Efficient detection of single nucleotide variants in targeted genomic loci.

Single nucleotide variants (SNVs), including single nucleotide polymorphisms, are often associated with morphological and/or physiological abnormalities in various organisms. Targeted genomic DNA can be amplified and directly sequenced to detect these mutations, but this method is relatively time consuming and expensive. We recently established the heteroduplex mobility assay to detect genetic mutations as an easy, low-cost method in genome editing, but detecting such small genetic differences remains difficult. Here, we developed a new, simple method to detect single nucleotide changes in the zebrafish genome by polymerase chain reaction (PCR) and electrophoresis. We first designed a specific single stranded DNA with four tandem guanine nucleotides inserted beside the mutation site, called guanine-inserted primer (GIP). When reannealing, hybridized complexes of GIP and PCR amplicons with or without 1-bp-mutated alleles form different bulge structures, presumably leading to different mobilities on a polyacrylamide gel. This GIP-interacting mobility assay is easy to use; therefore, it could contribute to the detection of SNVs in any organism.

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.

CRISPR-Cas9-Mediated Genome Modifications in Zebrafish.

CRISPR-Cas9 genome editing technology has been successfully applied to generate various genetic modifications in zebrafish. The CRISPR-Cas9 system, which originally consisted of three components, CRISPR RNA (crRNA), trans-activating crRNA (tracrRNA), and Cas9, efficiently induces DNA double-strand breaks (DSBs) at targeted genomic loci, often resulting in frameshift-mediated target gene disruption (knockout). However, it remains difficult to perform the targeted integration of exogenous DNA fragments (knock-in) with CRISPR-Cas9. DSBs can be restored through DNA repair mechanisms, such as nonhomologous end joining (NHEJ), microhomology-mediated end joining (MMEJ), and homology-directed repair (HDR). One of our two research groups established a method for the precise MMEJ-mediated targeted integrations of exogenous genes containing homologous microhomology sequences flanking a targeted genomic locus in zebrafish. The other group recently developed a method for knocking in ~200 nt sequences encoding composite tags using long single-stranded DNA (ssDNA) donors. This chapter summarizes the CRISPR-Cas9-mediated genome modification strategy in zebrafish.

Involvement of the zebrafish trrap gene in craniofacial development.

Trrap (transformation/transcription domain-associated protein) is a component shared by several histone acetyltransferase (HAT) complexes and participates in transcriptional regulation and DNA repair; however, the developmental functions of Trrap in vertebrates are not fully understood. Recently, it has been reported that human patients with genetic mutations in the TRRAP gene show various symptoms, including facial dysmorphisms, microcephaly and global developmental delay. To investigate the physiological functions of Trrap, we established trrap gene-knockout zebrafish and examined loss-of-function phenotypes in the mutants. The trrap zebrafish mutants exhibited smaller eyes and heads than the wild-type zebrafish. The size of the ventral pharyngeal arches was reduced and the mineralization of teeth was impaired in the trrap mutants. Whole-mount in situ hybridization analysis revealed that dlx3 expression was narrowly restricted in the developing ventral pharyngeal arches, while dlx2b expression was diminished in the trrap mutants. These results suggest that trrap zebrafish mutants are useful model organisms for a human disorder associated with genetic mutations in the human TRRAP gene.

Critical roles of the ddx5 gene in zebrafish sex differentiation and oocyte maturation.

DEAD-box helicase 5 (Ddx5) functions as an ATP-dependent RNA helicase and as a transcriptional coactivator for several transcription factors; however, the developmental function of the ddx5 gene in vertebrates is not fully understood. We found that the zebrafish ddx5 gene was expressed in developing gonads. Using the genome editing technology transcription activator-like effector nuclease, we established a ddx5-disrupted zebrafish and examined the morphological phenotypes of the mutant. We found that the majority of ddx5-deficient mutants developed as fertile males with normal testes and a small number of ddx5-deficient mutants developed as infertile females with small ovaries. Apoptotic cell death at 31 days post fertilization was increased in thick immature gonads (presumptive developing ovaries) of the ddx5-deficient mutant compared to those of heterozygous wild-type fish, while the number of apoptotic cells in thin immature gonads (presumptive developing testes) was comparable between the mutant and wild-type animals. Histological analysis revealed that ovaries of adult ddx5-deficient females had fewer vitellogenic oocytes and a larger number of stage I and II oocytes. The amount of cyclic adenosine monophosphate in the ddx5-deficient ovaries was high compared to that of wild-type ovaries, presumably leading to the mitotic arrest of oocyte maturation. Therefore, the ddx5 gene is dispensable for testis development, but it is essential for female sex differentiation and oocyte maturation in zebrafish.

Generation and characterization of keap1a- and keap1b-knockout zebrafish.

The Keap1-Nrf2 pathway is an evolutionarily conserved mechanism that protects cells from oxidative stress and electrophiles. Under homeostatic conditions, Keap1 interacts with Nrf2 and leads to its rapid proteasomal degradation, but when cells are exposed to oxidative stress/electrophiles, Keap1 senses them, resulting in an improper Keap1-Nrf2 interaction and Nrf2 stabilization. Keap1 is therefore considered both an "inhibitor" of and "stress sensor" for Nrf2 activation. Interestingly, fish and amphibians have two Keap1s (Keap1a and Keap1b), while there is only one in mammals, birds and reptiles. A phylogenetic analysis suggested that mammalian Keap1 is an ortholog of fish Keap1b, not Keap1a. In this study, we investigated the differences and similarities between Keap1a and Keap1b using zebrafish genetics. We generated zebrafish knockout lines of keap1a and keap1b. Homozygous mutants of both knockout lines were viable and fertile. In both mutant larvae, the basal expression of Nrf2 target genes and antioxidant activity were up-regulated in an Nrf2-dependent manner, suggesting that both Keap1a and Keap1b can function as Nrf2 inhibitors. We also analyzed the effects of the Nrf2 activator sulforaphane in these mutants and found that keap1a-, but not keap1b-, knockout larvae responded to sulforaphane, suggesting that the stress/chemical-sensing abilities of the two Keap1s are different.

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.

Involvement of the centrosomal protein 55 (cep55) gene in zebrafish head formation.

Mammalian CEP55 (centrosomal protein 55 kDa) is a coiled-coil protein localized to the centrosome in interphase cells and is required for cytokinesis. A homozygous non-sense mutation in human CEP55 has been recently identified in perinatal lethal MARCH (multinucleated neurons, anhydramnios, renal dysplasia, cerebellar hypoplasia and hydranencephaly) syndrome. We have isolated zebrafish cep55 mutants defective in head morphology. The zebrafish cep55 gene was expressed in the head including the retina and the pectoral fin at 1 day post-fertilization (dpf), and extensive cell death was widely observed in the head and tail of the cep55 mutant. In the cep55 mutant, the anterior-posterior distance of the ventral pharyngeal arches was short, and retinal lamination was disorganized. Neural cells, such as islet1-positive cells and pax2-positive cells, and fli1b-positive vascular cells were reduced in the head of the cep55 mutant. Thus, we propose that the zebrafish cep55 mutant is a model organism for human MARCH syndrome.

Essential role of prostaglandin E2 and the EP3 receptor in lymphatic vessel development during zebrafish embryogenesis.

Lymphatic endothelial cells arise from the venous endothelial cells in embryonic lymphatic development. However, the molecular mechanisms remain to be elucidated. We here report that prostaglandin (PG) E2 plays essential roles in the embryonic lymphatic development through the EP3 receptor, one of the PGE2 receptors. Knockdown of the EP3 receptor or inhibition of cyclooxygenases (COX; rate-limiting enzymes for PG synthesis) impaired lymphatic development by perturbing lymphatic specification during zebrafish development. These impairments by COX inhibition were recovered by treatment with sulprostone (EP1/3 agonist). Knockdown of the EP3 receptor further demonstrated its requirement in the expression of sex determining region Y-box 18 (sox18) and nuclear receptor subfamily 2, group F, member 2 (nr2f2), essential factors of the lymphatic specification. The EP3 receptor was expressed in the posterior cardinal vein (region of embryonic lymphatic development) and the adjacent intermediate cell mass (ICM) during the lymphatic specification. COX1 was expressed in the region more upstream of the posterior cardinal vein relative to the EP3 receptor, and the COX1-selective inhibitor impaired the lymphatic specification. On the other hand, two COX2 subtypes did not show distinct sites of expression around the region of expression of the EP3 receptor. Finally, we generated EP3-deficient zebrafish, which also showed defect in lymphatic specification and development. Thus, we demonstrated that COX1-derived PGE2-EP3 pathway is required for embryonic lymphatic development by upregulating the expression of key factors for the lymphatic specification.

Identification and biochemical characterization of a second zebrafish autotaxin gene.

Autotaxin (ATX) is a secreted enzyme that produces a bioactive lysophospholipid, lysophosphatidic acid (LPA). ATX plays a role in vascular and neural development in embryos but its mechanisms remain unclear. At the beginning of this study, only one zebrafish atx gene (atxa) was known and had been investigated. In this study, we generated ATX knockout (KO) fish by TALEN targeting atxa. Unexpectedly, atxa KO fish showed neither vascular defects nor reduction of ATX activity, implying the existence of one or more other ATXs in the genome. By a BLAST search using ATXa protein fragments as a query, we found a genomic sequence that closely resembled atxa exons 13, 14 and 15. Consequently, we cloned a cDNA encoding a second zebrafish autotaxin (ATXb), and found that it was transcribed in various tissues. The atxb gene encoded a protein of 832 amino acids (compared to 850 amino acids in ATXa) with 60% amino acid identity to ATXa and clustered with ATXs from other species. A recombinant ATXb protein showed lysophospholipase D (lysoPLD) activities with substrate specificities similar to those of ATXa and mammalian ATXs. These results indicate that ATXb is a second zebrafish ATX, which possibly shares redundant roles with ATXa in embryonic development.

The clock components Period2, Cryptochrome1a, and Cryptochrome2a function in establishing light-dependent behavioral rhythms and/or total activity levels in zebrafish.

The circadian clock generates behavioral rhythms to maximize an organism's physiological efficiency. Light induces the formation of these rhythms by synchronizing cellular clocks. In zebrafish, the circadian clock components Period2 (zPER2) and Cryptochrome1a (zCRY1a) are light-inducible, however their physiological functions are unclear. Here, we investigated the roles of zPER2 and zCRY1a in regulating locomotor activity and behavioral rhythms. zPer2/zCry1a double knockout (DKO) zebrafish displayed defects in total locomotor activity and in forming behavioral rhythms when briefly exposed to light for 3-h. Exposing DKO zebrafish to 12-h light improved behavioral rhythm formation, but not total activity. Our data suggest that the light-inducible circadian clock regulator zCRY2a supports rhythmicity in DKO animals exposed to 12-h light. Single cell imaging analysis revealed that zPER2, zCRY1a, and zCRY2a function in synchronizing cellular clocks. Furthermore, microarray analysis of DKO zebrafish showed aberrant expression of genes involved regulating cellular metabolism, including ATP production. Overall, our results suggest that zPER2, zCRY1a and zCRY2a help to synchronize cellular clocks in a light-dependent manner, thus contributing to behavioral rhythm formation in zebrafish. Further, zPER2 and zCRY1a regulate total physical activity, likely via regulating cellular energy metabolism. Therefore, these circadian clock components regulate the rhythmicity and amount of locomotor behavior.

Spatiotemporal expression of the cocaine- and amphetamine-regulated transcript-like (cart-like) gene during zebrafish embryogenesis.

The cocaine- and amphetamine-regulated transcript (CART) genes are involved in the neural regulation of energy homeostasis; however, their developmental expressions and functions are not fully understood in vertebrates. We have identified a novel zebrafish cart-like gene that encodes a protein of 105 amino acids possessing sequence similarity to zebrafish and mammalian CART proteins. RT-PCR analysis revealed that the cart-like transcripts were maternally supplied and gradually decreased during the cleavage, blastula and gastrula stages; then, transcripts subsequently reaccumulated at the segmentation, pharyngula and hatching stages. Based on a whole-mount in situ hybridization analysis using an antisense cart-like RNA probe, we found that the cart-like transcript was predominantly expressed in both the Rohon-Beard neurons and trigeminal ganglia, suggesting the involvement of the cart-like gene in zebrafish neural development.

Developmental expression of the slurp-like1/ly2.3/ly97.3 and slurp-like2/ly2.2/ly97.2 genes during zebrafish early embryogenesis.

Mammalian SLURP1 and SLURP2 belong to the Ly-6/uPAR superfamily and are involved in maintaining the physiological integrity of keratinocytes. However, the developmental expression and functions of other Ly-6/uPAR family genes in vertebrates are still obscure. We have isolated novel Ly-6/uPAR family genes slurp-like1 (ly2.3/ly97.3) and slurp-like2 (ly2.2/ly97.2) in zebrafish. Both the Slurp-like1 and Slurp-like2 proteins contain the typical signal sequence and carboxy-terminal CCXXXXCN (X: an arbitrary amino acid) consensus sequence of the Ly-6/uPAR family but lack a transmembrane domain and a GPI-anchoring signal sequence, suggesting that both proteins may function as secretory proteins. Whole-mount in situ hybridization analysis revealed that slurp-like1 was predominantly expressed in the floor plate of the neural tube and in the hypochord of the notochord at 24 h post-fertilization (hpf) and detected in the liver and intestinal bulb at 72 hpf, while slurp-like2 was expressed in the midbrain and hindbrain at 24 hpf and detected in the liver and pancreas at 72 hpf. Differential expression profiles of the slurp-like1 and slurp-like2 genes suggest the distinct physiological involvement of these genes in zebrafish early embryogenesis.

Roles of maternal wnt8a transcripts in axis formation in zebrafish.

In early zebrafish development, the program for dorsal axis formation begins soon after fertilization. Previous studies suggested that dorsal determinants (DDs) localize to the vegetal pole, and are transported to the dorsal blastomeres in a microtubule-dependent manner. The DDs activate the canonical Wnt pathway and induce dorsal-specific genes that are required for dorsal axis formation. Among wnt-family genes, only the wnt8a mRNA is reported to localize to the vegetal pole in oocytes and to induce the dorsal axis, suggesting that Wnt8a is a candidate DD. Here, to reveal the roles of maternal wnt8a, we generated wnt8a mutants by transcription activator-like effector nucleases (TALENs), and established zygotic, maternal, and maternal zygotic wnt8a mutants by germ-line replacement. Zebrafish wnt8a has two open reading frames (ORF1 and ORF2) that are tandemly located in the genome. Although the zygotic ORF1 or ORF2 wnt8a mutants showed little or no axis-formation defects, the ORF1/2 compound mutants showed antero-dorsalized phenotypes, indicating that ORF1 and ORF2 have redundant roles in ventrolateral and posterior tissue formation. Unexpectedly, the maternal wnt8a ORF1/2 mutants showed no axis-formation defects. The maternal-zygotic wnt8a ORF1/2 mutants showed more severe antero-dorsalized phenotypes than the zygotic mutants. These results indicated that maternal wnt8a is dispensable for the initial dorsal determination, but cooperates with zygotic wnt8a for ventrolateral and posterior tissue formation. Finally, we re-examined the maternal wnt genes and found that Wnt6a is an alternative candidate DD.

Medaka and zebrafish contactin1 mutants as a model for understanding neural circuits for motor coordination.

A spontaneous medaka ro mutant shows abnormal wobbling and rolling swimming behaviors. By positional cloning, we mapped the ro locus to a region containing the gene encoding Contactin1b (Cntn1b), which is an immunoglobulin (Ig)-superfamily domain-containing membrane-anchored protein. The ro mutant had a deletion in the cntn1b gene that introduced a premature stop codon. Furthermore, cntn1b mutants generated by the CRISPR/Cas9 system and trans-heterozygotes of the CRISPR mutant allele and ro had abnormal swimming behavior, indicating that the cntn1b gene was responsible for the ro-mutant phenotype. We also established zebrafish cntn1a and cntn1b mutants by transcription activator-like effector nucleases (TALENs). Zebrafish cntn1b but not cntn1a mutants showed abnormal swimming behaviors similar to those in the ro mutant, suggesting that Cntn1b plays a conserved role in the formation or function of the neural circuits that control swimming in teleosts. Although Cntn1-deficient mice have abnormal cerebellar neural circuitry, there was no apparent histological abnormality in the cerebellum of medaka or zebrafish cntn1b mutants. The medaka cntn1b mutants had defective optokinetic response (OKR) adaptation and abnormal rheotaxis (body positioning relative to water flow). Medaka and zebrafish cntn1b mutants are effective models for studying the neural circuits involved in motor learning and motor coordination.

CRISPR/Cas9-Mediated Targeted Knockin of Exogenous Reporter Genes in Zebrafish.

Genome editing technologies such as ZFN, TALEN, and CRISPR/Cas9 efficiently induce DNA double-stranded breaks (DSBs) at a targeted genomic locus, often resulting in a frameshift-mediated target gene disruption. It remains difficult to perform targeted integration of exogenous genes by genome editing technologies. DSBs can be restored through DNA repair mechanisms, such as non-homologous end joining (NHEJ), microhomology-mediated end joining (MMEJ), and homologous recombination (HR). It is well known that HR facilitates homology-dependent integration of donor DNA template into a targeted locus. Recently, both NHEJ-mediated and MMEJ-mediated targeted integrations of exogenous genes have been developed in zebrafish. This chapter summarizes the application of CRISPR/Cas9-mediated knock-in technology in zebrafish.

Exogenous gene integration mediated by genome editing technologies in zebrafish.

Genome editing technologies, such as transcription activator-like effector nuclease (TALEN) and the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein (Cas) systems, can induce DNA double-strand breaks (DSBs) at the targeted genomic locus, leading to frameshift-mediated gene disruption in the process of DSB repair. Recently, the technology-induced DSBs followed by DSB repairs are applied to integrate exogenous genes into the targeted genomic locus in various model organisms. In addition to a conventional knock-in technology mediated by homology-directed repair (HDR), novel knock-in technologies using refined donor vectors have also been developed with the genome editing technologies based on other DSB repair mechanisms, including non-homologous end joining (NHEJ) and microhomology-mediated end joining (MMEJ). Therefore, the improved knock-in technologies would contribute to freely modify the genome of model organisms.

Functional visualization and disruption of targeted genes using CRISPR/Cas9-mediated eGFP reporter integration in zebrafish.

The CRISPR/Cas9 complex, which is composed of a guide RNA (gRNA) and the Cas9 nuclease, is useful for carrying out genome modifications in various organisms. Recently, the CRISPR/Cas9-mediated locus-specific integration of a reporter, which contains the Mbait sequence targeted using Mbait-gRNA, the hsp70 promoter and the eGFP gene, has allowed the visualization of the target gene expression. However, it has not been ascertained whether the reporter integrations at both targeted alleles cause loss-of-function phenotypes in zebrafish. In this study, we have inserted the Mbait-hs-eGFP reporter into the pax2a gene because the disruption of pax2a causes the loss of the midbrain-hindbrain boundary (MHB) in zebrafish. In the heterozygous Tg[pax2a-hs:eGFP] embryos, MHB formed normally and the eGFP expression recapitulated the endogenous pax2a expression, including the MHB. We observed the loss of the MHB in homozygous Tg[pax2a-hs:eGFP] embryos. Furthermore, we succeeded in integrating the Mbait-hs-eGFP reporter into an uncharacterized gene epdr1. The eGFP expression in heterozygous Tg[epdr1-hs:eGFP] embryos overlapped the epdr1 expression, whereas the distribution of eGFP-positive cells was disorganized in the MHB of homozygous Tg[epdr1-hs:eGFP] embryos. We propose that the locus-specific integration of the Mbait-hs-eGFP reporter is a powerful method to investigate both gene expression profiles and loss-of-function phenotypes.

Detection of Multiple Genome Modifications Induced by the CRISPR/Cas9 System.

The recent remarkable innovation of an RNA-guided nuclease system, the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) system, enables us the modification of specific genomic loci in various model animals including zebrafish. With this system, multiple guide RNAs simultaneously injected with the Cas9 nuclease into zebrafish embryos cause multiple genome modifications at different genomic loci with high efficiency; therefore, a simple method to detect individual mutations at distinct loci is desired. In this chapter, we describe a procedure for inducing multiple CRISPR/Cas9-mediated genome modifications in zebrafish and a convenient method to detect CRISPR/Cas9-induced insertion and/or deletion (indel) mutations using a heteroduplex mobility assay (HMA).

Site-Specific Integration of Exogenous Genes Using Genome Editing Technologies in Zebrafish.

The zebrafish (Danio rerio) is an ideal vertebrate model to investigate the developmental molecular mechanism of organogenesis and regeneration. Recent innovation in genome editing technologies, such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated protein 9 (Cas9) system, have allowed researchers to generate diverse genomic modifications in whole animals and in cultured cells. The CRISPR/Cas9 and TALEN techniques frequently induce DNA double-strand breaks (DSBs) at the targeted gene, resulting in frameshift-mediated gene disruption. As a useful application of genome editing technology, several groups have recently reported efficient site-specific integration of exogenous genes into targeted genomic loci. In this review, we provide an overview of TALEN- and CRISPR/Cas9-mediated site-specific integration of exogenous genes in zebrafish.