Optical calcium sensors: development of a generic method for their introduction to the cell using conjugated cell penetrating peptides

Probes Encapsulated By Biologically Localised Embedding (PEBBLEs) are optical sensors with nanometer dimensions fabricated by microemulsion polymerisation. The most beneficial characteristic of these sensors is the protection offered by the sensor matrix which decreases interaction between the fluorophore and the cell. These sensors have been introduced to the cell by a number of methods; however this paper discusses the development of a generic method to facilitate inclusion of this type of sensor in the cell by a simple incubation step. This was achieved by covalent linkage of a synthetic Cell Penetrating Peptide (CPP) based on the Human Immuno-deficiency Virus (HIV) -1 Tat, to the external sensor matrix. Calcium sensors were used to demonstrate this approach to incorporate the sensors within the cell. Characterisation revealed the calcium sensors were approximately 30 ± 7 nm in diameter with a slightly negative zeta potential. The sensors demonstrated a linear range of 0–50 µM with negligible interference from a range of cellular ions and protein. Leaching of entrapped dyes from the calcium sensors was determined as 3% in a 24 h period, while photobleaching of the entrapped dye was minimal over a 40 min period. The sensors ability to cross the cell membrane using the covalently attached synthetic Tat peptide is demonstrated. Cellular inclusion of the sensors occurred within a 30 min incubation period.