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How NMR is Helping Us Understand Alzheimer's Disease

Alzheimer’s disease (AD) is the most common form of dementia and its prevalence is growing. There are currently around 5 million people affected in the United States, with this set to rise to 13 million by 2050 if we cannot develop therapies to effectively prevent or cure it. But, our ability to do so is held back by our relatively limited understanding of what causes the condition.

One theory is that AD is caused by the build-up of plaques in the brain called amyloid-β, and it is thought that these at least play a role in the disease at some stage. These plaques are formed from protein monomers that by themselves are harmless, but they can polymerize into a tangle of elongated fibers known as amyloid fibrils.

But, while imaging methods have helped us understand amyloid-β monomers and fibrils, they have been less useful in shedding light on intermediate oligomeric forms of amyloid-β. These can take on a whole range of structural conformations of varying toxicity in the brain, and are thought to play a greater role than fibrils in the disease course. Therefore, understanding their structure will be crucial to developing pharmaceutical therapies that could target them and alter AD progression.

A recent paper by Kotler et al. (2015) showed how combined NMR techniques could help researchers overcome this hurdle.

MAS spectra of the filtered disordered Aβ1-40 oligomer. An overlay of the assigned Hα regions from RFDR-based 2D 1H/1H (blue) and 2D 1H/1H TOCSY (red) spectra acquired at 298 K with 10 kHz MAS. The filtered oligomer sample was lyophilized and re-hydrated to double its initial concentration, making for a total Aβ1-40 of ~35 μM. The RFDR and TOCSY based 2D 1H/1H spectra were acquired with mixing times of 50 ms and 70 ms, respectively, at 600 MHz in 100% D2O, 10 mM sodium phosphate buffer, pH 7.4. RFDR spectra were acquired at 25 °C under 10 kHz MAS. Assignments are given for the RFDR-based 2D 1H/1H spectra. The acquisition time was 4 hours with non-uniform sampling.

The researchers employed a solid-state NMR technique called magic angle spinning (MAS) using a Bruker 600 MHz Avance II spectrometer. To increase the rate at which they could perform MAS, they coupled this to a Bruker comprehensive multiphase (CMP) probe.

They also used a technique in their experiments called 1H/1H radio frequency driven dipolar recoupling (RFDR). Together, these methods allowed the researchers to detect oligomers with a high degree of sensitivity and at a wide range of molecular weights.

Indeed, they were able to resolve the 2D spectra of the oligomers, in spite of them making up only 7% of the sample, against a background composed primarily of monomers and fibrils.

When coupled with other biophysical measurement techniques, the team were also able to identify and study the structure of a stable monomer found in the sample.

One advantage of the method, Kotler et al. note, is that, unlike other NMR approaches, it did not require them to freeze or radioactively label their sample. And, by using a spectral filter, they were able to detect specific oligomer signals without the need for a separate purification procedure, even though the oligomers made up such a small fraction of the overall sample.

The researchers say that these characteristics make the technique ideal for use on medically relevant samples that are not easily amenable to other techniques, such as amyloid oligomer samples taken from patients with Alzheimer’s disease. This study demonstrates the value of RFDR-based 2D 1H/1H experiments to enable researchers to obtain high-resolution data on such oligomeric structures, they conclude.

References

Alzheimer’s Association. 2015 Alzheimer’s Disease Facts and Figures. Alzheimer’s & Dementia 2015; 11(3): 332.

Eisenberg D & Jucker M. The Amyloid State of Proteins in Human Diseases. Cell 2012; 148: 1188-1203.

Karamanos TK, et al. Mechanisms of amyloid formation revealed by solution NMR. Progress in Nuclear Magnetic Resonance Spectroscopy 2015; 88-89: 86-104.

Kotler SA, et al. High-resolution NMR characterization of low abundance oligomers of amyloid-β without purification. Scientific Reports 2015; 5: 11811.

Ladiwala ARA, et al. Conformational differences between two amyloid β oligomers of similar size and dissimilar toxicity. Journal of Biological Chemistry 2012; 287: 24765-24773.