Increased Single-Fiber Jitter Level Is Associated With Reduction in Motor Function With Aging

Neuromuscular junction transmission failure in aging and sarcopenia: The nexus of the neurological and muscular systems

Sarcopenia, or age-related decline in muscle form and function, exerts high personal, societal, and economic burdens when untreated. Integrity and function of the neuromuscular junction (NMJ), as the nexus between the nervous and muscular systems, is critical for input and dependable neural control of muscle force generation. As such, the NMJ has long been a site of keen interest in the context of skeletal muscle function deficits during aging and in the context of sarcopenia. Historically, changes of NMJ morphology during aging have been investigated extensively but primarily in aged rodent models. Aged rodents have consistently shown features of NMJ endplate fragmentation and denervation. Yet, the presence of NMJ changes in older humans remains controversial, and conflicting findings have been reported. This review article describes the physiological processes involved in NMJ transmission, discusses the evidence that supports NMJ transmission failure as a possible contributor to sarcopenia, and speculates on the potential of targeting these defects for therapeutic development. The technical approaches that are available for assessment of NMJ transmission, whether each approach has been applied in the context of aging and sarcopenia, and the associated findings are summarized. Like morphological studies, age-related NMJ transmission deficits have primarily been studied in rodents. In preclinical studies, isolated synaptic electrophysiology recordings of endplate currents or potentials have been mostly used, and paradoxically, have shown enhancement, rather than failure, with aging. Yet, in vivo assessment of single muscle fiber action potential generation using single fiber electromyography and nerve-stimulated muscle force measurements show evidence of NMJ failure in aged mice and rats. Together these findings suggest that endplate response enhancement may be a compensatory response to post-synaptic mechanisms of NMJ transmission failure in aged rodents. Possible, but underexplored, mechanisms of this failure are discussed including the simplification of post-synaptic folding and altered voltage-gated sodium channel clustering or function. In humans, there is limited clinical data that has selectively investigated single synaptic function in the context of aging. If sarcopenic older adults turn out to exhibit notable impairments in NMJ transmission (this has yet to be examined but based on available evidence appears to be plausible) then these NMJ transmission defects present a well-defined biological mechanism and offer a well-defined pathway for clinical implementation. Investigation of small molecules that are currently available clinically or being testing clinically in other disorders may provide a rapid route for development of interventions for older adults impacted by sarcopenia.

Deletion of quinolinate phosphoribosyltransferase gene accelerates frailty phenotypes and neuromuscular decline with aging in a sex-specific pattern

Decline in neuromuscular function with aging is known to be a major determinant of disability and all-cause mortality in late life. Despite the importance of the problem, the neurobiology of age-associated muscle weakness is poorly understood. In a previous report, we performed untargeted metabolomics on frail older adults and discovered prominent alteration in the kynurenine pathway, the major route of dietary tryptophan degradation that produces neurotoxic intermediate metabolites. We also showed that neurotoxic kynurenine pathway metabolites are correlated with increased frailty score. For the present study, we sought to further examine the neurobiology of these neurotoxic intermediates by utilizing a mouse model that has a deletion of the quinolinate phosphoribosyltransferase (QPRT) gene, a rate-limiting step of the kynurenine pathway. QPRT-/- mice have elevated neurotoxic quinolinic acid level in the nervous system throughout their lifespan. We found that QPRT-/- mice have accelerated declines in neuromuscular function in an age- and sex-specific manner compared to control strains. In addition, the QPRT-/- mice show premature signs of frailty and body composition changes that are typical for metabolic syndrome. Our findings suggest that the kynurenine pathway may play an important role in frailty and age-associated muscle weakness.

Profiling age-related muscle weakness and wasting: neuromuscular junction transmission as a driver of age-related physical decline

Pathological age-related loss of skeletal muscle strength and mass contribute to impaired physical function in older adults. Factors that promote the development of these conditions remain incompletely understood, impeding development of effective and specific diagnostic and therapeutic approaches. Inconclusive evidence across species suggests disruption of action potential signal transmission at the neuromuscular junction (NMJ), the crucial connection between the nervous and muscular systems, as a possible contributor to age-related muscle dysfunction. Here we investigated age-related loss of NMJ function using clinically relevant, electrophysiological measures (single-fiber electromyography (SFEMG) and repetitive nerve stimulation (RNS)) in aged (26 months) versus young (6 months) F344 rats. Measures of muscle function (e.g., grip strength, peak plantarflexion contractility torque) and mass were assessed for correlations with physiological measures (e.g., indices of NMJ transmission). Other outcomes also included plantarflexion muscle contractility tetanic torque fade during 1-s trains of stimulation as well as gastrocnemius motor unit size and number. Profiling NMJ function in aged rats identified significant declines in NMJ transmission stability and reliability. Further, NMJ deficits were tightly correlated with hindlimb grip strength, gastrocnemius muscle weight, loss of peak contractility torque, degree of tetanic fade, and motor unit loss. Thus, these findings provide direct evidence for NMJ dysfunction as a potential mechanism of age-related muscle dysfunction pathogenesis and severity. These findings also suggest that NMJ transmission modulation may serve as a target for therapeutic development for age-related loss of physical function.

Behavioural Characterisation of Macrod1 and Macrod2 Knockout Mice

Adenosine diphosphate ribosylation (ADP-ribosylation; ADPr), the addition of ADP-ribose moieties onto proteins and nucleic acids, is a highly conserved modification involved in a wide range of cellular functions, from viral defence, DNA damage response (DDR), metabolism, carcinogenesis and neurobiology. Here we study MACROD1 and MACROD2 (mono-ADP-ribosylhydrolases 1 and 2), two of the least well-understood ADPr-mono-hydrolases. MACROD1 has been reported to be largely localized to the mitochondria, while the MACROD2 genomic locus has been associated with various neurological conditions such as autism, attention deficit hyperactivity disorder (ADHD) and schizophrenia; yet the potential significance of disrupting these proteins in the context of mammalian behaviour is unknown. Therefore, here we analysed both Macrod1 and Macrod2 gene knockout (KO) mouse models in a battery of well-defined, spontaneous behavioural testing paradigms. Loss of Macrod1 resulted in a female-specific motor-coordination defect, whereas Macrod2 disruption was associated with hyperactivity that became more pronounced with age, in combination with a bradykinesia-like gait. These data reveal new insights into the importance of ADPr-mono-hydrolases in aspects of behaviour associated with both mitochondrial and neuropsychiatric disorders.

Maintaining Muscle Function Across the Lifespan: The State of Science

Sarcopenia is a geriatric syndrome currently defined as pathological loss of muscle mass and function. Sarcopenia is not only a major contributor to loss of physical function in older adults but is also associated with increased risk of morbidity, mortality, and increased healthcare costs. As a complex and multifactorial syndrome, sarcopenia has been associated with numerous degenerative changes during the aging process, but there is building evidence for significant contributions to the development of sarcopenia from neurodegenerative changes in the peripheral nervous system. A variety of interventions have been investigated for the treatment of sarcopenia, but current management is primarily focused on nutrition and therapeutic exercise interventions. Great strides have been made to improve screening procedures and diagnostic criteria for sarcopenia, but continued optimization of diagnostic and screening strategies is needed to better identify individuals with sarcopenia or at risk of developing sarcopenia. Understanding and addressing the major drivers of sarcopenia pathogenesis will help develop therapeutics that can reduce the impact of sarcopenia on affected individuals and society.

Modulation of Microglia by Voluntary Exercise or CSF1R Inhibition Prevents Age-Related Loss of Functional Motor Units

Age-related loss of skeletal muscle innervation by motor neurons leads to impaired neuromuscular function and is a well-established clinical phenomenon. However, the underlying pathogenesis remains unclear. Studying mice, we find that the number of motor units (MUs) can be maintained by counteracting neurotoxic microglia in the aged spinal cord. We observe that marked innervation changes, detected by motor unit number estimation (MUNE), occur prior to loss of muscle function in aged mice. This coincides with gene expression changes indicative of neuronal remodeling and microglial activation in aged spinal cord. Voluntary exercise prevents loss of MUs and reverses microglia activation. Depleting microglia by CSF1R inhibition also prevents the age-related decline in MUNE and neuromuscular junction disruption, implying a causal link. Our results suggest that age-related changes in spinal cord microglia contribute to neuromuscular decline in aged mice and demonstrate that removal of aged neurotoxic microglia can prevent or reverse MU loss.

Neuromuscular junction transmission failure is a late phenotype in aging mice

Neurodegeneration has increasingly been considered an important factor in the pathogenesis of sarcopenia or age-related loss of muscle mass and strength. Experiments were designed to investigate the fidelity of neuromuscular junction (NMJ) transmission across the lifespan in hindlimb muscles of male and female C57BL/6J mice (at 12, 20, 24, 27, and 29 months of age). Single-fiber electromyography recordings demonstrated abrupt onset of NMJ transmission failure at 27 months of age. Failed NMJ transmission was a later onset phenotype as compared with other assessments of motor unit numbers, muscle contractility, and frailty which showed alterations at 20 months of age. Ex vivo NMJ recordings demonstrated no reduction of endplate current amplitude in support of reduced muscle fiber excitability as the cause of failed NMJ transmission in aged mice. Improved understanding of age-related neurodegeneration will likely have important implications in designing novel therapeutic interventions specific for different stages of sarcopenia. Our findings suggest reduced muscle excitability may be a potential therapeutic target for improvement of physical function in older adults.

A New Threshold for Appendicular Lean Mass Discriminates Muscle Weakness in Women With Hip Fracture: A Cross-Sectional Study

OBJECTIVE

The aim of the study was to investigate the relationship between measures of muscle mass and grip strength in women with subacute hip fracture. Firstly, we aimed to assess the capability of the current thresholds for appendicular lean mass, appendicular lean mass/body mass index ratio and appendicular lean mass/height to separate weak and nonweak women. Secondly, we aimed to explore alternative thresholds for the three measures of muscle mass to discriminate weakness.

DESIGN

This is cross-sectional study of 160 women with hip fracture admitted to a rehabilitation hospital. We assessed appendicular lean mass by dual-energy x-ray absorptiometry and grip strength by a Jamar hand dynamometer. Weakness was defined as grip strength of less than 16 kg.

RESULTS

Weakness was not significantly associated with appendicular lean mass of less than 15.02 kg, appendicular lean mass/body mass index ratio of less than 0.512 or appendicular lean mass/height of less than 5.67 kg/m. For appendicular lean mass (but not for the other 2 measures of muscle mass), an alternative threshold (11.87 kg instead of 15.02 kg) significantly discriminated weakness: χ (1, n = 160) = 10.77 (P = 0.001). The association between appendicular lean mass of less than 11.87 kg and grip strength of less than 16 kg persisted after adjustment for age and body mass index (odds ratio = 2.50, 95% confidence interval = 1.17-5.34, P = 0.018).

CONCLUSIONS

Data suggest that the current thresholds for measures of muscle mass do not discriminate weakness in women with subacute hip fracture. For appendicular lean mass, an alternative cutoff point actually separated weak and nonweak women.