The K+ channel, Kv2.1, is apposed to astrocytic processes and is associated with inhibitory postsynaptic membranes in hippocampal and cortical principal neurons and inhibitory interneurons

The Electrically Silent Kv5.1 Subunit Forms Heteromeric Kv2 Channels in Cortical Neurons and Confers Distinct Functional Properties

Voltage-gated K+ channels of the Kv2 family are highly expressed in brain and play dual roles in regulating neuronal excitability and in organizing endoplasmic reticulum-plasma membrane (ER-PM) junctions. Studies in heterologous cells suggest that Kv2.1 and Kv2.2 co-assemble with "electrically silent" KvS subunits to form heterotetrameric channels with distinct biophysical properties, but the prevalence and localization of these channels in native neurons are unknown. Here, using mass spectrometry-based proteomics, we identified five KvS subunits as components of native Kv2.1 channels immunopurified from mouse brain of both sexes, the most abundant being Kv5.1. We found that Kv5.1 co-immunoprecipitates with Kv2.1 and to a lesser extent with Kv2.2 from brain lysates and that Kv5.1 protein levels are decreased by 70% in Kv2.1 knock-out mice and 95% in Kv2.1/Kv2.2 double knock-out mice. RNAscope and immunolabeling revealed that Kv5.1 is prominently expressed in neocortex, where it is detected in a substantial fraction of Kv2.1/Kv2.2 positive neurons in layers 2/3, 5, and 6. At the subcellular level, Kv5.1 protein is coclustered with Kv2.1 and Kv2.2 at presumptive ER-PM junctions on the soma and proximal dendrites of cortical neurons. Moreover, in addition to modifying channel conductance, we found that Kv2/Kv5.1 channels are less phosphorylated and insensitive to RY785, a potent and selective Kv2 channel inhibitor. Together, these findings demonstrate that KvS subunits create multiple Kv2 channel subtypes in brain. Most notably, Kv2/Kv5.1 channels are highly expressed in cortical neurons, where their unique properties likely modulate the critical conducting and nonconducting roles of Kv2 channels.

The Shab family potassium channels are highly enriched at the presynaptic terminals of human neurons

The Shab family voltage-gated K+ channels (i.e., Kv2.1, Kv2.2) are widely expressed in mammalian brain and regulate neuronal action-potential firing. In addition to their canonical functions, the Kv2 proteins help establish direct attachments between plasma membrane and endoplasmic reticulum (ER), also known as ER-plasma membrane junctions. However, the biochemical properties and molecular organization of these ion channels have not yet been described in human neurons. Here, we have performed a systematic analysis of endogenous expression, post-translational modification, and subcellular distribution of the major components of Kv2 complex in neurons derived from human stem cells. We found that both Kv2.1, Kv2.2, and their auxiliary subunit AMIGO1 are significantly upregulated during early neurogenesis, localize at the cell surface, and already begin to assemble with each other. Human Kv2.1 and AMIGO1, but not Kv2.2, undergo substantial post-translational modification including phosphorylation and/or N-linked glycosylation. Acute pharmacological inhibition with Kv2 blockers also revealed their functional activation in human neurons. These proteins formed prominent clusters at cell bodies, dendritic branches, and axon initial segments. Interestingly, a large fraction of them also exhibited considerable accumulation at human presynaptic terminals, where they aggregated with the local ER network. This synaptic localization of Kv2 subunits was primarily restricted to presynaptic regions, as they demonstrated limited enrichment at postsynaptic densities. These results were highly reproducible in multiple stem cell lines used and alternative differentiation protocols tested, confirming that human presynaptic compartments can actively recruit the Shab family K+ ion channels.

A complex of the lipid transport ER proteins TMEM24 and C2CD2 with band 4.1 at cell-cell contacts

Junctions between the ER and plasma membrane (PM) are implicated in calcium homeostasis, non-vesicular lipid transfer, and other cellular functions. Two ER proteins that function both as tethers to the PM via a polybasic C-terminus motif and as phospholipid transporters are brain-enriched TMEM24 (C2CD2L) and its paralog C2CD2. We report that both proteins also form a complex with band 4.1 family members, which in turn bind PM proteins including cell adhesion molecules such as SynCAM 1. This complex enriches TMEM24 and C2CD2 containing ER/PM junctions at sites of cell contacts. Dynamic properties of TMEM24-dependent ER/PM junctions are impacted when band 4.1 is part of the junction, as TMEM24 at cell-adjacent ER/PM junctions is not shed from the PM by calcium rise, unlike TMEM24 at non-cell adjacent junctions. Lipid transport between the ER and the PM by TMEM24 and C2CD2 at sites where cells, including neurons, contact other cells may participate in adaptive responses to cell contact-dependent signaling.

Compartmentalized signaling in the soma: Coordination of electrical and protein kinase A signaling at neuronal ER-plasma membrane junctions

Neuronal information processing depends on converting membrane depolarizations into compartmentalized biochemical signals that can modify neuronal activity and structure. However, our understanding of how neurons translate electrical signals into specific biochemical responses remains limited, especially in the soma where gene expression and ion channel function are crucial for neuronal activity. Here, I emphasize the importance of physically compartmentalizing action potential-triggered biochemical reactions within the soma. Emerging evidence suggests that somatic endoplasmic reticulum-plasma membrane (ER-PM) junctions are specialized organelles that coordinate electrical and biochemical signaling. The juxtaposition of ion channels and signaling proteins at a prominent subset of these sites enables compartmentalized calcium and cAMP-dependent protein kinase (PKA) signaling. I explore the hypothesis that these PKA-containing ER-PM junctions serve as critical sites for translating membrane depolarizations into PKA signals and identify key gaps in knowledge of the assembly, regulation, and neurobiological functions of this somatic signaling system.

KCNB1-Leptin receptor complexes couple electric and endocrine function in the melanocortin neurons of the hypothalamus

The neurons of the melanocortin system regulate feeding and energy homeostasis through a combination of electrical and endocrine mechanisms. However, the molecular basis for this functional heterogeneity is poorly understood. Here, a voltage-gated potassium (Kv+) channel named KCNB1 (alias Kv2.1) forms stable complexes with the leptin receptor (LepR) in a subset of hypothalamic neurons including proopiomelanocortin (POMC) expressing neurons of the Arcuate nucleus (ARHPOMC). Mice lacking functional KCNB1 channels (NULL mice) have less adipose tissue and circulating leptin than WT animals and are insensitive to anorexic stimuli induced by leptin administration. NULL mice produce aberrant amounts of POMC at any developmental stage. Canonical LepR-STAT3 signaling-which underlies POMC production-is impaired, whereas non-canonical insulin receptor substrate PI3K/Akt/FOXO1 and ERK signaling are constitutively upregulated in NULL hypothalami. The levels of proto-oncogene c-Fos-that provides an indirect measure of neuronal activity-are higher in arcuate NULL neurons compared to WT and most importantly do not increase in the former upon leptin stimulation. Hence, a Kv channel provides a molecular link between neuronal excitability and endocrine function in hypothalamic neurons.

A novel loss-of-function KCNB1 gene variant in a twin with global developmental delay and seizures

Human voltage-gated potassium (Kv) channels are expressed by a 40-member gene family that is essential for normal electrical activity and is closely linked to various excitability disorders. Function-altering sequence variants in the KCNB1 gene, which encodes the neuronally expressed Kv2.1 channel, are associated with neurodevelopmental disorders including developmental delay with or without epileptic activity. In this study, we describe a 40-month-old fraternal twin who presented with severe neurodevelopmental delay. Electroencephalogram recordings at 19 months of age revealed poor sleep architecture and the presence of multifocal epileptiform discharges. The individual's fraternal twin was neurotypical, and there was no family history of neurodevelopmental delay or seizures. Whole genome sequencing at 33 months of age for the proband revealed a de novo variant in KCNB1 [c.1154C > T/p.Pro385Leu], encoding a proline-to-leucine substitution at residue 385, in the extracellular region immediately preceding Kv2.1 transmembrane segment 6 (S6). Cellular electrophysiological analysis of the effects of the gene variant in heterologously expressed Kv2.1 demonstrated that homozygous Kv2.1-P385L channels were completely non-functional. Channels generated by a 50/50 expression of wild-type Kv2.1 and Kv2.1-P385L, designed to mimic the proband's heterozygous status, revealed a partially dominant-negative effect, resulting in an 81% reduction in current magnitude. The dramatic loss of function in Kv2.1 is the most likely cause of the severe developmental delay and seizure activity in the proband, further enriching our phenotypic understanding of KCNB1 developmental encephalopathies.

GLT-1a glutamate transporter nanocluster localization is associated with astrocytic actin and neuronal Kv2 clusters at sites of neuron-astrocyte contact

Introduction: Astrocytic GLT-1 glutamate transporters ensure the fidelity of glutamic neurotransmission by spatially and temporally limiting glutamate signals. The ability to limit neuronal hyperactivity relies on the localization and diffusion of GLT-1 on the astrocytic surface, however, little is known about the underlying mechanisms. We show that two isoforms of GLT-1, GLT-1a and GLT-1b, form nanoclusters on the surface of transfected astrocytes and HEK-293 cells. Methods: We used both fixed and live cell super-resolution imaging of fluorescent protein and epitope tagged proteins in co-cultures of rat astrocytes and neurons. Immunofluorescence techniques were also used. GLT1 diffusion was assessed via single particle tracking and fluorescence recovery after photobleach (FRAP). Results: We found GLT-1a, but not GLT-1b, nanoclusters concentrated adjacent to actin filaments which was maintained after addition of glutamate. GLT-1a nanocluster concentration near actin filaments was prevented by expression of a cytosolic GLT-1a C-terminus, suggesting the C-terminus is involved in the localization adjacent to cortical actin. Using super-resolution imaging, we show that astrocytic GLT-1a and actin co-localize in net-like structures around neuronal Kv2.1 clusters at points of neuron/astrocyte contact. Conclusion: Overall, these data describe a novel relationship between GLT-1a and cortical actin filaments, which localizes GLT-1a near neuronal structures responsive to ischemic insult.

Distinct cellular expression and subcellular localization of Kv2 voltage-gated K+ channel subtypes in dorsal root ganglion neurons conserved between mice and humans

The distinct organization of Kv2 voltage-gated potassium channels on and near the cell body of brain neurons enables their regulation of action potentials and specialized membrane contact sites. Somatosensory neurons have a pseudounipolar morphology and transmit action potentials from peripheral nerve endings through axons that bifurcate to the spinal cord and the cell body within ganglia including the dorsal root ganglia (DRG). Kv2 channels regulate action potentials in somatosensory neurons, yet little is known about where Kv2 channels are located. Here we define the cellular and subcellular localization of the Kv2 paralogs, Kv2.1 and Kv2.2, in DRG somatosensory neurons with a panel of antibodies, cell markers, and genetically modified mice. We find that relative to spinal cord neurons, DRG neurons have similar levels of detectable Kv2.1, and higher levels of Kv2.2. In older mice, detectable Kv2.2 remains similar while detectable Kv2.1 decreases. Both Kv2 subtypes adopt clustered subcellular patterns that are distinct from central neurons. Most DRG neurons co-express Kv2.1 and Kv2.2, although neuron subpopulations show preferential expression of Kv2.1 or Kv2.2. We find that Kv2 protein expression and subcellular localization is similar between mouse and human DRG neurons. We conclude that the organization of both Kv2 channels is consistent with physiological roles in the somata and stem axons of DRG neurons. The general prevalence of Kv2.2 in DRG as compared to central neurons and the enrichment of Kv2.2 relative to detectable Kv2.1, in older mice, proprioceptors, and axons suggest more widespread roles for Kv2.2 in DRG neurons.

Stress-induced translation of KCNB1 contributes to the enhanced synaptic transmission of the lateral habenula

The lateral habenula (LHb) is a well-established brain region involved in depressive disorders. Synaptic transmission of the LHb neurons is known to be enhanced by stress exposure; however, little is known about genetic modulators within the LHb that respond to stress. Using recently developed molecular profiling methods by phosphorylated ribosome capture, we obtained transcriptome profiles of stress responsive LHb neurons during acute physical stress. Among such genes, we found that KCNB1 (Kv2.1 channel), a delayed rectifier and voltage-gated potassium channel, exhibited increased expression following acute stress exposure. To determine the roles of KCNB1 on LHb neurons during stress, we injected short hairpin RNA (shRNA) against the kcnb1 gene to block its expression prior to stress exposure. We observed that the knockdown of KCNB1 altered the basal firing pattern of LHb neurons. Although KCNB1 blockade did not rescue despair-like behaviors in acute learned helplessness (aLH) animals, we found that KCNB1 knockdown prevented the enhancement of synaptic strength in LHb neuron after stress exposure. This study suggests that KCNB1 may contribute to shape stress responses by regulating basal firing patterns and neurotransmission intensity of LHb neurons.

A complex of the lipid transport ER proteins TMEM24 and C2CD2 with band 4.1 at cell-cell contacts

Junctions between the ER and the plasma membrane (ER/PM junctions) are implicated in calcium homeostasis, non-vesicular lipid transfer and other cellular functions. Two ER proteins that function both as membrane tethers to the PM via a polybasic motif in their C-terminus and as phospholipid transporters are brain-enriched TMEM24 (C2CD2L) and its paralog C2CD2. Based on an unbiased proximity ligation analysis, we found that both proteins can also form a complex with band 4.1 family members, which in turn can bind a variety of plasma membrane proteins including cell adhesion molecules such as SynCAM 1. This complex results in the enrichment of TMEM24 and C2CD2 containing ER/PM junctions at sites of cell contacts. Dynamic properties of TMEM24-dependent ER/PM contacts are impacted when in complex as TMEM24 present at cell adjacent junctions is not shed by calcium rise, unlike TMEM24 at non-cell adjacent junctions. These findings suggest that cell-contact interactions control ER/PM junctions via TMEM24 complexes involving band 4.1 proteins.

The formation of KV2.1 macro-clusters is required for sex-specific differences in L-type CaV1.2 clustering and function in arterial myocytes

In arterial myocytes, the canonical function of voltage-gated CaV1.2 and KV2.1 channels is to induce myocyte contraction and relaxation through their responses to membrane depolarization, respectively. Paradoxically, KV2.1 also plays a sex-specific role by promoting the clustering and activity of CaV1.2 channels. However, the impact of KV2.1 protein organization on CaV1.2 function remains poorly understood. We discovered that KV2.1 forms micro-clusters, which can transform into large macro-clusters when a critical clustering site (S590) in the channel is phosphorylated in arterial myocytes. Notably, female myocytes exhibit greater phosphorylation of S590, and macro-cluster formation compared to males. Contrary to current models, the activity of KV2.1 channels seems unrelated to density or macro-clustering in arterial myocytes. Disrupting the KV2.1 clustering site (KV2.1S590A) eliminated KV2.1 macro-clustering and sex-specific differences in CaV1.2 cluster size and activity. We propose that the degree of KV2.1 clustering tunes CaV1.2 channel function in a sex-specific manner in arterial myocytes.

Neuronal ER-plasma membrane junctions couple excitation to Ca2+-activated PKA signaling

Junctions between the endoplasmic reticulum (ER) and the plasma membrane (PM) are specialized membrane contacts ubiquitous in eukaryotic cells. Concentration of intracellular signaling machinery near ER-PM junctions allows these domains to serve critical roles in lipid and Ca2+ signaling and homeostasis. Subcellular compartmentalization of protein kinase A (PKA) signaling also regulates essential cellular functions, however, no specific association between PKA and ER-PM junctional domains is known. Here, we show that in brain neurons type I PKA is directed to Kv2.1 channel-dependent ER-PM junctional domains via SPHKAP, a type I PKA-specific anchoring protein. SPHKAP association with type I PKA regulatory subunit RI and ER-resident VAP proteins results in the concentration of type I PKA between stacked ER cisternae associated with ER-PM junctions. This ER-associated PKA signalosome enables reciprocal regulation between PKA and Ca2+ signaling machinery to support Ca2+ influx and excitation-transcription coupling. These data reveal that neuronal ER-PM junctions support a receptor-independent form of PKA signaling driven by membrane depolarization and intracellular Ca2+, allowing conversion of information encoded in electrical signals into biochemical changes universally recognized throughout the cell.

The formation of K V 2.1 macro-clusters is required for sex-specific differences in L-type Ca V 1.2 clustering and function in arterial myocytes

In arterial myocytes, the canonical function of voltage-gated Ca V 1.2 and K V 2.1 channels is to induce myocyte contraction and relaxation through their responses to membrane depolarization, respectively. Paradoxically, K V 2.1 also plays a sex-specific role by promoting the clustering and activity of Ca V 1.2 channels. However, the impact of K V 2.1 protein organization on Ca V 1.2 function remains poorly understood. We discovered that K V 2.1 forms micro-clusters, which can transform into large macro-clusters when a critical clustering site (S590) in the channel is phosphorylated in arterial myocytes. Notably, female myocytes exhibit greater phosphorylation of S590, and macro-cluster formation compared to males. Contrary to current models, the activity of K V 2.1 channels seems unrelated to density or macro-clustering in arterial myocytes. Disrupting the K V 2.1 clustering site (K V 2.1 S590A ) eliminated K V 2.1 macro-clustering and sex-specific differences in Ca V 1.2 cluster size and activity. We propose that the degree of K V 2.1 clustering tunes Ca V 1.2 channel function in a sex-specific manner in arterial myocytes.

Domain and cell type-specific immunolocalisation of voltage-gated potassium channels in the mouse striatum

Diverse classes of voltage-gated potassium channels (Kv) are integral to the variety of electrical activity patterns that distinguish different classes of neurons in the brain. A feature of their heterogenous expression patterns is the highly precise manner in which specific cell types target their location within functionally specialised sub-cellular domains. Although Kv expression profiles in cortical brain regions are widely reported, their immunolocalisation in sub-cortical areas such as the striatum, and in associated diseases such as Parkinson's disease (PD), remain less well described. Therefore, the broad aims of this study were to provide a high resolution immunolocalisation analysis of various Kv subtypes within the mouse striatum and assess their potential plasticity in a model of PD. Immunohistochemistry and confocal microscopy revealed that immunoreactivity for Kv1.1, 1.2 and 1.4 overlapped to varying degrees with excitatory and inhibitory axonal marker proteins suggesting these Kv subtypes are targeted to axons innervating striatal medium spiny neurons (MSNs). Immunoreactivity for Kv1.3 strongly overlapped with signal for mitochondrial marker proteins in MSN somata and dendrites. Kv1.5 immunoreactivity was expressed in parvalbumin-immunopositive neurons whereas Kv1.6 was located in cells immunopositive for microglia. Signal for Kv2.1 was concentrated on the somatic and proximal dendritic plasma membrane of MSNs, whilst immunoreactivity for Kv4.2 was targeted to their distal dendritic regions. Finally, striatal Kv2.1 expression, at both the mRNA and protein levels, was decreased in alpha-synuclein overexpressing mice, yet increased in alpha-synuclein knockout mice, compared to wild-type counterparts. The data indicate a variety of Kv expression patterns that are distinctive to the striatum and susceptible to pathology that mirrors PD. Furthermore, these findings advance our understanding of the molecular diversity of various striatal cell types, and potentially have implications for the homeostatic changes of MSN excitability during associated medical conditions such as PD.

Transmembrane water-efflux rate measured by magnetic resonance imaging as a biomarker of the expression of aquaporin-4 in gliomas

The water-selective channel protein aquaporin-4 (AQP4) contributes to the migration and proliferation of gliomas, and to their resistance to therapy. Here we show, in glioma cell cultures, in subcutaneous and orthotopic gliomas in rats, and in glioma tumours in patients, that transmembrane water-efflux rate is a sensitive biomarker of AQP4 expression and can be measured via conventional dynamic-contrast-enhanced magnetic resonance imaging. Water-efflux rates correlated with stages of glioma proliferation as well as with changes in the heterogeneity of intra-tumoural and inter-tumoural AQP4 in rodent and human gliomas following treatment with temozolomide and with the AQP4 inhibitor TGN020. Regions with low water-efflux rates contained higher fractions of stem-like slow-cycling cells and therapy-resistant cells, suggesting that maps of water-efflux rates could be used to identify gliomas that are resistant to therapies.

Endoplasmic Reticulum-Plasma Membrane Junctions as Sites of Depolarization-Induced Ca2+ Signaling in Excitable Cells

Membrane contact sites between endoplasmic reticulum (ER) and plasma membrane (PM), or ER-PM junctions, are found in all eukaryotic cells. In excitable cells they play unique roles in organizing diverse forms of Ca2+ signaling as triggered by membrane depolarization. ER-PM junctions underlie crucial physiological processes such as excitation-contraction coupling, smooth muscle contraction and relaxation, and various forms of activity-dependent signaling and plasticity in neurons. In many cases the structure and molecular composition of ER-PM junctions in excitable cells comprise important regulatory feedback loops linking depolarization-induced Ca2+ signaling at these sites to the regulation of membrane potential. Here, we describe recent findings on physiological roles and molecular composition of native ER-PM junctions in excitable cells. We focus on recent studies that provide new insights into canonical forms of depolarization-induced Ca2+ signaling occurring at junctional triads and dyads of striated muscle, as well as the diversity of ER-PM junctions in these cells and in smooth muscle and neurons.

ER-PM Junctions on GABAergic Interneurons Are Organized by Neuregulin 2/VAP Interactions and Regulated by NMDA Receptors

Neuregulins (NRGs) signal via ErbB receptors to regulate neural development, excitability, synaptic and network activity, and behaviors relevant to psychiatric disorders. Bidirectional signaling between NRG2/ErbB4 and NMDA receptors is thought to homeostatically regulate GABAergic interneurons in response to increased excitatory neurotransmission or elevated extracellular glutamate levels. Unprocessed proNRG2 forms discrete clusters on cell bodies and proximal dendrites that colocalize with the potassium channel Kv2.1 at specialized endoplasmic reticulum-plasma membrane (ER-PM) junctions, and NMDA receptor activation triggers rapid dissociation from ER-PM junctions and ectodomain shedding by ADAM10. Here, we elucidate the mechanistic basis of proNRG2 clustering at ER-PM junctions and its regulation by NMDA receptors. Importantly, we demonstrate that proNRG2 promotes the formation of ER-PM junctions by directly binding the ER-resident membrane tether VAP, like Kv2.1. The proNRG2 intracellular domain harbors two non-canonical, low-affinity sites that cooperatively mediate VAP binding. One of these is a cryptic and phosphorylation-dependent VAP binding motif that is dephosphorylated following NMDA receptor activation, thus revealing how excitatory neurotransmission promotes the dissociation of proNRG2 from ER-PM junctions. Therefore, proNRG2 and Kv2.1 can independently function as VAP-dependent organizers of neuronal ER-PM junctions. Based on these and prior studies, we propose that proNRG2 and Kv2.1 serve as co-regulated downstream effectors of NMDA receptors to homeostatically regulate GABAergic interneurons.

Increased KV2.1 Channel Clustering Underlies the Reduction of Delayed Rectifier K+ Currents in Hippocampal Neurons of the Tg2576 Alzheimer's Disease Mouse

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by the progressive deterioration of cognitive functions. Cortical and hippocampal hyperexcitability intervenes in the pathological derangement of brain activity leading to cognitive decline. As key regulators of neuronal excitability, the voltage-gated K⁺ channels (K_V) might play a crucial role in the AD pathophysiology. Among them, the K_V2.1 channel, the main α subunit mediating the delayed rectifier K⁺ currents (I_DR) and controlling the intrinsic excitability of pyramidal neurons, has been poorly examined in AD. In the present study, we investigated the K_V2.1 protein expression and activity in hippocampal neurons from the Tg2576 mouse, a widely used transgenic model of AD. To this aim we performed whole-cell patch-clamp recordings, Western blotting, and immunofluorescence analyses. Our Western blotting results reveal that K_V2.1 was overexpressed in the hippocampus of 3-month-old Tg2576 mice and in primary hippocampal neurons from Tg2576 mouse embryos compared with the WT counterparts. Electrophysiological experiments unveiled that the whole I_DR were reduced in the Tg2576 primary neurons compared with the WT neurons, and that this reduction was due to the loss of the K_V2.1 current component. Moreover, we found that the reduction of the K_V2.1-mediated currents was due to increased channel clustering, and that glutamate, a stimulus inducing K_V2.1 declustering, was able to restore the I_DR to levels comparable to those of the WT neurons. These findings add new information about the dysregulation of ionic homeostasis in the Tg2576 AD mouse model and identify K_V2.1 as a possible player in the AD-related alterations of neuronal excitability.

Activity-dependent endoplasmic reticulum Ca2+ uptake depends on Kv2.1-mediated endoplasmic reticulum/plasma membrane junctions to promote synaptic transmission

The endoplasmic reticulum (ER) extends throughout the neuron as a continuous organelle, and its dysfunction is associated with several neurological disorders. During electrical activity, the ER takes up Ca²⁺ from the cytosol, which has been shown to support synaptic transmission. This close choreography of ER Ca²⁺ uptake with electrical activity suggests functional coupling of the ER to sources of voltage-gated Ca²⁺ entry through an unknown mechanism. We report that a nonconducting role for Kv2.1 through its ER binding domain is necessary for ER Ca²⁺ uptake during neuronal activity. Loss of Kv2.1 profoundly disables neurotransmitter release without altering presynaptic voltage. This suggests that Kv2.1-mediated signaling hubs play an important neurobiological role in Ca²⁺ handling and synaptic transmission independent of ion conduction.

The endoplasmic reticulum (ER) forms a continuous and dynamic network throughout a neuron, extending from dendrites to axon terminals, and axonal ER dysfunction is implicated in several neurological disorders. In addition, tight junctions between the ER and plasma membrane (PM) are formed by several molecules including Kv2 channels, but the cellular functions of many ER-PM junctions remain unknown. Recently, dynamic Ca²⁺ uptake into the ER during electrical activity was shown to play an essential role in synaptic transmission. Our experiments demonstrate that Kv2.1 channels are necessary for enabling ER Ca²⁺ uptake during electrical activity, as knockdown (KD) of Kv2.1 rendered both the somatic and axonal ER unable to accumulate Ca²⁺ during electrical stimulation. Moreover, our experiments demonstrate that the loss of Kv2.1 in the axon impairs synaptic vesicle fusion during stimulation via a mechanism unrelated to voltage. Thus, our data demonstrate that a nonconducting role of Kv2.1 exists through its binding to the ER protein VAMP-associated protein (VAP), which couples ER Ca²⁺ uptake with electrical activity. Our results further suggest that Kv2.1 has a critical function in neuronal cell biology for Ca²⁺ handling independent of voltage and reveals a critical pathway for maintaining ER lumen Ca²⁺ levels and efficient neurotransmitter release. Taken together, these findings reveal an essential nonclassical role for both Kv2.1 and the ER-PM junctions in synaptic transmission.

High spatial density is associated with non-conducting Kv channels from two families

Ion channels are well known for their ability to regulate the cell membrane potential. However, many ion channels also have functions that do not involve ion conductance. Kv2 channels are one family of ion channels whose non-conducting functions are central to mammalian cell physiology. Kv2.1 and Kv2.2 channels form stable contact sites between the endoplasmic reticulum and plasma membrane via an interaction with endoplasmic reticulum resident proteins. To perform this structural role, Kv2 channels are expressed at extremely high densities on the plasma membranes of many cell types, including central pyramidal neurons, α-motoneurons, and smooth muscle cells. Research from our lab and others has shown that the majority of these plasma membrane Kv2.1 channels do not conduct potassium in response to depolarization. The mechanism of this channel silencing is unknown but is thought to be dependent on channel density in the membrane. Furthermore, the prevalence of a non-conducting population of Kv2.2 channels has not been directly tested. In this work we make improved measurements of the numbers of conducting and non-conducting Kv2.1 channels expressed in HEK293 cells and expand the investigation of non-conducting channels to three additional Kv α-subunits: Kv2.2, Kv1.4, and Kv1.5. By comparing the numbers of gating and conducting channels in individual HEK293 cells, we found that on average, only 50% of both Kv2.1 and Kv2.2 channels conducted potassium and, as previously suggested, that fraction decreased with increased channel density in the plasma membrane. At the highest spatial densities tested, which are comparable with those found at Kv2 clusters in situ, only 20% of Kv2.1 and Kv2.2 channels conducted potassium. We also show for the first time that Kv1.4 and Kv1.5 exhibit density-dependent silencing, suggesting that this phenomenon has an underlying mechanism that is shared by Kv channels from multiple families.

Ion Channel Partnerships: Odd and Not-So-Odd Couples Controlling Neuronal Ion Channel Function

The concerted function of the large number of ion channels expressed in excitable cells, including brain neurons, shapes diverse signaling events by controlling the electrical properties of membranes. It has long been recognized that specific groups of ion channels are functionally coupled in mediating ionic fluxes that impact membrane potential, and that these changes in membrane potential impact ion channel gating. Recent studies have identified distinct sets of ion channels that can also physically and functionally associate to regulate the function of either ion channel partner beyond that afforded by changes in membrane potential alone. Here, we review canonical examples of such ion channel partnerships, in which a Ca²⁺ channel is partnered with a Ca²⁺-activated K⁺ channel to provide a dedicated route for efficient coupling of Ca²⁺ influx to K⁺ channel activation. We also highlight examples of non-canonical ion channel partnerships between Ca²⁺ channels and voltage-gated K⁺ channels that are not intrinsically Ca²⁺ sensitive, but whose partnership nonetheless yields enhanced regulation of one or the other ion channel partner. We also discuss how these ion channel partnerships can be shaped by the subcellular compartments in which they are found and provide perspectives on how recent advances in techniques to identify proteins in close proximity to one another in native cells may lead to an expanded knowledge of other ion channel partnerships.

Regulation of neuronal excitation-transcription coupling by Kv2.1-induced clustering of somatic L-type Ca2+ channels at ER-PM junctions

In hippocampal neurons, gene expression is triggered by electrical activity and Ca²⁺ entry via L-type Cav1.2 channels in a process called excitation–transcription coupling. We identified a domain on the voltage-gated K⁺ channel Kv2.1 that promotes the clustering of L-type Cav1.2 channels at endoplasmic reticulum–plasma membrane junctions in the soma of neurons. Importantly, we discovered by disrupting this domain that the Kv2.1-mediated clustering of Cav1.2 at this somatic microdomain is critical for depolarization-induced excitation–transcription coupling.

In mammalian brain neurons, membrane depolarization leads to voltage-gated Ca²⁺ channel-mediated Ca²⁺ influx that triggers diverse cellular responses, including gene expression, in a process termed excitation–transcription coupling. Neuronal L-type Ca²⁺ channels, which have prominent populations on the soma and distal dendrites of hippocampal neurons, play a privileged role in excitation–transcription coupling. The voltage-gated K⁺ channel Kv2.1 organizes signaling complexes containing the L-type Ca²⁺ channel Cav1.2 at somatic endoplasmic reticulum–plasma membrane junctions. This leads to enhanced clustering of Cav1.2 channels, increasing their activity. However, the downstream consequences of the Kv2.1-mediated regulation of Cav1.2 localization and function on excitation–transcription coupling are not known. Here, we have identified a region between residues 478 to 486 of Kv2.1’s C terminus that mediates the Kv2.1-dependent clustering of Cav1.2. By disrupting this Ca²⁺ channel association domain with either mutations or with a cell-penetrating interfering peptide, we blocked the Kv2.1-mediated clustering of Cav1.2 at endoplasmic reticulum–plasma membrane junctions and the subsequent enhancement of its channel activity and somatic Ca²⁺ signals without affecting the clustering of Kv2.1. These interventions abolished the depolarization-induced and L-type Ca²⁺ channel-dependent phosphorylation of the transcription factor CREB and the subsequent expression of c-Fos in hippocampal neurons. Our findings support a model whereby the Kv2.1-Ca²⁺ channel association domain-mediated clustering of Cav1.2 channels imparts a mechanism to control somatic Ca²⁺ signals that couple neuronal excitation to gene expression.

EVAP: A two-photon imaging tool to study conformational changes in endogenous Kv2 channels in live tissues

A primary goal of molecular physiology is to understand how conformational changes of proteins affect the function of cells, tissues, and organisms. Here, we describe an imaging method for measuring the conformational changes of the voltage sensors of endogenous ion channel proteins within live tissue, without genetic modification. We synthesized GxTX-594, a variant of the peptidyl tarantula toxin guangxitoxin-1E, conjugated to a fluorophore optimal for two-photon excitation imaging through light-scattering tissue. We term this tool EVAP (Endogenous Voltage-sensor Activity Probe). GxTX-594 targets the voltage sensors of Kv2 proteins, which form potassium channels and plasma membrane–endoplasmic reticulum junctions. GxTX-594 dynamically labels Kv2 proteins on cell surfaces in response to voltage stimulation. To interpret dynamic changes in fluorescence intensity, we developed a statistical thermodynamic model that relates the conformational changes of Kv2 voltage sensors to degree of labeling. We used two-photon excitation imaging of rat brain slices to image Kv2 proteins in neurons. We found puncta of GxTX-594 on hippocampal CA1 neurons that responded to voltage stimulation and retain a voltage response roughly similar to heterologously expressed Kv2.1 protein. Our findings show that EVAP imaging methods enable the identification of conformational changes of endogenous Kv2 voltage sensors in tissue.

Kcns3 deficiency disrupts Parvalbumin neuron physiology in mouse prefrontal cortex: Implications for the pathophysiology of schizophrenia

The unique fast spiking (FS) phenotype of cortical parvalbumin-positive (PV) neurons depends on the expression of multiple subtypes of voltage-gated potassium channels (Kv). PV neurons selectively express Kcns3, the gene encoding Kv9.3 subunits, suggesting that Kcns3 expression is critical for the FS phenotype. KCNS3 expression is lower in PV neurons in the neocortex of subjects with schizophrenia, but the effects of this alteration are unclear, because Kv9.3 subunit function is poorly understood. Therefore, to assess the role of Kv9.3 subunits in PV neuron function, we combined gene expression analyses, computational modeling, and electrophysiology in acute slices from the cortex of Kcns3-deficient mice. Kcns3 mRNA levels were ~ 50% lower in cortical PV neurons from Kcns3-deficient relative to wildtype mice. While silent per se, Kv9.3 subunits are believed to amplify the Kv2.1 current in Kv2.1-Kv9.3 channel complexes. Hence, to assess the consequences of reducing Kv9.3 levels, we simulated the effects of decreasing the Kv2.1-mediated current in a computational model. The FS cell model with reduced Kv2.1 produced spike trains with irregular inter-spike intervals, or stuttering, and greater Na+ channel inactivation. As in the computational model, PV basket cells (PVBCs) from Kcns3-deficient mice displayed spike trains with strong stuttering, which depressed PVBC firing. Moreover, Kcns3 deficiency impaired the recruitment of PVBC firing at gamma frequency by stimuli mimicking synaptic input observed during cortical UP states. Our data indicate that Kv9.3 subunits are critical for PVBC physiology and suggest that KCNS3 deficiency in schizophrenia could impair PV neuron firing, possibly contributing to deficits in cortical gamma oscillations in the illness.

Kv2 channel-AMIGO β-subunit assembly modulates both channel function and cell adhesion molecule surface trafficking

The Kv2 channels encode delayed rectifier currents that regulate membrane potential in many tissues. They also have a non-conducting function to form stable junctions between the endoplasmic reticulum and plasma membranes, creating membrane contact sites that mediate functions distinct from membrane excitability. Therefore, proteins that interact with Kv2.1 and Kv2.2 channels can alter conducting and/or non-conducting channel properties. One member of the AMIGO family of proteins is an auxiliary β-subunit for Kv2 channels and modulates Kv2.1 electrical activity. However, the AMIGO family has two additional members of ∼50% similarity that have not yet been characterized as Kv2 β-subunits. In this work, we show that the surface trafficking and localization of all three AMIGOs are controlled by their assembly with both Kv2 channels. Additionally, assembly of each AMIGO with either Kv2.1 or Kv2.2 hyperpolarizes the channel activation midpoint by -10 mV. However, only AMIGO2 significantly slows inactivation and deactivation, leading to a prolonged open state of Kv2 channels. The co-regulatory effects of Kv2s and AMIGOs likely fine-tune both the electrical and non-electrical properties of the cells in which they are expressed.

Current Pharmacologic Strategies for Treatment of Intractable Epilepsy in Children

Epileptic encephalopathy (EE) is a devastating pediatric disease that features medically resistant seizures, which can contribute to global developmental delays. Despite technological advancements in genetics, the neurobiological mechanisms of EEs are not fully understood, leaving few therapeutic options for affected patients. In this review, we introduce the most common EEs in pediatrics (i.e., Ohtahara syndrome, Dravet syndrome, and Lennox-Gastaut syndrome) and their molecular mechanisms that cause excitation/inhibition imbalances. We then discuss some of the essential molecules that are frequently dysregulated in EEs. Specifically, we explore voltage-gated ion channels, synaptic transmission-related proteins, and ligand-gated ion channels in association with the pathophysiology of Ohtahara syndrome, Dravet syndrome, and Lennox-Gastaut syndrome. Finally, we review currently available antiepileptic drugs used to treat seizures in patients with EEs. Since these patients often fail to achieve seizure relief even with the combination therapy, further extensive research efforts to explore the involved molecular mechanisms will be required to develop new drugs for patients with intractable epilepsy.

Fluorescent toxins as ion channel activity sensors

Voltage gated ion channels (VGICs) shape the electrical character of cells by undergoing structural changes in response to membrane depolarization. High-resolution techniques have provided a wealth of data on individual VGIC structures, but the conformational changes of endogenous channels in live cell membranes have remained unexplored. Here, we describe methods for imaging structural changes of voltage-gated K⁺ channels in living cells, using peptidyl toxins labeled with fluorophores that report specific protein conformations. These Endogenous Voltage-sensor Activity Probes (EVAPs) enable study of both VGIC allostery and function in the context of endogenous live-cell membranes under different physiological states. In this chapter, we describe methods for the synthesis, imaging, and analysis of dynamic EVAPs, which can report K⁺ channel activity in complex tissue preparations via 2-photon excitation microscopy, and environment-sensitive EVAPs, which report voltage-dependent conformational changes at the VGIC-toxin interface. The methods here present the utility of current EVAPs and lay the groundwork for the development of other probes that act by similar mechanisms. EVAPs can be correlated with electrophysiology, offering insight into the molecular details of endogenous channel function and allostery in live cells. This enables investigation of conformational changes of channels in their native, functional states, putting structures and models into a context of live-cell membranes. The expansive array of state-dependent ligands and optical probes should enable probes more generally for investigating the molecular motions of endogenous proteins.

ER-PM Contact Sites - SNARING Actors in Emerging Functions

The compartmentalisation achieved by confining cytoplasm into membrane-enclosed organelles in eukaryotic cells is essential for maintaining vital functions including ATP production, synthetic and degradative pathways. While intracellular organelles are highly specialised in these functions, the restricting membranes also impede exchange of molecules responsible for the synchronised and responsive cellular activities. The initial identification of contact sites between the ER and plasma membrane (PM) provided a potential candidate structure for communication between organelles without mixing by fusion. Over the past decades, research has revealed a far broader picture of the events. Membrane contact sites (MCSs) have been recognized as increasingly important actors in cell differentiation, plasticity and maintenance, and, upon dysfunction, responsible for pathological conditions such as cancer and neurodegenerative diseases. Present in multiple organelles and cell types, MCSs promote transport of lipids and Ca²⁺ homoeostasis, with a range of associated protein families. Interestingly, each MCS displays a unique molecular signature, adapted to organelle functions. This review will explore the literature describing the molecular components and interactions taking place at ER-PM contact sites, their functions, and implications in eukaryotic cells, particularly neurons, with emphasis on lipid transfer proteins and emerging function of SNAREs.

Epilepsy and neurobehavioral abnormalities in mice with a dominant-negative KCNB1 pathogenic variant

Developmental and epileptic encephalopathies (DEE) are a group of severe epilepsies that usually present with intractable seizures, developmental delay, and often have elevated risk for premature mortality. Numerous genes have been identified as a monogenic cause of DEE, including KCNB1. The voltage-gated potassium channel K_v2.1, encoded by KCNB1, is primarily responsible for delayed rectifier potassium currents that are important regulators of excitability in electrically excitable cells, including neurons. In addition to its canonical role as a voltage-gated potassium conductance, K_v2.1 also serves a highly conserved structural function organizing endoplasmic reticulum-plasma membrane junctions clustered in the soma and proximal dendrites of neurons. The de novo pathogenic variant KCNB1-p.G379R was identified in an infant with epileptic spasms, and atonic, focal and tonic-clonic seizures that were refractory to treatment with standard antiepileptic drugs. Previous work demonstrated deficits in potassium conductance, but did not assess non-conducting functions. To determine if the G379R variant affected K_v2.1 clustering at endoplasmic reticulum-plasma membrane junctions, K_v2.1-G379R was expressed in HEK293T cells. K_v2.1-G379R expression did not induce formation of endoplasmic reticulum-plasma membrane junctions, and co-expression of K_v2.1-G379R with K_v2.1-wild-type lowered induction of these structures relative to K_v2.1-WT alone, consistent with a dominant negative effect. To model this variant in vivo, we introduced Kcnb1^G379R into mice using CRISPR/Cas9 genome editing. We characterized neuronal expression, neurological and neurobehavioral phenotypes of Kcnb1^G379R/+ (Kcnb1^R/+) and Kcnb1^G379R/G379R (Kcnb1^R/R) mice. Immunohistochemistry studies on brains from Kcnb1^+/+, Kcnb1^R/+ and Kcnb1^R/R mice revealed genotype-dependent differences in the expression levels of K_v2.1 protein, as well as associated K_v2.2 and AMIGO-1 proteins. Kcnb1^R/+ and Kcnb1^R/R mice displayed profound hyperactivity, repetitive behaviors, impulsivity and reduced anxiety. Spontaneous seizures were observed in Kcnb1^R/R mice, as well as seizures induced by exposure to novel environments and/ or handling. Both Kcnb1^R/+ and Kcnb1^R/R mutants were more susceptible to proconvulsant-induced seizures. In addition, both Kcnb1^R/+ and Kcnb1^R/R mice exhibited abnormal interictal EEG activity, including isolated spike and slow waves. Overall, the Kcnb1^G379R mice recapitulate many features observed in individuals with DEE due to pathogenic variants in KCNB1. This new mouse model of KCNB1-associated DEE will be valuable for improving the understanding of the underlying pathophysiology and will provide a valuable tool for the development of therapies to treat this pharmacoresistant DEE.

Endoplasmic reticulum-plasma membrane contacts: Principals of phosphoinositide and calcium signaling

The endoplasmic reticulum (ER) forms an extensive network of membrane contact sites with intra-cellular organelles and the plasma membrane (PM). Interorganelle contacts have vital roles in membrane lipid and ion dynamics. In particular, ER-PM contacts are integral to numerous inter-cellular and intra-cellular signaling pathways including phosphoinositide lipid and calcium signaling, mechanotransduction, metabolic regulation, and cell stress responses. Accordingly, ER-PM contacts serve important signaling functions in excitable cells including neurons and muscle and endocrine cells. This review highlights recent advances in our understanding of the vital roles for ER-PM contacts in phosphoinositide and calcium signaling and how signaling pathways in turn regulate proteins that form and function at ER-PM contacts.

Potassium channels in the neuronal homeostasis and neurodegenerative pathways underlying Alzheimer's disease: An update

With more than 80 subunits, potassium (K⁺) channels represent a group of ion channels showing high degree of diversity and ubiquity. They play important role in the control of membrane depolarization and cell excitability in several tissues, including the brain. Controlling the intracellular and extracellular K⁺ flow in cells, they also modulate the hormone and neurotransmitter release, apoptosis and cell proliferation. It is therefore not surprising that an improper functioning of K⁺ channels in neurons has been associated with pathophysiology of a wide range of neurological disorders, especially Alzheimer's disease (AD). This review aims to give a comprehensive overview of the basic properties and pathophysiological functions of the main classes of K⁺ channels in the context of disease processes, also discussing the progress, challenges and opportunities to develop drugs targeting these channels as potential pharmacological approach for AD treatment.

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.

Kv2.1 mediates spatial and functional coupling of L-type calcium channels and ryanodine receptors in mammalian neurons

The voltage-gated K⁺ channel Kv2.1 serves a major structural role in the soma and proximal dendrites of mammalian brain neurons, tethering the plasma membrane (PM) to endoplasmic reticulum (ER). Although Kv2.1 clustering at neuronal ER-PM junctions (EPJs) is tightly regulated and highly conserved, its function remains unclear. By identifying and evaluating proteins in close spatial proximity to Kv2.1-containing EPJs, we discovered that a significant role of Kv2.1 at EPJs is to promote the clustering and functional coupling of PM L-type Ca²⁺ channels (LTCCs) to ryanodine receptor (RyR) ER Ca²⁺ release channels. Kv2.1 clustering also unexpectedly enhanced LTCC opening at polarized membrane potentials. This enabled Kv2.1-LTCC-RyR triads to generate localized Ca²⁺ release events (i.e., Ca²⁺ sparks) independently of action potentials. Together, these findings uncover a novel mode of LTCC regulation and establish a unique mechanism whereby Kv2.1-associated EPJs provide a molecular platform for localized somatodendritic Ca²⁺ signals in mammalian brain neurons.

Neuronal ER-plasma membrane junctions organized by Kv2-VAP pairing recruit Nir proteins and affect phosphoinositide homeostasis

The association of plasma membrane (PM)-localized voltage-gated potassium (Kv2) channels with endoplasmic reticulum (ER)-localized vesicle-associated membrane protein-associated proteins VAPA and VAPB defines ER-PM junctions in mammalian brain neurons. Here, we used proteomics to identify proteins associated with Kv2/VAP-containing ER-PM junctions. We found that the VAP-interacting membrane-associated phosphatidylinositol (PtdIns) transfer proteins PYK2 N-terminal domain-interacting receptor 2 (Nir2) and Nir3 specifically associate with Kv2.1 complexes. When coexpressed with Kv2.1 and VAPA in HEK293T cells, Nir2 colocalized with cell-surface-conducting and -nonconducting Kv2.1 isoforms. This was enhanced by muscarinic-mediated PtdIns(4,5)P₂ hydrolysis, leading to dynamic recruitment of Nir2 to Kv2.1 clusters. In cultured rat hippocampal neurons, exogenously expressed Nir2 did not strongly colocalize with Kv2.1, unless exogenous VAPA was also expressed, supporting the notion that VAPA mediates the spatial association of Kv2.1 and Nir2. Immunolabeling signals of endogenous Kv2.1, Nir2, and VAP puncta were spatially correlated in cultured neurons. Fluorescence-recovery-after-photobleaching experiments revealed that Kv2.1, VAPA, and Nir2 have comparable turnover rates at ER-PM junctions, suggesting that they form complexes at these sites. Exogenous Kv2.1 expression in HEK293T cells resulted in significant differences in the kinetics of PtdIns(4,5)P₂ recovery following repetitive muscarinic stimulation, with no apparent impact on resting PtdIns(4,5)P₂ or PtdIns(4)P levels. Finally, the brains of Kv2.1-knockout mice had altered composition of PtdIns lipids, suggesting a crucial role for native Kv2.1-containing ER-PM junctions in regulating PtdIns lipid metabolism in brain neurons. These results suggest that ER-PM junctions formed by Kv2 channel-VAP pairing regulate PtdIns lipid homeostasis via VAP-associated PtdIns transfer proteins.

Kv2 channels create endoplasmic reticulum / plasma membrane junctions: a brief history of Kv2 channel subcellular localization

The potassium channels Kv2.1 and Kv2.2 are widely expressed throughout the mammalian brain. Kv2.1 provides the majority of delayed rectifying current in rat hippocampus while both channels are differentially expressed in cortex. Particularly unusual is their neuronal surface localization pattern: while half the channel population is freely-diffusive on the plasma membrane as expected from the generalized Singer & Nicolson fluid mosaic model, the other half localizes into micron-sized clusters on the soma, dendrites, and axon initial segment. These clusters contain hundreds of channels, which for Kv2.1, are largely non-conducting. Competing theories of the mechanism underlying Kv2.1 clustering have included static tethering to being corralled by an actin fence. Now, recent work has demonstrated channel clustering is due to formation of endoplasmic reticulum/plasma membrane (ER/PM) junctions through interaction with ER-resident VAMP-associated proteins (VAPs). Interaction between surface Kv2 channels and ER VAPs groups channels together in clusters. ER/PM junctions play important roles in inter-organelle communication: they regulate ion flux, are involved in lipid transfer, and are sites of endo- and exocytosis. Kv2-induced ER/PM junctions are regulated through phosphorylation of the channel C-terminus which in turn regulates VAP binding, providing a rapid means to create or dismantle these microdomains. In addition, insults such as hypoxia or ischemia disrupt this interaction resulting in ER/PM junction disassembly. Kv2 channels are the only known plasma membrane protein to form regulated, injury sensitive junctions in this manner. Furthermore, it is likely that concentrated VAPs at these microdomains sequester additional interactors whose functions are not yet fully understood.

Lipid transporter TMEM24/C2CD2L is a Ca2+-regulated component of ER-plasma membrane contacts in mammalian neurons

Close appositions between the endoplasmic reticulum (ER) and the plasma membrane (PM) are a general feature of all cells and are abundant in neurons. A function of these appositions is lipid transport between the two adjacent bilayers via tethering proteins that also contain lipid transport modules. However, little is known about the properties and dynamics of these proteins in neurons. Here we focused on TMEM24/C2CD2L, an ER-localized SMP domain containing phospholipid transporter expressed at high levels in the brain, previously shown to be a component of ER-PM contacts in pancreatic β-cells. TMEM24 is enriched in neurons versus glial cells and its levels increase in parallel with neuronal differentiation. It populates ER-PM contacts in resting neurons, but elevations of cytosolic Ca²⁺ mediated by experimental manipulations or spontaneous activity induce its transient redistribution throughout the entire ER. Dissociation of TMEM24 from the plasma membrane is mediated by phosphorylation of an array of sites in the C-terminal region of the protein. These sites are only partially conserved in C2CD2, the paralogue of TMEM24 primarily expressed in nonneuronal tissues, which correspondingly display a much lower sensitivity to Ca²⁺ elevations. ER-PM contacts in neurons are also sites where Kv2 (the major delayed rectifier K⁺ channels in brain) and other PM and ER ion channels are concentrated, raising the possibility of a regulatory feedback mechanism between neuronal excitability and lipid exchange between the ER and the PM.

Remodeling neuronal ER-PM junctions is a conserved nonconducting function of Kv2 plasma membrane ion channels

The endoplasmic reticulum (ER) and plasma membrane (PM) form junctions crucial to ion and lipid signaling and homeostasis. The Kv2.1 ion channel is localized at ER–PM junctions in brain neurons and is unique among PM proteins in its ability to remodel these specialized membrane contact sites. Here, we show that this function is conserved between Kv2.1 and Kv2.2, which differ in their biophysical properties, modulation, and cellular expression. Kv2.2 ER–PM junctions are present at sites deficient in the actin cytoskeleton, and disruption of the actin cytoskeleton affects their spatial organization. Kv2.2-containing ER–PM junctions overlap with those formed by canonical ER–PM tethers. The ability of Kv2 channels to remodel ER–PM junctions is unchanged by point mutations that eliminate their ion conduction but eliminated by point mutations within the Kv2-specific proximal restriction and clustering (PRC) domain that do not impact their ion channel function. The highly conserved PRC domain is sufficient to transfer the ER–PM junction–remodeling function to another PM protein. Last, brain neurons in Kv2 double-knockout mice have altered ER–PM junctions. Together, these findings demonstrate a conserved in vivo function for Kv2 family members in remodeling neuronal ER–PM junctions that is distinct from their canonical role as ion-conducting channels shaping neuronal excitability.

Identification of VAPA and VAPB as Kv2 Channel-Interacting Proteins Defining Endoplasmic Reticulum-Plasma Membrane Junctions in Mammalian Brain Neurons

Membrane contacts between endoplasmic reticulum (ER) and plasma membrane (PM), or ER-PM junctions, are ubiquitous in eukaryotic cells and are platforms for lipid and calcium signaling and homeostasis. Recent studies have revealed proteins crucial to the formation and function of ER-PM junctions in non-neuronal cells, but little is known of the ER-PM junctions prominent in aspiny regions of mammalian brain neurons. The Kv2.1 voltage-gated potassium channel is abundantly clustered at ER-PM junctions in brain neurons and is the first PM protein that functions to organize ER-PM junctions. However, the molecular mechanism whereby Kv2.1 localizes to and remodels these junctions is unknown. We used affinity immunopurification and mass spectrometry-based proteomics on brain samples from male and female WT and Kv2.1 KO mice and identified the resident ER vesicle-associated membrane protein-associated proteins isoforms A and B (VAPA and VAPB) as prominent Kv2.1-associated proteins. Coexpression with Kv2.1 or its paralog Kv2.2 was sufficient to recruit VAPs to ER-PM junctions. Multiplex immunolabeling revealed colocalization of Kv2.1 and Kv2.2 with endogenous VAPs at ER-PM junctions in brain neurons from male and female mice in situ and in cultured rat hippocampal neurons, and KO of VAPA in mammalian cells reduces Kv2.1 clustering. The association of VAPA with Kv2.1 relies on a "two phenylalanines in an acidic tract" (FFAT) binding domain on VAPA and a noncanonical phosphorylation-dependent FFAT motif comprising the Kv2-specific clustering or PRC motif. These results suggest that Kv2.1 localizes to and organizes neuronal ER-PM junctions through an interaction with VAPs.SIGNIFICANCE STATEMENT Our study identified the endoplasmic reticulum (ER) proteins vesicle-associated membrane protein-associated proteins isoforms A and B (VAPA and VAPB) as proteins copurifying with the plasma membrane (PM) Kv2.1 ion channel. We found that expression of Kv2.1 recruits VAPs to ER-PM junctions, specialized membrane contact sites crucial to distinct aspects of cell function. We found endogenous VAPs at Kv2.1-mediated ER-PM junctions in brain neurons and other mammalian cells and that knocking out VAPA expression disrupts Kv2.1 clustering. We identified domains of VAPs and Kv2.1 necessary and sufficient for their association at ER-PM junctions. Our study suggests that Kv2.1 expression in the PM can affect ER-PM junctions via its phosphorylation-dependent association to ER-localized VAPA and VAPB.

Activity-dependent decrease in contact areas between subsurface cisterns and plasma membrane of hippocampal neurons

Subsurface cistern (SSC) in neuronal soma and primary dendrites is a specialized compartment of endoplasmic reticulum (ER) that is in close apposition (10 nm) with the plasma membrane (PM). ER-PM contact areas are thought to be involved in intracellular calcium regulation. Here, structural changes of SSC in hippocampal neurons were examined by electron microscopy upon depolarization with high K⁺ (90 mM) or application of NMDA (50 μM) in rat dissociated cultures as well as organotypic slice cultures. The number and average length of SSC-PM contact areas in neuronal somas significantly decreased within 30 s under excitatory condition. This decrease in SSC-PM contact area progressed with time and was reversible. These results demonstrate a structural decoupling between the SSC and the PM upon stimulation, suggesting that there may be a functional decoupling of the calcium regulation. Because SSC-PM contact areas may mediate calcium influx, the decrease in contact area may protect neurons from calcium overload upon heightened stimulation.

Paeonol promotes hippocampal synaptic transmission: The role of the Kv2.1 potassium channel

Paeonol is a major constituent of the Chinese herb Moutan cortex radices. Recent studies report that paeonol has neuroprotective effects and improves impaired learning and memory. However, its underlying mechanisms by which paeonol contributes to synaptic transmission remain unclear. In this study, we found that paeonol increased the frequency of miniature excitatory postsynaptic currents (mEPSCs) and spontaneous excitatory postsynaptic currents (sEPSCs), but had no effect on the amplitude in rat hippocampal CA1 neurons. Similarly, the acetylcholinesterase (AChE) inhibitor rivastigmine increased the frequency of mEPSCs, but had no effect upon amplitude in rat hippocampal neurons. Rivastigmine also inhibited the delayed outward K⁺ currents in rat hippocampal CA1 neurons, but had no effect in nucleus ambiguus (NA) neurons. The Kv2 blocker guangxitoxin-1E increased the frequency of both mEPSCs and sEPSCs of rat hippocampal CA1 neurons, without affecting their amplitude. Our results suggest that paeonol and rivastigmine enhance spontaneous presynaptic transmitter release, which may be associated with the inhibition of the hippocampal Kv2 current and with therapeutic potential in neurotransmitter deficits found in Alzheimer's disease (AD). Moreover, our data also show that paeonol protects against Aβ_25-35-induced impairment of long-term potentiation (LTP) in mouse hippocampal neurons. However, guangxitoxin-1E failed to potentiate the evoked field excitatory postsynaptic potentials (fEPSPs), LTP and Aβ_25-35-induced impairment of LTP. These results indicate that paeonol may has the potential to improve learning and memory in AD. Interestingly, this effect is not involved in the inhibition of the hippocampal Kv2 current.

Kv2 Ion Channels Determine the Expression and Localization of the Associated AMIGO-1 Cell Adhesion Molecule in Adult Brain Neurons

Voltage-gated K⁺ (Kv) channels play important roles in regulating neuronal excitability. Kv channels comprise four principal α subunits, and transmembrane and/or cytoplasmic auxiliary subunits that modify diverse aspects of channel function. AMIGO-1, which mediates homophilic cell adhesion underlying neurite outgrowth and fasciculation during development, has recently been shown to be an auxiliary subunit of adult brain Kv2.1-containing Kv channels. We show that AMIGO-1 is extensively colocalized with both Kv2.1 and its paralog Kv2.2 in brain neurons across diverse mammals, and that in adult brain, there is no apparent population of AMIGO-1 outside of that colocalized with these Kv2 α subunits. AMIGO-1 is coclustered with Kv2 α subunits at specific plasma membrane (PM) sites associated with hypolemmal subsurface cisternae at neuronal ER:PM junctions. This distinct PM clustering of AMIGO-1 is not observed in brain neurons of mice lacking Kv2 α subunit expression. Moreover, in heterologous cells, coexpression of either Kv2.1 or Kv2.2 is sufficient to drive clustering of the otherwise uniformly expressed AMIGO-1. Kv2 α subunit coexpression also increases biosynthetic intracellular trafficking and PM expression of AMIGO-1 in heterologous cells, and analyses of Kv2.1 and Kv2.2 knockout mice show selective loss of AMIGO-1 expression and localization in neurons lacking the respective Kv2 α subunit. Together, these data suggest that in mammalian brain neurons, AMIGO-1 is exclusively associated with Kv2 α subunits, and that Kv2 α subunits are obligatory in determining the correct pattern of AMIGO-1 expression, PM trafficking and clustering.

Ion Channels in Genetic Epilepsy: From Genes and Mechanisms to Disease-Targeted Therapies

Epilepsy is a common and serious neurologic disease with a strong genetic component. Genetic studies have identified an increasing collection of disease-causing genes. The impact of these genetic discoveries is wide reaching-from precise diagnosis and classification of syndromes to the discovery and validation of new drug targets and the development of disease-targeted therapeutic strategies. About 25% of genes identified in epilepsy encode ion channels. Much of our understanding of disease mechanisms comes from work focused on this class of protein. In this study, we review the genetic, molecular, and physiologic evidence supporting the pathogenic role of a number of different voltage- and ligand-activated ion channels in genetic epilepsy. We also review proposed disease mechanisms for each ion channel and highlight targeted therapeutic strategies.

Clinical features and outcome of 6 new patients carrying de novo KCNB1 gene mutations

Objective

To describe electroclinical features and outcome of 6 patients harboring KCNB1 mutations.

Methods

Clinical, EEG, neuropsychological, and brain MRI data analysis. Targeted next-generation sequencing of a 95 epilepsy gene panel.

Results

The mean age at seizure onset was 11 months. The mean follow-up of 11.3 years documented that 4 patients following an infantile phase of frequent seizures became seizure free; the mean age at seizure offset was 4.25 years. Epilepsy phenotypes comprised West syndrome in 2 patients, infantile-onset unspecified generalized epilepsy, myoclonic and photosensitive eyelid myoclonia epilepsy resembling Jeavons syndrome, Lennox-Gastaut syndrome, and focal epilepsy with prolonged occipital or clonic seizures in each and every one. Five patients had developmental delay prior to seizure onset evolving into severe intellectual disability with absent speech and autistic traits in one and stereotypic hand movements with impulse control disorder in another. The patient with Jeavons syndrome evolved into moderate intellectual disability. Mutations were de novo, 4 missense and 2 nonsense, 5 were novel, and 1 resulted from somatic mosaicism.

Conclusions

KCNB1-related manifestations include a spectrum of infantile-onset generalized or focal seizures whose combination leads to early infantile epileptic encephalopathy including West, Lennox-Gastaut, and Jeavons syndromes. Long-term follow-up highlights that following a stormy phase, seizures subside or cease and treatment may be eased or withdrawn. Cognitive and motor functions are almost always delayed prior to seizure onset and evolve into severe, persistent impairment. Thus, KCNB1 mutations are associated with diffuse brain dysfunction combining seizures, motor, and cognitive impairment.

Trafficking of Kv2.1 Channels to the Axon Initial Segment by a Novel Nonconventional Secretory Pathway

Kv2.1 is a major delayed-rectifier voltage-gated potassium channel widely expressed in neurons of the CNS. Kv2.1 localizes in high-density cell-surface clusters in the soma and proximal dendrites as well as in the axon initial segment (AIS). Given the crucial roles of both of these compartments in integrating signal input and then generating output, this localization of Kv2.1 is ideal for regulating the overall excitability of neurons. Here we used fluorescence recovery after photobleaching imaging, mutagenesis, and pharmacological interventions to investigate the molecular mechanisms that control the localization of Kv2.1 in these two different membrane compartments in cultured rat hippocampal neurons of mixed sex. Our data uncover a unique ability of Kv2.1 channels to use two molecularly distinct trafficking pathways to accomplish this. Somatodendritic Kv2.1 channels are targeted by the conventional secretory pathway, whereas axonal Kv2.1 channels are targeted by a nonconventional trafficking pathway independent of the Golgi apparatus. We further identified a new AIS trafficking motif in the C-terminus of Kv2.1, and show that putative phosphorylation sites in this region are critical for the restricted and clustered localization in the AIS. These results indicate that neurons can regulate the expression and clustering of Kv2.1 in different membrane domains independently by using two distinct localization mechanisms, which would allow neurons to precisely control local membrane excitability.SIGNIFICANCE STATEMENT Our study uncovered a novel mechanism that targets the Kv2.1 voltage-gated potassium channel to two distinct trafficking pathways and two distinct subcellular destinations: the somatodendritic plasma membrane and that of the axon initial segment. We also identified a distinct motif, including putative phosphorylation sites, that is important for the AIS localization. This raises the possibility that the destination of a channel protein can be dynamically regulated via changes in post-translational modification, which would impact the excitability of specific membrane compartments.

Kv2.1 Clustering Contributes to Insulin Exocytosis and Rescues Human β-Cell Dysfunction

Insulin exocytosis is regulated by ion channels that control excitability and Ca²⁺ influx. Channels also play an increasingly appreciated role in microdomain structure. In this study, we examine the mechanism by which the voltage-dependent K⁺ (Kv) channel Kv2.1 (KCNB1) facilitates depolarization-induced exocytosis in INS 832/13 cells and β-cells from human donors with and without type 2 diabetes (T2D). We find that Kv2.1, but not Kv2.2 (KCNB2), forms clusters of 6-12 tetrameric channels at the plasma membrane and facilitates insulin exocytosis. Knockdown of Kv2.1 expression reduces secretory granule targeting to the plasma membrane. Expression of the full-length channel (Kv2.1-wild-type) supports the glucose-dependent recruitment of secretory granules. However, a truncated channel (Kv2.1-ΔC318) that retains electrical function and syntaxin 1A binding, but lacks the ability to form clusters, does not enhance granule recruitment or exocytosis. Expression of KCNB1 appears reduced in T2D islets, and further knockdown of KCNB1 does not inhibit Kv current in T2D β-cells. Upregulation of Kv2.1-wild-type, but not Kv2.1-ΔC318, rescues the exocytotic phenotype in T2D β-cells and increases insulin secretion from T2D islets. Thus, the ability of Kv2.1 to directly facilitate insulin exocytosis depends on channel clustering. Loss of this structural role for the channel might contribute to impaired insulin secretion in diabetes.

Structural Similarities between Neuregulin 1-3 Isoforms Determine Their Subcellular Distribution and Signaling Mode in Central Neurons

The Neuregulin (NRG) family of ErbB ligands is comprised of numerous variants originating from the use of different genes, alternative promoters, and splice variants. NRGs have generally been thought to be transported to axons and presynaptic terminals where they signal via ErbB3/4 receptors in paracrine or juxtacrine mode. However, we recently demonstrated that unprocessed pro-NRG2 accumulates on cell bodies and proximal dendrites, and that NMDAR activity is required for shedding of its ectodomain by metalloproteinases. Here we systematically investigated the subcellular distribution and processing of major NRG isoforms in rat hippocampal neurons. We show that NRG1 isotypes I and II, which like NRG2 are single-pass transmembrane proteins with an Ig-like domain, share the same subcellular distribution and ectodomain shedding properties. We furthermore show that NRG3, like CRD-NRG1, is a dual-pass transmembrane protein that harbors a second transmembrane domain near its amino terminus. Both NRG3 and CRD-NRG1 cluster on axons through juxtacrine interactions with ErbB4 present on GABAergic interneurons. Interestingly, although single-pass NRGs accumulate as unprocessed proforms, axonal puncta of CRD-NRG1 and NRG3 are comprised of processed protein. Mutations of CRD-NRG1 and NRG3 that render them resistant to BACE cleavage, as well as BACE inhibition, result in the loss of axonal puncta and in the accumulation of unprocessed proforms in neuronal soma. Together, these results define two groups of NRGs with distinct membrane topologies and fundamentally different targeting and processing properties in central neurons. The implications of this functional diversity for the regulation of neuronal processes by the NRG/ErbB pathway are discussed.SIGNIFICANCE STATEMENT Numerous Neuregulins (NRGs) are generated through the use of different genes, promoters, and alternative splicing, but the functional significance of this evolutionary conserved diversity remains poorly understood. Here we show that NRGs can be categorized by their membrane topologies. Single-pass NRGs, such as NRG1 Types I/II and NRG2, accumulate as unprocessed proforms on cell bodies, and their ectodomains are shed by metalloproteinases in response to NMDA receptor activation. By contrast, dual-pass CRD-NRG1 and NRG3 are constitutively processed by BACE and accumulate on axons where they interact with ErbB4 in juxtacrine mode. These findings reveal a previously unknown functional relationship between membrane topology, protein processing, and subcellular distribution, and suggest that single- and dual-pass NRGs regulate neuronal functions in fundamentally different ways.

Neuregulin 1-ErbB module in C-bouton synapses on somatic motor neurons: molecular compartmentation and response to peripheral nerve injury

The electric activity of lower motor neurons (MNs) appears to play a role in determining cell-vulnerability in MN diseases. MN excitability is modulated by cholinergic inputs through C-type synaptic boutons, which display an endoplasmic reticulum-related subsurface cistern (SSC) adjacent to the postsynaptic membrane. Besides cholinergic molecules, a constellation of proteins involved in different signal-transduction pathways are clustered at C-type synaptic sites (M2 muscarinic receptors, Kv2.1 potassium channels, Ca2+ activated K+ [SK] channels, and sigma-1 receptors [S1R]), but their collective functional significance so far remains unknown. We have previously suggested that neuregulin-1 (NRG1)/ErbBs-based retrograde signalling occurs at this synapse. To better understand signalling through C-boutons, we performed an analysis of the distribution of C-bouton-associated signalling proteins. We show that within SSC, S1R, Kv2.1 and NRG1 are clustered in highly specific, non-overlapping, microdomains, whereas ErbB2 and ErbB4 are present in the adjacent presynaptic compartment. This organization may define highly ordered and spatially restricted sites for different signal-transduction pathways. SSC associated proteins are disrupted in axotomised MNs together with the activation of microglia, which display a positive chemotactism to C-bouton sites. This indicates that C-bouton associated molecules are also involved in neuroinflammatory signalling in diseased MNs, emerging as new potential therapeutic targets.