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1 The Water and Salt Research Center, University of Aarhus, Aarhus, Denmark; Department of Animal Physiology, University of Warmia and Mazury, Olsztyn, Poland
2 The Water and Salt Research Center, University of Aarhus, Aarhus, Denmark
3 Department of Molecular Biology, University of Aarhus, Denmark
* To whom correspondence should be addressed. E-mail: sn{at}ana.au.dk.
AQP7 is expressed in proximal tubules and is involved in glycerol uptake. The cellular expression and physiological function in other organs remains largely undefined. AQP7 knockout (KO) mice were generated and used for immunohistochemical analyses to define the organ and cellular expression of AQP7. AQP7 labeling was found in kidney proximal tubule, heart, skeletal muscle, testis, epididymis, as well as in white and brown adipose tissue (WAT and BAT) of wild type mice. Importantly, immunoreactivity was completely absent from these tissues in AQP7 KO mice. At the cellular level the capillary endothelium WAT and BAT displayed prominent staining, whereas AQP7 labeling in adipocyte membranes was undetectable. Double labeling confocal microscopy revealed co-expression of AQP7 with capillary AQP1 but not with adipocyte GLUT4. Moreover, immunoelectron microscopy and RT-PCR of isolated microvessels confirmed the vascular AQP7 expression. Distinct immunolabeling of the capillary endothelium was also observed in both skeletal and heart muscle with no staining of skeletal or cardiac myocytes. As previously reported, specific immunolabeling was confined to brush border in segment 3 renal proximal tubules and to spermatids and spermatozoa in male reproductive tract. The expression of AQP7 was induced up to 2.2-fold in WAT of mice with streptozotocin-induced diabetes mellitus (S-DM) compared to controls and fasting for 72 hr (but not 24 hr) induced significant increase in AQP7 expression. In conclusion, AQP7 is expressed in capillary endothelia of adipose tissue (and cardiac and striated muscle) and is up-regulated in WAT in response to S-DM supporting its role in glycerol metabolism.
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