Drebrin A regulates hippocampal LTP and hippocampus-dependent fear learning in adult mice

Structural plasticity of dendritic spines, which underlies higher brain functions including learning and memory, is dynamically regulated by the actin cytoskeleton and its associated proteins. Drebrin A is an F-actin-binding protein preferentially expressed in the brain and localized in the dendritic spines of mature neurons. Isoform conversion from drebrin E to drebrin A and accumulation of the latter in dendritic spines occurs during synapse maturation. We have previously demonstrated that drebrin A plays a pivotal role in spine morphogenesis and plasticity. However, it is unclear whether drebrin A plays a specific role in processes required for structural plasticity, and whether drebrin E can substitute in this role. To answer these questions, we analyzed mutant mice (named DAKO mice), in which isoform conversion from drebrin E to drebrin A is disrupted. In DAKO mouse brain, drebrin E continues to be expressed throughout life instead of drebrin A. Electrophysiological studies using hippocampal slices revealed that long-term potentiation of CA1 synapses was impaired in adult DAKO mice, but not in adolescents. In parallel with this age-dependent impairment, DAKO mice exhibited impaired hippocampus-dependent fear learning in an age-dependent manner; the impairment was evident in adult mice, but not in adolescents. In addition, histological investigation revealed that the spine length of the apical dendrite of CA1 pyramidal cells was significantly longer in adult DAKO mice than in wild-type mice. Our data indicate that the roles of drebrin E and drebrin A in brain function are different from each other, that the isoform conversion of drebrin is critical, and that drebrin A is indispensable for normal synaptic plasticity and hippocampus-dependent fear memory in the adult brain.

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.

The sulphydryl reagent, N-ethylmaleimide, disrupts sleep and blocks A1 adenosine receptor-mediated inhibition of intracellular calcium signaling in the in vitro ventromedial preoptic nucleus

To explore the neuronal signaling mechanisms underlying sleep regulation in the rat, the present study examined continuous intra-third ventricle infusion of N-ethylmaleimide (NEM), a sulphydryl reagent that inhibits G(i/o) protein-coupled receptor-mediated signaling pathways. The diurnal infusion of NEM (0.01-10 micromol/10 h) dose-dependently inhibited both non-rapid eye movement sleep and rapid eye movement sleep. A maximal dose of NEM (10 micromol/10 h) dramatically inhibited day-time sleep (-57% for non-rapid eye movement sleep and -89% for rapid eye movement sleep) with a compensatory increase of sleep during the subsequent night-time (+33% for non-rapid eye movement sleep and +259% for rapid eye movement sleep). The day-time brain temperature was also increased by NEM, demonstrating effects of NEM on both sleep and body temperature levels. Immunostaining of the rat hypothalamus with a monoclonal antibody against the A1 adenosine receptor (A1R) was used to explore the distribution of a sleep-related G(i/o) protein-coupled receptor. Robust A1R-like immunoreactivity was found in the ventromedial preoptic nucleus and the supraoptic nucleus. Fura-2-based Ca(2+) imaging analysis of acute hypothalamic slices further demonstrated that the A1R agonist N(6)-cyclopentyladenosine (CPA; 200 nM) inhibited spontaneous Ca(2+) oscillations and high potassium (80 mM)-induced Ca(2+) flux in the ventromedial preoptic nucleus, while NEM (100-300 microM) and an A1R antagonist 8-cyclopentyl-dipropylxanthine (300 nM) blocked the CPA actions and increased the high potassium-induced Ca(2+) flux. From these results we suggest that NEM-sensitive G protein-coupled receptor(s) may play an important role in the regulation of sleep and body temperature in the rat and one possible mechanism is an A1R-mediated regulation of intracellular Ca(2+) concentrations in the ventromedial preoptic nucleus.

Drebrin expression is increased in spinal motoneurons of rats after axotomy

Drebrin has been known to act on actin filaments at dendritic spines to cause morphological change, and might be related to the plasticity of synaptic transmission. In the present study, changes of drebrin were examined immunohistochemically in the spinal motoneurons of rats following unilateral sciatic nerve transection. Drebrin-immunoreactivity (-ir) in the motoneurons was significantly increased on the lesioned side after 3 days. Confocal laser-scanning microscopic images of the motoneurons revealed conspicuous increase in drebrin in the submembranous region of the cells. After 10 weeks, drebrin-ir on the lesioned side decreased to a level not significantly different from that on the unlesioned side. The results suggested that drebrin played important roles in synaptic restoration in axotomized motoneurons.

Molecular cloning and dendritic localization of rat SH3P7

SH3P7 was originally isolated by cloning SH3 domain ligand targets from a mouse embryo cDNA library. SH3P7 is an actin-binding protein implicated in antigen reception, JNK1 signalling, and Rac activation. It contains a drebrin homology sequence in its N-terminal region and a cortactin homology sequence (SH3 domain) in its C-terminal region. Both drebrin and cortactin are actin-binding proteins, and both have been suggested as possible regulators of the actin cytoskeleton in neurons. In the present study, we performed cDNA cloning of rat SH3P7, performed RT-PCR analysis, generated polyclonal antibodies against the recombinant rat SH3P7 protein, and examined the distribution of SH3P7 in the rat brain using immunohistochemistry. Sequence analysis revealed that there were at least four isoforms of the SH3P7 protein: SH3P7r1-SH3P7r4. RT-PCR analysis revealed that the predominant isoforms expressed in the brain were SH3P7r1 and SH3P7r3. The relative levels of isoform expression were similar among regions. Immunohistochemistry revealed that the most intense immunolabelling for SH3P7 was observed in the hippocampus and cerebellar cortex. Double-labelling studies with anti-SH3P7 antibody and other neuronal marker proteins revealed that SH3P7 was located primarily in dendrites, and in moderate amounts in cell bodies. Immunoreactivity was absent in the presynaptic terminals. In cultured astrocytes, SH3P7 was localized at protrusive structures of the cell periphery and in the cell body. We concluded that SH3P7 is ubiquitous in the rat brain, and occurs as several isoforms. Also, its dendritic localization suggests that SH3P7 is functionally linked to actin cytoskeleton organization in dendrites.

Brain Abeta amyloidosis in APPsw mice induces accumulation of presenilin-1 and tau

APPsw transgenic mice (Tg2576) overproducing mutant amyloid beta protein precursor (betaAPP) show substantial brain Abeta amyloidosis and behavioural abnormalities. To clarify the subsequent abnormalities, the disappearance of neurons and synapses and dystrophic neurite formation with accumulated proteins including hyperphosphorylated tau were examined. Tg2576 demonstrated substantial giant core plaques and diffuse plaques. The number of neurons was significantly decreased in the areas containing the amyloid cores compared with all other areas and corresponding areas in non-transgenic littermates in sections visualized by Nissl plus Congo red double staining (p<0.001). The presynaptic protein alpha-synuclein and postsynaptic protein drebrin were also absent in the amyloid cores. betaAPP and presenilin-1 were accumulated in dystrophic neurites in and around the core plaques. Tau phosphorylated at five independent sites was detected in the dystrophic neurites in the amyloid cores. Thus, the giant core plaques replaced normal brain tissues and were associated with subsequent pathological features such as dystrophic neurites and the appearance of hyperphosphorylated tau. These findings suggest a potential role for brain Abeta amyloidosis in the induction of secondary pathological steps leading to mental disturbance in Alzheimer's disease.

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

Recent technological progress has yielded great amounts of information about the molecular constituents of postsynaptic scaffolds in the dendritic spine. Actin filaments are major cytoskeletal elements in the dendritic spine, and they functionally interact with neurotransmitter receptors via regulatory actin-binding proteins. Drebrin A and alpha-actinin-2 are two major actin-binding proteins in dendritic spines. In adult brains, they are characteristically concentrated in spines, but not in dendritic shafts or cell bodies. Thus, they are part of a unique postsynaptic scaffold consisting of actin filaments, PSD protein family, and neurotransmitter receptors. Localization of NMDA receptors, actin filaments, and actin-binding proteins in spines changes in parallel with development, and in response to synaptic activity. This raises the possibility that clustering and anchoring of these characteristic molecular constituents at postsynaptic scaffolds play important roles in spine function. This article focuses on the clustering and anchoring mechanisms of NMDA receptors and actin filaments, and the involvement of actin-binding proteins, in dendritic spines, and the way in which characteristic postsynaptic scaffolds are built up.

The effects of neurotrophin-3 and brain-derived neurotrophic factor on cerebellar granule cell movement and neurite extension in vitro

Migration of the granule cells is a major stage of cerebellar maturation. Granule cells express neurotrophins and their receptors; however, their role in cell migration has not been defined. In this study we investigated the effects of exogenous neurotrophins on the movement and neurite extension of granule cells from glial-free cerebellar cell reaggregates in vitro. Our results provide direct evidence that neurotrophin-3 and brain-derived neurotrophic factor differentially affect the granule cells. Neurotrophin-3 significantly affected granule cell movements by decreasing the migration index (the ratio of the number of cells that moved further than half the neurite length) and the speed of cell soma movement, but did not affect neurite length or growth cone migration. In contrast, brain-derived neurotrophic factor and neurotrophin-4 acted on growing neurites and growth cones by significantly increasing neurite length and the speed of growth cone migration, but had no effect either on the migration index or on the speed of the cell soma movement. The results suggest that neurotrophins differentially affect neurite extension and the movements of cerebellar granule cells.

Non-muscle myosin IIB-like immunoreactivity is present at the drebrin-binding cytoskeleton in neurons

Dendritic spines are extremely motile, providing a structural mechanism for synaptic plasticity. Actin-myosin interaction is thought to be responsible for the change in the shape of spine. We have already reported that drebrin, an actin-binding protein, inhibits actin-myosin interaction and is enriched in the dendritic spine of mature neurons. In this study, we prepared the actin cytoskeleton of dendritic spines as an immunoprecipitate with anti-drebrin antibody from adult guinea-pig brain, immunized mice with the cytoskeleton, and obtained a monoclonal antibody (MAb) called MAb G650. MAb G650 reacted with non-muscle myosin IIB, but it did not react with muscle myosin II or non-muscle myosin IIA. Immunoblotting with this antibody revealed that drebrin-binding cytoskeleton contains this myosin IIB-like immunreactivity. Immunohistochemistry using MAb G650 demonstrated that this myosin IIB-like immunreactivity can be detected in the neuronal cell bodies and their apical dendrites, where drebrin is hardly detected. These data demonstrate that a myosin subtype associated with drebrin-binding actin filaments in the dendritic spines is myosin IIB, although this myosin is widely distributed in somato-dendritic subdomains of neurons. Furthermore, it is indicated that the cytoskeletons in dendritic spine were uniquely characterized with actin-binding proteins such as drebrin, but not with myosins.

Domain analysis of the actin-binding and actin-remodeling activities of drebrin

Drebrin is an actin-binding protein which is expressed at highly levels in neurons. When introduced into fibroblasts, it has been known to bind to F-actin and to cause remodeling of F-actin. Here, we performed a domain analysis of the actin-binding and actin-remodeling activities of drebrin. Various fragments of drebrin cDNA were fused with green fluorescent protein cDNA and introduced into Chinese hamster ovary cells. Association of the fusion protein with F-actin and remodeling of the F-actin were examined. We found that the central 85-amino-acid sequence (residues 233-317) was sufficient for the binding to and remodeling of F-actin. The binding activity of this fragment was relatively low compared with that of full-length drebrin, but all the types of abnormalities of F-actin that are observed with full-length drebrin were also observed with this fragment. When this sequence was further fragmented, the actin-binding activity was greatly reduced and the actin-remodeling activity disappeared. The actin-binding activity of the central region of drebrin was confirmed by a cosedimentation assay of chymotryptic fragments of drebrin with purified actin. These data indicate that the actin-binding domain and actin-remodeling domain are identical and that this domain is located at the central region of drebrin.

High level of adenosine A1 receptor-like immunoreactivity in the CA2/CA3a region of the adult rat hippocampus

We describe the immunocytochemical distribution of adenosine A1 receptors in the rat hippocampus. Adenosine A1 receptor-like immunoreactivity was seen on the cell soma and dendrites of pyramidal cells and the cell soma and proximal part of dendrites of granule cells, but not on glial cells. Developmentally, adenosine A1 receptor-like immunoreactivity was diffuse on postnatal day 7 and increased in intensity in individual cells by day 21. In the CA2/CA3a region, the adult pattern of A1 receptor distribution was established by day 28. In the adult rat hippocampus, rostrocaudal inspection revealed that immunoreactivity in CA2/CA3a was greatest. Confocal microscopy revealed differences in the staining patterns for the adenosine A receptor and synaptophysin, a marker of presynaptic terminals. This result suggests that the adenosine A1 receptor might have postsynaptic physiological functions. Double-labeling of adenosine A1 receptors and anterogradely-labeled fibers from the supramammillary nucleus showed that the fibers from the supramammillary nucleus terminate directly on the cell soma of the A1 receptor-immunopositive neurons in CA2/CA3a and the dentate gyrus. These results indicate that the adenosine A 1 receptor in CA2/CA3a and the dentate gyrus are in a position to regulate hippocampal theta activity and that resultant strong synaptic depression in CA2/CA3a could play a role in regulating the intrinsic signal flow between CA3 and CA1.

Loss of proteins regulating synaptic plasticity in normal aging of the human brain and in Alzheimer disease

Recent studies suggest that the cognitive impairment associated with normal aging is due to neuronal dysfunction rather than to loss of neurons or synapses. To characterize this dysfunction, molecular indices of neuronal function were quantified in autopsy samples of cerebral cortex. During normal aging, the most dramatic decline was found in levels of synaptic proteins involved in structural plasticity (remodeling) of axons and dendrites. Alzheimer disease, the most common cause of dementia in the elderly, was associated with an additional 81% decrease in levels of drebrin, a protein regulating postsynaptic plasticity. Disturbed mechanisms of plasticity may contribute to cognitive dysfunction during aging and in Alzheimer disease.

Change in the shape of dendritic spines caused by overexpression of drebrin in cultured cortical neurons

Dendritic spines are known to be extremely motile, providing a structural mechanism for synaptic plasticity. Actin filaments are thought to be responsible for the changes in the shape of spines. We tested our hypothesis that drebrin, an actin-binding protein, is a regulator of spine shape. In high-density long-term primary cultures of rat cerebral cortex neurons, drebrin was colocalized with actin filaments at spines. We introduced drebrin tagged with green fluorescent protein (GFP) into these neurons to test the ability of exogenous drebrin to localize at spines and the effect of overexpression of drebrin on spine shape. We observed that exogenous drebrin indeed accumulated in spines. But when the actin-binding domain of drebrin was deleted, the protein was distributed in both spines and dendritic shafts, indicating that accumulation of drebrin in the spines required its actin-binding activity. Statistical analysis of the lengths of spines as determined from confocal laser microscopic images revealed that the spines were significantly longer in GFP-drebrin-expressing neurons than in GFP-expressing neurons. The longer spines labeled with GFP-drebrin were demonstrated to be postsynaptic by double labeling of the presynaptic terminals with antibody against synaptophysin. These results directly indicate that drebrin binds to actin filaments at dendritic spines and alters spine shape.

Suppression of an actin-binding protein, drebrin, by antisense transfection attenuates neurite outgrowth in neuroblastoma B104 cells

Drebrins, actin-binding proteins, are dominantly expressed during embryogenesis and accumulated in neurite processes of postmigratory neurons. While the cytoskeletal proteins are the important factors for regulating neurite outgrowth, the cellular mechanism in neurons is still unclear. To address the role of drebrins in the neurite outgrowth, we have studied the effect of suppression of drebrin on a rat neuroblastoma B104 cell line, which constitutively expresses drebrin. Deprivation of serum or addition of gangliosides in the culture medium induced remarkable neurite outgrowth of B104 cells. We transfected B104 cells with an antisense construct of human drebrin E cDNA and found that the drebrin expression was significantly reduced in the stable antisense cell lines. In response to serum deprivation and gangliosides treatment, their ability to extend neurite processes was significantly attenuated. In contrast, the cell proliferation of the antisense transfectants was arrested by serum deprivation similar to control B104 cells. These data suggest that the drebrins are required for neurite outgrowth in neuronal cells.

Rapid conversion of drebrin isoforms during synapse formation in primary culture of cortical neurons

Easier methods to evaluate synapse formation in cultured neurons are desirable to investigate the regulatory mechanisms of synaptic development. We focused on drebrin, which changes from embryonic-type to adult-type isoform during postnatal development. The adult-type isoform of drebrin was detected by Western blotting after seventh day in primary cultured cortical neurons and the expression was coincidental with that of synaptophysin. Reverse transcription-polymerase chain reaction could demonstrate reversal of the predominant drebrin mRNA isoform from embryonic type to adult type between 8 and 13 days of culture. This is an easy and quick method to evaluate synapse formation in vitro.

Differential expression of rat brain synaptic proteins in development and aging

We have previously reported the differential involvement of synaptic proteins in Alzheimer's disease (AD). As AD is an aging-associated disease, in the present study we examined the developmental and aging-related changes in synaptic proteins such as synaptophysin, synaptobrevin, synaptotagmin, synaptosomal-associated protein 25 (SNAP-25), syntaxin 1/HPC-1 and drebrin in the rat brain. Immunoblot analyses of brain extracts from embryonic day 19 (E19) to postnatal 96-week-old rats indicated that the protein level of synaptophysin and synaptobrevin increased after birth, being highest at 24 weeks, and then decreased with aging. Synaptotagmin was detected at E19, with levels increasing after birth to 96 weeks. SNAP-25 levels were highest at 4 weeks, and then decreased with aging. Syntaxin 1/HPC-1 levels were high at E19 and 1 week, decreasing rapidly from 2 weeks onwards, and drebrin levels were highest at E19 and 1 week, and decreased during aging. The present results suggest that the expression of each synaptic protein is differentially regulated in development and aging.

Interactions of drebrin and gephyrin with profilin

Profilin is an actin monomer-binding protein which stimulates actin polymerization. Recent studies have revealed that profilin interacts with VASP, Mena, Bnilp, Bnrlp, and mDia, all of which have the proline-rich domain. Here, we isolated three profilin-binding proteins from rat brain cytosol by glutathione S-transferase-profilin affinity column chromatography and identified them as Mena, drebrin, and gephyrin. These proteins had a proline-rich domain and directly interacted with profilin.

Neurite formation induced in neuroblastoma cells and genetically altered non-neuronal cells

Ultrastructural changes associated with neurite formation were examined in suitably cultured neuroblastoma cells (Neuro-2a) and drebrin (developmentally regulated brain protein) gene-transfected fibroblasts (L cells) in culture. Both neuroblastoma cells and fibroblasts initially were flattened and epitheloid, with many microspikes or microvilli diffusely distributed over their surfaces. Intracellular organelles were abundant and diffusely arranged. In the transformed state, both cell types were round to oval with long processes where microspikes were concentrated. A constant arrangement of surface and intracellular structures was apparent, though drebrin gene-transfected fibroblasts retained some of their original characteristics. Neurite formation programmed by genes may be initiated by environmental factors in neuroblastoma cells. Neurite-like processes in fibroblasts may be formed due to changes in microfilaments resulting from transfection of the drebrin (actin-binding protein) gene.

Modulatory role of drebrin on the cytoskeleton within dendritic spines in the rat cerebral cortex

Morphological changes in the dendritic spines have been postulated to participate in the expression of synaptic plasticity. The cytoskeleton is likely to play a key role in regulating spine structure. Here we examine the molecular mechanisms responsible for the changes in spine morphology, focusing on drebrin, an actin-binding protein that is known to change the properties of actin filaments. We found that adult-type drebrin is localized in the dendritic spines of rat forebrain neurons, where it binds to the cytoskeleton. To identify the cytoskeletal proteins that associated with drebrin, we isolated drebrin-containing cytoskeletons using immunoprecipitation with a drebrin antibody. Drebrin, actin, myosin, and gelsolin were co-precipitated. We next examined the effect of drebrin on actomyosin interaction. In vitro, drebrin reduced the sliding velocity of actin filaments on immobilized myosin and inhibited the actin-activated ATPase activity of myosin. These results suggest that drebrin may modulate the actomyosin interaction within spines and may play a role in the structure-based plasticity of synapses.

Inhibition by drebrin of the actin-bundling activity of brain fascin, a protein localized in filopodia of growth cones

The purification of drebrin, an actin-binding protein that is specifically expressed in embryonic rat brain, was described previously. During the purification of drebrin, we found that an actin-binding protein of 54 kDa was also expressed at high levels in embryonic brain, and this protein was identified by immunoblotting as fascin. To explore the roles of fascin in brain development, we purified fascin from brains of infant rats and characterized it. We found that the actin-binding activity of fascin was strongly inhibited by drebrin. Fascin caused formation of actin bundles, a process that was inhibited in the presence of drebrin, as confirmed by electron microscopy and a low-speed centrifugation assay. In PC12 cells, fascin was localized in the filopodia of growth cones, whereas drebrin was localized in the basal region of growth cones. Our results suggest that fascin might play an important role in the organization of actin in filopodia and that this organization might be regulated by drebrin.

Stabilization of adhesion plaques by the expression of drebrin A in fibroblasts

The expression of drebrin A was induced in mouse fibroblasts (L cells) after transformation of cells with a vector that carried cDNA for rat drebrin A (developmentally regulated brain protein A) under the control of the promoter of the gene for metallothionein-I. When drebrin was expressed in the transformed cells (MTI-5 cells), the organization of actin filaments changed such that stress fibers were converted to a mesh-like structure. After subsequent treatment with 5 micrograms/ml cytochalasin D (a reagent that depolymerizes actin filaments), MTI-5 cells maintained their shape, while cells of a drebrin-negative cell line, MTI-11, formed retraction processes. Simultaneously, actin filaments changed into patchy dot-like aggregates in the cytoplasm of both MTI-5 and MTI-11 cells. These aggregates are known as cytoplasmic pools. In MTI-5 cells, adhesion plaques that were resistant to treatment with cytochalasin D appeared upon expression of drebrin. These adhesion plaques were immunostained with vinculin-specific antibodies, while those in MTI-11 cells were hardly immunostained. The amount of vinculin in MTI-5 cells increased in parallel with increase in the level of drebrin. These results suggest that expression of drebrin A induces changes in the assembly of actin filaments and adhesion plaques, with resultant modulation of cellular adhesion to the substratum.

Disappearance of actin-binding protein, drebrin, from hippocampal synapses in Alzheimer's disease

The actin-binding protein drebrin is localized in postsynaptic terminals in adult brain and is considered to be related to synaptic plasticity. Immunocytochemical study demonstrated that widespread drebrin immunoreactivity was observed in hippocampal formations of control human brains, while Alzheimer's disease (AD) brains showed remarkable reductions in this immunoreactivity. Western blot analysis demonstrated that drebrin E (116kD) as well as drebrin A (125 kD) presented in adult human brains, and that these isoforms were decreased in parallel in AD brains. On the other hand, synaptic vesicle-specific 38-kD protein (SVP-38), a presynaptic marker was not so changed in AD brains in comparison with control brains by both techniques. These findings suggest that drebrin E and A in the adult human brain may be co-localized in postsynaptic terminals, and that drebrin may be more sensitive as a marker of synaptic damage than SVP-38, and that the disappearance of drebrin may contribute to the pathogenesis of memory disturbance in AD.

K252a, a potent inhibitor of protein kinases, inhibits the migration of cerebellar granule cells in vitro

In order to elucidate the cellular mechanisms of migrating neurons, we developed an assay system in vitro, using an aggregation culture of developing granule cells from the rat cerebellum. This assay system allowed us to eliminate the effects of various factors other than neurons and to examine the direct effects of individual molecules on neuronal migration. In this assay system, we examined the effects of several protein kinase inhibitors on cerebellar granule cell migration, and revealed that K252a, an inhibitor of protein kinases and of the actions of neurotrophins, inhibited the migration. Within 5 min after the addition of K252a to the culture medium, most of the migrating spindle-shaped cells changed into non-migrating large and polygonal cells, which had many microspikes. Staining with rhodamine-phalloidin revealed the appearance of actin bundles that resembled stress fibers within these large cells. On the other hand, extension of neurites was not severely inhibited by the addition of K252a. These results suggest that the migration is regulated by a different mechanism from that of neurite growth.

Effect of a neuron-specific actin-binding protein, drebrin A, on cell-substratum adhesion

Drebrin A expression was induced in non-neuronal L cells via transfection with a vector containing the cDNA of rat drebrin A. Following treatment with colcemid (5 micrograms/ml) and cytochalasin D (0.5 micrograms/ml), most L cells collapsed into round cells, while drebrin expressing cells were resistant to the treatment, keeping their cell shapes. Simultaneously, actin filaments and microtubules were disrupted in both cell lines. By quantitative analysis, in the presence of cytochalasin D, the extent of cell spreading and cell attachment in drebrin expressing cells was significantly higher than that in control cells. These results suggest that drebrin A modulates cell-substratum adhesion.

The roles of microfilament-associated proteins, drebrins, in brain morphogenesis: a review

The cytoskeleton has been suggested to be one of the important endogenous factors that control neuronal morphogenesis. Analysis of the developmental changes in the protein composition of the brain led to the discovery of novel developmentally regulated actin-binding proteins, drebrins. Drebrins exhibit a number of characteristics that one might expect for an intracellular regulator of neuronal morphogenesis. Drebrin has three isoforms and the mRNA of each isoform is transcribed from a single gene through alternative RNA splicing mechanisms. The expression pattern of each isoform is regulated spatially and temporally in the developing brain. Drebrin and tropomyosin competitively bind to actin filaments, and the exclusion of tropomyosin from actin filaments by overexpression of drebrin in fibroblasts results in the appearance of thick, curving bundles of actin filaments, and the formation of cell processes. Taken together, these data indicate that drebrin is one of the intracellular regulators of the neuronal morphogenesis.

Drebrin, a development-associated brain protein from rat embryo, causes the dissociation of tropomyosin from actin filaments

Drebrin is a development-associated neuroprotein whose cDNA into fibroblasts causes the formation of dendrite-like structures (Shirao, T., Kojima, N., and Obata, K. (1992) Neuroreport 3, 109-112). To explore molecular functions of drebrin during brain development, we purified drebrin from brains of rat embryos. Drebrin bound to actin filaments at a stoichiometry of 1:5 with a dissociation constant (Kd) of 1.2 x 10(-7) M. It strongly inhibited the actin binding activity of tropomyosin. Excess amounts of tropomyosin also inhibited the drebrin binding to actin filaments, suggesting that drebrin and tropomyosin competitively bind to actin filaments. Further, drebrin inhibited not only the actin binding activity of alpha-actinin but also the actin cross-linking activity of alpha-actinin. Gene transfection experiments revealed that tropomyosin was dissociated from actin filaments in drebrin-overexpressing fibroblasts. Thus we hypothesize that drebrin may destabilize actin filaments by dissociating tropomyosin and alpha-actinin from actin filaments, resulting in the formation of axon and dendrites during neuronal development.

Formation of thick, curving bundles of actin by drebrin A expressed in fibroblasts

Drebrin A is a neuron-specific protein, the expression of which is regulated during development. Upon transfection of fibroblasts with drebrin A cDNA, the protein is expressed at high levels in fibroblasts and the outgrowth of highly branched, neurite-like cell processes is induced. In this report, we describe a biochemical examination of the binding of drebrin A to actin filaments. We also demonstrate by an immunocytochemical method that, when drebrin A is expressed in transfected cells, it binds to actin filaments and is concentrated in cell processes. Furthermore, we provide evidence that thick, curving bundles of actin together with drebrin are formed in some of the transfected cells. Our results suggest that the actin filaments that bind drebrin might be a novel class of actin filaments and might play a role in neuronal morphogenesis.

Actin-binding protein, drebrin, accumulates in submembranous regions in parallel with neuronal differentiation

Drebrins are developmentally regulated actin-binding proteins. In this study, we analyzed subcellular distribution of drebrin E in neuroblastoma cells (SH-SY5Y) in culture, especially in terms of its relationship to actin filaments. In undifferentiated cells, drebrin E was scattered as flocculus small dots along the stress fibers and also accumulated at adhesion plaques. In parallel with the neuronal differentiation following retinoic acid treatment, drebrin E was accumulated, accompanying filamentous (F) actin, in the submembranous cortical cytoplasm. Similar submembranous localization of drebrins was observed in primary cultured neurons. In the presence of drebrin E F-actin was more stable against cytochalasin D than F-actin lacking drebrin E. These results suggest that drebrin E plays a role in neuronal morphological differentiation by changing its subcellular localization with stabilized F-actin.

Molecular cloning of cDNA encoding human drebrin E and chromosomal mapping of its gene

Drebrins are novel actin-binding proteins in the brain which are developmentally regulated. Three isoforms: two embryonic types (E1 and E2) and an adult type (A) are generated by alternative RNA splicing from a single debrin gene in the chicken brain. A full length cDNA clone of human drebrin E has been isolated from a cDNA library of human fetus brain. The clone is 2596 base pairs in length and contains an open reading frame of 1947 nucleotides encoding a protein of 649 amino acids. The deduced amino acid sequence, except for the internal 138-nucleotide sequence (ins2), exhibits 88% homology with rat drebrin A. Spot blot hybridization using flow-sorted human chromosomes provides evidence that the gene encoding human drebrin protein locates on human chromosome 5.

Molecular cloning of a developmentally regulated brain protein, chicken drebrin A and its expression by alternative splicing of the drebrin gene

Drebrins are developmentally regulated proteins found in the chicken brain and are classified into three forms, E1, E2 and A. Previously we isolated two cDNAs corresponding to the embryonic drebrin mRNAs from a chick embryo cDNA library. They differed in that an internal 129-nucleotide sequence, designated ins1, was inserted in the cDNA encoding drebrin E2 and was deleted in the other cDNA encoding drebrin E1. To search for the cDNA clone encoding drebrin A, a cDNA library of 1-day-old chick brains was screened using embryonic drebrin cDNA fragments as probes. Consequently, a novel cDNA was isolated, the sequence of which was entirely identical with that of drebrin E2 except for the insertion of a 138-nucleotide sequence, designated ins2, in the 5' direction immediately upstream from ins1. Since the translation product of the entire coding region was similar to that of drebrin A, this cDNA should correspond to the mRNA for drebrin A. Sequencing analysis of three drebrin cDNAs clearly indicated that the heterogeneity of chicken drebrins was caused by the insertion or deletion of the two sequences, ins1 and ins2. The amino-terminal half region including ins2 and two short sequences in the carboxyl-terminal region of the predicted drebrin A were highly evolutionarily conserved. Cloning and sequencing of the drebrin gene revealed that ins1 and ins2 were independently encoded by separate exons and three drebrin isoforms were thought to arise by alternative splicing from a single drebrin gene. The difference in the time course of expression and tissue distribution of each drebrin suggests that the machinery of alternative splicing site selection of the drebrin gene is regulated in a developmental stage-dependent and tissue-specific manner.

[The role of neuronal cytoskeleton associated proteins in neuronal network formation]

There have been many studies to determine extrinsic factors that may regulate the neuronal migration and growth of axons and dendrites. However, the intracellular mechanism, especially the regulation of cytoskeleton, has not been clarified. It has been reported that actin filament crosslinking protein, MAR-CKS, play roles in cell motility through cytoskeletal rearrangement accompanied by rapid, PKC-dependent phosphorylation. Recently, we have demonstrated that neuron-specific actin binding protein, drebrin, changed the stability and distribution of microfilaments within the fibroblast and formed highly-branched dendrite-like cell processes from their cell perimeters. It has also been reported that overexpression of microtubule associated protein, tau, in a fibroblast induced long axon-like cellular processes. This review will focus on dynamic regulations of the microfilament by drebrin and those of the microtubules by MAP2 and tau. Since all kinds of cytoskeletons are related to each other, the binding ability of neurofilament H to microtubules and that of MAP2 to neurofilaments were also discussed.

Lesions of nigrostriatal pathway reduce expression of tyrosine hydroxylase gene in residual dopaminergic neurons of substantia nigra

The effects of unilateral mechanical transection of the nigrostriatal bundle of rat brain on the level of tyrosine hydroxylase (TH) mRNA and on the activity of TH enzyme in the substantia nigra (SN) were examined. Lesions resulted, by 14 days, in reductions of TH mRNA level to 10% of control and of TH enzyme activity to 39% of control in the ipsilateral SN. The percentage of TH mRNA is lower than either the percentage of surviving dopaminergic neurons or the remaining TH enzyme activity. In situ hybridization analyses also demonstrated the reduction of TH mRNA concentration in surviving dopaminergic neurons in the ipsilateral SN.

Changes of drebrin expression in the visual cortex of the cat during development

The expression of and developmental changes in drebrin were studied in cat visual cortex using immunohistochemistry and immunoblot analysis. Drebrin is a developmentally regulated brain protein which in the chicken has characteristic changes in expression related to developmental stage. A monoclonal antibody (MAb M2F6) raised against drebrin, was found to label the neuropil of the kitten visual cortex in the early postnatal period. At 1-3 weeks of age, the staining was prominent in layer IV of the visual cortex. The immunoreactivity, however, was found to be dramatically decreased around the end of the sensitive period for ocular dominance plasticity (approximately 3 months of age). In the adult visual cortex, almost no immunostaining was observed. These developmental changes revealed by an immunohistochemical method were confirmed using immunoblot analysis. Upon immunoblot analysis after SDS-PAGE of protein from the kitten visual cortex, MAb M2F6 was found to recognize two protein bands with molecular weights of 130 kDa (drebrin E) and 140 kDa (drebrin A). The developmental profile of the intensity of the two bands of the drebin closely parallels in time the postnatal changes in cortical susceptibility to visual deprivation. These results indicate that the expression of drebrin in kitten visual cortex is restricted to the early postnatal period and suggest that it may play an important role in the experience-dependent modification of cortical circuitry during the sensitive period.

Cloning of drebrin A and induction of neurite-like processes in drebrin-transfected cells

The developmentally-regulated neuron-specific protein, drebrin A, is expressed first at the time of outgrowth and maturation of dendrites, and is localized within dendrites of the adult brain. A cDNA clone of adult rat drebrin A was isolated and sequenced. There is no overall homology with other reported protein sequences except chicken drebrins. We constructed the expression vector MIW-DA containing the drebrin A cDNA. Transfection of nonneuronal cells with MIW-DA induced the formation of highly branched neurite-like cell processes. In these process-bearing transfectants, expressed debrin A is concentrated in submembraneous regions of the cell. Furthermore, actin concentration is higher in these cells than other fibroblasts. These results suggest a possible role of drebrin A in neurite outgrowth.

Expression of three drebrin isoforms in the developing nervous system

Drebrins are developmentally regulated brain proteins which were first isolated from brains of 10-day chick embryos. They are classified into three forms, drebrins E1, E2 and A. Cloning of drebrin cDNAs revealed that each drebrin isoform is encoded in an independent mRNA. Genomic Southern blot analysis and cloning of a drebrin gene revealed that the mRNAs of three drebrin isoforms are generated by alternative RNA splicing from a single gene. Immunohistochemistry and in situ hybridization analysis of the embryonic cerebellum indicated that drebrin mRNA is first transcribed in postmitotic neurons and that there seem relations between cell migration and expression of drebrin E1.

Nucleotide sequences of two embryonic drebrins, developmentally regulated brain proteins, and developmental change in their mRNAs

Drebrins are developmentally regulated proteins found in chicken brain and are classified into two forms of the embryonic type (E1 and E2) and one form of the adult type (A). Although the time courses of their appearance are different from each other, the structures of the 3 forms are closely related. Two kinds of drebrin cDNA, designated gDcw6 and gDcw17, were isolated from the cDNA library of the chicken embryo and their nucleotide sequences were determined. Their sequences were entirely identical except for a deletion of an internal 129-nucleotide sequence, and the gDcw17 insert contained an open reading frame capable of encoding 607 amino acids. These cDNAs seemed to correspond to two embryonic forms of drebrin mRNAs. The predicted drebrin molecules are highly hydrophilic and have proline-rich sequences and long stretches of glutamate in the carboxyl-terminal region. RNA dot-blot analysis using the drebrin cDNA as a probe demonstrated that the amounts of drebrin mRNAs were also developmentally regulated as those of drebrins. Southern blot analysis showed that the chicken genome has a single copy of the drebrin gene per haploid complement. These findings suggest that the multiple forms of drebrins result from alternative splicing of the single drebrin gene during neural development.

Molecular cloning of a cDNA for the developmentally regulated brain protein, drebrin

A lambda gt11 cDNA library from 10-day-old chicken embryo was screened immunologically using an antiserum against drebrins E1, E2 and A, proteins previously designated S5, S6 and S54, respectively. A cDNA clone for a common domain of drebrin was isolated. Northern blot analysis of chicken brain indicated that drebrin mRNAs are about 2.7 kilobases in molecular size and that expression of these proteins is developmentally regulated.

Localization of a developmentally regulated neuron-specific protein S54 in dendrites as revealed by immunoelectron microscopy

We sought to determine the ultrastructural localization of the developmentally regulated neuron-specific protein S54 in the chicken cerebellar cortex and optic tectum. The brains were fixed by perfusion with paraformaldehyde and glutaraldehyde. Frozen sections were immunocytochemically labeled with a monoclonal antibody to S54 protein. The immunoreactivity for S54 protein was localized in dendrites. No immunoreactivity for S54 protein was detected in axons and their presynaptic terminals.