The Chicken as a Model Organism to Study Heart Development.

Heart development is a complex process and begins with the long-range migration of cardiac progenitor cells during gastrulation. This culminates in the formation of a simple contractile tube with multiple layers, which undergoes remodeling into a four-chambered heart. During this morphogenesis, additional cell populations become incorporated. It is important to unravel the underlying genetic and cellular mechanisms to be able to identify the embryonic origin of diseases, including congenital malformations, which impair cardiac function and may affect life expectancy or quality. Owing to the evolutionary conservation of development, observations made in nonamniote and amniote vertebrate species allow us to extrapolate to human. This review will focus on the contributions made to a better understanding of heart development through studying avian embryos-mainly the chicken but also quail embryos. We will illustrate the classic and recent approaches used in the avian system, give an overview of the important discoveries made, and summarize the early stages of cardiac development up to the establishment of the four-chambered heart.

Cardiac injections of AntagomiRs as a novel tool for knockdown of miRNAs during heart development.

BACKGROUND: Studying microRNA networks during heart development is essential to obtain a better understanding of developmental defects and diseases associated with the heart and to identify novel opportunities for therapeutics. Here we highlight the advantages of chicken embryos as a vertebrate model for studying intermediate processes of heart development. Avians develop a four-chambered heart closely resembling human anatomy and they develop ex utero, which makes them easily accessible. Furthermore, embryos are available all year with a steady supply. RESULTS: In this report we established a novel method for the knockdown of microRNA function by microinjecting AntagomiRs into the chicken heart in ovo. Our approach enables the targeted delivery of antagomirs into a locally restricted area and is not impacted by circulation. After further embryo development the successful miRNA knockdown was confirmed. Loss of function phenotypes can be evaluated rapidly, compared to more time-consuming genetic ablation experiments. The local application avoids potential systemic effects of microRNA knockdown, therefore allowing the assessment of impacts on heart development only. The method can be adjusted for different stages of chicken embryos (HH13-HH18) as well as for knockdown or targeted overexpression of coding genes. CONCLUSION: In conclusion our method allows targeted and locally restricted delivery of Antagomirs to the heart leading to successful knockdown of microRNA function. This method enables rapid phenotypic assessment, for example by gene expression analysis of multiple cardiac genes.

Erythropoietin signaling regulates heme biosynthesis.

Heme is required for survival of all cells, and in most eukaryotes, is produced through a series of eight enzymatic reactions. Although heme production is critical for many cellular processes, how it is coupled to cellular differentiation is unknown. Here, using zebrafish, murine, and human models, we show that erythropoietin (EPO) signaling, together with the GATA1 transcriptional target, AKAP10, regulates heme biosynthesis during erythropoiesis at the outer mitochondrial membrane. This integrated pathway culminates with the direct phosphorylation of the crucial heme biosynthetic enzyme, ferrochelatase (FECH) by protein kinase A (PKA). Biochemical, pharmacological, and genetic inhibition of this signaling pathway result in a block in hemoglobin production and concomitant intracellular accumulation of protoporphyrin intermediates. Broadly, our results implicate aberrant PKA signaling in the pathogenesis of hematologic diseases. We propose a unifying model in which the erythroid transcriptional program works in concert with post-translational mechanisms to regulate heme metabolism during normal development.

The Pu.1 target gene Zbtb11 regulates neutrophil development through its integrase-like HHCC zinc finger.

In response to infection and injury, the neutrophil population rapidly expands and then quickly re-establishes the basal state when inflammation resolves. The exact pathways governing neutrophil/macrophage lineage outputs from a common granulocyte-macrophage progenitor are still not completely understood. From a forward genetic screen in zebrafish, we identify the transcriptional repressor, ZBTB11, as critical for basal and emergency granulopoiesis. ZBTB11 sits in a pathway directly downstream of master myeloid regulators including PU.1, and TP53 is one direct ZBTB11 transcriptional target. TP53 repression is dependent on ZBTB11 cys116, which is a functionally critical, metal ion-coordinating residue within a novel viral integrase-like zinc finger domain. To our knowledge, this is the first description of a function for this domain in a cellular protein. We demonstrate that the PU.1-ZBTB11-TP53 pathway is conserved from fish to mammals. Finally, Zbtb11 mutant rescue experiments point to a ZBTB11-regulated TP53 requirement in development of other organs.

The Early Stages of Heart Development: Insights from Chicken Embryos.

The heart is the first functioning organ in the developing embryo and a detailed understanding of the molecular and cellular mechanisms involved in its formation provides insights into congenital malformations affecting its function and therefore the survival of the organism. Because many developmental mechanisms are highly conserved, it is possible to extrapolate from observations made in invertebrate and vertebrate model organisms to humans. This review will highlight the contributions made through studying heart development in avian embryos, particularly the chicken. The major advantage of chick embryos is their accessibility for surgical manipulation and functional interference approaches, both gain- and loss-of-function. In addition to experiments performed in ovo, the dissection of tissues for ex vivo culture, genomic, or biochemical approaches is straightforward. Furthermore, embryos can be cultured for time-lapse imaging, which enables tracking of fluorescently labeled cells and detailed analysis of tissue morphogenesis. Owing to these features, investigations in chick embryos have led to important discoveries, often complementing genetic studies in mice and zebrafish. As well as including some historical aspects, we cover here some of the crucial advances made in understanding early heart development using the chicken model.

The mTORC1/4E-BP pathway coordinates hemoglobin production with L-leucine availability.

In multicellular organisms, the mechanisms by which diverse cell types acquire distinct amino acids and how cellular function adapts to their availability are fundamental questions in biology. We found that increased neutral essential amino acid (NEAA) uptake was a critical component of erythropoiesis. As red blood cells matured, expression of the amino acid transporter gene Lat3 increased, which increased NEAA import. Inadequate NEAA uptake by pharmacologic inhibition or RNAi-mediated knockdown of LAT3 triggered a specific reduction in hemoglobin production in zebrafish embryos and murine erythroid cells through the mTORC1 (mammalian target of rapamycin complex 1)/4E-BP (eukaryotic translation initiation factor 4E-binding protein) pathway. CRISPR-mediated deletion of members of the 4E-BP family in murine erythroid cells rendered them resistant to mTORC1 and LAT3 inhibition and restored hemoglobin production. These results identify a developmental role for LAT3 in red blood cells and demonstrate that mTORC1 serves as a homeostatic sensor that couples hemoglobin production at the translational level to sufficient uptake of NEAAs, particularly L-leucine.

Iron regulatory protein-1 protects against mitoferrin-1-deficient porphyria.

Mitochondrial iron is essential for the biosynthesis of heme and iron-sulfur ([Fe-S]) clusters in mammalian cells. In developing erythrocytes, iron is imported into the mitochondria by MFRN1 (mitoferrin-1, SLC25A37). Although loss of MFRN1 in zebrafish and mice leads to profound anemia, mutant animals showed no overt signs of porphyria, suggesting that mitochondrial iron deficiency does not result in an accumulation of protoporphyrins. Here, we developed a gene trap model to provide in vitro and in vivo evidence that iron regulatory protein-1 (IRP1) inhibits protoporphyrin accumulation. Mfrn1(+/gt);Irp1(-/-) erythroid cells exhibit a significant increase in protoporphyrin levels. IRP1 attenuates protoporphyrin biosynthesis by binding to the 5'-iron response element (IRE) of alas2 mRNA, inhibiting its translation. Ectopic expression of alas2 harboring a mutant IRE, preventing IRP1 binding, in Mfrn1(gt/gt) cells mimics Irp1 deficiency. Together, our data support a model whereby impaired mitochondrial [Fe-S] cluster biogenesis in Mfrn1(gt/gt) cells results in elevated IRP1 RNA-binding that attenuates ALAS2 mRNA translation and protoporphyrin accumulation.