The authors would like to thank Emilie Josse for the critical review of the manuscript. This work was supported by the foundation &#34;La Ligue contre le Cancer&#34;, the &#34;Fondation ARC pour la Recherche sur le Cancer&#34;, and the &#34;Centre National de la Recherche Scienti&#64257;que&#34; (CNRS).

1. Preparation of Large Unilamellar Vesicles (LUVs)
<ol>
	<li>Prepare LUVs for their use as cell membrane mimics for fluorescence leakage assay.</li>
	<li>Mix with a Hamilton glass syringe phosphatidylcholine (DOPC, 786.11 g/mol), sphingomyelin (SM, 760.22 g/mol) and Cholesterol (Chol, 386.65 g/mol) at the molar ratio 4:4:2. The lipid solution is obtained from a stock solution of each lipid solubilized in a methanol/chloroform (3/1; volume/volume) solvent at 25 mg/mL in a 25 mL glass round-bottom flask. Based on 4 &#18...

<ol>
	<li>Langel, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Handbook of Cell-Penetrating Peptides. , (2006).</li><li>Deshayes, S., Morris, M. C., Divita, G., Heitz, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cell-penetrating+peptides:+tools+for+intracellular+delivery+of+therapeutics.">Cell-penetrating peptides: tools for intracellular delivery of therapeutics.</a> Cellular and Molecular Life Sciences CMLS. 62 (16), 1839-1849 (2005).</li><li>Richard, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cell-penetrating+peptides.+A+reevaluation+of+the+mechanism+of+cellular+uptake.">Cell-penetrating peptides. A reevaluation of the mechanism of cellular uptake.</a> The Journal of Biological Chemistry. , (2003).</li><li>Jones, A., Sayers, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cell+entry+of+cell+penetrating+peptides:+tales+of+tails+wagging+dogs.">Cell entry of cell penetrating peptides: tales of tails wagging dogs.</a> Journal of Controlled Release Official Journal of the Controlled Release Society. , (2012).</li><li>T&#252;nnemann, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Live-cell+analysis+of+cell+penetration+ability+and+toxicity+of+oligo-arginines.">Live-cell analysis of cell penetration ability and toxicity of oligo-arginines.</a> Journal of Peptide Science. 14 (4), 469-476 (2008).</li><li>Jiao, C. -. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Translocation+and+endocytosis+for+cell-penetrating+peptide+internalization.">Translocation and endocytosis for cell-penetrating peptide internalization.</a> Journal of Biological Chemistry. 284 (49), 33957-33965 (2009).</li><li>Deshayes, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Deciphering+the+internalization+mechanism+of+WRAP:siRNA+nanoparticles.">Deciphering the internalization mechanism of WRAP:siRNA nanoparticles.</a> Biochimica Et Biophysica Acta. Biomembranes. 1862 (6), 183252 (2020).</li><li>Konate, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optimisation+of+vectorisation+property:+A+comparative+study+for+a+secondary+amphipathic+peptide.">Optimisation of vectorisation property: A comparative study for a secondary amphipathic peptide.</a> International Journal of Pharmaceutics. 509 (1-2), 71-84 (2016).</li><li>Lehto, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cellular+trafficking+determines+the+exon+skipping+activity+of+Pip6a-PMO+in+mdx+skeletal+and+cardiac+muscle+cells.">Cellular trafficking determines the exon skipping activity of Pip6a-PMO in mdx skeletal and cardiac muscle cells.</a> Nucleic Acids Research. 42 (5), 3207-3217 (2014).</li><li>Hoyer, J., Neundorf, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Peptide+vectors+for+the+nonviral+delivery+of+nucleic+acids.">Peptide vectors for the nonviral delivery of nucleic acids.</a> Accounts of Chemical Research. 45 (7), 1048-1056 (2012).</li><li>Milletti, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cell-penetrating+peptides:+classes,+origin,+and+current+landscape.">Cell-penetrating peptides: classes, origin, and current landscape.</a> Drug Discovery Today. 17 (15-16), 850-860 (2012).</li><li>Mueller, J., Kretzschmar, I., Volkmer, R., Boisguerin, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparison+of+cellular+uptake+using+22+CPPs+in+4+different+cell+lines.">Comparison of cellular uptake using 22 CPPs in 4 different cell lines.</a> Bioconjugate Chemistry. 19 (12), 2363-2374 (2008).</li><li>Ramaker, K., Henkel, M., Krause, T., R&#246;ckendorf, N., Frey, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cell+penetrating+peptides:+a+comparative+transport+analysis+for+474+sequence+motifs.">Cell penetrating peptides: a comparative transport analysis for 474 sequence motifs.</a> Drug Delivery. 25 (1), 928-937 (2018).</li><li>Maget-Dana, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+monolayer+technique:+a+potent+tool+for+studying+the+interfacial+properties+of+antimicrobial+and+membrane-lytic+peptides+and+their+interactions+with+lipid+membranes.">The monolayer technique: a potent tool for studying the interfacial properties of antimicrobial and membrane-lytic peptides and their interactions with lipid membranes.</a> Biochimica Et Biophysica Acta. 1462 (1-2), 109-140 (1999).</li><li>Alves, A. C., Ribeiro, D., Nunes, C., Reis, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biophysics+in+cancer:+The+relevance+of+drug-membrane+interaction+studies.">Biophysics in cancer: The relevance of drug-membrane interaction studies.</a> Biochimica et Biophysica Acta (BBA) - Biomembranes. 1858 (9), 2231-2244 (2016).</li><li>Konate, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Peptide-based+nanoparticles+to+rapidly+and+efficiently+"Wrap+'n+Roll"+siRNA+into+cells.">Peptide-based nanoparticles to rapidly and efficiently "Wrap 'n Roll" siRNA into cells.</a> Bioconjugate Chemistry. 30 (3), 592-603 (2019).</li><li>Seisel, Q., Pelletier, F., Deshayes, S., Boisguerin, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=How+to+evaluate+the+cellular+uptake+of+CPPs+with+fluorescence+techniques:+Dissecting+methodological+pitfalls+associated+to+tryptophan-rich+peptides.">How to evaluate the cellular uptake of CPPs with fluorescence techniques: Dissecting methodological pitfalls associated to tryptophan-rich peptides.</a> Biochimica Et Biophysica Acta. Biomembranes. 1861 (9), 1533-1545 (2019).</li><li>Derossi, D., Joliot, A. H., Chassaing, G., Prochiantz, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+third+helix+of+the+Antennapedia+homeodomain+translocates+through+biological+membranes.">The third helix of the Antennapedia homeodomain translocates through biological membranes.</a> Journal of Biological Chemistry. 269 (14), 10444-10450 (1994).</li><li>Takayama, M., Itoh, S., Nagasaki, T., Tanimizu, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+enzymatic+method+for+determination+of+serum+choline-containing+phospholipids.">A new enzymatic method for determination of serum choline-containing phospholipids.</a> Clinica Chimica Acta; International Journal of Clinical Chemistry. 79 (1), 93-98 (1977).</li><li>Notman, R., Noro, M., O'Malley, B., Anwar, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Molecular+basis+for+Dimethylsulfoxide+(DMSO)+action+on+lipid+membranes.">Molecular basis for Dimethylsulfoxide (DMSO) action on lipid membranes.</a> Journal of the American Chemical Society. 128 (43), 13982-13983 (2006).</li><li>Konate, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Insight+into+the+cellular+uptake+mechanism+of+a+secondary+amphipathic+cell-penetrating+peptide+for+siRNA+delivery.">Insight into the cellular uptake mechanism of a secondary amphipathic cell-penetrating peptide for siRNA delivery.</a> Biochemistry. 49 (16), 3393-3402 (2010).</li><li>Vaissi&#232;re, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+retro-inverso+cell-penetrating+peptide+for+siRNA+delivery.">A retro-inverso cell-penetrating peptide for siRNA delivery.</a> Journal of Nanobiotechnology. 15 (1), 34 (2017).</li><li>Eir&#237;ksd&#243;ttir, E., Konate, K., Langel, &. #. 2. 2. 0. ;., Divita, G., Deshayes, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Secondary+structure+of+cell-penetrating+peptides+controls+membrane+interaction+and+insertion.">Secondary structure of cell-penetrating peptides controls membrane interaction and insertion.</a> Biochimica et Biophysica Acta (BBA) - Biomembranes. 1798 (6), 1119-1128 (2010).</li><li>Ziegler, A., Li Blatter, X., Seelig, A., Seelig, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Protein+transduction+domains+of+HIV-1+and+SIV+TAT+interact+with+charged+lipid+vesicles.+Binding+mechanism+and+thermodynamic+analysis.">Protein transduction domains of HIV-1 and SIV TAT interact with charged lipid vesicles. Binding mechanism and thermodynamic analysis.</a> Biochemistry. 42 (30), 9185-9194 (2003).</li><li>Thor&#233;n, P. E. G., Persson, D., Karlsson, M., Nord&#233;n, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Antennapedia+peptide+penetratin+translocates+across+lipid+bilayers+-+the+first+direct+observation.">The Antennapedia peptide penetratin translocates across lipid bilayers - the first direct observation.</a> FEBS Letters. 482 (3), 265-268 (2000).</li><li>Mishra, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Translocation+of+HIV+TAT+peptide+and+analogues+induced+by+multiplexed+membrane+and+cytoskeletal+interactions.">Translocation of HIV TAT peptide and analogues induced by multiplexed membrane and cytoskeletal interactions.</a> Proceedings of the National Academy of Sciences. 108 (41), 16883-16888 (2011).</li><li>Rouser, G., Fleischer, S., Yamamoto, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two+dimensional+thin+layer+chromatographic+separation+of+polar+lipids+and+determination+of+phospholipids+by+phosphorus+analysis+of+spots.">Two dimensional thin layer chromatographic separation of polar lipids and determination of phospholipids by phosphorus analysis of spots.</a> Lipids. 5 (5), 494-496 (1970).</li><li>Bartlett, G. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Phosphorus+assay+in+column+chromatography.">Phosphorus assay in column chromatography.</a> Journal of Biological Chemistry. 234 (3), 466-468 (1959).</li><li>Stewart, J. C. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Colorimetric+determination+of+phospholipids+with+ammonium+ferrothiocyanate.">Colorimetric determination of phospholipids with ammonium ferrothiocyanate.</a> Analytical Biochemistry. 104 (1), 10-14 (1980).</li><li>Spinella, S. A., Nelson, R. B., Elmore, D. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measuring+peptide+translocation+into+large+unilamellar+vesicles.">Measuring peptide translocation into large unilamellar vesicles.</a> Journal of Visualized Experiments: JoVE. (59), e3571 (2012).</li><li>Wimley, W. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determining+the+effects+of+membrane-interacting+peptides+on+membrane+integrity.">Determining the effects of membrane-interacting peptides on membrane integrity.</a> Cell-Penetrating Peptides. 1324, 89-106 (2015).</li><li>B&#225;r&#225;ny-Wallje, E., Gaur, J., Lundberg, P., Langel, U., Gr&#228;slund, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Differential+membrane+perturbation+caused+by+the+cell+penetrating+peptide+Tp10+depending+on+attached+cargo.">Differential membrane perturbation caused by the cell penetrating peptide Tp10 depending on attached cargo.</a> FEBS Letters. 581 (13), 2389-2393 (2007).</li><li>van Rooijen, B. D., Claessens, M. M. A. E., Subramaniam, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Membrane+permeabilization+by+oligomeric+&#945;-Synuclein:+in+search+of+the+mechanism.">Membrane permeabilization by oligomeric &#945;-Synuclein: in search of the mechanism.</a> PloS One. 5 (12), 14292 (2010).</li><li>Hassane, F. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Insights+into+the+cellular+trafficking+of+splice+redirecting+oligonucleotides+complexed+with+chemically+modified+cell-penetrating+peptides.">Insights into the cellular trafficking of splice redirecting oligonucleotides complexed with chemically modified cell-penetrating peptides.</a> Journal of Controlled Release: Official Journal of the Controlled Release Society. 153 (2), 163-172 (2011).</li><li>Asciolla, J. J., Renault, T. T., Chipuk, J. 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The presented fluorescence leakage assay is a simple and fast method to address membrane destabilization by cell-penetrating peptide. Easy to do, it also enables an indirect comparison between different membrane-interacting peptides or other membrane-interacting molecules. Concerning critical steps of the protocol, as this assay provides relative values between the baseline (LUVs alone) and maximal fluorescence release (Triton condition), we usually evaluate the concentration of LUVs using the phospholipid quantification...

After their discovery in the nineties, cell-penetrating peptides (CPPs) were developed to promote an efficient cellular delivery of cargoes through the plasma membrane1,2. CPPs are usually short peptides, generally 8 to 30 amino acids, having a wide variety of origins. They were first defined as &#34;direct-translocating&#34; carriers, meaning they were able to cross the plasma membrane and to transfer a cargo into cells independently of any endocytotic pathway neither energy requirement nor receptor involvement. However, further investigations revealed that these first observations mainly came from fluorescence overestimation due to the experimental artefact and/or to fixation protocols using methanol3. Nowadays, it is widely accepted that CPP uptake takes place by both endocytosis and energy-independent translocation4,5,6,7 depending on different parameters such as the nature of cargo, the used link between CPP and cargo, the studied cell line, etc.
CPPs can be used as transfection agents according to two strategies, either involving a chemical link (covalent strategy) or electrostatic/hydrophobic interactions (non-covalent strategy) between the CPP and its cargo8,9,10,11. Although both strategies have shown their efficiency in the cell transfer of several cargoes, the understanding of the mechanism of internalization by CPPs is still under controversy and the balance between endocytosis pathways or direct penetration is still difficult to measure12,13. Although a set of experimental tools and strategies are available to clearly address the involvement of endocytic processes, the direct translocation seems, however, more difficult to characterize since it implies more discrete interactions with plasma membrane components. Biological membranes are usually composed of numerous components, from phospholipids to membrane proteins, which might vary according to the cellular type and/or the environment (stress conditions, cell division, etc.). This diversity of composition, and consequently the absence of a universal cellular membrane model does not enable studies in a single way. However, to circumvent these limitations step-by-step approaches were developed with artificial membrane or membrane extracts. From small unilamellar vesicles to monolayer approaches, every model was clearly pertinent to answer specific questions14,15. Among them, large unilamellar vesicles (LUVs) constitute an appropriate membrane mimicking model to study peptide/membrane interactions as being a key point in the internalization process.
In this context, the following protocol describes the investigation of the effects of peptides and peptide/membrane interactions on LUVs integrity through the monitoring of both an anionic fluorescent dye and its corresponding poly-cationic quencher encapsulated in liposomes. This tool is used to study CPP/membrane interactions in order to understand whether they are able to perform a direct membrane translocation. Although usually applied to compare different membrane-interacting peptides, this LUV fluorescence leakage assay could also be used for investigating both CPPs-cargo conjugates (covalent strategy) and CPP:cargo complexes (non-covalent strategy).
The present protocol will hence be first exemplified with the recently developed tryptophan (W)- and arginine (R)-rich Amphipathic Peptides (WRAP)16. WRAP is able to form peptide-based nanoparticles to rapidly and efficiently deliver small interfering RNA (siRNA) in several cell lines16. The fluorescence leakage properties of WRAP peptide alone or siRNA-loaded WRAP-based nanoparticles were monitored to characterize their mechanism of cellular internalization. We showed that their mechanism of internalization mainly involved direct translocation7. In a second example, the WRAP peptide was covalently conjugated to the protein/protein interfering peptide iCAL36 (WRAP-iCAL36)17 and its ability to destabilize membranes was compared in a fluorescence leakage assay to iCAL36 coupled to Penetratin18 (Penetratin-iCAL36), another CPP.
Finally, the advantages and limitations of the method will be discussed both from a technological point of view and with respect to biological relevance.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr >
			<td >25 mL glass round-bottom flask</td>
			<td >Pyrex</td>
			<td ></td>
			<td ></td>
		</tr>
		<tr >
			<td >8-aminonaphthalene-1, 3, 6-trisulfonic acid, disodium salt (ANTS)</td>
			<td >Invitrogen</td>
			<td >A350</td>
			<td>Protect from light&#160;</td>
		</tr>
		<tr >
			<td >Chloroform&#160;</td>
			<td >Sigma-Aldrich</td>
			<td >288306</td>
			<td></td>
		</tr>
		<tr >
			<td >Cholesterol</td>
			<td >Sigma-Aldrich</td>
			<td >C8667</td>
			<td></td>
		</tr>
		<tr >
			<td >DOPC (dioleoylphosphatidylcholine)</td>
			<td >Avanti Polar</td>
			<td>850375P</td>
			<td>Protect from air</td>
		</tr>
		<tr >
			<td >Extruder</td>
			<td >Avanti Polar</td>
			<td >610000</td>
			<td></td>
		</tr>
		<tr >
			<td >Fluorimeter</td>
			<td >PTI Serlabo</td>
			<td ></td>
			<td></td>
		</tr>
		<tr >
			<td >50 &#181;L glass syringe</td>
			<td >Hamilton</td>
			<td >705N</td>
			<td></td>
		</tr>
		<tr >
			<td >HEPES</td>
			<td >Sigma-Aldrich</td>
			<td >H3375</td>
			<td></td>
		</tr>
		<tr >
			<td >LabAssay Phospholipid&#160;</td>
			<td >WAKO&#160;</td>
			<td >296-63801</td>
			<td></td>
		</tr>
		<tr >
			<td >liquid chromatography column</td>
			<td >Sigma-Aldrich</td>
			<td ></td>
			<td></td>
		</tr>
		<tr >
			<td >Methanol</td>
			<td >Carlo Erba</td>
			<td >414902</td>
			<td></td>
		</tr>
		<tr >
			<td >Nuclepore polycarbonate membrane (0.1 &#181;m pore size, 25 mm diameter)</td>
			<td >Whatman</td>
			<td >800309</td>
			<td></td>
		</tr>
		<tr >
			<td >polystyrene cuvette, 10 x 10 x 45 mm</td>
			<td >Grener Bio-One</td>
			<td >614101</td>
			<td></td>
		</tr>
		<tr >
			<td >polystyrene semi-micro cuvette, DLS</td>
			<td >Fisher Scientific</td>
			<td >FB55924</td>
			<td></td>
		</tr>
		<tr >
			<td >p-xylene-bispyridinium bromide (DPX)</td>
			<td >Invitrogen</td>
			<td >X1525</td>
			<td>Protect from light&#160;</td>
		</tr>
		<tr >
			<td >quartz fluorescence cuvette</td>
			<td >Hellma</td>
			<td >109.004F-QS</td>
			<td></td>
		</tr>
		<tr >
			<td >rotavapor system&#160;</td>
			<td >Heidolph</td>
			<td >Z334898</td>
			<td></td>
		</tr>
		<tr >
			<td >Sephadex G-50 resin</td>
			<td >Amersham</td>
			<td >17-0042-01</td>
			<td></td>
		</tr>
		<tr >
			<td >Sodium azide (NaN3)</td>
			<td >Sigma-Aldrich</td>
			<td >S2002</td>
			<td></td>
		</tr>
		<tr >
			<td >Sodium chlorid (NaCl)</td>
			<td >Sigma-Aldrich</td>
			<td >S5886</td>
			<td></td>
		</tr>
		<tr >
			<td >Sonicator bath USC300T</td>
			<td >VWR</td>
			<td >142-6001</td>
			<td></td>
		</tr>
		<tr >
			<td >Sphingomyelin</td>
			<td >Avanti Polar</td>
			<td>860062P</td>
			<td>Protect from air</td>
		</tr>
		<tr >
			<td >Triton X-100&#160;</td>
			<td >Eromedex</td>
			<td >2000-B</td>
			<td></td>
		</tr>
		<tr >
			<td >Zetaziser NanoZS&#160;</td>
			<td >Malvern</td>
			<td >ZEN3500</td>
			<td></td>
		</tr>
	</tbody>
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fluorescent leakage assay to investigate membrane destabilization by cell-penetrating peptide

Cell-penetrating peptides (CPPs) are defined as carriers that are able to cross the plasma membrane and to transfer a cargo into cells. One of the main common features required for this activity resulted from the interactions of CPPs with the plasma membrane (lipids) and more particularly with components of the extracellular matrix of the membrane itself (heparan sulphate). Indeed, independent of the direct translocation or the endocytosis-dependent internalization, lipid bilayers are involved in the internalization process both at the level of the plasma membrane and at the level of intracellular traffic (endosomal vesicles). In this article, we present a detailed protocol describing the different steps of a large unilamellar vesicles (LUVs) formulation, purification, characterization, and application in fluorescence leakage assay in order to detect possible CPP-membrane destabilization/interaction and to address their role in the internalization mechanism. LUVs with a lipid composition reflecting the plasma membrane content are generated in order to encapsulate both a fluorescent dye and a quencher. The addition of peptides in the extravesicular medium and the induction of peptide-membrane interactions on the LUVs might thus induce in a dose-dependent manner a significant increase in fluorescence revealing a leakage. Examples are provided here with the recently developed tryptophan (W)- and arginine (R)-rich Amphipathic Peptides (WRAPs), which showed a rapid and efficient siRNA delivery in various cell lines. Finally, the nature of these interactions and the affinity for lipids are discussed to understand and to improve the membrane translocation and/or the endosomal escape.

The principle of the fluorescence leakage assay is shown in the Figure 1. In detail, large unilamellar vesicles (LUVs) encapsulating a fluorescent dye and a quencher (no fluorescence signal) are treated with the biomolecule of interest. Due to the interaction of the peptide with lipid membranes, which could imply membrane permeability, reorganization or even rupture, the fluorescence dye and the quencher are released from the LUVs. Subsequent dilutions in the...

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Fluorescent Leakage Assay to Investigate Membrane Destabilization by Cell-Penetrating Peptide

The fluorescence leakage assay is a simple method that enables the investigation of peptide/membrane interactions in order to understand their involvement in several biological processes and especially the ability of cell-penetrating peptides to disturb phospholipids bilayers during a direct cellular translocation process.

Cell-penetrating peptides (CPPs) are defined as carriers that are able to cross the plasma membrane and to transfer a cargo into cells. One of the main ...

This fluorescence leakage assay is a simple method for addressing membrane destabilization via biomolecules. Using liposomes encapsulated fluorescent dyes, it can be used to compare different membranes interacting peptides. This method is especially useful to investigate membrane destabilization by cell-penetrating peptides in order to understand the mechanism of cellular internalization and to assess their involvement in direct membrane translocation.Compared to biological membranes, liposomes enable control of the phospholipid composition in order to mimic different lipid values, such as plasma, endosomal, or mitochondrial membranes. To prepare LUVs for their use as cell membrane mimics for a florescence leakage assay, use a Hamilton glass syringe to mix phosphatidylcholine, sphingomyelin, and cholesterol at a 4:4:2 molar ratio. Use a rotary evaporator to evaporate the methanol chloroform under vacuum for 46 to 50 minutes at 60 degrees Celsius until a dried lipid film has formed.To reconstitute the multi-lamellar vesicles, resuspend the dried lipid film with one milliliter of lipid hydration solution and vortex thoroughly until the dried lipid film has dissolved. After confirming that all of the lipid has been incorporated and that no film is left on the sides of the flask, freeze and thaw the vesicles five times for 30 seconds in liquid nitrogen and two minutes in a 30 degree water bath per cycle. To prepare lipid extruder, insert two HEPES buffer-treated filter supports into each PTFE extruder piece within the metal extruder canister and place a 0.1 micron pore HEPES humidified polycarbonate membrane onto the top of one filter support.Screw the two metal extruder canisters together and place the assembled extruder into the holder. To check the extruder for leaks or other problems, introduce an empty one milliliter syringe into the appropriate hole at one extremity of the PTFE extruder piece and another one milliliter syringe loaded with one milliliter of HEPES buffer at the other extremity, then deliver the multi-lamellar vesicle sample to the extruder in the same manner and push the sample from one syringe to the other through the polycarbonate membrane at least 21 times to obtain uniform LUVs. To purify the LUVs, add freshly extruded LUVs onto a liquid chromatography column and let the vesicles enter the cross-linked dextran gel.Continuously add HEPES buffer two to the column and begin eluting approximately two milliliters of HEPES buffer two. The free yellow ANTS and DPX solution will migrate more slowly than the liposomes. After one milliliter has been eluted, start collecting the purified LUVs in 1.5 milliliter tubes.When the drops of eluent become opalescent, change the tube to recover the LUV-containing fraction. When the drops are no longer opalescent, elute another 500 microliters in a separate fraction before stopping the elution. To determine the LUV size and polydispersity index, indicate the viscosity of the solvent in the polystyrene semi-microcuvette to be used in the experiment and the dynamic light scattering system and add 500 microliters of the LUV solution to the cuvette, then load the cuvette onto the instrument and measure the size distribution in terms of the mean size of the particle distribution and the homogeneity.To measure the fluorescence leakage, dilute the LUVs in one milliliter of HEPES buffer two in a quartz fluorescence cuvette to a 100 micromolar final concentration and add a magnetic stirrer to facilitate homogenization of the solution during the experiment. Measure the LUVs alone during the first 100 seconds to assess the background fluorescence before measuring the leakage as an increase in fluorescence intensity upon the addition of aliquots of peptide solution over the next 900 seconds. To measure 100%florescence leakage as a positive control, add one microliter of Triton X-100 to the LUVs to solubilize the vesicles, resulting in a complete un-quenching of the probe during the last 100 seconds of the analysis.Then use the formula to calculate the leakage percentage at each time point. In the absence of peptides, no fluorescence leakage is observed from the LUVs. The addition of WRAP onto the LUVs induces a significant increase in fluorescence, revealing an important LUV leakage and ANTS release.After 15 minutes, a leakage of approximately 67%is obtained in the presence of 2.5 micromolar WRAP compared to the Triton positive control. In contrast, when WRAP is assembled at the same concentration with small interfering RNA to form peptide-based nanoparticles, the leakage is 1.5 fold weaker compared to the free peptide. When the conjugated Penetratin-iCAL peptide is used, no significant fluorescence increase is detected, whereas the addition of WRAP-iCAL peptide induces a net leakage characterized by very strong fluorescent signal at the same 2.5 micromolar concentration.These results indicate that the fluorescence leakage assay can be used to reveal the ability of some cell-penetrating peptides to perform peptide membrane interactions leading to a more or less pronounced membrane permeability. This method favors a rapid identification of membrane peptides interactions that lead to membrane destabilization in order to decipher the internalization mechanism of cell-penetrating peptides or other membrane active peptides such as photogenic or antimicrobial peptides. In case of peptide-lipid interaction without leakage induction, other approaches such as tryptophan fluorescence experiments are required.This fluorescence leakage assay is widely used within the CPP field and can provide important information about cell translocation or endosomal escape.

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5.8K vues.  INSERM U1046 - CNRS UMR 9214 - University of Montpellier. This fluorescence leakage assay is a simple method for addressing membrane destabilization via biomolecules. Using liposomes encapsulated fluorescent dyes, it can be used to compare different membranes interacting peptides. This method is especially useful to investigate membrane destabilization by cell-penetrating peptides in order to understand the mechanism of cellular internalization and to assess their involvement in direct membrane translocation.Compared to biological membranes, ...