The function of neural network is based on the regulated release of neurotransmitters by exocytosis and the subsequent recycling of synaptic vesicles (SV) membranes and proteins by endocytosis. The nascent SVs can be generated from plasma membrane or endosome-like vacuoles, however, the exact mechanism of SV recycling and the role of endosomes in this process are controversial. Furthermore, an essential element for maintaining neural networks is the regulation of neurotransmission through mechanisms of pre- and postsynaptic plasticity as well as homeostatic scaling, a process in which the activity-dependent changes of postsynaptic receptors lead to adaptive adjustment of the presynaptic neurotransmitter release. Whether and how the presynaptic neurotransmitter release is controlled intrinsically in an activity-dependent manner has not yet been fully understood. In my work, I examined the physiological function of the endosomal signal lipid phosphatidylinositol 3-phosphate [PI(3)P], a putative factor for regulation of SV recycling and control of presynaptic neurotransmission. By combining acute pharmacological and chemical-genetic approaches, I was able to show that excitatory neurotransmission and synaptic vesicle cycle are controlled by PI(3)P levels. Neuronal activity or pharmacological inhibition of PI 3-Kinase VPS34 lead to a drastic reduction in PI(3)P synthesis. PI(3)P reduction further inhibits excitatory neurotransmission and affects SV exo-/endocytosis. Conversely, silencing of neural network activity leads to increased PI(3)P levels. I was also able to show that the inhibition of exocytosis and endocytosis SV due to reduced PI(3)P levels is based on the Rab5-mediated hyperactivation of calcium-dependent protease calpain. Calpain-mediated cleavage and activation of the regulatory p35 subunit of the cyclin-dependent kinase 5 (Cdk5) is in turn responsible for the inhibition of exo- and endocytosis SV. As part of my doctorate, I was able to decipher an unexpected and novel function of the endosomal lipid PI(3)P in the control and scaling of the excitatory neurotransmission and thus neural network activity. This mechanism could explain the essential role of VPS34 in the development of the central nervous system as well as in neurological diseases and in neurodegeneration.
