Using Drosophila amyloid toxicity models to study Alzheimer's disease.

Alzheimer's disease (AD) is the most prevalent form of dementia and is characterised by a progressive loss of neurons, which manifests as gradual memory decline, followed by cognitive loss. Despite the significant progress in identifying novel biomarkers and understanding the prodromal pathology and symptomatology, AD remains a significant unmet clinical need. Lecanemab and aducanumab, the only Food and Drug Administration approved drugs to exhibit some disease-modifying clinical efficacy, target Aß amyloid, underscoring the importance of this protein in disease aetiology. Nevertheless, in the absence of a definitive cure, the utilisation of preclinical models remains imperative for the identification of novel therapeutic targets and the evaluation of potential therapeutic agents. Drosophila melanogaster is a model system that can be used as a research tool to investigate neurodegeneration and therapeutic interventions. The short lifespan, low price and ease of husbandry/rearing make Drosophila an advantageous model organism from a practical perspective. However, it is the highly conserved genome and similarity of Drosophila and human neurobiology which make flies a powerful tool to investigate neurodegenerative mechanisms. In addition, the ease of transgenic modifications allows for early proof of principle studies for future therapeutic approaches in neurodegenerative research. This mini review will specifically focus on utilising Drosophila as an in vivo model of amyloid toxicity in AD.

PolyGR and polyPR knock-in mice reveal a conserved neuroprotective extracellular matrix signature in C9orf72 ALS/FTD neurons.

Dipeptide repeat proteins are a major pathogenic feature of C9orf72 amyotrophic lateral sclerosis (C9ALS)/frontotemporal dementia (FTD) pathology, but their physiological impact has yet to be fully determined. Here we generated C9orf72 dipeptide repeat knock-in mouse models characterized by expression of 400 codon-optimized polyGR or polyPR repeats, and heterozygous C9orf72 reduction. (GR)400 and (PR)400 knock-in mice recapitulate key features of C9ALS/FTD, including cortical neuronal hyperexcitability, age-dependent spinal motor neuron loss and progressive motor dysfunction. Quantitative proteomics revealed an increase in extracellular matrix (ECM) proteins in (GR)400 and (PR)400 spinal cord, with the collagen COL6A1 the most increased protein. TGF-ß1 was one of the top predicted regulators of this ECM signature and polyGR expression in human induced pluripotent stem cell neurons was sufficient to induce TGF-ß1 followed by COL6A1. Knockdown of TGF-ß1 or COL6A1 orthologues in polyGR model Drosophila exacerbated neurodegeneration, while expression of TGF-ß1 or COL6A1 in induced pluripotent stem cell-derived motor neurons of patients with C9ALS/FTD protected against glutamate-induced cell death. Altogether, our findings reveal a neuroprotective and conserved ECM signature in C9ALS/FTD.

A monocarboxylate transporter rescues frontotemporal dementia and Alzheimer's disease models.

Brains are highly metabolically active organs, consuming 20% of a person's energy at resting state. A decline in glucose metabolism is a common feature across a number of neurodegenerative diseases. Another common feature is the progressive accumulation of insoluble protein deposits, it's unclear if the two are linked. Glucose metabolism in the brain is highly coupled between neurons and glia, with glucose taken up by glia and metabolised to lactate, which is then shuttled via transporters to neurons, where it is converted back to pyruvate and fed into the TCA cycle for ATP production. Monocarboxylates are also involved in signalling, and play broad ranging roles in brain homeostasis and metabolic reprogramming. However, the role of monocarboxylates in dementia has not been tested. Here, we find that increasing pyruvate import in Drosophila neurons by over-expression of the transporter bumpel, leads to a rescue of lifespan and behavioural phenotypes in fly models of both frontotemporal dementia and Alzheimer's disease. The rescue is linked to a clearance of late stage autolysosomes, leading to degradation of toxic peptides associated with disease. We propose upregulation of pyruvate import into neurons as potentially a broad-scope therapeutic approach to increase neuronal autophagy, which could be beneficial for multiple dementias.

Frontotemporal Dementia and Glucose Metabolism.

Frontotemporal dementia (FTD), hallmarked by antero-temporal degeneration in the human brain, is the second most common early onset dementia. FTD is a diverse disease with three main clinical presentations, four different identified proteinopathies and many disease-associated genes. The exact pathophysiology of FTD remains to be elucidated. One common characteristic all forms of FTD share is the dysregulation of glucose metabolism in patients' brains. The brain consumes around 20% of the body's energy supply and predominantly utilizes glucose as a fuel. Glucose metabolism dysregulation could therefore be extremely detrimental for neuronal health. Research into the association between glucose metabolism and dementias has recently gained interest in Alzheimer's disease. FTD also presents with glucose metabolism dysregulation, however, this remains largely an unexplored area. A better understanding of the link between FTD and glucose metabolism may yield further insight into FTD pathophysiology and aid the development of novel therapeutics. Here we review our current understanding of FTD and glucose metabolism in the brain and discuss the evidence of impaired glucose metabolism in FTD. Lastly, we review research potentially suggesting a causal relationship between FTD proteinopathies and impaired glucose metabolism in FTD.

Mitochondria Dysfunction in Frontotemporal Dementia/Amyotrophic Lateral Sclerosis: Lessons From Drosophila Models.

Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) are neurodegenerative disorders characterized by declining motor and cognitive functions. Even though these diseases present with distinct sets of symptoms, FTD and ALS are two extremes of the same disease spectrum, as they show considerable overlap in genetic, clinical and neuropathological features. Among these overlapping features, mitochondrial dysfunction is associated with both FTD and ALS. Recent studies have shown that cells derived from patients' induced pluripotent stem cells (iPSC)s display mitochondrial abnormalities, and similar abnormalities have been observed in a number of animal disease models. Drosophila models have been widely used to study FTD and ALS because of their rapid generation time and extensive set of genetic tools. A wide array of fly models have been developed to elucidate the molecular mechanisms of toxicity for mutations associated with FTD/ALS. Fly models have been often instrumental in understanding the role of disease associated mutations in mitochondria biology. In this review, we discuss how mutations associated with FTD/ALS disrupt mitochondrial function, and we review how the use of Drosophila models has been pivotal to our current knowledge in this field.

Activating transcription factor 4-dependent lactate dehydrogenase activation as a protective response to amyloid beta toxicity.

Accumulation of amyloid beta peptides is thought to initiate the pathogenesis of Alzheimer's disease. However, the precise mechanisms mediating their neurotoxicity are unclear. Our microarray analyses show that, in Drosophila models of amyloid beta 42 toxicity, genes involved in the unfolded protein response and metabolic processes are upregulated in brain. Comparison with the brain transcriptome of early-stage Alzheimer's patients revealed a common transcriptional signature, but with generally opposing directions of gene expression changes between flies and humans. Among these differentially regulated genes, lactate dehydrogenase (Ldh) was up-regulated by the greatest degree in amyloid beta 42 flies and the human orthologues (LDHA and LDHB) were down-regulated in patients. Functional analyses revealed that either over-expression or inhibition of Ldh by RNA interference (RNAi) slightly exacerbated climbing defects in both healthy and amyloid beta 42-induced Drosophila. This suggests that metabolic responses to lactate dehydrogenase must be finely-tuned, and that its observed upregulation following amyloid beta 42 production could potentially represent a compensatory protection to maintain pathway homeostasis in this model, with further manipulation leading to detrimental effects. The increased Ldh expression in amyloid beta 42 flies was regulated partially by unfolded protein response signalling, as ATF4 RNAi diminished the transcriptional response and enhanced amyloid beta 42-induced climbing phenotypes. Further functional studies are required to determine whether Ldh upregulation provides compensatory neuroprotection against amyloid beta 42-induced loss of activating transcription factor 4 activity and endoplasmatic reticulum stress. Our study thus reveals dysregulation of lactate dehydrogenase signalling in Drosophila models and patients with Alzheimer's disease, which may lead to a detrimental loss of metabolic homeostasis. Importantly, we observed that down-regulation of ATF4-dependent endoplasmic reticulum-stress signalling in this context appears to prevent Ldh compensation and to exacerbate amyloid beta 42-dependent neuronal toxicity. Our findings, therefore, suggest caution in the use of therapeutic strategies focussed on down-regulation of this pathway for the treatment of Alzheimer's disease, since its natural response to the toxic peptide may induce beneficial neuroprotective effects.

Enhanced insulin signalling ameliorates C9orf72 hexanucleotide repeat expansion toxicity in Drosophila.

G4C2 repeat expansions within the C9orf72 gene are the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The repeats undergo repeat-associated non-ATG translation to generate toxic dipeptide repeat proteins. Here, we show that insulin/IGF signalling is reduced in fly models of C9orf72 repeat expansion using RNA sequencing of adult brain. We further demonstrate that activation of insulin/IGF signalling can mitigate multiple neurodegenerative phenotypes in flies expressing either expanded G4C2 repeats or the toxic dipeptide repeat protein poly-GR. Levels of poly-GR are reduced when components of the insulin/IGF signalling pathway are genetically activated in the diseased flies, suggesting a mechanism of rescue. Modulating insulin signalling in mammalian cells also lowers poly-GR levels. Remarkably, systemic injection of insulin improves the survival of flies expressing G4C2 repeats. Overall, our data suggest that modulation of insulin/IGF signalling could be an effective therapeutic approach against C9orf72 ALS/FTD.

Cell type-specific modulation of healthspan by Forkhead family transcription factors in the nervous system.

Reduced activity of insulin/insulin-like growth factor signaling (IIS) increases healthy lifespan among diverse animal species. Downstream of IIS, multiple evolutionarily conserved transcription factors (TFs) are required; however, distinct TFs are likely responsible for these effects in different tissues. Here we have asked which TFs can extend healthy lifespan within distinct cell types of the adult nervous system in Drosophila Starting from published single-cell transcriptomic data, we report that forkhead (FKH) is endogenously expressed in neurons, whereas forkhead-box-O (FOXO) is expressed in glial cells. Accordingly, we find that neuronal FKH and glial FOXO exert independent prolongevity effects. We have further explored the role of neuronal FKH in a model of Alzheimer's disease-associated neuronal dysfunction, where we find that increased neuronal FKH preserves behavioral function and reduces ubiquitinated protein aggregation. Finally, using transcriptomic profiling, we identify Atg17, a member of the Atg1 autophagy initiation family, as one FKH-dependent target whose neuronal overexpression is sufficient to extend healthy lifespan. Taken together, our results underscore the importance of cell type-specific mapping of TF activity to preserve healthy function with age.

PICALM rescues glutamatergic neurotransmission, behavioural function and survival in a Drosophila model of Aß42 toxicity.

Alzheimer's disease (AD) is the most common form of dementia and the most prevalent neurodegenerative disease. Genome-wide association studies have linked PICALM to AD risk. PICALM has been implicated in Aß42 production and turnover, but whether it plays a direct role in modulating Aß42 toxicity remains unclear. We found that increased expression of the Drosophila PICALM orthologue lap could rescue Aß42 toxicity in an adult-onset model of AD, without affecting Aß42 level. Imbalances in the glutamatergic system, leading to excessive, toxic stimulation, have been associated with AD. We found that Aß42 caused the accumulation of presynaptic vesicular glutamate transporter (VGlut) and increased spontaneous glutamate release. Increased lap expression reversed these phenotypes back to control levels, suggesting that lap may modulate glutamatergic transmission. We also found that lap modulated the localization of amphiphysin (Amph), the homologue of another AD risk factor BIN1, and that Amph itself modulated postsynaptic glutamate receptor (GluRII) localization. We propose a model where PICALM modulates glutamatergic transmission, together with BIN1, to ameliorate synaptic dysfunction and disease progression.

RPS25 is required for efficient RAN translation of C9orf72 and other neurodegenerative disease-associated nucleotide repeats.

Nucleotide repeat expansions in the C9orf72 gene are the most common cause of amyotrophic lateral sclerosis and frontotemporal dementia. Unconventional translation (RAN translation) of C9orf72 repeats generates dipeptide repeat proteins that can cause neurodegeneration. We performed a genetic screen for regulators of RAN translation and identified small ribosomal protein subunit 25 (RPS25), presenting a potential therapeutic target for C9orf72-related amyotrophic lateral sclerosis and frontotemporal dementia and other neurodegenerative diseases caused by nucleotide repeat expansions.

C9orf72 arginine-rich dipeptide proteins interact with ribosomal proteins in vivo to induce a toxic translational arrest that is rescued by eIF1A.

A GGGGCC hexanucleotide repeat expansion within the C9orf72 gene is the most common genetic cause of both amyotrophic lateral sclerosis and frontotemporal dementia. Sense and antisense repeat-containing transcripts undergo repeat-associated non-AUG-initiated translation to produce five dipeptide proteins (DPRs). The polyGR and polyPR DPRs are extremely toxic when expressed in Drosophila neurons. To determine the mechanism that mediates this toxicity, we purified DPRs from the Drosophila brain and used mass spectrometry to identify the in vivo neuronal DPR interactome. PolyGR and polyPR interact with ribosomal proteins, and inhibit translation in both human iPSC-derived motor neurons, and adult Drosophila neurons. We next performed a screen of 81 translation-associated proteins in GGGGCC repeat-expressing Drosophila to determine whether this translational repression can be overcome and if this impacts neurodegeneration. Expression of the translation initiation factor eIF1A uniquely rescued DPR-induced toxicity in vivo, indicating that restoring translation is a potential therapeutic strategy. These data directly implicate translational repression in C9orf72 repeat-induced neurodegeneration and identify eIF1A as a novel modifier of C9orf72 repeat toxicity.

Sense and antisense RNA are not toxic in Drosophila models of C9orf72-associated ALS/FTD.

A GGGGCC hexanucleotide repeat expansion in the C9orf72 gene is the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. Neurodegeneration may occur via transcription of the repeats into inherently toxic repetitive sense and antisense RNA species, or via repeat-associated non-ATG initiated translation (RANT) of sense and antisense RNA into toxic dipeptide repeat proteins. We have previously demonstrated that regular interspersion of repeat RNA with stop codons prevents RANT (RNA-only models), allowing us to study the role of repeat RNA in isolation. Here we have created novel RNA-only Drosophila models, including the first models of antisense repeat toxicity, and flies expressing extremely large repeats, within the range observed in patients. We generated flies expressing ~ 100 repeat sense or antisense RNA either as part of a processed polyadenylated transcript or intronic sequence. We additionally created Drosophila expressing > 1000 RNA-only repeats in the sense direction. When expressed in adult Drosophila neurons polyadenylated repeat RNA is largely cytoplasmic in localisation, whilst intronic repeat RNA forms intranuclear RNA foci, as does > 1000 repeat RNA, thus allowing us to investigate both nuclear and cytoplasmic RNA toxicity. We confirmed that these RNA foci are capable of sequestering endogenous Drosophila RNA-binding proteins, and that the production of dipeptide proteins (poly-glycine-proline, and poly-glycine-arginine) is suppressed in our models. We find that neither cytoplasmic nor nuclear sense or antisense RNA are toxic when expressed in adult Drosophila neurons, suggesting they have a limited role in disease pathogenesis.

G-quadruplex-binding small molecules ameliorate C9orf72 FTD/ALS pathology in vitro and in vivo.

Intronic GGGGCC repeat expansions in C9orf72 are the most common known cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), which are characterised by degeneration of cortical and motor neurons, respectively. Repeat expansions have been proposed to cause disease by both the repeat RNA forming foci that sequester RNA-binding proteins and through toxic dipeptide repeat proteins generated by repeat-associated non-ATG translation. GGGGCC repeat RNA folds into a G-quadruplex secondary structure, and we investigated whether targeting this structure is a potential therapeutic strategy. We performed a screen that identified three structurally related small molecules that specifically stabilise GGGGCC repeat G-quadruplex RNA We investigated their effect in C9orf72 patient iPSC-derived motor and cortical neurons and show that they significantly reduce RNA foci burden and the levels of dipeptide repeat proteins. Furthermore, they also reduce dipeptide repeat proteins and improve survival in vivo, in GGGGCC repeat-expressing Drosophila Therefore, small molecules that target GGGGCC repeat G-quadruplexes can ameliorate the two key pathologies associated with C9orf72 FTD/ALS These data provide proof of principle that targeting GGGGCC repeat G-quadruplexes has therapeutic potential.

Ageing as a risk factor for ALS/FTD.

Like many other neurodegenerative diseases, age is a major risk factor in the development of ALS/FTD. But why is this the case? Recent genetic advances have highlighted some of pathways involved in the development of disease, and, strikingly, they appear to substantially overlap with those known to directly modulate the ageing process. Many ALS/FTD linked genes play a direct role in autophagy/lysosomal degradation, one of the most important pathways linked to ageing. However, systemic processes such as inflammation, as well as cellular maintenance pathways, including RNA splicing and nuclear-cytoplasmic transport have been increasingly linked both to disease and ageing. We highlight some of the shared mechanisms between the ageing process itself and emerging pathogenic mechanisms in ALS/FTD.

Increased Glucose Transport into Neurons Rescues Aß Toxicity in Drosophila.

Glucose hypometabolism is a prominent feature of the brains of patients with Alzheimer's disease (AD). Disease progression is associated with a reduction in glucose transporters in both neurons and endothelial cells of the blood-brain barrier. However, whether increasing glucose transport into either of these cell types offers therapeutic potential remains unknown. Using an adult-onset Drosophila model of Aß (amyloid beta) toxicity, we show that genetic overexpression of a glucose transporter, specifically in neurons, rescues lifespan, behavioral phenotypes, and neuronal morphology. This amelioration of Aß toxicity is associated with a reduction in the protein levels of the unfolded protein response (UPR) negative master regulator Grp78 and an increase in the UPR. We further demonstrate that genetic downregulation of Grp78 activity also protects against Aß toxicity, confirming a causal effect of its alteration on AD-related pathology. Metformin, a drug that stimulates glucose uptake in cells, mimicked these effects, with a concomitant reduction in Grp78 levels and rescue of the shortened lifespan and climbing defects of Aß-expressing flies. Our findings demonstrate a protective effect of increased neuronal uptake of glucose against Aß toxicity and highlight Grp78 as a novel therapeutic target for the treatment of AD.

C9orf72 repeat expansions cause neurodegeneration in Drosophila through arginine-rich proteins.

An expanded GGGGCC repeat in C9orf72 is the most common genetic cause of frontotemporal dementia and amyotrophic lateral sclerosis. A fundamental question is whether toxicity is driven by the repeat RNA itself and/or by dipeptide repeat proteins generated by repeat-associated, non-ATG translation. To address this question, we developed in vitro and in vivo models to dissect repeat RNA and dipeptide repeat protein toxicity. Expression of pure repeats, but not stop codon-interrupted "RNA-only" repeats in Drosophila caused adult-onset neurodegeneration. Thus, expanded repeats promoted neurodegeneration through dipeptide repeat proteins. Expression of individual dipeptide repeat proteins with a non-GGGGCC RNA sequence revealed that both poly-(glycine-arginine) and poly-(proline-arginine) proteins caused neurodegeneration. These findings are consistent with a dual toxicity mechanism, whereby both arginine-rich proteins and repeat RNA contribute to C9orf72-mediated neurodegeneration.

Cell-nonautonomous effects of dFOXO/DAF-16 in aging.

Drosophila melanogaster and Caenorhabditis elegans each carry a single representative of the Forkhead box O (FoxO) family of transcription factors, dFOXO and DAF-16, respectively. Both are required for lifespan extension by reduced insulin/Igf signaling, and their activation in key tissues can extend lifespan. Aging of these tissues may limit lifespan. Alternatively, FoxOs may promote longevity cell nonautonomously by signaling to themselves (FoxO to FoxO) or other factors (FoxO to other) in distal tissues. Here, we show that activation of dFOXO and DAF-16 in the gut/fat body does not require dfoxo/daf-16 elsewhere to extend lifespan. Rather, in Drosophila, activation of dFOXO in the gut/fat body or in neuroendocrine cells acts on other organs to promote healthy aging by signaling to other, as-yet-unidentified factors. Whereas FoxO-to-FoxO signaling appears to be required for metabolic homeostasis, our results pinpoint FoxO-to-other signaling as an important mechanism through which localized FoxO activity ameliorates aging.

Ageing as a risk factor for disease.

Age is the main risk factor for the prevalent diseases of developed countries: cancer, cardiovascular disease and neurodegeneration. The ageing process is deleterious for fitness, but can nonetheless evolve as a consequence of the declining force of natural selection at later ages, attributable to extrinsic hazards to survival: ageing can then occur as a side-effect of accumulation of mutations that lower fitness at later ages, or of natural selection in favour of mutations that increase fitness of the young but at the cost of a higher subsequent rate of ageing. Once thought of as an inexorable, complex and lineage-specific process of accumulation of damage, ageing has turned out to be influenced by mechanisms that show strong evolutionary conservation. Lowered activity of the nutrient-sensing insulin/insulin-like growth factor/Target of Rapamycin signalling network can extend healthy lifespan in yeast, multicellular invertebrates, mice and, possibly, humans. Mitochondrial activity can also promote ageing, while genome maintenance and autophagy can protect against it. We discuss the relationship between evolutionarily conserved mechanisms of ageing and disease, and the associated scientific challenges and opportunities.

Capu and Spire assemble a cytoplasmic actin mesh that maintains microtubule organization in the Drosophila oocyte.

Mutants in the actin nucleators Cappuccino and Spire disrupt the polarized microtubule network in the Drosophila oocyte that defines the anterior-posterior axis, suggesting that microtubule organization depends on actin. Here, we show that Cappuccino and Spire organize an isotropic mesh of actin filaments in the oocyte cytoplasm. capu and spire mutants lack this mesh, whereas overexpressed truncated Cappuccino stabilizes the mesh in the presence of Latrunculin A and partially rescues spire mutants. Spire overexpression cannot rescue capu mutants, but prevents actin mesh disassembly at stage 10B and blocks late cytoplasmic streaming. We also show that the actin mesh regulates microtubules indirectly, by inhibiting kinesin-dependent cytoplasmic flows. Thus, the Capu pathway controls alternative states of the oocyte cytoplasm: when active, it assembles an actin mesh that suppresses kinesin motility to maintain a polarized microtubule cytoskeleton. When inactive, unrestrained kinesin movement generates flows that wash microtubules to the cortex.