Reconfiguring the link between GAD65 and diabetes

The finding that the enzyme glutamic acid decarboxylase (GAD) 65 changes its shape when it turns itself on and off has reignited interest in this protein’s potential to provide a vaccine for type 1 diabetes.

GAD65 is found in high concentrations in the beta cells of the pancreas where it catalyses the conversion of L-glutamic acid into gamma-aminobutyric acid (GABA). GABA plays a primary role as an inhibitory neurotransmitter in the insulin-secreting pancreatic islets. GABA is a key inhibitory neurotransmitter in the brain, where GAD65 (and GAD67) also functions as its biosynthetic enzyme.

Previous studies have linked GAD65 to type 1 diabetes through the presence of GAD autoantibodies. This immune recognition of GAD65 results in the invasion and progressive loss of the beta cells of the pancreas, which is what ultimately causes type 1 diabetes.

The new finding that GAD65 regulates the production of GABA by changing its shape suggests that this is what causes GAD65 self-reactivity.

“GAD65 has an unpredictable, almost Jekyll and Hyde personality when it is turned on and off,” said Associate Professor Ashley Buckle, who led the research at Monash University.

“When active and making neurotransmitters, it is rigid and rather motionless. Ironically, when switched off, rather than resting as you might expect, it becomes mobile, dancing and jiggling around.

“We suspected that this dual personality might affect how antibodies ‘see’ it. This turns out to be true - antibodies interact with it very differently depending on whether it’s on or off,” he continued.

GAD65 has previously been used in clinical trials as a vaccine for type 1 diabetes, with limited success. Associate Professor Buckle said the discovery may ultimately lead to the development of better vaccines to potentially treat and prevent type 1 diabetes.

“The idea to immunise an individual with GAD65 to help the immune system develop a tolerance against it, to stop or at least dampen the immune reaction is a good one. But so far these attempts have not been very successful. This research could change that,” Associate Professor Buckle said.

The seven-year study used a combination of experimental and computational methods to understand what GAD65 looks like in its ‘off’ state and how human antibodies interact with both forms.

Associate Professor Buckle said access to the powerful beam lines at the Australian Synchrotron, as well as massive super computers at the Victorian Life Sciences Computation Initiative (VLSCI) and Monash, was critical to the research.

“Techniques like X-ray crystallography produce amazing, detailed pictures of large molecules, but these are often snapshots frozen in time. In order to ‘see’ the molecules in action we needed to combine other techniques, such as molecular simulation, to produce a movie. The Australian Synchrotron and the VLSCI played a major part in this work,” Associate Professor Buckle said.

In the next phase, the research team will visualise GAD65 as it interacts with a human antibody.

Understanding why GAD65 is recognised and targeted by antibodies is the next step. In addition to specific insights, it’s hoped this work will provide important basic knowledge that could be applied to broader aspects of health and medicine.

The study was published in the journal PNAS.