Challenges: Cell transplantation and gene therapy in muscular dystrophy

Systemic gene transfer reveals distinctive muscle transduction profile of tyrosine mutant AAV-1, -6, and -9 in neonatal dogs

The muscular dystrophies are a group of devastating genetic disorders that affect both skeletal and cardiac muscle. An effective gene therapy for these diseases requires bodywide muscle delivery. Tyrosine mutant adeno-associated virus (AAV) has been considered as a class of highly potent gene transfer vectors. Here, we tested the hypothesis that systemic delivery of tyrosine mutant AAV can result in bodywide muscle transduction in newborn dogs. Three tyrosine mutant AAV vectors (Y445F/Y731F AAV-1, Y445F AAV-6, and Y731F AAV-9) were evaluated. These vectors expressed the alkaline phosphatase reporter gene under transcriptional regulation of either the muscle-specific Spc5-12 promoter or the ubiquitous Rous sarcoma virus promoter. Robust skeletal and cardiac muscle transduction was achieved with Y445F/Y731F AAV-1. However, Y731F AAV-9 only transduced skeletal muscle. Surprisingly, Y445F AAV-6 resulted in minimal muscle transduction. Serological study suggests that the preexisting neutralization antibody may underlie the limited transduction of Y445F AAV-6. In summary, we have identified Y445F/Y731F AAV-1 as a potentially excellent systemic gene transfer vehicle to target both skeletal muscle and the heart in neonatal puppies. Our findings have important implications in exploring systemic neonatal gene therapy in canine models of muscular dystrophy.

Progress in therapy for Duchenne muscular dystrophy

Duchenne muscular dystrophy is a devastating muscular dystrophy of childhood. Mutations in the dystrophin gene destroy the link between the internal muscle filaments and the extracellular matrix, resulting in severe muscle weakness and progressive muscle wasting. There is currently no cure and, whilst palliative treatment has improved, affected boys are normally confined to a wheelchair by 12 years of age and die from respiratory or cardiac complications in their twenties or thirties. Therapies currently being developed include mutation-specific treatments, DNA- and cell-based therapies, and drugs which aim to modulate cellular pathways or gene expression. This review aims to provide an overview of the different therapeutic approaches aimed at reconstructing the dystrophin-associated protein complex, including restoration of dystrophin expression and upregulation of the functional homologue, utrophin.

Past, present and future of myoblast transplantation in the treatment of Duchenne muscular dystrophy

DMD is a genetic X-linked recessive disease that affects approximately one in 3500 male births. Boys with DMD have progressive and predictable muscle destruction because of the absence of Dys, a protein present under the muscle fiber membrane. Dys deficiency induces contraction-related membrane damages, activation of inflammatory-necrosis-fibrosis up to the cardiac-diaphragmatic failure and death. This review supports the therapeutic role of MT associated with immunosuppression in DMD patients, describing the history and the rationale of such approach. The authors underline the importance to evaluate a protocol of myoblast intradermal multi-injection to apply in young DMD patients

Myoblast transplantation: a possible surgical treatment for a severe pediatric disease

Duchenne muscular dystrophy (DMD) is a genetic X-linked recessive orphan disease that affects approximately 1 in 3 500 male births. Boys with DMD have progressive and predictable muscle destruction due to the absence of dystrophin, a protein present under the muscle fiber membrane. This absence induces contraction-related membrane damage and activation of inflammatory necrosis and fibrosis, leading to cardiac/diaphragmatic failure and death. The authors support the therapeutic role of myoblast transplantation in DMD, and describe the history and rationale for such an approach.

The dystrophin glycoprotein complex: signaling strength and integrity for the sarcolemma

The dystrophin glycoprotein complex (DGC) is a specialization of cardiac and skeletal muscle membrane. This large multicomponent complex has both mechanical stabilizing and signaling roles in mediating interactions between the cytoskeleton, membrane, and extracellular matrix. Dystrophin, the protein product of the Duchenne and X-linked dilated cardiomyopathy locus, links cytoskeletal and membrane elements. Mutations in additional DGC genes, the sarcoglycans, also lead to cardiomyopathy and muscular dystrophy. Animal models of DGC mutants have shown that destabilization of the DGC leads to membrane fragility and loss of membrane integrity, resulting in degeneration of skeletal muscle and cardiomyocytes. Vascular reactivity is altered in response to primary degeneration in striated myocytes and arises from a vascular smooth muscle cell-extrinsic mechanism.

Antigen processing and presentation in human muscle: cathepsin S is critical for MHC class II expression and upregulated in inflammatory myopathies

The immunological properties of muscle cells are of critical importance for both the pathogenesis of inflammatory muscle disorders as well as for understanding and controlling novel therapeutic strategies. Muscle cells can present antigens to both CD4 and CD8 cells. However, the cellular biochemistry of antigen processing and presentation by muscle cells is not clear. Cathepsins play a central role in the generation of antigenic peptide and control transport and maturation of MHC class II molecules. To further elucidate the molecular basis for the MHC class II-mediated antigen presentation by muscle cells, we here analyzed cultured human myoblasts and biopsies from inflammatory myopathies with respect to the expression and function of the constituents of the MHC class II antigen presentation machinery. We identified cathepsin S (CatS) as the dominant endocytic protease that is specifically upregulated under inflammatory conditions to significant mRNA levels, synchronously with HLA-DR, -DM and the class II invariant chain (Ii), both in muscle biopsies from affected individuals with inflammatory myopathies and in human myoblasts cultured in the presence of IFN-gamma. This led to translation of the mature CatS polypeptide that was enzymatically active in human myoblasts under inflammatory conditions. By contrast, expression of CatL and CatB was unaffected by IFN-gamma at both the expression and activity levels. CatS activity is required for efficient surface display of MHC class II in this cell type: functional inhibition of CatS using a CatS-selective inhibitor reduced the levels of surface class II alphabeta:peptide complexes on stimulated myoblasts by almost 50%. Surprisingly, and in contrast to B cells and dendritic cells, this was not due to inefficient processing of Ii in the absence of CatS, which was unaffected by the elimination of CatS activity. We therefore conclude that CatS is involved in the regulation of class II expression in human myoblasts independently from Ii processing.

Immunological capabilities of skeletal muscle cells

Muscle is the target of immunological injury in several muscle diseases. It is important therefore to understand the immunological capabilities of muscle cells themselves. Although it is conventional to discuss the effects of the immune system on other cells, tissues or organs, the system's boundaries cannot be sharply drawn, and in an increasing number of ways, the immunological capabilities of non-immune tissues are recognized as determining the course of immune-inflammatory processes. Muscle cells have an inherent ability to express and respond to a variety of immunologically relevant surface molecules, cytokines, and chemokines under inflammatory conditions. The ability of muscle cells to process and present antigens to the immune cells is currently debated; thus, this review is aimed at examining the immunological capabilities of skeletal muscle cells in vitro and in vivo.

Antisense-induced exon skipping and synthesis of dystrophin in the mdx mouse

Duchenne muscular dystrophy (DMD) is a severe muscle wasting disease arising from defects in the dystrophin gene, typically nonsense or frameshift mutations, that preclude the synthesis of a functional protein. A milder, allelic version of the disease, Becker muscular dystrophy, generally arises from in-frame deletions that allow synthesis of a shorter but still semifunctional protein. Therapies to introduce functional dystrophin into dystrophic tissue through either cell or gene replacement have not been successful to date. We report an alternative approach where 2'-O-methyl antisense oligoribonucleotides have been used to modify processing of the dystrophin pre-mRNA in the mdx mouse model of DMD. By targeting 2'-O-methyl antisense oligoribonucleotides to block motifs involved in normal dystrophin pre-mRNA splicing, we induced excision of exon 23, and the mdx nonsense mutation, without disrupting the reading frame. Exon 23 skipping was first optimized in vitro in transfected H-2K(b)-tsA58 mdx myoblasts and then induced in vivo. Immunohistochemical staining demonstrated the synthesis and correct subsarcolemmal localization of dystrophin and gamma-sarcoglycan in the mdx mouse after intramuscular delivery of antisense oligoribonucleotide:liposome complexes. This approach should reduce the severity of DMD by allowing a dystrophic gene transcript to be modified, such that it can be translated into a Becker-dystrophin-like protein.

Antisense-induced exon skipping and synthesis of dystrophin in the mdx mouse

Duchenne muscular dystrophy (DMD) is a severe muscle wasting disease arising from defects in the dystrophin gene, typically nonsense or frameshift mutations, that preclude the synthesis of a functional protein. A milder, allelic version of the disease, Becker muscular dystrophy, generally arises from in-frame deletions that allow synthesis of a shorter but still semifunctional protein. Therapies to introduce functional dystrophin into dystrophic tissue through either cell or gene replacement have not been successful to date. We report an alternative approach where 2'-O-methyl antisense oligoribonucleotides have been used to modify processing of the dystrophin pre-mRNA in the mdx mouse model of DMD. By targeting 2'-O-methyl antisense oligoribonucleotides to block motifs involved in normal dystrophin pre-mRNA splicing, we induced excision of exon 23, and the mdx nonsense mutation, without disrupting the reading frame. Exon 23 skipping was first optimized in vitro in transfected H-2K(b)-tsA58 mdx myoblasts and then induced in vivo. Immunohistochemical staining demonstrated the synthesis and correct subsarcolemmal localization of dystrophin and gamma-sarcoglycan in the mdx mouse after intramuscular delivery of antisense oligoribonucleotide:liposome complexes. This approach should reduce the severity of DMD by allowing a dystrophic gene transcript to be modified, such that it can be translated into a Becker-dystrophin-like protein.

Cell invasion is affected by differential expression of the urokinase plasminogen activator/urokinase plasminogen activator receptor system in muscle satellite cells from normal and dystrophic patients

The aim of this study was to evaluate the differential expression and the function in cell movement and proliferation of the urokinase plasminogen activator (u-PA) system in muscle satellite cells (MSC) of normal individuals and patients with Duchenne muscular dystrophy (DMD). By immunoenzymatic, zymographic, and radioligand binding methods and by quantitative polymerase chain reaction of the specific mRNA we have shown that both normal and DMD MSC produce u-PA and the plasminogen activator inhibitor-1 and express u-PA receptors (u-PAR). During the proliferation phase of their growth-differentiation program, MSC from DMD patients show more u-PAR than their normal counterpart, produce more plasminogen activator inhibitor-1, and release low amounts of u-PA into the culture medium. By Boyden chamber Matrigel invasion assays we have shown that normal MSC are more prone than DMD cells to spontaneous invasion but, when subjected to a chemotactic gradient of u-PA, DMD MSC sense the ligand much better and to a greater extent than normal MSC. u-PA also stimulates proliferation of MSC, but no difference is observable between normal and DMD patients. Antagonization of u-PA/u-PAR interaction with specific anti-u-PA and anti-u-PAR monoclonal antibodies and with antisense oligonucleotides inhibiting u-PAR expression indicates that u-PA/u-PAR interaction is required in spontaneous and u-PA-induced invasion, as well as in u-PA-induced proliferation.

Eukaryotic gene transfer with liposomes: effect of differences in lipid structure

Liposome mediated gene transfer has a great potential in gene therapy. In this review we discuss the physical and chemical properties of cationic liposomes that affect their abilities to mediate gene transfer into eukaryotic cells. The specific focus is on functional domains of cationic lipids. We address polar head variations, counterions, linker bonds, acyl chain variations, as well as composition of liposomes. We additionally discuss different functional groups of lipids affecting lipid bilayer packing, lipid association with DNA, fusion with the cellular membranes and the release of transferred DNA from endosomes into the cytoplasm.

Pluripotential mesenchymal cells repopulate bone marrow and retain osteogenic properties

Precursor cells, isolated from bone marrow, can develop into various cell types and may contribute to skeletal growth, remodeling, and repair. The D1 cell line was cloned from a multipotent mouse bone marrow stromal precursor and has osteogenic, chondrogenic, and adipogenic properties. The osteogenic phenotype of these precursor cells is relevant to the process of fracture healing and osteointegration of prosthetic implants. The D1 cells were labeled genetically using a replication incompetent retroviral vector encoding beta-galactosidase, an enzyme which is used as a marker. Labeled cells are readily identifiable by staining with 5-bromo-4-chloro-3-indoyl-beta-D-galactoside and by flow cytometry, and retain the desired osteogenic characteristics in vivo as shown by von Kossa staining, alkaline phosphatase assay, an increase in cyclic adenosine monophosphate in response to parathyroid hormone, osteocalcin messenger ribonucleic acid production, and bone formation in diffusion chambers. In addition, the cells cloned from marrow stroma repopulate the marrow of host mice, persist for several weeks, and retain their osteogenic potential ex vivo. The data suggest that such cells may be used to replenish the number of osteoprogenitors in marrow, which appear to decrease with age, thereby leading to recovery from bone loss and improved bone growth and repair. Labeling these cells creates a model in which to study the potential of such cells to participate in fracture repair, ingrowth around prosthetic implants, treatment of osteoporosis, and to explore the possibility of gene delivery to correct mutations or defects in metabolism that are responsible for certain skeletal abnormalities.

Forced MyHCIIB expression following targeted genetic manipulation of conditionally immortalized muscle precursor cells

The ability to carry out gene targeting in somatic stem cells while maintaining their stem cell characteristics would have important implications for gene therapy and for the analysis of gene function. Using mouse myoblasts, we have explored this possibility by attempting to alter the promoter of a myosin heavy chain gene (MyHCIIB) characteristic of physiologically "fast" muscle so as to force its unscheduled expression in physiologically "slow" muscle fibers. Conditionally immortalized muscle precursor cells were transfected with a gene targeting construct designed to replace the MyHCIIB promoter with that for the carbonic anhydrase III gene (CAIII), which is highly expressed in slow muscle. A potentially targeted clone was isolated and differentiated in culture to form myotubes which expressed MyHCIIB. Cells from the same clone were injected into both slow and fast muscle of host mice, where they contributed to fiber formation. In slow muscle, the fibers derived from this clone did not express MyHCIIB; this may reflect an instability of the targeted MyHCIIB locus and/or a failure of the hybrid promoter to function in slow fibers in vivo. Nonetheless, we have demonstrated that a "promoter knock-in" gene targeting procedure can be used to generate unique MyHCIIB-expressing myotubes in culture and that conditionally immortalized myoblasts can be subjected to extensive passaging and genetic manipulation without losing their ability to form fibers in culture and in vivo.

Transplantation of myoblasts from a transgenic mouse overexpressing dystrophin prduced only a relatively small increase of dystrophin-positive membrane

Myoblast cultures from normal and Tg-MDA (transgenic mouse overexpressing dystrophin 50-fold) mice were transplanted into dystrophin-deficient mdx mouse muscles. Four weeks after transplantation, dystrophin-positive fibers were observed four times more frequently in cross sections of muscles injected with Tg-MDA. Myoblasts from Tg-MDA mice also expressing the beta-gal transgene (Tg-MDA/beta-gal) and myoblasts from beta-gal transgenic mice containing one normal dystrophin gene (normal/beta-gal) were also transplanted into mdx mouse muscles. Four weeks after transplantation, the fiber length positive for dystrophin (nuclear domain) was shorter (439 +/- 326 microm) than the beta-gal nuclear domain (1466 +/- 713 microm) of the same fiber when normal/beta-gal myoblasts were transplanted, but increased (1302 +/- 487 microm) when Tg-MDA/beta-gal myoblasts were used. These experiments show that despite the presence in Tg-MDA myoblasts of constructions which lead in vivo in transgenic mice to an overexpression of dystrophin 50-fold, the membrane area over which dystrophin was expressed was increased only threefold. This observation is also expected for vector-mediated gene therapy.

Ciliary neurotrophic factor stimulates in vivo myotube formation in mice

It is known that ciliary neurotrophic factor (CNTF) administration reduces the atrophy observed with denervation, suggesting its role as a trophic factor for muscle cells. At the present, we studied the effects of 'in vivo' CNTF administration on the regenerative capacity of skeletal muscle fibres. Adult mice had their extensor digitorium longus muscle subjected to a denervation-devascularization lesion. CNTF (0.5 ng/microl) was administered using osmotic pumps implanted subcutaneously in unrestrained mice. CNTF was delivered into the muscle's region at a rate of 1 microl/h from 1 to 8 days after denervation. The results show that CNTF increased the number of regenerating myofibres by day 4. From day 7 on, the values seen on control and CNTF-treated groups were not significantly different. Our results show that 'in vivo' CNTF administration accelerates myotube differentiation.

Myonuclear apoptosis in dystrophic mdx muscle occurs by perforin-mediated cytotoxicity

Myonuclear apoptosis is an early event in the pathology of dystrophin-deficient muscular dystrophy in the mdx mouse. However, events that initiate apoptosis in muscular dystrophy are unknown, and whether elimination of apoptosis can ameliorate subsequent muscle wasting remains a major question. We have tested the hypothesis that cytotoxic T-lymphocytes initiate myonuclear apoptosis in dystrophic muscle, and examined whether perforin-mediated cytotoxicity plays a role in the pathophysiology of muscular dystrophy. Mdx mice showed muscle invasion by cytotoxic T cells and helper T cells at the onset of histologically detectable muscle fiber pathology. At this time, perforin-expressing cells were also present at elevated concentration. Mdx mice depleted of CD8(+) cells showed a significant reduction of apoptotic myonuclei concentration and a reduction in necrosis, judged by macrophage invasion of muscle fibers. Double-mutant mice, deficient in dystrophin and perforin, showed nearly complete absence of myonuclear apoptosis, and a significant reduction in the concentration of macrophages in the connective tissue surrounding muscle fibers. However, muscle fiber invasion by macrophages was not reduced significantly in double mutant mice. Thus, cytotoxic T-lymphocytes contribute significantly to apoptosis and necrosis in mdx dystrophy, and perforin-mediated killing is primarily responsible for myonuclear apoptosis.

Role of non-major histocompatibility complex antigens in the rejection of transplanted myoblasts

Myoblasts obtained from donors histoincompatible for several non-major histocompatibility complex antigens (i.e., including minor histocompatibility antigens) and from syngeneic donors were transplanted without any immunosuppression into the muscles of male dystrophic C57BL/10J mdx/mdx mice. Myoblasts from syngeneic mice resulted in the formation of a high percentage of dystrophin-positive fibers 16 weeks after the transplantation. There was no evidence of a cellular immune reaction against the donor myoblasts, i.e., no infiltration by CD4 or CD8 lymphocytes and no increased expression of granzyme B and interferon-gamma mRNAs. Transplantation of myoblasts obtained from donors histoincompatible only for non- major histocompatibility complex antigens produced a transient increase of dystrophin-positive fibers at 4 weeks after transplantation for some donor strains but not for others. For donor strains that did produce an increase at 4 weeks, the number of dystrophin-positive fibers was reduced 16 weeks after the transplantation. There was evidence of a cellular immune reaction-infiltration by CD4 and by CD8 lymphocytes and increased expression of granzyme B and interferon-gamma mRNAs. Transplantation of myoblasts obtained from male C57BL/10J +/+ mice into female C57BL/10J mdx/mdx mice also led to the presence of only a few dystrophin-positive fibers with the same signs of cellular immune reaction. In this later case, the cellular immune response was attributed to the H-Y minor antigens. Finally, antibodies against fetal calf serum were detected after both syngeneic and nonsyngeneic transplantations, indicating that the culture medium may also be a source of antigens. In mice, the presence of these antibodies against culture medium did not reduce the success of a first syngeneic transplantation.

Loss of the position-dependent reinnervation of regenerated toad (Bufo marinus) glutaeus muscle

The terminations of motor axons in the toad glutaeus muscle show a course dependency on the segmental origins of the axons on the spinal cord. Rostral axons in spinal nerve 8 innervate muscle fibres near the ventral surface of the muscle, while caudal axons in spinal nerve 9 innervate fibres mostly towards the opposing dorsal surface. Axons originating between these extremes tend to innervate the central regions of the muscle. A similar topographic projection is reestablished after denervation and when regenerating axons reinnervate the muscle via entirely novel pathways (Brown and Everett [1991] J. Comp. Neurol. 309:495-506). The findings are compatible with the graded expression of a determinant within the glutaeus muscle that biases the formation of synapses between positionally matched muscle fibres and motor axons. In the present work, we provide strong support for this view by showing that when the muscle is reinnervated by axons arising from only one spinal nerve, they expand their projection and form synapses in the muscle in a topographically appropriate manner. In a second experiment, we tested whether a muscle that had regenerated from its resident myogenic (satellite) cell population would be similarly reinnervated. This experiment was prompted by the work of others (Donoghue et al. [1992] Cell 69:67-77) showing that the myogenic precursor cells in adult muscle are a repository of "positional memory." In our experiments, a glutaeus muscle was removed from adult toads and soaked in bupivacaine for a brief period to destroy the muscle fibres before being sutured back into its normal position in the limb. The distribution of motor units in the muscles was determined by glycogen depletion after allowing 3-4 months for the muscles to regenerate from their satellite cell population and to become reinnervated. We found that muscle fibres belonging to single motor units were dispersed widely in the regenerated muscles and showed no topographic organisation. We conclude that the positional cues that direct topographic map formation are available to motor axons when they reinnervate a denervated mature muscle, but play no role in the reinnervation of a regenerated muscle.

Dystrophin-positive muscle fibers following C2 myoblast transplantation into mdx nude mice

To determine when and how the dystrophin-positive muscle fibers are formed after myoblast transplantation into dystrophin-negative muscles, the tibialis anterior (TA) muscle from mdx nude mouse was chronologically examined after C2 myoblast transplantation by immunohistochemical and glucose 6-phosphate isomerase (GPI) isoenzyme analyses. The host TA muscle transplanted with C2 myoblasts became necrotic with accumulation of basic fibroblast growth factor in the necrotic areas. This may stimulate concomitant proliferation of the host satellite cells and C2 myoblasts. Small dystrophin-positive muscle fibers appeared in the necrotic areas 3 days after transplantation. This TA muscle contained two different kinds of homodimer GPI isoenzymes but did not contain the heterodimer, suggesting rare fusion of host and donor cells. The dystrophin-positive muscle fibers in the necrotic areas rapidly increased in number and in size by 7 days, but they were smaller than the original host muscle fibers. They had central nuclei, indicating that they were regenerating fibers. The presence of heterodimer GPI isoenzyme in these muscles indicated that the regenerating fibers were mosaic host/donor muscle fibers. The dystrophin-positive muscle fibers are probably formed first by fusion of donor cells with each other and then later by the fusion of host satellite and donor cells.

Overexpression of genes in health and sickness. A bird's eye view

Many human disorders are associated with gene alterations, such as translocations, deletions, insertions, inversions, rearrangements and point mutations. However, an overexpression of certain normal genes could also contribute to the pathology of neurological disorders, retinal degeneration, diabetes, fibrosis of lung, cardiac and skin, programmed cell death and cancer. This implies that the regulated expression of normal genes is an important factor in determining human health. An understanding of the mechanisms involved in the control of expression of normal genes may provide a greater or more refined success in correcting, delaying or possibly preventing the disorders by a gene therapeutic approach.

"Bystander" damage of host muscle caused by implantation of MHC-compatible myogenic cells

Transplantation of normal myoblasts has been considered a potential therapy for muscle dystrophies. While survival of implanted cells has been described in animal experiments and in human trials, functional effects remained unclear. Here we report on survival of progenors of implanted C2nlsBAG cells in regenerating muscles but irreversible net loss in muscle tissue and contractile force. This is caused by immune rejection of implanted myoblasts despite MHC-compatibility and "bystander" damage of host muscle tissue.

Myoblast transfer and gene therapy in muscular dystrophies

Myoblast transfer therapy and gene therapy have both been proposed as potential treatments for inherited myopathies, such as Duchenne muscular dystrophy (DMD). The success of myoblast implantation in mouse models, where problems such as immune rejection are easily overcome, have led to similar experiments being attempted on Duchenne patients with limited, if any, success. Gene therapy, either by viral vectors or direct injection of the plasmid, has also had some success in animal models. Although both techniques, either separately or in combination, show some promise for the treatment of DMD, there are still many issues to be investigated in animal models, including the following: What is the best source of muscle precursor cells (mpc), and how may sufficient cells be obtained? What is the best vehicle for gene therapy? How far from the injection site can an implanted cell or gene have an effect? How can immune rejection of the injected cells or introduced protein be overcome? Does the introduced dystrophin lead to improved muscle function? Can cardiac muscle can be successfully treated by gene therapy? Can skeletal muscle which has undergone a great deal of damage be improved by either cell or gene therapy?

Functional effects of myoblast implantation into histoincompatible mice with or without immunosuppression

1. The goals of this study were to evaluate the immunogenicity of myogenic cells (MCs) (1) immediately after implantation into regenerating muscles, and (2) following their maturation under initial immunosuppression. Implanted mouse soleus muscles were evaluated by isometric tension recordings in vitro followed by histological investigations on frozen sections. 2. Implantation of non-histocompatible myoblasts into cryodamaged soleus muscles of CBA/J mice induced immune rejection which caused large and permanent deficits in muscle force: 4-42 weeks postimplantation maximal tetanic tension was 50-60% that of intact or regenerated cryodamaged control muscles without tendency for recovery or histological signs of muscle regeneration. Specific tension (force per unit muscle weight) was also significantly reduced. 3. On frozen sections, only 62 +/- 12% of the total area was desmin-positive, that is, occupied by muscle fibres, versus 90 +/- 4% in regenerated and 92 +/- 3% in intact muscles. Also, the total number of muscle fibre profiles was significantly reduced. 4. Under immune suppression with cyclosporin A (CsA), large muscles developed within 4 weeks. Following CsA withdrawal, muscle weight and force, in addition to desmin-positive areas on cross-sections, gradually declined over several months despite continual regeneration, indicating retarded immune rejection. 5. Initial application of CsA for 8 weeks after implantation, instead of 4 weeks, did not result in better survival of the implants, nor did a higher initial dose of CsA (100 instead of 50 mg kg-1 day-1). Prolonged continuous application of a reduced dose (25 mg kg-1 day-1) did not prevent muscle wasting but caused an additional delay. 6. It is concluded that histoincompatible myoblasts are highly immunogenic and that immune rejection causes large and permanent muscle deficits indicating elimination of host muscle tissue. Initial transient immunosuppression protects the incompatible cells, but after withdrawal, prolonged immune rejection and retarded muscle wasting occur.

Cellular and molecular reactions in mouse muscles after myoblast implantation

Implantation of skeletal muscle precursor cells is a potential means of cell-mediated gene therapy. One unresolved question is the degree of immunogenicity of such myoblasts. We designed the extreme situation of implanting cells of a non-histocompatible myoblast cell line into cryodamaged, but regeneration-capable, muscles of adult mice. Without immunosuppression donor cells are rejected within the first weeks. Immunosuppression with Cyclosporin A prevented invasion of T-lymphocytes and allowed differentiation of implanted myoblasts into myofibres as well as down-regulation of MHC expression. Still, withdrawal of Cyclosporin A after 4 weeks triggered lymphocyte invasion and cytotoxic cell reactions with rejection of donor tissue. Although the vast majority of muscle fibres was MHC-negative 1-4 days after Cyclosporin A withdrawal, single small desmin-positive profiles were weakly positive for donor MHC. Parallel with the increase in the number of lymphocytes, larger numbers of small and large muscle fibres expressed high levels of either donor, host or both, class I--but not class II--molecules. Surprisingly, immune reactions continued over several months, causing gradual loss of muscle tissue. Donor class I molecules persisted for more than 6 months after Cyclosporin A withdrawal, clearly indicating survival of donor muscle fibres despite ongoing rejection. Indirect evidence on the other hand suggests additional loss of host fibres, possibly caused by cytokine release from the immune cells (bystander damage). We conclude that transient treatment with Cyclosporin A induced a kind of tolerance related to the maturation and down-regulation of class I antigens in donor muscle fibres. It is suggested that the start of immune reaction following Cyclosporin A withdrawal is initiated by remaining small amounts of donor MHC molecules, possibly related to the continuous proliferation of the cell-lined-derived donor myoblasts.

Synthesis and secretion of matrix-degrading metalloproteases by human skeletal muscle satellite cells

The expression of matrix-degrading metalloproteases (MMPs) by human skeletal muscle satellite cells was investigated by zymography of cell culture media and by Northern blot analysis of mRNA prepared from satellite cells. Zymography in gelatin substrate gels revealed that satellite cells constitutively synthesize and secrete 72 kDa gelatinase (MMP-2). In addition, treatment of satellite cell cultures with phorbol ester resulted in an induction of 92 kDa gelatinase (MMP-9) activity. On casein substrate gels, little or no proteolytic activity was detectable in control or phorbol ester treated satellite cell cultures, suggesting that compared to fibroblasts, satellite cells secrete little or no interstitial collagenase (MMP-1) or stromelysin (MMP-3) activity. Northern blotting, however, revealed that there is detectable expression of mRNA transcripts encoding MMP-1 in satellite cell cultures, and that increased accumulation of MMP-1 mRNA transcripts occurs upon treatment of these cells with phorbol ester. In contrast, no constitutive, or induced expression of transcripts encoding MMP-3 was detectable in satellite cells. These findings show that satellite cells can synthesize and secrete selected members of the MMP family and suggest that skeletal muscle cells may participate directly in remodelling of the extracellular matrix during myogenesis and the regeneration of skeletal muscle.

Genetically modified skin to treat disease: potential and limitations

Molecular definition of disease at the level of the gene and advances in recombinant DNA technology suggest that many diseases are amenable to correction by genes not bearing the defective elements that result in disease. Many questions must be answered before this therapy can be used to correct chronic diseases. These questions fall into safety and efficacy categories. Experience with transplanting cellular elements of skin or skin substitutes (defined as skin that possess the cell types and a dermal structure to develop into a functioning skin) to athymic rodents is considerable and is seen as a system where these questions can be answered. This paper reviews these questions and presents our early analysis of genetically modified cells in skin substitutes in vivo and in vitro. Experimental data demonstrate that both a matrix of woven nylon, housing a fibroblast generated collage, and dead dermis can be utilized to shuttle genetically modified human fibroblasts from the laboratory to an in vivo setting. Genetically modified fibroblasts do not migrate from the shuttle to the surrounding tissue. The survival of significant numbers, approximately 70%, of genetically modified fibroblasts for at least 6 weeks in these shuttles, supports this general approach as having clinical utility. It is also concluded that skin substitute systems can be used to generate a genetically modified skin in vitro that has the capacity to develop into functional skin in vivo. Further, as genetically modified keratinocytes differentiate there is increased production by the transgene, supporting the concept that keratinocytes have true potential as shuttles for therapeutic genes. This work demonstrates that transplantation of systems containing genetically modified cells of the skin can be used to experimentally define many aspects of gene therapy using skin before this technology is taken to the clinic. Examples include determining the effect of gene transduction and expression on structure and function of the genetically modified skin as well as on distant skin and an assessment of the translational capacity of the transgene as function of time and cell number.

Enhancement of skeletal muscle regeneration

We have studied the effect of adding extra satellite cells or soluble factors from crushed muscle on regeneration of minced fragments from rat tibialis muscle. The muscle mince was wrapped in an artificial epimysium to prevent adhesions and cell immigration from adjacent muscles. Regeneration was quantitatively assessed by electrophoretic determination of the muscle-specific form of creatine kinase. Control minces exhibited three periods of change in creatine kinase activity during a 7-week regeneration period. Activity fell rapidly during the first week, then rose gradually from 1-3 weeks and increased more rapidly from 3-7 weeks. To augment the original complement of myogenic cells, satellite cells were isolated from the contralateral muscle, purified by density gradient centrifugation, and expanded in culture for 3 days before adding to the muscle mince. The added cells resulted in a 3-fold enhancement of creatine kinase activity throughout the regeneration period. Soluble muscle extract incorporated into a collagen matrix also stimulated regeneration when added to muscle mince. The extract accelerated the rate of creatine kinase increase during the 1-3 week period beyond that observed in the control or cell augmented mince, suggesting that factors in the extract may facilitate revascularization or reinnervation. The specific activity of creatine kinase was increased in regenerates augmented with both cells and extract, indicating that the effects enhance primarily myogenic processes.