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The earliest stages of embryonic development in all animals examined rely on maternal gene products that are generated during oogenesis and supplied to the egg. The period of maternal control of embryonic development varies among animals according to the onset of zygotic transcription and the persistence of maternal gene products. This maternal regulation has been little studied in vertebrates, due to the difficulty in manipulating maternal gene function and lack of basic molecular information. However, recent maternal-effect screens in the zebrafish have generated more than 40 unique mutants that are providing new molecular entry points to the maternal control of early vertebrate development. Here we discuss recent studies of 12 zebrafish mutant genes that illuminate the maternal molecular controls on embryonic development, including advances in the regulation of animal-vegetal polarity, egg activation, cleavage development, body plan formation, tissue morphogenesis, microRNA function and germ cell development.
The zebrafish has emerged as a premiere genetic tool for studying vertebrate development. In the 1990’s forward genetic screens identified numerous zygotic mutants defective in key molecules important in early embryonic development[1–7]. Recently, a major focus has shifted towards more specialized screens, including the identification of maternal-effect mutations in adult screens in the zebrafish. Since the earliest stages of development are driven primarily by maternal gene products, the identification of corresponding mutants is critical to provide genetic entry points to known maternally-controlled processes, which are still poorly understood in vertebrates.
Maternal-effect screens have identified over 40 mutants affecting many early developmental processes (Figure 1). Collectively, these mutants have defects in oocyte development, egg activation[8,9], embryonic cleavage[8–11], patterning and morphogenesis[9–11]. New studies have focused on the molecular identification of the corresponding mutant genes, a key step to understanding the molecular mechanisms governing the very earliest stages of embryogenesis. The results have unveiled novel and known genes with unanticipated roles in early development. Mutants generated by reverse genetic TILLING methods in the zebrafish have also recently revealed important roles for small noncoding RNA molecules, miRNAs and piRNAs[13,14], in the regulation of maternal processes in zebrafish development.
In this review, we highlight recent contributions to the molecular regulation of animal-vegetal polarity in the oocyte and egg, maternal gene regulation of early embryonic patterning, tissue morphogenesis, and small noncoding RNA molecules, which are emerging as important players in germ line development.
In frogs and fish, the first developmental asymmetry of the embryo is the animal-vegetal axis, which predicts the anterior-posterior axis of the embryo. This asymmetry is established during early stages of oogenesis and is first marked by formation of the Balbiani body (Bb; also referred to as the mitochondrial cloud) adjacent to the germinal vesicle (the oocyte nucleus) in stage I oocytes. The Bb position predicts the location of the vegetal pole, but its presence is only transient, as it disassembles by stage II of oogenesis. The Bb is composed of a collection of mitochondria, ER, germinal granules, and several germ plasm mRNAs (reviewed in). Recently, a thorough study recapitulated in zebrafish transgenic constructs the localization pattern of three Bb-localized transcripts. The 3’ UTRs of nanos, vasa, and dazl directed their localization initially to the Bb and then to the vegetal cortex of the oocyte, likely via the METRO pathway described in Xenopus. The ensuing distribution at the vegetal cortex differs among these transcripts: dazl persists at the vegetal cortex, vasa extends around the cortex, and nanos becomes unlocalized (Figure 2), suggesting, unexpectedly, that coordinate localization of germ plasm components is followed by their redistribution to distinct locations. However, after fertilization of the egg, these germ plasm RNAs reunite in the blastodisc at the animal pole, where they localize to the germ plasm at the cleavage furrows of the 4-cell stage embryo[16,18–21].
Three regions within the dazl 3’UTR are critical for, 1) localization to the Bb, 2) anchoring to the vegetal cortex, and 3) localization to the cleavage furrows in the early embryo. Interestingly, although the 3’UTR of the nanos-related gene Xcat2 in Xenopus, also localizes it to the Bb and vegetal cortex in zebrafish, little sequence similarity is evident in the 3’UTRs of Xcat2, and zebrafish nanos, dazl, and vasa, suggesting either small motifs, or secondary/tertiary structure of the UTRs are conserved. Thus, germ plasm mRNA localization occurs in a stepwise, spatiotemporal fashion, with conserved features to the transport systems in Xenopus.
Recent studies have provided genetic access into the mechanisms of Bb formation and germ plasm assembly in the oocyte. The maternal-effect mutant bucky ball (buc), identified through its animal-vegetal polarity egg phenotype, is defective in Bb formation and early oocyte polarity, as animal pole markers are expanded radially and vegetal pole markers are unlocalized[22,23]. Buc is required for early vegetal pole mRNA localization that occurs via the Bb, as well as late vegetal pole localization that occurs after Bb dissociation, which is postulated to depend on the early pathway[24,25]. Thus, the failure of late vegetal pole mRNA localization in buc mutant oocytes may be secondary to the lack of Bb formation. Buc functions to promote vegetal and exclude animal pole identity also in the surrounding follicle cell layer, preventing the formation of multiple micropylar cells, an animal pole-specific follicle cell fate. Thus, patterning of the oocyte and surrounding somatic follicle cell layer appears to be coordinated through an as yet unknown signaling mechanism .
Recently the molecular identity of buc was determined to be a novel 639 amino acid protein, first identified as XVelo1 in Xenopus. The predicted Buc protein contains no known functional motifs. The localization of buc mRNA is dynamic during oocyte development, initially localizing to the Bb, moving vegetally, and then ultimately localizing to the animal pole during late oogenesis (Figure 2). A Buc-GFP fusion also localizes to the Bb and, interestingly, to the germ plasm of the embryo and can induce ectopic germ cells in the embryo. Buc homologues in other vertebrates have low predicted amino acid conservation, indicating that it has evolved at a relatively accelerated rate. Interestingly, the human version lacks a complete open reading frame, suggesting that it has lost its function all together in humans or functions as an RNA. In zebrafish Buc clearly functions as a protein, since a nonsense mutation truncating it by just 30 amino acids causes a failure in Bb formation . However, the buc RNA could have additional functions in Bb formation and animal-vegetal polarity, similar to oskar in Drosophila, also a germ plasm component. Future structure-function experiments of the buc RNA/protein, together with the isolation of interacting partners will unravel mechanistically how this novel gene functions.
In zebrafish egg activation is marked by cortical granule exocytosis (CGE), chorion elevation and the segregation of cytoplasm from the yolk to the animal pole to form the single cell blastodisc. Embryos derived from brom bones (brb) mutant females are defective in this process . The egg activation defect is a result of failure of inositol 1,4,5-triphosphate (IP3) signaling, which induces a Ca2+ wave critical for normal egg activation in all animals examined. The reinstatement of either IP3 or Ca2+ in brb mutant eggs can rescue the egg activation defect. brb was shown to encode heterogeneous nuclear ribonucleoprotein I (hnRNP I), likely regulating during oogenesis the production of an egg activation signaling component. hnRNP I has been previous shown to be important in a variety of developmental processes including translational control in oogenesis, spermatogenesis and RNA localization[31–34]. Thus, brb mutants reveal yet another developmental function for hnRNP I, i.e. a role in egg activation.
Maternal-effect screens have yielded a significant number of mutations affecting cleavage development[8–11], although most of the mutant genes have yet to be cloned. cellular atoll (cea) mutants fail to undergo cleavages at the second cell division and beyond[8,35], and was recently shown to encode the centriolar component sas-6. Interestingly, as sperm normally provide the centriole to the zygote, cea also has a paternal-effect function, whereby wild-type eggs fertilized with cea mutant sperm result in inviable tetraploid embryos. Since the cleavage stage of development is primarily under maternal control, the eventual molecular identification of other maternal-effect mutant genes functioning during this period will enhance our understanding of this important stage of development.
The maternal-effect cellular island (cei) mutant displays an early defect in cleavage furrow formation . cei encodes a hypomorphic allele of Aurora B Kinase, a protein previously shown to be important in several aspects of cell division in other systems (reviewed in). Aurora B Kinase function is vital zygotically later in zebrafish embryogenesis, as a null retroviral insertional allele reveals furrow defects during this period[36,38]. Interestingly, the maternal-effect cei allele causes a specific defect in distal furrow formation during the early cleavage stage, while medially positioned furrows can form, likely due to intact mitotic spindle-derived signals mediating medial furrow formation. The ability of the cei mutant Aurora B Kinase protein to mediate all its cytokinesis functions zygotically, but its inability to mediate distal furrow formation in the cleavage embryo may be related to unique requirements for astral microtubules and Aurora B Kinase to divide the large cells of the very early embryo.
In fish and amphibians, the dorsoventral embryonic axis is established through a maternally-regulated Wnt/βcatenin pathway. Several zebrafish maternal-effect mutants with defects in dorsoventral axis formation have been identified. The maternal-effect ventralized mutants hecate and tokkaebe produce embryos lacking a dorsal organizer and consequently are radially ventralized. Although hecate and tokkaebe have not yet been molecular defined, injection/rescue experiments with Wnt signaling pathway components indicates that they act upstream of, or within a Wnt/βcatenin signaling pathway, respectively, to induce the dorsal organizer[39–41]. tokkaebe likely corresponds to a novel component of the Wnt pathway, since no known Wnt components are found within the 0.5 cM interval containing the tokkaebe mutation.
The maternal-effect ventralized mutant ichabod is caused by a specific loss of maternal βcatenin2 function. Two βcatenin genes have been identified in the zebrafish genome; however, only βcatenin2 is essential maternally to induce the dorsal organizer through a Wnt signaling pathway. Interestingly, the maternal, dorsal organizer Wnt/βcatenin2 pathway also functions to repress expression of a later Wnt8/βcatenin pathway that opposes dorsal specification ventrally (Figure 3). In zebrafish this late blastula Wnt pathway depends on both βcatenin1 and βcatenin2 function and is mediated by zygotic Wnt8 signaling ventrally[42–45]. Thus in the absence of both βcatenin1 and βcatenin2 function, early dorsal organizer formation fails; however, at later stages the ventral Wnt8 pathway also fails to oppose dorsal fate specification ventrally. The result is formation of dorsal tissues circumferentially. These results show, surprisingly, that at a late blastula stage circumferential organizer-like tissue can form in the absence of a maternal Wnt/βcatenin pathway and independently of Wnt/βcatenin signaling entirely.
The transcription factor pou2/oct4 also acts maternally in dorsoventral patterning and morphogenesis[46,47], revealing additional roles to its described maternal function in endoderm specification[48,49]. Loss of both maternal and zygotic (MZ) Pou2 causes severe dorsalization due to failure to induce bmp ligand gene expression, which functions in ventrolateral tissue specification. These results demonstrate that ventral specification via the BMP signaling pathway is not a default pathway, as previously thought, but instead is initiated by maternal Pou2/Oct4 in the early embryo (Figure 3). MZPou2 also regulates the morphogenic process of epiboly, the thinning and spreading of the blastoderm over the yolk cell, through functions in yolk cell microtubule formation, cell adhesive properties, and blastoderm cell movements via a cell non-autonomous mechanism [46,47]. Together, these studies reveal that the renowned pou2/oct4 stem cell gene in mammals is a key maternal regulator of early zebrafish development
The betty boop (bbp) mutant, identified in a maternal-effect screen, is a strictly maternally-acting gene regulating the morphogenic process of epiboly[10,50]. Embryos from bbp mothers develop normally until they reach 50% epiboly at which point the embryo abruptly bursts via a presumptive premature constriction of the actin cytoskeleton in the yolk cell. Interestingly, bbp was recently shown to encode the zebrafish homolog of Mitogen Activated Protein Kinase Activated Protein Kinase 2 (MAPKAPK2), a target of p38 MAP kinase (MAPK) in cell culture systems. During zebrafish epiboly, p38 MAPK also appears to activate MAPKAPK2, as a dominant-negative p38 MAPK causes the same epiboly defect as loss of bbp. Neither p38 MAPK, nor MAPKAPK2 have been previously implicated in tissue morphogenesis. Thus, the identification and cloning of the bbp gene is a model genetic case for an unexpected pathway being placed in a developmental process, in this case epiboly, which may not have been considered in a candidate-gene reverse genetic approach.
Small noncoding RNAs are emerging as important players in early zebrafish development. Maternal-zygotic (MZ) mutant embryos of the microRNA (miRNA)-processing enzyme Dicer exhibit early embryonic defects in gastrulation, somitogenesis, brain morphogenesis, and heart development . By removing the strong maternal component of dicer, MZ-dicer mutant embryos are completely devoid of miRNA processing and therefore, devoid of all miRNA function. miRNAs negatively regulate target genes by binding to their 3’UTR, promoting deadenylation, translational repression and/or ultimately degradation of the transcript (reviewed in ). In zebrafish, the predominantly expressed miRNA during early embryogenesis is miR-430, which is first expressed at the mid-blastula transition (MBT), and is not expressed maternally. A central finding revealed by the loss of miR-430 through MZ-dicer is its role in the clearance of maternal mRNAs at the MBT. In MZ-dicer mutant embryos, maternal mRNAs abnormally persist beyond the MBT. To date, no maternal miRNAs have been reported in the zebrafish embryo. Thus, it is unclear whether the maternal role of dicer is to primarily process miR-430 at the MBT or in addition, also process unidentified maternal miRNA(s).
Inhibition of miRNA function plays a role in germ line development. nanos1 is expressed early in germ line development and a mutant, generated through TILLING methods, demonstrates its maternal requirement for PGC survival and a function in the adult in maintaining oocyte production . nanos1 is resistant to miRNA repression in the germ line, but not in the soma, promoting its specific expression in the germ line. This resistance is conferred, at least in part, by the maternally-expressed germ cell-localized RNA binding protein Dead End (Dnd), which interacts with the nanos1 3’UTR, presumably blocking the binding of miR-430. Interestingly, this resistance to miRNA repression is also found in another germ plasm mRNA, Tudor-domain-containing-7 (Tdrd-7), suggesting a general mechanism involving Dnd in influencing germ cell-specific gene expression (Figure 4).
Germ plasm mRNA resistance to miRNA regulation is not universal, however, as the dnd 3’ UTR itself lacks a miR-430 site and therefore is presumably not repressed in the soma by miRNAs. Since dnd maternal transcripts are dramatically eliminated in the soma about one hour after the MBT, independently of miR-430 regulation, it suggests the existence of additional mode(s) of modulating maternal transcripts at this critical developmental transition. Likewise, vasa removal from the soma is also independent of miR-430 regulation. Thus, an unknown, possibly common additional mechanism eliminates dnd and vasa transcripts from the soma.
A Dicer-independent class of small noncoding RNAs, known as piRNAs or Piwi associated RNAs, appear to be germ line specific[13,14,60,61]. Piwi proteins are important for target gene silencing. Presumptive null, zygotic mutations in either of two piwi homologues in zebrafish, ziwi and zili, cause the progressive loss of germ cells between 3–7 weeks of age[13,14]. Mutant adults are phenotypically male, consistent with recent studies demonstrating that zebrafish develop phenotypically as male when eliminating the germ line[13,63,64]. Interestingly, there is a maternal-effect meiotic progression defect in zili hypomorphic mutants. Although mutant eggs can be fertilized, they fail to undergo meiosis I and II. Piwi proteins in the mouse function to repress transposon activity in the germ line[65,66]. The meiotic defects in zili hypomorphs mutants, however, are not linked to increased transposon activity as measured by quantitative RT-PCR and in situ hybridization. These findings reveal a novel function for piRNAs, whose function in vertebrates was previously thought confined to regulating transposon activity in the germ line.
A major hindrance to the molecular cloning of chemically-induced mutant genes in zebrafish is the incomplete assembly of the genomic sequence. This obstacle is becoming less of an issue with recent improvements to the assembly (currently Zv8; www.sanger.ac.uk). As existing gaps are eliminated and the genome sequence is completed, the molecular cloning of maternal-effect mutant genes will be greatly accelerated.
The investigation of the maternal functions of essential zygotic genes will be more difficult to study. Although germ line chimeric analysis is a successful method to examine the maternal function of zygotic lethal genes in zebrafish, it is quite labor intensive and would be impractical for large throughput analysis of such maternal function. This problem may be overcome by employing techniques that utilize mitotic recombination, a principle heavily relied upon in Drosophila genetics. RecQ helicases are known to prevent recombination during replication. Induced mitotic recombination through the suppression of RecQ helicases was recently demonstrated in zebrafish. This approach can generate mutant clones from heterozygous cells at a significant frequency (~1.7– 3.4% ). With further improvements in this technology, mitotic recombination could potentially be used to generate homozygous mutant germ line clones of zygotic lethal mutations from heterozygous individuals. Such germ line mosaic females would generate maternally-deficient eggs and embryos, allowing the study of the maternal gene function.
Reverse genetic techniques are very valuable complements to forward genetic approaches in the zebrafish. Antisense morpholino oligos are widely used to block translation or disrupt splicing[71,72]. However, the function of maternal protein already present in the egg cannot be blocked by morpholino injection into the egg, although advances in oocyte cell culture methods may make it possible to use this method in the future to examine maternal gene functions. Reverse genetic approaches that rely on induction of genomic sequence alterations are considerably more laborious, but generate robust loss-of-function reagents. Recently, the TILLING (Target Induced Local Lesions in Genomes) approach[13,14,55], as well as the zinc finger nuclease approach designed to mutate a specific sequence of the genome[73–75], have been very successful in generating mutants in zebrafish. Hence, TILLING and zinc finger nuclease strategies look very promising for eliminating gene function of suspected maternal-effect genes in zebrafish in the future.
We thank Lee Kapp for comments on the manuscript, and Eric Weinberg and Mate Varga for helpful discussion. Funding was provided by NIH grant HD050901 to MCM.
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