GPRGR22 (green) and GPRGR24 (pink) are co-expressed (white) in mosquito maxillary palps.
Hold your breath. That may be the only way to avoid detection by pesky mosquitoes, which use carbon dioxide to locate their prey. How do they sense a gas? Jones et al. report that two taste receptor genes mediate carbon dioxide sensation in a recent article in Nature.
Unlike mosquitoes, Drosophila avoid carbon dioxide, perhaps to stay away from dangerous sources of food. A group of sensory neurons in Drosophila antennae called ab1C respond to carbon dioxide. The chemosensory receptor Gr21a, which is related to Drosophila taste receptors, localizes to ab1C neurons. Drosophila lacking Gr21a do not respond to carbon dioxide.
The authors searched for other chemosensory receptors in ab1C neurons. Fluorescent RNA in situ hybridization showed that Gr63a colocalized with Gr21a in ab1C cells. Drosophila larvae also avoid carbon monoxide through a Gr21a-dependent mechanism. Gr21a and Gr63a promoters drove the expression of green fluorescent protein reporters in the same cells of Drosophila larvae.
Co-expression of Gr21a and Gr63a is sufficient to induce sensitivity to carbon dioxide in Drosophila. The authors ectopically expressed Gr21a and Gr63a in olfactory neurons that normally do not respond to carbon dioxide. Carbon dioxide increased firing only in neurons co-expressing Gr21a and Gr63a, not in neurons expressing either receptor alone. Like ab1C neurons, cells expressing both Gr21a and Gr63a showed dose-dependent responses to carbon dioxide. In Gr63a knockout Drosophila, ab1C neurons did not respond to carbon dioxide. Targeted expression of Gr63a in Gr21a-expressing cells rescued carbon dioxide response in ab1C cells of Gr63a knockouts.
A T-maze is a chamber shaped like a plus sign with different odors placed at the end of each arm. Drosophila avoid carbon dioxide in the T-maze. Gr63a knockouts were indifferent to carbon dioxide, as were Gr63a knockouts expressing either Gr63a under the control of the yeast upstream activation sequence or the yeast transcription factor GAL4 under the control of Gr21a promoter. However, carbon dioxide avoidance was rescued in Gr63a knockouts expressing both transgenes.
Do Gr21a and Gr63a mediate carbon dioxide sensation in other insects? The mosquito homologs for Gr21a and Gr63a are GPRGR22 and GPRGR24, respectively. The authors colocalized GPRGR22 and GPRGR24 in carbon dioxide-sensitive neurons of the mosquito maxillary palp. Therefore, GPRGR22 and GPRGR24 are likely to mediate carbon dioxide sensation in mosquitoes.
Mosquito-borne diseases, like malaria, certain forms of encephalitis, West Nile virus, yellow fever and dengue fever, cause approximately one million deaths each year. Therefore, blockade of mosquito carbon dioxide receptors might not only be a clever method of pest control, but also an important way to prevent disease.