Parkinson's disease is a debilitating, age-associated movement disorder. A central aspect of the pathophysiology of Parkinson's disease is the progressive demise of midbrain dopamine neurons and ...their axonal projections, but the underlying causes of this loss are unclear. Advances in genetics and experimental model systems have illuminated an important role for defects in intracellular transport pathways to lysosomes. The accumulation of altered proteins and damaged mitochondria, particularly at axon terminals, ultimately might overwhelm the capacity of intracellular disposal mechanisms. Cell-extrinsic mechanisms, including inflammation and prion-like spreading, are proposed to have both protective and deleterious functions in Parkinson's disease.
Astrocytes are the most abundant cell type in the central nervous system (CNS) and secrete various factors that regulate neuron development, function and connectivity. microRNAs (miRNAs) are small ...regulatory RNAs involved in posttranslational gene regulation. Recent findings showed that miRNAs are exchanged between cells via nanovesicles called exosomes. In this study, we sought to define which miRNAs are contained within exosomes secreted by astrocytes. We also explored whether astroglial miRNA secretion via exosomes is perturbed in a mouse model of amyotrophic lateral sclerosis (ALS), a neurodegenerative disease where astrocytes play a crucial role in driving disease progression.
By isolating and profiling the expression of miRNAs from primary mouse astrocytes and from the exosomes that astrocytes secrete, we compared miRNA expression in the cells and secreted vesicles. We established that miRNA expression profiles of astrocytes and their exosomes are vastly different. In addition, we determined that exosomal miRNA expression in astrocytes is not significantly perturbed in a mouse model of ALS.
Astrocytes secrete numerous miRNAs via exosomes and miRNA species contained in exosomes are considerably different from miRNAs detectable in astrocytes, suggesting the existence of a mechanism to select certain miRNAs for inclusion or exclusion from exosomes. The exosomal miRNA profiling dataset we have generated will provide a resource to aid in the investigation of this selection mechanism. Finally, the miRNA expression profile in astrocyte-secreted exosomes is not perturbed by expression of mutant SOD1-G93A.
Neurodegeneration is a feature of many debilitating, incurable diseases that are rapidly rising in prevalence, such as Parkinson's disease. There is an urgent need to develop new and more effective ...therapeutic strategies to combat these devastating diseases. Models - from cell-based systems, to unicellular organisms, to complex animals - have proven to be a useful tool to help the research community shed light on the mechanisms underlying neurodegenerative diseases, and these advances have now begun to provide promising therapeutic avenues. In this themed issue of Disease Models & Mechanisms, a special collection of articles focused on neurodegenerative diseases is introduced. The collection includes original research articles that provide new insights into the complex pathophysiology of such diseases, revealing candidate biomarkers or therapeutic targets. Some of the articles describe a new disease model that enables deeper exploration of key mechanisms. We also present a series of reviews that highlight some of the recent translational advances made in studies of neurodegenerative diseases. In this Editorial, we summarize the articles featured in this collection, emphasizing the impact that model-based studies have made in this exciting area of research.
TDP-43 and FUS are RNA-binding proteins that form cytoplasmic inclusions in some forms of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Moreover, mutations in ...TDP-43 and FUS are linked to ALS and FTLD. However, it is unknown whether TDP-43 and FUS aggregate and cause toxicity by similar mechanisms. Here, we exploit a yeast model and purified FUS to elucidate mechanisms of FUS aggregation and toxicity. Like TDP-43, FUS must aggregate in the cytoplasm and bind RNA to confer toxicity in yeast. These cytoplasmic FUS aggregates partition to stress granule compartments just as they do in ALS patients. Importantly, in isolation, FUS spontaneously forms pore-like oligomers and filamentous structures reminiscent of FUS inclusions in ALS patients. FUS aggregation and toxicity requires a prion-like domain, but unlike TDP-43, additional determinants within a RGG domain are critical for FUS aggregation and toxicity. In further distinction to TDP-43, ALS-linked FUS mutations do not promote aggregation. Finally, genome-wide screens uncovered stress granule assembly and RNA metabolism genes that modify FUS toxicity but not TDP-43 toxicity. Our findings suggest that TDP-43 and FUS, though similar RNA-binding proteins, aggregate and confer disease phenotypes via distinct mechanisms. These differences will likely have important therapeutic implications.
Abstract The discovery of C9orf72 mutations as the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) has awakened a surge of interest in deciphering ...how mutations in this mysterious gene cause disease and what can be done to stop it. C9orf72 harbors a hexanucleotide repeat, GGGGCC, in a non-coding region of the gene and a massive expansion of this repeat causes ALS, FTD, or both (FTD/ALS). Many questions lie ahead. What does this gene normally do? What is the consequence of an enormous GGGGCC repeat expansion on that gene's function? Could that hexanucleotide repeat expansion have additional pathological actions unrelated to C9orf72 function? There has been tremendous progress on all fronts in the quest to define how C9orf72 mutations cause disease. Many new experimental models have been constructed and unleashed in powerful genetic screens. Studies in mouse and human patient samples, including iPS-derived neurons, have provided unprecedented insights into pathogenic mechanisms. Three major hypotheses have emerged and are still being hotly debated in the field. These include 1) loss of function owing to decrease in the abundance of C9orf72 protein and its ability to carryout its still unknown cellular role; 2) RNA toxicity from bidirectionally transcribed sense (GGGGCC) and antisense (GGCCCC) transcripts that accumulate in RNA foci and might sequester critical RNA-binding proteins; 3) proteotoxicity from dipeptide repeat proteins produced by an unconventional form of translation from the expanded nucleotide repeats. Here we review the evidence in favor and against each of these three hypotheses. We also suggest additional experiments and considerations that we propose will help clarify which mechanism(s) are most important for driving disease and therefore most critical for considering during the development of therapeutic interventions.
Amyotrophic lateral sclerosis (ALS) is a fatal human neurodegenerative disease affecting primarily motor neurons. Two RNA-binding proteins, TDP-43 and FUS, aggregate in the degenerating motor neurons ...of ALS patients, and mutations in the genes encoding these proteins cause some forms of ALS. TDP-43 and FUS and several related RNA-binding proteins harbor aggregation-promoting prion-like domains that allow them to rapidly self-associate. This property is critical for the formation and dynamics of cellular ribonucleoprotein granules, the crucibles of RNA metabolism and homeostasis. Recent work connecting TDP-43 and FUS to stress granules has suggested how this cellular pathway, which involves protein aggregation as part of its normal function, might be coopted during disease pathogenesis.
Astrocytes regulate the response of the central nervous system to disease and injury and have been hypothesized to actively kill neurons in neurodegenerative disease
. Here we report an approach to ...isolate one component of the long-sought astrocyte-derived toxic factor
. Notably, instead of a protein, saturated lipids contained in APOE and APOJ lipoparticles mediate astrocyte-induced toxicity. Eliminating the formation of long-chain saturated lipids by astrocyte-specific knockout of the saturated lipid synthesis enzyme ELOVL1 mitigates astrocyte-mediated toxicity in vitro as well as in a model of acute axonal injury in vivo. These results suggest a mechanism by which astrocytes kill cells in the central nervous system.
Abstract Prions are self-templating protein conformers that are naturally transmitted between individuals and promote phenotypic change. In yeast, prion-encoded phenotypes can be beneficial, neutral ...or deleterious depending upon genetic background and environmental conditions. A distinctive and portable ‘prion domain’ enriched in asparagine, glutamine, tyrosine and glycine residues unifies the majority of yeast prion proteins. Deletion of this domain precludes prionogenesis and appending this domain to reporter proteins can confer prionogenicity. An algorithm designed to detect prion domains has successfully identified 19 domains that can confer prion behavior. Scouring the human genome with this algorithm enriches a select group of RNA-binding proteins harboring a canonical RNA recognition motif (RRM) and a putative prion domain. Indeed, of 210 human RRM-bearing proteins, 29 have a putative prion domain, and 12 of these are in the top 60 prion candidates in the entire genome. Startlingly, these RNA-binding prion candidates are inexorably emerging, one by one, in the pathology and genetics of devastating neurodegenerative disorders, including: amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U), Alzheimer's disease and Huntington's disease. For example, FUS and TDP-43, which rank 1st and 10th among RRM-bearing prion candidates, form cytoplasmic inclusions in the degenerating motor neurons of ALS patients and mutations in TDP-43 and FUS cause familial ALS. Recently, perturbed RNA-binding proteostasis of TAF15, which is the 2nd ranked RRM-bearing prion candidate, has been connected with ALS and FTLD-U. We strongly suspect that we have now merely reached the tip of the iceberg. We predict that additional RNA-binding prion candidates identified by our algorithm will soon surface as genetic modifiers or causes of diverse neurodegenerative conditions. Indeed, simple prion-like transfer mechanisms involving the prion domains of RNA-binding proteins could underlie the classical non-cell-autonomous emanation of neurodegenerative pathology from originating epicenters to neighboring portions of the nervous system. This article is part of a Special Issue entitled RNA-Binding Proteins.
Abstract Intra-neuronal protein aggregates made of fibrillar alpha-synuclein (α-syn) are the hallmark of Parkinson's disease (PD). With time, these aggregates spread through the brain following ...axonal projections. Understanding the mechanism of this spread is central to the study of the progressive nature of PD. Here we review data relevant to the uptake, transport and release of α-syn fibrils. We summarize several cell surface receptors that regulate the uptake of α-syn fibrils by neurons. The aggregates are then transported along axons, both in the anterograde and retrograde direction. The kinetics of transport suggests that they are part of the slow component b of axonal transport. Recent findings indicate that aggregated α-syn is secreted by neurons by non-canonical pathways that may implicate various molecular chaperones including USP19 and the DnaJ/Hsc70 complex. Additionally, α-syn fibrils may also be released and transmitted from neuron-to-neuron via exosomes and tunneling nanotubes. Understanding these different mechanisms and molecular players underlying α-syn spread is crucial for the development of therapies that could halt the progression of α-syn-related degenerative diseases.
Abstract
Reactive astrocytes have been implicated in the pathogenesis of neurodegenerative diseases, including a non-cell autonomous effect on motor neuron survival in ALS. We previously defined a ...mechanism by which microglia release three factors, IL-1α, TNFα, and C1q, to induce neurotoxic astrocytes. Here we report that knocking out these three factors markedly extends survival in the
SOD1
G93A
ALS mouse model, providing evidence for gliosis as a potential ALS therapeutic target.
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