Characterization of Seizure Induction Methods in Drosophila

KDM5-mediated transcriptional activation of ribosomal protein genes alters translation efficiency to regulate mitochondrial metabolism in neurons

Genes encoding the KDM5 family of transcriptional regulators are disrupted in individuals with intellectual disability (ID). To understand the link between KDM5 and ID, we characterized five Drosophila strains harboring missense alleles analogous to those observed in patients. These alleles disrupted neuroanatomical development, cognition and other behaviors, and displayed a transcriptional signature characterized by the downregulation of many ribosomal protein genes. A similar transcriptional profile was observed in KDM5C knockout iPSC-induced human glutamatergic neurons, suggesting an evolutionarily conserved role for KDM5 proteins in regulating this class of gene. In Drosophila, reducing KDM5 changed neuronal ribosome composition, lowered the translation efficiency of mRNAs required for mitochondrial function, and altered mitochondrial metabolism. These data highlight the cellular consequences of altered KDM5-regulated transcriptional programs that could contribute to cognitive and behavioral phenotypes. Moreover, they suggest that KDM5 may be part of a broader network of proteins that influence cognition by regulating protein synthesis.

Loss of the endoplasmic reticulum protein Tmem208 affects cell polarity, development, and viability

Nascent proteins destined for the cell membrane and the secretory pathway are targeted to the endoplasmic reticulum (ER) either posttranslationally or cotranslationally. The signal-independent pathway, containing the protein TMEM208, is one of three pathways that facilitates the translocation of nascent proteins into the ER. The in vivo function of this protein is ill characterized in multicellular organisms. Here, we generated a CRISPR-induced null allele of the fruit fly ortholog CG8320/Tmem208 by replacing the gene with the Kozak-GAL4 sequence. We show that Tmem208 is broadly expressed in flies and that its loss causes lethality, although a few short-lived flies eclose. These animals exhibit wing and eye developmental defects consistent with impaired cell polarity and display mild ER stress. Tmem208 physically interacts with Frizzled (Fz), a planar cell polarity (PCP) receptor, and is required to maintain proper levels of Fz. Moreover, we identified a child with compound heterozygous variants in TMEM208 who presents with developmental delay, skeletal abnormalities, multiple hair whorls, cardiac, and neurological issues, symptoms that are associated with PCP defects in mice and humans. Additionally, fibroblasts of the proband display mild ER stress. Expression of the reference human TMEM208 in flies fully rescues the loss of Tmem208, and the two proband-specific variants fail to rescue, suggesting that they are loss-of-function alleles. In summary, our study uncovers a role of TMEM208 in development, shedding light on its significance in ER homeostasis and cell polarity.

Drosophila expressing mutant human KCNT1 transgenes make an effective tool for targeted drug screening in a whole animal model of KCNT1-epilepsy

Mutations in the KCNT1 potassium channel cause severe forms of epilepsy which are poorly controlled with current treatments. In vitro studies have shown that KCNT1-epilepsy mutations are gain of function, significantly increasing K+ current amplitudes. To investigate if Drosophila can be used to model human KCNT1 epilepsy, we generated Drosophila melanogaster lines carrying human KCNT1 with the patient mutation G288S, R398Q or R928C. Expression of each mutant channel in GABAergic neurons gave a seizure phenotype which responded either positively or negatively to 5 frontline epilepsy drugs most commonly administered to patients with KCNT1-epilepsy, often with little or no improvement of seizures. Cannabidiol showed the greatest reduction of the seizure phenotype while some drugs increased the seizure phenotype. Our study shows that Drosophila has the potential to model human KCNT1- epilepsy and can be used as a tool to assess new treatments for KCNT1- epilepsy.

Polyvinylpyrrolidone-Capped Copper Oxide Nanoparticles-Anchored Pramipexole Attenuates the Rotenone-Induced Phenotypes in a Drosophila Parkinson's Disease Model

Parkinson's disease (PD) is a progressive, age-related neurodegenerative disease. The disease is characterized by the loss of dopaminergic neurons in the substantia nigra, pars compacta of the midbrain. Pramipexole (PPX) is a novel drug used for the treatment of PD. It has a high affinity for the dopamine (DA) D2 receptor subfamily and acts as a targeted mitochondrial antioxidant. It is less effective in the treatment of PD due to its short half-life, highly inconvenient dosing schedule, and long-term side effects. In recent years, PPX-loaded nanoformulations have been actively reported to overcome these limitations. In the current study, we focused on increasing the effectiveness of PPX by minimizing the dosing frequency and improving the treatment strategy for PD. Herein, we report the synthesis of biodegradable polyvinylpyrrolidone (PVP)-capped copper oxide nanoparticles (PVP-CuO NPs), followed by PPX anchoring on the surface of the PVP-CuO NPs (PPX-PVP-CuO NC), in a simple and inexpensive method. The newly formulated PPX-PVP-CuO NC complex was analyzed for its chemical and physical properties. The PPX-PVP-CuO NC was tested to protect against rotenone (RT)-induced toxicity in the Drosophila PD model. The in vivo studies using the RT-induced Drosophila PD model showed significant changes in negative geotaxis behavior and the level of DA and acetylcholinesterase. In addition, oxidative stress markers such as glutathione-S-transferase, total glutathione, thiobarbituric acid reactive species, and protein carbonyl content showed significant amelioration. The positive changes of PPX-PVP-CuO NC treatment in behavior, neurotransmitter level, and antioxidant level suggest its potential role in mitigating the PD phenotype. The formulation can be used for treatment or pharmacological intervention against PD.

Effects of valproate on seizure-like activity in Drosophila melanogaster with a knockdown of Ube3a in different neuronal populations as a model of Angelman Syndrome

Angelman Syndrome is a rare, genetically induced neurodevelopmental disorder. This disorder stems from a mutation or deletion of the maternal UBE3A gene. Characteristics of this disease include developmental delay, recurring seizures, and severe intellectual disabilities. We studied seizure activity in male Drosophila melanogaster with a knockdown of Ube3a in different neuronal populations (GABAergic, glutamatergic, mushroom body, and all neurons) and investigated the effects of the antiseizure medication (ASM) on seizure-like activity. Epileptiform activity was monitored in individual fruit flies using imaging chambers and mechanically induced seizures using a vortex assay. A positive control was also used: eas (easily shocked seizure phenotype). Seizure activity was analyzed for sums of seizure durations, number of seizures, and total time to return to normal activity. Ube3a knockdowns in GABAergic neurons elicited more seizure-like episodes than knockdowns in glutamatergic neurons and were on par with the positive control group and those with knockdowns in the mushroom bodies. We have established a method whereby valproate could be administered through food rather than through injections to effectively treat epileptiform activity. We demonstrated that if Ube3a is not knocked down pan-neuronally, Angelman Syndrome seizure-like activity can be studied using Drosophila melanogaster and therefore allows for high-throughput drug discovery.

A new neurodevelopmental disorder linked to heterozygous variants in UNC79

PURPOSE

The "NALCN channelosome" is an ion channel complex that consists of multiple proteins, including NALCN, UNC79, UNC80, and FAM155A. Only a small number of individuals with a neurodevelopmental syndrome have been reported with disease causing variants in NALCN and UNC80. However, no pathogenic UNC79 variants have been reported, and in vivo function of UNC79 in humans is largely unknown.

METHODS

We used international gene-matching efforts to identify patients harboring ultrarare heterozygous loss-of-function UNC79 variants and no other putative responsible genes. We used genetic manipulations in Drosophila and mice to test potential causal relationships between UNC79 variants and the pathology.

RESULTS

We found 6 unrelated and affected patients with UNC79 variants. Five patients presented with overlapping neurodevelopmental features, including mild to moderate intellectual disability and a mild developmental delay, whereas a single patient reportedly had normal cognitive and motor development but was diagnosed with epilepsy and autistic features. All displayed behavioral issues and 4 patients had epilepsy. Drosophila with UNC79 knocked down displayed induced seizure-like phenotype. Mice with a heterozygous loss-of-function variant have a developmental delay in body weight compared with wild type. In addition, they have impaired ability in learning and memory.

CONCLUSION

Our results demonstrate that heterozygous loss-of-function UNC79 variants are associated with neurologic pathologies.

Seizure-suppressor genes: can they help spearhead the discovery of novel therapeutic targets for epilepsy?

INTRODUCTION

Epilepsies are disorders of neuronal excitability characterized by spontaneously recurrent focal and generalized seizures, some of which result from genetic mutations. Despite the availability of antiseizure medications, pharmaco-resistant epilepsy is seen in about 23% of epileptic patients worldwide. Therefore, there is an urgent need to develop novel therapeutic strategies for epilepsies. Several epilepsy-associated genes have been found in humans. Seizure susceptibility can also be induced in Drosophila mutants, some showing features resembling human epilepsies. Interestingly, several second-site mutation gene products have been found to suppress seizure susceptibility in the seizure genetic model Drosophila. Thus, these so-called 'seizure-suppressor' gene variants may lead to developing a novel class of antiseizure medications.

AREA COVERED

This review evaluates the potential therapeutic of seizure-suppressor gene variants.

EXPERT OPINION

Studies on epilepsy-associated genes have allowed analyses of mutations linked to human epilepsy by reproducing these mutations in Drosophila using reverse genetics to generate potential antiseizure therapeutics. As a result, about fifteen seizure-suppressor gene mutants have been identified. Furthermore, some of these epilepsy gene mutations affect ligand-and voltage-gated ion channels. Therefore, a better understanding of the antiseizure activity of seizure-suppressor genes is essential in advancing gene therapy and precision medicine for epilepsy.

Drosophila melanogaster as a model for unraveling unique molecular features of epilepsy elicited by human GABA transporter 1 variants

Mutations in the human γ-aminobutyric acid (GABA) transporter 1 (hGAT-1) can instigate myoclonic-atonic and other generalized epilepsies in the afflicted individuals. We systematically examined fifteen hGAT-1 disease variants, all of which dramatically reduced or completely abolished GABA uptake activity. Many of these loss-of-function variants were absent from their regular site of action at the cell surface, due to protein misfolding and/or impaired trafficking machinery (as verified by confocal microscopy and de-glycosylation experiments). A modest fraction of the mutants displayed correct targeting to the plasma membrane, but nonetheless rendered the mutated proteins devoid of GABA transport, possibly due to structural alterations in the GABA binding site/translocation pathway. We here focused on a folding-deficient A288V variant. In flies, A288V reiterated its impeded expression pattern, closely mimicking the ER-retention demonstrated in transfected HEK293 cells. Functionally, A288V presented a temperature-sensitive seizure phenotype in fruit flies. We employed diverse small molecules to restore the expression and activity of folding-deficient hGAT-1 epilepsy variants, in vitro (in HEK293 cells) and in vivo (in flies). We identified three compounds (chemical and pharmacological chaperones) conferring moderate rescue capacity for several variants. Our data grant crucial new insights into: (i) the molecular basis of epilepsy in patients harboring hGAT-1 mutations, and (ii) a proof-of-principle that protein folding deficits in disease-associated hGAT-1 variants can be corrected using the pharmacochaperoning approach. Such innovative pharmaco-therapeutic prospects inspire the rational design of novel drugs for alleviating the clinical symptoms triggered by the numerous emerging pathogenic mutations in hGAT-1.

Drosophila melanogaster as a versatile model organism to study genetic epilepsies: An overview

Epilepsy is one of the most prevalent neurological disorders, affecting more than 45 million people worldwide. Recent advances in genetic techniques, such as next-generation sequencing, have driven genetic discovery and increased our understanding of the molecular and cellular mechanisms behind many epilepsy syndromes. These insights prompt the development of personalized therapies tailored to the genetic characteristics of an individual patient. However, the surging number of novel genetic variants renders the interpretation of pathogenetic consequences and of potential therapeutic implications ever more challenging. Model organisms can help explore these aspects in vivo. In the last decades, rodent models have significantly contributed to our understanding of genetic epilepsies but their establishment is laborious, expensive, and time-consuming. Additional model organisms to investigate disease variants on a large scale would be desirable. The fruit fly Drosophila melanogaster has been used as a model organism in epilepsy research since the discovery of "bang-sensitive" mutants more than half a century ago. These flies respond to mechanical stimulation, such as a brief vortex, with stereotypic seizures and paralysis. Furthermore, the identification of seizure-suppressor mutations allows to pinpoint novel therapeutic targets. Gene editing techniques, such as CRISPR/Cas9, are a convenient way to generate flies carrying disease-associated variants. These flies can be screened for phenotypic and behavioral abnormalities, shifting of seizure thresholds, and response to anti-seizure medications and other substances. Moreover, modification of neuronal activity and seizure induction can be achieved using optogenetic tools. In combination with calcium and fluorescent imaging, functional alterations caused by mutations in epilepsy genes can be traced. Here, we review Drosophila as a versatile model organism to study genetic epilepsies, especially as 81% of human epilepsy genes have an orthologous gene in Drosophila. Furthermore, we discuss newly established analysis techniques that might be used to further unravel the pathophysiological aspects of genetic epilepsies.

Generation and Characterization of the Drosophila melanogaster paralytic Gene Knock-Out as a Model for Dravet Syndrome

Dravet syndrome is a severe rare epileptic disease caused by mutations in the SCN1A gene coding for the Nav1.1 protein, a voltage-gated sodium channel alpha subunit. We have made a knock-out of the paralytic gene, the single Drosophila melanogaster gene encoding this type of protein, by homologous recombination. These flies showed a heat-induced seizing phenotype, and sudden death in long term seizures. In addition to seizures, neuromuscular alterations were observed in climbing, flight, and walking tests. Moreover, they also manifested some cognitive alterations, such as anxiety and problems in learning. Electrophysiological analyses from larval motor neurons showed a decrease in cell capacitance and membrane excitability, while persistent sodium current increased. To detect alterations in metabolism, we performed an NMR metabolomic profiling of heads, which revealed higher levels in some amino acids, succinate, and lactate; and also an increase in the abundance of GABA, which is the main neurotransmitter implicated in Dravet syndrome. All these changes in the paralytic knock-out flies indicate that this is a good model for epilepsy and specifically for Dravet syndrome. This model could be a new tool to understand the pathophysiology of the disease and to find biomarkers, genetic modifiers and new treatments.

Nitric oxide mediates activity-dependent change to synaptic excitation during a critical period in Drosophila

The emergence of coordinated network function during nervous system development is often associated with critical periods. These phases are sensitive to activity perturbations during, but not outside, of the critical period, that can lead to permanently altered network function for reasons that are not well understood. In particular, the mechanisms that transduce neuronal activity to regulating changes in neuronal physiology or structure are not known. Here, we take advantage of a recently identified invertebrate model for studying critical periods, the Drosophila larval locomotor system. Manipulation of neuronal activity during this critical period is sufficient to increase synaptic excitation and to permanently leave the locomotor network prone to induced seizures. Using genetics and pharmacological manipulations, we identify nitric oxide (NO)-signaling as a key mediator of activity. Transiently increasing or decreasing NO-signaling during the critical period mimics the effects of activity manipulations, causing the same lasting changes in synaptic transmission and susceptibility to seizure induction. Moreover, the effects of increased activity on the developing network are suppressed by concomitant reduction in NO-signaling and enhanced by additional NO-signaling. These data identify NO signaling as a downstream effector, providing new mechanistic insight into how activity during a critical period tunes a developing network.