SONIC HEDGEHOG PROMOTES ROD PHOTORECEPTOR DIFFERENTIATION IN MAMMALIAN RETINAL CELLS IN-VITRO

The hedgehog gene family encodes secreted proteins important in many d evelopmental patterning events in both vertebrates and invertebrates. In the Drosophila eye disk, hedgehog controls the progression of photo receptor differentiation in the morphogenetic furrow. To investigate w hether hedgehog proteins are also involved in the development of the v ertebrate retina at stages of photoreceptor differentiation, we analyz ed expression of the three known vertebrate hedgehog genes. We found t hat Sonic hedgehog and Desert hedgehog are expressed in the developing retina, albeit at very low levels, whereas Indian hedgehog (lhh) is e xpressed in the developing and mature retinal pigmented epithelium, be ginning at embryonic day 13. To determine whether hedgehog proteins ha ve activities on developing retinal cells, we used an in vitro system in which much of retinal histogenesis N-terminal recombinant Sonic Hed gehog protein (SHH-N) was added to rat retinal cultures for 3-12 d, an d tile numbers of retinal cells of various phenotypes were analyzed by immunohistochemistry. We found that SHH-N caused a transient in creas e in the number of retinal progenitor cells, and a 2- to 10-fold incre ase in the number of photoreceptors differentiating in the cultures wh en analyzed with three different photoreceptor-specific antigens, In c ontrast, the numbers of retinal ganglion cells and amacrine cells were similar to those in control cultures. These results show that Hedgeho g proteins can regulate mitogenesis and photoreceptor differentiation in the vertebrate retina, and Ihh is a candidate factor from the pigme nted epithelium to promote retinal progenitor proliferation and photor eceptor differentiation.