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Titel: LIN Layout
LIN: Forschungsabteilungen > Akkustik, Lernen, Sprache > Unterpunkt Ebene 3 > Unterpunkt Ebene 4
Titel: LIN Layout
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Molecular dynamics of the postsynaptic density The postsynaptic density (PSD) of spinous excitatory synapses is characterized by an electron-dense filamentous meshwork of cytoskeletal proteins that are thought to be crucially involved in the topological organization of synaptic signaling pathways. In recent years we have characterized a number of different PSD protein components. In a recent screen of the synaptic proteome in human chronic schizophrenia and the rat ketamine model of psychosis we found that the protein prohibitin is most likely causally involved in the synaptic pathology of schizophrenia (Smalla et al., 2008). Prohibitin is a tumor suppressor protein that forms large ring-shaped protein complexes in the inner mitochondrial membrane that appear to be essential for the viability of dendritic spines during synapto-dendritic morphogenesis. The precise molecular function of the synaptic prohibitin complex is not clear. Recently we also identified a novel GTPase activating protein (GAP) for Rap GTPases, termed SPAR2 (Spilker et al., 2008). SPAR2 shows significant sequence homology to SPAR (spine-associated RapGAP), a synaptic RapGAP that was reported to regulate spine morphology in cultured hippocampal neurons. Synapse-to-nucleus communication NMDA receptors and Ca2+ can exert multiple and very divergent effects within neuronal cells such as enhancing synaptic plasticity or triggering neuronal degeneration. This "Janus face" of the NMDA receptor function has attracted much interest during the past 20 years. Ca2+ influx through synaptic NMDA receptors triggers nuclear CREB phosphorylation via an ERK-dependent pathway, whereas Ca2+ influx through extrasynaptic NMDA receptors leads via an ERK-independent pathway to a dephosphorylation of CREB termed CREB shut-off. As opposed to the synaptic pathway the CREB shut-off signal is coupled to neuronal degeneration and cell death. Thus, CREB-regulated gene expression appears to be a shared mechanism in long-term plasticity and neuronal survival. We found a novel morphogenetic pathway linking the activity of NMDA receptors to CREB-dependent nuclear signaling events in primary neurons (Dieterich et al., 2008; Jordan & Kreutz, 2009). Important proteins on this pathway are the neuronal Ca2+ sensor caldendrin that is tightly associated with the PSD, the synapto-nuclear messenger Jacob and importin-a, a component of the nuclear import machinery. Strictly depending upon NMDA receptor stimulation Jacob translocates from distal dendrites to the nucleus as evidenced by time-lapse imaging. The translocation process requires the classical nuclear transport pathway since importin-a binding mutants of Jacob lacking the NLS do not move to the nucleus after glutamate stimulation. Caldendrin controls Jacob´s extra-nuclear localization by competing with the binding of importin-a to Jacob´s NLS in a Ca2+-dependent manner (Dieterich et al., 2008). This competition requires sustained Ca2+ levels, which presumably cannot be achieved by activation of NMDA receptors outside of the synaptic membrane, but are confined to Ca2+ microdomains in postsynaptic spines. Extrasynaptic NMDA receptors as opposed to their synaptic counterparts trigger the CREB shut-off pathway, and cell death. Indeed the nuclear accumulation of Jacob (Figure 1) results in a rapid stripping of synaptic contacts and in a drastically altered morphology of the dendritic tree. Nuclear knockdown of Jacob prevents CREB shut-off after extrasynaptic NMDA receptor activation, whereas its nuclear overexpression induces CREB shut-off without NMDA receptor stimulation.  Figure 1. Jacob immunoreactivity increases inside the nucleus after NMDA receptor stimulation. Image courtesy of A. Karpova. Neuronal Ca2+-binding proteins The broad range of different Ca2+-triggered phenomena in brain is reflected by the existence of a multitude of different Ca2+-binding proteins from which numerous belong to the EF-hand super-family. In recent years we have mainly analyzed the biophysical properties and cellular function of the neuronal Ca2+-binding proteins Caldendrin and Calneurons. Of utmost importance was the finding that Caldendrin plays an important role in synapse-to-nucleus communication by targeting the synapto-nuclear protein messenger Jacob to activated synapses (see above). Another major synaptic binding partner of Caldendrin appears to be the synaptic scaffolding protein PSD95. Calneuron 1 and 2 are highly homologous to each other and are most closely related to Caldendrin. We found that both Calneurons are tightly associated with neuronal Golgi. Phosphatidylinositol 4-OH kinase IIIß (PI-4Kß) is involved in the regulated local synthesis of phospholipids that are crucial for trans-Golgi network to plasma membrane trafficking. In a recent study we could show that Calneuron-1 and -2 physically associate with PI-4Kß, inhibit the enzyme profoundly at resting and low calcium levels and negatively interfere with Golgi-to-plasma membrane trafficking (Mikhaylova et al., 2009). At high calcium levels this inhibition is released and PI-4Kß is activated via a preferential association with the neuronal calcium sensor-1 (NCS-1). In accord to its supposed function as a filter for subthreshold Golgi calcium transients neuronal over-expression of Calneurons enlarge the size of the trans-Golgi network due to a built-up of vesicle proteins and reduces the number of axonal Piccolo-Bassoon transport vesicles, large dense core vesicles that carry a set of essential proteins for the formation of the presynaptic active zone during development (Figure 2). A corresponding protein knockdown has the opposite effect. The opposing roles of Calneurons and NCS-1 provide a molecular switch to decode Golgi local calcium transients at the Golgi and impose a calcium threshold for PI-4Kß activity and vesicle trafficking (Mikhaylova et al., 2009; Mikhaylova et al., 2010).  Figure 2. The calcium binding proteins Calneurons (in green) induce a prominent enlargement of the trans-Golgi network (in white) due to a block of TGN to plasma membrane trafficking and accumulation of vesicular proteins. Neuronal nuclei are shown in violet. The image was reconstructed in 3D using Imaris. Image courtesy of O. Kobler and M. Mikhaylova.
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