Expression of muscle genes in heterokaryons depends on gene dosage

Single-cell RNA-seq reveals activation of unique gene groups as a consequence of stem cell-parenchymal cell fusion

Fusion of donor mesenchymal stem cells with parenchymal cells of the recipient can occur in the brain, liver, intestine and heart following transplantation. The therapeutic benefit or detriment of resultant hybrids is unknown. Here we sought a global view of phenotypic diversification of mesenchymal stem cell-cardiomyocyte hybrids and associated time course. Using single-cell RNA-seq, we found hybrids consistently increase ribosome components and decrease genes associated with the cell cycle suggesting an increase in protein production and decrease in proliferation to accommodate the fused state. But in the case of most other gene groups, hybrids were individually distinct. In fact, though hybrids can express a transcriptome similar to individual fusion partners, approximately one-third acquired distinct expression profiles in a single day. Some hybrids underwent reprogramming, expressing pluripotency and cardiac precursor genes latent in parental cells and associated with developmental and morphogenic gene groups. Other hybrids expressed genes associated with ontologic cancer sets and two hybrids of separate experimental replicates clustered with breast cancer cells, expressing critical oncogenes and lacking tumor suppressor genes. Rapid transcriptional diversification of this type garners consideration in the context of cellular transplantation to damaged tissues, those with viral infection or other microenvironmental conditions that might promote fusion.

Macrophage traits in cancer cells are induced by macrophage-cancer cell fusion and cannot be explained by cellular interaction

Background

Cell fusion is a natural process in normal development and tissue regeneration. Fusion between cancer cells and macrophages generates metastatic hybrids with genetic and phenotypic characteristics from both maternal cells. However, there are no clinical markers for detecting cell fusion in clinical context. Macrophage-specific antigen CD163 expression in tumor cells is reported in breast and colorectal cancers and proposed being caused by macrophages-cancer cell fusion in tumor stroma. The purpose of this study is to examine the cell fusion process as a biological explanation for macrophage phenotype in breast.

Methods

Monocytes, harvested from male blood donor, were activated to M2 macrophages and co-cultured in ThinCert transwell system with GFP-labeled MCF-7 cancer cells. MCF7/macrophage hybrids were generated by spontaneous cell fusion, isolated by fluorescence-activated cell sorting and confirmed by fluorescence microscopy, short tandem repeats analysis and flow cytometry. CD163 expression was evaluated in breast tumor samples material from 127 women by immunohistochemistry.

Results

MCF-7/macrophage hybrids were generated spontaneously at average rate of 2 % and showed phenotypic and genetic traits from both maternal cells. CD163 expression in MCF-7 cells could not be induced by paracrine interaction with M2-activated macrophages. CD163 positive cancer cells in tumor sections grew in clonal collection and a cutoff point >25 % of positive cancer cells was significantly correlated to disease free and overall survival.

Conclusions

In conclusion, macrophage traits in breast cancer might be caused by cell fusion rather than explained by paracrine cellular interaction. These data provide new insights into the role of cell fusion in breast cancer and contributes to the development of clinical markers to identify cell fusion.

Cardiac Stem Cell Hybrids Enhance Myocardial Repair

RATIONALE

Dual cell transplantation of cardiac progenitor cells (CPCs) and mesenchymal stem cells (MSCs) after infarction improves myocardial repair and performance in large animal models relative to delivery of either cell population.

OBJECTIVE

To demonstrate that CardioChimeras (CCs) formed by fusion between CPCs and MSCs have enhanced reparative potential in a mouse model of myocardial infarction relative to individual stem cells or combined cell delivery.

METHODS AND RESULTS

Two distinct and clonally derived CCs, CC1 and CC2, were used for this study. CCs improved left ventricular anterior wall thickness at 4 weeks post injury, but only CC1 treatment preserved anterior wall thickness at 18 weeks. Ejection fraction was enhanced at 6 weeks in CCs, and functional improvements were maintained in CCs and CPC+MSC groups at 18 weeks. Infarct size was decreased in CCs, whereas CPC+MSC and CPC parent groups remained unchanged at 12 weeks. CCs exhibited increased persistence, engraftment, and expression of early commitment markers within the border zone relative to combinatorial and individual cell population-injected groups. CCs increased capillary density and preserved cardiomyocyte size in the infarcted regions suggesting CCs role in protective paracrine secretion.

CONCLUSIONS

CCs merge the application of distinct cells into a single entity for cellular therapeutic intervention in the progression of heart failure. CCs are a novel cell therapy that improves on combinatorial cell approaches to support myocardial regeneration.

Choosing Cell Fate Through a Dynamic Cell Cycle

A close relationship between proliferation and cell fate specification has been well documented in many developmental systems. In addition to the gradual cell fate changes accompanying normal development and tissue homeostasis, it is now commonly appreciated that cell fate could also undergo drastic changes, as illustrated by the induction of pluripotency from many differentiated somatic cell types during the process of Yamanaka reprogramming. Strikingly, the drastic cell fate change induced by Yamanaka factors (Oct4, Sox2, Klf4, and c-Myc) is preceded by extensive cell cycle acceleration. Prompted by our recent discovery that progression toward pluripotency from rare somatic cells could bypass the stochastic phase of reprogramming and that a key feature of these somatic cells is an ultrafast cell cycle (~8 h/cycle), we assess whether cell cycle dynamics could provide a general framework for controlling cell fate. Several potential mechanisms on how cell cycle dynamics may impact cell fate determination by regulating chromatin, key transcription factor concentration, or their interactions are discussed. Specific challenges and implications for studying and manipulating cell fate are considered.

Lineage conversion methodologies meet the reprogramming toolbox

Lineage conversion has recently attracted increasing attention as a potential alternative to the directed differentiation of pluripotent cells to obtain cells of a given lineage. Different means allowing for cell identity switch have been reported. Lineage conversion relied initially on the discovery of specific transcription factors generally enriched and characteristic of the target cell, and their forced expression in cells of a different fate. This approach has been successful in various cases, from cells of the hematopoietic systems to neurons and cardiomyocytes. Furthermore, recent reports have suggested the possibility of establishing a general lineage conversion approach bypassing pluripotency. This requires a first phase of epigenetic erasure achieved by short overexpression of the factors used to reprogram cells to a pluripotent state (such as a combination of Sox2, Klf4, c-Myc and Oct4), followed by exposure to specific developmental cues. Here we present these different direct conversion methodologies and discuss their potential as alternatives to using induced pluripotent stem cells and differentiation protocols to generate cell populations of a given fate.

Restricted Myogenic Potential of Mesenchymal Stromal Cells Isolated from Umbilical Cord

Nonhematopoietic cord blood cells and mesenchymal cells of umbilical cord Wharton's jelly have been shown to be able to differentiate into various cell types. Thus, as they are readily available and do not raise any ethical issues, these cells are considered to be a potential source of material that can be used in regenerative medicine. In our previous study, we tested the potential of whole mononucleated fraction of human umbilical cord blood cells and showed that they are able to participate in the regeneration of injured mouse skeletal muscle. In the current study, we focused at the umbilical cord mesenchymal stromal cells isolated from Wharton's jelly. We documented that limited fraction of these cells express markers of pluripotent and myogenic cells. Moreover, they are able to undergo myogenic differentiation in vitro, as proved by coculture with C2C12 myoblasts. They also colonize injured skeletal muscle and, with low frequency, participate in the formation of new muscle fibers. Pretreatment of Wharton's jelly mesenchymal stromal cells with SDF-1 has no impact on their incorporation into regenerating muscle fibers but significantly increased muscle mass. As a result, transplantation of mesenchymal stromal cells enhances the skeletal muscle regeneration.

Delineating nuclear reprogramming

Nuclear reprogramming is described as a molecular switch, triggered by the conversion of one cell type to another. Several key experiments in the past century have provided insight into the field of nuclear reprogramming. Previously deemed impossible, this research area is now brimming with new findings and developments. In this review, we aim to give a historical perspective on how the notion of nuclear reprogramming was established, describing main experiments that were performed, including (1) somatic cell nuclear transfer, (2) exposure to cell extracts and cell fusion, and (3) transcription factor induced lineage switch. Ultimately, we focus on (4) transcription factor induced pluripotency, as initiated by a landmark discovery in 2006, where the process of converting somatic cells to a pluripotent state was narrowed down to four transcription factors. The conception that somatic cells possess the capacity to revert to an immature status brings about huge clinical implications including personalized therapy, drug screening and disease modeling. Although this technology has potential to revolutionize the medical field, it is still impeded by technical and biological obstacles. This review describes the effervescent changes in this field, addresses bottlenecks hindering its advancement and in conclusion, applies the latest findings to overcome these issues.

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.

Nuclear reprogramming to a pluripotent state by three approaches

The stable states of differentiated cells are now known to be controlled by dynamic mechanisms that can easily be perturbed. An adult cell can therefore be reprogrammed, altering its pattern of gene expression, and hence its fate, to that typical of another cell type. This has been shown by three distinct experimental approaches to nuclear reprogramming: nuclear transfer, cell fusion and transcription-factor transduction. Using these approaches, nuclei from 'terminally differentiated' somatic cells can be induced to express genes that are typical of embryonic stem cells, which can differentiate to form all of the cell types in the body. This remarkable discovery of cellular plasticity has important medical applications.

Nuclear reprogramming to a pluripotent state by three approaches

The stable states of differentiated cells are now known to be controlled by dynamic mechanisms that can easily be perturbed. An adult cell can therefore be reprogrammed, altering its pattern of gene expression, and hence its fate, to that typical of another cell type. This has been shown by three distinct experimental approaches to nuclear reprogramming: nuclear transfer, cell fusion and transcription-factor transduction. Using these approaches, nuclei from 'terminally differentiated' somatic cells can be induced to express genes that are typical of embryonic stem cells, which can differentiate to form all of the cell types in the body. This remarkable discovery of cellular plasticity has important medical applications.

Reprogramming towards pluripotency requires AID-dependent DNA demethylation

Reprogramming of somatic cell nuclei to yield induced pluripotent stem (iPS) cells makes possible derivation of patient-specific stem cells for regenerative medicine. However, iPS cell generation is asynchronous and slow (2-3 weeks), the frequency is low (<0.1%), and DNA demethylation constitutes a bottleneck. To determine regulatory mechanisms involved in reprogramming, we generated interspecies heterokaryons (fused mouse embryonic stem (ES) cells and human fibroblasts) that induce reprogramming synchronously, frequently and fast. Here we show that reprogramming towards pluripotency in single heterokaryons is initiated without cell division or DNA replication, rapidly (1 day) and efficiently (70%). Short interfering RNA (siRNA)-mediated knockdown showed that activation-induced cytidine deaminase (AID, also known as AICDA) is required for promoter demethylation and induction of OCT4 (also known as POU5F1) and NANOG gene expression. AID protein bound silent methylated OCT4 and NANOG promoters in fibroblasts, but not active demethylated promoters in ES cells. These data provide new evidence that mammalian AID is required for active DNA demethylation and initiation of nuclear reprogramming towards pluripotency in human somatic cells.

Reprogramming to a muscle fate by fusion recapitulates differentiation

Fusion of mammalian cells to form stable, non-dividing heterokaryons results in nuclear reprogramming without the exchange of genetic material. In this report, we show that reprogramming in somatic cell heterokaryons involves activation of the canonical skeletal muscle transcription factors as well as contraction-excitation genes. Thus, the effect of heterokaryon formation on gene expression is to induce a recapitulation of differentiation. Heterokaryons formed with a relatively refractory cell type, the hepatocyte cell line HepG2, revealed the importance of both MyoD expression and other unidentified cytoplasmic components, neither of which are sufficient for efficient muscle gene activation, but are synergistic. We provide evidence that de-repression by transient histone deacetylase inhibition can induce MyoD expression and increase the extent and efficiency of muscle gene transcription. Taken together, the results suggest that understanding the mechanistic basis, using a combination of approaches, and taking into account cell history, will facilitate an increase in the efficiency and fidelity of conversion from one differentiated phenotype to another desired cell type. Inherent advantages of the heterokaryon system merit further investigation in the pursuit of directed cloning.

Nuclear reprogramming in heterokaryons is rapid, extensive, and bidirectional

An understanding of nuclear reprogramming is fundamental to the use of cells in regenerative medicine. Due to technological obstacles, the time course and extent of reprogramming of cells following fusion has not been assessed to date. Here, we show that hundreds of genes are activated or repressed within hours of fusion of human keratinocytes and mouse muscle cells in heterokaryons, and extensive changes are observed within 4 days. This study was made possible by the development of a broadly applicable approach, species-specific transcriptome amplification (SSTA), which enables global resolution of transcripts derived from the nuclei of two species, even when the proportions of species-specific transcripts are highly skewed. Remarkably, either phenotype can be dominant; an excess of primary keratinocytes leads to activation of the keratinocyte program in muscle cells and the converse is true when muscle cells are in excess. We conclude that nuclear reprogramming in heterokaryons is rapid, extensive, bidirectional, and dictated by the balance of regulators contributed by the cell types.

Climbing Mount Efficiency--small steps, not giant leaps towards higher cloning success in farm animals

Despite more than a decade of research efforts, farm animal cloning by somatic cell nuclear transfer (SCNT) is still frustratingly inefficient. Inefficiency manifests itself at different levels, which are currently not well integrated. At the molecular level, it leads to widespread genetic, epigenetic and transcriptional aberrations in cloned embryos. At the organismal level, these genome-wide abnormalities compromise development of cloned foetuses and offspring. Specific molecular defects need to be causally linked to specific cloned phenotypes, in order to design specific treatments to correct them. Cloning efficiency depends on the ability of the nuclear donor cell to be fully reprogrammed into an embryonic state and the ability of the enucleated recipient cell to carry out the reprogramming reactions. It has been postulated that reprogrammability of the somatic donor cell epigenome is influenced by its differentiation status. However, direct comparisons between cells of divergent differentiation status within several somatic lineages have found no conclusive evidence for this. Choosing somatic stem cells as donors has not improved cloning efficiency, indicating that donor cell type may be less critical for cloning success. Different recipient cells, on the other hand, vary in their reprogramming ability. In bovine, using zygotes instead of oocytes has increased cloning success. Other improvements in livestock cloning efficiency include better coordinating donor cell type with cell cycle stage and aggregating cloned embryos. In the future, it will be important to demonstrate if these small increases at every step are cumulative, adding up to an integrated cloning protocol with greatly improved efficiency.

Mediators of reprogramming: transcription factors and transitions through mitosis

It is thought that most cell types of the human body share the same genetic information as that contained in the zygote from which they originate. Consistent with this view, animal cloning studies demonstrated that the intact genome of a differentiated cell can be reprogrammed to support the development of an entire organism and allow the production of pluripotent stem cells. Recent progress in reprogramming research now points to an important role for transcription factors in the establishment and the maintenance of cellular phenotypes, and to cell division as a mediator of transitions between different states of gene expression.

Green fluorescent protein incorporation by mouse myoblasts may yield false evidence of myogenic differentiation of human haematopoietic stem cells

AIMS

Haematopoietic CD34+ stem cells are able to differentiate into skeletal muscle, a potentially invaluable tool for treating degenerative diseases such as muscular dystrophy. However, some studies argue that the differentiative potential of these cells might have been overestimated. In vitro studies provide a controlled environment in which to investigate this point.

METHODS

CD34+ stem cells from human peripheral blood, labelled with green fluorescent protein (GFP), were co-cultured with mouse myogenic C2C12 cells. The functional properties of mononucleated GFP+ cells were determined using electrophysiological techniques and were related to protein profiling determined by immunofluorescence staining and single-cell RT-PCR. Mouse mesoangioblasts co-cultured with human myotubes provided methodological controls.

RESULTS

After 2-4 days, mononucleated adherent GFP+ cells showed acetylcholine-evoked current responses, typical of myogenic cells, as if stem cells had integrated into the host environment. In contrast to this hypothesis, human nuclei could not be detected in adherent GFP+ cells by immunofluorescence. Moreover, single-cell RT-PCR showed that adherent GFP+ cells responsive to acetylcholine expressed mouse markers while loose unresponsive GFP+ cells were of human origin. The transcripts of the human alpha1 subunit of the acetylcholine muscle receptor were not amplified in co-cultures.

CONCLUSION

Single-cell analysis of functional properties combined with other markers revealed that, under the co-culture conditions used, GFP was transferred from human CD34+ stem cells to C2C12 myoblasts by mechanisms unrelated to myogenic stem cell differentiation. Our results emphasize the need for careful controls using several markers when investigating the myogenic differentiation of circulating stem cells.

Myogenic reprogramming of retina-derived cells following their spontaneous fusion with myotubes

Satellite cells are recognized as the main source for myoblasts in postnatal muscle. The possible participation of other cell types in myofiber maintenance remains a subject of debate. Here, we investigated the potential of vascular preparations from mouse retina to undergo myogenesis when cultured alone or with differentiated primary myogenic cultures. The choice of retina, an organ richly supplied with capillary network and anatomically separated from skeletal muscles, ensures that the vasculature preparation is devoid of satellite cells. We demonstrate that retina-derived cells spontaneously fuse with preexisting myotubes and contribute additional myonuclei, some of which initiate expression of muscle-specific genes after fusion. Myogenic differentiation of retinal cells prior to their fusion with preexisting myotubes was not detected. Although originating from vasculature preparations, nuclei undergoing myogenic reprogramming were contributed by cells that were neither endothelial nor blood borne. Our results suggest smooth muscle/pericytes as the possible source, and that myogenic reprogramming depends on the muscle specific transcription factor MyoD. Our studies provide insights into a novel avenue for myofiber maintenance, relying on nuclei of non-myogenic origin that undergo fusion and subsequent myogenic conversion within host myofibers. This process may support ongoing myofiber maintenance throughout life.

Active tissue-specific DNA demethylation conferred by somatic cell nuclei in stable heterokaryons

DNA methylation is among the most stable epigenetic marks, ensuring tissue-specific gene expression in a heritable manner throughout development. Here we report that differentiated mesodermal somatic cells can confer tissue-specific changes in DNA methylation on epidermal progenitor cells after fusion in stable multinucleate heterokaryons. Myogenic factors alter regulatory regions of genes in keratinocyte cell nuclei, demethylating and activating a muscle-specific gene and methylating and silencing a keratinocyte-specific gene. Because these changes occur in the absence of DNA replication or cell division, they are mediated by an active mechanism. Thus, the capacity to transfer epigenetic changes to other nuclei is not limited to embryonic stem cells and oocytes but is also a property of highly specialized mammalian somatic cells. These results suggest the possibility of directing the reprogramming of readily available postnatal human progenitor cells toward specific tissue cell types.

On experimental design and discourse in plasticity research

Communication in the stem cell field requires a common understanding of terminology and that "plasticity" phenomena are model- and, perhaps, species-dependent. Plasticity has generally been applied to unexpected differentiative events; will the term cease being useful when these unexpected pathways become recognized as normative? Four pathways of cell plasticity have now been recognized: (1) facultative, intraorgan self-renewing stem cells; (2) reversion of differentiated cells to blastema-like appearances, common in amphibians, perhaps restricted to neoplasia in mammals; (3) cells of one lineage directly changing to differentiation of another lineage cued by microenvironemental signals; (4) cell-cell fusion leading to changes in differentiation of the "incoming" cell in response to cytoplasmic and perhaps nuclear cues. In all of these, "differentiation" must be understood as a reflection of gene expression that is a highly intricate system of parallel, i.e., nonlinear molecular interactions. Present controversies regarding the plasticity of adult stem cells may be explained both by differences in experimental variables and techniques as well as by differing nonscientific, political, and/or polemical needs of investigators and commentators. Some of the variables in transplantation experiments, which are likely to be important in experimental outcome, but rarely addressed in interpretation of data, are the age of the cell donor and of the strain of mice or species used, the isolation technique used to obtain the putative stem cells, and the inherent effects of transgenic markers used to identify the donor or host cells. Also of great importance, but rarely controlled for in experimental design and interpretation, are the reproducibility and sensitivity of methods used to detect the markers of donor origin, the capacity of differentiated tissue to silence transgenes or alter marker expression, and--finally and most importantly--the different signals that influence plasticity phenomena in very different types of injury and regeneration. In different models of injury there are likely to be significant differences in promoting cell localization, proliferation, and predominance of "plasticity pathway," if any are involved, in determining outcome.

Cardiac stem cells and mechanisms of myocardial regeneration

This review discusses current understanding of the role that endogenous and exogenous progenitor cells may have in the treatment of the diseased heart. In the last several years, a major effort has been made in an attempt to identify immature cells capable of differentiating into cell lineages different from the organ of origin to be employed for the regeneration of the damaged heart. Embryonic stem cells (ESCs) and bone marrow-derived cells (BMCs) have been extensively studied and characterized, and dramatic advances have been made in the clinical application of BMCs in heart failure of ischemic and nonischemic origin. However, a controversy exists concerning the ability of BMCs to acquire cardiac cell lineages and reconstitute the myocardium lost after infarction. The recognition that the adult heart possesses a stem cell compartment that can regenerate myocytes and coronary vessels has raised the unique possibility to rebuild dead myocardium after infarction, to repopulate the hypertrophic decompensated heart with new better functioning myocytes and vascular structures, and, perhaps, to reverse ventricular dilation and wall thinning. Cardiac stem cells may become the most important cell for cardiac repair.

Nuclear reprogramming: A key to stem cell function in regenerative medicine

The goal of regenerative medicine is to restore form and function to damaged tissues. One potential therapeutic approach involves the use of autologous cells derived from the bone marrow (bone marrow-derived cells, BMDCs). Advances in nuclear transplantation, experimental heterokaryon formation and the observed plasticity of gene expression and phenotype reported in multiple phyla provide evidence for nuclear plasticity. Recent observations have extended these findings to show that endogenous cells within the bone marrow have the capacity to incorporate into defective tissues and be reprogrammed. Irrespective of the mechanism, the potential for new gene expression patterns by BMDCs in recipient tissues holds promise for developing cellular therapies for both proliferative and post-mitotic tissues.

Isolation and characterization of canine satellite cells

Satellite cells were isolated from biopsies of the biceps femoris of adult dogs. Virtually all cells expressed muscle-specific proteins. Proliferation of satellite cells increased as the concentration of fetal calf serum (FCS) was increased from 1 to 10% of the basal medium. The addition of mitogenic growth factors resulted in greater proliferation than that of cells cultured in basal medium alone. Maximum proliferation was obtained when fibroblast growth factor-basic (FGF2) was added to the medium, but differences existed between sources or types. Proliferation did not plateau when the concentration of recombinant human FGF2 was 75 ng/ml but reached maximum levels when 50 ng/ml of bovine FGF2 or 10 ng/ml of growth hormone or insulin-like growth factor-1 were added to the medium. Proliferation of satellite cells decreased when more than 5 ng/ml of transforming growth factor-alpha was included in the medium. Exposure of canine satellite cells to chemically defined media induced greater fusion of total nuclei (ODM-34%; 4F, ITT-CF, and SFG-23%) than exposure to other treatments, such as basal medium plus 2 mg/ml of 1-beta-d-arabinofuranosylcytosine, 5% chick embryo extract, 1% horse serum (average 9% fused nuclei), or 1% FCS (2% fused nuclei). Actin, myosin, desmin, neural cell adhesion molecule, MyoD1, and myogenin were expressed by canine satellite cells, but expression of major histocompatibility complex class II antigen was not detected. Reverse transcriptase-polymerase chain reaction detected expression of messenger ribonucleic acid for interleukin-6 (IL-6), IL-15, and leukemia inhibitory factor by canine satellite cells. Collectively, these data suggest that isolated canine satellite cells display properties of other types of myogenic cells and may be useful for further study of the regulation of postnatal myogenesis.

Have the bloody cells gone to our heads?

Recent studies have shown that cells expressing neuronal antigens can be derived from a bone marrow transplant. A new report lends support to and extends these previous results by presenting compelling morphological evidence for the generation and integration of highly differentiated bone marrow-derived neurons.

Plasticity of cell fate: insights from heterokaryons

Experiments with somatic cell hybrids and stable heterokaryons have demonstrated that differentiated cells exhibit a remarkable capacity to change. Heterokaryons have been particularly useful in determining the extent to which the differentiated state of a cell is plastic. Cell fate can be altered by a change in the balance of positive and negative trans-acting regulators. Although a single regulator may be sufficient in certain environments to trigger a change in cell fate, that regulator may be ineffective in other cell contexts where it encounters a different composition of regulators.

Induction of erythroid gene expression by microcell fusion

The molecular events which underlie lineage commitment and differentiation in hematopoietic cells are still incompletely understood. Microcell fusion is a versatile technique which has been utilized in characterizing and mapping genes involved in tumor suppression, cell senescence, and certain aspects of differentiation. Microcell fusion has the potential to contribute to the understanding of hematopoietic differentiation; however, application of this technique is limited by the need to use adherent cells as microcell donors, by the need to tag candidate chromosomes with a selectable marker, and by the need for prolonged selection of fused cells prior to characterization of their phenotype. We developed a modified technique of microcell fusion using square wave electroporation, which allows higher efficiency fusion than polyethylene glycol fusion. By using cross-species fusion and species-specific PCR primers, we were able to detect new gene induction events 48 h after microcell fusion. To study erythroid gene expression, we fused microcells from human erythroid K562 cells to murine B-lymphoid SP-2 cells. We found that microcell fusion induced the nonerythroid recipient cells to express alpha-globin mRNA in a dose-dependent manner. They also expressed RNA for beta-globin, GATA-1, and NF-E2. In contrast, there was no expression of heart- or liver-specific genes. We conclude that microcells from erythroid cells contain all the information necessary to induce expression of multiple erythroid genes. Analysis of the components of the microcells responsible for this new gene induction may allow the characterization of cellular factors responsible for erythroid-specific gene expression.

Positive regulators of the lineage-specific transcription factor GATA-1 in differentiating erythroid cells

The zinc finger transcription factor GATA-1 is a major regulator of gene expression in erythroid, megakaryocyte, and mast cell lineages. GATA-1 binds to WGATAR consensus motifs in the regulatory regions of virtually all erythroid cell-specific genes. Analyses with cultured cells and cell-free systems have provided strong evidence that GATA-1 is involved in control of globin gene expression during erythroid differentiation. Targeted mutagenesis of the GATA-1 gene in embryonic stem cells has demonstrated its requirement in normal erythroid development. Efficient rescue of the defect requires an intact GATA element in the distal promoter, suggesting autoregulatory control of GATA-1 transcription. To examine whether GATA-1 expression involves additional regulatory factors or is maintained entirely by an autoregulatory loop, we have used a transient heterokaryon system to test the ability of erythroid factors to activate the GATA-1 gene in nonerythroid nuclei. We show here that proerythroblasts and mature erythroid cells contain a diffusible activity (TAG) capable of transcriptional activation of GATA-1 and that this activity decreases during the terminal differentiation of erythroid cells. Nuclei from GATA-1- mutant embryonic stem cells can still be reprogrammed to express their globin genes in erythroid heterokaryons, indicating that de novo induction of GATA-1 is not required for globin gene activation following cell fusion.

Positive regulators of the lineage-specific transcription factor GATA-1 in differentiating erythroid cells

The zinc finger transcription factor GATA-1 is a major regulator of gene expression in erythroid, megakaryocyte, and mast cell lineages. GATA-1 binds to WGATAR consensus motifs in the regulatory regions of virtually all erythroid cell-specific genes. Analyses with cultured cells and cell-free systems have provided strong evidence that GATA-1 is involved in control of globin gene expression during erythroid differentiation. Targeted mutagenesis of the GATA-1 gene in embryonic stem cells has demonstrated its requirement in normal erythroid development. Efficient rescue of the defect requires an intact GATA element in the distal promoter, suggesting autoregulatory control of GATA-1 transcription. To examine whether GATA-1 expression involves additional regulatory factors or is maintained entirely by an autoregulatory loop, we have used a transient heterokaryon system to test the ability of erythroid factors to activate the GATA-1 gene in nonerythroid nuclei. We show here that proerythroblasts and mature erythroid cells contain a diffusible activity (TAG) capable of transcriptional activation of GATA-1 and that this activity decreases during the terminal differentiation of erythroid cells. Nuclei from GATA-1- mutant embryonic stem cells can still be reprogrammed to express their globin genes in erythroid heterokaryons, indicating that de novo induction of GATA-1 is not required for globin gene activation following cell fusion.

Muscle glucose-6-phosphate dehydrogenase deficiency: restoration of enzymatic activity in hybrid myotubes

A high level of glucose-6-phosphate dehydrogenase (G6PD) activity was observed in myoblasts and myotubes from normal human and mouse cell cultures. However, only a residual amount of activity was observed in myoblasts and myotubes obtained from G6PD-deficient patients (G6PD Mediterranean). Hybrids were formed by the fusion of normal (from human and mouse) and G6PD-deficient myoblasts (from the patients). These hybrids contained a high level of G6PD activity. Hoechst staining permitted to confirm that the enzymatic activity was not restrained to a domain near the competent nuclei. These results suggest that myoblast transplantation could be used to restore normal enzymatic activity in metabolic myopathies.

Trans-activation of the murine dystrophin gene in human-mouse hybrid myotubes

Myotube cultures of the myogenic cell line, C2, produce significantly lower levels of dystrophin than primary mouse cultures. We demonstrate that expression of the C2 dystrophin gene increases 10-fold in hybrid myotubes formed by fusion of C2 and dystrophin-deficient human myoblasts from a Duchenne muscular dystrophy patient. These results indicate that C2 cells are deficient in endogenous gene regulatory factors which enhance dystrophin expression, and that the C2 cell line may therefore be used to identify putative trans-acting factors involved in the regulation of dystrophin gene expression.

Stable hybrid myotubes: a new model for studying re-expression of enzymatic activities in vitro

Heterokaryons represent a stable and reproducible model system for the study of biochemical and molecular aspects responsible for muscle gene activation. Previous experiments have used this fusion system to demonstrate human gene activation in hybrids formed between human and non-human cells. The aim of this research was to apply this experimental model to the correction of a cytoplasmic activity, namely glucose-6-phosphate dehydrogenase (G6PD), in vitro, in hybrid myotubes formed between G6PD-negative and positive myoblasts. Different identification methods were used (Hoechst stain and Fluorescent Latex Microspheres, FLMs) to identify hybrid myotubes formed. We demonstrated the restoration of G6PD activity in all hybrid myotubes formed; we then tried to elucidate the mechanisms underlying the restoration of this specific activity and apply the results obtained to the understanding of more complex mechanisms involved in muscle gene activation.

Basement membrane and interstitial matrix components form separate matrices in heterokaryons of PYS-2 cells and fibroblasts

In order to gain further understanding of the spatial organization of interstitial and basement membrane matrices, we studied the expression of the interstitial matrix protein, fibronectin, and the basement membrane protein, laminin, in heterokaryons formed by the fusion of normal fibroblasts and teratocarcinoma-derived epithelial PYS-2 cells. These heterokaryons showed various distributions of the matrix proteins depending on the proportions of the different parental cell nuclei within the cytoplasm of the cell. Heterokaryons containing equal numbers of fibroblast and PYS-2 cell nuclei showed an abundant laminin matrix subcellularly and only minor amounts of fibronectin matrix at the periphery of the cells. Similar results were obtained in heterokaryons containing an excess of epithelial cell nuclei. In heterokaryons containing an excess of fibroblast nuclei, on the other hand, laminin matrix was reduced and a fibrillar fibronectin matrix was seen also on top of the cell body. The results suggest a gene dosage-type of effect on the expression of these proteins. Furthermore, extracellular laminin and fibronectin matrices did not codistribute around the heterokaryons but the two proteins were assembled into separate structures. The lack of codistribution of fibronectin and laminin matrices in heterokaryons suggests that the molecular interactions, which determine the assembly of basement membrane and interstitial matrices in these cells are highly type-specific. Similar mechanisms may also operate in the assembly of extracellular matrices in vivo.

Embryonic and fetal rat myoblasts express different phenotypes following differentiation in vitro

Myosin heavy chain (MHC) is encoded by a multigene family containing members which are expressed in developmental and fiber type-specific patterns. In developing rats, primary (1 degree) and secondary (2 degrees) myotubes can be distinguished by difference in MHC expression: 1 degree myotubes coexpress embryonic and slow MHC, while 2 degrees myotubes initially express only embryonic MHC. We have used monoclonal antibodies which recognize the embryonic, slow, neonatal, and adult fast IIB/IIX MHCs to examine MHC accumulation in myoblasts obtained from hindlimbs of embryonic day (ED) 14 and ED 20 Sprague-Dawley rats during differentiation in vitro. Embryonic myoblasts (ED 14), which develop into 1 degree myotubes in vivo, differentiate as myocytes or small myotubes (i.e., 1-4 nuclei) which express both embryonic and slow MHC. They do not accumulate detectable levels of neonatal or adult fast IIB/IIX MHC. Fetal myoblasts, which develop into secondary myotubes in vivo, fuse to form large myotubes (i.e., 10-50 nuclei) and express predominantly embryonic MHC at 3 days in culture. These myotubes accumulate neonatal and adult fast IIB/IIX isoforms of MHC and eventually contract spontaneously. In contrast to embryonic myotubes, they do not accumulate slow MHC. Our results demonstrate that embryonic and fetal rat myoblasts express different phenotypes in vitro and suggest that they represent distinct myoblast lineages similar to those previously described in chickens and mice. These two lineages may be responsible for the generation of distinct populations of 1 degree and 2 degrees myotubes in vivo.

Cooperation between the products of different nuclei in hybrid myotubes produces localized acetylcholine receptor clusters

Cultured myotubes form clusters of acetylcholine receptors (AChRs) spontaneously and at sites of nerve-muscle contact. To investigate the cellular mechanisms by which spontaneous clusters are formed, we have made hybrid myotubes between a mouse muscle cell variant, S27, that does not cluster AChRs, and one that does not make AChRs. We have also made hybrid myotubes using S27 and quail muscle cells. In both cases, clusters of AChRs were found near the non-S27 nuclei; in the case of the interspecific hybrids, mouse AChRs were associated with extracellular matrix components contributed by the quail nuclei. Our results suggest that AChRs made by one nucleus can be clustered by localized extracellular matrix produced by a different nucleus and provide an example of nuclear cooperation between the products of different nuclei within multinucleated muscle fibers.

Negative control of the helix-loop-helix family of myogenic regulators in the NFB mutant

We have characterized a nondifferentiating mouse muscle cell line, NFB, that represses the activity of the helix-loop-helix (HLH) family of myogenic regulators, yet expresses sarcomeric actins. The NFB MyoD gene is silent, but can be activated upon transfection of a long terminal region-controlled chicken MyoD cDNA, resulting in myogenesis. When NFB cells are fused with H9c2 rat muscle cells in heterokaryons, the level of rat MyoD transcripts declines. Thus, the stoichiometry of MyoD and the putative repressor controls myogenesis. Although NFB cells express myogenin and Myf-5 transcripts, the activity of these regulators is also repressed:myogenesis is not induced in 10T1/2 fibroblasts and is repressed in L6 muscle cells upon fusion with NFB cells. We conclude that the myogenic HLH regulators are not required for sarcomeric actin gene activation and that myogenesis is subject to dominant-negative control.

Effect of cell history on response to helix-loop-helix family of myogenic regulators

In multinucleated heterokaryons formed from the fusion of differentiated muscle cells to either hepatocytes or fibroblasts, muscle-specific gene expression is activated, liver-specific gene expression is repressed, and there are changes in the location of the Golgi apparatus. An understanding of the regulatory mechanisms that underlie this plasticity is of particular interest given the stability of the differentiated state in vivo. We have now investigated whether MyoD or myogenin, regulators of muscle-specific gene expression that have a helix-loop-helix motif, can induce the phenotypic conversion observed in heterokaryons. When these regulators were stably or transiently introduced into fibroblasts or hepatocytes by microinjection, transfection or retroviral infection with complementary DNA in expression vectors, fibroblasts expressed muscle-specific genes, whereas hepatocytes did not. However, fusion of hepatocytes stably expressing MyoD to fibroblasts resulted in activation in the heterokaryon of muscle-specific genes of both cell types. These results imply that other regulators, present in fibroblasts but not in hepatocytes, are necessary for the activation of muscle-specific genes, and indicate that the differentiated state of a cell is dictated by its history and a dynamic interaction among the proteins that it contains.

Uncoupling of muscle-specific protein expression in myocyte x myoblast heterokaryons

The inducibility of several rat skeletal muscle proteins was examined in heterokaryons formed by fusing differentiated chick myocytes to undifferentiated rat myoblasts. Chicken and rat proteins were distinguished using species-specific antibodies or by their different migrations in polyacrylamide or agarose gels. Both rat skeletal myosin light chain 1 and rat alpha-tropomyosin were induced in the heterokaryons. In contrast, neither rat acetylcholine receptors nor creatine kinase could be detected. These results suggest that chick myocytes may contain quantities of regulatory factors that are sufficient for the activation of some but not all of these rat muscle-specific proteins within the cellular context of the heterokaryon.

A myosin isoform repressed in hypertrophied ALD muscle of the chicken reappears during regeneration following cold injury

A library of monoclonal antibodies specific for myosin heavy chain (HC) was used to study myosin expression in regenerating fibers. The response to cold injury of slow skeletal ALD muscle previously induced to eliminate SM1 myosin by weight overload was compared to that of its contralateral control. Native gel electrophoresis combined with immunoblotting demonstrated that slow SM1 myosin HC eliminated from hypertrophic muscle reappeared both at the site of active regeneration and unexpectedly, also distal to the site of injury. The regeneration response of hypertrophied muscles was similar to that of the controls. In addition to SM1 myosin HC, ventricular-like and embryonic/fast isoforms were also expressed in both muscles during the early stages of regeneration and disappeared as the muscle fibers matured. These observations demonstrate that regenerating slow muscle fibers reexpress myosins' characteristic of developing muscle irrespective of the myosin phenotype prior to injury. The reappearance of repressed myosin HC in the hypertrophied ALD muscle is consistent with the presence of newly differentiated myonuclei.

How fixed is the differentiated state? Lessons from heterokaryons

The differentiated state is highly stable in vivo. Yet, in response to nuclear transplantation, tissue regeneration or cell fusion, the nuclei of differentiated cells exhibit a remarkable capacity to change. I review here the utility of heterokaryons, multinucleated cell hybrids, in elucidating the mechanisms that establish and maintain the differentiated state and yet allow such plasticity.

In vivo aging of human fibroblasts does not alter nuclear plasticity in heterokaryons

In vivo aging of human fibroblasts altered proliferative properties but not the potential for novel gene expression in response to muscle trans-acting factors. Heterokaryons produced by fusing fibroblasts with muscle cells permitted a dissociation of the effects of aging on cell division and other cell functions. Skin fibroblasts derived from fetal and adult stages of development were distinct cell types based on their doubling time, protein content, cell size, and specific binding of insulin and insulin-like growth factor I. Despite these differences in growth parameters, the two cell types were indistinguishable in heterokaryons. Muscle gene activation occurred in the absence of changes in chromatin structure requiring DNA replication. In addition, the time course, maximal efficiency, and effect of gene dosage on the expression of muscle gene products were similar for heterokaryons containing fetal and adult fibroblasts but distinct for heterokaryons containing keratinocytes. The difference between fibroblasts and keratinocytes in the time course of muscle gene expression is likely to reflect mechanisms of gene activation at the transcriptional level, since the kinetics of muscle protein accumulation paralleled that of muscle transcripts. These results indicate that nuclear plasticity is not altered in fibroblasts by in vivo aging.

Exaggerated triglyceride accretion in human preadipocyte-murine renal line hybrids composed of cells from massively obese subjects

To learn about adipose differentiation of precursors from postnatal adipose tissue of lean and massively obese subjects, human omental adipocyte precursor-murine renal adenocarcinoma cell (RAG) hybrids were formed by fusion with polyethylene glycol, and cultured selectively with 50 microM ouabain in hypoxanthine aminopterin thymidine (HAT) medium. Under conditions in which the parent cells did not differentiate, a number of hybrids, which were cloned, revealed morphologic and biochemical evidence of differentiation. In addition to activation of human genes within the common nucleus of the hybrids, murine cytoplasmic activators are probably also involved because heterocaryons (fused cells with two interspecific nuclei) revealed the same phenomenon. Hybrids composed of precursors from massively obese subjects disclosed more frequent and prominent differentiation. Since these hybrids, in contrast to those from the lean, recapitulate this phenomenon in subcultures, they provide the potential system for mapping the human gene(s) responsible for adipose differentiation and its exaggeration in massive obesity.

In vivo system for characterizing clonal variation and tissue-specific gene regulatory factors based on function

The inducibility of stably transfected alpha-cardiac actin genes differs among L cell clones. We examined the ability of muscle-specific factors to induce the expression of the human muscle alpha-cardiac actin gene promoter when stably transfected into mouse fibroblast L cells. This promoter is transcriptionally active in L cells at a low level, 2-5% of that in transfected muscle cells. Upon fusion with muscle cells to form heterokaryons, expression of the transfected alpha-cardiac actin gene promoter can be induced. However, induction is observed with only 10% of transfected L cell clones and the magnitude of this induction varies between 5- and 50-fold. These properties of the transfected L cell appear to be stably inherited. Our results are consistent with the hypothesis that muscle cells contain factors capable of increasing the transcription of the transfected gene, but that differences among L cell clones, possibly in the site of integration in the genome, determine the extent to which the gene can respond. By fusion into heterokaryons, transfectants with responsive genes can be identified. Such clones should prove useful in determining the basis for clonal variation. In addition, they provide an in vivo system for isolating functionally active tissue-specific transcription factors and the genes that encode them.

Muscle cell components dictate hepatocyte gene expression and the distribution of the Golgi apparatus in heterokaryons

Major changes in cytoarchitecture and gene expression were induced in short-term heterokaryons. When human hepatocytes were fused with mouse muscle cells, the hepatocyte Golgi apparatus changed from its usual polar location to a uniformly circumnuclear location typical of striated muscle. Human liver albumin ceased to be expressed, and expression of the human muscle cell-surface antigen 5.1H11 was induced without DNA replication or cell division. Coexpression of liver and muscle proteins was rarely observed. These novel findings provide insight into the regulation of gene expression and the targeting and localization of organelles with a central role in cell polarity, intracellular transport, and secretion.

Synthesis of fast myosin induced by fast ectopic innervation of rat soleus muscle is restricted to the ectopic endplate region

Skeletal muscle fibres, long multinucleated cells, arise by fusion of mononucleated myoblasts to form a myotube that matures into the adult fibre. The two major types of mature fibre, fast and slow fibres, differ physiologically in their rate of isotonic shortening. At the molecular level these type-specific physiological properties are ascribed to different isoforms of myosin, a major protein involved in shortening. Differentiation of fast and slow fibres seems to be under the control of motoneurones, and mature fibres are innervated by only one motoneurone. When rat soleus muscle (SOL, a slow muscle) is dually innervated with a fast nerve, it acquires some properties of a fast muscle, that is, low sensitivity to caffeine and high glycogen content. We report here that in dually innervated soleus muscle the foreign fast nerve induces synthesis of fast isoforms of myosin, but only in the segment of the muscle fibre that is close to the foreign endplate. The localized influence of the nerve endplates suggest that factors controlling the phenotypic expression of the muscle fibre have a short range of activity.

Plasticity of the differentiated state

Heterokaryons provide a model system in which to examine how tissue-specific phenotypes arise and are maintained. When muscle cells are fused with nonmuscle cells, muscle gene expression is activated in the nonmuscle cell type. Gene expression was studied either at a single cell level with monoclonal antibodies or in mass cultures at a biochemical and molecular level. In all of the nonmuscle cell types tested, including representatives of different embryonic lineages, phenotypes, and developmental stages, muscle gene expression was induced. Differences among cell types in the kinetics, frequency, and gene dosage requirements for gene expression provide clues to the underlying regulatory mechanisms. These results show that the expression of genes in the nuclei of differentiated cells is remarkably plastic and susceptible to modulation by the cytoplasm. The isolation of the genes encoding the tissue-specific trans-acting regulators responsible for muscle gene activation should now be possible.