The aberrant folding and subsequent aggregation of proteins into insoluble plaques known as amyloid fibrils is associated with a number of diseases, including Alzheimer’s and Parkinson’s diseases. The exact role that these aggregates play in the disease mechanisms is not yet well understood, in part due to the difficulties that arise when attempting to visualise the interactions between the carbon-rich protein aggregates and the carbon-rich cells due to a lack of contrast. Traditional strategies to overcome this lack of contrast have involved the use of stains or tags that potentially can be either unreliable or intrusive.
In this work we have taken a fragment of the Alzheimer’s-related Aβ peptide and replaced the naturally occurring sulfur that is present in the methionine amino acid with a selenium atom. Human phagocytic cells were exposed to different aggregate species formed from the selenium-labelled Aβ fragment and its selenium-free analogue to examine the toxicity, uptake and distribution of the aggregates. The monomeric protein and the fully aggregated mature amyloid fibrils did not show significant levels of toxicity whereas aggregation species occurring earlier in the aggregation process were found to be highly cytotoxic, in agreement with previous studies on similar species.
Cells exposed to the selenium-labelled aggregates were imaged using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), an electron microscopy technique in which only those electrons that are scattered to relatively high angles are used to generate an image. The majority of these electrons have undergone Rutherford scattering, the cross-section of which is dependent on Zn (n ~ 2). HAADF-STEM is therefore highly sensitive to local variations in atomic number. This technique has been used to visualise the selenium-labelled protein aggregates inside cells in two and three dimensions. The uptake and intracellular distributions of toxic and non-toxic aggregate species have been assessed and distinct differences have been observed correlating with the differences in toxicity.