Ontogeny, distribution, and possible functional implications of an unusual aquaporin, AQP8, in mouse liver

Aquaporins are channel proteins widely expressed in nature and known to facilitate the rapid movement of water across numerous cell membranes. A mammalian aquaporin, AQP8, was recently discovered and found to have a very distinct evolutionary pathway. To understand the reason for this divergence, here we define the ontogeny and exact subcellular localization of AQP8 in mouse liver, a representative organ transporting large volumes of water for secretion of bile. Northern blotting showed strong AQP8 expression between fetal day 17 and birth as well as at weaning and thereafter. Interestingly, this pattern was confirmed by immunohistochemistry and coincided both temporally and spatially with that of hepatic glycogen accumulation. As seen by reverse-transcription polymerase chain reaction (RTPCR) and immunohistochemistry, fasting was accompanied by remarkable down-regulation of hepatic AQP8 that paralleled the expected depletion of glycogen content. The level of hepatic AQP8 returned to be considerable after refeeding. Immunoelectron microscopy confirmed AQP8 in hepatocytes where labeling was over smooth endoplasmic reticulum (SER) membranes adjacent to glycogen granules and in canalicular membranes, subapical vesicles, and some mitochondria. In conclusion, in addition to supporting a role for AQP8 in canalicular water secretion, these findings also suggest an intracellular involvement of AQP8 in preserving cytoplasmic osmolality during glycogen metabolism and in maintaining mitochondrial volume. AQP8 may have evolved separately to feature these intracellular roles as no other known aquaporin shows this specialization.