A new protein curbs the hypertrophic effect of myostatin inhibition, adding remarkable endurance to motor performance in mice

Abstract

Current efforts to improve muscle performance are focused on muscle trophism via inhibition of the myostatin pathway: however they have been unsuccessful in the clinic to date. In this study, a novel protein has been created by combining the soluble activin receptor, a strong myostatin inhibitor, to the C-terminal agrin nLG3 domain (ActR-Fc-nLG3) involved in the development and maintenance of neuromuscular junctions. Both domains are connected via the constant region of an Igg1 monoclonal antibody. Surprisingly, young male mice treated with ActR-Fc-nLG3 showed a remarkably increased endurance in the rotarod test, significantly longer than the single domain compounds ActR-Fc and Fc-nLG3 treated animals. This increase in endurance was accompanied by only a moderate increase in body weights and wet muscle weights of ActR-Fc-nLG3 treated animals and were lower than expected. The myostatin inhibitor ActR-Fc induced, as expected, a highly significant increase in body and muscle weights compared to control animals and ActR-Fc-nLG3 treated animals. Moreover, the prolonged endurance effect was not observed when ActR-Fc and Fc-nLG3 were dosed simultaneously as a mixture and the body and muscle weights of these animals were very similar to ActR-Fc treated animals, indicating that both domains need to be on one molecule. Muscle morphology induced by ActR-Fc-nLG3 did not appear to be changed however, close examination of the neuromuscular junction showed significantly increased acetylcholine receptor surface area for ActR-Fc-nLG3 treated animals compared to controls. This result is consistent with published observations that endurance training in rats increased acetylcholine receptor quantity at neuromuscular junctions and provide evidence that improving nerve-muscle interaction could be an important factor for sustaining long term muscle activity.

Synergistically Acting on Myostatin and Agrin Pathways Increases Neuromuscular Junction Stability and Endurance in Old Mice

“…We previously described and tested [ 23 ] a novel fusion protein, ActR-Fc-nLG3, consisting in the ActR-IIB receptor coupled to the human neuronal laminin G3 domain of agrin (nLG3) by the c-terminus of the Fc part of an Igg1 mAb [ 25 , 35 ]. Based on the encouraging results we obtained on young mice [ 23 ], we investigated the effects of chronic (5 week) administration of ActR-Fc-nLG3 to aged mice, focusing on muscle mass, endurance and strength. Aging in mice reduces motor performance mimicking the loss of muscle strength and function that occurs in aged humans [ 30 ] .…”

“…Briefly, it consists of the mouse extracellular part of the ActR-IIB receptor coupled the Fc part of an Igg1 mAb. The ActR-Fc-nLG3 compound was described firstly in Boido et al, 2020 [ 23 ]. It consists of the neuronal laminin G3 domain of the neuronal form of agrin [ 25 , 26 ] -LG3- coupled to the c-terminus of the Fc part of an Igg1 mAb and to the myostatin inhibitor ActR-IIB receptor.…”

“…Animals were randomly assigned to one of three groups. During the first three weeks, PBS (vehicle) was administered by subcutaneous (s.c.) injections to all the animals to set baseline condition, then ActR-Fc (10mg/kg), ActR-Fc-nLG3 (10mg/kg), both dissolved in PBS, or the vehicle only were s.c. administered for the last five consecutive weeks, based on the belonging group [ 23 , 27 ]. Finally, at the end of week 8, mice received the last injection and were sacrificed 24h later ( Fig.…”

“…Thus, here we tested on 22 month-old mice a biological compound, ActR-Fc-nLG3, that combines the nLG3 domain from the C-terminus of human agrin (for its well-known role as a promoter of NMJs formation and maintenance [ 21 , 22 ]) with the potent myostatin inhibitor ActR-Fc, via the constant region of an Igg1 monoclonal antibody. We previously demonstrated that the administration of ActR-Fc-nLG3 to young mice resulted in a moderate increase in muscle mass, an increase in strength comparable to the one seen with myostatin inhibitors, but also a notable enhancement of motor performance in the rotarod [ 23 ].…”

The Failed Clinical Story of Myostatin Inhibitors against Duchenne Muscular Dystrophy: Exploring the Biology behind the Battle

“…Combined with myostatin inhibitor drugs, the likely outcome is muscular hypertrophy without the synergistic improvement of skeletal muscle function. To this effect, a novel dual pro-neuromuscular junction and soluble activin receptor myostatin inhibitor protein (ActR-Fc-nLG3), which forsakes the extent of hypertrophy normally shown with pharmacological myostatin inhibition to increase acetycholine receptor surface area at the neuromuscular junction, was shown to improve muscle functional endurance of mice in the rotarod test [ 77 ]. As yet, this drug is untested in preclinical animal models of DMD but demonstrates scope for combinatorial therapeutics that target both ACVR2 and neuromuscular junction signalling to elicit dual mass (albeit smaller) and strength gains.…”