Molecular organization and plasticity of the cytomatrix at the active zone
Highlights
► A complex proteinous cytomatrix, the CAZ, organizes presynaptic transmitter release. ► Topological localization of CAZ proteins relative to presynaptic specializations. ► Control of calcium channel positioning relative to release sites. ► Organization of SV clusters and actin cytoskeleton by the CAZ. ► Molecular remodeling of the CAZ during synaptic plasticity.
Introduction
Release of neurotransmitter from presynaptic boutons occurs at highly specialized regions of the presynaptic membrane: the active zone (AZ). At the AZ a series of membrane trafficking events, the synaptic vesicle (SV) cycle, underlies regulated release of neurotransmitter. Formally, the SV cycle can be divided into discrete steps including docking SVs to their release sites, priming of SVs to make them fusion competent and fusion of SVs with the AZ membrane allowing release of transmitter in response to Ca2+ influx through voltage-gated calcium channels (CaVs). Compensatory endocytosis of the SV membrane right next to the AZ followed by recycling and refilling of the SV with neurotransmitter complete the cycle [1]. A complex electron-dense meshwork of proteins, the cytomatrix at the AZ (CAZ), is assembled at the presynaptic membrane and is thought to spatially and functionally organize key steps of neurotransmitter release including the organization of SV pools (recently reviewed in [2]), tethering, docking and priming of SVs [3, 4] (see also Box 1), the physical coupling of exocytic and endocytic machineries [5, 6••] or the localization of presynaptic CaVs relative to SV docking sites (positional priming), which is an important determinant of release probability and presynaptic plasticity [7, 8]. Moreover, the CAZ proteins seem to be essential regulators of presynaptic assembly during synaptogenesis [9, 10, 11] and they are assumed to be involved in the recruitment and proper localization of cell adhesion molecules that mediate the proper alignment of the AZ with the postsynaptic neurotransmitter reception apparatus and determine synaptic specificity (for review see, e.g. [12, 13]).
The protein meshwork of the CAZ is organized by a small set of multi-domain scaffolding proteins including the Rab3-interacting molecules (RIMs), the RIM-binding proteins (RIM-BPs), Bassoon and Piccolo/Aczonin, the CAST/ELKS/Bruchpilot proteins, the Liprins-α and the UNC-13/Munc-13 proteins [11, 14, 15]. This review will discuss novel findings on the role of these proteins in organizing molecular structure and the functional plasticity of presynaptic neurotransmitter release sites.
Section snippets
Novel aspects on the molecular and ultra-structural organization of the CAZ
The ultra-structural organization of the presynaptic bouton of conventional synapses has been competently discussed in a recent article [2]. However, an even rough assignment of molecules to the complex structural elements identified by the electron microscopy (EM) has only been achieved for specializations of the CAZ, like the synaptic ribbon in retinal photoreceptor [16, 17••] or the T-bar in the Drosophila larval neuromuscular junction [18••]. A recent study has made a significant step
RIM and Munc13
At the molecular level, recruitment of SV to release sites and making them fusion competent (priming) are tightly connected and mediated by overlapping sets of molecules [1, 3, 4]. Munc13 is essential for SV priming most likely via interaction with the SNARE complex. The minimal region of Munc13-1 required for this function is the C-terminal portion of the molecule including the Munc-homology domains and the C2-domain [4, 24]. Ultra-structural reexamination of Munc13-1/2 double mutants revealed
A comparison with invertebrates
The NMJs of invertebrate models, like Drosophila and Caenorhabditis elegans, have contributed significantly to the understanding of assembly and plasticity mechanisms of the CAZ [11, 46]. As these organisms lack Bassoon and Piccolo and express merely one isoform of RIM/UNC-10, ELKS/CAST/Bruchpilot (BRP) and Liprin-α/SYD-2, a functional genetic analysis is much simpler than in vertebrates. BRP is a central organizer of T-bars, sophisticated CAZ structures at the Drosophila larval NMJ. It is
Remodeling of the CAZ during alterations of presynaptic efficacy
The likelihood by which an incoming action potential elicits SV fusion (release probability Pr) is an important determinant of synaptic strength. As such it is regulated during experience-induced (Hebbian) and homeostatic synaptic plasticity [57, 58]. Various recent studies suggest a role for the presynaptic CAZ in the control of Pr. In a live-imaging study using recombinant fluorescent protein-tagged Bassoon as an AZ marker Matz et al. [59•] demonstrated that Pr strongly correlates with the
Conclusions
Recent progress in the ultra-structural localization of particular protein components within the complex proteinaceous meshwork of the CAZ will help to understand the functioning of the molecular machinery at the AZ and to develop structural models of it (e.g. [36•]) — ideally in conjunction with the molecular anatomic model of the SV [65••]. Hand in hand with this ultra-structural information specific functional studies were performed for CAZ proteins that altogether imply a significant role for
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 in the authors’ laboratory on active zone function is supported by the Deutsche Forschungsgemeinsaft (DFG), the German Center for Neurodegenerative Diseases (DZNE), the European Commission (REPLACES, EFRE), the German Federal Government (ERANet Neuron) and the Center for Behavioral Brain Science (CBBS) Magdeburg.
References (67)
- et al.
Ultrastructural organization of presynaptic terminals
Curr Opin Neurobiol
(2011) - et al.
Regulation of membrane fusion in synaptic excitation-secretion coupling: speed and accuracy matter
Neuron
(2007) - et al.
Multiple roles of calcium ions in the regulation of neurotransmitter release
Neuron
(2008) - et al.
Molecular mechanisms of presynaptic differentiation
Annu Rev Cell Dev Biol
(2008) - et al.
Structural and functional plasticity of the cytoplasmic active zone
Curr Opin Neurobiol
(2011) - et al.
Cell adhesion, the backbone of the synapse: “vertebrate” and “invertebrate” perspectives
Cold Spring Harb Perspect Biol
(2009) - et al.
Molecular organization and assembly of the presynaptic active zone of neurotransmitter release
Results Probl Cell Differ
(2006) - et al.
Investigating interactions mediated by the presynaptic protein bassoon in living cells by Foerster's resonance energy transfer and fluorescence lifetime imaging microscopy
Biophys J
(2008) - et al.
RIM proteins activate vesicle priming by reversing autoinhibitory homodimerization of Munc13
Neuron
(2011) - et al.
RIM determines Ca(2+) channel density and vesicle docking at the presynaptic active zone
Neuron
(2011)