genetics & genomics of insect immunity

Question 1

activaiton of insect immune response to infection

Answer 1

• TM receptor activation
• cytoplasmic signal transduction
• TF activation and translocation to nucleus
• transcription of immune effector genes
• gene products combat infection
• signals can be amplified or dampened by other cascades undergoing the same process

Question 2

melanisation

Answer 2

• specific effector cascade of insects
  
  • orthological mechanisms in vertebrates but not for defence
• melanin production and secretion via phenylalanine pathway
• melanin polymers encapsulate pathogen
• ROS production

Question 3

pathogen recognition

Answer 3

• bacterial cell wall
• G+ → single lipid bilayer, thick peptidoglycan
  
  • NAM/NAG polymers connected by peptide bridges
• G- → 2 lipid membranes, thin PG, outer LPS layer
• PG and LPS differentiate bacteria from host
  
  • not TLR4 receptor for LPS in insects
• also porin in LPS recognised
• other species-specific structures
  
  • sugars

Question 4

peptidoglycan shield

Answer 4

• cross-links between terminal aa of one peptide in a bridge with penultimate of other peptide in same bridge
• D-alanine (terminal) connects to:
  
  • G+: Lys
  • G-: DAP
    
    • diaminopimelic acid
• used by insect to distinguish between G+/G-

Question 5

PGRPs

Answer 5

• PG recognition proteins, can act as receptors
• can be TM
  
  • PGRP-LC → recognises G-
• can be intracellular
  
  • PGRP-LE
• PGRP-SA/SD → recognise G+
• some cleave PG → no more recognition → dampen signal
  
  • lysozyme activity retained → lost in receptors

Question 6

GNBP

Answer 6

• family of recognition receptors
• GNB4 in Drosophila
• recognise sugars
  
  • fungal, on PG
• cooperate with PGRP to facilitate recognition
  
  • GNBP1 + PGRP-SA/SD → recognise Lys-type PG
• name = Gram negative binding proteins
  
  • misleading → more involved in G+ recognition
• involved in TLR pathways

Question 7

Toll-like receptors

Answer 7

• TM receptor first characterised in drosophila (developmental)
• then found in immunity
• no direct ligand recognition like in vertebrates
• recognition of Spz

Question 8

Spz

Answer 8

• cytokine like molecule
• final molecule in GNBP mediated pathway
• requires cleavage of ProSpz by SPE
  
  • SPE activated by protease action
  • stimulated by Persephone
    
    • activated by GNBP3 upon yeast/fungus ligand recognition
• also PGRP-SA/SD → bind GNBP1 → protease activation of SPE

Question 9

insect vs vertebrate TLR pathways

Answer 9

• homologous components and pathways
  
  • except no PAMP recognition in insects
• vertebrates:
  
  • IkB inhibition of NFkB
  • IkB phosphorylation → NFkB release
• insects:
  
  • Dif or Dorsal TF bound by Cactus
    
    • masks NLS
  • Cactus phosphorylated → ubiquitination & degradation
  • TF translocation

Question 10

Imd pathway

Answer 10

• PGRP-LC/LE recognises DAP type PG of G- bacteria
• receptor multimerises to initiate pathway
• Relish TF at final stage
  
  • similar to Dif/Dorsal/NFkB
  • inhibitory domain cleaved → activation

Question 11

AMPs

Answer 11

• antimicrobial peptides
• products of effector genes activated by immune signalling pathways
• at least 7 Drosophila families
  
  • e.g. defensins
• generally disrupt pathogen membrane
• flip-flop on membrane → intracellular → bind organelles/molecules → inhibit cell function

Question 12

melanisation pathway

Answer 12

• starts from phenylalanine
• many toxic intermediates
  
  • also attack pathogen
• eventual melanin production
  
  • encapsulate pathogen in hemolymph
  • prevent toxin release
• can be upregulated by signal transduction pathways

Question 13

complement pathway

Answer 13

• only well known in mosquitoes
• similar to vertebrate pathway
• TEP1 = equivalent to C3
  
  • both have buried thioester motif
  • conf change → exposure of reactive bond
  • oxidises nearby molecules (pathogen or host)
• common outcomes: lysis or phagocytosis (like vertebrates) as well as melanisation

Question 14

TEP1 transit

Answer 14

• carried around by leucine-rich repeat receptor complex
  
  • similar domains to TLRs
• thought to interact with pathogen and deliver TEP1 to surface
• similar complex to C3 convertase forms on pathogen surface
  
  • converts other TEP1 to more active form to bind surface

Question 15

insect antiviral immune pathways

Answer 15

• involve TLRs, maybe Imd
• could also be JAK/STAT
• RNAi pathway very important
  
  • dsRNA recognition of viral genetic material or replication intermediates
• results in activation of antiviral effectors

Question 16

comparative genomics

Answer 16

• drosophila, anopheles gambiae, aedes aegypti
• measure distance between orthologs
• innate immunity genes → more conserved than other genes
• signalling transduction pathways of immunity → some of most conserved genes of all insect genes