Phenotypic characterisation and genomic mapping of m865, a zebrafish mutation involved in the development of the dopaminergic system

The zebrafish mutation m865 was isolated during a large-scale mutagenesis screen (Holzschuh et al., 2003, Holzschuh and Driever, unpublished) aimed at identifying genes involved in the development and maintenance of subgroups of neurons in the zebrafish central nervous system. The phenotype of m865 mutant embryos shows defects in the development of dopaminergic neurons in the pretectum and of dopaminergic amacrine cells, as well as subtle size reduction of the caudal dopaminergic cluster in the diencephalon. The effects of the mutation appear not to be restricted to dopaminergic neurons, as development of other neurotransmitter systems (serotonergic and cholinergic) is impaired as well. Furthermore, increased apoptosis is localized to the m865 mutant retina and in the optic tectum starting at 24hpf, going along with a reduced size of the mutant head and eye. Retinal stratification is preserved in m865 mutant embryos. Crossing of the optic nerve at the optic chiasm and outgrowth of retinal ganglial cell axons towards the optic tectum generally appear unbiased, however retinal ganglial cells and their axons appear to be strongly reduced in size or number. Early patterning is not affected in m865 mutant embryos, and expression of genes known to play a role in dopaminergic cell differentiation is normal except for the expression of nurr1 in the mutant retina. Thus the m865 mutation does not specifically affect dopaminergic neuron development. Yet the defective development of the dopaminergic pretectal cluster may in part be explained by failure in availability of target-derived neurotrophic factors from optic tectum and epiphysis through increased apoptosis. The critical genomic interval of m865 could be narrowed down to a region of 110kb, containing four candidate genes. Two of them, spr and rrp4, are in focus of interest due to expression in morphological structures that are affected in m865 mutant embryos, or based on roles in dopaminergic development and function th