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Over the past decade, several molecules have been identified that influence neural cell fate in vertebrate embryos during gastrulation. The first neural inducers studied were proteins produced by dorsal mesoderm (the Spemann organizer); most of these proteins act by directly binding to and antagonizing the function of bone morphogenetic proteins (BMPs). Recent experiments have suggested that other secreted signals, such as Wnt and FGF, may neuralize ectoderm before organizer function by a different mechanism. Neural effector genes that mediate the response of ectoderm to secreted neuralizing signals have also been discovered. Interestingly, most of these newly identified neuralizing pathways continue the theme of BMP antagonism, but rather than antagonizing BMP protein function, they may neuralize tissue by suppressing Bmp expression. Down-regulation of Bmp expression in the prospective neural plate during gastrulation seems to be a shared feature of neural induction in vertebrate embryos. However, the signals used to accomplish this task seem to vary among vertebrates. Here, we will discuss the role of the recently identified secreted signals and neural effector genes in vertebrate neurogenesis.
Figure 1. Additive pathways may regulate Bmp expression to neuralize ectoderm. In the schematized Xenopus late blastulaembryo, FGF signaling (blue) is ubiquitous with potentially higher levels in the animal and marginal zones (LaBonne and Whitman, 1997; Uzgare et al., 1998; Christen and Slack, 1999; Curran and Grainger, 2000). Low levels of Bmp4 mRNA (yellow) are also found throughout the animal and marginal zones (Fainsod et al., 1994; Hemmati-Brivanlou and Thomsen, 1995; Nikaido et al., 1997; Streit et al., 1998; Dick et al., 2000; Wilson et al., 2000). Active Wnt pathway signaling (nuclear localized beta-catenin)(orange) occurs along the entire future dorsal side of the embryo (reviewed in Moon and Kimelman, 1998). BMP protein antagonists begin to be expressed in the organizer (white speckles)(Sasai, 1998; Chitnis, 1999; Streit and Stern, 1999c; Weinstein and Hemmati-Brivanlou, 1999; Harland, 2000; Robertis and Arechaga, 2001). By the early gastrula stage, Bmp4 mRNA levels have increased throughout the animal and marginal zones. The Wnt and FGF signaling pathways are still active as are BMP protein antagonists produced by the organizer. Neural effector genes begin to be expressed; some are localized to the presumptive neural territory (dark pink) whereas others are expressed throughout the animal hemisphere (light and dark pink)(Sasai, 1998; Chitnis, 1999; Streit and Stern, 1999c; Weinstein and Hemmati-Brivanlou, 1999; Harland, 2000; Robertis and Arechaga, 2001). By the late gastrula stages, Bmp4 mRNA has been cleared from the prospective neuroectoderm and dorsal marginal zone. Neural effector gene expression is localized to the prospective neural territory or subdomains within this territory. Lower right panel: the FGF and Wnt pathways and some of the neural effector genes can negatively regulate Bmp expression to neuralize tissue. As BMP activity can maintain Bmp expression, BMP protein antagonists could also indirectly affect Bmp expression levels (Hammerschmidt et al., 1996; Piccolo et al., 1997; Nguyen et al., 1998).Download figure to PowerPoint
