Asymmetry in Mechanosensitive Gene Expression during Aortic Arch Morphogenesis

Origins of Aortic Coarctation: A Vascular Smooth Muscle Compartment Boundary Model

Compartment boundaries divide the embryo into segments with distinct fates and functions. In the vascular system, compartment boundaries organize endothelial cells into arteries, capillaries, and veins that are the fundamental units of a circulatory network. For vascular smooth muscle cells (SMCs), such boundaries produce mosaic patterns of investment based on embryonic origins with important implications for the non-uniform distribution of vascular disease later in life. The morphogenesis of blood vessels requires vascular cell movements within compartments as highly-sensitive responses to changes in fluid flow shear stress and wall strain. These movements underline the remodeling of primitive plexuses, expansion of lumen diameters, regression of unused vessels, and building of multilayered artery walls. Although the loss of endothelial compartment boundaries can produce arterial-venous malformations, little is known about the consequences of mislocalization or the failure to form SMC-origin-specific boundaries during vascular development. We propose that the failure to establish a normal compartment boundary between cardiac neural-crest-derived SMCs of the 6th pharyngeal arch artery (future ductus arteriosus) and paraxial-mesoderm-derived SMCs of the dorsal aorta in mid-gestation embryos leads to aortic coarctation observed at birth. This model raises new questions about the effects of fluid flow dynamics on SMC investment and the formation of SMC compartment borders during pharyngeal arch artery remodeling and vascular development.

Congenital heart defects differ following left versus right avian cardiac neural crest ablation

The cardiac neural crest is critical for the normal development of the heart, as its surgical ablation in the chick recapitulates common human congenital heart defects such as 'Common Arterial Trunk' and 'Double Outlet Right Ventricle' (DORV). While left-right asymmetry is known to be important for heart development, little is known about potential asymmetric differences between right and left cardiac neural folds with respect to heart development. Here, through surgical ablation of either left or right cardiac neural crest, we find that right ablation results in more varied and more severe heart defects. Embryos with Common Arterial Trunk and with missing arteries occurred in right-ablated embryos but were not observed in left-ablated embryos; moreover, embryos with DORV and with misalignment of the arteries were more prevalent following right versus left cardiac crest ablation. In addition, survival of right-ablated embryos was lower than left-ablated embryos. Together, these data raise the intriguing possibility that there may be differences in left versus right cardiac neural crest during heart development.

A Rare Case of Polysplenia Syndrome Associated with Severe Cardiac Malformations and Congenital Alveolar Dysplasia in a One-Month-Old Infant: A Complete Macroscopic and Histopathologic Study

Polysplenia syndrome represents a type of left atrial isomerism characterized by multiple small spleens, often associated with cardiac malformations and with situs ambiguus of the abdominal organs. The case presented is of a one-month-old male infant, weighing approximately 3000 g, born at the County Clinical Emergency Hospital of Sibiu, who was hospitalized from birth until death. The patient suffered cardio-respiratory arrest due to severe hypoxia and septicemia on the background of a series of complex cardiac malformations associated with congenital abdominal organ anomalies. Examination of the body revealed a common atrium with complete atrioventricular canal defect, left ventricular hypertrophy, right ventricle hypoplasia, truncus arteriosus, superior vena cava duplication, bilobation of the lungs, situs ambiguous of the abdominal organs with right-sided stomach, a midline liver, gall bladder agenesis, multiple right-sided spleens and complete inversion of the intestines and pancreas. Histopathology concluded that the patient suffered cardiac lesions consistent with infantile lactic acidosis, as well as pulmonary modifications suggesting congenital alveolar dysplasia and altered hepatic architecture compatible with fibrosis.

Haemodynamic dependence of mechano-genetic evolution of the cardiovascular system in Japanese medaka

The progression of cardiac gene expression-wall shear stress (WSS) interplay is critical to identifying developmental defects during cardiovascular morphogenesis. However, mechano-genetics from the embryonic to larval stages are poorly understood in vertebrates. We quantified peak WSS in the heart and tail vessels of Japanese medaka from 3 days post fertilization (dpf) to 14 dpf using in vivo micro-particle image velocimetry flow measurements, and in parallel analysed the expression of five cardiac genes (fgf8, hoxb6b, bmp4, nkx2.5, smyd1). Here, we report that WSS in the atrioventricular canal (AVC), ventricular outflow tract (OFT), and the caudal vessels in medaka peak with inflection points at 6 dpf and 10-11 dpf instead of a monotonic trend. Retrograde flows are captured at the AVC and OFT of the medaka heart for the first time. In addition, all genes were upregulated at 3 dpf and 7 dpf, indicating a possible correlation between the two, with the cardiac gene upregulation preceding WSS increase in order to facilitate cardiac wall remodelling.

Fetal Blood Flow and Genetic Mutations in Conotruncal Congenital Heart Disease

In congenital heart disease, the presence of structural defects affects blood flow in the heart and circulation. However, because the fetal circulation bypasses the lungs, fetuses with cyanotic heart defects can survive in utero but need prompt intervention to survive after birth. Tetralogy of Fallot and persistent truncus arteriosus are two of the most significant conotruncal heart defects. In both defects, blood access to the lungs is restricted or non-existent, and babies with these critical conditions need intervention right after birth. While there are known genetic mutations that lead to these critical heart defects, early perturbations in blood flow can independently lead to critical heart defects. In this paper, we start by comparing the fetal circulation with the neonatal and adult circulation, and reviewing how altered fetal blood flow can be used as a diagnostic tool to plan interventions. We then look at known factors that lead to tetralogy of Fallot and persistent truncus arteriosus: namely early perturbations in blood flow and mutations within VEGF-related pathways. The interplay between physical and genetic factors means that any one alteration can cause significant disruptions during development and underscore our need to better understand the effects of both blood flow and flow-responsive genes.

Transcriptomic uniqueness and commonality of the ion channels and transporters in the four heart chambers

Myocardium transcriptomes of left and right atria and ventricles from four adult male C57Bl/6j mice were profiled with Agilent microarrays to identify the differences responsible for the distinct functional roles of the four heart chambers. Female mice were not investigated owing to their transcriptome dependence on the estrous cycle phase. Out of the quantified 16,886 unigenes, 15.76% on the left side and 16.5% on the right side exhibited differential expression between the atrium and the ventricle, while 5.8% of genes were differently expressed between the two atria and only 1.2% between the two ventricles. The study revealed also chamber differences in gene expression control and coordination. We analyzed ion channels and transporters, and genes within the cardiac muscle contraction, oxidative phosphorylation, glycolysis/gluconeogenesis, calcium and adrenergic signaling pathways. Interestingly, while expression of Ank2 oscillates in phase with all 27 quantified binding partners in the left ventricle, the percentage of in-phase oscillating partners of Ank2 is 15% and 37% in the left and right atria and 74% in the right ventricle. The analysis indicated high interventricular synchrony of the ion channels expressions and the substantially lower synchrony between the two atria and between the atrium and the ventricle from the same side.

Validating the Paradigm That Biomechanical Forces Regulate Embryonic Cardiovascular Morphogenesis and Are Fundamental in the Etiology of Congenital Heart Disease

The goal of this review is to provide a broad overview of the biomechanical maturation and regulation of vertebrate cardiovascular (CV) morphogenesis and the evidence for mechanistic relationships between function and form relevant to the origins of congenital heart disease (CHD). The embryonic heart has been investigated for over a century, initially focusing on the chick embryo due to the opportunity to isolate and investigate myocardial electromechanical maturation, the ability to directly instrument and measure normal cardiac function, intervene to alter ventricular loading conditions, and then investigate changes in functional and structural maturation to deduce mechanism. The paradigm of "Develop and validate quantitative techniques, describe normal, perturb the system, describe abnormal, then deduce mechanisms" was taught to many young investigators by Dr. Edward B. Clark and then validated by a rapidly expanding number of teams dedicated to investigate CV morphogenesis, structure-function relationships, and pathogenic mechanisms of CHD. Pioneering studies using the chick embryo model rapidly expanded into a broad range of model systems, particularly the mouse and zebrafish, to investigate the interdependent genetic and biomechanical regulation of CV morphogenesis. Several central morphogenic themes have emerged. First, CV morphogenesis is inherently dependent upon the biomechanical forces that influence cell and tissue growth and remodeling. Second, embryonic CV systems dynamically adapt to changes in biomechanical loading conditions similar to mature systems. Third, biomechanical loading conditions dynamically impact and are regulated by genetic morphogenic systems. Fourth, advanced imaging techniques coupled with computational modeling provide novel insights to validate regulatory mechanisms. Finally, insights regarding the genetic and biomechanical regulation of CV morphogenesis and adaptation are relevant to current regenerative strategies for patients with CHD.

Microstructure of early embryonic aortic arch and its reversibility following mechanically altered hemodynamic load release

In the embryonic heart, blood flow is distributed through a bilaterally paired artery system composed of the aortic arches (AAs). The purpose of this study is to establish an understanding of the governing mechanism of microstructural maturation of the AA matrix and its reversibility, toward the desired macroscopic vessel lumen diameter and thickness for healthy, abnormal, and in ovo repaired abnormal mechanical loading. While matrix-remodeling mechanisms were significantly different for normal versus conotruncal banding (CTB), both led to an increase in vessel lumen. Correlated with right-sided flow increase at Hamburger & Hamilton stages 21, intermittent load switching between collagen I and III with elastin and collagen-IV defines the normal process. However, decreases in collagen I, elastin, vascular endothelial growth factor-A, and fibrillin-1 in CTB were recovered almost fully following the CTB-release model, primarily because of the pressure load changes. The complex temporal changes in matrix proteins are illustrated through a predictive finite-element model based on elastin and collagen load-sharing mechanism to achieve lumen area increase and thickness increase resulting from wall shear stress and tissue strain, respectively. The effect of embryonic timing in cardiac interventions on AA microstructure was established where abnormal mechanical loading was selectively restored at the key stage of development. Recovery of the normal mechanical loading via early fetal intervention resulted in delayed microstructural maturation. Temporal elastin increase, correlated with wall shear stress, is required for continuous lumen area growth.NEW & NOTEWORTHY The present study undertakes comparative analyses of the mechanistic differences of the arterial matrix microstructure and dynamics in the three fundamental processes of control, conotruncal banded, and released conotruncal band in avian embryo. Among other findings, this study provides specific evidence on the restorative role of elastin during the early lumen growth process. During vascular development, a novel intermittent load-switching mechanism between elastin and collagen, triggered by a step increase in wall shear stress, governs the chronic vessel lumen cross-sectional area increase. Mimicking the fetal cardiovascular interventions currently performed in humans, the early release of the abnormal mechanical load rescues the arterial microstructure with time lag.

Mechanotransduction in cardiovascular morphogenesis and tissue engineering

Cardiovascular morphogenesis involves cell behavior and cell identity changes that are activated by mechanical forces associated with heart function. Recently, advances in in vivo imaging, methods to alter blood flow, and computational modelling have greatly advanced our understanding of how forces produced by heart contraction and blood flow impact different morphogenetic processes. Meanwhile, traditional genetic approaches have helped to elucidate how endothelial cells respond to forces at the cellular and molecular level. Here we discuss the principles of endothelial mechanosensitity and their interplay with cellular processes during cardiovascular morphogenesis. We then discuss their implications in the field of cardiovascular tissue engineering.