Jim McWhir

In vitro osteogenic differentiation of human ES cells.

Since their isolation in 1998, human embryonic stem (hES) cells have been shown to be capable of adopting various cell fates in vitro. Here, we present in vitro data demonstrating the directed commitment of human embryonic stem cells to the osteogenic lineage. Human ES cells are shown to respond to factors that promote osteogenesis, leading to activation of the osteogenic markers osteocalcin, parathyroid hormone receptor, bone sialoprotein, osteopontin, cbfa1, and collagen 1. Moreover, the mineralized nodules obtained are composed of hydroxyapatite, further establishing the similarity of osteoblasts in culture to bone. These results show that osteoblasts can be derived from human ES cultures in vitro and provide the basis for comparison of adult and embryonic-derived osteogenesis, and for an investigation of potential applications for hES cells in orthopaedic tissue repair.

Requirement for the heart-type fatty acid binding protein in cardiac fatty acid utilization

Nonenzymatic cytosolic fatty acid binding proteins (FABPs) are abundantly expressed in many animal tissues with high rates of fatty acid metabolism. No physiological role has been demonstrated for any FABP, although these proteins have been implicated in transport of free long‐chain fatty acids (LCFAs) and protection against LCFA toxicity. We report here that mice lacking heart‐type FABP (H‐FABP) exhibit a severe defect of peripheral (non‐hepatic, non‐fat) LCFA utilization. In these mice, the heart is unable to efficiently take up plasma LCFAs, which are normally its main fuel, and switches to glucose usage. Altered plasma levels of LCFAs, glucose, lactate and β‐hydroxybutyrate are consistent with depressed peripheral LCFA utilization, intensified carbohydrate usage, and increased hepatic LCFA oxidation; these changes are most pronounced under conditions favoring LCFA oxidation. H‐FABP deficiency is only incompletely compensated, however, causing acute exercise intolerance and, at old age, a localized cardiac hypertrophy. These data establish a requirement for H‐FABP in cardiac intracellular lipid transport and fuel selection and a major role in metabolic homeostasis. This new animal model should be particularly useful for investigating the significance of peripheral LCFA utilization for heart function, insulin sensitivity, and blood pressure.—Binas, B., Danneberg, H., McWhir, J., Mullins, L., Clark, A. J. Requirement for the heart‐type fatty acid binding protein in cardiac fatty acid utilization. FASEB J. 13, 805–812 (1999)

Immortalized Fibroblast‐Like Cells Derived from Human Embryonic Stem Cells Support Undifferentiated Cell Growth

Human embryonic stem cells (hESCs) have the potential to generate multiple cell types and hold promise for future therapeutic applications. Although undifferentiated hESCs can proliferate indefinitely, hESC derivatives significantly downregulate telomerase and have limited replication potential. In this study we examine whether the replicative lifespan of hESC derivatives can be extended by ectopic expression of human telomerase reverse transcriptase (hTERT), the catalytic component of the telomerase complex. To this end, we have derived HEF1 cells, a fibroblast‐like cell type, differentiated from hESCs. Infection of HEF1 cells with a retrovirus expressing hTERT extends their replicative capacity, resulting in immortal human HEF1‐hTERT cells. HEF1‐hTERT cells can be used to produce conditioned medium (CM) capable of supporting hESC growth under feeder‐free conditions. Cultures maintained in HEF1‐CM show characteristics similar to mouse embryonic fibroblast CM control cultures, including morphology, surface marker and transcription factor expression, telomerase activity, differentiation, and karyotypic stability. In addition, HEF1‐hTERT cells have the capacity to differentiate into cells of the osteogenic lineage. These results suggest that immortalized cell lines can be generated from hESCs and that cells derived from hESCs can be used to support their own growth, creating a genotypically homogeneous system for the culture of hESCs.

Infection efficiency of human and mouse embryonic stem cells using adenoviral and adeno-associated viral vectors.

Human and mouse embryonic stem (ES) cells have the capacity to differentiate into derivatives of all three germ layers, suggesting novel therapies for degenerative, metabolic, and traumatic disorders. ES-based regenerative medicine will be further advanced by the development of reliable methods for transgene introduction and expression. Here, we show infection of human and mouse embryonic stem (ES) cells with two of the most popular vectors in gene transfer, adenovirus type 5 (Ad5) and adeno-associated virus (AAV; serotypes 2, 4, and 5). All vectors express the nuclear-localized marker gene beta-galactosidase expressed from the Rous Sarcoma Virus long terminal repeat (RSV-LTR). Both Ad5 and AAV2 infected human and mouse ES cells and gave rise to beta-galactosidase expression. AAV4 and 5 did not yield detectable levels of beta-galactosidase expression. Quantitative PCR analysis of virally infected human and mouse ES cells revealed that only Ad5 and AAV2 are capable of transducing both cell-types. No viral DNA was detected in cells infected with either AAV4 or AAV5. Infection and subsequent differentiation of mouse and human ES cells with Ad5 showed that beta-galactosidase-expressing cells were restricted to cells in the interior of the embryoid body mass. No beta-galactosidase expression was observed in AAV-infected cells following differentiation. There was no difference in morphology or differentiation patterns between infected and noninfected differentiating mouse and human ES cells. Differentiation of hES cells prior to infection led to transduction of neuronally differentiated cells with good efficiency using all vectors. These data show that Ad5- and AAV2-based vectors are capable of infecting both human and mouse ES cells, in both their undifferentiated and differentiated states, whereas AAV4 and AAV5 can infect human and mouse ES cells only following differentiation.

Insertion of a foreign gene into the β-casein locus by Cre-mediated site-specific recombination

The expression of foreign genes in transgenic animals is generally unpredictable as transgenes are integrated at random after pro-nuclear injection into fertilized oocytes. In many cases, transgene expression is inhibited by neighbouring chromatin structures or by the repeated nature of the multiple transgene copies present at the integration site. A strategy involving homologous and site-specific recombination has been devised by which single copies of a foreign gene can be inserted specifically into the locus of a highly expressed gene. As a first step, a loxP recombination target site is introduced by homologous recombination into a predetermined gene locus such that the loxP sequence is placed next to the promoter region and replaces the translational initiation signal. In a subsequent site-specific recombination reaction, a gene of interest can be integrated into the pre-existing loxP site. This biphasic recombination strategy was used to integrate a luciferase reporter gene into the locus of the murine beta-casein gene in embryonic stem cells.

Effect of Cell Confluence on Production of Cloned Mice Using an Inbred Embryonic Stem Cell Line

Abstract Mice have been successfully cloned from both somatic cells and hybrid embryonic stem (ES) cells. Heterozygosity of the donor ES cell genome has been suggested as a crucial factor for long-term survival of cloned mice. In the present study, an inbred ES cell line, HM-1 (129/Ola), and a well-tested ES cell line, R1 (129/Sv × 129/Sv-CP), were used as donor cells to evaluate the developmental potential of nuclear transfer embryos. We found that ES cell confluence dramatically affects the developmental potential of reconstructed embryos. With the ES cell line HM-1 and 80–90% confluence, 49% of reconstructed embryos developed to the morula/blastocyst stage, 9% of these embryos developed to live pups when transferred to the surrogate mothers, and 5 of 18 live pups survived to adulthood. By contrast, at 60–70% confluence, only 22% of embryos developed to the morula/blastocyst stage, and after transfer, only a single fetus reached term. Consistent with previous reports, the nuclei of R1 ES cells were also shown to direct development to term, but no live pups were derived from cells at later passages (>20). Our results show that the developmental potential of reconstructed embryos is determined by both cell confluence and cell passage. These results also demonstrate that the inbred ES cell line, HM-1, can be used to produce viable cloned mice, although less efficiently than most heterozygous ES cell lines.

DNA microsatellite analysis of Dolly.

Dolly, the first animal cloned from an adult mammal, was produced by somatic cell nuclear transfer from a cell population derived from mammary tissue taken from a 6-year-old Finn Dorset ewe. Analysis of DNA from Dolly showed that she contained the same seven microsatellite alleles as those present in the cell population from which she was derived. Here we report a more detailed microsatellite analysis, which confirms the origin of Dolly.

Sequential Genetic Modification of the hprt Locus in Human ESCs Combining Gene Targeting and Recombinase-Mediated Cassette Exchange

Genetic modification of human embryonic stem cells (hESCs) will be an essential tool to allow full exploitation of these cells in regenerative medicine and in the study of hESC biology. Here we report multiple sequential modifications of an endogenous gene (hprt) in hESCs. A selectable marker flanked by heterospecific lox sites was first introduced by homologous recombination (HR) into the hprt gene. In a subsequent step, exchange of the selectable marker with another cassette was achieved by recombinase-mediated cassette exchange (RMCE). We show that 100% of the recovered clones were the result of RMCE using a promoter trap strategy at the hprt locus. hprt-targeted H1 cells maintained a diploid karyotype and expressed hESC surface markers before and after RMCE. Finally, we report a double replacement strategy using two sequential gene targeting steps resulting in the targeted correction of an hprt-mutated hESC line.

Human Embryonic Stem Cells Passaged Using Enzymatic Methods Retain a Normal Karyotype and Express CD30

Human embryonic stem cells (hESCs) are thought to be susceptible to chromosomal rearrangements as a consequence of single cell dissociation. Compared in this study are two methods of dissociation that do not generate single cell suspensions (collagenase and EDTA) with an enzymatic procedure using trypsin combined with the calcium-specific chelator EGTA (TEG), that does generate a single cell suspension, over 10 passages. Cells passaged by single cell dissociation using TEG retained a normal karyotype. However, cells passaged using EDTA, without trypsin, acquired an isochromosome p7 in three replicates of one experiment. In all of the TEG, collagenase and EDTA-treated cultures, cells retained consistent telomere length and potentiality, demonstrating that single cell dissociation can be used to maintain karyotypically and phenotypically normal hESCs. However, competitive genomic hybridization revealed that subkaryotypic deletions and amplifications could accumulate over time, reinforcing that present culture regimes remain suboptimal. In all cultures the cell surface marker CD30, reportedly expressed on embryonal carcinoma but not karyoptically normal ESCs, was expressed on hESCs with both normal and abnormal karyotype, but was upregulated on the latter.

Enhanced gene targeting frequency in ES cells with low genomic methylation levels

Increased methylation in promoter/enhancer regions typically results in transcriptional downregulation. The direct correlation between gene expression and homologous recombination (HR) is also widely acknowledged, and suggests that actively transcribed, hypomethylated targets may be more accessible to the HR machinery. Consistent with this hypothesis, we report that DNA methyltransferase 1 (Dnmt1)-knockout ES cells show a 2-fold increase in gene targeting frequency. However, the use of hypomethylated targeting vectors or the ectopic expression of a putative DNA demethylase did not enhance targeting frequency. These observations are discussed in the context of devising more efficient targeting protocols by transiently modifying genomic methylation levels.