Phototransduction from frogs to flies

1. Modeling signal transduction In this lecture i want to take a small bite at understanding how the eye works on the molecular level.In this lecture i want to take a small bite at understanding how the eye works on the molecular level.

2. Eye as an evolutionary challenge A fact recognaized by Darwin, who wrote� Lophotrochozoa (Molluscs); Ecdysozoa (Athropods); Cnidaria (Jelly fish); Deuterostomia (Vertebrates); Despite great anatomical similarity of vertebrate and sephalopod (mollusk) eyes in the former they develop as an outfrowth of the brain in the latter as a peripheral ectoderm. In cnidarian jellyfish, there is a sophisticated eye Cornea/lens/retina, but NO BRAIN at all. There are many fundamantal questions. E.g. there is a debate going on on whether eyes in different phyla have evolved From a common precursor or by convergent evolution. I will not get even close to answering these grand questions. I just want to bring them to your attention hoping that one of you will pick up the challenge and in a few years time enlighten us all. It is not possible to answer the question without getting intimately acquainted with the system. So i will spend the next hour telling you about photo-transduction in vertebrates and insects on the molecular level.A fact recognaized by Darwin, who wrote� Lophotrochozoa (Molluscs); Ecdysozoa (Athropods); Cnidaria (Jelly fish); Deuterostomia (Vertebrates); Despite great anatomical similarity of vertebrate and sephalopod (mollusk) eyes in the former they develop as an outfrowth of the brain in the latter as a peripheral ectoderm. In cnidarian jellyfish, there is a sophisticated eye Cornea/lens/retina, but NO BRAIN at all. There are many fundamantal questions. E.g. there is a debate going on on whether eyes in different phyla have evolved From a common precursor or by convergent evolution. I will not get even close to answering these grand questions. I just want to bring them to your attention hoping that one of you will pick up the challenge and in a few years time enlighten us all. It is not possible to answer the question without getting intimately acquainted with the system. So i will spend the next hour telling you about photo-transduction in vertebrates and insects on the molecular level.

3. Eyes are present in most metazoan phyla: how did they evolve? Now. Everybody (at least most metazoans) got eyes. Trilobites had compound eyes very reminiscent of those of insects. Even scallops apparently got some sort of a primitive eye, and octopus has a sophisticated camera eye very remiscient of vertebrate � except there�s no plausible common ancestor. Understanding eye evolution is a major challenge.Now. Everybody (at least most metazoans) got eyes. Trilobites had compound eyes very reminiscent of those of insects. Even scallops apparently got some sort of a primitive eye, and octopus has a sophisticated camera eye very remiscient of vertebrate � except there�s no plausible common ancestor. Understanding eye evolution is a major challenge.

4. The grand challenge:

5. More broadly: Molecular pathway(s) of phototransduction are similar to many other signaling pathways: olfaction, taste, etc �Comparative Systems Biology�

6. Modeling molecular mechanisms of  photo-transduction

7. Vertebrate Rods and Cones

8. Vertebrate photoreceptor cell

9. Intracellular Voltage in Rods and Cones

10. Measuring currents

11. Patch clamp

12. Single Photon Response

13. Patch clamp recordings

14. Electrophysiology of Rods

15. Light detector protein: Rhodopsin

16. cis/trans �retinal transition

17. Light-induced conformational transition

18. Rhodopsin is a G-Protein coupled receptor

20. Rhodopsin is a G-Protein coupled receptor

21. G- proteins

22. Steps in activation

24. �Effector�

25. Where did cGMP come from?

26. G Protein Effectors Include

27. Shut-off

28. Rhodopsin inactivation

29. Transducin is inactivated by the a intrinsic GTPase activity which hydrolyzes GTP to GDP

31. Recovery to the resting state requires resynthesis of the cGMP that had been lost to hydrolysis.

32. Ca- the second �2nd messenger�

33. Negative feedback

34. Signaling cascade

35. Phototransduction Cascadeas an enzymatic amplifier

36. Olfaction:

37. Olfactory receptorcascade

38. Invertebrate Phototransduction Cascade

39. Enzymatic amplifier modules This cascade picture patently omitted recover steps. When we remember those the amplifier modules start to look like this.This cascade picture patently omitted recover steps. When we remember those the amplifier modules start to look like this.

40. More examples of amp modules GEF �guanine nucleotide exchange factor and GAP GTPase activating proteinGEF �guanine nucleotide exchange factor and GAP GTPase activating protein

41. General �push-pull� amplifier circuit Homework: derive this using Michaelis-Menten. Of course there are all sorts of saturation effectsHomework: derive this using Michaelis-Menten. Of course there are all sorts of saturation effects

42. Amplifier gain and time constant

43. Linear analysis of vertebrate phototransduction

44. Linear analysis of the cascade

45. �Hard� and �Soft� parameters

46. General behavior Exlain what happens if tau is very smallExlain what happens if tau is very small

47. Case of feedback

48. Consequences of negative feedback

49. Signal and Noise: How much gain is enough?

50. Reaction shot noise

51. Channel opening noise

52. cGMP fluctuations

53. Locality of single photon response

54. Single photon response variability

55. What is the largest source of response variability?

56. Multistep deactivation of Rh*

57. Meta-Rhodopsin shut-off:

58. Summary:

59. Acknowledgements