Synthetic nanoassemblies restore disrupted intracellular homeostasis

Engineered E. coli cells can now “self-clear” synthetic nanoassemblies by switching on an internal redox repair program, according to a report published in Nature Chemical Biology. The study links photo-oxidative damage directly to an influx–efflux feedback loop in which bacterial enzymes both neutralize reactive oxygen species (ROS) and physically expel intracellular nanoparticles (NPs) to regain homeostasis.

The authors examined two related molecular states of a photosensitizing construct: the initially light-activated monomer (1) and a subsequent nanoassembly form (2). In engineered strain E. coli AB, which coexpresses OxyR-regulated methionine sulfoxide reductases (MsrA and MsrB), photo-illumination transformed extracellular 1 into 2, enabling uptake. Notably, before stress activation the constructs were not detected intracellularly, as confirmed by phasor-FLIM, setting a baseline for measuring regulated entry.

To determine whether the nanoassemblies overload cellular antioxidant capacity, they quantified intracellular ROS using DCFH-DA. In the engineered AB strain, ROS accumulation was strongly suppressed under light compared with a wild-type control, implying that photo-generated oxidants were rapidly buffered. The team attributes this protection to Met oxidation chemistry (Met → MetO) followed by enzymatic regeneration (MetO → Met), sustaining antioxidant availability rather than allowing runaway oxidative damage.

They further interrogated redox balance via the GSH/GSSG ratio. Under repeated light pulses, AB maintained the control-like ratio, whereas wild-type cells exposed to the same nanoassemblies experienced a sharp drop and failed to fully recover after successive cycles. This difference reinforced the idea that the intracellular repair machinery must remain functional for the feedback loop to complete.

Because oxidative stress can rapidly impair cellular energetics, ATP was monitored across the influx–efflux timing window. Wild-type cells showed a steep ATP decline, consistent with metabolism disruption, while AB displayed a milder decrease followed by recovery to roughly four-fifths of baseline, consistent with sustained redox correction.

The work then quantified multi-cycle clearance dynamics by flow cytometry. After 10 minutes of illumination, a large fraction of cells became PPIX-positive, indicating internal presence of 2. Over the next tens of minutes, PPIX-positive cells dropped markedly, consistent with enzymatic conversion that drives NP removal. By around 70 minutes, reduced 1 was expelled, closing the loop.

Critically, when an ATP synthase inhibitor (DCCD) was applied, no PPIX signal accumulation was observed during the second cycle. This indicates that uptake and cycling depend on active energy-consuming transport rather than passive diffusion.

Confocal microscopy corroborated the kinetics, showing PPIX fluorescence appearing after light exposure and disappearing as clearance proceeded. Finally, bacterial lysates were analyzed by analytical HPLC–MS to verify that compounds traced the intended regulatory feedback states, returning cells to a ready baseline for subsequent cycles.