Characterization and its implication of a novel taste receptor detecting nutrients in the honey bee, Apis mellifera

Abstract

Umami taste perception indicates the presence of amino acids, which are essential nutrients. Although the physiology of umami perception has been described in mammals, how insects detect amino acids remains unknown except in Drosophila melanogaster . We functionally characterized a gustatory receptor responding to L-amino acids in the western honey bee, Apis mellifera . Using a calcium-imaging assay and two-voltage clamp recording, we found that one of the honey bee’s gustatory receptors, AmGr10 , functions as a broadly tuned amino acid receptor responding to glutamate, aspartate, asparagine, arginine, lysine, and glutamine, but not to other sweet or bitter compounds. Furthermore, the sensitivity of AmGr10 to these L-amino acids was dramatically enhanced by purine ribonucleotides, like inosine-5′-monophosphate (IMP). Contact sensory hairs in the mouthpart of the honey bee responded strongly to glutamate and aspartate, which house gustatory receptor neurons expressing AmGr10 . Interestingly, AmGr10 protein is highly conserved among hymenopterans but not other insects, implying unique functions in eusocial insects.

An amino-acid mixture can be both rewarding and punishing to larval Drosophila

“…and sensory neuron physiology remain to be revealed. In other words, from the relatively little that is known to date about the organization of the afferent processing of amino acids in D. melanogaster, no sound argument can be derived about exactly how the parallel formation of appetitive and aversive reinforcement through the present 20 amino-acid mixture comes about (we note that in Grammia geneura caterpillars, methionine activates both feeding-stimulatory and -deterrent sensory neurons: Bernays and Chapman, 2001; see also Lim et al, 2019 reporting a novel aminoacid receptor in bees). Behavioural experiments in D. melanogaster larvae have so far only revealed that all 20 amino acids in the mixture can be rewarding when used individually (Kudow et al, 2017), but it is not known whether any of them can likewise have a punishing effect, or whether such a punishing effect can result from any specific combination of amino acids.…”

Peripheral taste detection in honey bees: What do taste receptors respond to?

“…From Lim et al. (2019). (c) Electrophysiological responses of antennal chaetic sensilla to salt or bitter tastes.…”

“…It was suggested that the fifth neuron could be responsive to amino acids, among other tastants (Whitehead & Larsen, 1976a). So far, only one study observed responses to amino acids delivered to the galea (Lim et al., 2019). Single‐ sensillum recordings from the 10 most distally located sensilla were performed upon stimulation with various concentrations of L‐glutamate and L‐aspartate (50, 100, and 200 mM), which are major components of pollen (Szczęsna, 2006).…”

“…Coupling molecular approaches with functional neurophysiology provides a valuable strategy to overcome the deficits in genetic‐tool availability in the honey bee. For instance, expressing GRs in Xenopus oocytes and coupling this expression with electrophysiological recordings (e.g., patch clamp recordings) enable the characterization of AmGr tuning (Değirmenci et al., 2018; Jung et al., 2015; Lim et al., 2019; Takada et al., 2018). Alternatively, the development of RNAi or CRISPR/Cas9 methods allows knocking out a GR gene and determining the consequences of its loss via electrophysiological and/or behavioral analyses (Değirmenci et al., 2020).…”

“…The basis for amino acid detection in honey bees is provided by AmGr10 as shown by a study in which this receptor gene was cloned and expressed in Xenopus oocytes or transfected in HEK cells, to obtain two independent measures of receptor sensitivity using electrophysiological recordings (Lim et al., 2019). These recordings showed that AmGr10 does not confer sensitivity to sweet and bitter tastants but confers sensitivity to a broad spectrum of amino acids such as aspartate, lysine, glutamate, glutamine, asparagine and arginine, and more particularly to L‐glutamate and L‐aspartate (Lim et al., 2019), which are major components of pollen (Szczęsna, 2006; Figure 3d). As in umami taste perception by humans, responses were enhanced by the addition of purine ribonucleotides such as IMP (inosine‐5′‐monophosphate) or GMP (Guanine 5′‐monophosphate).…”