Pax genes in development and maturation of the vertebrate visual system: Implications for optic nerve regeneration

Pax genes play a pivotal role in development of the vertebrate visual syste m. Pax6 is the master control gene for eye development: ectopic expression of Pax6 in Xenopus laevis and Drosophila melanogaster leads to the formatio n of differentiated eyes on the legs or wings. Pax6 is involved in formatio n of ganglion cells of the retina, as well as cells of the lens, iris and c ornea. In addition Pax6 may play a role in axon guidance in the visual syst em. Pax2 regulates differentiation of the optic disk through which retinal gang lion cell axons exit the eye. Furthermore, Pax2 plays a critical role in de velopment of the optic chiasm and in the guidance of axons along the contra lateral or ipsilateral tracts of the optic nerve to visual targets in the b rain. During development Pax7 is expressed in neuronal cells of one of the major visual targets in the brain, the optic tectum/superior colliculus. Neurons expressing Pax7 migrate towards the pia and concentrate in the stratum gris eum superficiale (SGFS), the target site for retinal axone. Together, expre ssion of Pax2, 6 and 7 may guide axons during formation of functional retin otectal/collicular projections. Highly regulated Pax gene expression is also observed in mature animals. Mo reover, evidence suggests that Pax genes are important for regeneration of the visual system. We are currently investigating Pax gene expression in sp ecies that display a range of outcomes of optic nerve regeneration. We pred ict that such information will provide valuable insights for the induction of successful. regeneration of the optic nerve and of other regions of the central nervous system in mammals including man.