The Interventricular Septum Is Biomechanically Distinct from the Ventricular Free Walls

The interventricular septum contributes to the pumping function of both ventricles. However, unlike the ventricular wall, its mechanical behavior remains largely unknown. To fill the knowledge gap, this study aims to characterize the biaxial and transmural variation of the mechanical properties of the septum and compare it to the free walls of the left and right ventricles (LV/RV). Fresh hearts were obtained from healthy, adult sheep. The septal wall was sliced along the mid-line into two septal sides and compared to the epicardial layers of the LV- and RV-free walls. Biaxial tensile mechanical tests and constitutive modeling were performed to obtain the passive mechanical properties of the LV- and RV-side of the septum and ventricular walls. We found that both sides of the septum were significantly softer than the respective ventricular walls, and that the septum presented significantly less collagen than the ventricular walls. At low strains, we observed the symmetric distribution of the fiber orientations and a similar anisotropic behavior between the LV-side and RV-side of the septum, with a stiffer material property in the longitudinal direction, rather than the circumferential direction. At high strains, both sides showed isotropic behavior. Both septal sides had similar intrinsic elasticity, as evidenced by experimental data and constitutive modeling. These new findings offer important knowledge of the biomechanics of the septum wall, which may deepen the understanding of heart physiology.

ERBB3-dependent AKT and ERK pathways are essential for atrioventricular cushion development in mouse embryos

ERBB family members and their ligands play an essential role in embryonic heart development and adult heart physiology. Among them, ERBB3 is a binding partner of ERBB2; the ERBB2/3 complex mediates downstream signaling for cell proliferation. ERBB3 has seven consensus binding sites to the p85 regulatory subunit of PI3K, which activates the downstream AKT pathway, leading to the proliferation of various cells. This study generated a human ERBB3 knock-in mouse expressing a mutant ERBB3 whose seven YXXM p85 binding sites were replaced with YXXA. Erbb3 knock-in embryos exhibited lethality between E12.5 to E13.5, and showed a decrease in mesenchymal cell numbers and density in AV cushions. We determined that the proliferation of mesenchymal cells in the atrioventricular (AV) cushion in Erbb3 knock-in mutant embryos was temporarily reduced due to the decrease of AKT and ERK1/2 phosphorylation. Overall, our results suggest that AKT/ERK activation by the ERBB3-dependent PI3K signaling is crucial for AV cushion morphogenesis during embryonic heart development.

Abstract P329: Exosomes Derived From Podoplanin Positive Cells Alter The Cellular Composition Of Healthy Mouse Hearts

Fibrosis and blood hypoperfusion stimulated by paracrine signals enhances the ventricular dysfunctionafter myocardial infarction (MI). We have earlier reported that within 2 days post-MI a cohort ofpodoplanin (PDPN) positive cells populate injured heart and enhance inflammatory response by physicalinteractions with monocytes. Here we explored whether exosomes from these cells could independentlyalter healthy heart physiology and structure. PDPN+ cells were isolated 2 days after MI, culture expandedand activated with TNFα and Angiotensin II. Exosomes derived from activated PDPN+cells conditionedmedia (PDPN+exo) were used in vitro for the treatment of mouse cardiac endothelial cells (mCECs) andmouse fibroblast (3T3) and in vivo for the treatment of healthy mouse hearts. In vitro, PDPN+exoinfluenced the phenotype of mCECs, stimulating their lineage into lymphatic endothelial cells andfacilitated fibroblasts transition to myofibroblast. Characterization of the protein content of PDPN+exoshowed high concentration of Notch receptors and γ-Secretase, suggesting these cellular transitions maydepend on exosome-mediated Notch translocation and cleavage. In fact, after exosomes treatmentcleaved notch (NICD) translocated in the nuclei of mCECs and 3T3 as early as 1h of treatment and eitherHes-1 or Hey-1, major transcription factors activated by NICD were enhanced within 2d of treatment.Using DAPT, a γSecretase inhibitor, notch cleavage was inhibited, and no phenotype switching in responseto exosome treatment was observed. In vivo, PDPN+exo were injected into the left ventricle of healthymouse hearts followed by boosters delivered by retro-orbital vein injection. Treated mice developed anextended epicardial fibrosis with a subsequent impairment in the contractility and increase of the enddiastolic and systolic volumes. The fibrotic area was characterized by vessels double positive toendothelial and lymphatic endothelial markers, and infiltrating CD45+ cells. Podoplanin positive cellsrepresent 80% of the scar’s cells of a chronic infarcted myocardium and the specific exosomes cargo highlyinfluence the lineage of cardiac cells altering the biology of endothelial cells and fibroblasts which mayfacilitate adverse remodeling.

Embryonic Onset of Sexually Dimorphic Heart Rates in the Viviparous Fish, Gambusia holbrooki

In fish, little is known about sex-specific differences in physiology and performance of the heart and whether these differences manifest during development. Here for the first time, the sex-specific heart rates during embryogenesis of Gambusia holbrooki, from the onset of the heart rates (HRs) to just prior to parturition, was investigated using light cardiogram. The genetic sex of the embryos was post-verified using a sex-specific genetic marker. Results reveal that heart rates and resting time significantly increase (p < 0.05) with progressive embryonic development. Furthermore, both ventricular and atrial frequencies of female embryos were significantly higher (p < 0.05) than those of their male sibs at the corresponding developmental stages and remained so at all later developmental stages (p < 0.05). In concurrence, the heart rate and ventricular size of the adult females were also significantly (p < 0.05) higher and larger respectively than those of males. Collectively, the results suggest that the cardiac sex-dimorphism manifests as early as late-organogenesis and persists through adulthood in this species. These findings suggest that the cardiac measurements can be employed to non-invasively sex the developing embryos, well in advance of when their phenotypic sex is discernible. In addition, G. holbrooki could serve as a better model to study comparative vertebrate cardiovascular development as well as to investigate anthropogenic and climatic impacts on heart physiology of this species, that may be sex influenced.

Impact of Maternal Food Restriction on Heart Proteome in Appropriately Grown and Growth-Restricted Wistar—Rat Offspring

Objective: Fetal growth restriction is associated with increased postnatal cardiovascular morbidity. The alterations in heart physiology and structure caused by in utero nutrient deprivation have not been extensively studied. We aim to investigate the impact of maternal food restriction on the cardiac proteome of newborn rats with normal (non-fetal growth-restricted (FGR)) and reduced (FGR) birth weight. Methods: On day 14 of gestation, 10 timed pregnant rats were randomized into two nutritional groups: (a) Standard laboratory diet and (b) 50% global food restriction. Pups born to food-restricted mothers were subdivided, based on birthweight, into fetal growth-restricted (FGR) and non-FGR, while pups born from normally nourished mothers were considered controls. Rat neonates were euthanized immediately after birth and the hearts of 11 randomly selected male offspring (n = 4 FGR, n = 4 non-FGR, n = 3 control group) were analyzed using quantitative proteomics. Results: In total, 7422 proteins were quantified (q < 0.05). Of these, 1175 were differentially expressed in FGR and 231 in non-FGR offspring vs. control with 151 common differentially expressed proteins (DEPs) between the two groups. Bioinformatics analysis of DEPs in FGR vs. control showed decreased integrin and apelin cardiac fibroblast signaling, decreased muscle contraction and glycolysis, and over-representation of a protein network related to embryonic development, and cell death and survival. Conclusion: Our study illustrates the distinct proteomic profile of FGR and non-FGR offspring of food-restricted dams underlying the importance of both prenatal adversities and birth weight in cardiac physiology and development.

The G4 resolvase RHAU modulates mRNA translation and stability to sustain postnatal heart function and regeneration

Post-transcriptional regulation of mRNA translation and stability is primarily achieved by RNA binding proteins (RBPs), which is of increasing importance for heart function. Furthermore, G-quadruplex (G4) and G4 resolvase activity are involved in a variety of biological processes. However, the role of G4 resolvase activity in heart function remains unknown. The present study aims to investigate the role of RHAU, an RBP with G4 resolvase activity in postnatal heart function through deletion of Rhau in the cardiomyocytes of postnatal mice. RHAU-deficient mice displayed progressive pathological remodeling leading to heart failure and mortality, and impaired neonatal heart regeneration. RHAU ablation reduced the protein levels but enhanced mRNA levels of Yap1 and Hexim1 that are important regulators for heart development and postnatal heart function. Furthermore, RHAU was found to associate with both the 5’- and 3’- UTRs of these genes to destabilize mRNA but to enhance translation. Thus, we have demonstrated the important functions of RHAU in the dual regulation of mRNA translation and stability, which is vital for heart physiology.

The right-sided heart failure

The right-sided heart failure (RSHF) is a complex clinical syndrome including different pathogenic mechanisms and processes resulted from the right ventricle (RV) dysfunction and manifested with signs of heart failure (HF). Recently, there is a growing scientific interest in the right-sided acute and chronic heart abnormalities; this is due to growing knowledge in this field and development of novel diagnostic, therapeutic and pharmacological approaches to treatment of pulmonary hypertension that is a common cause of RSHF. Cardiac embryogenesis, anatomic particularities, difference and interdependence of RV and the left ventricle (LV) are described in the article in order to improve the knowledge on structure and function of both the right heart and the left heart. Discussion on pathophysiology, causes and clinical manifestations of acute RSHF (aRSHF) and chronic RSHF (cRSHF) should consider the right heart physiology. Pharmacological treatment should be targeted to ventricle pre-load, myocardial contractility and RV post-load, correction of pulmonary circulation and LV volume resulting in post-load reduction and improvement in the LV function. Patients with biventricular dysfunction should be treated according to current clinical guidelines on therapy of chronic HF. Vasoactive agents and diuretics have an important role for the treatment of RSHF as this is the basic therapy of pulmonary congestion both in aRSHF and cRSHF. Step-by-step therapeutic algorithm is given in the article.