Diaphragm remodeling and compensatory respiratory mechanics in a canine model of Duchenne muscular dystrophy.

Ventilatory insufficiency remains the leading cause of death and late stage morbidity in Duchenne muscular dystrophy (DMD). To address critical gaps in our knowledge of the pathobiology of respiratory functional decline, we used an integrative approach to study respiratory mechanics in a translational model of DMD. In studies of individual dogs with the Golden Retriever muscular dystrophy (GRMD) mutation, we found evidence of rapidly progressive loss of ventilatory capacity in association with dramatic morphometric remodeling of the diaphragm. Within the first year of life, the mechanics of breathing at rest, and especially during pharmacological stimulation of respiratory control pathways in the carotid bodies, shift such that the primary role of the diaphragm becomes the passive elastic storage of energy transferred from abdominal wall muscles, thereby permitting the expiratory musculature to share in the generation of inspiratory pressure and flow. In the diaphragm, this physiological shift is associated with the loss of sarcomeres in series (∼ 60%) and an increase in muscle stiffness (∼ 900%) compared with those of the nondystrophic diaphragm, as studied during perfusion ex vivo. In addition to providing much needed endpoint measures for assessing the efficacy of therapeutics, we expect these findings to be a starting point for a more precise understanding of respiratory failure in DMD.

Myocardial gene delivery using molecular cardiac surgery with recombinant adeno-associated virus vectors in vivo.

We use a novel technique that allows for closed recirculation of vector genomes in the cardiac circulation using cardiopulmonary bypass, referred to here as molecular cardiac surgery with recirculating delivery (MCARD). We demonstrate that this platform technology is highly efficient in isolating the heart from the systemic circulation in vivo. Using MCARD, we compare the relative efficacy of single-stranded (ss) adeno-associated virus (AAV)6, ssAAV9 and self-complimentary (sc)AAV6-encoding enhanced green fluorescent protein, driven by the constitutive cytomegalovirus promoter to transduce the ovine myocardium in situ. MCARD allows for the unprecedented delivery of up to 48 green fluorescent protein genome copies per cell globally in the sheep left ventricular (LV) myocardium. We demonstrate that scAAV6-mediated MCARD delivery results in global, cardiac-specific LV gene expression in the ovine heart and provides for considerably more robust and cardiac-specific gene delivery than other available delivery techniques such as intramuscular injection or intracoronary injection; thus, representing a potential, clinically translatable platform for heart failure gene therapy.

Molecular approaches to therapy for Duchenne and limb-girdle muscular dystrophy.

The muscular dystrophies are a heterogeneous group of heritable disorders in which progressive muscle degeneration leads to regional or generalized weakness. Recent advances in molecular genetics, cell biology and vector discovery have improved the outlook for therapeutic intervention. This review focuses on novel approaches to the study of disease pathogenesis and refinements in gene- and cell-based strategies for protein restoration in Duchenne and limb-girdle muscular dystrophy, and concludes with a brief discussion of priorities for future clinical investigation.

Lung volume reduction surgery restores the normal diaphragmatic length-tension relationship in emphysematous rats.

OBJECTIVE: Improved respiratory muscle function is a major effect of a lung volume reduction surgery. We studied length adaptation in rat diaphragmatic muscle in an attempt to elucidate the mechanism by which diaphragmatic function improves after this controversial operation. METHODS: We developed a model of elastase-induced emphysema and bilateral volume reduction through median sternotomy in rats. Five months after emphysema induction, maximum exchangeable lung volume was determined in intubated and anesthetized control animals and animals with emphysema. Costal diaphragmatic length was measured in vivo, and the length at which maximal twitch force is generated was determined on muscle strips in vitro. Also 5 months after elastase administration, another cohort underwent volume reduction or sham sternotomy. Five months after the operation, these animals were similarly studied. RESULTS: Lung volume was increased in emphysematous rats versus control rats (50.9 +/- 1.7 vs 45.4 +/- 1.3 mL, P =.001). Lung volume was decreased in emphysematous animals that had undergone volume reduction versus sham sternotomy (44.7 +/- 0.60 vs 49.4 +/- 1.0 mL, P =.001). In situ diaphragm length (1.99 +/- 0.04 vs 2.24 +/- 0.07 cm, P =.001) and the length at which maximal twitch force is generated (2.25 +/- 0.06 vs 2.48 +/- 0.09 cm, P =.038) were shorter in emphysematous than control animals. After volume reduction, in situ diaphragm length (2.13 +/- 0.06 vs 1.83 +/- 0.02 cm, P <.001) and the length at which maximal twitch force is generated (2.50 +/- 0.08 vs 2.27 +/- 0.06 cm, P =.013) were longer than in animals undergoing sham sternotomy. CONCLUSIONS: In this experimental model of emphysema and lung volume reduction surgery, emphysema shortens the length at which maximal twitch force is generated and shifts the diaphragmatic length-tension curve to lower lengths; volume reduction returns the length at which maximal twitch force is generated toward normal and shifts the diaphragmatic length-tension curve back to longer lengths. This restoration toward normal physiology may enable the improvement in diaphragmatic function seen after lung volume reduction surgery. The mechanism by which these length adaptations occur merits further investigation.

Human skeletal myosin heavy chain genes are tightly linked in the order embryonic-IIa-IId/x-ILb-perinatal-extraocular.

Myosin heavy chain (MyHC) is the major contractile protein of muscle. We report the first complete cosmid cloning and definitive physical map of the tandemly linked human skeletal MyHC genes at 17p13.1. The map provides new information on the order, size, and relative spacing of the genes. and it resolves uncertainties about the two fastest twitch isoforms. The physical order of the genes is demonstrated to contrast with the temporal order of their developmental expression. Furthermore, nucleotide sequence comparisons allow an approximation of the relative timing of five ancestral duplications that created distinct genes for the six isoforms. A firm foundation is provided for molecular analysis in patients with suspected primary skeletal myosinopathies and for detailed modelling of the hypervariable surface loops which dictate myosin's kinetic properties.

Stable restoration of the sarcoglycan complex in dystrophic muscle perfused with histamine and a recombinant adeno-associated viral vector.

Limb-girdle muscular dystrophies 2C-F represent a family of autosomal recessive diseases caused by defects in sarcoglycan genes. The cardiomyopathic hamster is a naturally occurring model for limb-girdle muscular dystrophy caused by a primary deficiency in delta-sarcoglycan. We show here that acute sarcolemmal disruption occurs in this animal model during forceful muscle contraction. A recombinant adeno-associated virus vector encoding human delta-sarcoglycan conferred efficient and stable genetic reconstitution in the adult cardiomyopathic hamster when injected directly into muscle. A quantitative assay demonstrated that vector-transduced muscle fibers are stably protected from sarcolemmal disruption; there was no associated inflammation or immunologic response to the vector-encoded protein. Efficient gene transduction with rescue of the sarcoglycan complex in muscle fibers of the distal hindlimb was also obtained after infusion of recombinant adeno-associated virus into the femoral artery in conjunction with histamine-induced endothelial permeabilization. This study provides a strong rationale for the development of gene therapy for limb-girdle muscular dystrophy.

The myosin filament XV assembly: contributions of 195 residue segments of the myosin rod and the eight C-terminal residues.

A mixture of two peptides of approximately M(r) 13000 has been isolated from a papain digest of LC2 deficient myosin. The peptides assemble into highly ordered aggregates which in one view are made up of strands of pairs of dots with an average side to side spacing of 13.0 nm and an average axial repeat of 9.0 nm. In another view there are strands of single dots with a side-to-side spacing of 7.8 nm and an axial repeat of 9.1 nm. From N-terminal peptide sequencing, the two peptides have been shown to come from regions of the myosin rod displaced by 195 residues. We have shown that either peptide alone can assemble to form the same aggregates. The 195 residue displacement of the M(r) 13000 peptides corresponds closely to the 196 residue repeat of charges along the myosin rod. This finding permits us to designate 195 residue segments of the myosin rod and to relate assembly characteristics directly to the similar 195 residue segments and 196 residue charge repeat. The most C-terminal 195 residue segment carries information for assembly into helical strands. The contiguous 195 residue segment, in major part, carries information for the unipolar assembly, characteristic of the assembly in each half of the myosin filament. The next contiguous 195 residue segment, in major part, carries information for bipolar assembly which is characteristic of the bare zone region of the filament; and for the transition from the bipolar bare zone to unipolar assembly. The effect of the eight C-terminal residues of the myosin rod on the assembly of the contiguous 195 residues has also been studied. The entire fragment of 195 + eight C-terminal residues assembled to form helical strands with an axial repeat of 30 nm. Successive deletion of charged residues changed the axial repeat of the helical strands suggesting that the charged residues at the C-terminus are involved in determining the pitch in the helical assembly of the contiguous 195 residues.

In vivo expression of full-length human dystrophin from adenoviral vectors deleted of all viral genes.

Adenoviral vectors have been shown to effect efficient somatic gene transfer in skeletal muscle and thus offer potential for the development of therapy for Duchenne muscular dystrophy (DMD). Efficient transfer of recombinant genes has been demonstrated in skeletal muscle using recombinant adenoviruses deleted of E1. Application of this vector system to the treatment of DMD is limited by the vector immunogenicity, as well as by size constraints for insertion of recombinant genes, precluding the incorporation of a full-length dystrophin minigene construct. We describe in this study the use of helper adenovirus to generate a recombinant vector deleted of all viral open reading frames and containing a full-length dystrophin minigene. We show that this deleted vector (delta vector) is capable of efficiently transducing dystrophin in mdx mice, in myotubes in vitro and muscle fibers in vivo. Our modification of adenoviral vector technology may be useful for the development of gene therapies for DMD and other diseases.

Characterization of a human perinatal myosin heavy-chain transcript.

Using a monoclonal antibody specific to the neonatal myosin heavy chain, we have cloned the full-length heavy chain cDNA from an 18-week human fetal cDNA library. Ribonuclease protection assays were used to survey a human muscle collection ranging from 11 weeks gestation to 16 years. Expression of the RNA encoded by this cDNA was observed at 20 and 21 weeks gestation and at 2 days after birth. No expression was observed at 13.5 weeks, before 2 years, at 2 years, or after 2 years gestation. Due to the timing of its expression, this cDNA appears to represent of the human fetal myosin heavy chain. Sequencing of the entire 6010 bases showed high similarity to the rat perinatal myosin heavy chain [Periasamy, M., Wieczorek, D. F. & Nadal-Ginard, B. (1984) J. Biol. Chem. 21, 13,573-13,578]. However, moderate divergence was observed when compared to a previously described human perinatal myosin heavy chain [Karsch-Mizrachi, I., Feghali, R., Shows, T. B. & Leinwand, L. A. (1990) Gene 89, 289-294; Feghali, R. & Leinwand, L. A. (1989) J. Cell Biol. 108, 1791-1797]. Restriction fragment-length polymorphism analyses of sites in both the S1 and rod domains showed the presence of this fetal myosin heavy chain sequence in all 27 genomic samples examined. Restriction fragment-length polymorphism analysis failed to find the previously described perinatal isoform in any sample.

Percutaneous balloon catheter vascular control for infected axillary artery pseudoaneurysm.

The treatment of choice for the septic aortic prosthesis remains graft excision and extra-anatomic revascularization usually with axillofemoral bypass. Several recent retrospective series report secondary graft infection rates in these remedial procedures that range from 5-10%. In this setting, proximal axillary artery anastomotic disruption with pseudoaneurysm formation poses an especially vexing problem for the surgeon. We report a case of secondary graft infection presenting as unusually large, perianastomotic pseudoaneurysm formation. Our desire to avoid thoracotomy and the potential intrathoracic spread of infection prompted us to achieve proximal control by endovascular means. Balloon occlusion of the axillary artery proximal and distal to the graft anastomosis was achieved angiographically by percutaneous puncture of the ipsilateral brachial artery at two separate sites. This facilitated prompt removal of the infected graft with minimal intraoperative blood loss. Our favorable experience with this approach suggests a role for its more general application in the management of infected perianastomotic pseudoaneurysm.

Adaptations in myosin heavy chain expression and contractile function in dystrophic mouse diaphragm.

The X chromosome-linked muscular dystrophic (mdx) mouse lacks the subsarcolemmal protein dystrophin and thus represents a genetic homologue of human Duchenne muscular dystrophy. The present study examined alterations in diaphragm contractile properties and myosin heavy chain (MHC) expression in young (3-4 mo) and old (22-24 mo) control and mdx mice. In young mdx mice, maximum isometric tension (Po) was reduced to 50% of control values. An increase in fibers coexpressing types I (slow) and IIa MHC as well as regenerating fibers expressing embryonic MHC occurred, whereas IIx/b fibers were decreased. In the old mdx group, Po underwent a further reduction to 25% of control, and there was a slowing of twitch kinetics along with markedly increased diaphragm endurance. These changes were associated with an approximate sevenfold increase in type I MHC fibers and virtual elimination of the IIx/b fiber population; there was no detectable embryonic MHC expression. We conclude that the mdx diaphragm responds to progressive muscle degeneration with transition to a slower phenotype associated with reduced power output and augmented muscle endurance. In the setting of progressive muscle fiber destruction, these changes may help preserve contractile function and promote greater survival of remaining muscle fibers by decreasing cellular energy requirements.

Dystrophin protects the sarcolemma from stresses developed during muscle contraction.

The protein dystrophin, normally found on the cytoplasmic surface of skeletal muscle cell membranes, is absent in patients with Duchenne muscular dystrophy as well as mdx (X-linked muscular dystrophy) mice. Although its primary structure has been determined, the precise functional role of dystrophin remains the subject of speculation. In the present study, we demonstrate that dystrophin-deficient muscle fibers of the mdx mouse exhibit an increased susceptibility to contraction-induced sarcolemmal rupture. The level of sarcolemmal damage is directly correlated with the magnitude of mechanical stress placed upon the membrane during contraction rather than the number of activations of the muscle. These findings strongly support the proposition that the primary function of dystrophin is to provide mechanical reinforcement to the sarcolemma and thereby protect it from the membrane stresses developed during muscle contraction. Furthermore, the methodology used in this study should prove useful in assessing the efficacy of dystrophin gene therapy in the mdx mouse.

A PCR-based assay for the wild-type dystrophin gene transferred into the mdx mouse.

Myoblast transfer has emerged as a promising treatment for inherited myopathies such as Duchenne muscular dystrophy (DMD). Further development of the technique's therapeutic potential requires an experimental system in which issues of graft rejection can be clearly discriminated from those related to myoblast biology. Here we report the development and initial application of a quantitative assay for myogenic cells bearing a wild-type dystrophin gene following transfer into the mdx mouse. The technique relies upon the ability of a mutagenizing polymerase chain reaction (PCR) primer to create a new restriction site in the amplification production of the wild-type, but not the mdx dystrophin gene. The ratio of host to donor cells can be determined from muscle biopsies as small as 1 mg, regardless of donor H-2 background. This simple technique should allow a number of basic questions related to myoblast and direct gene transfer to be addressed using the mdx mouse model.

The mdx mouse diaphragm reproduces the degenerative changes of Duchenne muscular dystrophy.

Although murine X-linked muscular dystrophy (mdx) and Duchenne muscular dystrophy (DMD) are genetically homologous and both characterized by a complete absence of dystrophin, the limb muscles of adult mdx mice suffer neither the detectable weakness nor the progressive degeneration that are features of DMD. Here we show that the mdx mouse diaphragm exhibits a pattern of degeneration, fibrosis and severe functional deficit comparable to that of DMD limb muscle, although adult mice show no overt respiratory impairment. Progressive functional changes include reductions in strength (to 13.5% of control by two years of age), elasticity, twitch speed and fibre length. The collagen density rises to at least seven times that of control diaphragm and ten times that of mdx hind-limb muscle. By 1.5 years of age, similar but less severe histological changes emerge in the accessory muscles of respiration. On the basis of these findings, we propose that dystrophin deficiency alters the threshold for work-induced injury. Our data provide a quantitative framework for studying the pathogenesis of dystrophy and extend the application of the mdx mouse as an animal model.

The human embryonic myosin heavy chain. Complete primary structure reveals evolutionary relationships with other developmental isoforms.

We have isolated a single 6021-nucleotide cDNA fragment encoding the full length of the myosin heavy chain (MHC) isoform initially expressed in developing human limb muscle. The corresponding transcript is expressed in fetal, but not adult, human muscle, and the corresponding gene maps to human chromosome 17. Comparison of the full length nucleotide sequence with that of the orthologous rat gene transcript reveals 74, 90, and 80% similarities in the 5'-untranslated, coding, and 3'-untranslated regions, respectively. To precisely quantitate the degree of nucleotide sequence divergence between the human embryonic and other developmentally regulated MHC gene transcripts, we utilize the algorithm of Perler et al. (Perler, F., Efstratiadis, A., Lomedico, P., Gilbert, W., Kolodner, R. & Dodgson, J. (1980) Cell 20, 555-566) and make use of the codon-for-codon register attainable in alignments of the MHC rod encoding cDNA fragments. The results allow reconstruction of the order and relative timing of certain gene duplication events involved in the evolution of the multimembered mammalian MHC loci. By this analysis, the principal sarcomeric MHC gene expressed in the 14-day chick embryo is shown to be more distantly related to the mammalian embryonic MHC genes than to those expressed peri- and postnatally. Attention is focused on regional patterns of MHC sequence conservation, ordered with reference to the topology of our phylogenetic tree. We present a composite map depicting the deduced evolutionary age of various primary structural subdomains of the human embryonic MHC.

Isoform-specific cDNAs for human embryonic, neonatal, and slow skeletal myosin heavy chains.

A series of cDNA fragments encoding immunodetectable portions of the human slow/beta, neonatal, and embryonic isoforms of myosin heavy chain (MHC) were isolated from a human fetal muscle cDNA expression library. A 6 kb fragment isolated on a secondary screen represents the first cloned cDNA encoding a full-length vertebrate MHC (the human embryonic isoform). In the 3'-untranslated regions, 70-80% nucleotide sequence homology exists among orthologous human and rat cDNAs, whereas the homology is less than 65% among the paralogous cDNAs. Furthermore, approximately the same level of untranslated sequence conservation is observed at the 5'-terminus of the embryonic transcript. These results suggest that for both the 3'- and the 5'-untranslated domains, the rate of evolutionary sequence divergence is limited by functional constraints.

Human embryonic myosin heavy chain cDNA. Interspecies sequence conservation of the myosin rod, chromosomal locus and isoform specific transcription of the gene.

A 3.6 kilobase cDNA clone coding for the human embryonic myosin heavy chain has been isolated and characterized from an expression library prepared from human fetal skeletal muscle. The derived amino acid sequence for the entire rod part of myosin shows 97% sequence homology between human and rat and a striking interspecies sequence conservation among the charged amino acid residues. The single copy gene is localized to human chromosome 17 and its expression in fetal skeletal muscle is developmentally regulated. The sequence information permits the design of isoform-specific probes for studies on the structure of the gene and its role in normal and defective human myogenesis.