Role of insulin-related growth factors in the retina of the goldfish.

Abstract

Growth factors are important in the regulation of neurogenesis in the vertebrate retina. The teleost fish grows throughout its life with continual addition of new neurons in the retina; upon injury, the retina regenerates. Little is known about the role of growth factors in regulating neurogenesis in the normal or injured retina of adult goldfish. In eyecup preparations maintained in vitro, peptide growth factors were assayed for their ability to modulate the proliferation of three populations of neural progenitors: (1) pluripotent neuroepithelial cells at the circumferential germinal zone (CGZ), which generate all retinal neurons except rod photoreceptors, (2) rod precursors in the mature neural retina (MNR) that generate rod photoreceptors and (3) injury-induced neural progenitors. Dividing cells were counted and averaged in sections taken from normal and injured eyecups. The results demonstrated that nanomolar concentrations of insulin, IGF-I and IGF-II significantly stimulated proliferation of cells in the CGZ, but not rod precursors or injury-induced neural progenitors. In contrast, none of the other tested growth factors stimulated proliferation. Based on these results and published findings, IGF-I and the type 1 IGF receptors were hypothesized to be present in the goldfish retina. To test for the presence of type 1 IGF receptors (IGFR-1) in the goldfish retina, receptor binding, emulsion autoradiography experiments were performed by incubating normal and injured eyecups with $\sp{125}$I-IGF-I, processing for autoradiography, and examining the distribution of silver grains overlying retinal sections. Results showed binding of IGF-I to cells in the CGZ and processes within the inner plexiform layer of normal eyes, whereas, in injured eyes, IGF-I binding significantly increased in the MNR adjacent to the injury as compared to uninjured MNR. Finally, IGF-I treatment of isolated retinas increased phosphorylation of tyrosine residues on a putative IGFR-1, which could be immunoprecipitated. In conclusion, results suggest that IGF-I regulates the proliferation of retinal progenitors in the goldfish retina through the type 1 IGF receptor.