Electron microscopy reveals the presence of viruses in mouse embryonic fibroblasts but neither in human embryonic fibroblasts nor in human mesenchymal cells used for hESC maintenance: toward an implementation of microbiological quality assurance program in stem cell banks

Integration of xeno-free single-cell cloning in CRISPR-mediated DNA editing of human iPSCs improves homogeneity and methodological efficiency of cellular disease modeling

The capability to generate induced pluripotent stem cell (iPSC) lines, in tandem with CRISPR-Cas9 DNA editing, offers great promise to understand the underlying genetic mechanisms of human disease. The low efficiency of available methods for homogeneous expansion of singularized CRISPR-transfected iPSCs necessitates the coculture of transfected cells in mixed populations and/or on feeder layers. Consequently, edited cells must be purified using labor-intensive screening and selection, culminating in inefficient editing. Here, we provide a xeno-free method for single-cell cloning of CRISPRed iPSCs achieving a clonal survival of up to 70% within 7-10 days. This is accomplished through improved viability of the transfected cells, paralleled with provision of an enriched environment for the robust establishment and proliferation of singularized iPSC clones. Enhanced cell survival was accompanied by a high transfection efficiency exceeding 97%, and editing efficiencies of 50%-65% for NHEJ and 10% for HDR, indicative of the method's utility in stem cell disease modeling.

The Use of Stem Cell-Derived Organoids in Disease Modeling: An Update

Organoids represent one of the most important advancements in the field of stem cells during the past decade. They are three-dimensional in vitro culturing models that originate from self-organizing stem cells and can mimic the in vivo structural and functional specificities of body organs. Organoids have been established from multiple adult tissues as well as pluripotent stem cells and have recently become a powerful tool for studying development and diseases in vitro, drug screening, and host-microbe interaction. The use of stem cells-that have self-renewal capacity to proliferate and differentiate into specialized cell types-for organoids culturing represents a major advancement in biomedical research. Indeed, this new technology has a great potential to be used in a multitude of fields, including cancer research, hereditary and infectious diseases. Nevertheless, organoid culturing is still rife with many challenges, not limited to being costly and time consuming, having variable rates of efficiency in generation and maintenance, genetic stability, and clinical applications. In this review, we aim to provide a synopsis of pluripotent stem cell-derived organoids and their use for disease modeling and other clinical applications.

Cell-Based Therapy Manufacturing in Stirred Suspension Bioreactor: Thoughts for cGMP Compliance

Cell-based therapy (CBT) is attracting much attention to treat incurable diseases. In recent years, several clinical trials have been conducted using human pluripotent stem cells (hPSCs), and other potential therapeutic cells. Various private- and government-funded organizations are investing in finding permanent cures for diseases that are difficult or expensive to treat over a lifespan, such as age-related macular degeneration, Parkinson’s disease, or diabetes, etc. Clinical-grade cell manufacturing requiring current good manufacturing practices (cGMP) has therefore become an important issue to make safe and effective CBT products. Current cell production practices are adopted from conventional antibody or protein production in the pharmaceutical industry, wherein cells are used as a vector to produce the desired products. With CBT, however, the “cells are the final products” and sensitive to physico- chemical parameters and storage conditions anywhere between isolation and patient administration. In addition, the manufacturing of cellular products involves multi-stage processing, including cell isolation, genetic modification, PSC derivation, expansion, differentiation, purification, characterization, cryopreservation, etc. Posing a high risk of product contamination, these can be time- and cost- prohibitive due to maintenance of cGMP. The growing demand of CBT needs integrated manufacturing systems that can provide a more simple and cost-effective platform. Here, we discuss the current methods and limitations of CBT, based upon experience with biologics production. We review current cell manufacturing integration, automation and provide an overview of some important considerations and best cGMP practices. Finally, we propose how multi-stage cell processing can be integrated into a single bioreactor, in order to develop streamlined cGMP-compliant cell processing systems.

Clinical-Grade Human Pluripotent Stem Cells for Cell Therapy: Characterization Strategy

Human pluripotent stem cells have the potential to change the way in which human diseases are cured. Clinical-grade human embryonic stem cells and human induced pluripotent stem cells have to be created according to current good manufacturing practices and regulations. Quality and safety must be of the highest importance when humans' lives are at stake. With the rising number of clinical trials, there is a need for a consensus on hPSCs characterization. Here, we summarize mandatory and 'for information only' characterization methods with release criteria for the establishment of clinical-grade hPSC lines.

An Efficient Protocol for Single-Cell Cloning Human Pluripotent Stem Cells

Genomic manipulation of human pluripotent stem cells (hPSCs) has become essential to introduce genetic modifications and transgenes, and develop reporter lines. One of the major bottlenecks in gene editing is at the stage of single-cell cloning, which is thought to be variable across hPSC lines and is substantially reduced following a transfection. Due to the difficulty of performing fluorescent-assisted cell sorting (FACS) for single-cell isolation of hPSCs, previous approaches rely on manual colony picking, which is both time-consuming and labor-intensive. In this protocol, I provide a method for utilizing FACS to generate single-cell clones of hPSCs with efficiencies approaching 40% within 7–10 days. This can be achieved by sorting cells onto a feeder layer of MEFs in a stem cell defined medium with KSR and a Rock inhibitor, as early as 1–2 days following a transfection, streamlining the gene editing process. The approach described here provides a fundamental method for all researchers utilizing hPSCs for scientific studies.

Isolation, Culture, and Functional Characterization of Human Embryonic Stem Cells: Current Trends and Challenges

Human embryonic stem cells (hESCs) hold great potential for the treatment of various degenerative diseases. Pluripotent hESCs have a great ability to undergo unlimited self-renewal in culture and to differentiate into all cell types in the body. The journey of hESC research is not that smooth, as it has faced several challenges which are limited to not only tumor formation and immunorejection but also social, ethical, and political aspects. The isolation of hESCs from the human embryo is considered highly objectionable as it requires the destruction of the human embryo. The issue was debated and discussed in both public and government platforms, which led to banning of hESC research in many countries around the world. The banning has negatively affected the progress of hESC research as many federal governments around the world stopped research funding. Afterward, some countries lifted the ban and allowed the funding in hESC research, but the damage has already been done on the progress of research. Under these unfavorable conditions, still some progress was made to isolate, culture, and characterize hESCs using different strategies. In this review, we have summarized various strategies used to successfully isolate, culture, and characterize hESCs. Finally, hESCs hold a great promise for clinical applications with proper strategies to minimize the teratoma formation and immunorejection and better cell transplantation strategies.

Human foreskin fibroblasts: from waste bag to important biomedical applications

Circumcision is one of the most performed surgical procedures worldwide, and it is estimated that one in three men worldwide is circumcised, which makes the preputial skin removed after surgery an abundant material for possible applications. In particular, it is possible efficiently to isolate the cells of the foreskin, with fibroblasts being the most abundant cells of the dermis and the most used in biomedical research. This work aimed to review the knowledge and obtain a broad view of the main applications of human foreskin fibroblast cell culture. A literature search was conducted, including clinical trials, preclinical basic research studies, reviews and experimental studies. Several medical and laboratory applications of human foreskin fibroblast cell culture have been described, especially when it comes to the use of human foreskin fibroblasts as feeder cells for the cultivation of human embryonic stem cells, in addition to co-culture with other cell types. The culture of foreskin fibroblasts has also been used to: obtain induced pluripotent stem cells; the diagnosis of Clostridium difficile; to test the toxicity and effect of substances on normal cells, especially the toxicity of possible antineoplastic drugs; in viral culture, mainly of the human cytomegalovirus, study of the pathogenesis of other microorganisms; varied studies of cellular physiology and cellular interactions. Fibroblasts are important for cell models for varied application cultures, demonstrating how the preputial material can be reused, making possible new applications.

Level of evidence: Not applicable for this multicentre audit.

Effects of Feeder Cells on Dopaminergic Differentiation of Human Embryonic Stem Cells

Mouse embryonic fibroblasts (MEFs) and human foreskin fibroblasts (HFFs) are used for the culture of human embryonic stem cells (hESCs). MEFs and HFFs differed in their capacity to support the proliferation and pluripotency of hESCs and could affect cardiac differentiation potential of hESCs. The aim of this study was to evaluate the effect of MEFs and HFFs feeders on dopaminergic differentiation of hESCs lines. To minimize the impact of culture condition variation, two hESCs lines were cultured on mixed feeder cells (MFCs, MEFs: HFFs = 1:1) and HFFs feeder, respectively, and then were differentiated into dopaminergic (DA) neurons under the identical protocol. Dopaminergic differentiation was evaluated by immunocytochemistry, quantitative fluorescent real-time PCR, transmission and scanning electron microscopy, and patch clamp. Our results demonstrated that these hESCs-derived neurons were genuine and functional DA neurons. However, compared to hESCs line on MFCs feeder, hESCs line on HFFs feeder had a higher proportion of tyrosine hydroxylase (TH) positive cells and expressed higher levels of FOXA2, PITX3, NURR1, and TH genes. In addition, the values of threshold intensity and threshold membrane potential of DA neurons from hESCs line on HFFs feeder were lower than those of DA neurons from hESCs line on the MFCs feeder. In conclusion, HFFs feeder not only facilitated the differentiation of hESCs cells into dopaminergic neurons, but also induced hESCs-derived DA neurons to express higher electrophysiological excitability. Therefore, feeder cells could affect not only dopaminergic differentiation potential of different hESCs lines, but also electrophysiological properties of hESCs-derived DA neurons.

Human embryonic stem cell cultivation: historical perspective and evolution of xeno-free culture systems

Human embryonic stem cells (hESC) have emerged as attractive candidates for cell-based therapies that are capable of restoring lost cell and tissue function. These unique cells are able to self-renew indefinitely and have the capacity to differentiate in to all three germ layers (ectoderm, endoderm and mesoderm). Harnessing the power of these pluripotent stem cells could potentially offer new therapeutic treatment options for a variety of medical conditions. Since the initial derivation of hESC lines in 1998, tremendous headway has been made in better understanding stem cell biology and culture requirements for maintenance of pluripotency. The approval of the first clinical trials of hESC cells for treatment of spinal cord injury and macular degeneration in 2010 marked the beginning of a new era in regenerative medicine. Yet it was clearly recognized that the clinical utility of hESC transplantation was still limited by several challenges. One of the most immediate issues has been the exposure of stem cells to animal pathogens, during hESC derivation and during in vitro propagation. Initial culture protocols used co-culture with inactivated mouse fibroblast feeder (MEF) or human feeder layers with fetal bovine serum or alternatively serum replacement proteins to support stem cell proliferation. Most hESC lines currently in use have been exposed to animal products, thus carrying the risk of xeno-transmitted infections and immune reaction. This mini review provides a historic perspective on human embryonic stem cell culture and the evolution of new culture models. We highlight the challenges and advances being made towards the development of xeno-free culture systems suitable for therapeutic applications.

Morphological analysis of human induced pluripotent stem cells during induced differentiation and reverse programming

The fine analysis of cell components during the generation of pluripotent cells and their comparison to bone fide human embryonic stem cells (hESCs) are valuable tools to understand their biological behavior. In this report, human mesenchymal cells (hMSCs) generated from the human ES cell line H9, were reprogrammed back to induced pluripotent state using Oct-4, Sox2, Nanog, and Lin28 transgenes. Human induced pluripotent stem cells (hIPSCs) were analyzed using electron microscopy and compared with regard to the original hESCs and the hMSCs from which they were derived. This analysis shows that hIPSCs and the original hESCs are morphologically undistinguishable but differ from the hMSCs with respect to the presence of several morphological features of undifferentiated cells at both the cytoplasmic (ribosomes, lipid droplets, glycogen, scarce reticulum) and nuclear levels (features of nuclear plasticity, presence of euchromatin, reticulated nucleoli). We show that hIPSC colonies generated this way presented epithelial aspects with specialized junctions highlighting morphological criteria of the mesenchymal–epithelial transition in cells engaged in a successful reprogramming process. Electron microscopic analysis revealed also specific morphological aspects of partially reprogrammed cells. These results highlight the valuable use of electron microscopy for a better knowledge of the morphological aspects of IPSC and cellular reprogramming.

Autogenic feeder free system from differentiated mesenchymal progenitor cells, maintains pluripotency of the MEL-1 human embryonic stem cells

Human embryonic stem cells (hESc) are known for its pluripotency and self renewal capability, thus possess great potential in regenerative medicine. However, the lack of suitable xenofree extracellular matrix substrate inhibits further applications or the use of hESc in cell-based therapy. In this study, we described a new differentiation method, which generates a homogeneous population of mesenchymal progenitor cells (hESc-MPC) from hESc via epithelial-mesenchymal transition. The extracellular matrix (ECM) proteins from hESc-MPC had in turn supported the undifferentiated expansion of hESc. Immunocytochemistry and flow cytometry characterization of hESc-MPC revealed the presence of early mesenchymal markers. Tandem mass spectometry analysis of ECM produced by hESc-MPC revealed the presence of a mixture of extracellular proteins which includes tenascin C, fibronectin, and vitronectin. The pluripotency of hESc (MEL-1) cultured on the ECM was maintained as shown by the expression of pluripotent genes (FoxD3, Oct-4, Tdgf1, Sox-2, Nanog, hTERT, Rex1), protein markers (SSEA-3, SSEA-4, TRA-1-81, TRA-1-60, Oct-4) and the ability to differentiate into cells representative of ectoderm, endoderm and mesoderm. In summary, we have established a xeno-free autogenic feeder free system to support undifferentiated expansion of hESc, which could be of clinical relevance.

Derivation and propagation of human embryonic stem cell lines from frozen embryos in an animal product-free environment

The protocols described here are comprehensive instructions for deriving human embryonic stem (hES) cell lines in xeno-free conditions from cryopreserved embryos. Details are included for propagation, cryopreservation and characterization. Initial derivation is on feeder cells and is followed by adaptation to a feeder-free environment; competent technicians can perform these simplified methods easily. From derivation to cryopreservation of fully characterized initial stocks takes 3-4 months. These protocols served as the basis for standard operating procedures (SOPs), with both operational and technical components, that we set to meet good manufacturing practice (GMP) and UK regulatory body requirements for derivation of clinical-grade cells. As such, these SOPs are currently used in our current GMP compliant facility to derive hES cell lines ab initio, in an animal product-free environment; these lines are suitable for research and potentially for clinical use in cell therapy. So far, we have derived eight clinical-grade lines, which will be freely available to the scientific community after submission/accession to the UK Stem Cell Bank.

Efficient derivation and genetic modifications of human pluripotent stem cells on engineered human feeder cell lines

Derivation of pluripotent stem cells (iPSCs) induced from somatic cell types and the subsequent genetic modifications of disease-specific or patient-specific iPSCs are crucial steps in their applications for disease modeling as well as future cell and gene therapies. Conventional procedures of these processes require co-culture with primary mouse embryonic fibroblasts (MEFs) to support self-renewal and clonal growth of human iPSCs as well as embryonic stem cells (ESCs). However, the variability of MEF quality affects the efficiencies of all these steps. Furthermore, animal sourced feeders may hinder the clinical applications of human stem cells. In order to overcome these hurdles, we established immortalized human feeder cell lines by stably expressing human telomerase reverse transcriptase, Wnt3a, and drug resistance genes in adult mesenchymal stem cells. Here, we show that these immortalized human feeders support efficient derivation of virus-free, integration-free human iPSCs and long-term expansion of human iPSCs and ESCs. Moreover, the drug-resistance feature of these feeders also supports nonviral gene transfer and expression at a high efficiency, mediated by piggyBac DNA transposition. Importantly, these human feeders exhibit superior ability over MEFs in supporting homologous recombination-mediated gene targeting in human iPSCs, allowing us to efficiently target a transgene into the AAVS1 safe harbor locus in recently derived integration-free iPSCs. Our results have great implications in disease modeling and translational applications of human iPSCs, as these engineered human cell lines provide a more efficient tool for genetic modifications and a safer alternative for supporting self-renewal of human iPSCs and ESCs.

Maintenance of human embryonic stem cells in media conditioned by human mesenchymal stem cells obviates the requirement of exogenous basic fibroblast growth factor supplementation

Despite the improvements in the human embryonic stem cell (hESC) culture systems, very similar conditions to those used to maintain hESCs on mouse feeders are broadly applied to culture methods based on human feeders. Indeed, basic fibroblast growth factor (bFGF), a master hESC-sustaining factor, is still added in nearly all medium formulations for hESC propagation. Human foreskin fibroblasts (HFFs) and mesenchymal stem cells (MSCs) used as feeders have recently been reported to support hESC growth without exogenous bFGF. However, whether hESCs may be maintained undifferentiated without exogenous bFGF using media conditioned (CM) by human feeders remains elusive. We hypothesize that HFFs and hMSCs are likely to be functionally different and therefore the mechanisms by which HFF-CM and MSC-CM support undifferentiated growth of hESCs may differ. We have thus determined whether HFF-CM and/or MSC-CM sustain feeder-free undifferentiated growth of hESC without exogenous supplementation of bFGF. We report that hMSCs synthesize higher levels of endogenous bFGF than HFFs. Accordingly and in contrast to HFF-CM, MSC-CM produced without the addition of exogenous bFGF supports hESC pluripotency and culture homeostasis beyond 20 passages without the need of bFGF supplementation. hESCs maintained without exogenous bFGF in MSC-CM retained hESC morphology and expression of pluripotency surface markers and transcription factors, formed teratomas, and showed spontaneous and lineage-directed in vitro differentiation capacity. Our data indicate that MSC-CM, but not HFF-CM, provides microenvironment cues supporting feeder-free long-term maintenance of pluripotent hESCs and obviates the requirement of exogenous bFGF at any time.

Maintenance of human embryonic stem cells in mesenchymal stem cell-conditioned media augments hematopoietic specification

The realization of human embryonic stem cells (hESC) as a model for human developmental hematopoiesis and in potential cell replacement strategies relies on an improved understanding of the extrinsic and intrinsic factors regulating hematopoietic-specific hESC differentiation. Human mesenchymal stem cells (hMSCs) are multipotent cells of mesodermal origin that form a part of hematopoietic stem cell niches and have an important role in the regulation of hematopoiesis through production of secreted factors and/or cell-to-cell interactions. We have previously shown that hESCs may be successfully maintained feeder free using hMSC-conditioned media (MSC-CM). Here, we hypothesized that hESCs maintained in MSC-CM may be more prone to differentiation toward hematopoietic lineage than hESCs grown in standard human foreskin fibroblast-conditioned media. We report that specification into hemogenic progenitors and subsequent hematopoietic differentiation and clonogenic progenitor capacity is robustly enhanced in hESC lines maintained in MSC-CM. Interestingly, co-culture of hESCs on hMSCs fully abrogates hematopoietic specification of hESCs, thus suggesting that the improved hematopoietic differentiation is mediated by MSC-secreted factors rather than by MSC-hESC physical interactions. To investigate the molecular mechanism involved in this process, we analyzed global (LINE-1) methylation and genome-wide promoter DNA methylation. hESCs grown in MSC-CM showed a decrease of 17% in global DNA methylation and a promoter DNA methylation signature consisting of 45 genes commonly hypomethylated and 102 genes frequently hypermethylated. Our data indicate that maintenance of hESCs in MSC-CM robustly augments hematopoietic specification and that the process seems mediated by MSC-secreted factors conferring a DNA methylation signature to undifferentiated hESCs which may influence further predisposition toward hematopoietic specification.

New culture system for human embryonic stem cells: autologous mesenchymal stem cell feeder without exogenous fibroblast growth factor 2

Human embryonic stem (hES) cells have been successfully maintained using human-cell feeder systems or feeder-free systems. However, despite advances in culture techniques, hES cells require supplementation with fibroblast growth factor 2 (FGF-2), an exogenous stemness factor, which is needed to sustain the authentic undifferentiated status. We developed a new culture system for hES cells; this system does not require supplementation with FGF-2 to obtain hES cells that are suitable for tissue engineering and regenerative medicine. This culture system employed mesenchymal stem cells derived from hES cells (hESC-MSCs) as autologous human feeder cells in the absence of FGF-2. The hES cell line SNUhES3 cultured in this new autologous feeder culture system maintained the typical morphology of hES cells and expression of pluripotency-related proteins, SSEA-4, TRA-1-60, OCT4, and alkaline phosphatase, without development of abnormal karyotypes after more than 30 passages. RNA expression of the pluripotency-related genes OCT4 and NANOG was similar to the expression in SNUhES3 cells maintained on xenofeeder STO cells. To identify the mechanism that enables the cells to be maintained without exogenous FGF-2, we checked the secretion of FGF-2 from the mitomycin-C treated autofeeder hESC-MSCs versus xenofeeder STO cells, and confirmed that hESC-MSCs secreted FGF-2 whereas STO cells did not. The level of FGF-2 in the media from the autofeeder system without exogenous FGF-2 was comparable to that from the xenofeeder system with addition of FGF-2. In conclusion, our new culture system for hES cells, which employs a feeder layer of autologous hESC-MSCs, supplies sufficient amounts of secreted FGF-2 to eliminate the requirement for exogenous FGF-2.

The adaptation of human embryonic stem cells to different feeder-free culture conditions is accompanied by a mitochondrial response

The mitochondrial contribution to the maintenance of human embryonic stem cell (hESC) pluripotency and culture homeostasis remains poorly understood. Here, we sought to determine whether hESC adaptation to different feeder-free culture conditions is linked to a mitochondrial adaptation. The expression of ESC pluripotency factors and parameters of mitochondrial contribution including mitochondrial membrane potential, mtDNA content, and the expression of master mitochondrial genes implicated in replication, transcription, and biogenesis were determined in 8 hESC lines maintained in 2 distinct human feeders-conditioned media (CM): human foreskin fibroblast-CM (HFF-CM) and mesenchymal stem cell-CM (MSC-CM). We show a robust parallel trend between the expression of ESC pluripotency factors and the mitochondrial contribution depending on the culture conditions employed to maintain the hESCs, with those in MSC-CM consistently displaying increased levels of pluripotency markers associated to an enhanced mitochondrial contribution. The differences in the mitochondrial status between hESCs maintained in MSC-CM versus HFF-CM respond to coordinated changes in mitochondrial gene expression and biogenesis. Importantly, the culture conditions determine the mitochondrial distribution within the stage-specific embryonic antigen 3 positive (SSEA3(+)) and negative (SSEA3(-)) isolated cell subsets. hESC colonies in MSC-CM display an "intrinsic" high mitochondrial status which may suffice to support undifferentiated growth, whereas hESC colonies maintained in HFF-CM show low mitochondrial status, possibly relying on the production of autologous niche with higher mitochondrial status to support pluripotency and culture homeostasis. Pluripotency markers and mitochondrial status are concomitantly reverted on changing the culture conditions, supporting an unrecognized role of the mitochondria in response to hESC culture adaptation. We provide the first evidence supporting that hESCs adaptation to different feeder-free culture systems relies on a mitochondrial response.

The Nodal inhibitor Lefty is negatively modulated by the microRNA miR-302 in human embryonic stem cells

MicroRNAs (miRNAs) have been shown to be important in early development and maintenance of human embryonic stem cells (hESCs). The miRNA miR-302-367 is specifically expressed in hESCs, and its expression decays on differentiation. We previously identified the structure of the gene coding for the human miR-302-367 cluster and characterized its promoter. The promoter activity was functionally validated in hESCs, opening up new avenues to further investigate how these miRNA molecules fit in the complex molecular network conferring "stemness" properties to hESCs. The physiological roles of specific miRNA-mRNA interactions remain largely unknown. Here, we investigated putative miR-302-367 mRNA targets in hESCs, potentially relevant for ESC biology. We found that the Nodal inhibitors Lefty1 and Lefty2 are post-transcriptionally targeted by miR-302s in hESCs. Functional analyses indicate that miR-302s negatively modulate the level of lefties, and become upstream regulators of the TGFβ/Nodal pathway, functioning via Smad-2/3 signaling. Overexpression of the miR-302-367 cluster in hESCs causes a delay in early hESC differentiation, as measured by enhanced levels of ESC-specific transcription factors, coupled to a faster teratoma formation in mice transplanted with miR-302-367-expressing hESCs and a concomitant impairment of germ layer specification, displaying robust decreased levels of early mesodermal, endodermal, and ectodermal specific markers. These findings suggest that Lefty is negatively modulated by miR-302s in hESCs, which plays an important role in maintaining the balance between pluripotency and germ layer specification.

Insights into the cellular origin and etiology of the infant pro-B acute lymphoblastic leukemia with MLL-AF4 rearrangement

Infant acute lymphoblastic leukemia (ALL) involving mixed-lineage leukemia (MLL) fusions has attracted a huge interest in basic and clinical research because of its prenatal origin, mixed-lineage phenotype, dismal prognosis and extremely short latency. Over 90% of infant ALLs are pro-B ALL harboring the leukemic fusion MLL-AF4. Despite the fact that major achievements have provided a better understanding about the etiology of infant MLL-AF4+ ALL over the last two decades, key questions remain unanswered. Epidemiological and genetic studies suggest that the in utero origin of MLL rearrangements in infant leukemia may be the result of prenatal exposure to genotoxic compounds. In fact, chronic exposure of human embryonic stem cells (hESCs) to etoposide induces MLL rearrangements and makes hESC more prone to acquire subsequent chromosomal abnormalities than postnatal CD34(+) cells, linking embryonic exposure to topoisomerase II inhibitors to genomic instability and MLL rearrangements. Unfortunately, very little is known about the nature of the target cell for transformation. Neuron-glial antigen 2 expression was initially claimed to be specifically associated with MLL rearrangements and was recently shown to be readily expressed in CD34+CD38+, but not CD34+CD38- cells suggesting that progenitors rather than stem cells may be the target cell for transformation. Importantly, the recent findings showing that MLL-AF4 rearrangement is present and expressed in mesenchymal stem cells from infant patients with MLLAF4+ ALL challenged our current view of the etiology and cellular origin of this leukemia. It becomes therefore crucial to determine where the leukemia relapses come from and how the tumor-stroma relationship is defined at the molecular level. Finally, MLL-AF4 leukemogenesis has been particularly difficult to model and bona fide MLL-AF4 disease models do not exist so far. It is likely that the current disease models are missing some essential ingredients of leukemogenesis in the human embryo/fetus. We thus propose modeling MLL-AF4+ infant pro-B ALL using prenatal hESCs.

Endoplasmic reticulum stress signals in defined human embryonic stem cell lines and culture conditions

Human embryonic stem cells (hESCs) are especially resistant to several cellular stresses, but the existence and induction of Endoplasmic Reticulum (ER) stress by culture conditions are unknown. Using qPCR, here, we investigated the behavior of the principal sensors of ER stress and their relation with the feeder layer, the type of conditioned media used in feeder free systems and the upregulation of several differentiation markers. We observed the preservation of pluripotency, and detected differential expression of differentiation markers in HS181 and SHEF1 hESCs growing on Adipose-derived mesenchymal stem cells (ASCs) and feeder-free system with different conditioned media (HEF-CM and ASC-CM). Taken together, these results demonstrate evidence of ER stress events that cells must resolve to survive and maintenance of markers of pluripotency. The early differentiation status defined could progress into a more differentiated state, and may be influenced by culture conditions.

Maintenance of pluripotency in human embryonic stem cells cultured on a synthetic substrate in conditioned medium

Realizing the potential clinical and industrial applications of human embryonic stem cells (hESCs) is limited by the need for costly, labile, or undefined growth substrates. Here we demonstrate that trypsin passaging of the hESC lines, HUES7 and NOTT1, on oxygen plasma etched tissue culture polystyrene (PE-TCPS) in conditioned medium is compatible with pluripotency. This synthetic culture surface is stable at room temperature for at least a year and is readily prepared by placing polystyrene substrates in a radio frequency oxygen plasma generator for 5 min. Modification of the polystyrene surface chemistry by plasma etching was confirmed by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), which identified elemental and molecular changes as a result of the treatment. Pluripotency of hESCs cultured on PE-TCPS was gauged by consistent proliferation during serial passage, expression of stem cell markers (OCT4, TRA1-60, and SSEA-4), stable karyotype and multi-germlayer differentiation in vitro, including to pharmacologically responsive cardiomyocytes. Generation of cost-effective, easy-to-handle synthetic, defined, stable surfaces for hESC culture will expedite stem cell use in biomedical applications.

Establishment of an exogenous LIF-free culture system for mouse embryonic stem cells

Mouse embryonic stem cells (mESCs) have played a key role in the newly emerging fields of stem cell research. The traditional derivation and culture of mESCs have been based on the use of mouse embryonic fibroblasts (MEFs) treated with exogenous leukemia inhibitory factor (LIF). However, the rapid senescence of MEFs, coupled with the high cost of LIF, has significantly hampered the widespread use of mESCs in stem cell research. Thus, we present a novel exogenous LIF-free culture system for general mESCs applications, comprising fibroblast-like cells derived from the rabbit spleen (RSFs). We demonstrated that mESCs cultured on RSFs (mESCs-RSFs) maintained all mESC features after prolonged LIF-free culture, including alkaline phosphatase, cell surface markers (SSEA-1), molecular markers (OCT-4, NANOG, TERT, REX-1), karyotype, and pluripotency. The high expression level of both LIF and WNT3A in the RSFs may account for their ability to maintain mESCs without exogenous LIF. Moreover, this exogenous LIF-free culture system was verified to be of microbiological quality through analysis with electron transmission microscopy.

Proteomic profiling of human embryonic stem cell-derived microvesicles reveals a risk of transfer of proteins of bovine and mouse origin

BACKGROUND AIMS

Microvesicles (MV) shed from the plasma membrane of eukaryotic cells, including human embryonic stem cells (hESC), contain proteins, lipids and RNA and serve as mediators of cell-to-cell communication. However, they may also contain immunogenic membrane domains and infectious particles acquired from xenogenic components of the culture milieu. Therefore, MV represent a potential risk for clinical application of cell therapy.

METHODS

We tested the ability of hESC and their most commonly used feeder cells, mouse embryonic fibroblasts (MEF), to produce MV. We found that hESC are potent producers of MV, whereas mitotically inactivated MEF do not produce any detectable MV. We therefore employed a combined proteomic approach to identify the molecules that constitute the major components of MV from hESC maintained in a standard culture setting with xenogenic feeder cells.

RESULTS

In purified MV fractions, we identified a total of 22 proteins, including five unique protein species that are known to be highly expressed in invasive cancers and participate in cellular activation, metastasis and inhibition of apoptosis. Moreover, we found that hESC-derived MV contained the immunogenic agents apolipoprotein and transferrin, a source of Neu5Gc, as well as mouse retroviral Gag protein.

CONCLUSIONS

These findings indicate that MV represent a mechanism by which hESC communicate; however, they also serve as potential carriers of immunogenic and pathogenic compounds acquired from environment. Our results highlight a potential danger regarding the use of hESC that have previously been exposed to animal proteins and cells.

Feeder-free maintenance of hESCs in mesenchymal stem cell-conditioned media: distinct requirements for TGF-beta and IGF-II

A paracrine regulation was recently proposed in human embryonic stem cells (hESCs) grown in mouse embryonic fibroblast (MEF)-conditioned media (MEF-CM), where hESCs spontaneously differentiate into autologous fibroblast-like cells to maintain culture homeostasis by producing TGF-beta and insulin-like growth factor-II (IGF-II) in response to basic fibroblast growth factor (bFGF). Although the importance of TGF-beta family members in the maintenance of pluripotency of hESCs is widely established, very little is known about the role of IGF-II. In order to ease hESC culture conditions and to reduce xenogenic components, we sought (i) to determine whether hESCs can be maintained stable and pluripotent using CM from human foreskin fibroblasts (HFFs) and human mesenchymal stem cells (hMSCs) rather than MEF-CM, and (ii) to analyze whether the cooperation of bFGF with TGF-beta and IGF-II to maintain hESCs in MEF-CM may be extrapolated to hESCs maintained in allogeneic mesenchymal stem cell (MSC)-CM and HFF-CM. We found that MSCs and HFFs express all FGF receptors (FGFR1-4) and specifically produce TGF-beta in response to bFGF. However, HFFs but not MSCs secrete IGF-II. Despite the absence of IGF-II in MSC-CM, hESC pluripotency and culture homeostasis were successfully maintained in MSC-CM for over 37 passages. Human ESCs derived on MSCs and hESCs maintained in MSC-CM retained hESC morphology, euploidy, expression of surface markers and transcription factors linked to pluripotency and displayed in vitro and in vivo multilineage developmental potential, suggesting that IGF-II may be dispensable for hESC pluripotency. In fact, IGF-II blocking had no effect on the homeostasis of hESC cultures maintained either on HFF-CM or on MSC-CM. These data indicate that hESCs are successfully maintained feeder-free with IGF-II-lacking MSC-CM, and that the previously proposed paracrine mechanism by which bFGF cooperates with TGF-beta and IGF-II in the maintenance of hESCs in MEF-CM may not be fully extrapolated to hESCs maintained in CM from human MSCs.

Mesenchymal stem cells and their use as cell replacement therapy and disease modelling tool

Mesenchymal stem cells (MSCs) from adult somatic tissues may differentiate in vitro and in vivo into multiple mesodermal tissues including bone, cartilage, adipose tissue, tendon, ligament or even muscle. MSCs preferentially home to damaged tissues where they exert their therapeutic potential. A striking feature of the MSCs is their low inherent immunogenicity as they induce little, if any, proliferation of allogeneic lymphocytes and antigen-presenting cells. Instead, MSCs appear to be immunosuppressive in vitro. Their multi-lineage differentiation potential coupled to their immuno-privileged properties is being exploited worldwide for both autologous and allo-geneic cell replacement strategies. Here, we introduce the readers to the biology of MSCs and the mechanisms underlying immune tolerance. We then outline potential cell replacement strategies and clinical applications based on the MSCs immunological properties. Ongoing clinical trials for graft-versus-host-disease, haematopoietic recovery after co-transplantation of MSCs along with haematopoietic stem cells and tissue repair are discussed. Finally, we review the emerging area based on the use of MSCs as a target cell subset for either spontaneous or induced neoplastic transformation and, for modelling non-haematological mesenchymal cancers such as sarcomas.

Pluripotent stem cells as new drugs? The example of Parkinson's disease

Cell replacement therapy is a widely discussed novel concept of medical treatment. The increased knowledge in the stem cell field, particularly pluripotent stem cells, potentially provides powerful tools for this therapeutic concept. A large number of disease characterized by the loss of functional cells are potential candidates for cell replacement therapy and, in this regards, Parkinson's disease is of particular interest. It is one of the most prevalent neurodegenerative diseases caused by the loss of dopaminergic neurons in the Substantia nigra pars compacta. Pharmacological therapies are valuable but suffer from the progressive decline of efficacy as the disease progresses. Cell therapy application has emerged about two decades ago as a valid therapeutic alternative and recent advances in stem cell research suggest that pluripotent stem cell transplantation may be a promising approach to replace degenerated neurons in Parkinson's disease. Various sources of pluripotent stem cells (PSC) currently tested in animal models of Parkinson's disease have proven their efficacy in relieving symptoms and restoring damaged brain function. This review summarizes and discusses the important challenges that actually must be solved before the first studies of PSC transplantation can be undertaken into humans.

Genetic stability of human embryonic stem cells: A first-step toward the development of potential hESC-based systems for modeling childhood leukemia

Human ESCs provide an opportunity for modeling human-specific strategies to study the earliest events leading to normal hematopoietic specification versus leukemic transformation. Of interest, are the human childhood acute leukemias harboring specific fusion oncogenes such as MLL-AF4, TEL-AML1 or BCR-ABL wherein clinically significant manifestations arise in utero. The mechanisms of transformation are not amenable to analysis with patient samples and, many mouse models for pediatric leukemias have fallen short in illuminating the human disease because they do not recapitulate key aspects of the actual disease, suggesting that the mouse models are missing essential components of oncogenesis present in the human embryo. Prior to using hESCs as a tentative system for modeling leukemia, robust studies aimed at demonstrating their genetic stability are required; otherwise, cooperating mutations already present could prime hESCs susceptible to transformation. We performed an extensive molecular cytogenetic and cellular in vitro and in vivo analysis which reveals an overall genomic stability of HS181 and HS293 hESCs maintained long-term by mechanical dissociation in human feeders. Importantly, we show for the first time that the genetically stable HS181 hESC line differentiates into CD45+ hematopoietic cells and clonogenic hematopoietic progenitors. This data should encourage stem cell researchers to implement robust cytogenetic tools when assessing hESC genetic stability, in order to detect tiny but relevant biological functional or structural chromosome abnormalities and, paves the way for generating fusion oncogene-expressing transgenic hESCs as a human-specific system for studying the early in utero events leading to normal hematopoietic specification versus childhood leukemic transformation.

Reproductive medicine meets human embryonic stem cell (hESC) research: the need to adjust the regulatory framework to actual expectations and potential detrimental consequences of hESC research

Human embryonic stem cell (hESC)-based cell therapy depends on access to surplus embryos from IVF cycles and collaborative interactions between biomedical researchers and reproductive medicine professionals. It is becoming instrumental to achieve an international consensus about the standards that should regulate the manipulation of human embryonic tissue in two distinct settings: reproductive medicine and embryonic stem cell research. Within hESC research, the regulatory framework needs to be adjusted according to the actual expectations and potential detrimental consequences of hESC research.