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Distinct physiological stimuli are required for bidirectional synaptic plasticity in striatum and hippocampus, but differences in the underlying signaling mechanisms are poorly understood. We have begun to compare levels and interactions of key excitatory synaptic proteins in whole extracts and subcellular fractions isolated from micro-dissected striatum and hippocampus. Levels of multiple glutamate receptor subunits, calcium/calmodulin-dependent protein kinase II (CaMKII), a highly abundant serine/threonine kinase, and spinophilin, a F-actin and protein phosphatase 1 (PP1) binding protein, were significantly lower in striatal extracts, as well as in synaptic and/or extrasynaptic fractions, compared with similar hippocampal extracts/fractions. However, CaMKII interactions with spinophilin were more robust in striatum compared with hippocampus, and this enhanced association was restricted to the extrasynaptic fraction. NMDAR GluN2B subunits associate with both spinophilin and CaMKII, but spinophilin-GluN2B complexes were enriched in extrasynaptic fractions whereas CaMKII-GluN2B complexes were enriched in synaptic fractions. Notably, the association of GluN2B with both CaMKII and spinophilin was more robust in striatal extrasynaptic fractions compared with hippocampal extrasynaptic fractions. Selective differences in the assembly of synaptic and extrasynaptic signaling complexes may contribute to differential physiological regulation of excitatory transmission in striatum and hippocampus.
The striatum and hippocampus control different forms of learning (Amso et al. 2005; Berke et al. 2009). The majority (~95%) of neurons in the striatum are γ-aminobutyric acid-containing medium spiny neurons (MSNs) (Huang et al. 1992; Kreitzer and Malenka 2008), whereas glutamatergic pyramidal neurons predominate in hippocampus. Although bidirectional synaptic plasticity [i.e. long-term potentiation (LTP) and long-term depression (LTD)] is thought to play a key role in the function of both brain regions, there are substantial differences in the underlying mechanisms. For example, N-methyl-d-aspartate receptor (NMDAR)- and calcium/calmodulin-dependent protein kinase II (CaMKII)-dependent LTP has been extensively studied in hippocampal CA1 pyramidal neurons (Bear and Malenka 1994; Malenka 1994; Nicoll and Malenka 1995; Malenka and Bear 2004; Lisman et al. 2012), whereas LTP in striatal MSNs can only be reliably observed when NMDAR activity is enhanced (Calabresi et al. 1992; Jia et al. 2010). Moreover, these physiological synaptic differences between brain regions extend to pathological situations. For example, Rett Syndrome and Alzheimer disease are associated with a decrease in dendritic spine density in hippocampal neurons (Chapleau et al. 2009; Penzes et al. 2011), whereas Parkinson's disease is associated with decreased spine density in striatal MSNs (Stephens et al. 2005; Zaja-Milatovic et al. 2005). However, the molecular basis for these distinct synaptic properties is not well-understood.
Differences in the localization, expression, and/or interactions of proteins that modulate postsynaptic signaling may contribute to the unique physiological properties and pathological susceptibilities of striatal and hippocampal neurons. For example, transgenic mice lacking postsynaptic density-95 (PSD-95), the prototypical postsynaptic scaffolding protein, have decreased spine density in striatal MSNs, but increased spine density in CA1 hippocampal pyramidal neurons (Vickers et al. 2006). Total tissue levels of the alpha isoform of CaMKII are somewhat higher in hippocampus compared with striatum (Erondu and Kennedy 1985), whereas total levels of the actin- and CaMKII-binding protein, α-actinin-2, are higher in striatum compared with hippocampus (Wyszynski et al. 1998). However, to the best of our knowledge, there are no studies directly comparing interactions between signaling proteins in striatum and hippocampus.
We recently found that spinophilin targets protein phosphatase 1 (PP1) to CaMKII in adult striatum (Baucum et al. 2012). Here, we report that the association of CaMKII with the spinophilin-PP1 complex is significantly greater in adult striatum compared with hippocampus. The enhanced striatal association was detected in an extrasynaptic, but not synaptic fraction. Moreover, extrasynaptic NMDAR GluN2B subunits are more robustly associated with both spinophilin and CaMKII in striatum compared with hippocampus. These differences in protein-protein interactions in specific subcellular compartments may contribute to the distinct physiological properties and/or pathological susceptibilities of striatal and hippocampal neurons.