IMPORTANCE To understand the predilection for optic nerve and spinal cord pathologic changes in neuromyelitis optica (NMO). OBJECTIVE To evaluate tissue-specific expression (RNA and protein) and supramolecular aggregation of aquaporin 4 (AQP4) in mouse, rat, and human tissues. DESIGN Quantitative polymerase chain reaction, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and blue native polyacrylamide gel electrophoresis. SETTING Laboratory analysis of mouse, rat, and human tissue. PARTICIPANTS Tissues from control individuals and 1 patient with NMO obtained at autopsy. EXPOSURE Neuromyelitis optica in the patient. MAIN OUTCOMES AND MEASURES Tissue-specific messenger RNA and protein expression and supramolecular aggregation of AQP4. RESULTS The AQP4 messenger RNA and proteins were much more highly expressed in the optic nerve and spinal cord relative to other regions of the brain and to non-central nervous system tissues in all species evaluated. Large supramolecular aggregates of AQP4 were overrepresented in the optic nerve and spinal cord relative to other central nervous system tissue. There was no difference in AQP4 expression between an individual with NMO and the control samples. CONCLUSIONS AND RELEVANCE Optic nerve tissue from an individual with NMO did not differ in AQP4 expression from control samples. The relatively high levels of expression and of supramolecular aggregation in the optic nerve and spinal cord may contribute to the predilection of these structures to NMO pathologic changes.