Kaoru Mitsui

The homeoprotein Nanog is required for maintenance of pluripotency in mouse epiblast and ES cells.

Embryonic stem (ES) cells derived from the inner cell mass (ICM) of blastocysts grow infinitely while maintaining pluripotency. Leukemia inhibitory factor (LIF) can maintain self-renewal of mouse ES cells through activation of Stat3. However, LIF/Stat3 is dispensable for maintenance of ICM and human ES cells, suggesting that the pathway is not fundamental for pluripotency. In search of a critical factor(s) that underlies pluripotency in both ICM and ES cells, we performed in silico differential display and identified several genes specifically expressed in mouse ES cells and preimplantation embryos. We found that one of them, encoding the homeoprotein Nanog, was capable of maintaining ES cell self-renewal independently of LIF/Stat3. nanog-deficient ICM failed to generate epiblast and only produced parietal endoderm-like cells. nanog-deficient ES cells lost pluripotency and differentiated into extraembryonic endoderm lineage. These data demonstrate that Nanog is a critical factor underlying pluripotency in both ICM and ES cells.

A new protein containing an SH2 domain that inhibits JAK kinases

The proliferation and differentiation of cells of many lineages are regulated by secreted proteins known as cytokines. Cytokines exert their biological effect through binding to cell-surface receptors that are associated with one or more members of the JAK family of cytoplasmic tyrosine kinases. Cytokine-induced receptor dimerization leads to the activation of JAKs, rapid tyrosine-phosphorylation of the cytoplasmic domains, and subsequent recruitment of various signalling proteins, including members of the STAT family of transcription factors, to the receptor complex. Using the yeast two-hybrid system, we have now isolated a new SH2-domain-containing protein, JAB, which is a JAK-binding protein that interacts with the Jak2 tyrosine-kinase JH1 domain. JAB is structurally related to CIS, a cytokine-inducible SH2 protein,. Interaction of JAB with Jak1, Jak2 or Jak3 markedly reduces their tyrosine-kinase activity and suppresses the tyrosine-phosphorylation and activation of STATs. JAB and CIS appear to function as negative regulators in the JAK signalling pathway.

Role of ERas in promoting tumour-like properties in mouse embryonic stem cells.

Embryonic stem (ES) cells are pluripotent cells derived from early mammalian embryos. Their immortality and rapid growth make them attractive sources for stem cell therapies; however, they produce tumours (teratomas) when transplanted, which could preclude their therapeutic usage. Why ES cells, which lack chromosomal abnormalities, possess tumour-like properties is largely unknown. Here we show that mouse ES cells specifically express a Ras-like gene, which we have named ERas. We show that human HRasp, which is a recognized pseudogene, does not contain reported base substitutions and instead encodes the human orthologue of ERas. This protein contains amino-acid residues identical to those present in active mutants of Ras and causes oncogenic transformation in NIH 3T3 cells. ERas interacts with phosphatidylinositol-3-OH kinase but not with Raf. ERas-null ES cells maintain pluripotency but show significantly reduced growth and tumorigenicity, which are rescued by expression of ERas complementary DNA or by activated phosphatidylinositol-3-OH kinase. We conclude that the transforming oncogene ERas is important in the tumour-like growth properties of ES cells.

MDM2 interacts with MDMX through their RING finger domains.

The N‐terminus of MDM2 proto‐oncoprotein interacts with p53 and down modulates p53 activity by inhibiting transcriptional activity and promoting p53 degradation. MDMX is structurally related to MDM2 and also binds to p53. However, the function of MDMX has not been clarified yet. We found that MDM2 hetero‐oligomerized with MDMX through their C‐terminal RING finger domains. Yeast two‐hybrid analysis revealed that the hetero‐oligomerization between MDMX and MDM2 was more stable than the homo‐oligomerization of each protein. MDM2 has been shown to be degraded by the ubiquitin‐proteasome pathway, while MDMX was a stable protein. Interaction of MDMX with MDM2 through the C‐terminal RING finger domains resulted in inhibiting degradation of MDM2. These data indicate that MDMX functions as a regulator of MDM2.

Fbx15 Is a Novel Target of Oct3/4 but Is Dispensable for Embryonic Stem Cell Self-Renewal and Mouse Development

ABSTRACT Embryonic stem (ES) cells are immortal and pluripotent cells derived from early mammalian embryos. Transcription factor Oct3/4 is essential for self-renewal of ES cells and early mouse development. However, only a few Oct3/4 target genes have been identified. In this study, we found that F-box-containing protein Fbx15 was expressed predominantly in mouse undifferentiated ES cells. Inactivation of Oct3/4 in ES cells led to rapid extinction of Fbx15 expression. Reporter gene analyses demonstrated that this ES cell-specific expression required an 18-bp enhancer element located approximately 500 nucleotides upstream from the transcription initiation site. The enhancer contained an octamer-like motif and an adjacent Sox-binding motif. Deletion or point mutation of either motif abolished the enhancer activity. The 18-bp fragment became active in NIH 3T3 cells when Oct3/4 and Sox2 were coexpressed. A gel mobility shift assay demonstrated cooperative binding of Oct3/4 and Sox2 to the enhancer sequence. In mice having a β-galactosidase gene knocked into the Fbx15 locus, 5-bromo-4-chloro-3-indolyl-β-d-galactopyranoside staining was detected in ES cells, early embryos (two-cell to blastocyst stages), and testis tissue. Despite such specific expression of Fbx15, homozygous mutant mice showed no gross developmental defects and were fertile. Fbx15-null ES cells were normal in morphology, proliferation, and differentiation. These data demonstrate that Fbx15 is a novel target of Oct3/4 but is dispensable for ES cell self-renewal, development, and fertility.

APS, an adaptor protein containing Pleckstrin homology (PH) and Src homology-2 (SH2) domains inhibits the JAK-STAT pathway in collaboration with c-Cbl

We cloned a novel adaptor protein, APS (adaptor molecule containing Pleckstrin homology (PH) and Src Homology-2 (SH2) domains), which was tyrosine phosphorylated in response to c-kit or B cell receptor stimulation. Here, we report that APS was tyrosine phosphorylated by Janus kinase-2 (JAK2) at its C-terminal tyrosine residue and interacted with c-Cbl. Forced expression of APS in an erythropoietin (EPO)-dependent hematopoietic cell line resulted in reduced activation of STAT5 but not cell proliferation in response to EPO. APS bound to the phosphorylated tyrosine residue, Y343 of the erythropoietin receptor cytoplasmic domain. Co-expression of APS and c-Cbl, but not expression of either alone inhibited EPO-dependent STAT5 activation in 293 cells. This required the C-terminal phosphorylation site, as well as PH and SH2 domains of APS. Therefore, one of the major functions of APS is in recruitment of c-Cbl into the receptor/JAK complex, thereby inhibiting JAK signaling activity.

PHYLOGENETIC RELATIONSHIPS AND TAXONOMY OF SARCINOID GREEN ALGAE: CHLOROSARCINOPSIS, DESMOTETRA, SARCINOCHLAMYS GEN. NOV., NEOCHLOROSARCINA, AND CHLOROSPHAEROPSIS (CHLOROPHYCEAE, CHLOROPHYTA)1

We investigated the phylogenetic relationships and the taxonomy of 20 sarcinoid strains by their morphological features and using 18S rDNA sequence data. Nineteen strains of Chlorosarcinopsis (Gerneck) Herndon and allied genera were divided into four groups (A–D). Group A, comprising eight species, was concordant with the description of Chlorosarcinopsis and divided into four polyphyletic subgroups. Group B included two species of Chlorosarcinopsis and two strains of Desmotetra stigmatica (Deason) Deason et Floyd, all of which had parallel thylakoid membranes in pyrenoid matrices, and zoospores bearing apical stigma and subapical flagellar apparatus. The definition of Desmotetra Deason et Floyd was emended to include these features, and Chlorosarcinopsis delicata S. Watanabe was transferred to this genus. Group C comprised Chlorosarcina stigmatica Deason (ASIB.T105), for which we proposed Sarcinochlamys gen. nov., a genus distinct from Chlorosarcina Gerneck in having walled zoospores. Group D, comprising six species, corresponded to Neochlorosarcina and was divided into two subgroups. The presence of thin cell walls in Neochlorosarcina S. Watanabe was ascertained as a valid feature for circumscribing the sarcinoid genera. The physiological experiments on the species of Chlorosarcinopsis and Chlorosarcina by Groover and Bold (1968) were assessed based on the phylogenetic results. Groups (A, B, D) were roughly characterized by these features: algal mass color, utilization of selected nitrogen compounds and carbon sources under light/dark and aerobic/anaerobic conditions, and requirement for vitamin B12. Molecular analysis revealed that Chlorosphaeropsis alveolata Herndon had closer affinity with solitary Protosiphon Klebs and Spongiochloris Starr than with sarcinoid members.