Distinctive left-sided distribution of adrenergic-derived cells in the adult mouse heart

An update on the mechanisms of Takotsubo syndrome: "At the end an acute coronary syndrome"

Takotsubo syndrome (TTS) is an acute reversible form of myocardial dysfunction, often preceded by a physical or emotional stressful event, that acts as a trigger. Despite, recent advances in the comprehension of the mechanisms leading to TTS, its pathophysiology is far from being completely understood. However, several studies seem to suggest that an acute coronary microvascular dysfunction may represent a crucial pathogenic mechanism involved in TTS occurrence. In this article, we aim to review the complex pathophysiology of TTS and the possible different mechanisms underlying this clinical condition, focusing on the role of coronary microvascular dysfunction and the remaining knowledge's gaps in the field.

Takotsubo syndrome: getting closer to its causes

Takotsubo syndrome (TTS) accounts for between 1 and 4% of cases presenting clinically as an acute coronary syndrome. It typically presents as a transient cardiac phenotype of left ventricular dysfunction with spontaneous recovery. More dramatic presentations may include cardiogenic shock or cardiac arrest. Despite progress in the understanding of the condition since its first description in 1990, considerable questions remain into understanding underlying pathomechanisms. In this review article, we describe the current published data on potential underlying mechanisms associated with the onset of TTS including sympathetic nervous system over-stimulation, structural and functional alterations in the central nervous system, catecholamine secretion, alterations in the balance and distribution of adrenergic receptors, the additive impact of hormones including oestrogen, epicardial coronary or microvascular spasm, endothelial dysfunction, and genetics as potentially contributing to the cascade of events leading to the onset. These pathomechanisms provide suggestions for novel potential therapeutic strategies in patients with TTS including the role of cognitive behavioural therapy, beta-blockers, and endothelin-A antagonists. The underlying mechanism of TTS remains elusive. In reality, physical or emotional stressors likely trigger through the amygdala and hippocampus a central neurohumoral activation with the local and systemic secretion of excess catecholamine and other neurohormones, which exert its effect on the myocardium through a metabolic switch, altered cellular signalling, and endothelial dysfunction. These complex pathways exert a regional activation in the myocardium through the altered distribution of adrenoceptors and density of autonomic innervation as a protective mechanism from myocardial apoptosis. More research is needed to understand how these different complex mechanisms interact with each other to bring on the TTS phenotype.

Novel cardiac cell subpopulations: Pnmt-derived cardiomyocytes

Diversity among highly specialized cells underlies the fundamental biology of complex multi-cellular organisms. One of the essential scientific questions in cardiac biology has been to define subpopulations within the heart. The heart parenchyma comprises specialized cardiomyocytes (CMs). CMs have been canonically classified into a few phenotypically diverse subpopulations largely based on their function and anatomic localization. However, there is growing evidence that CM subpopulations are in fact numerous, with a diversity of genetic origin and putatively different roles in physiology and pathophysiology. In this chapter, we introduce a recently discovered CM subpopulation: phenylethanolamine-N-methyl transferase (Pnmt)-derived cardiomyocytes (PdCMs). We discuss: (i) canonical classifications of CM subpopulations; (ii) discovery of PdCMs; (iii) Pnmt and the role of catecholamines in the heart; similarities and dissimilarities of PdCMs and canonical CMs; and (iv) putative functions of PdCMs in both physiological and pathological states and future directions, such as in intra-cardiac adrenergic signalling.

Intricacies of the Molecular Machinery of Catecholamine Biosynthesis and Secretion by Chromaffin Cells of the Normal Adrenal Medulla and in Pheochromocytoma and Paraganglioma

The adrenal medulla is composed predominantly of chromaffin cells producing and secreting the catecholamines dopamine, norepinephrine, and epinephrine. Catecholamine biosynthesis and secretion is a complex and tightly controlled physiologic process. The pathways involved have been extensively studied, and various elements of the underlying molecular machinery have been identified. In this review, we provide a detailed description of the route from stimulus to secretion of catecholamines by the normal adrenal chromaffin cell compared to chromaffin tumor cells in pheochromocytomas. Pheochromocytomas are adrenomedullary tumors that are characterized by uncontrolled synthesis and secretion of catecholamines. This uncontrolled secretion can be partly explained by perturbations of the molecular catecholamine secretory machinery in pheochromocytoma cells. Chromaffin cell tumors also include sympathetic paragangliomas originating in sympathetic ganglia. Pheochromocytomas and paragangliomas are usually locally confined tumors, but about 15% do metastasize to distant locations. Histopathological examination currently poorly predicts future biologic behavior, thus long term postoperative follow-up is required. Therefore, there is an unmet need for prognostic biomarkers. Clearer understanding of the cellular mechanisms involved in the secretory characteristics of pheochromocytomas and sympathetic paragangliomas may offer one approach for the discovery of novel prognostic biomarkers for improved therapeutic targeting and monitoring of treatment or disease progression.

Pnmt-Derived Cardiomyocytes: Anatomical Localization, Function and Future Perspectives

In this mini-review, we provide an overview of phenylethanolamine-N-methyl transferase (Pnmt)-derived cardiomyocytes (PdCMs), a recently discovered cardiomyocyte subpopulation. We discuss their anatomical localization, physiological characteristics, possible function, and future perspectives. Their unique distribution in the heart, electrical activity, Ca2+ transient properties, and potential role in localized adrenergic signaling are discussed.

Three-dimensional image reconstruction of distribution of Pnmt+ cell-derived cells in murine heart

Elucidating the function of specific cell types in a highly complex multicellular system such as the heart often requires detailed anatomic reconstruction. We recently described a distinctive class of phenylethanolamine n-methyltransferase (Pnmt+) cell-derived cardiomyocytes (PdCMs), a new cardiomyocyte population with a potential endocrine role. In this dataset, a 3D reconstruction was carried out to visualise the distribution of PdCMs throughout the murine heart. Rigid registration (stiff rotation and translation) was applied to properly align the fused heart slice images based on landmarks using TrakEM2, an open source plug-in in Fiji. The registered slices were then analysed and reconstructed using MATLAB (MATLAB®. Version 8.3.0.532). The final reconstructed 3D volume was 561×866×48 pixels (corresponding to spatial resolutions of 5.8, 8.9 and 2.5 mm in the x-, y- and z-direction respectively), and visualised in Paraview. The reconstruction allows for detailed analyses of morphology, projections and cellular features of different cell types, enabling further geometrical and topological analyses. Image data can be accessed and viewed through Figshare.

Optogenetic Control of Heart Rhythm by Selective Stimulation of Cardiomyocytes Derived from Pnmt+ Cells in Murine Heart

In the present study, channelrhodopsin 2 (ChR2) was specifically introduced into murine cells expressing the Phenylethanolamine n-methyltransferase (Pnmt) gene, which encodes for the enzyme responsible for conversion of noradrenaline to adrenaline. The new murine model enabled the identification of a distinctive class of Pnmt-expressing neuroendocrine cells and their descendants (i.e. Pnmt⁺ cell derived cells) within the heart. Here, we show that Pnmt⁺ cells predominantly localized to the left side of the adult heart. Remarkably, many of the Pnmt⁺ cells in the left atrium and ventricle appeared to be working cardiomyocytes based on their morphological appearance and functional properties. These Pnmt⁺ cell derived cardiomyocytes (PdCMs) are similar to conventional myocytes in morphological, electrical and contractile properties. By stimulating PdCMs selectively with blue light, we were able to control cardiac rhythm in the whole heart, isolated tissue preparations and single cardiomyocytes. Our new murine model effectively demonstrates functional dissection of cardiomyocyte subpopulations using optogenetics, and opens new frontiers of exploration into their physiological roles in normal heart function as well as their potential application for selective cardiac repair and regeneration strategies.

Therapeutic potential of Pnmt+ primer cells for neuro/myocardial regeneration

Phenylethanolamine n-methyltransferase (Pnmt) catalyzes the conversion of norepinephrine into epinephrine, and thus serves as a marker of adrenergic cells. In adults, adrenergic cells are present in the adrenal medullae and the central and peripheral (sympathetic) nervous systems where they play key roles in stress responses and a variety of other functions. During early embryonic development, however, Pnmt first appears in the heart where it is associated with specialized myocytes in the pacemaking and conduction system. There is a transient surge in cardiac Pnmt expression beginning when the first myocardial contractions occur, before any nerve-like or neural crest cells appear in the heart. This early expression of Pnmt denotes a mesodermal origin of these "Instrinsic Cardiac Adrenergic" (ICA) cells. Interestingly, Pnmt+ cells are found in all four chambers of the developing heart, but by adult stages, are found primarily concentrated on the left side of the heart. This regionalized expression occurs in the left atrium and in specific regions of the left ventricle roughly corresponding to basal, mid, and apical sections. A second distinct population of Pnmt-expressing (Pnmt+) cells enters the embryonic heart from invading neural crest, and these "Neural Crest-Derived" (NCD) Pnmt+ cells appear to give rise to a subpopulation(s) of cardiac neurons. Pnmt expression thus serves as a marker not only for adrenergic cells, but also for precursor or "primer" cells destined to become specialized myocytes and neurons in the heart. This review discusses the distribution of Pnmt in the heart during development, including the types of cells where it is expressed, and their potential use for regenerative medicine therapies for cardiovascular disease.

Physiological and genomic consequences of adrenergic deficiency during embryonic/fetal development in mice: impact on retinoic acid metabolism

Adrenergic hormones are essential for early heart development. To gain insight into understanding how these hormones influence heart development, we evaluated genomic expression changes in embryonic hearts from adrenergic-deficient and wild-type control mice. To perform this study, we used a mouse model with targeted disruption of the Dopamine β-hydroxylase (Dbh) gene, whose product is responsible for enzymatic conversion of dopamine into norepinephrine. Embryos homozygous for the null allele (Dbh(-/-)) die from heart failure beginning as early as embryonic day 10.5 (E10.5). To assess underlying causes of heart failure, we isolated hearts from Dbh(-/-) and Dbh(+/+) embryos prior to manifestation of the phenotype and examined gene expression changes using genomic Affymetrix 430A 2.0 arrays, which enabled simultaneous evaluation of >22,000 genes. We found that only 22 expressed genes showed a significant twofold or greater change, representing ~0.1% of the total genes analyzed. More than half of these genes are associated with either metabolism (31%) or signal transduction (22%). Remarkably, several of the altered genes encode for proteins that are directly involved in retinoic acid (RA) biosynthesis and transport. Subsequent evaluation showed that RA concentrations were significantly elevated by an average of ~3-fold in adrenergic-deficient (Dbh(-/-)) embryos compared with controls, thereby suggesting that RA may be an important downstream mediator of adrenergic action during embryonic heart development.