Regulation of neural development and adult brain homeostasis in the zebrafish

Abstract

During vertebrate neural development, many genes and pathways are involved in order to properly pattern and maintain regional brain identities. This thesis documents the roles and pathways they are involved in of several genes that were identified from forward and reverse genetic screens in the zebrafish. These genes function in various aspects of neural development and of adult brain homeostasis. Wnt/?-catenin signaling is one of the best known signaling pathways that is involved in development of several organs and disease. The intracellular Wnt/?-catenin antagonist Adenomatous Polyposis Coli (Apc) was found to be important for proper pathfinding of retinal axons through the optic nerve towards the optic tectum using a zebrafish apc mutant from a forward screen. In addition, Apc was shown to be required for restricting activity of Wnt/?-catenin signaling in order to maintain secondary brain organizers such as the isthmic organizer and for dorsal midbrain survival during late maintenance stages of brain development. Two members of a relatively unknown family of nucleolar guanine nucleotide-binding like proteins, Gnl2 and nucleostemin/gnl3 (Ns) that were recovered from a forward and reverse genetic screen were found to be required for proper timing of neurogenesis in the retina. In zebrafish gnl2 and ns mutants, differentiation of retinal ganglion cells and other retinal cell types was delayed, due to prolonged cell cycle progression and reduced cell cycle exit. Since previous studies have implied that Ns may function at least partially through upregulation of the tumor suppressor p53, we evaluated contribution of p53 activation to the gnl2 and ns phenotype. We found that reduction of p53 levels did not restore proper differentiation in the retina, suggesting that the delayed differentiation is not due to p53 activation. On the other hand, the dramatic apoptosis that occurs in gnl2 and ns mutants was prevented upon reduction of p53. In familial early-onset Alzheimer’s disease, many missense mutations have been discovered in the ?-secretase component Presenilin 1 and 2 (Psen). These proteins are important for processing of many intramembrane proteins, such as Amyloid Precursor Protein and the Notch receptor. The Notch signaling pathway is important during development for maintenance of neural progenitor cells and is involved in differentiation of glial cells in adult brains. However, the exact molecular nature of the human mutations is unknown. We recovered two psen zebrafish mutants in a reverse genetic screen that in contrast to mouse Psen mutants are viable and fertile. We did not observe any gross morphological defects in adult psen mutants. However, proliferation of neural stem cells/progenitor cells and de novo neurogenesis of granule cells in the cerebellum was reduced. In addition, expression of neural stem/progenitor cells and radial glial cells was reduced. Expression of a Notch target gene was locally reduced, suggesting that reduced Notch signaling underlies the phenotype. These psen mutants represent a valuable tool for further study of Psen function in the adult and embryonic brain.