Retroviral vector-mediated gene transfer into human primary myogenic cells leads to expression in muscle fibers in vivo

Development of Approaches to Improve the Healing following Muscle Contusion

Muscle injuries are a challenging problem in traumatology, and the most frequent occurrence in sports medicine. Muscle contusions are among the most common muscle injuries. Although this injury is capable of healing, an incomplete functional recovery often occurs, depending on the severity of the blunt trauma. We have developed an animal model of muscle contusion in mice (high energy blunt trauma) and characterized the muscle's ability to heal following this injury using histology and immunohistochemistry to determine the level of muscle regeneration and the development of scar tissue. We have observed a massive muscle regeneration occurring in the first 2 wk postinjury that is subsequently followed by the development of muscle fibrosis. Based on these observations, we propose that the enhancement of muscle growth and regeneration, as well as the prevention of fibrotic development, could be used as approach(es) to improve the healing of muscle injuries. In fact, we have identified three growth factors (bFGF, IGF-1, and NGF) capable of enhancing myoblast proliferation and differentiation in vitro and improving the healing of the injured muscle in vivo. Furthermore, the ability of adenovirus to mediate direct and ex vivo gene transfer of β-galactosidase into the injured site opens possibilities of delivering an efficient and persistent expression of these growth factors in the injured muscle. These studies should help in the development of strategies to promote efficient muscle healing with complete functional recovery following muscle contusion. © 1998 Elsevier Science Inc.

Inhibition of CD26/DPP-IV enhances donor muscle cell engraftment and stimulates sustained donor cell proliferation

BACKGROUND

Transplantation of myogenic stem cells possesses great potential for long-term repair of dystrophic muscle. In murine-to-murine transplantation experiments, CXCR4 expression marks a population of adult murine satellite cells with robust engraftment potential in mdx mice, and CXCR4-positive murine muscle-derived SP cells home more effectively to dystrophic muscle after intra-arterial delivery in mdx5cv mice. Together, these data suggest that CXCR4 plays an important role in donor cell engraftment. Therefore, we sought to translate these results to a clinically relevant canine-to-canine allogeneic transplant model for Duchenne muscular dystrophy (DMD) and determine if CXCR4 is important for donor cell engraftment.

METHODS

In this study, we used a canine-to-murine xenotransplantation model to quantitatively compare canine muscle cell engraftment, and test the most effective cell population and modulating factor in a canine model of DMD using allogeneic transplantation experiments.

RESULTS

We show that CXCR4 expressing cells are important for donor muscle cell engraftment, yet FACS sorted CXCR4-positive cells display decreased engraftment efficiency. However, diprotin A, a positive modulator of CXCR4-SDF-1 binding, significantly enhanced engraftment and stimulated sustained proliferation of donor cells in vivo. Furthermore, the canine-to-murine xenotransplantation model accurately predicted results in canine-to-canine muscle cell transplantation.

CONCLUSIONS

Therefore, these results establish the efficacy of diprotin A in stimulating muscle cell engraftment, and highlight the pre-clinical utility of a xenotransplantation model in assessing the relative efficacy of muscle stem cell populations.

Cell proteomic footprint

The authentication of mammalian cell cultures and their subpopulations is of great demand in biotechnology and cell therapy. However, current techniques are either not efficient or can be very complex and expensive. Here we report a simple and straightforward approach for authentication of biological cells and their subpopulations with high speed, high throughput, low sample cost, and high sensitivity. We discovered that cell cultures treated with protease under soft, 'non-killing' conditions release fragments of cell surface proteins, whose composition is a strong characteristic of the cells. Mass spectrometric analysis of the released fragments allows a direct comparison of the produced mass spectrum with the mass spectrum of known cells. As an example, we applied this technique to verify subpopulations of human fibroblasts with different origins and which exhibit different medical characteristics.

Cell-lineage regulated myogenesis for dystrophin replacement: a novel therapeutic approach for treatment of muscular dystrophy

Duchenne muscular dystrophy (DMD) is characterized in skeletal muscle by cycles of myofiber necrosis and regeneration leading to loss of muscle fibers and replacement with fibrotic connective and adipose tissue. The ongoing activation and recruitment of muscle satellite cells for myofiber regeneration results in loss of regenerative capacity in part due to proliferative senescence. We explored a method whereby new myoblasts could be generated in dystrophic muscles by transplantation of primary fibroblasts engineered to express a micro-dystrophin/enhanced green fluorescent protein (muDys/eGFP) fusion gene together with a tamoxifen-inducible form of the myogenic regulator MyoD [MyoD-ER(T)]. Fibroblasts isolated from mdx(4cv) mice, a mouse model for DMD, were efficiently transduced with lentiviral vectors expressing muDys/eGFP and MyoD-ER(T) and underwent myogenic conversion when exposed to tamoxifen. These cells could also be induced to differentiate into muDys/eGFP-expressing myocytes and myotubes. Transplantation of transduced mdx(4cv) fibroblasts into mdx(4cv) muscles enabled tamoxifen-dependent regeneration of myofibers that express muDys. This lineage control method therefore allows replenishment of myogenic stem cells using autologous fibroblasts carrying an exogenous dystrophin gene. This strategy carries several potential advantages over conventional myoblast transplantation methods including: (i) the relative simplicity of culturing fibroblasts compared with myoblasts, (ii) a readily available cell source and ease of expansion and (iii) the ability to induce MyoD gene expression in vivo via administration of a medication. Our study provides a proof of concept for a novel gene/stem cell therapy technique and opens another potential therapeutic approach for degenerative muscle disorders.

Immortalization of human myogenic progenitor cell clone retaining multipotentiality

Human myogenic cells have limited ability to proliferate in culture. Although forced expression of telomerase can immortalize some cell types, telomerase alone delays senescence of human primary cultured myogenic cells, but fails to immortalize them. In contrast, constitutive expression of both telomerase and the E7 gene from human papillomavirus type 16 immortalizes primary human myogenic cells. We have established an immortalized primary human myogenic cell line preserving multipotentiality by ectopic expression of telomerase and E7. The immortalized human myogenic cells exhibit the phenotypic characteristics of their primary parent, including an ability to undergo myogenic, osteogenic, and adipogenic terminal differentiation under appropriate culture conditions. The immortalized cells will be useful for both basic and applied studies aimed at human muscle disorders. Furthermore, immortalization by transduction of telomerase and E7 represents a useful method by which to expand human myogenic cells in vitro without compromising their ability to differentiate.

Nerve growth factor improves the muscle regeneration capacity of muscle stem cells in dystrophic muscle

Researchers have attempted to use gene- and cell-based therapies to restore dystrophin and alleviate the muscle weakness that results from Duchenne muscular dystrophy (DMD). Our research group has isolated populations of muscle-derived stem cells (MDSCs) from the postnatal skeletal muscle of mice. In comparison with satellite cells, MDSCs display an improved transplantation capacity in dystrophic mdx muscle that we attribute to their ability to undergo long-term proliferation, self-renewal, and multipotent differentiation, including differentiation toward endothelial and neuronal lineages. Here we tested whether the use of nerve growth factor (NGF) improves the transplantation efficiency of MDSCs. We used two methods of in vitro NGF stimulation: retroviral transduction of MDSCs with a CL-NGF vector and direct stimulation of MDSCs with NGF protein. Neither method of NGF treatment changed the marker profile or proliferation behavior of the MDSCs, but direct stimulation with NGF protein significantly reduced the in vitro differentiation ability of the cells. NGF stimulation also significantly enhanced the engraftment efficiency of MDSCs transplanted within the dystrophic muscle of mdx mice, resulting in the regeneration of numerous dystrophin-positive muscle fibers. These findings highlight the importance of NGF as a modulatory molecule, the study of which will broaden our understanding of its biologic role in the regeneration and repair of skeletal muscle by musclederived cells.

Dystrophin Delivery in Dystrophin-Deficient DMDmdxSkeletal Muscle by Isogenic Muscle-Derived Stem Cell Transplantation

Duchenne's muscular dystrophy (DMD) is a lethal muscle disease caused by a lack of dystrophin expression at the sarcolemma of muscle fibers. We investigated retroviral vector delivery of dystrophin in dystrophin-deficient DMD(mdx) (hereafter referred to as mdx) mice via an ex vivo approach using mdx muscle-derived stem cells (MDSCs). We generated a retrovirus carrying a functional human mini-dystrophin (RetroDys3999) and used it to stably transduce mdx MDSCs obtained by the preplate technique (MD3999). These MD3999 cells expressed dystrophin and continued to express stem cell markers, including CD34 and Sca-1. MD3999 cells injected into mdx mouse skeletal muscle were able to deliver dystrophin. Though a relatively low number of dystrophin-positive myofibers was generated within the gastrocnemius muscle, these fibers persisted for up to 24 weeks postinjection. The injection of cells from additional MDSC/Dys3999 clones into mdx skeletal muscle resulted in varying numbers of dystrophin-positive myofibers, suggesting a differential regenerating capacity among the clones. At 2 and 4 weeks postinjection, the infiltration of CD4- and CD8-positive lymphocytes and a variety of cytokines was detected within the injected site. These data suggest that the transplantation of retrovirally transduced mdx MDSCs can enable persistent dystrophin restoration in mdx skeletal muscle; however, the differential regenerating capacity observed among the MDSC/Dys3999 clones and the postinjection immune response are potential challenges facing this technology.

Duchenne muscular dystrophy: current knowledge, treatment, and future prospects

The cloning of the dystrophin gene has led to major advances in the understanding of the molecular genetic basis of Duchenne, Becker, and other muscular dystrophies associated with mutations in genes encoding members of the dystrophin-associated glycoprotein complex. The recent introduction of pharmaceutical agents such as prednisone has shown great promise in delaying the progression of Duchenne muscular dystrophy but there remains a need to develop more long-term therapeutic interventions. Knowledge of the nature of the dystrophin gene and the glycoprotein complex has led many researchers to think that somatic gene replacement represents the most promising approach to treatment. The potential use of this strategy has been shown in the mdx mouse model of Duchenne muscular dystrophy, where germ line gene transfer of either a full-length or a smaller Becker-type dystrophin minigene prevents necrosis and restores normal muscle function.

Adenovirus mediated gene transfer to skeletal muscle

Transfer of therapeutic genes into muscle tissue has promise for the treatment of a variety of muscular dystrophies. Various vectors have been used to deliver genes to skeletal muscle but their application has faced several major limitations including: (1) the lack of transgene persistence caused by the immune rejection of transduced myofibers and/or vector toxicity, and (2) the maturation dependence of viral transduction. While the immunorejection and/or cytotoxic problems are being overcome with the development of new vectors, maturation-dependent viral transduction is still a major hurdle in gene transfer to skeletal muscle. Poor adenoviral transduction in mature myofibers has been attributed to: (1) the extracellular matrix of mature myofibers may form a physical barrier and prevent the passage of large viral particles; (2) viral receptors are down-regulated with muscle maturation; and (3) loss of myoblasts with muscle maturation, which serve as intermediaries in the viral transduction. In this review, we will focus on recent developments in overcoming those hurdles of gene therapy in skeletal muscle, especially to adenovirus (Ad), including: (1) new mutant vectors lacking all viral genes to decrease immunogenicity, and hence, improve persistence of transgene expression in muscle in vivo; (2) using tissue specific promoters to evade immunorejection; (3) permeabilization of the extracellular matrix; (4) modifying the viral receptors in mature myofibers; and (5) myoblast or muscle stem cell mediated ex vivo gene transfer.

High-efficiency adenovirus-mediated in vivo gene transfer into neonatal and adult rodent skeletal muscle

Several methodological limitations have emerged in the use of viral gene transfer into skeletal muscle. First, because the nuclei of mature muscle fibers do not undergo division, the use of strategies involving replicative integration of exogenous DNA is greatly limited. Another important limitation concerns the maturation-dependent loss in muscle fiber infectivity with adenoviral vectors. In this study, we investigated the possibility that high-titer infections with recombinant adenovirus, expressing a foreign marker gene under the control of a strong viral promoter, can significantly improve the efficiency of gene transfer in vivo into neonatal and adult rat skeletal muscle. High-titer (2 x 10(10) plaque forming units) intramuscular injection of replication-defective adenovirus vector, expressing green fluorescent protein (GFP) under the control of cytomegalovirus promoter, resulted in GFP expression in 99 +/- 0.34% of fibers in the adult soleus muscle and in approximately 85 +/- 1.44% of fibers in the adult tibialis anterior muscle. Interestingly, reduction in injected adenoviral dose significantly reduced the number of GFP-positive fibers in the adult tibialis anterior muscle, but not in the soleus muscle. However, in neonates, adenoviral infection resulted in GFP expression in 96-99% of the fibers in the tibialis anterior and the gastrocnemius muscles regardless of administered adenoviral dose.

The role of receptors in the maturation-dependent adenoviral transduction of myofibers

One of the major hurdles facing the application of adenoviral gene transfer to skeletal muscle is the maturation-dependent transduction of muscle myofibers. It was recently proposed that the viral receptors (Coxsackie and adenovirus receptor (CAR) and the integrins alphavbeta3/beta5) play a major role in the poor adenoviral transduction of mature myofibers. Here we report the findings of morphological studies designed to determine experimentally the role of receptors in the adenoviral transduction of mature myofibers. First, we observed that the expression of both attachment and internalization receptors did not change significantly during muscle development. Second, when an extended tropism adenoviral vector (AdPK) that attaches to heparan sulfate proteoglycan (HSP) is used, a significant reduction of adenoviral transduction still occurs in mature myofibers despite HSP's high expression in mature skeletal muscle fibers. Third, when the adeno-associated virus (AAV) is used, which also utilizes HSP as a viral receptor, muscle fibers at different maturities can be highly transduced. Fourth, the pre-irradiation of the skeletal muscle of newborn mice to inactivate myoblasts dramatically decreased the transduction level of Ad and AdPK, but had no effect on AAV-mediated viral transduction of immature myofibers. These results taken together suggest that the viral receptor(s) is not a major determinant in maturation-dependent adenoviral transduction of myofibers.

The influence of muscle fiber type in myoblast-mediated gene transfer to skeletal muscles

Myoblast transplantation has been hindered by immune rejection problems, as well as the poor survival and spread of transplanted cells. Our recent study has shown that the poor survival of the injected cells can be totally overcome by the use of specific populations of muscle-derived cells. In the present study, we have investigated whether a relationship exists between the fate of transplanted cells and the muscle fiber types. Four kinds of myogenic cells [primary myoblasts at a high purity (PMb), myoblasts isolated from fast single fibers (FMb), mdx (MCL), and MtMd-1 cell lines] were infected with an adenoviral vector carrying a LacZ reporter gene and injected into mdx hindlimb muscle. The LacZ transduced myofibers formed by the fusion of the injected myoblasts at 2-10 days postinjection were colocalized with MyHC stainings. The PMb cells, which expressed both slow and fast MyHCs in vitro, displayed the same phenotypes when injected into the m. soleus and m. gastrocnemius (white) muscles, which contained 70% and 0% of slow myofibers, respectively, and showed a high degree of fusion with host muscle fibers. In contrast, the FMb cells only expressed fast MyHCs in vitro and fused exclusively with each other or with host fast muscle fibers when injected in the m. gastrocnemius. Injected MCL and MtMd-1 fused predominantly with each other and displayed a similar expression of MyHCs to those they expressed in vitro. Just a few host myofibers were found to express the reporter gene product following implantation of both cell lines, indicating that these myogenic cell lines display an intrinsic potential to fuse together rather than with host myofibers. Based on the data, we concluded that 1) the essential key to survival is the ability of the donor cells to fuse with the host myofibers, and 2) the most successful combination is achieved between donor primary muscle cells that express both fast and slow MyHC and a host muscle type that facilitates fusion.

Preliminary results of myoblast injection into the urethra and bladder wall: a possible method for the treatment of stress urinary incontinence and impaired detrusor contractility

The purpose of this study is to explore the feasibility of myoblasts, the precursors of muscle fibers, injected periurethrally as a potential treatment of stress urinary incontinence. We also studied myoblast injection into the bladder wall to potentially improve detrusor contractility. A myoblast cell line was transduced with adenovirus carrying the expression of the beta-galactosidase reporter gene while in culture. The cells were incubated with fluorescent latex microspheres (FLMs) to follow the outcome of the injected cells. The tissue was harvested 3-4 days after injection; sectioned, fixed, assayed for beta-galactosidase expression, and counterstained with H+E. Photographs of the slides were taken under light and fluorescence microscopy. We have noted a large number of cells expressing beta-galactosidase and containing FLMs in the urethral and bladder walls under fluorescent microscopy (8 animals). Many regenerative myofibers expressing beta-galactosidase were also seen in the urethral and bladder walls. The fusion of injected myoblasts to form myotubes was seen in both the urethral and bladder walls. The introduction of myoblasts into the urethral and bladder wall is feasible and results in formation of myotubes and myofibers in the smooth muscle layers of the lower urinary tract. We hypothesize that myoblast injections can be used as a non-allergenic agent to enhance urethral closure and bladder function.

The use of adeno-associated virus to circumvent the maturation-dependent viral transduction of muscle fibers

Muscle-based gene therapy using adenovirus, retrovirus, and herpes simplex virus has been hindered by viral cytotoxicity, host immune response, and the maturation-dependent viral transduction of muscle fibers. The development of new mutant vectors has greatly reduced the toxicity and the immune rejection problems, but the inability of viral vectors to penetrate and transduce mature myofibers remains an important issue. Research has been focused on the characterization of barriers to viral transduction in mature myofibers to develop strategies to circumvent the maturation-dependent viral transduction of myofibers. Here, we report that adeno-associated virus (AAV) can be used to overcome the maturation-dependent viral transduction of myofibers. We have investigated by which mechanism AAV can penetrate and efficiently transduce mature muscle fibers, and have shown that this viral vector is not blocked by the basal lamina and that AAV transduction of myofibers is independent of myoblast mediation. Although AAV can efficiently transduce mature myofibers, a differential transduction is still observed among the different types of myofibers that correlates with the expression of the heparan sulfate proteoglycan receptors, the muscle maturity, the number of viral particles used, and the time postinjection. The identification of the mechanisms by which AAV transduces mature myofibers will help in the development of strategies to achieve an efficient muscle-based gene therapy for inherited and acquired diseases.

Use of growth factors to improve muscle healing after strain injury

Muscle injuries represent a large number of professional and recreational sports injuries. Muscle strains habitually occur after an eccentric contraction, which often leads to an injury located in the myotendinous junction. Treatment varies widely, depending on the severity of the trauma, but has remained limited mostly to rest, ice, compression, elevation, antiinflammatory drugs, and mobilization. The authors' research group aims to develop new biologic approaches to improve muscle healing after injuries, including muscle strains. To achieve this goal, the authors investigated several parameters that will lead to the development of new strategies to enhance muscle healing. The authors first evaluated natural muscle healing after strain injuries and showed that muscle regeneration occurs in the early phase of healing but becomes impaired with time by the development of tissue fibrosis. Several growth factors capable of improving muscle regeneration were investigated; basic fibroblast growth factor, insulin-like growth factor, and nerve growth factors were identified as substances capable of enhancing muscle regeneration and improving muscle force in the strained injured muscle. The current study should aid in the development of strategies to promote efficient muscle healing and complete recovery after strain injury.

Gene transfer and models of gene therapy for the myocardium

1. Gene transfer into the myocardium can be achieved through direct injection of plasmid DNA or through the delivery of viral vectors, either directly or through the coronary vasculature. Direct DNA injection has proven extremely valuable in studies aimed at characterizing the activities of promoter elements in cardiac tissue and for examining the influence of the pathophysiological state of the myocardium on expression of transferred foreign genes. 2. Viral vectors, in particular adenoviruses and adeno-associated virus, are capable of transfecting genetic material with high transduction efficiencies and have been applied to a range of model systems for in vivo gene transfer. Efficient gene transfer has been achieved into the coronary vessels and surrounding myocardium by intracoronary infusion of adenovirus. 3. Because the immunogenicity of viral vectors can limit transgene expression, much attention has been paid to strategies for circumventing this, including the development of new modified adenovirus and adeno-associated virus vectors that do not elicit significant inflammatory responses. While cellular transplantation may prove valuable for the repair of myocardial tissue, confirmation of its value awaits establishment of a functional improvement in the myocardium following cell grafting. 4. Because gene transfer into the myocardium can now be achieved with high efficiency in the absence of significant inflammatory responses, the ability to regulate foreign gene expression in response to an endogenous disease phenotype will enable the development of new effective viral vectors with direct clinical applicability for specified therapeutic targets.

Intramuscular grafts of myoblasts genetically modified to secrete glial cell line-derived neurotrophic factor prevent motoneuron loss and disease progression in a mouse model of familial amyotrophic lateral sclerosis

Effects of ex vivo GDNF gene delivery on the degeneration of motoneurons were studied in the G1H transgenic mouse model of familial ALS carrying a human superoxide dismutase (SOD1) with a Gly93Ala mutation (Gurney et al., 1994). Retroviral vectors were made to produce human GDNF or E. coli beta-galactosidase (beta-Gal) by transient transfection of the Phoenix cell line and used to infect primary mouse myoblasts. In 6-week-old G1H mice, 50,000 myoblasts per muscle were injected bilaterally into two hindlimb muscles. Untreated G1H and wild-type mice served as additional controls. At 17 weeks of age, 1 week before sacrifice, these muscles were injected with fluorogold (FG) to retrogradely label spinal motoneurons that maintained axonal projections to the muscles. There were significantly more large FG-labeled alpha motoneurons at 18 weeks in GDNF-treated G1H mice than in untreated and beta-Gal-treated G1H mice. A morphometric study of motoneuron size distribution showed that GDNF shifted the size distribution of motoneurons toward larger cells compared with control G1H mice, although the average size and number of large motoneurons in GDNF-treated mice were less than that in wild-type mice. GDNF also prolonged the onset of disease, delayed the deterioration of performance in tests of motor behavior, and slowed muscle atrophy. Quantitative, real-time RT-PCR and PCR showed persistence of transgene mRNA and DNA in muscle for up to 12 weeks postgrafting. These observations demonstrate that ex vivo GDNF gene therapy in a mouse model of FALS promotes the survival of functional motoneurons, suggesting that a similar approach might delay the progression of neurodegeneration in ALS.

In vitro and in vivo effects of glucocorticoids on gene transfer to skeletal muscle

As a pharmacological approach to potentially improve gene transfer efficiency into skeletal muscle cells, glucocorticoids were shown here to allow efficient transfection of cultured and mouse human myoblasts, human pulmonary A549 cells, but not dog myoblasts, independently of the transfection protocol, the reporter gene and the transcription promoter employed. Transduction with adenovirus was also increased by dexamethasone. Pretreatment of cells 48 h prior to transfection was most effective and was shown to be concentration-dependent. This effect is mediated by binding to the glucocorticoid receptor, but not by glucocorticoid responsive elements present in the vectors. The acute dexamethasone effect could be due to increased plasmid entry into the cells as suggested by Southern blot, whereas the sustained increase of luciferase activity in dexamethasone-treated cultures may be related to intracellular mechanisms following cell entry. In mice in vivo, a similar increase of luciferase activity upon glucocorticoid treatment was found.

Reversible immortalization of human myogenic cells by site-specific excision of a retrovirally transferred oncogene

Myogenic cells have a limited life span in culture, which prevents expansion at clinically relevant levels, and seriously limits any potential use in cell replacement or ex vivo gene therapy. We developed a strategy for reversibly immortalizing human primary myogenic cells, based on retrovirus-mediated integration of a wild-type SV40 large-T antigen (Tag), excisable by means of the Cre-Lox recombination system. Myogenic cells were transduced with a vector (LTTN-LoxP) expressing the SV40 Tag under the control of an LTR modified by the insertion of a LoxP site in the U3 region. Clonal isolates of Tag-positive cells showed modified growth characteristics and a significantly extended life span, while maintaining a full myogenic potential. Transient expression of Cre recombinase, delivered by transfection or adenoviral vector transduction, allowed excision of the entire provirus with up to >90% efficiency. Cultures of Cre-treated (Tag-) or untreated (Tag+) myogenic cells were genetically labeled with a lacZ retroviral vector, and injected into the regenerating muscle of SCID/bg immunodeficient mice. Tag- cells underwent terminal differentiation in vivo, giving rise to clusters of beta-Gal+ hybrid fibers with an efficiency comparable to that of control untransduced cells. Tag+ cells could not be detected after injection. Neither Tag+ nor Tag- cells formed tumor in this xenotransplantation model. Reversible immortalization by Tag therefore allows the expansion of primary myogenic cells in culture without compromising their ability to differentiate in vivo, and could represent a safe method by which to increase the availability of these cells for clinical application.

Development of approaches to improve cell survival in myoblast transfer therapy

Myoblast transplantation has been extensively studied as a gene complementation approach for genetic diseases such as Duchenne Muscular Dystrophy. This approach has been found capable of delivering dystrophin, the product missing in Duchenne Muscular Dystrophy muscle, and leading to an increase of strength in the dystrophic muscle. This approach, however, has been hindered by numerous limitations, including immunological problems, and low spread and poor survival of the injected myoblasts. We have investigated whether antiinflammatory treatment and use of different populations of skeletal muscle-derived cells may circumvent the poor survival of the injected myoblasts after implantation. We have observed that different populations of muscle-derived cells can be isolated from skeletal muscle based on their desmin immunoreactivity and differentiation capacity. Moreover, these cells acted differently when injected into muscle: 95% of the injected cells in some populations died within 48 h, while others richer in desmin-positive cells survived entirely. Since pure myoblasts obtained from isolated myofibers and myoblast cell lines also displayed a poor survival rate of the injected cells, we have concluded that the differential survival of the populations of muscle-derived cells is not only attributable to their content in desmin-positive cells. We have observed that the origin of the myogenic cells may influence their survival in the injected muscle. Finally, we have observed that myoblasts genetically engineered to express an inhibitor of the inflammatory cytokine, IL-1, can improve the survival rate of the injected myoblasts. Our results suggest that selection of specific muscle-derived cell populations or the control of inflammation can be used as an approach to improve cell survival after both myoblast transplantation and the myoblast-mediated ex vivo gene transfer approach.

High efficiency myogenic conversion of human fibroblasts by adenoviral vector-mediated MyoD gene transfer. An alternative strategy for ex vivo gene therapy of primary myopathies

Ex vivo gene therapy of primary myopathies, based on autologous transplantation of genetically modified myogenic cells, is seriously limited by the number of primary myogenic cells that can be isolated, expanded, transduced, and reimplanted into the patient's muscles. We explored the possibility of using the MyoD gene to induce myogenic conversion of nonmuscle, primary cells in a quantitatively relevant fashion. Primary human and murine fibroblasts from skin, muscle, or bone marrow were infected by an E1-deleted adenoviral vector carrying a retroviral long terminal repeat-promoted MyoD cDNA. Expression of MyoD caused irreversible withdrawal from the cell cycle and myogenic differentiation in the majority (from 60 to 90%) of cultured fibroblasts, as defined by activation of muscle-specific genes, fusion into contractile myotubes, and appearance of ultrastructurally normal sarcomagenesis in culture. 24 h after adenoviral exposure, MyoD-converted cultures were injected into regenerating muscle of immunodeficient (severe combined immunodeficiency/beige) mice, where they gave rise to beta-galactosidase positive, centrally nucleated fibers expressing human myosin heavy chains. Fibers originating from converted fibroblasts were indistinguishable from those obtained by injection of control cultures of lacZ-transduced satellite cells. MyoD-converted murine fibroblasts participated to muscle regeneration also in immunocompetent, syngeneic mice. Although antibodies from these mice bound to adenoviral infected cells in vitro, no inflammatory infiltrate was present in the graft site throughout the 3-wk study period. These data support the feasibility of an alternative approach to gene therapy of primary myopathies, based on implantation of large numbers of genetically modified primary fibroblasts massively converted to myogenesis by adenoviral delivery of MyoD ex vivo.

Muscle maturation: implications for gene therapy

Skeletal muscle is a promising target tissue for gene therapy, for both muscle and non-muscle disorders. A variety of methods have been studied to transfer genes into skeletal muscle, including retroviral, adenoviral and herpes simplex viral vectors. However, various factors impede muscle-based viral gene therapy. Here, we discuss why some viral vectors cannot efficiently transduce mature muscle fibers, and describe some new approaches to overcome this barrier.

Implications of maturation for viral gene delivery to skeletal muscle

Different viral vectors have been analyzed as gene delivery vehicles to skeletal muscle for potentially therapeutic purposes. In this review, we evaluate the application of retroviral, adenoviral, and herpes simplex viral vectors to deliver genes to skeletal muscle and focus on the dramatic loss of viral transduction detected throughout muscle maturation. Recent results suggested that there are several factors involved in the reduced viral transducibility of mature skeletal muscle: muscle cells become post-mitotic in an early stage, the extracellular matrix develops into a physical barrier, and a loss of myoblast mediation occurs since myoblasts progressively become quiescent. Approaches to improve viral gene delivery to mature skeletal muscle may include the use of particular enzymes to increase the permeability of the extracellular matrix, the pre-treatment of the muscle with a myonecrotic agent to induce myoblast mediation, or the application of the myoblast-mediated ex vivo gene transfer.

Muscle regeneration by bone marrow-derived myogenic progenitors

Growth and repair of skeletal muscle are normally mediated by the satellite cells that surround muscle fibers. In regenerating muscle, however, the number of myogenic precursors exceeds that of resident satellite cells, implying migration or recruitment of undifferentiated progenitors from other sources. Transplantation of genetically marked bone marrow into immunodeficient mice revealed that marrow-derived cells migrate into areas of induced muscle degeneration, undergo myogenic differentiation, and participate in the regeneration of the damaged fibers. Genetically modified, marrow-derived myogenic progenitors could potentially be used to target therapeutic genes to muscle tissue, providing an alternative strategy for treatment of muscular dystrophies.

Myoblast-mediated gene transfer to the joint

Several genetic and acquired pathologic conditions of the musculoskeletal system, such as arthritis and damage to ligament, cartilage, and meniscus, may be amenable to gene therapy. Even though ex vivo gene transfer with synovial cells has been shown to deliver genes encoding for anti-arthritic proteins into the rabbit knee joint, its success has been limited by a transient transgene expression. In this study, data were investigated regarding the use of muscle cells as an alternative gene-delivery vehicle to the joint in newborn rabbit and adult severe combined immunodeficiency mice. We demonstrated that myoblasts were transduced more efficiently than synovial cells with use of the same adenoviral preparation in vitro. After intra-articular injection, the engineered muscle cells adhered to several structures in the joint, including the ligament, capsule, and synovium. In addition, myoblasts fused to form many post-mitotic myotubes and myofibers at different locations of the joint of the newborn rabbit 5 days after the injection. In the knee of the adult mouse, myoblasts fused and expressed the reporter gene for at least 35 days after the injection. The presence of post-mitotic myofibers in the knee joint raises the possibility of long-term expression of the secreted protein. Currently, numerous tissues in the joint (ligament, meniscus, and cartilage) have poor intrinsic healing capacity and frequently need surgical corrections. A stable gene-delivery vehicle to the joint producing proteins that ameliorate these different musculoskeletal conditions may change the clinical implications of these pathologies.

A natural hepatocyte growth factor/scatter factor autocrine loop in myoblast cells and the effect of the constitutive Met kinase activation on myogenic differentiation

As a rule, hepatocyte growth factor/scatter factor (HGF/SF) is produced by mesenchymal cells, while its receptor, the tyrosine kinase encoded by the met proto-oncogene, is expressed by the neighboring epithelial cells in a canonical paracrine fashion. In the present work we show that both HGF/SF and met are coexpressed by undifferentiated C2 mouse myoblasts. In growing cells, the autocrine loop is active as the receptor exhibits a constitutive phosphorylation on tyrosine that can be abrogated by exogenously added anti-HGF/SF neutralizing antibodies. The transcription of HGF/SF and met genes is downregulated when myoblasts stop proliferating and differentiate. The coexpression of HGF/SF and met genes is not exclusive to C2 cells since it has been assessed also in other myogenic cell lines and in mouse primary satellite cells, suggesting that HGF/SF could play a role in muscle development through an autocrine way. To analyze the biological effects of HGF/SF receptor activation, we stably expressed the constitutively activated receptor catalytic domain (p65(tpr-met)) in C2 cells. This active kinase determined profound changes in cell shape and inhibited myogenesis at both morphological and biochemical levels. Notably, a complete absence of muscle regulatory markers such as MyoD and myogenin was observed in p65(tpr-met) highly expressing C2 clones. We also studied the effects of the ectopic expression of human isoforms of met receptor (h-met) and of HGF/SF (h-HGF/SF) in stable transfected C2 cells. Single constitutive expression of h-met or h-HGF/SF does not alter substantially the growth and differentiation properties of the myoblast cells, probably because of a species-specific ligand-receptor interaction. A C2 clone expressing simultaneously both h-met and h-HGF/SF is able to grow in soft agar and shows a decrease in myogenic potential comparable to that promoted by p65(tpr-met) kinase. These data indicate that a met kinase signal released from differentiation-dependent control provides a negative stimulus for the onset of myogenic differentiation.

LacZ gene transfer to skeletal muscle using a replication-defective herpes simplex virus type 1 mutant vector

Herpes simplex virus type 1 (HSV-1) represents a promising new viral vector capable of efficient transduction of myofibers in vivo. Here we report on the use of a replication-defective HSV-1 mutant vector (DZ) deleted for the essential immediate early (IE) gene ICP4 for studies of reporter gene transfer and expression following direct inoculation of mouse skeletal muscle. The recombinant vector was engineered to contain the Escherichia coli lacZ gene under transcriptional control of the strong human cytomegalovirus (HCMV) IE promoter. The effect of vector cytotoxicity on the durability of transgene expression following infection of muscle cells in culture and myofibers in vivo revealed that this first-generation HSV vector was cytopathic, limiting the persistence of vector-transduced cells. UV irradiation of vector preparations reduced viral cytotoxicity for myoblasts in culture without reducing significantly beta-galactosidase production. Moreover, muscle cell viability and the durability of transgene expression was enhanced by several days following UV inactivated-vector infection in vivo. Nevertheless, the viral DNA was subsequently lost from vector-inoculated muscle tissue within 2 weeks. This observation indicated that vector toxicity alone did not account for the lack of persistent transgene expression. Longer-term vector transduction and transgene expression was observed, however, following inoculation of immunodeficient SCID mice, indicating that host immunocompetence played an important role in determining the duration of transgene expression in animals. To support this hypothesis, cells expressing CD4 and CD8 antigens have been found in the HSV-1 injected muscle of immunocompetent mice. These data demonstrated that both vector toxicity and vector-induced immunity are significant obstacles to the use of HSV-1 vectors for muscle gene transfer. These impediments must be overcome to further develop HSV vectors for muscle gene therapy applications.

Clonal analysis of stably transduced human epidermal stem cells in culture

We have transduced normal human keratinocytes with retroviral constructs expressing a bacterial beta-galactosidase (beta-gal) gene or a human interleukin-6 (hIL-6) cDNA under control of a long terminal repeat. Efficiency of gene transfer averaged approximately 50% and 95% of clonogenic keratinocytes for beta-gal and hIL-6, respectively. Both genes were stably integrated and expressed for more than 150 generations. Clonal analysis showed that both holoclones and their transient amplifying progeny expressed the transgene permanently. Southern blot analysis on isolated clones showed that many keratinocyte stem cells integrated multiple proviral copies in their genome and that the synthesis of the exogenous gene product in vitro was proportional to the number of proviral integrations. When cohesive epidermal sheets prepared from stem cells transduced with hIL-6 were grafted on athymic animals, the serum levels of hIL-6 were strictly proportional to the rate of secretion in vitro and therefore to the number of proviral integrations. The possibility of specifying the level of transgene expression and its permanence in a homogeneous clone of stem cell origin opens new perspectives in the long-term treatment of genetic disorders.

IL-1alpha gene-transfected human melanoma cells increase tumor-cell adhesion to endothelial cells and their retention in the lung of nude mice

The interleukin-1alpha(IL-1) gene was introduced by retroviral gene transfer into the A375P human melanoma cell line. Two hygromycin-resistant colonies, colony 3 and colony 6, which respectively do not and do express and release IL-1, were selected on the basis of Northern blot and ELISA. Both colonies adhered to resting human endothelial cells (EC) to the same extent. Pre-treatment of EC for 6 hr with conditioned medium (CM) from colony 6, but not from colony 3, increased the adhesion of A375P melanoma and HT-29 colon-carcinoma cells to EC. This increase was blocked by adding interleukin-l-receptor antagonist (IL-1ra) to the EC monolayer. Treatment of EC with colony-6-CM increased the expression of intercellular-adhesion molecule I (ICAM-1), vascular-cell-adhesion molecule I (VCAM-1) and E-selectin. Co-cultivation of colony-6 but not colony-3 melanoma cells with EC caused time-dependent increased expression of these adhesion proteins, reflecting their kinetics of expression on EC. Treating the EC with monoclonal antibodies to VCAM-1 and E-selectin abolished the colony-6-CM-induced increase in adhesion respectively to A375P melanoma and HT-29 colon-carcinoma cells. In vivo, i.v. injection of colony-6 cells in nude mice increased the expression of VCAM-1 on lung microvascular EC. The retention of radiolabeled A375P melanoma cells in the lung was increased in nude mice primed with colony-6 cells, but not with colony-3 cells, injected 6 hr earlier. These results demonstrate that IL-1 produced constitutively by transformed A375P melanoma cells is functionally active, inducing adhesion molecules on EC that enhance their adhesiveness for tumor cells and increase tumor-cell retention in the lung of nude mice.

Construction of human factor IX expression vectors in retroviral vector frames optimized for muscle cells

Development of a highly refined human factor IX (hFIX) expression vector system is critical for establishing a durable hemophilia B gene therapy. Here we report construction of a series of retroviral vectors and identification of an optimal basic structure and components for expressing hFIX in skeletal muscle cells. These vectors, which are derived from Moloney murine leukemia virus (MoMLV) with its enhancer sequence in the 3' long terminal repeat (LTR) deleted, contained internal hFIX expression units inserted in forward configuration without or with a viral vector intron sequence (pdL or pdLIn vector frame, respectively) or in inverted configuration without a viral vector intron sequence (pdLi frame). Internal expression units contained a hFIX cDNA or hFIX minigene (hIXm1 or hIXm2) derived from the hFIX cDNA by insertion of a shortened first intron sequence of the hFIX gene. Regardless of the promoter and vector frame used, both hIXm1 and hIXm2 gave 10- to 14-fold higher hFIX expression compared to those with hFIX cDNA. Internal hFIX transcriptional control units of these vectors were composed of various promoters linked with or without the muscle creatine kinase enhancer (Me) sequence. Promoters tested included those of alpha-actin (alpha A775), beta-actin (beta A280), cytochrome oxidase (CO1250 and CO650), myogenin (Mg1031 and Mg353), and Rous sarcoma virus (RSV). beta A200, which was derived from beta A280 by eliminating potential polyadenylation sites, was also tested. As extensively examined with the myogenin promoter, presence of one or multiple copies of Me in the vectors elevated the expression activity in myotubes by 4.5- to 19-fold over those without Me, but not significantly in myoblasts. Similar enhancements in expression activity with Me were also observed with other promoters, except those of RSV and CO. The latter two showed only modest enhancements in the presence of Me. As assayed with myotubes in culture, the general order of hFIX expression activity of various promoters with four copies of Me in the three different vector frames was beta A280 approximately beta A200 > Mg353 > Mg1031 approximately RSV approximately CO650 approximately alpha A775 > CO1250. One exception was that CO650 showed significantly less activity in pdLi-type vectors than in the pdLIn vectors. Based on the systematic analyses of various structural components, a group of pdLi vectors consisting of beta A200, two to four copies of Me, and hIXm2 was identified to have the optimal basic vector structure to be used in retrovirus for hFIX expression in differentiated skeletal muscle cells. The present studies provide the critical first step for establishing a highly refined hemophilia B gene therapy based on skeletal muscle-targeted hFIX gene transfer.

Prolonged expression of therapeutic levels of human granulocyte colony-stimulating factor in rats following gene transfer to skeletal muscle

Gene transfer to skeletal muscle was examined as a means of gene therapy for neutropenia. A recombinant retrovirus containing a human granulocyte colony-stimulating factor (G-CSF) gene was introduced into primary human or rat myoblasts, which were then shown to produce biologically active G-CSF. Transplantation of G-CSF-producing rat myoblasts into the muscle of syngeneic rats resulted in a 15-fold increase in absolute neutrophil counts. This increase correlated with detection of circulating human G-CSF protein throughout the 6-month duration of the experiment. These results clearly demonstrate long-term production of therapeutically relevant amounts of a human protein by normal cells in vivo.

Murine dendritic cells loaded in vitro with soluble protein prime cytotoxic T lymphocytes against tumor antigen in vivo

The priming of an immune response against a major histocompatibility complex class I-restricted antigen expressed by nonhematopoietic cells involves the transfer of that antigen to a host bone marrow-derived antigen presenting cell (APC) for presentation to CD8+ T lymphocytes. Dendritic cells (DC), as bone marrow-derived APC, are first candidates for presentation of tumor-associated antigens (TAA). The aim of this study was to see whether DC are able to prime in vivo antigen-specific cytotoxic T lymphocytes after exposure to a soluble protein antigen in vitro. Lacking a well-defined murine TAA, we took advantage of beta-galactosidase (beta-gal)-transduced tumor cell lines as a model in which beta-gal operationally functions as TAA. For in vivo priming both a DC line, transduced or not transduced with the gene coding for murine GM-CSF, and fresh bone marrow-derived DC (bm-DC), loaded in vitro with soluble beta-gal, were used. Priming with either granulocyte macrophage colony-stimulating factor-transduced DC line or fresh bm-DC but not with untransduced DC line generated CTL able to lyse beta-gal-transfected target cells. Furthermore, GM-CSF was necessary for the DC line to efficiently present soluble beta-gal as an H-2Ld-restricted peptide to a beta-gal-specific CTL clone. Data also show that a long-lasting immunity against tumor challenge can be induced using beta-gal-pulsed bm-DC as vaccine. These results indicate that effector cells can be recruited and activated in vivo by antigen-pulsed DC, providing an efficient immune reaction against tumors.

Myogenic conversion of mammalian fibroblasts induced by differentiating muscle cells

Somite-derived skeletal myoblasts are supposed to be the sole source of muscle fibre nuclei during pre- and postnatal development, but evidence is accumulating for unorthodox contributions to muscle fibre nuclei from other cell types. For example, in tissue culture, fibroblasts can fuse with dysgenic myoblasts and restore correct membrane function. We report here the results of a series of experiments investigating this phenomenon and its possible mechanism. 10T1/2 cells, infected with a replication defective retrovirus encoding the bacterial enzyme beta-galactosidase, fused to form beta-galactosidase positive, differentiated myotubes when cocultured with differentiating uninfected C2C12 or primary myogenic cells, but this did not occur when they were cocultured with other cells such as 3T3 fibroblasts or PC12 pheochromocytoma cells. Myogenic conversion ranged from 1 to 10% of the 10T1/2 cell population and required close cell interaction between the different cells types: it was not induced by conditioned medium or extracellular matrix deposited by C2C12 cells. Myogenic conversion was also observed in vivo, after injection of similarly infected 10T1/2 cells into regenerating muscle. Conversion was seen also after coculture of uninfected 10T1/2 cells with primary chick myoblasts, thus demonstrating that it was not dependent upon viral infection and that there is no species or class barrier in this phenomenon. Primary fibroblasts, isolated from different organs of transgenic mice carrying a Lac Z marker under the control of a muscle-specific promoter, restricting beta-galactosidase expression to striated muscle cells, also underwent myogenic conversion, when cocultured with C2C12 myoblasts.(ABSTRACT TRUNCATED AT 250 WORDS)

A retroviral vector containing a muscle-specific enhancer drives gene expression only in differentiated muscle fibers

Genetically modified myogenic cells have a number of potentially relevant applications for gene therapy of genetic defects. Retroviral vectors proved to be a safe and efficient tool to transfer and express genes into satellite cells and their differentiated progeny, although muscle-specific regulation of the transferred gene is very difficult to achieve in a conventional vector framework. We modified a Moloney murine leukemia virus (MoMLV)-derived retroviral vector containing a bacterial beta-galactosidase (beta-Gal) reporter gene by inserting a muscle creatinine kinase (MCK) enhancer element into the U3 region of the viral long terminal repeat (LTR). The resulting vector (mLBSN) was transferred into cells of different histological origin, including undifferentiated murine and human myogenic cells, which were unable to express the transgene at detectable levels. Instead, gene expression from the modified LTR was obtained in a mouse myogenic cell line and in human primary satellite cells upon induction of differentiation into myotubes in culture, and correlated with the activation of the muscle differentiation program. beta-Gal-negative, mLBSN-transduced human satellite cells were also transplanted into the quadricep muscle of immunodeficient mice, where activation of the transgene expression was observed in vivo after differentiation and fusion into muscle fibers. These results show that retroviral vectors carrying LTRs modified in the enhancer sequences may be used to target tissue- and differentiation-specific gene expression into the muscle. For practical purposes, satellite cells engineered by muscle-specific retroviral vectors might represent an effective tool to deliver expression of a given gene product specifically into the muscle tissue, avoiding undesired protein accumulation in mononucleated cells. More generally, this type of vector might be useful whenever regulated expression of a transferred gene is necessary in a target cell or tissue.

Gene transfer into skeletal muscles by isogenic myoblasts

The best way to overcome immunorejection in heterologous myoblast transfer (HMT) is by the use of immunodeficient and/or highly immunosuppressed mice as hosts. The same may be attained by autologous myoblast transfer (AMT). In this paper, we describe myoblast transfer in mdx and normal mice where the donor myogenic cells originated from highly inbred litter mates that are considered to be isogenic and thus the procedure is analogous to AMT. The myoblasts were marked in vitro with Rous Sarcoma Virus (RSV)-luciferase (Lux) or RSV-beta-galactosidase (LacZ) reporter genes through transduction mediated by an autonomously replication-defective recombinant human adenovirus. This permitted us to follow their fate after transplantation. mdx and normal mice were irradiated with 20 Gray gamma rays; necrosis and regeneration were induced by intramuscular notexin prior to myoblast injection. In both mdx and normal mice, the expression of luciferase rapidly declined after the injection implying that a large portion of the injected myoblasts were lost by 48 hr, due to undetermined cause(s). The surviving, injected myoblasts well-mosaicized large groups of host fibers but only in the immediate vicinity of the injection. Substantial expression of the reporter gene continued up to 1 month post-transplantation in normal mice, but there was a gradual decline and eventual disappearance of the reporter gene expression in mdx mice. This latter phenomenon was due to the ongoing intense necrosis of muscle fibers in mdx. There was no evidence of immunorejection. These experiments indicate that even in the absence of immunorejection, myoblast transfer suffers from important negative features: major loss of myoblasts within 48 hr after the injection and lack of significant spread of the injected cells from the injection site in the host muscle. These factors, plus the limited proliferative and fusion capacity of Duchenne muscular dystrophy (DMD) myoblasts, make them less than an ideal vector for the dystrophin cDNA for dystrophin gene replacement therapy in DMD.