Essential Role for P62 in AMPA Receptor Trafficking and Synaptic Plasticity
Type of DegreeDissertation
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AMPA-type glutamate receptors (AMPARs) mediate major fast excitatory synaptic transmission in the mammalian central nerve system (CNS). Trafficking of AMPA receptors to and away from the postsynaptic membrane is fundamental to many forms of neuronal plasticity, including long-term potentiation (LTP) and long-term depression (LTD), which contribute to be the molecular and cellular basis for learning and memory. During LTP expression, more AMPARs are delivered to the postsynaptic membrane. In contrast, LTD induces receptor internalization. Therefore, alterations in synaptic strength are directly related to the receptor exocytosis and endocytosis. AMPA receptor trafficking is primarily regulated through receptor-associated proteins and post- translational modifications, principally phosphorylation of GluR1. The mechanism for regulation of AMPAR trafficking has been reviewed in detail in the chapter I. In the chapter II, the atypical protein kinase C (aPKC) scaffold, p62, was identified as the first non-C-terminal AMPA receptor interactor. AMPA receptor subunit intracellular loop L2-3 and the ZZ-type Zinc finger domain of p62 are essential for the interaction between these two proteins and for surface delivery of the receptor. This intracellular loop L2-3 is completely conserved throughout AMPA receptor subunits GluR1-4, but not other types of Glutamate receptor subunits. Furthermore, LTP was impaired in p62 knock-out mice in an age-dependent manner with normal basal synaptic transmission. Surface delivery of the AMPA receptor subunit GluR1 induced by cLTP (chemical LTP) was impaired and paralleled an absence of phosphorylation at S818, S831, and S845 in brain slices from p62 knock-out mice. These findings reveal that p62 plays a role in AMPA receptor trafficking and LTP expression. In addition, a possible conserved sequence motif (ISExSL) shared by all p62 interacting-aPKC substrates was discovered. Altogether, these findings indicate that p62 may regulate AMPAR trafficking and synaptic plasticity through recruiting AMPAR to aPKC for phosphorylation. A molecular model is presented depicting the mechanism whereby p62 may regulate AMPA receptor surface expression through aPKC-mediated phosphorylation. Finally, based on these findings, it is reasonable to hypothesize that overexpression of p62 in hippocampus, a brain region important for learning and memory, could facilitate aPKC-mediated GluR1 pS818 phosphorylation as well as trafficking, resulting in increased GluR1-containing AMPA receptors at the postsynaptic surface initiated by tetanic stimulation. Therefore, enhanced neuronal abilities including learning and memory, and synaptic plasticity are predicted for transgenic mice overexpressing p62. On the other hand, p62 has multiple functions in the cell and its expression is tightly regulated. Therefore, overexpression of p62 even in a mild manner could be detrimental. If this is the case, the p62 transgenic mice might develop unexpected developmental disorders. A p62 mutation P392L has recently been reported to increase osteoclastogenesis and cause a predisposition to the development of Paget disease. P392 is a major ubiquitin binding site in the UBA domain of p62 and P392L mutant p62 lacks of ubiquitin binding. As a result, ubiquitinated proteins cannot bind p62 for proteasome degradation. These ubiquitinated proteins accumulate in the cells and might induce hippocampal oxidative stress. Transgenic mice that overexpress this mutant p62 are likely to display disorders related to ubiquitination. To examine those possibilities, p62 transgenic mice which overexpress p62 in hippocampus will be generated. In this study, a Thy1 promoter was used to generate a p62 expression cassette. The full length p62 cDNA under the control of the Thy1 promoter will only be expressed in the central nervous system (CNS), specifically hippocampus and cortex, without affecting other tissues.