Knock-In Mice Lacking the PDZ-Ligand Motif of mGluR7a Show Impaired PKC-Dependent Autoinhibition of Glutamate Release, Spatial Working Memory Deficits, and Increased Susceptibility to Pentylenetetrazol

Abstract

The metabotropic glutamate receptor 7 (mGluR7) is widely expressed throughout the brain and primarily localized at presynaptic active zones, where it is thought to regulate neurotransmitter release. Protein interacting with C kinase 1 (PICK1), a postsynaptic density protein-95/disc-large tumor suppressor protein/zonula occludens-1 (PDZ)-domain protein, binds to the three C-terminal amino acids (-LVI) of the predominant mGluR7 splice variant, mGluR7a, and has been implicated in the synaptic clustering of this receptor. Here, we generated knock-in mice in which the C-terminal LVI coding sequence of exon 10 of the mGluR7 gene was replaced by three alanine codons (-AAA). Immunoprecipitation showed that the PICK1-mGluR7a interaction is disrupted in mGluR7a AAA/AAA mice. However, the synaptic localization of mGluR7a was not altered in cultured hippocampal neurons and brain sections prepared from the knock-in animals. In cerebellar granule cell cultures, the group III mGluR agonist L-AP-4 decreased the frequency of spontaneous excitatory currents in neurons derived from wild-type but not mGluR7a AAA/AAA mice, consistent with the interaction between mGluR7a and PICK1 being required for protein kinase C-mediated inhibition of glutamate release. At the behavioral level, the mGluR7a AAA/AAA mice showed no deficits in motor coordination, pain sensitivity, and anxiety but exhibited significant defects in hippocampus-dependent spatial working memory. In addition, they displayed a high susceptibility to the convulsant drug pentylenetetrazole. Together, these results indicate that PICK1 binding to the C-terminal region of mGluR7a is crucial for agonist-triggered presynaptic signaling in vivo.

The Metabotropic Glutamate Receptor mGlu7 Activates Phospholipase C, Translocates Munc-13-1 Protein, and Potentiates Glutamate Release at Cerebrocortical Nerve Terminals

“…Because the mGlu7 receptor is less efficiently coupled to release facilitation than to the release inhibition pathway, as evident by the prolonged exposure to agonist required for activation (at least 10 min), it seems that the major function of the receptor would appear to be to provide presynaptic inhibition control. This is consistent with data showing that the lack of functional mGlu7 receptor leads to a convulsive phenotype due to excessive glutamate release (44,45). A possible role for L-AP4-mediated release potentiation would be to preserve homeostasis by preventing synapses in saturating states of depression from accumulating glutamate.…”

PDZ Protein Regulation of G Protein–Coupled Receptor Trafficking and Signaling Pathways

“…PICK1 regulates PKC phosphorylation of mGluR7a, regulates the presynaptic clustering of mGluR7, and mediates stable mGluR7 cell surface expression (Boudin et al, 2000;Dev et al, 2000;Suh et al, 2008). mGluR7a knock-in mice lacking a PDZ-binding motif exhibit deficits in hippocampal-dependent spatial memory and are highly susceptible to the convulsant drugs, and the disruption of the mGluR7a-PICK1 complex induces epilepsy-like seizures (Bertaso et al, 2008;Zhang et al, 2008a). Taken together, it appears PICK1 may be important for regulating the trafficking of a subset of GPCRs and may prove important in regulating GPCR-mediated signaling pathways.…”

“…PICK1 interactions with mGluR7a have been shown to be important for presynaptic mGluR7a clustering. mGluR7a knock-in mice lacking a PDZ-binding motif exhibit deficits in hippocampal-dependent spatial memory, and the disruption of the mGluR7a-PICK1 complex induces epileptic-like seizures (Boudin et al, 2000;Bertaso et al, 2008;Zhang et al, 2008a). a-Syntrophin and b 2 -syntrophin knockout mice display normal systolic blood pressure and resting heart rate; however, a double knockout prevents a 1D AR-mediated blood pressure responses and exhibits a distinct hypotonic phenotype at rest, thereby demonstrating the capability for PDZ protein compensation in vivo (Lyssand et al, 2008).…”

Elfn1 recruits presynaptic mGluR7 in trans and its loss results in seizures

“…The presynaptic mGluR7 in local circuits involving OLM and HIPP cells is considered to control brain excitability and higher cognitive function [14][15][16][17] . mGluR7 exists in the presynaptic active zone, and interestingly, the protein is differentially concentrated at the single synaptic terminal, depending on the nature of the postsynaptic neurons 10 .…”

“…This finding suggests that some key molecules on OLM/HIPP cells trigger presynaptic mGluR7 accumulation. mGluR7 binds to the presynaptic scaffold protein PICK1 through its C terminus, and this interaction is essential for its presynaptic clustering 18,19 and suppression of seizures 15,16 . However, binding to PICK1 alone cannot explain the accumulation of mGluR7 onto specific interneurons.…”