Cardiac troponin T and fast skeletal muscle denervation in ageing

Abstract

BackgroundAgeing skeletal muscle undergoes chronic denervation, and the neuromuscular junction (NMJ), the key structure that connects motor neuron nerves with muscle cells, shows increased defects with ageing. Previous studies in various species have shown that with ageing, type II fast‐twitch skeletal muscle fibres show more atrophy and NMJ deterioration than type I slow‐twitch fibres. However, how this process is regulated is largely unknown. A better understanding of the mechanisms regulating skeletal muscle fibre‐type specific denervation at the NMJ could be critical to identifying novel treatments for sarcopenia. Cardiac troponin T (cTnT), the heart muscle‐specific isoform of TnT, is a key component of the mechanisms of muscle contraction. It is expressed in skeletal muscle during early development, after acute sciatic nerve denervation, in various neuromuscular diseases and possibly in ageing muscle. Yet the subcellular localization and function of cTnT in skeletal muscle is largely unknown.MethodsStudies were carried out on isolated skeletal muscles from mice, vervet monkeys, and humans. Immunoblotting, immunoprecipitation, and mass spectrometry were used to analyse protein expression, real‐time reverse transcription polymerase chain reaction was used to measure gene expression, immunofluorescence staining was performed for subcellular distribution assay of proteins, and electromyographic recording was used to analyse neurotransmission at the NMJ.ResultsLevels of cTnT expression in skeletal muscle increased with ageing in mice. In addition, cTnT was highly enriched at the NMJ region—but mainly in the fast‐twitch, not the slow‐twitch, muscle of old mice. We further found that the protein kinase A (PKA) RIα subunit was largely removed from, while PKA RIIα and RIIβ are enriched at, the NMJ—again, preferentially in fast‐twitch but not slow‐twitch muscle in old mice. Knocking down cTnT in fast skeletal muscle of old mice: (i) increased PKA RIα and reduced PKA RIIα at the NMJ; (ii) decreased the levels of gene expression of muscle denervation markers; and (iii) enhanced neurotransmission efficiency at NMJ.ConclusionsCardiac troponin T at the NMJ region contributes to NMJ functional decline with ageing mainly in the fast‐twitch skeletal muscle through interfering with PKA signalling. This knowledge could inform useful targets for prevention and therapy of age‐related decline in muscle function.

The sympathetic nervous system regulates skeletal muscle motor innervation and acetylcholine receptor stability

“…We ruled out the participation of several alternative pathways, and the potential effect of β2‐AR was also excluded because it was increased in the TA but produced no significant changes in the GA after SNS ablation. We also found an increase in PKA(RIα) but a decrease in the PKA(RIIα)/PKA(RIα) ratio, which was recently proposed as a marker of the failure of postsynaptic AChR recycling and membrane insertion . An alternative mechanism for the elevation of MuRF1 levels after SNS ablation could be decreased muscle autophagy.…”

“…The SNS signals through cAMP/protein kinase‐A (PKA) to regulate MuRF1 levels . After sympathectomy, PKA (RIα) increased, while PKA (RIIα) did not change, so the PKA (RIIa)/PKA (RIa) ratio decreased (Figure ), possibly contributing to the impaired neuromuscular transmission associated with increased cardiac troponin‐T in the NMJ postterminal, as previously proposed …”

“…All experiments and analyses were conducted blind to treatment group. No statistical methods were used to predetermine sample sizes; however, our sample sizes are similar to those reported in recently published studies . Sigma Plot, version 12.5 (Systat Software, Inc, San Jose, CA, USA), and Microsoft Excel software were used for statistical analysis.…”

Troponin through the looking-glass: emerging roles beyond regulation of striated muscle contraction

“…The authors went on to show that ECC dysfunction could be rescued in sedentary, old mice by administration of an inhibitor of TnT3 fragmentation—presumably by a mechanism that inhibits fragmented TnT3-dependent downregulation of Cav1.1 expression. Recent findings revealed that cTnT is enriched at the neuromuscular junction in fast-twitch muscle fibers of older mice [ 121 ]. Based on several observations in this study, the authors proposed that cTnT contributes to skeletal muscle dysfunction and denervation through mechanisms that involved PKA signaling in aging mice.…”

Novel Recessive TNNT1 Congenital Core‐Rod Myopathy in French Canadians

“…34 The cardiac isoform of troponin T (TnTc) is found at low levels in normal adult skeletal type I myofibers, consistent with its homology with TnTs. 19,35,36 Missense variants in the alpha-helical tropomyosin-binding domain of TnTc decrease its binding affinity for tropomyosin while rendering the thin filament complex hypersensitized to calcium. 37,38 The hypersensitized state arises as a consequence of a slower rate of calcium dissociation from the mutated troponin complex.…”