Dynamic Regulation of Homer Binding to Group I Metabotropic Glutamate Receptors by Preso1 and Converging Kinase Cascades

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Abstract
In rat sympathetic neurons from the superior cervical ganglia (SCG) expressing metabotropic glutamate receptor mGluR1 or mGluR5, overexpression of scaffolding Homer proteins, which bind to a Homer ligand in their C termini, cause receptor clustering and uncoupling from ion channel modulation. In the absence of recombinant Homer protein overexpression, uncoupling of mGluRs from voltage-dependent channels can be induced by expression of Preso1, an adaptor of proline-directed kinases that phosphorylates the Homer ligand and recruits binding of endogenous Homer proteins. Here we show that in SCG neurons expressing mGluR1 and the tyrosine receptor kinase B, treatment with brain-derived neurotrophic factor (BDNF) produces a similar uncoupling of the receptors from calcium channels. We investigated the pathways that mediate this uncoupling and compared it with uncoupling observed with Preso1 expression. Both BDNF- and Preso1-induced uncoupling require residues T1151 and S1154 in the mGluR1 Homer ligand (TPPSPF). Uncoupling via Preso1 but not BDNF was prevented by expression of a dominant negative Cdk5, suggesting that endogenous Cdk5 mediates Preso1-dependent phosphorylation of mGluR1. Dominant negative Cdk5 did not block the BDNF effect but this was sensitive to inhibitors of the mitogen-activated protein kinase kinase/extracellular signal–regulated kinase cascade. Interestingly, the BDNF pathway appeared to require native Preso1 binding to mGluR, because overexpression of the Preso1 FERM domain, which mediates the Preso1–mGluR interaction, prevented BDNF-induced uncoupling. These data suggest that the BDNF/tyrosine receptor kinase B and Cdk5 pathways converge at the level of mGluR to similarly induce Homer ligand phosphorylation, recruit Homer binding, and uncouple mGluRs from channel regulation.

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