Imd pathway
The Imd pathway is a broadly-conserved NF-κB immune signalling pathway of insects and some arthropods[1] that regulates a potent antibacterial defence response. The pathway is named after the discovery of a mutation causing severe immune deficiency (the gene was named "Imd" for "immune deficiency"). The Imd pathway was first discovered in 1995 using Drosophila fruit flies by Bruno Lemaitre and colleagues, who also later discovered that the Drosophila Toll gene regulated defence against Gram-positive bacteria and fungi.[2][3] Together the Toll and Imd pathways have formed a paradigm of insect immune signalling; as of September 2, 2019, these two landmark discovery papers have been cited collectively over 5000 times since publication on Google Scholar.[4][5]
The Imd pathway responds to signals produced by Gram-negative bacteria. Peptidoglycan recognition proteins (PGRPs) sense DAP-type peptidoglycan, which activates the Imd signalling cascade. This culminates in the translocation of the NF-κB transcription factor Relish, leading to production of antimicrobial peptides and other effectors.[6] Insects lacking Imd signalling either naturally or by genetic manipulation are extremely susceptible to infection by a wide variety of pathogens and especially bacteria.
Similarity to human pathways
The Imd pathway bears a number of similarities to mammalian
Similarity to TNFR signalling
The following genes are analogous or homologous between
- Imd: human orthologue = RIP1
- Tak1: human orthologue = Tak1
- TAB2: human orthologue = TAB2
- Dredd: human orthologue = caspase-8
- FADD: human orthologue = FADD
- Key/Ikkγ: human orthologue = NEMO[8]
- Ird5: human orthologue = IKK2
- Relish: human orthologues = p65/p50 and IκB
- Iap2: human orthologue = cIAP2
- UEV1a: human orthologue = UEV1a
- bend: human orthologue = UBC13
In Drosophila
While the exact
Peptidoglycan recognition proteins (PGRPs)
The sensing of bacterial signals is performed by peptidoglycan recognition protein LC (PGRP-LC), a transmembrane protein with an intracellular domain. Binding of bacterial peptidoglycan leads to dimerization of PGRP-LC which generates the conformation needed to bind and activate the Imd protein. However alternate
Other PGRPs can inhibit the activation of Imd signalling by binding bacterial signals or inhibiting host signalling proteins: PGRP-LF is a transmembrane PGRP that lacks an intracellular domain and does not bind peptidoglycan. Instead PGRP-LF forms dimers with PGRP-LC preventing PGRP-LC dimerization and consequently activation of Imd signalling. A number of secreted PGRPs have amidase activity that downregulate the Imd pathway by digesting peptidoglycan into short, non-immunogenic fragments. These include PGRP-LB, PGRP-SC1A, PGRP-SC1B, and PGRP-SC2. Additionally, PGRP-LB is the major regulator in the gut.[9]
Intracellular signalling components
The principle intracellular signalling protein is Imd, a death domain-containing protein that binds with FADD and Dredd to form a complex. Dredd is activated following
While Relish is integral for transcription of Imd pathway effectors, there is additional cooperation with other pathways such as
The antimicrobial response
Imd signalling regulates a number of effector peptides and proteins that are produced en masse following immune challenge.[11] This includes many of the major antimicrobial peptide genes of Drosophila, particularly: Diptericin, Attacin, Drosocin, Cecropin, and Defensin.[12] The Imd pathway regulates hundreds of genes after infection, however the antimicrobial peptides play one of the most essential roles of Imd signalling in defence. Flies lacking multiple antimicrobial peptide genes succumb to infections by a broad suite of Gram-negative bacteria.[13][14] Classical thinking suggested that antimicrobial peptides worked as a generalist cocktail in defence, where each peptide provided a small and somewhat redundant contribution.[15][6] However Hanson and colleagues found that single antimicrobial peptide genes displayed an unexpectedly high degree of specificity for defence against specific microbes.[13] The fly Diptericin A gene is essential for defence against the bacterium Providencia rettgeri (also suggested by an earlier evolutionary study[16]). A second specificity is encoded by Diptericin B, which defends flies against Acetobacter bacteria of the fly microbiome.[17] A third specificity is encoded by the gene Drosocin. Flies lacking Drosocin are highly susceptible to Enterobacter cloacae infection.[13][14][18] The Drosocin gene itself encodes two peptides (named Drosocin and Buletin), wherein it is specifically the Drosocin peptide that is responsible for defence against E. cloacae, while the Buletin peptide instead mediates a specific defence against another bacterium, Providencia burhodogranariea.[18] These works accompany others on antimicrobial peptides and effectors regulated by the Drosophila Toll pathway, which also display a specific importance in defence against certain fungi or bacteria.[19][20][21]
This work on Drosophila immune antimicrobial peptides and effectors has greatly revised the former view that such peptides are generalist molecules. The modern interpretation is now that specific molecules might provide a somewhat redundant layer of defence, but also single peptides can have critical importance, individually, against relevant microbes.[22][23][24][25]
Conservation in insects
The Imd pathway appears to have evolved in the last common ancestor of centipedes and insects.[1] However certain lineages of insects have since lost core components of Imd signalling. The first-discovered and most famous example is the pea aphid Acyrthosiphon pisum. It is thought that plant-feeding aphids have lost Imd signalling as they bear a number of bacterial endosymbionts, including both nutritional symbionts that would be disrupted by aberrant expression of antimicrobial peptides, and defensive symbionts that cover for some of the immune deficiency caused by loss of Imd signalling.[26] It has also been suggested that antimicrobial peptides, the downstream components of Imd signalling, may be detrimental to fitness and lost by insects with exclusively plant-feeding ecologies.[27]
Crosstalk between the Imd and Toll signalling pathways
While the Toll and Imd signalling pathways of Drosophila are commonly depicted as independent for explanatory purposes, the underlying complexity of Imd signalling involves a number of likely mechanisms wherein Imd signalling interacts with other signalling pathways including
Insects and arthropods lacking Imd signalling
- The pea aphid Acyrthosiphon pisum[26]
- The bed bug Cimex lectularius[29]
- The mite Tetranychus urticae[30]
References
- ^ PMID 25908671.
- PMID 7568155.
- S2CID 10736743.
- ^ "A recessive mutation, immune deficiency (imd), defines two distinct control pathways in the Drosophila host defense". Google Scholar. Retrieved 2 September 2019.
- ^ "The dorsoventral regulatory gene cassette spätzle/Toll/cactus controls the potent antifungal response in Drosophila adults". Google Scholar. Retrieved 2 September 2019.
- ^ PMID 17201680.
- ^ PMID 24706930.
- ^ a b "UniProtKB - Q9GYV5 (NEMO_DROME)". Uniprot.org.
Interpro family: IPR034735 NEMO_ZF
- PMID 16618604.
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- PMID 12032070.
- PMID 15976485.
- ^ PMID 30803481.
- ^ PMID 34791204.
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- PMID 26776733.
- S2CID 259115731.
- ^ PMID 35730150.
- PMID 25915418.
- PMID 32038657.
- PMID 34432851.
- PMID 32463841.
- S2CID 209357523.
- PMID 32355003.
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- ^ PMID 20178569.
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