Studies of Cdk5-Dependent Posttranslational Modifications of PSD-95

Maria Morabito, Department of Cell Biology, The University of Massachusetts Medical School

The kinase Cdk5 regulates dopamine neurotransmission, plays a key role in synaptic plasticity associated with learning and memory, and is an important player in the cellular and physiological responses to drugs of abuse by integrating a number of intracellular pathways that are targeted by psychostimulant drugs. Thus, understanding the molecular mechanisms by which Cdk5 regulates glutamatergic synaptic function is critical for understanding how drugs of abuse alter glutamatergic transmission and induce various forms of synaptic plasticity within the reward circuitry of the brain.

We have shown that Cdk5 phosphorylates PSD-95, a major postsynaptic scaffolding protein of glutamatergic synapses that interacts with NMDA and AMPA receptors and has been implicated in addiction and the regulation of synaptic strength and plasticity. The N-terminus of PSD-95 binds to the tyrosine kinase Src and contains a PEST motif implicated in the ubiquitination of PSD-95 and critical in regulating AMPA receptor surface expression, while the C-terminus of PSD-95 contains a tyrosine-based motif implicated in the trafficking of PSD-95. We have previously shown that Cdk5 phosphorylates PSD-95 within the PEST motif and this phosphorylation regulates the binding of Src to PSD-95. Furthermore, our preliminary studies indicate that Cdk5 regulates both tyrosine phosphorylation and ubiquitination of PSD-95, implicating Cdk5 in the regulation of the trafficking and stability of PSD-95.

We propose to identify the residues of PSD-95 that are phosphorylated and ubiquitinated in a Cdk5-dependent manner by Mass Spectrometry analysis. The proposed study will provide fundamental insights into the mechanisms by which Cdk5 regulates PSD-95 and will improve our understanding of the functions of both Cdk5 and PSD-95. Given the importance of PSD-95 in synaptic plasticity and drug addiction, the identification of the sites of phosphorylation and ubiquitination of PSD-95 is an essential step toward the characterization of the post-synaptic proteomic changes underlying synaptic plasticity and drug addition.