Characterization of aquaporin-4 in muscle and muscular dystrophy

Aquaporins are a growing family of transmembrane proteins that transport water and, in some cases, glycerol and urea across cellular membranes. Aquaporin-4 (AQP4) is enriched at the sarcolemma of skeletal muscle and may play a role in accommodating the rapid changes in cell volume and hydrostatic forces that occur during contraction in order to prevent damage to the sarcolemma. Recent evidence has shown that AQP4 is absent in dystrophin-deficient mdx mice, suggesting that AQP4 associates with dystrophin and has a role in the dystrophic process. To examine the relationship between aquaporins and muscle disease, and between aquaporins and dystrophin, we have investigated aquaporin expression in various mouse models of muscular dystrophy and cardiomyopathy before and after the onset of pathology. We find that AQP4 is expressed in prenecrotic mdx muscle despite the absence of dystrophin and that AQP4 is lost after the onset of muscle degeneration. Analysis of various dystrophin transgenic mice reveals that AQP4 is lost even when the dystrophin-glycoprotein complex is present, suggesting that loss of AQP4 is not directly resulting from loss of the DGC. AQP4 was also lost in muscular dystrophies caused by primary mutations in the sarcoglycan genes. Taken together, our data demonstrate that AQP4 loss in skeletal muscle correlates with muscular dystrophy and is a common feature of pathogenesis.