Sandra M. Bajjalieh

bajjalie@u.washington.edu
Professor, Department of Pharmacology

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The tightly regulated secretion of neurotransmitters is the primary form of communication in the nervous system. Transmitter secretion is mediated by a specialized membrane trafficking cycle that includes the synthesis, filling, targeting, priming, calcium-dependent fusion and recycling of transmitter-containing (synaptic) vesicles. Identifying the molecular events that produce and regulate this cycle is a first step toward understanding how genetic and environmental factors influence nervous system functioning. Towards this end, my lab is studying proteins localized to synaptic vesicles. Our current studies focus on Synaptic Vesicle Protein 2 (SV2) and synaptic vesicle ceramide kinase. Synaptic Vesicle Protein 2 (SV2) is a glycoprotein present in all synapticvesicles. To date, three isoforms, SV2A, SV2B and SV2C, have been characterized. All are, by sequence homology, members of the Major Facilitator Family of small molecule transporter proteins. This sequence homology suggests a functional homology, i.e. that SV2 mediates the transport of small molecules across the vesicle membrane. In addition, we have observed that SV2A binds synaptotagmin, a calcium binding protein required for fast, calcium-dependent neurosecretion. The SV2-synaptotagmin interaction is inhibited by calcium suggesting that dissociation of these proteins is one of the calcium-regulated events that controls vesicle fusion. Finally, SV2s have also been reported to interact with extracellular matrix proteins. Taken together, these observations suggest that SV2 performs multiple functions at the synapse including

1. transport of a common vesicle constituent,
2. regulation of exocytosis, and
3. establishment of synaptic connections.

We are currently testing these hypotheses by analyses of protein-protein interactions and by examining neurotransmitter release in SV2 "knockout" mice (mice in which the SV2 genes have been disrupted).

We have discovered two novel lipid modifying activities that are present at the synapse;

1. a calcium-stimulated kinase specific for the sphingolipid ceramide that is associated with synaptic vesicles and
2. a complementary ceramide 1-phosphate phosphatase activity present on synaptic plasma membranes.

The presence of these enzymes at nerve terminals suggests that ceramide phosphorylation and de-phosphorylation plays a role in neurotransmitter secretion. To identify that role we are examining the effect of exogenous ceramides on regulated secretion in cultured cell systems and characterizing synaptic vesicle ceramide kinase.