Combined MRI and ³¹P-MRS investigations of the ACTA1(H40Y) mouse model of nemaline myopathy show impaired muscle function and altered energy metabolism

Abstract

Nemaline myopathy (NM) is the most common disease entity among non-dystrophic skeletal muscle congenital diseases. Mutations in the skeletal muscle α-actin gene (ACTA1) account for ∼25% of all NM cases and are the most frequent cause of severe forms of NM. So far, the mechanisms underlying muscle weakness in NM patients remain unclear. Additionally, recent Magnetic Resonance Imaging (MRI) studies reported a progressive fatty infiltration of skeletal muscle with a specific muscle involvement in patients with ACTA1 mutations. We investigated strictly noninvasively the gastrocnemius muscle function of a mouse model carrying a mutation in the ACTA1 gene (H40Y). Skeletal muscle anatomy (hindlimb muscles and fat volumes) and energy metabolism were studied using MRI and (31)Phosphorus magnetic resonance spectroscopy. Skeletal muscle contractile performance was investigated while applying a force-frequency protocol (from 1-150 Hz) and a fatigue protocol (80 stimuli at 40 Hz). H40Y mice showed a reduction of both absolute (-40%) and specific (-25%) maximal force production as compared to controls. Interestingly, muscle weakness was associated with an improved resistance to fatigue (+40%) and an increased energy cost. On the contrary, the force frequency relationship was not modified in H40Y mice and the extent of fatty infiltration was minor and not different from the WT group. We concluded that the H40Y mouse model does not reproduce human MRI findings but shows a severe muscle weakness which might be related to an alteration of intrinsic muscular properties. The increased energy cost in H40Y mice might be related to either an impaired mitochondrial function or an alteration at the cross-bridges level. Overall, we provided a unique set of anatomic, metabolic and functional biomarkers that might be relevant for monitoring the progression of NM disease but also for assessing the efficacy of potential therapeutic interventions at a preclinical level.

Figure 1. Typical representative axial MR images obtained from WT (A) and H40Y (C) hindlimb muscles and the corresponding automatic segmentation allowing the separation of subcutaneous fat (red), intermuscular fat (blue), skeletal muscle (yellow) and bone/vessels/connective tissues (purple) for WT (B) and H40Y (D) hindlimb.

H40Y group showed large muscle atrophy as compared to WT group.

Figure 2. Absolute maximal force production (A), specific (B) and relative (C) force production during the force-frequency protocol in H40Y (○) and WT (•) groups.

Force was normalized to hindlimb muscles volume (B) and to maximal force obtained at 150 Hz (C). f₅₀ (inset fig. C) represents the frequency providing 50% of maximal force. Maximal force was lower in H40Y group as compared to WT group whereas f₅₀ was similar between the two groups. Values are presented as mean ± SEM. Significantly different between groups *P<0.05.

Figure 3. Specific force production during the stimulation protocol (A) and fatigue index (B) in H40Y (○) and WT (•) groups.

Force was normalized to hindlimb muscle volume. H40Y group showed a lower force production and an improved resistance to fatigue as compared to the WT group. Values are presented as mean ± SEM. Significantly different between groups *P<0.05.

Figure 4. Example of resting ³¹P-MR spectra in H40Y group (A) and WT group (B).

PCr to ATP ratios were calculated from the peak area of the PCr and β-ATP.

Figure 5. Changes in gastrocnemius PCr (% resting value; A), Pi (% resting value; B), and pHi (C) during the stimulation protocol were similar in H40Y (○) and WT (•) groups.

Values are presented as mean ± SEM.