Cardiac Looping Research Articles

Aryl organophosphate flame retardants induced cardiotoxicity during zebrafish embryogenesis: By disturbing expression of the transcriptional regulators

As a result of the ban on some brominated flame retardants (BFRs), the use of organophosphate flame retardants (OPFRs) increases, and they are detected in multi-environment media at higher frequency and concentrations. However, the toxicity data of OPFRs, especially those on developmental toxicology are quite limited, which prevents an accurate evaluation of their environmental and health risk. Because a previous study reported that two aryl-OPFRs induced cardiotoxicity during zebrafish embryogenesis, we designed experiments to compare the heart developmental toxicity of a series of aryl-OPFRs with alkyl-OPFRs and explored possible internal mechanism. First, acute toxicity of 9 frequently used OPFRs were studied with zebrafish embryos (2–96hpf). By comparing the LC50 and EC50 (pericardium edema) data, two aryl-OPFRs, triphenyl phosphate (TPhP) and cresyl diphenyl phosphate (CDP) showed greater heart developmental toxicity than the others. It was also found that the acute toxicity of OPFRs varied mainly depending on their hydrophobicity. Further study on the cardiotoxicty of TPhP and CDP showed that the cardiac looping progress can be impeded by 0.10mg/L TPhP or CDP exposure. Bradycardia and reduction of myocardium were also observed in 0.50 and 1.0mg/L TPhP groups and 0.10, 0.50, and 1.0mg/L CDP groups. 0–48hpf is the vulnerable window of zebrafish cardiogenesis that can be easily affected by TPhP and CDP. RT-qPCR measurement on the expressions of key transcriptional regulators in cardiogenesis showed that BMP4, NKX2–5, and TBX5 were significantly inhibited at the exposure points of 12hpf and 24hpf which may be the internal factors related to the heart developmental toxicity. As zebrafish is a good model organism for human health study, the present results call for a greater attention to the health risk of fetus in pregnant women exposed to such OPFRs.

Hemodynamic forces regulate developmental patterning of atrial conduction.

Anomalous action potential conduction through the atrial chambers of the heart can lead to severe cardiac arrhythmia. To date, however, little is known regarding the mechanisms that pattern proper atrial conduction during development. Here we demonstrate that atrial muscle functionally diversifies into at least two heterogeneous subtypes, thin-walled myocardium and rapidly conducting muscle bundles, during a developmental window just following cardiac looping. During this process, atrial muscle bundles become enriched for the fast conduction markers Cx40 and Nav1.5, similar to the precursors of the fast conduction Purkinje fiber network located within the trabeculae of the ventricles. In contrast to the ventricular trabeculae, however, atrial muscle bundles display an increased proliferation rate when compared to the surrounding myocardium. Interestingly, mechanical loading of the embryonic atrial muscle resulted in an induction of Cx40, Nav1.5 and the cell cycle marker Cyclin D1, while decreasing atrial pressure via in vivo ligation of the vitelline blood vessels results in decreased atrial conduction velocity. Taken together, these data establish a novel model for atrial conduction patterning, whereby hemodynamic stretch coordinately induces proliferation and fast conduction marker expression, which in turn promotes the formation of large diameter muscle bundles to serve as preferential routes of conduction.

Abstract 12438: Novel 4-Dimensional Optical Technique to Elucidate Hemodynamic Shear Forces and Initiation of Trabeculation During Cardiac Morphogenesis

Hemodynamic shear forces are intimately linked with cardiac development. However, the mechano-transduction mechanisms underlying this morphological event remain elusive. Trabeculation develops after cardiac looping, which is visualized in zebrafish embryos starting at 70 hours post-fertilization (hpf). We hypothesized that endocardial wall shear stress (WSS) up-regulates Notch signaling to initiate trabeculation. We developed a novel fast z-axis imaging acquisition system to reconstruct time-dependent 3-dimensional WSS in live zebrafish using single plane illuminating microscopy (SPIM). To assess the link between shear stress and trabeculation, we lowered the WSS by micro-injection of (1) gata1a-morpholino oligonucleotides (MO) at 2-cell stage to reduce erythropoiesis and viscosity by 70%, (2) tnnt2a-MO injection to inhibit cardiac contraction, and (3) use of weak atrium(wea) mutant to inhibit atrial contraction. To determine mechano-signal transduction underlying WSS and trabeculation, we assessed Notch signaling by in situ hybridization of NRG1 and notch1b. At 70 hpf, the control fish showed formation of trabeculation, whereas gata1 MO injected fish revealed a decrease in trabeculation in response to a reduction in shear stress by 90 %. Both wea mutant and tnnt2a MO injected fish exhibited smooth endocardial surfaces without trabecular networking. In situ hybridization of NRG1 and Notch 1b further showed a decrease in fluorescent intensity in the gata1 MO, wea mutant, and tnnt2a MO groups by 2.33±0.87-fold, 4.21±0.25-fold, and 4.96±0.17-fold, respectively (p < 0.05 vs. control fish, n=10). Thus, our findings indicate that hemodynamic shear forces are implicated in the initiation of cardiac trabecualtion via Notch signaling pathway, with a translational implication to understanding congenital noncompacted cardiomyopathy.

Orphan G-Protein Coupled Receptor 22 (Gpr22) Regulates Cilia Length and Structure in the Zebrafish Kupffer’s Vesicle

GPR22 is an orphan G protein-coupled receptor (GPCR). Since the ligand of the receptor is currently unknown, its biological function has not been investigated in depth. Many GPCRs and their intracellular effectors are targeted to cilia. Cilia are highly conserved eukaryotic microtubule-based organelles that protrude from the membrane of most mammalian cells. They are involved in a large variety of physiological processes and diseases. However, the details of the downstream pathways and mechanisms that maintain cilia length and structure are poorly understood. We show that morpholino knock down or overexpression of gpr22 led to defective left-right (LR) axis formation in the zebrafish embryo. Specifically, defective LR patterning included randomization of the left-specific lateral plate mesodermal genes (LPM) (lefty1, lefty2, southpaw and pitx2a), resulting in randomized cardiac looping. Furthermore, gpr22 inactivation in the Kupffer’s vesicle (KV) alone was still able to generate the phenotype, indicating that Gpr22 mainly regulates LR asymmetry through the KV. Analysis of the KV cilia by immunofluorescence and transmission electron microscopy (TEM), revealed that gpr22 knock down or overexpression resulted in changes of cilia length and structure. Further, we found that Gpr22 does not act upstream of the two cilia master regulators, Foxj1a and Rfx2. To conclude, our study characterized a novel player in the field of ciliogenesis.

Toxic effects of polychlorinated biphenyls on cardiac development in zebrafish.

Polychlorinated biphenyls (PCBs) are ubiquitous environmental pollutants that may pose significant health-risks to various organisms including humans. Although the mixed PCB Aroclor 1254 is widespread in the environment, its potential toxic effect on heart development and the mechanism underlying its developmental toxicity have not been previously studied. Here, we used the zebrafish as a toxicogenomic model to examine the effects of Aroclor 1254 on heart development. We found that PCB exposure during zebrafish development induced heart abnormalities including pericardial edema and cardiac looping defects. Further malformations of the zebrafish embryo were observed and death of the larvae occurred in a time- and dose-dependent manner. Our mechanistic studies revealed that abnormalities in the arylhydrocarbon receptor, Wnt and retinoic acid signaling pathways may underlie the effects of PCBs on zebrafish heart development. Interestingly, co-administration of Aroclor 1254 and diethylaminobenzaldehyde, an inhibitor of retinaldehyde dehydrogenase, partially rescued the toxic effects of PCBs on zebrafish heart development. In conclusion, PCBs can induce developmental defects in the zebrafish heart, which may be mediated by abnormal RA signaling.

Bending and twisting the embryonic heart: a computational model for c-looping based on realistic geometry.

The morphogenetic process of cardiac looping transforms the straight heart tube into a curved tube that resembles the shape of the future four-chambered heart. Although great progress has been made in identifying the molecular and genetic factors involved in looping, the physical mechanisms that drive this process have remained poorly understood. Recent work, however, has shed new light on this complicated problem. After briefly reviewing the current state of knowledge, we propose a relatively comprehensive hypothesis for the mechanics of the first phase of looping, termed c-looping, as the straight heart tube deforms into a c-shaped tube. According to this hypothesis, differential hypertrophic growth in the myocardium supplies the main forces that cause the heart tube to bend ventrally, while regional growth and cytoskeletal contraction in the omphalomesenteric veins (primitive atria) and compressive loads exerted by the splanchnopleuric membrane drive rightward torsion. A computational model based on realistic embryonic heart geometry is used to test the physical plausibility of this hypothesis. The behavior of the model is in reasonable agreement with available experimental data from control and perturbed embryos, offering support for our hypothesis. The results also suggest, however, that several other mechanisms contribute secondarily to normal looping, and we speculate that these mechanisms play backup roles when looping is perturbed. Finally, some outstanding questions are discussed for future study.

P316Expression and functional role of Ccdc80 in developing heart and in cardiomyopathies.

Purpose: The gene Coiled Coil Domain Containing 80 (Ccdc80) is highly expressed in the hearts of vertebrates and its role is well defined in tumor suppression, glucose homeostasis and adipocyte differentiation. Our aims are to define its functional role during embryonic development and in cardiomyopathies. Methods: We used 3 different models: 1. Morpholino gene knock-down in Zebrafish to characterize Ccdc80 role in embryonic development; 2. Immunohistochemistry on heart ventricles from monocrotaline (MCT) injected rats (6 decompensated vs 6 normal animals); and 3. on human four chambers hearts from patients affected by dilated cardiomyopathy (DCM) (14 samples vs 2 controls from donor hearts which were not transplanted). Results: In Zebrafish Ccdc80 was widely expressed in the forming heart, during all the developmental stages and Ccdc80-morphants showed defects in the developing heart, with impaired cardiac looping, atrium enlargement, blood stasis and peripheral congestion (heart failure). These phenotypical alterations were due to a disorder of the late phase of cardiac development, after myocytes differentiation. MCT injected rats showed evident cytosplasmic overexpression of Ccd80 protein in hypertrophic right ventricles (high pressure overload site) in comparison to controls. Human samples from DCM patients showed cytosplamic overexpression of Ccdc80 protein particularly evident at the level of atria, where the staining pattern from mild and patchy in controls became strong and homogeneous in diseased hearts. Conclusions: Our results demonstrated that Ccdc80 has an important role for correct heart development in a model of vertebrates. In adult hearts under stress and/or pressure overload, Ccdc80 showed increased expression as a likely adaptive role to be further investigated.

Cardiac looping may be driven by compressive loads resulting from unequal growth of the heart and pericardial cavity. Observations on a physical simulation model.

The transformation of the straight embryonic heart tube into a helically wound loop is named cardiac looping. Such looping is regarded as an essential process in cardiac morphogenesis since it brings the building blocks of the developing heart into an approximation of their definitive topographical relationships. During the past two decades, a large number of genes have been identified which play important roles in cardiac looping. However, how genetic information is physically translated into the dynamic form changes of the looping heart is still poorly understood. The oldest hypothesis of cardiac looping mechanics attributes the form changes of the heart loop (ventral bending → simple helical coiling → complex helical coiling) to compressive loads resulting from growth differences between the heart and the pericardial cavity. In the present study, we have tested the physical plausibility of this hypothesis, which we call the growth-induced buckling hypothesis, for the first time. Using a physical simulation model, we show that growth-induced buckling of a straight elastic rod within the confined space of a hemispherical cavity can generate the same sequence of form changes as observed in the looping embryonic heart. Our simulation experiments have furthermore shown that, under bilaterally symmetric conditions, growth-induced buckling generates left- and right-handed helices (D-/L-loops) in a 1:1 ratio, while even subtle left- or rightward displacements of the caudal end of the elastic rod at the pre-buckling state are sufficient to direct the buckling process toward the generation of only D- or L-loops, respectively. Our data are discussed with respect to observations made in biological “models.” We conclude that compressive loads resulting from unequal growth of the heart and pericardial cavity play important roles in cardiac looping. Asymmetric positioning of the venous heart pole may direct these forces toward a biased generation of D- or L-loops.

The atypical Rho GTPase, RhoU, regulates cell-adhesion molecules during cardiac morphogenesis

The vertebrate heart undergoes early complex morphologic events in order to develop key cardiac structures that regulate its overall function (Fahed et al., 2013). Although many genetic factors that participate in patterning the heart have been elucidated (Tu and Chi, 2012), the cellular events that drive cardiac morphogenesis have been less clear. From a chemical genetic screen to identify cellular pathways that control cardiac morphogenesis in zebrafish, we observed that inhibition of the Rho signaling pathways resulted in failure to form the atrioventricular canal and loop the linear heart tube. To identify specific Rho proteins that may regulate this process, we analyzed cardiac expression profiling data and discovered that RhoU was expressed at the atrioventricular canal during the time when it forms. Loss of RhoU function recapitulated the atrioventricular canal and cardiac looping defects observed in the ROCK inhibitor treated zebrafish. Similar to its family member RhoV/Chp (Tay et al., 2010), we discovered that RhoU regulates the cell junctions between cardiomyocytes through the Arhgef7b/Pak kinase pathway in order to guide atrioventricular canal development and cardiac looping. Inhibition of this pathway resulted in similar underlying cardiac defects and conversely, overexpression of a PAK kinase was able to rescue the loss of RhoU cardiac defect. Finally, we found that Wnt signaling, which has been implicated in atrioventricular canal development (Verhoeven et al., 2011), may regulate the expression of RhoU at the atrioventricular canal. Overall, these findings reveal a cardiac developmental pathway involving RhoU/Arhgef7b/Pak signaling, which helps coordinate cell junction formation between atrioventricular cardiomyocytes to promote cell adhesiveness and cell shapes during cardiac morphogenesis. Failure to properly form these cell adhesions during cardiac development may lead to structural heart defects and mechanistically account for the cellular events that occur in certain human congenital heart diseases.

Functional modelling of a novel mutation in BBS5

BackgroundBardet-Biedl syndrome (BBS) is an autosomal recessive ciliopathy disorder with 18 known causative genes (BBS1-18). The primary clinical features are renal abnormalities, rod-cone dystrophy, post-axial polydactyly, learning difficulties, obesity and male hypogonadism.ResultsWe describe the clinical phenotype in three Saudi siblings in whom we have identified a novel mutation in exon 12 of BBS5 (c.966dupT; p.Ala323CysfsX57). This single nucleotide duplication creates a frame shift results in a predicted elongated peptide. Translation blocking Morpholino oligonucleotides were used to create zebrafish bbs5 morphants. Morphants displayed retinal layering defects, abnormal cardiac looping and dilated, cystic pronephric ducts with reduced cilia expression. Morphants also displayed significantly reduced dextran clearance via the pronephros compared to wildtype embryos, suggesting reduced renal function in morphants. The eye, kidney and heart defects reported in morphant zebrafish resemble the human phenotype of BBS5 mutations. The pathogenicity of the novel BBS5 mutation was determined. Mutant mRNA was unable to rescue pleiotropic phenotypes of bbs5 morphant zebrafish and in cell culture we demonstrate a mislocalisation of mutant BBS5 protein which fails to localise discretely with the basal body.ConclusionsWe conclude that this novel BBS5 mutation has a deleterious function that accounts for the multisystem ciliopathy phenotype seen in affected human patients.

Autophagy is essential for cardiac morphogenesis during vertebrate development

Genetic analyses indicate that autophagy, an evolutionarily conserved lysosomal degradation pathway, is essential for eukaryotic differentiation and development. However, little is known about whether autophagy contributes to morphogenesis during embryogenesis. To address this question, we examined the role of autophagy in the early development of zebrafish, a model organism for studying vertebrate tissue and organ morphogenesis. Using zebrafish that transgenically express the fluorescent autophagy reporter protein, GFP-LC3, we found that autophagy is active in multiple tissues, including the heart, during the embryonic period. Inhibition of autophagy by morpholino knockdown of essential autophagy genes (including atg5, atg7, and becn1) resulted in defects in morphogenesis, increased numbers of dead cells, abnormal heart structure, and reduced organismal survival. Further analyses of cardiac development in autophagy-deficient zebrafish revealed defects in cardiac looping, abnormal chamber morphology, aberrant valve development, and ectopic expression of critical transcription factors including foxn4, tbx5, and tbx2. Consistent with these results, Atg5-deficient mice displayed abnormal Tbx2 expression and defects in valve development and chamber septation. Thus, autophagy plays an essential, conserved role in cardiac morphogenesis during vertebrate development.

Overexpression of miR-19b Impairs Cardiac Development in Zebrafish by Targeting ctnnb1

Background: MicroRNAs are broadly accepted as crucial regulators of cardiovascular development, and dysregulation of their expression has been linked to cardiac disease. MicroRNA cluster miR-17-92 has been implicated in cardiac development and function, yet its defined mechanisms of action in this context are uncertain. Here, we focused on miR-19b, a key component of the miR-17-92 cluster proven to induce cardiomyocyte proliferation in vitro. We aimed to identify the biological significance of miR-19b in cardiac development and its underlying molecular mechanism of action in vivo. Methods: We micro-injected zebrafish embryos with different concentrations (0, 2, 4 and 8 μm) of miR-19b mimics or a negative control, and assessed the embryo malformation rate, mortality rate, hatching rate and heart abnormalities at 72 hours post-fertilization (72 hpf). Results: We found that overexpression of miR-19b impacted left-right symmetry and cardiac development of zebrafish embryos, characterized by pericardial edema, slower heart rate and cardiac looping defects in a dose-dependent manner. Moreover, several important signaling molecules in the Wnt signaling pathway were abnormally expressed, suggesting that overexpression of miR-19b induces the inhibition of the Wnt signaling pathway by directly targeting ctnnb1. Interestingly, the deformed cardiac phenotype was partially rescued by treatment with the GSK3β inhibitor lithium chloride. Conclusion: Our findings suggest that miR-19b regulates laterality development and heart looping in zebrafish embryos by targeting ctnnb1.

The Effect of Paclobutrazol on the Development of Zebrafish (Danio Rerio) Embryos

Paclobutrazol (PBZ), a trazole-containing fungicide, is widely used on food crops. Frequent usage of PBZ may contaminate water, but its toxicity to aquatic organisms is understudied. Although the chronic effects of PBZ exposure on reproductive, antioxidant defense, and liver metabolism systems in rockfish have been reported, the toxic effects of PBZ on aquatic embryos are unknown. Here, we report that PBZ disrupts the development of heart and craniofacial cartilage in zebrafish embryos, and decreases their survival and hatching rates. PBZ affects the normal process of cardiac looping, which may lead to a slower heart beat accompanied by pericardia edema and apoptotic myocytes. PBZ also decreases the population of migratory neural crest cells, which give rise to craniofacial cartilage. Our results reveal high embryonic toxicity of PBZ on aquatic organisms, and thus hold significance for the impact of fungicides on public health and ecology.

Systems proteomics of cardiac chromatin identifies nucleolin as a regulator of growth and cellular plasticity in cardiomyocytes

Myocyte hypertrophy antecedent to heart failure involves changes in global gene expression, although the preceding mechanisms to coordinate DNA accessibility on a genomic scale are unknown. Chromatin-associated proteins alter chromatin structure by changing their association with DNA, thereby altering the gene expression profile. Little is known about the global changes in chromatin subproteomes that accompany heart failure, and the mechanisms by which these proteins alter chromatin structure. The present study tests the fundamental hypothesis that cardiac growth and plasticity in the setting of disease recapitulates conserved developmental chromatin remodeling events. We used quantitative proteomics to identify chromatin-associated proteins extracted via detergent and to quantify changes in their abundance during disease. Our study identified 321 proteins in this subproteome, demonstrating it to have modest conservation (37%) with that revealed using strong acid. Of these proteins, 176 exhibited altered expression during cardiac hypertrophy and failure; we conducted extensive functional characterization of one of these proteins, Nucleolin. Morpholino-based knockdown of nucleolin nearly abolished protein expression but surprisingly had little impact on gross morphological development. However, hearts of fish lacking Nucleolin displayed severe developmental impairment, abnormal chamber patterning and functional deficits, ostensibly due to defects in cardiac looping and myocyte differentiation. The mechanisms underlying these defects involve perturbed bone morphogenetic protein 4 expression, decreased rRNA transcription, and a shift to more heterochromatic chromatin. This study reports the quantitative analysis of a new chromatin subproteome in the normal and diseased mouse heart. Validation studies in the complementary model system of zebrafish examine the role of Nucleolin to orchestrate genomic reprogramming events shared between development and disease.

High-Content Screening Assay for Identification of Chemicals Impacting Cardiovascular Function in Zebrafish Embryos

Targeted assays are needed to better evaluate effects of chemicals on organogenesis and begin classification of chemicals by toxicologically relevant modes-of-action. Using transgenic zebrafish (fli1:egfp) that stably express eGFP within vascular endothelial cells, we have developed and optimized a 384-well-based high-content screening (HCS) assay that enables us to screen and identify chemicals affecting cardiovascular function at sublethal, nonteratogenic concentrations. Following static exposure of one embryo per well from 5 to 72 h postfertilization (hpf), automated image acquisition procedures and custom image analysis protocols are used to quantify body length, circulation, heart rate, pericardial area (a biomarker for cardiac looping defects), and intersegmental vessel area within freshly hatched live embryos. After optimizing 72 hpf anesthetization procedures, we evaluated each end point across four independent control plates containing 384 initial embryos per plate. Survival and imaging success rates across these plates ranged from 93 to 99% and 42 to 74%, respectively. Criteria were then defined for assay success and analysis of treatments, and 10 chemicals were screened for targeted effects on cardiovascular function. Compared to existing zebrafish-based assays, this method provides a comprehensive discovery platform with (1) increased sample sizes; (2) broad concentration-response format; and (3) the ability to identify chemicals that target cardiovascular function at nonteratogenic concentrations.

Moving Domain Computational Fluid Dynamics to Interface with an Embryonic Model of Cardiac Morphogenesis

Peristaltic contraction of the embryonic heart tube produces time- and spatial-varying wall shear stress (WSS) and pressure gradients (∇P) across the atrioventricular (AV) canal. Zebrafish (Danio rerio) are a genetically tractable system to investigate cardiac morphogenesis. The use of Tg(fli1a:EGFP)y1 transgenic embryos allowed for delineation and two-dimensional reconstruction of the endocardium. This time-varying wall motion was then prescribed in a two-dimensional moving domain computational fluid dynamics (CFD) model, providing new insights into spatial and temporal variations in WSS and ∇P during cardiac development. The CFD simulations were validated with particle image velocimetry (PIV) across the atrioventricular (AV) canal, revealing an increase in both velocities and heart rates, but a decrease in the duration of atrial systole from early to later stages. At 20-30 hours post fertilization (hpf), simulation results revealed bidirectional WSS across the AV canal in the heart tube in response to peristaltic motion of the wall. At 40-50 hpf, the tube structure undergoes cardiac looping, accompanied by a nearly 3-fold increase in WSS magnitude. At 110-120 hpf, distinct AV valve, atrium, ventricle, and bulbus arteriosus form, accompanied by incremental increases in both WSS magnitude and ∇P, but a decrease in bi-directional flow. Laminar flow develops across the AV canal at 20-30 hpf, and persists at 110-120 hpf. Reynolds numbers at the AV canal increase from 0.07±0.03 at 20-30 hpf to 0.23±0.07 at 110-120 hpf (p< 0.05, n=6), whereas Womersley numbers remain relatively unchanged from 0.11 to 0.13. Our moving domain simulations highlights hemodynamic changes in relation to cardiac morphogenesis; thereby, providing a 2-D quantitative approach to complement imaging analysis.

Ventricular septal defects in mice targeted deletion of versican in vascular smooth muscle cells

Purpose: To determine the function of versican (Vcan) in the cardiovascular system in vivo, we generated the versican conditional knockout mice, in which the Vcan gene was selectively ablated in vascular smooth muscle cells (SMCs). Versican is a large secreted chondroitin sulfate proteoglycan, which is important for cardiac development, and is also implicated in adult cardiovascular disease. Four splice variants of versican (V0, V1, V2, V3) have been identified, each containing two invariant domains, an N-terminal G1 domain that interact with hyaluronan, and a C-terminal G3 domain that interact with other extracellular matrix (ECM) molecules. As it has been reported that versican knockout mice died of cardiac defects at E10.5, the role of versican in cardiac development and disease has not been completely elucidated. Methods: SM22-Cre+:Vcanflox/flox conditional knockout mice were generated by combining Cre/loxP with a Flp/FRT system. In brief, a targeting vector harboring the Vcan flox allele was constructed by franking exon2 including start codon with loxP sites in the combination of a PGK-neoR cassette flanked by the FRT sequence. After obtaining Vcanflox/flox mice, they crossbred with SM22-Cre Tg mice, and the Vcan gene was selectively ablated in SMCs. The expression of different cardiac markers and ECM molecules were analyzed using histological and biochemical methods. Results: SM22-Cre+:Vcanflox/flox mice were viable and fertile, and they can reach their lifespan. The hearts in SM22-Cre+:Vcanflox/flox newborn mice showed normal cardiac looping; however, exhibited dilated heart and ventricular septal defects (VSD). The expressions of the a-actinin in their ventricular wall, which is a marker of cardiac muscle cell, decreased compared to wild type. Furthermore, though their arterial walls were thinner than wild type, elastic lamina was formed. Conclusions: Previously we generated the versican mutant mice (VcanΔ3/Δ3) whose versican lacked the A-subdomain of the G1 domain. The VcanΔ3/Δ3 mice died at neonatal period day 0, exhibiting the dilated heart and VSD. In this study, the targeted deletion of versican in SMCs leads to moderate cardiovascular defects with the dilated heart and VSD. SMCs transduce a variety of signals from a highly structured ECM that modulate cellular differentiation, migration, and proliferation. Our results suggest that the versican expression from SMCs is important for the cardiovascular development, and this study can lead to the treatment for congenital cardiovascular disease.

Grk5l Controls Heart Development by Limiting mTOR Signaling during Symmetry Breaking

The correct asymmetric placement of inner organs is termed situs solitus and is determined early during development. Failure in symmetry breaking results in conditions ranging from randomized organ arrangement to a complete mirror image, often accompanied by severe congenital heart defects (CHDs). We found that the zebrafish homolog of mammalian G protein-coupled receptor kinase 5 (GRK5) employs noncanonical, receptor-independent functions to secure symmetry breaking. Knockdown of GRK5's closest homolog in zebrafish embryos, Grk5l, is sufficient to randomize cardiac looping and left-right asymmetry. Mechanistically, we found that loss of GRK5 increases mammalian target of rapamycin complex 1 (mTORC1) activity. This causes elongation of motile cilia in the organ of laterality, a consequence that is known to be sufficient to trigger aberrant organ arrangement. By fine-tuning mTORC1, GRK5 thus serves an unanticipated function during early development, besides its well-characterized role in the adult heart. These findings could implicate GRK5 as a susceptibility allele for certain cases of CHD.

On the role of intrinsic and extrinsic forces in early cardiac S‐looping

Looping is a crucial phase during heart development when the initially straight heart tube is transformed into a shape that more closely resembles the mature heart. Although the genetic and biochemical pathways of cardiac looping have been well studied, the biophysical mechanisms that actually effect the looping process remain poorly understood. Using a combined experimental (chick embryo) and computational (finite element modeling) approach, we study the forces driving early s-looping when the primitive ventricle moves to its definitive position inferior to the common atrium. New results from our study indicate that the primitive heart has no intrinsic ability to form an s-loop and that extrinsic forces are necessary to effect early s-looping. They support previous studies that established an important role for cervical flexure in causing early cardiac s-looping. Our results also show that forces applied by the splanchnopleure cannot be ignored during early s-looping and shed light on the role of cardiac jelly. Using available experimental data and computer modeling, we successfully developed and tested a hypothesis for the force mechanisms driving s-loop formation. Forces external to the primitive heart tube are necessary in the later stages of cardiac looping. Experimental and model results support our proposed hypothesis for forces driving early s-looping.

A dual role for Nucleolin identified by systems analysis of cardiac chromatin remodelers

To elucidate mechanisms of chromatin reprogramming during cardiac development and disease, we used a systems approach to identify, screen, and validate changes to the nuclear proteome. Quantitative proteomic analysis of chromatin fractions from mouse hearts in basal, hypertrophy or failing conditions identified 338 proteins whose abundances change. One candidate was Nucleolin (Ncl), which increases 3‐fold in hypertrophy.Using a functional screen in zebrafish embryos, we characterized a novel role for Ncl to affect cardiac looping and differentiation. 48‐hour embryos express Ncl in the heart, with in situ hybridization demonstrating BMP4 restricted to the atrial/ventricular boundary. After Morpholino knockdown (KD) of Ncl, BMP4 is found throughout the ventricle, suggesting delayed myocyte differentiation. This is accompanied by defects in chamber looping, with 20% of fish failing to loop or reverse looping, a phenotype seen in 29% of fish after over expression; no changes in myocyte size were observed in KD fish. Delayed hearts exhibit a decreased heart rate and increased fractional shortening. Finally, in isolated rat myocytes, loss of Ncl alters transcription of SERCA, ANF, and β‐MHC, whose expression changes are hallmarks of cardiac hypertrophy. This suggests that changes in Ncl levels affect differentiation during development, whereas in disease it may drive the hypertrophic response. NIH funded.