👤 Mitinori Saitou

Signals from extraembryonic tissues in mice determine which proximal epiblast cells become primordial germ cells (PGCs). After their specification, approximately 40 PGCs appear at the base of the allantoic bud and migrate to the genital ridges, where they expand to about 25 000 cells by Embryonic Day (E)13.5. The heterochromatin protein 1 (HP1) family members HP1alpha, HP1beta, and HP1gamma (CBX5, CBX1, and CBX3, respectively) are thought to induce heterochromatin structure and to regulate gene expression by binding methylated histone H3 lysine 9. We found a dramatic loss of germ cells before meiosis in HP1gamma mutant (HP1gamma(-/-)) mice that we generated previously. The reduction in PGCs in HP1gamma(-/-) embryos was detectable from the early bud stage (E7.25), and the number of HP1gamma(-/-) PGCs was gradually reduced thereafter. Bromodeoxyuridine incorporation into PGCs was significantly reduced in E7.25 and E12.5 HP1gamma(-/-) embryos. Furthermore, a lower proportion of HP1gamma(-/-) PGCs than wild-type PGCs was in S phase, and a higher proportion, respectively, was in G1 phase at E12.5. Moreover, the proportion of p21 (Cip, official symbol CDKN1A)-positive HP1gamma(-/-) PGCs was increased, suggesting that the G1/S phase transition was inhibited. However, no differences were detected between fate determination, migration, apoptosis, or histone modification of PGCs of control embryos and those of HP1gamma(-/-) embryos. Therefore, the reduction in PGCs in HP1gamma(-/-) embryos could be caused by impaired cell cycle in PGCs. These results suggest that HP1gamma plays an important role in keeping enough germ cells by regulating the PGC cell cycle. Show less

The Schizosaccharomyces pombe cps1-12 (for chlorpropham supersensitive) mutant strain was originally isolated as hypersensitive to the spindle poison isopropyl N-3-chlorophenyl carbamate (chlorpropham) (J. Ishiguro and Y. Uhara, Jpn. J. Genet. 67:97-109, 1992). We have found that the cps1-12 mutation also confers (i) hypersensitivity to the immunosuppressant cyclosporin A (CsA), (ii) hypersensitivity to the drug papulacandin B, which specifically inhibits 1,3-beta-D-glucan synthesis both in vivo and in vitro, and (iii) thermosensitive growth at 37 degrees C. Under any of these restrictive treatments, cells swell up and finally lyse. With an osmotic stabilizer, cells do not lyse, but at 37 degrees C they become multiseptated and multibranched. The cps1-12 mutant, grown at a restrictive temperature, showed an increase in sensitivity to lysis by enzymatic cell wall degradation, in in vitro 1,3-beta-D-glucan synthase activity (173% in the absence of GTP in the reaction), and in cell wall biosynthesis (130% of the wild-type amount). Addition of Ca2+ suppresses hypersensitivity to papulacandin B and septation and branching phenotypes. All of these data suggest a relationship between the cps1+ gene and cell wall synthesis. A DNA fragment containing the cps1+ gene was cloned, and sequence analysis indicated that it encodes a predicted membrane protein of 1,729 amino acids with 15 to 16 transmembrane domains. S. pombe cps1p has overall 55% sequence identity with Fks1p or Fks2p, proposed to be catalytic or associated subunits of Saccharomyces cerevisiae 1,3-beta-D-glucan synthase. Thus, the cps1+ product might be a catalytic or an associated copurifying subunit of the fission yeast 1,3-beta-D-glucan synthase that plays an essential role in cell wall synthesis. Show less