Tomoaki Shirao

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.

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.

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

Abstract: 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.

Immunochemical homology of 3 developmentally regulated brain proteins and their developmental change in neuronal distribution

Proteins S5, S6, and S54 (mol. wts. 95,000, 100,000, and 110,000 Da) appear characteristically at certain developmental stages in the chick brain (Shirao, T. and Obata, K., J. Neurochem., 44 (1985) 1210-1216). In the present study polyclonal and monoclonal antibodies were developed against electrophoretically purified S5 and S6 proteins. Each polyclonal and monoclonal antibody specifically recognized all 3 proteins, S5, S6, and S54, by immunoblot analysis. The tissue specificities of these proteins were examined by immunoblot analysis with these antibodies. Proteins S5 and S6 were found in the neural tissue and in some non-neural tissues of chick embryo. In the adult chicken, however, they were detected neither in neural nor in non-neural tissues with the exception of the spinal ganglion. Protein S54, on the other hand, was found both in late embryonic and adult neural tissues. It was detected neither in embryonic nor in adult non-neural tissues. Immunohistochemical analysis of adult nervous system showed that S54 protein was present only in neurons. Therefore it is concluded that S54 protein is a neuron-specific protein. Developmental changes of localization of these proteins were then examined by immunohistochemistry. In the developing brain, immunostaining was first observed in newly differentiated neurons, later becoming localized in the neuronal processes. In the adult brain, the immunoreactivity was mainly localized in certain types of synaptic regions, but it was also observed in a small population of neuronal somata.

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 γ‐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. J. Comp. Neurol. 483:383–402, 2005.

Synaptic dysfunction and disruption of postsynaptic drebrin–actin complex: A study of neurological disorders accompanied by cognitive deficits

Many neurological disorders accompanied by cognitive deficits, including Alzheimers disease (AD) and Down syndrome, exhibit abnormal dendritic spine morphology. Actin-based cytoskeletal network dynamics is critical for the regulation of spine morphology and function. Recent experimental data from an AD animal model revealed that defects in intracellular signaling cascades related to the accumulation of amyloid beta (Abeta) peptide cause disruption of the postsynaptic actin-regulatory machinery, including cofilin and drebrin. The level of postsynaptic drebrin, a major F-actin-binding protein in dendritic spines, correlates well with the severity of cognitive impairment. We propose that an imbalanced regulation of the actin-regulatory machinery (loss of drebrin and increase of dephosphorylated cofilin) results in synaptic dysfunction, which underlies the cognitive impairment accompanying neurological disorders and normal aging.

Down‐regulation of drebrin A expression suppresses synaptic targeting of NMDA receptors in developing hippocampal neurones

Drebrin is a major F‐actin‐binding protein in the brain. We have recently demonstrated that drebrin A (neurone‐specific isoform) clusters at synapses and governs targeting of the post‐synaptic density 95 protein to synapses during development. To determine the role of drebrin A on excitatory synapse formation, we analysed whether the suppression of drebrin A expression affects filopodia‐spine morphology and synaptic targeting of NMDA receptors in cultured hippocampal neurones. Suppression of developmentally programmed up‐regulation of drebrin A by antisense treatment significantly decreased the density and width of filopodia‐spines. Immunocytochemistry showed that the antisense treatment did not attenuate synaptic clustering of NMDA receptors under conditions that permitted spontaneous activities but inhibited the accelerated targeting of NMDA receptors into synapses by its antagonist d‐(–)‐2‐amino‐5‐phosphonopentanoic acid. These results indicate that drebrin A up‐regulation plays a pivotal role in spine morphogenesis and activity‐dependent synaptic targeting of NMDA receptors.

Identification of a synaptic vesicle-specific 38,000-dalton protein by monoclonal antibodies

Synaptic vesicles were purified from the guinea pig cerebrum by sucrose density gradient centrifugation, and monoclonal antibodies (MAbs) were produced against this vesicle fraction. Seven MAbs (171B5, 171E8, 174D12, 174H11, 177A2, 177H11 and 178D4) recognized a novel acidic protein of about 38,000 daltons which was specific to synaptic vesicles. In immunofluorescence microscopy, the staining pattern of these MAbs corresponded to the distribution of the synapses in the guinea pig central nervous system. These MAbs appeared to stain all synaptic regions, irrespective of their synaptic function or type of neurotransmitters. MAb 171B5 and 174H11 stained the rat, rabbit and bovine synapses similarly to the guinea pig. Two other MAbs (171E8 and 177H11) stained other mammals weakly but the remaining 3 MAbs reacted only with the guinea pig. In immunoelectron microscopy of both the cerebellar tissue and isolated vesicle fraction, these MAbs selectively labeled the synaptic vesicles but not other structures. Immunoblot analysis was performed on electrophoretically separated proteins in vesicle fraction and brain homogenate. All of 7 MAbs reacted with a band at a molecular weight of about 38,000 from the guinea pig. Isoelectric focussing disclosed that this protein was acidic (pI 4.5-5).

Actin filaments and microtubules in dendritic spines.

Dendritic spines are small protrusions emerging from their parent dendrites, and their morphological changes are involved in synaptic plasticity. These tiny structures are composed of thousands of different proteins belonging to several subfamilies such as membrane receptors, scaffold proteins, signal transduction proteins, and cytoskeletal proteins. Actin filaments in dendritic spines consist of double helix of actin protomers decorated with drebrin and ADF/cofilin, and the balance of the two is closely related to the actin dynamics, which may govern morphological and functional synaptic plasticity. During development, the accumulation of drebrin‐binding type actin filaments is one of the initial events occurring at the nascent excitatory postsynaptic site, and plays a pivotal role in spine formation as well as small GTPases. It has been recently reported that microtubules transiently appear in dendritic spines in correlation with synaptic activity. Interestingly, it is suggested that microtubule dynamics might couple with actin dynamics. In this review, we will summarize the contribution of both actin filaments and microtubules to the formation and regulation of dendritic spines, and further discuss the role of cytoskeletal deregulation in neurological disorders.

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.