Accumulation of misfolded SOD1 outlines distinct patterns of motor neuron pathology and death during disease progression in a SOD1^G93A mouse model of amyotrophic lateral sclerosis

Abstract

Early misfolded superoxide dismutase 1 (mfSOD1) accumulation, motor neuron (MN) degeneration, and microgliosis are hallmark pathological features in SOD1 G93A amyotrophic lateral sclerosis (ALS) mice. Because of the different vulnerabilities of distinct MN subtypes, degenerating and surviving MNs coexist in different proportions during disease progression. By examining the expression of misfolded conformers of SOD1 using specific antibodies, we defined distinct MN phenotypes that were evaluated during disease progression and the local neuroinflammatory reaction. The most severe phenotype corresponded to somata of fast-twitch subtype MNs, which exhibited highly positive mfSOD1 immunostaining and an extreme degree of vacuolar degeneration. Vacuoles, which are of mitochondrial origin, contain mfSOD1 in conjunction with nonmitochondrial proteins, such as chromogranin, CD81, and flotillin. The fusion of ER-derived vesicles enriched in mfSOD1 with outer mitochondrial membranes is thought to be the primary mechanism for vacuole formation. In addition, the ulterior coalescence of enlarged mitochondria may lead to the formation of giant vacuoles. Vacuolar degeneration is a transient degenerative process occurring early during the presymptomatic stages of the disease in ALS mice. Some vacuolated MNs are also positive for pMLKL, the effector protein of necroptosis. This indicates a newly described mechanism in which extracellular vesicles derived from damaged MNs, via cellular secretion or necroptotic disruption, may be the triggers for initiating neuroinflammation, glial-mediated neurotoxicity, and disease spreading. Furthermore, as MN degeneration in mutant SOD1 mice is noncell autonomous, the effects of experimentally increasing or decreasing the microglial response on the expression of MN phenotypes were also evaluated, demonstrating bidirectional cross talk signaling between the degree of expression of mfSOD1 and local neuroinflammation. More detailed knowledge regarding these processes occurring long before the end stages of the disease is necessary to identify novel molecular targets for future preclinical testing.

Smart Nanoscale Extracellular Vesicles in the Brain: Unveiling their Biology, Diagnostic Potential, and Therapeutic Applications

“…A recent study identified a necroptotic motor neuron subtype in the SOD1G93A mutant mice, which displayed the presence of large vacuoles that were positive for the exosome marker CD81 and flotillin in immunofluorescence analysis. 158 However, a direct correlation of the observed exosome-positive vacuoles as the only source of ALS is not shown; the study confirms that the degenerating MNs employ EVs to mediate neuroinflammation and disease spread in ALS. Another aspect of ALS pathology is proteinopathy, which involves the aggregation of Tdp-43 in the brain.…”

“…Intriguingly, the degenerating MNs can act as a potent contributor to ALS progression and pathology in the later stages of the disease. A recent study identified a necroptotic motor neuron subtype in the SOD1G93A mutant mice, which displayed the presence of large vacuoles that were positive for the exosome marker CD81 and flotillin in immunofluorescence analysis . However, a direct correlation of the observed exosome-positive vacuoles as the only source of ALS is not shown; the study confirms that the degenerating MNs employ EVs to mediate neuroinflammation and disease spread in ALS.…”

Conformational fingerprinting with Raman spectroscopy reveals protein structure as a translational biomarker of muscle pathology

“…Protein misfolding is a feature of SOD1 ALS, with accumulations particularly prominent within the spinal cord in both human patients and rodent models. 41–43 Increases in misfolded proteins have also been reported in the context of denervation due to nerve ligation. 44 Thus, it is possible that the more modest structural alterations we observed in SOD1 G93A mice are a general phenomenon relating to denervation, rather than specific to ALS.…”

Trends in global amyotrophic lateral sclerosis research from 2000 to 2022: A bibliometric analysis

“…Although the relative frequency of SOD1 gradually decreased, its absolute frequency remained stable. This phenomenon may be attributed to the evolution of SOD1 research topics, from loss-of-function and oxidative stress in the beginning to prion-like transmission mechanisms in recent years ( Rotunno and Bosco, 2013 ; Silverman et al, 2016 ; Salvany et al, 2022 ). Some topics, such as excitotoxicity and apoptosis, have gradually attracted less attention.…”