Neutron beams help shed light on Alzheimer’s disease

University of Waterloo scientists are putting together pieces of the molecular puzzle behind Alzheimer’s disease by examining the role of disease-related biomolecules in model brain cell membranes.

Source: Canadian Neutron Beam Centre (CNBC)
Image: Shutterstock

An estimated 50 million people worldwide live with Alzheimer’s disease or similar forms of dementia. The disease, which primarily affects seniors, disrupts the normal functions of brain cells, leading to memory loss and cognitive impairment.

Professor Zoya Leonenko.

There are many changes that happen in our brains with age that correspond to changes at the molecular level. But which of these changes trigger the body’s amyloid stores to start building up in a disruptive way in people with Alzheimer’s disease? And what prevents this disruption from occurring in healthy people of the same age?

The answers may hold the key to preventing and treating this form of dementia. “The molecular mechanism behind Alzheimer’s disease is not well understood,” says Zoya Leonenko, a professor of biophysics at the University of Waterloo. Post-mortem analysis of diseased brain tissue find large buildups of a protein called amyloid. They accumulate at the surfaces of neuronal cellular membranes and damage them. Unfortunately, “we don’t have a way of studying these processes on molecular level non-invasively,” says Leonenko, “therefore we use model lipid membranes.”

Figuring out such molecular processes in a lab is a complex puzzle, and one that Leonenko’s research team is systematically working on. Elizabeth Drolle, one of Leonenko’s students, was pleased to help put a few of the pieces together through her PhD research.

Elizabeth Drolle at the E3 neutron beamline at the Canadian Neutron Beam Centre. The E3 beamline features some equipment from Nobel prize-winner Bertram Brockhouse’s original neutron beamline.

Leonenko’s team believes that the structure of the brain cellular membrane is critical to the molecular processes underlying amyloid binding and toxicity in Alzheimer’s. The structure of lipid membranes changes in age and in disease, and many factors affect its structure, for example cholesterol. “You have to simplify the problem and investigate in stages in order to make progress,” says Drolle.

Building on this hypothesis, one of Drolle’s first steps was to examine the effect of cholesterol on amyloid in simple model membranes using atomic force microscopy, a high-resolution imaging technique. By imaging the surface of the cell membrane, she observed that the cholesterol had created areas in the membrane surface that attracted amyloid [doi:10.1016/j.bpj.2012.06.053].

To help answer this question, Drolle used neutron beams to examine model cell membranes containing various amounts of melatonin and cholesterol. Neutron beams are one of the few tools that can be used to study such biomolecules inside a model membrane because they allow for realistic biological conditions to be carefully controlled during experiments. A second membrane-active molecule that Drolle investigated was melatonin, a sleep-related hormone. Previous research in mice has suggested that melatonin plays a protective role in Alzheimer’s disease. As the body ages, however, it produces less melatonin—which could explain the correlation between age and the onset of Alzheimer’s disease. But before a link can be established, scientists need to figure out what effect melatonin has on the structure of cellular lipid membranes, and how that could defend against the disease.

Illustration of Drolle’s model cell membranes, represented as gray balls with black tails. Cholesterol (orange) encouraged membrane molecules to pack tightly together, while melatonin (red) caused membrane molecules to spread out. The tightly packed membrane structure associated with an absence of melatonin could attract the amyloid buildups related to Alzheimer’s disease.

After accessing neutron beams at the Canadian Neutron Beam Centre (CNBC) with the support of CNBC scientist Norbert Kučerka, Drolle made a rather striking observation: cholesterol and melatonin had opposite effects on membrane structure. Indeed, while the cholesterol encouraged the membrane molecules to pack tightly together, melatonin made the membrane molecules spread out [doi:10.1016/j.bbamem.2013.05.015].

Leonenko’s collaborator Mikko Karttunen at the University of Waterloo reproduced Drolle’s experimental results using computer simulations of these model membranes. Drolle also conducted follow-up experiments using other methods, which provided complementary insights and added further evidence to the robustness of the neutron beam observations.

Overall, the results suggests that a tightly packed membrane structure, as associated with the presence of cholesterol, could play a role in attracting amyloid to build up on the cell membrane. Notably, the findings also suggest that the presence of melatonin may counteract the effects of cholesterol, thereby protecting brain cell membranes from amyloid-induced damage. These molecular-level insights are invaluable, as they can be used to help scientists better understand the mechanisms that lead to—and perhaps even defend against—Alzheimer’s disease.

Since then, Leonenko’s research team has designed more complex model membranes to imitate healthy and diseased brain cell membranes. Working with these new models, Drolle again used atomic force microscopy to observe differences in the membranes’ structures, particularly in terms of how they interact with amyloid. Her observations led to ideas about what changes that occur in brain cell membranes from normal aging and from Alzheimer’s disease may trigger amyloid to start building up and disrupt the membranes [doi:10.1371/journal.pone.0182194].

Today, Drolle has completed her PhD and is now a scientist with the Centre for Ocular Research and Education (CORE) within the School of Optometry & Vision Science at the University of Waterloo. CORE studies a variety of eye characteristics that could be used as indicators for a range of eye conditions, such as dry eye disease and contact lens discomfort, with the ultimate aim of enabling early diagnosis. It’s an area of investigation that builds naturally on her previous research experience: applying analytical skills and using experimental techniques to study the molecular chemistry of health conditions in a multidisciplinary environment.

Meanwhile, Leonenko’s research group has been continuing its investigations into the biomolecular mechanisms behind Alzheimer’s disease. These studies include neutron beam experiments at Oak Ridge National Laboratory in the United States to observe melatonin’s effects on both the healthy and the diseased model membranes. “Our research findings are all steps along the way that we hope will lead to development of a way to prevent or treat Alzheimer’s disease,” says Leonenko.