Critical role of biklf in erythroid cell differentiation in zebrafish

Creg1 Regulates Erythroid Development via TGF-β/Smad2-Klf1 Axis in Zebrafish

Understanding the regulation of normal erythroid development will help to develop new potential therapeutic strategies for disorders of the erythroid lineage. Cellular repressor of E1A-stimulated genes 1 (CREG1) is a glycoprotein that has been implicated in the regulation of tissue homeostasis. However, its role in erythropoiesis remains largely undefined. In this study, it is found that CREG1 expression increases progressively during erythroid differentiation. In zebrafish, creg1 mRNA is preferentially expressed within the intermediate cell mass (ICM)/peripheral blood island (PBI) region where primitive erythropoiesis occurs. Loss of creg1 leads to anemia caused by defective erythroid differentiation and excessive apoptosis of erythroid progenitors. Mechanistically, creg1 deficiency results in reduced activation of TGF-β/Smad2 signaling pathway. Treatment with an agonist of the Smad2 pathway (IDE2) could significantly restore the defective erythroid development in creg1-/- mutants. Further, Klf1, identified as a key target gene downstream of the TGF-β/Smad2 signaling pathway, is involved in creg1 deficiency-induced aberrant erythropoiesis. Thus, this study reveals a previously unrecognized role for Creg1 as a critical regulator of erythropoiesis, mediated at least in part by the TGF-β/Smad2-Klf1 axis. This finding may contribute to the understanding of normal erythropoiesis and the pathogenesis of erythroid disorders.

Cooperative contributions of the klf1 and klf17 genes in zebrafish primitive erythropoiesis

Krüppel-like transcription factors (Klfs), which are characterized by the three conserved C-terminal zinc fingers, are involved in various biological processes, such as haematopoiesis and angiogenesis. However, how the Klf family of transcription factors cooperate in organogenesis remains elusive. During zebrafish embryogenesis, both klf1 and klf17 are expressed in the intermediate cell mass (ICM), where primitive erythroid cells are produced. Using CRISPR-Cas9 genome editing technology, we established klf1-klf17 double mutant zebrafish to investigate the functionally interactive roles of the klf1 and klf17 genes. The klf1-klf17 mutant exhibited a diminished number of circulating primitive erythroid cells at 2 days postfertilization (dpf), while klf1 or klf17 single mutants and wild-type embryos produced comparable numbers of primitive erythroid cells. Circulating erythroid cells from the klf1-klf17 mutant possessed larger nuclei at 2 dpf than wild-type cells, suggesting the impairment of primitive erythroid cell maturation. The expression of the erythroid cell maturation markers band3 and mitoferrin, but not the haematopoietic progenitor markers c-myb and scl, was decreased in the klf1-klf17 mutant at 1 dpf. Thus, these results illustrate the cooperative function of klf1 and klf17 in the maturation processes of zebrafish primitive erythroid cells.

Characterization of biklf/klf17-deficient zebrafish in posterior lateral line neuromast and hatching gland development

Krüpple-like factors (Klfs) are highly conserved zinc-finger transcription factors that regulate various developmental processes, such as haematopoiesis and cardiovascular development. In zebrafish, transient knockdown analysis of biklf/klf17 using antisense morpholino suggests the involvement of biklf/klf17 in primitive erythropoiesis and hatching gland development; however, the continuous physiological importance of klf17 remains uncharacterized under the genetic ablation of the klf17 gene among vertebrates. We established the klf17-disrupted zebrafish lines using the CRISPR/Cas9 technology and performed phenotypic analysis throughout early embryogenesis. We found that the klf17-deficient embryos exhibited abnormal lateral line neuromast deposition, whereas the production of primitive erythrocytes and haemoglobin production were observed in the klf17-deficient embryos. The expression of lateral line neuromast genes, klf17 and s100t, in the klf17-deficient embryos was detected in posterior lateral line neuromasts abnormally positioned at short intervals. Furthermore, the klf17-deficient embryos failed to hatch and died without hatching around 15 days post-fertilization (dpf), whereas the dechorionated klf17-deficient embryos and wild-type embryos were alive at 15 dpf. The klf17-deficient embryos abolished hatching gland cells and Ctsl1b protein expression, and eliminated the expression of polster and hatching gland marker genes, he1.1, ctsl1b and cd63. Thus, the klf17 gene plays important roles in posterior lateral line neuromast and hatching gland development.

Kruppel-like factor family genes are expressed during Xenopus embryogenesis and involved in germ layer formation and body axis patterning

BACKGROUND

Kruppel-like factors (Klfs) are a family of transcription factors consisting of 17 members in mammals, Klf1-Klf17, which are involved in fundamental cellular physiological procedures, such as cell proliferation, differentiation, and apoptosis. However, their functions in embryonic development have been poorly understood. Our previous study has demonstrated that the pluripotency factor Klf4 participates in germ layer formation and axis patterning of Xenopus embryos by means of the regulation of key developmental signals. In the present study, we further investigated comprehensively the expression and functions of the klf family genes, klf2, klf5, klf6, klf7, klf8, klf11, klf15, and klf17, during the embryogenesis of Xenopus laevis.

RESULTS

Spatio-temporal expression analyses demonstrate that these genes are transcribed both maternally and zygotically in Xenopus embryos, and during organogenesis and tissue differentiation, they are localized to a variety of placodes and tissues. Gain and loss of function studies manifest that Klf factors play different roles in germ layer formation and body axis patterning. Moreover, each Klf factor exhibits distinct regulatory effects on the expression of genes that are essential for germ layer formation and body axis patterning.

CONCLUSIONS

These results suggest that Klf factors are involved in the fine-tuning of these genes during early embryogenesis.

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.

Epigenetic and genetic mechanisms in red cell biology

PURPOSE OF REVIEW

Erythropoiesis, in which hematopoietic stem cells (HSCs) generate lineage-committed progenitors that mature into erythrocytes, is regulated by numerous chromatin modifying and remodeling proteins. We will focus on how epigenetic and genetic mechanisms mesh to establish the erythroid transcriptome and how studying erythropoiesis can yield genomic principles.

RECENT FINDINGS

Trans-acting factor binding to small DNA motifs (cis-elements) underlies regulatory complex assembly at specific chromatin sites, and therefore unique transcriptomes. As cis-elements are often very small, thousands or millions of copies of a given element reside in a genome. Chromatin restricts factor access in a context-dependent manner, and cis-element-binding factors recruit chromatin regulators that mediate functional outputs. Technologies to map chromatin attributes of loci in vivo, to edit genomes and to sequence whole genomes have been transformative in discovering critical cis-elements linked to human disease.

SUMMARY

Cis-elements mediate chromatin-targeting specificity, and chromatin regulators dictate cis-element accessibility/function, illustrating an amalgamation of genetic and epigenetic mechanisms. Cis-elements often function ectopically when studied outside of their endogenous loci, and complex strategies to identify nonredundant cis-elements require further development. Facile genome-editing technologies provide a new approach to address this problem. Extending genetic analyses beyond exons and promoters will yield a rich pipeline of cis-element alterations with importance for red cell biology and disease.

csrnp1a is necessary for the development of primitive hematopoiesis progenitors in zebrafish

The CSRNP (cystein-serine-rich nuclear protein) transcription factors are conserved from Drosophila to human. Functional studies in mice, through knockout for each of their paralogs, have resulted insufficient to elucidate the function of this family of proteins in vertebrate development. Previously, we described the function of the zebrafish ortholog, Csnrp1/Axud1, showing its essential role in the survival and proliferation of cephalic progenitors. To extend our understanding of this family, we have studied the function of its paralog csrnp1a. Our results show that csrnp1a is expressed from 0 hpf, until larval stages, particularly in cephalic territories and in the intermediate cell mass (ICM). Using morpholinos in wild type and transgenic lines we observed that Csrnp1a knockdown generates a mild reduction in head size and a depletion of blood cells in circulation. This was combined with in situ hybridizations to analyze the expression of different mesodermal and primitive hematopoiesis markers. Morphant embryos have impaired blood formation without disruption of mesoderm specification, angiogenesis or heart development. The reduction of circulating blood cells occurs at the hematopoietic progenitor level, affecting both the erythroid and myeloid lineages. In addition, cell proliferation was also altered in hematopoietic anterior sites, specifically in spi1 expression domain. These and previous observations suggest an important role of Csnrps transcription factors in progenitor biology, both in the neural and hematopoietic linages.

Fishing pluripotency mechanisms in vivo

To understand the molecular mechanisms that regulate the biology of embryonic stem cells (ESCs) it is necessary to study how they behave in vivo in their natural environment. It is particularly important to study the roles and interactions of the different proteins involved in pluripotency and to use this knowledge for therapeutic purposes. The recent description of key pluripotency factors like Oct4 and Nanog in non-mammalian species has introduced other animal models, such as chicken, Xenopus, zebrafish and medaka, to the study of pluripotency in vivo. These animal models complement the mouse model and have provided new insights into the evolution of Oct4 and Nanog and their different functions during embryonic development. Furthermore, other pluripotency factors previously identified in teleost fish such as Klf4, STAT3, Sox2, telomerase and Tcf3 can now be studied in the context of a functional pluripotency network. The many experimental advantages of fish will fuel rapid analysis of the roles of pluripotency factors in fish embryonic development and the identification of new molecules and mechanisms governing pluripotency.

Involvement of Neptune in induction of the hatching gland and neural crest in the Xenopus embryo

Neptune, a Krüppel-like transcription factor, is expressed in various regions of the developing Xenopus embryo and it has multiple functions in the process of development in various organs. In situ hybridization analysis showed that Neptune is expressed in the boundary region between neural and non-neural tissues at the neurula stage, but little is known about the function of Neptune in this region. Here, we examined the expression and function of Neptune in the neural plate border (NPB) in the Xenopus embryo. Depletion of Neptune protein in developing embryos by using antisense MO caused loss of the hatching gland and otic vesicle as well as malformation of neural crest-derived cranial cartilages and melanocytes. Neptune MO also suppressed the expression of hatching gland and neural crest markers such as he, snail2, sox9 and msx1 at the neurula stage. Subsequent experiments showed that Neptune is necessary and sufficient for the differentiation of hatching gland cells and that it is located downstream of pax3 in the signal regulating the differentiation of these cells. Thus, Neptune is a new member of hatching gland specifier and plays a physiological role in determination and specification of multiple lineages derived from the NPB region.

Transcriptional regulation of a myeloid-lineage specific gene lysozyme C during zebrafish myelopoiesis

lysozyme C (lyz), a glycoside hydrolase expressed exclusively in myeloid cells, is involved in the host defense against bacterial infection. We isolated a 2.4kb zebrafish lyz promoter region and established transgenic lines that drive enhanced green fluorescent protein (EGFP) to examine how lyz expression is regulated during myelopoiesis. We found that the 2.4kb lyz promoter is sufficient to drive myeloid-specific expression of EGFP in zebrafish. We identified potential transcriptional regulatory elements including a Runx element (TGTGGT at -1.70kb) and a C/ebp element (TTTGGCAAT at -1.46kb) in the lyz promoter, and showed that they are required for myeloid-specific expression of EGFP. We found that the myeloid-lineage transcription factors C/ebp1, Runx1 and Pu.1 can bind to the 2.4kb lyz promoter. Forced expression of runx1, c/ebp1 and pu.1 together induced ectopic lyz expression in the intermediated cell mass (ICM). Thus, we propose that c/ebp1 and runx1 presumably cooperated with pu.1 in the transcriptional regulation of lyz during zebrafish myelopoiesis.

Mir-144 selectively regulates embryonic alpha-hemoglobin synthesis during primitive erythropoiesis

Precise transcriptional control of developmental stage-specific expression and switching of alpha- and beta-globin genes is significantly important to understand the general principles controlling gene expression and the pathogenesis of thalassemia. Although transcription factors regulating beta-globin genes have been identified, little is known about the microRNAs and trans-acting mechanism controlling alpha-globin genes transcription. Here, we show that an erythroid lineage-specific microRNA gene, miR-144, expressed at specific developmental stages during zebrafish embryogenesis, negatively regulates the embryonic alpha-globin, but not embryonic beta-globin, gene expression, through physiologically targeting klfd, an erythroid-specific Krüppel-like transcription factor. Klfd selectively binds to the CACCC boxes in the promoters of both alpha-globin and miR-144 genes to activate their transcriptions, thus forming a negative feedback circuitry to fine-tune the expression of embryonic alpha-globin gene. The selective effect of the miR-144-Klfd pathway on globin gene regulation may thereby constitute a novel therapeutic target for improving the clinical outcome of patients with thalassemia.

The receptor protein-tyrosine phosphatase, Dep1, acts in arterial/venous cell fate decisions in zebrafish development

Dep1 is a transmembrane protein-tyrosine phosphatase (PTP) that is expressed in vascular endothelial cells and has tumor suppressor activity. Mouse models with gene targeted Dep1 either show vascular defects, or do not show any defects at all. We used the zebrafish to investigate the role of Dep1 in early development. The zebrafish genome encodes two highly homologous Dep1 genes, Dep1a and Dep1b. Morpholinos specific for Dep1a and Dep1b induced defects in vasculature, resulting in defective blood circulation. However, Green Fluorescent Protein expression in fli1a::gfp1 transgenic embryos and cdh5 expression, markers of vascular endothelial cells, were normal upon Dep1a- and Dep1b-MO injection. Molecular markers indicated that arterial specification was reduced and venous markers were expanded in Dep1 morphants. Moreover, the Dep1a/Dep1b knockdowns were rescued by inhibition of Phosphatidylinositol-3 kinase (PI3K) and by expression of active Notch and Grl/Hey2. Our results suggest a model in which Dep1 acts upstream in a signaling pathway inhibiting PI3K, resulting in expression of Notch and Grl, thus regulating arterial specification in development.

Zebrafish in hematology: sushi or science?

After a decade of the "modern era" of zebrafish hematology research, what have been their major contributions to hematology and what challenges does the model face? This review argues that, in hematology, zebrafish have demonstrated their suitability, are proving their utility, have supplied timely and novel discoveries, and are poised for further significant contributions. It presents an overview of the anatomy, physiology, and genetics of zebrafish hematopoiesis underpinning their use in hematology research. Whereas reverse genetic techniques enable functional studies of particular genes of interest, forward genetics remains zebrafish's particular strength. Mutants with diverse and interesting hematopoietic defects are emerging from multiple genetic screens. Some mutants model hereditary blood diseases, occasionally leading to disease genes first; others provide insights into developmental hematology. Models of malignant hematologic disorders provide tools for drug-target and pharmaceutics discovery. Numerous transgenic zebrafish with fluorescently marked blood cells enable live-cell imaging of inflammatory responses and host-pathogen interactions previously inaccessible to direct observation in vivo, revealing unexpected aspects of leukocyte behavior. Zebrafish disease models almost uniquely provide a basis for efficient whole animal chemical library screens for new therapeutics. Despite some limitations and challenges, their successes and discovery potential mean that zebrafish are here to stay in hematology research.

Combinatorial regulation of novel erythroid gene expression in zebrafish

OBJECTIVE

The specification and differentiation of hematopoietic stem cells into red blood cells requires precise coordination by multiple transcription factors. Most genes important for erythroid maturation are regulated by the Gata family of DNA-binding proteins. Previously, we identified three novel genes kelch-repeat containing protein (krcp), kiaa0650, and testhymin/glucocorticoid inducible transcript 1 (glcci1) to be expressed in erythroid cells in a Gata-independent manner, and we sought to further understand how these transcripts are regulated during zebrafish hematopoiesis.

MATERIALS AND METHODS

We employed a loss-of-function approach, using combinations of antisense morpholinos to hematopoietic transcription factors and assayed for changes in gene expression in zebrafish embryos.

RESULTS

Upon examination of embryos deficient for Gata1, Gata2, Biklf, and/or Scl, we found distinct gene combinations were required for expression of the novel genes. While krcp expression was dependent upon Gata1 and Biklf, kiaa0650 expression was greatly reduced and glcci1 was maintained in Gata1/Gata2/Biklf-deficient embryos. As with the gata1 gene, kiaa0650 and krcp required Scl for blood expression. Although reduced, glcci1 was expressed in posterior blood precursors in the absence of Scl and Gata2.

CONCLUSIONS

This work identifies glcci1 as having Scl-independent expression in the posterior hematopoietic mesoderm, suggesting that its posterior expression is activated by factors upstream or parallel to Scl and Gata2. Additionally, these studies establish that blood gene expression programs are regulated by transcription factors acting in combination during erythroid maturation.

A global role for zebrafish klf4 in embryonic erythropoiesis

There are two waves of erythropoiesis, known as primitive and definitive waves in mammals and lower vertebrates including zebrafish. The founding member of the Kruppel-like factor (KLF) family of CACCC-box binding proteins, EKLF/Klf1, is essential for definitive erythropoiesis in mammals but only plays a minor role in primitive erythropoiesis. Morpholino knockdown experiments have shown a role for zebrafish klf4 in primitive erythropoiesis and hatching gland formation. In order to generate a global understanding of how klf4 might influence gene expression and differentiation, we have performed expression profiling of klf4 morphants, and then performed validation of many putative target genes by qRT-PCR and whole mount in situ hybridization. We found a critical role for klf4 in embryonic globin, heme synthesis and hatching gland gene expression. In contrast, there was an increase in expression of definitive hematopoietic specific genes such as larval globin genes, runx1 and c-myb from 24 hpf, suggesting a selective role for klf4 in primitive rather than definitive erythropoiesis. In addition, we show klf4 preferentially binds CACCC box elements in the primitive zebrafish beta-like globin gene promoters. These results have global implications for primitive erythroid gene regulation by KLF-CACCC box interactions.

Cloning and expression of zebrafish genes encoding the heme synthesis enzymes uroporphyrinogen III synthase (UROS) and protoporphyrinogen oxidase (PPO)

Heme is synthesized from glycine and succinyl CoA by eight heme synthesis enzymes. Although genetic defects in any of these enzymes are known to cause severe human blood diseases, their developmental expression in mammals is unknown. In this paper, we report two zebrafish heme synthesis enzymes, uroporphyrinogen III synthase (UROS) and protoporphyrinogen oxidase (PPO) that are well conserved in comparison to their human counterparts. Both UROS and PPO formed pairs of bilateral stripes in the lateral plate mesoderm at the 15-somite stage. At 24 h post-fertilization (hpf), UROS and PPO were predominantly expressed in the intermediate cell mass (ICM) that is the major site of primitive hematopoiesis. The expression of UROS and PPO was drastically suppressed in the bloodless mutants cloche and vlad tepes/gata 1 from 15-somite to 24hpf stages, indicating that both cloche and vlad tepes/gata 1 are required for the induction and maintenance of UROS and PPO expression in the ICM.

Identification of zygotic genes expressed at the midblastula transition in zebrafish

Early development of the embryo is directed by maternal gene products and characterised by limited zygotic gene activity, cell division synchrony and no cell motility in several vertebrates including fish and frogs. At the midblastula transition (MBT), zygotic transcription is grossly activated, cells become motile and cell divisions become asynchronous. The aim of this study was to identify genes whose expression is up-regulated at the MBT in zebrafish. Suppression subtractive hybridisation (SSH) was employed to isolate 48 unique cDNAs, 28 of which show significant similarity to known genes and 20 represent novel cDNAs. Twenty one of these genes, with potential roles in transcriptional regulation, cell cycle control, and embryonic patterning showed increased expression at the MBT. Our results demonstrate the value of SSH as a tool to clone novel, zygotic, developmentally regulated genes that may be important in the progression of the MBT and embryonic patterning.

Zebrafish KLF4 is essential for anterior mesendoderm/pre-polster differentiation and hatching

Gene knockout studies of Krüppel-like factors (KLFs) in mice have shown essential roles in organogenesis. A screen for KLF family members in zebrafish identified many KLFs. One of these, zebrafish KLF4 (zKLF4) is the homologue of neptune, a Xenopus laevis KLF. zKLF4 is expressed from approximately 80% epiboly a patch of dorsal/anterior mesendodermal cells called the pre-polster and, subsequently, in the polster and hatching gland. Here we investigate the function of zKLF4 using morpholino-based antisense oligonucleotides. Knockdown of zKLF4 resulted in complete absence of hatching gland formation and subsequent hatching in zebrafish. In addition, there was early knockdown of expression of the pre-polster/anterior mesendoderm markers CatL, cap1, and BMP4. These results indicate zKLF4 is expressed within the pre-polster, an early mesendodermal site, and that it plays a critical role in the differentiation of these cells into hatching gland cells.

Perturbation of rRNA Synthesis in the bap28 Mutation Leads to Apoptosis Mediated by p53 in the Zebrafish Central Nervous System

Zebrafish is a powerful vertebrate model system for using forward genetics to elucidate mechanisms of early development. We have used chemical mutagenesis to screen for mutants that show defects in the CNS. Here we describe the isolation of the bap28 mutation that leads to abnormalities in the brain starting at midsomitogenesis stages. Mutant embryos display excess apoptosis primarily in the central nervous system (CNS) and die by days 6-7 after fertilization. The mutation was positionally cloned and shown to affect a gene that encodes a large protein with high similarity to the uncharacterized human protein BAP28 and lower similarity to yeast Utp10. Utp10 is a component of a nucleolar U3 small nucleolar RNA-containing RNP complex that is required for transcription of ribosomal DNA and for processing of 18 S rRNA. We show that zebrafish Bap28 likewise is required for rRNA transcription and processing, with a major effect on 18 S rRNA maturation. We suggest that bap28 is required for cell survival in the CNS through its role in rRNA synthesis and processing. Inhibition of p53 protein expression in bap28 mutants led to embryos with morphologically normal appearance, suggesting that p53 is involved in triggering apoptosis in the bap28 mutant CNS. The bap28 mutation provides a genetic approach to study the role of ribosome biogenesis in the development of a vertebrate embryo.

Characterization of the heme synthesis enzyme coproporphyrinogen oxidase (CPO) in zebrafish erythrogenesis

Hemoglobin consists of heme and globin proteins and is essential for oxygen transport in all vertebrates. Although biochemical features of heme synthesis enzymes have been well characterized, the function of these enzymes in early embryogenesis is not fully understood. We found that the sixth heme synthesis enzyme, coproporphyrinogen oxidase (CPO), is predominantly expressed in the intermediate cell mass (ICM) that is a major site of zebrafish primitive hematopoiesis. Knockdown of zebrafish CPO using anti-sense morpholinos (CPO-MO) leads to a significant suppression of hemoglobin production without apparent reduction of blood cells. Injection of human CPO RNA, but not a mutant CPO RNA that is similar to a mutant responsible for a hereditary coproporphyria (HCP), restores hemoglobin production in the CPO-MO-injected embryos. Furthermore, expression of CPO in the ICM is severely suppressed in both vlad tepes/gata1 mutants and in biklf-MO-injected embryos. In contrast, over-expression of biklf and gata1 significantly induces ectopic CPO expression. The function of CPO in heme biosynthesis is apparently conserved between zebrafish and human, suggesting that CPO-MO-injected zebrafish embryos might be a useful in vivo assay system to measure the biological activity of human CPO mutations.

Characterisation of duplicate zinc finger like 2 erythroid precursor genes in zebrafish

In separate expression pattern and micro-array screens the zinc finger containing factor, znfl2, has been previously implicated in hematopoiesis. Here we analysed znfl2 expression in detail and performed genetic epistatic analysis in a series of hematopoietic mutants and transient gain-of-function models. znfl2 expression in the hematopoietic intermediate mesoderm and derived erythrocytes required early genes cloche and spadetail, but not gata1. Expression was up-regulated in scl gain-of-function embryos, identifying znfl2 as an early erythroid factor that is regulated upstream or independently of gata1. Furthermore, we identified a duplicate znfl2 gene in the genome (znfl2b) which was expressed in early mesendoderm and weakly in the lateral plate mesoderm, overlapping in expression with znfl2. The production of loss-of-function models for znfl2, znfl2b and znfl2/znfl2b together suggested that these erythrocyte specific zinc finger genes are dispensible for erythropoiesis.

Transcription factor KLF7 is important for neuronal morphogenesis in selected regions of the nervous system

The Krüppel-like transcription factors (KLFs) are important regulators of cell proliferation and differentiation in several different organ systems. The mouse Klf7 gene is strongly active in postmitotic neuroblasts of the developing nervous system, and the corresponding protein stimulates transcription of the cyclin-dependent kinase inhibitor p21waf/cip gene. Here we report that loss of KLF7 activity in mice leads to neonatal lethality and a complex phenotype which is associated with deficits in neurite outgrowth and axonal misprojection at selected anatomical locations of the nervous system. Affected axon pathways include those of the olfactory and visual systems, the cerebral cortex, and the hippocampus. In situ hybridizations and immunoblots correlated loss of KLF7 activity in the olfactory epithelium with significant downregulation of the p21waf/cip and p27kip1 genes. Cotransfection experiments extended the last finding by documenting KLF7's ability to transactivate a reporter gene construct driven by the proximal promoter of p27kip1. Consistent with emerging evidence for a role of Cip/Kip proteins in cytoskeletal dynamics, we also documented p21waf/cip and p27kip1 accumulation in the cytoplasm of differentiating olfactory sensory neurons. KLF7 activity might therefore control neuronal morphogenesis in part by optimizing the levels of molecules that promote axon outgrowth.

Microarray analysis of zebrafishcloche mutant using amplified cDNA and identification of potential downstream target genes

Zebrafish is an excellent model organism for studying vertebrate development and human disease. With the availability of increased numbers of zebrafish mutants and microarray chips, gene expression profiling has become a powerful tool for identification of downstream target genes perturbed by a specific mutation. One of the obstacles often encountered, however, is to isolate large numbers of zebrafish mutant embryos that are indistinguishable in morphology from the wild-type siblings for microarray analysis. Here, we report a method using amplified cDNA derived from five embryos for gene expression profiling of the 18-somite zebrafish cloche (clo) mutant, in which development of hematopoietic and endothelial lineages is severely impaired. In total, 31 differentially expressed target genes are identified, of which 13 have not been reported previously. We further determine that of these 13 new targets, 8 genes, including coproporphyrinogen oxidase (cpo), carbonic anhydrase (cahz), claudin g (cldn g), zinc-finger-like gene 2 (znfl2), neutrophil cytosol factor 1 (ncf1), matrix metalloproteinase 13 (mmp13), dual specificity phosphatase 5 (dusp5), and a novel gene referred as zebrafish vessel-specific gene 1 (zvsg1) are predominantly expressed in hematopoietic and endothelial cells. Comparative analysis demonstrates that this method is comparable and complementary to that of the conventional approach using unamplified sample. Our study provides valuable information for studying hematopoiesis and vessel formation. The method described here offers a powerful tool for gene expression profiling of zebrafish mutants in general.

Neptune is involved in posterior axis and tail formation inXenopus embryogenesis

In order to elucidate the molecular mechanisms underlying the posterior axis and tail formation in embryogenesis, the function of Neptune, a zinc-finger transcription factor, in Xenopus laevis embryos was investigated. Injection of neptune mRNA into the animal pole area of embryos resulted in the formation of an additional tail structure that included a neural tube and muscle tissue. This activity required FGF signaling since coinjection of a dominant-negative FGF receptor RNA (XFD) completely blocked the formation of a tail structure. A loss-of-function experiment using a fusion construct of neptune and Drosophila engrailed (en-neptune) RNA showed that endogenous Neptune is necessary for formation of the posterior trunk and tail. Furthermore, activity of Neptune was necessary for the endogenous expression of brachyury and fgf-8 at the late gastrula stage. These findings demonstrate a novel function of Neptune in the process of anterior-posterior axis formation through the FGF and brachyury signaling cascades. An experiment using a combination explant with ventral and dorsal marginal tissues showed that cooperation of these two distinct tissues is important for the tail formation and that expression of Neptune in prospective ventral cells may be involved in the activation of the process of tail formation.

A crucial interaction between embryonic red blood cell progenitors and paraxial mesoderm revealed in spadetail embryos

Zebrafish embryonic red blood cells (RBCs) develop in trunk intermediate mesoderm (IM), and early macrophages develop in the head, suggesting that local microenvironmental cues regulate differentiation of these two blood lineages. spadetail (spt) mutant embryos, which lack trunk paraxial mesoderm (PM) due to a cell-autonomous defect in tbx16, fail to produce embryonic RBCs but retain head macrophage development. In spt mutants, initial hematopoietic gene expression is absent in trunk IM, although endothelial and pronephric expression is retained, suggesting that early blood progenitor development is specifically disrupted. Using cell transplantation, we reveal that spt is required cell autonomously for early hematopoietic gene expression in trunk IM. Further, we uncover an interaction between embryonic trunk PM and blood progenitors that is essential for RBC development. Importantly, our data identify a hematopoietic microenvironment that allows embryonic RBC production in the zebrafish.

Identification of the Drosophila progenitor of mammalian Krüppel-like factors 6 and 7 and a determinant of fly development

The Krüppel-like transcription factors (KLFs) represent a family of 15 different zinc finger proteins of the C(2)H(2) type that are involved in vertebrate development and which control cell proliferation, growth and differentiation. Structural-functional considerations have segregated KLF6 and KLF7 into a phylogenetically distinct group. Here we report the identification of Luna, the Drosophila progenitor of the mammalian KLF6/KLF7 group. This conclusion is based on the near sequence identity, as well as the comparable location of the DNA-binding domains and nuclear localization signals of the insect and mammalian proteins. The homology extends to the composition and function of the amino-terminal segment of Luna which, similarly to the mammalian counterparts, stimulates transcription in a reporter gene assay. We also present preliminary in vivo evidence of Luna involvement in embryonic development and cell differentiation. First, luna RNA interference and luna overexpression during early Drosophila embryogenesis leads to developmental arrest at different embryonic stages. Second, targeted perturbation of luna expression in the forming compound eye interferes with terminal cell differentiation, but not cell specification. We therefore propose that Luna is a novel transcriptional determinant of Drosophila development.

Runx3 is required for hematopoietic development in zebrafish

We cloned zebrafish runx3/aml2/cbfa3 and examined its expression and function during embryogenesis. In the developing embryo, runx3 is dynamically expressed in hematopoietic, neuronal, and cartilaginous tissues. Hematopoietic expression of runx3 commences late in embryogenesis in the ventral tail intermediate cell mass and later colocalizes with spi1 and lyz in circulating blood cells. In the cloche mutant, hematopoietic expression was absent, suggesting that Runx3 functions downstream of cloche in a hematopoietic pathway. Neuronal tissues expressing runx3 include the trigeminal ganglia and Rohon-Beard neurons. Runx3 appears to contribute to normal development of primitive and definitive hematopoietic cells. When Runx3 function was compromised using morpholino oligonucleotides, a reduction in the number of mature blood cells was observed. Furthermore, Runx3 depletion decreased runx1 expression in the ventral wall of the dorsal aorta and reduced the number of spi1- and lyz-containing blood cells. Conversely, ubiquitous overexpression of runx3 led to an increase in primitive blood cell numbers, together with an increase in runx1-expressing cells in the ventral wall of the dorsal aorta. We propose a role for Runx3 in the regulation of blood cell numbers.

Complementary expression of AP-2 and AP-2rep in ectodermal derivatives of Xenopus embryos

In an attempt to define the pattern of developmental expression of AP-2rep and AP-2 in Xenopus embryos, we cloned a Xenopus AP-2rep cDNA. The AP-2rep message was localized in the organizer region at the gastrula stage whereas AP-2 was expressed ventro-laterally in the animal hemisphere. Later, AP-2rep was expressed in the entire neural tissue at the neurula stage while AP-2 was predominantly expressed in the cranial neural crest areas. The endogenous expression of AP-2 in the neural crest area was diminished by ectopic injection of AP-2rep RNA, suggesting a role for AP-2rep in the differentiation of neural tissues by restricting the expression of AP-2 in the Xenopus embryo.

Transcriptional regulation of erythropoiesis. Fine tuning of combinatorial multi-domain elements

Haematopoiesis, the differentiation of haematopoietic stem cells and progenitors into various lineages, involves complex interactions of transcription factors that modulate the expression of downstream genes and mediate proliferation and differentiation signals. Commitment of pluripotent haematopoietic stem cells to the erythroid lineage induces erythropoiesis, the production of red blood cells. This process involves a concerted progression through an erythroid burst forming unit (BFU-E), an erythroid colony forming unit (CFU-E), proerythroblast and an erythroblast. The terminally differentiated erythrocytes, in mammals, lose their nucleus yet function several more months. A well-coordinated cohort of transcription factors regulates the formation, survival, proliferation and differentiation of multipotent progenitor into the erythroid lineage. Here, we discuss broad-spectrum factors essential for self-renewal and/or differentiation of multipotent cells as well as specific factors required for proper erythroid development. These factors may operate solely or as part of transcriptional complexes, and exert activation or repression. Sequence comparisons reveal evolutionarily conserved modular composition for these factors; X-ray crystallography demonstrates that they include multidomain elements (e.g. HLH or zinc finger motifs), consistent with their complex interactions with other proteins. Finally, transfections and genomic studies show that the timing of each factor's expression during the hematopoietic process, the cell lineages affected and the existing combination of other factors determine the erythroid cell fate.

The homeobox gene mbx is involved in eye and tectum development

The homeobox gene mbx is first activated at the end of gastrulation in zebrafish in the presumptive forebrain and midbrain region. During somitogenesis stages, the anterior expression of mbx, which partly overlaps the future eye field, gradually decreases, while midbrain expression intensifies and becomes restricted to the presumptive tectum. Knockdown of mbx expression by morpholino antisense oligonucleotides (mbx-MO) leads to a reduction in the size of the eyes and tectum. Expression domains of rx1 and pax6 in the eye field and of mab21l2 in the eye field and tectum anlage were reduced in size in mbx-MO-injected embryos by somitogenesis stages. Further, induction of islet1 and lim3 expression in the eye at 2 days postfertilization (dpf) was suppressed in mbx-MO-injected embryos. In mbx-MO-injected embryos at 2-5 dpf, the lamination of the eye was disorganized and the number of retinal axons was substantially reduced, but the few remaining axons navigated appropriately to the contralateral tectum. A chimeric protein composed of the Mbx DNA-binding domain and the VP16 activation domain affected eye and tectum development similarly to mbx-MO knockdown, suggesting that Mbx acts as a transcriptional repressor in the zebrafish embryo. Based on these data, we propose that the mbx homeobox gene is required for the development of the eyes and tectum.

A nonsense mutation in zebrafish gata1 causes the bloodless phenotype in vlad tepes

Vlad tepes (vlt(m651)) is one of only five "bloodless" zebrafish mutants isolated through large-scale chemical mutagenesis screening. It is characterized by a severe reduction in blood cell progenitors and few or no blood cells at the onset of circulation. We now report characterization of the mutant phenotype and the identification of the gene mutated in vlt(m651). Embryos homozygous for the vlt(m651) mutation had normal expression of hematopoietic stem cell markers through 24 h postfertilization, as well as normal expression of myeloid and lymphoid markers. Analysis of erythroid development revealed variable expression of erythroid markers. Through positional and candidate gene cloning approaches we identified a nonsense mutation in the gata1 gene, 1015C --> T (Arg-339 --> Stop), in vlt(m651). The nonsense mutation was located C-terminal to the two zinc fingers and resulted in a truncated protein that was unable to bind DNA or mediate GATA-specific transactivation. A BAC clone containing the zebrafish gata1 gene was able to rescue the bloodless phenotype in vlt(m651). These results show that the vlt(m651) mutation is a previously uncharacterized gata1 allele in the zebrafish. The vlt(m651) mutation sheds new light on Gata1 structure and function in vivo, demonstrates that Gata1 plays an essential role in zebrafish hematopoiesis with significant conservation of function between mammals and zebrafish, and offers a powerful tool for future studies of the hematopoietic pathway.

neptune, a Krüppel-like transcription factor that participates in primitive erythropoiesis in Xenopus

The specification of the erythroid lineage from hematopoietic stem cells requires the expression and activity of lineage-specific transcription factors. One transcription factor family that has several members involved in hematopoiesis is the Krüppel-like factor (KLF) family [1]. For example, erythroid KLF (EKLF) regulates beta-globin expression during erythroid differentiation [2-6]. KLFs share a highly conserved zinc finger-based DNA binding domain (DBD) that mediates binding to CACCC-box and GC-rich sites, both of which are frequently found in the promoters of hematopoietic genes. Here, we identified a novel Xenopus KLF gene, neptune, which is highly expressed in the ventral blood island (VBI), cranial ganglia, and hatching and cement glands. neptune expression is induced in response to components of the BMP-4 signaling pathway in injected animal cap explants. Similar to its family member, EKLF, Neptune can bind CACCC-box and GC-rich DNA elements. We show that Neptune cooperates with the hematopoietic transcription factor XGATA-1 to enhance globin induction in animal cap explants. A fusion protein comprised of Neptune's DBD and the Drosophila engrailed repressor domain suppresses the induction of globin in ventral marginal zones and in animal caps. These studies demonstrate that Neptune is a positive regulator of primitive erythropoiesis in Xenopus.