Mass spectrometry analysis of tau and amyloid-beta in iPSC-derived models of Alzheimer's disease and dementia

Abstract

Induced pluripotent stem cell (iPSC) technology enables the generation of human neurons in vitro, which contain the precise genome of the cell donor, therefore permitting the generation of disease models from individuals with a disease-associated genotype of interest. This approach has been extensively used to model inherited forms of Alzheimer's disease and frontotemporal dementia. The combination of iPSC-derived neuronal models with targeted mass spectrometry analysis has provided unprecedented insights into the regulation of specific proteins in human neuronal physiology and pathology. For example enabling investigations into tau and APP/Aβ, specifically: protein isoform expression, relative levels of cleavage fragments, aggregated species and functionally critical post-translational modifications. The use of mass spectrometry has enabled a determination of how closely iPSC-derived models recapitulate disease profiles observed in the human brain. This review will highlight the progress to date in studies using iPSCs and mass spectrometry to model Alzheimer's disease and dementia. We go on to convey our optimism, as studies in the near future will make use of this precedent, together with novel techniques such as genome editing and stable isotope labelling, to provide real progress towards an in depth understanding of early neurodegenerative processes and development of novel therapeutic agents.

Keywords: Alzheimer's disease; amyloid-beta; induced pluripotent stem cells; mass spectrometry; tau.

FIGURE 1

Amyloid‐beta sequence, cleavage sites and insights from mass spectrometry. (a) The amino acid sequence of Aβ, with residues labile to post‐translational modifications highlighted: Green—phosphorylation and dityrosine/nitrotyrosine/nitration of serine and tyrosine residues. Gold—pyroglutamate modification of glutamic acid residues. Red—racemization/isomerization of aspartic acid residues. Arrows show the sites of secretase cleavage of Aβ. (b) Information learned from iPSC models subject to mass spectrometry analysis. Predominant forms of Aβ in different stages of neuronal differentiation (Bergström et al. 2016). The finding that N terminal cleavage can direct C terminal endoproteolysis (Siegel et al. 2017). The anti‐amyloidogenic role of BACE2 in reducing relative amounts of amyloidogenic species in Down's syndrome neurons (Alić et al. 2020). The specificity of avermectins, increasing the processivity of γ‐secretase while leaving short species of Aβ unchanged (Brownjohn et al. 2017)