A proto-aldehyde fluorescence quenched substrate for quantitative imaging of both protein and enzyme activity within cells

Fluorogenic substrates are emerging tools that enable studying enzymatic processes within their native cellular environments. However, fluorogenic substrates that function within live cells are generally incompatible with cellular fixation, preventing their tandem application with fundamental cell biology methods such as immunocytochemistry. Here we report a simple approach to enable the stable chemical fixation of the fluorescent product of a series of dark-to-light bis-acetal based substrates (BABS). Cleavage of the BABS glycosidic bond leads to liberation of a hemiacetal in which one branch consists of a proto-aldehyde bearing the fluorophore and the other branch consists of an alcohol bearing the quencher. Spontaneous breakdown of this hemiacetal leads to formation of a fluorescent aldehyde. Trapping of this aldehyde by standard reductive amination to intracellular proteins allows its stable retention within cells and concomitant imaging of cellular proteins by traditional immunocytochemistry. These bis-acetal substrates enable measuring changes in lysosomal GCase activity in response to both chemical and genetic perturbations. These tools will aid in studying the role of GCase activity in diseases and accelerate the creation of new therapeutic approaches targeting the GCase pathway. We also expect this strategy to be broadly useful for creating fixable substrates for other lysosomal enzymes.