Expression of the insulin receptor in the retina of the goldfish

Insulin is a peptide growth factor that is active in most tissues, both during development and in adulthood. The action of insulin is through its specific membrane receptor. Previously retinal progenitors in the adult goldfish were shown to proliferate vigorously when exposed to insulin in vitro.(1) The present study was undertaken to clone and characterize partial cDNAs that encode the goldfish's insulin receptor (IR) and to establish the cellular pattern of expression of this gene in the retina.

Standard methods were used for RNA isolation, reverse transcription-polymerase chain reaction, Northern blot analysis, and in situ hybridization.

Multiple clones were isolated that, based on sequence analysis, segregated into two groups, presumed to represent two genes that encode the IR. These clones were designated goldfish IR-1 (gfIR-1) and goldfish IR-2 (gfIR-2). Northern blot analysis showed that both genes are expressed in multiple tissues, including the retina. Both gfIR-1 and -2 give rise to a single, major transcript, but the sizes of the two transcripts are different. In situ hybridizations using digoxygenin-labeled riboprobes showed that gfIR-1 and -2 are expressed by all differentiated retinal neurons as well as neuronal progenitors in the circumferential germinal zone.

These data demonstrate that the IR is expressed in the retina of the goldfish, and, on the basis of the cellular pattern of expression, suggest that insulin may act both to regulate neurogenesis and influence the function of differentiated neurons. The cellular coexpression of the receptors for both insulin-like growth factor (IGF) 1 and insulin suggests that neurons and/or neuronal progenitors in the retina of the goldfish may contain hybrid IGF-1/insulin receptors.