Reviewing biophysical and cell biological methodologies in cell-penetrating peptide (CPP) research

The discovery of cell-penetrating peptides (CPPs), which have the ability to translocate across the plasma membranes of mammalian cells, has led to widespread optimism for delivery of problematic therapeutic cargoes to cells. These cargoes include peptide, protein, and nucleic acid biopharmaceuticals and even nano-sized vectors such as liposomes and nanoparticles. Research on CPPs includes biophysical studies of membrane models to investigate fundamental principles of CPP-lipid membrane interactions as well as cell studies focusing on the efficiency of uptake, mechanisms of translocation, and toxicity. However, both lines of research have suffered from misinterpretation as well as premature extrapolations. In this review, we provide a critical evaluation of the potential and limitations of selected biophysical methodologies such as fluorescence spectroscopy, circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy, atomic-force microscopy (AFM), and non-spectroscopic methods. We include a discussion of the most important bilayer membrane models in CPP research. We then evaluate important cell biological methodologies, in particular confocal laser scanning microscopy (CLSM) and fluorescence-associated cell sorting (FACS) in combination with various techniques to distinguish between translocated and non-translocated CPPs. Moreover, we discuss the diverse methodologies for tracing the pathways of CPP translocation and their routes of intracellular trafficking.