PLPP/CIN-mediated Mdm2 dephosphorylation increases seizure susceptibility via abrogating PSD95 ubiquitination

In the brain, murine double minute-2 (Mdm2), an E3-ubiquitin ligase, modulates neuronal excitability by regulating glutamate receptor and postsynaptic density 95 (PSD95) levels through ubiquitination. Thus, Mdm2 is relevant to epileptic seizures in human patients. Although phosphorylation at serine (S) 166 site by AKT increases Mdm2 activity, phosphatases of Mdm2 have been still elusive. Here, we demonstrate the novel function of pyridoxal-5'-phosphate phosphatase/chronophin (PLPP/CIN) in Mdm2 dephosphorylation that may negatively regulate PSD95 ubiquitination. As compared to wild-type mice, PLPP/CIN knockout (PLPP/CIN^-/-) mice showed the brief seizure activity and the higher Mdm2-S166 phosphorylation following kainic acid (KA) injection, independent of AKT activity. In addition, PLPP/CIN^-/- mice demonstrated the increases in Mdm2-PSD95 binding and PSD95 ubiquitination, accompanied by the decreases in Mdm2 ubiquitination and PSD95-NR2A (a subunit of N-methyl-d-aspartate receptor) bindings. Human PLPP/CIN over-expressing transgenic (PLPP/CIN^Tg) mice reversed these phenomena. In addition, Mdm2 knockdown abolished PSD95 ubiquitination and increased KA-induced seizure activity in PLPP/CIN^-/- mice without affecting AKT activity and protein levels of p53 and neural precursor cell expressed developmentally down-regulated gene 4-like (NEDD4-2). Therefore, we suggest that PLPP/CIN may be a potential therapeutic target for epilepsy and Mdm2-associated neurological diseases.