Aquaporin and aquaglyceroporin in silkworms, differently expressed in the hindgut and midgut of Bombyx mori

Two cDNAs similar to aquaporins (AQPs) from other insect species were identified and characterized from the silkworm larva, Bombyx mori. The first cDNA (AQP-Bom1) cloned from the anterior silk gland encodes a 25 900 Da protein similar to insect AQPs isolated from several liquid-feeding insects. The second cDNA (AQP-Bom2) cloned from the posterior midgut encodes a 27 694 Da protein. Northern blot analysis has revealed that the AQP-Bom1 mRNA (2.3 kb) is expressed predominantly in the hindgut (colon and rectum), and moderately or minimally in the silk gland, midgut and Malpighian tubules, while the AQP-Bom2 mRNA (1.3 kb) is mainly expressed in the posterior midgut and Malpighian tubules. Functional analysis in Xenopus oocytes microinjected with the cRNA of these AQPs revealed that the AQP-Bom1 mRNA encodes a water-specific aquaporin, likely involved in the water retrieval function of the hindgut, while the AQP-Bom2 mRNA encodes an aquaglyceroporin, increasing glycerol and urea uptake.