Olfactory system and MB development

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The Nobel Prize in Physiology or Medicine 2004

"for their discoveries of odorant receptors and the organization of the olfactory system"

Richard Axel Linda B. Buck

Olfactory receptor & different strategy

• Odorous ligands trigger G protein-coupled receptors• ~350 in human, ~1000 in C. elegans and mouse• 60 ORs in D.melanogaster, with 1300 receptor neurons

in flies.• there are ~60 gustatory receptors (GRs) in flies,

sensing sugar (attractant), bitter compound (repellent), salt and water.

C. elegans only 16 pairs sensory cell. Each with multiple kinds of receptor mice 2,000,000 receptor neurons. Each with one receptor

Odorant receptor (OR) genes: share as little as 20% identity

Drosophila Ordorant Receptors (DOR)

DOR expressions are conserved among individuals

DOR genes are expressed in distinct subsets of antennal neurons

Olfactory Sensilla

a: antenna, p:maxillaryB:basiconic (L and S, ~200), T:trichoid, C:coeloconic

1200 ORNs in antenna and 120 ORNs in maxillary pulp

Proneural genes in olfactory sensilla

Fig. 1 The insect antenna typically bears between several hundred and some thousand sensillar hairs, depending on the species, which are depicted on the left. Each sensillum contains between 1 and 4 ORNs (red and dark blue, respectively). These are bipolar neurons which, on one end, extend a dendrite that is bathed in sensillar lymph and interacts with odorants and on their other end project axons that terminate in the AL, where olfactory information is processed. ORNs are embedded in a layer of support cells (light green) that secrete proteins such as OBPs (orange and light blue) into the sensillum lymph. Odorants can enter the sensillum lymph via pores in the cuticle and cross the lymph in a hitherto still debated way that may involve OBPs as transport vehicles. Subsequent odorant-OR binding takes place along the dendrite of an ORN and may activate a heterotrimeric G-protein to targetone or more of many possible effectors and finally gates ion channels thereby creating APs

Expression of Or22a/b in the Dendrites of ORNs in Large Basiconica Sensilla

(D) Crosssection labeled with anti-22a/b antibody. Granules of immunogold are visible in the dendrites (labeled D).(E) Longitudinal section labeled with anti-22a/b antibody. Labels indicate the following: C, cuticle; D, dendrite; P, pore; and SL, sensillum lymph.(F) An example of a different morphological subtype of s. basiconica, which shows no labeling with anti-22a/b antibody.

Neuron, Vol. 37, 827–841, 2003

order binding protein

order degrading enzyme

sensory neuronmembrane protein

Fig. 2 Hypothetical model incorporating recent insights aboutmolecular interactions in the lumen and at the dendritic membraneof an insect ORN. Odorants entering through cuticular pores areimmediately loaded onto OBPs that transport chemicals toconventional ORs (ORx) and also protect them from degradationby ODEs (yellow). Transport of odorants is directed by a specificOBP receptor that is either constituted by (1) the conventional OR(interacting with the red odorant/ OBP) or (2) by a differentmolecule (SNMP?; interacting with the black odorant/ blue OBP),which may physically interact with the conventional and/or 83bfamily OR. SNMPs are candidate molecules that may function asOBP receptors. Conventional ORs physically interact with a highlyconserved 83b family OR which is expressed in a majority ofORNs. OR83b family proteins facilitate trafficking of conventionalORs to the dendritic membrane and may contribute to signaltransduction. A complex that consists of a conventional OR, anOr83b family protein and possibly additional molecules, may berequired to fully activate a heterotrimeric G-protein. Little isknown about the signal transduction events and ion channels thatare involved in the generation of APs in insects. Possible G proteineffectors involve phospholipases such as PLCb

Distribution of functional types of basiconica sensilla

Three types of large basiconic sensilla

18 classes of ORNs within eight functional types of basiconica sensilla

(ab1, ab2, ab3 are large basiconica sensilla)

Mapping Or 22a/b expression in ab3 sensilla

Drosophila olfactory system

Neurons expressing a given DOR gene converge on one or two spatially invariant

antennal lobe glomeruli

DOR-GAL4, UAS-nsyb-GFP

Olfactory neurons expressing a given DOR gene synapse with both

contralateral and ipsilateral glomeruli

Targeting specificity of ORN to glomeruli does not depend on Or genes expressed

UAS-GFP; Or22a-GAL4 halo mutants

(Or22a,b deleted)wild type

UAS-Or47a UAS-Or33cUASGFP; halo; Or22a-GAL4

Spatial maps of ORNs and glomeruli

Projection neurons

MARCM to generate single cell or NB clones

Targeting specificity of adPNs and lPNs to glomeruli

~50

~35

Drosophila olfactory system

The adult mushroom body: center for learning and memory

2,500 neurons

Home work

Principle and application of the MARCM technique

MB (2500 neurons) derived from 4 NBs

19 GAL4 strains reveal four folds of lineages in MB

WT HU

Hydroxyurea ablation of three NBs (only one left) gives rise to all MB structure

MARCM

Development of the MB

neuron pruning

TGF- signaling activates steroid hormone receptor expression

Expressions of Acj6 and Dfr, two POU domain homeobox proteins

Misexpression of Dfr in acj6 mutant clones reroutes DL1 innervation to another glomerulus