Ageing and neurotrophic signalling effects on diaphragm neuromuscular function

Abstract

The age-related mechanisms underlying sarcopenia are largely unknown. We hypothesize that age-related neuromuscular changes depend on brain-derived neurotrophic factor (BDNF) acting through the tropomyosin-related kinase receptor B (TrkB). Maximal specific force and neuromuscular transmission failure were assessed at 6, 18 and 24 months following control, BDNF or phosphoprotein phosphatase 1 derivative (1NMPP1) treatment in male TrkB(F616A) mice. Phosphoprotein phosphatase-1 derivatives such as 1NMPP1 inhibit TrkB kinase activity as a result of this single amino acid mutation in the ATP binding domain. Maximal twitch and isometric tetanic force were reduced at 24 months compared to 6 and 18 months (P < 0.001). Neuromuscular transmission failure significantly increased at 18 and 24 months compared to 6 months (age × treatment interaction: P < 0.001). Neuromuscular transmission was improved following BDNF at 6 and 18 months and was impaired only at 6 months following 1NMPP1 treatment. Age and inhibition of TrkB kinase activity had similar effects on neuromuscular transmission failure, supporting a critical role for BDNF/TrkB signalling on neuromuscular changes in ageing. These results suggest that an age-related loss of endogenous BDNF precedes reductions in TrkB kinase activity in the diaphragm muscle.

Figure 1

Maximal isometric twitch force (P_t) and maximal tetanic force (P_o), normalized to physiological cross-sectional area, of midcostal diaphragm muscles of mice across the lifespan (aged 6, 18 and 24 months) Data were analysed by one-way ANOVA (P < 0.001 for both P_t and P_o). *Significantly different from 6-and 18-month-old mice.

Figure 2

Muscle fatigue index of diaphragm muscle following 2 min of repetitive muscle stimulation across the lifespan in mice (aged 6, 18 and 24 months) Muscle fatigue index is calculated as the ratio of the force at 2 min to the initial force. Data were analysed by one-way ANOVA (P = 0.024). *Significantly different from 6-and 18-month-old mice.

Figure 3

The contribution of neuromuscular transmission failure to diaphragm muscle fatigue over a 2 min period of repetitive nerve stimulation and superimposed intermittent muscle stimulation across the lifespan in mice (aged 6, 18 and 24 months) A, representative tracings for control (vehicle-treated) diaphragm muscle–phrenic nerve preparations of each age group. B, time course of neuromuscular transmission failure during repetitive stimulation in control, and BDNF-or 1NMPP1-treated preparations at each age group. In all age and treatment groups, there is progressively greater neuromuscular transmission failure over time. Data were analysed by three-way repeated measures ANOVA (age × treatment × time – repeated; P < 0.001). C, neuromuscular transmission failure following 2 min of repetitive stimulation in control, and BDNF-or 1NMPP1-treated groups. Summary data are shown to facilitate comparisons across treatment and age groups. Data were analysed by two-way ANOVA (age × treatment; interaction P < 0.001). Post hoc comparisons: *significantly different from control at the same age; ^†significantly different from BDNF at the same age; ^‡significantly different from 6 months within the same treatment; ^§significantly different from 18 months within the same treatment.