Sex-specific role of myostatin signaling in neonatal muscle growth, denervation atrophy, and neuromuscular contractures

The Relative Efficacy of Available Proteasome Inhibitors in Preventing Muscle Contractures Following Neonatal Brachial Plexus Injury

BACKGROUND

Contractures following neonatal brachial plexus injury (NBPI) are associated with growth deficits in denervated muscles. This impairment is mediated by an increase in muscle protein degradation, as contractures can be prevented in an NBPI mouse model with bortezomib (BTZ), a proteasome inhibitor (PI). However, BTZ treatment causes substantial toxicity (0% to 80% mortality). The current study tested the hypothesis that newer-generation PIs can prevent contractures with less severe toxicity than BTZ.

METHODS

Unilateral brachial plexus injuries were surgically created in postnatal (5-day-old) mice. Following NBPI, mice were treated with either saline solution or various doses of 1 of 3 different PIs: ixazomib (IXZ), carfilzomib (CFZ), or marizomib (MRZ). Four weeks post-NBPI, mice were assessed for bilateral passive range of motion at the shoulder and elbow joints, with blinding to the treatment group, through an established digital photography technique to determine contracture severity. Drug toxicity was assessed with survival curves.

RESULTS

All PIs prevented contractures at both the elbow and shoulder (p < 0.05 versus saline solution controls), with the exception of IXZ, which did not prevent shoulder contractures. However, their efficacies and toxicity profiles differed. At lower doses, CFZ was limited by toxicity (30% to 40% mortality), whereas MRZ was limited by efficacy. At higher doses, CFZ was limited by loss of efficacy, MRZ was limited by toxicity (50% to 60% mortality), and IXZ was limited by toxicity (80% to 100% mortality) and loss of efficacy. Comparisons of the data on these drugs as well as data on BTZ generated in prior studies revealed BTZ to be optimal for preventing contractures, although it, too, was limited by toxicity.

CONCLUSIONS

All of the tested second-generation PIs were able to reduce NBPI-induced contractures, offering further proof of concept for a regulatory role of the proteasome in contracture formation. However, the narrow dose ranges of efficacy for all PIs highlight the necessity of precise proteasome regulation for preventing contractures. Finally, the substantial toxicity stemming from proteasome inhibition underscores the importance of identifying muscle-targeted strategies to suppress protein degradation and prevent contractures safely.

CLINICAL RELEVANCE

Although PIs offer unique opportunities to establish critical mechanistic insights into contracture pathophysiology, their clinical use is contraindicated in patients with NPBI at this time.

Heme oxygenase-1 therapy attenuates muscle atrophy following global brachial plexus avulsion in juvenile rats

INTRODUCTION/AIMS

Brachial plexus injury can seriously affect distal target muscle function, and long-term denervation leads to irreversible structural damage. In the present study, we examined the effect of hemin, a heme oxygenase-1 (HO-1) inducer, on intrinsic forepaw muscle atrophy induced by pan-plexus injury in juvenile rats, as well as its underlying mechanism.

METHODS

A global brachial plexus avulsion (GBPA) model of rat was established, and thirty 6-wk-old male rats were randomly divided into five groups: control, GBPA plus scramble small intering RNA (siRNA), GBPA plus scramble siRNA plus hemin, GBPA plus HO-1 siRNA, and GBPA plus HO-1 siRNA plus hemin. Hemin (50 mg/kg) was administered intraperitoneally once daily and the siRNA (5 μg) was injected intramuscularly twice a week. Intrinsic forepaw muscles were used for analysis. Myofiber cross-sectional area (CSA), capillary-to-fiber ratio (C/F), and fiber-type composition were assessed. The levels of inflammatory factors, ubiquitin-protein ligases, and autophagy-related proteins were also measured.

RESULTS

We found that hemin treatment could effectively ameliorate denervated intrinsic forepaw muscle atrophy and suppress type I to II myofiber-type conversion. Hemin treatment failed to prevent muscle capillary loss after denervation. The levels of inflammatory factors (tumor necrosis factor alpha [TNFα] and interleukin 6 [IL-6]), ubiquitin-protein ligases (MuRF-1 and MAFbx), and autophagy-related proteins (BNIP3 and LC3B-II/I ratio) were increased by denervation and HO-1 therapy attenuated the increment.

DISCUSSION

Upregulation of HO-1 might potentially be an effective strategy to alleviate denervation-related muscle atrophy and might be a promising adjunctive treatment to improve hand function in children with pan-plexus injury.

The role of sympathetic innervation in neonatal muscle growth and neuromuscular contractures

Neonatal brachial plexus injury (NBPI), a leading cause of pediatric upper limb paralysis, results in disabling and incurable muscle contractures that are driven by impaired longitudinal growth of denervated muscles. A rare form of NBPI, which maintains both afferent and sympathetic muscle innervation despite motor denervation, protects against contractures. We have previously ruled out a role for NRG/ErbB signaling, the predominant pathway governing antegrade afferent neuromuscular transmission, in modulating the formation of contractures. Our current study therefore investigated the contributions of sympathetic innervation of skeletal muscle in modulating NBPI-induced contractures. Through chemical sympathectomy and pharmacologic modification with a β2 -adrenergic agonist, we discovered that sympathetic innervation alone is neither required nor sufficient to modulate contracture formation in neonatal mice. Despite this, sympathetic innervation plays an intriguing sex-specific role in mediating neonatal muscle growth, as the cross-sectional area (CSA) and volume of normally innervated male muscles were diminished by ablation of sympathetic neurons and increased by β-adrenergic stimulation. Intriguingly, the robust alterations in CSA occurred with minimal changes to normal longitudinal muscle growth as determined by sarcomere length. Instead, β-adrenergic stimulation exacerbated sarcomere overstretch in denervated male muscles, indicating potentially discrete regulation of muscle width and length. Future investigations into the mechanistic underpinnings of these distinct aspects of muscle growth are thus essential for improving clinical outcomes in patients affected by muscle disorders in which both length and width are affected.