ABSTRACT
Water channel aquaporin-4 (AQP4) is predominantly expressed in glial cells
and has been implicated in brain edema resulting from water intoxication,
brain ischemia, or meningitis. An AQP4-speciic inhibitor suitable for
clinical use would thus be expected to help protect against brain edema. We
conirmed acetazolamide (AZA) speciically inhibits water permeation of the
rat AQP4 (rAQP4) channel, by measurement of water conduction through
AQP4-reconstituted liposomes. The structural study of this channel is also
important for understanding complex functions of the water channel in the
brain. The aqp4 gene is known to express two different isoforms, and only the shorter isoforms assemble into orthogonal arrays, while the longer isoforms
interfere with array formation. Two-dimensional (2D) crystals of AQP4
were revealed to be the same AQP4 packing as orthogonal arrays observed
in vivo. By sodium dodecyl sulfate (SDS)-digested freeze-fracture replica labeling (FRL), palmitoylation of the N-terminal cysteine was also identiied
to be involved in array inhibition of the longer isoform. The structure of the
double-layered 2D crystal of AQP4 suggested weak but speciic interactions
between tetramers in adjoining membranes, which may provide us insights
into an unknown and unexpected role for AQP4 in the adhesion of membrane
layers. Knowledge about multiplicity of AQP4 observed in structural studies
will provide an important building block for understanding the functional
complexity in our brain, while these functions remain to be elucidated.
