Central nervous system (CNS) signaling centers regionalize and pattern the vertebrate brain during embryogenesis. These centers are necessary and sufficient to induce nearby neural structures, and perturbing signaling center functions has disastrous consequences for brain development (1-4). Accordingly, investigating the developmental genetic mechanisms that mediate signaling center activities will enhance our understanding of genetic anomalies that underlie human neurodevelopmental diseases and disorders. Due to the exceedingly complex nature of vertebrate brain patterning, characterizing homologous genetic programs in simpler animals may provide new insights into basic principles of brain development. Uniquely among invertebrates, hemichordates possess signaling centers, which makes them a suitable group for developing new hypotheses to explain brain patterning mechanisms. Hemichordates are also located in an exceptional phylogenetic position to investigate signaling center evolution. Molecular mechanisms of brain patterning are the products of evolution, and cannot be fully understood without taking their origins into account (5). Beyond their essential roles in brain development, CNS signaling centers are top candidates for genetic novelties that might have facilitated morphological innovations during early brain evolution. The dominant hypothesis is that CNS signaling centers originated in concert with a complex, regionalized brain (6, 7). However, the finding that hemichordates utilize similar signaling centers for axial patterning, suggests that vertebrate signaling centers evolved by modifying components of an ancient molecular architecture. Furthermore, hemichordates utilize interactions between two gene families, FGFs and sFRPs, to mediate anterior patterning. sFRPs and FGFs are critical regulators of forebrain induction and patterning, but interactions between them have not been investigated in vertebrates (8-10). Based on expression patterns, FGFs and sFRPs could interact in the anterior neural ridge, ventral forebrain, and cortical hem. If FGF/sFRP interactions occur in these contexts then they would be relevant to many aspects of neocortical development.
Specific Aims I and II will utilize gene knockdowns, mRNA injections into blastomeres, and embryological manipulations of hemichordate embryos to further investigate the evolutionary origins of the anterior neural ridge and ancestral roles of fgf8/17/18, sfrp1/5, and apical ectoderm in anterior patterning.
Aim III will utilize the zebrafish as a vertebrate model to test the hypothesis that interactions between fgf3,8 and sfrp1a,5 regulate forebrain specification and patterning. The results of these experiments will be assessed by examining expression of potential target genes. If the hypothesized interactions occur, they could provide a novel mechanism to integrate two major signaling pathways involved in anterior forebrain establishment and patterning.
Deciphering the genetic processes that regulate brain development provides critical insights into the causes of genetic disorders and diseases of the human brain. Long-range signaling molecules, including FGFs and sFRPs, secreted from signaling centers orchestrate induction and patterning of many brain regions. This proposal aims to test the hypothesis that FGF/sFRP interactions are required to specify regional fate within the anterior forebrain and characterize their functions within an ancient genetic scaffold for axis patterning.
| Pani, Ariel M; Mullarkey, Erin E; Aronowicz, Jochanan et al. (2012) Ancient deuterostome origins of vertebrate brain signalling centres. Nature 483:289-94 |
