The presynaptic active zone

Abstract

Neurotransmitters are released by synaptic vesicle exocytosis at the active zone of a presynaptic nerve terminal. In this review, I discuss the molecular composition and function of the active zone. Active zones are composed of an evolutionarily conserved protein complex containing as core constituents RIM, Munc13, RIM-BP, α-liprin, and ELKS proteins. This complex docks and primes synaptic vesicles for exocytosis, recruits Ca(2+) channels to the site of exocytosis, and positions the active zone exactly opposite to postsynaptic specializations via transsynaptic cell-adhesion molecules. Moreover, this complex mediates short- and long-term plasticity in response to bursts of action potentials, thus critically contributing to the computational power of a synapse.

Figure 1. Location of the Active Zone in a Synapse

(A) Schematic drawing of a synapse. (B) Electron micrograph of a conventially fixed and stained synapse in a cultured hippocampal neuron. (C) Electron micrograph of a phosphotungstic acid stained synapse in a cultured hippocampal neuron to visualize pre- and postsynaptic specializations. (B) and (C) are reproduced with permission from Kaeser et al. (2011).

Figure 2. Domain Structures of Evolutionarily Conserved Core Active Zone Proteins

The diagram depicts the domain structures of the five core active zone proteins and their interactions with each other and with a series of other synaptic proteins. For each protein, isoforms generated by alternative promoters and major splice variants are also shown. Arrows indicate binding reactions. Proteins and second messengers binding to specific sites are shown in boxes; domain designations are indicated by standard abbreviations. For C2 domains, Ca²⁺-binding sites are indicated by a sphere and lack of Ca²⁺-binding by an “X.” Proteins and domains are drawn to scale. Identified C-terminal PDZ-domain binding sequences in ELKS, Liprins, and RIMs are shown; asterisks indicate positions of the C termini of a protein. For details, see text.

Figure 3. Molecular Model of the Active Zone Protein Complex and Its Relation to the Synaptic Vesicle Fusion Machinery, Ca²⁺ Channels, and Synaptic Cell-Adhesion Molecules

The various active zone proteins and their interactions are depicted schematically (see Figure 2 and the text for details).

Figure 4. Ultrastructure of the Mammalian and Drosophila Active Zone

(A) The left and central drawing depict a schematic view of the structure of the active zone as it emerges from studies of chemically fixed synapses, shown in a cross section (left) and top-down view (right; data from Akert et al. [1972] and Limbach et al. [2011]). The right drawing depicts a similarly schematic view of the active zone as it emerges from cryo-EM tomography of unfixed materials (based on Fernández-Busnadiego et al. [2010]). The nature and sizes of key features are explained below the drawings. (B) Structure of the Drosophila neuromuscular active zone and t-bar as revealed by high-resolution LM (adapted with permission from Liu et al. [2011]).

Figure 5. Ca²⁺ Binding to the Munc13 C2B Domain Mediates Short-Term Synaptic Plasticity

(A) Schematic drawing of the signaling domain architecture of Munc13-1 and ubMunc13-2 (see Figure 2). (B–D) Role of Ca²⁺-binding to the Munc13 C2B domain as analyzed in three different forms of short-term synaptic plasticity, use-dependent depression (B), augmentation (C), and diacylglycerol-induced synaptic facilitation (D). Graphs show normalized EPSC amplitudes in autapses from neurons lacking Munc13-1 and Munc13-2, and expressing wild-type ubMunc13-2 (Munc13-2^WT) or two different ubMunc13-2 mutants, namely a mutant in which the C2B domain is unable to bind Ca²⁺ (Munc13-2^DN), or in which the C2B domain Ca²⁺-binding site is activated to allow Ca²⁺-binding even in the absence of phosphatidylinositol-phosphates (Munc13-2^KW). Note that the activating mutation greatly increases the basal release probability in synapses, thus resulting in use-dependent depression because readily releasable vesicles become rapidly depleted. The inactivating mutation, conversely, has no effect on basal release probability but causes use-dependent depression and impairs augmentation because Ca²⁺-binding to the Munc13 C2B domain normally accelerates vesicle recruitment during stimulus trains. Note also that the activating mutation decreases the relative efficacy of phorbol esters in enhancing release because it already enhances the release probability, thereby partly occluding the phorbol ester effect. Modified with permission from Shin et al. (2010).