Role of subunit heteromerization and N-linked glycosylation in the formation of functional hyperpolarization-activated cyclic nucleotide-gated channels

Complex N-glycosylation stabilizes surface expression of transient receptor potential melastatin 4b protein

N-glycosylation is important for the function and regulation of ion channels. We examined the role of N-glycosylation of transient receptor potential melastatin (Trpm) 4b, a membrane glycoprotein that regulates calcium influx. Trpm4b was expressed in vivo in all rat tissues examined. In each tissue, Trpm4b had a different molecular mass, between ∼129 and ∼141 kDa, but all reverted to ∼120 kDa following treatment with peptide:N-glycosidase F, consistent with N-glycosylation being the principal form of post-translational modification of Trpm4b in vivo. In six stable isogenic cell lines that express different levels of Trpm4b, two forms were found, high mannose, core-glycosylated and complex, highly glycosylated (HG), with HG-Trpm4b comprising 85% of the total Trpm4b expressed. For both forms, surface expression was directly proportional to the total Trpm4b expressed. Complex N-glycosylation doubled the percentage of Trpm4b at the surface, compared with high mannose N-glycosylation. Mutation of the single N-glycosylation consensus sequence at Asn-988 (Trpm4b-N988Q), located near the pore-forming loop between transmembrane helices 5 and 6, prevented glycosylation, but did not prevent surface expression, impair formation of functional membrane channels, or alter channel conductance. In transfection experiments, the time courses for appearance of HG-Trpm4b and Trpm4b-N988Q on the surface were similar. In experiments with cycloheximide inhibition of protein synthesis, the time course for disappearance of HG-Trpm4b from the surface was much slower than that for Trpm4b-N988Q. We conclude that N-glycosylation is not required for surface expression or channel function, but that complex N-glycosylation plays a crucial role in stabilizing surface expression of Trpm4b.

Chronic atrial fibrillation alters the functional properties of If in the human atrium

INTRODUCTION

Despite the evidence that the hyperpolarization-activated current (If) is highly modulated in human cardiomyopathies, no definite data exist in chronic atrial fibrillation (cAF). We investigated the expression, function, and modulation of If in human cAF.

METHODS AND RESULTS

Right atrial samples were obtained from sinus rhythm (SR, n = 49) or cAF (duration >1 year, n = 31) patients undergoing corrective cardiac surgery. Among f-channel isoforms expressed in the human atrium (HCN1, 2 and 4), HCN4 mRNA levels measured by RT-PCR were significantly reduced. However, protein expression was preserved in cAF compared to SR (+85% for HCN4); concurrently, miR-1 expression was significantly reduced. In patch-clamped atrial myocytes, current-specific conductance (gf) was significantly increased in cAF at voltages around the threshold for If activation (-60 to -80 mV); accordingly, a 10-mV rightward shift of the activation curve occurred (P < 0.01). β-Adrenergic and 5-HT4 receptor stimulation exerted similar effects on If in cAF and SR cells, while the ANP-mediated effect was significantly reduced (P < 0.02), suggesting downregulation of natriuretic peptide signaling.

CONCLUSIONS

In human cAF modifications in transcriptional and posttranscriptional mechanisms of HCN channels occur, associated with a slight yet significant gain-of-function of If , which may contribute to enhanced atrial ectopy.

Up-regulation of hyperpolarization-activated cyclic nucleotide-gated channel 3 (HCN3) by specific interaction with K+ channel tetramerization domain-containing protein 3 (KCTD3)

Most ion channels consist of the principal ion-permeating core subunit(s) and accessory proteins that are assembled with the channel core. The biological functions of the latter proteins are diverse and include the regulation of the biophysical properties of the ion channel, its connection to signaling pathways and the control of its cell surface expression. There is recent evidence that native hyperpolarization-activated cyclic nucleotide-gated channel complexes (HCN1-4) also contain accessory subunits, among which TRIP8b (tetratricopeptide repeat-containing Rab8b-interacting protein) has been most extensively studied. Here, we identify KCTD3, a so far uncharacterized member of the potassium channel tetramerization-domain containing (KCTD) protein family as an HCN3-interacting protein. KCTD3 is widely expressed in brain and some non-neuronal tissues and colocalizes with HCN3 in specific regions of the brain including hypothalamus. Within the HCN channel family, KCTD3 specifically binds to HCN3 and leads to a profound up-regulation of cell surface expression and current density of this channel. HCN3 can also functionally interact with TRIP8b; however, we found no evidence for channel complexes containing both TRIP8b and KCTD3. The C terminus of HCN3 is crucially required for functional interaction with KCTD3. Replacement of the cytosolic C terminus of HCN2 by the corresponding domain of HCN3 renders HCN2 sensitive to regulation by KCTD3. The C-terminal-half of KCTD3 is sufficient for binding to HCN3. However, the complete protein including the N-terminal tetramerization domain is needed for HCN3 current up-regulation. Together, our experiments indicate that KCTD3 is an accessory subunit of native HCN3 complexes.

Peripheral hyperpolarization-activated cyclic nucleotide-gated channels contribute to inflammation-induced hypersensitivity of the rat temporomandibular joint

BACKGROUND

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels conduct an inward cation current (Ih ) that contributes to the maintenance of neuronal membrane potential and have been implicated in a number of animal models of neuropathic and inflammatory pain. In the current study, we investigated HCN channel involvement in inflammatory pain of the temporomandibular joint (TMJ).

METHODS

The contribution of HCN channels to inflammation (complete Freund's adjuvant; CFA)-induced mechanical hypersensitivity of the rat TMJ was tested with injections of the HCN channel blocker ZD7288. Retrograde labelling and immunohistochemistry was used to explore HCN channel expression in sensory neurons that innervate the TMJ.

RESULTS

Injection of CFA into the TMJ (n = 7) resulted in a significantly increased mechanical sensitivity relative to vehicle injection (n = 7) (p < 0.05). The mechanical hypersensitivity generated by CFA injection was blocked by co-injection of ZD7288 with the CFA (n = 7). Retrograde labelling and immunohistochemistry experiments revealed expression predominantly of HCN1 and HCN2 channel subunits in trigeminal ganglion neurons that innervate the TMJ (n = 3). No change in the proportion or intensity of HCN channel expression was found in inflamed (n = 6) versus control (n = 5) animals at the time point tested.

CONCLUSIONS

Our findings suggest a role for peripheral HCN channels in inflammation-induced pain of the TMJ. Peripheral application of a HCN channel blocker could provide therapeutic benefit for inflammatory TMJ pain and avoid side effects associated with activation of HCN channels in the central nervous system.

Cocaine sensitization increases I h current channel subunit 2 (HCN₂) protein expression in structures of the mesocorticolimbic system

Alteration of the biological activity among neuronal components of the mesocorticolimbic (MCL) system has been implicated in the pathophysiology of drug abuse. Changes in the electrophysiological properties of neurons involved in the reward circuit seem to be of utmost importance in addiction. The hyperpolarization-activated cyclic nucleotide current, I h, is a prominent mixed cation current present in neurons. The biophysical properties of the I h and its potential modulatory role in cell excitability depend on the expression profile of the hyperpolarization-activated cyclic nucleotide gated channel (HCN) subunits. We investigated whether cocaine-induced behavioral sensitization, an animal model of drug addiction, elicits region-specific changes in the expression of the HCN₂ channel's subunit in the MCL system. Tissue samples from the ventral tegmental area, prefrontal cortex, nucleus accumbens, and hippocampus were analyzed using Western blot. Our findings demonstrate that cocaine treatment induced a significant increase in the expression profile of the HCN₂ subunit in both its glycosylated and non-glycosylated protein isoforms in all areas tested. The increase in the glycosylated isoform was only observed in the ventral tegmental area. Together, these data suggest that the observed changes in MCL excitability during cocaine addiction might be associated with alterations in the subunit composition of their HCN channels.

Asymmetric divergence in structure and function of HCN channel duplicates in Ciona intestinalis

Hyperpolarization-activated Cyclic Nucleotide (HCN) channels are voltage-gated cation channels and are critical for regulation of membrane potential in electrically active cells. To understand the evolution of these channels at the molecular level, we cloned and examined two of three HCN homologs of the urochordate Ciona intestinalis (ciHCNa and ciHCNb). ciHCNa is like mammalian HCNs in that it possesses similar electrical function and undergoes N-glycosylation of a sequon near the pore. ciHCNb lacks the pore-associated N-glycosylation sequon and is predictably not N-glycosylated, and it also has an unusual gating phenotype in which the channel's voltage-sensitive gate appears to close incompletely. Together with previous findings, the data support an evolutionary trajectory in which an HCN ancestor underwent lineage-specific duplication in Ciona, to yield one HCN with most features that are conserved with the mammalian HCNs and another HCN that has been uniquely altered.

HCN1 and HCN2 proteins are expressed in cochlear hair cells: HCN1 can form a ternary complex with protocadherin 15 CD3 and F-actin-binding filamin A or can interact with HCN2

A unique coupling between HCN1 and stereociliary tip-link protein protocadherin 15 has been described for a teleost vestibular hair-cell model and mammalian organ of Corti (OC) (Ramakrishnan, N. A., Drescher, M. J., Barretto, R. L., Beisel, K. W., Hatfield, J. S., and Drescher, D. G. (2009) J. Biol. Chem. 284, 3227-3238). We now show that Ca(2+)-dependent interaction of the organ of Corti HCN1 and protocadherin 15 CD3 is mediated by amino-terminal sequence specific to HCN1 and is not replicated by analogous specific peptides for HCN2 or HCN4 nor by amino-terminal sequence conserved across HCN isoforms utilized in channel formation. Furthermore, the HCN1-specific peptide binds both phosphatidylinositol (3,4,5)-trisphosphate and phosphatidylinositol (4,5)-bisphosphate but not phosphatidylinositol 4-phosphate. Singly isolated cochlear inner and outer hair cells express HCN1 transcript, and HCN1 and HCN2 protein is immunolocalized to hair-cell stereocilia by both z-stack confocal and pre-embedding EM immunogold microscopy, with stereociliary tip-link and subcuticular plate sites. Quantitative PCR indicates HCN1/HCN2/HCN3/HCN4 = 9:9:1:89 in OC of the wild-type mouse, with HCN4 protein primarily attributable to inner sulcus cells. A mutant form of HCN1 mRNA and protein is expressed in the OC of an HCN1 mutant, corresponding to a full-length sequence with the in-frame deletion of pore-S6 domains, predicted by construct. The mutant transcript of HCN1 is ∼9-fold elevated relative to wild-type levels, possibly representing molecular compensation, with unsubstantial changes in HCN2, HCN3, and HCN4. Immunoprecipitation protocols indicate alternate interactions of full-length proteins; HCN1 can interact with protocadherin 15 CD3 and F-actin-binding filamin A forming a complex that does not include HCN2, or HCN1 can interact with HCN2 forming a complex without protocadherin 15 CD3 but including F-actin-binding fascin-2.

Distinct perinatal features of the hyperpolarization-activated non-selective cation current I(h) in the rat cortical plate

BACKGROUND

During neocortical development, multiple voltage- and ligand-gated ion channels are differentially expressed in neurons thereby shaping their intrinsic electrical properties. One of these voltage-gated ion channels, the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel and its current I(h), is an important regulator of neuronal excitability. Thus far, studies on an early I(h) appearance in rodent neocortex are missing or conflicting. Therefore, we focused our study on perinatal neocortical I(h) and its properties.

RESULTS

In the perinatal rat neocortex we observed a rapid increase in the number of neurons exhibiting I(h). Perinatal I(h) had unique properties: first, a pronounced cAMP sensitivity resulting in a marked shift of the voltage sufficient for half-maximum activation of the current towards depolarized voltages and second, an up to 10 times slower deactivation at physiological membrane potentials when compared to the one at postnatal day 30. The combination of these features was sufficient to suppress membrane resonance in our in silico and in vitro experiments. Although all four HCN subunits were present on the mRNA level we only detected HCN4, HCN3 and HCN1 on the protein level at P0. HCN1 protein at P0, however, appeared incompletely processed. At P30 glycosilated HCN1 and HCN2 dominated. By in silico simulations and heterologous co-expression experiments of a 'slow' and a 'fast' I(h) conducting HCN channel subunit in HEK293 cells, we mimicked most characteristics of the native current, pointing to a functional combination of subunit homo- or heteromeres.

CONCLUSION

Taken together, these data indicate a HCN subunit shift initiated in the first 24 hours after birth and implicate a prominent perinatal role of the phylogenetically older HCN3 and/or HCN4 subunits in the developing neocortex.

Morphological, neurochemical and electrophysiological features of parvalbumin-expressing cells: a likely source of axo-axonic inputs in the mouse spinal dorsal horn

Perception of normal bodily sensations relies on the precise regulation of sensory information entering the dorsal horn of the spinal cord. Inhibitory, axoaxonic, synapses provide a mechanism for this regulation, but the source of these important inhibitory connections remains to be elucidated. This study shows that a subpopulation of spinal interneurons that expresses parvalbumin and have specific morphological, connectivity and functional characteristics are a likely source of the inhibitory inputs that selectivity regulate non-noxious tactile input in the spinal cord. Our findings suggest that a loss of normal function in parvalbumin positive dorsal horn neurons may result in the development of tactile allodynia, where non-painful stimuli gain the capacity to evoke the sensation of pain.

The biological effects of ivabradine in cardiovascular disease

A large number of studies in healthy and asymptomatic subjects, as well as patients with already established cardiovascular disease (CAD) have demonstrated that heart rate (HR) is a very important and major independent cardiovascular risk factor for prognosis. Lowering heart rate reduces cardiac work, thereby diminishing myocardial oxygen demand. Several experimental studies in animals, including dogs and pigs, have clarified the beneficial effects of ivabradine associated with HR lowering. Ivabradine is a selective inhibitor of the hyperpolarisation activated cyclic-nucleotide-gated funny current (If) involved in pacemaker generation and responsiveness of the sino-atrial node (SAN), which result in HR reduction with no other apparent direct cardiovascular effects. Several studies show that ivabradine substantially and significantly reduces major risks associated with heart failure when added to guideline-based and evidence-based treatment. However the biological effect of ivabradine have yet to be studied. This effects can appear directly on myocardium or on a systemic level improving endothelial function and modulating immune cell migration. Indeed ivabradine is an 'open-channel' blocker of human hyperpolarization-activated cyclic nucleotide gated channels of type-4 (hHCN4), and a 'closed-channel' blocker of mouse HCN1 channels in a dose-dependent manner. At endothelial level ivabradine decreased monocyte chemotactin protein-1 mRNA expression and exerted a potent anti-oxidative effect through reduction of vascular NADPH oxidase activity. Finally, on an immune level, ivabradine inhibits the chemokine-induced migration of CD4-positive lymphocytes. In this review, we discuss the biological effects of ivabradine and highlight its effects on CAD.

Chronic inflammatory pain is associated with increased excitability and hyperpolarization-activated current (Ih) in C- but not Aδ-nociceptors

Inflammatory pain hypersensitivity results partly from hyperexcitability of nociceptive (damage-sensing) dorsal root ganglion (DRG) neurons innervating inflamed tissue. However, most of the evidence for this is derived from experiments using acute inflammatory states. Herein, we used several approaches to examine the impact of chronic or persistent inflammation on the excitability of nociceptive DRG neurons and on their expression of I(h) and the underlying hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which regulate neuronal excitability. Using in vivo intracellular recordings of somatic action potentials from L4/L5 DRG neurons in normal rats and rats with hindlimb inflammation induced by complete Freund's adjuvant (CFA), we demonstrate increased excitability of C- but not Aδ-nociceptors, 5 to 7 days after CFA. This included an afterdischarge response to noxious pinch, which may contribute to inflammatory mechanohyperalgesia, and increased incidence of spontaneous activity (SA) and decreased electrical thresholds, which are likely to contribute to spontaneous pain and nociceptor sensitization, respectively. We also show, using voltage clamp in vivo, immunohistochemistry and behavioral assays that (1) the inflammation-induced nociceptor hyperexcitability is associated, in C- but not Aδ-nociceptors, with increases in the mean I(h) amplitude/density and in the proportion of I(h) expressing neurons, (2) increased proportion of small DRG neurons (mainly IB4-negative) expressing HCN2 but not HCN1 or HCN3 channel protein, (3) increased HCN2- immunoreactivity in the spinal dorsal horn, and (4) attenuation of inflammatory mechanoallodynia with the selective I(h) antagonist, ZD7288. Taken together, the findings suggest that C- but not Aδ-nociceptors sustain chronic inflammatory pain and that I(h)/HCN2 channels contribute to inflammation-induced C-nociceptor hyperexcitability.

HCN channels expressed in the inner ear are necessary for normal balance function

HCN1-4 subunits form Na+/K+-permeable ion channels that are activated by hyperpolarization and carry the current known as I(h). I(h) has been characterized in vestibular hair cells of the inner ear, but its molecular correlates and functional contributions have not been elucidated. We examined Hcn mRNA expression and immunolocalization of HCN protein in the mouse utricle, a mechanosensitive organ that contributes to the sense of balance. We found that HCN1 is the most highly expressed subunit, localized to the basolateral membranes of type I and type II hair cells. We characterized I(h) using the whole-cell, voltage-clamp technique and found the current expressed in 84% of the cells with a mean maximum conductance of 4.4 nS. I(h) was inhibited by ZD7288, cilobradine, and by adenoviral expression of a dominant-negative form of HCN2. To determine which HCN subunits carried I(h), we examined hair cells from mice deficient in Hcn1, 2, or both. I(h) was completely abolished in hair cells of Hcn1⁻/⁻ mice and Hcn1/2⁻/⁻ mice but was similar to wild-type in Hcn2⁻/⁻ mice. To examine the functional contributions of I(h), we recorded hair cell membrane responses to small hyperpolarizing current steps and found that activation of I(h) evoked a 5-10 mV sag depolarization and a subsequent 15-20 mV rebound upon termination. The sag and rebound were nearly abolished in Hcn1-deficient hair cells. We also found that Hcn1-deficient mice had deficits in vestibular-evoked potentials and balance assays. We conclude that HCN1 contributes to vestibular hair cell function and the sense of balance.

Flexible architecture of IP3R1 by Cryo-EM

Inositol 1,4,5-trisphosphate receptors (IP3Rs) play a fundamental role in generating Ca2+ signals that trigger many cellular processes in virtually all eukaryotic cells. Thus far, the three-dimensional (3D) structure of these channels has remained extremely controversial. Here, we report a subnanometer resolution electron cryomicroscopy (cryo-EM) structure of a fully functional type 1 IP3R from cerebellum in the closed state. The transmembrane region reveals a twisted bundle of four α helices, one from each subunit, that form a funnel shaped structure around the 4-fold symmetry axis, strikingly similar to the ion-conduction pore of K+ channels. The lumenal face of IP3R1 has prominent densities that surround the pore entrance and similar to the highly structured turrets of Kir channels. 3D statistical analysis of the cryo-EM density map identifies high variance in the cytoplasmic region. This structural variation could be attributed to genuine structural flexibility of IP3R1.

Differential regulation of HCN channel isoform expression in thalamic neurons of epileptic and non-epileptic rat strains

Hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels represent the molecular substrate of the hyperpolarization-activated inward current (I(h)). Although these channels act as pacemakers for the generation of rhythmic activity in the thalamocortical network during sleep and epilepsy, their developmental profile in the thalamus is not yet fully understood. Here we combined electrophysiological, immunohistochemical, and mathematical modeling techniques to examine HCN gene expression and I(h) properties in thalamocortical relay (TC) neurons of the dorsal part of the lateral geniculate nucleus (dLGN) in an epileptic (WAG/Rij) compared to a non-epileptic (ACI) rat strain. Recordings of TC neurons between postnatal day (P) 7 and P90 in both rat strains revealed that I(h) was characterized by higher current density, more hyperpolarized voltage dependence, faster activation kinetics, and reduced cAMP-sensitivity in epileptic animals. All four HCN channel isoforms (HCN1-4) were detected in dLGN, and quantitative analyses revealed a developmental increase of protein expression of HCN1, HCN2, and HCN4 but a decrease of HCN3. HCN1 was expressed at higher levels in WAG/Rij rats, a finding that was correlated with increased expression of the interacting proteins filamin A (FilA) and tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b). Analysis of a simplified computer model of the thalamic network revealed that the alterations of I(h) found in WAG/Rij rats compensate each other in a way that leaves I(h) availability constant, an effect that ensures unaltered cellular burst activity and thalamic oscillations. These data indicate that during postnatal developmental the hyperpolarizing shift in voltage dependency (resulting in less current availability) is compensated by an increase in current density in WAG/Rij thereby possibly limiting the impact of I(h) on epileptogenesis. Because HCN3 is expressed higher in young versus older animals, HCN3 likely does not contribute to alterations in I(h) in older animals.

Colocalization of hyperpolarization-activated, cyclic nucleotide-gated channel subunits in rat retinal ganglion cells

The current-passing pore of mammalian hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels is formed by subunit isoforms denoted HCN1-4. In various brain areas, antibodies directed against multiple isoforms bind to single neurons, and the current (I(h)) passed during hyperpolarizations differs from that of heterologously expressed homomeric channels. By contrast, retinal rod, cone, and bipolar cells appear to use homomeric HCN channels. Here, we assess the generality of this pattern by examining HCN1 and HCN4 immunoreactivity in rat retinal ganglion cells, measuring I(h) in dissociated cells, and testing whether HCN1 and HCN4 proteins coimmunoprecipitate. Nearly half of the ganglion cells in whole-mounted retinae bound antibodies against both isoforms. Consistent with colocalization and physical association, 8-bromo-cAMP shifted the voltage sensitivity of I(h) less than that of HCN4 channels and more than that of HCN1 channels, and HCN1 coimmunoprecipitated with HCN4 from membrane fraction proteins. Finally, the immunopositive somata ranged in diameter from the smallest to the largest in rat retina, the dendrites of immunopositive cells arborized at various levels of the inner plexiform layer and over fields of different diameters, and I(h) activated with similar kinetics and proportions of fast and slow components in small, medium, and large somata. These results show that different HCN subunits colocalize in single retinal ganglion cells, identify a subunit that can reconcile native I(h) properties with the previously reported presence of HCN4 in these cells, and indicate that I(h) is biophysically similar in morphologically diverse retinal ganglion cells and differs from I(h) in rods, cones, and bipolar cells.

The fast and slow ups and downs of HCN channel regulation

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels (h channels) form the molecular basis for the hyperpolarization-activated current, I(h), and modulation of h channels contributes to changes in cellular properties critical for normal functions in the mammalian brain and heart. Numerous mechanisms underlie h channel modulation during both physiological and pathological conditions, leading to distinct changes in gating, kinetics, surface expression, channel conductance or subunit composition of h channels. Here we provide a focused review examining mechanisms of h channel regulation, with an emphasis on recent findings regarding interacting proteins such as TRIP8b. This review is intended to serve as a comprehensive resource for physiologists to provide potential molecular mechanisms underlying functionally important changes in I(h) in different biological models, as well as for molecular biologists to delineate the predicted h channel changes associated with complex regulatory mechanisms in both normal function and in disease states.

Towards an integrated view of HCN channel role in epilepsy

Epilepsy is the third most common brain disorder and affects millions of people. Epilepsy is characterized by the occurrence of spontaneous seizures, that is, bursts of synchronous firing of large populations of neurons. These are believed to result from abnormal regulation of neuronal excitability that favors hypersynchrony. Among the intrinsic conductances that govern neuronal excitability, the hyperpolarization-activated current (I(h)) plays complex and important roles in the fine-tuning of both cellular and network activity. Not surprisingly, dysregulation of I(h) and/or of its conducting ion-channels (HCN) has been strongly implicated in various experimental models of epilepsy, as well as in human epilepsy. Here we provide an overview of recent findings on the distinct physiological roles played by I(h) in specific contexts, and the cellular mechanisms that underlie these functions, including the subunit make-up of the channels. We further discuss current knowledge of dysregulation of I(h) and HCN channels in epilepsy in light of the multifaceted functions of I(h) in the brain.

PIP2-mediated HCN3 channel gating is crucial for rhythmic burst firing in thalamic intergeniculate leaflet neurons

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels generate a pacemaking current, I(h), which regulates neuronal excitability and oscillatory activity in the brain. Although all four HCN isoforms are expressed in the brain, the functional contribution of HCN3 is unknown. Using immunohistochemistry, confocal microscopy, and whole-cell patch-clamp recording techniques, we investigated HCN3 function in thalamic intergeniculate leaflet (IGL) neurons, as HCN3 is reportedly preferentially expressed in these cells. We observed that I(h) recorded from IGL, but not ventral geniculate nucleus, neurons in HCN2(+/+) mice and rats activated slowly and were cAMP insensitive, which are hallmarks of HCN3 channels. We also observed strong immunolabeling for HCN3, with no labeling for HCN1 and HCN4, and only very weak labeling for HCN2. Deletion of HCN2 did not alter I(h) characteristics in mouse IGL neurons. These data together indicate that the HCN3 channel isoform generated I(h) in IGL neurons. Intracellular phosphatidylinositol-4,5-bisphosphate (PIP(2)) shifted I(h) activation to more depolarized potentials and accelerated activation kinetics. Upregulation of HCN3 function by PIP(2) augmented low-threshold burst firing and spontaneous oscillations; conversely, depletion of PIP(2) or pharmacologic block of I(h) resulted in a profound inhibition of excitability. The results indicate that functional expression of HCN3 channels in IGL neurons is crucial for intrinsic excitability and rhythmic burst firing, and PIP(2) serves as a powerful modulator of I(h)-dependent properties via an effect on HCN3 channel gating. Since the IGL is a major input to the suprachiasmatic nucleus, regulation of pacemaking function by PIP(2) in the IGL may influence sleep and circadian rhythms.

Postnatal maturation of the hyperpolarization-activated cation current, I(h), in trigeminal sensory neurons

Hyperpolarization-activated inward currents (I(h)) contribute to neuronal excitability in sensory neurons. Four subtypes of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels generate I(h), with different activation kinetics and cAMP sensitivities. The aim of the present study was to examine the postnatal development of I(h) and HCN channel subunits in trigeminal ganglion (TG) neurons. I(h) was investigated in acutely dissociated TG neurons from rats aged between postnatal day (P)1 and P35 with whole cell patch-clamp electrophysiology. In voltage-clamp studies, I(h) was activated by a series of hyperpolarizing voltage steps from -40 mV to -120 mV in -10-mV increments. Tail currents from a common voltage step (-100 mV) were used to determine I(h) voltage dependence. I(h) activation was faster in older rats and occurred at more depolarized potentials; the half-maximal activation voltage (V(1/2)) changed from -89.4 mV (P1) to -81.6 mV (P35). In current-clamp studies, blocking I(h) with ZD7288 caused membrane hyperpolarization and increases in action potential half-duration at all postnatal ages examined. ZD7288 also reduced the action potential firing frequency in multiple-firing neurons. Western blot analysis of the TG detected immunoreactive bands corresponding to all HCN subtypes. HCN1 and HCN2 band density increased with postnatal age, whereas the low-intensity HCN3 and moderate-intensity HCN4 bands were not changed. This study suggests that functional I(h) are activated in rat trigeminal sensory neurons from P1 during postnatal development, have an increasing role with age, and modify neuronal excitability.

The glutamic acid-rich protein is a gating inhibitor of cyclic nucleotide-gated channels

The cyclic nucleotide-gated (CNG) cation channel of rod photoreceptors is a heterotetramer consisting of homologous CNGA1 and CNGB1a subunits. While CNGA1 is indispensable for channel activation, the specific role of CNGB1a in this process has remained elusive. Here, we show that the N-terminal glutamic acid-rich protein (GARP) domain of CNGB1a and soluble GARP2, which corresponds to the proximal portion of the GARP domain, act as autoinhibitory domains that decrease the opening probability of the CNG channel. In the presence of mutations that structurally impair the cyclic nucleotide-binding domain (CNBD) of CNGB1a, the GARP domain completely abolishes channel activity. In agreement with an inhibitory function of GARP, the activity of mutant CNG channels could be fully restored by deletion of the GARP domain. We identified two sequences within the GARP domain that confer most of the inhibitory effect and demonstrate that the profound inhibition imposed by the GARP domain is caused by direct and autonomous protein-protein interaction with the CNG channel complex. In wild-type rod CNG channels, this inhibitory effect can be relieved by binding of cGMP to the CNBD of CNGB1a. In conclusion, we propose that the N terminus of CNGB1a and soluble GARPs act as molecular gate keepers that control the activation of heteromeric rod CNG channels. Our results suggest that the GARP domain has evolved in rod photoreceptors to reduce current noise resulting from openings of CNG channels in the absence of cGMP.

Immunostaining for the α3 isoform of the Na+/K+-ATPase is selective for functionally identified muscle spindle afferents in vivo

Muscle spindle afferent (MSA) neurons can show rapid and sustained firing. Immunostaining for the α3 isoform of the Na⁺/K⁺-ATPase (α3) in some large dorsal root ganglion (DRG) neurons and large intrafusal fibres suggested α3 expression in MSAs (Dobretsov et al. 2003), but not whether α3-immunoreactive DRG neuronal somata were exclusively MSAs. We found that neuronal somata with high α3 immunointensity were neurofilament-rich, suggesting they have A-fibres; we therefore focussed on A-fibre neurons to determine the sensory properties of α3-immunoreactive neurons. We examined α3 immunointensity in 78 dye-injected DRG neurons whose conduction velocities and hindlimb sensory receptive fields were determined in vivo. A dense perimeter or ring of staining in a subpopulation of neurons was clearly overlying the soma membrane and not within satellite cells. Neurons with clear α3 rings (n = 23) were all MSAs (types I and II); all MSAs had darkly stained α3 rings, that tended to be darker in MSA1 than MSA2 units. Of 52 non-MSA A-fibre neurons including nociceptive and cutaneous low-threshold mechanoreceptive (LTM) neurons, 50 had no discernable ring, while 2 (Aα/β cutaneous LTMs) had weakly stained rings. Three of three C-nociceptors had no rings. MSAs with strong ring immunostaining also showed the strongest cytoplasmic staining. These findings suggest that α3 ring staining is a selective marker for MSAs. The α3 isoform of the Na⁺/K⁺-ATPase has previously been shown to be activated by higher Na⁺ levels and to have greater affinity for ATP than the α1 isoform (in all DRG neurons). The high α3 levels in MSAs may enable the greater dynamic firing range in MSAs.

Trafficking and surface expression of hyperpolarization-activated cyclic nucleotide-gated channels in hippocampal neurons

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels mediate the hyperpolarization-activated current I(h) and thus play important roles in the regulation of brain excitability. The subcellular distribution pattern of the HCN channels influences the effects that they exert on the properties and activity of neurons. However, little is known about the mechanisms that control HCN channel trafficking to subcellular compartments or that regulate their surface expression. Here we studied the dynamics of HCN channel trafficking in hippocampal neurons using dissociated cultures coupled with time lapse imaging of fluorophore-fused HCN channels. HCN1-green fluorescence protein (HCN1-GFP) channels resided in vesicle-like organelles that moved in distinct patterns along neuronal dendrites, and these properties were isoform-specific. HCN1 trafficking required intact actin and tubulin and was rapidly inhibited by activation of either NMDA or AMPA-type ionotropic glutamate receptors in a calcium-dependent manner. Glutamate-induced inhibition of the movement of HCN1-GFP-expressing puncta was associated with increased surface expression of both native and transfected HCN1 channels, and this surface expression was accompanied by augmented I(h). Taken together, the results reveal the highly dynamic nature of HCN1 channel trafficking in hippocampal neurons and provide a novel potential mechanism for rapid regulation of I(h), and hence of neuronal properties, via alterations of HCN1 trafficking and surface expression.

Hyperpolarization-activated and cyclic nucleotide-gated channels are differentially expressed in juxtaglomerular cells in the olfactory bulb of mice

In the olfactory bulb, input from olfactory receptor neurons is processed by neuronal networks before it is relayed to higher brain regions. In many neurons, hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels generate and control oscillations of the membrane potential. Oscillations also appear crucial for information processing in the olfactory bulb. Four channel isoforms exist (HCN1-HCN4) that can form homo- or heteromers. Here, we describe the expression pattern of HCN isoforms in the olfactory bulb of mice by using a novel and comprehensive set of antibodies against all four isoforms. HCN isoforms are abundantly expressed in the olfactory bulb. HCN channels can be detected in most cell populations identified by commonly used marker antibodies. The combination of staining with marker and HCN antibodies has revealed at least 17 different staining patterns in juxtaglomerular cells. Furthermore, HCN isoforms give rise to an unexpected wealth of co-expression patterns but are rarely expressed in the same combination and at the same level in two given cell populations. Therefore, heteromeric HCN channels may exist in several cell populations in vivo. Our results suggest that HCN channels play an important role in olfactory information processing. The staining patterns are consistent with the possibility that both homomeric and heteromeric HCN channels are involved in oscillations of the membrane potential of juxtaglomerular cells.

Evolutionary emergence of N-glycosylation as a variable promoter of HCN channel surface expression

All four mammalian hyperpolarization-activated cyclic nucleotide-modulated (HCN) channel isoforms have been shown to undergo N-linked glycosylation in the brain. With the mouse HCN2 isoform as a prototype, HCN channels have further been suggested to require N-glycosylation for function, a provocative finding that would make them unique in the voltage-gated potassium channel superfamily. Here, we show that both the HCN1 and HCN2 isoforms are also predominantly N-glycosylated in the embryonic heart, where they are found in significant amounts and where HCN-mediated currents are known to regulate beating frequency. Surprisingly, we find that N-glycosylation is not required for HCN2 function, although its cell surface expression is highly dependent on the presence of N-glycans. Comparatively, disruption of N-glycosylation only modestly impacts cell surface expression of HCN1 and leaves permeation and gating functions almost unchanged. This difference between HCN1 and HCN2 is consistent with evolutionary trajectories that diverged in an isoform-specific manner after gene duplication from a common HCN ancestor that lacked N-glycosylation and was able to localize efficiently to the cell surface.

The retinitis pigmentosa mutation c.3444+1G>A in CNGB1 results in skipping of exon 32

Retinitis pigmentosa (RP) is a severe hereditary eye disorder characterized by progressive degeneration of photoreceptors and subsequent loss of vision. Two of the RP associated mutations were found in the CNGB1 gene that encodes the B subunit of the rod cyclic nucleotide-gated channel (CNGB1a). One of them (c.3444+1G>A) is located at the donor site of exon 32 and has been proposed to result in a frameshift and truncation of the last 28 aa of the corresponding protein. However, this ambiguous conclusion was not verified by experimental data. Recently, another study reported that the last 28 aa of CNGB1a harbor a motif required for the proper targeting of this subunit to rod photoreceptor outer segments. This suggests that defective targeting is the major cause for the RP phenotype in affected patients. Here, we investigated the splicing of c.3444+1G>A by exon trapping experiments and could demonstrate that instead of the proposed truncation of the last 28 aa this mutation leads to replacement of the last 170 aa of CNGB1a by 68 unrelated amino acids. The 170 aa deletion covers the complete distal C-terminus including the last 10 aa of an important alpha (αC) helix within the ligand-binding domain of CNGB1a. When expressed in a heterologous expression system the corresponding mutant full-length CNGB1a subunit was more susceptible to proteosomal degradation compared to the wild-type counterpart. In conclusion, our experimental data do not support the hypothesis proposed by the original study on the c.3444+1G>A mutation. Based on this, we suggest that apart from the defective targeting other mechanisms may be responsible for the RP phenotype in affected individuals.

P-loop residues critical for selectivity in K channels fail to confer selectivity to rabbit HCN4 channels

HCN channels are thought to be structurally similar to K_v channels, but show much lower selectivity for K⁺. The ∼3.3 Å selectivity filter of K⁺ channels is formed by the pore-lining sequence XT(V/I)GYG, with X usually T, and is held stable by key residues in the P-loop. Differences in the P-loop sequence of HCN channels (eg. the pore-lining sequence L₄₇₈C₄₇₉IGYG) suggest these residues could account for differences in selectivity between these channel families. Despite being expressed, L478T/C479T HCN4 channels did not produce current. Since threonine in the second position is highly conserved in K⁺ channels, we also studied C479T channels. Based on permeability ratios (P_X/P_K), C479T HCN4 channels (K⁺(1)>Rb⁺(0.85)>Cs⁺(0.59)>Li⁺(0.50)≥Na⁺(0.49)) were less selective than WT rabbit HCN4 (K⁺(1)>Rb⁺(0.48)>Cs⁺(0.31)≥Na⁺(0.29)>Li⁺(0.03)), indicating that the TIGYG sequence is insufficient to confer K⁺ selectivity to HCN channels. C479T HCN4 channels had an increased permeability to large organic cations than WT HCN4 channels, as well as increased unitary K⁺ conductance, and altered channel gating. Collectively, these results suggest that HCN4 channels have larger pores than K⁺ channels and replacement of the cysteine at position 479 with threonine further increases pore size. Furthermore, selected mutations in other regions linked previously to pore stability in K⁺ channels (ie. S475D, S475E and F471W/K472W) were also unable to confer K⁺ selectivity to C479T HCN4 channels. Our findings establish the presence of the TIGYG pore-lining sequence does not confer K⁺ selectivity to rabbit HCN4 channels, and suggests that differences in selectivity of HCN4 versus K⁺ channels originate from differences outside the P-loop region.

Anatomical and electrophysiological comparison of CA1 pyramidal neurons of the rat and mouse

The study of learning and memory at the single-neuron level has relied on the use of many animal models, most notably rodents. Although many physiological and anatomical studies have been carried out in rats, the advent of genetically engineered mice has necessitated the comparison of new results in mice to established results from rats. Here we compare fundamental physiological and morphological properties and create three-dimensional compartmental models of identified hippocampal CA1 pyramidal neurons of one strain of rat, Sprague-Dawley, and two strains of mice, C57BL/6 and 129/SvEv. We report several differences in neuronal physiology and anatomy among the three animal groups, the most notable being that neurons of the 129/SvEv mice, but not the C57BL/6 mice, have higher input resistance, lower dendritic surface area, and smaller spines than those of rats. A surprising species-specific difference in membrane resonance indicates that both mouse strains have lower levels of the hyperpolarization-activated nonspecific cation current I(h). Simulations suggest that differences in I(h) kinetics rather than maximal conductance account for the lower resonance. Our findings indicate that comparisons of data obtained across strains or species will need to account for these and potentially other physiological and anatomical differences.

Hyperpolarization-activated cation channels: from genes to function

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels comprise a small subfamily of proteins within the superfamily of pore-loop cation channels. In mammals, the HCN channel family comprises four members (HCN1-4) that are expressed in heart and nervous system. The current produced by HCN channels has been known as I(h) (or I(f) or I(q)). I(h) has also been designated as pacemaker current, because it plays a key role in controlling rhythmic activity of cardiac pacemaker cells and spontaneously firing neurons. Extensive studies over the last decade have provided convincing evidence that I(h) is also involved in a number of basic physiological processes that are not directly associated with rhythmicity. Examples for these non-pacemaking functions of I(h) are the determination of the resting membrane potential, dendritic integration, synaptic transmission, and learning. In this review we summarize recent insights into the structure, function, and cellular regulation of HCN channels. We also discuss in detail the different aspects of HCN channel physiology in the heart and nervous system. To this end, evidence on the role of individual HCN channel types arising from the analysis of HCN knockout mouse models is discussed. Finally, we provide an overview of the impact of HCN channels on the pathogenesis of several diseases and discuss recent attempts to establish HCN channels as drug targets.

Postnatal expression pattern of HCN channel isoforms in thalamic neurons: relationship to maturation of thalamocortical oscillations

Hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels are the molecular substrate of the hyperpolarization-activated inward current (I(h)). Because the developmental profile of HCN channels in the thalamus is not well understood, we combined electrophysiological, molecular, immunohistochemical, EEG recordings in vivo, and computer modeling techniques to examine HCN gene expression and I(h) properties in rat thalamocortical relay (TC) neurons in the dorsal part of the lateral geniculate nucleus and the functional consequence of this maturation. Recordings of TC neurons revealed an approximate sixfold increase in I(h) density between postnatal day 3 (P3) and P106, which was accompanied by significantly altered current kinetics, cAMP sensitivity, and steady-state activation properties. Quantification on tissue levels revealed a significant developmental decrease in cAMP. Consequently the block of basal adenylyl cyclase activity was accompanied by a hyperpolarizing shift of the I(h) activation curve in young but not adult rats. Quantitative analyses of HCN channel isoforms revealed a steady increase of mRNA and protein expression levels of HCN1, HCN2, and HCN4 with reduced relative abundance of HCN4. Computer modeling in a simplified thalamic network indicated that the occurrence of rhythmic delta activity, which was present in the EEG at P12, differentially depended on I(h) conductance and modulation by cAMP at different developmental states. These data indicate that the developmental increase in I(h) density results from increased expression of three HCN channel isoforms and that isoform composition and intracellular cAMP levels interact in determining I(h) properties to enable progressive maturation of rhythmic slow-wave sleep activity patterns.

The cardiac pacemaker current

In mammals cardiac rate is determined by the duration of the diastolic depolarization of sinoatrial node (SAN) cells which is mainly determined by the pacemaker I(f) current. f-channels are encoded by four members of the hyperpolarization-activated cyclic nucleotide-gated gene (HCN1-4) family. HCN4 is the most abundant isoform in the SAN, and its relevance to pacemaking has been further supported by the discovery of four loss-of-function mutations in patients with mild or severe forms of cardiac rate disturbances. Due to its selective contribution to pacemaking, the I(f) current is also the pharmacological target of a selective heart rate-reducing agent (ivabradine) currently used in the clinical practice. Albeit to a minor extent, the I(f) current is also present in other spontaneously active myocytes of the cardiac conduction system (atrioventricular node and Purkinje fibres). In working atrial and ventricular myocytes f-channels are expressed at a very low level and do not play any physiological role; however in certain pathological conditions over-expression of HCN proteins may represent an arrhythmogenic mechanism. In this review some of the most recent findings on f/HCN channels contribution to pacemaking are described.

Activity-dependent regulation of HCN1 protein in cortical neurons

Homeostasis of neuronal activity is crucial to neuronal physiology. In dendrites, hyperpolarization-activated cyclic nucleotide-gated channel (HCN) 1 is considered to play critical roles in this process. While electrophysiological studies have demonstrated the dynamic modulation of I(h) current mediated by HCN1 proteins, little is known about the underlying molecular and cellular mechanisms. In this study, we utilized cortical cultured neurons and biochemical methods to identify molecular and cellular mechanisms that mediate the physiological regulation of HCN1 channel functions in cortical neurons. Pharmacological manipulations of neuronal activity resulted in changes in the expression level of HCN1. In addition, the surface expression of HCN1 was dynamically regulated by neuronal activity. Both of these changes led to functional modulations of HCN1 channels. Our study suggests that coordinated changes in protein expression and surface expression of HCN1 serve as the key regulatory mechanisms controlling the function of endogenous HCN1 protein in cortical neurons.

The two-pore channel TPCN2 mediates NAADP-dependent Ca(2+)-release from lysosomal stores

Second messenger-induced Ca²⁺-release from intracellular stores plays a key role in a multitude of physiological processes. In addition to 1,4,5-inositol trisphosphate (IP₃), Ca²⁺, and cyclic ADP ribose (cADPR) that trigger Ca²⁺-release from the endoplasmatic reticulum (ER), nicotinic acid adenine dinucleotide phosphate (NAADP) has been identified as a cellular metabolite that mediates Ca²⁺-release from lysosomal stores. While NAADP-induced Ca²⁺-release has been found in many tissues and cell types, the molecular identity of the channel(s) conferring this release remained elusive so far. Here, we show that TPCN2, a novel member of the two-pore cation channel family, displays the basic properties of native NAADP-dependent Ca²⁺-release channels. TPCN2 transcripts are widely expressed in the body and encode a lysosomal protein forming homomers. TPCN2 mediates intracellular Ca²⁺-release after activation with low-nanomolar concentrations of NAADP while it is desensitized by micromolar concentrations of this second messenger and is insensitive to the NAADP analog nicotinamide adenine dinucleotide phosphate (NADP). Furthermore, TPCN2-mediated Ca²⁺-release is almost completely abolished when the capacity of lysosomes for storing Ca²⁺ is pharmacologically blocked. By contrast, TPCN2-specific Ca²⁺-release is unaffected by emptying ER-based Ca²⁺ stores. In conclusion, these findings indicate that TPCN2 is a major component of the long-sought lysosomal NAADP-dependent Ca²⁺-release channel.

Alternatively spliced isoforms of TRIP8b differentially control h channel trafficking and function

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels (h channels) are the molecular basis for the current, I(h), which contributes crucially to intrinsic neuronal excitability. The subcellular localization and biophysical properties of h channels govern their function, but the mechanisms controlling these characteristics, and especially the potential role of auxiliary subunits or other binding proteins, remain unclear. We focused on TRIP8b, an h channel-interacting protein that colocalizes with HCN1 in cortical and hippocampal pyramidal neuron dendrites, and found that it exists in multiple alternative splice variants with distinct effects on h channel trafficking and function. The developmentally regulated splice variants of TRIP8b all shared dual, C terminus-located interaction sites with HCN1. When coexpressed with HCN1 in heterologous cells individual TRIP8b isoforms similarly modulated gating of I(h), causing a hyperpolarizing shift in voltage dependence of channel activation, but differentially upregulated or downregulated I(h) current density and HCN1 surface expression. In hippocampal neurons, coexpression of TRIP8b isoforms with HCN1 produced isoform-specific changes of HCN1 localization. Interestingly, the TRIP8b isoforms most abundant in the brain are those predicted to enhance h channel surface expression. Indeed, shRNA knockdown of TRIP8b in hippocampal neurons significantly reduced native I(h). Thus, although TRIP8b exists in multiple splice isoforms, our data suggest that the predominant role of this protein in brain is to promote h channel surface expression and enhance I(h). Because I(h) expression is altered in models of several diseases, including temporal lobe epilepsy, TRIP8b may play a role in both normal neuronal function and in aberrant neuronal excitability associated with neurological disease.

Inflammation-induced increase in hyperpolarization-activated, cyclic nucleotide-gated channel protein in trigeminal ganglion neurons and the effect of buprenorphine

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are active at resting membrane potential and thus contribute to neuronal excitability. Their increased activity has recently been demonstrated in models of nerve injury-induced pain. The major aim of the current study was to investigate altered HCN channel protein expression in trigeminal sensory neurons following inflammation of the dura. HCN1 and HCN2 channel immunoreactivity was observed on the membranes of medium- to large-sized trigeminal ganglion neurons with 76% and 85% of HCN1 and HCN2 expressing neurons also containing the 200 kDa neurofilament protein (associated with myelinated fibers). Western immunoblots of lysates from rat trigeminal ganglia also showed bands with appropriate molecular weights for HCN1 and HCN2. Three days after application of complete Freund's adjuvant (CFA) to the dura mater, Western blot band densities were significantly increased; compared to control, to 166% for HCN1 and 284% for HCN2 channel protein. The band densities were normalized against alpha-actin. In addition, the number of retrogradely labeled neurons from the dura expressing HCN1 and HCN2 was significantly increased to 247% (HCN1) and 171% (HCN2), three days after inflammation. When the opioid receptor partial agonist, buprenorphine, was given systemically, immediately after CFA, the inflammation-induced increase in HCN protein expression in both Western blot and immunohistochemical experiments was not observed. These results suggest that HCN1 and HCN2 are involved in inflammation-induced sensory neuron hyperexcitability, and indicate that an opioid receptor agonist can reverse the protein upregulation.

Alanine scanning of the S6 segment reveals a unique and cAMP-sensitive association between the pore and voltage-dependent opening in HCN channels

Hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels resemble Shaker K+ channels in structure and function. In both, changes in membrane voltage produce directionally similar movement of positively charged residues in the voltage sensor to alter the pore structure at the intracellular side and gate ion flow. However, HCNs open when hyperpolarized, whereas Shaker opens when depolarized. Thus, electromechanical coupling between the voltage sensor and gate is opposite. A key determinant of this coupling is the intrinsic stability of the pore. In Shaker, an alanine/valine scan of residues across the pore, by single point mutation, showed that most mutations made the channel easier to open and steepened the response of the channel to changes in voltage. Because most mutations likely destabilize protein packing, the Shaker pore is most stable when closed, and the voltage sensor works to open it. In HCN channels, the pore energetics and vector of work by the voltage sensor are unknown. Accordingly, we performed a 22-residue alanine/valine scan of the distal pore of the HCN2 isoform and show that the effects of mutations on channel opening and on the steepness of the response of the channel to voltage are mixed and smaller than those in Shaker. These data imply that the stabilities of the open and closed pore are similar, the voltage sensor must apply force to close the pore, and the interactions between the pore and voltage sensor are weak. Moreover, cAMP binding to the channel heightens the effects of the mutations, indicating stronger interactions between the pore and voltage sensor, and tips the energetic balance toward a more stable open state.

Associated changes in HCN2 and HCN4 transcripts and I(f) pacemaker current in myocytes

The time- and voltage-dependent inward current generated by the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels contributes to the tissue-specific rhythmic activities in the brain and heart. Four isoforms (HCN1-HCN4) have been identified. Previous studies showed that different HCN isoforms may form functional heteromeric channels. We report here that when HCN2 and HCN4 mRNA were injected into Xenopus oocytes with various ratios of HCN2 over HCN4 at 1:1, 10:1, and 1:10, respectively, the resultant channels showed a depolarized current activation and significantly faster activation kinetics near the midpoint of activation compared with HCN4 homomeric channels. In adult rat myocytes overexpressing HCN4, there was an associated increase in HCN2 mRNA. In neonatal rat myocytes in which HCN2 was knocked down, there was also a simultaneous decrease in HCN4 mRNA. Coimmunoprecipitation experiments showed that HCN2 and HCN4 channel proteins can associate with each other in adult rat ventricles. Finally, in adult myocytes overexpressing HCN4, the hyperpolarization-activated inward current activation, I(f), was shifted to physiological voltages from non-physiological voltages, associated with faster activation kinetics. These data suggested that different ratios of HCN2 and HCN4 transcripts overlapping in different tissues also contribute to the tissue-specific properties of I(f).

Cyclic nucleotide-regulated cation channels

Cyclic nucleotide-regulated cation channels are ion channels whose activation is regulated by the direct binding of cAMP or cGMP to the channel protein. Two structurally related families of channels regulated by cyclic nucleotides have been identified, the cyclic nucleotide-gated channels and the hyperpolarization-activated cyclic nucleotide-gated channels. Cyclic nucleotide-gated channels play a key role in visual and olfactory transduction. Hyperpolarization-activated cyclic nucleotide-gated channels are present in the conduction system of the heart and are involved in the control of cardiac automaticity. Moreover, these channels are widely expressed in central and peripheral neurons, where they control a variety of fundamental processes.

Hyperpolarization activated cyclic-nucleotide gated (HCN) channels in developing neuronal networks

Developing neuronal networks evolve continuously, requiring that neurons modulate both their intrinsic properties and their responses to incoming synaptic signals. Emerging evidence supports roles for the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in this neuronal plasticity. HCN channels seem particularly suited for fine-tuning neuronal properties and responses because of their remarkably large and variable repertoire of functions, enabling integration of a wide range of cellular signals. Here, we discuss the involvement of HCN channels in cortical and hippocampal network maturation, and consider potential roles of developmental HCN channel dysregulation in brain disorders such as epilepsy.

Novel mechanism for suppression of hyperpolarization-activated cyclic nucleotide-gated pacemaker channels by receptor-like tyrosine phosphatase-alpha

We have previously reported an important role of increased tyrosine phosphorylation activity by Src in the modulation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Here we provide evidence showing a novel mechanism of decreased tyrosine phosphorylation on HCN channel properties. We found that the receptor-like protein-tyrosine phosphatase-alpha (RPTPalpha) significantly inhibited or eliminated HCN2 channel expression in HEK293 cells. Biochemical evidence showed that the surface expression of HCN2 was remarkably reduced by RPTPalpha, which was in parallel to the decreased tyrosine phosphorylation of the channel protein. Confocal imaging confirmed that the membrane surface distribution of the HCN2 channel was inhibited by RPTPalpha. Moreover, we detected the presence of RPTPalpha proteins in cardiac ventricles with expression levels changed during development. Inhibition of tyrosine phosphatase activity by phenylarsine oxide or sodium orthovanadate shifted ventricular hyperpolarization-activated current (I(f), generated by HCN channels) activation from nonphysiological voltages into physiological voltages associated with accelerated activation kinetics. In conclusion, we showed a critical role RPTPalpha plays in HCN channel function via tyrosine dephosphorylation. These findings are also important to neurons where HCN and RPTPalpha are richly expressed.

Regulation of cell surface expression of functional pacemaker channels by a motif in the B-helix of the cyclic nucleotide-binding domain

Previous studies have suggested that a portion of the cyclic nucleotide-binding domain (CNBD) of the hyperpolarization-activated cyclic nucleotide-gated channel 2 (HCN2) "pacemaker" channel, composed of the A- and B-helices and the interceding beta-barrel, confers two functions: inhibition of channel opening in response to hyperpolarization and promotion of cell surface expression. The sequence determinants required for each of these functions are unknown. In addition, the mechanism underlying plasma membrane targeting by this subdomain has been limitedly explored. Here we identify a four-amino acid motif (EEYP) in the B-helix that strongly promotes channel export from the endoplasmic reticulum (ER) and cell surface expression but does not contribute to the inhibition of channel opening. This motif augments a step in the trafficking pathway and/or the efficiency of correct folding and assembly.