Mechanisms and consequences of regulating the spinophilin/NMDA receptor interaction

Abstract

Parkinson disease (PD) is the second most common neurodegenerative disease. It is characterized by loss of dopaminergic cells in the substantia nigra, which causes loss of dopaminergic synapses onto striatal medium spiny neurons (MSNs). Dendritic spines that are localized to these striatal MSNs receive synaptic inputs from both the nigral dopamine neurons and cortical glutamate neurons. Signaling downstream of excitatory, glutamatergic drive is modulated by dopamine. This tripartite connection: glutamate, dopamine, and MSN dendritic spine, is important for normal motor function. Glutamate released from presynaptic terminals binds to and activates two classes of inotropic glutamate receptors that are localized to dendritic spines on striatal MSNs: the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) and the N-methyl-D-aspartate receptor (NMDAR). Once these receptors are activated, they allow for Ca2+ influx, which in turn activates Ca2+-dependent processes that underlie neural plasticity, including long-term potentiation (LTP) and long-term depression (LTD). Proper machinery in the pre- and post-synaptic neurons is required for normal signal transduction. Moreover, this signal transduction requires proper organization of synaptic proteins, which is achieved by specific protein-protein interactions. These protein-protein interactions are dynamic and can be modulated under various conditions, including pathological changes in the phosphorylation status of a specific protein. Catalytically active proteins called phosphatases and kinases specifically regulate the phosphorylation status of synaptic proteins. Pathologically, in PD there is increased autophosphorylation and activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII). This increased phosphorylation may be due to changes in the activity of the serine/threonine protein phosphatase 1 (PP1), a highly conserved protein serine/threonine phosphatase that has a diverse set of functions in eukaryotes. Serine/threonine phosphatase substrate specificity is obtained via interactions with targeting and regulatory proteins. One such protein, spinophilin, is a scaffolding protein that targets PP1 to various synaptic substrates to regulate their phosphorylation. Interestingly, the association of PP1 with spinophilin is enhanced in a rat model of PD. The NMDAR is another protein that has altered phosphorylation in animal models of PD. We have found that there is a decrease in the NMDAR-spinophilin interaction in an animal model of PD. Here, we have found that spinophilin and the NMDAR interact in brain tissue and when overexpressed in a mammalian cell system. Moreover, we have identified novel mechanisms that regulate this interaction and have identified putative consequences of altering this association. These studies give us novel insight into mechanisms and consequences underlying pathological changes observed in an animal model of PD. Understanding these changes will inform novel therapeutic targets that may be useful in modulating striatal function.