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Evolutionary Conservation and Changes in Insect TRP Channels


Evolutionary Conservation and Changes in Insect TRP Channels

Hironori Matsuura et al. BMC Evol Biol.


Background: TRP (Transient Receptor Potential) channels respond to diverse stimuli and thus function as the primary integrators of varied sensory information. They are also activated by various compounds and secondary messengers to mediate cell-cell interactions as well as to detect changes in the local environment. Their physiological roles have been primarily characterized only in mice and fruit flies, and evolutionary studies are limited. To understand the evolution of insect TRP channels and the mechanisms of integrating sensory inputs in insects, we have identified and compared TRP channel genes in Drosophila melanogaster, Bombyx mori, Tribolium castaneum, Apis mellifera, Nasonia vitripennis, and Pediculus humanus genomes as part of genome sequencing efforts.

Results: All the insects examined have 2 TRPV, 1 TRPN, 1 TRPM, 3 TRPC, and 1 TRPML subfamily members, demonstrating that these channels have the ancient origins in insects. The common pattern also suggests that the mechanisms for detecting mechanical and visual stimuli and maintaining lysosomal functions may be evolutionarily well conserved in insects. However, a TRPP channel, the most ancient TRP channel, is missing in B. mori, A. mellifera, and N. vitripennis. Although P. humanus and D. melanogaster contain 4 TRPA subfamily members, the other insects have 5 TRPA subfamily members. T. castaneum, A. mellifera, and N. vitripennis contain TRPA5 channels, which have been specifically retained or gained in Coleoptera and Hymenoptera. Furthermore, TRPA1, which functions for thermotaxis in Drosophila, is missing in A. mellifera and N. vitripennis; however, they have other Hymenoptera-specific TRPA channels (AmHsTRPA and NvHsTRPA). NvHsTRPA expressed in HEK293 cells is activated by temperature increase, demonstrating that HsTRPAs function as novel thermal sensors in Hymenoptera.

Conclusion: The total number of insect TRP family members is 13-14, approximately half that of mammalian TRP family members. As shown for mammalian TRP channels, this may suggest that single TRP channels are responsible for integrating diverse sensory inputs to maintain the insect sensory systems. The above results demonstrate that there are both evolutionary conservation and changes in insect TRP channels. In particular, the evolutionary processes have been accelerated in the TRPA subfamily, indicating divergence in the mechanisms that insects use to detect environmental temperatures.


Figure 1
Figure 1
Phylogenetic tree of the insect TRP channels. Amino acid sequences of channel-forming six transmembrane domains of Drosophila melanogaster (Dm), Bombyx mori (Bm), Tribolium castaneum (Tc), Apis mellifera (Am), Nasonia vitripennis (Nv), and Pediculus humanus (Ph) TRP channels were aligned by the Muscle program, and then PhyML3.0 algorithm was applied for the maximum likelihood analyses, under the WAG amino acid substitution model and with 100 bootstrapped data sets using the PhyML Online server. The statistical confidence (bootstrap value) is indicated next to each interior branch. The insect TRP channels can be classified into 7 subfamilies, TRPC (red), TRPV (green), TRPM (light blue), TRPP (purple), TRPML (blue), TRPN (yellow), and TRPA (brown). The amino acid sequences of the insect TRP channels are listed in an Additional file 2.
Figure 2
Figure 2
Phylogenetic tree of D. melanogaster, C. elegans, and mouse TRPV and TRPM channels. The phylogenetic tree was constructed from the amino acid sequences of channel-forming six transmembrane domains of D. melanogaster TRPV (DmNan and DmIav), TRPM (DmTRPM), C. elegans TRPV (Ocr-1, 2, 3, 4, and Osm-9), TRPM (Gtl-1, 2, and Gon-2), and mouse TRPV (mTRPV1, 2, 3, 4, 5, and 6), TRPM (mTRPM1, 2, 3, 4, 5, 6, 7, and 8) as in Fig. 1. TRPV and TRPM subfamily members are shown by red and green, respectively. Nematode and mouse TRPV and TRPM channels form independent clades, demonstrating that nematode and mouse have independently expanded these TRP channels.
Figure 3
Figure 3
Intron positions and phases of TcTRPA5 and AmTRPA5. Protein-level alignment of TcTRPA5 and AmTRPA5 is shown. Identical amino acids are indicated by asterisks, and the conserved amino acids are shown by either dots or colons. Ankyrin repeat and ion transport domains are shown by blue and green, respectively. Intron positions are indicated by digits corresponding to the phase of the intron relative to the surrounding codons (phases 0, 1 and 2 introns fall before the first, second and third bases of a codon, respectively) in red. Delta indicates the absence of an intron. Introns 1, 2, and 4 of AmTRPA5 are located in the alignment gap regions. One intron in the ion transport domain-coding region of both TcTRPA5 and AmTRPA5 shares the same position and phase (indicated by red bold letters).
Figure 4
Figure 4
HsTRPA has evolved by duplication of the Wtrw gene. Positions of AmWtrw and AmHsTRPA genes on A. mellifera Group 2.21 scaffold as well as NvWtrw and NvHsTRPA on N. vitripennis scaffold 44 are shown. The exons of Wtrw and HsTRPA genes are indicated by red and green rectangles, respectively. The numbers indicate the sizes of DNA (bp), and the arrows show the direction of transcription of genes. AmWtrw, AmHsTRPA, and NvHsTRPA have no introns, while NvWtrw has 2 introns. The genes adjacent to Wtrw and HsTRPA are also conserved between Apis and Nasonia.
Figure 5
Figure 5
NvHsTRPA is a thermo-sensitive TRP channel. (A) Localization of V5 and His epitope-tagged NvHsTRPA protein in HEK293 cells visualized by immunofluorescence. (B) Detection of the epitope-tagged NvHsTRPA protein by Western blot. The lysates of HEK293 cells transfected with empty vector (Mock) and NvHsTRPA expression vector (NvHsTRPA) were analyzed by Western blot. Arrowhead indicates NvHsTRPA protein band, while asterisk shows the heat aggregated protein band. The NvHsTRPA expressing cells were treated with (+) or without (-) tunicamycin (Tun). The positions of MW markers are shown on the left in kDa. (C) Heat elicits inward current activation in a NvHsTRPA-expressing HEK293 cell (NvHsTRPA, left panel) at -60 mV holding potential in a whole-cell patch-clamp mode (n = 6). A standard bath solution and Cs-Asp/Ca2+(-) pipette solution were used. NvHsTRPA is activated by temperature increase, and quickly inactivated by constant heat application. Desensitization of the channel is also observed by repetitive heat applications. The current activation is not observed with the mock-transfected cells (Mock, right panel). (D) Current--voltage relationship of heat evoked current exhibits dual rectification with a slight positive reversal potential. Heat-dependent shifts of the liquid junction potentials (ΔJPH) were not corrected in the plot (n = 9). A standard bath solution and Cs-Asp/Ca2+(-) pipette solution were used.

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