Scheme of processes that maintain a certain receptor density on the postsynaptic membrane at the activated synapse. The right hand side shows signaling pathways for the regulation of FMRP-dependent translation through PP2A phosphatase and S6 kinase. On the left is the recycling of receptors through endocytosis (marked in light brown). The delay parameters τа and τb describe the time of activation (green arrow) and inhibition (dark red arrow) of de novo protein synthesis, τe is the time of protein translation and inclusion into the receptor complex (violet arrow), and τr is the time the receptor protein is localized in the endosome. The krz constant describes the efficiency of receptor recycling, the krx and kdx constants are the rates of receptor inclusion into the endosome and its degradation, respectively; kdz constant is the rate of receptor proteins degradation in the endosome. mGluR (metabotropic glutamate receptor); FMRP (fragile X mental retardation protein) – dendritic mRNA-binding protein; NRXNs (neurexins) – presynaptic membrane proteins; NLGNs (neuroligins), SHANK3 (SH3 domain and ankyrin repeat-containing protein), PCD-95 (postsynaptic density protein 95), HOMER – postsynaptic membrane proteins; PIKE (phosphoinositide-3 kinase enhancer), PI3K (phosphatidylinositol 3-kinase), Akt (protein kinase B), TSC1 (tumor suppressor hamartin), TSC2 (tumor suppressor tuberin), Rheb (Ras homolog enriched in brain), S6 kinase – mTOR (mammalian target of rapamycin) signaling pathway. Red color marks the names of proteins whose gene mutations are associated with neurodegenerative diseases
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