Melanopsin

Melanopsin was discovered by Ignacio Provencio as a new 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. 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. == Species distribution ==

Mammals have orthologous melanopsin genes named Opn4, which are approximately 50-55% conserved. Mammals lost the gene Opn4x relatively early in their evolution, leading to a general reduction in photosensory capability. The loss is estimated to have 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 Samer Hattar and colleagues. 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, rhodopsin and encephalopsin. Melanopsin, like all other animal opsins (e.g. rhodopsin), is a G-protein-coupled receptor (GPCR). The melanopsin protein has an extracellular N-terminal domain, an intracellular C-terminal domain, and seven alpha helices spanning through the plasma membrane. This lysine binds covalently retinal via a Schiff-base, which makes melanopsin light sensitive. In fact this is abolished if the lysine is replaced by an alanine. Melanopsin is more closely related to invertebrate visual opsins, which are rhabdomeric opsin, than to vertebrate visual opsins, which are cliary opsins. This is also reflected by the downstream signaling cascade, melanopsin couples in ipRGCs to the G-proteins G(q), G(11), and G(14), which are all of the G(q)-type. In fact, they can functionally replace each other, as a knocking out only two of them has no phenotypical effect. The G-proteins activate the phospholipase C PLCB4, Like other rhabdomeric opsins, Melanopsin has intrinsic photoisomerase activity. == Function ==

, a small percentage of which contain melanopsin. Light strikes the ganglia first, the rods and cones last. 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. 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 (closer to violet in the visible spectrum) are half as effective for melanopsin photoreceptor stimulation as light at 480 nm. The ipRGCs in the mammalian retina are one terminus of the retinohypothalamic tract that projects to the suprachiasmatic nucleus (SCN) 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. Melanopsin-containing ganglion cells are thought to influence these targets by releasing the neurotransmitters glutamate and pituitary adenylate cyclase activating polypeptide (PACAP) 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 (SCN) through the retinohypothalamic tract (RHT). 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. 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 (DA) is a factor in the regulation of melanopsin mRNA in ipRGCs. == Clinical significance ==

The discovery of the role of melanopsin in non-image forming vision has led to a growth in optogenetics. This field has shown promise in clinical applications, including the treatment of human eye diseases such as retinitis pigmentosa and diabetes. A missense mutation in Opn4, P10L, has been implicated in 5% of patients with Seasonal Affective Disorder (SAD). This is a condition in which people experience depressive thoughts in the winter due to decreased available light. Additionally, a melanopsin based receptor has been linked to migraine pain. Restoration of vision There has been recent research on the role of melanopsin in optogenetic therapy for patients with the degenerative eye disease retinitis pigmentosa (RP). Reintroducing functional melanopsin into the eyes of mice with retinal degeneration restores the pupillary light reflex (PLR). These same mice could also distinguish light stimuli from dark stimuli and showed increased sensitivity to room light. The higher sensitivity demonstrated by these mice shows promise for vision restoration that may be applicable to humans and human eye diseases. Control of sleep/wake patterns Melanopsin may aid in controlling sleep cycles and wakefulness. Tsunematsu and colleagues created transgenic mice that expressed melanopsin in hypothalamic orexin neurons. With a short 4-second pulse of blue light (guided by optical fibers), the transgenic mice could successfully transition from slow-wave sleep (SWS), which is commonly known as "deep sleep," to long-lasting wakefulness. After switching off the blue light, the hypothalamic orexin neurons showed activity for several tens of seconds. It has been shown that rods and cones play no role in the onset of sleep by light, distinguishing them from ipRGCs and melanopsin. This provides strong evidence that there is a link between ipRGCs in humans and alertness, particularly with high frequency light (e.g. blue light). Therefore, melanopsin can be used as a therapeutic target for controlling the sleep-wake cycle. Regulation of blood glucose levels In a paper published by Ye and colleagues in 2011, melanopsin was utilized to create an optogenetic synthetic transcription device that was tested in a therapeutic setting to produce Fc-glucagon-like peptide 1 (Fc-GLP-1), a fusion protein that helps control blood glucose levels in mammals with Type II Diabetes. The researchers subcutaneously implanted mice with microencapsulated transgenic HEK 293 cells that were cotransfected with two vectors including the melanopsin gene and the gene of interest under an NFAT (nuclear factor of activated T cells) promoter, respectively. It is through this engineered pathway that they successfully controlled the expression of Fc-GLP-1 in doubly recessive diabetic mice and reduced hyperglycemia, or high blood glucose levels, in these mice. This shows promise for the use of melanopsin as an optogenetic tool for the treatment of Type II diabetes. == See also ==