Advertisement
Browse Subject Areas
?

Click through the PLOS taxonomy to find articles in your field.

For more information about PLOS Subject Areas, click here.

  • Loading metrics

Open Access

Peer-reviewed

Research Article

ACE-031, a soluble activin type IIB receptor, increases muscle mass and strength in the common marmoset (Callithrix jacchus)

  • Samuel M. Cadena ,

    Contributed equally to this work with: Samuel M. Cadena, Sasha Bogdanovich

    Roles Data curation, Formal analysis, Methodology, Visualization, Writing – original draft, Writing – review & editing

    Affiliation Acceleron Pharma Inc., Cambridge, Massachusetts, United States of America

    ⨯
  • Sasha Bogdanovich ,

    Contributed equally to this work with: Samuel M. Cadena, Sasha Bogdanovich

    Roles Formal analysis, Methodology, Visualization, Writing – original draft, Writing – review & editing

    Affiliation Department of Physiology and Pennsylvania Muscle Institute, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, United States of America

    ⨯
  • Tejvir S. Khurana,

    Roles Supervision, Visualization, Writing – original draft, Writing – review & editing

    Affiliation Department of Physiology and Pennsylvania Muscle Institute, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, United States of America

    ⨯
  • Abigail Pullen,

    Roles Methodology, Writing – original draft, Writing – review & editing

    Affiliation Acceleron Pharma Inc., Cambridge, Massachusetts, United States of America

    ⨯
  • R. Scott Pearsall,

    Roles Formal analysis, Methodology, Writing – original draft, Writing – review & editing

    Affiliation Acceleron Pharma Inc., Cambridge, Massachusetts, United States of America

    ⨯
  • Elizabeth Curran,

    Roles Formal analysis, Methodology, Writing – original draft, Writing – review & editing

    Affiliation Harvard Medical School/New England Regional Primate Research Center, Southborough, Massachusetts, United States of America

    ⨯
  • Ryan Faucette,

    Roles Formal analysis, Methodology, Writing – original draft, Writing – review & editing

    Affiliation Acceleron Pharma Inc., Cambridge, Massachusetts, United States of America

    ⨯
  • Joan Lane,

    Roles Formal analysis, Methodology, Writing – original draft, Writing – review & editing

    Affiliation Acceleron Pharma Inc., Cambridge, Massachusetts, United States of America

    ⨯
  • Jasbir Seehra,

    Roles Conceptualization, Investigation, Methodology, Supervision, Writing – original draft, Writing – review & editing

    Affiliation Acceleron Pharma Inc., Cambridge, Massachusetts, United States of America

    ⨯
  • Jennifer L. Lachey,

    Roles Data curation, Formal analysis, Methodology, Writing – original draft, Writing – review & editing

    Affiliation Acceleron Pharma Inc., Cambridge, Massachusetts, United States of America

    ⨯
  • Alan D. Mizener,

    Roles Visualization, Writing – review & editing

    Affiliation West Virginia University Cancer Institute, West Virginia University School of Medicine, Morgantown, West Virginia, United States of America

    ⨯
  • Emidio E. Pistilli

    Roles Formal analysis, Investigation, Methodology, Visualization, Writing – original draft, Writing – review & editing

    epistilli2@hsc.wvu.edu

    Current address: Division of Exercise Physiology and West Virginia University Cancer Institute, West Virginia University School of Medicine, Morgantown, West Virginia, United States of America

    Affiliation Department of Physiology and Pennsylvania Muscle Institute, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, United States of America

    ⨯

Abstract

Pharmacological blockade of ligands for the activin receptor type IIB (ActRIIB) e.g., myostatin and activin A is associated with improvements in murine skeletal muscle mass and function. The efficacy of a similar treatment approach in a non-human primate (NHP) model would suggest a greater likelihood of success in the treatment of humans suffering from chronic myopathies. In the present study, we elucidate the potential therapeutic benefit of ACE-031, a therapeutic protein consisting of the ActRIIB extracellular region fused to human IgG1, in the common marmoset (Callithrix jacchus). Marmosets were randomized to receive ACE-031 or vehicle control (10 mM Tris buffered saline; TBS) for 14 weeks. Body composition was measured weekly throughout the experimental period and morphometric analysis and contractile properties of skeletal muscle were assessed terminally. There was a significant main effect of time and time x treatment interaction for lean body mass, such that marmosets administered ACE-031 were greater at euthanasia compared to baseline; this was not observed in the vehicle-treated controls. Biceps brachii exhibited a significant increase in the cross-sectional area of both type I and type II fibers and ex vivo contractile properties of the EDL showed an increase in absolute and specific force production. The efficacy of ACE-031 in non-human primates provides optimism that a therapeutic strategy that targets multiple negative regulators of skeletal muscle may be beneficial in treating myopathies in humans.

Citation: Cadena SM, Bogdanovich S, Khurana TS, Pullen A, Pearsall RS, Curran E, et al. (2026) ACE-031, a soluble activin type IIB receptor, increases muscle mass and strength in the common marmoset (Callithrix jacchus). PLoS One 21(2): e0342666. https://doi.org/10.1371/journal.pone.0342666

Editor: Keisuke Hitachi, Fujita Health University: Fujita Ika Daigaku, JAPAN

Received: October 7, 2025; Accepted: January 27, 2026; Published: February 13, 2026

Copyright: © 2026 Cadena et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting information files.

Funding: The author(s) received no specific funding for this work.

Competing interests: I have read the journal’s policy and the authors have declared no competing interests. Co-authors were former employees of a company Acceleron that holds the patent to the drug. However, the company is no longer active. This does not alter our adherence to PLOS ONE policies on sharing data and materials.

Introduction

Members of the transforming growth factor beta (TGF-β) superfamily, including myostatin (GDF-8) and activin A, are important regulators of skeletal muscle growth [1,2]. Overexpression of myostatin or activin A is associated with muscle wasting in both human chronic disease [3–7] and animal models [8–11]. In contrast, deficiency of either or both ligands in the form of genetic deletion [12–17] or pharmacological inhibition [17–22] results in increased muscle mass and rescue of muscle loss in models of muscle wasting. These findings suggest that therapeutics specifically targeting myostatin or activin A signaling to stimulate skeletal muscle growth may provide therapeutic benefit in human myopathies and conditions associated with muscle wasting.

In several mouse models of muscular dystrophy, myostatin inhibition is associated with modest improvements in dystrophic muscle, including increased muscle mass and fiber cross-sectional area, improved functional capacity and reduced degeneration and fibrosis [18,19,23–27]. However, when this same therapeutic strategy was applied clinically to patients afflicted with muscular dystrophies, the outcomes were not as promising [28–31]. While some improvements in force production were observed in a single muscle fiber preparation [28], investigators failed to observe any meaningful clinical benefit in patients [28–31]. It has been reported that circulating activin A levels in monkeys and humans are 3–4-fold higher than mice and rats [21], suggesting that myostatin specific treatments that appear promising in mice are not likely to be as impactful in humans. Thus, a more comprehensive treatment modality that targets both activin A and myostatin may be necessary to effectively counter the degeneration, weakness and functional decline of muscle associated with these myopathies in humans.

Myostatin and activin A signal through a heteromeric complex that consists of type I and II receptors of the TGF-β superfamily. Deletion or inhibition of the activin type IIB (ActRIIB) receptor in adult mice is associated with significant increases in skeletal muscle mass [32–36] that are greater than inhibition of myostatin alone [21,37], suggesting that the simultaneous inhibition of both myostatin and activin A could potentially present a more effective therapeutic strategy in the treatment of muscle disorders. Indeed, in a phase I trial evaluating anti-myostatin and anti-activin A antibodies in post-menopausal women, the dual antibody treatment was superior to either antibody alone in their ability to increase thigh muscle volume and total lean body mass [38]. Similarly, treatment with both antibodies significantly increased muscle mass in mice and non-human primates subjected to combined diet induced obesity and GLP-1-induced muscle loss [39]. Utilizing a ligand trap based treatment modality to inhibit myostatin and activin A with a single molecule, post-menopausal women treated with one dose of ACE-031 (a soluble form of the ActRIIB extracellular domain fused to the Fc portion of human IgG) exhibited a 3.3% increase in lean body mass and a 5.1% increase in thigh muscle volume, as assessed by DEXA and MRI, respectively [40]. In a follow-up study in boys with Duchenne muscular dystrophy, ACE-031 showed trends for increased lean body mass [41]. However, the study was terminated prematurely after the second dosing regimen due to safety concerns around epistaxis and telangiectasias. Later reports indicate that bleeding events were likely due to inhibition of BMP9, a ligand critical for vascular remodeling [42]. Thus, despite showing a tendency to increase muscle mass and prevent decline in the 6-minute walk time [41] the full clinical benefit of ACE-031 was never fully realized. Therefore, to better understand the full therapeutic potential of ACE-031 over a longer treatment duration and to bolster confidence that a similar treatment modality (i.e., a ligand trap inhibiting both myostatin and activin A) is likely to confer significant clinical benefit in muscular disorders, we herein report results from a 14-week dosing study in the common marmoset, characterizing the effects of ACE-031 on body weight, muscle mass, muscle fiber morphology and isometric strength.

Materials and methods

Expression and purification of ACE-031

The ActRIIB extracellular domain was produced by PCR amplification of the human ACVR2B gene. The primers used for the 5′ and 3′ ends included a SfoI and AgeI restriction site, respectively. PCR product was purified, digested with SfoI and AgeI and ligated into the pAID4 hFc vector (containing the tissue plasminogen activator signal sequence) to create the pAID4 ActRIIB.hFc expression construct. The sequences of ActRIIB extracellular domain and human immunoglobulin Fc region were confirmed by double strand dideoxy sequencing. ActRIIB.hFc plasmid was transfected into Chinese Hamster ovary (CHO) cells and a stable clone expressing ActRIIB.hFc was isolated. ActRIIB.hFc was purified from serum free media using affinity chromatography with Mab Select Sure Protein A (GE Healthcare, Piscataway NJ) and additional purification steps as needed. Purified ActRIIB.hFc was dialyzed into tris buffered saline (TBS; 10 mM tris, 137 mM NaCl and 2.7 mM KCl, pH 7.2).

Animals

Six male and six female healthy adult common marmosets (Callithrix jacchus) were included in this study. Animals were part of The New England Primate Research Center breeding colony and were maintained in accordance with the “Guide for the Care and Use of Laboratory Animals” of the Institute of Laboratory Resources, National Research Council. The facility is accredited by the Association for the Assessment and Accreditation of Laboratory Animal Care International and all work was approved by Harvard Medical School’s Standing Committee on Animals. Animals were randomized to control (n = 4) or treatment groups (n = 8), with an equal number of male and female animals in each group. Animals were dosed once weekly for 14 weeks by interscapular subcutaneous injection at a dose of 3.0 mg. kg-1 of ACE-031 or an equal volume of vehicle as a control (10 mM Tris buffered saline; TBS). Animals were euthanized at week 14 for tissue collection. Euthanasia was by pentobarbital overdose delivered intravenously under ketamine sedation.

Body composition

Measurements were obtained in conscious animals at baseline and every two weeks throughout dosing. Animals were habituated to entering an elongated holder for a food reward. Total body weight was obtained by weighing the holder and animal on an electronic scale. Fat mass and lean body mass were then quantified using an EchoMRI quantitative magnetic resonance analyzer (QMR; Echo Medical Systems, Houston, TX). The QMR analyzer was calibrated daily prior to analysis with a standard consisting of a measured amount of canola oil. Animals were measured three times during each session with the means used for further analyses.

Blood sampling

Blood collection for complete blood cell and serum chemistry analysis was performed under ketamine sedation (15 mg. kg-1 Ketaset IM; Fort Dodge, Overland Park, KS) at baseline and weeks 1, 4 and 8 following dosing. Terminal blood samples were taken at the time of necropsy.

Muscle fiber cross-sectional area (CSA) and fiber type distribution

Immediately following euthanasia, biceps brachii muscles were harvested, the long head was bisected near the mid-belly region, snap frozen in liquid-nitrogen-cooled isopentane and stored at −80 °C for subsequent analyses. Ten μm sections were fixed on glass slides with 10% formalin for 20 min. For CSA analyses, sections were incubated in wheat germ agglutinin (WGA) conjugated to Alexa Fluor 488 diluted in PBS (Invitrogen, Carlsbad, CA) for 60 min at room temperature. To distinguish between fiber types, sections were incubated in anti-fast (type II) MHC conjugated to alkaline phosphatase (Sigma, Saint Louis, MO) diluted in PBS for 60 min at room temperature then incubated for 10 minutes in Vector® Red Alkaline Phosphatase Substrate Kit (Vector Laboratories, Burlingame, CA). Stained sections were visualized with an Eclipse 80i fluorescent microscope and images captured with a Digital Sight DS-5Mc digital camera (Nikon Corporation, Melville, NY). Images were analyzed with NIS Elements imaging software (Nikon Corporation). For each sample an average of 942 fibers per section were measured to determine cross-sectional area and all fibers per section were counted to calculate fiber type distribution.

Ex vivo contractile properties

EDL muscles were dissected and tied around the proximal and distal tendons of the muscle with 6−0 sutures. The muscles were transferred to a stimulation chamber and muscle length was adjusted to achieve the maximal twitch response (Lo). Three twitch contractions were performed with muscle at Lo. Maintaining the muscle at Lo, the force-frequency relationship was determined. Tetanic contractions were performed at 30 Hz, 50 Hz, 80 Hz, 100 Hz, and 120 Hz and the maximal force plotted for each frequency. The maximal forces occurred at a stimulation frequency of 50 Hz, therefore, further analyses were performed at 50 Hz. Three tetanic contractions (50 Hz, 500ms) were performed with the muscle at Lo. All three twitches and tetani were analyzed and the maximal responses were averaged for each group.

Statistical analysis

Data are reported as means ± standard error of the mean unless stated otherwise. Non-normally distributed variables were log-transformed so that data were normally distributed prior to statistical analysis. Muscle weights, contractile properties and anthropometric measurements at euthanasia were analyzed using two-tailed unpaired Student’s t-test for between-group comparisons. Bi-weekly measures of body weight and lean body mass were analyzed using a 2-way repeated measures ANOVA and Bonferroni’s multiple comparisons post-hoc test. Absolute forces obtained in the force-frequency relationship were analyzed using a 2-way ANOVA and Bonferroni’s multiple comparisons post-hoc test. Significance was set at p < 0.05.

Results

Safety and tolerability, body weight and lean body mass

Marmosets received weekly interscapular subcutaneous injections of ACE-031 at a dose of 3.0 mg. kg-1 for 14 weeks. The drug was generally well tolerated, and marmosets showed no adverse effects associated with treatment. Blood samples were collected at several intervals throughout the course of the study to evaluate serum chemistries and all values remained within normal ranges (S1 Table). Upon necropsy, no gross safety concerns or morphological abnormalities in tissues or organs were observed. The 2-way ANOVA identified a main effect of time (p = 0.0092) for absolute body weight changes during the 14 weeks. However, the main effect of treatment (p = 0.6762) and the time x treatment interaction (p = 0.4406) were not significant. Compared to baseline, the body weight in ACE-031 treated marmosets were significantly greater at weeks 4–12. In contrast, compared to baseline the vehicle control group, treated with an equal volume of TBS, showed no significant differences in body weight (Fig 1A). The 2-way ANOVA identified a significant main effect of time (p < 0.0001), and a time x treatment interaction (p = 0.0012) for absolute changes in lean body mass during the 14 weeks. The time x treatment interaction suggests that lean body mass in ACE-031 treated marmosets was greater over time while lean body mass remained stable in control marmosets. However, the main effect of treatment (p = 0.7506) was not significant. Compared to baseline, lean body mass, as assessed by EchoMRI, in ACE-031 treated marmosets was significantly greater at weeks 2–14 (Fig 1B). There were no differences in fat mass over the 14 weeks of treatment (Fig 1C). Muscle mass analyzed using DEXA showed a significant difference in lean body mass of the arms of ACE-031 treated animals compared to controls (p = 0.0225) (Fig 1D). The same observation was not confirmed for total body mass or in other muscle compartments such as the trunk and legs.

thumbnail
Download:
Fig 1. Body weight and body composition analysis.

(A) Body weights of ACE-031 treated marmosets and control at the end of the treatment compared to baseline during the treatment period (week 0 versus week 14). (B) Lean body mass of ACE-031 treated marmosets was significantly greater at the end of the treatment (week 14) when compared to its baseline (week 0) while no differences were observed in control during the treatment period (week 0 versus week 14). (C) No differences in fat mass were observed in either group during the treatment period. (D) There was a significant difference in the muscle mass of the arms when comparing ACE-031 treated marmosets to control marmosets at the end of the treatment period. (E) The percent change in body weight for individual marmosets in the over the duration of the treatment period. (F) The percent change in body weight for individual marmosets in the over the duration of the treatment period.

https://doi.org/10.1371/journal.pone.0342666.g001

The percent change in body weight and lean body mass of individual marmosets are depicted in Fig 1E and 1F. The TBS-treated marmoset labeled as “A” presented with a relatively large increase in both body weight and lean body mass by the end of the experiment. Blood chemistries for this marmoset were all within normal ranges (S1 Table) and gross analysis upon necropsy revealed no apparent abnormalities that would explain a large degree of weight gain. In addition, pharmacokinetic assays confirmed that ACE-031 was not present in the serum of this marmoset (S2 Table), indicating the accumulation of body weight and lean body mass was not due to inadvertent ACE-031 exposure. We did not exclude this marmoset from statistical analyses and suggest that the overall change in body weight of this specific marmoset may have affected the group means and therefore the statistical comparisons between groups. Blood chemistries for all marmosets were within normal ranges (S1 and S2 Tables).

Muscle fiber CSA and fiber-type distribution

To further explore the anabolic propensity of ACE-031 in skeletal muscle and to corroborate changes in arm muscle mass observed by DEXA, a morphometric analysis of the biceps brachii was performed and muscle fibers were examined for differences in CSA and myosin heavy chain (MHC) isoform expression. Representative muscle images stained for MHC type II are shown in Fig 2A and 2B. Type II muscle fiber CSA was 20% greater and type I muscle fiber CSA was 34% greater in ACE-031 treated marmosets compared to TBS treated marmosets (Fig 2C and 2D). The type II and type I fiber CSA distributions in ACE-031 treated marmosets were shifted to the right toward larger muscle fiber CSA (Fig 2E and 2F). It is not known whether inhibition of ActRIIB signaling in skeletal muscle induces shifts in muscle fiber type. Therefore, the proportion of type I fibers per muscle section of the biceps brachii was determined. Biceps brachii muscles of both TBS- and ACE-031-treated marmosets consisted of 16% type I fibers (S1 File). Thus, up to a 14-week blockade of ActRIIB ligands does not alter the MHC fiber type profile of skeletal muscle in the common marmoset.

thumbnail
Download:
Fig 2. Biceps brachii fiber type analysis.

(A, B) Representative biceps brachii muscle sections from control and ACE-031 treated marmosets. (C, D) Type II fibers (fast) and type I fibers (slow) had significantly increased cross-sectional area. (E, F) Type II fibers and type I fibers exhibited a rightward shift of the fiber CSA distribution with ACE-031 treatment.

https://doi.org/10.1371/journal.pone.0342666.g002

Ex vivo contractile properties

To determine if ACE-031 treatment is associated with improvements in muscle function, we measured ex vivo contractile properties of the EDL. While absolute twitch force was not significantly different, specific twitch forces were significantly greater in EDL muscles from ACE-031 treated animals (Fig 3A and 3B). As the contractile properties of marmoset skeletal muscle have not been previously characterized, we established a force-frequency relationship for the EDL muscles to determine the stimulation frequency for optimal force production (Fig 3C). In both treatment groups, the greatest force was generated in response to a 50 Hz stimulus; therefore, the 50 Hz stimulation frequency was used to obtain maximal isometric tetanic force. While absolute tetanic isometric force was not significantly greater, specific tetanic force was greater in EDL muscles from ACE-031 treated animals (Fig 3D and 3E). Collectively, these data indicate an improvement in isometric muscle strength that is suggestive of enhanced muscle function.

thumbnail
Download:
Fig 3. Ex vivo muscle functional assessment.

(A) Absolute isometric twitch force of the EDL in ACE-031 treated marmosets compared to control marmosets. (B) Specific isometric twitch force of the EDL was significantly greater in ACE-031 treated marmosets compared to control marmosets. (C) The optimal stimulation frequency of the EDL was determined from the Force-Frequency relationship curve. (D) Absolute isometric tetanic force of the EDL was not significantly different when comparing ACE-031 treated to control marmosets. (E) Specific isometric tetanic force of the EDL was significantly greater in ACE-031 treated marmosets compared to control marmosets.

https://doi.org/10.1371/journal.pone.0342666.g003

Discussion

Myostatin has been shown to induce atrophy by inhibiting signaling pathways associated with protein synthesis and activating those associated with degradation [43–46]. Whereas, inhibition of myostatin and activin A has been shown to preserve muscle mass by preventing perturbations in these pathways [46]. Activin A levels in monkeys and humans are 3–4-fold higher than mice and rats [21] and myostatin levels in mice are more than 10-fold higher than humans [47] suggesting that myostatin specific treatments that appear promising in mice are not as likely to produce clinically meaningful outcomes in humans. Indeed, although myostatin specific treatments have been shown to be efficacious in a mouse model of muscular dystrophy [18,24,27], selective antagonism of myostatin has failed to show meaningful clinical benefit in patients with muscular dystrophies [29–31]. Further, a systematic analysis of the pharmacokinetic and pharmacodynamic profile of a myostatin neutralizing antibody in mice, rats, monkeys and humans suggest that the amount of drug needed to induce comparable increases in muscle mass in monkeys was 20-fold higher than what is needed in mice; indicating substantial species differences in sensitivity to myostatin specific treatments [48]. Thus, the use of NHPs is likely to be a superior predictor of efficacy for treatments targeting myostatin and activin A in humans.

Current standard drug therapies for muscular dystrophies predominantly rely on the use of corticosteroids [49]. While these treatments have been shown to slow disease progression by reducing inflammation and cell death, chronic corticosteroid use is associated with several side effects. These include behavioral abnormalities [50], increases in fat mass [51] and insulin resistance [52] and decreases in bone mineral density [53]. Recently, several novel treatments based on exon skipping and gene therapy have been approved. However, they are predicted to benefit only a small segment of the muscular dystrophy patient population and, in the case of the first approved gene therapy, have the possibility of serious adverse effects such as liver and heart injury [54–56]. Thus muscular dystrophy patients are in need of a new therapeutic approach that has the potential to benefit all patients without the risk of serious side effects.

Here we have described the effects of the pharmacological inhibition of ActRIIB ligands in the common marmoset by administration of ACE-031, a soluble form of ActRIIB fused to the Fc portion of human IgG. Administration of ACE-031 for 14 weeks produced a significant increase in marmoset body weight from baseline that was paralleled by an increase in lean body mass; suggesting that greater body mass associated with loss of ActRIIB-signaling is likely the result of increased skeletal muscle growth. This is consistent with several studies investigating myostatin inhibition alone [18,20,57] or in combination with inhibition of activin A [39,58–60], including the administration of a murine analog of ACE-031 (RAP-031, the extracellular region of ActRIIB fused to the Fc region of a murine IgG2) [32,33,59,61,62] and a monoclonal antibody against ActRIIA and ActRIIB [63,64] in models of muscle wasting and degeneration. In the current study, total fat mass did not differ between ACE-031 and TBS treated marmosets. This is consistent with previous reports showing that as opposed to obese mice, lean mice have a reduced tendency to shed fat mass when treated with ActRIIB-Fc [65]. The absence of changes in fat mass in this study further corroborates that the increase in body weight was the result of an increase in lean body mass.

Genetic deletion of myostatin is associated with a greater proportion of fast glycolytic fibers at the expense of slow oxidative fibers [66–68]; an effect that also persists in heterozygous myostatin KO animals [69]. This appears to be purely a developmental effect as mice administered an AAV expressing a dominant negative form of myostatin present with an increase in IIB fibers when treated as neonates but not when treated as adults [70]. Girgenrath et al., also showed that treatment of mature mice with a myostatin specific antibody for 12 weeks did not impact fiber-type distribution [68]. Similarly, we have shown here in the common marmoset and previously in mice that fiber type distribution is not altered by ACE-031 induced hypertrophy [71]. We have also shown here and previously [71] that both type I and II fibers respond to ACE-031 induced hypertrophy, exhibiting comparable increase in fiber cross-sectional area. Thus, it is expected that the inherent metabolic profiles of the muscle (i.e., fast/glycolytic vs slow/oxidative) are likely to remain intact and unaltered by ACE-031 treatment. This contrasts with other groups reporting that myostatin inhibition induced hypertrophy shows a clear selectivity for type IIB fibers [57,72,73]. Slow‐twitch fibers maintain oxidative capacity and fatigue resistance, supporting posture and endurance-based tasks, whereas fast‐twitch fibers provide high power output and rapid contractile kinetics needed for short bursts of energy. Therapeutics that induce fiber type transitions may result in functional trade-offs (e.g., resistance to fatigue at the expense of maximal force production, or vice versa). This has been demonstrated in myostatin KO mice where selective increase in type II fibers at the expense of type I is associated with reduced endurance capacity [74] and accelerated fatigue in isolated muscles [75]. Similarly, post-developmental KO of myostatin was associated with a significant reduction in voluntary wheel running and wild-type adult mice administered an AAV expressing the myostatin propeptide to antagonize myostatin activity presented with reduced forced distance running and greater fatigability in isolated muscles [76]. To date, a similar analysis in humans treated with muscle anabolics has not been completed but some insights may be garnered from examples of selective fiber type loss in the context of muscle atrophy. For example, elderly individuals with sarcopenia exhibit a selective atrophy of type II fibers [77–79], which has been shown to be associated with fracture related falls that may be attributed to inability to generate sufficient force needed to overcome loss of balance [77,79]. In contrast, heart failure patients present with a slow to fast fiber type transition that is associated with severe intolerance to physical exercise and accelerated fatigue during normal activities of daily living [80,81]. Thus, a muscle anabolic that preserves the pre-treatment fiber type profile is likely to benefit the patient by maintaining energy to perform normal activities of daily living while retaining the force generating capacity to prevent falls and injuries.

In the current study, ACE-031 administration was associated with a significant increase in both absolute and specific force of marmoset EDL muscles. In myostatin deficient mice, reports have shown moderate or no improvement in absolute force and a significant reduction in specific force [75,82]; an indication that improvements in skeletal muscle contractility do not occur proportionately to increases in skeletal muscle mass in myostatin-deficient mice. On the other hand, pharmacological inhibition of myostatin has been shown to increase absolute force of the EDL [18,23,24,57] and soleus [83] and Bogdanovich et al. reported a significant increase in specific force [24] in myostatin propeptide-mediated blockade. Consistent with ACE-031 administration, two groups, Minetti et al. [84] and Loro et al. [85] independently observed an increase in both absolute and specific force in dystrophic mice treated with trichostatin, a deacetylase inhibitor. The principal effector of deacetylase inhibitors in skeletal muscle is follistatin [86], which has been shown to inhibit myostatin as well as other negative regulators of skeletal muscle [12,87,88]. These observations further support the concept that a more broad-based therapeutic strategy, targeting multiple ActRIIB ligands, may be particularly beneficial in concurrently augmenting muscle mass and force production. Our findings indicate that ACE-031 is associated with greater lean body mass and muscle fiber cross-sectional area in a non-fiber-type specific manner. These skeletal muscle morphological effects translate to greater strength and functional capacity, manifested as an increase in both absolute and specific force. Consequently, this study confirms that concomitantly addressing multiple inhibitors of muscle growth, e.g., myostatin in combination with activins, through distinct approaches (antibody vs. ligand trap), might be more advantageous than targeting them independently. Assessing the impact of the dual-targeted pathways (myostatin and activins) in clinical trials resulted in much better changes in the body composition (increase lean body mass and reduction of fat mass), and in bone health [40,89] versus targeting myostatin alone [90]. Therefore, given the progressive muscle wasting nature of various human dystrophies and myopathies, a therapeutic strategy targeting multiple regulatory pathways of skeletal muscle could be more beneficial.

A limitation of this study is that the relatively small sample size, particularly in the control group (n = 4; 2 male, 2 female) compared with the treated group (n = 8; 4 male, 4 female), may have resulted in inadequate statistical power to detect subtle but potentially meaningful treatment effects. This limitation was underscored by a vehicle treated animal that showed an unexpected increase in body weight and muscle mass over the course of the study—an outcome anticipated in the ACE-031 treated group—which likely contributed to variability, limiting the ability to detect group differences. The authors concede that this limitation should be taken into account when interpreting the findings and their generalizability. An additional limitation was the use of the biceps brachii muscle for fiber typing and fiber CSA analysis and the use of the EDL muscle for functional analysis, which does not allow us to directly conclude on structure and function in the same muscle. The use of the EDL for functional measures was based on anatomical and methodological issues. The EDL muscle is a fusiform muscle, in which the muscle fibers run parallel to each other, with no pennation angle, and tapers at proximal and distal myotendinous junctions. Surgically, the EDL can be dissected with the proximal and distal tendons intact, leaving the muscle undamaged for subsequent functional assessment. This makes the EDL muscle ideal for functional studies as isometric force is directed in one direction between the connections to the force transducer and muscle stimulation chamber. Although the histological data in the biceps brachii and the functional data in the EDL are supportive in this study, systemic therapies in general may not produce similar effects in every muscle.

Despite the potential of ActRIIB ligand traps to ameliorate muscle wasting, the experience from the ACE-031 clinical program underscores important safety considerations for this drug class. ACE-031 was discontinued in a Phase II trial in Duchenne muscular dystrophy due to vascular-related adverse events, including epistaxis, telangiectasia, and gum bleeding, which were later attributed to inhibition of ligands beyond myostatin and activin A, particularly BMP9 [91], which plays a critical role in vascular homeostasis. This discovery highlights that broad inhibition of TGF-β family ligands can produce effects that extend beyond muscle. Thus, while ActRIIB-targeted therapies remain a promising approach to increasing muscle mass and function, the ACE-031 findings emphasize the need for more selective ligand targeting and the inclusion of monitoring of vascular endpoints in future clinical trials. Indeed, KER-065, an ActRIIA and ActRIIB chimera ligand trap targeting myostatin and activins recently completed phase I clinical trials, and has already demonstrated an ability to stimulate significant improvements in muscle mass and strength, and bone health in animal models [92–94]. Moreover, KER-065 was shown to have a 400-fold lower inhibitory activity against BMP9 [93]. As part of the phase I trial monitoring for epistaxis and telangiectasis was included and none reported in healthy volunteers [94]. Overall, the potential of dual inhibition of myostatin and activin pathways presents a compelling direction for future research and clinical applications in addressing muscle-wasting conditions. Further human studies are warranted to explore this therapeutic strategy and its implications for safely improving patient outcomes.

Supporting information

S1 Table. Serum chemistry panels.

https://doi.org/10.1371/journal.pone.0342666.s001

(XLSX)

S2 Table. ACE-031 pharmacokinetic analysis.

https://doi.org/10.1371/journal.pone.0342666.s002

(XLSX)

S1 File. Data used for statistical analyses.

https://doi.org/10.1371/journal.pone.0342666.s003

(XLSX)

Acknowledgments

We would like to thank Tim Ahern for his help in editing this manuscript.

References

  1. 1. Lee S-J, Reed LA, Davies MV, Girgenrath S, Goad MEP, Tomkinson KN, et al. Regulation of muscle growth by multiple ligands signaling through activin type II receptors. Proc Natl Acad Sci U S A. 2005;102(50):18117–22. pmid:16330774
  2. 2. Lee S-J, Lee Y-S, Zimmers TA, Soleimani A, Matzuk MM, Tsuchida K, et al. Regulation of muscle mass by follistatin and activins. Mol Endocrinol. 2010;24(10):1998–2008. pmid:20810712
  3. 3. Gonzalez-Cadavid NF, Taylor WE, Yarasheski K, Sinha-Hikim I, Ma K, Ezzat S, et al. Organization of the human myostatin gene and expression in healthy men and HIV-infected men with muscle wasting. Proc Natl Acad Sci U S A. 1998;95(25):14938–43. pmid:9843994
  4. 4. Yarasheski KE, Bhasin S, Sinha-Hikim I, Pak-Loduca J, Gonzalez-Cadavid NF. Serum myostatin-immunoreactive protein is increased in 60-92 year old women and men with muscle wasting. J Nutr Health Aging. 2002;6(5):343–8. pmid:12474026
  5. 5. Loumaye A, de Barsy M, Nachit M, Lause P, Frateur L, van Maanen A, et al. Role of Activin A and myostatin in human cancer cachexia. J Clin Endocrinol Metab. 2015;100(5):2030–8. pmid:25751105
  6. 6. Yaden BC, Wang YX, Wilson JM, Culver AE, Milner A, Datta-Mannan A, et al. Inhibition of activin A ameliorates skeletal muscle injury and rescues contractile properties by inducing efficient remodeling in female mice. Am J Pathol. 2014;184(4):1152–66. pmid:24655377
  7. 7. Zhou G, Gui X, Chen R, Fu X, Ji X, Ding H. Elevated serum Activin A in chronic obstructive pulmonary disease with skeletal muscle wasting. Clinics (Sao Paulo). 2019;74:e981. pmid:31271588
  8. 8. Durieux A-C, Amirouche A, Banzet S, Koulmann N, Bonnefoy R, Pasdeloup M, et al. Ectopic expression of myostatin induces atrophy of adult skeletal muscle by decreasing muscle gene expression. Endocrinology. 2007;148(7):3140–7. pmid:17395701
  9. 9. Reisz-Porszasz S, Bhasin S, Artaza JN, Shen R, Sinha-Hikim I, Hogue A, et al. Lower skeletal muscle mass in male transgenic mice with muscle-specific overexpression of myostatin. Am J Physiol Endocrinol Metab. 2003;285(4):E876-88. pmid:12824080
  10. 10. Zimmers TA, Davies MV, Koniaris LG, Haynes P, Esquela AF, Tomkinson KN, et al. Induction of cachexia in mice by systemically administered myostatin. Science. 2002;296(5572):1486–8. pmid:12029139
  11. 11. Ding H, Zhang G, Sin KWT, Liu Z, Lin R-K, Li M, et al. Activin A induces skeletal muscle catabolism via p38β mitogen-activated protein kinase. J Cachexia Sarcopenia Muscle. 2017;8(2):202–12. pmid:27897407
  12. 12. Lee S-J. Quadrupling muscle mass in mice by targeting TGF-beta signaling pathways. PLoS One. 2007;2(8):e789. pmid:17726519
  13. 13. McPherron AC, Lawler AM, Lee SJ. Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member. Nature. 1997;387(6628):83–90. pmid:9139826
  14. 14. McPherron AC, Lee SJ. Double muscling in cattle due to mutations in the myostatin gene. Proc Natl Acad Sci U S A. 1997;94(23):12457–61. pmid:9356471
  15. 15. Mosher DS, Quignon P, Bustamante CD, Sutter NB, Mellersh CS, Parker HG, et al. A mutation in the myostatin gene increases muscle mass and enhances racing performance in heterozygote dogs. PLoS Genet. 2007;3(5):e79. pmid:17530926
  16. 16. Schuelke M, Wagner KR, Stolz LE, Hübner C, Riebel T, Kömen W, et al. Myostatin mutation associated with gross muscle hypertrophy in a child. N Engl J Med. 2004;350(26):2682–8. pmid:15215484
  17. 17. Solagna F, Tezze C, Lindenmeyer MT, Lu S, Wu G, Liu S, et al. Pro-cachectic factors link experimental and human chronic kidney disease to skeletal muscle wasting programs. J Clin Invest. 2021;131(11):e135821. pmid:34060483
  18. 18. Bogdanovich S, Krag TOB, Barton ER, Morris LD, Whittemore L-A, Ahima RS, et al. Functional improvement of dystrophic muscle by myostatin blockade. Nature. 2002;420(6914):418–21. pmid:12459784
  19. 19. Bogdanovich S, McNally EM, Khurana TS. Myostatin blockade improves function but not histopathology in a murine model of limb-girdle muscular dystrophy 2C. Muscle Nerve. 2008;37(3):308–16. pmid:18041051
  20. 20. Whittemore L-A, Song K, Li X, Aghajanian J, Davies M, Girgenrath S, et al. Inhibition of myostatin in adult mice increases skeletal muscle mass and strength. Biochem Biophys Res Commun. 2003;300(4):965–71. pmid:12559968
  21. 21. Latres E, Mastaitis J, Fury W, Miloscio L, Trejos J, Pangilinan J, et al. Activin A more prominently regulates muscle mass in primates than does GDF8. Nat Commun. 2017;8:15153. pmid:28452368
  22. 22. Lee S-J, Lehar A, Meir JU, Koch C, Morgan A, Warren LE, et al. Targeting myostatin/activin A protects against skeletal muscle and bone loss during spaceflight. Proc Natl Acad Sci U S A. 2020;117(38):23942–51. pmid:32900939
  23. 23. Bartoli M, Poupiot J, Vulin A, Fougerousse F, Arandel L, Daniele N, et al. AAV-mediated delivery of a mutated myostatin propeptide ameliorates calpain 3 but not alpha-sarcoglycan deficiency. Gene Ther. 2007;14(9):733–40. pmid:17330087
  24. 24. Bogdanovich S, Perkins KJ, Krag TOB, Whittemore L-A, Khurana TS. Myostatin propeptide-mediated amelioration of dystrophic pathophysiology. FASEB J. 2005;19(6):543–9. pmid:15791004
  25. 25. Ohsawa Y, Hagiwara H, Nakatani M, Yasue A, Moriyama K, Murakami T, et al. Muscular atrophy of caveolin-3-deficient mice is rescued by myostatin inhibition. J Clin Invest. 2006;116(11):2924–34. pmid:17039257
  26. 26. Hammers DW, Hart CC, Patsalos A, Matheny MK, Wright LA, Nagy L, et al. Glucocorticoids counteract hypertrophic effects of myostatin inhibition in dystrophic muscle. JCI Insight. 2020;5(1):e133276. pmid:31830002
  27. 27. St Andre M, Johnson M, Bansal PN, Wellen J, Robertson A, Opsahl A, et al. A mouse anti-myostatin antibody increases muscle mass and improves muscle strength and contractility in the mdx mouse model of Duchenne muscular dystrophy and its humanized equivalent, domagrozumab (PF-06252616), increases muscle volume in cynomolgus monkeys. Skelet Muscle. 2017;7(1):25. pmid:29121992
  28. 28. Krivickas LS, Walsh R, Amato AA. Single muscle fiber contractile properties in adults with muscular dystrophy treated with MYO-029. Muscle Nerve. 2009;39(1):3–9. pmid:19086063
  29. 29. Wagner KR, Fleckenstein JL, Amato AA, Barohn RJ, Bushby K, Escolar DM, et al. A phase I/IItrial of MYO-029 in adult subjects with muscular dystrophy. Ann Neurol. 2008;63(5):561–71. pmid:18335515
  30. 30. Wagner KR, Abdel-Hamid HZ, Mah JK, Campbell C, Guglieri M, Muntoni F, et al. Randomized phase 2 trial and open-label extension of domagrozumab in Duchenne muscular dystrophy. Neuromuscul Disord. 2020;30(6):492–502. pmid:32522498
  31. 31. Muntoni F, Byrne BJ, McMillan HJ, Ryan MM, Wong BL, Dukart J, et al. The Clinical Development of Taldefgrobep Alfa: An Anti-Myostatin Adnectin for the Treatment of Duchenne Muscular Dystrophy. Neurol Ther. 2024;13(1):183–219. pmid:38190001
  32. 32. Pistilli EE, Bogdanovich S, Mosqueira M, Lachey J, Seehra J, Khurana TS. Pretreatment with a soluble activin type IIB receptor/Fc fusion protein improves hypoxia-induced muscle dysfunction. Am J Physiol Regul Integr Comp Physiol. 2010;298(1):R96–103. pmid:19864340
  33. 33. Pistilli EE, Bogdanovich S, Goncalves MD, Ahima RS, Lachey J, Seehra J, et al. Targeting the activin type IIB receptor to improve muscle mass and function in the mdx mouse model of Duchenne muscular dystrophy. Am J Pathol. 2011;178(3):1287–97. pmid:21356379
  34. 34. Alqallaf A, Engelbeen S, Palo A, Cutrupi F, Tanganyika-de Winter C, Plomp J, et al. The therapeutic potential of soluble activin type IIB receptor treatment in a limb girdle muscular dystrophy type 2D mouse model. Neuromuscul Disord. 2022;32(5):419–35. pmid:35465969
  35. 35. Béchir N, Pecchi E, Vilmen C, Le Fur Y, Amthor H, Bernard M, et al. ActRIIB blockade increases force-generating capacity and preserves energy supply in exercising mdx mouse muscle in vivo. FASEB J. 2016;30(10):3551–62. pmid:27416839
  36. 36. Liu M, Hammers DW, Barton ER, Sweeney HL. Activin Receptor Type IIB Inhibition Improves Muscle Phenotype and Function in a Mouse Model of Spinal Muscular Atrophy. PLoS One. 2016;11(11):e0166803. pmid:27870893
  37. 37. Chen JL, Walton KL, Hagg A, Colgan TD, Johnson K, Qian H, et al. Specific targeting of TGF-β family ligands demonstrates distinct roles in the regulation of muscle mass in health and disease. Proc Natl Acad Sci U S A. 2017;114(26):E5266–75. pmid:28607086
  38. 38. Gonzalez Trotter D, Donahue S, Wynne C, Ali S, Parasoglou P, Boyapati A, et al. GDF8 and activin A are the key negative regulators of muscle mass in postmenopausal females: a randomized phase I trial. Nat Commun. 2025;16(1):4376. pmid:40360471
  39. 39. Mastaitis JW, Gomez D, Raya JG, Li D, Min S, Stec M, et al. GDF8 and activin A blockade protects against GLP-1-induced muscle loss while enhancing fat loss in obese male mice and non-human primates. Nat Commun. 2025;16(1):4377. pmid:40360507
  40. 40. Attie KM, Borgstein NG, Yang Y, Condon CH, Wilson DM, Pearsall AE, et al. A single ascending-dose study of muscle regulator ACE-031 in healthy volunteers. Muscle Nerve. 2013;47(3):416–23. pmid:23169607
  41. 41. Campbell C, McMillan HJ, Mah JK, Tarnopolsky M, Selby K, McClure T, et al. Myostatin inhibitor ACE-031 treatment of ambulatory boys with Duchenne muscular dystrophy: Results of a randomized, placebo-controlled clinical trial. Muscle Nerve. 2017;55(4):458–64. pmid:27462804
  42. 42. Wagner KR. The elusive promise of myostatin inhibition for muscular dystrophy. Curr Opin Neurol. 2020;33(5):621–8. pmid:32773450
  43. 43. Trendelenburg AU, Meyer A, Rohner D, Boyle J, Hatakeyama S, Glass DJ. Myostatin reduces Akt/TORC1/p70S6K signaling, inhibiting myoblast differentiation and myotube size. Am J Physiol Cell Physiol. 2009;296(6):C1258-70. pmid:19357233
  44. 44. Welle S, Burgess K, Mehta S. Stimulation of skeletal muscle myofibrillar protein synthesis, p70 S6 kinase phosphorylation, and ribosomal protein S6 phosphorylation by inhibition of myostatin in mature mice. Am J Physiol Endocrinol Metab. 2009;296(3):E567-72. pmid:19141683
  45. 45. McFarlane C, Plummer E, Thomas M, Hennebry A, Ashby M, Ling N, et al. Myostatin induces cachexia by activating the ubiquitin proteolytic system through an NF-kappaB-independent, FoxO1-dependent mechanism. J Cell Physiol. 2006;209(2):501–14. pmid:16883577
  46. 46. Zhou X, Wang JL, Lu J, Song Y, Kwak KS, Jiao Q, et al. Reversal of cancer cachexia and muscle wasting by ActRIIB antagonism leads to prolonged survival. Cell. 2010;142(4):531–43. pmid:20723755
  47. 47. Lakshman KM, Bhasin S, Corcoran C, Collins-Racie LA, Tchistiakova L, Forlow SB, et al. Measurement of myostatin concentrations in human serum: Circulating concentrations in young and older men and effects of testosterone administration. Mol Cell Endocrinol. 2009;302(1):26–32. pmid:19356623
  48. 48. Singh P, Rong H, Gordi T, Bosley J, Bhattacharya I. Translational Pharmacokinetic/Pharmacodynamic Analysis of MYO-029 Antibody for Muscular Dystrophy. Clin Transl Sci. 2016;9(6):302–10. pmid:27700008
  49. 49. Ames WA, Hayes JA, Crawford MW. The role of corticosteroids in Duchenne muscular dystrophy: a review for the anesthetist. Paediatr Anaesth. 2005;15(1):3–8. pmid:15649156
  50. 50. Griggs RC, Miller JP, Greenberg CR, Fehlings DL, Pestronk A, Mendell JR, et al. Efficacy and safety of deflazacort vs prednisone and placebo for Duchenne muscular dystrophy. Neurology. 2016;87(20):2123–31. pmid:27566742
  51. 51. Wallace AM, Tucker P, Williams DM, Hughes IA, Ahmed SF. Short-term effects of prednisolone and dexamethasone on circulating concentrations of leptin and sex hormone-binding globulin in children being treated for acute lymphoblastic leukaemia. Clin Endocrinol (Oxf). 2003;58(6):770–6. pmid:12780755
  52. 52. Binnert C, Ruchat S, Nicod N, Tappy L. Dexamethasone-induced insulin resistance shows no gender difference in healthy humans. Diabetes Metab. 2004;30(4):321–6. pmid:15525874
  53. 53. Bianchi ML, Mazzanti A, Galbiati E, Saraifoger S, Dubini A, Cornelio F, et al. Bone mineral density and bone metabolism in Duchenne muscular dystrophy. Osteoporos Int. 2003;14(9):761–7. pmid:12897980
  54. 54. Mendell JR, Muntoni F, McDonald CM, Mercuri EM, Ciafaloni E, Komaki H, et al. AAV gene therapy for Duchenne muscular dystrophy: the EMBARK phase 3 randomized trial. Nat Med. 2025;31(1):332–41. pmid:39385046
  55. 55. Saad FA, Siciliano G, Angelini C. Advances in Dystrophinopathy Diagnosis and Therapy. Biomolecules. 2023;13(9):1319. pmid:37759719
  56. 56. Taylor L. Duchenne gene therapy halted after three deaths and company u turn. BMJ. 2025;390:r1548. pmid:40701635
  57. 57. Matsakas A, Foster K, Otto A, Macharia R, Elashry MI, Feist S, et al. Molecular, cellular and physiological investigation of myostatin propeptide-mediated muscle growth in adult mice. Neuromuscul Disord. 2009;19(7):489–99. pmid:19541486
  58. 58. Carpio Y, Acosta J, Morales R, Santisteban Y, Sanchéz A, Estrada MP. Regulation of body mass growth through activin type IIB receptor in teleost fish. Gen Comp Endocrinol. 2009;160(2):158–67. pmid:19056390
  59. 59. Morrison BM, Lachey JL, Warsing LC, Ting BL, Pullen AE, Underwood KW, et al. A soluble activin type IIB receptor improves function in a mouse model of amyotrophic lateral sclerosis. Exp Neurol. 2009;217(2):258–68. pmid:19285073
  60. 60. Nakatani M, Takehara Y, Sugino H, Matsumoto M, Hashimoto O, Hasegawa Y, et al. Transgenic expression of a myostatin inhibitor derived from follistatin increases skeletal muscle mass and ameliorates dystrophic pathology in mdx mice. FASEB J. 2008;22(2):477–87. pmid:17893249
  61. 61. Akpan I, Goncalves MD, Dhir R, Yin X, Pistilli EE, Bogdanovich S, et al. The effects of a soluble activin type IIB receptor on obesity and insulin sensitivity. Int J Obes (Lond). 2009;33(11):1265–73. pmid:19668253
  62. 62. George Carlson C, Bruemmer K, Sesti J, Stefanski C, Curtis H, Ucran J, et al. Soluble activin receptor type IIB increases forward pulling tension in the mdx mouse. Muscle Nerve. 2011;43(5):694–9. pmid:21462203
  63. 63. Morvan F, Rondeau J-M, Zou C, Minetti G, Scheufler C, Scharenberg M, et al. Blockade of activin type II receptors with a dual anti-ActRIIA/IIB antibody is critical to promote maximal skeletal muscle hypertrophy. Proc Natl Acad Sci U S A. 2017;114(47):12448–53. pmid:29109273
  64. 64. Lach-Trifilieff E, Minetti GC, Sheppard K, Ibebunjo C, Feige JN, Hartmann S, et al. An antibody blocking activin type II receptors induces strong skeletal muscle hypertrophy and protects from atrophy. Mol Cell Biol. 2014;34(4):606–18. pmid:24298022
  65. 65. Koncarevic A, Cornwall-Brady M, Pullen A, Davies M, Sako D, Liu J, et al. A soluble activin receptor type IIb prevents the effects of androgen deprivation on body composition and bone health. Endocrinology. 2010;151(9):4289–300. pmid:20573726
  66. 66. Ashmore CR, Robinson DW. Hereditary muscular hypertrophy in the bovine. I. Histological and biochemical characterization. Proc Soc Exp Biol Med. 1969;132(2):548–54. pmid:5355105
  67. 67. Hennebry A, Berry C, Siriett V, O’Callaghan P, Chau L, Watson T, et al. Myostatin regulates fiber-type composition of skeletal muscle by regulating MEF2 and MyoD gene expression. Am J Physiol Cell Physiol. 2009;296(3):C525-34. pmid:19129464
  68. 68. Girgenrath S, Song K, Whittemore L-A. Loss of myostatin expression alters fiber-type distribution and expression of myosin heavy chain isoforms in slow- and fast-type skeletal muscle. Muscle Nerve. 2005;31(1):34–40. pmid:15468312
  69. 69. Xing X-X, Xuan M-F, Jin L, Guo Q, Luo Z-B, Wang J-X, et al. Fiber-type distribution and expression of myosin heavy chain isoforms in newborn heterozygous myostatin-knockout pigs. Biotechnol Lett. 2017;39(12):1811–9. pmid:28861647
  70. 70. Morine KJ, Bish LT, Pendrak K, Sleeper MM, Barton ER, Sweeney HL. Systemic myostatin inhibition via liver-targeted gene transfer in normal and dystrophic mice. PLoS One. 2010;5(2):e9176. pmid:20161803
  71. 71. Cadena SM, Tomkinson KN, Monnell TE, Spaits MS, Kumar R, Underwood KW, et al. Administration of a soluble activin type IIB receptor promotes skeletal muscle growth independent of fiber type. J Appl Physiol (1985). 2010;109(3):635–42. pmid:20466801
  72. 72. Kota J, Handy CR, Haidet AM, Montgomery CL, Eagle A, Rodino-Klapac LR, et al. Follistatin gene delivery enhances muscle growth and strength in nonhuman primates. Sci Transl Med. 2009;1(6):6ra15. pmid:20368179
  73. 73. Ohsawa Y, Takayama K, Nishimatsu S, Okada T, Fujino M, Fukai Y, et al. The Inhibitory Core of the Myostatin Prodomain: Its Interaction with Both Type I and II Membrane Receptors, and Potential to Treat Muscle Atrophy. PLoS One. 2015;10(7):e0133713. pmid:26226340
  74. 74. Omairi S, Matsakas A, Degens H, Kretz O, Hansson K-A, Solbrå AV, et al. Enhanced exercise and regenerative capacity in a mouse model that violates size constraints of oxidative muscle fibres. Elife. 2016;5:e16940. pmid:27494364
  75. 75. Amthor H, Macharia R, Navarrete R, Schuelke M, Brown SC, Otto A, et al. Lack of myostatin results in excessive muscle growth but impaired force generation. Proc Natl Acad Sci U S A. 2007;104(6):1835–40. pmid:17267614
  76. 76. Mouisel E, Relizani K, Mille-Hamard L, Denis R, Hourdé C, Agbulut O, et al. Myostatin is a key mediator between energy metabolism and endurance capacity of skeletal muscle. Am J Physiol Regul Integr Comp Physiol. 2014;307(4):R444-54. pmid:24965795
  77. 77. Kramer IF, Snijders T, Smeets JSJ, Leenders M, van Kranenburg J, den Hoed M, et al. Extensive Type II Muscle Fiber Atrophy in Elderly Female Hip Fracture Patients. J Gerontol A Biol Sci Med Sci. 2017;72(10):1369–75. pmid:28329045
  78. 78. Nilwik R, Snijders T, Leenders M, Groen BBL, van Kranenburg J, Verdijk LB, et al. The decline in skeletal muscle mass with aging is mainly attributed to a reduction in type II muscle fiber size. Exp Gerontol. 2013;48(5):492–8. pmid:23425621
  79. 79. Tanganelli F, Meinke P, Hofmeister F, Jarmusch S, Baber L, Mehaffey S, et al. Type-2 muscle fiber atrophy is associated with sarcopenia in elderly men with hip fracture. Exp Gerontol. 2021;144:111171. pmid:33248151
  80. 80. Kaneguchi A, Sakitani N, Umehara T. Histological changes in skeletal muscle induced by heart failure in human patients and animal models: A scoping review. Acta Histochem. 2024;126(8):152210. pmid:39442432
  81. 81. Curcio F, Testa G, Liguori I, Papillo M, Flocco V, Panicara V, et al. Sarcopenia and Heart Failure. Nutrients. 2020;12(1):211. pmid:31947528
  82. 82. Mendias CL, Marcin JE, Calerdon DR, Faulkner JA. Contractile properties of EDL and soleus muscles of myostatin-deficient mice. J Appl Physiol (1985). 2006;101(3):898–905. pmid:16709649
  83. 83. Foster K, Graham IR, Otto A, Foster H, Trollet C, Yaworsky PJ, et al. Adeno-associated virus-8-mediated intravenous transfer of myostatin propeptide leads to systemic functional improvements of slow but not fast muscle. Rejuvenation Res. 2009;12(2):85–94. pmid:19405813
  84. 84. Minetti GC, Colussi C, Adami R, Serra C, Mozzetta C, Parente V, et al. Functional and morphological recovery of dystrophic muscles in mice treated with deacetylase inhibitors. Nat Med. 2006;12(10):1147–50. pmid:16980968
  85. 85. Loro E, Sengupta K, Bogdanovich S, Whig K, Schultz DC, Huryn DM, et al. High-throughput identification of post-transcriptional utrophin up-regulators for Duchenne muscle dystrophy (DMD) therapy. Sci Rep. 2020;10(1):2132. pmid:32034254
  86. 86. Iezzi S, Di Padova M, Serra C, Caretti G, Simone C, Maklan E, et al. Deacetylase inhibitors increase muscle cell size by promoting myoblast recruitment and fusion through induction of follistatin. Dev Cell. 2004;6(5):673–84. pmid:15130492
  87. 87. Amthor H, Nicholas G, McKinnell I, Kemp CF, Sharma M, Kambadur R, et al. Follistatin complexes Myostatin and antagonises Myostatin-mediated inhibition of myogenesis. Dev Biol. 2004;270(1):19–30. pmid:15136138
  88. 88. Gilson H, Schakman O, Kalista S, Lause P, Tsuchida K, Thissen J-P. Follistatin induces muscle hypertrophy through satellite cell proliferation and inhibition of both myostatin and activin. Am J Physiol Endocrinol Metab. 2009;297(1):E157-64. pmid:19435857
  89. 89. Campbell C, Escolar DM, Mah J, Tarnopolsky M, Selby K, McMillan H, et al. A phase 2, randomized, placebo-controlled, multiple ascending-dose study of ACE-031, a soluble activin receptor type IIB, in boys with Duchenne muscular dystrophy. Neurology. 2012.
  90. 90. Rybalka E, Timpani CA, Debruin DA, Bagaric RM, Campelj DG, Hayes A. The Failed Clinical Story of Myostatin Inhibitors against Duchenne Muscular Dystrophy: Exploring the Biology behind the Battle. Cells. 2020;9(12):2657. pmid:33322031
  91. 91. Ruiz S, Zhao H, Chandakkar P, Chatterjee PK, Papoin J, Blanc L, et al. A mouse model of hereditary hemorrhagic telangiectasia generated by transmammary-delivered immunoblocking of BMP9 and BMP10. Sci Rep. 2016;5:37366. pmid:27874028
  92. 92. Nathan R, Cahill M, Todorova R, Macaluso S, Tseng C, Fisher F, et al. P33 RKER-065 ameliorated muscle and bone loss in a progressive murine model of Duchenne muscular dystrophy. Neuromuscular Disord. 2023;33:S105.
  93. 93. Nathan R, Materna C, Welch D, Nurse T, Lema E, Gudelsky A, et al. P.191 RKER-065, a novel muscle and bone anabolic, increased muscle, grip strength and trabecular bone in a mouse model of Duchenne muscular dystrophy. Neuromuscular Disorders. 2022;32:S125.
  94. 94. Natarajan H, Taimi M, Hall S, Bobba S, Jiang Y, Grayson D, et al. Phase 1 trial in healthy participants of KER-065, modified activin receptor ligand trap, support development In DMD and other neuromuscular disorders. J Bone Min Res. 2025;40.