Modeling Cardiomyocyte Mechanics and Autoregulation of Contractility by Mechano-Chemo-Transduction Feedback

Modeling autoregulation of cardiac excitation-Ca-contraction and arrhythmogenic activities in response to mechanical load changes

The heart has intrinsic abilities to autoregulate contractile force in response to mechanical load. Recent experimental studies show that cardiomyocytes have mechano-chemo-transduction (MCT) mechanisms that form a closed feedback loop in the excitation-Ca2+ signaling-contraction (E-C) coupling. This closed feedback loop enables autoregulation of contraction in response to mechanical load changes. Here, we develop the first autoregulatory E-C coupling model that couples electrophysiology, Ca2+ signaling, force development and contraction, and MCT feedback. The model recapitulates the experimental data showing that the mechanical load on cardiomyocytes during contraction increases the L-type Ca2+ current, action potential duration, sarcoplasmic reticulum (SR) Ca2+ content, and SR Ca2+ release, giving rise to increased cytosolic Ca2+ transient (MCT-Ca2+ gain) and enhanced contraction. The model also makes non-trivial predictions on the autoregulation of contraction with moderate MCT-Ca2+ gain under a range of physiological load changes, but arrhythmogenic discordant alternans with excessive MCT-Ca2+ gain under pathological overload.

Dynamical effects of mechano-chemo-transduction on cardiac alternans

In every heartbeat, cardiac muscle cells perform excitation-Ca2+ signaling-contraction (EC) coupling to pump blood against the vascular resistance. Cardiomyocytes can sense the mechanical load and activate mechano-chemo-transduction (MCT) mechanism, which provides feedback regulation of EC coupling. MCT feedback is important for the heart to upregulate contraction in response to increased load to maintain cardiac output. MCT feedback enhances the L-type Ca2+ current, sensitizes ryanodine receptors (RyRs), and augments SERCA pump activity, thereby maintaining contraction amplitude despite increased load. However, under certain conditions, MCT feedback can also promote cardiac alternans, seen as beat-to-beat variations in action potential duration, Ca2+ transients, and contraction strength, which is a precursor to arrhythmias. While alternans can arise from instabilities in either membrane voltage or intracellular Ca2+ cycling, underlying mechanisms of MCT-induced alternans, particularly electromechanically discordant alternans where stronger beats are paradoxically associated with shorter action potentials, remain unclear. In this study, we used a mathematical model of the ventricular myocyte to investigate the effects of MCT feedback on the dynamical system that generates alternans. We systematically analyzed how MCT feedback, acting through L-type Ca2+ channels (LTCCs), RyRs, or SERCA, affects the stability of membrane voltage and Ca2+ cycling, as well as the coupling between them. Our results show that MCT feedback can generally promote both concordant and discordant alternans in action potential and Ca2+ transients, depending on the underlying instability mechanism. We found that MCT feedback through RyRs predominantly increases Ca2+ instability, while LTCC and SERCA feedback have complex effects due to the interplay between stability and coupling alterations. We also showed how to determine underlying mechanisms from experimental and clinical observations. Our modeling studies provide new insights into the complex dynamics underlying cardiac alternans and highlight the importance of MCT feedback in the development of life-threatening arrhythmias in the heart under mechanical load.

Deciphering mechanical cues in the microenvironment: from non-malignant settings to tumor progression

The tumor microenvironment functions as a dynamic and intricate ecosystem, comprising a diverse array of cellular and non-cellular components that precisely orchestrate pivotal tumor behaviors, including invasion, metastasis, and drug resistance. While unraveling the intricate interplay between the tumor microenvironment and tumor behaviors represents a tremendous challenge, recent research illuminates a crucial biological phenomenon known as cellular mechanotransduction. Within the microenvironment, mechanical cues like tensile stress, shear stress, and stiffness play a pivotal role by activating mechanosensitive effectors such as PIEZO proteins, integrins, and Yes-associated protein. This activation initiates cascades of intrinsic signaling pathways, effectively linking the physical properties of tissues to their physiological and pathophysiological processes like morphogenesis, regeneration, and immunity. This mechanistic insight offers a novel perspective on how the mechanical cues within the tumor microenvironment impact tumor behaviors. While the intricacies of the mechanical tumor microenvironment are yet to be fully elucidated, it exhibits distinct physical attributes from non-malignant tissues, including elevated solid stresses, interstitial hypertension, augmented matrix stiffness, and enhanced viscoelasticity. These traits exert notable influences on tumor progression and treatment responses, enriching our comprehension of the multifaceted nature of the microenvironment. Through this innovative review, we aim to provide a new lens to decipher the mechanical attributes within the tumor microenvironment from non-malignant contexts, broadening our knowledge on how these factors promote or inhibit tumor behaviors, and thus offering valuable insights to identify potential targets for anti-tumor strategies.

INNOVATIVE TECHNIQUES AND NEW INSIGHTS: Studying cardiac ionic currents and action potentials in physiologically relevant conditions

Cardiac arrhythmias are associated with various forms of heart diseases. Ventricular arrhythmias present a significant risk for sudden cardiac death. Atrial fibrillations predispose to blood clots leading to stroke and heart attack. Scientists have been developing patch-clamp technology to study ion channels and action potentials (APs) underlying cardiac excitation and arrhythmias. Beyond the traditional patch-clamp techniques, innovative new techniques were developed for studying complex arrhythmia mechanisms. Here we review the recent development of methods including AP-Clamp, Dynamic Clamp, AP-Clamp Sequential Dissection, and Patch-Clamp-in-Gel. These methods provide powerful tools for researchers to decipher how the dynamic systems in excitation-Ca2+ signaling-contraction feedforward and feedback to control cardiac function and how their dysregulations lead to heart diseases.