Homeostatic regulation of AMPA receptor trafficking and degradation by light-controlled single-synaptic activation

Diverging from the Norm: Reevaluating What Miniature Excitatory Postsynaptic Currents Tell Us about Homeostatic Synaptic Plasticity

The idea that the nervous system maintains a set point of network activity and homeostatically returns to that set point in the face of dramatic disruption-during development, after injury, in pathologic states, and during sleep/wake cycles-is rapidly becoming accepted as a key plasticity behavior, placing it alongside long-term potentiation and depression. The dramatic growth in studies of homeostatic synaptic plasticity of miniature excitatory synaptic currents (mEPSCs) is attributable, in part, to the simple yet elegant mechanism of uniform multiplicative scaling proposed by Turrigiano and colleagues: that neurons sense their own activity and globally multiply the strength of every synapse by a single factor to return activity to the set point without altering established differences in synaptic weights. We have recently shown that for mEPSCs recorded from control and activity-blocked cultures of mouse cortical neurons, the synaptic scaling factor is not uniform but is close to 1 for the smallest mEPSC amplitudes and progressively increases as mEPSC amplitudes increase, which we term divergent scaling. Using insights gained from simulating uniform multiplicative scaling, we review evidence from published studies and conclude that divergent synaptic scaling is the norm rather than the exception. This conclusion has implications for hypotheses about the molecular mechanisms underlying synaptic scaling.

Functional effects of haemoglobin can be rescued by haptoglobin in an in vitro model of subarachnoid haemorrhage

During subarachnoid haemorrhage, a blood clot forms in the subarachnoid space releasing extracellular haemoglobin (Hb), which causes oxidative damage and cell death in surrounding tissues. High rates of disability and cognitive decline in SAH survivors are attributed to loss of neurons and functional connections during secondary brain injury. Haptoglobin sequesters Hb for clearance, but this scavenging system is overwhelmed after a haemorrhage. Whilst exogenous haptoglobin application can attenuate cytotoxicity of Hb in vitro and in vivo, the functional effects of sub-lethal Hb concentrations on surviving neurons and whether cellular function can be protected with haptoglobin treatment remain unclear. Here we use cultured neurons to investigate neuronal health and function across a range of Hb concentrations to establish the thresholds for cellular damage and investigate synaptic function. Hb impairs ATP concentrations and cytoskeletal structure. At clinically relevant but sub-lethal Hb concentrations, we find that synaptic AMPAR-driven currents are reduced, accompanied by a reduction in GluA1 subunit expression. Haptoglobin co-application can prevent these deficits by scavenging free Hb to reduce it to sub-threshold concentrations and does not need to be present at stoichiometric amounts to achieve efficacy. Haptoglobin itself does not impair measures of neuronal health and function at any concentration tested. Our data highlight a role for Hb in modifying synaptic function in surviving neurons, which may link to impaired cognition or plasticity after SAH and support the development of haptoglobin as a therapy for subarachnoid haemorrhage.

The role of post-translational modifications in synaptic AMPA receptor activity

AMPA-type receptors for the neurotransmitter glutamate are very dynamic entities, and changes in their synaptic abundance underlie different forms of synaptic plasticity, including long-term synaptic potentiation (LTP), long-term depression (LTD) and homeostatic scaling. The different AMPA receptor subunits (GluA1-GluA4) share a common modular structure and membrane topology, and their intracellular C-terminus tail is responsible for the interaction with intracellular proteins important in receptor trafficking. The latter sequence differs between subunits and contains most sites for post-translational modifications of the receptors, including phosphorylation, O-GlcNAcylation, ubiquitination, acetylation, palmitoylation and nitrosylation, which affect differentially the various subunits. Considering that each single subunit may undergo modifications in multiple sites, and that AMPA receptors may be formed by the assembly of different subunits, this creates multiple layers of regulation of the receptors with impact in synaptic function and plasticity. This review discusses the diversity of mechanisms involved in the post-translational modification of AMPA receptor subunits, and their impact on the subcellular distribution and synaptic activity of the receptors.

Forebrain E-I balance controlled in cognition through coordinated inhibition and inhibitory transcriptome mechanism

Introduction

Forebrain neural networks are vital for cognitive functioning, and their excitatory-inhibitory (E-I) balance is governed by neural homeostasis. However, the homeostatic control strategies and transcriptomic mechanisms that maintain forebrain E-I balance and optimal cognition remain unclear.

Methods

We used patch-clamp and RNA sequencing to investigate the patterns of neural network homeostasis with suppressing forebrain excitatory neural activity and spatial training.

Results

We found that inhibitory transmission and receptor transcription were reduced in tamoxifen-inducible Kir2.1 conditional knock-in mice. In contrast, spatial training increased inhibitory synaptic connections and the transcription of inhibitory receptors.

Discussion

Our study provides significant evidence that inhibitory systems play a critical role in the homeostatic control of the E-I balance in the forebrain during cognitive training and E-I rebalance, and we have provided insights into multiple gene candidates for cognition-related homeostasis in the forebrain.

Advances in tethered photopharmacology for precise optical control of signaling proteins

To overcome the limitations of traditional pharmacology, the field of photopharmacology has developed around the central concept of using light to endow drug action with spatiotemporal precision. Tethered photopharmacology, where a photoswitchable ligand is covalently attached to a target protein, offers a particularly high degree of spatiotemporal control, as well as the ability to genetically target drug action and limit effects to specific protein subtypes. In this review, we describe the core engineering concepts of tethered pharmacology and highlight recent advances in harnessing the power of tethered photopharmacology for an expanded palette of targets and conjugation modes using new, complementary strategies. We also discuss the various applications, including mechanistic studies from the molecular biophysical realm to in vivo studies in behaving animals, that demonstrate the power of tethered pharmacology.

Memory destabilization during reconsolidation: a consequence of homeostatic plasticity?

Remembering is not a static process: When retrieved, a memory can be destabilized and become prone to modifications. This phenomenon has been demonstrated in a number of brain regions, but the neuronal mechanisms that rule memory destabilization and its boundary conditions remain elusive. Using two distinct computational models that combine Hebbian plasticity and synaptic downscaling, we show that homeostatic plasticity can function as a destabilization mechanism, accounting for behavioral results of protein synthesis inhibition upon reactivation with different re-exposure times. Furthermore, by performing systematic reviews, we identify a series of overlapping molecular mechanisms between memory destabilization and synaptic downscaling, although direct experimental links between both phenomena remain scarce. In light of these results, we propose a theoretical framework where memory destabilization can emerge as an epiphenomenon of homeostatic adaptations prompted by memory retrieval.

NGPF2 triggers synaptic scaling up through ALK-LIMK-cofilin-mediated mechanisms

Synaptic scaling is an extensively studied form of homeostatic plasticity critically involved in various brain functions. Although it is accepted that synaptic scaling is expressed through the postsynaptic accumulation of AMPA receptors (AMPARs), the induction mechanism remains elusive. In this study, we show that TTX treatment induces rapid but transient release of the neurite growth-promoting factor 2 (NGPF2), and this release is necessary and sufficient for TTX-induced scaling up. In addition, we show that inhibition of the anaplastic lymphoma kinase (ALK)-LIMK-cofilin signaling pathway blocks TTX- and NGPF2-induced synaptic scaling up. Furthermore, we show that TTX-induced release of NGPF2 is protein synthesis dependent and requires fragile X mental retardation protein 1 (FMRP1). These results indicate that activity blockade induces NGPF2 synthesis and release to trigger synaptic scaling up through LIMK-cofilin-dependent actin reorganization, spine enlargement, and stabilization of AMPARs at the synapse.

GRASP1 ubiquitination regulates AMPA receptor surface expression and synaptic activity in cultured hippocampal neurons

The synaptic expression of glutamate receptors of the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) type is dynamically controlled by interaction with binding partners and auxiliary proteins. These proteins can be regulated by posttranslational modifications, including ubiquitination. In this work, we investigated the regulation of glutamate receptor interacting protein-associated protein 1 (GRASP1) by ubiquitin-dependent mechanisms and its impact on surface expression and activity of synaptic AMPA receptors. Cotransfection of GFP-ubiquitin decreased myc-GRASP1 protein levels in HEK293T cells, and this effect was inhibited upon transfection of an ubiquitin mutant that cannot be ubiquitinated on Lys48. In addition, transfection of cultured hippocampal neurons with GFP-ubiquitin reduced the dendritic levels of endogenous GRASP1 and decreased the surface expression of GluA1 AMPA receptor subunits, an effect that was partly reversed by cotransfection with GRASP1. Similarly, transfection of hippocampal neurons with GFP-ubiquitin decreased the amplitude of miniature excitatory postsynaptic currents (mEPSCs) mediated by Ca²⁺ -impermeable AMPA receptors, and this effect was abrogated by cotransfection of GRASP1. Together, the results show a role for ubiquitination in the regulation of the postsynaptic protein GRASP1, which has an impact on the surface distribution of AMPA receptors and on their activity at the synapse.

Historical perspective and progress on protein ubiquitination at glutamatergic synapses

Transcription-translation coupling leads to the production of proteins that are key for controlling essential neuronal processes that include neuronal development and changes in synaptic strength. Although these events have been a prevailing theme in neuroscience, the regulation of proteins via posttranslational signaling pathways are equally relevant for these neuronal processes. Ubiquitin is one type of posttranslational modification that covalently attaches to its targets/substrates. Ubiquitination of proteins play a key role in multiple signaling pathways, the predominant being removal of its substrates by a large molecular machine called the proteasome. Here, I review 40 years of progress on ubiquitination in the nervous system at glutamatergic synapses focusing on axon pathfinding, synapse formation, presynaptic release, dendritic spine formation, and regulation of postsynaptic glutamate receptors. Finally, I elucidate emerging themes in ubiquitin biology that may challenge our current understanding of ubiquitin signaling in the nervous system.

The WD40-Repeat Protein WDR-20 and the Deubiquitinating Enzyme USP-46 Promote Cell Surface Levels of Glutamate Receptors

Reversible modification of AMPA receptors (AMPARs) with ubiquitin regulates receptor levels at synapses and controls synaptic strength. The conserved deubiquitinating enzyme (DUB) ubiquitin-specific protease-46 (USP-46) removes ubiquitin from AMPARs and protects them from degradation in both Caenorhabditis elegans and mammals. Although DUBs are critical for diverse physiological processes, the mechanisms that regulate DUBs, especially in the nervous system, are not well understood. We and others previously showed that the WD40-repeat proteins WDR-48 and WDR-20 bind to and stimulate the catalytic activity of USP-46. Here, we identify an activity-dependent mechanism that regulates WDR-20 expression and show that WDR-20 works together with USP-46 and WDR-48 to promote surface levels of the C. elegans AMPAR GLR-1. usp-46, wdr-48, and wdr-20 loss-of-function mutants exhibit reduced levels of GLR-1 at the neuronal surface and corresponding defects in GLR-1-mediated behavior. Increased expression of WDR-20, but not WDR-48, is sufficient to increase GLR-1 surface levels in an usp-46-dependent manner. Loss of usp-46, wdr-48, and wdr-20 function reduces the rate of local GLR-1 insertion in neurites, whereas overexpression of wdr-20 is sufficient to increase the rate of GLR-1 insertion. Genetic manipulations that chronically reduce or increase glutamate signaling result in reciprocal alterations in wdr-20 transcription and homeostatic compensatory changes in surface GLR-1 levels that are dependent on wdr-20 This study identifies wdr-20 as a novel activity-regulated gene that couples chronic changes in synaptic activity with increased local insertion and surface levels of GLR-1 via the DUB USP-46.SIGNIFICANCE STATEMENT Deubiquitinating enzymes (DUBs) are critical regulators of synapse development and function; however, the regulatory mechanisms that control their various physiological functions are not well understood. This study identifies a novel role for the DUB ubiquitin-specific protease-46 (USP-46) and its associated regulatory protein WD40-repeat protein-20 (WDR-20) in regulating local insertion of glutamate receptors into the neuronal cell surface. This work also identifies WDR-20 as an activity-regulated gene that couples chronic changes in synaptic activity with homeostatic compensatory increases in surface levels of GLR-1 via USP-46. Given that 35% of USP family DUBs associate with WDR proteins, understanding the mechanisms by which WDR proteins regulate USP-46 could have implications for a large number of DUBs in other cell types.

Remodeling without destruction: non-proteolytic ubiquitin chains in neural function and brain disorders

Ubiquitination is a fundamental posttranslational protein modification that regulates diverse biological processes, including those in the CNS. Several topologically and functionally distinct polyubiquitin chains can be assembled on protein substrates, modifying their fates. The classical and most prevalent polyubiquitin chains are those that tag a substrate to the proteasome for degradation, which has been established as a major mechanism driving neural circuit deconstruction and remodeling. In contrast, proteasome-independent non-proteolytic polyubiquitin chains regulate protein scaffolding, signaling complex formation, and kinase activation, and play essential roles in an array of signal transduction processes. Despite being a cornerstone in immune signaling and abundant in the mammalian brain, these non-proteolytic chains are underappreciated in neurons and synapses in the brain. Emerging studies have begun to generate exciting insights about some fundamental roles played by these non-degradative chains in neuronal function and plasticity. In addition, their roles in a number of brain diseases are being recognized. In this article, we discuss recent advances on these nonconventional ubiquitin chains in neural development, function, plasticity, and related pathologies.

Acetylation of AMPA Receptors Regulates Receptor Trafficking and Rescues Memory Deficits in Alzheimer's Disease

In Alzheimer's disease (AD), decreases in the amount and synaptic localization of AMPA receptors (AMPARs) result in weakened synaptic activity and dysfunction in synaptic plasticity, leading to impairments in cognitive functions. We have previously found that AMPARs are subject to lysine acetylation, resulting in higher AMPAR stability and protein accumulation. Here we report that AMPAR acetylation was significantly reduced in AD and neurons with Aβ incubation. We identified p300 as the acetyltransferase responsible for AMPAR acetylation and found that enhancing GluA1 acetylation ameliorated Aβ-induced reductions in total and cell-surface AMPARs. Importantly, expression of acetylation mimetic GluA1 (GluA1-4KQ) in APP/PS1 mice rescued impairments in synaptic plasticity and memory. These findings indicate that Aβ-induced reduction in AMPAR acetylation and stability contributes to synaptopathy and memory deficiency in AD, suggesting that AMPAR acetylation may be an effective molecular target for AD therapeutics.

Impaired State-Dependent Potentiation of GABAergic Synaptic Currents Triggers Seizures in a Genetic Generalized Epilepsy Model

Epileptic activity in genetic generalized epilepsy (GGE) patients preferentially appears during sleep and its mechanism remains unknown. Here, we found that sleep-like slow-wave oscillations (0.5 Hz SWOs) potentiated excitatory and inhibitory synaptic currents in layer V cortical pyramidal neurons from wild-type (wt) mouse brain slices. In contrast, SWOs potentiated excitatory, but not inhibitory, currents in cortical neurons from a heterozygous (het) knock-in (KI) Gabrg2+Q/390X model of Dravet epilepsy syndrome. This created an imbalance between evoked excitatory and inhibitory currents to effectively prompt neuronal action potential firings. Similarly, physiologically similar up-/down-state induction (present during slow-wave sleep) in cortical neurons also potentiated excitatory synaptic currents within brain slices from wt and het KI mice. Moreover, this state-dependent potentiation of excitatory synaptic currents entailed some signaling pathways of homeostatic synaptic plasticity. Consequently, in het KI mice, in vivo SWO induction (using optogenetic methods) triggered generalized epileptic spike-wave discharges (SWDs), being accompanied by sudden immobility, facial myoclonus, and vibrissa twitching. In contrast, in wt littermates, SWO induction did not cause epileptic SWDs and motor behaviors. To our knowledge, this is the first mechanism to explain why epileptic SWDs preferentially happen during non rapid eye-movement sleep and quiet-wakefulness in human GGE patients.

Regional transcriptome analysis of AMPA and GABAA receptor subunit expression generates E/I signatures of the human brain

Theoretical and experimental work has demonstrated that excitatory (E) and inhibitory (I) currents within cortical circuits stabilize to a balanced state. This E/I balance, observed from single neuron to network levels, has a fundamental role in proper brain function and its impairment has been linked to numerous brain disorders. Over recent years, large amount of microarray and RNA-Sequencing datasets have been collected, however few studies have made use of these resources for exploring the balance of global gene expression levels between excitatory AMPA receptors (AMPARs) and inhibitory GABA_A receptors. Here, we analyzed the relative relationships between these receptors to generate a basic transcriptional marker of E/I ratio. Using publicly available data from the Allen Brain Institute, we generated whole brain and regional signatures of AMPAR subunit gene expression in healthy human brains as well as the transcriptional E/I (tE/I) ratio. Then we refined the tE/I ratio to cell-type signatures in the mouse brain using data from the Gene Expression Omnibus. Lastly, we applied our workflow to developmental data from the Allen Brain Institute and revealed spatially and temporally controlled changes in the tE/I ratio during the embryonic and early postnatal stages that ultimately lead to the tE/I balance in adults.

The Ubiquitin System in Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, most prevalent in the elderly population and has a significant impact on individuals and their family as well as the health care system and the economy. While the number of patients affected by various forms of dementia including AD is on the increase, there is currently no cure. Although genome-wide association studies have identified genetic markers for familial AD, the molecular mechanisms underlying the initiation and development of both familial and sporadic AD remain poorly understood. Most neurodegenerative diseases and in particular those associated with dementia have been defined as proteinopathies due to the presence of intra- and/or extracellular protein aggregates in the brain of affected individuals. Although loss of proteostasis in AD has been known for decades, it is only in recent years that we have come to appreciate the role of ubiquitin-dependent mechanisms in brain homeostasis and in brain diseases. Ubiquitin is a highly versatile post-translational modification which regulates many aspects of protein fate and function, including protein degradation by the Ubiquitin-Proteasome System (UPS), autophagy-mediated removal of damaged organelles and proteins, lysosomal turnover of membrane proteins and of extracellular molecules brought inside the cell through endocytosis. Amyloid-β (Aβ) fragments as well as hyperphosphorylation of Tau are hallmarks of AD, and these are found in extracellular plaques and intracellular fibrils in the brain of individuals with AD, respectively. Yet, whether it is the oligomeric or the soluble species of Aβ and Tau that mediate toxicity is still unclear. These proteins impact on mitochondrial energy metabolism, inflammation, as well as a number of housekeeping processes including protein degradation through the UPS and autophagy. In this chapter, we will discuss the role of ubiquitin in neuronal homeostasis as well as in AD; summarise crosstalks between the enzymes that regulate protein ubiquitination and the toxic proteins Tau and Aβ; highlight emerging molecular mechanisms in AD as well as future strategies which aim to exploit the ubiquitin system as a source for next-generation therapeutics.

Hippocampal HECT E3 ligase inhibition facilitates consolidation, retrieval, and reconsolidation, and inhibits extinction of contextual fear memory

Ubiquitination is involved in synaptic plasticity and memory, but the involvement of HECT E3 ligases in these processes has not yet been established. Here, we bilaterally infused heclin, a specific inhibitor of some of these ligases, into the dorsal hippocampus of male Wistar rats that were trained in a contextual fear conditioning. Heclin improved short-term memory, consolidation, retrieval, and reconsolidation when administered immediately post training, prior to testing, or after memory reactivation, respectively. In addition, it impaired memory extinction when administered prior to a long reactivation session. Heclin infusion was also tested for locomotor activity and anxiety-like behavior in a circular arena, but no effect was seen. Taken together, these results indicate that HECT E3 ligases are involved in the modulation of fear memory.

Glutamatergic Signaling in the Central Nervous System: Ionotropic and Metabotropic Receptors in Concert

Glutamate serves as both the mammalian brain's primary excitatory neurotransmitter and as a key neuromodulator to control synapse and circuit function over a wide range of spatial and temporal scales. This functional diversity is decoded by two receptor families: ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs). The challenges posed by the complexity and physiological importance of each of these subtypes has limited our appreciation and understanding of how these receptors work in concert. In this review, by comparing both receptor families with a focus on their crosstalk, we argue for a more holistic understanding of neural glutamate signaling.

Non-scaling regulation of AMPA receptors in homeostatic synaptic plasticity

Homeostatic synaptic plasticity (HSP) as an activity-dependent negative feedback regulation of synaptic strength plays important roles in the maintenance of neuronal and neural circuitry stability. A primary cellular substrate for HSP expression is alterations in synaptic accumulation of glutamatergic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR). It is widely believed that during HSP, AMPAR accumulation changes with the same proportion at each synapse of a neuron, a process known as synaptic scaling. However, direct evidence on AMPAR synaptic scaling remains largely lacking. Here we report a direct examination of inactivity-induced homeostatic scaling of AMPAR at individual synapse by live-imaging. Surprisingly, instead of uniform up-scaling, a scattered pattern of changes in synaptic AMPAR was observed in cultured rat hippocampal neurons. While the majority of synapses showed up-regulation after activity inhibition, a reduction of AMPAR could be detected in certain synapses. More importantly, among the up-regulated synapses, a wide range of AMPAR changes was observed in synapses of the same neuron. We also found that synapses with higher levels of pre-existing AMPAR tend to be up-regulated by lesser extents, whereas the locations of synapses relative to the soma seem not affecting AMPAR scaling strengths. In addition, we observed strong competition between neighboring synapses during HSP. These results reveal that synaptic AMPAR may not be scaled during HSP, suggesting novel molecular mechanisms for information processing and storage at synapses.

Amyloid-β Induces AMPA Receptor Ubiquitination and Degradation in Primary Neurons and Human Brains of Alzheimer's Disease

As the primary mediator for synaptic transmission, AMPA receptors (AMPARs) are crucial for synaptic plasticity and higher brain functions. A downregulation of AMPAR expression has been indicated as one of the early pathological molecular alterations in Alzheimer's disease (AD), presumably via amyloid-β (Aβ). However, the molecular mechanisms leading to the loss of AMPARs remain less clear. We report that in primary neurons, application of Aβ triggers AMPAR internalization accompanied with a decrease in cell-surface AMPAR expression. Importantly, in both Aβ-treated neurons and human brain tissue from AD patients, we observed a significant decrease in total AMPAR amount and an enhancement in AMPAR ubiquitination. Consistent with facilitated receptor degradation, AMPARs show higher turnover rates in the presence of Aβ. Furthermore, AD brain lysates and Aβ-incubated neurons show increased expression of the AMPAR E3 ligase Nedd4 and decreased expression of AMPAR deubiquitinase USP46. Changes in these enzymes are responsible for the Aβ-dependent AMPAR reduction. These findings indicate that AMPAR ubiquitination acts as the key molecular event leading to the loss of AMPARs and thus suppressed synaptic transmission in AD.

Homeostatic scaling of active zone scaffolds maintains global synaptic strength

Synaptic terminals grow and retract throughout life, yet synaptic strength is maintained within stable physiological ranges. To study this process, we investigated Drosophila endophilin (endo) mutants. Although active zone (AZ) number is doubled in endo mutants, a compensatory reduction in their size homeostatically adjusts global neurotransmitter output to maintain synaptic strength. We find an inverse adaptation in rab3 mutants. Additional analyses using confocal, STED, and electron microscopy reveal a stoichiometric tuning of AZ scaffolds and nanoarchitecture. Axonal transport of synaptic cargo via the lysosomal kinesin adapter Arl8 regulates AZ abundance to modulate global synaptic output and sustain the homeostatic potentiation of neurotransmission. Finally, we find that this AZ scaling can interface with two independent homeostats, depression and potentiation, to remodel AZ structure and function, demonstrating a robust balancing of separate homeostatic adaptations. Thus, AZs are pliable substrates with elastic and modular nanostructures that can be dynamically sculpted to stabilize and tune both local and global synaptic strength.

Stargazin Dephosphorylation Mediates Homeostatic Synaptic Downscaling of Excitatory Synapses

Synaptic scaling is a form of homeostatic plasticity that is critical for maintaining neuronal activity within a dynamic range, and which alters synaptic strength through changes in postsynaptic AMPA-type glutamate receptors. Homeostatic scaling down of excitatory synapses has been shown to occur during sleep, and to contribute to synapse remodeling and memory consolidation, but the underlying mechanisms are only partially known. Here, we report that synaptic downscaling in cortical neurons is accompanied by dephosphorylation of the transmembrane AMPA receptor regulatory protein stargazin, and by an increase in its cell surface mobility. The changes in stargazin surface diffusion were paralleled by an increase in the mobility of GluA1-containing AMPA receptors at synaptic sites. In addition, stargazin dephosphorylation was required for the downregulation of surface levels of GluA1-containing AMPA receptors promoted by chronic elevation of neuronal activity, specifically by mediating the interaction with the adaptor proteins AP-2 and AP-3A. Disruption of the stargazin-AP-3A interaction was sufficient to prevent the decrease in cell surface GluA1-AMPA receptor levels associated with chronically enhanced synaptic activity, suggesting that scaling down is accomplished through decreased AMPA receptor recycling and enhanced lysosomal degradation. Thus, synaptic downscaling is associated with both increased stargazin and AMPA receptor cell surface diffusion, as well as with stargazin-mediated AMPA receptor endocytosis and lysosomal degradation.

Degradation for better survival? Role of ubiquitination in epithelial morphogenesis

As a prevalent post-translational modification, ubiquitination is essential for many developmental processes. Once covalently attached to the small and conserved polypeptide ubiquitin (Ub), a substrate protein can be directed to perform specific biological functions via its Ub-modified form. Three sequential catalytic reactions contribute to this process, among which E3 ligases serve to identify target substrates and promote the activated Ub to conjugate to substrate proteins. Ubiquitination has great plasticity, with diverse numbers, topologies and modifications of Ub chains conjugated at different substrate residues adding a layer of complexity that facilitates a huge range of cellular functions. Herein, we highlight key advances in the understanding of ubiquitination in epithelial morphogenesis, with an emphasis on the latest insights into its roles in cellular events involved in polarized epithelial tissue, including cell adhesion, asymmetric localization of polarity determinants and cytoskeletal organization. In addition, the physiological roles of ubiquitination are discussed for typical examples of epithelial morphogenesis, such as lung branching, vascular development and synaptic formation and plasticity. Our increased understanding of ubiquitination in epithelial morphogenesis may provide novel insights into the molecular mechanisms underlying epithelial regeneration and maintenance.

Fundamental Elements in Autism: From Neurogenesis and Neurite Growth to Synaptic Plasticity

Autism spectrum disorder (ASD) is a set of neurodevelopmental disorders with a high prevalence and impact on society. ASDs are characterized by deficits in both social behavior and cognitive function. There is a strong genetic basis underlying ASDs that is highly heterogeneous; however, multiple studies have highlighted the involvement of key processes, including neurogenesis, neurite growth, synaptogenesis and synaptic plasticity in the pathophysiology of neurodevelopmental disorders. In this review article, we focus on the major genes and signaling pathways implicated in ASD and discuss the cellular, molecular and functional studies that have shed light on common dysregulated pathways using in vitro, in vivo and human evidence.

Highlights

Autism spectrum disorder (ASD) has a prevalence of 1 in 68 children in the United States.

ASDs are highly heterogeneous in their genetic basis.

ASDs share common features at the cellular and molecular levels in the brain.

Most ASD genes are implicated in neurogenesis, structural maturation, synaptogenesis and function.

Ca2+/calmodulin binding to PSD-95 mediates homeostatic synaptic scaling down

Postsynaptic density protein-95 (PSD-95) localizes AMPA-type glutamate receptors (AMPARs) to postsynaptic sites of glutamatergic synapses. Its postsynaptic displacement is necessary for loss of AMPARs during homeostatic scaling down of synapses. Here, we demonstrate that upon Ca²⁺ influx, Ca²⁺/calmodulin (Ca²⁺/CaM) binding to the N-terminus of PSD-95 mediates postsynaptic loss of PSD-95 and AMPARs during homeostatic scaling down. Our NMR structural analysis identified E17 within the PSD-95 N-terminus as important for binding to Ca²⁺/CaM by interacting with R126 on CaM. Mutating E17 to R prevented homeostatic scaling down in primary hippocampal neurons, which is rescued via charge inversion by ectopic expression of CaM^R126E, as determined by analysis of miniature excitatory postsynaptic currents. Accordingly, increased binding of Ca²⁺/CaM to PSD-95 induced by a chronic increase in Ca²⁺ influx is a critical molecular event in homeostatic downscaling of glutamatergic synaptic transmission.

Regulation of AMPA Receptor Trafficking by Protein Ubiquitination

The molecular mechanisms underlying plastic changes in the strength and connectivity of excitatory synapses have been studied extensively for the past few decades and remain the most attractive cellular models of learning and memory. One of the major mechanisms that regulate synaptic plasticity is the dynamic adjustment of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor content on the neuronal plasma membrane. The expression of surface AMPA receptors (AMPARs) is controlled by the delicate balance between the biosynthesis, dendritic transport, exocytosis, endocytosis, recycling and degradation of the receptors. These processes are dynamically regulated by AMPAR interacting proteins as well as by various post-translational modifications that occur on their cytoplasmic domains. In the last few years, protein ubiquitination has emerged as a major regulator of AMPAR intracellular trafficking. Dysregulation of AMPAR ubiquitination has also been implicated in the pathophysiology of Alzheimer’s disease. Here we review recent advances in the field and provide insights into the role of protein ubiquitination in regulating AMPAR membrane trafficking and function. We also discuss how aberrant ubiquitination of AMPARs contributes to the pathogenesis of various neurological disorders, including Alzheimer’s disease, chronic stress and epilepsy.

The Transcellular Propagation and Intracellular Trafficking of α-Synuclein

Parkinson's disease is the second most common neurodegenerative disorder, with only partial symptomatic therapy and no mechanism-based therapies. The accumulation and aggregation of α-synuclein is causatively linked to the sporadic form of the disease, which accounts for 95% of cases. The pathology is a result of a gain of toxic function of misfolded α-synuclein conformers, which can template the aggregation of soluble monomers and lead to cellular dysfunction, at least partly by interfering with membrane fusion events at synaptic terminals. Here, we discuss the transcellular propagation and intracellular trafficking of α-synuclein and posit that endosomal processing could be a point of convergence between these two routes. Understanding these events will clarify the therapeutic potential of enzymes that regulate protein trafficking and degradation in synucleinopathies.

Lysosomes convene to keep the synapse clean

Kononenko highlights Goo et al.’s discovery that lysosomes can be positioned in neuronal dendritic spines in response to synapse activity​.

In neurons, lysosomes regulate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor levels at the plasma membrane, although their presence at distal dendrites is controversial. In this issue, Goo et al. (2017. J. Cell Biol.https://doi.org/10.1083/jcb.201704068) show for the first time that neuronal activity positions lysosomes at the dendritic spines to facilitate synaptic remodeling through local protein degradation.

Recent insights on principles of synaptic protein degradation

Maintaining synaptic integrity and function depends on the continuous removal and degradation of aged or damaged proteins. Synaptic protein degradation has received considerable attention in the context of synaptic plasticity and growing interest in relation to neurodegenerative and other disorders. Conversely, less attention has been given to constitutive, ongoing synaptic protein degradation and the roles canonical degradation pathways play in these processes. Here we briefly review recent progress on this topic and new experimental approaches which have expedited such progress and highlight several emerging principles. These include the realization that synaptic proteins typically have unusually long lifetimes, as might be expected from the remote locations of most synaptic sites; the possibility that degradation pathways can change with time from synthesis, cellular context, and physiological input; and that degradation pathways, other than ubiquitin-proteasomal-mediated degradation, might play key roles in constitutive protein degradation at synaptic sites. Finally, we point to the importance of careful experimental design and sufficiently sensitive techniques for studying synaptic protein degradation, which bring into account their slow turnover rates and complex life cycles.

Optogenetic Modulation of Intracellular Signalling and Transcription: Focus on Neuronal Plasticity

Several fields in neuroscience have been revolutionized by the advent of optogenetics, a technique that offers the possibility to modulate neuronal physiology in response to light stimulation. This innovative and far-reaching tool provided unprecedented spatial and temporal resolution to explore the activity of neural circuits underlying cognition and behaviour. With an exponential growth in the discovery and synthesis of new photosensitive actuators capable of modulating neuronal networks function, other fields in biology are experiencing a similar re-evolution. Here, we review the various optogenetic toolboxes developed to influence cellular physiology as well as the diverse ways in which these can be engineered to precisely modulate intracellular signalling and transcription. We also explore the processes required to successfully express and stimulate these photo-actuators in vivo before discussing how such tools can enlighten our understanding of neuronal plasticity at the systems level.

MAGUKs: multifaceted synaptic organizers

The PSD-95 family of proteins, known as MAGUKs, have long been recognized to be central building blocks of the PSD. They are categorized as scaffolding proteins, which link surface-expressed receptors to the intracellular signaling molecules. Although the four members of the PSD-95 family (PSD-95, PSD-93, SAP102, and SAP97) have many shared roles in regulating synaptic function, recent studies have begun to delineate specific binding partners and roles in plasticity. In the current review, we will highlight the conserved and unique roles of these proteins.

β-Amyloid triggers aberrant over-scaling of homeostatic synaptic plasticity

The over-production of β-amyloid (Aβ) has been strongly correlated to neuronal dysfunction and altered synaptic plasticity in Alzheimer’s disease (AD). Accordingly, it has been proposed that disrupted synaptic transmission and neuronal network instability underlie memory failure that is evident in the early phases of AD. Homeostatic synaptic plasticity (HSP) serves to restrain neuronal activity within a physiological range. Therefore a disruption of this mechanism may lead to destabilization in synaptic and neural circuit function. Here, we report that during HSP by neuronal activity deprivation, application of Aβ results in an aberrant over-response of the up-regulation of AMPA receptor (AMPAR)-mediated synaptic currents and cell-surface AMPAR expression. In the visual cortex, in vivo HSP induced by visual deprivation shows a similar over-response following an Aβ local injection. Aβ increases the expression of GluA2-lacking, calcium permeable AMPARs (CP-AMPARs), which are required for the initiation, but not maintenance of HSP. Both GluA2-lacking and GluA2-containing AMPARs contribute to the Aβ-mediated over-scaling of HSP. We also find that Aβ induces the dissociation of HDAC1 from the miR124 transcription factor EVI1, leading to an up-regulation of miR124 expression and increased amount of CP-AMPARs. Thus, via aberrant stimulation of miR124 expression and biogenesis of CP-AMPARs, Aβ is able to induce an over response in HSP. This Aβ-mediated dysregulation in homeostatic plasticity may play an important role in the pathogenesis of altered neural function and memory deficits in the early stages of AD.

Blast waves from detonated military explosive reduce GluR1 and synaptophysin levels in hippocampal slice cultures

Explosives create shockwaves that cause blast-induced neurotrauma, one of the most common types of traumatic brain injury (TBI) linked to military service. Blast-induced TBIs are often associated with reduced cognitive and behavioral functions due to a variety of factors. To study the direct effects of military explosive blasts on brain tissue, we removed systemic factors by utilizing rat hippocampal slice cultures. The long-term slice cultures were briefly sealed air-tight in serum-free medium, lowered into a 37°C water-filled tank, and small 1.7-gram assemblies of cyclotrimethylene trinitramine (RDX) were detonated 15cm outside the tank, creating a distinct shockwave recorded at the culture plate position. Compared to control mock-treated groups of slices that received equal submerge time, 1-3 blast impacts caused a dose-dependent reduction in the AMPA receptor subunit GluR1. While only a small reduction was found in hippocampal slices exposed to a single RDX blast and harvested 1-2days later, slices that received two consecutive RDX blasts 4min apart exhibited a 26-40% reduction in GluR1, and the receptor subunit was further reduced by 64-72% after three consecutive blasts. Such loss correlated with increased levels of HDAC2, a histone deacetylase implicated in stress-induced reduction of glutamatergic transmission. No evidence of synaptic marker recovery was found at 72h post-blast. The presynaptic marker synaptophysin was found to have similar susceptibility as GluR1 to the multiple explosive detonations. In contrast to the synaptic protein reductions, actin levels were unchanged, spectrin breakdown was not detected, and Fluoro-Jade B staining found no indication of degenerating neurons in slices exposed to three RDX blasts, suggesting that small, sub-lethal explosives are capable of producing selective alterations to synaptic integrity. Together, these results indicate that blast waves from military explosive cause signs of synaptic compromise without producing severe neurodegeneration, perhaps explaining the cognitive and behavioral changes in those blast-induced TBI sufferers that have no detectable neuropathology.

The effects of proteasomal inhibition on synaptic proteostasis

Synaptic function crucially depends on uninterrupted synthesis and degradation of synaptic proteins. While much has been learned on synaptic protein synthesis, little is known on the routes by which synaptic proteins are degraded. Here we systematically studied how inhibition of the ubiquitin-proteasome system (UPS) affects the degradation rates of thousands of neuronal and synaptic proteins. We identified a group of proteins, including several proteins related to glutamate receptor trafficking, whose degradation rates were significantly slowed by UPS inhibition. Unexpectedly, however, degradation rates of most synaptic proteins were not significantly affected. Interestingly, many of the differential effects of UPS inhibition were readily explained by a quantitative framework that considered known metabolic turnover rates for the same proteins. In contrast to the limited effects on protein degradation, UPS inhibition profoundly and preferentially suppressed the synthesis of a large number of synaptic proteins. Our findings point to the importance of the UPS in the degradation of certain synaptic proteins, yet indicate that under basal conditions most synaptic proteins might be degraded through alternative pathways.

Prolonged esophageal acid exposures induce synaptic downscaling of cortical membrane AMPA receptor subunits in rats

BACKGROUND

We recently reported the involvement of AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor subunit upregulation and phosphorylation in the rostral cingulate cortex (rCC) as the underlying mechanism of acute esophageal acid-induced cortical sensitization. Based on these findings, we proposed to investigate whether prolonged esophageal acid exposures in rats exhibit homeostatic synaptic scaling through downregulation of AMPA receptor expression in rCC neurons. We intended to study further whether this compensatory mechanism is impaired when rats are pre-exposed to repeated esophageal acid exposures neonatally during neuronal development.

METHODS

Two different esophageal acid exposure protocols in rats were used. Since AMPA receptor trafficking and channel conductance depend on CaMKIIα-mediated phosphorylation of AMPA receptor subunits, we examined the effect of esophageal acid on CaMKIIα activation and AMPA receptor expression in synaptoneurosomes and membrane preparations from rCCs.

KEY RESULTS

In cortical membrane preparations, GluA1 and pGluA1Ser(831) expression were significantly downregulated following prolonged acid exposures in adult rats; this was accompanied by the significant downregulation of cortical membrane pCaMKIIα expression. No change in GluA1 and pGluA1Ser(831) expression was observed in rCC membrane preparations in rats pre-exposed to acid neonatally followed by adult rechallenge.

CONCLUSIONS & INFERENCES

This study along with our previous findings suggests that synaptic AMPA receptor subunits expression and phosphorylation may be involved bidirectionally in both esophageal acid-induced neuronal sensitization and acid-dependent homeostatic plasticity in cortical neurons. The impairment of homeostatic compensatory mechanism as observed following early-in-life acid exposure could be the underlying mechanism of heightening cortical sensitization and esophageal hypersensitivity in patients with gastroesophageal reflux disease.

Plasticity in respiratory motor neurons in response to reduced synaptic inputs: A form of homeostatic plasticity in respiratory control?

For most individuals, the respiratory control system produces a remarkably stable and coordinated motor output-recognizable as a breath-from birth until death. Very little is understood regarding the processes by which the respiratory control system maintains network stability in the presence of changing physiological demands and network properties that occur throughout life. An emerging principle of neuroscience is that neural activity is sensed and adjusted locally to assure that neurons continue to operate in an optimal range, yet to date, it is unknown whether such homeostatic plasticity is a feature of the neurons controlling breathing. Here, we review the evidence that local mechanisms sense and respond to perturbations in respiratory neural activity, with a focus on plasticity in respiratory motor neurons. We discuss whether these forms of plasticity represent homeostatic plasticity in respiratory control. We present new analyses demonstrating that reductions in synaptic inputs to phrenic motor neurons elicit a compensatory enhancement of phrenic inspiratory motor output, a form of plasticity termed inactivity-induced phrenic motor facilitation (iPMF), that is proportional to the magnitude of activity deprivation. Although the physiological role of iPMF is not understood, we hypothesize that it has an important role in protecting the drive to breathe during conditions of prolonged or intermittent reductions in respiratory neural activity, such as following spinal cord injury or during central sleep apnea.

Histone Modification of Nedd4 Ubiquitin Ligase Controls the Loss of AMPA Receptors and Cognitive Impairment Induced by Repeated Stress

Stress and the major stress hormone corticosterone induce profound influences in the brain. Altered histone modification and transcriptional dysfunction have been implicated in stress-related mental disorders. We previously found that repeated stress caused an impairment of prefrontal cortex (PFC)-mediated cognitive functions by increasing the ubiquitination and degradation of AMPA-type glutamate receptors via a mechanism depending on the E3 ubiquitin ligase Nedd4. Here, we demonstrated that in PFC of repeatedly stressed rats, active glucocorticoid receptor had the increased binding to the glucocorticoid response element of histone deacetylase 2 (HDAC2) promoter, resulting in the upregulation of HDAC2. Inhibition or knock-down of HDAC2 blocked the stress-induced impairment of synaptic transmission, AMPAR expression, and recognition memory. Furthermore, we found that, in stressed animals, the HDAC2-dependent downregulation of histone methyltransferase Ehmt2 (G9a) led to the loss of repressive histone methylation at the Nedd4-1 promoter and the transcriptional activation of Nedd4. These results have provided an epigenetic mechanism and a potential treatment strategy for the detrimental effects of chronic stress.

SIGNIFICANCE STATEMENT

Prolonged stress exposure can induce altered histone modification and transcriptional dysfunction, which may underlie the profound influence of stress in regulating brain functions. We report an important finding about the epigenetic mechanism controlling the detrimental effects of repeated stress on synaptic transmission and cognitive function. First, it has revealed the stress-induced alteration of key epigenetic regulators HDAC2 and Ehmt2, which determines the synaptic and behavioral effects of repeated stress. Second, it has uncovered the stress-induced histone modification of the target gene Nedd4, an E3 ligase that is critically involved in the ubiquitination and degradation of AMPA receptors and cognition. Third, it has provided the epigenetic approach, HDAC2 inhibition or knock-down, to rescue synaptic and cognitive functions in stressed animals.

Opposing Effects of Neuronal Activity on Structural Plasticity

The connectivity of the brain is continuously adjusted to new environmental influences by several activity-dependent adaptive processes. The most investigated adaptive mechanism is activity-dependent functional or synaptic plasticity regulating the transmission efficacy of existing synapses. Another important but less prominently discussed adaptive process is structural plasticity, which changes the connectivity by the formation and deletion of synapses. In this review, we show, based on experimental evidence, that structural plasticity can be classified similar to synaptic plasticity into two categories: (i) Hebbian structural plasticity, which leads to an increase (decrease) of the number of synapses during phases of high (low) neuronal activity and (ii) homeostatic structural plasticity, which balances these changes by removing and adding synapses. Furthermore, based on experimental and theoretical insights, we argue that each type of structural plasticity fulfills a different function. While Hebbian structural changes enhance memory lifetime, storage capacity, and memory robustness, homeostatic structural plasticity self-organizes the connectivity of the neural network to assure stability. However, the link between functional synaptic and structural plasticity as well as the detailed interactions between Hebbian and homeostatic structural plasticity are more complex. This implies even richer dynamics requiring further experimental and theoretical investigations.

A Toolkit for Orthogonal and in vivo Optical Manipulation of Ionotropic Glutamate Receptors

The ability to optically manipulate specific neuronal signaling proteins with genetic precision paves the way for the dissection of their roles in brain function, behavior, and disease. Chemical optogenetic control with photoswitchable tethered ligands (PTLs) enables rapid, reversible and reproducible activation or block of specific neurotransmitter-gated receptors and ion channels in specific cells. In this study, we further engineered and characterized the light-activated GluK2 kainate receptor, LiGluR, to develop a toolbox of LiGluR variants. Low-affinity LiGluRs allow for efficient optical control of GluK2 while removing activation by native glutamate, whereas variant RNA edited versions enable the synaptic role of receptors with high and low Ca²⁺ permeability to be assessed and spectral variant photoswitches provide flexibility in illumination. Importantly, we establish that LiGluR works efficiently in the cortex of awake, adult mice using standard optogenetic techniques, thus opening the door to probing the role of specific synaptic receptors and cellular signals in the neural circuit operations of the mammalian brain in normal conditions and in disease. The principals developed in this study are widely relevant to the engineering and in vivo use of optically controllable proteins, including other neurotransmitter receptors.

MicroRNA miR124 is required for the expression of homeostatic synaptic plasticity

Homeostatic synaptic plasticity is a compensatory response to alterations in neuronal activity. Chronic deprivation of neuronal activity results in an increase in synaptic AMPA receptors (AMPARs) and postsynaptic currents. The biogenesis of GluA2-lacking, calcium-permeable AMPARs (CP-AMPARs) plays a crucial role in the homeostatic response; however, the mechanisms leading to CP-AMPAR formation remain unclear. Here we show that the microRNA, miR124, is required for the generation of CP-AMPARs and homeostatic plasticity. miR124 suppresses GluA2 expression via targeting its 3′-UTR, leading to the formation of CP-AMPARs. Blockade of miR124 function abolishes the homeostatic response, whereas miR124 overexpression leads to earlier induction of homeostatic plasticity. miR124 transcription is controlled by an inhibitory transcription factor EVI1, acting by association with the deacetylase HDAC1. Our data support a cellular cascade in which inactivity relieves EVI1/HDAC-mediated inhibition of miR124 gene transcription, resulting in enhanced miR124 expression, formation of CP-AMPARs and subsequent induction of homeostatic synaptic plasticity.

GluA2-lacking AMPA receptors are known to play a role in homeostatic plasticity. Here, the authors show that spiking activity blockade disinhibits mir124 transcription, which in turn suppresses GluA2 mRNA translation, thereby contributing to synaptic upscaling in hippocampal cells.

AMPA Receptor Phosphorylation and Synaptic Colocalization on Motor Neurons Drive Maladaptive Plasticity below Complete Spinal Cord Injury

Clinical spinal cord injury (SCI) is accompanied by comorbid peripheral injury in 47% of patients. Human and animal modeling data have shown that painful peripheral injuries undermine long-term recovery of locomotion through unknown mechanisms. Peripheral nociceptive stimuli induce maladaptive synaptic plasticity in dorsal horn sensory systems through AMPA receptor (AMPAR) phosphorylation and trafficking to synapses. Here we test whether ventral horn motor neurons in rats demonstrate similar experience-dependent maladaptive plasticity below a complete SCI in vivo. Quantitative biochemistry demonstrated that intermittent nociceptive stimulation (INS) rapidly and selectively increases AMPAR subunit GluA1 serine 831 phosphorylation and localization to synapses in the injured spinal cord, while reducing synaptic GluA2. These changes predict motor dysfunction in the absence of cell death signaling, suggesting an opportunity for therapeutic reversal. Automated confocal time-course analysis of lumbar ventral horn motor neurons confirmed a time-dependent increase in synaptic GluA1 with concurrent decrease in synaptic GluA2. Optical fractionation of neuronal plasma membranes revealed GluA2 removal from extrasynaptic sites on motor neurons early after INS followed by removal from synapses 2 h later. As GluA2-lacking AMPARs are canonical calcium-permeable AMPARs (CP-AMPARs), their stimulus- and time-dependent insertion provides a therapeutic target for limiting calcium-dependent dynamic maladaptive plasticity after SCI. Confirming this, a selective CP-AMPAR antagonist protected against INS-induced maladaptive spinal plasticity, restoring adaptive motor responses on a sensorimotor spinal training task. These findings highlight the critical involvement of AMPARs in experience-dependent spinal cord plasticity after injury and provide a pharmacologically targetable synaptic mechanism by which early postinjury experience shapes motor plasticity.

Ubiquitin-dependent trafficking and turnover of ionotropic glutamate receptors

Changes in synaptic strength underlie the basis of learning and memory and are controlled, in part, by the insertion or removal of AMPA-type glutamate receptors at the postsynaptic membrane of excitatory synapses. Once internalized, these receptors may be recycled back to the plasma membrane by subunit-specific interactions with other proteins or by post-translational modifications such as phosphorylation. Alternatively, these receptors may be targeted for destruction by multiple degradation pathways in the cell. Ubiquitination, another post-translational modification, has recently emerged as a key signal that regulates the recycling and trafficking of glutamate receptors. In this review, we will discuss recent findings on the role of ubiquitination in the trafficking and turnover of ionotropic glutamate receptors and plasticity of excitatory synapses.

Translational regulation of the expression of the ubiquitin E3 ligase Nedd4-1 by mTORC1-dependent signaling

We found recently that mTORC1 regulates the biosynthesis of the ubiquitin E3 ligase Nedd4–1, but not the close homolog Nedd4–2. This regulatory process plays a key role in promoting neurite growth in neurons of the mammalian central nervous system. The molecular mechanism underlying this rather specific regulation likely involves a pyrimidine-rich sequence stretch near the putative transcriptional start site within the 5′ untranslated region of the Nedd4–1 mRNA, which may play a crucial role in directing the assembly of the protein translation machinery. We postulate that the Nedd4–1 mRNA is a major target of the local translation machinery within neurons that can be translated in a spatially and temporally controlled manner in response to various stimuli. Based on this model, neuronal Nedd4–1 may not only be involved in the regulation of neurite growth but also in axon guidance, spine formation, and synaptic plasticity.

Differential role of the proteasome in the early and late phases of BDNF-induced facilitation of LTP

The neurotrophin brain-derived neurotrophic factor (BDNF) mediates activity-dependent long-term changes of synaptic strength in the CNS. The effects of BDNF are partly mediated by stimulation of local translation, with consequent alterations in the synaptic proteome. The ubiquitin-proteasome system (UPS) also plays an important role in protein homeostasis at the synapse by regulating synaptic activity. However, whether BDNF acts on the UPS to mediate the effects on long-term synaptic potentiation (LTP) has not been investigated. In the present study, we show similar and nonadditive effects of BDNF and proteasome inhibition on the early phase of synaptic potentiation (E-LTP) induced by theta-burst stimulation of rat hippocampal CA1 synapses. The effects of BDNF were blocked by the proteasome activator IU1, suggesting that the neurotrophin acts by decreasing proteasome activity. Accordingly, BDNF downregulated the proteasome activity in cultured hippocampal neurons and in hippocampal synaptoneurosomes. Furthermore, BDNF increased the activity of the deubiquitinating enzyme UchL1 in synaptoneurosomes and upregulated free ubiquitin. In contrast to the effects on posttetanic potentiation, proteasome activity was required for BDNF-mediated LTP. These results show a novel role for BDNF in UPS regulation at the synapse, which is likely to act together with the increased translation activity in the regulation of the synaptic proteome during E-LTP.

DNA methylation regulates neuronal glutamatergic synaptic scaling

Enhanced receptiveness at all synapses on a neuron that receive glutamatergic input is called cell-wide synaptic upscaling. We hypothesize that this type of synaptic plasticity may be critical for long-term memory storage within cortical circuits, a process that may also depend on epigenetic mechanisms, such as covalent chemical modification of DNA. We found that DNA cytosine demethylation mediates multiplicative synaptic upscaling of glutamatergic synaptic strength in cultured cortical neurons. Inhibiting neuronal activity with tetrodotoxin (TTX) decreased the cytosine methylation of and increased the expression of genes encoding glutamate receptors and trafficking proteins, in turn increasing the amplitude but not frequency of miniature excitatory postsynaptic currents (mEPSCs), indicating synaptic upscaling rather than increased spontaneous activity. Inhibiting DNA methyltransferase (DNMT) activity, either by using the small-molecule inhibitor RG108 or by knocking down Dnmt1 and Dnmt3a, induced synaptic upscaling to a similar magnitude as exposure to TTX. Moreover, upscaling induced by DNMT inhibition required transcription; the RNA polymerase inhibitor actinomycin D blocked upscaling induced by DNMT inhibition. Knocking down the cytosine demethylase TET1 also blocked the upscaling effects of RG108. DNMT inhibition induced a multiplicative increase in mEPSC amplitude, indicating that the alterations in glutamate receptor abundance occurred in a coordinated manner throughout a neuron and were not limited to individual active synapses. Our data suggest that DNA methylation status controls transcription-dependent regulation of glutamatergic synaptic homeostasis. Furthermore, covalent DNA modifications may contribute to synaptic plasticity events that underlie the formation and stabilization of memories.

Quantitative assessment of single-cell whole genome amplification methods for detecting copy number variation using hippocampal neurons

Single-cell genomic analysis has grown rapidly in recent years and finds widespread applications in various fields of biology, including cancer biology, development, immunology, pre-implantation genetic diagnosis, and neurobiology. To date, the amplification bias, amplification uniformity and reproducibility of the three major single cell whole genome amplification methods (GenomePlex WGA4, MDA and MALBAC) have not been systematically investigated using mammalian cells. In this study, we amplified genomic DNA from individual hippocampal neurons using three single-cell DNA amplification methods, and sequenced them at shallow depth. We then systematically evaluated the GC-bias, reproducibility, and copy number variations among individual neurons. Our results showed that single-cell genome sequencing results obtained from the MALBAC and WGA4 methods are highly reproducible and have a high success rate. The MALBAC displays significant biases towards high GC content. We then attempted to correct the GC bias issue by developing a bioinformatics pipeline, which allows us to call CNVs in single cell sequencing data, and chromosome level and sub-chromosomal level CNVs among individual neurons can be detected. We also proposed a metric to determine the CNV detection limits. Overall, MALBAC and WGA4 have better performance than MDA in detecting CNVs.

Mechanisms of oxidative stress resistance in the brain: Lessons learned from hypoxia tolerant extremophilic vertebrates

The Oxidative Stress Theory of Aging has had tremendous impact in research involving aging and age-associated diseases including those that affect the nervous system. With over half a century of accrued data showing both strong support for and against this theory, there is a need to critically evaluate the data acquired from common biomedical research models, and to also diversify the species used in studies involving this proximate theory. One approach is to follow Orgel's second axiom that "evolution is smarter than we are" and judiciously choose species that may have evolved to live with chronic or seasonal oxidative stressors. Vertebrates that have naturally evolved to live under extreme conditions (e.g., anoxia or hypoxia), as well as those that undergo daily or seasonal torpor encounter both decreased oxygen availability and subsequent reoxygenation, with concomitant increased oxidative stress. Due to its high metabolic activity, the brain may be particularly vulnerable to oxidative stress. Here, we focus on oxidative stress responses in the brains of certain mouse models as well as extremophilic vertebrates. Exploring the naturally evolved biological tools utilized to cope with seasonal or environmentally variable oxygen availability may yield key information pertinent for how to deal with oxidative stress and thereby mitigate its propagation of age-associated diseases.

Reconsolidation and the regulation of plasticity: moving beyond memory

Memory reconsolidation is a protein synthesis-dependent process that preserves, in some form, memories that have been destabilized through recall. Reconsolidation is a nearly universal phenomenon, occurring in a diverse array of species and learning tasks. The function of reconsolidation remains unclear but it has been proposed as a mechanism for updating or strengthening memories. Observations of an analog of reconsolidation in vitro and in sensory systems indicate that reconsolidation is unlikely to be a learning-specific phenomenon and may serve a broader function. We propose that reconsolidation arises from the activity-dependent induction of two coincident but opposing processes: the depotentiation and repotentiation of strengthened synapses. These processes suggest that reconsolidation reflects a fundamental mechanism that regulates and preserves synaptic strength.