Super-resolution imaging reveals the relationship between CaMKIIß and drebrin within dendritic spines.

Dendritic spines are unique postsynaptic structures that emerge from the dendrites of neurons. They undergo activity-dependent morphological changes known as structural plasticity. The changes involve actin cytoskeletal remodeling, which is regulated by actin-binding proteins. CaMKII is a crucial molecule in synaptic plasticity. Notably, CaMKIIß subtype is known to bind to filamentous-actin and is closely involved in structural plasticity. We have shown that CaMKIIß binds to drebrin, and is localized in spines as both drebrin-dependent and drebrin-independent pools. However, the nanoscale relationship between drebrin and CaMKIIß within dendritic spines has not been clarified. In this study, we used stochastic optical reconstruction microscopy (STORM) to examine the detailed localization of these proteins. STORM imaging showed that CaMKIIß co-localized with drebrin in the core region of spines, and localized in the submembrane region of spines without drebrin. Interestingly, the dissociation of CaMKIIß and drebrin in the core region was induced by NMDA receptor activation. In drebrin knockdown neurons, CaMKIIß was decreased in the core region but not in the submembrane region. Together it indicates that the clustering of CaMKIIß in the spine core region is dependent on drebrin. These findings suggest that drebrin-dependent CaMKIIß is in a standby state before its activation.

Nedd4-2-dependent regulation of astrocytic Kir4.1 and Connexin43 controls neuronal network activity.

Nedd4-2 is an E3 ubiquitin ligase in which missense mutation is related to familial epilepsy, indicating its critical role in regulating neuronal network activity. However, Nedd4-2 substrates involved in neuronal network function have yet to be identified. Using mouse lines lacking Nedd4-1 and Nedd4-2, we identified astrocytic channel proteins inwardly rectifying K+ channel 4.1 (Kir4.1) and Connexin43 as Nedd4-2 substrates. We found that the expression of Kir4.1 and Connexin43 is increased upon conditional deletion of Nedd4-2 in astrocytes, leading to an elevation of astrocytic membrane ion permeability and gap junction activity, with a consequent reduction of γ-oscillatory neuronal network activity. Interestingly, our biochemical data demonstrate that missense mutations found in familial epileptic patients produce gain-of-function of the Nedd4-2 gene product. Our data reveal a process of coordinated astrocytic ion channel proteostasis that controls astrocyte function and astrocyte-dependent neuronal network activity and elucidate a potential mechanism by which aberrant Nedd4-2 function leads to epilepsy.

Impacts of methotrexate on survival, dendrite development, and synapse formation of cortical neurons.

Methotrexate (MTX) is an anti-metabolite that has been used for the treatment of patients of acute lymphocytic leukemia or non-Hodgikin lymphoma for decades. In some cases, MTX-treated patients suffer from neurological side effects, including seizures and cognitive dysfunctions. While most patients are at developmental stages, information of the mechanisms of the side effects of MTX treatment on the developing neurons has been limited. Neurons develop in five steps in the human brain: neurogenesis, polarity formation, dendrite and axon development, synapse formation, and neuronal death. Except for neurogenesis, these processes can be recapitulated in the primary culture system of cortical neurons. Using primary cultured cortical neurons, we studied the impact of MTX treatment on dendrite development, synapse formation, and neuronal death in the present report. MTX treatment impaired neuronal survival, dendrite development, and synapse formation. Interestingly, half maximal effective concentrations (EC50 s) of MTX for all three processes are at the similar range and lower than the MTX concentration in the cerebrospinal fluid in treated patients. Our results provide possible mechanisms of neurological side effects in treated patients.

Easy and Reproducible Low-Density Primary Culture using Frozen Stock of Embryonic Hippocampal Neurons.

Neuronal culture is a valuable system for evaluating synaptic functions and drug screenings. In particular, a low-density culture of primary hippocampal neurons allows the study of individual neurons or subcellular components. We have shown subcellular protein localization within a neuron by immunocytochemistry, neuronal polarity, synaptic morphology, and its developmental change using a low-density primary hippocampal culture. Recently, ready-to-use frozen stocks of neurons have become commercially available. These frozen stocks of neurons reduce the time needed to prepare animal experiments and also contribute to the reduction of the number of animals used. Here, we introduce a reproducible low-density primary culture method using a 96-well plate. We used a commercially available frozen stock of neurons from the rat embryonic hippocampus. The neurons can be stably cultured long-term without media changes by reducing the growth of glial cells at particular timepoints. This high-throughput assay using low-density culture allows reproducible imaging-based evaluations of synaptic plasticity.

The murine ortholog of Kaufman oculocerebrofacial syndrome gene Ube3b is crucial for the maintenance of the excitatory synapses in the young adult stage.

Kaufman oculocerebrofacial syndrome (KOS) is an autosomal recessive developmental disorder. Inactivating mutations in UBE3B, an E3 ubiquitin ligase gene are causative for KOS. We have reported that towards postnatal week three, its murine ortholog, Ube3b, acts as a negative regulator of the number of dendritic spines. In this study, we investigated the role of Ube3b at the synapse in the young adult mice. With an improved estimation method, images from the hippocampal CA1 and CA2 regions acquired with 3D Stimulated Emission Depletion (3D-STED) microscopy were used to quantify the excitatory synapse numbers. In the young adult mice, the excitatory synapse density was decreased in brain-specific Ube3b conditional knockout mice as compared to the control. Our results indicate the novel role of Ube3b in the maintenance of synapse numbers in the young adult period.

Impairment of ciliary dynamics in an APP knock-in mouse model of Alzheimer's disease.

The primary cilium is a specialized microtubule-based sensory organelle that extends from the cell body of nearly all cell types. Neuronal primary cilia, which have their own unique signaling repertoire, are crucial for neuronal integrity and the maintenance of neuronal connectivity throughout adulthood. Dysfunction of cilia structure and ciliary signaling is associated with a variety of genetic syndromes, termed ciliopathies. One of the characteristic features of human ciliopathies is impairment of memory and cognition, which is also observed in Alzheimer's disease (AD). Amyloid ß peptide (Aß) is produced through the proteolytic processing of amyloid precursor protein (APP), and Aß accumulation in the brain is proposed to be an early toxic event in the pathogenesis of AD. To evaluate the effect of increased Aß level on primary cilia, we assessed ciliary dynamics in hippocampal neurons in an APP knock-in AD model (AppNL-G-F mice) compared to that in wild-type mice. Neuronal cilia length in the CA1, CA3, and dentate gyrus (DG) of wild-type mice increased significantly with age. In AppNL-G-F mice, such elongation was detected in the DG but not in the CA1 and CA3, where more Aß accumulation was observed. We further demonstrated that Aß1-42 treatment decreased cilia length both in hTERT-RPE1 cells and dissociated rat hippocampal neurons. There is growing evidence that reduced cilia length is associated with perturbations of synaptic connectivity and dendrite complexity. Thus, our observations raise the important possibility that structural alterations in neuronal cilia might have a role in AD development.

Super-resolved 3D-STED microscopy identifies a layer-specific increase in excitatory synapses in the hippocampal CA1 region of Neuroligin-3 KO mice.

The chemical synapse is one type of cell-adhesion system that transmits information from a neuron to another neuron in the complex neuronal network in the brain. Synaptic transmission is the rate-limiting step during the information processing in the neuronal network and its plasticity is involved in cognitive functions. Thus, morphological and electrophysiological analyses of synapses are of particular importance in neuroscience research. In the current study, we applied super-resolved three-dimensional stimulated emission depletion (3D-STED) microscopy for the morphological analyses of synapses. This approach allowed us to estimate the precise number of excitatory and inhibitory synapses in the mouse hippocampal tissue. We discovered a region-specific increase in excitatory synapses in a model mouse of autism spectrum disorder, Neuroligin-3 KO, with this method. This type of analysis will open a new field in developmental neuroscience in the future.

Ciliary GPCR-based transcriptome as a key regulator of cilia length control.

The primary cilium is a plasma membrane-protruding sensory organelle that efficiently conveys signaling cascades in a highly ordered microenvironment. Its signaling is mediated, in part, by a limited set of GPCRs preferentially enriched in the cilium membrane. This includes melanin-concentrating hormone (MCH) receptor 1 (MCHR1), which plays a role in feeding and mood. In addition to its receptor composition, the length of the cilium is a characteristic parameter that is implicated in its function. We previously found that MCH can dynamically shorten cilia length via the Gi/o and Akt pathways in both MCHR1-expressing hTERT-RPE1 cells (hRPE1 cells) and rat hippocampal neurons. However, the detailed mechanisms by which MCH regulates cilia length through ciliary MCHR1 remains unclear. In this study, we aimed to determine the transcriptome changes in MCHR1-expressing hRPE1 cells in response to MCH to identify the target molecules involved in cilia length regulation via MCHR1 activation. RNA sequencing analysis of ciliated cells subjected to MCH treatment showed upregulation of 424 genes and downregulation of 112 genes compared with static control cells. Validation by quantitative real-time PCR, knocking down, and CRISPR/Cas9-mediated knockout technology identified a molecule, PDZ and LIM domain-containing protein 5 (PDLIM5). Thus, it was considered as the most significant key factor for MCHR1-mediated shortening of cilia length. Additional analyses revealed that the actin-binding protein alpha-actinin 1/4 is a crucial downstream target of the PDLIM5 signaling pathway that exerts an effect on MCHR1-induced cilia shortening. In the endogenous MCHR1-expressing hippocampus, transcriptional upregulation of PDLIM5 and actinin 1/4, following the application of MCH, was detected when the MCHR1-positive cilia were shortened. Together, our transcriptome study based on ciliary MCHR1 function uncovered a novel and important regulatory step underlying cilia length control. These results will potentially serve as a basis for understanding the mechanism underlying the development of obesity and mood disorders.

NMDA receptor-dependent and -independent effects of natural compounds and crude drugs on synaptic states as revealed by drebrin imaging analysis.

Effective drugs that can cure cognitive impairments remain elusive. Because synaptic dysfunction has been correlated with cognitive impairments, drug development to target synaptic dysfunction is important. Recently, natural compounds and crude drugs have emerged as potential therapeutic agents for cognitive disorders. However, their effects on synaptic function remain unclear, because of lack of evaluation system with high reproducibility. We have recently developed highly reproducible in vitro high-content imaging analysis system for evaluation of synaptic function using drebrin as a marker for synaptic states. Therefore, we aimed to examine the direct effects of well-known natural compounds and crude drugs on synaptic states using this system. Rat hippocampal neurons were treated using natural compounds (nobiletin, diosgenin and tenuifolin) and crude drugs (Uncaria Hook [UH], Bezoar Bovis [BB], Coptis Rhizome [CR], Phellodendron Bark [PB] and Polygala Root [PR]). Immunocytochemical analysis was performed, and dendrite lengths and drebrin cluster densities were automatically quantified. We found that diosgenin, tenuifolin, CR, PB and PR decreased drebrin cluster densities, and the effects of PB and PR were partially dependent on N-methyl-D-aspartic acid-type glutamate receptors (NMDARs). Nobiletin and UH did not show any effects, whereas low-dose BB treatment increased drebrin cluster densities. Our results showed that diosgenin, tenuifolin, BB, CR, PB and PR appeared to directly change synaptic states. Particularly, the NMDAR dependency of PB and PR appears to affect synaptic plasticity.

Properties of primary cilia in melanin-concentrating hormone receptor 1-bearing hippocampal neurons in vivo and in vitro.

The primary cilium is a solitary organelle that organizes a sensitive signaling hub in a highly ordered microenvironment. Cilia are plastic structures, changing their length in response to bioactive substances, and ciliary length may be regulated to ensure efficient signaling capacity. Mammalian brain neurons possess primary cilia that are enriched in a set of G protein-coupled receptors (GPCRs), including the feeding-related melanin-concentrating hormone (MCH) receptor 1 (MCHR1). We previously demonstrated a novel biological phenomenon, ciliary MCHR1-mediated cilia length shortening through Gi/o and Akt signaling, using a simple cell culture model of human retinal pigmented epithelial RPE1 cells exogenously expressing MCHR1. In the present study, we characterized the properties of endogenous MCHR1-expressing primary cilia in hippocampal neurons in rodents. Using cultured dissociated rat hippocampal neurons in vitro, we showed that MCH triggered cilia length reduction involved in MCHR1-Gi/o and -Akt signaling. In rat hippocampal slice cultures with preservation of the cytoarchitecture and cell populations, ciliary MCHR1 was abundantly located in the CA1 and CA3 regions, but not in the dentate gyrus. Notably, treatment of slice cultures with MCH induced Gi/o- and Akt-dependent cilia shortening in the CA1 region without influencing cilia length in the CA3 region. Regarding the in vivo mouse brain, we observed higher levels of ciliary MCHR1 in the CA1 and CA3 regions as well as in slice cultures. In the starved state mice, a marked increase in MCH mRNA expression was detected in the lateral hypothalamus. Furthermore, MCHR1-positive cilia length in the hippocampal CA1 region was significantly shortened in fasted mice compared with fed mice. The present findings focused on the hippocampus provide a potential approach to investigate how MCHR1-driven cilia shortening regulates neuronal activity and physiological function toward feeding and memory tasks.

Postnatal Development of Functional Projections from Parasubiculum and Presubiculum to Medial Entorhinal Cortex in the Rat.

Neurons in parasubiculum (PaS), presubiculum (PrS), and medial entorhinal cortex (MEC) code for place (grid cells) and head direction. Directional input has been shown to be important for stable grid cell properties in MEC, and PaS and PrS have been postulated to provide this information to MEC. In line with this, head direction cells in those brain areas are present at postnatal day 11 (P11), having directional tuning that stabilizes shortly after eye opening, which is before premature grid cells emerge in MEC at P16. Whether functional connectivity between these structures exists at those early postnatal stages is unclear. Using anatomical tracing, voltage-sensitive dye imaging and single-cell patch recordings in female and male rat brain slices between P2 and P61, we determined when the pathways from PaS and PrS to MEC emerge, become functional, and how they develop. Anatomical connections from PaS and PrS to superficial MEC emerge between P4 and P6. Monosynaptic connectivity from PaS and PrS to superficial MEC was measurable from P9 to P10 onward, whereas connectivity with deep MEC was measurable from P11 to P12. From P14/P15 on, reactivity of MEC neurons to parasubicular and presubicular inputs becomes adult-like and continues to develop until P28-P30. The maturation of the efficacy of both inputs between P9 and P21 is paralleled by maturation of morphological properties, changes in intrinsic properties of MEC principal neurons, and changes in the GABAergic network of MEC. In conclusion, synaptic projections from PaS and PrS to MEC become functional and adult-like before the emergence of grid cells in MEC.SIGNIFICANCE STATEMENT Head direction information, crucial for grid cells in medial entorhinal cortex (MEC), is thought to enter MEC via parasubiculum (PaS) and presubiculum (PrS). Unraveling the development of functional connections between PaS, PrS, and MEC is key to understanding how spatial navigation, an important cognitive function, may evolve. To gain insight into the development, we used anatomical tracing techniques, voltage-sensitive dye imaging, and single-cell recordings. The combined data led us to conclude that synaptic projections from PaS and PrS to MEC become functional and adult-like before eye opening, allowing crucial head direction information to influence place encoding before the emergence of grid cells in rat MEC.

High-content imaging analysis for detecting the loss of drebrin clusters along dendrites in cultured hippocampal neurons.

INTRODUCTION: Detection of drug effects on neuronal synapses is important for predicting their adverse effects. We have used drebrin as a marker to detect the synaptic changes in cultured neurons. High concentration of glutamate decreases the amount of drebrin in synapses. To increase the availability of this method for high throughput analysis, we applied the drebrin-based evaluation of synapses to high-content imaging analysis using microplates. METHODS: Three weeks old cultured neurons were fixed and processed for immunocytochemistry to visualize drebrin clusters, dendrites and neuronal cell bodies. After automated image acquisition, total number of drebrin clusters per fields, linear density of drebrin cluster along dendrites, dendrite length and neuron number were automatically measured by a custom-designed protocol. RESULTS: Automated image acquisition and analysis showed that dendrite length and drebrin cluster density along dendrites are measured consistently and reproducibly. In addition, application of 10-100 µM glutamate for 10 min or 0.5-50 µM latrunculin A for 5 min significantly decreased drebrin cluster density without affecting neuron number. These results were consistent with our previous results using manual image acquisition and analysis with regular fluorescence microscope and image analysis software. Furthermore, 0.3 or 1.0 µM staurosporine for 24 h significantly decreased neuron number. DISCUSSION: The present study demonstrates that this high-throughput imaging analysis of drebrin cluster density along dendrites for detecting the effects of substances on synapses is sensitive enough to detect the effects of glutamate receptor activation and latrunculin A treatment, and indicates that this analysis will be useful for safety pharmacology study.

Characterization of Functional Primary Cilia in Human Induced Pluripotent Stem Cell-Derived Neurons.

Recent advances in human induced pluripotent stem cells (hiPSCs) offer new possibilities for biomedical research and clinical applications. Neurons differentiated from hiPSCs may be promising tools to develop novel treatment methods for various neurological diseases. However, the detailed process underlying functional maturation of hiPSC-derived neurons remains poorly understood. Here, we analyze the developmental architecture of hiPSC-derived cortical neurons, iCell GlutaNeurons, focusing on the primary cilium, a single sensory organelle that protrudes from the surface of most growth-arrested vertebrate cells. To characterize the neuronal cilia, cells were cultured for various periods and evaluated immunohistochemically by co-staining with antibodies against ciliary markers Arl13b and MAP2. Primary cilia were detected in neurons within days, and their prevalence and length increased with increasing days in culture. Treatment with the mood stabilizer lithium led to primary cilia length elongation, while treatment with the orexigenic neuropeptide melanin-concentrating hormone caused cilia length shortening in iCell GlutaNeurons. The present findings suggest that iCell GlutaNeurons develop neuronal primary cilia together with the signaling machinery for regulation of cilia length. Our approach to the primary cilium as a cellular antenna can be useful for both assessment of neuronal maturation and validation of pharmaceutical agents in hiPSC-derived neurons.

Assessment of NMDA receptor inhibition of phencyclidine analogues using a high-throughput drebrin immunocytochemical assay.

INTRODUCTION: In recent years, new psychoactive substances (NPS) have been widely distributed for abuse purposes. Effective measures to counter the spread of NPS are to promptly legislate them through the risk assessment. Phencyclidine analogues having inhibitory effects toward NMDA receptor (NMDAR) have recently emerged in Japan. Therefore, it is important to establish a high-throughput system for efficiently detecting NPS that can inhibit NMDAR activity. METHODS: Hippocampal neurons prepared from embryonic rats were incubated in 96-well microplates. After 3 weeks in vitro, cultured neurons were preincubated with phencyclidine (PCP) or PCP-analogues, including 3-methoxyphencyclidine (3-MeO-PCP) and 4-[1-(3-methoxyphenyl)cyclohexyl]morpholine (3-MeO-PCMo), and then treated with 100 µM glutamate for 10 min. After fixation, cultured neurons were immunostained with anti-drebrin and anti-MAP2 antibodies. The linear cluster density of drebrin along the dendrites was automatically quantified using a protocol that was originally developed by us. RESULTS: The high-throughput immunocytochemical assay, measuring drebrin cluster density of cultured neurons, demonstrated that glutamate-induced reduction of drebrin cluster density in 96-well plates is competitively inhibited by NMDAR antagonist, APV. The reduction was also antagonized by PCP, 3-MeO-PCP and 3-MeO-PCMo. The inhibitory activity of 3-MeO-PCMo was lower than that of PCP or 3-MeO-PCP, with IC50 values of 26.67 µM (3-MeO-PCMo), 2.02 µM (PCP) and 1.51 µM (3-MeO-PCP). DISCUSSION: The relative efficacy among PCP, 3-MeO-PCP and 3-MeO-PCMo calculated from IC50 are similar to those from Ki values. This suggests that the high-throughput imaging analysis is useful to speculate the Ki values of new PCP analogues without performing the kinetic studies.

Localization of Drebrin: Light Microscopy Study.

Developmental changes in the expression and localization of drebrin has been mainly analyzed in chick embryo and young rat by various anti-drebrin polyclonal and monoclonal antibodies. Immunoblot analysis demonstrated that the adult drebrin isoform (drebrin A) is restricted to neural tissues, while the embryonic drebrin isoforms (drebrin E1 and E2 in chicken and drebrin E in mammals) are found in a wide variety of tissues. In the developing brain, drebrin E (including chicken drebrin E2) is expressed in newly generated neurons. During neuronal migration, drebrin E is distributed ubiquitously within the neurons. Once drebrin A is expressed in the developing neuron, drebrin E is no longer present within the cell soma and accumulates in the growth cone of growing processes, resulting in the cessation of neuronal migration. The limited subcellular localization of drebrin A, which is possibly regulated by a drebrin A-specific mechanism, is likely to affect the localization of drebrin E. In the adult brain, drebrin is mainly localized in dendritic spines, but in some nuclei, drebrin can be detected in neuronal somata as well as dendritic spines. The fact that the developmental changes in drebrin expression highly correlate in time with the sensitive period of visual cortical plasticity in kittens suggests that synaptic plasticity depends on drebrin.

Role of Drebrin in Synaptic Plasticity.

Synaptic plasticity underlies higher brain function such as learning and memory, and the actin cytoskeleton in dendritic spines composing excitatory postsynaptic sites plays a pivotal role in synaptic plasticity. In this chapter, we review the role of drebrin in the regulation of the actin cytoskeleton during synaptic plasticity, under long-term potentiation (LTP) and long-term depression (LTD). Dendritic spines have two F-actin pools, drebrin-decorated stable F-actin (DF-actin) and drebrin-free dynamic F-actin (FF-actin). Resting dendritic spines change their shape, but are fairly constant over time at steady state because of the presence of DF-actin. Accumulation of DF-actin is inversely regulated by the intracellular Ca2+ concentration. However, LTP and LTD stimulation induce Ca2+ influx through N-methyl-D-aspartate (NMDA) receptors into the potentiated spines, resulting in drebrin exodus via myosin II ATPase activation. The potentiated spines change to excited state because of the decrease in DF-actin and thus change their shape robustly. In LTP, the Ca2+ increase via NMDA receptors soon returns to the basal level, and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) expression at the postsynaptic membrane is increased. The Ca2+ recovery and AMPAR increase coordinately induce the re-accumulation of DF-actin and change the dendritic spines from the excited state to steady state during LTP maintenance. During LTD, the prolonged intracellular Ca2+ increase inhibits the re-accumulation of DF-actin, resulting in facilitation of AMPAR endocytosis. Because of the positive feedback loop of the AMPAR decrease and drebrin re-accumulation inhibition, the dendritic spines are instable during LTD maintenance. Taken together, we propose the presence of resilient spines at steady state and plastic spines at excited state and discuss the physiological and pathological relevance of the two-state model to synaptic plasticity.

Drebrin E regulates neuroblast proliferation and chain migration in the adult brain.

F-actin-binding protein drebrin has two major isoforms: drebrin A and drebrin E. Drebrin A is the major isoform in the adult brain and is highly concentrated in dendritic spines, regulating spine morphology and synaptic plasticity. Conversely, drebrin E is the major isoform in the embryonic brain and regulates neuronal morphological differentiation, but it is also expressed in neurogenic regions of the adult brain. The subventricular zone (SVZ) is one of the brain regions where adult neurogenesis occurs. Neuroblasts migrate to the olfactory bulb (OB) and integrate into existing neuronal networks, after which drebrin expression changes from E to A, suggesting that drebrin E plays a specific role in neuroblasts in the adult brain. Therefore, to understand the role of drebrin E in the adult brain, we immunohistochemically analyzed adult neurogenesis using drebrin-null-mutant (DXKO) mice. In DXKO mice, the number of neuroblasts and cell proliferation decreased, although cell death remained unchanged. These results suggest that drebrin E regulates cell proliferation in the adult SVZ. Surprisingly, the decreased number of neuroblasts in the SVZ did not result in less neurons in the OB. This was because the survival rate of newly generated neurons in the OB increased in DXKO mice. Additionally, when neuroblasts reached the OB, the change in the migratory pathway from tangential to radial was partly disturbed in DXKO mice. These results suggest that drebrin E is involved in a chain migration of neuroblasts.

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".

The role of drebrin in dendritic spines.

Dendritic spines form typical excitatory synapses in the brain and their shapes vary depending on synaptic inputs. It has been suggested that the morphological changes of dendritic spines play an important role in synaptic plasticity. Dendritic spines contain a high concentration of actin, which has a central role in supporting cell motility, and polymerization of actin filaments (F-actin) is most likely involved in spine shape changes. Drebrin is an actin-binding protein that forms stable F-actin and is highly accumulated within dendritic spines. Drebrin has two isoforms, embryonic-type drebrin E and adult-type drebrin A, that change during development from E to A. Inhibition of drebrin A expression results in a delay of synapse formation and inhibition of postsynaptic protein accumulation, suggesting that drebrin A has an important role in spine maturation. In mature synapses, glutamate stimulation induces rapid spine-head enlargement during long-term potentiation (LTP) formation. LTP stimulation induces Ca2+ entry through N-methyl-d-aspartate (NMDA) receptors, which causes drebrin exodus from dendritic spines. Once drebrin exits from dendritic spine heads, the dynamic actin pool increases in spine heads to facilitate F-actin polymerization. To maintain enlarged spine heads, drebrin-decorated F-actin is thought to reform within the spine heads. Thus, drebrin plays a pivotal role in spine plasticity through regulation of F-actin.