Expression of multiple AQP4 pools in the plasma membrane and their association with the dystrophin complex

Altered aquaporin-4 (AQP4) expression has been reported in
brain edema, tumors, muscular dystrophy, and neuromyelitis
optica. However, the plasma membrane organization of AQP4
and its interaction with proteins such as the dystrophin-associated
protein complex are not well understood. In this study,
we used sucrose density gradient ultracentrifugation and 2D
blue native/sodium dodecyl sulfate-polyacrylamide gel electrophoresis
and showed the expression of several AQP4
multi-subunit complexes (pools) of different sizes, ranging
from >>1 MDa to 500 kDa and containing different ratios of
the 30/32 kDa AQP4 isoforms, indicative of orthogonal arrays
of particles of various sizes. A high molecular weight pool copurified
with dystrophin and beta-dystroglycan and was drastically
reduced in the skeletal muscle of mdx3cv mice, which
have no dystrophin. The number and size of the AQP4 pools
were the same in the kidney where dystrophin is not
expressed, suggesting the presence of dystrophin-like proteins
for their expression. We found that AQP2 is expressed
only in one major pool of 500 kDa, indicating that the
presence of different pools is a peculiarity of AQP4 rather than
a widespread feature in the AQP family. Finally, in skeletal
muscle caveolin-3 did not co-purify with any AQP4 pool,
indicating the absence of interaction of the two proteins and
confirming that caveolae and orthogonal arrays of particles
are two independent plasma membrane microdomains. These
results contribute to a better understanding of AQP4 membrane
organization and raise the possibility that abnormal
expression of specific AQP4 pools may be found in pathological
states.