Melanopsin
Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene Opn4. In the mammalian retina, there are two additional categories of opsins, both involved in the formation of visual images: rhodopsin and photopsin in the rod and cone photoreceptor cells, respectively.
In humans, melanopsin is found in intrinsically photosensitive retinal ganglion cells. It is also found in the iris of mice and primates. Melanopsin is also found in rats, amphioxus, and other chordates. ipRGCs are photoreceptor cells which are particularly sensitive to the absorption of short-wavelength visible light and communicate information directly to the area of the brain called the suprachiasmatic nucleus, also known as the central "body clock", in mammals. Melanopsin plays an important non-image-forming role in the setting of circadian rhythms as well as other functions. Mutations in the Opn4 gene can lead to clinical disorders, such as Seasonal Affective Disorder. According to one study, melanopsin has been found in eighteen sites in the human brain, intracellularly, in a granular pattern, in the cerebral cortex, the cerebellar cortex and several phylogenetically old regions, primarily in neuronal soma, not in nuclei. Melanopsin is also expressed in human cones. However, only 0.11% to 0.55% of human cones express melanopsin and are exclusively found in the peripheral regions of the retina. The human peripheral retina senses light at high intensities that is best explained by four different photopigment classes.
Discovery
Melanopsin was first discovered by Ignacio Provencio as a novel opsin in the melanophores, or light-sensitive skin cells, of the African clawed frog in 1998. A year later, researchers found that mice without any rods or cones, the cells involved in image-forming vision, still entrained to a light-dark cycle. This observation led to the conclusion that neither rods nor cones, located in the outer retina, are necessary for circadian entrainment and that a third class of photoreceptor exists in the mammalian eye. Provencio and colleagues then found in 2000 that melanopsin is also present in mouse retina, specifically in ganglion cells, and that it mediates non-visual photoreceptive tasks. Melanopsin was found to be encoded by Opn4 with orthologs in a variety of organisms.These retinal ganglion cells were found to be innately photosensitive, since they responded to light even while isolated, and were thus named intrinsically photosensitive Retinal Ganglion Cells. They constitute a third class of photoreceptor cells in the mammalian retina, besides the already known rods and cones, and were shown to be the principal conduit for light input to circadian photoentrainment. In fact, it was later demonstrated by Satchidananda Panda and colleagues that melanopsin pigment may be involved in entrainment of a circadian oscillator to light cycles in mammals since melanopsin was necessary for blind mice to respond to light.
Species distribution
Mammals have orthologous melanopsin genes named Opn4m, which are derived from one branch of the Opn4 family, and are approximately 50-55% conserved. However, non-mammalian vertebrates, including chickens and zebrafish, have another version of the melanopsin gene, Opn4x, which appears to have a distinct lineage that diverged from Opn4m about 360 million years ago. Mammals lost the gene Opn4x relatively early in their evolution, leading to a general reduction in photosensory capability. It is thought that this event can be explained by the fact that this occurred during the time in which nocturnal mammals were evolving.Structure
The human melanopsin gene, opn4, is expressed in ipRGCs, which comprises only 1-2% of RGCs in the inner mammalian retina, as studied by and colleagues. The gene spans approximately 11.8 kb and is mapped to the long arm of chromosome 10. The gene includes nine introns and ten exons compared to the four to seven exons typically found in other human opsins. In non-mammalian vertebrates, melanopsin is found in a wider subset of retinal cells, as well as in photosensitive structures outside the retina, such as the iris muscle of the eye, deep brain regions, the pineal gland, and the skin. Paralogs of Opn4 include OPN1LW, OPN1MW, RHO and OPN3 and were discovered by the Genome Project.Melanopsin, like all other animal opsins, is a member of the G-protein coupled receptor family. The melanopsin protein has seven alpha helices integrated in the plasma membrane, an N-terminal domain and a C-terminal domain. It resembles invertebrate opsins far more than vertebrate photopigments, especially in its amino acid sequence and downstream signaling cascade. Like invertebrate opsins, it appears to be a photopigment with intrinsic photoisomerase activity and signals through a G-protein of the Gq family.
Function
Melanopsin-containing ganglion cells, like rods and cones, exhibit both light and dark adaptation; they adjust their sensitivity according to the recent history of light exposure. However, while rods and cones are responsible for the reception of images, patterns, motion, and color, melanopsin-containing ipRGCs contribute to various reflexive responses of the brain and body to the presence of light.Evidence for melanopsin's physiological light detection has been tested in mice. A mouse cell line that is not normally photosensitive, Neuro-2a, is rendered light-sensitive by the addition of human melanopsin. The photoresponse is selectively sensitive to short-wavelength light, and has an intrinsic photoisomerase regeneration function that is chromatically shifted to longer wavelengths.
Melanopsin photoreceptors are sensitive to a range of wavelengths and reach peak light absorption at blue light wavelengths around 480 nanometers. Other wavelengths of light activate the melanopsin signaling system with decreasing efficiency as they move away from the optimum 480 nm. For example, shorter wavelengths around 445 nm are half as effective for melanopsin photoreceptor stimulation as light at 480 nm.
Melanopsin in the iris of some, primarily nocturnal, mammals closes the iris when it is exposed to light. This local pupil light reflex is absent from primates, even though their irises express melanopsin.
Mechanism
When light with an appropriate frequency enters the eye, it activates the melanopsin contained in intrinsically photosensitive retinal ganglion cells, triggering an action potential. These neuronal electrical signals travel through neuronal axons to specific brain targets, such as the center of pupillary control called the olivary pretectal nucleus of the midbrain. Consequently, stimulation of melanopsin in ipRGCs mediates behavioral and physiological responses to light, such as pupil constriction and inhibition of melatonin release from the pineal gland. The ipRGCs in the mammalian retina are one terminus of the retinohypothalamic tract that projects to the suprachiasmatic nucleus of the hypothalamus. The suprachiasmatic nucleus is sometimes described as the brain's "master clock", as it maintains the circadian rhythm, and nerve signals from ipRGCs to the SCN entrain the internal circadian rhythm to the rising and setting of the sun. The SCN also receives input from rods and cones through the retinohypothalamic tract, so information from all three photosensitive cell types in the mammalian retina are transmitted to the SCN.Melanopsin-containing ganglion cells are thought to influence these targets by releasing the neurotransmitters glutamate and pituitary adenylate cyclase activating polypeptide from their axon terminals. Melanopsin-containing ganglion cells also receive input from rods and cones that can add to the input to these pathways.
Effects on circadian rhythm
Melanopsin serves an important role in the photoentrainment of circadian rhythms in mammals. An organism that is photoentrained has aligned its activity to an approximately 24-hour cycle, the solar cycle on Earth. In mammals, melanopsin expressing axons target the suprachiasmatic nucleus through the retinohypothalamic tract.In mammals, the eye is the main photosensitive organ for the transmission of light signals to the brain. However, blind humans are still able to entrain to the environmental light-dark cycle, despite having no conscious perception of the light. One study exposed subjects to bright light for a prolonged duration of time and measured their melatonin concentrations. Melatonin was not only suppressed in visually unimpaired humans, but also in blind participants, suggesting that the photic pathway used by the circadian system is functionally intact despite blindness. Therefore, physicians no longer practice enucleation of blind patients, or removal of the eyes at birth, since the eyes play a critical role in the photoentrainment of the circadian pacemaker.
In mutant breeds of mice that lacked only rods, only cones, or both rods and cones, all breeds of mice still entrained to changing light stimuli in the environment, but with a limited response, suggesting that rods and cones are not necessary for circadian photoentrainment and that the mammalian eye must have another photopigment required for the regulation of the circadian clock.
Melanopsin-knockout mice display reduced photoentrainment. In comparison to wild-type mice that expressed melanopsin normally, deficits in light-induced phase shifts in locomotion activity were noted in melanopsin-null mice. These melanopsin-deficient mice did not completely lose their circadian rhythms, as they were still able to entrain to changing environmental stimuli, albeit more slowly than normal. This indicated that, although melanopsin is sufficient for entrainment, it must work in conjunction with other photopigments for normal photoentrainment activity. Triple-mutant mice that were rod-less, cone-less, and melanopsin-less display a complete loss in the circadian rhythms, so all three photopigments in these photoreceptors, rhodopsin, photopsin and melanopsin, are necessary for photoentrainment. Therefore, there is a functional redundancy between the three photopigments in the photoentrainment pathway of mammals. Deletion of only one photopigment does not eliminate the organism's ability to entrain to environmental light-dark cycles, but it does reduce the intensity of the response.
Regulation
Melanopsin undergoes phosphorylation on its intracellular carboxy tail as a way to deactivate its function. Compared to other opsins, melanopsin has an unusually long carboxy tail that contains 37 serine and threonine amino acid sites that could undergo phosphorylation. However, a cluster of seven amino acids are sufficient to deactivate zebrafish melanopsin. These sites are dephosphorylated when melanopsin is exposed to light and are unique from those that regulate rhodopsin. They are important for proper response to calcium ions in ipRGCs; lack of functional phosphorylation sites, particularly at serine-381 and serine-398, reduce the cell's response to light-induced calcium ion influx when voltage-gated calcium ion channels open.In terms of the gene Opn4, Dopamine is a factor in the regulation of melanopsin mRNA in ipRGCs.