Abstract
A subset of mammalian retinal ganglion cells expresses an opsin photopigment (melanopsin, Opn4) and is intrinsically photosensitive. The human retina contains melanopsin, but the literature lacks a direct investigation of its spectral sensitivity or G-protein selectivity. Here, we address this deficit by studying physiological responses driven by human melanopsin under heterologous expression in HEK293 cells. Luminescent reporters for common second messenger systems revealed that light induces a high amplitude increase in intracellular calcium and a modest reduction in cAMP in cells expressing human melanopsin, implying that this pigment is able to drive responses via both Gq and Gi/o class G-proteins. Melanopsins from mouse and amphioxus had a similar profile of G-protein coupling in HEK293 cells, but chicken Opn4m and Opn4x pigments exhibited some Gs activity in addition to a strong Gq/11 response. An action spectrum for the calcium response in cells expressing human melanopsin had the predicted form for an opsin : vitamin A1 pigment and peaked at 479 nm. The G-protein selectivity and spectral sensitivity of human melanopsin is similar to that previously described for rodents, supporting the utility of such laboratory animals for developing methods of manipulating this system using light or pharmacological agents.
Highlights
Animal photodetection relies on a class of G-protein-coupled receptors (GPCRs) composed of an apoprotein and associated light-sensitive chromophore
Melanopsins from a variety of species could be expressed in HEK293 cells at levels that were detectable by western blot and immunocytochemistry
In the case of human melanopsin, the majority of the protein produced was of expected size for a monomer, and much of it was localized to the cell membrane, with the remainder in small intracellular inclusions
Summary
Animal photodetection relies on a class of G-protein-coupled receptors (GPCRs) composed of an apoprotein (opsin) and associated light-sensitive chromophore (a retinaldehyde). These pigments initiate light-dependent intracellular cascades the nature of which are determined by the class of G-protein to which they couple. There are opsins which activate Ga subunits of the Gai/o (Gi) class, leading to a reduction in cyclic nucleotide concentrations [1,2,3]; Gas class, increasing in cyclic nucleotide concentrations [4]; and Gaq (Gaq/11) class, stimulating a phospholipase C-dependent cascade leading to a range of physiological events, including changes in intracellular calcium [5]. Melanopsin cells in mammals are specialized for measuring ambient illumination, contributing to visual discrimination [8,9], and driving a wide variety of physiological responses including, but not restricted to: synchronization of circadian clocks to light : dark cycles, regulation of pupil size, modulation of sleep and suppression of pineal melatonin production [10 –14]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: Proceedings of the Royal Society B: Biological Sciences
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
