Denervation versus pre‐ and postsynaptic muscle immobilization: Effects On acetylcholine‐ and muscle‐specific tyrosine kinase receptors

Heterologous fibrin biopolymer associated to a single suture stitch enables the return of neuromuscular junction to its mature pattern after peripheral nerve injury

Denervation leads to severe atrophy of neuromuscular junction (NMJ) structure including decrease of the expression of fundamental proteins. Up to now, conventional suture has been the gold standard method used to correct this injury. Fibrin sealant is one of the alternatives proposed to optimize this method. This study verified if the association of fibrin sealant - Heterologous Fibrin Biopolymer (HFB) and a single suture stitch promotes return of morphology and NMJ structure to mature pattern after peripheral nerve injury. Forty Wistar rats were distributed into 4 groups: Sham-Control (SC), Denervated-Control (DC), Suture-Lesion (SL) and Suture-Lesion + HFB (SFS). In SC group only the right sciatic nerve identification was done. In DC, SL and SFS groups fixation of nerve stumps on musculature immediately after neurotmesis was performed. After seven days, stump reconnection with 3 stitches in SL and a single stitch associated with HFB in SFS were done. After sixty days right soleus muscles were prepared for nicotinic acetylcholine receptors (nAChRs) and nerve terminal confocal analyses, and for nAChRs (α1, ε e γ), S100, Agrin, LRP-4, MMP-3, Rapsyn western blotting analyses. SC group presented normal morphology. In DC group it was observed flattening of NMJ, fragmentation of nAChRs and tangled nerve terminals. The majority of the parameters of SL and SFS groups presented values in between SC and DC groups. There was an increase of relative planar area in these groups (SL and SFS) highlighting that there was less nAChRs fragmentation and the values of protein expression showed return of nAChRs to mature pattern. Use of HFB associated with a single suture stitch decreased surgical time, minimized suture injuries, did not alter nerve regeneration and presented potential to reestablish the NMJ apparatus. These consolidated results encourage surgeons to develop future clinical trials to install definitively this new approach both for reconstructive surgery and neurosurgery.

Jitter evaluation in denervation and reinnervation in 32 cases of chronic radiculopathy

OBJECTIVE

To measure the jitter parameters in muscles with denervation/reinnervation in 32 chronic radiculopathy cases.

METHODS

Measurements were done in chronic denervated muscles by voluntary and electrical activation using a concentric needle electrode.

RESULTS

Mean jitter was abnormal in 87.5% (mean 49.2 µs) and 81.25% (mean 36.8 µs), for voluntary and electrical activation. In muscles with fibrillation potentials (FPs), the mean jitter was abnormal in all cases, and impulse blocking was frequent (53.4-92.3%). In muscles without FPs, the mean jitter was abnormal in 78.9% for voluntary activation and 68.4% for electrical activation. No correlation was found between jitter and motor unit action potential amplitude.

CONCLUSION

The muscles with FPs were associated with the immature spread of acetylcholine receptors (AChRs) throughout the muscle membrane. Conversely, the neuromuscular junctions (NMJs) assemble may be repressed by the already reinnervated muscles. For those, higher jitter may be due to the persistence of atrophic fibers expressing neonatal myosin heavy chain (MHC) and immaturity of NMJ composting instead of the overspread of immature AChRs.

SIGNIFICANCE

Jitter measurement must be avoided in chronic denervated muscles, regardless of FPs' presence. The activity of reinnervated muscle could maintain neonatal MHC and repress new NMJs development.

Effect of Inhibiting p38 on HuR Involving in β-AChR Post-transcriptional Mechanisms in Denervated Skeletal Muscle

Previous studies reported that RNA-binding protein human antigen R (HuR) mediates changes in the stability of AChR β-subunit mRNA after skeletal muscle denervation; also, p38 pathway regulated the stability of AChR β-subunit mRNA in C2C12 myotubes. However, the relationship between HuR and p38 in regulating the stability of AChR β-subunit mRNA have not been clarified. In this study, we wanted to examine the effect of inhibiting p38 on HuR in denervated skeletal muscle. Denervation model was built and 10% DMSO or SB203580 were administered respectively follow denervation. Tibialis muscles were collected in 10% DMSO-administered contralateral (undenervated) leg, 10% DMSO-administered denervated leg, SB203580-administered contralateral (undenervated) leg, and SB203580-administered denervated leg, respectively. P38 protein, β-AChR mRNA and protein, HuR protein, β-AChR mRNA stability, and HuR binding with AChR β-subunit mRNAs were measured. Results demonstrated that the administration of SB203580 can inhibit the increase of β-AChR protein expression and mRNA expression and stability, and RNA-binding protein human antigen R (HuR) expression, in cytoplasmic and nuclear fractions in skeletal muscle cells following denervation. Importantly, we observed that SB203580 also inhibited the increased level of binding activity between HuR and AChR β-subunit mRNAs following denervation. Collectively, these results suggested that inhibition of p38 can post-transcriptionally inhibit β-AChR upregulation via HuR in denervated skeletal muscle.

Two Pathways Regulate Differential Expression of nAChRs Between the Orbicularis Oris and Gastrocnemius

BACKGROUND

We previously demonstrated differential expression of nicotinic acetylcholine receptors (nAChRs) in the facial nerve-innervated orbicularis oris and somatic nerve-innervated gastrocnemius, which contribute to different sensitivities to muscle relaxants. Furthermore, the orbicularis oris exhibits less sensitivity to muscle relaxants after facial nerve injury, which is also related to upregulation of nAChRs. Here, we explored the regulatory mechanism for the different expression patterns. Because the agrin/Lrp4/MuSK/rapsyn and neuregulin1/ErbB signaling pathways are indispensable for maintaining the expression of nAChRs, we examined the activity of these two signaling pathways in gastrocnemius and orbicularis oris innervated by normal or injured facial nerves.

MATERIALS AND METHODS

A quantitative analysis of these two signaling pathways was realized by immunofluorescence, and immunoprecipitation was applied to detect the level of phosphorylated MuSK in the gastrocnemius and orbicularis oris innervated by normal or injured facial nerves in adult rats.

RESULTS

ErbB and the phosphorylated MuSK were expressed more in orbicularis oris than in gastrocnemius (P < 0.05). No significant difference was found in the expression of agrin/Lrp4/MuSK/rapsyn. After facial nerve injury, the level of agrin and the percentage of phosphorylated MuSK decreased significantly, although the expression levels of MuSK, rapsyn, and neuregulin1/ErbB were highly upregulated.

CONCLUSIONS

Differential expression of the neuregulin1/ErbB signaling pathway may account for the different expression patterns of nAChRs at the neuromuscular junctions of the orbicularis oris and gastrocnemius. Overexpression of MuSK and rapsyn may contribute to upregulation of nAChRs after facial nerve injury.

Alleviating Sepsis-Induced Neuromuscular Dysfunction Linked With Acetylcholine Receptors by Agrin

BACKGROUND

Abnormal expression and distribution of nicotinic acetylcholine receptors (nAChRs) in skeletal muscle caused by sepsis can lead to neuromuscular dysfunction. Here, we asked whether neural agrin regulates nAChRs to ameliorate muscle function, which could be associated with the agrin/muscle-specific kinase pathway.

METHODS

Rats were subjected to cecal ligation and puncture (CLP) group, sham group, or control group to observe the alteration caused by sepsis. To verify the effect of improving function, rats were injected with agrin or normal saline intramuscularly after CLP. Electromyogram was used to measure neuromuscular function. Cytokines levels of serum and the expression of related proteins and mRNA were tested after treatment.

RESULTS

Compared with the rats in control or sham group, CLP-treated rats showed an acute inflammatory status and a reduction of neuromuscular dysfunction in tibialis anterior muscle, which was associated with abnormal expression in agrin/muscle-specific kinase pathway and increased expression of γ- and α7-nAChR. Exogenous agrin alleviated neuromuscular dysfunction and decreased the expression of γ- and α7-nAChR through agrin-related signaling pathway.

CONCLUSIONS

The decreased expression of agrin may lead to skeletal muscle dysfunction. Early enhancement of intramuscular agrin levels after sepsis may be a potential strategy for the treatment of sepsis-induced muscle dysfunction.

Muscle satellite cells are functionally impaired in myasthenia gravis: consequences on muscle regeneration

Myasthenia gravis (MG) is a neuromuscular disease caused in most cases by anti-acetyl-choline receptor (AChR) autoantibodies that impair neuromuscular signal transmission and affect skeletal muscle homeostasis. Myogenesis is carried out by muscle stem cells called satellite cells (SCs). However, myogenesis in MG had never been explored. The aim of this study was to characterise the functional properties of myasthenic SCs as well as their abilities in muscle regeneration. SCs were isolated from muscle biopsies of MG patients and age-matched controls. We first showed that the number of Pax7+ SCs was increased in muscle sections from MG and its experimental autoimmune myasthenia gravis (EAMG) mouse model. Myoblasts isolated from MG muscles proliferate and differentiate more actively than myoblasts from control muscles. MyoD and MyoG were expressed at a higher level in MG myoblasts as well as in MG muscle biopsies compared to controls. We found that treatment of control myoblasts with MG sera or monoclonal anti-AChR antibodies increased the differentiation and MyoG mRNA expression compared to control sera. To investigate the functional ability of SCs from MG muscle to regenerate, we induced muscle regeneration using acute cardiotoxin injury in the EAMG mouse model. We observed a delay in maturation evidenced by a decrease in fibre size and MyoG mRNA expression as well as an increase in fibre number and embryonic myosin heavy-chain mRNA expression. These findings demonstrate for the first time the altered function of SCs from MG compared to control muscles. These alterations could be due to the anti-AChR antibodies via the modulation of myogenic markers resulting in muscle regeneration impairment. In conclusion, the autoimmune attack in MG appears to have unsuspected pathogenic effects on SCs and muscle regeneration, with potential consequences on myogenic signalling pathways, and subsequently on clinical outcome, especially in the case of muscle stress.