Abstract
Overcoming the nonspecific cellular uptake of cell-penetrating peptides (CPPs) is a major hurdle in their clinical application. Using pH as the activation switch, histidine-glutamic acid (HE) dipeptide repeats were fused to CPPs to trigger the membrane-penetrating activity at mildly acidic pH environments (i.e., pH 6.5 or below) while masking the internalization at neutral pH (i.e., pH 7.0 or above). In this study, a series of recombinant GST-fusion proteins containing an HE oligopeptide sequence (i.e., (HE)n with n = 8, 10, or 12) and a cationic CPP (i.e., YG(RG)6, YGR6G6, or Tat) were engineered for a pH-sensitive study comparing their cellular uptake and surface binding in cultured HeLa cells. Circular dichroism (CD) spectroscopy was performed to correlate differences between CPPs in secondary structure with the pH sensitivity. YGR6G6 with clustered arginine residues exhibited greater pH sensitivity in cellular uptake than YG(RG)6 with separated arginine residues. Increasing the stretch of HE repeats decreased cellular uptake and surface binding for both YG(RG)6 and YGR6G6. The ratio of cellular internalization at pH 7.5 vs 6.0 was not changed by the presence of serum. CD spectral data revealed that both (HE)10-Tat and (HE)10-YGR6G6 exhibited an unordered secondary structure, whereas (HE)10-YG(RG)6 adopted an antiparallel β-sheet conformation. This β-sheet conformation presumably stabilized the association of (HE)10 with YG(RG)6, leading to weakened pH sensitivity of (HE)10-YG(RG)6. On the other hand, the random-coiled structures, that is, (HE)10-YGR6G6 and (HE)10-Tat, both showed higher pH sensitivity as determined in cell experiments. The data presented in this study provide a basis for the future design of pH-sensitive HE-CPP carrier for targeted drug delivery.