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Abstract:
Cell penetrating peptides (CPP) are a special class of peptides that possess the property to traverse the formidable barrier of the plasma membrane and deliver cargos into cells. Using CPP as vectors and DNA, mRNA or proteins/enzymes as potential intracellular targets, a new generation of intracellular contrast agents (CAs) can be developed. These agents have prospective use for molecular imaging (both optical and magnetic resonance imaging) by targeted labeling of cells. Aiming to image the presence of specific mRNAs or enzymes, two mRNA targeting (contains a PNA sequence antisense or non-sense to the target mRNA of DsRed) and one enzyme targeted (contains a unit cleavable by -galactosidase) CAs were tested for their activity in the presence and absence of respective targets. The antisense targeting CA, their nonsense derivative and the enzyme targeted CA were taken up efficiently into cells by an exclusively endosomal mechanism as observed by fluorescence microscopy. Cell free binding assays proved a specific interaction with a synthetic target for the antisense but not for non-sense CA. Magnetic Resonance studies showed a higher uptake in transgenic DsRed expressing cells than the parent cells. However, no difference was observable for antisense versus non-sense CA in DsRed cells, due to the vesicular entrapment which is preventing the specific interaction between CA and cytosolic target. Since a comparable cellular distribution was visible for the enzyme targeted agent, a specific accumulation in -galactosidase containing cells is also unlikely. The results show that even though the designed CAs were efficiently taken up into cells, they can interact specifi cally with the target only if colocalization is achieved. However, a lack of specificity is caused by the endosomal entrapment. Further modifications are required to achieve the release from endosomes or a direct uptake into the cytosol.
