Differential control of presynaptic efficacy by postsynaptic N-cadherin and β-catenin

Phosphorylation of Doc2 by EphB2 modulates Munc13-mediated SNARE complex assembly and neurotransmitter release

At the synapse, presynaptic neurotransmitter release is tightly controlled by release machinery, involving the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and Munc13. The Ca2+ sensor Doc2 cooperates with Munc13 to regulate neurotransmitter release, but the underlying mechanisms remain unclear. In our study, we have characterized the binding mode between Doc2 and Munc13 and found that Doc2 originally occludes Munc13 to inhibit SNARE complex assembly. Moreover, our investigation unveiled that EphB2, a presynaptic adhesion molecule (SAM) with inherent tyrosine kinase functionality, exhibits the capacity to phosphorylate Doc2. This phosphorylation attenuates Doc2 block on Munc13 to promote SNARE complex assembly, which functionally induces spontaneous release and synaptic augmentation. Consistently, application of a Doc2 peptide that interrupts Doc2-Munc13 interplay impairs excitatory synaptic transmission and leads to dysfunction in spatial learning and memory. These data provide evidence that SAMs modulate neurotransmitter release by controlling SNARE complex assembly.

Glial Cx43 hemichannels and neuronal Panx1 hemichannels and P2X7 receptors orchestrate presynaptic homeostatic plasticity

The emerging role of glial cells in modulating neuronal excitability and synaptic strength is a growing field in neuroscience. In recent years, a pivotal role of gliotransmission in homeostatic presynaptic plasticity has been highlighted and glial-derived ATP arises as a key contributor. However, very little is known about the glial non-vesicular ATP-release pathway and how ATP participates in the modulation of synaptic strength. Here, we investigated the functional changes occurring in neurons upon chronic inactivity and the role of the purinergic signaling, connexin43 and pannexin1 hemichannels in this process. By using hippocampal dissociated cultures, we showed that blocking connexin43 and pannexin1 hemichannels decreases the amount of extracellular ATP. Moreover, Ca2+ imaging assays using Fluo-4/AM revealed that blocking connexin43, neuronal P2X7Rs and pannexin1 hemichannels decreases the amount of basal Ca2+ in neurons. A significant impairment in synaptic vesicle pool size was also evidenced under these conditions. Interestingly, rescue experiments where Panx1HCs are blocked showed that the compensatory adjustment of cytosolic Ca2+ was recovered after P2X7Rs activation, suggesting that Panx1 acts downstream P2X7Rs. These changes were accompanied by a modulation of neuronal permeability, as revealed by ethidium bromide uptake experiments. In particular, the permeability of neuronal P2X7Rs and pannexin1 hemichannels is increased upon 24 h of inactivity. Taken together, we have uncovered a role for connexin43-dependent ATP release and neuronal P2X7Rs and pannexin1 hemichannels in the adjustment of presynaptic strength by modulating neuronal permeability, the entrance of Ca2+ into neurons and the size of the recycling pool of synaptic vesicles.

Microglial CD300f immune receptor contributes to the maintenance of neuron viability in vitro and after a penetrating brain injury

Emerging evidences suggest that immune receptors participate in diverse microglial and macrophage functions by regulating their immunometabolism, inflammatory phenotype and phagocytosis. CD300f, a TREM2-like lipid sensing immune receptor, that integrates activating and inhibitory cell-signalling pathways, modulates inflammation, efferocytosis and microglial metabolic fitness. In particular, CD300f overexpression was described to be neuroprotective after an acute brain injury, suggesting a role for this immune receptor in neurotrophic interactions. Thus, we hypothesised that CD300f modulates neuronal survival through neuron-microglial interactions. In order to study its biological function, we used in vitro and in vivo approaches, CD300f-/- animals and rCD300f-Fc, a fusion protein that interrupts the endogen interaction between CD300f receptor-ligands. In hippocampal cocultures containing neurons and mixed glia, we observed that rCD300f-Fc, but not control IgGs induced neuronal death. In accordance, in vivo studies performed by injecting rCD300f-Fc or control IgGs into rat or WT or CD300 KO mice neocortex, showed an increased lesioned area after a penetrating brain injury. Interestingly, this neuronal death was dependent on glia, and the neurotoxic mechanism did not involve the increase of proinflammatory cytokines, the participation of NMDA receptors or ATP release. However, exogenous addition of glial cell line-derived neurotrophic factor (GDNF) prevented this process. Taken together, our results suggest that CD300f modulates neuronal survival in vitro and after a penetrating brain injury in vivo and that CD300f inhibition alters microglial phenotype generating a neurotoxic microenvironment.

Target cell-specific synaptic dynamics of excitatory to inhibitory neuron connections in supragranular layers of human neocortex

Rodent studies have demonstrated that synaptic dynamics from excitatory to inhibitory neuron types are often dependent on the target cell type. However, these target cell-specific properties have not been well investigated in human cortex, where there are major technical challenges in reliably obtaining healthy tissue, conducting multiple patch-clamp recordings on inhibitory cell types, and identifying those cell types. Here, we take advantage of newly developed methods for human neurosurgical tissue analysis with multiple patch-clamp recordings, post-hoc fluorescent in situ hybridization (FISH), machine learning-based cell type classification and prospective GABAergic AAV-based labeling to investigate synaptic properties between pyramidal neurons and PVALB- vs. SST-positive interneurons. We find that there are robust molecular differences in synapse-associated genes between these neuron types, and that individual presynaptic pyramidal neurons evoke postsynaptic responses with heterogeneous synaptic dynamics in different postsynaptic cell types. Using molecular identification with FISH and classifiers based on transcriptomically identified PVALB neurons analyzed by Patch-seq, we find that PVALB neurons typically show depressing synaptic characteristics, whereas other interneuron types including SST-positive neurons show facilitating characteristics. Together, these data support the existence of target cell-specific synaptic properties in human cortex that are similar to rodent, thereby indicating evolutionary conservation of local circuit connectivity motifs from excitatory to inhibitory neurons and their synaptic dynamics.

Cortical Parvalbumin-Positive Interneuron Development and Function Are Altered in the APC Conditional Knockout Mouse Model of Infantile and Epileptic Spasms Syndrome

Infantile and epileptic spasms syndrome (IESS) is a childhood epilepsy syndrome characterized by infantile or late-onset spasms, abnormal neonatal EEG, and epilepsy. Few treatments exist for IESS, clinical outcomes are poor, and the molecular and circuit-level etiologies of IESS are not well understood. Multiple human IESS risk genes are linked to Wnt/β-catenin signaling, a pathway that controls developmental transcriptional programs and promotes glutamatergic excitation via β-catenin's role as a synaptic scaffold. We previously showed that deleting adenomatous polyposis coli (APC), a component of the β-catenin destruction complex, in excitatory neurons (APC cKO mice, APCfl/fl x CaMKIIαCre) increased β-catenin levels in developing glutamatergic neurons and led to infantile behavioral spasms, abnormal neonatal EEG, and adult epilepsy. Here, we tested the hypothesis that the development of GABAergic interneurons (INs) is disrupted in APC cKO male and female mice. IN dysfunction is implicated in human IESS, is a feature of other rodent models of IESS, and may contribute to the manifestation of spasms and seizures. We found that parvalbumin-positive INs (PV+ INs), an important source of cortical inhibition, were decreased in number, underwent disproportionate developmental apoptosis, and had altered dendrite morphology at P9, the peak of behavioral spasms. PV+ INs received excessive excitatory input, and their intrinsic ability to fire action potentials was reduced at all time points examined (P9, P14, P60). Subsequently, GABAergic transmission onto pyramidal neurons was uniquely altered in the somatosensory cortex of APC cKO mice at all ages, with both decreased IPSC input at P14 and enhanced IPSC input at P9 and P60. These results indicate that inhibitory circuit dysfunction occurs in APC cKOs and, along with known changes in excitation, may contribute to IESS-related phenotypes.SIGNIFICANCE STATEMENT Infantile and epileptic spasms syndrome (IESS) is a devastating epilepsy with limited treatment options and poor clinical outcomes. The molecular, cellular, and circuit disruptions that cause infantile spasms and seizures are largely unknown, but inhibitory GABAergic interneuron dysfunction has been implicated in rodent models of IESS and may contribute to human IESS. Here, we use a rodent model of IESS, the APC cKO mouse, in which β-catenin signaling is increased in excitatory neurons. This results in altered parvalbumin-positive GABAergic interneuron development and GABAergic synaptic dysfunction throughout life, showing that pathology arising in excitatory neurons can initiate long-term interneuron dysfunction. Our findings further implicate GABAergic dysfunction in IESS, even when pathology is initiated in other neuronal types.

Regulation of Presynaptic Release Machinery by Cell Adhesion Molecules

The synapse is a highly specialized asymmetric structure that transmits and stores information in the brain. The size of pre- and postsynaptic structures and function is well coordinated at the individual synapse level. For example, large postsynaptic dendritic spines have a larger postsynaptic density with higher α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) number on their surface, while juxtaposing presynaptic terminals have a larger active zone and higher release probability. This indicates that pre- and postsynaptic domains bidirectionally communicate to coordinate assembly of specific molecules on both sides of the synaptic cleft. Cell adhesion molecules (CAMs) that localize at synapses form transsynaptic protein interactions across the synaptic cleft and play important roles in synapse formation and regulation. The extracellular domain of CAMs is essential for specific synapse formation and function. In contrast, the intracellular domain is necessary for binding with synaptic molecules and signal transduction. Therefore, CAMs play an essential role on synapse function and structure. In fact, ample evidence indicates that transsynaptic CAMs instruct and modulate functions at presynaptic sites. This chapter focuses on transsynaptic protein interactions that regulate presynaptic functions emphasizing the role of neuronal CAMs and the intracellular mechanism of their regulation.

Involvement of CaV 2.2 channels and α2 δ-1 in homeostatic synaptic plasticity in cultured hippocampal neurons

In the mammalian brain, presynaptic CaV 2 channels play a pivotal role in synaptic transmission by mediating fast neurotransmitter exocytosis via influx of Ca2+ into the active zone of presynaptic terminals. However, the distribution and modulation of CaV 2.2 channels at plastic hippocampal synapses remains to be elucidated. Here, we assess CaV 2.2 channels during homeostatic synaptic plasticity, a compensatory form of homeostatic control preventing excessive or insufficient neuronal activity during which extensive active zone remodelling has been described. We show that chronic silencing of neuronal activity in mature hippocampal cultures resulted in elevated presynaptic Ca2+ transients, mediated by increased levels of CaV 2.2 channels at the presynaptic site. This work focused further on the role of α2 δ-1 subunits, important regulators of synaptic transmission and CaV 2.2 channel abundance at the presynaptic membrane. We found that α2 δ-1 overexpression reduces the contribution of CaV 2.2 channels to total Ca2+ flux without altering the amplitude of the Ca2+ transients. Levels of endogenous α2 δ-1 decreased during homeostatic synaptic plasticity, whereas the overexpression of α2 δ-1 prevented homeostatic synaptic plasticity in hippocampal neurons. Together, this study reveals a key role for CaV 2.2 channels and novel roles for α2 δ-1 during synaptic plastic adaptation. KEY POINTS: The roles of CaV 2.2 channels and α2 δ-1 in homeostatic synaptic plasticity in hippocampal neurons in culture were examined. Chronic silencing of neuronal activity resulted in elevated presynaptic Ca2+ transients, mediated by increased levels of CaV 2.2 channels at presynaptic sites. The level of endogenous α2 δ-1 decreased during homeostatic synaptic plasticity, whereas overexpression of α2 δ-1 prevented homeostatic synaptic plasticity in hippocampal neurons. Together, this study reveals a key role for CaV 2.2 channels and novel roles for α2 δ-1 during synaptic plastic adaptation.

miR ‐124‐dependent tagging of synapses by synaptopodin enables input‐specific homeostatic plasticity

Homeostatic synaptic plasticity is a process by which neurons adjust their synaptic strength to compensate for perturbations in neuronal activity. Whether the highly diverse synapses on a neuron respond uniformly to the same perturbation remains unclear. Moreover, the molecular determinants that underlie synapse‐specific homeostatic synaptic plasticity are unknown. Here, we report a synaptic tagging mechanism in which the ability of individual synapses to increase their strength in response to activity deprivation depends on the local expression of the spine‐apparatus protein synaptopodin under the regulation of miR‐124. Using genetic manipulations to alter synaptopodin expression or regulation by miR‐124, we show that synaptopodin behaves as a “postsynaptic tag” whose translation is derepressed in a subpopulation of synapses and allows for nonuniform homeostatic strengthening and synaptic AMPA receptor stabilization. By genetically silencing individual connections in pairs of neurons, we demonstrate that this process operates in an input‐specific manner. Overall, our study shifts the current view that homeostatic synaptic plasticity affects all synapses uniformly to a more complex paradigm where the ability of individual synapses to undergo homeostatic changes depends on their own functional and biochemical state.

The Shaping of AMPA Receptor Surface Distribution by Neuronal Activity

Neurotransmission is critically dependent on the number, position, and composition of receptor proteins on the postsynaptic neuron. Of these, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) are responsible for the majority of postsynaptic depolarization at excitatory mammalian synapses following glutamate release. AMPARs are continually trafficked to and from the cell surface, and once at the surface, AMPARs laterally diffuse in and out of synaptic domains. Moreover, the subcellular distribution of AMPARs is shaped by patterns of activity, as classically demonstrated by the synaptic insertion or removal of AMPARs following the induction of long-term potentiation (LTP) and long-term depression (LTD), respectively. Crucially, there are many subtleties in the regulation of AMPARs, and exactly how local and global synaptic activity drives the trafficking and retention of synaptic AMPARs of different subtypes continues to attract attention. Here we will review how activity can have differential effects on AMPAR distribution and trafficking along with its subunit composition and phosphorylation state, and we highlight some of the controversies and remaining questions. As the AMPAR field is extensive, to say the least, this review will focus primarily on cellular and molecular studies in the hippocampus. We apologise to authors whose work could not be cited directly owing to space limitations.

Transsynaptic N-Cadherin Adhesion Complexes Control Presynaptic Vesicle and Bulk Endocytosis at Physiological Temperature

At mammalian glutamatergic synapses, most basic elements of synaptic transmission have been shown to be modulated by specific transsynaptic adhesion complexes. However, although crucial for synapse homeostasis, a physiological regulation of synaptic vesicle endocytosis by adhesion molecules has not been firmly established. The homophilic adhesion protein N-cadherin is localized at the peri-active zone, where the highly temperature-dependent endocytosis of vesicles occurs. Here, we demonstrate an important modulatory role of N-cadherin in endocytosis at near physiological temperature by synaptophysin-pHluorin imaging. Different modes of endocytosis including bulk endocytosis were dependent on N-cadherin expression and function. N-cadherin modulation might be mediated by actin filaments because actin polymerization ameliorated the knockout-induced endocytosis defect. Using super-resolution imaging, we found strong recruitment of N-cadherin to glutamatergic synapses upon massive vesicle release, which might in turn enhance vesicle endocytosis. This provides a novel, adhesion protein-mediated mechanism for efficient coupling of exo- and endocytosis.

ADAM10 hyperactivation acts on piccolo to deplete synaptic vesicle stores in Huntington's disease

Synaptic dysfunction and cognitive decline in Huntington's disease (HD) involve hyperactive A disintegrin and metalloproteinase domain-containing protein 10 (ADAM10). To identify the molecular mechanisms through which ADAM10 is associated with synaptic dysfunction in HD, we performed an immunoaffinity purification-mass spectrometry (IP-MS) study of endogenous ADAM10 in the brains of wild-type and HD mice. We found that proteins implicated in synapse organization, synaptic plasticity, and vesicle and organelles trafficking interact with ADAM10, suggesting that it may act as hub protein at the excitatory synapse. Importantly, the ADAM10 interactome is enriched in presynaptic proteins and ADAM10 co-immunoprecipitates with piccolo (PCLO), a key player in the recycling and maintenance of synaptic vesicles. In contrast, reduced ADAM10/PCLO immunoprecipitation occurs in the HD brain, with decreased density of synaptic vesicles in the reserve and docked pools at the HD presynaptic terminal. Conditional heterozygous deletion of ADAM10 in the forebrain of HD mice reduces active ADAM10 to wild-type level and normalizes ADAM10/PCLO complex formation and synaptic vesicle density and distribution. The results indicate that presynaptic ADAM10 and PCLO are a relevant component of HD pathogenesis.

Synapse development and maturation at the drosophila neuromuscular junction

Synapses are the sites of neuron-to-neuron communication and form the basis of the neural circuits that underlie all animal cognition and behavior. Chemical synapses are specialized asymmetric junctions between a presynaptic neuron and a postsynaptic target that form through a series of diverse cellular and subcellular events under the control of complex signaling networks. Once established, the synapse facilitates neurotransmission by mediating the organization and fusion of synaptic vesicles and must also retain the ability to undergo plastic changes. In recent years, synaptic genes have been implicated in a wide array of neurodevelopmental disorders; the individual and societal burdens imposed by these disorders, as well as the lack of effective therapies, motivates continued work on fundamental synapse biology. The properties and functions of the nervous system are remarkably conserved across animal phyla, and many insights into the synapses of the vertebrate central nervous system have been derived from studies of invertebrate models. A prominent model synapse is the Drosophila melanogaster larval neuromuscular junction, which bears striking similarities to the glutamatergic synapses of the vertebrate brain and spine; further advantages include the simplicity and experimental versatility of the fly, as well as its century-long history as a model organism. Here, we survey findings on the major events in synaptogenesis, including target specification, morphogenesis, and the assembly and maturation of synaptic specializations, with a emphasis on work conducted at the Drosophila neuromuscular junction.

Distinct Target-Specific Mechanisms Homeostatically Stabilize Transmission at Pre- and Post-synaptic Compartments

Neurons must establish and stabilize connections made with diverse targets, each with distinct demands and functional characteristics. At Drosophila neuromuscular junctions (NMJs), synaptic strength remains stable in a manipulation that simultaneously induces hypo-innervation on one target and hyper-innervation on the other. However, the expression mechanisms that achieve this exquisite target-specific homeostatic control remain enigmatic. Here, we identify the distinct target-specific homeostatic expression mechanisms. On the hypo-innervated target, an increase in postsynaptic glutamate receptor (GluR) abundance is sufficient to compensate for reduced innervation, without any apparent presynaptic adaptations. In contrast, a target-specific reduction in presynaptic neurotransmitter release probability is reflected by a decrease in active zone components restricted to terminals of hyper-innervated targets. Finally, loss of postsynaptic GluRs on one target induces a compartmentalized, homeostatic enhancement of presynaptic neurotransmitter release called presynaptic homeostatic potentiation (PHP) that can be precisely balanced with the adaptations required for both hypo- and hyper-innervation to maintain stable synaptic strength. Thus, distinct anterograde and retrograde signaling systems operate at pre- and post-synaptic compartments to enable target-specific, homeostatic control of neurotransmission.

Glial ATP and Large Pore Channels Modulate Synaptic Strength in Response to Chronic Inactivity

A key feature of neurotransmission is its ability to adapt to changes in neuronal environment, which is essential for many brain functions. Homeostatic synaptic plasticity (HSP) emerges as a compensatory mechanism used by neurons to adjust their excitability in response to changes in synaptic activity. Recently, glial cells emerged as modulators for neurotransmission by releasing gliotransmitters into the synaptic cleft through pathways that include P2X₇ receptors (P2X₇R), connexons, and pannexons. However, the role of gliotransmission in the activity-dependent adjustment of presynaptic strength is still an open question. Here, we investigated whether glial cells participate in HSP upon chronic inactivity and the role of adenosine triphosphate (ATP), connexin43 hemichannels (Cx43HCs), and pannexin1 (Panx1) channels in this process. We used immunocytochemistry against vesicular glutamate transporter 1 (vGlut1) to estimate changes in synaptic strength in hippocampal dissociated cultures. Pharmacological manipulations indicate that glial-derived ATP and P2X₇R are required for HSP. In addition, inhibition of Cx43 and Panx1 channels reveals a pivotal role for these channels in the compensatory adjustment of synaptic strength, emerging as new pathways for ATP release upon inactivity. The involvement of Panx1 channels was confirmed by using Panx1-deficient animals. Lacking Panx1 in neurons is sufficient to prevent the P2X₇R-dependent upregulation of presynaptic strength; however, the P2X₇R-dependent compensatory adjustment of synapse density requires both neuronal and glial Panx1. Together, our data supports an essential role for glial ATP signaling and Cx43HCs and Panx1 channels in the homeostatic adjustment of synaptic strength in hippocampal cultures upon chronic inactivity.

Axonal autophagosome maturation defect through failure of ATG9A sorting underpins pathology in AP-4 deficiency syndrome

Adaptor protein (AP) complexes mediate key sorting decisions in the cell through selective incorporation of transmembrane proteins into vesicles. Little is known of the roles of AP-4, despite its loss of function leading to a severe early onset neurological disorder, AP-4 deficiency syndrome. Here we demonstrate an AP-4 epsilon subunit knockout mouse model that recapitulates characteristic neuroanatomical phenotypes of AP-4 deficiency patients. We show that ATG9A, critical for autophagosome biogenesis, is an AP-4 cargo, which is retained within the trans-Golgi network (TGN) in vivo and in culture when AP-4 function is lost. TGN retention results in depletion of axonal ATG9A, leading to defective autophagosome generation and aberrant expansions of the distal axon. The reduction in the capacity to generate axonal autophagosomes leads to defective axonal extension and de novo generation of distal axonal swellings containing accumulated ER, underlying the impaired axonal integrity in AP-4 deficiency syndrome.Abbreviations: AP: adaptor protein; AP4B1: adaptor-related protein complex AP-4, beta 1; AP4E1: adaptor-related protein complex AP-4, epsilon 1; ATG: autophagy-related; EBSS: Earle's balanced salt solution; ER: endoplasmic reticulum; GFAP: glial fibrillary acidic protein; GOLGA1/Golgin-97/GOLG97: golgi autoantigen, golgin subfamily a, 1; GOLGA2/GM130: golgi autoantigen, golgin subfamily a, 2; HSP: hereditary spastic paraplegia; LC3/MAP1LC3B: microtubule-associated protein 1 light chain 3 beta; MAP2: microtubule-associated protein 2; MAPK8IP1/JIP1: mitogen-acitvated protein kinase 8 interacting protein 1; NEFH/NF200: neurofilament, heavy polypeptide; RBFOX3/NeuN (RNA binding protein, fox-1 homolog [C. elegans] 3); SQSTM1/p62: sequestosome 1; TGN: trans-Golgi network; WIPI2: WD repeat domain, phosphoinositide interacting protein 2.

miRNA-Dependent Control of Homeostatic Plasticity in Neurons

Homeostatic plasticity is a form of plasticity in which neurons compensate for changes in neuronal activity through the control of key physiological parameters such as the number and the strength of their synaptic inputs and intrinsic excitability. Recent studies revealed that miRNAs, which are small non-coding RNAs repressing mRNA translation, participate in this process by controlling the translation of multiple effectors such as glutamate transporters, receptors, signaling molecules and voltage-gated ion channels. In this review, we present and discuss the role of miRNAs in both cell-wide and compartmentalized forms of homeostatic plasticity as well as their implication in pathological processes associated with homeostatic failure.

β-Catenin in the Adult Visual Cortex Regulates NMDA-Receptor Function and Visual Responses

The formation, plasticity and maintenance of synaptic connections is regulated by molecular and electrical signals. β-Catenin is an important protein in these events and regulates cadherin-mediated cell adhesion and the recruitment of pre- and postsynaptic proteins in an activity-dependent fashion. Mutations in the β-catenin gene can cause cognitive disability and autism, with life-long consequences. Understanding its synaptic function may thus be relevant for the treatment of these disorders. So far, β-catenin's function has been studied predominantly in cell culture and during development but knowledge on its function in adulthood is limited. Here, we show that ablating β-catenin in excitatory neurons of the adult visual cortex does not cause the same synaptic deficits previously observed during development. Instead, it reduces NMDA-receptor currents and impairs visual processing. We conclude that β-catenin remains important for adult cortical function but through different mechanisms than during development.

Differential role of pre- and postsynaptic neurons in the activity-dependent control of synaptic strengths across dendrites

Neurons receive a large number of active synaptic inputs from their many presynaptic partners across their dendritic tree. However, little is known about how the strengths of individual synapses are controlled in balance with other synapses to effectively encode information while maintaining network homeostasis. This is in part due to the difficulty in assessing the activity of individual synapses with identified afferent and efferent connections for a synapse population in the brain. Here, to gain insights into the basic cellular rules that drive the activity-dependent spatial distribution of pre- and postsynaptic strengths across incoming axons and dendrites, we combine patch-clamp recordings with live-cell imaging of hippocampal pyramidal neurons in dissociated cultures and organotypic slices. Under basal conditions, both pre- and postsynaptic strengths cluster on single dendritic branches according to the identity of the presynaptic neurons, thus highlighting the ability of single dendritic branches to exhibit input specificity. Stimulating a single presynaptic neuron induces input-specific and dendritic branchwise spatial clustering of presynaptic strengths, which accompanies a widespread multiplicative scaling of postsynaptic strengths in dissociated cultures and heterosynaptic plasticity at distant synapses in organotypic slices. Our study provides evidence for a potential homeostatic mechanism by which the rapid changes in global or distant postsynaptic strengths compensate for input-specific presynaptic plasticity.

The cell adhesion protein CAR is a negative regulator of synaptic transmission

The Coxsackievirus and adenovirus receptor (CAR) is essential for normal electrical conductance in the heart, but its role in the postnatal brain is largely unknown. Using brain specific CAR knockout mice (KO), we discovered an unexpected role of CAR in neuronal communication. This includes increased basic synaptic transmission at hippocampal Schaffer collaterals, resistance to fatigue, and enhanced long-term potentiation. Spontaneous neurotransmitter release and speed of endocytosis are increased in KOs, accompanied by increased expression of the exocytosis associated calcium sensor synaptotagmin 2. Using proximity proteomics and binding studies, we link CAR to the exocytosis machinery as it associates with syntenin and synaptobrevin/VAMP2 at the synapse. Increased synaptic function does not cause adverse effects in KO mice, as behavior and learning are unaffected. Thus, unlike the connexin-dependent suppression of atrioventricular conduction in the cardiac knockout, communication in the CAR deficient brain is improved, suggesting a role for CAR in presynaptic processes.

Intracellular protein complexes involved in synapse assembly in presynaptic neurons

The presynaptic active zone, composed of evolutionarily conserved protein complexes, is a specialized area that serves to orchestrate precise and efficient neurotransmitter release by organizing various presynaptic proteins involved in mediating docking and priming of synaptic vesicles, recruiting voltage-gated calcium channels, and modulating presynaptic nerve terminals with aligned postsynaptic structures. Among membrane proteins localized to active zone, presynaptic neurexins and LAR-RPTPs (leukocyte common antigen-related receptor tyrosine phosphatase) have emerged as hubs that orchestrate both shared and distinct extracellular synaptic adhesion pathways. In this chapter, we discuss intracellular signaling cascades involved in recruiting various intracellular proteins at both excitatory and inhibitory synaptic sites. In particular, we highlight recent studies on key active zone proteins that physically and functionally link these cascades with neurexins and LAR-RPTPs in both vertebrate and invertebrate model systems. These studies allow us to build a general, universal view of how presynaptic active zones operate together with postsynaptic structures in neural circuits.

Social Stimulus Causes Aberrant Activation of the Medial Prefrontal Cortex in a Mouse Model With Autism-Like Behaviors

Autism spectrum disorder (ASD) is a highly prevalent and genetically heterogeneous brain disorder. Developing effective therapeutic interventions requires knowledge of the brain regions that malfunction and how they malfunction during ASD-relevant behaviors. Our study provides insights into brain regions activated by a novel social stimulus and how the activation pattern differs between mice that display autism-like disabilities and control littermates. Adenomatous polyposis coli (APC) conditional knockout (cKO) mice display reduced social interest, increased repetitive behaviors and dysfunction of the β-catenin pathway, a convergent target of numerous ASD-linked human genes. Here, we exposed the mice to a novel social vs. non-social stimulus and measured neuronal activation by immunostaining for the protein c-Fos. We analyzed three brain regions known to play a role in social behavior. Compared with control littermates, APC cKOs display excessive activation, as evidenced by an increased number of excitatory pyramidal neurons stained for c-Fos in the medial prefrontal cortex (mPFC), selectively in the infralimbic sub-region. In contrast, two other social brain regions, the medial amygdala and piriform cortex show normal levels of neuron activation. Additionally, APC cKOs exhibit increased frequency of miniature excitatory postsynaptic currents (mEPSCs) in layer 5 pyramidal neurons of the infralimbic sub-region. Further, immunostaining is reduced for the inhibitory interneuron markers parvalbumin (PV) and somatostatin (SST) in the APC cKO mPFC. Our findings suggest aberrant excitatory-inhibitory balance and activation patterns. As β-catenin is a core pathway in ASD, we identify the infralimbic sub-region of the mPFC as a critical brain region for autism-relevant social behavior.

PTPσ Drives Excitatory Presynaptic Assembly via Various Extracellular and Intracellular Mechanisms

Leukocyte common antigen-receptor protein tyrosine phosphatases (LAR-RPTPs) are hub proteins that organize excitatory and inhibitory synapse development through binding to various extracellular ligands. Here, we report that knockdown (KD) of the LAR-RPTP family member PTPσ reduced excitatory synapse number and transmission in cultured rat hippocampal neurons, whereas KD of PTPδ produced comparable decreases at inhibitory synapses, in both cases without altering expression levels of interacting proteins. An extensive series of rescue experiments revealed that extracellular interactions of PTPσ with Slitrks are important for excitatory synapse development. These experiments further showed that the intracellular D2 domain of PTPσ is required for induction of heterologous synapse formation by Slitrk1 or TrkC, suggesting that interaction of LAR-RPTPs with distinct intracellular presynaptic proteins, drives presynaptic machinery assembly. Consistent with this, double-KD of liprin-α2 and -α3 or KD of PTPσ substrates (N-cadherin and p250RhoGAP) in neurons inhibited Slitrk6-induced, PTPσ-mediated heterologous synapse formation activity. We propose a synaptogenesis model in presynaptic neurons involving LAR-RPTP-organized retrograde signaling cascades, in which both extracellular and intracellular mechanisms are critical in orchestrating distinct synapse types.SIGNIFICANCE STATEMENT In this study, we sought to test the unproven hypothesis that PTPσ and PTPδ are required for excitatory and inhibitory synapse formation/transmission, respectively, in cultured hippocampal neurons, using knockdown-based loss-of-function analyses. We further performed extensive structure-function analyses, focusing on PTPσ-mediated actions, to address the mechanisms of presynaptic assembly at excitatory synaptic sites. Using interdisciplinary approaches, we systematically applied a varied set of PTPσ deletion variants, point mutants, and splice variants to demonstrate that both extracellular and intracellular mechanisms are involved in organizing presynaptic assembly. Strikingly, extracellular interactions of PTPσ with heparan sulfates and Slitrks, intracellular interactions of PTPσ with liprin-α and its associated proteins through the D2 domain, as well as distinct substrates are all critical.

Feeding and GLP-1 receptor activation stabilize β-catenin in specific hypothalamic nuclei in male rats

β-catenin is a multifunctional protein that can act in the canonical Wnt/β-catenin pathway to regulate gene expression but can also bind to cadherin proteins in adherens junctions where it plays a key role in regulating cytoskeleton linked with these junctions. Recently, evidence has been presented indicating an essential role for β-catenin in regulating trafficking of insulin vesicles in β-cells and showing that changes in nutrient levels rapidly alter levels of β-catenin in these cells. Given the importance of neuroendocrine hormone secretion in the regulation of whole body glucose homeostasis, the objective of this study was to investigate whether β-catenin signalling is regulated in the hypothalamus during the normal physiological response to food intake. Rats were subjected to a fasting/re-feeding paradigm, and then samples collected at specific timepoints for analysis of β-catenin expression by immunohistochemistry and Western blotting. Changes in gene expression were assessed by RT-qPCR. Using immunohistochemistry, feeding acutely increased detectable cytoplasmic levels of β-catenin ('stabilized β-catenin') in neurons in specific regions of the hypothalamus involved in metabolic regulation, including the arcuate, dorsomedial and paraventricular nuclei of the hypothalamus. Feeding-induced elevations in β-catenin in these nuclei were associated with increased transcription of several genes that are known to be responsive to Wnt/β-catenin signalling. The effect of feeding was mimicked by administration of the GLP-1 agonist exendin-4, and was characterized by cAMP-dependent phosphorylation of β-catenin at serine residues 552 and 675. The data suggest that β-catenin/TCF signalling is involved in metabolic sensing in the hypothalamus. This article is protected by copyright. All rights reserved.

Synapse-specific and compartmentalized expression of presynaptic homeostatic potentiation

Postsynaptic compartments can be specifically modulated during various forms of synaptic plasticity, but it is unclear whether this precision is shared at presynaptic terminals. Presynaptic homeostatic plasticity (PHP) stabilizes neurotransmission at the Drosophila neuromuscular junction, where a retrograde enhancement of presynaptic neurotransmitter release compensates for diminished postsynaptic receptor functionality. To test the specificity of PHP induction and expression, we have developed a genetic manipulation to reduce postsynaptic receptor expression at one of the two muscles innervated by a single motor neuron. We find that PHP can be induced and expressed at a subset of synapses, over both acute and chronic time scales, without influencing transmission at adjacent release sites. Further, homeostatic modulations to CaMKII, vesicle pools, and functional release sites are compartmentalized and do not spread to neighboring pre- or post-synaptic structures. Thus, both PHP induction and expression mechanisms are locally transmitted and restricted to specific synaptic compartments.

Everything we think and do is the result of communication between neurons. This communication takes place at junctions called synapses. When two nerve cells or neurons communicate at a synapse, the output terminal of the first cell releases a chemical called a neurotransmitter. This binds to receiver proteins, or receptors, on the second cell. When this communication is interrupted, synapses can adapt to maintain a stable dialogue between them. This can occur in two ways. Either the first neuron starts to release more neurotransmitter from its output terminal, or the second neuron produces extra receptors with which to detect the neurotransmitter.

But how specific are these changes? The brain contains far more synapses than neurons because each neuron can form synapses with many other cells. Can a neuron adjust how much of the neurotransmitter it releases at some of its synapses while leaving the others unchanged?

Li et al. have now addressed this question by studying a special type of synapse that forms between neurons and muscles, known as a neuromuscular junction. At one particular neuromuscular junction in fruit flies, a single neuron splits into two output terminals, each of which forms a synapse with a different muscle. Li et al. show that when the number of neurotransmitter receptors in one of the muscles is artificially reduced, the associated output terminal compensates by increasing its neurotransmitter release. By contrast, the other output terminal remains unaffected.

This suggests that a neuron can induce remarkably specific changes in a subset of its synapses. This discovery paves the way towards identifying the smallest possible unit of change that can occur in the neurons’ ability to communicate. This unit may in turn be the smallest change that can support learning. Such knowledge will help us understand how the nervous system processes and stabilizes information transfer, both in health and after injury or disease.

Neuropilin-2/PlexinA3 Receptors Associate with GluA1 and Mediate Sema3F-Dependent Homeostatic Scaling in Cortical Neurons

Regulation of AMPA-type glutamate receptor (AMPAR) number at synapses is a major mechanism for controlling synaptic strength during homeostatic scaling in response to global changes in neural activity. We show that the secreted guidance cue semaphorin 3F (Sema3F) and its neuropilin-2 (Npn-2)/plexinA3 (PlexA3) holoreceptor mediate homeostatic plasticity in cortical neurons. Sema3F-Npn-2/PlexA3 signaling is essential for cell surface AMPAR homeostatic downscaling in response to an increase in neuronal activity, Npn-2 associates with AMPARs, and Sema3F regulates this interaction. Therefore, Sema3F-Npn-2/PlexA3 signaling controls both synapse development and synaptic plasticity.

Nanoscale Structural Plasticity of the Active Zone Matrix Modulates Presynaptic Function

The active zone (AZ) matrix of presynaptic terminals coordinates the recruitment of voltage-gated calcium channels (VGCCs) and synaptic vesicles to orchestrate neurotransmitter release. However, the spatial organization of the AZ and how it controls vesicle fusion remain poorly understood. Here, we employ super-resolution microscopy and ratiometric imaging to visualize the AZ structure on the nanoscale, revealing segregation between the AZ matrix, VGCCs, and putative release sites. Long-term blockade of neuronal activity leads to reversible AZ matrix unclustering and presynaptic actin depolymerization, allowing for enrichment of AZ machinery. Conversely, patterned optogenetic stimulation of postsynaptic neurons retrogradely enhanced AZ clustering. In individual synapses, AZ clustering was inversely correlated with local VGCC recruitment and vesicle cycling. Acute actin depolymerization led to rapid (5 min) nanoscale AZ matrix unclustering. We propose a model whereby neuronal activity modulates presynaptic function in a homeostatic manner by altering the clustering state of the AZ matrix.

Transcellular Nanoalignment of Synaptic Function

At each of the brain's vast number of synapses, the presynaptic nerve terminal, synaptic cleft, and postsynaptic specialization form a transcellular unit to enable efficient transmission of information between neurons. While we know much about the molecular machinery within each compartment, we are only beginning to understand how these compartments are structurally registered and functionally integrated with one another. This review will describe the organization of each compartment and then discuss their alignment across pre- and postsynaptic cells at a nanometer scale. We propose that this architecture may allow for precise synaptic information exchange and may be modulated to contribute to the remarkable plasticity of brain function.

Transcellular Nanoalignment of Synaptic Function

At each of the brain's vast number of synapses, the presynaptic nerve terminal, synaptic cleft, and postsynaptic specialization form a transcellular unit to enable efficient transmission of information between neurons. While we know much about the molecular machinery within each compartment, we are only beginning to understand how these compartments are structurally registered and functionally integrated with one another. This review will describe the organization of each compartment and then discuss their alignment across pre- and postsynaptic cells at a nanometer scale. We propose that this architecture may allow for precise synaptic information exchange and may be modulated to contribute to the remarkable plasticity of brain function.

Heterophilic Type II Cadherins Are Required for High-Magnitude Synaptic Potentiation in the Hippocampus

Hippocampal CA3 neurons form synapses with CA1 neurons in two layers, stratum oriens (SO) and stratum radiatum (SR). Each layer develops unique synaptic properties but molecular mechanisms that mediate these differences are unknown. Here, we show that SO synapses normally have significantly more mushroom spines and higher-magnitude long-term potentiation (LTP) than SR synapses. Further, we discovered that these differences require the Type II classic cadherins, cadherins-6, -9, and -10. Though cadherins typically function via trans-cellular homophilic interactions, our results suggest presynaptic cadherin-9 binds postsynaptic cadherins-6 and -10 to regulate mushroom spine density and high-magnitude LTP in the SO layer. Loss of these cadherins has no effect on the lower-magnitude LTP typically observed in the SR layer, demonstrating that cadherins-6, -9, and -10 are gatekeepers for high-magnitude LTP. Thus, Type II cadherins may uniquely contribute to the specificity and strength of synaptic changes associated with learning and memory.

The malleable brain: plasticity of neural circuits and behavior - a review from students to students

One of the most intriguing features of the brain is its ability to be malleable, allowing it to adapt continually to changes in the environment. Specific neuronal activity patterns drive long-lasting increases or decreases in the strength of synaptic connections, referred to as long-term potentiation and long-term depression, respectively. Such phenomena have been described in a variety of model organisms, which are used to study molecular, structural, and functional aspects of synaptic plasticity. This review originated from the first International Society for Neurochemistry (ISN) and Journal of Neurochemistry (JNC) Flagship School held in Alpbach, Austria (Sep 2016), and will use its curriculum and discussions as a framework to review some of the current knowledge in the field of synaptic plasticity. First, we describe the role of plasticity during development and the persistent changes of neural circuitry occurring when sensory input is altered during critical developmental stages. We then outline the signaling cascades resulting in the synthesis of new plasticity-related proteins, which ultimately enable sustained changes in synaptic strength. Going beyond the traditional understanding of synaptic plasticity conceptualized by long-term potentiation and long-term depression, we discuss system-wide modifications and recently unveiled homeostatic mechanisms, such as synaptic scaling. Finally, we describe the neural circuits and synaptic plasticity mechanisms driving associative memory and motor learning. Evidence summarized in this review provides a current view of synaptic plasticity in its various forms, offers new insights into the underlying mechanisms and behavioral relevance, and provides directions for future research in the field of synaptic plasticity. Read the Editorial Highlight for this article on page 788. Cover Image for this issue: doi: 10.1111/jnc.13815.

A Critical Role of Presynaptic Cadherin/Catenin/p140Cap Complexes in Stabilizing Spines and Functional Synapses in the Neocortex

The formation of functional synapses requires coordinated assembly of presynaptic transmitter release machinery and postsynaptic trafficking of functional receptors and scaffolds. Here, we demonstrate a critical role of presynaptic cadherin/catenin cell adhesion complexes in stabilizing functional synapses and spines in the developing neocortex. Importantly, presynaptic expression of stabilized β-catenin in either layer (L) 4 excitatory neurons or L2/3 pyramidal neurons significantly upregulated excitatory synaptic transmission and dendritic spine density in L2/3 pyramidal neurons, while its sparse postsynaptic expression in L2/3 neurons had no such effects. In addition, presynaptic β-catenin expression enhanced release probability of glutamatergic synapses. Newly identified β-catenin-interacting protein p140Cap is required in the presynaptic locus for mediating these effects. Together, our results demonstrate that cadherin/catenin complexes stabilize functional synapses and spines through anterograde signaling in the neocortex and provide important molecular evidence for a driving role of presynaptic components in spinogenesis in the neocortex.

Electrical stimulation delays reinnervation in denervated rat muscle

INTRODUCTION

It is not clear if electrical stimulation (ES) can affect muscle reinnervation. This study aimed to verify if ES affects neuromuscular recovery after nerve crush injury in rats.

METHODS

Denervated muscles were electrically stimulated daily for 6 or 14 days. Neuromuscular performance and excitability, and muscle morphology were determined. Muscle trophism markers (atrogin-1, MuRF-1, and myoD), as well as neuromuscular junction (NMJ) organization (muscle-specific receptor tyrosine kinase [MuSK], cytoplasmic protein downstream of kinase-7 [Dok-7], nicotinic ACh receptor [nAChR], and neural cell adhesion molecule [N-CAM]) were assessed.

RESULTS

ES impaired neuromuscular recovery at day 14 postdenervation. Muscle hypoexcitability was accentuated by ES at 6 and 14 days postdenervation. Although ES reduced the accumulation of atrogin-1, MuRF1, and myoD mRNAs, it increased muscle atrophy. Gene expression of MuSK, Dok-7, nAChR, and the content of N-CAM protein were altered by ES.

DISCUSSION

ES can delay the reinnervation process by modulating factors related to NMJ stability and organization, and inducing dysfunction, hypoexcitability, and muscle atrophy. Muscle Nerve 56: E108-E118, 2017.

Release activity-dependent control of vesicle endocytosis by the synaptic adhesion molecule N-cadherin

At synapses in the mammalian brain, continuous information transfer requires the long-term maintenance of homeostatic coupling between exo- and endocytosis of synaptic vesicles. Because classical endocytosis is orders of magnitude slower than the millisecond-range exocytosis of vesicles, high frequency vesicle fusion could potentially compromise structural stability of synapses. However, the molecular mechanisms mediating the tight coupling of exo- and endocytosis are largely unknown. Here, we investigated the role of the transsynaptic adhesion molecules N-cadherin and Neuroligin1 in regulating vesicle exo- and endocytosis by using activity-induced FM4–64 staining and by using synaptophysin-pHluorin fluorescence imaging. The synaptic adhesion molecules N-cadherin and Neuroligin1 had distinct impacts on exo- and endocytosis at mature cortical synapses. Expression of Neuroligin1 enhanced vesicle release in a N-cadherin-dependent way. Most intriguingly, expression of N-cadherin enhanced both vesicle exo- and endocytosis. Further detailed analysis of N-cadherin knockout neurons revealed that the boosting of endocytosis by N-cadherin was largely dependent on preceding high levels of vesicle release activity. In summary, regulation of vesicle endocytosis was mediated at the molecular level by N-cadherin in a release activity-dependent manner. Because of its endocytosis enhancing function, N-cadherin might play an important role in the coupling of vesicle exo- and endocytosis.

Postsynaptic Y654 dephosphorylation of β-catenin modulates presynaptic vesicle turnover through increased n-cadherin-mediated transsynaptic signaling

Synaptic adhesion molecules, which coordinately control structural and functional changes at both sides of synapses, are important for synaptogenesis and synaptic plasticity. Because they physically form homophilic or heterophilic adhesions across synaptic junctions, these molecules can initiate transsynaptic communication in both anterograde and retrograde directions. Using optical imaging approaches, we investigated whether an increase in postsynaptic N-cadherin could correspondingly alter the function of connected presynaptic terminals. Postsynaptic expression of β-catenin Y654F, a phosphorylation-defective form with enhanced binding to N-cadherin, is sufficient to increase postsynaptic surface levels of N-cadherin and consequently promote presynaptic reorganizations. Such reorganizations include increases in the densities of the synaptic vesicle protein, Synaptotagmin 1 and the active zone scaffold protein, Bassoon, the number of active boutons and the size of the total recycling vesicle pool. In contrast, synaptic vesicle turnover is significantly impaired, preventing the exchange of synaptic vesicles with adjacent boutons. Together, N-cadherin-mediated retrograde signaling, governed by phosphoregulation of postsynaptic β-catenin Y654, coordinately modulates presynaptic vesicle dynamics to enhance synaptic communication in mature neurons. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 61-74, 2017.

Mechanisms of homeostatic plasticity in the excitatory synapse

Brain development, sensory information processing, and learning and memory processes depend on Hebbian forms of synaptic plasticity, and on the remodeling and pruning of synaptic connections. Neurons in networks implicated in these processes carry out their functions while facing constant perturbation; homeostatic responses are therefore required to maintain neuronal activity within functional ranges for proper brain function. Here, we will review in vitro and in vivo studies demonstrating that several mechanisms underlie homeostatic plasticity of excitatory synapses, and identifying participant molecular players. Emerging evidence suggests a link between disrupted homeostatic synaptic plasticity and neuropsychiatric and neurologic disorders. Hebbian forms of synaptic plasticity, such as long-term potentiation (LTP), induce long-lasting changes in synaptic strength, which can be destabilizing and drive activity to saturation. Conversely, homeostatic plasticity operates to compensate for prolonged activity changes, stabilizing neuronal firing within a dynamic physiological range. We review mechanisms underlying homeostatic plasticity, and address how neurons integrate distinct forms of plasticity for proper brain function. This article is part of a mini review series: "Synaptic Function and Dysfunction in Brain Diseases".

Hippocampal Wnt Signaling: Memory Regulation and Hormone Interactions

Wnt signaling has emerged in recent years as a major player in both nervous system development and adult synaptic plasticity. Of particular relevance to researchers studying learning and memory, Wnt signaling is critical for normal functioning of the hippocampus, a brain region that is essential for many types of memory formation and whose dysfunction is implicated in numerous neurodegenerative and psychiatric conditions. Impaired hippocampal Wnt signaling is implicated in several of these conditions, however, little is known about how Wnt signaling mediates hippocampal memory formation. This review will provide a general overview of Wnt signaling and discuss evidence demonstrating a key role for Wnt signaling in hippocampal memory formation in both normal and disease states. The regulation of Wnt signaling by ovarian sex steroid hormones will also be highlighted, given that the neuroprotection afforded by Wnt-hormone interactions may have significant implications for cognitive function in aging, neurodegenerative disease, and ischemic injury.

The postsynaptic t-SNARE Syntaxin 4 controls traffic of Neuroligin 1 and Synaptotagmin 4 to regulate retrograde signaling

Postsynaptic cells can induce synaptic plasticity through the release of activity-dependent retrograde signals. We previously described a Ca(2+)-dependent retrograde signaling pathway mediated by postsynaptic Synaptotagmin 4 (Syt4). To identify proteins involved in postsynaptic exocytosis, we conducted a screen for candidates that disrupted trafficking of a pHluorin-tagged Syt4 at Drosophila neuromuscular junctions (NMJs). Here we characterize one candidate, the postsynaptic t-SNARE Syntaxin 4 (Syx4). Analysis of Syx4 mutants reveals that Syx4 mediates retrograde signaling, modulating the membrane levels of Syt4 and the transsynaptic adhesion protein Neuroligin 1 (Nlg1). Syx4-dependent trafficking regulates synaptic development, including controlling synaptic bouton number and the ability to bud new varicosities in response to acute neuronal stimulation. Genetic interaction experiments demonstrate Syx4, Syt4, and Nlg1 regulate synaptic growth and plasticity through both shared and parallel signaling pathways. Our findings suggest a conserved postsynaptic SNARE machinery controls multiple aspects of retrograde signaling and cargo trafficking within the postsynaptic compartment.

Emerging Link between Alzheimer's Disease and Homeostatic Synaptic Plasticity

Alzheimer's disease (AD) is an irreversible brain disorder characterized by progressive cognitive decline and neurodegeneration of brain regions that are crucial for learning and memory. Although intracellular neurofibrillary tangles and extracellular senile plaques, composed of insoluble amyloid-β (Aβ) peptides, have been the hallmarks of postmortem AD brains, memory impairment in early AD correlates better with pathological accumulation of soluble Aβ oligomers and persistent weakening of excitatory synaptic strength, which is demonstrated by inhibition of long-term potentiation, enhancement of long-term depression, and loss of synapses. However, current, approved interventions aiming to reduce Aβ levels have failed to retard disease progression; this has led to a pressing need to identify and target alternative pathogenic mechanisms of AD. Recently, it has been suggested that the disruption of Hebbian synaptic plasticity in AD is due to aberrant metaplasticity, which is a form of homeostatic plasticity that tunes the magnitude and direction of future synaptic plasticity based on previous neuronal or synaptic activity. This review examines emerging evidence for aberrant metaplasticity in AD. Putative mechanisms underlying aberrant metaplasticity in AD will also be discussed. We hope this review inspires future studies to test the extent to which these mechanisms contribute to the etiology of AD and offer therapeutic targets.

C-terminal Src Kinase Gates Homeostatic Synaptic Plasticity and Regulates Fasciclin II Expression at the Drosophila Neuromuscular Junction

Forms of homeostatic plasticity stabilize neuronal outputs and promote physiologically favorable synapse function. A well-studied homeostatic system operates at the Drosophila melanogaster larval neuromuscular junction (NMJ). At the NMJ, impairment of postsynaptic glutamate receptor activity is offset by a compensatory increase in presynaptic neurotransmitter release. We aim to elucidate how this process operates on a molecular level and is preserved throughout development. In this study, we identified a tyrosine kinase-driven signaling system that sustains homeostatic control of NMJ function. We identified C-terminal Src Kinase (Csk) as a potential regulator of synaptic homeostasis through an RNAi- and electrophysiology-based genetic screen. We found that Csk loss-of-function mutations impaired the sustained expression of homeostatic plasticity at the NMJ, without drastically altering synapse growth or baseline neurotransmission. Muscle-specific overexpression of Src Family Kinase (SFK) substrates that are negatively regulated by Csk also impaired NMJ homeostasis. Surprisingly, we found that transgenic Csk-YFP can support homeostatic plasticity at the NMJ when expressed either in the muscle or in the nerve. However, only muscle-expressed Csk-YFP was able to localize to NMJ structures. By immunostaining, we found that Csk mutant NMJs had dysregulated expression of the Neural Cell Adhesion Molecule homolog Fasciclin II (FasII). By immunoblotting, we found that levels of a specific isoform of FasII were decreased in homeostatically challenged GluRIIA mutant animals–but markedly increased in Csk mutant animals. Additionally, we found that postsynaptic overexpression of FasII from its endogenous locus was sufficient to impair synaptic homeostasis, and genetically reducing FasII levels in Csk mutants fully restored synaptic homeostasis. Based on these data, we propose that Csk and its SFK substrates impinge upon homeostatic control of NMJ function by regulating downstream expression or localization of FasII.

Homeostasis is a fundamental topic in biology. Individual cells and systems of cells constantly monitor their environments and adjust their outputs in order to maintain physiological properties within ranges that can support life. The nervous system is no exception. Synapses and circuits are endowed with a capacity to respond to environmental challenges in a homeostatic fashion. As a result, synaptic output stays within an appropriate physiological range. We know that homeostasis is a fundamental form of regulation in animal nervous systems, but we have very little information about how it works. In this study, we examine the fruit fly Drosophila melanogaster and its ability to maintain normal levels of synaptic output over long periods of developmental time. We identify new roles in this process for classical signaling molecules called C-terminal Src kinase, Src family kinases, as well as a neuronal cell adhesion molecule called Fasciclin II, which was previously shown to stabilize synaptic contacts between neurons and muscles. Our work contributes to a broader understanding of how neurons work to maintain stable outputs. Ultimately, this type of knowledge could have important implications for neurological disorders in which stability is lost, such as forms of epilepsy or ataxia.

Emergent Synapse Organizers: LAR-RPTPs and Their Companions

Leukocyte common antigen-related receptor tyrosine phosphatases (LAR-RPTPs) have emerged as key players that organize various aspects of neuronal development, including axon guidance, neurite extension, and synapse formation and function. Recent research has highlighted the roles of LAR-RPTPs at neuronal synapses in mediating distinct synaptic adhesion pathways through interactions with a host of extracellular ligands and in governing a variety of intracellular signaling cascades through binding to various scaffolds and signaling proteins. In this chapter, we review and update current research progress on the extracellular ligands of LAR-RPTPs, regulation of their extracellular interactions by alternative splicing and heparan sulfates, and their intracellular signaling machineries. In particular, we review structural insights on complexes of LAR-RPTPs with their various ligands. These studies lend support to general molecular mechanisms underlying LAR-RPTP-mediated synaptic adhesion and signaling pathways.

Target-specific regulation of presynaptic release properties at auditory nerve terminals in the avian cochlear nucleus

Short-term synaptic plasticity (STP) acts as a time- and firing rate-dependent filter that mediates the transmission of information across synapses. In the auditory brain stem, the divergent pathways that encode acoustic timing and intensity information express differential STP. To investigate what factors determine the plasticity expressed at different terminals, we tested whether presynaptic release probability differed in the auditory nerve projections to the two divisions of the avian cochlear nucleus, nucleus angularis (NA) and nucleus magnocellularis (NM). Estimates of release probability were made with an open-channel blocker ofN-methyl-d-aspartate (NMDA) receptors. Activity-dependent blockade of NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) with application of 20 μM (+)-MK801 maleate was more rapid in NM than in NA, indicating that release probability was significantly higher at terminals in NM. Paired-pulse ratio (PPR) was tightly correlated with the blockade rate at terminals in NA, suggesting that PPR was a reasonable proxy for relative release probability at these synapses. To test whether release probability was similar across convergent inputs onto NA neurons, PPRs of different nerve inputs onto the same postsynaptic NA target neuron were measured. The PPRs, as well as the plasticity during short trains, were tightly correlated across multiple inputs, further suggesting that release probability is coordinated at auditory nerve terminals in a target-specific manner. This highly specific regulation of STP in the auditory brain stem provides evidence that the synaptic dynamics are tuned to differentially transmit the auditory information in nerve activity into parallel ascending pathways.

Mechanism of Activity-Dependent Cargo Loading via the Phosphorylation of KIF3A by PKA and CaMKIIa

A regulated mechanism of cargo loading is crucial for intracellular transport. N-cadherin, a synaptic adhesion molecule that is critical for neuronal function, must be precisely transported to dendritic spines in response to synaptic activity and plasticity. However, the mechanism of activity-dependent cargo loading remains unclear. To elucidate this mechanism, we investigated the activity-dependent transport of N-cadherin via its transporter, KIF3A. First, by comparing KIF3A-bound cargo vesicles with unbound KIF3A, we identified critical KIF3A phosphorylation sites and specific kinases, PKA and CaMKIIa, using quantitative phosphoanalyses. Next, mutagenesis and kinase inhibitor experiments revealed that N-cadherin transport was enhanced via phosphorylation of the KIF3A C terminus, thereby increasing cargo-loading activity. Furthermore, N-cadherin transport was enhanced during homeostatic upregulation of synaptic strength, triggered by chronic inactivation by TTX. We propose the first model of activity-dependent cargo loading, in which phosphorylation of the KIF3A C terminus upregulates the loading and transport of N-cadherin in homeostatic synaptic plasticity.

The role of AMPA receptors in postsynaptic mechanisms of synaptic plasticity

In the mammalian central nervous system, excitatory glutamatergic synapses harness neurotransmission that is mediated by ion flow through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). AMPARs, which are enriched in the postsynaptic membrane on dendritic spines, are highly dynamic, and shuttle in and out of synapses in an activity-dependent manner. Changes in their number, subunit composition, phosphorylation state, and accessory proteins can all regulate AMPARs and thus modify synaptic strength and support cellular forms of learning. Furthermore, dysregulation of AMPAR plasticity has been implicated in various pathological states and has important consequences for mental health. Here we focus on the mechanisms that control AMPAR plasticity, drawing particularly from the extensive studies on hippocampal synapses, and highlight recent advances in the field along with considerations for future directions.

Homeostatic control of presynaptic neurotransmitter release

It is well established that the active properties of nerve and muscle cells are stabilized by homeostatic signaling systems. In organisms ranging from Drosophila to humans, neurons restore baseline function in the continued presence of destabilizing perturbations by rebalancing ion channel expression, modifying neurotransmitter receptor surface expression and trafficking, and modulating neurotransmitter release. This review focuses on the homeostatic modulation of presynaptic neurotransmitter release, termed presynaptic homeostasis. First, we highlight criteria that can be used to define a process as being under homeostatic control. Next, we review the remarkable conservation of presynaptic homeostasis at the Drosophila, mouse, and human neuromuscular junctions and emerging parallels at synaptic connections in the mammalian central nervous system. We then highlight recent progress identifying cellular and molecular mechanisms. We conclude by reviewing emerging parallels between the mechanisms of homeostatic signaling and genetic links to neurological disease.

Presynaptic NCAM is required for motor neurons to functionally expand their peripheral field of innervation in partially denervated muscles

The function of neural cell adhesion molecule (NCAM) expression in motor neurons during axonal sprouting and compensatory reinnervation was explored by partially denervating soleus muscles in mice lacking presynaptic NCAM (Hb9(cre)NCAM(flx)). In agreement with previous studies, the contractile force of muscles in wild-type (NCAM(+/+)) mice recovered completely 2 weeks after 75% of the motor innervation was removed because motor unit size increased by 2.5 times. In contrast, similarly denervated muscles in Hb9(cre)NCAM(flx) mice failed to recover the force lost due to the partial denervation because motor unit size did not change. Anatomical analysis indicated that 50% of soleus end plates were completely denervated 1-4 weeks post-partial denervation in Hb9(cre)NCAM(flx) mice, while another 25% were partially reinnervated. Synaptic vesicles (SVs) remained at extrasynaptic regions in Hb9(cre)NCAM(flx) mice rather than being distributed, as occurs normally, to newly reinnervated neuromuscular junctions (NMJs). Electrophysiological analysis revealed two populations of NMJs in partially denervated Hb9(cre)NCAM(flx) soleus muscles, one with high (mature) quantal content, and another with low (immature) quantal content. Extrasynaptic SVs in Hb9(cre)NCAM(flx) sprouts were associated with L-type voltage-dependent calcium channel (L-VDCC) immunoreactivity and maintained an immature, L-VDCC-dependent recycling phenotype. Moreover, acute nifedipine treatment potentiated neurotransmission at newly sprouted NMJs, while chronic intraperitoneal treatment with nifedipine during a period of synaptic consolidation enhanced functional motor unit expansion in the absence of presynaptic NCAM. We propose that presynaptic NCAM bridges a critical link between the SV cycle and the functional expansion of synaptic territory through the regulation of L-VDCCs.

Activity-dependent Wnt 7 dendritic targeting in hippocampal neurons: plasticity- and tagging-related retrograde signaling mechanism?

Wnt proteins have emerged as transmembrane signaling molecules that regulate learning and memory as well as synaptic plasticity at central synapses (Inestrosa and Arenas (2010) Nat Rev Neurosci 11:77-86; Maguschak and Ressler (2011) J Neurosci 31:13057-13067; Tabatadze et al. (2012) Hippocampus 22: 1228-1241; Fortress et al. (2013) J Neurosci 33:12619-12626). For example, there is both a training-selective and Wnt isoform-specific increase in Wnt 7 levels in hippocampus seven days after spatial learning in rats (Tabatadze et al. (2012) Hippocampus 22: 1228-1241). Despite growing interest in Wnt signaling pathways in the adult brain, intracellular distribution and release of Wnt molecules from synaptic compartments as well as their influence on synaptic strength and connectivity remain less well understood. As a first step in such an analysis, we show here that Wnt 7 levels in primary hippocampal cells are elevated by potassium or glutamate activation in a time-dependent manner. Subsequent Wnt 7 elevation in dendrites suggests selective somato-dendritic trafficking followed by transport from dendrites to their spines. Wnt 7 elevation is also TTX-reversible, establishing that its elevation is indeed an activity-dependent process. A second stimulation given 6 h after the first significantly reduces Wnt 7 levels in dendrites 3 h later as compared to non-stimulated controls suggesting activity-dependent Wnt 7 release from dendrites and spines. In a related experiment designed to mimic the release of Wnt 7, exogenous recombinant Wnt 7 increased the number of active zones in presynaptic terminals as indexed by bassoon. This suggests the formation of new presynaptic release sites and/or presynaptic terminals. Wnt signaling inhibitor sFRP-1 completely blocked this Wnt 7-induced elevation of bassoon cluster number and cluster area. We suggest that Wnt 7 is a plasticity-related protein involved in the regulation of presynaptic plasticity via a retrograde signaling mechanism as previously proposed (Routtenberg (1999) Trends in Neuroscience 22:255-256). These findings provide support for this proposal, which offers a new perspective on the synaptic tagging mechanism (Redondo and Morris (2011) Nat Rev Neurosci 12:17-30).

Cell biology in neuroscience: the interplay between Hebbian and homeostatic synaptic plasticity

Synaptic plasticity, a change in the efficacy of synaptic signaling, is a key property of synaptic communication that is vital to many brain functions. Hebbian forms of long-lasting synaptic plasticity-long-term potentiation (LTP) and long-term depression (LTD)-have been well studied and are considered to be the cellular basis for particular types of memory. Recently, homeostatic synaptic plasticity, a compensatory form of synaptic strength change, has attracted attention as a cellular mechanism that counteracts changes brought about by LTP and LTD to help stabilize neuronal network activity. New findings on the cellular mechanisms and molecular players of the two forms of plasticity are uncovering the interplay between them in individual neurons.

Target-cell-specific short-term plasticity in local circuits

Short-term plasticity (STP) denotes changes in synaptic strength that last up to tens of seconds. It is generally thought that STP impacts information transfer across synaptic connections and may thereby provide neurons with, for example, the ability to detect input coherence, to maintain stability and to promote synchronization. STP is due to a combination of mechanisms, including vesicle depletion and calcium accumulation in synaptic terminals. Different forms of STP exist, depending on many factors, including synapse type. Recent evidence shows that synapse dependence holds true even for connections that originate from a single presynaptic cell, which implies that postsynaptic target cell type can determine synaptic short-term dynamics. This arrangement is surprising, since STP itself is chiefly due to presynaptic mechanisms. Target-specific synaptic dynamics in addition imply that STP is not a bug resulting from synapses fatiguing when driven too hard, but rather a feature that is selectively implemented in the brain for specific functional purposes. As an example, target-specific STP results in sequential somatic and dendritic inhibition in neocortical and hippocampal excitatory cells during high-frequency firing. Recent studies also show that the Elfn1 gene specifically controls STP at some synapse types. In addition, presynaptic NMDA receptors have been implicated in synapse-specific control of synaptic dynamics during high-frequency activity. We argue that synapse-specific STP deserves considerable further study, both experimentally and theoretically, since its function is not well known. We propose that synapse-specific STP has to be understood in the context of the local circuit, which requires combining different scientific disciplines ranging from molecular biology through electrophysiology to computer modeling.