Distinct Patterns of Retinal Expression from Five Zebrafish Melanopsin-related Genes

Abstract

Purpose: : Melanopsin-expressing cells are intrinsically photosensitive and responsible for mediating circadian and other non-image forming visual responses to light in vertebrates. Phylogenetic analysis and sequence similarity demonstrates that vertebrates evolved two melanopsin groups, one more similar to the first melanopsin gene discovered in Xenopus (opn4x) and the other to the mammalian gene (opn4m). While mammals contain only one melanopsin gene expressed exclusively in retinal ganglion cells, non-mammalian vertebrates possess at least two genes that show more widespread expression in the retina, and also in the brain. We are interested in understanding the function of melanopsin-expressing cells in the teleost brain and retina.

Methods: : DNA sequences were identified in the zebrafish genome that contain high homology to the Melanopsin subclass of Opsins. Full-length clones of five putative melanopsin-related genes were isolated. A heterologous cell culture system is being used to determine their absorption spectra and assay whether the zebrafish proteins are capable of inducing a light response mediated by G-protein activation. RNA in situ hybridization analyses were performed to characterize the expression pattern during development and the identity of the zebrafish melanopsin-expressing cells.

Results: : All five zebrafish melanopsin-related genes encode a seven transmembrane domain protein with the characteristic hallmarks of this specialized opsin subclass, including tyrosine and glutamate residues that serve as counter ions essential for phototransduction and highly conserved amino acids in the third and fourth cytoplasmic loops. The zebrafish genes are expressed in unique and non-overlapping patterns in the retina, in a small subset of ganglion cells as in mammals, but also in bipolar and horizontal cells. In addition, all of the genes are expressed transiently in distinct regions of the developing zebrafish brain, including in cells adjacent to the pineal.

Keywords: circadian rhythms • ganglion cells • retina