An AIE-driven fluorescent polysaccharide polymersome as an enzyme-responsive FRET nanoprobe to study the real-time delivery aspects in live cells

Abstract

We report aggregation induced emission (AIE) driven polysaccharide polymersomes as fluorescence resonance energy transfer (FRET) nanoprobes to study their intracellular enzyme-responsive delivery by real-time live-cell confocal microscopy bio-imaging techniques. An AIE active tetraphenylethylene (TPE) optical chromophore and plant-based vesicular directing hydrophobic unit were grafted on clinically relevant polysaccharide-dextran via enzyme-cleavable aliphatic ester chemical linkages. The TPE-tagged dextran self-assembled as 180 ± 20 nm blue-luminescent polymersomes in aqueous medium and exhibited excellent encapsulation capabilities for water soluble Rose Bengal (RB) and water insoluble Nile red (NR) fluorophores. The selective photoexcitation of the TPE chromophore enabled the FRET process between the TPE donor and RB (or NR) acceptor molecule in <50 Å Förster distance afforded by the polymersome. The FRET probe was very stable under extracellular conditions and it exclusively underwent lysosomal esterase enzymatic biodegradation at the intracellular compartments to release RB. The enzyme-trigger enabled the FRET probe to function as an extracellular turn-ON → intracellular turn-Off red-fluorescent signal (Probe-1). In this process, the AIE self-emission was also simultaneously restored on the TPE chromophore (blue-luminescent, Probe-2) followed by the isolation of donor and acceptor in the cytosol. As a result, this new design enabled the visualization of real-time enzyme-responsive delivery by monitoring the dual fluorescent signals from both the polymer host (blue) and encapsulated guest (red) in a single nano-platform. In vitro cytotoxicity studies established that the polymersome probe was non-toxic to cells up to 300 μg mL⁻¹. Lyso-tracker staining experiments supported the FRET probe internalization in the lysosomal compartments for enzymatic-biodegradation. Live cell confocal microscopy with selective photo-excitation was used to directly monitor the enzyme-responsive FRET action in human breast cancer MCF 7 and wild-type mouse embryonic fibroblast cell lines (WT-MEFs). It was found that the tailor-made polymersome FRET probe was efficient to deliver the loaded cargo in <3 h in live cells which predicts the usefulness of the probe in biomedical research.