Download Bio200 Signaling and Fertilization

Bio200
Signaling and Fertilization
Outline Questions
Understanding Fertilization
•  How do cells signal to each other?
–  What molecules are released, and what molecules are used to
recognize signals?
–  Why would a cell need to signal to another cell?
•  What are the three cell-cell signaling events necessary
for successful sea urchin fertilization?
•  How does the acrosomal reaction allow the sperm to
fertilize the oocyte?
•  What protections are used by the oocyte to defend itself?
Cell-to-Cell Signaling
•  We ve already talked about cell-to-cell
signaling several contexts,
–  Growth factors, hormones, secreted proteins, etc
•  There is a vast array of cell signaling methods,
which include
– 
– 
– 
– 
– 
Lipid-soluble signals
Cell-to-cell contact mediated by receptors
Signal receptors
Second messengers and enzymes
Direct cytoplasmic contact
– A quick tour is in order…
Lipid-soluble signals,
(the easy way)
Steroid
hormone
1. Lipid-soluble (small, hydrophobic)
signal diffuses across plasma
membrane into cell.
Plasma
membrane
2. Signal binds to
receptor, inducing
conformational change or
binding to a carrier.
Receptor
in cytosol
No membrane
proteins are
required
Target gene DNA
Nuclear
membrane
Nucleus
3. (example) Complex
binds to DNA, inducing a
change in gene activity.
Intermembrane
Signal Receptors
Intercellular
signal
Cell binding
receptor
1. Signal is received.
Outside of cell
Inside of cell
Second
messengers
Intracellular
signals
Receptor protein
in membrane
2. Signal is
transduced.
3. Signal is amplified.
4. Cell responds.
Direct cell contact/signalling
Tubule of ER passing through
plasmodesmata
Smooth
endoplasmic
reticulum
Direct cell contact/signalling
Tubule of ER passing through
plasmodesmata
Smooth
endoplasmic
reticulum
Gap
junctions
Membrane proteins
from adjacent cells
line up to form
a channel
What does this have to do with fertilization?
Fertilization in the open ocean
Gametes
Adult sea
urchin
•  Advantages of this method,
–  Cheap
–  Long distance
–  Synchronizable
Jelly coat
layer
•  Problems,
–  Low probability of success in
massive expanse of water
–  Many predators
–  Confusion between species
Sperm
Nucleus
Vitelline
envelope
Plasma
membrane
Cortical
granules
Yolk granules
Homing in on the Egg, Resact and resact receptor
•  To help the sperm find the egg
Egg
–  Egg releases a small 10-aa peptide
resact
–  Resact diffuse from the egg into nearby
water
–  Sperm use this concentration gradient to
guide towards egg
•  Sperm have two swim motions,
–  Low ligand binding = turn
–  High ligand binding = straight
–  Why does this set of rules get the
sperm to the egg?
–  Chemotaxis
Acrosome Packaging,
Sea Urchin
Sperm Anatomy
Nucleus
Acrosome
Centriole
Actin monomer pool
Actin polymerase
enzyme
Acrosome Packaging,
Sea Urchin
Sperm Anatomy
Y
Nucleus
Acrosome
Y
Centriole
Actin monomer pool
Actin polymerase
enzyme
Digestive Enzyme
Jelly coat factor
receptor
Bindin
The acrosomal reaction
Animations to watch at home
http://worms.zoology.wisc.edu/urchins/acrosome2.html
An excellent longer video:
http://www.youtube.com/watch?v=_5OvgQW6FG4
from 2:14 for the acrosome reaction
Bindin and Fertilizin
Bindin is exposed
along the acrosomal
process
Y
Y
Plasma membranes fuse,
nucleus/centriole are inserted
Binds to the
fertilizin
receptor on
the egg
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Possibly the first well‐characterized protein‐protein
binding event: bindin and fertilizin.
How would mutations in these proteins affect
speciation?
Gene Mutated
Normal location of
the protein
Fertilzation rate
of mutant
Other phenotype?
“Bindin” receptor
protein
Exterior of sperm
membrane
4.4%
Sperm pass jelly coat
layer well
“Resact” receptor
protein
Exterior of sperm
membrane
21.3%
Fertilization in test
tube is 100%
Sperm-specific
receptor protein
Exterior of sperm
membrane
8.5%
Sperm on vitelline
envelope fertilize well
Jelly coat proteins
Forming jelly coat
layer on oocyte
8.5%
No visible difference in
oocyte layers
Wild-type sperm
and oocytes
n/a
100%
Sperm find oocyte and
reach vitelline
envelope layer quickly
Motivations for Sperm and Egg
Sperm
Egg
No
Sperm
Entry
1 Sperm
Enters
2+ Sperm
Enter
Polyspermy is a major concern for the egg
Blocking polyspermy, The Calcium Wave
Calcium Wave Video
http://www.youtube.com/watch?v=BH06WgFua_4
Blocking polyspermy: Cortical Granules
•  Calcium wave causes cortical
granules to fuse with egg plasma
membrane
•  Carbohydrates are released into the
inter-layer space
•  This thick fluid absorbs water and
swells, pushing away the vitelline
envelope.
Blocking polyspermy, Fertilization Membrane Pulls Away
This clears other sperm away from the plasma
membrane by ~6 seconds after sperm entry
Slow Block Video
http://www.youtube.com/watch?v=MHWkltLbJp0
Blocking polyspermy, the Fast block
•  The egg is still vulnerable to polyspermy in
the first few seconds
–  Especially problematic with many sperm
reaching the jelly coat at nearly the same time
•  The Fast Block system uses the change
in membrane potential in the egg to help
protect against a second sperm
–  Change in membrane potential rapidly and
temporarily distorts fertilizin structure
–  This inactivates all fertilizin molecules for ~6
seconds
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Key Concepts
•  Cell-cell signaling is a widespread and varied molecular interaction
that helps cells define their role, position, and communication.
•  Cells can release small molecule signals (like hormones) and have
specific receptors for each signal that they need to receive.
•  Sea urchin fertilization models cell-cell signaling in three separate
interactions between signaling molecules (resact, jelly coat factor,
bindin) and their respective receptors.
•  These reactions are species specific
•  Communication is 2-way through these process
•  This ensures that the correct binding is reached to minimize waste
•  The acrosomal reaction allow the pre-packaged components of sea
urchin sperm to facilitate a rapid and accurate fertilization.
•  Oocytes use both a fast-block and a slow-block mechanism to
prevent fatal polyspermy fertilizations.