title: Hippocampal mossy fibers induce assembly and clustering of PSD95-containing postsynaptic densities independent of glutamate receptor activation.
abstract: Factors that regulate the formation, spatial patterning, and maturation of CNS synapses are poorly understood. We used organotypic hippocampal slice cultures derived from developing (P5-P7) rat to test whether synaptic activity regulates the development and organization of postsynaptic structures at mossy fiber (MF) giant synapses. Antibodies to a prominent postsynaptic density (PSD) scaffold protein, PSD95, identified large (>1 microm) and irregularly shaped PSD assemblies that codistributed with synapsin-I or metabotropic glutamate receptor 7b (mGluR7b) -immunolabeled MF terminals in area CA3. To investigate the spatial organization of synaptic PSDs on individual pyramidal cells, neurons in slice cultures were transfected with a vector encoding a GFP-PSD95 fusion protein. Confocal three-dimensional reconstructions revealed clusters of PSDs along proximal dendrites of transfected pyramidal neurons in area CA3, but not in CA1. Clusters averaged 7.6 microm in length (range, 2.2-29 microm) and contained up to 35 individual PSDs (mean, 8.3). PSD clusters failed to form when slices were cultured without MFs, indicating that MFs induce cluster assembly. Chronic blockade of N-methyl-D-apartate- and AMPA/kainate-type glutamate receptors did not disrupt MF targeting or de novo formation of PSD clusters with a normal distribution on target cells. Additionally, glutamate receptor blockers did not alter the ultrastructural development of MF giant synapses containing multiple puncta adherens-like junctions and asymmetric synaptic junctions at dendritic shaft and spine domains, respectively. The results indicate that MF axons can induce the assembly and clustering of PSD95-containing postsynaptic complexes, displaying a normal subcellular and tissue distribution, by mechanisms that are independent of ionotropic glutamate receptor activation.
authors.affiliation: Department of Biological Sciences, University of Iowa, Iowa City, Iowa 52242-1324, USA.
authors.forename: L
authors.initials: L
authors.lastname: Qin
authors.forename: G S
authors.initials: GS
authors.lastname: Marrs
authors.forename: R
authors.initials: R
authors.lastname: McKim
authors.forename: M E
authors.initials: ME
authors.lastname: Dailey
mesh_terms: D000818:Animals; D003712:Dendrites / Q000378:metabolism; D006624:Hippocampus / Q000166:cytology / Q000378:metabolism; D066298:In Vitro Techniques; D019599:Mossy Fibers, Hippocampal / Q000502:physiology* / Q000648:ultrastructure; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D017966:Pyramidal Cells / Q000378:metabolism; D051381:Rats; D017470:Receptors, Glutamate / Q000502:physiology*; D013569:Synapses / Q000502:physiology* / Q000648:ultrastructure
chemical_list: D009419:Nerve Tissue Proteins; D017470:Receptors, Glutamate; C033652:postsynaptic density proteins