[Show abstract][Hide abstract]ABSTRACT: X-linked myotubular myopathy is a congenital myopathy caused by deficiency of myotubularin. Patients often present with severe perinatal weakness, requiring mechanical ventilation to prevent death from respiratory failure. We recently reported that an activin receptor type IIB inhibitor produced hypertrophy of type 2b myofibers and modest increases of strength and life span in the severely myopathic Mtm1δ4 mouse model of X-linked myotubular myopathy. We have now performed a similar study in the less severely symptomatic Mtm1 p.R69C mouse in hopes of finding greater treatment efficacy. Activin receptor type IIB inhibitor treatment of Mtm1 p.R69C animals produced behavioral and histological evidence of hypertrophy in gastrocnemius muscles but not in quadriceps or triceps. The ability of the muscles to respond to activin receptor type IIB inhibitor treatment correlated with treatment-induced increases in satellite cell number and several muscle-specific abnormalities of hypertrophic signaling. Treatment-responsive Mtm1 p.R69C gastrocnemius muscles displayed lower levels of phosphorylated ribosomal protein S6 and higher levels of phosphorylated eukaryotic elongation factor 2 kinase than were observed in Mtm1 p.R69C quadriceps muscle or in muscles from wild-type littermates. Hypertrophy in the Mtm1 p.R69C gastrocnemius muscle was associated with increased levels of phosphorylated ribosomal protein S6. Our findings indicate that muscle-, fiber type-, and mutation-specific factors affect the response to hypertrophic therapies that will be important to assess in future therapeutic trials.
Full-text · Article · Apr 2014 · American Journal Of Pathology
[Show abstract][Hide abstract]ABSTRACT: Obesity results from disproportionately high energy intake relative to energy expenditure. Many therapeutic strategies have focused on the intake side of the equation, including pharmaceutical targeting of appetite and digestion. An alternative approach is to increase energy expenditure through physical activity or adaptive thermogenesis. A pharmacological way to increase muscle mass and hence exercise capacity is through inhibition of the activin receptor type IIB (ActRIIB). Muscle mass and strength is regulated, at least in part, by growth factors that signal via ActRIIB. Administration of a soluble ActRIIB protein comprised of a form of the extracellular domain of ActRIIB fused to a human Fc (ActRIIB-Fc) results in a substantial muscle mass increase in normal mice. However, ActRIIB is also present on and mediates the action of growth factors in adipose tissue, although the function of this system is poorly understood. In the current study, we report the effect of ActRIIB-Fc to suppress diet-induced obesity and linked metabolic dysfunctions in mice fed a high-fat diet. ActRIIB-Fc induced a brown fat-like thermogenic gene program in epididymal white fat, as shown by robustly increased expression of the thermogenic genes uncoupling protein 1 and peroxisomal proliferator-activated receptor-γ coactivator 1α. Finally, we identified multiple ligands capable of reducing thermogenesis that represent likely target ligands for the ActRIIB-Fc effects on the white fat depots. These data demonstrate that novel therapeutic ActRIIB-Fc improves obesity and obesity-linked metabolic disease by both increasing skeletal muscle mass and by inducing a gene program of thermogenesis in the white adipose tissues.
[Show abstract][Hide abstract]ABSTRACT: Introduction:
In this study we investigated the action of RAP-031, a soluble activin receptor type IIB (ActRIIB) comprised of a form of the ActRIIB extracellular domain linked to a murine Fc, and the NF-κB inhibitor, ursodeoxycholic acid (UDCA), on the whole body strength of mdx mice.
The whole body tension (WBT) method of assessing the forward pulling tension (FPT) exerted by dystrophic (mdx) mice was used.
RAP-031 produced a 41% increase in body mass and a 42.5% increase in FPT without altering the FPT normalized for body mass (WBT). Coadministration of RAP-031 with UDCA produced increases in FPT that were associated with an increase in WBT.
Myostatin inhibition increases muscle mass without altering the fundamental weakness characteristic of dystrophic muscle. Cotreatment with an NF-κB inhibitor potentiates the effects of myostatin inhibition in improving FPT in mdx mice.
[Show abstract][Hide abstract]ABSTRACT: The activin receptor type IIB (ActRIIB) is a transmembrane receptor for transforming growth factor-β superfamily members, including myostatin, that are involved in the negative regulation of skeletal muscle mass. We tested the translational hypothesis that blocking ligand binding to ActRIIB for 12 weeks would stimulate skeletal muscle growth and improve muscle function in the mdx mouse. ActRIIB was targeted using a novel inhibitor comprised of the extracellular portion of the ActRIIB fused to the Fc portion of murine IgG (sActRIIB), at concentrations of 1.0 and 10.0 mg/kg(-1) body weight. After 12 weeks of treatment, the 10.0 mg/kg(-1) dose caused a 27% increase in body weight with a concomitant 33% increase in lean muscle mass. Absolute force production of the extensor digitorum longus muscle ex vivo was higher in mice after treatment with either dose of sActRIIB, and the specific force was significantly higher after the lower dose (1.0 mg/kg(-1)), indicating functional improvement in the muscle. Circulating creatine kinase levels were significantly lower in mice treated with sActRIIB, compared with control mice. These data show that targeting the ActRIIB improves skeletal muscle mass and functional strength in the mdx mouse model of DMD, providing a therapeutic rationale for use of this molecule in treating skeletal myopathies.
Full-text · Article · Mar 2011 · American Journal Of Pathology
[Show abstract][Hide abstract]ABSTRACT: X-linked myotubular myopathy (XLMTM) is a congenital disorder caused by deficiency of the lipid phosphatase, myotubularin. Patients with XLMTM often have severe perinatal weakness that requires mechanical ventilation to prevent death from respiratory failure. Muscle biopsy specimens from patients with XLMTM exhibit small myofibers with central nuclei and central aggregations of organelles in many cells. It was postulated that therapeutically increasing muscle fiber size would cause symptomatic improvement in myotubularin deficiency. Recent studies have elucidated an important role for the activin-receptor type IIB (ActRIIB) in regulation of muscle growth and have demonstrated that ActRIIB inhibition results in significant muscle hypertrophy. To evaluate whether promoting muscle hypertrophy can attenuate symptoms resulting from myotubularin deficiency, the effect of ActRIIB-mFC treatment was determined in myotubularin-deficient (Mtm1δ4) mice. Compared with wild-type mice, untreated Mtm1δ4 mice have decreased body weight, skeletal muscle hypotrophy, and reduced survival. Treatment of Mtm1δ4 mice with ActRIIB-mFC produced a 17% extension of lifespan, with transient increases in weight, forelimb grip strength, and myofiber size. Pathologic analysis of Mtm1δ4 mice during treatment revealed that ActRIIB-mFC produced marked hypertrophy restricted to type 2b myofibers, which suggests that oxidative fibers in Mtm1δ4 animals are incapable of a hypertrophic response in this setting. These results support ActRIIB-mFC as an effective treatment for the weakness observed in myotubularin deficiency.
Full-text · Article · Feb 2011 · American Journal Of Pathology
[Show abstract][Hide abstract]ABSTRACT: Studies in humans and rodents indicate that a minimum amount of stored energy is required for normal pubertal development. The adipocyte-derived hormone leptin is a key metabolic signal to the neuroendocrine reproductive axis. Humans and mice lacking leptin or the leptin receptor (LepR) (ob/ob and db/db mice, respectively) are infertile and fail to enter puberty. Leptin administration to leptin-deficient subjects and ob/ob mice induces puberty and restores fertility, but the exact site or sites of leptin action are unclear. Here, we found that genetic deletion of LepR selectively from hypothalamic Kiss1 neurons in mice had no effect on puberty or fertility, indicating that direct leptin signaling in Kiss1 neurons is not required for these processes. However, bilateral lesions of the ventral premammillary nucleus (PMV) of ob/ob mice blunted the ability of exogenous leptin to induce sexual maturation. Moreover, unilateral reexpression of endogenous LepR in PMV neurons was sufficient to induce puberty and improve fertility in female LepR-null mice. This LepR reexpression also normalized the increased hypothalamic GnRH content characteristic of leptin-signaling deficiency. These data suggest that the PMV is a key site for leptin's permissive action at the onset of puberty and support the hypothesis that the multiple actions of leptin to control metabolism and reproduction are anatomically dissociated.
Full-text · Article · Jan 2011 · The Journal of clinical investigation
[Show abstract][Hide abstract]ABSTRACT: Akt is a critical mediator of developmental skeletal muscle growth. Treatment with a soluble ActRIIB fusion protein (ActRIIB-mFc) increases skeletal muscle mass and strength by inhibiting myostatin and related peptides. Recent in vitro studies have suggested that Akt signaling is necessary for the ability of ActRIIB inhibition to induce muscle hypertrophy. Thus, we hypothesized that mice deficient in either Akt1 or Akt2 would not respond to in vivo inhibition of ActRIIB with ActRIIB-mFc treatment.
We analyzed body composition and muscle parameters in wild-type C57BL/6J and Akt1 and Akt2 knockout mice, and compared the responses to blockade of ActRIIB signaling via ActRIIB-mFc treatment. Mice lacking Akt1 or Akt2 had reduced muscle mass, grip strength and contractile force. However, deficiency of Akt1 or Akt2 did not prevent the ability of ActRIIB-mFc treatment to induce muscle hypertrophy, or increase grip strength and contractile force. Akt1 and Akt2 deficient mice responded similarly as wild type mice to ActRIIB-mFc treatment by increasing fiber size.
Akt1 and Akt2 are important for the regulation of skeletal muscle mass and function. However, these Akt isoforms are not essential for the ability of ActRIIB inhibition to regulate muscle size, fiber type, strength or contractile force.
[Show abstract][Hide abstract]ABSTRACT: Effects of Akt deficiency and ActRIIB inhibition on MAFbx and MuRF1 expression. Effects of genotype and ActRIIB-mFc treatment (Rx, black bar) or vehicle (Veh, white bar) in wild-type (WT), Akt1 knockout mice (Akt1ko), and Akt2 knockout mice (Akt2ko) on MAFbx (Fbxo32) and MuRF1 (Trim63) expression. Expression values are normalized to phosphoriboprotein (36B4; Rplp0). Data are mean ± SEM, n = 5. **P<0.01 vs. same genotype treated vehicle.
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[Show abstract][Hide abstract]ABSTRACT: Androgen deprivation, a consequence of hypogonadism, certain cancer treatments, or normal aging in men, leads to loss of muscle mass, increased adiposity, and osteoporosis. In the present study, using a soluble chimeric form of activin receptor type IIB (ActRIIB) we sought to offset the adverse effects of androgen deprivation on muscle, adipose tissue, and bone. Castrated (ORX) or sham-operated (SHAM) mice received either TBS [vehicle-treated (VEH)] or systemic administration of ActRIIB-mFc, a soluble fusion protein comprised of a form of the extracellular domain of ActRIIB fused to a murine IgG2aFc subunit. In vivo body composition imaging demonstrated that ActRIIB-mFc treatment results in increased lean tissue mass of 23% in SHAM mice [19.02 +/- 0.42 g (VEH) versus 23.43 +/- 0.35 g (ActRIIB-mFc), P < 0.00001] and 26% in ORX mice [15.59 +/- 0.26 g (VEH) versus 19.78 +/- 0.26 g (ActRIIB-mFc), P < 0.00001]. Treatment also caused a decrease in adiposity of 30% in SHAM mice [5.03 +/- 0.48 g (VEH) versus 3.53 +/- 0.19 g (ActRIIB-mFc), NS] and 36% in ORX mice [7.12 +/- 0.53 g (VEH) versus 4.57 +/- 0.28 g (ActRIIB-mFc), P < 0.001]. These changes were also accompanied by altered serum levels of leptin, adiponectin, and insulin, as well as by prevention of steatosis (fatty liver) in ActRIIB-mFc-treated ORX mice. Finally, ActRIIB-mFc prevented loss of bone mass in ORX mice as assessed by whole body dual x-ray absorptiometry and micro-computed tomography of proximal tibias. The data demonstrate that treatment with ActRIIB-mFc restored muscle mass, adiposity, and bone quality to normal levels in a mouse model of androgen deprivation, thereby alleviating multiple adverse consequences of such therapy.
[Show abstract][Hide abstract]ABSTRACT: This is the first report that inhibition of negative regulators of skeletal muscle by a soluble form of activin type IIB receptor (ACE-031) increases muscle mass independent of fiber-type expression. This finding is distinct from the effects of selective pharmacological inhibition of myostatin (GDF-8), which predominantly targets type II fibers. In our study 8-wk-old C57BL/6 mice were treated with ACE-031 or vehicle control for 28 days. By the end of treatment, mean body weight of the ACE-031 group was 16% greater than that of the control group, and wet weights of soleus, plantaris, gastrocnemius, and extensor digitorum longus muscles increased by 33, 44, 46 and 26%, respectively (P<0.05). Soleus fiber-type distribution was unchanged with ACE-031 administration, and mean fiber cross-sectional area increased by 22 and 28% (P<0.05) in type I and II fibers, respectively. In the plantaris, a predominantly type II fiber muscle, mean fiber cross-sectional area increased by 57% with ACE-031 treatment. Analysis of myosin heavy chain (MHC) isoform transcripts by real-time PCR indicated no change in transcript levels in the soleus, but a decline in MHC I and IIa in the plantaris. In contrast, electrophoretic separation of total soleus and plantaris protein indicated that there was no change in the proportion of MHC isoforms in either muscle. Thus these data provide optimism that ACE-031 may be a viable therapeutic in the treatment of musculoskeletal diseases. Future studies should be undertaken to confirm that the observed effects are not age dependent or due to the relatively short study duration.
Preview · Article · May 2010 · Journal of Applied Physiology
[Show abstract][Hide abstract]ABSTRACT: Hypoxia, or reduced oxygen, occurs in a variety of clinical and environmental situations. Hypoxic exposure is associated with decreased muscle mass and a concomitant reduction in exercise capacity, although the exact mechanisms are not completely understood. The activin type IIB receptor (ActRIIB) is a receptor for transforming growth factor-beta (TGFbeta) superfamily members that are involved in the negative regulation of lean tissue mass. Given that hypoxia has negative effects on muscle mass and function and that modulation of the ActRIIB has been shown to increase muscle mass, we tested the hypothesis that pharmacological targeting of the ActRIIB for 2 wk would attenuate the loss of muscle mass and function in mice after exposure to normobaric hypoxia. ActRIIB modulation was achieved using a soluble activin receptor/Fc fusion protein (sActRIIB) in mice housed in a hypoxic chamber for 1 or 2 wk. Hypoxia induced a reduction in body weight in PBS- and sActRIIB-treated mice, although sActRIIB-treated mice remained larger throughout the hypoxic exposure. The absolute forces generated by extensor digitorum longus muscles were also significantly greater in sActRIIB- than PBS-treated mice and were more resistant to eccentric contraction-induced force drop after eccentric lengthening contractions. In summary, sActRIIB pretreatment attenuated hypoxia-induced muscle dysfunction. These data suggest that targeting the ActRIIB is an effective strategy to counter hypoxia-induced muscle dysfunction and to preacclimatize to hypoxia in clinical or high-altitude settings.
Full-text · Article · Oct 2009 · AJP Regulatory Integrative and Comparative Physiology
[Show abstract][Hide abstract]ABSTRACT: Myostatin, also known as Growth and Differentiation Factor 8, is a secreted protein that inhibits muscle growth. Disruption of myostatin signaling increases muscle mass and decreases glucose, but it is unclear whether these changes are related. We treated mice on chow and high-fat diets with a soluble activin receptor type IIB (ActRIIB, RAP-031), which is a putative endogenous signaling receptor for myostatin and other ligands of the TGF-beta superfamily.
After 4 weeks, RAP-031 increased lean and muscle mass, grip strength and contractile force. RAP-031 enhanced the ability of insulin to suppress glucose production under clamp conditions in high-fat fed mice, but did not significantly change insulin-mediated glucose disposal. The hepatic insulin-sensitizing effect of RAP-031 treatment was associated with increased adiponectin levels. RAP-031 treatment for 10 weeks further increased muscle mass and drastically reduced fat content in mice on either chow or high-fat diet. RAP-031 suppressed hepatic glucose production and increased peripheral glucose uptake in chow-fed mice. In contrast, RAP-031 suppressed glucose production with no apparent change in glucose disposal in high-fat-diet mice.
Our findings show that disruption of ActRIIB signaling is a viable pharmacological approach for treating obesity and diabetes.
Full-text · Article · Sep 2009 · International journal of obesity (2005)
[Show abstract][Hide abstract]ABSTRACT: The central actions of leptin are essential for homeostatic control of adipose tissue mass, glucose metabolism, and many autonomic and neuroendocrine systems. In the brain, leptin acts on numerous different cell types via the long-form leptin receptor (LepRb) to elicit its effects. The precise identification of leptin's cellular targets is fundamental to understanding the mechanism of its pleiotropic central actions. We have systematically characterized LepRb distribution in the mouse brain using in situ hybridization in wildtype mice as well as by EYFP immunoreactivity in a novel LepRb-IRES-Cre EYFP reporter mouse line showing high levels of LepRb mRNA/EYFP coexpression. We found substantial LepRb mRNA and EYFP expression in hypothalamic and extrahypothalamic sites described before, including the dorsomedial nucleus of the hypothalamus, ventral premammillary nucleus, ventral tegmental area, parabrachial nucleus, and the dorsal vagal complex. Expression in insular cortex, lateral septal nucleus, medial preoptic area, rostral linear nucleus, and in the Edinger-Westphal nucleus was also observed and had been previously unreported. The LepRb-IRES-Cre reporter line was used to chemically characterize a population of leptin receptor-expressing neurons in the midbrain. Tyrosine hydroxylase and Cre reporter were found to be coexpressed in the ventral tegmental area and in other midbrain dopaminergic neurons. Lastly, the LepRb-IRES-Cre reporter line was used to map the extent of peripheral leptin sensing by central nervous system (CNS) LepRb neurons. Thus, we provide data supporting the use of the LepRb-IRES-Cre line for the assessment of the anatomic and functional characteristics of neurons expressing leptin receptor.
Full-text · Article · Jun 2009 · The Journal of Comparative Neurology
[Show abstract][Hide abstract]ABSTRACT: Amyotrophic lateral sclerosis (ALS) is a neurologic disease characterized by progressive weakness that results in death within a few years of onset by respiratory failure. Myostatin is a member of the TGF-beta superfamily that is predominantly expressed in muscle and acts as a negative regulator of muscle growth. Attenuating myostatin has previously been shown to produce increased muscle mass and strength in normal and disease animal models. In this study, a mouse model of ALS (SOD1(G93A) transgenic mice) was treated with a soluble activin receptor, type IIB (ActRIIB.mFc) which is a putative endogenous signaling receptor for myostatin in addition to other ligands of the TGF-beta superfamily. ActRIIB.mFc treatment produces a delay in the onset of weakness, an increase in body weight and grip strength, and an enlargement of muscle size whether initiated pre-symptomatically or after symptom onset. Treatment with ActRIIB.mFc did not increase survival or neuromuscular junction innervation in SOD1(G93A) transgenic mice. Pharmacologic treatment with ActRIIB.mFc was superior in all measurements to genetic deletion of myostatin in SOD1(G93A) transgenic mice. The improved function of SOD1(G93A) transgenic mice following treatment with ActRIIB.mFc is encouraging for the development of TGF-beta pathway inhibitors to increase muscle strength in patients with ALS.
No preview · Article · Apr 2009 · Experimental Neurology