Tubulyzine, a novel tri-substituted triazine, prevents the early cell death of transplanted myogenic cells and improves transplantation success

Orienting Muscle Stem Cells for Regeneration in Homeostasis, Aging, and Disease

Muscle stem cells, or satellite cells, are required for skeletal muscle maintenance, growth, and repair. Following satellite cell activation, several factors drive asymmetric cell division to generate a stem cell and a proliferative progenitor that forms new muscle. The balance between symmetric self-renewal and asymmetric division significantly impacts the efficiency of regeneration. In this Review, we discuss the relationship of satellite cell heterogeneity and the establishment of polarity to asymmetric division, as well as how these processes are impacted in homeostasis, aging, and disease. We also highlight therapeutic opportunities for targeting satellite cell polarity and self-renewal to stimulate muscle regeneration.

Adipose Tissue-Derived Stromal Cells in Matrigel Impacts the Regeneration of Severely Damaged Skeletal Muscles

In case of large injuries of skeletal muscles the pool of endogenous stem cells, i.e., satellite cells, might be not sufficient to secure proper regeneration. Such failure in reconstruction is often associated with loss of muscle mass and excessive formation of connective tissue. Therapies aiming to improve skeletal muscle regeneration and prevent fibrosis may rely on the transplantation of different types of stem cell. Among such cells are adipose tissue-derived stromal cells (ADSCs) which are relatively easy to isolate, culture, and manipulate. Our study aimed to verify applicability of ADSCs in the therapies of severely injured skeletal muscles. We tested whether 3D structures obtained from Matrigel populated with ADSCs and transplanted to regenerating mouse gastrocnemius muscles could improve the regeneration. In addition, ADSCs used in this study were pretreated with myoblasts-conditioned medium or anti-TGFβ antibody, i.e., the factors modifying their ability to proliferate, migrate, or differentiate. Analyses performed one week after injury allowed us to show the impact of 3D cultured control and pretreated ADSCs at muscle mass and structure, as well as fibrosis development immune response of the injured muscle.

1,25-Dihydroxyvitamin D3 Increases the Transplantation Success of Human Muscle Precursor Cells in SCID Mice

Human muscle precursor cell (hMPC) transplantation is a potential therapy for severe muscle trauma or myopathies. Some previous studies demonstrated that 1,25-dihydroxyvitamin-D3 (1,25-D3) acted directly on myoblasts, regulating their proliferation and fusion. 1,25-D3 is also involved in apoptosis modulation of other cell types and may thus contribute to protect the transplanted hMPCs. We have therefore investigated whether 1,25-D3 could improve the hMPC graft success. The 1,25-D3 effects on hMPC proliferation, fusion, and survival were initially monitored in vitro. hMPCs were also grafted in the tibialis anterior of SCID mice treated or not with 1,25-D3 to determine its in vivo effect. Graft success, proliferation, and viability of transplanted hMPCs were evaluated. 1,25-D3 enhanced proliferation and fusion of hMPCs in vitro and in vivo. However, 1,25-D3 did not protect hMPCs from various proapoptotic factors (in vitro) or during the early posttransplantation period. 1,25-D3 enhanced hMPC graft success because the number of muscle fibers expressing human dystrophin was significantly increased in the TA sections of 1,25-D3-treated mice (166.75 ± 20.64) compared to the control mice (97.5 ± 16.58). This result could be partly attributed to the improvement of the proliferation and differentiation of hMPCs in the presence of 1,25-D3. Thus, 1,25-D3 administration could improve the clinical potential of hMPC transplantation currently developed for muscle trauma or myopathies.

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.

Protective effect of sodium ascorbate on efficacy of intramuscular transplantation of autologous muscle-derived cells

INTRODUCTION

The possible reason for elimination of myogenic cells after transplantation is inflammation at the injection site associated with oxidative stress. The aim of this study was to determine whether preconditioning of muscle-derived cells with an antioxidant, sodium ascorbate, can influence the fate of transplanted cells.

METHODS

Autologous transplantation of muscle-derived cells was performed in rabbits. Isolated cells were identified, lipofected with β-galactosidase, preincubated or not with sodium ascorbate, and injected intramuscularly.

RESULTS

Two weeks after autologous transplantation in the edge of a previous muscle defect, donor cells formed multinucleated young myotubes. Pretreatment of cells with sodium ascorbate before injection resulted in a significant increase of donor cells at the injection site 2 weeks after transfer.

CONCLUSIONS

These results show that: (1) preincubation with antioxidant can increase the efficacy of myogenic cell transplantation; and (2) oxidative stress may play a role in elimination of cells after autologous transplantation.

Skeletal myoblasts for cardiac repair

Stem cells provide an alternative curative intervention for the infarcted heart by compensating for the cardiomyocyte loss subsequent to myocardial injury. The presence of resident stem and progenitor cell populations in the heart, and nuclear reprogramming of somatic cells with genetic induction of pluripotency markers are the emerging new developments in stem cell-based regenerative medicine. However, until safety and feasibility of these cells are established by extensive experimentation in in vitro and in vivo experimental models, skeletal muscle-derived myoblasts, and bone marrow cells remain the most well-studied donor cell types for myocardial regeneration and repair. This article provides a critical review of skeletal myoblasts as donor cells for transplantation in the light of published experimental and clinical data, and indepth discussion of the advantages and disadvantages of skeletal myoblast-based therapeutic intervention for augmentation of myocardial function in the infarcted heart. Furthermore, strategies to overcome the problems of arrhythmogenicity and failure of the transplanted skeletal myoblasts to integrate with the host cardiomyocytes are discussed.

Combinatorial Solid-Phase Synthesis of 6-Aryl-1,3,5-triazines via Suzuki Coupling

A synthetic methodology to prepare collections of trisubstituted aryl 1,3,5-triazines with broad structural diversity via Suzuki coupling has been developed. We first optimized the combinatorial derivatization of the triazine core using Suzuki cross-coupling. Second, in order to further expand the methodology for the preparation of negatively charged triazines, we adapted this approach to polymer-supported amino acids and prepared aryl triazines with different charge distribution. With a collection of 160 aryl triazine derivatives in good purities and without any purification step, we proved the viability of this orthogonal scheme for the preparation of triazine libraries using amine/amino acid-captured solid supports and Suzuki cross-coupling.

Novel Orthogonal Synthesis of a Tagged Combinatorial Triazine Library via Grignard Reaction

To expand the diversity of 1,3,5-triazine libraries to aryl and alkyl functionalities through the C–C bond, we employed a novel orthogonal synthesis via Grignard monoalkylation or monoarylation of cyanuric chloride in solution to prepare aryl- or alkyl-substituted triazine building blocks. These aryl- or alkyl-substituted triazine building blocks were captured by a resin-bound amine, followed by amination and acidic cleavage with high purity. Herein, we demonstrate a novel orthogonal synthesis of a tagged aryl- and alkyl-triazine library on solid support, utilizing building blocks prepared via Grignard reaction in solution. Through incorporation of a triethylene glycol linker at one of the alternate sites on the triazine scaffold we explored an intrinsic tagged library approach.

A synthetic mechano growth factor E Peptide enhances myogenic precursor cell transplantation success

Myogenic precursor cell (MPC) transplantation is a good strategy to introduce dystrophin expression in muscles of Duchenne muscular dystrophy (DMD) patients. Insulin-like growth factor (IGF-1) promotes MPC activities, such as survival, proliferation, migration and differentiation, which could enhance the success of their transplantation. Alternative splicing of the IGF-1 mRNA produces different muscle isoforms. The mechano growth factor (MGF) is an isoform, especially expressed after a mechanical stress. A 24 amino acids peptide corresponding to the C-terminal part of the MGF E domain (MGF-Ct24E peptide) was synthesized. This peptide had been shown to enhance the proliferation and delay the terminal differentiation of C(2)C(12) myoblasts. The present study showed that the MGF-Ct24E peptide improved human MPC transplantation by modulating their proliferation and differentiation. Indeed, intramuscular or systemic delivery of this synthetic peptide significantly promoted engraftment of human MPCs in mice. In vitro experiments demonstrated that the MGF-Ct24E peptide enhanced MPC proliferation by a different mechanism than the binding to the IGF-1 receptor. Moreover, MGF-Ct24E peptide delayed human MPC differentiation while having no outcome on survival. Those combined effects are probably responsible for the enhanced transplantation success. Thus, the MGF-Ct24E peptide is an interesting agent to increase MPC transplantation success in DMD patients.

Characterization of hereditary inclusion body myopathy myoblasts: possible primary impairment of apoptotic events

Hereditary inclusion body myopathy (HIBM) is a unique muscular disorder caused by mutations in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene. GNE encodes a bi-functional enzyme acting in the biosynthetic pathway of sialic acid. Since the underlying myopathological mechanism leading to the disease phenotype is poorly understood, we have established human myoblasts cultures, derived from HIBM satellite cells carrying the homozygous M712T mutation, and identified cellular and molecular characteristics of these cells. HIBM and control myoblasts showed similar heterogeneous patterns of proliferation and differentiation. Upon apoptosis induction, phosphatidylserine externalization was similar in HIBM and controls. In contrast, the active forms of caspase-3 and -9 were strongly enhanced in most HIBM cultures compared to controls, while pAkt, downregulated in controls, remained high in HIBM cells. These results could indicate impaired apoptotic signaling in HIBM cells. Since satellite cells enable partial regeneration of the post-mitotic muscle tissue, these altered processes could contribute to the muscle mass loss seen in patients. The identification of survival defects in HIBM affected muscle cells could disclose new functions for GNE in muscle cells.

Asymmetric self-renewal and commitment of satellite stem cells in muscle

Satellite cells play a central role in mediating the growth and regeneration of skeletal muscle. However, whether satellite cells are stem cells, committed progenitors, or dedifferentiated myoblasts has remained unclear. Using Myf5-Cre and ROSA26-YFP Cre-reporter alleles, we observed that in vivo 10% of sublaminar Pax7-expressing satellite cells have never expressed Myf5. Moreover, we found that Pax7(+)/Myf5(-) satellite cells gave rise to Pax7(+)/Myf5(+) satellite cells through apical-basal oriented divisions that asymmetrically generated a basal Pax7(+)/Myf5(-) and an apical Pax7(+)/Myf5(+) cells. Prospective isolation and transplantation into muscle revealed that whereas Pax7(+)/Myf5(+) cells exhibited precocious differentiation, Pax7(+)/Myf5(-) cells extensively contributed to the satellite cell reservoir throughout the injected muscle. Therefore, we conclude that satellite cells are a heterogeneous population composed of stem cells and committed progenitors. These results provide critical insights into satellite cell biology and open new avenues for therapeutic treatment of neuromuscular diseases.

Synthesis and biological activity of 8-azapurine and pyrazolo[4,3-d]pyrimidine analogues of myoseverin

The trisubstituted purine myoseverin has been recently identified as a novel inhibitor of microtubule assembly. To analyze the effects of modifying its heterocyclic skeleton, we prepared 8-azapurine and pyrazolo[4,3-d]pyrimidine analogues of myoseverin and compared their biological activities. Rearrangement of nitrogen atoms in the heterocycle changes the affinity of the compounds to purified tubulin, as demonstrated by the results of polymerization assays, and affects the proliferation of cancer cell lines. Surprisingly, compound E2GG, a pyrazolo[4,3-d]pyrimidine analogue of myoseverin, displayed inhibitory activity towards both tubulin polymerization and the activity of cyclin-dependent kinases 1, 2 and 7. Such a dual specificity-inhibitor offers a starting point for developing a novel class of antiproliferative agents.

A Single-Cell Analysis of Myogenic Dedifferentiation Induced by Small Molecules

An important direction in chemical biology is the derivation of compounds that affect cellular differentiation or its reversal. The fragmentation of multinucleate myofibers into viable mononucleates (called cellularization) occurs during limb regeneration in urodele amphibians, and the isolation of myoseverin, a trisubstituted purine that could apparently activate this pathway of myogenic dedifferentiation in mammalian cells, generated considerable interest. We have explored the mechanism and outcome of cellularization at a single-cell level, and we report findings that significantly extend the previous work with myoseverin. Using a panel of compounds, including a triazine compound with structural similarity and comparable activity to myoseverin, we have identified microtubule disruption as critical for activation of the response. Time-lapse microscopy has enabled us to analyze the fate of identified mononucleate progeny, and directly assess the extent of dedifferentiation.

Real-time imaging of myoblast transplantation using the human sodium iodide symporter

The quantification of the graft success is a key element to evaluate the efficiency of cellular therapies for several pathologies such as Duchenne muscular dystrophy. This study describes an approach to evaluate the success of myoblast transplantation (i.e., survival of the transplanted cells and the muscle fibers formed) by real-time imaging. C2C12 myoblasts were first transfected with a plasmid containing the human sodium iodide symporter (hNIS) gene. Specific uptake of the radioactive sodium pertechnetate (Na99mTcO4) by the hNIS-positive myoblasts was demonstrated in vitro, while only background level of Na99mTcO4 was observed within the control cells. The cells were then transplanted into the tibialis anterior (TA) muscle of mdx (X-linked dystrophic) mice. Following intraperitoneal administration of Na99mTcO4, scintigraphies were performed to detect hNIS-dependent Na99mTcO4 uptake within the TA. This approach permitted to evaluate the progression of the transplantation and the graft success without having to biopsy the animals during the follow-up period.

Autologous transplantation of porcine myogenic precursor cells in skeletal muscle

Myoblast transplantation is a potential therapy for severe muscle trauma, myopathies and heart infarct. Success with this therapy relies on the ability to obtain cell preparations enriched in myogenic precursor cells and on their survival after transplantation. To define myoblast transplantation strategies applicable to patients, we used a large animal model, the pig. Muscle dissociation procedures adapted to porcine tissue gave high yields of cells containing at least 80% myogenic precursor cells. Autologous transplantation of 3[H]-thymidine labeled porcine myogenic precursor cells indicated 60% survival at day 1 followed by a decay to 10% at day 5 post-injection. Nuclei of myogenic precursor cells transduced with a lentivirus encoding the nls-lacZ reporter gene were present in host myotubes 8 days post-transplantation, indicating that injected myogenic precursor cells contribute to muscle regeneration. This work suggests that pig is an adequate large animal model for exploring myogenic precursor cells transplantation strategies applicable in patients.

Muscle-fiber apoptosis in neuromuscular diseases

Muscle-fiber loss is a characteristic of many progressive neuromuscular disorders. Over the past decade, identification of a growing number of apoptosis-associated factors and events in pathological skeletal muscle provided increasing evidence that apoptotic cell-death mechanisms account significantly for muscle-fiber atrophy and loss in a wide spectrum of neuromuscular disorders. It became obvious that there is not one specific pathway for muscle fibers to undergo apoptotic degradation. In contrast, certain neuromuscular diseases seem to involve characteristic expression patterns of apoptosis-related factors and pathways. Furthermore, there are some characteristics of muscle-fiber apoptosis that rely on the muscle fiber itself as an extremely specified cell type. Multinucleated muscle fibers with successive muscle-fiber segments controlled by individual nuclei display some specifics different from apoptosis of mononucleated cells. This review focuses on the expression patterns of apoptosis-associated factors in different primary and secondary neuromuscular disorders and gives a synopsis of current knowledge.