Molecular diversity of ion channels and cell function.
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Animals↗
Biomedical subjects
B Rudy
Publications and source records attributed to B Rudy.
Molecular diversity of K+ channels.
K+ channel principal subunits are by far the largest and most diverse of the ion channels. This diversity originates partly from the large number of genes coding for K+ channel principal subunits, but also from other processes such as alternative splicing, generating multiple mRNA transcripts from a single gene, heteromeric assembly of different principal subunits, as well as possible RNA editing and posttranslational modifications. In this chapter, we attempt to give an overview (mostly in tabular format) of the different genes coding for K+ channel principal and accessory subunits and their genealogical relationships. We discuss the possible correlation of different principal subunits with native K+ channels, the biophysical and pharmacological properties of channels formed when principal subunits are expressed in heterologous expression systems, and their patterns of tissue expression. In addition, we devote a section to describing how diversity of K+ channels can be conferred by heteromultimer formation, accessory subunits, alternative splicing, RNA editing and posttranslational modifications. We trust that this collection of facts will be of use to those attempting to compare the properties of new subunits to the properties of others already known or to those interested in a comparison between native channels and cloned candidates.
Alternative Splicing↗
Contributions of Kv3 channels to neuronal excitability.
Four mammalian Kv3 genes have been identified, each of which generates, by alternative splicing, multiple protein products differing in their C-terminal sequence. Products of the Kv3.1 and Kv3.2 genes express similar delayed-rectifier type currents in heterologous expression systems, while Kv3.3 and Kv3.4 proteins express A-type currents. All Kv3 currents activate relatively fast at voltages more positive than -10 mV, and deactivate very fast. The distribution of Kv3 mRNAs in the rodent CNS was studied by in situ hybridization, and the localization of Kv3.1 and Kv3.2 proteins has been studied by immunohistochemistry. Most Kv3.2 mRNAs (approximately 90%) are present in thalamic-relay neurons throughout the dorsal thalamus. The protein is expressed mainly in the axons and terminals of these neurons. Kv3.2 channels are thought to be important for thalamocortical signal transmission. Kv3.1 and Kv3.2 proteins are coexpressed in some neuronal populations such as in fast-spiking interneurons of the cortex and hippocampus, and neurons in the globus pallidus. Coprecipitation studies suggest that in these cells the two types of protein form heteromeric channels. Kv3 proteins appear to mediate, in native neurons, similar currents to those seen in heterologous expression systems. The activation voltage and fast deactivation rates are believed to allow these channels to help repolarize action potentials fast without affecting the threshold for action potential generation. The fast deactivating current generates a quickly recovering after hyperpolarization, thus maximizing the rate of recovery of Na+ channel inactivation without contributing to an increase in the duration of the refractory period. These properties are believed to contribute to the ability of neurons to fire at high frequencies and to help regulate the fidelity of synaptic transmission. Experimental evidence has now become available showing that Kv3.1-Kv3.2 channels play critical roles in the generation of fast-spiking properties in cortical GABAergic interneurons.
Alternative Splicing↗
The role of Kir2.1 in the genesis of native cardiac inward-rectifier K+ currents during pre- and postnatal development.
Our results demonstrate that (a) the Kir2.1 gene encodes a native K+ channel protein with a 21-pS conductance; (b) this channel has an important role in the genesis of adult ventricular 1K1; and (c) the contribution of Kir2.1 channel proteins to 1K1 changes during development. The lack of contribution of Kir2.1 to fetal 1K1 channels is interesting from the point of view of possible future generation of knockout mice lacking Kir2.1, since cardiac abnormalities would not be expected to result in fetal lethality. These observations provide further support for a generalized hypothesis that different genes may code for 1K1 channel proteins at various developmental stages. However, the effects of these AS-oligos must first be examined on native 1K1 channels in cardiac myocytes before definite conclusions can be reached.
Animals↗
Chromosomal mapping of the potassium channel genes Kcnq2 and Kcnq3 in mouse.
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Animals↗
Sexual activity, substance use, and HIV/STD knowledge among detained male adolescents with multiple versus first admissions.
BACKGROUND: Incarcerated and detained youth are at high risk for sexually transmitted diseases (STD), including human immunodeficiency virus (HIV). GOAL OF THE STUDY: To compare the level of sexual activity and substance use-related risk and knowledge regarding HIV/STD among male adolescents with multiple (YMA) versus first admissions (YFA) to a detention facility as a basis for the development of specific intervention strategies. STUDY DESIGN: Sexual and substance use histories, HIV/STD knowledge, and perceived risk were collected through structured interviews of a consecutive sample of detained youth. Human immunodeficiency virus antibody seroprevalence was determined using a blind study of discarded blood. RESULTS: Overall, these youth (N = 486) reported high levels of noninjection drug use, sexual risk activities, and knowledge regarding HIV/STD prevention. Furthermore, most of these youth reported that their risk for HIV infection was low (68%). Eighty-one percent of all youth reported recent (past 6 months) vaginal sex, and 14% reported insertive anal sex. Controlling for age, YMA were more likely to initiate sex at age 13 or younger (OR 1.38; 95% CI, 1.11-1.70), to report eight or more lifetime sex partners (OR 1.36; 95% CI, 1.13-1.63), and to have ever exchanged drugs or money for sex (OR 1.54; 95% CI, 1.08-2.19). However, these youth were less likely to report condom use with their last sex partner (OR 0.74; 95% CI, 0.60-0.93). More than one third (34%) of all youth felt that consistent use of condoms would not provide a high level of protection against HIV. CONCLUSION: Youth with multiple versus first admissions are at higher risk of HIV/STD infections through their lifetime and recent sexual activities. Interventions targeted to this population will need to address the barriers to and facilitators of condom use, strategies to promote positive attitudes toward condoms, and strategies to reduce the high level of alcohol and substance use.
Adolescent↗
Kv3.1-Kv3.2 channels underlie a high-voltage-activating component of the delayed rectifier K+ current in projecting neurons from the globus pallidus.
The globus pallidus plays central roles in the basal ganglia circuitry involved in movement control as well as in cognitive and emotional functions. There is therefore great interest in the anatomic and electrophysiological characterization of this nucleus. Most pallidal neurons are GABAergic projecting cells, a large fraction of which express the calcium binding protein parvalbumin (PV). Here we show that PV-containing pallidal neurons coexpress Kv3. 1 and Kv3.2 K+ channel proteins and that both Kv3.1 and Kv3.2 antibodies coprecipitate both channel proteins from pallidal membrane extracts solubilized with nondenaturing detergents, suggesting that the two channel subunits are forming heteromeric channels. Kv3.1 and Kv3.2 channels have several unusual electrophysiological properties when expressed in heterologous expression systems and are thought to play special roles in neuronal excitability including facilitating sustained high-frequency firing in fast-spiking neurons such as interneurons in the cortex and the hippocampus. Electrophysiological analysis of freshly dissociated pallidal neurons demonstrates that these cells have a current that is nearly identical to the currents expressed by Kv3.1 and Kv3.2 proteins in heterologous expression systems, including activation at very depolarized membrane potentials (more positive than -10 mV) and very fast deactivation rates. These results suggest that the electrophysiological properties of native channels containing Kv3.1 and Kv3.2 proteins in pallidal neurons are not significantly affected by factors such as associated subunits or postranslational modifications that result in channels having different properties in heterologous expression systems and native neurons. Most neurons in the globus pallidus have been reported to fire sustained trains of action potentials at high-frequency. Kv3.1-Kv3.2 voltage-gated K+ channels may play a role in helping maintain sustained high-frequency repetitive firing as they probably do in other neurons.
Animals↗
Function of specific K(+) channels in sustained high-frequency firing of fast-spiking neocortical interneurons.
Fast-spiking GABAergic interneurons of the neocortex and hippocampus fire high-frequency trains of brief action potentials with little spike-frequency adaptation. How these striking properties arise is unclear, although recent evidence suggests K(+) channels containing Kv3.1-Kv3.2 proteins play an important role. We investigated the role of these channels in the firing properties of fast-spiking neocortical interneurons from mouse somatosensory cortex using a pharmacological and modeling approach. Low tetraethylammonium (TEA) concentrations (</=1 mM), which block only a few known K(+) channels including Kv3.1-Kv3.2, profoundly impaired action potential repolarization and high-frequency firing. Analysis of the spike trains evoked by steady depolarization revealed that, although TEA had little effect on the initial firing rate, it strongly reduced firing frequency later in the trains. These effects appeared to be specific to Kv3.1 and Kv3.2 channels, because blockade of dendrotoxin-sensitive Kv1 channels and BK Ca(2+)-activated K(+) channels, which also have high TEA sensitivity, produced opposite or no effects. Voltage-clamp experiments confirmed the presence of a Kv3.1-Kv3.2-like current in fast-spiking neurons, but not in other interneurons. Analysis of spike shape changes during the spike trains suggested that Na(+) channel inactivation plays a significant role in the firing-rate slowdown produced by TEA, a conclusion that was supported by computer simulations. These findings indicate that the unique properties of Kv3.1-Kv3.2 channels enable sustained high-frequency firing by facilitating the recovery of Na(+) channel inactivation and by minimizing the duration of the afterhyperpolarization in neocortical interneurons.
Action Potentials↗
FGF-2 potentiates Ca(2+)-dependent inactivation of NMDA receptor currents in hippocampal neurons.
Peptide growth factors such as the neurotrophins and fibroblast growth factors have potent effects on synaptic transmission, development, and cell survival. We report that chronic (hours) treatment with basic fibroblast growth factor (FGF-2) potentiates Ca(2+)-dependent N-methyl-D-aspartate (NMDA) receptor inactivation in cultured hippocampal neurons. This effect is specific for the NMDA-subtype of ionotropic glutamate receptor and FGF-2. The potentiated inactivation requires ongoing protein synthesis during growth factor treatment and the activity of protein phosphatase 2B (PP2B or calcineurin) during agonist application. These results suggest a mechanism by which FGF-2 receptor signaling may regulate neuronal survival and synaptic plasticity.
Actinin↗
Inhibition of rat ventricular IK1 with antisense oligonucleotides targeted to Kir2.1 mRNA.
The cardiac inward rectifying K+ current (IK1) is important in maintaining the maximum diastolic potential. We used antisense oligonucleotides to determine the role of Kir2.1 channel proteins in the genesis of native rat ventricular IK1. A combination of two antisense phosphorothioate oligonucleotides inhibited heterologously expressed Kir2.1 currents in Xenopus oocytes, either when coinjected with Kir2.1 cRNA or when applied in the incubation medium. Specificity was demonstrated by the lack of inhibition of Kir2.2 and Kir2.3 currents in oocytes. In rat ventricular myocytes (4-5 days culture), these oligonucleotides caused a significant reduction of whole cell IK1 (without reducing the transient outward K+ current or the L-type Ca2+ current). Cell-attached patches demonstrated the occurrence of multiple channel events in control myocytes (8, 14, 21, 35, 43, and 80 pS). The 21-pS channel was specifically knocked down in antisense-treated myocytes (fewer patches contained this channel, and its open frequency was reduced). These results demonstrate that the Kir2.1 gene encodes a specific native 21-pS K(+)-channel protein and that this channel has an essential role in the genesis of cardiac IK1.
Animals↗
Differential expression of Kv4 K+ channel subunits mediating subthreshold transient K+ (A-type) currents in rat brain.
The mammalian Kv4 gene subfamily and its Drosophila Shal counterpart encode proteins that form fast inactivating K+ channels that activate and inactivate at subthreshold potentials and recover from inactivation at a faster rate than other inactivating Kv channels. Taken together, the properties of Kv4 channels compare best with those of low-voltage activating "A-currents" present in the neuronal somatodendritic compartment and widely reported across several types of central and peripheral neurons, as well as the (Ca2+-independent) transient outward potassium conductance of heart cells (Ito). Three distinct genes have been identified that encode mammalian Shal homologs (Kv4. 1, Kv4.2, and Kv4.3), of which the latter two are abundant in rat adult brain and heart tissues. The distribution in the adult rat brain of the mRNA transcripts encoding the three known Kv4 subunits was studied by in situ hybridization histochemistry. Kv4.1 signals are very faint, suggesting that Kv4.1 mRNAs are expressed at very low levels, but Kv4.2 and Kv4.3 transcripts appear to be abundant and each produces a unique pattern of expression. Although there is overlap expression of Kv4.2 and Kv4.3 transcripts in several neuronal populations, the dominant feature is one of differential, and sometimes reciprocal expression. For example, Kv4.2 transcripts are the predominant form in the caudate-putamen, pontine nucleus and several nuclei in the medula, whereas the substantia nigra pars compacta, the restrosplenial cortex, the superior colliculus, the raphe, and the amygdala express mainly Kv4.3. Some brain structures contain both Kv4.2 and Kv4.3 mRNAs but each dominates in distinct neuronal subpopulations. For example, in the olfactory bulb Kv4.2 dominates in granule cells and Kv4.3 in periglomerular cells. In the hippocampus Kv4.2 is the most abundant isoform in CA1 pyramidal cells, whereas only Kv4.3 is expressed in interneurons. Both are abundant in CA2-CA3 pyramidal cells and in granule cells of the dentate gyrus, which also express Kv4.1. In the dorsal thalamus strong Kv4.3 signals are seen in several lateral nuclei, whereas medial nuclei express Kv4.2 and Kv4.3 at moderate to low levels. In the cerebellum Kv4.3, but not Kv4.2, is expressed in Purkinje cells and molecular layer interneurons. In the cerebellar granule cell layer, the reciprocity between Kv4.2 and Kv4.3 is observed in subregions of the same neuronal population. In fact, the distribution of Kv4 channel transcripts in the cerebellum defines a new pattern of compartmentation of the cerebellar cortex and the first one involving molecules directly involved in signal processing.
Action Potentials↗
beta subunits influence the biophysical and pharmacological differences between P- and Q-type calcium currents expressed in a mammalian cell line.
Human epithelial kidney cells (HEK) were prepared to coexpress alpha1A, alpha2delta with different beta calcium channel subunits and green fluorescence protein. To compare the calcium currents observed in these cells with the native neuronal currents, electrophysiological and pharmacological tools were used conjointly. Whole-cell current recordings of human epithelial kidney alpha1A-transfected cells showed small inactivating currents in 80 mM Ba2+ that were relatively insensitive to calcium blockers. Coexpression of alpha1A, betaIb, and alpha2delta produced a robust inactivating current detected in 10 mM Ba2+, reversibly blockable with low concentration of omega-agatoxin IVA (omega-Aga IVA) or synthetic funnel-web spider toxin (sFTX). Barium currents were also supported by alpha1A, beta2a, alpha2delta subunits, which demonstrated the slowest inactivation and were relatively insensitive to omega-Aga IVA and sFTX. Coexpression of beta3 with the same combination as above produced inactivating currents also insensitive to low concentration of omega-Aga IVA and sFTX. These data indicate that the combination alpha1A, betaIb, alpha2delta best resembles P-type channels given the rate of inactivation and the high sensitivity to omega-Aga IVA and sFTX. More importantly, the specificity of the channel blocker is highly influenced by the beta subunit associated with the alpha1A subunit.
Barium↗
K+ channel subunit isoforms with divergent carboxy-terminal sequences carry distinct membrane targeting signals.
Kv3 K+ channel genes encode multiple products by alternative splicing of 3' ends resulting in the expression of K+ channel proteins that differ only in their C-termini. This divergence does not affect the electrophysiological properties of the channels expressed by these proteins. A similar alternative splicing with unknown function is seen in K+ channel genes of other families. We have investigated the possibility that the alternative splicing serves to generate channel subunits with different membrane targeting signals by examining the sorting behavior of three alternatively-spliced Kv3.2 isoforms when expressed in polarized MDCK cells. Two Kv3.2 proteins, Kv3.2b and Kv3.2c were expressed predominantly in the apical membrane, while Kv3.2a was localized mainly to the basolateral side (thought to be equivalent to the axonal and somatodendritic compartments in neurons, respectively). The Kv3.2 mRNA transcripts used in these studies are identical except for their 3' sequence, encoding the extreme C-terminal domain of the protein and the 3'UTR of the mRNA. However, the proteins achieve the same localizations in MDCK cells when expressed from constructs containing or lacking the 3'UTR, indicating that the differential localization is due to targeting signals present in the C' terminal domain of the protein. These results suggest that the alternative splicing of Kv3 genes is involved in channel localization. Since the precise localization of any given ion channel on the neuronal surface has significant functional implications, the results shown here suggest an important function for the alternative splicing of 3' ends seen in many K+ channel genes.
Alternative Splicing↗
Subcellular localization of the K+ channel subunit Kv3.1b in selected rat CNS neurons.
Voltage-gated potassium channels constitute the largest group of heteromeric ion channels discovered to date. Over 20 genes have been isolated, encoding different channel subunit proteins which form functional tetrameric K+ channels. We have analyzed the subcellular localization of subunit Kv3.1b, a member of the Kv3 (Shaw-like) subfamily, in rat brain at the light and electron microscopic level, using immunocytochemical detection. Detailed localization was carried out in specific neurons of the neocortex, hippocampus and cerebellum. The identity of Kv3.1b-positive neurons was established using double labeling with markers for specific neuronal populations. In the neocortex, the Kv3.1b subunit was expressed in most parvalbumin-containing bipolar, basket or chandelier cells, and in some bipolar or double bouquet neurons containing calbindin. In the hippocampus, Kv3.1b was expressed in many parvalbumin-containing basket cells, as well as in calbindin-positive neurons in the stratum oriens, and in a small number of interneurons that did not stain for either parvalbumin or calbindin. Kv3.1b protein was not present in pyramidal cells in the neocortex and the hippocampus, but these cells were outlined by labeled presynaptic terminals from interneuron axons that surround the postsynaptic cell. In the cerebellar cortex, granule cells were the only population expressing the channel protein. Careful examination of individual granule cells revealed a non-uniform distribution of Kv3.1 staining on the somata: circular bands of labeling were present in the vicinity of the axon hillock. In cortical and hippocampal interneurons, as well as in cerebellar granule cells, the Kv3.1b subunit was present in somatic and unmyelinated axonal membranes and adjacent cytoplasm, as well as in the most proximal portion of dendritic processes, but not throughout most of the dendrite. Labeling was also seen in the terminals of labeled axons, but not at a higher concentration than in other parts of the axon. The distribution in the cells analyzed supports a role in action potential transmission by regulating action potential duration.
Animals↗
Molecular characterization of the sodium channel subunits expressed in mammalian cerebellar Purkinje cells.
Inactivating and noninactivating Na+ conductances are known to generate, respectively, the rising phase and the prolonged plateau phase of cerebellar Purkinje cell (PC) action potentials. These conductances have different voltage activation levels, suggesting the possibility that two distinct types of ion channels are involved. Single Purkinje cell reverse transcription-PCR from guinea pig cerebellar slices identified two Na+ channel alpha subunit transcripts, the orthologs of RBI (rat brain I) and Nach6/Scn8a. The latter we shall name CerIII. In situ hybridization histochemistry in rat brain demonstrated broad CerIII expression at high levels in many neuronal groups in the brain and spinal cord, with little if any expression in white matter, or nerve tracts. RBII (rat brain II), the most commonly studied recombinant Na+ channel alpha subunit is not expressed in PCs. As the absence of Scn8a has been correlated with motor endplate disease (med), in which transient sodium currents are spared, RBI appears to be responsible for the transient sodium current in PC. Conversely, jolting mice with a mutated Scn8a message demonstrates PC abnormalities in rapid, simple spike generation, linking CerIII to the persistent sodium current.
Action Potentials↗
Progressive multifocal leukoencephalopathy in an HIV-infected child.
A child with perinatally acquired HIV infection presented with acute neurologic deterioration. A cerebellar white matter lesion seen on CT and MRI later proved to be progressive multifocal leukoencephalopathy (PML) by histology. Although a recognized disease of HIV-infected adults, PML is certain to be seen with more frequency in HIV-infected children who are surviving longer as a result of improved medical care. Recognition of the clinical and radiographic manifestations is important because of the dismal prognosis.
AIDS Dementia Complex↗
Protease inhibitor therapy in children with perinatally acquired HIV infection.
OBJECTIVE: To review the short-term response and safety of protease inhibitor therapy in HIV-infected children. DESIGN: Retrospective chart review of open-label protease inhibitor-containing combination therapy. SETTING: Two urban pediatric HIV centers. PATIENTS: Twenty-eight HIV-infected children were prescribed 30 protease inhibitor-containing antiretroviral therapy combinations. The median age at initiation of protease inhibitor antiretroviral therapy was 79 months. Patients had been on previous antiretroviral therapy for a mean of 45.5 months. RESULTS: Of the 28 children who completed at least 1 month of therapy, 26 experienced marked virologic and immunologic improvement (mean maximal decrease in viral load 1.90 log10 copies/ml; SD, 0.8; mean maximal rise in CD4+ lymphocytes of 279 x 10(6)/l; SD, 300 x 10(6)/l). Eleven patients achieved a viral nadir of < 400 copies/ml, and seven sustained this level of viral suppression for a mean of 6 months. Indinavir use was associated with a high incidence of renal side-effects, including two patients who developed interstitial nephritis. Two patients on ritonavir experienced a significant elevation of liver enzymes. CONCLUSIONS: Protease inhibitor therapy was associated with substantial short-term virologic and immunologic improvement in this primarily heavily pretreated cohort, with 25% maintaining a viral load of < 400 copies/ml after 6 months of therapy. There was a significant rate of adverse events. Pharmacokinetic and safety data are needed to guide aggressive antiretroviral therapy in HIV-infected children, and further treatment options are required for those failing or intolerant to the available protease inhibitors.
Anti-HIV Agents↗
Modulation of Kv4 channels, key components of rat ventricular transient outward K+ current, by PKC.
Current evidence suggests that members of the Kv4 subfamily may encode native cardiac transient outward current (I(to)). Antisense hybrid-arrest with oligonucleotides targeted to Kv4 mRNAs specifically inhibited rat ventricular I(to), supporting this hypothesis. To determine whether protein kinase C (PKC) affects I(to) by an action on these molecular components, we compared the effects of PKC activation on Kv4.2 and Kv4.3 currents expressed in Xenopus oocytes and rat ventricular I(to). Phorbol 12-myristate 13-acetate (PMA) suppressed both Kv4.2 and Kv4.3 currents as well as native I(to), but not after preincubation with PKC inhibitors (e.g., chelerythrine). An inactive stereoisomer of PMA had no effect. Phenylephrine or carbachol inhibited Kv4 currents only when coexpressed, respectively, with alpha1C-adrenergic or M1 muscarinic receptors (this inhibition was also prevented by chelerythrine). The voltage dependence and inactivation kinetics of Kv4.2 were unchanged by PKC, but small effects on the rates of inactivation and recovery from inactivation of native I(to) were observed. Thus Kv4.2 and Kv4.3 proteins are important subunits of native rat ventricular I(to), and PKC appears to reduce this current by affecting the molecular components of the channels mediating I(to).
Animals↗