Structure Parameters of Synaptic Vesicles Quantified by Small-Angle X-Ray Scattering

Abstract

Synaptic vesicles (SVs) are small, membrane-bound organelles that are found in the synaptic terminal of neurons, and which are crucial in neurotransmission. After a rise in internal [Ca(2+)] during neuronal stimulation, SVs fuse with the plasma membrane releasing their neurotransmitter content, which then signals neighboring neurons. SVs are subsequently recycled and refilled with neurotransmitter for further rounds of release. Recently, tremendous progress has been made in elucidating the molecular composition of SVs, as well as putative protein-protein interactions. However, what is lacking is an empirical description of SV structure at the supramolecular level-which is necessary to enable us to fully understand the processes of membrane fusion, retrieval, and recycling. Using small-angle x-ray scattering, we have directly investigated the size and structure of purified SVs. From this information, we deduced detailed size and density parameters for the protein layers responsible for SV function, as well as information about the lipid bilayer. To achieve a convincing model fit, a laterally anisotropic structure for the protein shell is needed, as a rotationally symmetric density profile does not explain the data. Not only does our model confirm many of the preexisting ideas concerning SV structure, but also for the first time, to our knowledge, it indicates structural refinements, such as the presence of protein microdomains.

Stalk structures in lipid bilayer fusion studied by x-ray diffraction

“…4.12a as well, using a typical radius of a synaptic vesicle of R = 20 nm [178,179] merely by a factor of ∼ 2 and corresponds to 3-4 times the energy of 35±7 k B T released during formation of a single SNARE complex [180]. Since synaptic vesicle membranes contain about 40 mol% cholesterol and considerable amounts of PE lipids and PIP 2 [178], the value W vesicle hyd for the physiologically relevant case is probably lower than those shown in Fig.…”

Synaptic vesicles studied by dynamic light scattering

“…These particles may be formed by larger membrane aggregates, possibly originating from early endosomes. For further details see [5,6].…”

“…The first value, which should be compared to the value of 45% obtained by the regularisation fit, shows that these numbers depend strongly on the choice of the model. The cryo-EM data is taken from [6]. Note the different scalings of the two components of p n (R h ), indicating that the large particles can be viewed as a small contamination when considering the number of particles.…”

“…The transport of the neurotransmitter, the fusion at the synaptic membrane, and the release of the stored neurotransmitters into the synaptic cleft are since long known as an essential step in nerve conduction of the chemical synapse [1][2][3]. A detailed structural view of these molecular mechanisms is still lacking, notwithstanding the enormous progress in the field during recent years [4][5][6][7]. For example, it is now known that proteins cluster and form micro-domains on the SV membrane.…”

“…The lowresolution structure and size distribution of SV populations have been characterized by means of cryogenic electron microscopy (cryo-EM) [5] and synchrotron-based small-angle X-ray scattering (SAXS) data recorded from SV dispersions under quasi-physiological conditions [6].…”