The role of aquaporins in excretion in insects

Abstract

SummaryOne of the aspects of insect osmoregulation that has most intrigued researchers is the ability of a simple tubular epithelium, such as the Malpighian tubule, to create both hypo-and hyperosmotic urine. Indeed, Ramsayʼs initial observation that isolated tubules could secrete a hypoosmotic urine led him to attribute the phenomenon to the active transport of water. In the ensuing decades several models for solute recycling have been proposed, but only in the last 15 years has it become clear that tubule water permeability is due to the presence of aquaporins (AQPs), the ubiquitous water transport proteins. There are 13 known human AQPs, and they are tissue and even membrane specific. It is now clear that the number and type of AQPs within a membrane are the major determinants of its water transport capacity. There are many gene homologs for the AQPs, so proof of function requires expression of the protein in a defined system. Within the insects, only seven AQPs have been functionally expressed and, of these, four directly or indirectly function in excretion. In this paper we review the basic structure and general function of AQPs and then examine the source, localization and functional attributes of those isolated from insects.

Identification and validation of a gene causing cross-resistance between insecticide classes inAnopheles gambiaefrom Ghana

“…The closest protein orthologs are putative aquaporins from mosquitoes Aedes aegypti (AAEL005001, 55% amino acid identify), Culex quinquefasciatus (CPIJ009225, 42% amino acid identity), and the louse Pediculus humanus (PHUM474700, 38% amino acid identity). Aquaporins are membrane proteins involved in the movement of water and other solutes across biological membranes, and in insects are often associated with the excretory/osmoregulatory Malpighian tubule system (42,43).…”

Hypotonicity Stimulates Potassium Flux through the WNK-SPAK/OSR1 Kinase Cascade and the Ncc69 Sodium-Potassium-2-Chloride Cotransporter in the Drosophila Renal Tubule

“…Hypotonicity activates the WNK-Fray-Ncc69 pathway because no stimulation of K ϩ flux is seen in tubules with decreased wnk, fray, or Ncc69. Increased KCl secretion drives an increase in osmotically obliged water flux, presumably through aquaporins expressed in the tubule (64), resulting in increased fluid secretion.…”

The role of the gut in insect chilling injury: cold-induced disruption of osmoregulation in the fall field cricket,Gryllus pennsylvanicus

“…Typically, the Malpighian tubules produce primary urine that is roughly isosmotic with the hemolymph by coupling the action of a V-ATPase and H + -cation exchangers, ultimately driving K + , Cl -(and Na + , in blood-feeding insects) transcellularly from the hemolymph into the lumen. This process maintains a gradient favoring movement of water (through aquaporins) and waste products (through dedicated transporters) into the lumen of the tubules (Ramsay, 1954;Pannabecker, 1995;O'Donnell, 2009;Spring et al, 2009). Although some resorption of water and ions occurs along the length of the Malpighian tubule, most water and ion resorption in terrestrial insects (specifically locusts) occurs in the hindgut, where Na + -K + -ATPase and an electrogenic Cl -pump maintain steep local osmotic gradients driving the movement of water and Na + back into the hemolymph (Ramsay, 1971;Hanrahan and Phillips, 1982).…”

“…The bulk movement of ions down their electrochemical gradient is facilitated by ion-specific channels that may or may not be gated (Heimlich et al, 2004;Lang et al, 2007), whereas water movement is facilitated by the presence of aquaporins (Spring et al, 2009).…”