Discovery of seven hox genes in zebrafish thrombopoiesis

Thrombopoiesis is the production of platelets from megakaryocytes in the bone marrow of mammals. In fish, thrombopoiesis involves the formation of thrombocytes without megakaryocyte-like precursors but derived from erythrocyte thrombocyte bi-functional precursor cells. One unique feature of thrombocyte differentiation involves the maturation of young thrombocytes in circulation. In this study, we investigated the role of hox genes in zebrafish thrombopoiesis to model platelet production. We selected hoxa10b, hoxb2a, hoxc5a, hoxd3a, and hoxc11b from thrombocyte RNA expression data, and checked whether they are expressed in young or mature thrombocytes. We found hoxa10b, hoxb2a, hoxc5a, and hoxd3a were expressed in both young and mature thrombocytes and hoxc11b was expressed in only young thrombocytes. We then performed knockdowns of these 5 hox genes and found hoxc11b knockdown resulted in thrombocytosis and the rest showed thrombocytopenia. To identify hox genes that could have been missed by the above datasets, we performed knockdowns 47 hox genes in the zebrafish genome and found hoxa9a, and hoxb1a knockdowns resulted in thrombocytopenia and they were expressed in both young and mature thrombocytes. In conclusion, our comprehensive knockdown study identified Hoxa10b, Hoxb2a, Hoxc5a, Hoxd3a, Hoxa9a, and Hoxb1a, as positive regulators and Hoxc11b, as a negative regulator for thrombocyte development.

Role of microRNAs and their downstream target transcription factors in zebrafish thrombopoiesis

Previous studies have shown that human platelets and megakaryocytes carry microRNAs suggesting their role in platelet function and megakaryocyte development, respectively. However, a comprehensive study on the microRNAs and their targets has not been undertaken. Zebrafish thrombocytes could be used as a model to study their role in megakaryocyte maturation and platelet function because thrombocytes have both megakaryocyte features and platelet properties. In our laboratory, we identified 15 microRNAs in thrombocytes using single-cell RNA sequencing. We knocked down each of these 15 microRNAs by the piggyback method and found knockdown of three microRNAs, mir-7148, let-7b, and mir-223 in adult zebrafish led to an increase in the percentage of thrombocytes. Functional thrombocyte analysis using plate tilt assay showed no modulatory effect of the three microRNAs on thrombocyte aggregation/agglutination. We also found enhanced thrombosis using arterial laser thrombosis assay in a group of zebrafish larvae after mir-7148, let-7b, and mir-223 knockdowns. These results suggested mir-7148, let-7b, and mir-223 are repressors for thrombocyte production. We then explored miRWalk database for let-7b downstream targets and then selected those that are expressed in thrombocytes, and from this list based on their role in differentiation selected 14 genes, rorca, tgif1, rfx1a, deaf1, zbtb18, mafba, cebpa, spi1a, spi1b, fhl3b, ikzf1, irf5, irf8, and lbx1b that encode transcriptional regulators. The qRT-PCR analysis of expression levels of the above genes following let-7b knockdown showed changes in the expression of 13 targets. We then studied the effect of the 13 targets on thrombocyte production and identified 5 genes, irf5, tgif1, irf8, cebpa, and rorca that showed thrombocytosis and one gene, ikzf1 that showed thrombocytopenia. Furthermore, we tested whether mir-223 regulates any of the above 13 transcription factors after mir-223 knockdown using qRT-PCR. Six of the 13 genes showed similar gene expression as observed with let-7b knockdown and 7 genes showed opposing results. Thus, our results suggested a possible regulatory network in common with both let-7b and mir-223. We also identified that tgif1, cebpa, ikzf1, irf5, irf8, and ikzf1 play a role in thrombopoiesis. Since the ikzf1 gene showed a differential expression profile in let-7b and mir-223 knockdowns but resulted in thrombocytopenia in ikzf1 knockdown in both adults and larvae we also studied an ikzf1 mutant and showed the mutant had thrombocytopenia. Taken together, these studies showed that thrombopoiesis is controlled by a network of transcription regulators that are regulated by multiple microRNAs in both positive and negative manner resulting in overall inhibition of thrombopoiesis.

Role of MicroRNAs and their Downstream Target Transcription Factors in Zebrafish Thrombopoiesis

Previous studies have shown that human platelets and megakaryocytes carry microRNAs suggesting their role in platelet function and megakaryocyte development, respectively. However, a comprehensive study on the microRNAs and their targets has not been undertaken. Zebrafish thrombocytes could be used as a model to study their role in megakaryocyte maturation and platelet function because thrombocytes have both megakaryocyte features and platelet properties. In our laboratory, we identified 15 microRNAs in thrombocytes using single-cell RNA sequencing. We knocked down each of these 15 microRNAs by the piggyback method and found knockdown of three microRNAs, mir-7148, let-7b , and mir-223 in adult zebrafish led to an increase in the percentage of thrombocytes. Functional thrombocyte analysis using plate tilt assay showed no modulatory effect of the three microRNAs on thrombocyte aggregation/agglutination. We also found enhanced thrombosis using arterial laser thrombosis assay in a group of zebrafish larvae after mir-7148, let-7b , and mir-223 knockdowns. These results suggested mir-7148, let-7b , and mir-223 are repressors for thrombocyte production. We then explored miRWalk database for let-7b downstream targets and then selected those that are expressed in thrombocytes, and from this list based on their role in differentiation selected 14 genes, rorca, tgif1, rfx1a, deaf1, zbtb18, mafba, cebpa, spi1a, spi1b, fhl3b, ikzf1, irf5, irf8 , and lbx1b that encode transcriptional regulators. The qRT-PCR analysis of expression levels of the above genes following let-7b knockdown showed changes in the expression of 13 targets. We then studied the effect of the 13 targets on thrombocyte production and identified 5 genes, irf5, tgif1, irf8, cebpa , and rorca that showed thrombocytosis and one gene, ikzf1 that showed thrombocytopenia. Furthermore, we tested whether mir-223 regulates any of the above 13 transcription factors after mir-223 knockdown using qRT-PCR. Six of the 13 genes showed similar gene expression as observed with let-7b knockdown and 7 genes showed opposing results. Thus, our results suggested a possible regulatory network in common with both let-7b and mir-223 . We also identified that tgif1, cebpa, ikzf1, irf5 , irf8 , and ikzf1 play a role in thrombopoiesis. Since the ikzf1 gene showed a differential expression profile in let-7b and mir-223 knockdowns but resulted in thrombocytopenia in ikzf1 knockdown in both adults and larvae we also studied an ikzf1 mutant and showed the mutant had thrombocytopenia. Taken together, these studies showed that thrombopoiesis is controlled by a network of transcription regulators that are regulated by multiple microRNAs in both positive and negative manner resulting in overall inhibition of thrombopoiesis.

Analysis of transcribed sequences from young and mature zebrafish thrombocytes

The zebrafish is an excellent model system to study thrombocyte function and development. Due to the difficulties in separating young and mature thrombocytes, comparative transcriptomics between these two cell types has not been performed. It is important to study these differences in order to understand the mechanism of thrombocyte maturation. Here, we performed single-cell RNA sequencing of the young and mature zebrafish thrombocytes and compared the two datasets for young and mature thrombocyte transcripts. We found a total of 9143 genes expressed cumulatively in both young and mature thrombocytes, and among these, 72% of zebrafish thrombocyte-expressed genes have human orthologs according to the Ensembl human genome annotation. We also found 397 uniquely expressed genes in young and 2153 uniquely expressed genes in mature thrombocytes. Of these 397 and 2153 genes, 272 and 1620 corresponded to human orthologous genes, respectively. Of all genes expressed in both young and mature thrombocytes, 4224 have been reported to be expressed in human megakaryocytes, and 1603 were found in platelets. Among these orthologs, 156 transcription factor transcripts in thrombocytes were found in megakaryocytes and 60 transcription factor transcripts were found in platelets including a few already known factors such as Nfe2 and Nfe212a (related to Nfe2) that are present in both megakaryocytes, and platelets. These results indicate that thrombocytes have more megakaryocyte features and since platelets are megakaryocyte fragments, platelets also appear to be thrombocyte equivalents. In conclusion, our study delineates the differential gene expression patterns of young and mature thrombocytes, highlighting the processes regulating thrombocyte maturation. Future knockdown studies of these young and mature thrombocyte-specific genes are feasible and will provide the basis for understanding megakaryocyte maturation.

Pancytopenia and thrombosis defects in zebrafish mutants of Fanconi anemia genes

Progressive pancytopenia is a common feature observed in DNA crosslink repair deficiency disorder, Fanconi anemia (FA). However, this phenotype has not been recapitulated in single FA gene knockout animal models. In this study, we analyzed hematological characteristics in zebrafish null mutants for two FA genes, fanca and fanco. In adult mutants, we demonstrate age-associated reduction in blood cell counts for all lineages, resembling progressive pancytopenia in FA patients. In larval mutants, we demonstrate vascular injury-induced thrombosis defects, particularly upon treatment with crosslinking agent diepoxybutane (DEB), indicating DNA damage induced inefficiency of thrombocytes.

Splenectomy in zebrafish: a new model for immune thrombocytopenia

In humans, splenectomy is performed to treat many clinical disorders, including immune thrombocytopenia. However, the incidence of splenectomies for immune thrombocytopenia as a therapeutic has significantly declined over the past decade due to the availability of new therapies. Infection and sepsis as a result of splenectomies are well documented, but other long-term effects are not well characterized. Evidence suggests that persons who have had a prior splenectomy may be at an increased risk of vascular conditions. Also, elevated levels of cell-derived microparticles appear to contribute to an increased risk of thrombosis and cardiovascular disease. However, in vivo studies on the increased levels of microparticles following splenectomy are limited. In order to understand the effects of splenectomies, we developed a protocol for splenectomy in adult zebrafish. After anesthesia, the spleen was removed under a stereomicroscope after making an incision on the ventral side of the fish. The spleen was removed by pulling with forceps. The incision was closed by Vetbond tissue glue. Blood collected from both splenectomized zebrafish and those that underwent sham surgeries was immunolabeled with polyclonal antisera against αIIb, followed by flow cytometry. We observed elevated levels of thrombocytes and their microparticles in splenectomized zebrafish. Finally, by injecting αIIb antibody intravenously into zebrafish, we found the thrombocyte counts decreased, suggesting the fish developed immune thrombocytopenia like conditions, which were then reversed by splenectomy. In summary, the model developed here should be useful to study molecular changes due to splenectomy. Also, the zebrafish will be useful in modeling treatment of immune thrombocytopenia like conditions.

Knockdown and Knockout of Tissue Factor Pathway Inhibitor in Zebrafish

Tissue Factor Pathway Inhibitor (TFPI) is an anticoagulant that inhibits factor VIIa and Xa in the blood coagulation pathways. TFPI contains three Kunitz domains, K1, K2, and K3. K1 and K2 inhibit factor VIIa and Xa, respectively. However, the regulation of TFPI is poorly studied. Since zebrafish has become an alternate model to discover novel actors in hemostasis, we hypothesized that TFPI regulation could be studied using this model. As a first step, we confirmed the presence of tfpia in zebrafish using RT-PCR. We then performed piggyback knockdowns of tfpia and found increased coagulation activity in tfpia knockdown. We then created a deletion mutation in tfpia locus using CRISPR/Cas9 method. The tfpia homozygous deletion mutants showed increased coagulation activities similar to that found in tfpia knockdown. Taken together, our data suggest that tfpia is a negative regulator for zebrafish coagulation, and silencing it leads to thrombotic phenotype. Also, the zebrafish tfpia knockout model could be used for reversing this thrombotic phenotype to identify antithrombotic novel factors by the genome-wide piggyback knockdown method.

Identification of zebrafish ortholog for human coagulation factor IX and its age-dependent expression

BACKGROUND

Coagulation factor IX (FIX) is a serine protease zymogen involved in the intrinsic blood coagulation pathway, and its deficiency causes hemophilia B. Zebrafish has three f9 genes, and the ortholog to human F9 is unknown.

OBJECTIVE

To identify the zebrafish ortholog to F9 using sequence analysis and piggyback knockdown technology.

METHODS

Gene and protein sequence analysis for three f9 genes, f9a, f9b, and f9l, present in the zebrafish genome was performed. In vivo and in vitro assays after knockdown of each gene and immunodepletion using specific antibodies were carried out.

RESULTS

Sequence analysis revealed that f9a and f9b are similar to human F9, whereas f9l is similar to human F10. RNA analysis showed an age-dependent increase in expression of all three genes. Zebrafish f9a gene knockdown and Fixa immunodepletion prolonged kinetic partial thromboplastin time (kPTT), whereas f9l knockdown and Fixl immunodepletion prolonged kPTT, kinetic prothrombin time, and kinetic Russell viper venom activation time. Laser-assisted venous thrombosis increased time to occlusion after f9a and f9l knockdown and antibody inhibition of Fixa and Fixl. Further, analysis of plasma proteins by mass spectrometry and immunohistochemistry detected all three proteins.

CONCLUSIONS

Our findings suggest that zebrafish f9a has functional activity similar to human F9. Fixl is functionally similar to Fx. The age-dependent increases of these factors are comparable to those observed in mice and humans. Thus, the zebrafish model could be used to study factors involved in increasing f9a expression during aging. It could also be used to test whether normal human Factor IX and Factor IX Leyden promoter work in zebrafish background.

Knockdown screening of chromatin binding and regulatory proteins in zebrafish identified Suz12b as a regulator of tfpia and an antithrombotic drug target

Tissue factor pathway inhibitor (TFPI) is an anticoagulant protein that inhibits factor VIIa and Xa in the coagulation cascade. It has been shown that forkhead box P3 protein is a TFPI transcriptional repressor. However, there are no studies on chromatin remodeling that control TFPI expression. We hypothesized that the genome-wide knockdowns of the chromatin binding and regulatory proteins (CBRPs) in zebrafish could identify novel tfpia gene regulators. As an initial step, we selected 69 CBRP genes from the list of zebrafish thrombocyte-expressed genes. We then performed a 3-gene piggyback knockdown screen of these 69 genes, followed by quantification of tfpia mRNA levels. The results revealed that knockdown of brd7, ing2, ing3, ing4, and suz12b increased tfpia mRNA levels. The simultaneous knockdown of these 5 genes also increased tfpia mRNA levels. We also performed individual gene and simultaneous 5-gene knockdowns on the 5 genes in zebrafish larvae. We found that after laser injury, it took a longer time for the formation of the thrombus to occlude the caudal vessel compared to the control larvae. We then treated the larvae and adults with a chemical UNC6852 known to proteolytically degrade polycomb repressor complex 2, where SUZ12 is a member, and observed prolongation of time to occlude (TTO) the caudal vein after laser injury and increased tfpia mRNA levels in larvae and adults, respectively. In summary, our results have identified novel epigenetic regulators for tfpia and exploited this information to discover a drug that enhances tfpia mRNA levels and prolongation of TTO. This discovery provides the basis for testing whether UNC6852 could be used as an antithrombotic drug. This approach could be used to study the regulation of other plasma proteins, including coagulant and anticoagulant factors.

Microkinetic coagulation assays for human and zebrafish plasma

Coagulation assays, prothrombin time (PT), and partial thromboplastin time (PTT) are tests to measure the clotting ability of plasma and used in evaluating patients suffering from bleeding disorders. These assays require 100 μl of human plasma. In zebrafish, dilute plasma with exogenously added human fibrinogen was used. Our objective is to create a microkinetic coagulation assay for human and zebrafish plasmas using 1 μl plasma under conditions similar to PT and PTTs. Here, we developed an assay using the Take3 plate with wells holding up to 6 μl, which can be loaded in a microplate reader for measuring the absorbance of fibrin formation. In this assay, we used 1 μl of citrated zebrafish or human plasma followed by the addition of either thromboplastin or Dade ACTIN or factor X activator from Russell viper venom as an activating agent and CaCl2. We found 4 or 3 μl of the final volume of reaction was optimal. Our results showed both zebrafish and human plasmas yielded kinetic PT, kinetic PTT, and kinetic Russel's viper venom time curves similar to previously established curves using dilute plasma. This kinetic coagulation was inhibited by heparin and was reduced significantly in coagulation factor deficient plasmas. These results validated our microkinetic coagulation assays. Moreover, we derived clotting times from these kinetic curves, which were identical to human PT, PTT, and Russel's viper venom time. In conclusion, we established a microkinetic assay that could measure blood coagulation activity in models like zebrafish and human blood samples obtained from a finger prick in adults or heel prick in infants.

Prostasin and hepatocyte growth factor B in factor VIIa generation: Serine protease knockdowns in zebrafish

BACKGROUND

Blood clotting in humans is initiated by the binding of tissue factor to activated coagulation factor VII (FVIIa) in the plasma. Previous studies have reported that hepsin and factor VII (FVII)-activating protease are responsible for generating FVIIa.

OBJECTIVES

We aimed to identify other proteases that may activate FVII using zebrafish as a model.

METHODS

We screened 179 genes encoding serine protease domains using the piggyback knockdown method to identify genes involved in the activation of zebrafish Fvii. A prolonged kinetic prothrombin time (kPT) assay was used to detect gene knockdown effects.

RESULTS

In the primary screen, 21 genes showed prolonged kPT. In the secondary screen, 14 of 21 genes showed positive results. In the tertiary screen, all 14 genes showed prolonged kPT. These 14 genes were knocked down again to estimate relative levels of zebrafish Fviia. Six genes, including known genes, such as f10 and novel prostasin and hepatocyte growth factor B (hgfb), showed lower Fviia levels. Fvii levels were affected only by the knockdown of f7 and not by the knockdown of the other five genes.

CONCLUSIONS

Prostasin and hgfb are involved in generating Fviia. We hypothesize that prostasin exerts serine protease activity directly or indirectly to activate Fvii. As Hgfb has a mutated serine protease domain, it may not cleave Fvii but may bind to Fvii to induce autoactivation. The approach developed here may be extended to design other large-scale knockdown screens.

Generation of transgenic zebrafish with 2 populations of RFP- and GFP-labeled thrombocytes: analysis of their lipids

Zebrafish thrombocytes are similar to mammalian platelets. Mammals have young platelets (also called reticulated platelets) and mature platelets. Likewise, zebrafish have 2 populations of thrombocytes; one is DiI-C18 (DiI)+ (DP), and the other is DiI- (DN). However, the mechanism of selective thrombocyte labeling by DiI is unknown. Furthermore, there is no transgenic zebrafish line where DP and DN thrombocytes are differentially labeled with fluorescent proteins. In this study, we found that Glo fish, in which the myosin light chain 2 promoter drives the rfp gene, have a population of thrombocytes that are red fluorescent protein (RFP) labeled. We also generated transgenic GloFli fish in which DP and DN thrombocytes are labeled with RFP and green fluorescent protein (GFP), respectively. Single-cell lipid analysis showed a twofold increase in phosphatidylethanolamine (PE) and a twofold decrease in phosphatidylcholine (PC) in RFP+ thrombocytes compared with GFP+ thrombocytes, suggesting that lipid composition may be important for DiI differential labeling. Therefore, we tested liposomes prepared with different ratios of PC and PE and observed that liposomes prepared with higher amounts of PE favor DiI labeling, whereas the PC concentration had a modest effect. In liposomes prepared using only PE or PC, increased concentrations of PE resulted in increased DiI binding. These results suggest that because RFP+ thrombocytes have higher PE concentrations, DiI may bind to them efficiently, thus explaining the selective labeling of thrombocytes by DiI. This work also provides GloFli fish that should be useful in understanding the mechanism of thrombocyte maturation.

Effect of MS222 on Hemostasis in Zebrafish

MS222 is a compound used in anesthetizing vertebrates, including fish and frogs. Several side effects of this anesthetic have been reported, but its effect on hemostasis has not been studied. In our laboratory, we have used zebrafish for more than 2 decades as a model system to study hemostasis. During this period, we have had trouble in collecting blood from anesthetized zebrafish and observed more rapid blood clotting than in nonanesthetized counterparts. However, no systematic studies regarding the effect of MS222 on zebrafish hemostasis are available. In this study, we performed various assays such as gill bleeding, measurement of Hct, total blood cell counts, thrombocyte counts, thrombocyte aggregation, and coagula- tion and measured the amount of blood collected. We found that Hct values, the amount of blood collected, bleeding, and coagulation differed significantly between anesthetized and nonanesthetized fish. Our results suggest that blood collected after MS222 anesthesia of zebrafish has altered hemostasis.

G protein-coupled receptor gpr34l mutation affects thrombocyte function in zebrafish

Haemostasis is a defence mechanism that has evolved to protect organisms from losing their circulating fluid. We have previously introduced zebrafish as a model to study the genetics of haemostasis to identify novel genes that play a role in haemostasis. Here, we identify a zebrafish mutant that showed prolonged time to occlusion (TTO) in the laser injury venous thrombosis assay. By linkage analysis and fine mapping, we found a mutation in the orphan G protein-coupled receptor 34 like gene (gpr34l) causing a change of Val to Glu in the third external loop of Gpr34l. We have shown that injection of zebrafish gpr34l RNA rescues the prolonged TTO defect. The thrombocytes from the mutant showed elevated levels of cAMP that supports the defective thrombocyte function. We also have demonstrated that knockdown of this gene by intravenous Vivo-Morpholino injections yielded a phenotype similar to the gpr34l mutation. These results suggest that the lack of functional Gpr34l leads to increased cAMP levels that result in defective thrombocyte aggregation.

Intraflagellar transport proteins are involved in thrombocyte filopodia formation and secretion

Intraflagellar transport (IFT) proteins are vital for the genesis and maintenance of cilia. Our identification of ift122 transcripts in zebrafish thrombocytes that lack primary cilia was unexpected. IFT proteins serve transport in cilia, whose narrow dimensions may have necessitated the evolution of IFT from vesicular transport in ancestral eukaryotes. We hypothesized that IFTs might also facilitate transport within the filopodia that form when thrombocytes are activated. To test this possibility, we knocked down ift122 expression by injecting antisense Morpholino oligonucleotides (MOs) into zebrafish embryos. Laser-induced arterial thrombosis showed prolonged time to occlusion (TTO) of the vessel, as would be expected with defective thrombocyte function. Acute effects in adult zebrafish were evaluated by Vivo-Morpholino (Vivo-MO) knockdown of ift122. Vivo-MO morphants showed a prolonged time to thrombocyte aggregation (TTA) in the plate tilt assay after thrombocyte activation by the following agonists: ADP, collagen, PAR1 peptide, and epinephrine. A luminescence assay for ATP revealed that ATP secretion by thrombocytes was reduced in collagen-activated blood of Vivo-MO ift122 morphants. Moreover, DiI-C18 labeled morphant thrombocytes exposed to collagen showed reductions in filopodia number and length. Analysis of ift mutants, in which cilia defects have been noted, also showed prolongation of TTO in our arterial laser thrombosis assay. Additionally, collagen activation of wild-type thrombocytes led to a concentration of IFT122 both within and at the base of filopodia. Taken together these results, suggest that IFT proteins are involved in both the extension of filopodia and secretion of ATP, which are critical in thrombocyte function.

Animal Models of Thrombosis From Zebrafish to Nonhuman Primates: Use in the Elucidation of New Pathologic Pathways and the Development of Antithrombotic Drugs

Thrombosis is a leading cause of morbidity and mortality worldwide. Animal models are used to understand the pathological pathways involved in thrombosis and to test the efficacy and safety of new antithrombotic drugs. In this review, we will first describe the central role a variety of animal models of thrombosis and hemostasis has played in the development of new antiplatelet and anticoagulant drugs. These include the widely used P2Y12 antagonists and the recently developed orally available anticoagulants that directly target factor Xa or thrombin. Next, we will describe the new players, such as polyphosphate, neutrophil extracellular traps, and microparticles, which have been shown to contribute to thrombosis in mouse models, particularly venous thrombosis models. Other mouse studies have demonstrated roles for the factor XIIa and factor XIa in thrombosis. This has spurred the development of strategies to reduce their levels or activities as a new approach for preventing thrombosis. Finally, we will discuss the emergence of zebrafish as a model to study thrombosis and its potential use in the discovery of novel factors involved in thrombosis and hemostasis. Animal models of thrombosis from zebrafish to nonhuman primates are vital in identifying pathological pathways of thrombosis that can be safely targeted with a minimal effect on hemostasis. Future studies should focus on understanding the different triggers of thrombosis and the best drugs to prevent each type of thrombotic event.

Zebrafish thrombocyte aggregation by whole blood aggregometry and flow cytometry

Zebrafish has become an excellent model system to study mammalian hemostasis. Despite our extensive efforts to develop technologies to measure zebrafish hemostasis and even with previously established thrombocyte qualitative and quantitative functional assays, quantifying thrombocyte function for high throughput applications has been a challenge. In this paper, we have developed two quantitative methods to estimate thrombocyte aggregation: one by whole blood aggregometry and the other by flow cytometry. We found that it is possible to conduct whole blood aggregometry using only 2 µl of blood and the currently available aggregometer. Each of three agonists, arachidonic acid, ADP, and collagen yielded impedance curves similar to those obtained with human blood. We were also able to use flow cytometry to indirectly quantify the extent of thrombocyte aggregation by labeling whole blood with mepacrine, aggregating in the presence of each of the above agonists, separating the aggregates from the white blood cells by centrifugation, and then sorting the resulting white cell fraction for thrombocyte numbers. These methods have high throughput capabilities and have the potential to be used in large scale screens to detect and characterize mutants with thrombocyte functional defects or to identify genes involved in thrombocyte function by large scale knockdowns.

Knockdown of αIIb by RNA degradation by delivering deoxyoligonucleotides piggybacked with control vivo-morpholinos into zebrafish thrombocytes

Morpholino and vivo-morpholino gene knockdown methods have been used to study thrombocyte function in zebrafish. However, a large-scale knockdown of the entire zebrafish genome using these technologies to study thrombocyte function is prohibitively expensive. We have developed an inexpensive gene knockdown method, which uses a hybrid of a control vivo-morpholino and a standard antisense oligonucleotide specific for a gene. This hybrid molecule is able to deliver antisense deoxyoligonucleotides into zebrafish thrombocytes because it piggybacks on a control vivo-morpholino. To validate use of this hybrid molecule in gene knockdowns, we targeted the thrombocyte specific αIIb gene with a hybrid of a control vivo-morpholino and an oligonucleotide antisense to αIIb mRNA. The use of this piggyback technology resulted in degradation of αIIb mRNA and led to thrombocyte functional defect. This piggyback method to knockdown genes is inexpensive since one control vivo-morpholino can be used to target many different genes by making many independent gene-specific oligonucleotide hybrids. Thus, this novel piggyback technology can be utilized for cost-effective large-scale knockdowns of genes to study thrombocyte function in zebrafish.

Dioxin-induced thrombocyte aggregation in zebrafish

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a canonical member of a group of dioxins which are byproducts of industrial combustion and are dangerous environmental pollutants. TCDD has been shown to cause several abnormalities in humans and wildlife, and recently, some dioxins have been found to activate platelets. However, TCDD-mediated platelet activation pathways are elusive and virtually nothing is known about TCDD activation of fish thrombocytes. To investigate TCDD effect on thrombocyte function, we tested zebrafish blood in presence of TCDD using a thrombocyte functional assay. We found that TCDD activated thrombocytes. Further experiments showed that thrombocytes of fish treated with TCDD formed both aggregates and filopodia. To investigate the mechanism of TCDD-mediated activation of thrombocytes we used inhibitors for Gq, cyclooxygenase-1, aryl hydrocarbon receptor (AHR), c-src, Akt, and ERK1/2. We found that TCDD induces AHR which activates c-src and signals the activation of Akt and ERK1/2 which are ultimately involved in generation of thromboxane A2. Furthermore, we found that ADP potentiates TCDD action, which led to the discovery that ADP itself activates AHR in the absence of TCDD. Taken together, these results resolved the pathway of TCDD activation of thrombocytes and led to the finding that ADP is an activator of AHR.

Role of hepsin in factor VII activation in zebrafish

Factor VII, the initiator of the extrinsic coagulation cascade, circulates in human plasma mainly in its zymogen form, factor VII and in small amounts in its activated form, factor VIIa. However, the mechanism of initial generation of factor VIIa is not known despite intensive research using currently available model systems. Earlier findings suggested serine proteases factor VII activating protease and hepsin play a role in activating factor VII, however, it has remained controversial. In this paper we estimated the levels of factor VIIa and factor VII for the first time in zebrafish adult population and also reevaluated the role of the above two serine proteases in activating factor VII in vivo using zebrafish as a model system. Knockdown of factor VII activating protease and hepsin was performed followed by assaying for their effect on factor VIIa concentration and extrinsic coagulation as measured by the kinetic prothrombin time. Factor VII activating protease knockdown showed no change in kinetic prothrombin time and no effect on factor VIIa levels while hepsin knockdown increased the kinetic prothrombin time and significantly reduced the factor VIIa plasma levels. Our results thus indicate that hepsin plays a physiologically important role in factor VII activation and hemostasis in zebrafish.

G6f-like is an ITAM-containing collagen receptor in thrombocytes

Collagen activates mammalian platelets through a complex of the immunoglobulin (Ig) receptor GPVI and the Fc receptor γ-chain, which has an immunoreceptor tyrosine-based activation motif (ITAM). Cross-linking of GPVI mediates activation through the sequential activation of Src and Syk family kinases and activation of PLCγ2. Nucleated thrombocytes in fish are activated by collagen but lack an ortholog of GPVI. In this study we show that collagen activates trout thrombocytes in whole blood and under flow conditions through a Src kinase driven pathway. We identify the Ig receptor G6f-like as a collagen receptor and demonstrate in a cell line assay that it signals through its cytoplasmic ITAM. Using a morpholino for in vivo knock-down of G6f-like levels in zebrafish, we observed a marked delay or absence of occlusion of the venous and arterial systems in response to laser injury. Thus, G6f-like is a physiologically relevant collagen receptor in fish thrombocytes which signals through the same ITAM-based signalling pathway as mammalian GPVI, providing a novel example of convergent evolution.

Role of zebrafish thrombocyte and non-thrombocyte microparticles in hemostasis

Hemostasis is a defense mechanism that protects an organism from bleeding in the event of injury. We have previously demonstrated the utility of the zebrafish as a model to study human hemostasis. However, there are no studies on the role of microparticles in hemostasis in early vertebrates. Studying microparticles in zebrafish may provide insight into the evolution of microparticle function in hemostasis and may lead to direct observation of these microparticles in zebrafish larvae due to transparency of the vessels. In this investigation we demonstrate the presence of cellular microparticles in fish blood by both immunostaining as well as by using zebrafish whose thrombocytes are labeled with green fluorescent protein. Further investigation showed that microparticles were also labeled by fluorescein isothiocyanate annexin V, suggesting that these particles are derived via apoptosis. A portion of the fluorescein isothiocyanate annexin V labeled microparticles was also labeled by DiI-C18. Labeling by DiI-C18 suggests that some microparticles are derived from young thrombocytes. Additionally, GpIIb antibody labels almost all thrombocyte-derived microparticles and a greater percentage of microparticles are labeled by GpIIb antibody than by DiI-C18. This suggests that thrombocyte microparticles are derived from both young and mature thrombocytes. Furthermore, the increase of microparticles by adding excessive microparticles into blood in vitro and through intravenous injections led to an increased hemostatic response. In addition, treatment with tumor necrosis factor alpha resulted in an increased number of thrombocyte microparticles and enhanced hemostasis; in contrast, treatment with zVAD-FMK, a caspase inhibitor, resulted in a decrease in thrombocyte microparticles and decreased hemostasis. We also found that thrombocyte microparticles agglutinate, along with other cells and cellular microparticles, in the presence of an excess of either ristocetin or ultra-large von Willebrand factor. Also, stimulation of von Willebrand factor release in vivo resulted in clusters of thrombocyte microparticles in the veins. Moreover, thrombocyte microparticles were the first to appear at the site of arterial injury. We found that thrombocyte microparticles are functionally equivalent to platelet microparticles. The microparticles initiate arterial thrombus formation in a von Willebrand factor-dependent manner and further enhance thrombus formation by forming clusters of microparticles in venous thrombosis. This finding may have applications for understanding the role of platelet microparticles in humans and may have diagnostic applications.

Separation of young and mature thrombocytes by a novel immuno-selection method

Our earlier studies on the structural and functional properties of zebrafish thrombocytes have shown that they have many similarities to mammalian platelets. We have also shown that zebrafish have both young and mature thrombocytes as do mammalian platelets. In addition, we have distinguished young thrombocytes from mature thrombocytes microscopically using lipophilic DiI-C18, and have shown that young thrombocytes have higher GPIIb receptor levels. However, at present, there is no immunoselection method to separate young thrombocytes from mature thrombocytes in order to study differences among them, such as mRNA expression levels of thrombocyte specific genes. We developed a novel technique employing specific biotinylated anti-Cy3 antibody against the chromophore of DiI-C18 and using streptavidin magnetic beads to separate young thrombocytes from mature thrombocytes. Our technique separates and differentiates young and mature thrombocytes from whole blood. This method is specific and is effective with small amounts of blood.

Crystal structure of thrombin in complex with S-variegin: insights of a novel mechanism of inhibition and design of tunable thrombin inhibitors

The inhibition of thrombin is one of the important treatments of pathological blood clot formation. Variegin, isolated from the tropical bont tick, is a novel molecule exhibiting a unique ‘two-modes’ inhibitory property on thrombin active site (competitive before cleavage, noncompetitive after cleavage). For the better understanding of its function, we have determined the crystal structure of the human α-thrombin:synthetic-variegin complex at 2.4 Å resolution. The structure reveals a new mechanism of thrombin inhibition by disrupting the charge relay system. Based on the structure, we have designed 17 variegin variants, differing in potency, kinetics and mechanism of inhibition. The most active variant is about 70 times more potent than the FDA-approved peptidic thrombin inhibitor, hirulog-1/bivalirudin. In vivo antithrombotic effects of the variegin variants correlate well with their in vitro affinities for thrombin. Our results encourage that variegin and the variants show strong potential for the development of tunable anticoagulants.

Abstract 4377: Proper migration of mammalian prostate stem cells to the area of the prostate gland in zebrafish

Androgen independent prostate cancer contributes to the maximum mortality in prostate cancer patients. One school of thought is that these cancer cells arise in metastatic sites where the cancer cells take on stem cell behavior similar to the stem cell behavior observed in normal prostate gland. Thus, understanding prostate gland stem cells and their migration pattern may provide clues to their biology which may have applications in their migration during metastasis. To understand the migration of mammalian prostate stem cells we used zebrafish as a model. We have previously established zebrafish as a model to study prostate cancer because we found the presence of prostate-like- cells around the zebrafish testes. In this abstract we further characterized location of normal prostate-like cells. We found that they are present in the anterior and posterior aspects of testes. Furthermore, we cloned 2 kb prostate specific antigen promoter into pDsRed 2.1 plasmid and injected this plasmid DNA into single cell-stage zebrafish embryo with I-Sce1 to enhance transgenesis frequency and germline transmission. Larvae injected with PSA promoter-DSRed 2 constructs were screened three days post fertilization (dpf) for the expression of red fluorescence, which was observed in bilaterally arranged spots in gonads. We then used 5 dpf zebrafish larvae and IV-injected cells from four different cell lines expressing either GFP or RFP: two with stem cell phenotype RWPE-1-shDAB2IP (GFP), PZ-HPV7-shDAB2IP (GFP), and two as controls RWPE-1-shCon (GFP) and PC-3 (RFP). After two days, microscopy for green or red fluorescence signal revealed RWPE-1-shDAB2IP and PZ-HPV7-shDAB2IP cells bilaterally migrated to the yolk, i.e. the similar area where PSA promoter expression was found. This specific migration was not observed with RWPE-1-shCon and PC-3 cells lines. These same fluorescence reporter experiments were performed in parallel in 48 hr and 72 hr post fertilization (hpf) dechorinated larvae. We observed similar results in the 72 hpf larvae as were found in 5 dpf larvae but not in the 48 hpf larvae. Taken together, these results suggest that RWPE-1-shDAB2IP and PZ-HPV7-shDAB2IP cells have the homing receptors necessary for migrating to the same area in gonads. Understanding these homing mechanisms may be relevant for controlling the abnormal migration in metastasis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4377. doi:10.1158/1538-7445.AM2011-4377

Laser-induced thrombosis in zebrafish

In the event of injury to the vasculature in vertebrate organisms bleeding is stopped by a defense mechanism called hemostasis. Even though biochemical studies characterized a number of factors, classical genetic methods have not been applied to study hemostasis. We introduced zebrafish as an animal model to study genetics of hemostasis. To conduct genetic studies of hemostasis, we required a global screening method to address all the factors of hemostasis such as those present in plasma, in platelets or those present in the endothelium. Therefore, we developed a global laser induced thrombosis method which can assay all these components. In this paper, we describe the principle of this method as well as provide the detailed protocol so this could be used as a screening tool to measure hemostasis in any laboratory.

Zebrafish von Willebrand factor

von Willebrand factor (vWF) is a large protein involved in primary hemostasis. A dysfunction in this protein or an insufficient production of the protein leads to improper platelet adhesion/aggregation, resulting in a bleeding phenotype known as von Willebrand disease (vWD). To gain a better understanding of vWF interactions in vivo, the use of zebrafish as a model is ideal because of the transparency of the embryos and larvae. In this article, we examined the presence and function of vWF in hemostasis of zebrafish utilizing a variety of molecular methods. Using RT-PCR and antibody staining, we have shown that vWF mRNA is present in thrombocytes. Through antibody staining, we demonstrated vWF is synthesized in blood vessels. The role of zebrafish vWF in hemostasis was established through knockdown methods using vWF morpholino (vWF MO) antisense oligonucleotides. Embryos injected with vWF MO at the one to four cell stages resulted in a bleeding phenotype. Injection of embryos with vWF MO also caused an increase in time to occlusion within arteries in larvae upon laser induced injury. We then used vWF-specific Vivo-morpholinos (VMO) to induce vWF knockdown in adult zebrafish by targeting the exon homologous to the human exon 28 of the vWF gene. The reduced ristocetin-mediated agglutination of thrombocytes in a plate tilting assay, using blood from adult zebrafish injected with VMO, provided evidence that vWF is involved in the hemostatic process. We also administered desmopressin acetate to larvae and adults which resulted in enhanced aggregation/agglutination of thrombocytes. Zebrafish genome database analysis revealed the presence of GPIbβ gene. It also revealed the exon of zebrafish vWF gene corresponding to exon 28 of human vWF gene is highly similar to the exon 28 of human vWF gene, except that it has an insertion that leads to a translated peptide sequence that separates the two A domains coded by this exon. This exon is also conserved in other fishes. In summary, we established that zebrafish vWF has a role similar to that of vWF found in humans, thus, making zebrafish a useful model for studying the cell biology of vWF in vivo.

Abstract 3251: Zebrafish: Is it a viable model for prostate cancer

Since its introduction as a vertebrate model organism for developmental studies, the Zebrafish model has continued to improve in its useful applications for studying human disease and more recently cancer. However, because there has been no evidence for the presence of prostate glands in fish, this model has not been exploited to study cancer of the prostate gland. In this abstract, we present evidence that fish carry prostate-like cells and, therefore, we postulate that zebrafish could be used as a model for prostate cancer research.

We hypothesized that even though a well-defined prostate gland is not present in fish, there may be a few cells similar to thyroid gland. As a first approach, we developed an experimental design for detection of Prostate Specific Antigen (PSA) gene and protein expression, as well as PSA gene mapping. PSA is a serine protease that binds in seminal plasma to protein C inhibitor, which is involved in the clotting cascade in humans. When we analyzed the zebrafish genome, we found a PSA gene that is syntenic to human PSA and at the protein level zebrafish PSA is 70% homologous to human PSA. We also found PSA mRNA by RT-PCR analysis of the total testes RNA. In situ hybridization of the 4 day old zebrafish larva revealed expression of PSA in gonad. Treatment of zebrafish larvae with the anti-androgen, hydroxyl-flutamide, showed a PSA response profile similar to that in mammalian models in which there is a reduction in PSA mRNA at low doses and an increase at high doses with a return to baseline levels. Based on these findings we contend zebrafish have cells which are functionally equivalent to the prostate gland. We further believe that this information supports the notion of using zebrafish as a model to generate prostate cancer -like symptoms and propose for future studies expression of SV40 large T antigen under the PSA promoter to be used to demonstrate proof of principle. Such a model would be useful to explore not only early detection of prostate cancer but also to provide the opportunity for large scale studies of chemical suppression of cancer progression.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3251.

Loss of GATA1 and gain of FLI1 expression during thrombocyte maturation

In this paper, we characterized expression of GATA1 and FLI1 gene promoters in thrombocytes of zebrafish transgenic lines, G1-GM2 and TG(fli1:EGFP)y1 that carry transgenes of GATA1 and FLI1 gene promoters driving GFP. We found two discrete populations of thrombocytes verified by morphology, labeled with GFP in both G1-GM2 and TG(fli1:EGFP)y1 lines: (1) the more intensely labeled GFP+ thrombocyte, and (2) the less intensely labeled GFP+ thrombocytes. The more intensely labeled GFP+ thrombocyte in G1-GM2 line and the less intensely labeled GFP+ thrombocytes in the TG(fli1:EGFP)y1 line corresponded to young thrombocytes. These results showed that young thrombocytes have higher GATA1 promoter activity, while mature thrombocytes have more FLI1 gene promoter transcription. This finding suggests that there is a gradual loss of GATA1 and gain of FLI1 expression as the thrombocytes mature, and this overexpression of FLI1 may help maintain the thrombocyte lineage. Furthermore, the presence of transcriptional factors similar to those found in megakaryocytes raises the possibility that vertebrate thrombocytes may be the forerunners of mammalian megakaryocytes and, therefore, could serve as a model to study megakaryocyte maturation.

Vivo-Morpholino knockdown of alphaIIb: A novel approach to inhibit thrombocyte function in adult zebrafish

Knockdown of protein function by antisense oligonucleotides has been used to understand the protein function not only in development but also in human diseases. Recently, Vivo-Morpholinos, chemically modified morpholinos which penetrate the cells, have been used in adult experimental animal models to alter the splicing and thereby change the protein expression. Until now, there have been no such studies using Vivo-Morpholinos to evaluate hemostatic function in adult animals. We injected alphaIIb Vivo-Morpholinos intravenously into adult zebrafish. Thrombocyte function was assayed by time to aggregation assay of the citrated blood, annexin V binding to thrombocytes, and gill bleeding. The thrombocyte functional inhibition occurred in 24 h after alphaIIb Vivo-Morpholinos injection and reached a maximum in 48 h. However, in 72 h, the inhibition was no longer observed. Reduction of annexin V binding to thrombocytes and increased gill bleeding were observed 48 h after alphaIIb Vivo-Morpholino injections. The action of the alphaIIb Vivo-Morpholino was demonstrated by the presence of an alternatively spliced alphaIIb mRNA and the reduction of alphaIIb in thrombocytes of fish treated with alphaIIb Vivo-Morpholino. These results provide the first proof of principle that thrombocyte function can be inhibited by thrombocyte-specific Vivo-Morpholinos in adult zebrafish and presents an approach to knockdown thrombocyte-specific genes to conduct biochemical studies in thrombocytes. This study also provides the first antisense antithrombotic approach to inhibit thrombocyte function in adult zebrafish.

Evolution of primary hemostasis in early vertebrates

Hemostasis is a defense mechanism which protects the organism in the event of injury to stop bleeding. Recently, we established that all the known major mammalian hemostatic factors are conserved in early vertebrates. However, since their highly vascularized gills experience high blood pressure and are exposed to the environment, even very small injuries could be fatal to fish. Since trypsins are forerunners for coagulation proteases and are expressed by many extrapancreatic cells such as endothelial cells and epithelial cells, we hypothesized that trypsin or trypsin-like proteases from gill epithelial cells may protect these animals from gill bleeding following injuries. In this paper we identified the release of three different trypsins from fish gills into water under stress or injury, which have tenfold greater serine protease activity compared to bovine trypsin. We found that these trypsins activate the thrombocytes and protect the fish from gill bleeding. We found 27 protease-activated receptors (PARs) by analyzing zebrafish genome and classified them into five groups, based on tethering peptides, and two families, PAR1 and PAR2, based on homologies. We also found a canonical member of PAR2 family, PAR2-21A which is activated more readily by trypsin, and PAR2-21A tethering peptide stops gill bleeding just as trypsin. This finding provides evidence that trypsin cleaves a PAR2 member on thrombocyte surface. In conclusion, we believe that the gills are evolutionarily selected to produce trypsin to activate PAR2 on thrombocyte surface and protect the gills from bleeding. We also speculate that trypsin may also protect the fish from bleeding from other body injuries due to quick contact with the thrombocytes. Thus, this finding provides evidence for the role of trypsins in primary hemostasis in early vertebrates.

Modular, easy-to-assemble, low-cost zebrafish facility

We have constructed a standalone, modular, low-cost water recirculation zebrafish facility in our laboratory. This is easy to maintain and will take only approximately $500 and a half a day to build an 80-tank module. The system described here will be useful to even investigators who are new to zebrafish aquaculture.

Microarray analysis of prothrombin knockdown in zebrafish

The serine protease thrombin is generated from its precursor, prothrombin, in the coagulation cascade and plays a central role in fibrin deposition and platelet activation mediated through the protease activated receptors. Knockdown of prothrombin in the zebrafish was previously shown to recapitulate the phenotype observed in prothrombin knockout mice, such as an absence of blood pericardial edema, and hemorrhage. However, the role of thrombin during embryogenesis is not fully understood. To find genes affected by potential thrombin signaling in embryogenesis before blood circulation, microarray analysis was performed using total RNA prepared from antisense-injected, knockdown embryos versus mismatch-injected at 20 h post fertilization. A total of 63 upregulated and downregulated genes were identified with duplicate microarrays using dye reversal and a two-fold difference limitation. Real time RT-PCR for 10 selected genes identified by the microarray confirmed the expression changes in these genes. One particular gene, phlda3, was at least eleven fold upregulated, and in situ hybridization revealed expansion of phlda3 expression in the central nervous system, branchial arches, and head endoderm in knockdown embryos. The identification of these genes regulated by thrombin according to microarray analysis should provide a greater understanding of the effects of thrombin activity in the early vertebrate embryo.

pak2a mutations cause cerebral hemorrhage in redhead zebrafish

The zebrafish is a powerful model for studying vascular development, demonstrating remarkable conservation of this process with mammals. Here, we identify a zebrafish mutant, redhead (rhd(mi149)), that exhibits embryonic CNS hemorrhage with intact gross development of the vasculature and normal hemostatic function. We show that the rhd phenotype is caused by a hypomorphic mutation in p21-activated kinase 2a (pak2a). PAK2 is a kinase that acts downstream of the Rho-family GTPases CDC42 and RAC and has been implicated in angiogenesis, regulation of cytoskeletal structure, and endothelial cell migration and contractility among other functions. Correction of the Pak2a-deficient phenotype by Pak2a overexpression depends on kinase activity, implicating Pak2 signaling in the maintenance of vascular integrity. Rescue by an endothelial-specific transgene further suggests that the hemorrhage seen in Pak2a deficiency is the result of an autonomous endothelial cell defect. Reduced expression of another PAK2 ortholog, pak2b, in Pak2a-deficient embryos results in a more severe hemorrhagic phenotype, consistent with partially overlapping functions for these two orthologs. These data provide in vivo evidence for a critical function of Pak2 in vascular integrity and demonstrate a severe disease phenotype resulting from loss of Pak2 function.

Zebrafish: from hematology to hydrology

Hemostasis is a defense mechanism that protects an organism from bleeding. Abnormal hemostasis results in bleeding disorders and thrombosis. Several factors are known to control hemostasis in mammals. Despite this progress, more factors remain to be identified. Classical genetic approaches have resolved physiological pathways. However, classical genetics could not be applied completely to hemostasis pathways a decade ago. We introduced zebrafish as a model system to study hemostasis and thrombosis using classical genetic methods. First, we established that zebrafish hemostasis is essentially similar to mammalian hemostasis. Secondly, we developed screening tools for isolating hemostatic mutants. These tools enabled us to identify hemostatic mutants, as well as providing a means to study hemostasis by knockdown methods. Continued characterization of the physiology of thrombus formation led to the novel finding of thrombocyte clustering. Recently, we have discovered that fish secrete proteases, which participate in hemostasis and may have direct relevance in humans. Future work to identify most of the players in thrombus formation is underway.

Laser-induced thrombosis in zebrafish larvae: a novel genetic screening method for thrombosis

Classical genetic approaches to study hemostasis and thrombosis have not been available until our recent introduction of the teleost, Danio rerio (the zebrafish), as an effective genetic model for in vivo coagulation assays. The genetic screen for this model is carried out using the genome saturation mutagenesis approach. The resulting mutants are screened for hemostatic or thrombotic defects. We developed a global physiological screening method for thrombosis by utilizing a laser to induce thrombosis in a specifically targeted area of the major artery and vein. Using this assay, we have screened many fish for abnormal hemostasis, and have isolated a number of mutants with abnormal coagulation parameters. These mutants can be grown, bred, and further evaluated for the genetic etiology of their abnormal hemostatic pathways.

The Zebrafish fade out mutant: a novel genetic model for Hermansky-Pudlak syndrome

PURPOSE

To characterize retinal morphology and visual system function in the zebrafish mutant fade out (fad) and to establish the mutant as a lower vertebrate model for Hermansky-Pudlak syndrome (HPS).

METHODS

Retinal morphology of fad larvae was examined between 3 and 9 days postfertilization (dpf) by standard histology, transmission electron microscopy, and immunohistochemistry examination. Apoptotic cells were visualized by TdT-mediated dUTP nick-end labeling (TUNEL) staining. Visual system function was probed by electroretinography and behavioral assessment by optokinetic response measurements. Blood clotting was evaluated by time to occlusion testing of blood vessels as an arterial thrombosis assay. The chromosomal location of fad was determined by simple sequence-length polymorphism mapping. Genomic fragments of candidate genes were cloned by standard molecular techniques and mapped to the zebrafish genome by radiation hybrid mapping.

RESULTS

Mutant fad larvae are hypopigmented and show structural defects in the outer retina. Melanosomes of these larvae in the retinal pigment epithelium are hypopigmented, generally smaller, and progressively reduced in number compared to nonmutant larvae. Progressive microvilli protrusions into the photoreceptor cell layer are not detectable, and photoreceptor outer segments get shorter and are misaligned. Photoreceptors subsequently undergo apoptosis, with a peak of cell death at 6 dpf. Electrical responses of the retina and visual performance are severely reduced. Blood clotting is prolonged in mutant fad larvae. Genomic mapping of fad reveals distinct genomic positions of the mutant gene from known human HPS genes.

CONCLUSIONS

The fad mutant shows syndromic defects in pigmentation, outer retinal structure and function, and blood clotting. This syndrome is characteristic of Hermansky-Pudlak syndrome (HPS), making fad a novel genetic model of HPS. The gene does not cosegregate with the known human HPS genes, suggesting a novel molecular cause of HPS.

SLC24A5, a putative cation exchanger, affects pigmentation in zebrafish and humans

Lighter variations of pigmentation in humans are associated with diminished number, size, and density of melanosomes, the pigmented organelles of melanocytes. Here we show that zebrafish golden mutants share these melanosomal changes and that golden encodes a putative cation exchanger slc24a5 (nckx5) that localizes to an intracellular membrane, likely the melanosome or its precursor. The human ortholog is highly similar in sequence and functional in zebrafish. The evolutionarily conserved ancestral allele of a human coding polymorphism predominates in African and East Asian populations. In contrast, the variant allele is nearly fixed in European populations, is associated with a substantial reduction in regional heterozygosity, and correlates with lighter skin pigmentation in admixed populations, suggesting a key role for the SLC24A5 gene in human pigmentation.

Young thrombocytes initiate the formation of arterial thrombi in zebrafish

The zebrafish system is an excellent vertebrate genetic model to study hemostasis and thrombosis because saturation mutagenesis screens can identify novel genes that play a role in this vital physiologic pathway. To study hemostatic mutations, it is important to understand the physiology of zebrafish hemostasis and thrombosis. Previously, we identified zebrafish thrombocytes and have shown that they participate in arterial thrombus formation. Here, we recognized 2 populations of thrombocytes distinguishable by DiI-C18 (DiI) staining. DiI+ thrombocytes have a high density of adhesive receptors and are functionally more active than DiI- thrombocytes. We classified DiI+ thrombocytes as young and DiI- thrombocytes as mature thrombocytes. We found young and mature thrombocytes each formed independent clusters and that young thrombocytes clustered first. We have also shown that young thrombocytes initiate arterial thrombus formation. We propose that due to the increased adhesive receptor density on young thrombocytes, they adhere first to the subendothelial matrix, get activated rapidly, release agonists, and recruit more young thrombocytes, which further release more agonists. This increase in agonists activates the less active mature thrombocytes, drawing them to the growing thrombus. Since arterial thrombus formation is a fundamental hemostatic event, this mechanism may be conserved in mammals and may open new avenues for prevention of arterial thrombosis.

Zebrafish: a tool to study hemostasis and thrombosis

PURPOSE OF REVIEW

In the past eight years our laboratory has developed the zebrafish model to study hemostasis and thrombosis. The purpose of this review is to explore current developments involving the zebrafish model in the study of hemostasis and thrombosis because the time is now ripe to apply this model to identify novel players that participate in hemostasis and thrombosis.

RECENT FINDINGS

In the past twelve months, three papers appeared in the hemostasis and thrombosis area using the zebrafish model. The first one is a review article that summarizes establishment of the zebrafish model to study hemostasis and thrombosis. The second study is a methodological paper describing assays for measuring hemostasis and thrombosis by inducing vascular occlusion in zebrafish larvae. The third paper describes a knockdown of prothrombin in zebrafish, which recapitulates knockout studies in mouse, and marks the beginning of studies in the hemostasis and thrombosis area by this new knockdown technology. In addition to the above papers, there is one abstract that describes kinetics of thrombocyte and thrombocyte-microparticle recruitment in laser-induced arterial thrombus formation in zebrafish.

SUMMARY

With the above advances, the zebrafish model has now matured to the point that it can address more important questions in the hemostasis and thrombosis area using genetic approaches. This review therefore summarizes the issues described in the above papers along with thoughts about future progress of the zebrafish model as a tool to study hemostasis and thrombosis.

Zebrafish: a genetic model for hemostasis and thrombosis

Here we review the zebrafish hemostatic system, its relevance to mammalian hemostasis, and its efficacy as a vertebrate genetic model to further the understanding of hemostasis and thrombosis.

Knockdown of prothrombin in zebrafish

Thrombin is a serine protease generated from its zymogen, prothrombin, and plays a central role in the coagulation cascade. It is also important for mammalian development. The zebrafish has now been established as an excellent genetic model for studies on mammalian hemostasis and development. In this report, we used prothrombin-specific antisense morpholinos to knock down the levels of prothrombin to characterize the effects of prothrombin deficiency in the zebrafish embryo. Prothrombin morpholino-injected zebrafish embryos yielded an early phenotype exhibiting severe abnormalities that later showed occasional bleeding. In a second late phenotype, the embryos had no observable morphological abnormalities in early stages, but showed occasional bleeding at later stages. These phenotypes resembled characteristics shown by prothrombin knockout mice. Laser-induced vascular injury on some of the normal appearing phenotypic larvae showed a prolonged time to occlusion, and recombinant zebrafish prothrombin injected into these larvae restored a normal time to occlusion thus showing the specificity of the morpholino effect. The system developed here should be useful for investigation of the role of thrombin in vertebrate development.

Annual Fish as a Genetic Model for Aging

Advancement in the genetics of aging and identification of longevity genes has been largely due to the model organisms such as Caenorhabditis elegans and Drosophila melanogaster. However, knowledge gained from these invertebrates will not be able to identify vertebrate-specific longevity genes. The mouse has a relatively long life span of about 3 years, which limits its utility for screening of longevity genes. Fish have been used in aging studies. However, systematic comparison of survivorship curves for fish is lacking. In this study, we compared the survivorship curves of zebrafish and 2 different annual fish, namely, Cynolebias nigripinnis and Nothobranchius rachovii. These studies established that Nothobranchius rachovii has the shortest life span (8.5 months, at which time 10% of population remains). We also established that it is possible to breed Nothobranchius rachovii under laboratory conditions, and showed that their embryos can be stored for several months and hatched at any time by adding water. In addition, we have isolated 31 cDNA markers out of 71 attempted amplifications based on corresponding homologous genomic sequences in zebrafish and Fugu available from public databases, suggesting that approximately 40% of the genes from Nothobranchius rachovii could be easily isolated. Thus, the ability to be bred under laboratory conditions and the availability of cDNA markers for mapping, along with the major advantage of a relatively short life span, make Nothobranchius rachovii an attractive vertebrate genetic model for aging over other available vertebrate models.

Radiographic analysis of zebrafish skeletal defects

Systematic identification of skeletal dysplasias in model vertebrates provides insight into the pathogenesis of human skeletal disorders and can aid in the identification of orthologous human genes. We are undertaking a mutagenesis screen for skeletal dysplasias in adult zebrafish, using radiography to detect abnormalities in skeletal anatomy and bone morphology. We have isolated chihuahua, a dominant mutation causing a general defect in bone growth. Heterozygous chihuahua fish have phenotypic similarities to human osteogenesis imperfecta, a skeletal dysplasia caused by mutations in the type I collagen genes. Mapping and molecular characterization of the chihuahua mutation indicates that the defect resides in the gene encoding the collagen I(alpha1) chain. Thus, chihuahua accurately models osteogenesis imperfecta at the biologic and molecular levels, and will prove an important resource for studies on the disease pathophysiology. Radiography is a practical screening tool to detect subtle skeletal abnormalities in the adult zebrafish. The identification of chihuahua demonstrates that mutant phenotypes analogous to human skeletal dysplasias will be discovered.

Zebrafish-Mycobacterium marinum model for mycobacterial pathogenesis

We report here the development of a pathogenesis model utilizing Mycobacterium marinum infection of zebrafish (Danio rerio) for the study of mycobacterial disease. The zebrafish model mimics certain aspects of human tuberculosis, such as the formation of granuloma-like lesions and the ability to establish either an acute or a chronic infection based upon inoculum. This model allows the genetics of mycobacterial disease to be studied in both pathogen and host.