Noggin Expression in the Adult Retina Suggests a Conserved Role during Vertebrate Evolution

Vertebrates share common mechanisms in the control of development and in the maintenance of neural and retinal function. The secreted factor Noggin, a BMP inhibitor, plays a crucial role in neural induction during embryonic development. Moreover, we have shown its involvement in retinal differentiation of pluripotent cells. Here we show Noggin expression in the adult retina in three vertebrate species. Four Noggin genes are present in zebrafish (Danio rerio; ZbNog1, 2, 3, 5), three in frog (Xenopus laevis; XenNog1, 2 and 4), and one in mouse (Mus musculus; mNog). Quantitative RT-PCR experiments show the presence of ZbNog3 and ZbNog5 mRNAs, but not ZbNog1 and ZbNog2, in the adult zebrafish retina. All three genes are expressed in the frog retina, and mNog in the mouse. Immunohistochemistry data show that Noggin proteins are predominantly localized in the Golgi apparatus of photoreceptors and in the fibers of the outer plexiform layer. Lower expression levels are also found in inner plexiform layer fibers, in ganglion cells, in the ciliary marginal zone, and in retinal pigmented epithelium. Our results show that Noggin has a specific cellular and sub-cellular expression in the adult vertebrate retina, which is conserved during evolution. In addition to its established role during embryonic development, we postulate that Noggin also exerts a functional role in the adult retina.