Aquaporin 4 Forms a Macromolecular Complex with Glutamate Transporter 1 and Mu Opioid Receptor in Astrocytes and Participates in Morphine Dependence

The water channel aquaporin 4 (AQP4) is abundantly expressed in astrocytes and provides a mechanism by which water permeability of the plasma membrane can be regulated. Evidence suggests that AQP4 is associated with glutamate transporter-1 (GLT-1) for glutamate clearance and contributes to morphine dependence. Previous studies show that AQP4 deficiency changed the mu opioid receptor expression and opioid receptors’ characteristics as well. In this study, we focused on whether AQP4 could form macromolecular complex with GLT-1 and mu opioid receptor (MOR) and participates in morphine dependence. By using immunofluorescence staining, fluorescence resonance energy transfer, and co-immunoprecipitation, we demonstrated that AQP4 forms protein complexes with GLT-1 and MOR in both brain tissue and primary cultured astrocytes. We then showed that the C-terminus of AQP4 containing the amino acid residues 252 to 323 is the site of interaction with GLT-1. Protein kinase C, activated by morphine, played an important role in regulating the expression of these proteins. These findings may help to reveal the mechanism that AQP4, GLT-1, and MOR form protein complex and participate in morphine dependence, and deeply understand the reason that AQP4 deficiency maintains extracellular glutamate homeostasis and attenuates morphine dependence, moreover emphasizes the function of astrocyte in morphine dependence.