An imaging process could help shed light on cellular processes in medical disorders such as Creutzfeldt-Jacob disease (CJD).

CJD is one of several diseases, including type II diabetes and Alzheimer's disease, associated with proteins that polymerise into fibres inside cells. The self-assembly processes involved in these diseases are similar to those when peptide-based hydrogels are formed. Now, Bing Xu and colleagues at the Hong Kong University of Science and Technology in China have used sulfonated azo dye Congo red to detect this hydrogel formation in cells.

Supramolecular hydrogels are large, water insoluble networks of nanofibres formed when molecules called hydrogelators self-assemble. Xu and his colleagues were able to use Congo red to stain hydrogels in two types of mammalian cell and one strain of bacterium. The team exploited the capacity of enzymes within the tested cells to convert added substrates to hydrogelators and so induce nanofibre formation. When the dye is added to these treated cells, it permeates the cells and binds to the nanofibres. In green light, the dye gives a distinctive red fluorescent emission peak but only if hydrogelation has occurred. 

'The Congo red assay is an effective and easy method for determining intracellular hydrogelation,' said Xu. It removes the need for laborious sample preparation and expensive instrumentation, he added.

'The assay should also assist the exploration of the intracellular self-assembly of small molecules,' said Xu. But he cautioned that further work could be required to help improve the specificity of this type of assay. 'It is conceivable that Congo red can stain other supramolecular hydrogels intracellularly,' he said.

Kathryn Lees