Vertebrate neural induction

Control of neurogenesis--lessons from frogs, fish and flies

Two types of genes activated by neural inducers have been identified, those that lead to the activation of proneural genes and those that limit the activity of these genes to specific domains in the neural plate. The analysis of these genes has begun to fill gaps in our understanding of events that lead from neural induction to the generation of neurons within three longitudinal columns in the Xenopus and zebrafish neural plate.

Programmed cell death during Xenopus development: a spatio-temporal analysis

Programmed cell death (PCD) is an integral part of many developmental processes. In vertebrates little is yet known on the patterns of PCD and its role during the early phases of development, when embryonic tissue layers migrate and pattern formation takes place. We describe the spatio-temporal patterns of cell death during early Xenopus development, from fertilization to the tadpole stage (stage 35/36). Cell death was analyzed by a whole-mount in situ DNA end-labeling technique (the TUNEL protocol), as well as by serial sections of paraffin-embedded TUNEL-stained embryos. The first cell death was detected during gastrulation, and as development progressed followed highly dynamic and reproducible patterns, strongly suggesting it is an important component of development at these stages. The detection of PCD during neural induction, neural plate patterning, and later during the development of the nervous system highlights the role of PCD throughout neurogenesis. Additionally, high levels of cell death were detected in the developing tail and sensory organs. This is the first detailed description of PCD throughout early development of a vertebrate, and provides the basis for further studies on its role in the patterning and morphogenesis of the embryo.

cyclops encodes a nodal-related factor involved in midline signaling

Ventral structures in the central nervous system are patterned by signals emanating from the underlying mesoderm as well as originating within the neuroectoderm. Mutations in the zebrafish, Danio rerio, are proving instrumental in dissecting these midline signals. The cyclops mutation leads to a loss of medial floor plate and to severe deficits in ventral forebrain development, leading to cyclopia. Here, we report that the cyclops locus encodes the nodal-related protein Ndr2, a member of the transforming growth factor type beta superfamily of factors. The evidence includes identification of a missense mutation in the initiation codon and rescue of the cyclops phenotype by expression of ndr2 RNA, here renamed "cyclops." Thus, in interaction with other molecules implicated in these processes such as sonic hedgehog and one-eyed-pinhead, cyclops is required for ventral midline patterning of the embryonic central nervous system.

The zebrafish organizer

Signals from the organizer play a crucial role in patterning the vertebrate embryo. Recent molecular analysis of zebrafish mutations has established an essential role for BMP2 and chordin in organizer function and has identified one-eyed pinhead as a novel EGF-like gene involved in prechordal plate and endoderm formation. In addition, embryological studies have suggested that the zebrafish organizer is induced by extraembryonic cues and have defined two novel organizing centers that pattern the nervous system along the anterior-posterior axes.

A role for the vegetally expressed Xenopus gene Mix.1 in endoderm formation and in the restriction of mesoderm to the marginal zone

We have studied the role of the activin immediate-early response gene Mix.1 in mesoderm and endoderm formation. In early gastrulae, Mix.1 is expressed throughout the vegetal hemisphere, including marginal-zone cells expressing the trunk mesodermal marker Xbra. During gastrulation, the expression domains of Xbra and Mix.1 become progressively exclusive as a result of the establishment of a negative regulatory loop between these two genes. This mutual repression is important for the specification of the embryonic body plan as ectopic expression of Mix.1 in the Xbra domain suppresses mesoderm differentiation. The same effect was obtained by overexpressing VP16Mix.1, a fusion protein comprising the strong activator domain of viral VP16 and the homeodomain of Mix.1, suggesting that Mix.1 acts as a transcriptional activator. Mix.1 also has a role in endoderm formation. It cooperates with the dorsal vegetal homeobox gene Siamois to activate the endodermal markers edd, Xlhbox8 and cerberus in animal caps. Conversely, vegetal overexpression of enRMix.1, an antimorphic Mix.1 mutant, leads to a loss of endoderm differentiation. Finally, by targeting enRMix.1 expression to the anterior endoderm, we could test the role of this tissue during embryogenesis and show that it is required for head formation.

Lens induction by Pax-6 in Xenopus laevis

Despite extensive study following the pioneering work of Spemann on lens development (Spemann, H. (1901) Verh. Anat. Ges. 15, 61-79) and the subsequent establishment of the concept of embryonic induction, the molecular mechanism of vertebrate lens induction remains largely unknown. Here we report that in Xenopus expression of Pax-6 results in lens formation in a cell autonomous manner. In animal cap experiments, Pax-6 induced expression of the lens-specific marker beta B1-crystallin without inducing the general neural marker NCAM. Ectopic Pax-6 expression also resulted in the formation of ectopic lenses in whole embryos as well as in animal cap explants indicating that in vertebrates, as well as Drosophila (Halder, G., Callaerts, P., and Gehring, W.J. (1995) Science 267, 1788-1792), Pax-6 can direct the development of major components of the eye. Interestingly, ectopic lenses formed in whole embryos without association with neural tissue. Treatments giving rise to anterior neural tissue in animal cap explants resulted in the expression of both beta B1-crystallin and Pax-6. Given the ability of Pax-6 to direct lens formation, we propose that the establishment of Pax-6 expression in the presumptive lens ectoderm during normal development is likely to be a critical response of lens-competent ectoderm to early lens inducers.