Myogenic fusion of human bone marrow stromal cells, but not hematopoietic cells

Hypoxic preconditioning-engineered bone marrow mesenchymal stem cell-derived exosomes promote muscle satellite cell activation and skeletal muscle regeneration via the miR-210-3p/KLF7 mechanism

Sarcopenia is a gradual and widespread decline in muscle mass and function in skeletal muscle, leading to significant implications for individuals and society. Currently, there is a lack of effective treatment methods for sarcopenia. Muscle satellite cells(SCs) play a crucial role in the occurrence and development of sarcopenia, and their proliferation and differentiation abilities are closely related to the progression of disease. This study evaluated the effects of exosomes derived from hypoxic preconditioning bone marrow mesenchymal stem cells (BMSCs) on the proliferation of SCs and skeletal muscle regeneration. We found that the capacity for the proliferation and differentiation of SCs in elderly rats was notably diminished, leading us to create a sarcopenia model in elderly rats. By separating and extracting exosomes from BMSCs treated with normoxic (N-Exos) and hypoxic (H-Exos) conditions, in vivo and in vitro studies showed that both N-Exos and H-Exos can regulate the proliferation and differentiation of SCs in elderly rats, and promote skeletal muscle regeneration and functional recovery. The beneficial effects of H-Exos were also more significant than those of the N-Exos group. In vitro studies demonstrated that H-Exos could influence the expression of the KLF7 gene and protein in SCs by delivering miR-210-3P. This, in turn, impacted the phosphorylation of the PI3K/AKT signaling pathway and contributed to the function of SCs. H-Exos stimulated SCs and promoted skeletal muscle regeneration during sarcopenia by delivering miR-210-3P to target the KLF7/PI3K/AKT signaling pathway. This may serve as a possible treatment option for sarcopenia.

The miR151 and miR5100 Transfected Bone Marrow Stromal Cells Increase Myoblast Fusion in IGFBP2 Dependent Manner

Background

Bone marrow stromal cells (BMSCs) form a perivascular cell population in the bone marrow. These cells do not present naïve myogenic potential. However, their myogenic identity could be induced experimentally in vitro or in vivo. In vivo, after transplantation into injured muscle, BMSCs rarely fused with myofibers. However, BMSC participation in myofiber reconstruction increased if they were modified by NICD or PAX3 overexpression. Nevertheless, BMSCs paracrine function could play a positive role in skeletal muscle regeneration. Previously, we showed that SDF-1 treatment and coculture with myofibers increased BMSC ability to reconstruct myofibers. We also noticed that SDF-1 treatment changed selected miRNAs expression, including miR151 and miR5100.

Methods

Mouse BMSCs were transfected with miR151 and miR5100 mimics and their proliferation, myogenic differentiation, and fusion with myoblasts were analyzed.

Results

We showed that miR151 and miR5100 played an important role in the regulation of BMSC proliferation and migration. Moreover, the presence of miR151 and miR5100 transfected BMSCs in co-cultures with human myoblasts increased their fusion. This effect was achieved in an IGFBP2 dependent manner.

Conclusions

Mouse BMSCs did not present naïve myogenic potential but secreted proteins could impact myogenic cell differentiation. miR151 and miR5100 transfection changed BMSC migration and IGFBP2 and MMP12 expression in BMSCs. miR151 and miR5100 transfected BMSCs increased myoblast fusion in vitro.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s12015-022-10350-y.

The Role of MSCs and Cell Fusion in Tissue Regeneration

Regenerative medicine is concerned with the investigation of therapeutic agents that can be used to promote the process of regeneration after injury or in different diseases. Mesenchymal stem/stromal cells (MSCs) and their secretome—including extracellular vesicles (EVs) are of great interest, due to their role in tissue regeneration, immunomodulatory capacity and low immunogenicity. So far, clinical studies are not very conclusive as they show conflicting efficacies regarding the use of MSCs. An additional process possibly involved in regeneration might be cell fusion. This process occurs in both a physiological and a pathophysiological context and can be affected by immune response due to inflammation. In this review the role of MSCs and cell fusion in tissue regeneration is discussed.

Hypoxia preconditioned bone marrow-derived mesenchymal stromal/stem cells enhance myoblast fusion and skeletal muscle regeneration

Background

The skeletal muscle reconstruction occurs thanks to unipotent stem cells, i.e., satellite cells. The satellite cells remain quiescent and localized between myofiber sarcolemma and basal lamina. They are activated in response to muscle injury, proliferate, differentiate into myoblasts, and recreate myofibers. The stem and progenitor cells support skeletal muscle regeneration, which could be disturbed by extensive damage, sarcopenia, cachexia, or genetic diseases like dystrophy. Many lines of evidence showed that the level of oxygen regulates the course of cell proliferation and differentiation.

Methods

In the present study, we analyzed hypoxia impact on human and pig bone marrow-derived mesenchymal stromal cell (MSC) and mouse myoblast proliferation, differentiation, and fusion. Moreover, the influence of the transplantation of human bone marrow-derived MSCs cultured under hypoxic conditions on skeletal muscle regeneration was studied.

Results

We showed that bone marrow-derived MSCs increased VEGF expression and improved myogenesis under hypoxic conditions in vitro. Transplantation of hypoxia preconditioned bone marrow-derived MSCs into injured muscles resulted in the improved cell engraftment and formation of new vessels.

Conclusions

We suggested that SDF-1 and VEGF secreted by hypoxia preconditioned bone marrow-derived MSCs played an essential role in cell engraftment and angiogenesis. Importantly, hypoxia preconditioned bone marrow-derived MSCs more efficiently engrafted injured muscles; however, they did not undergo myogenic differentiation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02530-3.

Muscular Contribution to Adolescent Idiopathic Scoliosis from the Perspective of Stem Cell-Based Regenerative Medicine

Adolescent idiopathic scoliosis (AIS) is a relatively frequent disease within a range 0.5%-5.0% of population, with higher frequency in females. While a resultant spinal deformity is usually medically benign condition, it produces far going psychosocial consequences, which warrant attention. The etiology of AIS is unknown and current therapeutic approaches are symptomatic only, and frequently inconvenient or invasive. Muscular contribution to AIS is widely recognized, although it did not translate to clinical routine as yet. Muscle asymmetry has been documented by pathological examinations as well as systemic muscle disorders frequently leading to scoliosis. It has been also reported numerous genetic, metabolic and radiological alterations in patients with AIS, which are linked to muscular and neuromuscular aspects. Therefore, muscles might be considered an attractive and still insufficiently exploited therapeutic target for AIS. Stem cell-based regenerative medicine is rapidly gaining momentum based on the tremendous progress in understanding of developmental biology. It comes also with a toolbox of various stem cells such as satellite cells or mesenchymal stem cells, which could be transplanted; also, the knowledge acquired in research on regenerative medicine can be applied to manipulation of endogenous stem cells to obtain desired therapeutic goals. Importantly, paravertebral muscles are located relatively superficially; therefore, they can be an easy target for minimally invasive approaches to treatment of AIS. It comes in pair with a fast progress in image guidance, which allows for precise delivery of therapeutic agents, including stem cells to various organs such as brain, muscles, and others. Summing up, it seems that there is a link between AIS, muscles, and stem cells, which might be worth of further investigations with a long-term goal of setting foundations for eventual bench-to-bedside translation.

Cell Fusion-Mediated Tissue Regeneration as an Inducer of Polyploidy and Aneuploidy

The biological phenomenon of cell fusion plays a crucial role in several physiological processes, including wound healing and tissue regeneration. Here, it is assumed that bone marrow-derived stem cells (BMSCs) could adopt the specific properties of a different organ by cell fusion, thereby restoring organ function. Cell fusion first results in the production of bi- or multinucleated hybrid cells, which either remain as heterokaryons or undergo ploidy reduction/heterokaryon-to-synkaryon transition (HST), thereby giving rise to mononucleated daughter cells. This process is characterized by a merging of the chromosomes from the previously discrete nuclei and their subsequent random segregation into daughter cells. Due to extra centrosomes concomitant with multipolar spindles, the ploidy reduction/HST could also be associated with chromosome missegregation and, hence, induction of aneuploidy, genomic instability, and even putative chromothripsis. However, while the majority of such hybrids die or become senescent, aneuploidy and genomic instability appear to be tolerated in hepatocytes, possibly for stress-related adaption processes. Likewise, cell fusion-induced aneuploidy and genomic instability could also lead to a malignant conversion of hybrid cells. This can occur during tissue regeneration mediated by BMSC fusion in chronically inflamed tissue, which is a cell fusion-friendly environment, but is also enriched for mutagenic reactive oxygen and nitrogen species.

Lack of a skeletal muscle phenotype in adult human bone marrow stromal cells following xenogeneic-free expansion

Background

Many studies have elegantly shown that murine and rat bone marrow-derived mesenchymal stromal cells (bmMSCs) contribute to muscle regeneration and improve muscle function. Yet, the ability of transplanted human bmMSCs to manifest myogenic potential shows conflicting results. While human adipose- and umbilical cord-derived MSCs can be differentiated into a skeletal muscle phenotype using horse serum (HS), bmMSCs have only been shown to differentiate towards the skeletal muscle lineage using a complex mixture of cytokines followed by transfection with notch intracellular domain.

Methods

Since xenogeneic-free growth supplements are increasingly being used in the expansion of bmMSCs in clinical trials, we investigated the effects of human plasma and platelet lysate (P/PL) on the expression of neuromuscular markers and whether P/PL-expanded human bmMSCs could be differentiated towards a skeletal myogenic phenotype. Neuromuscular markers were measured using the highly sensitive droplet digital polymerase chain reaction for measuring the expression of Myf5, MyoD, MyoG, ACTA1, Desmin, GAP-43, and Coronin 1b transcripts, by performing immunofluorescence for the expression of Desmin, GAP-43, and MEF2, and flow cytometry for the expression of CD56/neural cell adhesion molecule (NCAM).

Results

Despite that bmMSCs expressed the myogenic regulatory factor (MRF) MEF2 after expansion in P/PL, bmMSCs cultured under such conditions did not express other essential MRFs including Myf5, MyoD, MyoG, or ACTA1 needed for myogenesis. Moreover, HS did not induce myogenesis of bmMSCs and hence did not induce the expression of any of these myogenic markers. P/PL, however, did lead to a significant increase in neurogenic GAP-43, as well as Desmin expression, and resulted in a high baseline expression of the neurogenic gene Coronin 1b which was sustained under further P/PL or HS culture conditions. Fetal bovine serum resulted in equally high levels of GAP-43 and Coronin 1b. Moreover, the proportion of CD56/NCAM-positive bmMSCs cultured in P/PL was 5.9 ± 2.1.

Conclusions

These data suggest that P/PL may prime a small portion of bmMSCs towards an early neural precursor cell type. Collectively, this shows that P/PL partially primes the cells towards a neurogenic phenotype, but does not prime adult human bmMSCs towards the skeletal muscle lineage.

The factors present in regenerating muscles impact bone marrow-derived mesenchymal stromal/stem cell fusion with myoblasts

BACKGROUND

Satellite cells, a population of unipotent stem cells attached to muscle fibers, determine the excellent regenerative capability of injured skeletal muscles. Myogenic potential is also exhibited by other cell populations, which exist in the skeletal muscles or come from other niches. Mesenchymal stromal/stem cells inhabiting the bone marrow do not spontaneously differentiate into muscle cells, but there is some evidence that they are capable to follow the myogenic program and/or fuse with myoblasts.

METHODS

In the present study we analyzed whether IGF-1, IL-4, IL-6, and SDF-1 could impact human and porcine bone marrow-derived mesenchymal stromal/stem cells (hBM-MSCs and pBM-MSCs) and induce expression of myogenic regulatory factors, skeletal muscle-specific structural, and adhesion proteins. Moreover, we investigated whether these factors could induce both types of BM-MSCs to fuse with myoblasts. IGF-1, IL-4, IL-6, and SDF-1 were selected on the basis of their role in embryonic myogenesis as well as skeletal muscle regeneration.

RESULTS

We found that hBM-MSCs and pBM-MSCs cultured in vitro in the presence of IGF-1, IL-4, IL-6, or SDF-1 did not upregulate myogenic regulatory factors. Consequently, we confirmed the lack of their naïve myogenic potential. However, we noticed that IL-4 and IL-6 impacted proliferation and IL-4, IL-6, and SDF-1 improved migration of hBM-MSCs. IL-4 treatment resulted in the significant increase in the level of mRNA encoding CD9, NCAM, VCAM, and m-cadherin, i.e., proteins engaged in cell fusion during myotube formation. Additionally, the CD9 expression level was also driven by IGF-1 treatment. Furthermore, the pre-treatment of hBM-MSCs either with IGF-1, IL-4, or SDF-1 and treatment of pBM-MSCs either with IGF-1 or IL-4 increased the efficacy of hybrid myotube formation between these cells and C2C12 myoblasts.

CONCLUSIONS

To conclude, our study revealed that treatment with IGF-1, IL-4, IL-6, or SDF-1 affects BM-MSC interaction with myoblasts; however, it does not directly promote myogenic differentiation of these cells.

Induction of bone marrow-derived cells myogenic identity by their interactions with the satellite cell niche

Background

Skeletal muscle regeneration is possible thanks to unipotent stem cells, which are satellite cells connected to the myofibers. Populations of stem cells other than muscle-specific satellite cells are considered as sources of cells able to support skeletal muscle reconstruction. Among these are bone marrow-derived mesenchymal stem cells (BM-MSCs), which are multipotent, self-renewing stem cells present in the bone marrow stroma. Available data documenting the ability of BM-MSCs to undergo myogenic differentiation are not definitive. In the current work, we aimed to check if the satellite cell niche could impact the ability of bone marrow-derived cells to follow a myogenic program.

Methods

We established a new in-vitro method for the coculture of bone marrow-derived cells (BMCs) that express CXCR4 (CXCR4⁺BMCs; the stromal-derived factor-1 (Sdf-1) receptor) with myofibers. Using various tests, we analyzed the myogenic identity of BMCs and their ability to fuse with myoblasts in vitro and in vivo.

Results

We showed that Sdf-1 treatment increased the number of CXCR4⁺BMCs able to bind the myofiber and occupy the satellite cell niche. Moreover, interaction with myofibers induced the expression of myogenic regulatory factors (MRFs) in CXCR4⁺BMCs. CXCR4⁺BMCs, pretreated by the coculture with myofibers and Sdf-1, participated in myotube formation in vitro and also myofiber reconstruction in vivo. We also showed that Sdf-1 overexpression in vivo (in injured and regenerating muscles) supported the participation of CXCR4⁺BMCs in new myofiber formation.

Conclusion

We showed that CXCR4⁺BMC interaction with myofibers (that is, within the satellite cell niche) induced CXCR4⁺BMC myogenic commitment. CXCR4⁺BMCs, pretreated using such a method of culture, were able to participate in skeletal muscle regeneration.

Differentiation and sarcomere formation in skeletal myocytes directly prepared from human induced pluripotent stem cells using a sphere-based culture

Human induced-pluripotent stem cells (iPSCs) are a promising resource for propagation of myogenic progenitors. Our group recently reported a unique protocol for the derivation of myogenic progenitors directly (without genetic modification) from human pluripotent cells using free-floating spherical culture. Here we expand our previous efforts and attempt to determine how differentiation duration, culture surface coatings, and nutrient supplements in the medium influence progenitor differentiation and formation of skeletal myotubes containing sarcomeric structures. A long differentiation period (over 6 weeks) promoted the differentiation of iPSC-derived myogenic progenitors and subsequent myotube formation. These iPSC-derived myotubes contained representative sarcomeric structures, consisting of organized myosin and actin filaments, and could spontaneously contract. We also found that a bioengineering approach using three-dimensional (3D) artificial muscle constructs could facilitate the formation of elongated myotubes. Lastly, we determined how culture surface coating matrices and different supplements would influence terminal differentiation. While both Matrigel and laminin coatings showed comparable effects on muscle differentiation, B27 serum-free supplement in the differentiation medium significantly enhanced myogenesis compared to horse serum. Our findings support the possibility to create an in vitro model of contractile sarcomeric myofibrils for disease modeling and drug screening to study neuromuscular diseases.

Effect of Bone Marrow Mesenchymal Stem Cells on Satellite Cell Proliferation and Apoptosis in Immobilization-Induced Muscle Atrophy in Rats

BACKGROUND Muscle atrophy due to disuse occurs along with adverse physiological and functional changes, but bone marrow stromal cells (MSCs) may be able to act as muscle satellite cells to restore myofibers. Thus, we investigated whether MSCs could enhance the proliferation of satellite cells and suppress myonuclear apoptosis during immobilization. MATERIAL AND METHODS We isolated, purified, amplified, and identified MSCs. Rats (n=48) were randomized into 3 groups: WB group (n=16), IM-PBS group (n=16), and IM-MSC (n=16). Rat hind limbs were immobilized for 14 d, treated with MSCs or phosphate-buffered saline (PBS), and then studied using immunohistochemistry and Western blot analysis to characterize the proteins involved. Apoptosis was assessed by terminal deoxynucleotidyl transferase (TdT)-mediated deoxy-UTP nick end labeling (TUNEL) method. RESULTS We compared muscle mass, cross-sectional areas, and peak tetanic forces and noted insignificant differences between PBS- and MSC-treated animals, but satellite cell proliferation was significantly greater after MSC treatment (p<0.05). Apoptotic myonuclei were reduced (p<0.05) after MSC treatment as well. Pro-apoptotic Bax was down-regulated and anti-apoptotic Bcl-2 and p-Akt protein were upregulated (p<0.05). CONCLUSIONS MSCs injected during hind limb immobilization can maintain satellite cell activity by suppressing myonuclear apoptosis.

Stem cells migration during skeletal muscle regeneration - the role of Sdf-1/Cxcr4 and Sdf-1/Cxcr7 axis

The skeletal muscle regeneration occurs due to the presence of tissue specific stem cells - satellite cells. These cells, localized between sarcolemma and basal lamina, are bound to muscle fibers and remain quiescent until their activation upon muscle injury. Due to pathological conditions, such as extensive injury or dystrophy, skeletal muscle regeneration is diminished. Among the therapies aiming to ameliorate skeletal muscle diseases are transplantations of the stem cells. In our previous studies we showed that Sdf-1 (stromal derived factor −1) increased migration of stem cells and their fusion with myoblasts in vitro. Importantly, we identified that Sdf-1 caused an increase in the expression of tetraspanin CD9 - adhesion protein involved in myoblasts fusion. In the current study we aimed to uncover the details of molecular mechanism of Sdf-1 action. We focused at the Sdf-1 receptors - Cxcr4 and Cxcr7, as well as signaling pathways induced by these molecules in primary myoblasts, as well as various stem cells - mesenchymal stem cells and embryonic stem cells, i.e. the cells of different migration and myogenic potential. We showed that Sdf-1 altered actin organization via FAK (focal adhesion kinase), Cdc42 (cell division control protein 42), and Rac-1 (Ras-Related C3 Botulinum Toxin Substrate 1). Moreover, we showed that Sdf-1 modified the transcription profile of genes encoding factors engaged in cells adhesion and migration. As the result, cells such as primary myoblasts or embryonic stem cells, became characterized by more effective migration when transplanted into regenerating muscle.

Immunomodulatory capacity of the local mesenchymal stem cells transplantation after severe skeletal muscle injury in female rats

CONTEXT

Cell therapy technique with stem cells is a very attractive strategy for the treatment of muscle disorders.

OBJECTIVE

The objective of this study was to investigate the mechanism of local transplantation of mesenchymal stem cells (MSCs) which could contribute to skeletal muscle healing.

MATERIALS AND METHODS

Female rats were divided into three equal groups as the following: group 1, the negative control group (untreated group), group 2, sham-treated group, rats with muscle injuries involving volumetric muscle loss (VML) of adductor brevis muscle and injected locally with phosphate-buffered saline (PBS) 0.5 ml without stem cells after 7 d of muscle injury, group 3, treated group, rats with VML and injected locally (intramuscular) with 1.5 × 10⁶ bone marrow MSCs suspended in PBS 0.5 ml (1) after 7 d of muscle tissue injury. All animals were sacrificed after 4 weeks of stem cell transplantation.

RESULTS

In vitro culture the morphology of MSCs reached confluence and appeared as long spindle in shape on 9-14 d. Most of the cells did not express the hematopoietic cell marker, CD34 and CD45 but expressed MSCs marker CD44, CD90 and CD105. The remarkable increase of proliferating cell nuclear antigen positive nucleus was recorded in MSCs group as compared to PBS group. After 28 d of injection, administration of only PBS into the site of muscle injury caused up-regulation in the levels of interleukins IL-1β, IL-6, tumor necrosis factor alpha (TNF-α), transforming growth factor beta (TGF-β1), interferon alpha (IFN-α) and down-regulate the level of IL-10 in muscular tissue comparing to the untreated control. Bone marrow MSCs + PBS injected at the site of muscle injury significantly down-regulate the inflammatory cytokines levels IL-1β and IL-6 and TNF-α, TGF-β1 and IFN-α and up-regulate the level of IL-10. Collagen concentrations in the injured skeletal muscle estimated by enzyme-linked immuno sorbent assay and stained with Masson trichrome stain were increased with PBS group and decreased after transplantation of bone marrow MSCs in the site of injury. Muscle sections stained with H&E showed a higher number of centronucleated regenerating myofibers in the stem-cell-treated group than in the (PBS) and untreated control group. Microvasculature of skeletal muscle was decreased as demonstrated by immunostaining technique for CD34 in PBS group from untreated control. The MSCs group showed angiogenesis and marked increase of skeletal muscle microvasculature than PBS group.

CONCLUSION

MSCs can modify the local immunological responses and improve muscle regeneration by suppressing of inflammatory cytokines, activating of the anti-inflammatory cytokine, restoration of muscle fibers and angiogenesis. By means of increase in TGF-β production in response to muscle injury prevent the repair of injured fibers and increase connective tissue production (collagen fibers), thus propagating skeletal muscle weakness and fibrosis whereas MSCs + PBS injected at the site of muscle injury significantly down-regulate (TGF-β1) and hence the level of collagen (fibrosis or scar areas). MSCs are able to block the fibrotic signaling cascade by declining TGF-β1 and scar areas in the injured muscle.

The Mutual Interactions between Mesenchymal Stem Cells and Myoblasts in an Autologous Co-Culture Model

Both myoblasts and mesenchymal stem cells (MSC) take part in the muscle tissue regeneration and have been used as experimental cellular therapy in muscular disorders treatment. It is possible that co-transplantation approach could improve the efficacy of this treatment. However, the relations between those two cell types are not clearly defined. The aim of this study was to determine the reciprocal interactions between myoblasts and MSC in vitro in terms of the features important for the muscle regeneration process. Primary caprine muscle-derived cells (MDC) and bone marrow-derived MSC were analysed in autologous settings. We found that MSC contribute to myotubes formation by fusion with MDC when co-cultured directly, but do not acquire myogenic phenotype if exposed to MDC-derived soluble factors only. Experiments with exposure to hydrogen peroxide showed that MSC are significantly more resistant to oxidative stress than MDC, but a direct co-culture with MSC does not diminish the cytotoxic effect of H2O2 on MDC. Cell migration assay demonstrated that MSC possess significantly greater migration ability than MDC which is further enhanced by MDC-derived soluble factors, whereas the opposite effect was not found. MSC-derived soluble factors significantly enhanced the proliferation of MDC, whereas MDC inhibited the division rate of MSC. To conclude, presented results suggest that myogenic precursors and MSC support each other during muscle regeneration and therefore myoblasts-MSC co-transplantation could be an attractive approach in the treatment of muscular disorders.

Muscle Satellite Cells: Exploring the Basic Biology to Rule Them

Adult skeletal muscle is a postmitotic tissue with an enormous capacity to regenerate upon injury. This is accomplished by resident stem cells, named satellite cells, which were identified more than 50 years ago. Since their discovery, many researchers have been concentrating efforts to answer questions about their origin and role in muscle development, the way they contribute to muscle regeneration, and their potential to cell-based therapies. Satellite cells are maintained in a quiescent state and upon requirement are activated, proliferating, and fusing with other cells to form or repair myofibers. In addition, they are able to self-renew and replenish the stem pool. Every phase of satellite cell activity is highly regulated and orchestrated by many molecules and signaling pathways; the elucidation of players and mechanisms involved in satellite cell biology is of extreme importance, being the first step to expose the crucial points that could be modulated to extract the optimal response from these cells in therapeutic strategies. Here, we review the basic aspects about satellite cells biology and briefly discuss recent findings about therapeutic attempts, trying to raise questions about how basic biology could provide a solid scaffold to more successful use of these cells in clinics.

Strategies for skeletal muscle tissue engineering: seed vs. soil

The most commonly used tissue engineering approach includes the ex vivo combination of site-appropriate cell(s) and scaffold material(s) to create three-dimensional constructs for tissue replacement or reconstruction. These three-dimensional combinations are typically subjected to a period of culture and conditioning (i.e., self-assembly and maturation) to promote the development of ex vivo constructs which closely mimic native target tissue. This cell-based approach is challenged by the host response to the engineered tissue construct following surgical implantation. As an alternative to the cell-based approach, acellular biologic scaffolds attract endogenous cells and remodel into partially functional mimics of native tissue upon implantation. The present review examines cell-types (i.e., seed), scaffold materials (i.e., soil), and challenges associated with functional tissue engineering. Skeletal muscle is used as the target tissue prototype but the discussed principles will largely apply to most body systems.

Barriers in contribution of human mesenchymal stem cells to murine muscle regeneration

AIM: To study regeneration of damaged human and murine muscle implants and the contribution of added xenogeneic mesenchymal stem cells (MSCs).

METHODS: Minced human or mouse skeletal muscle tissues were implanted together with human or mouse MSCs subcutaneously on the back of non-obese diabetic/severe combined immunodeficient mice. The muscle tissues (both human and murine) were minced with scalpels into small pieces (< 1 mm³) and aliquoted in portions of 200 mm³. These portions were either cryopreserved in 10% dimethylsulfoxide or freshly implanted. Syngeneic or xenogeneic MSCs were added to the minced muscles directly before implantation. Implants were collected at 7, 14, 30 or 45 d after transplantation and processed for (immuno)histological analysis. The progression of muscle regeneration was assessed using a standard histological staining (hematoxylin-phloxin-saffron). Antibodies recognizing Pax7 and von Willebrand factor were used to detect the presence of satellite cells and blood vessels, respectively. To enable detection of the bone marrow-derived MSCs or their derivatives we used MSCs previously transduced with lentiviral vectors expressing a cytoplasmic LacZ gene. X-gal staining of the fixed tissues was used to detect β-galactosidase-positive cells and myofibers.

RESULTS: Myoregeneration in implants of fresh murine muscle was evident as early as day 7, and progressed with time to occupy 50% to 70% of the implants. Regeneration of fresh human muscle was slower. These observations of fresh muscle implants were in contrast to the regeneration of cryopreserved murine muscle that proceeded similarly to that of fresh tissue except for day 45 (P < 0.05). Cryopreserved human muscle showed minimal regeneration, suggesting that the freezing procedure was detrimental to human satellite cells. In fresh and cryopreserved mouse muscle supplemented with LacZ-tagged mouse MSCs, β-galactosidase-positive myofibers were identified early after grafting at the well-vascularized periphery of the implants. The contribution of human MSCs to murine myofiber formation was, however, restricted to the cryopreserved mouse muscle implants. This suggests that fresh murine muscle tissue provides a suboptimal environment for maintenance of human MSCs. A detailed analysis of the histological sections of the various muscle implants revealed the presence of cellular structures with a deviating morphology. Additional stainings with alizarin red and alcian blue showed myofiber calcification in 50 of 66 human muscle implants, and encapsulated cartilage in 10 of 81 of murine muscle implants, respectively.

CONCLUSION: In mouse models the engagement of human MSCs in myoregeneration might be underestimated. Furthermore, our model permits the dissection of species-specific factors in the microenvironment.

Uniaxial cyclic strain of human adipose-derived mesenchymal stem cells and C2C12 myoblasts in coculture

Tissue engineering skeletal muscle in vitro is of great importance for the production of tissue-like constructs for treating tissue loss due to traumatic injury or surgery. However, it is essential to find new sources of cells for muscle engineering as efficient in vitro expansion and culture of primary myoblasts are problematic. Mesenchymal stem cells may be a promising source of myogenic progenitor cells and may be harvested in large numbers from adipose tissue. As skeletal muscle is a mechanically dynamic tissue, we have investigated the effect of cyclic mechanical strain on the myogenic differentiation of a coculture system of murine C2C12 myoblasts and human adipose-derived mesenchymal stem cells. Fusion of mesenchymal stem cells with nascent myotubes and expression of human sarcomeric proteins was observed, indicating the potential for myogenic differentiation of human mesenchymal stem cells. Cyclic mechanical strain did not affect the fusion of mesenchymal stem cells, but maturation of myotubes was perturbed.

Recent Patents Pertaining to Immune Modulation and Musculoskeletal Regeneration with Wharton's Jelly Cells

Umbilical cord mesenchymal stromal cells (UCMSCs) are isolated from Wharton's jelly in the umbilical cord at birth, and offer advantages over adult mesenchymal stromal cells (MSCs) such as highly efficient isolation, faster proliferation in vitro, a broader differentiation potential, and non-invasive harvesting procedure. Their expansion and differentiation potential renders them a promising cell source for tissue engineering and clinical applications. This review discusses recent updates on the differentiation strategies for musculoskeletal tissue engineering including cartilage, bone, and muscle. In addition to tissue engineering applications, UCMSCs can be utilized to support hematopoiesis and modulate immune response. We review the patents relevant to the application of MSCs including UCMSCs in hematopoiesis and immune modulation. Finally, the current hurdles in the clinical translation of UCMSCs are discussed. During clinical translation, it is critical to develop large-scale manufacturing of UCMSCs as well as the composition of expansion and differentiation media. Four clinical trials to date have examined the safety and efficacy of UCMSCs. Once public banking of UCMSCs is available to supply matched allogeneic units and once UCMSC manufacturing is standardized, we anticipate that UCMSCs will be more widely used in clinical trials.

Skeletal muscle tissue engineering: which cell to use?

Tissue-engineered skeletal muscle is urgently required to treat a wide array of devastating congenital and acquired conditions. Selection of the appropriate cell type requires consideration of several factors which amongst others include, accessibility of the cell source, in vitro myogenicity at high efficiency with the ability to maintain differentiation over extended periods of time, susceptibility to genetic manipulation, a suitable mode of delivery and finally in vivo differentiation giving rise to restoration of structural morphology and function. Potential stem-progenitor cell sources include and are not limited to satellite cells, myoblasts, mesoangioblasts, pericytes, muscle side-population cells, CD133(+) cells, in addition to embryonic stem cells, mesenchymal stem cells, amniotic fluid stem cells and induced pluripotent stem (iPS) cells. The relative merits and inherent limitations of these cell types within the field of tissue-engineering are discussed in the light of current research. Recent advances in the field of iPS cells should bear the fruits for some exciting developments within the field in the forthcoming years.

Satellite cells and the muscle stem cell niche

Adult skeletal muscle in mammals is a stable tissue under normal circumstances but has remarkable ability to repair after injury. Skeletal muscle regeneration is a highly orchestrated process involving the activation of various cellular and molecular responses. As skeletal muscle stem cells, satellite cells play an indispensible role in this process. The self-renewing proliferation of satellite cells not only maintains the stem cell population but also provides numerous myogenic cells, which proliferate, differentiate, fuse, and lead to new myofiber formation and reconstitution of a functional contractile apparatus. The complex behavior of satellite cells during skeletal muscle regeneration is tightly regulated through the dynamic interplay between intrinsic factors within satellite cells and extrinsic factors constituting the muscle stem cell niche/microenvironment. For the last half century, the advance of molecular biology, cell biology, and genetics has greatly improved our understanding of skeletal muscle biology. Here, we review some recent advances, with focuses on functions of satellite cells and their niche during the process of skeletal muscle regeneration.

Reprogramming adult Schwann cells to stem cell-like cells by leprosy bacilli promotes dissemination of infection

Differentiated cells possess a remarkable genomic plasticity that can be manipulated to reverse or change developmental commitments. Here, we show that the leprosy bacterium hijacks this property to reprogram adult Schwann cells, its preferred host niche, to a stage of progenitor/stem-like cells (pSLC) of mesenchymal trait by downregulating Schwann cell lineage/differentiation-associated genes and upregulating genes mostly of mesoderm development. Reprogramming accompanies epigenetic changes and renders infected cells highly plastic, migratory, and immunomodulatory. We provide evidence that acquisition of these properties by pSLC promotes bacterial spread by two distinct mechanisms: direct differentiation to mesenchymal tissues, including skeletal and smooth muscles, and formation of granuloma-like structures and subsequent release of bacteria-laden macrophages. These findings support a model of host cell reprogramming in which a bacterial pathogen uses the plasticity of its cellular niche for promoting dissemination of infection and provide an unexpected link between cellular reprogramming and host-pathogen interaction.

Pharmacologically active microcarriers influence VEGF-A effects on mesenchymal stem cell survival

Resistance of transplanted mesenchymal stem cells (MSCs) in post-ischemic heart is limited by their poor vitality. Vascular-endothelial-growth-factor-A (VEGF-A) as such or slowly released by fibronectin-coated pharmacologically-active-microcarriers (FN-PAM-VEGF) could differently affect survival kinases and anti-apoptotic mediator (e.g. Bcl-2). Therefore VEGF-A or FN-PAM-VEGF could differently enhance cell proliferation, and/or resistance to hypoxia/reoxygenation (H/R) of MSCs. To test these hypotheses MSCs were incubated for 6-days with VEGF-A alone or with FN-PAM-VEGF. In addition, MSCs pre-treated for 24-hrs with VEGF-A or FN-PAM-VEGF were subsequently exposed to H/R (72-hrs 3% O₂ and 3-hrs of reoxygenation). Cell-proliferation and post-hypoxic vitality were determined. Kinases were studied at 30-min., 1- and 3-days of treatment. Cell-proliferation increased about twofold (P < 0.01) 6-days after VEGF-A treatment, but by a lesser extent (55% increase) with FN-PAM-VEGF (P < 0.05). While MSC pre-treatment with VEGF-A confirmed cell-proliferation, pre-treatment with FN-PAM-VEGF protected MSCs against H/R. In the early phase of treatments, VEGF-A increased phospho-Akt, phospho-ERK-1/2 and phospho-PKCε compared to the untreated cells or FN-PAM-VEGF. Afterword, kinase phosphorylations were higher with VGEF, except for ERK-1/2, which was similarly increased by both treatments at 3 days. Only FN-PAM-VEGF significantly increased Bcl-2 levels. After H/R, lactate dehydrogenase release and cleaved Caspase-3 levels were mainly reduced by FN-PAM-VEGF. While VEGF-A enhances MSC proliferation in normoxia, FN-PAM-VEGF mainly hampers post-hypoxic MSC death. These different effects underscore the necessity of approaches suited to the various conditions. The use of FN-PAM-VEGF could be considered as a novel approach for enhancing MSC survival and regeneration in hostile environment of post-ischemic tissues.

Adipose-derived stem-cell treatment of skeletal muscle injury

BACKGROUND

The aim of the present study was to investigate whether adipose-derived stem cells could contribute to skeletal muscle-healing.

METHODS

Adipose-derived stem cells of male rats were cultured and injected into the soleus muscles of female rats. Two and four weeks after injections, muscles were tested for tetanic force (50 Hz). Histological analysis was performed to evaluate muscle collagen deposition and the number of centronucleated muscle fibers. In order to track donor cells, chimerism was detected with use of real-time polymerase chain reaction targeting the male sex-determining region Y (SRY) gene.

RESULTS

Two weeks after cell injection, tetanus strength and the number of centronucleated regenerating myofibers, as well as the number of centronucleated regenerating myofibers, were higher in the treated group than they were in the control group (mean and standard error of the mean, 79.2 ± 5.0% versus 58.3 ± 8.1%, respectively [p < 0.05]; and 145 ± 36 versus 273 ± 18 per 10³ myofibers, respectively [p < 0.05]). However, there were no significant differences at four weeks. Treatment did not decrease collagen deposition. Male gene was not detected in female host tissue at two and four weeks after engraftment by polymerase chain reaction analysis.

CONCLUSIONS

Adipose-derived stem-cell therapy increased muscle repair and force at two weeks, but not four weeks, after injection, suggesting that adipose-derived stem-cell administration may accelerate muscle repair; however, the rapid disappearance of injected cells suggests a paracrine mechanism of action.

Myogenic Properties of Human Mesenchymal Stem Cells Derived from Three Different Sources

Mesenchymal stem cells (MSCs) of mammals have been isolated from many tissues and are characterized by their aptitude to differentiate into bone, cartilage, and fat. Differentiation into cells of other lineages like skeletal muscle, tendon/ligament, nervous tissue, and epithelium has been attained with MSCs derived from some tissues. Whether such abilities are shared by MSCs of all tissues is unknown. We therefore compared for three human donors the myogenic properties of MSCs from adipose tissue (AT), bone marrow (BM), and synovial membrane (SM). Our data show that human MSCs derived from the three tissues differ in phenotype, proliferation capacity, and differentiation potential. The division rate of AT-derived MSCs (AT-MSCs) was distinctly higher than that of MSCs from the other two tissue sources. In addition, clear donor-specific differences in the long-term maintenance of MSC proliferation ability were observed. Although similar in their in vitro fusogenic capacity with murine myoblasts, MSCs of the three sources contributed to a different extent to skeletal muscle regeneration in vivo. Transplanting human AT-, BM-, or SM-MSCs previously transduced with a lentiviral vector encoding β-galactosidase into cardiotoxin-damaged tibialis anterior muscles (TAMs) of immunodeficient mice revealed that at 30 days after treatment the frequency of hybrid myofibers was highest in the TAMs treated with AT-MSCs. Our finding of human-specific β-spectrin and dystrophin in hybrid myofibers containing human nuclei argues for myogenic programming of MSCs in regenerating murine skeletal muscle. For the further development of MSC-based treatments of myopathies, AT-MSCs appear to be the best choice in view of their efficient contribution to myoregeneration, their high ex vivo expansion potential, and because their harvesting is less demanding than that of BM- or SM-MSCs.

Mouse and human pluripotent stem cells and the means of their myogenic differentiation

Pluripotent stem cells, such as embryonic stem cells and induced pluripotent stem cells, are an important tool in the studies focusing at the differentiation of various cell types, including skeletal myoblasts. They are also considered as a source of the cells that due to their pluripotent character and availability could be turned into any required tissue and then used in future in regenerative medicine. However, the methods of the derivation of some of cell types from pluripotent cells still need to be perfected. This chapter summarizes the history and current advancements in the derivation and testing of pluripotent stem cells-derived skeletal myoblasts. It focuses at the in vitro methods allowing the differentiation of stem cells grown in monolayer or propagated as embryoid bodies, and also at in vivo tests allowing the verification of the functionality of obtained skeletal myoblasts.

Dental follicle cells and treated dentin matrix scaffold for tissue engineering the tooth root

Tissue engineering strategies to reconstruct tooth roots are an effective therapy for the treatment of tooth loss. However, strategies to successfully regenerate tooth roots have not been developed and optimized. In the present study, rat dental follicle stem cells (DFCs) were characterized, followed by a thorough investigation of tooth roots regeneration for a combination of DFCs seeding cells, treated dentin matrix (TDM) scaffolds, and an inductive alveolar fossa microenvironment. Eighteen clones derived from single DFCs were harvested; however, only three clones were amplified successfully more than five passages and 90-95 days in culture. Following 270 days or 30 passages, the heterogeneous DFCs showed suitable characteristics for seeding cells to regenerate tooth roots. However, various features, such as variable proliferation rates, differentiation characteristics, apoptosis rates, and total lifespan were observed in DFCs and the three clones. Importantly, upon transplantation of DFCs combined with TDM for four weeks, root-like tissues stained positive for markers of dental pulp and periodontal tissues were regenerated in the alveolar fossa, but not in the skull and omental pockets. These results indicate that tooth roots were successfully regenerated and suggest that the combination of DFCs with TDM in the alveolar fossa is a feasible strategy for tooth roots regeneration. This strategy could be a promising approach for the treatment of clinical tooth loss and provides a perspective with potential applications to regeneration of other tissues and organs.

Potential benefits of allogeneic bone marrow mesenchymal stem cells for wound healing

INTRODUCTION

It is becoming increasingly evident that select adult stem cells have the capacity to participate in repair and regeneration of damaged and/or diseased tissues. Mesenchymal stem cells have been among the most studied adult stem cells for the treatment of a variety of conditions, including wound healing.

AREAS COVERED

Mesenchymal stem cell features potentially beneficial to cutaneous wound healing applications are reviewed.

EXPERT OPINION

Given their potential for in vitro expansion and immune modulatory effects, both autologous and allogeneic mesenchymal stem cells appear to be well suited as wound healing therapies. Allogeneic mesenchymal stem cells derived from young healthy donors could have particular advantage over autologous sources where age and systemic disease can be significant factors.

Human epicardium-derived cells fuse with high efficiency with skeletal myotubes and differentiate toward the skeletal muscle phenotype: a comparison study with stromal and endothelial cells

EPDCs fuse with skeletal myotubes with higher efficiency when compared to MSCs and endothelial cells. Independently from the cell origin, all nuclei recruited inside myotubes express muscle-specific genes. VCAM1 expression in nonmuscle cells is induced by soluble factors secreted by myotubes, and its function is required for fusion to occur.

Recent studies have underscored a role for the epicardium as a source of multipotent cells. Here, we investigate the myogenic potential of adult human epicardium-derived cells (EPDCs) and analyze their ability to undergo skeletal myogenesis when cultured with differentiating primary myoblasts. Results are compared to those obtained with mesenchymal stromal cells (MSCs) and with endothelial cells, another mesodermal derivative. We demonstrate that EPDCs spontaneously fuse with pre-existing myotubes with an efficiency that is significantly higher than that of other cells. Although at a low frequency, endothelial cells may also contribute to myotube formation. In all cases analyzed, after entering the myotube, nonmuscle nuclei are reprogrammed to express muscle-specific genes. The fusion competence of nonmyogenic cells in vitro parallels their ability to reconstitute dystrophin expression in mdx mice. We additionally show that vascular cell adhesion molecule 1 (VCAM1) expression levels of nonmuscle cells are modulated by soluble factors secreted by skeletal myoblasts and that VCAM1 function is required for fusion to occur. Finally, treatment with interleukin (IL)-4 or IL-13, two cytokines released by differentiating myotubes, increases VCAM1 expression and enhances the rate of fusion of EPDCs and MSCs, but not that of endothelial cells.

Transcription factor rational design improves directed differentiation of human mesenchymal stem cells into skeletal myocytes

There is great interest in transdifferentiating cells from one lineage into those of another and in dedifferentiating mature cells back into a stem/progenitor cell state by deploying naturally occurring transcription factors (TFs). Often, however, steering cellular differentiation pathways in a predictable and efficient manner remains challenging. Here, we investigated the principle of combining domains from different lineage-specific TFs to improve directed cellular differentiation. As proof-of-concept, we engineered the whole-human TF MyoDCD, which has the NH(2)-terminal transcription activation domain (TAD) and adjacent DNA-binding motif of MyoD COOH-terminally fused to the TAD of myocardin (MyoCD). We found via reporter gene and marker protein assays as well as by a cell fusion readout system that, targeting the TAD of MyoCD to genes normally responsive to the skeletal muscle-specific TF MyoD enforces more robust myogenic reprogramming of nonmuscle cells than that achieved by the parental, prototypic master TF, MyoD. Human mesenchymal stem cells (hMSCs) transduced with a codon-optimized microdystrophin gene linked to a synthetic striated muscle-specific promoter and/or with MyoD or MyoDCD were evaluated for complementing the genetic defect in Duchenne muscular dystrophy (DMD) myocytes through heterotypic cell fusion. Cotransduction of hMSCs with MyoDCD and microdystrophin led to chimeric myotubes containing the highest dystrophin levels.

Stem cell homing in musculoskeletal injury

The regenerative potential of injured adult tissue suggests the physiological existence of cells capable of participating in the reparative process. Recent studies indicate that stem-like cells residing in tissues contribute to tissue repair and are replenished by precursor bone marrow-derived cells. Mesenchymal stromal cells (MSC) are among the candidates for reparative cells. These cells can potentially be mobilized into the circulation in response to injury signals and exert their reparative effects at the site of injury. Current therapies for musculoskeletal injuries pose unavoidable risks which can impede full recovery. Trafficking of MSC to the injury site and their subsequent participation in the regenerative process is thought to be a natural healing response that can be imitated or augmented by enhancing the endogenous MSC pool with exogenously administered MSC. Therefore, a promising alternative to the existing strategies employed in the treatment of musculoskeletal injuries is to reinforce the inherent reparative capacity of the body by delivering MSC harvested from the patient's own tissues to the site of injury. The aim of this review is to inform the reader of studies that have evaluated the intrinsic homing and regenerative abilities of MSC, with particular emphasis on the repair of musculoskeletal injuries. Research that supports the direct use of MSC (without in vitro differentiation into tissue-specific cells) will also be reported. Based on accruing evidence that the natural healing mechanism involves the recruitment of MSC and their subsequent reparative actions at the site of injury, as well as documented therapeutic response after the exogenous administration of MSC, the feasibility of the emerging strategy of instant stem-cell therapy will be proposed.

Evolution of programmed cell fusion: common mechanisms and distinct functions

Eukaryotic cells have evolved diverged mechanisms to merge cells. Here, we discuss three types of cell fusion: (1) Non-self-fusion, cells with different genetic contents fuse to start a new organism and fusion between enveloped viruses and host cells; (2) Self-fusion, genetically identical cells fuse to form a multinucleated cell; and (3) Auto-fusion, a single cell fuses with itself by bringing specialized cell membrane domains into contact and transforming itself into a ring-shaped cell. This is a new type of selfish fusion discovered in C. elegans. We divide cell fusion into three stages: (1) Specification of the cell-fusion fate; (2) Cell attraction, attachment, and recognition; (3) Execution of plasma membrane fusion, cytoplasmic mixing and cytoskeletal rearrangements. We analyze cell fusion in diverse biological systems in development and disease emphasizing the mechanistic contributions of C. elegans to the understanding of programmed cell fusion, a genetically encoded pathway to merge specific cells.

Long-term contribution of human bone marrow mesenchymal stromal cells to skeletal muscle regeneration in mice

Mesenchymal stromal cells (MSCs) are attractive for cellular therapy of muscular dystrophies as they are easy to procure, can be greatly expanded ex vivo, and contribute to skeletal muscle repair in vivo. However, detailed information about the contribution of bone marrow (BM)-derived human MSCs (BM-hMSCs) to skeletal muscle regeneration in vivo is very limited. Here, we present the results of a comprehensive study of the fate of LacZ-tagged BM-hMSCs following implantation in cardiotoxin (CTX)-injured tibialis anterior muscles (TAMs) of immunodeficient mice. β-Galactosidase-positive (β-gal(+)) human-mouse hybrid myofibers (HMs) were counted in serial cross sections over the full length of the treated TAMs of groups of mice at monthly intervals. The number of human cells was estimated using chemiluminescence assays. While the number of human cells declined gradually to about 10% of the injected cells at 60 days after transplantation, the number of HMs increased from day 10 onwards, reaching 104 ± 39.1 per TAM at 4 months postinjection. β-gal(+) cells and HMs were distributed over the entire muscle, indicating migration of the former from the central injection site to the ends of the TAMs. The identification of HMs that stained positive for human spectrin suggests myogenic reprogramming of hMSC nuclei. In summary, our findings reveal that BM-hMSCs continue to participate in the regeneration/remodeling of CTX-injured TAMs, resulting in ±5% HMs at 4 months after damage induction. Moreover, donor-derived cells were shown to express genetic information, both endogenous and transgenic, in recipient myofibers.

Bimolecular fluorescence complementation analysis of eukaryotic fusion products

BACKGROUND INFORMATION

Cell fusion is known to underlie key developmental processes in humans and is postulated to contribute to tissue maintenance and even carcinogenesis. The mechanistic details of cell fusion, especially between different cell types, have been difficult to characterize because of the dynamic nature of the process and inadequate means to track fusion products over time. Here we introduce an inducible system for detecting and tracking live cell fusion products in vitro and potentially in vivo. This system is based on BiFC (bimolecular fluorescence complementation) analysis. In this approach, two proteins that can interact with each other are joined to fragments of a fluorescent protein and are expressed in separate cells. The interaction of said proteins after cell fusion produces a fluorescent signal, enabling the identification and tracking of fusion products over time.

RESULTS

Long-term tracking of fused p53-deficient cells revealed that hybrid cells were capable of proliferation. In some cases, proliferation was preceded by nuclear fusion and division was asymmetric (69%+/-2% of proliferating hybrids), suggesting chromosomal instability. In addition, asymmetric division following proliferation could give rise to progeny indistinguishable from unfused counterparts.

CONCLUSIONS

These results support the possibility that the chromosomal instability characteristic of tumour cells may be incurred as a consequence of cell fusion and suggest that the role of cell fusion in carcinogenesis may have been masked to this point for lack of an inducible method to track cell fusion. In sum, the BiFC-based approach described here allows for comprehensive studies of the mechanism and biological impact of cell fusion in nature.

I-Z-I complexes in congenital myopathy

A 3-month-old boy with hypotonia at birth succumbed to a congenital myopathy. The major finding in his muscle biopsy corresponded to I-Z-I complexes described previously in embryonic skeletal muscle. A few previous myopathy cases have described findings suggestive of I-Z-I-like complexes. A mutation affecting mononuclear myoblasts or early myotubes was suspected, although an acquired lesion could not be ruled out. The findings may also have been altered by secondary events in this unusual case.

Bone marrow transplantation results in human donor blood cells acquiring and displaying mouse recipient class I MHC and CD45 antigens on their surface

Background

Mouse models of human disease are invaluable for determining the differentiation ability and functional capacity of stem cells. The best example is bone marrow transplants for studies of hematopoietic stem cells. For organ studies, the interpretation of the data can be difficult as transdifferentiation, cell fusion or surface antigen transfer (trogocytosis) can be misinterpreted as differentiation. These events have not been investigated in hematopoietic stem cell transplant models.

Methodology/Principal Findings

In this study we investigated fusion and trogocytosis involving blood cells during bone marrow transplantation using a xenograft model. We report that using a standard SCID repopulating assay almost 100% of the human donor cells appear as hybrid blood cells containing both mouse and human surface antigens.

Conclusion/Significance

Hybrid cells are not the result of cell-cell fusion events but appear to be due to efficient surface antigen transfer, a process referred to as trogocytosis. Antigen transfer appears to be non-random and includes all donor cells regardless of sub-type. We also demonstrate that irradiation preconditioning enhances the frequency of hybrid cells and that trogocytosis is evident in non-blood cells in chimera mice.

The origin, molecular regulation and therapeutic potential of myogenic stem cell populations

Satellite cells, originating in the embryonic dermamyotome, reside beneath the myofibre of mature adult skeletal muscle and constitute the tissue-specific stem cell population. Recent advances following the identification of markers for these cells (including Pax7, Myf5, c-Met and CD34) (CD, cluster of differentiation; c-Met, mesenchymal epithelial transition factor) have led to a greater understanding of the role played by satellite cells in the regeneration of new skeletal muscle during growth and following injury. In response to muscle damage, satellite cells harbour the ability both to form myogenic precursors and to self-renew to repopulate the stem cell niche following myofibre damage. More recently, other stem cell populations including bone marrow stem cells, skeletal muscle side population cells and mesoangioblasts have also been shown to have myogenic potential in culture, and to be able to form skeletal muscle myofibres in vivo and engraft into the satellite cell niche. These cell types, along with satellite cells, have shown potential when used as a therapy for skeletal muscle wasting disorders where the intrinsic stem cell population is genetically unable to repair non-functioning muscle tissue. Accurate understanding of the mechanisms controlling satellite cell lineage progression and self-renewal as well as the recruitment of other stem cell types towards the myogenic lineage is crucial if we are to exploit the power of these cells in combating myopathic conditions. Here we highlight the origin, molecular regulation and therapeutic potential of all the major cell types capable of undergoing myogenic differentiation and discuss their potential therapeutic application.

Enhancement of myogenic and muscle repair capacities of human adipose-derived stem cells with forced expression of MyoD

Muscle disorders such as Duchenne muscular dystrophy (DMD) still need effective treatments, and mesenchymal stem cells (MSCs) may constitute an attractive cell therapy alternative because they are multipotent and accessible in adult tissues. We have previously shown that human multipotent adipose-derived stem (hMADS) cells were able to restore dystrophin expression in the mdx mouse. The goal of this work was to improve the myogenic potential of hMADS cells and assess the impact on muscle repair. Forced expression of MyoD in vitro strongly induced myogenic differentiation while the adipogenic differentiation was inhibited. Moreover, MyoD-expressing hMADS cells had the capacity to fuse with DMD myoblasts and to restore dystrophin expression. Importantly, transplantation of these modified hMADS cells into injured muscles of immunodepressed Rag2(-/-)gammaC(-/-) mice resulted in a substantial increase in the number of hMADS cell-derived fibers. Our approach combined the easy access of MSCs from adipose tissue, the highly efficient lentiviral transduction of these cells, and the specific improvement of myogenic differentiation through the forced expression of MyoD. Altogether our results highlight the capacity of modified hMADS cells to contribute to muscle repair and their potential to deliver a repairing gene to dystrophic muscles.

Myogenic stem cells

Both skeletal muscle and bone marrow tissue contain myogenic stem cells. The population residing in muscles is heterogenic. Predominant in number are "typical" satellite cells - muscle progenitors migrating from somites during embryonic life. Another population is group of multipotent muscle stem cells which, at least in part, are derived from bone marrow. These cells are tracked by gradient of growth factors releasing from muscle during injury or exercise. Recruited bone marrow-derived cells gradually change their phenotype becoming muscle stem cells and eventually can attain satellite cell position and express Pax7 protein. Mesenchymal stem cells (MSC) isolated directly from bone marrow also display myogenic potential, although methods of induction of myogenic differentiation in vitro have not been optimized yet. Concerning efforts of exploiting myogenic stem cells in cell-mediated therapies it is important to understand the cause of impaired regenerative potential of aged muscle. Up to now, most of research data suggest that majority of age related changes in skeletal muscles are reversible, thus depending on extrinsic factors. However, irreversible intrinsic features of muscle stem cells are also taken into consideration.

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.

Multipotent adult progenitor cells sustain function of ischemic limbs in mice

Despite progress in cardiovascular research, a cure for peripheral vascular disease has not been found. We compared the vascularization and tissue regeneration potential of murine and human undifferentiated multipotent adult progenitor cells (mMAPC-U and hMAPC-U), murine MAPC-derived vascular progenitors (mMAPC-VP), and unselected murine BM cells (mBMCs) in mice with moderate limb ischemia, reminiscent of intermittent claudication in human patients. mMAPC-U durably restored blood flow and muscle function and stimulated muscle regeneration, by direct and trophic contribution to vascular and skeletal muscle growth. This was in contrast to mBMCs and mMAPC-VP, which did not affect muscle regeneration and provided only limited and transient improvement. Moreover, mBMCs participated in a sustained inflammatory response in the lower limb, associated with progressive deterioration in muscle function. Importantly, mMAPC-U and hMAPC-U also remedied vascular and muscular deficiency in severe limb ischemia, representative of critical limb ischemia in humans. Thus, unlike BMCs or vascular-committed progenitors, undifferentiated multipotent adult progenitor cells offer the potential to durably repair ischemic damage in peripheral vascular disease patients.

Genetic complementation of human muscle cells via directed stem cell fusion

Duchenne muscular dystrophy (DMD) is caused by mutations in the X chromosome-linked DMD gene, which encodes the sarcolemma-stabilizing protein-dystrophin. Initial attempts at DMD therapy deployed muscle progenitor cells from healthy donors. The utilization of these cells is, however, hampered by their immunogenicity, while those from DMD patients are scarce and display limited ex vivo replication. Nonmuscle cells with myogenic capacity may offer valuable alternatives especially if, to allow autologous transplantation, they are amenable to genetic intervention. As a paradigm for therapeutic gene transfer by heterotypic cell fusion we are investigating whether human mesenchymal stem cells (hMSCs) can serve as donors of recombinant DMD genes for recipient human muscle cells. Here, we show that forced MyoD expression in hMSCs greatly increases their tendency to participate in human myotube formation turning them into improved DNA delivery vehicles. Efficient loading of hMSCs with recombinant DMD was achieved through a new tropism-modified high-capacity adenoviral (hcAd) vector directing striated muscle-specific synthesis of full-length dystrophin. This study introduces the principle of genetic complementation of gene-defective cells via directed cell fusion and provides an initial framework to test whether transient MyoD synthesis in autologous, gene-corrected hMSCs increases their potential for treating DMD and, possibly, other muscular dystrophies.

Cells isolated from the epidermis by Hoechst dye exclusion, small size, and negative selection for hematopoietic markers can generate B lymphocyte precursors

Transdifferentiation has become a common claim for somatic stem cells, yet how such cells can be directed toward a specified cell lineage has not been well investigated. We previously demonstrated that when isolated epidermal stem cells were placed into an embryonic environment, their potential was extended beyond the keratinocyte lineage. Here, we present evidence that cells isolated using a modification of our published method for epidermal stem cells can be specifically directed to differentiate into B lymphocyte precursors. We found that these isolated cells co-cultured with S17 bone marrow stromal cells in cytokine-supplemented medium changed their cell surface marker profile and gene expression pattern to one characteristic of B lymphocyte precursors. Such cells also underwent variable, diversity, joining rearrangement at the immunoglobulin heavy-chain locus, a permanent genetic change unique to lymphocytes. This feature is limited to the cells isolated using the modified epidermal stem cell method, as cells isolated using the modified transit amplifying cell method could not be re-directed or reprogrammed. Such results demonstrate that cells from the epidermis can be directed to cross lineage boundaries to become mesodermally derived lymphocytes.

Therapeutic pathways of adult stem cell repair

The use of adult stem cells as therapeutic agents to treat disease has become increasingly prevalent. During the last decade, isolated and expanded stem and progenitor cells have demonstrated the capacity to differentiate into multiple cell types. Early optimism that in vitro differentiation capacity would translate into in vivo tissue regeneration has lessened and identifying the mechanisms that underlie the benefit of stem cell repair is an emerging area of investigation. This review considers several of the pathways and mechanisms required for adult stem cell repair. These mechanisms include the mobilization and the homing of stem cells to sites of injury, immunomodulatory effect of stem cells, and the association of stem cells with increased vascularization of injured tissue. These data suggest that the unique properties of adult stem cells can be utilized to treat a wide variety of diseases that cannot be treated with existing pharmacological agents, and prompt new paradigms for the bio-pharmacokinetics of biological expressed by efficacious stem cells.

TPA-induced multinucleation of a mesenchymal stem cell-like clone is mediated primarily by karyokinesis without cytokinesis, although cell-cell fusion also occurs

The 5F9A cell, which is a mesenchymal stem cell-like clone established from rat bone marrow substrate adherent cells, can differentiate into adipocytes and osteoblasts in vitro under the appropriate conditions. Multinucleated cells could be also induced by 12-O-tetradecanoylphorbol 13-acetate (TPA) in 5F9A cells. This effect was mediated by protein kinase C. Possible mechanisms of multinucleation by TPA were hypothesized to be either karyokinesis without cytokinesis or cell-cell fusion. By observation using time-lapse phase-contrast microscopy, we determined that the multinucleated cells were generated mainly by karyokinesis without cytokinesis. Cell fusion was studied using time-lapse photography, and confocal laser scanning microscopy using two differentially labeled cells. These techniques demonstrated that multinucleated 5F9A cells could be produced by cell fusion, albeit at a low frequency. We conclude that multinucleated 5F9A cells are formed primarily by karyokinesis without cytokinesis, although some cells are also formed by cell-cell fusion.

Evaluation of Sca-1 and c-Kit As Selective Markers for Muscle Remodelling by Nonhemopoietic Bone Marrow Cells

Bone marrow (BM)-derived cells (BMCs) have demonstrated a myogenic tissue remodeling capacity. However, because the myoremodeling is limited to approximately 1%-3% of recipient muscle fibers in vivo, there is disagreement regarding the clinical relevance of BM for therapeutic application in myodegenerative conditions. This study sought to determine whether rare selectable cell surface markers (in particular, c-Kit) could be used to identify a BMC population with enhanced myoremodeling capacity. Dystrophic mdx muscle remodeling has been achieved using BMCs sorted by expression of stem cell antigen-1 (Sca-1). The inference that Sca-1 is also a selectable marker associated with myoremodeling capacity by muscle-derived cells prompted this study of relative myoremodeling contributions from BMCs (compared with muscle cells) on the basis of expression or absence of Sca-1. We show that myoremodeling activity does not differ in cells sorted solely on the basis of Sca-1 from either muscle or BM. In addition, further fractionation of BM to a more mesenchymal-like cell population with lineage markers and CD45 subsequently revealed a stronger selectability of myoremodeling capacity with c-Kit/Sca-1 (p < .005) than with Sca-1 alone. These results suggest that c-Kit may provide a useful selectable marker that facilitates selection of cells with an augmented myoremodeling capacity derived from BM and possibly from other nonmuscle tissues. In turn, this may provide a new methodology for rapid isolation of myoremodeling capacities from muscle and nonmuscle tissues. Disclosure of potential conflicts of interest is found at the end of this article.