Molecular Approaches to Olfaction 
Randall R. Reed, Ph.D. — Associate Investigator 
Dr. Reed is also Associate Professor in the Departments of Molecular Biology and Genetics and of 
Neuroscience at the Johns Hopkins University School of Medicine. He received his bachelor's degree in 
biophysics from Johns Hopkins and his Ph.D. degree from Yale University. His postdoctoral research was 
done with Philip Leder at Harvard Medical School. 
OLFACTION is among the oldest of the sen- 
sory systems. All multicellular and many un- 
icellular organisms have evolved sensitive che- 
mosensory systems able to detect and identify 
natural chemical substances. The olfactory sys- 
tem of vertebrates and analogous systems for the 
other senses — vision, hearing, taste, and touch 
— allow the conversion of external stimuli into 
nerve impulses. In mammals, the olfactory sys- 
tem is exquisitely sensitive, capable of detecting 
some odorants present at a concentration of only 
a few parts per trillion. 
The ability of the olfactory system to discrimi- 
nate thousands of different odorants suggests a 
complex coding mechanism. However, the bio- 
logical basis for this coding remains a mystery. 
Unlike the visual and auditory systems, which 
need only encode information on frequency (or 
wavelength) and intensity, the olfactory sense re- 
quires multidimensional information. These con- 
siderations suggest a complex signal transductory 
process. 
The signal transductory pathway for olfaction 
can be divided temporally and spatially into sev- 
eral distinct steps. The first consists of the solubi- 
lization and concentration of airborne odorants. 
Considerable experimental data suggest that com- 
ponents of the mucus are able to concentrate 
odorants several thousandfold. We previously 
identified cDNA clones encoding proteins that 
are present at high concentration in the mucus 
and appear to bind odorants. These proteins, 
from rat and frog, are members of the retinol- 
binding protein family, many of which have been 
shown to solubilize hydrophobic ligands in 
serum. They are likely to play a similar role in the 
olfactory system. 
The recognition of the chemical structure of an 
odorant and transduction of that information 
across the plasma membrane is a poorly under- 
stood process. Some investigators hypothesize 
that odorants interact directly with the lipid 
membrane, but it is difficult to see how the abil- 
ity to discriminate stereoisomeric compounds 
could be accommodated by such a system. Sev- 
eral years ago we began to test an alternative hy- 
pothesis: that binding of odorants to specific 
membrane-associated proteins leads to intracel- 
lular changes in the primary sensory neuron. 
The detection of odorant-stimulated activation 
of second messengers in olfactory neurons sug- 
gested an analogy to sensory systems. In visual 
transduction, which is the best characterized of 
these systems, sensitivity is achieved through a 
second messenger cascade consisting of the 
membrane-bound receptor rhodopsin, a rod 
photoreceptor-specific GTP-binding protein, 
transducin, and a cyclic GMP phosphodiesterase. 
Light-stimulated decrease in the concentration of 
intracellular second messenger leads to modula- 
tion of the cellular membrane. Activation of 
adenylyl cyclase in preparations of rat olfactory 
cilia depends on the presence of guanine nucleo- 
tides. The observation of odorant-stimulated 
GTP-dependent adenylyl cyclase activity argues 
strongly for a similar GTP-binding protein- 
coupled signal transductory pathway in olfaction. 
We have identified a GTP-binding protein as 
well as an adenylyl cyclase expressed exclusively 
in olfactory sensory neurons. Moreover, these 
components are localized to the olfactory cilia, 
where the initial events in olfactory signal trans- 
duction are thought to occur. The Go,f protein, 
which is highly homologous to a GTP-binding 
protein that stimulates adenylyl cyclase in other 
systems, is expected to interact directly with the 
olfactory receptors. Additionally, we have demon- 
strated that Golf can couple receptor activation to 
increases in intracellular cAMP in cell lines defi- 
cient for the stimulatory G protein, G^. 
Identification of genes encoding olfactory re- 
ceptors may reveal how these structures are able 
to detect thousands of different odors. Recently, 
Linda Buck and Richard Axel (HHMI, Columbia 
University College of Physicians and Surgeons) 
have described a gene family that encodes pro- 
teins expressed in olfactory tissue. We have iden- 
tified additional members of this large family, 
utilizing the polymerase chain reaction tech- 
nique. These experiments reveal that expression 
of the mRNA that encodes these putative olfac- 
tory receptors is confined to the sensory neurons 
of the olfactory epithelium. 
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