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Signaling proteins in the post-synaptic density

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  • Biology
  • Medicine


The ability of an organism to respond to its environment stems from synapses and signaling in the post-synaptic density (PSD). Neurological disorders often occur at the level of faulty signal transduction in the PSD. Here we describe the behavioral characterization of Densin 180, a PSD-enriched scaffold protein. We also report on the regulation of Ras guanine release factor1 (RasGRF1), a guanine exchange factor that promotes activation of Ras and thus the ERK pathway as part of an NMDA (N-methyl-D-aspartate) receptor complex with CaMKII (Ca2+/calmodulin-dependent kinase). The Densin KO exhibits severe nest building deficits, elevated anxiety and aggressiveness, impaired sensorimotor gating, hyperlocomotion to novel objects, and short-term memory (hippocampal- and cortical- dependent) deficits. These behavioral abnormalities resemble schizophrenia and autism. Decreases in the schizophrenia susceptibility gene products, DISC1 and mGluR5, are observed in the KO relative to the WT and may be a result of a decrease in their common binding partner α-actinin. α-actinin is known to regulate mGluR5 surface levels. Cross-linking and stabilization of PSD protein architecture by scaffold proteins like Densin may contribute to some of the observed behavioral abnormalities. The Densin KO also has blunted activity-dependent gene expression. Steady-state levels of the immediate early genes Arc and c-fos are decreased in the hippocampus and cortex of brain sections. Levels of Arc induced in response to stimulation by the neurotrophin BDNF is significantly decreased in the Densin KO neurons after 8 hours of treatment. Impairments in BDNF signaling can lead to affective and cognitive disorder and has a role in cortical inhibition. Dysfunction in BDNF signaling and DISC1 signaling have been previously implicated in autism. The Densin KO also is susceptible to seizures, in particular when injected with Nembutal, a GABA(A) agonist. One point of intersection between signaling pathways that involve DISC1, mGluR, and BDNF is at the level of ERK signaling, which if impaired may establish a hypoglutamatergic state in the Densin KO. In addition to characterizing the Densin KO, we studied possible interactions of RasGRF1 and CaMKII with the NR2B subunit tail of the NMDA receptor. CaMKII phosphorylation sites on RasGRF1 were identified, including Ser916, by mass spectrometry. Immunoprecipitation from HEK cells revealed that RasGRF1 enhances CaMKII association with NR2B.

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