Presynaptic short- and long-term enhancement of neurotransmitter release: Molecular mechanisms and behavioral relevance
Final Report Abstract
In this Emmy Noether project, we investigated how the structure and molecular composition of synapses –highly specialized cell-to-cell contact sites- determine neural information transfer. This relies on neurotransmitter liberation from the presynaptic cell and on transmitter detection by the postsynaptic cell. Dynamic changes of this transmission are called synaptic plasticity and enable the nervous system to stabilize information flow, to process activity patterns, or to store information. We here used electrophysiology, microscopy, and mathematical modeling to characterize the function and composition of the Drosophila melanogaster neuromuscular junction as a model synapse. This was combined with genetic and pharmacological perturbations to dissect the contribution of synaptic components to basic function and plastic changes. We found that evolutionarily conserved (M)Unc13 proteins are limiting components to generate the highly specific “release sites” to which the liberation of neurotransmitters from the presynaptic cell is restricted. For neural processing transmission is typically evoked by action potentials, brief de- and repolarizations of the cellular membrane potentials. However, all known synapses also transmit spontaneously – without action potentials. While the functional relevance of this transmission mode remains debated, we could show that the same machinery – including the release sites – is involved in both principal transmission modes. In evoked transmission, the release of neurotransmitters is induced by synaptic Ca2+ influx through voltage gated Ca2+ ion channels. We could show that the distance between release sites and those channels is differentially regulated and that this has a profound impact on the temporal transmission profile. We furthermore showed that the typically heterogeneous distribution of release sites with respect to those channels makes it particularly difficult to achieve a successive strengthening of responses to repeated stimulation. As a solution to this problem, we propose a model in which the number of participating release sites quickly increases to achieve this. Further investigations supported the idea that the number of participating release sites also increased on longer timescales to homeostatically restore function when the synaptic sensitivity to transmitters was reduced. We found that this adaptation required Unc13 proteins and appears to happen in two waves: Within minutes synapses functionally adapt without detectable structural changes, but structural changes are needed to stabilize homeostasis on longer timescales. A novel research focus that unexpectedly developed in this project was the investigation of signaling lipids that interact with synaptic proteins and influence neurotransmitter release. Using optical uncaging to change lipid levels and mathematical modeling to estimate biochemical parameters of lipid-protein interaction, we found that the molecular composition and levels of the lipids influenced protein interactions and neurotransmitter release. Together, this project contributed to understanding the principal function and dynamic regulation of synapses. Ongoing and future research could test the generality of our observations and how changes in release site numbers might help avoid other forms of synaptic dysfunction or contribute to other types of plasticity.
Publications
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(2016) Active zone scaffolds differentially accumulate Unc13 isoforms to tune Ca(2+) channel-vesicle coupling. Nat Neurosci.
Bohme, M. A., Beis, C., Reddy-Alla, S., Reynolds, E., Mampell, M. M., Grasskamp, A. T., Lutzkendorf, J., Bergeron, D. D., Driller, J. H., Babikir, H., Gottfert, F., Robinson, I. M., O'Kane, C. J., Hell, S. W., Wahl, M. C., Stelzl, U., Loll, B., Walter, A. M., and Sigrist, S. J.
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(2017) Phosphatidylinositol 4,5-bisphosphate optical uncaging potentiates exocytosis. eLife
Walter, A. M., Muller, R., Tawfik, B., Wierda, K. D., Pinheiro, P. S., Nadler, A., McCarthy, A. W., Ziomkiewicz, I., Kruse, M., Reither, G., Rettig, J., Lehmann, M., Haucke, V., Hille, B., Schultz, C., and Sorensen, J. B.
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(2017) Stable Positioning of Unc13 Restricts Synaptic Vesicle Fusion to Defined Release Sites to Promote Synchronous Neurotransmission. Neuron
Reddy-Alla, S., Bohme, M. A., Reynolds, E., Beis, C., Grasskamp, A. T., Mampell, M. M., Maglione, M., Jusyte, M., Rey, U., Babikir, H., McCarthy, A. W., Quentin, C., Matkovic, T., Bergeron, D. D., Mushtaq, Z., Gottfert, F., Owald, D., Mielke, T., Hell, S. W., Sigrist, S. J., and Walter, A. M.
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(2018). Active zone scaffold protein ratios tune functional diversity across brain synapses. Cell Reports
Fulterer, A., Andlauer, T.F.M., Ender, A., Maglione, M., Eyring, K., Woitkuhn, J., Lehmann, Matkovic, T., Geiger, J.R.P., Walter, A.M., Nagel, K., and Sigrist, S.J.
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(2018). Regulation of synaptic release-site-Ca2+-channel coupling as a mechanism to control release probability and short-term plasticity. FEBS Lett
Bohme, M. A., Grasskamp, A. T., and Walter, A. M.
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(2019). Rapid active zone remodeling consolidates presynaptic potentiation. Nat. Commun.
Böhme, M. A., McCarthy, A. W., Grasskamp, A. T., Beuschel, C. B., Goel, P., Jusyte, M., Laber, D., Huang, S., Rey, U., Petzold, A., Lehmann, M., Göttfert, F., Haghighi, P., Hell, S. W., Owald, D., Dickman, D., Sigrist, S. J., and Walter, A.M.
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(2020). Live-cell lipid biochemistry reveals a role of diacylglycerol side-chain composition for cellular lipid dynamics and protein affinities. PNAS
Schumacher, M., Grasskamp, A.T., Barahtjan, P., Wagner, N., Lombardot, B., Schumacher, J., S., Lohmann, A., Henry, I., Shevchenko, A., Coskun, Ü., Walter, A.M., and Nadler, A.
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(2020). Rapid regulation of vesicle priming explains synaptic facilitation despite heterogeneous vesicle:Ca2+ channel distances. eLife
Kobbersmed, J.R.L., Grasskamp, A.T., Jusyte, M., Böhme, M.A., Ditlevsen, S., Sørensen, J.B., and Walter, A.,M.
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(2021). Glial Synaptobrevin mediates peripheral nerve insulation, neural metabolic supply, and is required for motor function. Glia
Bohme, M. A., McCarthy, A. W., Blaum, N., Berezeckaja, M., Ponimaskine, K., Schwefel, D., and Walter, A. M.
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(2022) Allosteric stabilization of calcium and phosphoinositide dual binding engages several synaptotagmins in fast exocytosis. eLife
Kobbersmed, J. R. L., Berns, M. M. M., Ditlevsen, S., Sorensen, J. B., and Walter, A. M.
