General and strain-specific age changes at mouse limb neuromuscular junctions

doi:10.1016/0197-4580(87)90069-8

Neurobiology of Aging

Volume 8, Issue 4, July–August 1987, Pages 309-318

https://doi.org/10.1016/0197-4580(87)90069-8 Get rights and content

Abstract

Age changes at limb neuromuscular junctions (NMJs) of CB57 and BALB/C mice were investigated and compared with previous results in a hydrid strain, in order to identify common characteristics of neuromuscular transmission and to assess the prevalence of signs of denervation and disuse. Resting membrane potential decreased with age in soleus muscles with no change in passive membrane properties. Miniature end-plate potential, amplitude and quantal content were greatly increased, with no change in muscle fiber diameter or nerve terminal morphometry. The various observed patterns of age changes were not those of disuse or denervation. Although there were diverse age changes in spontaneous transmitter release in different mouse strains, increased transmitter release per unit length of nerve terminal in limb muscle was a notably consistent finding across strains. It is apparently less subject to epigenetic variation during aging than most other reported neuromuscular physiological parameters.

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Homeostatic plasticity induced by increased acetylcholine release at the mouse neuromuscular junction
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Citation Excerpt :
These results indicated similar quantal content of EPPs in both genotypes (Quantal Content, 3 months, WT: 28 ± 9.8 Quanta/EPP vs. HV: 30 ± 10 Quanta/EPP, t36 = 0.6, p = 0.5514, Fig. 2C). Several previous studies describe changes with age to neurotransmission at wild type rat and mouse NMJ (Smith, 1984; Anis and Robbins, 1987; Willadt et al., 2016; reviewed by Willadt et al., 2018). To test for age differences in WT and HV mice, we next examined quantal parameters of neurotransmission in older animals.
At the neuromuscular junction (NMJ), changes to the size of the postsynaptic potential induce homeostatic compensation. At the Drosophila NMJ, increased glutamate release causes a compensatory decrease in quantal content, but it is unknown if this mechanism operates at the cholinergic mammalian NMJ. We addressed this question by recording endplate potentials (EPP) and muscle contraction in 3-month and 24-month ChAT-ChR2-EYFP mice that overexpress vesicular acetylcholine transporter and release more acetylcholine per vesicle. At 3 months, the quantal content of EPPs from ChAT-ChR2-EYFP mice were not different from WT controls, however tetanic depression was greater, and quantal size during high-frequency stimulation and the size of the readily releasable pool (RRP) were decreased. At 24 months of age, quantal content was reduced in ChAT-ChR2-EYFP mice, which normalized synaptic depression despite smaller RRP. The effect of pancuronium on indirect evoked muscle twitch was not different between groups. These results indicate that an increase in the amount of acetylcholine per vesicle induces two distinct age-dependent homeostatic mechanisms compensating excessive acetylcholine release.
‘Fragmentation’ of NMJs: a sign of degeneration or regeneration? A long journey with many junctions
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Citation Excerpt :
It is a commonly held view that as mammals age, their NMJs take on an increasingly ‘fragmented’ form, and it has been suggested that this is likely to be associated with impaired neuromuscular transmission (Valdez et al., 2010; Chai et al., 2011; Deschenes, 2011; Rudolf et al., 2014). Clear evidence of increased fragmentation comes from mice (Anis and Robbins, 1987; Valdez et al., 2010; Li et al., 2011; Valdez et al., 2012; Willadt et al., 2016). While similar changes have been reported in humans, (Oda, 1984), a recent, very detailed, study of NMJs in many human muscles found no evidence for increased fragmentation (Jones et al., 2017).
Mammalian neuromuscular junctions (NMJs) often consist of curved bands of synaptic contact, about 3–6 μm wide, which resemble pretzels. This contrasts with the NMJs of most animal species which consist of a cluster of separate synaptic spots, each of which is also about 3–6 μm across. In a number of situations, including a variety of disease states as well as normal ageing, mammalian NMJs acquire a more ‘fragmented’ appearance that resembles somewhat that of other species. This ‘fragmentation’ of the NMJ has sometimes been interpreted as a ‘disintegration’ or ‘degeneration’, with the suggestion that it might be associated with impaired neuromuscular transmission. An alternative view is that NMJ fragmentation is the outcome of a normal process by which the NMJ is maintained in an effective state. In this highly personal commentary, I cite a number of examples of this and point out that although the ‘pretzel’ form arises during normal development as a result of the sculpting of an immature synaptic ‘plaque’, in virtually all situations where new synaptic contact is established in adult mammals this occurs by the addition of new synaptic ‘spots’ rather than by the extension, or neoformation, of ‘pretzels’. Further, where appropriate studies have been performed, no evidence of a correlation between the degree of fragmentation and the efficacy of transmission has emerged. It may therefore be more appropriate to consider NMJ ‘fragmentation’ as a form of regeneration, rather than of degeneration. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.
Cellular and Molecular Anatomy of the Human Neuromuscular Junction
2017, Cell Reports
Citation Excerpt :
One area of research that is currently receiving significant interest concerns an apparent age-related decline in synaptic stability at the NMJ, manifesting as degenerative changes affecting both the pre-synaptic motor nerve terminal and the post-synaptic motor endplate (Gonzalez-Freire et al., 2014; Liu et al., 2017). Although findings from animal models (Anis and Robbins, 1987; Balice-Gordon and Lichtman, 1990; Valdez et al., 2010; Willadt et al., 2016) suggest that NMJs are inherently unstable with age, it is unclear whether a similar phenomenon occurs across the longer human lifespan. We were able to address this important question as our human tissue samples incorporated patients from the fourth to the tenth decades of life, with the individual ages of patients distributed approximately evenly across the age range (Table S2).
The neuromuscular junction (NMJ) plays a fundamental role in transferring information from lower motor neuron to skeletal muscle to generate movement. It is also an experimentally accessible model synapse routinely studied in animal models to explore fundamental aspects of synaptic form and function. Here, we combined morphological techniques, super-resolution imaging, and proteomic profiling to reveal the detailed cellular and molecular architecture of the human NMJ. Human NMJs were significantly smaller, less complex, and more fragmented than mouse NMJs. In contrast to mice, human NMJs were also remarkably stable across the entire adult lifespan, showing no signs of age-related degeneration or remodeling. Super-resolution imaging and proteomic profiling revealed distinctive distribution of active zone proteins and differential expression of core synaptic proteins and molecular pathways at the human NMJ. Taken together, these findings reveal human-specific cellular and molecular features of the NMJ that distinguish them from comparable synapses in other mammalian species.
The effects of pre-habilitative conditioning on unloading-induced adaptations in young and aged neuromuscular systems
2012, Experimental Gerontology
The capacity of pre-habilitative conditioning – exercise performed a priori – to mitigate neuromuscular maladaptations to disuse is unclear. This study evaluated pre-habilitation by examining neuromuscular junctions (NMJs) and the myofibers they innervate in young adult and aged muscles. Within each age category, 40 rats were divided into four treatment groups: 1) control, 2) hindlimb suspended (unloaded), 3) prehabilitative conditioning preceding hindlimb suspension, and 4) pre-habilitative conditioning alone. Cytofluorescent staining was used to visualize NMJs, and histochemical staining to assess myofiber profiles (size and type). Statistical analysis featured 2-way ANOVA with main effects for age and treatment, along with interaction. NMJs consistently revealed significant (P ≤ 0.05) main effects for age, but not treatment, or interaction. Typically, aged NMJs showed elongated nerve terminal branching, and more dispersed post-synaptic clusters of ACh receptors, resulting in reduced post-synaptic area per given length of pre-synaptic branching. Analysis of myofiber profiles showed significant main effects for age, treatment, and their interaction. Aged myofibers were smaller than the young ones and a higher percentage of them were Type I. Aged fibers experienced significantly greater unloading-induced atrophy than the young ones. Pre-habilitative conditioning significantly attenuated unloading-induced atrophy among aged, but not young myofibers. It was also observed that pre-habilitative conditioning alone increased myofiber size among aged, but not young adult muscles. In summary, myofibers were more sensitive than NMJs to the treatment interventions implemented. Although more sensitive to the negative effects of muscle unloading, aged myofibers were also more responsive to the hypertrophic effects of pre-habilitative conditioning.
Effect of exercise on physiological age-related change at mouse neuromuscular junctions
1990, Neurobiology of Aging
To determine the effect of endurance exercise on physiological age-related change at the mouse neuromuscular junction (NMJ), synaptic function was studied for extensor digitorum longus (EDL) and soleus muscles of three C57BL/6J mouse groups, 1) young adult control (YC: 10 months), 2) old control (OC: 20 months), and 3) old mice which exercised (OE: 20 months) since young-adulthood. Electrophysiological properties were studied with intracellular recording techniques. Safety margin was studied by measuring indirect isometric twitch tension in different calcium concentrations. With sedentary aging, EDL and soleus quantal contents increased. Following aging combined with 10 months of exercise, the EDL quantal contents in OE and YC animals were similar. In contrast, soleus quantal content was greater in OE than in YC animals. Determined safety margins were OC>YC=OE for EDL, and OC=YC=OE for soleus. This is the first study to indicate that physiological age-related changes at NMJs of EDL and soleus muscles are affected differently by endurance exercise. Exercise prevented all physiological age-related changes in EDL NMJs but not in soleus NMJs, this suggests that EDL changes are associated with inactivity during aging, while soleus changes are “fundamental” age changes.
Aging increases calcium influx at motor nerve terminal
1990, International Journal of Developmental Neuroscience
To determine whether increased transmitter release from soleus nerve terminals of old C57BL/6J mice is caused by an altered Ca²⁺ regulation, the time course of post-tetanic potentiation of miniature endplate potential (MEPP) frequency was used as an indicator of the kinetics of Ca²⁺ metabolism in young (10 months) and old (24 months) mice. Post-tetanic potentiation properties were studied in either (1) 0.2 mM Ca²⁺, 5.0 mM Mg²⁺ Krebs; or (2) Ca²⁺-free/EGTA Krebs to eliminate Ca²⁺ influx, and thereby isolated Ca²⁺ buffering. In the 0.2 mM Ca²⁺ Krebs, the time constants of decay of augmentation (T_A) and potentiation (T_P) were longer in old (T_A = 10.3 ± 1.0 sec, T_P = 195.3 ± 5.4 sec) than in young (T_A = 7.0 ± 0.7 sec, T_P = 78.8 ± 6.6 sec) nerve terminals. Evoked transmitter release was measured in 0.4 mM Ca²⁺, 2.75 mM Mg²⁺ Krebs. Quantal content of the endplate potential was positively correlated with T_A (r = 0.95) and with T_P (r = 0.98). In the Ca²⁺-free/EGTA Krebs, there was no difference in post-tetanic potentiation properties between young and old terminals. These results suggest that Ca²⁺ influx into the soleus nerve terminal increases with aging. This may explain, at least in part, the increased quantal content observed at old terminals.

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General and strain-specific age changes at mouse limb neuromuscular junctions

Abstract

Neuroscience

Brain Res

Exp Neurol

Exp Neurol

Neuroscience

Dev Biol

Brain Res

Exp Neurol

Exp Neurol

J. Biol Chem

An electron-microscopic study of zinc iodide-osmium impregnation of neurons. I. Staining of synaptic vesicles at cholinergic junctions

Brain Res

Microlocalization of receptors in young and old mouse muscle

Soc Neurosci Abstr

Use of the aging mouse sternocleidomastoid muscle in the search for generalities about neuromuscular synaptic aging

Soc Neurosci Abstr

Nerve terminal currents in young and old CBF-1 mouse neuromuscular junction

Soc Neurosci Abstr

Neuromuscular transmission and correlated morphology in young and old mice

J Physiol (Lond)

Physiological and structural changes at the amphibian myoneural junction in the course of nerve degeneration

J Physiol (Lond)

The trophic influence of tetradotoxin-inactive nerves on normal and deinnervated rat skeletal muscle

J Physiol (Lond)

The effect of disuse on cholinergic enzymes

J Physiol (Lond)

Morphology and physiology of skeletal muscle in aging rodents

Muscle Nerve

Statistical analysis of factors involved in neuromuscular facilitation and depression

J Physiol (Lond)

Age changes in neuromuscular junction morphology and acetylcholine receptor distribution in rat skeletal muscle fibres

J Physiol (Lond)