The Importance of MALDI Imaging to Reveal the Complexities of Neurological Disorders. By Professor Masaya Ikegawa, Genomics and Proteomics for Human and Model Organisms, Department of Medical Life Systems at Doshisha University
Dementia traditionally is seen as affecting older people, although around 5% of dementia cases are known as ‘young onset’ cases in people aged between 35-65 – with over 9.9 million new cases of dementia each year worldwide. Alzheimer’s disease (AD) is the most common cause of dementia and leads to protein build-up in the brain in the form of plaques, which interfere with nerve cell connections. These can ultimately lead to nerve cell death and brain tissue loss.
The ‘amyloid hypothesis’ has been described as the leading cause of Alzheimer’s – amyloid-β (Aβ), an insoluble peptide, has been characterised in the plaques found in the brains of patients with Alzheimer’s. The formation of Aβ arises from the cleavage of a much larger protein called amyloid precursor protein (APP). Before becoming plaques, Aβ monomers (individual peptides) clump together into oligomers, which can be seen in very early onset AD. These oligomers progress to form the neurotoxic plaques. Although well-understood, the characterisation of Aβ structural assembly requires more in-depth research to improve understanding of Alzheimer’s pathogenesis, and better develop appropriate diagnostic and therapeutic biomarkers.
In typical Alzheimer’s neuropathy, immunohistochemistry (IHC) methods have been used in the past to determine the localisation of Aβ peptides in brain tissues. However, this technique can introduce bias because it cannot discriminate between different variants when a number of epitopes are used simultaneously. Mass spectrometry-based proteomic analysis has gained popularity as an alternative method for characterising the variety of Aβ species in the brain and most recently, matrix-assisted laser desorption/ionisation (MALDI)-imaging mass spectrometry has emerged as an important tool for investigating protein and small molecule distribution within biological systems. MALDI imaging can individually track the whole distribution of complex molecules which have multiple modifications, which is an advantage over IHC. Previously, the signal could be quite vague but even with a muddy biological matrix at 100µm resolution, MALDI imaging provides high quality results.
Here at the Department of Medical Life Systems, we have been able to characterise a broad range of Aβ species deposits in brains with Alzheimer’s disease and cerebral amyloid angiopathy (CAA) using MALDI imaging, and found that Aβ structure determines the deposition location in brains with Alzheimer’s disease. Characterising and visualising the many Aβ species is necessary for the understanding of Aβ-production (metabolism and deposition) and may help elucidate the pathogenesis of Alzheimer’s disease and CAA.
This work cannot be completed alone. In our case, we work closely with a wide range of scientists on cell biology, neurology, and pathology all over Japan, most especially with Professor Shigeo Murayama, Neuropathology, The Brain Bank for Aging Research, Tokyo Metropolitan Geriatric Hospital and Institute of Gerontology, Tokyo, Japan.
Additionally my colleague, Assistant Professor Nobuto Kakuda, and I work closely with the Japan Agency for Medical Research and Development (AMED), which is aiming to launch a biomarker study to screen out the pre-clinical situation of Alzheimer’s disease.
AMED aims to merge cutting-edge basic neuroscience with clinical research, and elucidate the molecular pathophysiology of dementia and develop diagnostic and therapeutic methods, e.g., analysing metabolic and inflammatory stress, Aβ degradation in Alzheimers’s disease, developing novel immunotherapies, diagnostic and therapeutic drugs against dementia with Lewy bodies and molecular targeted therapies in frontotemporal lobar degeneration (FTLD), thereby finding a cure for dementia by 2025.
We are working collaboratively with AMED on three key themes: The first theme involved establishing the very high sensitivity ELISA system to detect Aβ peptide in plasma (with Assistant Professor Kakuda), the second was forming a novel therapeutic strategy to inhibit production of Aβ (with Associate Professor Funamoto). The third theme is centred around the laboratory’s key research project of using imaging mass spectrometry for detecting Aβ peptide in the circulation of patients affected by Alzheimer’s disease.
Although there is currently no cure for Alzheimer’s disease, many organisations including ours, are using innovative technology to research new treatments for this form of dementia. It is clear to me that mass spectrometry-based proteomic analysis is a valuable approach to characterise the variety of Aβ species in brain tissues.
MALDI imaging has shown advantages over traditional immunohistochemistry methods to determine the localisation of Aβs in brain tissue. MALDI imaging using Bruker’s rapifleX can reveal the distribution of various Aβ species within the same sections of human autopsied brains, without specific probes, at a high resolution.
In the near future, we are now looking to target tau proteins using imaging mass spectrometry, to understand how the ‘tangles’ that these proteins form in the brain correlate to the cognitive problems patients with Alzheimer’s Disease experience. By working together we are moving towards understanding this disease better, to ultimately find a cure.