Date Published: January 29, 2019
Publisher: Public Library of Science
Author(s): Katherine J. Turner, Jacqueline Hoyle, Leonardo E. Valdivia, Kara L. Cerveny, Wendy Hart, Maryam Mangoli, Robert Geisler, Michele Rees, Corinne Houart, Richard J. Poole, Stephen W. Wilson, Gaia Gestri, Cecilia Moens.
Through forward genetic screening for mutations affecting visual system development, we identified prominent coloboma and cell-autonomous retinal neuron differentiation, lamination and retinal axon projection defects in eisspalte (ele) mutant zebrafish. Additional axonal deficits were present, most notably at midline axon commissures. Genetic mapping and cloning of the ele mutation showed that the affected gene is slbp, which encodes a conserved RNA stem-loop binding protein involved in replication dependent histone mRNA metabolism. Cells throughout the central nervous system remained in the cell cycle in ele mutant embryos at stages when, and locations where, post-mitotic cells have differentiated in wild-type siblings. Indeed, RNAseq analysis showed down-regulation of many genes associated with neuronal differentiation. This was coincident with changes in the levels and spatial localisation of expression of various genes implicated, for instance, in axon guidance, that likely underlie specific ele phenotypes. These results suggest that many of the cell and tissue specific phenotypes in ele mutant embryos are secondary to altered expression of modules of developmental regulatory genes that characterise, or promote transitions in, cell state and require the correct function of Slbp-dependent histone and chromatin regulatory genes.
Mutations in a wide variety of genes are known to lead to congenital abnormalities of eye formation [1,2]. Some of these genes, such as pax6 and rx3, show temporally and spatially restricted expression within developing visual system structures and consequently, a priori, are obvious candidates for roles in eye formation . However, other genes, such as hdac1  and yap , are more ubiquitously expressed and consequently visual system specific phenotypes observed upon aberrant gene function are not so easily explained. Forward genetic screens in animal models provide a relatively unbiased approach to identify the full spectrum of genes involved in specific developmental processes, as the initial selection is based upon phenotypes of interest . To this end, we have been using a forward genetic approach in which we screen existing and new zebrafish lines carrying randomly induced mutations for phenotypes affecting visual system development.
In this study we describe the cloning of the slbpty77e mutation and characterization of nervous system phenotypes in slbpty77e mutants. We show that although early born neurons are present and elaborate axons in slbpty77e mutants, later born neurons are severely depleted and consequently late forming commissures are absent and early tracts and commissures fail to grow. These results are consistent with the observation that many proliferative neural cells fail to transition to differentiation and consequently there are major alterations in the spatial and temporal distribution of proliferative versus differentiated cells in the developing nervous system. One result of this phenotype is that early born neurons differentiate in a very abnormal environment and this no doubt contributes to the axon guidance and other defects present in mutants. Such phenotypes may be a consequence of the role of Slbp in regulating histones that modulate chromatin structure thereby influencing expression of modules of developmental regulatory genes.