Radixin regulates synaptic GABAA receptor density and is essential for reversal learning and short-term memory.

Neurotransmitter receptor density is a major variable in regulating synaptic strength. Receptors rapidly exchange between synapses and intracellular storage pools through endocytic recycling. In addition, lateral diffusion and confinement exchanges surface membrane receptors between synaptic and extrasynaptic sites. However, the signals that regulate this transition are currently unknown. GABAA receptors containing α5-subunits (GABAAR-α5) concentrate extrasynaptically through radixin (Rdx)-mediated anchorage at the actin cytoskeleton. Here we report a novel mechanism that regulates adjustable plasma membrane receptor pools in the control of synaptic receptor density. RhoA/ROCK signalling regulates an activity-dependent Rdx phosphorylation switch that uncouples GABAAR-α5 from its extrasynaptic anchor, thereby enriching synaptic receptor numbers. Thus, the unphosphorylated form of Rdx alters mIPSCs. Rdx gene knockout impairs reversal learning and short-term memory, and Rdx phosphorylation in wild-type mice exhibits experience-dependent changes when exposed to novel environments. Our data suggest an additional mode of synaptic plasticity, in which extrasynaptic receptor reservoirs supply synaptic GABAARs.

Synaptic stability and plasticity in a floating world.

A fundamental feature of membranes is the lateral diffusion of lipids and proteins. Control of lateral diffusion provides a mechanism for regulating the structure and function of synapses. Single-particle tracking (SPT) has emerged as a powerful way to directly visualize these movements. SPT can reveal complex diffusive behaviors, which can be regulated by neuronal activity over time and space. Such is the case for neurotransmitter receptors, which are transiently stabilized at synapses by scaffolding molecules. This regulation provides new insight into mechanisms by which the dynamic equilibrium of receptor-scaffold assembly can be regulated. We will briefly review here recent data on this mechanism, which ultimately tunes the number of receptors at synapses and therefore synaptic strength.

Receptors look outward: revealing signals that bring excitation to synapses.

Defining the molecular mechanisms that govern the trafficking of glutamate receptors to excitatory synaptic contacts is fundamental to understanding the mechanisms that regulate synapse maturation and neuronal excitability. Previous studies have identified several scaffolding molecules and adaptor proteins that regulate glutamate receptor trafficking and retention at the synapse. Recent work, however, has elucidated new players such as the N-cadherin adhesion complex, and members of the pentraxin family that regulate clustering of glutamate receptors through extracellular protein interactions. Here, we highlight recently identified modes that regulate glutamate receptor clustering, and discuss their relevance to synapse maturation.

Cell adhesion molecules at the synapse.

Synapses are specialized intercellular junctions whose specificity and plasticity provide the structural and functional basis for the formation and maintenance of the complex neural network in the brain. The number, location, and type of synapses formed are well controlled, since synaptic circuits are formed in a highly reproducible way. This implies the existence of cellular and molecular properties that determine the connectivity of each neuron in the nervous system. Recent evidence has elucidated that these key features of the synapse are regulated by several families of cell-adhesion molecules (CAMs) enriched at synaptic junctions, including neuroligins, SynCAM, NCAM, L1-CAM, cadherins, protocadherins, and integrins. In this review we will discuss the various stages of synaptogenesis from the perspective of CAMs: Contact initiation, recruitment of presynaptic and postsynaptic proteins, synapse maturation/stabilization or elimination, and synaptic plasticity. We will also highlight some of the factors that regulate the function of these CAMs at the synapse, and discuss how dysfunction of these adhesive systems may contribute to several neurological disorders.

A monovalent streptavidin with a single femtomolar biotin binding site.

Streptavidin and avidin are used ubiquitously because of the remarkable affinity of their biotin binding, but they are tetramers, which disrupts many of their applications. Making either protein monomeric reduces affinity by at least 10(4)-fold because part of the binding site comes from a neighboring subunit. Here we engineered a streptavidin tetramer with only one functional biotin binding subunit that retained the affinity, off rate and thermostability of wild-type streptavidin. In denaturant, we mixed a streptavidin variant containing three mutations that block biotin binding with wild-type streptavidin in a 3:1 ratio. Then we generated monovalent streptavidin by refolding and nickel-affinity purification. Similarly, we purified defined tetramers with two or three biotin binding subunits. Labeling of site-specifically biotinylated neuroligin-1 with monovalent streptavidin allowed stable neuroligin-1 tracking without cross-linking, whereas wild-type streptavidin aggregated neuroligin-1 and disrupted presynaptic contacts. Monovalent streptavidin should find general application in biomolecule labeling, single-particle tracking and nanotechnology.

A preformed complex of postsynaptic proteins is involved in excitatory synapse development.

Nonsynaptic clusters of postsynaptic proteins have been documented; however, their role remains elusive. We monitored the trafficking of several candidate proteins implicated in synaptogenesis, when nonsynaptic clusters of scaffold proteins are most abundant. We find a protein complex consisting of two populations that differ in their content, mobility, and involvement in synapse formation. One subpopulation is mobile and relies on actin transport for delivery to nascent and existing synapses. These mobile clusters contain the scaffolding proteins PSD-95, GKAP, and Shank. A proportion of mobile clusters that exhibits slow movement and travels short distances contains neuroligin-1. The second group consists of stationary nonsynaptic scaffold complexes that mainly contain neuroligin-1, can recruit synaptophysin-containing axonal transport vesicles, and are readily transformed to functional presynaptic contacts that recycle the vital dye FM 4-64. These results postulate a mechanism whereby preformed scaffold protein complexes serve as predetermined postsynaptic hotspots for establishment of new functional excitatory synapses.

Neuroligins mediate excitatory and inhibitory synapse formation: involvement of PSD-95 and neurexin-1beta in neuroligin-induced synaptic specificity.

The balance between excitatory and inhibitory synapses is a tightly regulated process that requires differential recruitment of proteins that dictate the specificity of newly formed contacts. However, factors that control this process remain unidentified. Here we show that members of the neuroligin (NLG) family, including NLG1, NLG2, and NLG3, drive the formation of both excitatory and inhibitory presynaptic contacts. The enrichment of endogenous NLG1 at excitatory contacts and NLG2 at inhibitory synapses supports an important in vivo role for these proteins in the development of both types of contacts. Immunocytochemical and electrophysiological analysis showed that the effects on excitatory and inhibitory synapses can be blocked by treatment with a fusion protein containing the extracellular domain of neurexin-1beta. We also found that overexpression of PSD-95, a postsynaptic binding partner of NLGs, resulted in a shift in the distribution of NLG2 from inhibitory to excitatory synapses. These findings reveal a critical role for NLGs and their synaptic partners in controlling the number of inhibitory and excitatory synapses. Furthermore, relative levels of PSD-95 alter the ratio of excitatory to inhibitory synaptic contacts by sequestering members of the NLG family to excitatory synapses.

A balance between excitatory and inhibitory synapses is controlled by PSD-95 and neuroligin.

Factors that control differentiation of presynaptic and postsynaptic elements into excitatory or inhibitory synapses are poorly defined. Here we show that the postsynaptic density (PSD) proteins PSD-95 and neuroligin-1 (NLG) are critical for dictating the ratio of excitatory-to-inhibitory synaptic contacts. Exogenous NLG increased both excitatory and inhibitory presynaptic contacts and the frequency of miniature excitatory and inhibitory synaptic currents. In contrast, PSD-95 overexpression enhanced excitatory synapse size and miniature frequency, but reduced the number of inhibitory synaptic contacts. Introduction of PSD-95 with NLG augmented synaptic clustering of NLG and abolished NLG effects on inhibitory synapses. Interfering with endogenous PSD-95 expression alone was sufficient to reduce the ratio of excitatory-to-inhibitory synapses. These findings elucidate a mechanism by which the amounts of specific elements critical for synapse formation control the ratio of excitatory-to-inhibitory synaptic input.

Presynaptic trafficking of synaptotagmin I is regulated by protein palmitoylation.

Protein palmitoylation plays a critical role in sorting and targeting of several proteins to pre- and postsynaptic sites. In this study, we have analyzed the role of palmitoylation in trafficking of synaptotagmin I and its modulation by synaptic activity. We found that palmitoylation of N-terminal cysteines contributed to sorting of synaptotagmin I to an intracellular vesicular compartment at the presynaptic terminal. Presynaptic targeting is a unique feature of N-terminal sequences of synaptotagmin I because the palmitoylated N terminus of synaptotagmin VII failed to localize to presynaptic sites. We also found that palmitate was stably associated with both synaptotagmin I and SNAP-25 and that rapid neuronal depolarization did not affect palmitate turnover on these proteins. However, long-term treatment with drugs that either block synaptic activity or disrupt SNARE complex assembly modulated palmitoylation and accumulation of synaptotagmin I at presynaptic sites. We conclude that palmitoylation is involved in trafficking of specific elements involved in transmitter release and that distinct mechanisms regulate addition and removal of palmitate on select neuronal proteins.

The tibial-1 pioneer pathway: an in vivo model for neuronal outgrowth and guidance.

As neurons extend axons to their targets during development, growth cones must reorient their direction of migration in response to extracellular guidance cues. A variety of model systems have been employed in order to dissect the cellular and molecular mechanisms that underlie this complex process. One preparation, the developing grasshopper limb bud, has proved to offer a number of advantages in which to examine mechanisms of growth cone guidance and motility in vivo. First, the relatively large size of the embryonic nervous system allows for straightforward imaging of both fixed and live neurons in vivo. Second, the peripheral nerves generated in the limb bud are highly stereotyped. Third, intact embryos can be cultured for a period of days, allowing for fairly easy perturbations at precise developmental stages. Fourth, due to the ease of dissection, numerous cell biological and molecular techniques can be utilized in the limb bud. Finally, axon guidance molecules and mechanisms are conserved between grasshoppers and other organism, including vertebrates.