Multimodal MRI and 31P-MRS Investigations of the ACTA1(Asp286Gly) Mouse Model of Nemaline Myopathy Provide Evidence of Impaired In Vivo Muscle Function, Altered Muscle Structure and Disturbed Energy Metabolism

Charlotte Gineste,David Bendahan,Christophe Vilmen,Kristen J Nowak,Yann Le Fur,Julien Gondin,Patrick J Cozzone,Guillaume Duhamel,Ruben Artero

doi:10.1371/journal.pone.0072294

Abstract

Nemaline myopathy (NM), the most common non-dystrophic congenital disease of skeletal muscle, can be caused by mutations in the skeletal muscle α-actin gene (ACTA1) (~25% of all NM cases and up to 50% of severe forms of NM). Muscle function of the recently generated transgenic mouse model carrying the human Asp286Gly mutation in the ACTA1 gene (Tg(ACTA1)Asp286Gly) has been mainly investigated in vitro. Therefore, we aimed at providing a comprehensive picture of the in vivo hindlimb muscle function of Tg(ACTA1)Asp286Gly mice by combining strictly noninvasive investigations. Skeletal muscle anatomy (hindlimb muscles, intramuscular fat volumes) and microstructure were studied using multimodal magnetic resonance imaging (Dixon, T2, Diffusion Tensor Imaging [DTI]). Energy metabolism was studied using 31-phosphorus Magnetic Resonance Spectroscopy (31P-MRS). Skeletal muscle contractile performance was investigated while applying a force-frequency protocol (1–150 Hz) and a fatigue protocol (6 min–1.7 Hz). Tg(ACTA1)Asp286Gly mice showed a mild muscle weakness as illustrated by the reduction of both absolute (30%) and specific (15%) maximal force production. Dixon MRI did not show discernable fatty infiltration in Tg(ACTA1)Asp286Gly mice indicating that this mouse model does not reproduce human MRI findings. Increased T2 values were observed in Tg(ACTA1)Asp286Gly mice and might reflect the occurrence of muscle degeneration/regeneration process. Interestingly, T2 values were linearly related to muscle weakness. DTI experiments indicated lower λ2 and λ3 values in Tg(ACTA1)Asp286Gly mice, which might be associated to muscle atrophy and/or the presence of histological anomalies. Finally 31P-MRS investigations illustrated an increased anaerobic energy cost of contraction in Tg(ACTA1)Asp286Gly mice, which might be ascribed to contractile and non-contractile processes. Overall, we provide a unique set of information about the anatomic, metabolic and functional consequences of the Asp286Gly mutation that might be considered as relevant biomarkers for monitoring the severity and/or the progression of NM and for assessing the efficacy of potential therapeutic interventions.

Highlights

Nemaline myopathy (NM) is the most common form of nondystrophic congenital myopathies and is characterized by muscle dysfunction and the presence of rod shaped structures in the muscle fibers [1]
On the basis of quantitative information related to high-energy phosphorylated compounds and pH in exercising muscle using 31-phosphorus Magnetic Resonance Spectroscopy (31P-MRS) [29,30,31], we recently demonstrated that the energy cost of contraction was higher for the Acta1(H40Y) mice as compared to controls [32]
Considering the rarity of NM, the well-known limitations linked to the analysis of biopsy samples and the possible differences between in vitro and in vivo measurements of muscle function, we aimed at investigating in vivo the functional, anatomical and metabolic consequences of the ACTA1 Asp286Gly mutation in a recently generated transgenic NM mouse model by utilising a strictly noninvasive approach

Summary

Introduction

Nemaline myopathy (NM) is the most common form of nondystrophic congenital myopathies and is characterized by muscle dysfunction (almost always weakness) and the presence of rod shaped structures in the muscle fibers [1]. The more recent model expresses the ACTA1 (Asp286Gly) transgene containing a mutation previously identified in a patient with a severe form of NM [5]. This model has been generated either with [11] or without [10] an enhanced green fluorescent protein (EGFP)-tag and both reproduce the usual main clinical feature of NM, namely muscle weakness, as illustrated by the reduced maximal force production in isolated soleus and EDL muscles [10,11]. It becomes questionable as to whether and to what extent the Asp286Gly mutation affects the in vivo muscle function in Tg(ACTA1)Asp286Gly mice

Methods

Results

Conclusion