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

Mechanisms and Implications of Electrical Heterogeneity in Cardiac Function in Ischemic Heart Disease

A healthy heart shows intrinsic electrical heterogeneities that play a significant role in cardiac activation and repolarization. However, cardiac diseases may perturb the baseline electrical properties of the healthy cardiac tissue, leading to increased arrhythmic risk and compromised cardiac functions. Moreover, biological variability among patients produces a wide range of clinical symptoms, which complicates the treatment and diagnosis of cardiac diseases. Ischemic heart disease is usually caused by a partial or complete blockage of a coronary artery. The onset of the disease begins with myocardial ischemia, which can develop into myocardial infarction if it persists for an extended period. The progressive regional tissue remodeling leads to increased electrical heterogeneities, with adverse consequences on arrhythmic risk, cardiac mechanics, and mortality. This review aims to summarize the key role of electrical heterogeneities in the heart on cardiac function and diseases. Ischemic heart disease has been chosen as an example to show how adverse electrical remodeling at different stages may lead to variable manifestations in patients. For this, we have reviewed the dynamic electrophysiological and structural remodeling from the onset of acute myocardial ischemia and reperfusion to acute and chronic stages post-myocardial infarction. The arrhythmic mechanisms, patient phenotypes, risk stratification at different stages, and patient management strategies are also discussed. Finally, we provide a brief review on how computational approaches incorporate human electrophysiological heterogeneity to facilitate basic and translational research.

The life cycle of a capillary: Mechanisms of angiogenesis and rarefaction in microvascular physiology and pathologies

Capillaries are the smallest blood vessels (<10 μm in diameter) in the body and their walls are lined by endothelial cells. These microvessels play a crucial role in nutrient and gas exchange between blood and tissues. Capillary endothelial cells also produce vasoactive molecules and initiate the electrical signals that underlie functional hyperemia and neurovascular coupling. Accordingly, capillary function and density are critical for all cell types to match blood flow to cellular activity. This begins with the process of angiogenesis, when new capillary blood vessels emerge from pre-existing vessels, and ends with rarefaction, the loss of these microvascular structures. This review explores the mechanisms behind these processes, emphasizing their roles in various microvascular diseases and their impact on surrounding cells in health and disease. We discuss recent work on the mechanisms controlling endothelial cell proliferation, migration, and tube formation that underlie angiogenesis under physiological and pathological conditions. The mechanisms underlying functional and anatomical rarefaction and the role of pericytes in this process are also discussed. Based on this work, a model is proposed in which the balance of angiogenic and rarefaction signaling pathways in a particular tissue match microvascular density to the metabolic demands of the surrounding cells. This negative feedback loop becomes disrupted during microvascular rarefaction: angiogenic mechanisms are blunted, reactive oxygen species accumulate, capillary function declines and eventually, capillaries disappear. This, we propose, forms the foundation of the reciprocal relationship between vascular density, blood flow, and metabolic needs and functionality of nearby cells.

Pacemaker Channels and the Chronotropic Response in Health and Disease

Loss or dysregulation of the normally precise control of heart rate via the autonomic nervous system plays a critical role during the development and progression of cardiovascular disease-including ischemic heart disease, heart failure, and arrhythmias. While the clinical significance of regulating changes in heart rate, known as the chronotropic effect, is undeniable, the mechanisms controlling these changes remain not fully understood. Heart rate acceleration and deceleration are mediated by increasing or decreasing the spontaneous firing rate of pacemaker cells in the sinoatrial node. During the transition from rest to activity, sympathetic neurons stimulate these cells by activating β-adrenergic receptors and increasing intracellular cyclic adenosine monophosphate. The same signal transduction pathway is targeted by positive chronotropic drugs such as norepinephrine and dobutamine, which are used in the treatment of cardiogenic shock and severe heart failure. The cyclic adenosine monophosphate-sensitive hyperpolarization-activated current (If) in pacemaker cells is passed by hyperpolarization-activated cyclic nucleotide-gated cation channels and is critical for generating the autonomous heartbeat. In addition, this current has been suggested to play a central role in the chronotropic effect. Recent studies demonstrate that cyclic adenosine monophosphate-dependent regulation of HCN4 (hyperpolarization-activated cyclic nucleotide-gated cation channel isoform 4) acts to stabilize the heart rate, particularly during rapid rate transitions induced by the autonomic nervous system. The mechanism is based on creating a balance between firing and recently discovered nonfiring pacemaker cells in the sinoatrial node. In this way, hyperpolarization-activated cyclic nucleotide-gated cation channels may protect the heart from sinoatrial node dysfunction, secondary arrhythmia of the atria, and potentially fatal tachyarrhythmia of the ventricles. Here, we review the latest findings on sinoatrial node automaticity and discuss the physiological and pathophysiological role of HCN pacemaker channels in the chronotropic response and beyond.

A novel conceptual model of heart rate autonomic modulation based on a small-world modular structure of the sinoatrial node

The present view on heartbeat initiation is that a primary pacemaker cell or a group of cells in the sinoatrial node (SAN) center paces the rest of the SAN and the atria. However, recent high-resolution imaging studies show a more complex paradigm of SAN function that emerges from heterogeneous signaling, mimicking brain cytoarchitecture and function. Here, we developed and tested a new conceptual numerical model of SAN organized similarly to brain networks featuring a modular structure with small-world topology. In our model, a lower rate module leads action potential (AP) firing in the basal state and during parasympathetic stimulation, whereas a higher rate module leads during β-adrenergic stimulation. Such a system reproduces the respective shift of the leading pacemaker site observed experimentally and a wide range of rate modulation and robust function while conserving energy. Since experimental studies found functional modules at different scales, from a few cells up to the highest scale of the superior and inferior SAN, the SAN appears to feature hierarchical modularity, i.e., within each module, there is a set of sub-modules, like in the brain, exhibiting greater robustness, adaptivity, and evolvability of network function. In this perspective, our model offers a new mainframe for interpreting new data on heterogeneous signaling in the SAN at different scales, providing new insights into cardiac pacemaker function and SAN-related cardiac arrhythmias in aging and disease.

Interstitial cells of Cajal - pacemakers of the gastrointestinal tract

Gastrointestinal (GI) organs display spontaneous, non-neurogenic electrical, and mechanical rhythmicity that underlies fundamental motility patterns, such as peristalsis and segmentation. Electrical rhythmicity (aka slow waves) results from pacemaker activity generated by interstitial cells of Cajal (ICC). ICC express a unique set of ionic conductances and Ca2+ handling mechanisms that generate and actively propagate slow waves. GI smooth muscle cells lack these conductances. Slow waves propagate actively within ICC networks and conduct electrotonically to smooth muscle cells via gap junctions. Slow waves depolarize smooth muscle cells and activate voltage-dependent Ca2+ channels (predominantly CaV1.2), causing Ca2+ influx and excitation-contraction coupling. The main conductances responsible for pacemaker activity in ICC are ANO1, a Ca2+ -activated Cl- conductance, and CaV3.2. The pacemaker cycle, as currently understood, begins with spontaneous, localized Ca2+ release events in ICC that activate spontaneous transient inward currents due to activation of ANO1 channels. Depolarization activates CaV 3.2 channels, causing the upstroke depolarization phase of slow waves. The upstroke is transient and followed by a long-duration plateau phase that can last for several seconds. The plateau phase results from Ca2+ -induced Ca2+ release and a temporal cluster of localized Ca2+ transients in ICC that sustains activation of ANO1 channels and clamps membrane potential near the equilibrium potential for Cl- ions. The plateau phase ends, and repolarization occurs, when Ca2+ stores are depleted, Ca2+ release ceases and ANO1 channels deactivate. This review summarizes key mechanisms responsible for electrical rhythmicity in gastrointestinal organs.

SanPy: A whole-cell electrophysiology analysis pipeline

The analysis of action potentials and other membrane voltage fluctuations provide a powerful approach for interrogating the function of excitable cells. Yet, a major bottleneck in the interpretation of this critical data is the lack of intuitive, agreed upon software tools for its analysis. Here, we present SanPy, a Python-based open-source and freely available software pipeline for the analysis and exploration of whole-cell current-clamp recordings. SanPy provides a robust computational engine with an application programming interface. Using this, we have developed a cross-platform graphical user interface that does not require programming. SanPy is designed to extract common parameters from action potentials including threshold time and voltage, peak, half-width, and interval statistics. In addition, several cardiac parameters are measured including the early diastolic duration and rate. SanPy is built to be fully extensible by providing a plugin architecture for the addition of new file loaders, analysis, and visualizations. A key feature of SanPy is its focus on quality control and data exploration. In the desktop interface, all plots of the data and analysis are linked allowing simultaneous data visualization from different dimensions with the goal of obtaining ground truth analysis. We provide documentation for all aspects of SanPy including several use cases and examples. To test SanPy, we have performed analysis on current-clamp recordings from heart and brain cells. Taken together, SanPy is a powerful tool for whole-cell current-clamp analysis and lays the foundation for future extension by the scientific community.

Emergent activity, heterogeneity, and robustness in a calcium feedback model of the sinoatrial node

The sinoatrial node (SAN) is the primary pacemaker of the heart. SAN activity emerges at an early point in life and maintains a steady rhythm for the lifetime of the organism. The ion channel composition and currents of SAN cells can be influenced by a variety of factors. Therefore, the emergent activity and long-term stability imply some form of dynamical feedback control of SAN activity. We adapt a recent feedback model-previously utilized to describe control of ion conductances in neurons-to a model of SAN cells and tissue. The model describes a minimal regulatory mechanism of ion channel conductances via feedback between intracellular calcium and an intrinsic target calcium level. By coupling a SAN cell to the calcium feedback model, we show that spontaneous electrical activity emerges from quiescence and is maintained at steady state. In a 2D SAN tissue model, spatial variability in intracellular calcium targets lead to significant, self-organized heterogeneous ion channel expression and calcium transients throughout the tissue. Furthermore, multiple pacemaking regions appear, which interact and lead to time-varying cycle length, demonstrating that variability in heart rate is an emergent property of the feedback model. Finally, we demonstrate that the SAN tissue is robust to the silencing of leading cells or ion channel knockouts. Thus, the calcium feedback model can reproduce and explain many fundamental emergent properties of activity in the SAN that have been observed experimentally based on a minimal description of intracellular calcium and ion channel regulatory networks.

Pericytes and the Control of Blood Flow in Brain and Heart

Pericytes, attached to the surface of capillaries, play an important role in regulating local blood flow. Using optogenetic tools and genetically encoded reporters in conjunction with confocal and multiphoton imaging techniques, the 3D structure, anatomical organization, and physiology of pericytes have recently been the subject of detailed examination. This work has revealed novel functions of pericytes and morphological features such as tunneling nanotubes in brain and tunneling microtubes in heart. Here, we discuss the state of our current understanding of the roles of pericytes in blood flow control in brain and heart, where functions may differ due to the distinct spatiotemporal metabolic requirements of these tissues. We also outline the novel concept of electro-metabolic signaling, a universal mechanistic framework that links tissue metabolic state with blood flow regulation by pericytes and vascular smooth muscle cells, with capillary K_ATP and Kir2.1 channels as primary sensors. Finally, we present major unresolved questions and outline how they can be addressed.

The paradigm shift: Heartbeat initiation without "the pacemaker cell"

The current dogma about the heartbeat origin is based on "the pacemaker cell," a specialized cell residing in the sinoatrial node (SAN) that exhibits spontaneous diastolic depolarization triggering rhythmic action potentials (APs). Recent high-resolution imaging, however, demonstrated that Ca signals and APs in the SAN are heterogeneous, with many cells generating APs of different rates and rhythms or even remaining non-firing (dormant cells), i.e., generating only subthreshold signals. Here we numerically tested a hypothesis that a community of dormant cells can generate normal automaticity, i.e., "the pacemaker cell" is not required to initiate rhythmic cardiac impulses. Our model includes 1) non-excitable cells generating oscillatory local Ca releases and 2) an excitable cell lacking automaticity. While each cell in isolation was not "the pacemaker cell", the cell system generated rhythmic APs: The subthreshold signals of non-excitable cells were transformed into respective membrane potential oscillations via electrogenic Na/Ca exchange and further transferred and integrated (computed) by the excitable cells to reach its AP threshold, generating rhythmic pacemaking. Cardiac impulse is an emergent property of the SAN cellular network and can be initiated by cells lacking intrinsic automaticity. Cell heterogeneity, weak coupling, subthreshold signals, and their summation are critical properties of the new pacemaker mechanism, i.e., cardiac pacemaker can operate via a signaling process basically similar to that of "temporal summation" happening in a neuron with input from multiple presynaptic cells. The new mechanism, however, does not refute the classical pacemaker cell-based mechanism: both mechanisms can co-exist and interact within SAN tissue.

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.

Molecular and Functional Remodeling of Superior and Inferior SAN in a Rat Model of HCM

BACKGROUND

Recently, our laboratory presented functional and molecular evidence for the presence of 2 competing sinoatrial node (SAN) pacemakers in healthy human and rat hearts. Anatomically localized near the superior vena cava and inferior vena cava, the superior and inferior SANs (sSAN and iSAN, respectively) preferentially control fast and slow normal heart rates. However, only 1 dominant pacemaker, primarily the sSAN, was functional in the failing rat heart with hypertrophic cardiomyopathy.

OBJECTIVES

This study aimed to determine the transcriptional basis of functional silencing of 1 of 2 dominant pacemakers in failing rat hearts.

METHODS

Ascending aortic constriction was performed on 1-week-old male Sprague-Dawley rat pups to induce left ventricular hypertrophy and heart failure. The dominant pacemaker was anatomically mapped in adult (10-12 weeks old) healthy and failing rat hearts using optical mapping in isolated right atrial tissue preparations. RNA sequencing was used to identify regional sSAN/iSAN gene expression differences between healthy and failing rat hearts.

RESULTS

In all failing rat hearts optically mapped in this study (n = 4), only the sSAN pacemaker was functional, while the iSAN was silent. Compared to healthy rat hearts, a total of 3,640 genes were downregulated, and 4,518 genes were upregulated in failing rat hearts. The functional quiescence of the iSAN in these failing rat hearts may be explained by their downregulation of sodium, potassium, and calcium ion channels as well as their downregulation of specific structural genes, including ankyrin, titin, and myosin heavy chain. Moreover, the iSAN showed predominant downregulation of several key transcription factors such as Tbx5, Tbx3, Shox2, and Smad9.

CONCLUSIONS

Pressure-overload-induced heart failure resulted in significant downregulation of critical transcription factors, ion channels, and structural transcripts of the iSAN, which could explain the functional silencing of the iSAN in failing rat hearts.

The Heart's Pacemaker Mimics Brain Cytoarchitecture and Function: Novel Interstitial Cells Expose Complexity of the SAN

BACKGROUND

The sinoatrial node (SAN) of the heart produces rhythmic action potentials, generated via calcium signaling within and among pacemaker cells. Our previous work has described the SAN as composed of a hyperpolarization-activated cyclic nucleotide-gated potassium channel 4 (HCN4)-expressing pacemaker cell meshwork, which merges with a network of connexin 43⁺/F-actin⁺ cells. It is also known that sympathetic and parasympathetic innervation create an autonomic plexus in the SAN that modulates heart rate and rhythm. However, the anatomical details of the interaction of this plexus with the pacemaker cell meshwork have yet to be described.

OBJECTIVES

This study sought to describe the 3-dimensional cytoarchitecture of the mouse SAN, including autonomic innervation, peripheral glial cells, and pacemaker cells.

METHODS

The cytoarchitecture of SAN whole-mount preparations was examined by three-dimensional confocal laser-scanning microscopy of triple immunolabeled with combinations of antibodies for HCN4, S100 calcium-binding protein B (S100B), glial fibrillary acidic protein (GFAP), choline acetyltransferase, or vesicular acetylcholine transporter, and tyrosine hydroxylase, and transmission electron microscopy.

RESULTS

The SAN exhibited heterogeneous autonomic innervation, which was accompanied by a web of peripheral glial cells and a novel S100B⁺/GFAP^- interstitial cell population, with a unique morphology and a distinct distribution pattern, creating complex interactions with other cell types in the node, particularly with HCN4-expressing cells. Transmission electron microscopy identified a similar population of interstitial cells as telocytes, which appeared to secrete vesicles toward pacemaker cells. Application of S100B to SAN preparations desynchronized Ca²⁺ signaling in HCN4-expressing cells and increased variability in SAN impulse rate and rhythm.

CONCLUSIONS

The autonomic plexus, peripheral glial cell web, and a novel S100B⁺/GFAP^- interstitial cell type embedded within the HCN4⁺ cell meshwork increase the structural and functional complexity of the SAN and provide a new regulatory pathway of rhythmogenesis.

Biological noise is a key determinant of the reproducibility and adaptability of cardiac pacemaking and EC coupling

Each heartbeat begins with the generation of an action potential in pacemaking cells in the sinoatrial node. This signal triggers contraction of cardiac muscle through a process termed excitation-contraction (EC) coupling. EC coupling is initiated in dyadic structures of cardiac myocytes, where ryanodine receptors in the junctional sarcoplasmic reticulum come into close apposition with clusters of CaV1.2 channels in invaginations of the sarcolemma. Cooperative activation of CaV1.2 channels within these clusters causes a local increase in intracellular Ca2+ that activates the juxtaposed ryanodine receptors. A salient feature of healthy cardiac function is the reliable and precise beat-to-beat pacemaking and amplitude of Ca2+ transients during EC coupling. In this review, we discuss recent discoveries suggesting that the exquisite reproducibility of this system emerges, paradoxically, from high variability at subcellular, cellular, and network levels. This variability is attributable to stochastic fluctuations in ion channel trafficking, clustering, and gating, as well as dyadic structure, which increase intracellular Ca2+ variance during EC coupling. Although the effects of these large, local fluctuations in function and organization are sometimes negligible at the macroscopic level owing to spatial-temporal summation within and across cells in the tissue, recent work suggests that the "noisiness" of these intracellular Ca2+ events may either enhance or counterintuitively reduce variability in a context-dependent manner. Indeed, these noisy events may represent distinct regulatory features in the tuning of cardiac contractility. Collectively, these observations support the importance of incorporating experimentally determined values of Ca2+ variance in all EC coupling models. The high reproducibility of cardiac contraction is a paradoxical outcome of high Ca2+ signaling variability at subcellular, cellular, and network levels caused by stochastic fluctuations in multiple processes in time and space. This underlying stochasticity, which counterintuitively manifests as reliable, consistent Ca2+ transients during EC coupling, also allows for rapid changes in cardiac rhythmicity and contractility in health and disease.

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.

Mechanisms and Physiological Implications of Cooperative Gating of Ion Channels Clusters

Ion channels play a central role in the regulation of nearly every cellular process. Dating back to the classic 1952 Hodgkin-Huxley model of the generation of the action potential, ion channels have always been thought of as independent agents. A myriad of recent experimental findings exploiting advances in electrophysiology, structural biology, and imaging techniques, however, have posed a serious challenge to this long-held axiom as several classes of ion channels appear to open and close in a coordinated, cooperative manner. Ion channel cooperativity ranges from variable-sized oligomeric cooperative gating in voltage-gated, dihydropyridine-sensitive Ca_v1.2 and Ca_v1.3 channels to obligatory dimeric assembly and gating of voltage-gated Na_v1.5 channels. Potassium channels, transient receptor potential channels, hyperpolarization cyclic nucleotide-activated channels, ryanodine receptors (RyRs), and inositol trisphosphate receptors (IP₃Rs) have also been shown to gate cooperatively. The implications of cooperative gating of these ion channels range from fine tuning excitation-contraction coupling in muscle cells to regulating cardiac function and vascular tone, to modulation of action potential and conduction velocity in neurons and cardiac cells, and to control of pace-making activity in the heart. In this review, we discuss the mechanisms leading to cooperative gating of ion channels, their physiological consequences and how alterations in cooperative gating of ion channels may induce a range of clinically significant pathologies.