Transfer of pathogenic and nonpathogenic cytosolic proteins between spinal cord motor neurons in vivo in chimeric mice

Abstract

Recent studies have reported spread of pathogenic proteins in the mammalian nervous system, but whether nonpathogenic ones spread is unknown. We initially investigated whether spread of a mutant amyotrophic lateral sclerosis-associated cytosolic superoxide dismutase 1 (SOD1) protein between motor neurons could be detected in intact chimeric mice. Eight-cell embryos from G85R SOD1YFP and G85R SOD1CFP mice were aggregated, and spinal cords of adult chimeric progeny were examined for motor neurons with cytosolic double fluorescence. By 3 mo of age, we observed extensive double fluorescence, including in amyotrophic lateral sclerosisaffected cranial nerve motor nuclei but not in the relatively spared extraocular nuclei. Chimeras of nonpathogenic wtSOD1YFP and G85R SOD1CFP also exhibited double fluorescence. In a third chimera, mitochondrial mCherry did not transfer to G85R SOD1YFP motor neurons, suggesting that neither RNA nor organelles transfer, but mitomCherry neurons received G85R SOD1YFP. In a chimera of ChAT promoter-EGFP and mito-mCherry, EGFP efficiently transferred to mito-mCherry + cells. Thus, nonpathogenic cytosolic proteins appear capable of transfer. During study of both the SOD1FP and EGFP chimeras, we observed fluorescence also in small cells neighboring the motor neurons, identified as mature gray matter oligodendrocytes. Double fluorescence in the G85R SOD1FP chimera and observation of the temporal development of fluorescence first in motor neurons and then in these oligodendrocytes suggest that they may be mediators of transfer of cytosolic proteins between motor neurons.

Large SOD1 aggregates, unlike trimeric SOD1, do not impact cell viability in a model of amyotrophic lateral sclerosis

“…Instead, our results suggested that the SOD1 trimer, which may escape immunohistochemistry detection due to its small size and transient nature, was a primary origin of motor neuron impairment. Furthermore, research with chimeric mouse embryos expressing SOD1-GFP fusion proteins unveiled that pathogenic cytosolic SOD1 could transfer between spinal cord motor neurons (30). We posit that in comparison with insoluble fibrils, SOD1 trimers can translocate between cells more readily.…”

Prion‐like protein aggregates exploit the RHO GTPase to cofilin‐1 signaling pathway to enter cells

“…The symptoms then spread contiguously to adjacent regions by an orderly process (Gowers, ; Ravits & La Spada, ). The spreading of SOD1 aggregates from cell to cell by a prion‐like mechanism, which has not only been observed in cells (Grad et al , , ; Münch et al , ) but also in mice (Ayers et al , , ; Thomas et al , ), could explain the defining pathological hallmarks of ALS. With this model, the site of disease corresponds to the misfolding of the disease‐causing protein and the spreading of the symptoms results from the spreading of the SOD1 prion.…”

Aggregated SOD1 causes selective death of cultured human motor neurons

“…Interestingly, both the genetically-driven familial and the sporadic forms of ALS are clinically and pathologically similar, suggesting a possible common pathogenesis and a shared pathway of neurodegeneration. Indeed, SOD1 and TDP-43 protein aggregates, often associated with ubiquitin, can be found in postmortem spinal cord samples from both sporadic and familial ALS patients, as well as in animal models 21 – 23 .…”

“…The SOD1 protein is ubiquitously expressed but, in ALS, it causes specific MN degeneration despite the presence of SOD1 aggregates not only in MNs, but also in the nuclei of ventral horn astrocytes, microglia, and oligodendrocytes of familial ALS, as well as sporadic ALS, patients 24 . There is experimental evidence supporting uptake and propagation of pathological conformations of both SOD1 21 , 25 – 28 and TDP-43 29 , 30 , but the precise mechanism of SOD1 aggregate transfer is unknown. Consistent with the prion hypothesis, these proteins have been shown to induce a pathologic conformation on their natively folded counterparts in a template-directed manner 19 , 26 , 28 .…”

“…Studies performed in ALS mice found that the injection of mutant SOD1 spinal cord homogenates into the spinal cords of G85R-YFP-SOD1 mice induces ALS like pathology that spreads through neuroanatomical pathways 25 . In another study, mutant and wild-type (WT) SOD1 proteins can transfer between MNs in a process that might be mediated through other cell types, possibly oligodendrocytes, which in turn, might even enhance the aggregate toxicity 21 . Recent work further suggests that exosomes and tunneling nanotubes may be responsible for cell-to-cell transmission 29 , 30 .…”