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Presynaptic Calcium Channels
 
 
Review

Synaptic Vesicles Having Large Contact Areas with the Presynaptic Membrane are Preferentially Hemifused at Active Zones of Frog Neuromuscular Junctions Fixed during Synaptic Activity

by 1,2
1
Department of Biology, Texas A&M University, College Station, TX 77845, USA
2
Department of Physics, Stanford University School of Humanities and Sciences, Stanford, CA 94309, USA
Int. J. Mol. Sci. 2019, 20(11), 2692; https://doi.org/10.3390/ijms20112692
Received: 24 April 2019 / Revised: 23 May 2019 / Accepted: 29 May 2019 / Published: 31 May 2019
(This article belongs to the Special Issue Neurotransmitter Secretion and Release)
Synaptic vesicles dock on the presynaptic plasma membrane of axon terminals and become ready to fuse with the presynaptic membrane or primed. Fusion of the vesicle membrane and presynaptic membrane results in the formation of a pore between the membranes, through which the vesicle’s neurotransmitter is released into the synaptic cleft. A recent electron tomography study on frog neuromuscular junctions fixed at rest showed that there is no discernible gap between or merging of the membrane of docked synaptic vesicles with the presynaptic membrane, however, the extent of the contact area between the membrane of docked synaptic vesicles and the presynaptic membrane varies 10-fold with a normal distribution. The study also showed that when the neuromuscular junctions are fixed during repetitive electrical nerve stimulation, the portion of large contact areas in the distribution is reduced compared to the portion of small contact areas, suggesting that docked synaptic vesicles with the largest contact areas are greatly primed to fuse with the membrane. Furthermore, the finding of several hemifused synaptic vesicles among the docked vesicles was briefly reported. Here, the spatial relationship of 81 synaptic vesicles with the presynaptic membrane at active zones of the neuromuscular junctions fixed during stimulation is described in detail. For the most of the vesicles, the combined thickness of each of their contact sites was not different from the sum of the membrane thicknesses of the vesicle membrane and presynaptic membrane, similar to the docked vesicles at active zones of the resting neuromuscular junctions. However, the combined membrane thickness of a small portion of the vesicles was considerably less than the sum of the membrane thicknesses, indicating that the membranes at their contact sites were fixed in a state of hemifusion. Moreover, the hemifused vesicles were found to have large contact areas with the presynaptic membrane. These findings support the recently proposed hypothesis that, at frog neuromuscular junctions, docked synaptic vesicles with the largest contact areas are most primed for fusion with the presynaptic membrane, and that hemifusion is a fusion intermediate step of the vesicle membrane with the presynaptic membrane for synaptic transmission. View Full-Text
Keywords: electron tomography; active zone; synaptic vesicle; hemifusion; synaptic transmission; nerve terminal; synapse electron tomography; active zone; synaptic vesicle; hemifusion; synaptic transmission; nerve terminal; synapse
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MDPI and ACS Style

Jung, J.H. Synaptic Vesicles Having Large Contact Areas with the Presynaptic Membrane are Preferentially Hemifused at Active Zones of Frog Neuromuscular Junctions Fixed during Synaptic Activity. Int. J. Mol. Sci. 2019, 20, 2692. https://doi.org/10.3390/ijms20112692

AMA Style

Jung JH. Synaptic Vesicles Having Large Contact Areas with the Presynaptic Membrane are Preferentially Hemifused at Active Zones of Frog Neuromuscular Junctions Fixed during Synaptic Activity. International Journal of Molecular Sciences. 2019; 20(11):2692. https://doi.org/10.3390/ijms20112692

Chicago/Turabian Style

Jung, Jae Hoon. 2019. "Synaptic Vesicles Having Large Contact Areas with the Presynaptic Membrane are Preferentially Hemifused at Active Zones of Frog Neuromuscular Junctions Fixed during Synaptic Activity" International Journal of Molecular Sciences 20, no. 11: 2692. https://doi.org/10.3390/ijms20112692

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