%0 Journal Article %A Lauren E. Olinski %A Ayumi C. Tsuda %A Julie A. Kauer %A Elena Oancea %T Endogenous opsin 3 (OPN3) protein expression in the adult brain using a novel OPN3 -mCherry knock-in mouse model %D 2020 %R 10.1523/ENEURO.0107-20.2020 %J eneuro %P ENEURO.0107-20.2020 %X The opsins have been studied extensively for their functions in visual phototransduction; however, the mechanisms underlying extraocular opsin signaling remain poorly understood. The first mammalian extraocular opsin to be discovered—opsin 3 (OPN3)—was found in the brain more than two decades ago, yet its function remains unknown. A significant hindrance to studying OPN3 has been a lack of specific antibodies against mammalian OPN3, resulting in an incomplete understanding of its expression in the brain. Al though Opn3 promoter-driven reporter mice have been generated to examine general OPN3 localization, they lack the regulated expression of the endogenous protein and the ability to study its subcellular localization. To circumvent these issues, we have created a knock-in OPN3-mCherry mouse model in which the fusion protein OPN3- mCherry is expressed under the endogenous Opn3 promoter. Viable and fertile homozygotes for the Opn3- mCherry allele were used to create an extensive map of OPN3-mCherry expression across the adult mouse brain. OPN3-mCherry was readily visualized in distinct layers of the cerebral cortex, the hippocampal formation, distinct nuclei of the thalamus, as well as many other regions in both neuronal and non -neuronal cells. Our mouse model offers a new platform to investigate the function of OPN3 in the brain.SIGNIFICANCE STATEMENT Before the current report, there had been a significant lack of encephalic OPN3 protein characterization, likely driven by the absence of murine OPN3 antibodies. This study describes a novel OPN3-mCherry knock-in mouse that we used to analyze endogenous OPN3 protein expression across the brain. We have uncovered new aspects of OPN3: localization to previously undocumented brain subregions, expression in GABAergic neurons and non-neuronal cells, and punctate subcellular localization in the soma. Our OPN3 expression map is an invaluable step towards discovering its elusive encephalic functions. The OPN3-mCherry mouse will facilitate investigation of OPN3 not only in the brain, but also across the entire organism, a useful feature as OPN3 is emerging as a possible mediator of phototransduction outside the brain. %U https://www.eneuro.org/content/eneuro/early/2020/07/31/ENEURO.0107-20.2020.full.pdf