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

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          Abstract

          Mechanisms that regulate the number of cell division rounds during embryogenesis have remained largely elusive. To investigate this issue, we used the ascidian, which develops into a tadpole larva with a small number of cells. The embryonic cells divide 11.45 times on average from fertilization to hatching. The number of cell division rounds varies depending on embryonic lineages. Notochord and muscle consist of large postmitotic cells and stop dividing early in developing embryos. Here we show that conversion of mesenchyme to muscle cell fates by inhibition of inductive FGF signaling or mis-expression of a muscle-specific key transcription factor for muscle differentiation, Tbx6, changed the number of cell divisions in accordance with the altered fate. Tbx6 likely activates a putative mechanism to halt cell division at a specific stage. However, precocious expression of Tbx6 has no effect on progression of the developmental clock itself. Zygotic expression of a cyclin-dependent kinase inhibitor, CKI-b, is initiated in muscle and then in notochord precursors. CKI-b is possibly downstream of tissue-specific key transcription factors of notochord and muscle. In the two distinct muscle lineages, postmitotic muscle cells are generated after 9 and 8 rounds of cell division depending on lineage, but the final cell divisions occur at a similar developmental stage. CKI-b gene expression starts simultaneously in both muscle lineages at the 110-cell stage, suggesting that CKI-b protein accumulation halts cell division at a similar stage. The difference in the number of cell divisions would be due to the cumulative difference in cell cycle length. These results suggest that muscle cells do not count the number of cell division rounds, and that accumulation of CKI-b protein triggered by tissue-specific key transcription factors after cell fate determination might act as a kind of timer that measures elapsed time before cell division termination.

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          CDK inhibitors: positive and negative regulators of G1-phase progression.

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            Identification of a novel inhibitor of mitogen-activated protein kinase kinase.

            The compound U0126 (1,4-diamino-2,3-dicyano-1, 4-bis[2-aminophenylthio]butadiene) was identified as an inhibitor of AP-1 transactivation in a cell-based reporter assay. U0126 was also shown to inhibit endogenous promoters containing AP-1 response elements but did not affect genes lacking an AP-1 response element in their promoters. These effects of U0126 result from direct inhibition of the mitogen-activated protein kinase kinase family members, MEK-1 and MEK-2. Inhibition is selective for MEK-1 and -2, as U0126 shows little, if any, effect on the kinase activities of protein kinase C, Abl, Raf, MEKK, ERK, JNK, MKK-3, MKK-4/SEK, MKK-6, Cdk2, or Cdk4. Comparative kinetic analysis of U0126 and the MEK inhibitor PD098059 (Dudley, D. T., Pang, L., Decker, S. J., Bridges, A. J., and Saltiel, A. R. (1995) Proc. Natl. Acad. Sci U. S. A. 92, 7686-7689) demonstrates that U0126 and PD098059 are noncompetitive inhibitors with respect to both MEK substrates, ATP and ERK. We further demonstrate that the two compounds bind to deltaN3-S218E/S222D MEK in a mutually exclusive fashion, suggesting that they may share a common or overlapping binding site(s). Quantitative evaluation of the steady state kinetics of MEK inhibition by these compounds reveals that U0126 has approximately 100-fold higher affinity for deltaN3-S218E/S222D MEK than does PD098059. We further tested the effects of these compounds on the activity of wild type MEK isolated after activation from stimulated cells. Surprisingly, we observe a significant diminution in affinity of both compounds for wild type MEK as compared with the deltaN3-S218E/S222D mutant enzyme. These results suggest that the affinity of both compounds is mediated by subtle conformational differences between the two activated MEK forms. The MEK affinity of U0126, its selectivity for MEK over other kinases, and its cellular efficacy suggest that this compound will serve as a powerful tool for in vitro and cellular investigations of mitogen-activated protein kinase-mediated signal transduction.
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              CDK inhibitors: cell cycle regulators and beyond.

              First identified as cell cycle inhibitors mediating the growth inhibitory cues of upstream signaling pathways, the cyclin-CDK inhibitors of the Cip/Kip family p21Cip1, p27Kip1, and p57Kip2 have emerged as multifaceted proteins with functions beyond cell cycle regulation. In addition to regulating the cell cycle, Cip/Kip proteins play important roles in apoptosis, transcriptional regulation, cell fate determination, cell migration and cytoskeletal dynamics. A complex phosphorylation network modulates Cip/Kip protein functions by altering their subcellular localization, protein-protein interactions, and stability. These functions are essential for the maintenance of normal cell and tissue homeostasis, in processes ranging from embryonic development to tumor suppression.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2014
                7 March 2014
                : 9
                : 3
                : e90188
                Affiliations
                [1]Department of Biological Sciences, Graduate School of Science, Osaka University, Toyonaka, Osaka, Japan
                University of Minnesota Medical School, United States of America
                Author notes

                Competing Interests: The authors have declared that no competing interests exist.

                Conceived and designed the experiments: HN MK GK. Performed the experiments: MK. Analyzed the data: HN MK GK. Wrote the paper: HN MK GK.

                [¤]

                Current address: Asamushi Research Center for Marine Biology, Graduate School of Life Science, Tohoku University, Aomori, Japan

                Article
                PONE-D-13-48224
                10.1371/journal.pone.0090188
                3946487
                ea10a04d-b0a7-470c-a1e0-b428a2086bcb
                Copyright @ 2014

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                History
                : 22 November 2013
                : 31 January 2014
                Page count
                Pages: 9
                Funding
                This work was supported by Grants in Aid from JSPS and MEXT to H.N. and G.K. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Research Article
                Biology
                Anatomy and Physiology
                Endocrine System
                Endocrine Physiology
                Growth Factors
                Biochemistry
                Proteins
                Growth Factors
                Developmental Biology
                Morphogenesis
                Growth Control
                Cell Fate Determination
                Embryology
                Molecular Development
                Molecular Cell Biology
                Cell Division
                Cell Growth
                Medicine
                Anatomy and Physiology
                Endocrine System
                Endocrine Physiology
                Growth Factors
                Endocrinology
                Endocrine Physiology
                Growth Factors

                Uncategorized
                Uncategorized

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