Tissue-specific regulation of the number of cell division rounds by inductive cell interaction and transcription factors during ascidian embryogenesis

Abstract

Mechanisms that regulate the number of cells constituting the body have remained largely elusive. We approached this issue in the ascidian, Halocynthia roretzi, which develops into a tadpole larva with a small number of cells. The embryonic cells divide 11 times on average from fertilization to hatching. The number of cell division rounds varies among tissue types. For example, notochord cells divide 9 times and give rise to large postmitotic cells in the tadpole. The number of cell division rounds in partial embryos derived from tissue-precursor blastomeres isolated at the 64-cell stage also varied between tissues and coincided with their counterparts in the intact whole embryos to some extent, suggesting tissue-autonomous regulation of cell division. Manipulation of cell fates in notochord, nerve cord, muscle, and mesenchyme lineage cells by inhibition or ectopic activation of the inductive FGF signal changed the number of cell divisions according to the altered fate. Knockdown and missexpression of Brachyury (Bra), an FGF-induced notochord-specific key transcription factor for notochord differentiation, indicated that Bra is also responsible for regulation of the number of cell division rounds, suggesting that Bra activates a putative mechanism to halt cell division at a specific stage. The outcome of precocious expression of Bra suggests that the mechanism involves a putative developmental clock that is likely shared in blastomeres other than those of notochord and functions to terminate cell division at three rounds after the 64-cell stage. Precocious expression of Bra has no effect on progression of the developmental clock itself.

Brachyury regulates proliferation of cancer cells via a p27Kip1-dependent pathway

“…Interestingly, HCT116 cells appeared to be nullipotent in differentiation assays of CSCs [47] suggesting again, that the presence of Brachyury might be required to regulate the interface between proliferation and differentiation. In accordance, evidence also suggests that Brachyury is critical in controlling both cell differentiation and division during normal development of the notochord in ascidians [48]. …”

Regulation of the Number of Cell Division Rounds by Tissue-Specific Transcription Factors and Cdk Inhibitor during Ascidian Embryogenesis

“…To inhibit the FGF signaling pathway, embryos were treated with 2 µM U0126 (Promega) after the 16-cell stage [7], [15]. U0126 is MEK inhibitor that inhibits both the activation of MEK by Raf and the activation of ERK by MEK [16].…”

“…1A, B). Manipulation of cell fates in notochord, nerve cord, muscle, and mesenchyme lineage cells by inhibition or ectopic activation of the inductive FGF signal results in conversion of the number of cell divisions to that of the altered fate [7]. FGF signaling in notochord promotes expression of a notochord-specific transcription factor, Brachyury (Bra), which is essential for notochord differentiation.…”

“…Knockdown or mis-expression of Bra indicates that Bra is responsible for regulation of the number of cell division rounds, suggesting that Bra activates a putative mechanism to halt cell division at a specific stage. However, precocious expression of Bra does not put the developmental clock forward that controls the developmental stage at which cell division is eventually terminated [7].…”

Evolution and development of budding by stem cells: Ascidian coloniality as a case study

“…In larger eggs increased cell division in specific cell lineages after gastrulation leads to higher numbers of cells in many tissues of the tadpole larvae. Time and lineage fate dependent FGF signaling as a regulatory mechanism for increased or decreased cell divisions in the ascidian gastrula has recently been reported (Fujikawa et al, 2011). Therefore, one non-autonomous modification to allow increased cell divisions in brooded colonial ascidian embryos is likely mediated by the upregulation of FGF signaling.…”