Long-Term Temporal Dynamics of Synaptic Vesicles

Long-Term Temporal Dynamics of Synaptic Vesicles

Sven Truckenbrodt

Doctoral thesis

Date of Examination: 2016-10-17

Date of issue: 2017-09-11

Advisor: Rizzoli, Silvio Prof. Dr.

Referee: Rizzoli, Silvio Prof. Dr.

Referee: Jahn, Reinhard Prof. Dr.

Persistent Address: http://hdl.handle.net/11858/00-1735-0000-0023-3EFA-E

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Abstract

English

Neurotransmission requires the release of neurotransmitters from synaptic vesicles. This occurs via fusion of the vesicle to the pre-synaptic membrane upon stimulation. However, not all synaptic vesicles are equally releasable, and it has long been debated why the majority of synaptic vesicles do not respond to physiological levels of stimulation. I demonstrate here, using live-cell antibody-tagging in rat hippocampal cultures, that only young synaptic vesicles are releasing neurotransmitter, and that they become more reluctant to release as they age. This inactivation of synaptic vesicles is not strictly an ageing-dependent process, but conditional upon vesicle usage. I report here that synaptic vesicles release ~260 times, on average, before becoming inactive, and that increasing usage frequency speeds up inactivation. The inactivation is caused by contamination of synaptic vesicles with the cell membrane protein SNAP25. SNAP25 can interact with the vesicle protein CSPα in cis- complexes on the vesicle itself. This sequesters CSPα and prevents the formation of trans- complexes with SNAP25 on the cell membrane. This trans-interaction, however, promotes vesicle fusion to the cell membrane. The more often a vesicle has fused to the cell membrane, the higher its chance is to be contaminated with SNAP25, and the less competent it is for future rounds of release. The inactivation of ageing synaptic vesicles is presumably coupled to usage to remove potentially damaged synaptic vesicles from neurotransmission. This hypothesis is strengthened by the observation of endocytosis defects and neurite degeneration when aged vesicles are forced to release. I further provide several timing parameters for key events in the life of synaptic vesicles, which can serve as a framework towards a quantitative model of the synaptic vesicle life cycle.

Keywords: neuroscience, neurobiology, synaptic vesicles, synaptic vesicle life cycle, ageing