Traffic of Synaptic Vesicle Proteins in Neurons and Endocrine Cells 
microvesicles has demonstrated that they are simi- 
lar in composition to bona fide neuronal SVs. 
Consistent with these findings, SLMVs, like SVs, 
undergo exocytosis, endocytosis, and recycling. 
We have therefore explored the possibility that 
SLMVs may share with SV the property to store 
and secrete nonpeptide neurotransmitters. The 
model endocrine cell we have used for these stud- 
ies is the insulin-secreting cell of the endocrine 
pancreas, also called pancreatic |8-cell. 
Previous work had suggested that the nonpep- 
tide neurotransmitter GABA, the major inhibitory 
neurotransmitter in the brain, may play a para- 
crine role in the physiology of the endocrine pan- 
creas. GABA was reported to be concentrated in 
pancreatic jS-cells and to act on surrounding pan- 
creatic endocrine cells. Work we have carried out 
so far has suggested that SLMVs of /3-cells may in- 
deed store GABA. These findings support the hy- 
pothesis that the neuronal secretory pathway in- 
volving SVs represents an amplification and an 
adaptation of a secretory pathway that is ex- 
pressed in a more rudimentary form in peptide- 
secreting endocrine cells. 
Such results, in addition to having important 
implications for the field of endocrine physiol- 
ogy, raise the possibility of using endocrine cells 
to answer questions on the life cycle of SVs that 
would not be feasible to address in neurons. Us- 
ing endocrine cell lines as a model system, we 
have obtained evidence indicating that the bio- 
genesis of SLMVs (and presumably of SVs) is ef- 
fected by protein sorting from endosomal 
membranes. 
Molecular Mechanisms of Exocytosis 
Third, we are interested in elucidating the mo- 
lecular mechanisms of SV exocytosis. The ap- 
proach we are currently taking is based on the 
assumption that at least some basic feature of the 
exocytotic process may have been conserved in 
evolution. We are exploring the potential of yeast 
genetics to identify some of the proteins in- 
volved. A variety of temperature-sensitive yeast 
mutants defective in vesicle exocytosis {sec mu- 
tants), and therefore in growth, have been iso- 
lated in recent years. We are currently searching 
for gene products that rescue sec mutations in a 
functional assay, by transfecting yeast sec mutants 
with mammalian brain cDNAs. 
Stiflf-Man Syndrome and Diabetes 
The presence of SV-like organelles in endocrine 
cells suggests that pathological processes affecting 
SV proteins (mutations, autoimmunity) might af- 
fect both the nervous and endocrine systems. 
These considerations led us to identify a possible 
link between stiff-man syndrome and insulin- 
dependent diabetes mellitus (IDDM). Stiff-man 
syndrome is a rare and severe disease of the central 
nervous system characterized by a progressive ri- 
gidity of the body musculature. In a series of stud- 
ies that involved a collaboration with the Univer- 
sity of Milano, we have obtained evidence for an 
autoimmune pathogenesis of stiff-man syndrome 
and identified the GABA-synthesizing enzyme, 
GAD, as the major autoantigen. 
GAD is concentrated at the cytoplasmic surface 
of SVs of GABAergic neurons and of SLMVs of pan- 
creatic jS-cells. IDDM, which results from an au- 
toimmune destruction of (S-cells, was found to 
occur at high frequency in stiff-man syndrome 
patients. This observation then led us to hypothe- 
size an identity between GAD and the 64-kDa 
autoantigen of IDDM previously identified by 
Steinunn Baekkeskov, Ake Lemmark, and their co- 
workers (Hagedorn Research Laboratory, Den- 
mark). A collaboration between our group and 
that of Dr. Baekkeskov (now at the University of 
California, San Francisco) confirmed that the 64- 
kDa antigen is GAD. 
These findings may lead to a diagnostic assay 
for IDDM and for preclinical stages of the disease. 
In addition, it opens the possibility of investigat- 
ing molecular mechanisms of autoimmunity in 
IDDM. 
During the next year the work on SV exocytosis 
and on GAD autoimmunity in stiff-man syndrome 
will be partially supported by the National Insti- 
tutes of Health. 
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