Production of Olfactory Receptors and Nanosomes Using Yeast System for Bioelectronic Nose - Bioelectronic Nose: Integration of Biotechnology and Nanotechnology - page 135

Fig. 8.6 Differential bioluminescence dose-response upon odorant stimulation of yeast-expressed
olfactory receptors. Measurements were performed on yeast transformed to co-express hOR17-40,
Golf and the luciferase reporter. This strain was induced with 2% galactose at 15°C. Dose-
response curves are plotted as a difference of bioluminescence response to odorants relative to
controls obtained by replacing odorants with water. (Adapted from Fig. 3 in [ 53 ] with permission
from the Royal Society of Chemistry)
expression level reached for ORs is noteworthy, since it could easily be monitored
by direct Western Blotting from cell lysates without prior purification or immuno-
precipitation, even if this level may vary from receptor to receptor and depends on
the construct. Moreover, it is a relatively cheap and easy to use system. However,
some aspects still require extensive studies. The first question is related to the speci-
ficity of ORs expressed in heterologous systems. Indeed, Touhara et al. pointed
out the potential influence of a different cellular environment in an heterologous
system relative to the native one to modulate OR specificity [ 54 ]. The preferential
ligand of an OR might thus depend on the heterologous system used, an hypothesis
that has not been systematically tested. The second question is about the OR sen-
sitivity to odorants in heterologous systems, and the odorant concentration used to
monitor the functional response of the ORs expressed. These two points are quite
interconnected. For instance, ORI7 preferential ligand is reported to be octanal by
Zhao et al. [ 55 ] after adenovirus-mediated expression in olfactory epithelium, but
heptanal in both COS cells and S. cerevisiae [ 16 , 17 ]. However, Zhao's experiments
on adenovirus-infected olfactory epithelium were conducted with varying carbon
chain length aldehydes using a single odorant concentration of 10 −3 M [ 55 ]. At
this concentration, they reported that octanal exhibited the largest response. On the
contrary, Levasseur et al. used concentrations down to 10 −13 M, and demonstrated
that COS-I7 cells exhibit response to heptanal in a low concentration range (10 −14 to
10 −12 M), to nonanal in an intermediate concentration range (10 −13 to 10 −10 M), and
to octanal for higher odorant concentrations over a broader range (10 −12 to 10 −7 M),
thus defining heptanal as the preferential odorant ligand for rat ORI7, inducing a
response at the lowest concentration [ 5 ]. Concentrations down to 10 −11 or 10 −17 M