- Email: [email protected]
Olfaction: Mosquitoes Love Your Acid Odors
Current Biology
Dispatches transfer and generalization after perceptual learning. For example, previous research had demonstrated that transfer is critically affected by initial training conditions, such as training in more than one condition, reducing visual adaptation, or training at different difficulty levels [12–16]. In addition, mere exposure to repetitive stimulation using an oriented stimulus has been shown to result in improved discrimination over a wide range of orientations [17]. In their cleverly designed experiments, Tan et al. [7] were able to create a novel paradigm that links feature learning to category learning. Their results imply a close interaction between category learning and visual perceptual learning. Category learning is assumed to involve higher processing stages [9], so the findings of Tan et al. [7] can best be explained by a crucial role of top-down influences arising from higher category-processing stages. Given that category learning was not location specific, in this scenario, it is conceivable that after an orientation has been allocated to a category, the training of this orientation connects to the category-processing units, thereby eliminating its location specificity. Historically, the use of simple features like orientations goes back to the seminal work of Hubel and Wiesel [18], who suggested that single neurons are triggered by basic features relevant for perception. There is a controversial debate about the use of natural scenes versus artificial stimuli [5,19]. The findings of Tan et al. [7] provide the first lines of evidence for perceptual learning under more generalized conditions getting close to natural, everyday environments. In fact, analysis of neural processing by means of voltagesensitive dye imaging has shown that natural scenes create different states of neural dynamics compared to those seen when using simple stimuli [20]. Extrapolating from the new work of Tan et al. [7], therefore, a next logical step would be to use natural scenes for visual perceptual learning that contain both features and categories. It is conceivable that further research into this direction might show quite different properties of visual perceptual learning, with a broader generalization than observed so far. The latter would be a highly desired prerequisite for a broader use of visual
perceptual learning applications in rehabilitation and clinical interventions. REFERENCES 1. Seitz, A.R., and Dinse, H.R. (2007). A common framework for perceptual learning. Curr. Opin. Neurobiol. 17, 148–153. 2. Sagi, D. (2011). Perceptual learning in vision research. Vis. Res. 51, 1552–1566. 3. Watanabe, T., and Sasaki, Y. (2015). Perceptual learning: toward a comprehensive theory. Annu. Rev. Psychol. 66, 197–221. 4. Seitz, A.R. (2017). Perceptual learning. Curr. Biol. 27, R631–R636. 5. Rust, N.C., and Movshon, J.A. (2005). In praise of artifice. Nat. Neurosci. 8, 1647–1650. 6. Levi, D.M. (2012). Prentice award lecture 2011: Removing the brakes on plasticity in the amblyopic brain. Optom. Vis. Sci. 89, 827–838. 7. Tan, Q., Wang, Z., Sasaki, Y., and Watanabe, T. (2019). Category-induced transfer of visual perceptual learning. Curr. Biol. 29, 1374–1378. 8. Gibson, J.J. (1986). The Ecological Approach to Visual Perception (Psychology Press). 9. Ashby, F.G., and Maddox, W.T. (2005). Human category learning. Annu. Rev. Psychol. 56, 149–178. 10. Fahle, M., and Poggio, T. (2002). Perceptual Learning (Cambridge, MA: MIT Press). 11. Kahnt, T., Grueschow, M., Speck, O., and Haynes, J.-D. (2011). Perceptual learning and decision-making in human medial frontal cortex. Neuron 70, 549–559.
12. Xiao, L.-Q., Zhang, J.-Y., Wang, R., Klein, S.A., Levi, D.M., and Yu, C. (2008). Complete transfer of perceptual learning across retinal locations enabled by double training. Curr. Biol. 18, 1922–1926. 13. Jeter, P.E., Dosher, B.A., Liu, S.-H., and Lu, Z.-L. (2010). Specificity of perceptual learning increases with increased training. Vis. Res. 50, 1928–1940. 14. Hung, S.-C., and Seitz, A.R. (2014). Prolonged training at threshold promotes robust retinotopic specificity in perceptual kearning. J. Neurosci. 34, 8423–8431. 15. Maniglia, M., and Seitz, A.R. (2019). A new look at visual system plasticity. Trends Cogn. Sci. 23, 82–83. 16. Xiong, Y.-Z., Zhang, J.-Y., and Yu, C. (2016). Bottom-up and top-down influences at untrained conditions determine perceptual learning specificity and transfer. eLife 5, e14614. 17. Marzoll, A., Saygi, T., and Dinse, H.R. (2018). The effect of LTP- and LTD-like visual stimulation on modulation of human orientation discrimination. Sci. Rep. 8, 16156. 18. Hubel, D.H., and Wiesel, T.N. (1962). Receptive fields, binocular interaction and functional architecture in the cat’s visual cortex. J. Physiol. 160, 106–154. 19. Olshausen, B.A., and Field, D.J. (1996). Natural image statistics and efficient coding. Network 7, 333–339. 20. Onat, S., Ko¨nig, P., and Jancke, D. (2011). Natural scene evoked population dynamics across cat primary visual cortex captured with voltage-sensitive dye imaging. Cereb. Cortex 21, 2542–2554.
Olfaction: Mosquitoes Love Your Acid Odors Christopher J. Potter The Solomon H. Snyder Department of Neuroscience, The Center for Sensory Biology, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA Correspondence: [email protected] https://doi.org/10.1016/j.cub.2019.03.010
Mosquitoes use their sense of smell to find humans. A new study shows that the ionotropic receptor 8a (IR8a) plays a primary and nonredundant role in human host-seeking behaviors. Humans are smelly. To female Aedes aegypti mosquitoes, the human scent is absolutely wonderful, offering the promise of a delicious blood meal full of
R282 Current Biology 29, R280–R299, April 22, 2019 ª 2019 Elsevier Ltd.
nutrients to support egg production. Mosquitoes have existed since before the dawn of humans, and mosquitoes that evolved to feed on human blood have
Current Biology
Dispatches adapted their incredible sense of smell to seek out and distinguish humans from amongst other animals [1]. How do they do this? A primary long-distance cue that suggests the nearby presence of a warmblooded animal is carbon dioxide (CO2). With each breath, humans exhale 4% CO2 — ten-fold the background level. To detect and respond to CO2, Aedes mosquitoes use a gustatory receptor complex comprised of the chemosensory receptor proteins Gr1, Gr2, and Gr3 [2]. Indeed, CO2 can trigger mosquitoes to enter an active host-seeking state in which human odors like lactic acid become even more attractive [3–5]. Mutation of Gr3 in Aedes renders mosquitoes defective in host-seeking attraction [5,6]. But since CO2 is emitted by most animals, it alone is insufficient to specify human targets. The leading chemoreceptor family for this purpose was originally thought to be the insect odorant receptors (ORs) — ion channels that function with an obligate coreceptor called Orco to respond to a range of odorants found in human skin (such as acetates, alcohols, and ketones) [7–9]. Surprisingly, mutation of Orco in Aedes mosquitoes (and thereby loss of function of all odor-activated OR neurons) did not lead to a decrease in human attraction [10]. Instead, Orco-mutant Aedes mosquitoes exhibited problems distinguishing humans from other animals, but were nonetheless still attracted to humans [10]. This suggested that a third family of insect chemosensory receptors, the ionotropic receptors (IRs), which respond mainly to acids and amines, might be key to odor-guided host attraction [10–12]. But a direct role of IRs in host attraction remained to be tested. A new study from Matthew DeGennaro, Joshua Raji, and coworkers now shows that IRs are indeed major chemosensory receptors for driving host-seeking attraction and biting in Aedes mosquitoes [13]. IRs are an ancient family of insect sensory receptors, and members of the IR family can function as chemoreceptors (detecting odorants and tastants), thermoreceptors, or hygroreceptors [14,15]. For the chemoreceptors, there are 30 potential odor-activated IRs expressed in the Aedes antennae, and each likely requires at least one IR coreceptor to function: Ir8a, Ir25a, or Ir76b
[12,14,16]. Unlike ORs, which typically have only one OR expressed along with its co-receptor Orco, different odorbinding IRs can be co-expressed in the same olfactory neuron, and the combination of IRs likely leads to changes in odor responses [11,12]. This combination of expressed IRs likely allows a larger set of odorants to be detected by a small number of IRs [12]. Of the IR co-receptors, Ir8a is specifically expressed in the Aedes antennae, and thus likely plays an exclusive role in olfaction [16]. With this in mind, Raji et al. used CRISPR/Cas9 to mutate the Ir8a gene in Aedes aegypti mosquitoes, and performed an elegant set of experiments testing its function in mediating attraction to humans [13]. To determine which odorants were no longer detected by Ir8a mutants, the authors performed electroantennagrams to record the odor-induced activities from Ir8a mutant antennae. The responses to typical OR/Orco odorants (alcohols like 1-octen-3-ol and aldehydes like nonanal) were unaffected, whereas Ir8a mutant antennae no longer responded to a range of volatile acids, including lactic acid. Human skin presents more lactic acid than many other animals, and could be an important human-specific chemosensory cue [3]. Lactic acid and CO2, when presented individually, are minimally attractive to Aedes mosquitoes, but become more attractive when mixed [3]. The authors found that Ir8a-mutant mosquitoes were no longer attracted to the lactic acid and CO2 mixture, suggesting that Ir8a is required for guiding attraction towards this important human odor. To determine if Ir8a plays a role in attraction to humans, the authors performed a series of olfactory behavioral experiments. They used a uniport olfactory assay that consisted of a large cylindrical tube with an odor box attached upwind. If mosquitoes are attracted to what they smell or sense in the upwind box, they will fly towards the box and be trapped in a collection chamber. In the presence of CO2, when a human arm or a nylon sleeve containing human odors is placed in the odor box, wild-type mosquitoes were robustly attracted; 75% of all mosquitoes on average were found in the collection chamber within 8 minutes. Consistent with previous results, Orco mutant mosquitoes were
just as attracted to the human odor samples as wild-type mosquitoes. Strikingly, the Ir8a mutants showed a dramatic decrease in attraction (to 50% on average). These behavioral results strongly suggest that Ir8a, and Ir8aexpressing receptor neurons, are playing a large and non-redundant role in guiding attraction behaviors to human odors (Figure 1). The authors next conducted attraction experiments to human arms using Gr3 single mutants and Gr3/Ir8a doublemutant Aedes mosquitoes. Interestingly, the Gr3 mutant mosquitoes were slightly less attracted (35% attracted) to the human arm than the Ir8a mutant mosquitoes (50% attracted, on average). Strikingly, human attraction was not further reduced in the Gr3/Ir8a double-mutant mosquitoes, and remained at 35%, which was also the same attraction as the Ir8a/Orco double mutant. One interpretation of these results is that CO2 activation of the Gr1/ Gr2/Gr3 complex is required to activate both Orco and Ir8a-mediated odor attraction. The absence of CO2 mimics mutation of both Orco and Ir8a. In other words, CO2 activation acts to prime the mosquito for odorant-guided host seeking and likely serves to integrate other sensory cues — like heat or humidity — as well. How might Ir8a-mediated activity influence mosquito biting? To address this question, the authors utilized an artificial blood feeder that allowed the variables of human odor, heat, and CO2 to be independently changed. In the presence of all three sensory cues, 75% of Aedes mosquitoes will take a blood meal. If human odors are removed, this drops to 25%. If only CO2 is removed, then 15% of the mosquitoes will feed. And finally, if the blood is at room temperature, 0% of the mosquitoes will feed. These data suggest that heat is a vital component to blood feeding, followed by CO2 presence, and finally human odors. The authors also tested the effects of the Ir8a mutant on blood feeding. In this case, about 25% of the mutant mosquitoes would feed, which closely resembled the behavior of mosquitoes in the absence of human odor. Together, this suggests Ir8a activities might be major contributors to human-odor detection during blood feeding.
Current Biology 29, R280–R299, April 22, 2019 R283
Current Biology
Dispatches aegypti mosquitoes using RNA interference. Insect Mol. Biol. 21, 119–127.
Searching for blood meal
3. Acree, F., Jr., Turner, R.B., Gouck, H.K., Beroza, M., and Smith, N. (1968). L-Lactic acid: a mosquito attractant isolated from humans. Science 161, 1346–1347.
Are CO2 (Gr1/Gr2/Gr3) sensors active? YES
NO
Are Ir8a-expressing olfactory neurons activated? YES
4. Gillies, M.T. (1980). The role of carbon dioxide in host-finding by mosquitoes (Diptera: Culicidae): a review. Bull. Entomol. Res. 70, 525–532.
No animals nearby. Keep searching.
NO
5. McMeniman, C.J., Corfas, R.A., Matthews, B.J., Ritchie, S.A., and Vosshall, L.B. (2014). Multimodal integration of carbon dioxide and other sensory cues drives mosquito attraction to humans. Cell 156, 1060–1071. 6. Potter, C.J. (2014). Stop the biting: targeting a mosquito’s sense of smell. Cell 156, 878–881.
Probably not an animal. Check Orco neurons or keep searching.
Are Orco-expressing olfactory neurons detecting human-centric odors? NO
YES Probably human! Do thermoreceptors detect a warm (~36˚C) object? YES
Live human! Bite!
7. Hallem, E.A., Nicole Fox, A., Zwiebel, L.J., and Carlson, J.R. (2004). Olfaction: mosquito receptor for human-sweat odorant. Nature 427, 212–213. 8. Hallem, E.A., and Carlson, J.R. (2006). Coding of odors by a receptor repertoire. Cell 125, 143–160.
Probably an animal. Do thermoreceptors detect a warm (~36˚C) object?
NO
Dead human? Keep searching.
YES
Live animal! Bite!
9. Larsson, M.C., Domingos, A.I., Jones, W.D., Chiappe, M.E., Amrein, H., and Vosshall, L.B. (2004). Or83b encodes a broadly expressed odorant receptor essential for Drosophila olfaction. Neuron 43, 703–714.
NO
Dead animal? Keep searching. Current Biology
Figure 1. A mosquito’s guide to biting.
10. DeGennaro, M., McBride, C.S., Seeholzer, L., Nakagawa, T., Dennis, E.J., Goldman, C., Jasinskiene, N., James, A.A., and Vosshall, L.B. (2013). Orco mutant mosquitoes lose strong preference for humans and are not repelled by volatile DEET. Nature 498, 487–491.
A decision tree for the sensory decisions guiding host-seeking by mosquitoes. Ir8a functions at a pivotal position in host-seeking behaviors.
11. Benton, R., Vannice, K.S., Gomez-Diaz, C., and Vosshall, L.B. (2009). Variant ionotropic glutamate receptors as chemosensory receptors in Drosophila. Cell 136, 149–162.
The study by Raji et al. provides strong evidence that acid odors make humans smell wonderful to mosquitoes. It also raises a number of future questions and experiments. Although it is clear that Ir8a is a key co-receptor required to mediate detection of human acid odors such as lactic acid, the IR receptors directly activated by these odors remain to be identified. The low level of remaining attraction in the Ir8a/Gr3 double mutant further suggests the involvement of additional chemoreceptors in human attraction. One possibility is that the remaining 20% attraction is mediated by Ir25a co-receptor complexes that remain functional in the absence of Ir8a, and which likely respond to amine odors present on human skin [11,17]. The role of Ir25a in human attraction might be addressed in future studies by examining olfactory host-seeking behaviors of Ir25a mutants. If Ir25a is not mediating human attraction, this strongly suggests that
12. Abuin, L., Bargeton, B., Ulbrich, M.H., Isacoff, E.Y., Kellenberger, S., and Benton, R. (2011). Functional architecture of olfactory ionotropic glutamate receptors. Neuron 69, 44–60.
unknown receptor(s) remain to be identified, and that they non-redundantly guide mosquitoes to human odors. Furthermore, although the study provides strong evidence for Ir8a function in human attraction, it was not demonstrated whether Ir8a functions in host discrimination. That is, is Ir8a used to distinguish humans from other animals, as appears to be a role for the OR/Orco chemoreceptors? Future olfactory assays that test Ir8a mutant mosquito preferences for human vs non-human animals will likely address this question. REFERENCES 1. McBride, C.S., Baier, F., Omondi, A.B., Spitzer, S.A., Lutomiah, J., Sang, R., Ignell, R., and Vosshall, L.B. (2014). Evolution of mosquito preference for humans linked to an odorant receptor. Nature 515, 222–227. 2. Erdelyan, C.N., Mahood, T.H., Bader, T.S., and Whyard, S. (2012). Functional validation of the carbon dioxide receptor genes in Aedes
R284 Current Biology 29, R280–R299, April 22, 2019
13. Raji, J.I., Melo, N., Castillo, J.S., Gonzalez, S., Saldana, V., Stensmyr, M.C., and DeGennaro, M. (2019). Aedes aegypti mosquitoes detect acidic volatiles found in human odor using the IR8a pathway. Curr. Biol. 29, 1253–1262. 14. Croset, V., Rytz, R., Cummins, S.F., Budd, A., Brawand, D., Kaessmann, H., Gibson, T.J., and Benton, R. (2010). Ancient protostome origin of chemosensory ionotropic glutamate receptors and the evolution of insect taste and olfaction. PLoS Genet. 6, e1001064. 15. Montell, C., and Zwiebel, L.J. (2016). Mosquito sensory systems. Adv. In Insect Phys. 51, 293–328. 16. Matthews, B.J., McBride, C.S., DeGennaro, M., Despo, O., and Vosshall, L.B. (2016). The neurotranscriptome of the Aedes aegypti mosquito. BMC Genomics 17, 32. 17. Hussain, A., Zhang, M., Ucpunar, H.K., Svensson, T., Quillery, E., Gompel, N., Ignell, R., and Grunwald Kadow, I.C. (2016). Ionotropic chemosensory receptors mediate the taste and smell of polyamines. PLoS Biol. 14, e1002454.
Dispatches transfer and generalization after perceptual learning. For example, previous research had demonstrated that transfer is critically affected by initial training conditions, such as training in more than one condition, reducing visual adaptation, or training at different difficulty levels [12–16]. In addition, mere exposure to repetitive stimulation using an oriented stimulus has been shown to result in improved discrimination over a wide range of orientations [17]. In their cleverly designed experiments, Tan et al. [7] were able to create a novel paradigm that links feature learning to category learning. Their results imply a close interaction between category learning and visual perceptual learning. Category learning is assumed to involve higher processing stages [9], so the findings of Tan et al. [7] can best be explained by a crucial role of top-down influences arising from higher category-processing stages. Given that category learning was not location specific, in this scenario, it is conceivable that after an orientation has been allocated to a category, the training of this orientation connects to the category-processing units, thereby eliminating its location specificity. Historically, the use of simple features like orientations goes back to the seminal work of Hubel and Wiesel [18], who suggested that single neurons are triggered by basic features relevant for perception. There is a controversial debate about the use of natural scenes versus artificial stimuli [5,19]. The findings of Tan et al. [7] provide the first lines of evidence for perceptual learning under more generalized conditions getting close to natural, everyday environments. In fact, analysis of neural processing by means of voltagesensitive dye imaging has shown that natural scenes create different states of neural dynamics compared to those seen when using simple stimuli [20]. Extrapolating from the new work of Tan et al. [7], therefore, a next logical step would be to use natural scenes for visual perceptual learning that contain both features and categories. It is conceivable that further research into this direction might show quite different properties of visual perceptual learning, with a broader generalization than observed so far. The latter would be a highly desired prerequisite for a broader use of visual
perceptual learning applications in rehabilitation and clinical interventions. REFERENCES 1. Seitz, A.R., and Dinse, H.R. (2007). A common framework for perceptual learning. Curr. Opin. Neurobiol. 17, 148–153. 2. Sagi, D. (2011). Perceptual learning in vision research. Vis. Res. 51, 1552–1566. 3. Watanabe, T., and Sasaki, Y. (2015). Perceptual learning: toward a comprehensive theory. Annu. Rev. Psychol. 66, 197–221. 4. Seitz, A.R. (2017). Perceptual learning. Curr. Biol. 27, R631–R636. 5. Rust, N.C., and Movshon, J.A. (2005). In praise of artifice. Nat. Neurosci. 8, 1647–1650. 6. Levi, D.M. (2012). Prentice award lecture 2011: Removing the brakes on plasticity in the amblyopic brain. Optom. Vis. Sci. 89, 827–838. 7. Tan, Q., Wang, Z., Sasaki, Y., and Watanabe, T. (2019). Category-induced transfer of visual perceptual learning. Curr. Biol. 29, 1374–1378. 8. Gibson, J.J. (1986). The Ecological Approach to Visual Perception (Psychology Press). 9. Ashby, F.G., and Maddox, W.T. (2005). Human category learning. Annu. Rev. Psychol. 56, 149–178. 10. Fahle, M., and Poggio, T. (2002). Perceptual Learning (Cambridge, MA: MIT Press). 11. Kahnt, T., Grueschow, M., Speck, O., and Haynes, J.-D. (2011). Perceptual learning and decision-making in human medial frontal cortex. Neuron 70, 549–559.
12. Xiao, L.-Q., Zhang, J.-Y., Wang, R., Klein, S.A., Levi, D.M., and Yu, C. (2008). Complete transfer of perceptual learning across retinal locations enabled by double training. Curr. Biol. 18, 1922–1926. 13. Jeter, P.E., Dosher, B.A., Liu, S.-H., and Lu, Z.-L. (2010). Specificity of perceptual learning increases with increased training. Vis. Res. 50, 1928–1940. 14. Hung, S.-C., and Seitz, A.R. (2014). Prolonged training at threshold promotes robust retinotopic specificity in perceptual kearning. J. Neurosci. 34, 8423–8431. 15. Maniglia, M., and Seitz, A.R. (2019). A new look at visual system plasticity. Trends Cogn. Sci. 23, 82–83. 16. Xiong, Y.-Z., Zhang, J.-Y., and Yu, C. (2016). Bottom-up and top-down influences at untrained conditions determine perceptual learning specificity and transfer. eLife 5, e14614. 17. Marzoll, A., Saygi, T., and Dinse, H.R. (2018). The effect of LTP- and LTD-like visual stimulation on modulation of human orientation discrimination. Sci. Rep. 8, 16156. 18. Hubel, D.H., and Wiesel, T.N. (1962). Receptive fields, binocular interaction and functional architecture in the cat’s visual cortex. J. Physiol. 160, 106–154. 19. Olshausen, B.A., and Field, D.J. (1996). Natural image statistics and efficient coding. Network 7, 333–339. 20. Onat, S., Ko¨nig, P., and Jancke, D. (2011). Natural scene evoked population dynamics across cat primary visual cortex captured with voltage-sensitive dye imaging. Cereb. Cortex 21, 2542–2554.
Olfaction: Mosquitoes Love Your Acid Odors Christopher J. Potter The Solomon H. Snyder Department of Neuroscience, The Center for Sensory Biology, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA Correspondence: [email protected] https://doi.org/10.1016/j.cub.2019.03.010
Mosquitoes use their sense of smell to find humans. A new study shows that the ionotropic receptor 8a (IR8a) plays a primary and nonredundant role in human host-seeking behaviors. Humans are smelly. To female Aedes aegypti mosquitoes, the human scent is absolutely wonderful, offering the promise of a delicious blood meal full of
R282 Current Biology 29, R280–R299, April 22, 2019 ª 2019 Elsevier Ltd.
nutrients to support egg production. Mosquitoes have existed since before the dawn of humans, and mosquitoes that evolved to feed on human blood have
Current Biology
Dispatches adapted their incredible sense of smell to seek out and distinguish humans from amongst other animals [1]. How do they do this? A primary long-distance cue that suggests the nearby presence of a warmblooded animal is carbon dioxide (CO2). With each breath, humans exhale 4% CO2 — ten-fold the background level. To detect and respond to CO2, Aedes mosquitoes use a gustatory receptor complex comprised of the chemosensory receptor proteins Gr1, Gr2, and Gr3 [2]. Indeed, CO2 can trigger mosquitoes to enter an active host-seeking state in which human odors like lactic acid become even more attractive [3–5]. Mutation of Gr3 in Aedes renders mosquitoes defective in host-seeking attraction [5,6]. But since CO2 is emitted by most animals, it alone is insufficient to specify human targets. The leading chemoreceptor family for this purpose was originally thought to be the insect odorant receptors (ORs) — ion channels that function with an obligate coreceptor called Orco to respond to a range of odorants found in human skin (such as acetates, alcohols, and ketones) [7–9]. Surprisingly, mutation of Orco in Aedes mosquitoes (and thereby loss of function of all odor-activated OR neurons) did not lead to a decrease in human attraction [10]. Instead, Orco-mutant Aedes mosquitoes exhibited problems distinguishing humans from other animals, but were nonetheless still attracted to humans [10]. This suggested that a third family of insect chemosensory receptors, the ionotropic receptors (IRs), which respond mainly to acids and amines, might be key to odor-guided host attraction [10–12]. But a direct role of IRs in host attraction remained to be tested. A new study from Matthew DeGennaro, Joshua Raji, and coworkers now shows that IRs are indeed major chemosensory receptors for driving host-seeking attraction and biting in Aedes mosquitoes [13]. IRs are an ancient family of insect sensory receptors, and members of the IR family can function as chemoreceptors (detecting odorants and tastants), thermoreceptors, or hygroreceptors [14,15]. For the chemoreceptors, there are 30 potential odor-activated IRs expressed in the Aedes antennae, and each likely requires at least one IR coreceptor to function: Ir8a, Ir25a, or Ir76b
[12,14,16]. Unlike ORs, which typically have only one OR expressed along with its co-receptor Orco, different odorbinding IRs can be co-expressed in the same olfactory neuron, and the combination of IRs likely leads to changes in odor responses [11,12]. This combination of expressed IRs likely allows a larger set of odorants to be detected by a small number of IRs [12]. Of the IR co-receptors, Ir8a is specifically expressed in the Aedes antennae, and thus likely plays an exclusive role in olfaction [16]. With this in mind, Raji et al. used CRISPR/Cas9 to mutate the Ir8a gene in Aedes aegypti mosquitoes, and performed an elegant set of experiments testing its function in mediating attraction to humans [13]. To determine which odorants were no longer detected by Ir8a mutants, the authors performed electroantennagrams to record the odor-induced activities from Ir8a mutant antennae. The responses to typical OR/Orco odorants (alcohols like 1-octen-3-ol and aldehydes like nonanal) were unaffected, whereas Ir8a mutant antennae no longer responded to a range of volatile acids, including lactic acid. Human skin presents more lactic acid than many other animals, and could be an important human-specific chemosensory cue [3]. Lactic acid and CO2, when presented individually, are minimally attractive to Aedes mosquitoes, but become more attractive when mixed [3]. The authors found that Ir8a-mutant mosquitoes were no longer attracted to the lactic acid and CO2 mixture, suggesting that Ir8a is required for guiding attraction towards this important human odor. To determine if Ir8a plays a role in attraction to humans, the authors performed a series of olfactory behavioral experiments. They used a uniport olfactory assay that consisted of a large cylindrical tube with an odor box attached upwind. If mosquitoes are attracted to what they smell or sense in the upwind box, they will fly towards the box and be trapped in a collection chamber. In the presence of CO2, when a human arm or a nylon sleeve containing human odors is placed in the odor box, wild-type mosquitoes were robustly attracted; 75% of all mosquitoes on average were found in the collection chamber within 8 minutes. Consistent with previous results, Orco mutant mosquitoes were
just as attracted to the human odor samples as wild-type mosquitoes. Strikingly, the Ir8a mutants showed a dramatic decrease in attraction (to 50% on average). These behavioral results strongly suggest that Ir8a, and Ir8aexpressing receptor neurons, are playing a large and non-redundant role in guiding attraction behaviors to human odors (Figure 1). The authors next conducted attraction experiments to human arms using Gr3 single mutants and Gr3/Ir8a doublemutant Aedes mosquitoes. Interestingly, the Gr3 mutant mosquitoes were slightly less attracted (35% attracted) to the human arm than the Ir8a mutant mosquitoes (50% attracted, on average). Strikingly, human attraction was not further reduced in the Gr3/Ir8a double-mutant mosquitoes, and remained at 35%, which was also the same attraction as the Ir8a/Orco double mutant. One interpretation of these results is that CO2 activation of the Gr1/ Gr2/Gr3 complex is required to activate both Orco and Ir8a-mediated odor attraction. The absence of CO2 mimics mutation of both Orco and Ir8a. In other words, CO2 activation acts to prime the mosquito for odorant-guided host seeking and likely serves to integrate other sensory cues — like heat or humidity — as well. How might Ir8a-mediated activity influence mosquito biting? To address this question, the authors utilized an artificial blood feeder that allowed the variables of human odor, heat, and CO2 to be independently changed. In the presence of all three sensory cues, 75% of Aedes mosquitoes will take a blood meal. If human odors are removed, this drops to 25%. If only CO2 is removed, then 15% of the mosquitoes will feed. And finally, if the blood is at room temperature, 0% of the mosquitoes will feed. These data suggest that heat is a vital component to blood feeding, followed by CO2 presence, and finally human odors. The authors also tested the effects of the Ir8a mutant on blood feeding. In this case, about 25% of the mutant mosquitoes would feed, which closely resembled the behavior of mosquitoes in the absence of human odor. Together, this suggests Ir8a activities might be major contributors to human-odor detection during blood feeding.
Current Biology 29, R280–R299, April 22, 2019 R283
Current Biology
Dispatches aegypti mosquitoes using RNA interference. Insect Mol. Biol. 21, 119–127.
Searching for blood meal
3. Acree, F., Jr., Turner, R.B., Gouck, H.K., Beroza, M., and Smith, N. (1968). L-Lactic acid: a mosquito attractant isolated from humans. Science 161, 1346–1347.
Are CO2 (Gr1/Gr2/Gr3) sensors active? YES
NO
Are Ir8a-expressing olfactory neurons activated? YES
4. Gillies, M.T. (1980). The role of carbon dioxide in host-finding by mosquitoes (Diptera: Culicidae): a review. Bull. Entomol. Res. 70, 525–532.
No animals nearby. Keep searching.
NO
5. McMeniman, C.J., Corfas, R.A., Matthews, B.J., Ritchie, S.A., and Vosshall, L.B. (2014). Multimodal integration of carbon dioxide and other sensory cues drives mosquito attraction to humans. Cell 156, 1060–1071. 6. Potter, C.J. (2014). Stop the biting: targeting a mosquito’s sense of smell. Cell 156, 878–881.
Probably not an animal. Check Orco neurons or keep searching.
Are Orco-expressing olfactory neurons detecting human-centric odors? NO
YES Probably human! Do thermoreceptors detect a warm (~36˚C) object? YES
Live human! Bite!
7. Hallem, E.A., Nicole Fox, A., Zwiebel, L.J., and Carlson, J.R. (2004). Olfaction: mosquito receptor for human-sweat odorant. Nature 427, 212–213. 8. Hallem, E.A., and Carlson, J.R. (2006). Coding of odors by a receptor repertoire. Cell 125, 143–160.
Probably an animal. Do thermoreceptors detect a warm (~36˚C) object?
NO
Dead human? Keep searching.
YES
Live animal! Bite!
9. Larsson, M.C., Domingos, A.I., Jones, W.D., Chiappe, M.E., Amrein, H., and Vosshall, L.B. (2004). Or83b encodes a broadly expressed odorant receptor essential for Drosophila olfaction. Neuron 43, 703–714.
NO
Dead animal? Keep searching. Current Biology
Figure 1. A mosquito’s guide to biting.
10. DeGennaro, M., McBride, C.S., Seeholzer, L., Nakagawa, T., Dennis, E.J., Goldman, C., Jasinskiene, N., James, A.A., and Vosshall, L.B. (2013). Orco mutant mosquitoes lose strong preference for humans and are not repelled by volatile DEET. Nature 498, 487–491.
A decision tree for the sensory decisions guiding host-seeking by mosquitoes. Ir8a functions at a pivotal position in host-seeking behaviors.
11. Benton, R., Vannice, K.S., Gomez-Diaz, C., and Vosshall, L.B. (2009). Variant ionotropic glutamate receptors as chemosensory receptors in Drosophila. Cell 136, 149–162.
The study by Raji et al. provides strong evidence that acid odors make humans smell wonderful to mosquitoes. It also raises a number of future questions and experiments. Although it is clear that Ir8a is a key co-receptor required to mediate detection of human acid odors such as lactic acid, the IR receptors directly activated by these odors remain to be identified. The low level of remaining attraction in the Ir8a/Gr3 double mutant further suggests the involvement of additional chemoreceptors in human attraction. One possibility is that the remaining 20% attraction is mediated by Ir25a co-receptor complexes that remain functional in the absence of Ir8a, and which likely respond to amine odors present on human skin [11,17]. The role of Ir25a in human attraction might be addressed in future studies by examining olfactory host-seeking behaviors of Ir25a mutants. If Ir25a is not mediating human attraction, this strongly suggests that
12. Abuin, L., Bargeton, B., Ulbrich, M.H., Isacoff, E.Y., Kellenberger, S., and Benton, R. (2011). Functional architecture of olfactory ionotropic glutamate receptors. Neuron 69, 44–60.
unknown receptor(s) remain to be identified, and that they non-redundantly guide mosquitoes to human odors. Furthermore, although the study provides strong evidence for Ir8a function in human attraction, it was not demonstrated whether Ir8a functions in host discrimination. That is, is Ir8a used to distinguish humans from other animals, as appears to be a role for the OR/Orco chemoreceptors? Future olfactory assays that test Ir8a mutant mosquito preferences for human vs non-human animals will likely address this question. REFERENCES 1. McBride, C.S., Baier, F., Omondi, A.B., Spitzer, S.A., Lutomiah, J., Sang, R., Ignell, R., and Vosshall, L.B. (2014). Evolution of mosquito preference for humans linked to an odorant receptor. Nature 515, 222–227. 2. Erdelyan, C.N., Mahood, T.H., Bader, T.S., and Whyard, S. (2012). Functional validation of the carbon dioxide receptor genes in Aedes
R284 Current Biology 29, R280–R299, April 22, 2019
13. Raji, J.I., Melo, N., Castillo, J.S., Gonzalez, S., Saldana, V., Stensmyr, M.C., and DeGennaro, M. (2019). Aedes aegypti mosquitoes detect acidic volatiles found in human odor using the IR8a pathway. Curr. Biol. 29, 1253–1262. 14. Croset, V., Rytz, R., Cummins, S.F., Budd, A., Brawand, D., Kaessmann, H., Gibson, T.J., and Benton, R. (2010). Ancient protostome origin of chemosensory ionotropic glutamate receptors and the evolution of insect taste and olfaction. PLoS Genet. 6, e1001064. 15. Montell, C., and Zwiebel, L.J. (2016). Mosquito sensory systems. Adv. In Insect Phys. 51, 293–328. 16. Matthews, B.J., McBride, C.S., DeGennaro, M., Despo, O., and Vosshall, L.B. (2016). The neurotranscriptome of the Aedes aegypti mosquito. BMC Genomics 17, 32. 17. Hussain, A., Zhang, M., Ucpunar, H.K., Svensson, T., Quillery, E., Gompel, N., Ignell, R., and Grunwald Kadow, I.C. (2016). Ionotropic chemosensory receptors mediate the taste and smell of polyamines. PLoS Biol. 14, e1002454.