Biol168_10F_Lecture 18_3Nov2010

Biol168_10F_Lecture 18_3Nov2010 - Dr Morris Maduro UC...

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Dr. Morris Maduro, UC Riverside BIOL 168 (10F) – Lecture 18, page 1 Lecture 18: Xenopus : Organizer pathways, neurulation Textbook: 2nd Ed.: pp. 33-35; pp. 99-105; 125-133; 278-80; 282; 313-315. 3rd Ed.: 95-96; 260-262; 283- 288. Some figures redrawn from Wolpert et al. (2nd Ed.) Frzb: A Wnt Inhibitor The most anterior head structures are induced by the leading edge of the dorsal blastopore lip (see below) and function by blocking the Wnt pathway in addition to antagonizing BMP signaling. The Xwnt8 protein is capable of inhibiting neural induction. It is normally expressed throughout the marginal (equatorial) mesoderm, except in the region of the dorsal lip of the blastopore. Therefore, the organizer must express factors that antagonize Wnt signaling. The protein Frzb (“frizbee”) is a soluble form of the Frizzled receptor expressed in the organizer. It is capable of binding Wnt proteins in solution, and acts to ‘mop up’ secreted Xwnt8 so that it cannot bind normal Frizzled receptors. Excess Frzb expression in the marginal zone blocks Wnt signaling, and the resultant embryos lack ventral structures (similar to overexpression of noggin). A second Wnt inhibitor, Dickkopf (German for ‘big head’) can also antagonize Wnt signaling. Depletion of Dickkopf by injection of anti-Dickkopf antibodies into the blastocoel results in embryos that have underdeveloped heads with abnormally small brains. Cerberus: A three-way inhibitor Induction of the anteriormost head structures (cement gland, eyes, olfactory placodes) can be achieved by injection of the secreted protein Cerberus . If Cerberus is injected into a vegetal ventral blastomere at the 32-cell stage, ectopic head structures are generated. Cerberus has widespread inhibitory activity, blocking BMPs, Nodal-related proteins, and Xwnt8. It is expressed by the deepest tissue of the organizer (see below). The Model for Organizer Genes Expressed In the Organizer Recall the generation of the Nodal gradient by overlapping Wnt and VegT signals. Wnt signaling activates siamois . Where siamois and high levels of Nodals are expressed, the regulator goosecoid is activated, which is one of the genes that marks the organizer. In the organizer, the BMP antagonists Xnr3 , Noggin and Chordin are activated, leading to repression of BMP4 and Wnt8 . ( Xnr3 is an unusual Nodal-related gene as the other Xnr s do not antagonize BMP signaling.) BMP4 is expressed throughout the late blastula, while Xwnt-8 is expressed in the future mesoderm; both specify ventral mesodermal structures. Thus, a BMP4/Xwnt8 gradient is established that is high ventrally, and whose activity is repressed in the organizer. The organizer has three domains The organizer itself can be subdivided into three parts, based on their inductive capacity (i.e. the inducers they express), their morphogenetic movements, and the tissues they make autonomously: the head organizer , the trunk-tail organizer, and the deep deep endo-mesoderm .
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Dr. Morris Maduro, UC Riverside
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Biol168_10F_Lecture 18_3Nov2010 - Dr Morris Maduro UC...

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