Myostatin/Activin Receptor Ligands in Muscle and the Development Status of Attenuating Drugs

Abstract

Muscle wasting disease indications are among the most debilitating and often deadly noncommunicable disease states. As a comorbidity, muscle wasting is associated with different neuromuscular diseases and myopathies, cancer, heart failure, chronic pulmonary and renal diseases, peripheral neuropathies, inflammatory disorders, and, of course, musculoskeletal injuries. Current treatment strategies are relatively ineffective and can at best only limit the rate of muscle degeneration. This includes nutritional supplementation and appetite stimulants as well as immunosuppressants capable of exacerbating muscle loss. Arguably, the most promising treatments in development attempt to disrupt myostatin and activin receptor signaling because these circulating factors are potent inhibitors of muscle growth and regulators of muscle progenitor cell differentiation. Indeed, several studies demonstrated the clinical potential of "inhibiting the inhibitors," increasing muscle cell protein synthesis, decreasing degradation, enhancing mitochondrial biogenesis, and preserving muscle function. Such changes can prevent muscle wasting in various disease animal models yet many drugs targeting this pathway failed during clinical trials, some from serious treatment-related adverse events and off-target interactions. More often, however, failures resulted from the inability to improve muscle function despite preserving muscle mass. Drugs still in development include antibodies and gene therapeutics, all with different targets and thus, safety, efficacy, and proposed use profiles. Each is unique in design and, if successful, could revolutionize the treatment of both acute and chronic muscle wasting. They could also be used in combination with other developing therapeutics for related muscle pathologies or even metabolic diseases.

Keywords: ACVR2; ACVR2B; ActRIIa; ActRIIb; GDF11; activin; growth/differentiation factor (GDF)8; muscle atrophy; muscle wasting; myostatin.

Graphical Abstract

Figure 1.

Complexity of the TGF-β superfamily network. (A) General overview of the ligand-mediated signaling pathway starting with (1) secretion of the ligand:latency associated protein (LAP [prodomain]) complex and its association with binding proteins in the extracellular matrix. (2) Cytoskeletal forces and/or proteolysis release the ligand:LAP complex, often dissociating each. (3) Mature ligands are then free to associate with protein antagonists in the extracellular space or in circulation. (4) LAPs are also released with ligand binding to type II receptors followed by recruitment of type I receptors and their transphosphorylation by type II. (5) The serine kinase domain of type 1 receptors then phosphorylates receptor (R)-Smads that then bind Co-Smads allowing for (6) nuclear translocation, binding of transcription factors (TF) and coregulators and ultimately, (7) gene transactivation. (8) Inhibitory (I)-Smads are among the target genes and attenuate both receptor activation and Co-Smad complex formation. (B) Color-coding indicates the different ligands (purple and blue) that bind ActRII/IIb type II receptors (orange) that associate with different type I receptors (yellow and red) to separately activate distinct Smad signaling pathways.

Figure 2.

Anabolic and catabolic regulation of muscle. (A) Parsing of general physiological and pathological conditions as well as the primary factors that differentially regulate skeletal muscle hypertrophy and atrophy (BMP, bone morphogenic protein; COPD, chronic obstructive pulmonary disorder; ESRD/CKD, end-stage renal disease/chronic kidney disease; GDF, growth/differentiation factor; HF, heart failure; MSTN, myostatin; MSI, musculoskeletal injury). (B) Model for MSTN interactions with the GH/IGF1 axis. Arrows represent stimulation, blocked lines inhibition. Arrow/line thickness is relative to influence. (C) Model for the paradoxical actions of IL-6 on skeletal muscle satellite cells and hypertrophy as well as on muscle protein degradation and atrophy. Colored arrows correspond to labeled factor, black arrows indicate increase (CD8+, cluster of differentiation 8 positive T-helper immune cell; MuRF1, muscle RING finger 1 [Trim63]; MAFbx, muscle atrophy F-Box [Atrogin-1]).

Figure 3.

Canonical intracellular signaling pathways activated by ActRII ligands. (A) Endogenous ActRII signaling via phosphorylation (P) of Smad2 and Smad3. Includes receptor activation by myostatin (MSTN), an activin (ActA shown), growth/differentiating factor (GDF)11, or bone morphogenic protein (BMP)9. Each number represents the intracellular signaling locations and red dots represent ubiquitin. Dashed arrows represent movement of FKBP12 and methylated Smad7; green arrows direct pathway activation; red arrows direct negative feedback and signal termination. (B) Responses to pharmacological antagonism of Smad2/3 signaling in muscle. Arrows indicate activation; blocked lines inhibition. Silenced pathways are grayed, whereas blue symbols and green arrows represent pathways activated as a result of agents that attenuate ActRIIa/b activation and/or Smad2/3 signaling. These agents (red) include antibodies and ligand traps or the overexpression of Smad7, the endogenous pathway inhibitor. (C) Relative expression of the indicated genes was plotted using publicly available data from the Gene Expression Omnibus, record GSE67326 (61). This record was obtained from human skeletal muscle-derived cells (hSkMDCs) that were differentiated in vitro from primary satellite cells and then stimulated for 8 or 24 hours with 0, 10, 30, or 300 ng/mL myostatin (M) or GDF11 (G). Raw expression values were transformed to percent of 0 controls for each probe/spot. These values were then used to calculate group means (n = 4). Significant differences between means were determined using a 2-way ANOVA and Tukey’s post hoc test and are indicated by asterisks (compared with 0 controls: *P < 0.05, **0.01, ***0.001).

Figure 4.

Dystrophin/costamere functional relationship. The costamere is composed of 2 protein complexes: the dystrophin-associated glycoprotein complex (DGC) and the integrin complex (IC). (A) Structural components of the DGC using color-coded labels for individual proteins or protein classes (DG, dystroglycan; DTNA, dystrobrevin-a; FKRP, Fukutin-related protein; NOS, nitric oxide synthase; SYCN, syncoilin). Dystrophin binds filamentous (F)-actin that in turn binds Z-line components of sarcomeres, physically linking the contractile machinery to the costamere and the extracellular matrix. Anchoring the costamere to the basement membrane depends upon proper glycosylation of different proteins including α-dystroglycan and the sarcoglycans. (B, C) Model for longitudinal and lateral force transmission based on contractile studies of healthy and dystrophic (mdx) mice. Color-coded labels in panel B apply to all panels representing the eccentric (lengthening) contraction cycle (Dys, dystrophin; ECM, extracellular matrix). The percentage of total specific force (numbers on right) transmitted laterally or longitudinally are represented by upper and lower yellow arrows, respectively, in each panel.

Figure 5.

Pathophysiology of IBM. Cartoon representation of muscle fascicle cross-sections from healthy subjects and from IBM patients before and after treatment with corticosteroids. Different structural components of mature muscle fascicles are labeled to the left of each panel. Dotted lines aid in representing the change in fascicle size with disease progression.