A Quantitative Fluorescence Microscopy-based Single Liposome Assay for Detecting the Compositional Inhomogeneity Between Individual Liposomes

Summary

This protocol describes the fabrication of liposomes and how these can be immobilized on a surface and imaged individually in a massive parallel manner using fluorescence microscopy. This allows for the quantification of the size and compositional inhomogeneity between single liposomes of the population.

Abstract

Most research employing liposomes as membrane model systems or drug delivery carriers relies on bulk read-out techniques and thus intrinsically assumes all liposomes of the ensemble to be identical. However, new experimental platforms able to observe liposomes at the single-particle level have made it possible to perform highly sophisticated and quantitative studies on protein-membrane interactions or drug carrier properties on individual liposomes, thus avoiding errors from ensemble averaging. Here we present a protocol for preparing, detecting, and analyzing single liposomes using a fluorescence-based microscopy assay, facilitating such single-particle measurements. The setup allows for imaging individual liposomes in a massive parallel manner and is employed to reveal intra-sample size and compositional inhomogeneities. Additionally, the protocol describes the advantages of studying liposomes at the single liposome level, the limitations of the assay, and the important features to be considered when modifying it to study other research questions.

Introduction

Liposomes are spherical phospholipid-based vesicles that are heavily used both in basic and applied research. They function as excellent membrane model systems, because their physiochemical properties can be easily manipulated by varying the lipid components making up the liposome^1,2. Also, liposomes constitute the most used drug delivery nanocarrier system, offering improved pharmacokinetics and pharmacodynamics as well as high biocompatibility³.

For many years, liposomes have primarily been studied using bulk techniques, giving only access to ensemble avera....

Protocol

1. Liposome Preparation

NOTE: Briefly, preparation of liposomes usually includes three crucial steps: 1) preparation of dry lipid films of the desired lipid composition; 2) rehydration of the lipids for formation of liposomes; and 3) controlling the size and lamellarity of the liposome population.

1. Weigh out the lipids and dissolve them in tert-butanol:water (9:1) in glass vials.
   1. Dissolve POPC (1-palmitoyl-2-oleoyl-glycero-3-phosphocholine; MW = 760 g/mol) to 50 mM.
   2. Dissolve cholesterol (MW = 387 g/mol) to 25 mM.
   3. Dissolve DOPE-Atto488 (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-Atto488; MW = 1....

Results

Following the protocol described makes it possible to image single liposomes in a massive parallel manner (Figure 1). The successful surface immobilization of liposomes should be immediately apparent upon the addition of the liposome solution to the chamber (step 3.6 in the protocol) as diffraction limited intensity spots should appear in the image (Figure 1B and Figure 1C

Discussion

It is important to note that while we describe in detail how the single liposomes assay can be used to study the compositional inhomogeneity between individual liposomes, the platform is very versatile. As previously shown and discussed in the introduction, the protocol can easily be adapted to study aspects of membrane-membrane fusion, protein-membrane interactions, or liposomal drug carrier characterization. For any scientific questions being addressed, the power of the single liposome assay lies in the ability to dete.......

Materials

Name	Company	Catalog Number	Comments
8-well microscopy slides (µ slides)	Ibidi	80827	Microscopy slides with glass bottom
Avanti Mini Extrusion kit	Avanti Polar Lipids	610000	Consumables (Whatman filters) can be aquired from GE Healthcare
BSA	Sigma	A9418
BSA-Biotin	Sigma	A8549
Cholesterol	Avanti Polar Lipids	700000	Traded trough Sigma
Computer with FIJI (Fiji Is Just ImageJ)			ComDet plugin must be installed. Also, a data handling software (Excel, MatLab, OpenOffice, GraphPad Prism etc.) able to load .txt files will be needed to plot the data
DOPE-Atto488	Atto-Tech	AD488-165
DOPE-Atto655	Atto-Tech	AD655-165
DOPE-PEG-Biotin	Avanti Polar Lipids	880129	Traded trough Sigma
D-Sorbitol	Sigma	S-6021
Freeze-dryer			e.g. ScanVac Coolsafe from Labogene
Glass vials	Brown Chromatography	150903	Glass vials that can resist snap-freezing in liquid nitrogen. The 8 mL version of the vials has a size that also fits with the syringes of the extrusion kit
HCl	Honeywell Fluka	258148
Heating bath			Capable of heating to minimum 65C
Heating plate w. Magnet stirring			Capable of heating to minimum 65C
HEPES	Sigma	H3375
Liquid nitrogen			Including container for storage, e.g. Rubber-bath
Magnetic stirring bars	VWR	442-4520 (EU)
Microcentrifuge tubes 1.5 mL	Eppendorf	0030 120.086 (EU)
Microscope			For the images in this protocol a Leica SP5 confocal microscope has been used
Na HEPES	Sigma	H7006
NaCl	Sigma	S9888
NaOH	Honeywell Fluka	71686
POPC	Avanti Polar Lipids	850457	Traded trough Sigma
Streptavidin	Sigma	S4762
tert-Butanol (2-methyl-2-propanol)	Honeywell Riedel-de Haën	24127
Ultrapure water			e.g. MilliQ