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Biol168_10F_Lecture 17_1Nov2010

Biol168_10F_Lecture 17_1Nov2010 - Dr Morris Maduro UC...

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Dr. Morris Maduro, UC Riverside BIOL 168 (10F) – Lecture 17, page 1 Lecture 17: Xenopus : TGF- β pathway, Spemann-Mangold organizer Textbook: 2nd Ed.: p. 8 (Fig 1.10); p. 72; pp. 93-99. 3rd Ed.: 115-116; 130-139; 166-168. Some figures redrawn from Wolpert et al. (2nd Ed.) The Mid-Blastula Transition Most animals have a point at which cell cycles slow, and zygotic transcription is initiated: the mid-blastula transition (MBT). In Xenopus , the MBT occurs at ~4000 cells (12 cleavages), just before gastrulation, and at Cycle 14 in Drosophila . Cell cleavages henceforth become asynchronous as gap phases are added to the cell cycle. The transition is triggered by the ratio of DNA to cytoplasm. Although the exact mechanism by which this ratio is measured is not known, we can hypothesize a cytoplasmic factor (e.g. a repressor) whose concentration per nucleus decreases as repeated cell divisions occur. Initiation of Gastrulation: The Nieuwkoop center We had earlier described cortical rotation, which gives rise to the grey crescent and a region of the embryo that specifies the dorsal axis – the Nieuwkoop center . The Nieuwkoop center is present in the blastula, before gastrulation. At the 32-cell stage, transplantation of the future Nieuwkoop center to the ventral side of another embryo results in the generation of a second axis. Recall that cortical rotation is required for specification of the dorsal axis, and that the vegetal region of the egg contains information that can specify the mesoderm (mesoderm induction). How are these events related to the formation of the Nieuwkoop center? TGF- β signaling A large amount of data suggest that TGF- β (TGF: Transforming Growth Factor) signaling is critical to mesoderm induction (fig. from Wu and Hill, Dev Cell 16: 329-343.):
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Dr. Morris Maduro, UC Riverside BIOL 168 (10F) – Lecture 17, page 2 TGF- β ligands are first proteolytically processed. During TGF- β signaling, a Type II receptor dimer binds the TGF- β ligand, recruiting a Type I receptor dimer, forming a TypeI-II heterotetramer. This leads to phosphorylation of receptor-activated SMADs (e.g. SMAD2, SMAD3). These bind with co-SMADs (e.g. SMAD4) and the complex enters the nucleus, where it interacts with other transcription factors to regulate target genes. TGF- β superfamily members include BMP , Nodal , Vg1 and activin . Injection of mRNAs that encode dominant-negative Type II receptors (lacking internal kinase domain; these prevent formation of normal heterodimers) can block mesoderm formation. However, it is not clear which ligands are affected by such treatment. VegT and Vg1 function in mesoderm induction
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Biol168_10F_Lecture 17_1Nov2010 - Dr Morris Maduro UC...

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