PAX-6 IS INVOLVED IN THE SPECIFICATION OF HINDBRAIN MOTOR-NEURON SUBTYPE

Pax-6 is a member of the vertebrate Pax gene family, which is structur ally related to the Drosophila pair-rule gene, paired, In mammals, Pax -6 is expressed in several discrete domains of the developing CNS and has been implicated in neural development, although its precise role r emains elusive, We found a novel Small eye rat strain (rSey(2)) with p henotypes similar to mouse and rat Small eye, Analyses of the Pax-6 ge ne revealed one base (C) insertion in an exon encoding the region down stream of the paired box: of the Pax-6 gene, resulting in generation o f truncated protein due to the frame shift, To explore the roles of Pa x-6 in neural development, we searched for abnormalities in the nervou s system in rSey(2) homozygous embryos, rSey(2)/rSey(2) exhibited abno rmal development of motor neurons in the hindbrain, The Islet-I-positi ve motor neurons were generated just ventral to the Pax-6-expressing d omain both in the wildtype and mutant embryos, However, two somatic mo tor (SM) nerves, the abducent and hypoglossal nerves, were missing in homozygous embryos, By retrograde and anterograde labeling, we found n o SM-type axonogenesis (ventrally growing) in the mutant postotic hind brain, though branchiomotor and visceral motor (BM/VM)-type axons (dor sally growing) mere observed within the neural tube, To discover wheth er the identity of these motor neuron subtypes was changed in the muta nt, we examined expression of LIM homeobox genes, Islet-1, Islet-2 and Lim-3, At the postotic levels of the hindbrain, SM neurons expressed all the three LIM genes, whereas BM/VM-type neurons were marked by Isl et-1 only. In the Pax-6 mutant hindbrain, Islet-2 expression was speci fically missing, which resulted in the loss of the cells harboring the postotic hindbrain SM-type LIM code (Islet-1 + Islet-2 + Lim-3), Furt hermore, we found that expression of Wnt-7b, which overlapped with Pax -6 in the ventrolateral domain of the neural tube, was also specifical ly missing in the mutant hindbrain, while it remained intact in the do rsal non-overlapping domain, These results strongly suggest that Pax-6 is involved in the specification of subtypes of hindbrain motor neuro ns, presumably through the regulation of Islet-2 and Wnt-7b expression .