The zebrafish Fgf-3 gene: cDNA sequence, transcript structure and genomic organization

Primer on FGF3

Though initially discovered as a proto-oncogene in virally induced mouse mammary tumors, FGF3 is primarily active in prenatal stages, where it is found at various sites at specific times. FGF3 is crucial during development, as its roles include tail formation, inner ear development and hindbrain induction and patterning. FGF3 expression and function are highly conserved in vertebrates, while it also interacts with other FGFs in various developmental processes. Intriguingly, while it is classified as a classical paracrine signaling factor, murine FGF3 was uniquely found to also act in an intracrine manner, depending on alternative translation initiation sites. Corresponding with its conserved role in inner ear morphogenesis, mutations in FGF3 in humans are associated with LAMM syndrome, a disorder that include hearing loss and inner ear malformations. While recent studies indicate of some FGF3 presence in post-natal stages, emerging evidences of its upregulation in various human tumors and cariogenic processes in mouse models, highlights the importance of its close regulation in adult tissues. Altogether, the broad and dynamic expression pattern and regulation of FGF3 in embryonic and adult tissues together with its link to congenital malformations and cancer, calls for further discoveries of its diverse roles in health and disease.

Fibroblast growth factor pathway component expression in the regenerating zebrafish fin

Adult zebrafish regenerate their appendages (fins) after amputation including the regeneration of bone structures (fin rays). Fibroblast growth factor (Fgf) signaling, which is involved in morphogenetic processes during development, has been shown to be essential for the process of fin regeneration. Moreover, mutations in Fgf pathway component genes lead to abnormal skeletal growth in teleosts and mammals, including humans, illustrating the importance of Fgf signaling in the growth control of tissues. Here, we revisited Fgf signaling pathway component expression by RNA in situ hybridization to test for the expression of about half of the ligands and all receptors of the pathway in the regenerating zebrafish fin. Expression patterns of fgf7, fgf10b, fgf12b, fgf17b and fgfr1b have not been reported in the literature before. We summarize and discuss known and novel localization of expression and find that all five Fgf receptors (fgfr1a, fgfr1b, fgfr2, fgfr3 and fgfr4) and most of the tested ligands are expressed in specific regions of the regenerate. Our work provides a basis to study domain specific functions of Fgf signaling in the regenerating teleost appendage.

Different Fgfs have distinct roles in regulating neurogenesis after spinal cord injury in zebrafish

Background

Despite conserved developmental processes and organization of the vertebrate central nervous system, only some vertebrates including zebrafish can efficiently regenerate neural damage including after spinal cord injury. The mammalian spinal cord shows very limited regeneration and neurogenesis, resulting in permanent life-long functional impairment. Therefore, there is an urgent need to identify the cellular and molecular mechanisms that can drive efficient vertebrate neurogenesis following injury. A key pathway implicated in zebrafish neurogenesis is fibroblast growth factor signaling.

Methods

In the present study we investigated the roles of distinct fibroblast growth factor members and their receptors in facilitating different aspects of neural development and regeneration at different timepoints following spinal cord injury. After spinal cord injury in adults and during larval development, loss and/or gain of Fgf signaling was combined with immunohistochemistry, in situ hybridization and transgenes marking motor neuron populations in in vivo zebrafish and in vitro mammalian PC12 cell culture models.

Results

Fgf3 drives neurogenesis of Islet1 expressing motor neuron subtypes and mediate axonogenesis in cMet expressing motor neuron subtypes. We also demonstrate that the role of Fgf members are not necessarily simple recapitulating development. During development Fgf2, Fgf3 and Fgf8 mediate neurogenesis of Islet1 expressing neurons and neuronal sprouting of both, Islet1 and cMet expressing motor neurons. Strikingly in mammalian PC12 cells, all three Fgfs increased cell proliferation, however, only Fgf2 and to some extent Fgf8, but not Fgf3 facilitated neurite outgrowth.

Conclusions

This study demonstrates differential Fgf member roles during neural development and adult regeneration, including in driving neural proliferation and neurite outgrowth of distinct spinal cord neuron populations, suggesting that factors including Fgf type, age of the organism, timing of expression, requirements for different neuronal populations could be tailored to best drive all of the required regenerative processes.

β-Amyloid precursor protein-b is essential for Mauthner cell development in the zebrafish in a Notch-dependent manner

Amyloid precursor protein (APP) is a transmembrane glycoprotein that has been the subject of intense research because of its implication in Alzheimer's disease. However, the physiological function of APP in the development and maintenance of the central nervous system remains largely unknown. We have previously shown that the APP homologue in zebrafish (Danio rerio), Appb, is required for motor neuron patterning and formation. Here we study the function of Appb during neurogenesis in the zebrafish hindbrain. Partial knockdown of Appb using antisense morpholino oligonucleotides blocked the formation of the Mauthner neurons, uni- or bilaterally, with an aberrant behavior as a consequence of this cellular change. The Appb morphants had decreased neurogenesis, increased notch signaling and notch1a expression at the expense of deltaA/D expression. The Mauthner cell development could be restored either by a general decrease in Notch signaling through γ-secretase inhibition or by a partial knock down of Notch1a. Together, this demonstrates the importance of Appb in neurogenesis and for the first time shows the essential requirement of Appb in the formation of a specific cell type, the Mauthner cell, in the hindbrain during development. Our results suggest that Appb-regulated neurogenesis is mediated through balancing the Notch1a signaling pathway and provide new insights into the development of the Mauthner cell.

A mutation in cnot8, component of the Ccr4-not complex regulating transcript stability, affects expression levels of developmental regulators and reveals a role of Fgf3 in development of caudal hypothalamic dopaminergic neurons

While regulation of the activity of developmental control genes at the transcriptional level as well as by specific miRNA-based degradation are intensively studied, little is known whether general cellular mechanisms controlling mRNA decay may contribute to differential stability of mRNAs of developmental control genes. Here, we investigate whether a mutation in the deadenylation dependent mRNA decay pathway may reveal differential effects on developmental mechanisms, using dopaminergic differentiation in the zebrafish brain as model system. In a zebrafish genetic screen aimed at identifying genes controlling dopaminergic neuron development we isolated the m1061 mutation that selectively caused increased dopaminergic differentiation in the caudal hypothalamus, while other dopaminergic groups were not affected. Positional cloning revealed that m1061 causes a premature stop codon in the cnot8 open reading frame. Cnot8 is a component of the Ccr4-Not complex and displays deadenylase activity, which is required for removal of the poly (A) tail in bulk mRNA turnover. Analyses of expression of developmental regulators indicate that loss of Cnot8 activity results in increased mRNA in situ hybridization signal levels for a subset of developmental control genes. We show that in the area of caudal hypothalamic dopaminergic differentiation, mRNA levels for several components of the FGF signaling pathway, including Fgf3, FGF receptors, and FGF target genes, are increased. Pharmacological inhibition of FGF signaling or a mutation in the fgf3 gene can compensate the gain of caudal hypothalamic dopaminergic neurons in cnot8m1061 mutants, indicating a role for Fgf3 in control of development of this dopaminergic population. The cnot8m1061 mutant phenotype provides an in vivo system to study roles of the Cnot8 deadenylase component of the mRNA decay pathway in vertebrate development. Our data indicate that attenuation of Cnot8 activity differentially affects mRNA levels of developmental control genes.

Targeted germ line disruptions reveal general and species-specific roles for paralog group 1 hox genes in zebrafish

Background

The developing vertebrate hindbrain is transiently segmented into rhombomeres by a process requiring Hox activity. Hox genes control specification of rhombomere fates, as well as the stereotypic differentiation of rhombomere-specific neuronal populations. Accordingly, germ line disruption of the paralog group 1 (PG1) Hox genes Hoxa1 and Hoxb1 causes defects in hindbrain segmentation and neuron formation in mice. However, antisense-mediated interference with zebrafish hoxb1a and hoxb1b (analogous to murine Hoxb1 and Hoxa1, respectively) produces phenotypes that are qualitatively and quantitatively distinct from those observed in the mouse. This suggests that PG1 Hox genes may have species-specific functions, or that anti-sense mediated interference may not completely inactivate Hox function in zebrafish.

Results

Using zinc finger and TALEN technologies, we disrupted hoxb1a and hoxb1b in the zebrafish germ line to establish mutant lines for each gene. We find that zebrafish hoxb1a germ line mutants have a more severe phenotype than reported for Hoxb1a antisense treatment. This phenotype is similar to that observed in Hoxb1 knock out mice, suggesting that Hoxb1/hoxb1a have the same function in both species. Zebrafish hoxb1b germ line mutants also have a more severe phenotype than reported for hoxb1b antisense treatment (e.g. in the effect on Mauthner neuron differentiation), but this phenotype differs from that observed in Hoxa1 knock out mice (e.g. in the specification of rhombomere 5 (r5) and r6), suggesting that Hoxa1/hoxb1b have species-specific activities. We also demonstrate that Hoxb1b regulates nucleosome organization at the hoxb1a promoter and that retinoic acid acts independently of hoxb1b to activate hoxb1a expression.

Conclusions

We generated several novel germ line mutants for zebrafish hoxb1a and hoxb1b. Our analyses indicate that Hoxb1 and hoxb1a have comparable functions in zebrafish and mouse, suggesting a conserved function for these genes. In contrast, while Hoxa1 and hoxb1b share functions in the formation of r3 and r4, they differ with regards to r5 and r6, where Hoxa1 appears to control formation of r5, but not r6, in the mouse, whereas hoxb1b regulates formation of r6, but not r5, in zebrafish. Lastly, our data reveal independent regulation of hoxb1a expression by retinoic acid and Hoxb1b in zebrafish.

Fgf3 and Fgf10a work in concert to promote maturation of the epibranchial placodes in zebrafish

Essential cellular components of the paired sensory organs of the vertebrate head are derived from transient thickenings of embryonic ectoderm known as cranial placodes. The epibranchial (EB) placodes give rise to sensory neurons of the EB ganglia that are responsible for relaying visceral sensations form the periphery to the central nervous system. Development of EB placodes and subsequent formation of EB ganglia is a multistep process regulated by various extrinsic factors, including fibroblast growth factors (Fgfs). We discovered that two Fgf ligands, Fgf3 and Fgf10a, cooperate to promote EB placode development. Whereas EB placodes are induced in the absence of Fgf3 and Fgf10a, they fail to express placode specific markers Pax2a and Sox3. Expression analysis and mosaic rescue experiments demonstrate that Fgf3 signal is derived from the endoderm, whereas Fgf10a is emitted from the lateral line system and the otic placode. Further analyses revealed that Fgf3 and Fgf10a activities are not required for cell proliferation or survival, but are required for placodal cells to undergo neurogenesis. Based on these data, we conclude that a combined loss of these Fgf factors results in a failure of the EB placode precursors to initiate a transcriptional program needed for maturation and subsequent neurogenesis. These findings highlight the importance and complexity of reiterated Fgf signaling during cranial placode formation and subsequent sensory organ development.

Gene responses in the central nervous system of zebrafish embryos exposed to the neurotoxicant methyl mercury

Methyl mercury (MeHg) is a neurotoxicant with adverse effects on the development of the nervous system from fish to man. Despite a detailed understanding of the molecular mechanisms by which MeHg affects cellular homeostasis, it is still not clear how MeHg causes developmental neurotoxicity. We performed here a genome-wide transcriptional analysis of MeHg-exposed zebrafish embryos and combined this with a whole-mount in situ expression analysis of 88 MeHg-affected genes. The majority of the analyzed genes showed tissue- and region-restricted responses in various organs and tissues. The genes were linked to gene ontology terms like oxidative stress, transport and cell protection. Areas even within the central nervous system (CNS) are affected differently resulting in distinct cellular stress responses. Our study revealed an unexpected heterogeneity in gene responses to MeHg exposure in different tissues and neuronal subregions, even though the known molecular action of MeHg would predict a similar burden of exposed cells. The overall structure of the developing brain of MeHg-exposed embryos appeared normal, suggesting that the mechanism leading to differentiation of the CNS is not overtly affected by exposure to MeHg. We propose that MeHg disturbs the function of the CNS by disturbing the cellular homeostasis. As these cellular stress responses comprise genes that are also involved in normal neuronal activity and learning, MeHg may affect the developing CNS in a subtle manner that manifests itself in behavioral deficits.

Development of hypothalamic serotoninergic neurons requires Fgf signalling via the ETS-domain transcription factor Etv5b

Serotonin is a monoamine neurotransmitter that is involved in numerous physiological functions and its dysregulation is implicated in various psychiatric diseases. In all non-placental vertebrates, serotoninergic (5-HT) neurons are present in several regions of the brain, including the hypothalamus. In placental mammals, however, 5-HT neurons are located in the raphe nuclei only. In all species, though, 5-HT neurons constitute a functionally and molecularly heterogeneous population. How the non-raphe 5-HT populations are developmentally encoded is unknown. Using the zebrafish model we show that, in contrast to the raphe populations, hypothalamic 5-HT neurons are generated independently of the ETS-domain transcription factor Pet1 (Fev). By applying a combination of pharmacological tools and gene knockdown and/or overexpression experiments, we demonstrate that Fgf signalling acts via another ETS-domain transcription factor, Etv5b (Erm), to induce hypothalamic 5-HT neurons. We provide evidence that Etv5b exerts its effects by regulating cell cycle parameters in 5-HT progenitors. Our results highlight a novel role for Etv5b in neuronal development and provide support for the existence of a developmental heterogeneity among 5-HT neurons in their requirement for ETS-domain transcription factors.

Graded levels of Pax2a and Pax8 regulate cell differentiation during sensory placode formation

Pax gene haploinsufficiency causes a variety of congenital defects. Renal-coloboma syndrome, resulting from mutations in Pax2, is characterized by kidney hypoplasia, optic nerve malformation, and hearing loss. Although this underscores the importance of Pax gene dosage in normal development, how differential levels of these transcriptional regulators affect cell differentiation and tissue morphogenesis is still poorly understood. We show that differential levels of zebrafish Pax2a and Pax8 modulate commitment and behavior in cells that eventually contribute to the otic vesicle and epibranchial placodes. Initially, a subset of epibranchial placode precursors lie lateral to otic precursors within a single Pax2a/8-positive domain; these cells subsequently move to segregate into distinct placodes. Using lineage-tracing and ablation analyses, we show that cells in the Pax2a/8+ domain become biased towards certain fates at the beginning of somitogenesis. Experiments involving either Pax2a overexpression or partial, combinatorial Pax2a and Pax8 loss of function reveal that high levels of Pax favor otic differentiation whereas low levels increase cell numbers in epibranchial ganglia. In addition, the Fgf and Wnt signaling pathways control Pax2a expression: Fgf is necessary to induce Pax2a, whereas Wnt instructs the high levels of Pax2a that favor otic differentiation. Our studies reveal the importance of Pax levels during sensory placode formation and provide a mechanism by which these levels are controlled.

A screen for hoxb1-regulated genes identifies ppp1r14al as a regulator of the rhombomere 4 Fgf-signaling center

Segmentation of the vertebrate hindbrain into multiple rhombomeres is essential for proper formation of the cerebellum, cranial nerves and cranial neural crest. Paralog group 1 (PG1) hox genes are expressed early in the caudal hindbrain and are required for rhombomere formation. Accordingly, loss of PG1 hox function disrupts development of caudal rhombomeres in model organisms and causes brainstem defects, associated with cognitive impairment, in humans. In spite of this important role for PG1 hox genes, transcriptional targets of PG1 proteins are not well characterized. Here we use ectopic expression together with embryonic dissection to identify novel targets of the zebrafish PG1 gene hoxb1b. Of 100 genes up-regulated by hoxb1b, 54 were examined and 25 were found to represent novel hoxb1b regulated hindbrain genes. The ppp1r14al gene was analyzed in greater detail and our results indicate that Hoxb1b is likely to directly regulate ppp1r14al expression in rhombomere 4. Furthermore, ppp1r14al is essential for establishment of the earliest hindbrain signaling-center in rhombomere 4 by regulating expression of fgf3.

Heterogeneity and Fgf dependence of adult neural progenitors in the zebrafish telencephalon

Adult telencephalic neurogenesis is a conserved trait of all vertebrates studied. It has been investigated in detail in rodents, but very little is known about the composition of neurogenic niches and the cellular nature of progenitors in nonmammalian vertebrates. To understand the components of the progenitor zones in the adult zebrafish telencephalon and the link between glial characteristics and progenitor state, we examined whether canonical glial markers are colocalized with proliferation markers. In the adult zebrafish telencephalon, we identify heterogeneous progenitors that reside in two distinct glial domains. We find that the glial composition of the progenitor zone is linked to its proliferative behavior. Analyzing both fast-cycling proliferating cells as well as slowly cycling progenitors, we find four distinct progenitor types characterized by differential expression of glial markers. Importantly, a significant proportion of progenitors do not display typical radial glia characteristics. By blocking or activating Fgf signaling by misexpression of a dominant negative Fgf-receptor 1 or Fgf8a, respectively, we find that ventral and dorsal progenitors in the telencephalon also differ in their requirement for Fgf signaling. Together with data on the expression of Fgf signaling components in the ventricular zone of the telencephalon, this suggests that Fgf signaling directly regulates proliferation of specific subsets of adult telencephalic progenitors in vivo. Taken together our results show that adult neural progenitor cells are heterogeneous with their respect to distribution into two distinct glial domains and their dependence upon Fgf signaling as a proliferative cue in the zebrafish telencephalon.

prep1.2 and aldh1a2 participate to a positive loop required for branchial arches development in zebrafish

Segmentation is a key step in embryonic development. Acting in all germ layers, it is responsible for the generation of antero-posterior asymmetries. Hox genes, with their diverse expression in individual segments, are fundamental players in the determination of different segmental fates. In vertebrates, Hox gene products gain specificity for DNA sequences by interacting with Pbx, Prep and Meis homeodomain transcription factors. In this work we cloned and analysed prep1.2 in zebrafish. In-situ hybridization experiments show that prep1.2 is maternally and ubiquitously expressed up to early somitogenesis when its expression pattern becomes more restricted to the head and trunk mesenchyme. Experiments of loss of function with prep1.2 morpholinos change the shape of the hyoid and third pharyngeal cartilages while arches 4-7 and pectoral fins are absent, a phenotype strikingly similar to that caused by loss of retinoic acid (RA). In fact, we show that prep1.2 is positively regulated by RA and required for the normal expression of aldh1a2 at later stages, particularly in tissues involved in the development of the branchial arches and pectoral fins. Thus, prep1.2 and aldh1a2 are members of an indirect positive feedback loop required for pharyngeal endoderm and posterior branchial arches development. As the paralogue gene prep1.1 is more important in hindbrain patterning and neural crest chondrogenesis, we provide evidence of a functional specialization of prep genes in zebrafish head segmentation and morphogenesis.

FGF-dependent mechanosensory organ patterning in zebrafish

During development, organ primordia reorganize to form repeated functional units. In zebrafish (Danio rerio), mechanosensory organs called neuromasts are deposited at regular intervals by the migrating posterior lateral line (pLL) primordium. The pLL primordium is organized into polarized rosettes representing proto-neuromasts, each with a central atoh1a-positive focus of mechanosensory precursors. We show that rosettes form cyclically from a progenitor pool at the leading zone of the primordium as neuromasts are deposited from the trailing region. fgf3/10 signals localized to the leading zone are required for rosette formation, atoh1a expression, and primordium migration. We propose that the fibroblast growth factor (FGF) source controls primordium organization, which, in turn, regulates the periodicity of neuromast deposition. This previously unrecognized mechanism may be applicable to understanding segmentation and morphogenesis in other organ systems.

The zebrafish fgf family

Fibroblast growth factors (FGFs) are secreted polypeptide growth factors. The FGF signaling system plays crucial roles in multiple developmental processes in vertebrates. The human FGF family comprises 22 members. Although 16 zebrafish fgfs have been reported, the zebrafish fgf family has not been well elucidated. We have identified 11 additional zebrafish fgfs by conducting a homology-based search in the zebrafish genome and cDNA databases. The zebrafish fgf family now comprises at least 27 members. By conducting phylogenetic and gene location analyses, we examined relationships of zebrafish fgf genes with human FGF genes. All the zebrafish orthologs of human FGFs except for FGF9 have been identified. Zebrafish fgf9 might have been lost from the genome during evolution. In addition, six paralogs of zebrafish fgf genes have been identified. The phylogenetic analysis suggests that the zebrafish fgf gene family can be divided into seven subfamilies. The zebrafish fgf subfamilies are essentially consistent with the human FGF subfamilies, although some include potential paralogs. As the zebrafish system has proved useful for studying gene functions and genetic diseases, the present findings will be useful for elucidation of roles of FGFs in zebrafish and humans.

Specification of epibranchial placodes in zebrafish

In all vertebrates, the neurogenic placodes are transient ectodermal thickenings that give rise to sensory neurons of the cranial ganglia. Epibranchial (EB) placodes generate neurons of the distal facial, glossopharyngeal and vagal ganglia, which convey sensation from the viscera, including pharyngeal endoderm structures, to the CNS. Recent studies have implicated signals from pharyngeal endoderm in the initiation of neurogenesis from EB placodes; however, the signals underlying the formation of placodes are unknown. Here, we show that zebrafish embryos mutant for fgf3 and fgf8 do not express early EB placode markers, including foxi1 and pax2a. Mosaic analysis demonstrates that placodal cells must directly receive Fgf signals during a specific crucial period of development. Transplantation experiments and mutant analysis reveal that cephalic mesoderm is the source of Fgf signals. Finally, both Fgf3 and Fgf8 are sufficient to induce foxi1-positive placodal precursors in wild-type as well as Fgf3-plus Fgf8-depleted embryos. We propose a model in which mesoderm-derived Fgf3 and Fgf8 signals establish both the EB placodes and the development of the pharyngeal endoderm, the subsequent interaction of which promotes neurogenesis. The coordinated interplay between craniofacial tissues would thus assure proper spatial and temporal interactions in the shaping of the vertebrate head.

Zebrafish pax5 regulates development of the utricular macula and vestibular function

The zebrafish otic vesicle initially forms with only two sensory epithelia, the utricular and saccular maculae, which primarily mediate vestibular and auditory function, respectively. Here, we test the role of pax5, which is preferentially expressed in the utricular macula. Morpholino knockdown of pax5 disrupts vestibular function but not hearing. Neurons of the statoacoustic ganglion (SAG) develop normally. Utricular hair cells appear to form normally but a variable number subsequently undergo apoptosis and are extruded from the otic vesicle. Dendrites of the SAG persist in the utricle but become disorganized after hair cell loss. Hair cells in the saccule develop and survive normally. Otic expression of pax5 requires pax2a and fgf3, mutations in which cause vestibular defects, albeit by distinct mechanisms. Thus, pax5 works in conjunction with fgf3 and pax2a to establish and/or maintain the utricular macula and is essential for vestibular function.

Vhnf1 acts downstream of Bmp, Fgf, and RA signals to regulate endocrine beta cell development in zebrafish

Bmp, Fgf, and retinoic acid (RA) signals have been implicated as regulators of pancreas development. However, the integration of these signaling pathways in vivo is not fully understood. Variant hnf1 (Vhnf1) is a transcription factor involved in pancreas, liver, and kidney development and its mutation in zebrafish causes underdeveloped pancreas and liver. We investigated the signaling pathways that regulate vhnf1 expression during pancreas development. First, we showed that Bmp activity is required for vhnf1 expression in the endoderm. In chordin (a Bmp antagonist) morpholino (MO)-injected embryos, vhnf1 expression in endoderm and in endocrine beta cells is expanded. On the other hand, in alk8 (a type I TGFbeta receptor) MO-injected embryos, vhnf1 expression in the endoderm is significantly reduced. Second, we showed that Fgf signaling participates in regulation of pancreas development through the vhnf1 pathway. Third, we demonstrated that RA fails to rescue reduction of insulin expression in vhnf1 mutants, whereas overexpression of vhnf1 restores insulin expression that is repressed by treatment with a RA receptor inhibitor. And finally, we revealed that both Bmp and Fgf signals act genetically upstream of RA in directing pancreas development. Taken together, our data establish that vhnf1 acts downstream of the signaling pathways of RA, Bmp, and Fgf to regulate pancreas development in zebrafish.

Endoderm-derived Fgf3 is necessary and sufficient for inducing neurogenesis in the epibranchial placodes in zebrafish

In vertebrates, epibranchial placodes are transient ectodermal thickenings that contribute sensory neurons to the epibranchial ganglia. These ganglia innervate internal organs and transmit information on heart rate, blood pressure and visceral distension from the periphery to the central nervous system. Despite their importance, the molecular mechanisms that govern the induction and neurogenesis of the epibranchial placodes are only now being elucidated. In this study, we demonstrate that endoderm is required for neurogenesis of the zebrafish epibranchial placodes. Mosaic analyses confirm that endoderm is the source of the neurogenic signal. Using a morpholino knockdown approach, we find that fgf3 is required for the majority of placode cells to undergo neurogenesis. Tissue transplants demonstrate that fgf3 activity is specifically required in the endodermal pouches. Furthermore, ectopic fgf3 expression is sufficient for inducing phox2a-positive neurons in wild-type and endoderm-deficient embryos. Surprisingly, ectodermal foxi1 expression, a marker for the epibranchial placode precursors, is present in both endoderm-deficient embryos and fgf3 morphants, indicating that neither endoderm nor Fgf3 is required for initial placode induction. Based on these findings, we propose a model for epibranchial placode development in which Fgf3 is a major endodermal determinant required for epibranchial placode neurogenesis.

Chordin, FGF signaling, and mesodermal factors cooperate in zebrafish neural induction

The ectoderm gives rise to both neural tissue and epidermis. In vertebrates, specification of the neural plate requires repression of bone morphogenetic protein (BMP) signaling in the dorsal ectoderm. The extracellular BMP antagonist Chordin and other signals from the dorsal mesoderm play important roles in this process. We utilized zebrafish mutant combinations that disrupt Chordin and mesoderm formation to reveal additional signals that contribute to the establishment of the neural domain. We demonstrate that fibroblast growth factor (FGF) signaling accounts for the additional activity in neural specification. Impeding FGF signaling results in a shift of ectodermal markers from neural to epidermal. However, following inhibition of FGF signaling, expression of anterior neural markers recovers in a Nodal-dependent fashion. Simultaneously blocking, Chordin, mesoderm formation, and FGF signaling eliminates neural marker expression during gastrula stages. We observed that FGF signaling is required for chordin expression but that it also acts via other mechanisms to repress BMP transcription during late blastula stages. Activation of FGF signaling was also able to repress BMP transcription in the absence of protein synthesis. Our results support a model in which specification of anterior neural tissue requires early FGF-mediated repression of BMP transcript levels and later activities of Chordin and mesodermal factors.

Genetic interactions underlying otic placode induction and formation

The formation of the otic placode is a complex process requiring multiple inductive signals. In zebrafish, fgf3 and fgf8, dlx3b and dlx4b, and foxi1 have been identified as the earliest-acting genes in this process. fgf3 and fgf8 are required as inductive signals, whereas dlx3b, dlx4b, and foxi1 appear to act directly within otic primordia. We have investigated potential interactions among these genes. Depletion of either dlx3b and dlx4b or foxi1 leads to a delay of pax2a expression in the otic primordia and reduction of the otic vesicle. Depletion of both foxi1 and dlx3b results in a complete ablation of otic placode formation. A strong synergistic interaction is also observed among foxi1, fgf3, and fgf8, and a weaker interaction among dlx3b, fgf3, and fgf8. Misexpression of foxi1 can induce expression of pax8, an early marker for the otic primordia, in embryos treated with an inhibitor of fibroblast growth factor (FGF) signaling. Conversely, morpholino knockdown of foxi1 blocks ectopic pax8 expression and otic vesicle formation induced by misexpression of fgf3 and/or fgf8. The observed genetic interactions suggest a model in which foxi1 and dlx3b/dlx4b act in independent pathways together with distinct phases of FGF signaling to promote otic placode induction and development.

fgf17b, a novel member of Fgf family, helps patterning zebrafish embryos

Fibroblast growth factors (Fgfs) play important roles in the pattern formation of early vertebrate embryos. We have identified a zebrafish ortholog of human FGF17, named fgf17b. The first phase of fgf17b expression occurs in the blastodermal margin of late blastulae and in the embryonic shield of early gastrulae. The second phase starts after the onset of segmentation, mainly in the presomitic mesoderm and newly formed somites. Injection of fgf17b mRNA into one-cell embryos induces expression of the mesodermal marker no tail (ntl) and rescues ntl expression suppressed by overexpression of lefty1 (lft1). Overexpression of fgf17b dorsalizes zebrafish gastrulae by enhancing expression of chordin (chd), which is an antagonist of the ventralizing signals BMPs. In addition, overexpression of fgf17b posteriorizes the neuroectoderm. Simultaneous knockdown of fgf17b and fgf8 with antisense morpholinos results in reduction of chd and ntl. Knockdown of fgf17b can alleviate inhibitory effect of ectopic expression of fgf3 on otx1. These data together suggest that Fgf17b plays a role in early embryonic patterning. We also demonstrate that fgf17b and fgf8 have stronger mesoderm inducting activity than fgf3, whereas fgf17b and fgf3 have stronger activity in posteriorizing the neuroectoderm than fgf8. Like fgf8, activation of fgf17b expression depends on Nodal signaling.

Fgf3 signaling from the ventral diencephalon is required for early specification and subsequent survival of the zebrafish adenohypophysis

The pituitary gland consists of two major parts: the neurohypophysis, which is of neural origin; and the adenohypophysis, which is of non-neural ectodermal origin. Development of the adenohypophysis is governed by signaling proteins from the infundibulum, a ventral structure of the diencephalon that gives rise to the neurohypophysis. In mouse, the fibroblast growth factors Fgf8, Fgf10 and Fgf18 are thought to affect multiple processes of pituitary development: morphogenesis and patterning of the adenohypophyseal anlage; and survival, proliferation and differential specification of adenohypophyseal progenitor cells. Here, we investigate the role of Fgf3 during pituitary development in the zebrafish, analyzing lia/fgf3 null mutants. We show that Fgf3 signaling from the ventral diencephalon is required in a non-cell autonomous fashion to induce the expression of lim3,pit1 and other pituitary-specific genes in the underlying adenohypophyseal progenitor cells. Despite the absence of such early specification steps, fgf3 mutants continue to form a distinct pituitary anlage of normal size and shape, until adenohypophyseal cells die by apoptosis. We further show that Sonic Hedgehog (Shh) cannot rescue pituitary development, although it is able to induce adenohypophyseal cells in ectopic placodal regions of fgf3 mutants, indicating that Fgf3 does not act via Shh, and that Shh can act independently of Fgf3. In sum, our data suggest that Fgf3 signaling primarily promotes the transcriptional activation of genes regulating early specification steps of adenohypophyseal progenitor cells. This early specification seems to be essential for the subsequent survival of pituitary cells, but not for pituitary morphogenesis or pituitary cell proliferation.

The roles of Hedgehogs and Fibroblast Growth Factors in eye development and retinal cell rescue

Knowledge of normal eye development is crucial for the development of retinal rescue strategies. I shall focus on two signalling pathways that affect retinal development. Fibroblast growth factors function in retinal cell proliferation, retinal ganglion cell axon guidance and target recognition, craniofacial patterning and lens induction. Hedgehog proteins are required for progression of the neurogenic wave, cell proliferation, photoreceptor differentiation, retinal ganglion cell axon growth and craniofacial patterning. These signalling pathways have pleiotropic effects, can interact and have the potential to be used therapeutically. The zebrafish model organism may be well suited to studying how signalling pathways interact.

Fgf8 and Fgf3 are required for zebrafish ear placode induction, maintenance and inner ear patterning

The vertebrate inner ear develops from initially 'simple' ectodermal placode and vesicle stages into the complex three-dimensional structure which is necessary for the senses of hearing and equilibrium. Although the main morphological events in vertebrate inner ear development are known, the genetic mechanisms controlling them are scarcely understood. Previous studies have suggested that the otic placode is induced by signals from the chordamesoderm and the hindbrain, notably by fibroblast growth factors (Fgfs) and Wnt proteins. Here we study the role of Fgf8 as a bona-fide hindbrain-derived signal that acts in conjunction with Fgf3 during placode induction, maintenance and otic vesicle patterning. Acerebellar (ace) is a mutant in the fgf8 gene that results in a non-functional Fgf8 product. Homozygous mutants for acerebellar (ace) have smaller ears that typically have only one otolith, abnormal semi-circular canals, and behavioral defects. Using gene expression markers for the otic placode, we find that ace/fgf8 and Fgf-signaling are required for normal otic placode formation and maintenance. Conversely, misexpression of fgf8 or Fgf8-coated beads implanted into the vicinity of the otic placode can increase ear size and marker gene expression, although competence to respond to the induction appears restricted. Cell transplantation experiments and expression analysis suggest that Fgf8 is required in the hindbrain in the rhombomere 4-6 area to restore normal placode development in ace mutants, in close neighbourhood to the forming placode, but not in mesodermal tissues. Fgf3 and Fgf8 are expressed in hindbrain rhombomere 4 during the stages that are critical for placode induction. Joint inactivation of Fgf3 and Fgf8 by mutation or antisense-morpholino injection causes failure of placode formation and results in ear-less embryos, mimicking the phenotype we observe after pharmacological inhibition of Fgf-signaling. Fgf8 and Fgf3 together therefore act during induction and differentiation of the ear placode. In addition to the early requirement for Fgf signaling, the abnormal differentiation of inner ear structures and mechanosensory hair cells in ace mutants, pharmacological inhibition of Fgf signaling, and the expression of fgf8 and fgf3 in the otic vesicle demonstrate independent Fgf function(s) during later development of the otic vesicle and lateral line organ. We furthermore addressed a potential role of endomesomerm by studying mzoep mutant embryos that are depleted of head endomesodermal tissue, including chordamesoderm, due to a lack of Nodal-pathway signaling. In these embryos, early placode induction proceeds largely normally, but the ear placode extends abnormally to midline levels at later stages, suggesting a role for the midline in restricting placode development to dorsolateral levels. We suggest a model of zebrafish inner ear development with several discrete steps that utilize sequential Fgf signals during otic placode induction and vesicle patterning.

Genomic structure and embryonic expression of estrogen receptor beta a (ERbetaa) in zebrafish (Danio rerio)

Estrogenic steroid hormones mediate complex actions important in both embryonic and adult life. The hormones signal through ligand-inducible transcription factors known as estrogen receptors (ERs). In this study, we have isolated a zebrafish estrogen receptor with homology to human estrogen receptor beta (ERbeta). This zebrafish ERbeta (ERbetaa) has a conserved genomic structure of eight coding exons with boundaries similar to those of human ERbeta. The coding exon structures of two other zebrafish estrogen receptors (ERalpha and ERbetab) are presented as well. We also analyzed 3.3 kb of the promoter region and identified numerous putative transcription factor binding sites, including SP1 and ER half sites. Zebrafish ERbetaa message RNA is maternally loaded, but quickly degraded after fertilization, as detected by reverse transcriptase polymerase chain reaction. ERbetaa transcripts are detected again between 24 and 48 h post fertilization. These results indicate that ERbeta has been highly conserved during evolution and is likely used during later embryogenesis in zebrafish. Future identification of the expression levels and patterns of this and other estrogen receptors in zebrafish will allow a better understanding of estrogen signaling during embryogenesis.

FGF3 and FGF8 mediate a rhombomere 4 signaling activity in the zebrafish hindbrain

The segmentation of the vertebrate hindbrain into rhombomeres is highly conserved, but how early hindbrain patterning is established is not well understood. We show that rhombomere 4 (r4) functions as an early-differentiating signaling center in the zebrafish hindbrain. Time-lapse analyses of zebrafish hindbrain development show that r4 forms first and hindbrain neuronal differentiation occurs first in r4. Two signaling molecules, FGF3 and FGF8, which are both expressed early in r4, are together required for the development of rhombomeres adjacent to r4, particularly r5 and r6. Transplantation of r4 cells can induce expression of r5/r6 markers, as can misexpression of either FGF3 or FGF8. Genetic mosaic analyses also support a role for FGF signaling acting from r4. Taken together, our findings demonstrate a crucial role for FGF-mediated inter-rhombomere signaling in promoting early hindbrain patterning and underscore the significance of organizing centers in patterning the vertebrate neural plate.

Fgf3 and Fgf8 are required together for formation of the otic placode and vesicle

Fgf3 has long been implicated in otic placode induction and early development of the otocyst; however, the results of experiments in mouse and chick embryos to determine its function have proved to be conflicting. In this study, we determined fgf3 expression in relation to otic development in the zebrafish and used antisense morpholino oligonucleotides to inhibit Fgf3 translation. Successful knockdown of Fgf3 protein was demonstrated and this resulted in a reduction of otocyst size together with reduction in expression of early markers of the otic placode.

fgf3 is co-expressed with fgf8 in the hindbrain prior to otic induction and, strikingly, when Fgf3 morpholinos were co-injected together with Fgf8 morpholinos, a significant number of embryos failed to form otocysts. These effects were made manifest at early stages of otic development by an absence of early placode markers (pax2.1 and dlx3) but were not accompanied by effects on cell division or death. The temporal requirement for Fgf signalling was established as being between 60% epiboly and tailbud stages using the Fgf receptor inhibitor SU5402. However, the earliest molecular event in induction of the otic territory, pax8 expression, did not require Fgf signalling, indicating an inductive event upstream of signalling by Fgf3 and Fgf8. We propose that Fgf3 and Fgf8 are required together for formation of the otic placode and act during the earliest stages of its induction.

Inhibition of BMP activity by the FGF signal promotes posterior neural development in zebrafish

The expression patterns of region-specific neuroectodermal genes and fate-map analyses in zebrafish gastrulae suggest that posterior neural development is initiated by nonaxial signals, distinct from organizer-derived secreted bone morphogenetic protein (BMP) antagonists. This notion is further supported by the misexpression of a constitutively active form of zebrafish BMP type IA receptor (CA-BRIA) in the zebrafish embryos. It effectively suppressed the anterior neural marker, otx2, but not the posterior marker, hoxb1b. Furthermore, we demonstrated that the cells in the presumptive posterior neural region lose their neural fate only when CA-BRIA and Xenopus dominant-negative fibroblast growth factor (FGF) receptors (XFD) are coexpressed. The indications are that FGF signaling is involved in the formation of the posterior neural region, counteracting the BMP signaling pathway within the target cells. We then examined the functions of Fgf3 in posterior neural development. Zebrafish fgf3 is expressed in the correct place (dorsolateral margin) and at the correct time (late blastula to early gastrula stages), the same point that the most precocious posterior neural marker, hoxb1b, is first activated. Unlike other members of the FGF family, Fgf3 had little mesoderm-inducing activity. When ectopically expressed, Fgf3 expands the neural region with suppression of anterior neural fate. However, this effect was mediated by Chordino (zebrafish Chordin), because Fgf3 induces chordino expression in the epiblast and Fgf3-induced neural expansion was substantially suppressed in dino mutants with mutated chordino genes. The results obtained in the present study reveal multiple actions of the FGF signal on neural development: it antagonizes BMP signaling within posterior neural cells, induces the expression of secreted BMP antagonists, and suppresses anterior neural fate.

Tight transcriptional control of the ETS domain factors Erm and Pea3 by Fgf signaling during early zebrafish development

Several molecules of the Fibroblast growth factor family have been implicated in the development of the vertebrate brain, but the effectors of these molecules remain largely unknown. Here we study Erm and Pea3, two ETS domain transcription factors, and show that their expression correlates closely with the domains of fgf8 and fgf3 expression. In situ hybridization analysis in wild-type and acerebellar (ace) mutant embryos defective for fgf8 demonstrates a requirement of Fgf8 for normal expression levels of erm and pea3 transcripts in and close to various domains of Fgf8 action, including the prospective midbrain-hindbrain region, the somites, the neural crest, the forebrain, and developing eyes. Morpholino-oligomer-assisted gene knock-down experiments targeted against fgf8 and fgf3 suggest that Fgf3 and Fgf8 are co-regulators of these genes in the early forebrain anlage. Furthermore, inhibition of Fgf signaling by overexpression of sprouty4 or application of the Fgf inhibitor SU5402 leads to a loss of all erm and pea3 expression domains. Conversely, ectopically provided fgf3 mRNA or implanted beads coated with Fgf8 elicit ectopic transcription of erm and pea3. Both activation and loss of transcripts can be observed within short time frames. We conclude that both the transcriptional onset and maintenance of these factors are tightly coupled to Fgf signaling and propose that erm and pea3 transcription is a direct readout of cells to Fgf levels. Given the knowledge that has accumulated on the posttranslational control of ETS domain factors and their combinatorial interactions with other transcription factors, we suggest that the close coupling of erm and pea3 transcription to Fgf signaling might serve to integrate Fgf signaling with other signals to establish refined patterns in embryonic development.

Zebrafish fgf3 and fgf8 encode redundant functions required for otic placode induction

Members of the fibroblast growth factor (FGF) family of peptide ligands have been implicated in otic placode induction in several vertebrate species. Here, we have functionally analyzed the roles of fgf3 and fgf8 in zebrafish otic development. The role of fgf8 was assessed by analyzing acerebellar (ace) mutants. fgf3 function was disrupted by injecting embryos with antisense morpholino oligomers (MO) specifically designed to block translation of fgf3 transcripts. Disruption of either fgf3 or fgf8 causes moderate reduction in the size of the otic vesicle. Injection of fgf3-MO into ace/ace mutants causes much more severe reduction or complete loss of otic tissue. Moreover, preplacode cells fail to express pax8 and pax2.1, indicating disruption of early stages of otic induction in fgf3-depleted ace/ace mutants. Both fgf3 and fgf8 are normally expressed in the germring by 50% epiboly and are induced in the primordium of rhombomere 4 by 80% epibloy. In addition, fgf3 is expressed during the latter half of gastrulation in the prechordal plate and paraxial cephalic mesendoderm, tissues that either pass beneath or persist near the prospective otic ectoderm. Conditions that alter the pattern of expression of fgf3 and/or fgf8 cause corresponding changes in otic induction. Loss of maternal and zygotic one-eyed pinhead (oep) does not alter expression of fgf3 or fgf8 in the hindbrain, but ablates mesendodermal sources of fgf signaling and delays otic induction by several hours. Conversely, treatment of wild-type embryos with retinoic acid greatly expands the periotic domains of expression of fgf3, fgf8, and pax8 and leads to formation of supernumerary and ectopic otic vesicles. These data support the hypothesis that fgf3 and fgf8 cooperate during the latter half of gastrulation to induce differentiation of otic placodes.

sprouty4 acts in vivo as a feedback-induced antagonist of FGF signaling in zebrafish

In looking for novel factors involved in the regulation of the fibroblast growth factor (FGF) signaling pathway, we have isolated a zebrafish sprouty4 gene, based on its extensive similarities with the expression patterns of both fgf8 and fgf3. Through gain- and loss-of-function experiments, we demonstrate that Fgf8 and Fgf3 act in vivo to induce the expression of Spry4, which in turn can inhibit activity of these growth factors. When overexpressed at low doses, Spry4 induces loss of cerebellum and reduction in size of the otic vesicle, thereby mimicking the fgf8/acerebellar mutant phenotype. Injections of high doses of Spry4 cause ventralization of the embryo, an opposite phenotype to the dorsalisation induced by overexpression of Fgf8 or Fgf3. Conversely we have shown that inhibition of Spry4 function through injection of antisense morpholino oligonucleotide leads to a weak dorsalization of the embryo, the phenotype expected for an upregulation of Fgf8 or Fgf3 signaling pathway. Finally, we show that Spry4 interferes with FGF signaling downstream of the FGF receptor 1 (FGFR1). In addition, our analysis reveals that signaling through FGFR1/Ras/mitogen-activated protein kinase pathway is involved, not in mesoderm induction, but in the control of the dorsoventral patterning via the regulation of bone morphogenetic protein (BMP) expression.

Overlapping and distinct functions provided by fgf17, a new zebrafish member of the Fgf8/17/18 subgroup of Fgfs

Members of the fibroblast growth factor (Fgf) family are important signaling molecules in several inductive and patterning processes, and act as brain organizer-derived signals during formation of the early vertebrate nervous system. We isolated a new member of the Fgf8/17/18 subgroup of Fgfs from the zebrafish, and studied its expression and function during somitogenesis, optic stalk and midbrain-hindbrain boundary (MHB) development. In spite of a slightly higher aminoacid similarity to Fgf8, expression analysis and mapping to a chromosome stretch that is syntenic with mammalian chromosomes shows that this gene is orthologous to mammalian Fgf17. These data provide a further example of conserved chromosomal organization between zebrafish and mammalian genomes. Using an mRNA injection assay, we show that fgf17 can act similar to fgf8 during gastrulation, when fgf17 is not normally expressed. Direct comparison of the expression patterns of fgf17 and fgf8 suggest however a possible cooperation of these Fgfs at later stages in several tissues requiring Fgf signaling. Analysis of zebrafish MHB mutants demonstrates a gene-dosage dependent requirement of fgf17 expression for the no isthmus// pax2.1 gene, showing that no isthmus/pax2.1 functions upstream of fgf17 at the MHB in a haplo-insufficient manner, similar to what has been reported for mammalian pax2 mutants. In contrast, only maintenance of fgf17 expression is disturbed at the MHB of acerebellar/fgf8 mutants. Consistent with a requirement for fgf8 function, implantation of FGF8-soaked beads induces fgf17 expression, and expression is upregulated in aussicht mutants, which display upregulation of the Fgf8 signaling pathway. Taken together, our results argue that Fgf8 and Fgf17 act as hierarchically organized signaling molecules during development of the MHB organizer and possibly other organizers in the developing nervous system.

Identification of a fibroblast growth factor 6 (FGF6) gene in a non-mammalian vertebrate: continuous expression of FGF6 accompanies muscle fiber hyperplasia

FGF6, a member of the fibroblast growth factor (FGF) family, is specifically expressed in developing skeletal muscle and may participate in muscle maintenance and regeneration. Until now, no convincing evidence for the existence of an FGF6 gene in non-mammalian vertebrates has been put forward. Only a hybrid growth factor containing features characteristic of both FGF4 and FGF6 has been identified in frogs and chickens, suggesting that the step of duplication which created FGF4 and FGF6 took place with the emergence of mammals. In this study, we report the isolation and characterization of a genomic clone encoding the trout (Oncorhynchus mykiss) fibroblast growth factor 6 (TFGF6). An initial cDNA clone was generated by PCR amplification using degenerate oligo primers corresponding to a conserved region of protein found in the mouse and human homologs. The screening of a genomic library with the cloned PCR product led to the isolation of a clone composed of three exons encoding a putative protein of 206 amino acids which exhibits a potential signal peptide and shows 64.6 and 63.6% similarity with mouse and human FGF6, respectively (77% over the carboxy two-thirds of the protein) and only 46.5% similarity with mouse and human FGF4 (62% over the carboxy two-thirds of the protein). The splice position of the three exons was found to be analogous to the human and mouse FGF6 and the start translation site of TFGF6 was preceded by a long stretch of nucleotides that is highly and specifically conserved in mammalian FGF6. Furthermore, a comparative reverse transcriptase-linked PCR assay revealed that the expression pattern of TFGF6 is close to that of mammals, TFGF6 transcripts being present in muscle (fast-twitch and to a lesser extent slow-twitch fibers), heart, testis and brain. Interestingly, the prolonged phase of muscle fiber hyperplasia which occurs in trout is accompanied by the lasting expression of TFGF6 up to the adult stage suggesting that TFGF6 may participate in the continuous generation of muscle fibers within the myotomal musculature of post larval animals.

Fibroblast growth factors as multifunctional signaling factors

The fibroblast growth factor (FGF) family consists of at least 15 structurally related polypeptide growth factors. Their expression is controlled at the levels of transcription, mRNA stability, and translation. The bioavailability of FGFs is further modulated by posttranslational processing and regulated protein trafficking. FGFs bind to receptor tyrosine kinases (FGFRs), heparan sulfate proteoglycans (HSPG), and a cysteine-rich FGF receptor (CFR). FGFRs are required for most biological activities of FGFs. HSPGs alter FGF-FGFR interactions and CFR participates in FGF intracellular transport. FGF signaling pathways are intricate and are intertwined with insulin-like growth factor, transforming growth factor-beta, bone morphogenetic protein, and vertebrate homologs of Drosophila wingless activated pathways. FGFs are major regulators of embryonic development: They influence the formation of the primary body axis, neural axis, limbs, and other structures. The activities of FGFs depend on their coordination of fundamental cellular functions, such as survival, replication, differentiation, adhesion, and motility, through effects on gene expression and the cytoskeleton.

Fibroblast growth factor 3, a protein with dual subcellular localization, is targeted to the nucleus and nucleolus by the concerted action of two nuclear localization signals and a nucleolar retention signal

The major isoform of fibroblast growth factor 3 (FGF3) is initiated from a CUG codon, and the resultant product is distributed to the nucleus/nucleolus and secretory pathway. This dual subcellular localization is achieved in part by the competing effects of two classical intracellular targeting signals located near the amino terminus. At the extreme amino terminus is a short stretch of 29 amino acids before a signal peptide necessary for translocation into the endoplasmic reticulum, which is next to an adjacent bipartite nuclear localization signal. The carboxyl-terminal region of FGF3 is also implicated in nuclear/nucleolar localization. We describe here the characterization of carboxyl-terminal signals by showing they are capable of directing a heterologous protein, beta-galactosidase, to the nucleus. Furthermore, appending both the amino- and carboxyl-terminal domains onto beta-galactosidase, reproduces the dual subcellular localization properties of FGF3. Nuclear uptake of FGF3 appears to be signal-mediated since it binds to karyopherin alpha, the nuclear localization signal binding subunit of a heterodimeric receptor of the nuclear import machinery. The import of FGF3 into the nucleus is energy-dependent, and the inhibition of this process has demonstrated the importance of the nucleolar retention signal in nucleoplasmic and nucleolar accumulation.

Identification of a MMTV insertion mutation within the coding region of the Fgf-3 protooncogene

The Fgf-3 protooncogene (previously called int-2) is a target of proviral insertion mutations in mammary tumors induced by the mouse mammary tumor virus (MMTV). These insertion mutations result in the transcriptional activation of Fgf-3, which is not normally expressed in the adult mammary gland. Previous mapping studies of numerous Fgf-3 insertion mutations have failed to reveal any provirus integrations within the gene coding region. This finding is consistent with the hypothesis that oncogenesis occurs in this system as a consequence of up-regulation of Fgf-3 transcription, rather than from alterations of the gene product. During an analysis of a new cohort of tumors from the WXG-2 mouse strain, a breast tumor was identified which had a MMTV provirus integrated 24 bp upstream of the Fgf-3 stop codon. This insertion mutation generated a fusion transcript which was readily detectable in tumor RNA by RT-PCR. The predicted protein product of this fusion transcript is missing 8 aa of native sequence and contains an additional 8 aa of cryptic MMTV-encoded sequence. These data document the first exception to the generalization that the Fgf-3 coding region is not disrupted by MMTV insertion mutation.