Molecular cloning of a novel Xenopus spalt gene (Xsal-3)

Induction of nephron progenitors and glomeruli from human pluripotent stem cells

Studies of kidney regeneration using stem cells have progressed rapidly in recent years. Our group has developed a protocol to induce nephron progenitors from both mouse and human pluripotent stem cells which is based on a revised model of early stage kidney specification. The induced progenitors readily reconstitute three-dimensional nephron structures, including glomeruli and renal tubules, in vitro. We can further generate human induced pluripotent stem cells (iPSCs), in which nephrin-expressing glomerular podocytes are tagged with green fluorescent protein (GFP). The sorted GFP-positive cells retain the podocyte-specific molecular and structural features. Upon transplantation, mouse endothelial cells of the host animals are integrated into the human iPSC-derived glomeruli, and the podocytes show further maturation. Other laboratories have reported different protocols to induce nephron structures from human iPSCs in vitro. These findings will accelerate our understanding of kidney development and diseases in humans.

Spalt-like 4 promotes posterior neural fates via repression of pou5f3 family members in Xenopus

Amphibian neural development occurs as a two-step process: (1) induction specifies a neural fate in undifferentiated ectoderm; and (2) transformation induces posterior spinal cord and hindbrain. Signaling through the Fgf, retinoic acid (RA) and Wnt/β-catenin pathways is necessary and sufficient to induce posterior fates in the neural plate, yet a mechanistic understanding of the process is lacking. Here, we screened for factors enriched in posterior neural tissue and identify spalt-like 4 (sall4), which is induced by Fgf. Knockdown of Sall4 results in loss of spinal cord marker expression and increased expression of pou5f3.2 (oct25), pou5f3.3 (oct60) and pou5f3.1 (oct91) (collectively, pou5f3 genes), the closest Xenopus homologs of mammalian stem cell factor Pou5f1 (Oct4). Overexpression of the pou5f3 genes results in the loss of spinal cord identity and knockdown of pou5f3 function restores spinal cord marker expression in Sall4 morphants. Finally, knockdown of Sall4 blocks the posteriorizing effects of Fgf and RA signaling in the neurectoderm. These results suggest that Sall4, activated by posteriorizing signals, represses the pou5f3 genes to provide a permissive environment allowing for additional Wnt/Fgf/RA signals to posteriorize the neural plate.

Kidney regeneration: common themes from the embryo to the adult

The vertebrate kidney has an inherent ability to regenerate following acute damage. Successful regeneration of the injured kidney requires the rapid replacement of damaged tubular epithelial cells and reconstitution of normal tubular function. Identifying the cells that participate in the regeneration process as well as the molecular mechanisms involved may reveal therapeutic targets for the treatment of kidney disease. Renal regeneration is associated with the expression of genetic pathways that are necessary for kidney organogenesis, suggesting that the regenerating tubular epithelium may be "reprogrammed" to a less-differentiated, progenitor state. This review will highlight data from various vertebrate models supporting the hypothesis that nephrogenic genes are reactivated as part of the process of kidney regeneration following acute kidney injury (AKI). Emphasis will be placed on the reactivation of developmental pathways and how our understanding of the resulting regeneration process may be enhanced by lessons learned in the embryonic kidney.

Targeting transcription factor SALL4 in acute myeloid leukemia by interrupting its interaction with an epigenetic complex

An exciting recent approach to targeting transcription factors in cancer is to block formation of oncogenic complexes. We investigated whether interfering with the interaction of the transcription factor SALL4, which is critical for leukemic cell survival, and its epigenetic partner complex represents a novel therapeutic approach. The mechanism of SALL4 in promoting leukemogenesis is at least in part mediated by its repression of the tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN) through its interaction with a histone deacetylase (HDAC) complex. In this study, we demonstrate that a peptide can compete with SALL4 in interacting with the HDAC complex and reverse its effect on PTEN repression. Treating SALL4-expressing malignant cells with this peptide leads to cell death that can be rescued by a PTEN inhibitor. The antileukemic effect of this peptide can be confirmed on primary human leukemia cells in culture and in vivo, and is identical to that of down-regulation of SALL4 in these cells using an RNAi approach. In summary, our results demonstrate a novel peptide that can block the specific interaction between SALL4 and its epigenetic HDAC complex in regulating its target gene, PTEN. Furthermore, targeting SALL4 with this approach could be an innovative approach in treating leukemia.

A novel spalt gene expressed in branchial arches affects the ability of cranial neural crest cells to populate sensory ganglia

Cranial neural crest cells differentiate into diverse derivatives including neurons and glia of the cranial ganglia, and cartilage and bone of the facial skeleton. Here, we explore the function of a novel transcription factor of the spalt family that might be involved in early cell-lineage decisions of the avian neural crest. The chicken spalt4 gene (csal4) is expressed in the neural tube, migrating neural crest, branchial arches and, transiently, in the cranial ectoderm. Later, it is expressed in the mesectodermal, but not neuronal or glial, derivatives of midbrain and hindbrain neural crest. After over-expression by electroporation into the cranial neural tube and neural crest, we observed a marked redistribution of electroporated neural crest cells in the vicinity of the trigeminal ganglion. In control-electroporated embryos, numerous, labeled neural crest cells (approximately 80% of the population) entered the ganglion, many of which differentiated into neurons. By contrast, few (approximately 30% of the population) spalt-electroporated neural crest cells entered the trigeminal ganglion. Instead, they localized in the mesenchyme around the ganglionic periphery or continued further ventrally to the branchial arches. Interestingly, little or no expression of differentiation markers for neurons or other cell types was observed in spalt-electroporated neural crest cells.

The role of stem cell factor SALL4 in leukemogenesis

SALL4, a member of the SALL gene family, is one of the most important transcriptional regulators of stem cells. It is of particular interest to stem cell biologists because it is linked to the self-renewal of both embryonic stem cells (ESCs) and hematopoietic stem cells (HSCs), and it is involved in human leukemia. In ESCs, the Sall4/Oct4/Nanog core transcriptional network governs the self-renewal and pluripotent properties of human and murine ESCs. In normal HSCs and leukemic stem cells (LSCs), SALL4 is linked to three known pathways that are involved in self-renewal: Wnt/β-catenin, Bmi-1, and Pten. Despite the important shared role of SALL4 in self-renewal of HSCs and LSCs, our recent studies obtained through correlating global downstream target genes and unique functional studies in normal versus leukemic cells have demonstrated that SALL4 has differential effects on both pro- and anti-apoptotic pathways in normal and leukemic cells. Targeting SALL4, particularly when combined with the use of ABT-737, a BCL2 antagonist, could lead to leukemic cell-specific apoptosis. This review summarizes our current knowledge on the SALL gene family development, particularly on the role of SALL4 in stem cells, as well as tumorigenesis, especially leukemogenesis.

The role of HSAL (SALL) genes in proliferation and differentiation in normal hematopoiesis and leukemogenesis

The National Blood Foundation (NBF) support was critical in the author's research career development. The NBF support came in the form of a start-up seed grant that she got from the American Association of Blood Banks, an organization that advances the practice and standards of transfusion medicine and cellular therapies and an organization in which she is a proud member. The NBF grant enabled her to keep up with her transfusion medicine practice while pursuing her passion to be a physician scientist. During its funding period, she was able to obtain critical preliminary bench data and to secure several National Institutes of Health grants with over a million dollars direct cost. In addition, the knowledge gained from the NBF-supported projects is currently being translated into medical practice in her lab by testing on cord blood expansion. She is looking forward to spending the upcoming years of her professional career bridging bedside observations on transfusion medicine with bench experiences and then utilizing that bench-derived knowledge in the practice of transfusion medicine.

Nephron progenitors in the metanephric mesenchyme

The kidney is formed by a reciprocally inductive interaction between two precursor tissues, the metanephric mesenchyme and the ureteric bud. This interaction can be divided into three processes: attraction of the ureteric bud toward the mesenchyme, maintenance of the mesenchyme in an undifferentiated state versus transition to an epithelial state, and further differentiation into multiple epithelial lineages, such as glomeruli and renal tubules. In this review we describe our recent findings related to each process. A mesenchymal nuclear zinc finger protein, Sall1, controls ureteric bud attraction by regulating a novel kinesin, Kif26b. The Sall1 gene is highly expressed in multipotent nephron progenitors in the mesenchyme, and these cells can partially reconstitute a three-dimensional structure in organ cultures following Wnt4 stimulation. While Notch2 is required for further differentiation of proximal nephron structures, ectopic Notch2 activation in the embryonic kidney depletes nephron progenitors, suggesting that Notch2 stabilizes--rather than dictates--nephron fate by shutting down the maintenance of undifferentiated progenitor cells.

In vitro organogenesis using multipotent cells

Abstract The establishment of efficient methods for promoting stem cell differentiation into target cells is important not only in regenerative medicine, but also in drug discovery. In addition to embryonic stem (ES) cells and various somatic stem cells, such as mesenchymal stem cells derived from bone marrow, adipose tissue, and umbilical cord blood, a novel dedifferentiation technology that allows the generation of induced pluripotent stem (iPS) cells has been recently developed. Although an increasing number of stem cell populations are being described, there remains a lack of protocols for driving the differentiation of these cells. Regeneration of organs from stem cells in vitro requires precise blueprints for each differentiation step. To date, studies using various model organisms, such as zebrafish, Xenopus laevis, and gene-targeted mice, have uncovered several factors that are critical for the development of organs. We have been using X. laevis, the African clawed frog, which has developmental patterns similar to those seen in humans. Moreover, Xenopus embryos are excellent research tools for the development of differentiation protocols, since they are available in high numbers and are sufficiently large and robust for culturing after simple microsurgery. In addition, Xenopus eggs are fertilized externally, and all stages of the embryo are easily accessible, making it relatively easy to study the functions of individual gene products during organogenesis using microinjection into embryonic cells. In the present review, we provide examples of methods for in vitro organ formation that use undifferentiated Xenopus cells. We also describe the application of amphibian differentiation protocols to mammalian stem cells, so as to facilitate the development of efficient methodologies for in vitro differentiation.

Sall1, sall2, and sall4 are required for neural tube closure in mice

Four homologs to the Drosophila homeotic gene spalt (sal) exist in both humans and mice (SALL1 to SALL4/Sall1 to Sall4, respectively). Mutations in both SALL1 and SALL4 result in the autosomal-dominant developmental disorders Townes-Brocks and Okihiro syndrome, respectively. In contrast, no human diseases have been associated with SALL2 to date, and Sall2-deficient mice have shown no apparent abnormal phenotype. We generated mice deficient in Sall2 and, contrary to previous reports, 11% of our Sall2-deficient mice showed background-specific neural tube defects, suggesting that Sall2 has a role in neurogenesis. To investigate whether Sall4 may compensate for the absence of Sall2, we generated compound Sall2 knockout/Sall4 genetrap mutant mice. In these mutants, the incidence of neural tube defects was significantly increased. Furthermore, we found a similar phenotype in compound Sall1/4 mutant mice, and in vitro studies showed that SALL1, SALL2, and SALL4 all co-localized in the nucleus. We therefore suggest a fundamental and redundant function of the Sall proteins in murine neurulation, with the heterozygous loss of a particular SALL protein also possibly compensated in humans during development.

In Vitro organogenesis using amphibian pluripotent cells

Mesoderm induction as a result of the interaction between endoderm and ectoderm is one of the most crucial events in vertebrate development. We identified activin as a strong mesoderm-inducing factor in an animal cap assay, an in vitro assay system using amphibian pluripotential cell mass. Activin induces mesodermal tisswes including most dorsal mesodermal tissue, notochord (which has important roles in neural induction, somite segmentation, and endodermal organogenesis), and its effects are concentration-dependent. Animal cap cells treated with high concentrations of activin differentiate into anterior endoderm, which can act as an organizer, or center of body patterning. We have established an in vitro induction system for 22 different organs and tissues using animal cap cells, and have isolated many organ-specific genes. With these useful methods, and analysis of newly isolated tissue- and organ-specific genes, the molecular biological "road map" for organogenesis is being established.

The vertebrate spalt genes in development and disease

The spalt proteins are encoded by a family of evolutionarily conserved genes found in species as diverse as Drosophila, C. elegans and vertebrates. In humans, mutations in some of these genes are associated with several congenital disorders which underscores the importance of spalt gene function in embryonic development. Recent studies have begun to cast light on the functions of this family of proteins with increasing understanding of the developmental processes regulated and the molecular mechanisms used. Here we review what is currently known about the role of spalt genes in vertebrate development and human disease.

Germ layers to organs: Using Xenopus to study “later” development

The amphibian embryo is a highly successful model system with great promise for organogenesis research. Since the late 1800s, amphibians have been employed to understand vertebrate development and since the 1950s, the African clawed frog Xenopus laevis has been the amphibian of choice. In the past two decades, Xenopus has led the way forward in, among other things, identifying transcription factors, gene regulatory networks and inter- and intracellular signaling pathways that control early development (from fertilization through gastrulation and neurulation). Perhaps the best measure of how successful Xenopus has been as a model for early mammalian development is the observation that much of the knowledge gleaned from Xenopus studies has subsequently directly translated to discoveries of similar mechanisms operating in mouse development. Despite this great success in early development, research on organogenesis in Xenopus has lagged behind the mouse. However, recent technical advances now make Xenopus amenable for studies on later development, including organogenesis. Here, we discuss why Xenopus is well suited for such research and, we believe, permits addressing questions that have been difficult to approach using other model systems. We also highlight how Xenopus researchers have already begun studying a number of major organs, pancreas, liver, kidney and heart, and suggest how Xenopus might contribute more to these areas in the near future.

Defining the heterochromatin localization and repression domains of SALL1

SALL1 has been identified as one of four human homologues of the Drosophila region-specific homeotic gene spalt (sal), encoding zinc finger proteins of characteristic structure. Mutations of SALL1 on chromosome 16q12.1 cause Townes-Brocks syndrome (TBS, OMIM 107480). We have shown previously that SALL1 acts as a strong transcriptional repressor in mammalian cells when fused to a heterologous DNA-binding domain. Here, we report that SALL1 contains two repression domains, one located at the extreme N-terminus of the protein and the other in the central region. SALL1 fragments with the central repression domain exhibited a punctate nuclear distribution pattern at pericentromeric heterochromatin foci in murine NIH-3T3 cells, suggesting an association between repression and heterochromatin localization. The implications of these findings for the pathogenesis of Townes-Brocks syndrome are discussed.

Essential Roles of Sall Family Genes in Kidney Development

We isolated a mouse Sall1, a mammalian homologue of the Drosophila region-specific homeotic gene spalt (sal), and found that mice deficient in Sall1 die in the perinatal period from kidney agenesis. Sall1 is expressed in the metanephric mesenchyme surrounding the ureteric bud, and the homozygous deletion of Sall1 results in an incomplete ureteric bud outgrowth. Therefore Sall1 is essential for ureteric bud invasion, the initial key step for metanephros development. We also set up an in vitro culture system, using NIH3T3 cells stably expressing Wnt4 as a feeder layer, to identify kidney progenitors in the metanephric mesenchyme. In this culture condition, a single renal progenitor in the mesenchyme forms colonies consisting of several types of epithelial cells that exist in glomeruli and renal tubules. We found that only cells strongly expressing Sall1 (Sall1-GFP(high) cells) form colonies and that they reconstitute a three-dimensional kidney structure in an organ culture setting. Thus our colony-forming assay, which identifies multipotent progenitors in the embryonic mouse kidney, can be used for examining mechanisms of renal progenitor differentiation.

Nodal-related geneXnr5 is amplified in theXenopus genome

In Xenopus, six nodal-related genes (Xnrs) have been identified to date. We found numerous tandem duplications of Xnr5 in the Xenopus laevis and Xenopus tropicalis genomes that involve highly conserved copies of coding and regulatory regions. The duplicated versions of Xnr5 were expressed in both the superficial and deep layer of dorsal endoderm and in the deep layer of ventral endoderm, where the initial inducers of mesendoderm formation would be expected to be localized. Overexpression of secreted inhibitors of Xnrs led to a substantially enhanced transcription of the duplicated Xnr5 genes and Xnr6 in embryos. Therefore, Xnr5 and Xnr6 have a novel feedback loop to inhibit transcription of Xnr5 and Xnr6. These results suggest that the initialization of a strong Xnr5 and Xnr6 signal is enabled by the rapid transcription from multiple genes. The novel feedback loop may negatively regulate transcription of Xnr5s and Xnr6 to limit overproduction of these potent inducers, with the Xnr5/Xnr6 signal then activating positive (Xnrs) and negative (Xlefty) loops, which regulate the range of mesodermal tissues produced.

Xnr2 and Xnr5 unprocessed proteins inhibit Wnt signaling upstream of dishevelled

Nodal and Nodal-related proteins activate the Activin-like signal pathway and play a key role in the formation of mesoderm and endoderm in vertebrate development. Recent studies have shown additional activities of Nodal-related proteins apart from the canonical Activin-like signal pathway. Here we report a novel function of Nodal-related proteins using cleavage mutants of Xenopus nodal-related genes (cmXnr2 and cmXnr5), which are known to be dominant-negative inhibitors of nodal family signaling. cmXnr2 and cmXnr5 inhibited both BMP signaling and Wnt signaling without activating the Activin-like signal in animal cap assays. Pro region construct of Xnr2 and Xnr5 did not inhibit Xwnt8, and pro/mature region chimera mutant cmActivin-Xnr2 and cmActivin-Xnr5 also did not inhibit Xwnt8 activity. These results indicate that the pro domains of Xnr2 and Xnr5 are necessary, but not sufficient, for Wnt inhibition, by Xnr family proteins. In addition, Western blot analysis and immunohistochemistry analysis revealed that the unprocessed Xnr5 protein is stably produced and secreted as effectively as mature Xnr5 protein, and that the unprocessed Xnr5 protein diffused in the extracellular space. These results suggest that unprocessed Xnr2 and Xnr5 proteins may be involved in inhibiting both BMP and Wnt signaling and are able to be secreted to act on somewhat distant target cells, if these are highly produced.

Essential roles of Sall1 in kidney development

SALL1 is a mammalian homologue of the Drosophila region-specific homeotic gene spalt (sal) and heterozygous mutations in SALL1 in humans lead to Townes-Brocks syndrome. We isolated a mouse homologue of SALL1 (Sall1) and found that mice deficient in Sall1 die in the perinatal period with kidney agenesis. Sall1 is expressed in the metanephric mesenchyme surrounding ureteric bud and homozygous deletion of Sall1 results in an incomplete ureteric bud outgrowth. Therefore, Sall1 is essential for ureteric bud invasion, the initial key step for metanephros development. We also generated mice in which a green fluorescent protein (GFP) gene was inserted into the Sall1 locus and we isolated the GFP-positive population from embryonic kidneys of these mice by fluorescence-activated cell sorting (FACS). We then compared gene expression profiles in the GFP-positive and -negative population using microarray analysis, followed by in situ hybridization. We detected many genes known to be important for metanephros development, and genes expressed abundantly in the metanephric mesenchyme. We also found groups of genes which are not known to be expressed in the metanephric mesenchyme. Thus a combination of microarray technology and Sall1-GFP mice is useful for systematic identification of genes expressed in the developing kidney.

Expression of Xenopus XlSALL4 during limb development and regeneration

The multi-C2H2 zinc-finger domain containing transcriptional regulators of the spalt (SAL) family plays important developmental regulatory roles. In a competitive subtractive hybridization screen of genes expressed in Xenopus laevis hindlimb regeneration blastemas, we identified a SAL family member that, by phylogenetic analysis, falls in the same clade as human SALL4 and have designated it as XlSALL4. Mutations of human SALL4 have been linked to Okihiro syndrome, which includes preaxial (anterior) limb defects. The expression pattern of XlSALL4 transcripts during normal forelimb and hindlimb development and during hindlimb regeneration at the regeneration-competent and regeneration-incompetent stages is temporally and regionally dynamic. We show for the first time that a SAL family member (XlSALL4) is expressed at the right place and time to play a role regulating both digit identity along the anterior/posterior axis and epimorphic limb regeneration.

SALL4mutations in Okihiro syndrome (Duane-radial ray syndrome), acro-renal-ocular syndrome, and related disorders

Okihiro/Duane-radial ray syndrome (DRRS) is an autosomal dominant condition characterized by radial ray defects and Duane anomaly (a form of strabismus). Other abnormalities reported in this condition are anal, renal, cardiac, ear, and foot malformations, and hearing loss. The disease is the result of a mutation in the SALL4 gene, a human gene related to the developmental regulator spalt (sal) of Drosophila melanogaster. SALL4 mutations may also cause acro-renal-ocular syndrome (AROS), which differs from DRRS by the presence of structural eye anomalies, and phenotypes similar to thalidomide embryopathy and Holt-Oram syndrome (HOS). The SALL4 gene product is a zinc finger protein that is thought to act as a transcription factor. It contains three highly conserved C2H2 double zinc finger domains, which are evenly distributed. A single C2H2 motif is attached to the second domain, and at the amino terminus SALL4 contains a C2HC motif. Seventeen of the 22 SALL4 mutations known to date (five of which are presented here for the first time) are located in exon 2, and five are located in exon 3. These are nonsense mutations, short duplications, and short deletions. All of the mutations lead to preterminal stop codons and are thought to cause the phenotype via haploinsufficiency. This assumption is supported by the detection of six larger deletions involving the whole gene or single exons. This article summarizes the current knowledge about SALL4 defects and associated syndromes, and describes the clinical distinctions with similar phenotypes caused by other gene defects.

Loss of the Sall3 gene leads to palate deficiency, abnormalities in cranial nerves, and perinatal lethality

Members of the Spalt gene family encode putative transcription factors characterized by seven to nine C2H2 zinc finger motifs. Four genes have been identified in mice--Spalt1 to Spalt4 (Sall1 to Sall4). Spalt homologues are widely expressed in neural and mesodermal tissues during early embryogenesis. Sall3 is normally expressed in mice from embryonic day 7 (E7) in the neural ectoderm and primitive streak and subsequently in the brain, peripheral nerves, spinal cord, limb buds, palate, heart, and otic vesicles. We have generated a targeted disruption of Sall3 in mice. Homozygous mutant animals die on the first postnatal day and fail to feed. Examination of the oral structures of these animals revealed that abnormalities were present in the palate and epiglottis from E16.5. In E10.5 embryos, deficiencies in cranial nerves that normally innervate oral structures, particularly the glossopharyngeal nerve (IX), were observed. These studies indicate that Sall3 is required for the development of nerves that are derived from the hindbrain and for the formation of adjacent branchial arch derivatives.

Tissue generation from amphibian animal caps

Formation of three germ layers is the most important event in early vertebrate development. Animal cap assays can be used to reproduce the in vivo induction of amphibian tissues in order to investigate the differentiation processes that occur in normal embryonic development. Activin treatment strongly and dose-dependently induces various types of mesodermal and endodermal tissue in cultured animal caps. Beating heart, pronephros, pancreas and cartilage can be induced by microsurgical manipulation and simultaneous treatment with activin and other factors. These in vitro induction systems will be helpful for elucidating the mechanisms of tissue induction and organ formation in vertebrate development.

Current state of and outlook for organogenesis from undifferentiated cells

PURPOSE

The aims of regenerative medicine and artificial internal organ development are, respectively, to regenerate and provide for transplant tissue or organs.

METHODS

We summarize recent research on tissue differentiation and organogenesis using stem cells and report our laboratory's research using amphibian undifferentiated cells.

RESULTS

We have successfully induced differentiation in cells from Xenopus laevis and generated structures in vitro that function in a similar way to organs when transplanted to Xenopus and newt embryos. We are attempting to establish a system to induce sensory organs, including eyes.

CONCLUSION

Our experimental systems in amphibians are useful for organogenetic research, and it is hoped that our techniques can, in the future, be applied to mammals.

Kidney development conserved over species: essential roles of Sall1

The kidney develops in three stages: pronephros, mesonephros, and metanephros. Molecular mechanisms underlying these three steps are similar, and we can thus examine genetic cascades occurring during development. The induction system for pronephros in vitro has been established in Xenopus. Using this system, we isolated Sall1 that is essential for the initial step in metanephros formation. The potential mechanisms involved and future directions regarding kidney development are discussed.

Drosophila spalt/spalt-related mutants exhibit Townes-Brocks' syndrome phenotypes

Mutations in SALL1, the human homolog of the Drosophila spalt gene, result in Townes-Brocks' syndrome, which is characterized by hand/foot, anogenital, renal, and ear anomalies, including sensorineural deafness. spalt genes encode zinc finger transcription factors that are found in animals as diverse as worms, insects, and vertebrates. Here, we examine the effect of losing both of the spalt genes, spalt and spalt-related, in the fruit fly Drosophila melanogaster, and report defects similar to those in humans with Townes-Brocks' syndrome. Loss of both spalt and spalt-related function in flies yields morphological defects in the testes, genitalia, and the antenna. Furthermore, spalt/spalt-related mutant antennae show severe reductions in Johnston's organ, the major auditory organ in Drosophila. Electrophysiological analyses confirm that spalt/spalt-related mutant flies are deaf. These commonalities suggest that there is functional conservation for spalt genes between vertebrates and insects.

Clinical significance of transmembrane 4 superfamily in colon cancer

Cell motility is an important cellular function closely related to the processes of tumour progression and metastasis. Several members of transmembrane 4 superfamily (TM4SF) have been reported to be associated with cell motility and metastatic potential of solid tumour. The aim of this study is to clarify the clinical significance of the member of TM4SF (MRP-1/CD9, KAI1/CD82 and CD151) in human colon cancer. We studied 146 colon cancer patients who underwent curative surgery and studied the expression of MRP-1/CD9, KAI1/CD82 and CD151 using reverse transcriptase - polymerase chain reaction and immunohistochemistry. We found that 64 patients (43.8%) had MRP-1/CD9-positive tumours and that the overall survival rate of patients with MRP-1/CD9-positive tumours was much higher than that of patients with MRP-1/CD9-negative tumours (89.8 vs 50.8%, P<0.001). In contrast, 63 patients (43.2%) had KAI1/CD82-positive tumours and the overall survival rate of patients with KAI1/CD82-positive tumours was also higher than that of patients with KAI1/CD82-negative tumours (84.8 vs 54.9%, P=0.002). On the other hand, positive CD151 expression had a bad effect on the overall survival rate of patients with colon cancer (61.2 vs 74.9%, P=0.022). In a multivariate analysis, MRP-1/CD9 status was a good indicator of the overall survival (P=0.007). We have shown that the reduction of MRP-1/CD9 and KAI1/CD82 expression, and the increasing CD151 expression are indicators for a poor prognosis in patients with colon cancer. This is a first report describing about the relation between CD151 and colon cancer.

Mutations at the SALL4 locus on chromosome 20 result in a range of clinically overlapping phenotypes, including Okihiro syndrome, Holt-Oram syndrome, acro-renal-ocular syndrome, and patients previously reported to represent thalidomide embryopathy

We have recently shown that Okihiro syndrome results from mutation in the putative zinc finger transcription factor gene SALL4 on chromosome 20q13.13-13.2. There is considerable overlap of clinical features of Okihiro syndrome with other conditions, most notably Holt-Oram syndrome, a condition in part resulting from mutation of the TBX5 locus, as well as acro-renal-ocular syndrome. We analysed further families/patients with the clinical diagnosis of Holt-Oram syndrome and acro-renal-ocular syndrome for SALL4 mutations. We identified a novel SALL4 mutation in one family where the father was originally thought to have thalidomide embryopathy and had a daughter with a similar phenotype. We also found two novel mutations in two German families originally diagnosed as Holt-Oram syndrome and a further mutation in one out of two families carrying the diagnosis acro-renal-ocular syndrome. Our results show that some cases of "thalidomide embryopathy" might be the result of SALL4 mutations, resulting in an increased risk for similarly affected offspring. Furthermore we confirm the overlap of acro-renal-ocular syndrome with Okihiro syndrome at the molecular level and expand the phenotype of SALL4 mutations.

The conserved glutamine-rich region of chick csal1 and csal3 mediates protein interactions with other spalt family members. Implications for Townes-Brocks syndrome

Members of the spalt family of zinc finger-containing proteins have been implicated in development and disease. However, very little is known about the molecular function of spalt proteins. We have used biochemical approaches to characterize functional domains of two chick spalt homologs, csal1 and csal3. We show that csal1 and csal3 proteins repress transcription and that they can interact with each other. Furthermore, we found that truncated chick spalt proteins, similar to the truncated spalt protein expressed in the human congenital disorder Townes-Brocks syndrome, affect the nuclear localization of full-length spalt. Our findings have implications for the understanding of Townes-Brocks syndrome and the role of spalt genes in normal development. We propose that truncated spalt can exert a dominant negative effect and is able to interfere with the correct function of full-length protein, by causing its displacement from the nucleus. This could affect the transcriptional repressor activity of spalt and DNA binding. Spalt protein truncations could also affect the function of other spalt family members in various tissues.

Identification of genes expressed during Xenopus laevis limb regeneration by using subtractive hybridization

Suppression polymerase chain reaction-based subtractive hybridization was used to identify genes that are expressed during Xenopus laevis hindlimb regeneration. Subtractions were done by using RNAs extracted from the regeneration-competent stage (stage 53) and regeneration-incompetent stage (stage 59) of limb development. Forward and reverse subtractions were done between stage 53 7-day blastema and stage 53 contralateral limb (competent stage), stage 59 7-day pseudoblastema and stage 59 contralateral limb (incompetent stage), and stage 53 7-day blastema and stage 59 7-day pseudoblastema. Several thousand clones were analyzed from the various subtracted libraries, either by random selection and sequencing (1,920) or by screening subtracted cDNA clones (6,150), arrayed on nylon membranes, with tissue-specific probes. Several hundred clones were identified from the array screens whose expression levels were at least twofold higher in experimental tissue vs. control tissue (e.g., blastema vs. limb) and selected for sequencing. In addition, primers were designed to assay several of the randomly selected clones and used to assess the level of expression of these genes during regeneration and normal limb development. Approximately half of the selected clones were differentially expressed, as expected, including several that demonstrate blastema-specific enhancement of expression. Three distinct categories of expression were identified in our screens: (1) clones that are expressed in both regeneration-competent blastemas and -incompetent pseudoblastemas, (2) clones that are expressed at highest levels in regeneration-competent blastemas, and (3) clones that are expressed at highest levels in regeneration-incompetent pseudoblastemas. Characterizing the role of each of these three categories of genes will be important in furthering our understanding of the process of tissue regeneration.

Expression of three spalt (sal) gene homologues in zebrafish embryos

Three homologues of the Drosophilaregion-specific homeotic gene spalt (sal) have been isolated in zebrafish, sall1a, sall1b and sall3. Phylogenetic analysis of these genes against known salDNA sequences showed zebrafish sall1aand sall1b to be orthologous to other vertebrate sal-1 genes and zebrafish sall3to be orthologous to other vertebrate sal-3 genes, except Xenopus sall3. Phylogenetic reconstruction suggests that zebrafish sall1a and sall1bresulted from a gene duplication event occurring prior to the divergence of the ray-finned and lobe-finned fish lineages. Analysis of the expression pattern of the zebrafish sal genes shows that sall1a and sall3 share expression domains with both orthologous and non-orthologous vertebrate sal genes. Both are expressed in various regions of the CNS, including in primary motor neurons. Outside of the CNS, sall1a expression is observed in the otic vesicle (ear), heart and in a discrete region of the pronephric ducts. These analyses indicate that orthologies between zebrafish sal genes and other vertebrate sal genes do not imply equivalence of expression pattern and, therefore, that biological functions are not entirely conserved. However we suggest that, like other vertebrate sal genes, zebrafish sal genes have a role in neural development. Also, expression of zebrafish sall1a in the otic vesicle, heart sac and the pronephric ducts of zebrafish embryos is possibly consistent with some of the abnormalities seen in Sall1-deficient mice and in Townes-Brocks Syndrome, a human disorder which is caused by mutations in the human spalt gene SALL1.

In vitro induction of the pronephric duct in Xenopus explants

The earliest form of embryonic kidney, the pronephros, consists of three components: glomus, tubule and duct. Treatment of the undifferentiated animal pole ectoderm of Xenopus laevis with activin A and retinoic acid (RA) induces formation of the pronephric tubule and glomus. In this study, the rate of induction of the pronephric duct, the third component of the pronephros, was investigated in animal caps treated with activin A and RA. Immunohistochemistry using pronephric duct-specific antibody 4A6 revealed that a high proportion of the treated explants contained 4A6-positive tubular structures. Electron microscopy showed that the tubules in the explants were similar to the pronephric ducts of normal larvae, and they also expressed Gremlin and c-ret, molecular markers for pronephric ducts. These results suggest that the treatment of Xenopus ectoderm with activin A and RA induces a high rate of differentiation of pronephric ducts, in addition to the differentiation of the pronephric tubule and glomus, and that this in vitro system can serve as a simple and effective model for analysis of the mechanism of pronephros differentiation.

Multiple nodal-related genes act coordinately in Xenopus embryogenesis

Four nodal-related genes (Xnr1-4) have been isolated in Xenopus to date, and we recently further identified two more, Xnr5 and Xnr6. In the present functional study, we constructed cleavage mutants of Xnr5 (cmXnr5) and Xnr6 (cmXnr6) which were expected to act in a dominant-negative manner. Both cmXnr5 and cmXnr6 inhibited the activities of Xnr5 and Xnr6 in co-overexpression experiments. cmXnr5 also inhibited the activity of Xnr2, Xnr4, Xnr6, derrière, and BVg1, but did not inhibit the activity of Xnr1 or activin. Misexpression of cmXnr5 led to a severe delay in initiation of gastrulation and phenotypic changes, including defects in anterior structures, which were very similar to those seen in maternal VegT-depleted embryos. Further, although the expression of Xnr1, Xnr2, and Xnr4 was not delayed in these embryos, it was markedly reduced. Injection of cmXnr5 had no notable effect on expression of Xnr3, Xnr6, derrière, or siamois. Several mesodermal and endodermal markers also showed delayed and decreased expression during gastrulation in cmXnr5-injected embryos. These results suggest that, in early Xenopus embryogenesis, nodal-related genes may heterodimerize with other TGF-beta ligands, and further that one nodal-related gene alone is insufficient for mesendoderm formation, which may require the cooperative interaction of multiple nodal-related genes.

Murine homolog of SALL1 is essential for ureteric bud invasion in kidney development

SALL1 is a mammalian homolog of the Drosophilaregion-specific homeotic gene spalt (sal); heterozygous mutations in SALL1 in humans lead to Townes-Brocks syndrome. We have isolated a mouse homolog of SALL1 (Sall1) and found that mice deficient in Sall1 die in the perinatal period and that kidney agenesis or severe dysgenesis are present. Sall1 is expressed in the metanephric mesenchyme surrounding ureteric bud; homozygous deletion ofSall1 results in an incomplete ureteric bud outgrowth, a failure of tubule formation in the mesenchyme and an apoptosis of the mesenchyme. This phenotype is likely to be primarily caused by the absence of the inductive signal from the ureter, as the Sall1-deficient mesenchyme is competent with respect to epithelial differentiation. Sall1 is therefore essential for ureteric bud invasion, the initial key step for metanephros development.

Cloning and expression of CSAL2, a new member of the spalt gene family in chick

In this study we describe cloning and expression of CSAL2, a second member of the spalt gene family in chick. All spalt proteins are characterized by the presence of multiple zinc-finger motifs, which are highly conserved. Mutations in HSAL1, a human spalt gene result in Townes-Brocks syndrome (TBS). We show here that CSAL2 is expressed in many of the tissues affected in TBS, including neural tissue, limb buds, mesonephros and cloaca.