Localization of the calcium-binding protein secretagogin in cone bipolar cells of the mammalian retina

Secretagogin, a recently cloned member of the EF-hand family of calcium binding proteins, was localized in the mouse, rat, and rabbit retina using immunofluorescence immunohistochemistry. Secretagogin is expressed in subpopulations of ON and OFF cone bipolar cells; however, no immunoreactivity was observed in rod bipolar cells in any of these species. Using subtype-specific markers and mice expressing green fluorescent protein (GFP) within specific cell classes, we found that secretagogin is expressed in Types 2, 3, 4, 5, 6 and possibly Type 8 cone bipolar cells in the mouse retina. The expression pattern in the rat retina differs slightly with expression in cone bipolar cell Types 2, 5, 6, 7, and 8. Evaluation of secretagogin in the developing mouse retina revealed expression as early as postnatal day 6, with OFF cone bipolar cells showing secretagogin expression prior to the ON cone bipolar cells. Secretagogin is a useful marker of cone bipolar cells for studying alterations in bipolar cell morphology during development and degeneration. Further work will be necessary to elucidate the functional role of this protein in bipolar cells.