Facile Membrane Flow and Tension Equilibration at a Presynaptic Nerve Terminal

Abstract

Synaptic transmission is the process that underlies most neuronal communication. In the pre-synaptic neuron, membrane depolarization opens calcium channels and the calcium influx triggers exocytosis, in which synaptic vesicles (SVs) fuse with the presynaptic plasma membrane and release neurotransmitters to the synaptic cleft. Membrane added to the presynaptic terminal, via exocytosis, must be recovered by endocytosis to maintain a releasable pool of SVs and to preserve the terminal surface area. That is, exocytosis and endocytosis are coupled. Because at least some endocytosis occurs some distance away from exocytosis sites, SVs turnover implies generation of membrane tension gradients and membrane flow from exocytic sites to endocytic ones. Plasma membrane flows in non- neuronal cells and non-terminal regions of neurons are known to be extremely slow. Such slow flows would impose constraints on the spatio-temporal coupling of exo- and endocytosis. However, membrane tension dynamics and membrane flows have never been quantified at nerve terminals. Here we use goldfish retinal bipolar neurons which possess a giant synaptic terminal to address this issue. We use optical tweezers to pull a thin membrane tether whose restoring force reports membrane tension. We observe that tension can propagate rapidly (seconds) over long distances (∼10 µm) in the terminal. In addition, tethers can be dragged around the terminal with little resistance, in sharp contrast to the soma where high friction between the membrane and the cytoskeleton prevents tether sliding. Overall, our results suggest that facile membrane flow and tension equilibration at presynaptic terminals are tuned for rapid turnover of synaptic vesicles, thus playing a key role in neurotransmission.