Odorant-Receptor Interaction - Bioelectronic Nose: Integration of Biotechnology and Nanotechnology - page 72

heterologously-expressed ORs and OSNs expressing the corresponding ORs are not
exactly the same, the discrepancy can also be explained by other reasons including
the difference in the size of the odorant library, odor concentrations, and perhaps a
biased expression of certain ORs in mouse OSNs.

4.4 Role of Mucus in Odorant Recognition

The functional studies done in heterologous system or isolated OSNs represent ef-
ficient and replicable means of identifying odorant-receptor interactions; however,
there are concerns that the functional profiles generated in these systems may not
precisely reflect in vivo odorant perception. Alternatively, in vivo methodologies
based upon glomeruli activity in the OB may circumvent the uncertainties and help
to establish a combinatorial code that more accurately reflects sensory activities
under physiological conditions. By combining calcium imaging, retrograde dye
labeling, and single cell RT-PCR, Oka et al. successfully isolated the most sensitive
ORs for eugenol, methyl isoeugenol, and isovaleric acid using glomeruli activation
pattern in dorsal OB [ 19 ]. Notably, in some cases, OR sensitivity and ligand speci-
ficity obtained from olfactory bulbar activation patterns are found to be different
from that in isolated OSNs or cell-based heterologous expression systems [ 19 ]. One
possibility is that events prior to receptor-ligand interaction may influence which
and how ORs are activated. Indeed, when nasal mucus was eliminated from the
nasal cavity, the specificity of glomeruli in vivo became more repeatable and more
similar to OR activity in vitro [ 19 ]. It is known that ORs are expressed on the cilia
on the dendritic end of OSNs and the cilia on this epithelial surface are immersed
in nasal mucus, so odorants from external environment must cross the mucosal
perireceptor space to interact with ORs [ 20 ]. Proteomic analyses revealed that the
nasal mucus contains odorant-binding proteins and various metabolic enzymes [ 21 ,
22 ]. The nasal mucus may, therefore, play an important role in odorant recognition
by influencing the ways ORs interact with odorants.

Odorant-binding proteins Odorant-binding proteins (OBPs) were first discovered
in the nasal olfactory mucosa of bovines as a soluble protein capable of binding the
odorant 2-isobutyl-3-methoxypyrazine [ 23 ]. OBPs belong to the lipocalin family,
which are extracellular transport proteins for small hydrophobic molecules in aque-
ous solutions [ 24 ]. OBPs are present at high levels in nasal mucus (100 μM-1 mM)
[ 25 ] and can reversibly bind odorants with dissociation constants in the micromolar
range and thus effectively solubilizing volatile odorants, facilitating their diffusion
through the mucus barrier towards ORs [ 26 ]. Though it has been speculated that
OBPs may be responsible for carrying and concentrating odorants for subsequent
presentation to ORs, their exact role in olfaction remains to be explored. Some
studies have demonstrated that OBPs have a broad binding spectrum toward hydro-
phobic molecules [ 27 - 29 ] since there are numerous hydrophobic molecules but
only limited numbers of OBP genes found in mammals (four each in human [ 30 ]
and mouse [ 31 ]). Moreover, it is unclear whether OBPs could act as co-activators