Klf2 is an essential regulator of vascular hemodynamic forces in vivo

The force within: endocardial development, mechanotransduction and signalling during cardiac morphogenesis

Endocardial cells are cardiac endothelial cells that line the interior of the heart tube. Historically, their contribution to cardiac development has mainly been considered from a morphological perspective. However, recent studies have begun to define novel instructive roles of the endocardium, as a sensor and signal transducer of biophysical forces induced by blood flow, and as an angiocrine signalling centre that is involved in myocardial cellular morphogenesis, regeneration and reprogramming. In this Review, we discuss how the endocardium develops, how endocardial-myocardial interactions influence the developing embryonic heart, and how the dysregulation of blood flow-responsive endocardial signalling can result in pathophysiological changes.

Reduced Krüppel-Like Factor 2 Aggravates Glomerular Endothelial Cell Injury and Kidney Disease in Mice with Unilateral Nephrectomy

Loss of functional nephrons induces compensatory glomerular hyperfiltration and hypertrophy, leading to the progression of chronic kidney disease. Krüppel-like factor 2 (KLF2), a shear-stress-inducible transcription factor, confers protection against endothelial injury. Because glomerular hyperfiltration is associated with shear stress, we hypothesized that KLF2 may be an important factor in the compensatory response to unilateral nephrectomy (UNX). To test this hypothesis, endothelial cell-specific Klf2 heterozygous knockout mice (KO) and their wild-type littermate control (WT) underwent either UNX or sham-operation. WT-UNX mice developed compensatory renal hypertrophy as expected, whereas KO-UNX mice did not. KO-UNX mice exhibited higher blood pressure, reduced glomerular filtration rate, and significant increase in proteinuria and glomerulosclerosis compared to WT-UNX. Expression of endothelial nitric oxide synthase (official name Nos3), a known transcriptional target gene of KLF2, was significantly reduced and dysregulation of other endothelial genes was also observed in the glomeruli of KO-UNX when compared to WT-UNX and sham-operated mice. Furthermore, both podocyte number and expression of podocyte markers were also significantly reduced in KO-UNX glomeruli, indicating a potential cross talk between glomerular endothelial cells and podocytes. Finally, decreased renal expression of KLF2 in nephrectomy patients was associated with the progression of kidney disease. Taken together, our data demonstrate a protective role of KLF2 against glomerular endothelial cell injury and progression of chronic kidney disease in the model of compensatory renal hypertrophy.

Cerebral cavernous malformations arise from endothelial gain of MEKK3-KLF2/4 signalling

Cerebral cavernous malformations (CCMs) are common inherited and sporadic vascular malformations that cause stroke and seizures in younger individuals¹. CCMs arise from endothelial cell loss of KRIT1, CCM2, or PDCD10, non-homologous proteins that form an adaptor complex². How disruption of the CCM complex results in disease remains controversial, with numerous signaling pathways (including Rho^3,4, SMAD⁵ and Wnt/β-catenin⁶) and processes such as endothelial-mesenchymal transition (EndMT)⁵ proposed to play causal roles. CCM2 binds MEKK3^7–11, and we have recently demonstrated that CCM complex regulation of MEKK3 is essential during vertebrate heart development¹². Here, we investigate this mechanism in CCM disease pathogenesis. Using a neonatal mouse model of CCM disease, we find that expression of the MEKK3 target genes KLF2 and KLF4, as well as Rho and ADAMTS protease activity, are increased in the endothelial cells of early CCM lesions. In contrast, we find no evidence of EndMT or increased SMAD or Wnt signaling during early CCM formation. Endothelial-specific loss of Mekk3, Klf2, or Klf4 dramatically prevents lesion formation, reverses the increase in Rho activity, and rescues lethality. Consistent with these findings in mice, we demonstrate that endothelial expression of KLF2 and KLF4 is elevated in human familial and sporadic CCM lesions, and that a disease-causing human CCM2 mutation abrogates MEKK3 interaction without affecting CCM complex formation. These studies identify gain of MEKK3 signaling and KLF2/4 function as causal mechanisms for CCM pathogenesis that may be targeted to develop new CCM therapeutics.

Characterization of Hemodynamics in Great Arteries of Wild-Type Mouse Using Computational Fluid Dynamics Based on Ultrasound Images

Hemodynamic factors in cardiovascular system are hypothesized to play a significant role in causing structural heart development. It is thus important to improve our understanding of velocity characteristics and parameters. We present such a study on wild-type mouse to characterize the vessel geometry, flow pattern, and wall shear stress in great arteries. Microultrasound imaging for small animals was used to measure blood boundary and velocity of the great arteries. Subsequently, specimens' flow boundary conditions were used for 3-dimensional reconstructions of the great artery and aortic arch dimensions, and blood flow velocity data were input into subject-specific computational fluid dynamics for modeling hemodynamics. Measurement by microultrasound imaging showed that blood velocities in the great artery and aortic arch had strong correlations with vascular sizes, whereas blood pressure had a weak trend in relation to vascular size. Wall shear stress magnitude increased when closer to arterial branches and reduced proximally in the aortic root and distally in the descending aorta, and the parameters were related to the fluid mechanics in branches in some degree. We developed a method to investigate fluid mechanics in mouse arteries, using a combination of microultrasound and computational fluid dynamics, and demonstrated its ability to reveal detailed geometric, kinematic, and fluid mechanics parameters.

Interplay between cardiac function and heart development

Mechanotransduction refers to the conversion of mechanical forces into biochemical or electrical signals that initiate structural and functional remodeling in cells and tissues. The heart is a kinetic organ whose form changes considerably during development and disease. This requires cardiomyocytes to be mechanically durable and able to mount coordinated responses to a variety of environmental signals on different time scales, including cardiac pressure loading and electrical and hemodynamic forces. During physiological growth, myocytes, endocardial and epicardial cells have to adaptively remodel to these mechanical forces. Here we review some of the recent advances in the understanding of how mechanical forces influence cardiac development, with a focus on fluid flow forces. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

The expression dynamics of mechanosensitive genes in extra-embryonic vasculature after heart starts to beat in chick embryo

BACKGROUND

Fluid flow plays an important role in vascular development. However, the detailed mechanisms, particularly the link between flow and modulation of gene expression during vascular development, remain unexplored. In chick embryo, the key events of vascular development from initiation of heart beat to establishment of effective blood flow occur between the stages HH10 and HH13. Therefore, we propose a novel in vivo model to study the flow experienced by developing endothelium.

OBJECTIVE

Using this model, we aimed to capture the transcriptome dynamics of the pre- and post-flow conditions.

METHODS

RNA was isolated from extra embryonic area vasculosa (EE-AV) pooled from three chick embryos between HH10-HH13 and RNA sequencing was performed.

RESULTS

The whole transcriptome sequencing of chick identified up-regulation of some of the previously well-known mechanosensitive genes including NFR2, HAND1, CTGF and KDR. GO analyses of the up-regulated genes revealed enrichment of several biological processes including heart development, extracellular matrix organization, cell-matrix adhesion, cell migration, blood vessel development, patterning of blood vessels, collagen fibril organization. Genes encoding for gap junctions proteins which are involved in vascular remodeling and arterial-venous differentiation, and genes involved in cell-cell adhesion, and ECM interactions were significantly up-regulated. Validation of selected genes through semi quantitative PCR was performed.

CONCLUSION

The study indicates that shear stress plays a major role in development. Through appropriate validation, this platform can serve as an in vivo model to study conditions of disturbed flow in pathology as well as normal flow during development.

Importance of Endogenous Atrial and Brain Natriuretic Peptides in Murine Embryonic Vascular and Organ Development

Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) bind to the receptor guanylyl cyclase (GC)-A, leading to diuresis, natriuresis, and blood vessel dilation. In addition, ANP and BNP have various angiogenic properties in ischemic tissue. When breeding mice devoid of GC-A, we noted significant skewing of the Mendelian ratio in the offspring, suggesting embryonic lethality due to knockout of GC-A. Consequently, we here investigated the roles of endogenous ANP and BNP in embryonic neovascularization and organ morphogenesis. Embryos resulting from GC-A(-/-) × GC-A(+/-) crosses developed hydrops fetalis (HF) beginning at embryonic day (E)14.5. All embryos with HF had the genotype GC-A(-/-). At E17.5, 33.3% (12 of 36) of GC-A(-/-) embryos had HF, and all GC-A(-/-) embryos with HF were dead. Beginning at E16.0, HF-GC-A(-/-) embryos demonstrated poorly developed superficial vascular vessels and sc hemorrhage, the fetal side of the placenta appeared ischemic, and vitelline vessels on the yolk sac were poorly developed. Furthermore, HF-GC-A(-/-) embryos also showed abnormal constriction of umbilical cord vascular vessels, few cardiac trabeculae and a thin compact zone, hepatic hemorrhage, and poor bone development. Electron microscopy of E16.5 HF-GC-A(-/-) embryos revealed severe vacuolar degeneration in endothelial cells, and the expected 3-layer structure of the smooth muscle wall of the umbilical artery was indistinct. These data demonstrate the importance of the endogenous ANP/BNP-GC-A system not only in the neovascularization of ischemic tissues but also in embryonic vascular development and organ morphogenesis.

K. Rothman A, Elworthy S, van Eeden FJM, Chico TJA. klf2ash317 Mutant Zebrafish Do Not Recapitulate Morpholino-Induced Vascular and Haematopoietic Phenotypes

Introduction and Objectives

The zinc-finger transcription factor Krϋppel-like factor 2 (KLF2) transduces blood flow into molecular signals responsible for a wide range of responses within the vasculature. KLF2 maintains a healthy, quiescent endothelial phenotype. Previous studies report a range of phenotypes following morpholino antisense oligonucleotide-induced klf2a knockdown in zebrafish. Targeted genome editing is an increasingly applied method for functional assessment of candidate genes. We therefore generated a stable klf2a mutant zebrafish and characterised its cardiovascular and haematopoietic development.

Methods and Results

Using Transcription Activator-Like Effector Nucleases (TALEN) we generated a klf2a mutant (klf2a^sh317) with a 14bp deletion leading to a premature stop codon in exon 2. Western blotting confirmed loss of wild type Klf2a protein and the presence of a truncated protein in klf2a^sh317 mutants. Homozygous klf2a^sh317 mutants exhibit no defects in vascular patterning, survive to adulthood and are fertile, without displaying previously described morphant phenotypes such as high-output cardiac failure, reduced haematopoetic stem cell (HSC) development or impaired formation of the 5^th accessory aortic arch. Homozygous klf2a^sh317 mutation did not reduce angiogenesis in zebrafish with homozygous mutations in von Hippel Lindau (vhl), a form of angiogenesis that is dependent on blood flow. We examined expression of three klf family members in wildtype and klf2a^sh317 zebrafish. We detected vascular expression of klf2b (but not klf4a or biklf/klf4b/klf17) in wildtypes but found no differences in expression that might account for the lack of phenotype in klf2a^sh317 mutants. klf2b morpholino knockdown did not affect heart rate or impair formation of the 5^th accessory aortic arch in either wildtypes or klf2a^sh317 mutants.

Conclusions

The klf2a^sh317 mutation produces a truncated Klf2a protein but, unlike morpholino induced klf2a knockdown, does not affect cardiovascular development.

A Decade of Discovery in the Genetic Understanding of Thoracic Aortic Disease

Aortic aneurysms are responsible for a significant number of all deaths in Western countries. In this review we provide a perspective on the important progress made over the past decade in the understanding of the genetics of this condition, with an emphasis on the more frequent forms of vascular smooth muscle and transforming growth factor β (TGF-β) signalling alterations. For several nonsyndromic and syndromic forms of thoracic aortic disease, a genetic basis has now been identified, with 3 main pathomechanisms that have emerged: perturbation of the TGF-β signalling pathway, disruption of the vascular smooth muscle cell (VSMC) contractile apparatus, and impairment of extracellular matrix synthesis. Because smooth muscle cells and proteins of the extracellular matrix directly regulate TGF-β signalling, this latter pathway emerges as a key component of thoracic aortic disease initiation and progression. These discoveries have revolutionized our understanding of thoracic aortic disease and provided inroads toward gene-specific stratification of treatment. Last, we outline how these genetic findings are translated into novel pharmaceutical approaches for thoracic aortic disease.

Lymph flow regulates collecting lymphatic vessel maturation in vivo

Fluid shear forces have established roles in blood vascular development and function, but whether such forces similarly influence the low-flow lymphatic system is unknown. It has been difficult to test the contribution of fluid forces in vivo because mechanical or genetic perturbations that alter flow often have direct effects on vessel growth. Here, we investigated the functional role of flow in lymphatic vessel development using mice deficient for the platelet-specific receptor C-type lectin-like receptor 2 (CLEC2) as blood backfills the lymphatic network and blocks lymph flow in these animals. CLEC2-deficient animals exhibited normal growth of the primary mesenteric lymphatic plexus but failed to form valves in these vessels or remodel them into a structured, hierarchical network. Smooth muscle cell coverage (SMC coverage) of CLEC2-deficient lymphatic vessels was both premature and excessive, a phenotype identical to that observed with loss of the lymphatic endothelial transcription factor FOXC2. In vitro evaluation of lymphatic endothelial cells (LECs) revealed that low, reversing shear stress is sufficient to induce expression of genes required for lymphatic valve development and identified GATA2 as an upstream transcriptional regulator of FOXC2 and the lymphatic valve genetic program. These studies reveal that lymph flow initiates and regulates many of the key steps in collecting lymphatic vessel maturation and development.

An allelic series of miR-17 ∼ 92-mutant mice uncovers functional specialization and cooperation among members of a microRNA polycistron

Polycistronic microRNA clusters are a common feature of vertebrate genomes. The coordinated expression of miRNAs belonging to different seed families from a single transcription unit suggests functional cooperation, but this hypothesis has not been experimentally tested. Here we report the characterization of an allelic series of genetically engineered mice harboring selective targeted deletions of individual components of miR-17~92. Our results demonstrate the co-existence of functional cooperation and specialization among members of this cluster, identify a novel function for the miR-17 seed family in controlling axial patterning in vertebrates, and show that loss of miR-19 selectively impairs Myc-driven tumorigenesis in two models of human cancer. By integrating phenotypic analysis and gene expression profiling, we provide a genome-wide view of how components of a polycistronic miRNA-cluster affect gene expression in vivo. The reagents and datasets reported here will accelerate exploration of the complex biological functions of this important miRNA cluster.

Genome-wide analysis of the zebrafish Klf family identifies two genes important for erythroid maturation

Krüppel-like transcription factors (Klfs), each of which contains a CACCC-box binding domain, have been investigated in a variety of developmental processes, such as angiogenesis, neurogenesis and somatic-cell reprogramming. However, the function and molecular mechanism by which the Klf family acts during developmental hematopoiesis remain elusive. Here, we report identification of 24 Klf family genes in zebrafish using bioinformatics. Gene expression profiling shows that 6 of these genes are expressed in blood and/or vascular endothelial cells during embryogenesis. Loss of function of 2 factors (klf3 or klf6a) leads to a decreased number of mature erythrocytes. Molecular studies indicate that both Klf3 and Klf6a are essential for erythroid cell differentiation and maturation but that these two proteins function in distinct manners. We find that Klf3 inhibits the expression of ferric-chelate reductase 1b (frrs1b), thereby promoting the maturation of erythroid cells, whereas Klf6a controls the erythroid cell cycle by negatively regulating cdkn1a expression to determine the rate of red blood cell proliferation. Taken together, our study provides a global view of the Klf family members that contribute to hematopoiesis in zebrafish and sheds new light on the function and molecular mechanism by which Klf3 and Klf6a act during erythropoiesis in vertebrates.

Vascular smooth muscle cell in atherosclerosis

Vascular smooth muscle cells (VSMCs) exhibit phenotypic and functional plasticity in order to respond to vascular injury. In case of the vessel damage, VSMCs are able to switch from the quiescent 'contractile' phenotype to the 'proinflammatory' phenotype. This change is accompanied by decrease in expression of smooth muscle (SM)-specific markers responsible for SM contraction and production of proinflammatory mediators that modulate induction of proliferation and chemotaxis. Indeed, activated VSMCs could efficiently proliferate and migrate contributing to the vascular wall repair. However, in chronic inflammation that occurs in atherosclerosis, arterial VSMCs become aberrantly regulated and this leads to increased VSMC dedifferentiation and extracellular matrix formation in plaque areas. Proatherosclerotic switch in VSMC phenotype is a complex and multistep mechanism that may be induced by a variety of proinflammatory stimuli and hemodynamic alterations. Disturbances in hemodynamic forces could initiate the proinflammatory switch in VSMC phenotype even in pre-clinical stages of atherosclerosis. Proinflammatory signals play a crucial role in further dedifferentiation of VSMCs in affected vessels and propagation of pathological vascular remodelling.

The cerebral cavernous malformation pathway controls cardiac development via regulation of endocardial MEKK3 signaling and KLF expression

The cerebral cavernous malformation (CCM) pathway is required in endothelial cells for normal cardiovascular development and to prevent postnatal vascular malformations, but its molecular effectors are not well defined. Here we show that loss of CCM signaling in endocardial cells results in mid-gestation heart failure associated with premature degradation of cardiac jelly. CCM deficiency dramatically alters endocardial and endothelial gene expression, including increased expression of the Klf2 and Klf4 transcription factors and the Adamts4 and Adamts5 proteases that degrade cardiac jelly. These changes in gene expression result from increased activity of MEKK3, a mitogen-activated protein kinase that binds CCM2 in endothelial cells. MEKK3 is both necessary and sufficient for expression of these genes, and partial loss of MEKK3 rescues cardiac defects in CCM-deficient embryos. These findings reveal a molecular mechanism by which CCM signaling controls endothelial gene expression during cardiovascular development that may also underlie CCM formation.

Blood flow mechanics in cardiovascular development

Hemodynamic forces are fundamental to development. Indeed, much of cardiovascular morphogenesis reflects a two-way interaction between mechanical forces and the gene network activated in endothelial cells via mechanotransduction feedback loops. As these interactions are becoming better understood in different model organisms, it is possible to identify common mechanogenetic rules, which are strikingly conserved and shared in many tissues and species. Here, we discuss recent findings showing how hemodynamic forces potentially modulate cardiovascular development as well as the underlying fluid and tissue mechanics, with special attention given to the flow characteristics that are unique to the small scales of embryos.

ICOS maintains the T follicular helper cell phenotype by down-regulating Krüppel-like factor 2

ICOS signaling is required for inhibition of the transcription factor Klf2, which controls expression of genes expressed by follicular T helper (Tfh) cells. When ICOS signaling is blocked, Tfh cells lose expression of characteristic Tfh genes and revert to an effector phenotype, resulting in disruption of the germinal center response.

The co-stimulators ICOS (inducible T cell co-stimulator) and CD28 are both important for T follicular helper (TFH) cells, yet their individual contributions are unclear. Here, we show that each molecule plays an exclusive role at different stages of TFH cell development. While CD28 regulated early expression of the master transcription factor Bcl-6, ICOS co-stimulation was essential to maintain the phenotype by regulating the novel TFH transcription factor Klf2 via Foxo1. Klf2 directly binds to Cxcr5, Ccr7, Psgl-1, and S1pr1, and low levels of Klf2 were essential to maintain this typical TFH homing receptor pattern. Blocking ICOS resulted in relocation of fully developed TFH cells back to the T cell zone and reversion of their phenotype to non-TFH effector cells, which ultimately resulted in breakdown of the germinal center response. Our study describes for the first time the exclusive role of ICOS and its downstream signaling in the maintenance of TFH cells by controlling their anatomical localization in the B cell follicle.

Shear stress modulates endothelial KLF2 through activation of P2X4

Vascular endothelial cells that are in direct contact with blood flow are exposed to fluid shear stress and regulate vascular homeostasis. Studies report endothelial cells to release ATP in response to shear stress that in turn modulates cellular functions via P2 receptors with P2X4 mediating shear stress-induced calcium signaling and vasodilation. A recent study shows that a loss-of-function polymorphism in the human P2X4 resulting in a Tyr315>Cys variant is associated with increased pulse pressure and impaired endothelial vasodilation. Although the importance of shear stress-induced Krüppel-like factor 2 (KLF2) expression in atheroprotection is well studied, whether ATP regulates KLF2 remains unanswered and is the objective of this study. Using an in vitro model, we show that in human umbilical vein endothelial cells (HUVECs), apyrase decreased shear stress-induced KLF2, KLF4, and NOS3 expression but not that of NFE2L2. Exposure of HUVECs either to shear stress or ATPγS under static conditions increased KLF2 in a P2X4-dependent manner as was evident with both the receptor antagonist and siRNA knockdown. Furthermore, transient transfection of static cultures of human endothelial cells with the Tyr315>Cys mutant P2X4 construct blocked ATP-induced KLF2 expression. Also, P2X4 mediated the shear stress-induced phosphorylation of extracellular regulated kinase-5, a known regulator of KLF2. This study demonstrates a major physiological finding that the shear-induced effects on endothelial KLF2 axis are in part dependent on ATP release and P2X4, a previously unidentified mechanism.

Downregulation of Kruppel-like factor 2 is associated with poor prognosis for nonsmall-cell lung cancer

Kruppel-like factor 2 (KLF2) expression is diminished in many malignancies. However, its expression and role in nonsmall-cell lung cancer (NSCLC) remain unknown. In this study, we found that KLF2 levels were decreased in NSCLC tissues compared with adjacent normal tissues. Its expression level was significantly correlated with TNM stages, tumor size, and lymph node metastasis. Moreover, patients with low levels of KLF2 expression had a relatively poor prognosis. Furthermore, knockdown of KLF2 expression by siRNA could promote cell proliferation, while ectopic expression of KLF2 inhibited cell proliferation and promoted apoptosis in NSCLC cells partly via regulating CDKN1A/p21 and CDKN2B/p15 protein expression. Our findings present that decreased KLF2 could be identified as a poor prognostic biomarker in NSCLC and regulate cell proliferation and apoptosis.

Blood flow and Bmp signaling control endocardial chamber morphogenesis

During heart development, the onset of heartbeat and blood flow coincides with a ballooning of the cardiac chambers. Here, we have used the zebrafish as a vertebrate model to characterize chamber ballooning morphogenesis of the endocardium, a specialized population of endothelial cells that line the interior of the heart. By combining functional manipulations, fate mapping studies, and high-resolution imaging, we show that endocardial growth occurs without an influx of external cells. Instead, endocardial cell proliferation is regulated, both by blood flow and by Bmp signaling, in a manner independent of vascular endothelial growth factor (VEGF) signaling. Similar to myocardial cells, endocardial cells obtain distinct chamber-specific and inner- versus outer-curvature-specific surface area sizes. We find that the hemodynamic-sensitive transcription factor Klf2a is involved in regulating endocardial cell morphology. These findings establish the endocardium as the flow-sensitive tissue in the heart with a key role in adapting chamber growth in response to the mechanical stimulus of blood flow.

The hemodynamically-regulated vascular microenvironment promotes migration of the steroidogenic tissue during its interaction with chromaffin cells in the zebrafish embryo

Background

While the endothelium-organ interaction is critical for regulating cellular behaviors during development and disease, the role of blood flow in these processes is only partially understood. The dorsal aorta performs paracrine functions for the timely migration and differentiation of the sympatho-adrenal system. However, it is unclear how the adrenal cortex and medulla achieve and maintain specific integration and whether hemodynamic forces play a role.

Methodology and Principal Findings

In this study, the possible modulation of steroidogenic and chromaffin cell integration by blood flow was investigated in the teleostean counterpart of the adrenal gland, the interrenal gland, in the zebrafish (Danio rerio). Steroidogenic tissue migration and angiogenesis were suppressed by genetic or pharmacologic inhibition of blood flow, and enhanced by acceleration of blood flow upon norepinephrine treatment. Repressed steroidogenic tissue migration and angiogenesis due to flow deficiency were recoverable following restoration of flow. The regulation of interrenal morphogenesis by blood flow was found to be mediated through the vascular microenvironment and the Fibronectin-phosphorylated Focal Adhesion Kinase (Fn-pFak) signaling. Moreover, the knockdown of krüppel-like factor 2a (klf2a) or matrix metalloproteinase 2 (mmp2), two genes regulated by the hemodynamic force, phenocopied the defects in migration, angiogenesis, the vascular microenvironment, and pFak signaling of the steroidogenic tissue observed in flow-deficient embryos, indicating a direct requirement of mechanotransduction in these processes. Interestingly, epithelial-type steroidogenic cells assumed a mesenchymal-like character and downregulated β-Catenin at cell-cell junctions during interaction with chromaffin cells, which was reversed by inhibiting blood flow or Fn-pFak signaling. Blood flow obstruction also affected the migration of chromaffin cells, but not through mechanosensitive or Fn-pFak dependent mechanisms.

Conclusions and Significance

These results demonstrate that hemodynamically regulated Fn-pFak signaling promotes the migration of steroidogenic cells, ensuring their interaction with chromaffin cells along both sides of the midline during interrenal gland development.

Reduced Krüppel-like factor 2 expression may aggravate the endothelial injury of diabetic nephropathy

Kruppel-like Factor 2 (KLF2), a shear-stress inducible transcription factor, has endoprotective effects. In streptozotocin-induced diabetic rats, we found that glomerular Klf2 expression was reduced in comparison to non-diabetic rats. However, normalization of hyperglycemia by insulin treatment increased Klf2 expression to a level higher than that of non-diabetic rats. Consistent with this, we found that Klf2 expression was suppressed by high glucose but increased by insulin in cultured endothelial cells. To determine the role of KLF2 in streptozotocin-induced diabetic nephropathy, we used endothelial cell-specific Klf2 heterozygous knockout mice and found that diabetic knockout mice developed more kidney/glomerular hypertrophy and proteinuria than diabetic wide type mice. Glomerular expression of Vegfa, Flk1, and angiopoietin 2 increased but expression of Flt1, Tie2, and angiopoietin 1 decreased in diabetic knockout compared to diabetic wide type mice. Glomerular expression of ZO-1, glycocalyx, and eNOS was also decreased in diabetic knockout compared to diabetic wide type mice. These data suggest knockdown of Klf2 expression in the endothelial cells induced more endothelial cell injury. Interestingly, podocyte injury was also more prominent in diabetic knockout compared to diabetic wide type mice, indicating a crosstalk between these two cell types. Thus, KLF2 may play a role in glomerular endothelial cell injury in early diabetic nephropathy.

Connecting the coronaries: how the coronary plexus develops and is functionalized

The establishment of the coronary circulation is one of the final critical steps during heart development. Despite decades of research, our understanding of how the coronary vasculature develops and connects to the aorta remains limited. This review serves two specific purposes: it addresses recent advances in understanding the origin of the coronary endothelium, and it then focuses on the last crucial step of coronary vasculature development, the connection of the coronary plexus to the aorta. The chick and quail animal models have yielded most of the information for how these connections form, starting with a fine network of vessels that penetrate the aorta and coalesce to form two distinct ostia. Studies in mouse and rat confirm that at least some of these steps are conserved in mammals, but gaps still exist in our understanding of mammalian coronary ostia formation. The signaling cues necessary to guide the coronary plexus to the aorta are also incompletely understood. Hypoxia-inducible transcription factor-1 and its downstream targets are among the few identified genes that promote the formation of the coronary stems. Together, this review summarizes our current knowledge of coronary vascular formation and highlights the significant gaps that remain. In addition, it highlights some of the coronary artery anomalies known to affect human health, demonstrating that even seemingly subtle defects arising from incorrect coronary plexus formation can result in significant health crises.

Krüppel-like transcription factors KLF1 and KLF2 have unique and coordinate roles in regulating embryonic erythroid precursor maturation

The Krüppel-like transcription factors KLF1 and KLF2 are essential for embryonic erythropoiesis. They can partially compensate for each other during mouse development, and coordinately regulate numerous erythroid genes, including the β-like globins. Simultaneous ablation of KLF1 and KLF2 results in earlier embryonic lethality and severe anemia. In this study, we determine that this anemia is caused by a paucity of blood cells, and exacerbated by diminished β-like globin gene expression. The anemia phenotype is dose-dependent, and, interestingly, can be ameliorated by a single copy of the KLF2, but not the KLF1 gene. The roles of KLF1 and KLF2 in maintaining normal peripheral blood cell numbers and globin mRNA amounts are erythroid cell-specific. Mechanistic studies led to the discovery that KLF2 has an essential function in erythroid precursor maintenance. KLF1 can partially compensate for KLF2 in this role, but is uniquely crucial for erythroid precursor proliferation through its regulation of G1- to S-phase cell cycle transition. A more drastic impairment of primitive erythroid colony formation from embryonic progenitor cells occurs with simultaneous loss of KLF1 and KLF2 than with loss of a single factor. KLF1 and KLF2 coordinately regulate several proliferation-associated genes, including Foxm1. Differential expression of FoxM1, in particular, correlates with the observed KLF1 and KLF2 gene dosage effects on anemia. Furthermore, KLF1 binds to the FoxM1 gene promoter in blood cells. Thus KLF1 and KLF2 coordinately regulate embryonic erythroid precursor maturation through the regulation of multiple homeostasis-associated genes, and KLF2 has a novel and essential role in this process.

Intracardiac flow dynamics regulate atrioventricular valve morphogenesis

AIMS

Valvular heart disease is responsible for considerable morbidity and mortality. Cardiac valves develop as the heart contracts, and they function throughout the lifetime of the organism to prevent retrograde blood flow. Their precise morphogenesis is crucial for cardiac function. Zebrafish is an ideal model to investigate cardiac valve development as it allows these studies to be carried out in vivo through non-invasive imaging. Accumulating evidence suggests a role for contractility and intracardiac flow dynamics in cardiac valve development. However, these two factors have proved difficult to uncouple, especially since altering myocardial function affects the intracardiac flow pattern.

METHODS AND RESULTS

Here, we describe novel zebrafish models of developmental valve defects. We identified two mutant alleles of myosin heavy chain 6 that can be raised to adulthood despite having only one functional chamber-the ventricle. The adult mutant ventricle undergoes remodelling, and the atrioventricular (AV) valves fail to form four cuspids. In parallel, we characterized a novel mutant allele of southpaw, a nodal-related gene involved in the establishment of left-right asymmetry, which exhibits randomized heart and endoderm positioning. We first observed that in southpaw mutants the relative position of the two cardiac chambers is altered, affecting the geometry of the heart, while myocardial function appears unaffected. Mutant hearts that loop properly or exhibit situs inversus develop normally, whereas midline, unlooped hearts exhibit defects in their transvalvular flow pattern during AV valve development as well as defects in valve morphogenesis.

CONCLUSION

Our data indicate that intracardiac flow dynamics regulate valve morphogenesis independently of myocardial contractility.

KLF2 is a rate-limiting transcription factor that can be targeted to enhance regulatory T-cell production

Regulatory T cells (Tregs) are a specialized subset of CD4(+) T cells that maintain self-tolerance by functionally suppressing autoreactive lymphocytes. The Treg compartment is composed of thymus-derived Tregs (tTregs) and peripheral Tregs (pTregs) that are generated in secondary lymphoid organs after exposure to antigen and specific cytokines, such as TGF-β. With regard to this latter lineage, pTregs [and their ex vivo generated counterparts, induced Tregs (iTregs)] offer particular therapeutic potential because these cells can be raised against specific antigens to limit autoimmunity. We now report that transcription factor Krüppel-like factor 2 (KLF2) is necessary for the generation of iTregs but not tTregs. Moreover, drugs that limit KLF2 proteolysis during T-cell activation enhance iTreg development. To the authors' knowledge, this study identifies the first transcription factor to distinguish between i/pTreg and tTreg ontogeny and demonstrates that KLF2 is a therapeutic target for the production of regulatory T cells.

KLF2--a negative regulator of pre-B cell clonal expansion and B cell activation

Maturation as well as antigen-dependent activation of B cells is accompanied by alternating phases of proliferation and quiescence. We and others have previously shown that Krüppel-like factor 2 (KLF2), a regulator of T cell quiescence and migration, is upregulated in small resting precursor (pre)-B cells after assembly of the immature pre-B cell receptor (pre-BCR) and is downregulated upon antigen-induced proliferation of mature B cells. These findings suggest that KLF2, besides its function in maintaining follicular B cell identity, peripheral B cell homeostasis and homing of antigen-specific plasma cells to the bone marrow, also controls clonal expansion phases in the B cell lineage. Here, we demonstrate that enforced expression of KLF2 in primary pre-B cells results in a severe block of pre-BCR-induced proliferation, upregulation of the cell cycle inhibitors p21 and p27 and downregulation of c-myc. Furthermore, retroviral KLF2 transduction of primary B cells impairs LPS-induced activation, favors apoptosis and results in reduced abundance of factors, such as AID, IRF4 and BLIMP1, that control the antigen-dependent phase of B cell activation and plasma cell differentiation. Hence, we conclude that KLF2 is not only a key player in terminating pre-B cell clonal expansion but also a potent suppressor of B cell activation.

Lymphatic function is required prenatally for lung inflation at birth

Neonatal mice lacking lymphatic vessels due to loss of lymphangiogenic factor CCBE1 or VEGFR3 function fail to inflate their lungs, suggestive of respiratory failure in infants with congenital pulmonary lymphangiectasia.

Mammals must inflate their lungs and breathe within minutes of birth to survive. A key regulator of neonatal lung inflation is pulmonary surfactant, a lipoprotein complex which increases lung compliance by reducing alveolar surface tension (Morgan, 1971). Whether other developmental processes also alter lung mechanics in preparation for birth is unknown. We identify prenatal lymphatic function as an unexpected requirement for neonatal lung inflation and respiration. Mice lacking lymphatic vessels, due either to loss of the lymphangiogenic factor CCBE1 or VEGFR3 function, appear cyanotic and die shortly after birth due to failure of lung inflation. Failure of lung inflation is not due to reduced surfactant levels or altered development of the lung but is associated with an elevated wet/dry ratio consistent with edema. Embryonic studies reveal active lymphatic function in the late gestation lung, and significantly reduced total lung compliance in late gestation embryos that lack lymphatics. These findings reveal that lymphatic vascular function plays a previously unrecognized mechanical role in the developing lung that prepares it for inflation at birth. They explain respiratory failure in infants with congenital pulmonary lymphangiectasia, and suggest that inadequate late gestation lymphatic function may also contribute to respiratory failure in premature infants.

Regulation of an inflammatory disease: Krüppel-like factors and atherosclerosis

This invited review summarizes work presented in the Russell Ross lecture delivered at the 2012 proceedings of the American Heart Association. We begin with a brief overview of the structural, cellular, and molecular biology of Krüppel-like factors. We then focus on discoveries during the past decade, implicating Krüppel-like factors as key determinants of vascular cell function in atherosclerotic vascular disease.

Echocardiographic assessment of embryonic and fetal mouse heart development: a focus on haemodynamics and morphology

Background. Heart development is a complex process, and abnormal development may result in congenital heart disease (CHD). Currently, studies on animal models mainly focus on cardiac morphology and the availability of hemodynamic data, especially of the right heart half, is limited. Here we aimed to assess the morphological and hemodynamic parameters of normal developing mouse embryos/fetuses by using a high-frequency ultrasound system. Methods. A timed breeding program was initiated with a WT mouse line (Swiss/129Sv background). All recordings were performed transabdominally, in isoflurane sedated pregnant mice, in hearts of sequential developmental stages: 12.5, 14.5, and 17.5 days after conception (n = 105). Results. Along development the heart rate increased significantly from 125 ± 9.5 to 219 ± 8.3 beats per minute. Reliable flow measurements could be performed across the developing mitral and tricuspid valves and outflow tract. M-mode measurements could be obtained of all cardiac compartments. An overall increase of cardiac systolic and diastolic function with embryonic/fetal development was observed. Conclusion. High-frequency echocardiography is a promising and useful imaging modality for structural and hemodynamic analysis of embryonic/fetal mouse hearts.

Characterization of the vessel geometry, flow mechanics and wall shear stress in the great arteries of wildtype prenatal mouse

Introduction

Abnormal fluid mechanical environment in the pre-natal cardiovascular system is hypothesized to play a significant role in causing structural heart malformations. It is thus important to improve our understanding of the prenatal cardiovascular fluid mechanical environment at multiple developmental time-points and vascular morphologies. We present such a study on fetal great arteries on the wildtype mouse from embryonic day 14.5 (E14.5) to near-term (E18.5).

Methods

Ultrasound bio-microscopy (UBM) was used to measure blood velocity of the great arteries. Subsequently, specimens were cryo-embedded and sectioned using episcopic fluorescent image capture (EFIC) to obtain high-resolution 2D serial image stacks, which were used for 3D reconstructions and quantitative measurement of great artery and aortic arch dimensions. EFIC and UBM data were input into subject-specific computational fluid dynamics (CFD) for modeling hemodynamics.

Results

In normal mouse fetuses between E14.5–18.5, ultrasound imaging showed gradual but statistically significant increase in blood velocity in the aorta, pulmonary trunk (with the ductus arteriosus), and descending aorta. Measurement by EFIC imaging displayed a similar increase in cross sectional area of these vessels. However, CFD modeling showed great artery average wall shear stress and wall shear rate remain relatively constant with age and with vessel size, indicating that hemodynamic shear had a relative constancy over gestational period considered here.

Conclusion

Our EFIC-UBM-CFD method allowed reasonably detailed characterization of fetal mouse vascular geometry and fluid mechanics. Our results suggest that a homeostatic mechanism for restoring vascular wall shear magnitudes may exist during normal embryonic development. We speculate that this mechanism regulates the growth of the great vessels.

Haemodynamically dependent valvulogenesis of zebrafish heart is mediated by flow-dependent expression of miR-21

Heartbeat is required for normal development of the heart, and perturbation of intracardiac flow leads to morphological defects resembling congenital heart diseases. These observations implicate intracardiac haemodynamics in cardiogenesis, but the signalling cascades connecting physical forces, gene expression and morphogenesis are largely unknown. Here we use a zebrafish model to show that the microRNA, miR-21, is crucial for regulation of heart valve formation. Expression of miR-21 is rapidly switched on and off by blood flow. Vasoconstriction and increasing shear stress induce ectopic expression of miR-21 in the head vasculature and heart. Flow-dependent expression of mir-21 governs valvulogenesis by regulating the expression of the same targets as mouse/human miR-21 (sprouty, pdcd4, ptenb) and induces cell proliferation in the valve-forming endocardium at constrictions in the heart tube where shear stress is highest. We conclude that miR-21 is a central component of a flow-controlled mechanotransduction system in a physicogenetic regulatory loop.

Decoding the Pluripotency Network: The Emergence of New Transcription Factors

Since the successful isolation of mouse and human embryonic stem cells (ESCs) in the past decades, massive investigations have been conducted to dissect the pluripotency network that governs the ability of these cells to differentiate into all cell types. Beside the core Oct4-Sox2-Nanog circuitry, accumulating regulators, including transcription factors, epigenetic modifiers, microRNA and signaling molecules have also been found to play important roles in preserving pluripotency. Among the various regulations that orchestrate the cellular pluripotency program, transcriptional regulation is situated in the central position and appears to be dominant over other regulatory controls. In this review, we would like to summarize the recent advancements in the accumulating findings of new transcription factors that play a critical role in controlling both pluripotency network and ESC identity.

The interplay between cell signalling and mechanics in developmental processes

Force production and the propagation of stress and strain within embryos and organisms are crucial physical processes that direct morphogenesis. In addition, there is mounting evidence that biomechanical cues created by these processes guide cell behaviours and cell fates. In this Review we discuss key roles for biomechanics during development to directly shape tissues, to provide positional information for cell fate decisions and to enable robust programmes of development. Several recently identified molecular mechanisms suggest how cells and tissues might coordinate their responses to biomechanical cues. Finally, we outline long-term challenges in integrating biomechanics with genetic analysis of developing embryos.

Ethanol exposure alters early cardiac function in the looping heart: a mechanism for congenital heart defects?

Alcohol-induced congenital heart defects are frequently among the most life threatening and require surgical correction in newborns. The etiology of these defects, collectively known as fetal alcohol syndrome, has been the focus of much study, particularly involving cellular and molecular mechanisms. Few studies have addressed the influential role of altered cardiac function in early embryogenesis because of a lack of tools with the capability to assay tiny beating hearts. To overcome this gap in our understanding, we used optical coherence tomography (OCT), a nondestructive imaging modality capable of micrometer-scale resolution imaging, to rapidly and accurately map cardiovascular structure and hemodynamics in real time under physiological conditions. In this study, we exposed avian embryos to a single dose of alcohol/ethanol at gastrulation when the embryo is sensitive to the induction of birth defects. Late-stage hearts were analyzed using standard histological analysis with a focus on the atrio-ventricular valves. Early cardiac function was assayed using Doppler OCT, and structural analysis of the cardiac cushions was performed using OCT imaging. Our results indicated that ethanol-exposed embryos developed late-stage valvuloseptal defects. At early stages, they exhibited increased regurgitant flow and developed smaller atrio-ventricular cardiac cushions, compared with controls (uninjected and saline-injected embryos). The embryos also exhibited abnormal flexion/torsion of the body. Our evidence suggests that ethanol-induced alterations in early cardiac function have the potential to contribute to late-stage valve and septal defects, thus demonstrating that functional parameters may serve as early and sensitive gauges of cardiac normalcy and abnormalities.

The Krüppel-like factor 2 and Krüppel-like factor 4 genes interact to maintain endothelial integrity in mouse embryonic vasculogenesis

Background

Krüppel-like Factor 2 (KLF2) plays an important role in vessel maturation during embryonic development. In adult mice, KLF2 regulates expression of the tight junction protein occludin, which may allow KLF2 to maintain vascular integrity. Adult tamoxifen-inducible Krüppel-like Factor 4 (KLF4) knockout mice have thickened arterial intima following vascular injury. The role of KLF4, and the possible overlapping functions of KLF2 and KLF4, in the developing vasculature are not well-studied.

Results

Endothelial breaks are observed in a major vessel, the primary head vein (PHV), in KLF2-/-KLF4-/- embryos at E9.5. KLF2-/-KLF4-/- embryos die by E10.5, which is earlier than either single knockout. Gross hemorrhaging of multiple vessels may be the cause of death. E9.5 KLF2-/-KLF4+/- embryos do not exhibit gross hemorrhaging, but cross-sections display disruptions of the endothelial cell layer of the PHV, and these embryos generally also die by E10.5. Electron micrographs confirm that there are gaps in the PHV endothelial layer in E9.5 KLF2-/-KLF4-/- embryos, and show that the endothelial cells are abnormally bulbous compared to KLF2-/- and wild-type (WT). The amount of endothelial Nitric Oxide Synthase (eNOS) mRNA, which encodes an endothelial regulator, is reduced by 10-fold in E9.5 KLF2-/-KLF4-/- compared to KLF2-/- and WT embryos. VEGFR2, an eNOS inducer, and occludin, a tight junction protein, gene expression are also reduced in E9.5 KLF2-/-KLF4-/- compared to KLF2-/- and WT embryos.

Conclusions

This study begins to define the roles of KLF2 and KLF4 in the embryonic development of blood vessels. It indicates that the two genes interact to maintain an intact endothelial layer. KLF2 and KLF4 positively regulate the eNOS, VEGFR2 and occludin genes. Down-regulation of these genes in KLF2-/-KLF4-/- embryos may result in the observed loss of vascular integrity.

Endothelial junctional adhesion molecule-a guides monocytes into flow-dependent predilection sites of atherosclerosis

BACKGROUND

Junctional adhesion molecule (JAM)-A expressed in endothelial, epithelial, and blood cells can regulate permeability and leukocyte extravasation. Atherosclerosis develops at sites of disturbed flow in large arteries, but the mechanisms guiding inflammatory cells into these predilection sites remain unknown.

METHODS AND RESULTS

To characterize cell-specific functions of JAM-A in atherosclerosis, we used apolipoprotein E-deficient mice with a somatic or endothelium-specific deficiency in JAM-A and bone marrow chimeras with JAM-A-deficient leukocytes. We show that impaired JAM-A expression in endothelial cells reduced mononuclear cell recruitment into the arterial wall and limited atherosclerotic lesion formation in hyperlipidemic mice. In contrast, JAM-A deficiency in bone marrow cells impeded monocyte de-adhesion, thereby increasing vascular permeability and lesion formation, whereas somatic JAM-A deletion revealed no significant effects. Regions with disturbed flow displayed a focal enrichment and luminal redistribution of endothelial JAM-A and were preferentially protected by its deficiency. The functional expression and redistribution of endothelial JAM-A was increased by oxidized low-density lipoprotein, but confined by atheroprotective laminar flow through an upregulation of microRNA (miR)-145, which repressed JAM-A.

CONCLUSIONS

Our data identify endothelial JAM-A as an important effector molecule integrating atherogenic conditions to direct inflammatory cell entry at predilection sites of atherosclerosis.

ENU-induced mutation in the DNA-binding domain of KLF3 reveals important roles for KLF3 in cardiovascular development and function in mice

KLF3 is a Krüppel family zinc finger transcription factor with widespread tissue expression and no previously known role in heart development. In a screen for dominant mutations affecting cardiovascular function in N-ethyl-N-nitrosourea (ENU) mutagenized mice, we identified a missense mutation in the Klf3 gene that caused aortic valvular stenosis and partially penetrant perinatal lethality in heterozygotes. All homozygotes died as embryos. In the first of three zinc fingers, a point mutation changed a highly conserved histidine at amino acid 275 to arginine (Klf3^H275R). This change impaired binding of the mutant protein to KLF3's canonical DNA binding sequence. Heterozygous Klf3^H275R mutants that died as neonates had marked biventricular cardiac hypertrophy with diminished cardiac chambers. Adult survivors exhibited hypotension, cardiac hypertrophy with enlarged cardiac chambers, and aortic valvular stenosis. A dominant negative effect on protein function was inferred by the similarity in phenotype between heterozygous Klf3^H275R mutants and homozygous Klf3 null mice. However, the existence of divergent traits suggested the involvement of additional interactions. We conclude that KLF3 plays diverse and important roles in cardiovascular development and function in mice, and that amino acid 275 is critical for normal KLF3 protein function. Future exploration of the KLF3 pathway provides a new avenue for investigating causative factors contributing to cardiovascular disorders in humans.

Cardiac defects are among the most common malformations in humans. Most causative genetic mutations remain unknown. To discover new causative genes important in cardiovascular development and function, we examined 1770 mice with randomly mutated genes and found a mutant with aortic valvular stenosis, and increased risk of fetal and neonatal death. Using linkage analysis and sequencing, we identified a protein-altering point mutation in the gene regulatory protein KLF3. Mice that survived into adulthood with one mutant copy of the Klf3 gene had low arterial blood pressure, enlarged hearts, and increased mortality due to heart failure. When both copies of the Klf3 gene was mutant, then embryos had heart defects, and all died before birth. KLF3 had no previously known role in heart development so to confirm these findings, we (1) knocked down klf3 expression in zebrafish embryos and (2) examined mice with a mutation that effectively eliminated the KLF3 protein. In both cases, cardiovascular dysfunction was observed. In conclusion, we have discovered that KLF3 plays diverse and important roles in cardiovascular development and function in mice. Future exploration of the KLF3 pathway provides a new avenue for investigating causative factors contributing to cardiovascular disorders in humans.

The transcription factor Atonal homolog 8 regulates Gata4 and Friend of Gata-2 during vertebrate development

GATA and Friend of GATA (FOG) form a transcriptional complex that plays a key role in cardiovascular development in both fish and mammals. In the present study we demonstrate that the basic helix-loop-helix transcription factor Atonal homolog 8 (Atoh8) is required for development of the heart in fish but not in mice. Genetic studies reveal that Atoh8 interacts specifically with Gata4 and Fog1 during development of the heart and swim bladder in the fish. Biochemical studies reveal that ATOH8, GATA4, and FOG2 associate in a single complex in vitro. In contrast to fish, ATOH8-deficient mice exhibit normal cardiac development and loss of ATOH8 does not alter cardiac development in Gata4(+/-) mice. This species difference in the role of ATOH8 is explained in part by LacZ and GFP reporter alleles that reveal restriction of Atoh8 expression to atrial but not ventricular myocardium in the mouse. Our findings identify ATOH8 as a novel regulator of GATA-FOG function that is required for cardiac development in the fish but not the mouse. Whether ATOH8 modulates GATA-FOG function at other sites or in more subtle ways in mammals is not yet known.

Ubiad1 is an antioxidant enzyme that regulates eNOS activity by CoQ10 synthesis

Protection against oxidative damage caused by excessive reactive oxygen species (ROS) by an antioxidant network is essential for the health of tissues, especially in the cardiovascular system. Here, we identified a gene with important antioxidant features by analyzing a null allele of zebrafish ubiad1, called barolo (bar). bar mutants show specific cardiovascular failure due to oxidative stress and ROS-mediated cellular damage. Human UBIAD1 is a nonmitochondrial prenyltransferase that synthesizes CoQ10 in the Golgi membrane compartment. Loss of UBIAD1 reduces the cytosolic pool of the antioxidant CoQ10 and leads to ROS-mediated lipid peroxidation in vascular cells. Surprisingly, inhibition of eNOS prevents Ubiad1-dependent cardiovascular oxidative damage, suggesting a crucial role for this enzyme and nonmitochondrial CoQ10 in NO signaling. These findings identify UBIAD1 as a nonmitochondrial CoQ10-forming enzyme with specific cardiovascular protective function via the modulation of eNOS activity.

Cardiac outflow tract anomalies

The mature outflow tract (OFT) is, in basic terms, a short conduit. It is a simple, although vital, connection situated between contracting muscular heart chambers and a vast embryonic vascular network. Unfortunately, it is also a focal point underlying many multifactorial congenital heart defects (CHDs). Through the use of various animal models combined with human genetic investigations, we are beginning to comprehend the molecular and cellular framework that controls OFT morphogenesis. Clear roles of neural crest cells (NCC) and second heart field (SHF) derivatives have been established during OFT formation and remodeling. The challenge now is to determine how the SHF and cardiac NCC interact, the complex reciprocal signaling that appears to be occurring at various stages of OFT morphogenesis, and finally how endocardial progenitors and primary heart field (PHF) communicate with both these colonizing extra-cardiac lineages. Although we are beginning to understand that this dance of progenitor populations is wonderfully intricate, the underlying pathogenesis and the spatiotemporal cell lineage interactions remain to be fully elucidated. What is now clear is that OFT alignment and septation are independent processes, invested via separate SHF and cardiac neural crest (CNC) lineages. This review will focus on our current understanding of the respective contributions of the SHF and CNC lineage during OFT development and pathogenesis.

Krüppel-like factor 2 is required for normal mouse cardiac development

Krüppel-like factor 2 (KLF2) is expressed in endothelial cells in the developing heart, particularly in areas of high shear stress, such as the atrioventricular (AV) canal. KLF2 ablation leads to myocardial thinning, high output cardiac failure and death by mouse embryonic day 14.5 (E14.5) in a mixed genetic background. This work identifies an earlier and more fundamental role for KLF2 in mouse cardiac development in FVB/N mice. FVB/N KLF2−/− embryos die earlier, by E11.5. E9.5 FVB/N KLF2−/− hearts have multiple, disorganized cell layers lining the AV cushions, the primordia of the AV valves, rather than the normal single layer. By E10.5, traditional and endothelial-specific FVB/N KLF2−/− AV cushions are hypocellular, suggesting that the cells accumulating at the AV canal have a defect in endothelial to mesenchymal transformation (EMT). E10.5 FVB/N KLF2−/− hearts have reduced glycosaminoglycans in the cardiac jelly, correlating with the reduced EMT. However, the number of mesenchymal cells migrating from FVB/N KLF2−/− AV explants into a collagen matrix is reduced considerably compared to wild-type, suggesting that the EMT defect is not due solely to abnormal cardiac jelly. Echocardiography of E10.5 FVB/N KLF2−/− embryos indicates that they have abnormal heart function compared to wild-type. E10.5 C57BL/6 KLF2−/− hearts have largely normal AV cushions. However, E10.5 FVB/N and C57BL/6 KLF2−/− embryos have a delay in the formation of the atrial septum that is not observed in a defined mixed background. KLF2 ablation results in reduced Sox9, UDP-glucose dehydrogenase (Ugdh), Gata4 and Tbx5 mRNA in FVB/N AV canals. KLF2 binds to the Gata4, Tbx5 and Ugdh promoters in chromatin immunoprecipitation assays, indicating that KLF2 could directly regulate these genes. In conclusion, KLF2−/− heart phenotypes are genetic background-dependent. KLF2 plays a role in EMT through its regulation of important cardiovascular genes.

Overexpression of human amyloidogenic light chains causes heart failure in embryonic zebrafish: a preliminary report

AL cardiomyopathy leading to heart failure (HF) represents a significant cause of morbidity and mortality in systemic amyloidosis. However, the paucity of robust in vivo models of AL-induced cardiac dysfunction has limited our ability to probe the mechanisms of AL heart disease. To address this problem, we have developed a model of AL HF in zebrafish embryos by injection of in vitro transcribed mRNA encoding amyloidogenic light chain (aLC) into fertilized oocytes. We demonstrate that expression of aLC causes cardiomyopathy in developing zebrafish without significantly impairing extracardiac development. The cardiac ventricle of embryos expressing aLC exhibit impaired contractility, smaller size, and increased myocardial thickness which result in congestion and edema, features paralleling the clinical manifestations of amyloid cardiomyopathy. Phosphorylated p38, a marker of oxidative stress, was increased in response to aLC expression. No evidence of amyloid fibril deposition was identified. Thus, expression of aLC mRNA in zebrafish results in cardio toxic effects without fibril deposition. This is consistent with prior evidence indicating that aLC oligomers mediate cardiac dysfunction in vitro. This model will allow exploration of amyloid pathophysiology and testing of interventions to reduce and reverse the deleterious effects of amyloidosis on myocardial function.

Dynamic regulation of the cerebral cavernous malformation pathway controls vascular stability and growth

Cardiovascular growth must balance stabilizing signals required to maintain endothelial connections and network integrity with destabilizing signals that enable individual endothelial cells to migrate and proliferate. The cerebral cavernous malformation (CCM) signaling pathway utilizes the adaptor protein CCM2 to strengthen endothelial cell junctions and stabilize vessels. Here we identify a CCM2 paralog, CCM2L, that is expressed selectively in endothelial cells during periods of active cardiovascular growth. CCM2L competitively blocks CCM2-mediated stabilizing signals biochemically, in cultured endothelial cells, and in developing mice. Loss of CCM2L reduces endocardial growth factor expression and impairs tumor growth and wound healing. Our studies identify CCM2L as a molecular mechanism by which endothelial cells coordinately regulate vessel stability and growth during cardiovascular development, as well as postnatal vessel growth.