Research

Advanced Imaging Mass Spectrometry in Neurochemistry

The incredible advances in medical sciences and hygiene during the last fifty years have led to improvements in healthcare standards and a dramatic increase the average lifespan of the world’s population. As societies, in particular those in developed countries are getting older the prevalence for age related disease, including neurodegenerative diseases is on the rise. The current efforts of dealing with common age-related diseases, such as Alzheimer’s disease, thus, pose an immense challenge to mankind with respect to social and economic factors. The major reason for this problem is that there are still no curative treatments for these diseases. As the pathogenic mechanisms underlying these diseases remain largely elusive, cures do not seem to be on the immediate horizon. 1

The major pathological hallmarks of Alzheimer’s disease (AD) is the progressive accumulation and aggregation of beta-amyloid (Aβ) and hyperphosphorylated-tau, into neurotoxic deposits. Aβ aggregation has been suggested as a possibly critical, early inducer driving the disease progression. However, the exact molecular processes underlying Aβ plaque pathology remains unknown, which hampers the development of effective AD treatment strategies. 2,3

The primary goal of our research is to develop and employ advanced molecular imaging mass spectrometry (IMS) to probe the chemical and structural aspects of Aβ plaque pathology in Alzheimer’s disease. In detail, the aim is elucidate chemical properties of diffuse and mature Aβ plaques and their molecular environment in human brain tissue and transgenic AD mice brain. Moreover, we aim to establish mechanistic insight in subcellular Aβ aggregation dynamics in transgenic AD mice using metabolic labeling and ultrahigh resolution imaging MS for elucidating spatial and temporal changes in plaque chemistry.

Imaging Mass Spectrometry

Imaging mass spectrometry is a powerful approach for comprehensive analysis of spatial intensity distribution profiles of molecular species in biological tissue and single cells.3,4 In contrast to common biological imaging techniques imaging MS does not require any a priori knowledge of the potential target species and is not dependent on antibody or primer availability and specificity. Imaging MS features high molecular specificity and allows multiplexed detection, localization, identification and quantification of hundreds of peptides, metabolites and lipids in situ.3,4 The technique employs different probes to desorb and ionize intact molecular species directly from a biological sample, where the most prominent approaches include matrix assisted laser desorption/ionization (MALDI) 3 and secondary ion mass spectrometry (SIMS) (Fig.1).5

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Figure 1. Imaging Mass Spectometry Principle (a) For tissue-imaging MS, frozen brain is sectioned (~12μm) using a cryostat microtome and mounted onto conductive glass slides. For MALDI IMS, an UV absorbing crystalline matrix is applied using e.g. a nebulizer based spray-coating approach (right). SIMS and MALDI based desorption and ionization is performed using an ionbeam (SIMS) or laser (MALDI) as projectile followed typically by time of flight mass spectrometry (ToF SIMS, MALDI TOF), where ions are separated based on their mass to charge (m/z) (b) IMS is performed on tissue sections in a predefined raster. The individual intensity distribution pattern can then be mapped for each molecular peaks (m/z) revealing characteristic distributions that are in line with anatomical features (c).

These different IMS modalities are complementary, since they have various strengths and limitations with respect to spatial resolution and molecular information. 4,6 Therefore, multimodal IMS strategies provide a particularly powerful approach for comprehensive investigation of complex biomolecular structures in situ and in vitro.6 MALDI for instance, is well suited for imaging large molecular species such as intact glycolipids, peptides and intact proteins (m<100kDa) at 10μm lateral resolution.4 SIMS in turn employs ion beams for probing chemical surfaces and facilitates high-resolution imaging (<1µm). It is well suited for analysis of elemental and other inorganic species as well as organic low molecular weight compounds (m<1000Da) including metabolites and lipids (Fig.1). 5 Standard SIMS, (time of flight SIMS, ToF-SIMS) does not require any special sample preparation. Therefore, subsequent MALDI IMS can be performed on the same section. 6 A particularly powerful SIMS technique is nanoSIMS, where a highly focused ion beam is used for imaging at subcellular length scales down to 50nm. 7 This technique employs magnetic sector analyzers with high transmission and hence high sensitivity, allowing quantification of target compounds. However, nanoSIMS does require extensive sample preparation (embedding and polishing), which permits follow up analysis on these samples.

In Sweden, this technology will be available shortly at the Swedish National Center for Imaging Mass Spectrometry at the University of Gothenburg and Chalmers University of Technology, headed by Prof. Andrew Ewing. (www.ncims.se)

References

  1. J. Hardy and D.J. Selkoe Science, 2003. 297(5580):353-6
  2. D.R. Thal et al., Neurology, 2002. 58(12):1791-800
  3. R. M. Caprioli, T. B. Farmer, J. Gile, Anal Chem 1997, 69. 4751-60D.
  4. L. A. McDonnell, R. M. A. Heeren, Mass Spectrom Rev 2007, 26. 606-643
  5. J.C Vickerman and D. Briggs. ToF-SIMS: surface analysis by mass spectrometry. IM, 2001.
  6. J. Hanrieder, N. T. N. Phan, M. E. Kurczy, et al., ACS Chem Neurosci 2013, 4. 666-679
  7. M. L. Steinhauser, A. P. Bailey, S. E. Senyo, et al., Nature 2012, 481. 516-9

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