Changes In Synaptic Structure Research Articles

Why Do Hair Cells Have Ribbons? Focus on “Synaptic Ribbon Enables Temporal Precision of Hair Cell Afferent Synapse by Increasing the Number of Readily Releasable Vesicles: A Modeling Study”

Hair cells, like retinal photoreceptors and bipolar cells, encode stimuli with graded changes in membrane potential and transmit that signal through ribbon synapses to drive action potentials in associated afferent neurons ([Sterling and Matthews 2005][1]). Thus one would expect transmitter release

Rapid Activity-Dependent Modifications in Synaptic Structure and Function Require Bidirectional Wnt Signaling

Activity-dependent modifications in synapse structure play a key role in synaptic development and plasticity, but the signaling mechanisms involved are poorly understood. We demonstrate that glutamatergic Drosophila neuromuscular junctions undergo rapid changes in synaptic structure and function in response to patterned stimulation. These changes, which depend on transcription and translation, include formation of motile presynaptic filopodia, elaboration of undifferentiated varicosities, and potentiation of spontaneous release frequency. Experiments indicate that a bidirectional Wnt/Wg signaling pathway underlies these changes. Evoked activity induces Wnt1/Wg release from synaptic boutons, which stimulates both a postsynaptic DFz2 nuclear import pathway as well as a presynaptic pathway involving GSK-3beta/Shaggy. Our findings suggest that bidirectional Wg signaling operates downstream of synaptic activity to induce modifications in synaptic structure and function. We propose that activation of the postsynaptic Wg pathway is required for the assembly of the postsynaptic apparatus, while activation of the presynaptic Wg pathway regulates cytoskeletal dynamics.

Genes and Circuits for Olfactory-Associated Long-Term Memory in Drosophila

One of the formidable challenges in modern neuroscience is to identify the physical basis of long-term memory (LTM) storage—the engram. Cellular and molecular experiments have suggested that the engram for a particular behavioral task is encoded as changes in synaptic structure and function, yet distributed in an unknown fashion across an ill-defined neural circuit or network. Accumulating genetic and circuitry information has provided some clues toward resolving this engram puzzle. This review will focus on recent discoveries of genes and circuits involved in the formation of olfactory-associated LTM in Drosophila.

TARP Subtypes Differentially and Dose-Dependently Control Synaptic AMPA Receptor Gating

A family of transmembrane AMPA receptor regulatory proteins (TARPs) profoundly affects the trafficking and gating of AMPA receptors (AMPARs). Although TARP subtypes are differentially expressed throughout the CNS, it is unclear whether this imparts functional diversity to AMPARs in distinct neuronal populations. Here, we examine the effects of each TARP subtype on the kinetics of AMPAR gating in heterologous cells and in neurons. We report a striking heterogeneity in the effects of TARP subtypes on AMPAR deactivation and desensitization, which we demonstrate controls the time course of synaptic transmission. In addition, we find that some TARP subtypes dramatically slow AMPAR activation kinetics. Synaptic AMPAR kinetics also depend on TARP expression level, suggesting a variable TARP/AMPAR stoichiometry. Analysis of quantal synaptic transmission in a TARP gamma-4 knockout (KO) mouse corroborates our expression data and demonstrates that TARP subtype-specific gating of AMPARs contributes to the kinetics of native AMPARs at central synapses.

Extracellular Interactions between GluR2 and N-Cadherin in Spine Regulation

Via its extracellular N-terminal domain (NTD), the AMPA receptor subunit GluR2 promotes the formation and growth of dendritic spines in cultured hippocampal neurons. Here we show that the first N-terminal 92 amino acids of the extracellular domain are necessary and sufficient for GluR2's spine-promoting activity. Moreover, overexpression of this extracellular domain increases the frequency of miniature excitatory postsynaptic currents (mEPSCs). Biochemically, the NTD of GluR2 can interact directly with the cell adhesion molecule N-cadherin, in cis or in trans. N-cadherin-coated beads recruit GluR2 on the surface of hippocampal neurons, and N-cadherin immobilization decreases GluR2 lateral diffusion on the neuronal surface. RNAi knockdown of N-cadherin prevents the enhancing effect of GluR2 on spine morphogenesis and mEPSC frequency. Our data indicate that in hippocampal neurons N-cadherin and GluR2 form a synaptic complex that stimulates presynaptic development and function as well as promoting dendritic spine formation.

α2A-Adrenoceptors Strengthen Working Memory Networks by Inhibiting cAMP-HCN Channel Signaling in Prefrontal Cortex

Spatial working memory (WM; i.e., "scratchpad" memory) is constantly updated to guide behavior based on representational knowledge of spatial position. It is maintained by spatially tuned, recurrent excitation within networks of prefrontal cortical (PFC) neurons, evident during delay periods in WM tasks. Stimulation of postsynaptic alpha2A adrenoceptors (alpha2A-ARs) is critical for WM. We report that alpha2A-AR stimulation strengthens WM through inhibition of cAMP, closing Hyperpolarization-activated Cyclic Nucleotide-gated (HCN) channels and strengthening the functional connectivity of PFC networks. Ultrastructurally, HCN channels and alpha2A-ARs were colocalized in dendritic spines in PFC. In electrophysiological studies, either alpha2A-AR stimulation, cAMP inhibition or HCN channel blockade enhanced spatially tuned delay-related firing of PFC neurons. Conversely, delay-related network firing collapsed under conditions of excessive cAMP. In behavioral studies, either blockade or knockdown of HCN1 channels in PFC improved WM performance. These data reveal a powerful mechanism for rapidly altering the strength of WM networks in PFC.

Various Dendritic Abnormalities Are Associated with Fibrillar Amyloid Deposits in Alzheimer's Disease

Dystrophic neurites are associated with fibrillar amyloid deposition in Alzheimer's disease (AD), but the frequency and types of changes in synaptic structures near amyloid deposits have not been well characterized. Using high-resolution confocal microscopy to image lipophilic dye-labeled dendrites and thioflavin-S-labeled amyloid plaques, we systematically analyzed the structural changes of dendrites associated with amyloid deposition in both a transgenic mouse model of AD (PSAPP) and in human postmortem brain. We found that in PSAPP mice, dendritic branches passing through or within 40 mum from amyloid deposits displayed various dendritic abnormalities such as loss of dendritic spines, shaft atrophy, bending, abrupt branch endings, varicosity formation, and sprouting. Similar structural alterations of dendrites were seen in postmortem human AD tissue, with spine loss as the most common abnormality in both PSAPP mice and human AD brains. These results demonstrate that fibrillar amyloid deposits and their surrounding microenvironment are toxic to dendrites and likely contribute to significant disruption of neuronal circuits in AD.

Ischemia-induced modifications in hippocampal CA1 stratum radiatum excitatory synapses

Relatively mild ischemic episode can initiate a chain of events resulting in delayed cell death and significant lesions in the affected brain regions. We studied early synaptic modifications after brief ischemia modeled in rats by transient vessels' occlusion in vivo or oxygen-glucose deprivation in vitro and resulting in delayed death of hippocampal CA1 pyramidal cells. Electron microscopic analysis of excitatory spine synapses in CA1 stratum radiatum revealed a rapid increase of the postsynaptic density (PSD) thickness and length, as well as formation of concave synapses with perforated PSD during the first 24 h after ischemic episode, followed at the long term by degeneration of 80% of synaptic contacts. In presynaptic terminals, ischemia induced a depletion of synaptic vesicles and changes in their spatial arrangement: they became more distant from active zones and had larger intervesicle spacing compared to controls. These rapid structural synaptic changes could be implicated in the mechanisms of cell death or adaptive plasticity. Comparison of the in vivo and in vitro model systems used in the study demonstrated a general similarity of these early morphological changes, confirming the validity of the in vitro model for studying synaptic structural plasticity.

Endocannabinoids regulate interneuron migration and morphogenesis by transactivating the TrkB receptor.

In utero exposure to Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the active component from marijuana, induces cognitive deficits enduring into adulthood. Although changes in synaptic structure and plasticity may underlie Delta(9)-THC-induced cognitive impairments, the neuronal basis of Delta(9)-THC-related developmental deficits remains unknown. Using a Boyden chamber assay, we show that agonist stimulation of the CB(1) cannabinoid receptor (CB(1)R) on cholecystokinin-expressing interneurons induces chemotaxis that is additive with brain-derived neurotrophic factor (BDNF)-induced interneuron migration. We find that Src kinase-dependent TrkB receptor transactivation mediates endocannabinoid (eCB)-induced chemotaxis in the absence of BDNF. Simultaneously, eCBs suppress the BDNF-dependent morphogenesis of interneurons, and this suppression is abolished by Src kinase inhibition in vitro. Because sustained prenatal Delta(9)-THC stimulation of CB(1)Rs selectively increases the density of cholecystokinin-expressing interneurons in the hippocampus in vivo, we conclude that prenatal CB(1)R activity governs proper interneuron placement and integration during corticogenesis. Moreover, eCBs use TrkB receptor-dependent signaling pathways to regulate subtype-selective interneuron migration and specification.

Activity-Dependent Dendritic Spine Structural Plasticity Is Regulated by Small GTPase Rap1 and Its Target AF-6

Activity-dependent remodeling of dendritic spines is essential for neural circuit development and synaptic plasticity, but the mechanisms that coordinate synaptic structural and functional plasticity are not well understood. Here we investigate the signaling pathways that enable excitatory synapses to undergo activity-dependent structural modifications. We report that activation of NMDA receptors in cultured cortical neurons induces spine morphogenesis and activation of the small GTPase Rap1. Rap1 bimodally regulates spine morphology: activated Rap1 recruits the PDZ domain-containing protein AF-6 to the plasma membrane and induces spine neck elongation, while inactive Rap1 dissociates AF-6 from the membrane and induces spine enlargement. Rap1 also regulates spine content of AMPA receptors: thin spines induced by Rap1 activation have reduced GluR1-containing AMPA receptor content, while large spines induced by Rap1 inactivation are rich in AMPA receptors. These results identify a signaling pathway that regulates activity-dependent synaptic structural plasticity and coordinates it with functional plasticity.

Arousal from hibernation alters contextual learning and memory

Hibernation is a unique and highly regulated physiological state characterized by profound, albeit periodically reversible, depression in body temperature, metabolism, and consciousness. Hippocampal synapses undergo pronounced remodeling in concert with torpor and arousal. During hibernation, the number of postsynaptic densities, apical dendritic branches, and spine densities decreases substantially in the hippocampus. Upon arousal these parameters increase beyond pre-hibernation levels and peak within 2–3 h. By 24 h after arousal, dendritic parameters remain elevated but have started to subside, consistent with pruning and differentiation. The present study examined the functional consequences of these natural changes in synaptic structure. Wild-caught Arctic ground squirrels (AGS) were trained in a hippocampal-dependent contextual fear conditioning task at 3 h, 24 h, or 4 weeks after arousal (warm-adapted euthermic control group). All groups acquired the fear conditioned response similarly on the training day. During a subsequent retention test session, AGS in the 24 h group exhibited enhanced expression of contextual fear compared to the other two groups. These data suggest that the morphological and biochemical changes occurring at 24 h after arousal from hibernation affect hippocampal-dependent learning and memory. The natural change in synaptic structure during hibernation may provide a unique opportunity to assess the neural substrates underlying cognitive enhancement.

Tumor necrosis factor alpha, interleukin-1 beta, interleukin-6 and major histocompatibility complex molecules in the normal brain and after peripheral immune challenge

The capacity of the brain to activate an inflammatory reaction involving the production of cytokines in response to an immune challenge in the periphery has been well established. Interleukin-1 beta is a cytokine that responds with the most widespread pattern of expression followed by tumor necrosis factor alpha and interleukin-6. In addition, our laboratory has shown that class I major histocompatibility complex molecules are upregulated in the brain in response to peripheral administration of bacterial products. Remarkably, during recent years, all these immune genes have been shown to participate in activity-dependent structural synaptic changes in specific neurochemical circuitries in the normal brain. These processes range from the refinement of synaptic connections in sensory systems to learning and memory storage functions of the hippocampus. Therefore, the mechanisms of defense against pathogens can dramatically affect brain structure and function-inducing changes in cognition, mood and behavior. The immune reactions initiated by viruses, bacteria and parasites may result in latent vulnerabilities which could become manifest with future stressors or challenges. Other inflammatory challenges may function as triggers for uncovering pre-existing vulnerabilities or exacerbation of previous functional deficits, or clinical symptoms of neurological or psychiatric conditions. This review will discuss the importance of infections on basic neuronal processes owing to the alteration in the brain of the balance of cytokines involved in higher brain functions.

The birth (and adolescence) of LTP

The birth (and adolescence) of LTP

Changes in synaptic structure underlie the developmental speeding of AMPA receptor–mediated EPSCs

At many excitatory and inhibitory synapses throughout the nervous system, postsynaptic currents become faster as the synapse matures, primarily owing to changes in receptor subunit composition. The origin of the developmental acceleration of AMPA receptor (AMPAR)-mediated excitatory postsynaptic currents (EPSCs) remains elusive. We used patch-clamp recordings, electron microscopic immunogold localization of AMPARs, partial three-dimensional reconstruction of the neuropil and numerical simulations of glutamate diffusion and AMPAR activation to examine the factors underlying the developmental speeding of miniature EPSCs in mouse cerebellar granule cells. We found that the main developmental change that permits submillisecond transmission at mature synapses is an alteration in the glutamate concentration waveform as experienced by AMPARs. This can be accounted for by changes in the synaptic structure and surrounding neuropil, rather than by a change in AMPAR properties. Our findings raise the possibility that structural alterations could be a general mechanism underlying the change in the time course of AMPAR-mediated synaptic transmission.

Phosphodiesterase 4B (PDE4B) and cAMP-level regulation within different tissue fractions of rat hippocampal slices during long-term potentiation in vitro

Molecular events associated with mnemonic processes and neuronal plasticity are postulated to result in functional changes in synaptic structure. One possible site is the post-synaptic density, where activity-dependent changes modulate signal transduction cascades. In this report, we detail spatial–temporal changes for phosphodiesterase 4B (PDE4B) proteins and their substrate cAMP within three neuronal fractions during early and late long-term potentiation (LTP). The cAMP-dependent protein kinase A cascade – which can be regulated by distinct PDE4B activity – is required for mnemonic processes as well as mechanisms of neuronal plasticity, such as those during the maintenance or late-LTP. Fluorescence in situ hybridization studies (FISH) identified no translocation of PDE4B3 from the soma after late-LTP induction indicating a subtle, local control of PDE4B activity. Protein changes were detected within the PSD-enriched fraction. From these results, we conclude that either the changes in PDE4B are due to modulation of pre-existing mRNA, or that the protein is specifically translocated to activated synaptic structures. Furthermore, we report late changes in cAMP levels in the somato-dendritic fraction and discuss this result with the increased PDE4B1/3 doublet in the PSD-enriched fraction.

Regulation ofcpg15 expression during single whisker experience in the barrel cortex of adult mice

Regulation of gene transcription by neuronal activity is thought to be key to the translation of sensory experience into long-term changes in synaptic structure and function. Here we show that cpg15, a gene encoding an extracellular signaling molecule that promotes dendritic and axonal growth and synaptic maturation, is regulated in the somatosensory cortex by sensory experience capable of inducing cortical plasticity. Using in situ hybridization, we monitored cpg15 expression in 4-week-old mouse barrel cortex after trimming all whiskers except D1. We found that cpg15 expression is depressed in the deprived barrels and enhanced in the barrel column corresponding to the spared D1 whisker. Changes in cpg15 mRNA levels first appear in layer IV, peak 12 h after deprivation, and then decline rapidly. In layers II/III, changes in cpg15 expression appear later, peak at 24 h, and persist for days. Induction of cpg15 expression is significantly diminished in adolescent as well as adult CREB knockout mice. cpg15's spatio-temporal expression pattern and its regulation by CREB are consistent with a role in experience-dependent plasticity of cortical circuits. Our results suggest that local structural and/or synaptic changes may be a mechanism by which the adult cortex can adapt to peripheral manipulations.

Brain plasticity and remodeling of AMPA receptor properties by calcium-dependent enzymes.

Long-term potentiation (LTP) and long-term depression (LTD) are two experimental models of synaptic plasticity that have been studied extensively in the last 25 years, as they may represent basic mechanisms to store certain types of information in neuronal networks. In several brain regions, these two forms of synaptic plasticity require dendritic depolarization, and the amplitude and duration of the depolarization-induced calcium signal are crucial parameters for the generation of either LTP or LTD. The rise in calcium concentration mediated by activation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors has been proposed to stimulate various calcium-dependent processes that could convert the induction signal into long-lasting changes in synaptic structure and function. According to several lines of experimental evidence, alterations in synaptic function observed with LTP and LTD are thought to be the result of modifications of postsynaptic currents mediated by the a-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) subtype of glutamate receptors. The question of which type(s) of receptor changes constitutes the basis for the expression of synaptic plasticity is still very much open. Here, we review data relevant to the issue of selective modulation of AMPA receptor properties occurring after learning and memory, environmental enrichment, and synaptic plasticity. We also discuss potential cellular mechanisms whereby calcium-dependent enzymes might regulate AMPA receptor properties during LTP and LTD, focusing on protein kinases, proteases and lipases.

A Minimal Clustering Motif

Assembly of proteins in macromolecular complexes is critical for many signaling processes, including organization of the postsynaptic membrane proteins. Christopherson et al. investigated how PSD-95, a palmitoylated scaffolding protein of the membrane-associated guanylate kinase (MAGUK) family, promoted protein clustering. Using tagged versions of PSD-95 or fragments of PSD-95, they determined that the first 13 amino acids, which also include two palmitoylated cysteine residues, were necessary and sufficient to produce clustering of PSD-95 or chimeric proteins [attaching the 13 amino acids to green fluorescent protein (GFP) and maltose, for example]. Oligomerization was detected by coimmunoprecipitation and required palmitoylation, because 2-bromopalmitate, an inhibitor of the palmitate transferase, or mutation of the sequence to prevent palmitoylation blocked oligomerization. Other palmitoylated sequences, such as the one from GAP43, did not induce oligomerization of chimeric proteins or of PSD-95 when used in place of the PSD-95 native sequence. In cells cotransfected with PSD-95 and Kv1.4 K + channels, only those channels that were oligomeric clustered with PSD-95 in membrane microdomains. When the channel subunits were fused into a single subunit, then the channels were diffusely expressed on the cell surface. Surprisingly, the GAP43-PSD-95 chimera that cannot homooligomerize was capable of stimulating clustering of the Kv1.4 subunits. Elimination of the PSD-95 N-terminal palmitoylation sequence and addition of a C-terminal prenylation sequence allowed PSD-95 to promote Kv1.4 clustering, but did not promote homomultimerization. These data identify a short, palmitoylation-dependent sequence that is sufficient for PSD-95 homomultimerization, but also show that homomultimerization is not necessary for PSD-95-mediated clustering of the Kv1.4 channels. Lipid modification is essential for clustering activity, and reversible palmitoylation may represent one model for assembly and disassembly of postsynaptic complexes during activity-dependent changes in synaptic structure. K. S. Christopherson, N. T. Sweeney, S. E. Craven, R. Kang, A. E.-D. El-Husseini, D. S. Bredt, Lipid- and protein-mediated multimerization of PSD-95: Implications for receptor clustering and assembly of synaptic protein networks. J. Cell Sci. 116 , 3213-3219 (2003). [Abstract] [Full Text]

Selectively Reduced Expression of Synaptic Plasticity-Related Genes in Amyloid Precursor Protein + Presenilin-1 Transgenic Mice

A critical question in Alzheimer's disease (AD) research is the cause of memory loss that leads to dementia. The amyloid precursor protein + presenilin-1 (APP+PS1) transgenic mouse is a model for amyloid deposition, and like AD, the mice develop memory deficits as amyloid deposits accumulate. We profiled gene expression in these transgenic mice by microarray and quantitative RT-PCR (qRT-PCR). At the age when these animals developed cognitive dysfunction, they had reduced mRNA expression of several genes essential for long-term potentiation and memory formation (Arc, Zif268, NR2B, GluR1, Homer-1a, Nur77/TR3). These changes appeared to be related to amyloid deposition, because mRNA expression was unchanged in the regions that did not accumulate amyloid. Transgene expression was similar in both amyloid-containing and amyloid-free regions of the brain. Interestingly, these changes occurred without apparent changes in synaptic structure, because a number of presynaptic marker mRNAs (growth-associated protein-43, synapsin, synaptophysin, synaptopodin, synaptotagmin, syntaxin) remained stable. Additionally, a number of genes related to inflammation were elevated in transgenic mice, primarily in the regions containing amyloid. In AD cortical tissue, the same memory-associated genes were downregulated. However, all synaptic and neuronal transcripts were reduced, implying that the loss of neurons and synapses contributed to these changes. We conclude that reduced expression of selected genes associated with memory consolidation are linked to memory loss in both circumstances. This suggests that the memory loss in APP+PS1 transgenic mice may model the early memory dysfunction in AD before the degeneration of synapses and neurons.

Activity-dependent elimination of neuromuscular synapses.

At developing neuromuscular synapses in vertebrates, different motor axon inputs to muscle fibers compete for maintenance of their synapses. Competition results in progressive changes in synaptic structure and strength that lead to the weakening and loss of some inputs, a process that has been called synapse elimination. At the same time, a single input is strengthened and maintained throughout adult life, consistently recruiting muscle fibers to contract even at rapid firing rates. Work over the last decade has led to an understanding of some of the cell biological mechanisms that underlie competition and how these culminate in synapse elimination. We discuss current ideas about how activity modulates neuromuscular synaptic competition, how competition leads to synapse loss, and how these processes are modulated by cell-cell signaling. A common feature of competition at neuromuscular as well as CNS synapses is that temporally correlated activity seems to slow or prevent competition, while uncorrelated activity seems to trigger or enhance competition. Important questions that remain to be addressed include how patterns of motor neuron activity affect synaptic strength, what is the temporal relationship between changes in synaptic strength and structure, and what cellular signals mediate synapse loss. Answers to these questions will expand our understanding of the mechanisms by which activity edits synaptic structure and function, writing permanent changes in neural circuitry.