Altered gravity downregulates aquaporin-1 protein expression in choroid plexus

Aquaporin-1 (AQP1) is a water channel expressed abundantly at the apical po le of choroidal epithelial cells. The protein expression was quantified by immunocytochemistry and confocal microscopy in adult rats adapted to altere d gravity. AQP1 expression was decreased by 64% at the apical pole of choro idal cells in rats dissected 5.5-8 h after a 14-day spaceflight. AQP1 was s ignificantly overexpressed in rats readapted for 2 days to Earth's gravity after an 11-day flight (48% overshoot, when compared with the value measure d in control rats). In a ground-based model that simulates some effects of weightlessness and alters choroidal structures and functions, apical AQP1 e xpression was reduced by 44% in choroid plexus from rats suspended head dow n for 14 days and by 69% in rats suspended for 28 days. Apical AQP1 was rap idly enhanced in choroid plexus of rats dissected 6 h after a 14-day suspen sion (57% overshoot, in comparison with control rats) and restored to the c ontrol level when rats were dissected 2 days after the end of a 14-day susp ension. Decreases in the apical expression of choroidal AQP1 were also note d in rats adapted to hypergravity in the NASA 24-ft centrifuge: AQP1 expres sion was reduced by 47% and 85% in rats adapted for 14 days to 2 G and 3 G, respectively. AQP1 is downregulated in the apical membrane of choroidal ce lls in response to altered gravity and is rapidly restored after readaptati on to normal gravity. This suggests that water transport, which is partly i nvolved in the choroidal production of cerebrospinal fluid, might be decrea sed during spaceflight and after chronic hypergravity.