SOD1 activity threshold and TOR signalling modulate VAP(P58S) aggregation via reactive oxygen species-induced proteasomal degradation in a Drosophila model of amyotrophic lateral sclerosis

Age-dependent dynamics of neuronal VAPBALS inclusions in the adult brain

Amyotrophic Lateral Sclerosis (ALS) is a relentlessly progressive and fatal disease, caused by the degeneration of upper and lower motor neurons within the brain and spinal cord in the ageing human. The dying neurons contain cytoplasmic inclusions linked to the onset and progression of the disease. Here, we use a Drosophila model of ALS8 (VAPP58S) to understand the modulation of these inclusions in the ageing adult brain. The adult VAPP58S fly shows progressive deterioration in motor function till its demise 25 days post-eclosion. The density of VAPP58S-positive brain inclusions is stable for 5-15 days of age. In contrast, adding a single copy of VAPWT to the VAPP58S animal leads to a large decrease in inclusion density with concomitant rescue of motor function and lifespan. ER stress, a contributing factor in disease, shows reduction with ageing for the disease model. Autophagy, rather than the Ubiquitin Proteasome system, is the dominant mechanism for aggregate clearance. We explored the ability of Drosophila Valosin-containing protein (VCP/TER94), the ALS14 locus, which is involved in cellular protein clearance, to regulate age-dependent aggregation. Contrary to expectation, TER94 overexpression increased VAPP58S punctae density, while its knockdown led to enhanced clearance. Expression of a dominant positive allele, TER94R152H, further stabilised VAPP58S puncta, cementing roles for an ALS8-ALS14 axis. Our results are explained by a mechanism where autophagy is modulated by TER94 knockdown. Our study sheds light on the complex regulatory events involved in the neuronal maintenance of ALS8 aggregates, suggesting a context-dependent switch between proteasomal and autophagy-based mechanisms as the larvae develop into an adult. A deeper understanding of the nucleation and clearance of the inclusions, which affect cellular stress and function, is essential for understanding the initiation and progression of ALS.

Downregulation of oxidative stress-mediated glial innate immune response suppresses seizures in a fly epilepsy model

Previous work in our laboratory has shown that mutations in prickle (pk) cause myoclonic-like seizures and ataxia in Drosophila, similar to what is observed in humans carrying mutations in orthologous PRICKLE genes. Here, we show that pk mutant brains show elevated, sustained neuronal cell death that correlates with increasing seizure penetrance, as well as an upregulation of mitochondrial oxidative stress and innate immune response (IIR) genes. Moreover, flies exhibiting more robust seizures show increased levels of IIR-associated target gene expression suggesting they may be linked. Genetic knockdown in glia of either arm of the IIR (Immune Deficiency [Imd] or Toll) leads to a reduction in neuronal death, which in turn suppresses seizure activity, with oxidative stress acting upstream of IIR. These data provide direct genetic evidence that oxidative stress in combination with glial-mediated IIR leads to progression of an epilepsy disorder.

Overexpression of mTOR in Leukocytes from ALS8 Patients

BACKGROUND

The mutated VAPBP56S (vesicle B associated membrane protein - P56S) protein has been described in a Brazilian family and classified as Amyotrophic Lateral Sclerosis type 8 (ALS8).

OBJECTIVE

We aimed to study altered biochemical and immunological parameters in cells from ALS8 patients to identify possible biomarkers or therapeutic targets.

METHODS

Wild-type VAPB, VAPBP56S, mTOR, proinflammatory cytokines, and oxidant/reducing levels in serum, leucocytes, and cellular lysate from ALS8 patients and health Controls were performed by ELISA, fluorimetry, and spectrophotometry.

RESULTS

Our results showed similar levels of mutant and wild-type VAPB in serum and intracellular lysate (p > 0.05) when ALS8 patients and Controls were compared. IL-1β, IL-6, and IL-18 levels in patients and Controls showed no difference, suggesting an absence of peripheral inflammation (p > 0.05). Oxidative metabolic response, assessed by mitochondrial ROS production, and reductive response by MTT reduction, were higher in the ALS8 group compared to Controls (p < 0.05), although not characterizing typical oxidative stress in ALS8 patients. Total mTOR levels (phosphorylated or non-phosphorylated) of ALS8 patients were significantly lower in serum and higher in intracellular lysate than the mean equivalents in Controls (p < 0.05). A similar result was observed when we quantified the phosphorylated protein (p < 0.05).

CONCLUSION

We demonstrate the possibility of using these biochemical and immunological parameters as potential therapeutic targets or biomarkers. Furthermore, by hypothesis, we suggest a hormetic response in which both VAPB forms could coexist in different proportions throughout life. The mutated VAPBP56S production would increase with aging and predominate over the wild-type VAPB levels, determining the onset of symptoms and aggravating the disease.

Loss of Activity-Induced Mitochondrial ATP Production Underlies the Synaptic Defects in a Drosophila Model of ALS

Mutations in the gene encoding vesicle-associated membrane protein B (VAPB) cause a familial form of amyotrophic lateral sclerosis (ALS). Expression of an ALS-related variant of vapb (vapbP58S ) in Drosophila motor neurons results in morphologic changes at the larval neuromuscular junction (NMJ) characterized by the appearance of fewer, but larger, presynaptic boutons. Although diminished microtubule stability is known to underlie these morphologic changes, a mechanism for the loss of presynaptic microtubules has been lacking. By studying flies of both sexes, we demonstrate the suppression of vapbP58S -induced changes in NMJ morphology by either a loss of endoplasmic reticulum (ER) Ca2+ release channels or the inhibition Ca2+/calmodulin (CaM)-activated kinase II (CaMKII). These data suggest that decreased stability of presynaptic microtubules at vapbP58S NMJs results from hyperactivation of CaMKII because of elevated cytosolic [Ca2+]. We attribute the Ca2+ dyshomeostasis to delayed extrusion of cytosolic Ca2+ Suggesting that this defect in Ca2+ extrusion arose from an insufficient response to the bioenergetic demand of neural activity, depolarization-induced mitochondrial ATP production was diminished in vapbP58S neurons. These findings point to bioenergetic dysfunction as a potential cause for the synaptic defects in vapbP58S -expressing motor neurons.SIGNIFICANCE STATEMENT Whether the synchrony between the rates of ATP production and demand is lost in degenerating neurons remains poorly understood. We report that expression of a gene equivalent to an amyotrophic lateral sclerosis (ALS)-causing variant of vesicle-associated membrane protein B (VAPB) in fly neurons decouples mitochondrial ATP production from neuronal activity. Consequently, levels of ATP in mutant neurons are unable to keep up with the bioenergetic burden of neuronal activity. Reduced rate of Ca2+ extrusion, which could result from insufficient energy to power Ca2+ ATPases, results in the accumulation of residual Ca2+ in mutant neurons and leads to alterations in synaptic vesicle (SV) release and synapse development. These findings suggest that synaptic defects in a model of ALS arise from the loss of activity-induced ATP production.

Deconvoluting the Complexity of Reactive Oxygen Species (ROS) in Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are becoming a serious public health concern as the world’s population continues to age, demanding the discovery of more effective therapies. Excessive formation of reactive oxygen species (ROS) can result in oxidative stress (OS), which can be regarded as one of the common causes of neurodegenerative diseases (NDs). Thus, in this review, we focus on summarizing the consequences of ROS NDs, while taking the four prevalent NDs as examples, including Alzheimer’s disease (AD), Parkinson’s disease (PD), Amyotrophic lateral sclerosis (ALS), and Huntington’s disease (HD), to illustrate the key signaling pathways and relevant drugs. Together, these findings may shed new light on a field in which ROS-related pathways play a key role; thereby setting the groundwork for the future therapeutic development of neurodegenerative diseases.

Caspar, an adapter for VAPB and TER94, modulates the progression of ALS8 by regulating IMD/NFκB mediated glial inflammation in a drosophila model of human disease

Amyotrophic lateral sclerosis (ALS) is a fatal, late-onset, progressive motor neurodegenerative disorder. A key pathological feature of the disease is the presence of heavily ubiquitinated protein inclusions. Both the unfolded protein response and the ubiquitin–proteasome system appear significantly impaired in patients and animal models of ALS. We have studied cellular and molecular mechanisms involved in ALS using a vesicle-associated membrane protein-associated protein B (VAPB/ALS8) Drosophila model [Moustaqim-Barrette, A., Lin, Y.Q., Pradhan, S., Neely, G.G., Bellen, H.J. and Tsuda, H. (2014) The ALS 8 protein, VAP, is required for ER protein quality control. Hum. Mol. Genet., 23, 1975–1989], which mimics many systemic aspects of the human disease. Here, we show that VAPB, located on the cytoplasmic face of the endoplasmic reticulum membrane, interacts with Caspar, an orthologue of human fas associated factor 1 (FAF1). Caspar, in turn, interacts with transitional endoplasmic reticulum ATPase (TER94), a fly orthologue of ALS14 (VCP/p97, valosin-containing protein). Caspar overexpression in the glia extends lifespan and also slows the progression of motor dysfunction in the ALS8 disease model, a phenomenon that we ascribe to its ability to restrain age-dependent inflammation, which is modulated by Relish/NFκB signalling. Caspar binds to VAPB via an FFAT motif, and we find that Caspar’s ability to negatively regulate NFκB signalling is not dependent on the VAPB:Caspar interaction. We hypothesize that Caspar is a key molecule in the pathogenesis of ALS. The VAPB:Caspar:TER94 complex appears to be a candidate for regulating both protein homeostasis and NFκB signalling, with our study highlighting a role for Caspar in glial inflammation. We project human FAF1 as an important protein target to alleviate the progression of motor neuron disease.

Activation of the ubiquitin-proteasome system contributes to oculopharyngeal muscular dystrophy through muscle atrophy

Oculopharyngeal muscular dystrophy (OPMD) is a late-onset disorder characterized by progressive weakness and degeneration of specific muscles. OPMD is due to extension of a polyalanine tract in poly(A) binding protein nuclear 1 (PABPN1). Aggregation of the mutant protein in muscle nuclei is a hallmark of the disease. Previous transcriptomic analyses revealed the consistent deregulation of the ubiquitin-proteasome system (UPS) in OPMD animal models and patients, suggesting a role of this deregulation in OPMD pathogenesis. Subsequent studies proposed that UPS contribution to OPMD involved PABPN1 aggregation. Here, we use a Drosophila model of OPMD to address the functional importance of UPS deregulation in OPMD. Through genome-wide and targeted genetic screens we identify a large number of UPS components that are involved in OPMD. Half dosage of UPS genes reduces OPMD muscle defects suggesting a pathological increase of UPS activity in the disease. Quantification of proteasome activity confirms stronger activity in OPMD muscles, associated with degradation of myofibrillar proteins. Importantly, improvement of muscle structure and function in the presence of UPS mutants does not correlate with the levels of PABPN1 aggregation, but is linked to decreased degradation of muscle proteins. Oral treatment with the proteasome inhibitor MG132 is beneficial to the OPMD Drosophila model, improving muscle function although PABPN1 aggregation is enhanced. This functional study reveals the importance of increased UPS activity that underlies muscle atrophy in OPMD. It also provides a proof-of-concept that inhibitors of proteasome activity might be an attractive pharmacological approach for OPMD.

Oculopharyngeal muscular dystrophy (OPMD) is a genetic disease characterized by progressive weakness of specific muscles, leading to swallowing difficulties (dysphagia), eyelid drooping (ptosis) and walking difficulties at later stages. No drug treatments are currently available. OPMD is due to mutations in a nuclear protein called poly(A) binding protein nuclear 1 (PABPN1) that is involved in processing of different classes of RNAs in the nucleus. We have used an animal model of OPMD that we have developed in the fly Drosophila to investigate the role in OPMD of the ubiquitin-proteasome system, a pathway specialized in protein degradation. We report an increased activity of the ubiquitin-proteasome system that is associated with degradation of muscular proteins in the OPMD Drosophila model. We propose that higher activity of the ubiquitin-proteasome system leads to muscle atrophy in OPMD. Importantly, oral treatment of this OPMD animal model with an inhibitor of proteasome activity reduces muscle defects. A number of proteasome inhibitors are approved drugs used in clinic against cancers, therefore our results provide a proof-of-concept that inhibitors of proteasome might be of interest in future treatments of OPMD.

Exploring the alternative: Fish, flies and worms as preclinical models for ALS

Amyotrophic lateral sclerosis (ALS) is an incurable neurodegenerative disorder characterized by the loss of upper and lower motor neurons. In general, patients succumb to respiratory insufficiency due to respiratory muscle weakness. Despite many promising therapeutic strategies primarily identified in rodent models, patient trials remain rather unsuccessful. There is a clear need for alternative approaches, which could provide directions towards the justified use of rodents and which increase the likelihood to identify new promising clinical candidates. In the last decades, the use of fast genetic approaches and the development of high-throughput screening platforms in the nematode Caenorhabditis elegans, in the fruit fly (Drosophila melanogaster) and in zebrafish (Danio rerio) have contributed to new insights into ALS pathomechanisms, disease modifiers and therapeutic targets. In this mini-review, we provide an overview of these alternative small animal studies, modeling the most common ALS genes and discuss the most recent preclinical discoveries. We conclude that small animal models will not replace rodent models, yet they clearly represent an important asset for preclinical studies.

A transition to degeneration triggered by oxidative stress in degenerative disorders

Although the activities of many signaling pathways are dysregulated during the progression of neurodegenerative and muscle degeneration disorders, the precise sequence of cellular events leading to degeneration has not been fully elucidated. Two kinases of particular interest, the growth-promoting Tor kinase and the energy sensor AMPK, appear to show reciprocal changes in activity during degeneration, with increased Tor activity and decreased AMPK activity reported. These changes in activity have been predicted to cause degeneration by attenuating autophagy, leading to the accumulation of unfolded protein aggregates and dysfunctional mitochondria, the consequent increased production of reactive oxygen species (ROS), and ultimately oxidative damage. Here we propose that this increased ROS production not only causes oxidative damage but also ultimately induces an oxidative stress response that reactivates the redox-sensitive AMPK and activates the redox-sensitive stress kinase JNK. Activation of these kinases reactivates autophagy. Because at this late stage, cells have become filled with dysfunctional mitochondria and protein aggregates, which are autophagy targets, this autophagy reactivation induces degeneration. The mechanism proposed here emphasizes that the process of degeneration is dynamic, that dysregulated signaling pathways change over time and can transition from deleterious to beneficial and vice versa as degeneration progresses.

Build-UPS and break-downs: metabolism impacts on proteostasis and aging

Perturbation of metabolism elicits cellular stress which profoundly modulates the cellular proteome and thus protein homeostasis (proteostasis). Consequently, changes in the cellular proteome due to metabolic shift require adaptive mechanisms by molecular protein quality control. The mechanisms vitally controlling proteostasis embrace the entire life cycle of a protein involving translational control at the ribosome, chaperone-assisted native folding, and subcellular sorting as well as proteolysis by the proteasome or autophagy. While metabolic imbalance and proteostasis decline have been recognized as hallmarks of aging and age-associated diseases, both processes are largely considered independently. Here, we delineate how proteome stability is governed by insulin/IGF1 signaling (IIS), mechanistic target of Rapamycin (TOR), 5′ adenosine monophosphate-activated protein kinase (AMPK), and NAD-dependent deacetylases (Sir2-like proteins known as sirtuins). This comprehensive overview is emphasizing the regulatory interconnection between central metabolic pathways and proteostasis, indicating the relevance of shared signaling nodes as targets for future therapeutic interventions.

Modeling Neurodegenerative Disorders in Drosophila melanogaster

Drosophila melanogaster provides a powerful genetic model system in which to investigate the molecular mechanisms underlying neurodegenerative diseases. In this review, we discuss recent progress in Drosophila modeling Alzheimer's Disease, Parkinson's Disease, Amyotrophic Lateral Sclerosis (ALS), Huntington's Disease, Ataxia Telangiectasia, and neurodegeneration related to mitochondrial dysfunction or traumatic brain injury. We close by discussing recent progress using Drosophila models of neural regeneration and how these are likely to provide critical insights into future treatments for neurodegenerative disorders.

First person – Kriti Chaplot

First Person is a series of interviews with the first authors of a selection of papers published in Disease Models & Mechanisms, helping early-career researchers promote themselves alongside their papers. Kriti Chaplot is first author on ‘SOD1 activity threshold and TOR signalling modulate VAP(P58S) aggregation via reactive oxygen species-induced proteasomal degradation in a Drosophila model of amyotrophic lateral sclerosis’, published in DMM. Kriti is a PhD student in the lab of Dr Girish Ratnaparkhi at the Indian Institute of Science Education and Research, Pune, India. Her main research interest is delineating cellular mechanisms that perturb aggregation in neurodegenerative diseases.