Original published in Neuron 21, 123-136, 1998; copyright by Cell Press. Full size image available through contact |
Transmitter release at synapses is a complex process that involves a large number of proteins. These regulate the turnover of synaptic vesicles, small membranous intracellular organelles that store neurotransmitter and release it upon stimulation by fusing with the cell membrane. The protein machinery that mediates this release process is subject to regulation by numerous factors. Regulation occurs either directly, e.g. through proteins that interact with the release machinery, or indirectly, e.g. through second messengers that activate protein kinases which in turn modify components of the release apparatus.
One of the most intriguing second messenger pathways in the regulation of neurotransmitter release involves the generation of diacylglycerol, a metabolite of membrane phospholipids. The "textbook" target of the diacylglycerol second messenger pathway is protein kinase C. This kinase enhances transmitter release by phosphorylating proteins of the release machinery and ion channels in the cell membrane. In addition, protein kinase C is involved in numerous biological processes, including cell division and tumorogenesis.
Protein kinase C is activated by a group of tumor-promoting reagents called phorbol esters. These bind to the same site in protein kinase C that interacts with diacylglycerol under physiological conditions. Numerous studies in the past employed phorbol esters as key reagents in the analysis of protein kinase C function, and almost invariably, their effects were attributed to protein kinase C activation. Andrea Betz and Iris Augustin in the laboratory of Nils Brose, head of a research group at the Max Planck Institute for Experimental Medicine, Göttingen/Germany, identified Munc13-1 as a novel brain specific target of phorbol esters whose pharmacological characteristics are almost indistinguishable from those of protein kinase C. In collaboration with Thomas C. Südhof, University of Texas Southwestern Medical Center, Dallas/Texas, the Göttingen group observed that in living cells, phorbol esters recruit Munc13-1 to the cell membrane and activate it, a feature that so far was thought to be unique to protein kinase C. Together with Michael Rickmann, assistant professor at the University Center for Anatomy, Göttingen/Germany, the researchers were able to show that Munc13-1 is specifically targeted to synaptic transmitter release sites. There it acts as an enhancer of the release process, as U. Ashery and J. Rettig, two electrophysiologists in the group of Erwin Neher at the Max Planck Institute for Biophysical Chemistry, Göttingen/Germany, discovered in an elegant approach. They expressed Munc13-1 in neuromuscular synapses of Xenopus frogs. When they measured the characteristics of these synapses, they discovered that these were much more responsive to phorbol esters than control synapses.
Our data establish Munc13-1 as a novel presynaptic target of the diacylglycerol second messenger pathway that acts in parallel with protein kinase C to regulate transmitter release, says Brose. He names two main reasons for the importance of the group's findings: The study adds a new regulatory process to our understanding of synaptic transmission, Brose says "and it's going to rattle a few cages". Hundreds of published studies on protein kinase C function employed pharmacological reagents that do not allow to distinguish between the kinase and Munc13-proteins. Given that Munc13-proteins are present in almost all cells of the body, the Göttingen group is convinced that at least some of the published data have to be reevaluated in view of their new findings.
Journal
Neuron