Sod1 gene ablation in adult mice leads to physiological changes at the neuromuscular junction similar to changes that occur in old wild-type mice

Abstract

Reactive oxygen species (ROS) are believed to be important mediators of muscle atrophy and weakness in aging and many degenerative conditions. However, the mechanisms and physiological processes specifically affected by elevated ROS in neuromuscular units that contribute to muscle weakness during aging are not well defined. Here we investigate the effects of chronic oxidative stress on neurotransmission and excitation-contraction (EC) coupling mechanisms in the levator auris longus (LAL) muscle from young (4–8 months) and old (22–28 months) wild type mice and young adult Sod1−/− mice. The frequency of spontaneous neurotransmitter release and the amplitude of evoked neurotransmitter release in young Sod1−/− and old wild type LAL neuromuscular junctions were significantly reduced from the young wild type values, and those declines were mirrored by decreases in synaptic vesicle pool size. Presynaptic cytosolic calcium concentration and mitochondrial calcium uptake amplitudes showed substantial increases in stimulated young Sod1−/− and old axon terminals. Surprisingly, LAL muscle fibers from old mice showed a greater excitability than fibers from either young wild type or young Sod1−/− LAL muscle fibers. Both evoked excitatory junction potential (EJP) and spontaneous mini EJP amplitudes were considerably higher in LAL muscles from old mice than in fibers from young Sod1−/− LAL muscle. Despite a greater excitability, sarcoplasmic calcium influx in both old wild type and young Sod1−/− LAL muscle fibers was significantly smaller. Sarcoplasmic reticulum calcium levels were also smaller in both mice, but the difference was not statistically significant in muscle fibers from old wild type mice. The protein ratio of triad calcium channels – RyR1/DHPR was not different in all groups. However, fibers from both Sod1−/− and old mice had substantially elevated levels of protein carbonylation and S-nitrosylation modifications. Overall, our results suggest that young Sod1−/− recapitulate many neuromuscular and muscle fiber changes seen in old mice. We also conclude that muscle weakness in old mice might in part be driven by ROS mediated EC uncoupling, while both EC uncoupling and reduced neurotransmitter release contribute to muscle weakness in Sod1−/− mice.

Redox homeostasis and age‐related deficits in neuromuscular integrity and function

“…Moreover, although indistinguishable from WT mice at birth, by 5–8 months of age, SOD1 −/− mice show an accelerated neuromuscular ageing phenotype associated with myofibre atrophy (Figure 5), neurological impairments (Figure 6) and functional deficits 275. The features of the SOD1 −/− mouse model mimic those observed in 30 month old WT mice27, 277 and in older humans 6, 277.…”

“…Moreover, although indistinguishable from WT mice at birth, by 5-8 months of age, SOD1 À/À mice show an accelerated neuromuscular ageing phenotype associated with myofibre atrophy (Figure 5), neurological impairments ( Figure 6) and functional deficits. 275 The features of the SOD1 À/À mouse model mimic those observed in 30 month old WT mice 27,277 and in older humans. 6,277 In addition, in common with old WT mice, skeletal muscle from SOD1 À/À rodents exhibits increased levels of oxidative damage 27,[29][30][31]33,34,276,277 and a constitutive activation of redox-sensitive transcription factors 33 ; hence, it has been suggested that this knockout murine model represents a useful model for the study of chronic oxidative damage in the context of neuromuscular ageing in an effort to identify potential mechanisms and pathways that underlie sarcopenia in humans.…”

Age‐related changes in the structure and function of mammalian neuromuscular junctions

“…Most intracellular studies of the impact of age on NMJ function have found that the amplitude of EPPs/EPCs is greater in old than in young animals (while the opposite result—a decline in EPP amplitude with age—has been referred to in passing, we are unaware that a full account of this work has ever been published). In the majority of cases, this effect is associated with an increase in QC, with quantal size remaining unchanged . One counter‐example is a study that found an unchanged EPP amplitude but a large, unexplained increase in mEPP amplitude …”

“…In the majority of cases, this effect is associated with an increase in QC, with quantal size remaining unchanged . One counter‐example is a study that found an unchanged EPP amplitude but a large, unexplained increase in mEPP amplitude …”

Age-induced oxidative stress: how does it influence skeletal muscle quantity and quality?

“…Muscle fiber activation. At the neuromuscular junction, ROS production was recently demonstrated to be greater in the levator auris longus muscle in old mice compared with young mice (26). Interestingly, these changes were associated with less neurotransmitter being released at the synaptic cleft, which could cause failure in the generation of an action potential by the sarcolemma.…”