Molecular Characterisation of the Interaction of Microbes with the Insulin Pathway
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Exposure to microorganisms is considered an environmental factor which can contribute to diabetes mellitus via cytotoxicity or autoimmune responses against pancreatic cells. Firstly, the effects on rat insulinoma pancreatic β-cell line of secondary metabolites pyrrolnitrin (Burkholderia spp), phenazine compounds (Pseudomonas aeruginosa and Burkholderia spp) were investigated. Both compounds separately showed significant cytotoxicity after 24 h and at concentrations of 10 & 100 ng/ml potentiated insulin gene transcription, Ca2+ content and glucose-stimulated insulin secretion (GSIS). Furthermore, the outward membrane current was inhibited by phenazine (100 ng/ml) or pyrrolnitrin (10 or 100 ng/ml). Secondly, the capacity of 45 microbial species to bind insulin was screened in order to assess how common insulin binding was amongst microorganisms Burkholderia multivorans, B. cenocepacia and Aeromonas salmonicida bound insulin. A genomic library of B. multivorans was constructed in λ Zap Express and screened successfully for insulin binding recombinants. Recombinant phagemids p1 & p2 were excised, p1, encoded an insulin binding protein (IBP1 30 kDa) with homology to the iron complex siderophore receptor. For p2, two IBPs were detected at 20 & 30 kDa (IBP2 & IBP3), representing an intracellular and outer membrane peptide transporter. Comparison of IBP1 and human insulin receptor (HIR) produced 6 linear epitopes, and for IBP2 & IBP3 produced 3 epitopes. Thirdly, glutamic acid decarboxylase GAD65 is a major pancreatic autoantigen contributing to autoimmune diabetes. To assess the likelihood that microorganisms possess epitopes that mimic regions on GAD, 45 microbial species were tested for homology. This was facilitated by purifying recombinant GAD protein which was used to produce GAD antiserum. Four E. coli cross-reacting proteins were identified using mass spectrometry, outer-membrane protein A, formate dehydrogenase, superoxide dismutase and DNA starvation protein. Epitopes occurred at the C-terminal region of GAD65 (residues 419–565), a region previously reported to be targeted by autoantibodies. This study suggests that pyrrolnitrin and phenazine are cytotoxic to pancreatic β-cells and B. multivorans IBPs linear epitopes may be diabetogenic, particularly in patients with cystic fibrosis related diabetes (CFRD) who suffer a long term infections with Pseudomonas and Burkholderia species. Furthermore, microbial GAD epitopes could potentially induce an autoimmune response leading to diabetes.
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