Unequal segregation of parental chromosomes in embryonic stem cell hybrids

Electrophysiological Properties of Tetraploid Cardiomyocytes Derived from Murine Pluripotent Stem Cells Generated by Fusion of Adult Somatic Cells with Embryonic Stem Cells

Most cardiomyocytes (CMs) in the adult mammalian heart are either binucleated or contain a single polyploid nucleus. Recent studies have shown that polyploidy in CMs plays an important role as an adaptive response to physiological demands and environmental stress and correlates with poor cardiac regenerative ability after injury. However, knowledge about the functional properties of polyploid CMs is limited. In this study, we generated tetraploid pluripotent stem cells (PSCs) by fusion of murine embryonic stem cells (ESCs) and somatic cells isolated from bone marrow or spleen and performed a comparative analysis of the electrophysiological properties of tetraploid fusion-derived PSCs and diploid ESC-derived CMs. Fusion-derived PSCs exhibited characteristics of genuine ESCs and contained a near-tetraploid genome. Ploidy features and marker expression were also retained during the differentiation of fusion-derived cells. Fusion-derived PSCs gave rise to CMs, which were similar to their diploid ESC counterparts in terms of their expression of typical cardiospecific markers, sarcomeric organization, action potential parameters, response to pharmacologic stimulation with various drugs, and expression of functional ion channels. These results suggest that the state of ploidy does not significantly affect the structural and electrophysiological properties of murine PSC-derived CMs. These results extend our knowledge of the functional properties of polyploid CMs and contribute to a better understanding of their biological role in the adult heart.

Establishment of the Primary Avian Gonadal Somatic Cell Lines for Cytogenetic Studies

Simple Summary

We developed a simple method for primary somatic cell culture establishment from the ovaries of the great tits and testes of ten Passerine species. The ovary-derived cell cultures were cultivated until the tenth passage without any noticeable decrease in their proliferative activity, while testis-derived cell cultures demonstrated a decreased proliferation potential. However, sufficient material was available from both cell cultures originating from the ovary and testis to make excellent mitotic metaphase chromosomal preparations. We demonstrated the high efficiency of electroporation for genetic modification of the ovary-derived cell line. Thus, the established ovary-derived cell line could be efficiently used in cytogenetic and genomic studies.

Abstract

The last decade was marked by a steep rise in avian studies at genomic and cellular levels. Cell lines are important tools for in vitro studies in cell biology and cytogenetics. We developed a simple method of primary somatic cell culture establishment from the ovaries of the great tits (Parus major) and testes of ten Passerine species, characterized the cellular composition of the ovary-derived lines using RT-PCR and immunolocalization of the tissue-specific markers and tested the efficiency of two methods of genetic transformation of the ovary-derived cell line. We found that the ovary-derived cell cultures of the great tit were composed of fibroblasts mainly, but also contained interstitial and granulosa cells. They were cultivated until the 10th passage without any noticeable decrease in their proliferative activity. The testis-derived cell cultures had lower proliferative potential. However, both ovary- and testis-derived cell cultures provided enough material for high quality mitotic metaphase chromosome preparations. The efficiency of its transduction with lentivirus containing a GFP reporter was very low, while electroporation with episomal vectors expressing GFP resulted in a high yield of GFP-positive cells. The proposed method could be used for the generation of high quality material for various cytogenetic and genomic studies.

Alternative dominance of the parental genomes in hybrid cells generated through the fusion of mouse embryonic stem cells with fibroblasts

For the first time, two types of hybrid cells with embryonic stem (ES) cell-like and fibroblast-like phenotypes were produced through the fusion of mouse ES cells with fibroblasts. Transcriptome analysis of 2,848 genes differentially expressed in the parental cells demonstrated that 34-43% of these genes are expressed in hybrid cells, consistent with their phenotypes; 25-29% of these genes display intermediate levels of expression, and 12-16% of these genes maintained expression at the parental cell level, inconsistent with the phenotype of the hybrid cell. Approximately 20% of the analyzed genes displayed unexpected expression patterns that differ from both parents. An unusual phenomenon was observed, namely, the illegitimate activation of Xist expression and the inactivation of one of two X-chromosomes in the near-tetraploid fibroblast-like hybrid cells, whereas both Xs were active before and after in vitro differentiation of the ES cell-like hybrid cells. These results and previous data obtained on heterokaryons suggest that the appearance of hybrid cells with a fibroblast-like phenotype reflects the reprogramming, rather than the induced differentiation, of the ES cell genome under the influence of a somatic partner.

Generation of mouse chimeras with high contribution of tetraploid embryonic stem cells and embryonic stem cell-fibroblast hybrid cells

The in vitro long-term cultivation of embryonic stem (ES) cells derived from pre-implantation embryos offers the unique possibility of combining ES cells with pre-implantation embryos to generate chimeras, thus facilitating the creation of a bridge between in vitro and in vivo investigations. Genomic manipulation using ES cells and homologous recombination is one of the most outstanding scientific achievements, resulting in the generation of animals with desirable genome modifications. As such, the generation of ES cells with different ploidy via cell fusion also deserves much attention because this approach allows for the production of chimeras that contain somatic cells with various ploidy. Therefore, this is a powerful tool that can be used to study the role of polyploidy in the normal development of mammals.

Reprogramming mediated by cell fusion technology

This review is focused on recent advances in fusion-based reprogramming of cells of different pluripotent statuses or lineage origins. Recent findings are discussed from standpoints of both the developmental potency of hybrid cells generated by fusion of pluripotent embryonic stem (ES) cells, embryonal carcinoma (EC) cells, and somatic cells and epigenetic mechanisms and other aspects involved in the reprogramming process. Complete reprogramming occurs at least 5-7 days after fusion and includes at least two steps. (i) initiation at the heterokaryon stage and choice of the direction of reprogramming using an "all-or-none principle" to establish the dominance of one parental genome and (ii) "fixation" of the newly acquired expression profile by epigenetic mechanisms. The first step is realized without cell division, whereas the second requires cell proliferation. Reprogramming in hybrid cells is rapid and complete. Thus, cell fusion is a powerful tool for reprogramming.

DNA-unstable decaploid mouse H1 (ES) cells established from DNA-stable pentaploid H1 (ES) cells polyploidized using demecolcine

OBJECTIVES

DNA content of diploid H1 (ES) cells (2H1 cells) has been shown to be stable in long-term culture; however, tetraploid and octaploid H1 (ES) cells (4H1 and 8H1 cells, respectively) were DNA-unstable. Pentaploid H1 (ES) cells (5H1 cells) established recently have been found to be DNA-stable; how, then is cell DNA stability determined? To discuss ploidy stability, decaploid H1 (ES) cells (10H1 cells) were established from 5H1 cells and examined for DNA stability.

MATERIALS AND METHODS

5H1 cells were polyploidized using demecolcine (DC) and 10H1 cells were obtained by one-cell cloning.

RESULTS

Number of chromosomes of 10H1 cells was 180 and durations of their G(1), S, and G(2)/M phases were 3, 7 and 6 h respectively. Volume of 10H1 cells was double that of 5H1 cells and morphology of 10H1 cells was flagstone-like in shape. 10H1 cells exhibited alkaline phosphatase activity and their DNA content decayed in 91 days of culture. 10H1 cells injected into mouse abdomen formed solid tumours that contained several kinds of differentiated cells with lower DNA content, suggesting that 10H1 cells were pluripotent and DNA-unstable. Loss of DNA stability was explained using a hypothesis concerning DNA structure of polyploid cells as DNA reconstructed through ploidy doubling was arranged in mirror symmetry in a new configuration.

CONCLUSION

In the pentaploid-decaploid transition of H1 cells, cell cycle parameters and pluripotency were retained, but morphology and DNA stability were altered.

Reprogramming factors involved in hybrids and cybrids of human embryonic stem cells fused with hepatocytes

Embryonic stem cells (ESCs) have the potential to reprogram somatic cells into ESC-like cells through cell fusion. In the present study, the potential of human (h)ESC cytoplasts and karyoplasts to reprogram human hepatocytes was evaluated. Green fluorescent protein (GFP) transfected hESCs (ENVY cells) were fused with SNARF-1 (CellTracker)-labeled human hepatocytes using polyethylene glycol (PEG) and fluorescence-activated cell sorting (FACS) to produce hESC-hepatocyte hybrids. Immunocytochemical analysis of ESC markers showed that the hybrids expressed OCT4, TRA-1-60, TRA-1-81, SSEA-4, and GCTM-2. However, SSEA-1, which is typically low or absent on hESCs, was detected on hESC–hepatocyte hybrids. Moreover, reverse transcriptase polymerase chain reaction (RT-PCR) showed that alpha-fetoprotein, which is highly expressed in hepatocytes, was erased in the hybrids. These results indicated that hESCs have the potential to reprogram hepatocyte phenotype to a relatively undifferentiated state, but such hybrid cells are not identical to hESCs. Although hESC–hepatocyte hybrids were aneuploid, they were able to differentiate into embryoid bodies and some types of somatic cells. Furthermore, cybrids of enucleated hESCs and hepatocytes were produced by cell fusion, but the cybrids were unable to self-renew in the same way as hESCs. Presumably, the reprogramming factors are associated with the karyoplast and not the cytoplast of hESCs.

Hexaploid H1 (ES) cells established from octaploid H1 cells are as DNA stable as pentaploid H1 cells

Hexaploid H1 (ES) cells (6H1 cells) were established from octaploid H1 cells (8H1 cells), as were pentaploid H1 cells (5H1 cells). 6H1 cells were compared with 5H1 cells. The number of chromosomes of 6H1 cells was 115, 20 more than the 95 of 5H1 cells. The durations of G(1), S, and G(2)/M phases of 6H1 cells were 3, 7, and 6 h, respectively, almost the same as those of 5H1 cells. The cell volume of 6H1 cells was equivalent that of 5H1 cells. The morphology of 6H1 cells was flattened circular cluster, different from the spherical cluster of 5H1 cells. 6H1 cells exhibited alkaline phosphatase activity as well as 5H1 cells. The DNA content of 6H1 cells was stable and maintained for 300 days of culturing, the same as that of 5H1 cells. The DNA stability of 6H1 cells was explained using a hypothesis concerning the DNA structure of polyploid cells because the asymmetric configuration of homologous chromosomes in 6H1 cells inhibited chromosome loss.

Bidirectional reprogramming of mouse embryonic stem cell/fibroblast hybrid cells is initiated at the heterokaryon stage

Immunofluorescent analysis of markers specific for parental genomes was used to study heterokaryons and hybrid cells soon after the fusion of diploid embryonic stem (ES) cells marked with green fluorescent protein and diploid fibroblasts labeled by blue fluorescent beads. Heterokaryons were identified by an analysis of parental mitochondrial DNAs. Within 20 h after fusion, most heterokaryons (up to 80%) had a fibroblast-like phenotype, being positive for typical fibroblast markers (collagen type I, fibronectin, lamin A/C) and for the modification me3H3K27 chromatin marking the inactive X chromosome but being negative for Oct4 and Nanog. Approximately 20% of heterokaryons had an alternative ES-like phenotype being positive for Oct4 and Nanog, with signs of reactivation of the previously inactive X-chromosome but negative for fibroblast markers. Hybrid cells having alternative phenotypes were easily identified from 24-48 h. The level of DNA methylation at the promoter of the fibroblast Oct4 allele in the ES-like hybrid cells at day 4 was similar to that of ES cells but at the same time, both parental Oct4 alleles were heavily methylated in fibroblast-like hybrid cells. Thus, bidirectional reprogramming initiated at the heterokaryon stage seems to lead to the formation of two types of hybrid cells with alternative dominance of the parental genomes. However, the further fates of two types of hybrid cells are different: ES-like hybrid cells form colonies at 4-6 days but no colonies are derived from the fibroblast-like hybrid cells. The latter grow as disconnected single cells and are unable to form colonies, like mouse embryonic fibroblasts.

DNA stable pentaploid H1 (ES) cells obtained from an octaploid cell induced from tetraploid cells polyploidized using demecolcine

Pentaploid H1 (ES) cells (5H1 cells) were accidentally obtained through one-cell cloning of octaploid H1 (ES) cells (8H1 cells) that were established from tetraploid H1 (ES) cells (4H1 cells) polyploidized using demecolcine. The number of chromosomes of 5H1 cells was 100, unlike the 40 of diploid H1 (ES) cells (2H1 cells), 80 of 4H1, and 160 of 8H1 cells. The durations of G(1), S, and G(2)/M phases of 5H1 cells were 3, 7, and 6 h, respectively, almost the same as those of 2H1, 4H1, and 8H1 cells. The cell volume of 5H1 cells was half of that of 8H1 cells, suggesting that 5H1 cells were created through abnormal cell divisions of 8H1 cells. The morphology of growing 5H1 cells was a spherical cluster similar to that of 2H1 cells and differing from the flagstone-like shape of 4H1 and 8H1 cells. Pentaploid solid tumors were formed from 5H1 cells after interperitoneal injection into the mouse abdomen, and they contained endodermal, mesodermal, and ectodermal cells as well as undifferentiated cells, suggesting both that the DNA content of 5H1 cells was retained during tumor formation and that the 5H1 cells were pluripotent. The DNA content of 5H1 cells was stable in long-term culturing as 2H1 cells, meaning that 5H1 and 2H1 cells shared similarities in DNA structure. The excellent stability of the DNA content of 5H1 cells was explained using a hypothesis for the DNA structure of polyploid cells because the pairing of homologous chromosomes in 5H1 cells is spatially forbidden.

Chromosomal and telomeric reprogramming following ES-somatic cell fusion

Chromosomal and telomeric reprogramming was assessed in intraspecies hybrids obtained by fusion of embryonic stem (ES) cells and mouse embryonic fibroblasts. Evaluation of the ploidy of ES-somatic hybrids revealed that 21 of 59 clones had a tetraploid DNA profile while the remaining clones showed deviations from the expected profile of fusion between two diploid cells. Microsatellite polymerase chain reaction analysis of four of these clones demonstrated no random loss of somatic chromosome pairs in the ES-somatic cell hybrids. Pluripotential of ES-somatic hybrids was assessed by gene expression analysis, antibody staining for Oct4 and SSEA-1 and teratoma formation containing derivatives of the three germ layers. Reprogramming of telomeric maintenance was observed with ES-somatic hybrids showing high telomerase activity and increased telomere lengths. However, we detected no significant increase in the expression of the three critical telomerase subunits: telomerase reverse transcriptase (TERT), telomerase RNA component (TERC), and dyskerin. This indicates that activation of telomerase and telomere maintenance is not reliant on changes in gene expression of TERT, TERC, and dyskerin following ES-somatic cell fusion or sister chromatid recombination and may arise through elimination of negative regulation of telomerase activity. This is the first demonstration of telomere lengthening following cell fusion and offers a new model for studying and identifying new regulators of telomere maintenance.

Allelic expression and DNA methylation profiles of promoters at the parental Oct4 and Nanog genes in Mus musculus ES cell/Mus caroli splenocyte hybrid cells

Expression of the parental Oct4 and Nanog alleles and DNA methylation of their promoters were studied in a set of Mus musculus embryonic stem (ES) cell/M. caroli splenocyte hybrid cells containing a variable ratio of parental chromosomes 6 and 17. The transcripts of the reactivated splenocyte Oct4 and Nanog genes were revealed in all hybrid cell clones positive for M. caroli chromosomes 6 and 17. We found that 11 CpG sites in the Oct4 promoter were heavily methylated in M. caroli splenocytes (>80%), whereas M. musculus ES cells were essentially unmethylated (<1%). Analysis of the methylation status of the Oct4 promoter in seven hybrid cell clones showed that the splenocyte-derived promoter sequence lost DNA methylation so that its methylation level was comparable with that of the ES cells. Additionally, no preferential de novo methylation was seen in the Oct4 promoters of M. musculus and M. caroli in teratomas developed from two independent hybrid clones. The upstream region of Nanog was heavily methylated in mouse embryonic fibroblasts (66%) and less methylated in M. caroli splenocytes (24%). The Nanog promoter region was completely unmethylated in M. musculus ES cells. We found that both parental alleles of the Nanog gene promoter were essentially unmethylated in five examined hybrid clones. Thus, we have demonstrated that (1) the Oct4 and Nanog genes of splenocytes are activated, and their promoters undergo demethylation in ES cell hybrids; (2) these events are independent of the number and ratio of parental chromosomes carrying these genes.

Embryonic stem cell/fibroblast hybrid cells with near-tetraploid karyotype provide high yield of chimeras

Ten primary clones of hybrid cells were produced by the fusion of diploid embryonic stem (ES) cells, viz., line E14Tg2aSc4TP6.3 marked by green fluorescent protein (GFP), with diploid embryonic or adult fibroblasts derived from DD/c mice. All the hybrid clones had many characteristics similar to those of ES cells and were positive for GFP. Five hybrid clones having ploidy close to tetraploidy (over 80% of cells had 76-80 chromosomes) were chosen for the generation of chimeras via injection into C57BL blastocysts. These hybrid clones also contained microsatellites marking all ES cell and fibroblast chromosomes judging from microsatellite analysis. Twenty chimeric embryos at 11-13 days post-conception were obtained after injection of hybrid cells derived from two of three clones. Many embryos showed a high content of GFP-positive descendents of the tested hybrid cells. Twenty one adult chimeras were generated by the injection of hybrid cells derived from three clones. The contribution of GFP-labeled hybrid cells was significant and comparable with that of diploid E14Tg2aSc4TP6.3 cells. Cytogenetic and microsatellite analyses of cell cultures derived from chimeric embryos or adults indicated that the initial karyotype of the tested hybrid cells remained stable during the development of the chimeras, i.e., the hybrid cells were mainly responsible for the generation of the chimeras. Thus, ES cell/fibroblast hybrid cells with near-tetraploid karyotype are able to generate chimeras at a high rate, and many adult chimeras contain a high percentage of descendants of the hybrid cells.

Alteration and preservation of cellular characteristics in long-term culture of tetraploid H-1 (ES) cells

To examine the alteration in cellular characteristics of polyploid ES cells during long-term culturing, tetraploid H-1 (ES) cells were continuously cultured for 180 days. Cellular DNA content of the tetraploid cells decreased and reached a plateau of 3.3 C, where C represents the complement of haploid chromosomes. The chromosome number also decreased, indicating that the DNA loss was induced by chromosome loss. Cell volume was maintained, suggesting that the DNA loss did not involve cytoplasmic loss. The cell cycle parameters were almost the same during the DNA decay process, indicating that cell cycle progression was independent of the quantity of homologous chromosomes. Hypotetraploid cells showed alkaline phosphatase activity and formed teratocarcinomas in mouse abdomens, suggesting that the pluripotent potential was maintained. Cellular morphology was also retained, suggesting that the gene expression specifying morphological characteristics was conserved. We conclude that these initial cellular characteristics of tetraploid H1 (ES) cells were preserved in long-term culture, irrespective of chromosome loss.

Stem cells and nuclear reprogramming

Derivation of human embryonic stem (ES) cells from preimplantation embryos ten years ago raised great hopes that they may be an excellent source of cells for cell replacement therapy. However, serious ethical concerns and the risk of immune rejection of allotransplanted cells have hindered the translation of ES cell-based therapies into the clinic. In an attempt to circumvent these barriers, a number of methods have been developed for converting adult somatic cells into a pluripotent state from which ethically acceptable patient-specific mature cells of interest could be derived. These efforts, backed by advances in elucidating the molecular basis of pluripotency, have culminated in successful reprogramming of fibroblasts into ES cell-like cells, termed induced pluripotent stem (iPS) cells, by ectopic expression of only a handful of "stemness" factors. iPS cells possess morphological, molecular and developmental features of conventional blastocyst-derived ES cells and have the potential to serve as a source of therapeutic cells for customized tissue repair, gene therapy, drug discovery, toxicological testing and for studying the molecular basis of human disease. The goal of this review is to provide the current state-of-the-art in this very exciting and dynamic field and to discuss barriers that remain to be removed before the therapeutic potential of iPS cells can be fully realized.

X inactivation counting and choice is a stochastic process: evidence for involvement of an X-linked activator

Female mammalian cells achieve dosage compensation of X-encoded genes by X chromosome inactivation (XCI). This process is thought to involve X chromosome counting and choice. To explore how this process is initiated, we analyzed XCI in tetraploid XXXX, XXXY, and XXYY embryonic stem cells and found that every X chromosome within a single nucleus has an independent probability to initiate XCI. This finding suggests a stochastic mechanism directing XCI counting and choice. The probability is directly proportional to the X chromosome:ploidy ratio, indicating the presence of an X-encoded activator of XCI, that itself is inactivated by the XCI process. Deletion of a region including Xist, Tsix, and Xite still results in XCI on the remaining wild-type X chromosome in female cells. This result supports a stochastic model in which each X chromosome in a nucleus initiates XCI independently and positions an X-encoded trans-acting XCI-activator outside the deleted region.

Dominant manifestation of pluripotency in embryonic stem cell hybrids with various numbers of somatic chromosomes

Developmental potential was assessed in 8 intra-specific and 20 inter-specific hybrid clones obtained by fusion of embryonic stem (ES) cells with either splenocytes or fetal fibroblasts. Number of chromosomes derived from ES cells in these hybrid clones was stable while contribution of somatic partner varied from single chromosomes to complete complement. This allowed us to compare pluripotency of the hybrid cells with various numbers of somatic chromosomes. Three criteria were used for the assessment: (i) expression of Oct-4 and Nanog genes; (ii) analyses of teratomas generated by subcutaneous injections of the tested cells into immunodeficient mice; (iii) contribution of the hybrid cells in chimeras generated by injection of the tested cells into C57BL blastocysts. All tested hybrid clones showed expression of Oct-4 and Nanog at level comparable to ES cells. Histological and immunofluorescent analyses demonstrated that most teratomas formed from the hybrid cells with different number of somatic chromosomes contained derivatives of three embryonic layers. Tested hybrid clones make similar contribution in various tissues of chimeras in spite of significant differences in the number of somatic chromosomes they contained. The data indicate that pluripotency is manifested as a dominant trait in the ES hybrid cells and does not depend substantially on the number of somatic chromosomes. The latter suggests that the developmental potential derived from ES cells is maintained in ES-somatic cell hybrids by cis-manner and is rather resistant to trans-acting factors emitted from the somatic one.

Generation of somatic cell hybrids for the production of biologically active factors that stimulate proliferation of other cells

OBJECTIVE

Some normal somatic cells in culture divide a limited number of times before entering a non-dividing state called replicative senescence and fusion of normal cells with immortal cells claimed to produce hybrid cells of limited proliferation. We reinvestigated the proliferative capacity of hybrid cells between normal cell and immortal cell.

MATERIALS AND METHODS

Normal pig fibroblast cells and cells of immortal mouse fibroblast cell line F7, a derivative of GM05267, were fused by polyethylene glycol treatment and subsequently the fused cells were cultured in a selective medium containing hypoxanthine-aminopterin-thymidine in order to enrich the hybrid cells. The hybrid cells were then monitored for chromosome content and proliferation.

RESULTS

Cytogenetic analysis revealed that the hybrid cells contained polyploidy chromosomes derived from normal pig fibroblasts. These hybrid cells exhibit no sign of replicative senescence after more than 190 population doublings in vitro. Instead, these hybrid cells have an accelerated growth and proliferate even in the complete absence of glutamine. In addition, these hybrids produce biologically active factors in the conditioned media, which not only can accelerate their own proliferation but also can reinitiate mitotic activity in the senescent-like normal fibroblast cells.

CONCLUSIONS

Our results question the validity of cellular senescence as a dominant trait. Additionally, the generation of hybrid cells using the specific mouse cell line can be applied to the generation of hybrids with other normal cell types and can be used to produce tissue-specific growth-factor(s) to extend the lifespan and/or improve the proliferation of various normal cells, including adult stem cells.

Checkpoint-apoptosis uncoupling in human and mouse embryonic stem cells: a source of karyotpic instability

Karyotypic abnormalities in cultured embryonic stem cells (ESCs), especially near-diploid aneuploidy, are potential obstacles to ESC use in regenerative medicine. Events causing chromosomal abnormalities in ESCs may be related to events in tumor cells causing chromosomal instability (CIN) in human disease. However, the underlying mechanisms are unknown. Using multiparametric permeabilized-cell flow cytometric analysis, we found that the mitotic-spindle checkpoint, which helps maintain chromosomal integrity during all cell divisions, functions in human and mouse ESCs, but does not initiate apoptosis as it does in somatic cells. This allows an unusual tolerance to polyploidy resulting from failed mitosis, which is common in rapidly proliferating cell populations and which is reduced to near-diploid aneuploidy, which is also common in human neoplastic disease. Checkpoint activation in ESC-derived early-differentiated cells results in robust apoptosis without polyploidy/aneuploidy similar to that in somatic cells. Thus, the spindle checkpoint is "uncoupled" from apoptosis in ESCs and is a likely source of karyotypic abnormalities. This natural behavior of ESCs to tolerate/survive varying degrees of ploidy change could complicate genome-reprogramming studies and stem-cell plasticity studies, but could also reveal clues about the mechanisms of CIN in human tumors.

Cell fusion for reprogramming pluripotency

Embryonic stem cell (ESC) technology should enable the generation of specific cell types for the study and treatment of human diseases. Therapeutic cloning provides a way to generate ESCs genetically matched to diseased individuals through nuclear reprogramming of the somatic genome. However, practical and ethical limitations associated with therapeutic cloning are calling for the development of oocyte- and-embryo-free alternatives for obtaining of autologous pluripotent cells for transplantation therapy. An alternative approach to reprogram the somatic genome involves fusion between somatic and pluripotent cells. Potential fusion partners with reprogramming activities include embryonal carcinoma cells, embryonic germ cells, and ESCs. Experimental evidence is now available, which demonstrates that mouse and human somatic cells can be reprogrammed by fusion to form pluripotent hybrid cells. Recent progress infusion-based reprogramming is reviewed with reference to the developmental potency of hybrid cells as well as genetic and epigenetic correlates of reprogramming. However, hybrid cells lack therapeutic potential because of their abnormal ploidy and the presence of nonautologous genes from the pluripotent parent. We discuss the potential of fusion-based reprogramming for the generation of diploid, autologous pluripotent cells using two alternative routes: the enucleation of ESCs and the fusion of such cytoplasts to somatic cell karyoplasts or intact somatic cells, and the selective elimination of the pluripotent genome following fusion to the somatic partner. Finally, these approaches are discussed in the light of recent progress showing that overexpression of embryonic transcription factors can restore a state of pluripotency to somatic cells.

Polyethylene Glycol-Mediated Fusion between Primary Mouse Mesenchymal Stem Cells and Mouse Fibroblasts Generates Hybrid Cells with Increased Proliferation and Altered Differentiation

Bone marrow-derived mesenchymal stem cells (MSCs) can differentiate into different cell lineages with the appropriate stimulation in vitro. Transplantation of MSCs in human and other animal models was found to repair tissues through the fusion of transplanted MSCs with indigenous cells. We have generated mouseâmouse hybrid cell lines in vitro by polyethylene glycol-mediated fusion of primary mouse MSCs with mouse fibroblasts to investigate the characteristics of hybrid cells, including their potentials for proliferation and differentiation. Similar to the parental MSCs, hybrid cells are positive for the cell-surface markers CD29, CD44, CD49e, and Sca-1, and negative for Gr-1, CD11b, CD13, CD18, CD31, CD43, CD45, CD49d, CD90.2, CD445R/B220, and CD117 markers. The hybrid cells also produce a high level of tissue nonspecific alkaline phosphatase compared to the parental cells. Conditioned medium of hybrid cells contain biologically active factors that are capable of stimulating proliferation of other cells. Although the parental MSCs can differentiate into adipogenic and osteogenic lineages, hybrid cells held disparate differentiation capacity. Hybrid cell lines in general have increased proliferative capacity than the primary MSCs. Our study demonstrates that proliferative hybrid cell lines can be generated in vitro by induced fusion of both immortal and primary somatic cells with primary MSCs.

Concise review: scientific and ethical roadblocks to human embryonic stem cell therapy

Despite the identified therapeutic potential of embryonic stem cells for treating human disease and injury, a number of roadblocks, scientific and ethical, stand in the way of progress toward this goal. We identify six areas of particular interest: tumorigenicity, animal product contamination, genetic compatibility, funding, cell type for transplantation, "embryo-friendly" derivation methods and discuss avenues for moving beyond the difficulties.

Tetraploid embryonic stem cells contribute to the inner cell mass of mouse blastocysts

The demonstration that mouse somatic cells can be reprogrammed following fusion with embryonic stem (ES) cells may provide an alternative to somatic cell nuclear transfer (therapeutic cloning) to generate autologous stem cells. In an attempt to produce cells with an increased pool of reprogramming factors, tetraploid ES cells were produced by polyethylene glycol mediated fusion of two ES cell lines transfected with plasmids carrying puromycin or neomycin resistance cassettes, respectively, followed by double antibiotic selection. Tetraploid ES cells retain properties characteristic of diploid ES cells, including the expression of pluripotent gene markers Oct4 and Rex1. On injection into the testis capsule of severe combined immunodeficient (SCID) mice, tetraploid ES cells are able to form teratomas containing cells representative of all three germ layers. Further, these cells demonstrated the ability to integrate into the inner cell mass of blastocysts. This study indicates that tetraploid ES cells are promising candidates as cytoplasm donors for reprogramming studies.