Traffic of Synaptic Vesicle Proteins in Neurons 
and Endocrine Cells 
Pietro De Camilli, M.D. — Investigator 
Dr. De Camilli is also Professor of Cell Biology at Yale University School of Medicine. He received his M.D. 
degree from the University of Milano, Italy, where he worked with Jacopo Meldolesi. He did his 
postdoctoral studies with Paul Greengard at Yale University. Prior to his current position, he held 
appointments both at Yale and the University of Milano. Dr. De Camilli is a recipient of a McKnight 
Scholars Award and a member of the European Molecular Biology Organization (EMBO ). 
WE are interested in the mechanisms of sig- 
naling between nerve cells, with emphasis 
on neuronal secretion of neurotransmitter mole- 
cules. Most neurotransmitters are stored in vesi- 
cles within nerve terminals and are secreted in 
response to depolarization of the terminal by fu- 
sion of the vesicle with the plasmalemma, a pro- 
cess called exocytosis. Most, and possibly all, 
nerve terminals contain two classes of secretory 
vesicles: synaptic vesicles (SVs) and the so-called 
large dense-core vesicles (LDCVs), which differ 
in a variety of properties, including neurotrans- 
mitter content. 
SVs are small, morphologically homogeneous 
vesicles (50-nm diameter), containing nonpep- 
tide neurotransmitters only. They are very abun- 
dant in nerve terminals, and clusters of SVs ap- 
posed to the plasmalemma represent a typical 
structural feature of axon endings. Their exocyto- 
sis takes place selectively at the presynaptic plas- 
malemma, is faithfully linked to nerve terminal 
depolarization, and plays the dominant role in 
the fast, point-to-point intercellular signaling typ- 
ical of the nervous system (the so-called synap- 
tic transmission). SV membranes are rapidly re- 
internalized after exocytosis and are used to 
re-form SVs, which are loaded locally with 
neurotransmitters . 
LDCVs are larger vesicles with a dense protein- 
rich core. Their contents are predominantly pep- 
tide neurotransmitters, though they may also 
contain amines. Their exocytosis is preferentially 
triggered by trains of action potentials and is in- 
volved primarily in a modulatory type of signal- 
ing. They have long been recognized as the neuro- 
nal organelles equivalent to secretory granules of 
endocrine cells, which secrete peptide hormones 
and amines. In contrast, many of the properties of 
SVs set them apart from other secretory organ- 
elles. The elucidation of these properties repre- 
sents the main focus of our laboratory. We are 
following three major lines of research. (Several 
aspects of this research have previously been sup- 
ported by the National Institutes of Health.) 
Biogenesis and Traffic of Synaptic Vesicles 
First, we are investigating the biogenesis and 
traffic of SVs, using in vivo and in vitro systems. 
The large body of information recently accumu- 
lated on the molecular structure of SVs has made 
available numerous molecular probes that can be 
used in these studies (some of which are carried 
out in collaboration with Reinhard Jahn [HHMI, 
Yale University] and Thomas Siidhof [HHMI, Uni- 
versity of Texas Southwestern Medical Center at 
Dallas]). We have developed an immunocyto- 
chemical assay to detect SV exocytosis. Using this 
assay on hippocampal neurons in primary cul- 
ture, we have shown that SVs undergo a high rate 
of constitutive exocytosis and recycling in devel- 
oping neurons before synaptic contacts are 
formed. 
These findings suggest that synapse formation 
coincides with a reorganization of the exocytotic 
machinery (rather than with its de novo assem- 
bly) and that signaling via SV exocytosis may play 
an important role in nervous system development 
prior to synapse formation. We are working at de- 
veloping assays, some cell free, to define specific 
steps of the exo-endocytotic pathway of SVs. Such 
assays aid greatly in elucidating the underlying 
molecular mechanisms. 
Synapse-like Microvesicles 
in Endocrine Cells 
Second, we are exploring the relationship of 
SVs to organelles found in nonneuronal cells. Do 
SVs represent the adaptation of recycling vesicles 
to organelles found in other cells? How is the 
exo-endocytotic recycling of SVs related to the 
plasmalemma-endosome recycling that operates 
in all cells and occurs, for example, in the recy- 
cling of receptors? 
We and others have found that a variety of SV 
proteins are expressed at significant levels by 
peptide-secreting endocrine cells. Investigating 
the subcellular localization of these proteins, we 
have found that at least some of them (synapto- 
physin, protein p29) are concentrated on a popu- 
lation of microvesicles (synapse-like microvesi- 
cles, SLMVs) distinct from the secretory granules 
that store and secrete peptide hormones. 
A first biochemical characterization of these 
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