Exploring the role of TRPM4 in calcium-dependent triggered activity and cardiac arrhythmias

Ca2+ Signaling in Cardiac Fibroblasts: An Emerging Signaling Pathway Driving Fibrotic Remodeling in Cardiac Disorders

Cardiac fibrosis is a scarring event that occurs in the myocardium in response to multiple cardiovascular disorders, such as acute myocardial infarction (AMI), ischemic cardiomyopathy, dilated cardiomyopathy, hypertensive heart disease, inflammatory heart disease, diabetic cardiomyopathy, and aortic stenosis. Fibrotic remodeling is mainly sustained by the differentiation of fibroblasts into myofibroblasts, which synthesize and secrete most of the extracellular matrix (ECM) proteins. An increase in the intracellular Ca2+ concentration ([Ca2+]i) in cardiac fibroblasts is emerging as a critical mediator of the fibrogenic signaling cascade. Herein, we review the mechanisms that may shape intracellular Ca2+ signals involved in fibroblast transdifferentiation into myofibroblasts. We focus our attention on the functional interplay between inositol-1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) and store-operated Ca2+ entry (SOCE). In accordance with this, InsP3Rs and SOCE drive the Ca2+ response elicited by Gq-protein coupled receptors (GqPCRs) that promote fibrotic remodeling. Then, we describe the additional mechanisms that sustain extracellular Ca2+ entry, including receptor-operated Ca2+ entry (ROCE), P2X receptors, Transient Receptor Potential (TRP) channels, and Piezo1 channels. In parallel, we discuss the pharmacological manipulation of the Ca2+ handling machinery as a promising approach to mitigate or reverse fibrotic remodeling in cardiac disorders.

TRPM4 contributes to cell death in prostate cancer tumor spheroids, and to extravasation and metastasis in a zebrafish xenograft model system

Transient receptor potential melastatin-4 (TRPM4) ion channel expression is upregulated in prostate cancer (PCa), contributing to increased cell proliferation, migration, adhesion, epithelial-to-mesenchymal transition, cell cycle shift, and alterations of intracellular Ca2+ signaling. GEO2R platform analysis of messenger RNA (mRNA) expression of ~ 6350 genes in normal and malignant prostate tissue samples from 15 PCa patients demonstrates that TRPM4 expression is upregulated sixfold and is among the most significantly upregulated genes in PCa. We find that absence of TRPM4 reduced PCa tumor spheroid size and decreased PCa tumor spheroid outgrowth. In addition, lack of TRPM4 increased cell death in PCa tumor spheroids, a phenotype that is absent in two-dimensional (2D) cancer cell systems. Lastly, absence of TRPM4 in PCa cells reduced extravasation and metastatic burden in a preclinical zebrafish cancer model. Taken together, our findings show that TRPM4 is an attractive therapeutic target in PCa and highlights the need for future development of pharmacological tools.

Thermo-TRP regulation by endogenous factors and its physiological function at core body temperature

Transient receptor potential (TRP) channels with temperature sensitivities (thermo-TRPs) are involved in various physiological processes. Thermo-TRPs that detect temperature changes in peripheral sensory neurons possess indispensable functions in thermosensation, eliciting defensive behavior against noxious temperatures and driving autonomic/behavioral thermoregulatory responses to maintain body temperature in mammals. Moreover, most thermo-TRPs are functionally expressed in cells and tissues where the temperature is maintained at a constant core body temperature. To perform physiological functions, the activity of each thermo-TRP channel must be regulated by endogenous mechanisms at body temperature. Dysregulation of this process can lead to various diseases. This review highlights the endogenous factors regulating thermo-TRP activity and physiological functions at constant core body temperature.

A frozen portrait of a warm channel

In order to understand protein function, the field of structural biology makes extensive use of cryogenic electron microscopy (cryo-EM), a technique that enables structure determination at atomic resolution following embedding of protein particles in vitreous ice. Considering the profound effects of temperature on macromolecule function, an important-but often neglected-question is how the frozen particles relate to the actual protein conformations at physiological temperatures. In a recent study, Hu et al. compare structures of the cation channel TRPM4 "frozen" at 4 °C versus 37 °C, revealing how temperature critically affects the binding of activating Ca2+ ions and other channel modulators.

Uncoupling cytosolic calcium from membrane voltage by transient receptor potential melastatin 4 channel (TRPM4) modulation: A novel strategy to treat ventricular arrhythmias

The current antiarrhythmic paradigm is mainly centered around modulating membrane voltage. However, abnormal cytosolic calcium (Ca2+) signaling, which plays an important role in driving membrane voltage, has not been targeted for therapeutic purposes in arrhythmogenesis. There is clear evidence for bidirectional coupling between membrane voltage and intracellular Ca2+. Cytosolic Ca2+ regulates membrane voltage through Ca2+-sensitive membrane currents. As a component of Ca2+-sensitive currents, Ca2+-activated nonspecific cationic current through the TRPM4 (transient receptor potential melastatin 4) channel plays a significant role in Ca2+-driven changes in membrane electrophysiology. In myopathic and ischemic ventricles, upregulation and/or enhanced activity of this current is associated with the generation of afterdepolarization (both early and delayed), reduction of repolarization reserve, and increased propensity to ventricular arrhythmias. In this review, we describe a novel concept for the management of ventricular arrhythmias in the remodeled ventricle based on mechanistic concepts from experimental studies, by uncoupling the Ca2+-induced changes in membrane voltage by inhibition of this TRPM4-mediated current.