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Synaptic scaffolding proteins from membrane-associated guanylate kinases (MAGUK) family are implicated in synapse formation and functioning. To better understand the role of one of the proteins of this family, SAP97, we studied with electron microscopy the effects of its overexpression on spine and synapse morphology in CA1 pyramidal neurons of rat organotypic hippocampal slice cultures. Dramatic spine enlargement induced by SAP97 overexpression was accompanied by marked morphological changes, with spines enwrapping and engulfing presynaptic terminals. The size and complexity of the PSD was also significantly increased. Similar to PSD-95, SAP97 promoted formation of multi-innervated spines (MIS). In addition, both MAGUK proteins induced multiple excitatory contacts on dendritic shafts suggesting a mechanism for shaft synapse formation. Formation of MIS and shaft synapses was blocked by the nitric oxide synthase (NOS) inhibitor L-NAME. Immunochemistry revealed that overexpression of SAP97 was associated with overexpression of PSD-95 and recruitment of nNOS to the synapse. These data provide evidence for both common and distinct structural alterations produced by overexpression of SAP97 and PSD-95 and demonstrate strong interactions between these two proteins to regulate contact formation through nitric oxide signaling. © 2010 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Patterns of activity that induce synaptic plasticity at excitatory synapses, such as long-term potentiation, result in structural remodeling of the postsynaptic spine, comprising an enlargement of the spine head and reorganization of the postsynaptic density (PSD). Furthermore, spine synapses represent complex functional units in which interaction between the presynaptic varicosity and the postsynaptic spine is also modulated by surrounding astroglial processes. To investigate how activity patterns could affect the morphological interplay between these three partners, we used an electron microscopic (EM) approach and 3D reconstructions of excitatory synapses to study the activity-related morphological changes underlying induction of synaptic potentiation by theta burst stimulation or brief oxygen/glucose deprivation episodes in hippocampal organotypic slice cultures. EM analyses demonstrated that the typical glia-synapse organization described in in vivo rat hippocampus is perfectly preserved and comparable in organotypic slice cultures. Three-dimensional reconstructions of synapses, classified as simple or complex depending upon PSD organization, showed significant changes following induction of synaptic potentiation using both protocols. The spine head volume and the area of the PSD significantly enlarged 30 min and 1 h after stimulation, particularly in large synapses with complex PSD, an effect that was associated with a concomitant enlargement of presynaptic terminals. Furthermore, synaptic activity induced a pronounced increase of the glial coverage of both pre- and postsynaptic structures, these changes being prevented by application of the NMDA receptor antagonist D-2-amino-5-phosphonopentanoic acid. These data reveal dynamic, activity-dependent interactions between glial processes and pre- and postsynaptic partners and suggest that glia can participate in activity-induced structural synapse remodeling. © 2009 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]