Melanopsin-Encoded Response Properties of Intrinsically Photosensitive Retinal Ganglion Cells

Abstract

SUMMARY Melanopsin photopigment expressed in intrinsically photosensitive retinal ganglion cells (ipRGCs) plays a crucial role in the adaptation of mammals to their ambient light environment through both image-forming and non-image-forming (NIF) visual responses. ipRGCs are structurally and functionally distinct from classical rod/cone photoreceptors and have unique properties including single-photon response, long response latency, photon integration over time, and slow deactivation. Here we discovered that amino-acid sequence features of melanopsin protein contribute to the functional properties of the ipRGCs. Phosphorylation of a cluster of Ser/Thr residues in the C-terminal cytoplasmic region of melanopsin contributes to deactivation, which in turn determines response latency and threshold sensitivity of the ipRGCs. The poorly conserved region distal to phosphorylation cluster inhibits functional role of phosphorylation, thereby constituting a unique delayed-deactivation mechanism. Concerted action of both regions sustains responses to dim light, allows for the integration of light over time, and results in precise signal duration.

The C-terminus and Third Cytoplasmic Loop Cooperatively Activate Mouse Melanopsin Phototransduction

“…Calcium imaging of this mutant reveals delayed activation kinetics (Figure 4A) compared to wildtype melanopsin, supporting a functional role of this C-terminal region in mediating signaling activation. Previous studies that examined melanopsin C-terminal truncations (29) suggest that the distal C-terminus (after D396) doesn’t contribute to the production of rapid signaling activation and can potentially sterically hinder the G-protein from accessing the cytoplasmic side of the receptor (32). To disrupt the predicted proximal C-terminal conformation, we introduced proline mutations (H377P L380P Y382P) in the critical region predicted to work synergistically with the cytoplasmic loops to regulate phototransduction activation.…”

“…Mouse melanopsin’s C-terminus possesses 38 serine and threonine residues (Figure 1B) that may serve as potential phosphorylation sites. It is well established that melanopsin C-terminal phosphorylation is critical for signaling deactivation and the lifetime of melanopsin-driven behaviors such as pupil constriction and jet-lag photoentrainment (28, 29, 31, 32). Given the enrichment of serine and threonine residues at the proximal region (from residues H351-T385) of mouse melanopsin’s C-terminus, we tested if phosphorylation of these sites contributes to regulation of phototransduction activation, in addition to deactivation.…”

“…GPCR regions important for G-protein binding selectivity include the 2 nd and 3 rd intracellular loops, and cytoplasmic extensions of transmembrane helices 5 and 6, which form critical contacts with helices 4 and 5 on Gα (26). GPCR C-termini can regulate G-protein activation as well as serve as substrates for GPCR Kinase (GRK) to mediate arrestin binding (for melanopsin C-terminal phosphorylation and deactivation, see 27, 28, 29, 30, 31, 32, 33). Structural analysis of rhodopsin—G-protein complexes reveals contacts between helix 8 on rhodopsin’s C-terminus and the C-terminal helix 5 on Gαi (34) or transducin (35), and contacts have been found on rhodopsin’s C-terminus with Gβ when in complex with Gαi (36).…”

Light‐Controlled Mammalian Cells and Their Therapeutic Applications in Synthetic Biology

“…These membrane-bound GPC photoreceptors are retinal-dependent. [45] During signal transduction, exposure to light induces isomerization of the retinal chromophore from 11-cis-retinal to all-trans-retinal, which subsequently changes the conformation of the melanopsin receptor [38] (Figure 1b). While microbes incorporate mainly all-transretinal that is activated to 13-cis-retinal in melanopsin, mammals use 11-cis-retinal that is transformed into its all-trans-conformer upon illumination.…”

Ectopic Expression of Mouse Melanopsin in Drosophila Photoreceptors Reveals Fast Response Kinetics and Persistent Dark Excitation

“…in the Limulus (45)), a relatively small net amount of photopigment molecules can be shifted from one dark stable pigment state to the other. In contrast, in a tristable photopigment system, even when large spectral overlap exists between the photopigment 11-cis and all-trans states, a considerable amount of photopigment can be shifted between the 11-cis and all-trans pigment states (5,46). In opn4;ninaE I17 photoreceptors, when maximal intensity blue (ϳ430 nm) light was applied to dark-raised opn4; ninaE I17 flies, in some cells with relatively large peak LIC amplitude, the blue illuminated cells maintained their current response to the blue light, long after light off (Fig.…”