Altered phosphorylation and localization of the A-type channel, Kv4.2 in status epilepticus

SUMOylation of the Kv4.2 Ternary Complex Increases Surface Expression and Current Amplitude by Reducing Internalization in HEK 293 Cells

Kv4 α-subunits exist as ternary complexes (TC) with potassium channel interacting proteins (KChIP) and dipeptidyl peptidase-like proteins (DPLP); multiple ancillary proteins also interact with the α-subunits throughout the channel’s lifetime. Dynamic regulation of Kv4.2 protein interactions adapts the transient potassium current, IA, mediated by Kv4 α-subunits. Small ubiquitin-like modifier (SUMO) is an 11 kD peptide post-translationally added to lysine (K) residues to regulate protein–protein interactions. We previously demonstrated that when expressed in human embryonic kidney (HEK) cells, Kv4.2 can be SUMOylated at two K residues, K437 and K579. SUMOylation at K437 increased surface expression of electrically silent channels while SUMOylation at K579 reduced IA maximal conductance (Gmax) without altering surface expression. KChIP and DPLP subunits are known to modify the pattern of Kv4.2 post-translational decorations and/or their effects. In this study, co-expressing Kv4.2 with KChIP2a and DPP10c altered the effects of enhanced Kv4.2 SUMOylation. First, the effect of enhanced SUMOylation was the same for a TC containing either the wild-type Kv4.2 or the mutant K437R Kv4.2, suggesting that either the experimental manipulation no longer enhanced K437 SUMOylation or K437 SUMOylation no longer influenced Kv4.2 surface expression. Second, instead of decreasing IA Gmax, enhanced SUMOylation at K579 now produced a significant ∼37–70% increase in IA maximum conductance (Gmax) and a significant ∼30–50% increase in Kv4.2g surface expression that was accompanied by a 65% reduction in TC internalization. Blocking clathrin-mediated endocytosis (CME) in HEK cells expressing the Kv4.2 TC mimicked and occluded the effect of SUMO on IA Gmax; however, the amount of Kv4.2 associated with the major adaptor for constitutive CME, adaptor protein 2 (AP2), was not SUMO dependent. Thus, SUMOylation reduced Kv4.2 internalization by acting downstream of Kv4.2 recruitment into clathrin-coated pits. In sum, the two major findings of this study are: SUMOylation of Kv4.2 at K579 regulates TC internalization most likely by promoting channel recycling. Additionally, there is a reciprocity between Kv4.2 SUMOylation and the Kv4.2 interactome such that SUMOylation regulates the interactome and the interactome influences the pattern and effect of SUMOylation.

KCND2 variants associated with global developmental delay differentially impair Kv4.2 channel gating

Here, we report on six unrelated individuals, all presenting with early-onset global developmental delay, associated with impaired motor, speech and cognitive development, partly with developmental epileptic encephalopathy and physical dysmorphisms. All individuals carry heterozygous missense variants of KCND2, which encodes the voltage-gated potassium (Kv) channel α-subunit Kv4.2. The amino acid substitutions associated with the variants, p.(Glu323Lys) (E323K), p.(Pro403Ala) (P403A), p.(Val404Leu) (V404L) and p.(Val404Met) (V404M), affect sites known to be critical for channel gating. To unravel their likely pathogenicity, recombinant mutant channels were studied in the absence and presence of auxiliary β-subunits under two-electrode voltage-clamp in Xenopus oocytes. All channel mutants exhibited slowed and incomplete macroscopic inactivation, and the P403A variant in addition slowed activation. Co-expression of KChIP2 or DPP6 augmented the functional expression of both wild-type and mutant channels, however, the auxiliary β-subunit-mediated gating modifications differed from wild-type and among mutants. To simulate the putative setting in the affected individuals, heteromeric Kv4.2 channels (wild-type + mutant) were studied as ternary complexes (containing both KChIP2 and DPP6). In the heteromeric ternary configuration, the E323K variant exhibited only marginal functional alterations compared to homomeric wild-type ternary, compatible with mild loss-of-function. By contrast, the P403A, V404L and V404M variants displayed strong gating impairment in the heteromeric ternary configuration, compatible with loss or gain-of-function. Our results support the etiological involvement of Kv4.2 channel gating impairment in early-onset monogenic global developmental delay. In addition, they suggest that gain-of-function mechanisms associated with a substitution of V404 increase epileptic seizure susceptibility.

On the Origin of Paroxysmal Depolarization Shifts: The Contribution of Cav1.x Channels as the Common Denominator of a Polymorphous Neuronal Discharge Pattern

Since their discovery in the 1960s, the term paroxysmal depolarization shift (PDS) has been applied to a wide variety of reinforced neuronal discharge patterns. Occurrence of PDS as cellular correlates of electrographic spikes during latent phases of insult-induced rodent epilepsy models and their resemblance to giant depolarizing potentials (GDPs) nourished the idea that PDS may be involved in epileptogenesis. Both GDPs and - in analogy - PDS may lead to progressive changes of neuronal properties by generation of pulsatile intracellular Ca²⁺ elevations. Herein, a key element is the gating of L-type voltage gated Ca²⁺ channels (LTCCs, Cav1.x family), which may convey Ca²⁺ signals to the nucleus. Accordingly, the present study investigates various insult-associated neuronal challenges for their propensities to trigger PDS in a LTCC-dependent manner. Our data demonstrate that diverse disturbances of neuronal function are variably suited to induce PDS-like events, and the contribution of LTCCs is essential to evoke PDS in rat hippocampal neurons that closely resemble GDPs. These PDS appear to be initiated in the dendritic sub-compartment. Their morphology critically depends on the position of recording electrodes and on their rate of occurrence. These results provide novel insight into induction mechanisms, origin, variability, and co-existence of PDS with other discharge patterns and thereby pave the way for future investigations regarding the role of PDS in epileptogenesis.

Rapamycin, but not minocycline, significantly alters ultrasonic vocalization behavior in C57BL/6J pups in a flurothyl seizure model

Epilepsy is one of the most common neurological disorders, with individuals having an increased susceptibility of seizures in the first few years of life, making children at risk of developing a multitude of cognitive and behavioral comorbidities throughout development. The present study examined the role of PI3K/Akt/mTOR pathway activity and neuroinflammatory signaling in the development of autistic-like behavior following seizures in the neonatal period. Male and female C57BL/6J mice were administered 3 flurothyl seizures on postnatal (PD) 10, followed by administration of minocycline, the mTOR inhibitor rapamycin, or a combined treatment of both therapeutics. On PD12, isolation-induced ultrasonic vocalizations (USVs) of mice were examined to determine the impact of seizures and treatment on communicative behaviors, a component of the autistic-like phenotype. Seizures on PD10 increased the quantity of USVs in female mice and reduced the amount of complex call types emitted in males compared to controls. Inhibition of mTOR with rapamycin significantly reduced the quantity and duration of USVs in both sexes. Changes in USVs were associated with increases in mTOR and astrocyte levels in male mice, however, three PD10 seizures did not result in enhanced proinflammatory cytokine expression in either sex. Beyond inhibition of mTOR activity by rapamycin, both therapeutics did not demonstrate beneficial effects. These findings emphasize the importance of differences that may exist across preclinical seizure models, as three flurothyl seizures did not induce as drastic of changes in mTOR activity or inflammation as observed in other rodent models.

Increased expression of Fragile X mental retardation protein in malformative lesions of patients with focal cortical dysplasia

OBJECTIVE

Focal cortical dysplasia (FCD) accounts for nearly half of all cases of medically refractory epilepsy in the pediatric and adult patient populations. This neurological disorder stems from localized malformations in cortical brain tissue due to impaired neuronal proliferation, differentiation, and migration patterns. Recent studies in animal models have highlighted the potential role of the Fragile X mental retardation protein (FMRP) levels in FCD. The purpose of this study was to investigate the status of FMRP activation in cortical brain tissues surgically resected from patients with FCD. In parallel, this study also investigated protein levels within the PI3K/AKT/mTOR and canonical Wnt signaling pathways.

METHODS

Pathologic tissue from malformative lesions of FCD patients with medically refractory epilepsy was compared to relatively normal control non-epileptic tissue from patients with intracranial neoplasms. A series of western blotting assays were performed to assess key proteins in the PI3K/AKT/mTOR, canonical Wnt signaling pathways, and FMRP.

RESULTS

There was suppression of S235/236-phosphorylated S6, GSK3α, and GSK3β protein levels in samples derived from FCD patients, compared to non-epileptic controls. FCD samples also had significantly greater levels of total and S499-phosphorylated FMRP.

CONCLUSION

These findings support our hypothesis that malformative lesions associated with FCD are characterized by high levels of FMRP activation along with dysregulation of both PI3K/AKT/mTOR and canonical Wnt signaling. These novel clinical findings extend previous work in animal models, further suggesting a potential unforeseen role of GSK3α and GSK3β in the pathophysiology of FCD and refractory epilepsy.

Modulation of Kv4.2/KChIP3 interaction by the ceroid lipofuscinosis neuronal 3 protein CLN3

Voltage-gated potassium (Kv) channels of the Kv4 subfamily associate with Kv channel-interacting proteins (KChIPs), which leads to enhanced surface expression and shapes the inactivation gating of these channels. KChIP3 has been reported to also interact with the late endosomal/lysosomal membrane glycoprotein CLN3 (ceroid lipofuscinosis neuronal 3), which is modified because of gene mutation in juvenile neuronal ceroid lipofuscinosis (JNCL). The present study was undertaken to find out whether and how CLN3, by its interaction with KChIP3, may indirectly modulate Kv4.2 channel expression and function. To this end, we expressed KChIP3 and CLN3, either individually or simultaneously, together with Kv4.2 in HEK 293 cells. We performed co-immunoprecipitation experiments and found a lower amount of KChIP3 bound to Kv4.2 in the presence of CLN3. In whole-cell patch-clamp experiments, we examined the effects of CLN3 co-expression on the KChIP3-mediated modulation of Kv4.2 channels. Simultaneous co-expression of CLN3 and KChIP3 with Kv4.2 resulted in a suppression of the typical KChIP3-mediated modulation; i.e. we observed less increase in current density, less slowing of macroscopic current decay, less acceleration of recovery from inactivation, and a less positively shifted voltage dependence of steady-state inactivation. The suppression of the KChIP3-mediated modulation of Kv4.2 channels was weaker for the JNCL-related missense mutant CLN3R334C and for a JNCL-related C-terminal deletion mutant (CLN3ΔC). Our data support the notion that CLN3 is involved in Kv4.2/KChIP3 somatodendritic A-type channel formation, trafficking, and function, a feature that may be lost in JNCL.

Lipopolysaccharide-induced inflammation leads to acute elevations in pro-inflammatory cytokine expression in a mouse model of Fragile X syndrome

Fragile X syndrome (FXS) is a neurodevelopmental disorder caused by a single genetic mutation in the Fmr1 gene, serving as the largest genetic cause of intellectual disability. Trinucleotide expansion mutations in Fmr1 result in silencing and hypermethylation of the gene, preventing synthesis of the RNA binding protein Fragile X mental retardation protein which functions as a translational repressor. Abnormal immune responses have been demonstrated to play a role in FXS pathophysiology, however, whether these alterations impact how those with FXS respond to an immune insult behaviorally is not entirely known. In the current study, we examine how Fmr1 knockout (KO) and wild type (WT) mice respond to the innate immune stimulus lipopolysaccharide (LPS), both on a molecular and behavioral level, to determine if Fmr1 mutations impact the normal physiological response to an immune insult. In response to LPS, Fmr1 KO mice had elevated hippocampal IL-1β and IL-6 mRNA levels 4 h post-treatment compared to WT mice, with no differences detected in any cytokines at baseline or between genotypes 24 h post-LPS administration. Fmr1 KO mice also had upregulated hippocampal BDNF gene expression 4 h post-treatment compared to WT mice, which was not dependent on LPS administration. There were no differences in hippocampal protein expression between genotypes in microglia (Iba1) or astrocyte (GFAP) reactivity. Further, both genotypes displayed the typical sickness response following LPS stimulation, demonstrated by a significant reduction in food burrowed by LPS-treated mice in a burrowing task. Additional investigation is critical to determine if the transient increases in cytokine expression could lead to long-term changes in downstream molecular signaling in FXS.

Neuronal deletion of phosphatase and tensin homolog results in cerebellar motor learning dysfunction and alterations in intracellular signaling

The purpose of this investigation was to examine cerebellar levels of several molecular signaling pathways, including PI3K/AKT/mammalian target of rapamycin (mTOR) signaling and markers of neuronal migration, following loss of the phosphatase and tensin homolog (PTEN) gene in a subset of neurons, as well as the accompanying behavior phenotype in mice. Motor coordination and learning were measured by the sticker removal task and the accelerating rotarod. Western blots were conducted on cerebellar tissue samples. We demonstrated that neuron subset-specific deletion of PTEN in mice led to deficits in motor coordination. These changes were accompanied by alterations in many different proteins, including the PI3K/AKT/mTOR signaling pathway, FMRP, glutamate receptors, and neuronal migration markers. These data firstly support a role for hyperactivation of mTOR in the cerebellum following the loss of PTEN, accompanied by behavioral deficits. Moreover, the results of the current study support a broader role for PTEN signaling in early neuronal migration and organization of the cerebellum, and point to a putative role for PTEN in many neuropsychiatric conditions.

Madecassic Acid Reduces Fast Transient Potassium Channels and Promotes Neurite Elongation in Hippocampal CA1 Neurons

BACKGROUND AND OBJECTIVE

Madecassic Acid (MA) is well known to induce neurite elongation. However, its correlation with the expression of fast transient potassium (AKv) channels during neuronal development has not been well studied. Therefore, the present study was designed to investigate the effects of MA on the modulation of AKv channels during neurite outgrowth.

METHODS

Neurite outgrowth was measured with morphometry software, and Kv4 currents were recorded by using the patch clamp technique.

RESULTS

The ability of MA to promote neurite outgrowth is dose-dependent and was blocked by using the mitogen/extracellular signal-regulated kinase (MEK) inhibitor U0126. MA reduced the peak current density and surface expression of the AKv channel Kv4.2 with or without the presence of NaN3. The surface expression of Kv4.2 channels was also reduced after MA treatment of growing neurons. Ethylene glycol tetraacetic acid (EGTA) and an N-methyl-D-aspartate (NMDA) receptor blocker, MK801 along with MA prevented the effect of MA on neurite length, indicating that calcium entry through NMDA receptors is necessary for MA-induced neurite outgrowth.

CONCLUSION

The data demonstrated that MA increased neurite outgrowth by internalizing AKv channels in neurons. Any alterations in the precise density of ion channels can lead to deleterious consequences on health because it changes the electrical and mechanical function of a neuron or a cell. Modulating ion channel's density is exciting research in order to develop novel drugs for the therapeutic treatment of various diseases of CNS.

Transient Potassium Channels: Therapeutic Targets for Brain Disorders

Transient potassium current channels (I_A channels), which are expressed in most brain areas, have a central role in modulating feedforward and feedback inhibition along the dendroaxonic axis. Loss of the modulatory channels is tightly associated with a number of brain diseases such as Alzheimer’s disease, epilepsy, fragile X syndrome (FXS), Parkinson’s disease, chronic pain, tinnitus, and ataxia. However, the functional significance of I_A channels in these diseases has so far been underestimated. In this review, we discuss the distribution and function of I_A channels. Particularly, we posit that downregulation of I_A channels results in neuronal (mostly dendritic) hyperexcitability accompanied by the imbalanced excitation and inhibition ratio in the brain’s networks, eventually causing the brain diseases. Finally, we propose a potential therapeutic target: the enhanced action of I_A channels to counteract Ca²⁺-permeable channels including NMDA receptors could be harnessed to restore dendritic excitability, leading to a balanced neuronal state.

Molecular interplay between hyperactive mammalian target of rapamycin signaling and Alzheimer's disease neuropathology in the NS-Pten knockout mouse model

Dysregulation of the PI3K/Akt/mTOR signaling cascade has been associated with the pathology of neurodegenerative disorders, specifically Alzheimer's disease (AD). Both in-vivo models and post-mortem brain samples of individuals with AD have commonly shown hyperactivation of the pathway. In the present study, we examine how neuron subset-specific deletion of Pten (NS-Pten) in mice, which presents with hyperactive mammalian target of rapamycin (mTOR) activity, affects the hippocampal protein levels of key neuropathological hallmarks of AD. We found NS-Pten knockout (KO) mice to have elevated levels of amyloid-β, α-synuclein, neurofilament-L, and pGSK3α in the hippocampal synaptosome compared with NS-Pten wild type mice. In contrast, there was a decreased expression of amyloid precursor protein, tau, GSK3α, and GSK3β in NS-Pten KO hippocampi. Overall, there were significant alterations in levels of proteins associated with AD pathology in NS-Pten KO mice. This study provides novel insight into how altered mTOR signaling is linked to AD pathology, without the use of an in-vivo AD model that already displays neuropathological hallmarks of the disease.

Altered A-type potassium channel function in the nucleus tractus solitarii in acquired temporal lobe epilepsy

Sudden unexpected death in epilepsy (SUDEP) is among the leading causes of death in people with epilepsy. Individuals with temporal lobe epilepsy (TLE) have a high risk for SUDEP because the seizures are often medically intractable. Neurons in the nucleus tractus solitarii (NTS) have been implicated in mouse models of SUDEP and play a critical role in modulating cardiorespiratory and autonomic output. Increased neuronal excitability of inhibitory, GABAergic neurons in the NTS develops during epileptogenesis, and NTS dysfunction has been implicated in mouse models of SUDEP. In this study we used the pilocarpine-induced status epilepticus model of TLE (i.e., pilo-SE mice) to investigate the A-type voltage-gated K⁺ channel as a potential contributor to increased excitability in GABAergic NTS neurons during epileptogenesis. Compared with age-matched control mice, pilo-SE mice displayed an increase in spontaneous action potential frequency and half-width 9-12 wk after treatment. Activity of GABAergic NTS neurons from pilo-SE mice showed less sensitivity to 4-aminopyridine. Correspondingly, reduced A-type K⁺ current amplitude was detected in these neurons, with no change in activation or inactivation kinetics. No changes were observed in K_v4.1, K_v4.2, K_v4.3, KChIP1, KChIP3, or KChIP4 mRNA expression. These changes contribute to the increased excitability in GABAergic NTS neurons that develops in TLE and may provide insight into potential mechanisms contributing to the increased risk for cardiorespiratory collapse and SUDEP in this model. NEW & NOTEWORTHY Sudden unexpected death in epilepsy (SUDEP) is a leading cause of death in epilepsy, and dysfunction in central autonomic neurons may play a role. In a mouse model of acquired epilepsy, GABAergic neurons in the nucleus tractus solitarii developed a reduced amplitude of the A-type current, which contributes to the increased excitability seen in these neurons during epileptogenesis. Neuronal excitability changes in inhibitory central vagal circuitry may increase the risk for cardiorespiratory collapse and SUDEP.

A single seizure selectively impairs hippocampal-dependent memory and is associated with alterations in PI3K/Akt/mTOR and FMRP signaling

Objective

A single brief seizure before learning leads to spatial and contextual memory impairment in rodents without chronic epilepsy. These results suggest that memory can be impacted by seizure activity in the absence of epilepsy pathology. In this study, we investigated the types of memory affected by a seizure and the time course of impairment. We also examined alterations to mammalian target of rapamycin (mTOR) and fragile X mental retardation protein (FMRP) signaling, which modulate elements of the synapse and may underlie impairment.

Methods

We induced a single seizure and investigated hippocampal and nonhippocampal memory using trace fear conditioning, novel object recognition (NOR), and accelerating rotarod to determine the specificity of impairment in mice. We used western blot analysis to examine for changes to cellular signaling and synaptic proteins 1 h, 24 h, and 1 week after a seizure. We also included a histologic examination to determine if cell loss or gross lesions might alternatively explain memory deficits.

Results

Behavioral results indicated that a seizure before learning leads to impairment of trace fear memory that worsens over time. In contrast, nonhippocampal memory was unaffected by a seizure in the NOR and rotarod tasks. Western analysis indicated increased hippocampal phospho‐S6 and total FMRP 1 h following a seizure. Tissue taken 24 h after a seizure indicated increased hippocampal GluA1, suggesting increased α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) receptor expression. Histologic analysis indicated that neither cell loss nor lesions are present after a single seizure.

Significance

The presence of memory impairment in the absence of damage suggests that memory impairment caused by seizure activity differs from general memory impairment in epilepsy. Instead, memory impairment after a single seizure is associated with alterations to mTOR and FMRP signaling, which leads to a disruption of synaptic proteins involved in consolidation of long‐term memory. These results have implications for understanding memory impairment in epilepsy.

Modifying Rap1-signalling by targeting Pde6δ is neuroprotective in models of Alzheimer's disease

Background

Neuronal Ca²⁺ dyshomeostasis and hyperactivity play a central role in Alzheimer’s disease pathology and progression. Amyloid-beta together with non-genetic risk-factors of Alzheimer’s disease contributes to increased Ca²⁺ influx and aberrant neuronal activity, which accelerates neurodegeneration in a feed-forward fashion. As such, identifying new targets and drugs to modulate excessive Ca²⁺ signalling and neuronal hyperactivity, without overly suppressing them, has promising therapeutic potential.

Methods

Here we show, using biochemical, electrophysiological, imaging, and behavioural tools, that pharmacological modulation of Rap1 signalling by inhibiting its interaction with Pde6δ normalises disease associated Ca²⁺ aberrations and neuronal activity, conferring neuroprotection in models of Alzheimer’s disease.

Results

The newly identified inhibitors of the Rap1-Pde6δ interaction counteract AD phenotypes, by reconfiguring Rap1 signalling underlying synaptic efficacy, Ca²⁺ influx, and neuronal repolarisation, without adverse effects in-cellulo or in-vivo. Thus, modulation of Rap1 by Pde6δ accommodates key mechanisms underlying neuronal activity, and therefore represents a promising new drug target for early or late intervention in neurodegenerative disorders.

Conclusion

Targeting the Pde6δ-Rap1 interaction has promising therapeutic potential for disorders characterised by neuronal hyperactivity, such as Alzheimer’s disease.

Electronic supplementary material

The online version of this article (10.1186/s13024-018-0283-3) contains supplementary material, which is available to authorized users.

Saikosaponin A modulates remodeling of Kv4.2-mediated A-type voltage-gated potassium currents in rat chronic temporal lobe epilepsy

Background

Chronic temporal lobe epilepsy (cTLE) is the most common intractable epilepsy. Recent studies have shown that saikosaponin A (SSa) could inhibit epileptiform discharges induced by 4 action potentials and selectively increase the transient inactivating K⁺ currents (I_A). However, the mechanisms of SSa on I_A remain unclear. In this study, we comprehensively evaluated the anticonvulsant activities of SSa and explored whether or not it plays an anti-epileptic role in a Li-pilocarpine induced epilepsy rat model via remodeling Kv4.2-mediated A-type voltage-gated potassium currents (Kv4.2-mediated I_A).

Materials and methods

All in vitro spontaneous recurrent seizures (SRS) were recorded with continuous video monitoring. Nissl’s staining was used to evaluate the SSa protection of neurons and immunohistochemistry, Western blot, and quantitative reverse transcription PCR were used to quantify the expression of Kchip1 and Kv4.2 in the hippocampal CA1 field and the adjacent cortex following Li-pilocarpine induced status epilepticus. We used whole-cell current-clamp recordings to evaluate the anticonvulsant activities of SSa in a hippocampal neuronal culture model of cTLE, while whole-cell voltage-clamp recordings were used to evaluate the modulatory effects of SSa on Kv4.2-mediated I_A.

Results

SSa treatment significantly reduced the frequency and duration of SRS over the course of eight weeks and increased the production of Kchip1 and Kv4.2. In addition, SSa attenuated spontaneous recurrent epileptiform discharges (SREDs) in the hippocampal neuronal model and up-regulated Kv4.2-mediated I_A.

Conclusions

SSa exerted a disease-modifying effect in our cTLE rat model both in vivo and in vitro; the increase in Kv4.2-mediated I_A may contribute to the anticonvulsant mechanisms of SSa.

Neuroinflammation Alters Integrative Properties of Rat Hippocampal Pyramidal Cells

Neuroinflammation is consistently found in many neurological disorders, but whether or not the inflammatory response independently affects neuronal network properties is poorly understood. Here, we report that intracerebroventricular injection of the prototypical inflammatory molecule lipopolysaccharide (LPS) in rats triggered a strong and long-lasting inflammatory response in hippocampal microglia associated with a concomitant upregulation of Toll-like receptor (TLR4) in pyramidal and hilar neurons. This, in turn, was associated with a significant reduction of the dendritic hyperpolarization-activated cyclic AMP-gated channel type 1 (HCN1) protein level while Kv4.2 channels were unaltered as assessed by western blot. Immunohistochemistry confirmed the HCN1 decrease in CA1 pyramidal neurons and showed that these changes were associated with a reduction of TRIP8b, an auxiliary subunit for HCN channels implicated in channel subcellular localization and trafficking. At the physiological level, this effect translated into a 50% decrease in HCN1-mediated currents (Ih) measured in the distal dendrites of hippocampal CA1 pyramidal cells. At the functional level, the band-pass-filtering properties of dendrites in the theta frequency range (4-12 Hz) and their temporal summation properties were compromised. We conclude that neuroinflammation can independently trigger an acquired channelopathy in CA1 pyramidal cell dendrites that alters their integrative properties. By directly changing cellular function, this phenomenon may participate in the phenotypic expression of various brain diseases.

Neuronal subset-specific deletion of Pten results in aberrant Wnt signaling and memory impairments

The canonical Wnt and PI3K/Akt/mTOR pathways both play critical roles in brain development early in life. There is extensive evidence of how each pathway is involved in neuronal and synaptic maturation, however, how these molecular networks interact requires further investigation. The present study examines the effect of neuronal subset-specific deletion of phosphatase and tensin homolog (Pten) in mice on Wnt signaling protein levels and associated cognitive impairments. PTEN functions as a negative regulator of the PI3K/Akt/mTOR pathway, and mutations in Pten can result in cognitive and behavioral impairments. We found that deletion of Pten resulted in elevated Dvl2, Wnt5a/b, and Naked2, along with decreased GSK3β hippocampal synaptosome protein expression compared to wild type mice. Aberrations in the canonical Wnt pathway were associated with learning and memory deficits in Pten knockout mice, specifically in novel object recognition and the Lashley maze. This study demonstrates that deletion of Pten not only significantly impacts PI3K/Akt/mTOR signaling, but affects proper functioning of the Wnt signaling pathway. Overall, these findings will help elucidate how the PI3K/Akt/mTOR pathway intersects with Wnt signaling to result in cognitive impairments, specifically in memory.

ERK5 Phosphorylates Kv4.2 and Inhibits Inactivation of the A-Type Current in PC12 Cells

Extracellular signal-regulated kinase 5 (ERK5) regulates diverse physiological responses such as proliferation, differentiation, and gene expression. Previously, we demonstrated that ERK5 is essential for neurite outgrowth and catecholamine biosynthesis in PC12 cells and sympathetic neurons. However, it remains unclear how ERK5 regulates the activity of ion channels, which are important for membrane excitability. Thus, we examined the effect of ERK5 on the ion channel activity in the PC12 cells that overexpress both ERK5 and the constitutively active MEK5 mutant. The gene and protein expression levels of voltage-dependent Ca²⁺ and K⁺ channels were determined by RT-qPCR or Western blotting. The A-type K⁺ current was recorded using the whole-cell patch clamp method. In these ERK5-activated cells, the gene expression levels of voltage-dependent L- and P/Q-type Ca²⁺ channels did not alter, but the N-type Ca²⁺ channel was slightly reduced. In contrast, those of K_v4.2 and K_v4.3, which are components of the A-type current, were significantly enhanced. Unexpectedly, the protein levels of K_v4.2 were not elevated by ERK5 activation, but the phosphorylation levels were increased by ERK5 activation. By electrophysiological analysis, the inactivation time constant of the A-type current was prolonged by ERK5 activation, without changes in the peak current. Taken together, ERK5 inhibits an inactivation of the A-type current by phosphorylation of K_v4.2, which may contribute to the neuronal differentiation process.

A-type K+ channels impede supralinear summation of clustered glutamatergic inputs in layer 3 neocortical pyramidal neurons

A-type K⁺ channels restrain the spread of incoming signals in tufted and apical dendrites of pyramidal neurons resulting in strong compartmentalization. However, the exact subunit composition and functional significance of K⁺ channels expressed in small diameter proximal dendrites remain poorly understood. We focus on A-type K⁺ channels expressed in basal and oblique dendrites of cortical layer 3 pyramidal neurons, in ex vivo brain slices from young adult mice. Blocking putative Kv4 subunits with phrixotoxin-2 enhances depolarizing potentials elicited by uncaging RuBi-glutamate at single dendritic spines. A concentration of 4-aminopyridine reported to block Kv1 has no effect on such responses. 4-aminopyridine and phrixotoxin-2 increase supralinear summation of glutamatergic potentials evoked by synchronous activation of clustered spines. The effect of 4-aminopyridine on glutamate responses is simulated in a computational model where the dendritic A-type conductance is distributed homogeneously or in a linear density gradient. Thus, putative Kv4-containing channels depress excitatory inputs at single synapses. The additional recruitment of Kv1 subunits might require the synchronous activation of multiple inputs to regulate the gain of signal integration.

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We focus on A-type K+ channels expressed in oblique and basal dendrites.

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Putative Kv4 subunits depress excitatory signals generated by single spine excitation.

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Kv4 and Kv1 regulate supralinear signal integration at clustered dendritic spines.

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A computational model simulates Kv-mediated modulation of dendritic integration.

Status epilepticus: Role for etiology in determining response to benzodiazepines

OBJECTIVE

Clinical factors contributing to benzodiazepine failure in treating status epilepticus (SE) include suboptimal dosing and seizure duration. As many benzodiazepine-refractory episodes of SE arise from acute etiologies, we sought to determine whether etiology impacts SE treatment.

METHODS

The potency of diazepam to terminate SE induced by lithium-pilocarpine (LiPilo-SE) or kainic acid (KA-SE) in 3-week-old rats was studied by video-electroencephalography. Synaptic γ-aminobutyric acid type A receptor (GABAR)-mediated currents were recorded from dentate granule cells using voltage-clamp electrophysiology. Surface expression of γ2 subunit-containing GABARs and Kv4.2 potassium channels in hippocampal slices was determined using a biotinylation assay. Expression of phosphorylated forms of β2/3 and γ2 subunits was determined using phosphospecific antibodies and Western blotting.

RESULTS

Diazepam failed to terminate late SE in LiPilo-SE animals but was successful in terminating KA-SE of 1- and 3-hour duration. One hour after SE onset, GABAR-mediated synaptic inhibition and γ2 subunit-containing GABAR surface expression were reduced in LiPilo-SE animals. These were unchanged in KA-SE animals at 1 and 3 hours. Phosphorylation of γ2 subunit residue S327 was unchanged in both models, although GABAR β3 subunit S408/409 residues were dephosphorylated in the LiPilo-SE animals. Kv4.2 potassium channel surface expression was increased in LiPilo-SE animals but reduced in KA-SE animals.

INTERPRETATION

SE-model-dependent differences support a novel hypothesis that the development of benzodiazepine pharmacoresistance may be etiologically predetermined. Further studies are required to investigate the mechanisms that underlie such etiological differences during SE and whether etiology-dependent protocols for the treatment of SE need to be developed. Ann Neurol 2018;83:830-841.

Somatodendritic surface expression of epitope-tagged and KChIP binding-deficient Kv4.2 channels in hippocampal neurons

Kv4.2 channels mediate a subthreshold-activating somatodendritic A-type current (ISA) in hippocampal neurons. We examined the role of accessory Kv channel interacting protein (KChIP) binding in somatodendritic surface expression and activity-dependent decrease in the availability of Kv4.2 channels. For this purpose we transfected cultured hippocampal neurons with cDNA coding for Kv4.2 wild-type (wt) or KChIP binding-deficient Kv4.2 mutants. All channels were equipped with an externally accessible hemagglutinin (HA)-tag and an EGFP-tag, which was attached to the C-terminal end. Combined analyses of EGFP self-fluorescence, surface HA immunostaining and patch-clamp recordings demonstrated similar dendritic trafficking and functional surface expression for Kv4.2[wt]HA,EGFP and the KChIP binding-deficient Kv4.2[A14K]HA,EGFP. Coexpression of exogenous KChIP2 augmented the surface expression of Kv4.2[wt]HA,EGFP but not Kv4.2[A14K]HA,EGFP. Notably, activity-dependent decrease in availability was more pronounced in Kv4.2[wt]HA,EGFP + KChIP2 coexpressing than in Kv4.2[A14K]HA,EGFP + KChIP2 coexpressing neurons. Our results do not support the notion that accessory KChIP binding is a prerequisite for dendritic trafficking and functional surface expression of Kv4.2 channels, however, accessory KChIP binding may play a potential role in Kv4.2 modulation during intrinsic plasticity processes.

The stochastic nature of action potential backpropagation in apical tuft dendrites

In cortical pyramidal neurons, backpropagating action potentials (bAPs) supply Ca²⁺ to synaptic contacts on dendrites. To determine whether the efficacy of AP backpropagation into apical tuft dendrites is stable over time, we performed dendritic Ca²⁺ and voltage imaging in rat brain slices. We found that the amplitude of bAP-Ca²⁺ in apical tuft branches was unstable, given that it varied from trial to trial (termed "bAP-Ca²⁺ flickering"). Small perturbations in dendritic physiology, such as spontaneous synaptic inputs, channel inactivation, or temperature-induced changes in channel kinetics, can cause bAP flickering. In the tuft branches, the density of Na⁺ and K⁺ channels was sufficient to support local initiation of fast spikelets by glutamate iontophoresis. We quantified the time delay between the somatic AP burst and the peak of dendritic Ca²⁺ transient in the apical tuft, because this delay is important for induction of spike-timing dependent plasticity. Depending on the frequency of the somatic AP triplets, Ca²⁺ signals peaked in the apical tuft 20-50 ms after the 1st AP in the soma. Interestingly, at low frequency (<20 Hz), the Ca²⁺ peaked sooner than at high frequency, because only the 1st AP invaded tuft. Activation of dendritic voltage-gated Ca²⁺ channels is sensitive to the duration of the dendritic voltage transient. In apical tuft branches, small changes in the duration of bAP voltage waveforms cause disproportionately large increases in dendritic Ca²⁺ influx (bAP-Ca²⁺ flickering). The stochastic nature of bAP-Ca²⁺ adds a new perspective on the mechanisms by which pyramidal neurons combine inputs arriving at different cortical layers.NEW & NOTEWORTHY The bAP-Ca²⁺ signal amplitudes in some apical tuft branches randomly vary from moment to moment. In repetitive measurements, successful AP invasions are followed by complete failures. Passive spread of voltage from the apical trunk into the tuft occasionally reaches the threshold for local Na⁺ spike, resulting in stronger Ca²⁺ influx. During a burst of three somatic APs, the peak of dendritic Ca²⁺ in the apical tuft occurs with a delay of 20-50 ms depending on AP frequency.

The Segregated Expression of Voltage-Gated Potassium and Sodium Channels in Neuronal Membranes: Functional Implications and Regulatory Mechanisms

Neurons are highly polarized cells with apparent functional and morphological differences between dendrites and axon. A critical determinant for the molecular and functional identity of axonal and dendritic segments is the restricted expression of voltage-gated ion channels (VGCs). Several studies show an uneven distribution of ion channels and their differential regulation within dendrites and axons, which is a prerequisite for an appropriate integration of synaptic inputs and the generation of adequate action potential (AP) firing patterns. This review article will focus on the signaling pathways leading to segmented expression of voltage-gated potassium and sodium ion channels at the neuronal plasma membrane and the regulatory mechanisms ensuring segregated functions. We will also discuss the relevance of proper ion channel targeting for neuronal physiology and how alterations in polarized distribution contribute to neuronal pathology.

Superimposing Status Epilepticus on Neuron Subset-Specific PTEN Haploinsufficient and Wild Type Mice Results in Long-term Changes in Behavior

We evaluated the effects of superimposing seizures on a genetic mutation with known involvement in both Autism Spectrum Disorder and in epilepsy. Neuron-subset specific (NS)-Pten heterozygous (HT) and wildtype (WT) adult mice received either intraperitoneal injections of kainic acid (20 mg/kg) to induce status epilepticus or the vehicle (saline). Animals then received a battery of behavioral tasks in order to evaluate activity levels, anxiety, repetitive-stereotyped behavior, social behavior, learning and memory. In the open field task, we found that HT mice after seizures showed a significant increase in total activity and total distance in the surround region of the open field. In the elevated plus maze task, we found that HT mice after seizures displayed increased total distance and velocity as compared to HT mice that did not undergo seizures and WT controls. In the social chamber test, we found the HT mice after seizures displayed an impairment in social behavior. These findings demonstrate that superimposing seizures on a genetic mutation can result in long-term alterations in activity and social behavior in mice.

The effect of early life status epilepticus on ultrasonic vocalizations in mice

OBJECTIVE

Infant crying is a series of innate vocal patterns intended to elicit the attention of adult caregivers for fulfillment of specific needs such as pain, hunger, or hypostimulation. It is one of the earliest forms of observable communication. In neonatal rodents, this behavior has recently been investigated as a potential early behavioral marker of neural deficits in neurodevelopmental disorders. However, few studies have examined the effects of seizures on vocalization behavior during the neonatal period. The purpose of this study is to investigate the effect of a single kainate-induced early life seizure on vocalization behavior in mice. This study also investigates the subsequent effect of seizures on two pathways critical for early neural development and epileptogenesis: the phosphoinositide 3-kinase|serine/threonine kinase|mammalian target of rapamycin (PI3K-Akt-mTOR) and canonical (Wingless-Int Wnt) intracellular signaling pathways.

METHODS

On postnatal day 10, male and female 129SvEvTac mice received a single intraperitoneal injection of kainic acid (2.5 mg/kg) or vehicle injection. The kainate administration resulted in 1-2 h of status epilepticus. On postnatal days 11 and 12, the quantity and duration of isolation-induced ultrasonic vocalizations were recorded. Western blotting analyses were performed using male and female pups on postnatal day 12.

RESULTS

There was significant, male-specific suppression in the quantity and total duration of 50-kHz calls on postnatal day 12 following seizures. The hippocampi of male mice on this postnatal day also revealed male-specific changes in the PI3K-Akt-mTOR intracellular signaling pathway, as well as changes in phosphorylated fragile × mental retardation protein.

SIGNIFICANCE

These findings demonstrate that early life seizures can disrupt communication behavior in neonatal mice.

Spatiotemporal profile of Map2 and microglial changes in the hippocampal CA1 region following pilocarpine-induced status epilepticus

Status epilepticus (SE) triggers pathological changes to hippocampal dendrites that may promote epileptogenesis. The microtubule associated protein 2 (Map2) helps stabilize microtubules of the dendritic cytoskeleton. Recently, we reported a substantial decline in Map2 that coincided with robust microglia accumulation in the CA1 hippocampal region after an episode of SE. A spatial correlation between Map2 loss and reactive microglia was also reported in human cortex from refractory epilepsy. New evidence supports that microglia modulate dendritic structures. Thus, to identify a potential association between SE-induced Map2 and microglial changes, a spatiotemporal profile of these events is necessary. We used immunohistochemistry to determine the distribution of Map2 and the microglia marker IBA1 in the hippocampus after pilocarpine-induced SE from 4 hrs to 35 days. We found a decline in Map2 immunoreactivity in the CA1 area that reached minimal levels at 14 days post-SE and partially increased thereafter. In contrast, maximal microglia accumulation occurred in the CA1 area at 14 days post-SE. Our data indicate that SE-induced Map2 and microglial changes parallel each other’s spatiotemporal profiles. These findings may lay the foundation for future mechanistic studies to help identify potential roles for microglia in the dendritic pathology associated with SE and epilepsy.

Potassium Channels in Epilepsy

This review attempts to give a concise and up-to-date overview on the role of potassium channels in epilepsies. Their role can be defined from a genetic perspective, focusing on variants and de novo mutations identified in genetic studies or animal models with targeted, specific mutations in genes coding for a member of the large potassium channel family. In these genetic studies, a demonstrated functional link to hyperexcitability often remains elusive. However, their role can also be defined from a functional perspective, based on dynamic, aggravating, or adaptive transcriptional and posttranslational alterations. In these cases, it often remains elusive whether the alteration is causal or merely incidental. With ∼80 potassium channel types, of which ∼10% are known to be associated with epilepsies (in humans) or a seizure phenotype (in animals), if genetically mutated, a comprehensive review is a challenging endeavor. This goal may seem all the more ambitious once the data on posttranslational alterations, found both in human tissue from epilepsy patients and in chronic or acute animal models, are included. We therefore summarize the literature, and expand only on key findings, particularly regarding functional alterations found in patient brain tissue and chronic animal models.

Status Epilepticus

Although the majority of seizures are brief and cause no long-term consequences, a subset is sufficiently prolonged that long-term consequences can result. These very prolonged seizures are termed "status epilepticus" (SE) and are considered a neurological emergency. The clinical presentation of SE can be diverse. SE can occur at any age but most commonly occurs in the very young and the very old. There are numerous studies on SE in animals in which the pathophysiology, medication responses, and pathology can be rigorously studied in a controlled fashion. Human data are consistent with the animal data. In particular, febrile status epilepticus (FSE), a form of SE common in young children, is associated with injury to the hippocampus and subsequent temporal lobe epilepsy (TLE) in both animals and humans.

Comparison of Equivalence between Two Commercially Available S499-Phosphorylated FMRP Antibodies in Mice

Fragile X syndrome (FXS) develops from excessive trinucleotide CGG repeats in the 5’-untranslated region at Xq27.3 of the Fmr-1 gene, which functionally silences its expression and prevents transcription of its protein. This disorder is the most prominent form of heritable intellectual deficiency, affecting roughly 1 in 5,000 males and 1 in 10,000 females globally. Antibody specificity and selectivity are essential for investigating changes in intracellular protein signaling and phosphorylation status of the Fragile X Mental Retardation Protein (FMRP). Currently, both PhosphoSolutions^® and abcam® produce commercially available S499-phosphorylated FMRP specific antibodies. The antibody from PhosphoSolutions^® has been validated in previous studies; however, the antibody from abcam^® antibody has yet to receive similar validation. This study aims to determine whether these two antibodies are true equivalents through western blot analysis of both NS-Pten knockout (KO) and Fmr-1 KO mice strains. We prepared hippocampal synaptosomal preparations and probed the samples using total FMRP, abcam^® phosphorylated FMRP, and PhosphoSolutions^® phosphorylated FMRP antibodies. We found that there was a significant increase in phosphorylated FMRP levels using the abcam® and PhosphoSolutions^® antibodies in the NS-Pten KO mice compared to wildtype mice. However, there was much more variability using the abcam^® antibody. Furthermore, there was a band present in the Fmr-1 KO for the phosphorylated FMRP site using the abcam^® antibody for western blotting but not for the PhosphoSolutions^® antibody. Our findings strongly suggest that the antibody from abcam^® is neither specific nor selective for its advertised targeted substrate, S499-phosphorylated FMRP.

The pervasive reduction of GABA-mediated synaptic inhibition of principal neurons in the hippocampus during status epilepticus

The goal of this study was to determine whether there are region-specific or time-dependent changes in GABA-mediated synaptic inhibition of principal neurons in the hippocampus during in vivo status epilepticus. Standard whole cell patch clamp electrophysiological techniques were used to characterize miniature inhibitory postsynaptic currents (mIPSCs) in recordings from the principal neurons (PNs) of the dentate gyrus, CA1, and CA3 in acutely-obtained hippocampal slices from control and lithium/pilocarpine-induced status epilepticus(SE)-treated animals. The reduction in mIPSC amplitude was pervasive across the 3 regions examined in hippocampal slices obtained after 60 min (late) or just 15 min after the onset of SE. The mIPSC frequency was reduced in all 3 regions after 60 min and 2 regions (dentate, CA1) after 15 min. These findings lend further support to the hypothesis that a rapid modification of the postsynaptic GABAA receptor population leads to a widespread decline in GABA-mediated inhibition that, in part, contributes to both the self-sustaining nature of SE and to the decrease in the efficacy of benzodiazepines.

Convergent phosphomodulation of the major neuronal dendritic potassium channel Kv4.2 by pituitary adenylate cyclase-activating polypeptide

The endogenous neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is secreted by both neuronal and non-neuronal cells in the brain and spinal cord, in response to pathological conditions such as stroke, seizures, chronic inflammatory and neuropathic pain. PACAP has been shown to exert various neuromodulatory and neuroprotective effects. However, direct influence of PACAP on the function of intrinsically excitable ion channels that are critical to both hyperexcitation as well as cell death, remain largely unexplored. The major dendritic K(+) channel Kv4.2 is a critical regulator of neuronal excitability, back-propagating action potentials in the dendrites, and modulation of synaptic inputs. We identified, cloned and characterized the downstream signaling originating from the activation of three PACAP receptor (PAC1) isoforms that are expressed in rodent hippocampal neurons that also exhibit abundant expression of Kv4.2 protein. Activation of PAC1 by PACAP leads to phosphorylation of Kv4.2 and downregulation of channel currents, which can be attenuated by inhibition of either PKA or ERK1/2 activity. Mechanistically, this dynamic downregulation of Kv4.2 function is a consequence of reduction in the density of surface channels, without any influence on the voltage-dependence of channel activation. Interestingly, PKA-induced effects on Kv4.2 were mediated by ERK1/2 phosphorylation of the channel at two critical residues, but not by direct channel phosphorylation by PKA, suggesting a convergent phosphomodulatory signaling cascade. Altogether, our findings suggest a novel GPCR-channel signaling crosstalk between PACAP/PAC1 and Kv4.2 channel in a manner that could lead to neuronal hyperexcitability.

Contrasting features of ERK1/2 activity and synapsin I phosphorylation at the ERK1/2-dependent site in the rat brain in status epilepticus induced by kainic acid in vivo

Extracellular signal-regulated kinase 1/2 (ERK1/2) plays diverse roles in the central nervous system. Activation of ERK1/2 has been observed in various types of neuronal excitation, including seizure activity in vivo and in vitro. However, studies examining ERK1/2 activity and its substrate phosphorylation in parallel are scarce especially in seizure models. We have been studying the phosphorylation state of the presynaptic protein, synapsin I at ERK1/2-dependent and -independent sites in various types of seizure models and showed that ERK1/2-dependent phosphorylation of synapsin I was indeed under control of ERK1/2 activity in vivo. To further expand our study, here we examined the effects of prolonged seizure activity on ERK1/2 activity and synapsin I phosphorylation by using status epilepticus induced by kainic acid (KA-SE) in rats in vivo. In KA-SE, robust ERK1/2 activation was observed in the hippocampus, a representative limbic structure, with lesser activation in the parietal cortex, a representative non-limbic structure. In contrast, the phosphorylation level of synapsin I at ERK1/2-dependent phospho-site 4/5 was profoundly decreased, the extent of which was much larger in the hippocampus than in the parietal cortex. In addition, phosphorylation at other ERK1/2-independent phospho-sites in synapsin I also showed an even larger decrease. All these changes disappeared after recovery from KA-SE. These results indicate that the phosphorylation state of synapsin I is dynamically regulated by the balance between kinase and phosphatase activities. The contrasting features of robust ERK1/2 activation yet synapsin I dephosphorylation may be indicative of an irreversible pathological outcome of the epileptic state in vivo.

Single versus combinatorial therapies in status epilepticus: Novel data from preclinical models

Drug-refractory status epilepticus (RSE) is a major medical emergency with a mortality of up to 40% and the risk of severe long-term consequences. The mechanisms involved in RSE are incompletely understood. Animal models are important in developing treatment strategies for more effective termination of SE and prevention of its long-term outcomes. The pilocarpine and lithium-pilocarpine rat models are widely used in this respect. In these models, resistance to diazepam and other antiseizure drugs (ASDs) develops during SE so that an SE that is longer than 30 min is difficult to suppress. Furthermore, because all ASDs used in SE treatment are much more rapidly eliminated by rodents than by humans, SE recurs several hours after ASD treatment. Long-term consequences include hippocampal damage, behavioral alterations, and epilepsy with spontaneous recurrent seizures. In this review, different rational polytherapies for SE, which are more effective than monotherapies, are discussed, including a novel polytherapy recently developed by our group. Based on data from diverse seizure models, we hypothesized that cholinergic mechanisms are involved in the mechanisms underlying ASD resistance of SE. We, therefore, developed an intravenous drug cocktail, consisting of diazepam, phenobarbital, and the anticholinergic scopolamine. This drug combination irreversibly terminated SE when administered 60, 90, or 120 min after SE onset. The efficacy of this cocktail in terminating SE was comparable with the previously reported efficacy of polytherapies with the glutamate receptor antagonist ketamine. Furthermore, when injected 60 min after SE onset, the scopolamine-containing cocktail prevented development of epilepsy and hippocampal neurodegeneration, which was not observed with high doses of diazepam or a combination of phenobarbital and diazepam. Our data add to the existing preclinical evidence that rational polytherapy can be more effective than monotherapy in the treatment of SE and that combinatorial therapy may offer a clinically useful option for the treatment of RSE. This article is part of a Special Issue entitled "Status Epilepticus".

Protein kinase C bidirectionally modulates Ih and hyperpolarization-activated cyclic nucleotide-gated (HCN) channel surface expression in hippocampal pyramidal neurons

KEY POINTS

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, particularly that of the HCN1 isoform, are enriched in the distal dendrites of hippocampal CA1 pyramidal neurons; these channels have physiological functions with respect to decreasing neuronal excitability. In the present study, we aimed to investigate phosphorylation as a mechanism controlling Ih amplitude and HCN1 surface expression in hippocampal principal neurons under normal physiological conditions. Tyrosine phosphorylation decreased Ih amplitude at maximal activation (maximal Ih ), without altering HCN1 surface expression, in two classes of hippocampal principal neurons. Inhibition of serine/threonine protein phosphatases 1 and 2A decreased maximal Ih and HCN1 surface expression in hippocampal principal neurons. Protein kinase C (PKC) activation irreversibly diminished Ih and HCN1 surface expression, whereas PKC inhibition augmented Ih and HCN1 surface expression. PKC activation increased HCN1 channel phosphorylation. These results demonstrate the novel finding of a phosphorylation mechanism, dependent on PKC activity, which bidirectionally modulates Ih amplitude and HCN1channel surface expression in hippocampal principal neurons under normal physiological conditions.

ABSTRACT

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) ion channels attenuate excitability in hippocampal pyramidal neurons. Loss of HCN channel-mediated current (Ih ), particularly that mediated by the HCN1 isoform, occurs with the development of epilepsy. Previously, we showed that, following pilocarpine-induced status epilepticus, there are two independent changes in HCN function in dendrites: decreased Ih amplitude associated with a loss of HCN1 surface expression and a hyperpolarizing shift in voltage-dependence of activation (gating). The hyperpolarizing shift in gating was attributed to decreased phosphorylation as a result of a loss of p38 mitogen-activated protein kinase activity and increased calcineurin activity; however, the mechanisms controlling Ih amplitude and HCN1 surface expression under epileptic or normal physiological conditions are poorly understood. We aimed to investigate phosphorylation as a mechanism regulating Ih amplitude and HCN1 surface expression (i.e. as is the case for HCN gating) in hippocampal principal neurons under normal physiological conditions. We discovered that inhibition of either tyrosine phosphatases or the serine/threonine protein phosphatases 1 and 2A decreased Ih at maximal activation in hippocampal CA1 pyramidal dendrites and pyramidal-like principal neuron somata from naïve rats. Furthermore, we found that inhibition of PP1/PP2A decreased HCN1 surface expression, whereas tyrosine phosphatase inhibition did not. Protein kinase C (PKC) activation reduced Ih amplitude and HCN1 surface expression, whereas PKC inhibition produced the opposite effect. Inhibition of protein phosphatases 1 and 2A and activation of PKC increased the serine phosphorylation state of the HCN1 protein. The effect of PKC activation on Ih was irreversible. These results indicate that PKC bidirectionally modulates Ih amplitude and HCN1 surface expression in hippocampal principal neurons.

Tau-dependent Kv4.2 depletion and dendritic hyperexcitability in a mouse model of Alzheimer's disease

Neuronal hyperexcitability occurs early in the pathogenesis of Alzheimer's disease (AD) and contributes to network dysfunction in AD patients. In other disorders with neuronal hyperexcitability, dysfunction in the dendrites often contributes, but dendritic excitability has not been directly examined in AD models. We used dendritic patch-clamp recordings to measure dendritic excitability in the CA1 region of the hippocampus. We found that dendrites, more so than somata, of hippocampal neurons were hyperexcitable in mice overexpressing Aβ. This dendritic hyperexcitability was associated with depletion of Kv4.2, a dendritic potassium channel important for regulating dendritic excitability and synaptic plasticity. The antiepileptic drug, levetiracetam, blocked Kv4.2 depletion. Tau was required, as crossing with tau knock-out mice also prevented both Kv4.2 depletion and dendritic hyperexcitability. Dendritic hyperexcitability induced by Kv4.2 deficiency exacerbated behavioral deficits and increased epileptiform activity in hAPP mice. We conclude that increased dendritic excitability, associated with changes in dendritic ion channels including Kv4.2, may contribute to neuronal dysfunction in early stages AD.

Homeostasis or channelopathy? Acquired cell type-specific ion channel changes in temporal lobe epilepsy and their antiepileptic potential

Neurons continuously adapt the expression and functionality of their ion channels. For example, exposed to chronic excitotoxicity, neurons homeostatically downscale their intrinsic excitability. In contrast, the “acquired channelopathy” hypothesis suggests that proepileptic channel characteristics develop during epilepsy. We review cell type-specific channel alterations under different epileptic conditions and discuss the potential of channels that undergo homeostatic adaptations, as targets for antiepileptic drugs (AEDs). Most of the relevant studies have been performed on temporal lobe epilepsy (TLE), a widespread AED-refractory, focal epilepsy. The TLE patients, who undergo epilepsy surgery, frequently display hippocampal sclerosis (HS), which is associated with degeneration of cornu ammonis subfield 1 pyramidal cells (CA1 PCs). Although the resected human tissue offers insights, controlled data largely stem from animal models simulating different aspects of TLE and other epilepsies. Most of the cell type-specific information is available for CA1 PCs and dentate gyrus granule cells (DG GCs). Between these two cell types, a dichotomy can be observed: while DG GCs acquire properties decreasing the intrinsic excitability (in TLE models and patients with HS), CA1 PCs develop channel characteristics increasing intrinsic excitability (in TLE models without HS only). However, thorough examination of data on these and other cell types reveals the coexistence of protective and permissive intrinsic plasticity within neurons. These mechanisms appear differentially regulated, depending on the cell type and seizure condition. Interestingly, the same channel molecules that are upregulated in DG GCs during HS-related TLE, appear as promising targets for future AEDs and gene therapies. Hence, GCs provide an example of homeostatic ion channel adaptation which can serve as a primer when designing novel anti-epileptic strategies.

Deletion of PTEN produces autism-like behavioral deficits and alterations in synaptic proteins

Many genes have been implicated in the underlying cause of autism but each gene accounts for only a small fraction of those diagnosed with autism. There is increasing evidence that activity-dependent changes in neuronal signaling could act as a convergent mechanism for many of the changes in synaptic proteins. One candidate signaling pathway that may have a critical role in autism is the PI3K/AKT/mTOR pathway. A major regulator of this pathway is the negative repressor phosphatase and tensin homolog (PTEN). In the current study we examined the behavioral and molecular consequences in mice with neuron subset-specific deletion of PTEN. The knockout (KO) mice showed deficits in social chamber and social partition test. KO mice demonstrated alterations in repetitive behavior, as measured in the marble burying test and hole-board test. They showed no changes in ultrasonic vocalizations emitted on postnatal day 10 or 12 compared to wildtype (WT) mice. They exhibited less anxiety in the elevated-plus maze test and were more active in the open field test compared to WT mice. In addition to the behavioral alterations, KO mice had elevation of phosphorylated AKT, phosphorylated S6, and an increase in S6K. KO mice had a decrease in mGluR but an increase in total and phosphorylated fragile X mental retardation protein. The disruptions in intracellular signaling may be why the KO mice had a decrease in the dendritic potassium channel Kv4.2 and a decrease in the synaptic scaffolding proteins PSD-95 and SAP102. These findings demonstrate that deletion of PTEN results in long-term alterations in social behavior, repetitive behavior, activity, and anxiety. In addition, deletion of PTEN significantly alters mGluR signaling and many synaptic proteins in the hippocampus. Our data demonstrates that deletion of PTEN can result in many of the behavioral features of autism and may provide insights into the regulation of intracellular signaling on synaptic proteins.

Voltage-gated potassium channels at the crossroads of neuronal function, ischemic tolerance, and neurodegeneration

Voltage-gated potassium (Kv) channels are widely expressed in the central and peripheral nervous system and are crucial mediators of neuronal excitability. Importantly, these channels also actively participate in cellular and molecular signaling pathways that regulate the life and death of neurons. Injury-mediated increased K(+) efflux through Kv2.1 channels promotes neuronal apoptosis, contributing to widespread neuronal loss in neurodegenerative disorders such as Alzheimer's disease and stroke. In contrast, some forms of neuronal activity can dramatically alter Kv2.1 channel phosphorylation levels and influence their localization. These changes are normally accompanied by modifications in channel voltage dependence, which may be neuroprotective within the context of ischemic injury. Kv1 and Kv7 channel dysfunction leads to neuronal hyperexcitability that critically contributes to the pathophysiology of human clinical disorders such as episodic ataxia and epilepsy. This review summarizes the neurotoxic, neuroprotective, and neuroregulatory roles of Kv channels and highlights the consequences of Kv channel dysfunction on neuronal physiology. The studies described in this review thus underscore the importance of normal Kv channel function in neurons and emphasize the therapeutic potential of targeting Kv channels in the treatment of a wide range of neurological diseases.

Deletion of PTEN produces deficits in conditioned fear and increases fragile X mental retardation protein

The phosphatase and tensin homolog detected on chromosome 10 (PTEN) gene product modulates activation of the phosphatidylinositol 3-kinase (PI3K)/AKT pathway. The PI3K pathway has been found to be involved in the regulation of the fragile X mental retardation protein, which is important for long-term depression and in the formation of new memories. We used delayed fear conditioning and trace fear conditioning to determine learning and memory deficits in neuron subset-specific Pten (NS-Pten) conditional knockout (KO) mice. We found that NS-Pten KO mice had deficits in contextual learning and trace conditioning, but did not have deficits in the ability to learn a conditioned stimulus. Furthermore, we found increased levels in the total and phosphorylated forms of the fragile X mental retardation protein (FMRP) in the hippocampus of NS-Pten KO mice.

Role of the mTOR signaling pathway in epilepsy

Epilepsy, a common neurological disorder and cause of significant morbidity and mortality, places an enormous burden on the individual and society. Presently, most drugs for epilepsy primarily suppress seizures as symptomatic therapies but do not possess actual antiepileptogenic or disease-modifying properties. The mTOR (mammalian target of rapamycin) signaling pathway is involved in major multiple cellular functions, including protein synthesis, cell growth and proliferation and synaptic plasticity, which may influence neuronal excitability and be responsible for epileptogenesis. Intriguing findings of the frequent hyperactivation of mTOR signaling in epilepsy make it a potential mechanism in the pathogenesis as well as an attractive target for the therapeutic intervention, and have driven the significant ongoing efforts to pharmacologically target this pathway. This review explores the relevance of the mTOR pathway to epileptogenesis and its potential as a therapeutic target in epilepsy treatment by presenting the current results on mTOR inhibitors, in particular, rapamycin, in animal models of diverse types of epilepsy. Limited clinical studies in human epilepsy, some paradoxical experimental data and outstanding questions have also been discussed.

Rapamycin reverses status epilepticus-induced memory deficits and dendritic damage

Cognitive impairments are prominent sequelae of prolonged continuous seizures (status epilepticus; SE) in humans and animal models. While often associated with dendritic injury, the underlying mechanisms remain elusive. The mammalian target of rapamycin complex 1 (mTORC1) pathway is hyperactivated following SE. This pathway modulates learning and memory and is associated with regulation of neuronal, dendritic, and glial properties. Thus, in the present study we tested the hypothesis that SE-induced mTORC1 hyperactivation is a candidate mechanism underlying cognitive deficits and dendritic pathology seen following SE. We examined the effects of rapamycin, an mTORC1 inhibitor, on the early hippocampal-dependent spatial learning and memory deficits associated with an episode of pilocarpine-induced SE. Rapamycin-treated SE rats performed significantly better than the vehicle-treated rats in two spatial memory tasks, the Morris water maze and the novel object recognition test. At the molecular level, we found that the SE-induced increase in mTORC1 signaling was localized in neurons and microglia. Rapamycin decreased the SE-induced mTOR activation and attenuated microgliosis which was mostly localized within the CA1 area. These findings paralleled a reversal of the SE-induced decreases in dendritic Map2 and ion channels levels as well as improved dendritic branching and spine density in area CA1 following rapamycin treatment. Taken together, these findings suggest that mTORC1 hyperactivity contributes to early hippocampal-dependent spatial learning and memory deficits and dendritic dysregulation associated with SE.

Dendritic ion channelopathy in acquired epilepsy

Ion channel dysfunction or "channelopathy" is a proven cause of epilepsy in the relatively uncommon genetic epilepsies with Mendelian inheritance. But numerous examples of acquired channelopathy in experimental animal models of epilepsy following brain injury have also been demonstrated. Our understanding of channelopathy has grown due to advances in electrophysiology techniques that have allowed the study of ion channels in the dendrites of pyramidal neurons in cortex and hippocampus. The apical dendrites of pyramidal neurons comprise the vast majority of neuronal surface membrane area, and thus the majority of the neuronal ion channel population. Investigation of dendritic ion channels has demonstrated remarkable plasticity in ion channel localization and biophysical properties in epilepsy, many of which produce hyperexcitability and may contribute to the development and maintenance of the epileptic state. Herein we review recent advances in dendritic physiology and cell biology, and their relevance to epilepsy.

Kv4 potassium channels modulate hippocampal EPSP-spike potentiation and spatial memory in rats

Kv4 channels regulate the backpropagation of action potentials (b-AP) and have been implicated in the modulation of long-term potentiation (LTP). Here we showed that blockade of Kv4 channels by the scorpion toxin AmmTX3 impaired reference memory in a radial maze task. In vivo, AmmTX3 intracerebroventricular (i.c.v.) infusion increased and stabilized the EPSP-spike (E-S) component of LTP in the dentate gyrus (DG), with no effect on basal transmission or short-term plasticity. This increase in E-S potentiation duration could result from the combination of an increase in excitability of DG granular cells with a reduction of GABAergic inhibition, leading to a strong reduction of input specificity. Radioactive in situ hybridization (ISH) was used to evaluate the amounts of Kv4.2 and Kv4.3 mRNA in brain structures at different stages of a spatial learning task in naive, pseudoconditioned, and conditioned rats. Significant differences in Kv4.2 and Kv4.3 mRNA levels were observed between conditioned and pseudoconditioned rats. Kv4.2 and Kv4.3 mRNA levels were transiently up-regulated in the striatum, nucleus accumbens, retrosplenial, and cingulate cortices during early stages of learning, suggesting an involvement in the switch from egocentric to allocentric strategies. Spatial learning performance was positively correlated with the levels of Kv4.2 and Kv4.3 mRNAs in several of these brain structures. Altogether our findings suggest that Kv4 channels could increase the signal-to-noise ratio during information acquisition, thereby allowing a better encoding of the memory trace.

A-type K+ channels encoded by Kv4.2, Kv4.3 and Kv1.4 differentially regulate intrinsic excitability of cortical pyramidal neurons

Rapidly activating and rapidly inactivating voltage-gated A-type K+ currents, IA, are key determinants of neuronal excitability and several studies suggest a critical role for the Kv4.2 pore-forming α subunit in the generation of IA channels in hippocampal and cortical pyramidal neurons. The experiments here demonstrate that Kv4.2, Kv4.3 and Kv1.4 all contribute to the generation of IA channels in mature cortical pyramidal (CP) neurons and that Kv4.2-, Kv4.3- and Kv1.4-encoded IA channels play distinct roles in regulating the intrinsic excitability and the firing properties of mature CP neurons. In vivo loss of Kv4.2, for example, alters the input resistances, current thresholds for action potential generation and action potential repolarization of mature CP neurons. Elimination of Kv4.3 also prolongs action potential duration, whereas the input resistances and the current thresholds for action potential generation in Kv4.3−/− and WT CP neurons are indistinguishable. In addition, although increased repetitive firing was observed in both Kv4.2−/− and Kv4.3−/− CP neurons, the increases in Kv4.2−/− CP neurons were observed in response to small, but not large, amplitude depolarizing current injections, whereas firing rates were higher in Kv4.3−/− CP neurons only with large amplitude current injections. In vivo loss of Kv1.4, in contrast, had minimal effects on the intrinsic excitability and the firing properties of mature CP neurons. Comparison of the effects of pharmacological blockade of Kv4-encoded currents in Kv1.4−/− and WT CP neurons, however, revealed that Kv1.4-encoded IA channels do contribute to controlling resting membrane potentials, the regulation of current thresholds for action potential generation and repetitive firing rates in mature CP neurons.

Differential dorso-ventral distributions of Kv4.2 and HCN proteins confer distinct integrative properties to hippocampal CA1 pyramidal cell distal dendrites

The dorsal and ventral regions of the hippocampus perform different functions. Whether the integrative properties of hippocampal cells reflect this heterogeneity is unknown. We focused on dendrites where most synaptic input integration takes place. We report enhanced backpropagation and theta resonance and decreased summation of synaptic inputs in ventral versus dorsal CA1 pyramidal cell distal dendrites. Transcriptional Kv4.2 down-regulation and post-transcriptional hyperpolarization-activated cyclic AMP-gated channel (HCN1/2) up-regulation may underlie these differences, respectively. Our results reveal differential dendritic integrative properties along the dorso-ventral axis, reflecting diverse computational needs.

Kv4.2 knockout mice have hippocampal-dependent learning and memory deficits

Kv4.2 channels contribute to the transient, outward K(+) current (A-type current) in hippocampal dendrites, and modulation of this current substantially alters dendritic excitability. Using Kv4.2 knockout (KO) mice, we examined the role of Kv4.2 in hippocampal-dependent learning and memory. We found that Kv4.2 KO mice showed a deficit in the learning phase of the Morris water maze (MWM) and significant impairment in the probe trial compared with wild type (WT). Kv4.2 KO mice also demonstrated a specific deficit in contextual learning in the fear-conditioning test, without impairment in the conditioned stimulus or new context condition. Kv4.2 KO mice had normal activity, anxiety levels, and prepulse inhibition compared with WT mice. A compensatory increase in tonic inhibition has been previously described in hippocampal slice recordings from Kv4.2 KO mice. In an attempt to decipher whether increased tonic inhibition contributed to the learning and memory deficits in Kv4.2 KO mice, we administered picrotoxin to block GABA(A) receptors (GABA(A)R), and thereby tonic inhibition. This manipulation had no effect on behavior in the WT or KO mice. Furthermore, total protein levels of the α5 or δ GABA(A)R subunits, which contribute to tonic inhibition, were unchanged in hippocampus. Overall, our findings add to the growing body of evidence, suggesting an important role for Kv4.2 channels in hippocampal-dependent learning and memory.

Mutant LGI1 inhibits seizure-induced trafficking of Kv4.2 potassium channels

Activity-dependent redistribution of ion channels mediates neuronal circuit plasticity and homeostasis, and could provide pro-epileptic or compensatory anti-epileptic responses to a seizure. Thalamocortical neurons transmit sensory information to the cerebral cortex and through reciprocal corticothalamic connections are intensely activated during a seizure. Therefore, we assessed whether a seizure alters ion channel surface expression and consequent neurophysiologic function of thalamocortical neurons. We report a seizure triggers a rapid (<2h) decrease of excitatory postsynaptic current (EPSC)-like current-induced phasic firing associated with increased transient A-type K(+) current. Seizures also rapidly redistributed the A-type K(+) channel subunit Kv4.2 to the neuronal surface implicating a molecular substrate for the increased K(+) current. Glutamate applied in vitro mimicked the effect, suggesting a direct effect of glutamatergic transmission. Importantly, leucine-rich glioma-inactivated-1 (LGI1), a secreted synaptic protein mutated to cause human partial epilepsy, regulated this seizure-induced circuit response. Human epilepsy-associated dominant-negative-truncated mutant LGI1 inhibited the seizure-induced suppression of phasic firing, increase of A-type K(+) current, and recruitment of Kv4.2 surface expression (in vivo and in vitro). The results identify a response of thalamocortical neurons to seizures involving Kv4.2 surface recruitment associated with dampened phasic firing. The results also identify impaired seizure-induced increases of A-type K(+) current as an additional defect produced by the autosomal dominant lateral temporal lobe epilepsy gene mutant that might contribute to the seizure disorder.

Rapid loss of dendritic HCN channel expression in hippocampal pyramidal neurons following status epilepticus

Epilepsy is associated with loss of expression and function of hyperpolarization-activated, cyclic nucleotide-gated (HCN) ion channels. Previously, we showed that loss of HCN channel-mediated current (I(h)) occurred in the dendrites of CA1 hippocampal pyramidal neurons after pilocarpine-induced status epilepticus (SE), accompanied by loss of HCN1 channel protein expression. However, the precise onset and mechanistic basis of HCN1 channel loss post-SE was unclear, particularly whether it preceded the onset of spontaneous recurrent seizures and could contribute to epileptogenesis or development of the epileptic state. Here, we found that loss of I(h) and HCN1 channel expression began within an hour after SE and involved sequential processes of dendritic HCN1 channel internalization, delayed loss of protein expression, and later downregulation of mRNA expression. We also found that an in vitro SE model reproduced the rapid loss of dendritic I(h), demonstrating that this phenomenon was not specific to in vivo SE. Together, these results show that HCN1 channelopathy begins rapidly and persists after SE, involves both transcriptional and nontranscriptional mechanisms, and may be an early contributor to epileptogenesis.