Genetic control for development of cerebellar neurons and neural circuits in zebrafish

○ Miki Takeuchi1, Takashi Shimizu1,2, Shuichi Kani3, Young-Ki Bae3, Koji Tanabe3, Ryo Kusuda1,2, Kazuhide Asawaka4, Koichi Kawakami4, and Masahiko Hibi1,2

1 Division of Molecular and Developmental Biology, National Institute of Genetics, Mishima, Japan
2 Department of Genetics, Graduate University for Advanced Studies, Mishima, Japan

The cerebellum forms in the dorsal part of the most anterior hindbrain and functions in integrating sensory information and motor control. Our previous anatomical analysis revealed that the structure of cerebellum and cerebellar neural circuits is evolutionarily conserved between mammals and zebrafish. We also recently reported that a similar set of proneural genes are used for development of cerebellar neurons in zebrafish. These findings indicate that zebrafish cerebellum and cerebellar neural circuit provide a good model for understanding the formation and functions of complex brain structures. To reveal the gene cascades that control the development of cerebellar neurons and neural circuits, we isolated zebrafish mutants for defects in the development of Purkinje and granule cells and in the formation of their neurites. Among the mutants, gazami mutant larvae displayed a strong reduction of granule cells but not of Purkinje cells. gazami locus encoded cp27 gene, the function of which has not been revealed. While the expressions of atoh1 genes were not affected, Neurod+ granule cells were strongly reduced in the gazami mutant larvae. These data indicate that CP27 functions during differentiation of granule cells from their progenitors. shiomaneki mutant larvae showed shortened and/or mistargeted parallel fibers, which are axons of the granule cells. We found that the shiomaneki gene encodes Col4a6, a subunit of type IV collagen and a major component of the basement membrane. The same parallel fiber defects were observed in the zebrafish Col4a5 mutant (dragnet) larvae. As Col4a6 forms a trimer complex with Col4a5, our data indicate that the Col4a5/6 complex plays an important role in axogenesis of the parallel fibers. We will report current status of our studies on CP27 and Col4a5/6. We also isolated several Gal4 enhancer/gene-trap lines that mark cerebellar neurons or precerebellar nuclei. We will also discuss our future plan for functional analyses of cerebellar neural circuits using Gal4 lines.