Bio200 Signaling and Fertilization

Transcription

1 Bio200 Signaling and Fertilization

2 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?

3 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

4 Lipid-soluble signals, (the easy way) Steroid hormone 1. Lipid-soluble (small, hydrophobic) signal diffuses across plasma membrane into cell. Plasma membrane Receptor in cytosol 2. Signal binds to receptor, inducing conformational change or binding to a carrier. No membrane proteins are required Nucleus 3. (example) Complex binds to DNA, inducing a change in gene activity. Nuclear membrane Target gene DNA

5 Intermembrane Signal Receptors Outside of cell Intercellular signal Cell binding receptor 1. Signal is received. Receptor protein in membrane Inside of cell Second messengers Intracellular signals 2. Signal is transduced. 3. Signal is amplified. 4. Cell responds.

6 Direct cell contact/signalling Tubule of ER passing through plasmodesmata Smooth endoplasmic reticulum

7 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

8 What does this have to do with fertilization?

9 Fertilization in the open ocean Advantages of this method, Cheap Long distance Synchronizable Problems, Low probability of success in massive expanse of water Many predators Confusion between species Sperm Nucleus Gametes Jelly coat layer Vitelline envelope Plasma membrane Cortical granules Yolk granules Adult sea urchin

10 Homing in on the Egg, Resact and resact receptor Egg To help the sperm find the 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

11 Acrosome Packaging, Sea Urchin Sperm Anatomy Nucleus Acrosome Centriole Actin monomer pool Actin polymerase enzyme

12 Acrosome Packaging, Sea Urchin Sperm Anatomy Nucleus Y Acrosome Y Centriole Actin monomer pool Actin polymerase enzyme Digestive Enzyme Jelly coat factor receptor Bindin

13 The acrosomal reaction

14 Animations to watch at home An excellent longer video: from 2:14 for the acrosome reaction

15 Plasma membranes fuse, nucleus/centriole are inserted Y Bindin and Fertilizin Bindin is exposed along the acrosomal process Y Binds to the fertilizin receptor on the egg

16 Possibly the first well characterized protein protein binding event: bindin and fertilizin. How would mutations in these proteins affect speciation?

17 Gene Mutated Normal location of the protein Fertilzation rate of mutant Other phenotype? Bindin receptor protein Resact receptor protein Sperm-specific receptor protein Jelly coat proteins Exterior of sperm membrane 4.4% Exterior of sperm membrane 21.3% Exterior of sperm membrane 8.5% Forming jelly coat layer on oocyte 8.5% Sperm pass jelly coat layer well Fertilization in test tube is 100% Sperm on vitelline envelope fertilize well No visible difference in oocyte layers Wild-type sperm and oocytes n/a 100% Sperm find oocyte and reach vitelline envelope layer quickly

18 Motivations for Sperm and Egg Sperm Egg No Sperm Entry 1 Sperm Enters 2+ Sperm Enter Polyspermy is a major concern for the egg

19 Blocking polyspermy, The Calcium Wave

20 Calcium Wave Video

21 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.

22 Blocking polyspermy, Fertilization Membrane Pulls Away This clears other sperm away from the plasma membrane by ~6 seconds after sperm entry

23 Slow Block Video

24 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

25 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.