Second Messenger Pathways in Identified Neurons 
James H. Schwartz, M.D., Ph.D. — Investigator 
Dr. Schwartz is also Professor of Physiology and Cellular Biophysics and of Neurology at Columbia Uni- 
versity College of Physicians and Surgeons. He received his M.D. degree from New York University Medical 
Center and his Ph.D. degree from the Rockefeller University for work with Fritz Lipmann on the catalytic 
site of alkaline phosphatase in Escherichia coli. He then joined the faculty of the Department of Microbi- 
ology at New York University. While there, he joined the Division of Neurobiology and Behavior, which 
later moved to Columbia's College of Physicians and Surgeons. Dr. Schwartz's honors include the Selman 
A. Waksman Award in Microbiology. He has been a Visiting Scholar at the New York Psychoanalytic In- 
stitute and at the NIH. 
STIMULATION of a nerve cell can result in 
long-lasting changes in its biochemistry. 
These are produced by a process called signal 
transduction. The stimulus, delivered by either a 
neurotransmitter or a hormone, acts upon spe- 
cific receptors on the nerve cell's surface; and 
these, in turn, induce the formation of "second 
messengers" inside the cell. 
Second messengers usually work by binding to 
proteins that might be called secondary effectors. 
In many instances the effectors are enzymes 
termed protein kinases. When activated, protein 
kinases catalyze the transfer of a phosphoryl 
group from ATP to a large number of different 
cellular proteins. As a consequence of the phos- 
phorylation reactions, the functions of these cel- 
lular proteins are changed. Thus production of 
the second messenger alters the way the neuron 
behaves. 
How long the nerve cell remains changed ini- 
tially depends on the lifetime of the second mes- 
senger. The secondary effector remains activated 
as long as there is enough second messenger 
within the cell. After brief stimulation, second 
messengers are formed for only a few minutes, 
the action of the secondary effector is brief, and 
the altered state of the neuron persists for less 
than an hour. If the stimulation is prolonged, 
however, the result is quite different. An interest- 
ing and important phenomenon occurs. 
In such cases, the activity of the secondary ef- 
fector enzyme persists even after formation of the 
second messenger stops. One might say fancifully 
that the enzyme is taught to behave differently. 
Like the animal, the molecule behaves as if it has 
learned. This phenomenon underlies the en- 
hancement of synaptic transmission believed to 
be the basis of several simple kinds of learning 
and memory. 
We have been interested in the mechanisms by 
which secondary effector enzymes become edu- 
cated. In previous years, we reported progress in 
understanding the mechanisms for three of the 
best known second messenger pathways. After dis- 
covering in 1985 the phenomenon for the Ca^V 
calmodulin-dependent protein kinase (a second- 
ary effector enzyme for Ca^"^), we described 
similar phenomena for the cAMP-dependent pro- 
tein kinase (PKA) and for protein kinase C. This 
year we made significant advances in understand- 
ing how PKA is made to persist for many hours 
after stimulation ceases. 
For all of these protein kinases (which are dis- 
tantly related in evolution) , the second messen- 
ger activates the enzyme by releasing an inhibi- 
tion built into the latter molecule. This process is 
reversible. With PKA the inactive form is a tetra- 
meric complex consisting of two identical regula- 
tory (R) subunits that fit precisely together with 
two catalytic (C) subunits. The R subunits, upon 
binding the second messenger, cAMP, release the 
two C subunits for action. 
In earlier experiments we demonstrated that 
the ratio of R to C changes in sensory neurons of 
the marine moUusk Aplysia californica to cause 
the enhanced protein phosphorylation that oc- 
curs in the presynaptic facilitation of sensory-to- 
motor neuron synapses in long-term sensitization 
of defensive reflexes. The change consists of a 
decrease in the amount of R subunit, which re- 
quires the synthesis of new protein. 
What is the mechanism for producing the 
change in the R to C ratio observed in sensory 
neurons of Aplysia trained behaviorally to ex- 
hibit the long-term sensitization? To answer this 
question, we cloned cDNAs encoding both R and 
C subunits and therefore can determine the 
amounts of their messenger RNAs. In addition, we 
can raise antibodies to measure the amounts of 
subunit protein in sensory cells before, during, 
and after training. We find that the PKA changes 
after long-term training because R subunits in the 
neurons disappear. Furthermore, there is no 
change in the amount of C subunits, nor in R or C 
subunit messenger RNAs. 
Disappearance of proteins is commonly caused 
by enzymes that digest them. In the sequence of 
amino acids of the R subunits inferred from the 
cDNA cloning experiments, there is an amino- 
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