Roles of iroquois homeobox genes in vertebrate eye development
Student thesis: Doctoral Thesis
Eye is a delicate organ which perceives light, processes and relays the signal to the central nerve system. The understanding of eye development has advanced considerately in recent years, primarily due to the study of the model organisms across different phyla. Although the eyes of vertebrates and fruitfly (Drosophila) have tremendous anatomical and physiological differences, it is now evident that they share surprisingly conserved regulatory pathways to direct eye development. The Iroquois (Iro) family of genes was first characterized in Drosophila and it is essential for the growth and patterning of the eye/antenna imaginal disc. However, little is known about the roles of lroquois genes in vertebrate eye. In this project, we examined the functional roles of Iroquois genes in vertebrate eye development by cloning, studying the developmental expression patterns and perturbing the gene functions using the mouse and the zebrafish as models. The mouse Iroquois gene lrx5 was identified from screening a murine adult brain cDNA library with the Drosophila Iro homologues. Sequence analysis revealed that 1rx5 is one of the six mouse Iroquois genes and displays similarity to the Drosophila homologues. It is located in mouse chromosome 15 in a cluster with lrx311rx6. We examined the developmental expression of In5 and other members. All of the murine lrx genes are expressed in the neural retina from E12.5, which is the initiation stage of retinal neuron differentiation. Specifically lrx5 is expressed in the ganglion cell and inner nuclear layers of postnatal murine retina. Targeted deletion of 11x5 affects the expression of the terminal ganglion cell differentiation marker Thy- 1 but not the formation of ganglion cells. Similarly, lrx5 mutant retinas also express the early cone bipolar cell marker Vsx1 but not the terminal differentiation marker recoverin. Our data reveals a novel role for lrx5 in the differentiation of both retinal ganglion and cone bipolar cells. We also examined the role of Iro genes in zebrafish eye development. One of the zebrafish novel Iro homologues is identified with embryonic cDNA library screening. This novel Iro homolog shares highest homology to mouse, human, frog and chick Iro1llrxl. Interestingly, two other research groups also identified another zebrafish Iro1llrxl homologue and we named the homologue identified here as ziro1b while the other Iro1llrxl homologue as ziro1a. Genomic marker analysis showed that these two Iro1llrxl genes are two distinct genes and located in different chromosomes. Ziro1b is expressed in the neural retina during embryogenesis but not zim1a. Zebrafish larvae with targeted gene inactivation of zim1b showed severe retinal deformation and visual impairment. Although most of the retinal neurons can initially differentiate, the propagation of the differentiation is arrested. In addition, cells expressing sonic hedgehog did not spread across the retina in the typical manner of retinal neurogenesis in ziro1b inactivated retina. Hedgehog signalling is proved to control the propagation of the retinal neurogenesis in both Drosophila and zebrafish. These results suggest that zim1b is required for the propagation of neurogenic wave through controlling the spread of the sonic hedgehog wave. On the basis of the data obtained from mouse and zebrafish studies, Iroquois genes play multiple essential roles in vertebrate eye development. These data also provide new insights in the conservation of regulatory networks and pathways across different phyla.
- Vertebrates, Eye, Homeobox genes, Embryology