The defining feature of eukaryotic cells is the presence of numerous organelles, each of which provides the optimal membrane surface and enclosed environment for specific cellular processes. The precise lipid and protein composition of membranes and the formation of sub-domains within them play essential roles in regulating organelle-specific reactions. Membrane composition is also crucial to inter-organelle trafficking and signaling.
The Bajjalieh Lab studies how organelle membrane composition influences fundamental cellular processes. Our current work is focused on understanding how the composition of synaptic vesicle membranes modulates the release of neurotransmitter from neurons and how lipid modifying enzymes contribute to vesicle fusion and intracellular signaling. On-going projects include:
Determining the role of Synaptic Vesicle Protein 2 (SV2) in synaptic vesicle fusion
SV2 is a family of three membrane glycoproteins specifically expressed in the regulated secretory vesicles of vertebrate neurons and endocrine cells. The most widely expressed SV2 isoform, SV2A, is the binding site of the anti-epilepsy drug levetiracetam. We are using a combination of reverse genetics, biochemistry, and physiology to understand SV2's action in neural synapses. We find that neurotransmitter release is significantly reduced in neurons that lack SV2 and that this reduction is associated with a decrease in the number of neurotransmitter-containing vesicles that are "primed" for fusion with the plasma membrane. Our studies are now focused on how SV2 contributes to vesicle priming. To determine this we are assessing the ability of mutant versions of SV2 to rescue neurotransmission in neurons cultured from SV2 knockout mice.
Measuring the protein composition of synaptic vesicle membranes and how it varies in diseases of the nervous system
Although the composition of synaptic vesicles is predicted to influence the efficacy of synaptic vesicle fusion, little is known about the consistency of vesicle protein composition. Together with Daniel Chiu's group in the Department of Chemistry, we are developing approaches to image single synaptic vesicles and measure the number of specific membrane proteins on them. Initial results suggest that vesicles contain two populations of proteins, those whose numbers are essentially constant between vesicles and those that vary in number between vesicles. We will use this approach in future studies to determine how changes in one protein's expression affects the number of other vesicle proteins and how vesicle composition varies in models of neurological disease.
Characterizing the role of lipid modifying enzymes and signaling lipids in synaptic vesicle fusion and other cellular processes
In early work we discovered two lipid modifying enzymes present at the synapse; 1) synaptic vesicle ceramide kinase and 2) a complementary ceramide 1-phosphate phosphatase that is present on synaptic plasma membranes. The presence of these enzymes at nerve terminals suggests that ceramide phosphorylation and de-phosphorylation play a role in synaptic vesicle fusion. We would like to identify the isoform of ceramide kinase associated with synaptic vesicles and determine its role in neurotransmission.
Our search for ceramide kinase led to the identification of a novel lipid kinase that phosphorylates multiple substrates. Multi-Lipid Kinase (MuLK) is expressed in all tissues, with especially high levels in the brain. We are now working to identify the role of MuLK in cellular functioning.
Inter-conversion of signaling lipids.
Nearly all simple lipids have a signaling role in cells. Shown are signaling lipids, some of their cellular functions and how they are synthesized and converted. Inter-conversion of lipids is mediated by a number of specific enzymes. MuLK has the unique property of catalyzing multiple reactions (shown in pink).