ROMK1 (Kir1.1) causes apoptosis and chronic silencing of hippocampal neurons

N-acetyl cysteine turns EPAC activators into potent killers of acute lymphoblastic leukemia cells

Today, the majority of patients with pediatric B cell precursor acute lymphoblastic leukemia (BCP-ALL, hereafter ALL) survive their disease, but many of the survivors suffer from life-limiting late effects of the treatment. ALL develops in the bone marrow, where the cells are exposed to cAMP-generating prostaglandin E2. We have previously identified the cAMP signaling pathway as a putative target for improved efficacy of ALL treatment, based on the ability of cAMP signaling to reduce apoptosis induced by DNA damaging agents. In the present study, we have identified the antioxidant N-acetyl cysteine (NAC) as a powerful modifier of critical events downstream of the cell-permeable cAMP analog 8-(4-chlorophenylthio) adenosine-3', 5'- cyclic monophosphate (8-CPT). Accordingly, we found NAC to turn 8-CPT into a potent killer of ALL cells in vitro both in the presence and absence of DNA damaging treatment. Furthermore, we revealed that NAC in combination with 8-CPT is able to delay the progression of ALL in a xenograft model in NOD-scid IL2Rγnull mice. NAC was shown to rely on the ability of 8-CPT to activate the guanine-nucleotide exchange factor EPAC, and we demonstrated that the ALL cells are killed by apoptosis involving sustained elevated levels of calcium imposed by the combination of the two drugs. Taken together, we propose that 8-CPT in the presence of NAC might be utilized as a novel strategy for treating pediatric ALL patients, and that this powerful combination might be exploited to enhance the therapeutic index of current ALL targeting therapies.

Optogenetic control of neural activity: The biophysics of microbial rhodopsins in neuroscience

Optogenetics, the use of microbial rhodopsins to make the electrical activity of targeted neurons controllable by light, has swept through neuroscience, enabling thousands of scientists to study how specific neuron types contribute to behaviors and pathologies, and how they might serve as novel therapeutic targets. By activating a set of neurons, one can probe what functions they can initiate or sustain, and by silencing a set of neurons, one can probe the functions they are necessary for. We here review the biophysics of these molecules, asking why they became so useful in neuroscience for the study of brain circuitry. We review the history of the field, including early thinking, early experiments, applications of optogenetics, pre-optogenetics targeted neural control tools, and the history of discovering and characterizing microbial rhodopsins. We then review the biophysical attributes of rhodopsins that make them so useful to neuroscience - their classes and structure, their photocycles, their photocurrent magnitudes and kinetics, their action spectra, and their ion selectivity. Our hope is to convey to the reader how specific biophysical properties of these molecules made them especially useful to neuroscientists for a difficult problem - the control of high-speed electrical activity, with great precision and ease, in the brain.

Single-Cell Labeling Strategies to Dissect Neuronal Structures and Local Functions

The brain network consists of ten billion neurons and is the most complex structure in the universe. Understanding the structure of complex brain networks and neuronal functions is one of the main goals of modern neuroscience. Since the seminal invention of Golgi staining, single-cell labeling methods have been among the most potent approaches for dissecting neuronal structures and neural circuits. Furthermore, the development of sparse single-cell transgenic methods has enabled single-cell gene knockout studies to examine the local functions of various genes in neural circuits and synapses. Here, we review non-transgenic single-cell labeling methods and recent advances in transgenic strategies for sparse single neuronal labeling. These methods and strategies will fundamentally contribute to the understanding of brain structure and function.

Intracellular potassium depletion enhances apoptosis induced by staurosporine in cultured trigeminal satellite glial cells

PURPOSE

Satellite glial cells (SGC) surrounding neurons in sensory ganglia can buffer extracellular potassium, regulating the excitability of injured neurons and possibly influencing a shift from acute to neuropathic pain. SGC apoptosis may be a key component in this process. This work evaluated induction or enhancement of apoptosis in cultured trigeminal SGC following changes in intracellular potassium [K]ic.

MATERIALS AND METHODS

We developed SGC primary cultures from rat trigeminal ganglia (TG). Purity of our cultures was confirmed using immunofluorescence and western blot analysis for the presence of the specific marker of SGC, glutamine synthetase (GS). SGC [K]ic was depleted using hypo-osmotic shock and 4 mM bumetanide plus 10 mM ouabain. [K]ic was measured using the K⁺ fluorescent indicator potassium benzofuran isophthalate (PBFI-AM).

RESULTS

SGC tested positive for GS and hypo-osmotic shock induced a significant decrease in [K]ic at every evaluated time. Cells were then incubated for 5 h with either 2 mM staurosporine (STS) or 20 ng/ml of TNF-α and evaluated for early apoptosis and late apoptosis/necrosis by flow cytometry using annexin V and propidium iodide. A significant increase in early apoptosis, from 16 to 38%, was detected in SGC with depleted [K]ic after incubation with STS. In contrast, TNF-α did not increase early apoptosis in normal or [K]ic depleted SGC.

CONCLUSION

Hypo-osmotic shock induced a decrease in intracellular potassium in cultured trigeminal SGC and this enhanced apoptosis induced by STS that is associated with the mitochondrial pathway. These results suggest that K⁺ dysregulation may underlie apoptosis in trigeminal SGC.

Interactions of nanomaterials with ion channels and related mechanisms

The pharmacological potential of nanotechnology, especially in drug delivery and bioengineering, has developed rapidly in recent decades. Ion channels, which are easily targeted by external agents, such as nanomaterials (NMs) and synthetic drugs, due to their unique structures, have attracted increasing attention in the fields of nanotechnology and pharmacology for the treatment of ion channel-related diseases. NMs have significant effects on ion channels, and these effects are manifested in many ways, including changes in ion currents, kinetic characteristics and channel distribution. Subsequently, intracellular ion homeostasis, signalling pathways, and intracellular ion stores are affected, leading to the initiation of a range of biological processes. However, the effect of the interactions of NMs with ion channels is an interesting topic that remains obscure. In this review, we have summarized the recent research progress on the direct and indirect interactions between NMs and ion channels and discussed the related molecular mechanisms, which are crucial to the further development of ion channel-related nanotechnological applications.

Neurocalcin regulates nighttime sleep and arousal in Drosophila

Sleep-like states in diverse organisms can be separated into distinct stages, each with a characteristic arousal threshold. However, the molecular pathways underlying different sleep stages remain unclear. The fruit fly, Drosophila melanogaster, exhibits consolidated sleep during both day and night, with night sleep associated with higher arousal thresholds compared to day sleep. Here we identify a role for the neuronal calcium sensor protein Neurocalcin (NCA) in promoting sleep during the night but not the day by suppressing nocturnal arousal and hyperactivity. We show that both circadian and light-sensing pathways define the temporal window in which NCA promotes sleep. Furthermore, we find that NCA promotes sleep by suppressing synaptic release from a dispersed wake-promoting neural network and demonstrate that the mushroom bodies, a sleep-regulatory center, are a module within this network. Our results advance the understanding of how sleep stages are genetically defined.

Practical Considerations for the Use of DREADD and Other Chemogenetic Receptors to Regulate Neuronal Activity in the Mammalian Brain

Chemogenetics is the process of genetically expressing a macromolecule receptor capable of modulating the activity of the cell in response to selective chemical ligand. This chapter will cover the chemogenetic technologies that are available to date, focusing on the commonly available engineered or otherwise modified ligand-gated ion channels and G-protein-coupled receptors in the context of neuromodulation. First, we will give a brief overview of each chemogenetic approach as well as in vitro/in vivo applications, then we will list their strengths and weaknesses. Finally, we will provide tips for ligand application in each case.Each technology has specific limitations that make them more or less suitable for different applications in neuroscience although we will focus mainly on the most commonly used and versatile family named designer receptors exclusively activated by designer drugs or DREADDs. We here describe the most common cases where these can be implemented and provide tips on how and where these technologies can be applied in the field of neuroscience.

Oxygen-glucose deprivation enhancement of cell death/apoptosis in PC12 cells and hippocampal neurons correlates with changes in neuronal excitatory amino acid neurotransmitter signaling and potassium currents

Neuronal death is a pathophysiological process that is often caused by hypoxia/ischemia. However, the causes of hypoxia/ischemia-induced neuronal death are debated, and additional experimental data are needed to resolve this debate. In the present study, we applied oxygen-glucose deprivation (OGD) to PC12 cells and hippocampal neurons to establish a hypoxia/ischemia model. We evaluated the effects of OGD on cell death/apoptosis and on the levels of two excitatory amino acid neurotransmitters, aspartic acid and glutamic acid, in both hippocampal neurons and the medium used to culture the hippocampal neurons. We also evaluated GluR2 expression in hippocampal neurons as well as the effects of OGD on whole-cell potassium currents in PC12 cells and hippocampal neurons. Our experimental results showed that OGD significantly decreased cell viability and markedly enhanced apoptosis in PC12 cells and hippocampal neurons. OGD treatment for 3 h increased the levels of Asp and Glu in the medium used to culture hippocampal neurons, but decreased both the levels of Asp and Glu and GluR2 expression in hippocampal neurons. Furthermore, OGD altered the electrophysiological properties of voltage-dependent potassium channels in PC12 cells and hippocampal neurons in different ways; OGD decreased the voltage-dependent potassium current in PC12 cells, but increased this current in hippocampal neurons. On the basis of these results, we concluded that OGD enhanced neuronal cell death/apoptosis in addition to altering neuronal excitatory amino acid neurotransmitter signaling and whole-cell voltage-dependent potassium currents.

Genistein inhibition of OGD-induced brain neuron death correlates with its modulation of apoptosis, voltage-gated potassium and sodium currents and glutamate signal pathway

In the present study, we established an in vitro model of hypoxic-ischemia via exposing primary neurons of newborn rats to oxygen-glucose deprivation (OGD) and observing the effects of genistein, a soybean isoflavone, on hypoxic-ischemic neuron viability, apoptosis, voltage-activated potassium (Kv) and sodium (Nav) currents, and glutamate receptor subunits. The results indicated that OGD exposure reduced the viability and increased the apoptosis of brain neurons. Meanwhile, OGD exposure caused changes in the current-voltage curves and current amplitude values of voltage-activated potassium and sodium currents; OGD exposure also decreased GluR2 expression and increased NR2 expression. However, genistein at least partially reversed the effects caused by OGD. The results suggest that hypoxic-ischemia-caused neuronal apoptosis/death is related to an increase in K(+) efflux, a decrease in Na(+) influx, a down-regulation of GluR2, and an up-regulation of NR2. Genistein may exert some neuroprotective effects via the modulation of Kv and Nav currents and the glutamate signal pathway, mediated by GluR2 and NR2.

Oxidation of K(+) Channels in Aging and Neurodegeneration

Reversible regulation of proteins by reactive oxygen species (ROS) is an important mechanism of neuronal plasticity. In particular, ROS have been shown to act as modulatory molecules of ion channels-which are key to neuronal excitability-in several physiological processes. However ROS are also fundamental contributors to aging vulnerability. When the level of excess ROS increases in the cell during aging, DNA is damaged, proteins are oxidized, lipids are degraded and more ROS are produced, all culminating in significant cell injury. From this arose the idea that oxidation of ion channels by ROS is one of the culprits for neuronal aging. Aging-dependent oxidative modification of voltage-gated potassium (K(+)) channels was initially demonstrated in the nematode Caenorhabditis elegans and more recently in the mammalian brain. Specifically, oxidation of the delayed rectifier KCNB1 (Kv2.1) and of Ca(2+)- and voltage sensitive K(+) channels have been established suggesting that their redox sensitivity contributes to altered excitability, progression of healthy aging and of neurodegenerative disease. Here I discuss the implications that oxidation of K(+) channels by ROS may have for normal aging, as well as for neurodegenerative disease.

Calstabin 2: An important regulator for learning and memory in mice

Calstabin2, also named FK506 binding protein 12.6 (FKBP12.6), is a subunit of ryanodine receptor subtype 2 (RyR2) macromolecular complex, which is an intracellular calcium channel and abundant in the brain. Previous studies identified a role of leaky neuronal RyR2 in posttraumatic stress disorder (PTSD). However, the functional role of Calstabin2 in the cognitive function remains unclear. Herein, we used a mouse model of genetic deletion of Calstabin2 to investigate the function of Calstabin2 in cognitive dysfunction. We found that Calstabin2 knockout (KO) mice showed significantly reduced performance in Morris Water Maze (MWM), long-term memory (LTM) contextual fear testing, and rotarod test when compared to wild type (WT) littermates. Indeed, genetic deletion of Calstabin2 reduced long-term potentiation (LTP) at the hippocampal CA3-CA1 connection, increased membrane excitability, and induced RyR2 leak. Finally, we demonstrated that the increase in cytoplasmic calcium activated Ca(2+) dependent potassium currents and led to neuronal apoptosis in KO hippocampal neurons. Thus, these results suggest that neuronal RyR2 Ca(2+) leak due to Calstabin2 deletion contributes to learning deficiency and memory impairment.

Increase in the titer of lentiviral vectors expressing potassium channels by current blockade during viral vector production

Background

High titers of lentiviral vectors are required for the efficient transduction of a gene of interest. During preparation of lentiviral the vectors, the protein of interest is inevitably expressed in the viral vector-producing cells. This expression may affect the production of the lentiviral vector.

Methods

We prepared lentiviral vectors expressing inwardly rectifying potassium channel (Lv-Kir2.1), its dominant-negative form (Lv-Kir-DN), and other K⁺ channels, using the ubiquitously active β-actin and neuron-specific synapsin I promoters.

Results

The titer of Lv-Kir-DN was higher than that of Lv-Kir2.1, suggesting a negative effect of induced K⁺ currents on viral titer. We then blocked Kir2.1 currents with the selective blocker Ba²⁺ during Lv-Kir2.1 production, and obtained about a 5-fold increase in the titer. Higher extracellular K⁺ concentrations increased the titer of Lv-Kir2.1 about 9-fold. With a synapsin I promoter Ba²⁺ increased the titer because of the moderate expression of Kir2.1 channel. Channel blockade also increased the titers of the lentivirus expressing Kv1.4 and TREK channels, but not HERG. The increase in titer correlated with the K⁺ currents generated by the channels expressed.

Conclusion

In the production of lentivirus expressing K⁺ channels, titers are increased by blocking K⁺ currents in the virus-producing cells. This identifies a crucial issue in the production of viruses expressing membrane channels, and should facilitate basic and gene therapeutic research on channelopathies.

Ion channels in the regulation of apoptosis

Apoptosis, a type of genetically controlled cell death, is a fundamental cellular mechanism utilized by multicellular organisms for disposal of cells that are no longer needed or potentially detrimental. Given the crucial role of apoptosis in physiology, deregulation of apoptotic machinery is associated with various diseases as well as abnormalities in development. Acquired resistance to apoptosis represents the common feature of most and perhaps all types of cancer. Therefore, repairing and reactivating apoptosis represents a promising strategy to fight cancer. Accumulated evidence identifies ion channels as essential regulators of apoptosis. However, the contribution of specific ion channels to apoptosis varies greatly depending on cell type, ion channel type and intracellular localization, pathology as well as intracellular signaling pathways involved. Here we discuss the involvement of major types of ion channels in apoptosis regulation. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Chemogenetic tools to interrogate brain functions

Elucidating the roles of neuronal cell types for physiology and behavior is essential for understanding brain functions. Perturbation of neuron electrical activity can be used to probe the causal relationship between neuronal cell types and behavior. New genetically encoded neuron perturbation tools have been developed for remotely controlling neuron function using small molecules that activate engineered receptors that can be targeted to cell types using genetic methods. Here we describe recent progress for approaches using genetically engineered receptors that selectively interact with small molecules. Called "chemogenetics," receptors with diverse cellular functions have been developed that facilitate the selective pharmacological control over a diverse range of cell-signaling processes, including electrical activity, for molecularly defined cell types. These tools have revealed remarkably specific behavioral physiological influences for molecularly defined cell types that are often intermingled with populations having different or even opposite functions.

In vivo expression of a light-activatable potassium channel using unnatural amino acids

Optical control of protein function provides excellent spatial-temporal resolution for studying proteins in situ. Although light-sensitive exogenous proteins and ligands have been used to manipulate neuronal activity, a method for optical control of neuronal proteins using unnatural amino acids (Uaa) in vivo is lacking. Here, we describe the genetic incorporation of a photoreactive Uaa into the pore of an inwardly rectifying potassium channel Kir2.1. The Uaa occluded the pore, rendering the channel nonconducting, and, on brief light illumination, was released to permit outward K(+) current. Expression of this photoinducible inwardly rectifying potassium (PIRK) channel in rat hippocampal neurons created a light-activatable PIRK switch for suppressing neuronal firing. We also expanded the genetic code of mammals to express PIRK channels in embryonic mouse neocortex in vivo and demonstrated a light-activated PIRK current in cortical neurons. These principles could be generally expanded to other proteins expressed in the brain to enable optical regulation.

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

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

Heterologous expression of a glial Kir channel (KCNJ10) in a neuroblastoma spinal cord (NSC-34) cell line

Heterologous expression of Kir channels offers a tool to modulate excitability of neurons which provide insight into Kir channel functions in general. Inwardly-rectifying K+ channels (Kir channels) are potential candidate proteins to hyperpolarize neuronal cell membranes. However, heterologous expression of inwardly-rectifying K+ channels has previously proven to be difficult. This was mainly due to a high toxicity of the respective Kir channel expression. We investigated the putative role of a predominantly glial-expressed, weakly rectifying Kir channel (Kir4.1 channel subunit; KCNJ10) in modulating electrophysiological properties of a motoneuron-like cell culture (NSC-34). Transfection procedures using an EGFP-tagged Kir4.1 protein in this study proved to have no toxic effects on NSC-34 cells. Using whole cell-voltage clamp, a substantial increase of inward rectifying K+ currents as well as hyperpolarization of the cell membrane was observed in Kir4.1-transfected cells. Na+ inward currents, observed in NSC-34 controls, were absent in Kir4.1/EGFP motoneuronal cells. The Kir4.1-transfection did not influence the NaV1.6 sodium channel expression. This study demonstrates the general feasibility of a heterologous expression of a weakly inward-rectifying K+ channel (Kir4.1 subunit) and shows that in vitro overexpression of Kir4.1 shifts electrophysiological properties of neuronal cells to a more glial-like phenotype and may therefore be a candidate tool to dampen excitability of neurons in experimental paradigms.

Adult-specific electrical silencing of pacemaker neurons uncouples molecular clock from circadian outputs

BACKGROUND

Circadian rhythms regulate physiology and behavior through transcriptional feedback loops of clock genes running within specific pacemaker cells. In Drosophila, molecular oscillations in the small ventral lateral neurons (sLNvs) command rhythmic behavior under free-running conditions releasing the neuropeptide PIGMENT DISPERSING FACTOR (PDF) in a circadian fashion. Electrical activity in the sLNvs is also required for behavioral rhythmicity. Yet, how temporal information is transduced into behavior remains unclear.

RESULTS

Here we developed a new tool for temporal control of gene expression to obtain adult-restricted electrical silencing of the PDF circuit, which led to reversible behavioral arrhythmicity. Remarkably, PERIOD (PER) oscillations during the silenced phase remained unaltered, indicating that arrhythmicity is a direct consequence of the silenced activity. Accordingly, circadian axonal remodeling and PDF accumulation were severely affected during the silenced phase.

CONCLUSIONS

Although electrical activity of the sLNvs is not a clock component, it coordinates circuit outputs leading to rhythmic behavior.

Small-molecule modulators of inward rectifier K+ channels: recent advances and future possibilities

Inward rectifier potassium (Kir) channels have been postulated as therapeutic targets for several common disorders including hypertension, cardiac arrhythmias and pain. With few exceptions, however, the small-molecule pharmacology of this family is limited to nonselective cardiovascular and neurologic drugs with off-target activity toward inward rectifiers. Consequently, the actual therapeutic potential and 'drugability' of most Kir channels has not yet been determined experimentally. The purpose of this review is to provide a comprehensive summary of publicly disclosed Kir channel small-molecule modulators and highlight recent targeted drug-discovery efforts toward Kir1.1 and Kir2.1. The review concludes with a brief speculation on how the field of Kir channel pharmacology will develop over the coming years and a discussion of the increasingly important role academic laboratories will play in this progress.

Involvement of the 4-aminopyridine-sensitive transient A-type K+ current in macrophage-induced neuronal injury

Through their capacity to secrete, upon activation, a variety of bioactive molecules, brain macrophages (and resident microglia) play an important role in brain immune and inflammatory responses. To test our hypothesis that activated macrophages induce neuronal injury by enhancing neuronal outward K(+) current, we studied the effects of lipopolysaccharide (LPS)-stimulated human monocyte-derived macrophage (MDM) on neuronal transient A-type K(+) current (I(A)) and resultant neuronal injury in primary rat hippocampal neuronal cultures. Bath application of LPS-stimulated MDM-conditioned media (MCM+) enhanced neuronal I(A) in a concentration-dependent manner. Non-stimulated MCM (MCM-) failed to alter I(A). The enhancement of neuronal I(A) was recapitulated in neurons co-cultured with macrophages. The link of MCM(+)-induced enhancement of I(A) to MCM(+)-associated neuronal injury, as detected by propidium iodide and 4'',6-diamidino-2-phenylindol staining (DAPI) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay, was demonstrated by experimental results showing that addition of I(A) blocker 4-aminopyridine to the cultures protected hippocampal neurons from MCM(+)-induced neuronal injury. Further investigation revealed that glutamate was involved in MCM(+)-induced enhancement of neuronal I(A). These results suggest that during brain inflammation macrophages (and microglia) might mediate neuronal injury via enhancement of neuronal I(A), and that neuronal K(v) channel might be a potential target for the development of therapeutic strategies for some neurodegenerative disorders by which immune and inflammatory responses are believed to be involved in the pathogenesis.

High-throughput screening reveals a small-molecule inhibitor of the renal outer medullary potassium channel and Kir7.1

The renal outer medullary potassium channel (ROMK) is expressed in the kidney tubule and critically regulates sodium and potassium balance. The physiological functions of other inward rectifying K(+) (Kir) channels expressed in the nephron, such as Kir7.1, are less well understood in part due to the lack of selective pharmacological probes targeting inward rectifiers. In an effort to identify Kir channel probes, we performed a fluorescence-based, high-throughput screen (HTS) of 126,009 small molecules for modulators of ROMK function. Several antagonists were identified in the screen. One compound, termed VU590, inhibits ROMK with submicromolar affinity, but has no effect on Kir2.1 or Kir4.1. Low micromolar concentrations inhibit Kir7.1, making VU590 the first small-molecule inhibitor of Kir7.1. Structure-activity relationships of VU590 were defined using small-scale parallel synthesis. Electrophysiological analysis indicates that VU590 is an intracellular pore blocker. VU590 and other compounds identified by HTS will be instrumental in defining Kir channel structure, physiology, and therapeutic potential.

Ion channels to inactivate neurons in Drosophila

Ion channels are the determinants of excitability; therefore, manipulation of their levels and properties provides an opportunity for the investigator to modulate neuronal and circuit function. There are a number of ways to suppress electrical activity in Drosophila neurons, for instance, over-expression of potassium channels (i.e. Shaker Kv1, Shaw Kv3, Kir2.1 and DORK) that are open at resting membrane potential. This will result in increased potassium efflux and membrane hyperpolarisation setting resting membrane potential below the threshold required to fire action potentials. Alternatively over-expression of other channels, pumps or co-transporters that result in a hyperpolarised membrane potential will also prevent firing. Lastly, neurons can be inactivated by, disrupting or reducing the level of functional voltage-gated sodium (Nav1 paralytic) or calcium (Cav2 cacophony) channels that mediate the depolarisation phase of action potentials. Similarly, strategies involving the opposite channel manipulation should allow net depolarisation and hyperexcitation in a given neuron. These changes in ion channel expression can be brought about by the versatile transgenic (i.e. Gal4/UAS based) systems available in Drosophila allowing fine temporal and spatial control of (channel) transgene expression. These systems are making it possible to electrically inactivate (or hyperexcite) any neuron or neural circuit in the fly brain, and much like an exquisite lesion experiment, potentially elucidate whatever interesting behaviour or phenotype each network mediates. These techniques are now being used in Drosophila to reprogram electrical activity of well-defined circuits and bring about robust and easily quantifiable changes in behaviour, allowing different models and hypotheses to be rapidly tested.

Bradykinin inhibits the transient outward K+ current in mouse Schwann cells via the cAMP/PKA pathway

Bradykinin (BK) is an endogenous peptide with diverse biological actions and is considered to be an important mediator of the inflammatory response in both the peripheral and the central nervous systems. BK has attracted recent interest as a potential mediator of K(+) conductance, Cl(-) channels, and Ca(2+)-activated K(+) channels. However, few reports have associated BK with the voltage-gated K(+) current. In this study, we demonstrated that BK suppressed the transient outward potassium current (I(A)) in mouse Schwann cells using whole cell recording techniques. At a concentration of 0.1 muM to 5 muM, BK reversibly inhibited I(A) in a dose-dependent manner with the modulation of steady-state activation and inactivation properties. The effect of BK on I(A) current was abolished after preincubation with a B(2) receptor antagonist but could not be eliminated by B(1) receptor antagonist. Intracellular application of GTP-gammaS induced an irreversible decrease in I(A), and the inhibition of G(s) using NF449 provoked a gradual augmentation in I(A) and eliminated the BK-induced effect on I(A,) while the G(i)/(o) antagonist NF023 did not. The application of forskolin or dibutyryl-cAMP mimicked the inhibitory effect of BK on I(A) and abolished the BK-induced effect on I(A). H-89, an inhibitor of PKA, augmented I(A) amplitude and completely eliminated the BK-induced inhibitory effect on I(A). In contrast, activation of PKC by PMA augmented I(A) amplitude. A cAMP assay revealed that BK significantly increased intracellular cAMP level. It is therefore concluded that BK inhibits the I(A) current in Schwann cells by cAMP/PKA-dependent pathways via activation of the B(2) receptor.

The Kir channel immunoglobulin domain is essential for Kir1.1 (ROMK) thermodynamic stability, trafficking and gating

The renal inward rectifying potassium channel Kir1.1 plays key roles in regulating electrolyte homeostasis and blood pressure. Loss-of-function mutations in the channel cause a life-threatening salt and water balance disorder in infants called antenatal Bartter syndrome (ABS). Of more than 30 ABS mutations identified, approximately half are located in the intracellular domain of the channel. The mechanisms underlying channel dysfunction for most of these mutations are unknown. By mapping intracellular mutations onto an atomic model of Kir1.1, we found that several of these are localized to a phylogenetically ancient immunoglobulin (Ig)-like domain (IgLD) that has not been characterized previously, prompting us to examine this structure in detail. The IgLD is assembled from two beta-pleated sheets packed face-to-face, creating a beta-sheet interface or core, populated by highly conserved side chains. Thermodynamic calculations on computationally mutated channels suggest that IgLD core residues are among the most important residues for determining cytoplasmic domain stability. Consistent with this notion, we show that two ABS mutations (A198T and Y314C) located within the IgLD core impair channel biosynthesis and trafficking in mammalian cells. A fraction of core mutant channels reach the cell surface, but are electrically silent due to closure of the helix-bundle gate. Compensatory mutation-induced rescue of channel function revealed that IgLD core mutants fail to rectify. Our study sheds new light on the pathogenesis of ABS and establishes the IgLD as an essential structure within the Kir channel family.

Flupirtine as neuroprotective add-on therapy in autoimmune optic neuritis

Multiple sclerosis (MS) is a common inflammatory disease of the central nervous system that results in persistent impairment in young adults. During chronic progressive disease stages, there is a strong correlation between neurodegeneration and disability. Current therapies fail to prevent progression of neurological impairment during these disease stages. Flupirtine, a drug approved for oral use in patients suffering from chronic pain, was used in a rat model of autoimmune optic neuritis and significantly increased the survival of retinal ganglion cells, the neurons that form the axons of the optic nerve. When flupirtine was combined with interferon-beta, an established immunomodulatory therapy for MS, visual functions of the animals were improved during the acute phase of optic neuritis. Furthermore, flupirtine protected retinal ganglion cells from degeneration in a noninflammatory animal model of optic nerve transection. Although flupirtine was shown previously to increase neuronal survival by Bcl-2 up-regulation, this mechanism does not appear to play a role in flupirtine-mediated protection of retinal ganglion cells either in vitro or in vivo. Instead, we showed through patch-clamp investigations that the activation of inwardly rectifying potassium channels is involved in flupirtine-mediated neuroprotection. Considering the few side effects reported in patients who receive long-term flupirtine treatment for chronic pain, our results indicate that this drug is an interesting candidate for further evaluation of its neuroprotective potential in MS.

Kv 1.1 is associated with neuronal apoptosis and modulated by protein kinase C in the rat cerebellar granule cell

Previously, we reported that apoptosis of cerebellar granular neurons induced by low-K+ and serum-free (LK-S) was associated with an increase in the A-type K+ channel current (I(A)), and an elevated expression of main alpha-subunit of the I(A) channel, which is known as Kv4.2 and Kv4.3. Here, we show, as assessed by quantitative RT-PCR and whole-cell recording, that besides Kv4.2 and Kv4.3, Kv1.1 is very important for I(A) channel. The expression of Kv1.1 was elevated in the apoptotic neurons, while silencing Kv1.1 expression by siRNA reduced the I(A) amplitude of the apoptotic neuron, and increased neuron viability. Inhibiting Kv1.1 current by dendrotoxin-K evoked a similar effect of reduction of I(A) amplitude and protection of neurons. Applying a protein kinase C (PKC) activator, phorbol ester acetate A (PMA) mimicked the LK-S-induced neuronal apoptotic effect, enhanced the I(A) amplitude and reduced the granule cell viability. The PKC inhibitor, bisindolylmaleimide I and Gö6976 protected the cell against apoptosis induced by LK-S. After silencing the Kv1.1 gene, the effect of PMA on the residual K+ current was reduced significantly. Quantitative RT-PCR and Western immunoblot techniques revealed that LK-S treatment and PMA increased the level of the expression of Kv1.1, in contrast, bisindolylmaleimide I inhibited Kv1.1 expression. In addition, the activation of the PKC isoform was identified in apoptotic neurons. We thus conclude that in the rat cerebellar granule cell, the I(A) channel associated with apoptotic neurons is encoded mainly by the Kv1.1 gene, and that the PKC pathway promotes neuronal apoptosis by a brief modulation of the I(A) amplitude and a permanent increase in the levels of expression of the Kv1.1 alpha-subunit.

Water and ion channels: crucial in the initiation and progression of apoptosis in central nervous system?

Programmed cell death (PCD), is a highly regulated and sophisticated cellular mechanism that commits cell to isolated death fate. PCD has been implicated in the pathogenesis of numerous neurodegenerative disorders. Countless molecular events underlie this phenomenon, with each playing a crucial role in death commitment. A precedent event, apoptotic volume decrease (AVD), is ubiquitously observed in various forms of PCD induced by different cellular insults. Under physiological conditions, cells when subjected to osmotic fluctuations will undergo regulatory volume increase/decrease (RVI/RVD) to achieve homeostatic balance with neurons in the brain being additionally protected by the blood-brain-barrier. However, during AVD following apoptotic trigger, cell undergoes anistonic shrinkage that involves the loss of water and ions, particularly monovalent ions e.g. K(+), Na(+) and Cl(-). It is worthwhile to concentrate on the molecular implications underlying the loss of these cellular components which posed to be significant and crucial in the successful propagation of the apoptotic signals. Microarray and real-time PCR analyses demonstrated several ion and water channel genes are regulated upon the onset of lactacystin (a proteosomal inhibitor)-mediated apoptosis. A time course study revealed that gene expressions of water and ion channels are being modulated just prior to apoptosis, some of which are aquaporin 4 and 9, potassium channels and chloride channels. In this review, we shall looked into the molecular protein machineries involved in the execution of AVD in the central nervous system (CNS), and focus on the significance of movements of each cellular component in affecting PCD commitment, thus provide some pharmacological advantages in the global apoptotic cell death.

Rational optimization and imaging in vivo of a genetically encoded optical voltage reporter

The hybrid voltage sensor (hVOS) combines membrane-targeted green fluorescent protein and the hydrophobic anion dipicrylamine (DPA) to provide a promising tool for optical recording of electrical activity from genetically defined populations of neurons. However, large fluorescence signals are obtained only at high DPA concentrations (>3 mum) that increase membrane capacitance to a level that suppresses neural activity. Here, we develop a quantitative model of the sensor to guide its optimization and achieved an approximate threefold increase in fractional fluorescence change at a lower DPA concentration of 2 mum. Using this optimized voltage reporter, we perform optical recordings of evoked activity in the Drosophila antennal lobe with millisecond temporal resolution but fail to detect action potentials, presumably because spike initiation and/or propagation are inhibited by the capacitive load added even at reduced DPA membrane densities. We evaluate strategies for potential further improvement of hVOS quantitatively and derive theoretical performance limits for optical voltage reporters in general.

Over-expression of the potassium channel Kir2.3 using the dopamine-1 receptor promoter selectively inhibits striatal neurons

Dysfunction of basal ganglia circuits underlies a variety of movement disorders and neuropsychiatric conditions. Selective control of the electrical activity of striatal outflow pathways by manipulation of ion channel function presents a novel therapeutic approach. Toward this end, we have constructed and studied in vitro an adenoviral gene transfer vector that employs the promoter region of the dopamine-1 receptor to drive expression of the inward rectifier K(+) channel Kir2.3. The use of this neuronal promoter confers cell-type specificity and a physiological level of trans-gene expression in rat primary striatal cultures. The electrophysiological properties were confirmed in transfected human embryonic kidney cells, in which an inwardly-rectifying, Cs(+)-sensitive current was measured by voltage clamp. Current clamp studies of transduced striatal neurons demonstrated an increase in the firing threshold, latency to first action potential and decrease in neuronal excitability. Neurotoxin-induced activation of c-Fos, a marker of neuronal activity, was blocked in transduced neurons indicating that the decrease in electrical excitability was physiologically significant. When used in vivo, this strategy may have the potential to positively impact movement disorders by selectively changing activity of neurons belonging to the direct striatal pathway, characterized by the expression of dopamine-1 receptors.

Gabapentin activates ROMK1 channels by a protein kinase A (PKA)-dependent mechanism

BACKGROUND AND PURPOSE

Gabapentin is an effective anticonvulsant. The major physiological function of renal outer medullary potassium (ROMK1) channels is to maintain the resting membrane potential (RMP). We investigated the effect of gabapentin on ROMK1 channels and the mechanism involved.

EXPERIMENTAL APPROACH

Xenopus oocytes were injected with mRNA coding for wild-type or mutant ROMK1 channels and giant inside-out patch-clamp recordings were performed.

KEY RESULTS

Gabapentin increased the activity of ROMK1 channels, concentration-dependently and enhanced the activity of wild-type and an intracellular pH (pH(i))-gating residue mutant (K80M) channels over a range of pH(i). Gabapentin also increased activity of channels mutated at phosphatidylinositol 4,5-bisphosphate (PIP(2))-binding sites (R188Q, R217A and K218A). However, gabapentin failed to enhance channel activity in the presence of protein kinase A (PKA) inhibitors and did not activate phosphorylation site mutants (S44A, S219A or S313A), mutants that mimicked the negative charge carried by a phosphate group bound to a serine (S44D, S219D or S313D), or a mutated channel with a positive charge (S219R). These findings show that gabapentin activates ROMK1 channels independently of the pH(i) and not via a PIP(2)-dependent pathway. The effects of gabapentin on ROMK1 channels may be due to a PKA-mediated phosphorylation-induced conformational change, but not to charge-charge interactions.

CONCLUSIONS AND IMPLICATIONS

ROMK1 channels are the main channels responsible for maintaining the RMP during cellular excitation. Gabapentin increased the activity of ROMK1 channels by a PKA-dependent mechanism, reducing neuronal excitability, and this may play an important role in its antiepileptic effect.

Reversible silencing of neuronal excitability in behaving mice by a genetically targeted, ivermectin-gated Cl- channel

Several genetic strategies for inhibiting neuronal function in mice have been described, but no system that directly suppresses membrane excitability and is triggered by a systemically administered drug, has been validated in awake behaving animals. We expressed unilaterally in mouse striatum a modified heteromeric ivermectin (IVM)-gated chloride channel from C. elegans (GluClalphabeta), systemically administered IVM, and then assessed amphetamine-induced rotational behavior. Rotation was observed as early as 4 hr after a single intraperitoneal IVM injection (10 mg/kg), reached maximal levels by 12 hr, and was almost fully reversed by 4 days. Multiple cycles of silencing and recovery could be performed in a single animal. In striatal slice preparations from GluClalphabeta-expressing animals, IVM rapidly suppressed spiking. The two-subunit GluCl/IVM system permits "intersectional" strategies designed to increase the cellular specificity of silencing in transgenic animals.

Axons mediate the distribution of arylsulfatase A within the mouse hippocampus upon gene delivery

Axonal transport of the lysosomal enzyme arylsulfatase A (ARSA) may be an additional mechanism of enzyme distribution after in vivo brain gene transfer in an animal model of metachromatic leukodystrophy (MLD). Direct molecular demonstration of the movement of this lysosomal enzyme within axonal networks was missing. We generated lentiviral vectors carrying the ARSA cDNA tagged with hemagglutinin or the green fluorescent protein and examined the subcellular localization and anatomical distribution of the tagged enzymes within the MLD hippocampus after in vivo lentiviral gene transfer. The use of tagged ARSA allowed direct real-time observation and tracking of axon-dendritic transport of the enzyme after lentiviral gene therapy. Tagged ARSA was expressed in transduced pyramidal, granule, and hilar neurons within the lentiviral-injected side and was robustly contained in vesicles within ipsilateral axon-dendritic processes as well as in vesicles associated with contralateral axons and commissural axons of the ventral hippocampal commissure. Axonal transport of tagged ARSA led to the correction of hippocampal defects in long-term treated MLD mice, which was accompanied by enzyme uptake in nontransduced contralateral neurons, enzyme accumulation within the lysosomal compartment, and clearance of sulfatide storage deposits in this region of the MLD brain. These results contribute to the understanding of the mechanisms of distribution of lysosomal enzymes within the mammalian brain after direct gene therapy, demonstrating the use of neural processes for enzyme transport.

Selective and quickly reversible inactivation of mammalian neurons in vivo using the Drosophila allatostatin receptor

Genetic strategies for perturbing activity of selected neurons hold great promise for understanding circuitry and behavior. Several such strategies exist, but there has been no direct demonstration of reversible inactivation of mammalian neurons in vivo. We previously reported quickly reversible inactivation of neurons in vitro using expression of the Drosophila allatostatin receptor (AlstR). Here, adeno-associated viral vectors are used to express AlstR in vivo in cortical and thalamic neurons of rats, ferrets, and monkeys. Application of the receptor's ligand, allatostatin (AL), leads to a dramatic reduction in neural activity, including responses of visual neurons to optimized visual stimuli. Additionally, AL eliminates activity in spinal cords of transgenic mice conditionally expressing AlstR. This reduction occurs selectively in AlstR-expressing neurons. Inactivation can be reversed within minutes upon washout of the ligand and is repeatable, demonstrating that the AlstR/AL system is effective for selective, quick, and reversible silencing of mammalian neurons in vivo.

Comparative gene expression analysis reveals a characteristic molecular profile of the superior olivary complex

The superior olivary complex (SOC) is a very conspicuous structure in the mammalian auditory brainstem. It represents the first binaural processing center and is important for sound localization in the azimuth and in feedback regulation of cochlear function. In order to define molecular determinants of the SOC, which are of potential functional relevance, we have performed a comprehensive analysis of its transcriptome by serial analysis of gene expression in adult rats. Here, we performed a detailed analysis of the SOC's gene expression profile compared to that of two other neural tissues, the striatum and the hippocampus, and with extraocular muscle tissue. This tested the hypothesis that SOC-specific or significantly upregulated transcripts provide candidates for the specific function of auditory neurons. Thirty-three genes were significantly upregulated in the SOC when compared to the two other neural tissues. Thirteen encoded proteins involved in neurotransmission, including action potential propagation, exocytosis, and myelination; five genes are important for the energy metabolism, and five transcripts are unknown or poorly characterized and have yet to be described in the nervous system. The comparison of functional gene classes indicates that the SOC has the highest energy demand of the three neural tissues, yet protein turnover is apparently not increased. This suggests a high energy demand for fueling auditory neurotransmission. Such a demand may have implications on auditory-specific tasks and relate to central auditory processing disorders. Ultimately, these data provide new avenues to foster investigations of auditory function and to advance molecular physiology in the central auditory system.

Rapid and reversible chemical inactivation of synaptic transmission in genetically targeted neurons

Inducible and reversible silencing of selected neurons in vivo is critical to understanding the structure and dynamics of brain circuits. We have developed Molecules for Inactivation of Synaptic Transmission (MISTs) that can be genetically targeted to allow the reversible inactivation of neurotransmitter release. MISTs consist of modified presynaptic proteins that interfere with the synaptic vesicle cycle when crosslinked by small molecule "dimerizers." MISTs based on the vesicle proteins VAMP2/Synaptobrevin and Synaptophysin induced rapid ( approximately 10 min) and reversible block of synaptic transmission in cultured neurons and brain slices. In transgenic mice expressing MISTs selectively in Purkinje neurons, administration of dimerizer reduced learning and performance of the rotarod behavior. MISTs allow for specific, inducible, and reversible lesions in neuronal circuits and may provide treatment of disorders associated with neuronal hyperactivity.

Neurochemical and electrophysiological characteristics of rat striatal neurons in primary culture

Neurons maintained in dispersed primary culture offer a number of advantages as a model system and are particularly well-suited for studies of the intrinsic electrical properties of neurons by patch clamp. We have characterized the immunocytochemical and electrophysiological properties of cultured rat striatal neurons as they develop in vitro in order to compare this model system with the known properties found in vivo. We found a high abundance of cells in vitro corresponding to the principal striatal output neuron, the medium spiny neuron. Immunocytochemical studies indicate that these cells have both dopamine-1 and dopamine-2 receptors and that there is overlap in their expression within the population of neurons. Semiquantitative analysis revealed bimodal distributions of dopamine receptor expression among the population of neurons. The principal peptide neurotransmitters substance P and enkephalin were present but at reduced levels compared with adult preparations. Other striatal markers such as calbindin, calretinin, and the cannabinoid-1 receptor were abundant. An immunocytochemical survey of voltage-gated K(+) channel subunits characteristic of adult tissue demonstrated the presence in vitro of Kv1.1, Kv1.4, Kv4.2, Kv4.3, and Kvbeta1.1, which have been associated with the rapidly inactivating currents. Electrophysiological studies employing voltage clamp revealed that outward currents had a large inactivating (A-type) component characteristic of mature basal ganglia. Current clamp studies reveal complex spontaneous firing patterns in a subset of neurons, including bursting behaviors superimposed on a slow depolarization. The inward rectifying channels Kir2.1 and Kir2.3, which are specific to particular compartments in adult striatum, were present in culture.

Attenuation of apoptosis in enterocytes by blockade of potassium channels

Apoptosis plays an important role in maintaining the balance between proliferation and cell loss in the intestinal epithelium. Apoptosis rates may increase in intestinal pathologies such as inflammatory bowel disease and necrotizing enterocolitis, suggesting pharmacological prevention of apoptosis as a therapy for these conditions. Here, we explore the feasibility of this approach using the rat epithelial cell line IEC-6 as a model. On the basis of the known role of K+ efflux in apoptosis in various cell types, we hypothesized that K+ efflux is essential for apoptosis in enterocytes and that pharmacological blockade of this efflux would inhibit apoptosis. By probing intracellular [K+] with the K+-sensitive fluorescent dye and measuring the efflux of 86Rb+, we found that apoptosis-inducing treatment with the proteasome inhibitor MG-132 leads to a twofold increase in K+ efflux from IEC-6 cells. Blockade of K+ efflux with tetraethylammonium, 4-aminopyridine, stromatoxin, chromanol 293B, and the recently described K+ channel inhibitor 48F10 prevents DNA fragmentation, caspase activation, release of cytochrome c from mitochondria, and loss of mitochondrial membrane potential. Thus K+ efflux occurs early in the apoptotic program and is required for the execution of later events. Apoptotic K+ efflux critically depends on activation of p38 MAPK. These results demonstrate for the first time the requirement of K+ channel-mediated K+ efflux for progression of apoptosis in enterocytes and suggest the use of K+ channel blockers to prevent apoptotic cell loss occurring in intestinal pathologies.

Inhibitory effect of ganglioside GD1b on K+ current in hippocampal neurons and its involvement in apoptosis suppression

Gangliosides are endogenous membrane components enriched in neuronal cells. They have been shown to play regulatory roles in many cellular processes. Here, we show for the first time that ganglioside GD1b plays an antiapoptotic role in cultured hippocampal neurons. GD1b inhibited the voltage-dependent outward delayed rectifier current (I(K)) but not the transient outward A-type current in a dose-dependent manner, with an IC50 value of 15.2 microM. This effect appears to be somehow specific, because GD1b, but not GM1, GM2, GM3, GD1a, GD3, or GT1b, was effective in inhibiting I(K). Intracellular application of staurosporine (STS; 0.1 microM) resulted in rapid activation of I(K), which was partially reversed upon addition of the K+ channel blocker tetraethylammonium (TEA; 5 mM) and GD1b (10 microM). Furthermore, GD1b (10 microM) attenuated STS-induced neuronal apoptosis by nearly the same amount as 5 mM TEA. In addition, GD1b suppressed the apoptosis-associated caspase 3 activation that was activated by STS. Collectively, these findings suggest that GD1b plays an antiapoptotic role in cultured hippocampal neurons through its inhibitory effect on the I(K) and caspase activity.

Nuclear factor-κB activation by reactive oxygen species mediates voltage-gated K+ current enhancement by neurotoxic β-amyloid peptides in nerve growth factor-differentiated PC-12 cells and hippocampal neurones

Increased activity of plasma membrane K+ channels, leading to decreased cytoplasmic K+ concentrations, occurs during neuronal cell death. In the present study, we showed that the neurotoxic beta-amyloid peptide Abeta(25-35) caused a dose-dependent (0.1-10 microm) and time-dependent (> 12 h) enhancement of both inactivating and non-inactivating components of voltage-dependent K+ (VGK) currents in nerve growth factor-differentiated rat phaeochromocytoma (PC-12) cells and primary rat hippocampal neurones. Similar effects were exerted by Abeta(1-42), but not by the non-neurotoxic Abeta(35-25) peptide. Abeta(25-35) and Abeta(1-42) caused an early (15-20 min) increase in intracellular Ca(2+) concentration. This led to an increased production of reactive oxygen species (ROS), which peaked at 3 h and lasted for 24 h; ROS production seemed to trigger the VGK current increase as vitamin E (50 microm) blocked both the Abeta(25-35)- and Abeta(1-42)-induced ROS increase and VGK current enhancement. Inhibition of protein synthesis (cycloheximide, 1 microg/mL) and transcription (actinomycin D, 50 ng/mL) blocked Abeta(25-35)-induced VGK current enhancement, suggesting that this potentiation is mediated by transcriptional activation induced by ROS. Interestingly, the specific nuclear factor-kappaB inhibitor SN-50 (5 microm), but not its inactive analogue SN-50M (5 microm), fully counteracted Abeta(1-42)- or Abeta(25-35)-induced enhancement of VGK currents, providing evidence for a role of this family of transcription factors in regulating neuronal K+ channel function during exposure to Abeta.