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(The FASEB Journal. 2005;19:543-549.)
© 2005 FASEB

Myostatin propeptide-mediated amelioration of dystrophic pathophysiology

Sasha Bogdanovich*, Kelly J. Perkins*, Thomas O. B. Krag*,{dagger}, Lisa-Anne Whittemore{ddagger} and Tejvir S. Khurana*,1

* Department of Physiology and Pennsylvania Muscle Institute, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, USA;
{dagger} Department of Experimental Medicine, Glostrup Hospital, Glostrup, Denmark; and;
{ddagger} Cardiovascular and Metabolic Disease Department, Wyeth Research, Cambridge, Massachusetts, USA

1Correspondence: Department of Physiology and Pennsylvania Muscle Institute, A-601 Richards Building, University of Pennsylvania School of Medicine, 3700 Hamilton Walk, Philadelphia, PA 19104-6085, USA. E-mail: tsk{at}mail.med.upenn.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Mutations in myostatin (GDF8) cause marked increases in muscle mass, suggesting that this transforming growth factor-ß (TGF-ß) superfamily member negatively regulates muscle growth. Myostatin blockade therefore offers a strategy for reversing muscle wasting in Duchenne’s muscular dystrophy (DMD) without resorting to genetic manipulation. Here, we demonstrate that pharmacological blockade using a myostatin propeptide stabilized by fusion to IgG-Fc improved pathophysiology of the mdx mouse model of DMD. Functional benefits evidenced by specific force improvement, exceeded those reported previously using myostatin antibody-mediated blockade. More importantly, use of a propeptide blockade strategy obviates possibilities of anti-idiotypic responses that could potentially limit the effectiveness of antibody-mediated myostatin blockade strategies over time. This study provides a novel pharmacological strategy for treatment of diseases associated with muscle wasting such as DMD and since it uses an endogenous inhibitor of myostatin should help circumvent technical hurdles and toxicity associated with conventional gene or cell based therapies.


Key Words: DMD • dystroglycan complex • neuromuscular disease • dystrophin • extracellular matrix


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
DUCHENNE’S MUSCULAR DYSTROPHY (DMD) is the most common X-linked neuromuscular disease and is estimated to affect 1 in 3500 newborn males. DMD is characterized by progressive and severe muscle loss that leads to loss of ambulation, with those affected often becoming wheelchair dependent toward the end of the first decade of life. The disease is caused by mutations in the DMD gene resulting in quantitative and/or qualitative disturbances in expression of the gene product, dystrophin (1 , 2) . Dystrophin is associated with the membrane-bound dystroglycan complex (DGC), which forms an important link with laminin, a constituent of the extracellular matrix. The DGC itself is part of a larger complex of evolutionary conserved proteins associated with dystrophin, including nNOS, dystroglycan, the sarcoglycans, syntrophins, dystrobrevin, and utrophin/DRP (3 4 5) . Mutations in the genes encoding various members of the complex (and proteins binding members of the complex, e.g., {alpha}-2 laminin) are thought to disrupt sarcolemmal integrity, resulting in a variety of X-linked and limb girdle muscular dystrophies (LGMDs) (6) . Mutations in genes not directly linked to the complex (e.g., emerin, dysferlin) can also result in certain types of muscular dystrophies (7 8 9) . In the case of LGMD 2B, defective sarcolemmal repair due to dysferlin deficiency, rather than defective sarcolemmal integrity per se is thought to be mechanistic (10) . Although DMD remains incurable, steady advances using gene-based, cell-based, and pharmacological strategies in experimental models of the disease continue to be made (11 12 13) .

Myostatin (GDF8) is a member of the transforming growth factor-ß (TGF-ß) superfamily of growth/developmental factors. Myostatin is a negative regulator of muscle growth, with gene mutations leading to increased musculature in vivo. This is exemplified by the 20% increase in muscle mass seen in the naturally occurring "double-muscled" cattle first noted >200 years ago. Increased muscle mass due to myostatin mutations has been reported in mice (14 15 16) , and more recently in humans (17) . In common with other members of the TGF-ß superfamily, myostatin consists of a ~26 kDa inactive N-terminal (also called the propeptide) region, an invariant tetrapeptide (RSRR) cleavage site, and an ~12 kDa active C-terminal (or mature) region. The gene and its modular organization have been extremely well conserved across evolution. The first 300 amino acids (encoding the inactive propeptide region) are 90% identical, whereas the tetrapeptide cleavage site in addition to the last 100 amino acids (encoding the active peptide) are 100% identical among human, mouse, chicken, and pig species. Analogous to other members of the TGF-ß superfamily, the myostatin propeptide can bind and inhibit the activity of the active myostatin peptide in vitro. Furthermore, attenuation of myostatin activity using blocking antibodies, peptides, or pseudo-ligands has been demonstrated to increase muscle mass in mice in vivo (18 19 20) .

We and others have hypothesized that increasing the myogenic/regenerative process by modulation of myostatin activity may improve the muscle weakness that is pathognomic of DMD (19 , 21) . Indeed, myostatin antibody-mediated blockade in vivo (19) was recently shown to functionally improve the phenotype of the mdx mouse model (22 , 23) of DMD. In these experiments, 4-wk-old male mdx mice were treated for 3 months with weekly intraperitoneal (i.p.) injections of JA16 mouse monoclonal antibodies against myostatin at 60 mg/kg. The anatomical, biochemical, and physiological improvement achieved using blocking antibodies was considered promising since it circumvented technical hurdles and toxicity associated with conventional gene or cell-based therapies (19) .

However, a complete reversal of phenotype was not achieved, as evidenced by lack of improvement in specific force. In an attempt to overcome these limitations as well as to extend previous studies, we describe the use of a novel propeptide-based myostatin inhibitor to investigate the effects of myostatin blockade on mdx mice in vivo.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Injection of mice
Four-week-old male mdx mice were treated for 3 months with weekly i.p. injections of either recombinantly synthesized myostatin propeptide (14 , 20 , 24 25 26) fused with mouse Fc to improve stability in vivo (dose 10 mg/kg; treated mdx group) or equimolar amounts of mouse Fc alone (dose 5.7 mg/kg; control mdx group).

Physiological and biochemical studies
The myostatin propeptide fused to a murine IgG-Fc region (20 , 24) and control mouse Fc peptide were obtained from Wyeth Research (Cambridge, MA, USA). The endogenous propeptide inhibits binding of myostatin to its receptor ActRIIB with an IC50 of 1 nM and the propeptide-Fc fusion inhibits myostatin with a similar affinity (24) . Physiological properties of muscle were analyzed using freshly dissected ex vivo muscle from 16 wk-old-male mdx (C57BL/10ScSn-DMDmdx/J) mice, as described previously (19 , 27 28 29) . Muscle length (Lo) was adjusted to achieve maximal twitch response. Eccentric contraction (ECC) force drop was calculated from the first and fifth tetanus of the standard ECC protocol, supramaximal stimulus of 700 ms (500 ms isometric phase, 200 ms eccentric phase), total lengthening Lo/10; lengthening velocity 0.5 Lo/s. At the end of physiological studies muscles were flash-frozen in liquid nitrogen cooled isopentane and stored at –80°C before sectioning. For biochemical assessment, serum was separated by centrifugation from tail vein blood that had previously been allowed to clot. Serum creatine kinase (CK) was measured using the indirect CK colorimetric assay kit and standards (Sigma, St. Louis, MO, USA).

Muscle morphology
Serial frozen sections (10 µm thickness) were cut at midbelly of muscle, subjected to gentle fixation using 100% ice cold methanol for 5 min, and stored in airtight containers at –80°C before analysis. Sections were processed for hematoxylin and eosin (H&E) staining for histology. For morphometric analysis sections were double labeled with anti-laminin monoclonal antibodies (Sigma) and the DNA binding dye Hoechst 33825, washed, incubated with Alexa 488-labeled goat anti-mouse antibodies, washed, mounted, and visualized using epi-fluorescence illumination on an Olympus BX 51 microscope. Pictures were taken using an Olympus Magnafire or Nikon Coolpix 950 Digital camera. Morphometric measurements were made on these digitized images using the Scion 4.02 image processing software (www.scioncorp.com). All myofibers (17,225) contained in the muscles physiologically evaluated in this study were imaged and scored for centrally nucleated fiber (CNF) determination. 5656 fibers were measured to calculate single fiber area.

Semiquantitative RT-PCR
For semiquantitative RT-PCR, total RNA isolation from mdx mouse tissue, subsequent cDNA synthesis and radiolabeled PCR analysis of utrophin and GAPDH were performed using primers and conditions as described previously (30) .

Statistical analysis
ANOVA (Fig. 1 a) and Student’s t test (all other figures) were used to determine statistical significance of results. The following convention is used for graphical representation: mean ± SD; treated mdx mice, red; control mdx mice, blue; dashed line, data from age-matched C57BL/10 normal mice.



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  		Figure 1. Consequences of propeptide-mediated myostatin blockade in mdx mice. Comparisons of growth curves (a), endurance time on a rota-rod (b) and EDL weight (c), between treated mdx mice (red) and control mdx mice (blue). Treated mdx mice had significantly different growth curves (n=5; ANOVA P≤0.001), endurance time (9.2±2.8 vs. 5.2±1.9 s; n=5; t test P<0.05) and EDL weights (17.0±4.5 vs. 12.9±3.8 mg; n=10; t test P<0.01) compared with mdx controls.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
To determine overall consequences of propeptide-based myostatin blockade on mdx mice in vivo, we plotted growth curves of mice injected with the myostatin propeptide-Fc (treated group) or control Fc peptide (control group) after weighing the mice weekly. Figure 1a illustrates that growth was significantly accelerated in treated mice compared with controls. Previous studies have shown that mdx mice have diminished rota-rod performance (31) . As demonstrated in Fig. 1b , the treated mdx mice had improved rota-rod performance, consistent with increased functional muscle and appropriately coordinated neuromuscular function in vivo. Increase in muscle mass was quantified by dissecting out and weighing individual extensor digitorum longus (EDL) muscles after sacrificing the animals. As shown in Fig. 1c , EDL muscles from the treated group of animals weighed significantly more than controls, consistent with previous observations on myostatin blockade (19 , 25 , 32) .

To quantify physiological improvement of myostatin propeptide-treated muscle, we recorded maximum force produced upon field depolarization of freshly dissected EDL muscles ex vivo. Figure 2 a–e illustrates a significant increase in maximal force produced during twitch and tetanic contraction in treated mdx mice. No significant differences were detected in twitch contraction time or half-relaxation time. Additional mechanical parameters are detailed in Table 1 . Compared with antibody-mediated blockade (19) , where tetanic force was increased proportional to increased muscle size (i.e., absolute force) but remained unchanged when normalized for increased size (i.e., specific force), propeptide-mediated blockade led to an increase in both absolute and specific tetanic force (Fig. 2d, e ), demonstrating an improvement in an important index of disease pathophysiology.



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  		Figure 2. Increase of muscle strength by propeptide-mediated blockade in mdx mice. Comparisons of twitch (a, b) and tetanic force (c–e) between treated mdx mice (red) and control mdx mice (blue). Treated mdx mice generated greater absolute force during twitch a) representative traces; b) 146 ± 26.0 vs. 106 ± 22.3 mN; n = 10; t test P< 0.0005 and tetanic contraction; c) representative traces; d) 412.8 ± 81.3 vs. 250.6. ± 53.4 mN; n = 10; t test P< 0.0007. They generated greater specific force during tetanic contraction e) 263.9 ± 95.7 vs. 189.1 ± 62.4 mN/mm2; n=10; t test P≤ 0.01.


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  		Table 1. Contractile and morphometric properties of EDL musclea

Detailed morphometric analysis was performed to determine whether the increase in muscle mass and strength caused by propeptide-mediated myostatin blockade occurred due to hypertrophy or hyperplasia (see Table 1 ). As shown in Fig. 3 a, there was a significant increase in cross sectional area (CSA) of EDL from the treated group of mice. The average single fiber area was significantly increased compared with controls (Fig. 3b ), suggesting true hypertrophy at the single myofiber level as evidenced by an overall shift of distribution. The average number of myofibers in the EDL remained relatively constant (Table 1) , suggesting the increase in muscle mass was due to hypertrophy rather than hyperplasia. This is similar to results obtained with mice transgenically expressing a dominant negative myostatin propeptide molecule mutated at the cleavage site (16) , as well as using blocking antibodies (19) . A slight increase was noted in the number of CNF in the treated group of mdx mice (Table 1) , indicating fibers that had undergone prior regeneration (23) . As CNFs are considered more resistant to necrosis, this finding suggests a potential mechanism by which myostatin blockade exerts beneficial effects in mdx mice (33) . Indeed, increased regeneration is consistent with the predicted role of myostatin blockade enhancing proliferation of skeletal muscle precursors in vivo (34 35 36) . Alternatively, increased myofiber stability may be a consequence of myostatin blockade enabling muscle growth above the level that can be compromised by normal workloads (37) .



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  		Figure 3. Increase of muscle size by propeptide-mediated blockade in mdx mice. Treated mdx mice (red bars) had larger CSA (a); 1.8 ± 0.5 vs. 1.4 ± 0.8 mm2; n = 10; t test P ≤ 0.05 and single fiber areas (b); 1616.3 ± 674.9 vs. 1317.1 ± 615.1 µm2; nfibers = 2000; t test P < 0.0001 compared with mdx controls (blue bars).

To determine whether propeptide-mediated myostatin blockade resulted in histological improvement, we performed additional analysis of muscle using H&E staining. Examination of EDL was equivocal; whereas evidence of prior degeneration and regeneration (in the form of CNFs) was evident in both groups, insufficient foci of degeneration were observed to comment on improvement (unpublished data). We therefore examined the diaphragm, since this muscle group, in contrast to EDL, shows greater degenerative changes at 16 wk (38) , the age at which this trial was terminated. A reduction in degenerative changes was observed in the diaphragm of treated mdx mice (Fig. 4 ), suggesting improvement of muscle pathology. We analyzed serum CK concentrations, since extremely high serum CK levels are noted in mdx mice and are considered a biochemical marker for sarcolemmal damage (39) . At trial initiation, both groups of mdx mice had elevations of serum CK compared with normal mice. After 3 months of propeptide-mediated myostatin blockade, a decline in serum CK concentrations in treated mdx mice was noted (Fig. 4c ), providing biochemical evidence for improvement due to myostatin propeptide-mediated blockade in vivo. To determine whether improvement was dependent on utrophin up-regulation, we analyzed utrophin mRNA expression levels in muscle; however, no difference was observed between treated and control mdx mouse muscle (Fig. 5 ). Improvement of the major functional indices, including increase in body weight, single fiber area, absolute and specific force generation, and reduction of serum CK, was noted in an independent pilot trial undertaken by us using 5 mdx-treated and 5 mdx-control mice (unpublished data).



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  		Figure 4. Reduction in muscle damage by propeptide-mediated myostatin blockade in mdx mice. Control mdx mice (a) had significantly greater pathological changes in the diaphragm compared with treated (b) mdx mice. Inset shows normal C57BL/10 mice. H&E staining; scale bar, 100 µm. c) Treated mdx mice (red) had significantly decreased serum CK concentrations compared with mdx controls (blue) (10491.7±7072.5 vs. 1611.7±723.6 U/l; n=5; t test P≤0.05). The levels were reduced significantly, but not to the low levels noted in normal age-matched C57BL/10 mice (yellow; 738.9±246.8 U/l; n=9).



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  		Figure 5. Propeptide-mediated myostatin blockade does not increase utrophin message levels. Treated mdx mice (top panel, right lanes) show no detectable increase of utrophin mRNA expression compared with control mdx mice (top panel, left lanes). Bottom panel shows GAPDH controls.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Correction of the muscle wasting pathognomic of DMD constitutes an important goal for a variety of therapeutic strategies (11 12 13 , 40 , 41) . It is becoming increasingly apparent that this can be achieved not only by positive effectors of muscle growth (e.g., IGF-1) (42 , 43) , but also via repression of "negative" growth factors such as myostatin (GDF8) (19 , 21) , which inhibit muscle growth. We used the latter strategy in this study to demonstrate that myostatin blockade achieved by i.p. injections of a stabilized version of the myostatin propeptide resulted in a functional improvement of dystrophic pathophysiology in mdx mice. This strategy provides a novel pharmacological approach for treatment of diseases associated with muscle wasting and circumvents technical hurdles and toxicity associated with conventional gene or cell-based therapy. The use of this endogenously expressed molecule obviates the possibility of an anti-idiotypic response that could potentially limit effectiveness of antibody-mediated myostatin blockade strategy (19) over time. As sequence information for canine (K. J. Perkins and T. S. Khurana; AY367768) and human (19 , 25) myostatin is available, species-specific propeptide molecule(s) can be readily generated for conducting preclinical studies in the canine DMD model. If issues regarding the significantly higher costs, breeding difficulties, and phenotypic variability associated with the canine model can be satisfactorily circumvented by experimental design, preclinical testing of the myostatin propeptide approach in dystrophic dogs could facilitate efficient progression to clinical studies in patients, since the progressive muscle weakness and fibrosis observed for the canine DMD model more closely resembles human DMD than mice. In princi-ple, synthesis of therapeutic propeptide molecules would overcome the potential delays and difficulties associated with identification and isolation of functional canine or humanized antibodies against myostatin for use in these species.

The degree of physiological improvement achieved using the propeptide approach exceeded the improvement achieved using murine antibody-mediated blockade (19) , as evidenced by an improvement in specific force (Table 2 ). This may be related to the ~ 500-fold higher binding affinity of the propeptide moiety to myostatin compared with the blocking antibodies and/or the potential for the propeptide moiety to target GDF8 (myostatin) and closely related GDF11, rather than GDF8 alone. Improvement of specific force is a significant finding since it provides physiological evidence of a fundamental improvement in muscle mechanics using this pharmacological strategy. Though extremely encouraging, it is important to point out that no improvement was observed when using provocative ex vivo lengthening contraction protocols (Table 1) , suggesting that the muscle remained susceptible to contraction-mediated damage (27 , 28) . This may be related to late onset and/or inadequate dosage of propeptide or a shared feature of strategies that seek to increase muscle mass in order to compensate for the dystrophic phenotype rather than directly replace the missing gene product (19 , 37 , 42 , 43) . These limitations could be overcome by combinatorial use with conventional gene/cell-based therapies or pharmacological approaches designed to correct specific functional deficits (44) . The fact that such significant benefits were achieved independent of increased utrophin expression suggests that myostatin blockade could be further potentiated by combination with effectors of utrophin up-regulation known to improve ECC force drop (13 , 29 , 45) . Experiments are currently under way to test these possibilities.


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  		Table 2. Comparison of antibody and propeptide-mediated myostatin blockade in mdx mice


   ACKNOWLEDGMENTS
 
T.S.K. was a paid consultant for Wyeth Research during the study. The study was supported in part by a grant from Wyeth Research to Glostrup Hospital (Denmark). S.B. was supported by a grant from the Muscular Dystrophy Association, USA. T.O.B.K. was supported by a predoctoral fellowship from the Faculty of Medicine, University of Copenhagen, Denmark.

Received for publication September 14, 2004. Accepted for publication December 2, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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