Oxidation of NADPH on Kvbeta1 inhibits ball-and-chain type inactivation by restraining the chain

Resurgent current in context: Insights from the structure and function of Na and K channels

Discovered just over 25 years ago in cerebellar Purkinje neurons, resurgent Na current was originally described operationally as a component of voltage-gated Na current that flows upon repolarization from relatively depolarized potentials and speeds recovery from inactivation, increasing excitability. Its presence in many excitable cells and absence from others has raised questions regarding its biophysical and molecular mechanisms. Early studies proposed that Na channels capable of generating resurgent current are subject to a rapid open-channel block by an endogenous blocking protein, which binds upon depolarization and unblocks upon repolarization. Since the time that this mechanism was suggested, many physiological and structural studies of both Na and K channels have revealed aspects of gating and conformational states that provide insights into resurgent current. These include descriptions of domain movements for activation and inactivation, solution of cryo-EM structures with pore-blocking compounds, and identification of native blocking domains, proteins, and modulatory subunits. Such results not only allow the open-channel block hypothesis to be refined but also link it more clearly to research that preceded it. This review considers possible mechanisms for resurgent Na current in the context of earlier and later studies of ion channels and suggests a framework for future research.

Diversification of Potassium Currents in Excitable Cells via Kvβ Proteins

Excitable cells of the nervous and cardiovascular systems depend on an assortment of plasmalemmal potassium channels to control diverse cellular functions. Voltage-gated potassium (Kv) channels are central to the feedback control of membrane excitability in these processes due to their activation by depolarized membrane potentials permitting K⁺ efflux. Accordingly, Kv currents are differentially controlled not only by numerous cellular signaling paradigms that influence channel abundance and shape voltage sensitivity, but also by heteromeric configurations of channel complexes. In this context, we discuss the current knowledge related to how intracellular Kvβ proteins interacting with pore complexes of Shaker-related Kv1 channels may establish a modifiable link between excitability and metabolic state. Past studies in heterologous systems have indicated roles for Kvβ proteins in regulating channel stability, trafficking, subcellular targeting, and gating. More recent works identifying potential in vivo physiologic roles are considered in light of these earlier studies and key gaps in knowledge to be addressed by future research are described.

Myocardial Blood Flow Control by Oxygen Sensing Vascular Kvβ Proteins

[Figure: see text].

The mono-ADP-ribosyltransferase ARTD10 regulates the voltage-gated K+ channel Kv1.1 through protein kinase C delta

Background

ADP-ribosylation is a ubiquitous post-translational modification that involves both mono- and poly-ADP-ribosylation. ARTD10, also known as PARP10, mediates mono-ADP-ribosylation (MARylation) of substrate proteins. A previous screen identified protein kinase C delta (PKCδ) as a potential ARTD10 substrate, among several other kinases. The voltage-gated K⁺ channel Kv1.1 constitutes one of the dominant Kv channels in neurons of the central nervous system and the inactivation properties of Kv1.1 are modulated by PKC. In this study, we addressed the role of ARTD10-PKCδ as a regulator of Kv1.1.

Results

We found that ARTD10 inhibited PKCδ, which increased Kv1.1 current amplitude and the proportion of the inactivating current component in HeLa cells, indicating that ARTD10 regulates Kv1.1 in living cells. An inhibitor of ARTD10, OUL35, significantly decreased peak amplitude together with the proportion of the inactivating current component of Kv1.1-containing channels in primary hippocampal neurons, demonstrating that the ARTD10-PKCδ signaling cascade regulates native Kv1.1. Moreover, we show that the pharmacological blockade of ARTD10 increases excitability of hippocampal neurons.

Conclusions

Our results, for the first time, suggest that MARylation by ARTD10 controls neuronal excitability.

Metabolism and Redox in Pulmonary Vascular Physiology and Pathophysiology

Significance: This review considers how some systems controlling pulmonary vascular function are potentially regulated by redox processes to examine how and why conditions such as prolonged hypoxia, pathological mediators, and other factors promoting vascular remodeling contribute to the development of pulmonary hypertension (PH). Recent Advances and Critical Issues: Aspects of vascular remodeling induction mechanisms described are associated with shifts in glucose metabolism through the pentose phosphate pathway and increased cytosolic NADPH generation by glucose-6-phosphate dehydrogenase, increased glycolysis generation of cytosolic NADH and lactate, mitochondrial dysfunction associated with superoxide dismutase-2 depletion, changes in reactive oxygen species and iron metabolism, and redox signaling. Future Directions: The regulation and impact of hypoxia-inducible factor and the function of cGMP-dependent and redox regulation of protein kinase G are considered for their potential roles as key sensors and coordinators of redox and metabolic processes controlling the progression of vascular pathophysiology in PH, and how modulating aspects of metabolic and redox regulatory systems potentially function in beneficial therapeutic approaches.

Coronary microvascular Kv1 channels as regulatory sensors of intracellular pyridine nucleotide redox potential

Smooth muscle voltage-gated potassium (Kv) channels are important regulators of microvascular tone and tissue perfusion. Recent studies indicate that Kv1 channels represent a key component of the physiological coupling between coronary blood flow and myocardial oxygen demand. While the mechanisms by which metabolic changes in the heart are transduced to alter coronary Kv1 channel gating and promote vasodilation are unclear, a growing body of evidence underscores a pivotal role of Kv1 channels in sensing the cellular redox status. Here, we discuss current knowledge of mechanisms of Kv channel redox regulation with respect to pyridine nucleotide modulation of Kv1 function via ancillary Kvβ proteins as well as direct modulation of channel activity via reactive oxygen and nitrogen species. We identify areas of additional research to address the integration of regulatory processes under altered physiological and pathophysiological conditions that may reveal insights into novel treatment strategies for conditions in which the matching of coronary blood supply and myocardial oxygen demand is compromised.

Coupling Drosophila melanogaster Cryptochrome Light Activation and Oxidation of the Kvβ Subunit Hyperkinetic NADPH Cofactor

Motivated by the observations on the involvement of light-induced processes in the Drosophila melanogaster cryptochrome (DmCry) in regulation of the neuronal firing rate, which is achieved by a redox-state change of its voltage-dependent K⁺ channel Kvβ subunit hyperkinetic (Hk) reduced nicotinamide adenine dinucleotide phosphate (NADPH) cofactor, we propose in this work two hypothetical pathways that may potentially enable such coupling. In the first pathway, triggered by blue-light-induced formation of a radical pair [FAD^•-TRP^•+] in DmCry, the hole (TRP^•+) may hop to Hk, for example, through a tryptophan chain and oxidize NADPH, possibly leading to inhibition of the N-terminus inactivation in the K⁺ channel. In a second possible pathway, DmCry's FAD^•- is reoxidized by molecular oxygen, producing H₂O₂, which then diffuses to Hk and oxidizes NADPH. In this work, by applying a combination of quantum and empirical-based methods for free-energy calculations, we find that the oxidation of NADPH by TRP^•+ or H₂O₂ and the reoxidation of FAD^•- by O₂ are thermodynamically feasible. Our results may have an implication in identifying a magnetic sensing signal transduction pathway, specifically upon Drosophila's Hk NADPH cofactor oxidation, with a subsequent inhibition of the K⁺ channel N-terminus inactivation gate, permitting K⁺ flux.

Circadian redox rhythms in the regulation of neuronal excitability

Oxidation-reduction reactions are essential to life as the core mechanisms of energy transfer. A large body of evidence in recent years presents an extensive and complex network of interactions between the circadian and cellular redox systems. Recent advances show that cellular redox state undergoes a ~24-h (circadian) oscillation in most tissues and is conserved across the domains of life. In nucleated cells, the metabolic oscillation is dependent upon the circadian transcription-translation machinery and, vice versa, redox-active proteins and cofactors feed back into the molecular oscillator. In the suprachiasmatic nucleus (SCN), a hypothalamic region of the brain specialized for circadian timekeeping, redox oscillation was found to modulate neuronal membrane excitability. The SCN redox environment is relatively reduced in daytime when neuronal activity is highest and relatively oxidized in nighttime when activity is at its lowest. There is evidence that the redox environment directly modulates SCN K⁺ channels, tightly coupling metabolic rhythms to neuronal activity. Application of reducing or oxidizing agents produces rapid changes in membrane excitability in a time-of-day-dependent manner. We propose that this reciprocal interaction may not be unique to the SCN. In this review, we consider the evidence for circadian redox oscillation and its interdependencies with established circadian timekeeping mechanisms. Furthermore, we will investigate the effects of redox on ion-channel gating dynamics and membrane excitability. The susceptibility of many different ion channels to modulation by changes in the redox environment suggests that circadian redox rhythms may play a role in the regulation of all excitable cells.

Fly DPP10 acts as a channel ancillary subunit and possesses peptidase activity

Mammalian DPP6 (DPPX) and DPP10 (DPPY) belong to a family of dipeptidyl peptidases, but lack enzyme activity. Instead, these proteins form complexes with voltage-gated K(+) channels in Kv4 family to control their gating and other properties. Here, we find that the fly DPP10 ortholog acts as an ancillary subunit of Kv4 channels and digests peptides. Similarly to mammalian DPP10, the fly ortholog tightly binds to rat Kv4.3 protein. The association causes negative shifts in voltage dependence of channel activation and steady state inactivation. It also results in faster inactivation and recovery from inactivation. In addition to its channel regulatory role, fly DPP10 exhibits significant dipeptidyl peptidase activity with Gly-Pro-MCA (glycyl-L-proline 4-methylcoumaryl-7-amide) as a substrate. Heterologously expressed Flag-tagged fly DPP10 and human DPP4 show similar Km values towards this substrate. However, fly DPP10 exhibits approximately a 6-times-lower relative kcat value normalized with anti-Flag immunoreactivity than human DPP4. These results demonstrate that fly DPP10 is a dual functional protein, controlling Kv4 channel gating and removing bioactive peptides.

Mitochondrial Ultrastructure and Glucose Signaling Pathways Attributed to the Kv1.3 Ion Channel

Gene-targeted deletion of the potassium channel Kv1.3 (Kv1.3^−∕−) results in “Super-smeller” mice with a sensory phenotype that includes an increased olfactory ability linked to changes in olfactory circuitry, increased abundance of olfactory cilia, and increased expression of odorant receptors and the G-protein, G_olf. Kv1.3^−∕− mice also have a metabolic phenotype including lower body weight and decreased adiposity, increased total energy expenditure (TEE), increased locomotor activity, and resistance to both diet- and genetic-induced obesity. We explored two cellular aspects to elucidate the mechanism by which loss of Kv1.3 channel in the olfactory bulb (OB) may enhance glucose utilization and metabolic rate. First, using in situ hybridization we find that Kv1.3 and the insulin-dependent glucose transporter type 4 (GLUT4) are co-localized to the mitral cell layer of the OB. Disruption of Kv1.3 conduction via construction of a pore mutation (W386F Kv1.3) was sufficient to independently translocate GLUT4 to the plasma membrane in HEK 293 cells. Because olfactory sensory perception and the maintenance of action potential (AP) firing frequency by mitral cells of the OB is highly energy demanding and Kv1.3 is also expressed in mitochondria, we next explored the structure of this organelle in mitral cells. We challenged wildtype (WT) and Kv1.3^−∕− male mice with a moderately high-fat diet (MHF, 31.8 % kcal fat) for 4 months and then examined OB ultrastructure using transmission electron microscopy. In WT mice, mitochondria were significantly enlarged following diet-induced obesity (DIO) and there were fewer mitochondria, likely due to mitophagy. Interestingly, mitochondria were significantly smaller in Kv1.3^−∕− mice compared with that of WT mice. Similar to their metabolic resistance to DIO, the Kv1.3^−∕− mice had unchanged mitochondria in terms of cross sectional area and abundance following a challenge with modified diet. We are very interested to understand how targeted disruption of the Kv1.3 channel in the OB can modify TEE. Our study demonstrates that Kv1.3 regulates mitochondrial structure and alters glucose utilization; two important metabolic changes that could drive whole system changes in metabolism initiated at the OB.

Fe2O3 nanoparticles suppress Kv1.3 channels via affecting the redox activity of Kvβ2 subunit in Jurkat T cells

Superparamagnetic iron oxide nanoparticles (SPIONs) are promising nanomaterials in medical practice due to their special magnetic characteristics and nanoscale size. However, their potential impacts on immune cells are not well documented. This study aims to investigate the effects of Fe2O3 nanoparticles (Fe2O3-NPs) on the electrophysiology of Kv1.3 channels in Jurkat T cells. Using the whole-cell patch-clamp technique, we demonstrate that incubation of Jurkat cells with Fe2O3-NPs dose- and time-dependently decreased the current density and shifted the steady-state inactivation curve and the recovery curve of Kv1.3 channels to a rightward direction. Fe2O3-NPs increased the NADP level but decreased the NADPH level of Jurkat cells. Direct induction of NADPH into the cytosole of Jurkat cells via the pipette abolished the rightward shift of the inactivation curve. In addition, transmission electron microscopy showed that Fe2O3-NPs could be endocytosed by Jurkat cells with relatively low speed and capacity. Fe2O3-NPs did not significantly affect the viability of Jurkat cells, but suppressed the expressions of certain cytokines (TNFα, IFNγ and IL-2) and interferon responsive genes (IRF-1 and PIM-1), and the time courses of Fe2O3-NPs endocytosis and effects on the expressions of cytokines and interferon responsive genes were compatible. We conclude that Fe2O3-NPs can be endocytosed by Jurkat cells and act intracellularly. Fe2O3-NPs decrease the current density and delay the inactivation and recovery kinetics of Kv1.3 channels in Jurkat cells by oxidizing NADPH and therefore disrupting the redox activity of the Kvβ2 auxiliary subunit, and as a result, lead to changes of the Kv1.3 channel function. These results suggest that iron oxide nanoparticles may affect T cell function by disturbing the activity of Kv1.3 channels. Further, the suppressing effects of Fe2O3-NPs on the expressions of certain inflammatory cytokines and interferon responsive genes suggest that iron oxide nanoparticles may exert modulatory effects on T cell immune activities and anti-inflammation effects.

Ca(2+)/calmodulin regulates Kvβ1.1-mediated inactivation of voltage-gated K(+) channels

A-type K⁺ channels open on membrane depolarization and undergo subsequent rapid inactivation such that they are ideally suited for fine-tuning the electrical signaling in neurons and muscle cells. Channel inactivation mostly follows the so-called ball-and-chain mechanism, in which the N-terminal structures of either the K⁺ channel’s α or β subunits occlude the channel pore entry facing the cytosol. Inactivation of Kv1.1 and Kv1.4 channels induced by Kvβ1.1 subunits is profoundly decelerated in response to a rise in the intracellular Ca²⁺ concentration, thus making the affected channel complexes negative feedback regulators to limit neuronal overexcitation. With electrophysiological and biochemical experiments we show that the Ca²⁺ dependence is gained by binding of calmodulin to the “chain” segment of Kvβ1.1 thereby compromising the mobility of the inactivation particle. Furthermore, inactivation regulation via Ca²⁺/calmodulin does not interfere with the β subunit’s enzymatic activity as an NADPH-dependent oxidoreductase, thus rendering the Kvβ1.1 subunit a multifunctional receptor that integrates cytosolic signals to be transduced to altered electrical cellular activity.

Redox regulation of ion channels in the pulmonary circulation

SIGNIFICANCE

The pulmonary circulation is a low-pressure, low-resistance, highly compliant vasculature. In contrast to the systemic circulation, it is not primarily regulated by a central nervous control mechanism. The regulation of resting membrane potential due to ion channels is of integral importance in the physiology and pathophysiology of the pulmonary vasculature.

RECENT ADVANCES

Redox-driven ion conductance changes initiated by direct oxidation, nitration, and S-nitrosylation of the cysteine thiols and indirect phosphorylation of the threonine and serine residues directly affect pulmonary vascular tone.

CRITICAL ISSUES

Molecular mechanisms of changes in ion channel conductance, especially the identification of the sites of action, are still not fully elucidated.

FUTURE DIRECTIONS

Further investigation of the interaction between redox status and ion channel gating, especially the physiological significance of S-glutathionylation and S-nitrosylation, could result in a better understanding of the physiological and pathophysiological importance of these mediators in general and the implications of such modifications in cellular functions and related diseases and their importance for targeted treatment strategies.

Hitting the Bull's-Eye in Metastatic Cancers-NSAIDs Elevate ROS in Mitochondria, Inducing Malignant Cell Death

Tumor metastases that impede the function of vital organs are a major cause of cancer related mortality. Mitochondrial oxidative stress induced by hypoxia, low nutrient levels, or other stresses, such as genotoxic events, act as key drivers of the malignant changes in primary tumors to enhance their progression to metastasis. Emerging evidence now indicates that mitochondrial modifications and mutations resulting from oxidative stress, and leading to OxPhos stimulation and/or enhanced reactive oxygen species (ROS) production, are essential for promoting and sustaining the highly metastatic phenotype. Moreover, the modified mitochondria in emerging or existing metastatic cancer cells, by their irreversible differences, provide opportunities for selectively targeting their mitochondrial functions with a one-two punch. The first blow would block their anti-oxidative defense, followed by the knockout blow—promoting production of excess ROS, capitulating the terminal stage—activation of the mitochondrial permeability transition pore (mPTP), specifically killing metastatic cancer cells or their precursors. This review links a wide area of research relevant to cellular mechanisms that affect mitochondria activity as a major source of ROS production driving the pro-oxidative state in metastatic cancer cells. Each of the important aspects affecting mitochondrial function are discussed including: hypoxia, HIFs and PGC1 induced metabolic changes, increased ROS production to induce a more pro-oxidative state with reduced antioxidant defenses. It then focuses on how the mitochondria, as a major source of ROS in metastatic cancer cells driving the pro-oxidative state of malignancy enables targeting drugs affecting many of these altered processes and why the NSAIDs are an excellent example of mitochondria-targeted agents that provide a one-two knockout activating the mPTP and their efficacy as selective anticancer metastasis drugs.

Oxidative modulation of voltage-gated potassium channels

SIGNIFICANCE

Voltage-gated K+ channels are a large family of K+-selective ion channel protein complexes that open on membrane depolarization. These K+ channels are expressed in diverse tissues and their function is vital for numerous physiological processes, in particular of neurons and muscle cells. Potentially reversible oxidative regulation of voltage-gated K+ channels by reactive species such as reactive oxygen species (ROS) represents a contributing mechanism of normal cellular plasticity and may play important roles in diverse pathologies including neurodegenerative diseases.

RECENT ADVANCES

Studies using various protocols of oxidative modification, site-directed mutagenesis, and structural and kinetic modeling provide a broader phenomenology and emerging mechanistic insights.

CRITICAL ISSUES

Physicochemical mechanisms of the functional consequences of oxidative modifications of voltage-gated K+ channels are only beginning to be revealed. In vivo documentation of oxidative modifications of specific amino-acid residues of various voltage-gated K+ channel proteins, including the target specificity issue, is largely absent.

FUTURE DIRECTIONS

High-resolution chemical and proteomic analysis of ion channel proteins with respect to oxidative modification combined with ongoing studies on channel structure and function will provide a better understanding of how the function of voltage-gated K+ channels is tuned by ROS and the corresponding reducing enzymes to meet cellular needs.

New inhibitors of the Kvβ2 subunit from mammalian Kv1 potassium channels

The role of the redox state of Kvβ subunits in the modulation of Kv1 potassium channels has been well documented over the past few years. It has been suggested that a molecule that binds to or inhibits the aldo-keto reductase activity of Kvβ might affect the modulation of channel properties. Previous studies of possible modulators of channel activity have shown that cortisone and some related compounds are able to physically dissociate the channel components by binding to a site at the interface between α and β subunits. Herein, we describe some new inhibitors of rat brain Kvβ2, identified using an assay based on multiple substrate turnover. This approach allows one to focus on molecules that specifically block NADPH oxidation. These studies showed that, at 0.5mM, 3,4-dihydroxphenylacetic acid (DOPAC) was an inhibitor of Kvβ2 turnover yielding a ∼ 40-50% reduction in the aldehyde reductase activity of this subunit. Other significant inhibitors include the bioflavinoid, rutin and the polyphenol resveratrol; some of the known cardioprotective effects of these molecules may be attributable to Kv1 channel modulation. Cortisone or catechol caused moderate inhibition of Kvβ2 turnover, and the aldo-keto reductases inhibitor valproate had an even smaller effect. Despite the importance of the Kv1 channels in a number of disease states, there have been few Kvβ2 inhibitors reported. While the ones identified in this study are only effective at high concentrations, they could serve as tools to decipher the role of Kvβ2 in vivo and, eventually, inform the development of novel therapeutics.

Brain circadian oscillators and redox regulation in mammals

SIGNIFICANCE

Functional states of organisms vary rhythmically with a period of about a day (i.e., circadian). This endogenous dynamic is shaped by day-night alternations in light and energy. Mammalian circadian rhythms are orchestrated by the hypothalamic suprachiasmatic nucleus (SCN), a brain region specialized for timekeeping. These autonomous ~24-h oscillations are cell-based, requiring transcription-translation-based regulation. SCN circadian oscillations include the maintenance of intrinsic rhythms, sensitivities to input signals, and generation of output signals. These change predictably as time proceeds from dawn to day, dusk, and through the night. SCN neuronal excitability, a highly energy-demanding process, also oscillates over ~24 h. The nature of the relationship of cellular metabolism and excitability had been unknown.

RECENT ADVANCES

Global SCN redox state was found to undergo an autonomous circadian rhythm. Redox state is relatively reduced in daytime, when neuronal activity is high, and oxidized during nighttime, when neurons are relatively inactive. Redox modulates neuronal excitability via tight coupling: imposed reducing or oxidizing shifts immediately alter membrane excitability. Whereas an intact transcription-translation oscillator is necessary for the redox oscillation, metabolic modulation of excitability is too rapid to be under clockwork control.

CRITICAL ISSUES

Our observations lead to the hypothesis that redox state and neuronal activity are coupled nontranscriptional circadian oscillators in SCN neurons. Critical issues include discovering molecular and cellular substrates and functional consequences of this redox oscillator.

FUTURE DIRECTIONS

Understanding interdependencies between cellular energy metabolism, neuronal activity, and circadian rhythms is critical to developing therapeutic strategies for treating neurodegenerative diseases and brain metabolic syndromes.

Sequestosome1/p62: a regulator of redox-sensitive voltage-activated potassium channels, arterial remodeling, inflammation, and neurite outgrowth

Sequestosome1/p62 (SQSTM1) is an oxidative stress-inducible protein regulated by the redox-sensitive transcription factor Nrf2. It is not an antioxidant but known as a multifunctional regulator of cell signaling with an ability to modulate targeted or selective degradation of proteins through autophagy. SQSTM1 implements these functions through physical interactions with different types of proteins including atypical PKCs, nonreceptor-type tyrosine kinase p56(Lck) (Lck), polyubiquitin, and autophagosomal factor LC3. One of the notable physiological functions of SQSTM1 is the regulation of redox-sensitive voltage-gated potassium (Kv) channels which are composed of α and β subunits: (Kvα)4 (Kvβ)4. Previous studies have established that SQSTM1 scaffolds PKCζ, enhancing phosphorylation of Kvβ which induces inhibition of pulmonary arterial Kv1.5 channels under acute hypoxia. Recent studies reveal that Lck indirectly interacts with Kv1.3 α subunits and plays a key role in acute hypoxia-induced Kv1.3 channel inhibition in T lymphocytes. Kv1.3 channels provide a signaling platform to modulate the migration and proliferation of arterial smooth muscle cells and activation of T lymphocytes, and hence have been recognized as a therapeutic target for treatment of restenosis and autoimmune diseases. In this review, we focus on the functional interactions of SQSTM1 with Kv channels through two key partners aPKCs and Lck. Furthermore, we provide molecular insights into the functions of SQSTM1 in suppression of proliferation of arterial smooth muscle cells and neointimal hyperplasia following carotid artery ligation, in T lymphocyte differentiation and activation, and in NGF-induced neurite outgrowth in PC12 cells.

Regulation of ion channels by pyridine nucleotides

Recent research suggests that in addition to their role as soluble electron carriers, pyridine nucleotides [NAD(P)(H)] also regulate ion transport mechanisms. This mode of regulation seems to have been conserved through evolution. Several bacterial ion-transporting proteins or their auxiliary subunits possess nucleotide-binding domains. In eukaryotes, the Kv1 and Kv4 channels interact with pyridine nucleotide-binding β-subunits that belong to the aldo-keto reductase superfamily. Binding of NADP(+) to Kvβ removes N-type inactivation of Kv currents, whereas NADPH stabilizes channel inactivation. Pyridine nucleotides also regulate Slo channels by interacting with their cytosolic regulator of potassium conductance domains that show high sequence homology to the bacterial TrkA family of K(+) transporters. These nucleotides also have been shown to modify the activity of the plasma membrane K(ATP) channels, the cystic fibrosis transmembrane conductance regulator, the transient receptor potential M2 channel, and the intracellular ryanodine receptor calcium release channels. In addition, pyridine nucleotides also modulate the voltage-gated sodium channel by supporting the activity of its ancillary subunit-the glycerol-3-phosphate dehydrogenase-like protein. Moreover, the NADP(+) metabolite, NAADP(+), regulates intracellular calcium homeostasis via the 2-pore channel, ryanodine receptor, or transient receptor potential M2 channels. Regulation of ion channels by pyridine nucleotides may be required for integrating cell ion transport to energetics and for sensing oxygen levels or metabolite availability. This mechanism also may be an important component of hypoxic pulmonary vasoconstriction, memory, and circadian rhythms, and disruption of this regulatory axis may be linked to dysregulation of calcium homeostasis and cardiac arrhythmias.

Subgroups of parvalbumin-expressing interneurons in layers 2/3 of the visual cortex

The input, processing, and output characteristics of inhibitory interneurons help shape information flow through layers 2/3 of the visual cortex. Parvalbumin (PV)-positive interneurons modulate and synchronize the gain and dynamic responsiveness of pyramidal neurons. To define the diversity of PV interneurons in layers 2/3 of the developing visual cortex, we characterized their passive and active membrane properties. Using Ward's and k-means multidimensional clustering, we identified four PV interneuron subgroups. The most notable difference between the subgroups was their firing patterns in response to moderate stimuli just above rheobase. Two subgroups showed regular and continuous firing at all stimulus intensities above rheobase. The difference between these two continuously firing subgroups was that one fired at much higher frequencies and transitioned into this high-frequency firing rate at or near rheobase. The two other subgroups showed irregular, stuttering firing patterns just above rheobase. Both of these subgroups typically transitioned to regular and continuous firing at intense stimulations, but one of these subgroups, the strongly stuttering subgroup, showed irregular firing across a wider range of stimulus intensities and firing frequencies. The four subgroups also differed in excitatory synaptic input, providing independent support for the classification of subgroups. The subgroups of PV interneurons identified here would respond differently to inputs of varying intensity and frequency, generating diverse patterns of PV inhibition in the developing neural circuit.

Mitochondrial ATP-sensitive potassium channel activity and hypoxic preconditioning are independent of an inwardly rectifying potassium channel subunit in Caenorhabditis elegans

Hypoxic preconditioning (HP) is an evolutionarily-conserved mechanism that protects an organism against stress. The mitochondrial ATP-sensitive K(+) channel (mK(ATP)) plays an essential role in the protective signaling, but remains molecularly undefined. Several lines of evidence suggest that mK(ATP) may arise from an inward rectifying K(+) channel (Kir). The genetic model organism Caenorhabditis elegans exhibits HP and displays mK(ATP) activity. Here, we investigate the tissue expression profile of the three C. elegans Kir genes and demonstrate that mutant strains where the irk genes have been deleted either individually or in combination can be protected by HP and exhibit robust mK(ATP) channel activity in purified mitochondria. These data suggest that the mK(ATP) in C. elegans does not arise from a Kir derived channel.

Modulation of Kv3.4 channel N-type inactivation by protein kinase C shapes the action potential in dorsal root ganglion neurons

Fast inactivation of heterologously expressed Kv3.4 channels is dramatically slowed upon phosphorylation of the channel's N-terminal (N-type) inactivation gate by protein kinase C (PKC). However, the presence and physiological importance of this exquisite modulation in excitable tissues were unknown. Here, we employed minimally invasive cell-attached patch-clamping, single-cell qPCR and specific siRNAs to unambiguously demonstrate that fast-inactivating Kv3.4 channels underlie a robust high voltage-activated A-type K(+) current (I(AHV)) in nociceptive dorsal root ganglion neurons from 7-day-old rats. We also show that PKC activation with phorbol 12,13-dibutyrate (PDBu) causes a 4-fold slowing of Kv3.4 channel inactivation and, consequently, accelerates the repolarization of the action potential (AP) by 22%, which shortens the AP duration by 14%. G-protein coupled receptor (GPCR) agonists eliminate I(AHV) fast inactivation in a membrane-delimited manner, suggesting a Kv3.4 channel signalling complex. Preincubation of the neurons with the PKC inhibitor bisindolylmaleimide II inhibits the effect of GPCR agonists and PDBu. Furthermore, activation of PKC via GPCR agonists recapitulates the effects of PDBu on the AP. Finally, transfection of the neurons with Kv3.4 siRNA prolongs the AP by 25% and abolishes the GPCR agonist-induced acceleration of the AP repolarization. These results show that Kv3.4 channels help shape the repolarization of the nociceptor AP, and that modulation of Kv3.4 channel N-type inactivation by PKC regulates AP repolarization and duration. We propose that the dramatic modulation of I(AHV) fast inactivation by PKC represents a novel mechanism of neural plasticity with potentially significant implications in the transition from acute to chronic pain.

Diverse roles for auxiliary subunits in phosphorylation-dependent regulation of mammalian brain voltage-gated potassium channels

Voltage-gated ion channels are a diverse family of signaling proteins that mediate rapid electrical signaling events. Among these, voltage-gated potassium or Kv channels are the most diverse partly due to the large number of principal (or α) subunits and auxiliary subunits that can assemble in different combinations to generate Kv channel complexes with distinct structures and functions. The diversity of Kv channels underlies much of the variability in the active properties between different mammalian central neurons and the dynamic changes that lead to experience-dependent plasticity in intrinsic excitability. Recent studies have revealed that Kv channel α subunits and auxiliary subunits are extensively phosphorylated, contributing to additional structural and functional diversity. Here, we highlight recent studies that show that auxiliary subunits exert some of their profound effects on dendritic Kv4 and axonal Kv1 channels through phosphorylation-dependent mechanisms, either due to phosphorylation on the auxiliary subunit itself or by influencing the extent and/or impact of α subunit phosphorylation. The complex effects of auxiliary subunits and phosphorylation provide a potent mechanism to generate additional diversity in the structure and function of Kv4 and Kv1 channels, as well as allowing for dynamic reversible regulation of these important ion channels.