A Near-Infrared Ratiometric Fluorescent Probe for Detecting Endogenous Cu2+ in the Brain

Copper participates in a range of critical functions in the nervous system in the human brain. Disturbances in brain copper content is strongly associated with neurological disease. For example, changes in the level and distribution of copper are reported in neuroblastoma, Alzheimer's disease and Lewy body disorders. There is a need for more sensitive techniques to measure intracellular copper levels to have a better understanding of the role of copper homeostasis in neuronal disorders. Here we report a reaction-based near-infrared (NIR) ratiometric fluorescent probe CyCu1 for imaging Cu2+ in biological samples. High stability and selectivity of CyCu1 enabled the probe to be deployed as a sensor in a range of systems, including SH-SY5Y and neuroblastoma cells. Furthermore, it can be used in plant cells, reporting copper added to Arabidopsis roots. We also used CyCu1 to explore Cu2+ levels and distribution in postmortem brain tissues from patients with the Lewy body disorder, Dementia with Lewy bodies (DLB). We found significant decreases in Cu2+ content in the nuclei, cytoplasm, neurons and extraneuronal space in the degenerating substantia nigra (SN) in DLB compared with healthy age-matched control tissues. These findings enhance our understanding of Cu2+ dysregulation in Lewy body disorders. Our probe also shows promise as a photoacoustic imaging agent, with potential for applications in bimodal imaging.