Kiss and Run movie

The kiss-and-run model
of neurotransmitter secretion
The clear synaptic vesicles of neurons release their content at the presynaptic membrane and are then quickly retrieved.
It is unclear whether a complete cycle of exocytosis and endocytosis is always involved [Heuser and Reese (1973) J. Cell Biol. 57, 315] or whether neurotransmitter can be released by a transient interaction [Ceccarelli, Hurlbut and Mauro (1973) J. Cell Biol. 57, 499 ].
Findings in chromaffin and mast cells suggest that exocytosis is preceded by the formation of a pore that has similar conductance properties to ion channels. The content of the secretory organelle partially escapes at this early step, but the pore can close before the vesicle fuses fully [Chow, Von Ruden and Neher (1992) Nature 356,60; Alvarez de Toledo, Fernandez-Chacon and Fernandez (1993) Nature 363, 554]
May quantal release of neurotransmitter from clear synaptic vesicles occur by a similar 'kiss and-run' mechanism? [Neher (1993) Nature 363,497; Fesce, Grohovaz, Valtorta and Meldolesi (1994) Trends in Cell Biol. 4, 1].
Klingauf et al. [Nature 394, 381, 1998] detected at least two components of recycling in hippocampal neurons: a slow process (20-30 s) and a much faster process, which was interpreted as kiss-and-run recycling, the role of the latter process greatly increasing with stimulus strength and elevated [Ca2+]. And recently, Alés and colleagues [Nature Cell Biology 1, 40,1999], by means of cell-attached capacitance measurements coupled to catecholamine assay by a carbon fibre positioned within the pipette, showed that in chromaffin cells the full vesicle content is discharged through reversible fusion. Furthermore, the frequency of kiss-and-run fusions rises from 5% to almost 80% as calcium concentration increases, and the duration of the transient opening falls to a few tens of ms.

Thus, the cells appear to have the choice among at least two processes for membrane recycling: slow conventional endocytosis predominates when [Ca2+] is low, fast kiss-and-run when it is high. If a chromaffin granule, which has a volume about 10 times as large and is filled with an organised macro-molecular matrix trapping the biogenic amines, is fully discharged in few ms, it is well possible that a synaptic vesicle releases its soluble neurotransmitter in a fraction of a millisecond and rapidly pinches off from the presynaptic membrane [Fesce and Meldolesi (1999) Nature Cell Biology 1, E3].

Trends in Cell Biology Coverpage: The Kiss

Local recycling of synaptic vesicles first proposed by B. Ceccarelli et al.

Electron Microscopy: vesicle recycling on the spot

High power electron micrograph of portions of three different neuromuscular junctions at the level of the "active zone". The preparations were stimulated for 2 hr at 2/sec in curare plus horseradish peroxidase. Junctional clefts contain rich deposits of peroxidase reaction products. The figures show three degrees of association between peroxidase-labeled vesicles and the prejunctional membrane (arrows). In A the membrane of the vesicle is completely fused with the prejunctional membrane; in B the continuity of the two membranes is mantained through a short stalk; and in C the vesicle appears to be in the process of losing contact with the axolemma.  [calibration bar: 0.25 mm]

Vesicle fusions frozen during transmitter release:
fusion pores can be seen, wide-open vesicles are rare, late events
Electron Microscopy after Quick Freezing

Electron micrograph of a cross section from a neuromuscular preparation quick-frozen 10 ms after a single stimulus in the presence of 4-aminopyridine (which prolongs the time course of the evoked release) and processed as described in [Torri Tarelli, Grohovaz, Fesce and Ceccarelli (1985) J. Cell Biol. 101,1386].

  • The bracket highlights the structure of an active zone, facing the post-junctional infolding.
  • The arrow indicates a vesicle open to the synaptic cleft through a narrow pore.
  • The arrow head indicates a wide-open vesicle fusion (omega shape) as could occasionally be seen only at late times (>5 ms)

  • The kiss-and-run model
    A schematic drawing of the model
    Diagrammatic representation of three possible pathways for vesicle fusion after the vesicle has docked with to the presynaptic membrane
  • top: Protein-protein interactions mediate the formation of a supramolecular complex that operates as the  reversible fusion pore. upon widening of the pore the complex disassembles by the inclusion of mobile lipids, and full fusion occurs
  • middle: A scaffold of proteins produces a local perturbation of the bilayers that leads to hemifusion; a reversible pore opens across the the resulting single bilayer and eventually leads to complete fusion
  • bottom: The classical apposition-fusion-fission sequence through the formation of a pentalaminar structure.

  • In all cases the fusion is followed by collapse of the vesicle membrane into the axolemma, intermixing of the component, and subsequent sorting and retrieval by coated pits and vesicles.
  • The gray bar highlights the reversible opening that may account for kiss-and-run release.

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