Mitochondrial impairment induced by postnatal ActRIIB blockade does not alter function and energy status in exercising mouse glycolytic muscle in vivo

Abstract

-Because it leads to a rapid and massive muscle hypertrophy, postnatal blockade of the activin type IIB receptor (ActRIIB) is a promising therapeutic strategy for counteracting muscle wasting. However, the functional consequences remain very poorly documented in vivo. Here, we have investigated the impact of 8-wk ActRIIB blockade with soluble receptor (sActRIIB-Fc) on gastrocnemius muscle anatomy, energy metabolism, and force-generating capacity in wild-type mice, using totally noninvasive magnetic resonance imaging (MRI) and dynamic 31 P-MRS. Compared with vehicle (PBS) control, sActRIIB-Fc treatment resulted in a dramatic increase in body weight (ϩ29%) and muscle volume (ϩ58%) calculated from hindlimb MR imaging, but did not alter fiber type distribution determined via myosin heavy chain isoform analysis. In resting muscle, sActRIIB-Fc treatment induced acidosis and PCr depletion, thereby suggesting reduced tissue oxygenation. During an in vivo fatiguing exercise (6-min repeated maximal isometric contraction electrically induced at 1.7 Hz), maximal and total absolute forces were larger in sActRIIB-Fc treated animals (ϩ26 and ϩ12%, respectively), whereas specific force and fatigue resistance were lower (Ϫ30 and Ϫ37%, respectively). Treatment with sActRIIB-Fc further decreased the maximal rate of oxidative ATP synthesis (Ϫ42%) and the oxidative capacity (Ϫ34%), but did not alter the bioenergetics status in contracting muscle. Our findings demonstrate in vivo that sActRIIB-Fc treatment increases absolute force-generating capacity and reduces mitochondrial function in glycolytic gastrocnemius muscle, but this reduction does not compromise energy status during sustained activity. Overall, these data support the clinical interest of postnatal ActRIIB blockade. skeletal muscle hypertrophy; activin type IIB receptor; myostatin; force; fatigue ACTIVIN TYPE IIB RECEPTOR (ActRIIB) is a transmembrane serinethreonine kinase receptor highly expressed in mammalian skeletal muscle. It mediates signaling for myostatin (GDF8) and other members of the transforming growth factor-␤ family that are involved in the negative regulation of skeletal muscle development (16,37). Disruption of the ActRIIB signaling pathway leads to a rapid and massive increase in muscle mass resulting from fiber hypertrophy with increased myofiber protein synthesis (10, 58). In that context, several pharmacological approaches based on the postnatal ActRIIB signaling blockade have been considered for counteracting muscle wasting commonly associated to aging, neuromuscular disorders and various catabolic diseases (17,33,38). These approaches mainly include the systemic delivery of neutralizing antibodies against ActRIIB or myostatin (31, 36), and injection of a soluble recombinant form of ActRIIB (sActRIIB-Fc), which acts as a decoy receptor disrupting the interaction of endogenous ActRIIB receptor with its ligands (5,38,44).Although ActRIIB signaling blockade has been already tested in patients (30, 57) and animal models (10,44,47) suffering from dystrophi...

Soluble activin receptor type IIB decoy receptor differentially impacts murine osteogenesis imperfecta muscle function

“…5). Although other studies of mouse models with muscle atrophy showed that soluble ActRIIB treatment increased absolute muscle contractile function of compromised muscle, 23,24,28,44 there were also reports of earlier muscle fatigability, 26,45 decreased gene expression of components involved in mitochondrial function and oxidative phosphorylation, 46,47 and reduced myofiberspecific function in mdx mice (mouse model of Duchenne muscular dystrophy) with treatment of higher concentrations of soluble ActRIIB. 25 Consistent with these studies, myostatin knockout (MSTN 2/2 ) mice exhibited centralized nuclei, a sign of muscle degeneration, and reduced relative muscle contractile function compared with WT and heterozygous myostatin-deficient (MSTN 1/2 ) littermates, 48 suggesting that a complete absence of myostatin is detrimental to muscle myofiber function.…”

“…56 Several studies have raised concerns about impairment of mitochondrial function with myostatin inhibition. A limited group of studies in MSTN 2/2 mice demonstrated increased force production that was not proportional to their muscle hypertrophy, which may have reflected decreased mitochondrial function, [45][46][47]57 muscle fiber type switching, 57,58 deficits in oxidative metabolism that can lead to decreased endurance capacity, [57][58][59][60] and/or decreased capillary density in muscle that alters the aerobic metabolism. 58 Unlike MSTN 2/2 mice, Bechir and colleagues found that energy status and function of glycolytic muscles were not disturbed during 6-minute fatigue exercise with ActRIIB blockade in both WT and mdx mice.…”

Activin type IIB receptor blockade does not limit adenosine triphosphate supply in mouse skeletal muscle in Vivo

“…This was unlikely in the present study for at least 2 reasons. First, we recently demonstrated that an 8‐week treatment with sActRIIB‐Fc does not affect fiber type distribution in the gastrocnemius muscle of wild‐type mice . Second, our methodology allows stimulation of the whole gastrocnemius muscle, and there is no reason that 1 type of fiber would be recruited more than the other in sActRIIB‐Fc‐treated animals compared with controls.…”

“…Shift in the pattern of fiber type recruitment and alteration of activation–relaxation processes have been proposed to explain the reduction of contractile ATP cost throughout a fatiguing exercise . Mouse gastrocnemius muscle is a mixed muscle composed mainly of fast glycolytic type‐2b fibers (about 85%) with low oxidative capacity; the remaining muscle is composed of fast‐oxidative type‐2a fibers with higher oxidative capacity . Because a fiber is more fatigue resistant and contracts more economically when its metabolism is oxidative, change in muscle typology and/or preferential recruitment of type‐2a fibers could explain the decrease in contractile ATP cost in fatigued muscle.…”

“…Activin type IIB receptor mediates the signaling for secreted molecules of the transforming growth factor‐β family involved in the negative regulation of muscle development, the most important being myostatin (GDF‐8) . Postnatal blockade of ActRIIB signaling by using systemic injection of soluble ActRIIB decoy receptor (sActRIIB‐Fc) or neutralizing antibodies against ActRIIB has been shown to induce a rapid and massive muscle hypertrophy leading to absolute strength improvement in healthy human and in normal and dystrophic mice . These findings have raised the possibility of pharmaceutical blockade of ActRIIB as a promising means for improving the quality of life and reducing mortality of patients with muscle wasting associated with conditions such as neuromuscular disorders, cancer, human immunodeficiency virus, and sarcopenia …”