Modeling the endosomal escape of cell-penetrating peptides using a transmembrane pH gradient

Component: Membrane Model; Description: Large unilamellar vesicles (LUVs) composed of POPC (zwitterionic) and POPG (20% negatively charged), mimicking endosomal membrane composition

Component: Proton Pump; Description: Bacteriorhodopsin (BR) — light-driven proton pump; reconstituted into LUV membranes via detergent-mediated reconstitution (n-octyl-β-D-glucopyranoside); pumps protons from outside to inside upon illumination

Component: pH Gradient; Description: Light-induced acidification inside LUVs (~2 pH units drop), mimicking late endosomal pH (acidic interior, neutral exterior)

Component: CPP Payload; Description: Fluorescein-labeled CPPs (20 μM) entrapped inside LUVs

Component: Fluorescence Quencher; Description: KI (100 mM) entrapped with CPPs to quench fluorescence of peptides remaining inside LUVs

Component: CPPs Tested; Description: • Non-amphipathic (hydrophilic): R9 (nona-arginine), TAT(48-60)<br>• Intermediately amphipathic: M918, pVEC, penetratin (from prior work)<br>• Hydrophobic: TP10 (from prior work)

Component: Endosomal Escape Enhancers; Description: • Chloroquine (CQ) — 75 μM entrapped inside LUVs; weak base, proton absorber, membrane destabilizer<br>• Pyrenebutyrate (PB) — 50 μM added outside; hydrophobic counter-anion, membrane destabilizer

Component: Alternative Vesicle Compositions; Description: Zwitterionic POPC only; endosome-mimetic (50% PC, 20% PE, 10% PI, 20% BMP)

Parameter: Vesicle Preparation; Details: POPC/POPG (4:1 molar ratio) LUVs; extrusion through 100 nm polycarbonate filters; BR reconstitution via detergent-mediated method (OG solubilization + Bio-Bead detergent removal)

Parameter: CPP Encapsulation; Details: 20 μM fluorescein-labeled CPP + 100 mM KI entrapped inside LUVs during vesicle formation; external quencher/peptide removed by PD-10 columns

Parameter: pH Gradient Induction; Details: Illumination with 150 W xenon arc lamp; BR pumps protons from outside to inside; outside pH monitored with pH meter

Parameter: Translocation Measurement; Details: Fluorescence spectroscopy (Horiba Jobin Yvon Fluorolog-3); excitation 494 nm, emission 505-550 nm; 20°C; fluorescence increase indicates peptide escape to outside (higher pH, no quencher)

Parameter: Controls; Details: • Dark (no pH gradient)<br>• 4°C (no proton pumping)<br>• KI quenching (Stern-Volmer plots) to confirm peptide outside<br>• Free fluorescein pH dependence (Fig. S2)

Parameter: Vesicle Compositions Tested; Details: POPC/POPG (20% negative charge), pure POPC (zwitterionic), endosome-mimetic (PC/PE/PI/BMP)

Parameter: Escape Enhancer Experiments; Details: CQ encapsulated (75 μM); PB added externally (50 μM); pH gradient measured over time

Parameter: Replicates; Details: Repeated measurements in identically prepared samples; good reproducibility (Fig. S1)

Analysis Category: pH Gradient Establishment; Methods: pH meter (Thermo Scientific model 320); outside pH measured during dark and illumination; BR proton pumping efficiency

Analysis Category: CPP Translocation; Methods: Fluorescence spectroscopy (Horiba Jobin Yvon Fluorolog-3); fluorescein excitation 494 nm, emission 505-550 nm; relative fluorescence intensity changes indicate peptide escape

Analysis Category: Quenching Studies; Methods: Stern-Volmer plots: F₀/F vs. KI concentration (mM); slope increase under illumination indicates peptide outside LUVs

Analysis Category: Vesicle Characterization; Methods: DLS (dynamic light scattering) for size and polydispersity of BR-LUVs

Analysis Category: pH Dependence of Fluorescein; Methods: Free fluorescein fluorescence as function of pH (Fig. S2); decreased fluorescence at lower pH

Analysis Category: Membrane Stability; Methods: pH gradient maintenance as proxy for membrane integrity; effect of CQ and PB on pH gradient

Vesicle Composition: POPC/POPG (20% negative); pH Gradient Efficiency: Strongest; Comment: Used for all translocation experiments

Vesicle Composition: Pure POPC (zwitterionic); pH Gradient Efficiency: Weaker; Comment: Lower proton barrier

Vesicle Composition: Endosome-mimetic (PC/PE/PI/BMP); pH Gradient Efficiency: Weaker; Comment: Less efficient proton pumping

CPP Class: Non-amphipathic (hydrophilic); CPP: R9; Fluorescence Change (Light vs. Dark): Decrease; Interpretation: Peptides remain inside; pH drop quenches fluorescence

CPP Class: Non-amphipathic (hydrophilic); CPP: TAT(48-60); Fluorescence Change (Light vs. Dark): Decrease (small); Interpretation: Mostly inside; slight translocation

CPP Class: Intermediately amphipathic; CPP: M918; Fluorescence Change (Light vs. Dark): Increase; Interpretation: Escape to outside; pH gradient promotes translocation

CPP Class: Intermediately amphipathic; CPP: pVEC; Fluorescence Change (Light vs. Dark): Increase; Interpretation: Escape to outside; pH gradient promotes translocation

CPP Class: Intermediately amphipathic; CPP: Penetratin; Fluorescence Change (Light vs. Dark): Increase (prior work); Interpretation: Similar to M918/pVEC

CPP Class: Hydrophobic; CPP: TP10; Fluorescence Change (Light vs. Dark): Destabilizes gradient; Interpretation: Membrane disruption at ≥0.5 μM

CPP Class: All CPPs tested; Result: Unable to translocate (weaker pH gradient)

Agent: Chloroquine (CQ); Condition: 75 μM inside LUVs; Effect on pH Gradient: ~10× weaker pH increase; Interpretation: Proton absorption + transient membrane destabilization; proton leakage

Agent: Pyrenebutyrate (PB); Condition: 50 μM outside LUVs; Effect on pH Gradient: Rapid loss of pH gradient; Interpretation: Membrane destabilization; short inward pumping then collapse

Agent: TP10 (hydrophobic CPP); Condition: 0.5 μM outside; Effect on pH Gradient: No reproducible gradient; Interpretation: Membrane destabilization

Condition: Dark; Stern-Volmer Slope: Low; Interpretation: Peptides inside LUVs

Condition: Illumination; Stern-Volmer Slope: Increased slope; Interpretation: Small amount of peptide outside; majority remain inside

1. Synthetic model system: The study uses LUVs, not living cells; lacks cellular components (proteins, cytoskeleton, receptors) that may influence endosomal escape in vivo.

2. pH gradient magnitude may differ from in vivo: The light-induced pH gradient (~2 units) approximates late endosomes but may not fully recapitulate the complex pH dynamics of maturing endosomes.

3. Fluorescein pH sensitivity complicates interpretation: The authors note that "there may also be intermediate situations where the two effects more or less can cancel one another" (pH quenching vs. escape fluorescence increase).

4. Rhodamine label unsuitable: TMR labeling was attempted but "the more hydrophobic rhodamine seems to interact with the bilayer independent of the nature of the peptide, giving rise to fluorescence quenching."

5. Limited CPP panel: Only five CPPs were tested; conclusions about hydrophobicity dependence are based on this limited set.

6. Vesicle composition not fully optimized: The endosome-mimetic composition (PC/PE/PI/BMP) gave weaker pH gradients than POPC/POPG, limiting direct extrapolation to biological endosomes.

7. No direct measurement of peptide translocation: Translocation was inferred from fluorescence changes rather than directly measured (e.g., by separating vesicles from external solution).

8. Time scale differences: The translocation time scale (20-100 min) may differ from endosomal escape kinetics in living cells.

9. KI quenching potential artifacts: KI may affect membrane stability or peptide-membrane interactions beyond fluorescence quenching.

10. No cargo delivery demonstrated: The study measures peptide translocation, not functional delivery of biologically active cargo.

11. pH gradient dependence on vesicle composition: The authors note that "BR proton pumping is less efficient with these vesicles or the vesicles form less of a membrane proton barrier compared to the POPC-containing ones," limiting generalizability.

Report prepared based on the published Biochimica et Biophysica Acta (BBA) - Biomembranes article. For full experimental details, supplementary figures, and complete references, please refer to the original publication.