Mitochondrial Defects in the Respiratory Complex I Contribute to Impaired Translational Initiation via ROS and Energy Homeostasis in SMA Motor Neurons

Abstract

BackgroundSpinal muscular atrophy (SMA) is a neuromuscular disease, characterized by loss of lower alpha motor neurons, which leads to proximal muscle weakness. SMA is caused by reduced levels of Survival of Motor Neuron (SMN) due to biallelic deletions or mutations in the SMN1 gene and mainly non-functional SMN2 copy gene. When SMN levels fall under a certain threshold, a plethora of cellular pathways are disturbed including RNA processing, protein synthesis, metabolic defects and dysfunctional mitochondria. Dysfunctional mitochondria can harm cells by decreased ATP production, but also by increased oxidative stress due to elevated production of reactive oxygen species (ROS). Since neurons mainly produce energy via mitochondrial oxidative phosphorylation to cover their high energy demands, restoring metabolic/ oxidative homeostasis can be beneficial in SMA pathology.MethodsWe performed whole proteome analysis of murine primary motor neurons using mass spectrometry to identify molecular mechanisms altered by SMN deficiency, which contribute to SMA pathology. Identified pathways were independently confirmed by biochemical and molecular biological methods as well as imaging analysis. Furthermore, cellular energy and ROS levels were biochemically measured in WT and SMA motor neurons.ResultsWe report that primary SMA motor neurons show disturbed energy homeostasis such as a reduced number of functional mitochondria, impaired glucose uptake and overall lower basal ATP concentrations. In addition, elevated ROS levels cause an increase of protein carbonylation and impaired protein synthesis efficiency in SMA motor neurons. Counteracting these cellular impairments with supplemented pyruvate reduced elevated ROS levels, increased ATP and SMN protein levels in SMA motor neurons. Furthermore, we found that pyruvate-mediated SMN protein synthesis is mTOR-dependent. Most importantly, we show that ROS regulates global protein synthesis at the translational initiation step, which is impaired in SMA.ConclusionIn summary, we found that excessive amount of cellular ROS caused by defective mitochondria inhibits initiation of mRNA translation, which results in pathological phenotypes often observed in degenerative neurons. As many neuropathies share patho-phenotypes such as dysfunctional mitochondria, excessive ROS and impaired protein synthesis.Our findings suggest a new molecular networking system among these pathways.

Genomic variants causing mitochondrial dysfunction are common in hereditary lower motor neuron disease

“…Naturally, the high metabolic requirements of neuronal cells result in a high dependency on mitochondrial ATP production through oxidative phosphorylation to uphold neuronal homeostasis and function. As a consequence, motor neurons are extremely sensitive to disbalances of mitochondrial physiology, and it is not surprising that mitochondrial dysfunction is a pathological process shared by ALS (Smith et al, 2019) and SMA (Acsadi et al, 2009; Miller et al, 2016; Thelen et al, 2020). Many neuromuscular disorders where mitochondrial dysfunction is not the primary underlying defect present with accompanying mitochondrial abnormalities (Katsetos et al, 2013), the most notable being SMN1 ‐related SMA (Acsadi et al, 2009; Miller et al, 2016; Thelen et al, 2020).…”