Molecular Mechanisms in the Regulation 
of Synaptic Transmission 
Richard L. Huganir, Ph.D. — Associate Investigator 
Dr. Huganir is also Associate Professor of Neuroscience at the Johns Hopkins University School of 
Medicine. He completed his undergraduate work in biochemistry at Vassar College and received his Ph.D. 
degree in biochemistry and molecular and cell biology from Cornell University, where he performed his 
thesis research in the laboratory of Efraim Packer. After completing a postdoctoral fellowship with Paul 
Greengard at Yale University School of Medicine, Dr. Huganir moved to the Rockefeller University. 
INFORMATION processing in the brain de- 
pends on the transmission of signals between 
neurons at specialized areas of contact, called 
synapses. At synapses, ion channel proteins in the 
neuronal cell membrane generate an electrical 
current, which triggers the release of chemical 
signals from the first neuron, called the presynap- 
tic neuron. These chemical signals, or neurotrans- 
mitters, bind to specific receptor proteins in the 
membrane of the second neuron, called the post- 
synaptic neuron. The neurotransmitter receptors 
then generate electrical currents in the postsyn- 
aptic neuron and thus complete the process of 
synaptic transmission. 
The efficiency of synaptic transmission at any 
given synapse is constantly changing in response 
to a variety of factors; this synaptic plasticity 
plays a major role in the function of the nervous 
system. Both the amount of neurotransmitter re- 
leased from the presynaptic neuron in response 
to a given electrical signal and the sensitivity of 
the postsynaptic receptor system for a given 
amount of neurotransmitter can be modulated. 
The molecular mechanisms that underlie the 
modulation of synaptic transmission have only 
begun to be defined. Recent studies have pro- 
vided evidence that protein phosphorylation is 
an important mechanism in the regulation of syn- 
aptic transmission. 
Protein phosphorylation systems consist of 
three primary components, a protein kinase, a 
substrate protein, and a phosphoprotein phos- 
phatase. Protein kinases are enzymes that catalyze 
the chemical transfer of phosphate molecules 
from ATP to specific substrate proteins. The activ- 
ities of many protein kinases are regulated by neu- 
rotransmitters and hormones through the actions 
of substances called second messengers, such as 
cAMP, calcium, and diacylglycerol. Substrate 
proteins include many cellular components, 
among them enzymes, ion channels, and neuro- 
transmitter receptors. The addition of the nega- 
tively charged phosphate group alters the struc- 
ture of these substrate proteins, thereby 
regulating their functional properties. Phospho- 
protein phosphatases are enzymes that reverse 
the process of protein phosphorylation, remove 
the phosphate group from the substrate protein, 
and return it to its basal state. 
My laboratory is concerned with the structure 
and function of neurotransmitter receptors and 
the role of protein phosphorylation in the regula- 
tion of the properties of the neurotransmitter re- 
ceptors. We have used as a model system the best- 
characterized neurotransmitter receptor and ion 
channel in neurobiology today, the nicotinic ace- 
tylcholine receptor. In addition, we have been 
studying the major inhibitory neurotransmitter 
receptors in the brain, the GABA^ receptors, and 
the major excitatory neurotransmitter receptors 
in the brain, the glutamate receptors. These re- 
ceptors are neurotransmitter-dependent ion 
channels that generate electrical currents in the 
postsynaptic membrane of the synapse in re- 
sponse to their neurotransmitter. 
To study the molecular mechanisms involved 
in neurotransmitter receptor and ion channel 
function, it is essential to identify chemically the 
specific proteins required for this activity. We 
began by defining the molecular components re- 
quired for the functioning of the nicotinic acetyl- 
choline receptor ion channel. Using membrane 
reconstitution techniques, we solubilized the nic- 
otinic receptor and its ion channel from isolated 
postsynaptic membranes, purified it, and recon- 
stituted it into phospholipid vesicles. These stud- 
ies demonstrated that the purified receptor, con- 
sisting of four types of protein subunits (a, /?, 7, 
5) , contains the ion channel and has all the biolog- 
ical properties of the nicotinic receptor in the 
intact cell. 
We next began to characterize the protein 
phosphorylation of these structural components. 
Postsynaptic membranes isolated from synapses 
highly enriched in the nicotinic acetylcholine re- 
ceptor contain at least three different types of 
protein kinases that phosphorylate the nicotinic 
receptor on six different phosphorylation sites: 
cAMP-dependent protein kinase phosphorylates 
the 7- and 6-subunits of the receptor; a calcium- 
and diacylglycerol-dependent protein kinase 
phosphorylates the 5-subunit; and a protein- 
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