How to Fill a Synaptic Vesicle with Neurotransmitters? Transport Mechanisms and Ion Balance

Abstract

Signaling between neurons is mediated by neurotransmitters that are released by exocytosis of synaptic vesicles upon stimulation. To maintain effective transmission over time, synaptic vesicles are rapidly recycled within nerve terminals and re-filled with transmitters by the action of vesicular neurotransmitter transporters. These transporters draw on an electrochemical proton gradient generated by a vacuolar ATPase. Unlike transporters at the plasma membrane, the ion and solute content of the vesicle interior changes dramatically during transport. Furthermore, charge and osmotic balance need to be maintained during transport, particularly when considering that thousands of uncharged (GABA, glycine) or charged (glutamate, monoamines) transmitter molecules must be translocated within seconds while only 50 charges will result in a membrane potential of 100 mV. Here we have used reconstitution of purified transporters in artificial vesicles, newly developed hybrid vesicles in which native synaptic vesicles were fused with liposomes of defined content, and single vesicle imaging to shed light on the transport mechanisms of glutamate and GABA. Our results show that the transporters cope with the challenge by influencing the driving force and by flexible adapting their transport mode during vesicle filling.