs; addition of resveratrol to the osteogenic medium (OM) and irradiatiation after osteogenic induction reduce teratoma formation in animal models; transfection with lentiviral bone morphogenetic protein 2 results in higher mineralization compared to osteo-induction in OM; addition of TGF-β, IGF-1 and FGF-β to OM increases osteogenic capability of iPSCs.

In Pursuit of Authenticity: Induced Pluripotent Stem Cell-Derived Retinal Pigment Epithelium for Clinical Applications

For effective treatment, induced pluripotent stem cell (iPSC)-retinal pigment epithelium (RPE) must recapitulate the physiology of native human RPE cells. A set of physiologically relevant functional assays that assess the polarized functional activity and maturation state of the intact RPE monolayer is provided. The study data show that donor-to-donor variability exceeds the tissue-to-tissue variability for a given donor and provides, for the first time, criteria necessary to identify iPSC-RPE cells most suitable for clinical application.

Induced pluripotent stem cells (iPSCs) can be efficiently differentiated into retinal pigment epithelium (RPE), offering the possibility of autologous cell replacement therapy for retinal degeneration stemming from RPE loss. The generation and maintenance of epithelial apical-basolateral polarity is fundamental for iPSC-derived RPE (iPSC-RPE) to recapitulate native RPE structure and function. Presently, no criteria have been established to determine clonal or donor based heterogeneity in the polarization and maturation state of iPSC-RPE. We provide an unbiased structural, molecular, and physiological evaluation of 15 iPSC-RPE that have been derived from distinct tissues from several different donors. We assessed the intact RPE monolayer in terms of an ATP-dependent signaling pathway that drives critical aspects of RPE function, including calcium and electrophysiological responses, as well as steady-state fluid transport. These responses have key in vivo counterparts that together help determine the homeostasis of the distal retina. We characterized the donor and clonal variation and found that iPSC-RPE function was more significantly affected by the genetic differences between different donors than the epigenetic differences associated with different starting tissues. This study provides a reference dataset to authenticate genetically diverse iPSC-RPE derived for clinical applications.

Significance

The retinal pigment epithelium (RPE) is essential for maintaining visual function. RPE derived from human induced pluripotent stem cells (iPSC-RPE) offer a promising cell-based transplantation therapy for slowing or rescuing RPE-induced visual function loss. For effective treatment, iPSC-RPE must recapitulate the physiology of native human RPE. A set of physiologically relevant functional assays are provided that assess the polarized functional activity and maturation state of the intact RPE monolayer. The present data show that donor-to-donor variability exceeds the tissue-to-tissue variability for a given donor and provides, for the first time, criteria necessary to identify iPSC-RPE most suitable for clinical application.

Establishment of Human Neural Progenitor Cells from Human Induced Pluripotent Stem Cells with Diverse Tissue Origins

Human neural progenitor cells (hNPCs) have previously been generated from limited numbers of human induced pluripotent stem cell (hiPSC) clones. Here, 21 hiPSC clones derived from human dermal fibroblasts, cord blood cells, and peripheral blood mononuclear cells were differentiated using two neural induction methods, an embryoid body (EB) formation-based method and an EB formation method using dual SMAD inhibitors (dSMADi). Our results showed that expandable hNPCs could be generated from hiPSC clones with diverse somatic tissue origins. The established hNPCs exhibited a mid/hindbrain-type neural identity and uniform expression of neural progenitor genes.

Induced Pluripotent Stem Cells as a new Strategy for Osteogenesis and Bone Regeneration

Induced pluripotent stem (iPS) cells, possess high proliferation and differentiation ability, are now considered an attractive option for osteogenic differentiation and bone regeneration. In fact, recent discoveries have demonstrated that iPS cells can be differentiated into osteoblasts, suggesting that iPS cells have the potential to advance future bone regenerative therapies. Herein, we provide an overview of the recent findings on osteogenic characteristics and differentiation potential of iPS cells. In addition, we discuss current methods for inducing their specification towards osteogenic phenotype as well as in vivo evidence supporting the therapeutic benefit of iPS-derived osteoblasts. Finally, we describe recent findings regarding the use of iPS-derived cells for osteogenic differentiation and bone regeneration, which have indicated that these pluripotent cells represent an ideal tool for regenerative cell therapies and might contribute to the development of future bone tissue engineering.

Angiogenic activity mediates bone repair from human pluripotent stem cell-derived osteogenic cells

Human pluripotent stem cells provide a standardized resource for bone repair. However, criteria to determine which exogenous cells best heal orthopedic injuries remain poorly defined. We evaluated osteogenic progenitor cells derived from both human embryonic stem cells (hESCs) and induced pluripotent stem cells (hiPSCs). Phenotypic and genotypic analyses demonstrated that these hESCs/hiPSCs are similar in their osteogenic differentiation efficiency and they generate osteogenic cells comparable to osteogenic cells derived from mesenchymal stromal cells (BM-MSCs). However, expression of angiogenic factors, such as vascular endothelial growth factor and basic fibroblast growth factor in these osteogenic progenitor cells are markedly different, suggesting distinct pro-angiogenic potential of these stem cell derivatives. Studies to repair a femur non-union fracture demonstrate only osteogenic progenitor cells with higher pro-angiogenic potential significantly enhance bone repair in vivo. Together, these studies highlight a key role of pro-angiogenic potential of transplanted osteogenic cells for effective cell-mediated bone repair.

New Protocol to Optimize iPS Cells for Genome Analysis of Fibrodysplasia Ossificans Progressiva

Successful in vitro disease-recapitulation using patient-specific induced pluripotent stem cells (iPSCs) requires two fundamental technical issues: appropriate control cells and robust differentiation protocols. To investigate fibrodysplasia ossificans progressiva (FOP), a rare genetic disease leading to extraskeletal bone formation through endochondral ossification, gene-corrected (rescued) iPSC clones (resFOP-iPSC) were generated from patient-derived iPSC (FOP-iPSC) as genetically matched controls, and the stepwise induction method of mesenchymal stromal cells (iMSCs) through neural crest cell (NCC) lineage was used to recapitulate the disease phenotype. FOP-iMSCs possessing enhanced chondrogenic ability were transcriptionally distinguishable from resFOP-iMSCs and activated the SMAD1/5/8 and SMAD2/3 pathways at steady state. Using this method, we identified MMP1 and PAI1 as genes responsible for accelerating the chondrogenesis of FOP-iMSCs. These data indicate that iMSCs through NCC lineage are useful for investigating the molecular mechanism of FOP and corresponding drug discovery.

Selection via pluripotency-related transcriptional screen minimizes the influence of somatic origin on iPSC differentiation propensity

The value of induced pluripotent stem cells (iPSCs) within regenerative medicine is contingent on predictable and consistent iPSC differentiation. However, residual influence of the somatic origin or reprogramming technique may variegate differentiation propensity and confound comparative genotype/phenotype analyses. The objective of this study was to define quality control measures to select iPSC clones that minimize the influence of somatic origin on differentiation propensity independent of the reprogramming strategy. More than 60 murine iPSC lines were derived from different fibroblast origins (embryonic, cardiac, and tail tip) via lentiviral integration and doxycycline-induced transgene expression. Despite apparent equivalency according to established iPSC histologic and cytomorphologic criteria, clustering of clonal variability in pluripotency-related gene expression identified transcriptional outliers that highlighted cell lines with unpredictable cardiogenic propensity. Following selection according to a standardized gene expression profile calibrated by embryonic stem cells, the influence of somatic origin on iPSC methylation and transcriptional patterns was negated. Furthermore, doxycycline-induced iPSCs consistently demonstrated earlier differentiation than lentiviral-reprogrammed lines using contractile cardiac tissue as a measure of functional differentiation. Moreover, delayed cardiac differentiation was predominately associated with upregulation in pluripotency-related gene expression upon differentiation. Starting from a standardized pool of iPSCs, relative expression levels of two pluripotency genes, Oct4 and Zfp42, statistically correlated with enhanced cardiogenicity independent of somatic origin or reprogramming strategy (R(2)  = 0.85). These studies demonstrate that predictable iPSC differentiation is independent of somatic origin with standardized gene expression selection criteria, while the residual impact of reprogramming strategy greatly influences predictable output of tissue-specification required for comparative genotype/phenotype analyses.

Generation of iPSCs as a Pooled Culture Using Magnetic Activated Cell Sorting of Newly Reprogrammed Cells

Although significant advancement has been made in the induced pluripotent stem cell (iPSC) field, current methods for iPSC derivation are labor intensive and costly. These methods involve manual selection, expansion, and characterization of multiple clones for each reprogrammed cell sample and therefore significantly hampers the feasibility of studies where a large number of iPSCs need to be derived. To develop higher throughput iPSC reprogramming methods, we generated iPSCs as a pooled culture using rigorous cell surface pluripotent marker selection with TRA-1-60 or SSEA4 antibodies followed by Magnetic Activated Cell Sorting (MACS). We observed that pool-selected cells are similar or identical to clonally derived iPSC lines from the same donor by all criteria examined, including stable expression of endogenous pluripotency genes, normal karyotype, loss of exogenous reprogramming factors, and in vitro spontaneous and lineage directed differentiation potential. This strategy can be generalized for iPSC generation using both integrating and non-integrating reprogramming methods. Our studies provide an attractive alternative to clonal derivation of iPSCs using rigorously selected cell pools and is amenable to automation.

Enhanced chondrogenesis of induced pluripotent stem cells from patients with neonatal-onset multisystem inflammatory disease occurs via the caspase 1-independent cAMP/protein kinase A/CREB pathway

OBJECTIVE

Neonatal-onset multisystem inflammatory disease (NOMID) is a dominantly inherited autoinflammatory disease caused by NLRP3 mutations. NOMID pathophysiology is explained by the NLRP3 inflammasome, which produces interleukin-1β (IL-1β). However, epiphyseal overgrowth in NOMID is resistant to anti-IL-1 therapy and may therefore occur independently of the NLRP3 inflammasome. This study was undertaken to investigate the effect of mutated NLRP3 on chondrocytes using induced pluripotent stem cells (iPSCs) from patients with NOMID.

METHODS

We established isogenic iPSCs with wild-type or mutant NLRP3 from 2 NOMID patients with NLRP3 somatic mosaicism. The iPSCs were differentiated into chondrocytes in vitro and in vivo. The phenotypes of chondrocytes with wild-type and mutant NLRP3 were compared, particularly the size of the chondrocyte tissue produced.

RESULTS

Mutant iPSCs produced larger chondrocyte masses than wild-type iPSCs owing to glycosaminoglycan overproduction, which correlated with increased expression of the chondrocyte master regulator SOX9. In addition, in vivo transplantation of mutant cartilaginous pellets into immunodeficient mice caused disorganized endochondral ossification. Enhanced chondrogenesis was independent of caspase 1 and IL-1, and thus the NLRP3 inflammasome. Investigation of the human SOX9 promoter in chondroprogenitor cells revealed that the CREB/ATF-binding site was critical for SOX9 overexpression caused by mutated NLRP3. This was supported by increased levels of cAMP and phosphorylated CREB in mutant chondroprogenitor cells.

CONCLUSION

Our findings indicate that the intrinsic hyperplastic capacity of NOMID chondrocytes is dependent on the cAMP/PKA/CREB pathway, independent of the NLRP3 inflammasome.

Improvement of In Vitro Osteogenic Potential through Differentiation of Induced Pluripotent Stem Cells from Human Exfoliated Dental Tissue towards Mesenchymal-Like Stem Cells

Constraints for the application of MSCs for bone reconstruction include restricted self-renewal and limited cell amounts. iPSC technology presents advantages over MSCs, providing homogeneous cellular populations with prolonged self-renewal and higher plasticity. However, it is unknown if the osteogenic potential of iPSCs differs from that of MSCs and if it depends on the iPSCs originating cellular source. Here, we compared the in vitro osteogenesis between stem cells from human deciduous teeth (SHED) and MSC-like cells from iPSCs from SHED (iPS-SHED) and from human dermal fibroblasts (iPS-FIB). MSC-like cells from iPS-SHED and iPS-FIB displayed fibroblast-like morphology, downregulation of pluripotency markers and upregulation of mesenchymal markers. Comparative in vitro osteogenesis analysis showed higher osteogenic potential in MSC-like cells from iPS-SHED followed by MSC-like cells from iPS-FIB and SHED. CD105 expression, reported to be inversely correlated with osteogenic potential in MSCs, did not display this pattern, considering that SHED presented lower CD105 expression. Higher osteogenic potential of MSC-like cells from iPS-SHED may be due to cellular homogeneity and/or to donor tissue epigenetic memory. Our findings strengthen the rationale for the use of iPSCs in bone bioengineering. Unveiling the molecular basis behind these differences is important for a thorough use of iPSCs in clinical scenarios.

Derivation of mesenchymal stromal cells from pluripotent stem cells through a neural crest lineage using small molecule compounds with defined media

Neural crest cells (NCCs) are an embryonic migratory cell population with the ability to differentiate into a wide variety of cell types that contribute to the craniofacial skeleton, cornea, peripheral nervous system, and skin pigmentation. This ability suggests the promising role of NCCs as a source for cell-based therapy. Although several methods have been used to induce human NCCs (hNCCs) from human pluripotent stem cells (hPSCs), such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), further modifications are required to improve the robustness, efficacy, and simplicity of these methods. Chemically defined medium (CDM) was used as the basal medium in the induction and maintenance steps. By optimizing the culture conditions, the combination of the GSK3β inhibitor and TGFβ inhibitor with a minimum growth factor (insulin) very efficiently induced hNCCs (70-80%) from hPSCs. The induced hNCCs expressed cranial NCC-related genes and stably proliferated in CDM supplemented with EGF and FGF2 up to at least 10 passages without changes being observed in the major gene expression profiles. Differentiation properties were confirmed for peripheral neurons, glia, melanocytes, and corneal endothelial cells. In addition, cells with differentiation characteristics similar to multipotent mesenchymal stromal cells (MSCs) were induced from hNCCs using CDM specific for human MSCs. Our simple and robust induction protocol using small molecule compounds with defined media enabled the generation of hNCCs as an intermediate material producing terminally differentiated cells for cell-based innovative medicine.

Nuclear receptor gene alteration in human induced pluripotent stem cells with hepatic differentiation propensity

AIM

Human induced pluripotent stem (hiPS) cells are an alternative cell source of regenerative medicine for liver disease. Because variations in hepatic differentiation efficacy among hiPS cells exist, it is important to select a hiPS cell line with hepatic differentiation propensity. In addition, nuclear receptors (NR) regulate essential biological processes including differentiation and development. In this study, we identified the hiPS cell line with hepatic differentiation propensity and examined expression levels of 48 NR during this process.

METHODS

We screened 28 hiPS cell lines, which are established from various tissues of healthy persons with various reprogramming methods, using a three-step differentiation method, and examined expression levels of 48 NR by quantitative real-time polymerase chain reaction during the differentiation process in the selected cells.

RESULTS

hiPS-RIKEN-2B and hiPS-RIKEN-2F cells have hepatic differentiation propensity. Differentiation propensity towards endoderm was affected by donor origin but not by reprogramming methods or cell type of origins. Expression levels of NR were closely associated with those of hepatic differentiation markers. Furthermore, expression patterns of NR were categorized as five patterns. In particular, seven NR such as chicken ovalbumin upstream promoter transcription factor 1, retinoic acid receptor α, peroxisome proliferator-activated receptor-γ, progesterone receptor, photoreceptor cell-specific nuclear receptor, tailless homolog orphan receptor and glucocorticoid receptor were identified as the genes of which expression gradually goes up with differentiation.

CONCLUSION

These findings will be useful for not only elucidating mechanisms of hepatic differentiation of hiPS cells but also cell-based therapy for liver diseases.

Cancer-like epigenetic derangements of human pluripotent stem cells and their impact on applications in regeneration and repair

A growing body of work has raised concern that many human pluripotent stem cell (hPSC) lines possess tumorigenic potential following differentiation to clinically relevant lineages. In this review, we highlight recent work characterizing the spectrum of cancer-like epigenetic derangements in human embryonic stem cells (hESC) and human induced pluripotent stem cells (hiPSC) that are associated with reprogramming errors or prolonged culture that may contribute to such tumorigenicity. These aberrations include cancer-like promoter DNA hypermethylation and histone marks associated with pluripotency, as well as aberrant X-chromosome regulation. We also feature recent work that suggests optimized high-fidelity reprogramming derivation methods can minimize cancer-associated epigenetic aberrations in hPSC, and thus ultimately improve the ultimate clinical utility of hiPSC in regenerative medicine.

"Mesenchymal" stem cells

Two opposing descriptions of so-called mesenchymal stem cells (MSCs) exist at this time. One sees MSCs as the postnatal, self-renewing, and multipotent stem cells for the skeleton. This cell coincides with a specific type of bone marrow perivascular cell. In skeletal physiology, this skeletal stem cell is pivotal to the growth and lifelong turnover of bone and to its native regeneration capacity. In hematopoietic physiology, its role as a key player in maintaining hematopoietic stem cells in their niche and in regulating the hematopoietic microenvironment is emerging. In the alternative description, MSCs are ubiquitous in connective tissues and are defined by in vitro characteristics and by their use in therapy, which rests on their ability to modulate the function of host tissues rather than on stem cell properties. Here, I discuss how the two views developed, conceptually and experimentally, and attempt to clarify the confusion arising from their collision.

Functional comparison of human-induced pluripotent stem cell-derived mesenchymal cells and bone marrow-derived mesenchymal stromal cells from the same donor

Mesenchymal stem cells (MSCs) have a high potential for therapeutic efficacy in treating diverse musculoskeletal injuries and cardiovascular diseases, and for ameliorating the severity of graft-versus-host and autoimmune diseases. While most of these clinical applications require substantial cell quantities, the number of MSCs that can be obtained initially from a single donor is limited. Reports on the derivation of MSC-like cells from pluripotent stem cells (PSCs) are, thus, of interest, as the infinite proliferative capacity of PSCs opens the possibility to generate large amounts of uniform batches of MSCs. However, characterization of such MSC-like cells is currently inadequate, especially with regard to the question of whether these cells are equivalent or identical to MSCs. In this study, we have derived MSC-like cells [induced PSC-derived MSC-like progenitor cells (iMPCs)] using four different methodologies from a newly established induced PSC line reprogrammed from human bone marrow stromal cells (BMSCs), and compared the iMPCs directly with the originating parental BMSCs. The iMPCs exhibited typical MSC/fibroblastic morphology and MSC-typical surface marker profile, and they were capable of differentiation in vitro along the osteogenic, chondrogenic, and adipogenic lineages. However, compared with the parental BMSCs, iMPCs displayed a unique expression pattern of mesenchymal and pluripotency genes and were less responsive to traditional BMSC differentiation protocols. We, therefore, conclude that iMPCs generated from PSCs via spontaneous differentiation represent a distinct population of cells which exhibit MSC-like characteristics.

Suitability of porcine chondrocyte micromass culture to model osteoarthritis in vitro

In vitro tissue models are useful tools for the development of novel therapy strategies in cartilage repair and care. The limited availability of human primary tissue and high costs of animal models hamper preclinical tests of innovative substances and techniques. In this study we tested the potential of porcine chondrocyte micromass cultures to mimic human articular cartilage and essential aspects of osteoarthritis (OA) in vitro. Primary chondrocytes were enzymatically isolated from porcine femoral condyles and were maintained in 96-multiwell format to establish micromass cultures in a high-throughput scale. Recombinant porcine tumor necrosis factor alpha (TNF-α) was used to induce OA-like changes documented on histological (Safranin O, collagen type II staining), biochemical (hydroxyproline assay, dimethylmethylene blue method), and gene expression level (Affymetrix porcine microarray, real time PCR) and were compared with published data from human articular cartilage and human micromass cultures. After 14 days in micromass culture, porcine primary chondrocytes produced ECM rich in proteoglycans and collagens. On gene expression level, significant correlations of detected genes with porcine cartilage (r = 0.90), human cartilage (r = 0.71), and human micromass culture (r = 0.75) were observed including 34 cartilage markers such as COL2A1, COMP, and aggrecan. TNF-α stimulation led to significant proteoglycan (-75%) and collagen depletion (-50%). Comparative expression pattern analysis revealed the involvement of catabolic enzymes (MMP1, -2, -13, ADAM10), chemokines (IL8, CCL2, CXCL2, CXCL12, CCXL14), and genes associated with cell death (TNFSF10, PMAIPI, AHR) and skeletal development (GPNMB, FRZB) including transcription factors (WIF1, DLX5, TWIST1) and growth factors (IGFBP1, -3, TGFB1) consistent with published data from human OA cartilage. Expression of genes related to cartilage ECM formation (COL2A1, COL9A1, COMP, aggrecan) as well as hypertrophic bone formation (COL1A1, COL10A1) was predominantly found decreased. These findings indicating significant parallels between human articular cartilage and the presented porcine micromass model and vice versa confirm the applicability of known cartilage marker and their characteristics in the porcine micromass model. TNF-α treatment enabled the initiation of typical OA reaction patterns in terms of extensive ECM loss, cell death, formation of an inflammatory environment through the induction of genes coding for chemokines and enzymes, and the modulation of genes involved in skeletal development such as growth factors, transcription factors, and cartilage ECM-forming genes. In conclusion, the porcine micromass model represents an alternative tissue platform for the evaluation of innovative substances and techniques for the treatment of OA.

Stem cells for osteodegenerative diseases: current studies and future outlook

As the worldwide population grows and life expectancies continue to increase, degenerative diseases of the bones, muscles, and connective tissue are a growing problem for society. Current therapies for osteodegenerative disorders such as hormone replacement therapies, calcium/vitamin D supplements and oral bisphosphonates are often inadequate to stop degeneration and/or have serious negative side effects. Thus, there is an urgent need in the medical community for more effective and safer treatments. Stem cell therapies for osteodegenerative disorders have been rigorously explored over the last decade and are yielding some promising results in animal models and clinical trials. Although much work still needs to be done to ensure the safety and efficacy of these therapies, stem cells represent a new frontier of exciting possibilities for bone and cartilage regeneration.

The aging signature: a hallmark of induced pluripotent stem cells?

The discovery that somatic cells can be induced into a pluripotent state by the expression of reprogramming factors has enormous potential for therapeutics and human disease modeling. With regard to aging and rejuvenation, the reprogramming process resets an aged, somatic cell to a more youthful state, elongating telomeres, rearranging the mitochondrial network, reducing oxidative stress, restoring pluripotency, and making numerous other alterations. The extent to which induced pluripotent stem cell (iPSC)s mime embryonic stem cells is controversial, however, as iPSCs have been shown to harbor an epigenetic memory characteristic of their tissue of origin which may impact their differentiation potential. Furthermore, there are contentious data regarding the extent to which telomeres are elongated, telomerase activity is reconstituted, and mitochondria are reorganized in iPSCs. Although several groups have reported that reprogramming efficiency declines with age and is inhibited by genes upregulated with age, others have successfully generated iPSCs from senescent and centenarian cells. Mixed findings have also been published regarding whether somatic cells generated from iPSCs are subject to premature senescence. Defects such as these would hinder the clinical application of iPSCs, and as such, more comprehensive testing of iPSCs and their potential aging signature should be conducted.

Induced pluripotent stem cells from patients with human fibrodysplasia ossificans progressiva show increased mineralization and cartilage formation

Background

Abnormal activation of endochondral bone formation in soft tissues causes significant medical diseases associated with disability and pain. Hyperactive mutations in the bone morphogenetic protein (BMP) type 1 receptor ACVR1 lead to fibrodysplasia ossificans progressiva (FOP), a rare genetic disorder characterized by progressive ossification in soft tissues. However, the specific cellular mechanisms are unclear. In addition, the difficulty obtaining tissue samples from FOP patients and the limitations in mouse models of FOP hamper our ability to dissect the pathogenesis of FOP.

Methods

To address these challenges and develop a “disease model in a dish”, we created human induced pluripotent stem cells (iPS cells) derived from normal and FOP dermal fibroblasts by two separate methods, retroviral integration or integration-free episomal vectors. We tested if the ability to contribute to different steps of endochondral bone formation was different in FOP vs. control iPS cells.

Results

Remarkably, FOP iPS cells showed increased mineralization and enhanced chondrogenesis in vitro. The mineralization phenotypes could be suppressed with a small-molecule inhibitor of BMP signaling, DMH1. Our results indicate that the FOP ACVR1 R206H mutation favors chondrogenesis and increases mineral deposition in vitro.

Conclusions

Our findings establish a FOP disease cell model for in vitro experimentation and provide a proof-of-concept for using human iPS cell models to understand human skeletal disorders.

The application of induced pluripotent stem cells for bone regeneration: current progress and prospects

Loss of healthy bone tissue and dysosteogenesis are still common and significant problems in clinics. Cell-based therapy using mesenchymal stem cells (MSCs) has been performed in patients for quite some time, but the inherent drawbacks of these cells, such as the reductions in proliferation rate and osteogenic differentiation potential that occur with aging, greatly limit their further application. Moreover, embryonic stem cells (ESCs) have brought new hope to osteoregenerative medicine because of their full pluripotent differentiation potential and excellent performance in bone regeneration. However, the ethical issues involved in destroying human embryos and the immune reactions that occur after transplantation are two major stumbling blocks impeding the clinical application of ESCs. Instead, induced pluripotent stem cells (iPSCs), which are ESC-like pluripotent cells that are reprogrammed from adult somatic cells using defined transcription factors, are considered a more promising source of cells for regenerative medicine because they present no ethical or immunological issues. Here, we summarize the primary technologies for generating iPSCs and the biological properties of these cells, review the current advances in iPSC-based bone regeneration and, finally, discuss the remaining challenges associated with these cells, particularly safety issues and their potential application for osteoregenerative medicine.