A nanoparticle probe for the imaging of autophagic flux in live mice via magnetic resonance and near-infrared fluorescence.

Autophagy-the lysosomal degradation of cytoplasmic components via their sequestration into double-membraned autophagosomes-has not been detected non-invasively. Here we show that the flux of autophagosomes can be measured via magnetic resonance imaging or serial near-infrared fluorescence imaging of intravenously injected iron oxide nanoparticles decorated with cathepsin-cleavable arginine-rich peptides functionalized with the near-infrared fluorochrome Cy5.5 (the peptides facilitate the uptake of the nanoparticles by early autophagosomes, and are then cleaved by cathepsins in lysosomes).