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NESC 2570 > Phototransduction > Flashcards

Phototransduction Flashcards

1
Q

Anatomy of the eye - Neural components

A
  • retina
    • fovea
  • optic disk
  • optic nerve
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2
Q

Optical components

A
  • cornea
  • aqueous humor
  • lens
  • vitreous humor
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3
Q

Supporting components

A
  • uveal tract
    • choroid
      • pigmented epithelium
    • ciliary body
    • iris
  • sclera
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4
Q

Focusing images on the retina

A
  • cornea: provides 80% of the focusing power
    • cannot change shape
  • lens: provides 20% of focussing power
    • thin lens: less light bending (far objects)
    • thick lens: more light bending (near objects)
  • shape of the lens is controlled by the ciliary muscles
    • relax: thin lens (far sight)
    • contracted: fat lens (near sight) accommodation
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5
Q

Organization of the retina

A 
Distal
-pigment epithelium
-photoreceptor outer segments (rods and cones)
-outer nuclear layer
-outer plexiform layer
-inner nuclear layer
-inner plexiform layer (amacrine, bipolar, horizontal cells)
-ganglion cell layer
-nerve fibre
Proximal
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6
Q

Phototransduction

A
  • transduction=transformation of light energy into neuron activity
  • light causes hyperpolarization of photoreceptors
  • more intense flash response causes larger hyperpolarization
  • dark: Na+ influx, K+ efflux, depolarization
  • light: reduced Na+ influx, K+ efflux, hyperpolarization
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7
Q

Light transduction

A
  • outer segment of photoreceptors are filled with stacks of disc membranes
  • disc membranes covered with opsins
    • Rods have rhodopsin
    • cone have opsins (S, M, L)
      • vertebrate opsins are closesly relation to metabotropic NT receptors (7TMD)
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8
Q

Rhodopsin

A
  • rhodopsin has a chromophore called retinal covalently bound to its 7th transmembrane domain
  • activation: isomerization of retinal causes conformation change in rhodopsin
    • activated rhodopsin has a conformation that exposes a binding pocked which interacts with the G protein Transducin (Gat)
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9
Q

Transducin activation

A
  • transducin is a heterotrimeric G protein
  • conformational change in rhodopsin triggers a conformational change in Transducins a subunit
    • results in:
      1. Decrease in the affinity of the a unit for GDP causing dissociation of GDP from the a subunit, and binding of GTP
      2. Dissociation of BY from a subunit
      3. Release of G proteins from rhodopsin
  • amplification: more than one transducin can be activated during the time the rhodopsin is bound to all-trans retinal
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10
Q

G proteins: conformational changes

A
  • a subunit:
    • nucleotide binding site (also the GTPase region) interacts with 3 switch regions
    • when GDP is exchanged for GTP the terminal phosphate group of GTP forms hydrogen bones with side chains of switch 1 and 2 to prevent it from interacting with the loops at the bottom of the GY propeller

-BY subunit doesnt change conformation

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11
Q

G protein modulation

A
  • GEFs: facilitate release of GDP
    • increases activity due to more Ga-GTP
    • ligand bound GPCR acts as a GEF
  • GDIs: inhibit release of GDP
    • decrease activity: less Ga-GTP
  • GAPs: activate intrinsic GTPase activity
    • decrease activity: less Ga-GTP
  • GIPs: stop intrinsic GTPase from working
    • increase activity: more Ga-GTP
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12
Q

Process

A
  • light activated opsin causes Transducin to Exchange GDP for GTP
  • transducin dissociates
  • a subunit activated phosphodiesterase
  • decreased cGMP causes cGMP gated channels to close
  • causes hyperpolarization because Na+ and Ca2+ cannot enter cell
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13
Q

Rods can detect a single photon of light

A
  • a single isomerization of retinal starts an enzymatic cascade
  • 1 activated rhodopsin can close 2% of rods cGMP gated channels
  • changes membrane potential by 1mV
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14
Q

Photo-cascade inactivation

A
  • activated rhodopsin (R*) is phosphorylated on 3 different sites by rhodopsin kinase
    • multi site inactivation may yield more uniform Tim course of inactivation (less variability = better signal)
  • phosphorylated rhodopsin is bound by arresting
  • arresting binding causes conformation change in R* so that it cannot active Transducin anymore
  • Transducin is inactivated by RGS9-GB5-R9AP complex
    • RGS9 = GAP
    • GB5 = Regulatory subunit
    • R9AP = membrane anchor
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15
Q

Light adaption

A
  • phototransduction adjusts its magnitude to prevailing light levels
  • prevents signal saturation, where all cGMP gated channels are closed
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16
Q

Light adaption mechanism 1

A
  • goal: keep some cGMP gated channels open
  • action: make more cGMP
  1. GUANYLATE CYCLASE
    - dark: intracellular Ca2+ inhibits guanylate cyclase activating proteins (GCAPs) (which produces cGMP)
    - light: induced closing of cGMP-channels decreases intracellular Ca2+
    • drop in Ca2+ up-regulates GCAPs
    • GCAPs up regulate guanylate cyclase to produce cGMP
      - fastest and most powerful mechanism
      - also assists in photo cascade inactivation
17
Q

Light adaption mechanism 2

A
  • goal: keep some cGMP gated channels open
  • action: ensure less PDE activity
  1. RECOVERIN
    - Ca2+ biding protein similar to calmoduin
    - dark: Ca2+-Recoverin inhibits rhodopsin kinase from phosphorylation great rhodopsin
    - light induced drop in Ca2+ relieves this inhibition
    • low Ca2+ = more rapid R* inactivation
    • less PDE activation
    • more cGMP
18
Q

Light adaption mechanism 3

A
  • goal: keep some cGMP gated channels open
  • action: make channels more sensitive to cGMP
  1. CALMODULIN
  • dark: Ca2+-Calmodulin binds to cGMP gated channels and desensitizes it to cGMP
  • light induced drop in Ca2+ = calmodulin dissociates from the channel making it more sensitive to cGMP
    • stays open with fewer bound cGMP
19
Q

Dark adaption

A
  • eyes become more sensitive
  • due to replenishing of 11-cis retinal opsins
  • retinoids cycle in photoreceptor and pigment epithelium
  • interphotoreceptor binding proteins (IRBP) chaperones retinoids between photoreceptors and pigment epithelial cells
  • convert all-trans retinal back to 11-cis retinal in pigment epithelium so they can be reactivated by light
20
Q

Disc maintenance

A
  • photoreceptor disks are continuous being produced
  • disks migrate from soma toward end of outer segment
  • old disks removed by pigment epithelium
21
Q

Specialization of rod and cone systems

A
  • rods have high sensitivity, specialized for low intensity vision
  • cone have low sensitivity, specialized for high sensitive vision
  • cone transduction specialized for bright light
    • reduced amplification in their phototransduction cascade
      • slower/less effective Gat and PDE
    • higher expression of RGS9 (GAP)
      • faster inactivation
    • cone specific opsin kinase (GRK7)
      • more efficient R* inactivation
    • much larger changes in Ca2+
      • engage in Ca2+ feedback mechanisms more rapidly
22
Q

Specialization of rod and cone systems continued

A
  • rods send converging inputs to bipolar cells
    • pooling of input leads to greater sensitivity and less acuity
  • cones have less convergence
    • less sensitivity, greater acuity
  • only cones at fovea
  • less cones everywhere else
  • no rods at fovea
23
Q

Fovea

A
  • specialized for high acuity
  • avascular - no overlaying blood vessels obscuring path of light to the cones
  • inner retinal cells (bipolar, ganglion, horizontal, amacrine) swept to side so light has unobstructed path to cones
  • foveal cones thinner than elsewhere
  • these factors contribute to pit-like structure
24
Q

Cones and colour vision

A
  • human colour vision is trichromatic
  • short, medium and long cones
  • short: blue (400 range)
  • medium: green-yellow (450-550)
  • long: yellow-red (500-600)
S: only 5-10% of cone population 
   -absent from fovea
   -important for circadian rhythms
M & L: predominant rental cone types
     -proportions vary between humans yet all have normal colour vision
25
Q

Cones and colour deficiency

A
  • x linked mutations can cause dichromatism
  • protonopia: missing L cones
  • deuteranopia: missing M cones
  • amino acid sequence of each cone opsin determine the wavelength of light its most sensitive to
    • gradual mutation gave rise to Rhodopsin plus 3 cone types
  • M and L both on X chromosome and are genetically similar indication recent evolutionary origin - probably from gene duplication
  • errors in crossing over during meiosis can produce dichromatism

NESC 2570 (9 decks)

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