Mechanisms controlling assembly and plasticity of presynaptic active zone scaffolds Astrid G Petzoldt 1,2,3 , Janine Lu ¨ tzkendorf 1,3 and Stephan J Sigrist 1,2 Cognitive processes including memory formation and learning rely on a precise, local and dynamic control of synapse functionality executed by molecular changes within both presynaptic and postsynaptic compartments. Recently, the size of the presynaptic active zone scaffold, a cluster of large multi-domain proteins decorating the presynaptic plasma membrane, was found to directly scale with the action potential evoked release of synaptic vesicles. The challenge now is to constitute an integrated picture of how long-range axonal transport, local exchange and localization mechanisms at the scaffold and degradation processes are integrated to allow for dynamic and controlled scaffold rearrangements. Here we discuss findings from multiple model systems emphasizing both short-term and long-term regulations of active zone composition and function. Addresses 1 Neurogenetik, Institut fu ¨r Biologie, Freie Universita ¨t Berlin, 14195 Berlin, Germany 2 Cluster of Excellence NeuroCure, Charite ´ – Universita ¨ tsmedizin Berlin, 10117 Berlin, Germany Corresponding author: Sigrist, Stephan J (stephan.sigrist@fu-berlin.de) 3 These authors contributed equally to the work. Current Opinion in Neurobiology 2016, 39:69–76 This review comes from a themed issue on Cellular neuroscience Edited by Bettina Winckler and Mikael Simons http://dx.doi.org/10.1016/j.conb.2016.04.009 0959-4388/Published by Elsevier Ltd. Introduction Functionality of the nervous system is based on a rapid communication between neurons and their target cells through specialized cell–cell contacts generically termed synapses. Appropriate synaptic function is essential for all types of cognitive processes, including memory formation and learning. Chemical synapses are asymmetrically or- ganized with a presynaptic ‘active zone’ (AZ) capable of neurotransmitter release upon action potential arrival and a postsynaptic compartment able to receive and further process this signal. The presynaptic compartment usually accumulates large numbers of synaptic vesicles (SVs). The cytoplasm of the presynaptic bouton moreover is populated with several hundred protein species in copy numbers ranging over several orders of magnitude [1  ]. However, the AZ scaffold, an electron dense structure essential for synapse tenacity, localization of SV fusion and positioning of voltage-dependent calcium channels, involves only a few canonical protein families: ELKS/ CAST family, RIM-superfamily, including the mammalian Piccolo and Bassoon, RIM-BP, (M)UNC-13, Liprin-a and SYD-1 (Table 1) [2–5]. The use of electron tomography and super-resolution light microscopy revealed underlying macromolecular ‘architectures’ within presynaptic scaf- folds [6–8]. Scaffold assembly is based on defined and dynamically regulated protein–protein interactions using a conserved set of interaction surfaces including both intramolecular and intermolecular coiled-coil interactions, SAM and PDZ domain interactions [2]. The possibility of multiple potentially parallel interactions results in at least partial functional ‘redundancy’ between AZ scaffold com- ponents, thus complicating stringent functional analysis and necessitating the simultaneous manipulations of sev- eral genetic loci. This biochemical and genetic complexity is likely a direct reflection of the tailoring of these crucial neuronal compartments towards robustness, combining the high stability of a lifelong structure with the demand for dynamic changes adapting to plasticity requirements. In fact, recent data show that at individual AZs, scaffold size scales with the probability of SV release on the time scale of several minutes only. The difficulty now is to elucidate the detailed cell-biological mechanisms integrating assembly and maintenance with dynamic plasticity processes in different biological contexts concerning neuron type, de- velopmental state and age of the organism. Here we review and try to conceptualize recent findings to create an inte- grated picture of the regulatory processes determining AZ scaffold architecture, size and function. Dynamic control of active zone scaffold size and release function Work at both mammalian and Drosophila synapses pro- vides evidence for a tight link between AZ size and complexity and the resulting functional synaptic output. A recently developed assay monitoring the Ca 2+ influx through postsynaptic glutamate receptors of neuromus- cular synapses of Drosophila larvae allows for the detec- tion of single AZ release events [9]. This assay provides the possibility to relate the microscopic organization of an individual AZ to its functional properties concerning spontaneous and evoked release. Interestingly it could Available online at www.sciencedirect.com ScienceDirect www.sciencedirect.com Current Opinion in Neurobiology 2016, 39:69–76