Neuron. "Decision Tree" at the neurotransmitter release site

Synaptic transmission is the most basic way of information communication between neurons.its core function is to convert electrical signals (action potentials) into chemical signals (neurotransmitters) and act on the next level neurons.neurotransmitters are very dangerous to the nervous system, so their synthesis, storage, recovery and degradation are strictly regulated.in synaptic terminals, neurotransmitters are encapsulated in small droplets with a diameter of about 40 nm at very high concentrations (100-1000 mm).the surface of vesicles is composed of lipid bilayer, and the essence of transmitter release is the fusion process of vesicles and presynaptic membranes.when the action potential reaches the synaptic terminals, extracellular calcium ions enter the synapse through the calcium channels of nerve terminals, triggering vesicular fusion, and the whole process is completed in tens to hundreds of microseconds.then the contradiction comes: calcium ion can only carry out thermal movement after entering the synapse. However, due to the low body temperature (only dozens of degrees higher than the freezing point), the diffusion of calcium ion is slow, and it can only affect extremely limited space in a short time. How can nerve terminals sense calcium signal in an instant? In order to guarantee the response speed, neurons have evolved a very sophisticated system.its principle is very simple: time = distance / speed. Since the speed is limited by the laws of physics, it is simply to compress the space distance.the secret weapon of synaptic terminals is a set of molecular machines called active zones. It is actually a giant protein complex around the calcium channel, which can capture vesicles and make them close to calcium channels (10-50 nm).in this way, once the calcium channel is activated, the influx of calcium ions will find vesicles in a very short distance and trigger the release of neurotransmitters.the role of the active region is very much like a matchmaker. She holds the calcium channel in one hand and the vesicles in the other hand, creating opportunities for the two to fuse (transmitter release).so there is a question of role orientation in this process: is the active area an accessory produced around the calcium channel demonoxidation? Or is it more like a core social platform, attracting calcium channels and vesicles to communicate online? On June 16, 2020, Pascal s. KAESER research team of Harvard Medical School published the article synapse and active zone assembly in the absense of presynaptic Ca2 + channels and Ca2 in the journal Neuron+ The active region is more like a social platform, which determines the location and functional characteristics of synaptic connections, and calcium channels are just VIP users of the platform.this role discrimination is of great significance for answering how neurons determine the location of neurotransmitters.there are two main hypotheses for this problem: one is that the generation of release sites is an active process mediated by calcium signal, that is, calcium channel generates local high concentration calcium signal under the repeated driving of action potential, starts the signal pathway, recruits proteins in active region, and captures vesicles through active region to form release site; the other hypothesis holds that release site is active region Core proteins (such as rim, elks, Munc13, etc.) are assembled randomly by phase separation.once the assembly is completed, the release position is defined.to test the first hypothesis, the researchers decided to cut off the core signaling pathway by removing calcium signal.however, there are two risks in this operation: 1) calcium channel and calcium signal are essential for organisms, and rash intervention often leads to death; 2) calcium channel is a random combination of multiple subunits, with a wide variety and complementary functions, which poses a major challenge to gene manipulation. in order to cope with the above risks, the researchers used the method of culturing neural networks in vitro to conduct functional tests, which avoided the risk of animal death due to organ failure. on the other hand, to avoid compensatory effects, they knocked out all calcium channels that could enter the axon at the same time. when these operations were completed, they were surprised to find that although synaptic transmission was completely stopped, the synaptic density, microstructure, vesicle distribution and active regions were intact from any angle. to further verify the results, the team used the virus to perform the same operation on a class of giant synapses (calyx of held) of the auditory system in mice brain (animals survived because of the local operation on specific circuits), and the experimental results were completely consistent with those before. the results directly refute the first hypothesis, indicating that the generation of release sites is not dependent on calcium channels or calcium signals. to test the second hypothesis, that is, whether the active region has the ability to determine the release location. the researchers first made neurons develop into active regions in these environments. subsequently, to test whether these "beautiful" active regions have the proper function, the researchers used viruses to re express calcium channels. results synaptic transmission was immediately restored. this indicates that these active regions have all the functions of self-assembly, recruitment of calcium channels and vesicles, sensing of calcium signal and mediating the release of neurotransmitters. after this experiment, the researchers believe that the self-assembly of active region defines the location of transmitter release. After the assembly, the active region protein recruits functional elements (calcium channel and vesicle) to mediate the transmitter release. an interesting question is: among the many subtypes of calcium channels, only a small part can be enriched in axons. If calcium channels are completely recruited by synaptic active regions, does this mean that axons' selectivity for calcium channels comes from active regions? In order to verify this hypothesis, researchers have carried out a series of modifications to the calcium channel, gradually knocking out the interaction site between calcium channel and active region protein. the results showed that the abundance of calcium channels entering axon terminals was highly correlated with these action sites. because these sites are located at the C-terminal of calcium channel α subunit, the researchers believe that the amino acid sequence of the C-terminal of calcium channel is likely to have calcium channel localization signal. this study confirmed the role of active region and calcium channel in the decision-making of transmitter release site, and strongly proved that the generation of active region was the premise of calcium channel enrichment, and on this basis, the localization sequence of calcium channel was identified. synaptic transmission is the basis of all neural coding. Calcium channels and active regions control the basic properties of synaptic transmission and play an important role in neural computing. a deep understanding of the system will undoubtedly provide a strong theoretical support for the rapid development of neuroscience. the study was conducted by Pascal KAESER team, Harvard Medical School. Richard held (now Stanford University), Liu Changliang (Harvard Medical School) and Ma Kun Peng (Harvard Medical School & Chinese Academy of Sciences) are the co authors of this study. Br / after the completion of the synaptic binding site between the synaptic domain and the receptor, the active region of the synaptic region of was defined by the post synaptic release site of the receptor