Clustering and anchoring mechanisms of molecular constituents of postsynaptic scaffolds in dendritic spines

ISGylation is induced in neurons by demyelination driving ISG15-dependent microglial activation

The causes of grey matter pathology and diffuse neuron injury in MS remain incompletely understood. Axonal stress signals arising from white matter lesions has been suggested to play a role in initiating this diffuse grey matter pathology. Therefore, to identify the most upstream transcriptional responses in neurons arising from demyelinated axons, we analyzed the transcriptome of actively translating neuronal transcripts in mouse models of demyelinating disease. Among the most upregulated genes, we identified transcripts associated with the ISGylation pathway. ISGylation refers to the covalent attachment of the ubiquitin-like molecule interferon stimulated gene (ISG) 15 to lysine residues on substrates targeted by E1 ISG15-activating enzyme, E2 ISG15-conjugating enzymes and E3 ISG15-protein ligases. We further confirmed that ISG15 expression is increased in MS cortical and deep gray matter. Upon investigating the functional impact of neuronal ISG15 upregulation, we noted that ISG15 expression was associated changes in neuronal extracellular vesicle protein and miRNA cargo. Specifically, extracellular vesicle-associated miRNAs were skewed toward increased frequency of proinflammatory and neurotoxic miRNAs and decreased frequency of anti-inflammatory and neuroprotective miRNAs. Furthermore, we found that ISG15 directly activated microglia in a CD11b-dependent manner and that microglial activation was potentiated by treatment with EVs from neurons expressing ISG15. Further study of the role of ISG15 and ISGylation in neurons in MS and neurodegenerative diseases is warranted.

One Brain-All Cells: A Comprehensive Protocol to Isolate All Principal CNS-Resident Cell Types from Brain and Spinal Cord of Adult Healthy and EAE Mice

In experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis, the role of each central nervous system (CNS)-resident cell type during inflammation, neurodegeneration, and remission has been frequently addressed. Although protocols for the isolation of different individual CNS-resident cell types exist, none can harvest all of them within a single experiment. In addition, isolation of individual cells is more demanding in adult mice and even more so from the inflamed CNS. Here, we present a protocol for the simultaneous purification of viable single-cell suspensions of all principal CNS-resident cell types (microglia, oligodendrocytes, astrocytes, and neurons) from adult mice-applicable in healthy mice as well as in EAE. After dissociation of the brain and spinal cord from adult mice, microglia, oligodendrocytes, astrocytes and, neurons were isolated via magnetic-activated cell sorting (MACS). Validations comprised flow cytometry, immunocytochemistry, as well as functional analyses (immunoassay and Sholl analysis). The purity of each cell isolation averaged 90%. All cells displayed cell-type-specific morphologies and expressed specific surface markers. In conclusion, this new protocol for the simultaneous isolation of all major CNS-resident cell types from one CNS offers a sophisticated and comprehensive way to investigate complex cellular networks ex vivo and simultaneously reduce mice numbers to be sacrificed.

L-3-n-Butylphthalide improves synaptic and dendritic spine plasticity and ameliorates neurite pathology in Alzheimer's disease mouse model and cultured hippocampal neurons

Alzheimer's disease (AD) is the most common cause of dementia among elderly people. Despite enormous efforts, the pathogenesis of AD still remains unclear and no drug has yet been proved to be disease-modifying. As the basis of learning and memory, the plasticity of synapse and dendritic spine has been impaired during AD progression. Previous studies have showed a protective effect of L-3-n-butylphthalide (L-NBP) on cognitive deficits in AD, we wonder whether this protective effect is associated with positive alterations on synapse and dendritic spines. In this study, we first of all confirmed the anti-dementia effect of L-NBP in 13-month-old APP/PS1 mice, and then investigated the alterations in synaptic and dendritic spine plasticity due to L-NBP treatment both in vivo and in vitro. We also conducted preliminary studies and found the possible mechanisms related to the inhibition of over-activated complement cascade and the remodeling of actin cytoskeleton. Besides, we also found extra benefits of L-NBP on presynaptic dystrophic neurites and attempted to give explanations from the view of autophagy regulation. Taken together, our study added some new evidence to the application of L-NBP in AD treatment and provided deeper insight into the relevant mechanisms for future study.

Ligand-dependent spatiotemporal signaling profiles of the μ-opioid receptor are controlled by distinct protein-interaction networks

Ligand-dependent differences in the regulation and internalization of the μ-opioid receptor (MOR) have been linked to the severity of adverse effects that limit opiate use in pain management. MOR activation by morphine or [d-Ala²,N-MePhe⁴, Gly-ol]enkephalin (DAMGO) causes differences in spatiotemporal signaling dependent on MOR distribution at the plasma membrane. Morphine stimulation of MOR activates a Gα_i/o–Gβγ–protein kinase C (PKC) α phosphorylation pathway that limits MOR distribution and is associated with a sustained increase in cytosolic extracellular signal-regulated kinase (ERK) activity. In contrast, DAMGO causes a redistribution of the MOR at the plasma membrane (before receptor internalization) that facilitates transient activation of cytosolic and nuclear ERK. Here, we used proximity biotinylation proteomics to dissect the different protein-interaction networks that underlie the spatiotemporal signaling of morphine and DAMGO. We found that DAMGO, but not morphine, activates Ras-related C3 botulinum toxin substrate 1 (Rac1). Both Rac1 and nuclear ERK activity depended on the scaffolding proteins IQ motif-containing GTPase-activating protein-1 (IQGAP1) and Crk-like (CRKL) protein. In contrast, morphine increased the proximity of the MOR to desmosomal proteins, which form specialized and highly-ordered membrane domains. Knockdown of two desmosomal proteins, junction plakoglobin or desmocolin-1, switched the morphine spatiotemporal signaling profile to mimic that of DAMGO, resulting in a transient increase in nuclear ERK activity. The identification of the MOR-interaction networks that control differential spatiotemporal signaling reported here is an important step toward understanding how signal compartmentalization contributes to opioid-induced responses, including anti-nociception and the development of tolerance and dependence.

EFAD transgenic mice as a human APOE relevant preclinical model of Alzheimer's disease

Identified in 1993, APOE4 is the greatest genetic risk factor for sporadic Alzheimer's disease (AD), increasing risk up to 15-fold compared with APOE3, with APOE2 decreasing AD risk. However, the functional effects of APOE4 on AD pathology remain unclear and, in some cases, controversial. In vivo progress to understand how the human (h)-APOE genotypes affect AD pathology has been limited by the lack of a tractable familial AD-transgenic (FAD-Tg) mouse model expressing h-APOE rather than mouse (m)-APOE. The disparity between m- and h-apoE is relevant for virtually every AD-relevant pathway, including amyloid-β (Aβ) deposition and clearance, neuroinflammation, tau pathology, neural plasticity and cerebrovascular deficits. EFAD mice were designed as a temporally useful preclinical FAD-Tg-mouse model expressing the h-APOE genotypes for identifying mechanisms underlying APOE-modulated symptoms of AD pathology. From their first description in 2012, EFAD mice have enabled critical basic and therapeutic research. Here we review insights gleaned from the EFAD mice and summarize future directions.

Incorrect dosage of IQSEC2, a known intellectual disability and epilepsy gene, disrupts dendritic spine morphogenesis

There is considerable genetic and phenotypic heterogeneity associated with intellectual disability (ID), specific learning disabilities, attention-deficit hyperactivity disorder, autism and epilepsy. The intelligence quotient (IQ) motif and SEC7 domain containing protein 2 gene (IQSEC2) is located on the X-chromosome and harbors mutations that contribute to non-syndromic ID with and without early-onset seizure phenotypes in both sexes. Although IQ and Sec7 domain mutations lead to partial loss of IQSEC2 enzymatic activity, the in vivo pathogenesis resulting from these mutations is not known. Here we reveal that IQSEC2 has a key role in dendritic spine morphology. Partial loss-of-function mutations were modeled using a lentiviral short hairpin RNA (shRNA) approach, which achieved a 57% knockdown of Iqsec2 expression in primary hippocampal cell cultures from mice. Investigating gross morphological parameters after 8 days of in vitro culture (8DIV) identified a 32% reduction in primary axon length, in contrast to a 27% and 31% increase in the number and complexity of dendrites protruding from the cell body, respectively. This increase in dendritic complexity and spread was carried through dendritic spine development, with a 34% increase in the number of protrusions per dendritic segment compared with controls at 15DIV. Although the number of dendritic spines had normalized by 21DIV, a reduction was noted in the number of immature spines. In contrast, when modeling increased dosage, overexpression of wild-type IQSEC2 led to neurons with shorter axons that were more compact and displayed simpler dendritic branching. Disturbances to dendritic morphology due to knockdown of Iqsec2 were recapitulated in neurons from Iqsec2 knockout mice generated in our laboratory using CRISPR/Cas9 technology. These observations provide evidence of dosage sensitivity for IQSEC2, which normally escapes X-inactivation in females, and links these disturbances in expression to alterations in the morphology of developing neurons.

The role of drebrin in neurons

Drebrin is an actin-binding protein that changes the helical pitch of actin filaments (F-actin), and drebrin-decorated F-actin shows slow treadmilling and decreased rate of depolymerization. Moreover, the characteristic morphology of drebrin-decorated F-actin enables it to respond differently to the same signals from other actin cytoskeletons. Drebrin consists of two major isoforms, drebrin E and drebrin A. In the developing brain, drebrin E appears in migrating neurons and accumulates in the growth cones of axons and dendrites. Drebrin E-decorated F-actin links lamellipodium F-actin to microtubules in the growth cones. Then drebrin A appears at nascent synapses and drebrin A-decorated F-actin facilitates postsynaptic molecular assembly. In the adult brain, drebrin A-decorated F-actin is concentrated in the central region of dendritic spines. During long-term potentiation initiation, NMDA receptor-mediated Ca²⁺ influx induces the transient exodus of drebrin A-decorated F-actin via myosin II ATPase activation. Because of the unique physical characteristics of drebrin A-decorated F-actin, this exodus likely contributes to the facilitation of F-actin polymerization and spine enlargement. Additionally, drebrin reaccumulation in dendritic spines is observed after the exodus. In our drebrin exodus model of structure-based synaptic plasticity, reestablishment of drebrin A-decorated F-actin is necessary to keep the enlarged spine size during long-term potentiation maintenance. In this review, we introduce the genetic and biochemical properties of drebrin and the roles of drebrin in early stage of brain development, synaptic formation and synaptic plasticity. Further, we discuss the pathological relevance of drebrin loss in Alzheimer's disease. This article is part of the mini review series "60th Anniversary of the Japanese Society for Neurochemistry".

Connexin43 Forms Supramolecular Complexes through Non-Overlapping Binding Sites for Drebrin, Tubulin, and ZO-1

Gap junctions are membrane specialization domains identified in most tissue types where cells abut each other. The connexin channels found in these membrane domains are conduits for direct cell-to-cell transfer of ions and molecules. Connexin43 (Cx43) is the most ubiquitous connexin, with critical roles in heart, skin, and brain. Several studies described the interaction between Cx43 and the cytoskeleton involving the actin binding proteins Zonula occludens (ZO-1) and drebrin, as well as with tubulin. However, a direct interaction has not been identified between drebrin and Cx43. In this study, co-IP and NMR experiments were used to demonstrate that the Cx43-CT directly interacts with the highly conserved N-terminus region of drebrin. Three Cx43-CT areas were found to be involved in drebrin binding, with residues 264–275 being critical for the interaction. Mimicking Src phosphorylation within this region (Y265) significantly disrupted the interaction between the Cx43-CT and drebrin. Immunofluorescence showed colocalization of Cx43, drebrin, and F-actin in astrocytes and Vero cells membrane, indicating that Cx43 forms a submembrane protein complex with cytoskeletal and scaffolding proteins. The co-IP data suggest that Cx43 indirectly interacts with F-actin through drebrin. Along with the known interaction of the Cx43-CT with ZO-1 and tubulin, the data presented here for drebrin indicate non-overlapping and separated binding sites for all three proteins for which simultaneous binding could be important in regulating cytoskeleton rearrangements, especially for neuronal migration during brain development.

Novel Applications of Magnetic Cell Sorting to Analyze Cell-Type Specific Gene and Protein Expression in the Central Nervous System

The isolation and study of cell-specific populations in the central nervous system (CNS) has gained significant interest in the neuroscience community. The ability to examine cell-specific gene and protein expression patterns in healthy and pathological tissue is critical for our understanding of CNS function. Several techniques currently exist to isolate cell-specific populations, each having their own inherent advantages and shortcomings. Isolation of distinct cell populations using magnetic sorting is a technique which has been available for nearly 3 decades, although rarely used in adult whole CNS tissue homogenate. In the current study we demonstrate that distinct cell populations can be isolated in rodents from early postnatal development through adulthood. We found this technique to be amendable to customization using commercially available membrane-targeted antibodies, allowing for cell-specific isolation across development and animal species. This technique yields RNA which can be utilized for downstream applications—including quantitative PCR and RNA sequencing—at relatively low cost and without the need for specialized equipment or fluorescently labeled cells. Adding to its utility, we demonstrate that cells can be isolated largely intact, retaining their processes, enabling analysis of extrasomatic proteins. We propose that magnetic cell sorting will prove to be a highly useful technique for the examination of cell specific CNS populations.

Unpredictable chronic mild stress induces anxiety and depression-like behaviors and inactivates AMP-activated protein kinase in mice

The unpredictable chronic mild stress (UCMS) model was developed based upon the stress-diathesis hypothesis of depression. Most effects of UCMS can be reversed by antidepressants, demonstrating a strong predictive validity of this model for depression. However, the mechanisms underlying the effects induced by UCMS remain incompletely understood. Increasing evidence has shown that AMP-activated protein kinase (AMPK) regulates intracellular energy metabolism and is especially important for neurons because neurons are known to have small energy reserves. Abnormalities in the AMPK pathway disturb normal brain functions and synaptic integrity. In the present study, we first investigated the effects of UCMS on a battery of different tests measuring anxiety and depression-like behaviors in female C57BL/6N mice after 4 weeks of UCMS exposure. Stressed mice showed suppressed body weight gain, heightened anxiety, and increased immobility in the forced swim and tail suspension tests. These results are representative of some of the core symptoms of depression. Simultaneously, we observed decrease of synaptic proteins in the cortex of mice subjected to UCMS, which is associated with decreased levels of phosphorylated AMP-activated protein kinase α (AMPKα) and 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase). Our findings suggest that AMPKα inactivation might be a mechanism by which UCMS causes anxiety/depression-like behaviors in mice.

Expression of Dbn1 during mouse brain development and neural stem cell differentiation

Dbn1 is a newly discovered gene in the drebrin gene family of mice. Previous studies have reported that Dbn1 is specifically expressed in the mouse brain suggesting its potential role in brain development. However, a detailed analysis of Dbn1 expression during mouse brain development has not been demonstrated. Here, we describe the expression pattern of Dbn1 and the coexpression of Dbn1 and actin during the development of the mouse brain from embryonic day 14 (E14) to adulthood and during the differentiation of neural stem cells (NSCs), as determined using immunohistochemistry, double-labeling immunofluorescence, and quantitative real-time polymerase chain reaction. During mouse brain development, Dbn1 expression level was high at E14, attenuated postnatally, reached its highest point at postnatal day 7 (P7), and showed a very low level at adulthood. Imaging data showed that Dbn1 was mainly expressed in the hippocampus, ventricular zone, and cortex, where NSCs are densely distributed, and that the intracellular distribution of Dbn1 was predominantly located in the cytoplasm edges and neurites. Moreover, the signal for colocalization of Dbn1 with actin was intense at E14, P0, and P7, but it was weak at adulthood. During NSC differentiation, Dbn1 mRNA expression increased after the onset of differentiation and reached its highest point at 3days, followed by a decrease in expression. The imaging data showed that Dbn1 was increasingly expressed in the extending neurites in accordance with the cell morphological changes that occur during differentiation. Furthermore, obvious colocalization signals of Dbn1 with actin were found in the neurites and dendritic spines. Collectively, these results suggest that Dbn1 may play a key role in mouse brain development and may regulate NSC differentiation by filamentous actin.

14-3-3 proteins are required for hippocampal long-term potentiation and associative learning and memory

14-3-3 is a family of regulatory proteins highly expressed in the brain. Previous invertebrate studies have demonstrated the importance of 14-3-3 in the regulation of synaptic functions and learning and memory. However, the in vivo role of 14-3-3 in these processes has not been determined using mammalian animal models. Here, we report the behavioral and electrophysiological characterization of a new animal model of 14-3-3 proteins. These transgenic mice, considered to be a 14-3-3 functional knock-out, express a known 14-3-3 inhibitor in various brain regions of different founder lines. We identify a founder-specific impairment in hippocampal-dependent learning and memory tasks, as well as a correlated suppression in long-term synaptic plasticity of the hippocampal synapses. Moreover, hippocampal synaptic NMDA receptor levels are selectively reduced in the transgenic founder line that exhibits both behavioral and synaptic plasticity deficits. Collectively, our findings provide evidence that 14-3-3 is a positive regulator of associative learning and memory at both the behavioral and cellular level.

Bisphenol A promotes dendritic morphogenesis of hippocampal neurons through estrogen receptor-mediated ERK1/2 signal pathway

Bisphenol A (BPA), an environmental endocrine disruptor, has attracted increasing attention to its adverse effects on brain developmental process. The previous study indicated that BPA rapidly increased motility and density of dendritic filopodia and enhanced the phosphorylation of N-methyl-d-aspartate (NMDA) receptor subunit NR2B in cultured hippocampal neurons within 30min. The purpose of the present study was further to investigate the effects of BPA for 24h on dendritic morphogenesis and the underlying mechanisms. After cultured for 5d in vitro, the hippocampal neurons from 24h-old rat were infected by AdV-EGFP to indicate time-lapse imaging of living neurons. The results demonstrated that the exposure of the cultured hippocampal neurons to BPA (10, 100nM) or 17β-estradiol (17β-E2, 10nM) for 24h significantly promoted dendritic development, as evidenced by the increased total length of dendrite and the enhanced motility and density of dendritic filopodia. However, these changes were suppressed by an ERs antagonist, ICI182,780, a non-competitive NMDA receptor antagonist, MK-801, and a mitogen-activated ERK1/2-activating kinase (MEK1/2) inhibitor, U0126. Meanwhile, the increased F-actin (filamentous actin) induced by BPA (100nM) was also completely eliminated by these blockers. Furthermore, the result of western blot analyses showed that, the exposure of the cultures to BPA or 17β-E2 for 24h promoted the expression of Rac1/Cdc42 but inhibited that of RhoA, suggesting Rac1 (Ras related C3 botulinum toxinsubstrate 1)/Cdc42 (cell divisioncycle 42) and RhoA (Ras homologous A), the Rho family of small GTPases, were involved in BPA- or 17β-E2-induced changes in the dendritic morphogenesis of neurons. These BPA- or 17β-E2-induced effects were completely blocked by ICI182,780, and were partially suppressed by U0126. These results reveal that, similar to 17β-E2, BPA exerts its effects on dendritic morphogenesis by eliciting both nuclear actions and extranuclear-initiated actions that are integrated to influence the development of dendrite in hippocampal neurons.

Embryonic and larval neural connectivity: progressive changes in synapse form and function at the neuromuscular junction mediated by cytoskeletal regulation

During development, precise formation of millions of synaptic connections is critical for the formation of a functional nervous system. Synaptogenesis is a complex multistep process in which axons follow gradients of secreted and cell surface guidance cues to reach their target area, at which point they must accurately distinguish their specific target. Upon target recognition, the axonal growth cone undergoes rapid growth and morphological changes, ultimately forming a functional synapse that continues to remodel during activity-dependent plasticity. Significant evidence suggests that the underlying actin and microtubule (MT) cytoskeletons are key effectors throughout synaptogenesis downstream of numerous receptors and signaling pathways. An increasing number of cytoskeletal-associated proteins have been shown to influence actin and MT stability and dynamics and many of these regulators have been implicated during synaptic morphogenesis using both mammalian and invertebrate model systems. In this review, we present an overview of the role cytoskeletal regulators play during the formation of the Drosophila neuromuscular junction.

Presenilin conditional double knockout mice exhibit decreases in drebrin a at hippocampal CA1 synapses

Presenilin conditional double knockout (PScDKO) mice have been used as animal models to study the development of Alzheimer's disease (AD) phenotypes. Studies to date indicate that these animals exhibit memory dysfunction and decreased synaptic plasticity before the onset of neurodegeneration. Therefore, the current study sought to examine how the loss of presenilin expression leads to these defects. Drebrin A, a neuron-specific actin-binding protein, has been shown to play an important role in the activity-dependent redistribution of the NMDA type of glutamate receptors at the synapse which, in turn, is a critical step for enabling synaptic plasticity. It is hypothesized that defects in the activity dependent redistribution of NMDA receptors in PScDKO mice may be due to loss of drebrin A. In this study, electron microscopic immunocytochemistry (EM-ICC) was used to quantify and locate drebrin A in the CA1 field of the hippocampus of PScDKO mice. The high resolution of EM-ICC allowed for differentiation between drebrin A at the synapse and at nonsynaptic sites, the latter of which would reflect the protein's role in regulating the reserve or degradative pool of NMDA receptors. The results here demonstrate that loss of function of presenilin in mice leads to a decrease in immunoreactivity for drebrin A at both synaptic (54% decrease, P < 0.01) and nonsynaptic areas (40% decrease, P < 0.01) and overall (44% decrease, P < 0.01). The reduction of drebrin A in both synaptic and nonsynaptic locations of the spine may implicate impairment in glutamate receptor trafficking to and from the postsynaptic plasma membrane. In addition, because of reduced drebrin A at nonsynaptic sites, the regulation of the reserve and degradative pools of glutamate receptors may also be impaired, leading to further synaptic dysfunction. Therefore, this study provides evidence to the theory that drebrin A has a causal role in compromising activity-dependent glutamate receptor trafficking in PScDKO mice.

Actin-binding Rho activating protein is expressed in the central nervous system of normal adult rats

Previous studies show that actin-binding Rho activating protein (Abra) is expressed in cardiomyocytes and vascular smooth muscle cells. In this study, we investigated the expression profile of Abra in the central nervous system of normal adult rats by confocal immunofluorescence. Results showed that Abra immunostaining was located in neuronal nuclei, cytoplasm and processes in the central nervous system, with the strongest staining in the nuclei; in the cerebral cortex, Abra positive neuronal bodies and processes were distributed in six cortical layers including molecular layer, external granular layer, external pyramidal layer, internal granular layer, internal pyramidal layer and polymorphic layer; in the hippocampus, the cell bodies of Abra positive neurons were distributed evenly in pyramidal layer and granular layer, with positive processes in molecular layer and orien layer; in the cerebellar cortex, Abra staining showed the positive neuronal cell bodies in Purkinje cell layer and granular layer and positive processes in molecular layer; in the spinal cord, Abra-immunopositive products covered the whole gray matter and white matter; co-localization studies showed that Abra was co-stained with F-actin in neuronal cytoplasm and processes, but weakly in the nuclei. In addition, in the hippocampus, Abra was co-stained with F-actin only in neuronal processes, but not in the cell body. This study for the first time presents a comprehensive overview of Abra expression in the central nervous system, providing insights for further investigating the role of Abra in the mature central nervous system.

Potential role of drebrin a, an f-actin binding protein, in reactive synaptic plasticity after pilocarpine-induced seizures: functional implications in epilepsy

Several neurological disorders characterized by cognitive deficits, including Alzheimer's disease, down syndrome, and epilepsy exhibit abnormal spine density and/or morphology. Actin-based cytoskeleton network dynamics is critical for the regulation of spine morphology and synaptic function. In this paper, I consider the functions of drebrin A in cell shaping, spine plasticity, and synaptic function. Developmentally regulated brain protein (drebrin A) is one of the most abundant neuron-specific binding proteins of F-actin and its expression is increased in parallel with synapse formation. Drebrin A is particularly concentrated in dendritic spines receiving excitatory inputs. Our recent findings point to a critical role of DA in dendritic spine structural integrity and stabilization, likely via regulation of actin cytoskeleton dynamics, and glutamatergic synaptic function that underlies the development of spontaneous recurrent seizures in pilocarpine-treated animals. Further research into this area may provide useful insights into the pathology of status epilepticus and epileptogenic mechanisms and ultimately may provide the basis for future treatment options.

Quantitative estimates of the cytoplasmic, PSD, and NMDAR-bound pools of CaMKII in dendritic spines

CaMKII plays a critical role in long-term potentiation (LTP). The kinase is a major component of the postsynaptic density (PSD); however, it is also contained in the spine cytoplasm. CaMKII can now be monitored optically in living neurons, and it is therefore important to understand the contribution of the PSD and cytoplasmic pools to optical signals. Here, we estimate the size of these pools under basal conditions. From EM immunolabeling data, we calculate that the PSD/cytoplasmic ratio is ~5%. A second independent estimate is derived from measurements indicating that the average mushroom spine PSD contains 90 to 240 holoenzymes. A cytoplasmic concentration of 16 μM (~2590 holoenzymes) in the spine can be estimated from the total measured CaMKII content of hippocampal tissue, the relative volume of different compartments, and the spine-dendrite ratio of CaMKII (2:1). These numbers yield a second estimate (6%) of the PSD/spine ratio in good agreement with the first. The CaMKII bound to the NMDAR is important because preventing the formation of this complex blocks LTP induction. We estimate that the percentage of spine CaMKII held active by binding to the NMDAR is ~0.2%. Implications of the high spine concentration of CaMKII (> 100 μM alpha subunits) and the small fraction within the PSD are discussed. Of particular note, the finding that the CaMKII signal in spines shows only transient activation (open state) after LTP induction is subject to the qualification that it does not reflect the small but important pool bound to the NMDAR.

LIM kinase mediates estrogen action on the actin depolymerization factor Cofilin

The ovarian hormone estrogen increases the axospinous synapse density in the hippocampal CA1 region of young female rats but fails to do so in aged rats. This estrogen-mediated alteration of spine synapse structures suggests the coincident requirement for the structural reorganization of the underlying actin cytoskeleton network. Actin reorganization is known to require the deactivation of Cofilin, an actin depolymerization factor. Cofilin is deactivated by LIM kinase (LIMK), and LIMK activity is modulated by the phosphorylation of specific residues. We have previously demonstrated that estrogen is able to increase phosphorylated LIMK (pLIMK) immunoreactivity (IR) in the hippocampus in vivo and that this estrogen-stimulated pLIMK-IR is decreased in the aged brain. Because Cofilin phosphorylation allows for actin filament elongation and spine synapse growth, we sought to determine if estrogen acts through Cofilin and if such estrogen action requires the observed LIMK activity. Using both hippocampal neurons and the NG108-15 neuroblastoma cell line, we demonstrate here that estrogen stimulates the phosphorylation of Cofilin in vitro. Furthermore, this estrogen action on Cofilin requires LIMK. Lastly, while initiating the phosphorylation of LIMK and Cofilin, estrogen can also stimulate the formation of filopodial extensions, an early step in the formation of nascent spines, demonstrating that estrogen can alter the actin-dependent neuronal morphology. This linkage of estrogen communication to Cofilin via LIMK provides the functionality to the age-sensitive pLIMK-IR that we have observed in vivo.

Drosophila twinfilin is required for cell migration and synaptic endocytosis

Precise actin regulation is essential for diverse cellular processes such as axonal growth, cell migration and endocytosis. twinfilin (twf) encodes a protein that sequesters actin monomers, but its in vivo functions are unclear. In this study, we characterized twf-null mutants in a metazoan for the first time and found that Drosophila twf negatively regulates F-actin formation in subcellular regions of rapid actin turnover in three different systems, namely postsynaptic neuromuscular junction (NMJ) synapses, migratory border cells and epithelial follicle cells. Loss of twf function results in defects in axonal growth in the brain and border cell migration in the ovary. Additionally, we found that the actin-dependent postsynaptic localization of glutamate receptor GluRIIA, but not GluRIIB, was specifically reduced in twf mutants. More importantly, we showed that twf mutations caused significantly reduced presynaptic endocytosis at NMJ synapses, as detected using the fluorescent dye FM1-43 uptake assay. Furthermore, electrophysiological analysis under high-frequency stimulation showed compromised neurotransmission in twf mutant synapses, confirming an insufficient replenishment of synaptic vesicles. Together, our results reveal that twinfilin promotes actin turnover in multiple cellular processes that are highly dependent on actin dynamics.

Mapping of drebrin binding site on F-actin

Drebrin is a filament-binding protein involved in organizing the dendritic pool of actin. Previous in vivo studies identified the actin-binding domain of drebrin (DrABD), which causes the same rearrangements in the cytoskeleton as the full-length protein. Site-directed mutagenesis, electron microscopic reconstruction, and chemical cross-linking combined with mass spectrometry analysis were employed here to map the DrABD binding interface on actin filaments. DrABD could be simultaneously attached to two adjacent actin protomers using the combination of 2-iminothiolane (Traut's reagent) and MTS1 [1,1-methanediyl bis(methanethiosulfonate)]. Site-directed mutagenesis combined with chemical cross-linking revealed that residue 238 of DrABD is located within 5.4 A from C374 of actin protomer 1 and that native cysteine 308 of drebrin is near C374 of actin protomer 2. Mass spectrometry analysis revealed that a zero-length cross-linker, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, can link the N-terminal G-S extension of the recombinant DrABD to E99 and/or E100 on actin. Efficient cross-linking of drebrin residues 238, 248, 252, 270, and 271 to actin residue 51 was achieved with reagents of different lengths (5.4-19 A). These results suggest that the "core" DrABD is centered on actin subdomain 2 and may adopt a folded conformation upon binding to F-actin. The results of electron microscopic reconstruction, which are in a good agreement with the cross-linking data, revealed polymorphism in DrABD binding to F-actin and suggested the existence of two binding sites. These results provide new structural insight into the previously observed competition between drebrin and several other F-actin-binding proteins.

Drebrin a knockout eliminates the rapid form of homeostatic synaptic plasticity at excitatory synapses of intact adult cerebral cortex

Homeostatic synaptic plasticity (HSP) is important for maintaining neurons' excitability within the dynamic range and for protecting neurons from unconstrained long-term potentiation that can cause breakdown of synapse specificity (Turrigiano [2008] Cell 135:422-435). Knowledge of the molecular mechanism underlying this phenomenon remains incomplete, especially for the rapid form of HSP. To test whether HSP in adulthood depends on an F-actin binding protein, drebrin A, mice deleted of the adult isoform of drebrin (DAKO) but retaining the embryonic isoform (drebrin E) were generated. HSP was assayed by determining whether the NR2A subunit of N-methyl-D-aspartate receptors (NMDARs) can rise rapidly within spines following the application of an NMDAR antagonist, D-APV, onto the cortical surface. Electron microscopic immunocytochemistry revealed that, as expected, the D-APV treatment of wild-type (WT) mouse cortex increased the proportion of NR2A-immunolabeled spines within 30 minutes relative to basal levels in hemispheres treated with an inactive enantiomer, L-APV. This difference was significant at the postsynaptic membrane and postsynaptic density (i.e., synaptic junction) as well as at nonsynaptic sites within spines and was not accompanied by spine size changes. In contrast, the D-APV treatment of DAKO brains did not augment NR2A labeling within the spine cytoplasm or at the synaptic junction, even though basal levels of NR2A were not significantly different from those of WT cortices. These findings indicate that drebrin A is required for the rapid (<30 minutes) form of HSP at excitatory synapses of adult cortices, whereas drebrin E is sufficient for maintaining basal NR2A levels within spines.

Combinatorial morphogenesis of dendritic spines and filopodia by SPAR and alpha-actinin2

Rap small GTPases regulate excitatory synaptic strength and morphological plasticity of dendritic spines. Changes in spine structure are mediated by the F-actin cytoskeleton, but the link between Rap activity and actin dynamics is unclear. Here, we report a novel interaction between SPAR, a postsynaptic inhibitor of Rap, and alpha-actinin, a family of actin-cross-linking proteins. SPAR and alpha-actinin engage in bidirectional structural plasticity of dendritic spines: SPAR promotes spine head enlargement, whereas increased alpha-actinin2 expression favors dendritic spine elongation and thinning. Surprisingly, SPAR and alpha-actinin2 can function in an additive rather than antagonistic fashion at the same dendritic spine, generating combination spine/filopodia hybrids. These data identify a molecular pathway bridging the actin cytoskeleton and Rap at synapses, and suggest that formation of spines and filopodia are not necessarily opposing forms of structural plasticity.

Decreased drebrin mRNA expression in Alzheimer disease: correlation with tau pathology

To investigate the mRNA expression of the dendritic spine protein drebrin in Alzheimer's disease (AD), we performed post-mortem in situ hybridization studies in brain sections from 20 AD patients and 21 controls. AD diagnosis was confirmed by decreased drebrin protein and increased Abeta(40) (+464%; P < 0.05), Abeta(42) (+369%; P < 0.0001), Abeta(42/40) ratio (+226%; P < 0.01), total tau (+2,725%; P < 0.0001), and paired helical filament tau (PHFtau; +867%; P < 0.001) compared with controls. We found significant decreases in drebrin mRNA in the parietal cortex (-27%; P < 0.01), the temporal cortex (-22%; P < 0.05), and the hippocampus (-25%; P < 0.05) of AD patients compared with controls. Cortical levels of drebrin mRNA correlated positively with soluble total tau (r(2) = +0.244) but negatively with duration of symptoms (r(2) = -0.357) and PHFtau (r(2) = -0.248). Drebrin mRNA levels were correlated to a lesser degree with the drebrin protein content (r(2) = +0.136) and with sim2 (r(2) = +0.176), a potential modulator of drebrin transcription. Our results suggest that the down-regulation of drebrin mRNA expression plays an important role in AD and is closely related to the progression of the disease.

Targets of caspase-6 activity in human neurons and Alzheimer disease

Caspase-6 activation occurs early in Alzheimer disease and sometimes precedes the clinical manifestation of the disease in aged individuals. The active Caspase-6 is localized in neuritic plaques, in neuropil threads, and in neurofibrillary tangles containing neurons that are not morphologically apoptotic in nature. To investigate the potential consequences of the activation of Caspase-6 in neurons, we conducted a proteomics analysis of Caspase-6-mediated cleavage of human neuronal proteins. Proteins from the cytosolic and membrane subcellular compartments were treated with recombinant active Caspase-6 and compared with undigested proteins by two-dimensional gel electrophoresis. LC/MS/MS analyses of the proteins that were cleaved identified 24 different potential protein substrates. Of these, 40% were cytoskeleton or cytoskeleton-associated proteins. We focused on the cytoskeleton proteins because these are critical for neuronal structure and function. Caspase-6 cleavage of alpha-Tubulin, alpha-Actinin-4, Spinophilin, and Drebrin was confirmed. At least one Caspase-6 cleavage site was identified for Drebrin, Spinophilin, and alpha-Tubulin. A neoepitope antiserum to alpha-Tubulin cleaved by Caspase-6 immunostained neurons, neurofibrillary tangles, neuropil threads, and neuritic plaques in Alzheimer disease and co-localized with active Caspase-6. These results imply that the early and neuritic activation of Caspase-6 in Alzheimer disease could disrupt the cytoskeleton network of neurons, resulting in impaired neuronal structure and function in the absence of cell death. This study provides novel insights into the pathophysiology of Alzheimer disease.

Do we need molecular tomography of a cell and how can it be achieved?

1. The spatial relationship between intracellular molecules and their local concentrations are two critical parameters required for a better understanding of protein-protein interactions in the cell. 2. Determination of the local concentration of proteins in individual cells using more sophisticated techniques and determination of the spatial relationship between a molecular platform and its partners is essential for allow us to obtain more convincing and concrete scientific conclusions. 3. As a reasonable goal, development of molecular tomography of the cell is proposed.

Chemical and morphological alterations of spines within the hippocampus and entorhinal cortex precede the onset of Alzheimer's disease pathology in double knock-in mice

Mice with knock-in of two mutations that affect beta amyloid processing and levels (2xKI) exhibit impaired spatial memory by 9-12 months of age, together with synaptic plasticity dysfunction in the hippocampus. The goal of this study was to identify changes in the molecular and structural characteristics of synapses that precede and thus could exert constraints upon cellular mechanisms underlying synaptic plasticity. Drebrin A is one protein reported to modulate spine sizes and trafficking of proteins to and from excitatory synapses. Thus, we examined levels of drebrin A within postsynaptic spines in the hippocampus and entorhinal cortex. Our electron microscopic immunocytochemical analyses reveal that, by 6 months, the proportion of hippocampal spines containing drebrin A is reduced and this change is accompanied by an increase in the mean size of spines and decreased density of spines. In the entorhinal cortex of 2xKI brains, we detected no decrement in the proportion of spines labeled for drebrin A and no significant change in spine density at 6 months, but rather a highly significant reduction in the level of drebrin A immunoreactivity within each spine. These changes are unlike those observed for the somatosensory cortex of 2xKI mice, in which synapse density and drebrin A immunoreactivity levels remain unchanged at 6 months and older. These results indicate that brains of 2xKI mice, like those of humans, exhibit regional differences of vulnerability, with the hippocampus exhibiting the first signatures of structural changes that, in turn, may underlie the emergent inability to update spatial memory in later months.

Distinct effects of individual opioids on the morphology of spines depend upon the internalization of mu opioid receptors

This study has examined the relationship between the effects of opioids on the internalization of mu opioid receptors (MORs) and the morphology of dendritic spines. Several opioids (morphine, etorphine, DAMGO or methadone) were applied to cultured hippocampal neurons. Live imaging and biochemical techniques were used to examine the dynamic changes in MOR internalization and spine morphology. This study reveals that MOR internalization can regulate opioid-induced morphological changes in dendritic spines: (1) Chronic treatment with morphine, which induced minimal receptor internalization, caused collapse of dendritic spines. In contrast, "internalizing" opioids such as DAMGO and etorphine induced the emergence of new spines. It reveals that opioid-induced changes in spines vary greatly depending on how the applied opioid agonist affects MOR internalization. (2) The blockade of receptor internalization by dominant negative mutant of dynamin, K44E, reversed the effects of DAMGO and etorphine. It indicates that receptor internalization is necessary for the distinct effects of DAMGO and etorphine on spines. (3) In neurons that were cultured from MOR knock-out mice and had been co-transfected with DsRed and MOR-GFP, morphine caused collapse of spines whereas DAMGO induced emergence of new spines, indicating that opioids can alter the structure of spines via postsynaptic MORs. (4) Methadone at a low concentration induced minimal internalization and had effects that were similar to morphine. At a high concentration, methadone induced robust internalization and had effects that are opposite to morphine. The concentration-dependent opioid-induced changes in dendritic spines might also contribute to the variation in the effects of individual opioids.

Role of actin cytoskeleton in dendritic spine morphogenesis

Dendritic spines are the postsynaptic receptive regions of most excitatory synapses, and their morphological plasticity play a pivotal role in higher brain functions, such as learning and memory. The dynamics of spine morphology is due to the actin cytoskeleton concentrated highly in spines. Filopodia, which are thin and headless protrusions, are thought to be precursors of dendritic spines. Drebrin, a spine-resident side-binding protein of filamentous actin (F-actin), is responsible for recruiting F-actin and PSD-95 into filopodia, and is suggested to govern spine morphogenesis. Interestingly, some recent studies on neurological disorders accompanied by cognitive deficits suggested that the loss of drebrin from dendritic spines is a common pathognomonic feature of synaptic dysfunction. In this review, to understand the importance of actin-binding proteins in spine morphogenesis, we first outline the well-established knowledge pertaining to the actin cytoskeleton in non-neuronal cells, such as the mechanism of regulation by small GTPases, the equilibrium between globular actin (G-actin) and F-actin, and the distinct roles of various actin-binding proteins. Then, we review the dynamic changes in the localization of drebrin during synaptogenesis and in response to glutamate receptor activation. Because side-binding proteins are located upstream of the regulatory pathway for actin organization via other actin-binding proteins, we discuss the significance of drebrin in the regulatory mechanism of spine morphology through the reorganization of the actin cytoskeleton. In addition, we discuss the possible involvement of an actin-myosin interaction in the morphological plasticity of spines.

Many faces of drebrin: from building dendritic spines and stabilizing gap junctions to shaping neurite-like cell processes

In this review we consider the multiple functions of developmentally regulated brain protein (drebrin), an actin-binding protein, in the formation of cellular polarity in different cell types. Drebrin has a well-established role in the morphogenesis, patterning and maintenance of dendritic spines in neurons. We have recently shown that drebrin also stabilizes Connexin-43 containing gap junctions at the plasma membrane. The latest literature and our own data suggest that drebrin may be broadly involved in shaping cell processes and in the formation of stabilized plasma membrane domains, an effect that is likely to be of crucial significance for formation of cell polarity in both neuronal and non-neuronal types.

Targeted deletion of a single Sca8 ataxia locus allele in mice causes abnormal gait, progressive loss of motor coordination, and Purkinje cell dendritic deficits

Spinocerebellar ataxia type 8 (SCA8) patients typically have a slowly progressive, adult-onset ataxia. SCA8 is dominantly inherited and is caused by large CTG repeat expansions in the untranslated antisense RNA of the Kelch-like 1 gene (KLHL1), but the molecular mechanism through which this expansion leads to disease is still unknown. To more fully characterize the underlying molecular mechanisms involved in SCA8, we developed a mouse model in which Klhl1 is deleted in either all tissues or is deleted specifically in Purkinje cells only. We found that mice that are either homozygous or heterozygous for the Klhl1 deletion have significant gait abnormalities at an early age and develop a significant loss of motor coordination by 24 weeks of age. This loss progresses more rapidly in homozygous knock-outs. Mice with Klhl1 specifically deleted in only Purkinje cells had a loss of motor coordination that was almost identical to the total-tissue deletion mice. Finally, we found significant Purkinje cell dendritic deficits, as measured by the thickness of the molecular layer, in all mice in which Klhl1 was deleted (both total and Purkinje cell-specific deletions) and an intermediate reduction in molecular layer thickness in mice with reduced levels of Klhl1 expression (heterozygous deletions). The results from this mouse model show that even a partial loss of Klhl1 function leads to degeneration of Purkinje cell function and indicates that loss of KLHL1 activity is likely to play a significant part in the underlying pathophysiology of SCA8.

Involvement of mesangial cells expressing alpha-smooth muscle actin during restorative glomerular remodeling in Thy-1.1 nephritis

The function of actin cytoskeleton in mesangial cells (MCs) during the recovering process of injured glomeruli is not fully understood. MCs in injured glomeruli express alpha-smooth muscle actin (alpha-SMA), which is not detected in normal glomeruli. We focused on the localization of alpha-SMA in MCs of Thy-1.1 nephritic rat. Expression of alpha-SMA in the injured glomeruli peaked at day 5 after antibody injection and then declined gradually. At day 5, MCs, where alpha-SMA was localized at their cytoplasmic processes situated in various positions, occupied the expanded mesangium. MCs expressing alpha-SMA tended to be located at the peripheral region close to the glomerular basement membrane (GBM) or endothelial cells at day 8. Localization of alpha-SMA within the peripheral MCs was restricted to the cytoplasmic processes radiating toward the GBM and touching it with their tips at day 8. These alpha-SMA-containing processes are suitable to transmit the contractile force to GBM and may contribute to normalize the expanded glomerular volume. In addition, an actin-binding protein, drebrin, was localized in all MC processes extending toward various directions throughout the course of nephritis, suggesting that drebrin is involved in the formation of MC processes.

The postsynaptic density

Glutamatergic synapses in the central nervous system are characterized by an electron-dense web underneath the postsynaptic membrane; this web is called the postsynaptic density (PSD). PSDs are composed of a dense network of several hundred proteins, creating a macromolecular complex that serves a wide range of functions. Prominent PSD proteins such as members of the MaGuk or ProSAP/Shank family build up a dense scaffold that creates an interface between clustered membrane-bound receptors, cell adhesion molecules and the actin-based cytoskeleton. Moreover, kinases, phosphatases and several proteins of different signalling pathways are specifically localized within the spine/PSD compartment. Small GTPases and regulating proteins are also enriched in PSDs being the molecular basis for regulated structural changes of cytoskeletal components within the synapse in response to external or internal stimuli, e.g. synaptic activation. This synaptic rearrangement (structural plasticity) is a rapid process and is believed to underlie learning and memory formation. The characterization of synapse/PSD proteins is especially important in the light of recent data suggesting that several mental disorders have their molecular defect at the synapse/PSD level.

In vivo, competitive blockade of N-methyl-D-aspartate receptors induces rapid changes in filamentous actin and drebrin A distributions within dendritic spines of adult rat cortex

In vitro studies have demonstrated that prolonged N-methyl-D-aspartate receptor (NMDAR) blockade triggers a homeostatic up-regulation of NMDARs at synapses. Such upregulation can also be seen within 30 min in vivo in adult rats, implicating trafficking of reserve pools of NMDARs. Here, we evaluated the involvement of filamentous actin (F-actin), the major cytoskeletal component in spines, in this rapid in vivo homeostatic response, using biotinylated phalloidin as its probe. We also immuno-labeled spines for drebrin A, an F-actin-binding protein found at excitatory synapses and with a proposed role of modulating F-actin's cross-linking with one another and interactions with NMDARs. Quantitative 2-D analysis of ultrastructural images revealed that NMDAR blockade increased filamentous actin labeling per spine by 62.5% (P<0.005). The proportion of dendritic spines immuno-labeled for drebrin A also increased significantly, from 67.5% to 85% following NMDAR blockade (P<0.001), especially among larger spines. The frequency distributions of spine widths and postsynaptic density lengths were not affected by the D-(+)-2-amino-5-phosphonopentanoic acid (D-APV) treatment. However, the average postsynaptic density length was reduced by 25 nm among the fewer, drebrin A immuno-negative spines, indicating that drebrin A confers stability to synapse size. We propose that, in a homeostatic in vivo response, increases of drebrin A and F-actin within spines can enhance NMDAR trafficking by reducing cytoskeletal rigidity within the spine cytoplasm without changing the overt morphology of axo-spinous synapses. Alternatively or in addition, the cytoskeletal redistribution within spine cytoplasm may be triggered by the D-APV-induced, homeostatic up-regulation of NMDAR.

alpha-Actinin-dependent cytoskeletal anchorage is important for ICAM-5-mediated neuritic outgrowth

Intercellular adhesion molecule-5 (ICAM-5, telencephalin) is a dendrite-expressed membrane glycoprotein of telencephalic neurons in the mammalian brain. By deletion of the cytoplasmic and membrane-spanning domains of ICAM-5, we observed that the membrane distribution of ICAM-5 was determined by the cytoplasmic portion. Therefore we have characterized the intracellular associations of ICAM-5 by using a bacterially expressed glutathione S-transferase (GST) fusion protein encompassing the cytoplasmic part of ICAM-5. One of the main proteins in the neuronal cell line Paju that bound to the ICAM-5 cytodomain was alpha-actinin. ICAM-5 expressed in transfected Paju cells was found in alpha-actinin immunoprecipitates, and ICAM-5 colocalized with alpha-actinin both in Paju cells and in dendritic filopodia and spines of primary hippocampal neurons. We were also able to coprecipitate alpha-actinin from rat brain homogenate. Binding to alpha-actinin appeared to be mediated mainly through the N-terminal region of the ICAM-5 cytodomain, as the ICAM-5(857-861) cytoplasmic peptide (KKGEY) mediated efficient binding to alpha-actinin. Surface plasmon resonance analysis showed that the turnover of the interaction was rapid. In a mutant cell line, Paju-ICAM-5-KK/AA, the distribution was altered, which implies the importance of the lysines in the interaction. Furthermore, we found that the ICAM-5/alpha-actinin interaction is involved in neuritic outgrowth and the ICAM-5(857-861) cytoplasmic peptide induced morphological changes in Paju-ICAM-5 cells. In summary, these results show that the interaction between ICAM-5 and alpha-actinin is mediated through binding of positively charged amino acids near the transmembrane domain of ICAM-5, and this interaction may play an important role in neuronal differentiation.

Cytoskeleton changes following differentiation of N1E-115 neuroblastoma cell line

No systematic approach to detect expression of differentiation-related elements was published so far. The undifferentiated N1E-115 neuroblastoma cell line was switched into a neuronal phenotype by DMSO treatment and used for proteomic experiments. We used two-dimensional gel electrophoresis followed by unambiguous mass spectrometrical identification of proteins to generate a map of cytoskeleton proteins (CPs), i.e., to search for differentiation-related structures. Alpha-actin, actin-like protein 6A, gamma-tubulin complex component 2, tubulin alpha 3/alpha 7, CLIP associating protein 2, B4 integrin interactor homolog were detectable in the undifferentiated cell line exclusively and neuron-specific CPs drebrin and presynaptic density protein 95, actin-related protein 2/3, alpha and beta-centractin, PDZ-domain actin binding protein, actinin alpha 1, profilin II, ezrin, coactosin-like protein, transgelin 2, myosin light polypeptide 6, tubulin alpha 2, 6 and 7, beta tubulin (94% similar with tubulin beta-2), tubulin beta 3, tubulin tyrosine ligase-like protein 1, lamin B1 and keratin 20 were observed in the differentiated cell line only. We herein identified differentiation-related expressional patterns thus providing new evidence for the role of CPs in the process of neuronal differentiation.

Rac1 induces the clustering of AMPA receptors during spinogenesis

Glutamatergic synapses switch from nonspiny synapses to become dendritic spines during early neuronal development. Here, we report that the lack of sufficient Rac1, a small RhoGTPase, contributes to the absence of spinogenesis in immature neurons. The overexpression of green fluorescence protein-tagged wild-type Rac1 initiated the formation of dendritic spines in cultured dissociated hippocampal neurons younger than 11 d in vitro, indicating that Rac1 is likely one of the missing pieces responsible for the lack of spines in immature neurons. The overexpression of wild-type Rac1 also induced the clustering of AMPA receptors (AMPARs) and increased the amplitude of miniature EPSCs (mEPSCs). The expression of constitutively active Rac1 induced the formation of unusually large synapses with large amounts of AMPAR clusters. Also, our live imaging experiments revealed that the contact of an axon induced the clustering of Rac1, and subsequent morphological changes led to spinogenesis. Additionally, the overexpression of wild-type Rac1 and constitutively active Rac1 increased the size of preexisting spines and the amplitude of mEPSCs in mature neurons (>21 d in vitro) within 24 h after transfection. Together, these results indicate that activation of Rac1 enhances excitatory synaptic transmission by recruiting AMPARs to synapses during spinogenesis, thus providing a mechanistic link between presynaptic and postsynaptic developmental changes. Furthermore, we show that Rac1 has two distinct roles at different stages of neuronal development. The activation of Rac1 initiates spinogenesis at an early stage and regulates the function and morphology of preexisting spines at a later stage.

Activation of N-methyl-d-aspartate receptor induces a shift of drebrin distribution: Disappearance from dendritic spines and appearance in dendritic shafts

Drebrin is a major actin-filament-binding protein localized in mature dendritic spines. A recent in vivo immunoelectron microscopic study suggests that drebrin content at each dendritic spine is regulated by some unknown mechanisms. In the present in vitro study, we examined whether glutamate stimulation alters drebrin content in dendritic spines. Glutamate stimulation induced disappearance of drebrin immunostaining from dendritic spines but led to appearance of drebrin immunostaining in dendritic shafts and somata. The glutamate-induced shift of drebrin immunostaining was blocked by an NMDA receptor antagonist. Immunoblot analyses showed that both the total and the cytosolic drebrin remained unchanged and revealed that the drebrin shift was not due to drebrin degradation. These findings indicate that NMDA receptor activation induces a shift in subcellular distribution of drebrin associated with actin filaments, and that the shift might be a molecular basis for actin reorganization accompanied with synaptic plasticity.

The 4.1 protein coracle mediates subunit-selective anchoring of Drosophila glutamate receptors to the postsynaptic actin cytoskeleton

Glutamatergic Drosophila neuromuscular junctions contain two spatially, biophysically, and pharmacologically distinct subtypes of postsynaptic glutamate receptor (GluR). These receptor subtypes appear to be molecularly identical except that A receptors contain the subunit GluRIIA (but not GluRIIB), and B receptors contain the subunit GluRIIB (but not GluRIIA). A- and B-type receptors are coexpressed in the same cells, in which they form homotypic clusters. During development, A- and B-type receptors can be differentially regulated. The mechanisms that allow differential segregation and regulation of A- and B-type receptors are unknown. Presumably, A- and B-type receptors are differentially anchored to the membrane cytoskeleton, but essentially nothing is known about how Drosophila glutamate receptors are localized or anchored. We identified coracle, a homolog of mammalian brain 4.1 proteins, in yeast two-hybrid and genetic screens for proteins that interact with and localize Drosophila glutamate receptors. Coracle interacts with the C terminus of GluRIIA but not GluRIIB. To test whether coracle is required for glutamate receptor localization, we immunocytochemically and electrophysiologically examined receptors in coracle mutants. In coracle mutants, synaptic A-type receptors are lost, but there is no detectable change in B-type receptor function or localization. Pharmacological disruption of postsynaptic actin phenocopies the coracle mutants, suggesting that A-type receptors are anchored to the actin cytoskeleton via coracle, whereas B-type receptors are anchored at the synapse by another (yet unknown) mechanism.

Stability of the distribution of spines containing drebrin A in the sensory cortex layer I of mice expressing mutated APP and PS1 genes

Post-mortem cortices from patients diagnosed with Alzheimer's disease (AD) exhibit reduced levels of drebrin, an F-actin binding protein of dendritic spines and shafts. We used a mouse model of familial AD (FAD) to determine whether the density of cortical spines engaged in asymmetric (presumably excitatory) synapses and containing drebrin A is reduced and if so, whether this occurs prior to the emergence of beta amyloid deposits, when only soluble beta amyloid (Abeta) is present. Quantitative electron microscopic immunocytochemistry revealed that by 6 months, the proportion of postsynaptic spines with drebrin A within somatosensory cortex layer I was smaller for the FAD model mice, when compared to the corresponding region of WT mice (P < 0.0005). However, the areal density of postsynaptic spines containing drebrin A was relatively constant from 3 to 18 months and beyond for both genotypes, suggesting that drebrin A confers stability to postsynaptic spines. Further measurements confirmed that the reduced proportion of drebrin A-containing spines in brains of FAD mice at 6 months is due to the greater size and areal density of spine profiles lacking drebrin A. Thus, soluble Abeta could affect spines lacking drebrin A more strongly than spines containing drebrin A. At 6 months and older, a larger fraction of spinous drebrin A in 2xKI mice was located near the synaptic membrane, as compared to those of WT mice. This pattern may reflect an altered trafficking of synaptic molecules within spines, a factor adding to the decline of synaptic function and plasticity.

Overexpression of drebrin A in immature neurons induces the accumulation of F-actin and PSD-95 into dendritic filopodia, and the formation of large abnormal protrusions

Drebrin A is a neuron-specific F-actin binding protein, and plays a pivotal role in the spine formation. In this study, we expressed drebrin A tagged with green fluorescent protein (GFP-DA) in hippocampal neurons at 7-9 days in vitro when presynaptic terminals are not fully maturated. GFP-DA was accumulated in dendritic protrusions and formed large abnormal structures. Since these structures were similar to filopodia in terms of lack of MAP2 immunostaining, we named them "megapodia" meaning large dendritic filopodia. F-actin and PSD-95 were also accumulated in megapodia, and their amounts were significantly correlated with that of GFP-DA. However, the expression of GFP-DA did not result in the promotion of the morphological change from filopodia into spines. These results demonstrate that drebrin A accumulates spine-resident proteins via protein-protein interaction in filopodia, and suggest that the spine formation requires the concurrence of the increase of drebrin-A expression and the functional presynaptic contact.

Proteomic analysis of TRPC5- and TRPC6-binding partners reveals interaction with the plasmalemmal Na+/K+-ATPase

Mammalian transient receptor potential canonical (TRPC) genes encode a family of nonselective cation channels that are activated following stimulation of G-protein-coupled membrane receptors linked to phospholipase C. In Drosophila photoreceptor cells, TRP channels are found in large, multimolecular signaling complexes in association with the PDZ-containing scaffolding protein, INAD. A similar mammalian TRPC "signalplex" has been proposed, but has yet to be defined. In the present study, affinity-purified polyclonal antibodies against TRPC5 and TRPC6 were used to immunoprecipitate signalplex components from rat brain lysates. Immunoprecipitated proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, digested with trypsin, and sequenced by mass spectrometry. Proteins identified in the immunoprecipitates included cytoskeletal proteins spectrin, myosin, actin, drebrin, tubulin, and neurabin; endocytic vesicle-associated proteins clathrin, dynamin and AP-2; and the plasmalemmal Na(+)/K(+)-ATPase (NKA) pump. Several of these interactions were confirmed by reciprocal immunoprecipitation followed by Western blot analysis. In lysates from rat kidney, TRPC6, but not TRPC3, was found to coimmunoprecipitate with the NKA pump. Likewise, TRPC6, stably expressed in human embryonic kidney (HEK) cells, coimmunoprecipitated with endogenous NKA and colocalized with the pump to the plasmalemma when examined by immunofluorescence microscopy. Cell surface biotinylation experiments in intact HEK cells, confirmed that both the Na(+) pump and TRPC6 were present in the surface membrane and appeared to interact. Lastly, TRPC6 coimmunoprecipitated with the NKA pump when the proteins were coexpressed in Spodoptera frugiperda insect cells using recombinant baculoviruses. These observations suggest that TRPC6 and the Na(+) pump are part of a functional complex that may be involved in ion transport and homeostasis in both the brain and kidney.

Quantitative analysis of both protein expression and serine / threonine post-translational modifications through stable isotope labeling with dithiothreitol

While phosphorylation and O-GlcNAc (cytoplasmic and nuclear glycosylation) are linked to normal and pathological changes in cell states, these post-translational modifications have been difficult to analyze in proteomic studies. We describe advances in beta-elimination / Michael addition-based approaches which allow for mass spectrometry-based identification and comparative quantification of O-phosphate or O-GlcNAc-modified peptides, as well as cysteine-containing peptides for expression analysis. The method (BEMAD) involves differential isotopic labeling through Michael addition with normal dithiothreitol (DTT) (d0) or deuterated DTT (d6), and enrichment of these peptides by thiol chromatography. BEMAD was comparable to isotope-coded affinity tags (ICAT; a commercially available differential isotopic quantification technique) in protein expression analysis, but also provided the identity and relative amounts of both O-phosphorylation and O-GlcNAc modification sites. Specificity of O-phosphate vs. O-GlcNAc mapping is achieved through coupling enzymatic dephosphorylation or O-GlcNAc hydrolysis with differential isotopic labeling. Blocking of cysteine labeling by prior oxidation of a cytosolic lysate from mouse brain allowed specific targeting of serine / threonine post-translational modifications as demonstrated through identification of 21 phosphorylation sites (5 previously reported) in a single mass spectrometry analysis. These results demonstate BEMAD is suitable for large-scale quantitative analysis of both protein expression and serine / threonine post-translational modifications.

Drebrin A is a postsynaptic protein that localizes in vivo to the submembranous surface of dendritic sites forming excitatory synapses

Drebrin A is a neuron-specific, actin binding protein. Evidence to date is from in vitro studies, consistently supporting the involvement of drebrin A in spinogenesis and synaptogenesis. We sought to determine whether drebrin A arrives at the plasma membrane of neurons, in vivo, in time to orchestrate spinogenesis and synaptogenesis. To this end, a new antibody was used to locate drebrin A in relation to electron microscopically imaged synapses during early postnatal days. Western blotting showed that drebrin A emerges at postnatal day (PNd) 6 and becomes progressively more associated with F-actin in the pellet fraction. Light microscopy showed high concentrations of drebrin A in the synaptic layers of the hippocampus and cortex. Electron microscopy revealed that drebrin A in these regions is located exclusively in dendrites both neonatally and in adulthood. In adulthood, nearly all of the synaptic drebrin A is within spines forming asymmetric excitatory synapses, verified by gamma-aminobutyric acid (GABA) negativity. At PNd7, patches of drebrin A immunoreactivity were discretely localized to the submembranous surfaces of dendrites forming slight protrusions-protospines. The drebrin A sites exhibited only thin postsynaptic densities and lacked axonal associations or were contacted by axons that contained only a few vesicles. Yet, because of their immunoreactivity to the NR2B subunit of N-methyl-D-aspartate receptors and immunonegativity of axon terminals to GABA, these could be presumed to be nascent, excitatory synapses. Thus, drebrin A may be involved in organizing the dendritic pool of actin for the formation of spines and of axospinous excitatory synapses during early postnatal periods.

Molecular mechanisms of dendritic spine development and remodeling

Dendritic spines are small protrusions that cover the surface of dendrites and bear the postsynaptic component of excitatory synapses. Having an enlarged head connected to the dendrite by a narrow neck, dendritic spines provide a postsynaptic biochemical compartment that separates the synaptic space from the dendritic shaft and allows each spine to function as a partially independent unit. Spines develop around the time of synaptogenesis and are dynamic structures that continue to undergo remodeling over time. Changes in spine morphology and density influence the properties of neural circuits. Our knowledge of the structure and function of dendritic spines has progressed significantly since their discovery over a century ago, but many uncertainties still remain. For example, several different models have been put forth outlining the sequence of events that lead to the genesis of a spine. Although spines are small and apparently simple organelles with a cytoskeleton mainly composed of actin filaments, regulation of their morphology and physiology appears to be quite sophisticated. A multitude of molecules have been implicated in dendritic spine development and remodeling, suggesting that intricate networks of interconnected signaling pathways converge to regulate actin dynamics in spines. This complexity is not surprising, given the likely importance of dendritic spines in higher brain functions. In this review, we discuss the molecules that are currently known to mediate the exquisite sensitivity of spines to perturbations in their environment and we outline how these molecules interface with each other to mediate cascades of signals flowing from the spine surface to the actin cytoskeleton.