Antibody Therapy Targeting RAN Proteins Rescues C9 ALS/FTD Phenotypes in C9orf72 Mouse Model

High-throughput screen of 100 000 small molecules in C9ORF72 ALS neurons identifies spliceosome modulators that mobilize G4C2 repeat RNA into nuclear export and repeat associated non-canonical translation

An intronic G4C2 repeat expansion in the C9ORF72 gene is the major known cause for Amyotrophic Lateral Sclerosis (ALS), with current evidence for both, loss of function and pathological gain of function disease mechanisms. We screened 96 200 small molecules in C9ORF72 patient iPS neurons for modulation of nuclear G4C2 RNA foci and identified 82 validated hits, including the Brd4 inhibitor JQ1 as well as novel analogs of Spliceostatin-A, a known modulator of SF3B1, the branch point binding protein of the U2-snRNP. Spliceosome modulation by these SF3B1 targeted compounds recruits SRSF1 to nuclear G4C2 RNA, mobilizing it from RNA foci into nucleocytoplasmic export. This leads to increased repeat-associated non-canonical (RAN) translation and ultimately, enhanced cell toxicity. Our data (i) provide a new pharmacological entry point with novel as well as known, publicly available tool compounds for dissection of C9ORF72 pathobiology in C9ORF72 ALS models, (ii) allowing to differentially modulate RNA foci versus RAN translation, and (iii) suggest that therapeutic RNA foci elimination strategies warrant caution due to a potential storage function, counteracting translation into toxic dipeptide repeat polyproteins. Instead, our data support modulation of nuclear export via SRSF1 or SR protein kinases as possible targets for future pharmacological drug discovery.

Neuronal polyunsaturated fatty acids are protective in ALS/FTD

Here we report a conserved transcriptomic signature of reduced fatty acid and lipid metabolism gene expression in a Drosophila model of C9orf72 repeat expansion, the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD), and in human postmortem ALS spinal cord. We performed lipidomics on C9 ALS/FTD Drosophila, induced pluripotent stem (iPS) cell neurons and postmortem FTD brain tissue. This revealed a common and specific reduction in phospholipid species containing polyunsaturated fatty acids (PUFAs). Feeding C9 ALS/FTD flies PUFAs yielded a modest increase in survival. However, increasing PUFA levels specifically in neurons of C9 ALS/FTD flies, by overexpressing fatty acid desaturase enzymes, led to a substantial extension of lifespan. Neuronal overexpression of fatty acid desaturases also suppressed stressor-induced neuronal death in iPS cell neurons of patients with both C9 and TDP-43 ALS/FTD. These data implicate neuronal fatty acid saturation in the pathogenesis of ALS/FTD and suggest that interventions to increase neuronal PUFA levels may be beneficial.

Microglia in ALS: Insights into Mechanisms and Therapeutic Potential

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by the loss of motor neurons, leading to escalating muscle weakness, atrophy, and eventually paralysis. While neurons are the most visibly affected, emerging data highlight microglia-the brain's resident immune cells-as key contributors to disease onset and progression. Rather than existing in a simple beneficial or harmful duality, microglia can adopt multiple functional states shaped by internal and external factors, including those in ALS. Collectively, these disease-specific forms are called disease-associated microglia (DAM). Research using rodent models, patient-derived cells, and human postmortem tissue shows that microglia can transition into DAM phenotypes, driving inflammation and neuronal injury. However, these cells can also fulfill protective roles under certain conditions, revealing their adaptable nature. This review explores recent discoveries regarding the multifaceted behavior of microglia in ALS, highlights important findings that link these immune cells to motor neuron deterioration, and discusses emerging therapies-some already used in clinical trials-that aim to recalibrate microglial functions and potentially slow disease progression.

Amyotrophic lateral sclerosis caused by hexanucleotide repeat expansions in C9orf72: from genetics to therapeutics

GGGGCC repeat expansions in C9orf72 are a common genetic cause of amyotrophic lateral sclerosis in people of European ancestry; however, substantial variability in the penetrance of the mutation, age at disease onset, and clinical presentation can complicate diagnosis and prognosis. The repeat expansion is bidirectionally transcribed in the sense and antisense directions into repetitive RNAs and translated into dipeptide repeat proteins, and both accumulate in the cortex, cerebellum, and the spinal cord. Furthermore, neuropathological aggregates of phosphorylated TDP-43 are observed in motor cortex and other cortical regions, and in the spinal cord of patients at autopsy. C9orf72 repeat expansions can also cause frontotemporal dementia. The GGGGCC repeat induces a complex interplay of loss-of-function and gain-of-function pathological mechanisms. Clinical trials using antisense oligonucleotides to target the GGGGCC repeat RNA have not been successful, potentially because they only target a single gain-of-function mechanism. Novel therapeutic approaches targeting the DNA repeat expansion, multiple repeat-derived RNA species, or downstream targets of TDP-43 dysfunction are, however, on the horizon, together with the development of diagnostic and prognostic biomarkers.

CASP8 intronic expansion identified by poly-glycine-arginine pathology increases Alzheimer's disease risk

Alzheimer's disease (AD) affects more than 10% of the population ≥65 y of age, but the underlying biological risks of most AD cases are unclear. We show anti-poly-glycine-arginine (a-polyGR) positive aggregates frequently accumulate in sporadic AD autopsy brains (45/80 cases). We hypothesize that these aggregates are caused by one or more polyGR-encoding repeat expansion mutations. We developed a CRISPR/deactivated-Cas9 enrichment strategy to identify candidate GR-encoding repeat expansion mutations directly from genomic DNA isolated from a-polyGR(+) AD cases. Using this approach, we isolated an interrupted (GGGAGA)n intronic expansion within a SINE-VNTR-Alu element in CASP8 (CASP8-GGGAGAEXP). Immunostaining using a-polyGR and locus-specific C-terminal antibodies demonstrate that the CASP8-GGGAGAEXP expresses hybrid poly(GR)n(GE)n(RE)n proteins that accumulate in CASP8-GGGAGAEXP(+) AD brains. In cells, expression of CASP8-GGGAGAEXP minigenes leads to increased p-Tau (Ser202/Thr205) levels. Consistent with other types of repeat-associated non-AUG (RAN) proteins, poly(GR)n(GE)n(RE)n protein levels are increased by stress. Additionally, levels of these stress-induced proteins are reduced by metformin. Association studies show specific aggregate promoting interrupted CASP8-GGGAGAEXP sequence variants found in ~3.6% of controls and 7.5% AD cases increase AD risk [CASP8-GGGAGA-AD-R1; OR 2.2, 95% CI (1.5185 to 3.1896), P = 3.1 × 10-5]. Cells transfected with a high-risk CASP8-GGGAGA-AD-R1 variant show increased toxicity and increased levels of poly(GR)n(GE)n(RE)n aggregates. Taken together, these data identify polyGR(+) aggregates as a frequent and unexpected type of brain pathology in AD and CASP8-GGGAGA-AD-R1 alleles as a relatively common AD risk factor. Taken together, these data support a model in which CASP8-GGGAGAEXP alleles combined with stress increase AD risk.

A bacterial artificial chromosome mouse model of amyotrophic lateral sclerosis manifests 'space cadet syndrome' on two FVB backgrounds

C9orf72-related amyotrophic lateral sclerosis (ALS)/frontotemporal dementia (FTD) has proven difficult to model in mice. Liu et al. (2016) reported a bacterial artificial chromosome (BAC) transgenic mouse displaying behavioural, motor and pathological abnormalities. This was followed by multiple laboratories independently refuting and confirming phenotypes. A proposed explanation centred on the use of different FVB background lines (from The Jackson Laboratory and Janvier Labs). We studied C9orf72 BAC mice on both backgrounds and found significantly elevated levels of dipeptide repeat proteins, but no evidence of a transgene-associated phenotype. We observed seizures and a gradual decline in functional performance in transgenic and non-transgenic mice, irrespective of genetic background. The phenotype was in keeping with the so-called 'space cadet syndrome'. Our findings indicate that the differences previously reported are not due to C9orf72 status and highlight the importance of using genetic backgrounds that do not confound interpretation of neurodegenerative phenotypes.

CRISPR/Cas13d targeting suppresses repeat-associated non-AUG translation of C9orf72 hexanucleotide repeat RNA

A hexanucleotide GGGGCC repeat expansion in the non-coding region of the C9orf72 gene is the most common genetic mutation identified in patients with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The resulting repeat RNA and dipeptide repeat proteins from non-conventional repeat translation have been recognized as important markers associated with the diseases. CRISPR/Cas13d, a powerful RNA-targeting tool, has faced challenges in effectively targeting RNA with stable secondary structures. Here we report that CRISPR/Cas13d can be optimized to specifically target GGGGCC repeat RNA. Our results demonstrate that the CRISPR/Cas13d system can be harnessed to significantly diminish the translation of poly-dipeptides originating from the GGGGCC repeat RNA. This efficacy has been validated in various cell types, including induced pluripotent stem cells and differentiated motor neurons originating from C9orf72-ALS patients, as well as in C9orf72 repeat transgenic mice. These findings demonstrate the application of CRISPR/Cas13d in targeting RNA with intricate higher-order structures and suggest a potential therapeutic approach for ALS and FTD.

The prion-like effect and prion-like protein targeting strategy in amyotrophic lateral sclerosis

Pathological proteins in amyotrophic lateral sclerosis (ALS), such as superoxide dismutase 1, TAR DNA-binding protein 43, and fused in sarcoma, exhibit a prion-like pattern. All these proteins have a low-complexity domain and seeding activity in cells. In this review, we summarize the studies on the prion-like effect of these proteins and list six prion-like protein targeting strategies that we believe have potential for ALS therapy, including antisense oligonucleotides, antibody-based technology, peptide, protein chaperone, autophagy enhancement, and heteromultivalent compounds. Considering the pathological complexity and heterogeneity of ALS, we believe that the final solution to ALS therapy is most likely to be an individualized cocktail therapy, including clearance of toxicity, blockage of pathological progress, and protection of neurons.

Pin1-catalyzed conformational regulation after phosphorylation: A distinct checkpoint in cell signaling and drug discovery

Protein phosphorylation is one of the most common mechanisms regulating cellular signaling pathways, and many kinases and phosphatases are proven drug targets. Upon phosphorylation, protein functions can be further regulated by the distinct isomerase Pin1 through cis-trans isomerization. Numerous protein targets and many important roles have now been elucidated for Pin1. However, no tools are available to detect or target cis and trans conformation events in cells. The development of Pin1 inhibitors and stereo- and phospho-specific antibodies has revealed that cis and trans conformations have distinct and often opposing cellular functions. Aberrant conformational changes due to the dysregulation of Pin1 can drive pathogenesis but can be effectively targeted in age-related diseases, including cancers and neurodegenerative disorders. Here, we review advances in understanding the roles of Pin1 signaling in health and disease and highlight conformational regulation as a distinct signal transduction checkpoint in disease development and treatment.

Updates on Disease Mechanisms and Therapeutics for Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS), or Lou Gehrig's disease, is a motor neuron disease. In ALS, upper and lower motor neurons in the brain and spinal cord progressively degenerate during the course of the disease, leading to the loss of the voluntary movement of the arms and legs. Since its first description in 1869 by a French neurologist Jean-Martin Charcot, the scientific discoveries on ALS have increased our understanding of ALS genetics, pathology and mechanisms and provided novel therapeutic strategies. The goal of this review article is to provide a comprehensive summary of the recent findings on ALS mechanisms and related therapeutic strategies to the scientific audience. Several highlighted ALS research topics discussed in this article include the 2023 FDA approved drug for SOD1 ALS, the updated C9orf72 GGGGCC repeat-expansion-related mechanisms and therapeutic targets, TDP-43-mediated cryptic splicing and disease markers and diagnostic and therapeutic options offered by these recent discoveries.

RNP granules in ALS and neurodegeneration: From multifunctional membraneless organelles to therapeutic opportunities

Amyotrophic lateral sclerosis (ALS) and related neurodegenerative diseases are characterised by dysfunction of a host of RNA-binding proteins (RBPs) and a severely disrupted RNA metabolism. Recently, RBP-harbouring phase-separated complexes, ribonucleoprotein (RNP) granules, have come into the limelight as "crucibles" of neuronal pathology in ALS. RNP granules are indispensable for the multitude of regulatory processes underlying cellular RNA metabolism and serve as critical organisers of cellular biochemistry. Neurons, highly specialised cells, heavily rely on RNP granules for efficient trafficking, signalling and stress responses. Multiple RNP granule components, primarily RBPs such as TDP-43 and FUS, are affected by ALS mutations. However, even in the absence of mutations, RBP proteinopathies represent pathophysiological hallmarks of ALS. Given the high local concentrations of RBPs and RNAs, their weakened or enhanced interactions within RNP granules disrupt their homeostasis. Thus, the physiological process of phase separation and RNP granule formation, vital for maintaining the high-functioning state of neuronal cells, becomes their Achilles heel. Here, we will review the recent literature on the causes and consequences of abnormal RNP granule functioning in ALS and related disorders. In particular, we will summarise the evidence for the network-level dysfunction of RNP granules in these conditions and discuss considerations for therapeutic interventions to target RBPs, RNP granules and their network as a whole.

Development and Preclinical Evaluation of a Copper-64-Labeled Antibody Targeting Glycine-Alanine Dipeptides for PET Imaging of C9orf72-Associated Amyotrophic Lateral Sclerosis/Frontotemporal Dementia

Aggregating poly(glycine-alanine) (poly-GA) is derived from the unconventional translation of the pathogenic intronic hexanucleotide repeat expansion in the C9orf72 gene, which is the most common genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Poly-GA accumulates predominantly in neuronal cytoplasmic inclusions unique to C9orf72 ALS/FTD patients. Poly-GA is, therefore, a promising target for PET/CT imaging of FTD/ALS to monitor disease progression and therapeutic interventions. A novel 64Cu-labeled anti-GA antibody (mAb1A12) targeting the poly-GA protein was developed and evaluated in a transgenic mouse model. It was obtained with high radiochemical purity (RCP), radiochemical yield (RCY), and specific activity, and showed high stability in vitro and ex vivo and specifically bound to poly-GA. The affinity of NODAGA-mAb1A12 for poly-GA was not affected by this modification. [64Cu]Cu-NODAGA-mAb1A12 was injected into transgenic mice expressing GFP-(GA)175 in excitatory neurons driven by Camk2a-Cre and in control littermates. PET/CT imaging was performed at 2, 20, and 40 h post-injection (p.i.) and revealed a higher accumulation in the cortex in transgenic mice than in wild-type mice, as reflected by higher standardized uptake value ratios (SUVR) using the cerebellum as the reference region. The organs were isolated for biodistribution and ex vivo autoradiography. Autoradiography revealed a higher cortex-to-cerebellum ratio in the transgenic mice than in the controls. Results from autoradiography were validated by immunohistochemistry and poly-GA immunoassays. Moreover, we confirmed antibody uptake in the CNS in a pharmacokinetic study of the perfused tissues. In summary, [64Cu]Cu-NODAGA-mAb1A12 demonstrated favorable in vitro characteristics and an increased relative binding in poly-GA transgenic mice compared to wild-type mice in vivo. Our results with this first-in-class radiotracer suggested that targeting poly-GA is a promising approach for PET/CT imaging in FTD/ALS.

C9ORF72 hexanucleotide repeat expansion: From ALS and FTD to a broader pathogenic role?

The major gene underlying monogenic forms of amyotrophic lateral sclerosis (ALS) and fronto-temporal dementia (FTD) is C9ORF72. The causative mutation in C9ORF72 is an abnormal hexanucleotide (G4C2) repeat expansion (HRE) located in the first intron of the gene. The aim of this review is to propose a comprehensive update on recent developments on clinical, biological and therapeutics aspects related to C9ORF72 in order to highlight the current understanding of genotype-phenotype correlations, and also on biological machinery leading to neuronal death. We will particularly focus on the broad phenotypic presentation of C9ORF72-related diseases, that goes well beyond the classical phenotypes observed in ALS and FTD patients. Last, we will comment the possible therapeutical hopes for patients carrying a C9ORF72 HRE.

Mapping of repeat-associated non-AUG (RAN) translation knowledge: A bibliometric analysis

Over 50 genetic human disorders are attributed to the irregular expansion of microsatellites. These expanded microsatellite sequences can experience bidirectional transcription, leading to new reading frames. Beyond the standard AUG initiation or adjacent start codons, they are translated into proteins characterized by disease-causing amino acid repeats through repeat-associated non-AUG translation. Despite its significance, there's a discernible gap in comprehensive and objective articles on RAN translation. This study endeavors to evaluate and delineate the contemporary landscape and progress of RAN translation research via a bibliometric analysis. We sourced literature on RAN translation from the Web of Science Core Collection. Utilizing two bibliometric analysis tools, CiteSpace and VOSviewer, we gauged individual impacts and interactions by examining annual publications, journals, co-cited journals, countries/regions, institutions, authors, and co-cited authors. Following this, we assessed the co-occurrence and bursts of keywords and co-cited references to pinpoint research hotspots and trending in RAN translation. Between 2011 and 2022, 1317 authors across 359 institutions from 34 countries/regions contributed to 250 publications on RAN translation, spread across 118 academic journals. This article presents a systematic, objective, and comprehensive analysis of the current literature on RAN translation. Our findings emphasize that mechanisms related to C9orf72 ALS/FTD are pivotal topics in the realm of RAN translation, with cellular stress and the utilization of small molecule marking the trending research areas.

Pin1-Catalyzed Conformation Changes Regulate Protein Ubiquitination and Degradation

The unique prolyl isomerase Pin1 binds to and catalyzes cis-trans conformational changes of specific Ser/Thr-Pro motifs after phosphorylation, thereby playing a pivotal role in regulating the structure and function of its protein substrates. In particular, Pin1 activity regulates the affinity of a substrate for E3 ubiquitin ligases, thereby modulating the turnover of a subset of proteins and coordinating their activities after phosphorylation in both physiological and disease states. In this review, we highlight recent advancements in Pin1-regulated ubiquitination in the context of cancer and neurodegenerative disease. Specifically, Pin1 promotes cancer progression by increasing the stabilities of numerous oncoproteins and decreasing the stabilities of many tumor suppressors. Meanwhile, Pin1 plays a critical role in different neurodegenerative disorders via the regulation of protein turnover. Finally, we propose a novel therapeutic approach wherein the ubiquitin-proteasome system can be leveraged for therapy by targeting pathogenic intracellular targets for TRIM21-dependent degradation using stereospecific antibodies.

Role of C9orf72 hexanucleotide repeat expansions in ALS/FTD pathogenesis

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are progressive neurological disorders that share neurodegenerative pathways and features. The most prevalent genetic causes of ALS/FTD is the GGGGCC hexanucleotide repeat expansions in the first intron region of the chromosome 9 open reading frame 72 (C9orf72) gene. In this review, we comprehensively summarize the accumulating evidences elucidating the pathogenic mechanism associated with hexanucleotide repeat expansions in ALS/FTD. These mechanisms encompass the structural polymorphism of DNA and transcribed RNA, the formation of RNA foci via phase separation, and the cytoplasmic accumulation and toxicities of dipeptide-repeat proteins. Additionally, the formation of G-quadruplex structures significantly impairs the expression and normal function of the C9orf72 protein. We also discuss the sequestration of specific RNA binding proteins by GGGGCC RNA, which further contributes to the toxicity of C9orf72 hexanucleotide repeat expansions. The deeper understanding of the pathogenic mechanism of hexanucleotide repeat expansions in ALS/FTD provides multiple potential drug targets for these devastating diseases.

Tau Immunotherapies for Alzheimer's Disease and Related Tauopathies: Status of Trials and Insights from Preclinical Studies

 The tau protein undergoes pathological changes in Alzheimer's disease and other tauopathies that eventually lead to functional impairments. Over the years, several therapeutic approaches have been examined to slow or halt the progression of tau pathology but have yet to lead to an approved disease-modifying treatment. Of the drugs in clinical trials that directly target tau, immunotherapies are the largest category and mostly consist of antibodies in different stages of development. There is a reasonable optimism that at least some of these compounds will have a clinically meaningful efficacy. This view is based on the significant although modest efficacy of some antibodies targeting amyloid-β in Alzheimer's disease and the fact that tau pathology correlates much better with the degree of dementia than amyloid-β lesions. In Alzheimer's disease, clearing pathological tau may therefore improve function later in the disease process than when removing amyloid-β. This review provides a brief update on the active and passive clinical tau immunization trials with insight from preclinical studies. Various epitopes are being targeted and some of the antibodies are said to target extracellular tau but because almost all of pathological tau is found intracellularly, the most efficacious antibodies should be able to enter the cell.

Tau-targeting therapies for Alzheimer disease: current status and future directions

Alzheimer disease (AD) is the most common cause of dementia in older individuals. AD is characterized pathologically by amyloid-β (Aβ) plaques and tau neurofibrillary tangles in the brain, with associated loss of synapses and neurons, which eventually results in dementia. Many of the early attempts to develop treatments for AD focused on Aβ, but a lack of efficacy of these treatments in terms of slowing disease progression led to a change of strategy towards targeting of tau pathology. Given that tau shows a stronger correlation with symptom severity than does Aβ, targeting of tau is more likely to be efficacious once cognitive decline begins. Anti-tau therapies initially focused on post-translational modifications, inhibition of tau aggregation and stabilization of microtubules. However, trials of many potential drugs were discontinued because of toxicity and/or lack of efficacy. Currently, the majority of tau-targeting agents in clinical trials are immunotherapies. In this Review, we provide an update on the results from the initial immunotherapy trials and an overview of new therapeutic candidates that are in clinical development, as well as considering future directions for tau-targeting therapies.

CLIP-Seq analysis enables the design of protective ribosomal RNA bait oligonucleotides against C9ORF72 ALS/FTD poly-GR pathophysiology

Amyotrophic lateral sclerosis and frontotemporal dementia patients with a hexanucleotide repeat expansion in C9ORF72 (C9-HRE) accumulate poly-GR and poly-PR aggregates. The pathogenicity of these arginine-rich dipeptide repeats (R-DPRs) is thought to be driven by their propensity to bind low-complexity domains of multivalent proteins. However, the ability of R-DPRs to bind native RNA and the significance of this interaction remain unclear. Here, we used computational and experimental approaches to characterize the physicochemical properties of R-DPRs and their interaction with RNA. We find that poly-GR predominantly binds ribosomal RNA (rRNA) in cells and exhibits an interaction that is predicted to be energetically stronger than that for associated ribosomal proteins. Critically, modified rRNA "bait" oligonucleotides restore poly-GR-associated ribosomal deficits and ameliorate poly-GR toxicity in patient neurons and Drosophila models. Our work strengthens the hypothesis that ribosomal function is impaired by R-DPRs, highlights a role for direct rRNA binding in mediating ribosomal dysfunction, and presents a strategy for protecting against C9-HRE pathophysiological mechanisms.

A human antibody against pathologic IAPP aggregates protects beta cells in type 2 diabetes models

In patients with type 2 diabetes, pancreatic beta cells progressively degenerate and gradually lose their ability to produce insulin and regulate blood glucose. Beta cell dysfunction and loss is associated with an accumulation of aggregated forms of islet amyloid polypeptide (IAPP) consisting of soluble prefibrillar IAPP oligomers as well as insoluble IAPP fibrils in pancreatic islets. Here, we describe a human monoclonal antibody selectively targeting IAPP oligomers and neutralizing IAPP aggregate toxicity by preventing membrane disruption and apoptosis in vitro. Antibody treatment in male rats and mice transgenic for human IAPP, and human islet-engrafted mouse models of type 2 diabetes triggers clearance of IAPP oligomers resulting in beta cell protection and improved glucose control. These results provide new evidence for the pathological role of IAPP oligomers and suggest that antibody-mediated removal of IAPP oligomers could be a pharmaceutical strategy to support beta cell function in type 2 diabetes.

Microglia and Astrocytes in Amyotrophic Lateral Sclerosis: Disease-Associated States, Pathological Roles, and Therapeutic Potential

Microglial and astrocytic reactivity is a prominent feature of amyotrophic lateral sclerosis (ALS). Microglia and astrocytes have been increasingly appreciated to play pivotal roles in disease pathogenesis. These cells can adopt distinct states characterized by a specific molecular profile or function depending on the different contexts of development, health, aging, and disease. Accumulating evidence from ALS rodent and cell models has demonstrated neuroprotective and neurotoxic functions from microglia and astrocytes. In this review, we focused on the recent advancements of knowledge in microglial and astrocytic states and nomenclature, the landmark discoveries demonstrating a clear contribution of microglia and astrocytes to ALS pathogenesis, and novel therapeutic candidates leveraging these cells that are currently undergoing clinical trials.

Toxicity of C9orf72-associated dipeptide repeat peptides is modified by commonly used protein tags

Hexanucleotide repeat expansions in the C9orf72 gene are the most prevalent genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. Transcripts of the expansions are translated into toxic dipeptide repeat (DPR) proteins. Most preclinical studies in cell and animal models have used protein-tagged polyDPR constructs to investigate DPR toxicity but the effects of tags on DPR toxicity have not been systematically explored. Here, we used Drosophila to assess the influence of protein tags on DPR toxicity. Tagging of 36 but not 100 arginine-rich DPRs with mCherry increased toxicity, whereas adding mCherry or GFP to GA100 completely abolished toxicity. FLAG tagging also reduced GA100 toxicity but less than the longer fluorescent tags. Expression of untagged but not GFP- or mCherry-tagged GA100 caused DNA damage and increased p62 levels. Fluorescent tags also affected GA100 stability and degradation. In summary, protein tags affect DPR toxicity in a tag- and DPR-dependent manner, and GA toxicity might be underestimated in studies using tagged GA proteins. Thus, including untagged DPRs as controls is important when assessing DPR toxicity in preclinical models.

Repeat length of C9orf72-associated glycine-alanine polypeptides affects their toxicity

G4C2 hexanucleotide repeat expansions in a non-coding region of the C9orf72 gene are the most common cause of familial amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). G4C2 insertion length is variable, and patients can carry up to several thousand repeats. Dipeptide repeat proteins (DPRs) translated from G4C2 transcripts are thought to be a main driver of toxicity. Experiments in model organisms with relatively short DPRs have shown that arginine-rich DPRs are most toxic, while polyGlycine-Alanine (GA) DPRs cause only mild toxicity. However, GA is the most abundant DPR in patient brains, and experimental work in animals has generally relied on the use of low numbers of repeats, with DPRs often tagged for in vivo tracking. Whether repeat length or tagging affect the toxicity of GA has not been systematically assessed. Therefore, we generated Drosophila fly lines expressing GA100, GA200 or GA400 specifically in adult neurons. Consistent with previous studies, expression of GA100 and GA200 caused only mild toxicity. In contrast, neuronal expression of GA400 drastically reduced climbing ability and survival of flies, indicating that long GA DPRs can be highly toxic in vivo. This toxicity could be abolished by tagging GA400. Proteomics analysis of fly brains showed a repeat-length-dependent modulation of the brain proteome, with GA400 causing earlier and stronger changes than shorter GA proteins. PolyGA expression up-regulated proteins involved in ER to Golgi trafficking, and down-regulated proteins involved in insulin signalling. Experimental down-regulation of Tango1, a highly conserved regulator of ER-to Golgi transport, partially rescued GA400 toxicity, suggesting that misregulation of this process contributes to polyGA toxicity. Experimentally increasing insulin signaling also rescued GA toxicity. In summary, our data show that long polyGA proteins can be highly toxic in vivo, and that they may therefore contribute to ALS/FTD pathogenesis in patients.

Intercellular transmission of pathogenic proteins in ALS: Exploring the pathogenic wave

In patients with amyotrophic lateral sclerosis (ALS), disease symptoms and pathology typically spread in a predictable spatiotemporal pattern beginning at a focal site of onset and progressing along defined neuroanatomical tracts. Like other neurodegenerative diseases, ALS is characterized by the presence of protein aggregates in postmortem patient tissue. Cytoplasmic, ubiquitin-positive aggregates of TDP-43 are observed in approximately 97% of sporadic and familial ALS patients, while SOD1 inclusions are likely specific to cases of SOD1-ALS. Additionally, the most common subtype of familial ALS, caused by a hexanucleotide repeat expansion in the first intron of the C9orf72 gene (C9-ALS), is further characterized by the presence of aggregated dipeptide repeat proteins (DPRs). As we will describe, cell-to-cell propagation of these pathological proteins tightly correlates with the contiguous spread of disease. While TDP-43 and SOD1 are capable of seeding protein misfolding and aggregation in a prion-like manner, C9orf72 DPRs appear to induce (and transmit) a 'disease state' more generally. Multiple mechanisms of intercellular transport have been described for all of these proteins, including anterograde and retrograde axonal transport, extracellular vesicle secretion, and macropinocytosis. In addition to neuron-to-neuron transmission, transmission of pathological proteins occurs between neurons and glia. Given that the spread of ALS disease pathology corresponds with the spread of symptoms in patients, the various mechanisms by which ALS-associated protein aggregates propagate through the central nervous system should be closely examined.

C9orf72 poly-PR helps p53 escape from the ubiquitin-proteasome system and promotes its stability

C9orf72-derived dipeptide repeats (DPRs) proteins have been regarded as the pathogenic cause of neurodegeneration in amyotrophic lateral sclerosis and frontotemporal dementia (C9-ALS/FTD). As the most toxic DPRs in C9-ALS/FTD, poly-proline-arginine (poly-PR) is associated with the stability and accumulation of p53, which consequently induces neurodegeneration. However, the exact molecular mechanism via which C9orf72 poly-PR stabilizes p53 remains unclear. In this study, we showed that C9orf72 poly-PR induces not only neuronal damage but also p53 accumulation and p53 downstream gene activation in primary neurons. C9orf72 (PR)₅₀ also slows down p53 protein turnover without affecting the p53 transcription level and thus promotes its stability in N2a cells. Interestingly, the ubiquitin-proteasome system but not the autophagy function was impaired in (PR)₅₀ transfected N2a cells, resulting in defective p53 degradation. Moreover, we found that (PR)₅₀ induces mdm2 mistranslocation from the nucleus to the cytoplasm and competitively binds to p53, reducing mdm2-p53 interactions in the nucleus in two different (PR)₅₀ transfected cells. Our data strongly indicate that (PR)₅₀ reduces mdm2-p53 interactions and causes p53 to escape from the ubiquitin-proteasome system, promoting its stability and accumulation. Inhibiting or at least downregulating (PR)₅₀ binding with p53 may be therapeutically exploited for the treatment of C9-ALS/FTD.

C9orf72 poly-GA proteins impair neuromuscular transmission

Amyotrophic lateral sclerosis (ALS) is a devastating motoneuron disease, in which lower motoneurons lose control of skeletal muscles. Degeneration of neuromuscular junctions (NMJs) occurs at the initial stage of ALS. Dipeptide repeat proteins (DPRs) from G4C2 repeat-associated non-ATG (RAN) translation are known to cause C9orf72-associated ALS (C9-ALS). However, DPR inclusion burdens are weakly correlated with neurodegenerative areas in C9-ALS patients, indicating that DPRs may exert cell non-autonomous effects, in addition to the known intracellular pathological mechanisms. Here, we report that poly-GA, the most abundant form of DPR in C9-ALS, is released from cells. Local administration of poly-GA proteins in peripheral synaptic regions causes muscle weakness and impaired neuromuscular transmission in vivo. The NMJ structure cannot be maintained, as evidenced by the fragmentation of postsynaptic acetylcholine receptor (AChR) clusters and distortion of presynaptic nerve terminals. Mechanistic study demonstrated that extracellular poly-GA sequesters soluble Agrin ligands and inhibits Agrin-MuSK signaling. Our findings provide a novel cell non-autonomous mechanism by which poly-GA impairs NMJs in C9-ALS. Thus, targeting NMJs could be an early therapeutic intervention for C9-ALS.

The era of cryptic exons: implications for ALS-FTD

TDP-43 is an RNA-binding protein with a crucial nuclear role in splicing, and mislocalises from the nucleus to the cytoplasm in a range of neurodegenerative disorders. TDP-43 proteinopathy spans a spectrum of incurable, heterogeneous, and increasingly prevalent neurodegenerative diseases, including the amyotrophic lateral sclerosis and frontotemporal dementia disease spectrum and a significant fraction of Alzheimer's disease. There are currently no directed disease-modifying therapies for TDP-43 proteinopathies, and no way to distinguish who is affected before death. It is now clear that TDP-43 proteinopathy leads to a number of molecular changes, including the de-repression and inclusion of cryptic exons. Importantly, some of these cryptic exons lead to the loss of crucial neuronal proteins and have been shown to be key pathogenic players in disease pathogenesis (e.g., STMN2), as well as being able to modify disease progression (e.g., UNC13A). Thus, these aberrant splicing events make promising novel therapeutic targets to restore functional gene expression. Moreover, presence of these cryptic exons is highly specific to patients and areas of the brain affected by TDP-43 proteinopathy, offering the potential to develop biomarkers for early detection and stratification of patients. In summary, the discovery of cryptic exons gives hope for novel diagnostics and therapeutics on the horizon for TDP-43 proteinopathies.

Autophagy and its mediated mitochondrial quality control maintain pollen tube growth and male fertility in Arabidopsis

Macroautophagy/autophagy, a major catabolic pathway in eukaryotes, participates in plant sexual reproduction including the processes of male gametogenesis and the self-incompatibility response. Rapid pollen tube growth is another essential reproductive process that is metabolically highly demanding to drive the vigorous cell growth for delivery of male gametes for fertilization in angiosperms. Whether and how autophagy operates to maintain the homeostasis of pollen tubes remains unknown. Here, we provide evidence that autophagy is elevated in growing pollen tubes and critically required during pollen tube growth and male fertility in Arabidopsis. We demonstrate that SH3P2, a critical non-ATG regulator of plant autophagy, colocalizes with representative ATG proteins during autophagosome biogenesis in growing pollen tubes. Downregulation of SH3P2 expression significantly disrupts Arabidopsis pollen germination and pollen tube growth. Further analysis of organelle dynamics reveals crosstalk between autophagosomes and prevacuolar compartments following the inhibition of phosphatidylinositol 3-kinase. In addition, time-lapse imaging and tracking of ATG8e-labeled autophagosomes and depolarized mitochondria demonstrate that they interact specifically via the ATG8-family interacting motif (AIM)-docking site to mediate mitophagy. Ultrastructural identification of mitophagosomes and two additional forms of autophagosomes imply that multiple types of autophagy are likely to function simultaneously within pollen tubes. Altogether, our results suggest that autophagy is functionally crucial for mediating mitochondrial quality control and canonical cytoplasm recycling during pollen tube growth.Abbreviations: AIM: ATG8-family interacting motif; ATG8: autophagy related 8; ATG5: autophagy related 5; ATG7: autophagy related 7; BTH: acibenzolar-S-methyl; DEX: dexamethasone; DNP: 2,4-dinitrophenol; GFP: green fluorescent protein; YFP: yellow fluorescent protein; PtdIns3K: phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol-3-phosphate; PVC: prevacuolar compartment; SH3P2: SH3 domain-containing protein 2.

Treatment of hereditary amyotrophic lateral sclerosis

Currently, only four molecules can be prescribed for amyotrophic lateral sclerosis (ALS), of which only one is approved worldwide for this indication, riluzole. Although progress in the therapeutic field remains unsatisfactory, we have to notice that genetics have undergone impressive improvements over the last three decades and, by extension, our knowledge of ALS cases linked to a pathogenic mutation that accounts for 10% of all cases (either sporadic or familiar) and is currently called hereditary ALS (hALS). In many neurological diseases treatment targeting pathogenic genes have significatively improved the natural profile of the disease: this is perfectly illustrated for familial amyloid neuropathy and spinal muscular atrophy. Because of these findings and the urgent need to find a cure for ALS, many trials have focused on familial ALS targeting the four most important genes linked to the disease: C9orf72, SOD1, TARDBP and FUS. We propose in this review an update on the perspectives of treatment that may be available in mid-term in hALS and will discuss in the last part the potential consequences for asymptomatic relatives of patients with a hALS and for ALS patients.

Comprehensive evaluation of human-derived anti-poly-GA antibodies in cellular and animal models of C9orf72 disease

Hexanucleotide G₄C₂ repeat expansions in the C9orf72 gene are the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. Dipeptide repeat proteins (DPRs) generated by translation of repeat-containing RNAs show toxic effects in vivo as well as in vitro and are key targets for therapeutic intervention. We generated human antibodies that bind DPRs with high affinity and specificity. Anti-GA antibodies engaged extra- and intra-cellular poly-GA and reduced aggregate formation in a poly-GA overexpressing human cell line. However, antibody treatment in human neuronal cultures synthesizing exogenous poly-GA resulted in the formation of large extracellular immune complexes and did not affect accumulation of intracellular poly-GA aggregates. Treatment with antibodies was also shown to directly alter the morphological and biochemical properties of poly-GA and to shift poly-GA/antibody complexes to more rapidly sedimenting ones. These alterations were not observed with poly-GP and have important implications for accurate measurement of poly-GA levels including the need to evaluate all centrifugation fractions and disrupt the interaction between treatment antibodies and poly-GA by denaturation. Targeting poly-GA and poly-GP in two mouse models expressing G₄C₂ repeats by systemic antibody delivery for up to 16 mo was well-tolerated and led to measurable brain penetration of antibodies. Long-term treatment with anti-GA antibodies produced improvement in an open-field movement test in aged C9orf72⁴⁵⁰ mice. However, chronic administration of anti-GA antibodies in AAV-(G₄C₂)₁₄₉ mice was associated with increased levels of poly-GA detected by immunoassay and did not significantly reduce poly-GA aggregates or alleviate disease progression in this model.

Rapid and comprehensive diagnostic method for repeat expansion diseases using nanopore sequencing

We developed a diagnostic method for repeat expansion diseases using a long-read sequencer to improve currently available, low throughput diagnostic methods. We employed the real-time target enrichment system of the nanopore GridION sequencer using the adaptive sampling option, in which software-based target assignment is available without prior sample enrichment, and built an analysis pipeline that prioritized the disease-causing loci. Twenty-two patients with various neurological and neuromuscular diseases, including 12 with genetically diagnosed repeat expansion diseases and 10 manifesting cerebellar ataxia, but without genetic diagnosis, were analyzed. We first sequenced the 12 molecularly diagnosed patients and accurately confirmed expanded repeats in all with uniform depth of coverage across the loci. Next, we applied our method and a conventional method to 10 molecularly undiagnosed patients. Our method corrected inaccurate diagnoses of two patients by the conventional method. Our method is superior to conventional diagnostic methods in terms of speed, accuracy, and comprehensiveness.

CRISPR/Cas9-mediated excision of ALS/FTD-causing hexanucleotide repeat expansion in C9ORF72 rescues major disease mechanisms in vivo and in vitro

A GGGGCC₂₄₊ hexanucleotide repeat expansion (HRE) in the C9ORF72 gene is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), fatal neurodegenerative diseases with no cure or approved treatments that substantially slow disease progression or extend survival. Mechanistic underpinnings of neuronal death include C9ORF72 haploinsufficiency, sequestration of RNA-binding proteins in the nucleus, and production of dipeptide repeat proteins. Here, we used an adeno-associated viral vector system to deliver CRISPR/Cas9 gene-editing machineries to effectuate the removal of the HRE from the C9ORF72 genomic locus. We demonstrate successful excision of the HRE in primary cortical neurons and brains of three mouse models containing the expansion (500-600 repeats) as well as in patient-derived iPSC motor neurons and brain organoids (450 repeats). This resulted in a reduction of RNA foci, poly-dipeptides and haploinsufficiency, major hallmarks of C9-ALS/FTD, making this a promising therapeutic approach to these diseases.

The contribution of proteasomal impairment to autophagy activation by C9orf72 poly-GA aggregates

BACKGROUND

Poly-GA, a dipeptide repeat protein unconventionally translated from GGGGCC (G4C2) repeat expansions in C9orf72, is abundant in C9orf72-related amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (C9orf72-ALS/FTD). Although the poly-GA aggregates have been identified in C9orf72-ALS/FTD neurons, the effects on UPS (ubiquitin-proteasome system) and autophagy and their exact molecular mechanisms have not been fully elucidated.

RESULTS

Herein, our in vivo experiments indicate that the mice expressing ploy-GA with 150 repeats instead of 30 repeats exhibit significant aggregates in cells. Mice expressing 150 repeats ploy-GA shows behavioral deficits and activates autophagy in the brain. In vitro findings suggest that the poly-GA aggregates influence proteasomal by directly binding proteasome subunit PSMD2. Subsequently, the poly-GA aggregates activate phosphorylation and ubiquitination of p62 to recruit autophagosomes. Ultimately, the poly-GA aggregates lead to compensatory activation of autophagy. In vivo studies further reveal that rapamycin (autophagy activator) treatment significantly improves the degenerative symptoms and alleviates neuronal injury in mice expressing 150 repeats poly-GA. Meanwhile, rapamycin administration to mice expressing 150 repeats poly-GA reduces neuroinflammation and aggregates in the brain.

CONCLUSION

In summary, we elucidate the relationship between poly-GA in the proteasome and autophagy: when poly-GA forms complexes with the proteasome, it recruits autophagosomes and affects proteasome function. Our study provides support for further promoting the comprehension of the pathogenesis of C9orf72, which may bring a hint for the exploration of rapamycin for the treatment of ALS/FTD.

Enhanced motor cortex output and disinhibition in asymptomatic female mice with C9orf72 genetic expansion

Information and action coding by cortical circuits relies on a balanced dialogue between excitation and inhibition. Circuit hyperexcitability is considered a potential pathophysiological mechanism in various brain disorders, but the underlying deficits, especially at early disease stages, remain largely unknown. We report that asymptomatic female mice carrying the chromosome 9 open reading frame 72 (C9orf72) repeat expansion, which represents a high-prevalence genetic abnormality for human amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) spectrum disorder, exhibit abnormal motor cortex output. The number of primary motor cortex (M1) layer 5 pyramidal neurons is reduced in asymptomatic mice, with the surviving neurons receiving a decreased inhibitory drive that results in a higher M1 output, specifically during high-speed animal locomotion. Importantly, using deep-learning algorithms revealed that speed-dependent M1 output predicts the likelihood of C9orf72 genetic expansion. Our data link early circuit abnormalities with a gene mutation in asymptomatic ALS/FTLD carriers.

Targeting tau only extracellularly is likely to be less efficacious than targeting it both intra- and extracellularly

Aggregation of the tau protein is thought to be responsible for the neurodegeneration and subsequent functional impairments in diseases that are collectively named tauopathies. Alzheimer's disease is the most common tauopathy, but the group consists of over 20 different diseases, many of which have tau pathology as their primary feature. The development of tau therapies has mainly focused on preventing the formation of and/or clearing these aggregates. Of these, immunotherapies that aim to either elicit endogenous tau antibodies or deliver exogenous ones are the most common approach in clinical trials. While their mechanism of action can involve several pathways, both extra- and intracellular, pharmaceutical companies have primarily focused on antibody-mediated clearance of extracellular tau. As we have pointed out over the years, this is rather surprising because it is well known that most of pathological tau protein is found intracellularly. It has been repeatedly shown by several groups over the past decades that antibodies can enter neurons and that their cellular uptake can be enhanced by various means, particularly by altering their charge. Here, we will briefly describe the potential extra- and intracellular mechanisms involved in antibody-mediated clearance of tau pathology, discuss these in the context of recent failures of some of the tau antibody trials, and finally provide a brief overview of how the intracellular efficacy of tau antibodies can potentially be further improved by certain modifications that aim to enhance tau clearance via specific intracellular degradation pathways.

Poly(GR) and poly(GA) in cerebrospinal fluid as potential biomarkers for C9ORF72-ALS/FTD

GGGGCC repeat expansion in C9ORF72, which can be translated in both sense and antisense directions into five dipeptide repeat (DPR) proteins, including poly(GP), poly(GR), and poly(GA), is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Here we developed sensitive assays that can detect poly(GA) and poly(GR) in the cerebrospinal fluid (CSF) of patients with C9ORF72 mutations. CSF poly(GA) and poly(GR) levels did not correlate with age at disease onset, disease duration, or rate of decline of ALS Functional Rating Scale, and the average levels of these DPR proteins were similar in symptomatic and pre-symptomatic patients with C9ORF72 mutations. However, in a patient with C9ORF72-ALS who was treated with antisense oligonucleotide (ASO) targeting the aberrant C9ORF72 transcript, CSF poly(GA) and poly(GR) levels decreased approximately 50% within 6 weeks, indicating they may serve as sensitive fluid-based biomarkers in studies directed against the production of GGGGCC repeat RNAs or DPR proteins.

Cracking the cryptic code in amyotrophic lateral sclerosis and frontotemporal dementia: Towards therapeutic targets and biomarkers

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are two devastating human neurodegenerative diseases. A hallmark pathological feature of both diseases is the depletion of the RNA-binding protein TDP-43 from the nucleus in the brain and spinal cord of patients. A major function of TDP-43 is to repress the inclusion of cryptic exons during RNA splicing. When it becomes depleted from the nucleus in disease, this function is lost, and recently, several key cryptic splicing targets of TDP-43 have emerged, including STMN2, UNC13A, and others. UNC13A is a major ALS/FTD risk gene, and the genetic variations that increase the risk for disease seem to do so by making the gene more susceptible to cryptic exon inclusion when TDP-43 function is impaired. Here, we discuss the prospects and challenges of harnessing these cryptic splicing events as novel therapeutic targets and biomarkers. Deciphering this new cryptic code may be a touchstone for ALS and FTD diagnosis and treatment.

Proximity proteomics of C9orf72 dipeptide repeat proteins identifies molecular chaperones as modifiers of poly-GA aggregation

The most common inherited cause of two genetically and clinico-pathologically overlapping neurodegenerative diseases, amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), is the presence of expanded GGGGCC intronic hexanucleotide repeats in the C9orf72 gene. Aside from haploinsufficiency and toxic RNA foci, another non-exclusive disease mechanism is the non-canonical translation of the repeat RNA into five different dipeptide repeat proteins (DPRs), which form neuronal inclusions in affected patient brains. While evidence from cellular and animal models supports a toxic gain-of-function of pathologic poly-GA, poly-GR, and poly-PR aggregates in promoting deposition of TDP-43 pathology and neurodegeneration in affected brain areas, the relative contribution of DPRs to the disease process in c9FTD/ALS patients remains unclear. Here we have used the proximity-dependent biotin identification (BioID) proximity proteomics approach to investigate the formation and collective composition of DPR aggregates using cellular models. While interactomes of arginine rich poly-GR and poly-PR aggregates overlapped and were enriched for nucleolar and ribosomal proteins, poly-GA aggregates demonstrated a distinct association with proteasomal components, molecular chaperones (HSPA1A/HSP70, HSPA8/HSC70, VCP/p97), co-chaperones (BAG3, DNAJA1A) and other factors that regulate protein folding and degradation (SQSTM1/p62, CALR, CHIP/STUB1). Experiments in cellular models of poly-GA pathology show that molecular chaperones and co-chaperones are sequestered to the periphery of dense cytoplasmic aggregates, causing depletion from their typical cellular localization. Their involvement in the pathologic process is confirmed in autopsy brain tissue, where HSPA8, BAG3, VCP, and its adapter protein UBXN6 show a close association with poly-GA aggregates in the frontal cortex, temporal cortex, and hippocampus of c9FTLD and c9ALS cases. The association of heat shock proteins and co-chaperones with poly-GA led us to investigate their potential role in reducing its aggregation. We identified HSP40 co-chaperones of the DNAJB family as potent modifiers that increased the solubility of poly-GA, highlighting a possible novel therapeutic avenue and a central role of molecular chaperones in the pathogenesis of human C9orf72-linked diseases.

HDAC6 Interacts With Poly (GA) and Modulates its Accumulation in c9FTD/ALS

The aberrant translation of a repeat expansion in chromosome 9 open reading frame 72 (C9orf72), the most common cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), results in the accumulation of toxic dipeptide repeat (DPR) proteins in the central nervous system We have found that, among the sense DPR proteins, HDAC6 specifically interacts with the poly (GA) and co-localizes with inclusions in both patient tissue and a mouse model of this disease (c9FTD/ALS). Overexpression of HDAC6 increased poly (GA) levels in cultured cells independently of HDAC6 deacetylase activity, suggesting that HDAC6 can modulate poly (GA) pathology through a mechanism that depends upon their physical interaction. Moreover, decreasing HDAC6 expression by stereotaxic injection of antisense oligonucleotides significantly reduced the number of poly (GA) inclusions in c9FTD/ALS mice. These findings suggest that pharmacologically reducing HDAC6 levels could be of therapeutic value in c9FTD/ALS.

Altered Blood-Brain Barrier and Blood-Spinal Cord Barrier Dynamics in Amyotrophic Lateral Sclerosis: Impact on Medication Efficacy and Safety

The access of drugs into the central nervous system (CNS) is regulated by the blood-brain barrier (BBB) and blood-spinal cord barrier (BSCB). A large body of evidence supports perturbation of these barriers in neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. Modifications to the BBB and BSCB are also reported in amyotrophic lateral sclerosis (ALS), albeit these modifications have received less attention relative to those in other neurodegenerative diseases. Such alterations to the BBB and BSCB have the potential to impact on CNS exposure of drugs in ALS, modulating the effectiveness of drugs intended to reach the brain and the toxicity of drugs that are not intended to reach the brain. Given the clinical importance of these phenomena, this review will summarise reported modifications to the BBB and BSCB in ALS, discuss their impact on CNS drug exposure and suggest further research directions so as to optimise medicine use in people with ALS.

The molecular pathogenesis of repeat expansion diseases

Expanded short tandem repeats in the genome cause various monogenic diseases, particularly neurological disorders. Since the discovery of a CGG repeat expansion in the FMR1 gene in 1991, more than 40 repeat expansion diseases have been identified to date. In the coding repeat expansion diseases, in which the expanded repeat sequence is located in the coding regions of genes, the toxicity of repeat polypeptides, particularly misfolding and aggregation of proteins containing an expanded polyglutamine tract, have been the focus of investigation. On the other hand, in the non-coding repeat expansion diseases, in which the expanded repeat sequence is located in introns or untranslated regions, the toxicity of repeat RNAs has been the focus of investigation. Recently, these repeat RNAs were demonstrated to be translated into repeat polypeptides by the novel mechanism of repeat-associated non-AUG translation, which has extended the research direction of the pathological mechanisms of this disease entity to include polypeptide toxicity. Thus, a common pathogenesis has been suggested for both coding and non-coding repeat expansion diseases. In this review, we briefly outline the major pathogenic mechanisms of repeat expansion diseases, including a loss-of-function mechanism caused by repeat expansion, repeat RNA toxicity caused by RNA foci formation and protein sequestration, and toxicity by repeat polypeptides. We also discuss perturbation of the physiological liquid-liquid phase separation state caused by these repeat RNAs and repeat polypeptides, as well as potential therapeutic approaches against repeat expansion diseases.

RAN proteins in neurodegenerative disease: Repeating themes and unifying therapeutic strategies

Microsatellite-expansion mutations cause >50 neurological diseases but there are no effective treatments. Mechanistic studies have historically focused on protein loss-of-function and protein or RNA gain-of-function effects. It is now clear that many expansion mutations are bidirectionally transcribed producing two toxic expansion RNAs, which can produce up to six mutant proteins by repeat associated non-AUG (RAN) translation. Multiple types of RAN proteins have been shown to be toxic in cell and animal models, to lead to common types of neuropathological changes, and to dysregulate key pathways. How RAN proteins are produced without the canonical AUG or close-cognate AUG-like initiation codons is not yet completely understood but RNA structure, flanking sequences and stress pathways have been shown to be important. Here, we summarize recent progress in understanding the role of RAN proteins, mechanistic insights into their production, and the identification of novel therapeutic strategies that may be applicable across these neurodegenerative disorders.

Neurologic Therapeutics in 2035: The Neurology Future Forecasting Series

Therapeutic development has accelerated rapidly in the past 5 years in many neurologic and neurodegenerative diseases. The therapeutic categories of development include small molecules, genetic therapies, and cell-based therapies. Current development has provided novel treatment approaches to disorders without available treatment. However, the regulatory procedures to allow for access to these therapies is challenging, as is the ability to provide wide access to increasingly expensive therapies. By 2035, these challenges are likely to have accelerated and have the potential to create bottlenecks in drug approval and reduced access to patients. Innovative regulatory and payer solutions are required. In addition, ethical considerations around genetic therapies should be considered in current and future development. These approaches will ensure that patients with neurologic disease have broad access to highly innovative therapies.

Antibody-Based Therapeutic Interventions for Amyotrophic Lateral Sclerosis: A Systematic Literature Review

Amyotrophic Lateral Sclerosis (ALS) is a mid-life onset neurodegenerative disease that manifests its symptomatology with motor impairments and cognitive deficits overlapping with Frontotemporal Lobar Degeneration (FTLD). The etiology of ALS remains elusive, with various mechanisms and cellular targets implicated, and no treatment can reverse or stop the progression of the pathology. Therapeutic interventions based on passive immunization are gaining attention for neurodegenerative diseases, and FDA recently approved the first antibody-based approach for Alzheimer's disease. The present systematic review of the literature aims to highlight the efforts made over the past years at developing antibody-based strategies to cure ALS. Thirty-one original research papers have been selected where the therapeutic efficacy of antibodies were investigated and described in patients and animal models of ALS. Antibody-based interventions analyzed, target both extracellular molecules implicated in the pathology and intracellular pathogenic proteins known to drive the disease, such as SOD1, TDP-43 or C9ORF72 repeats expansions. The potentials and limitations of these therapeutic interventions have been described and discussed in the present review.

The molecular mechanisms that drive intracellular neutralization by the antibody-receptor and RING E3 ligase TRIM21

The cytosolic antibody receptor and RING E3 ligase TRIM21 targets intracellular, antibody-coated immune complexes for degradation and activates the immune system. Here we review how TRIM21 degrades diverse targets and how this activity can be exploited in molecular biology and for the development of new therapeutics. In addition, we compare what is known about TRIM21's mechanism to other TRIM proteins and RING E3 ligases.

Genetically encoded intrabodies as high-precision tools to visualize and manipulate neuronal function

Basic neuroscience research employs numerous forms of antibodies as key reagents in diverse applications. While the predominant use of antibodies is as immunolabeling reagents, neuroscientists are making increased use of intracellular antibodies or intrabodies. Intrabodies are recombinant antibodies genetically encoded for expression within neurons. These can be used to target various cargo (fluorescent proteins, reporters, enzymes, etc.) to specific molecules and subcellular domains to report on and manipulate neuronal function with high precision. Intrabodies have the advantages inherent in all genetically encoded recombinant antibodies but represent a distinct subclass in that their structure allows for their expression and function within cells. The high precision afforded by the ability to direct their expression to specific cell types, and the selective binding of intrabodies to targets within these allows intrabodies to offer unique advantages for neuroscience research, given the tremendous molecular, cellular and morphological complexity of brain neurons. Intrabodies expressed within neurons have been used for a variety of purposes in basic neuroscience research. Here I provide a general background to intrabodies and their development, and examples of their emerging utility as valuable basic neuroscience research tools.

The Evolving Concept of Neuro-Thromboinflammation for Neurodegenerative Disorders and Neurotrauma: A Rationale for PAR1-Targeting Therapies

Interest in the role of coagulation and fibrinolysis in the nervous system was active in several laboratories dating back before cloning of the functional thrombin receptor in 1991. As one of those, our attention was initially on thrombin and plasminogen activators in synapse formation and elimination in the neuromuscular system, with orientation towards diseases such as amyotrophic lateral sclerosis (ALS) and how clotting and fibrinolytic pathways fit into its pathogenesis. This perspective is on neuro-thromboinflammation, emphasizing this emerging concept from studies and reports over more than three decades. It underscores how it may lead to novel therapeutic approaches to treat the ravages of neurotrauma and neurodegenerative diseases, with a focus on PAR1, ALS, and parmodulins.

A C. elegans model of C9orf72-associated ALS/FTD uncovers a conserved role for eIF2D in RAN translation

A hexanucleotide repeat expansion GGGGCC in the non-coding region of C9orf72 is the most common cause of inherited amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Toxic dipeptide repeats (DPRs) are synthesized from GGGGCC via repeat-associated non-AUG (RAN) translation. Here, we develop C. elegans models that express, either ubiquitously or exclusively in neurons, 75 GGGGCC repeats flanked by intronic C9orf72 sequence. The worms generate DPRs (poly-glycine-alanine [poly-GA], poly-glycine-proline [poly-GP]) and poly-glycine-arginine [poly-GR]), display neurodegeneration, and exhibit locomotor and lifespan defects. Mutation of a non-canonical translation-initiating codon (CUG) upstream of the repeats selectively reduces poly-GA steady-state levels and ameliorates disease, suggesting poly-GA is pathogenic. Importantly, loss-of-function mutations in the eukaryotic translation initiation factor 2D (eif-2D/eIF2D) reduce poly-GA and poly-GP levels, and increase lifespan in both C. elegans models. Our in vitro studies in mammalian cells yield similar results. Here, we show a conserved role for eif-2D/eIF2D in DPR expression.

Targeted protein degradation using intracellular antibodies and its application to neurodegenerative disease

Antibodies mediate the majority of their effects in the extracellular domain, or in intracellular compartments isolated from the cytosol. Under a growing list of circumstances, however, antibodies are found to gain access to the cytoplasm. Cytosolic immune complexes are bound by the atypical antibody receptor TRIM21, which mediates the rapid degradation of the immune complexes at the proteasome. These discoveries have informed the development of TRIM-Away, a technique to selectively deplete proteins using delivery of antibodies into cells. A range of related approaches that elicit selective protein degradation using intracellular constructs linking antibody fragments to degradative effector functions have also been developed. These methods hold promise for inducing the degradation of proteins as both research tools and as a novel therapeutic approach. Protein aggregates are a pathophysiological feature of neurodegenerative diseases and are considered to have a causal role in pathology. Immunotherapy is emerging as a promising route towards their selective targeting, and a role of antibodies in the cytosol has been demonstrated in cell-based assays. This review will explore the mechanisms by which therapeutic antibodies engage and eliminate intracellularly aggregated proteins. We will discuss how future developments in intracellular antibody technology may enhance the therapeutic potential of such antibody-derived therapies.

Therapeutic strategies for C9orf72 amyotrophic lateral sclerosis and frontotemporal dementia

PURPOSE OF REVIEW

An intronic G4C2 expansion mutation in C9orf72 is the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia (C9-ALS/FTD). Although there are currently no treatments for this insidious, fatal disease, intense research has led to promising therapeutic strategies, which will be discussed here.

RECENT FINDINGS

Therapeutic strategies for C9-ALS/FTD have primarily focused on reducing the toxic effects of mutant expansion RNAs or the dipeptide repeat proteins (DPRs). The pathogenic effects of G4C2 expansion transcripts have been targeted using approaches aimed at promoting their degradation, inhibiting nuclear export or silencing transcription. Other promising strategies include immunotherapy to reduce the DPRs themselves, reducing RAN translation, removing the repeats using DNA or RNA editing and manipulation of downstream disease-altered stress granule pathways. Finally, understanding the molecular triggers that lead to pheno-conversion may lead to opportunities that can delay symptomatic disease onset.

SUMMARY

A large body of evidence implicates RAN-translated DPRs as a main driver of C9-ALS/FTD. Promising therapeutic strategies for these devastating diseases are being rapidly developed with several approaches already in or approaching clinical trials.