Assembly of the presynaptic active zone
Introduction
Transmission and processing of information in the brain mostly occur at synapses, sites of communication between neurons at which a presynaptic nerve terminal contacts a postsynaptic cell. Synaptic transmission is initiated by the fusion of neurotransmitter-containing synaptic vesicles with the plasma membrane. This fusion step is mediated by a set of conserved fusion proteins including SNAREs, their regulators, and Ca2+ sensors. While presynaptic fusion rates are low at rest, they dramatically increase upon opening of voltage-gated Ca2+ channels in response to action potential firing. Synaptic vesicle exocytosis is executed within less than one millisecond upon presynaptic depolarization, is precisely targeted toward postsynaptic receptors, is remarkably plastic, and is heterogeneous across synapse and neuronal types. These important features of synaptic transmission are mediated by sophisticated protein machinery called the active zone (Box 1) [1,2].
In this review, we discuss recent progress and current models of the mechanisms of active zone assembly. The focus of this review is on ‘primary’ active zone proteins, which we narrowly define as the proteins that are preferentially localized to the active zone membrane despite the lack of transmembrane domains, and function together in coupling vesicle fusion to Ca2+ influx through Ca2+ channels (Box 1, Figure 1). These proteins include RIM, Munc13, Bassoon/Piccolo, Liprin-α, ELKS and RIM-BP. Notably, this definition does not include SNARE proteins or Ca2+ channels, but instead highlights that one active zone function is to control the relative positioning of these essential proteins [1,2]. We further define active zone assembly as the process that positions and maintains these proteins in a complex that supports exocytotic functions at the presynaptic plasma membrane. Active zone assembly and function are closely linked. Any given component may have assembly roles in recruiting other proteins to the active zone and direct roles in mediating exocytosis of vesicles. The following steps are essential for active zone assembly (Figure 2):
• the generation of active zone proteins, their sorting into axonal transport, and their capture in a nerve terminal
• the assembly of these active zone constituents into protein complexes
• the positioning and anchoring of these complexes at the presynaptic target membrane precisely opposed to postsynaptic receptors
These steps are interconnected and do not necessarily occur in the same order for each protein. We highlight recent progress in dissecting each of these steps, first focusing on the best understood step, assembly of the protein complex, and we explore key challenges that lie ahead.
Section snippets
The active zone protein complex and its assembly
Superresolution microscopy has revolutionized the characterization of active zone protein complexes and uncovered striking patterning of the active zone. In the vertebrate brain, the large scaffold protein Bassoon is oriented such that its C-terminus is close to the presynaptic plasma membrane, and its N-terminus reaches tens of nanometers into the presynaptic cytoplasm [3]. Other active zone proteins localize between the Bassoon N-termini and C-termini [4•], indicating that they are part of
Anchoring of the active zone at the target membrane
Active zone protein complexes are anchored at the presynaptic plasma membrane opposed to postsynaptic specializations. At excitatory synapses, alignment between active zone complexes and AMPA receptors is executed with remarkable precision, within trans-synaptic nanodomains of ∼80 nm in diameter [7••]. Notably, none of the primary active zone proteins have a transmembrane domain, indicating that other molecules must control active zone anchoring and alignment. A recent review [2] speculated on
Axonal transport and presynaptic capture of active zone proteins
Active zone protein constituents are made and packaged in the soma, sorted into and transported along the axon, and captured at nerve terminals. Despite their central importance, these are the least understood processes in active zone assembly.
Initial work proposed that active zone precursors form in the soma and are transported as discrete units or together with other presynaptic proteins [48,49]. However, the content of these precursor complexes and how many different precursors exist has
Outlook
Research has started to shed light on how active zones are assembled, but the understanding is incomplete and lags behind studies of active zone protein function. Throughout this short review, we have discussed plausible working models for the transport, assembly and membrane anchoring of the active zone protein complex, along with pressing questions that remain open. The key overall goals for future research on active zone assembly beyond these points should include the following:
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Conflict of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
Work on active zone assembly and function in the Kaeser laboratory is supported by the National Institutes of Health (R01NS083898, R01NS103484 and R01MH113349 to P.S.K.). We apologize to our colleagues that we could not cite all important active zone work due to space restrictions. We thank Dr Shan Shan Wang for electron micrographs presented in Figure 1a and all members of the Kaeser laboratory for insightful discussions.
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Complexin Membrane Interactions: Implications for Synapse Evolution and Function
2023, Journal of Molecular BiologyCitation Excerpt :More than two decades of research into the molecules underlying exocytosis have revolutionized our understanding of synaptic transmission, and a coherent picture of the core presynaptic release machinery is beginning to take shape.1–4 Briefly, neurotransmitter-containing synaptic vesicles (SVs) become tightly associated with the synaptic bouton plasma membrane in part through interactions at the protein-dense active zone (AZ) where voltage-gated calcium (Ca2+) channels also localize.5–6 Like many other forms of eukaryotic lipid bilayer fusion, SNARE proteins (Synaptobrevin/VAMP2 on the vesicle and Syntaxin 1 and SNAP25 on the plasma membrane) are required for SV fusion.
Interactions between Membraneless Condensates and Membranous Organelles at the Presynapse: A Phase Separation View of Synaptic Vesicle Cycle
2023, Journal of Molecular BiologyCitation Excerpt :These seven proteins likely form the basic architecture of the active zone to support essentially all known activities occurring within this confined space. Besides, many other proteins, such as the SNARE fusion machinery and cofactors, ion channels, synaptic adhesion molecules and cytoskeletons are also known to locate within or in peripheral to active zones,118,121,124 forming a highly intricate molecular interaction network right beneath the plasma membrane (Figure 2(B)).117,125 Decades of genetic studies have elucidated key roles of individual scaffold proteins in controlling SV exocytosis.117–118
Ultrastructural analysis of wild-type and RIM1α knockout active zones in a large cortical synapse
2022, Cell ReportsCitation Excerpt :Several RIM1α domains bind to VGCCs and its Zinc finger domain to SVs, therefore RIM is an essential link (Kaeser et al., 2011; Schoch et al., 2002). In addition, RIM1α interacts with core AZ components like ELKS/CAST, RIM-BP, and α-liprin, generating a multifunctional scaffold (Emperador-Melero and Kaeser, 2020). Therefore, the absence of RIM1α results in a disassembly of AZ scaffold leading to an increase in AZ surface area and variability.
Vesicle choreographies keep up cell-to-extracellular matrix adhesion dynamics in polarized epithelial and endothelial cells
2022, Matrix BiologyCitation Excerpt :During embryonic development, the binding of fibronectin to its major receptor α5β1 integrin induces apico-basal polarity of endothelial cells and the formation of the single lumen of blood vessels [69,70]. We [11] and others [71] revealed that, similarly to exocytosis in neuron presynaptic active zone [72], the polarized secretion of PGCs carrying freshly synthesized fibronectin at the basal surface of endothelial cells relies on the protein tyrosine phosphatase receptor type f polypeptide (PTPRF, also named LAR for leukocyte common antigen related) and its directly interacting adaptor PTPRF interacting protein α1 (PPFIA1), also known as liprin-α1. Of note both proteins were identified as components of integrin adhesome complexes in different cell types [73–76].