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

Microbiota affects mitochondria and immune cell infiltrations via alternative polyadenylation during postnatal heart development

There is a growing body of evidence supporting the significant impact of microbiota on heart development. Alternative polyadenylation (APA) is a crucial mechanism for gene expression regulation and has been implicated in postnatal heart development. Nonetheless, whether microbiota can influence postnatal heart development through the regulation of APA remains unclear. Therefore, we conducted APA sequencing on heart tissues collected from specific pathogen-free (SPF) mice and germ-free (GF) mice at three different developmental stages: within the first 24 h after birth (P1), 7-day-old SPF mice, and 7-day-old GF mice. This approach allowed us to obtain a comprehensive genome-wide profile of APA sites in the heart tissue samples. In this study, we made a significant observation that GF mice exhibited noticeably longer 3' untranslated region (3' UTR) lengths. Furthermore, we confirmed significant alterations in the 3' UTR lengths of mitochondria-related genes, namely Rala, Timm13, and Uqcc3. Interestingly, the GF condition resulted in a marked decrease in mitochondrial cristae density and a reduction in the level of Tomm20 in postnatal hearts. Moreover, we discovered a connection between Rala and Src, which further implicated their association with other differentially expressed genes (DEGs). Notably, most of the DEGs were significantly downregulated in GF mice, with the exceptions being Thbs1 and Egr1. Importantly, the GF condition demonstrated a correlation with a lower infiltration of immune cells, whereby the levels of resting NK cells, Th17 cells, immature dendritic cells, and plasma cells in GF mice were comparable to those observed in P1 mice. Furthermore, we established significant correlations between these immune cells and Rala as well as the related DEGs. Our findings clearly indicated that microbiota plays a vital role in postnatal heart development by affecting APA switching, mitochondria and immune cell infiltrations.

APJ+ cells in the SHF contribute to the cells of aorta and pulmonary trunk through APJ signaling

Outflow tract develops from cardiac progenitor cells in the second heart field (SHF) domain. APJ, a G-Protein Coupled Receptor, is expressed by cardiac progenitor cells in the SHF. By lineage tracing APJ + SHF cells, we show that these cardiac progenitor cell contribute to the cells of outflow tract (OFT), which eventually give rise to aorta and pulmonary trunk/artery upon its morphogenesis. Furthermore, we show that early APJ + cells give rise to both aorta and pulmonary cells but late APJ + cells predominantly give rise to pulmonary cells. APJ is expressed by the outflow tract progenitors but its role in the SHF is unclear. We performed knockout studies to determine the role of APJ in SHF cell proliferation and survival. Our data suggested that APJ knockout in the SHF reduced the proliferation of SHF progenitors, while there was no significant impact on survival of the SHF progenitors. In addition, we show that ectopic overexpression of WNT in these cells disrupted aorta and pulmonary morphogenesis from outflow tract. Overall, our study have identified APJ + progenitor population within the SHF that give rise to aorta and pulmonary trunk/artery cells. Furthermore, we show that APJ signaling stimulate proliferation of these cells in the SHF.

Modulation of APLNR Signaling Is Required during the Development and Maintenance of the Hematopoietic System

Apelin receptor (APLNR/AGTRLl1/APJ) marks a transient cell population during the differentiation of hematopoietic stem and progenitor cells (HSPCs) from pluripotent stem cells, but its function during the production and maintenance of hematopoietic stem cells is not clear. We generated an Aplnr-tdTomato reporter mouse embryonic stem cell (mESC) line and showed that HSPCs are generated exclusively from mesodermal cells that express Aplnr-tdTomato. HSPC production from mESCs was impaired when Aplnr was deleted, implying that this pathway is required for their production. To address the role of APLNR signaling in HSPC maintenance, we added APELIN ligands to ex vivo AGM cultures. Activation of the APLNR pathway in this system impaired the generation of long-term reconstituting HSPCs and appeared to drive myeloid differentiation. Our data suggest that the APLNR signaling is required for the generation of cells that give rise to HSCs, but that its subsequent downregulation is required for their maintenance.

The SARS-CoV-2 receptor and other key components of the Renin-Angiotensin-Aldosterone System related to COVID-19 are expressed in enterocytes in larval zebrafish

People with underlying conditions, including hypertension, obesity, and diabetes, are especially susceptible to negative outcomes after infection with coronavirus SARS-CoV-2, which causes COVID-19. Hypertension and respiratory inflammation are exacerbated by the Renin-Angiotensin-Aldosterone System (RAAS), which normally protects from rapidly dropping blood pressure via Angiotensin II (Ang II) produced by the enzyme Ace. The Ace paralog Ace2 degrades Ang II, counteracting its chronic effects, and serves as the SARS-CoV-2 receptor. Ace, the coronavirus, and COVID-19 comorbidities all regulate Ace2, but we do not yet understand how. To exploit zebrafish (Danio rerio) to help understand the relationship of the RAAS to COVID-19, we must identify zebrafish orthologs and co-orthologs of human RAAS genes and understand their expression patterns. To achieve these goals, we conducted genomic and phylogenetic analyses and investigated single cell transcriptomes. Results showed that most human RAAS genes have one or more zebrafish orthologs or co-orthologs. Results identified a specific type of enterocyte as the specific site of expression of zebrafish orthologs of key RAAS components, including Ace, Ace2, Slc6a19 (SARS-CoV-2 co-receptor), and the Angiotensin-related peptide cleaving enzymes Anpep (receptor for the common cold coronavirus HCoV-229E), and Dpp4 (receptor for the Middle East Respiratory Syndrome virus, MERS-CoV). Results identified specific vascular cell subtypes expressing Ang II receptors, apelin, and apelin receptor genes. These results identify genes and cell types to exploit zebrafish as a disease model for understanding mechanisms of COVID-19.

The role of Elabela in kidney disease

Elabela, also known as Toddler or Apela, is a recently discovered hormonal peptide containing 32 amino acids. Elabela is a ligand of the apelin receptor (APJ). APJ is a G protein-coupled receptor widely expressed throughout body, and together with its cognate ligand, apelin, it plays an important role in various physiological processes including cardiovascular functions, angiogenesis and fluid homeostasis. Elabela also participates in embryonic development and pathophysiological processes in adulthood. Elabela is highly expressed in undifferentiated embryonic stem cells and regulates endoderm differentiation and cardiovascular system development. During differentiation, Elabela is highly expressed in pluripotent stem cells and in adult renal collecting ducts and loops, where it functions to maintain water and sodium homeostasis. Other studies have also shown that Elabela plays a crucial role in the pathogenesis of kidney diseases. This review addresses the role of Elabela in kidney diseases including renal ischemia/reperfusion injury, hypertensive nephropathy, diabetic nephropathy, and cardiorenal syndrome.

An intestinal cell type in zebrafish is the nexus for the SARS-CoV-2 receptor and the Renin-Angiotensin-Aldosterone System that contributes to COVID-19 comorbidities

People with underlying conditions, including hypertension, obesity, and diabetes, are especially susceptible to negative outcomes after infection with the coronavirus SARS-CoV-2. These COVID-19 comorbidities are exacerbated by the Renin-Angiotensin-Aldosterone System (RAAS), which normally protects from rapidly dropping blood pressure or dehydration via the peptide Angiotensin II (Ang II) produced by the enzyme Ace. The Ace paralog Ace2 degrades Ang II, thus counteracting its chronic effects. Ace2 is also the SARS-CoV-2 receptor. Ace , the coronavirus, and COVID-19 comorbidities all regulate Ace2 , but we don't yet understand how. To exploit zebrafish ( Danio rerio ) as a disease model to understand mechanisms regulating the RAAS and its relationship to COVID-19 comorbidities, we must first identify zebrafish orthologs and co-orthologs of human RAAS genes, and second, understand where and when these genes are expressed in specific cells in zebrafish development. To achieve these goals, we conducted genomic analyses and investigated single cell transcriptomes. Results showed that most human RAAS genes have an ortholog in zebrafish and some have two or more co-orthologs. Results further identified a specific intestinal cell type in zebrafish larvae as the site of expression for key RAAS components, including Ace, Ace2, the coronavirus co-receptor Slc6a19, and the Angiotensin-related peptide cleaving enzymes Anpep and Enpep. Results also identified specific vascular cell subtypes as expressing Ang II receptors, apelin , and apelin receptor genes. These results identify specific genes and cell types to exploit zebrafish as a disease model for understanding the mechanisms leading to COVID-19 comorbidities.

SUMMARY STATEMENT

Genomic analyses identify zebrafish orthologs of the Renin-Angiotensin-Aldosterone System that contribute to COVID-19 comorbidities and single-cell transcriptomics show that they act in a specialized intestinal cell type.

Characterization of the Apelin/Elabela Receptors (APLNR) in Chickens, Turtles, and Zebrafish: Identification of a Novel Apelin-Specific Receptor in Teleosts

Apelin receptor(s) (APLNR) are suggested to mediate the actions of apelin and Elabela (ELA) peptides in many physiological processes, including cardiovascular development and food intake in vertebrates. However, the functionality of APLNR has not been examined in most vertebrate groups. Here, we characterized two APLNRs APLNR1, APLNR2) in chickens and red-eared sliders, and three APLNRs in zebrafish (APLNR2a, APLNR2b, APLNR3a), which are homologous to human APLNR. Using luciferase-reporter assays or Western blot, we demonstrated that in chickens, APLNR1 (not APLNR2) expressed in HEK293 cells was potently activated by chicken apelin-36 and ELA-32 and coupled to Gi-cAMP and MAPK/ERK signaling pathways, indicating a crucial role of APLNR1 in mediating apelin/ELA actions; in red-eared sliders, APLNR2 (not APLNR1) was potently activated by apelin-36/ELA-32, suggesting that APLNR2 may mediate apelin/ELA actions; in zebrafish, both APLNR2a and APLNR2b were potently activated by apelin-36/ELA-32 and coupled to Gi-cAMP signaling pathway, as previously proposed, whereas the novel APLNR3a was specifically and potently activated by apelin. Similarly, an apelin-specific receptor (APLNR3b) sharing 57% sequence identity with zebrafish APLNR3a was identified in Nile tilapia. Collectively, our data facilitates the uncovering of the roles of APLNR signaling in different vertebrate groups and suggests a key functional switch between APLNR1 and APLNR2/3 in mediating the actions of ELA and apelin during vertebrate evolution.

Apelin attenuates the osteoblastic differentiation of aortic valve interstitial cells via the ERK and PI3-K/Akt pathways

Aortic valve calcification (AVC), which used to be recognized as a passive and irreversible process, is now widely accepted as an active and regulated process characterized by osteoblastic differentiation of aortic valve interstitial cells (AVICs). Apelin, the endogenous ligand for G-protein-coupled receptor APJ, was found to have protective cardiovascular effects in several studies. However, the effects and mechanisms of apelin on osteoblastic differentiation of AVICs have not been elucidated. Using a pro-calcific medium, we devised a method to produce calcific human AVICs. These cells were used to study the relationship between apelin and the osteoblastic calcification of AVICs and the involved signaling pathways. Alkaline phosphatase (ALP) activity/expression and runt-related transcription factor 2 (Runx2) expression were examined as hallmark proteins in this research. The involved signaling pathways were studied using the extracellular signal-regulated kinase (ERK) inhibitor, PD98059, and the phosphatidylinositol 3-kinase (PI3-K) inhibitor, LY294002. The results indicate that apelin attenuates the expression and activity of ALP, the expression of Runx2, and the formation of mineralized nodules. This protective effect was dependent on the dose of apelin, reaching the maximum at 100 pM, and was connected to activity of ERK and Akt (a downstream effector of PI3-K). The activation of ERK and PI3-K initiated the effects of apelin on ALP activity/expression and Runx2, but PD98059 and LY294002 abolished the effect. These results demonstrate that apelin attenuates the osteoblastic differentiation of AVICs via the ERK and PI3-K/Akt pathway.

Genetic targeting of sprouting angiogenesis using Apln-CreER

Under pathophysiological conditions in adults, endothelial cells (ECs) sprout from pre-existing blood vessels to form new ones by a process termed angiogenesis. During embryonic development, Apelin (APLN) is robustly expressed in vascular ECs. In adult mice, however, APLN expression in the vasculature is significantly reduced. Here we show that APLN expression is reactivated in adult ECs after ischaemia insults. In models of both injury ischaemia and tumor angiogenesis, we find that Apln-CreER genetically labels sprouting but not quiescent vasculature. By leveraging this specific activity, we demonstrate that abolishment of the VEGF-VEGFR2 signalling pathway as well as ablation of sprouting ECs diminished tumour vascularization and growth without compromising vascular homeostasis in other organs. Collectively, we show that Apln-CreER distinguishes sprouting vessels from stabilized vessels in multiple pathological settings. The Apln-CreER line described here will greatly aid future mechanistic studies in both vascular developmental biology and adult vascular diseases.

Comparatively examining of the apelin-13 levels in the Capoeta trutta (Heckel, 1843) and Cyprinus carpio (Linnaeus, 1758)

Apelin is a recently discovered peptide produced by several tissues in the various vertebrates and fish. Apelin has been suggested to have role in regulation of many diverse physiological functions including food intake, energy homoeostasis, immunity, osmoregulation and reproduction. In this study, apelin-13 levels in the blood serum of Cyprinus carpio and Capoetta trutta were determined. Then the results were compared between two species and sexes of each species. Apelin-13 level was analysed using the enzyme-linked immunoassay (ELISA) kit (Rat apelin-13 ELISA kit, catalog no: CSB-E14367r). Apelin-13 level in the blood serum of C. trutta was significantly higher than those of the C. carpio (p < 0.05). However, its levels were observed to be no significant difference (p > 0.05) that compared to between sexes of each species. There was a significant negative correlation (r = -0.829, p = 0.0001) between the apelin-13 level and body weight of C. carpio. However, no significant correlation (r = -0.022, p = 0.924) between the apelin-13 level and weight of C. trutta observed.

Apelin-APJ signaling is a critical regulator of endothelial MEF2 activation in cardiovascular development

RATIONALE

The peptide ligand apelin and its receptor APJ constitute a signaling pathway with numerous effects on the cardiovascular system, including cardiovascular development in model organisms such as xenopus and zebrafish.

OBJECTIVE

This study aimed to characterize the embryonic lethal phenotype of the Apj-/- mice and to define the involved downstream signaling targets.

METHODS AND RESULTS

We report the first characterization of the embryonic lethality of the Apj-/- mice. More than half of the expected Apj-/- embryos died in utero because of cardiovascular developmental defects. Those succumbing to early embryonic death had markedly deformed vasculature of the yolk sac and the embryo, as well as poorly looped hearts with aberrantly formed right ventricles and defective atrioventricular cushion formation. Apj-/- embryos surviving to later stages demonstrated incomplete vascular maturation because of a deficiency of vascular smooth muscle cells and impaired myocardial trabeculation and ventricular wall development. The molecular mechanism implicates a novel, noncanonical signaling pathway downstream of apelin-APJ involving Gα13, which induces histone deacetylase (HDAC) 4 and HDAC5 phosphorylation and cytoplasmic translocation, resulting in activation of myocyte enhancer factor 2. Apj-/- mice have greater endocardial Hdac4 and Hdac5 nuclear localization and reduced expression of the myocyte enhancer factor 2 (MEF2) transcriptional target Krüppel-like factor 2. We identify a number of commonly shared transcriptional targets among apelin-APJ, Gα13, and MEF2 in endothelial cells, which are significantly decreased in the Apj-/- embryos and endothelial cells.

CONCLUSIONS

Our results demonstrate a novel role for apelin-APJ signaling as a potent regulator of endothelial MEF2 function in the developing cardiovascular system.

Activin and BMP4 synergistically promote formation of definitive endoderm in human embryonic stem cells

Human embryonic stem cells (hESCs) herald tremendous promise for the production of clinically useful cell types for the treatment of injury and disease. Numerous reports demonstrate their differentiation into definitive endoderm (DE) cells, the germ layer from which pancreatic β cells and hepatocytes arise, solely from exposure to a high dose of recombinant Activin/Nodal. We show that combining a second related ligand, BMP4, in combination with Activin A yields 15%-20% more DE as compared with Activin A alone. The addition of recombinant BMP4 accelerates the downregulation of pluripotency genes, particularly SOX2, and results in upregulation of endogenous BMP2 and BMP4, which in turn leads to elevated levels of phospho-SMAD1/5/8. Combined Activin A and BMP4 treatment also leads to an increase in the expression of DE genes CXCR4, SOX17, and FOXA2 when compared with Activin A addition alone. Comparative microarray studies between DE cells harvested on day 3 of differentiation further reveal a novel set of genes upregulated in response to initial BMP4 exposure. Several of these, including APLNR, LRIG3, MCC, LEPREL1, ROR2, and LZTS1, are expressed in the mouse primitive streak, the site of DE formation. Thus, this synergism between Activin A and BMP4 during the in vitro differentiation of hESC into DE suggests a complex interplay between BMP and Activin/Nodal signaling during the in vivo allocation and expansion of the endoderm lineage.

Apelin enhances cardiac neovascularization after myocardial infarction by recruiting aplnr+ circulating cells

RATIONALE

Neovascularization stimulated by local or recruited stem cells after ischemia is a key process that salvages damaged tissue and shows similarities with embryonic vascularization. Apelin receptor (Aplnr) and its endogenous ligand apelin play an important role in cardiovascular development. However, the role of apelin signaling in stem cell recruitment after ischemia is unknown.

OBJECTIVE

To investigate the role of apelin signaling in recruitment after ischemia.

METHODS AND RESULTS

Aplnr was specifically expressed in circulating cKit+/Flk1+ cells but not in circulating Sca1+/Flk1+ and Lin+ cells. cKit+/Flk1+/Aplnr+ cells increased significantly early after myocardial ischemia but not after hind limb ischemia, indicative of an important role for apelin/Aplnr in cell recruitment during the nascent biological repair response after myocardial damage. In line with this finding, apelin expression was upregulated in the infarcted myocardium. Injection of apelin into the ischemic myocardium resulted in accelerated and increased recruitment of cKit+/Flk1+/Aplnr+ cells to the heart. Recruited Aplnr+/cKit+/Flk1+ cells promoted neovascularization in the peri-infarct area by paracrine activity rather than active transdifferentiation, resulting into cardioprotection as indicated by diminished scar formation and improved residual cardiac function. Aplnr knockdown in the bone marrow resulted in aggravation of myocardial ischemia-associated damage, which could not be rescued by apelin.

CONCLUSIONS

We conclude that apelin functions as a new and potent chemoattractant for circulating cKit+/Flk1+/Aplnr+ cells during early myocardial repair, providing myocardial protection against ischemic damage by improving neovascularization via paracine action.

Effect of apelin-apelin receptor system in postischaemic myocardial protection: a pharmacological postconditioning tool?

In the heart, a great part of ischaemia and reperfusion injuries occurs mainly during the first minutes of reperfusion. The opening of the mitochondrial permeability transition pores is the end point of the cascade to myocardial damage. Also, oxidative stress contributes to cell death. Postconditioning is a protective maneuver that can be selectively timed at the beginning of reperfusion. It is hypothesized that it acts via the reperfusion injury salvage kinase pathway, which includes nitric oxide-dependent and nitric oxide-independent cascades. Apelin is an endogenous peptide that can protect the heart from reperfusion injury if given at the beginning of reperfusion but not before ischaemia. It is hypothesized that it may trigger the reperfusion injury salvage kinase pathway via a specific apelin receptor. Apelin can also limit the oxidative stress by the activation of superoxide dismutase. Apelin and apelin receptor expression increase early after ischaemia and at the beginning of an ischaemic heart failure. These observations suggest that the endogenous release of the peptide can limit the severity of an infarction and ameliorate myocardial contractility compromised by the appearance of the failure. Due to its protective activities, apelin could be a therapeutic tool if administered with the same catheter used for angioplasty or after the maneuvers aimed at bypassing a coronary occlusion.

Tissue renin-angiotensin-aldosterone systems: Targets for pharmacological therapy

The renin-angiotensin-aldosterone system is one of the most important systems in cardiovascular control and in the pathogenesis of cardiovascular diseases. Therefore, it is already a very successful drug target for the therapy of these diseases. However, angiotensins are generated not only in the plasma but also locally in tissues from precursors and substrates either locally expressed or imported from the circulation. In most areas of the brain, only locally generated angiotensins can exert effects on their receptors owing to the blood-brain barrier. Other tissue renin-angiotensin-aldosterone systems are found in cardiovascular organs such as kidney, heart, and vessels and play important roles in the function of these organs and in the deleterious actions of hypertension and diabetes on these tissues. Novel components with mostly opposite actions to the classical renin-angiotensin-aldosterone systems have been described and need functional characterization to evaluate their suitability as novel drug targets.

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.

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.

A microarray screen for direct targets of Zic1 identifies an aquaporin gene, aqp-3b, expressed in the neural folds

The Zic1 transcription factor plays multiple roles during early development, for example, in patterning the early neural plate and formation of the neural crest, somites, and cerebellum. To identify direct downstream target genes of Zic1, a microarray screen was conducted in Xenopus laevis that identified 85 genes upregulated twofold or more. These include transcription factors, receptors, enzymes, proteins involved in retinoic acid signaling, and an aquaglyceroporin (aqp-3b), but surprisingly no genes known to be involved in cell proliferation. We show that both aqp-3 and aqp-3b were expressed in adult tissues, while during early embryonic development, only aqp-3b was transcribed. During neurula stages, aqp-3b was expressed specifically in the neural folds. This pattern of aqp-3b expression closely resembled that of NF-protocadherin (NFPC), which is involved in cell adhesion and neural tube closure. Aqp-3b may also be involved in neural tube closure, since mammalian Aqp-3 promotes cell migration and proliferation.

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.

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.

Emerging role of the apelin system in cardiovascular homeostasis

The angiotensin receptor-like 1 (APJ) and its novel ligand, apelin, share similarities in structure and anatomical distribution with that of angiotensin II and the angiotensin II type 1 receptor. However, apelin has positive inotropic, vasodilatory and diuretic properties. Differential expression and synthesis of apelin and the APJ receptor in normal and failing hearts suggest that the apelin system may contribute to the pathophysiology of human heart failure and has potential therapeutic use in treatment of heart failure.