Motoneuron survival is promoted by specific exercise in a mouse model of amyotrophic lateral sclerosis

Abstract

Several studies using transgenic mouse models of familial amyotrophic lateral sclerosis (ALS) have reported a life span increase in exercised animals, as long as animals are submitted to a moderate-intensity training protocol. However, the neuroprotective potential of exercise is still questionable. To gain further insight into the cellular basis of the exercise-induced effects in neuroprotection, we compared the efficiency of a swimming-based training, a high-frequency and -amplitude exercise that preferentially recruits the fast motor units, and of a moderate running-based training, that preferentially triggers the slow motor units, in an ALS mouse model. Surprisingly, we found that the swimming-induced benefits sustained the motor function and increased the ALS mouse life span by about 25 days. The magnitude of this beneficial effect is one of the highest among those induced by any therapeutic strategy in this disease. We have shown that, unlike running, swimming significantly delays spinal motoneuron death and, more specifically, the motoneurons of large soma area. Analysis of the muscular phenotype revealed a swimming-induced relative maintenance of the fast phenotype in fast-twitch muscles. Furthermore, the swimming programme preserved astrocyte and oligodendrocyte populations in ALS spinal cord. As a whole, these data are highly suggestive of a causal relationship not only linking motoneuron activation and protection, but also motoneuron protection and the maintenance of the motoneuron surrounding environment. Basically, exercise-induced neuroprotective mechanisms provide an example of the molecular adaptation of activated motoneurons.

Figure 1

Differential effects of either a swimming- or a running-based training in ALS mice Only the swimming programme delays disease onset (A) and extends survival (B) in ALS mice as shown by Kaplan–Meier curves. The mean time to disease onset was delayed by 2 weeks (P < 0.001) and the mean time to death was delayed by 3 weeks (P < 0.01) for the swimming ALS mice compared to the sedentary and the running ALS mice. Swimming delays the onset of disease symptoms, including a stabilization of the body weight (C), a delay in disease-related weakness as shown by the performance assessment of forelimb strength (grip test) (D) and the level of spontaneous exploratory activity (open field) (E). The behavioural data are shown as mean ±s.e.m. (n= 8; *0.01 < P < 0.05), *control versus ALS mice or ⁺swimming ALS versus other ALS groups).

Figure 2

Only the swimming-based training significantly protects lumbar motoneurons in ALS mice A, representative images showing motoneuron populations identified using ChAT immunostaining in the ventral horn of the lumbar spinal cord from control mice and sedentary, running and swimming ALS mice (scale bar: 50 μm). B, quantification of surviving motoneurons in control mice and sedentary, running and swimming ALS mice at 115 days of age. Data are shown as mean ±s.d. C, morphometric evaluation of surviving motoneurons in control mice and sedentary, running and swimming ALS mice at 115 days of age. Percentage of motoneurons by soma area (distribution: 200 μm²) are shown as mean ±s.d. (n= 8; ***P < 0.001, **0.001 < P < 0.01, *0.01 < P < 0.05), *control versus ALS mice or ⁺swimming ALS versus other ALS groups.

Figure 3

Differential effects of swimming- and running-based training programmes in sustaining the skeletal muscle phenotype in ALS mice Only the swimming programme limits the hypoplasia affecting the two extensor muscles, namely the slow-twitch soleus and the fast-twitch plantaris from ALS mice at 115 days of age (A). Both exercises limit the hypoplasia detected in the fast-twitch flexor tibialis. B, representative image showing a type I fibre appearance in a fast-twitch tibialisin sedentary ALS mice. Scale bar: 200 μm. Analysis of the muscle typology in ALS muscles compared to controls (C) reveals a slow-to-fast myofibre transition in the slow-twitch soleus, and a fast-to-slow myofibre transition in the fast-twitch plantaris and tibialis that are significantly limited by the swimming programme. The running- and the swimming-based training programmes significantly decrease the severe muscular atrophy detected in all myofibre types in soleus, and in IIb myofibre in plantaris and tibialis ALS muscles as shown by the evolution of the cross-section area (CSA) of each fibre type (D). Data are shown as mean ±s.d. (n= 5; an average of 3 sections of each muscle per animal at 115 days old was used; *0.01 < P < 0.05, **0.001 < P < 0.01, ***P < 0.001, *control versus ALS groups, ‡sedentary ALS versus training ALS groups, +swimming ALS versus other ALS groups).

Figure 4

Differential effects of swimming- and running-based training programmes in maintaining the non-neuronal cell populations in the lumbar spinal cord of ALS mice A, representative images showing a population of astrocytes in control mice and in sedentary, running and swimming ALS mice using glial fibrillary acidic protein (GFAP) immunostaining.The quantification of astrocytes in the ventral horn of the lumbar spinal cord reveals a severe astrogliosis in sedentary and running ALS mice that is limited by swimming exercise (B). C, representative images showing a population of oligodendrocytes in control, sedentary, running and swimming ALS mice using CAII immunostaining. The unexpected ALS-induced decrease of oligodendrocyte population is limited by swimming exercise (D). E, representative images showing a population of neural progenitor cells (NPC) in control mice and sedentary, running and swimming ALS mice using nestin immunostaining. Cell quantification in hemi-spinal cord reveals a limited NPC proliferation in swimming-exercised mice (E). All scale bars: 100 μm. Number of cells are shown as mean ±s.d.; (n= 5; 10 sections of spinal cord per animal. ***P < 0.001, *control versus ALS mice or +swimming ALS versus other ALS groups or ‡sedentary ALS versus training ALS groups).

Figure 5

The swimming-based training limits cell death The swimming programme induces a reduction of apoptotic cells as shown by immunodetection of active caspase-3 in the ventral horn of the lumbar spinal cord from control mice and sedentary, running and swimming ALS mice at 115 days of age (A) and by Western blot (B) (n= 3 in each group). Scale bar: 100 μm.

Figure 6

Effects of swimming and running-based training programmes on the core body temperature of control mice Monitoring of core body temperature before (T0) and after training (T30). Chart representation of core body temperature for each animal (n= 12, ○); means are indicated by a bar. No significant difference could be observed before and after swimming- (P= 0.190) or running-based (P= 0.264) training.