GPAT2 is required for piRNA biogenesis, transposon silencing, and maintenance of spermatogonia in mice†.

PIWI-interacting RNAs (piRNAs), a subclass of germ cell-specific noncoding small RNAs, are essential for de novo DNA methylation of retrotransposon genes in embryonic testes. PIWIL2/MILI, one of three mouse PIWI family members, is indispensable for piRNA production, DNA methylation of retrotransposons presumably via piRNA, and normal spermatogenesis. In vitro analysis using germline stem cells (GS cells) revealed that glycerol-3-phosphate acyltransferase 2 (GPAT2), which is a mitochondrial outer membrane protein involved in generation of lysophosphatidic acid (LPA) and highly expressed in testes, plays important roles in spermatogenesis. Namely, GPAT2 binds to PIWIL2 and is closely involved in the biogenesis of piRNAs; this process is independent of its enzymatic activity on LPA. However, GS cells recapitulate only a limited phase of spermatogenesis and the biological functions of GPAT2 remain largely unknown. In this study, we generated GPAT2-deficient mice and conducted comprehensive analyses. The deficient mice showed defective piRNA production and subsequent de-silencing of IAP and Line-1 retrotransposons in fetal testes. In addition, apoptosis of pachytene spermatocytes was observed. These abnormalities were all common to the phenotype of PIWIL2-deficient mice, in which piRNA production was impaired. GPAT2-deficient mice exhibited apoptosis in spermatogonia at the neonatal stage, which was not observed in PIWIL2-deficient mice. These data show that GPAT2 plays a critical role in preventing apoptosis in spermatogonia.

Stella controls chromocenter formation through regulation of Daxx expression in 2-cell embryos.

In mammals, the structure of the pericentromeric region alters from a ring structure to a dot-like structure during the 2-cell stage. This structural alteration is termed chromocenter formation (CF) and is required for preimplantation development. Although reverse transcripts of major satellite repeats at pericentromeric regions are known to play roles in CF, its underlying mechanism is not fully understood. We previously reported that Stella (also known as PGC7 and Dppa3) deficiency led to developmental arrest at the preimplantation stage, accompanied by frequent chromosome segregation. In this study, we further investigated the effect of Stella deficiency on chromatin reorganization. The Stella-null embryos exhibited impaired CF and reduced expression of the reverse strand of major satellite repeats. In addition, the accumulation of H3.3, a histone H3 variant associated with transcriptional activation, at the pericentromeric regions and expression of the H3.3-specific chaperone Daxx were reduced in Stella-null embryos. These abnormalities were restored by the enforced expression of Daxx in Stella-null embryos. Thus, Stella controls the expression of Daxx and ensures chromatin reorganization in early embryos.

A novel marker for undifferentiated human embryonic stem cells.

Human embryonic stem cells (hESCs) are pluripotent stem cells from early embryos, and their self-renewal capacity depends on the sustained expression of hESC-specific molecules and the suppressed expression of differentiation-associated genes. To discover novel molecules expressed on hESCs, we generated a panel of monoclonal antibodies against undifferentiated hESCs. The antigen recognized by MAb2 is expressed on the cell surface of undifferentiated hESCs; three diffused bands with molecular mass between 30 and 60 kDa in the lysates of hESCs were diminished during hESC differentiation into neural cells. The expression of MAb2 antigen was also observed on the plasma membrane of lung cancer cells, and MAb2 detected 55, 50, and 35 kDa protein bands in the cell lysates. Immunoprecipitation followed by proteomics analyses identified CD147/basigin as a MAb2 antigen. Finally, the positive expression of CD147/basigin protein in undifferentiated hESCs was confirmed. These results suggested that CD147/basigin could be another undifferentiated hESC marker.

Stella preserves maternal chromosome integrity by inhibiting 5hmC-induced γH2AX accumulation.

In the mouse zygote, Stella/PGC7 protects 5-methylcytosine (5mC) of the maternal genome from Tet3-mediated oxidation to 5-hydroxymethylcytosine (5hmC). Although ablation of Stella causes early embryonic lethality, the underlying molecular mechanisms remain unknown. In this study, we report impaired DNA replication and abnormal chromosome segregation (ACS) of maternal chromosomes in Stella-null embryos. In addition, phosphorylation of H2AX (γH2AX), which has been reported to inhibit DNA replication, accumulates in the maternal chromatin of Stella-null zygotes in a Tet3-dependent manner. Cell culture assays verified that ectopic appearance of 5hmC induces abnormal accumulation of γH2AX and subsequent growth retardation. Thus, Stella protects maternal chromosomes from aberrant epigenetic modifications to ensure early embryogenesis.

Expression of the clustered NeuAcα2-3Galß O-glycan determines the cell differentiation state of the cells.

Human embryonic stem cells (hESCs) are pluripotent stem cells from early embryos, and their self-renewal capacity depends on the sustained expression of hESC-specific molecules and the suppressed expression of differentiation-associated genes. To discover novel molecules expressed on hESCs, we generated a panel of monoclonal antibodies against undifferentiated hESCs and evaluated their ability to mark cancer cells, as well as hESCs. MAb7 recognized undifferentiated hESCs and showed a diffuse band with molecular mass of >239 kDa in the lysates of hESCs. Although some amniotic epithelial cells expressed MAb7 antigen, its expression was barely detected in normal human keratinocytes, fibroblasts, or endothelial cells. The expression of MAb7 antigen was observed only in pancreatic and gastric cancer cells, and its levels were elevated in metastatic and poorly differentiated cancer cell lines. Analyses of MAb7 antigen suggested that the clustered NeuAcα2-3Galß O-linked oligosaccharides on DMBT1 (deleted in malignant brain tumors 1) were critical for MAb7 binding in cancer cells. Although features of MAb7 epitope were similar with those of TRA-1-60, distribution of MAb7 antigen in cancer cells was different from that of TRA-1-60 antigen. Exposure of a histone deacetylase inhibitor to differentiated gastric cancer MKN74 cells evoked the expression of MAb7 antigen, whereas DMBT1 expression remained unchanged. Cell sorting followed by DNA microarray analyses identified the down-regulated genes responsible for the biosynthesis of MAb7 antigen in MKN74 cells. In addition, treatment of metastatic pancreatic cancer cells with MAb7 significantly abrogated the adhesion to endothelial cells. These results raised the possibility that MAb7 epitope is a novel marker for undifferentiated cells such as hESCs and cancer stem-like cells and plays a possible role in the undifferentiated cells.

Production of a monoclonal antibody specific for Pou5f1/Oct4.

Pou5f1/Oct4, a member of the POU transcription factor family, is exclusively expressed in embryonic stem cells, which are involved in self-renewal and maintaining pluripotency. In the present study, we report on the establishment of a monoclonal antibody that is specific for Oct4 using the rat medial iliac lymph node method. In an immunoblotting analysis, our antibody detected endogenous Oct4. In addition, immunocytochemical staining using the antibody revealed the nuclear localization of Oct4. This monoclonal antibody has the potential for use in the further analysis of Oct4 function in stem cells.

Contribution of Leydig and Sertoli cells to testosterone production in mouse fetal testes.

Testosterone is a final product of androgenic hormone biosynthesis, and Leydig cells are known to be the primary source of androgens. In the mammalian testis, two distinct populations of Leydig cells, the fetal and the adult Leydig cells, develop sequentially, and these two cell types differ both morphologically and functionally. It is well known that the adult Leydig cells maintain male reproductive function by producing testosterone. However, it has been controversial whether fetal Leydig cells can produce testosterone, and the synthetic pathway of testosterone in the fetal testis is not fully understood. In the present study, we generated transgenic mice in which enhanced green fluorescence protein was expressed under the control of a fetal Leydig cell-specific enhancer of the Ad4BP/SF-1 (Nr5a1) gene. The transgene construct was prepared by mutating the LIM homeodomain transcription factor (LHX9)-binding sequence in the promoter, which abolished promoter activity in the undifferentiated testicular cells. These transgenic mice were used to collect highly pure fetal Leydig cells. Gene expression and steroidogenic enzyme activities in the fetal Leydig cells as well as in the fetal Sertoli cells and adult Leydig cells were analyzed. Our results revealed that the fetal Leydig cells synthesize only androstenedione because they lack expression of Hsd17b3, and fetal Sertoli cells convert androstenedione to testosterone, whereas adult Leydig cells synthesize testosterone by themselves. The current study demonstrated that both Leydig and Sertoli cells are required for testosterone synthesis in the mouse fetal testis.