Karyomeres in early cleavage embryos of ophryotrocha labronica lagreca and bacci

Summary Karyomeres or chromosome vesicles occur regularly at all cell divisions in cleavage embryos ofOphryotrocha labronica up to the 16-cell stage. They are formed as separate units, containing one or several nucleolus-like bodies (NLB) as well as intranuclear annulate lamellae (IAL), but coalesce later into a compound nucleus, in connection with copious blebbing and simultaneous appearance of cytoplasmic annulate lamellae (CAL). Labelling of the early embryos with³H-thymidine revealed marked localization of the synthesized DNA to the karyomere envelope region, whereas³H-uridine incorporation, indicating RNA synthesis, was sparse and notably absent in the NLB. On the other hand the latter structure like the envelopes preferentially incorporated³H-myoinositol, and displayed considerable labelling with³H-leucine. The mechanism and general significance of karyomere formation is discussed with particular attention to the NLB and their possible involvement in nuclear membrane formation.This work has been supported by the Swedish Natural Science Research Council and Kungliga Fysiografiska Sällskapet, Lund.The excellent technical assistance of Mrs Annagreta Petersen and Mrs Lena Olsson is gratefully acknowledged.     

Roles of POLD4, smallest subunit of DNA polymerase δ, in nuclear structures and genomic stability of human cells

Mammalian DNA polymerase δ (pol δ) is essential for DNA replication, though the functions of this smallest subunit of POLD4 have been elusive. We investigated pol δ activities in vitro and found that it was less active in the absence of POLD4, irrespective of the presence of the accessory protein PCNA. shRNA-mediated reduction of POLD4 resulted in a marked decrease in colony formation activity by Calu6, ACC-LC-319, and PC-10 cells. We also found that POLD4 reduction was associated with an increased population of karyomere-like cells, which may be an indication of DNA replication stress and/or DNA damage. The karyomere-like cells retained an ability to progress through the cell cycle, suggesting that POLD4 reduction induces modest genomic instability, while allowing cells to grow until DNA damage reaches an intolerant level. Our results indicate that POLD4 is required for the in vitro pol δ activity, and that it functions in cell proliferation and maintenance of genomic stability of human cells.