In the modern age of genomes and phylogenetics, the odds can be stacked against you if you work in a non-model organism. So when Ann Stuart was curious to find out more about one group of transporter proteins (known as SLC6 transporters) in her favourite organism, the barnacle, she knew she would have little luck unless she moved to a better understood creature. Which is why she shifted her gaze to the recently sequenced fruit fly, Drosophila. With the benefit of the insect's intact genome, Stuart knew that she and her team would be able to discover which of the SLC6 transporters the insect carried and where they were expressed to get a better understanding of their physiological role(p. 3383).
SLC6 transporters are a large group of proteins that transport a whole suite of physiologically significant molecules across cell membranes. Powered by sodium and chloride gradients, many of these transporters are involved in neurotransmitter transport and are found in the central nervous system, where the ionic environment is dominated by sodium. Matthew Thimgan and bioinformaticist Jonathan Berg began searching the insect's genome with the sequences of serotonin, dopamine and norepinephrine SLC6 transporters.
Thimgan remembers that he and Berg swiftly drew up a list of 21 Drosophila SLC6 genes, but refining the phylogenetic tree showing the relationships between various members of the diverse family took much longer. Out of the 21 genes, Thimgan and Berg found members of six of the SLC6 subfamilies, including the monoamine transporters, GABA transporters and amino acid transporters.
Curious to know which tissues express the Drosophila SLC6 genes,Thimgan began exploring microscopically thin insect body sections with RNA probes, designed to recognise mRNA from all 21 SLC6 genes, to find out where each gene is expressed. Thimgan remembers that planing the frozen insects'tiny bodies was complicated by their exoskeletons, but after a few weeks, he'd mastered the technique.
Knowing that many of the genes were involved in neurotransmitter transport,Thimgan concentrated on searching for SLC6 genes in the insect's head, and sure enough, the two main neurotransmitter (seratonin and dopamine)transporters were expressed in the head and thorax. Meanwhile, the Drosophila GABA transporter turned up in the glia cells surrounding neurones in the central nervous system; glia cells are often responsible for GABA reuptake from synapses. Surprisingly, Thimgan only found one amino acid transporter in the insect's central nervous system. He explains that some amino acids are key neurotransmitters with dedicated transporters in mammals,but he suspects that the sole insect amino acid transporter is not specific and probably transports a wide range of amino acids.
The main surprise came when Thimgan began looking for the recently identified group of insect amino acid transporters (IAAT) that are driven by either sodium or potassium gradients, allowing them to function in tissues other than the central nervous system. Drosophila has six IAATs,which cropup throughout the insect's body including in the digestive tract,Malpighian tubules and the female's reproductive system. While the IAATs probably transport neurotransmitter amino acids in the central nervous system,Thimgan suspects that they transport digested amino acids in the digestive tract and could be involved in loading nutrients into females' gametes ready to fuel embryos through gestation.
Thimgan hopes that this thorough analysis of SLC6 transporters in Drosophila could be `a jumping off point in Drosophilaphysiology... improving its function as a physiological model organism' he adds.