Transcript Vision + Desensitization

OUR 5 MAJOR SENSORY
SYSTEMS
Vision - the detection of light
Olfaction- (sense of smell) the detection of small molecules
in the air
Taste or Gustation- the detection of selected organic
compounds and ions by the tongue
Hearing-The detection of sound (or pressure wave in the air)
Touch- the detection of changes in pressure, temp. and other
factors by the skin
SENSORY SYSTEMS
When fully adapted to darkness our eyes
allow us to sense very low levels of light,
down to a limit of less than 10 photons.
With more light we are able to distinguish
millions of colors.
Through our senses of smell and taste we
are able to detect thousands of chemicals
and sort them into distinct categories
Each of these primary sensory systems contains
specialized sensory neurons that transmit nerve
impulses to the CNS
In the CNS theses signals are processed
and combined with other information to
yield a perception that may trigger a
change in behavior.
By these means, our senses allow us to
detect changes in our environments and
adjust our behavior appropriately
Photoreceptor molecules in the eye
detect visible light
Vision is based on the absorption of light by photoreceptor
cells in the eye
Photoreceptor cells are sensitive to light in a relatively
narrow region of the electromagnetic spectrum between
300-850nm
Two kinds of photoreceptors
Rods (100 million) and Cons (3 million)
Rods function in dim light and do not perceive color
Cons function in bright light and are responsible for color vision
VISION
Pigment epithelium
Neuronal layers
The Retina
• Contains photoreceptor cells (rods
and cones) and associated
interneurones and sensory neurones
light
t oopt
ic gango
il n
nerve cels
bp
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neurones
rod cone pigm
ent
ed
cels
cels
ret ina
Vision---rod/cones
The neural circuits in the retina
of a primate
-The incoming light reaches the
photoreceptor cells (rods and cones) only
after passing through several thin,
transparent layers of other neurons.
-The pigment epithelium absorbs the light
that is not absorbed by the photoreceptor
cells and thus minimizes reflections of
stray light.
The ganglion cells communicate to the
thalamus by sending action potentials
down their axons.
However, the photoreceptor cells and
other neurons communicate by graded
synaptic potentials that are conducted
electronically.
The Rod Cell
Scanning electron micrographs of retinal rod cells
Schematic representation
of a rod cell
100,000,000 rod cells
in human retina
Photoperception
1000 disks, 16nm thick
Rod cell (1x40µm)
Biochemistry. L. Stryer
The disks which are membrane enclosed sacs are
densely packed with photoreceptor molecules
The photosensitive molecule is called the
visual pigment because it is highly colored due to
light absorption
The photoreceptor molecule in the rods is rhodopsin
consists of opsin linked to 11-cis-retinal
300-850nm
The electromagnetic spectrum
Absorption spectrum of rhodopsin
Questions
How does the cell respond to
photons?
What mechanism converts light
into a cellular signal?
Ligand-activated
Receptor
Light-activated
Receptor
Rhodopsin
(polyene- with 6 alternating double and single bonds)
Illustration of
Rhodopsin (blue)
with
11-cis retinal (red)
(440nm absorption)
The protonated form of the 11-cis retinal absorbs at 440nm
Unlike 380nm of the non-protonated.
The positive charge of Lys296(VII) is compensated by Glu113(II)
Activation of rhodopsin by a photon-converting a light energy of
A photon into atomic motion
-The isomerization causes the Shiff-base nitrogen to move
approximately 5A, assuming that the cyclohexane ring
of the cis-retinal group remains fixed/
-Inverse agonist- 108 Rhodopsin molecules /cell
RHODOPSIN
In
Glu113
D(E)RY
Cys322
Cys323
Helix VIII
(311-321)
Asp2
Out
Cys 110
Cys 187
Met1
Lys 296
Glu181
Asp15
The three dimensional structure of rhodopsin
Rhodopsin 2.8A resolution;
Science 289, 739-745 (2000)
Science 389,739 (2000)
Three dimensional Model of Rhodopsin
Palmitoyl
at Helix 8
Retinal
Rhodopsin photoactivation
Alcohol dehydrogenases
Transducin at 39kD; b 36kD;  8kD
a
b

In the dark transducin is in the GDP form
the binding of GTP to transducin leads to the
release of R* which enables it to catalyze the
Activation of another molecule of transducin
A single R* catalyzes the activation of 500
molecules of transducin, the first stage in
the amplification of vision
Schematic diagram of the cyclic GMP cascade of vision
Activation of phosphodiesterase
by Gat
The binding of GTP switches on the phosphodiesterase (PDE)
by relieving an inhibitory constraint. In the dark the two
catalytic subunits a and b are held in check by a pair of
inhibitory subunits ().
By binding of Gat to the enzyme it removes the inhibitory
subunits and the enzyme is activated


a
Inactive
b
Gat
 Gat

a
b
Gat
Active
The hydrolysis of cGMP by phosphodiesterase is the second stage of
of amplification
11-cis-retinal
11-trans-retinal
Membrane potential
Light hyperpolarizes the plasma membrane of a retinal rod
cell
The light induced hyperpolarization is transmitted by the plasma
membrane from the outer segment to the synaptic body.
A single photon closes hundreds of cation specific channels (~500)
and leads to a hyperpolarization of about 1-5mV
Cation channels (~500) in the rod cell
close following the transduction of a
single photon.
These represent 3% of the total number
of channels that are open in the dark. The
resultant hyperpolarization is about 1mV
and lasts about 1 sec.
This is sufficient to depress the rate of
neurotransmitter release that transmits the
onward signal
The high-degree of co-operativity (3 molecules of cGMP) to open
the channel increases the sensitivity of the channel for small
changes in cGMP which enable it to act as a switch.
CNG- Cyclic nucleotide-gated channels
Cyclic nucleotide
binding domain
Dark Current
BiologyMad.com
In the Dark…
• In the dark the channel is open  Na+ flow in can
cause rod cells to depolarise.
– Therefore in total darkness, the membrane of a rod cell is
polarised
• Therefore rod cells release neurotransmitter in the
dark
• However the synapse with bipolar cells is an
inhibitory synapse i.e. the neurotransmitter stops
impulse
BiologyMad.com
BiologyMad.com
In the Light…
As cis retinal is converted to trans retinal, the
Na+ channels begin to close
i
less neurotransmitter is produced. If the
threshold is reached, the bipolar cell will be
depolarised
i
forms an impulse which is then passed to the
ganglion cells and then to the brain
BiologyMad.com
BiologyMad.com
Rods and Cones
Rods
Cones
Outer segment is rod
shaped
109 cells per eye,
distributed throughout the
retina, so used for
peripheral vision.
Good sensitivity
Only 1 type 
monochromatic vision
Outer segment is cons
shaped
106 cells per eye, found
mainly in the fovea, so can
only detect images in
centre of retina.
Poor sensitivity
3 types (R, G & B) 
colour vision
Many rods connected to
one bipolar cell  poor
acuity = poor resolution
Each cone is connected to
one bipolar cell  good
acuity = good resolution
One rhodopsin molecule
Absorbs one photon
500 Transducin molecules are activated
500 Phospodiesterase molecules
are activated
105 cGMP molecules are hydrolyzed
250 Na+ channels closed
106-107 ions/sec are prevented from entering
the cell for a period of 1 sec
Rod cell membrane is
hyperpolarized by 1 mV
Guanylate cyclase
GTP
cGMP +PPi
The enzyme Guanylate cyclase
looses its activity in high Ca2+
BiologyMad.com
Color Vision
• 3 different cone cells. Each have a
different form of opsin (they have the
same retinal)
• 3 forms of rhodopsin are sensitive to
different parts of the spectrum
– 10% red cones
– 45% blue cones
– 45% blue cones
Con Cells
The absorption spectra of the cone visual
pigment responsible for color vision
The cone photoreceptors are 7TM domain receptors that utilize
11-cis-retinal as chromophore. Absorption maxima (nm)
in human are 426 (blue), 530 (green) and 560 (red)
Comparison of the amino acid sequence of
the green and red photoreceptors
Color Vision
• Colored light will stimulate these 3 cells differently - by
comparing the nerve impulses from the 3 kinds of cones
the brain can detect any colour
–
–
–
–
Red light  stimulates R cones
Yellow light  stimulates R and G cones equally
Cyan light  stimulates B and G cones equally
White light  stimulates all 3 cones equally
• Called the trichromatic theory of color vision
Color Vision
• When we look at something the image falls on the
fovea and we see it in color and sharp detail.
• Objects in the periphery of our field of view are not
seen in colour, or detail.
• The fovea has high density of cones.
• Each cone has a synapse with one bipolar cell and
one ganglion  each cone sends impulses to the
brain about its own small area of the retina  high
visual acuity
Evolutionary relationships among visual pigments
Visual pigments have evolved by gene duplication
Color blindness
The genes for the green and red pigments
lie adjacent on the human X chromosome.
Are 98% identical in nucleotide sequence
including introns and UTR
-Therefore, are susceptible for to unequal
homologous recombination
-5% of males have this form of blindness
Recombination pathways leading to color blindness
Rearrangements in the course of DNA replication
A) Loss of visual pigment B) The formation of hybrid
pigemnt genes that encode photoreceptors with anomalous
abs. spectra
A homologous recombination: the exchange of DNA segment at
equivalent positions between chromosomes with substantial similarity
Termination of the signal
One of the most important part of the signaling machinery
is termination of the signal even in the presence of the stimulus
This phenomenon is referred to as “desensitization”
Such mechanisms operate at both the level of the receptor
as well as down stream at the level of G-protein
Rapid termination of the receptor signal is controlled
by receptor phosphorylation which is mediated by second
messenger-kinases PKA and PKC or by a distinct
Receptor-kinsases (GRKs) together with arrestins
Heterologous desensitization
Second-messenger kinase regulation
PKA and PKC uncouple receptors from their
respective G-proteins and serve as negative-feedback regulatory loops.
Feed back regulation by the 2nd messengerstimulated kinases PKA and PKC.
The phosphorylated receptor changes its
conformation and no longer can activate the Gproteins.
It is an agonist non-specific desensitization
Homologous desensitization
GRK(G-ptrotein-receptor kinase)-mediated desensitization
A complex mechanism for regulating 7TM-receptor
activity called GRK-barrestin system
It is also called an agonist-specific desensitization
because only the activated agonist-occupied
conformation of the receptor is phosphorylated by by
GRK.
A two step process in which agonist-occupied
receptor is phosphorylated by GRK and then binds
an arrestin proteins. This leads to a rapid-agonist
specific desensitization
Heterogous and homologous desensitization
The major GPCR regulatory pathway involves
phosphorylation of activated receptors by G
protein–coupled receptor kinases (GRKs),
followed by binding of arrestin proteins, which
1)
prevent receptors from activating
downstream heterotrimeric G protein
pathways while
2)
allowing activation of arrestin-dependent
signaling pathways.
GRK - G-protein–coupled receptor kinase
As long as the agonist remains bound to the receptor, the activated
receptor can continue to activate G proteins.
GRK which is catalytically activated by this interaction, also
recognizes the activated conformation of the receptor.
Activated GRKs phosphorylate (P) intracellular domains of the
receptor and are then released. The agonist-activated, GRKphosphorylated receptor binds tightly to an arrestin protein, which
desensitizes further G protein activation and couples the receptor to
the clathrin-coated-pit internalization pathway and to arrestinscaffolded (and G protein–independent) signaling pathways.
GRK-GPCR-kinase
The role of GRK-phosphorylation of the receptors in the
sequestration process is to facilitate arrestin binding
Experiments to prove this idea
1)A mutated b-adrenergic receptor Y326A is a poor
substrate for b-Adrenergic receptor-kinase, and is
not sequestered. Over-expression of b-arrestin
restores sequestration
2) Removal of C-terminal tail (sites for GRK sites)
prevents sequestration
Arrestins
The arrestin family includes > 6 members several of
which undergo alternative splicing
The affinity of b-arrestin (selective for the breceptors) increases 10-30 fold by GRK-catalyzed
phosphorylation, whereas agonist occupancy has a
much less significant effect.
The b-arrestins promote internalization by binding to
clatherin
Science, 297, 529 (2002)
badrenergic receptor
PKA
PKA
Homologous desensitization
bark
Rhodopsin
Rhodopsin kinase
Termination
Activation
GTP
RGS and GAP Activities
GDP
GTP
GDP
Neuron 20, 11-14 (1999)
11-cis vs. all-trans retinal