Melanopsins: Localization and Phototransduction in Xenopus laevis Melanophores

Abstract

Xenopus laevis melanophores express two melanopsins, Opn4x and Opn4m. We identified Opn4x immunoreactivity throughout the melanophore cytoplasm and in the cell membrane. The strongest immunopositivity for Opn4m was observed in the nuclear region, and no labeling was seen in the cell membrane. This immunodistribution suggests Opn4x as the functional photopigment. In X. laevis melanophores, light triggers pigment dispersion and clock gene induction at blue wavelength, which maximally activates melanopsins. Although light stimulation activates phospholipase C and increases intracellular calcium and cGMP, this nucleotide does not participate in photo-induced melanin dispersion. Nevertheless, the guanylyl cyclase activator YC-1 stimulates Per1 expression, similar to blue light pulse, and the use of pharmacological inhibitors indicates the participation of the phosphoinositide cascade. Since cAMP levels does not change after blue light stimulation, the cAMP/PKA pathway most probably is not involved in blue light induction of Per in X. laevis melanophores. Given the localization of melanopsins and our pharmacological data, the light-induced clock gene expression seems to be mediated by Opn4x through phosphoinositide cascade and rise in cGMP, thus leading to the reset of the biological clock in our model.

OPN4 belongs to the photosensitive system of the human skin

“…Furthermore, OPN4 was also found to localize to the cytoplasm. This was similar to results for the melanophore of X. laevis (Moraes, Ramos, Poletini, & Castrucci, ) and in human brain cells (Nissilä et al, ).…”

“…Furthermore, OPN4 was also found to localize to the cytoplasm. This was similar to results for the melanophore of X. laevis (Moraes, Ramos, Poletini, & Castrucci, 2015) and in human brain cells (Nissilä et al, 2017). As the wavelength (488 nm) of B excitation light is similar to the active wavelength of OPN4, we performed live imaging experiments to directly confirm the Ca 2+ influx in human skin cultured cells upon BL (488 nm) irradiation.…”

Dexamethasone Modulates Nonvisual Opsins, Glucocorticoid Receptor, and Clock Genes inDanio rerioZEM-2S Cells

“…Control preparation incubated with preserum showed no immunolabeling (Figure 2(a)). Although it is commonly accepted that opsins are typically located in the cell membrane, similar results were also reported in Xenopus laevis melanophores [16]. These cells express two melanopsins, Opn4x and Opn4m , which are also differentially distributed in the cell: immunoreactivity for Opn4x was seen in the cytoplasm and cell membrane whereas Opn4m is strongly expressed in the nucleus [16].…”

“…Although it is commonly accepted that opsins are typically located in the cell membrane, similar results were also reported in Xenopus laevis melanophores [16]. These cells express two melanopsins, Opn4x and Opn4m , which are also differentially distributed in the cell: immunoreactivity for Opn4x was seen in the cytoplasm and cell membrane whereas Opn4m is strongly expressed in the nucleus [16]. The precise explanation for this distinct distribution is elusive and further investigation is needed to solve this question.…”

Seeing the light to change colour: An evolutionary perspective on the role of melanopsin in neuroendocrine circuits regulating light‐mediated skin pigmentation

“…While melanopsin might play a direct role in the primary response, the data to date are mainly correlative. More substantive evidence come from studies with a melanophore cell line established from stage 30–35 X. laevis tadpoles that shows Opn4x and Opn4m have a distinct subcellular localization, with Opn4x found in the cell membrane and cytoplasm, and Opn4m in the nucleus (Moraes, Ramos, Poletini, & Castrucci, ). In these cells, light induces melanosome dispersion with a maximum response at 460 nm (Daniolos, Lerner, & Lerner, ), and only when all trans retinal is present (Rollag, ), suggesting an opsin‐mediated response.…”