Changes in fluid secretion rate alter net transepithelial transport of MRP2 and P-glycoprotein substrates in Malpighian tubules of Drosophila melanogaster

Transepithelial transport of P-glycoprotein substrate by the Malpighian tubules of the desert locust

Extrusion of xenobiotics is essential for allowing animals to remove toxic substances present in their diet or generated as a biproduct of their metabolism. By transporting a wide range of potentially noxious substrates, active transporters of the ABC transporter family play an important role in xenobiotic extrusion. One such class of transporters are the multidrug resistance P-glycoprotein transporters. Here, we investigated P-glycoprotein transport in the Malpighian tubules of the desert locust (Schistocerca gregaria), a species whose diet includes plants that contain toxic secondary metabolites. To this end, we studied transporter physiology using a modified Ramsay assay in which ex vivo Malpighian tubules are incubated in different solutions containing the P-glycoprotein substrate dye rhodamine B in combination with different concentrations of the P-glycoprotein inhibitor verapamil. To determine the quantity of the P-glycoprotein substrate extruded we developed a simple and cheap method as an alternative to liquid chromatography–mass spectrometry, radiolabelled alkaloids or confocal microscopy. Our evidence shows that: (i) the Malpighian tubules contain a P-glycoprotein; (ii) tubule surface area is positively correlated with the tubule fluid secretion rate; and (iii) as the fluid secretion rate increases so too does the net extrusion of rhodamine B. We were able to quantify precisely the relationships between the fluid secretion, surface area, and net extrusion. We interpret these results in the context of the life history and foraging ecology of desert locusts. We argue that P-glycoproteins contribute to the removal of xenobiotic substances from the haemolymph, thereby enabling gregarious desert locusts to maintain toxicity through the ingestion of toxic plants without suffering the deleterious effects themselves.

The insect excretory system as a target for novel pest control strategies

The insect excretory system plays essential roles in osmoregulation, ionoregulation and toxin elimination. Understanding the mechanisms of fluid and ion transport by the epithelial cells of the excretory system provides a foundation for development of novel pest management strategies. In the present review, we focus on two such strategies: first, impairment of osmoregulation by manipulation of diuretic or antidiuretic signaling pathways and second, interference with toxin elimination by inhibition of toxin transport systems.

Quantitative fluorescence microscopy provides high resolution imaging of passive diffusion and P-gp mediated efflux at the in vivo blood-brain barrier

Quantitative fluorescent microscopy is an emerging technology that has provided significant insight into cellular dye accumulation, organelle function, and tissue physiology. However, historically dyes have only been used to qualitatively or semi-quantitatively (fold change) determine changes in blood-brain barrier (BBB) integrity. Herein, we present a novel method to calculate the blood to brain transfer rates of the dyes rhodamine 123 and Texas red across the in situ BBB. We observed that rhodamine 123 is subject to p-glycoprotein mediated efflux at the rat BBB and can be increased nearly 20-fold with p-glycoprotein inhibition. However, Texas Red appears to not be subject to MRP2 mediated efflux at the rat BBB, agreeing with literature reports suggesting MRP2 may lack functionality at the normal rat BBB. Lastly, we present data demonstrating that once dyes have crossed the BBB, diffusion of the dye molecule is not as instantaneous as has been previously suggested. We propose that future work can now be completed to (1) match BBB transfer coefficients to interstitial diffusion constants and (2) use dyes with specific affinities to cellular organelles or that have specific properties (e.g., subject to efflux transporters) to more fully understand BBB physiology.

First evidence of epithelial transport in tardigrades: a comparative investigation of organic anion transport

We investigated transport of the organic anion Chlorophenol Red (CPR) in the tardigrade Halobiotus crispae using a new method for quantifying non-fluorescent dyes. We compared the results acquired from the tardigrade with CPR transport data obtained from Malpighian tubules of the desert locust Schistocerca gregaria. CPR accumulated in the midgut lumen of H. crispae, indicating that organic anion transport takes place here. Our results show that CPR transport is inhibited by the mitochondrial un-coupler DNP (1 mmol l–1; 81% reduction), the Na+/K+-ATPase inhibitor ouabain (10 mmol l–1; 21% reduction) and the vacuolar H+-ATPase inhibitor bafilomycin (5 μmol l–1; 21% reduction), and by the organic anions PAH (10 mmol l–1; 44% reduction) and probenecid (10 mmol l–1; 61% reduction, concentration-dependent inhibition). Transport by locust Malpighian tubules exhibits a similar pharmacological profile, albeit with markedly higher concentrations of CPR being reached in S. gregaria. Immunolocalization of the Na+/K+-ATPase α-subunit in S. gregaria revealed that this transporter is abundantly expressed and localized to the basal cell membranes. Immunolocalization data could not be obtained from H. crispae. Our results indicate that organic anion secretion by the tardigrade midgut is transporter mediated with likely candidates for the basolateral entry step being members of the Oat and/or Oatp transporter families. From our results, we cautiously suggest that apical H+ and possibly basal Na+/K+ pumps provide the driving force for the transport; the exact coupling between electrochemical gradients generated by the pumps and transport of ions, as well as the nature of the apical exit step, are unknown. This study is, to our knowledge, the first to show active epithelial transport in tardigrades.

Effects of acute or chronic exposure to dietary organic anions on secretion of methotrexate and salicylate by Malpighian tubules of Drosophila melanogaster larvae

The effects of dietary exposure to organic anions on the physiology of isolated Malpighian tubules and on tubule gene expression were examined using larvae of Drosophila melanogaster. Acute (24 h) or chronic (7 d) exposure to type I organic anions (fluorescein or salicylate) was associated with increased fluid secretion rates and increased fluxes of both salicylate and the type II organic anion methotrexate. By contrast, chronic exposure to dietary methotrexate was associated with increased fluid secretion rate and increased flux of methotrexate, but not salicylate. Exposure to methotrexate in the diet resulted in increases in the expression of a multidrug efflux transporter gene (MET; CG30344) in the Malpighian tubules. There were also increases in expression of genes for either a Drosophila multidrug resistance-associated protein (dMRP; CG6214) or an organic anion transporting polypeptide (OATP; CG3380), depending on the concentration of methotrexate in the diet. Exposure to salicylate in the diet was associated with an increase in expression of dMRP and with decreases of MET and OATP. Exposure to dietary salicylate or methotrexate was also associated with different patterns of expression of heat shock protein genes. The results suggest that exposure to specific type I or type II organic anions has multiple effects and results not only in increased organic anion transport but also in increased rates of inorganic ion transport, which drives osmotically-obliged fluid secretion. Increased fluid secretion may enhance secretion of organic anions by eliminating diffusive backflux from the tubule lumen to the hemolymph.

Physiological and molecular characterization of methotrexate transport by Malpighian tubules of adult Drosophila melanogaster

A radioisotope tracer technique and quantitative PCR were used to study the mechanisms and regulation of transepithelial transport of the type II organic anion methotrexate (MTX) by the Malpighian tubules of Drosophila melanogaster. Transport of MTX was saturable and Na(+)-independent; the kinetic parameters J(max) and K(t) were 437fmolmin(-1) and 23.5microM, respectively. The transport of MTX was competitively inhibited by phenol red and probenecid; non-competitively inhibited by salicylate, verapamil and MK-571; and uncompetitively inhibited by Texas Red. Dietary exposure to 0.1mM MTX led to dramatic increases in gene expression for several members of the ABC family of transporters in both the Malpighian tubules and the gut. Our results suggest that multiple transporters are upregulated in response to dietary exposure to MTX. Increased levels of the protein products which may result from expression of these genes may enhance elimination of toxic compounds such as MTX or its metabolites.

Too much of a good thing: how insects cope with excess ions or toxins in the diet

Much of our understanding of the ionoregulatory and excretory physiology of blood-feeding insects can be traced to a series of papers by Simon Maddrell and colleagues in the 1970s and 1980s. These studies of the Malpighian (renal)tubules of Rhodnius prolixus revealed a number of physiological adaptations to the short-term and long-term stresses associated with blood feeding. More recent electrophysiological studies using voltage- and ion-selective microelectrodes have extended our understanding of the mechanisms and control of ion transport by the secretory and reabsorptive segments of the Rhodnius Malpighian tubule. The discovery that the rates of transport of organic anions, urates and Ca2+ are synchronized to coincide with the appearance of the products of blood meal digestion in the haemolymph of Rhodnius has stimulated parallel studies in Drosophila. This recent research has examined how excretory mechanisms for organic cations and organic anions are altered by exposure to such compounds in the diet. These studies also show that the Drosophila Malpighian tubule provides a useful model for analysis of the roles of transporters such as P-glycoproteins and multidrug resistance-associated proteins in the excretion of toxins.

Neuroarchitecture of aminergic systems in the larval ventral ganglion of Drosophila melanogaster

Biogenic amines are important signaling molecules in the central nervous system of both vertebrates and invertebrates. In the fruit fly Drosophila melanogaster, biogenic amines take part in the regulation of various vital physiological processes such as feeding, learning/memory, locomotion, sexual behavior, and sleep/arousal. Consequently, several morphological studies have analyzed the distribution of aminergic neurons in the CNS. Previous descriptions, however, did not determine the exact spatial location of aminergic neurite arborizations within the neuropil. The release sites and pre-/postsynaptic compartments of aminergic neurons also remained largely unidentified. We here used gal4-driven marker gene expression and immunocytochemistry to map presumed serotonergic (5-HT), dopaminergic, and tyraminergic/octopaminergic neurons in the thoracic and abdominal neuromeres of the Drosophila larval ventral ganglion relying on Fasciclin2-immunoreactive tracts as three-dimensional landmarks. With tyrosine hydroxylase- (TH) or tyrosine decarboxylase 2 (TDC2)-specific gal4-drivers, we also analyzed the distribution of ectopically expressed neuronal compartment markers in presumptive dopaminergic TH and tyraminergic/octopaminergic TDC2 neurons, respectively. Our results suggest that thoracic and abdominal 5-HT and TH neurons are exclusively interneurons whereas most TDC2 neurons are efferent. 5-HT and TH neurons are ideally positioned to integrate sensory information and to modulate neuronal transmission within the ventral ganglion, while most TDC2 neurons appear to act peripherally. In contrast to 5-HT neurons, TH and TDC2 neurons each comprise morphologically different neuron subsets with separated in- and output compartments in specific neuropil regions. The three-dimensional mapping of aminergic neurons now facilitates the identification of neuronal network contacts and co-localized signaling molecules, as exemplified for DOPA decarboxylase-synthesizing neurons that co-express crustacean cardioactive peptide and myoinhibiting peptides.

Effects of chronic exposure to dietary salicylate on elimination and renal excretion of salicylate by Drosophila melanogaster larvae

The effects of chronic exposure to dietary salicylate on elimination and renal excretion of salicylate by D. melanogaster larvae were evaluated using salicylate-selective microelectrodes. Larvae chronically exposed to dietary salicylate showed 25% less salicylate in the haemolymph compared to the control group after feeding on a salicylate-enriched diet. By 1 h after transfer to a salicylate-free diet the levels of salicylate in the haemolymph of larvae raised on dietary salicylate were 46% lower than in the control group. Salicylate flux increased dramatically across Malpighian tubules but not across midgut or hindgut isolated from larvae chronically exposed to dietary salicylate, relative to the control group. Malpighian tubules isolated from experimental larvae showed a 4.7-fold increase in Kt and a nearly 5-fold increase in Jmax relative to the control. These changes in salicylate transport were accompanied by a 3.2-fold increase in fluid secretion rate. Moreover, the high rates of fluid secretion by the Malpighian tubules isolated from experimental larvae were stimulated 2.1-fold and 2.8-fold when tubules were challenged with 1 mmol l(-1) cAMP and 10 micromol l(-1) leucokinin I, respectively. Taken together, these results indicate that chronic exposure of D. melanogaster larvae to dietary salicylate alters elimination of such toxins from the haemolymph and increases the basal rate of fluid secretion and excretion of salicylate by the Malpighian tubules.