Presynaptic active zone is a layer of electron-dense material beneath the plasma membranes in the presynaptic compartments. These electron-dense materials are made up of densely packed proteins which function in docking and priming readily releasable synaptic vesicles (SVs) and clustering and positioning voltage-gated Ca2+ channels (VGCCs) at subregions of the presynaptic active zone membrane to regulate the pace and strength of neurotransmitter releases (reviewed in 32015539).
Formation, Composition & dynamics (Assembly and disassembly)
Every presynapse develops a 'active zone' structure in which ion channels cluster and synaptic vesicles release neurotransmitters. Active zones contain a number of scaffold proteins, including RIM, RIM-BP, ELKS, Munc13, Liprin, and CASKn (33208945, 32015539).
It is found that during the early stages of synapse development, the main active-zone scaffold proteins Liprin and ELKS-1 undergo phase separation and later mature into a solid structure. Liprin and ELKS-1 mutant proteins which lack phase separation activity are shown to be still abundant in synapses, but show abnormalities in active-zone formation and synapse function (33208945.
Furthermore, a recent super-resolution microscopic imaging investigation revealed that, rather than a randomly diffused distribution, RIM forms nanocluster distributions inside concentrated active zones. Multiple proline-rich sequences found throughout RIM can selectively bind to the three SH3 domains of RIM-BP, causing the complex to phase separate. The phase separation of the RIM–RIM-BP complex is also aided by many intrinsically disordered sequences in RIM (reviewed in 32015539).
Relation to human disease**
The density of clustered VGCC on presynaptic plasma membranes, as well as the closeness of clustered VGCC to calcium sensors at SV fusion sites, are crucial for fast and accurate neurotransmitter release. RIM and RIM-BP are known to be required for the clustering and localization of VGCC at active zones, as well as the coupling of clustered VGCC to release sites9. The cytosolic tail of VGCC can be recruited to the RIM–RIM-BP condensates via direct Ca2+ channel tail binds to both RIM and RIM-BP, which fits with our functional data. The Ca2+ channel tail, in turn, cpromotes the RIM–RIM–BP mixture phase separate(reviewed in 32015539).
Chen X, Wu X, Wu H, Zhang M. Phase separation at the synapse. Nat Neurosci. 2020 Mar;23(3):301-310. doi: 10.1038/s41593-019-0579-9. Epub 2020 Feb 3. PMID: 32015539.
McDonald NA, Fetter RD, Shen K. Assembly of synaptic active zones requires phase separation of scaffold molecules. Nature. 2020 Dec;588(7838):454-458. doi: 10.1038/s41586-020-2942-0. Epub 2020 Nov 18. Erratum in: Nature. 2021 Jul;595(7866):E35. PMID: 33208945.