Molecular Approaches to Olfaction 
We are now engaged in characterizing these re- 
ceptors biochemically and in screening for the 
specific ligands that activate them. The subcellu- 
lar localization of the receptor proteins is being 
examined by means of antibodies directed against 
conserved regions. These molecular tools should 
allow us to address some important questions. 
T)oes each of the several million olfactory sensory 
cells express a single receptor protein species? 
How are the genes that encode these receptors 
organized, and how is their expression regulated? 
The final step in the transduction of odorant 
stimuli is the generation of the intracellular sig- 
nal and the firing of an action potential. Special- 
ized forms of second messenger-generating en- 
zymes and novel ion channels play important 
roles in this process. We have identified cDNA 
clones encoding three distinct forms of adenylyl 
cyclase and are investigating the regulation of 
this important enzyme in olfactory tissue. Olfac- 
tory neuronal adenylyl cyclase type III has bio- 
chemical properties that would be advantageous 
for an enzyme involved in sensory transduction. 
Electrophysiologic experiments have identified 
cyclic nucleotide-gated ion channels in olfac- 
tory neurons, and we have recently isolated and 
characterized cDNA clones from olfactory tissue 
that encode ion channels with properties similar 
to those found in the visual system. 
The olfactory system is also interesting as a 
model for neuron differentiation and develop- 
ment. The olfactory neuroepithelium is the only 
neuronal tissue in adult mammals that undergoes 
continual regeneration. The lifetime of sensory 
neurons is approximately 40 days, after which 
they are shed from the epithelium and replaced 
from a population of neuroblast-like precursor 
cells. Moreover, if the nerve leading from the sen- 
sory neurons to the olfactory bulb is severed, all 
1 0 million receptor cells are rapidly lost and sub- 
sequently replaced in a relatively synchronous 
fashion. My laboratory is beginning to address the 
mechanism of regulation of olfactory neuron- 
specific genes. 
We have identified a specific DNA sequence 
upstream of all the genes known to be expressed 
specifically by the olfactory neurons and have ob- 
served in olfactory homogenates a binding activ- 
ity specific for that sequence. This putative tran- 
scriptional regulator might direct the expression 
of the entire repertoire of genes involved in olfac- 
tory signal transduction and neuronal maturation. 
We have recently identified a number of cDNA 
clones that encode proteins expressed only in the 
mature sensory neurons and are attempting to 
elucidate their role in the cell. A novel group of 
proteins, those expressed transiently during neu- 
ron maturation, may include receptors for neuro- 
genic as well as neurotrophic factors. Several of 
the genes we have identified appear to encode 
membrane-bound or cell surface proteins. 
We are continuing to use several techniques to 
elucidate the mechanism of signal transduction. 
Likewise, the identification of proteins asso- 
ciated with the replacement of olfactory neurons 
provides the tools to study neural development, 
not just in the olfactory system but also in other 
areas of the brain. In the future we will focus on 
the molecular components that underlie the 
complex mechanisms of signal transduction, sig- 
nal processing, and the formation of neural 
connections. 
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