| The plasma membrane transporters comprise a class of proteins that mediate the flux of substrates into cells from the extracellular space. They serve as cellular gatekeepers, and control most substances that go in (or out) of the cell. Unsurprisingly, any defect in the transport activity will lead to cellular dysfunction and disease. Here, I use the fruit fly Drosophila melanogaster as a genetic model to study the function of neurotransmitter and polyamine transporters. First, a screen for novel plasma membrane neurotransmitter transporters is performed, and reveals three different transporters (CG4476, CG13794 and CG13795) that are preferentially expressed in the fly eye. In vivo expression of these transporter candidates was confirmed by RT-PCR, and their developmental expression profile elucidated. One of these transporter candidates, CG4476, was further analyzed and a deletion mutant created to test for function. These mutants appeared to be blind in a fast phototaxis assay, although their electroretinograms were normal, suggesting that CG4476 functions in vision modulation rather than light perception. Second, the splice variant B form of the Drosophila vesicular monoamine transporter (dVMAT) was further characterized and found to be exclusively expressed in a subset of subretinal glia. A mutation of the VMAT gene not only significantly reduces the expression of dVMAT-B, but also of histamine in these glia. Interestingly, the decrease in global histamine correlates to a marked increase in light attraction in a fast phototaxis assay, suggesting a direct role of dVMAT-B in vision. Last, the first polyamine uptake activity described in Drosophila is presented. The spermine and spermidine uptake activity is temperature dependent, saturable, and inhibited by the polyamine transporter specific blockers methylglyoxal bis(guanylhydrazone)(MGBG) and paraquat. As in mammals, polyamine uptake in Drosophila S2 cells is solely H+-dependent. Interestingly, in the fruit fly, polyamine transport presents a pharmacological profile similar, if not identical to the Solute carrier (Slc) family 22 of organic cation transporters. The genetic tools available in the fruit fly should permit the complete characterization of these transporters, as well as the elucidation of their regulatory pathways. |
