Mouse hippocampal phosphorylation footprint induced by generalized seizures: Focus on ERK, mTORC1 and Akt/GSK-3 pathways

Integrated computational approaches for identification of potent pyrazole-based glycogen synthase kinase-3β (GSK-3β) inhibitors: 3D-QSAR, virtual screening, docking, MM/GBSA, EC, MD simulation studies

Glycogen synthase kinase-3β (GSK-3β) has emerged as a crucial target due to its substantial contribution in various cellular processes. Dysfunctional GSK-3β activity can lead to ion channel disturbances, sustain abnormal excitability, and contribute to the pathogenesis of epilepsy and other GSK-3β-related disorders. A set of 82 pyrazole analogs was utilized to study its structural features using a three-dimensional quantitative structure-activity relationship (3D-QSAR), virtual screening, molecular docking, and molecular dynamics. The QSAR model, validated using internal and external methods, demonstrated robustness with a high correlation coefficient r2training = 0.99, cross-validation coefficient q2 = 0.79, r2test = 0.69, and r2external = 0.74. The "Average of Actives" in the Activity Atlas model identified 17 molecules as active. Subsequent pharmacophore-based virtual screening of 17 actives yielded 70 compounds, which were selected as the prediction set to determine the potential GSK-3β inhibitors. Docking studies pinpointed compound P66 as the promising lead compound, with a docking score of - 10.555 kcal/mol. These findings were further supported by electrostatic potential (ESP), electrostatic complementarity (EC), and Molecular Mechanics/Generalized Born Surface Area (MM/GBSA) analyses. Furthermore, a 500 ns molecular dynamics (MD) simulation confirmed the structural and conformational stability of the lead complex throughout the simulation period. As a result, this study suggests that compound P66 holds the potential to be a potent lead candidate for the inhibition of GSK-3β, offering a novel therapeutic approach for GSK-3β related disorders, including epilepsy.

Temporal changes in mouse hippocampus transcriptome after pilocarpine-induced seizures

Introduction

Status epilepticus (SE) is a seizure lasting more than 5 min that can have lethal consequences or lead to various neurological disorders, including epilepsy. Using a pilocarpine-induced SE model in mice we investigated temporal changes in the hippocampal transcriptome.

Methods

We performed mRNA-seq and microRNA-seq analyses at various times after drug treatment.

Results

At 1 h after the start of seizures, hippocampal cells upregulated transcription of immediate early genes and genes involved in the IGF-1, ERK/MAPK and RNA-PolII/transcription pathways. At 8 h, we observed changes in the expression of genes associated with oxidative stress, overall transcription downregulation, particularly for genes related to mitochondrial structure and function, initiation of a stress response through regulation of ribosome and translation/EIF2 signaling, and upregulation of an inflammatory response. During the middle of the latent period, 36 h, we identified upregulation of membrane components, cholesterol synthesis enzymes, channels, and extracellular matrix (ECM), as well as an increased inflammatory response. At the end of the latent period, 120 h, most changes in expression were in genes involved in ion transport, membrane channels, and synapses. Notably, we also elucidated the involvement of novel pathways, such as cholesterol biosynthesis pathways, iron/BMP/ferroptosis pathways, and circadian rhythms signaling in SE and epileptogenesis.

Discussion

These temporal changes in metabolic reactions indicate an immediate response to injury followed by recovery and regeneration. CREB was identified as the main upstream regulator. Overall, our data provide new insights into molecular functions and cellular processes involved at different stages of seizures and offer potential avenues for effective therapeutic strategies.

Differential Regulation of Wnt/β-catenin Signaling in Acute and Chronic Epilepsy in Repeated Low Dose Lithium-Pilocarpine Rat Model of Status Epilepticus

Epilepsy is a chronic neurological complication characterized by unprovoked seizure episodes due to the imbalance between excitatory and inhibitory neurons. The epileptogenesis process has been reported to be involved in chronic epilepsy however, the mechanism underlying epileptogenesis remains unclear. Recent studies have shown the possible involvement of Wnt/β-catenin signaling in the neurogenesis and neuronal reorganization in epileptogenesis. In this study, we used repeated low dose lithium-pilocarpine model of status epilepsy (SE) to study the involvement of Wnt/β-catenin signaling at acute and chronic stages post SE induction. The acute study ranged from day 0 to day 28 post SE induction and the chronic study ranged from day 0 to day 56 post SE induction. Several neurobehavioral parameters and seizure score and seizure frequency was analysed until the end of the study. The proteins involved in the regulation of Wnt/β-catenin signaling and downstream cascading were analysed using western blot and quantitative real-time PCR analysis. The Wnt/β-catenin pathway was found inactive in acute SE, while the same was found activated at the chronic stage. Our findings suggest that the activated Wnt/β-catenin signaling in chronic epilepsy might be the possible mechanism underlying epileptogenesis as indicated by increased neuronal count, increased synaptic density, astrogliosis and apoptosis in chronic epilepsy. These findings can help target the Wnt/β-catenin pathway differentially depending upon the type of epilepsy. The acute stage characterized by SE can be improved by targeting GSK-3β levels and the chronic stage characterized by temporal lobe epilepsy can be improved by targeting β-catenin and disheveled proteins.

Electroconvulsive seizures regulate various stages of hippocampal cell genesis and mBDNF at different times after treatment in adolescent and adult rats of both sexes

Electroconvulsive therapy, a fast-acting option for treatment-resistant depression, is modeled at the preclinical level through the induction of electroconvulsive seizures (ECS) in rodents. Recent studies from our group proved sex- and age-differences in the antidepressant-like response elicited by ECS in rats; while an antidepressant-like response was observed in male adolescent and adult rats (although with greater efficacy in adulthood), the same parameters rendered inefficacious in females of any age. To better understand the potential sex differences taking place at the molecular level that might be mediating these behavioral disparities, we evaluated the impact of a repeated treatment with ECS (95 mA for 0.6 s, 100 Hz, 0.6 ms) in adolescent and adult rats of both sexes. Several hippocampal markers of neuroplasticity, commonly regulated by most antidepressants, such as those of neurogenesis (cell proliferation, neurogenic differentiation, long-term cell survival) or mBDNF and associated signaling (e.g., mTOR and ERK1/2) were evaluated at different time-points after treatment (1-, 8-, 15- and up to 30-days post-treatment). The main results demonstrated that ECS improved the survival rate of new cells born in the dentate gryus before treatment. Moreover, ECS increased cell proliferation and neurogenic differentiation at different times post-treatment, paired with persistent increases in mBDNF, observed long after treatment. In general, effects were different for each sex and varied with the age of the animal (adolescent vs. adulthood). The present study is the first-one to demonstrate that such persistent molecular changes induced by ECS in hippocampus, some of them observed up to 30-days post-treatment, also occurred in female rats and adolescence. Although these molecular changes could not justify the lack of ECS efficacy described by these same parameters of ECS in female rats (vs. male rats), they proposed certain beneficial effects common to both sexes, and age periods studied, opening the avenue for further studies. Based on these neurochemical effects, ECS should have displayed similar efficacies for both biological sexes. Therefore, the reason behind these disparities should be further explored to better translate efficacious treatments specific and/or personalized for each sex to the clinic.

Administration of Kainic Acid Differentially Alters Astrocyte Markers and Transiently Enhanced Phospho-tau Level in Adult Rat Hippocampus

Kainic acid (KA), an analogue of the excitatory neurotransmitter glutamate, when administered systemically can trigger seizures and neuronal loss in a manner that mirrors the neuropathology of human mesial temporal lobe epilepsy (mTLE), which affects ∼50 million people globally. Evidence suggests that changes in astrocytes which precede neuronal damage play an important role in the degeneration of neurons and/or development of seizures in TLE pathogenesis. Additionally, a role for microtubule associated tau protein, involved in various neurodegenerative diseases including Alzheimer's disease, has also been suggested in the development of seizure and/or neurodegeneration in TLE pathogenesis. At present, possible alterations of different subtypes of astrocytes and their association, if any, with tau protein in TLE remain unclear. In this study, we evaluated alterations of different subtypes of astrocytes and phospho-/cleaved-tau levels in KA-treated rat model of TLE. Our results reveal that levels/expression of various astrocyte markers such as GFAP, vimentin, S100B, Aldh1L1, but not GS, are increased in the hippocampus of KA-treated rats. The levels/expression of both A1(C3+) and A2(S100A10+)-like astrocytes are also increased in KA-treated rats. Concurrently, the total (Tau1 and Tau5) and phospho-tau (AT270 and PHF1) levels are transiently enhanced following KA administration. Furthermore, the level/expression of cleaved-tau, which is apparent in a subset of GFAP-, S100B- and A2-positive astrocytes, are increased in KA-treated rats. These results, taken together, suggest a differential role for various astrocytic subpopulations and tau protein in the development of seizure and/or loss of neurons in KA model of TLE and possibly in human mTLE pathogenesis.

A Genetic Model of Epilepsy with a Partial Alzheimer's Disease-Like Phenotype and Central Insulin Resistance

Studies have suggested an important connection between epilepsy and Alzheimer's disease (AD), mostly due to the high number of patients diagnosed with AD who develop epileptic seizures later on. However, this link is not well understood. Previous studies from our group have identified memory impairment and metabolic abnormalities in the Wistar audiogenic rat (WAR) strain, a genetic model of epilepsy. Our goal was to investigate AD behavioral and molecular alterations, including brain insulin resistance, in naïve (seizure-free) animals of the WAR strain. We used the Morris water maze (MWM) test to evaluate spatial learning and memory performance and hippocampal tissue to verify possible molecular and immunohistochemical alterations. WARs presented worse performance in the MWM test (p < 0.0001), higher levels of hyperphosphorylated tau (S396) (p < 0.0001) and phosphorylated glycogen synthase kinase 3 (S21/9) (p < 0.05), and lower insulin receptor levels (p < 0.05). Conversely, WARs and Wistar controls present progressive increase in amyloid fibrils (p < 0.0001) and low levels of soluble amyloid-β. Interestingly, the detected alterations were age-dependent, reaching larger differences in aged than in young adult animals. In summary, the present study provides evidence of a partial AD-like phenotype, including altered regulation of insulin signaling, in a genetic model of epilepsy. Together, these data contribute to the understanding of the connection between epilepsy and AD as comorbidities. Moreover, since both tau hyperphosphorylation and altered insulin signaling have already been reported in epilepsy and AD, these two events should be considered as important components in the interconnection between epilepsy and AD pathogenesis and, therefore, potential therapeutic targets in this field.

Altered Protein Profiles During Epileptogenesis in the Pilocarpine Mouse Model of Temporal Lobe Epilepsy

Epilepsy is characterized by recurrent, spontaneous seizures and is a major contributor to the global burden of neurological disease. Although epilepsy can result from a variety of brain insults, in many cases the cause is unknown and, in a significant proportion of cases, seizures cannot be controlled by available treatments. Understanding the molecular alterations that underlie or are triggered by epileptogenesis would help to identify therapeutics to prevent or control progression to epilepsy. To this end, the moderate throughput technique of Reverse Phase Protein Arrays (RPPA) was used to profile changes in protein expression in a pilocarpine mouse model of acquired epilepsy. Levels of 54 proteins, comprising phosphorylation-dependent and phosphorylation-independent components of major signaling pathways and cellular complexes, were measured in hippocampus, cortex and cerebellum of mice at six time points, spanning 15 min to 2 weeks after induction of status epilepticus. Results illustrate the time dependence of levels of the commonly studied MTOR pathway component, pS6, and show, for the first time, detailed responses during epileptogenesis of multiple components of the MTOR, MAPK, JAK/STAT and apoptosis pathways, NMDA receptors, and additional cellular complexes. Also noted are time- and brain region- specific changes in correlations among levels of functionally related proteins affecting both neurons and glia. While hippocampus and cortex are primary areas studied in pilocarpine-induced epilepsy, cerebellum also shows significant time-dependent molecular responses.

Increased epileptogenicity in a mouse model of neurofibromatosis type 1

RATIONALE

Neurofibromatosis type 1 (NF1) is associated with higher rates of epilepsy compared to the general population. Some NF1 patients with epilepsy do not have intracranial lesions, suggesting the genetic mutation itself may contribute to higher rates of epilepsy in these patients. We have recently demonstrated increased seizure susceptibility in the Nf1^+/- mouse, but it is unknown whether this model displays altered epileptogenicity, as has been reported in patients with NF1. The aim of this study was to determine whether the Nf1^+/- mouse is more susceptible to electrical kindling-induced epileptogenesis.

METHODS

Young male or female adult Nf1^+/- or Nf1^+/+ (wild-type; WT) mice were implanted with electrodes for neocortical or hippocampal kindling paradigms. Neocortical kindling was performed for 40 stimulation sessions followed by baseline EEG monitoring to detect possible SRSs. Hippocampal kindling was performed with a modified extended kindling paradigm, completed to a maximum of 80 sessions to try to induce spontaneous repetitive seizures (SRSs). Western blot assays were performed in naïve and kindled mice to compare levels of Akt and MAPK (ERK1/2), proteins downstream of the NF1 mutation.

RESULTS

The average initial neocortical after-discharge threshold (ADT) was significantly lower in the Nf1^+/- group, which also required fewer stimulations to reach stage 5 seizure, had greater average seizure severity across all kindling sessions, had a greater number of convulsive seizures, and had a faster progression of after-discharge duration and Racine score during kindling. No WT mice exhibited SRS after neocortical kindling, versus 33% of Nf1^+/- mice. The average initial hippocampal ADT was not significantly different between the WT and Nf1^+/- groups, nor was there a difference in the number of stimulations required to reach the kindled state. The WT group had a significantly higher average seizure severity across all kindling sessions as compared with the Nf1^+/- mice. The WT group also had faster progression of the Racine seizure score over the kindling sessions, mainly due to a faster increase in seizures severity early during the kindling process. However, SRSs were seen in 50% of Nf1^+/- mice after modified extended kindling and in no WT mice. Western blots showed hippocampal kindling increased the ratio of phosphorylated/total Akt in both the WT and Nf1^+/- mice, while neocortical kindling led to increased ratios of phosphorylated/total Akt and MAPK in Nf1^+/- mice only.

CONCLUSIONS

We have demonstrated for the first time an increased rate of epileptogenesis in an animal model of NF1 with no known macroscopic/neoplastic brain lesions. This work provides evidence for the genetic mutation itself playing a role in seizures and epilepsy in patients with NF1, and supports the use of the Nf1^+/- mouse model in future mechanistic studies.

Unravelling the Role of Glycogen Synthase Kinase-3 in Alzheimer's Disease-Related Epileptic Seizures

Alzheimer’s disease (AD) is the most common form of dementia. An increasing body of evidence describes an elevated incidence of epilepsy in patients with AD, and many transgenic animal models of AD also exhibit seizures and susceptibility to epilepsy. However, the biological mechanisms that underlie the occurrence of seizure or increased susceptibility to seizures in AD is unknown. Glycogen synthase kinase-3 (GSK-3) is a serine/threonine kinase that regulates various cellular signaling pathways, and plays a crucial role in the pathogenesis of AD. It has been suggested that GSK-3 might be a key factor that drives epileptogenesis in AD by interacting with the pathological hallmarks of AD, amyloid precursor protein (APP) and tau. Furthermore, seizures may also contribute to the progression of AD through GSK-3. In this way, GSK-3 might be involved in initiating a vicious cycle between AD and seizures. This review aims to summarise the possible role of GSK-3 in the link between AD and seizures. Understanding the role of GSK-3 in AD-associated seizures and epilepsy may help researchers develop new therapeutic approach that can manage seizure and epilepsy in AD patients as well as decelerate the progression of AD.

Control of neuronal excitability by GSK-3beta: Epilepsy and beyond

Glycogen synthase kinase 3beta (GSK-3β) is an enzyme with a variety of cellular functions in addition to the regulation of glycogen metabolism. In the central nervous system, different intracellular signaling pathways converge on GSK-3β through a cascade of phosphorylation events that ultimately control a broad range of neuronal functions in the development and adulthood. In mice, genetically removing or increasing GSK-3β cause distinct functional and structural neuronal phenotypes and consequently affect cognition. Precise control of GSK-3β activity is important for such processes as neuronal migration, development of neuronal morphology, synaptic plasticity, excitability, and gene expression. Altered GSK-3β activity contributes to aberrant plasticity within neuronal circuits leading to neurological, psychiatric disorders, and neurodegenerative diseases. Therapeutically targeting GSK-3β can restore the aberrant plasticity of neuronal networks at least in animal models of these diseases. Although the complete repertoire of GSK-3β neuronal substrates has not been defined, emerging evidence shows that different ion channels and their accessory proteins controlling excitability, neurotransmitter release, and synaptic transmission are regulated by GSK-3β, thereby supporting mechanisms of synaptic plasticity in cognition. Dysregulation of ion channel function by defective GSK-3β activity sustains abnormal excitability in the development of epilepsy and other GSK-3β-linked human diseases.

GSK3β and Tau Protein in Alzheimer's Disease and Epilepsy

Alzheimer's disease (AD) is the most common form of dementia present in older adults; its etiology involves genetic and environmental factors. In recent years, epidemiological studies have shown a correlation between AD and chronic epilepsy since a considerable number of patients with AD may present seizures later on. Although the pathophysiology of seizures in AD is not completely understood, it could represent the result of several molecular mechanisms linked to amyloid beta-peptide (Aβ) accumulation and the hyperphosphorylation of tau protein, which may induce an imbalance in the release and recapture of excitatory and inhibitory neurotransmitters, structural alterations of the neuronal cytoskeleton, synaptic loss, and neuroinflammation. These changes could favor the recurrent development of hypersynchronous discharges and epileptogenesis, which, in a chronic state, favor the neurodegenerative process and influence the cognitive decline observed in AD. Supporting this correlation, histopathological studies in the brain tissue of temporal lobe epilepsy (TLE) patients have revealed the presence of Aβ deposits and the accumulation of tau protein in the neurofibrillary tangles (NFTs), accompanied by an increase of glycogen synthase kinase-3 beta (GSK3β) activity that may lead to an imminent alteration in posttranslational modifications of some microtubule-associated proteins (MAPs), mainly tau. The present review is focused on understanding the pathological aspects of GSK3β and tau in the development of TLE and AD.

Tau Phosphorylation in a Mouse Model of Temporal Lobe Epilepsy

Hyperphosphorylation of the microtubule-associated protein tau and its resultant aggregation into neurofibrillary tangles (NFT) is a pathological characteristic of neurodegenerative disorders known as tauopathies. Tau is a neuronal protein involved in the stabilization of microtubule structures of the axon and the aberrant phosphorylation of tau is associated with several neurotoxic effects. The discovery of tau pathology and aggregates in the cortex of Temporal lobe epilepsy (TLE) patients has focused interest on hyperphosphorylation of tau as a potential mechanism contributing to increased states of hyperexcitability and cognitive decline. Previous studies using animal models of status epilepticus and tissue from patients with TLE have shown increased tau phosphorylation in the brain following acute seizures and during epilepsy, with tau phosphorylation correlating with cognitive deficits in patients. Suggesting a functional role of tau during epilepsy, studies in tau-deficient and tau-overexpressing mice have demonstrated a causal role of tau during seizure generation. Previous studies, analyzing the impact of seizures on tau hyperphosphorylation, have mainly used animal models of acute seizures. These models, however, do not replicate all aspects of chronic epilepsy. In this study, we investigated the effects of acute seizures (status epilepticus) and chronic epilepsy upon the expression and phosphorylation of tau using the intra-amygdala kainic acid (KA)-induced status epilepticus mouse model. Status epilepticus resulted in an immediate increase in total tau levels in the hippocampus, in particular, the dentate gyrus, and phosphorylation of the AT8 epitope (Ser202, Thr205), with phosphorylated tau mainly localizing to the mossy fibers of the dentate gyrus. During epilepsy, abnormal phosphorylation of tau was detected again at the AT8 epitope with lower total tau levels in the CA3 and CA1 subfields of the hippocampus. Chronic epilepsy in mice also resulted in a strong localization of AT8 phospho-tau to microglia, indicating a distinct pattern of tau hyperphosphorylation during chronic epilepsy compared to status epilepticus. Our results reaffirm previous observations of tau phosphorylation post-status epilepticus, but also elaborate on tau alterations in epileptic mice which more faithfully mimic TLE. Our results confirm seizures affect tau hyperphosphorylation, however, suggest epitope-specific phosphorylation of tau and differences in cell-specific localization according to disease progression.

Prevention of post-ischemic seizure by rapamycin is associated with deactivation of mTOR and ERK1/2 pathways in hyperglycemic rats

Pre-ischemic hyperglycemia increases the occurrence of post-ischemic seizures both in experimental and clinical settings. The underlying mechanisms are not fully delineated; however, activation of mammalian target of rapamycin (mTOR) has been shown to be engaged in the pathogenesis of epilepsy, in which seizures are a regular occurrence. Therefore, we wanted to explore specifically the capacity of an mTOR inhibitor, rapamycin, in preventing post-ischemic seizures in hyperglycemic rats and to explore the underlying molecular mechanisms. The results showed that none of the rats in the sham control, EG ischemic, or within 3 h of I/R in hyperglycemic ischemic groups experienced seizures. Generalized tonic-clonic seizures were observed in all 8/8 of hyperglycemic ischemic rats at 16 h of I/R. Treatment with rapamycin successfully blocked post-ischemic seizures in 7/8 hyperglycemic ischemic animals. Rapamycin also lessened the neuronal death extraordinarily in hyperglycemic ischemic animals as revealed by histopathological studies. Protein analysis revealed that transient ischemia resulted in increases in p-mTOR and p-S6, especially in the hippocampi of the hyperglycemic ischemic rats. Rapamycin treatment completely blocked mTOR activation. Furthermore, hyperglycemic ischemia induced a much prominent rise of p-ERK1/2 both in the cortex and the hippocampi compared with EG counterparts; whereas rapamycin suppressed it. We conclude that the development of post-ischemic seizures in the hyperglycemic animals may be associated with activations of mTOR and ERK1/2 pathways and that rapamycin treatment inhibited the post-ischemic seizures effectively by suppressing the mTOR and ERK1/2 signaling.

GSK-3β at the Intersection of Neuronal Plasticity and Neurodegeneration

In neurons, Glycogen Synthase Kinase-3β (GSK-3β) has been shown to regulate various critical processes underlying structural and functional synaptic plasticity. Mouse models with neuron-selective expression or deletion of GSK-3β present behavioral and cognitive abnormalities, positioning this protein kinase as a key signaling molecule in normal brain functioning. Furthermore, mouse models with defective GSK-3β activity display distinct structural and behavioral abnormalities, which model some aspects of different neurological and neuropsychiatric disorders. Equalizing GSK-3β activity in these mouse models by genetic or pharmacological interventions is able to rescue some of these abnormalities. Thus, GSK-3β is a relevant therapeutic target for the treatment of many brain disorders. Here, we provide an overview of how GSK-3β is regulated in physiological synaptic plasticity and how aberrant GSK-3β activity contributes to the development of dysfunctional synaptic plasticity in neuropsychiatric and neurodegenerative disorders.

Bi-directional genetic modulation of GSK-3β exacerbates hippocampal neuropathology in experimental status epilepticus

Glycogen synthase kinase-3 (GSK-3) is ubiquitously expressed throughout the brain and involved in vital molecular pathways such as cell survival and synaptic reorganization and has emerged as a potential drug target for brain diseases. A causal role for GSK-3, in particular the brain-enriched GSK-3β isoform, has been demonstrated in neurodegenerative diseases such as Alzheimer’s and Huntington’s, and in psychiatric diseases. Recent studies have also linked GSK-3 dysregulation to neuropathological outcomes in epilepsy. To date, however, there has been no genetic evidence for the involvement of GSK-3 in seizure-induced pathology. Status epilepticus (prolonged, damaging seizure) was induced via a microinjection of kainic acid into the amygdala of mice. Studies were conducted using two transgenic mouse lines: a neuron-specific GSK-3β overexpression and a neuron-specific dominant-negative GSK-3β (GSK-3β-DN) expression in order to determine the effects of increased or decreased GSK-3β activity, respectively, on seizures and attendant pathological changes in the hippocampus. GSK-3 inhibitors were also employed to support the genetic approach. Status epilepticus resulted in a spatiotemporal regulation of GSK-3 expression and activity in the hippocampus, with decreased GSK-3 activity evident in non-damaged hippocampal areas. Consistent with this, overexpression of GSK-3β exacerbated status epilepticus-induced neurodegeneration in mice. Surprisingly, decreasing GSK-3 activity, either via overexpression of GSK-3β-DN or through the use of specific GSK-3 inhibitors, also exacerbated hippocampal damage and increased seizure severity during status epilepticus. In conclusion, our results demonstrate that the brain has limited tolerance for modulation of GSK-3 activity in the setting of epileptic brain injury. These findings caution against targeting GSK-3 as a treatment strategy for epilepsy or other neurologic disorders where neuronal hyperexcitability is an underlying pathomechanism.

Profiling the MAPK/ERK dependent and independent activity regulated transcriptional programs in the murine hippocampus in vivo

Activity-dependent alteration of the transcriptional program is central for shaping neuronal connectivity. Constitutively expressed transcription factors orchestrate the initial response to neuronal stimulation and serve as substrates for second messenger-regulated kinase signalling cascades. The mitogen-activated protein kinase ERK conveys signalling from the synapse to the nucleus but its genetic signature following neuronal activity has not been revealed. The goal of the present study was to identify ERK dependent and independent activity regulated transcriptional programs in the murine hippocampus. We used generalized seizures combined with the pharmacological intervention of MEK activation as an in vivo model to determine the complete transcriptional program initiated by ERK after neuronal activity. Our survey demonstrates that the induction of a large number of activity-regulated genes, including Arc/Arg3.1, Arl5b, Gadd45b, Homer1, Inhba and Zwint, is indeed dependent on ERK phosphorylation. In contrast, expression of a small group of genes, including Npas4, Arl4d, Errfi1, and Rgs2, is only partially dependent or completely independent (Ppp1r15a) of this signalling pathway. Among the identified transcripts are long non-coding (lnc) RNAs and induction of LincPint and splice variants of NEAT1 are ERK dependent. Our survey provides a comprehensive analysis of the transcriptomic response conveyed by ERK signalling in the hippocampus.

mTOR and MAPK: from localized translation control to epilepsy

Background

Epilepsy is one of the most common neurological diseases characterized by excessive hyperexcitability of neurons. Molecular mechanisms of epilepsy are diverse and not really understood. All in common is the misregulation of proteins that determine excitability such as potassium and sodium channels as well as GABA receptors; which are all known as biomarkers for epilepsy. Two recently identified key pathways involve the kinases mechanistic target of rapamycin (mTOR) and mitogen-activated protein kinases (MAPK). Interestingly, mRNAs coding for those biomarkers are found to be localized at or near synapses indicating a local misregulation of synthesis and activity.

Results

Research in the last decade indicates that RNA-binding proteins (RBPs) responsible for mRNA localization, stability and translation mediate local expression control. Among others, they are affected by mTOR and MAPK to guide expression of epileptic factors. These results suggest that mTOR/MAPK act on RBPs to regulate the fate of mRNAs, indicating a misregulation of protein expression at synapses in epilepsy.

Conclusion

We propose that mTOR and MAPK regulate RBPs, thereby guiding the local expression of their target-mRNAs encoding for markers of epilepsy. Thus, misregulated mTOR/MAPK-RBP interplay may result in excessive local synthesis of ion channels and receptors thereby leading to hyperexcitability. Continuous stimulation of synapses further activates mTOR/MAPK pathway reinforcing their effect on RBP-mediated expression control establishing the basis for epilepsy. Here, we highlight findings showing the tight interplay between mTOR as well as MAPK with RBPs to control expression for epileptic biomarkers.