Skeletal Muscle Mitochondrial and Autophagic Dysregulation Are Modifiable in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a health- and life-limiting neuromuscular disorder. Although varying degrees of mitochondrial abnormalities have been documented in SMA skeletal muscle, the influence of disease progression on pathways that govern organelle turnover and dynamics are poorly understood. Thus, the purpose of this study was to investigate skeletal muscle mitochondria during SMA disease progression and determine the effects of therapeutic modalities on organelle biology.

Smn^2B/+ and Smn^2B/- severe SMA-like mice were used to investigate mitochondrial turnover and dynamics signalling. Muscles were analysed at postnatal day 9 (P9), P13 or P21 to address pre-symptomatic, early symptomatic and late symptomatic periods of the disorder. Additionally, we utilized an acute dose of exercise and urolithin A (UA) to stimulate organelle remodelling in skeletal muscle of SMA mice in vivo and in SMA patient-derived myotubes in vitro, respectively.

Smn^2B/+ and Smn^2B/- mice demonstrated similar levels of muscle mitochondrial oxidative phosphorylation (OxPhos) proteins throughout disease progression. In contrast, at P21 the mRNA levels of upstream factors important for the transcription of mitochondrial genes encoded by the nuclear and mitochondrial DNA, including nuclear respiratory factor 2, sirtuin 1, mitochondrial transcription factor A and tumour protein 53, were upregulated (+31%-195%, p < 0.05) in Smn^2B/- mice relative to Smn^2B/+. Early and late symptomatic skeletal muscle from SMA-like mice showed greater autophagosome formation as denoted by more phosphorylated autophagy related 16-like 1 (p-ATG16L1^Ser278) puncta (+60%-80%, p < 0.05), along with a build-up of molecules indicative of damaged mitochondria such as BCL2 interacting protein 3, Parkin and PTEN-induced kinase 1 (+100%-195%, p < 0.05). Furthermore, we observed a fragmented mitochondrial phenotype at P21 that was concomitant with abnormal splicing of Optic atrophy 1 transcripts (-53%, p < 0.05). A single dose of exercise augmented the expression of citrate synthase (+43%, p < 0.05) and corrected the over-assembly of autophagosomes (-64%, p < 0.05). In patient muscle cells, UA treatment stimulated autophagic flux, increased the expression of OxPhos proteins (+15%-47%, p < 0.05) and improved maximal oxygen consumption (+84%, p < 0.05).

Abnormal skeletal muscle mitochondrial turnover and dynamics are associated with disease progression in Smn^2B/- mice despite compensatory elevations in upstream factors important for organelle synthesis and recycling. Exercise and UA enhance mitochondrial health in skeletal muscle, which indicates that lifestyle-based and pharmacological interventions may be effective countermeasures targeting the organelle for therapeutic remodelling in SMA.