Evidence for involvement of activin A and bone morphogenetic protein 4 in mammalian mesoderm and hematopoietic development

Protocol for efficient generation of human artery and vein endothelial cells from pluripotent stem cells

Blood vessels permeate all organs and execute myriad roles in health and disease. Here, we present a protocol to efficiently generate human artery and vein endothelial cells (ECs) from pluripotent stem cells within 3-4 days of differentiation. We delineate how to seed human pluripotent stem cells and sequentially differentiate them into primitive streak, lateral mesoderm, and either artery or vein ECs. We differentiate stem cells in defined, serum-free culture media in monolayers, without feeder cells or genetic manipulations. For complete details on the use and execution of this protocol, please refer to Ang et al. 1.

Mouse embryonic stem cell-derived blood-brain barrier model: applicability to studying antibody triggered receptor mediated transcytosis

Blood brain barrier (BBB) models in vitro are an important tool to aid in the pre-clinical evaluation and selection of BBB-crossing therapeutics. Stem cell derived BBB models have recently demonstrated a substantial advantage over primary and immortalized brain endothelial cells (BECs) for BBB modeling. Coupled with recent discoveries highlighting significant species differences in the expression and function of key BBB transporters, the field is in need of robust, species-specific BBB models for improved translational predictability. We have developed a mouse BBB model, composed of mouse embryonic stem cell (mESC-D3)-derived brain endothelial-like cells (mBECs), employing a directed monolayer differentiation strategy. Although the mBECs showed a mixed endothelial-epithelial phenotype, they exhibited high transendothelial electrical resistance, inducible by retinoic acid treatment up to 400 Ω cm². This tight cell barrier resulted in restricted sodium fluorescein permeability (1.7 × 10^-5 cm/min), significantly lower than that of bEnd.3 cells (1.02 × 10^-3 cm/min) and comparable to human induced pluripotent stem cell (iPSC)-derived BECs (2.0 × 10^-5 cm/min). The mBECs expressed tight junction proteins, polarized and functional P-gp efflux transporter and receptor mediated transcytosis (RMT) receptors; collectively important criteria for studying barrier regulation and drug delivery applications in the CNS. In this study, we compared transport of a panel of antibodies binding species selective or cross-reactive epitopes on BBB RMT receptors in both the mBEC and human iPSC-derived BEC model, to demonstrate discrimination of species-specific BBB transport mechanisms.

How might we build limbs in vitro informed by the modular aspects and tissue-dependency in limb development?

Building limb morphogenesis in vitro would substantially open up avenues for research and applications of appendage development. Recently, advances in stem cell engineering to differentiate desired cell types and produce multicellular structures in vitro have enabled the derivation of limb-like tissues from pluripotent stem cells. However, in vitro recapitulation of limb morphogenesis is yet to be achieved. To formulate a method of building limbs in vitro, it is critically important to understand developmental mechanisms, especially the modularity and the dependency of limb development on the external tissues, as those would help us to postulate what can be self-organized and what needs to be externally manipulated when reconstructing limb development in vitro. Although limbs are formed on the designated limb field on the flank of embryo in the normal developmental context, limbs can also be regenerated on the amputated stump in some animals and experimentally induced at ectopic locations, which highlights the modular aspects of limb morphogenesis. The forelimb-hindlimb identity and the dorsal-ventral, proximal-distal, and anterior-posterior axes are initially instructed by the body axis of the embryo, and maintained in the limb domain once established. In contrast, the aspects of dependency on the external tissues are especially underscored by the contribution of incoming tissues, such as muscles, blood vessels, and peripheral nerves, to developing limbs. Together, those developmental mechanisms explain how limb-like tissues could be derived from pluripotent stem cells. Prospectively, the higher complexity of limb morphologies is expected to be recapitulated by introducing the morphogen gradient and the incoming tissues in the culture environment. Those technological developments would dramatically enhance experimental accessibility and manipulability for elucidating the mechanisms of limb morphogenesis and interspecies differences. Furthermore, if human limb development can be modeled, drug development would be benefited by in vitro assessment of prenatal toxicity on congenital limb deficiencies. Ultimately, we might even create a future in which the lost appendage would be recovered by transplanting artificially grown human limbs.

The negative regulation of gene expression by microRNAs as key driver of inducers and repressors of cardiomyocyte differentiation

Cardiac muscle damage-induced loss of cardiomyocytes (CMs) and dysfunction of the remaining ones leads to heart failure, which nowadays is the number one killer worldwide. Therapies fostering effective cardiac regeneration are the holy grail of cardiovascular research to stop the heart failure epidemic. The main goal of most myocardial regeneration protocols is the generation of new functional CMs through the differentiation of endogenous or exogenous cardiomyogenic cells. Understanding the cellular and molecular basis of cardiomyocyte commitment, specification, differentiation and maturation is needed to devise innovative approaches to replace the CMs lost after injury in the adult heart. The transcriptional regulation of CM differentiation is a highly conserved process that require sequential activation and/or repression of different genetic programs. Therefore, CM differentiation and specification have been depicted as a step-wise specific chemical and mechanical stimuli inducing complete myogenic commitment and cell-cycle exit. Yet, the demonstration that some microRNAs are sufficient to direct ESC differentiation into CMs and that four specific miRNAs reprogram fibroblasts into CMs show that CM differentiation must also involve negative regulatory instructions. Here, we review the mechanisms of CM differentiation during development and from regenerative stem cells with a focus on the involvement of microRNAs in the process, putting in perspective their negative gene regulation as a main modifier of effective CM regeneration in the adult heart.

Cpmer: A new conserved eEF1A2-binding partner that regulates Eomes translation and cardiomyocyte differentiation

Previous studies have shown that eukaryotic elongation factor 1A2 (eEF1A2) serves as an essential heart-specific translation elongation element and that its mutation or knockout delays heart development and causes congenital heart disease and death among species. However, the function and regulatory mechanisms of eEF1A2 in mammalian heart development remain largely unknown. Here we identified the long noncoding RNA (lncRNA) Cpmer (cytoplasmic mesoderm regulator), which interacted with eEF1A2 to co-regulate differentiation of mouse and human embryonic stem cell-derived cardiomyocytes. Mechanistically, Cpmer specifically recognized Eomes mRNA by RNA-RNA pairing and facilitated binding of eEF1A2 with Eomes mRNA, guaranteeing Eomes mRNA translation and cardiomyocyte differentiation. Our data reveal a novel functionally conserved lncRNA that can specifically regulate Eomes translation and cardiomyocyte differentiation, which broadens our understanding of the mechanism of lncRNA involvement in the subtle translational regulation of eEF1A2 during mammalian heart development.

Single-cell transcriptomic characterization of a gastrulating human embryo

Gastrulation is the fundamental process in all multicellular animals through which the basic body plan is first laid down1-4. It is pivotal in generating cellular diversity coordinated with spatial patterning. In humans, gastrulation occurs in the third week after fertilization. Our understanding of this process in humans is relatively limited and based primarily on historical specimens5-8, experimental models9-12 or, more recently, in vitro cultured samples13-16. Here we characterize in a spatially resolved manner the single-cell transcriptional profile of an entire gastrulating human embryo, staged to be between 16 and 19 days after fertilization. We use these data to analyse the cell types present and to make comparisons with other model systems. In addition to pluripotent epiblast, we identified primordial germ cells, red blood cells and various mesodermal and endodermal cell types. This dataset offers a unique glimpse into a central but inaccessible stage of our development. This characterization provides new context for interpreting experiments in other model systems and represents a valuable resource for guiding directed differentiation of human cells in vitro.

Generation of Human Induced Pluripotent Stem Cells and Differentiation into Cardiomyocytes

Failure to regenerate myocardium after injury is a major cause of mortality and morbidity in humans. Direct differentiation of human induced pluripotent stem cells (iPSCs) into cardiomyocytes provides an invaluable resource to pursue cardiac regeneration based on cellular transplantation. Beyond the potential for clinical therapies, iPSC technology also enables the generation of cardiomyocytes to recapitulate patient-specific phenotypes, thus presenting a powerful in vitro cell-based model to understand disease pathology and guide precision medicine. Here, we describe protocols for reprogramming of human dermal fibroblasts and blood cells into iPSCs using the non-integrative Sendai virus system and for the monolayer differentiation of iPSCs to cardiomyocytes using chemically defined media.

Enhanced Induction of Definitive Endoderm Differentiation of Mouse Embryonic Stem Cells in Simulated Microgravity

Directed in vitro differentiation of pluripotent stem cells toward definitive endoderm (DE) offers great research and therapeutic potential since these cells can further differentiate into cells of the respiratory and gastrointestinal tracts, as well as associated organs such as pancreas, liver, and thyroid. We hypothesized that culturing mouse embryonic stem cells (mESCs) under simulated microgravity (SMG) conditions in rotary bioreactors (BRs) will enhance the induction of directed DE differentiation. To test our hypothesis, we cultured the cells for 6 days in two-dimensional monolayer colony cultures or as embryoid bodies (EBs) in either static conditions or, dynamically, in the rotary BRs. We used flow cytometry and quantitative polymerase chain reaction to analyze the expression of marker proteins and genes, respectively, for pluripotency (Oct3/4) and mesendodermal (Brachyury T), endodermal (FoxA2, Sox17, CxCr4), and mesodermal (Vimentin, Meox1) lineages. Culture in the form of EBs in maintenance media in the presence of leukemia inhibitory factor, in static or SMG conditions, induced expression of some of the differentiation markers, suggesting heterogeneity of the cells. This is in line with previous studies showing that differentiation is initiated as cells are aggregated into EBs even without supplementing differentiation factors to the media. Culturing EBs in static conditions in differentiation media (DM) in the presence of activin A reduced Oct3/4 expression and significantly increased Brachyury T and CxCr4 expression, but downregulated FoxA2 and Sox17. However, culturing in SMG BRs in DM upregulated Brachyury T and all of the DE markers and reduced Oct3/4 expression, indicating the advantage of dynamic cultures in BRs to specifically enhance directed DE differentiation. Given the potential discrepancies between the SMG conditions on earth and actual microgravity conditions, as observed in other studies, future experiments in space flight are required to validate the effects of reduced gravity on mESC differentiation.

Loss of Myeloid BMPR1a Alters Differentiation and Reduces Mouse Prostate Cancer Growth

The Bone Morphogenetic Protein (BMP) pathway is a member of the TGFβ signaling family and has complex roles in cancer. BMP signaling is rarely mutated and can be frequently overexpressed in many human cancers. The dichotomous role of BMPs as both tumor promoters and suppressors appears to be largely context based in both the cancer cell and the surrounding microenvironment. Myeloid cells including macrophages and neutrophils have been shown to be tumor promoting when stimulated from BMPs. We found that conditional deletion of BMPR1a in myeloid cells (LysMCre) restricts tumor progression in a syngeneic mouse prostate cancer model. Specific changes occurred in myeloid cells both in tumor bearing mice and tumor naïve mice throughout multiple tissues. We profiled myeloid subsets in the bone marrow, spleen and primary tumor and found myeloid BMPR1a loss altered the differentiation and lineage capability of distinct populations by histologic, flow cytometry and high dimensional mass cytometry analysis. We further confirmed the requirement for BMP signaling with pharmacologic inhibition of THP-1 and Raw264.7 activated into M2 macrophages with the BMP inhibitor DMH1. M2 polarized primary bone marrow derived cells from LysMCre BMPR1a knockout mice indicated a distinct requirement for BMP signaling in myeloid cells during M2 activation. These results indicate a unique necessity for BMP signaling in myeloid cells during tumor progression.

Cellular Plasticity in Musculoskeletal Development, Regeneration, and Disease

In this review, we highlight themes from a recent workshop focused on "Plasticity of Cell Fate in Musculoskeletal Tissues" held at the Orthopaedic Research Society's 2019 annual meeting. Experts in the field provided examples of mesenchymal cell plasticity during normal musculoskeletal development, regeneration, and disease. A thorough understanding of the biology underpinning mesenchymal cell plasticity may offer a roadmap for promoting regeneration while attenuating pathologic differentiation. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:708-718, 2020.

Analysis of pooled genome sequences from Djallonke and Sahelian sheep of Ghana reveals co-localisation of regions of reduced heterozygosity with candidate genes for disease resistance and adaptation to a tropical environment

BACKGROUND

The Djallonke sheep is well adapted to harsh environmental conditions, and is relatively resistant to Haemonchosis and resilient to animal trypanosomiasis. The larger Sahelian sheep, which cohabit the same region, is less well adapted to these disease challenges. Haemonchosis and Trypanosomiasis collectively cost the worldwide animal industry billions of dollars in production losses annually.

RESULTS

Here, we separately sequenced and then pooled according to breed the genomes from five unrelated individuals from each of the Djallonke and Sahelian sheep breeds (sourced from Ghana), at greater than 22-fold combined coverage for each breed. A total of approximately 404 million (97%) and 343 million (97%) sequence reads from the Djallonke and Sahelian breeds respectively, were successfully mapped to the sheep reference genome Oar v3.1. We identified approximately 11.1 million and 10.9 million single nucleotide polymorphisms (SNPs) in the Djallonke and Sahelian breeds, with approximately 15 and 16% respectively of these not previously reported in sheep. Multiple regions of reduced heterozygosity were also found; 70 co-localised within genomic regions harbouring genes that mediate disease resistance, immune response and adaptation in sheep or cattle. Thirty- three of the regions of reduced heterozygosity co-localised with previously reported genes for resistance to haemonchosis and trypanosomiasis.

CONCLUSIONS

Our analyses suggest that these regions of reduced heterozygosity may be signatures of selection for these economically important diseases.

An iPSC-Derived Myeloid Lineage Model of Herpes Virus Latency and Reactivation

Herpesviruses undergo life-long latent infection which can be life-threatening in the immunocompromised. Models of latency and reactivation of human cytomegalovirus (HCMV) include primary myeloid cells, cells known to be important for HCMV latent carriage and reactivation in vivo. However, primary cells are limited in availability, and difficult to culture and to genetically modify; all of which have hampered our ability to fully understand virus/host interactions of this persistent human pathogen. We have now used iPSCs to develop a model cell system to study HCMV latency and reactivation in different cell types after their differentiation down the myeloid lineage. Our results show that iPSCs can effectively mimic HCMV latency/reactivation in primary myeloid cells, allowing molecular interrogations of the viral latent/lytic switch. This model may also be suitable for analysis of other viruses, such as HIV and Zika, which also infect cells of the myeloid lineage.

Improved methods for the differentiation of hypothalamic vasopressin neurons using mouse induced pluripotent stem cells

High differentiation efficiency is one of the most important factors in developing an in vitro model from pluripotent stem cells. In this report, we improved the handling technique applied to mouse-induced pluripotent stem (iPS) cells, resulting in better differentiation into hypothalamic vasopressin (AVP) neurons. We modified the culture procedure to make the maintenance of iPS cells in an undifferentiated state much easier. Three-dimensional floating culture was demonstrated to be effective for mouse iPS cells. We also improved the differentiation method with regards to embryology, resulting in a greater number of bigger colonies of AVP neurons differentiating from mouse iPS cells. Fgf8, which was not used in the original differentiation method, increased iPS differentiation into AVP neurons. These refinements will be useful as a valuable tool for the modeling of degenerative disease in AVP neurons in vitro using disease-specific iPS cells in future studies.

Allele-specific RNA-seq expression profiling of imprinted genes in mouse isogenic pluripotent states

BACKGROUND

Genomic imprinting, resulting in parent-of-origin specific gene expression, plays a critical role in mammalian development. Here, we apply allele-specific RNA-seq on isogenic B6D2F1 mice to assay imprinted genes in tissues from early embryonic tissues between E3.5 and E7.25 and in pluripotent cell lines to evaluate maintenance of imprinted gene expression. For the cell lines, we include embryonic stem cells (ESCs) and epiblast stem cells (EpiSCs) derived from fertilized embryos and from embryos obtained after nuclear transfer (NT) or parthenogenetic activation (PGA).

RESULTS

As homozygous genomic regions of PGA-derived cells are not compatible with allele-specific RNA-seq, we developed an RNA-seq-based genotyping strategy allowing identification of informative heterozygous regions. Global analysis shows that proper imprinted gene expression as observed in embryonic tissues is largely lost in the ESC lines included in this study, which mainly consisted of female ESCs. Differentiation of ESC lines to embryoid bodies or NPCs does not restore monoallelic expression of imprinted genes, neither did reprogramming of the serum-cultured ESCs to the pluripotent ground state by the use of 2 kinase inhibitors. Fertilized EpiSC and EpiSC-NT lines largely maintain imprinted gene expression, as did EpiSC-PGA lines that show known paternally expressed genes being silent and known maternally expressed genes consistently showing doubled expression. Notably, two EpiSC-NT lines show aberrant silencing of Rian and Meg3, two critically imprinted genes in mouse iPSCs. With respect to female EpiSC, most of the lines displayed completely skewed X inactivation suggesting a (near) clonal origin.

CONCLUSIONS

Altogether, our analysis provides a comprehensive overview of imprinted gene expression in pluripotency and provides a benchmark to allow identification of cell lines that faithfully maintain imprinted gene expression and therefore retain full developmental potential.

It Takes Two: Endothelial-Perivascular Cell Cross-Talk in Vascular Development and Disease

The formation of new blood vessels is a crucial step in the development of any new tissue both during embryogenesis and in vitro models as without sufficient perfusion the tissue will be unable to grow beyond the size where nutrition and oxygenation can be managed by diffusion alone. Endothelial cells are the primary building block of blood vessels and are capable of forming tube like structures independently however they are unable to independently form functional vasculature which is capable of conducting blood flow. This requires support from other structures including supporting perivascular cells and the extracellular matrix. The crosstalk between endothelial cells and perivascular cells is vital in regulating vasculogenesis and angiogenesis and the consequences when this is disrupted can be seen in a variety of congenital and acquired disease states. This review details the mechanisms of vasculogenesis in vivo during embryogenesis and compares this to currently employed in vitro techniques. It also highlights clinical consequences of defects in the endothelial cell-pericyte cross-talk and highlights therapies which are being developed to target this pathway. Improving the understanding of the intricacies of endothelial-pericyte signaling will inform pathophysiology of multiple vascular diseases and allow the development of effective in vitro models to guide drug development and assist with approaches in tissue engineering to develop functional vasculature for regenerative medicine applications.

Regenerating the kidney using human pluripotent stem cells and renal progenitors

INTRODUCTION

Chronic kidney disease is a major health-care problem worldwide and its cost is becoming no longer affordable. Indeed, restoring damaged renal structures or building a new kidney represents an ambitious and ideal alternative to renal replacement therapy. Streams of research have explored the possible application of pluripotent stem cells (SCs) (embryonic SCs and induced pluripotent SCs) in different strategies aimed at regenerate functioning nephrons and at understanding the mechanisms of kidney regeneration.

AREAS COVERED

In this review, we will focus on the main potential applications of human pluripotent SCs to kidney regeneration, including those leading to rebuilding new kidneys or part of them (organoids, scaffolds, biological microdevices) as well as those aimed at understanding the pathophysiological mechanisms of renal disease and regenerative processes (modeling of kidney disease, genome editing). Moreover, we will discuss the role of endogenous renal progenitors cells in order to understand and promote kidney regeneration, as an attractive alternative to pluripotent SCs.

EXPERT OPINION

Opportunities and pitfalls of all these strategies will be underlined, finally leading to the conclusion that a deeper knowledge of the biology of pluripotent SCs is mandatory, in order to allow us to hypothesize their clinical application.

High cell density suppresses BMP4-induced differentiation of human pluripotent stem cells to produce macroscopic spatial patterning in a unidirectional perfusion culture chamber

Spatial pattern formation is a critical step in embryogenesis. Bone morphogenetic protein 4 (BMP4) and its inhibitors are major factors for the formation of spatial patterns during embryogenesis. However, spatial patterning of the human embryo is unclear because of ethical issues and isotropic culture environments resulting from conventional culture dishes. Here, we utilized human pluripotent stem cells (hiPSCs) and a simple anisotropic (unidirectional perfusion) culture chamber, which creates unidirectional conditions, to measure the cell community effect. The influence of cell density on BMP4-induced differentiation was explored during static culture using a conventional culture dish. Immunostaining of the early differentiation marker SSEA-1 and the mesendoderm marker BRACHYURY revealed that high cell density suppressed differentiation, with small clusters of differentiated and undifferentiated cells formed. Addition of five-fold higher concentration of BMP4 showed similar results, suggesting that suppression was not caused by depletion of BMP4 but rather by high cell density. Quantitative RT-PCR array analysis showed that BMP4 induced multi-lineage differentiation, which was also suppressed under high-density conditions. We fabricated an elongated perfusion culture chamber, in which proteins were transported unidirectionally, and hiPSCs were cultured with BMP4. At low density, the expression was the same throughout the chamber. However, at high density, SSEA-1 and BRACHYURY were expressed only in upstream cells, suggesting that some autocrine/paracrine factors inhibited the action of BMP4 in downstream cells to form the spatial pattern. Human iPSCs cultured in a perfusion culture chamber might be useful for studying in vitro macroscopic pattern formation in human embryogenesis.

The BMP pathway: A unique tool to decode the origin and progression of leukemia

The microenvironment (niche) governs the fate of stem cells (SCs) by balancing self-renewal and differentiation. Increasing evidence indicates that the tumor niche plays an active role in cancer, but its important properties for tumor initiation progression and resistance remain to be identified. Clinical data show that leukemic stem cell (LSC) survival is responsible for disease persistence and drug resistance, probably due to their sustained interactions with the tumor niche. Bone morphogenetic protein (BMP) signaling is a key pathway controlling stem cells and their niche. BMP2 and BMP4 are important in both the normal and the cancer context. Several studies have revealed profound alterations of the BMP signaling in cancer SCs, with major deregulations of the BMP receptors and their downstream signaling elements. This was illustrated in the hematopoietic system by pioneer studies in chronic myelogenous leukemia that may now be expanded to acute myeloid leukemia and lymphoid leukemia, as reviewed here. At diagnosis, cells from the leukemic microenvironment are the major providers of soluble BMPs. Conversely, LSCs display altered receptors and downstream BMP signaling elements accompanied by altered functional responses to BMPs. These studies reveal the role of BMPs in tumor initiation, in addition to their known effects in later stages of transformation and progression. They also reveal the importance of BMPs in fueling cell transformation and expansion by overamplifying a natural SC response. This mechanism may explain the survival of LSCs independently of the initial oncogenic event and therefore may be involved in resistance processes.

Making muscle: skeletal myogenesis in vivo and in vitro

Skeletal muscle is the largest tissue in the body and loss of its function or its regenerative properties results in debilitating musculoskeletal disorders. Understanding the mechanisms that drive skeletal muscle formation will not only help to unravel the molecular basis of skeletal muscle diseases, but also provide a roadmap for recapitulating skeletal myogenesis in vitro from pluripotent stem cells (PSCs). PSCs have become an important tool for probing developmental questions, while differentiated cell types allow the development of novel therapeutic strategies. In this Review, we provide a comprehensive overview of skeletal myogenesis from the earliest premyogenic progenitor stage to terminally differentiated myofibers, and discuss how this knowledge has been applied to differentiate PSCs into muscle fibers and their progenitors in vitro.

Markers associated with neuron-specific Ube3a imprinting during neuronal differentiation of mouse embryonic stem cells

Understanding gene expression in the brain requires allele-specific transcriptome analysis because of the presence of neuron-specific imprinted genes, which are expressed in a neuron-specific and parent-of-origin-specific manner. Ube3a is a neuron-specific imprinted gene with an expression pattern that changes from biallelic to maternal only (Ube3a imprinting) during differentiation. Because Ube3a imprinting occurs only in neurons, it has the potential to be a marker to assess the quality of neurons produced by in vitro neuronal differentiation of embryonic stem cells (ESCs). For the analysis of Ube3a imprinting, genetic polymorphisms between the two alleles are necessary to identify the parental origin of each. However, ESCs derived from commonly used inbred mouse strains have no genetic polymorphisms. To overcome this problem, we examined 10 markers of neurogenesis to determine whether they were associated with Ube3a imprinting. We measured the relative expression levels of these 10 gene markers and assessed the Ube3a imprinting status of 54 neuron samples differentiated under various in vitro conditions. Then we divided the samples into two groups depending on their Ube3a imprinting status and selected markers statistically associated with Ube3a imprinting. The identified markers included the antisense noncoding transcript of Ube3a and a mature neuron marker Mtap2, consistent with the markers we used empirically in our previous study to assess the quality of differentiated neurons. These findings provide new quality control criteria for differentiated neurons, and could also be applied to human ESCs and induced pluripotent stem cells.

Thrombopoietin induces hematopoiesis from mouse ES cells via HIF-1α-dependent activation of a BMP4 autoregulatory loop

Understanding the molecular mechanisms underlying hematopoietic differentiation of embryonic stem (ES) cells may help to ascertain the conditions for the in vitro generation of hematopoietic cells. Previously, we found that patients with congenital amegakaryocytic thrombocytopenia (CAMT), who develop pancytopenia early after birth, harbor mutations within the thrombopoietin (TPO) receptor, c-MPL. This knowledge, together with observations in vitro and in vivo, suggests that TPO/c-MPL signaling promotes early hematopoiesis. However, the mechanisms underlying TPO signaling are not fully elucidated. Here, we describe a direct connection between TPO and bone morphogenetic protein 4 (BMP4) signaling pathways in determining the hematopoietic fate of ES cells. Morphogen BMP4 is known to induce early hematopoietic differentiation of ES cells. Treatment of ES cells with TPO induced the autocrine production of BMP4 with concomitant upregulation of the BMP receptor BMPR1A, phosphorylation of SMAD1, 5, 8, and activation of specific BMP4 target genes; this was mediated by TPO-dependent binding of transcription factor HIF-1α to the BMP4 gene promoter. Treatment of ES cells with the BMP antagonist noggin substantially reduced TPO-dependent hematopoietic differentiation of ES cells. Thus, our findings contribute to the establishment of techniques for generating hematopoietic cells from ES cells.

The many faces of Pluripotency: in vitro adaptations of a continuum of in vivo states

Pluripotency defines the propensity of a cell to differentiate into, and generate, all somatic, as well as germ cells. The epiblast of the early mammalian embryo is the founder population of all germ layer derivatives and thus represents the bona fide in vivo pluripotent cell population. The so-called pluripotent state spans several days of development and is lost during gastrulation as epiblast cells make fate decisions towards a mesoderm, endoderm or ectoderm identity. It is now widely recognized that the features of the pluripotent population evolve as development proceeds from the pre- to post-implantation period, marked by distinct transcriptional and epigenetic signatures. During this period of time epiblast cells mature through a continuum of pluripotent states with unique properties. Aspects of this pluripotent continuum can be captured in vitro in the form of stable pluripotent stem cell types. In this review we discuss the continuum of pluripotency existing within the mammalian embryo, using the mouse as a model, and the cognate stem cell types that can be derived and propagated in vitro. Furthermore, we speculate on embryonic stage-specific characteristics that could be utilized to identify novel, developmentally relevant, pluripotent states.

Generation of thalamic neurons from mouse embryonic stem cells

The thalamus is a diencephalic structure that plays crucial roles in relaying and modulating sensory and motor information to the neocortex. The thalamus develops in the dorsal part of the neural tube at the level of the caudal forebrain. However, the molecular mechanisms that are essential for thalamic differentiation are still unknown. Here, we have succeeded in generating thalamic neurons from mouse embryonic stem cells (mESCs) by modifying the default method that induces the most-anterior neural type in self-organizing culture. A low concentration of the caudalizing factor insulin and a MAPK/ERK kinase inhibitor enhanced the expression of the caudal forebrain markers Otx2 and Pax6. BMP7 promoted an increase in thalamic precursors such as Tcf7l2⁺/Gbx2⁺ and Tcf7l2⁺/Olig3⁺ cells. mESC thalamic precursors began to express the glutamate transporter vGlut2 and the axon-specific marker VGF, similar to mature projection neurons. The mESC thalamic neurons extended their axons to cortical layers in both organotypic culture and subcortical transplantation. Thus, we have identified the minimum elements sufficient for in vitro generation of thalamic neurons. These findings expand our knowledge of thalamic development.

Multifactorial Optimizations for Directing Endothelial Fate from Stem Cells

Embryonic stem cells (ESC) and induced pluripotent stem (iPS) cells are attractive in vitro models of vascular development, therapeutic angiogenesis, and tissue engineering. However, distinct ESC and iPS cell lines respond differentially to the same microenvironmental factors. Developing improved/optimized differentiation methodologies tailored/applicable in a number of distinct iPS and ESC lines remains a challenge in the field. Currently published methods for deriving endothelial cells (EC) robustly generate high numbers of endothlelial progenitor cells (EPC) within a week, but their maturation to definitive EC is much more difficult, taking up to 2 months and requiring additional purification. Therefore, we set out to examine combinations/levels of putative EC induction factors—utilizing our stage-specific chemically-defined derivation methodology in 4 ESC lines including: kinetics, cell seeding density, matrix signaling, as well as medium treatment with vascular endothelial growth factor (VEGF), and basic fibroblast growth factor (bFGF). The results indicate that temporal development in both early and late stages is the most significant factor generating the desired cells. The generation of early Flk-1⁺/KDR⁺ vascular progenitor cells (VPC) from pluripotent ESC is directed predominantly by high cell seeding density and matrix signaling from fibronectin, while VEGF supplementation was NOT statistically significant in more than one cell line, especially with fibronectin matrix which sequesters autocrine VEGF production by the differentiating stem cells. Although some groups have shown that the GSK3-kinase inhibitor (CHIR) can facilitate EPC fate, it hindered the generation of KDR+ cells in our preoptimized medium formulations. The methods summarized here significantly increased the production of mature vascular endothelial (VE)-cadherin+ EC, with up to 93% and 57% purity from mouse and human ESC, respectively, before VE-cadherin+ EC purification.

Role of the bone morphogenic protein pathway in developmental haemopoiesis and leukaemogenesis

Myeloid leukaemias share the common characteristics of being stem cell-derived clonal diseases, characterised by excessive proliferation of one or more myeloid lineage. Chronic myeloid leukaemia (CML) arises from a genetic alteration in a normal haemopoietic stem cell (HSC) giving rise to a leukaemic stem cell (LSC) within the bone marrow (BM) ‘niche’. CML is characterised by the presence of the oncogenic tyrosine kinase fusion protein breakpoint cluster region-abelson murine leukaemia viral oncogene homolog 1 (BCR-ABL), which is responsible for driving the disease through activation of downstream signal transduction pathways. Recent evidence from our group and others indicates that important regulatory networks involved in establishing primitive and definitive haemopoiesis during development are reactivated in myeloid leukaemia, giving rise to an LSC population with altered self-renewal and differentiation properties. In this review, we explore the role the bone morphogenic protein (BMP) signalling plays in stem cell pluripotency, developmental haemopoiesis, HSC maintenance and the implication of altered BMP signalling on LSC persistence in the BM niche. Overall, we emphasise how the BMP and Wnt pathways converge to alter the Cdx–Hox axis and the implications of this in the pathogenesis of myeloid malignancies.

Hepatocyte-like cells derived from induced pluripotent stem cells

The discovery that coordinated expression of a limited number of genes can reprogram differentiated somatic cells to induced pluripotent stem cells (iPSC) has opened novel possibilities for developing cell-based models of diseases and regenerative medicine utilizing cell reprogramming or cell transplantation. Directed differentiation of iPSCs can potentially generate differentiated cells belonging to any germ layer, including cells with hepatocyte-like morphology and function. Such cells, termed iHeps, can be derived by sequential cell signaling using available information on embryological development or by forced expression of hepatocyte-enriched transcription factors. In addition to the translational aspects of iHeps, the experimental findings have provided insights into the mechanisms of cell plasticity that permit one cell type to transition to another. However, iHeps generated by current methods do not fully exhibit all characteristics of mature hepatocytes, highlighting the need for additional research in this area. Here we summarize the current approaches and achievements in this field and discuss some existing hurdles and emerging approaches for improving iPSC differentiation, as well as maintaining such cells in culture for increasing their utility in disease modeling and drug development.

Expression of Bone Morphogenetic Proteins in Stromal Cells from Human Bone Marrow Long-term Culture

Highly purified primitive hemopoietic stem cells express BMP receptors but do not synthesize bone morphogenetic proteins (BMPs). However, exogenously added BMPs regulate their proliferation, differentiation, and survival. To further explore the mechanism by which BMPs might be involved in hemopoietic differentiation, we tested whether stromal cells from long-term culture (LTC) of normal human bone marrow produce BMPs, BMP receptors, and SMAD signaling molecules. Stromal cells were immunohistochemically characterized by the presence of lyzozyme, CD 31, factor VIII, CD 68, S100, alkaline phosphatase, and vimentin. Gene expression was analyzed by RT-PCR and the presence of BMP protein was confirmed by immunohistochemistry (IHC). The supportive role of the stromal cell layer in hemopoiesis in vitro was confirmed by a colony assay of clonogenic progenitors. Bone marrow stromal cells express mRNA and protein for BMP-3, -4, and -7 but not for BMP-2, -5, and -6 from the first to the eighth week of culture. Furthermore, stromal cells express the BMP type I receptors, activin-like kinase-3 (ALK-3), ALK-6, and the downstream transducers SMAD-1, -4, and -5. Thus, human bone marrow stromal cells synthesize BMPs, which might exert their effects on hemopoietic stem cells in a paracrine manner through specific BMP receptors.

Capturing Identity and Fate Ex Vivo: Stem Cells from the Mouse Blastocyst

During mouse preimplantation development, three molecularly, morphologically, and spatially distinct lineages are formed, the embryonic epiblast, the extraembryonic primitive endoderm, and the trophectoderm. Stem cell lines representing each of these lineages have now been derived and can be indefinitely maintained and expanded in culture, providing an unlimited source of material to study the interplay of tissue-specific transcription factors and signaling pathways involved in these fundamental cell fate decisions. Here we outline our current understanding of the derivation, maintenance, and properties of these in vitro stem cell models representing the preimplantation embryonic lineages.

Comparison of two types of non-adherent plate for neuronal differentiation of mouse embryonic stem cells

In vitro differentiation systems of mouse embryonic stem cells (ESCs) are widely used as tools for studies of cell differentiation, organogenesis, and regenerative medicine. We have studied the regulation of neuron-specific imprinting genes, Ube3a and its antisense transcripts (Ube3a ATS), using in vitro neuronal differentiation of F1 hybrid ESCs. Each different non-adherent plate used for embryoid body (EB) formation during differentiation is associated with different costs; notably, plates coated with 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer are more expensive than untreated polystyrene plates. Here, we assessed whether the polymer-coated plates gave better results than the untreated plates. The first stage of differentation was performed in the MPC polymer-coated or untreated plates. The formed EBs were then passaged onto laminin-coated plates for further differentiation into neurons. Neither the neuron-specific imprinting status of Ube3a nor the expression levels of the neuron-specific markers Ube3a ATS and Mtap2 differed between neurons prepared on untreated plates and those prepared on MPC polymer-coated plates. These results suggest that the two non-adherent plates displayed almost the same characteristics for inducing neuronal differentiation of mouse ESCs and EB formation. Our study proved that untreated polystyrene plates are a cost-effective choice for EB formation in in vitro differentiation systems of mouse ESCs.

SCL/TAL1 in Hematopoiesis and Cellular Reprogramming

SCL, a transcription factor of the basic helix-loop-helix family, is a master regulator of hematopoiesis. Scl specifies lateral plate mesoderm to a hematopoietic fate and establishes boundaries by inhibiting the cardiac lineage. A combinatorial interaction between Scl and Vegfa/Flk1 sets in motion the first wave of primitive hematopoiesis. Subsequently, definitive hematopoietic stem cells (HSCs) emerge from the embryo proper via an endothelial-to-hematopoietic transition controlled by Runx1, acting with Scl and Gata2. Past this stage, Scl in steady state HSCs is redundant with Lyl1, a highly homologous factor. However, Scl is haploinsufficient in stress response, when a rare subpopulation of HSCs with very long term repopulating capacity is called into action. SCL activates transcription by recruiting a core complex on DNA that necessarily includes E2A/HEB, GATA1-3, LIM-only proteins LMO1/2, LDB1, and an extended complex comprising ETO2, RUNX1, ERG, or FLI1. These interactions confer multifunctionality to a complex that can control cell proliferation in erythroid progenitors or commitment to terminal differentiation through variations in single component. Ectopic SCL and LMO1/2 expression in immature thymocytes activates of a stem cell gene network and reprogram cells with a finite lifespan into self-renewing preleukemic stem cells (pre-LSCs), an initiating event in T-cell acute lymphoblastic leukemias. Interestingly, fate conversion of fibroblasts to hematoendothelial cells requires not only Scl and Lmo2 but also Gata2, Runx1, and Erg, indicating a necessary collaboration between these transcription factors for hematopoietic reprogramming. Nonetheless, full reprogramming into self-renewing multipotent HSCs may require additional factors and most likely, a permissive microenvironment.

Developmental Competence for Primordial Germ Cell Fate

During mammalian embryonic development, the trophectoderm and primitive endoderm give rise to extraembryonic tissues, while the epiblast differentiates into all somatic lineages and the germline. Remarkably, only a few classes of signaling pathways induce the differentiation of these progenitor cells into diverse lineages. Accordingly, the functional outcome of a particular signal depends on the developmental competence of the target cells. Thus, developmental competence can be defined as the ability of a cell to integrate intrinsic and extrinsic cues to execute a specific developmental program toward a specific cell fate. Downstream of signaling, there is the combinatorial activity of transcription factors and their cofactors, which is modulated by the chromatin state of the target cells. Here, we discuss the concept of developmental competence, and the factors that regulate this state with reference to the specification of mammalian primordial germ cells.

Techniques for the induction of human pluripotent stem cell differentiation towards cardiomyocytes

The derivation of pluripotent stem cells from human embryos and the generation of induced pluripotent stem cells (iPSCs) from somatic cells opened a new chapter in studies on the regeneration of the post-infarction heart and regenerative medicine as a whole. Thus, protocols for obtaining iPSCs were enthusiastically adopted and widely used for further experiments on cardiac differentiation. iPSC-mediated cardiomyocytes (iPSC-CMs) under in vitro culture conditions are generated by simulating natural cardiomyogenesis and involve the wingless-type mouse mammary tumour virus integration site family (WNT), transforming growth factor beta (TGF-β) and fibroblast growth factor (FGF) signalling pathways. New strategies have been proposed to take advantage of small chemical molecules, organic compounds and even electric or mechanical stimulation. There are three main approaches to support cardiac commitment in vitro: embryoid bodis (EBs), monolayer in vitro cultures and inductive co-cultures with visceral endoderm-like (END2) cells. In EB technique initial uniform size of pluripotent stem cell (PSC) colonies has a pivotal significance. Hence, some methods were designed to support cells aggregation. Another well-suited procedure is based on culturing cells in monolayer conditions in order to improve accessibility of growth factors and nutrients. Other distinct tactics are using visceral endoderm-like cells to culture them with PSCs due to secretion of procardiac cytokines. Finally, the appropriate purification of the obtained cardiomyocytes is required prior to their administration to a patient under the prospective cellular therapy strategy. This goal can be achieved using non-genetic methods, such as the application of surface markers and fluorescent dyes. Copyright © 2016 John Wiley & Sons, Ltd.

Pluripotency Factors on Their Lineage Move

Pluripotent stem cells are characterised by continuous self-renewal while maintaining the potential to differentiate into cells of all three germ layers. Regulatory networks of maintaining pluripotency have been described in great detail and, similarly, there is great knowledge on key players that regulate their differentiation. Interestingly, pluripotency has various shades with distinct developmental potential, an observation that coined the term of a ground state of pluripotency. A precise interplay of signalling axes regulates ground state conditions and acts in concert with a combination of key transcription factors. The balance between these transcription factors greatly influences the integrity of the pluripotency network and latest research suggests that minute changes in their expression can strengthen but also collapse the network. Moreover, recent studies reveal different facets of these core factors in balancing a controlled and directed exit from pluripotency. Thereby, subsets of pluripotency-maintaining factors have been shown to adopt new roles during lineage specification and have been globally defined towards neuroectodermal and mesendodermal sets of embryonic stem cell genes. However, detailed underlying insights into how these transcription factors orchestrate cell fate decisions remain largely elusive. Our group and others unravelled complex interactions in the regulation of this controlled exit. Herein, we summarise recent findings and discuss the potential mechanisms involved.

Collaborative rewiring of the pluripotency network by chromatin and signalling modulating pathways

Reprogramming of somatic cells to induced pluripotent stem cells (iPSCs) represents a profound change in cell fate. Here, we show that combining ascorbic acid (AA) and 2i (MAP kinase and GSK inhibitors) increases the efficiency of reprogramming from fibroblasts and synergistically enhances conversion of partially reprogrammed intermediates to the iPSC state. AA and 2i induce differential transcriptional responses, each leading to the activation of specific pluripotency loci. A unique cohort of pluripotency genes including Esrrb require both stimuli for activation. Temporally, AA-dependent histone demethylase effects are important early, whereas Tet enzyme effects are required throughout the conversion. 2i function could partially be replaced by depletion of components of the epidermal growth factor (EGF) and insulin growth factor pathways, indicating that they act as barriers to reprogramming. Accordingly, reduction in the levels of the EGF receptor gene contributes to the activation of Esrrb. These results provide insight into the rewiring of the pluripotency network at the late stage of reprogramming.

Modeling murine yolk sac hematopoiesis with embryonic stem cell culture systems

The onset of hematopoiesis in mammals is defined by generation of primitive erythrocytes and macrophage progenitors in embryonic yolk sac. Laboratories have met the challenge of transient and swiftly changing specification events from ventral mesoderm through multipotent progenitors and maturing lineage-restricted hematopoietic subtypes, by developing powerful in vitro experimental models to interrogate hematopoietic ontogeny. Most importantly, studies of differentiating embryonic stem cell derivatives in embryoid body and stromal coculture systems have identified crucial roles for transcription factor networks (e.g. Gata1, Runx1, Scl) and signaling pathways (e.g. BMP, VEGF, WNT) in controlling stem and progenitor cell output. These and other relevant pathways have pleiotropic biological effects, and are often associated with early embryonic lethality in knockout mice. Further refinement in subsequent studies has allowed conditional expression of key regulatory genes, and isolation of progenitors via cell surface markers (e.g. FLK1) and reporter-tagged constructs, with the purpose of measuring their primitive and definitive hematopoietic potential. These observations continue to inform attempts to direct the differentiation, and augment the expansion, of progenitors in human cell culture systems that may prove useful in cell replacement therapies for hematopoietic deficiencies. The purpose of this review is to survey the extant literature on the use of differentiating murine embryonic stem cells in culture to model the developmental process of yolk sac hematopoiesis.

Tissue-engineered vascular grafts created from human induced pluripotent stem cells

The utility of human induced pluripotent stem cells (hiPSCs) to create tissue-engineered vascular grafts was evaluated in this study. hiPSC lines were first induced into a mesenchymal lineage via a neural crest intermediate using a serum-free, chemically defined differentiation scheme. Derived cells exhibited commonly known mesenchymal markers (CD90, CD105, and CD73 and negative marker CD45) and were shown to differentiate into several mesenchymal lineages (osteogenic, chondrogenic, and adipogenic). Functional vascular grafts were then engineered by culturing hiPSC-derived mesenchymal progenitor cells in a pulsatile bioreactor system over 8 weeks to induce smooth muscle cell differentiation and collagenous matrix generation. Histological analyses confirmed layers of calponin-positive smooth muscle cells in a collagen-rich matrix. Mechanical tests revealed that grafts had an average burst pressure of 700 mmHg, which is approximately half that of native veins. Additionally, studies revealed that karyotypically normal mesenchymal stem cell clones led to generation of grafts with predicted features of engineered vascular grafts, whereas derived clones having chromosomal abnormalities generated calcified vessel constructs, possibly because of cell apoptosis during culture. Overall, these results provide significant insight into the utility of hiPS cells for vascular graft generation. They pave the way for creating personalized, patient-specific vascular grafts for surgical applications, as well as for creating experimental models of vascular development and disease.

BMP4 promotes EMT and mesodermal commitment in human embryonic stem cells via SLUG and MSX2

Bone morphogenetic proteins (BMPs) initiate differentiation in human embryonic stem cells (hESCs) but the exact mechanisms have not been fully elucidated. We demonstrate here that SLUG and MSX2, transcription factors involved in epithelial-mesenchymal transitions, essential features of gastrulation in development and tumor progression, are important mediators of BMP4-induced differentiation in hESCs. Phosphorylated Smad1/5/8 colocalized with the SLUG protein at the edges of hESC colonies where differentiation takes place. The upregulation of the BMP target SLUG was direct as shown by the binding of phosphorylated Smad1/5/8 to its promoter, which interrupted the formation of adhesion proteins, resulting in migration. Knockdown of SLUG by short hairpin RNA blocked these changes, confirming an important role for SLUG in BMP-mediated mesodermal differentiation. Furthermore, BMP4-induced MSX2 expression leads to mesoderm formation and then preferential differentiation toward the cardiovascular lineage.

Directed differentiation of pluripotent stem cells to kidney cells

Regenerative medicine affords a promising therapeutic strategy for the treatment of patients with chronic kidney disease. Nephron progenitor cell populations exist only during embryonic kidney development. Understanding the mechanisms by which these populations arise and differentiate is integral to the challenge of generating new nephrons for therapeutic purposes. Pluripotent stem cells (PSCs), comprising embryonic stem cells, and induced pluripotent stem cells (iPSCs) derived from adults, have the potential to generate functional kidney cells and tissue. Studies in mouse and human PSCs have identified specific approaches to the addition of growth factors, including Wnt and fibroblast growth factor, that can induce PSC differentiation into cells with phenotypic characteristics of nephron progenitor populations with the capacity to form kidney-like structures. Although significant progress has been made, further studies are necessary to confirm the production of functional kidney cells and to promote their three-dimensional organization into bona fide kidney tissue. Human PSCs have been generated from patients with kidney diseases, including polycystic kidney disease, Alport syndrome, and Wilms tumor, and may be used to better understand phenotypic consequences of naturally occurring genetic mutations and to conduct "clinical trials in a dish". The capability to generate human kidney cells from PSCs has significant translational applications, including the bioengineering of functional kidney tissue, use in drug development to test compounds for efficacy and toxicity, and in vitro disease modeling.

BMP signaling balances murine myeloid potential through SMAD-independent p38MAPK and NOTCH pathways

Bone morphogenetic protein (BMP) signaling regulates early hematopoietic development, proceeding from mesoderm patterning through the progressive commitment and differentiation of progenitor cells. The BMP pathway signals largely through receptor-mediated activation of Mothers Against Decapentaplegic homolog (SMAD) proteins, although alternate pathways are modulated through various components of mitogen-activated protein kinase (MAPK) signaling. Using a conditional, short hairpin RNA (shRNA)-based knockdown system in the context of differentiating embryonic stem cells (ESCs), we demonstrated previously that Smad1 promotes hemangioblast specification, but then subsequently restricts primitive progenitor potential. Here we show that co-knockdown of Smad5 restores normal progenitor potential of Smad1-depleted cells, suggesting opposing functions for Smad1 and Smad5. This balance was confirmed by cotargeting Smad1/5 with a specific chemical antagonist, LDN193189 (LDN). However, we discovered that LDN treatment after hemangioblast commitment enhanced primitive myeloid potential. Moreover, inhibition with LDN (but not SMAD depletion) increased expression of Delta-like ligands Dll1 and Dll3 and NOTCH activity; abrogation of NOTCH activity restored LDN-enhanced myeloid potential back to normal, corresponding with expression levels of the myeloid master regulator, C/EBPα. LDN but not SMAD activity was also associated with activation of the p38MAPK pathway, and blocking this pathway was sufficient to enhance myelopoiesis. Therefore, NOTCH and p38MAPK pathways balance primitive myeloid progenitor output downstream of the BMP pathway.

Lysophosphatidic acid acts as a nutrient-derived developmental cue to regulate early hematopoiesis

Primitive hematopoiesis occurs in the yolk sac blood islands during vertebrate embryogenesis, where abundant phosphatidylcholines (PC) are available as important nutrients for the developing embryo. However, whether these phospholipids also generate developmental cues to promote hematopoiesis is largely unknown. Here, we show that lysophosphatidic acid (LPA), a signaling molecule derived from PC, regulated hemangioblast formation and primitive hematopoiesis. Pharmacological and genetic blockage of LPA receptor 1 (LPAR1) or autotoxin (ATX), a secretory lysophospholipase that catalyzes LPA production, inhibited hematopoietic differentiation of mouse embryonic stem cells and impaired the formation of hemangioblasts. Mechanistic experiments revealed that the regulatory effect of ATX-LPA signaling was mediated by PI3K/Akt-Smad pathway. Furthermore, during in vivo embryogenesis in zebrafish, LPA functioned as a developmental cue for hemangioblast formation and primitive hematopoiesis. Taken together, we identified LPA as an important nutrient-derived developmental cue for primitive hematopoiesis as well as a novel mechanism of hemangioblast regulation.

Building and repairing the heart: what can we learn from embryonic development?

Mammalian heart formation is a complex morphogenetic event that depends on the correct temporal and spatial contribution of distinct cell sources. During cardiac formation, cellular specification, differentiation, and rearrangement are tightly regulated by an intricate signaling network. Over the last years, many aspects of this network have been uncovered not only due to advances in cardiac development comprehension but also due to the use of embryonic stem cells (ESCs) in vitro model system. Additionally, several of these pathways have been shown to be functional or reactivated in the setting of cardiac disease. Knowledge withdrawn from studying heart development, ESCs differentiation, and cardiac pathophysiology may be helpful to envisage new strategies for improved cardiac repair/regeneration. In this review, we provide a comparative synopsis of the major signaling pathways required for cardiac lineage commitment in the embryo and murine ESCs. The involvement and possible reactivation of these pathways following heart injury and their role in tissue recovery will also be discussed.

Modulators of erythropoiesis: emerging therapies for hemoglobinopathies and disorders of red cell production

Use of new compound such as inhibitors of JAK2 or transforming growth factor β-like molecules might soon revolutionize the treatment of β-thalassemia and related disorders. However, this situation requires careful optimization, noting the potential for off-target immune suppression for JAK2 inhibitors and the lack of mechanistic insights for the use of the ligand trap soluble molecules that sequester ligands of activin receptor IIA and B.

Activin A supplement in the hESCs culture enhances the endoderm differentiation efficiency

Activin A is a critical regulator in human embryonic stem cells (hESCs) maintenance and differentiation. Different concentrations of Activin A affect hESC maintenance and differentiation in different ways. A high concentration favors anterior primitive streak and gives rise to DE if the stimulation persists. hESCs were cultured with and without 10 ng/mL Activin A supplement respectively. The two groups of cells were differentiated into endoderm cells with 100 ng/mL Activin A and other reagents. Microarray-based DNA methylation was analyzed with the Infinium Human Methylation450 BeadChip on these two groups. There was a significant difference in endoderm differentiation efficiency (average efficiency: 71 vs. 58.5%, P < 0.05). hESCs cultured with Activin A supplement had an increased propensity to form definitive endoderm cells in response to Activin A and Wnt signal. Differentially Methylated Regions (DMRs) between these two groups were found. DMRs were related to the stem cell maintenance and gene regulation by peroxisome proliferators via PPARα, indicating that hESCs maintained with Activin A supplement had stronger "stemness."

Mesenchymal morphogenesis of embryonic stem cells dynamically modulates the biophysical microtissue niche

Stem cell fate and function are dynamically modulated by the interdependent relationships between biochemical and biophysical signals constituting the local 3D microenvironment. While approaches to recapitulate the stem cell niche have been explored for directing stem cell differentiation, a quantitative relationship between embryonic stem cell (ESC) morphogenesis and intrinsic biophysical cues within three-dimensional microtissues has not been established. In this study, we demonstrate that mesenchymal embryonic microtissues induced by BMP4 exhibited increased stiffness and viscosity accompanying differentiation, with cytoskeletal tension significantly contributing to multicellular stiffness. Perturbation of the cytoskeleton during ESC differentiation led to modulation of the biomechanical and gene expression profiles, with the resulting cell phenotype and biophysical properties being highly correlated by multivariate analyses. Together, this study elucidates the dynamics of biophysical and biochemical signatures within embryonic microenvironments, with broad implications for monitoring tissue dynamics, modeling pathophysiological and embryonic morphogenesis and directing stem cell patterning and differentiation.

Regulation of pluripotent cell differentiation by a small molecule, staurosporine

Research in the embryo and in culture has resulted in a sophisticated understanding of many regulators of pluripotent cell differentiation. As a consequence, protocols for the differentiation of pluripotent cells generally rely on a combination of exogenous growth factors and endogenous signalling. Little consideration has been given to manipulating other pathways to achieve pluripotent cell differentiation. The integrity of cell:cell contacts has been shown to influence lineage choice during pluripotent cell differentiation, with disruption of cell:cell contacts promoting mesendoderm formation and maintenance of cell:cell contacts resulting in the preferential formation of neurectoderm. Staurosporine is a broad spectrum inhibitor of serine/threonine kinases which has several effects on cell function, including interruption of cell:cell contacts, decreasing focal contact size, inducing epithelial to mesenchyme transition (EMT) and promoting cell differentiation. The possibility that staurosporine could influence lineage choice from pluripotent cells in culture was investigated. The addition of staurosporine to differentiating mouse EPL resulted in preferential formation of mesendoderm and mesoderm populations, and inhibited the formation of neurectoderm. Addition of staurosporine to human ES cells similarly induced primitive streak marker gene expression. These data demonstrate the ability of staurosporine to influence lineage choice during pluripotent cell differentiation and to mimic the effect of disrupting cell:cell contacts. Staurosporine induced mesendoderm in the absence of known inducers of formation, such as serum and BMP4. Staurosporine induced the expression of mesendoderm markers, including markers that were not induced by BMP4, suggesting it acted as a broad spectrum inducer of molecular gastrulation. This approach has identified a small molecule regulator of lineage choice with potential applications in the commercial development of ES cell derivatives, specifically as a method for forming mesendoderm progenitors or as a culture adjunct to prevent the formation of ectoderm progenitors during pluripotent cell differentiation.

Soluble factors secreted by differentiating embryonic stem cells stimulate exogenous cell proliferation and migration

INTRODUCTION

Stem cells are being investigated as catalysts of tissue regeneration to either directly replace or promote cellularity lost as a result of traumatic injury or degenerative disease. In many reports, despite low numbers of stably integrated cells, the transient presence of cells delivered or recruited to sites of tissue remodeling globally benefits functional recovery. Such findings have motivated the need to determine how paracrine factors secreted from transplanted cells may be capable of positively impacting endogenous repair processes and somatic cell responses.

METHODS

Embryonic stem cells were differentiated as embryoid bodies (EBs) in vitro and media conditioned by EBs were collected at different intervals of time. Gene and protein expression analysis of several different growth factors secreted by EBs were examined by polymerase chain reaction and enzyme-linked immunosorbent assay analysis, respectively, as a function of time. The proliferation and migration of fibroblasts and endothelial cells treated with EB conditioned media was examined compared with unconditioned and growth media controls.

RESULTS

The expression of several growth factors, including bone morphogenic protein-4, insulin-like growth factors and vascular endothelial growth factor-A, increased during the course of embryonic stem cell (ESC) differentiation as EBs. Conditioned media collected from EBs at different stages of differentiation stimulated proliferation and migration of both fibroblasts and endothelial cells, based on 5-bromo-2'-deoxyuridine incorporation and transwell assays, respectively.

CONCLUSIONS

Overall, these results demonstrate that differentiating ESCs express increasing amounts of various growth factors over time that altogether are capable of stimulating mitogenic and motogenic activity of exogenous cell populations.

Induced pluripotent stem cells as a source of hepatocytes

During the past decade, a series of discoveries has established the potential of the so called terminally differentiated cells to transition to more primitive progenitor cells. The dramatic demonstration of the ability to reprogram differentiated somatic cells to induced pluripotent stem cells (iPSC) that can then give rise to cells of all three germ layers has opened the possibility of generating virtually any cell type in culture, from any given individual. Taking advantage of these concepts, researchers have generated iPSCs by reprogramming a wide variety of somatic cells. In addition to their practical implications, these studies have provided crucial insights into the mechanism of cell plasticity that underlies the transition from one cell type to another. Using concepts derived from research on embryological development, investigators have differentiated iPSCs to cells resembling hepatocytes in many ways. Such hepatocyte-like cells could be of enormous value in disease modeling, drug discovery and regenerative medicine. However, the currently available methods do not yield cells that fully reproduce the characteristics of adult primary hepatocytes. Thus generating hepatocytes from iPSCs is very much a work in progress. In addition to chronicling these exciting developments, this review will discuss the emergent new approaches to generating iPSCs, improving their differentiation to hepatocyte-like cells and maintaining the hepatocyte-like cells in culture for longer survival and better function.