Novel pharmacological activity of loperamide and CP-339,818 on human HCN channels characterized with an automated electrophysiology assay

The Contribution of HCN Channelopathies in Different Epileptic Syndromes, Mechanisms, Modulators, and Potential Treatment Targets: A Systematic Review

Background

Hyperpolarization-activated cyclic nucleotide-gated (HCN) current reduces dendritic summation, suppresses dendritic calcium spikes, and enables inhibitory GABA-mediated postsynaptic potentials, thereby suppressing epilepsy. However, it is unclear whether increased HCN current can produce epilepsy. We hypothesized that gain-of-function (GOF) and loss-of-function (LOF) variants of HCN channel genes may cause epilepsy.

Objectives

This systematic review aims to summarize the role of HCN channelopathies in epilepsy, update genetic findings in patients, create genotype–phenotype correlations, and discuss animal models, GOF and LOF mechanisms, and potential treatment targets.

Methods

The review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement, for all years until August 2021.

Results

We identified pathogenic variants of HCN1 (n = 24), HCN2 (n = 8), HCN3 (n = 2), and HCN4 (n = 6) that were associated with epilepsy in 74 cases (43 HCN1, 20 HCN2, 2 HCN3, and 9 HCN4). Epilepsy was associated with GOF and LOF variants, and the mechanisms were indeterminate. Less than half of the cases became seizure-free and some developed drug-resistant epilepsy. Of the 74 cases, 12 (16.2%) died, comprising HCN1 (n = 4), HCN2 (n = 2), HCN3 (n = 2), and HCN4 (n = 4). Of the deceased cases, 10 (83%) had a sudden unexpected death in epilepsy (SUDEP) and 2 (16.7%) due to cardiopulmonary failure. SUDEP affected more adults (n = 10) than children (n = 2). HCN1 variants p.M234R, p.C329S, p.V414M, p.M153I, and p.M305L, as well as HCN2 variants p.S632W and delPPP (p.719–721), were associated with different phenotypes. HCN1 p.L157V and HCN4 p.R550C were associated with genetic generalized epilepsy. There are several HCN animal models, pharmacological targets, and modulators, but precise drugs have not been developed. Currently, there are no HCN channel openers.

Conclusion

We recommend clinicians to include HCN genes in epilepsy gene panels. Researchers should explore the possible underlying mechanisms for GOF and LOF variants by identifying the specific neuronal subtypes and neuroanatomical locations of each identified pathogenic variant. Researchers should identify specific HCN channel openers and blockers with high binding affinity. Such information will give clarity to the involvement of HCN channelopathies in epilepsy and provide the opportunity to develop targeted treatments.

Loperamide Inhibits Replication of Severe Fever with Thrombocytopenia Syndrome Virus

BACKGROUND

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne infectious disease caused by the SFTS virus (SFTSV). SFTS is mainly prevalent in East Asia. It has a mortality rate of up to 30%, and there is no approved treatment against the disease. In this study, we evaluated the effect of loperamide, an antidiarrheal and antihyperalgesic agent, on the propagation of SFTSV in a cell culture system.

METHODS

SFTSV-infected human cell lines were exposed to loperamide, and viral titers were evaluated. To clarify the mode of action of loperamide, several chemical compounds having shared targets with loperamide were used. Calcium imaging was also performed to understand whether loperamide treatment affected calcium influx.

RESULTS

Loperamide inhibited SFTSV propagation in several cell lines. It inhibited SFTSV in the post-entry step and restricted calcium influx into the cell. Furthermore, nifedipine, a calcium channel inhibitor, also blocked post-entry step of SFTSV infection.

CONCLUSIONS

Loperamide inhibits SFTSV propagation mainly by restraining calcium influx into the cytoplasm. This indicates that loperamide, a Food and Drug Administration (FDA)-approved drug, has the potential for being used as a treatment option against SFTS.

The HCN Channel Blocker ZD7288 Induces Emesis in the Least Shrew (Cryptotis parva)

Subtypes (1-4) of the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are widely expressed in the central and peripheral nervous systems, as well as the cells of smooth muscles in many organs. They mainly serve to regulate cellular excitability in these tissues. The HCN channel blocker ZD7288 has been shown to reduce apomorphine-induced conditioned taste aversion on saccharin preference in rats suggesting potential antinausea/antiemetic effects. Currently, in the least shew model of emesis we find that ZD7288 induces vomiting in a dose-dependent manner, with maximal efficacies of 100% at 1 mg/kg (i.p.) and 83.3% at 10 µg (i.c.v.). HCN channel subtype (1-4) expression was assessed using immunohistochemistry in the least shrew brainstem dorsal vagal complex (DVC) containing the emetic nuclei (area postrema (AP), nucleus tractus solitarius and dorsal motor nucleus of the vagus). Highly enriched HCN1 and HCN4 subtypes are present in the AP. A 1 mg/kg (i.p.) dose of ZD7288 strongly evoked c-Fos expression and ERK1/2 phosphorylation in the shrew brainstem DVC, but not in the in the enteric nervous system in the jejunum, suggesting a central contribution to the evoked vomiting. The ZD7288-evoked c-Fos expression exclusively occurred in tryptophan hydroxylase 2-positive serotonin neurons of the dorsal vagal complex, indicating activation of serotonin neurons may contribute to ZD7288-induced vomiting. To reveal its mechanism(s) of emetic action, we evaluated the efficacy of diverse antiemetics against ZD7288-evoked vomiting including the antagonists/inhibitors of: ERK1/2 (U0126), L-type Ca2+ channel (nifedipine); store-operated Ca2+ entry (MRS 1845); T-type Ca2+ channel (Z944), IP3R (2-APB), RyR receptor (dantrolene); the serotoninergic type 3 receptor (palonosetron); neurokinin 1 receptor (netupitant), dopamine type 2 receptor (sulpride), and the transient receptor potential vanilloid 1 receptor agonist, resiniferatoxin. All tested antiemetics except sulpride attenuated ZD7288-evoked vomiting to varying degrees. In sum, ZD7288 has emetic potential mainly via central mechanisms, a process which involves Ca2+ signaling and several emetic receptors. HCN channel blockers have been reported to have emetic potential in the clinic since they are currently used/investigated as therapeutic candidates for cancer therapy related- or unrelated-heart failure, pain, and cognitive impairment.

Pharmacology of A-Type K+ Channels

Transient outward potassium currents were first described nearly 60 years ago, since then major strides have been made in understanding their molecular basis and physiological roles. From the large family of voltage-gated potassium channels members of 3 subfamilies can produce such fast-inactivating A-type potassium currents. Each subfamily gives rise to currents with distinct biophysical properties and pharmacological profiles and a simple workflow is provided to aid the identification of channels mediating A-type currents in native cells. Their unique properties and regulation enable A-type K⁺ channels to perform varied roles in excitable cells including repolarisation of the cardiac action potential, controlling spike and synaptic timing, regulating dendritic integration and long-term potentiation as well as being a locus of neural plasticity.

Pharmacology of Ivabradine and the Effect on Chronic Heart Failure

Chronic Heart Failure (CHF) is a complex clinical syndrome with a high incidence worldwide. Although various types of pharmacological and device therapies are available for CHF, the prognosis is not ideal, for which, the control of increased Heart Rate (HR) is critical. Recently, a bradycardic agent, ivabradine, is found to reduce HR by inhibiting the funny current (If). The underlying mechanism states that ivabradine can enter the Hyperpolarization-activated Cyclic Nucleotide-gated (HCN) channels and bind to the intracellular side, subsequently inhibiting the If. This phenomenon can prolong the slow spontaneous phase in the diastolic depolarization, and thus, reduce HR. The clinical trials demonstrated the significant effects of the drug on reducing HR and improving the symptoms of CHF with fewer adverse effects. This review primarily introduces the chemical features and pharmacological characteristics of ivabradine and the mechanism of treating CHF. Also, some expected therapeutic effects on different diseases were also concluded. However, ivabradine, as a typical If channel inhibitor, necessitates additional research to verify its pharmacological functions.

HCN Channels: New Therapeutic Targets for Pain Treatment

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are highly regulated proteins which respond to different cellular stimuli. The HCN currents (I_h) mediated by HCN1 and HCN2 drive the repetitive firing in nociceptive neurons. The role of HCN channels in pain has been widely investigated as targets for the development of new therapeutic drugs, but the comprehensive design of HCN channel modulators has been restricted due to the lack of crystallographic data. The three-dimensional structure of the human HCN1 channel was recently reported, opening new possibilities for the rational design of highly-selective HCN modulators. In this review, we discuss the structural and functional properties of HCN channels, their pharmacological inhibitors, and the potential strategies for designing new drugs to block the HCN channel function associated with pain perception.

High throughput screening technologies for ion channels

Ion channels are involved in a variety of fundamental physiological processes, and their malfunction causes numerous human diseases. Therefore, ion channels represent a class of attractive drug targets and a class of important off-targets for in vitro pharmacological profiling. In the past decades, the rapid progress in developing functional assays and instrumentation has enabled high throughput screening (HTS) campaigns on an expanding list of channel types. Chronologically, HTS methods for ion channels include the ligand binding assay, flux-based assay, fluorescence-based assay, and automated electrophysiological assay. In this review we summarize the current HTS technologies for different ion channel classes and their applications.

HCN Channels Modulators: The Need for Selectivity

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels, the molecular correlate of the hyperpolarization-activated current (If/Ih), are membrane proteins which play an important role in several physiological processes and various pathological conditions. In the Sino Atrial Node (SAN) HCN4 is the target of ivabradine, a bradycardic agent that is, at the moment, the only drug which specifically blocks If. Nevertheless, several other pharmacological agents have been shown to modulate HCN channels, a property that may contribute to their therapeutic activity and/or to their side effects. HCN channels are considered potential targets for developing drugs to treat several important pathologies, but a major issue in this field is the discovery of isoform-selective compounds, owing to the wide distribution of these proteins into the central and peripheral nervous systems, heart and other peripheral tissues. This survey is focused on the compounds that have been shown, or have been designed, to interact with HCN channels and on their binding sites, with the aim to summarize current knowledge and possibly to unveil useful information to design new potent and selective modulators.

Expression and function of a CP339,818-sensitive K⁺ current in a subpopulation of putative nociceptive neurons from adult mouse trigeminal ganglia

Trigeminal ganglion (TG) neurons are functionally and morphologically heterogeneous, and the molecular basis of this heterogeneity is still not fully understood. Here we describe experiments showing that a subpopulation of neurons expresses a delayed-rectifying K(+) current (IDRK) with a characteristically high (nanomolar) sensitivity to the dihydroquinoline CP339,818 (CP). Although submicromolar CP has previously been shown to selectively block Kv1.3 and Kv1.4 channels, the CP-sensitive IDRK found in TG neurons could not be associated with either of these two K(+) channels. It could neither be associated with Kv2.1 channels homomeric or heteromerically associated with the Kv9.2, Kv9.3, or Kv6.4 subunits, whose block by CP, tested using two-electrode voltage-clamp recordings from Xenopus oocytes, resulted in the low micromolar range, nor to the Kv7 subfamily, given the lack of blocking efficacy of 3 μM XE991. Within the group of multiple-firing neurons considered in this study, the CP-sensitive IDRK was preferentially expressed in a subpopulation showing several nociceptive markers, such as small membrane capacitance, sensitivity to capsaicin, and slow afterhyperpolarization (AHP); in these neurons the CP-sensitive IDRK controls the membrane resting potential, the firing frequency, and the AHP duration. A biophysical study of the CP-sensitive IDRK indicated the presence of two kinetically distinct components: a fast deactivating component having a relatively depolarized steady-state inactivation (IDRKf) and a slow deactivating component with a more hyperpolarized V1/2 for steady-state inactivation (IDRKs).

Local loperamide injection reduces mechanosensitivity of rat cutaneous, nociceptive C-fibers

Loperamide reverses signs of mechanical hypersensitivity in an animal model of neuropathic pain suggesting that peripheral opioid receptors may be suitable targets for the treatment of neuropathic pain. Since little is known about loperamide effects on the responsiveness of primary afferent nerve fibers, in vivo electrophysiological recordings from unmyelinated afferents innervating the glabrous skin of the hind paw were performed in rats with an L5 spinal nerve lesion or sham surgery. Mechanical threshold and responsiveness to suprathreshold stimulation were tested before and after loperamide (1.25, 2.5 and 5 µg in 10 µl) or vehicle injection into the cutaneous receptive field. Loperamide dose-dependently decreased mechanosensitivity in unmyelinated afferents of nerve-injured and sham animals, and this effect was not blocked by naloxone pretreatment. We then investigated loperamide effects on nerve conduction by recording compound action potentials in vitro during incubation of the sciatic nerve with increasing loperamide concentrations. Loperamide dose-dependently decreased compound action potentials of myelinated and unmyelinated fibers (ED50 = 8 and 4 µg/10 µl, respectively). This blockade was not prevented by pre-incubation with naloxone. These results suggest that loperamide reversal of behavioral signs of neuropathic pain may be mediated, at least in part, by mechanisms independent of opioid receptors, most probably by local anesthetic actions.

Hyperpolarization-activated cyclic nucleotide-gated channels in olfactory sensory neurons regulate axon extension and glomerular formation

Mechanisms influencing the development of olfactory bulb glomeruli are poorly understood. While odor receptors (ORs) play an important role in olfactory sensory neuron (OSN) axon targeting/coalescence (Mombaerts et al., 1996; Wang et al., 1998; Feinstein and Mombaerts, 2004), recent work showed that G protein activation alone is sufficient to induce OSN axon coalescence (Imai et al., 2006; Chesler et al., 2007), suggesting an activity-dependent mechanism in glomerular development. Consistent with these data, OSN axon projections and convergence are perturbed in mice deficient for adenylyl cyclase III, which is downstream from the OR and catalyzes the conversion of ATP to cAMP. However, in cyclic nucleotide-gated (CNG) channel knock-out mice OSN axons are only transiently perturbed (Lin et al., 2000), suggesting that the CNG channel may not be the sole target of cAMP. This prompted us to investigate an alternative channel, the hyperpolarization-activated, cyclic nucleotide-gated cation channel (HCN), as a potential developmental target of cAMP in OSNs. Here, we demonstrate that HCN channels are developmentally precocious in OSNs and therefore are plausible candidates for affecting OSN axon development. Inhibition of HCN channels in dissociated OSNs significantly reduced neurite outgrowth. Moreover, in HCN1 knock-out mice the formation of glomeruli was delayed in parallel with perturbations of axon organization in the olfactory nerve. These data support the hypothesis that the outgrowth and coalescence of OSN axons is, at least in part, subject to activity-dependent mechanisms mediated via HCN channels.

Screening technologies for ion channel drug discovery

For every movement, heartbeat and thought, ion channels need to open and close. It is therefore not surprising that their malfunctioning leads to serious diseases. Currently, only approximately 10% of drugs, with a market value in excess of US$10 billion, act on ion channels. The systematic exploitation of this target class has started, enabled by novel assay technologies and fundamental advances of the structural and mechanistic understanding of channel function. The latter, which was rewarded with the Nobel Prize in 2003, has opened up an avenue for rational drug design. In this review we provide an overview of the current repertoire of screening technologies that has evolved to drive ion channel-targeted drug discovery towards new medicines of the future.

A Novel High-Throughput Screening Assay for HCN Channel Blocker Using Membrane Potential-Sensitive Dye and FLIPR

Hyperpolarization-activated cation nonselective (HCN) channels represent an interesting group of targets for drug development. In this study, the authors report the development of a novel membrane potential-sensitive dye (MPSD) assay for HCN channel modulators that has been miniaturized into 384-well fluorescent imaging plate reader (FLIPR) high-throughput screening (HTS) format. When optimized (by cell plating density, plate type, cell recovery from cryopreservation), the wellto-well signal variability was low, with a Z' = 0.73 and coefficient of variation = 6.4%, whereas the MPSD fluorescence signal amplitude was -23,700 ± 1500 FLIPR3 relative fluorescence units (a linear relationship was found between HCN1 MPSD fluorescence signal and the cell plating density) and was completely blocked by 30 µM ZD7288. The assay tolerated up to 1% DMSO, inclusion of which did not significantly change the signal kinetics or amplitude. A single-concentration screening of an ion channel-focused library composed of 4855 compounds resulted in 89 HCN1 blocker hits, 51 of which were subsequently analyzed with an 8-point concentration-response analysis on the IonWorks HT electrophysiology platform. The correlation between MPSD and the electrophysiology assay was moderate, as shown by the linear regression analysis (r2 = 0.56) between the respective IC50s obtained using these 2 assays. The reported HTS-compatible HCN channel blocker assay can serve as a tool in drug discovery in the pursuit of HCN channel isoform-selective small molecules that could be used in the development of clinically relevant compounds. (Journal of Biomolecular Screening 2009:1119-1128)

Recent advances in electrophysiology-based screening technology and the impact upon ion channel discovery research

Ion channels are recognised as an increasingly tractable class of targets for the discovery and development of new drugs, with a diverse range of ion channel proteins now implicated across a wide variety of disease states and potential therapeutic applications. Whilst the field now ranks as one of the most dynamic fields for drug discovery research, it has historically been regarded by many researchers as a class of proteins associated with numerous technical challenges. Recent advances in our understanding of molecular biology and the increasing acceptance of electrophysiology-based screening methodology mean that ion channels are rapidly progressing towards universal acceptance as worthy and approachable targets for drug discovery. This chapter will outline the commercially available electrophysiology-based screening technologies and give an overview of the range of options for progressing pharmaceutical research and development against this important target class.