Similarities between bacterial GAD and human GAD65: Implications in gut mediated autoimmune type 1 diabetes

PLoS One. 2022 Feb 23;17(2):e0261103. doi: 10.1371/journal.pone.0261103. eCollection 2022.

Abstract

A variety of islet autoantibodies (AAbs) can predict and possibly dictate eventual type 1 diabetes (T1D) diagnosis. Upwards of 75% of those with T1D are positive for AAbs against glutamic acid decarboxylase (GAD65 or GAD), a producer of gamma-aminobutyric acid (GABA) in human pancreatic beta cells. Interestingly, bacterial populations within the human gut also express GAD and produce GABA. Evidence suggests that dysbiosis of the microbiome may correlate with T1D pathogenesis and physiology. Therefore, autoimmune linkages between the gut microbiome and islets susceptible to autoimmune attack need to be further elucidated. Utilizing in silico analyses, we show that 25 GAD sequences from human gut bacterial sources show sequence and motif similarities to human beta cell GAD65. Our motif analyses determined that most gut GAD sequences contain the pyroxical dependent decarboxylase (PDD) domain of human GAD65, which is important for its enzymatic activity. Additionally, we showed overlap with known human GAD65 T cell receptor epitopes, which may implicate the immune destruction of beta cells. Thus, we propose a physiological hypothesis in which changes in the gut microbiome in those with T1D result in a release of bacterial GAD, thus causing miseducation of the host immune system. Due to the notable similarities we found between human and bacterial GAD, these deputized immune cells may then target human beta cells leading to the development of T1D.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Antigen-Presenting Cells / immunology
  • Autoantibodies / immunology*
  • Bacteria / enzymology*
  • Computer Simulation
  • Diabetes Mellitus, Type 1 / enzymology
  • Diabetes Mellitus, Type 1 / immunology*
  • Diabetes Mellitus, Type 1 / microbiology*
  • Epitopes, T-Lymphocyte / immunology
  • Gastrointestinal Microbiome / immunology*
  • Genes, Bacterial
  • Glutamate Decarboxylase / genetics*
  • Glutamate Decarboxylase / immunology*
  • Humans
  • Islets of Langerhans / enzymology
  • Islets of Langerhans / immunology
  • Mice
  • Pan troglodytes / microbiology
  • Phylogeny
  • Protein Domains
  • Sequence Alignment / methods
  • gamma-Aminobutyric Acid / metabolism

Substances

  • Autoantibodies
  • Epitopes, T-Lymphocyte
  • anti-GAD65 autoantibody
  • gamma-Aminobutyric Acid
  • Glutamate Decarboxylase
  • glutamate decarboxylase 2

Grants and funding

The(sugar)science is an all-volunteer 501c3 non-profit organization. B.J. was supported by a Canadian Institutes of Health Research (CIHR) doctoral scholarship. T.M.R. was supported by a National Science Foundation Graduate Research Fellowship, and F.S.L.B. has a Simon Fraser University distinguished professorship and support from the Natural Sciences and Engineering Research Council of Canada (NSERC).