Visual phototransduction

Last updated

Visual phototransduction is the sensory transduction process of the visual system by which light is detected by photoreceptor cells (rods and cones) in the vertebrate retina. A photon is absorbed by a retinal chromophore (each bound to an opsin), which initiates a signal cascade through several intermediate cells, then through the retinal ganglion cells (RGCs) comprising the optic nerve.

Contents

Overview

Light enters the eye, passes through the optical media, then the inner neural layers of the retina before finally reaching the photoreceptor cells in the outer layer of the retina. The light may be absorbed by a chromophore bound to an opsin, which photoisomerizes the chromophore, initiating both the visual cycle, which "resets" the chromophore, and the phototransduction cascade, which transmits the visual signal to the brain. The cascade begins with graded polarization (an analog signal) of the excited photoreceptor cell, as its membrane potential increases from a resting potential of −70 mV, proportional to the light intensity. At rest, the photoreceptor cells are continually releasing glutamate at the synaptic terminal to maintain the potential. [1] The transmitter release rate is lowered (hyperpolarization) as light intensity increases. Each synaptic terminal makes up to 500 contacts with horizontal cells and bipolar cells. [1] These intermediate cells (along with amacrine cells) perform comparisons of photoreceptor signals within a receptive field, but their precise functionalities are not well understood. The signal remains as a graded polarization in all cells until it reaches the RGCs, where it is converted to an action potential and transmitted to the brain. [1]

Photoreceptors

The photoreceptor cells involved in vertebrate vision are the rods, the cones, and the photosensitive ganglion cells (ipRGCs). These cells contain a chromophore (11-cis-retinal, the aldehyde of vitamin A1 and light-absorbing portion) that is bound to a cell membrane protein, opsin. Rods are responsible for vision under low light intensity and contrast detections. Because they all have the same response across frequencies, no colour information can be deduced from the rods only, as in low light conditions for example. Cones, on the other hand, are of different kinds with different frequency response, such that colour can be perceived through comparison of the outputs of different kinds of cones. Each cone type responds best to certain wavelengths, or colours, of light because each type has a slightly different opsin. The three types of cones are L-cones, M-cones and S-cones that respond optimally to long wavelengths (reddish colour), medium wavelengths (greenish colour), and short wavelengths (bluish colour) respectively. Humans have trichromatic photopic vision consisting of three opponent process channels that enable colour vision. [2] Rod photoreceptors are the most common cell type in the retina and develop quite late. Most cells become postmitotic before birth, but differentiation occurs after birth. In the first week after birth, cells mature and the eye becomes fully functional at the time of opening. The visual pigment rhodopsin (rho) is the first known sign of differentiation in rods. [3]

Transduction process

To understand the photoreceptor's behaviour to light intensities, it is necessary to understand the roles of different currents.

There is an ongoing outward potassium current through nongated K+-selective channels. This outward current tends to hyperpolarize the photoreceptor at around −70 mV (the equilibrium potential for K+).

There is also an inward sodium current carried by cGMP-gated sodium channels. This "dark current" depolarizes the cell to around −40 mV. This is significantly more depolarized than most other neurons.

A high density of Na+-K+ pumps enables the photoreceptor to maintain a steady intracellular concentration of Na+ and K+.

When light intensity increases, the potential of the membrane decreases (hyperpolarization). Because as the intensity increases, the release of the stimulating neurotransmitter glutamate of the photoreceptors is reduced. When light intensity decreases, that is, in the dark environment, glutamate release by photoreceptors increases. This increases the membrane potential and produces membrane depolarization. [1]

In the dark

Photoreceptor cells are unusual cells in that they depolarize in response to absence of stimuli or scotopic conditions (darkness). In photopic conditions (light), photoreceptors hyperpolarize to a potential of −60 mV.

In the dark, cGMP levels are high and keep cGMP-gated sodium channels open allowing a steady inward current, called the dark current. This dark current keeps the cell depolarized at about −40 mV, leading to glutamate release which inhibits excitation of neurons.

The depolarization of the cell membrane in scotopic conditions opens voltage-gated calcium channels. An increased intracellular concentration of Ca2+ causes vesicles containing glutamate, a neurotransmitter, to merge with the cell membrane, therefore releasing glutamate into the synaptic cleft, an area between the end of one cell and the beginning of another neuron. Glutamate, though usually excitatory, functions here as an inhibitory neurotransmitter.

In the cone pathway, glutamate:

In the light

In summary: Light closes cGMP-gated sodium channels, reducing the influx of both Na+ and Ca2+ ions. Stopping the influx of Na+ ions effectively switches off the dark current. Reducing this dark current causes the photoreceptor to hyperpolarise, which reduces glutamate release which thus reduces the inhibition of retinal nerves, leading to excitation of these nerves. This reduced Ca2+ influx during phototransduction enables deactivation and recovery from phototransduction, as discussed below in § Deactivation of the phototransduction cascade.

Representation of molecular steps in photoactivation (modified from Leskov et al., 2000 ). Depicted is an outer membrane disk in a rod. Step 1: Incident photon (hn) is absorbed and activates a rhodopsin (likewise photopsin) by conformational change in the disk membrane to R*. Step 2: Next, R* makes repeated contacts with transducin molecules, catalysing its activation to G* by the release of bound GDP in exchange for cytoplasmic GTP, which expels its b and g subunits. Step 3: G* binds inhibitory g subunits of the phosphodiesterase (PDE), activating its a and b subunits. Step 4: Activated PDE hydrolyses cGMP. Step 5: Guanylyl cyclase (GC) synthesizes cGMP, the second messenger in the phototransduction cascade. Reduced levels of cytosolic cGMP cause cyclic nucleotide-gated channels to close, preventing further influx of Na and Ca . Phototransduction.png
Representation of molecular steps in photoactivation (modified from Leskov et al., 2000 ). Depicted is an outer membrane disk in a rod. Step 1: Incident photon (hν) is absorbed and activates a rhodopsin (likewise photopsin) by conformational change in the disk membrane to R*. Step 2: Next, R* makes repeated contacts with transducin molecules, catalysing its activation to G* by the release of bound GDP in exchange for cytoplasmic GTP, which expels its β and γ subunits. Step 3: G* binds inhibitory γ subunits of the phosphodiesterase (PDE), activating its α and β subunits. Step 4: Activated PDE hydrolyses cGMP. Step 5: Guanylyl cyclase (GC) synthesizes cGMP, the second messenger in the phototransduction cascade. Reduced levels of cytosolic cGMP cause cyclic nucleotide-gated channels to close, preventing further influx of Na and Ca .
  1. A photon interacts with a retinal molecule in an opsin complex in a photoreceptor cell. The retinal undergoes isomerization, changing from the 11-cis-retinal to the all-trans-retinal configuration.
  2. Opsin therefore undergoes a conformational change to metarhodopsin II.
  3. Metarhodopsin II activates a G protein known as transducin. This causes transducin to dissociate from its bound GDP, and bind GTP; then the alpha subunit of transducin dissociates from the beta and gamma subunits, with the GTP still bound to the alpha subunit.
  4. The alpha subunit-GTP complex activates phosphodiesterase, also known as PDE6. It binds to one of two regulatory subunits of PDE (which itself is a tetramer) and stimulates its activity.
  5. PDE hydrolyses cGMP, forming GMP. This lowers the intracellular concentration of cGMP and therefore the sodium channels close. [5]
  6. Closure of the sodium channels causes hyperpolarization of the cell due to the ongoing efflux of potassium ions.
  7. Hyperpolarization of the cell causes voltage-gated calcium channels to close.
  8. As the calcium level in the photoreceptor cell drops, the amount of the neurotransmitter glutamate that is released by the cell also drops. This is because calcium is required for the glutamate-containing vesicles to fuse with cell membrane and release their contents (see SNARE proteins).
  9. A decrease in the amount of glutamate released by the photoreceptors causes depolarization of on-centre bipolar cells (rod and cone On bipolar cells) and hyperpolarization of cone off-centre bipolar cells.

Deactivation of the phototransduction cascade

In light, low cGMP levels close Na+ and Ca2+ channels, reducing intracellular Na+ and Ca2+. During recovery (dark adaptation), the low Ca2+ levels induce recovery (termination of the phototransduction cascade), as follows:

  1. Low intracellular Ca2+ causes Ca2+ to dissociate from guanylate cyclase activating protein (GCAP). The liberated GCAP ultimately restores depleted cGMP levels, which re-opens the cGMP-gated cation channels (restoring dark current).
  2. Low intracellular Ca2+ causes Ca2+ to dissociate from GTPase-activating protein (GAP), also known as regulator of G protein signalling. The liberated GAP deactivates transducin, terminating the phototransduction cascade (restoring dark current).
  3. Low intracellular Ca2+ makes intracellular Ca-recoverin-RK dissociate into Ca2+ and recoverin and rhodopsin kinase (RK). The liberated RK then phosphorylates the Metarhodopsin II, reducing its binding affinity for transducin. Arrestin then completely deactivates the phosphorylated-metarhodopsin II, terminating the phototransduction cascade (restoring dark current).
  4. Low intracellular Ca2+ make the Ca2+/calmodulin complex within the cGMP-gated cation channels more sensitive to low cGMP levels (thereby, keeping the cGMP-gated cation channel open even at low cGMP levels, restoring dark current) [6]

In more detail:

GTPase Accelerating Protein (GAP) of RGS (regulators of G protein signalling) interacts with the alpha subunit of transducin, and causes it to hydrolyse its bound GTP to GDP, and thus halts the action of phosphodiesterase, stopping the transformation of cGMP to GMP. This deactivation step of the phototransduction cascade (the deactivation of the G protein transducer) was found to be the rate limiting step in the deactivation of the phototransduction cascade. [7]

In other words: Guanylate Cyclase Activating Protein (GCAP) is a calcium binding protein, and as the calcium levels in the cell have decreased, GCAP dissociates from its bound calcium ions, and interacts with Guanylate Cyclase, activating it. Guanylate Cyclase then proceeds to transform GTP to cGMP, replenishing the cell's cGMP levels and thus reopening the sodium channels that were closed during phototransduction.

Finally, Metarhodopsin II is deactivated. Recoverin, another calcium binding protein, is normally bound to Rhodopsin Kinase when calcium is present. When the calcium levels fall during phototransduction, the calcium dissociates from recoverin, and rhodopsin kinase is released and phosphorylates metarhodopsin II, which decreases its affinity for transducin. Finally, arrestin, another protein, binds the phosphorylated metarhodopsin II, completely deactivating it. Thus, finally, phototransduction is deactivated, and the dark current and glutamate release is restored. It is this pathway, where Metarhodopsin II is phosphorylated and bound to arrestin and thus deactivated, which is thought to be responsible for the S2 component of dark adaptation. The S2 component represents a linear section of the dark adaptation function present at the beginning of dark adaptation for all bleaching intensities.

Visual cycle

The absorption of light leads to an isomeric change in the retinal molecule. 1415 Retinal Isomers.jpg
The absorption of light leads to an isomeric change in the retinal molecule.

The visual cycle occurs via G-protein coupled receptors called retinylidene proteins which consists of a visual opsin and a chromophore 11-cis-retinal. The 11-cis-retinal is covalently linked to the opsin receptor via Schiff base. When it absorbs a photon, 11-cis-retinal undergoes photoisomerization to all-trans-retinal, which changes the conformation of the opsin GPCR leading to signal transduction cascades which causes closure of cyclic GMP-gated cation channel, and hyperpolarization of the photoreceptor cell. Following photoisomerization, all-trans-retinal is released from the opsin protein and reduced to all-trans-retinol, which travels to the retinal pigment epithelium to be "recharged". It is first esterified by lecithin retinol acyltransferase (LRAT) and then converted to 11-cis-retinol by the isomerohydrolase RPE65. The isomerase activity of RPE65 has been shown; it is uncertain whether it also acts as the hydrolase. [8] Finally, it is oxidized to 11-cis-retinal before travelling back to the photoreceptor cell outer segment where it is again conjugated to an opsin to form new, functional visual pigment (retinylidene protein), namely photopsin or rhodopsin.

In invertebrates

Visual phototransduction in invertebrates like the fruit fly differs from that of vertebrates, described up to now. The primary basis of invertebrate phototransduction is the PI(4,5)P2 cycle. Here, light induces the conformational change into rhodopsin and converts it into meta-rhodopsin. This helps in dissociation of G-protein complex. Alpha sub-unit of this complex activates the PLC enzyme (PLC-beta) which hydrolyse the PIP2 into DAG. This hydrolysis leads to opening of TRP channels and influx of calcium.[ citation needed ]

Invertebrate photoreceptor cells differ morphologically and physiologically from their vertebrate counterparts. Visual stimulation in vertebrates causes a hyperpolarization (weakening) of the photoreceptor membrane potential, whereas invertebrates experience a depolarization with light intensity. Single-photon events produced under identical conditions in invertebrates differ from vertebrates in time course and size. Likewise, multi-photon events are longer than single-photon responses in invertebrates. However, in vertebrates, the multi-photon response is similar to the single-photon response. Both phyla have light adaptation and single-photon events are smaller and faster. Calcium plays an important role in this adaptation. Light adaptation in vertebrates is primarily attributable to calcium feedback, but in invertebrates cyclic AMP is another control on dark adaptation. [9] [ verification needed ]

Opsins and spectral sensitivity

The arrangement of the photoreceptor cells in a pale and yellow ommatidia of Drosophila melanogaster: The top row shows two of the six outer photoreceptor cells (R1-R6) and the inner R7 and R8 cells. The bottom row shows the different opsins (Rh1, Rh3, Rh4, Rh5, and Rh6) the cells express. Figure from Sharkey et al. (2020). Arrangement of photoreceptor cells with their opsins in the Drosophila melanogaster omatidium.png
The arrangement of the photoreceptor cells in a pale and yellow ommatidia of Drosophila melanogaster: The top row shows two of the six outer photoreceptor cells (R1-R6) and the inner R7 and R8 cells. The bottom row shows the different opsins (Rh1, Rh3, Rh4, Rh5, and Rh6) the cells express. Figure from Sharkey et al. (2020).
Expression of the opsin Rh1 in the photoreceptor cells R1-R6 Expression of Rhodopsin-1 in Drosophila eye.jpg
Expression of the opsin Rh1 in the photoreceptor cells R1-R6

The genome of Drosophila encodes seven opsins, [11] five of those are expressed in the omatidia of the eye. The photoreceptor cells R1-R6 express the opsin Rh1, [12] which absorbs maximally blue light (around 480 nm), [13] [14] [15] however the R1-R6 cells cover a broader range of the spectrum than an opsin would allow due to a sensitising pigment [16] [17] that adds two sensitivity maxima in the UV-range (355 and 370 nm). [15] The R7 cells come in two types with yellow and pale rhabdomeres (R7y and R7p). [18] [19] The pale R7p cells express the opsin Rh3, [20] [21] which maximally absorbs UV-light (345 nm). [22] The R7p cells are strictly paired with the R8p cells that express Rh5, [21] which maximally absorbs violet light (437 nm). [15] The other, the yellow R7y cells express a blue-absorbing screening pigment [18] and the opsin Rh4, [23] which maximally absorbs UV-light (375 nm). [22] The R7y cells are strictly paired with R8y cells that express Rh6, [24] which maximally absorbs UV-light (508 nm). [15] In a subset of omatidia both R7 and R8 cells express the opsin Rh3. [21]

However, these absorption maxima of the opsins where measured in white eyed flies without screening pigments (Rh3-Rh6), [22] [15] or from the isolated opsin directly (Rh1). [13] Those pigments reduce the light that reaches the opsins depending on the wavelength. Thus in fully pigmented flies, the effective absorption maxima of opsins differs and thus also the sensitivity of their photoreceptor cells. With screening pigment, the opsin Rh3 is short wave shifted from 345 nm [a] to 330 nm and Rh4 from 375 nm to 355 nm. Whether screening pigment is present does not make a practical difference for the opsin Rh5 (435 nm and 437 nm), while the opsin R6 is long wave shifted by 92 nm from 508 nm to 600 nm. [10]

Additionally of the opsins of the eye, Drosophila has two more opsins: The ocelli express the opsin Rh2, [25] [26] which maximally absorbs violet light (~420 nm). [26] And the opsin Rh7, which maximally absorbs UV-light (350 nm) with an unusually long wavelength tail up to 500 nm. The long tail disappears if a lysine at position 90 is replaced by glutamic acid. This mutant then absorbs maximally violet light (450 nm). [27] The opsin Rh7 entrains with cryptochrome the circadian rhythm of Drosophila to the day-night-cycle in the central pacemaker neurons. [28]

Each Drosophila opsin binds the carotenoid chromophore 11-cis-3-hydroxyretinal via a lysine. [29] [30] This lysine is conserved in almost all opsins, only a few opsins have lost it during evolution. [31] Opsins without it are not light sensitive. [32] [33] [34] In particular, the Drosophila opsins Rh1, Rh4, and Rh7 function not only as photoreceptors, but also as chemoreceptors for aristolochic acid. These opsins still have the lysine like other opsins. However, if it is replaced by an arginine in Rh1, then Rh1 loses light sensitivity but still responds to aristolochic acid. Thus, the lysine is not needed for Rh1 to function as chemoreceptor. [33]

Phototransduction

As in vertebrate vision, visual transduction in invertebrates occurs via a G protein-coupled pathway. However, in vertebrates, the G protein is transducin, while the G protein in invertebrates is Gq (dgq in Drosophila). When rhodopsin (Rh) absorbs a photon of light its chromophore, 11-cis-3-hydroxyretinal, is isomerized to all-trans-3-hydroxyretinal. Rh undergoes a conformational change into its active form, metarhodopsin. Metarhodopsin activates Gq, which in turn activates a phospholipase Cβ (PLCβ) known as NorpA. [35]

PLCβ hydrolyses phosphatidylinositol (4,5)-bisphosphate (PIP2), a phospholipid found in the cell membrane, into soluble inositol triphosphate (IP3) and diacylglycerol (DAG), which stays in the cell membrane. DAG, a derivative of DAG, or PIP2 depletion cause a calcium-selective ion channel known as transient receptor potential (TRP) to open and calcium and sodium flows into the cell. [36] IP3 is thought to bind to IP3 receptors in the subrhabdomeric cisternae, an extension of the endoplasmic reticulum, and cause release of calcium, but this process does not seem to be essential for normal vision. [35]

Calcium binds to proteins such as calmodulin (CaM) and an eye-specific protein kinase C (PKC) known as InaC. These proteins interact with other proteins and have been shown to be necessary for shut off of the light response. In addition, proteins called arrestins bind metarhodopsin and prevent it from activating more Gq. A sodium-calcium exchanger known as CalX pumps the calcium out of the cell. It uses the inward sodium gradient to export calcium at a stoichiometry of 3 Na+/ 1 Ca++. [37]

TRP, InaC, and PLC form a signalling complex by binding a scaffolding protein called InaD. InaD contains five binding domains called PDZ domain proteins, which specifically bind the C termini of target proteins. Disruption of the complex by mutations in either the PDZ domains or the target proteins reduces the efficiency of signalling. For example, disruption of the interaction between InaC, the protein kinase C, and InaD results in a delay in inactivation of the light response.

Unlike vertebrate metarhodopsin, invertebrate metarhodopsin can be converted back into rhodopsin by absorbing a photon of orange light (580 nm).

Notes

  1. ↑ Sharkey et al. [10] give the absorption maximum of Rh3 as 334 nm in their result section. However, in the introduction and the material and methods section they give it as 345 nm. For both values, they cite Feiler et al., who reported 345 nm only. [22] Therefore, this seems to be a mistake and they probably meant there 345 nm, too.

References

  1. 1 2 3 4 Bertalmío, Marcelo (2020). "The biological basis of vision: the retina". Vision Models for High Dynamic Range and Wide Colour Gamut Imaging: 11–46. doi:10.1016/B978-0-12-813894-6.00007-7. ISBN   978-0-12-813894-6. S2CID   209571302.
  2. ↑ Ebrey, Thomas; Koutalos, Yiannis (January 2001). "Vertebrate Photoreceptors". Progress in Retinal and Eye Research. 20 (1): 49–94. doi:10.1016/S1350-9462(00)00014-8. PMID   11070368. S2CID   2789591.
  3. ↑ E.Y. Popova; C.J. Barnstable;(2019). "Chapter 15 - Insights Into the Epigenetics of Retinal Development and Diseases". https://doi.org/10.1016/B978-0-12-814879-2.00016-9
  4. ↑ Leskov, Ilya; Klenchin, Handy, Whitlock, Govardovskii, Bownds, Lamb, Pugh, Arshavsky (September 2000). "The Gain of Rod Phototransduction: Reconciliation of Biochemical and Electrophysiological Measurements". Neuron. 27 (3): 525–537. doi: 10.1016/S0896-6273(00)00063-5 . PMID   11055435. S2CID   15573966.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ↑ Arshavsky, Vadim Y.; Lamb, Trevor D.; Pugh, Edward N. (2002). "G Proteins and Phototransduction". Annual Review of Physiology. 64 (1): 153–187. doi:10.1146/annurev.physiol.64.082701.102229. PMID   11826267.
  6. ↑ Hsu, Yi-Te; Molday, Robert S. (1993). "Modulation of the CGMP-gated channel of rod photoreceptor cells by calmodulin". Nature. 361 (6407): 76–79. Bibcode:1993Natur.361...76H. doi:10.1038/361076a0. PMID   7678445. S2CID   4362581.
  7. ↑ Krispel, CM; Chen, D; Melling, D; Chen, YJ; Martemyanov, KA; Quillinan, N; Arshavsky, VY; Wensel, TG; Chen, CK; Burns, ME (2006). "RGS expression rate-limits recovery of rod photoresponses". Neuron. 51 (4): 409–416. Bibcode:2006Neuro.51...409K. doi: 10.1016/j.neuron.2006.07.010 . PMID   16908407..
  8. ↑ Moiseyev, Gennadiy; Chen, Ying; Takahashi, Yusuke; Wu, Bill X.; Ma, Jian-xing (30 August 2005). "RPE65 is the isomerohydrolase in the retinoid visual cycle". Proceedings of the National Academy of Sciences. 102 (35): 12413–12418. Bibcode:2005PNAS..10212413M. doi: 10.1073/pnas.0503460102 . PMC   1194921 . PMID   16116091.
  9. ↑ Rayer, B.; Naynert, M.; Stieve, H. (November 1990). "New trends in photobiology". Journal of Photochemistry and Photobiology B: Biology. 7 (2–4): 107–148. doi:10.1016/1011-1344(90)85151-L. PMID   2150859.
  10. 1 2 3 4 Sharkey CR, Blanco J, Leibowitz MM, Pinto-Benito D, Wardill TJ (October 2020). "The spectral sensitivity of Drosophila photoreceptors". Scientific Reports. 10 (1): 18242. Bibcode:2020NatSR..1018242S. doi:10.1038/s41598-020-74742-1. PMC   7588446 . PMID   33106518. S2CID   215551298. CC-BY icon.svg Material was copied and adapted from this source, which is available under a Creative Commons Attribution 4.0 International License.
  11. ↑ Feuda R, Goulty M, Zadra N, Gasparetti T, Rosato E, Pisani D, et al. (August 2021). "Phylogenomics of Opsin Genes in Diptera Reveals Lineage-Specific Events and Contrasting Evolutionary Dynamics in Anopheles and Drosophila". Genome Biology and Evolution. 13 (8): evab170. doi:10.1093/gbe/evab170. PMC   8369074 . PMID   34270718.
  12. ↑ Harris WA, Stark WS, Walker JA (April 1976). "Genetic dissection of the photoreceptor system in the compound eye of Drosophila melanogaster". The Journal of Physiology. 256 (2): 415–439. doi:10.1113/jphysiol.1976.sp011331. PMC   1309314 . PMID   16992509. S2CID   45888026.
  13. 1 2 Ostroy SE, Wilson M, Pak WL (August 1974). "Drosophila rhodopsin: photochemistry, extraction and differences in the norp AP12 phototransduction mutant". Biochemical and Biophysical Research Communications. 59 (3): 960–966. doi:10.1016/s0006-291x(74)80073-2. PMID   4213042.
  14. ↑ Ostroy SE (November 1978). "Characteristics of Drosophila rhodopsin in wild-type and norpA vision transduction mutants". The Journal of General Physiology. 72 (5): 717–732. doi:10.1085/jgp.72.5.717. PMC   2228556 . PMID   105082. S2CID   5802525.
  15. 1 2 3 4 5 6 7 Salcedo E, Huber A, Henrich S, Chadwell LV, Chou WH, Paulsen R, Britt SG (December 1999). "Blue- and green-absorbing visual pigments of Drosophila: ectopic expression and physiological characterization of the R8 photoreceptor cell-specific Rh5 and Rh6 rhodopsins". The Journal of Neuroscience. 19 (24): 10716–10726. doi:10.1523/jneurosci.19-24-10716.1999. PMC   6784940 . PMID   10594055. S2CID   17575850.
  16. ↑ Kirschfeld K, Franceschini N, Minke B (September 1977). "Evidence for a sensitising pigment in fly photoreceptors". Nature. 269 (5627): 386–390. Bibcode:1977Natur.269..386K. doi:10.1038/269386a0. PMID   909585. S2CID   28890008.
  17. ↑ Minke B, Kirschfeld K (May 1979). "The contribution of a sensitizing pigment to the photosensitivity spectra of fly rhodopsin and metarhodopsin". The Journal of General Physiology. 73 (5): 517–540. doi:10.1085/jgp.73.5.517. PMC   2215190 . PMID   458418. S2CID   12451748.
  18. 1 2 Kirschfeld K, Feiler R, Franceschini N (1978). "A photostable pigment within the rhabdomere of fly photoreceptors no. 7". Journal of Comparative Physiology A. 125 (3): 275–284. doi:10.1007/BF00656606. S2CID   40233531.
  19. ↑ Kirschfeld K, Franceschini N (June 1977). "Photostable pigments within the membrane of photoreceptors and their possible role". Biophysics of Structure and Mechanism. 3 (2): 191–194. doi:10.1007/BF00535818. PMID   890056. S2CID   5846094.
  20. ↑ Zuker CS, Montell C, Jones K, Laverty T, Rubin GM (May 1987). "A rhodopsin gene expressed in photoreceptor cell R7 of the Drosophila eye: homologies with other signal-transducing molecules". The Journal of Neuroscience. 7 (5): 1550–1557. doi:10.1523/jneurosci.07-05-01550.1987. PMC   6568820 . PMID   2437266. S2CID   1490332.
  21. 1 2 3 Chou WH, Hall KJ, Wilson DB, Wideman CL, Townson SM, Chadwell LV, Britt SG (December 1996). "Identification of a novel Drosophila opsin reveals specific patterning of the R7 and R8 photoreceptor cells". Neuron. 17 (6): 1101–1115. doi: 10.1016/s0896-6273(00)80243-3 . PMID   8982159. S2CID   18294965.
  22. 1 2 3 4 Feiler R, Bjornson R, Kirschfeld K, Mismer D, Rubin GM, Smith DP, et al. (October 1992). "Ectopic expression of ultraviolet-rhodopsins in the blue photoreceptor cells of Drosophila: visual physiology and photochemistry of transgenic animals". The Journal of Neuroscience. 12 (10): 3862–3868. doi:10.1523/jneurosci.12-10-03862.1992. PMC   6575971 . PMID   1403087.
  23. ↑ Montell C, Jones K, Zuker C, Rubin G (May 1987). "A second opsin gene expressed in the ultraviolet-sensitive R7 photoreceptor cells of Drosophila melanogaster". The Journal of Neuroscience. 7 (5): 1558–1566. doi:10.1523/JNEUROSCI.07-05-01558.1987. PMC   6568825 . PMID   2952772. S2CID   17003459.
  24. ↑ Huber A, Schulz S, Bentrop J, Groell C, Wolfrum U, Paulsen R (April 1997). "Molecular cloning of Drosophila Rh6 rhodopsin: the visual pigment of a subset of R8 photoreceptor cells". FEBS Letters. 406 (1–2): 6–10. doi:10.1016/s0014-5793(97)00210-x. PMID   9109375. S2CID   18368117.
  25. ↑ Pollock JA, Benzer S (June 1988). "Transcript localization of four opsin genes in the three visual organs of Drosophila; RH2 is ocellus specific". Nature. 333 (6175): 779–782. Bibcode:1988Natur.333..779P. doi:10.1038/333779a0. PMID   2968518. S2CID   4303934.
  26. 1 2 Feiler R, Harris WA, Kirschfeld K, Wehrhahn C, Zuker CS (June 1988). "Targeted misexpression of a Drosophila opsin gene leads to altered visual function". Nature. 333 (6175): 737–741. Bibcode:1988Natur.333..737F. doi:10.1038/333737a0. PMID   2455230. S2CID   4248264.
  27. ↑ Sakai K, Tsutsui K, Yamashita T, Iwabe N, Takahashi K, Wada A, Shichida Y (August 2017). "Drosophila melanogaster rhodopsin Rh7 is a UV-to-visible light sensor with an extraordinarily broad absorption spectrum". Scientific Reports. 7 (1): 7349. Bibcode:2017NatSR...7.7349S. doi:10.1038/s41598-017-07461-9. PMC   5544684 . PMID   28779161. S2CID   3276084.
  28. ↑ Ni JD, Baik LS, Holmes TC, Montell C (May 2017). "A rhodopsin in the brain functions in circadian photoentrainment in Drosophila". Nature. 545 (7654): 340–344. Bibcode:2017Natur.545..340N. doi:10.1038/nature22325. PMC   5476302 . PMID   28489826. S2CID   4468254.
  29. ↑ Vogt K (1 February 1984). "The Chromophore of the Visual Pigment in Some Insect Orders". Zeitschrift für Naturforschung C. 39 (1–2): 196–197. doi: 10.1515/znc-1984-1-236 . S2CID   88980658.
  30. ↑ Vogt K, Kirschfeld K (April 1984). "Chemical identity of the chromophores of fly visual pigment". Naturwissenschaften. 71 (4): 211–213. Bibcode:1984NW.....71..211V. doi:10.1007/BF00490436. S2CID   24205801.
  31. ↑ Gühmann M, Porter ML, Bok MJ (August 2022). "The Gluopsins: Opsins without the Retinal Binding Lysine". Cells. 11 (15): 2441. doi: 10.3390/cells11152441 . PMC   9368030 . PMID   35954284.
  32. ↑ Katana R, Guan C, Zanini D, Larsen ME, Giraldo D, Geurten BR, et al. (September 2019). "Chromophore-Independent Roles of Opsin Apoproteins in Drosophila Mechanoreceptors". Current Biology. 29 (17): 2961–2969.e4. Bibcode:2019CBio...29E2961K. doi: 10.1016/j.cub.2019.07.036 . PMID   31447373. S2CID   201420079.
  33. 1 2 Leung NY, Thakur DP, Gurav AS, Kim SH, Di Pizio A, Niv MY, Montell C (April 2020). "Functions of Opsins in Drosophila Taste". Current Biology. 30 (8): 1367–1379.e6. Bibcode:2020CBio...30E1367L. doi:10.1016/j.cub.2020.01.068. PMC   7252503 . PMID   32243853.
  34. ↑ Kumbalasiri T, Rollag MD, Isoldi MC, Castrucci AM, Provencio I (March 2007). "Melanopsin triggers the release of internal calcium stores in response to light". Photochemistry and Photobiology. 83 (2): 273–279. doi:10.1562/2006-07-11-RA-964. PMID   16961436. S2CID   23060331.
  35. 1 2 Raghu P, Colley NJ, Webel R, James T, Hasan G, Danin M, et al. (May 2000). "Normal phototransduction in Drosophila photoreceptors lacking an InsP(3) receptor gene". Molecular and Cellular Neurosciences. 15 (5): 429–45. doi:10.1006/mcne.2000.0846. PMID   10833300. S2CID   23861204.
  36. ↑ Hardie RC, Juusola M (October 2015). "Phototransduction in Drosophila". Current Opinion in Neurobiology. 34: 37–45. doi:10.1016/j.conb.2015.01.008. PMID   25638280. S2CID   140206989.
  37. ↑ Wang T, Xu H, Oberwinkler J, Gu Y, Hardie RC, Montell C (February 2005). "Light activation, adaptation, and cell survival functions of the Na+/Ca2+ exchanger CalX". Neuron. 45 (3): 367–78. doi: 10.1016/j.neuron.2004.12.046 . PMID   15694324.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.