Aquaporin-4-binding autoantibodies in patients with neuromyelitis optica impair glutamate transport by down-regulating EAAT2.
Neuromyelitis optica (NMO)-immunoglobulin G (IgG) is a clinically validated serum biomarker that distinguishes relapsing central nervous system (CNS) inflammatory demyelinating disorders related to NMO from multiple sclerosis. This autoantibody targets astrocytic aquaporin-4 (AQP4) water channels. Clinical, radiological, and immunopathological data suggest that NMO-IgG might be pathogenic. Characteristic CNS lesions exhibit selective depletion of AQP4, with and without associated myelin loss; focal vasculocentric deposits of IgG, IgM, and complement; prominent edema; and inflammation. The effect of NMO-IgG on astrocytes has not been studied. In this study, we demonstrate that exposure to NMO patient serum and active complement compromises the membrane integrity of CNS-derived astrocytes. Without complement, astrocytic membranes remain intact, but AQP4 is endocytosed with concomitant loss of Na(+)-dependent glutamate transport and loss of the excitatory amino acid transporter 2 (EAAT2) . Our data suggest that EAAT2 and AQP4 exist in astrocytic membranes as a macromolecular complex. Transport-competent EAAT2 protein is up-regulated in differentiating astrocyte progenitors and in nonneural cells expressing AQP4 transgenically. Marked reduction of EAAT2 in AQP4-deficient regions of NMO patient spinal cord lesions supports our immunocytochemical and immunoprecipitation data. Thus, binding of NMO-IgG to astrocytic AQP4 initiates several potentially neuropathogenic mechanisms: complement activation, AQP4 and EAAT2 down-regulation, and disruption of glutamate homeostasis.
Department of Laboratory Medicine and Pathology, Mayo Clinic College of Medicine, Rochester, MN 55905, USA.
Shannon R
SR
Hinson
Shanu F
SF
Roemer
Claudia F
CF
Lucchinetti
James P
JP
Fryer
Thomas J
TJ
Kryzer
Jayne L
JL
Chamberlain
Charles L
CL
Howe
Sean J
SJ
Pittock
Vanda A
VA
Lennon
D051401:Aquaporin 4 / Q000276:immunology* / Q000378:metabolism; D001253:Astrocytes / Q000166:cytology / Q000378:metabolism*; D001323:Autoantibodies / Q000276:immunology / Q000378:metabolism*; D001692:Biological Transport / Q000276:immunology; D015153:Blotting, Western; D002462:Cell Membrane / Q000187:drug effects; D003165:Complement System Proteins / Q000633:toxicity; D017931:DNA Primers / Q000235:genetics; D027342:Excitatory Amino Acid Transporter 2; D005786:Gene Expression Regulation / Q000276:immunology*; D050491:Glutamate Plasma Membrane Transport Proteins / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism*; D006801:Humans; D007074:Immunoglobulin G / Q000378:metabolism; D047468:Immunoprecipitation; D009471:Neuromyelitis Optica / Q000276:immunology* / Q000473:pathology; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013116:Spinal Cord / Q000378:metabolism
C496030:AQP4 protein, human; D051401:Aquaporin 4; D001323:Autoantibodies; D017931:DNA Primers; D027342:Excitatory Amino Acid Transporter 2; D050491:Glutamate Plasma Membrane Transport Proteins; D007074:Immunoglobulin G; C493397:SLC1A2 protein, human; D018698:Glutamic Acid; D003165:Complement System Proteins