Tottori University, Tottori 680-8553, Japan.
J Insect Physiol. 2012 Apr;58(4):523-33. doi: 10.1016/j.jinsphys.2012.01.007. Epub 2012 Jan 21.
Larval lepidopteran and coleopteran insects have evolved a specialised cryptonephric system in the hindgut in which water is constantly and rapidly taken up before defecation. In the silkworm, Bombyx mori, the movement of water through the epithelia within the cryptonephric rectal complex is likely facilitated by the two aquaporins, AQP-Bom1 and AQP-Bom3. Both are functionally water-specific and are predominantly expressed in the hindgut (colon and rectum). Phylogenetically, AQP-Bom1 and AQP-Bom3 belong to the DRIP (Drosophila integral protein) and PRIP (Pyrocoelia rufa integral protein) subfamilies, respectively, of the insect AQP clade. In immunoblot analyses using antipeptide antibodies for each Bombyx AQP, the predicted molecular mass for the respective AQPs were around 25 kDa, and further indicated that both tended to be oligomerised as a homotetramer (∼110 kDa). AQP-Bom1 [DRIP] was exclusively expressed at the apical plasma membrane of colonic and rectal epithelial cells, whereas AQP-Bom3 [PRIP] was expressed at the basal plasma membrane of these cells. This polarised localisation of DRIP/PRIP was also observed in the outer cryptonephric Malpighian tubules (outer cMT) and in the six tubules just outside the cryptonephric rectal complex (rectal lead MT). In the rectal epithelia, water is transported from the rectal lumen to the perinephric space and then deposited into the lumen of the outer cMT; the water then goes through the tubular lumen to exit the complex and is finally transported across the rectal lead MT. We conclude that rectal water retrieval into the haemocoele occurs at the very limited region of the water-permeable sites in MT epithelia after passing the rectal and cMT epithelia and that the high osmotic permeability is due to the presence of two distinct water-specific AQPs (DRIP and PRIP) in the epithelial cells of lepidopteran hindgut.
鳞翅目和鞘翅目幼虫昆虫在其后肠中进化出一种特殊的隐肾系统，在排泄前不断快速地吸收水分。在桑蚕（Bombyx mori）中，AQP-Bom1 和 AQP-Bom3 这两种水通道蛋白可能促进了水在隐肾直肠复合体上皮细胞内的运动。这两种蛋白都是功能上的水通道蛋白，主要在昆虫的后肠（结肠和直肠）中表达。从系统发生的角度来看，AQP-Bom1 和 AQP-Bom3 分别属于昆虫 AQP 类群的 DRIP（果蝇整合蛋白）和 PRIP（Pyrocoelia rufa 整合蛋白）亚家族。在使用针对每个 Bombyx AQP 的肽抗体进行的免疫印迹分析中，预测的相应 AQP 的分子质量约为 25 kDa，并进一步表明两者都倾向于作为同源四聚体（~110 kDa）聚合。AQP-Bom1 [DRIP] 仅在结肠和直肠上皮细胞的顶质膜上表达，而 AQP-Bom3 [PRIP] 则在这些细胞的基底质膜上表达。这种 DRIP/PRIP 的极性定位也在外部隐肾马尔皮基氏管（外 cMT）和隐肾直肠复合体外部的六根管（直肠前 MT）中观察到。在直肠上皮中，水从直肠腔运输到肾周腔，然后沉积到外 cMT 的腔中；然后水通过管状腔离开复合体，最后通过直肠前 MT 运输。我们得出结论，水从直肠进入血腔是在穿过直肠和 cMT 上皮后，在外 MT 上皮的水通透部位非常有限的区域发生的，高渗透压通透性是由于在鳞翅目昆虫的后肠上皮细胞中存在两种不同的水通道蛋白（DRIP 和 PRIP）。
