Spinal Muscular Atrophy autophagy profile is tissue-dependent: differential regulation between muscle and motoneurons

Abstract

Spinal muscular atrophy (SMA) is a neuromuscular genetic disease caused by reduced survival motor neuron (SMN) protein. SMN is ubiquitous and deficient levels cause spinal cord motoneurons (MNs) degeneration and muscle atrophy. Nevertheless, the mechanism by which SMN reduction in muscle contributes to SMA disease is not fully understood. Therefore, studies evaluating atrophy mechanisms in SMA muscles will contribute to strengthening current knowledge of the pathology. Here we propose to evaluate autophagy in SMA muscle, a pathway altered in myotube atrophy. We analized autophagy proteins and mTOR in muscle biopsies, fibroblasts, and lymphoblast cell lines from SMA patients and in gastrocnemius muscles from a severe SMA mouse model. Human MNs differentiated from SMA and unaffected control iPSCs were also included in the analysis of the autophagy. Muscle biopsies, fibroblasts, and lymphoblast cell lines from SMA patients showed reduction of the autophagy marker LC3-II. In SMA mouse gastrocnemius, we observed lower levels of LC3-II, Beclin 1, and p62/SQSTM1 proteins at pre-symptomatic stage. mTOR phosphorylation at Ser2448 was decreased in SMA muscle cells. However, in mouse and human cultured SMA MNs mTOR phosphorylation and LC3-II levels were increased. These results suggest a differential regulation in SMA of the autophagy process in muscle cells and MNs. Opposite changes in autophagy proteins and mTOR phosphorylation between muscle cells and neurons were observed. These differences may reflect a specific response to SMN reduction, which could imply diverse tissue-dependent reactions to therapies that should be taken into account when treating SMA patients.

A Compound Heterozygous Mutation in Calpain 1 Identifies a New Genetic Cause for Spinal Muscular Atrophy Type 4 (SMA4)

“…Accordingly, quantification of the immunofluorescent staining shows no changes in the levels of cytosolic LC3B positive puncta between the SMA4 patient fibroblasts and controls (Figure 4B). Although recent data indicates a tissue-specific regulation in SMA of the autophagy process (Sansa et al, 2021b), our data suggest that the CAPN1 compound heterozygous mutation doesn't impact the autophagy process in SMA4-derived cells. Mitochondrial dysfunction has been extensively associated with SMA pathophysiology and it is evident across the clinical spectrum of SMA [reviewed at (James et al, 2021)].…”

“…Patient fibroblasts have proven to be a suitable model to investigate pathomechanisms in neurodegenerative diseases (Auburger et al, 2012), however the precise pathways activated by genetic mutations can be overlooked when not using complementary approaches, including spinal cord motor neurons and in vivo models (Juneja et al, 2019). In SMA, this has been recently illustrated by the vastly different autophagy profile between muscle, motor neurons and skin fibroblasts from a SMA patient (Sansa et al, 2021b). Further studies using neuronal in vitro and in vivo models bearing the p. G492R/p.…”

Brain-protective mechanisms of autophagy associated circRNAs: Kick starting self-cleaning mode in brain cells via circRNAs as a potential therapeutic approach for neurodegenerative diseases

“…SMA is a devastating genetic neuromuscular condition and the primary cause of infant death, which weakens the muscles resulting in low amounts of the Survival Motor Neuron (SMN) protein ( Sansa et al, 2021 ). There are five types of SMA: Types 0, 1, 2, 3, and 4.…”

An isogenic human iPSC model unravels neurodevelopmental abnormalities in SMA

“…The nuclear chaperone role of SMN was the first described and is the best molecularly characterized SMN function to date. However, SMN has been involved in numerous other intracellular pathways, such as axonal transport of mRNAs and RNPs, ribosomal dynamics and translation (Lauria et al, 2020), mitochondrial trafficking (James et al, 2021; Zilio et al, 2022), endosomal and membrane recycling pathways (Dimitriadi et al, 2016; Riessland et al, 2017; Hosseinibarkooie et al, 2016) and autophagy (Rodriguez-Muela et al, 2018; Sansa et al, 2021). SMN binds to a large variety of proteins engaged in multiple cellular mechanisms; it is therefore expected that a better understanding of the protein’s biology will uncover additional SMN functions.…”