A feedback loop between dipeptide-repeat protein, TDP-43 and karyopherin-α mediates C9orf72-related neurodegeneration

Chimeric Peptide Species Contribute to Divergent Dipeptide Repeat Pathology in c9ALS/FTD and SCA36

GGGGCC hexanucleotide repeat expansions (HREs) in C9orf72 cause amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) and lead to the production of aggregating dipeptide repeat proteins (DPRs) via repeat associated non-AUG (RAN) translation. Here, we show the similar intronic GGCCTG HREs that causes spinocerebellar ataxia type 36 (SCA36) is also translated into DPRs, including poly(GP) and poly(PR). We demonstrate that poly(GP) is more abundant in SCA36 compared to c9ALS/FTD patient tissue due to canonical AUG-mediated translation from intron-retained GGCCTG repeat RNAs. However, the frequency of the antisense RAN translation product poly(PR) is comparable between c9ALS/FTD and SCA36 patient samples. Interestingly, in SCA36 patient tissue, poly(GP) exists as a soluble species, and no TDP-43 pathology is present. We show that aggregate-prone chimeric DPR (cDPR) species underlie the divergent DPR pathology between c9ALS/FTD and SCA36. These findings reveal key differences in translation, solubility, and protein aggregation of DPRs between c9ALS/FTD and SCA36.

Dipeptide repeat derived from C9orf72 hexanucleotide expansions forms amyloids or natively unfolded structures in vitro

The abnormal repetition of the hexanucleotide GGGGCC within the C9orf72 gene is the most common genetic cause of both Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD). Different hypothesis have been proposed to explain the pathogenicity of this mutation. Among them, the production of aberrant proteins called Dipeptide Repeat Proteins (DPR) from the repeated sequence. Those proteins are of interest, as they are toxic and form insoluble deposits in patient brains. In this study, we characterized the structural features of three different DPR encoded by the hexanucleotide repeat GGGGCC, namely poly-GA, poly-GP and poly-PA. We showed that DPR are natively unstructured proteins and that only poly-GA forms in vitro fibrillary aggregates. Poly-GA fibrils are of amyloid nature as revealed by their high content in beta sheets. They neither bind Thioflavin T nor Primuline, the commonly used amyloid fluorescent dyes. Remarkably, not all of the poly-GA primary structure was part of fibrils amyloid core.

Cell-to-cell transmission of C9orf72 poly-(Gly-Ala) triggers key features of ALS/FTD

The C9orf72 repeat expansion causes amyotrophic lateral sclerosis and frontotemporal dementia, but the poor correlation between C9orf72‐specific pathology and TDP‐43 pathology linked to neurodegeneration hinders targeted therapeutic development. Here, we addressed the role of the aggregating dipeptide repeat proteins resulting from unconventional translation of the repeat in all reading frames. Poly‐GA promoted cytoplasmic mislocalization and aggregation of TDP‐43 non‐cell‐autonomously, and anti‐GA antibodies ameliorated TDP‐43 mislocalization in both donor and receiver cells. Cell‐to‐cell transmission of poly‐GA inhibited proteasome function in neighboring cells. Importantly, proteasome inhibition led to the accumulation of TDP‐43 ubiquitinated within the nuclear localization signal (NLS) at lysine 95. Mutagenesis of this ubiquitination site completely blocked poly‐GA‐dependent mislocalization of TDP‐43. Boosting proteasome function with rolipram reduced both poly‐GA and TDP‐43 aggregation. Our data from cell lines, primary neurons, transgenic mice, and patient tissue suggest that poly‐GA promotes TDP‐43 aggregation by inhibiting the proteasome cell‐autonomously and non‐cell‐autonomously, which can be prevented by inhibiting poly‐GA transmission with antibodies or boosting proteasome activity with rolipram.

Traffic jam at the nuclear pore: All roads lead to nucleocytoplasmic transport defects in ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a fatal late-onset neurodegenerative disease that specifically affects the function and survival of spinal and cortical motor neurons. ALS shares many genetic, clinical, and pathological characteristics with frontotemporal dementia (FTD), and these diseases are now recognized as presentations of a disease spectrum known as ALS/FTD. The molecular determinants of neuronal loss in ALS/FTD are still debated, but the recent discovery of nucleocytoplasmic transport defects as a common denominator of most if not all forms of ALS/FTD has dramatically changed our understanding of the pathogenic mechanisms of this disease. Loss of nuclear pores and nucleoporin aggregation, altered nuclear morphology, and impaired nuclear transport are some of the most prominent features that have been identified using a variety of animal, cellular, and human models of disease. Here, we review the experimental evidence linking nucleocytoplasmic transport defects to the pathogenesis of ALS/FTD and propose a unifying view on how these defects may lead to a vicious cycle that eventually causes neuronal death.

Implications of Selective Autophagy Dysfunction for ALS Pathology

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disorder that progressively affects motor neurons in the brain and spinal cord. Due to the biological complexity of the disease, its etiology remains unknown. Several cellular mechanisms involved in the neurodegenerative process in ALS have been found, including the loss of RNA and protein homeostasis, as well as mitochondrial dysfunction. Insoluble protein aggregates, damaged mitochondria, and stress granules, which contain RNA and protein components, are recognized and degraded by the autophagy machinery in a process known as selective autophagy. Autophagy is a highly dynamic process whose dysregulation has now been associated with neurodegenerative diseases, including ALS, by numerous studies. In ALS, the autophagy process has been found deregulated in both familial and sporadic cases of the disease. Likewise, mutations in genes coding for proteins involved in the autophagy machinery have been reported in ALS patients, including selective autophagy receptors. In this review, we focus on the role of selective autophagy in ALS pathology.

Importin-α2 mediates brain development, learning and memory consolidation in Drosophila

Neuronal development and memory consolidation are conserved processes that rely on nuclear-cytoplasmic transport of signaling molecules to regulate gene activity and initiate cascades of downstream cellular events. Surprisingly, few reports address and validate this widely accepted perspective. Here we show that Importin-α2 (Imp-α2), a soluble nuclear transporter that shuttles cargoes between the cytoplasm and nucleus, is vital for brain development, learning and persistent memory in Drosophila melanogaster. Mutations in importin-α2 (imp-α2, known as Pendulin or Pen and homologous with human KPNA2) are alleles of mushroom body miniature B (mbmB), a gene known to regulate aspects of brain development and influence adult behavior in flies. Mushroom bodies (MBs), paired associative centers in the brain, are smaller than normal due to defective proliferation of specific intrinsic Kenyon cell (KC) neurons in mbmB mutants. Extant KCs projecting to the MB β-lobe terminate abnormally on the contralateral side of the brain. mbmB adults have impaired olfactory learning but normal memory decay in most respects, except that protein synthesis-dependent long-term memory (LTM) is abolished. This observation supports an alternative mechanism of persistent memory in which mutually exclusive protein-synthesis-dependent and -independent forms rely on opposing cellular mechanisms or circuits. We propose a testable model of Imp-α2 and nuclear transport roles in brain development and conditioned behavior. Based on our molecular characterization, we suggest that mbmB is hereafter referred to as imp-α2^mbmB.

Glycine-alanine dipeptide repeats spread rapidly in a repeat length- and age-dependent manner in the fly brain

Hexanucleotide repeat expansions of variable size in C9orf72 are the most prevalent genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. Sense and antisense transcripts of the expansions are translated by repeat-associated non-AUG translation into five dipeptide repeat proteins (DPRs). Of these, the polyGR, polyPR and, to a lesser extent, polyGA DPRs are neurotoxic, with polyGA the most abundantly detected DPR in patient tissue. Trans-cellular transmission of protein aggregates has recently emerged as a major driver of toxicity in various neurodegenerative diseases. In vitro evidence suggests that the C9 DPRs can spread. However, whether this phenomenon occurs under more complex in vivo conditions remains unexplored. Here, we used the adult fly brain to investigate whether the C9 DPRs can spread in vivo upon expression in a subset of neurons. We found that only polyGA can progressively spread throughout the brain, which accumulates in the shape of aggregate-like puncta inside recipient cells. Interestingly, GA transmission occurred as early as 3 days after expression induction. By comparing the spread of 36, 100 and 200 polyGA repeats, we found that polyGA spread is enhanced upon expression of longer GA DPRs. Transmission of polyGA is greater in older flies, indicating that age-associated factors exacerbate the spread. These data highlight a unique propensity of polyGA to spread throughout the brain, which could contribute to the greater abundance of polyGA in patient tissue. In addition, we present a model of early GA transmission that is suitable for genetic screens to identify mechanisms of spread and its consequences in vivo.

Mechanisms of Immune Activation by c9orf72-Expansions in Amyotrophic Lateral Sclerosis and Frontotemporal Dementia

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are neurodegenerative disorders with overlapping pathomechanisms, neurobehavioral features, and genetic etiologies. Individuals diagnosed with either disorder exhibit symptoms within a clinical spectrum. Symptoms of ALS involve neuromusculature deficits, reflecting upper and lower motor neurodegeneration, while the primary clinical features of FTD are behavioral and cognitive impairments, reflecting frontotemporal lobar degeneration. An intronic G₄C₂ hexanucleotide repeat expansion (HRE) within the promoter region of chromosome 9 open reading frame 72 (C9orf72) is the predominant monogenic cause of both ALS and FTD. While the heightened risk to develop ALS/FTD in response to C9orf72 expansions is well-established, studies continue to define the precise mechanisms by which this mutation elicits neurodegeneration. Studies show that G₄C₂ expansions undergo repeat-associated non-ATG dependent (RAN) translation, producing dipeptide repeat proteins (DRPs) with varying toxicities. Accumulation of DRPs in neurons, in particular arginine containing DRPs, have neurotoxic effects by potently impairing nucleocytoplasmic transport, nucleotide metabolism, lysosomal processes, and cellular metabolic pathways. How these pathophysiological effects of C9orf72 expansions engage and elicit immune activity with additional neurobiological consequences is an important line of future investigations. Immunoreactive microglia and elevated levels of peripheral inflammatory cytokines noted in individuals with C9orf72 ALS/FTD provide evidence that persistent immune activation has a causative role in the progression of each disorder. This review highlights the current understanding of the cellular, proteomic and genetic substrates through which G₄C₂ HREs may elicit detrimental immune activity, facilitating region-specific neurodegeneration in C9orf72 mediated ALS/FTD. We in particular emphasize interactions between intracellular pathways induced by C9orf72 expansions and innate immune inflammasome complexes, intracellular receptors responsible for eliciting inflammation in response to cellular stress. A further understanding of the intricate, reciprocal relationship between the cellular and molecular pathologies resulting from C9orf72 HREs and immune activation may yield novel therapeutics for ALS/FTD, which currently have limited treatment strategies.

C9orf72-generated poly-GR and poly-PR do not directly interfere with nucleocytoplasmic transport

Repeat expansions in the C9orf72 gene cause amyotrophic lateral sclerosis and frontotemporal dementia characterized by dipeptide-repeat protein (DPR) inclusions. The toxicity associated with two of these DPRs, poly-GR and poly-PR, has been associated with nucleocytoplasmic transport. To investigate the causal role of poly-GR or poly-PR on active nucleocytoplasmic transport, we measured nuclear import and export in poly-GR or poly-PR expressing Hela cells, neuronal-like SH-SY5Y cells and iPSC-derived motor neurons. Our data strongly indicate that poly-GR and poly-PR do not directly impede active nucleocytoplasmic transport.

Repeat-associated non-AUG (RAN) translation mechanisms are running into focus for GGGGCC-repeat associated ALS/FTD

Many human diseases are associated with the expansion of repeat sequences within the genes. It has become clear that expressed disease transcripts bearing such long repeats can undergo translation, even in the absence of a canonical AUG start codon. Termed "RAN translation" for repeat associated non-AUG translation, this process is becoming increasingly prominent as a contributor to these disorders. Here we discuss mechanisms and variables that impact translation of the repeat sequences associated with the C9orf72 gene. Expansions of a G4C2 repeat within intron 1 of this gene are associated with the motor neuron disease ALS and dementia FTD, which comprise a clinical and pathological spectrum. RAN translation of G4C2 repeat expansions has been studied in cells in culture (ex vivo) and in the fly in vivo. Cellular states that lead to RAN translation, like stress, may be critical contributors to disease progression. Greater elucidation of the mechanisms that impact this process and the factors contributing will lead to greater understanding of the repeat expansion diseases, to the potential development of novel approaches to therapeutics, and to a greater understanding of how these players impact biological processes in the absence of disease.

Repeat-associated non-AUG (RAN) translation: insights from pathology

More than 40 different neurological diseases are caused by microsatellite repeat expansions. Since the discovery of repeat-associated non-AUG (RAN) translation by Zu et al. in 2011, nine expansion disorders have been identified as RAN-positive diseases. RAN proteins are translated from different types of nucleotide repeat expansions and can be produced from both sense and antisense transcripts. In some diseases, RAN proteins have been shown to accumulate in affected brain regions. Here we review the pathological and molecular aspects associated with RAN protein accumulation for each particular disorder, the correlation between disease pathology and the available in vivo models and the common aspects shared by some of the newly discovered RAN proteins.

The coming-of-age of nucleocytoplasmic transport in motor neuron disease and neurodegeneration

The nuclear pore is the gatekeeper of nucleocytoplasmic transport and signaling through which a vast flux of information is continuously exchanged between the nuclear and cytoplasmic compartments to maintain cellular homeostasis. A unifying and organizing principle has recently emerged that cements the notion that several forms of amyotrophic lateral sclerosis (ALS), and growing number of other neurodegenerative diseases, co-opt the dysregulation of nucleocytoplasmic transport and that this impairment is a pathogenic driver of neurodegeneration. The understanding of shared pathomechanisms that underpin neurodegenerative diseases with impairments in nucleocytoplasmic transport and how these interface with current concepts of nucleocytoplasmic transport is bound to illuminate this fundamental biological process in a yet more physiological context. Here, I summarize unresolved questions and evidence and extend basic and critical concepts and challenges of nucleocytoplasmic transport and its role in the pathogenesis of neurodegenerative diseases, such as ALS. These principles will help to appreciate the roles of nucleocytoplasmic transport in the pathogenesis of ALS and other neurodegenerative diseases, and generate a framework for new ideas of the susceptibility of motoneurons, and possibly other neurons, to degeneration by dysregulation of nucleocytoplasmic transport.

Repeat-Associated Non-ATG Translation: Molecular Mechanisms and Contribution to Neurological Disease

Microsatellite mutations involving the expansion of tri-, tetra-, penta-, or hexanucleotide repeats cause more than 40 different neurological disorders. Although, traditionally, the position of the repeat within or outside of an open reading frame has been used to focus research on disease mechanisms involving protein loss of function, protein gain of function, or RNA gain of function, the discoveries of bidirectional transcription and repeat-associated non-ATG (RAN) have blurred these distinctions. Here we review what is known about RAN proteins in disease, the mechanisms by which they are produced, and the novel therapeutic opportunities they provide.

C9orf72-FTD/ALS pathogenesis: evidence from human neuropathological studies

What are the most important and treatable pathogenic mechanisms in C9orf72-FTD/ALS? Model-based efforts to address this question are forging ahead at a blistering pace, often with conflicting results. But what does the human neuropathological literature reveal? Here, we provide a critical review of the human studies to date, seeking to highlight key gaps or uncertainties in our knowledge. First, we engage the C9orf72-specific mechanisms, including C9orf72 haploinsufficiency, repeat RNA foci, and dipeptide repeat protein inclusions. We then turn to some of the most prominent C9orf72-associated features, such as TDP-43 loss-of-function, TDP-43 aggregation, and nuclear transport defects. Finally, we review potential disease-modifying epigenetic and genetic factors and the natural history of the disease across the lifespan. Throughout, we emphasize the importance of anatomical precision when studying how candidate mechanisms relate to neuronal, regional, and behavioral findings. We further highlight methodological approaches that may help address lingering knowledge gaps and uncertainties, as well as other logical next steps for the field. We conclude that anatomically oriented human neuropathological studies have a critical role to play in guiding this fast-moving field toward effective new therapies.