We report on how a dimer of the cell-penetrating peptide TAT, dfTAT, penetrates live cells by escaping from endosomes with a particularly high efficiency. By mediating endosomal leakage, dfTAT also delivers proteins into cultured cells after a simple co-incubation procedure. Cytosolic delivery is achieved in most cells in a culture and only a relatively small amount of material remains trapped inside endosomes. Delivery does not require binding interactions between dfTAT and a protein, multiple molecules can be delivered at once, and delivery can be repeated. Remarkably, dfTAT-mediated delivery does not noticeably impact cell viability, proliferation, or gene expression. This new delivery strategy should be extremely useful for cell-based assays, cellular imaging applications, and the ex vivo manipulation of cells.

Cell penetrating peptides (CPPs) can deliver cell-impermeable therapeutic cargos into cells. In particular, CPP-cargo conjugates tend to accumulate inside cells by endocytosis. However, they often remain trapped inside endocytic organelles and fail to reach the cytosolic space of cells efficiently. In this review, the evidence for CPP-mediated endosomal escape is discussed. In addition, several strategies that have been utilized to enhance the endosomal escape of CPP-cargos are described. The recent development of branched systems that display multiple copies of a CPP is presented. The use of viral or synthetic peptides that can disrupt the endosomal membrane upon activation by the low pH of endosomes is also discussed. Finally, we survey how CPPs labeled with chromophores can be used in combination with light to stimulate endosomal lysis. The mechanisms and challenges associated with these intracellular delivery methodologies are discussed.

Endosomal entrapment is a severely limiting bottleneck in the delivery of biologics into cells. The compound dfTAT was recently found to circumvent this problem by mediating endosomal leakage efficiently and without toxicity. Herein, we report on the mechanism of endosomal escape of this cell-penetrating peptide. By modulating the trafficking of the peptide within the endocytic pathway, we identify late endosomes as the organelles rendered leaky by dfTAT. We establish that dfTAT binds bis(monoacylglycero)phosphate (BMP), a lipid found in late endosomes, and that the peptide causes the fusion and leakage of bilayers containing BMP. Together, these data identify late endosomes as desirable gateways for cell penetration and BMP as a cellular factor that can be exploited for the development of future delivery agents.

Cell delivery or cell killing processes often involve the crossing or disruption of cellular membranes. We review how, by modifying the composition and properties of membranes, membrane oxidation can be exploited to enhance the delivery of macromolecular cargos into live human cells. We also describe how membrane oxidation can be utilized to achieve efficient killing of bacteria by antimicrobial peptides. Finally, we present recent evidence highlighting how membrane oxidation is intimately engaged in natural biological processes such as antigen delivery in dendritic cells and in the killing of bacteria by human macrophages. Overall, the insights that have been recently gained in this area should facilitate the development of more effective delivery technologies and antimicrobial therapeutic approaches.

Edited by F. Peter GuengerichCell-penetrating peptides (CPPs) are well established as delivery agents for otherwise cell-impermeable cargos. CPPs can also theoretically be used to modulate intracellular processes. However, their susceptibility to proteolytic degradation often limits their utility in these applications. Previous studies have explored the consequences for cellular uptake of converting the residues in CPPs from L-to D-stereochemistry, but conflicting results have been reported and specific steps en route to intracellular activity have not been explored. Here we use dimeric fluorescence TAT as a model CPP to explore the broader consequences of L-to D-stereochemical conversion. We show that inversion of chirality provides protease resistance without altering the overall mode of cellular entry, a process involving endocytic uptake followed by endosomal escape and cytosolic access. However, whereas inversion of chirality reduces endocytic uptake, the D-peptide, once in the endosome, is significantly more prone to escape than its L-counterpart. Moreover, the D-peptide is retained in the cytosol of cells for several days, whereas the L-peptide is degraded within hours. Notably, while the L-peptide is relatively innocuous to cells, the D-peptide exerts a prolonged antiproliferative activity. Together, our results establish connections between chirality, protease resistance, cellular penetration, and intracellular activity that may be useful for the development of future delivery agents with improved properties. Cell-penetrating peptides (CPPs)2 have become important tools for the delivery of macromolecular cargoes inside cells (1-3). These delivery agents show promise in therapeutic applications and are useful reagents for cell biology assays (4 -6). For instance, CPPs (TAT, penetratin, etc.) are currently being tested in several preclinical and clinical trials (7,8). However, CPPs exposed to cells or serum are rapidly degraded, and this can consequently render these compounds less effective in vivo or in vitro (9 -13). To protect CPPs from degradation, a common strategy has been to employ D-amino acids instead of their L-amino acid counterparts. D-Peptides are protease-resistant, and this approach has been applied to CPPs such as TAT, R9, penetratin, hLF, pVEC, and sweet arrow peptide (10, 14 -17). In addition, the extended in vivo half-lives of D-peptides over L-peptides have contributed to the successful development of D-polyarginine CPPs as cancer contrast agents (18,19). How inversion of chirality impacts the multifaceted functions of CPPs, however, remains unclear.Several reports have indicated that cellular uptake of CPPs is independent of peptide backbone chirality (16,20). Uptake of the CPPs studied was thought to involve direct plasma membrane translocation. This is because uptake persisted at 4°C, a condition that typically abolishes endocytosis (16). In many cases, however, the penetration of CPPs into cells involves different routes of cellular entry (21). Instead of crossing the plasma membr...