The electrochemical jelly inside your head contains something like one quadrillion synapses, the junctions at which nerve cells talk to one other by converting electrical signals into chemical ones and then back again. They have two components (sometimes three): the nerve terminal of one cell, which stores and releases neurotransmitter molecules, and the 'post-synaptic' membrane of another cell, which contains binding sites for the neurotransmitters.
Synapses are miniscule – nerve terminals are about one thousandth of a millimetre in diameter, and the space between them and the membrane they contact a mere 20-40 millionths of a millimetre wide – and are densely packed in the grey matter of the brain tissue, making them notoriously difficult to study. But researchers in Germany have now created an exquisitely detailed model of a nerve terminal, which shows the distribution of 300,000 individual protein molecules in the nerve terminal in atomic detail, and hints at how neurotransmitter release is regulated.
Inside the nerve terminal, neurotransmitter molecules are stored in tiny spheres called synaptic vesicles, which are "docked" in an "active zone" just beneath the cell membrane. When a nervous impulse arrives at the terminal, it causes a few of the vesicles to fuse with the membrane and release their contents. Later on, the spent vesicles are recycled – they are pulled back out of the membrane, re-filled with neurotransmitters, and eventually re-used.
At any given terminal, vesicle fusion can occur hundreds of times per second, as trains of impulses arrive one after the other. The whole process of vesicle docking, fusion and recycling is therefore tightly regulated, to ensure that there is a ready supply of vesicles that can fuse in quick succession and maintain the rapid bursts of neuronal activity.
The model was created by Benjamin Wilhelm of the University of Göttingen Medical Center and his colleagues, who isolated millions of nerve terminals from rat brains, and used a combination of electron microscopy, mass spectrometry, antibody staining, and super-resolution fluorescence microscopy to examine them closely, and determine the abundance and distribution of 62 different proteins known to be crucial for the synaptic vesicle cycle.
3D model of an average nerve ending in the rat brain shows the distribution of 300,000 individual protein molecules involved in neurotransmitter release. Created by Burkhard Rammner, from Wilhelm, et al.
“Our model shows that the proteins involved in neurotransmitter release can be enormously abundant, with up to 27,000 copies per synapse,” says Silvio Rizzoli, senior author of the study, “whereas proteins involved in recycling are present in only 1,000-4,000 copies.” The high number of vesicle-release proteins isn’t entirely surprising, because nerve terminals are thought to contain hundreds of vesicles docked at release sites.
The numbers of proteins involved in recycling are much lower than expected, however. They give the terminal the capacity to recycle just 10% of its entire vesicle pool, so it seems that recycling takes place slowly, in the seconds following vesicle fusion, and so does not contribute when multiple vesicles fuse in quick succession. “The advantage is that the cell does not need to produce huge numbers of recycling proteins, which saves energy.”
The model also shows that the proteins involved in the same steps of the vesicle cycle are present in roughly equal amounts within the terminal, raising the question of how their synthesis is regulated such that each set of proteins is present at the right concentration. “I don’t know how this happens,” says Rizzoli, “but it opens a whole new area of research.”
Talvinder Sihra of University College London says the study is “a real technical tour de force that allows us to get a feel for what the nerve terminal looks like from the inside.” Sihra helped develop the nerve terminal purification technique, and points out its limitations: There are various types of synapses in the brain, which differ in size, shape, and the neurotransmitters they contain. “The preparation contains all manner of neurotransmitters, but excitatory and inhibitory terminals differ in structure, so the picture provided by the study is that of an ‘average’ terminal without regard for the neurotransmitter.”
Rizzoli readily acknowledges this. “We do have data dealing with synapses of different sizes,” he says, “but our model is nevertheless an average synapse that may not be fully representative of synapses in specialised neuronal pathways.” He and his colleagues are now looking to generate a similar model of an entire neuron, and hope to complete it within the next five years.
Reference: Wilhelm, B. G., et al. (2014). Composition of isolated synaptic boutons reveals the amounts of vesicle trafficking proteins.Science, 344: 1023-1028. DOI: 10.1126/science.1252884