A rapid and efficient method for freezing and recovering clones of embryonic stem cells

Rationally optimized cryopreservation of multiple mouse embryonic stem cell lines: I--Comparative fundamental cryobiology of multiple mouse embryonic stem cell lines and the implications for embryonic stem cell cryopreservation protocols

The post-thaw recovery of mouse embryonic stem cells (mESCs) is often assumed to be adequate with current methods. However as this publication will show, this recovery of viable cells actually varies significantly by genetic background. Therefore there is a need to improve the efficiency and reduce the variability of current mESC cryopreservation methods. To address this need, we employed the principles of fundamental cryobiology to improve the cryopreservation protocol of four mESC lines from different genetic backgrounds (BALB/c, CBA, FVB, and 129R1 mESCs) through a comparative study characterizing the membrane permeability characteristics and membrane integrity osmotic tolerance limits of each cell line. In the companion paper, these values were used to predict optimal cryoprotectants, cooling rates, warming rates, and plunge temperatures, and then these predicted optimal protocols were validated against standard freezing protocols.

Surface-based cryopreservation strategies for human embryonic stem cells: a comparative study

Human embryonic stem cells (hESC) hold tremendous potential in the emerging fields of gene and cell therapy as well as in basic scientific research. One of the major challenges regarding their application is the development of efficient cryopreservation protocols for hESC since current methods present poor recovery rates and/or technical difficulties which impair the development of effective processes that can handle bulk quantities of pluripotent cells. The main focus of this work was to compare different strategies for the cryopreservation of adherent hESC colonies. Slow-rate freezing protocols using intact hESC colonies was evaluated and compared with a surface-based vitrification approach. Entrapment within ultra-high viscous alginate was investigated as the main strategy to avoid the commonly observed loss of viability and colony fragmentation during slow-rate freezing. Our results indicate that entrapment beneath a layer of ultra-high viscous alginate does not provide further protection to hESC cryopreserved through slow-rate freezing, irrespectively of the cryomedium used. Vitrification of adherent hESC colonies on culture dishes yielded significantly higher recovery rates when compared to the slow-rate freezing approaches investigated. The pluripotency of hESC was not changed after a vitrification/thawing cycle and during further propagation in culture. In conclusion, from the cryopreservation methods investigated in this study, surface-based vitrification of hESC has proven to be the most efficient for the cryopreservation of intact hESC colonies, reducing the time required to amplify frozen stocks thus supporting the widespread use of these cells in research and clinical applications.

Improved viability of freeze-thawed embryonic stem cells after exposure to glutathione

Adding a potent antioxidant, glutathione (GSH), to a cryoprotective solution consisting of dimethyl sulfoxide and ethylene glycol and/or postthaw culture medium significantly improved the postthaw viability of mouse embryonic stem cells. This effect, which was caused by a decrease in reactive oxygen species, was only induced by exposure of embryonic stem cells during cryopreservation.

An improved cryopreservation method for a mouse embryonic stem cell line

Embryonic stem (ES) cell lines including the C57BL/6 genetic background are central to projects such as the Knock-Out Mouse Project, North American Conditional Mouse Mutagenesis Program, and European Conditional Mouse Mutagenesis Program, which seek to create thousands of mutant mouse strains using ES cells for the production of human disease models in biomedical research. Crucial to the success of these programs is the ability to efficiently cryopreserve these mutant cell lines for storage and transport. Although the ability to successfully cryopreserve mouse ES cells is often assumed to be adequate, the percent post-thaw recovery of viable cells varies greatly among genetic backgrounds and individual cell lines within a genetic background. Therefore, there is a need to improve the efficiency and reduce the variability of current mouse ES cell cryopreservation methods. To address this need, we employed the principles of fundamental cryobiology to improve the cryopreservation protocol of a C57BL/6 mouse ES cell line by characterizing the membrane permeability characteristics and osmotic tolerance limits. These values were used to predict optimal cooling rates, warming rates, and type of cryoprotectant, which were then verified experimentally. The resulting protocol, generated through this hypothesis-driven approach, resulted in a 2-fold increase in percent post-thaw recovery of membrane-intact ES cells as compared to the standard freezing protocol, as measured by propidium iodide exclusion. Additionally, our fundamental cryobiological approach to improving cryopreservation protocols provides a model system by which additional cryopreservation protocols may be improved in future research for both mouse and human ES cell lines.

Cryopreservation of rhesus macaque embryonic stem cells

Studies using nonhuman primate embryonic stem (ES) cells will facilitate the translation of basic ES cell research to clinical use by providing a large animal model for in vivo testing. Unfortunately, nonhuman primate ES cells do not survive well following cryopreservation, a problem that limits the quantity and quality of these cells for research. More lines have to be established to fulfill demand, and thawed aliquots must go through more passages to generate adequate numbers. In addition, suboptimal cryopreservation can induce epigenetic changes and impose a selection bias for their outgrowth. Therefore, defining the optimal cryopreservation technique for nonhuman primate ES cells is critical for the further development of this research. To address this problem, we tested various cryoprotectants as well as cryopreservation procedures in an attempt to define a protocol that yields high viability with retention of ES cell phenotype and function. Here, we report a freezing protocol that preserves the intercellular attachments that are vital to primate ES cell function. We describe a slow, controlled-rate cooling protocol with ice crystal induction that increased the survival rate of ES cells from <22% to >90%. Preserved cells retained a normal karyotype and did not lose their ability to express markers of undifferentiated ES cells.

Cryopreservation of hematopoietic stem cells

Stem cell transplantation represents a critical approach for the treatment of many malignant and non-malignant diseases. The foundation for these approaches is the ability to cryopreserve marrow cells for future use. This technique is routinely employed in all autologous settings and is critical for cord blood transplantation. A variety of cryopreservatives have been used with multiple freezing and thawing techniques as outlined in the later chapters. Freezing efficiency has been proven repeatedly and the ability of long-term stored marrow to repopulate has been established. Standard approaches outlined here are used in many labs as the field continues to evolve.

Kinetics of cell death of frozen-thawed human embryonic stem cell colonies is reversibly slowed down by exposure to low temperature

A major challenge in the widespread application of hES (human embryonic stem) cells in clinical therapy and basic scientific research is the development of efficient cryopreservation protocols. Conventional slow-cooling protocols utilizing standard cryoprotectant concentrations i.e. 10% (v/v) DMSO, yield extremely low survival rates of less than 5% as reported by previous studies. This study characterized cell death in frozen-thawed hES colonies that were cryopreserved under standard conditions. Surprisingly, our results showed that immediately after post-thaw washing, the overwhelming majority of hES cells were viable (approximately 98%), as assessed by the trypan blue exclusion test. However, when the freshly thawed hES colonies were placed in a 37 degrees C incubator, there was a gradual reduction in cell viability over time. The kinetics of cell death was drastically slowed down by keeping the freshly thawed hES colonies at 4 degrees C, with more than 90% of cells remaining viable after 90 min of incubation at 4 degrees C. This effect was reversible upon re-exposing the cells to physiological temperatures. The vast majority of low temperature-exposed hES colonies gradually underwent cell death upon incubation for a further 90 min at 37 degrees C. Hence, our observations would strongly suggest involvement of a self-induced apoptotic mechanism, as opposed to cellular necrosis arising from cryoinjury.

Cryopreservation of human embryonic stem cell lines

Two different approaches have been adopted for the cryopreservation of human embryonic stem cells (hESCs): vitrification and conventional slow cooling/rapid warming. The vitrification method described here is designed for hESCs that grow as discrete colonies on a feeder cell monolayer, and are subcultured by manual subdivision of the colonies into multicellular clumps. hESCs that are subcultured by enzymatic dissociation can more conveniently be cryopreserved by conventional slow cooling/rapid warming methods. Although both methods are suitable for use in a research context, neither is suitable for cryopreservation of embryonic stem cells destined for clinical diagnostic or therapeutic uses without modification.

Loss of viability during freeze-thaw of intact and adherent human embryonic stem cells with conventional slow-cooling protocols is predominantly due to apoptosis rather than cellular necrosis

A major challenge in the widespread application of human embryonic stem (hES) cells in clinical therapy and basic scientific research is the development of efficient cryopreservation protocols. Conventional slow-cooling protocols utilizing standard cryoprotectant concentrations i.e. 10% (v/v) DMSO, yield extremely low survival rates of <5% as reported by previous studies. This study characterized cell death within frozen-thawed hES colonies that were cryopreserved under standard conditions. Surprisingly, our results showed that immediately after post-thaw washing, the overwhelming majority of hES cells were viable (approximately 98%), as assessed by the trypan blue exclusion test. However, when the freshly-thawed hES colonies were incubated within a 37 degrees C incubator, there was observed to be a gradual reduction in cell viability over time. The kinetics of cell death was drastically slowed-down by keeping the freshly-thawed hES colonies at 4 degrees C, with >90% of cells remaining viable after 90 min of incubation at 4 degrees C. This effect was reversible upon re-exposing the cells to physiological temperature. The vast majority of low temperature-exposed hES colonies gradually underwent cell death upon incubation for a further 90 min at 37 degrees C. Terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end-labeling (TUNEL) assay confirmed apoptosis-induced nuclear DNA fragmentation in frozen-thawed hES cells after incubation at 37 degrees C for 90 min. Expression of active caspase-3 enzyme, which is another prominent marker of apoptosis, was confirmed by immunocytochemical staining, while transmission electron microscopy showed typical ultrastructural features of apoptosis such as chromatin condensation and margination to the nuclear membrane. Hence, our results demonstrated that apoptosis instead of cellular necrosis, is the major mechanism of the loss of viability of cryopreserved hES cells during freeze-thawing with conventional slow-cooling protocols.

A PROPOSED DESIGN FOR THE CRYOPRESERVATION OF INTACT AND ADHERENT HUMAN EMBRYONIC STEM CELL COLONIES

Recently, it was demonstrated that the application of slow-cooling cryopreservation protocols to adherent human embryonic stem (hES) cell colonies, cultured on matrigel or murine embryonic fibroblast feeder layers, resulted in marked improvement in postthaw viability and reduction in cell differentiation. However, the use of commercially available culture plates for this purpose presents several limitations. Most obviously, these plates are not designed for cryopreservation or to withstand the low temperatures encountered during liquid nitrogen cryopreservation, or both. The physical storage of cryopreserved plates is another consideration, in addition to difficulty in maintaining sterile conditions in liquid nitrogen storage and during the thaw phase in a water bath. Hence, a redesign of the cell culture plate for the cryopreservation of adherent hES cell colonies is proposed. In this model, a culture plate made of synthetic materials resistant to storage at -196 degrees C of liquid nitrogen is designed, with readily attachable screw-cap culture wells that function as a replacement for cryovial storage. The detachable wells facilitate storage and after thawing can easily be reattached to a specially designed holding plate. Currently, there are no commercially available cell culture plates using this design concept. The proposed design is envisioned to facilitate the cryopreservation of intact adherent hES cell colonies that could assist the development of automated systems for handling bulk quantities of cells.

Differentiation of Osteoblasts from Murine Embryonic Stem Cells by Overexpression of the Transcriptional Factor Osterix

Osterix is a transcription factor crucial for the normal development of the osteoblast. Here we have investigated whether the osteogenic differentiation of murine embryonic stem (ES) cells can be induced by overexpression of osterix. Differentiation was initiated by formation of embryoid bodies (EB) which were then dispersed and cultured in alpha-minimum essential medium supplemented with L-ascorbate phosphate and alpha-glycerophosphate for up to 21 days. osterix was found to induce expression of several osteoblast-specific markers, as confirmed by immunostaining and real-time RT-PCR. The expression of genes encoding osteocalcin and Cbfa1 was upregulated and the formation of mineralized bone nodules was significantly increased by osterix transfection. In combination with dexamethasone, bone nodule formation was further increased in osterix-transfected cells. Expression of both Sox-9 and PPAR-gamma, genes that are associated with chondrocyte and adipocyte differentiation, was initially increased in the osterix-transfected cells but was downregulated after day 7. This suggests that the process of osterix-induced differentiation of ES cells involves transition through an intermediate bi- or tripotential progenitor cell population. In conclusion, this cell differentiation strategy is useful not only for generating osteoblastic cells from ES cells, but also for investigating factors that influence this process and potentially delineating the ontogeny of the osteoblast.

Paracrine induction of stem cell renewal by LIF-deficient cells: a new ES cell regulatory pathway

The propagation of pluripotential mouse embryonic stem (ES) cells is sustained by leukemia inhibitory factor (LIF) or related cytokines that act through a common receptor complex comprising the LIF receptor subunit (LIF-R) and the signal transducer gp130. However, the findings that embryos lacking LIF-R or gp130 can develop beyond gastrulation argue for the existence of an alternative pathway(s) governing the maintenance of pluripotency in vivo. In order to define those factors that contribute to self-renewal in ES cell cultures, we have generated ES cells in which both copies of the lif gene are deleted. These cells showed a significantly reduced capacity for regeneration of stem cell colonies when induced to differentiate, confirming that LIF is the major endogenous regulatory cytokine in ES cell cultures. However, self-renewal was not abolished and undifferentiated ES cell colonies were still obtained in the complete absence of LIF. A differentiated, LIF-deficient, parietal endoderm-like cell line was derived and shown to support ES cell propagation via production of a soluble, macromolecular, trypsin-sensitive activity. This activity, which we name ES cell renewal factor (ESRF), is distinct from members of the IL-6/LIF family because (i) it is effective on ES cells lacking LIF-R; (ii) it is not blocked by anti-gp130 neutralizing antibodies; and (iii) it acts without activation of STAT3. ES cells propagated clonally using ESRF alone can contribute fully to chimaeras and engender germline transmission. These findings establish that ES cell pluripotency can be sustained via a LIF-R/gp130-independent, STAT-3 independent, signaling pathway. Operation of this pathway in vivo could play an important role in the regulation of pluripotency in the epiblast and account for the viability of lifr -/- and gp130 -/- embryos.

Erythroid 5-aminolevulinate synthase is required for erythroid differentiation in mouse embryonic stem cells

We have examined the induction of the enzymes of the heme biosynthetic pathway during erythroid differentiation of mouse embryonic stem (ES) cells. Following transfer to appropriate medium all of the pathway enzymes are induced within three days. Unlike differentiating mouse erythroleukemia cells (Lake-Bullock, H. and Dailey, H.A. Mol Cell Biol 13:7122-7132, 1993), all of the enzymes appear to be induced simultaneously and not sequentially in differentiating ES cells. The role of erythroid 5-aminolevulinate synthase (ALAS-2) in this differentiation process was examined by disruption of the ALAS-2 gene. The targeting vector used for disruption replaced all of exons 4 to 6 with a selectable neomycin resistance gene. The resulting genetically modified (ALAS-2 knockout) cells, as well as normal ES cells were used to study induction of heme biosynthesis. Following 10 days of culture in methylcellulose media significant morphological differences between the embryoid bodies (EBs) of the two cell lines were observed. ES cells exhibited morphology of typical EBs with a dark field (blood island) in the center, while ALAS-2 knockout ES cells developed very poorly both in size and shape. At 8 days of differentiation, only 3% of all EBs contained visible erythropoietic cells (i.e., stained positively for hemoglobin) in the ALAS-2 knockout cell line, compared with 50% in ES cells. Most of the genes in the heme synthetic pathway were expressed to a stable level within 3 to 6 days after induction in normal ES cells, while the ALAS-2 knockout cell line failed to significantly increase the level of expression of these genes. Fetal beta-globin mRNA was not detectable in the differentiating ALAS-2 knockout cells, whereas mRNA for this gene was detected in normal ES cells within 3 days of differentiation. These results suggest that ALAS-2 is necessary for ES cell erythroid differentiation and that there is an interrelationship between heme and globin synthesis in differentiating ES cells.

YAC transgenesis: a study of conditions to protect YAC DNA from breakage and a protocol for transfection

Yeast artificial chromosomes (YACs) are providing a great boon to transgene technology by allowing the easy mutagenesis and study of very large DNAs. The large insert sizes of these vectors permit more accurate analysis of the regulation of transgene expression than smaller, more artificially assembled constructs. Transfection of mammalian cells by YACs can be accomplished by a number of methods; the most prevalent, using gel-purified DNA, is dependent upon compaction by salts to protect the large YAC DNA from breakage. We show that the common reliance on NaCl to compact YAC DNA sufficiently to protect it from breakage is not well-founded. Even the use of mixtures of polyamines and NaCl allows substantial damage to purified YACs. The use of polyamines alone in low salt buffers to compact YAC DNA provides the best protection from breakage and allows very effective transfection of murine embryonic stem (ES) cells. We provide a detailed method for ES cell transfection by YACs utilizing the DOTAP lipofection reagent that optimizes transfection efficiency and recovery of intact YACs.

The methyl-CpG binding protein MeCP2 is essential for embryonic development in the mouse

Vertebrate genomes are heavily methylated at cytosines in the sequence CpG. The biological role of this modification is probably mediated by DNA binding proteins that are either attracted to or repelled by methyl-CpG. MeCP2 is an abundant chromosomal protein that binds specifically to methylated DNA in vitro, and depends upon methyl-CpG for its chromosomal distribution in vivo. To assess the functional significance of MeCP2, the X-linked gene was mutated in male mouse embryonic stem (ES) cells using a promoterless gene-targeting construct containing a lacZ reporter gene. Mutant ES cells lacking MeCP2 grew with the same vigour as the parental line and were capable of considerable differentiation. Chimaeric embryos derived from several independent mutant lines, however, exhibited developmental defects whose severity was positively correlated with the contribution of mutant cells. The results demonstrate that MeCP2, like DNA methyltransferase, is dispensable in stem cells, but essential for embryonic development.

Dicistronic targeting constructs: reporters and modifiers of mammalian gene expression

To investigate the activity of candidate regulatory molecules in mammalian embryogenesis, we have developed a general strategy for modifying and reporting resident chromosomal gene expression. The picornaviral internal ribosome-entry site was incorporated into gene targeting constructs to provide cap-independent translation of a selectable marker from fusion transcripts generated following homologous recombination. These promoterless constructs were highly efficient and have been used both to inactivate the stem-cell-specific transcription factor Oct-4 and to introduce a quantitative regulatory modification into the gene for a stem-cell maintenance factor, differentiation-inhibiting activity. In addition, the inclusion of a beta-galactosidase reporter gene in the constructs enabled accurate and sensitive detection of cellular sites of transcription. This has allowed visualization of putative "stem-cell niches" in which sources of elevated expression of differentiation-inhibiting activity were localized to the differentiated cells surrounding colonies of stem cells.