Kv3.3 Channels Bind Hax-1 and Arp2/3 to Assemble a Stable Local Actin Network that Regulates Channel Gating

Direct prediction of intermolecular interactions driven by disordered regions

Intrinsically disordered regions (IDRs) are critical for a wide variety of cellular functions, many of which involve interactions with partner proteins. Molecular recognition is typically considered through the lens of sequence-specific binding events. However, a growing body of work has shown that IDRs often interact with partners in a manner that does not depend on the precise order of the amino acid order, instead driven by complementary chemical interactions leading to disordered bound-state complexes. Despite this emerging paradigm, we lack tools to describe, quantify, predict, and interpret these types of structurally heterogeneous interactions from the underlying amino acid sequences. Here, we repurpose the chemical physics developed originally for molecular simulations to develop an approach for predicting intermolecular interactions between IDRs and partner proteins. Our approach enables the direct prediction of phase diagrams, the identification of chemically-specific interaction hotspots on IDRs, and a route to develop and test mechanistic hypotheses regarding IDR function in the context of molecular recognition. We use our approach to examine a range of systems and questions to highlight its versatility and applicability.

HAX-1 interferes in assembly of NLRP3-ASC to block microglial pyroptosis in cerebral I/R injury

Acute cerebral ischemia has a high rate of disability and death. Although timely recanalization therapy may rescue the ischemic brain tissue, cerebral ischemia-reperfusion injury has been shown to limit the therapeutic effects of vascular recanalization. Protein HAX-1 has been reported as a pro-survival protein that plays an important role in various disorders, particularly in association with the nervous system. However, the effects and mechanisms of HAX-1 in cerebral IR injury have yet to be elucidated. So, we aimed to investigate the effect of HAX-1 on microglial pyroptosis and explore its potential neuroprotective effects in ischemia-reperfusion injury. Our results show that the expression of HAX-1 decreased after cerebral IR injury, accompanied by an increase in pyroptosis pathway activation. In addition, HAX-1 could inhibit microglial pyroptosis both in vivo and in vitro and reduce the release of inflammatory mediators. The above neuroprotective effects might be partially mediated by inhibiting of interaction of NLRP3 and ASC through competitive binding, followed by the attenuation of NLRP3 inflammasome formation. In conclusion, Our findings support that HAX-1 exhibits a protective role in cerebral I/R injury, and further study on HAX-1 expression regulation will contribute to cerebral infarction therapy.

NOD1 cooperates with HAX-1 to promote cell migration in a RIPK2- and NF-ĸB-independent manner

The human Nod-like receptor protein NOD1 is a well-described pattern-recognition receptor (PRR) with diverse functions. NOD1 associates with F-actin and its protein levels are upregulated in metastatic cancer cells. A hallmark of cancer cells is their ability to migrate, which involves actin remodelling. Using chemotaxis and wound healing assays, we show that NOD1 expression correlated with the migration rate and chemotactic index in the cervical carcinoma cell line HeLa. The effect of NOD1 in cell migration was independent of the downstream kinase RIPK2 and NF-ĸB activity. Additionally, NOD1 negatively regulated the phosphorylation status of cofilin, which inhibits actin turnover. Co-immunoprecipitation assays identified HCLS1-associated protein X-1 (HAX-1) as a previously unknown interaction partner of NOD1. Silencing of HAX-1 expression reduced the migration behaviour to similar levels as NOD1 knockdown, and simultaneous knockdown of NOD1 and HAX-1 showed no additive effect, suggesting that both proteins act in the same pathway. In conclusion, our data revealed an important role of the PRR NOD1 in regulating cell migration as well as chemotaxis in human cervical cancer cells and identified HAX-1 as a protein that interacts with NOD1 and is involved in this signalling pathway.

Small Extracellular Vesicles Maintain Homeostasis of Senescent Mesenchymal Stem Cells at Least Through Excreting Harmful Lipids

Mesenchymal stem cells (MSCs) play an essential role in multiple physiological processes in vivo and a promising cell-based therapy for various diseases. Nonetheless, MSCs suffer from senescence with expansion culture, leading to a limitation for their clinical application. Recently, it was reported that small extracellular vesicles (sEVs) are involved in regulation of senescence in tumor cells and fibroblasts. However, the biological roles of sEVs in senescent MSCs (Sen MSCs) are poorly understood. In this study, we established a replicative senescence model of MSCs by successive passages and compared the phenotypic changes between presenescent MSCs (Pre-Sen MSCs) and Sen MSCs and found that Sen MSCs exhibited a diminished adipogenic and osteogenic differentiation potential and elevated senescence-associated secretory phenotype levels. In addition, we found that sEV secretion was increased in Sen MSCs, and inhibition of sEV secretion led to apoptosis, DNA damage, and decreased cell viability, suggesting that increased sEV secretion plays an important role in maintaining Sen MSC homeostasis. To further investigate the molecular mechanisms, metabolomic profiling of Pre-Sen MSC-derived sEVs (Pre-Sen-sEVs) and Sen MSC-derived sEVs (Sen-sEVs) was performed. The results showed that lipid metabolites were significantly increased in Sen-sEVs and these significantly upregulated lipid metabolites were shown to be toxic for inducing cellular senescence and apoptosis in previous studies. Kyoto Encyclopedia of Genes and Genomes analysis revealed enrichment of differential metabolites between Pre-Sen-sEVs and Sen-sEVs mainly in 25 signaling pathways, of which 21 metabolic pathways have been shown to be closely associated with senescence. Taken together, our findings suggested that increased sEV secretion maintains Sen MSC homeostasis, at least in part, by excreting harmful lipids, thus providing new insights into the regulation of senescence by sEVs.

Development, circuitry, and function of the zebrafish cerebellum

The cerebellum represents a brain compartment that first appeared in gnathostomes (jawed vertebrates). Besides the addition of cell numbers, its development, cytoarchitecture, circuitry, physiology, and function have been highly conserved throughout avian and mammalian species. While cerebellar research in avian and mammals is extensive, systematic investigations on this brain compartment in zebrafish as a teleostian model organism started only about two decades ago, but has provided considerable insight into cerebellar development, physiology, and function since then. Zebrafish are genetically tractable with nearly transparent small-sized embryos, in which cerebellar development occurs within a few days. Therefore, genetic investigations accompanied with non-invasive high-resolution in vivo time-lapse imaging represents a powerful combination for interrogating the behavior and function of cerebellar cells in their complex native environment.

A missense mutation in Kcnc3 causes hippocampal learning deficits in mice

Although a wide variety of genetic tools has been developed to study learning and memory, the molecular basis of memory encoding remains incompletely understood. Here, we undertook an unbiased approach to identify novel genes critical for memory encoding. From a large-scale, in vivo mutagenesis screen using contextual fear conditioning, we isolated in mice a mutant, named Clueless, with spatial learning deficits. A causative missense mutation (G434V) was found in the voltage-gated potassium channel, subfamily C member 3 (Kcnc3) gene in a region that encodes a transmembrane voltage sensor. Generation of a Kcnc3^G434V CRISPR mutant mouse confirmed this mutation as the cause of the learning defects. While G434V had no effect on transcription, translation, or trafficking of the channel, electrophysiological analysis of the G434V mutant channel revealed a complete loss of voltage-gated conductance, a broadening of the action potential, and decreased neuronal firing. Together, our findings have revealed a role for Kcnc3 in learning and memory.

Identification of the ataxin-1 interaction network and its impact on spinocerebellar ataxia type 1

Background

Spinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disease caused by a polyglutamine expansion in the ataxin-1 protein. The pathogenic mechanism resulting in SCA1 is still unclear. Protein–protein interactions affect the function and stability of ataxin-1.

Methods

Wild-type and mutant ataxin-1 were expressed in HEK-293T cells. The levels of expression were assessed using real-time polymerase chain reaction (PCR) and Western blots. Co-immunoprecipitation was done in HEK-293T cells expressing exogenous wild-type and mutant ataxin-1 using anti-Flag antibody following by tandem affinity purification in order to study protein–protein interactions. The candidate interacting proteins were validated by immunoprecipitation. Chromatin immunoprecipitation and high-throughput sequencing and RNA immunoprecipitation and high-throughput sequencing were performed using HEK-293T cells expressing wild-type or mutant ataxin-1.

Results

In this study using HEK-293T cells, we found that wild-type ataxin-1 interacted with MCM2, GNAS, and TMEM206, while mutant ataxin-1 lost its interaction with MCM2, GNAS, and TMEM206. Two ataxin-1 binding targets containing the core GGAG or AAAT were identified in HEK-293T cells using ChIP-seq. Gene Ontology analysis of the top ataxin-1 binding genes identified SLC6A15, NTF3, KCNC3, and DNAJC6 as functional genes in neurons in vitro. Ataxin-1 also was identified as an RNA-binding protein in HEK-293T cells using RIP-seq, but the polyglutamine expansion in the ataxin-1 had no direct effects on the RNA-binding activity of ataxin-1.

Conclusions

An expanded polyglutamine tract in ataxin-1 might interfere with protein–protein or protein–DNA interactions but had little effect on protein–RNA interactions. This study suggested that the dysfunction of protein–protein or protein–DNA interactions is involved in the pathogenesis of SCA1.

Kv3.3 subunits control presynaptic action potential waveform and neurotransmitter release at a central excitatory synapse

Kv3 potassium currents mediate rapid repolarisation of action potentials (APs), supporting fast spikes and high repetition rates. Of the four Kv3 gene family members, Kv3.1 and Kv3.3 are highly expressed in the auditory brainstem and we exploited this to test for subunit-specific roles at the calyx of Held presynaptic terminal in the mouse. Deletion of Kv3.3 (but not Kv3.1) reduced presynaptic Kv3 channel immunolabelling, increased presynaptic AP duration and facilitated excitatory transmitter release; which in turn enhanced short-term depression during high-frequency transmission. The response to sound was delayed in the Kv3.3KO, with higher spontaneous and lower evoked firing, thereby reducing signal-to-noise ratio. Computational modelling showed that the enhanced EPSC and short-term depression in the Kv3.3KO reflected increased vesicle release probability and accelerated activity-dependent vesicle replenishment. We conclude that Kv3.3 mediates fast repolarisation for short precise APs, conserving transmission during sustained high-frequency activity at this glutamatergic excitatory synapse.

Multiple Roles of Actin in Exo- and Endocytosis

Cytoskeletal filamentous actin (F-actin) has long been considered a molecule that may regulate exo- and endocytosis. However, its exact roles remained elusive. Recent studies shed new light on many crucial roles of F-actin in regulating exo- and endocytosis. Here, this progress is reviewed from studies of secretory cells, particularly neurons and endocrine cells. These studies reveal that F-actin is involved in mediating all kinetically distinguishable forms of endocytosis, including ultrafast, fast, slow, bulk, and overshoot endocytosis, likely via membrane pit formation. F-actin promotes vesicle replenishment to the readily releasable pool most likely via active zone clearance, which may sustain synaptic transmission and overcome short-term depression of synaptic transmission during repetitive firing. By enhancing plasma membrane tension, F-actin promotes fusion pore expansion, vesicular content release, and a fusion mode called shrink fusion involving fusing vesicle shrinking. Not only F-actin, but also the F-actin assembly pathway, including ATP hydrolysis, N-WASH, and formin, are involved in mediating these roles of exo- and endocytosis. Neurological disorders, including spinocerebellar ataxia 13 caused by Kv3.3 channel mutation, may involve impairment of F-actin and its assembly pathway, leading in turn to impairment of exo- and endocytosis at synapses that may contribute to neurological disorders.

Hsa-miR223-3p circulating level is upregulated in Friedreich's ataxia and inversely associated with HCLS1 associated protein X-1, HAX-1

Frataxin (FXN) deficiency is responsible for Friedreich's ataxia (FRDA) in which, besides the characteristic features of spinocerebellar ataxia, two thirds of patients develop hypertrophic cardiomyopathy that often progresses to heart failure and premature death. Different mechanisms might underlie FRDA pathogenesis. Among them, the role of miRNAs deserves investigations. We carried out a miRNA PCR-array analysis of plasma samples of early-, intermediate- and late-onset FRDA groups, defining a set of 30 differentially expressed miRNAs. Hsa-miR223-3p is the only miRNA shared between the three patient groups and appears upregulated in all of them. The upregulation of hsa-miR223-3p was further validated in all enrolled patients (n = 37, Fc = +2.3; p < 0.0001). Using a Receiver Operating Characteristic (ROC) curve analysis, we quantified the predictive value of circulating hsa-miR223-3p for FRDA, obtaining an AUC (Area Under the ROC Curve) value of 0.835 (p < 0.0001) for all patients. Interestingly, we found a significant positive correlation between hsa-miR223-3p expression and cardiac parameters in typical FRDA patients (onset < 25 years). Moreover, a significant negative correlation between hsa-miR223-3p expression and HAX-1 (HCLS1 associated protein X-1) at mRNA and protein level was observed in all FRDA patients. In silico analyses suggested HAX-1 as a target gene of hsa-miR223-3p. Accordingly, we report that HAX-1 is negatively regulated by hsa-miR223-3p in cardiomyocytes (AC16) and neurons (SH-SY5Y), which are critically affected cell types in FRDA. This study describes for the first time the association between hsa-miR223-3p and HAX-1 expression in FRDA, thus supporting a potential role of this microRNA as non-invasive epigenetic biomarker for FRDA.

Suppression of Kv3.3 channels by antisense oligonucleotides reverses biochemical effects and motor impairment in spinocerebellar ataxia type 13 mice

Mutations in KCNC3, the gene that encodes the Kv3.3 voltage dependent potassium channel, cause Spinocerebellar Ataxia type 13 (SCA13), a disease associated with disrupted motor behaviors, progressive cerebellar degeneration, and abnormal auditory processing. The Kv3.3 channel directly binds Hax-1, a cell survival protein. A disease-causing mutation, Kv3.3-G592R, causes overstimulation of Tank Binding Kinase 1 (Tbk1) in the cerebellum, resulting in the degradation of Hax-1 by promoting its trafficking into multivesicular bodies and then to lysosomes. We have now tested the effects of antisense oligonucleotides (ASOs) directed against the Kv3.3 channel on both wild type mice and those bearing the Kv3.3-G592R-encoding mutation. Intracerebroventricular infusion of the Kcnc3-specific ASO suppressed both mRNA and protein levels of the Kv3.3 channel. In wild-type animals, this produced no change in levels of activated Tbk1, Hax-1 or Cd63, a tetraspanin marker for late endosomes/multivesicular bodies. In contrast, in mice homozygous for the Kv3.3-G592R-encoding mutation, the same ASO reduced Tbk1 activation and levels of Cd63, while restoring the expression of Hax-1 in the cerebellum. The motor behavior of the mice was tested using a rotarod assay. Surprisingly, the active ASO had no effects on the motor behavior of wild type mice but restored the behavior of the mutant mice to those of age-matched wild type animals. Our findings indicate that, in mature intact animals, suppression of Kv3.3 expression can reverse the deleterious effects of a SCA13 mutation while having little effect on wild type animals. Thus, targeting Kv3.3 expression may prove a viable therapeutic approach for SCA13.

Spinocerebellar ataxias (SCAs) caused by common mutations

The term SCA refers to a phenotypically and genetically heterogeneous group of autosomal dominant spinocerebellar ataxias. Phenotypically they present as gait ataxia frequently in combination with dysarthria and oculomotor problems. Additional signs and symptoms are common and can include various pyramidal and extrapyramidal signs and intellectual impairment. Genetic causes of SCAs are either repeat expansions within disease genes or common mutations (point mutations, deletions, insertions etc.). Frequently the two types of mutations cause indistinguishable phenotypes (locus heterogeneity). This article focuses on SCAs caused by common mutations. It describes phenotype and genotype of the presently 27 types known and discusses the molecular pathogenesis in those 21 types where the disease gene has been identified. Apart from the dominant types, the article also summarizes findings in a variant caused by mutations in a mitochondrial gene. Possible common disease mechanisms are considered based on findings in the various SCAs described.

Action potential-coupled Rho GTPase signaling drives presynaptic plasticity

In contrast to their postsynaptic counterparts, the contributions of activity-dependent cytoskeletal signaling to presynaptic plasticity remain controversial and poorly understood. To identify and evaluate these signaling pathways, we conducted a proteomic analysis of the presynaptic cytomatrix using in vivo biotin identification (iBioID). The resultant proteome was heavily enriched for actin cytoskeleton regulators, including Rac1, a Rho GTPase that activates the Arp2/3 complex to nucleate branched actin filaments. Strikingly, we find Rac1 and Arp2/3 are closely associated with synaptic vesicle membranes in adult mice. Using three independent approaches to alter presynaptic Rac1 activity (genetic knockout, spatially restricted inhibition, and temporal optogenetic manipulation), we discover that this pathway negatively regulates synaptic vesicle replenishment at both excitatory and inhibitory synapses, bidirectionally sculpting short-term synaptic depression. Finally, we use two-photon fluorescence lifetime imaging to show that presynaptic Rac1 activation is coupled to action potentials by voltage-gated calcium influx. Thus, this study uncovers a previously unrecognized mechanism of actin-regulated short-term presynaptic plasticity that is conserved across excitatory and inhibitory terminals. It also provides a new proteomic framework for better understanding presynaptic physiology, along with a blueprint of experimental strategies to isolate the presynaptic effects of ubiquitously expressed proteins.

Progressive myoclonus epilepsy KCNC1 variant causes a developmental dendritopathy

OBJECTIVE

Mutations in KCNC1 can cause severe neurological dysfunction, including intellectual disability, epilepsy, and ataxia. The Arg320His variant, which occurs in the voltage-sensing domain of the channel, causes a highly penetrant and specific form of progressive myoclonus epilepsy with severe ataxia, designated myoclonus epilepsy and ataxia due to potassium channel mutation (MEAK). KCNC1 encodes the voltage-gated potassium channel KV 3.1, a channel that is important for enabling high-frequency firing in interneurons, raising the possibility that MEAK is associated with reduced interneuronal function.

METHODS

To determine how this variant triggers MEAK, we expressed KV 3.1bR320H in cortical interneurons in vitro and investigated the effects on neuronal function and morphology. We also performed electrophysiological recordings of oocytes expressing KV 3.1b to determine whether the mutation introduces gating pore currents.

RESULTS

Expression of the KV 3.1bR320H variant profoundly reduced excitability of mature cortical interneurons, and cells expressing these channels were unable to support high-frequency firing. The mutant channel also had an unexpected effect on morphology, severely impairing neurite development and interneuron viability, an effect that could not be rescued by blocking KV 3 channels. Oocyte recordings confirmed that in the adult KV 3.1b isoform, R320H confers a dominant negative loss-of-function effect by slowing channel activation, but does not introduce potentially toxic gating pore currents.

SIGNIFICANCE

Overall, our data suggest that, in addition to the regulation of high-frequency firing, KV 3.1 channels play a hitherto unrecognized role in neuronal development. MEAK may be described as a developmental dendritopathy.

Optimized Tuning of Auditory Inner Hair Cells to Encode Complex Sound through Synergistic Activity of Six Independent K+ Current Entities

Auditory inner hair cells (IHCs) convert sound vibrations into receptor potentials that drive synaptic transmission. For the precise encoding of sound qualities, receptor potentials are shaped by K⁺ conductances tuning the properties of the IHC membrane. Using patch-clamp and computational modeling, we unravel this membrane specialization showing that IHCs express an exclusive repertoire of six voltage-dependent K⁺ conductances mediated by K_v1.8, K_v7.4, K_v11.1, K_v12.1, and BK_Ca channels. All channels are active at rest but are triggered differentially during sound stimulation. This enables non-saturating tuning over a far larger potential range than in IHCs expressing fewer current entities. Each conductance contributes to optimizing responses, but the combined activity of all channels synergistically improves phase locking and the dynamic range of intensities that IHCs can encode. Conversely, hypothetical simpler IHCs appear limited to encode only certain aspects (frequency or intensity). The exclusive channel repertoire of IHCs thus constitutes an evolutionary adaptation to encode complex sound through multifaceted receptor potentials.

The interactome of multifunctional HAX1 protein suggests its role in the regulation of energy metabolism, de-aggregation, cytoskeleton organization and RNA-processing

HCLS1-associated protein X-1 (HAX1) is a multifunctional protein involved in many cellular processes, including apoptosis, cell migration and calcium homeostasis, but its mode of action still remains obscure. Multiple HAX1 protein partners have been identified, but they are involved in many distinct pathways, form different complexes and do not constitute a coherent group. By characterizing HAX1 protein interactome using targeted approach, we attempt to explain HAX1 multiple functions and its role in the cell. Presented analyses indicate that HAX1 interacts weakly with a wide spectrum of proteins and its interactome tends to be cell-specific, which conforms to a profile of intrinsically disordered protein (IDP). Moreover, we have identified a mitochondrial subset of HAX1 protein partners and preliminarily characterized its involvement in the cellular response to oxidative stress and aggregation.

Cerebellar Kv3.3 potassium channels activate TANK-binding kinase 1 to regulate trafficking of the cell survival protein Hax-1

Mutations in KCNC3, which encodes the Kv3.3 potassium channel, cause degeneration of the cerebellum, but exactly how the activity of an ion channel is linked to the survival of cerebellar neurons is not understood. Here, we report that Kv3.3 channels bind and stimulate Tank Binding Kinase 1 (TBK1), an enzyme that controls trafficking of membrane proteins into multivesicular bodies, and that this stimulation is greatly increased by a disease-causing Kv3.3 mutation. TBK1 activity is required for the binding of Kv3.3 to its auxiliary subunit Hax-1, which prevents channel inactivation with depolarization. Hax-1 is also an anti-apoptotic protein required for survival of cerebellar neurons. Overactivation of TBK1 by the mutant channel leads to the loss of Hax-1 by its accumulation in multivesicular bodies and lysosomes, and also stimulates exosome release from neurons. This process is coupled to activation of caspases and increased cell death. Our studies indicate that Kv3.3 channels are directly coupled to TBK1-dependent biochemical pathways that determine the trafficking of cellular constituents and neuronal survival.

How the activity of the neuronal Kv3.3 voltage-dependent channel is regulated is unclear. Here, the authors show that the known Kv3.3 channel complex with Hax1, which affects spinal cerebellar ataxia, regulates the enzyme Tank Binding Kinase 1, modulating survival of cerebellar neurons.

Presynaptic Kv3 channels are required for fast and slow endocytosis of synaptic vesicles

Since their discovery decades ago, the primary physiological and pathological effects of potassium channels have been attributed to their ion conductance, which sets membrane potential and repolarizes action potentials. For example, Kv3 family channels regulate neurotransmitter release by repolarizing action potentials. Here we report a surprising but crucial function independent of potassium conductance: by organizing the F-actin cytoskeleton in mouse nerve terminals, the Kv3.3 protein facilitates slow endocytosis, rapid endocytosis, vesicle mobilization to the readily releasable pool, and recovery of synaptic depression during repetitive firing. A channel mutation that causes spinocerebellar ataxia inhibits endocytosis, vesicle mobilization, and synaptic transmission during repetitive firing by disrupting the ability of the channel to nucleate F-actin. These results unmask novel functions of potassium channels in endocytosis and vesicle mobilization crucial for sustaining synaptic transmission during repetitive firing. Potassium channel mutations that impair these "non-conducting" functions may thus contribute to generation of diverse neurological disorders.

Kv3 channels contribute to cancer cell migration via vimentin regulation

Cell migration is a complex and important process in cancer progression. Vimentin has pivotal roles in cancer cell migration, and various signaling pathways including the AKT pathway are involved in cancer cell migration via vimentin regulation. Recent studies have revealed that voltage-gated potassium (Kv) channels have important functions in cancer cell migration; however, the exact mechanism is still unclear. In the present study, we focused on Kv3 channels with vimentin in cancer migration using human cervical cancer cells (HeLa) and canine mammary tumor cells (CHMp). Cancer cell migration was significantly inhibited, and vimentin expression was significantly decreased by Kv3 blocker, BDS-II. The Kv3 blocker also inactivated the AKT pathway in HeLa cells. In addition, reduced expressions of vimentin and Kv3.4 were observed in HeLa cells when treated with AKT blocker, MK2206. These results suggest that Kv3 channels play important roles in cancer cell migration by regulating vimentin and having closely related with the AKT pathway in human cervical cancer cells.

Modulation of Neuronal Potassium Channels During Auditory Processing

The extraction and localization of an auditory stimulus of interest from among multiple other sounds, as in the ‘cocktail-party’ situation, requires neurons in auditory brainstem nuclei to encode the timing, frequency, and intensity of sounds with high fidelity, and to compare inputs coming from the two cochleae. Accurate localization of sounds requires certain neurons to fire at high rates with high temporal accuracy, a process that depends heavily on their intrinsic electrical properties. Studies have shown that the membrane properties of auditory brainstem neurons, particularly their potassium currents, are not fixed but are modulated in response to changes in the auditory environment. Here, we review work focusing on how such modulation of potassium channels is critical to shaping the firing pattern and accuracy of these neurons. We describe how insights into the role of specific channels have come from human gene mutations that impair localization of sounds in space. We also review how short-term and long-term modulation of these channels maximizes the extraction of auditory information, and how errors in the regulation of these channels contribute to deficits in decoding complex auditory information.

Modulation of Actin Filament Dynamics by Inward Rectifying of Potassium Channel Kir2.1

Apart from its ion channel properties, the Kir2.1 channel has been found in tumors and cancer cells to facilitate cancer cell motility. It is assumed that Kir2.1 might be associated with cell actin filament dynamics. With the help of structured illumination microscopy (SIM), we show that Kir2.1 overexpression promotes actin filament dynamics, cell invasion, and adhesion. Mutated Kir2.1 channels, with impaired membrane expression, present much weaker actin regulatory effects, which indicates that precise Kir2.1 membrane localization is key to its actin filament remolding effect. It is found that Kir2.1 membrane expression and anchoring are associated with PIP₂ affinity, and PIP₂ depletion inhibits actin filament dynamics. We also report that membrane-expressed Kir2.1 regulates redistribution and phosphorylation of FLNA (filamin A), which may be the mechanism underlying Kir2.1 and actin filament dynamics. In conclusion, Kir2.1 membrane localization regulates cell actin filaments, and not the ion channel properties. These data indicate that Kir2.1 may have additional cellular functions distinct from the regulation of excitability, which provides new insight into the study of channel proteins.

Haploinsufficiency of Cyfip2 Causes Lithium-Responsive Prefrontal Dysfunction

OBJECTIVE

Genetic variants of the cytoplasmic FMR1-interacting protein 2 (CYFIP2) encoding an actin-regulatory protein are associated with brain disorders, including intellectual disability and epilepsy. However, specific in vivo neuronal defects and potential treatments for CYFIP2-associated brain disorders remain largely unknown. Here, we characterized Cyfip2 heterozygous (Cyfip2^+/- ) mice to understand their neurobehavioral phenotypes and the underlying pathological mechanisms. Furthermore, we examined a potential treatment for such phenotypes of the Cyfip2^+/- mice and specified a neuronal function mediating its efficacy.

METHODS

We performed behavioral analyses of Cyfip2^+/- mice. We combined molecular, ultrastructural, and in vitro and in vivo electrophysiological analyses of Cyfip2^+/- prefrontal neurons. We also selectively reduced CYFIP2 in the prefrontal cortex (PFC) of mice with virus injections.

RESULTS

Adult Cyfip2^+/- mice exhibited lithium-responsive abnormal behaviors. We found increased filamentous actin, enlarged dendritic spines, and enhanced excitatory synaptic transmission and excitability in the adult Cyfip2^+/- PFC that was restricted to layer 5 (L5) neurons. Consistently, adult Cyfip2^+/- mice showed increased seizure susceptibility and auditory steady-state responses from the cortical electroencephalographic recordings. Among the identified prefrontal defects, lithium selectively normalized the hyperexcitability of Cyfip2^+/- L5 neurons. RNA sequencing revealed reduced expression of potassium channel genes in the adult Cyfip2^+/- PFC. Virus-mediated reduction of CYFIP2 in the PFC was sufficient to induce L5 hyperexcitability and lithium-responsive abnormal behavior.

INTERPRETATION

These results suggest that L5-specific prefrontal dysfunction, especially hyperexcitability, underlies both the pathophysiology and the lithium-mediated amelioration of neurobehavioral phenotypes in adult Cyfip2^+/- mice, which can be implicated in CYFIP2-associated brain disorders. ANN NEUROL 2020;88:526-543.

Functional interactions of ion channels with the actin cytoskeleton: does coupling to dynamic actin regulate NMDA receptors?

Synapses are enriched in the cytoskeletal protein actin, which determines the shape of the pre- and postsynaptic compartments, organizes the neurotransmitter release machinery, and provides a framework for trafficking of components. In the postsynaptic compartment, interactions with actin or its associated proteins are also critical for the localization and activity of synaptic neurotransmitter receptors and ion channels. Actin binding proteins, including spectrin and α-actinin, serve as molecular linkages between the actin cytoskeleton and a diverse collection of receptors, including the NMDA receptor (NMDAR) and voltage-gated Na⁺ channels. The actin cytoskeleton can regulate neurotransmitter receptors and ion channels by controlling their trafficking and localization at the synapse and by directly gating receptor channel opening. We highlight evidence that synaptic actin couples physically and functionally to the NMDAR and supports its activity. The molecular mechanisms by which actin regulates NMDARs are only just emerging, and recent advancements in light and electron microscopy-based imaging techniques should aide in elucidating these mechanisms.

Mechanisms underlying auditory processing deficits in Fragile X syndrome

Autism spectrum disorders (ASD) are strongly associated with auditory hypersensitivity or hyperacusis (difficulty tolerating sounds). Fragile X syndrome (FXS), the most common monogenetic cause of ASD, has emerged as a powerful gateway for exploring underlying mechanisms of hyperacusis and auditory dysfunction in ASD. This review discusses examples of disruption of the auditory pathways in FXS at molecular, synaptic, and circuit levels in animal models as well as in FXS individuals. These examples highlight the involvement of multiple mechanisms, from aberrant synaptic development and ion channel deregulation of auditory brainstem circuits, to impaired neuronal plasticity and network hyperexcitability in the auditory cortex. Though a relatively new area of research, recent discoveries have increased interest in auditory dysfunction and mechanisms underlying hyperacusis in this disorder. This rapidly growing body of data has yielded novel research directions addressing critical questions regarding the timing and possible outcomes of human therapies for auditory dysfunction in ASD.

Frataxin deficiency in Friedreich's ataxia is associated with reduced levels of HAX-1, a regulator of cardiomyocyte death and survival

Frataxin deficiency, responsible for Friedreich's ataxia (FRDA), is crucial for cell survival since it critically affects viability of neurons, pancreatic beta cells and cardiomyocytes. In FRDA, the heart is frequently affected with typical manifestation of hypertrophic cardiomyopathy, which can progress to heart failure and cause premature death. A microarray analysis performed on FRDA patient's lymphoblastoid cells stably reconstituted with frataxin, indicated HS-1-associated protein X-1 (HAX-1) as the most significantly upregulated transcript (FC = +2, P < 0.0006). quantitative Reverse Transcription-Polymerase Chain Reaction (qRT-PCR) and western blot analysis performed on (I) HEK293 stably transfected with empty vector compared to wild-type frataxin and (II) lymphoblasts from FRDA patients show that low frataxin mRNA and protein expression correspond to reduced levels of HAX-1. Frataxin overexpression and silencing were also performed in the AC16 human cardiomyocyte cell line. HAX-1 protein levels are indeed regulated through frataxin modulation. Moreover, correlation between frataxin and HAX-1 was further evaluated in peripheral blood mononuclear cells (PBMCs) from FRDA patients and from non-related healthy controls. A regression model for frataxin which included HAX-1, group membership and group* HAX-1 interaction revealed that frataxin and HAX-1 are associated both at mRNA and protein levels. Additionally, a linked expression of FXN, HAX-1 and antioxidant defence proteins MnSOD and Nrf2 was observed both in PBMCs and AC16 cardiomyocytes. Our results suggest that HAX-1 could be considered as a potential biomarker of cardiac disease in FRDA and the evaluation of its expression might provide insights into its pathogenesis as well as improving risk stratification strategies.

Intrinsically disordered HAX-1 regulates Ca2+ cycling by interacting with lipid membranes and the phospholamban cytoplasmic region

Hematopoietic-substrate-1 associated protein X-1 (HAX-1) is a 279 amino acid protein expressed ubiquitously. In cardiac muscle, HAX-1 was found to modulate the sarcoendoplasmic reticulum calcium ATPase (SERCA) by shifting its apparent Ca2+ affinity (pCa). It has been hypothesized that HAX-1 binds phospholamban (PLN), enhancing its inhibitory function on SERCA. HAX-1 effects are reversed by cAMP-dependent protein kinase A that phosphorylates PLN at Ser16. To date, the molecular mechanisms for HAX-1 regulation of the SERCA/PLN complex are still unknown. Using enzymatic, in cell assays, circular dichroism, and NMR spectroscopy, we found that in the absence of a binding partner HAX-1 is essentially disordered and adopts a partial secondary structure upon interaction with lipid membranes. Also, HAX-1 interacts with the cytoplasmic region of monomeric and pentameric PLN as detected by NMR and in cell FRET assays, respectively. We propose that the regulation of the SERCA/PLN complex by HAX-1 is mediated by its interactions with lipid membranes, adding another layer of control in Ca2+ homeostatic balance in the heart muscle.

Current Opinions and Consensus for Studying Tremor in Animal Models

Tremor is the most common movement disorder; however, we are just beginning to understand the brain circuitry that generates tremor. Various neuroimaging, neuropathological, and physiological studies in human tremor disorders have been performed to further our knowledge of tremor. But, the causal relationship between these observations and tremor is usually difficult to establish and detailed mechanisms are not sufficiently studied. To overcome these obstacles, animal models can provide an important means to look into human tremor disorders. In this manuscript, we will discuss the use of different species of animals (mice, rats, fruit flies, pigs, and monkeys) to model human tremor disorders. Several ways to manipulate the brain circuitry and physiology in these animal models (pharmacology, genetics, and lesioning) will also be discussed. Finally, we will discuss how these animal models can help us to gain knowledge of the pathophysiology of human tremor disorders, which could serve as a platform towards developing novel therapies for tremor.

Gamma oscillations in the pedunculopontine nucleus are regulated by F-actin: neuroepigenetic implications

The pedunculopontine nucleus (PPN) is part of the reticular activating system (RAS) in charge of arousal and rapid eye movement sleep. The presence of high-frequency membrane oscillations in the gamma-band range in the PPN has been extensively demonstrated both in vivo and in vitro. Our group previously described histone deacetylation (HDAC) inhibition in vitro induced protein changes in F-actin cytoskeleton and intracellular Ca²⁺ concentration regulation proteins in the PPN. Here, we present evidence that supports the presence of a fine balance between HDAC function and calcium calmodulin kinase II-F-actin interactions in the PPN. We modified F-actin polymerization in vitro by using jasplakinolide (1 μM, a promoter of F-actin stabilization), or latrunculin-B (1 μM, an inhibitor of actin polymerization). Our results showed that shifting the balance in either direction significantly reduced PPN gamma oscillation as well as voltage-dependent calcium currents.

Complexes formed with integrin-α5 and KCNB1 potassium channel wild type or epilepsy-susceptibility variants modulate cellular plasticity via Ras and Akt signaling

Voltage-gated potassium (K⁺) channel subfamily B member 1 (KCNB1, Kv2.1) and integrin-α5 form macromolecular complexes-named integrin-α5-KCNB1 complexes (IKCs)-in the human brain, but their function was poorly understood. Here we report that membrane depolarization triggered IKC intracellular signals mediated by small GTPases of the Ras subfamily and protein kinase B (Akt) to advance the development of filopodia and lamellipodia in Chinese hamster ovary cells, stimulate their motility, and enhance neurite outgrowth in mouse neuroblastoma Neuro2a cells. Five KCNB1 mutants (L211P, R312H G379R, G381R, and F416L) linked to severe infancy or early-onset epileptic encephalopathy exhibited markedly defective conduction. However, although L211P, G379R, and G381R normally engaged Ras/Akt and stimulated cell migration, R312H and F416L failed to activate Ras/Akt signaling and did not enhance cell migration. Taken together, these data suggest that IKCs modulate cellular plasticity via Ras and Akt signaling. As such, defective IKCs may cause epilepsy through mechanisms other than dysregulated excitability such as, for example, abnormal neuronal development and resulting synaptic connectivity.-Yu, W., Shin, M. R., Sesti, F. Complexes formed with integrin-α5 and KCNB1 potassium channel wild type or epilepsy-susceptibility variants modulate cellular plasticity via Ras and Akt signaling.

HAX1 impact on collective cell migration, cell adhesion, and cell shape is linked to the regulation of actomyosin contractility

HAX1 protein is involved in the regulation of apoptosis, cell motility and calcium homeostasis. Its overexpression was reported in several tumors, including breast cancer. This study demonstrates that HAX1 has an impact on collective, but not single-cell migration, thus indicating the importance of cell–cell contacts for the HAX1-mediated effect. Accordingly, it was shown that HAX1 knockdown affects cell–cell junctions, substrate adhesion, and epithelial cell layer integrity. As demonstrated here, these effects can be attributed to the modulation of actomyosin contractility through changes in RhoA and septin signaling. Additionally, it was shown that HAX1 does not influence invasive potential in the breast cancer cell line, suggesting that its role in breast cancer progression may be linked instead to collective invasion of the epithelial cells but not single-cell dissemination.

Neurological Disease Modelling for Spinocerebellar Ataxia Using Zebrafish

The cerebellum integrates sensory information and motor actions. Increasing experimental evidence has revealed that these functions as well as the cerebellar cytoarchitecture are highly conserved in zebrafish compared with mammals. However, the potential of zebrafish for modelling human cerebellar diseases remains to be addressed. Spinocerebellar ataxias (SCAs) represent a group of genetically inherited cerebellar diseases leading to motor discoordination that is most often caused by affected cerebellar Purkinje cells (PCs). Towards modelling SCAs in zebrafish we identified a small-sized PC-specific regulatory element that was used to develop coexpression vectors with tunable expression strength. These vectors allow for in vivo imaging of SCA-affected PCs by high-resolution fluorescence imaging. Next, zebrafish with SCA type 13 (SCA13) transgene expression were established, revealing that SCA13-induced cell-autonomous PC degeneration results in eye movement deficits. Thus, SCA13 zebrafish mimic the neuropathology of an SCA-affected brain as well as the involved loss of motor control and hence provide a powerful approach to unravel SCA13-induced cell biological pathogenic and cytotoxic mechanisms.

Hematopoietic-substrate-1 associated protein X-1 (HAX-1) regulates liver cancer cells growth, metastasis, and angiogenesis through Akt

The aim of this study was to investigate the effects and mechanisms of hematopoietic-substrate-1-associated protein X-1 (HAX-1) on liver cancer cells. Information on HAX-1 from liver cancer patients was analyzed by the Cancer Genome Atlas (TCGA) program. Cell migration and invasion abilities were respectively tested by scratch assay and transwell assay. Tube formation assay was applied to detect angiogenesis protein and mRNA was determined using quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot. We found that the median month survival of HAX-1 overexpressing liver cancer patients was shorter than that of HAX-1 normal liver cancer patients. HAX-1 was overexpressed in liver cancer tissues and cells, and HAX-1 overexpression promoted the liver cancer cells growth, migration, and invasion, whereas silencing HAX-1 produced the opposite results. Inhibition of Akt by LY294002 reversed the migration and invasion abilities of liver cancer cells, and inhibited the ability of cells growth and angiogenesis. Silencing PIK3CA enhanced the inhibitory effects of HAX-1 silencing on the viability, migration, and invasion of liver cancer cells. HAX-1 affected liver cancer cells metastasis and angiogenesis by affecting Akt phosphorylation and FOXO3A expression.

Cytoskeleton disruption affects Kv2.1 channel function and its modulation by PIP2

Voltage-gated potassium channels are expressed in a wide variety of excitable and non-excitable cells and regulate numerous cellular functions. The activity of ion channels can be modulated by direct interaction or/and functional coupling with other proteins including auxiliary subunits, scaffold proteins and the cytoskeleton. Here, we evaluated the influence of the actin-based cytoskeleton on the Kv2.1 channel using pharmacological and electrophysiological methods. We found that disruption of the actin-based cytoskeleton by latrunculin B resulted in the regulation of the Kv2.1 inactivation mechanism; it shifted the voltage of half-maximal inactivation toward negative potentials by approximately 15 mV, accelerated the rate of closed-state inactivation, and delayed the recovery rate from inactivation. The actin cytoskeleton stabilizing agent phalloidin prevented the hyperpolarizing shift in the half-maximal inactivation potential when co-applied with latrunculin B. Additionally, PIP2 depletion (a strategy that regulates Kv2.1 inactivation) after cytoskeleton disruption does not regulate further the inactivation of Kv2.1, which suggests that both factors could be regulating the Kv2.1 channel by a common mechanism. In summary, our results suggest a role for the actin-based cytoskeleton in regulating Kv2.1 channels.

Vector Analysis of Cytoskeletal Structural Tension and the Mechanisms that Underpin Spectrin-Related Forces in Pyroptosis

Aims: Pyroptotic cells are characterized by plasma swelling, membrane blebbing, and disintegration of the cell membrane mediated by spectrin-based membrane skeleton and intercellular competitive tension activities. The spectrin-based membrane skeleton is involved in membrane organization through the regulation of intercellular tension. Using genetically encoded tension sensors to attain noninvasive force measurements in structural proteins, we investigated how cytoskeletal structural tension influences changes in plasma morphology during pyroptosis and the regulatory mechanism of cytoskeletal structural tension that underpins pyroptosis. Results: The results indicate that increasing spectrin tension is caused by osmotic swelling. Hightened tension of spectrin was closely associated with the shrink tension transmitted synergistically by microfilaments (MFs) and microtubules (MTs). However, the increment of spectrin tension in pyroptotic cells was controlled antagonistically by MF and MT forces. Different from MF tension, outward MT forces participated in the formation of membrane blebs. Spectrin tension caused by inward MF forces resisted pyroptosis swelling. Stabilization of MF and MT structure had little influence on intracellular tension and pyroptosis deformation. Pyroptosis-induced cytoskeletal structural tension was highly dependent on calcium signaling and reactive oxygen species generation. Blocking of membrane pores, nonselective ion flux, or elimination of caspase-1 cleavage resulted in the remission of structural forces associated with pyroptosis failure. Innovation and Conclusions: The data suggest that subcellular tension, in terms of magnitude and vector, is integral to pyroptosis through the mediation of swelling and blebbing and the elimination of structural tension, especially MT forces, may result in pyroptosis inhibition.

Cell Responses to Extracellular α-Synuclein

Synucleins are small naturally unfolded proteins involved in neurodegenerative diseases and cancer. The family contains three members: α-, β-, and -synuclein. α-Synuclein is the most thoroughly investigated because of its close association with Parkinson's disease (PD), dementia with Lewy bodies and multiple system atrophy. Until recently, the synuclein's research was mainly focused on their intracellular forms. However, new studies highlighted the important role of extracellular synucleins. Extracellular forms of synucleins propagate between various types of cells, bind to cell surface receptors and transmit signals, regulating numerous intracellular processes. Here we give an update of the latest results about the mechanisms of action of extracellular synucleins, their binding to cell surface receptors, effect on biochemical pathways and the role in neurodegeneration and neuroinflammation.

Balanced Activity between Kv3 and Nav Channels Determines Fast-Spiking in Mammalian Central Neurons

Fast-spiking (FS) neurons can fire action potentials (APs) up to 1,000 Hz and play key roles in vital functions such as sound location, motor coordination, and cognition. Here we report that the concerted actions of Kv3 voltage-gated K⁺ (Kv) and Na⁺ (Nav) channels are sufficient and necessary for inducing and maintaining FS. Voltage-clamp analysis revealed a robust correlation between the Kv3/Nav current ratio and FS. Expressing Kv3 channels alone could convert ∼30%-60% slow-spiking (SS) neurons to FS in culture. In contrast, co-expression of either Nav1.2 or Nav1.6 together with Kv3.1 or Kv3.3, but not alone or with Kv1.2, converted SS to FS with 100% efficiency. Furthermore, RNA-sequencing-based genome-wide analysis revealed that the Kv3/Nav ratio and Kv3 expression levels strongly correlated with the maximal AP frequencies. Therefore, FS is established by the properly balanced activities of Kv3 and Nav channels and could be further fine-tuned by channel biophysical features and localization patterns.

C-terminal proline deletions in KCNC3 cause delayed channel inactivation and an adult-onset progressive SCA13 with spasticity

Mutations in the potassium channel gene KCNC3 (Kv3.3) cause the autosomal dominant neurological disease, spinocerebellar ataxia 13 (SCA13). In this study, we expand the genotype-phenotype repertoire of SCA13 by describing the novel KCNC3 deletion p.Pro583_Pro585del highlighting the allelic heterogeneity observed in SCA13 patients. We characterize adult-onset, progressive clinical symptoms of two afflicted kindred and introduce the symptom of profound spasticity not previously associated with the SCA13 phenotype. We also present molecular and electrophysiological characterizations of the mutant protein in mammalian cell culture. Mechanistically, the p.Pro583_Pro585del protein showed normal membrane trafficking with an altered electrophysiological profile, including slower inactivation and decreased sensitivity to the inactivation-accelerating effects of the actin depolymerizer latrunculin B. Taken together, our results highlight the clinical importance of the intracellular C-terminal portion of Kv3.3 and its association with ion channel function.

Cell type-specific CLIP reveals that NOVA regulates cytoskeleton interactions in motoneurons

Background

Alternative RNA processing plays an essential role in shaping cell identity and connectivity in the central nervous system. This is believed to involve differential regulation of RNA processing in various cell types. However, in vivo study of cell type-specific post-transcriptional regulation has been a challenge. Here, we describe a sensitive and stringent method combining genetics and CLIP (crosslinking and immunoprecipitation) to globally identify regulatory interactions between NOVA and RNA in the mouse spinal cord motoneurons.

Results

We developed a means of undertaking motoneuron-specific CLIP to explore motoneuron-specific protein–RNA interactions relative to studies of the whole spinal cord in mouse. This allowed us to pinpoint differential RNA regulation specific to motoneurons, revealing a major role for NOVA in regulating cytoskeleton interactions in motoneurons. In particular, NOVA specifically promotes the palmitoylated isoform of the cytoskeleton protein Septin 8 in motoneurons, which enhances dendritic arborization.

Conclusions

Our study demonstrates that cell type-specific RNA regulation is important for fine tuning motoneuron physiology and highlights the value of defining RNA processing regulation at single cell type resolution.

Electronic supplementary material

The online version of this article (10.1186/s13059-018-1493-2) contains supplementary material, which is available to authorized users.

Noonan Syndrome-Associated SHP2 Dephosphorylates GluN2B to Regulate NMDA Receptor Function

Hyperactivating mutations in the non-receptor tyrosine phosphatase SHP2 cause Noonan syndrome (NS). NS is associated with cognitive deficits, but how hyperactivation of SHP2 in NS changes neuron function is not well understood. We find that mice bearing an NS-associated SHP2 allele (NS mice) have selectively impaired Schaffer collateral-CA1 NMDA (N-methyl-D-aspartate) receptor (NMDAR)-mediated neurotransmission and that residual NMDAR-mediated currents decay faster in NS mice because of reduced contribution of GluN1:GluN2B diheteromers. Consistent with altered GluN2B function, we identify GluN2B Y1252 as an NS-associated SHP2 substrate both in vitro and in vivo. Mutation of Y1252 does not alter recombinant GluN1:GluN2B receptor kinetics. Instead, phospho-Y1252 binds the actin-regulatory adaptor protein Nck2, and this interaction is required for proper NMDAR function. These results establish SHP2 and Nck2 as NMDAR regulatory proteins and strongly suggest that NMDAR dysfunction contributes to NS cognitive deficits.

Membrane bending by actin polymerization

Actin polymerization provides driving force to aid several types of processes that involve pulling the plasma membrane into the cell, including phagocytosis, cellular entry of large viruses, and endocytosis. In endocytosis, actin polymerization is especially important under conditions of high membrane tension or high turgor pressure. Recent modeling efforts have shown how actin polymerization can give rise to a distribution of forces around the endocytic site, and explored how these forces affect the shape dynamics; experiments have revealed the structure of the endocytic machinery in increasing detail, and demonstrated key feedback interactions between actin assembly and membrane curvature. Here we provide a perspective on these findings and suggest avenues for future research.

Kv3 Channels: Enablers of Rapid Firing, Neurotransmitter Release, and Neuronal Endurance

The intrinsic electrical characteristics of different types of neurons are shaped by the K+ channels they express. From among the more than 70 different K+ channel genes expressed in neurons, Kv3 family voltage-dependent K+ channels are uniquely associated with the ability of certain neurons to fire action potentials and to release neurotransmitter at high rates of up to 1,000 Hz. In general, the four Kv3 channels Kv3.1-Kv3.4 share the property of activating and deactivating rapidly at potentials more positive than other channels. Each Kv3 channel gene can generate multiple protein isoforms, which contribute to the high-frequency firing of neurons such as auditory brain stem neurons, fast-spiking GABAergic interneurons, and Purkinje cells of the cerebellum, and to regulation of neurotransmitter release at the terminals of many neurons. The different Kv3 channels have unique expression patterns and biophysical properties and are regulated in different ways by protein kinases. In this review, we cover the function, localization, and modulation of Kv3 channels and describe how levels and properties of the channels are altered by changes in ongoing neuronal activity. We also cover how the protein-protein interaction of these channels with other proteins affects neuronal functions, and how mutations or abnormal regulation of Kv3 channels are associated with neurological disorders such as ataxias, epilepsies, schizophrenia, and Alzheimer's disease.

Analysis of the expression of HAX-1 gene in human glioma

Glioma, as the most common aggressive malignant tumor in the central nervous system, is still an insurmountable issue in neural diseases. The proliferation and survival mechanism of glioma cells need to be explored further for the development of glioma treatment. Hematopoietic cell-specific protein 1 associated protein X-1 (HAX-1) is well known for its anti-apoptotic effect. It was reported to play an important role in several malignant tumors. However, the effect of HAX-1 in glioma still remains unknown. This study aimed to investigate the expression of HAX-1 in glioma and the correlation between HAX-1 and the clinicopathological characteristics and prognosis of glioma. Quantitative reverse transcription polymerase chain reaction and Western blot analysis showed that HAX-1 was overexpressed in glioma cell lines compared with normal human astrocytes. This trend was confirmed by comparing the expression of HAX-1 in glioma tissues and nontumorous tissues. The study also analyzed the correlation between the expression of HAX-1 and clinicopathological characteristics of glioma and found the expression of HAX-1 to be highly related to the differentiation and World Health Organization stage of glioma tissues. The survival analysis revealed that HAX-1 was an independent prognostic factor. In conclusion, this novel study suggested that the overexpression of HAX-1 might contribute to the malignant progression of glioma.

Local Turgor Pressure Reduction via Channel Clustering

The primary drivers of yeast endocytosis are actin polymerization and curvature-generating proteins, such as clathrin and BAR domain proteins. Previous work has indicated that these factors may not be capable of generating the forces necessary to overcome turgor pressure. Thus local reduction of the turgor pressure, via localized accumulation or activation of solute channels, might facilitate endocytosis. The possible reduction in turgor pressure was calculated numerically, by solving the diffusion equation through a Legendre polynomial expansion. It was found that for a region of increased permeability having radius 45 nm, as few as 60 channels with a spacing of 10 nm could locally decrease the turgor pressure by 50%. We identified a key dimensionless parameter, p = P₁a/D, where P₁ is the increased permeability, a is the radius of the permeable region, and D is the solute diffusion coefficient. When p > 0.44, the turgor pressure is locally reduced by >50%. An approximate analytic theory was used to generate explicit formulas for the turgor pressure reduction in terms of key parameters. These findings may also be relevant to plants, where the mechanisms that allow endocytosis to proceed despite high turgor pressure are largely unknown.

K+ Channel Kv3.4 Is Essential for Axon Growth by Limiting the Influx of Ca2+ into Growth Cones

Membrane excitability in the axonal growth cones of embryonic neurons influences axon growth. Voltage-gated K⁺ (Kv) channels are key factors in controlling membrane excitability, but whether they regulate axon growth remains unclear. Here, we report that Kv3.4 is expressed in the axonal growth cones of embryonic spinal commissural neurons, motoneurons, dorsal root ganglion neurons, retinal ganglion cells, and callosal projection neurons during axon growth. Our in vitro (cultured dorsal spinal neurons of chick embryos) and in vivo (developing chick spinal commissural axons and rat callosal axons) findings demonstrate that knockdown of Kv3.4 by a specific shRNA impedes axon initiation, elongation, pathfinding, and fasciculation. In cultured dorsal spinal neurons, blockade of Kv3.4 by blood depressing substance II suppresses axon growth via an increase in the amplitude and frequency of Ca²⁺ influx through T-type and L-type Ca²⁺ channels. Electrophysiological results show that Kv3.4, the major Kv channel in the axonal growth cones of embryonic dorsal spinal neurons, is activated at more hyperpolarized potentials and inactivated more slowly than it is in postnatal and adult neurons. The opening of Kv3.4 channels effectively reduces growth cone membrane excitability, thereby limiting excessive Ca²⁺ influx at subthreshold potentials or during Ca²⁺-dependent action potentials. Furthermore, excessive Ca²⁺ influx induced by an optogenetic approach also inhibits axon growth. Our findings suggest that Kv3.4 reduces growth cone membrane excitability and maintains [Ca²⁺]_i at an optimal concentration for normal axon growth.SIGNIFICANCE STATEMENT Accumulating evidence supports the idea that impairments in axon growth contribute to many clinical disorders, such as autism spectrum disorders, corpus callosum agenesis, Joubert syndrome, Kallmann syndrome, and horizontal gaze palsy with progressive scoliosis. Membrane excitability in the growth cone, which is mainly controlled by voltage-gated Ca²⁺ (Cav) and K⁺ (Kv) channels, modulates axon growth. The role of Cav channels during axon growth is well understood, but it is unclear whether Kv channels control axon outgrowth by regulating Ca²⁺ influx. This report shows that Kv3.4, which is transiently expressed in the axonal growth cones of many types of embryonic neurons, acts to reduce excessive Ca²⁺ influx through Cav channels and thus permits normal axon outgrowth.

Neurite outgrowth is driven by actin polymerization even in the presence of actin polymerization inhibitors

Actin polymerization is a universal mechanism to drive plasma membrane protrusion in motile cells. One apparent exception to this rule is continuing, or even accelerated outgrowth of neuronal processes in the presence of actin polymerization inhibitors. This fact together with a key role of microtubule dynamics in neurite outgrowth led to the concept that microtubules directly drive plasma membrane protrusion, either in the course of polymerization or motor-driven sliding. Surprisingly, a possibility that unextinguished actin polymerization drives neurite outgrowth in the presence of actin drugs was not explored. We show that cultured hippocampal neurons treated with cytochalasin D or latrunculin B contained dense accumulations of branched actin filaments at ∼50% of neurite tips at all tested drug concentrations (1-10 μM). Actin polymerization was required for neurite outgrowth, because only low concentrations of either inhibitor increased the length and/or a number of neurites, whereas high concentrations inhibited neurite outgrowth. Importantly, neurites undergoing active elongation invariably contained a bright F-actin patch at the tip, whereas actin-depleted neurites never elongated, even though they still contained dynamic microtubules. Stabilization of microtubules by taxol treatment did not stop elongation of cytochalasin d-treated neurites. We conclude that actin polymerization is indispensable for neurite elongation.

Larval application of sodium channel homologous dsRNA restores pyrethroid insecticide susceptibility in a resistant adult mosquito population

BACKGROUND

Mosquitoes host and pass on to humans a variety of disease-causing pathogens such as infectious viruses and other parasitic microorganisms. The emergence and spread of insecticide resistance is threatening the effectiveness of current control measures for common mosquito vector borne diseases, such as malaria, dengue and Zika. Therefore, the emerging resistance to the widely used pyrethroid insecticides is an alarming problem for public health. Herein we demonstrated the use of RNA interference (RNAi) to increase susceptibility of adult mosquitoes to a widely used pyrethroid insecticide.

METHODS

Experiments were performed on a field-collected pyrethroid resistant strain of Ae. aegypti (Rio de Janeiro; RJ). Larvae from the resistant Ae. aegypti population were soaked with double-stranded RNAs (dsRNAs) that correspond either to voltage-gate sodium channel (VGSC), P-glycoprotein, or P450 detoxification genes and reared to adulthood. Adult mortality rates in the presence of various Deltamethrin pyrethroid concentrations were used to assess mosquito insecticide susceptibility.

RESULTS

We characterized the RJ Ae. aegypti strain with regard to its level of resistance to a pyrethroid insecticide and found that it was approximately 6 times more resistant to Deltamethrin compared to the laboratory Rockefeller strain. The RJ strain displayed a higher frequency of Val1016Ile and Phe1534Cys substitutions of the VGSC gene. The resistant strain also displayed a higher basal expression level of VGSC compared to the Rockefeller strain. When dsRNA-treated mosquitoes were subjected to a standard pyrethroid contact bioassay, only dsRNA targeting VGSC increased the adult mortality of the pyrethroid resistant strain. The dsRNA treatment proved effective in increasing adult mosquito susceptibility over a range of pyrethroid concentrations and these results were associated with dsRNA-specific small interfering RNAs in treated adults, and the corresponding specific down regulation of VGSC gene expression level. Finally, we demonstrated that the efficiency of our approach was further improved by 'tiling' along the VGSC gene in order to identify the most potent dsRNA sequences.

CONCLUSIONS

These results demonstrate that dsRNA applied to mosquito larvae retains its biological activity into adulthood. Thus, the RNAi system reported here could be a useful approach to control the widespread insecticide resistance in mosquitoes and other insect vectors of human diseases.

Actin'g against the Ball and Chain

Spinocerebellar ataxia type 13 is a rare autosomal-dominant neurodegenerative disease induced by mutations in the voltage-dependent Kv3.3 potassium channel. Recently in Cell, Zhang et al. (2016) provide new insights into how Arp2/3-dependent actin polymerization modulates both Kv3.3 activity and its ability to stimulate actin polymerization via Hax-1.