Pax-2 controls multiple steps of urogenital development

Urogenital system development in mammals requires the coordinated differentiation of two distinct tissues, the ductal epithelium and the nephrogenic mesenchyme, both derived from the intermediate mesoderm of the early embryo. The former give rise to the genital tracts, ureters and kidney collecting duct system, whereas mesenchymal components undergo epithelial transformation to form nephrons in both the mesonephric (embryonic) and metanephric (definitive) kidney. Pax-2 is a transcriptional regulator of the paired-box family and is widely expressed during the development of both ductal and mesenchymal components of the urogenital system. We report here that Pax-2 homozygous mutant newborn mice lack kidneys, ureters and genital tracts. We attribute these defects to dysgenesis of both ductal and mesenchymal components of the developing urogenital system. The Wolffian and Mullerian ducts, precursors of male and female genital tracts, respectively, develop only partially and degenerate during embryogenesis. The ureters, inducers of the metanephros are absent and therefore kidney development does not take place. Mesenchyme of the nephrogenic cord fails to undergo epithelial transformation and is not able to form tubules in the mesonephros. In addition, we show that the expression of specific markers for each of these components is de-regulated in Pax-2 mutants. These data show that Pax-2 is required for multiple steps during the differentiation of intermediate mesoderm. In addition, Pax-2 mouse mutants provide an animal model for human hereditary kidney diseases.

Eya1-deficient mice lack ears and kidneys and show abnormal apoptosis of organ primordia

Haploinsufficiency for human EYA1, a homologue of the Drosophila melanogaster gene eyes absent (eya), results in the dominantly inherited disorders branchio-oto-renal (BOR) syndrome and branchio-oto (BO) syndrome, which are characterized by craniofacial abnormalities and hearing loss with (BOR) or without (BO) kidney defects. To understand the developmental pathogenesis of organs affected in these syndromes, we inactivated the gene Eya1 in mice. Eya1 heterozygotes show renal abnormalities and a conductive hearing loss similar to BOR syndrome, whereas Eya1 homozygotes lack ears and kidneys due to defective inductive tissue interactions and apoptotic regression of the organ primordia. Inner ear development in Eya1 homozygotes arrests at the otic vesicle stage and all components of the inner ear and specific cranial sensory ganglia fail to form. In the kidney, Eya1 homozygosity results in an absence of ureteric bud outgrowth and a subsequent failure of metanephric induction. Gdnf expression, which is required to direct ureteric bud outgrowth via activation of the c-ret Rtk (refs 5, 6, 7, 8), is not detected in Eya1-/- metanephric mesenchyme. In Eya1-/- ear and kidney development, Six but not Pax expression is Eya1 dependent, similar to a genetic pathway elucidated in the Drosophila eye imaginal disc. Our results indicate that Eya1 controls critical early inductive signalling events involved in ear and kidney formation and integrate Eya1 into the genetic regulatory cascade controlling kidney formation upstream of Gdnf. In addition, our results suggest that an evolutionarily conserved Pax-Eya-Six regulatory hierarchy is used in mammalian ear and kidney development.

Isolation of renal progenitor cells from adult human kidney

We describe here isolation and characterization of CD133+ cells derived from normal adult human kidney. These cells lacked the expression of hematopoietic markers and expressed PAX-2, an embryonic renal marker, suggesting their renal origin. Renal tissue-derived CD133+ cells and clones of individual cells were capable of expansion and limited self-renewal and differentiated in vitro into epithelial or endothelial cells. On subcutaneous implantation in SCID mice, the undifferentiated cells formed tubular structures expressing renal epithelial markers. At variance, when differentiated in endothelial cells, these cells formed functional vessels. On intravenous injection in SCID mice with glycerol-induced tubulonecrosis, the in vitro expanded renal-derived CD133+ cells homed into the injured kidney and integrated in tubules. We propose that CD133+ cells from kidney represent a multipotent adult resident stem cell population that may contribute to the repair of renal injury.

Mutation of the PAX2 gene in a family with optic nerve colobomas, renal anomalies and vesicoureteral reflux

Paired box (PAX) genes play a critical role in human development and disease. The PAX2 gene is expressed in primitive cells of the kidney, ureter, eye, ear and central nervous system. We have conducted a mutational analysis of PAX2 in a family with optic nerve colobomas, renal hypoplasia, mild proteinuria and vesicoureteral reflux. We report a single nucleotide deletion in exon five, causing a frame-shift of the PAX2 coding region in the octapeptide domain. The phenotype resulting from the PAX2 mutation in this family was very similar to abnormalities that have been reported in Krd mutant mice. These data suggest that PAX2 is required for normal kidney and eye development.

Nephric lineage specification by Pax2 and Pax8

The mammalian kidney develops in three successive steps from the initial pronephros via the mesonephros to the adult metanephros. Although the nephric lineage is specified during pronephros induction, no single regulator, including the transcription factor Pax2 or Pax8, has yet been identified to control this initial phase of kidney development. In this paper, we demonstrate that mouse embryos lacking both Pax2 and Pax8 are unable to form the pronephros or any later nephric structures. In these double-mutant embryos, the intermediate mesoderm does not undergo the mesenchymal-epithelial transitions required for nephric duct formation, fails to initiate the kidney-specific expression of Lim1 and c-Ret, and is lost by apoptosis 1 d after failed pronephric induction. Conversely, retroviral misexpression of Pax2 was sufficient to induce ectopic nephric structures in the intermediate mesoderm and genital ridge of chick embryos. Together, these data identify Pax2 and Pax8 as critical regulators that specify the nephric lineage.

Pax2 contributes to inner ear patterning and optic nerve trajectory

During gestation, the paired box-containing gene Pax2 is expressed in the mid-hindbrain area, developing eye and inner ear. We generated Pax2 null mutant mice, which show the requirement of Pax2 for the establishment of axonal pathways along the optic stalks and ventral diencephalon. In mutant brains, the optic tracts remain totally ipsilateral due to agenesis of the optic chiasma. Furthermore, Pax2 mutants show extension of the pigmented retina into the optic stalks and failure of the optic fissure to close resulting in coloboma. In the inner ear, Pax2 mutants show agenesis of the cochlea and the spiral ganglion, i.e., the parts of the organ responsible for auditory function and in whose primordium Pax2 is expressed. Our results identify Pax2 as a major regulator of patterning during organogenesis of the eye and inner ear and indicate its function in morphogenetic events required for closure of the optic fissure and neural tube.

The SCL gene specifies haemangioblast development from early mesoderm

The SCL gene encodes a basic helix-loop-helix (bHLH) transcription factor that is essential for the development of all haematopoietic lineages. SCL is also expressed in endothelial cells, but its function is not essential for specification of endothelial progenitors and the role of SCL in endothelial development is obscure. We isolated the zebrafish SCL homologue and show that it was co-expressed in early mesoderm with markers of haematopoietic, endothelial and pronephric progenitors. Ectopic expression of SCL mRNA in zebrafish embryos resulted in overproduction of common haematopoietic and endothelial precursors, perturbation of vasculogenesis and concomitant loss of pronephric duct and somitic tissue. Notochord and neural tube formation were unaffected. These results provide the first evidence that SCL specifies formation of haemangioblasts, the proposed common precursor of blood and endothelial lineages. Our data also underline the striking similarities between the role of SCL in haematopoiesis/vasculogenesis and the function of other bHLH proteins in muscle and neural development.

Early development of the zebrafish pronephros and analysis of mutations affecting pronephric function

The zebrafish pronephric kidney provides a simplified model of nephron development and epithelial cell differentiation which is amenable to genetic analysis. The pronephros consists of two nephrons with fused glomeruli and paired pronephric tubules and ducts. Nephron formation occurs after the differentiation of the pronephric duct with both the glomeruli and tubules being derived from a nephron primordium. Fluorescent dextran injection experiments demonstrate that vascularization of the zebrafish pronephros and the onset of glomerular filtration occurs between 40 and 48 hpf. We isolated fifteen recessive mutations that affect development of the pronephros. All have visible cysts in place of the pronephric tubule at 2–2.5 days of development. Mutants were grouped in three classes: (1) a group of twelve mutants with defects in body axis curvature and manifesting the most rapid and severe cyst formation involving the glomerulus, tubule and duct, (2) the fleer mutation with distended glomerular capillary loops and cystic tubules, and (3) the mutation pao pao tang with a normal glomerulus and cysts limited to the pronephric tubules. double bubble was analyzed as a representative of mutations that perturb the entire length of the pronephros and body axis curvature. Cyst formation begins in the glomerulus at 40 hpf at the time when glomerular filtration is established suggesting a defect associated with the onset of pronephric function. Basolateral membrane protein targeting in the pronephric duct epithelial cells is also severely affected, suggesting a failure in terminal epithelial cell differentiation and alterations in electrolyte transport. These studies reveal the similarity of normal pronephric development to kidney organogenesis in all vertebrates and allow for a genetic dissection of genes needed to establish the earliest renal function.

Midline signalling is required for Pax gene regulation and patterning of the eyes

Pax6 and Pax2 are members of the Pax family of transcription factors that are both expressed in the developing visual system of zebrafish embryos. Pax6 protein is present in all cells that form the neural retina and pigment epithelium, whereas Pax2 is located primarily in cells that will give rise to the optic stalk. In this study, we have addressed the role of midline signalling in the regulation of Pax2 and Pax6 distributions and in the subsequent morphogenesis of the eyes. Midline signalling is severely perturbed in cyclops mutant embryos resulting in an absence of ventral midline CNS tissue and fusion of the eyes. Mutant embryos ectopically express Pax6 in a bridge of tissue around the anterior pole of the neural keel in the position normally occupied by cells that form the optic stalks. In contrast, Pax2 protein is almost completely absent from this region in mutant embryos. Concommitant with the changes in Pax protein distribution, cells in the position of the optic stalks differentiate as retina. These results suggest that a signal emanating from the midline, which is absent in cyclops mutant embryos, may be required to promote Pax2 and inhibit Pax6 expression in cells destined to form the optic stalks. Sonic hedgehog (Shh also known as Vhh-1 and Hhg-1) is a midline signalling molecule that is absent from the neuroepithelium of cyclops mutant embryos at early developmental stages. To test the possibility that Shh might be able to regulate the spatial expression of Pax6 and Pax2 in the optic primordia, it was overexpressed in the developing CNS. The number of cells containing Pax2 was increased following shh overexpression and embryos developed hypertrophied optic stalk-like structures. Complimentary to the changes in Pax2 distribution, there were fewer Pax6-containing cells and pigment epithelium and neural retina were reduced. Our results suggest that Shh or a closely related signalling molecule emanating from midline tissue in the ventral forebrain either directly or indirectly induces the expression of Pax2 and inhibits the expression of Pax6 and thus may regulate the partitioning of the optic primordia into optic stalks and retinal tissue.

Pax-2 is a DNA-binding protein expressed in embryonic kidney and Wilms tumor

The murine Pax-2 gene contains a protein coding domain homologous to the Drosophila paired-box, first described in certain developmental control genes of the segmentation type. Polyclonal antibodies recognize two Pax-2 proteins that are encoded by differentially spliced mRNAs. The Pax-2 proteins can bind a DNA sequence known to interact with the paired domain of a Drosophila protein. By immunocytochemistry, expression of Pax-2 could be localized to the nuclei of condensing mesenchyme cells and their epithelial derivatives in the developing kidney. Expression is abruptly down-regulated as the tubular epithelium differentiates. High levels of Pax-2 expression could also be detected in the epithelial cells of human Wilms tumors. These data suggest that Pax-2 is a transcription factor active during the mesenchyme-to-epithelium transition in early kidney development and in Wilms tumor.

The mouse Pax2(1Neu) mutation is identical to a human PAX2 mutation in a family with renal-coloboma syndrome and results in developmental defects of the brain, ear, eye, and kidney

We describe a new mouse frameshift mutation (Pax2(1Neu)) with a 1-bp insertion in the Pax2 gene. This mutation is identical to a previously described mutation in a human family with renal-coloboma syndrome [Sanyanusin, P., McNoe, L. A., Sullivan, M. J., Weaver, R. G. & Eccles, M. R. (1995) Hum. Mol. Genet. 4, 2183-2184]. Heterozygous mutant mice exhibit defects in the kidney, the optic nerve, and retinal layer of the eye, and in homozygous mutant embryos, development of the optic nerve, metanephric kidney, and ventral regions of the inner ear is severely affected. In addition, we observe a deletion of the cerebellum and the posterior mesencephalon in homozygous mutant embryos demonstrating that, in contrast to mutations in Pax5, which is also expressed early in the mid-hindbrain region, loss of Pax2 gene function alone results in the early loss of the mid-hindbrain region. The mid-hindbrain phenotype is similar to Wnt1 and En1 mutant phenotypes, suggesting the conservation of gene regulatory networks between vertebrates and Drosophila.

Tlx3 and Tlx1 are post-mitotic selector genes determining glutamatergic over GABAergic cell fates

Glutamatergic and GABAergic neurons mediate much of the excitatory and inhibitory neurotransmission, respectively, in the vertebrate nervous system. The process by which developing neurons select between these two cell fates is poorly understood. Here we show that the homeobox genes Tlx3 and Tlx1 determine excitatory over inhibitory cell fates in the mouse dorsal spinal cord. First, we found that Tlx3 was required for specification of, and expressed in, glutamatergic neurons. Both generic and region-specific glutamatergic markers, including VGLUT2 and the AMPA receptor Gria2, were absent in Tlx mutant dorsal horn. Second, spinal GABAergic markers were derepressed in Tlx mutants, including Pax2 that is necessary for GABAergic differentiation, Gad1/2 and Viaat that regulate GABA synthesis and transport, and the kainate receptors Grik2/3. Third, ectopic expression of Tlx3 was sufficient to suppress GABAergic differentiation and induce formation of glutamatergic neurons. Finally, excess GABA-mediated inhibition caused dysfunction of central respiratory circuits in Tlx3 mutant mice.

Integrin alpha8beta1 is critically important for epithelial-mesenchymal interactions during kidney morphogenesis

We present genetic evidence that integrins regulate epithelial-mesenchymal interactions during organogenesis. Mice with a mutation in the alpha8 gene do not express the integrin alpha8 beta1 and exhibit profound deficits in kidney morphogenesis. In wild-type animals, inductive interactions between the ureteric epithelium and metanephric mesenchyme are essential for kidney morphogenesis. In alpha8 mutant homozygotes, growth and branching of the ureteric bud and recruitment of mesenchymal cells into epithelial structures are defective. Consistent with these phenotypes, alpha8 expression is induced in mesenchymal cells upon contact with the ureter. Since none of its previously identified ligands appears likely to mediate the essential functions of alpha8 beta1 in kidney morphogenesis, we have used an alpha8 beta1-alkaline phosphatase chimera to localize novel ligand(s) in the growing ureter. The distribution of these ligand(s) makes them strong candidates for regulators of kidney morphogenesis.

Six1 is required for the early organogenesis of mammalian kidney

The murine Six gene family, homologous to Drosophila sine oculis (so) which encodes a homeodomain transcription factor, is composed of six members (Six1-6). Among the six members, only the Six2 gene has been previously shown to be expressed early in kidney development, but its function is unknown. We have recently found that the Six1 gene is also expressed in the kidney. In the developing kidney, Six1 is expressed in the uninduced metanephric mesenchyme at E10.5 and in the induced mesenchyme around the ureteric bud at E11.5. At E17.5 to P0, Six1 expression became restricted to a subpopulation of collecting tubule epithelial cells. To study its in vivo function, we have recently generated Six1 mutant mice. Loss of Six1 leads to a failure of ureteric bud invasion into the mesenchyme and subsequent apoptosis of the mesenchyme. These results indicate that Six1 plays an essential role in early kidney development. In Six1(-/-) kidney development, we have found that Pax2, Six2 and Sall1 expression was markedly reduced in the metanephric mesenchyme at E10.5, indicating that Six1 is required for the expression of these genes in the metanephric mesenchyme. In contrast, Eya1 expression was unaffected in Six1(-/-) metanephric mesenchyme at E10.5, indicating that Eya1 may function upstream of Six1. Moreover, our results show that both Eya1 and Six1 expression in the metanephric mesenchyme is preserved in Pax2(-/-) embryos at E10.5, further indicating that Pax2 functions downstream of Eya1 and Six1 in the metanephric mesenchyme. Thus, the epistatic relationship between Pax, Eya and Six genes in the metanephric mesenchyme during early kidney development is distinct from a genetic pathway elucidated in the Drosophila eye imaginal disc. Finally, our results show that Eya1 and Six1 genetically interact during mammalian kidney development, because most compound heterozygous embryos show hypoplastic kidneys. These analyses establish a role for Six1 in the initial inductive step for metanephric development.

The role of Six1 in mammalian auditory system development

The homeobox Six genes, homologues to Drosophila sine oculis (so) gene, are expressed in multiple organs during mammalian development. However, their roles during auditory system development have not been studied. We report that Six1 is required for mouse auditory system development. During inner ear development, Six1 expression was first detected in the ventral region of the otic pit and later is restricted to the middle and ventral otic vesicle within which, respectively, the vestibular and auditory epithelia form. By contrast, Six1 expression is excluded from the dorsal otic vesicle within which the semicircular canals form. Six1 is also expressed in the vestibuloacoustic ganglion. At E15.5, Six1 is expressed in all sensory epithelia of the inner ear. Using recently generated Six1 mutant mice, we found that all Six1(+/-) mice showed some degree of hearing loss because of a failure of sound transmission in the middle ear. By contrast, Six1(-/-) mice displayed malformations of the auditory system involving the outer, middle and inner ears. The inner ear development in Six1(-/-) embryos arrested at the otic vesicle stage and all components of the inner ear failed to form due to increased cell death and reduced cell proliferation in the otic epithelium. Because we previously reported that Six1 expression in the otic vesicle is Eya1 dependent, we first clarified that Eya1 expression was unaffected in Six1(-/-) otic vesicle, further demonstrating that the Drosophila Eya-Six regulatory cassette is evolutionarily conserved during mammalian inner ear development. We also analyzed several other otic markers and found that the expression of Pax2 and Pax8 was unaffected in Six1(-/-) otic vesicle. By contrast, Six1 is required for the activation of Fgf3 expression and the maintenance of Fgf10 and Bmp4 expression in the otic vesicle. Furthermore, loss of Six1 function alters the expression pattern of Nkx5.1 and Gata3, indicating that Six1 is required for regional specification of the otic vesicle. Finally, our data suggest that the interaction between Eya1 and Six1 is crucial for the morphogenesis of the cochlea and the posterior ampulla during inner ear development. These analyses establish a role for Six1 in early growth and patterning of the otic vesicle.

Pax genes and organogenesis

Pax genes are a family of developmental control genes that encode nuclear transcription factors. They are characterized by the presence of the paired domain, a conserved amino acid motif with DNA-binding activity. Originally, paired-box-containing genes were detected in Drosophila melanogaster, where they exert multiple functions during embryogenesis. In vertebrates, Pax genes are also involved in embryogenesis. Mutations in four out of nine characterized Pax genes have been associated with either congenital human diseases such as Waardenburg syndrome (PAX3), Aniridia (PAX6), Peter's anomaly (PAX6), renal coloboma syndrome (PAX2) or spontaneous mouse mutants (undulated (Pax1), Splotch (Pax3), Small eye (Pax6), Pax2(1)Neu), which all show defects in development. Recently, analysis of spontaneous and transgenic mouse mutants has revealed that vertebrate pax genes are key regulators during organogenesis of kidney, eye, ear, nose, limb muscles, vertebral column and brain. Like their Drosophila counterparts, vertebrate Pax genes are involved in pattern formation during embryogenesis, possibly by determining the time and place of organ initiation or morphogenesis. For most tissues, however, the nature of the primary developmental action of Pax transcription factors remains to be elucidated. One predominant theme is signal transduction during tissue interactions, which may lead to a position-specific regulation of cell proliferation.

Sonic hedgehog is not required for the induction of medial floor plate cells in the zebrafish

Sonic hedgehog (Shh) is a secreted protein that is involved in the organization and patterning of several tissues in vertebrates. We show that the zebrafish sonic-you (syu) gene, a member of a group of five genes required for somite patterning, is encoding Shh. Embryos mutant for a deletion of syu display defects in patterning of the somites, the lateral floor plate cells, the pectoral fins, the axons of motorneurons and the retinal ganglion cells. In contrast to mouse embryos lacking Shh activity, syu mutant embryos do form medial floor plate cells and motorneurons. Since ectopic overexpression of shh in zebrafish embryos does not induce ectopic medial floor plate cells, we conclude that shh is neither required nor sufficient to induce this cell type in the zebrafish.

Characterization of three novel members of the zebrafish Pax2/5/8 family: dependency of Pax5 and Pax8 expression on the Pax2.1 (noi) function

The mammalian Pax2, Pax5 and Pax8 genes code for highly related transcription factors, which play important roles in embryonic development and organogenesis. Here we report the characterization of all members of the zebrafish Pax2/5/8 family. These genes have arisen by duplications before or at the onset of vertebrate evolution. Due to an additional genome amplification in the fish lineage, the zebrafish contains two Pax2 genes, the previously known Pax[b] gene (here renamed as Pax2.1) and a novel Pax2.2 gene. The zebrafish Pax2.1 gene most closely resembles the mammalian Pax2 gene in its expression pattern, as it is transcribed first in the midbrain-hindbrain boundary region, then in the optic stalk, otic system, pronephros and nephric ducts, and lastly in specific interneurons of the hindbrain and spinal cord. Pax2.2 differs from Pax2.1 by the absence of expression in the nephric system and by a delayed onset of transcription in other Pax2.1 expession domains. Pax8 is also expressed in the same domains as Pax2.1, but its transcription is already initiated during gastrulation in the primordia of the otic placode and pronephric anlage, thus identifying Pax8 as the earliest developmental marker of these structures. The zebrafish Pax5 gene, in contrast to its mouse orthologue, is transcribed in the otic system in addition to its prominent expression at the midbrain-hindbrain boundary. The no isthmus (noi) mutation is known to inactivate the Pax2.1 gene, thereby affecting the development of the midbrain-hindbrain boundary region, pronephric system, optic stalk and otic region. Although the different members of the Pax2/5/8 family may potentially compensate for the loss of Pax2.1 function, we demonstrate here that only the expression of the Pax2.2 gene remains unaffected in noi mutant embryos. The expression of Pax5 and Pax8 is either not initiated at the midbrain-hindbrain boundary or is later not maintained in other expression domains. Consequently, the noi mutation of zebrafish is equivalent to combined inactivation of the mouse Pax2 and Pax5 genes with regard to the loss of midbrain-hindbrain boundary development.

Pax genes and their roles in cell differentiation and development

Members of the Pax gene family are expressed in various tissues during ontogenesis. Evidence for their crucial role in morphogenesis, organogenesis, cell differentiation and oncogenesis is provided by rodent mutants and human diseases. Additionally, recent experimental in vivo and in vitro approaches have led to the identification of molecules that interact with Pax proteins.

Generation of Pax2-Cre mice by modification of a Pax2 bacterial artificial chromosome

The Pax2 gene is expressed in the developing otocyst, kidney, and midbrain-hindbrain boundary. We generated Pax2-Cre transgenic lines by modification of a Pax2 bacterial artificial chromosome (BAC). In one Pax2-Cre line, Cre mRNA starts to be expressed in the otic placode at the late presomite stage. R26R reporter mouse analysis revealed that the Cre expression is sufficient to delete the loxP-flanked sequences in most of the cells in the inner ear. Reporter-positive cells are also detected in other Pax2-expressing tissues such as midbrain, cerebellum, olfactory bulb, and kidney, suggesting that these cells are the descendants of Pax2-expressing cells in these tissues and that Pax2-Cre transgenic mice can delete genes efficiently in these tissues.

Tripartite organization of the ancestral chordate brain and the antiquity of placodes: insights from ascidian Pax-2/5/8, Hox and Otx genes

Ascidians and vertebrates belong to the Phylum Chordata and both have dorsal tubular central nervous systems. The structure of the ascidian neural tube is extremely simple, containing less than 400 cells, among which less than 100 cells are neurons. Recent studies suggest that, despite its simple organization, the mechanisms patterning the ascidian neural tube are similar to those of the more complex vertebrate brain. Identification of homologous regions between vertebrate and ascidian nervous systems, however, remains to be resolved. Here we report the expression of HrPax-258 gene: an ascidian homologue of vertebrate Pax-2, Pax-5 and Pax-8 genes. Molecular phylogenetic analyses indicate that HrPax-258 is descendant from a single precursor gene that gave rise to the three vertebrate genes. The expression pattern of HrPax-258 suggests that this subfamily of Pax genes has conserved roles in regional specification of the brain. Comparison with expression of ascidian Otx (Hroth) and a Hox gene (HrHox1) by double-staining in situ hybridizations indicate that the ascidian brain region can be subdivided into three regions; the anterior region marked by Hroth probably homologous to the vertebrate forebrain and midbrain, the middle region marked by HrPax-258 probably homologous to the vertebrate anterior hindbrain (and maybe also midbrain) and the posterior region marked by Hox genes which is homologous to the vertebrate hindbrain and spinal cord. Later expression of HrPax-258 in atrial primordia implies that basal chordates such as ascidians have already acquired a sensory organ that develops from epidermal thickenings (placodes) and expresses HrPax-258; we suggest it is homologous to the vertebrate ear. Therefore, placodes are not likely to be a newly acquired feature in vertebrates, but may have already been possessed by the earliest chordates.

Combinatorial signaling in the specification of unique cell fates

How multifunctional signals combine to specify unique cell fates during pattern formation is not well understood. Here, we demonstrate that together with the transcription factor Lozenge, the nuclear effectors of the EGFR and Notch signaling pathways directly regulate D-Pax2 transcription in cone cells of the Drosophila eye disc. Moreover, the specificity of D-Pax2 expression can be altered upon genetic manipulation of these inputs. Thus, a relatively small number of temporally and spatially controlled signals received by a set of pluripotent cells can create the unique combinations of activated transcription factors required to regulate target genes and ultimately specify distinct cell fates within this group. We expect that similar mechanisms may specify pattern formation in vertebrate developmental systems that involve intercellular communication.

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.

Mesenchymal to epithelial conversion in rat metanephros is induced by LIF

Inductive signals cause conversion of mesenchyme into epithelia during the formation of many organs. Yet a century of study has not revealed the inducing molecules. Using a standard model of induction, we found that ureteric bud cells secrete factors that convert kidney mesenchyme to epithelia that, remarkably, then form nephrons. Purification and sequencing of one such factor identified it as leukemia inhibitory factor (LIF). LIF acted on epithelial precursors that we identified by the expression of Pax2 and Wnt4. Other IL-6 type cytokines acted like LIF, and deletion of their shared receptor reduced nephron development. In situ, the ureteric bud expressed LIF, and metanephric mesenchyme expressed its receptors. The data suggest that IL-6 cytokines are candidate regulators of mesenchymal to epithelial conversion during kidney development.