RT Journal Article
SR Electronic
T1 Synaptotagmin-7 Is Essential for Ca<sup>2+</sup>-Triggered Delayed Asynchronous Release But Not for Ca<sup>2+</sup>-Dependent Vesicle Priming in Retinal Ribbon Synapses
JF The Journal of Neuroscience
JO J. Neurosci.
FD Society for Neuroscience
SP 11024
OP 11033
DO 10.1523/JNEUROSCI.0759-15.2015
VO 35
IS 31
A1 Fujun Luo
A1 Taulant Bacaj
A1 Thomas C. Südhof
YR 2015
UL http://www.jneurosci.org/content/35/31/11024.abstract
AB Most synapses release neurotransmitters in two phases: (1) a fast synchronous phase lasting a few milliseconds; and (2) a delayed “asynchronous” phase lasting hundreds of milliseconds. Ca2+ triggers fast synchronous neurotransmitter release by binding to synaptotagmin-1, synaptotagmin-2, or synaptotagmin-9, but how Ca2+ triggers delayed asynchronous release has long remained enigmatic. Recent results suggested that consistent with the Ca2+-sensor function of synaptotagmin-7 in neuroendocrine exocytosis, synaptotagmin-7 also functions as a Ca2+ sensor for synaptic vesicle exocytosis but operates during delayed asynchronous release. Puzzlingly, a subsequent study postulated that synaptotagmin-7 is not a Ca2+ sensor for release but mediates Ca2+-dependent vesicle repriming after intense stimulation. To address these issues, we here analyzed synaptic transmission at rod bipolar neuron–AII amacrine cell synapses in acute mouse retina slices as a model system. Using paired recordings, we show that knock-out of synaptotagmin-7 selectively impairs delayed asynchronous release but not fast synchronous release. Delayed asynchronous release was blocked in wild-type synapses by intracellular addition of high concentrations of the slow Ca2+-chelator EGTA, but EGTA had no effect in synaptotagmin-7 knock-out neurons because delayed asynchronous release was already impaired. Moreover, direct measurements of vesicle repriming failed to uncover an effect of the synaptotagmin-7 knock-out on vesicle repriming. Our data demonstrate that synaptotagmin-7 is selectively essential for Ca2+-dependent delayed asynchronous release in retinal rod bipolar cell synapses, that its function can be blocked by simply introducing a slow Ca2+ buffer into the cells, and that synaptotagmin-7 is not required for normal vesicle repriming.SIGNIFICANCE STATEMENT How Ca2+ triggers delayed asynchronous release has long remained enigmatic. Synaptotagmin-7 has been implicated recently as Ca2+ sensor in mediating delayed asynchronous release, or vesicle repriming, in cultured neurons. To test the precise function of synaptotagmin-7 in a physiologically important synapse in situ, we have used pair recordings to study the synaptic transmission between retinal rod bipolar cells and AII amacrine cells. Our data demonstrate that the knock-out of synaptotagmin-7 selectively impaired delayed asynchronous release but not synchronous release. In contrast, the readily releasable vesicles after depletion recover normally in knock-out mice. Therefore, our findings extend our knowledge of synaptotagmins as Ca2+ sensors in vesicle fusion and support the idea that synapses are governed universally by different synaptotagmin Ca2+ sensors mediating distinct release.