GDNF is regulated in an activity-dependent manner in rat skeletal muscle

Expression of Glial-Cell-Line-Derived Neurotrophic Factor Family Ligands in Human Intervertebral Discs

Glial-cell-line-derived neurotrophic factor (GDNF) family ligands (GFLs) contribute to the sensitization of primary afferents and are involved in the pathogenesis of inflammatory pain. The purpose of this preliminary study was to examine the expression of other GFLs (neurturin (NRTN), artemin (ARTN), persephin (PSPN)) and receptors in human IVD cells and tissues exhibiting early and advanced stages of degeneration. Human IVD cells were cultured as a monolayer after isolation from the nucleus pulposus (NP) and anulus fibrosus (AF) tissues. The mRNA expression of NRTN, ARTN, PSPN, and their receptors (GFRA2-GFRA4) was quantified using real-time PCR. Protein expression was evaluated using immunohistochemistry and Western blotting. The expression of NRTN, ARTN, PSPN, and their co-receptors (GFRA2-GFRA4) was identified in human IVD cells at both mRNA and protein levels. A trend was noted wherein the mRNA expression of ARTN, PSPN, and GFRA2 was upregulated by IL-1β treatment in a dose-dependent manner. The percentages of immunopositive cells in the advanced degenerate stage of ARTN, PSPN, and GFRA2 were significantly higher than those in the early degenerate stage. Their expression was enhanced in advanced tissue degeneration, which suggests that GFLs (ARTN and PSPN) may be involved in the pathogenesis of discogenic pain.

A cross-sectional study of ageing at the mouse neuromuscular junction and effects of an experimental therapeutic approach for dynapenia

Despite prior efforts to understand and target dynapenia (age-induced loss of muscle strength), this condition remains a major challenge that reduces the quality of life in the aged population. We have focused on the neuromuscular junction (NMJ) where changes in structure and function have rarely been systematically studied as a dynamic and progressive process. Our cross-sectional study found neurotransmission at the male mouse NMJ to be biphasic, displaying an early increase followed by a later decrease, and this phenotype was associated with structural changes to the NMJ. A cross-sectional characterization showed that age-induced alterations fell into four age groups: young adult (3-6 months), adult (7-18 months), early aged (19-24 months), and later aged (25-30 months). We then utilized a small molecule therapeutic candidate, GV-58, applied acutely during the later aged stage to combat age-induced reductions in transmitter release by increasing calcium influx during an action potential, which resulted in a significant increase in transmitter release. This comprehensive study of neuromuscular ageing at the NMJ will enable future research to target critical time points for therapeutic intervention. KEY POINTS: Age-induced frailty and falls are the leading causes of injury-related death and are caused by an age-induced loss of muscle strength due to a combination of neurological and muscular changes. A cross-sectional approach was used to study age-induced changes to the neuromuscular junction in a mouse model, and physiological changes that were biphasic over the ageing time course were found. Changes in physiology at the neuromuscular junction were correlated with alterations in neuromuscular junction morphology. An acutely applied positive allosteric gating modifier of presynaptic voltage-gated calcium channels was tested as a candidate therapeutic strategy that could increase transmitter release at aged neuromuscular junctions. These results provide a detailed time course of age-induced changes at the neuromuscular junction in a mouse model and test a candidate therapeutic strategy for weakness.

Influence of altered serum and muscle concentrations of BDNF on electrophysiological properties of spinal motoneurons in wild-type and BDNF-knockout rats

The purpose of this study was to determine whether altered serum and/or muscle concentrations of brain-derived neurotrophic factor (BDNF) can modify the electrophysiological properties of spinal motoneurons (MNs). This study was conducted in wild-type and Bdnf heterozygous knockout rats (HET, SD-BDNF). Rats were divided into four groups: control, knockout, control trained, and knockout trained. The latter two groups underwent moderate-intensity endurance training to increase BDNF levels in serum and/or hindlimb muscles. BDNF and other neurotrophic factors (NFs), including glial cell-derived neurotrophic factor (GDNF), neurotrophin-3 (NT-3), nerve growth factor (NGF), and neurotrophin-4 (NT-4) were assessed in serum and three hindlimb muscles: the tibialis anterior (TA), medial gastrocnemius (MG), and soleus (Sol). The concentrations of tropomyosin kinase receptor B (Trk-B), interleukin-15 (IL-15), and myoglobin (MYO/MB) were also evaluated in these muscles. The electrophysiological properties of lumbar MNs were studied in vivo using whole-cell current-clamp recordings. Bdnf knockout rats had reduced levels of all studied NFs in serum but not in hindlimb muscles. Interestingly, decreased serum NF levels did not influence the electrophysiological properties of spinal MNs. Additionally, endurance training did not change the serum concentrations of any of the NFs tested but significantly increased BDNF and GDNF levels in the TA and MG muscles in both trained groups. Furthermore, the excitability of fast MNs was reduced in both groups of trained rats. Thus, changes in muscle (but not serum) concentrations of BDNF and GDNF may be critical factors that modify the excitability of spinal MNs after intense physical activity.

Spinal Irisin Gene Delivery Attenuates Burn Injury-Induced Muscle Atrophy by Promoting Axonal Myelination and Innervation of Neuromuscular Junctions

Muscle loss and weakness after a burn injury are typically the consequences of neuronal dysregulation and metabolic change. Hypermetabolism has been noted to cause muscle atrophy. However, the mechanism underlying the development of burn-induced motor neuropathy and its contribution to muscle atrophy warrant elucidation. Current therapeutic interventions for burn-induced motor neuropathy demonstrate moderate efficacy and have side effects, which limit their usage. We previously used a third-degree burn injury rodent model and found that irisin-an exercise-induced myokine-exerts a protective effect against burn injury-induced sensory and motor neuropathy by attenuating neuronal damage in the spinal cord. In the current study, spinal irisin gene delivery was noted to attenuate burn injury-induced sciatic nerve demyelination and reduction of neuromuscular junction innervation. Spinal overexpression of irisin leads to myelination rehabilitation and muscular innervation through the modulation of brain-derived neurotrophic factor and glial-cell-line-derived neurotrophic factor expression along the sciatic nerve to the muscle tissues and thereby modulates the Akt/mTOR pathway and metabolic derangement and prevents muscle atrophy.

Effects of regenerative peripheral nerve interface on dorsal root ganglia neurons following peripheral axotomy

Background

Long-term delayed reconstruction of injured peripheral nerves always results in poor recovery. One important reason is retrograde cell death among injured sensory neurons of dorsal root ganglia (DRG). A regenerative peripheral nerve interface (RPNI) was capable of generating new synaptogenesis between the proximal nerve stump and free muscle graft. Meanwhile, sensory receptors within the skeletal muscle can also be readily reinnervated by donor sensory axons, which allows the target muscles to become sources of sensory information for function reconstruction. To date, the effect of RPNI on injured sensory neurons is still unclear. Here, we aim to investigate the potential neuroprotective role of RPNI on sensory DRG neurons after sciatic axotomy in adult rats.

Materials and methods

The sciatic nerves of sixty rats were transected. The rats were randomly divided into three groups following this nerve injury: no treatment (control group, n = 20), nerve stump implantation inside a fully innervated muscle (NSM group, n = 20), or nerve stump implantation inside a free muscle graft (RPNI group, n = 20). At 8 weeks post-axotomy, ipsilateral L4 and L5 DRGs were harvested in each group. Toluidine blue staining was employed to quantify the neuronal densities in DRGs. The neuronal apoptosis index was quantified with TUNEL assay. Western blotting was applied to measure the expressions of Bax, Bcl-2, and neurotrophins (NTs) in ipsilateral DRGs.

Results

There were significantly higher densities of neurons in ipsilateral DRGs of RPNI group than NSM and control groups at 8 weeks post-axotomy (p < 0.01). Meanwhile, neuronal apoptosis index and the expressions of pro-apoptotic Bax within the ipsilateral DRGs were significantly lower in the RPNI group than those in the control and NSM groups (p < 0.05), while the opposite result was observed in the expression of pro-survival Bcl-2. Furthermore, the expressions of NGF, NT-3, BDNF, and GDNF were also upregulated in the ipsilateral DRGs in the RPNI group (p < 0.01).

Conclusion

The present results demonstrate that RPNI could prevent neuronal loss after peripheral axotomy. And the neuroprotection effect has a relationship with the upregulation of NTs in DRGs, such as NGF, NT-3, BDNF, and GDNF. These findings provide an effective therapy for neuroprotection in the delayed repair of the peripheral nerve injury.

GDNF to the rescue: GDNF delivery effects on motor neurons and nerves, and muscle re-innervation after peripheral nerve injuries

Peripheral nerve injuries commonly occur due to trauma, like a traffic accident. Peripheral nerves get severed, causing motor neuron death and potential muscle atrophy. The current golden standard to treat peripheral nerve lesions, especially lesions with large (≥ 3 cm) nerve gaps, is the use of a nerve autograft or reimplantation in cases where nerve root avulsions occur. If not tended early, degeneration of motor neurons and loss of axon regeneration can occur, leading to loss of function. Although surgical procedures exist, patients often do not fully recover, and quality of life deteriorates. Peripheral nerves have limited regeneration, and it is usually mediated by Schwann cells and neurotrophic factors, like glial cell line-derived neurotrophic factor, as seen in Wallerian degeneration. Glial cell line-derived neurotrophic factor is a neurotrophic factor known to promote motor neuron survival and neurite outgrowth. Glial cell line-derived neurotrophic factor is upregulated in different forms of nerve injuries like axotomy, sciatic nerve crush, and compression, thus creating great interest to explore this protein as a potential treatment for peripheral nerve injuries. Exogenous glial cell line-derived neurotrophic factor has shown positive effects in regeneration and functional recovery when applied in experimental models of peripheral nerve injuries. In this review, we discuss the mechanism of repair provided by Schwann cells and upregulation of glial cell line-derived neurotrophic factor, the latest findings on the effects of glial cell line-derived neurotrophic factor in different types of peripheral nerve injuries, delivery systems, and complementary treatments (electrical muscle stimulation and exercise). Understanding and overcoming the challenges of proper timing and glial cell line-derived neurotrophic factor delivery is paramount to creating novel treatments to tend to peripheral nerve injuries to improve patients’ quality of life.

Sixty days of head-down tilt bed rest with or without artificial gravity do not affect the neuromuscular secretome

Artificial gravity is a potential countermeasure to attenuate effects of weightlessness during long-term spaceflight, including losses of muscle mass and function, possibly to some extent attributable to disturbed neuromuscular interaction. The 60-day AGBRESA bed-rest study was conducted with 24 participants (16 men, 8 women; 33 ± 9 years; 175 ± 9 cm; 74 ± 10 kg; 8 control group, 8 continuous (cAG) and 8 intermittent (iAG) centrifugation) to assess the impact of bed rest with or without daily 30-min continuous/intermittent centrifugation with 1G at the centre of mass. Fasting blood samples were collected before and on day 6, 20, 40 and 57 during 6° head-down tilt bed rest. Concentrations of circulating markers of muscle wasting (GDF-8/myostatin; slow skeletal muscle troponin T; prostaglandin E2), neurotrophic factors (BDNF; GDNF) and C-terminal Agrin Fragment (CAF) were determined by ELISAs. Creatine kinase activity was assessed by colorimetric enzyme assay. Repeated-measures ANOVAs were conducted with TIME as within-subject, and INTERVENTION and SEX as between-subject factors. The analyses revealed no significant effect of bed rest or sex on any of the parameters. Continuous or intermittent artificial gravity is a safe intervention that does not have a negative impact of the neuromuscular secretome.

How to Build and to Protect the Neuromuscular Junction: The Role of the Glial Cell Line-Derived Neurotrophic Factor

The neuromuscular junction (NMJ) is at the crossroad between the nervous system (NS) and the muscle. Following neurotransmitter release from the motor neurons (MNs), muscle contraction occurs and movement is generated. Besides eliciting muscle contraction, the NMJ represents a site of chemical bidirectional interplay between nerve and muscle with the active participation of Schwann cells. Indeed, signals originating from the muscle play an important role in synapse formation, stabilization, maintenance and function, both in development and adulthood. We focus here on the contribution of the Glial cell line-Derived Neurotrophic Factor (GDNF) to these processes and to its potential role in the protection of the NMJ during neurodegeneration. Historically related to the maintenance and survival of dopaminergic neurons of the substantia nigra, GDNF also plays a fundamental role in the peripheral NS (PNS). At this level, it promotes muscle trophism and it participates to the functionality of synapses. Moreover, compared to the other neurotrophic factors, GDNF shows unique peculiarities, which make its contribution essential in neurodegenerative disorders. While describing the known structural and functional changes occurring at the NMJ during neurodegeneration, we highlight the role of GDNF in the NMJ–muscle cross-talk and we review its therapeutic potential in counteracting the degenerative process occurring in the PNS in progressive and severe diseases such as Alzheimer’s disease (AD), Amyotrophic Lateral Sclerosis (ALS) and Spinal Muscular Atrophy (SMA). We also describe functional 3D neuromuscular co-culture systems that have been recently developed as a model for studying both NMJ formation in vitro and its involvement in neuromuscular disorders.

Mechanical, topographical and chemical cues combined with physical therapy for peripheral nerve injuries

Aim: The aim of this paper is to evaluate biomaterial cues combined with physical therapy (PT) on functional recovery in a rat sciatic nerve injury model. Materials & methods: Nerve growth conduits were filled with longitudinally aligned hyaluronic acid fibers and microspheres containing neurotrophic factor (growth factor [GF]). All animals received behavior and functional testing throughout the study, which concluded with measurement of compound muscle action potentials and contractile force of the gastrocnemius muscle. Results & conclusion: Including GF improved recovery of gross motor function and increased sensory pain sensation. During the 4 weeks that animals participated in PT, these groups showed higher static sciatic index scores. Including GF and PT has the potential to improve clinical outcomes following peripheral nerve injury.

GDNF synthesis, signaling, and retrograde transport in motor neurons

Glial cell line–derived neurotrophic factor (GDNF) is a 134 amino acid protein belonging in the GDNF family ligands (GFLs). GDNF was originally isolated from rat glial cell lines and identified as a neurotrophic factor with the ability to promote dopamine uptake within midbrain dopaminergic neurons. Since its discovery, the potential neuroprotective effects of GDNF have been researched extensively, and the effect of GDNF on motor neurons will be discussed herein. Similar to other members of the TGF-β superfamily, GDNF is first synthesized as a precursor protein (pro-GDNF). After a series of protein cleavage and processing, the 211 amino acid pro-GDNF is finally converted into the active and mature form of GDNF. GDNF has the ability to trigger receptor tyrosine kinase RET phosphorylation, whose downstream effects have been found to promote neuronal health and survival. The binding of GDNF to its receptors triggers several intracellular signaling pathways which play roles in promoting the development, survival, and maintenance of neuron-neuron and neuron-target tissue interactions. The synthesis and regulation of GDNF have been shown to be altered in many diseases, aging, exercise, and addiction. The neuroprotective effects of GDNF may be used to develop treatments and therapies to ameliorate neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). In this review, we provide a detailed discussion of the general roles of GDNF and its production, delivery, secretion, and neuroprotective effects on motor neurons within the mammalian neuromuscular system.

A Chemically Defined Common Medium for Culture of C2C12 Skeletal Muscle and Human Induced Pluripotent Stem Cell Derived Spinal Spheroids

Introduction

Multicellular platforms and linked multi organ on chip devices are powerful tools for drug discovery, and basic mechanistic studies. Often, a critical constraint is defining a culture medium optimal for all cells present in the system. In this study, we focused on the key cells of the neuromuscular junction i.e., skeletal muscle and motor neurons.

Methods

Formulation of a chemically defined medium for the co-culture of C2C12 skeletal muscle cells and human induced pluripotent stem cell (hiPSC) derived spinal spheroids (SpS) was optimized. C2C12 cells in 10 experimental media conditions and 2 topographies were evaluated over a 14-day maturation period to determine the ideal medium formulation for skeletal muscle tissue development.

Results

During early maturation, overexpression of genes for myogenesis and myopathy was observed for several media conditions, corresponding to muscle delamination and death. Together, we identified 3 media formulations that allowed for more controlled differentiation, healthier muscle tissue, and long-term culture duration. This evidence was then used to select media formulations to culture SpS and subsequently assessed axonal growth. As axonal growth in SpS cultures was comparable in all selected media conditions, our data suggest that the neuronal basal medium with no added supplements is the ideal medium formulation for both cell types.

Conclusions

Optimization using both topographical cues and culture media formulations provides a comprehensive analyses of culture conditions that are vital to future applications for in vitro NMJ models.

Exercise-Related Oxidative Stress as Mechanism to Fight Physical Dysfunction in Neuromuscular Disorders

Neuromuscular diseases (NMDs) are a group of often severely disabling disorders characterized by dysfunction in one of the main constituents of the motor unit, the cardinal anatomic-functional structure behind force and movement production. Irrespective of the different pathogenic mechanisms specifically underlying these disease conditions genetically determined or acquired, and the related molecular pathways involved in doing that, oxidative stress has often been shown to play a relevant role within the chain of events that induce or at least modulate the clinical manifestations of these disorders. Due to such a putative relevance of the imbalance of redox status occurring in contractile machinery and/or its neural drive in NMDs, physical exercise appears as one of the most important conditions able to positively interfere along an ideal axis, going from a deranged metabolic cell homeostasis in motor unit components to the reduced motor performance profile exhibited by the patient in everyday life. If so, it comes out that it would be important to identify a proper training program, suitable for load and type of exercise that is able to improve motor performance in adaptation and response to such a homeostatic imbalance. This review therefore analyzes the role of different exercise trainings on oxidative stress mechanisms, both in healthy and in NMDs, also including preclinical studies, to elucidate at which extent these can be useful to counteract muscle impairment associated to the disease, with the final aim of improving physical functions and quality of life of NMD patients.

Neuromuscular Activity Induces Paracrine Signaling and Triggers Axonal Regrowth after Injury in Microfluidic Lab-On-Chip Devices

Peripheral nerve injuries, including motor neuron axonal injury, often lead to functional impairments. Current therapies are mostly limited to surgical intervention after lesion, yet these interventions have limited success in restoring functionality. Current activity-based therapies after axonal injuries are based on trial-error approaches in which the details of the underlying cellular and molecular processes are largely unknown. Here we show the effects of the modulation of both neuronal and muscular activity with optogenetic approaches to assess the regenerative capacity of cultured motor neuron (MN) after lesion in a compartmentalized microfluidic-assisted axotomy device. With increased neuronal activity, we observed an increase in the ratio of regrowing axons after injury in our peripheral-injury model. Moreover, increasing muscular activity induces the liberation of leukemia inhibitory factor and glial cell line-derived neurotrophic factor in a paracrine fashion that in turn triggers axonal regrowth of lesioned MN in our 3D hydrogel cultures. The relevance of our findings as well as the novel approaches used in this study could be useful not only after axotomy events but also in diseases affecting MN survival.

The Role of Exercise as a Non-pharmacological Therapeutic Approach for Amyotrophic Lateral Sclerosis: Beneficial or Detrimental?

Amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease, involves the rapid deterioration of motor neurons resulting in severe muscle atrophy and respiratory insufficiency. It is considered a "multisystemic" disease with many potential mechanisms responsible for its pathology. Currently, there is no cure for ALS. Exercise training is suggested as a potential approach to reduce ALS pathology, but its beneficial role remains controversial. This review provides an overview of the effects of exercise training in ALS-affected mice and patients. It will compare the intensity, duration, and type of exercise on the health of SOD1^G93A mice, a mouse model of familial ALS, and review clinical studies involving ALS patients undergoing both endurance and resistance training. In summary, mild-to-moderate swimming-based endurance training appears the most advantageous mode of exercise in SOD1^G93A mice, improving animal survival, and delaying the onset and progression of disease. Furthermore, clinical studies show that both endurance and resistance training have an advantageous impact on the quality of life of ALS patients without extending life expectancy. However, small sample sizes, non-representative control populations, heterogeneous disease stage of patients, and the presence of confounders often exist in the exercise studies conducted with ALS patients. This raises concerns about the interpretation of these findings and, therefore, these results should be considered with caution. While promising, more pre-clinical and clinical studies with improved experimental design and fewer limitations are still necessary to confirm the impact of exercise training on the health of ALS patients.

Rejuvenation of the aged neuromuscular junction by exercise

Age-dependent declines in muscle function are observed across species. The loss of mobility resulting from the decline in muscle function represents an important health issue and a key determinant of quality of life for the elderly. It is believed that changes in the structure and function of the neuromuscular junction are important contributors to the observed declines in motor function with increased age. Numerous studies indicate that the aging muscle is an important contributor to the deterioration of the neuromuscular junction but the cellular and molecular mechanisms driving the degeneration of the synapse remain incompletely described. Importantly, growing data from both animal models and humans indicate that exercise can rejuvenate the neuromuscular junction and improve motor function. In this review we will focus on the role of muscle-derived neurotrophin signaling in the rejuvenation of the aged neuromuscular junction in response to exercise.

Application of differentiated human tonsil-derived stem cells to trembler-J mice

INTRODUCTION

Mesenchymal stem cells (MSCs) can differentiate into various cell types.

METHODS

In this study we investigated the potential of human tonsil-derived MSCs (T-MSCs) for neuromuscular regeneration in trembler-J (Tr-J) mice, a model for Charcot-Marie-Tooth disease type 1A (CMT1A).

RESULTS

T-MSCs differentiated toward skeletal myocytes with increased expression of skeletal muscle-related markers (including troponin I type 1, and myogenin), and the formation of myotubes in vitro. In-situ transplantation of T-MSC-derived myocytes (T-MSC myocytes) into the gastrocnemius muscle in Tr-J mice enhanced motor function, with recovery of compound muscle action potential amplitudes. Morphology of the sciatic nerve and skeletal muscle recovered without the formation of teratomas, and the expression levels of nerve growth factor and glial-cell-line-derived neurotrophic factor were increased significantly in T-MSC myocytes compared with T-MSCs in vitro.

DISCUSSION

Transplantation of T-MSC myocytes could enable neuromuscular regeneration in patients with CMT1A. Muscle Nerve 57: 478-486, 2018.

Neuroprotective Effects of Exercise Treatments After Injury: The Dual Role of Neurotrophic Factors

BACKGROUND

Shared connections between physical activity and neuroprotection have been studied for decades, but the mechanisms underlying this effect of specific exercise were only recently brought to light. Several evidences suggest that physical activity may be a reasonable and beneficial method to improve functional recovery in both peripheral and central nerve injuries and to delay functional decay in neurodegenerative diseases. In addition to improving cardiac and immune functions, physical activity may represent a multifunctional approach not only to improve cardiocirculatory and immune functions, but potentially modulating trophic factors signaling and, in turn, neuronal function and structure at times that may be critical for neurodegeneration and regeneration.

METHODS

Research content related to the effects of physical activity and specific exercise programs in normal and injured nervous system have been reviewed.

RESULTS

Sustained exercise, particularly if applied at moderate intensity and early after injury, exerts anti-inflammatory and pro-regenerative effects, and may boost cognitive and motor functions in aging and neurological disorders. However, newest studies show that exercise modalities can differently affect the production and function of brain-derived neurotrophic factor and other neurotrophins involved in the generation of neuropathic conditions. These findings suggest the possibility that new exercise strategies can be directed to nerve injuries with therapeutical benefits.

CONCLUSION

Considering the growing burden of illness worldwide, understanding of how modulation of neurotrophic factors contributes to exercise-induced neuroprotection and regeneration after peripheral nerve and spinal cord injuries is a relevant topic for research, and represents the beginning of a new non-pharmacological therapeutic approach for better rehabilitation of neural disorders.

Electrical muscle stimulation elevates intramuscular BDNF and GDNF mRNA following peripheral nerve injury and repair in rats

Despite advances in surgery, patients with nerve injuries frequently have functional deficits. We previously demonstrated in a rat model that daily electrical muscle stimulation (EMS) following peripheral nerve injury and repair enhances reinnervation, detectable as early as two weeks post-injury. In this study, we explain the enhanced early reinnervation observed with electrical stimulation. In two groups of rats, the tibial nerve was transected and immediately repaired. Gastrocnemius muscles were implanted with intramuscular electrodes for sham or muscle stimulation. Muscles were stimulated daily, eliciting 600 contractions for one hour/day, repeated five days per week. Sixteen days following nerve injury, muscles were assessed for functional reinnervation by motor unit number estimation methods using electromyographic recording. In a separate cohort of rats, surgical and electrical stimulation procedures were identical but muscles and distal nerve stumps were harvested for molecular analysis. We observed that stimulated muscles had significantly higher motor unit number counts. Intramuscular levels of brain-derived and glial cell line-derived neurotrophic factor (BDNF and GDNF) mRNA were significantly upregulated in muscles that underwent daily electrical stimulation compared to those without stimulation. The corresponding levels of trophic factor mRNA within the distal stump were not different from one another, indicating that the intramuscular electrical stimulus does not modulate Schwann cell-derived trophic factor transcription. Stimulation over a three-month period maintained elevated muscle-derived GDNF but not BDNF mRNA. In conclusion, EMS elevates intramuscular trophic factor mRNA levels which may explain how EMS enhances neural regeneration following nerve injury.

Long-term physical activity: an exogenous risk factor for sporadic amyotrophic lateral sclerosis?

OBJECTIVES

To conduct a geographically defined, UK-based case-control study, to examine any association between physical activity (PA) and amyotrophic lateral sclerosis (ALS).

METHODS

A novel historical PA questionnaire was designed, validated, and subsequently administered in individual face-to-face interviews of 175 newly diagnosed sporadic ALS cases and 317 age- and sex-matched community controls. Historical PA energy expenditure and time spent in vigorous-intensity PA were derived from questionnaire data and compared between cases and controls.

RESULTS

Participation in an extra 10kJ/kg/day of PA (equivalent to approximately 45minutes brisk walking) was consistently associated with an increased risk of ALS, with the strongest association observed for adulthood exercise-related PA (OR 1.47, 95% CI 1.10-1.97). An extra 10mins/day of vigorous PA was also associated with the odds of ALS (OR 1.03, 95% CI 1·01-1·05). Results were slightly attenuated following adjustment for smoking and educational attainment.

CONCLUSIONS

To our knowledge this is the first study to demonstrate a positive association between ALS and PA participation using a specifically designed and validated historical PA questionnaire. Despite the well-established health benefits of PA, a high activity lifestyle may also be associated with elevated risk of ALS. Large-scale prospective studies in the future may help to confirm this association.

Effects of exercise on neuromuscular junction components across age: systematic review of animal experimental studies

BACKGROUND

During almost one-third of our life, maturation of the nervous system promotes strength and muscle mass increase. However, as age advances, the nervous system begins to suffer a slow and continue reduction of its functions. Neuromuscular junction (NMJ) is one of the structures of which change due to aging process. Physical training leads to significant adjustments in NMJs of young and aged animals. Nevertheless, studies that aimed to investigate this effect have, in many cases, methodological variables that may have some influence on the result. Thus, this study aimed to carry out a systematic review about the effects of exercise training on the NMJ compartments of young, adult and aged animals.

RESULTS

We searched PubMed, Google Scholar, Science Direct, Scielo and Lilacs databases for animal experimental studies that studied exercise effects on the NMJs components across age. After inclusion and exclusion criteria, we included nine articles in systematic review and two for meta-analysis (young/adult NMJ).

CONCLUSIONS

We identified that exercise training cause NMJ hypertrophy on young animals and NMJ compression on aged ones. However, many methodological issues such as age, skeletal muscle and fibers type, and type of exercise and training protocol might influence the results. Graphical abstract: Flow gram is actually to be show at results section as Fig 1.

Daily Electrical Muscle Stimulation Enhances Functional Recovery Following Nerve Transection and Repair in Rats

Background. Incomplete recovery following surgical reconstruction of damaged peripheral nerves is common. Electrical muscle stimulation (EMS) to improve functional outcomes has not been effective in previous studies. Objective. To evaluate the efficacy of a new, clinically translatable EMS paradigm over a 3-month period following nerve transection and immediate repair. Methods. Rats were divided into 6 groups based on treatment (EMS or no treatment) and duration (1, 2, or 3 months). A tibial nerve transection injury was immediately repaired with 2 epineurial sutures. The right gastrocnemius muscle in all rats was implanted with intramuscular electrodes. In the EMS group, the muscle was electrically stimulated with 600 contractions per day, 5 days a week. Terminal measurements were made after 1, 2, or 3 months. Rats in the 3-month group were assessed weekly using skilled and overground locomotion tests. Neuromuscular junction reinnervation patterns were also examined. Results. Muscles that received daily EMS had significantly greater numbers of reinnervated motor units with smaller average motor unit sizes. The majority of muscle endplates were reinnervated by a single axon arising from a nerve trunk with significantly fewer numbers of terminal sprouts in the EMS group, the numbers being small. Muscle mass and force were unchanged but EMS improved behavioral outcomes. Conclusions. Our results demonstrated that EMS using a moderate stimulation paradigm immediately following nerve transection and repair enhances electrophysiological and behavioral recovery.

Calcitonin gene-related peptide regulation of glial cell-line derived neurotrophic factor in differentiated rat myotubes

Glial cell-line derived neurotrophic factor (GDNF) is the most potent trophic factor for motoneuron survival and neuromuscular junction formation. GDNF is upregulated in injured or denervated skeletal muscle and returns to normal levels following reinnervation. However, the mechanism by which GDNF is regulated in denervated muscle is not well understood. The nerve-derived neurotransmitter calcitonin gene-related peptide (CGRP) is upregulated following neuromuscular injury and is subsequently released from motoneurons at the neuromuscular junction. CGRP also promotes nerve regeneration, but the mechanism is not well understood. The current study investigates whether this increase in CGRP regulates GDNF, thus playing a key role in promoting regeneration of injured nerves. This study demonstrates that CGRP increases GDNF secretion without affecting its transcription or translation. Rat L6 myoblasts were differentiated into myotubes and subsequently treated with CGRP. GDNF mRNA expression levels were quantified by quantitative real-time reverse transcription-polymerase chain reaction, and secreted GDNF was quantified in the conditioned medium by ELISA. CGRP treatment increased secreted GDNF protein without altering GDNF mRNA levels. The translational inhibitor cycloheximide did not affect CGRP-induced GDNF secreted protein levels, whereas the secretional inhibitor brefeldin A blocked the CGRP-induced increase in GDNF. This study highlights the importance of injury-induced upregulation of CGRP by exposing its ability to increase GDNF levels and demonstrates a secretional mechanism for regulation of this key regeneration-promoting neurotrophic factor.

Effects of acetylcholine and electrical stimulation on glial cell line-derived neurotrophic factor production in skeletal muscle cells

Glial cell line-derived neurotrophic factor (GDNF) is a neurotrophic factor required for survival of neurons in the central and peripheral nervous system. Specifically, GDNF has been characterized as a survival factor for spinal motor neurons. GDNF is synthesized and secreted by neuronal target tissues, including skeletal muscle in the peripheral nervous system; however, the mechanisms by which GDNF is synthesized and released by skeletal muscle are not fully understood. Previous results suggested that cholinergic neurons regulate secretion of GDNF by skeletal muscle. In the current study, GDNF production by skeletal muscle myotubes following treatment with acetylcholine was examined. Acetylcholine receptors on myotubes were identified with labeled alpha-bungarotoxin and were blocked using unlabeled alpha-bungarotoxin. The question of whether electrical stimulation has a similar effect to that of acetylcholine was also investigated. Cells were stimulated with voltage pulses; at 1 and 5 Hz frequencies for times ranging from 30 min to 48 h. GDNF content in myotubes and GDNF in conditioned culture medium were quantified by enzyme-linked immunosorbant assay. Results suggest that acetylcholine and short-term electrical stimulation reduce GDNF secretion, while treatment with carbachol or long-term electrical stimulation enhances GDNF production by skeletal muscle.

Accuracy of regenerating motor neurons: influence of diffusion in denervated nerve

Following injury to a peripheral nerve the denervated distal nerve segment undergoes remarkable changes including loss of the blood-nerve barrier, Schwann cell proliferation, macrophage invasion, and the production of many cytokines and neurotrophic factors. The aggregate consequence of such changes is that the denervated nerve becomes a permissive and even preferred target for regenerating axons from the proximal nerve segment. The possible role that an original end-organ target (e.g. muscle) may play in this phenomenon during the regeneration period is largely unexplored. We used the rat femoral nerve as an in vivo model to begin to address this question. We also examined the effects of disrupting communication with muscle in terms of accuracy of regenerating motor neurons as judged by their ability to correctly project to their original terminal nerve branch. Our results demonstrate that the accuracy of regenerating motor neurons is dependent upon the denervated nerve segment remaining in uninterrupted continuity with muscle. We hypothesized that this influence of muscle on the denervated nerve might be via diffusion-driven movement of biomolecules or the active axonal transport that continues in severed axons for several days in the rat, so we devised experiments to separate these two possibilities. Our data show that disrupting ongoing diffusion-driven movement in a denervated nerve significantly reduces the accuracy of regenerating motor neurons.

GDNF content and NMJ morphology are altered in recruited muscles following high-speed and resistance wheel training

Glial cell line‐derived neurotrophic factor (GDNF) may play a role in delaying the onset of aging and help compress morbidity by preventing motor unit degeneration. Exercise has been shown to alter GDNF expression differently in slow‐ and fast‐twitch myofibers. The aim was to examine the effects of different intensities (10, 20, ~30, and ~40 m·min⁻¹) of wheel running on GDNF expression and neuromuscular junction (NMJ) plasticity in slow‐ and fast‐twitch myofibers. Male Sprague‐Dawley Rats (4 weeks old) were divided into two sedentary control groups (CON4 week, n = 5 and CON6 week, n = 5), two involuntary running groups, one at a low velocity; 10 m/min (INVOL‐low, n = 5), and one at a higher velocity; 20 m/min (INVOL‐high, n = 5), and two voluntary running groups with resistance (VOL‐R, n = 5, 120 g), and without resistance (VOL‐NR, n = 5, 4.5 g). GDNF protein content, determined by enzyme‐linked immunosorbent assay (ELISA), increased significantly in the recruited muscles. Plantaris (PLA) GDNF protein content increased 174% (P <0.05) and 161% (P <0.05) and end plate‐stained area increased 123% (P <0.05) and 72% (P <0.05) following VOL‐R, and VOL‐NR training, respectively, when compared to age‐matched controls. A relationship exists between GDNF protein content and end plate area (r = 0.880, P < 0.01, n = 15). VOL‐R training also resulted in more dispersed synapses in the PLA muscle when compared to age‐matched controls (P <0.05). Higher intensity exercise (>30 m/min) can increase GDNF protein content in fast‐twitch myofibers as well as induce changes in the NMJ morphology. These findings help to inform exercise prescription to preserve the integrity of the neuromuscular system through aging and disease.

Higher intensity exercise (>30 m/min) can increase glial cell line‐derived neurotrophic factor (GDNF) protein content in fast‐twitch myofibers as well as induce changes in the neuromuscular junction (NMJ) morphology. These findings help to inform exercise prescription to preserve the integrity of the neuromuscular system through aging and disease.

Extracellular vesicles from a muscle cell line (C2C12) enhance cell survival and neurite outgrowth of a motor neuron cell line (NSC-34)

Introduction

There is renewed interest in extracellular vesicles over the past decade or 2 after initially being thought of as simple cellular garbage cans to rid cells of unwanted components. Although there has been intense research into the role of extracellular vesicles in the fields of tumour and stem cell biology, the possible role of extracellular vesicles in nerve regeneration is just in its infancy.

Background

When a peripheral nerve is damaged, the communication between spinal cord motor neurons and their target muscles is disrupted and the result can be the loss of coordinated muscle movement. Despite state-of-the-art surgical procedures only approximately 10% of adults will recover full function after peripheral nerve repair. To improve upon such results will require a better understanding of the basic mechanisms that influence axon outgrowth and the interplay between the parent motor neuron and the distal end organ of muscle. It has previously been shown that extracellular vesicles are immunologically tolerated, display targeting ligands on their surface, and can be delivered in vivo to selected cell populations. All of these characteristics suggest that extracellular vesicles could play a significant role in nerve regeneration.

Methods

We have carried out studies using 2 very well characterized cell lines, the C2C12 muscle cell line and the motor neuron cell line NSC-34 to ask the question: Do extracellular vesicles from muscle influence cell survival and/or neurite outgrowth of motor neurons?

Conclusion

Our results show striking effects of extracellular vesicles derived from the muscle cell line on the motor neuron cell line in terms of neurite outgrowth and survival.

Glial cell line-derived neurotrophic factor (GDNF) expression and NMJ plasticity in skeletal muscle following endurance exercise

Glial cell line-derived neurotrophic factor (GDNF) supports and maintains the neuromuscular system during development and through adulthood by promoting neuroplasticity. The aim of this study was to determine if different modes of exercise can promote changes in GDNF expression and neuromuscular junction (NMJ) morphology in slow- and fast-twitch muscles. Rats were randomly assigned to a run training (run group), swim training (swim group), or sedentary control group. GDNF protein content was determined by enzyme-linked immunosorbant assay. GDNF protein content increased significantly in soleus (SOL) following both training protocols (P<0.05). Although not significant, an increase of 60% in the extensor digitorum longus (EDL) followed swim-training (NS; P<0.06). NMJ morphology was analyzed by measuring α-bungarotoxin labeled post-synaptic end plates. GDNF content and total end plate area were positively correlated. End plate area decreased in EDL of the run group and increased in SOL of the swim group. The results indicate that GDNF expression and NMJ morphological changes are activity dependent and that different changes may be observed by varying the exercise intensity in slow- and fast-twitch fibers.

Short-term exercise increases GDNF protein levels in the spinal cord of young and old rats

Neurotrophic factors may play a role in exercise-induced neuroprotective effects, however it is not known if exercise mediates changes in glial cell line-derived neurotrophic factor (GDNF) protein levels in the spinal cord. The aim of the current study was to determine if 2 weeks of exercise alters GDNF protein content in the lumbar spinal cord of young and old rats. GDNF protein was quantified via an enzyme-linked immunosorbent assay and Western blot. Immunohistochemical analysis localized GDNF in choline acetyltransferase (ChAT)-positive motor neurons and cell body areas were measured. Involuntary running in the young animals appeared to elicit the greatest increase in GDNF protein content (sixfold increase), followed by swimming (threefold increase) and voluntary running (twofold increase); however there was no significant difference between the modalities of exercise. Low-intensity running of the old animals significantly increased GDNF protein content in the spinal cord. Both young and old exercised animals showed a doubling in ChAT-positive motor neuron cell body areas. These results suggest that GDNF protein content in the spinal cord is modulated by exercise.

Neuroprotection for amyotrophic lateral sclerosis: role of stem cells, growth factors, and gene therapy

Various molecular mechanisms including apoptosis, inflammation, oxidative stress, mitochondrial dysfunction and excitotoxicity have been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS), though the exact mechanisms have yet to be specified. Furthermore, the underlying restorative molecular mechanisms resulting in neuronal and/or non-neuronal regeneration have to be yet elucidated. Therapeutic agents targeting one or more of these mechanisms to combat either initiation or progression of the disease are under research. Novel treatments including stem cell therapy, growth factors, and gene therapy might prolong survival and delay progression of symptoms. Harnessing the regenerative potential of the central nervous system would be a novel approach for the treatment of motor neuron death resulting from ALS. Endogenous neural replacement, if augmented with administration of exogenous growth factors or with pharmaceuticals that increase the rate of neural progenitor formation, neural migration, and neural maturation could slow the rate of cell loss enough to result in clinical improvement. In this review, we discuss the impact of therapeutic treatment involving stem cell therapy, growth factors, gene therapy, and combination therapy on disease onset and progression of ALS. In addition, we summarize human clinical trials of stem cell therapy, growth factor therapy, and gene therapy in individuals with ALS.

Differential effects of activity dependent treatments on axonal regeneration and neuropathic pain after peripheral nerve injury

Activity treatments are useful strategies to increase axonal regeneration and functional recovery after nerve lesions. They are thought to benefit neuropathy by enhancing neurotrophic factor expression. Nevertheless the effects on sensory function are still unclear. Since neurotrophic factors also play a fundamental role in peripheral and central sensitization, we studied the effects of acute electrical stimulation and early treadmill exercise on nerve regeneration and on neuropathic pain, and the relation with the expression of neurotrophins. After sciatic nerve section and suture repair, rats were subjected to electrical stimulation (ES) for 4h after injury, forced treadmill running (TR) for 5 days, or both treatments combined. Sciatic nerve section induced hyperalgesia in the medial area of the plantar skin in the injured paw. TR and ES differently but positively reduced adjacent neuropathic pain before and after sciatic reinnervation. ES enhanced motor and sensory reinnervation, and combination with TR induced strong agonistic effects in relieving pain. The differential effects of these activity treatments were related to changes in neurotrophic factor mRNA levels in sensory and motor neurons. ES speeded up expression of BDNF and GDNF in DRG, and of BDNF and NT3 in the ventral horn. TR reduced the levels of pro-nociceptive factors such as BDNF, NGF and GDNF in DRG. Combination of ES and TR induced intermediate levels suggesting an optimal balancing of treatment effects.

IB4(+) nociceptors mediate persistent muscle pain induced by GDNF

Skeletal muscle is a well-known source of glial cell line-derived neurotrophic factor (GDNF), which can produce mechanical hyperalgesia. Since some neuromuscular diseases are associated with both increased release of GDNF and intense muscle pain, we explored the role of GDNF as an endogenous mediator in muscle pain. Intramuscularly injected GDNF induced a dose-dependent (0.1-10 ng/20 μl) persistent (up to 3 wk) mechanical hyperalgesia in the rat. Once hyperalgesia subsided, injection of prostaglandin E(2) at the site induced a prolonged mechanical hyperalgesia (>72 h) compared with naïve rats (<4 h; hyperalgesic priming). Selective neurotoxic destruction of IB4(+) nociceptors attenuated both GDNF hyperalgesia and hyperalgesic priming. Ergonomic muscular injury induced by eccentric exercise or mechanical vibration increased muscle GDNF levels at 24 h, a time point where rats also exhibited marked muscle hyperalgesia. Intrathecal antisense oligodeoxynucleotides to mRNA encoding GFRα1, the canonical binding receptor for GDNF, reversibly inhibited eccentric exercise- and mechanical vibration-induced muscle hyperalgesia. Finally, electrophysiological recordings from nociceptors innervating the gastrocnemius muscle in anesthetized rats, revealed significant increase in response to sustained mechanical stimulation after local GDNF injection. In conclusion, these data indicate that GDNF plays a role as an endogenous mediator in acute and induction of chronic muscle pain, an effect likely to be produced by GDNF action at GFRα1 receptors located in IB4(+) nociceptors.

IB4-saporin attenuates acute and eliminates chronic muscle pain in the rat

The function of populations of nociceptors in muscle pain syndromes remain poorly understood. We compared the contribution of two major classes, isolectin B4-positive (IB4(+)) and IB4-negative (IB4(-)) nociceptors, in acute and chronic inflammatory and ergonomic muscle pain. Baseline mechanical nociceptive threshold was assessed in the gastrocnemius muscle of rats treated with IB4-saporin, which selectively destroys IB4(+) nociceptors. Rats were then submitted to models of acute inflammatory (intramuscular carrageenan)- or ergonomic intervention (eccentric exercise or vibration)-induced muscle pain, and each of the three models also evaluated for the transition from acute to chronic pain, manifest as prolongation of prostaglandin E2 (PGE(2))-induced hyperalgesia, after recovery from the hyperalgesia induced by acute inflammation or ergonomic interventions. IB4-saporin treatment did not affect baseline mechanical nociceptive threshold. However, compared to controls, IB4-saporin treated rats exhibited shorter duration mechanical hyperalgesia in all three models and attenuated peak hyperalgesia in the ergonomic pain models. And, IB4-saporin treatment completely prevented prolongation of PGE(2)-induced mechanical hyperalgesia. Thus, IB4(+) and IB4(-) neurons contribute to acute muscle hyperalgesia induced by diverse insults. However, only IB4+ nociceptors participate in the long term consequence of acute hyperalgesia.

The recent understanding of the neurotrophin's role in skeletal muscle adaptation

This paper summarizes the various effects of neurotrophins in skeletal muscle and how these proteins act as potential regulators of the maintenance, function, and regeneration of skeletal muscle fibers. Increasing evidence suggests that this family of neurotrophic factors influence not only the survival and function of innervating motoneurons but also the development and differentiation of myoblasts and muscle fibers. Muscle contractions (e.g., exercise) produce BDNF mRNA and protein in skeletal muscle, and the BDNF seems to play a role in enhancing glucose metabolism and may act for myokine to improve various brain disorders (e.g., Alzheimer's disease and major depression). In adults with neuromuscular disorders, variations in neurotrophin expression are found, and the role of neurotrophins under such conditions is beginning to be elucidated. This paper provides a basis for a better understanding of the role of these factors under such pathological conditions and for treatment of human neuromuscular disease.

Glial cell line-derived neurotrophic factor protein content in rat skeletal muscle is altered by increased physical activity in vivo and in vitro

Current evidence suggests that exercise and glial cell line-derived neurotrophic factor (GDNF) independently cause significant morphological changes in the neuromuscular system. The aim of the current study was to determine if increased physical activity regulates GDNF protein content in rat skeletal muscle. Extensor Digitorum Longus (EDL) and Soleus (SOL) hind limb skeletal muscles were analyzed following 2 weeks of involuntary exercise and 4 h of field stimulation or stretch in muscle bath preparations. GDNF protein content was measured via enzyme-linked immunosorbent assay (ELISA). Two weeks of exercise increased GDNF protein content in SOL as compared to sedentary controls (4.4±0.3 pg GDNF/mg tissue and 3.1±0.6 pg GDNF/mg tissue, respectively) and decreased GDNF protein content in EDL as compared to controls (1.0±0.1 pg GDNF/mg tissue and 2.3±0.7 pg GDNF/mg tissue, respectively). GDNF protein content in the EDL decreased following both field stimulation (56%±18% decrease from controls) and stretch (66%±10% decrease from controls). SOL responded to field stimulation with a 38%±7% increase from controls in GDNF protein content, but showed no change following stretch. Pre-treatment with α-bungarotoxin abolished the effects of field stimulation in both muscles and blocked the effect of stretch in EDL. α-bungarotoxin pre-treatment and stretch increased GDNF protein content to 240%±10% of controls in the SOL. Exposure to carbamylcholine decreased GDNF protein content to 51%±28% of controls in the EDL but not SOL. These results suggest that GDNF protein content in skeletal muscle may be controlled by stretch, where it may increase GDNF protein content, and membrane depolarization/acetylcholine (ACh) which acts to decrease GDNF protein content.

Melatonin plus exercise-based neurorehabilitative therapy for spinal cord injury

Spinal cord injury (SCI) is damage to the spinal cord caused by the trauma or disease that results in compromised or loss of body function. Subsequent to SCI in humans, many individuals have residual motor and sensory deficits that impair functional performance and quality of life. The available treatments for SCI are rehabilitation therapy, activity-based therapies, and pharmacological treatment using antioxidants and their agonists. Among pharmacological treatments, the most efficient and commonly used antioxidant for experimental SCI treatment is melatonin, an indolamine secreted by pineal gland at night. Melatonin's receptor-independent free radical scavenging action and its broad-spectrum antioxidant activity makes it an ideal antioxidant to protect tissue from oxidative stress-induced secondary damage after SCI. Owing to the limitations of an activity-based therapy and antioxidant treatment singly on the functional recovery and oxidative stress-induced secondary damages after SCI, a melatonin plus exercise treatment may be a more effective therapy for SCI. As suggested herein, supplementation with melatonin in conjunction with exercise not only would improve the functional recovery by enhancing the beneficial effects of exercise but would reduce the secondary tissue damage simultaneously. Finally, melatonin may protect against exercise-induced fatigue and impairments. In this review, based on the documented evidence regarding the beneficial effects of melatonin, activity-based therapy and the combination of both on functional recovery, as well as reduction of secondary damage caused by oxidative stress after SCI, we suggest the melatonin combined with exercise would be a novel neurorehabilitative strategy for the faster recovery after SCI.

Synergistic effect of melatonin on exercise-induced neuronal reconstruction and functional recovery in a spinal cord injury animal model

Nitric oxide (NO) may aggravate neuronal damage after spinal cord injury (SCI). We hypothesized that NO produced by inducible nitric oxide synthase (iNOS) accelerates secondary damage to spinal tissue, which may be reversed by the neuroprotectant, melatonin. This study investigated the effects of combination therapy with melatonin (10 mg/kg) and exercise (10 m/min) on recovery from SCI caused by contusion. We examined locomotor recovery, iNOS gene expression, autophagic and apoptotic signaling, including Beclin-1, LC3, p53 and IKKalpha protein expression and histological alterations in the ventral horn of the spinal cord. Melatonin in combination with exercise resulted in significantly increased hindlimb movement (P < 0.05), a reduced level of iNOS mRNA (P < 0.05) and more motor neurons in the ventral horn, versus control SCI and SCI plus exercise alone, with no effect on the other signaling molecules examined. This study shows that combined therapy with melatonin and exercise reduces the degree of secondary damage associated with SCI in rats and supports the possible use of melatonin in combination with exercise to reduce the side effects related to exercise-induced fatigue and impairment.

Trophic factor expression in phrenic motor neurons

The function of a motor neuron and the muscle fibers it innervates (i.e., a motor unit) determines neuromotor output. Unlike other skeletal muscles, respiratory muscles (e.g., the diaphragm, DIAm) must function from birth onwards in sustaining ventilation. DIAm motor units are capable of both ventilatory and non-ventilatory behaviors, including expulsive behaviors important for airway clearance. There is significant diversity in motor unit properties across different types of motor units in the DIAm. The mechanisms underlying the development and maintenance of motor unit diversity in respiratory muscles (including the DIAm) are not well understood. Recent studies suggest that trophic factor influences contribute to this diversity. Remarkably little is known about the expression of trophic factors and their receptors in phrenic motor neurons. This review will focus on the contribution of trophic factors to the establishment and maintenance of motor unit diversity in the DIAm, during development and in response to injury or disease.

Activation of microglia by endotoxin suppresses the secretion of glial cell line-derived neurotrophic factor (GDNF) through the action of protein kinase C alpha (PKCalpha) and mitogen-activated protein kinases (MAPKS)

The ability of microglia to produce/secrete glial cell line-derived neurotrophic factor (GDNF) in vitro was examined. Immunoblotting analysis revealed that nonstimulated microglia release limited amounts of GDNF with molecular sizes of 14 and 17 kDa. However, the secreted amounts significantly decreased when the microglia were activated with the endotoxin lipopolysaccharide (LPS). Comparison of the amounts of GDNF in the cells and the conditioned medium between the nonstimulated microglia and LPS-stimulated microglia clarified that the secretion of GDNF, but not its production, is strongly suppressed when the microglia are activated with LPS. The inhibitor experiments suggested that the GDNF secretion is depressed by a signaling cascade associated with protein kinase C alpha (PKCalpha) and/or mitogen-activated protein kinases (MAPKs). As expected from the above results, a PKC activator suppressed the secretion of GDNF in nonstimulated microglia. Taken together, these results demonstrated that microglia have the ability to produce and secrete GDNF in vitro, and that the secretion is suppressed by stimulation with endotoxin, probably due to a signaling mechanism involving PKCalpha and/or MAPKs.

Bioengineered nerve regeneration and muscle reinnervation

The peripheral nervous system has the intrinsic capacity to regenerate but the reinnervation of muscles is often suboptimal and results in limited recovery of function. Injuries to nerves that innervate complex organs such as the larynx are particularly difficult to treat. The many functions of the larynx have evolved through the intricate neural regulation of highly specialized laryngeal muscles. In this review, we examine the responses of nerves and muscles to injury, focusing on changes in the expression of neurotrophic factors, and highlight differences between the skeletal limb and laryngeal muscle systems. We also describe how artificial nerve conduits have become a useful tool for delivery of neurotrophic factors as therapeutic agents to promote peripheral nerve repair and might eventually be useful in the treatment of laryngeal nerve injury.

Effects of high-intensity endurance exercise training in the G93A mouse model of amyotrophic lateral sclerosis

The G93A transgenic mouse has a mutation in copper/zinc superoxide dismutase (CuZnSOD) that results in oxidative stress and motor neuron loss. Endurance exercise training is known to increase antioxidant capacity in skeletal muscle. Therefore, we hypothesized that endurance training may extend onset of disease or survival in the G93A mouse. We examined the effects of high-intensity endurance exercise training (45 min/day, 5 times/week, progressive increase from 9 to 22 m/min) on disease onset and survival in G93A mice. Endurance training did not affect clinical onset, although it hastened death in male mice (P < 0.05). Endurance-trained males had a statistically significant decrease in rotarod performance at 112 days (P < 0.05), whereas sedentary males decreased at 119 days (P < 0.05). Endurance-trained and sedentary females decreased at 126 days and 129 days, respectively (P < 0.05). Female mice lived longer than males (P < 0.05), and there was a trend for hastened clinical onset in males (P = 0.062). We conclude that high-intensity endurance exercise training does not affect onset of clinical symptoms in G93A mice but hastens a decrease in motor performance and death following onset of clinical symptoms in male mice only. In light of a recent report describing increased survival following low-intensity endurance training, it appears that training intensity is an important determinant of survival in the G93A mouse.

Exercise-induced gene expression in soleus muscle is dependent on time after spinal cord injury in rats

Cycling exercise attenuates atrophy in hindlimb muscles and causes changes in spinal cord properties after spinal cord injury in rats. We hypothesized that exercising soleus muscle expresses genes that are potentially beneficial to the injured spinal cord. Rats underwent spinal cord injury at T10 and were exercised on a motor-driven bicycle. Soleus muscle and lumbar spinal cord tissue were used for messenger RNA (mRNA) analysis. Gene expression of brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) was elevated 11- and 14-fold, respectively, in soleus muscle after one bout of exercise performed 5 days after spinal cord transection. Also, c-fos and heat shock protein-27 (HSP27) mRNA abundance were increased 11- and 7-fold, respectively. When exercise was started 2 days after the injury, the changes in gene expression were not observed. By contrast, at 2 but not at 5 days after transection, expression of the HSP27 gene was elevated sixfold in the lumbar spinal cord, independent of exercise. Electromyographic activity in soleus muscles was also decreased at 2 days, indicating that the spinal cord was less permissive to exercise at this early time. Long-term exercise for 4 weeks attenuated muscle atrophy equally well in rats started at 2 days or 5 days after injury. We conclude that BDNF and GDNF released from exercising muscle may be involved in exercise-induced plasticity of the spinal cord. Furthermore, the data suggest that the lumbar spinal cord undergoes time-dependent changes that temporarily impede the ability of the muscle to respond to exercise.

Invited review: Mechanisms underlying motor unit plasticity in the respiratory system

Neuromotor control of skeletal muscles, including respiratory muscles, is ultimately dependent on the function of the motor unit (comprising an individual motoneuron and the muscle fibers it innervates). Considerable diversity exists across diaphragm motor units, yet remarkable homogeneity is present (and maintained) within motor units. In recent years, the mechanisms underlying the development and adaptability of respiratory motor units have received great attention, leading to significant advances in our understanding of diaphragm motor unit plasticity. For example, following imposed inactivity of the diaphragm muscle, there are changes at phrenic motoneurons, neuromuscular junctions, and muscle fibers that tend to restore the ability of the diaphragm to sustain ventilation. The role of activity, neurotrophins, and other growth factors in modulating this adaptability is discussed.