Synergy and specificity of two Na+–aromatic amino acid symporters in the model alimentary canal of mosquito larvae

Abstract

SUMMARYThe nutrient amino acid transporter (NAT) subfamily is the largest subdivision of the sodium neurotransmitter symporter family (SNF; also known as SLC6; HUGO). There are seven members of the NAT population in the African malaria mosquito Anopheles gambiae, two of which, AgNAT6 and AgNAT8, preferably transport indole-and phenyl-branched substrates, respectively. The relative expression and distribution of these aromatic NATs were examined with transporter-specific antibodies in Xenopus oocytes and mosquito larval alimentary canal, representing heterologous and tissue expression systems, respectively. NATspecific aromatic-substrate-induced currents strongly corresponded with specific accumulation of both transporters in the plasma membrane of oocytes. Immunolabeling revealed elevated expressions of both transporters in specific regions of the larval alimentary canal, including salivary glands, cardia, gastric caeca, posterior midgut and Malpighian tubules. Differences in relative expression densities and spatial distribution of the transporters were prominent in virtually all of these regions, suggesting unique profiles of the aromatic amino acid absorption. For the first time reversal of the location of a transporter between apical and basal membranes was identified in posterior and anterior epithelial domains corresponding with secretory and absorptive epithelial functions, respectively. Both aromatic NATs formed putative homodimers in the larval gut whereas functional monomers were over-expressed heterologously in Xenopus oocytes. The results unequivocally suggest functional synergy between substrate-specific AgNAT6 and AgNAT8 in intracellular absorption of aromatic amino acids. More broadly, they suggest that the specific selectivity, regional expression and polarized membrane docking of NATs represent key adaptive traits shaping functional patterns of essential amino acid absorption in the metazoan alimentary canal and other tissues.

A SLC6 transporter of the novel B0,- system aids in absorption and detection of nutrient amino acids in Caenorhabditis elegans

“…As expected, we found strong constitutive expression of a transcriptional snf-5-specific EGFP reporter in the absorptive epithelial cells of the alimentary canal. This finding is consistent with the anticipated NAT role of SNF-5 in the nutrient absorption of amino acids, as was similarly identified for insect NATs via in situ hybridization and immunolabeling (Meleshkevitch et al, 2006;Meleshkevitch et al, 2009a;Okech et al, 2008). The transcriptional marker cannot address membrane localization of the transporter.…”

“…The transcriptional marker cannot address membrane localization of the transporter. However, we expect apical docking of this transporter, as was reported from immunolabeling of insect and mammalian NATs in the insect gut and mammalian intestinal and renal epithelia (Okech et al, 2008;Romeo et al, 2006;Vanslambrouck et al, 2010). In contrast, SNF-5 expression in rectal gland cells may indicate basolateral localization, supporting the secretory function of the organ, as was previously identified for insect NATs in the salivary gland and other secretory tissues (Okech et al, 2008).…”

“…However, we expect apical docking of this transporter, as was reported from immunolabeling of insect and mammalian NATs in the insect gut and mammalian intestinal and renal epithelia (Okech et al, 2008;Romeo et al, 2006;Vanslambrouck et al, 2010). In contrast, SNF-5 expression in rectal gland cells may indicate basolateral localization, supporting the secretory function of the organ, as was previously identified for insect NATs in the salivary gland and other secretory tissues (Okech et al, 2008). Previous studies in insects showed unique patterns of NAT expression in neurons of the central ganglia and sensory system, which, together with the aromatic amino acid specificity of these transporters, suggests their role as substrate providers for the synthesis of monoamine neurotransmitters (Meleshkevitch et al, 2006;Meleshkevitch et al, 2009a).…”

Tissue specific ionomotive enzyme activity and K+ reabsorption reveal the rectum as an important ionoregulatory organ in larvalChironomous ripariusexposed to varying salinity

“…The physiological consequences of decreased NKA and VA activity in the PMG of larval C. riparius in response to IPW rearing are unclear (see Fig.2C, Fig.3C). In mosquitoes, the PMG is implicated in Na + absorption through VA-driven cation/amino acid symport across the apical membrane and subsequent NKA-driven Na + transport across the basal membrane into the hemolymph (Patrick et al, 2006;Okech et al, 2008b;Boudko et al, 2005;Rheault et al, 2007). If similar transport processes were present in the PMG of C. riparius, then the observed enzymatic activity decrease in IPW rearing conditions would not be consistent with conserving ions or nutrient uptake in dilute conditions.…”

“…The foregut, encompassed by the esophagus, receives the food bolus and moves it to the midgut (Clements, 1992;Dow, 1986). The midgut, which is further divided into the gastric caeca, anterior midgut and posterior midgut, is important for maintaining ion, fluid and acid-base balance in aquatic insect larvae as it is the main uptake site of minerals, water and nutrients, which are ingested as or with food (Dow, 1986;Clark et al, 1999;Clark et al, 2005;Khodabandeh, 2006;Boudko, 2012;Okech et al, 2008a;Okech et al, 2008b;Linser et al, 2009;Jagadeshwaran et al, 2010). The hindgut, which includes an ileum and a rectum, and the Malpighian tubules together constitute the excretory system.…”