Neuromuscular Dysfunction Precedes Cognitive Impairment in a Mouse Model of Alzheimer's Disease

Abstract

Alzheimer's disease (AD) develops along a continuum that spans years prior to diagnosis. Decreased muscle function and mitochondrial respiration occur years earlier in those that develop AD; however, it is unknown what causes these peripheral phenotypes in a disease of the brain. Exercise promotes muscle, mitochondria, and cognitive health and is proposed to be a potential therapeutic for AD, but no study has investigated how skeletal muscle adapts to exercise training in an AD-like context. Utilizing 5xFAD mice, an AD model that develops ad-like pathology and cognitive impairments around 6 mo of age, we examined in vivo neuromuscular function and exercise adapations (mitochondrial respiration and RNA sequencing) before the manifestation of overt cognitive impairment. We found 5xFAD mice develop neuromuscular dysfunction beginning as early as 4 mo of age, characterized by impaired nerve-stimulated muscle torque production and compound nerve action potential of the sciatic nerve. Furthermore, skeletal muscle in 5xFAD mice had altered, sex-dependent, adaptive responses (mitochondrial respiration and gene expression) to exercise training in the absence of overt cognitive impairment. Changes in peripheral systems, specifically neural communication to skeletal muscle, may be harbingers for AD and have implications for lifestyle interventions, like exercise, in AD.

Keywords: 5xFAD; alzheimer’s disease; exercise; mitochondria; neuromuscular; sciatic nerve.

Graphical Abstract

Integrated model for the development of neuromuscular dysfunction in the AD-like pathology of 5xFAD mice. (A) At 3 mo of age, nerve-stimulated muscle function is normal across genotypes and sexes. (B) By as early as 4 mo of age, nerve-stimulated muscle function declines, and (C) corresponds to impaired sciatic nerve function in 5xFAD mice. Peripheral neuromuscular dysfunction (D) corresponds to an altered mitochondrial and transcriptional response of skeletal muscle to exercise training. (E) These peripheral phenotypes of the early AD-like pathology of 5xFAD mice were present in the absence of overt cognitive decline, particularly in male mice. Created in Biorender.

Figure 1.

Development of neuromuscular dysfunction in male and female 5xFAD mice. (A) Study the design of tibial nerve-stimulated plantar flexor torque production followed by direct muscle stimulation 48 h later, performed monthly from 3 to 6 mo of age. B-I Torque-Frequency Curves; (B) Male WT tibial nerve stimulation. (C) Male 5xFAD tibial nerve stimulation. (D) Female WT tibial nerve stimulation. (E) Female 5xFAD tibial nerve stimulation. (F) Male WT direct muscle stimulation. (G) Male 5xFAD direct muscle stimulation. (H) Female WT direct muscle stimulation. (I) Female 5xFAD direct muscle stimulation. J-M Torque at 100 Hz; (J) Male torque production at 100 Hz via tibial nerve stimulation. (K) Female torque production at 100 Hz via tibial nerve stimulation. (L) Male torque production at 100 Hz via direct muscle stimulation. (M) Female torque production at 100 Hz via direct muscle stimulation. (N) Male body mass. (O) Female body mass. n = 5 per group and sex. Data presented as mean ± SEM and repeated measures two-way ANOVA was performed and Sidak’s post-hoc test performed when significant interaction between variables was observed (*P < 0.05 and **P < 0.01). Panel A created in Biorender.

Figure 2.

Impaired compound nerve action potential (CNAP) in the sciatic nerve of male and female 5xFAD mice: (A) Graphic of sciatic CNAP stimulation. (B) Reresentative WT versus 5xFAD CNAP waveform. (C) Male and female WT and 5xFAD sciatic CNAP duration. n = 5 per group and sex. Data presented as mean ± SEM and students T-test performed (**P < 0.01). Panel A created in Biorender.

Figure 3.

Running wheel exercise and skeletal muscle adaptations. (A) Male WT and 5xFAD Ex average daily running. (B) Male exercise capacity. (C) Male blood lactate pre- and post-exercise capacity test. (D) Female WT and 5xFAD Ex average daily. (E) Female exercise capacity. (F) Female blood lactate pre- and post-exercise capacity test. (G) Male ADP-stimulated State 3 respiration. (H) Male SUIT protocol and respiration rates area under the curve. (I) Female ADP-stimulated State 3 respiration. (J) Female SUIT protocol and respiration rates area under the curve. Males n = 6 per group; females n = 4-7 per group. Data presented as mean ± SEM and analyzed via Student’s t-test (one variable) or two-way ANOVA (two variables) with post-hoc analysis performed when significant interaction effect was found (*P < 0.05, **P < 0.01, ***P < 0.001).

Figure 4.

Altered transcriptional changes in skeletal muscle with exercise training between male and female 5xFAD mice. (A) Heat map of global gene changes in male mice. (B) Differently expressed gene (DEG) between male groups. (C) Chord diagram of representative gene pathway differences between male WT Ex versus WT sed and 5xFAD Ex versus WT Ex. (D) Heat map of global gene changes in female mice. (E) Differently expressed gene (DEG) between female groups. (F) Chord diagram of representative gene pathway differences between female WT Ex versus WT sed and 5xFAD Ex versus WT Ex. Males n = 6 per group; females n = 4-7 per group.

Figure 5.

T-maze assessment of cognitive function in male and female 5xFAD mice following exercise training: Mice were introduced to the T-maze and tested 10 times over 3 d. (A) Male percent success rate in alternation from initial T-arm choice. (B) Male average time to T-arm commitment. (C) Male time to commitment difference between trial and sample test. (D) Female percent success rate in alternation from initial T-arm choice. (E) Female average time to T-arm commitment. (F) Female time to commitment difference between trial and sample test. Males n = 6 per group; females n = 4-7 per group. Data presented as mean ± SEM and analyzed via two-way ANOVA with post-hoc analysis performed with significant interaction effect was found (*P < 0.05 and **P < 0.01).