Sensory/ Motor Systems March 10, 2008

Transcription

1 Sensory/ Motor Systems March 10, 2008 Greg Suh Title: Chemical Senses- periphery

2 Richard Axel and Linda Buck Win Nobel Prize for Olfactory System Research 2

3 3 After Leslie Vosshal

4 Fruit Odors Pear Pear Banana Orange Peach Pineapple Apple Lemon 4 After Leslie Vosshal

5 Stereochemistry and Olfaction Menthol Caraway Spearmint 5 After Leslie Vosshal

6 A multitude of vertebrate chemosensory receptor genes MOE VNO 6 After Leslie Vosshal

7 Topographic maps Retinotopy y x x Spatial information Olfactory projections y? x T y p e o f r e c e p t o r y 7

8 Our brain is consisted of a series of maps 8 Kandel et al.: Principles of Neural Science Purves et al.: Neuroscience

9 Sensory Perception One receptor per cell allows Brain discrimination Qualitative map? Blue Red Green 9

10 Space is Used in Sense Organs and the Brain to Encode Quality System Visual Somatosensory Auditory Olfactory Quality Position in the external world Position on the body Tone Chemical identity 10

11 Comparisons of Vision and Olfaction 3 Literature use Sensory stimuli Vision Complimentary: He has a vision. Well defined; extremely well organized spatially Olfaction Instinctive: This smells right. Poorly defined; poor spatial organization Signaling mechanism Brain processing Photon to rhodopsin to G protein to cgmp gated channel closure; fast Best studied because of precise stimuli and sophisticated physiology & psychophysics Odorant to OR to G protein to camp gated channel opening; slow Poorly understood until molecular recent advance Nobel Prize to Richard Axel & Linda 11 Buck

12 Olfaction 2 Odorants bind to Olfactory Receptors (ORs) in the dendritic tip of Olfactory Receptor Neurons In mouse, there are ~1000 OR genes (~ 3% of genes!) Each OR neuron expresses only one type of OR: Exclusion ORs activate G protein, adenylate cyclase & camp-gated channels: Sensory stimuli are sent as action potentials to the brain Olfaction is a combinatorial recognition process: each OR is activated by a number of odorants; each odorant activates a number of ORs (ORNs). A vast number of odorants can be distinguished How is the olfactory information processed in the brain? 12

13 Anatomy of the Olfaction System A: Olfactory receptor neuron B: Glomerulus C: Mitral cell D: Granule cell E: Mitral cell axons sending olfactory information to the olfactory cortex 13 from Ramon y Cajal (1911) Histology

14 Anatomy of the Olfaction System 14 from Axel (1995) Sci. Am. 273(4), 154

15 Anatomy of the Olfaction System 6 15 Axel (1995) Sci. Am. 273(4), 154

16 Three models of information processing 16 Axel (1995) Sci. Am. 273(4), 154

17 Olfactory receptor expression in nasal epithelia Vassar et al. (1993) Cell 74:309 Random expression of receptors within one of the four zones: 17 The ORNs expressing common receptors are NOT grouped

18 ORN axon convergence I: in situ hybridization of receptor expression In situ hybridization for a particular OR gene: two bilaterally glomeruli in the olfactory bulb 1- what is OR s function in the OB? 2- why is OR expressed in a single glomeruli? ORNs expressing the same receptor appear to target their axons to the same glomeruli 18 (usually one receptor, two glomeruli per hemisphere) Axel (1995) Sci. Am. 273(4), 154

19 Is the model B correct? 19

20 ORN axon convergence II - IRES: translation of the P2 receptor and tau-lacz - tau-lacz: Microtubule binding protein tau and lacz, for axon labeling - KI: insertion of a transgene into a defined genomic locus (here the P2 locus): 20 transgene is expressed in the exact same pattern as the endogenous gene

21 All P2 receptor expressing neurons converge their axons onto the same glomeruli P2 in situ tau-lacz 21

22 olfactory bulb nose 22 Mombaerts et al. (1996) Cell 87, 675

23 A glomerular map for odor representation Each of the 1000 ORs in mouse is expressed randomly in ~1000 ORNs The ~1000 ORNs expressing a given OR gene converge their axons onto ~2 of the ~2000 glomeruli The olfactory information is represented as a two -dimensional map in the olfactory bulb; a given odor should activate a defined set of olfactory receptors and a defined set of glomeruli 23

24 The Wiring Specificity Problem Human Brain: ~10 12 neurons, ~10 15 synapses Human genome: ~3x10 9 bp, ~3-5 x 10 4 genes There are far few genes (proteins) than needed to make cytochemical tags that specify every connection precisely! Possible Solutions Make many proteins from a single gene Use combinatorial code to specify connections Use different amount of the same protein to specify different connections: gradients Leave some wiring unspecified allow experience to determine the final connections Olfactory maps are qualitatively different from visual 24 /somatosensory maps: digital vs. continuous

25 Roles of olfactory receptors in ORN axon targeting Knock-in of IRES-tau-lacZ in P2 receptor ORN now express only lacz Axon convergence is disrupted Thus, OR are required for axon convergence to glomeruli 25

26 P2-IRES-tauLacZ P2-deletion-IRES-tauLacZ 26 Wang et al. (1998) Cell 93, 47

27 Receptor swap restores ORN axon convergence 43 P2 receptor is replaced with a different OR-IRES-tau-lacZ ORNs normally expressing P2 now express M50 along with the tau-lacz marker Axon convergence is restored 27 Where do the axons go? P2? M50?

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30 Olfactory receptors play an instructive although not deterministic role in ORN axon targeting M50->P2 swap results in targeting of ORN axons to some intermediate glomerulus! (So did several other swaps) Some other as-yet unidentified cues must work together with olfactory receptors to instruct targeting specificity The ligands that these ORN axons recognize are completely mysterious Could post-synaptic target cells play a role? 30 Wang et al. (1998) Cell 93, 47

31 How Are Neural Maps Made? Determination partially by connections Independent specification 31

32 Post-synaptic target cells are not involved in the map - In Tbr-/- mice, mitral cells (second order projection neurons ) are not formed. - In Dlx-1,2 -/- mice, the majority of GABAergic local interneurons (e.g. granule cells) are not formed. - Yet ORN axon convergence are normal Hence, ORN axon convergence appears not to be dependent on their major post-synaptic targets 32 or Tbr-/-, P2-IRES-lau-lacZ

33 How Are Neural Maps Made? Determination partially by connections Independent specification 33

34 Cyclic Nucleotide-Gated Channel KO Mice are Totally Anosmic Knockout Wildtype Electroolfactograms (EOG) show that response to all odorants is absent in CNG channel KO mice. (The residual opposite sign response to triethylamine arises from non-neuronal stimulation) (Brunet et al., Neuron 17:681 (1996) 34

35 Neuronal activity is not involved in the map In the absence of odor-induced electrical activity, 35 ORN axon targeting is normal

36 Antennae Maxillary Palps 36

37 The Drosophila Brain Mushroom body Lateral horn Antennal lobe Antenna Maxillary palp Antennal lobe Mushroom body37 Lateral horn

38 The Antennal Lobe (fly Olfactory bulb) comprises ~ 43 glomeruli, occupied in stereotypic positions. D A V P 38

39 Neurons expressing a given OR converge upon a single pair of glomeruli. Antenna Antenna lobe Or47a Or47b Or22a Vosshall et al., 2000

40 Molecular biology of Drosophila olfaction There are ~50 olfactory receptor genes in Drosophila (DOR); each DOR is expressed in a stereotypical subset of olfactory receptor neurons (ORNs, ~ 1300 total) in the third antennal segment, or in the maxillary palp Each ORN expresses only one DOR; the exception to this rule is one distant member of DOR: OR83b, which is expressed in 80% of all ORNs. OR83b may serve as a co -receptor ORNs expressing the same receptor converge their axons onto the same glomerulus (~ 40-50) Hence, the 1 receptor, 1-2 glomerulus rule derived from the mouse study also applies in Drosophila, at least to the first degree of approximation Clyne et al. (1999) Neuron 22, 327; Vosshall et al. (1999) Cell 96, 725; Vosshall et al. (2000) Cell 102, ; Gao et al. (2000) Nat. Neurosci. 3, 780

41 How Are Neural Maps Made? Determination partially by connections Independent specification 41 Are the identities of projection (OR target) neurons instructed by ORNs, or are they independently specified?

42 MARCM: Mosaic Analysis with a Repressible Cell Marker Developed by Luo and 42 Lee

43 Dorsal and lateral neuroblast clones contain PNs with stereotypical & complementary projections. Greg Jefferis, Lisa Marin

44 Projection Neurons Connect the Antennal Lobe to the Mushroom Body and lateral horn. 44

45 Expression of CD2 in a Population of Projection Neurons 45

46 Expression of CD8-GFP in Single Projection Neurons 46

47 Individual Projection Neurons Innervate a Single Glomerulus 47

48 Different Projection Neurons Show Distinct Patterns in the lateral horn. 48

49 Similarities of Projection Patterns

50 Spatial Map in the Higher Brain

51 G-CaMP: A Calcium Sensitive GFP N M 13 C CaM Ca nm 509 nm cpegfp G-CaMP Nakai et al., 2001

52 The Antennae-Brain Preparation

53 Glomerular Response to Caproic Acid and Pyridine Caproic Acid Pyridine

54 Odor Representations in the Fly Brain

55 C. elegans olfaction Cori Bargmann and colleagues Of the 959 cells in C. elegans, 302 are neurons; 3 pairs (AWAs, AWBs, AWCs) are responsible for sensing volatile stimuli (olfaction) AWAs and AWCs mediate odor-induced attraction; AWBs mediate odor-induced repulsion (laser ablation experiments) ~500 G-protein coupled receptors are expressed in sensory neurons, ~100 of which are likely expressed in olfactory neurons; thus individual neurons express multiple olfactory receptors 19 Odr-10 is a receptor for diacetyl, is expressed in AWA neurons, and mediates diacetyl induced attraction Misexpress Odr-10 in AWB neurons in Odr-10 mutants resulted in diacetyl induced repulsion! Thus, a simple logic likely applies to C. elegans olfaction: all attractive receptors are expressed in AWAs and AWCs; all repulsive receptors are expressed in AWBs. Activation of a particular class of sensory neurons determines the behavioral consequence. Space code in the simplest form! Sengupta et al. (1996) Cell 84, 899 Troemel et al. (1997) Cell 91,

56 Mammalian Taste

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