The G Protein-Coupled Receptor Agtrl1b Regulates Early Development of Myocardial Progenitors

G protein coupled receptors in embryonic stem cells: a role for Gs-alpha signaling

Background

Identification of receptor mediated signaling pathways in embryonic stem (ES) cells is needed to facilitate strategies for cell replacement using ES cells. One large receptor family, largely uninvestigated in ES cells, is G protein coupled receptors (GPCRs). An important role for these receptors in embryonic development has been described, but little is known about GPCR expression in ES cells.

Methodology/Principal Findings

We have examined the expression profile of 343 different GPCRs in mouse ES cells demonstrating for the first time that a large number of GPCRs are expressed in undifferentiated and differentiating ES cells, and in many cases at high levels. To begin to define a role for GPCR signaling in ES cells, the impact of activating Gs-alpha, one of the major alpha subunits that couples to GPCRs, was investigated. Gs-alpha activation resulted in larger embryoid bodies (EBs), due, in part, to increased cell proliferation and prevented the time-related decline in expression of transcription factors important for maintaining ES cell pluripotency.

Significance/Conclusions

These studies suggest that Gs-alpha signaling contributes to ES cell proliferation and pluripotency and provide a framework for further investigation of GPCRs in ES cells.

G protein betagamma subunits regulate cell adhesion through Rap1a and its effector Radil

The activation of several G protein-coupled receptors is known to regulate the adhesive properties of cells in different contexts. Here, we reveal that Gbetagamma subunits of heterotrimeric G proteins regulate cell-matrix adhesiveness by activating Rap1a-dependent inside-out signals and integrin activation. We show that Gbetagamma subunits enter in a protein complex with activated Rap1a and its effector Radil and establish that this complex is required downstream of receptor stimulation for the activation of integrins and the positive modulation of cell-matrix adhesiveness. Moreover, we demonstrate that Gbetagamma and activated Rap1a promote the translocation of Radil to the plasma membrane at sites of cell-matrix contacts. These results add to the molecular understanding of how G protein-coupled receptors impinge on cell adhesion and suggest that the Gbetagamma x Rap1 x Radil complex plays important roles in this process.

Identification and pharmacological properties of E339-3D6, the first nonpeptidic apelin receptor agonist

Apelin plays a prominent role in body fluid and cardiovascular homeostasis. To explore further upstream the role played by this peptide, nonpeptidic agonists and antagonists of the apelin receptor are required. To identify such compounds that do not exist to date, we used an original fluorescence resonance energy transfer-based assay to screen a G-protein-coupled receptor-focused library of fluorescent compounds on the human EGFP-tagged apelin receptor. This led to isolated E339-3D6 that displayed a 90 nM affinity and behaved as a partial agonist with regard to cAMP production and as a full agonist with regard to apelin receptor internalization. Finally, E339-3D6 induced vasorelaxation of rat aorta precontracted with noradrenaline and potently inhibited systemic vasopressin release in water-deprived mice when intracerebroventricularly injected. This compound represents the first nonpeptidic agonist of the apelin receptor, the optimization of which will allow development of a new generation of vasodilator and aquaretic agents.

The effects of apelin treatment on skeletal muscle mitochondrial content

Adipose tissue is recognized as a key player in the regulation of whole body metabolism. Apelin, is a recently identified adipokine that when given to mice results in increases in skeletal muscle uncoupling protein 3 (UCP3) content. Similarly, acute apelin treatment has been shown to increase the activity of 5′-AMP-activated protein kinase (AMPK), a reputed mediator of skeletal muscle mitochondrial biogenesis. Given these findings, we sought to determine the effects of apelin on skeletal muscle mitochondrial content. Male Wistar rats were given daily intraperitoneal injections of apelin-13 (100 nmol/kg) for 2 wk. We made the novel observation that the activities of citrate synthase, cytochrome c oxidase, and β-hydroxyacyl coA dehydrogenase (βHAD) were increased in triceps but not heart and soleus muscles from apelin-treated rats. When confirming these results we found that both nuclear and mitochondrial-encoded subunits of the respiratory chain were increased in triceps from apelin-treated rats. Similarly, apelin treatment increased the protein content of components of the mitochondrial import and assembly pathway. The increases in mitochondrial marker proteins were associated with increases in proliferator-activated receptor-γ coactivator-1 (PGC-1β) but not PGC-1α or Pgc-1-related co-activator (PRC) mRNA expression. Chronic and acute apelin treatment did not increase the protein content and/or phosphorylation status of AMPK and its downstream substrate acetyl-CoA carboxylase. These findings are the first to demonstrate that apelin treatment can induce skeletal muscle mitochondrial content. Given the lack of an effect of apelin on AMPK signaling and PGC-1α mRNA expression, these results suggest that apelin increases skeletal muscle mitochondrial content through a mechanism that is distinct from that of more robust physiological stressors.

Zebrafish aplnra functions in epiboly

Background

The zebrafish, Danio rerio, possesses the paralogous genes aplnra and aplnrb that are duplicates of an ancestral orthologue of the human APLNR gene encoding a G-protein coupled receptor that binds the peptide ligand APELIN and is required for normal cardiovascular function. aplnrb is required for migration of cells contributing to heart development in zebrafish embryos. aplnra is transcribed in a complex pattern during early development but its function in embryogenesis is largely unknown.

Findings

Blockage of translation of aplnra mRNA in zebrafish embryos results in retarded or failed epiboly with the blastoderm apparently disconnected from the nuclei of the yolk syncytial layer. Gastrulation is also defective. Failure of correct tail extension is observed with ectopic structures resembling somites positioned dorsal to the spinal cord.

Conclusion

aplnra, unlike its duplicate aplnrb, is essential for normal epiboly, although this function appears to be independent of signalling activated by zebrafish Apelin. The defects in epiboly caused by loss of aplnra activity appear, at least partially, to be due to a requirement for aplnra activity in the yolk syncytial layer.

G(i)-coupled GPCR signaling controls the formation and organization of human pluripotent colonies

Background

Reprogramming adult human somatic cells to create human induced pluripotent stem (hiPS) cell colonies involves a dramatic morphological and organizational transition. These colonies are morphologically indistinguishable from those of pluripotent human embryonic stem (hES) cells. G protein-coupled receptors (GPCRs) are required in diverse developmental processes, but their role in pluripotent colony morphology and organization is unknown. We tested the hypothesis that G_i-coupled GPCR signaling contributes to the characteristic morphology and organization of human pluripotent colonies.

Methodology/Principal Findings

Specific and irreversible inhibition of G_i-coupled GPCR signaling by pertussis toxin markedly altered pluripotent colony morphology. Wild-type hES and hiPS cells formed monolayer colonies, but colonies treated with pertussis toxin retracted inward, adopting a dense, multi-layered conformation. The treated colonies were unable to reform after a scratch wound insult, whereas control colonies healed completely within 48 h. In contrast, activation of an alternative GPCR pathway, G_s-coupled signaling, with cholera toxin did not affect colony morphology or the healing response. Pertussis toxin did not alter the proliferation, apoptosis or pluripotency of pluripotent stem cells.

Conclusions/Significance

Experiments with pertussis toxin suggest that G_i signaling plays a critical role in the morphology and organization of pluripotent colonies. These results may be explained by a G_i-mediated density-sensing mechanism that propels the cells radially outward. GPCRs are a promising target for modulating the formation and organization of hiPS and hES cell colonies and may be important for understanding somatic cell reprogramming and for engineering pluripotent stem cells for therapeutic applications.

Increased apelin following bone marrow mononuclear cell transplantation contributes to the improvement of cardiac function in patients with severe heart failure

We previously reported that intracoronary implantation of bone marrow mononuclear cells (BMMC) into ischemic hearts improved cardiac function after myocardial infarction. However, the mechanisms have not been elucidated. The present study investigates whether apelin, a newly described inotropic peptide with important cardiovascular regulatory properties, contributes to the functional improvement in patients with severe heart failure after cell transplantation. Forty consecutive patients with severe heart failure secondary to myocardial infarction were assigned to the BMMC therapy group or the standard medication group according to each patient's decision on a signed consent document. In 20 patients intracoronary cell infusion was performed, and another 20 patients were matched to receive standard medication as therapeutic controls. An additional 20 healthy subjects were designated as normal controls. Clinical manifestations, echocardiograms, and biochemical assays were recorded. Plasma apelin and brain natriuretic protein (BNP) levels were determined by enzyme immunoassay. Baseline levels of plasma apelin were significantly lower in all heart failure patients compared to normal subjects. In patients who underwent cell transplantation, apelin increased significantly from 3 to 21 days after operation, followed by significant improvement in cardiac function. In parallel, BNP varied inversely with the increase of apelin. In patients receiving standard medical treatment, apelin remained at a lower level. Our findings indicated that increased apelin levels following cell therapy may act as a paracrine mediator produced from BMMCs and play an important role in the treatment of heart failure through autocrine and paracrine mechanisms.

Endogenous regulation of cardiovascular function by apelin-APJ

Studies have shown significant cardiovascular effects of exogenous apelin administration, including the potent activation of cardiac contraction. However, the role of the endogenous apelin-APJ pathway is less clear. To study the loss of endogenous apelin-APJ signaling, we generated mice lacking either the ligand (apelin) or the receptor (APJ). Apelin-deficient mice were viable, fertile, and showed normal development. In contrast, APJ-deficient mice were not born in the expected Mendelian ratio, and many showed cardiovascular developmental defects. Under basal conditions, both apelin and APJ null mice that survived to adulthood manifested modest decrements in contractile function. However, with exercise stress both mutant lines demonstrated consistent and striking decreases in exercise capacity. To explain these findings, we explored the role of autocrine signaling in vitro using field stimulation of isolated left ventricular cardiomyocytes lacking either apelin or APJ. Both groups manifested less sarcomeric shortening and impaired velocity of contraction and relaxation with no difference in calcium transient. Taken together, these results demonstrate that endogenous apelin-APJ signaling plays a modest role in maintaining basal cardiac function in adult mice with a more substantive role during conditions of stress. In addition, an autocrine pathway seems to exist in myocardial cells, the ablation of which reduces cellular contraction without change in calcium transient. Finally, differences in the developmental phenotype between apelin and APJ null mice suggest the possibility of undiscovered APJ ligands or ligand-independent effects of APJ.

Apelin in goldfish (Carassius auratus): cloning, distribution and role in appetite regulation

Apelin is a recently discovered peptide produced by several tissues including brain and adipose tissue. In mammals and zebrafish, apelin regulates cardiovascular functions. Recent evidence in mammals suggest that apelin might also regulate food intake. In this study, we cloned a cDNA encoding apelin and examined apelin mRNA distribution within the brain and in peripheral tissues. We also assessed the effects of fasting on apelin brain mRNA abundance. Apelin mRNA was expressed throughout the brain as well as in several peripheral tissues including brain, spleen, heart and fat. Apelin mRNA abundance in both hypothalamus and telencephalon was significant higher in fasted fish than in fed fish. In order to further characterize apelin in goldfish, we assessed the effects of central (intracerebroventricular, icv) and peripheral (intraperitoneal, ip) injections of apelin-13 on food intake in goldfish. Apelin injected ip at a dose of 100ng/g or icv at a dose of 10ng/g induced a significant increase in food intake compared to saline-injected fish. Our results suggest that apelin acts as an orexigenic factor in goldfish. Its widespread distribution in the brain and the periphery also suggests that apelin might have multiple physiological regulating roles in fish.

G protein-coupled receptor APJ and its ligand apelin act downstream of Cripto to specify embryonic stem cells toward the cardiac lineage through extracellular signal-regulated kinase/p70S6 kinase signaling pathway

RATIONALE

Pluripotent stem cells represent a powerful model system to study the early steps of cardiac specification for which the molecular control is largely unknown. The EGF-CFC (epidermal growth factor-Cripto/FRL-1/Cryptic) Cripto protein is essential for cardiac myogenesis in embryonic stem cells (ESCs).

OBJECTIVE

Here, we study the role of apelin and its G protein-coupled receptor, APJ, as downstream targets of Cripto both in vivo and in ESC differentiation.

METHODS AND RESULTS

Gain-of-function experiments show that APJ suppresses neuronal differentiation and restores the cardiac program in Cripto(-/-) ESCs. Loss-of-function experiments point for a central role for APJ/apelin in the gene regulatory cascade promoting cardiac specification and differentiation in ESCs. Remarkably, we show for the first time that apelin promotes mammalian cardiomyogenesis via activation of mitogen-activated protein kinase/p70S6 through coupling to a Go/Gi protein.

CONCLUSIONS

Together our data provide evidence for a previously unrecognized function of APJ/apelin in the Cripto signaling pathway governing mesoderm patterning and cardiac specification in mammals.

[Apelin signalisation and vascular physiopathology]

The formation of the vascular system is an early step in organogenesis that involves the participation of various signalling pathways. Integration of the extracellular signals decoded by their cognate membrane receptors orchestrate the cell events, which act at different stages, from the primitive network formed by vasculogenesis to the arborescent network remodeled by angiogenesis. Our laboratory showed the participation of a new signalling pathway in physiological angiogenesis and tumour neovascularisation. This signalling pathway named apelin comprises a G protein-coupled receptor and a peptide ligand. Expression of apelin receptors is observed during the embryonic formation of blood vessels where it is localized in the endothelium. In HUVECs, which endogenously express apelin receptors, apelin promotes the phosphorylation of ERKs, Akt and p70 S6 Kinase. In addition, apelin increases in vitro the proliferation of these endothelial cells. Finally, injection of apelin in the vitreous induces in vivo the sprouting and the proliferation of endothelial cells from the retinal vascular network. Accordingly, all these results led us to study the role of apelin signalling in tumour neovascularisation. In two tumoral cell lines, we showed that hypoxia induces the expression of apelin gene. In addition, the overexpression of apelin gene resulting from stable transfection of these cell lines clearly accelerates in vivo tumour growth, as a consequence of an increased number of vessels irrigating these tumours. The pathological relevance of these data has been validated by the characterization of an overexpression of apelin gene in one third of human tumours. Taken together, apelin signalling is both involved in physiological angiogenesis and pathological neoangiogenesis, and therefore represents an interesting pharmacological target for anti-angiogenic therapies.

Differential requirement for BMP signaling in atrial and ventricular lineages establishes cardiac chamber proportionality

The function of an organ relies upon the proper relative proportions of its individual operational components. For example, effective embryonic circulation requires the appropriate relative sizes of each of the distinct pumps created by the atrial and ventricular cardiac chambers. Although the differences between atrial and ventricular cardiomyocytes are well established, little is known about the mechanisms regulating production of proportional numbers of each cell type. We find that mutation of the zebrafish type I BMP receptor gene alk8 causes reduction of atrial size without affecting the ventricle. Loss of atrial tissue is evident in the lateral mesoderm prior to heart tube formation and results from the inhibition of BMP signaling during cardiac progenitor specification stages. Comparison of the effects of decreased and increased BMP signaling further demonstrates that atrial cardiomyocyte production correlates with levels of BMP signaling while ventricular cardiomyocyte production is less susceptible to manipulation of BMP signaling. Additionally, mosaic analysis provides evidence for a cell-autonomous requirement for BMP signaling during cardiomyocyte formation and chamber fate assignment. Together, our studies uncover a new role for BMP signaling in the regulation of chamber size, supporting a model in which differential reception of cardiac inductive signals establishes chamber proportion.

Arterial-venous specification during development

The major arteries and veins of the vertebrate circulatory system are formed early in embryonic development, before the onset of circulation, following de novo aggregation of "angioblast" progenitors in a process called vasculogenesis. Initial embryonic determination of artery or vein identity is regulated by variety of genetic factors that work in concert to specify endothelial cell fate, giving rise to 2 distinct components of the circulatory loop possessing unique structural characteristics. Work in multiple in vivo animal model systems has led to a detailed examination of the interacting partners that determine arterial and venous specification. We discuss the hierarchical arrangement of many signaling molecules, including Hedgehog (Hh), vascular endothelial growth factor (VEGF), Notch, and chicken ovalbumin upstream-transcription factor II (COUP-TFII) that promote or inhibit divergent pathways of endothelial cell fate. Elucidation of the functional role of these genetic determinants of blood vessel specification together with the epigenetic factors involved in subsequent modification of arterial-venous identity will allow for potential new therapeutic targets for vascular disorders.

The outflow tract of the heart in fishes: anatomy, genes and evolution

A large number of congenital heart defects associated with mortality in humans are those that affect the cardiac outflow tract, and this provides a strong imperative to understand its development during embryogenesis. While there is wide phylogenetic variation in adult vertebrate heart morphology, recent work has demonstrated evolutionary conservation in the early processes of cardiogenesis, including that of the outflow tract. This, along with the utility and high reproductive potential of fish species such as Danio rerio, Oryzias latipes etc., suggests that fishes may provide ideal comparative biological models to facilitate a better understanding of this poorly understood region of the heart. In this review, the authors present the current understanding of both phylogeny and ontogeny of the cardiac outflow tract in fishes and examine how new molecular studies are informing the phylogenetic relationships and evolutionary trajectories that have been proposed. The authors also attempt to address some of the issues of nomenclature that confuse this area of research.

Hedgehog signaling plays a cell-autonomous role in maximizing cardiac developmental potential

Elucidation of the complete roster of signals required for myocardial specification is crucial to the future of cardiac regenerative medicine. Prior studies have implicated the Hedgehog (Hh) signaling pathway in the regulation of multiple aspects of heart development. However, our understanding of the contribution of Hh signaling to the initial specification of myocardial progenitor cells remains incomplete. Here, we show that Hh signaling promotes cardiomyocyte formation in zebrafish. Reduced Hh signaling creates a cardiomyocyte deficit, and increased Hh signaling creates a surplus. Through fate-mapping, we find that Hh signaling is required at early stages to ensure specification of the proper number of myocardial progenitors. Genetic inducible fate mapping in mouse indicates that myocardial progenitors respond directly to Hh signals, and transplantation experiments in zebrafish demonstrate that Hh signaling acts cell autonomously to promote the contribution of cells to the myocardium. Thus, Hh signaling plays an essential early role in defining the optimal number of cardiomyocytes, making it an attractive target for manipulation of multipotent progenitor cells.

Chemokine signaling controls endodermal migration during zebrafish gastrulation

Directed cell movements during gastrulation establish the germ layers of the vertebrate embryo and coordinate their contributions to different tissues and organs. Anterior migration of the mesoderm and endoderm has largely been interpreted to result from epiboly and convergent-extension movements that drive body elongation. We show that the chemokine Cxcl12b and its receptor Cxcr4a restrict anterior migration of the endoderm during zebrafish gastrulation, thereby coordinating its movements with those of the mesoderm. Depletion of either gene product causes disruption of integrin-dependent cell adhesion, resulting in separation of the endoderm from the mesoderm; the endoderm then migrates farther anteriorly than it normally would, resulting in bilateral duplication of endodermal organs. This process may have relevance to human gastrointestinal bifurcations and other organ defects.

The hepatic apelin system: a new therapeutic target for liver disease

Apelin is a peptide that plays an important role in heart physiology and pathophysiology, inflammation, and angiogenesis. We evaluated whether the endogenous apelin system is involved in the pathogenesis of the hepatic remodeling and cardiovascular and renal complications occurring in advanced liver disease. The circulating levels of apelin, the messenger RNA (mRNA) and protein expression of apelin and apelin receptor, the immunohistological detection of apelin and apelin receptor, and the effects induced by the chronic administration of an apelin receptor antagonist on fibrosis and vessel density were evaluated in rats with cirrhosis and ascites and in control rats. The serum levels of apelin in patients with cirrhosis were also measured. Apelin levels were higher in rats with cirrhosis than in controls. Apelin mRNA showed a four-fold rise only in hepatic tissue, but not in the lung, heart, or kidney of rats with cirrhosis. These animals also showed hepatic apelin receptor mRNA levels 300 times higher than controls. Apelin was highly expressed by stellate cells, whereas apelin receptor was overexpressed in the hepatic parenchyma of animals with cirrhosis. Rats with cirrhosis treated with the apelin receptor antagonist showed diminished hepatic fibrosis and vessel density, improved cardiovascular performance, and renal function and lost ascites. Human patients also showed a marked increase in apelin levels.

CONCLUSION

The selective hepatic activation of the apelin system, together with the drop in fibrosis and neoangiogenesis and the improvement in cardiovascular and excretory function resulting from apelin receptor blockade, points to the hepatic apelin system as a novel therapeutic target in liver disease.

Sphingosine-1-phosphate receptors regulate individual cell behaviours underlying the directed migration of prechordal plate progenitor cells during zebrafish gastrulation

During vertebrate gastrulation, cells forming the prechordal plate undergo directed migration as a cohesive cluster. Recent studies revealed that E-cadherin-mediated coherence between these cells plays an important role in effective anterior migration, and that platelet-derived growth factor (Pdgf) appears to act as a guidance cue in this process. However, the mechanisms underlying this process at the individual cell level remain poorly understood. We have identified miles apart (mil) as a suppressor of defective anterior migration of the prospective prechordal plate in silberblick (slb)/wnt11 mutant embryos, in which E-cadherin-mediated coherence of cell movement is reduced. mil encodes Edg5, a sphingosine-1-phosphate (S1P) receptor belonging to a family of five G-protein-coupled receptors (S1PRs). S1P is a lipid signalling molecule that has been implicated in regulating cytoskeletal rearrangements, cell motility and cell adhesion in a variety of cell types. We examined the roles of Mil in anterior migration of prechordal plate progenitor cells and found that, in slb embryos injected with mil-MO, cells migrate with increased motility but decreased directionality, without restoring the coherence of cell migration. This indicates that prechordal plate progenitor cells can migrate effectively as individuals, as well as in a coherent cluster of cells. Moreover, we demonstrate that Mil regulates cell motility and polarisation through Pdgf and its intracellular effecter PI3K, but modulates cell coherence independently of the Pdgf/PI3K pathway, thus co-ordinating cell motility and coherence. These results suggest that the net migration of prechordal plate progenitors is determined by different parameters, including motility, persistence and coherence.

The orphan G protein-coupled receptor 161 is required for left-right patterning

Gpr161 (also known as RE2) is an orphan G protein-coupled receptor (GPCR) that is expressed during embryonic development in zebrafish. Determining its biological function has proven difficult due to lack of knowledge regarding its natural or synthetic ligands. Here, we show that targeted knockdown of gpr161 disrupts asymmetric gene expression in the lateral plate mesoderm, resulting in aberrant looping of the heart tube. This is associated with elevated Ca(2+) levels in cells lining the Kupffer's vesicle and normalization of Ca(2+) levels, by over-expression of ncx1 or pmca-RNA, is able to partially rescue the cardiac looping defect in gpr161 knockdown embryos. Taken together, these data support a model in which gpr161 plays an essential role in left-right (L-R) patterning by modulating Ca(2+) levels in the cells surrounding the Kupffer's vesicle.

Back and forth between cell fate specification and movement during vertebrate gastrulation

Animal body plan arises during gastrulation and organogenesis by the coordination of inductive events and cell movements. Several signaling pathways, such as BMP, FGF, Hedgehog, Nodal, and Wnt have well-recognized instructive roles in cell fate specification during vertebrate embryogenesis. Growing evidence indicates that BMP, Nodal, and FGF signaling also regulate cell movements, and that they do so through mechanisms distinct from those that specify cell fates. Moreover, pathways controlling cell movements can also indirectly influence cell fate specification by regulating dimensions and relative positions of interacting tissues. The current challenge is to delineate the molecular mechanisms via which the major signaling pathways regulate cell fate specification and movements, and how these two processes are coordinated to ensure normal development.

The role of apelin in cardiovascular function and heart failure

Apelin is a novel peptide that acts through the APJ receptor, sharing similarities with the angiotensin II-angiotensin II type 1 receptor pathway. It is a peripheral vasodilator, powerful inotrope and may affect central fluid homeostasis. Animal and human studies suggest that it may play a role in the pathogenesis of heart failure by modulating the harmful effects of angiotensin II. Apelin is reduced in patients with heart failure and up regulated following favourable left ventricular remodelling. It is widely distributed in a number of tissues, mainly restricted to vascular endothelium. This comprehensive review of the literature highlights the important studies that have led to the discovery of apelin and its role in cardiovascular function and heart failure.

Sdf1/Cxcr4 signaling controls the dorsal migration of endodermal cells during zebrafish gastrulation

During vertebrate gastrulation, both mesodermal and endodermal cells internalize through the blastopore beneath the ectoderm. In zebrafish, the internalized mesodermal cells move towards the dorsal side of the gastrula and, at the same time, they extend anteriorly by convergence and extension (C&E) movements. Endodermal cells showing characteristic filopodia then migrate into the inner layer within the hypoblast next to the yolk syncytial layer (YSL). However, little is known about how the movement of endodermal cells is regulated during gastrulation. Here we show that sdf1a- and sdf1b-expressing mesodermal cells control the movements of the cxcr4a-expressing endodermal cells. The directional migration of endodermal cells during gastrulation is inhibited by knockdown of either cxcr4a or sdf1a/sdf1b (sdf1). We also show that misexpressed Sdf1 acts as a chemoattractant for cxcr4a-expressing endodermal cells. We further found, using the endoderm-specific transgenic line Tg(sox17:EGFP), that Sdf1/Cxcr4 signaling regulates both the formation and orientation of filopodial processes in endodermal cells. Moreover, the accumulation of phosphoinositide 3,4,5-trisphosphate (PIP(3)), which is known to occur at the leading edge of migrating cells, is not observed at the filopodia of endodermal cells. Based on our results, we propose that sdf1-expressing mesodermal cells, which overlie the endodermal layer, guide the cxcr4a-expressing endodermal cells to the dorsal side of the embryo during gastrulation, possibly through a PIP(3)-independent pathway.

Spatial and temporal role of the apelin/APJ system in the caliber size regulation of blood vessels during angiogenesis

Blood vessels change their caliber to adapt to the demands of tissues or organs for oxygen and nutrients. This event is mainly organized at the capillary level and requires a size-sensing mechanism. However, the molecular regulatory mechanism involved in caliber size modification in blood vessels is not clear. Here we show that apelin, a protein secreted from endothelial cells under the activation of Tie2 receptor tyrosine kinase on endothelial cells, plays a role in the regulation of caliber size of blood vessel through its cognate receptor APJ, which is expressed on endothelial cells. During early embryogenesis, APJ is expressed on endothelial cells of the new blood vessels sprouted from the dorsal aorta, but not on pre-existing endothelial cells of the dorsal aorta. Apelin-deficient mice showed narrow blood vessels in intersomitic vessels during embryogenesis. Apelin enhanced endothelial cell proliferation in the presence of vascular endothelial growth factor and promoted cell-to-cell aggregation. These results indicated that the apelin/APJ system is involved in the regulation of blood vessel diameter during angiogenesis.

In vivo genetic profiling and cellular localization of apelin reveals a hypoxia-sensitive, endothelial-centered pathway activated in ischemic heart failure

Signaling by the peptide ligand apelin and its cognate G protein-coupled receptor APJ has a potent inotropic effect on cardiac contractility and modulates systemic vascular resistance through nitric oxide-dependent signaling. In addition, there is evidence for counterregulation of the angiotensin and vasopressin pathways. Regulatory stimuli of the apelin-APJ pathway are of obvious importance but remain to be elucidated. To better understand the physiological response of apelin-APJ to disease states such as heart failure and to elucidate the mechanism by which such a response might occur, we have used the murine model of left anterior descending coronary artery ligation-induced ischemic cardiac failure. To identify the key cells responsible for modulation and production of apelin in vivo, we have created a novel apelin-lacZ reporter mouse. Data from these studies demonstrate that apelin and APJ are upregulated in the heart and skeletal muscle following myocardial injury and suggest that apelin expression remains restricted to the endothelium. In cardiac failure, endothelial apelin expression correlates with other hypoxia-responsive genes, and in healthy animals both apelin and APJ are markedly upregulated in various tissues following systemic hypoxic exposure. Experiments with cultured endothelial cells in vitro show apelin mRNA and protein levels to be increased by hypoxia, through a hypoxia-inducible factor-mediated pathway. These studies suggest that apelin-expressing endothelial cells respond to conditions associated with heart failure, possibly including local tissue hypoxia, and modulate apelin-APJ expression to regulate cardiovascular homeostasis. The apelin-APJ pathway may thus provide a mechanism for systemic endothelial monitoring of tissue perfusion and adaptive regulation of cardiovascular function.

Expanding role for the apelin/APJ system in physiopathology

Apelin is a bioactive peptide known as the ligand of the G protein-coupled receptor APJ. Diverse active apelin peptides exist under the form of 13, 17 or 36 amino acids, originated from a common 77-amino-acid precursor. Both apelin and APJ mRNA are widely expressed in several rodent and human tissues and have functional effects in both the central nervous system and peripheral tissues. Apelin has been shown to be involved in the regulation of cardiovascular functions, fluid homeostasis, vessel formation and cell proliferation. More recently, apelin has been described as an adipocyte-secreted factor (adipokine), up-regulated in obesity. By acting as circulating hormone or paracrine factor, adipokines are involved in physiological regulations (fat depot development, energy storage, metabolism or eating behavior) or in the promotion of obesity-associated disorders (type 2 diabetes and cardiovascular dysfunctions). In this regard, expression of apelin gene in adipose tissue is increased by insulin and TNFα. This review will consider the main roles of apelin in physiopathology with particular attention on its role in energy balance regulation and in obesity-associated disorders.

Small heat shock protein Hsp27 is required for proper heart tube formation

The small heat shock protein Hsp27 has been shown to be involved in a diverse array of cellular processes, including cellular stress response, protein chaperone activity, regulation of cellular glutathione levels, apoptotic signaling, and regulation of actin polymerization and stability. Furthermore, mutation within Hsp27 has been associated with the human congenital neuropathy Charcot-Marie Tooth (CMT) disease. Hsp27 is known to be expressed in developing embryonic tissues; however, little has been done to determine the endogenous requirement for Hsp27 in developing embryos. In this study, we show that depletion of XHSP27 protein results in a failure of cardiac progenitor fusion resulting in cardia bifida. Furthermore, we demonstrate a concomitant disorganization of actin filament organization and defects in myofibril assembly. Moreover, these defects are not associated with alterations in specification or differentiation. We have thus demonstrated a critical requirement for XHSP27 in developing cardiac and skeletal muscle tissues.

Apelin and its g protein-coupled receptor regulate cardiac development as well as cardiac function

The Apelin pathway has only recently emerged as an important regulator of cardiac and vascular function, mediating adaptation to physiological stress and disease. In this issue of Developmental Cell, experiments in zebrafish convincingly show a critical role for this pathway in myocardial cell specification and heart development.