Second Messengers and Cell Regulation 
David L. Carters, Ph.D. — Investigator 
Dr. Garters is also Professor of Pharmacology at the University of Texas Southwestern Medical Center at 
Dallas. He received his B.S. degree in agriculture and his Ph.D. degree in biochemistry at the University 
of Wisconsin. His postdoctoral research was done at Vanderbilt University. Before assuming his present 
position, Dr. Garbers was Professor of Pharmacology and of Molecular Physiology and Biophysics 
at Vanderbilt University School of Medicine. 
THE focus of the research in this laboratory 
centers on the mechanisms by which ceils 
communicate with each other — specifically the 
mechanisms by which sea urchin or mammalian 
spermatozoa detect signals from the egg. These 
studies have turned out to be applicable to so- 
matic cells and have led to the identification of a 
new cell surface receptor family in humans and 
other mammals. Members of this family serve as 
receptors for molecules that regulate blood pres- 
sure, as well as a large number of other physiolog- 
ical processes. 
Around 1981 our laboratory reported the puri- 
fication of a peptide that can stimulate sperm mo- 
tility. This peptide was derived from media in 
which sea urchin eggs had been allowed to stand. 
Subsequently it was demonstrated that different 
species of sea urchins contain different peptides 
and that the molecules from one species do not 
necessarily stimulate the sperm cells from an- 
other. In later research it became clear that sperm 
cells detect higher concentrations of peptide and 
swim toward them. Since the highest concentra- 
tions are around the egg, the sperm cell swims 
directly toward the egg under normal conditions. 
This laboratory set out to determine how the 
cell detects the egg peptide. We found that a par- 
ticular protein on the sperm plasma membrane 
specifically bound the egg peptide. This protein 
appeared to serve as the detector or receptor mol- 
ecule, but how it signaled to spermatozoa that a 
specific egg peptide had been bound was not 
clear. 
To help resolve the question, we purified the 
receptor protein on the plasma membrane. It was 
identified as the enzyme guanylyl cyclase, which 
catalyzes the formation of cyclic GMP, a small 
molecule that causes a change in the behavior of 
many different cells. It seemed possible that the 
receptor is situated with part of it outside the 
cell, where it could bind the egg peptide, and the 
other part inside, where it forms cGMP. The 
cGMP thus formed would then serve as a signal 
that egg peptide is being detected, lending 
greater speed and direction to the spermatozoon. 
To provide evidence that the membrane recep- 
tor protein is in fact guanylyl cyclase, we isolated 
complementary DNA clones for this enzyme. 
Such clones allow one not only to predict the 
primary structure of a protein, the receptor in 
this case, but to direct protein synthesis in quan- 
tity. Unfortunately, the sea urchin sperm receptor 
was not formed in the proper manner, and re- 
search continues on its expression. 
Under appropriate conditions, DNA will bind 
(hybridize) to closely related DNA. Therefore sea 
urchin DNA was used to determine whether 
mammals contain a related protein that might 
serve as a receptor. Clones containing comple- 
mentary DNA were isolated from rat brain. The 
DNA sequence revealed that a rat brain guanylyl 
cyclase is a component of the plasma membrane, 
with approximately one-half of the protein out- 
side and one-half inside the cell. The intracellu- 
lar region of the sea urchin sperm enzyme is very 
similar to the part of the rat brain enzyme inside 
the cell. In the regions outside the cell, however, 
the two proteins show little similarity. This 
would be expected if guanylyl cyclase serves as a 
cell surface receptor for peptides and the peptide 
of mammals is different from that of the sea ur- 
chin. That is, the detector part of the molecule 
would need to change to recognize a different 
peptide, but intracellular regions of the receptor 
could remain unaltered. 
Subsequent binding studies demonstrated that 
the cloned rat brain guanylyl cyclase could specif- 
ically bind and be activated by certain peptides 
synthesized in the heart and brain. These atrial 
natriuretic peptides (ANPs) regulate blood pres- 
sure, as well as various other physiological 
events. 
Since sea urchin sperm cells respond to pep- 
tides that do not resemble ANP, other animals 
may be expected to contain yet other peptides 
that interact with guanylyl cyclase. In addition, 
multiple membrane forms of guanylyl cyclase 
may exist within mammals. We have used the 
DNA that encodes the ANP receptor to determine 
whether other guanylyl cyclase receptors exist. 
Another receptor with properties similar to the 
one described above has been identified. This 
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