Mitochondria are remarkably plastic organelles constantly changing their shape to fulfil their various functional activities. Although the osmotic movement of water into and out of the mitochondrion is central for its morphology and activity, the molecular mechanisms and the pathways for water transport across the inner mitochondrial membrane (IMM), the main barrier for molecules moving into and out of the organelle, are completely unknown. Here, we show the presence of a member of the aquaporin family of water channels, AQP8, and demonstrate the strikingly high water permeability (Pf) characterizing the rat liver IMM. Immunoblotting, electron microscopy, and biophysical studies show that the largest mitochondria feature the highest AQP8 expression and IMM Pf. AQP8 was also found in the mitochondria of other organs, whereas no other known aquaporins were seen. The osmotic water transport of liver IMM was partially inhibited by the aquaporin blocker Hg2+, while the related activation energy remained low, suggesting the presence of a Hg2+-insensitive facilitated pathway in addition to AQP8. It is suggested that AQP8-mediated water transport may be particularly important for rapid expansions of mitochondrial volume such as those occurring during active oxidative phosphorylation and those following apoptotic signals.

Objective: Glycerol constitutes an important metabolite for the control of lipid accumulation and glucose homeostasis. The impact of obesity and obesity-associated type 2 diabetes as well as the potential regulatory role of insulin and leptin on aquaglyceroporins (AQP) 3, 7, and 9 were analyzed. Research Design and Methods: The tissue distribution and expression of AQP in biopsies of omental and sc adipose tissue as well as liver were analyzed in lean and obese Caucasian volunteers (n = 63). The effect of insulin (1, 10, and 100 nmol/liter) and leptin (0.1, 1, and 10 nmol/liter) on the expression of the glycerol channels was determined in vitro in human omental adipocytes and HepG2 hepatocytes. The translocation of AQP in response to insulin and isoproterenol was analyzed by immunocytochemistry. Results: In addition to the well-known expression of AQP7 in adipose tissue, AQP3 and AQP9 were also expressed in both omental and sc adipose tissue. Obese type 2 diabetes patients showed higher expression of AQP in visceral adipose tissue and lower expression of AQP7 in sc adipose tissue and hepatic AQP9. The staining of AQP9 in the plasma membrane of adipocytes was reinforced by insulin, whereas isoproterenol induced the translocation of AQP3 and AQP7 from the lipid droplets to the plasma membrane. Insulin up-regulated all AQP, whereas leptin up-regulated AQP3 and down-regulated AQP7 and AQP9 in adipocytes and hepatocytes. These effects were abrogated by both the phosphatidylinositol 3-kinase inhibitor wortmannin and the mammalian target of rapamycin inhibitor rapamycin. Conclusions: Our findings show, for the first time, that insulin and leptin regulate the AQP through the phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin pathway in human visceral adipocytes and hepatocytes. AQP3 and AQP7 may facilitate glycerol efflux from adipose tissue while reducing the glycerol influx into hepatocytes via AQP9 to prevent the excessive lipid accumulation and the subsequent aggravation of hyperglycemia in human obesity.

The aquaporin family of molecular water channels is widely expressed throughout the plant and animal kingdoms. No bacterial aquaporins are known; however, sequence-related bacterial genes have been identified that encode glycerol facilitators (glpF). By homology cloning, a novel aquaporin-related DNA (aqpZ) was identified that contained no surface N-glycosylation consensus. The aqpZ RNA was not identified in mammalian mRNA by Northern analysis and exhibited bacterial codon usage preferences. Southern analysis failed to demonstrate aqpZ in mammalian genomic DNA, whereas a strongly reactive DNA was present in chromosomal DNA from Escherichia coli and other bacterial species and did not correspond to glpF. The aqpZ DNA isolated from E. coli contained a 693-base pair open reading frame encoding a polypeptide 28-38% identical to known aquaporins. When compared with other aquaporins, aqpZ encodes a 10-residue insert preceding exofacial loop C, truncated NH2 and COOH termini, and no cysteines at known mercury-sensitive sites. Expression of aqpZ cRNA conferred Xenopus oocytes with a 15-fold increase in osmotic water permeability, which was maximal after 5 days of expression, was not inhibited with HgCl2, exhibited a low activation energy (Ea = 3.8 kcal/mol), and failed to transport nonionic solutes such as urea and glycerol. In contrast, oocytes expressing glpF transported glycerol but exhibited limited osmotic water permeability. Phylogenetic comparison of aquaporins and homologs revealed a large separation between aqpZ and glpF, consistent with an ancient gene divergence.