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

Abstract

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.

The Opioid-Addicted Tetrapartite Synapse

“…In addition to cystine-glutamate exchange, other mechanisms of glutamate release by astrocytes have been reported (119), but none have yet been found to contribute to glutamate plasticity observed after extinction from opioids or during reinstated seeking. For instance, whereas stimulating astroglial MORs increases glutamate transmission in hippocampus via the glutamate channel TREK-1 (120), this channel is also downregulated after long-term opioid use (121).…”

Computational modeling of opioid-induced synaptic plasticity in hippocampus

“…As a result, it can be concluded that, regardless of the role of astrocyte in synapse, the pathologic LTP cannot be controlled. Some researchers believe that astrocyte dysfunction can lead to addiction [ 28 , 30 ]. Thus, it can be concluded that without the manipulation of astrocyte function, we may not be able to inhibit the pathological LTP induction.…”

“…Further excitation of pyramidal neurons leads to more glutamate release, which means CA1 pyramidal neurons and astrocytes are going to be more excited than normal level. Furthermore, opioids decrease astrocytic glutamate transporter1 (GLT1) activation which is responsible for the glutamate uptake from the synaptic cleft [ 28 , 30 ]. As a result, reuptake of glutamate is reduced by astrocyte, and therefore more glutamate can be found in the synaptic cleft.…”

“…Furthermore, astrocyte involves in opioid dependency [ 28 , 29 ]. It has been shown that the main player of astrocyte’s contribution to opioids dependency is its important role in homeostasis, which mainly mediates by astrocytic transporters [ 28 – 30 ].…”

Chansporter complexes in cell signaling

“…The coassembly between AQP4 and EAAT2 is mediated by a region within amino acids 252–323 of the AQP4 C terminus. Interestingly, morphine was found to regulate expression of the complex via activation of protein kinase C, and it is suggested that the complex might play a role in morphine dependence . In another recent study, the astrocytic Fc receptor was found to be required for autoantibody‐induced internalization of AQP4 and EAAT2, a finding which may aid development of therapies to treat neuromyelitis optica, an autoimmune disease in which AQP4 autoantibodies cause CNS immunopathology and secondary CNS demyelination .…”