Intercellular propagated misfolding of wild-type Cu/Zn superoxide dismutase occurs via exosome-dependent and -independent mechanisms.

Amyotrophic lateral sclerosis (ALS) is predominantly sporadic, but associated with heritable genetic mutations in 5-10% of cases, including those in Cu/Zn superoxide dismutase (SOD1). We previously showed that misfolding of SOD1 can be transmitted to endogenous human wild-type SOD1 (HuWtSOD1) in an intracellular compartment. Using NSC-34 motor neuron-like cells, we now demonstrate that misfolded mutant and HuWtSOD1 can traverse between cells via two nonexclusive mechanisms: protein aggregates released from dying cells and taken up by macropinocytosis, and exosomes secreted from living cells. Furthermore, once HuWtSOD1 propagation has been established, misfolding of HuWtSOD1 can be efficiently and repeatedly propagated between HEK293 cell cultures via conditioned media over multiple passages, and to cultured mouse primary spinal cord cells transgenically expressing HuWtSOD1, but not to cells derived from nontransgenic littermates. Conditioned media transmission of HuWtSOD1 misfolding in HEK293 cells is blocked by HuWtSOD1 siRNA knockdown, consistent with human SOD1 being a substrate for conversion, and attenuated by ultracentrifugation or incubation with SOD1 misfolding-specific antibodies, indicating a relatively massive transmission particle which possesses antibody-accessible SOD1. Finally, misfolded and protease-sensitive HuWtSOD1 comprises up to 4% of total SOD1 in spinal cords of patients with sporadic ALS (SALS). Propagation of HuWtSOD1 misfolding, and its subsequent cell-to-cell transmission, is thus a candidate process for the molecular pathogenesis of SALS, which may provide novel treatment and biomarker targets for this devastating disease.

Extracellular wildtype and mutant SOD1 induces ER-Golgi pathology characteristic of amyotrophic lateral sclerosis in neuronal cells.

Amyotrophic lateral sclerosis (ALS) is a fatal and rapidly progressing neurodegenerative disorder and the majority of ALS is sporadic, where misfolding and aggregation of Cu/Zn-superoxide dismutase (SOD1) is a feature shared with familial mutant-SOD1 cases. ALS is characterized by progressive neurospatial spread of pathology among motor neurons, and recently the transfer of extracellular, aggregated mutant SOD1 between cells was demonstrated in culture. However, there is currently no evidence that uptake of SOD1 into cells initiates neurodegenerative pathways reminiscent of ALS pathology. Similarly, whilst dysfunction to the ER-Golgi compartments is increasingly implicated in the pathogenesis of both sporadic and familial ALS, it remains unclear whether misfolded, wildtype SOD1 triggers ER-Golgi dysfunction. In this study we show that both extracellular, native wildtype and mutant SOD1 are taken up by macropinocytosis into neuronal cells. Hence uptake does not depend on SOD1 mutation or misfolding. We also demonstrate that purified mutant SOD1 added exogenously to neuronal cells inhibits protein transport between the ER-Golgi apparatus, leading to Golgi fragmentation, induction of ER stress and apoptotic cell death. Furthermore, we show that extracellular, aggregated, wildtype SOD1 also induces ER-Golgi pathology similar to mutant SOD1, leading to apoptotic cell death. Hence extracellular misfolded wildtype or mutant SOD1 induce dysfunction to ER-Golgi compartments characteristic of ALS in neuronal cells, implicating extracellular SOD1 in the spread of pathology among motor neurons in both sporadic and familial ALS.

Extracellular aggregated Cu/Zn superoxide dismutase activates microglia to give a cytotoxic phenotype.

A large body of literature suggests that amyotrophic lateral sclerosis (ALS) pathology is intimately linked with neuroinflammation, specifically activation and recruitment of microglia and astrocytes. The actual cause of gliosis is unclear. Extracellular Cu/Zn superoxide dismutase (SOD1) has recently been shown to activate microglia in a CD14 dependant mechanism providing one potential pathway by which glial cells become activated. As protein inclusions are thought to be an important part of ALS pathology and are associated with all forms of ALS, we sought to determine if aggregated SOD1 would activate microglia. Recombinant SOD1 was aggregated and this, or monomeric forms of SOD1 were then added to EOC.13 microglial cells or primary microglial cells in culture. Although monomeric mutant SOD1 has been shown to promote microglial activation in the past, we found that aggregated SOD1 was able to much more efficiently activate microglia in culture when compared with the unaggregated form of mutant SOD1. In addition to CD14 dependant pathways, aggregated SOD1 also bound to the surface of glial cells and was internalized in a lipid raft and scavenger receptor dependent manner. We have for the first time shown that aggregated mutant SOD1 potently activates microglia. These results suggest that there may be a potential link between protein aggregation and microglial activation in ALS.

Addition of exogenous SOD1 aggregates causes TDP-43 mislocalisation and aggregation.

ALS is characterised by a focal onset of motor neuron loss, followed by contiguous outward spreading of pathology throughout the nervous system, resulting in paralysis and death generally within a few years after diagnosis. The aberrant release and uptake of toxic proteins including SOD1 and TDP-43 and their subsequent propagation, accumulation and deposition in motor neurons may explain such a pattern of pathology. Previous work has suggested that the internalization of aggregates triggers stress granule formation. Given the close association of stress granules and TDP-43, we wondered whether internalisation of SOD1 aggregates stimulated TDP-43 cytosolic aggregate structures. Addition of recombinant mutant G93A SOD1 aggregates to NSC-34 cells was found to trigger a rapid shift of TDP-43 to the cytoplasm where it was still accumulated after 48 h. In addition, SOD1 aggregates also triggered cleavage of TDP-43 into fragments including a 25 kDa fragment. Collectively, this study suggests a role for protein aggregate uptake in TDP-43 pathology.

Flow cytometric measurement of the cellular propagation of TDP-43 aggregation.

Amyotrophic lateral sclerosis is a devastating neuromuscular degenerative disease characterized by a focal onset of motor neuron loss, followed by contiguous outward spreading of pathology including TAR DNA-binding protein of 43 kDa (TDP-43) aggregates. Previous work suggests that TDP-43 can move between cells. Here we used a novel flow cytometry technique (FloIT) to analyze TDP-43 inclusions and propagation. When cells were transfected to express either mutant G294A TDP-43 fused to GFP or wild type TDP-43fused to tomato red and then co-cultured, flow cytometry detected intact cells containing both fusion proteins and using FloIT detected an increase in the numbers of inclusions in lysates from cells expressing wild type TDP-43-tomato. Furthermore, in this same model, FloIT analyses detected inclusions containing both fusion proteins. These results imply the transfer of TDP-43 fusion proteins between cells and that this process can increase aggregation of wild-type TDP-43 by a mechanism involving co-aggregation with G294A TDP-43.

The role of macropinocytosis in the propagation of protein aggregation associated with neurodegenerative diseases.

With the onset of the rapidly aging population, the impact of age related neurodegenerative diseases is becoming a predominant health and economic concern. Neurodegenerative diseases such as Alzheimer's disease, Creutzfeldt-Jakob disease (CJD), Parkinson's disease, Huntington's disease, frontotemporal dementia (FTD), and amyotrophic lateral sclerosis (ALS) result from the loss of a specific subsets of neurons, which is closely associated with accumulation and deposition of specific protein aggregates. Protein aggregation, or fibril formation, is a well-studied phenomenon that occurs in a nucleation-dependent growth reaction. Recently, there has been a swell of literature implicating protein aggregation and its ability to propagate cell-to-cell in the rapid progression of these diseases. In order for protein aggregation to be kindled in recipient cells it is a requisite that aggregates must be able to be released from one cell and then taken up by others. In this article we will explore the relationship between protein aggregates, their propagation and the role of macropinocytosis in their uptake. We highlight the ability of neurons to undergo stimulated macropinocytosis and identify potential therapeutic targets.

SOD1 protein aggregates stimulate macropinocytosis in neurons to facilitate their propagation.

BACKGROUND: Amyotrophic Lateral Sclerosis is characterized by a focal onset of symptoms followed by a progressive spread of pathology that has been likened to transmission of infectious prions. Cell-to-cell transmission of SOD1 protein aggregates is dependent on fluid-phase endocytosis pathways, although the precise molecular mechanisms remain to be elucidated. RESULTS: We demonstrate in this paper that SOD1 aggregates interact with the cell surface triggering activation of Rac1 and subsequent membrane ruffling permitting aggregate uptake via stimulated macropinocytosis. In addition, other protein aggregates, including those associated with neurodegenerative diseases (TDP-43, Httex146Q, α-synuclein) also trigger membrane ruffling to gain entry into the cell. Aggregates are able to rupture unstructured macropinosomes to enter the cytosol allowing propagation of aggregation to proceed. CONCLUSION: Thus, we conclude that in addition to basic proteostasis mechanisms, pathways involved in the activation of macropinocytosis are key determinants in the spread of pathology in these misfolding diseases.