Abstract
Aluminium is biologically reactive and its ability to potentiate the immune response has driven its inclusion in both veterinary and human vaccines. Consequently, the need for unequivocal visualisation of aluminium in vivo has created a focused research effort to establish fluorescent molecular probes for this purpose. The most commonly used direct fluorescent labels for the detection of aluminium are morin (2′,3,4′,5,7-pentahydroxyflavone) and lumogallion [4-chloro-3-(2,4-dihydroxyphenylazo)-2-hydroxybenzene-1-sulphonic acid]. While the former has gained popularity in the detection of aluminium in plants and predominantly within root tips, the latter boasts greater sensitivity and selectivity for the detection of aluminium in human cells and tissues. Herein, we have developed a simplified morin staining protocol using the autofluorescence quenching agent, Sudan Black B. This modified protocol improves tissue morphology and increases analytical sensitivity, which allows intracellular aluminium to be detected in monocytes and when co-localised with senile plaques in human brain tissue of donors diagnosed with familial Alzheimer’s disease. Overall, our results demonstrate a simple approach to minimise false positives in the use of morin to unequivocally detect aluminium in vivo.
Highlights
Aluminium is the third most abundant element and the most abundant metal in the Earth’s crust
Further attempts to reduce intrinsic cellular autofluorescence of morin-stained T helper 1 (THP-1) cells were made via their pre-treatment using the hexadentate ligand, ethylenediaminetetraacetic acid (EDTA)
Native THP-1 cells stained with lumogallion in an identical manner revealed a weak brown/orange fluorescence emission that was clearly distinguishable against internalised lumogallionreactive particulates
Summary
Aluminium is the third most abundant element and the most abundant metal in the Earth’s crust. While low molecular weight neutral complexes of aluminium are predicted to traverse cells via passive transport and enter via passive diffusion (Exley and Mold 2015), particulate forms of the metal ion are transported intracellularly via non-receptor mediated endocytosis (Mold et al 2014). The latter has relevance for the cellular uptake of aluminium adjuvants that are included in human vaccinations to potentiate and shape the immune response (Reed et al 2013; Shardlow et al 2018). Understanding the mechanisms driving the cellular uptake and fate of aluminium are crucial in the development of both new and existing vaccines (Shardlow et al 2018)
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