Comparative Study of Structural and Functional Rearrangements in Skeletal Muscle Mitochondria of SOD1-G93A Transgenic Mice at Pre-, Early-, and Late-Symptomatic Stages of ALS Progression

Background: Amyotrophic lateral sclerosis (ALS) is a progressive multisystem disease characterized by limb and trunk muscle weakness that is attributed, in part, to abnormalities in mitochondrial ultrastructure and impaired mitochondrial functions. This study investigated the time course of structural and functional rearrangements in skeletal muscle mitochondria in combination with motor impairments in Tg (copper-zinc superoxide dismutase enzyme (SOD1) G93A) dl1/GurJ (referred to as SOD1-G93A/low) male mice, a familial ALS model, as compared with non-transgenic littermates. Methods: The neurological status and motor functions were assessed weekly using the paw grip endurance method and the grid suspension test with two-limb and four-limb suspension tasks. Transmission electron microscopy followed by quantitative analysis was performed to study ultrastructural alterations in the quadriceps femoris. Functional analysis of skeletal muscle mitochondria was performed using high-resolution Oxygraph-2k (O2K) respirometry and methods for assessing the calcium retention capacity index and the content of lipid peroxidation products in freshly isolated preparations. Results: Based on the behavioral phenotyping data, specific age groups were identified: postnatal day 56 (P56) (n = 10–11), 84 (P84) (n = 10–11), and 156 (P154) (n = 10–12), representing the pre-symptomatic, early-symptomatic and late-symptomatic stages of ALS progression in SOD1-G93A/low mice, respectively. Electron microscopy showed mosaic destructive changes in subsarcolemmal mitochondria in fibers of the quadriceps femoris from 84-day-old SOD1-G93A/low mice. Morphometric analysis revealed an elevation in the mean size of the mitochondria in SOD1-G93A mice at P84 and P154. In addition, the P154 transgenic group demonstrated a decrease in sarcomere width and the number of mitochondria per unit area. At the symptomatic stage, SOD1-G93A mice exhibited a decreased respiratory control ratio, ADP-stimulated, and uncoupled respiration rates of mitochondria isolated from the quadriceps femoris muscle, as measured by high-resolution respirometry. In parallel, the mitochondria showed lower calcium retention capacity and increased levels of lipid peroxidation products compared with the control. Conclusions: Taken together, these results indicate stage-dependent changes in skeletal muscle mitochondrial ultrastructure and functions associated with defective oxidative phosphorylation, impaired calcium homeostasis, and oxidative damage in the SOD1-G93A/low mouse model, which appears to be a promising direction for the development of combination therapies for ALS.