Overexpression of a Slit homologue impairs convergent extension of the mesoderm and causes cyclopia in embryonic zebrafish

Slit is expressed in the midline of the central nervous system both in vert ebrates and invertebrates. In Drosophila, it is the midline repellent actin g as a ligand for the Roundabout (Robo) protein, the repulsive receptor whi ch is expressed on the growth cones of the commissural neurons. We have iso lated cDNA fragments of the zebrafish slit2 and slit3 homologues and found that both genes start to be expressed by the midgastrula stage well before the axonogenesis begins in the nervous system, both in the axial mesoderm, and slit2 in the anterior margin of the neural plate and slit3 in the polst er at the anterior end of the prechordal mesoderm. Later, expression of sli t2 mRNA is detected mainly in midline structures such as the floor plate ce lls and the hypochord, and in the anterior margins of the neural plates in the zebrafish embryo, while slit3 expression is observed in the anterior ma rgin of the prechordal plate, the floorplate cells in the hindbrain, and th e motor neurons both in the hindbrain and the spinal cord. To study the rol e of Slit in early embryos, we overexpressed Slit2 in the whole embryos eit her by injection of its mRNA into one-cell stage embryos or by heat-shock t reatment of the transgenic embryos which carries the slit2 gene under contr ol of the heat-shock promoter. Overexpression of Slit2 in such ways impaire d the convergent extension movement of the mesoderm and the rostral migrati on of the cells in the dorsal diencephalon and resulted in cyclopia. Our re sults shed light on a novel aspect of Slit function as a regulatory factor of mesodermal cell movement during gastrulation. (C) 2001 Academic Press.