Development and Function of the Synapse 
Richard H. Scheller, Ph.D. — Associate Investigator 
Dr. Scheller is also Associate Professor of Molecular and Cellular Physiology and Associate Professor of 
Biological Sciences (by courtesy) at Stanford University. He received his B.S. degree from the University 
of Wisconsin- Madison and his Ph.D. degree in chemistry from the California Institute of Technology. 
Dr. Scheller was a postdoctoral student with Richard Axel and Eric Kandel at Columbia University. 
THE nervous system is composed of large num- 
bers of unique cells that communicate with 
each other via the regulated release of chemical 
neurotransmitters. These synaptic interactions 
govern animal behavior, and modulation of the 
efficacy of synaptic communication is thought to 
underlie learning and memory. We are interested 
in understanding the molecular mechanisms of 
synaptic formation during development and re- 
generation in the peripheral nervous system after 
nerve injury. It is also our goal to contribute to an 
understanding of how the nerve terminal func- 
tions in the regulation of neurotransmitter 
release. 
Processing and Packaging of Neuropeptides 
Many synapses release two types of chemical 
messengers: fast-acting, or classical transmitters, 
and slower-acting messengers, or neuromodula- 
tors. Most of the various chemicals used as mes- 
sengers in the brain are neuropeptides. These 
molecules are synthesized as larger precursors 
that are processed to smaller active peptides. One 
interesting neuropeptide precursor is expressed 
in an identified set of neurons, the bag cells, in 
the marine snail Aplysia. When these neurons 
fire, they release a set of neuropeptides derived 
from a single precursor. These peptides act on 
neurons and peripheral tissues to regulate egg 
laying, a stereotyped behavior. 
Interestingly, the peptides produced on the 
egg-laying hormone (ELH) precursor are pack- 
aged in two types of vesicles. These vesicles con- 
tain different sets of peptides and are differen- 
tially localized within the neurons. We are 
interested in understanding how the peptides ini- 
tially synthesized on a single precursor are sorted 
into different vesicles. We are also interested in 
understanding the physiological significance of 
the differential packaging and localization. 
When the ELH precursor is transfected into 
mammalian pituitary tumor cells (AtT-20 cells), 
ELH is packaged with the endogenous hormone. 
The amino-terminal region of the precursor is de- 
graded within the secretory cell, probably in the 
lysosomes. Thus the AtT-20 cells, like the bag 
cells, differentially route the two regions of the 
ELH prohormone. Mutating the first cleavage site 
from a set of four basic residues to two basic resi- 
dues results in constitutive secretion of the 
amino-terminal region of the precursor, not intra- 
cellular degradation. These results suggest that 
the subcellular location of the first endoproteoly- 
tic processing event is critical in determining the 
routing of the processing intermediates. 
Mechanisms of Synaptic Transmission 
When the action potential travels down the 
nerve and enters a release zone, changes in the 
membrane potential open channels that allow cal- 
cium to enter the cell. The calcium promotes 
transmitter release and membrane fusion. The 
membrane then recycles, forming new vesicles, 
which are then replenished with chemical trans- 
mitter. This cycle might be considered the funda- 
mental process that underlies nervous system 
function, yet little is known about the molecular 
mechanisms involved. In an attempt to define the 
molecular mechanisms that regulate membrane 
flow in the nerve, our laboratory and others have 
begun to characterize the proteins associated 
with the critical organelle in the process, the syn- 
aptic vesicle. For these studies, we use mamma- 
lian brain and the electric organs of marine rays. 
These electric organs have a concentration of syn- 
apses approximately 100-fold higher than that of 
skeletal muscle. In addition, these synapses are 
homogeneous; they all use the neurotransmitter 
acetylcholine. 
Purified synaptic vesicles contain about 20-50 
protein bands when fractionated on acrylamide 
gels. Genes encoding many of these proteins have 
been characterized and the primary sequence of 
the molecules determined. Some of the proteins 
show interesting homologies to other molecules, 
and others are turning out to have counterparts in 
yeast where genetic studies of membrane traffick- 
ing have provided insight into the secretory pro- 
cess. It has also become apparent that many of the 
synaptic vesicle proteins are members of small 
gene families. Individual members of these gene 
families are differentially expressed through the 
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