Presynaptic and Postsynaptic Scaffolds

NROXXX10.1177/1073858414523321The NeuroscientistZiv and Fisher-Lavie 523321research-article2014 Review The Neuroscientist 2014, Vol. 20(5) 439 –452 Presynaptic and Postsynaptic Scaffolds: © The Author(s) 2014 Reprints and permissions: sagepub.com/journalsPermissions.nav Dynamics Fast and Slow DOI: 10.1177/1073858414523321 nro.sagepub.com Noam E. Ziv1 and Arava Fisher-Lavie1 Abstract The development of methods to follow the dynamics of synaptic molecules in living neurons has radically altered our view of the synapse, from that of a generally static structure to that of a dynamic molecular assembly at steady state. This view holds not only for relatively labile synaptic components, such as synaptic vesicles, cytoskeletal elements, and neurotransmitter receptors, but also for the numerous synaptic molecules known as scaffolding molecules, a generic name for a diverse class of molecules that organize synaptic function in time and space. Recent studies reveal that these molecules, which confer a degree of stability to synaptic assemblies over time scales of hours and days, are themselves subject to significant dynamics. Furthermore, these dynamics are probably not without effect; wherever studied, these seem to be associated with spontaneous changes in scaffold molecule content, synaptic size, and possibly synaptic function. This review describes the dynamics exhibited by synaptic scaffold molecules, their typical time scales, and the potential implications to our understanding of synaptic function. Keywords synaptic tenacity, synaptic dynamics, synaptic scaffolds, FRAP, photoactivation Chemical synapses are sites of cell-cell contact special- held in register by trans-synaptic cell adhesion molecules ized for the rapid transmission of signals between neu- and extracellular matrix proteins. rons and their targets: muscles, glands, or other neurons. Historically, much of what was known about CNS The vast majority of synapses in mammals are found in synapses and, in particular, about AZs, and PSDs was the central nervous system (CNS), where they typically based on electron microscopy (EM). However, an explo- connect the axon of one neuron to the dendrite or soma of sive amount of discoveries has been made during the last another neuron. Synaptic transmission is a directional 20 years on the molecular composition of CNS syn- process, and this directionality is manifested in the asym- apses—on the families, types, and subtypes of molecules metric structure of the presynaptic and postsynaptic com- found at excitatory and inhibitory synapses and their pri- partments. The axonal presynaptic compartment is mary roles: receptors, scaffolding molecules, proteins characterized by the presence of dozens to hundreds of involved in membrane trafficking, posttranslational mod- neurotransmitter-filled synaptic vesicles (SVs) and by ifications, protein synthesis and degradation, and many active zones (AZs), which are specialized regions of the more. It would be fair to say, however, that during the presynaptic axonal plasma membrane where SVs dock, initial years, a major concept carried over from the era of fuse, and release neurotransmitters into the synaptic cleft EM primacy was the notion that the synapse is a struc- (Fig. 1). The AZ is characterized by an electron-dense ture—a rather rigid and static molecular organization that matrix or lattice of proteins (the cytoskeleton of the AZ can undergo change when instructed to do so by physio- [CAZ]), which is thought to define the AZ as the site of logical signals but is otherwise quite stable and SV docking and fusion. The postsynaptic reception appa- unchanging. ratus is also characterized by an electron-dense thicken- ing referred to as the postsynaptic density (PSD), the central function of which is to confine receptors of the 1Technion–Israel Institute of Technology, Haifa, Israel appropriate type in front of the AZ. At excitatory glutamatergic synapses, PSDs are typically found at the tip of Corresponding Author: small dendritic protrusions known as dendritic spines, Noam E. Ziv, Technion–Israel Institute of Technology, Faculty of Medicine and Network Biology Research Laboratories, Fishbach whereas PSDs of inhibitory synapses are typically located Building, Technion City, Haifa 32000, Israel on dendritic shafts or cell bodies. The PSD and CAZ are Email: [email protected] Downloaded from nro.sagepub.com at TECHNION 34965 IND on November 3, 2015 440 The Neuroscientist 20(5) Zhang 2009; Gundelfinger and Fejtova 2012; Kneussel and Loebrich 2007; Sheng and Hoogenraad 2007; Südhof 2012; Verpelli and others 2012). Instead, the review will discuss the dynamics of these molecules, the relationships of these dynamics with those of other molecules (such as neurotransmitter receptors), the relationships with other processes such as protein synthesis and degradation, and finally, the functional implications that these dynamics might have on synaptic function. The Active Zone Presynaptic sites in the mammalian CNS typically appear as small varicosities (presynaptic boutons) distributed along axons (en passant synapses) in an irregular, near- random fashion (Hellwig and others 1994; Shepherd and others 2002 and references within) or, less commonly, at Figure 1. A schematic illustration of a mammalian central the tip of axonal branchlets. When observed at the EM nervous system, en passant glutamatergic synapse. The level, presynaptic boutons are found to contain clusters of presynaptic active zone, including molecules of the cytoskeleton SVs, opposed to the specialized region of the presynaptic 2+ of the active zone, voltage-gated Ca channels, and docked plasma membrane known as the AZ. In mammalian CNS vesicles, is shaded in gray. The active zone is juxtaposed and synapses, AZs are disc-like structures with diameters of connected to the postsynaptic density (PSD), located at the approximately 200 to 500 nm. The AZ is surrounded by a tip of a dendritic spine, via adhesion molecule pairs. Glutamate receptors are concentrated in the postsynaptic membrane via perisynaptic zone, which seems to be the major site of interactions with PSD scaffolding molecules. endocytosis, the essential regenerative upstroke of the SV cycle. Moreover, EM studies suggest that en passant presynaptic boutons lack obvious physical barriers that sepa- The advent of methods that allowed one to study the rate their contents from the cytoplasm and membrane of molecular dynamics of synaptic molecules, from large the axon proper (Shepherd and Harris 1998) or from those populations of synaptic molecules to single molecules of neighboring boutons. The lack of such barriers would within and outside of synapses, as well as long-term seem to challenge the ability of individual synapses to pre- imaging methodologies that allowed synapses to be fol- serve their complement of presynaptic molecules and SVs. lowed for many hours and days, has radically altered this Moreover, both spontaneous and, in particular, evoked notion (Choquet and Triller 2013). It is now clear that neurotransmitter release are associated with tremendous synapses are not structures in a strict sense but dynamic membrane trafficking, which involves SV mobilization, molecular assemblies at complex steady states, with syn- exocytosis, endocytosis, and SV reformation, suggesting aptic molecules moving in, out, and between synapses that activity further exacerbates this challenge. over a variety of time scales—from subseconds to The advent of new imaging techniques has allowed hours—and with synaptic compartments changing their these dynamics to be studied in living neurons and to eval- instantaneous contents and probably their fine functional uate how well this challenge is addressed. In this respect, characteristics on similar and longer time scales. several key techniques proved to be most valuable (Box 1). The main focus of this review is to describe the key Most of these are based on 1) the expression of fusion pro- concepts that have emerged over the last years concern- teins of specific synaptic molecules and green fluorescent ing the dynamic nature of synapses in the mammalian protein (GFP), one of its spectral variants or photoactivat- CNS. Specifically, the review will focus on synaptic able/photoswitchable derivatives; 2) confined photo- molecular complexes that are generally referred to as bleaching or photoactivation; and 3) time-lapse imaging synaptic scaffolds due to the roles that these are believed for monitoring changes in the fluorescence of these mole- to play in organizing presynaptic and postsynaptic orga- cules at individual synapses or changes in the distribution nization on a molecular scale. This generic role assign- of the fluorescently tagged molecules among nearby syn- ment clearly does not do justice to the specific properties apses (Kim and others 2010; Okabe 2012). and functions of these molecules; however, a comprehen- A large series of experiments using these and related sive review of their particular functions will not be pro- methods has revealed that under baseline conditions, vided here, and the reader is invited to refer to several SVs, traveling as individual vesicles or small packets, as recent reviews on these topics if so desired (Feng and well as SV-associated molecules, are continuously Downloaded from nro.sagepub.com at TECHNION 34965 IND on November 3, 2015 Ziv and Fisher-Lavie 441 Box 1. Measuring the Dynamics of Synaptic Scaffold Molecules (A) Fluorescence recovery after photobleaching. This illustration shows

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