The Regulation of Self-Renewal in Human Embryonic Stem Cells

Cryopreservation of organoids: Strategies, innovation, and future prospects

Organoid technology has demonstrated unique advantages in multidisciplinary fields such as disease research, tumor drug sensitivity, clinical immunity, drug toxicology, and regenerative medicine. It will become the most promising research tool in translational research. However, the long preparation time of organoids and the lack of high-quality cryopreservation methods limit the further application of organoids. Although the high-quality cryopreservation of small-volume biological samples such as cells and embryos has been successfully achieved, the existing cryopreservation methods for organoids still face many bottlenecks. In recent years, with the development of materials science, cryobiology, and interdisciplinary research, many new materials and methods have been applied to cryopreservation. Several new cryopreservation methods have emerged, such as cryoprotectants (CPAs) of natural origin, ice-controlled biomaterials, and rapid rewarming methods. The introduction of these technologies has expanded the research scope of cryopreservation of organoids, provided new approaches and methods for cryopreservation of organoids, and is expected to break through the current technical bottleneck of cryopreservation of organoids. This paper reviews the progress of cryopreservation of organoids in recent years from three aspects: damage factors of cryopreservation of organoids, new protective agents and loading methods, and new technologies of cryopreservation and rewarming.

Cryopreservation of Organoids

Organoids represent indispensable opportunities for biomedicine, including drug discovery, cancer biology, regenerative and personalised medicine or tissue and organ transplantation. However, the lack of optimised preservation strategies limits the wide use of organoids in research or clinical fields. In this review, we present a short outline of the recent developments in organoid research and current cryopreservation strategies for organoid systems. While both vitrification and slow controlled freezing have been utilized for the cryopreservation of organoid structures or their precursor components, the controlled-rate slow freezing under protection of Me2 SO remains the most common approach. The application of appropriate pre- or post-treatment strategies, like the addition of Rho-kinase or myosin inhibitors into cell culture or cryopreservation medium, can increase the recovery of complex organoid constructs post-thaw. However, the high complexity of the organoid structure and heterogeneity of cellular composition bring challenges associated with cryoprotectant distribution, distinct response of cells to the solution and freezing-induced injuries. The deficit of adequate quality control methods, which may ensure the assessment of organoid recovery in due term without prolonged re-cultivation process, represents another challenge limiting the reproducibility of current cryobanking technology. In this review, we attempt to assess the current demands and achievements in organoid cryopreservation and highlight the key questions to focus on during the development of organoid preservation technologies.

Schistosoma mansoni Fibroblast Growth Factor Receptor A Orchestrates Multiple Functions in Schistosome Biology and in the Host-Parasite Interplay

Stem cells play significant roles in driving the complex life cycle of Schistosoma mansoni. Fibroblast growth factor (FGF) receptor A (SmFGFRA) is essential for maintaining the integrity of schistosome stem cells. Using immunolocalization, we demonstrated that SmFGFRA was distributed abundantly in germinal/stem cells of different S. mansoni life stages including eggs, miracidia, cercariae, schistosomula and adult worms. Indeed, SmFGFRA was also localized amply in embryonic cells and in the perinuclear region of immature eggs; von Lichtenberg's layer and the neural mass of mature eggs; the ciliated surface and neural mass of miracidia; the tegument cytosol of cercariae, schistosomula and adult worms; and was present in abundance in the testis and vitellaria of adult worms of S. mansoni. The distribution pattern of SmFGFRA illustrates the importance of this molecule in maintaining stem cells, development of the nervous and reproductive system of schistosomes, and in the host-parasite interplay. We showed SmFGFRA can bind human FGFs, activating the mitogen activated protein kinase (MAPK) pathway of adult worms in vitro. Inhibition of FGF signaling by the specific tyrosine kinase inhibitor BIBF 1120 significantly reduced egg hatching ability and affected the behavior of miracidia hatched from the treated eggs, emphasizing the importance of FGF signaling in driving the life cycle of S. mansoni. Our findings provide increased understanding of the complex schistosome life cycle and host-parasite interactions, indicating components of the FGF signaling pathway may represent promising targets for developing new interventions against schistosomiasis.

Xeno-Free Materials for Stabilizing Basic Fibroblast Growth Factor and Enhancing Cell Proliferation in Human Pluripotent Stem Cell Cultures

Induced pluripotent stem cells (iPSCs) are widely considered important for developing novel regenerative therapies. A major challenge to the growth and proliferation of iPSCs is the maintenance of their undifferentiated status in xeno- and feeder-free conditions. Basic fibroblast growth factor (bFGF) is known to contribute to the expansion of stem cells; however, bFGF is notoriously heat-labile and easily denatured. Here, we investigate the effects of a series of synthetic sulfated/sulfonated polymers and saccharides on the growth of iPSCs. We observed that these materials effectively prevented the reduction of bFGF levels in iPSC culture media during storage at 37 °C. Some of the tested materials also suppressed heat-induced decline in medium performance and maintained cell proliferation. Our results suggest that these sulfated materials can be used to improve the expansion culture of undifferentiated iPSCs and show the potential of cost effective, chemically defined materials for improvement of medium performance while culturing iPSCs.

FOLFOX Therapy Induces Feedback Upregulation of CD44v6 through YB-1 to Maintain Stemness in Colon Initiating Cells

Cancer initiating cells (CICs) drive tumor formation and drug-resistance, but how they develop drug-resistance characteristics is not well understood. In this study, we demonstrate that chemotherapeutic agent FOLFOX, commonly used for drug-resistant/metastatic colorectal cancer (CRC) treatment, induces overexpression of CD44v6, MDR1, and oncogenic transcription/translation factor Y-box-binding protein-1 (YB-1). Our study revealed that CD44v6, a receptor for hyaluronan, increased the YB-1 expression through PGE2/EP1-mTOR pathway. Deleting CD44v6, and YB-1 by the CRISPR/Cas9 system attenuates the in vitro and in vivo tumor growth of CICs from FOLFOX resistant cells. The results of DNA:CD44v6 immunoprecipitated complexes by ChIP (chromatin-immunoprecipitation) assay showed that CD44v6 maintained the stemness traits by promoting several antiapoptotic and stemness genes, including cyclin-D1,BCL2,FZD1,GINS-1, and MMP9. Further, computer-based analysis of the clones obtained from the DNA:CD44v6 complex revealed the presence of various consensus binding sites for core stemness-associated transcription factors “CTOS” (c-Myc, TWIST1, OCT4, and SOX2). Simultaneous expressions of CD44v6 and CTOS in CD44v6 knockout CICs reverted differentiated CD44v6-knockout CICs into CICs. Finally, this study for the first time describes a positive feedback loop that couples YB-1 induction and CD44 alternative splicing to sustain the MDR1 and CD44v6 expressions, and CD44v6 is required for the reversion of differentiated tumor cells into CICs.

Cancer stem cell subpopulations in primary colon adenocarcinoma

Aims

The cancer stem cell concept proposes that tumor growth and recurrence is driven by a small population of cancer stem cells (CSCs). In this study we investigated the expression of induced-pluripotent stem cell (iPSC) markers and their localization in primary low-grade adenocarcinoma (LGCA) and high-grade adenocarcinoma (HGCA) and their patient-matched normal colon samples.

Materials and methods

Transcription and translation of iPSC markers OCT4, SOX2, NANOG, KLF4 and c-MYC were investigated using immunohistochemical (IHC) staining, RT-qPCR and in-situ hybridization (ISH).

Results

All five iPSC markers were detected at the transcriptional and translational levels. Protein abundance was found to be correlated with tumor grade. Based on their protein expression within the tumors, two sub-populations of cells were identified: a NANOG⁺/OCT4^- epithelial subpopulation and an OCT4⁺/NANOG^- stromal subpopulation. All cases were accurately graded based on four pieces of iPSC marker-related data.

Conclusions

This study suggests the presence of two putative sub-populations of CSCs: a NANOG⁺/OCT4^- epithelial subpopulation and an OCT4⁺/NANOG^- stromal subpopulation. Normal colon, LGCA and HGCA could be accurately distinguished from one another using iPSC marker expression. Once validated, novel combinations of iPSC markers may provide diagnostic and prognostic value to help guide patient management.

Expression of Embryonic Stem Cell Markers in Microcystic Lymphatic Malformation

Aim: To investigate the expression of embryonic stem cell (ESC) markers in microcystic lymphatic malformation (mLM). Methods and Results: Cervicofacial mLM tissue samples from nine patients underwent 3,3'-diaminobenzidine (DAB) immunohistochemical (IHC) staining for ESC markers octamer-binding protein 4 (OCT4), homeobox protein NANOG, sex determining region Y-box 2 (SOX2), Krupple-like factor (KLF4), and proto-oncogene c-MYC. Transcriptional activation of these ESC markers was investigated using real-time polymerase chain reaction (RT-qPCR) and colorimetric in situ hybridization (CISH) on four and five of these mLM tissue samples, respectively. Immunofluorescence (IF) IHC staining was performed on three of these mLM tissue samples to investigate localization of these ESC markers. DAB and IF IHC staining demonstrated the expression of OCT4, SOX2, NANOG, KLF4, and c-MYC on the endothelium of lesional vessels with abundant expression of c-MYC and SOX2, which was also present on the cells within the stroma, in all nine mLM tissue samples. RT-qPCR and CISH confirmed transcriptional activation of all these ESC markers investigated. Conclusions: These findings suggest the presence of a primitive population on the endothelium of lesional vessels and the surrounding stroma in mLM. The abundant expression of the progenitor-associated markers SOX2 and c-MYC suggests that the majority are of progenitor phenotype with a small number of ESC-like cells.

Characterization of Cancer Stem Cells in Colon Adenocarcinoma Metastasis to the Liver

Background

Fifty percent of colorectal cancer (CRC) patients develop liver metastasis. This study identified and characterized cancer stem cells (CSCs) within colon adenocarcinoma metastasis to the liver (CAML).

Methods

3,3-Diaminobenzidine immunohistochemical (IHC) staining was performed on nine CAML samples for embryonic stem cell (ESC) markers OCT4, SOX2, NANOG, c-Myc, and KLF4. Immunofluorescence (IF) IHC staining was performed to investigate coexpression of two markers. NanoString mRNA expression analysis and colorimetric in situ hybridization (CISH) were performed on four snap-frozen CAML tissue samples for transcript expression of these ESC markers. Cells stained positively and negatively for each marker by IHC and CISH staining were counted and analyzed.

Results

3,3-Diaminobenzidine IHC staining, and NanoString and CISH mRNA analyses demonstrated the expression of OCT4, SOX2, NANOG, c-Myc, and KLF4 within in all nine CAML samples, except for SOX2 which was below detectable levels on NanoString mRNA analysis. IF IHC staining showed the presence of a SOX2⁺/NANOG⁺/KLF4⁺/c-Myc⁺/OCT⁻ CSC subpopulation within the tumor nests, and a SOX2⁺/NANOG⁺/KLF4⁺/c-Myc⁺/OCT4⁻ CSC subpopulation and a SOX2⁺/NANOG⁺/KLF4⁺/c-Myc⁺/OCT4⁺ CSC subpopulation within the peritumoral stroma.

Conclusion

The novel finding of three CSC subpopulations within CAML provides insights into the biology of CRC.

Cancer stem cells in colorectal cancer: a review

Colorectal cancer (CRC) is the second most common cancer in women and the third most common in men. Adenocarcinoma accounts for 90% of CRC cases. There has been accumulating evidence in support of the cancer stem cell (CSC) concept of cancer which proposes that CSCs are central in the initiation of cancer. CSCs have been the focus of study in a range of cancers, including CRC. This has led to the identification and understanding of genes involved in the induction and maintenance of pluripotency of stem cells, and markers for CSCs, including those investigated specifically in CRC. Knowledge of the expression pattern of CSCs in CRC has been increasing in recent years, revealing a heterogeneous population of cells within CRC ranging from pluripotent to differentiated cells, with overlapping and sometimes unique combinations of markers. This review summarises current literature on the understanding of CSCs in CRC, including evidence of the presence of CSC subpopulations, and the stem cell markers currently used to identify and localise these CSC subpopulations. Future research into this field may lead to improved methods for early detection of CRC, novel therapy and monitoring of treatment for CRC and other cancer types.

Ectopic Gastric and Intestinal Phenotypes, Neuroendocrine Cell Differentiation, and SOX2 Expression Correlated With Early Tumor Progression in Colorectal Laterally Spreading Tumors

INTRODUCTION

The significance of the ectopic gastric phenotype remains unclear in patients with colorectal laterally spreading tumors (LSTs). We investigated clinicopathologic differences among LST subtypes, aiming to identify factors indicative of malignant transformation and invasion that are linked to ectopic gastric phenotype and tumor progression.

MATERIALS AND METHODS

We analyzed the morphologic characteristics of 105 colorectal LSTs resected by endoscopic submucosal dissection. LSTs were classified into 2 subtypes: granular (G-LST) and nongranular (NG-LST). Resected LSTs were analyzed histologically and were immunohistochemically stained for MUC5AC, MUC6, chromogranin A, CD10, and SOX2.

RESULTS

The 105 LSTs included 60 G-LSTs and 45 NG-LSTs. By histology, G-LSTs comprised 5 adenomas with low-grade dysplasia (LAs), 45 adenomas with high-grade dysplasia (HAs), and 10 adenocarcinomas invading the submucosa (SMs). NG-LSTs comprised 8 LAs, 25 HAs, and 12 SMs. MUC5AC positivity was significantly higher in G-LSTs compared to NG-LSTs (P = .002), and MUC5AC positivity in HA lesions was significantly higher than in LA lesions (P = .01). MUC6 and SOX2 positivity in SM G-LSTs, and chromogranin A positivity in SM NG-LSTs were significantly higher than in HAs (P = .01, .01, and .03, respectively). CD10 positivity in SM NG-LSTs was significantly higher than in HAs and LAs (P = .02 and .01, respectively).

CONCLUSION

Ectopic gastric and intestinal phenotypes, neuroendocrine cell differentiation, and SOX2 expression differ according to tumor grade in colorectal LSTs, and these markers are correlated with early tumor progression in each LST subtype.

Comparison of gene expression of SOX2 and OCT4 in normal tissue, polyps, and colon adenocarcinoma using immunohistochemical staining

BACKGROUND

Cancer stem cells have been isolated and characterized in all common cancers. SOX2 and OCT4 are important genes to enhance the self-renewal ability as activate stem cells and inhibit the genes that start differentiation and thus maintain the self-renewal ability of stem cells. Also, the aim of this study is "Comparison of gene expression of SOX2 and OCT4 in normal tissue, polyps, and colon adenocarcinoma using immunohistochemical staining."

MATERIALS AND METHODS

This cross-sectional study conducted on 20 patients so that for each patient, a sample of healthy tissue, dysplastic polyp tissue, and colon adenocarcinoma were provided as microscopic sections and staining on each tissue was performed through immunohistochemistry method by markers OCT4 and SOX2. The collected data were interred into SPSS version 18.0, (SPSS Inc., Chicago, IL, USA) software and the level of significance were considered as <0.05.

RESULTS

The study sample consisted of 20 patients including 11 men (55%) and 9 women (45%) with a mean age of 55.6 ± 9.88 years. There was no association between Oct4 and colorectal cancer (CRC) patients (P > 0.05), but there was a significant correlation between Sox2 expression and CRC (P < 0.05). Patients in many aspects such as race, type of polyp, presence of lymph node, grade and intensity of Sox2 in different types of patients' tissues (P < 0.05).

CONCLUSION

Regarding our findings, the expression of Sox2 would be a liable marker for evaluating of cancer progression and could be a treatment target of CRC cells.

A stable chimeric fibroblast growth factor (FGF) can successfully replace basic FGF in human pluripotent stem cell culture

Fibroblast growth factors (FGFs) are essential for maintaining self-renewal in human embryonic stem cells and induced pluripotent stem cells. Recombinant basic FGF (bFGF or FGF2) is conventionally used to culture pluripotent stem cells; however, because of the instability of bFGF, repeated addition of fresh bFGF into the culture medium is required in order to maintain its concentration. In this study, we demonstrate that a heat-stable chimeric variant of FGF, termed FGFC, can be successfully used for maintaining human pluripotent stem cells. FGFC is a chimeric protein composed of human FGF1 and FGF2 domains that exhibits higher thermal stability and protease resistance than do both FGF1 and FGF2. Both human embryonic stem cells and induced pluripotent stem cells were maintained in ordinary culture medium containing FGFC instead of FGF2. Comparison of cells grown in FGFC with those grown in conventional FGF2 media showed no significant differences in terms of the expression of pluripotency markers, global gene expression, karyotype, or differentiation potential in the three germ lineages. We therefore propose that FGFC may be an effective alternative to FGF2, for maintenance of human pluripotent stem cells.

Protein kinase C-induced early growth response protein-1 binding to SNAIL promoter in epithelial-mesenchymal transition of human embryonic stem cells

Epithelial-mesenchymal transition (EMT) has been thought to occur during early embryogenesis, and also the differentiation process of human embryonic stem (hES) cells. Spontaneous differentiation is sometimes observed at the peripheral of the hES cell colonies in conventional culture conditions, indicating that EMT occurs in hES cell culture. However, the triggering mechanism of EMT is not yet fully understood. The balance between self-renewal and differentiation of human pluripotent stem (hPS) cells is controlled by various signal pathways, including the fibroblast growth factor (FGF)-2. However, FGF-2 has a complex role for self-renewal of hES cells. FGF-2 activates phosphatidylinositol-3 kinase/AKT, mitogen-activated protein kinase/extracellular signal-regulated kinase-1/2 kinase, and also protein kinase C (PKC). Here, we showed that a PKC rapidly induced an early growth response protein-1 (EGR-1) in hES cells, which was followed by upregulation of EMT-related genes. Before the induction of EMT-related genes, EGR-1 was translocated into the nucleus, and then bound directly to the promoter region of SNAIL, which is a master regulator of EMT. SNAIL expression was attenuated by knockdown of EGR-1, but upregulated by ectopic expression of EGR-1. EGR-1 as the downstream signal of PKC might play a key role in EMT initiation during early differentiation of hES cells. This study would lead to a more robust understanding of the mechanisms underlying the balance between self-renewal and initiation of differentiation in hPS cells.

Delphinidin prevents hypoxia-induced mouse embryonic stem cell apoptosis through reduction of intracellular reactive oxygen species-mediated activation of JNK and NF-κB, and Akt inhibition

Delphinidin, gallic acid, betulinic acid, and ursolic acid, which are bio-active ingredients in a variety of fruits, vegetables, and herbs, have potent antioxidant activity and various biological activities. However, it is not clear whether these bio-active ingredients can significantly contribute to the protection of embryonic stem (ES) cells from hypoxia-induced apoptosis. In the present study, hypoxia-induced ES cells apoptosis with time, which were abrogated by pretreatment with all ingredients. Hypoxia-induced ROS generation was blocked by pretreatment with all ingredients in a dose-dependent manner, with the maximum ROS scavenging effect observed for delphinidin. Hypoxia increased phosphorylation of JNK and NF-κB were blocked by pretreatment of delphinidin as well as NAC. Hypoxia decreased phosphorylation of Akt(thr308) and (ser473); these decreases were reversed by pretreatment with delphinidin or NAC. However, Akt inhibition did not affect NF-κB phosphorylation. Delphinidin attenuated the hypoxia-induced increase in Bax, cleaved caspase-9, cleaved caspase-3, and decrease in Bcl-2, which were diminished by pretreatment of Akt inhibitor. Hypoxia induced Bax translocation from the cytosol to mitochondria. Furthermore, hypoxia induced mitochondria membrane potential loss and cytochrome c release in cytosol, which were blocked by delphinidin pretreatment. Hypoxia induced cleavage of procaspase-9 and procaspase-3 which were blocked by delphinidin or SP600125, but Akt inhibitor abolished the protection effect of delphinidin. Moreover, inhibition of JNK and NF-κB abolished hypoxia-induced ES cell apoptosis and inhibition of Akt attenuated delphinidin-induced blockage of apoptosis. The results indicate that delphinidin can prevent hypoxia-induced apoptosis of ES cells through the inhibition of JNK and NF-κB phosphorylation, and restoration of Akt phosphorylation.

Engineered micromechanical cues affecting human pluripotent stem cell regulations and fate

The survival, growth, self-renewal, and differentiation of human pluripotent stem cells (hPSCs) are influenced by their microenvironment, or so-called "niche," consisting of particular chemical and physical cues. Previous studies on mesenchymal stem cells and other stem cells have collectively uncovered the importance of physical cues and have begun to shed light on how stem cells sense and process such cues. In an attempt to support similar progress in mechanobiology of hPSCs, we review mechanosensory machinery, which plays an important role in cell-extracellular matrix interactions, cell-cell interactions, and subsequent intracellular responses. In addition, we review recent studies on the mechanobiology of hPSCs, in which engineered micromechanical environments were used to investigate effects of specific physical cues. Identifying key physical cues and understanding their mechanism will ultimately help in harnessing the full potential of hPSCs for clinical applications.

Involvement of crosstalk between Oct4 and Meis1a in neural cell fate decision

Oct4 plays a critical role both in maintaining pluripotency and the cell fate decision of embryonic stem (ES) cells. Nonetheless, in the determination of the neuroectoderm (NE) from ES cells, the detailed regulation mechanism of the Oct4 gene expression is poorly understood. Here, we report that crosstalk between Oct4 and Meis1a, a Pbx-related homeobox protein, is required for neural differentiation of mouse P19 embryonic carcinoma (EC) cells induced by retinoic acid (RA). During neural differentiation, Oct4 expression was transiently enhanced during 6–12 h of RA addition and subsequently disappeared within 48 h. Coinciding with up-regulation of Oct4 expression, the induction of Meis1a expression was initiated and reached a plateau at 48 h, suggesting that transiently induced Oct4 activates Meis1a expression and the up-regulated Meis1a then suppresses Oct4 expression. Chromatin immunoprecipitation (ChIP) and luciferase reporter analysis showed that Oct4 enhanced Meis1a expression via direct binding to the Meis1 promoter accompanying histone H3 acetylation and appearance of 5-hydoxymethylcytosine (5hmC), while Meis1a suppressed Oct4 expression via direct association with the Oct4 promoter together with histone deacetylase 1 (HDAC1). Furthermore, ectopic Meis1a expression promoted neural differentiation via formation of large neurospheres that expressed Nestin, GLAST, BLBP and Sox1 as neural stem cell (NSC)/neural progenitor markers, whereas its down-regulation generated small neurospheres and repressed neural differentiation. Thus, these results imply that crosstalk between Oct4 and Meis1a on mutual gene expressions is essential for the determination of NE from EC cells.

Protein kinase C regulates human pluripotent stem cell self-renewal

Background

The self-renewal of human pluripotent stem (hPS) cells including embryonic stem and induced pluripotent stem cells have been reported to be supported by various signal pathways. Among them, fibroblast growth factor-2 (FGF-2) appears indispensable to maintain self-renewal of hPS cells. However, downstream signaling of FGF-2 has not yet been clearly understood in hPS cells.

Methodology/Principal Findings

In this study, we screened a kinase inhibitor library using a high-throughput alkaline phosphatase (ALP) activity-based assay in a minimal growth factor-defined medium to understand FGF-2-related molecular mechanisms regulating self-renewal of hPS cells. We found that in the presence of FGF-2, an inhibitor of protein kinase C (PKC), GF109203X (GFX), increased ALP activity. GFX inhibited FGF-2-induced phosphorylation of glycogen synthase kinase-3β (GSK-3β), suggesting that FGF-2 induced PKC and then PKC inhibited the activity of GSK-3β. Addition of activin A increased phosphorylation of GSK-3β and extracellular signal-regulated kinase-1/2 (ERK-1/2) synergistically with FGF-2 whereas activin A alone did not. GFX negated differentiation of hPS cells induced by the PKC activator, phorbol 12-myristate 13-acetate whereas Gö6976, a selective inhibitor of PKCα, β, and γ isoforms could not counteract the effect of PMA. Intriguingly, functional gene analysis by RNA interference revealed that the phosphorylation of GSK-3β was reduced by siRNA of PKCδ, PKCε, and ζ, the phosphorylation of ERK-1/2 was reduced by siRNA of PKCε and ζ, and the phosphorylation of AKT was reduced by PKCε in hPS cells.

Conclusions/Significance

Our study suggested complicated cross-talk in hPS cells that FGF-2 induced the phosphorylation of phosphatidylinositol-3 kinase (PI3K)/AKT, mitogen-activated protein kinase/ERK-1/2 kinase (MEK), PKC/ERK-1/2 kinase, and PKC/GSK-3β. Addition of GFX with a MEK inhibitor, U0126, in the presence of FGF-2 and activin A provided a long-term stable undifferentiated state of hPS cells even though hPS cells were dissociated into single cells for passage. This study untangles the cross-talk between molecular mechanisms regulating self-renewal and differentiation of hPS cells.

Pluripotency and nuclear reprogramming

Pluripotency is a "blank" cellular state characteristic of specific cells within the early embryo (e.g., epiblast cells) and of certain cells propagated in vitro (e.g., embryonic stem cells, ESCs). The terms pluripotent cell and stem cell are often used interchangeably to describe cells capable of differentiating into multiple cell types. In this review, we discuss the prevailing molecular and functional definitions of pluripotency and the working parameters employed to describe this state, both in the context of cells residing within the early embryo and cells propagated in vitro.

Investigating the role of FGF-2 in stem cell maintenance by global phosphoproteomics profiling

Human embryonic stem cells (hESCs) are of immense interest for regenerative medicine as a source of tissue replacement. Expansion of hESCs as a pluripotent population requires a balance between survival, proliferation and self-renewal signals. One of the key growth factors that maintains this balance is fibroblast growth factor-2 (FGF-2). However, the underlying molecular mechanisms are poorly understood. We recently profiled specifically tyrosine phosphorylation events that occur during FGF-2 stimulation of hESCs (Ding et al., J. Cell. Physiol. 2010, 225, 417-428). Here, we complement this phosphoproteome profiling by analyzing temporal dynamics of mostly serine and threonine protein phosphorylation events. Our multi-dimensional strategy combines strong cation exchange chromatography to reduce the sample complexity followed by titanium dioxide off-line for the enrichment of phosphopeptides and dimethylation-based stable isotope labeling for quantification. This approach allowed us to identify and quantify 3261 unique proteins from which 1064 proteins were found to be phosphorylated in one or more residues (representing 1653 unique phosphopeptides). Approximately 40% of the proteins (553 unique phosphopeptides) showed differential phosphorylation upon FGF-2 treatment. Among those are several members of the canonical pathways involved in pluripotency and self-renewal (e.g. Wnt and PI3K/AKT), hESC-associated proteins such as SOX2, RIF1, SALL4, DPPA4, DNMT3B and 53 proteins that are target genes of the pluripotency transcription factors SOX2, OCT4 and NANOG. These findings complement existing pluripotency analyses and provide new insights into how FGF-2 assists in maintaining the undifferentiated state of hESCs.

Stem cell culture engineering - process scale up and beyond

Advances in stem cell research and recent work on clinical trials employing stem cells have heightened the prospect of stem cell applications in regenerative medicine. The eventual clinical application of stem cells will require transforming cell production from laboratory practices to robust processes. Most stem cell applications will require extensive ex vivo handling of cells, from isolation, cultivation, and directed differentiation to product cell separation, cell derivation, and final formulation. Some applications require large quantities of cells in each defined batch for clinical use in multiple patients; others may be for autologous use and require only small-scale operations. All share a common requirement: the production must be robust and generate cell products of consistent quality. Unlike the established manufacturing process of recombinant protein biologics, stem cell applications will likely see greater variability in their cell source and more fluctuations in product quality. Nevertheless, in devising stem cell-based bioprocesses, much insight could be gained from the manufacturing of biological materials, including recombinant proteins and anti-viral vaccines. The key to process robustness is thus not only the control of traditional process chemical and physical variables, but also the sustenance of cells in the desired potency or differentiation state through controlling non-traditional variables, such as signaling pathway modulators.

Tyrosine phosphorylation profiling in FGF-2 stimulated human embryonic stem cells

The role of fibroblast growth factor-2 (FGF-2) in maintaining undifferentiated human embryonic stem cells (hESC) was investigated using a targeted phosphoproteomics approach to specifically profile tyrosine phosphorylation events following FGF-2 stimulation. A cumulative total number of 735 unique tyrosine phosphorylation sites on 430 proteins were identified, by far the largest inventory to date for hESC. Early signaling events in FGF-2 stimulated hESC were quantitatively monitored using stable isotope dimethyl labeling, resulting in temporal tyrosine phosphorylation profiles of 316 unique phosphotyrosine peptides originating from 188 proteins. Apart from the rapid activation of all four FGF receptors, trans-activation of several other receptor tyrosine kinases (RTKs) was observed as well as induced tyrosine phosphorylation of downstream proteins such as PI3-K, MAPK and several Src family members. Both PI3-K and MAPK have been linked to hESC maintenance through FGF-2 mediated signaling. The observed activation of the Src kinase family members by FGF-2 and loss of pluripotent marker expression post Src kinase inhibition may point to the regulation of cytoskeletal and actin depending processes to maintain undifferentiated hESC.

An optimized small molecule inhibitor cocktail supports long-term maintenance of human embryonic stem cells

A major challenge in stem cell-mediated regenerative medicine is the development of defined culture systems for the maintenance of clinical-grade human embryonic stem (hES) cells. Here, we identify, using a feedback system control scheme, a unique combination of three small molecule inhibitors that enables the maintenance of hES cells on a fibronectin-coated surface through single cell passaging. After 20 passages, the undifferentiated state of the hES cells was confirmed by OCT4, SSEA4 and NANOG expressions, whereas their pluripotent potential and genetic integrity were demonstrated by teratoma formation and normal karyotype, respectively. Our study attests to the power of the feedback system control scheme to quickly pinpoint optimal conditions for desired biological activities, and provides a chemically defined, scalable and single cell passaging culture system for hES cells.

Human stem cell research and regenerative medicine--present and future

INTRODUCTION

Stem cells are cells with the ability to grow and differentiate into more than 200 cell types.

SOURCES OF DATA

We review here the characteristics and potential of human embryonic stem cells (hESCs), induced pluripotent stem cells (iPSCs) and adult stem cells (ASCs).

AREAS OF AGREEMENT

The differentiation ability of all stem cell types could be stimulated to obtain specialized cells that represent renewable sources of functional cells useful for cell-based therapy.

AREAS OF CONTROVERSY

The proof of functional differentiated cells needs to be investigated in more detail using both in vitro and in vivo assays including animal disease models and clinical studies.

GROWING POINTS

Much progress has been made in the ASCs-based therapies. Meanwhile hESCs and iPSCs have dramatically emerged as novel approaches to understand pathogenesis of different diseases.

AREAS TIMELY FOR DEVELOPING RESEARCH

A number of new strategies become very important in regenerative medicine. However, we discuss the limitations of stem cells and latest development in the reprogramming research.

FGF-2 modulates Wnt signaling in undifferentiated hESC and iPS cells through activated PI3-K/GSK3beta signaling

Fibroblast growth factor-2 (FGF-2) is widely used to culture human embryonic stem cells (hESC) and induced pluripotent stem (iPS) cells. Despite its importance in maintaining undifferentiated hESC phenotype, a lack of understanding in the role of FGF-2 still exists. Here, we investigate the signaling events in hESC following the addition of exogenous FGF-2. In this study, we show that hESC express all forms of fibroblast growth factor receptors (FGFRs) which co-localize on Oct3/4 positive cells. Furthermore, downregulation of Oct3/4 in hESC occurs following treatment with an FGFR inhibitor, suggesting that FGF signaling may regulate Oct3/4 expression. This is also observed in iPS cells. Also, downstream of FGF signaling, both mitogen activated protein kinase (MAPK) and phosphoinositide 3-kinase pathways (PI3-K) are activated following FGF-2 stimulation. Notably, inhibition of MAPK and PI3-K signaling using specific kinase inhibitors revealed that activated PI3-K, rather than MAPK, can mediate pluripotent marker expression. To understand the importance of PI3-K activation, activation of Wnt/beta-catenin by FGF-2 was investigated. Wnt signaling had been implicated to have a role in maintaining of pluripotent hESC. We found that upon FGF-2 stimulation, GSK3beta is phosphorylated following which nuclear translocation of beta-catenin and TCF/LEF activation occurs. Interestingly, inhibition of the Wnt pathway with Dikkopf-1 (DKK-1) resulted in only partial suppression of the FGF-2 induced TCF/LEF activity. Prolonged culture of hESC with DKK-1 did not affect pluripotent marker expression. These results suggest that FGF-2 mediated PI3-K signaling may have a direct role in modulating the downstream of Wnt pathway to maintain undifferentiated hESC.

Growth of primary embryo cells in a microculture system

We present optimal perfusion conditions for the growth of primary mouse embryonic fibroblasts (mEFs) and mouse embryonic stem cells (mESCs) using a microfluidic perfusion culture system. In an effort to balance nutrient renewal while ensuring the presence of cell secreted factors, we found that the optimal perfusion rate for culturing primary embryonic fibroblasts (mEFs) in our experimental setting is 10 nL/min with an average flow velocity 0.55 microm/s in the microchannel. Primary mEFs may have a greater dependence on cell secreted factors when compared to their immortalized counterpart 3T3 fibroblasts cultured under similar conditions. Both the seeding density and the perfusion rate are critical for the proliferation of primary cells. A week long cultivation of mEFs and mESCs using the microculture system exhibited similar morphology and viability to those grown in a petri dish. Both mEFs and mESCs were analyzed using fluorescence immunoassays to determine their proliferative status and protein expression. Our results demonstrate that a perfusion-based microculture environment is capable of supporting the highly proliferative status of pluripotent embryonic stem cells.

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.

A human stem cell-based model for identifying adverse effects of organic and inorganic chemicals on the developing nervous system

The aim of our study was to investigate whether a human neural stem cell line derived from umbilical cord blood (HUCB-NSC) can serve as a reliable test model for developmental neurotoxicity (DNT). We assessed the sensitivity of HUCB-NSCs at different developmental stages to a panel of neurotoxic (sodium tellurite, methylmercury chloride, cadmium chloride, chlorpyrifos, and L-glutamate) and non-neurotoxic (acetaminophen, theophylline, and D-glutamate) compounds. In addition, we investigated the effect of some compounds on key neurodevelopmental processes like cell proliferation, apoptotic cell death, and neuronal and glial differentiation. Less differentiated HUCB-NSCs were generally more sensitive to neurotoxicants, with the notable exception of L-glutamate, which showed a higher toxicity to later stages. The relative potencies of the compounds were: cadmium chloride > methylmercury chloride >> chlorpyrifos >> L-glutamate. Fifty nanomolar methylmercury chloride (MeHgCl) inhibited proliferation and induced apoptosis in early-stage cells. At the differentiated stage, 1 muM MeHgCl induced selective loss of S100 beta-expressing astrocytic cells. One millimolar L-glutamate did not influence the early stages of HUCB-NSC development, but it affected late stages of neuronal differentiation. A valuable system for in vitro DNT assessment should be able to discriminate between neurotoxic and non-neurotoxic compounds and show different susceptibilities to chemicals according to developmental stage and cell lineage. Although not exhaustive, this work shows that the HUCB-NSC model fulfils these criteria and may serve as a human in vitro model for DNT priority setting.

Ly-1 antibody reactive clone is an important nucleolar protein for control of self-renewal and differentiation in embryonic stem cells

Embryonic stem cells (ESCs) possess the capacity to self-renew and differentiate into all cell types of an organism. It is essential to understand how these properties are controlled for the potential usage of their derivatives in clinical settings and reprogramming of differentiated somatic cells. Although transcriptional factors, such as Oct4, Sox2, and Nanog, have been considered as a part of the core regulatory circuitry, a growing body of evidence suggests that additional factors exist and contribute to the control of ESC self-renewal and differentiation. Here, we report that Ly-1 antibody reactive clone (LYAR), a zinc finger nucleolar protein highly expressed in undifferentiated ESCs, plays a critical role in maintaining ESC identity. Its downregulation significantly reduces the rate of ESC growth and increases their apoptosis. Moreover, reduced expression of LYAR in ESCs impairs their differentiation capacity, failing to rapidly silence pluripotency markers and to activate differentiation genes upon differentiation. Mechanistically, LYAR forms a complex with another nucleolar protein, nucleolin, and prevents its self-cleavage, maintaining a normal steady-state level of nucleolin protein in undifferentiated ESCs. Interestingly, the downregulation of nucleolin is detrimental to the growth of ESCs and increases the rate of apoptosis, similarly to the knockdown of LYAR. Thus, our data emphasize the fact that other genes besides Oct4 and Nanog are uniquely required for ESC self-renewal and differentiation and demonstrate that LYAR functions to control the stability of nucleolin protein, which in turn is essential for maintaining the self-renewal of ESCs.

Induced pluripotent stem cells in regenerative medicine: an argument for continued research on human embryonic stem cells

Human embryonic stem cells (ESCs) can be induced to differentiate into a wide range of tissues that soon could be used for therapeutic applications in regenerative medicine. Despite their developmental potential, sources used to generate human ESC lines raise serious ethical concerns, which recently prompted efforts to reprogram somatic cells back to a pluripotent state. These efforts resulted in the generation of induced pluripotent stem (iPS) cells that are functionally similar to ESCs. However, the genetic manipulations required to generate iPS cells may complicate their growth and developmental characteristics, which poses serious problems in predicting how they will behave when used for tissue-regenerative purposes. In this article we summarize the recently developed methodologies used to generate iPS cells, including those that minimize their genetic manipulation, and discuss several important complicating features of iPS cells that may compromise their future use for therapies in regenerative medicine.

Automated, scalable culture of human embryonic stem cells in feeder-free conditions

Large-scale manufacture of human embryonic stem cells (hESCs) is prerequisite to their widespread use in biomedical applications. However, current hESC culture strategies are labor-intensive and employ highly variable processes, presenting challenges for scaled production and commercial development. Here we demonstrate that passaging of the hESC lines, HUES7, and NOTT1, with trypsin in feeder-free conditions, is compatible with complete automation on the CompacT SelecT, a commercially available and industrially relevant robotic platform. Pluripotency was successfully retained, as evidenced by consistent proliferation during serial passage, expression of stem cell markers (OCT4, NANOG, TRA1-81, and SSEA-4), stable karyotype, and multi-germlayer differentiation in vitro, including to pharmacologically responsive cardiomyocytes. Automation of hESC culture will expedite cell-use in clinical, scientific, and industrial applications.

Modeling of asymmetric cell division in hematopoietic stem cells--regulation of self-renewal is essential for efficient repopulation

Hematopoietic stem cells (HSCs) are characterized by their ability of self-renewal to replenish the stem cell pool and differentiation to more mature cells. The subsequent stages of progenitor cells also share some of this dual ability. It is yet unknown whether external signals modulate proliferation rate or rather the fraction of self-renewal. We propose three multicompartment models, which rely on a single external feedback mechanism. In Model 1 the signal enhances proliferation, whereas the self-renewal rates in all compartments are fixed. In Model 2 the signal regulates the rate of self-renewal, whereas the proliferation rate is unchanged. In Model 3, the signal regulates both proliferation and self-renewal rates. This study demonstrates that a unique strictly positive stable steady state can only be achieved by regulation of the rate of self-renewal. Furthermore, it requires a lower number of effective cell doublings. In order to maintain the stem cell pool, the self-renewal ratio of the HSC has to be > or =50% and it has to be higher than the self-renewal ratios of all downstream compartments. Interestingly, the equilibrium level of mature cells depends only on the parameters of self-renewal of HSC and it is independent of the parameters of dynamics of all upstream compartments. The model is compatible with the increase of leukocyte numbers following HSC transplantation. This study demonstrates that extrinsic regulation of the self-renewal rate of HSC is most essential in the process of hematopoiesis.

Human embryonic stem cells: 10 years on

Substantial advances in the biology of human embryonic stem (ES) cells, and the technology for working with them, have been made over the past 10 years. Regulatory frameworks for their study are well developed, although some countries remain particularly restrictive. Markers and criteria for characterising human ES cells are also generally agreed, and protocols for promoting their differentiation are being established, providing the groundwork for the development of applications over the next 10 years. The recent appearance of technology to convert somatic cells to 'induced Pluripotent Stem Cells' closely resembling ES cells will certainly speed up these developments.

Sphingosine-1-phosphate mediates transcriptional regulation of key targets associated with survival, proliferation, and pluripotency in human embryonic stem cells

Human embryonic stem cells (hESCs) replicate in vitro by the process of self-renewal, whilst maintaining their pluripotency. Understanding the pathways involved in the regulation of this process will assist in developing fully-defined conditions for the robust proliferation of hESCs necessary for therapeutic applications. We previously demonstrated that sphingosine-1-phosphate (S1P) plays an important role in survival and proliferation of hESCs. and here the key signaling pathways and downstream targets of S1P were investigated in a representative cell line (Shef 4). A significant rise in ERK1/2 activation with S1P treatment was witnessed in hESCs maintained on murine embryonic fibroblasts (MEFs) exhibiting significantly higher levels of active ERK1/2 than those grown on Matrigel. RT-PCR and microarray analysis of micro-dissected, non-differentiated hESC revealed 1049 differentially expressed genes in S1P treated preparations compared with controls (n = 3). S1P regulated apoptosis through several BCL-2 family members, including BAX and BID, with increased expression of cell cycle progression genes associated with proliferation of hESC cultures. S1P treatment also increased expression of cell adhesion genes specifically cadherins and integrins. However, gene expression associated with pluripotency was decreased with S1P treatment indicating that an increased rate of hESC turnover (higher proliferation and lower apoptosis) may be balanced by an increased susceptibility to differentiate.

Human embryonic stem cells: origins, characteristics and potential for regenerative therapy

Stem cells and their use in regenerative therapies are currently hot topics in both biology and medicine. For transfusion scientists the concept of cell therapy is not a new idea but rather a fundamental practice in this field. Bone marrow transplantation was pioneered in the 1960s and relies on the capacity of haemopoietic stem cells in the donated bone marrow to completely reconstitute the blood system of the recipient. Although this capacity of adult (or somatic) stem cells to regenerate the tissue from which they arise is extremely important, the isolation and cultivation of human embryonic stem cells (hESCs) have opened up the possibility to generate any cell or tissue of the body. This characteristic of hESC offers the hope of cell replacement and regenerative therapy for a whole array of diseases, many of which are currently untreatable. However, in order to understand the potential advantages and disadvantages of using stem cells in regenerative medicine, it is necessary to fully understand their origin, characteristics and potential. This review will concentrate particularly on hESCs and their derivation, characterization and capacity to differentiate into clinically useful tissue including haemopoietic lineages.

Plasma membrane proteomics of human embryonic stem cells and human embryonal carcinoma cells

Human embryonic stem cells (hESCs) are of immense interest in regenerative medicine as they can self-renew indefinitely and can give rise to any adult cell type. Human embryonal carcinoma cells (hECCs) are the malignant counterparts of hESCs found in testis tumors. hESCs that have acquired chromosomal abnormalities in culture are essentially indistinguishable from hECC. Direct comparison of karyotypically normal hESCs with hECCs could lead to understanding differences between their mechanisms of growth control and contribute to implementing safe therapeutic use of stem cells without the development of germ cell cancer. While several comparisons of hECCs and hESCs have been reported, their cell surface proteomes are largely unknown, partly because plasma membrane proteomics is still a major challenge. Here, we present a strategy for the identification of plasma membrane proteins that has been optimized for application to the relatively small numbers of stem cells normally available, and that does not require tedious cell fractionation. The method led to the identification of 237 and 219 specific plasma membrane proteins in the hESC line HUES-7 and the hECC line NT2/D1, respectively. In addition to known stemness-associated cell surface markers like ALP, CD9, and CTNNB, a large number of receptors, transporters, signal transducers, and cell-cell adhesion proteins were identified. Our study revealed that several Hedgehog and Wnt pathway members are differentially expressed in hESCs and hECCs including NPC1, FZD2, FZD6, FZD7, LRP6, and SEMA4D, which play a pivotal role in stem cell self-renewal and cancer growth. Various proteins encoded on chromosome 12p, duplicated in testicular cancer, were uniquely identified in hECCs. These included GAPDH, LDHB, YARS2, CLSTN3, CSDA, LRP6, NDUFA9, and NOL1, which are known to be upregulated in testicular cancer. Distinct HLA molecules were revealed on the surface of hESCs and hECCs, despite their low abundance. Results were compared with genomic and proteomic data sets reported previously for mouse ESCs, hECCs, and germ cell tumors. Our data provides a surface signature for HUES-7 and NT2/D1 cells and distinguishes normal hESCs from hECCs, helping explain their 'benign' versus 'malignant' nature.

Insulin redirects differentiation from cardiogenic mesoderm and endoderm to neuroectoderm in differentiating human embryonic stem cells

Human embryonic stem cells (hESC) can proliferate indefinitely while retaining the capacity to form derivatives of all three germ layers. We have reported previously that hESC differentiate into cardiomyocytes when cocultured with a visceral endoderm-like cell line (END-2). Insulin/insulin-like growth factors and their intracellular downstream target protein kinase Akt are known to protect many cell types from apoptosis and to promote proliferation, including hESC-derived cardiomyocytes. Here, we show that in the absence of insulin, a threefold increase in the number of beating areas was observed in hESC/END-2 coculture. In agreement, the addition of insulin strongly inhibited cardiac differentiation, as evidenced by a significant reduction in beating areas, as well as in alpha-actinin and beta-myosin heavy chain (beta-MHC)-expressing cells. Real-time reverse transcription-polymerase chain reaction and Western blot analysis showed that insulin inhibited cardiomyogenesis in the early phase of coculture by suppressing the expression of endoderm (Foxa2, GATA-6), mesoderm (brachyury T), and cardiac mesoderm (Nkx2.5, GATA-4). In contrast to previous reports, insulin was not sufficient to maintain hESC in an undifferentiated state, since expression of the pluripotency markers Oct3/4 and nanog declined independently of the presence of insulin during coculture. Instead, insulin promoted the expression of neuroectodermal markers. Since insulin triggered sustained phosphorylation of Akt in hESC, we analyzed the effect of an Akt inhibitor during coculture. Indeed, the inhibition of Akt or insulin-like growth factor-1 receptor reversed the insulin-dependent effects. We conclude that in hESC/END-2 cocultures, insulin does not prevent differentiation but favors the neuroectodermal lineage at the expense of mesendodermal lineages.

Stem cell regulation by lysophospholipids

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) regulate a diverse range of mammalian cell processes, largely through engaging multiple G protein-coupled receptors specific for these lysophospholipids. LPA and S1P have been clearly identified to have widespread physiological and pathophysiological actions, controlling events within the reproductive, gastrointestinal, vascular, nervous and immune systems, and also having a prominent role in cancer. Here we review the recent literature showing the additional emerging role for LPA and S1P in the regulation of stem cells and their progenitors. We discuss the role of these lysophospholipids in regulating the proliferation, survival, differentiation and migration of a range of adult and embryonic stem cells and progenitors, and thus are likely to play a substantial role in the maintenance, generation, mobilisation and homing of stem cell and progenitor populations in the body.

Human embryonic stem cell derivation and nuclear transfer: impact on regenerative therapeutics and drug discovery

This review aims to introduce current and future uses of human embryonic stem cells derived from in vitro-fertilized embryos. These and stem cells derived from parthenogenetic and nuclear transfer embryos could be used for cell therapy, as in vitro cell models for drug discovery/screening, and for studying early human development and pathogenesis of human diseases. However, development of therapeutic and screening applications and products from embryonic stem cells is hampered by several barriers. Therefore, gaps in our current understanding of the basic science of stem cells need to be filled before either application can move forward with confidence.

A chemical approach to stem cell biology

Small molecule libraries have been used successfully to probe several biological systems. Recent work has translated these successes across to the field of stem cell biology. Stem cells hold promise for both modeling of early development as well as having therapeutic potential. Enhanced understanding of the molecular mechanisms that control stem cell fates as well as an improved ability to manipulate cell populations are required. Known mechanistic chemical compounds have been used with stem cells to accomplish these two goals. More recently, through the utilization of high fitness libraries in phenotype-based screens, several small molecules that control self-renewal and differentiation in stem cells have been identified. These small molecules provide useful chemical tools for both basic research and practical applications.