Characterization of the role of HCN channels in β3-adrenoceptor mediated rat bladder relaxation

Loss of resilience contributes to detrusor underactivity in advanced age

Volume hyposensitivity resulting from impaired sympathetic detrusor relaxation during bladder filling contributes to detrusor underactivity (DU) associated with aging. Detrusor tension regulation provides an adaptive sensory input of bladder volume to the brainstem and is challenged by physiological stressors superimposed upon biological aging. We recently showed that HCN channels have a stabilizing role in detrusor sympathetic relaxation. While mature mice maintain homeostasis in the face of stressors, old mice are not always capable. In old mice, there is a dichotomous phenotype, in which resilient mice adapt and maintain homeostasis, while non-resilient mice fail to maintain physiologic homeostasis. In this DU model, we used cystometry as a stressor to categorize mice as old-responders (old-R, develop a filling/voiding cycle) or old-non-responders (old-NR, fail to develop a filling/voiding cycle; fluctuating high pressures and continuous leaking), while also assessing functional and molecular differences. Lamotrigine (HCN activator)-induced bladder relaxation is diminished in old-NR mice following HCN-blockade. Relaxation responses to NS 1619 were reduced in old-NR mice, with the effect lost following HCN-blockade. However, RNA-sequencing revealed no differences in HCN gene expression and electrophysiology studies showed similar percentage of detrusor myocytes expressing HCN (I_h) current between old-R and old-NR mice. Our murine model of DU further defines a role for HCN, with failure of adaptive recalibration of HCN participation and intensity of HCN-mediated stabilization, while genomic studies show upregulated myofibroblast and fibrosis pathways and downregulated neurotransmitter-degradation pathways in old-NR mice. Thus, the DU phenotype is multifactorial and represents the accumulation of age-associated loss in homeostatic mechanisms.

The hyperpolarization-activated, cyclic nucleotide-gated channel resides on myocytes in mouse bladders and contributes to adrenergic-induced detrusor relaxation

Control of urinary continence is predicated on sensory signaling about bladder volume. Bladder sensory nerve activity is dependent on tension, implicating autonomic control over detrusor myocyte activity during bladder filling. Hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels are known contributors to bladder control, but their mechanism of action is not well understood. The lack of a definitive identification of cell type(s) expressing HCN in the bladder presents a significant knowledge gap. We recently reported a complete transcriptomic atlas of the C57BL/6 mouse bladder showing the dominant HCN paralog in mouse bladder, Hcn1, is limited to a subpopulation of detrusor smooth myocytes (DSMs). Here, we report details of these findings, along with results of patch-clamp experiments, immunohistochemistry, and functional myobath/tension experiments in bladder strips. With the use of a transgenic mouse expressing fluorescence-tagged α-smooth muscle actin, our data confirmed location and function of DSM HCN channels. Despite previous associations of HCN with postulated bladder interstitial cells, neither evidence of specific interstitial cell types nor an association of nonmyocytes with HCN was discovered. We confirm that HCN activation participates in reducing sustained (tonic) detrusor tension via cAMP, with no effect on intermittent (phasic) detrusor activity. In contrast, blockade of HCN increases phasic activity induced by a protein kinase A (PKA) blocker or a large-conductance Ca²⁺-activated K⁺ (BK) channel opener. Our findings, therefore, suggest a central role for detrusor myocyte HCN in regulating and constraining detrusor myocyte activity during bladder filling.

The funny current: Even funnier than 40 years ago. Uncanonical expression and roles of HCN/f channels all over the body

Discovered some 40 years ago, the I_f current has since been known as the "pacemaker" current due to its role in the initiation and modulation of the heartbeat and of neuronal excitability. But this is not all, the funny current keeps entertaining the researchers; indeed, several data discovering novel and uncanonical roles of f/HCN channel are quickly accumulating. In the present review, we provide an overview of the expression and cellular functions of HCN/f channels in a variety of systems/organs, and particularly in sour taste transduction, hormones secretion, activation of astrocytes and microglia, inhibition of osteoclastogenesis, renal ammonium excretion, and peristalsis in the gastrointestinal and urine systems. We also analyzed the role of HCN channels in sustaining cellular respiration in mitochondria and their participation to mitophagy under specific conditions. The relevance of HCN currents in undifferentiated cells, and specifically in the control of stem cell cycle and in bioelectrical signals driving left/right asymmetry during zygote development, is also considered. Finally, we present novel data concerning the expression of HCN mRNA in human leukocytes. We can thus conclude that the emerging evidence presented in this review clearly points to an increasing interest and importance of the "funny" current that goes beyond its role in cardiac sinoatrial and neuronal excitability regulation.

Aging Changes in Bladder Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels Are Associated With Increasing Heterogeneity of Adrenergic/Mucosal Influence on Detrusor Control in the Mouse

A geroscience-informed approach to the increasing prevalence of bladder control problems in older adults requires understanding the impact of aging on dynamic mechanisms that ensure resilience in response to stressors challenging asymptomatic voluntary control over urine storage and voiding. Bladder control is predicated on sensory neural information about bladder volume. Modulation of volume-induced bladder wall tensions by autonomic and mucosal factors controls neural sensitivity to bladder volume. We hypothesized that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels integrate these factors and thereby mediate adrenergic detrusor tension control. Furthermore, loss of HCN expression compromises that integration and could result in loss of precision of detrusor control. Using a life-span mouse model, reverse transcription quantitative real-time PCR and pharmacologic studies in pretensioned intact and mucosa-denuded bladder strips were made. The dominant hcn1 expression declines with maturation and aging; however, aging is also associated with increased variance around mean values. In strips from Mature animals, isoproterenol had less effect in denuded muscle strips than in intact strips, and HCN blockade diminished isoproterenol responsiveness. With aging, variances about mean response values significantly increased, paralleling hcn1 expression. Our findings support a role for HCN in providing neuroendocrine/paracrine integration and suggest an association of increased heterogeneity of HCN expression in aging with reductions in response precision to neuroendocrine control. The functional implication is an increased risk of dysfunction of brainstem/bladder regulation of neuronal sensitivity to bladder volume. This supports the clinical model of the aging bladder phenotype as an expression of loss of resilience, and not as emerging bladder pathology with aging.

Constitutively active HCN channels constrain detrusor excitability and modulate evoked contractions of human bladder

OBJECTIVE

Expression of Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is reported in bladder, but the functional role remains unsettled. Here, we immunolocalized the HCN1 and HCN4 subtype in human bladder and investigated their functional significance.

METHODS

Bladder procured from ten organ donors was dissected into mucosa (containing urothelium and submucosa) and detrusor for double immunofluorescence of HCN1 and 4 subtypes with gap junction and neural proteins together with isometric tension recordings. Mucosa intact and denuded detrusor strips were stretched to a basal tension of 10 mN for eliciting either tetrodotoxin (TTX) resistant spontaneous, carbachol evoked contractions and TTX sensitive electrical field stimulated (EFS), pre and post-addition of HCN blocker, ZD7288 or the activator, Lamotrigine or the cholinesterase inhibitor, Neostigmine.

RESULTS

Double immunofluorescence revealed immunolocalization of HCN1 and HCN4 subtype with calcitonin gene related peptide (CGRP), choline acetyl transferase and gap junction proteins in mucosa and detrusor. Removal of mucosa significantly raised the resting tension and the force of spontaneous contractions upon cumulative addition of ZD7288 in micromolar range relative to Lamotrigine treated strips (P<0.05). ZD7288 [10 nM] did not affect the contractile response evoked by EFS or carbachol, but the addition of ZD7288 [10 nM] in presence of Neostigmine [1 µM] significantly enhanced the atropine and TTX sensitive EFS evoked contractions of mucosa denuded strips.

CONCLUSIONS

Overall, HCN channels immunolocalized in mucosa, smooth muscle, gap junctions and nerve fibers exert a tonic constraint on detrusor excitability, enable spatio-temporal integration of evoked contractions and constrain the release of neurotransmitters, respectively. In contrast to the pacemaker role in other organs, findings argue for a non-pacemaking role of HCN channels in human bladder.

HCN as a Mediator of Urinary Homeostasis: Age-Associated Changes in Expression and Function in Adrenergic Detrusor Relaxation

The Hyperpolarization activated, cyclic nucleotide gated (HCN) channel is a candidate mediator of neuroendocrine influence over detrusor tonus during filling. In other tissues, HCN loss with aging is linked to declines in rhythmicity and function. We hypothesized that HCN has an age-sensitive expression profile and functional role in adrenergic bladder relaxation. HCN was examined in bladders from young (2-6 months) and old (18-24 months) C57BL/6 female mice, using qRT-PCR, RNAScope, and Western blots. Isometric tension studies were conducted using bladder strips from young wild-type (YWT), old wild-type (OWT), and young HCN1 knock-out (YKO) female mice to test the role HCN in effects of β-adrenergic stimulation. Hcn1 is the dominant HCN isoform RNA in the mouse bladder wall, and is diminished with age. Location of Hcn RNA within the mouse bladder wall is isoform-specific, with HCN1 limited to the detrusor layer. Passively-tensioned YWT bladder strips are relaxed by isoproterenol in the presence of HCN function, where OWT strips are relaxed only in the presence of HCN blockade. HCN has an age-specific expression and function in adrenergic detrusor relaxation in mouse bladder strips.

β3-Adrenergic receptor-dependent modulation of the medium afterhyperpolarization in rat hippocampal CA1 pyramidal neurons

Action potential firing in hippocampal pyramidal neurons is regulated by generation of an afterhyperpolarization (AHP). Three phases of AHP are recognized, with the fast AHP regulating action potential firing at the onset of a burst and the medium and slow AHPs supressing action potential firing over hundreds of milliseconds and seconds, respectively. Activation of β-adrenergic receptors suppresses the slow AHP by a protein kinase A-dependent pathway. However, little is known regarding modulation of the medium AHP. Application of the selective β-adrenergic receptor agonist isoproterenol suppressed both the medium and slow AHPs evoked in rat CA1 hippocampal pyramidal neurons recorded from slices maintained in organotypic culture. Suppression of the slow AHP was mimicked by intracellular application of cAMP, with the suppression of the medium AHP by isoproterenol still being evident in cAMP-dialyzed cells. Suppression of both the medium and slow AHPs was antagonized by the β-adrenergic receptor antagonist propranolol. The effect of isoproterenol to suppress the medium AHP was mimicked by two β₃-adrenergic receptor agonists, BRL37344 and SR58611A. The medium AHP was mediated by activation of small-conductance calcium-activated K⁺ channels and deactivation of H channels at the resting membrane potential. Suppression of the medium AHP by isoproterenol was reduced by pretreating cells with the H-channel blocker ZD7288. These data suggest that activation of β₃-adrenergic receptors inhibits H channels, which suppresses the medium AHP in CA1 hippocampal neurons by utilizing a pathway that is independent of a rise in intracellular cAMP. This finding highlights a potential new target in modulating H-channel activity and thereby neuronal excitability. NEW & NOTEWORTHY The noradrenergic input into the hippocampus is involved in modulating long-term synaptic plasticity and is implicated in learning and memory. We demonstrate that activation of functional β₃-adrenergic receptors suppresses the medium afterhyperpolarization in hippocampal pyramidal neurons. This finding provides an additional mechanism to increase action potential firing frequency, where neuronal excitability is likely to be crucial in cognition and memory.

Hyperpolarization-Activated Cyclic Nucleotide-Gated Non-selective (HCN) Ion Channels Regulate Human and Murine Urinary Bladder Contractility

Purpose: Hyperpolarization-activated cyclic nucleotide gated non-selective (HCN) channels have been demonstrated in the urinary bladder in various species. Since they play a major role in governing rhythmic activity in pacemaker cells like in the sinoatrial node, we explored the role of these channels in human and murine detrusor smooth muscle.

Methods: In an organ bath, human and murine detrusor smooth muscle specimens were challenged with the HCN channel blocker ZD7288. In human tissue derived from macroscopically tumor-free cancer resections, the urothelium was removed. In addition, HCN1-deficient mice were used to identify the contribution of this particular isoform. Expression of HCN channels in the urinary bladder was analyzed using histological and ultrastructural analyses as well as quantitative reverse transcriptase polymerase chain reaction (RT-PCR).

Results: We found that the HCN channel blocker ZD7288 (50 μM) both induced tonic contractions and increased phasic contraction amplitudes in human and murine detrusor specimens. While these responses were not sensitive to tetrodotoxin, they were significantly reduced by the gap junction inhibitor 18β-glycyrrhetic acid suggesting that HCN channels are located within the gap junction-interconnected smooth muscle cell network rather than on efferent nerve fibers. Immunohistochemistry suggested HCN channel expression on smooth muscle tissue, and immunoelectron microscopy confirmed the scattered presence of HCN2 on smooth muscle cell membranes. HCN channels seem to be down-regulated with aging, which is paralleled by an increasing effect of ZD7288 in aging detrusor tissue. Importantly, the anticonvulsant and HCN channel activator lamotrigine relaxed the detrusor which could be reversed by ZD7288.

Conclusion: These findings demonstrate that HCN channels are functionally present and localized on smooth muscle cells of the urinary bladder. Given the age-dependent decline of these channels in humans, activation of HCN channels by compounds such as lamotrigine opens up the opportunity to combat detrusor hyperactivity in the elderly by drugs already approved for epilepsy.

BDNF overexpression in the bladder induces neuronal changes to mediate bladder overactivity

Elevated levels of brain-derived neurotrophic factor (BDNF) in urine of overactive bladder (OAB) patients support the association of BDNF with OAB symptoms, but the causality is not known. Here, we investigated the functionality of BDNF overexpression in rat bladder following bladder wall transfection of either BDNF or luciferase (luciferase) transgenes (10 µg). One week after transfection, BDNF overexpression in bladder tissue and elevation of urine BDNF levels were observed together with increased transcript of BDNF, its cognate receptors (TrkB and p75^NTR), and downstream PLCγ isoforms in bladder. BDNF overexpression can induce the bladder overactivity (BO) phenotype which is demonstrated by the increased voiding pressure and reduced intercontractile interval during transurethral open cystometry under urethane anesthesia. A role for BDNF-mediated enhancement of prejunctional cholinergic transmission in BO is supported by the significant increase in the atropine- and neostigmine-sensitive component of nerve-evoked contractions and upregulation of choline acetyltransferase, vesicular acetylcholine transporter, and transporter Oct2 and -α1 receptors. In addition, higher expression of transient receptor channels (TRPV1 and TRPA1) and pannexin-1 channels in conjunction with elevation of ATP and neurotrophins in bladder and also in L6/S1 dorsal root ganglia together support a role for sensitized afferent nerve terminals in BO. Overall, genomic changes in efferent and afferent neurons of bladder induced by the overexpression of BDNF per se establish a mechanistic link between elevated BDNF levels in urine and dysfunctional voiding observed in animal models and in OAB patients.

The Dome Wall of Bladder Acts as a Pacemaker Site in Detrusor Instability in Rats

BACKGROUND The aim of this study was to confirm that the interstitial cells of Cajal (ICCs) in the dome wall of the bladder are pacemaker cells, and that the dome wall of the bladder acts as a pacemaker site in the detrusor instability (DI) rat model. MATERIAL AND METHODS The model of DI in Wistar rats was established and urodynamic studies measuring the bladder volume and pressure were performed. The detrusor excitability was investigated using the amplitude and frequency of phasic contraction of strips. The localization and quantity of ICCs was identified by immunohistochemistry and c-KIT protein expression in the rat bladder. PCR assay and Western blot were used to assess the expression of HCN2 and Cx43. RESULTS The bladder capacity, residual volume, voiding volume, and maximum voiding pressure were significantly increased in the DI group. The contraction frequency and amplitude of the strips from the dome of the bladder in the DI group were higher than the triangle, body, and base parts. Both the concentration of c-KIT positive ICCs cells and expression of the c-KIT protein in the dome wall were higher than in other parts of the bladder. The expression of HCN2 and Cx43 in each part of the DI rat group were obviously higher than each part in the control group. Compared to the body, base, and triangle parts, the expression of HCN2 and Cx43 in the dome wall were obviously higher in the DI group. CONCLUSIONS The quantity of ICCs was higher in the dome wall and the dome wall of bladder acts as a pacemaker site in the DI rat model.