Maternal Thyroid Hormone Programs Cardiovascular Functions in the Offspring

Background: Maternal thyroid hormone (TH) plays an essential role for fetal development, especially for the cardiovascular system and its central control. However, the precise consequences of altered TH action during the different periods in pregnancy remain poorly understood. Methods: To address this question, we used mice heterozygous for a mutant thyroid hormone receptor α1 (TRα1) and wild-type controls that were born to wild-type mothers treated with 3,3',5-triiodothyronine (T3) during the first or the second half of pregnancy. We then phenotyped the offspring animals as adults by in vivo measurements and postmortem tissue analyses. Results: Maternal T3 treatment in either half of the pregnancy did not affect postnatal growth development. Serum thyroxine and hypophyseal thyrotropin subunit beta or deiodinase type II expression was also not affected in any group, only TRα1 mutant males exhibited a reduction in serum T3 levels after the treatment. Likewise, hepatic deiodinase type I was not altered, but serum selenium levels were reduced by the maternal treatment in wild-type offspring of both genders. Most interestingly, a significant increase in heart weight was found in adult wild-types born to mothers that received T3 during the first or second half of pregnancy, while TRα1 mutant males were protected from this effect. Moreover, we detected a significant increase in heart rate selectively in male mice that were exposed to elevated maternal T3 in the second half of the pregnancy. Conclusion: Taken together, our findings demonstrate that maternal TH is of particular relevance during the second half of pregnancy for establishing cardiac properties, with specific effects depending on TRα1 or gender. The data advocate routinely monitoring TH levels during pregnancy to avoid adverse cardiac effects in the offspring.

Cation leak underlies neuronal excitability in an HCN1 developmental and epileptic encephalopathy

Pathogenic variants in HCN1 are associated with developmental and epileptic encephalopathies. The recurrent de novo HCN1 M305L pathogenic variant is associated with severe developmental impairment and drug-resistant epilepsy. We engineered the homologue Hcn1 M294L heterozygous knock-in (Hcn1M294L) mouse to explore the disease mechanism underlying an HCN1 developmental and epileptic encephalopathy. The Hcn1M294L mouse recapitulated the phenotypic features of patients with the HCN1 M305L variant, including spontaneous seizures and a learning deficit. Active epileptiform spiking on the electrocorticogram and morphological markers typical of rodent seizure models were observed in the Hcn1M294L mouse. Lamotrigine exacerbated seizures and increased spiking, whereas sodium valproate reduced spiking, mirroring drug responses reported in a patient with this variant. Functional analysis in Xenopus laevis oocytes and layer V somatosensory cortical pyramidal neurons in ex vivo tissue revealed a loss of voltage dependence for the disease variant resulting in a constitutively open channel that allowed for cation 'leak' at depolarized membrane potentials. Consequently, Hcn1M294L layer V somatosensory cortical pyramidal neurons were significantly depolarized at rest. These neurons adapted through a depolarizing shift in action potential threshold. Despite this compensation, layer V somatosensory cortical pyramidal neurons fired action potentials more readily from rest. A similar depolarized resting potential and left-shift in rheobase was observed for CA1 hippocampal pyramidal neurons. The Hcn1M294L mouse provides insight into the pathological mechanisms underlying hyperexcitability in HCN1 developmental and epileptic encephalopathy, as well as being a preclinical model with strong construct and face validity, on which potential treatments can be tested.

Gating movements and ion permeation in HCN4 pacemaker channels

The HCN1-4 channel family is responsible for the hyperpolarization-activated cation current If/Ih that controls automaticity in cardiac and neuronal pacemaker cells. We present cryoelectron microscopy (cryo-EM) structures of HCN4 in the presence or absence of bound cAMP, displaying the pore domain in closed and open conformations. Analysis of cAMP-bound and -unbound structures sheds light on how ligand-induced transitions in the channel cytosolic portion mediate the effect of cAMP on channel gating and highlights the regulatory role of a Mg2+ coordination site formed between the C-linker and the S4-S5 linker. Comparison of open/closed pore states shows that the cytosolic gate opens through concerted movements of the S5 and S6 transmembrane helices. Furthermore, in combination with molecular dynamics analyses, the open pore structures provide insights into the mechanisms of K+/Na+ permeation. Our results contribute mechanistic understanding on HCN channel gating, cyclic nucleotide-dependent modulation, and ion permeation.

Structural Identification of the Pacemaker Cells and Expression of Hyperpolarization-Activated Cyclic Nucleotide-Gated (HCN) Channels in the Heart of the Wild Atlantic Cod, Gadus morhua (Linnaeus, 1758)

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are proteins that contain highly conserved functional domains and sequence motifs that are correlated with their unique biophysical activities, to regulate cardiac pacemaker activity and synaptic transmission. These pacemaker proteins have been studied in mammalian species, but little is known now about their heart distribution in lower vertebrates and c-AMP modulation. Here, we characterized the pacemaker system in the heart of the wild Atlantic cod (Gadus morhua), with respect to primary pacemaker molecular markers. Special focus is given to the structural, ultrastructural and molecular characterization of the pacemaker domain, through the expression of HCN channel genes and the immunohistochemistry of HCN isoforms, including the location of intracardiac neurons that are adjacent to the sinoatrial region of the heart. Similarly to zebrafish and mammals, these neurons are immunoreactive to ChAT, VAChT and nNOS. It has been shown that cardiac pacemaking can be modulated by sympathetic and parasympathetic pathways, and the existence of intracardiac neurons projecting back to the central nervous system provide a plausible link between them.

Bidirectional flow of the funny current (If) during the pacemaking cycle in murine sinoatrial node myocytes

Sinoatrial node myocytes (SAMs) act as cardiac pacemaker cells by firing spontaneous action potentials (APs) that initiate each heartbeat. The funny current (If) is critical for the generation of these spontaneous APs; however, its precise role during the pacemaking cycle remains unresolved. Here, we used the AP-clamp technique to quantify If during the cardiac cycle in mouse SAMs. We found that If is persistently active throughout the sinoatrial AP, with surprisingly little voltage-dependent gating. As a consequence, it carries both inward and outward current around its reversal potential of -30 mV. Despite operating at only 2 to 5% of its maximal conductance, If carries a substantial fraction of both depolarizing and repolarizing net charge movement during the firing cycle. We also show that β-adrenergic receptor stimulation increases the percentage of net depolarizing charge moved by If, consistent with a contribution of If to the fight-or-flight increase in heart rate. These properties were confirmed by heterologously expressed HCN4 channels and by mathematical models of If Modeling further suggested that the slow rates of activation and deactivation of the HCN4 isoform underlie the persistent activity of If during the sinoatrial AP. These results establish a new conceptual framework for the role of If in pacemaking, in which it operates at a very small fraction of maximal activation but nevertheless drives membrane potential oscillations in SAMs by providing substantial driving force in both inward and outward directions.

cAMP binding to closed pacemaker ion channels is non-cooperative

Electrical activity in the brain and heart depends on rhythmic generation of action potentials by pacemaker ion channels (HCN) whose activity is regulated by cAMP binding1. Previous work has uncovered evidence for both positive and negative cooperativity in cAMP binding2,3, but such bulk measurements suffer from limited parameter resolution. Efforts to eliminate this ambiguity using single-molecule techniques have been hampered by the inability to directly monitor binding of individual ligand molecules to membrane receptors at physiological concentrations. Here we overcome these challenges using nanophotonic zero-mode waveguides4 to directly resolve binding dynamics of individual ligands to multimeric HCN1 and HCN2 ion channels. We show that cAMP binds independently to all four subunits when the pore is closed, despite a subsequent conformational isomerization to a flip state at each site. The different dynamics in binding and isomerization are likely to underlie physiologically distinct responses of each isoform to cAMP5 and provide direct validation of the ligand-induced flip-state model6-9. This approach for observing stepwise binding in multimeric proteins at physiologically relevant concentrations can directly probe binding allostery at single-molecule resolution in other intact membrane proteins and receptors.

Intrinsic Electrical Remodeling Underlies Atrioventricular Block in Athletes

[Figure: see text].

Promotion of Differentiating Bone Marrow Mesenchymal Stromal Cells (BMSCs) into Cardiomyocytes via HCN2 and HCN4 Cotransfection

AIM

Investigation of the influences HCN2 and HCN4 has on bone marrow mesenchymal stromal cells (BMSCs) on cardiomyocyte differentiation.

METHODS

Miniature adult pigs were used for bone marrow extraction and isolation of BMSCs. The identification of these BMSCs was done by using flow cytometry for the detection of expressed surface antigens CD45, CD11B, CD44, and CD90. Using HCN2 and HCN4 genes cotransfected into BMSCs as group HCN2+HCN4 while myocardial induction solution was used to induced BMSC differentiation in the BMSC induction group. Myocardial marker proteins α-actin and cTnT were detected by immunofluorescence staining, while α-actin, cTnT, and Desmin myocardial marker proteins expressed were detected by Western blot. The whole-cell patch-clamp technique was used to identify and detect cellular HCN2 channels, HCN4 channel current activation curve, and the inhibitory effect of CsCl on heterologous expression currents.

RESULTS

Flow cytometry results showed that CD45 and CD11B were expressed negatively while CD90 and CD44 were positive. Post HCN2 and HCN4 gene transfection, immunofluorescence staining, and Western blot showed significantly increased HCN2, HCN4, α-actin, and cTnT expressed in group HCN2+HCN4 were, which could be compared to the expression levels in the BMSC-induced group. The HCN2+HCN4 group was able to document cell membrane channel ion currents that were similar to If properties.

CONCLUSION

HCN2 and HCN4 overexpression can considerably enhance the MSC ability to differentiate into cardiomyocytes in vitro and restore the ionic current.

Cohesin-protein Shugoshin-1 controls cardiac automaticity via HCN4 pacemaker channel

Endogenous cardiac pacemaker function regulates the rate and rhythm of cardiac contraction. The mutation p.Lys23Glu in the cohesin protein Shugoshin-1 causes severe heart arrhythmias due to sinoatrial node dysfunction and a debilitating gastrointestinal motility disorder, collectively termed the Chronic Atrial and Intestinal Dysrhythmia Syndrome, linking Shugoshin-1 and pacemaker activity. Hyperpolarization-activated, cyclic nucleotide-gated cation channel 4 (HCN4) is the predominant pacemaker ion-channel in the adult heart and carries the majority of the "funny" current, which strongly contributes to diastolic depolarization in pacemaker cells. Here, we study the mechanism by which Shugoshin-1 affects cardiac pacing activity with two cell models: neonatal rat ventricular myocytes and Chronic Atrial and Intestinal Dysrhythmia Syndrome patient-specific human induced pluripotent stem cell derived cardiomyocytes. We find that Shugoshin-1 interacts directly with HCN4 to promote and stabilize cardiac pacing. This interaction enhances funny-current by optimizing HCN4 cell-surface expression and function. The clinical p.Lys23Glu mutation leads to an impairment in the interaction between Shugoshin-1 and HCN4, along with depressed funny-current and dysrhythmic activity in induced pluripotent stem cell derived cardiomyocytes derived from Chronic Atrial and Intestinal Dysrhythmia Syndrome patients. Our work reveals a critical non-canonical, cohesin-independent role for Shugoshin-1 in maintaining cardiac automaticity and identifies potential therapeutic avenues for cardiac pacemaking disorders, in particular Chronic Atrial and Intestinal Dysrhythmia Syndrome.

A detailed characterization of the hyperpolarization-activated "funny" current (If) in human-induced pluripotent stem cell (iPSC)-derived cardiomyocytes with pacemaker activity

Properties of the funny current (I_f) have been studied in several animal and cellular models, but so far little is known concerning its properties in human pacemaker cells. This work provides a detailed characterization of I_f in human-induced pluripotent stem cell (iPSC)–derived pacemaker cardiomyocytes (pCMs), at different time points. Patch-clamp analysis showed that I_f density did not change during differentiation; however, after day 30, it activates at more negative potential and with slower time constants. These changes are accompanied by a slowing in beating rate. I_f displayed the voltage-dependent block by caesium and reversed (E_rev) at − 22 mV, compatibly with the 3:1 K⁺/Na⁺ permeability ratio. Lowering [Na⁺]_o (30 mM) shifted the E_rev to − 39 mV without affecting conductance. Increasing [K⁺]_o (30 mM) shifted the E_rev to − 15 mV with a fourfold increase in conductance. pCMs express mainly HCN4 and HCN1 together with the accessory subunits CAV3, KCR1, MiRP1, and SAP97 that contribute to the context-dependence of I_f. Autonomic agonists modulated the diastolic depolarization, and thus rate, of pCMs. The adrenergic agonist isoproterenol induced rate acceleration and a positive shift of I_f voltage-dependence (EC₅₀ 73.4 nM). The muscarinic agonists had opposite effects (Carbachol EC₅₀, 11,6 nM). Carbachol effect was however small but it could be increased by pre-stimulation with isoproterenol, indicating low cAMP levels in pCMs. In conclusion, we demonstrated that pCMs display an I_f with the physiological properties expected by pacemaker cells and may thus represent a suitable model for studying human I_f-related sinus arrhythmias.

Ivabradine prevents deleterious effects of dopamine therapy in heart failure: No role for HCN4 overexpression

BACKGROUND

Exacerbations of chronic heart failure (CHF) are often treated with catecholamines to provide short term inotropic support, but this strategy is associated with long-term detrimental hemodynamic effects and increased ventricular arrhythmias (VA), possibly related to increased heart rate (HR). We hypothesized that ivabradine may prevent adverse effects of short-term dopamine treatment in CHF.

METHODS

Rats with post-myocardial infarction CHF received 2-week infusion of saline, dopamine(D), ivabradine(I) or D&I; cardiac function was assessed using echocardiography and pressure-volume loops while VA were assessed using telemetric ECG recording. Expression of HCN4, a potentially proarrhythmic channel blocked by ivabradine, was assessed in left ventricular (LV) myocardium. HCN4 expression was also assessed in human explanted normal and failing hearts and correlated with VA.

FINDINGS

Dopamine infusion had detrimental effects on hemodynamic parameters and LV remodeling and induced VA in CHF rats, while ivabradine completely prevented these effects. CHF rats demonstrated HCN4 overexpression in LV myocardium, and ivabradine and, unexpectedly, dopamine prevented this. Failing human hearts also exhibited HCN4 overexpression in LV myocardium that was unrelated to patient's sex, CHF etiology, VA severity or plasma NT-proBNP.

INTERPRETATION

HR reduction offered by ivabradine may be a feasible strategy to extract benefits of inotropic support in CHF exacerbations, avoiding detrimental effects on CHF biology or VA. Ivabradine may offer additional beneficial effects in this setting, going beyond pure HR reduction, however prevention of ventricular HCN4 overexpression is unlikely to play a major role.

Isolated downregulation of HCN2 in ventricles of rats with streptozotocin-induced diabetic cardiomyopathy

BACKGROUND

In spite of disrupted repolarization of diabetic heart, some studies report less tendency of diabetic heart to develop ventricular arrhythmias suggesting effective compensatory mechanism. We hypothesized that myocardial alterations in HCN2 and HCN4 channels occur under hyperglycaemia.

METHODS

Diabetes was induced in rats using a single injection of streptozotocin (STZ; 55 mg/kg body weight, i.p.). Basal ECG was measured. Expression of mRNA for HCN channels, potassium channels and microRNA 1 and 133a were measured in ventricular tissues. Protein expression of HCN2 channel isoform was assessed in five different regions of the heart by western blotting. Differentiated H9c2 cell line was used to examine HCN channels expression under hyperglycaemia in vitro.

RESULTS

Six weeks after STZ administration, heart rate was reduced, QRS complex duration, QT interval and T-wave were prolonged in diabetic rats compared to controls. mRNA and protein expressions of HCN2 decreased exclusively in the ventricles of diabetic rats. HCN2 expression levels in atria of STZ rats and H9c2 cells treated with excess of glucose were not changed. MicroRNA levels were stable in STZ rat hearts. We found significantly decreased mRNA levels of several potassium channels participating in repolarization, namely Kcnd2 (Ito1), Kcnh2 (IKr), Kcnq1 (IKs) and Kcnj11 (IKATP).

CONCLUSIONS

This result together with downregulated HCN2 channels suggest that HCN channels might be an integral part of ventricular electric remodelling and might play a role in cardiac repolarization projected in altered arrhythmogenic profile of diabetic heart.

cAMP-PKA signaling is involved in regulation of spinal HCN channels function in diabetic neuropathic pain

The cyclic adenosine monophosphate-protein kinase A (cAMP-PKA) signaling acts a pivotal part in hyperpolarization-activated cyclic nucleotide-gated (HCN) channels-mediated neuropathic and inflammatory pain. However, there has been no evidence of cAMP-PKA signaling is involved in regulation of spinal HCN channels function in the occurrence of diabetic neuropathic pain (DNP). The study aimed to elucidate the impact of HCN channels on neuropathic pain in a rat model of diabetes induced by streptozotocin, and whether cAMP-PKA signaling is involved in regulation of HCN channels function. In this report, we evaluated the effect of intrathecal administration of HCN channel blockers ZD7288, cAMP inhibitor SQ22536 and PKA inhibitor H-89 on nociceptive behavior in DNP rats. The mechanical withdrawal threshold (MWT) was measured to evaluate pain behavior in rats. Protein expression levels of HCN2, HCN4 channels and PKA in the spinal dorsal horn of rats were assessed. Furthermore, the levels of cAMP in rat spinal dorsal horn was analyzed. We discovered that DNP rats showed significant mechanical allodynia and are related to the increased HCN2 and HCN4 channels expression, enhanced cAMP production and elevated the expression of PKA protein in the spinal dorsal horn, which were attenuated by intrathecal ZD7288. Furthermore, intrathecal injection of SQ22536 and H-89 significantly reduced the HCN2 and HCN4 channels expression in the spinal dorsal horn of DNP rats. Our findings indicate that HCN channels of the spinal dorsal horn participate in the pathogenesis of allodynia in rats with DNP, which could be regulated by cAMP-PKA signaling. Therefore, HCN channels and cAMP-PKA signaling are potential targets for hyperalgesia treatment in DNP patients.

AAV-Mediated CRISPRi and RNAi Based Gene Silencing in Mouse Hippocampal Neurons

Uncovering the physiological role of individual proteins that are part of the intricate process of cellular signaling is often a complex and challenging task. A straightforward strategy of studying a protein's function is by manipulating the expression rate of its gene. In recent years, the Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)/Cas9-based technology was established as a powerful gene-editing tool for generating sequence specific changes in proliferating cells. However, obtaining homogeneous populations of transgenic post-mitotic neurons by CRISPR/Cas9 turned out to be challenging. These constraints can be partially overcome by CRISPR interference (CRISPRi), which mediates the inhibition of gene expression by competing with the transcription machinery for promoter binding and, thus, transcription initiation. Notably, CRISPR/Cas is only one of several described approaches for the manipulation of gene expression. Here, we targeted neurons with recombinant Adeno-associated viruses to induce either CRISPRi or RNA interference (RNAi), a well-established method for impairing de novo protein biosynthesis by using cellular regulatory mechanisms that induce the degradation of pre-existing mRNA. We specifically targeted hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels, which are widely expressed in neuronal tissues and play essential physiological roles in maintaining biophysical characteristics in neurons. Both of the strategies reduced the expression levels of three HCN isoforms (HCN1, 2, and 4) with high specificity. Furthermore, detailed analysis revealed that the knock-down of just a single HCN isoform (HCN4) in hippocampal neurons did not affect basic electrical parameters of transduced neurons, whereas substantial changes emerged in HCN-current specific properties.

Inhibition of hyperpolarization-activated cyclic nucleotide-gated channels with natural flavonoid quercetin

Quercetin is a natural flavonoid which has been reported to be analgesic in different animal models of pain. However, the mechanism underlying the pain-relieving effects is still unclear. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels play critical roles in controlling pacemaker activity in cardiac and nervous systems, making the channel a new target for therapeutic exploration. In this study, we explored a series of flavonoids for their modulation on HCN channels. Among all tested flavonoids, quercetin was the most potent inhibitor for HCN channels with an IC₅₀ value of 27.32 ± 1.19 μM for HCN2. Furthermore, quercetin prominently left shifted the voltage-dependent activation curves of HCN channels and decelerated deactivation process. The results presented herein firstly characterize quercetin as a novel and potent inhibitor for HCN channels, which represents a novel structure for future drug design of HCN channel inhibitors.

Electrical stimulation applied during differentiation drives the hiPSC-CMs towards a mature cardiac conduction-like cells

Human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) resemble fetal cardiomyocytes and electrical stimulation (ES) has been explored to mature the differentiated cells. Here, we hypothesize that ES applied at the beginning of the differentiation process, triggers both differentiation of the hiPSC-CMs into a specialized conduction system (CS) phenotype and cell maturation. We applied ES for 15 days starting on day 0 of the differentiation process and found an increased expression of transcription factors and proteins associated with the development and function of CS including Irx3, Nkx2.5 and contactin 2, Hcn4 and Scn5a, respectively. We also found activation of intercalated disc proteins (Nrap and β-catenin). We detected ES-induced CM maturation as indicated by increased Tnni1 and Tnni3 expression. Confocal micrographs showed a shift towards expression of the gap junction protein connexin 40 in ES hiPSC-CM compared to the more dominant expression of connexin 43 in controls. Finally, analysis of functional parameters revealed that ES hiPSC-CMs exhibited faster action potential (AP) depolarization, longer intracellular Ca2+ transients, and slower AP duration at 90% of repolarization, resembling fast conducting fibers. Altogether, we provided evidence that ES during the differentiation of hiPSC to cardiomyocytes lead to development of cardiac conduction-like cells with more mature cytoarchitecture. Thus, hiPSC-CMs exposed to ES during differentiation can be instrumental to develop CS cells for cardiac disease modelling, screening individual drugs on a precison medicine type platform and support the development of novel therapeutics for arrhythmias.

The structure and function of TRIP8b, an auxiliary subunit of hyperpolarization-activated cyclic-nucleotide gated channels

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are expressed throughout the mammalian central nervous system (CNS). These channels have been implicated in a wide range of diseases, including Major Depressive Disorder and multiple subtypes of epilepsy. The diversity of functions that HCN channels perform is in part attributable to differences in their subcellular localization. To facilitate a broad range of subcellular distributions, HCN channels are bound by auxiliary subunits that regulate surface trafficking and channel function. One of the best studied auxiliary subunits is tetratricopeptide-repeat containing, Rab8b-interacting protein (TRIP8b). TRIP8b is an extensively alternatively spliced protein whose only known function is to regulate HCN channels. TRIP8b binds to HCN pore-forming subunits at multiple interaction sites that differentially regulate HCN channel function and subcellular distribution. In this review, we summarize what is currently known about the structure and function of TRIP8b isoforms with an emphasis on the role of this auxiliary subunit in health and disease.

Deep posteromedial cortical rhythm in dissociation

Advanced imaging methods now allow cell-type-specific recording of neural activity across the mammalian brain, potentially enabling the exploration of how brain-wide dynamical patterns give rise to complex behavioural states1-12. Dissociation is an altered behavioural state in which the integrity of experience is disrupted, resulting in reproducible cognitive phenomena including the dissociation of stimulus detection from stimulus-related affective responses. Dissociation can occur as a result of trauma, epilepsy or dissociative drug use13,14, but despite its substantial basic and clinical importance, the underlying neurophysiology of this state is unknown. Here we establish such a dissociation-like state in mice, induced by precisely-dosed administration of ketamine or phencyclidine. Large-scale imaging of neural activity revealed that these dissociative agents elicited a 1-3-Hz rhythm in layer 5 neurons of the retrosplenial cortex. Electrophysiological recording with four simultaneously deployed high-density probes revealed rhythmic coupling of the retrosplenial cortex with anatomically connected components of thalamus circuitry, but uncoupling from most other brain regions was observed-including a notable inverse correlation with frontally projecting thalamic nuclei. In testing for causal significance, we found that rhythmic optogenetic activation of retrosplenial cortex layer 5 neurons recapitulated dissociation-like behavioural effects. Local retrosplenial hyperpolarization-activated cyclic-nucleotide-gated potassium channel 1 (HCN1) pacemakers were required for systemic ketamine to induce this rhythm and to elicit dissociation-like behavioural effects. In a patient with focal epilepsy, simultaneous intracranial stereoencephalography recordings from across the brain revealed a similarly localized rhythm in the homologous deep posteromedial cortex that was temporally correlated with pre-seizure self-reported dissociation, and local brief electrical stimulation of this region elicited dissociative experiences. These results identify the molecular, cellular and physiological properties of a conserved deep posteromedial cortical rhythm that underlies states of dissociation.

Rational design of a mutation to investigate the role of the brain protein TRIP8b in limiting the cAMP response of HCN channels in neurons

TRIP8b (tetratricopeptide repeat-containing Rab8b-interacting protein) is the neuronal regulatory subunit of HCN channels, a family of voltage-dependent cation channels also modulated by direct cAMP binding. TRIP8b interacts with the C-terminal region of HCN channels and controls both channel trafficking and gating. The association of HCN channels with TRIP8b is required for the correct expression and subcellular targeting of the channel protein in vivo. TRIP8b controls HCN gating by interacting with the cyclic nucleotide-binding domain (CNBD) and competing for cAMP binding. Detailed structural knowledge of the complex between TRIP8b and CNBD was used as a starting point to engineer a mutant channel, whose gating is controlled by cAMP, but not by TRIP8b, while leaving TRIP8b-dependent regulation of channel trafficking unaltered. We found two-point mutations (N/A and C/D) in the loop connecting the CNBD to the C-linker (N-bundle loop) that, when combined, strongly reduce the binding of TRIP8b to CNBD, leaving cAMP affinity unaltered both in isolated CNBD and in the full-length protein. Proof-of-principle experiments performed in cultured cortical neurons confirm that the mutant channel provides a genetic tool for dissecting the two effects of TRIP8b (gating versus trafficking). This will allow the study of the functional role of the TRIP8b antagonism of cAMP binding, a thus far poorly investigated aspect of HCN physiology in neurons.

Silencing miR-370-3p rescues funny current and sinus node function in heart failure

Bradyarrhythmias are an important cause of mortality in heart failure and previous studies indicate a mechanistic role for electrical remodelling of the key pacemaking ion channel HCN4 in this process. Here we show that, in a mouse model of heart failure in which there is sinus bradycardia, there is upregulation of a microRNA (miR-370-3p), downregulation of the pacemaker ion channel, HCN4, and downregulation of the corresponding ionic current, If, in the sinus node. In vitro, exogenous miR-370-3p inhibits HCN4 mRNA and causes downregulation of HCN4 protein, downregulation of If, and bradycardia in the isolated sinus node. In vivo, intraperitoneal injection of an antimiR to miR-370-3p into heart failure mice silences miR-370-3p and restores HCN4 mRNA and protein and If in the sinus node and blunts the sinus bradycardia. In addition, it partially restores ventricular function and reduces mortality. This represents a novel approach to heart failure treatment.

PDK1-AKT signaling pathway regulates the expression and function of cardiac hyperpolarization-activated cyclic nucleotide-modulated channels

AIM

The enzyme 3-phosphoinositide-dependent protein kinase-1 (PDK1) is associated with cardiac and pathological remodeling and ion channel function regulation. However, whether it regulates hyperpolarization-activated cyclic nucleotide-modulated channels (HCNs) remains unclear.

MAIN METHODS

In the atrial myocytes of heart-specific PDK1 "knockout" mouse model and neonatal mice, protein kinase B (AKT)-related inhibitors or agonists as well as knockdown or overexpression plasmids were used to study the relationship between PDK1 and HCNs.

KEY FINDINGS

HCN1 expression and AKT phosphorylation at the Thr308 site were significantly decreased in atrial myocytes after PDK1 knockout or inhibition; in contrast, HCN2 and HCN4 levels were significantly increased. Also, a similar trend of HCNs expression has been observed in cultured atrial myocytes after PDK1 inhibition, as further demonstrated via immunofluorescence and patch-clamp experiments. Moreover, these results of PDK1 overexpression indicate an opposite trend compared with the previous experimental results. However, the results of PDK1 inhibition or overexpression could be reversed by activating or inhibiting AKT, respectively.

SIGNIFICANCE

These results indicate that the PDK1-AKT signaling pathway is involved in the regulation of HCN mRNA transcription, protein expression, HCN current density, and cell membrane location.

The conserved DNMT1-dependent methylation regions in human cells are vulnerable to neurotoxicant rotenone exposure

BACKGROUND

Allele-specific DNA methylation (ASM) describes genomic loci that maintain CpG methylation at only one inherited allele rather than having coordinated methylation across both alleles. The most prominent of these regions are germline ASMs (gASMs) that control the expression of imprinted genes in a parent of origin-dependent manner and are associated with disease. However, our recent report reveals numerous ASMs at non-imprinted genes. These non-germline ASMs are dependent on DNA methyltransferase 1 (DNMT1) and strikingly show the feature of random, switchable monoallelic methylation patterns in the mouse genome. The significance of these ASMs to human health has not been explored. Due to their shared allelicity with gASMs, herein, we propose that non-traditional ASMs are sensitive to exposures in association with human disease.

RESULTS

We first explore their conservancy in the human genome. Our data show that our putative non-germline ASMs were in conserved regions of the human genome and located adjacent to genes vital for neuronal development and maturation. We next tested the hypothesized vulnerability of these regions by exposing human embryonic kidney cell HEK293 with the neurotoxicant rotenone for 24 h. Indeed,14 genes adjacent to our identified regions were differentially expressed from RNA-sequencing. We analyzed the base-resolution methylation patterns of the predicted non-germline ASMs at two neurological genes, HCN2 and NEFM, with potential to increase the risk of neurodegeneration. Both regions were significantly hypomethylated in response to rotenone.

CONCLUSIONS

Our data indicate that non-germline ASMs seem conserved between mouse and human genomes, overlap important regulatory factor binding motifs, and regulate the expression of genes vital to neuronal function. These results support the notion that ASMs are sensitive to environmental factors such as rotenone and may alter the risk of neurological disease later in life by disrupting neuronal development.

Involvement of Hyperpolarization-Activated Cyclic Nucleotide-Gated Channel 3 in Oxytocin Neuronal Activity in Lactating Rats With Pup Deprivation

Oxytocin, a hypothalamic neuropeptide essential for breastfeeding, is mainly produced in oxytocin neurons in the supraoptic nucleus (SON) and paraventricular nucleus. However, mechanisms underlying oxytocin secretion, specifically the involvement of hyperpolarization-activated cyclic nucleotide-gated channel 3 (HCN3) in oxytocin neuronal activity, remain unclear. Using a rat model of intermittent and continuous pup deprivation (PD) at the middle stage of lactation, we analyzed the contribution of HCN3 in oxytocin receptor (OTR)-associated signaling cascade to oxytocin neuronal activity in the SON. PD caused maternal depression, anxiety, milk shortage, involution of the mammary glands, and delays in uterine recovery, particularly in continuous PD. PD increased hypothalamic but not plasma oxytocin levels in enzyme-linked immunosorbent assay. In the SON, PD increased c-Fos expression but reduced expressions of cyclooxygenase-2 and HCN3 in Western blots and/or immunohistochemistry. Moreover, PD significantly increased the molecular association of OTR with HCN3 in coimmunoprecipitation. In brain slices, inhibition of HCN3 activity with DK-AH269 blocked prostaglandin E₂-evoked increase in the firing activity and burst discharge in oxytocin neurons in patch-clamp recordings. In addition, oxytocin-evoked increase in the molecular association between OTR and HCN3 in brain slices of the SON was blocked by pretreatment with indomethacin, an inhibitor of cyclooxygenase-2. These results indicate that normal activity of oxytocin neurons is under the regulation of an oxytocin receptor-cyclooxygenase-2-HCN3 pathway and that PD disrupts maternal behavior through increasing intranuclear oxytocin secretion in the SON but likely reducing bolus oxytocin release into the blood through inhibition of HCN3 activity.

Chronic ethanol exposure alters prelimbic prefrontal cortical Fast-Spiking and Martinotti interneuron function with differential sex specificity in rat brain

Chronic ethanol exposure results in numerous neurobiological adaptations that promote deficits in medial prefrontal cortical (mPFC) function associated with blunted inhibitory control and elevated anxiety during withdrawal. Studies exploring alcohol dependence-related changes in this region have largely investigated adaptations in glutamatergic signaling, with inhibitory neurotransmission remaining relatively understudied. To address this, we used biochemical and electrophysiological methods to evaluate the effects of ethanol on the activity of deep-layer prelimbic mPFC Fast-Spiking (FS) and Martinotti interneurons after chronic ethanol exposure in male and female rats. We report that chronic alcohol exposure significantly impairs FS neuron excitability in both males and females. Interestingly, we observed a marked sex difference in the baseline activity of Martinotti cells that furthermore displayed differential sex-specific responses to alcohol exposure. In addition, alcohol effects on Martinotti neuron excitability negatively correlated with hyperpolarization-activated currents mediated by hyperpolarization-activated cyclic nucleotide gated (HCN) channels, indicative of a causal relationship. Analysis of HCN1 protein expression also revealed a substantial sex difference, although no effect of ethanol on HCN1 protein expression was observed. Taken together, these findings further elucidate the complex adaptations that occur in the mPFC after chronic ethanol exposure and reveal fundamental differences in interneuron activity between sexes. Furthermore, this disparity may reflect innate differences in intracortical microcircuit function between male and female rats, and offers a tenable circuit-level explanation for sex-dependent behavioral responses to alcohol.

Facilitation of spinal α-motoneuron excitability by histamine and the underlying ionic mechanisms

Spinal α-motoneurons directly innervate skeletal muscles and function as the final common path for movement and behavior. The processes that determine the excitability of motoneurons are critical for the execution of motor behavior. In fact, it has been noted that spinal motoneurons receive various neuromodulatory inputs, especially monoaminergic one. However, the roles of histamine and hypothalamic histaminergic innervation on spinal motoneurons and the underlying ionic mechanisms are still largely unknown. In the present study, by using the method of intracellular recording on rat spinal slices, we found that activation of either H₁ or H₂ receptor potentiated repetitive firing behavior and increased the excitability of spinal α-motoneurons. Both of blockage of K⁺ channels and activation of Na⁺-Ca²⁺ exchangers were involved in the H₁ receptor-mediated excitation on spinal motoneurons, whereas the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels were responsible for the H₂ receptor-mediated excitation. The results suggest that, through switching functional status of ion channels and exchangers coupled to histamine receptors, histamine effectively biases the excitability of the spinal α-motoneurons. In this way, the hypothalamospinal histaminergic innervation may directly modulate final motor outputs and actively regulate spinal motor reflexes and motor execution.