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

Abstract

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.

Keywords: Dynapenia; Mobility; Muscle weakness; Older adults; Physical function; Skeletal muscle function deficits.

Fig. 1.. Structure of the mammalian neuromuscular junction (NMJ).

A: Illustration of a motor axon extensively branching to innervate two muscle fibers at the respective NMJs. The arrow indicates a section where the nerve has been removed to allow better visualization of the postsynaptic folds. B: A human NMJ illustrating the nerve terminal in green and the post-synaptic acetylcholine receptors (AChRs) in red. The nerve terminals enlarged boutons, from which ACh is released, are readily seen. (The scale bar is 20 μm). C: An electron micrograph section through a single human bouton highlighting the extensive infoldings of the postsynaptic myofiber membrane. (The scale bar is 1 μm). Reprinted from Slater via an open access Creative Common CC BY license (Slater, 2017). See section II for further discussion.

Fig. 2.. Neuromuscular junction transmission (NMJ) at the motor unit and single muscle fiber levels in health and disease.

A. In the healthy state, a motor unit, the muscles fibers innervated by a single motor neuron, discharges as summated, all-or-none responses. NMJ transmission can be assessed on the motor unit level (motor unit action potentials) as indicated by relatively stable size and shape between discharges. Similarly, NMJ transmission can be assessed at the single fiber level (single fiber electromyography, SFEMG) as indicated by consistent timing and occurrence of action potentials relative between two synapses of the same motor unit (in voluntary SFEMG) or relative to axonal stimulus (in stimulated SFEMG) indicate integrity of NMJ transmission. B. In contrast, in conditions of NMJ failure, the size and shape of motor unit action potential responses vary between discharges, termed “jiggle” and SFEMG shows increased variability of action potential latency (jitter) and intermittent all-or-none action potential failure (blocking). Of note, voluntary SFEMG can only be performed with low intensity contractions as such only interrogates muscle fiber pairs of low threshold motor units. Conversely, stimulated SFEMG interrogates synapses independent of threshold. Conceptual figure not drawn to scale. Created with biorender.com.

Fig. 3.. Representative Single Fiber Electromyography recordings.

A-B. Normal jitter and no blocking at a normal synapse (right: superimposed, left: rastered). C-D. Increased jitter and blocking at a failing synapse (right: superimposed, left: rastered). SFEMG recorded at an axonal stimulation frequency of 10 Hz from the gastrocnemius muscle in a young (A-B) and aged (C-D) C57BL/6 mouse. Sweep speed of 500 μs and a sensitivity of 500 μV. Reprinted with permission from Chugh et al., 2020.

Fig. 4.. Conceptual model of age-related changes in NMJ form and function.

The available preclinical physiological data support that there is a loss of the safety factor and failure of NMJ transmission during aging. Yet, direct evidence from clinical populations, specifically those impacted by sarcopenia, are lacking. Here we present a conceptual model of the age-related changes in presynaptic (upper panel), synaptic (middle panel), and post-synaptic (lower panel) form and function. See sections III and IV for further discussion. Created with biorender.com. ACh: Acetylcholine, AChR: Acetylcholine receptor, VGCC: voltage-gated calcium channel, CLC-1: ClC-1 chloride channel, VGSC: voltage-gated sodium channel.

Fig. 5.. Small molecule treatments that could, conceptually, be used to enhance neuromuscular junction transmission in sarcopenia.

These small molecules are already approved by the FDA or in the pipeline for approval for a variety of neuromuscular diseases that are characterized by muscle weakness. Presynaptic (left panel): 3,4 Diaminopyridine blocks presynaptic voltage gated potassium channels to prolong presynaptic nerve action potential to increase Ca²⁺ influx. Synaptic (center panel): The acetylcholinesterase inhibitor (AChE) pyridostigmine increases persistence of acetylcholine (ACh) at the synapse. Postsynaptic (right Panel): The CLC-1 channel inhibitor blocks Cl⁻ ion conductance and increases sarcolemmal excitability. See section V for further discussion. Created with biorender.com. ACh: Acetylcholine, AChR: Acetylcholine receptor, VGCC: voltage-gated calcium channel, VGKC: voltage-gated potassium channel; AChE: Acetylcholinesterase; CLC-1: ClC-1 chloride channel.