Motor activity affects adult skeletal muscle re-innervation acting via tyrosine kinase receptors

The effects of aerobic exercise on neuroimmune responses in animals with traumatic peripheral nerve injury: a systematic review with meta-analyses

BACKGROUND

Increasing pre-clinical evidence suggests that aerobic exercise positively modulates neuroimmune responses following traumatic nerve injury. However, meta-analyses on neuroimmune outcomes are currently still lacking. This study aimed to synthesize the pre-clinical literature on the effects of aerobic exercise on neuroimmune responses following peripheral nerve injury.

METHODS

MEDLINE (via Pubmed), EMBASE and Web of Science were searched. Controlled experimental studies on the effect of aerobic exercise on neuroimmune responses in animals with a traumatically induced peripheral neuropathy were considered. Study selection, risk of bias assessment and data extraction were performed independently by two reviewers. Results were analyzed using random effects models and reported as standardized mean differences. Outcome measures were reported per anatomical location and per class of neuro-immune substance.

RESULTS

The literature search resulted in 14,590 records. Forty studies were included, reporting 139 comparisons of neuroimmune responses at various anatomical locations. All studies had an unclear risk of bias. Compared to non-exercised animals, meta-analyses showed the following main differences in exercised animals: (1) in the affected nerve, tumor necrosis factor-α (TNF-α) levels were lower (p = 0.003), while insulin-like growth factor-1 (IGF-1) (p < 0.001) and Growth Associated Protein 43 (GAP43) (p = 0.01) levels were higher; (2) At the dorsal root ganglia, brain-derived neurotrophic factor (BDNF)/BDNF mRNA levels (p = 0.004) and nerve growth factor (NGF)/NGF mRNA (p < 0.05) levels were lower; (3) in the spinal cord, BDNF levels (p = 0.006) were lower; at the dorsal horn, microglia (p < 0.001) and astrocyte (p = 0.005) marker levels were lower; at the ventral horn, astrocyte marker levels (p < 0.001) were higher, and several outcomes related to synaptic stripping were favorably altered; (4) brainstem 5-HT2A receptor levels were higher (p = 0.001); (5) in muscles, BDNF levels (p < 0.001) were higher and TNF-α levels lower (p < 0.05); (6) no significant differences were found for systemic neuroimmune responses in blood or serum.

CONCLUSION

This review revealed widespread positive modulatory effects of aerobic exercise on neuroimmune responses following traumatic peripheral nerve injury. These changes are in line with a beneficial influence on pro-inflammatory processes and increased anti-inflammatory responses. Given the small sample sizes and the unclear risk of bias of the studies, results should be interpreted with caution.

The impact of different exercise protocols on rat soleus muscle reinnervation and recovery following peripheral nerve lesion and regeneration

Background: Incomplete functional recovery following traumatic peripheral nerve injury is common, mainly because not all axons successfully regenerate and reinnervate target muscles. Exercise can improve functional outcomes increasing the terminal sprouting during the muscle reinnervation. However, exercise is not a panacea per se. Indeed, the type of exercise adopted dramatically impacts the outcomes of rehabilitation therapy. To gain insight into the therapeutic effects of different exercise regimens on reinnervation following traumatic nerve lesion, we evaluated the impact of different clinically transferable exercise protocols (EPs) on metabolic and functional muscle recovery following nerve crush.

Methods: The reinnervation of soleus muscle in adult nerve-crushed rats was studied following 6 days of different patterns (continuous or intermittent) and intensities (slow, mid, and fast) of treadmill running EPs. The effects of EPs on muscle fiber multiple innervation, contractile properties, metabolic adaptations, atrophy, and autophagy were assessed using functional and biochemical approaches.

Results: Results showed that an intermittent mid-intensity treadmill EP improves soleus muscle reinnervation, whereas a slow continuous running EP worsens the functional outcome. However, the mid-intensity intermittent EP neither enhanced the critical mediators of exercise-induced metabolic adaptations, namely, PGC-1α, nor improved muscle atrophy. Conversely, the autophagy-related marker LC3 increased exclusively in the mid-intensity intermittent EP group.

Conclusion: Our results demonstrated that an EP characterized by a mid-intensity intermittent activity enhances the functional muscle recovery upon a nerve crush, thus representing a promising clinically transferable exercise paradigm to improve recovery in humans following peripheral nerve injuries.

mGluR5 Allosteric Modulation Promotes Neurorecovery in a 6-OHDA-Toxicant Model of Parkinson's Disease

Parkinson's disease is a neurodegenerative disease characterized by a loss of dopaminergic substantia nigra neurons and depletion of dopamine. To date, current therapeutic approaches focus on managing motor symptoms and trying to slow neurodegeneration, with minimal capacity to promote neurorecovery. mGluR5 plays a key role in neuroplasticity, and altered mGluR5 signaling contributes to synucleinopathy and dyskinesia in patients with Parkinson's disease. Here, we tested whether the mGluR5-negative allosteric modulator, (2-chloro-4-[2[2,5-dimethyl-1-[4-(trifluoromethoxy) phenyl] imidazol-4-yl] ethynyl] pyridine (CTEP), would be effective in improving motor deficits and promoting neural recovery in a 6-hydroxydopamine (6-OHDA) mouse model. Lesions were induced by 6-ODHA striatal infusion, and 30 days later treatment with CTEP (2 mg/kg) or vehicle commenced for either 1 or 12 weeks. Animals were subjected to behavioral, pathological, and molecular analyses. We also assessed how long the effects of CTEP persisted, and finally, using rapamycin, determined the role of the mTOR pathway. CTEP treatment induced a duration-dependent improvement in apomorphine-induced rotation and performance on rotarod in lesioned mice. Moreover, CTEP promoted a recovery of striatal tyrosine hydroxylase-positive fibers and normalized FosB levels in lesioned mice. The beneficial effects of CTEP were paralleled by an activation of mammalian target of rapamycin (mTOR) pathway and elevated brain-derived neurotrophic factor levels in the striatum of lesioned mice. The mTOR inhibitor, rapamycin (sirolimus), abolished CTEP-induced neurorecovery and rescue of motor deficits. Our findings indicate that mTOR pathway is a useful target to promote recovery and that mGluR5 allosteric regulators may potentially be repurposed to selectively target this pathway to enhance neuroplasticity in patients with Parkinson's disease.

The effects of neuregulin-1β on intrafusal muscle fiber formation in neuromuscular coculture of dorsal root ganglion explants and skeletal muscle cells

Background

The formation of intrafusal muscle (IM) fibers and their contact with afferent proprioceptive axons is critical for construction, function, and maintenance of the stretch reflex. Many factors affect the formation of IM fibers. Finding new factors and mechanisms of IM fiber formation is essential for the reconstruction of stretch reflex arc after injury.

Methods

We established a coculture system of organotypic dorsal root ganglion (DRG) explants and dissociated skeletal muscle (SKM) cells. The formation of IM fibers was observed in this coculture system after neuregulin-1β (NRG-1β) incubation.

Results

We found that NRG-1β promoted outgrowth of neurites and migration of neurons from the organotypic DRG explants and that this correlated with an induction of growth-associated protein 43 (GAP-43) expression. NRG-1β also increased the amount of nuclear bag fibers and nuclear chain fibers by elevating the proportion of tyrosine kinase receptor C (TrkC) phenotypic DRG neurons. In addition, we found that the effects of NRG-1β could be blocked by inhibiting ERK1/2, PI3K/Akt, and JAK2/STAT3 signaling pathways.

Conclusion

These data imply that NRG-1β promoted neurite outgrowth and neuronal migration from the organotypic DRG explants and that this correlated with an induction of GAP-43 expression. The modulating effects of NRG-1β on TrkC DRG neuronal phenotype may link to promote IM fiber formation. The effects produced by NRG-1β in this neuromuscular coculture system provide new data for the therapeutic potential on IM fiber formation after muscle injury.

Electronic supplementary material

The online version of this article (10.1186/s13395-018-0175-9) contains supplementary material, which is available to authorized users.

Specific effects of neuregulin-1β on the communication between DRG neurons and skeletal muscle cells in vitro

The communication between primary afferent neuron and skeletal muscle (SKM) is one of the important factors on maintaining the structure and function of SKM cells. Neuregulin-1β (NRG-1β) signaling is essential for regulating synaptic neurotransmission. Here, we established a neuromuscular coculture model of dorsal root ganglion (DRG) sensory neurons and SKM cells to explore the nerve-muscle communication in the presence of exogenous NRG-1β. The expression of three distinct subtypes (TrkA, TrkB, and TrkC) of tyrosine kinase receptors was monitored for the phenotypical alterations of the neurons. The aggregation extent of acetylcholine receptor (AChR) represents the specific changes of SKM cells after NRG-1β incubation in this neuromuscular coculture model. The results showed that NRG-1β not only enhanced neurite outgrowth of DRG neurons but also increased the length and branches of SKM cells. NRG-1β treatment not only induced expression of all the three subtypes of Trk receptors in neurons but also promoted AChR aggregation on the surface of SKM cells. The effects of NRG-1β could be blocked by administration of ERK1/2 inhibitor PD98059, PI3K inhibitor LY294002, and JAK2 inhibitor AG490, respectively. These data imply that NRG-1β is essential for the nerve-muscle communication by enhancing growth and modifying phenotypes of the two different kinds of cells. The specific effects produced by NRG-1β add novel interpretation for nerve-muscle communication between sensory neurons and SKM cells.

Failed reinnervation in aging skeletal muscle

BACKGROUND

Skeletal muscle displays a marked accumulation of denervated myofibers at advanced age, which coincides with an acceleration of muscle atrophy.

METHODS

In this study, we evaluated the hypothesis that the accumulation of denervated myofibers in advanced age is due to failed reinnervation by examining muscle from young adult (YA) and very old (VO) rats and from a murine model of sporadic denervation secondary to neurotrypsin over-expression (Sarco mouse).

RESULTS

Both aging rat muscle and Sarco mouse muscle exhibited marked fiber-type grouping, consistent with repeating cycles of denervation and reinnervation, yet in VO muscle, rapsyn at the endplate increased and was associated with only a 10 % decline in acetylcholine receptor (AChR) intensity, whereas in Sarco mice, there was a decline in rapsyn and a 25 % decrease in AChR intensity. Transcripts of muscle-specific kinase (21-fold), acetylcholine receptor subunits α (68-fold), ε (threefold) and γ (47-fold), neural cell adhesion molecule (66-fold), and runt-related transcription factor 1 (33-fold) were upregulated in VO muscle of the rat, consistent with the marked persistent denervation evidenced by a large proportion of very small fibers (>20 %). In the Sarco mice, there were much smaller increases in denervation transcripts (0-3.5-fold) and accumulation of very small fibers (2-6 %) compared to the VO rat, suggesting a reduced capacity for reinnervation in aging muscle. Despite the marked persistent denervation in the VO rat muscle, transcripts of neurotrophins involved in promoting axonal sprouting following denervation exhibited no increase, and several miRNAs predicted to suppress neurotrophins were elevated in VO rat.

CONCLUSIONS

Our results support the hypothesis that the accumulation of denervated fibers with aging is due to failed reinnervation and that this may be affected by a limited neurotrophin response that mediates axonal sprouting following denervation.

Effect of Different Exercise Intensities on the Myotendinous Junction Plasticity

Myotendinous junctions (MTJs) are anatomical regions specialized in transmission of contractile strength from muscle to tendon and, for this reason, a common site where acute injuries occur during sport activities. In this work we investigated the influence of exercise intensity on MTJ plasticity, as well as on the expression of insulin-like growth factor 1 (IGF-1) and transforming growth factor beta (TGF-β) and their receptors in muscle and tendon. Three groups of rats were analyzed: control (CTRL), slow-runner (RUN-S) and fast-runner (RUN-F) trained using a treadmill. Ultrastructural and morphometric analyses of distal MTJs from extensor digitorum longus muscles have been performed. Contractile strength and hypertrophy were investigated by using in vivo tension recordings and muscle cross-sectional area (CSA) analysis, respectively. mRNA levels of PGC-1α, vinculin, IGF-1Ea and TGF-β have been quantified in muscle belly, while IGF-1Ea, TGF-β and their receptors in tendon. Morphometry revealed an increased MTJ complexity and interaction surface between tissues in trained rats according to training intensity. CSA analysis excluded hypertrophy among groups, while muscle strength was found significantly enhanced in exercised rats in comparison to controls. In muscle tissue, we highlighted an increased mRNA expression of PGC-1α and vinculin in both trained conditions and of TGF-β in RUN-F. In tendon, we mainly noted an enhancement of TGF-β mRNA expression only in RUN-F group and a raise of Betaglycan tendon receptor mRNA levels proportional to exercise intensity. In conclusion, MTJ plasticity appears to be related to exercise intensity and molecular analysis suggests a major role played by TGF-β.

Effects of moderate- and high-intensity chronic exercise on brain-derived neurotrophic factor expression in fast and slow muscles

INTRODUCTION

Brain-derived neurotrophic factor (BDNF) protein expression is sensitive to cellular activity. In the sedentary state, BDNF expression is affected by the muscle phenotype.

METHODS

Eighteen Wistar rats were divided into the following 3 groups: sedentary (S); moderate-intensity training (MIT); and high-intensity training (HIT). The training protocol lasted 8 weeks. Forty-eight hours after training, total RNA and protein levels in the soleus and plantaris muscles were obtained.

RESULTS

In the plantaris, the BDNF protein level was lower in the HIT than in the S group (P < 0.05). A similar effect was found in the soleus (without significant difference). In the soleus, higher Bdnf mRNA levels were found in the HIT group (P < 0.001 vs. S and MIT groups). In the plantaris muscle, similar Bdnf mRNA levels were found in all groups.

CONCLUSIONS

These results indicate that high-intensity chronic exercise reduces BDNF protein level in fast muscles and increases Bdnf mRNA levels in slow muscles.

Functional evaluation of peripheral nerve regeneration and target reinnervation in animal models: a critical overview

Peripheral nerve injuries usually lead to severe loss of motor, sensory and autonomic functions in the patients. Due to the complex requirements for adequate axonal regeneration, functional recovery is often poorly achieved. Experimental models are useful to investigate the mechanisms related to axonal regeneration and tissue reinnervation, and to test new therapeutic strategies to improve functional recovery. Therefore, objective and reliable evaluation methods should be applied for the assessment of regeneration and function restitution after nerve injury in animal models. This review gives an overview of the most useful methods to assess nerve regeneration, target reinnervation and recovery of complex sensory and motor functions, their values and limitations. The selection of methods has to be adequate to the main objective of the research study, either enhancement of axonal regeneration, improving regeneration and reinnervation of target organs by different types of nerve fibres, or increasing recovery of complex sensory and motor functions. It is generally recommended to use more than one functional method for each purpose, and also to perform morphological studies of the injured nerve and the reinnervated targets.

Effect of fat free mass on serum and plasma BDNF concentrations during exercise and recovery in healthy young men

Exercise results in release of brain derived neurotrophic factor into the circulation; however, little is known about the changes in serum and plasma brain derived neurotrophic factor concentrations and factors influencing brain derived neurotrophic factor during exercise and recovery. Serum (n=23) and plasma (n=10) brain derived neurotrophic factor concentrations were measured in healthy young men at rest, during steady-rate and after exercise to determine the maximum aerobic power. A two-way analysis of variance was used to investigate brain derived neurotrophic factor levels in blood during exercise and recovery, with one between-subject factor (a median split on: age, height, body mass, fat free mass, body mass index and aerobic fitness), and one within-subject factor (time). Serum brain derived neurotrophic factor concentrations increased in response to exercise and declined rapidly in recovery. Plasma brain derived neurotrophic factor had a greater proportional increase relative to exhaustive exercise compared with serum brain derived neurotrophic factor and was slower to return to near baseline values. There was a significant group-by-time interaction indicating a greater release and faster recovery for serum brain derived neurotrophic factor in high- compared with low-fat free mass individuals.