Comprehensive multilevel in vivo and in vitro analysis of heart rate fluctuations in mice by ECG telemetry and electrophysiology

Feeding behavior modifies the circadian variation in RR and QT intervals by distinct mechanisms in mice

Rhythmic feeding behavior is critical for regulating phase and amplitude in the ≈24-h variation of heart rate (RR intervals), ventricular repolarization (QT intervals), and core body temperature in mice. We hypothesized changes in cardiac electrophysiology associated with feeding behavior were secondary to changes in core body temperature. Telemetry was used to record electrocardiograms and core body temperature in mice during ad libitum-fed conditions and after inverting normal feeding behavior by restricting food access to the light cycle. Light cycle-restricted feeding modified the phase and amplitude of 24-h rhythms in RR and QT intervals, and core body temperature to realign with the new feeding time. Changes in core body temperature alone could not account for changes in phase and amplitude in the ≈24-h variation of the RR intervals. Heart rate variability analysis and inhibiting β-adrenergic and muscarinic receptors suggested that changes in the phase and amplitude of 24-h rhythms in RR intervals were secondary to changes in autonomic signaling. In contrast, changes in QT intervals closely mirrored changes in core body temperature. Studies at thermoneutrality confirmed that the daily variation in QT interval, but not RR interval, primarily reflected daily changes in core body temperature (even in ad libitum-fed conditions). Correcting the QT interval for differences in core body temperature helped unmask QT interval prolongation after starting light cycle-restricted feeding and in a mouse model of long QT syndrome. We conclude feeding behavior alters autonomic signaling and core body temperature to regulate phase and amplitude in RR and QT intervals, respectively.NEW & NOTEWORTHY We used time-restricted feeding and thermoneutrality to demonstrate that different mechanisms regulate the 24-h rhythms in heart rate and ventricular repolarization. The daily rhythm in heart rate reflects changes in autonomic input, whereas daily rhythms in ventricular repolarization reflect changes in core body temperature. This novel finding has major implications for understanding 24-h rhythms in mouse cardiac electrophysiology, arrhythmia susceptibility in transgenic mouse models, and interpretability of cardiac electrophysiological data acquired in thermoneutrality.

Feeding Behavior Modifies the Circadian Variation in RR and QT intervals by Distinct Mechanisms in Mice

Rhythmic feeding behavior is critical for regulating the phase and amplitude in the ≍24-hour variation of the heart rate (RR intervals), ventricular repolarization (QT intervals), and core body temperature in mice. We hypothesized the changes in cardiac electrophysiology associated with feeding behavior were secondary to changes in core body temperature. Telemetry was used to record electrocardiograms and core body temperature in mice during ad libitum-fed conditions and after inverting normal feeding behavior by restricting food access to the light cycle. Light cycle-restricted feeding quickly modified the phase and amplitude of the 24-hour rhythms in RR intervals, QT intervals, and core body temperature to realign with the new feeding time. Heart rate variability analysis and inhibiting β-adrenergic and muscarinic receptors suggested that the changes in the phase and amplitude of the 24-hour rhythms in RR intervals were secondary to changes in autonomic signaling. In contrast, the changes in the QT intervals closely mirrored changes in core body temperature. Studies at thermoneutrality confirmed the daily variation in the QT interval, but not the RR interval, and reflected daily changes in core body temperature (even in ad libitum-fed conditions). Correcting the QT interval for differences in core body temperature helped to unmask QT interval prolongation after starting light cycle-restricted feeding and in a mouse model of long QT syndrome. We conclude feeding behavior alters autonomic signaling and core body temperature to regulate the phase and amplitude in RR and QT intervals, respectively.

Membrane Lipid Domains Modulate HCN Channels in Nociceptor DRG Neurons

Cell membranes consist of heterogeneous lipid domains that influence key cellular processes, including signal transduction, endocytosis, and electrical excitability. Using FRET-based fluorescent assays and fluorescence lifetime imaging microscopy (FLIM), we found that the dimension of cholesterol-enriched ordered membrane domains (OMD) varies considerably, depending on specific cell types. The size of OMDs is also dependent on cholesterol levels and the structure of lipid tails. Particularly, nociceptor dorsal root ganglion (DRG) neurons exhibit large OMDs. Disruption of OMDs potentiated action potential firing in nociceptor DRG neurons and facilitated opening of native hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. This increased neuronal firing could be partially due to an increased open probability of HCN channels. In animal models of neuropathic pain, we observed shrunken OMDs and relocalization of HCN channels from OMDs to disordered lipid domains. The gating effect on HCN channels was likely a result of direct modulation of the voltage sensor by OMDs. These findings suggest that disturbances in lipid domains play a role in regulating HCN channels within nociceptor DRG neurons, influencing pain modulation.

Cholecystokinin-A signaling regulates automaticity of pacemaker cardiomyocytes

Aims: The behavior of pacemaker cardiomyocytes (PCs) in the sinoatrial node (SAN) is modulated by neurohormonal and paracrine factors, many of which signal through G-protein coupled receptors (GPCRs). The aims of the present study are to catalog GPCRs that are differentially expressed in the mammalian SAN and to define the acute physiological consequences of activating the cholecystokinin-A signaling system in isolated PCs. Methods and results: Using bulk and single cell RNA sequencing datasets, we identify a set of GPCRs that are differentially expressed between SAN and right atrial tissue, including several whose roles in PCs and in the SAN have not been thoroughly characterized. Focusing on one such GPCR, Cholecystokinin-A receptor (CCKAR), we demonstrate expression of Cckar mRNA specifically in mouse PCs, and further demonstrate that subsets of SAN fibroblasts and neurons within the cardiac intrinsic nervous system express cholecystokinin, the ligand for CCKAR. Using mouse models, we find that while baseline SAN function is not dramatically affected by loss of CCKAR, the firing rate of individual PCs is slowed by exposure to sulfated cholecystokinin-8 (sCCK-8), the high affinity ligand for CCKAR. The effect of sCCK-8 on firing rate is mediated by reduction in the rate of spontaneous phase 4 depolarization of PCs and is mitigated by activation of beta-adrenergic signaling. Conclusion: (1) PCs express many GPCRs whose specific roles in SAN function have not been characterized, (2) Activation of the cholecystokinin-A signaling pathway regulates PC automaticity.

Cholecystokinin-A Signaling Regulates Automaticity of Pacemaker Cardiomyocytes

Aims

The behavior of pacemaker cardiomyocytes (PCs) in the sinoatrial node (SAN) is modulated by neurohormonal and paracrine factors, many of which signal through G-protein coupled receptors (GPCRs). The aims of the present study are to catalog GPCRs that are differentially expressed in the mammalian SAN and to define the acute physiological consequences of activating the cholecystokinin-A signaling system in isolated PCs.

Methods and Results

Using bulk and single cell RNA sequencing datasets, we identify a set of GPCRs that are differentially expressed between SAN and right atrial tissue, including several whose roles in PCs and in the SAN have not been thoroughly characterized. Focusing on one such GPCR, Cholecystokinin-A receptor (CCK A R), we demonstrate expression of Cckar mRNA specifically in mouse PCs, and further demonstrate that subsets of SAN fibroblasts and neurons within the cardiac intrinsic nervous system express cholecystokinin, the ligand for CCK A R. Using mouse models, we find that while baseline SAN function is not dramatically affected by loss of CCK A R, the firing rate of individual PCs is slowed by exposure to sulfated cholecystokinin-8 (sCCK-8), the high affinity ligand for CCK A R. The effect of sCCK-8 on firing rate is mediated by reduction in the rate of spontaneous phase 4 depolarization of PCs and is mitigated by activation of beta-adrenergic signaling.

Conclusions

(1) PCs express many GPCRs whose specific roles in SAN function have not been characterized, (2) Activation of the the cholecystokinin-A signaling pathway regulates PC automaticity.

What to consider for ECG in mice-with special emphasis on telemetry

Genetically or surgically altered mice are commonly used as models of human cardiovascular diseases. Electrocardiography (ECG) is the gold standard to assess cardiac electrophysiology as well as to identify cardiac phenotypes and responses to pharmacological and surgical interventions. A variety of methods are used for mouse ECG acquisition under diverse conditions, making it difficult to compare different results. Non-invasive techniques allow only short-term data acquisition and are prone to stress or anesthesia related changes in cardiac activity. Telemetry offers continuous long-term acquisition of ECG data in conscious freely moving mice in their home cage environment. Additionally, it allows acquiring data 24/7 during different activities, can be combined with different challenges and most telemetry systems collect additional physiological parameters simultaneously. However, telemetry transmitters require surgical implantation, the equipment for data acquisition is relatively expensive and analysis of the vast number of ECG data is challenging and time-consuming. This review highlights the limits of non-invasive methods with respect to telemetry. In particular, primary screening using non-invasive methods can give a first hint; however, subtle cardiac phenotypes might be masked or compensated due to anesthesia and stress during these procedures. In addition, we detail the key differences between the mouse and human ECG. It is crucial to consider these differences when analyzing ECG data in order to properly translate the insights gained from murine models to human conditions.

Adenylyl cyclase isoform 1 contributes to sinoatrial node automaticity via functional microdomains

Sinoatrial node (SAN) cells are the heart's primary pacemaker. Their activity is tightly regulated by β-adrenergic receptor (β-AR) signaling. Adenylyl cyclase (AC) is a key enzyme in the β-AR pathway that catalyzes the production of cAMP. There are current gaps in our knowledge regarding the dominant AC isoforms and the specific roles of Ca2+-activated ACs in the SAN. The current study tests the hypothesis that distinct AC isoforms are preferentially expressed in the SAN and compartmentalize within microdomains to orchestrate heart rate regulation during β-AR signaling. In contrast to atrial and ventricular myocytes, SAN cells express a diverse repertoire of ACs, with ACI as the predominant Ca2+-activated isoform. Although ACI-KO (ACI-/-) mice exhibit normal cardiac systolic or diastolic function, they experience SAN dysfunction. Similarly, SAN-specific CRISPR/Cas9-mediated gene silencing of ACI results in sinus node dysfunction. Mechanistically, hyperpolarization-activated cyclic nucleotide-gated 4 (HCN4) channels form functional microdomains almost exclusively with ACI, while ryanodine receptor and L-type Ca2+ channels likely compartmentalize with ACI and other AC isoforms. In contrast, there were no significant differences in T-type Ca2+ and Na+ currents at baseline or after β-AR stimulation between WT and ACI-/- SAN cells. Due to its central characteristic feature as a Ca2+-activated isoform, ACI plays a unique role in sustaining the rise of local cAMP and heart rates during β-AR stimulation. The findings provide insights into the critical roles of the Ca2+-activated isoform of AC in sustaining SAN automaticity that is distinct from contractile cardiomyocytes.

Disruption of mitochondria-sarcoplasmic reticulum microdomain connectomics contributes to sinus node dysfunction in heart failure

The sinoatrial node (SAN), the leading pacemaker region, generates electrical impulses that propagate throughout the heart. SAN dysfunction with bradyarrhythmia is well documented in heart failure (HF). However, the underlying mechanisms are not completely understood. Mitochondria are critical to cellular processes that determine the life or death of the cell. The release of Ca2+ from the ryanodine receptors 2 (RyR2) on the sarcoplasmic reticulum (SR) at mitochondria-SR microdomains serves as the critical communication to match energy production to meet metabolic demands. Therefore, we tested the hypothesis that alterations in the mitochondria-SR connectomics contribute to SAN dysfunction in HF. We took advantage of a mouse model of chronic pressure overload-induced HF by transverse aortic constriction (TAC) and a SAN-specific CRISPR-Cas9-mediated knockdown of mitofusin-2 (Mfn2), the mitochondria-SR tethering GTPase protein. TAC mice exhibited impaired cardiac function with HF, cardiac fibrosis, and profound SAN dysfunction. Ultrastructural imaging using electron microscope (EM) tomography revealed abnormal mitochondrial structure with increased mitochondria-SR distance. The expression of Mfn2 was significantly down-regulated and showed reduced colocalization with RyR2 in HF SAN cells. Indeed, SAN-specific Mfn2 knockdown led to alterations in the mitochondria-SR microdomains and SAN dysfunction. Finally, disruptions in the mitochondria-SR microdomains resulted in abnormal mitochondrial Ca2+ handling, alterations in localized protein kinase A (PKA) activity, and impaired mitochondrial function in HF SAN cells. The current study provides insights into the role of mitochondria-SR microdomains in SAN automaticity and possible therapeutic targets for SAN dysfunction in HF patients.

Age and Gender Differences in Cardiovascular Autonomic Failure in the Transgenic PLP-syn Mouse, a Model of Multiple System Atrophy

Multiple system atrophy (MSA) is a rare and progressive neurodegenerative disorder. Autonomic failure (AF) is one main clinical feature which has a significant impact on health-related quality of life. The neuropathological hallmark of MSA is the abnormal accumulation of α-synuclein in oligodendrocytes forming glial cytoplasmic inclusions. Only little is known about gender and age differences in AF in MSA. This study was carried out in 6 and 12 months old transgenic PLP-α-syn and WT male and female mice. Heart rate variability (HRV) was assessed both in time, frequential and non-linear domains. Baroreflex sensitivity (BRS) was estimated by the sequence method. Duration of ventricular depolarization and repolarization (QT/QTc intervals) were evaluated from the ECG signals. Three-way ANOVA (genotype x gender x age) with Sidak's method post-hoc was used to analyze data. BRS was significantly changed in PLP-α-syn mice and was age-dependent. QT and QTc intervals were not significantly modified in PLP-α-syn mice. An impaired HRV was observed at 12 months of age in PLP-α-syn female but not in male mice, indicative of cardiovascular AF.

Aging Alters the Formation and Functionality of Signaling Microdomains Between L-type Calcium Channels and β2-Adrenergic Receptors in Cardiac Pacemaker Cells

Heart rate is accelerated to match physiological demands through the action of noradrenaline on the cardiac pacemaker. Noradrenaline is released from sympathetic terminals and activates β1-and β2-adrenergic receptors (ΑRs) located at the plasma membrane of pacemaker cells. L-type calcium channels are one of the main downstream targets potentiated by the activation of β-ARs. For this signaling to occur, L-type calcium channels need to be located in close proximity to β-ARs inside caveolae. Although it is known that aging causes a slowdown of the pacemaker rate and a reduction in the response of pacemaker cells to noradrenaline, there is a lack of in-depth mechanistic insights into these age-associated changes. Here, we show that aging affects the formation and function of adrenergic signaling microdomains inside caveolae. By evaluating the β1 and β2 components of the adrenergic regulation of the L-type calcium current, we show that aging does not alter the regulation mediated by β1-ARs but drastically impairs that mediated by β2-ARs. We studied the integrity of the signaling microdomains formed between L-type calcium channels and β-ARs by combining high-resolution microscopy and proximity ligation assays. We show that consistent with the electrophysiological data, aging decreases the physical association between β2-ARs and L-type calcium channels. Interestingly, this reduction is associated with a decrease in the association of L-type calcium channels with the scaffolding protein AKAP150. Old pacemaker cells also have a reduction in caveolae density and in the association of L-type calcium channels with caveolin-3. Together the age-dependent alterations in caveolar formation and the nano-organization of β2-ARs and L-type calcium channels result in a reduced sensitivity of the channels to β2 adrenergic modulation. Our results highlight the importance of these signaling microdomains in maintaining the chronotropic modulation of the heart and also pinpoint the direct impact that aging has on their function.

Heart Rate Variability Reveals Altered Autonomic Regulation in Response to Myocardial Infarction in Experimental Animals

The analysis of beating rate provides information on the modulatory action of the autonomic nervous system on the heart, which mediates adjustments of cardiac function to meet hemodynamic requirements. In patients with myocardial infarction, alterations of heart rate variability (HRV) have been correlated to the occurrence of arrhythmic events and all-cause mortality. In the current study, we tested whether experimental rodent models of myocardial infarction recapitulate dynamics of heart rate variability observed in humans, and constitute valid platforms for understanding mechanisms linking autonomic function to the development and manifestation of cardiovascular conditions. For this purpose, HRV was evaluated in two engineered mouse lines using electrocardiograms collected in the conscious, restrained state, using a tunnel device. Measurements were obtained in naïve mice and animals at 3–∼28 days following myocardial infarction, induced by permanent coronary artery ligation. Two mouse lines with inbred and hybrid genetic background and, respectively, homozygous (Homo) and heterozygous (Het) for the MerCreMer transgene, were employed. In the naïve state, Het female and male mice presented prolonged RR interval duration (∼9%) and a ∼4-fold increased short- and long-term RR interval variability, with respect to sex-matched Homo mice. These differences were abrogated by pharmacological interventions inhibiting the sympathetic and parasympathetic axes. At 3–∼14 days after myocardial infarction, RR interval duration increased in Homo mice, but was not affected in Het animals. In contrast, Homo mice had minor modifications in HRV parameters, whereas substantial (> 50%) reduction of short- and long-term RR interval variation occurred in Het mice. Interestingly, ex vivo studies in isolated organs documented that intrinsic RR interval duration increased in infarcted vs. non-infarcted Homo and Het hearts, whereas RR interval variation was not affected. In conclusion, our study documents that, as observed in humans, myocardial infarction in rodents is associated with alterations in heart rhythm dynamics consistent with sympathoexcitation and parasympathetic withdrawal. Moreover, we report that mouse strain is an important variable when evaluating autonomic function via the analysis of HRV.

In vivo and ex vivo electrophysiological study of the mouse heart to characterize the cardiac conduction system, including atrial and ventricular vulnerability

The mouse is a common and cost-effective animal model for basic research, and the number of genetically engineered mouse models with cardiac phenotype is increasing. In vivo electrophysiological study in mice is similar to that performed in humans. It is indispensable for acquiring intracardiac electrocardiogram recordings and determining baseline cardiac cycle intervals. Furthermore, the use of programmed electrical stimulation enables determination of parameters such as sinoatrial conduction time, sinus node recovery time, atrioventricular-nodal conduction properties, Wenckebach periodicity, refractory periods and arrhythmia vulnerability. This protocol describes specific procedures for determining these parameters that were adapted from analogous human protocols for use in mice. We include details of ex vivo electrophysiological study, which provides detailed insights into intrinsic cardiac electrophysiology without external influences from humoral and neural factors. In addition, we describe a heart preparation with intact innervation by the vagus nerve that can be used as an ex vivo model for vagal control of the cardiac conduction system. Data acquisition for in vivo and ex vivo electrophysiological study takes ~1 h per mouse, depending on the number of stimulation protocols applied during the procedure. The technique yields highly reliable results and can be used for phenotyping of cardiac disease models, elucidating disease mechanisms and confirming functional improvements in gene therapy approaches as well as for drug and toxicity testing.

Deciphering cellular signals in adult mouse sinoatrial node cells

Sinoatrial node (SAN) cells are the pacemakers of the heart. This study describes a method for culturing and infection of adult mouse SAN cells with FRET-based biosensors that can be exploited to examine signaling events. SAN cells cultured in media with blebbistatin or (S)-nitro-blebbistatin retain their morphology, protein distribution, action potential (AP) waveform, and cAMP dynamics for at least 40 h. SAN cells expressing targeted cAMP sensors show distinct β-adrenergic-mediated cAMP pools. Cyclic GMP, protein kinase A, Ca2+/CaM kinase II, and protein kinase D in SAN cells also show unique dynamics to different stimuli. Heart failure SAN cells show a decrease in cAMP and cGMP levels. In summary, a reliable method for maintaining adult mouse SAN cells in culture is presented, which facilitates studies of signaling networks and regulatory mechanisms during physiological and pathological conditions.

The Role of POPDC Proteins in Cardiac Pacemaking and Conduction

The Popeye domain-containing (POPDC) gene family, consisting of Popdc1 (also known as Bves), Popdc2, and Popdc3, encodes transmembrane proteins abundantly expressed in striated muscle. POPDC proteins have recently been identified as cAMP effector proteins and have been proposed to be part of the protein network involved in cAMP signaling. However, their exact biochemical activity is presently poorly understood. Loss-of-function mutations in animal models causes abnormalities in skeletal muscle regeneration, conduction, and heart rate adaptation after stress. Likewise, patients carrying missense or nonsense mutations in POPDC genes have been associated with cardiac arrhythmias and limb-girdle muscular dystrophy. In this review, we introduce the POPDC protein family, and describe their structure function, and role in cAMP signaling. Furthermore, the pathological phenotypes observed in zebrafish and mouse models and the clinical and molecular pathologies in patients carrying POPDC mutations are described.

Moving average and standard deviation thresholding (MAST): a novel algorithm for accurate R-wave detection in the murine electrocardiogram

Advances in implantable radio-telemetry or diverse biologging devices capable of acquiring high-resolution ambulatory electrocardiogram (ECG) or heart rate recordings facilitate comparative physiological investigations by enabling detailed analysis of cardiopulmonary phenotypes and responses in vivo. Two priorities guiding the meaningful adoption of such technologies are: (1) automation, to streamline and standardize large dataset analysis, and (2) flexibility in quality-control. The latter is especially relevant when considering the tendency of some fully automated software solutions to significantly underestimate heart rate when raw signals contain high-amplitude noise. We present herein moving average and standard deviation thresholding (MAST), a novel, open-access algorithm developed to perform automated, accurate, and noise-robust single-channel R-wave detection from ECG obtained in chronically instrumented mice. MAST additionally and automatically excludes and annotates segments where R-wave detection is not possible due to artefact levels exceeding signal levels. Customizable settings (e.g. window width of moving average) allow for MAST to be scaled for use in non-murine species. Two expert reviewers compared MAST's performance (true/false positive and false negative detections) with that of a commercial ECG analysis program. Both approaches were applied blindly to the same random selection of 270 3-min ECG recordings from a dataset containing varying amounts of signal artefact. MAST exhibited roughly one quarter the error rate of the commercial software and accurately detected R-waves with greater consistency and virtually no false positives (sensitivity, Se: 98.48% ± 4.32% vs. 94.59% ± 17.52%, positive predictivity, +P: 99.99% ± 0.06% vs. 99.57% ± 3.91%, P < 0.001 and P = 0.0274 respectively, Wilcoxon signed rank; values are mean ± SD). Our novel, open-access approach for automated single-channel R-wave detection enables investigators to study murine heart rate indices with greater accuracy and less effort. It also provides a foundational code for translation to other mammals, ectothermic vertebrates, and birds.

Cardiomyocyte Deletion of Bmal1 Exacerbates QT- and RR-Interval Prolongation in Scn5a +/ΔKPQ Mice

Circadian rhythms are generated by cell autonomous circadian clocks that perform a ubiquitous cellular time-keeping function and cell type-specific functions important for normal physiology. Studies show inducing the deletion of the core circadian clock transcription factor Bmal1 in adult mouse cardiomyocytes disrupts cardiac circadian clock function, cardiac ion channel expression, slows heart rate, and prolongs the QT-interval at slow heart rates. This study determined how inducing the deletion of Bmal1 in adult cardiomyocytes impacted the in vivo electrophysiological phenotype of a knock-in mouse model for the arrhythmogenic long QT syndrome (Scn5a^+/ΔKPQ). Electrocardiographic telemetry showed inducing the deletion of Bmal1 in the cardiomyocytes of mice with or without the ΔKPQ-Scn5a mutation increased the QT-interval at RR-intervals that were ≥130 ms. Inducing the deletion of Bmal1 in the cardiomyocytes of mice with or without the ΔKPQ-Scn5a mutation also increased the day/night rhythm-adjusted mean in the RR-interval, but it did not change the period, phase or amplitude. Compared to mice without the ΔKPQ-Scn5a mutation, mice with the ΔKPQ-Scn5a mutation had reduced heart rate variability (HRV) during the peak of the day/night rhythm in the RR-interval. Inducing the deletion of Bmal1 in cardiomyocytes did not affect HRV in mice without the ΔKPQ-Scn5a mutation, but it did increase HRV in mice with the ΔKPQ-Scn5a mutation. The data demonstrate that deleting Bmal1 in cardiomyocytes exacerbates QT- and RR-interval prolongation in mice with the ΔKPQ-Scn5a mutation.

The Organization of the Sinoatrial Node Microvasculature Varies Regionally to Match Local Myocyte Excitability

The cardiac cycle starts when an action potential is produced by pacemaking cells in the sinoatrial node. This cycle is repeated approximately 100 000 times in humans and 1 million times in mice per day, imposing a monumental metabolic demand on the heart, requiring efficient blood supply via the coronary vasculature to maintain cardiac function. Although the ventricular coronary circulation has been extensively studied, the relationship between vascularization and cellular pacemaking modalities in the sinoatrial node is poorly understood. Here, we tested the hypothesis that the organization of the sinoatrial node microvasculature varies regionally, reflecting local myocyte firing properties. We show that vessel densities are higher in the superior versus inferior sinoatrial node. Accordingly, sinoatrial node myocytes are closer to vessels in the superior versus inferior regions. Superior and inferior sinoatrial node myocytes produce stochastic subthreshold voltage fluctuations and action potentials. However, the intrinsic action potential firing rate of sinoatrial node myocytes is higher in the superior versus inferior node. Our data support a model in which the microvascular densities vary regionally within the sinoatrial node to match the electrical and Ca²⁺ dynamics of nearby myocytes, effectively determining the dominant pacemaking site within the node. In this model, the high vascular density in the superior sinoatrial node places myocytes with metabolically demanding, high-frequency action potentials near vessels. The lower vascularization and electrical activity of inferior sinoatrial node myocytes could limit these cells to function to support sinoatrial node periodicity with sporadic voltage fluctuations via a stochastic resonance mechanism.

Beyond pacemaking: HCN channels in sinoatrial node function

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are key proteins involved in the initiation and regulation of the heartbeat. Pacemaker cells within the sinoatrial node generate the electrical impulse that underlies the contraction of all atrial and ventricular cardiomyocytes. To generate a stable heart rhythm, it is necessary that the spontaneous activity of pacemaker cells is synchronized. Entrainment processes in the sinoatrial node create synchrony and also mediate heart rate regulation. In the past years it has become clear that the role of HCN channels goes beyond just pacemaking and that the channels play pivotal roles in these entrainment processes that coordinate and balance sinoatrial node network activity. Here, we review the role of HCN channels in the central pacemaker process and highlight new aspects of the contribution of HCN channels to stabilizing the electrical activity of the sinoatrial node network, especially during heart rate regulation by the autonomic nervous system.

Cellular and molecular landscape of mammalian sinoatrial node revealed by single-cell RNA sequencing

Bioelectrical impulses intrinsically generated within the sinoatrial node (SAN) trigger the contraction of the heart in mammals. Though discovered over a century ago, the molecular and cellular features of the SAN that underpin its critical function in the heart are uncharted territory. Here, we identify four distinct transcriptional clusters by single-cell RNA sequencing in the mouse SAN. Functional analysis of differentially expressed genes identifies a core cell cluster enriched in the electrogenic genes. The similar cellular features are also observed in the SAN from both rabbit and cynomolgus monkey. Notably, Vsnl1, a core cell cluster marker in mouse, is abundantly expressed in SAN, but is barely detectable in atrium or ventricle, suggesting that Vsnl1 is a potential SAN marker. Importantly, deficiency of Vsnl1 not only reduces the beating rate of human induced pluripotent stem cell - derived cardiomyocytes (hiPSC-CMs) but also the heart rate of mice. Furthermore, weighted gene co-expression network analysis (WGCNA) unveiled the core gene regulation network governing the function of the SAN in mice. Overall, these findings reveal the whole transcriptome profiling of the SAN at single-cell resolution, representing an advance toward understanding of both the biology and the pathology of SAN.

The spontaneous bioelectrical activity of pacemaker cells in sinoatrial node (SAN) triggers the heartbeats. Here, the authors perform single-cell RNA sequencing in the mouse SAN and identify molecular and cellular features of the SAN conserved in rabbit and cynomolgus monkey, identifying a new potential SAN marker.

cAMP-dependent regulation of HCN4 controls the tonic entrainment process in sinoatrial node pacemaker cells

It is highly debated how cyclic adenosine monophosphate-dependent regulation (CDR) of the major pacemaker channel HCN4 in the sinoatrial node (SAN) is involved in heart rate regulation by the autonomic nervous system. We addressed this question using a knockin mouse line expressing cyclic adenosine monophosphate-insensitive HCN4 channels. This mouse line displayed a complex cardiac phenotype characterized by sinus dysrhythmia, severe sinus bradycardia, sinus pauses and chronotropic incompetence. Furthermore, the absence of CDR leads to inappropriately enhanced heart rate responses of the SAN to vagal nerve activity in vivo. The mechanism underlying these symptoms can be explained by the presence of nonfiring pacemaker cells. We provide evidence that a tonic and mutual interaction process (tonic entrainment) between firing and nonfiring cells slows down the overall rhythm of the SAN. Most importantly, we show that the proportion of firing cells can be increased by CDR of HCN4 to efficiently oppose enhanced responses to vagal activity. In conclusion, we provide evidence for a novel role of CDR of HCN4 for the central pacemaker process in the sinoatrial node.

The involvement of cAMP-dependent regulation of HCN4 in the chronotropic heart rate response is a matter of debate. Here the authors use a knockin mouse model expressing cAMP-insensitive HCN4 channels to discover an inhibitory nonfiring cell pool in the sinoatrial node and a tonic and mutual interaction between firing and nonfiring pacemaker cells that is controlled by cAMP-dependent regulation of HCN4, with implications in chronotropic heart rate responses.

ATAC-Seq Reveals an Isl1 Enhancer That Regulates Sinoatrial Node Development and Function

RATIONALE

Cardiac pacemaker cells (PCs) in the sinoatrial node (SAN) have a distinct gene expression program that allows them to fire automatically and initiate the heartbeat. Although critical SAN transcription factors, including Isl1 (Islet-1), Tbx3 (T-box transcription factor 3), and Shox2 (short-stature homeobox protein 2), have been identified, the cis-regulatory architecture that governs PC-specific gene expression is not understood, and discrete enhancers required for gene regulation in the SAN have not been identified.

OBJECTIVE

To define the epigenetic profile of PCs using comparative ATAC-seq (assay for transposase-accessible chromatin with sequencing) and to identify novel enhancers involved in SAN gene regulation, development, and function.

METHODS AND RESULTS

We used ATAC-seq on sorted neonatal mouse SAN to compare regions of accessible chromatin in PCs and right atrial cardiomyocytes. PC-enriched assay for transposase-accessible chromatin peaks, representing candidate SAN regulatory elements, were located near established SAN genes and were enriched for distinct sets of TF (transcription factor) binding sites. Among several novel SAN enhancers that were experimentally validated using transgenic mice, we identified a 2.9-kb regulatory element at the Isl1 locus that was active specifically in the cardiac inflow at embryonic day 8.5 and throughout later SAN development and maturation. Deletion of this enhancer from the genome of mice resulted in SAN hypoplasia and sinus arrhythmias. The mouse SAN enhancer also directed reporter activity to the inflow tract in developing zebrafish hearts, demonstrating deep conservation of its upstream regulatory network. Finally, single nucleotide polymorphisms in the human genome that occur near the region syntenic to the mouse enhancer exhibit significant associations with resting heart rate in human populations.

CONCLUSIONS

(1) PCs have distinct regions of accessible chromatin that correlate with their gene expression profile and contain novel SAN enhancers, (2) cis-regulation of Isl1 specifically in the SAN depends upon a conserved SAN enhancer that regulates PC development and SAN function, and (3) a corresponding human ISL1 enhancer may regulate human SAN function.

Rhythm dynamics of the aging heart: an experimental study using conscious, restrained mice

Heart rate variability (HRV) is a measure of variation in time interval between heartbeats and reflects the influence of autonomic nervous system and circulating/locally released factors on sinoatrial node discharge. Here, we tested whether electrocardiograms (ECGs) obtained in conscious, restrained mice, a condition that affects sympathovagal balance, reveal alterations of heart rhythm dynamics with aging. Moreover, based on emergence of sodium channels as modulators of pacemaker activity, we addressed consequences of altered sodium channels on heart rhythm. C57Bl/6 mice and mice with enhanced late sodium current due to Nav1.5 mutation at Ser571 (S571E) at ~4 to ~24 mo of age, were studied. HRV was assessed using time- and frequency-domain and nonlinear parameters. For C57Bl/6 and S571E mice, standard deviation of RR intervals (SDRR), total power of RR interval variation, and nonlinear standard deviation 2 (SD2) were maximal at ~4 mo and decreased at ~18 and ~24 mo, together with attenuation of indexes of sympathovagal balance. Modulation of sympathetic and/or parasympathetic divisions revealed attenuation of autonomic tone at ~24 mo. At ~4 mo, S571E mice presented lower heart rate and higher SDRR, total power, and SD2 with respect to C57Bl/6, properties reversed by late sodium current inhibition. At ~24 mo, heart rate decreased in C57Bl/6 but increased in S571E, a condition preserved after autonomic blockade. Collectively, our data indicate that aging is associated with reduced HRV. Moreover, sodium channel function conditions heart rate and its age-related adaptations, but does not interfere with HRV decline occurring with age.NEW & NOTEWORTHY We have investigated age-associated alterations of heart rate properties in mice using conscious electrocardiographic recordings. Our findings support the notion that aging is coupled with altered sympathovagal balance with consequences on heart rate variability. Moreover, by using a genetically engineered mouse line, we provide evidence that sodium channels modulate heart rate and its age-related adaptations.

A Physiolomics Approach to Reveal Systemic Organ Dynamics in a Rodent

The central nervous system controls the activity states of the peripheral organs in response to various environmental changes. However, the physiological interactions across multiple organs remain largely unknown. Recently, we have developed an electrophysiological recording system that simultaneously captures neuronal population activity patterns in the brain, heartbeat signals, muscle contraction signals, respiratory signals, and vagus nerve action potentials in freely moving rodents. This paper summarizes several recent insights obtained from this recording system, including the observations that some but not all brain activity patterns are associated with peripheral organ activity in a behavioral test, and that functions across cortical networks can predict stress-induced changes in cardiac function in rats. The evidence suggests that adding information on peripheral physiological signals to behavioral data assists in a more accurate estimation of animals' mental states. The concept of such a research approach opens a new field of large-scale analysis of systemic physiological signals, termed "physiolomics," which is expected to unveil further physiological issues involving mind-body associations in health and disease.

Abolishing cAMP sensitivity in HCN2 pacemaker channels induces generalized seizures

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are dually gated channels that are operated by voltage and by neurotransmitters via the cAMP system. cAMP-dependent HCN regulation has been proposed to play a key role in regulating circuit behavior in the thalamus. By analyzing a knockin mouse model (HCN2EA), in which binding of cAMP to HCN2 was abolished by 2 amino acid exchanges (R591E, T592A), we found that cAMP gating of HCN2 is essential for regulating the transition between the burst and tonic modes of firing in thalamic dorsal-lateral geniculate (dLGN) and ventrobasal (VB) nuclei. HCN2EA mice display impaired visual learning, generalized seizures of thalamic origin, and altered NREM sleep properties. VB-specific deletion of HCN2, but not of HCN4, also induced these generalized seizures of the absence type, corroborating a key role of HCN2 in this particular nucleus for controlling consciousness. Together, our data define distinct pathological phenotypes resulting from the loss of cAMP-mediated gating of a neuronal HCN channel.

The Role of the Autonomic Nervous System on Cardiac Rhythm during the Evolution of Diabetes Mellitus Using Heart Rate Variability as a Biomarker

Heart rate variability (HRV) is highly influenced by the Autonomic Nervous System (ANS). Several illnesses have been associated with changes in the ANS, thus altering the pattern of HRV. However, the variability of the heart rhythm is originated within the Sinus Atrial Node (SAN) which has its own variability. Still, although both oscillators produce HRV, the influence of the SAN on HRV has not yet been exhaustively studied. On the other hand, the complications of diabetes mellitus (DM), for instance, nephropathy, retinopathy, and neuropathy, increase cardiovascular morbidity and mortality. Traditionally, these complications are diagnosed only when the patient is already suffering from the negative symptoms these complications implicate. Consequently, it is of paramount importance to develop new techniques for early diagnosis prior to any deterioration on healthy patients. HRV has been proved to be a valuable, noninvasive clinical evidence for evaluating diseases and even for describing aging and behavior. In this study, several ECGs were recorded and their RR and PP intervals were analyzed to detect the interpotential interval (ii) of the SAN. Additionally, HRV reduction was quantified to identify alterations in the nervous system within the nodal tissue via measuring the SD1/SD2 ratio in a Poincaré plot. With 15 years of DM development, the data showed an age-dependent increase in HRV due to the axon retraction of ANS neurons from its effectors. In addition, these alterations modify the heart rhythm-producing fatal arrhythmias. Therefore, it is possible to avoid the consequences of DM identifying alterations in SAN previous to its symptomatic appearance. This could be used as an early diagnosis indicator.

A tissue-engineered scale model of the heart ventricle

Laboratory studies of the heart use cell and tissue cultures to dissect heart function yet rely on animal models to measure pressure and volume dynamics. Here, we report tissue-engineered scale models of the human left ventricle, made of nanofibrous scaffolds that promote native-like anisotropic myocardial tissue genesis and chamber-level contractile function. Incorporating neonatal rat ventricular myocytes or cardiomyocytes derived from human induced pluripotent stem cells, the tissue-engineered ventricles have a diastolic chamber volume of ~500 μL (comparable to that of the native rat ventricle and approximately 1/250 the size of the human ventricle), and ejection fractions and contractile work 50–250 times smaller and 10⁴–10⁸ times smaller than the corresponding values for rodent and human ventricles, respectively. We also measured tissue coverage and alignment, calcium-transient propagation and pressure/volume loops in the presence or absence of test compounds. Moreover, we describe an instrumented bioreactor with ventricular-assist capabilities, and provide a proof-of-concept disease model of structural arrhythmia. The model ventricles can be evaluated with the same assays used in animal models and in clinical settings.

Postnatal undernutrition in mice causes cardiac arrhythmogenesis which is exacerbated when pharmacologically stressed

Growth restriction caused by postnatal undernutrition increases risk for cardiovascular disease in adulthood with the potential to induce arrhythmogenesis. Thus, the purpose was to determine if undernutrition during development produced arrhythmias at rest and when stressed with dobutamine in adulthood. Mouse dams were fed (CON: 20% protein), or low-protein (LP: 8%) diet before mating. A cross-fostering model was used where pups nursed by dams fed LP diet in early [EUN; postnatal day (PN) 1–10], late (LUN; PN11–21) and whole (PUN; 1–21) phases of postnatal life. Weaned pups were switched to CON diets for the remainder of the study (PN80). At PN80, body composition (magnetic resonance imaging), and quantitative electrocardiogram (ECG) measurements were obtained under 1% isoflurane anesthesia. After baseline ECG, an IP injection (1.5 µg/g body weight) of dobutamine was administered and ECG repeated. Undernutrition significantly (P<0.05) reduced body weight in LUN (22.68±0.88 g) and PUN (19.96±0.32 g) but not in CON (25.05±0.96 g) and EUN (25.28±0.9207 g). Fat mass decreased in all groups compared with controls (CON: 8.00±1.2 g, EUN: 6.32±0.65 g, LUN: 5.11±1.1 g, PUN: 3.90±0.25 g). Lean mass was only significantly reduced in PUN (CON: 17.99±0.26 g, EUN: 17.78±0.39 g, LUN: 17.34±0.33 g, PUN: 15.85±0.28 g). Absolute heart weights were significantly less from CON, with PUN having the smallest. ECG showed LUN had occurrences of atrial fibrillation; EUN had increases of 1st degree atrioventricular block upon stimulation, and PUN had increased risk for ventricular depolarization arrhythmias. CON did not display arrhythmias. Undernutrition in early life resulted in ventricular arrhythmias under stressed conditions, but undernutrition occurring in later postnatal life there is an increased incidence of atrial arrhythmias.

Simultaneous Recordings of Cortical Local Field Potentials, Electrocardiogram, Electromyogram, and Breathing Rhythm from a Freely Moving Rat

Monitoring the physiological dynamics of the brain and peripheral tissues is necessary for addressing a number of questions about how the brain controls body functions and internal organ rhythms when animals are exposed to emotional challenges and changes in their living environments. In general experiments, signals from different organs, such as the brain and the heart, are recorded by independent recording systems that require multiple recording devices and different procedures for processing the data files. This study describes a new method that can simultaneously monitor electrical biosignals, including tens of local field potentials in multiple brain regions, electrocardiograms that represent the cardiac rhythm, electromyograms that represent awake/sleep-related muscle contraction, and breathing signals, in a freely moving rat. The recording configuration of this method is based on a conventional micro-drive array for cortical local field potential recordings in which tens of electrodes are accommodated, and the signals obtained from these electrodes are integrated into a single electrical board mounted on the animal's head. Here, this recording system was improved so that signals from the peripheral organs are also transferred to an electrical interface board. In a single surgery, electrodes are first separately implanted into the appropriate body parts and the target brain areas. The open ends of all of these electrodes are then soldered to individual channels of the electrical board above the animal's head so that all of the signals can be integrated into the single electrical board. Connecting this board to a recording device allows for the collection of all of the signals into a single device, which reduces experimental costs and simplifies data processing, because all data can be handled in the same data file. This technique will aid the understanding of the neurophysiological correlates of the associations between central and peripheral organs.

Altered heart rate regulation by the autonomic nervous system in mice lacking natriuretic peptide receptor C (NPR-C)

Natriuretic peptides (NPs) play essential roles in the regulation of cardiovascular function. NP effects are mediated by receptors known as NPR-A, NPR-B or NPR-C. NPs have potent effects on regulation of heart rate (HR) by the autonomic nervous system (ANS), but the role of NPR-C in these effects has not been investigated. Accordingly, we have used telemetric ECG recordings in awake, freely moving wildtype and NPR-C knockout (NPR-C^−/−) mice and performed heart rate variability (HRV) analysis to assess alterations in sympatho-vagal balance on the heart following loss of NPR-C. Our novel data demonstrate that NPR-C^−/− mice are characterized by elevations in HR, reductions in circadian changes in HR and enhanced occurrence of sinus pauses, indicating increased arrhythmogenesis and a loss of HRV. Time domain and frequency domain analyses further demonstrate that HRV is reduced in NPR-C^−/− mice in association with a reduction in parasympathetic activity. Importantly, the low frequency to high frequency ratio was increased in NPR-C^−/− mice indicating that sympathetic activity is also enhanced. These changes in autonomic regulation were confirmed using atropine and propranolol to antagonize the ANS. These findings illustrate that loss of NPR-C reduces HRV due to perturbations in the regulation of the heart by the ANS.

Suppression of ryanodine receptor function prolongs Ca2+ release refractoriness and promotes cardiac alternans in intact hearts

Beat-to-beat alternations in the amplitude of the cytosolic Ca²⁺ transient (Ca²⁺ alternans) are thought to be the primary cause of cardiac alternans that can lead to cardiac arrhythmias and sudden death. Despite its important role in arrhythmogenesis, the mechanism underlying Ca²⁺ alternans remains poorly understood. Here, we investigated the role of cardiac ryanodine receptor (RyR2), the major Ca²⁺ release channel responsible for cytosolic Ca²⁺ transients, in cardiac alternans. Using a unique mouse model harboring a suppression-of-function (SOF) RyR2 mutation (E4872Q), we assessed the effect of genetically suppressing RyR2 function on Ca²⁺ and action potential duration (APD) alternans in intact hearts, and electrocardiogram (ECG) alternans in vivo We found that RyR2-SOF hearts displayed prolonged sarcoplasmic reticulum Ca²⁺ release refractoriness and enhanced propensity for Ca²⁺ alternans. RyR2-SOF hearts/mice also exhibited increased propensity for APD and ECG alternans. Caffeine, which enhances RyR2 activity and the propensity for catecholaminergic polymorphic ventricular tachycardia (CPVT), suppressed Ca²⁺ alternans in RyR2-SOF hearts, whereas carvedilol, a β-blocker that suppresses RyR2 activity and CPVT, promoted Ca²⁺ alternans in these hearts. Thus, RyR2 function is an important determinant of Ca²⁺, APD, and ECG alternans. Our data also indicate that the activity of RyR2 influences the propensity for cardiac alternans and CPVT in an opposite manner. Therefore, overly suppressing or enhancing RyR2 function is pro-arrhythmic.

A new device for the simultaneous recording of cerebral, cardiac, and muscular electrical activity in freely moving rodents

We present a new technique for the simultaneous capture of bioelectrical time signals from the brain and peripheral organs of freely moving rodents. The recording system integrates all systemic signals into an electrical interface board that is mounted on an animal's head for an extended period. The interface board accommodates up to 48 channels, enabling us to analyze neuronal activity patterns in multiple brain regions by comparing a variety of physiological body states over weeks and months. This technique will advance the understanding of the neurophysiological correlate of mind-body associations in health and disease.

Heart Rate Variability as Early Biomarker for the Evaluation of Diabetes Mellitus Progress

According to the American Diabetes Association (ADA), the side effects of diabetes mellitus have recently increased the global health expenditure each year. Of these, the early diagnostic can contribute to the decrease on renal, cardiovascular, and nervous systems complications. However, the diagnostic criteria, which are commonly used, do not suggest the diabetes progress in the patient. In this study, the streptozotocin model in mice (cDM) was used as early diagnostic criterion to reduce the side effects related to the illness. The results showed some clinical signs similarly to five-year diabetes progress without renal injury, neuropathies, and cardiac neuropathy autonomic in the cDM-model. On the other hand, the electrocardiogram was used to determine alterations in heart rate and heart rate variability (HRV), using the Poincaré plot to quantify the HRV decrease in the cDM-model. Additionally, the SD1/SD2 ratio and ventricular arrhythmias showed increase without side effects of diabetes. Therefore, the use of HRV as an early biomarker contributes to evaluating diabetes mellitus complications from the diagnostic.