Alteration of Mitochondrial Integrity as Upstream Event in the Pathophysiology of SOD1-ALS

Abstract

Little is known about the early pathogenic events by which mutant superoxide dismutase 1 (SOD1) causes amyotrophic lateral sclerosis (ALS). This lack of mechanistic understanding is a major barrier to the development and evaluation of efficient therapies. Although protein aggregation is known to be involved, it is not understood how mutant SOD1 causes degeneration of motoneurons (MNs). Previous research has relied heavily on the overexpression of mutant SOD1, but the clinical relevance of SOD1 overexpression models remains questionable. We used a human induced pluripotent stem cell (iPSC) model of spinal MNs and three different endogenous ALS-associated SOD1 mutations (D90Ahom, R115Ghet or A4Vhet) to investigate early cellular disturbances in MNs. Although enhanced misfolding and aggregation of SOD1 was induced by proteasome inhibition, it was not affected by activation of the stress granule pathway. Interestingly, we identified loss of mitochondrial, but not lysosomal, integrity as the earliest common pathological phenotype, which preceded elevated levels of insoluble, aggregated SOD1. A super-elongated mitochondrial morphology with impaired inner mitochondrial membrane potential was a unifying feature in mutant SOD1 iPSC-derived MNs. Impaired mitochondrial integrity was most prominent in mutant D90Ahom MNs, whereas both soluble disordered and detergent-resistant misfolded SOD1 was more prominent in R115Ghet and A4Vhet mutant lines. Taking advantage of patient-specific models of SOD1-ALS in vitro, our data suggest that mitochondrial dysfunction is one of the first crucial steps in the pathogenic cascade that leads to SOD1-ALS and also highlights the need for individualized medical approaches for SOD1-ALS.

Downstream Effects of Mutations in SOD1 and TARDBP Converge on Gene Expression Impairment in Patient-Derived Motor Neurons

“…Patients carrying D90A SOD1 mutation showed less phenotypic variability than patients with other Cu/Zn SOD1 mutations, with the exception of the variability of the age of onset of first symptoms (patients show the widest span of 74 years) [ 41 ]. Cellularly, D90A mutant iPSC-derived motor neurons yield also different mitochondrial and aggregation phenotypes compared to other SOD1 mutations [ 42 ]. Since our intention was to search and address for overarching phenotypes we thus included also a D90A cell line.…”

“…All these cell lines have been previously characterized including the acquisition of classical spinal motor neurons markers (described above), electrophysiological function and the sequential appearance of progressive neurodegeneration. Please refer to the citations for detailed information [ 42 , 46 , 47 ]. In summary, immunolabeling detected the presence of neuronal (TuJ1 80–90%, MAP2 80–90%) and motor neuron specific markers (SMI32 70–75%) in the cells without significant differences in neuron morphologies between wildtype controls ( n = 3 subjects, 1 clone each) and ALS mutants (SOD1 n = 2 subjects, 1 clone each; TDP43 n = 2 subjects, 2 clones of one subject and 1 clone of second subject).…”

“…In summary, immunolabeling detected the presence of neuronal (TuJ1 80–90%, MAP2 80–90%) and motor neuron specific markers (SMI32 70–75%) in the cells without significant differences in neuron morphologies between wildtype controls ( n = 3 subjects, 1 clone each) and ALS mutants (SOD1 n = 2 subjects, 1 clone each; TDP43 n = 2 subjects, 2 clones of one subject and 1 clone of second subject). There was no difference between wildtype and mutants SOD1 [ 42 , 46 ] and TDP43 [ 47 ].…”

“…In parallel, the medium constitution was changed: Instead of PMA and RA, 10 ng/µL BDNF, 500 µM dbcAMP and 10 ng/µL GDNF was added to N2B27 ensuring neuronal maturation. Motor neurons were harvested after being cultured for 30 days [ 42 , 46 , 47 ].…”

Mitochondrial dysfunction of induced pluripotent stem cells-based neurodegenerative disease modeling and therapeutic strategy

“…Moreover, the transcriptomic analysis showed decreased gene expression of the mitochondrial ETC, which was restored by overexpression of PGC1α to stimulate mitochondrial biogenesis and improve mitochondrial function ( Mehta et al, 2021 ). Günther et al reported that SOD1 mutant iPSC-derived MNs displayed low levels of ATP without other mitochondrial metabolic changes ( Günther et al, 2022 ). Hor and colleagues used three sALS iPSC line-derived MNs and three fALS iPSC line-derived MNs carrying SOD1, TDP43, and C9ORF72 mutants to find reductions in basal respiration, ATP production, and spare respiratory capacity, causing increased glycolysis and lactate.…”