Emerging roles of Lys63-linked polyubiquitination in neuronal excitatory postsynapses

In the mammalian brain, neuronal excitatory synaptic development, function, and plasticity largely rely on dynamic, activity-dependent changes in the macromolecular protein complex called the postsynaptic density (PSD). Activity-dependent Lys48-linked polyubiquitination and subsequent proteasomal degradation of key proteins in the PSD have been reported. However, investigations into the functions and regulatory mechanisms of Lys63-linked polyubiquitination, the second most abundant polyubiquitin form in synapses, have recently begun. Recent studies showed that a Lys63 linkage-specific deubiquitinase (DUB), cylindromatosis-associated DUB (CYLD) localizes to the PSD where its DUB activity is regulated by different kinases. In addition, Lys63-linked polyubiquitination of postsynaptic density 95 (PSD-95), a core scaffolding protein of the PSD, was identified and its functional significance in synaptic plasticity was characterized. In this review, we summarize these recent findings on Lys63-linked polyubiquitination in excitatory postsynapses, and also propose key questions and prospects about this emerging type of posttranslational modification of the PSD proteome.