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Time-gated FRET nanoassemblies for rapid and sensitive intra- and extracellular fluorescence imaging

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Science Advances  10 Jun 2016:
Vol. 2, no. 6, e1600265
DOI: 10.1126/sciadv.1600265
  • Fig. 1 Schematic presentation of the FRET imaging approaches used in this study.

    (1) Extracellular FRET between Tb- and QD-functionalized antibodies that bind to different epitopes of EGFR on the cell membrane. (2) Intracellular (cytosol) FRET from Tb-to-QD and FRET relays from Tb-to-QD-to-dye using microinjected QDs conjugated with Alexa Fluor 647 (AF) and Lumi4-Tb peptides via hexahistidine (His6) self-assembly. (3) Intracellular (endosomes/lysosomes) FRET from Tb-to-QD using CPP-mediated endocytosis of QDs conjugated with Lumi4-Tb peptides via hexahistidine self-assembly.

  • Fig. 2 Extracellular Tb-to-QD FRET using immunostaining of EGFR.

    (A and B) Tb-to-QD FRET detected on A431 cells with QD- and Tb-conjugated antibodies (A) and nanobodies (B). For both immunostaining approaches, the time-gated Tb (TG Tb PL) and QD (TG QD PL) channels reveal bright PL signals originating mainly from the cell membranes. (C and D) In contrast, staining with only QD-antibodies (C) or only Tb-antibodies (D) does not result in TG PL in the QD channel (TG QD PL, right), and only the pure QD SS PL (C, SS QD PL, left) or the pure TG Tb PL (D, left) become visible. Excitation and emission wavelengths for the different detection channels were as follows: λex = 365 nm and λem = 494 ± 10 nm for TG Tb PL, λex = 365 nm and λem = 655 ± 20 nm for TG QD PL, and λex = 545 ± 15 nm and λem = 610 ± 35 nm for SS QD PL. For TG images, the number of integrations was 220 and 110 for the TG Tb PL and TG QD PL channels, respectively. Tb-to-QD FRET channel images (TG QD PL) were corrected for spectral crosstalk, and each TG QD PL channel image in this figure is presented at identical contrast. Scale bars, 20 μm.

  • Fig. 3 Intracellular Tb-to-QD FRET increase with increasing Tb per QD valencies.

    (A) PL images showing cells after injection with Tb15-QD solution (QD concentration, 0.5 μM). Excitation and emission wavelengths for the different detection channels were as follows: λex = 545 ± 15 nm and λem = 610 ± 35 nm for SS QD (SS QD PL), λex = 365 nm and λem = 494 ± 10 nm for TG Tb (TG Tb PL), and λex = 365 nm and λem = 605 ± 8 nm for TG Tb-to-QD FRET (TG QD PL). For TG images, the number of integrations was 660. TG QD PL channel images were corrected for spectral crosstalk. Scale bar, 20 μm. (B) Ratio of spectral crosstalk-corrected TG QD PL signal to SS QD PL signal plotted as a function of Tb per QD molar ratio in the injection solution. The FRET ratio clearly increases with the number of Tb on the QD surface, indicating QD-FRET sensitization by multiple Tbs. PL intensities of TG and SS QD channels were taken from several regions of interest (ROIs) of ≥10 cells under each condition. Error bars, SEM.

  • Fig. 4 Intracellular Tb-to-QD FRET and Tb-to-QD-to-dye FRET relays after microinjection into live HeLa cells.

    (A) PL images showing cells after injection with Tb20-QD-AFy nanoassemblies with AF valencies (y) of 0, 12.5, or 20 (from top to bottom). Excitation and emission wavelengths for the different detection channels were as follows: λex = 545 ± 15 nm and λem = 605 ± 8 nm for SS QD (SS QD PL), λex = 365 nm and λem = 494 ± 10 nm for TG Tb (TG Tb PL), λex = 365 nm and λem = 605 ± 8 nm for TG Tb-to-QD FRET (TG QD PL), and λex = 365 nm and λem = 710 ± 20 nm for TG Tb-to-QD-to-AF FRET (TG AF PL). For TG images, the number of integrations was 660. TG QD PL channel images were corrected for spectral crosstalk. Scale bars, 20 μm. (B) Ratio of spectral crosstalk–corrected TG QD PL signal to SS QD PL signal (top) and TG AF PL signal to SS QD PL signal (bottom) plotted as a function of AF per QD molar ratio in injection solution. The ratio of TG QD PL to SS QD PL decreases slightly as the number of AF on the QD surface is increased, whereas the TG AF PL–to–SS QD PL ratio increases substantially. This indicates a Tb-to-AF FRET relayed by the central QD. PL intensities of Tb-to-QD FRET, Tb-to-QD-to-AF FRET, and QD channels were taken from several ROIs of ≥10 cells for each condition. Error bars, SEM.

  • Fig. 5 Intracellular Tb-to-QD FRET after CPP-mediated endocytosis.

    TG images of different CPP25-QD-Tbx FRET complexes (with x = 12.5, 25, or 35 from top to bottom) internalized in HeLa cells. All samples were excited at 349 nm with a pulsed (100-Hz) laser. The signals were collected with a 60× objective [UPLSAPO, numerical aperture (NA) = 1.35]. Emission wavelengths for the different detection channels were λem = 542 ± 20 nm for TG Tb PL and λem = 620 ± 14 nm for TG Tb-to-QD FRET (TG QD PL). Images were acquired with an intensified charge-coupled device (ICCD) camera, and the gating parameters were fixed at 10 μs for a gate delay, 2.5 ms for gate width, and 400 gates per exposure. Scale bars, 20 μm.

  • Fig. 6 Influence of different CPP and Tb per QD valency.

    (A) PL images of CPP20-QD-Tb20 nanoassemblies internalized into HeLa cells. Excitation and emission wavelengths for the different detection channels were as follows: λex = 349 nm and λem = 542 ± 20 nm for TG Tb [(A1) TG Tb PL], λex = 349 nm and λem = 605 ± 15 nm for TG Tb-to-QD FRET [(A2) TG QD PL], and λex = 494 ± 20 nm and λem = 605 ± 15 nm for SS QD [(A3) SS QD PL]. The signals were collected with a 60× objective (Uplsapo, NA = 1.35). SS QD images were acquired with a complementary metal-oxide semiconductor (CMOS) camera (acquisition time of 5 ms), and TG Tb PL and TG QD PL images were acquired with an ICCD camera (gating parameters were fixed at 10 μs for a gate delay, 2.5 ms for gate width, and 800 gates per exposure). Scale bars, 20 μm. (B) FRET ratios (intensities of TG QD PL to TG Tb PL channels taken from several ROIs of ≥20 cells for each system) of pure Tb, CPP40-QD-Tb40, and CPP20-QD-Tb20 nanoassemblies. Error bars, SEM.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/6/e1600265/DC1

    fig. S1. PL images of A431 cells labeled with anti–EGFR-Tb and anti–HER2-QD antibody conjugates.

    fig. S2. Intracellular Tb-to-QD FRET at 0.9 μM QD concentrations.

    fig. S3. Calibration of microinjection system.

    fig. S4. Cellular uptake of CPP-QDs at different QD concentrations.

    fig. S5. Cellular uptake of QD at different CPP per QD valencies.

    fig. S6. PL images of CPP40-QD-Tb40 nanoassemblies internalized into HeLa cells.

    fig. S7. QD ligand structures.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. PL images of A431 cells labeled with anti–EGFR-Tb and anti–HER2-QD antibody conjugates.
    • fig. S2. Intracellular Tb-to-QD FRET at 0.9 μM QD concentrations.
    • fig. S3. Calibration of microinjection system.
    • fig. S4. Cellular uptake of CPP-QDs at different QD concentrations.
    • fig. S5. Cellular uptake of QD at different CPP per QD valencies.
    • fig. S6. PL images of CPP40-QD-Tb40 nanoassemblies internalized into HeLa cells.
    • fig. S7. QD ligand structures.

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