Biomedical Engineering Reference
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Tat (Wadia et al. 2004 ), M918 (El-Andaloussi et al. 2007 ) and other permeant
peptides (Sawant and Torchilin 2010 ).
Clathrin-mediated internalization was initially reported for Tat (Richard et al.
2003 ) but different results were obtained in further studies. It was first reported that
knock down of clathrin-mediated endocytosis or knockout of caveolin-mediated
endocytosis did not affect the ability of Tat to enter cells (Ter-Avetisyan et al.
2009 ). In addition, it was suggested that Tat could internalize at 4°C through a
direct translocation mechanism (Jiao et al. 2009 ; Ter-Avetisyan et al. 2009 ).
For oligoarginine peptides, it was first reported that the peptide internalized
through macropinocytosis (Nakase et al. 2004 ) or that direct translocation driven by
the membrane potential occurred (Rothbard et al. 2004 ; 2005 ). It was further pro-
posed by a single-molecule motion study that the mode by which octaarginine
penetrates the cell membrane could be either a multi-mechanism uptake process or
a mechanism different from passive diffusion and endocytosis (Lee et al. 2008 ).
Other studies suggested that this peptide induces the formation of transient pores in
cell membranes in the presence of an electrostatic potential gradient (Herce et al.
2009 ; Cahill 2010 ). A recent work reports that oligoarginine can change the lipid
composition of cell membrane through the translocation in the outer membrane
leaflet of sphingomyelinase and ceramide formation (Verdurmen et al. 2010 ).
Finally, internalization of the pAntp homeobox and of the derived penetratin
peptide was originally described as a temperature and energy-independent process
(Joliot et al. 1991 ; Derossi et al. 1994 ). Further studies suggested that penetratin
enters via an endocytosis pathway rather than a translocation mechanism (Drin
et al. 2003 ; Jones et al. 2005 ). A recent study highlighted the possibility that pen-
etratin could internalize through transient pores and activate a resealing mecha-
nism, known as a membrane repair response (Palm-Apergi et al. 2009 )
6
Model Membrane Perturbation by CPP
To evaluate the energy-independent contribution to CPP uptake, the so-called direct
translocation, several biophysical studies using different lipid models systems and
a panoply of techniques have been employed in an attempt to elucidate the role of
proteoglycans and lipids in the uptake mechanism as well as the peptide and lipid
restructuration taking place upon their contact.
The direct translocation of CPPs through liposomes, has been investigated by
several laboratories and conflicting results have been obtained. Uptake studies on
giant unilamellar vesicles (GUVs) reported a transbilayer movement of penetratin
(Thoren et al. 2000 ) or Tat(48-59) (Curnow et al. 2005 ), which contradicted studies
on smaller lipid vesicles or planar membranes where these CPPs were found not to
cross the membrane. Studies on LUVs have established that translocation is depen-
dent on membrane potential and is modulated by the lipid composition (Terrone
et al. 2003 ). One of the reasons for the divergent results may come from the different
membrane curvature of the different lipid model systems used.
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