Ito channels are octomeric complexes with four subunits of each Kv4.2 and K+ channel-interacting protein 2

Impaired Human Cardiac Cell Development due to NOTCH1 Deficiency

BACKGROUND

NOTCH1 pathogenic variants are implicated in multiple types of congenital heart defects including hypoplastic left heart syndrome, where the left ventricle is underdeveloped. It is unknown how NOTCH1 regulates human cardiac cell lineage determination and cardiomyocyte proliferation. In addition, mechanisms by which NOTCH1 pathogenic variants lead to ventricular hypoplasia in hypoplastic left heart syndrome remain elusive.

METHODS

CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats)/Cas9 genome editing was utilized to delete NOTCH1 in human induced pluripotent stem cells. Cardiac differentiation was carried out by sequential modulation of WNT signaling, and NOTCH1 knockout and wild-type differentiating cells were collected at day 0, 2, 5, 10, 14, and 30 for single-cell RNA-seq.

RESULTS

Human NOTCH1 knockout induced pluripotent stem cells are able to generate functional cardiomyocytes and endothelial cells, suggesting that NOTCH1 is not required for mesoderm differentiation and cardiovascular development in vitro. However, disruption of NOTCH1 blocks human ventricular-like cardiomyocyte differentiation but promotes atrial-like cardiomyocyte generation through shortening the action potential duration. NOTCH1 deficiency leads to defective proliferation of early human cardiomyocytes, and transcriptomic analysis indicates that pathways involved in cell cycle progression and mitosis are downregulated in NOTCH1 knockout cardiomyocytes. Single-cell transcriptomic analysis reveals abnormal cell lineage determination of cardiac mesoderm, which is manifested by the biased differentiation toward epicardial and second heart field progenitors at the expense of first heart field progenitors in NOTCH1 knockout cell populations.

CONCLUSIONS

NOTCH1 is essential for human ventricular-like cardiomyocyte differentiation and proliferation through balancing cell fate determination of cardiac mesoderm and modulating cell cycle progression. Because first heart field progenitors primarily contribute to the left ventricle, we speculate that pathogenic NOTCH1 variants lead to biased differentiation of first heart field progenitors, blocked ventricular-like cardiomyocyte differentiation, and defective cardiomyocyte proliferation, which collaboratively contribute to left ventricular hypoplasia in hypoplastic left heart syndrome.

Kv Channel Ancillary Subunits: Where Do We Go from Here?

Voltage-gated potassium (Kv) channels each comprise four pore-forming α-subunits that orchestrate essential duties such as voltage sensing and K+ selectivity and conductance. In vivo, however, Kv channels also incorporate regulatory subunits-some Kv channel specific, others more general modifiers of protein folding, trafficking, and function. Understanding all the above is essential for a complete picture of the role of Kv channels in physiology and disease.

Heritable arrhythmias associated with abnormal function of cardiac potassium channels

Cardiomyocytes express a surprisingly large number of potassium channel types. The primary physiological functions of the currents conducted by these channels are to maintain the resting membrane potential and mediate action potential repolarization under basal conditions and in response to changes in the concentrations of intracellular sodium, calcium, and ATP/ADP. Here, we review the diversity and functional roles of cardiac potassium channels under normal conditions and how heritable mutations in the genes encoding these channels can lead to distinct arrhythmias. We briefly review atrial fibrillation and J-wave syndromes. For long and short QT syndromes, we describe their genetic basis, clinical manifestation, risk stratification, traditional and novel therapeutic approaches, as well as insights into disease mechanisms provided by animal and cellular models.

Targeting of Potassium Channels in Cardiac Arrhythmias

Cardiomyocytes are endowed with a complex repertoire of ion channels, responsible for the generation of action potentials (APs), travelling waves of electrical excitation, propagating throughout the heart and leading to cardiac contractions. Cardiac AP waveforms are shaped by a striking diversity of K⁺ channels. The pivotal role of K⁺ channels in cardiac health and disease is underscored by the dramatic impact that K⁺ channel dysfunction has on cardiac arrhythmias. The development of drugs targeted to specific K⁺ channels is expected to provide an optimized approach to antiarrhythmic therapy. Here, we review the functional roles of cardiac potassium channels under normal and diseased states. We survey current antiarrhythmic drugs (AADs) targeted to voltage-gated and Ca²⁺-activated K⁺ channels and highlight future research opportunities.

Nocturnal Atrial Fibrillation Caused by Mutation in KCND2, Encoding Pore-Forming (α) Subunit of the Cardiac Kv4.2 Potassium Channel

BACKGROUND

Paroxysmal atrial fibrillation (AF) can be caused by gain-of-function mutations in genes, encoding the cardiac potassium channel subunits KCNJ2, KCNE1, and KCNH2 that mediate the repolarizing potassium currents I_k1, I_ks, and I_kr, respectively.

METHODS

Linkage analysis, whole-exome sequencing, and Xenopus oocyte electrophysiology studies were used in this study.

RESULTS

Through genetic studies, we showed that autosomal dominant early-onset nocturnal paroxysmal AF is caused by p.S447R mutation in KCND2, encoding the pore-forming (α) subunit of the Kv4.2 cardiac potassium channel. Kv4.2, along with Kv4.3, contributes to the cardiac fast transient outward K⁺ current, I_to. I_to underlies the early phase of repolarization in the cardiac action potential, thereby setting the initial potential of the plateau phase and governing its duration and amplitude. In Xenopus oocytes, the mutation increased the channel's inactivation time constant and affected its regulation: p.S447 resides in a protein kinase C (PKC) phosphorylation site, which normally allows attenuation of Kv4.2 membrane expression. The mutant Kv4.2 exhibited impaired response to PKC; hence, Kv4.2 membrane expression was augmented, enhancing potassium currents. Coexpression of mutant and wild-type channels (recapitulating heterozygosity in affected individuals) showed results similar to the mutant channel alone. Finally, in a hybrid channel composed of Kv4.3 and Kv4.2, simulating the mature endogenous heterotetrameric channel underlying I_to, the p.S447R Kv4.2 mutation exerted a gain-of-function effect on Kv4.3.

CONCLUSIONS

The mutation alters Kv4.2's kinetic properties, impairs its inhibitory regulation, and exerts gain-of-function effect on both Kv4.2 homotetramers and Kv4.2-Kv4.3 heterotetramers. These effects presumably increase the repolarizing potassium current I_to, thereby abbreviating action potential duration, creating arrhythmogenic substrate for nocturnal AF. Interestingly, Kv4.2 expression was previously shown to demonstrate circadian variation, with peak expression at daytime in murine hearts (human nighttime), with possible relevance to the nocturnal onset of paroxysmal AF symptoms in our patients. The atrial-specific phenotype suggests that targeting Kv4.2 might be effective in the treatment of nocturnal paroxysmal AF, avoiding adverse ventricular effects.

Modulation of human Kv4.3/KChIP2 channel inactivation kinetics by cytoplasmic Ca2

The transient outward current (I _to) in the human heart is mediated by Kv4.3 channels complexed with Kv channel interacting protein (KChIP) 2, a cytoplasmic Ca²⁺-binding EF-hand protein known to modulate Kv4.3 inactivation gating upon heterologous co-expression. We studied Kv4.3 channels co-expressed with wild-type (wt) or EF-hand-mutated (ΔEF) KChIP2 in human embryonic kidney (HEK) 293 cells. Co-expression took place in the absence or presence of BAPTA-AM, and macroscopic currents were recorded in the whole-cell patch-clamp configuration with different free Ca²⁺ concentrations in the patch-pipette. Our data indicate that Ca²⁺ is not necessary for Kv4.3/KChIP2 complex formation. The Kv4.3/KChIP2-mediated current decay was faster and the recovery of Kv4.3/KChIP2 channels from inactivation slower with 50 μM Ca²⁺ than with BAPTA (nominal Ca²⁺-free) in the patch-pipette. The apparent Ca²⁺-mediated slowing of recovery kinetics was still observed when EF-hand 4 of KChIP2 was mutated (ΔEF4) but not when EF-hand 2 (ΔEF2) was mutated, and turned into a Ca²⁺-mediated acceleration of recovery kinetics when EF-hand 3 (ΔEF3) was mutated. In the presence of the Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93 cytoplasmic Ca²⁺ (50 μM) induced an acceleration of Kv4.3/KChIP2 recovery kinetics, which was still observed when EF-hand 2 was mutated (ΔEF2) but not when EF-hand 3 (ΔEF3) or EF-hand 4 (ΔEF4) was mutated. Our results support the notion that binding of Ca²⁺ to KChIP2 EF-hands can acutely modulate Kv4.3/KChIP2 channel inactivation gating, but the Ca²⁺-dependent gating modulation depends on CaMKII action. Our findings speak for an acute modulation of I _to kinetics and frequency-dependent I _to availability in cardiomyocytes under conditions with elevated Ca²⁺ levels and CaMKII activity.

KChIP2 genotype dependence of transient outward current (Ito) properties in cardiomyocytes isolated from male and female mice

The transient outward current (I_to) in cardiomyocytes is largely mediated by Kv4 channels associated with Kv Channel Interacting Protein 2 (KChIP2). A knockout model has documented the critical role of KChIP2 in I_to expression. The present study was conducted to characterize in both sexes the dependence of I_to properties, including current magnitude, inactivation kinetics, recovery from inactivation and voltage dependence of inactivation, on the number of functional KChIP2 alleles. For this purpose we performed whole-cell patch-clamp experiments on isolated left ventricular cardiomyocytes from male and female mice which had different KChIP2 genotypes; i.e., wild-type (KChIP2^+/+), heterozygous knockout (KChIP2^+/-) or complete knockout of KChIP2 (KChIP2^-/-). We found in both sexes a KChIP2 gene dosage effect (i.e., a proportionality between number of alleles and phenotype) on I_to magnitude, however, concerning other I_to properties, KChIP2^+/- resembled KChIP2^+/+. Only in the total absence of KChIP2 (KChIP2^-/-) we observed a slowing of I_to kinetics, a slowing of recovery from inactivation and a negative shift of a portion of the voltage dependence of inactivation. In a minor fraction of KChIP2^-/- myocytes I_to was completely lost. The distinct KChIP2 genotype dependences of I_to magnitude and inactivation kinetics, respectively, seen in cardiomyocytes were reproduced with two-electrode voltage-clamp experiments on Xenopus oocytes expressing Kv4.2 and different amounts of KChIP2. Our results corroborate the critical role of KChIP2 in controlling I_to properties. They demonstrate that the Kv4.2/KChIP2 interaction in cardiomyocytes is highly dynamic, with a clear KChIP2 gene dosage effect on Kv4 channel surface expression but not on inactivation gating.

Genetic Control of Potassium Channels

Approximately 80 genes in the human genome code for pore-forming subunits of potassium (K(+)) channels. Rare variants (mutations) in K(+) channel-encoding genes may cause heritable arrhythmia syndromes. Not all rare variants in K(+) channel-encoding genes are necessarily disease-causing mutations. Common variants in K(+) channel-encoding genes are increasingly recognized as modifiers of phenotype in heritable arrhythmia syndromes and in the general population. Although difficult, distinguishing pathogenic variants from benign variants is of utmost importance to avoid false designations of genetic variants as disease-causing mutations.

Effect and mechanism of fluoxetine on electrophysiology in vivo in a rat model of postmyocardial infarction depression

BACKGROUND

Major depression is diagnosed in 18% of patients following myocardial infarction (MI), and the antidepressant fluoxetine is shown to effectively decrease depressive symptoms and improve coronary heart disease prognosis. We observed the effect of fluoxetine on cardiac electrophysiology in vivo in a rat model of post-MI depression and the potential mechanism.

METHODS AND RESULTS

Eighty adult male Sprague Dawley rats (200-250 g) were randomly assigned to five groups: normal control (control group), MI (MI group), depression (depression group), post-MI depression (model group), and post-MI depression treated with intragastric administration of 10 mg/kg fluoxetine (fluoxetine group). MI was induced by left anterior descending coronary artery ligation. Depression was developed by 4-week chronic mild stress (CMS). Behavior measurement was done before and during the experiment. Electrophysiology study in vivo and Western blot analysis were carried on after 4 weeks of CMS. After 4 weeks of CMS, depression-like behaviors were observed in the MI, depression, and model groups, and chronic fluoxetine administration could significantly improve those behaviors (P<0.05 vs model group). Fluoxetine significantly increased the ventricular fibrillation threshold compared with the model group (20.20±9.32 V vs 14.67±1.85 V, P<0.05). Expression of Kv4.2 was significantly reduced by 29%±12%, 24%±6%, and 41%±15%, respectively, in the MI group, CMS group, and model group, which could be improved by fluoxetine (30%±9%). But fluoxetine showed no improvement on the MI-induced loss of Cx43.

CONCLUSION

The susceptibility to ventricular arrhythmias was increased in depression and post-MI depression rats, and fluoxetine may reduce the incidence of ventricular arrhythmia in post-MI depression rats and thus improve the prognosis. This may be related in part to the upregulation of Kv4.2 by fluoxetine.

The stoichiometry and biophysical properties of the Kv4 potassium channel complex with K+ channel-interacting protein (KChIP) subunits are variable, depending on the relative expression level

Kv4 is a voltage-gated K(+) channel, which underlies somatodendritic subthreshold A-type current (ISA) and cardiac transient outward K(+) (Ito) current. Various ion channel properties of Kv4 are known to be modulated by its auxiliary subunits, such as K(+) channel-interacting protein (KChIP) or dipeptidyl peptidase-like protein. KChIP is a cytoplasmic protein and increases the current amplitude, decelerates the inactivation, and accelerates the recovery from inactivation of Kv4. Crystal structure analysis demonstrated that Kv4 and KChIP form an octameric complex with four Kv4 subunits and four KChIP subunits. However, it remains unknown whether the Kv4·KChIP complex can have a different stoichiometry other than 4:4. In this study, we expressed Kv4.2 and KChIP4 with various ratios in Xenopus oocytes and observed that the biophysical properties of Kv4.2 gradually changed with the increase in co-expressed KChIP4. The tandem repeat constructs of Kv4.2 and KChIP4 revealed that the 4:4 (Kv4.2/KChIP4) channel shows faster recovery than the 4:2 channel, suggesting that the biophysical properties of Kv4.2 change, depending on the number of bound KChIP4s. Subunit counting by single-molecule imaging revealed that the bound number of KChIP4 in each Kv4.2·KChIP4 complex was dependent on the expression level of KChIP4. Taken together, we conclude that the stoichiometry of Kv4·KChIP complex is variable, and the biophysical properties of Kv4 change depending on the number of bound KChIP subunits.

T-type channels buddy up

The electrical output of neurons relies critically on voltage- and calcium-gated ion channels. The traditional view of ion channels is that they operate independently of each other in the plasma membrane in a manner that could be predicted according to biophysical characteristics of the isolated current. However, there is increasing evidence that channels interact with each other not just functionally but also physically. This is exemplified in the case of Cav3 T-type calcium channels, where new work indicates the ability to form signaling complexes with different types of calcium-gated and even voltage-gated potassium channels. The formation of a Cav3-K complex provides the calcium source required to activate KCa1.1 or KCa3.1 channels and, furthermore, to bestow a calcium-dependent regulation of Kv4 channels via associated KChIP proteins. Here, we review these interactions and discuss their significance in the context of neuronal firing properties.

Functional stoichiometry underlying KChIP regulation of Kv4.2 functional expression

K channel-interacting proteins (KChIPs) enhance functional expression of Kv4 channels by binding to an N-terminal regulatory region located in the first 40 amino acids of Kv4.2 that we call the functional expression regulating N-terminal (FERN) domain. Mutating two residues in the FERN domain to alanines, W8A and F11A, disrupts KChIP binding and regulation of Kv4.2 without eliminating the FERN domain's control of basal expression level or regulation by DPP6. When Kv4.2(W8A,F11A) is co-expressed with wild type Kv4.2 and KChIP3 subunits, a dominant negative effect is seen where the current expression is reduced to levels normally seen without KChIP addition. The dominant negative effect correlates with heteromultimeric channels remaining on intracellular membranes despite KChIP binding to non-mutant Kv4.2 subunits. In contrast, the deletion mutant Kv4.2(Δ1-40), eliminating both KChIP binding and the FERN domain, has no dominant negative effect even though the maximal conductance level is 5x lower than seen with KChIP3. The 5x increased expression seen with KChIP integration into the channel is fully apparent even when a reduced number of KChIP subunits are incorporated as long as all FERN domains are bound. Our results support the hypothesis that KChIPs enhances Kv4.2 functional expression by a 1 : 1 suppression of the N-terminal FERN domain and by producing additional positive regulatory effects on functional channel expression.

Molecular structure and target recognition of neuronal calcium sensor proteins

BACKGROUND

Neuronal calcium sensor (NCS) proteins, a sub-branch of the calmodulin superfamily, are expressed in the brain and retina where they transduce calcium signals and are genetically linked to degenerative diseases. The amino acid sequences of NCS proteins are highly conserved but their physiological functions are quite distinct. Retinal recoverin and guanylate cyclase activating proteins (GCAPs) both serve as calcium sensors in retinal rod cells, neuronal frequenin (NCS1) modulate synaptic activity and neuronal secretion, K+ channel interacting proteins (KChIPs) regulate ion channels to control neuronal excitability, and DREAM (KChIP3) is a transcriptional repressor that regulates neuronal gene expression.

SCOPE OF REVIEW

Here we review the molecular structures of myristoylated forms of NCS1, recoverin, and GCAP1 that all look very different, suggesting that the sequestered myristoyl group helps to refold these highly homologous proteins into very different structures. The molecular structure of NCS target complexes have been solved for recoverin bound to rhodopsin kinase, NCS-1 bound to phosphatidylinositol 4-kinase, and KChIP1 bound to A-type K+ channels.

MAJOR CONCLUSIONS

We propose the idea that N-terminal myristoylation is critical for shaping each NCS family member into a unique structure, which upon Ca2+-induced extrusion of the myristoyl group exposes a unique set of previously masked residues, thereby exposing a distinctive ensemble of hydrophobic residues to associate specifically with a particular physiological target. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Impact of KChIP2 on Cardiac Electrophysiology and the Progression of Heart Failure

Electrophysiological remodeling of cardiac potassium ion channels is important in the progression of heart failure. A reduction of the transient outward potassium current (I_to) in mammalian heart failure is consistent with a reduced expression of potassium channel interacting protein 2 (KChIP2, a K_V4 subunit). Approaches have been made to investigate the role of KChIP2 in shaping cardiac I_to, including the use of transgenic KChIP2 deficient mice and viral overexpression of KChIP2. The interplay between I_to and myocardial calcium handling is pivotal in the development of heart failure, and is further strengthened by the dual role of KChIP2 as a functional subunit on both K_V4 and Ca_V1.2. Moreover, the potential arrhythmogenic consequence of reduced I_to may contribute to the high relative incidence of sudden death in the early phases of human heart failure. With this review, we offer an overview of the insights into the physiological and pathological roles of KChIP2 and we discuss the limitations of translating the molecular basis of electrophysiological remodeling from animal models of heart failure to the clinical setting.

Molecular structure and target recognition of neuronal calcium sensor proteins

Neuronal calcium sensor (NCS) proteins, a sub-branch of the EF-hand superfamily, are expressed in the brain and retina where they transduce calcium signals and are genetically linked to degenerative diseases. The amino acid sequences of NCS proteins are highly conserved but their physiological functions are quite distinct. Retinal recoverin and guanylate cyclase activating proteins (GCAPs) both serve as calcium sensors in retinal rod cells, neuronal frequenin (NCS1) modulates synaptic activity and neuronal secretion, K⁺ channel interacting proteins (KChIPs) regulate ion channels to control neuronal excitability, and DREAM (KChIP3) is a transcriptional repressor that regulates neuronal gene expression. Here we review the molecular structures of myristoylated forms of NCS1, recoverin, and GCAP1 that all look very different, suggesting that the sequestered myristoyl group helps to refold these highly homologous proteins into very different structures. The molecular structure of NCS target complexes have been solved for recoverin bound to rhodopsin kinase (RK), NCS-1 bound to phosphatidylinositol 4-kinase, and KChIP1 bound to A-type K⁺ channels. We propose that N-terminal myristoylation is critical for shaping each NCS family member into a different structure, which upon Ca²⁺-induced extrusion of the myristoyl group exposes a unique set of previously masked residues that interact with a particular physiological target.

Mechanisms of closed-state inactivation in voltage-gated ion channels

Inactivation of voltage-gated ion channels is an intrinsic auto-regulatory process necessary to govern the occurrence and shape of action potentials and establish firing patterns in excitable tissues. Inactivation may occur from the open state (open-state inactivation, OSI) at strongly depolarized membrane potentials, or from pre-open closed states (closed-state inactivation, CSI) at hyperpolarized and modestly depolarized membrane potentials. Voltage-gated Na(+), K(+), Ca(2+) and non-selective cationic channels utilize both OSI and CSI. Whereas there are detailed mechanistic descriptions of OSI, much less is known about the molecular basis of CSI. Here, we review evidence for CSI in voltage-gated cationic channels (VGCCs) and recent findings that shed light on the molecular mechanisms of CSI in voltage-gated K(+) (Kv) channels. Particularly, complementary observations suggest that the S4 voltage sensor, the S4S5 linker and the main S6 activation gate are instrumental in the installment of CSI in Kv4 channels. According to this hypothesis, the voltage sensor may adopt a distinct conformation to drive CSI and, depending on the stability of the interactions between the voltage sensor and the pore domain, a closed-inactivated state results from rearrangements in the selectivity filter or failure of the activation gate to open. Kv4 channel CSI may efficiently exploit the dynamics of the subthreshold membrane potential to regulate spiking properties in excitable tissues.

Trafficking of neuronal two pore domain potassium channels

The activity of two pore domain potassium (K2P) channels regulates neuronal excitability and cell firing. Post-translational regulation of K2P channel trafficking to the membrane controls the number of functional channels at the neuronal membrane affecting the functional properties of neurons. In this review, we describe the general features of K channel trafficking from the endoplasmic reticulum (ER) to the plasma membrane via the Golgi apparatus then focus on established regulatory mechanisms for K2P channel trafficking. We describe the regulation of trafficking of TASK channels from the ER or their retention within the ER and consider the competing hypotheses for the roles of the chaperone proteins 14-3-3, COP1 and p11 in these processes and where these proteins bind to TASK channels. We also describe the localisation of TREK channels to particular regions of the neuronal membrane and the involvement of the TREK channel binding partners AKAP150 and Mtap2 in this localisation. We describe the roles of other K2P channel binding partners including Arf6, EFA6 and SUMO for TWIK1 channels and Vpu for TASK1 channels. Finally, we consider the potential importance of K2P channel trafficking in a number of disease states such as neuropathic pain and cancer and the protection of neurons from ischemic damage. We suggest that a better understanding of the mechanisms and regulations that underpin the trafficking of K2P channels to the plasma membrane and to localised regions therein may considerably enhance the probability of future therapeutic advances in these areas.

The membrane protein MiRP3 regulates Kv4.2 channels in a KChIP-dependent manner

MiRP3, the single-span membrane protein encoded by KCNE4, is localized by immunofluorescence microscopy to the transverse tubules of murine cardiac myocytes. MiRP3 is found to co-localize with Kv4.2 subunits that contribute to cardiac transient outward potassium currents (I(to)). Whole-cell, voltage-clamp recordings of human MiRP3 and Kv4.2 expressed in a clonal cell line (tsA201) reveal MiRP3 to modulate Kv4.2 current activation, inactivation and recovery from inactivation. MiRP3 shifts the half-maximal voltage for activation (V(1/2)) approximately 20 mV and slows time to peak approximately 100%. In addition, MiRP3 slows inactivation approximately 100%, speeds recovery from inactivation approximately 30%, and enhances restored currents so they 'overshoot' baseline levels. The cytoplasmic accessory subunit KChIP2 also assembles with Kv4.2 in tsA201 cells to increase peak current, shift V(1/2) approximately 5 mV, slow time to peak approximately 10%, slow inactivation approximately 100%, and speed recovery from inactivation approximately 250% without overshoot. Simultaneous expression of all three subunits yields a biophysical profile unlike either accessory subunit alone, abolishes MiRP3-induced overshoot, and allows biochemical isolation of the ternary complex. Thus, regional heterogeneity in cardiac expression of MiRP3, Kv4.2 and KChIP2 in health and disease may establish the local attributes and magnitude of cardiac I(to).

Convergent modulation of Kv4.2 channel alpha subunits by structurally distinct DPPX and KChIP auxiliary subunits

Kv4.2 is the major voltage-gated K(+) (Kv) channel alpha subunit responsible for the somatodendritic transient or A-type current I(SA) that activates at subthreshold membrane potentials. Stable association of Kv4.2 with diverse auxiliary subunits and reversible Kv4.2 phosphorylation regulate I(SA) function. Two classes of auxiliary subunits play distinct roles in modulating the biophysical properties of Kv4.2: dipeptidyl-peptidase-like type II transmembrane proteins typified by DPPX-S, and cytoplasmic Ca(2+) binding proteins known as K(+) channel interacting proteins (KChIPs). Here, we characterize the convergent roles that DPPX-S and KChIPs play as component subunits of Kv4.2 channel complexes. We coexpressed DPPX-S with Kv4.2 in heterologous cells and found a dramatic redistribution of Kv4.2, releasing it from intracellular retention and allowing plasma membrane expression, as well as altered Kv4.2 phosphorylation, detergent solubility, and stability. These changes are remarkably similar to those obtained upon coexpression of Kv4.2 with the structurally distinct KChIPs1-3 auxiliary subunits. KChIP4a, which negatively affects the impact of other KChIPs on Kv4.2, also inhibits the effects of DPPX-S, consistent with the formation of a ternary complex of Kv4.2, DPPX-S, and KChIPs early in channel biosynthesis. Tandem MS analyses reveal that coexpression with DPPX-S or KChIP2 leads to a pattern of Kv4.2 phosphorylation in heterologous cells similar to that observed in brain, but lacking in cells expressing Kv4.2 alone. In conclusion, transmembrane DPPX-S and cytoplasmic KChIPs, despite having distinct structures and binding sites on Kv4.2, exert similar effects on Kv4.2 trafficking, but distinct effects on Kv4.2 gating.

The role of calsenilin/DREAM/KChIP3 in contextual fear conditioning

Potassium channel interacting proteins (KChIPs) are members of a family of calcium binding proteins that interact with Kv4 potassium (K(+)) channel primary subunits and also act as transcription factors. The Kv4 subunit is a primary K(+) channel pore-forming subunit, which contributes to the somatic and dendritic A-type currents throughout the nervous system. These A-type currents play a key role in the regulation of neuronal excitability and dendritic processing of incoming synaptic information. KChIP3 is also known as calsenilin and as the transcription factor, downstream regulatory element antagonist modulator (DREAM), which regulates a number of genes including prodynorphin. KChIP3 and Kv4 primary channel subunits are highly expressed in hippocampus, an area of the brain important for learning and memory. Through its various functions, KChIP3 may play a role in the regulation of synaptic plasticity and learning and memory. We evaluated the role of KChIP3 in a hippocampus-dependent memory task, contextual fear conditioning. Male KChIP3 knockout (KO) mice showed significantly enhanced memory 24 hours after training as measured by percent freezing. In addition, we found that membrane association and interaction with Kv4.2 of KChIP3 protein was significantly decreased and nuclear KChIP3 expression was increased six hours after the fear conditioning training paradigm with no significant change in KChIP3 mRNA. In addition, prodynorphin mRNA expression was significantly decreased six hours after fear conditioning training in wild-type (WT) but not in KO animals. These data suggest a role for regulation of gene expression by KChIP3/DREAM/calsenilin in consolidation of contextual fear conditioning memories.

Pentameric assembly of potassium channel tetramerization domain-containing protein 5

We report the X-ray crystal structure of human potassium channel tetramerization domain-containing protein 5 (KCTD5), the first member of the family to be so characterized. Four findings were unexpected. First, the structure reveals assemblies of five subunits while tetramers were anticipated; pentameric stoichiometry is observed also in solution by scanning transmission electron microscopy mass analysis and analytical ultracentrifugation. Second, the same BTB (bric-a-brac, tramtrack, broad complex) domain surface mediates the assembly of five KCTD5 and four voltage-gated K(+) (Kv) channel subunits; four amino acid differences appear crucial. Third, KCTD5 complexes have well-defined N- and C-terminal modules separated by a flexible linker that swivels by approximately 30 degrees; the C-module shows a new fold and is required to bind Golgi reassembly stacking protein 55 with approximately 1 microM affinity, as judged by surface plasmon resonance and ultracentrifugation. Fourth, despite the homology reflected in its name, KCTD5 does not impact the operation of Kv4.2, Kv3.4, Kv2.1, or Kv1.2 channels.

Localization and targeting of voltage-dependent ion channels in mammalian central neurons

The intrinsic electrical properties and the synaptic input-output relationships of neurons are governed by the action of voltage-dependent ion channels. The localization of specific populations of ion channels with distinct functional properties at discrete sites in neurons dramatically impacts excitability and synaptic transmission. Molecular cloning studies have revealed a large family of genes encoding voltage-dependent ion channel principal and auxiliary subunits, most of which are expressed in mammalian central neurons. Much recent effort has focused on determining which of these subunits coassemble into native neuronal channel complexes, and the cellular and subcellular distributions of these complexes, as a crucial step in understanding the contribution of these channels to specific aspects of neuronal function. Here we review progress made on recent studies aimed to determine the cellular and subcellular distribution of specific ion channel subunits in mammalian brain neurons using in situ hybridization and immunohistochemistry. We also discuss the repertoire of ion channel subunits in specific neuronal compartments and implications for neuronal physiology. Finally, we discuss the emerging mechanisms for determining the discrete subcellular distributions observed for many neuronal ion channels.

Contribution of electrostatic and structural properties of Kv4.3 S4 arginine residues to the regulation of channel gating

Previous work has demonstrated that replacing individual arginine (R) residues in the S4 domain of Kv4.3 with alanine (A) not only altered activation and deactivation processes, but also those of closed-state inactivation (CSI) and recovery. R-->A mutants eliminated individual positive charge while substantially reducing side chain volume and hydrophilic character. Their novel effects on gating may thus have been the result of electrostatic and/or structural perturbations. To address this issue, and to gain further insights into the roles that S4 plays in the regulation of Kv4.3 gating transitions, we comparatively analyzed arginine to glutamine (R-->Q) mutations at positions 290, 293, and 296. This maneuver maintained positive charge elimination of the R-->A mutants, while partially restoring native side chain volume and hydrophilic properties. R-->A and R-->Q mutant pairs produced similar effects on the forward gating process of activation. In contrast, significant differences between the two substitutions were discovered on deactivation, CSI, and recovery, with the R-->Q mutants partially restoring wild type characteristics. Our results argue that modification of individual S4 residue properties may result in altered localized interactions within unique microenvironments encountered during forward and reverse gating transitions. As such, predominant effects appear on the reverse gating transitions of deactivation and recovery. These results are consistent with the proposal that arginine residues in S4 are involved in regulating Kv4.3 CSI and recovery.

Gating charge immobilization in Kv4.2 channels: the basis of closed-state inactivation

Kv4 channels mediate the somatodendritic A-type K+ current (I(SA)) in neurons. The availability of functional Kv4 channels is dynamically regulated by the membrane potential such that subthreshold depolarizations render Kv4 channels unavailable. The underlying process involves inactivation from closed states along the main activation pathway. Although classical inactivation mechanisms such as N- and P/C-type inactivation have been excluded, a clear understanding of closed-state inactivation in Kv4 channels has remained elusive. This is in part due to the lack of crucial information about the interactions between gating charge (Q) movement, activation, and inactivation. To overcome this limitation, we engineered a charybdotoxin (CTX)-sensitive Kv4.2 channel, which enabled us to obtain the first measurements of Kv4.2 gating currents after blocking K+ conduction with CTX (Dougherty and Covarrubias. 2006J. Gen. Physiol. 128:745-753). Here, we exploited this approach further to investigate the mechanism that links closed-state inactivation to slow Q-immobilization in Kv4 channels. The main observations revealed profound Q-immobilization at steady-state over a range of hyperpolarized voltages (-110 to -75 mV). Depolarization in this range moves <5% of the observable Q associated with activation and is insufficient to open the channels significantly. The kinetics and voltage dependence of Q-immobilization and ionic current inactivation between -153 and -47 mV are similar and independent of the channel's proximal N-terminal region (residues 2-40). A coupled state diagram of closed-state inactivation with a quasi-absorbing inactivated state explained the results from ionic and gating current experiments globally. We conclude that Q-immobilization and closed-state inactivation at hyperpolarized voltages are two manifestations of the same process in Kv4.2 channels, and propose that inactivation in the absence of N- and P/C-type mechanisms involves desensitization to voltage resulting from a slow conformational change of the voltage sensors, which renders the channel's main activation gate reluctant to open.

I SA channel complexes include four subunits each of DPP6 and Kv4.2

Kv4 potassium channels produce rapidly inactivating currents that regulate excitability of muscles and nerves. To reconstitute the neuronal A-type current I(SA), Kv4 subunits assemble with DPP6, a single transmembrane domain accessory subunit. DPP6 alters function-accelerating activation, inactivation, and recovery from inactivation-and increases surface expression. We sought here to determine the stoichiometry of Kv4 and DPP6 in complexes using functional and biochemical methods. First, wild type channels formed from subunit monomers were compared with channels carrying subunits linked in tandem to enforce 4:4 and 4:2 assemblies (Kv4.2-DPP6 and Kv4.2-Kv4.2-DPP6). Next, channels were overexpressed and purified so that the molar ratio of subunits in complexes could be assessed by direct amino acid analysis. Both biophysical and biochemical methods indicate that I(SA) channels carry four subunits each of Kv4.2 and DPP6.

MiRP3 acts as an accessory subunit with the BK potassium channel

MinK-related peptides (MiRPs) are single-span membrane proteins that assemble with specific voltage-gated K+ (Kv) channel alpha-subunits to establish gating kinetics, unitary conductance, expression level, and pharmacology of the mixed complex. MiRP3 (encoded by the KCNE4 gene) has been shown to alter the behavior of some Kv alpha-subunits in vitro but its natural partners and physiologic functions are unknown. Seeking in vivo partners for MiRP3, immunohistochemistry was used to localize its expression to a unique subcellular site, the apical membrane of renal intercalated cells, where one potassium channel type has been recorded, the calcium- and voltage-gated channel BK. Overlapping staining of these two proteins was found in rabbit intercalated cells, and MiRP3 and BK subunits expressed in tissue culture cells were found to form detergent-stable complexes. Electrophysiologic and biochemical evaluation showed MiRP3 to act on BK to reduce current density in two fashions: shifting the current-voltage relationship to more depolarized voltages in a calcium-dependent fashion ( approximately 10 mV at normal intracellular calcium levels) and accelerating degradation of MiRP3-BK complexes. The findings suggest a role for MiRP3 modulation of BK-dependent urinary potassium excretion.

Potassium channels: newly found players in synaptic plasticity

One of the major issues for modern neuroscience research concerns the molecular and cellular mechanisms that underlie the acquisition, storage, and recollection of memories by the brain. Regulation of the strength of individual synaptic inputs (synaptic plasticity) has, for decades, been the front-running candidate mechanism for cellular information storage, with some direct supporting evidence recently obtained. Research into the molecular mechanisms responsible for changing synaptic strength has, to date, primarily focused on trafficking and properties of the neurotransmitter receptors themselves (AMPARs and NMDARs). However, recent evidence indicates that, subsequent to receptor activation, synaptic inputs are subject to regulation by synaptically located K+ channels. It is therefore critical to understand the biophysical properties and subcellular localization (density and distribution) of these channels and how their properties are modulated. Here we will review recent findings showing that two different classes of K+ channels (A-type and small conductance, Ca2+ -activated K+ channels), beyond their traditional role in regulating action potential firing, contribute to the regulation of synaptic strength in the hippocampus. In addition, we discuss how modulation of these channels' properties and expression might contribute to synaptic plasticity.

Voltage-gated ion channel Kv4.3 is associated with Rap guanine nucleotide exchange factors and regulates angiotensin receptor type 1 signaling to small G-protein Rap

The voltage-gated potassium channel Kv4.3 was coexpressed with its beta-subunit Kv channel-interacting protein 2 and the angiotensin type 1 receptor in HEK-293 cells. Proteomic analysis of proteins coimmunoprecipitated with Kv4.3 revealed that Kv4.3 is associated with Rap guanine nucleotide exchange factors MR-GEF and EPAC-1. Previously, we demonstrated that Kv4.3 interacts with the angiotensin type 1 receptor in HE293 cells and cardiac myocytes. On the basis of this, we investigated the angiotensin type 1 receptor signaling to small G-proteins Ras and Rap-1 in the presence and absence of the Kv4.3-Kv channel-interacting protein 2 macromolecular complex. Ras activation was not significantly affected by coexpression of Kv4.3 and Kv channel-interacting protein 2. Ras exhibited a rapid activation-inactivation pattern with maximum activity at 2.5 min after addition of angiotensin II. In contrast, activation of Rap-1 was affected dramatically by coexpression of Kv4.3 and Kv channel-interacting protein 2 with the angiotensin type 1 receptor. In the absence of Kv4.3 and Kv channel-interacting protein 2, stimulation of the angiotensin type 1 receptor resulted in steady activation of Rap-1 that reached a plateau 25 min after addition of angiotensin II. In the presence of Kv4.3 and Kv channel-interacting protein 2, Rap-1 reaches a maximum activity 2.5 min after addition of angiotensin II and then deactivates rapidly, demonstrating a pattern of activation similar to that of Ras. Our findings show that Kv4.3 regulates angiotensin type 1 receptor signaling to the small G-protein Rap-1.

Total chemical synthesis and biophysical characterization of the minimal isoform of the KChIP2 potassium channel regulatory subunit

The potassium channel accessory subunit KChIP2 associates with Kv4.2 channels in the cardiac myocyte and is involved in the regulation of the transient outward current (I(to)) during the early phase of repolarization of the action potential. As a first step to biophysically probe the mechanism of KChIP2, we have chemically synthesized its minimal isoform, KChIP2d, using Boc chemistry solid phase peptide synthesis in conjunction with native chemical ligation. The synthetic KChIP2d protein is primarily alpha-helical as predicted and becomes more structured upon binding calcium as assessed by (1)H-NMR and CD spectroscopy. Synthetic KChIP2d is in a monomer-dimer equilibrium in solution, and there is evidence for two monomer binding sites on an N-terminal peptide of Kv4.2. Planned future studies include the incorporation of fluorescent and spin labeled probes in KChIP2d to yield structural information in parallel with electrophysiologic studies to elucidate KChIP2d's mechanism of action.

A dipeptidyl aminopeptidase-like protein remodels gating charge dynamics in Kv4.2 channels

Dipeptidyl aminopeptidase–like proteins (DPLPs) interact with Kv4 channels and thereby induce a profound remodeling of activation and inactivation gating. DPLPs are constitutive components of the neuronal Kv4 channel complex, and recent observations have suggested the critical functional role of the single transmembrane segment of these proteins (Zagha, E., A. Ozaita, S.Y. Chang, M.S. Nadal, U. Lin, M.J. Saganich, T. McCormack, K.O. Akinsanya, S.Y. Qi, and B. Rudy. 2005. J. Biol. Chem. 280:18853–18861). However, the underlying mechanism of action is unknown. We hypothesized that a unique interaction between the Kv4.2 channel and a DPLP found in brain (DPPX-S) may remodel the channel's voltage-sensing domain. To test this hypothesis, we implemented a robust experimental system to measure Kv4.2 gating currents and study gating charge dynamics in the absence and presence of DPPX-S. The results demonstrated that coexpression of Kv4.2 and DPPX-S causes a −26 mV parallel shift in the gating charge-voltage (Q-V) relationship. This shift is associated with faster outward movements of the gating charge over a broad range of relevant membrane potentials and accelerated gating charge return upon repolarization. In sharp contrast, DPPX-S had no effect on gating charge movements of the Shaker B Kv channel. We propose that DPPX-S destabilizes resting and intermediate states in the voltage-dependent activation pathway, which promotes the outward gating charge movement. The remodeling of gating charge dynamics may involve specific protein–protein interactions of the DPPX-S's transmembrane segment with the voltage-sensing and pore domains of the Kv4.2 channel. This mechanism may determine the characteristic fast operation of neuronal Kv4 channels in the subthreshold range of membrane potentials.

KCNE3 is an inhibitory subunit of the Kv4.3 potassium channel

The mammalian Kv4.3 potassium channel is a fast activating and inactivating K+ channel widely distributed in mammalian tissues. Kv4.3 is the major component of various physiologically important currents ranging from A-type currents in the CNS to the transient outward potassium conductance in the heart (I(to)). Here we show that the KCNE3 beta-subunit has a strong inhibitory effect on current conducted by heterologously expressed Kv4.3 channels. KCNE3 reduces the Kv4.3 current amplitude, and it slows down the channel activation and inactivation as well as the recovery from inactivation. KCNE3 also inhibits currents generated by Kv4.3 in complex with the accessory subunit KChIP2. We find the inhibitory effect of KCNE3 to be specific for Kv4.3 within the Kv4 channel family. Kv4.3 has previously been shown to interact with a number of beta-subunits, but none of the described subunit-interactions exert an inhibitory effect on the Kv4.3 current.

Molecular characterization of a novel dipeptidyl peptidase like 2-short form (DPL2-s) that is highly expressed in the brain and lacks dipeptidyl peptidase activity

DPL2 (DPP10) found at chromosome 2q14.1 is a member of the dipeptidyl peptidase IV (DPIV) gene family. Here we characterize a novel short DPL2 isoform (DPL2-s), a 789-amino acid protein, that differs from the previously described long DPL2 isoform (DPL2-l) at the N-terminal cytoplasmic domain by 13 amino acids. The two DPL2 isoforms use alternate first exons. DPL2 mRNA was expressed mainly in the brain and pancreas. Multiple forms of recombinant DPL2-s protein were observed in 293T cells, having mobilities 96 kDa, 100 kDa, and approximately 250 kDa which may represent soluble DPL2, transmembrane DPL2 and multimeric DPL2 respectively. DPL2 is glycosylated as a band shift is observed following PNGase F deglycosylation. DPL2-s was expressed primarily on the cell surface of transfected 293T and PC12 cells. DPL2-s exhibits high sequence homology with other DPIV peptidases, but lacks a catalytic serine residue and lacks dipeptidyl peptidase activity. Substitutions of Gly(644)-->Ser, Lys(643)Gly(644)-->TrpSer, or Asp(561)Lys(643)Gly(644)-->TyrTrpSer in the catalytic motif did not confer dipeptidyl peptidase activity upon DPL2-s. Thus, although DPL2 is similar in structure and sequence to the other dipeptidyl peptidases, it lacks vital residues required to confer dipeptidyl peptidase activity and has instead evolved features that enable it to act as an important component of voltage-gated potassium channels.

Transient outward potassium current, 'Ito', phenotypes in the mammalian left ventricle: underlying molecular, cellular and biophysical mechanisms

At least two functionally distinct transient outward K(+) current (I(to)) phenotypes can exist across the free wall of the left ventricle (LV). Based upon their voltage-dependent kinetics of recovery from inactivation, these two phenotypes are designated 'I(to,fast)' (recovery time constants on the order of tens of milliseconds) and 'I(to,slow)' (recovery time constants on the order of thousands of milliseconds). Depending upon species, either I(to,fast), I(to,slow) or both current phenotypes may be expressed in the LV free wall. The expression gradients of these two I(to) phenotypes across the LV free wall are typically heterogeneous and, depending upon species, may consist of functional phenotypic gradients of both I(to,fast) and I(to,slow) and/or density gradients of either phenotype. We review the present evidence (molecular, biophysical, electrophysiological and pharmacological) for Kv4.2/4.3 alpha subunits underlying LV I(to,fast) and Kv1.4 alpha subunits underlying LV I(to,slow) and speculate upon the potential roles of each of these currents in determining frequency-dependent action potential characteristics of LV subepicardial versus subendocardial myocytes in different species. We also review the possible functional implications of (i) ancillary subunits that regulate Kv1.4 and Kv4.2/4.3 (Kvbeta subunits, DPPs), (ii) KChIP2 isoforms, (iii) spider toxin-mediated block of Kv4.2/4.3 (Heteropoda toxins, phrixotoxins), and (iv) potential mechanisms of modulation of I(to,fast) and I(to,slow) by cellular redox state, [Ca(2)(+)](i) and kinase-mediated phosphorylation. I(to) phenotypic activation and state-dependent gating models and molecular structure-function relationships are also discussed.

Contribution of N- and C-terminal Kv4.2 channel domains to KChIP interaction [corrected]

Association of Shal gene-related voltage-gated potassium (Kv4) channels with cytoplasmic Kv channel interacting proteins (KChIPs) influences inactivation gating and surface expression. We investigated both functional and biochemical consequences of mutations in cytoplasmic N and C-terminal Kv4.2 domains to characterize structural determinants for KChIP interaction. We performed a lysine-scanning mutagenesis within the proximal 40 amino acid portion and a structure-based mutagenesis in the tetramerization 1 (T1) domain of Kv4.2. In addition, the cytoplasmic Kv4.2 C-terminus was truncated at various positions. Wild-type and mutant Kv4.2 channels were coexpressed with KChIP2 isoforms in mammalian cell lines. The KChIP2-induced modulation of Kv4.2 currents was studied with whole-cell patch clamp and the binding of KChIP2 isoforms to Kv4.2 channels with coimmunoprecipitation experiments. Our results define one major interaction site for KChIPs, including amino acids in the proximal N-terminus between residues 11 and 23, where binding and functional modulation are essentially equivalent. A further interaction site includes residues in the T1 domain. Notably, C-terminal deletions also had marked effects on KChIP2-dependent gating modulation and KChIP2 binding, revealing a previously unknown involvement of domains within the cytoplasmic Kv4.2 C-terminus in KChIP interaction. Less coincidence of binding and functional modulation indicates a more loose 'anchoring' at T1- and C-terminal interaction sites. Our results refine and extend previously proposed structural models for Kv4.2/KChIP complex formation.

Identification and characterization of small molecule modulators of KChIP/Kv4 function

Potassium channels and their associated subunits are important contributors to electrical excitability in many cell types. In this study, a yeast two-hybrid assay was used to identify inhibitors such as a diaryl-urea compound (CL-888) that binds to and modulates the formation of the Kv4/KChIP complex. CL-888 altered the apparent affinity of KChIP1 to Kv4.3-N in a Biacore assay, but did not dissociate the two proteins in size-exclusion chromatography experiments. Kv4.2/KChIP1 current amplitude and kinetics were altered with compound exposure, supporting the hypothesis of a compound-induced conformational change in the protein complex. Fluorescence spectroscopy of a unique tryptophan residue in KChIP1 was consistent with compound binding to the protein. Molecular modeling using the KChIP1 crystal structure indicates that compound binding may occur in a small tryptophan-containing binding pocket located on the hydrophilic side of the protein.

Functionally active t1-t1 interfaces revealed by the accessibility of intracellular thiolate groups in kv4 channels

Gating of voltage-dependent K(+) channels involves movements of membrane-spanning regions that control the opening of the pore. Much less is known, however, about the contributions of large intracellular channel domains to the conformational changes that underlie gating. Here, we investigated the functional role of intracellular regions in Kv4 channels by probing relevant cysteines with thiol-specific reagents. We find that reagent application to the intracellular side of inside-out patches results in time-dependent irreversible inhibition of Kv4.1 and Kv4.3 currents. In the absence or presence of Kv4-specific auxiliary subunits, mutational and electrophysiological analyses showed that none of the 14 intracellular cysteines is essential for channel gating. C110, C131, and C132 in the intersubunit interface of the tetramerization domain (T1) are targets responsible for the irreversible inhibition by a methanethiosulfonate derivative (MTSET). This result is surprising because structural studies of Kv4-T1 crystals predicted protection of the targeted thiolate groups by constitutive high-affinity Zn(2+) coordination. Also, added Zn(2+) or a potent Zn(2+) chelator (TPEN) does not significantly modulate the accessibility of MTSET to C110, C131, or C132; and furthermore, when the three critical cysteines remained as possible targets, the MTSET modification rate of the activated state is approximately 200-fold faster than that of the resting state. Biochemical experiments confirmed the chemical modification of the intact alpha-subunit and the purified tetrameric T1 domain by MTS reagents. These results conclusively demonstrate that the T1--T1 interface of Kv4 channels is functionally active and dynamic, and that critical reactive thiolate groups in this interface may not be protected by Zn(2+) binding.

Trafficking of potassium channels

Recent progress in our understanding of the trafficking of potassium channels can be seen in particular when considering the Kv-type channels. To date, we have discovered that folding of the Kv1.3 T1 domain begins in the ribosomal exit tunnel, and that the cell surface expression of Kv4 channels is enhanced by the presence of two recently identified accessory subunits. Current advances are beginning to enable us to understand the Kv supermolecular complex containing these subunits in crystallographic detail. In addition, determinants that govern the dendritic or axonal targeting of Kv channels have also been identified. In terms of the bigger picture, the careful analysis of gene expression patterns in the brain paves the way for studying trafficking in a physiological context. Indeed, neuronal activity has recently been shown to fine-tune the localization of Kv2.1 channels in microdomains of the neuronal plasma membrane.

KChIPs and Kv4 alpha subunits as integral components of A-type potassium channels in mammalian brain

Voltage-gated potassium (Kv) channels from the Kv4, or Shal-related, gene family underlie a major component of the A-type potassium current in mammalian central neurons. We recently identified a family of calcium-binding proteins, termed KChIPs (Kv channel interacting proteins), that bind to the cytoplasmic N termini of Kv4 family alpha subunits and modulate their surface density, inactivation kinetics, and rate of recovery from inactivation (An et al., 2000). Here, we used single and double-label immunohistochemistry, together with circumscribed lesions and coimmunoprecipitation analyses, to examine the regional and subcellular distribution of KChIPs1-4 and Kv4 family alpha subunits in adult rat brain. Immunohistochemical staining using KChIP-specific monoclonal antibodies revealed that the KChIP polypeptides are concentrated in neuronal somata and dendrites where their cellular and subcellular distribution overlaps, in an isoform-specific manner, with that of Kv4.2 and Kv4.3. For example, immunoreactivity for KChIP1 and Kv4.3 is concentrated in the somata and dendrites of hippocampal, striatal, and neocortical interneurons. Immunoreactivity for KChIP2, KChIP4, and Kv4.2 is concentrated in the apical and basal dendrites of hippocampal and neocortical pyramidal cells. Double-label immunofluorescence labeling revealed that throughout the forebrain, KChIP2 and KChIP4 are frequently colocalized with Kv4.2, whereas in cortical, hippocampal, and striatal interneurons, KChIP1 is frequently colocalized with Kv4.3. Coimmunoprecipitation analyses confirmed that all KChIPs coassociate with Kv4 alpha subunits in brain membranes, indicating that KChIPs 1-4 are integral components of native A-type Kv channel complexes and are likely to play a major role as modulators of somatodendritic excitability.

Molecular physiology and modulation of somatodendritic A-type potassium channels

The somatodendritic subthreshold A-type K+ current (ISA) in nerve cells is a critical component of the ensemble of voltage-gated ionic currents that determine somatodendritic signal integration. The underlying K+ channel belongs to the Shal subfamily of voltage-gated K+ channels. Most Shal channels across the animal kingdom share a high degree of structural conservation, operate in the subthreshold range of membrane potentials, and exhibit relatively fast inactivation and recovery from inactivation. Mammalian Shal K+ channels (Kv4) undergo preferential closed-state inactivation with features that are generally inconsistent with the classical mechanisms of inactivation typical of Shaker K+ channels. Here, we review (1) the physiological and genetic properties of ISA, 2 the molecular mechanisms of Kv4 inactivation and its remodeling by a family of soluble calcium-binding proteins (KChIPs) and a membrane-bound dipeptidase-like protein (DPPX), and (3) the modulation of Kv4 channels by protein phosphorylation.

Determinants of voltage-gated potassium channel surface expression and localization in Mammalian neurons

Neurons strictly regulate expression of a wide variety of voltage-dependent ion channels in their surface membranes to achieve precise yet dynamic control of intrinsic membrane excitability. Neurons also exhibit extreme morphological complexity that underlies diverse aspects of their function. Most ion channels are preferentially targeted to either the axonal or somatodendritic compartments, where they become further localized to discrete membrane subdomains. This restricted accumulation of ion channels enables local control of membrane signaling events in specific microdomains of a given compartment. Voltage-dependent K+ (Kv) channels act as potent modulators of diverse excitatory events such as action potentials, excitatory synaptic potentials, and Ca2+ influx. Kv channels exhibit diverse patterns of cellular expression, and distinct subtype-specific localization, in mammalian central neurons. Here we review the mechanisms regulating the abundance and distribution of Kv channels in mammalian neurons and discuss how dynamic regulation of these events impacts neuronal signaling.

The MinK-related peptides

Voltage-gated potassium (Kv) channels mediate rapid, selective diffusion of K+ ions through the plasma membrane, controlling cell excitability, secretion and signal transduction. KCNE genes encode a family of single transmembrane domain proteins called MinK-related peptides (MiRPs) that function as ancillary or beta subunits of Kv channels. When co-expressed in heterologous systems, MiRPs confer changes in Kv channel conductance, gating kinetics and pharmacology, and are fundamental to recapitulation of the properties of some native currents. Inherited mutations in KCNE genes are associated with diseases of cardiac and skeletal muscle, and the inner ear. This article reviews our current understanding of MiRPs--their functional roles, the mechanisms underlying their association with Kv alpha subunits, their patterns of native expression and emerging evidence of the potential roles of MiRPs in the brain. The ubiquity of MiRP expression and their promiscuous association with Kv alpha subunits suggest a prominent role for MiRPs in channel dependent systems.

Transmural expression of transient outward potassium current subunits in normal and failing canine and human hearts

The transient outward current (I(to)), an important contributor to transmural electrophysiological heterogeneity, is significantly remodelled in congestive heart failure (CHF). The molecular bases of transmural I(to) gradients and CHF-dependent ionic remodelling are incompletely understood. To elucidate these issues, we studied mRNA and protein expression of Kv4.3 and KChIP2, the principal alpha and beta subunits believed to form I(to), in epicardial and endocardial tissues and in isolated cardiomyocytes from control dogs and dogs with CHF induced by 240 beats min(-1) ventricular tachypacing. CHF decreased I(to) density in both epicardium and endocardium (by 73 and 55% at +60 mV, respectively), without a significant change in relative current density (endocardium/epicardium 0.11 control, 0.17 CHF). There were transmural gradients in mRNA expression of both Kv4.3 (endocardium/epicardium ratio 0.3 under control conditions) and KChIP2 (endocardium/epicardium ratio 0.2 control), which remained in the presence of CHF (Kv4.3 endocardium/epicardium ratio 0.4; KChIP2 0.4). There were qualitatively similar protein expression gradients in human and canine cardiac tissues and isolated canine cardiomyocytes; however, the KChIP2 gradient was only detectable with a highly selective monoclonal antibody and closely approximated the I(to) density gradient. Kv4.3 mRNA expression was reduced by CHF, but KChIP2 mRNA was not significantly changed. CHF decreased Kv4.3 protein expression in canine cardiac tissues and cardiomyocytes, as well as in terminally failing human heart tissue samples, but KChIP2 protein was not down-regulated in any of the corresponding sample sets. We conclude that both Kv4.3 and KChIP2 may contribute to epicardial-endocardial gradients in I(to), and that I(to) down-regulation in human and canine CHF appears due primarily to changes in Kv4.3.

Modulation of Kv4.2 channel expression and gating by dipeptidyl peptidase 10 (DPP10)

The dipeptidyl aminopeptidase-like protein DPPX (DPP6) associates with Kv4 potassium channels, increasing surface trafficking and reconstituting native neuronal ISA-like properties. Dipeptidyl peptidase 10 (DPP10) shares with DPP6 a high amino acid identity, lack of enzymatic activity, and expression predominantly in the brain. We used a two-electrode voltage-clamp and oocyte expression system to determine if DPP10 also interacts with Kv4 channels and modulates their expression and function. Kv4.2 coimmunoprecipitated with HA/DPP10 from extracts of oocytes heterologously expressing both proteins. Coexpression with DPP10 and HA/DPP10 enhanced Kv4.2 current by approximately fivefold without increasing protein level. DPP10 also remodeled Kv4.2 kinetic and steady-state properties by accelerating time courses of inactivation and recovery (taurec: WT = 200 ms, +DPP10 = 78 ms). Furthermore, DPP10 introduced hyperpolarizing shifts in the conductance-voltage relationship (approximately 19 mV) as well as steady-state inactivation (approximately 7 mV). The effects of DPP10 on Kv4.1 were similar to Kv4.2; however, distinct biophysical differences were observed. Additional experiments suggested that the cytoplasmic N-terminal domain of DPP10 determines the acceleration of inactivation. In summary, DPP10 is a potent modulator of Kv4 expression and biophysical properties and may be a critical component of somatodendritic ISA channels in the brain.

Structure of a Human A-type Potassium Channel Interacting Protein DPPX, a Member of the Dipeptidyl Aminopeptidase Family

It has recently been reported that dipeptidyl aminopeptidase X (DPPX) interacts with the voltage-gated potassium channel Kv4 and that co-expression of DPPX together with Kv4 pore forming alpha-subunits, and potassium channel interacting proteins (KChIPs), reconstitutes properties of native A-type potassium channels in vitro. Here we report the X-ray crystal structure of the extracellular domain of human DPPX determined at 3.0A resolution. This structure reveals the potential for a surface electrostatic change based on the protonation state of histidine. Subtle changes in extracellular pH might modulate the interaction of DPPX with Kv4.2 and possibly with other proteins. We propose models of DPPX interaction with the voltage-gated potassium channel complex. The dimeric structure of DPPX is highly homologous to the related protein DPP-IV. Comparison of the active sites of DPPX and DPP-IV reveals loss of the catalytic serine residue but the presence of an additional serine near the "active" site. However, the arrangement of residues is inconsistent with that of canonical serine proteases and DPPX is unlikely to function as a protease (dipeptidyl aminopeptidase).

Protein–protein interactions of KChIP proteins and Kv4.2

To prove heteromeric assembly of KChIP proteins, the present study is carried out. The results of chemical crosslinking and pull down assay revealed that KChIP1, KChIP2.1, and KChIP2.2 could form homo- as well as hetero-oligomer, and this oligomerization exhibited a Ca(2+)-dependent manner. Moreover, homomeric and heteromeric assembly of KChIPs did not perturb their interaction with Kv4.2 K(+) channel, indicating that the region associated with oligomerization of KChIPs was distinct from that for binding with Kv4.2. Together with previous findings that the net effects of KChIP proteins on the molecular properties and trafficking of Kv channel were different, these observations open a fascinating possibility that the electrophysiological properties of Kv channel may be differently regulated by homomeric and heteromeric assembly of KChIPs.

Three-dimensional structure of I(to); Kv4.2-KChIP2 ion channels by electron microscopy at 21 Angstrom resolution

Regulatory KChIP2 subunits assemble with pore-forming Kv4.2 subunits in 4:4 complexes to produce native voltage-gated potassium (Kv) channels like cardiac I(to) and neuronal I(A) subtypes. Here, negative stain electron microscopy (EM) and single particle averaging reveal KChIP2 to create a novel approximately 35 x 115 x 115 Angstrom, intracellular fenestrated rotunda: four peripheral columns that extend down from the membrane-embedded portion of the channel to enclose the Kv4.2 "hanging gondola" (a platform held beneath the transmembrane conduction pore by four internal columns). To reach the pore from the cytosol, ions traverse one of four external fenestrae to enter the rotundal vestibule and then cross one of four internal windows in the gondola.