Received for publication January 19, 2005.
Revised May 23, 2005.
Accepted for publication May 24, 2005.

NMDA attenuates CXCR2-mediated neuroprotection through enhancing the receptor phosphorylation and blocking the receptor recycling

Abstract

Abnormal extracellular accumulations of -amyloid, a major component of the senile plaques, and of the excitatory amino acid glutamate are both thought to be associated with degeneration of nerve cells in the central nervous system of Alzheimer's disease (AD) patients. The chemokine receptor CXCR2 has been shown to play a role in protecting neurons against -amyloid induced injury in vitro, it remains unclear whether CXCR2-mediated neuroprotection is affected by glutamate. We demonstrated that pre-treatment of hippocampal neurons with a sublethal concentration of N-methyl-D-aspartate (NMDA) attenuated the macrophage inflammatory protein 2 (MIP2) induced protection against -amyloid induced neuronal death. The NMDA induced inhibition was blocked by (+)-5-methyl-10, 11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801), a non-competitive NMDA receptor antagonist, indicating involvement of NMDA receptors in this process. A sublethal dose of NMDA pre-treatment induced CXCR2 phosphorylation, although in a less extent compared to the receptor phosphorylation induced by MIP2, and differential serine residues were involved in NMDA-and MIP2-induced CXCR2 phosphorylation. Moreover, NMDA treatment reduced the CXCR2-mediated Ca²⁺ mobilization, suggesting that NMDA induces cross-desensitization of CXCR2. CXCR2 underwent dephosphorylation after removal of the extracellular ligand, but the dephosphorylation rate was significantly reduced in the cells pretreated with NMDA. Treatment of the neuronal cells with NMDA retarded the recycling of CXCR2. In view of the critical role of receptor phosphorylation and recycling in the functional responsiveness of the chemokine receptor, these observations indicate a novel pathway through which glutamate may interfere with the neuroprotective function of chemokines.

Key words: Chemotactic peptides, Glutamate, Protein Phosphatases (other), Receptor synthesis/trafficking, Desensitization/uncoupling, Phosphorylation/Dephosphorylation, Recycling, Fluorescence techniques