Odor Receptors Govern the Formation of a Precise Topographic Map

Department of Biochemistry and Molecular BiophysicsColumbia UniversityNew York, NY 10032

Wang, F., Nemes, A., Mendelsohn, M., Axel, R.

Odorant receptor proteins in olfactory receptor neurons are key components in guiding axon terminal location. This means that the receptors help to provide a topographic map within the olfactory bulb that distinguishes odorants.

• Construction of P2-deletion plasmids

• Generation of targeted mutations

P2-Deletion-IRES-tau-lacZ

M71P2-IRES-tau-lacZ

M50P2-IRES-tau-lacZ

P3P2-IRES-tau-lacZ

• Immunohistochemistry to detect β-galactosidase was performed.

• 28 µm coronal sections were dissected and frozen in OCT

• Tissues were fixed and then reacted with anti-β-galactosidase

antibody at 1:1000 dilution in PTS

• The bound antibody was visualized using Cy3-conjugated anti-rabbit

IgG

• X-gal staining was performed

MethodsMethods

Results – P2-DeletionResults – P2-Deletion•P2 ORNs converge on exactly 2 out of 1800

glomeruli (lateral and medial)

•P2-Deletion-IRES-tau-lacZ mutants pass through cribiform plate into OB but do not converge onto glomeruli

Results – M71 substitutionResults – M71 substitution• M71 receptor gene resides in different

chromosome and different epithelial zone than P2 (radical substitution)

• Axons of wild-type M71 receptor converge on opposite end of OB compared to P2 axons

• ORNs expressing M71P2-IRES-tau-lacZ allele still reside in P2 zone in epithelium

• ORNs expressing M71P2-IRES-tau-lacZ allele converge on glomeruli other than M71 or P2

Results – M50 SubstitutionResults – M50 Substitution• M50 gene located on same chromosome (#7)

as P2 gene – however different epithelial zone• ORNs expressing M50P2-IRES-tau-lacZ

allele reside in P2 zone in epithelium.• ORNs expressing M50P2-IRES-tau-lacZ

allele project axons to glomeruli distinct from either M50 or P2 wildtype

Results – P3 substitutionResults – P3 substitution• P3 gene located on same chromosome and

same epithelial zone as P2• P3 glomerulus lies 200 um anterior to P2

glomerulus (about 3 glomeruli separation)• ORNs expressing P3P2-IRES-tau-lacZ allele

converge on glomerulus adjacent to P3 and distinct from P2

P3 wtP3 wtP3 wt

ConclusionsConclusions• Odorant receptor is required for axon to terminate onto

glomerulus but not required to enter OB

• Other molecules may also be involved in guidance of axons

• Receptor may determine the anterior-posterior position of axon projection in OB while epithelial zone of a particular receptor type may determine the dorso-ventral location

• Proposals for how receptor guides axons

receptor may also be expressed along with other guidance molecules on the axon terminals which interact with OB

receptor activation in dendrite may result in expression of guidance molecules on axon terminals

• Activity dependent processes are unlikely to establish topographical map – but may be important in maintenance of the map