Improved media for normal human muscle satellite cells: serum-free clonal growth and enhanced growth with low serum

Challenges and Considerations of Preclinical Development for iPSC-Based Myogenic Cell Therapy

Cell therapies derived from induced pluripotent stem cells (iPSCs) offer a promising avenue in the field of regenerative medicine due to iPSCs' expandability, immune compatibility, and pluripotent potential. An increasing number of preclinical and clinical trials have been carried out, exploring the application of iPSC-based therapies for challenging diseases, such as muscular dystrophies. The unique syncytial nature of skeletal muscle allows stem/progenitor cells to integrate, forming new myonuclei and restoring the expression of genes affected by myopathies. This characteristic makes genome-editing techniques especially attractive in these therapies. With genetic modification and iPSC lineage specification methodologies, immune-compatible healthy iPSC-derived muscle cells can be manufactured to reverse the progression of muscle diseases or facilitate tissue regeneration. Despite this exciting advancement, much of the development of iPSC-based therapies for muscle diseases and tissue regeneration is limited to academic settings, with no successful clinical translation reported. The unknown differentiation process in vivo, potential tumorigenicity, and epigenetic abnormality of transplanted cells are preventing their clinical application. In this review, we give an overview on preclinical development of iPSC-derived myogenic cell transplantation therapies including processes related to iPSC-derived myogenic cells such as differentiation, scaling-up, delivery, and cGMP compliance. And we discuss the potential challenges of each step of clinical translation. Additionally, preclinical model systems for testing myogenic cells intended for clinical applications are described.

Development of a Chemically Defined Medium for in vitro Expansion of Primary Bovine Satellite Cells

The use of fetal bovine serum (FBS) in animal cell culture media is widely spread since it provides a broad spectrum of molecules that are known to support cell attachment and growth. However, the harvest and collection procedures of FBS raise ethical concerns and serum is an ill-defined and expensive component. This is especially problematic when it comes to regulatory approval for food applications like cultured meat. The aim of this study is to develop a chemically defined, cost efficient serum-free and animal-free medium that supports the attachment and expansion of bovine myoblasts while maintaining their differentiation capacity. Bovine satellite cells were harvested and isolated from a fresh sample of skeletal muscle tissue and cultured in planar systems. The efficacy of the tested formulations was assessed with metabolic assays and cell counting techniques. Optical microscopy was used to observe cellular morphology and statistical analysis was applied. Based on a comprehensive literature analysis, a defined serum-free medium (SFM) composition was developed consisting of DMEM/F12 as basal medium, supplemented with L-ascorbic acid 2-phosphate, fibronectin, hydrocortisone, GlutaMAX™, albumin, ITS-X, hIL-6, α-linolenic acid, and growth factors such as FGF-2, VEGF, IGF-1, HGF, and PDGF-BB. To our knowledge, this is the first defined serum-free and animal free medium formulation specific for bovine myoblasts to date. We conclude that the SFM formulation supported exponential cell growth up to 97% of the serum-containing golden standard growth medium. All reagents used in this study are chemically defined.

Sarcolemmal Complement Membrane Attack Complex Deposits During Acute Rejection of Myofibers in Nonhuman Primates

We have previously studied in nonhuman primates several aspects of the acute rejection of myofibers, including the histological characteristics, the mechanisms of myofiber elimination by the T cells, and the development of anti-donor antibodies. Here, we report the participation of the complement membrane attack complex (MAC) in this context. We used muscle sections of macaques from experiments of allogeneic muscle precursor cell transplantation with confirmed rejection of the graft-derived myofibers. Sections were stained with hematoxylin and eosin, alizarin red and for immunodetection of MAC, CD8, CD4, C3, C4d, and immunoglobulins. The prominent finding was the presence of sarcolemmal MAC (sMAC) deposits in biopsies with ongoing acute rejection or with recent acute rejection. The numbers of sMAC-positive myofibers were variable, being higher when there was an intense lymphocyte infiltration. Few sMAC-positive myofibers were necrotic or had evidence of sarcolemma permeation. The immunodetection of C3, C4d, and immunoglobulins did not provide significant elements. In conclusion, sMAC deposits were related to myofiber rejection. The fact that the vast majority of sMAC-positive myofibers had no signs of necrosis or sarcolemmal permeation suggests that MAC would not be harmful to myofibers by itself.

De Novo Circulating Antidonor's Cell Antibodies During Induced Acute Rejection of Allogeneic Myofibers in Myogenic Cell Transplantation: A Study in Nonhuman Primates

Background

Transplantation of myogenic cells has potential applications in the treatment of muscle pathologies. Excluding purely autologous cell transplantation, graft viability depends on an adequate control of acute rejection (AR). To contribute in understanding AR in this context, we analyzed whether de novo circulating antibodies against donor’s cells are detected during induced AR of graft-derived myofibers in nonhuman primates.

Methods

We allotransplanted satellite cell-derived myoblasts in macaques immunosuppressed with tacrolimus. To induce AR of graft-derived myofibers, we administered tacrolimus for 4 weeks to allow complete myofiber formation, and then we stopped tacrolimus administration. Cell-grafted sites were biopsied at tacrolimus withdrawal and then every 2 weeks and analyzed by histology until AR completion. Blood samples were taken before immunosuppression, at tacrolimus withdrawal and then every 2 weeks to detect antibodies against the donor’s cells by flow cytometry.

Results

There was an increase of antibodies against the donor’s cells related to AR in all monkeys. This increase was variable in intensity, and preceded, coincided or followed the histological evidence of AR (focal accumulations of lymphocytes) and/or the loss of myofibers of donor origin, and remained until the end of the follow-up (up to 8 weeks after tacrolimus withdrawal).

Conclusions

Flow cytometry detection of de novo circulating antibodies against the donor’s cells was consistently associated with AR. A clear increase in this antibody detection indicated current or recent AR. Smaller increases in comparison to the preimmunosuppression values were not associated with AR.

The Process of Engraftment of Myogenic Cells in Skeletal Muscles of Primates: Understanding Clinical Observations and Setting Directions in Cell Transplantation Research

We studied in macaques the evolution of the intramuscular transplantation of muscle precursor cells between the time of administration and the time at which the graft is considered stable. Satellite cell–derived myoblasts labeled with ß-galactosidase were transplanted into 1 cm³ muscle regions following cell culture and transplantation protocols similar to our last clinical trials. These regions were biopsied 1 h, 1, 3, 7 d, and 3 wk later and analyzed by histology. We observed that the cell suspension leaks from the muscle bundles during injection toward the epimysium and perimysium, where most cells accumulate after transplantation. We observed evidence of necrosis, apoptosis, and mitosis in the accumulations of grafted cells, and of potential migration to participate in myofiber regeneration in the surrounding muscle bundles. After 3 wk, the compact accumulations of grafted cells left only some graft-derived myotubes and small myofibers in the perimysium. Hybrid myofibers were abundant in the muscle fascicles at 3 wk posttransplantation, and they most likely occur by grafted myoblasts that migrated from the peripheral accumulations than by the few remaining within the fascicles immediately after injection. These observations explain the findings in clinical trials of myoblast transplantation and provide information for the future research in cell therapy in myology.

Cell Therapy in Myology: Dynamics of Muscle Precursor Cell Death after Intramuscular Administration in Non-human Primates

Cell therapy could be useful for the treatment of myopathies. A problem observed in mice, with different results and interpretations, is a significant death among the transplanted cells. We analyzed this problem in non-human primates, the animal model more similar to humans. Autologous or allogeneic myoblasts (with or without a reporter gene) were proliferated in vitro, labeled with [¹⁴C]thymidine, and intramuscularly injected in macaques. Some monkeys were immunosuppressed for long-term follow-up. Cell-grafted regions were biopsied at different intervals and analyzed by radiolabel quantification and histology. Most radiolabel was lost during the first week after injection, regardless of whether the cells were allogeneic or autologous, the culture conditions, and the use or not of immunosuppression. There was no significant difference between 1 hr and 1 day post-transplantation, a significant decrease between days 1 and 3 (45% to 83%), a significant decrease between days 3 and 7 (80% to 92%), and no significant differences between 7 days and 3 weeks. Our results confirmed in non-human primates a progressive and significant death of the grafted myoblasts during the first week after administration, relatively similar to some observations in mice but with different kinetics.

Design, evaluation, and application of engineered skeletal muscle

For over two decades, research groups have been developing methods to engineer three-dimensional skeletal muscle tissues. These tissues hold great promise for use in disease modeling and pre-clinical drug development, and have potential to serve as therapeutic grafts for functional muscle repair. Recent advances in the field have resulted in the engineering of regenerative muscle constructs capable of survival, vascularization, and functional maturation in vivo as well as the first-time creation of functional human engineered muscles for screening of therapeutics in vitro. In this review, we will discuss the methodologies that have progressed work in the muscle tissue engineering field to its current state. The emphasis will be placed on the existing procedures to generate myogenic cell sources and form highly functional muscle tissues in vitro, techniques to monitor and evaluate muscle maturation and performance in vitro and in vivo, and surgical strategies to both create diseased environments and ensure implant survival and rapid integration into the host. Finally, we will suggest the most promising methodologies that will enable continued progress in the field.

Interstitial Outburst of Angiogenic Factors During Skeletal Muscle Regeneration After Acute Mechanical Trauma

Angiogenesis is a key event during tissue regeneration, but the intimate mechanisms controlling this process are still largely unclear. Therefore, the cellular and molecular interplay along normal tissue regeneration should be carefully unveiled. To this matter, we investigated by xMAP assay the dynamics of some angiogenic factors known to be involved in tissue repair, such as follistatin (FST), Placental Growth Factor-2 (PLGF-2), epidermal growth factor (EGF), betacellulin (BTC), and amphiregulin (AREG) using an animal model that mimics acute muscle contusion injuries. In situ immunofluorescence was used for the evaluation and tissue distribution of their cellular sources. Tissue levels of explored factors increased significantly during degeneration and inflammatory stage of regeneration, peaking first week postinjury. However, except for PLGF-2 and EGF, their levels remained significantly elevated after the inflammatory process started to fade. Serum levels were significantly increased only after 24 h for AREG and EGF. Though, for all factors except FST, the levels in injured samples did not correlate with serum or contralateral tissue levels, excluding the systemic influence. We found significant correlations between the levels of EGF and AREG, BTC, FST and FST and AREG in injured samples. Interstitial cells expressing these factors were highlighted by in situ immunolabeling and their number correlated with measured levels dynamics. Our study provides evidence of a dynamic level variation along the regeneration process and a potential interplay between selected angiogenic factors. They are synthesized, at least partially, by cell populations residing in skeletal muscle interstitium during regeneration after acute muscle trauma.

Bioengineered human myobundles mimic clinical responses of skeletal muscle to drugs

Existing in vitro models of human skeletal muscle cannot recapitulate the organization and function of native muscle, limiting their use in physiological and pharmacological studies. Here, we demonstrate engineering of electrically and chemically responsive, contractile human muscle tissues (‘myobundles’) using primary myogenic cells. These biomimetic constructs exhibit aligned architecture, multinucleated and striated myofibers, and a Pax7⁺ cell pool. They contract spontaneously and respond to electrical stimuli with twitch and tetanic contractions. Positive correlation between contractile force and GCaMP6-reported calcium responses enables non-invasive tracking of myobundle function and drug response. During culture, myobundles maintain functional acetylcholine receptors and structurally and functionally mature, evidenced by increased myofiber diameter and improved calcium handling and contractile strength. In response to diversely acting drugs, myobundles undergo dose-dependent hypertrophy or toxic myopathy similar to clinical outcomes. Human myobundles provide an enabling platform for predictive drug and toxicology screening and development of novel therapeutics for muscle-related disorders.

DOI:http://dx.doi.org/10.7554/eLife.04885.001

Scientists have developed realistic models of the human liver, lung, and heart that allow them to observe living tissue in the laboratory. These models have helped us to better understand how these organs work and what goes wrong in diseases that affect these organs. The models can also be used to test how new drugs may affect a particular organ without the risk of exposing patients to the drug.

Efforts to develop a realistic laboratory model of human muscle tissues that can contract like real muscles have not been as successful to date. This shortcoming has potentially hindered the development of drugs to treat numerous disorders that affect muscles and movement in humans—such as muscular dystrophies, which are diseases in which people progressively lose muscle strength.

Some important drugs, like cholesterol-lowering statins, have detrimental effects on muscle tissue; one statin was so harmful to muscles that it had to be withdrawn from the market. As such, it would be useful to have experimental models that would allow scientists to test whether potential drugs damage or treat muscle tissue.

Madden et al. have now bioengineered a three-dimensional laboratory model of living muscle tissue made of cells taken from biopsies of several different human patients. These tissues were grown into bundles of muscle fibers on special polymer frames in the laboratory. The bioengineered muscle bundles respond to electrical and chemical signals and contract just like normal muscle. They also exhibit the same structure and signaling as healthy muscle tissue in humans.

Madden et al. exposed the muscle tissue bundles to three drugs known to affect muscles to determine if the model could be used to test whether drugs have harmful effects. This revealed that the bundles had weaker contractions in response to statins and the malaria drug chloroquine, just like normal muscles do—and that this effect worsened if more of each drug was used. Madden et al. also found that a drug that strengthens muscle contractions at low doses and damages muscle at high doses in humans has similar effects in the model.

As well as this model being used to screen for harmful effects of drugs before clinical trials, the technique used to create the model could be used to grow muscle tissue from patients with muscle diseases. This would help researchers and doctors to better understand the patient's condition and potentially develop more efficient therapies. Also, the technique could be eventually developed to grow healthy muscle tissue to implant in patients who have been injured.

DOI:http://dx.doi.org/10.7554/eLife.04885.002

First study of intra-arterial delivery of myogenic mononuclear cells to skeletal muscles in primates

The main challenge of cell transplantation as a treatment of myopathies is the large amount of tissue to treat. Intravascular delivery of cells may be an ideal route if proven to be effective and safe. Given the importance of nonhuman primates for preclinical research in transplantation, we tested the intra-arterial injection of β-galactosidase (β-Gal)-labeled myoblasts in macaques. Cells were injected into one of the femoral arteries in seven monkeys. Some muscle sites were damaged concomitantly in three monkeys. Various organs and muscles were sampled 1 h, 1 day, 12 days, 3 weeks, and 5 weeks after transplantation. Samples were analyzed by histology. Most β-Gal(+) cells were observed in the capillaries and arterioles of muscles and other tissues of the leg homolateral to the cell injection. Groups of necrotic myofibers in the proximity of an arteriole plugged by a β-Gal(+) embolus were interpreted as microinfarcts. Scarce β-Gal(+) cells were observed in the lungs 1 h and 1 day posttransplantation. No β-Gal(+) cells were observed in other organs or muscles. β-Gal(+) myofibers were observed 12 days, 3 weeks, and 5 weeks after transplantation in muscles of the leg after the cell injection, in sites that were damaged at the time of cell injection. In conclusion, most intra-arterially injected myoblasts were retained in vessels of the leg homolateral to the cell injection site, and they fused with myofibers in regions in which there was a process of myofiber regeneration. This manuscript is published as part of the International Association of Neurorestoratology (IANR) special issue of Cell Transplantation.

Efficient myoblast expansion for regenerative medicine use

Cellular therapy using expanded autologous myoblasts is a treatment modality for a variety of diseases. In the present study, we compared the commercial skeletal muscle cell growth medium-2 (SKGM-2) with a medium designed by our group for the expansion of skeletal myoblasts. The use of an in-house medium [DMEM/F12 medium supplemented with EGF, bFGF, HGF, insulin and dexamethasone (DFEFH)] resulted in a greater number of myoblast colonies (>50%) and a 3-, 4- and 9‑fold higher proliferation rate, eventually resulting in a 3-, 7- and 87-fold greater number of cells at the 1st, 2nd and 3rd passage, respectively, compared with the cells grown in SKGM-2 medium. The average CD56 expression level was higher in the myoblasts cultured in DFEFH than in those culturd in SKGM-2 medium. At the 3rd passage, lower expression levels of myostatin and considerably higher expression levels of myogenin were observed in the cells that were grown in DFEFH medium. The results of our study indicated that myoblasts cultured in both medium types displayed fusogenic potential at the 3rd passage. Furthermore, it was shown that cells cultured in DFEFH medium created myotubes with a considerably higher number of nuclei. Additionally, we observed that the fusion potential of the cells markedly decreased with the subsequent passages and that the morphology of the myoblasts differed between the 2 cultured media. Our data demonstrate that culture in the DFEFH medium leads to an approximately 90‑fold greater number of myoblasts, with improved morphology and greater fusion potential, compared with culture in the commercial SKGM-2 medium.

Electroporation as a method to induce myofiber regeneration and increase the engraftment of myogenic cells in skeletal muscles of primates

Engraftment of intramuscularly transplanted myogenic cells in mice can be optimized after induction of massive myofiber damage that triggers myofiber regeneration and recruitment of grafted cells; this generally involves either myotoxin injection or cryodamage. There are no effective methods to produce a similar process in the muscles of large mammals such as primates. In this study, we tested the use of intramuscular electroporation for this purpose in 11 macaques. The test sites were 1 cm of skeletal muscle. Each site was treated with 3 penetrations of a 2-needle electrode with 1 cm spacing, applying 3 pulses of 400 V/cm, for a duration of 5 milliseconds and a delay of 200 milliseconds during each penetration. Transplantation of β-galactosidase-labeled myoblasts was done in electroporated and nonelectroporated sites. Electroporation induced massive myofiber necrosis that was followed by efficient muscle regeneration. Myoblast engraftment was substantially increased in electroporated compared with nonelectroporated sites. This suggests that electroporation may be a useful tool to study muscle regeneration in primates and other large mammals and as a method for increasing the engraftment of myoblasts and other myogenic cells in intramuscular transplantation.

Selection and tuning of a fast and simple phase-contrast microscopy image segmentation algorithm for measuring myoblast growth kinetics in an automated manner

Acquiring and processing phase-contrast microscopy images in wide-field long-term live-cell imaging and high-throughput screening applications is still a challenge as the methodology and algorithms used must be fast, simple to use and tune, and as minimally intrusive as possible. In this paper, we developed a simple and fast algorithm to compute the cell-covered surface (degree of confluence) in phase-contrast microscopy images. This segmentation algorithm is based on a range filter of a specified size, a minimum range threshold, and a minimum object size threshold. These parameters were adjusted in order to maximize the F-measure function on a calibration set of 200 hand-segmented images, and its performance was compared with other algorithms proposed in the literature. A set of one million images from 37 myoblast cell cultures under different conditions were processed to obtain their cell-covered surface against time. The data were used to fit exponential and logistic models, and the analysis showed a linear relationship between the kinetic parameters and passage number and highlighted the effect of culture medium quality on cell growth kinetics. This algorithm could be used for real-time monitoring of cell cultures and for high-throughput screening experiments upon adequate tuning.

Intramuscular transplantation of myogenic cells in primates: importance of needle size, cell number, and injection volume

The aim of this study was to quantitatively define the main measurable technical parameters for the intramuscular transplantation of myogenic cells in primates. Myoblasts transduced with the gene coding for β-galactosidase were injected into the skeletal muscles of 15 monkeys. The following parameters were studied: needle size, number of cells per injection, and volume of cell suspension per injection. Monkeys were immunosuppressed with tacrolimus. The cell-injected sites were biopsied 1 or 2 months later. Biopsies were examined histologically to assess the myoblast engraftment and the muscle structure. The conclusions were as follows: (1) Needles should be thin enough to avoid important tissue damage and allow muscle regeneration as satisfactory as possible. Among those tested, 27G should be the choice if the length is consistent with depth of injection. (2) At least 100,000 cells should be delivered per centimeter of needle trajectory. (3) The smallest volumes of cell suspension per injection should be used. In this study, 1 µl/cm of injection trajectory was sufficient. In principle, these parameters apply to muscles in which no damage occurred other than the injections.

Necrosis, sarcolemmal damage and apoptotic events in myofibers rejected by CD8+ lymphocytes: Observations in nonhuman primates

To detect the mechanisms of death in allogeneic myofibers rejected by the immune system, myoblasts were allotransplanted in muscles of macaques immunosuppressed with tacrolimus. Immunosuppression was stopped 1month later to induce a massive rejection of allogeneic myofibers. Grafted sites were biopsied at 2-week intervals and analyzed by histology. The loss of allogeneic myofibers was rapid and concomitant with an intense infiltration of CD8+ lymphocytes. Several necrotic myofibers were observed in the lymphocyte accumulations by intracellular complement immunodetection. Dystrophin and spectrin immunodetection showed sarcolemmal damage in myofibers surrounded and invaded by CD8+ lymphocytes. Active caspase-3 was immunodetected in some myofibers surrounded by CD8+ lymphocytes. This is the first evidence that the collapse of myofibers attacked by T lymphocytes occurs by necrosis possibly due to damage of the sarcolemma. Caspase 3 is activated at least in some myofibers, but there was no evidence of a complete classical process of apoptosis.

Fetuin-A ({alpha}2HS-glycoprotein) is a major serum adhesive protein that mediates growth signaling in breast tumor cells

The identity of the cell adhesive factors in fetal bovine serum, commonly used to supplement growth media, remains a mystery due to the plethora of serum proteins. In the present analyses, we showed that fetuin-A, whose function in cellular attachment in tissue culture has been debated for many years, is indeed a major serum cell attachment factor particularly for tumor cells. We are able to report this because of a new purification strategy that has for the first time given us a homogeneous protein band in colloidal Coomassie-stained gels that retains biological activity. The tumor cells adhered to immobilized fetuin-A and not α(2)-macroglobulin, its major contaminant. The interaction of cells with fetuin-A was driven mainly by Ca(2+) ions, and cells growing in regular medium supplemented with fetal bovine serum were just as sensitive to loss of extracellular Ca(2+) ions as cells growing in fetuin-A. Fractionation of human serum revealed that cell attachment was confined to the fractions that had fetuin-A. Interestingly, the tumor cells also took up fetuin-A and secreted it back to the medium using an unknown mechanism that can be observed in live cells. The attachment of tumor cells to fetuin-A was accompanied by phosphatidylinositol 3-kinase/Akt activation that was down-regulated in cells that lack annexin-A6, one of the cell surface receptors for fetuin-A. Taken together, our data show the significance of fetuin-A in tumor cell growth mechanisms in vitro and open new research vistas for this protein.

Characterization of distinct mesenchymal-like cell populations from human skeletal muscle in situ and in vitro

Human skeletal muscle is an essential source of various cellular progenitors with potential therapeutic perspectives. We first used extracellular markers to identify in situ the main cell types located in a satellite position or in the endomysium of the skeletal muscle. Immunohistology revealed labeling of cells by markers of mesenchymal (CD13, CD29, CD44, CD47, CD49, CD62, CD73, CD90, CD105, CD146, and CD15 in this study), myogenic (CD56), angiogenic (CD31, CD34, CD106, CD146), hematopoietic (CD10, CD15, CD34) lineages. We then analysed cell phenotypes and fates in short- and long-term cultures of dissociated muscle biopsies in a proliferation medium favouring the expansion of myogenic cells. While CD56(+) cells grew rapidly, a population of CD15(+) cells emerged, partly from CD56(+) cells, and became individualized. Both populations expressed mesenchymal markers similar to that harboured by human bone marrow-derived mesenchymal stem cells. In differentiation media, both CD56(+) and CD15(+) cells shared osteogenic and chondrogenic abilities, while CD56(+) cells presented a myogenic capacity and CD15(+) cells presented an adipogenic capacity. An important proportion of cells expressed the CD34 antigen in situ and immediately after muscle dissociation. However, CD34 antigen did not persist in culture and this initial population gave rise to adipogenic cells. These results underline the diversity of human muscle cells, and the shared or restricted commitment abilities of the main lineages under defined conditions.

Differentiation potential of human muscle-derived cells towards chondrogenic phenotype in alginate beads culture

OBJECTIVE

The aim of this study was to evaluate the differentiation potential of two populations of muscle-derived cells (CD56- and CD56+) towards chondrogenic phenotype in alginate beads culture and to compare the effect of transforming growth factor beta 1 (TGFbeta1) on the differentiation process in these populations.

METHODS

Muscle CD56- and CD56+ cells were cultured in alginate beads, in a chondrogenic medium, containing or not TGFbeta1 (10 ng/ml). Cultures were maintained for 3, 7, 14 or 21 days in a humidified culture incubator. At harvest, one culture of each set was fixed for alcian blue staining and aggrecan detection. The steady-state level of matrix macromolecules mRNA was assessed by real-time polymerase chain reaction (PCR). Protein detection was performed by western-blot analysis. The binding activity of nuclear extracts to Cbfa1 DNA sequence was also evaluated by electrophoretic mobility shift assays (EMSA).

RESULTS

Chondrogenic differentiation of both CD56+ and CD56- muscle-derived cells was improved in alginate scaffold, even without growth factor, as suggested by increased chondrogenesis markers expression during the culture. Furthermore, TGFbeta1 enhanced the differentiation process and allowed to maintain a high expression of markers of mature chondrocytes. Of importance, the combination of alginate and TGFbeta1 treatment resulted in a further down-regulation of collagen type I and type X, as well as Cbfa1 both expression and binding activity.

CONCLUSIONS

Thus, alginate scaffold and chondrogenic medium are sufficient to lead both populations CD56+ and CD56- towards chondrogenic differentiation. Moreover, TGFbeta1 enhances this process and allows to maintain the chondrogenic phenotype by inhibiting terminal differentiation, particularly for CD56- cells.

A novel in vitro three-dimensional skeletal muscle model

A novel three-dimensional (3D) skeletal muscle model composed of C2C12 mouse myoblasts is described. This model was generated by cultivating myoblasts in suspension using the rotary cell culture system (RCCS), a unique culture environment. Single-cell suspensions of myoblasts were seeded at 5 x 10(5)/ml in growth medium without exogenous support structures or substrates. Cell aggregation occurred in both RCCS and suspension control (SC) conditions within 12 h but occurred more rapidly in the SC at all time intervals examined. RCCS-cultured myoblasts fused and differentiated into a 3D construct without serum deprivation or alterations. Syncitia were quantified at 3 and 6+ d in stained thin sections. A significantly greater number of syncitia was found at 6+ d in the RCCS cultures compared to the SC. The majority of syncitia were localized to the periphery of the cell constructs for all treatments. The expression of sarcomeric myosin heavy chain (MHC) was localized at or near the periphery of the 3D construct. The majority of MHC was associated with the large cells (syncitia) of the 6+-d aggregates. These results show, for the first time, that myoblasts form syncitia and express MHC in the presence of growth factors and without the use of exogenous supports or substrates. This model test system is useful for investigating initial cell binding, myoblast fusion and syncitia formation, and differentiation processes.

Craniofacial muscle engineering using a 3-dimensional phosphate glass fibre construct

The current technique to replace missing craniofacial skeletal muscle is the surgical transfer of local or free flaps. This is associated with donor site morbidity, possible tissue rejection and limited supply. The alternative is to engineer autologous skeletal muscle in vitro, which can then be re-implanted into the patient. A variety of biomaterials have been used to engineer skeletal muscle with limited success. This study investigated the use of phosphate-based glass fibres as a potential scaffold material for the in vitro engineering of craniofacial skeletal muscle. Human masseter (one of the muscles of mastication)--derived cell cultures were used to seed the glass fibres, which were arranged into various configurations. Growth factors and matrix components were to used to manipulate the in vitro environment. Outcome was determined with the aid of microscopy, time-lapse footage, immunofluorescence imaging and CyQUANT proliferation, creatine kinase and protein assays. A 3-dimensional mesh arrangement of the glass fibres was the best at encouraging cell attachment and proliferation. In addition, increasing the density of the seeded cells and using Matrigel and insulin-like growth factor I enhanced the formation of prototypic muscle fibres. In conclusion, phosphate-based glass fibres can support the in vitro engineering of human craniofacial muscle.

An Overview of Myoblast Transplantation for Myocardial Regeneration

Regenerative medicine represents a new frontier in treatment of disease, particularly cardiovascular disease. The contractile elements of the heart, cardiomyocytes, lack the capacity for any postnatal proliferation or regeneration. Therefore, repair of heart damage can be achieved only by manipulating cardiomyocytes to regrow or by introducing exogenous cells with the capacity to restore function to the myocardium. Many attempts have been made with various cell types to repair the damaged myocardium. We will present here a summary of some of those studies and also present in detail studies utilizing a promising, near-term, and practical source of cells for treatment of heart disease: autologous skeletal myoblasts.

Antisense oligonucleotides and short interfering RNAs silencing the cyclin-dependent kinase inhibitor p21 improve proliferation of Duchenne muscular dystrophy patients' primary skeletal myoblasts

Increased levels of the cyclin-dependent kinase inhibitor p21 associated with decreased myoblast proliferation may be involved in the dystrophic process in Duchenne muscular dystrophy (DMD). Therefore we are interested to improve the proliferation of primary myoblasts of DMD patients by a reduction in p21 using either antisense oligonucleotides (ASO) or short interfering RNAs (siRNA). After transient transfection of myoblasts in cell culture proliferation was analyzed using a 5-bromo-2'-deoxyuridine assay comparing specific transfected cells with untransfected cells and cells transfected with scrambled ASO and luciferase siRNA, respectively. Four of five Dystrophin-deficient (Dys(-)) cell culture samples revealed an increase in proliferation between 7% and 18% compared to untransfected cells and between 8% and 36% compared to cells transfected with scrambled ASO. Transfection with siRNA was performed for selected samples to determine whether siRNA is more effective in gene silencing than ASO. The increase in proliferation using luciferase siRNA as reference was comparable to or less than ASO data using scrambled ASO as reference. Using untransfected cells as reference, the increase in proliferation was higher for siRNA than ASO (20-47% vs. 7-18%), but the data must be carefully interpreted with respect to nonspecific effects on gene expression by siRNA. Our findings of transient p21 gene silencing represent a basis for viral vector-mediated drug-inducible p21 shRNA expression in Dys(-) myoblasts which might enhance, prolong and regulate the proliferation effect.

Dysferlin expression after normal myoblast transplantation in SCID and in SJL mice

Limb girdle muscular dystrophy type 2B form and Miyoshi myopathy are both caused by mutations in the recently cloned gene dysferlin. In the present study, we have investigated whether cell transplantation could permit dysferlin expression in vivo. Two transplantation models were used: SCID mice transplanted with normal human myoblasts, and SJL mice, the mouse model for limb girdle muscular dystrophy type 2B and Miyoshi myopathy, transplanted with allogeneic primary mouse muscle cell cultures expressing the beta-galactosidase gene under control of a muscle promoter of Troponin I. FK506 immunosuppression was used in the non-compatible allogeneic model. One month after transplantation, human and mouse dysferlin proteins were detected in all transplanted SCID and SJL muscles, respectively. Co-localization of dysferlin and human dystrophin or beta-galactosidase-positive fibers was observed following the transplantation of myoblasts. Dysferlin proteins were monitored by immunocytochemistry and Western blot. The number of dysferlin-positive fibers was 40-50% and 20-30% in SCID and SJL muscle sections, respectively. Detection of dysferlin in both SCID mice and dysferlin-deficient SJL mouse shows that myoblast transplantation permits the expression of the donor dysferlin protein.

Scale-up of a myoblast culture process

The effects of different types of cell carriers, strategies for cell transfer on carriers, and of several fusion inhibitors on the growth kinetics of primary human myoblasts culture were studied in order to develop a bioprocess suitable for the treatment of Duchenne muscular dystrophy based on the transplantation of unfused cells. Our results indicate that myoblast production is larger on Cytodex 1 and 3 than on polypropylene or polyester fabrics and on a commercial porous macrocarrier. Myoblast growth conditions with Cytodex 1 were further investigated to establish the bioprocess operating conditions. It was found that microcarrier density of 3 g DW l(-1), inoculum density of 2x10(5) cells ml(-1), and continuous agitation speed of 30-rpm result in final myoblast production comparable to static cultures. However, for all the culture conditions used, myoblasts growth kinetics exhibited a lag phase that lasted a minimum of 1 week prior to growth, the end of the lag phase correlating with the appearance of microcarrier aggregates. Based on this observation, we propose that aggregation promotes cell growth by offering a network of very large inter-particular pores that protect cells from mechanical stress. We took advantage of the presence of these aggregates for the scale-up of the culture process. Indeed, using myoblast-loaded microcarrier-aggregates instead of myoblast suspension to inoculate a fresh suspension of microcarriers significantly reduced the duration of the lag phase and allowed the scale-up of the bioprocess at the 500-ml scale. In order to ensure the production of unfused myoblasts, the efficiency of five different fusion inhibitors was investigated. Only calpeptin (9.1 microg ml(-1)) significantly inhibited the fusion of the myoblasts, while TGFbeta (50 ng ml(-1)) and LPA (10 microg ml(-1)) increased myoblasts growth but did not affect fusion, sphingosine (30 microg ml(-1)) induced a 50% death and NMMA (25 microg ml(-1)) had no effect on either growth or fusion. Finally, transplantation trials on severe combined immunodeficient mice showed that microcarrier-cultured human myoblasts grown using the optimized bioprocess resulted in grafts as successful as myoblasts grown in static cultures. The bioprocess, therefore, prove to be suitable for the large-scale production of myoblasts required for muscular dystrophy treatment.

Transfection of large plasmids in primary human myoblasts

The ex vivo gene therapy approach for Duchenne muscular dystrophy is promising since myoblast transplantation in primates is now very efficient. One obstacle to this treatment is the low transfection efficiency of large DNA constructs in human primary myoblasts. Small plasmids can be easily transfected with the new phosphonolipid described in this study. However, a dramatic drop in transfection efficiency is observed with plasmids of 12 kb or more containing EGFP minidystrophin and EGFP dystrophin fusion genes. The transfection of human primary myoblasts with such large plasmids could only be achieved when the DNA was linked to an adenovirus with the use of polyethylenimine (PEI), with efficiencies ranging between 3 and 5% of transitory transfection. Branched 2 kDa PEI was less toxic in PEI adenofection than branched 25 kDa PEI or linear 22 kDa PEI. The adenovirus was an absolute necessity for an efficient transfection. An integrin-binding peptide, a nuclear localization signal peptide, chloroquine, glycerol or cell cycle synchronization using aphidicolin did not enhance PEI adenofection. Following PEI adenofection, the adenoviral proteins were detected using a polyclonal antibody. The detected antigens fell below the detectable level after 12 days in culture. We thus provide in this study an efficient and reproducible method to permit efficient delivery of large plasmids to human primary myoblasts for the ex vivo gene therapy of Duchenne muscular dystrophy.

Intracellular delivery of a Tat-eGFP fusion protein into muscle cells

The Tat protein from HIV-1, when fused with heterologous proteins or peptides, can traverse biological membranes in a process called "protein transduction," delivering its cargo into cells. A Tat-eGFP fusion protein was purified from bacteria to study the transduction kinetics of Tat fusion proteins into cultured myoblasts and in the muscle tissue. Correctly folded Tat-eGFP reaches a maximum intracellular level in nearly 30 min, while its endogenous fluorescence is first detected only after 14 h. The nuclear localization signal from the basic domain of Tat was not sufficient to confer nuclear localization to Tat-eGFP, suggesting that the nuclear import pathway used by the exogenously added Tat-eGFP might be sensitive to the folding state of eGFP. In mice, the direct delivery to the muscle tissue using subcutaneous injections or the intra-arterial pathway led to few positive fibers in the muscle periphery or surrounding the blood vessels. Muscles injected with Tat-eGFP showed intense labeling of the extracellular matrix (ECM), suggesting that, although Tat fusion proteins can transduce muscle fibers, their binding by components of the ECM surrounding myofibers could interfere with the intracellular transduction process.

The uncoupling protein-3 gene is transcribed from tissue-specific promoters in humans but not in rodents

Uncoupling protein-3 (UCP3), a mitochondrial membrane transporter, is a candidate effector of thermogenesis. Even though mice with targeted disruption of the UCP3 gene are not obese, indirect evidence suggests that this protein contributes to the control of energy expenditure in humans. We therefore characterized the human UCP3 gene and compared it with its rodent homologues with respect to tissue-specific expression and regulatory regions. Like rodent UCP3, human UCP3 was expressed in skeletal muscle and brown adipose tissue (BAT). The short mRNA isoform, UCP3(S), which is absent in rodents, was relatively more abundant in human skeletal muscle in comparison to human BAT. Two tissue-specific transcription start sites for each skeletal muscle and BAT were delineated for human UCP3. Tissue-specific transcript initiation was maintained in both tissues and cultured cells over a wide range of expression levels. In contrast, rodent transcripts were initiated at the same site in BAT and muscle tissue. Comparison of human and rodent promoters indicated a rapid phylogenetic evolution suggesting functional diversification. The transcription from tissue-specific promoters in humans is a novel finding that may provide the basis for therapeutic interventions aimed at regulating energy expenditure in a tissue-specific fashion.

Exposure to tissue culture conditions can adversely affect myoblast behavior in vivo in whole muscle grafts: implications for myoblast transfer therapy

The effects of tissue culture conditions on the viability of myoblasts in whole muscles transplanted in vivo were investigated. Whole male (SJL/J) donor muscles were exposed to various tissue culture reagents and proteolytic enzymes, and allografted into female (SJL/J) host mice. Desmin immunohistochemistry was used to assess the numbers of myogenic cells (as an index of myoblast viability and the extent of regeneration) in tissue sections of whole-muscle grafts sampled on days 7 and 14. DNA quantitation with a Y-chromosome-specific probe was used to determine the total Y-1 sequence DNA (as an index of myoblast survival and proliferation) in whole-muscle grafts sampled on days 1, 3, and 7. In grafts exposed to serum-free medium, there was a delay in myoblast fusion at 7 days that was recovered by 14 days, but exposure to serum (10% or 20%) had a prolonged adverse effect on myotube formation at 14 days. DNA quantitation demonstrated that either serum-free culture medium or 10% serum enhanced the number of male cells within whole-muscle grafts at 7 days. Proteolytic digestion (even for 5 min) of whole muscles prior to grafting was extremely detrimental to myoblast survival and viability at 7 and 14 days. The unexpected finding of adverse effects of tissue culture conditions on the regeneration of whole-muscle grafts in vivo appears to parallel the major problem of the rapid death of isolated cultured donor myoblasts after injection in myoblast transfer therapy. The use of whole-muscle grafts provides an alternative and sensitive model to analyze the crucial effects of various tissue culture components on the subsequent survival and proliferation of myogenic cells in vivo.

Increased myogenic potential and fusion of matrilysin-expressing myoblasts transplanted in mice

The success of myoblast transplantation in clinical trials has been limited in part by the low dispersion of grafted cells outside the injection site. Our research group previously reported that the culture of myoblasts with concanavalin A, a stimulator of metalloproteinase production, increased their migration. Several lines of evidence also suggested that muscle cell fusion involves metalloproteinase-sensitive mechanisms. To determine whether the increased expression of metalloproteinases had an influence on myoblast fusion and dispersion through the muscle following transplantation, we generated a myoblast cell line expressing human matrilysin (MMP-7). The MMP-7-expressing myoblasts were obtained by the stable transfection of a matrilysin expression vector in a TnILacZ immortomouse myoblast clone. Matrilysin-expressing myoblasts showed a highly increased in vitro fusion index, forming seven times (p < 0.001) more myotubes than the control cell line and three times (p < 0.001) more myotubes than the Immortomyoblast parental clone. Single-site transplantation of matrilysin-expressing myoblasts generated more fibers (p < 0.001), over a greater surface (p < 0.001) than the control cell line. The cotransplantation of matrilysin-expressing myoblasts and of normal human myoblasts in SCID mice increased the number of human dystrophin-positive fibers and myotubes by sixfold. Although no significant increased migration of myoblasts outside the injection sites was observed, our results show that the metalloproteinase activity can improve the myogenic potential of myoblasts in vitro and the fusion of myoblasts with host fibers in vivo. MMP-7 expression may be useful in increasing myoblast transplantation success.

Serum-free culture conditions for analysis of secretory proteinases during myogenic differentiation of mouse C2C12 myoblasts

We have been studying extracellular proteins such as proteinases and attachment factors under serum-free culture conditions. A number of studies on myogenesis using an in vitro culture system have reported that proteinases and ECM components play significant roles in muscle differentiation. However, most of the studies were performed in the presence of serum. Serum is abundant in the aforementioned proteins and its use in serum-free culture affects many cellular functions significantly. In this study, we tried to establish serum-free culture conditions for analyzing extracellular proteins involved in mouse myogenic differentiation. By evaluating media, supplements, and procedure of cell inoculation under serum-free conditions and by comparing the resultant conditions with conventional conditions on differentiated characteristics of the cells, it was revealed that serum-free Dulbecco's modified Eagle's medium/Ham's F-12 plus insulin more efficiently supported myogenesis morphologically and biochemically than conventional 2% horse serum-containing culture and that secretory proteinases obtained from our serum-free culture were different from those obtained utilizing conventional serum-free cultures in their activities and patterns. Since our serum-free medium consists of simple components, the medium is low cost and easy to prepare. Furthermore, the results suggest that our culture conditions are superior to conventional conditions biochemically and morphologically and will provide more precise and accurate information on extracellular proteins involved in myogenesis.

Cloning of novel injury-regulated genes. Implications for an important role of the muscle-specific protein skNAC in muscle repair

To gain insight into the molecular mechanisms underlying the wound repair process, we searched for genes that are regulated by skin injury. Using the differential display reverse transcription-polymerase chain reaction technique, we identified a gene that was strongly induced as early as 12 h after wounding. Sequence analysis revealed the identity of the corresponding protein with skeletal muscle nascent polypeptide-associated complex (skNAC), a recently identified muscle-specific transcription factor. By in situ hybridization and immunohistochemistry, we demonstrated the specific expression of skNAC in skeletal muscle cells of the panniculus carnosus at the wound edge. Furthermore, in vitro studies with cultured myoblasts revealed expression of skNAC in differentiating and differentiated, but not in proliferating, nondifferentiated cells. Differentiation of cultured myoblasts was accompanied by simultaneous expression of skNAC and the muscle-specific transcription factor myogenin. Our results provide the first evidence for a role of skNAC in muscle repair processes. Furthermore, they demonstrate the usefulness of our approach in identifying new players in wound repair.

Tissue-engineered human bioartificial muscles expressing a foreign recombinant protein for gene therapy

Murine skeletal muscle cells transduced with foreign genes and tissue engineered in vitro into bioartificial muscles (BAMs) are capable of long-term delivery of soluble growth factors when implanted into syngeneic mice (Vandenburgh et al., 1996b). With the goal of developing a therapeutic cell-based protein delivery system for humans, similar genetic tissue-engineering techniques were designed for human skeletal muscle stem cells. Stem cell myoblasts were isolated, cloned, and expanded in vitro from biopsied healthy adult (mean age, 42 +/- 2 years), and elderly congestive heart failure patient (mean age, 76 +/- 1 years) skeletal muscle. Total cell yield varied widely between biopsies (50 to 672 per 100 mg of tissue, N = 10), but was not significantly different between the two patient groups. Percent myoblasts per biopsy (73 +/- 6%), number of myoblast doublings prior to senescence in vitro (37 +/- 2), and myoblast doubling time (27 +/- 1 hr) were also not significantly different between the two patient groups. Fusion kinetics of the myoblasts were similar for the two groups after 20-22 doublings (74 +/- 2% myoblast fusion) when the biopsy samples had been expanded to 1 to 2 billion muscle cells, a number acceptable for human gene therapy use. The myoblasts from the two groups could be equally transduced ex vivo with replication-deficient retroviral expression vectors to secrete 0.5 to 2 microg of a foreign protein (recombinant human growth hormone, rhGH)/10(6) cells/day, and tissue engineered into human BAMs containing parallel arrays of differentiated, postmitotic myofibers. This work suggests that autologous human skeletal myoblasts from a potential patient population can be isolated, genetically modified to secrete foreign proteins, and tissue engineered into implantable living protein secretory devices for therapeutic use.

Recombinant Human Albumin in Cell Culture: Evaluation of Growth-Promoting Potential for NRK and SCC-9 Cells In Vitro

Serum-derived albumin has for a long time been used in cell culture media, but the exact role of albumin and/or impurities bound to albumin has not been precisely defined. In this study, recombinant human albumin was evaluated for its growth-promoting activity on two cell lines, NRK and SCC-9. For NRK cells, the recombinant human albumin was found to exert an inhibitory effect. The fact that fatty acid free HSA was also inhibitory while HSA fraction V was stimulatory suggested a role for fatty acids or some other bound moieties in growth stimulation by HSA fraction V. Addition of oleic acid, cholesterol, phosphatidylcholine, phosphatidylserine or a combination of these lipids, however, did not significantly improve the growth stimulating activity of either fatty acid free HSA or the recombinant human albumin. For SCC-9 cells, both recombinant human albumin and fatty acid free HSA showed slight stimulation (although they were not as active as HSA fraction V), suggesting that in some cell systems, the albumin molecule per se may promote cell growth and survival.

Myoblast alpha v beta3 integrin levels are controlled by transcriptional regulation of expression of the beta3 subunit and down-regulation of beta3 subunit expression is required for skeletal muscle cell differentiation

The expression of alpha v beta3 integrin was examined in human skeletal muscle cells grown in vitro. The alpha v and beta3 subunits showed different patterns of expression during myogenesis. Expression of the alphav subunit did not change significantly during the growth and differentiation of muscle cells, whereas expression of the beta3 subunit was markedly down-regulated at both the message and protein levels. Down-regulation of beta3 subunit expression did not occur when cultures were treated with 5-bromo-2'-deoxyuridine to inhibit myoblast terminal differentiation, but did occur in cultures in which fusion was selectively inhibited by growth in EGTA-containing medium. These results suggest that the regulation of integrin beta3 subunit expression is tightly coupled to the myogenic terminal differentiation program, but is not simply a consequence of membrane reorganization due to the fusion process. Several stably transfected lines of mouse C2 myoblasts were derived from cultures transfected with a cDNA encoding the complete human beta3 integrin sequence under the control of the CMV promoter. Lines which constitutively expressed high levels of the human beta3 integrin subunit did not fuse or biochemically differentiate, whereas lines expressing moderate levels of the beta3 integrin subunit showed delayed fusion and differentiation. Lines expressing very low to undetectable levels of the human beta3 integrin subunit exhibited unimpaired fusion and differentiation. Taken together these results suggest (i) that down-regulation of alpha v beta3 integrin normally occurs as part of the myogenic terminal differentiation program, (ii) that this is mediated by regulation of the expression of the beta3 subunit, and (iii) that down-regulation of expression of the beta3 integrin subunit appears essential for myoblast differentiation.

Rapid death of injected myoblasts in myoblast transfer therapy

Myoblast transplantation has been proposed as a potential therapy for Duchenne muscular dystrophy (DMD). A Y-chromosome-specific probe was used to track the fate of donor male myoblasts injected into dystrophic muscles of female mdx mice (which are an animal model for DMD). In situ analysis with the Y-probe showed extremely poor survival of isolated normal male (C57B1/10Sn) donor myoblasts after injection into injured or uninjured muscles of dystrophic (mdx) and normal (C57B1/10Sn) female host mice. A decrease in the numbers of donor (male) myoblasts was seen from 2 days and was marked by 7 days after injection: few or no donor myoblasts were detected in host muscles examined at 3-12 months. There was limited movement of the injected donor myoblasts and fusion into host myofibers was rare. The results of this study strongly suggest that the failure of clinical trials of myoblast transplantation in boys with DMD may have been due to rapid and massive death of the donor myoblasts soon after myoblast injection.

Extrinsic regulation of domestic animal-derived satellite cells

Satellite cells are the postnatal myogenic cells, as they provide myonuclei to support skeletal muscle hypertrophy and are principal cells responsible for myofiber repair and regeneration. Even though research with satellite cells from meat animals is new, considerable data exist to suggest that these cells are regulated through both intrinsic and extrinsic mechanisms. This review covers the present status of the extrinsic factors known or postulated to modulate meat animal satellite cell growth and development.

Substance P and neurokinin A metabolism by cultured human skeletal muscle myocytes and fibroblasts

A recent study determined that cultured human skeletal muscle adult myoblasts, myotubes, and fibroblasts degraded angiotensins and kinins via neutral endopeptidase-24.11 (NEP-24.11: EC 3.4.24.11) and aminopeptidase N (APN: EC 3.4.11.2). Due to the possible importance of other peptides to skeletal muscle blood flow and function, the present study looked specifically at the metabolism of the neurokinins substance P (SP) and neurokinin A (NKA) by skeletal muscle peptidases. The results show that SP is degraded not only by NEP-24.11, but also sequentially by dipeptidyl(amino)peptidase IV (DAP IV: EC 3.4.14.5)/APN. NKA is unaffected by DAP IV but is metabolized by NEP-24.11 and APN. NEP-24.11 was inhibited by phosphoramidon (IC50 = 80 nM), thiorphan and ZINCOV, DAP IV by diprotin A (IC50 = 8 microM), and APN by amastatin (IC50 = 50 nM) and bestatin (IC50 = 100 microM). Skeletal muscle myocyte and fibroblast metabolism of SP and NKA may regulate local skeletal muscle vascular and extravascular functions including SP- and NKA-mediated nerve-induced vasodilation. Inhibition of both NEP-24.11 and DAP IV/APN may increase skeletal muscle blood flow and decrease peripheral vascular resistance via potentiation of local neurokinin levels.

Glucose transport in cultured human skeletal muscle cells. Regulation by insulin and glucose in nondiabetic and non-insulin-dependent diabetes mellitus subjects

A primary human skeletal muscle culture (HSMC) system, which retains cellular integrity and insulin responsiveness for glucose transport was employed to evaluate glucose transport regulation. As previously reported, cells cultured from non-insulin-dependent diabetic (NIDDM) subjects displayed significant reductions in both basal and acute insulin-stimulated transport compared to nondiabetic controls (NC). Fusion/differentiation of NC and NIDDM HSMC in elevated media insulin (from 22 pM to 30 microM) resulted in increased basal transport activities but reduced insulin-stimulated transport, so that cells were no longer insulin responsive. After fusion under hyperinsulinemic conditions, GLUT1 protein expression was elevated in both groups while GLUT4 protein level was unaltered. Fusion of HSMC under hyperglycemic conditions (10 and 20 mM) decreased glucose transport in NC cells only when combined with hyperinsulinemia. Hyperglycemia alone down-regulated transport in HSMC of NIDDM, while the combination of hyperglycemia and hyperinsulinemia had greater effects. In summary: (a) insulin resistance of glucose transport can be induced in HSMC of both NC and NIDDM by hyperinsulinemia and is accompanied by unaltered GLUT4 but increased GLUT1 levels; and (b) HSMC from NIDDM subjects demonstrate an increased sensitivity to impairment of glucose transport by hyperglycemia. These results indicate that insulin resistance in skeletal muscle can be acquired in NC and NIDDM from hyperinsulinemia alone but that NIDDM is uniquely sensitive to the additional influence of hyperglycemia.

Stimulation of protein and DNA synthesis in mouse C2C12 satellite cells: evidence for phospholipase D-dependent and -independent pathways

In C2C12 myoblasts, 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulated a phospholipase D (PLD) to degrade phosphatidylcholine (PC) as measured by the release of choline and an increase in the formation of phosphatidic acid (PA) (or phosphatidylbutanol [PtdBuOH] in the presence of 0.5% butanol). Exogenous PLD also stimulated choline release, PA and PtdBuOH formation. The protein kinase C (PKC) inhibitor, Ro-31-8220, and PKC downregulation significantly inhibited the effects of TPA but Ro-31-8220 had no effect on PLD action. Neither basic Fibroblast Growth Factor (bFGF) or Epidermal Growth Factor (EGF) increased PLD activity. All agonists stimulated protein synthesis during both a 90 min and a 6 hr incubation and increased RNA accretion after 6 hr. The response at 90 min was not inhibited by the transcription inhibitor, actinomycin D. Ro-31-8220 and PKC downregulation significantly inhibited all the effects of TPA. In contrast, Ro-31-8220 significantly inhibited the increase in RNA accretion elicited by PLD but had no effect on the ability of agonists other than TPA to enhance protein synthesis. All agonists also stimulated thymidine incorporation into DNA. The effects of EGF, bFGF, and PLD were rapid and transient whereas that of TPA was delayed and sustained. Ro-31-8220 and PKC downregulation significantly inhibited the response due to TPA. Furthermore, Ro-31-8220 also significantly inhibited the effects elicited by EGF and PLD but not that induced by bFGF. In differentiated myotubes, TPA and PLD, but not bFGF or EGF, again stimulated choline release and PtdBuOH formation. However, all agents failed to stimulate protein synthesis and RNA accretion. The data demonstrate the presence in C2C12 myoblasts, but not differentiated myotubes, of both a PLD-dependent and PLD-independent pathway(s) leading to the stimulation of protein synthesis, RNA accretion, and DNA synthesis.