Phototransduction in insects.
So why then did ciliary photoreceptors come to dominate as mediators of vision in vertebrates?
Fain et al (2010) propose that it was the evolution of rod-type cells, which are much more sensitive to light than cones and other ciliary-type photoreceptors. Thus, the evolution of rods enabled vision in dim light, with cones enabling vision (and in some cases colour vision) in bright light.
Describe quantum bumps?
(B) Quantum-bumps from Drosophila photoreceptor. A two second-long dim light flash (bar: about four effective photons s−1), elicits a train of discrete single photon responses, variable in amplitude but on average about 10 pA.
Describe the Superimposed responses from a Drosophila
C) Superimposed responses from a Drosophila photoreceptor to six 1 ms dim flashes (arrow) each containing only one effective photon. The single photon responses arise abruptly following a finite and variable latency and vary in amplitude. The summed response is consequently much noisier than is the bipolar cell response.
Describe the Evolution of opsins.
C-opsins are found in ciliary-type photoreceptors. R-opsins are found in rhabdomeric-type photoreceptors. Both types are found in some species.
What does this electron micrograph show?
Electron micrograph showing the rhabdomeric compartment of a Drosophila photoreceptor cell.
What is Under each facet of the compound eye
ommatidium
Describe the role of histamine as a neurotransmitter
Histamine acts as an inhibitory neurotransmitter, triggering inhibitory postsynaptic potentials (IPSPs) in cells that are post-synaptic to the photoreceptor cells. The histamine receptor that mediates the inhibitory effects of histamine is a ligand-gated chloride channel, which is homologous to the GABA-A-type receptors and glycine receptors that mediate inhibitory transmission in the vertebrate CNS.
Describe the photoreceptors in scallops?
In animals such as scallops where ciliary-type photoreceptors and microvillar-type photoreceptors occur together in eyes, albeit in different layers, the ciliary photoreceptors are much less sensitive than the microvillar photoreceptors but are thought to serve the useful function of detecting decrements in illumination to mediate shadow responses.
What happens to photoreceptors in mammals under light?
In mammals light causes hyperpolarisation of photoreceptor cells. Light causes depolarisation of Drosophila photoreceptor cells
What did the Analysis of mutant strains of Drosophila with visual defects lead to?
It led to the identification of genes encoding proteins that mediate phototransduction in Drosophila.
So why did the two types of photoreceptor cells evolve originally and why has one type been lost in some lineages (e.g. c-opsins and ciliary-type photoreceptors are not present in Drosophila).
Microvillar-type photoreceptors (like those found in Drosophila) are much more sensitive to light than ciliary-type photoreceptors (e.g. rods/cones) - for example, a single photon will elicit a response.
Describe teh characteristics of Microvillar-type photoreceptors
Microvillar-type photoreceptors can therefore support vision in dim light. However, they can also adapt to very bright light, enabling vision in daylight. In this respect they are more versatile than ciliary-type photoreceptors.
What is included in the scaffolding protien
PKC, protein kinase C NINAC, class III myosin INAD, a protein containing PDZ binding domains responsible for forming the signaling complex
Describe the GPCR effector: phospholipase C (PLC)
PLC catalyses formation of the second messengers inositoltriphosphate (IP3) and diacylglycerol (DAG) from the membrane lipid phosphatidylinositol- 4,5-biphosphate (PIP2).
What is pax6?
Pax6 is a transcription factor and mutations in pax6 cause defects in eye development in vertebrates and invertebrates.
Describe the similarities between the phototransduction in vertebrate and Drosophila photoreceptors
Photon-activated protein is a G-protein coupled receptor = rhodopsin Photons induce conformational change in a bound-ligand (retinal) Activation of rhodopsin leads to activation of G-proteins.
Which photoreceptor cells are the first optic neuropile, the lamina (La).
R1-R6
Describe the photoreceptor R1-R8 cells
R1-R8 cells are not identical. R1-R6 are all anatomically similar and are shown in green R7 is shown as blue. R8 is shown as red.
Which photoreceptor cells are the the second optic neuropile, the medulla (Me).
R7-R8
What light is RH2 sensitive to and where is it expressed in?
Rh2 is sensitive to violet light and is expressed in the photoreceptor cells of ocelli ("simple" eyes located on other parts of the head)
What are RH3 and RH4 sensitive to and where are they expressed?
Rh3 and Rh4 are UV sensitive and are expressed in non- overlapping sets of R7 cells.
What are Rh5 and Rh6 sensitive to? and where are they expressed
Rh5 and Rh6 are blue/green sensitive and are expressed in non-overlapping sets of R8 cells.
Describe the differences between the phototransduction in vertebrate and Drosophila photoreceptors
Rhodopsin is located in intracellular membranes (disks) in vertebrate rods, whereas it is located in microvillar cell membranes in Drosophila photoreceptors. Retinal is covalently bound to rhodopsin in Drosophila, whereas in vertebrates it becomes dissociated from rhodopsin following activation of rhodopsin Downstream effector of G-protein (transducin) in vertebrate rods is cGMP-phosphodiesterase, whereas in Drosophila it is phospholipase C. Light causes hyperpolarisation of vertebrate photoreceptor cells by causing in a decrease in currents carried by cGMP-gated cation channels, whereas in Drosophila photoreceptors light causes depolarisation by causing activation of TRP-type cation channels. Hyperpolarisation of vertebrate photoreceptors causes a decrease in release of glutamate, which acts as an excitatory or inhibitory transmitter postsynaptically (depending on bipolar cell type). Depolarisation of Drosophila photoreceptors causes an increase in release of histamine, which acts as an inhibitory transmitter postsynaptically.
Describe teh differences between Scallop microvillar-type and ciliary-type photoreceptors
Scallop microvillar-type photoreceptors in the proximal layer of retina (see previous slide) are depolarised by light (A below), whereas ciliary-type photoreceptors in the distal retina (see previous slide) are hyperpolarised by light. (B below). However, the microvillar-type photoreceptors are 2 orders of magnitude more sensitive to light than the ciliary-type photoreceptors.
What did the Sequencing of the ninaE gene reveal
revealed that it encodes a G-protein coupled receptor. ninaE encodes a Drosophila rhodopsin known as Rh1.
What is Rh1 is maximally sensitive to?
to blue light, as are the R1-R6 photoreceptor cells that express Rh1
What are ion is trpl permeable to?
to sodium ions.
What is trp
transient receptor potential
How can trp channels mediate light-induced depolarisation of Drosophila photo- receptor cells.
trp channels are permeable to calcium
Describe the differences between trp and trpl
The Drosophila "transient receptor potential" channel is the prototypical TRP channel, belonging to and defining the TRPC subfamily. Together with a second TRPC channel, trp-like (TRPL), TRP mediates the transducer current in the fly's photoreceptors. TRP and TRPL are also implicated in olfaction and Malpighian tubule function. In photoreceptors, TRP and TRPL are localised in the ~30,000 packed microvilli that form the photosensitive "rhabdomere"-a light-guiding rod, housing rhodopsin and the rest of the phototransduction machinery. TRP (but not TRPL) is assembled into multimolecular signalling complexes by a PDZ-domain scaffolding protein (INAD). TRPL (but not TRP) undergoes light-regulated translocation between cell body and rhabdomere. TRP and TRPL are also found in photoreceptor synapses where they may play a role in synaptic transmission. Like other TRPC channels, TRP and TRPL are activated by a G protein-coupled phospholipase C (PLCβ4) cascade. Although still debated, recent evidence indicates the channels can be activated by a combination of PIP2 depletion and protons released by the PLC reaction. PIP2 depletion may act mechanically as membrane area is reduced by cleavage of PIP2's bulky inositol headgroup. TRP, which dominates the light-sensitive current, is Ca(2+) selective (P Ca:P Cs >50:1), whilst TRPL has a modest Ca(2+) permeability (P Ca:P Cs ~5:1). Ca(2+) influx via the channels has profound positive and negative feedback roles, required for the rapid response kinetics, with Ca(2+) rapidly facilitating TRP (but not TRPL) and also inhibiting both channels. In trp mutants, stimulation by light results in rapid depletion of microvillar PIP2 due to lack of Ca(2+) influx required to inhibit PLC. This accounts for the "transient receptor potential" phenotype that gives the family its name and, over a period of days, leads to light-dependent retinal degeneration. Gain-of-function trp mutants with uncontrolled Ca(2+) influx also undergo retinal degeneration due to Ca(2+) cytotoxicity. In vertebrate retina, mice knockout studies suggest that TRPC6 and TRPC7 mediate a PLCβ4-activated transducer current in intrinsically photosensitive retinal ganglion cells, expressing melanopsin. TRPA1 has been implicated as a "photo-sensing" TRP channel in human melanocytes and light-sensitive neurons in the body wall of Drosophila.
What do the The densely packed microvilli do to the rhabdomeric compartment of a Drosophila photoreceptor cell.
The densely packed microvilli increase the surface area for phototransduction, maximising sensitivity to light.
How is the rhabdom formed?
The rhabdom is formed by microvillar projections from photoreceptor cells
Describe the The visual system of box jellyfish.
The visual system of box jellyfish. (a) A Chiropsella bronzie medusa. Each medusa has four sensory clubs known as rhopalia. (b) Each rhopalium possesses six eyes; two pit eyes (pe), two slit eyes (se), one upper lens eye (ule) and one lower lens eye (lle). (c) The retina of the eyes consist of three layers; a ciliary layer (cl), a pigment layer (pl) and a neural layer. All photoreceptors are ciliary, demonstrated here by the presence of a ciliary rootlet (cr).
How are Drosophila rhodopsin activated?
They are activated by a light induced conformational change in the structure of a bound retinal ligand.
What happens when Light causes photoisomerisation of Drosophila rhodopsin to metarhodopsin
This activates a G-protein complex.
What occured when light was exposed to Drosophila photoreceptors
We found that light exposure evoked rapid PLC-mediated contractions of the photoreceptor cells and modulated the activity of mechanosensitive channels introduced into photoreceptor cells. Furthermore, photoreceptor light responses were facilitated by membrane stretch and were inhibited by amphipaths, which alter lipid bilayer properties. These results indicate that, by cleaving PIP(2), PLC generates rapid physical changes in the lipid bilayer that lead to contractions of the microvilli, and suggest that the resultant mechanical forces contribute to gating the light-sensitive channels.
Phototransduction in Drosophila microvillar photoreceptor cells is mediated by what?
a G protein-activated phospholipase C (PLC). PLC hydrolyzes the minor membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP(2)), leading by an unknown mechanism to activation of the prototypical transient receptor potential (TRP) and TRP-like (TRPL) channels.
What are opsins
a large family of closely related G-protein receptors that mediate phototransduction in all known metazoans. (A) Crystal structure of bovine rhodopsin with seven transmembrane helical domains labeled with Roman numerals, (B) Lysine in the seventh transmembrane domain of opsin forms a covalent bond with the aldehyde of the chromophore retinal. Light produces a photoisomerization of 11-cis retinal to all-trans retinal and changes the conformation of the rest of the opsin protein, facilitating the binding of G-protein and triggering the phototransduction cascade. Most metazoans use retinal, but 3-dehydroretinal is found in some fresh-water vertebrates, and 3-hydroxyretinal in many insects. (C) Phylogenetic tree of opsins; ciliary photoreceptors (red lines) have c-opsins and G0 opsins, and microvillar (blue lines) have r-opsins. Proteins in these three subfamilies show differences in amino acid sequence that are thought to be responsible for their different properties and interactions with different G proteins. The three different forms of photopigment (and others not shown) diverged very early probably among primitive metazoans.
What are Each ommatidium is comprised of
a ring of 8 photoreceptor cells (R1-R8) arranged around a central phototransducing structure called the rhabdom.
How does Light causes depolarisation of Drosophila photoreceptor cells
by activation of cation channels that are permeable to sodium and calcium ions.
The expression of melanopsin in these cells is not required for normal vision but is required for what?
entrainment to light-dark cycles (circadian rhythms) and the pupillary light reflex.
some invertebrates (e.g. cnidarians) the two photoreceptor types are employed for what?
for vision at different stages of development (e.g. larval v adult).
What neurotransmitter does Arthropod photoreceptors release
histamine
Where was Melanopsin originally found in
in light sensitive cells in frog skin.
How are rhodopsin proteins activated in mammals
in mammals retinal has to dissociate from the rhodopsin protein for reisomerization, in Drosophila retinal is permanently covalently linked to rhodopsins.
What did the sequencing of the trp gene reveal
it encodes an ion channel protein that resembles the superfamily of ion channel proteins that include voltage-gated ion channels and cGMP-gated ion channels.
What did the Sequencing of the norpA gene reveal
it encodes the enzyme phospholipase C.
what does NINA mean?
no inactivation, no afterpotential
photoreceptor cells from trp mutants display what?
normal responses to weak stimulation but are unable to maintain the receptor potential in response to maintained bright light
