Imaging single synaptic vesicles undergoing repeated fusion events: kissing, running, and kissing again.

At synapses of the mammalian central nervous system, release of neurotransmitter occurs at rates transiently as high as 100 Hz, putting extreme demands on nerve terminals with only tens of functional vesicles at their disposal. Thus, the presynaptic vesicle cycle is particularly critical to maintain neurotransmission. To understand vesicle cycling at the most fundamental level, we studied single vesicles undergoing exo/endocytosis and tracked the fate of newly retrieved vesicles. This was accomplished by minimally stimulating boutons in the presence of the membrane-fluorescent styryl dye FM1-43, then selecting for terminals that contained only one dye-filled vesicle. We then observed the kinetics of dye release during single action potential stimulation. We found that most vesicles lost only a portion of their total dye during a single fusion event, but were able to fuse again soon thereafter. We interpret this as direct evidence of "kiss-and-run" followed by rapid reuse. Other interpretations such as "partial loading" and "endosomal splitting" were largely excluded on the basis of multiple lines of evidence. Our data placed an upper bound of <1.4 s on the lifetime of the kiss-and-run fusion event, based on the assumption that aqueous departitioning is rate limiting. The repeated use of individual vesicles held over a range of stimulus frequencies up to 30 Hz and was associated with neurotransmitter release. A small percentage of fusion events did release a whole vesicle's worth of dye in one action potential, consistent with a classical picture of exocytosis as fusion followed by complete collapse or at least very slow retrieval.

Single synaptic vesicles fusing transiently and successively without loss of identity.

Vesicle fusion and recycling are particularly critical for ongoing neurotransmitter release in the small nerve terminals of the brain, which typically contain about 30 functional vesicles. However, the modes of exocytosis and endocytosis that operate at synapses of the central nervous system are incompletely understood. Here we show real-time visualization of a single vesicle fusing at a small synapse of the central nervous system, made possible by highly intensified charge-coupled device imaging of hippocampal synaptic terminals, in which a single vesicle was labelled with the fluorescent membrane marker FM1-43 (ref. 6). In a small number of cases, full loss of fluorescent membrane dye was elicited by a single action potential, consistent with classical complete collapse. In most cases, however, action potentials triggered only partial loss of fluorescence, suggesting vesicular retention of membrane marker, consistent with 'kiss-and-run' vesicle cycling. An alternative hypothesis of independent fusion of partially stained vesicles arising from endosomal splitting could be excluded by observations on the size and timing of successive fusion events. Thus, our experimental evidence supports a predominance of kiss-and-run fusion events and rapid vesicular re-use.

Limited numbers of recycling vesicles in small CNS nerve terminals: implications for neural signaling and vesicular cycling.

The tiny nerve terminals of central synapses contain far fewer vesicles than preparations commonly used for analysis of neurosecretion. Photoconversion of vesicles rendered fluorescent with the dye FM1-43 directly identified vesicles capable of engaging in exo-endocytotic recycling following stimulated Ca(2+) entry. This recycling pool typically contained 30-45 vesicles, only a minority fraction (15-20% on average) of the total vesicle population. The smallness of the recycling pool would severely constrain rates of quantal neurotransmission if classical pathways were solely responsible for vesicle recycling. Fortunately, vesicles can undergo rapid retrieval and reuse in addition to conventional slow recycling, to the benefit of synaptic information flow and neuronal signaling.

A genetically engineered cell-based biosensor for functional classification of agents.

Cell-based biosensors (CBBs) utilize whole cells to detect biologically active agents. Although CBBs have shown success in detecting the presence of biological agents, efforts to classify the type of agent based on functional activity have proven difficult because multiple biochemical pathways can lead to the same cellular response. However, a new approach using a genetically-engineered cell-based biosensor (GECBB) described in this paper translates this cross-talk noise into common-mode noise that can be rejected. The GECBB operates by assaying for an agent's ability to differentially activate two populations of cells, wild-type (WT) cells and cells genetically engineered to lack a specific receptor, knockout (KO) cells. Any biological agent that targets the knocked out receptor will evoke a response in the WT but not in the KO. Thus, the GECBB is exquisitely sensitive to agents that effect the engineered pathway. This approach provides the benefits of an assay for specific functional activity while simplifying signal analysis. The GECBB implemented was designed to be sensitive to agents that activate the beta 1-adrenergic receptor (beta 1-AR). This was achieved by using mouse cardiomyocytes in which the beta 1-AR had been knocked out. The cellular signal used in the GECBB was the spontaneous beat rate of the two cardiomyocyte syncitia as measured with microelectrode arrays. The GECBB was able to detect the beta-AR agonist isoproterenol (ISO) at a concentration of 10 microM (P<0.005).