Epilepsy-associated gene Nedd4-2 mediates neuronal activity and seizure susceptibility through AMPA receptors

Pervasive expostulation of p53 gene promoting the precipitation of neurogenic convulsions: A journey in therapeutic advancements

Epilepsy, a neurological disorder characterized by prolonged and excessive seizures, has been linked to elevated levels of the tumor suppressor gene p53, which contributes to neuronal dysfunction. This review explores the molecular mechanisms of p53 in epilepsy and discusses potential future therapeutic strategies. Research indicates that changes in p53 expression during neuronal apoptosis, neuroinflammation, and oxidative stress play a significant role in the pathogenesis of epilepsy. Elevated p53 disrupts glutamatergic neurotransmission and hyperactivates NMDA and AMPA receptors, leading to increased neuronal calcium influx, mitochondrial oxidative stress, and activation of apoptotic pathways mediated neuronal dysfunction, exacerbating epileptogenesis. The involvement of p53 in epilepsy suggests that targeting this protein could be beneficial in mitigating neuronal damage and preventing seizure recurrence. Pharmacological agents like pifithrin-α have shown promise in reducing p53-mediated apoptosis and seizure severity. Gene therapy approaches, such as viral vector-mediated delivery of wild-type p53 or RNA interference targeting mutant p53, have also been effective in restoring normal p53 function and reducing seizure susceptibility. Despite these advances, the heterogeneous nature of epilepsy and potential long-term side effects of p53 modulation present challenges. Future research should focus on elucidating the precise molecular mechanisms of p53 and developing personalized therapeutic strategies. Modulating p53 activity holds promise for reducing seizure susceptibility and improving the quality of life for individuals with epilepsy. The current review provides the understanding the intricate role of p53 in neuroinflammatory pathways, including JAK-STAT, JNK, NF-κB, Sonic Hedgehog, and Wnt, is crucial for developing targeted therapies.

Understanding neurodevelopmental proteasomopathies as new rare disease entities: A review of current concepts, molecular biomarkers, and perspectives

The recent advances in high throughput sequencing technology have drastically changed the practice of medical diagnosis, allowing for rapid identification of hundreds of genes causing human diseases. This unprecedented progress has made clear that most forms of intellectual disability that affect more than 3% of individuals worldwide are monogenic diseases. Strikingly, a substantial fraction of the mendelian forms of intellectual disability is associated with genes related to the ubiquitin-proteasome system, a highly conserved pathway made up of approximately 1200 genes involved in the regulation of protein homeostasis. Within this group is currently emerging a new class of neurodevelopmental disorders specifically caused by proteasome pathogenic variants which we propose to designate "neurodevelopmental proteasomopathies". Besides cognitive impairment, these diseases are typically associated with a series of syndromic clinical manifestations, among which facial dysmorphism, motor delay, and failure to thrive are the most prominent ones. While recent efforts have been made to uncover the effects exerted by proteasome variants on cell and tissue landscapes, the molecular pathogenesis of neurodevelopmental proteasomopathies remains ill-defined. In this review, we discuss the cellular changes typically induced by genomic alterations in proteasome genes and explore their relevance as biomarkers for the diagnosis, management, and potential treatment of these new rare disease entities.

Serum/glucocorticoid regulated kinase 1 (SGK1) in neurological disorders: pain or gain

Serum/Glucocorticoid Regulated Kinase 1 (SGK1), a serine/threonine kinase, is ubiquitous across a wide range of tissues, orchestrating numerous signaling pathways and associated with various human diseases. SGK1 has been extensively explored in diverse types of immune and inflammatory diseases, cardiovascular disorders, as well as cancer metastasis. These studies link SGK1 to cellular proliferation, survival, metabolism, membrane transport, and drug resistance. Recently, increasing research has focused on SGK1's role in neurological disorders, including a variety of neurodegenerative diseases (e.g., Alzheimer's disease, Huntington's disease and Parkinson's disease), brain injuries (e.g., cerebral ischemia and traumatic brain injury), psychiatric conditions (e.g., depression and drug addiction). SGK1 is emerging as an increasingly compelling therapeutic target across the spectrum of neurological disorders, supported by the availability of several effective agents. However, the conclusions of many studies observing the prevalence and function of SGK1 in neurological disorders are contradictory, necessitating a review of the SGK1 research within neurological disorders. Herein, we review recent literature on SGK1's primary functions within the nervous system and its impacts within different neurological disorders. We summarize significant findings, identify research gaps, and outline possible future research directions based on the current understanding of SGK1 to help further progress the understanding and treatment of neurological disorders.

Involvement of ubiquitination in Alzheimer's disease

The hallmark pathological features of Alzheimer's disease (AD) consist of senile plaques, which are formed by extracellular β-amyloid (Aβ) deposition, and neurofibrillary tangles, which are formed by the hyperphosphorylation of intra-neuronal tau proteins. With the increase in clinical studies, the in vivo imbalance of iron homeostasis and the dysfunction of synaptic plasticity have been confirmed to be involved in AD pathogenesis. All of these mechanisms are constituted by the abnormal accumulation of misfolded or conformationally altered protein aggregates, which in turn drive AD progression. Proteostatic imbalance has emerged as a key mechanism in the pathogenesis of AD. Ubiquitination modification is a major pathway for maintaining protein homeostasis, and protein degradation is primarily carried out by the ubiquitin-proteasome system (UPS). In this review, we provide an overview of the ubiquitination modification processes and related protein ubiquitination degradation pathways in AD, focusing on the microtubule-associated protein Tau, amyloid precursor protein (APP), divalent metal transporter protein 1 (DMT1), and α-amino-3-hyroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors. We also discuss recent advances in ubiquitination-based targeted therapy for AD, with the aim of contributing new ideas to the development of novel therapeutic interventions for AD.

Modulating Endoplasmic Reticulum Chaperones and Mutant Protein Degradation in GABRG2(Q390X) Associated with Genetic Epilepsy with Febrile Seizures Plus and Dravet Syndrome

A significant number of patients with genetic epilepsy do not obtain seizure freedom, despite developments in new antiseizure drugs, suggesting a need for novel therapeutic approaches. Many genetic epilepsies are associated with misfolded mutant proteins, including GABRG2(Q390X)-associated Dravet syndrome, which we have previously shown to result in intracellular accumulation of mutant GABAA receptor γ2(Q390X) subunit protein. Thus, a potentially promising therapeutic approach is modulation of proteostasis, such as increasing endoplasmic reticulum (ER)-associated degradation (ERAD). To that end, we have here identified an ERAD-associated E3 ubiquitin ligase, HRD1, among other ubiquitin ligases, as a strong modulator of wildtype and mutant γ2 subunit expression. Overexpressing HRD1 or knockdown of HRD1 dose-dependently reduced the γ2(Q390X) subunit. Additionally, we show that zonisamide (ZNS)-an antiseizure drug reported to upregulate HRD1-reduces seizures in the Gabrg2+/Q390X mouse. We propose that a possible mechanism for this effect is a partial rescue of surface trafficking of GABAA receptors, which are otherwise sequestered in the ER due to the dominant-negative effect of the γ2(Q390X) subunit. Furthermore, this partial rescue was not due to changes in ER chaperones BiP and calnexin, as total expression of these chaperones was unchanged in γ2(Q390X) models. Our results here suggest that leveraging the endogenous ERAD pathway may present a potential method to degrade neurotoxic mutant proteins like the γ2(Q390X) subunit. We also demonstrate a pharmacological means of regulating proteostasis, as ZNS alters protein trafficking, providing further support for the use of proteostasis regulators for the treatment of genetic epilepsies.

Unraveling the Neural Circuits: Techniques, Opportunities and Challenges in Epilepsy Research

Epilepsy, a prevalent neurological disorder characterized by high morbidity, frequent recurrence, and potential drug resistance, profoundly affects millions of people globally. Understanding the microscopic mechanisms underlying seizures is crucial for effective epilepsy treatment, and a thorough understanding of the intricate neural circuits underlying epilepsy is vital for the development of targeted therapies and the enhancement of clinical outcomes. This review begins with an exploration of the historical evolution of techniques used in studying neural circuits related to epilepsy. It then provides an extensive overview of diverse techniques employed in this domain, discussing their fundamental principles, strengths, limitations, as well as their application. Additionally, the synthesis of multiple techniques to unveil the complexity of neural circuits is summarized. Finally, this review also presents targeted drug therapies associated with epileptic neural circuits. By providing a critical assessment of methodologies used in the study of epileptic neural circuits, this review seeks to enhance the understanding of these techniques, stimulate innovative approaches for unraveling epilepsy's complexities, and ultimately facilitate improved treatment and clinical translation for epilepsy.

Hyperfunction of post-synaptic density protein 95 promotes seizure response in early-stage aβ pathology

Accumulation of amyloid-beta (Aβ) can lead to the formation of aggregates that contribute to neurodegeneration in Alzheimer's disease (AD). Despite globally reduced neural activity during AD onset, recent studies have suggested that Aβ induces hyperexcitability and seizure-like activity during the early stages of the disease that ultimately exacerbate cognitive decline. However, the underlying mechanism is unknown. Here, we reveal an Aβ-induced elevation of postsynaptic density protein 95 (PSD-95) in cultured neurons in vitro and in an in vivo AD model using APP/PS1 mice at 8 weeks of age. Elevation of PSD-95 occurs as a result of reduced ubiquitination caused by Akt-dependent phosphorylation of E3 ubiquitin ligase murine-double-minute 2 (Mdm2). The elevation of PSD-95 is consistent with the facilitation of excitatory synapses and the surface expression of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors induced by Aβ. Inhibition of PSD-95 corrects these Aβ-induced synaptic defects and reduces seizure activity in APP/PS1 mice. Our results demonstrate a mechanism underlying elevated seizure activity during early-stage Aβ pathology and suggest that PSD-95 could be an early biomarker and novel therapeutic target for AD.

NEDD4-2 and the CLC-2 channel regulate neuronal excitability in the pathogenesis of mesial temporal lobe epilepsy

An increasing number of studies have focused on the role of NEDD4-2 in regulating neuronal excitability and the mechanism of epilepsy. However, the exact mechanism has not yet been elucidated. Here, we explored the roles of NEDD4-2 and the CLC-2 channel in regulating neuronal excitability and mesial temporal lobe epilepsy (MTLE) pathogenesis. First, chronic MTLE models were induced by lithium-pilocarpine in developmental rats. Coimmunoprecipitation analysis revealed that the interaction between CLC-2 and NEDD4-2. Western blot analyses indicated that NEDD4-2 expression was downregulated, while phosphorylated (P-) NEDD4-2 and CLC-2 expression was upregulated in adult MTLE rats. Then, the primary hippocampal neuronal cells were isolated and cultured, and the NEDD4-2 was knocked down by shRNA vector, resulting in decreased protein levels of CLC-2. While CLC-2 absence caused increased NEDD4-2 in cells. Next, in an epileptic cell model induced by a Mg2+-free culture, whole-cell current-clamp recording demonstrated that NEDD4-2 deficiency inhibited the spontaneous action potentials of cells, and CLC-2 absence caused more significant decrease in the spontaneous action potentials of cells. In conclusion, we herein revealed that NEDD4-2 regulates the expression of CLC-2, which is involved in neuronal excitability, and participates in the pathogenesis of MTLE.

AMPK role in epilepsy: a promising therapeutic target?

Epilepsy is a complex and multifaceted neurological disorder characterized by spontaneous and recurring seizures. It poses significant therapeutic challenges due to its diverse etiology and often-refractory nature. This comprehensive review highlights the pivotal role of AMP-activated protein kinase (AMPK), a key metabolic regulator involved in cellular energy homeostasis, which may be a promising therapeutic target for epilepsy. Current therapeutic strategies such as antiseizure medication (ASMs) can alleviate seizures (up to 70%). However, 30% of epileptic patients may develop refractory epilepsy. Due to the complicated nature of refractory epilepsy, other treatment options such as ketogenic dieting, adjunctive therapy, and in limited cases, surgical interventions are employed. These therapy options are only suitable for a select group of patients and have limitations of their own. Current treatment options for epilepsy need to be improved. Emerging evidence underscores a potential association between impaired AMPK functionality in the brain and the onset of epilepsy, prompting an in-depth examination of AMPK's influence on neural excitability and ion channel regulation, both critical factors implicated in epileptic seizures. AMPK activation through agents such as metformin has shown promising antiepileptic effects in various preclinical and clinical settings. These effects are primarily mediated through the inhibition of the mTOR signaling pathway, activation of the AMPK-PI3K-c-Jun pathway, and stimulation of the PGC-1α pathway. Despite the potential of AMPK-targeted therapies, several aspects warrant further exploration, including the detailed mechanisms of AMPK's role in different brain regions, the impact of AMPK under various conditional circumstances such as neural injury and zinc toxicity, the long-term safety and efficacy of chronic metformin use in epilepsy treatment, and the potential benefits of combination therapy involving AMPK activators. Moreover, the efficacy of AMPK activators in refractory epilepsy remains an open question. This review sets the stage for further research with the aim of enhancing our understanding of the role of AMPK in epilepsy, potentially leading to the development of more effective, AMPK-targeted therapeutic strategies for this challenging and debilitating disorder.

Nedd4-2-dependent regulation of astrocytic Kir4.1 and Connexin43 controls neuronal network activity

Nedd4-2 is an E3 ubiquitin ligase in which missense mutation is related to familial epilepsy, indicating its critical role in regulating neuronal network activity. However, Nedd4-2 substrates involved in neuronal network function have yet to be identified. Using mouse lines lacking Nedd4-1 and Nedd4-2, we identified astrocytic channel proteins inwardly rectifying K+ channel 4.1 (Kir4.1) and Connexin43 as Nedd4-2 substrates. We found that the expression of Kir4.1 and Connexin43 is increased upon conditional deletion of Nedd4-2 in astrocytes, leading to an elevation of astrocytic membrane ion permeability and gap junction activity, with a consequent reduction of γ-oscillatory neuronal network activity. Interestingly, our biochemical data demonstrate that missense mutations found in familial epileptic patients produce gain-of-function of the Nedd4-2 gene product. Our data reveal a process of coordinated astrocytic ion channel proteostasis that controls astrocyte function and astrocyte-dependent neuronal network activity and elucidate a potential mechanism by which aberrant Nedd4-2 function leads to epilepsy.

Physiological Functions of the Ubiquitin Ligases Nedd4-1 and Nedd4-2

The Nedd4 family of E3 ubiquitin ligases, consisting of a C2-WW(n)-HECT domain architecture, includes the closely related Nedd4/Nedd4-1 and Nedd4L/Nedd4-2, which play critical roles in human physiology and pathophysiology.This review focuses on the regulation of enzymatic activity of these Nedd4 proteins, as well as on their roles in regulating stability and function of membrane and other signaling proteins, such as ion channels, ion transporters, and growth factor receptors. The diseases caused by impairment of such regulation are discussed, as well as opportunities and challenges for targeting these enzymes for therapy.

Molecular Mechanisms of AMPA Receptor Trafficking in the Nervous System

Synaptic plasticity enhances or reduces connections between neurons, affecting learning and memory. Postsynaptic AMPARs mediate greater than 90% of the rapid excitatory synaptic transmission in glutamatergic neurons. The number and subunit composition of AMPARs are fundamental to synaptic plasticity and the formation of entire neural networks. Accordingly, the insertion and functionalization of AMPARs at the postsynaptic membrane have become a core issue related to neural circuit formation and information processing in the central nervous system. In this review, we summarize current knowledge regarding the related mechanisms of AMPAR expression and trafficking. The proteins related to AMPAR trafficking are discussed in detail, including vesicle-related proteins, cytoskeletal proteins, synaptic proteins, and protein kinases. Furthermore, significant emphasis was placed on the pivotal role of the actin cytoskeleton, which spans throughout the entire transport process in AMPAR transport, indicating that the actin cytoskeleton may serve as a fundamental basis for AMPAR trafficking. Additionally, we summarize the proteases involved in AMPAR post-translational modifications. Moreover, we provide an overview of AMPAR transport and localization to the postsynaptic membrane. Understanding the assembly, trafficking, and dynamic synaptic expression mechanisms of AMPAR may provide valuable insights into the cognitive decline associated with neurodegenerative diseases.

Genome Sequencing of Consanguineous Family Implicates Ubiquitin-Specific Protease 53 (USP53) Variant in Psychosis/Schizophrenia: Wild-Type Expression in Murine Hippocampal CA 1-3 and Granular Dentate with AMPA Synapse Interactions

Psychosis is a severe mental disorder characterized by abnormal thoughts and perceptions (e.g., hallucinations) occurring quintessentially in schizophrenia and in several other neuropsychiatric disorders. Schizophrenia is widely considered as a neurodevelopmental disorder that onsets during teenage/early adulthood. A multiplex consanguineous Pakistani family was afflicted with severe psychosis and apparent autosomal recessive transmission. The first-cousin parents and five children were healthy, whereas two teenage daughters were severely affected. Structured interviews confirmed the diagnosis of DSM-V schizophrenia. Probands and father underwent next-generation sequencing. All available relatives were subjected to confirmatory Sanger sequencing. Homozygosity mapping and directed a priori filtering identified only one rare variant [MAF < 5(10)-5] at a residue conserved across vertebrates. The variant was a non-catalytic deubiquitinase, USP53 (p.Cys228Arg), predicted in silico as damaging. Genome sequencing did not identify any other potentially pathogenic single nucleotide variant or structural variant. Since the literature on USP53 lacked relevance to mental illness or CNS expression, studies were conducted which revealed USP53 localization in regions of the hippocampus (CA 1-3) and granular dentate. The staining pattern was like that seen with GRIA2/GluA2 and GRIP2 antibodies. All three proteins coimmunoprecipitated. These findings support the glutamate hypothesis of schizophrenia as part of the AMPA-R interactome. If confirmed, USP53 appears to be one of the few Mendelian variants potentially causal to a common-appearing mental disorder that is a rare genetic form of schizophrenia.

Ubiquitination of the GluA1 Subunit of AMPA Receptors Is Required for Synaptic Plasticity, Memory, and Cognitive Flexibility

Activity-dependent changes in the number of AMPA-type glutamate receptors (AMPARs) at the synapse underpin the expression of LTP and LTD, cellular correlates of learning and memory. Post-translational ubiquitination has emerged as a key regulator of the trafficking and surface expression of AMPARs, with ubiquitination of the GluA1 subunit at Lys-868 controlling the post-endocytic sorting of the receptors into the late endosome for degradation, thereby regulating their stability at synapses. However, the physiological significance of GluA1 ubiquitination remains unknown. In this study, we generated mice with a knock-in mutation in the major GluA1 ubiquitination site (K868R) to investigate the role of GluA1 ubiquitination in synaptic plasticity, learning, and memory. Our results reveal that these male mice have normal basal synaptic transmission but exhibit enhanced LTP and deficits in LTD. They also display deficits in short-term spatial memory and cognitive flexibility. These findings underscore the critical roles of GluA1 ubiquitination in bidirectional synaptic plasticity and cognition in male mice.SIGNIFICANCE STATEMENT Subcellular targeting and membrane trafficking determine the precise number of AMPA-type glutamate receptors at synapses, processes that are essential for synaptic plasticity, learning, and memory. Post-translational ubiquitination of the GluA1 subunit marks AMPARs for degradation, but its functional role in vivo remains unknown. Here we demonstrate that the GluA1 ubiquitin-deficient mice exhibit an altered threshold for synaptic plasticity accompanied by deficits in short-term memory and cognitive flexibility. Our findings suggest that activity-dependent ubiquitination of GluA1 fine-tunes the optimal number of synaptic AMPARs required for bidirectional synaptic plasticity and cognition in male mice. Given that increases in amyloid-β cause excessive ubiquitination of GluA1, inhibiting that GluA1 ubiquitination may have the potential to ameliorate amyloid-β-induced synaptic depression in Alzheimer's disease.

KCTD13-mediated ubiquitination and degradation of GluN1 regulates excitatory synaptic transmission and seizure susceptibility

Temporal lobe epilepsy (TLE) is the most common and severe form of epilepsy in adults; however, its underlying pathomechanisms remain elusive. Dysregulation of ubiquitination is increasingly recognized to contribute to the development and maintenance of epilepsy. Herein, we observed for the first time that potassium channel tetramerization domain containing 13 (KCTD13) protein, a substrate-specific adapter for cullin3-based E3 ubiquitin ligase, was markedly down-regulated in the brain tissue of patients with TLE. In a TLE mouse model, the protein expression of KCTD13 dynamically changed during epileptogenesis. Knockdown of KCTD13 in the mouse hippocampus significantly enhanced seizure susceptibility and severity, whereas overexpression of KCTD13 showed the opposite effect. Mechanistically, GluN1, an obligatory subunit of N-methyl-D-aspartic acid receptors (NMDARs), was identified as a potential substrate protein of KCTD13. Further investigation revealed that KCTD13 facilitates lysine-48-linked polyubiquitination of GluN1 and its degradation through the ubiquitin-proteasome pathway. Besides, the lysine residue 860 of GluN1 is the main ubiquitin site. Importantly, dysregulation of KCTD13 affected membrane expression of glutamate receptors and impaired glutamate synaptic transmission. Systemic administration of the NMDAR inhibitor memantine significantly rescued the epileptic phenotype aggravated by KCTD13 knockdown. In conclusion, our results demonstrated an unrecognized pathway of KCTD13-GluN1 in epilepsy, suggesting KCTD13 as a potential neuroprotective therapeutic target for epilepsy.

The role of post-translational modifications in synaptic AMPA receptor activity

AMPA-type receptors for the neurotransmitter glutamate are very dynamic entities, and changes in their synaptic abundance underlie different forms of synaptic plasticity, including long-term synaptic potentiation (LTP), long-term depression (LTD) and homeostatic scaling. The different AMPA receptor subunits (GluA1-GluA4) share a common modular structure and membrane topology, and their intracellular C-terminus tail is responsible for the interaction with intracellular proteins important in receptor trafficking. The latter sequence differs between subunits and contains most sites for post-translational modifications of the receptors, including phosphorylation, O-GlcNAcylation, ubiquitination, acetylation, palmitoylation and nitrosylation, which affect differentially the various subunits. Considering that each single subunit may undergo modifications in multiple sites, and that AMPA receptors may be formed by the assembly of different subunits, this creates multiple layers of regulation of the receptors with impact in synaptic function and plasticity. This review discusses the diversity of mechanisms involved in the post-translational modification of AMPA receptor subunits, and their impact on the subcellular distribution and synaptic activity of the receptors.

The recycling of AMPA receptors/GABAa receptors is related to neuronal excitation/inhibition imbalance and may be regulated by KIF5A

BACKGROUND

Excitation/inhibition imbalance (E/I imbalance), which involves an increase of alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) receptors (AMPARs) and decrease of gamma-aminobutyric acid type A (GABA) type A receptors (GABAaRs) on the neuron surface, has been documented in the pathogenesis of seizures. Notably, it has been established that both the glutamate receptor subunit 2 (GluR2) of AMPARs and beta 2/3 subunits of GABAaRs (Gabrb2+3) participate in the recycling mechanism mediated by the kinesin heavy chain isoform 5A (KIF5A), which determines the number of neuron surface receptors. However, it remains unclear whether receptor recycling is involved in the pathogenesis of seizures.

METHODS

Twelve adult male Sprague-Dawley rats were randomly allocated to the normal control (Ctl) group (n=6) and the pentylenetetrazol (PTZ)-induced seizure (Sez) group (n=6). The rats in the Ctl group were treated with saline. The rats in the Sez group received an intraperitoneal injection of PTZ at an initial dose of 40 mg/kg. Primary cultured neurons were obtained from newborn rats (24-hour-old). The neurons were exposed to magnesium-free (Mg2+-free) extracellular fluid for 3 hours to establish the seizure model in vitro. We detected the electrophysiology of the seizure model, the expression levels of KIF5A, GluR2, and Gabrb2+3, the recycling ratio of GluR2 and Gabrb2+3, the interaction between KIF5A and GluR2, and the interaction between KIF5A and Gabrb2+3.

RESULTS

In the Sez group, the expression of GluR2 on the cell surface was increased and the expression of Gabrb2+3 on the cell surface was decreased. The amplitude and frequency of action potentials were significantly increased in the Mg2+-free group. The amplitude and decay time of AMPAR-mediated miniature excitatory postsynaptic currents were increased in the Mg2+-free group. The amplitude and decay time of miniature inhibitory postsynaptic currents were decreased in the Mg2+-free group. The recycling ratio of GluR2 was increased and the recycling ratio of Gabrb2+3 was decreased in the Mg2+-free group. The interaction between KIF5A and GluR2 was increased, and the interaction between KIF5A and Gabrb2+3 was decreased in the seizure model in vivo and in vitro.

CONCLUSIONS

The recycling of AMPA receptors/GABAa receptors is related to E/I imbalance and may be regulated by KIF5A.

RNF220 is an E3 ubiquitin ligase for AMPA receptors to regulate synaptic transmission

The accurate expression of postsynaptic AMPA receptors (AMPARs) is critical for information processing in the brain, and ubiquitination is a key regulator for this biological process. However, the roles of E3 ubiquitin ligases in the regulation of AMPARs are poorly understood. Here, we find that RNF220 directly interacts with AMPARs to meditate their polyubiquitination, and RNF220 knockout specifically increases AMPAR protein levels, thereby enhancing basal synaptic activity while impairing synaptic plasticity. Moreover, depending on its E3 ubiquitin ligase activity, RNF220 represses AMPAR-mediated excitatory synaptic responses and their neuronal surface expression. Furthermore, learning and memory are altered in forebrain RNF220-deficient mice. In addition, two neuropathology-related RNF220 variants fail to repress excitatory synaptic activity because of the incapability to regulate AMPAR ubiquitination due to their attenuated interaction. Together, we identify RNF220 as an E3 ubiquitin ligase for AMPARs and establish its substantial role in excitatory synaptic transmission and brain function.

The interplay of epilepsy with impaired mitophagy and autophagy linked dementia (MAD): A review of therapeutic approaches

The duration and, age of dementia have been linked to a higher risk of seizures. The exact mechanism that drives epileptogenesis in impaired mitophagy and autophagy linked dementia (MAD) is fully defined after reviewing the Scopus, Publon, and Pubmed databases. The epileptogenesis in patients with Alzheimer's disease dementia (ADD) and Parkinson's disease dementia (PDD) is due to involvement of amyloid plaques (Aβ), phosphorylated tau (pTau), Parkin, NF-kB and NLRP3 inflammasome. Microglia, the prime protective and inflammatory cells in the brain exert crosstalk between mitophagy and inflammation. Several researchers believed that the inflammatory brain cells microglia could be a therapeutic target for the treatment of a MAD associated epilepsy. There are conventional antiepileptic drugs such as gabapentin, lamotrigine, phenytoin sodium, carbamazepine, oxcarbazepine, felbamate, lamotrigine, valproate sodium, and topiramate are prescribed by a psychiatrist to suppress seizure frequency. Also, the conventional drugs generate serious adverse effects and synergises dementia characteristics. The adverse effect of carbamazepine is neurotoxic and also, damages haemopoietic system and respiratory tract. The phenytoin treatment causes cerebellar defect and anemia. Dementia and epilepsy have a complicated relationship, thus targeting mitophagy for cure of epileptic dementia makes sense. Complementary and alternative medicine (CAM) is one of the rising strategies by many patients of the world, not only to suppress seizure frequency but also to mitigate dementia characteristics of patients. Therefore our present review focus on the interplay between epilepsy and MAD and their treatment with CAM approaches.

Nedd4-2 Haploinsufficiency in Mice Impairs the Ubiquitination of Rer1 and Increases the Susceptibility to Endoplasmic Reticulum Stress and Seizures

Neural precursor cell expressed developmentally downregulated gene 4-like (NEDD4-2) is an epilepsy-associated gene encoding an E3 ligase that ubiquitinates neuroactive substrates. An involvement of NEDD4-2 in endoplasmic reticulum (ER) stress has been recently found with mechanisms needing further investigations. Herein, Nedd4-2+/− mice were found intolerant to thapsigargin (Tg) to develop ER stress in the brain. Pretreatment of Tg aggravated the pentylenetetrazole (PTZ)-induced seizures. Retention in endoplasmic reticulum 1 (Rer1), an ER retrieval receptor, was upregulated through impaired ubiquitination in Nedd4-2+/− mouse brain. Nedd4-2 interacted with Rer1 more strongly in mice with Tg administration. The negative regulation and NEDD4-2-mediated ubiquitination on RER1 were evaluated in cultured neurocytes and gliacytes by NEDD4-2 knockdown and overexpression. NEDD4-2 interacted with RER1 at higher levels in the cells with Tg treatment. Disruption of the 36STPY39 motif of RER1 attenuated the interaction with NEDD4-2, and the ubiquitinated RER1 underwent proteasomal degradation. Furthermore, the interactome of Rer1 was screened by immunoprecipitation-mass spectrometry in PTZ-induced mouse hippocampus, showing multiple potential ER retrieval cargoes that mediate neuroexcitability. The α1 subunit of the GABAA receptor was validated to interact with Rer1 and retain in ER more heavily in Nedd4-2+/− mouse brain by Endo-H digestion. In conclusion, Nedd4-2 deficiency in mice showed impaired ubiquitination of Rer1 and increased ER stress and seizures. These data indicate a protective effect of NEDD4-2 in ER stress and seizures possibly via RER1. We also provided potential ER retention cargoes of Rer1 awaiting further investigation.

Synapse Formation and Function Across Species: Ancient Roles for CCP, CUB, and TSP-1 Structural Domains

The appearance of synapses was a crucial step in the creation of the variety of nervous systems that are found in the animal kingdom. With increased complexity of the organisms came a greater number of synaptic proteins. In this review we describe synaptic proteins that contain the structural domains CUB, CCP, or TSP-1. These domains are found in invertebrates and vertebrates, and CUB and CCP domains were initially described in proteins belonging to the complement system of innate immunity. Interestingly, they are found in synapses of the nematode C. elegans, which does not have a complement system, suggesting an ancient function. Comparison of the roles of CUB-, CCP-, and TSP-1 containing synaptic proteins in various species shows that in more complex nervous systems, these structural domains are combined with other domains and that there is partial conservation of their function. These three domains are thus basic building blocks of the synaptic architecture. Further studies of structural domains characteristic of synaptic proteins in invertebrates such as C. elegans and comparison of their role in mammals will help identify other conserved synaptic molecular building blocks. Furthermore, this type of functional comparison across species will also identify structural domains added during evolution in correlation with increased complexity, shedding light on mechanisms underlying cognition and brain diseases.

Long-term efficacy of low-dose perampanel for progressive myoclonus epilepsy in a patient with Gaucher disease type 3

PURPOSE

We report the case of a patient with progressive myoclonus epilepsy due to Gaucher disease type 3 whose seizures and ability to perform activities of daily living were significantly improved after starting low-dose perampanel therapy.

CASE

Our patient's generalized tonic-clonic seizures and myoclonus did not improve despite the administration of multiple antiseizure medications and enzyme replacement therapy. The myoclonus reduced following pharmacological chaperone therapy, but this effect was temporary, and the generalized tonic-clonic seizures continued to occur. However, the generalized tonic-clonic seizures disappeared following treatment with 2 mg/day of perampanel. In addition, the decrease in myoclonus dramatically improved motor function such as talking, eating, and walking and stabilized the patient's mental status. These effects have been sustained for more than 4 years.

CONCLUSION

Perampanel is expected to be effective in the treatment of progressive myoclonus epilepsy associated with Gaucher disease type 3 and should be considered the drug of choice for this condition.

The Role of NEDD4 E3 Ubiquitin–Protein Ligases in Parkinson’s Disease

Parkinson’s disease (PD) is a debilitating neurodegenerative disease that causes a great clinical burden. However, its exact molecular pathologies are not fully understood. Whilst there are a number of avenues for research into slowing, halting, or reversing PD, one central idea is to enhance the clearance of the proposed aetiological protein, oligomeric α-synuclein. Oligomeric α-synuclein is the main constituent protein in Lewy bodies and neurites and is considered neurotoxic. Multiple E3 ubiquitin-protein ligases, including the NEDD4 (neural precursor cell expressed developmentally downregulated protein 4) family, parkin, SIAH (mammalian homologues of Drosophila seven in absentia), CHIP (carboxy-terminus of Hsc70 interacting protein), and SCFFXBL5 SCF ubiquitin ligase assembled by the S-phase kinase-associated protein (SKP1), cullin-1 (Cul1), a zinc-binding RING finger protein, and the F-box domain/Leucine-rich repeat protein 5-containing protein FBXL5), have been shown to be able to ubiquitinate α-synuclein, influencing its subsequent degradation via the proteasome or lysosome. Here, we explore the link between NEDD4 ligases and PD, which is not only via α-synuclein but further strengthened by several additional substrates and interaction partners. Some members of the NEDD4 family of ligases are thought to crosstalk even with PD-related genes and proteins found to be mutated in familial forms of PD. Mutations in NEDD4 family genes have not been observed in PD patients, most likely because of their essential survival function during development. Following further in vivo studies, it has been thought that NEDD4 ligases may be viable therapeutic targets in PD. NEDD4 family members could clear toxic proteins, enhancing cell survival and slowing disease progression, or might diminish beneficial proteins, reducing cell survival and accelerating disease progression. Here, we review studies to date on the expression and function of NEDD4 ubiquitin ligases in the brain and their possible impact on PD pathology.

E3 ubiquitin ligase Nedd4-2 exerts neuroprotective effects during endoplasmic reticulum stress

The neural precursor cell expressed developmentally down-regulated protein 4-like (Nedd4-2) is an E3 ubiquitin ligase critical for neurodevelopment and homeostasis of neural circuit excitability. While dysregulation of Nedd4-2 has been linked to elevated seizure susceptibility through impaired ubiquitination of multiple direct substrates, it remains largely unclear whether Nedd4-2 interconnects other cellular pathways that affect neuronal activity and seizure susceptibility. Here, we first showed that Nedd4-2 associates with the endoplasmic reticulum (ER) and regulates the expression of multiple ER-resident proteins. Furthermore, utilizing Nedd4-2 conditional knockout mice, we showed that Nedd4-2 is required for the maintenance of spontaneous neural activity and excitatory synapses following the induction of ER stress. When analyzing activation of the canonical pathways of ER stress response, we found that Nedd4-2 is required for phosphorylation of eIF2α. While phosphorylation of eIF2α has been shown to reduce seizure susceptibility, attempts to facilitate phosphorylation of eIF2α in Nedd4-2 conditional knockout mice failed to produce such a beneficial function, suggesting a role for Nedd4-2 in integrating the ER stress response to modulate seizure susceptibility. Altogether, our study demonstrates neuroprotective functions of Nedd4-2 during ER stress in neurons and could provide insight into neurological diseases in which the expression or activity of Nedd4-2 is impaired.

Nedd4-2 binding to 14-3-3 modulates the accessibility of its catalytic site and WW domains

Neural precursor cells expressed developmentally downregulated protein 4-2 (Nedd4-2), a homologous to the E6-AP carboxyl terminus (HECT) ubiquitin ligase, triggers the endocytosis and degradation of its downstream target molecules by regulating signal transduction through interactions with other targets, including 14-3-3 proteins. In our previous study, we found that 14-3-3 binding induces a structural rearrangement of Nedd4-2 by inhibiting interactions between its structured domains. Here, we used time-resolved fluorescence intensity and anisotropy decay measurements, together with fluorescence quenching and mass spectrometry, to further characterize interactions between Nedd4-2 and 14-3-3 proteins. The results showed that 14-3-3 binding affects the emission properties of AEDANS-labeled WW3, WW4, and, to a lesser extent, WW2 domains, and reduces their mobility, but not those of the WW1 domain, which remains mobile. In contrast, 14-3-3 binding has the opposite effect on the active site of the HECT domain, which is more solvent exposed and mobile in the complexed form than in the apo form of Nedd4-2. Overall, our results suggest that steric hindrance of the WW3 and WW4 domains combined with conformational changes in the catalytic domain may account for the 14-3-3 binding-mediated regulation of Nedd4-2.

Kainate and AMPA receptors in epilepsy: Cell biology, signalling pathways and possible crosstalk

Epilepsy is caused when rhythmic neuronal network activity escapes normal control mechanisms, resulting in seizures. There is an extensive and growing body of evidence that the onset and maintenance of epilepsy involves alterations in the trafficking, synaptic surface expression and signalling of kainate and AMPA receptors (KARs and AMPARs). The KAR subunit GluK2 and AMPAR subunit GluA2 are key determinants of the properties of their respective assembled receptors. Both subunits are subject to extensive protein interactions, RNA editing and post-translational modifications. In this review we focus on the cell biology of GluK2-containing KARs and GluA2-containing AMPARs and outline how their regulation and dysregulation is implicated in, and affected by, seizure activity. Further, we discuss role of KARs in regulating AMPAR surface expression and plasticity, and the relevance of this to epilepsy. This article is part of the special issue on 'Glutamate Receptors - Kainate receptors'.

AMPA Receptor Antagonists Facilitate NEDD4-2-Mediated GRIA1 Ubiquitination by Regulating PP2B-ERK1/2-SGK1 Pathway in Chronic Epilepsy Rats

The neural precursor cell expressed by developmentally downregulated gene 4-2 (NEDD4-2) is a ubiquitin E3 ligase that has a high affinity toward binding and ubiquitinating glutamate ionotropic receptor α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type subunit 1 (GRIA1, also referred to GluR1 or GluA1). Since dysregulation of GRIA1 surface expression is relevant to the responsiveness to AMPA receptor (AMPAR) antagonists (perampanel and GYKI 52466) in chronic epilepsy rats, it is likely that NEDD4-2 may be involved in the pathogenesis of intractable epilepsy. However, the role of NEDD4-2-mediated GRIA1 ubiquitination in refractory seizures to AMPAR antagonists is still unknown. In the present study, both AMPAR antagonists recovered the impaired GRIA1 ubiquitination by regulating protein phosphatase 2B (PP2B)-extracellular signal-regulated kinase 1/2 (ERK1/2)-serum and glucocorticoid-regulated kinase 1 (SGK1)-NEDD4-2 signaling pathway in responders (whose seizure activities are responsive to AMPAR), but not non-responders (whose seizure activities were uncontrolled by AMPAR antagonists). In addition, cyclosporin A (CsA, a PP2B inhibitor) co-treatment improved the effects of AMPAR antagonists in non-responders, independent of AKT signaling pathway. Therefore, our findings suggest that dysregulation of PP2B-ERK1/2-SGK1-NEDD4-2-mediated GRIA1 ubiquitination may be responsible for refractory seizures and that this pathway may be a potential therapeutic target for improving the treatment of intractable epilepsy in response to AMPAR antagonists.

Infantile spasms-linked Nedd4-2 mediates hippocampal plasticity and learning via cofilin signaling

Individuals affected by infantile spasms (IS), such as those carrying mutations in an IS-linked gene, neural precursor cell expressed developmentally downregulated gene 4-like (Nedd4-2), exhibit developmental delays and learning disabilities, but the underlying mechanism is unknown. Using conditional Nedd4-2 knockout mice, we uncover that Nedd4-2 functions to maintain the excitatory synapses in hippocampal neurons and allows for late-phase long-term synaptic potentiation (L-LTP) at Schaffer collateral synapses in the hippocampus. We also find that Nedd4-2 is required for multiple forms of hippocampus-dependent learning and memory. Mechanistically, we show that loss of Nedd4-2 leads to a decrease in actin polymerization caused by reduced phosphorylation of the actin depolymerizing protein cofilin. A cell-permeable peptide promoting phosphorylation of endogenous cofilin in Nedd4-2 knockout neurons restores the number of hippocampal excitatory synapses and hippocampal L-LTP and partially restores hippocampus-dependent learning in mice. Taken together, our results reveal a novel mechanism underlying IS-associated learning disabilities and may provide information for future therapeutic strategies for IS.

ER stress-induced modulation of neural activity and seizure susceptibility is impaired in a fragile X syndrome mouse model

Imbalanced neuronal excitability homeostasis is commonly observed in patients with fragile X syndrome (FXS) and the animal model of FXS, the Fmr1 KO. While alterations of neuronal intrinsic excitability and synaptic activity at the steady state in FXS have been suggested to contribute to such a deficit and ultimately the increased susceptibility to seizures in FXS, it remains largely unclear whether and how the homeostatic response of neuronal excitability following extrinsic challenges is disrupted in FXS. Our previous work has shown that the acute response following induction of endoplasmic reticulum (ER) stress can reduce neural activity and seizure susceptibility. Because many signaling pathways associated with ER stress response are mediated by Fmr1, we asked whether acute ER stress-induced reduction of neural activity and seizure susceptibility are altered in FXS. Our results first revealed that acute ER stress can trigger a protein synthesis-dependent prevention of neural network synchronization in vitro and a reduction of susceptibility to kainic acid-induced seizures in vivo in wild-type but not in Fmr1 KO mice. Mechanistically, we found that acute ER stress-induced activation of murine double minute-2 (Mdm2), ubiquitination of p53, and the subsequent transient protein synthesis are all impaired in Fmr1 KO neurons. Employing a p53 inhibitor, Pifithrin-α, to mimic p53 inactivation, we were able to blunt the increase in neural network synchronization and reduce the seizure susceptibility in Fmr1 KO mice following ER stress induction. In summary, our data revealed a novel cellular defect in Fmr1 KO mice and suggest that an impaired response to common extrinsic challenges may contribute to imbalanced neuronal excitability homeostasis in FXS.

14-3-3-protein regulates Nedd4-2 by modulating interactions between HECT and WW domains

Neural precursor cell expressed developmentally down-regulated 4 ligase (Nedd4-2) is an E3 ubiquitin ligase that targets proteins for ubiquitination and endocytosis, thereby regulating numerous ion channels, membrane receptors and tumor suppressors. Nedd4-2 activity is regulated by autoinhibition, calcium binding, oxidative stress, substrate binding, phosphorylation and 14-3-3 protein binding. However, the structural basis of 14-3-3-mediated Nedd4-2 regulation remains poorly understood. Here, we combined several techniques of integrative structural biology to characterize Nedd4-2 and its complex with 14-3-3. We demonstrate that phosphorylated Ser³⁴² and Ser⁴⁴⁸ are the key residues that facilitate 14-3-3 protein binding to Nedd4-2 and that 14-3-3 protein binding induces a structural rearrangement of Nedd4-2 by inhibiting interactions between its structured domains. Overall, our findings provide the structural glimpse into the 14-3-3-mediated Nedd4-2 regulation and highlight the potential of the Nedd4-2:14-3-3 complex as a pharmacological target for Nedd4-2-associated diseases such as hypertension, epilepsy, kidney disease and cancer.

Pohl et al. investigated the structural basis of Nedd4-2 regulation by 14-3-3 and found that phosphorylated Ser342 and Ser448 are the main residues that facilitate 14-3-3 binding to Nedd4-2. The authors propose that the Nedd4-2:14-3-3 complex then stimulates a structural rearrangement of Nedd4-2 through inhibiting interaction of its structured domains.

Historical perspective and progress on protein ubiquitination at glutamatergic synapses

Transcription-translation coupling leads to the production of proteins that are key for controlling essential neuronal processes that include neuronal development and changes in synaptic strength. Although these events have been a prevailing theme in neuroscience, the regulation of proteins via posttranslational signaling pathways are equally relevant for these neuronal processes. Ubiquitin is one type of posttranslational modification that covalently attaches to its targets/substrates. Ubiquitination of proteins play a key role in multiple signaling pathways, the predominant being removal of its substrates by a large molecular machine called the proteasome. Here, I review 40 years of progress on ubiquitination in the nervous system at glutamatergic synapses focusing on axon pathfinding, synapse formation, presynaptic release, dendritic spine formation, and regulation of postsynaptic glutamate receptors. Finally, I elucidate emerging themes in ubiquitin biology that may challenge our current understanding of ubiquitin signaling in the nervous system.

Nedd4-2 haploinsufficiency in mice causes increased seizure susceptibility and impaired Kir4.1 ubiquitination

Neural precursor cell expressed developmentally down-regulated gene 4-like (NEDD4-2) encodes a ubiquitin E3 ligase that is involved in epileptogenesis with mechanisms needing further investigation. We constructed a novel Nedd4-2^+/- mouse model with half level of both Nedd4-2 long and short isoforms in the brain. Nedd4-2 haploinsufficiency caused increased susceptibility and severity of pentylenetetrazole (PTZ)-induced seizures. Of the 3379 proteins identified by the hippocampal proteomic analysis, 55 were considered altered in Nedd4-2^+/- mice compared with wild-type control, among which the inwardly rectifying K⁺ channel Kir4.1 was up-regulated by 1.83-fold. Kir4.1 was subsequently confirmed to be less ubiquitinated in response to comprised Nedd4-2 in mouse brains and C6 cells. Kir4.1 associated with Nedd4-2 through the threonine³¹²-proline motif in the intracellular domain by target mutagenesis. Adaptor protein 14-3-3 facilitated Nedd4-2-mediated ubiquitination of Kir4.1. Our data consolidate the detailed molecular mechanism of Nedd4-2-mediated Kir4.1 ubiquitination, and provide a possible relationship between increased seizure susceptibility and impaired Kir4.1 ubiquitination in the brain.

Regulation of Notch signaling by E3 ubiquitin ligases

Notch signaling is an evolutionarily conserved pathway that is widely used for multiple cellular events during development. Activation of the Notch pathway occurs when the ligand from a neighboring cell binds to the Notch receptor and induces cleavage of the intracellular domain of Notch, which further translocates into the nucleus to activate its downstream genes. The involvement of the Notch pathway in diverse biological events is possible due to the complexity in its regulation. In order to maintain tight spatiotemporal regulation, the Notch receptor, as well as its ligand, undergoes a series of physical and biochemical modifications that, in turn, helps in proper maintenance and fine-tuning of the signaling outcome. Ubiquitination is the post-translational addition of a ubiquitin molecule to a substrate protein, and the process is regulated by E3 ubiquitin ligases. The present review describes the involvement of different E3 ubiquitin ligases that play an important role in the regulation and maintenance of proper Notch signaling and how perturbation in ubiquitination results in abnormal Notch signaling leading to a number of human diseases.

Unbiased proteomic screening identifies a novel role for the E3 ubiquitin ligase Nedd4-2 in translational suppression during ER stress

Endoplasmic reticulum (ER) stress occurs when protein folding or maturation is disrupted. A malfunction in the ER stress response can lead to cell death and has been observed in many neurological diseases. However, how the ER stress response is regulated in neuronal cells remains largely unclear. Here, we studied an E3 ubiquitin ligase named neural precursor cell expressed developmentally down-regulated protein 4-like (Nedd4-2). Nedd4-2 is highly expressed in the brain and has a high affinity toward ubiquitinating membrane-bound proteins. We first utilized unbiased proteomic profiling with ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) of isolated membrane fractions from mouse whole brains to identify novel targets of Nedd4-2. Through this screen, we found that the expression and ubiquitination of ribosomal proteins are regulated by Nedd4-2 and we confirmed an association between Nedd4-2 and ribosomes through ribosome sedimentation and polysome profiling. Further, we utilized immunoprecipitation and western blotting to show that induction of ER stress promotes an association between Nedd4-2 and ribosomal proteins, which is mediated through dephosphorylation of Nedd4-2 at serine-342. This increased interaction between Nedd4-2 and ribosomal proteins in turn mediates ER stress-associated translational suppression. In summary, the results of this study demonstrate a novel regulatory mechanism underlying the ER stress response and a novel function of Nedd4-2 in translational control. Our findings may shed light on neurological diseases in which the ER stress response or the function of Nedd4-2 is dysregulated.

Chronic Activation of Gp1 mGluRs Leads to Distinct Refinement of Neural Network Activity through Non-Canonical p53 and Akt Signaling

Group 1 metabotropic glutamate receptors (Gp1 mGluRs), including mGluR1 and mGluR5, are critical regulators for neuronal and synaptic plasticity. Dysregulated Gp1 mGluR signaling is observed with various neurologic disorders, including Alzheimer’s disease, Parkinson’s disease, epilepsy, and autism spectrum disorders (ASDs). It is well established that acute activation of Gp1 mGluRs leads to elevation of neuronal intrinsic excitability and long-term synaptic depression. However, it remains unknown how chronic activation of Gp1 mGluRs can affect neural activity and what molecular mechanisms might be involved. In the current study, we employed a multielectrode array (MEA) recording system to evaluate neural network activity of primary mouse cortical neuron cultures. We demonstrated that chronic activation of Gp1 mGluRs leads to elevation of spontaneous spike frequency while burst activity and cross-electrode synchronization are maintained at the baseline. We further showed that these neural network properties are achieved through proteasomal degradation of Akt that is dependent on the tumor suppressor p53. Genetically knocking down p53 disrupts the elevation of spontaneous spike frequency and alters the burst activity and cross-electrode synchronization following chronic activation of Gp1 mGluRs. Importantly, these deficits can be restored by pharmacologically inhibiting Akt to mimic inactivation of Akt mediated by p53. Together, our findings reveal the effects of chronic activation of Gp1 mGluRs on neural network activity and identify a unique signaling pathway involving p53 and Akt for these effects. Our data can provide insights into constitutively active Gp1 mGluR signaling observed in many neurologic and psychiatric disorders.

Ionotropic Glutamate Receptors in Epilepsy: A Review Focusing on AMPA and NMDA Receptors

It is widely accepted that glutamate-mediated neuronal hyperexcitation plays a causative role in eliciting seizures. Among glutamate receptors, the roles of N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors in physiological and pathological conditions represent major clinical research targets. It is well known that agonists of NMDA or AMPA receptors can elicit seizures in animal or human subjects, while antagonists have been shown to inhibit seizures in animal models, suggesting a potential role for NMDA and AMPA receptor antagonists in anti-seizure drug development. Several such drugs have been evaluated in clinical studies; however, the majority, mainly NMDA-receptor antagonists, failed to demonstrate adequate efficacy and safety for therapeutic use, and only an AMPA-receptor antagonist, perampanel, has been approved for the treatment of some forms of epilepsy. These results suggest that a misunderstanding of the role of each glutamate receptor in the ictogenic process may underlie the failure of these drugs to demonstrate clinical efficacy and safety. Accumulating knowledge of both NMDA and AMPA receptors, including pathological gene mutations, roles in autoimmune epilepsy, and evidence from drug-discovery research and pharmacological studies, may provide valuable information enabling the roles of both receptors in ictogenesis to be reconsidered. This review aimed to integrate information from several studies in order to further elucidate the specific roles of NMDA and AMPA receptors in epilepsy.

Hippocampal HECT E3 ligase inhibition facilitates consolidation, retrieval, and reconsolidation, and inhibits extinction of contextual fear memory

Ubiquitination is involved in synaptic plasticity and memory, but the involvement of HECT E3 ligases in these processes has not yet been established. Here, we bilaterally infused heclin, a specific inhibitor of some of these ligases, into the dorsal hippocampus of male Wistar rats that were trained in a contextual fear conditioning. Heclin improved short-term memory, consolidation, retrieval, and reconsolidation when administered immediately post training, prior to testing, or after memory reactivation, respectively. In addition, it impaired memory extinction when administered prior to a long reactivation session. Heclin infusion was also tested for locomotor activity and anxiety-like behavior in a circular arena, but no effect was seen. Taken together, these results indicate that HECT E3 ligases are involved in the modulation of fear memory.

Novel roles of ER stress in repressing neural activity and seizures through Mdm2- and p53-dependent protein translation

Seizures can induce endoplasmic reticulum (ER) stress, and sustained ER stress contributes to neuronal death after epileptic seizures. Despite the recent debate on whether inhibiting ER stress can reduce neuronal death after seizures, whether and how ER stress impacts neural activity and seizures remain unclear. In this study, we discovered that the acute ER stress response functions to repress neural activity through a protein translation-dependent mechanism. We found that inducing ER stress promotes the expression and distribution of murine double minute-2 (Mdm2) in the nucleus, leading to ubiquitination and down-regulation of the tumor suppressor p53. Reduction of p53 subsequently maintains protein translation, before the onset of translational repression seen during the latter phase of the ER stress response. Disruption of Mdm2 in an Mdm2 conditional knockdown (cKD) mouse model impairs ER stress-induced p53 down-regulation, protein translation, and reduction of neural activity and seizure severity. Importantly, these defects in Mdm2 cKD mice were restored by both pharmacological and genetic inhibition of p53 to mimic the inactivation of p53 seen during ER stress. Altogether, our study uncovered a novel mechanism by which neurons respond to acute ER stress. Further, this mechanism plays a beneficial role in reducing neural activity and seizure severity. These findings caution against inhibition of ER stress as a neuroprotective strategy for seizures, epilepsies, and other pathological conditions associated with excessive neural activity.

One-third of epilepsy patients respond poorly to current anti-epileptic drugs. Thus, there is an urgent need to characterize cellular behavior during seizures, and the corresponding molecular mechanisms in order to develop better therapies. Seizures are known to induce ER stress but how the ER stress response functions to modulate seizure activity is unknown. Our study provides evidence to demonstrate a novel and beneficial role for the ER stress response in reducing neural activity and seizure severity. Mechanistically, we found that these beneficial effects are mediated by elevated protein translation, which is triggered by the activation of Mdm2-p53 signaling, during the early ER stress response. Our findings suggest that therapeutic attempts to reduce ER stress in epilepsies may result in worsening seizure activity and therefore caution against inhibition of ER stress as a neuroprotective strategy for epilepsies.

C2-lacking isoform of Nedd4-2 regulates excitatory synaptic strength through GluA1 ubiquitination-independent mechanisms

Neural precursor cell expressed developmentally downregulated gene 4-like (Nedd4-2) is an epilepsy-associated gene, which encodes a ubiquitin E3 ligase that is highly expressed in the brain. Nedd4-2's substrates include many ion channels and receptors because its N-terminal C2 domain guides Nedd4-2 to the cell membrane. We previously found that Nedd4-2 ubiquitinates the glutamate receptor subunit 1 (GluA1) subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor, which leads to reduction of neuronal excitability and seizures in mice. However, despite awareness of a Nedd4-2 isoform with no C2 domain, the functions of this isoform remain elusive. In this study, we showed that the C2-lacking Nedd4-2 has reduced membrane distribution and exhibits reduced affinity toward ubiquitinating GluA1. However, when expressed in primary cortical neurons, we found that the C2-lacking Nedd4-2 exhibits a similar activity toward reducing excitatory synaptic strength as does the C2-containing Nedd4-2. In an attempt to identify novel Nedd4-2 substrates that could mediate excitatory synaptic strength, we used unbiased proteomic screening and found multiple synaptic regulators that were up-regulated in the brain of conditional Nedd4-2 knockout mice, including protein phosphatase 3 catalytic subunit-α (PPP3CA; alternately called calcineurin A-α). We confirmed PPP3CA as a substrate of the C2-lacking Nedd4-2 and showed that all three epilepsy-associated missense mutations of Nedd4-2 disrupted PPP3CA ubiquitination. Altogether, our results revealed novel potential Nedd4-2 substrates and suggest that the C2-lacking Nedd4-2 represses excitatory synaptic strength most likely through GluA1 ubiquitination-independent mechanisms. These findings provide novel information to further our knowledge about Nedd4-2-dependent neuronal excitability homeostasis and pathological hyperexcitability when Nedd4-2 is compromised.

PLPP/CIN-mediated NEDD4-2 S448 dephosphorylation regulates neuronal excitability via GluA1 ubiquitination

Neuronal precursor cell expressed developmentally downregulated 4-2 (NEDD4-2) is an E3 ubiquitin ligase to regulate ion transport by controlling cellular trafficking/endocytosis and lysosomal degradation of ion channels and transporters. Thus, NEDD4-2 is relevant to neuronal excitability and epileptic encephalopathies in human patients. However, the regulatory molecules for NEDD4-2 dephosphorylation have been still elusive. Here, we demonstrate that pyridoxal-5′-phosphate phosphatase/chronophin (PLPP/CIN) specifically dephosphorylated NEDD4-2 serine (S) 448 site. PLPP/CIN deletion inhibited NEDD4-2 ubiquitination, and diminished the responsiveness of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptor (AMPAR) by facilitating NEDD4-2-mediated ubiquitination of GluA1 subunit under physiological condition. PLPP/CIN overexpression reversed these effects. These PLPP/CIN-mediated processes were required for the increased seizure severity and its progression in response to kainic acid (KA). Therefore, we suggest the novel function of PLPP/CIN as a NEDD4-2 phosphatase, which may be a potential therapeutic target for NEDD4-2-associated diseases as well as various neurological and psychiatric disorders, including epilepsy.

Amyloid-β Induces AMPA Receptor Ubiquitination and Degradation in Primary Neurons and Human Brains of Alzheimer's Disease

As the primary mediator for synaptic transmission, AMPA receptors (AMPARs) are crucial for synaptic plasticity and higher brain functions. A downregulation of AMPAR expression has been indicated as one of the early pathological molecular alterations in Alzheimer's disease (AD), presumably via amyloid-β (Aβ). However, the molecular mechanisms leading to the loss of AMPARs remain less clear. We report that in primary neurons, application of Aβ triggers AMPAR internalization accompanied with a decrease in cell-surface AMPAR expression. Importantly, in both Aβ-treated neurons and human brain tissue from AD patients, we observed a significant decrease in total AMPAR amount and an enhancement in AMPAR ubiquitination. Consistent with facilitated receptor degradation, AMPARs show higher turnover rates in the presence of Aβ. Furthermore, AD brain lysates and Aβ-incubated neurons show increased expression of the AMPAR E3 ligase Nedd4 and decreased expression of AMPAR deubiquitinase USP46. Changes in these enzymes are responsible for the Aβ-dependent AMPAR reduction. These findings indicate that AMPAR ubiquitination acts as the key molecular event leading to the loss of AMPARs and thus suppressed synaptic transmission in AD.

Loss of fragile X protein FMRP impairs homeostatic synaptic downscaling through tumor suppressor p53 and ubiquitin E3 ligase Nedd4-2

Synaptic scaling allows neurons to homeostatically readjust synaptic strength upon chronic neural activity perturbations. Although altered synaptic scaling has been implicated to underlie imbalanced brain excitability in neurological disorders such as autism spectrum disorders and epilepsy, the molecular dysregulation and restoration of synaptic scaling in those diseases have not been demonstrated. Here, we showed that the homeostatic synaptic downscaling is absent in the hippocampal neurons of Fmr1 KO mice, the mouse model of the most common inherited autism, fragile X syndrome (FXS). We found that the impaired homeostatic synaptic downscaling in Fmr1 KO neurons is caused by loss-of-function dephosphorylation of an epilepsy-associated ubiquitin E3 ligase, neural precursor cell expressed developmentally down-regulated gene 4-2, Nedd4-2. Such dephosphorylation of Nedd4-2 is surprisingly caused by abnormally stable tumor suppressor p53 and subsequently destabilized kinase Akt. Dephosphorylated Nedd4-2 fails to elicit 14-3-3-dependent ubiquitination and down-regulation of the GluA1 subunit of AMPA receptor, and therefore impairs synaptic downscaling. Most importantly, using a pharmacological inhibitor of p53, Nedd4-2 phosphorylation, GluA1 ubiquitination and synaptic downscaling are all restored in Fmr1 KO neurons. Together, our results discover a novel cellular mechanism underlying synaptic downscaling, and demonstrate the dysregulation and successful restoration of this mechanism in the FXS mouse model.

Drug repositioning in epilepsy reveals novel antiseizure candidates

Objective

Epilepsy treatment falls short in ~30% of cases. A better understanding of epilepsy pathophysiology can guide rational drug development in this difficult to treat condition. We tested a low-cost, drug-repositioning strategy to identify candidate epilepsy drugs that are already FDA-approved and might be immediately tested in epilepsy patients who require new therapies.

Methods

Biopsies of spiking and nonspiking hippocampal brain tissue from six patients with unilateral mesial temporal lobe epilepsy were analyzed by RNA-Seq. These profiles were correlated with transcriptomes from cell lines treated with FDA-approved drugs, identifying compounds which were tested for therapeutic efficacy in a zebrafish seizure assay.

Results

In spiking versus nonspiking biopsies, RNA-Seq identified 689 differentially expressed genes, 148 of which were previously cited in articles mentioning seizures or epilepsy. Differentially expressed genes were highly enriched for protein-protein interactions and formed three clusters with associated GO-terms including myelination, protein ubiquitination, and neuronal migration. Among the 184 compounds, a zebrafish seizure model tested the therapeutic efficacy of doxycycline, metformin, nifedipine, and pyrantel tartrate, with metformin, nifedipine, and pyrantel tartrate all showing efficacy.

Interpretation

This proof-of-principle analysis suggests our powerful, rapid, cost-effective approach can likely be applied to other hard-to-treat diseases.

Physiological Functions of Nedd4-2: Lessons from Knockout Mouse Models

Protein modification by ubiquitination plays a key evolutionarily conserved role in regulating membrane proteins. Nedd4-2, a ubiquitin ligase, targets membrane proteins such as ion channels and transporters for ubiquitination. This Nedd4-2-mediated ubiquitination provides a crucial step in controlling the membrane availability of these proteins, thus affecting their signaling and physiological outcomes. In one well-studied example, Nedd4-2 fine-tunes the physiological function of the epithelial sodium channel (ENaC), thus modulating Na⁺ reabsorption by epithelia to maintain whole-body Na⁺ homeostasis. This review summarizes the key signaling pathways regulated by Nedd4-2 and the possible implications of such regulation in various pathologies.

Mdm2 mediates FMRP- and Gp1 mGluR-dependent protein translation and neural network activity

Activating Group 1 (Gp1) metabotropic glutamate receptors (mGluRs), including mGluR1 and mGluR5, elicits translation-dependent neural plasticity mechanisms that are crucial to animal behavior and circuit development. Dysregulated Gp1 mGluR signaling has been observed in numerous neurological and psychiatric disorders. However, the molecular pathways underlying Gp1 mGluR-dependent plasticity mechanisms are complex and have been elusive. In this study, we identified a novel mechanism through which Gp1 mGluR mediates protein translation and neural plasticity. Using a multi-electrode array (MEA) recording system, we showed that activating Gp1 mGluR elevates neural network activity, as demonstrated by increased spontaneous spike frequency and burst activity. Importantly, we validated that elevating neural network activity requires protein translation and is dependent on fragile X mental retardation protein (FMRP), the protein that is deficient in the most common inherited form of mental retardation and autism, fragile X syndrome (FXS). In an effort to determine the mechanism by which FMRP mediates protein translation and neural network activity, we demonstrated that a ubiquitin E3 ligase, murine double minute-2 (Mdm2), is required for Gp1 mGluR-induced translation and neural network activity. Our data showed that Mdm2 acts as a translation suppressor, and FMRP is required for its ubiquitination and down-regulation upon Gp1 mGluR activation. These data revealed a novel mechanism by which Gp1 mGluR and FMRP mediate protein translation and neural network activity, potentially through de-repressing Mdm2. Our results also introduce an alternative way for understanding altered protein translation and brain circuit excitability associated with Gp1 mGluR in neurological diseases such as FXS.

Regulation of AMPA Receptor Trafficking by Protein Ubiquitination

The molecular mechanisms underlying plastic changes in the strength and connectivity of excitatory synapses have been studied extensively for the past few decades and remain the most attractive cellular models of learning and memory. One of the major mechanisms that regulate synaptic plasticity is the dynamic adjustment of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor content on the neuronal plasma membrane. The expression of surface AMPA receptors (AMPARs) is controlled by the delicate balance between the biosynthesis, dendritic transport, exocytosis, endocytosis, recycling and degradation of the receptors. These processes are dynamically regulated by AMPAR interacting proteins as well as by various post-translational modifications that occur on their cytoplasmic domains. In the last few years, protein ubiquitination has emerged as a major regulator of AMPAR intracellular trafficking. Dysregulation of AMPAR ubiquitination has also been implicated in the pathophysiology of Alzheimer’s disease. Here we review recent advances in the field and provide insights into the role of protein ubiquitination in regulating AMPAR membrane trafficking and function. We also discuss how aberrant ubiquitination of AMPARs contributes to the pathogenesis of various neurological disorders, including Alzheimer’s disease, chronic stress and epilepsy.