Thyroid hormone-dependent and independent processes of red blood cell transition from larval to adult type during metamorphosis in Xenopus laevis.

Amphibian metamorphosis results in drastic whole-body remodeling. Thyroid hormone (TH) drives most of these metamorphic changes. A prominent event during this remodeling is the red blood cell (RBC) transition from larval to adult forms, which exclusively contain larval and adult hemoglobin, respectively. However, the role of TH in RBC transition remains unclear. Here we reconfirmed that RBC transition of Xenopus laevis is completed much later than morphological metamorphosis. Further, larval and adult RBCs/erythroblasts proliferated both in the erythropoietic liver and in circulation during metamorphic climax. RBC transition was also confirmed in Rana ornativentris, but in contrast to X. laevis, adult RBC-specific proliferation was observed from the early climax stages. We also revealed in either species that RBC transition occurs in the liver prior to circulating RBCs. Moreover, anemia induction using phenylhydrazine during the prometamorphosis of X. laevis caused precocious RBC transition even when TH synthesis was blocked, resulting in metamorphosis-arrested larvae in which most of RBCs were of adult type. These results indicate that a decline in larval RBCs facilitates RBC transition during metamorphosis in a TH-independent manner. Further, combined administration of phenylhydrazine and TH induced precocious appearance of adult RBCs in early prometamorphic X. laevis tadpoles, whereas individual treatment with phenylhydrazine or TH did not cause precocious RBC transition; this suggests that TH is required to initiate RBC transition by promoting the differentiation of adult erythroblasts during early prometamorphosis in X. laevis. These results show that TH-dependent and independent processes are present in RBC transition in X. laevis.

Genome evolution in the allotetraploid frog Xenopus laevis.

To explore the origins and consequences of tetraploidy in the African clawed frog, we sequenced the Xenopus laevis genome and compared it to the related diploid X. tropicalis genome. We characterize the allotetraploid origin of X. laevis by partitioning its genome into two homoeologous subgenomes, marked by distinct families of 'fossil' transposable elements. On the basis of the activity of these elements and the age of hundreds of unitary pseudogenes, we estimate that the two diploid progenitor species diverged around 34 million years ago (Ma) and combined to form an allotetraploid around 17-18 Ma. More than 56% of all genes were retained in two homoeologous copies. Protein function, gene expression, and the amount of conserved flanking sequence all correlate with retention rates. The subgenomes have evolved asymmetrically, with one chromosome set more often preserving the ancestral state and the other experiencing more gene loss, deletion, rearrangement, and reduced gene expression.

Derivation of proliferative islet1-positive cells during metamorphosis and wound response in Xenopus.

In mammalian hearts, cardiomyocytes retain a transient capacity to proliferate and regenerate following injury before birth, whereas they lose proliferative capacity immediately after birth. It has also been known that cardiac progenitor cells including islet1-positive cells do not contribute to the cardiac repair and regeneration in mammals. In contrast, hearts of zebrafish, amphibians and reptiles maintain a regenerative ability throughout life. Here, we analyzed proliferative capacity of cardiac cells during cardiac development and post-ventricular resection using Xenopus laevis, especially focusing on islet1. Immunohistochemical examination showed that islet1-positive cells were present in a wide range of the ventricle and maintained high dividing ability after metamorphosis. Interestingly, the islet1-positive cells were preserved even at 1 year after metamorphosis, some of which showed tropomyosin expression. To assess the possibility of islet1-positive cells as a cellular resource, islet1 response to cardiac resection was analyzed, using adult hearts of 3 months after metamorphosis. Transient gene activation of islet1 in apical region was detected within 1 day after amputation. Histological analyses revealed that islet1-positive cells appeared in the vicinity of resection plane at 1 day post-amputation (dpa) and increased at 3 dpa in both tropomyosin-positive and tropomyosin-negative regions. Vascular labeling analysis by biotinylated dextran amine (BDA) indicated that the islet1-positive cells in a tropomyosin-negative region were closely associated with cardiac vessels. Moreover, dividing ability at this time point was peaked. The resected region was healed with tropomyosin-positive cardiomyocytes until 3 months post-amputation. These results suggest a role of islet1-positive cells as a cellular resource for vascularization and cardiogenesis in Xenopus laevis.

Identification and characterization of POU class V family genes in Japanese red bellied newt, Cynops pyrrhogaster.

Mammalian Pou5f1 encodes the POU family class V (POU-V) transcription factor which is essential for the pluripotency of embryonic cells and germ cells. In vertebrates, various POU-V family genes have been identified and classified into the POU5F1 family or its paralogous POU5F3 family. In this study, we cloned two cDNAs named CpPou5f1 and CpPou5f3, which encode POU-V family proteins of the Japanese red bellied newt Cynops pyrrhogaster. In the predicted amino acid sequence encoded by CpPou5f1, the typical MAGH sequence at the N-terminus and deletion of arginine at the fifth position of POU-homeodomain were recognized, but not in the sequence encoded by CpPou5f3. Phylogenetic analysis using Clustal Omega software indicated that CpPou5f1 and CpPou5f3 are classified into the clade of the POU5F1 and POU5F3 families, respectively. In a real-time polymerase chain reaction (RT-PCR) analysis, the marked gene expression of CpPou5f1 was observed during oogenesis and early development up to the tail-bud stage, whereas weak gene expression of CpPou5f3 was detected only in the early stages of oogenesis and gastrula. In adult organs, CpPou5f1 was expressed only in the ovary, while gene expression of CpPou5f3 was recognized in various organs. A regeneration experiment using larval forelimb revealed that transient gene expression of CpPou5f1 occurred at the time of wound healing, followed by gene activation of CpPou5f3 during the period of blastema formation. These results suggest that CpPou5f1 and CpPou5f3 might play different roles in embryogenesis and limb regeneration.

High variability of expression profiles of homeologous genes for Wnt, Hh, Notch, and Hippo signaling pathways in Xenopus laevis.

Cell signaling pathways, such as Wnt, Hedgehog (Hh), Notch, and Hippo, are essential for embryogenesis, organogenesis, and tissue homeostasis. In this study, we analyzed 415 genes involved in these pathways in the allotetraploid frog, Xenopus laevis. Most genes are retained in two subgenomes called L and S (193 homeologous gene pairs and 29 singletons). This conservation rate of homeologs is much higher than that of all genes in the X. laevis genome (86.9% vs 60.2%). Among singletons, 24 genes are retained in the L subgenome, a rate similar to the average for all genes (82.8% vs 74.6%). In addition, as general components of signal transduction, we also analyzed 32 heparan sulfate proteoglycan (HSPG)-related genes and eight TLE/Groucho transcriptional corepressors-related genes. In these gene sets, all homeologous pairs have been retained. Transcriptome analysis using RNA-seq data from developmental stages and adult tissues demonstrated that most homeologous pairs of signaling components have variable expression patterns, in contrast to the conservative expression profiles of homeologs for transcription factors. Our results indicate that homeologous gene pairs for cell signaling regulation have tended to become subfunctionalized after allotetraploidization. Diversification of signaling pathways by subfunctionalization of homeologs may enhance environmental adaptability. These results provide insights into the evolution of signaling pathways after polyploidization.

Conservatism and variability of gene expression profiles among homeologous transcription factors in Xenopus laevis.

Xenopus laevis has an allotetraploid genome of 3.1Gb, in contrast to the diploid genome of a closely related species, Xenopus tropicalis. Here, we identified 412 genes (189 homeolog pairs, one homeologous gene cluster pair, and 28 singletons) encoding transcription factors (TFs) in the X. laevis genome by comparing them with their orthologs from X. tropicalis. Those genes include the homeobox gene family (Mix/Bix, Lhx, Nkx, Paired, POU, and Vent), Sox, Fox, Pax, Dmrt, Hes, GATA, T-box, and some clock genes. Most homeolog pairs for TFs are retained in two X. laevis subgenomes, named L and S, at higher than average rates (87.1% vs 60.2%). Among the 28 singletons, 82.1% were deleted from chromosomes of the S subgenome, a rate similar to the genome-wide average (82.1% vs 74.6%). Interestingly, nkx2-1, nkx2-8, and pax9, which reside consecutively in a postulated functional gene cluster, were deleted from the S chromosome, suggesting cluster-level gene regulation. Transcriptome correlation analysis demonstrated that TF homeolog pairs tend to have more conservative developmental expression profiles than most other types of genes. In some cases, however, either of the homeologs may show strongly different spatio-temporal expression patterns, suggesting neofunctionalization, subfunctionalization, or nonfunctionalization after allotetraploidization. Analyses of otx1 suggests that homeologs with much lower expression levels have undergone greater amino acid sequence diversification. Our comprehensive study implies that TF homeologs are highly conservative after allotetraploidization, possibly because the DNA sequences that they bind were also duplicated, but in some cases, they differed in expression levels or became singletons due to dosage-sensitive regulation of their target genes.

Mitochondrial trafficking through Rhot1 is involved in the aggregation of germinal granule components during primordial germ cell formation in Xenopus embryos.

In many animals, the germ plasm is sufficient and necessary for primordial germ cell (PGC) formation. It contains germinal granules and abundant mitochondria (germline-Mt). However, the role of germline-Mt in germ cell formation remains poorly understood. In Xenopus, the germ plasm is distributed as many small islands at the vegetal pole, which gradually aggregates to form a single large mass in each of the four vegetal pole cells at the early blastula stage. Polymerized microtubules and the adapter protein kinesin are required for the aggregation of germ plasm. However, it remains unknown whether germline-Mt trafficking is important for the cytoplasmic transport of germinal granules during germ plasm aggregation. In this study, we focused on the mitochondrial small GTPase protein Rhot1 to inhibit mitochondrial trafficking during the germ plasm aggregation. Expression of Rhot1ΔC, which lacks the C-terminal mitochondrial transmembrane domain, inhibited the aggregation of germline-Mt during early development. In Rhot1-inhibited embryos, germinal granule components did not aggregate during cleavage stages, which reduced the number of PGCs on the genital ridge at tail-bud stage. These results suggest that mitochondrial trafficking is involved in the aggregation of germinal granule components, which are essential for the formation of PGCs.

Pou5f3.2-induced proliferative state of embryonic cells during gastrulation of Xenopus laevis embryo.

POU class V (POU-V) transcription factors play the important role in maintenance of pluripotency and cell differentiation. Pou5f3.2 (Oct25), one of Xenopus POU-V transcription factors, shows the zygotic expression prior to gastrulation. In order to know the molecular mechanism of pou5f3.2 expression at gastrula stage, we examined a responsiveness of pou5f3.2 to Nodal signaling. Animal cap assay demonstrated that Xnr2 activates the gene expression of pou5f3.2. In comparative analysis of the 5'-flanking region of pou5f3.2 between Xenopus laevis and X. tropicalis, two conserved regions were detected within the flanking region. Reporter analyses showed that one of the conserved regions contained an enhancer region, which had several Smad2/3 and FoxH1 binding motifs. ChIP assay demonstrated that Smad2 binds to the enhancer region. These results suggest that Nodal signaling induces zygotic expression of pou5f3.2 at gastrula stage. To understand a role of pou5f3.2 in gastrula embryos, morpholino oligo DNA of pou5f3.2 was injected into the lateral side of one blastomere at the 2-cell stage. The morphant embryos showed diminution of Xbra1 expression and gastrulation defect in the injection side, suggesting the essential role of pou5f3.2 at the gastrula stage. Xbra1 expression and gastrulation were also inhibited by injecting with the synthesized RNAs of pou5f3.2. Furthermore, in the pou5f3.2-injected embryo, gene expression of p27Xic1 was drastically suppressed, and the number of dividing cells increased in the injection side. These results suggest that one role of pou5f3.2 is to keep the embryonic cells in undifferentiated and proliferative state during gastrulation.

Possible regulation of Oct60 transcription by a positive feedback loop in Xenopus oocytes.

The POU family subclass V (POU-V) proteins have important roles in maintaining cells in an undifferentiated state. In Xenopus, expression of the POU-V protein Oct60 was detected in oocytes and was found to decrease in blastula- to gastrula-stage embryos. In addition, Oct60 overexpression inhibits some signals in early embryogenesis, including Activin/Nodal, BMP, and Wnt signalling. In this report, we analysed mechanisms of Oct60 promoter activation and discovered that Oct60 transcription was activated ectopically in somatic nuclei by oocyte extract treatment. Promoter assays demonstrated that Oct60 transcription was activated in oocytes specifically and that this activation was dependent on an Octamer-Sox binding motif. ChIP assays showed that the Oct60 protein binds the motif. These results suggest that Oct60 transcription is regulated by a positive-feedback loop in Xenopus oocytes.

The structure and development of Xenopus laevis cornea.

The African clawed frog, Xenopus laevis, is a widely used model organism for tissue development. We have followed the process of corneal development closely in Xenopus and examined the corneal ultrastructure at each stage during its formation. Xenopus cornea development starts at stage 25 from a simple embryonic epidermis overlying the developing optic vesicle. After detachment of the lens placode which takes place around stage 30, cranial neural crest cells start to invade the space between the lens and the embryonic epidermis to construct the corneal endothelium. At stage 41, a second wave of migratory cells containing presumptive keratocytes invades the matrix leading to the formation of inner cornea and outer cornea. Three-dimensional electron microscopic examination shows that a unique cell mass, the stroma attracting center, connects the two layers like the center pole of a tent. After stage 48, many secondary stromal keratocytes individually migrate to the center and form the stroma layer. At stage 60, the stroma space is largely filled by collagen lamellae and keratocytes, and the stroma attracting center disappears. At early metamorphosis, the embryonic epithelium gradually changes to the adult corneal epithelium, which is covered by microvilli. Around stage 62 the embryonic epithelium thickens and a massive cell death is observed in the epithelium, coinciding with eyelid opening. After metamorphosis, the frog cornea has attained the adult structure of three cellular layers, epithelium, stroma, and endothelium, and two acellular layers between the cellular layers, namely the Bowman's layer and Descemet's membrane. After initial completion, Xenopus cornea, in particular the stroma, continues to thicken and enlarge throughout the lifetime of the animal. In the adult, a p63 positive limbus-like wavy structure is observed at the peripheral edge of the cornea. Proliferation analysis shows that the basal corneal epithelial cells actively divide and there are a small number of proliferating cells among the stroma and endothelial cells. This study shows that the development and structure of Xenopus cornea is largely conserved with human although there are some unique processes in Xenopus.

The analysis of the expression of a novel gene, Xenopus polka dots, which was expressed in the embryonic and larval epidermis during early development.

The epidermis serves as a barrier protecting organs and tissues from the environment, and comprises many types of cells. A cell renewal system is established in epidermis: old epithelial cells are replaced by newly differentiated cells, which are derived from epidermal stem cells located near basement membrane. In order to examine the mechanism of epidermal development, we isolated a novel gene expressed in Xenopus epidermis and named the gene Xenopus polka dots (Xpod) from its polka dot-like expression pattern throughout larval periods. Several immunohistochemical examinations showed that the Xpod-expressing cell type is neither p63-positive epidermal stem cells, nor the α-tubulin-positive ciliated cells, but a subset of the foxi1e-positive ionocytes. The forced gene expression of foxi1e caused the suppression of Xpod expression, while Xpod showed no effect on foxi1e expression. In a comparison of several osmotic conditions, we found that hypertonic culture caused the increase in number of the Xpod-expressing cell, whereas number of the foxi1e-expressing cells was reduced under the hypertonic condition. These results show the possibility that Xpod is involved in the establishment of a certain subpopulation of ionocytes under hypertonic conditions.

Transcriptional regulatory elements of hif1α in a distal locus of islet1 in Xenopus laevis.

Adult mammalian hearts are not regenerative. However, recent studies have evidenced that hypoxia enhances their regeneration. Islet1 (isl1) is known as a cardiac progenitor marker, which is quiescent in adult mammal hearts. In Xenopus hearts, transcriptional activation of isl1 was shown during cardiac regeneration of froglets at 3 months after metamorphosis. In this study, we examined transcriptional regulation of isl1 focusing on hypoxia-inducible factor 1α (hif1α) in Xenopus heart. We found that hif1α expression was increased in response to cardiac injury and overexpression of hif1α upregulated mRNA expression of isl1. Multiple conservation analysis including 9 species revealed that 8 multiple conserved regions (MCRs) were present upstream of isl1. DNA sequence analysis using JASPAR showed hif1α binding motifs in MCRs. By luciferase reporter assay and chromatin immunoprecipitation analysis, we found that hif1α directly bound to hif1α motifs in the most distant MCR8 and showed a specific transcriptional activity on the MCR8. In the luciferase assay using constructs carrying MCR8 without a responsive motif of hif1α, the reporter activity was lost. Pharmacologically inhibition of hif1α affected isl1 transcription and downstream events including cardiac phenotypes, suggesting functional defects of islet1. Contrarily in murine hearts, transcription of isl1 was unresponsive even after cryoinjury to adult hearts while hif1α mRNA was induced. In comparative analysis of multiple alignment, hif1α elements present in MCR8 of Xenopus or zebrafish were found to be disrupted as species are evolutionarily distant from Xenopus and zebrafish. Our results suggested an altered switch of isl1 transcription between mammals and Xenopus laevis.

Pou5f3.3 is involved in establishment and maintenance of hematopoietic cells during Xenopus development.

Three POU family class V gene homologues are expressed in the development of Xenopus. In contrast to the expression of Pou5f3.1 and Pou5f3.2 in organogenesis, Pou5f3.3 is expressed during oogenesis in ovary. We investigated the expression and function of Pou5f3.3 in organogenesis of Xenopus laevis. RT-PCR and immunohistochemical analysis indicated that Pou5f3.3 was expressed in a small number of adult liver cells and blood cells. Immunocytochemical investigation proved that Bmi1, a marker for hematopoietic progenitor cells, was co-expressed in Pou5f3.3-expressing small spherical cells in the peripheral blood. In anemic induction by intraperitoneal injection of phenyl hydrazine, the number of Pou5f3.3-expressing cells significantly increased within 3 days after phenyl hydrazine injection. In CRISPR/Cas mutagenesis of Pou5f3.3, Bmi1-positive hematopoietic progenitor cell count decreased in the hematopoietic dorsal-lateral plate (DLP) region, resulting in a considerable reduction in peripheral blood cells. CRISPR/Cas-induced hematopoietic deficiency was completely rescued by Pou5f3.3 supplementation, but not by Pou5f3.1 or Pou5f3.2. Transplantation experiments using the H2B-GFP transgenic line demonstrated that DLP-derived Pou5f3.3-positive and Bmi1-positive cells were translocated into the liver and bone through the bloodstream. These results suggest that Pou5f3.3 plays an essential role in the establishment and maintenance of hematopoietic progenitor cells during Xenopus development.

Perturbation of Notch/Suppressor of Hairless pathway disturbs migration of primordial germ cells in Xenopus embryo.

Primordial germ cells (PGCs) in Xenopus embryo are specified in the endodermal cell mass and migrate dorsally toward the future gonads. The role of the signal mediated by Notch and Suppressor of Hairless [Su(H)] was analyzed on the migrating PGCs at the tailbud stage. X-Notch-1 and X-Delta-1 are expressed in the migrating PGCs and surrounding endodermal cells, whereas X-Delta-2 and X-Serrate-1 are expressed preferentially in the PGCs. Suppression and constitutive activation of the Notch/Su(H) signaling in the whole endoderm region or selectively in the PGCs resulted in an increase in ectopic PGCs located in lateral or ventral regions. Knocking down of the Notch ligands by morpholino oligonucleotides revealed that X-Delta-2 was indispensable for the correct PGC migration. The ectopic PGCs seemed to have lost their motility in the Notch/Su(H) signal-manipulated embryos. Our results suggest that a cell-to-cell interaction via the Notch/Su(H) pathway has a significant role in the PGC migration by regulating cell motility.

[Usefulness of epidural catheter with distal subcutaneous reservoir for cancer pain control].

When an effective pain relief cannot be achieved by systemic administration of analgesics, neuraxial opioid therapy such as epidural (EPI) and subarachnoid (SA) catheters should be offered. During the period of 2004 to 2008, EPI (117 patients) and SA (1 patients) with an epidural catheter with subcutaneous reservoir also showed a significant improvement in their pain level calculated by numerical rating scale (NRS). Two cases of infection were caused by a subcutaneous reservoir, however, no serious infections, such as epidural abscess, were observed. Long-term catheter trouble occurred in 15 patients (12.8%) which was considered to be the catheter obstruction caused by epidural fibrosis. It is necessary to establish the common guidelines between hospital doctors and general practitioners to prevent the incidence of the catheter troubles and infections.

XSu(H)2 is an essential factor for gene expression and morphogenesis of the Xenopus gastrula embryo.

The CSL (CBF-1, Suppressor of Hairless, Lag-1) transcriptional factor is an important mediator of Notch signal transduction. It plays a key role in cell fate determination by cell-cell interaction. CSL functions as a transcriptional repressor before the activation of Notch signaling. However, once Notch signaling is activated, CSL is converted into a transcriptional activator. It remains unclear if CSL has any function during early development before neurogenesis, while transcriptional products exist from the maternal stage. Here, we analyzed the function of Xenopus Suppressor of Hairless (XSu(H)) using morpholino antisense oligonucleotides (MO), which interfere with the translation of transcripts. In Xenopus embryos, maternal transcripts of both XSu(H)1 and XSu(H)2 were ubiquitously observed until the blastula stage and thereafter only XSu(H)1 was zygotically transcribed. Knockdown experiments with MO demonstrated that XSu(H)2 depletion caused a decrease in the expression of the Xbrachyury, MyoD and JNK1 genes. Morphological and histological examinations indicated that XSu(H)2 depletion caused abnormal gastrulation, which resulted in severe defects of the notochord and somitic mesoderm. The effect of XSu(H)2-MO was completely rescued by co-injection of XSu(H)2 mRNAs, but not by XSu(H)1 mRNAs. XESR-1, a Notch signaling target gene, inhibited Xbrachyury expression. However, expression of the XESR-1 gene was not induced by depletion of XSu(H)2. Co-injection of the dominant-negative form of XESR-1 could not rescue the suppression of Xbrachyury expression in the XSu(H)2-depleted embryo. These results suggest that XSu(H)2 is involved in mesoderm formation and the cell movement of gastrula embryos in a different manner from the XESR-1-mediated Notch signaling pathway.

Novel erythrocyte clumps revealed by an orphan gene Newtic1 in circulating blood and regenerating limbs of the adult newt.

The newt, a group of urodele amphibians, has outstanding ability to repeatedly regenerate various body parts, even in the terrestrial life-stage. In this animal, when the limb is amputated, a cell mass named the blastema appears on the stump and eventually gives rise to a new functional limb. Erythrocytes (red blood cells) in most non-mammalian vertebrates, including the newt, preserve their nucleus throughout their life-span, although physiological roles of such nucleated erythrocytes, other than oxygen delivery, are not known. Here we report novel behavior of erythrocytes in the newt. We identified an orphan gene Newtic1, whose transcripts significantly increased in the blastema. Newtic1 was expressed in a subset of erythrocytes that formed a novel clump (EryC). EryC formed a complex with monocytes and was circulating throughout the body. When the limb was amputated, EryCs were newly generated in the stump and accumulated into a distal portion of the growing blastema. Our data suggested that the newt erythrocytes carried multiple secretory molecules including growth factors and matrix metalloproteases, and were capable of delivering these molecules into the blastema as a form of EryCs. This study provides insight into regulations and roles of nucleated erythrocytes, that are independent of oxygen delivery.

XMam1, Xenopus Mastermind1, induces neural gene expression in a Notch-independent manner.

Mastermind, which is a Notch signal component, is a nuclear protein and is thought to contribute to the transactivation of target genes. Previously we showed that XMam1, Xenopus Mastermind1, was essential in the transactivation of a Notch target gene, XESR-1, and was involved in primary neurogenesis. To examine the function of XMam1 during Xenopus early development in detail, XMam1-overexpressed embryos were analyzed. Overexpression of XMam1 ectopically caused the formation of a cell mass with pigmentation on the surface of embryos and expressed nrp-1. The nrp-1-positive cell mass was produced by XMam1 without expression of the Notch target gene, XESR-1, and not by the activation form of Notch, NICD. The ectopic expression of nrp-1 was not inhibited by co-injection of XMam1 with a molecule known to inhibit Notch signaling. The nrp-1 expression was also recognized in the animal cap injected with XMam1DeltaN, which lacks the basic domain necessary for interacting with NICD and Su(H). These results show that XMam1 has the ability to induce the cell fate into the neurogenic lineage in a Notch-independent manner.

Purification of NADPH-P450 reductase (NPR) from Xenopus laevis and the developmental change in NPR expression.

NADPH-P450 reductase (NPR) was purified from hepatic microsomes of Xenopus laevis. The electron transfer activity of purified NPR was 23.8 units/min/mg with horse cytochrome c. The aminopyrine demethylation activity of rat CYP2B1 with Xenopus NPR was 58.1 nmol/min/nmol. The corresponding cDNA was isolated from Xenopus liver. The homology in amino acid sequence deduced from NPR cDNA isolated from Xenopus liver was 80%, 78%, and 81% with human, rat, and rabbit NPR, respectively. Antibody against Xenopus NPR was prepared. The expression of NPR was investigated in various tissues and in early development by Western blotting. NPR was most abundantly expressed in the kidney, followed by the liver, lung, and heart. The brain had very low levels of NPR. The level of NPR protein was almost the same at all stages, 2-cell stage (st. 2), blastula (st. 8), gastrula (st. 12), tail bud (st. 26) and larva (st.35), examined in this study. We further investigated the distribution of NPR using whole-mount in situ hybridization. NPR mRNA was expressed in cement gland, lens placode, ear vesicle, mesencephalon, rhombencephalon, lymphatic vessel, and heart anlage in the embryo at stage 29. Xenopus NPR has similar properties to mouse and rat NPRs. Localization of NPR in Xenopus embryo was consistent with the abnormal region caused by NPR deficiency in mice.

The intracellular domain of X-Serrate-1 is cleaved and suppresses primary neurogenesis in Xenopus laevis.

The Notch ligands, Delta/Serrate/Lag-2 (DSL) proteins, mediate the Notch signaling pathway in a numerous developmental processes in multicellular organisms. Although the ligands induce the activation of the Notch receptor, the intracellular domain-deleted forms of the ligands cause dominant-negative phenotypes, implying that the intracellular domain is necessary for the Notch signal transduction. Here we examined the role of the intracellular domain of Xenopus Serrate (XSICD) in Xenopus embryos. X-Serrate-1 has the putative nuclear localization sequence (NLS) in downstream of the transmembrane domain. Biochemical analysis revealed that XSICD fragments are cleaved from the C-terminus side of X-Serrate-1. Fluorescence microscopic analysis showed that the nuclear localization of XSICD occurs in the neuroectoderm of the embryo injected with the full-length X-Serrate-1/GFP. Overexpression of XSICD showed the inhibitory effect on primary neurogenesis. However, a point mutation in the NLSs of XSICD inhibited the nuclear localization of XSICD, which caused the induction of a neurogenic phenotype. The animal cap assay revealed that X-Serrate-1 suppresses primary neurogenesis in neuralized animal cap, but X-Delta-1 does not. Moreover, XSICD could not activate the expression of the canonical Notch target gene, XESR-1 in contrast to the case of full-length X-Serrate-1. These results suggest that the combination of XSICD-mediated intracellular signaling and the extracellular domain of Notch ligands-mediated activation of Notch receptor is involved in the primary neurogenesis. Moreover, we propose a bi-directional signaling pathway mediated by X-Serrate-1 in Notch signaling.