Cyclin A, cyclin B and stringlike are regulated separately in cell cycle arrested trochoblasts of Patella vulgata embryos

Trochoblasts are the first cells to differentiate during the development of spiralian embryos. Differentiation is accompanied by a cell division arrest. In embryos of the limpet Patella vulgata, the participation of cell cycle-regulating factors in trochoblast arrest was analysed as a first step to unravel its cause. We determined the cell cycle phase in which the trochoblasts are arrested by analysing the subcellular locations of mitotic cyclins. The results show that the trochoblasts are most likely arrested in the G2 phase. This was supported by measurement of the DNA content in trochoblast nuclei after the last division. Trochoblasts complete their final division at the sixth mitotic cycle. This mitotic cycle resembles the first postblastoderm cell cycle of Drosophila, in which mitotic activity is controlled by expression of the string gene. As failure of string expression results in cell cycle arrest in the G2 phase, negative regulation of a Patella string homolog could be responsible for trochoblast arrest. Although Stl messengers disappeared from trochoblasts during their final division, expression was observed again 20 min later. Messengers remained present in all trochoblasts at low levels during further development. Thus, expression of the stringlike gene allows the cell cycle arrest of these cells, whereas in Drosophila cells arrested in division lack string messengers.     

Role of baculovirus IE2 and its RING finger in cell cycle arrest

The ie2 gene of Autographa californica nuclear polyhedrosis virus (AcMNPV) is known to transactivate transient expression from viral promoters in a host cell-specific manner. We report that transfection of Spodoptera frugiperda (SF-21) cells with ie2 was sufficient to arrest the cell cycle, resulting in the accumulation of enlarged cells with abnormally high DNA contents. By 72 h posttransfection, more than 50% of ie2-transfected cells had DNA contents greater than 4N. There was no evidence of mitotic spindle formation in these cells, and expression of ie2 appeared to block cell cycle progression in S phase. Several ie2 mutants were analyzed to further define the region of IE2 responsible for arresting the cell cycle. Analysis of these mutants showed that deletion of the RING finger motif eliminated the ability of IE2 to arrest the cell cycle but did not affect its ability to transactivate the ie1 promoter. Moreover, mutation of a single conserved cysteine (C251) of the RING finger motif abolished the ability of IE2 to block cell cycle progression but had no apparent effect on its transregulatory activity. In contrast, a mutant of IE2 containing a deletion of residues 94 to 173 was able to block cell division but lacked trans-regulatory activity. Thus, the ability of IE2 to arrest the cell cycle depended on the integrity of the RING finger motif and was distinct from and independent of its ability to trans-activate the ie1 promoter. IE2 also arrested the division of cells derived from other insect species, Trichoplusia ni (TN-368 and BTI-TN-5B1-4) and Helicoverpa zea (Hz-AM1).     

Transcatheter occlusion of residual patent ductus arteriosus after surgical ligation

It is known that cellular signals produced in response to an inappropriate spindle formation cause the cell to be arrested at metaphase (M) in the cell cycle. We report here that the 42-kDa isoform of MAPK (ERK2) was tyrosyl-phosphorylated and activated in response to epidermal growth factor (EGF) in interphase but not in M-arrested HeLa cells. However, the basal level of activity of M-arrested cells was higher than that of interphase, although the overall tyrosyl phosphorylation content was small. Further, the EGF receptor and its associated proteins GTPase-activating protein and phospholipase C were phosphorylated in M-arrested cells to a lower extent than they were in interphase. This implies that in spite of its high level of basal activity, the scarcity of MAPK activation in mitosis in response to EGF stems from an early impairment of phosphorylation of the receptor and neighboring proteins. The biological significance of these results underlies the importance of keeping the cell sheltered from extracellular signals when it undergoes division.