The mitogen-activated protein kinase (MAPK) c-Jun N-terminal kinase (JNK) has been implicated in a number of neuropathologies, but a clear role for JNK in physiological processes in the brain has not been well defined. The effects of JNK in neuropathologies often do not correlate with the phosphorylation and activation of the transcription factor c-Jun, a target of activated JNK, which suggested to Thomas et al. that other neuronal substrates of JNK might exist. The authors searched a database of protein sequences to identify potential JNK substrates based on a consensus phosphorylation sequence and identified the AMPA-type glutamate receptor (GluR) subunits GluR2L and GluR4 as JNK targets. In vitro kinase assays showed the JNK1-mediated phosphorylation of GST fusion proteins that contained either the C terminus of GluR2L or that of GluR4. JNK1 is constitutively active in the brain, and the authors found that both GluR2L and GluR4 were constitutively phosphorylated in unstimulated rat cortical neurons in culture. This phosphorylation was blocked by an inhibitor of JNK but not by inhibitors of other MAPKs. Coimmunoprecipitation experiments showed the presence of complexes that contained JNK1, the JNK activator MAPK kinase 7, and either GluR2L or GluR4. Stimulation of neurons with NMDA, which activates NMDA-type glutamate receptors, resulted in the rapid dephosphorylation of GluR2L and GluR4, without affecting JNK activity, whereas inhibition of glutamatergic signaling by treatment of cells with tetrodotoxin resulted in the rapid phosphorylation of the receptor subunits. NMDA treatment also caused the internalization of GluR2L. JNK-mediated rephosphorylation of a fluorescently tagged GluR2L was necessary for reinsertion of the receptor into the plasma membrane following NMDA removal. Thus, JNK plays a physiological role in regulating AMPA receptor trafficking as a consequence of neuronal activity.
G. M. Thomas, D.-T. Lin, M. Nuriya, R. L. Huganir, Rapid and bi-directional regulation of AMPA receptor phosphorylation and trafficking by JNK. EMBO J. 27, 361-372 (2008). [PubMed]