The expression of melanopsin and clock genes in Xenopus laevis melanophores and their modulation by melatonin

Abstract

Vertebrates have a central clock and also several peripheral clocks. Light responses might result from the integration of light signals by these clocks. The dermal melanophores of Xenopus laevis have a photoreceptor molecule denominated melanopsin (OPN4x). The mechanisms of the circadian clock involve positive and negative feedback. We hypothesize that these dermal melanophores also present peripheral clock characteristics. Using quantitative PCR, we analyzed the pattern of temporal expression of Opn4x and the clock genes Per1, Per2, Bmal1, and Clock in these cells subjected to a 14-h light:10-h dark (14L:10D) regime or constant darkness (DD). Also, in view of the physiological role of melatonin in the dermal melanophores of X. laevis, we determined whether melatonin modulates the expression of these clock genes. These genes show a time-dependent expression pattern when these cells are exposed to 14L:10D, which differs from the pattern observed under DD. Cells kept in DD for 5 days exhibited overall increased mRNA expression for Opn4x and Clock, and a lower expression for Per1, Per2, and Bmal1. When the cells were kept in DD for 5 days and treated with melatonin for 1 h, 24 h before extraction, the mRNA levels tended to decrease for Opn4x and Clock, did not change for Bmal1, and increased for Per1 and Per2 at different Zeitgeber times (ZT). Although these data are limited to one-day data collection, and therefore preliminary, we suggest that the dermal melanophores of X. laevis might have some characteristics of a peripheral clock, and that melatonin modulates, to a certain extent, melanopsin and clock gene expression.

Regulation of Melanopsins andPer1byα-MSH and Melatonin in PhotosensitiveXenopus laevisMelanophores

“…During the photophase, melatonin seems to promote a disruption of the light sensor —melanopsins in this model— represented by the dramatic inhibition of both photopigment expressions and loss of changes in the temporal profile (Figures 2(a), 2(b), 3(a), and 3(b)). This inhibitory effect of melatonin on gene expression was also seen after only 1 hour melatonin treatment of X. laevis melanophores in DD [61]. X. laevis melanophores, therefore, is an interesting model to study melatonin regulation of the circadian peripheral systems.…”

Melatonin modulates M4-type ganglion-cell photoreceptors

“…6A). Indeed, melatonin has been shown to downregulate melanopsin mRNA levels in Xenopus dermal melanophores (Bluhm et al, 2012). However, exogenous melatonin did not significantly alter the amplitude of M4 cells’ intrinsic light responses (Fig.…”

Melanopsins: Localization and Phototransduction in Xenopus laevis Melanophores

“…Besides the notable and visually detectable phenomenon of granule migration in the cytoplasm of X. laevis melanophores, these cells are a robust model to study peripheral clocks, as they encode photopigments (melanopsins) , clock genes and tyrosinase , an enzyme demonstrated to be clock regulated since its promoter has E‐box motifs in several species . Here, we detail the cell compartments where the melanopsin proteins are located, and demonstrate the role of key signaling molecules in the blue light evoked path to the clock molecular machinery.…”