Electron Microscopy of Excitatory and Inhibitory Synapses: A Brief Review

Gray, Edward G.

doi:10.1016/s0079-6123(08)63235-5

Cited by 190 publications

(74 citation statements)

References 34 publications

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“…Early electron microscopy studies uncovered a fundamental dichotomy in the ultrastructure of synapses in vertebrate CNS, classified as type I (asymmetric) and type II (symmetric) synapses, based on the width of their electron-dense postsynaptic density (PSD) [1]. Subsequently, identification of glutamate (and aspartate) and GABA (and glycine) as neurotransmitters in the CNS, combined with the advent of immunohistochemistry, showed that this dichotomy corresponds to excitatory and inhibitory synapses, respectively.…”

Section: Introductionmentioning

confidence: 99%

Molecular and functional heterogeneity of GABAergic synapses

Fritschy

Panzanelli

Tyagarajan

2012

Cell. Mol. Life Sci.

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Knowledge of the functional organization of the GABAergic system, the main inhibitory neurotransmitter system, in the CNS has increased remarkably in recent years. In particular, substantial progress has been made in elucidating the molecular mechanisms underlying the formation and plasticity of GABAergic synapses. Evidence available ascribes a key role to the cytoplasmic protein gephyrin to form a postsynaptic scaffold anchoring GABA(A) receptors along with other transmembrane proteins and signaling molecules in the postsynaptic density. However, the mechanisms of gephyrin scaffolding remain elusive, notably because gephyrin can auto-aggregate spontaneously and lacks PDZ protein interaction domains found in a majority of scaffolding proteins. In addition, the structural diversity of GABA(A) receptors, which are pentameric channels encoded by a large family of subunits, has been largely overlooked in these studies. Finally, the role of the dystrophin-glycoprotein complex, present in a subset of GABAergic synapses in cortical structures, remains ill-defined. In this review, we discuss recent results derived mainly from the analysis of mutant mice lacking a specific GABA(A) receptor subtype or a core protein of the GABAergic postsynaptic density (neuroligin-2, collybistin), highlighting the molecular diversity of GABAergic synapses and its relevance for brain plasticity and function. In addition, we discuss the contribution of the dystrophin-glycoprotein complex to the molecular and functional heterogeneity of GABAergic synapses. AbstractKnowledge of the functional organization of the GABAergic system, the main inhibitory neurotransmitter system, in the CNS has increased remarkably in recent years. In particular, substantial progress has been made in elucidating the molecular mechanisms underlying formation and plasticity of GABAergic synapses. Evidence available ascribes a key role to the cytoplasmic protein gephyrin to form a postsynaptic scaffold anchoring GABA A receptors along with other transmembrane proteins and signaling molecules in the postsynaptic density. However, the mechanisms of gephyrin scaffolding remain elusive, notably because gephyrin can auto-aggregate spontaneously and lacks PDZ protein interaction domains found in a majority of scaffolding proteins. In addition, the structural diversity of GABA A receptors, which are pentameric channels encoded by a large family of subunits, has been largely overlooked in these studies. Finally, the role of the dystrophin-glycoprotein complex, present in a subset of GABAergic synapses in cortical structures, remains ill-defined. In this review, we discuss recent results derived mainly from the analysis of mutant mice lacking a specific GABA A receptor subtype or a core protein of the GABAergic postsynaptic density (neuroligin-2, collybistin), highlighting the molecular diversity of GABAergic synapses and its relevance for brain plasticity and function. In addition, we discuss the contribution of the dystrophinglycoprotein complex to the molecular ...

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Section: Introductionmentioning

confidence: 99%

Molecular and functional heterogeneity of GABAergic synapses

Fritschy

Panzanelli

Tyagarajan

2012

Cell. Mol. Life Sci.

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“…Some investigators have suggested that different types of densities are associated with synapses of particular types. For example, type I PSDs most often occur in synapses that are thought to be excitatory (1,7,8), whereas type II PSDs are seen in synapses thought to be inhibitory (9, 10). This hypothesis suggests.that morphologically distinct PSDs may also contain distinct proteins that serve specialized functions associated with their particular transmitter type.…”

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confidence: 99%

Biochemical and immunochemical evidence that the "major postsynaptic density protein" is a subunit of a calmodulin-dependent protein kinase.

Kennedy¹,

Bennett²,

Erondu³

1983

Proc. Natl. Acad. Sci. U.S.A.

516

261

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By three criteria, two biochemical and one immunochemical, the major postsynaptic density protein (mPSDp) is indistinguishable from the 50-kilodalton (kDa) a subunit of a brain calmodulin-dependent protein kinase. First, the two proteins comigrate on NaDodSO4/polyacrylamide gels. Second, iodinated tryptic peptide maps of the two -are identical. Finally, a monoclonal antibody (6G9) that was raised against the protein kinase binds on immunoblots to a single 50 kDa band in crude brain homogenates and to both the a subunit of the purified kinase and the mPSDp from postsynaptic density fractions. The purified kinase holoenzyme also contains a 60-kDa subunit termed 13. A comparison of the peptide map of .8 with the maps of 60-kDa proteins from the postsynaptic density fraction suggests that ,B is present there but is not the only protein present in this molecular weight range. These results indicate that the calmodulin-dependent protein kinase is a major constituent of the postsynaptic density fraction and thus may be a component of type I postsynaptic densities.Many synaptic junctions in the central nervous system contain a prominent specialized structure called the "postsynaptic density" (PSD) (1-5). When viewed by electron microscopy, it is a fibrous, electron-opaque thickening lying opposite the presynaptic terminal, on the cytoplasmic side of the postsynaptic membrane. The morphology of postsynaptic densities is variable. Some are thick [20-60 nm (5)] and appear to cover the entire postsynaptic surface area, whereas others are thin, often discontinuous, patches (1, 6). The former have been termed type I PSDs, while the latter are called-type II (1). Some investigators have suggested that different types of densities are associated with synapses of particular types. For example, type I PSDs most often occur in synapses that are thought to be excitatory (1,7,8), whereas type II PSDs are seen in synapses thought to be inhibitory (9, 10). This hypothesis suggests.that morphologically distinct PSDs may also contain distinct proteins that serve specialized functions associated with their particular transmitter type.In order to understand the structure and function of PSDs, various research groups have developed subcellular fractionation methods to isolate highly enriched preparations of PSDlike material (11-13). These procedures involve osmotic lysis of a subcellular fraction enriched in synaptosomes, followed by purification of junctional membrane complexes and extraction of membrane components by treatment with detergent. The detergent-insoluble residue, purified by density gradient fractionation, consists of electron-opaque, fibrous, disk-shaped structures, 20-60 nm thick and 200-500 nm in diameter (11-15). These structures are -similar in appearance and staining characteristics to type I PSDs in intact fixed tissue, and they are not produced by detergent treatment of other subcellular organelles such as mitochondria or myelin (11). Thus, this subcellular fraction is considered to be highly enriched in type...

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“…Previously, we showed that of two y-aminobutyrate binding sites present in brain SPMs, only one is associated with the postsynaptic density (PSD) (16). The PSD is a proteinaceous organelle associated with the postjunctional membrane and is thought to provide a framework for the attachment of transmitter receptors and other elements participating in the postsynaptic neuronal response (14,15,17,18). It is an especially prominent component of excitatory (type I) synapses (17).…”

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confidence: 99%

“…The PSD is a proteinaceous organelle associated with the postjunctional membrane and is thought to provide a framework for the attachment of transmitter receptors and other elements participating in the postsynaptic neuronal response (14,15,17,18). It is an especially prominent component of excitatory (type I) synapses (17). In the present study, we have examined the binding of L-[3H]-glutamate to isolated PSDs as a means of (i) better defining binding site heterogeneity and (ii) determining the nature of those transmitter recognition sites that initiate the excitatory response in the postsynaptic neuron.…”

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confidence: 99%

“…USA 81 (1984) 6877 trifugation in 0.5 mM Hepes/KOH buffer (pH 7.2) and finally were resuspended in a small volume of the same buffer for protein determination (21). The morphology and composition of SPMs and PSDs previously has been documented in detail (14,15,(17)(18)(19)(20) (8,23), is down-regulated by 1-5 mM Na+ (24), and is selectively inhibited by the phosphonic acid analogue APB (6). Therefore, these characteristics were utilized to determine whether these sites were present in PSDs.…”

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Selective association of N-methyl aspartate and quisqualate types of L-glutamate receptor with brain postsynaptic densities.

Fagg¹,

Matus²

1984

Proc. Natl. Acad. Sci. U.S.A.

182

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Recognition sites for the excitatory neurotransmitter, L-glutamate, were studied in synaptic plasma membranes and postsynaptic densities (PSDs) isolated from rat brains. The results demonstrate (i) that L-glutamate binding sites may be resolved into three distinct subtypes (categories Al, A2, and A4), each corresponding to an electrophysiologically identified receptor class, and (it) that the N-methyl aspartate (Al) ), which is the major population present in SPMs, is absent from the PSD. Those sites present in PSDs may be subdivided into two populations which, as in the case of the electrophysiologically derived receptor classification scheme, are differentially sensitive to N-methyl DL-aspartate (NMeAsp) and quisqualate (sites Al and A2, respectively).MATERIALS AND METHODS Chemicals. L-[G-3H]Glutamic acid of specific radioactivity of 29-45 Ci/mmol (1 Ci = 37 GBq) was purchased from Amersham. Triton X-100 (gas chromatographic grade) and the L and D isomers of glutamic and aspartic acids were obtained from Merck; NMeAsp, quinolinic, and kainic acids, from Sigma; quisqualic, ibotenic, DL-2-amino-5-phosphonovaleric (DL-APV), and DL-2-amino-7-phosphonoheptanoic (DL-APH) acids, from Cambridge Research Biochemicals (Harston, U.K.); and DL-2-amino-3-phosphonopropionic acid and DL-APB, from Calbiochem (La Jolla, CA). DL-2-amino-6-phosphonohexanoic acid was kindly provided by J. F. Collins (London). Subcellular Fractionation. SPMs and PSDs were isolated from fresh brains (excluding brainstem) of adult male rats (strain RAI, 180-250 g; Ciba-Geigy). The SPM fraction was prepared by the method of Jones and Matus (19), and this was treated with either 0.5% or 1.0% Triton X-100, followed by centrifugation through 1.0 M sucrose, to yield PSDs (18,20) (the results obtained with either concentration of Triton X-100 were indistinguishable and were pooled). Fractions were washed four times by resuspension and cenAbbreviations: PSD, postsynaptic density; SPM, synaptic plasma membrane; APB, 2-amino-4-phosphonobutyrate; APH, 2-amino-7-phosphonoheptanoate; APV, 2-amino-5-phosphonovalerate; NMeAsp, N-methyl DL-aspartate; NMeDAsp, N-methyl D-aspartate. 6876The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "adiertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

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Electron Microscopy of Excitatory and Inhibitory Synapses: A Brief Review

Cited by 190 publications

References 34 publications

Molecular and functional heterogeneity of GABAergic synapses

Molecular and functional heterogeneity of GABAergic synapses

Biochemical and immunochemical evidence that the "major postsynaptic density protein" is a subunit of a calmodulin-dependent protein kinase.

Selective association of N-methyl aspartate and quisqualate types of L-glutamate receptor with brain postsynaptic densities.

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