Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 301Bi 1 “Drugs and the Brain” Lecture 28 Tuesday, May 30, 2006Testing our understanding of evolution: the eye2“Nothing in biology makes sense except in the light of evolution”Theodosius DobzhanskyAll modern biological processes evolved from related processes.Every modern gene evolved from other genes.Every gene has an ortholog in related species, and most genes have paralogs in the same species.3Myr BPHemoglobin paralogs in the human genomeMyr BPHemoglobin paralogs in the mouse genomechromosome 7 orthologs resemble each otheracross species (mouse  vs human ) human  vs mouse  vs paralogs resemble each other, distant or closely, within a species G vs AWe lack the time to discuss the origin of life in Bi 144. Connections to the brain3. retina1. lens andoptics 2. photoreception event5. A master switchthat controls differentiation6. Time frame for evolution of the major features5Because all vertebrate eyes are quite similar, the hunt for orthologs is straightforward and successful in most cases. That two organisms share many orthologs is powerful evidence for the view that those organisms are descended from a common ancestor—a central aspect of evolution.6The lens has an index of refraction greater than water, because it contains a high concentration of protein. Many proteins have been used in various animals, in a process termed “gene sharing”. Some of these proteins, termed crystallins, are also enzymes that perform metabolic functions in other tissues.Apparently the only requirement is that the protein have good solubility and no attached groups (such as vitamins) that might absorb light.1. Lens and opticsfrom Lecture 5 How much is 4 mM protein?A typical protein has 500 amino acid residues.An average residue has a molecular mass of 110.Therefore the average protein has a molecular mass of 55,000.( 4 x 10-3 mol/liter) x (5.5 x 104 g/mol) = 2.2 x 102 g/l = 220 g/l.The cell is ~22% protein!7Pax-6 orthologs occur in phyla as diverse as as mammals, insects, and molluscs. Many genes, including crystallins, have acquired a “Pax-6 responsive element”similar to Lecture 14CrystallinPax-6, a transcription factor with orthologs in many speciesPax-6 (vertebrates)Ey (Drosophila)8We went hunting for orthologs and paralogs, but we found a molecular example of the classical principal that “structure comes before function”. Existing proteins have been used for an additional functions. Which way were they adopted? Probably the use in the lens came second. Evidently several distinct transcription factors can “share” activation of a given gene.from Lecture 14:9The aperture mechanism: controlled by smooth musclessinglesmooth muscle cellinextensible fibersContracts and thickens: leads to smaller pupilnerve from brain; muscarinic synapseInnervated smooth muscles control:diameter of blood vessels, peristaltic activity of the intestinal tract, diameter of the bladder neckIn each case, the nervous system has evolved circuits that(1) extract and integrate information from sensors and (2) employ smooth muscles in a homeostatic loop.blocker: atropine from Atropa belladonna10Photoreceptor organs have evolved independently at least 40 times, each time responding to the visible spectrum and near-UV.How do we explain the use of a limited part of the spectrum?Infrared light is not sufficiently energetic to provoke photochemistry such as cis-trans isomerization.Shorter-wavelength ultraviolet light is too energetic and would destroy organic molecules. 2. The photoreception event11rodcone Rhodopsin Rhodopsin h12cytosolThe usual GPCR pathwaynucleuskinasephosphorylatedproteincAMPCa2+intracellularmessengerreceptortsqiG proteinenzymechannel effectormembranefrom Lecture 1213Intracellular messengers bind to proteinskinasesphosphorylatedproteinA few ion channels(olfactory system, retina)NNNNNH2OOHOHHOP-OOcyclic AMP (cAMP)Ca2+ andbut back in lecture 12, we said . . .intracellularmessengerCa2+cAMPcGMP14The GPCR pathway in a photoreceptorchannelreceptortsqiG proteinenzymechannel effectorcytosolintracellularmessengerCa2+cAMPcGMP15GTP GDP + PiEffector: enzyme or channeloutsideinsideNeurotransmitter or hormonebinds to receptoractivatesG proteinThe Central Dogma of Drugs and the Brain, Part 2:The G Protein-Coupled Receptor PathwayHow fast?100 ms to 10 sHow far?Probably less 1 mlike Lecture 1216Photon isomerizesretinal bound to rhodopsinSpecial aspects of the G protein-coupled receptor pathway in photoreceptionHow fast?< 100 msHow far?< 1 mGTPactivatesG proteinEffector is an enzymeGDP + Pilike Lecture 12hAlthough the components are not membrane-bound, the membranes effectively restrict their motionIn rods and cones, these proteins lack lipid tails17Each opsin interacts distinctly with retinal, producing a distinct absorption spectrum. Absorption spectra of cone pigmentsThere are at least 4 paralogs of opsin in the human genome.18cAMP ATP Effector enzyme“cyclase”Breakdown enzyme“phosphodiesterase”Inhibited by caffeineuninterestingcGMP GTP Enzyme“cyclase”Breakdown enzyme“phosphodiesterase”uninterestingThe effector for Gtlike Lecture 12A paralog expressed elsewhere in the body is inhibited by ViagrachannelreceptortsqiG proteinenzymechannel effectorintracellularmessengerCa2+cAMPcGMP“Viagra . . . may cause a perception of bluish haze or increased light sensitivity in some patients.”19Rods and Cones have cGMP-activated Na+ Channels Excised “inside-out” patch allows access to the inside surface of the membraneno cGMPno channel openings+cGMP*+cGMP*closedopenlike Lecture 12receptorqiG proteinchanneltsenzymechannel effectorintracellularmessengerCa2+cAMPcGMP20RodConeSynapsesHorizontal cellBipolar cellSynapsesGanglion cell is unique in firing impulsesoptic nerve3. Neurons of the retinaGlutamate is the major transmitter; modulation by dopamine, acetylcholine.Many paralogs to genes expressed elsewhere:channels,receptors,transporters.21Roger Sperry’s Nobel prize-winning experiments (1948) (goldfish): After he cut the optic nerve, individual fibers grew back to their original destination in the brain. Sperry also conducted the “Split brain” experiments that