Translocation of molecules into cells by pH-dependent insertion of a transmembrane helix

Abstract

We have previously observed the spontaneous, pH-dependent insertion of a water-soluble peptide to form a helix across lipid bilayers [Hunt, J. F., Rath, P., Rothschild, K. J. & Engelman, D. M. (1997) Biochemistry 36, 15177-15192]. We now use a related peptide, pH (low) insertion peptide, to translocate cargo molecules attached to its C terminus across the plasma membranes of living cells. Translocation is selective for low pH, and various types of cargo molecules attached by disulfides can be released by reduction in the cytoplasm, including peptide nucleic acids, a cyclic peptide (phalloidin), and organic compounds. Because a high extracellular acidity is characteristic of a variety of pathological conditions (such as tumors, infarcts, stroke-afflicted tissue, atherosclerotic lesions, sites of inflammation or infection, or damaged tissue resulting from trauma) or might be created artificially, pH (low) insertion peptide may prove a useful tool for selective delivery of agents for drug therapy, diagnostic imaging, genetic control, or cell regulation.

Fig. 1.

pHLIP insertion and topology. (a) Schematic diagram of cargo molecule delivery into a cell. At physiological pH, the peptide–cargo conjugate interacts weakly with a membrane. At low pH, the peptide forms a transmembrane helix with its C terminus inserted in the cytoplasm. Reduction of the disulfide bond releases a drug. (b) The topology of the pHLIP peptide in a lipid bilayer was determined by using the NBD–dithionite quenching reaction. The fluorescence signal of NBD attached to the N terminus of peptide was monitored at 530 nm when excited at 470 nm. Sodium dithionite was added to the NBD peptide inserted into POPC large unilamellar vesicles. (c) POPC large unilamellar vesicles containing dithionite ion inside were added to the NBD peptide at pH 8.0, and then decreasing the pH triggered insertion of the peptide in liposomes. Triton X-100 was used for the disruption of liposomes. The concentration of the peptide used in the experiments was 7 μM.

Fig. 2.

pHLIP transport of dansyl into cells. (a) Fluorescence images of HeLa cells incubated (for 15 min) with cleavable pHLIP–S–S–dansyl construct (7 μM) at pH 5.5, 6.5, 7.0, and 7.4 and washed with PBS buffer at pH 7.4 are shown. (b) Quantification of fluorescence images. The fluorescence signal of cells at pH 5.5 was taken as 100%. The uptake of dansyl strongly decreases with an increase of extracellular incubation pH.

Fig. 3.

The delivery of PNA into cells by pHLIP. (a) Fluorescence and phase-contrast images of HeLa cells incubated (for 30 min) with a pHLIP–S–S–PNA–TAMRA cleavable construct (1 μM) at pH 7.4 (Left) and 6.5 (Right) are shown. No translocation was observed of pHLIP–S–S–PNA–TAMRA at pH 7.4 or PNA–TAMRA at pH 7.4 or 6.5 (data not shown). (b) HeLa cells labeled with PNA–TAMRA translocated by pHLIP at pH 6.5 (Left) and the same cells treated with SYTOX–Green (Right). The majority of cells have only PNA–TAMRA, whereas only one cell had both PNA–TAMRA and SYTOX–Green.

Fig. 4.

The delivery of phalloidin into cells by pHLIP. (a) Fluorescence images of HeLa cells incubated (for 1 h) with a pHLIP–S–S–Ph–TRITC cleavable construct (2 μM) at pH 7.4 (Left) and 6.5 (Right) are shown. The fluorescence was extremely weak after pH 7.4 incubation and localized to the plasma membrane. Strong fluorescence of actin filaments was observed after pH 6.5 incubation. (b) Images of HeLa (Left), breast JC (Center), and prostate TRAMP-C1 (Right) cancer cells with fluorescent actin filaments are shown. Cells were incubated for 1 h with the cleavable pHLIP–S–S–Ph–TRITC (0.5–1 μM) at pH 6.5 followed by washing with PBS at pH 7.4.

Fig. 5.

Cytofluorometry of HeLa cells. (a and b) The untreated cells (a) and cells treated with Ph–TRITC (b) at pH 6.5 and 37°C are shown. (c–e) Cells treated (for 1 h) with 6 μM pHLIP–S–S–Ph–TRITC at pH 7.4 and 37°C (c), pH 6.5 and 37°C (d), and pH 6.5 and 4°C (e) are shown.

Fig. 6.

Cell phenotypes induced by phalodin transport. (a) Phase-contrast images of HeLa cells incubated (for 1 h) with pHLIP–S–S–Ph–TRITC (1 μM) at pH 6.5 and 7.4 followed by washing with PBS (pH 7.4) before (Left) and 5 min after adding of the dissociation solution (Right). Cells treated with the peptide–phalloidin at low pH remained unchanged, consistent with stabilization of the cytoskeleton by Ph–TRITC delivered by the pHLIP. (b) Fluorescence images of nuclei stained with DAPI (0.5 μM) and corresponding phase-contrast images of the multinucleated HeLa cells are presented. Multinucleation was observed at 48 h after treatment of cells with of pHLIP–S–S–Ph–TRITC (1 μM) at pH 6.5 for 1 h.