Novel Identification of Ankyrin-R in Cardiac Fibroblasts and a Potential Role in Heart Failure.

Altered ankyrin-R (AnkR; encoded by ANK1) expression is associated with diastolic function, left ventricular remodeling, and heart failure with preserved ejection fraction (HFpEF). First identified in erythrocytes, the role of AnkR in other tissues, particularly the heart, is less studied. Here, we identified the expression of both canonical and small isoforms of AnkR in the mouse myocardium. We demonstrate that cardiac myocytes primarily express small AnkR (sAnkR), whereas cardiac fibroblasts predominantly express canonical AnkR. As canonical AnkR expression in cardiac fibroblasts is unstudied, we focused on expression and localization in these cells. AnkR is expressed in both the perinuclear and cytoplasmic regions of fibroblasts with considerable overlap with the trans-Golgi network protein 38, TGN38, suggesting a potential role in trafficking. To study the role of AnkR in fibroblasts, we generated mice lacking AnkR in activated fibroblasts (Ank1-ifKO mice). Notably, Ank1-ifKO mice fibroblasts displayed reduced collagen compaction, supportive of a novel role of AnkR in normal fibroblast function. At the whole animal level, in response to a heart failure model, Ank1-ifKO mice displayed an increase in fibrosis and T-wave inversion compared with littermate controls, while preserving cardiac ejection fraction. Collagen type I fibers were decreased in the Ank1-ifKO mice, suggesting a novel function of AnkR in the maturation of collagen fibers. In summary, our findings illustrate the novel expression of AnkR in cardiac fibroblasts and a potential role in cardiac function in response to stress.

Molecular Pathways and Animal Models of Arrhythmias.

Arrhythmias account for over 300,000 annual deaths in the United States, and approximately half of all deaths are associated with heart disease. Mechanisms underlying arrhythmia risk are complex; however, work in humans and animal models over the past 25 years has identified a host of molecular pathways linked with both arrhythmia substrates and triggers. This chapter will focus on select arrhythmia pathways solved by linking human clinical and genetic data with animal models.

Phosphorylation of cardiac sodium channel at Ser571 anticipates manifestations of the aging myopathy.

Diastolic dysfunction and delayed ventricular repolarization are typically observed in the elderly, but whether these defects are intimately associated with the progressive manifestation of the aging myopathy remains to be determined. In this regard, aging in experimental animals is coupled with increased late Na+ current (INa,L) in cardiomyocytes, raising the possibility that INa,L conditions the modality of electrical recovery and myocardial relaxation of the aged heart. For this purpose, aging male and female wild-type (WT) C57Bl/6 mice were studied together with genetically engineered mice with phosphomimetic (gain of function, GoF) or ablated (loss of function, LoF) mutations of the sodium channel Nav1.5 at Ser571 associated with, respectively, increased and stabilized INa,L. At ∼18 mo of age, WT mice developed prolonged duration of the QT interval of the electrocardiogram and impaired diastolic left ventricular (LV) filling, defects that were reversed by INa,L inhibition. Prolonged repolarization and impaired LV filling occurred prematurely in adult (∼5 mo) GoF mutant mice, whereas these alterations were largely attenuated in aging LoF mutant animals. Ca2+ transient decay and kinetics of myocyte shortening/relengthening were delayed in aged (∼24 mo) WT myocytes, with respect to adult cells. In contrast, delayed Ca2+ transients and contractile dynamics occurred at adult stage in GoF myocytes and further deteriorated in old age. Conversely, myocyte mechanics were minimally affected in aging LoF cells. Collectively, these results document that Nav1.5 phosphorylation at Ser571 and the late Na+ current modulate the modality of myocyte relaxation, constituting the mechanism linking delayed ventricular repolarization and diastolic dysfunction.NEW & NOTEWORTHY We have investigated the impact of the late Na current (INa,L) on cardiac and myocyte function with aging by using genetically engineered animals with enhanced or stabilized INa,L, due to phosphomimetic or phosphoablated mutations of Nav1.5. Our findings support the notion that phosphorylation of Nav1.5 at Ser571 prolongs myocardial repolarization and impairs diastolic function, contributing to the manifestations of the aging myopathy.

MicroRNA-1 Deficiency Is a Primary Etiological Factor Disrupting Cardiac Contractility and Electrophysiological Homeostasis.

BACKGROUND: MicroRNA-1 (miR1), encoded by the genes miR1-1 and miR1-2, is the most abundant microRNA in the heart and plays a critical role in heart development and physiology. Dysregulation of miR1 has been associated with various heart diseases, where a significant reduction (>75%) in miR1 expression has been observed in patient hearts with atrial fibrillation or acute myocardial infarction. However, it remains uncertain whether miR1-deficiency acts as a primary etiological factor of cardiac remodeling. METHODS: miR1-1 or miR1-2 knockout mice were crossbred to produce 75%-miR1-knockdown (75%KD; miR1-1+/-:miR1-2-/- or miR1-1-/-:miR1-2+/-) mice. Cardiac pathology of 75%KD cardiomyocytes/hearts was investigated by ECG, patch clamping, optical mapping, transcriptomic, and proteomic assays. RESULTS: In adult 75%KD hearts, the overall miR1 expression was reduced to ≈25% of the normal wild-type level. These adult 75%KD hearts displayed decreased ejection fraction and fractional shortening, prolonged QRS and QT intervals, and high susceptibility to arrhythmias. Adult 75%KD cardiomyocytes exhibited prolonged action potentials with impaired repolarization and excitation-contraction coupling. Comparatively, 75%KD cardiomyocytes showcased reduced Na+ current and transient outward potassium current, coupled with elevated L-type Ca2+ current, as opposed to wild-type cells. RNA sequencing and proteomics assays indicated negative regulation of cardiac muscle contraction and ion channel activities, along with a positive enrichment of smooth muscle contraction genes in 75%KD cardiomyocytes/hearts. miR1 deficiency led to dysregulation of a wide gene network, with miR1's RNA interference-direct targets influencing many indirectly regulated genes. Furthermore, after 6 weeks of bi-weekly intravenous tail-vein injection of miR1 mimics, the ejection fraction and fractional shortening of 75%KD hearts showed significant improvement but remained susceptible to arrhythmias. CONCLUSIONS: miR1 deficiency acts as a primary etiological factor in inducing cardiac remodeling via disrupting heart regulatory homeostasis. Achieving stable and appropriate microRNA expression levels in the heart is critical for effective microRNA-based therapy in cardiovascular diseases.

Biochemical Structure and Function of TRAPP Complexes in the Cardiac System.

Trafficking protein particle (TRAPP) is well reported to play a role in the trafficking of protein products within the Golgi and endoplasmic reticulum. Dysfunction in TRAPP has been associated with disorders in the nervous and cardiovascular systems, but the majority of literature focuses on TRAPP function in the nervous system solely. Here, we highlight the known pathways of TRAPP and hypothesize potential impacts of TRAPP dysfunction on the cardiovascular system, particularly the role of TRAPP as a guanine-nucleotide exchange factor for Rab1 and Rab11. We also review the various cardiovascular phenotypes associated with changes in TRAPP complexes and their subunits.