Fluorescence imaging with single-molecule sensitivity and fluorescence correlation spectroscopy of cell-penetrating neuropeptides

Quantification of protein delivery in live cells using fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is a quantitative single-molecule method that measures the concentration and rate of diffusion of fluorophore-tagged molecules, both large and small, in vitro and within live cells, and even within discrete cellular compartments. FCS is exceptionally well-suited to directly quantify the efficiency of intracellular protein delivery-specifically, how well different "cell-penetrating" proteins and peptides guide proteinaceous materials into the cytosol and nuclei of live mammalian cells. This article provides an overview of the procedures necessary to execute robust FCS experiments and evaluate endosomal escape efficiencies: preparation of fluorophore-tagged proteins, incubation with mammalian cells and preparation of FCS samples, setup and execution of an FCS experiment, and a detailed discussion of and custom MATLAB® script for analyzing the resulting autocorrelation curves in the context of appropriate diffusion models.

Molecular partners for interaction and cell internalization of cell-penetrating peptides: how identical are they?

Cell-penetrating peptides are short basic peptide sequences that might display amphipathic properties. These positively charged peptides internalize into all cell types, albeit with different efficiency. Cell-penetrating peptides use all routes of pinocytosis to internalize, in addition to direct membrane translocation that requires interaction with lipid membrane domains. These differences in internalization efficiency according to the peptide sequence and cell type suggest that the cell-penetrating peptides interact with different molecular partners at the cell surface. This review will first report on data that describe the molecular interaction of the most popular cell-penetrating peptides (penetratin, Tat and oligoarginine) with carbohydrates and lipids. The second part of the review will be dedicated to cell studies that have reported how cell surface composition influences cell internalization. Discussion will focus on the gap between in vitro and in cellulo studies, and more specifically to which extent the interaction with molecules found in membranes reflect the internalization efficiency of the peptides.