Uptake of cell-penetrating peptides is dependent on peptide-to-cell ratio rather than on peptide concentration

Component: Peptides (CPPs); Description: • MAP (model amphipathic peptide): KLALKLALKALKAALKLA-amide; fluos-labeled (FLUOS) at N-terminus<br>• Penetratin: RQIKIWFQNRRMKWKK-amide; fluos-labeled

Component: Control Peptides; Description: • KLGL (non-amphipathic MAP analogue): KLGLKLGLKLGLKGLKLGL-amide; fluos-labeled<br>• PNA (peptide nucleic acid, 12-mer); fluos-labeled

Component: Cell Model; Description: CHO (Chinese hamster ovary) cells; cultured in 24-well plates

Component: Cell Densities Tested; Description: 20,000 – 200,000 cells/well (for 2-day cultures); varied split ratios yielding 200,000 – 1,300,000 cells/well

Component: Incubation Conditions; Description: DPBS with glucose (DPBSG); 37°C for 1 h (unless otherwise indicated); peptide concentrations: 1 μM MAP, 10 μM penetratin, 0.5 μM PNA, 50 μM KLGL

Component: Energy Depletion; Description: 30 min preincubation + peptide incubation in DPBS with 25 mM 2-deoxyglucose and 10 mM sodium azide

Component: Differentiation Method; Description: Diazotized 2-nitroaniline treatment—modifies primary amines of surface-bound (extracellular) peptide, increasing HPLC retention time; distinguishes internalized from surface-bound peptide

Component: Quantification; Description: HPLC with fluorescence detection; Bradford protein assay; LDH leakage assay (membrane damage); cell volume: 1.2 pL (Coulter counter)

Parameter: Cell Culture; Details: CHO cells seeded at various densities; used 2, 4, or 6 days post-seeding (media changed every 2 days); experiments in 24-well plates

Parameter: Peptide Incubation; Details: 200 μL peptide solution in DPBSG; 1 h at 37°C (unless stated); cells washed twice; surface peptide modified with diazo reagent; lysed with 0.1% Triton X-100/0.01 M TFA

Parameter: Uptake Measurement; Details: HPLC (Polyencap A300 column); fluorescence detection (ex 445 nm, em 520 nm); intracellular concentration calculated from lysate fluorescence and cell volume

Parameter: Cell Number Variation; Details: Fixed peptide concentration and volume; varying cell number to test peptide-to-cell ratio dependence

Parameter: Incubation Volume Variation; Details: Fixed cell number; 200-800 μL peptide solution; varying total peptide amount per cell

Parameter: Competition Experiments; Details: 1 μM labeled MAP + 1 μM unlabeled MAP; or 2 μM labeled MAP; comparing intracellular label content

Parameter: Controls; Details: LDH release (membrane damage); cell synchronization (serum starvation, DMSO) to rule out cell cycle effects; addition of FL-MAP to lysate to rule out protease artifacts

Parameter: Energy Depletion Controls; Details: 2-deoxyglucose + sodium azide; comparison of degradation products (intact peptide vs. metabolites)

Parameter: Replicates; Details: Not explicitly stated; representative experiments with multiple samples

Analysis Category: Peptide Synthesis; Methods: Solid-phase Fmoc chemistry (MilliGen 9050); N-terminal FLUOS conjugation; preparative HPLC; MALDI-MS confirmation

Analysis Category: Surface vs. Internalized Peptide; Methods: Diazotized 2-nitroaniline treatment (0°C, 10 min); modifies primary amines on surface-bound peptide; HPLC shifts modified peptide retention time; only internalized peptide remains unmodified

Analysis Category: HPLC Analysis; Methods: Bischoff gradient system; Polyencap A300 column; 0.01 M TFA/acetonitrile gradient; fluorescence detection; quantitation vs. calibration curves

Analysis Category: Capillary Electrophoresis (PNA); Methods: P/ACE MDQ with LIF detector; neutral coated capillary; 200 mM Tris/borate, pH 7.5, 5 M urea, 0.1% SDS; 645 V/cm

Analysis Category: Protein Quantification; Methods: Bradford assay (Sigma) for cell number normalization; average protein content: 180 μg per 10⁶ cells

Analysis Category: Cell Volume; Methods: Coulter-ZM counter; average volume: 1.2 pL; independent of cell density

Analysis Category: LDH Leakage; Methods: LDH-L reagent (Sigma); spectrophotometric 340 nm; 0.1% Triton X-100 for maximum leakage

Analysis Category: Cell Synchronization; Methods: 24-h serum starvation or 96-h preincubation with 1.5% DMSO (G1 arrest); compared to free-growing cells

Culture Age: 2 days; Control Cells (μM): 24; Energy-Depleted Cells (μM): 20

Culture Age: 4 days; Control Cells (μM): ~15; Energy-Depleted Cells (μM): ~5

Culture Age: 6 days; Control Cells (μM): 8.5; Energy-Depleted Cells (μM): 0.6

Culture Age: Interpretation; Control Cells (μM): Uptake decreases with culture age; Energy-Depleted Cells (μM): Energy depletion effect more pronounced

Cells per Well: ~200,000; Intracellular MAP (μM): ~40; Accumulation Fold: 40×

Cells per Well: ~500,000; Intracellular MAP (μM): ~10-15; Accumulation Fold: 10-15×

Cells per Well: ~1,300,000; Intracellular MAP (μM): <2; Accumulation Fold: <2×

Cells per Well: Correlation; Intracellular MAP (μM): Negative with cell number; Accumulation Fold: -

Cells per Well: ~300,000; Intracellular Penetratin (μM): ~250

Cells per Well: ~1,000,000; Intracellular Penetratin (μM): ~120

Cells per Well: Conclusion; Intracellular Penetratin (μM): Penetratin uptake also inversely correlates with cell number

Parameter: 200 μL → 400 μL; MAP: Linear increase; Penetratin: Linear increase

Parameter: 400 μL → 600 μL; MAP: Linear increase; Penetratin: Linear increase

Parameter: 600 μL → 800 μL; MAP: Linear increase; Penetratin: Deviated from linear (possible saturation)

Parameter: Doubling volume vs. doubling concentration; MAP: Volume increase more effective; Penetratin: -

Treatment: Control (1 μM labeled); Intact MAP: Low; Energy-Dependent Metabolite (Fluos-Lys): Present; Energy-Independent Metabolites: Present

Treatment: +1 μM unlabeled MAP; Intact MAP: Increased; Energy-Dependent Metabolite (Fluos-Lys): Decreased (~50%); Energy-Independent Metabolites: Unchanged

Treatment: 2 μM labeled MAP; Intact MAP: Increased; Energy-Dependent Metabolite (Fluos-Lys): Decreased (relative); Energy-Independent Metabolites: Unchanged

Treatment: Interpretation; Intact MAP: Energy-dependent metabolite can be competitively inhibited; Energy-Dependent Metabolite (Fluos-Lys): -; Energy-Independent Metabolites: -

Parameter: Intracellular label content (1 μM MAP); Control Cells: Higher; Energy-Depleted Cells: Lower

Parameter: Fluos-Lys (energy-dependent metabolite); Control Cells: Present; Energy-Depleted Cells: Completely absent

Parameter: Energy-independent metabolites; Control Cells: Present; Energy-Depleted Cells: Present (unchanged)

Parameter: Intact MAP contribution; Control Cells: Low (<10-20%); Energy-Depleted Cells: Similar

Parameter: Supernatant Fluos-Lys; Control Cells: Not detected; Energy-Depleted Cells: -

Parameter: Membrane permeability; Control Cells: Fluos-Lys is membrane-impermeable; Energy-Depleted Cells: -

Substance: KLGL (non-amphipathic); Uptake Dependence on Cell Number: Not significant; Degradation: Comparable to MAP

Substance: PNA (endocytic uptake); Uptake Dependence on Cell Number: Not significant; Degradation: Remained intact

Substance: Conclusion; Uptake Dependence on Cell Number: Peptide-to-cell ratio dependence is specific to amphipathic CPPs; Degradation: -

Condition: Serum starvation (24 h); Uptake vs. Control: No significant difference

Condition: DMSO (96 h, G1 arrest); Uptake vs. Control: No significant difference

Condition: Conclusion; Uptake vs. Control: Cell cycle stage is not the cause of density-dependent uptake differences

1. Single cell type: All experiments performed with CHO cells; the peptide-to-cell ratio dependence may not generalize to other cell lines or primary cells.

2. Model CPPs only: The study focuses on MAP and penetratin; other CPPs (TAT, transportan, etc.) may behave differently.

3. Degradation complicates uptake interpretation: The authors note that "more than 90% of the intracellular fluorescence is associated with degradation products" in some experiments, making it difficult to distinguish true peptide uptake from metabolite accumulation.

4. No functional cargo delivery demonstrated: The study measures peptide uptake and degradation only; delivery of biologically active cargo was not assessed.

5. Energy depletion may affect multiple pathways: 2-deoxyglucose and sodium azide deplete ATP broadly, potentially affecting many cellular processes beyond endocytosis.

6. Cell volume measurement assumptions: Cell volume was measured by Coulter counter and assumed constant across densities; any density-dependent changes in cell size would affect intracellular concentration calculations.

7. No in vivo studies: All experiments performed in vitro; relevance to in vivo CPP delivery is not addressed.

8. Possible protease artifacts: The authors note that FL-MAP added to lysate did not degrade, confirming that proteases were not active during lysis, but this does not rule out degradation during peptide incubation or cell processing.

9. Limited characterization of degradation products: While Fluos-Lys was identified, the "energy-independent metabolites" (>10 peaks) were not structurally characterized.

10. No exploration of peptide-to-lipid ratio at membrane level: The authors reference peptide-to-vesicle ratio in liposome studies but do not measure membrane-associated peptide or peptide-to-membrane lipid ratios directly.

11. Potential for diazo reagent artifacts: The diazotized 2-nitroaniline treatment may have incomplete labeling efficiency or affect membrane integrity; the authors validated this method previously but limitations remain.

12. No investigation of peptide uptake kinetics: Uptake was measured at a single time point (1 h) for most experiments; time-dependent effects were not fully characterized.

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