Elucidating the mechanism of cellular uptake

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The discovery of these natural peptides was followed by the development of various synthetic analogues of tat peptides such as penetratin, a peptide of 16 amino acids, derived from the DNA binding domain of the Antennapedia homeoprotein, which is currently the second most commonly used CPP in NP research after the HIV tat peptide [20].In addition to the use of CPPs, the efficacy of m NP internalisation may be further enhanced by the use of external static magnetic fields [21] through a process termed ‘magnetofection’, which is capable of increasing the transfection rates to about 100 fold using extremely low concentrations of m NPs [22].To overcome this difficulty, and to further enhance the cellular uptake, NPs can be conjugated with cell penetrating peptides (CPPs), which are vectors employed for to enhance cell internalisation [9,10].

Alternatively, clathrin-dependent endocytosis is a receptor-mediated endocytosis, which involves the formation of plasma membrane vesicles membrane invagination, containing receptors that are highly specific to the molecule being internalised.Magnetic NPs (m NPs), such as superparamagnetic iron oxide nanoparticles (Fe), are of such a size that they are easily magnetised under an applied field, but lose their magnetism as soon as the magnetic field is removed (thus preventing NP aggregation), and provide an excellent platform for use in clinic.Currently m NPs are employed in clinic in MRI, which allows intra-tissue and intracellular detection.The common structural feature of these CPPs is the presence of basic or cationic amino acids, in particular lysine and arginine, conferring the translocation properties [12].The first discovered CPP was the HIV transactivator of transcription (tat) peptide, which is a non-toxic curtailed version of the naturally existing protein [13,14].

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