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Neurons are arranged in networks in which they communicate between each other by the means of synapses. Synaptic transmission can undergo activity-dependent and short-lived changes in strength, known as short-term synaptic plasticity. Short-term plasticity is an important mechanism for neural networks in living organisms, which plays a role in adaptation to sensory inputs and information processing. One of the interesting forms of short-term enhancement in transmitter release is post-tetanic potentiation (PTP). PTP is generated upon repetitive activity of the synapse and lasts tens of seconds to minutes. A more detailed understanding of the molecular mechanisms of short-term plasticity will be helpful to understand information processing in neural networks. Here, I studied the cellular and molecular basis of post-tetanic potentiation at the calyx of Held model synapse. Pharmacological inhibition of conventional protein kinase C (cPKC) isoforms resulted in a near- complete block of PTP. This showed that cPKC dependent phosphorylation of a target presynaptic protein is a key process underlying PTP. Upon inhibition of phosphatases by calyculin, PTP was remarkably prolonged, by the suppression of dephosphorylation. By expressing a FRET (Fluorescence Resonance Energy Transfer) based PKC activity probe, I obtained independent evidence for transient activation of cPKC during PTP. This refers to a transient phosphorylation/dephosphorylation, which underlies the short-term changes in the release probability during PTP. To investigate the target protein, which becomes phosphorylated by PKC during PTP, I studied phosphorylation of the presynaptic protein Munc18-1. By establishing a virus-mediated in vivo gene-replacement approach using floxed Munc18-1 mice, we replaced endogenous Munc18-1 with phosphorylation-deficient form of Munc18-1. I showed that in the nerve terminals expressing the phosphorylation deficient form of Munc18-1, PTP was strongly suppressed as compared to the wild type form of Munc18-1. This PhD thesis work proposes a molecular model for PTP in which a transient phosphorylation/dephosphorylation of Munc18-1 by conventional PKCs and phosphatases, respectively modulates the calcium sensitivity of vesicle fusion and determines the kinetics of PTP.
Eilif Benjamin Muller, Michael Reimann, James Gonzalo King, Marwan Muhammad Ahmed Abdellah, Pramod Shivaji Kumbhar, András Ecker, Sirio Bolaños Puchet, James Bryden Isbister, Daniela Egas Santander, Jorge Blanco Alonso, Giuseppe Chindemi, Ioannis Magkanaris