Identification of a human brain-specific gene, calneuron 1, a new member of the calmodulin superfamily.

The calmodulin superfamily includes the calmodulins, calcium-binding proteins, and related genes. Herein, we describe the cloning and characterization of human calneuron 1 (CALN1). CALN1 encodes a novel neuron-specific protein that maps to chromosome 7q11. CALN1 spans a large genomic region (>360 kb). Sequence comparison shows significant similarity with the calmodulin superfamily of genes, especially in the two conserved EF-hand motifs. The mouse orthologous gene (Caln1) shows little prenatal expression, with highest expression at Postnatal Day 21. In situ hybridization to adult mouse brain shows high expression in the cerebellum, hippocampus, and cortex. The high expression of this gene exclusively in brain, the developmental changes in expression levels, the high homology with calmodulin which indicates a potential role in signal transduction, and the cellular localization of the mRNA suggest that CALN1 has a significant role in the physiology of neurons and is potentially important in memory and learning.

Fox (forkhead) genes are involved in the dorso-ventral patterning of the Xenopus mesoderm.

Fox (forkhead/winged helix) genes encode a family of transcription factors that are involved in embryonic pattern formation, regulation of tissue specific gene expression and tumorigenesis. Several of them are transcribed during Xenopus embryogenesis and are important for the patterning of ectoderm, mesoderm and endoderm. We have isolated three forkhead genes that are activated during gastrulation and play an important role in the dorso-ventral patterning of the mesoderm. XFKH1 (FoxA4b), the first vertebrate forkhead gene to be implicated in embryonic pattern formation, is expressed in the Spemann-Mangold organizer region and later in the embryonic notochord. XFKH7, the Xenopus orthologue of the murine Mfh1(Foxc2), is expressed in the presomitic mesoderm, but not in the notochord or lateral plate mesoderm. Finally, XFD-13'(FoxF1b)1 is expressed in the lateral plate mesoderm, but not in the notochord or presomitic mesoderm. Expression pattern and functional experiments indicate that these three forkhead genes are involved in the dorso-ventral patterning of the mesoderm.

Mutations in the human forkhead transcription factor FOXE3 associated with anterior segment ocular dysgenesis and cataracts.

Dysgenesis of the anterior segment of the eye delineates a spectrum of human developmental disorders that show wide phenotypic and genetic heterogeneity. It is also frequently associated with cataracts and glaucoma resulting in visual disability in childhood. The recently described forkhead transcription factor gene Foxe3 was shown to be involved in the dysgenetic lens phenotype in mice, which is characterized by small cataractic lens and anterior segment anomalies. Here we report an identification and characterization of the human ortholog of this gene, FOXE3. The gene was found to be expressed in the anterior lens epithelium and to be mutated in patients with ocular disorders. An insertion of G in the coding region of the FOXE3 gene that occurred 15 nucleotides upstream of the stop codon was identified in a family with anterior segment ocular dysgenesis and cataracts. The mutation causes a frameshift that results in an abnormal sequence of five terminal amino acids and an addition of 111 amino acids to the predicted protein. The mutation was present in two affected individuals from this family and was not identified in 180 normal control chromosomes.

Function of Rx, but not Pax6, is essential for the formation of retinal progenitor cells in mice.

Rx plays a critical role in eye formation. Targeted elimination of Rx results in embryos that do not develop eyes. In this study, we have investigated the expression of Otx2, Six3, and Pax6 in Rx deficient embryos. We find that these genes show normal activation in the anterior neural plate in Rx-/- embryos, but they are not upregulated in the area of the neural plate that would form the primordium of the optic vesicle. In contrast, in homozygous Small eye embryos that lack Pax6 function, Rx shows normal activation in the anterior neural plate and normal upregulation in the optic vesicle/retinal progenitor cells. This suggests that neither Rx expression nor the formation of retinal progenitor cells is dependent on a functional copy of the Pax6 gene, but that Pax6 expression and the formation of the progenitor cells of the optic cup is dependent on a functional copy of the Rx gene.

Modules in the photoreceptor RGS9-1.Gbeta 5L GTPase-accelerating protein complex control effector coupling, GTPase acceleration, protein folding, and stability.

RGS (regulators of G protein signaling) proteins regulate G protein signaling by accelerating GTP hydrolysis, but little is known about regulation of GTPase-accelerating protein (GAP) activities or roles of domains and subunits outside the catalytic cores. RGS9-1 is the GAP required for rapid recovery of light responses in vertebrate photoreceptors and the only mammalian RGS protein with a defined physiological function. It belongs to an RGS subfamily whose members have multiple domains, including G(gamma)-like domains that bind G(beta)(5) proteins. Members of this subfamily play important roles in neuronal signaling. Within the GAP complex organized around the RGS domain of RGS9-1, we have identified a functional role for the G(gamma)-like-G(beta)(5L) complex in regulation of GAP activity by an effector subunit, cGMP phosphodiesterase gamma and in protein folding and stability of RGS9-1. The C-terminal domain of RGS9-1 also plays a major role in conferring effector stimulation. The sequence of the RGS domain determines whether the sign of the effector effect will be positive or negative. These roles were observed in vitro using full-length proteins or fragments for RGS9-1, RGS7, G(beta)(5S), and G(beta)(5L). The dependence of RGS9-1 on G(beta)(5) co-expression for folding, stability, and function has been confirmed in vivo using transgenic Xenopus laevis. These results reveal how multiple domains and regulatory polypeptides work together to fine tune G(talpha) inactivation.

Xlim-1 and LIM domain binding protein 1 cooperate with various transcription factors in the regulation of the goosecoid promoter.

The homeobox genes Xlim-1 and goosecoid (gsc) are coexpressed in the Spemann organizer and later in the prechordal plate that acts as head organizer. Based on our previous finding that gsc is a possible target gene for Xlim-1, we studied the regulation of gsc transcription by Xlim-1 and other regulatory genes expressed at gastrula stages, by using gsc-luciferase reporter constructs injected into animal explants. A 492-bp upstream region of the gsc promoter responds to Xlim-1/3m, an activated form of Xlim-1, and to a combination of wild-type Xlim-1 and Ldb1, a LIM domain binding protein, supporting the view that gsc is a direct target of Xlim-1. Footprint and electrophoretic mobility shift assays with GST-homeodomain fusion proteins and embryo extracts overexpressing FLAG-tagged full-length proteins showed that the Xlim-1 homeodomain or Xlim-1/Ldb1 complex recognize several TAATXY core elements in the 492-bp upstream region, where XY is TA, TG, CA, or GG. Some of these elements are also bound by the ventral factor PV.1, whereas a TAATCT element did not bind Xlim-1 or PV.1 but did bind the anterior factors Otx2 and Gsc. These proteins modulate the activity of the gsc reporter in animal caps: Otx2 activates the reporter synergistically with Xlim-1 plus Ldb1, whereas Gsc and PV.1 strongly repress reporter activity. We show further, using animal cap assays, that the endogenous gsc gene was synergistically activated by Xlim-1, Ldb1, and Otx2 and that the endogenous otx2 gene was activated by Xlim-1/3m, and this activation was suppressed by the posterior factor Xbra. Based on these data, we propose a model for gene interactions in the specification of dorsoventral and anteroposterior differences in the mesoderm during gastrulation.

Forkhead Foxe3 maps to the dysgenetic lens locus and is critical in lens development and differentiation.

Here we report the isolation of a novel forkhead gene, Foxe3, that plays an important role in lens formation. During development Foxe3 is expressed in all undifferentiated lens tissues, and is turned off upon fiber cell differentiation. Foxe3 maps to a chromosomal region containing the dysgenetic lens (dyl) mutation. Mice homozygous for dyl display several defects in lens development. dyl mice also show altered patterns of crystallin expression suggesting a dysregulation of lens differentiation. We have identified mutations in Foxe3 that cosegregate with the dyl phenotype and are a likely cause of the mutant phenotype. Head ectoderm expression of Foxe3 is absent in Rx-/- and Small eye embryos indicating that Rx and Pax6 activity are necessary for Foxe3 expression.

Regulation of the pancreatic islet-specific gene BETA2 (neuroD) by neurogenin 3.

The BETA2 (neuroD) gene is expressed in endocrine cells during pancreas development and is essential for proper islet morphogenesis. The objective of this study is to identify potential upstream regulators of the BETA2 gene during pancreas development. We demonstrated that the expression of neurogenin 3 (ngn3), an islet- and neuron-specific basic-helix-loop-helix transcription factor, partially overlaps that of BETA2 during early mouse development. More importantly, overexpression of ngn3 can induce the ectopic expression of BETA2 in Xenopus embryos and stimulate the endogenous RNA of BETA2 in endocrine cell lines. Furthermore, overexpression of ngn3 could cause a dose-dependent activation on the 1.0-kb BETA2 promoter in islet-derived cell lines. Deletion and mutation analyses revealed that two proximal E box sequences, E1 and E3, could bind to ngn3-E47 heterodimer and mediate ngn3 activation. Based on these results, we hypothesize that ngn3 is involved in activating the expression of BETA2 at an early stage of islet cell differentiation through the E boxes in the BETA2 promoter.

Regulation of eye formation by the Rx and pax6 homeobox genes.

The Rx and pax6 homeobox genes are among the earliest genes expressed in the eye primordia and play crucial roles in the specification of ocular fate. These genes exhibit strong conservation of sequence and expression patterns among vertebrates. As transcription factors, Rx and Pax6 exert their effects through the activation and repression of downstream targets. Overexpression of each protein induces ectopic retinal tissue formation, as well as induction of the other. pax6 gene mutations have been correlated with an array of human diseases, and a similar array of mutations potentially exists for the human Rx gene. Based on functional studies, the vertebrate Rx and pax6 genes are likely to regulate cell proliferation and are required for the initial commitment to retinal and lens cell fate, respectively, while pax6 appears to play additional roles in the formation of the retina and cornea.

A novel fork head gene mediates early steps during Xenopus lens formation.

Xlens1 is a novel Xenopus member of the fork head gene family, named for its nearly restricted expression in the anterior ectodermal placode, presumptive lens ectoderm (PLE), and anterior epithelium of the differentiated lens. The temporal and spatial restriction of its expression suggests that: (1) Xlens1 is transcribed initially at neural plate stages in response to putative signals from the anterior neural plate that transform lens-competent ectoderm to lens-biased ectoderm; (2) further steps in the process of lens-forming bias restrict Xlens1 expression to the presumptive lens ectoderm (PLE) during later neural plate stages; (3) interactions with the optic vesicle maintain Xlens1 expression in the lens placode; and (4) Xlens1 expression is downregulated as committed lens cells undergo terminal differentiation. Induction assays demonstrate that pax6 induces Xlens1 expression, but unlike pax6, Xlens1 cannot induce the expression of the lens differentiation marker beta-crystallin. In the whole embryo, overexpression of Xlens1 in the lens ectoderm causes it to thicken and maintain gene expression characteristics of the PLE. Also, this overexpression suppresses differentiation in the lens ectoderm, suggesting that Xlens1 functions to maintain specified lens ectoderm in an undifferentiated state. Misexpression of Xlens1 in other regions causes hypertrophy of restricted tissues but only occasionally leads ectopic sites of gamma-crystallin protein expression in select anterior head regions. These results indicate that Xlens1 expression alone does not specify lens ectoderm. Lens specification and differentiation likely depends on a combination of other gene products and an appropriate level of Xlens1 activity.

The Rx homeobox gene is essential for vertebrate eye development.

Development of the vertebrate eye requires a series of steps including specification of the anterior neural plate, evagination of the optic vesicles from the ventral forebrain, and the cellular differentiation of the lens and retina. Homeobox-containing genes, especially the transcription regulator Pax6, play a critical role in vertebrate and invertebrate eye formation. Mutations in Pax6 function result in eye malformations known as Aniridia in humans and Small eye syndrome in mice. The Drosophila homologue of Pax6, eyeless, is also necessary for correct invertebrate eye development, and its misexpression leads to formation of ectopic eyes in Drosophila. Here we show that a conserved vertebrate homeobox gene, Rx, is essential for normal eye development, and that its misexpression has profound effects on eye morphology. Xenopus embryos injected with synthetic Rx RNA develop ectopic retinal tissue and display hyperproliferation in the neuroretina. Mouse embryos carrying a null allele of this gene do not form optic cups and so do not develop eyes. The Rx gene family plays an important role in the establishment and/or proliferation of retinal progenitor cells.

Expression pattern of an axolotl floor plate-specific fork head gene reflects early developmental differences between frogs and salamanders.

Gastrulation is one of the most important stages of animal development and, as such, tends to be remarkably conserved. Therefore it is interesting to see that the two amphibian species, Xenopus laevis (frog) and Ambystoma mexicanum (axolotl), are different in the arrangement of cell types just before and during gastrulation. In Xenopus, the cells that will form dorsal mesoderm are located deep in the dorsal marginal zone, while in the axolotl, these are on the surface of the embryo. In this study we investigated whether homologous genes known to be involved in the formation of dorsal structures show a different pattern of expression in these two species. For this purpose, we isolated a fork head gene (AxFKH 1) from the axolotl, which is likely to be the homologue of the Xenopus fork head gene, XFKH 1 (Pintallavis, XFD-1). We find that AxFKH 1 and XFH 1 have a similar pattern of expression, but there are some important differences. In early gastrulae, transcripts are detected in the organizer region of both species. In late gastrulae, the transcripts in Xenopus are located in both the superficial and deep layers, but they are only found in the superficial layer of axolotl embryos. During neurulation, XFKH 1 is expressed in notochord and neural floor plate, whereas AxFKH 1 is expressed in the neural floor plate only. We propose that the differences in expression pattern of these two genes are due to a difference in formation of dorsal structures between these two species. Furthermore, the expression pattern of these two genes early in gastrulation is consistent with the idea that at least some of the neural floor plate cells are already determined at this time.

The homeobox gene PV.1 mediates specification of the prospective neural ectoderm in Xenopus embryos.

Bone morphogenetic protein 4 (BMP4), a member of the TGF beta superfamily, has been implicated in the dorsoventral specification of both mesoderm and ectoderm. High levels of BMP4 signaling appear to specify ventral lineages, while lower levels are causally associated with the development of dorsal lineages. We have previously identified a homeobox-containing transcription factor (PV. 1) which is a likely mediator of the ventralizing effects of BMP4 in the mesoderm. Here we provide evidence that PV.1 also functions downstream of BMP4 in the patterning of ectoderm, specifying epidermal and suppressing neural gene expression. PV.1 is expressed in the prospective neuroectoderm at the time of ectodermal fate determination. BMP4 and xSmad1 (a downstream effector of BMP4) induce PV.1 in uncommitted ectoderm and the dominant negative form of the BMP4 receptor (DN-BR) blocks PV.1 expression. In animal pole explants PV.1 counteracts the neuralizing effects of chordin and the DN-BR and restores them to their original epidermal fate. To address the physiological significance of these observations we employed an animal cap transplantation system and demonstrated that overexpression of PV.1 in the prospective neuroectoderm specifically blocks neurogenesis in intact embryos. Thus, PV.1 plays an important role in the ventralization of both mesoderm and ectoderm. We have previously shown that PV.1 is also preferentially expressed in the ventral endoderm, suggesting that this transcription factor may be involved in the ventralization of all three germ layers.

Budhead, a fork head/HNF-3 homologue, is expressed during axis formation and head specification in hydra.

Accumulating evidence indicates that a common set of genes and mechanisms regulates the developmental processes of a variety of triploblastic organisms despite large variation in their body plans. To what extent these same genes and mechanisms are also conserved among diploblasts, which arose earlier in metazoan evolution, is unclear. We have characterized a hydra homologue of the fork head/HNF-3 class of winged-helix proteins, termed budhead, whose expression patterns suggest a role(s) similar to that found in vertebrates. The vertebrate HNF-3 beta homologues are expressed early in embryogenesis in regions that have organizer properties, and later they have several roles, among them an important role in rostral head formation. In the adult hydra, where axial patterning processes are continuously active, budhead is expressed in the upper part of the head, which has organizer properties. It is also expressed during the formation of a new axis as part of the development of a bud, hydra's asexual form of reproduction. Expression during later stages of budding, during head regeneration and the formation of ectopic heads, indicates a role in head formation. It is likely that budhead plays a critical role in head as well as axis formation in hydra.

A novel homeobox gene PV.1 mediates induction of ventral mesoderm in Xenopus embryos.

The formation of ventral mesoderm has been traditionally viewed as a result of a lack of dorsal signaling and therefore assumed to be a default state of mesodermal development. The discovery that bone morphogenetic protein 4 (BMP4) can induce ventral mesoderm led to the suggestion that the induction of the ventral mesoderm requires a different signaling pathway than the induction of the dorsal mesoderm. However, the individual components of this pathway remained largely unknown. Here we report the identification of a novel Xenopus homeobox gene PV.1 (posterior-ventral 1) that is capable of mediating induction of ventral mesoderm. This gene is activated in blastula stage Xenopus embryos, its expression peaks during gastrulation and declines rapidly after neurulation is complete. PV.1 is expressed in the ventral marginal zone of blastulae and later in the posterior ventral area of gastrulae and neurulae. PV.1 is inducible in uncommited ectoderm by the ventralizing growth factor BMP4 and counteracts the dorsalizing effects of the dominant negative BMP4 receptor. Overexpression of PV.1 yields ventralized tadpoles and rescues embryos partially dorsalized by LiCl treatment. In animal caps, PV.1 ventralizes induction by activin and inhibits expression of dorsal specific genes. All of these effects mimic those previously reported for BMP4. These observations suggest that PV.1 is a critical component in the formation of ventral mesoderm and possibly mediates the effects of BMP4.

Initiation of anterior head-specific gene expression in uncommitted ectoderm of Xenopus laevis by ammonium chloride.

The role of homeobox-containing genes in the regional specification of the vertebrate embryo has been an area of intense research over the last decade. Whereas it appears that the homeobox genes of the Hox gene family play an important role in the specification of the trunk, the genes and processes involved in the specification of the head are less well understood. We have isolated a new head-specific homeobox gene, XANF-2, that appears to be involved in the regional specification of the anterior head of Xenopus embryos. This gene is initially expressed in the anterior dorsal region of early embryos and later exclusively in the primordium of the anterior pituitary gland. XANF-2 represents the earliest marker for the anterior pituitary lineage. Ammonium chloride is able to induce the expression of XANF-2 in uncommitted ectoderm. These and other data indicate that ammonium chloride is capable of inducing a large portion of the anterior dorsal region of the embryo which includes, but is not limited to, the anterior pituitary gland and cement gland anlagen. This implies that changes in intracellular ionic conditions play an important role in the formation of the anterior head region. In addition to NH4Cl, injection of follistatin RNA can induce transcription of XANF-2, suggesting that these two unrelated compounds can activate a chain of events leading to the formation of the amphibian head. Furthermore, we demonstrate that planar induction in Keller sandwiches can induce XANF-2 expression as well as the expression of the cement gland-specific gene, XCG 13, indicating that planar signaling can account for induction of even the most anterior regions of the embryo.

Differential expression of fork head genes during early Xenopus and zebrafish development.

Intense efforts have been devoted to the identification of genes that are causatively involved in pattern-forming events of invertebrates and vertebrates. Several gene families involved in this process have been identified. Here we focus on the Xenopus fork head domain gene family. One of its members, XFKH1/Pintallavis/XFD1, has been shown previously to be involved in axial formation, and the expression patterns of the other family members discussed below suggest that they too play a major role in the initial steps of patterning and axial organization. In this report, we describe four Xenopus fork head genes (XFKH3, 4, 5, and 6) and analyze the distribution of their transcripts during early development. XFKH3 is expressed in developing somites but not notochord, XFKH4 in forebrain, anterior retina, and neural crest cells, and XFKH5 in a subset of epidermal cells and the neural floor plate. Finally, transcripts of XFKH6 are seen in neural crest-derived cranial ganglia. In addition, we show that at least some of the zebrafish fork head genes might serve a comparable function. Zebrafish zf-FKH1 has a similar expression pattern as Xenopus XFKH1/Pintallavis/XFD1. It is transcribed in the notochord and neural floor plate. The polster or "pillow" also shows very high levels of zf-FKH1 mRNA.

Expression of the LIM class homeobox gene Xlim-1 in pronephros and CNS cell lineages of Xenopus embryos is affected by retinoic acid and exogastrulation.

The LIM class homeobox gene Xlim-1 is expressed in Xenopus embryos in the lineages leading to (i) the notochord, (ii) the pronephros, and (iii) certain cells of the central nervous system (CNS). In its first expression phase, Xlim-1 mRNA arises in the Spemann organizer region, accumulates in prechordal mesoderm and notochord during gastrulation, and decays in these tissues during neurula stages except that it persists in the posterior tip of the notochord. In the second phase, expression in lateral mesoderm begins at late gastrula, and converges to the pronephros at tailbud stages. Expression in a central location of the neural plate also initiates at late gastrula, expands anteriorly and posteriorly, and becomes established in the lateral regions of the spinal cord and hindbrain at tailbud stages. Thus Xlim-1 expression precedes morphogenesis, suggesting that it may be involved in cell specification in these lineages. Enhancement of Xlim-1 expression by retinoic acid (RA) was first detectable in the dorsal mesoderm at initial gastrula. During gastrulation and early neurulation, RA strongly enhanced Xlim-1 expression in all three lineages and also expanded its expressing domains; this overexpression correlated well with RA phenotypes such as enlarged pronephros and hindbrain-like structure. Exogastrulation reduced Xlim-1 expression in the lateral mesoderm and ectoderm but not in the notochord, suggesting that the second phase of Xlim-1 expression requires mesoderm/ectoderm interactions. RA treatment of exogastrulae did not revert this reduction.

Differential expression of a Distal-less homeobox gene Xdll-2 in ectodermal cell lineages.

Neural induction in Xenopus requires the activation of new sets of genes that are necessary for cellular and regional specification of the neural tube. It has been reported earlier that members of the Distal-less homeobox gene family are specifically activated in distinct regions of the central nervous system (CNS) of Xenopus embryos (Dirksen et al., 1993; Papalopulu and Kintner, 1993). In this paper we describe in detail a Xenopus homeobox containing gene Xdll-2, which belongs to the Distal-less gene family. In contrast to other previously described Xenopus family members, Xdll-2 is expressed in the embryonic ectoderm and is specifically repressed in the CNS. This repression can be mimicked in isolated animal caps by treatment with activin. Expression of Xdll-2 persists in the epidermis and some neural crest cells. Because of its spatial and temporal expression pattern this gene is a good candidate to have a regulatory function in the initial formation of the epidermis. Its high level of expression in adult skin indicates that its function is continuously required in this tissue.

The homeodomain gene Xenopus Distal-less-like-2 (Xdll-2) is regulated by a conserved mechanism in amphibian and mammalian epidermis.

Xenopus Distal-less-like-2 (Xdll-2) is a gene encoding a homeodomain protein expressed predominantly in the epidermis of frog embryos. We report here that this epidermal expression is specified by a regulatory 5' flanking DNA region located within 933 bp of the start of transcription. This regulatory DNA also confers upon a globin reporter gene calcium-inducible expression in cultured murine keratinocytes and induction-dependent repression in frog ectodermal cells treated in vitro with activin A. These results reveal a new example of phylogenetically conserved, tissue-specific transcriptional regulation of a homeodomain gene.