Olfactory system and MB development

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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
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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

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Odorant receptor (OR) genes share as little as
20 identity
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Drosophila Ordorant Receptors (DOR)
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DOR expressions are conserved among individuals
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DOR genes are expressed in distinct subsets of
antennal neurons
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Olfactory Sensilla
a antenna, pmaxillary Bbasiconic (L and S,
200), Ttrichoid, Ccoeloconic
1200 ORNs in antenna and 120 ORNs in maxillary
pulp
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Proneural genes in olfactory sensilla
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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 target one or more of many possible
effectors and finally gates ion channels thereby
creating APs
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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, 827841, 2003
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order degrading enzyme
order binding protein
sensory neuron membrane protein
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Fig. 2 Hypothetical model incorporating recent
insights about molecular interactions in the
lumen and at the dendritic membrane of an insect
ORN. Odorants entering through cuticular pores
are immediately loaded onto OBPs that transport
chemicals to conventional ORs (ORx) and also
protect them from degradation by ODEs (yellow).
Transport of odorants is directed by a
specific OBP receptor that is either constituted
by (1) the conventional OR (interacting with the
red odorant/ OBP) or (2) by a different molecule
(SNMP? interacting with the black odorant/ blue
OBP), which may physically interact with the
conventional and/or 83b family OR. SNMPs are
candidate molecules that may function as OBP
receptors. Conventional ORs physically interact
with a highly conserved 83b family OR which is
expressed in a majority of ORNs. OR83b family
proteins facilitate trafficking of
conventional ORs to the dendritic membrane and
may contribute to signal transduction. A complex
that consists of a conventional OR, an Or83b
family protein and possibly additional molecules,
may be required to fully activate a
heterotrimeric G-protein. Little is known about
the signal transduction events and ion channels
that are involved in the generation of APs in
insects. Possible G protein effectors involve
phospholipases such as PLCb
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Distribution of functional types of basiconica
sensilla
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Three types of large basiconic sensilla
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18 classes of ORNs within eight functional types
of basiconica sensilla (ab1, ab2, ab3 are large
basiconica sensilla)
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Mapping Or 22a/b expression in ab3 sensilla
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Drosophila olfactory system
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Neurons expressing a given DOR gene converge on
one or two spatially invariant antennal lobe
glomeruli
DOR-GAL4, UAS-nsyb-GFP
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Olfactory neurons expressing a given DOR gene
synapse with both contralateral and ipsilateral
glomeruli
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Targeting specificity of ORN to glomeruli does
not depend on Or genes expressed
UAS-GFP Or22a-GAL4
halo mutants (Or22a,b deleted)
wild type
UASGFP halo Or22a-GAL4
UAS-Or47a
UAS-Or33c
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Spatial maps of ORNs and glomeruli
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Projection neurons
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MARCM to generate single cell or NB clones
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Targeting specificity of adPNs and lPNs to
glomeruli
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Drosophila olfactory system
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The adult mushroom body center for learning and
memory

• 2,500 neurons

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Home work

• Principle and application of the MARCM technique

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MB (2500 neurons) derived from 4 NBs
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19 GAL4 strains reveal four folds of lineages in
MB
Hydroxyurea ablation of three NBs (only one left)
gives rise to all MB structure
WT
HU
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MARCM
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Development of the MB
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g neuron pruning
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TGF-b signaling activates steroid hormone
receptor expression
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Expressions of Acj6 and Dfr, two POU domain
homeobox proteins
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Misexpression of Dfr in acj6 mutant clones
reroutes DL1 innervation to another glomerulus