Both aging and exercise training alter the rate of recovery of neuromuscular performance of male soleus muscles

Mature and Juvenile Neuromuscular Plasticity in Response to Unloading

The neuromuscular junction (NMJ) is the synapse that enables the requisite electrical communication between the motor nervous system and the myofibers that respond to such electrical stimulation with movement and force development. Changes in an NMJ's normal activity pattern have been demonstrated to remodel both the synapse and the myofibers that comprise the NMJ. Significant amounts of research have been devoted to the study of aging on the neuromuscular system. Far less, however, has been focused on revealing the effects of reduced activity on the NMJ and myofibers comprising juvenile neuromuscular systems. In the present investigation, the consequences of decreased activity imposed by muscle unloading (UL) via hindlimb suspension for 2 weeks (a period known to induce muscle remodeling) were examined in both young adult, that is, mature (8 mo), and juvenile (3 mo) neuromuscular systems. In total, 4 treatment groups comprised of 10 animals (Juvenile-Control, Juvenile-Unloaded, Mature-Control, and Mature-Unloaded) were studied. Immunofluorescent procedures, coupled with confocal microscopy, were used to quantify remodeling of both the pre- and postsynaptic features of NMJs, as well as assessing the myofiber profiles of the soleus muscles housing the NMJs of interest. Results of ANOVA procedures revealed that there were significant (p < 0.05) main effects for both treatment, whereby UL consistently led to expanded size of the NMJ, and Age where expanded NMJ dimensions were consistently linked with mature compared to juvenile neuromuscular systems. Moreover, only sporadically was interaction between the main effects of Age and Treatment noted. Importantly, one variable that remained impressively resistant to the effects of both Age and Treatment was the critical parameter of pre- to postsynaptic coupling suggesting stability in effective communication at the NMJ throughout the lifespan and despite changes in activity patterns. The data presented here suggest that further inquiry must be performed regarding disuse-related plasticity of the neuromuscular system in adolescent individuals as those individuals regularly suffer injuries resulting in periods of muscle UL.

Loss of neuromuscular junction integrity and muscle atrophy in skeletal muscle disuse

Physical inactivity (PI) is a major risk factor of chronic diseases. A major aspect of PI is loss of muscle mass and strength. The latter phenomenon significantly impacts daily life and represent a major issue for global health. Understandably, skeletal muscle itself has been the major focus of studies aimed at understanding the mechanisms underlying loss of mass and strength. Relatively lesser attention has been given to the contribution of alterations in somatomotor control, despite the fact that these changes can start very early and can occur at multiple levels, from the cortex down to the neuromuscular junction (NMJ). It is well known that exposure to chronic inactivity or immobilization causes a disproportionate loss of force compared to muscle mass, i.e. a loss of specific or intrinsic whole muscle force. The latter phenomenon may be partially explained by the loss of specific force of individual muscle fibres, but several other players are very likely to contribute to such detrimental phenomenon. Irrespective of the length of the disuse period, the loss of force is, in fact, more than two-fold greater than that of muscle size. It is very likely that somatomotor alterations may contribute to this loss in intrinsic muscle force. Here we review evidence that alterations of one component of somatomotor control, namely the neuromuscular junction, occur in disuse. We also discuss some of the novel players in NMJ stability (e.g., homer, bassoon, pannexin) and the importance of new established and emerging molecular markers of neurodegenerative processes in humans such as agrin, neural-cell adhesion molecule and light-chain neurofilaments.

Adaptive Remodeling of the Neuromuscular Junction with Aging

Aging is associated with gradual degeneration, in mass and function, of the neuromuscular system. This process, referred to as “sarcopenia”, is considered a disease by itself, and it has been linked to a number of other serious maladies such as type II diabetes, osteoporosis, arthritis, cardiovascular disease, and even dementia. While the molecular causes of sarcopenia remain to be fully elucidated, recent findings have implicated the neuromuscular junction (NMJ) as being an important locus in the development and progression of that malady. This synapse, which connects motor neurons to the muscle fibers that they innervate, has been found to degenerate with age, contributing both to senescent-related declines in muscle mass and function. The NMJ also shows plasticity in response to a number of neuromuscular diseases such as amyotrophic lateral sclerosis (ALS) and Lambert-Eaton myasthenic syndrome (LEMS). Here, the structural and functional degradation of the NMJ associated with aging and disease is described, along with the measures that might be taken to effectively mitigate, if not fully prevent, that degeneration.

Effects of exercise training on neuromuscular junctions and their active zones in young and aged muscles

The neuromuscular junction (NMJ) connects the motor neuron with myofibers allowing muscle contraction. Both aging and increased activity result in NMJ remodeling. Here, the effects of exercise were examined in young and aged soleus muscles. Using immunofluorescent staining procedures, cellular and active zone components of the NMJ were quantified following a treadmill running program. Immunofluorescence was employed to determine myofiber profiles (size and type). Two-way analysis of variance procedures with main effects of age and treatment showed that when analyzing NMJs at the cellular level, significant (p ≤ 0.05) effects were identified for age, but not treatment. However, when examining subcellular active zones, effects for exercise, but not for age, were detected. Myofiber cross-sectional area showed that aging elicited atrophy and that among younger muscles endurance exercise training yielded decrements in myofiber size. Conversely, among aged muscles training elicited whole muscle and myofiber trends (p < 0.10) toward hypertrophy. Thus, different components of the neuromuscular system harbor unique sensitivities to various stimuli enabling proper adaptations to attain optimal function under differing conditions.

Muscle gene expression of CGRP-α, CGRP receptor, nAchR-β, and GDNF in response to different endurance training protocols of Wistar rats

The neuromuscular junction underwent adaptations to meet the demands of muscles following increased muscle activity. This study aimed to investigate the effects of high-intensity interval training (HIIT), endurance training (END), and mixed interval training (MIX) on the gene expression of the calcitonin gene-related peptide-α (CGRP-α), CGRP receptor, nicotinic acetylcholine receptors (nAchR)-β and glial-derived neurotrophic factor (GDNF) among different muscle types. Male Wistar rats were randomly divided into four groups: Control (n = 8), END (n = 8), HIIT (n = 8), and MIX (n = 8). The animals run each training protocol for 8 weeks (five sessions/week). Forty-eight hours after the last training session, the muscles gastrocnemius and soleus were excised under the sterilized situation. After collection, the material was prepared for RNA extraction, Reverse Transcriptase reaction, and qPCR assay. The HIIT training up-regulated the CGRP-α (p < 0.01), CGRP-Rec (p < 0.01), and GDNF (p < 0.01) in soleus as well as the nAchR-β (p < 0.01) and GDNF (p < 0.01) in gastrocnemius muscles. END training down-regulated the gene expression of CGRP-α (p < 0.01), and nAchR-β (p < 0.01) in gastrocnemius but up-regulated nAchR-β (p = 0.037) in soleus and GDNF (p  < 0.01) in gastrocnemius muscles. MIX training did not show any significant up or down-regulation. The endurance performance of HIIT and MIX groups was higher than the END group (p < 0.01). All studied genes up-regulated by HIIT training in a muscle type-specific manner. It seems that the improvement of some synaptic indices induced by HIIT resulted in the improvement of endurance performance.

New Insights into the Lactate Shuttle: Role of MCT4 in the Modulation of the Exercise Capacity

Lactate produced by muscle during high-intensity activity is an important end product of glycolysis that supports whole body metabolism. The lactate shuttle model suggested that lactate produced by glycolytic muscle fibers is utilized by oxidative fibers. MCT4 is a proton coupled monocarboxylate transporter preferentially expressed in glycolytic muscle fibers and facilitates the lactate efflux. Here we investigated the exercise capacity of mice with disrupted lactate shuttle due to global deletion of MCT4 (MCT4^-/-) or muscle-specific deletion of the accessory protein Basigin (iMSBsg^-/-). Although MCT4^-/- and iMSBsg^-/- mice have normal muscle morphology and contractility, only MCT4^-/- mice exhibit an exercise intolerant phenotype. In vivo measurements of compound muscle action potentials showed a decrement in the evoked response in the MCT4^-/- mice. This was accompanied by a significant structural degeneration of the neuromuscular junctions (NMJs). We propose that disruption of the lactate shuttle impacts motor function and destabilizes the motor unit.