KChIP1 and frequenin modify shal-evoked potassium currents in pyloric neurons in the lobster stomatogastric ganglion

KV Channel-Interacting Proteins in the Neurological and Cardiovascular Systems: An Updated Review

K_V channel-interacting proteins (KChIP1-4) belong to a family of Ca²⁺-binding EF-hand proteins that are able to bind to the N-terminus of the K_V4 channel α-subunits. KChIPs are predominantly expressed in the brain and heart, where they contribute to the maintenance of the excitability of neurons and cardiomyocytes by modulating the fast inactivating-K_V4 currents. As the auxiliary subunit, KChIPs are critically involved in regulating the surface protein expression and gating properties of K_V4 channels. Mechanistically, KChIP1, KChIP2, and KChIP3 promote the translocation of K_V4 channels to the cell membrane, accelerate voltage-dependent activation, and slow the recovery rate of inactivation, which increases K_V4 currents. By contrast, KChIP4 suppresses K_V4 trafficking and eliminates the fast inactivation of K_V4 currents. In the heart, I_Ks, I_Ca,L, and I_Na can also be regulated by KChIPs. I_Ca,L and I_Na are positively regulated by KChIP2, whereas I_Ks is negatively regulated by KChIP2. Interestingly, KChIP3 is also known as downstream regulatory element antagonist modulator (DREAM) because it can bind directly to the downstream regulatory element (DRE) on the promoters of target genes that are implicated in the regulation of pain, memory, endocrine, immune, and inflammatory reactions. In addition, all the KChIPs can act as transcription factors to repress the expression of genes involved in circadian regulation. Altered expression of KChIPs has been implicated in the pathogenesis of several neurological and cardiovascular diseases. For example, KChIP2 is decreased in failing hearts, while loss of KChIP2 leads to increased susceptibility to arrhythmias. KChIP3 is increased in Alzheimer's disease and amyotrophic lateral sclerosis, but decreased in epilepsy and Huntington's disease. In the present review, we summarize the progress of recent studies regarding the structural properties, physiological functions, and pathological roles of KChIPs in both health and disease. We also summarize the small-molecule compounds that regulate the function of KChIPs. This review will provide an overview and update of the regulatory mechanism of the KChIP family and the progress of targeted drug research as a reference for researchers in related fields.

Multiple roles for frequenin/NCS-1 in synaptic function and development

The calcium-binding protein frequenin (Frq), discovered in the fruit fly Drosophila, and its mammalian homologue neuronal calcium sensor 1 (NCS-1) have been reported to affect several aspects of synaptic transmission, including basal levels of neurotransmission and short- and long-term synaptic plasticities. However, discrepant reports leave doubts about the functional roles of these conserved proteins. In this review, we attempt to resolve some of these seemingly contradictory reports. We discuss how stimulation protocols, sources of calcium (voltage-gated channels versus internal stores), and expression patterns (presynaptic versus postsynaptic) of Frq may result in the activation of various protein targets, leading to different synaptic effects. In addition, the potential interactions of Frq's C-terminal and N-terminal domains with other proteins are discussed. Frq also has a role in regulating neurite outgrowth, axonal regeneration, and synaptic development. We examine whether the effects of Frq on neurotransmitter release and neurite outgrowth are distinct or interrelated through homeostatic mechanisms. Learning and memory are affected by manipulations of Frq probably through changes in synaptic transmission and neurite outgrowth, raising the possibility that Frq may be implicated in human pathological conditions, including schizophrenia, bipolar disorder, and X-linked mental retardation.

Localization and function of Ih channels in a small neural network

Subthreshold ionic currents, which activate below the firing threshold and shape the cell's firing properties, play important roles in shaping neural network activity. We examined the distribution and synaptic roles of the hyperpolarization-activated inward current (I(h)) in the pyloric network of the lobster stomatogastric ganglion (STG). I(h) channels are expressed throughout the STG in a patchy distribution and are highly expressed in the fine neuropil, an area that is rich in synaptic contacts. We performed double labeling for I(h) protein and for the presynaptic marker synaptotagmin. The large majority of labeling in the fine neuropil was adjacent but nonoverlapping, suggesting that I(h) is localized in close proximity to synapses but not in the presynaptic terminals. We compared the pattern of I(h) localization with Shal transient potassium channels, whose expression is coregulated with I(h) in many STG neurons. Unlike I(h), we found significant levels of Shal protein in the soma membrane and the primary neurite. Both proteins were found in the synaptic fine neuropil, but with little evidence of colocalization in individual neurites. We performed electrophysiological experiments to study a potential role for I(h) in regulating synaptic transmission. At a synapse between two identified pyloric neurons, the amplitude of inhibitory postsynaptic potentials (IPSPs) decreased with increasing postsynaptic activation of I(h). Pharmacological block of I(h) restored IPSP amplitudes to levels seen when I(h) was not activated. These experiments suggest that modulation of postsynaptic I(h) might play an important role in the control of synaptic strength in this rhythmogenic neural network.

Model calcium sensors for network homeostasis: sensor and readout parameter analysis from a database of model neuronal networks

In activity-dependent homeostatic regulation (ADHR) of neuronal and network properties, the intracellular Ca(2+) concentration is a good candidate for sensing activity levels because it is correlated with the electrical activity of the cell. Previous ADHR models, developed with abstract activity sensors for model pyloric neurons and networks of the crustacean stomatogastric ganglion, showed that functional activity can be maintained by a regulation mechanism that senses activity levels solely from Ca(2+). At the same time, several intracellular pathways have been discovered for Ca(2+)-dependent regulation of ion channels. To generate testable predictions for dynamics of these signaling pathways, we undertook a parameter study of model Ca(2+) sensors across thousands of model pyloric networks. We found that an optimal regulation signal can be generated for 86% of model networks with a sensing mechanism that activates with a time constant of 1 ms and that inactivates within 1 s. The sensor performed robustly around this optimal point and did not need to be specific to the role of the cell. When multiple sensors with different time constants were used, coverage extended to 88% of the networks. Without changing the sensors, it extended to 95% of the networks by letting the sensors affect the readout nonlinearly. Specific to this pyloric network model, the sensor of the follower pyloric constrictor cell was more informative than the pacemaker anterior burster cell for producing a regulatory signal. Conversely, a global signal indicating network activity that was generated by summing the sensors in individual cells was less informative for regulation.

Fast inactivation of Shal (K(v)4) K+ channels is regulated by the novel interactor SKIP3 in Drosophila neurons

Shal K+ (K(v)4) channels across species carry the major A-type K+ current present in neurons. Shal currents are activated by small EPSPs and modulate post-synaptic potentials, backpropagation of action potentials, and induction of LTP. Fast inactivation of Shal channels regulates the impact of this post-synaptic modulation. Here, we introduce SKIP3, as the first protein interactor of Drosophila Shal K+ channels. The SKIP gene encodes three isoforms with multiple protein-protein interaction domains. SKIP3 is nervous system specific and co-localizes with Shal channels in neuronal cell bodies, and in puncta along processes. Using a genetic deficiency of SKIP, we show that the proportion of neurons displaying a very fast inactivation, consistent with Shal channels exclusively in a "fast" gating mode, is increased in the absence of SKIP3. As a scaffold-like protein, SKIP3 is likely to lead to the identification of a novel regulatory complex that modulates Shal channel inactivation.

A potassium channel (Kv4) cloned from the heart of the tunicate Ciona intestinalis and its modulation by a KChIP subunit

Voltage-gated ion channels of the Kv4 subfamily produce A-type currents whose properties are tuned by accessory subunits termed KChIPs, which are a family of Ca2+ sensor proteins. By modifying expression levels and the intrinsic biophysical properties of Kv4 channels, KChIPs modulate the excitability properties of neurons and myocytes. We studied how a Kv4 channel from a tunicate, the first branching clade of the chordates, is modulated by endogenous KChIP subunits. BLAST searches in the genome of Ciona intestinalis identified a single Kv4 gene and a single KChIP gene, implying that the diversification of both genes occurred during early vertebrate evolution, since the corresponding mammalian gene families are formed by several paralogues. In this study we describe the cloning and characterization of a tunicate Kv4 channel, CionaKv4, and a tunicate KChIP subunit, CionaKChIP. We demonstrate that CionaKChIP strongly modulates CionaKv4 by producing larger currents that inactivate more slowly than in the absence of the KChIP subunit. Furthermore, CionaKChIP shifted the midpoints of activation and inactivation and slowed deactivation and recovery from inactivation of CionaKv4. Modulation by CionaKChIP requires the presence of the intact N terminus of CionaKv4 because, except for a minor effect on inactivation, CionaKChIP did not modulate CionaKv4 channels that lacked amino acids 2-32. In summary, our results suggest that modulation of Kv4 channels by KChIP subunits is an ancient mechanism for modulating electrical excitability.

Structural requirements for rapid inactivation and voltage dependence in splice variants of lobster Shaker potassium channels

We studied the properties of currents generated in Xenopus oocytes by nine splice variants of the spiny lobster Shaker gene. These isoforms differ in their amino termini and in the P-loop region of the pore. Both the voltage dependence and kinetic properties of the currents varied significantly, depending on which amino terminus was present. A cluster of net positive charges at the N-terminus was not necessary for rapid inactivation: negatively charged N-termini also inactivated rapidly. There was no obvious correlation between N-terminus length and inactivation rate. These N-terminal effects were additive with a separate set of voltage and kinetic properties controlled by the two alternative P-loop exons.

Activity-independent coregulation of IA and Ih in rhythmically active neurons

The fast transient potassium or A current (IA) plays an important role in determining the activity of central pattern generator neurons. We have previously shown that the shal K+ channel gene encodes IA in neurons of the pyloric network in the spiny lobster. To further study how IA shapes pyloric neuron and network activity, we microinjected RNA for a shal-GFP fusion protein into four identified pyloric neuron types. Neurons expressing shal-GFP had a constant increase in IA amplitude, regardless of cell type. This increase in IA was paralleled by a concomitant increase in the hyperpolarization-activated cation current Ih in all pyloric neurons. Despite significant increases in these currents, only modest changes in cell firing properties were observed. We used models to test two hypotheses to explain this failure to change firing properties. First, this may reflect the mislocalization of the expressed shal protein solely to the somata and initial neurites of injected neurons, rendering it electrically remote from the integrating region in the neuropil. To test this hypothesis, we generated a multicompartment model where increases in IA could be localized to the soma, initial neurite, or neuropil/axon compartments. Although spike activity was somewhat more sensitive to increases in neuropil/axon versus somatic/primary neurite IA, increases in IA limited to the soma and primary neurite still evoked much more dramatic changes than were seen in the shal-GFP-injected neurons. Second, the effect of the increased IA could be compensated by the endogenous increase in Ih. To test this, we modeled the compensatory increases of IA and Ih with a cycling two-cell model. We found that the increase in Ih was sufficient to compensate the effects of increased IA, provided that they increase in a constant ratio, as we observed experimentally in both shal-injected and noninjected neurons. Thus an activity-independent homeostatic mechanism maintains constant neuronal activity in the face of dramatic increases in IA.

Differential distribution of KChIPs mRNAs in adult mouse brain

The K(+) channel interacting proteins (KChIPs1-4) interact with and modulate activity and trafficking of Kv4 potassium channels. We report here the distribution of KChIPs in adult mouse brain. KChIP1 was expressed in a subpopulation of neurons widely distributed in brain and enriched in Purkinje cells of the cerebellum and in the reticular thalamic and medial habenular nuclei. KChIP2 transcripts were highly expressed in layer IV of the cerebral cortex and in striatum and hippocampus, but expressed at low levels in cerebellum. KChIP3 transcripts were detected primarily in the layer V and deep layer VI of the cerebral cortex, the hippocampus, and the entire cerebellum. KChIP4 was highly expressed by neurons in layers II-IV of cortex and in hippocampus, thalamus and the Purkinje cell layer of the cerebellum. Collectively, different KChIPs appear to be expressed by selected neuronal populations in different brain regions where expression of Kv4.2 and Kv4.3 have been reported. These findings support the likelihood of functional interactions between KChIPs and Kv4 K(+) channels in brain.

Residues within the myristoylation motif determine intracellular targeting of the neuronal Ca2+ sensor protein KChIP1 to post-ER transport vesicles and traffic of Kv4 K+ channels

KChIPs (K+ channel interacting proteins) regulate the function of A-type Kv4 potassium channels by modifying channel properties and by increasing their cell surface expression. We have explored factors affecting the localisation of Kv4.2 and the targeting of KChIP1 and other NCS proteins by using GFP-variant fusion proteins expressed in HeLa cells. ECFP-Kv4.2 expressed alone was not retained in the ER but reached the Golgi complex. In cells co-expressing ECFP-Kv4.2 and KChIP1-EYFP, the two proteins were co-localised and were mainly present on the plasma membrane. When KChIP1-EYFP was expressed alone it was instead targeted to punctate structures. This was distinct from the localisation of the NCS proteins NCS-1 and hippocalcin, which were targeted to the trans-Golgi network (TGN) and plasma membrane. The membrane localisation of each NCS protein required myristoylation and minimal myristoylation motifs of hippocalcin or KChIP1 were sufficient to target fusion proteins to either TGN/plasma membrane or to punctate structures. The existence of targeting information within the N-terminal motifs was confirmed by mutagenesis of residues corresponding to three conserved basic amino acids in hippocalcin and NCS-1 at positions 3, 7 and 9. Residues at these positions determined intracellular targeting to the different organelles. Myristoylation and correct targeting of KChIP1 was required for the efficient traffic of ECFP-Kv4.2 to the plasma membrane. Expression of KChIP1(1-11)-EYFP resulted in the formation of enlarged structures that were positive for ERGIC-53 and beta-COP. ECFP-Kv4.2 was also accumulated in these structures suggesting that KChIP1(1-11)-EYFP inhibited traffic out of the ERGIC. We suggest that KChIP1 is targeted by its myristoylation motif to post-ER transport vesicles where it could interact with and regulate the traffic of Kv4 channels to the plasma membrane under the influence of localised Ca2+ signals.

Electrophysiological and Morphological Characterization of Identified Motor Neurons in theDrosophilaThird Instar Larva Central Nervous System

We have used dye fills and electrophysiological recordings to identify and characterize a cluster of motor neurons in the third instar larval ventral ganglion. This cluster of neurons is similar in position to the well-studied embryonic RP neurons. Dye fills of larval dorsomedial neurons demonstrate that individual neurons within the cluster can be reproducibly identified by observing their muscle targets and bouton morphology. The terminal targets of these five neurons are body wall muscles 6/7, 1, 14, and 30 and the intersegmental nerve (ISN) terminal muscles (1, 2, 3, 4, 9, 10, 19, 20). All cells except the ISN neuron, which has a type Is ending, display type Ib boutons. Two of these neurons appear to be identical to the embryonic RP3 and aCC cells, which define the most proximal and distal innervations within a hemisegment. The targets of the other neurons in the larval dorsomedial cluster do not correspond to embryonic targets of the neurons in the RP cluster, suggesting rewiring of this circuit during early larval stages. Electrophysiological studies of the five neurons in current clamp revealed that type Is neurons have a longer delay in the appearance of the first spike compared with type Ib neurons. Genetic, biophysical, and pharmacological studies in current and voltage clamp show this delay is controlled by the kinetics and voltage sensitivity of inactivation of a current whose properties suggest that it may be the Shal IAcurrent. The combination of genetic identification and whole cell recording allows us to directly explore the cellular substrates of neural and locomotor behavior in an intact system.

An ER export signal accelerates the surface expression of shal potassium channels in pyloric neurons of the lobster stomatogastric ganglion

The shal gene encoding the transient potassium current, I(A), plays important roles in shaping the firing properties of neurons in the pyloric network in the stomatogastric ganglion (STG) of the spiny lobster, Panulirus interruptus. However, when we overexpressed the shal protein in pyloric dilator (PD) neurons, the effect of increased I(A )was compensated by a parallel upregulation of the hyperpolarization activated inward current ( I(h)). In an attempt to temporally separate the overexpression of shal from the compensatory up-regulation of I(h) channels, we inserted an endoplasmic reticulum (ER) export signal sequence, FCYENE, into the shal gene. This signal sequence accelerated the surface expression of shal protein in Xenopus oocytes and PD neurons. However, the accelerated expression of shal still did not alter the firing properties of the injected neuron, suggesting that the compensatory upregulation of I(h) occurs simultaneously with the upregulation of I(A).