Lipids, curvature, and nano-medicine

Abstract

The physical properties of the lamellar lipid-bilayer component of biological membranes are controlled by a host of thermodynamic forces leading to overall tensionless bilayers with a conspicuous lateral pressure profile and build-in curvature-stress instabilities that may be released locally or globally in terms of morphological changes. In particular, the average molecular shape and the propensity of the different lipid and protein species for forming non-lamellar and curved structures are a source of structural transitions and control of biological function. The effects of different lipids, sterols, and proteins on membrane structure are discussed and it is shown how one can take advantage of the curvature-stress modulations brought about by specific molecular agents, such as fatty acids, lysolipids, and other amphiphilic solutes, to construct intelligent drug-delivery systems that function by enzymatic triggering via curvature.Practical applications: The simple concept of lipid molecular shape and how it impacts on the structure of lipid aggregates, in particular the curvature and curvature stress in lipid bilayers and liposomes, can be exploited to construct liposome-based drug-delivery systems, e.g., for use as nano-medicine in cancer therapy. Non-lamellar-forming lysolipids and fatty acids, some of which may be designed to be prodrugs, can be created by phospholipase action in diseased tissues thereby providing for targeted drug release and proliferation of molecular entities with conical shape that break down the permeability barrier of the target cells and may hence enhance efficacy.

Figure 1

Schematic illustration of fusion and fission processes involving transport processes of vesicles that are trafficking proteins between the endoplasmic reticulum (bottom) and the Golgi apparatus (top). Courtesy of Dr. Matthias Weiss.

Figure 2

Schematic illustration of the self-assembly process of lipids in water forming aggregates such as (a) a monolayer on the air–water interface, (b) a lipid bilayer, (c) a micelle, (d) a unilamellar liposome (vesicle), and (e) a multi-lamellar liposome.

Figure 3

Snapshots from a large-scale computer simulation of the self-assembly process of lipid vesicles (unilamellar liposomes) in water based on dissipative particle dynamics calculations . The simulation box is 90 nm³ and contains 50.000 lipid molecules in water. The simulation covers a time span of 128 µs. Courtesy of Dr. Julian Shillcock.

Figure 4

Schematic illustration of lamellar and non-lamellar lipid aggregates formed by self-assembly processes in water. The different structures have different senses of curvature and are arranged in accordance with the value of the phenomenological molecular packing parameter, P = v/al, where v is the molecular volume, a is the cross-sectional area of the head group, and l is the length of the molecule. Adapted from .

Figure 5

(a) Lipid monolayers with negative, zero, and positive (from left to right) curvature determined by the conicity of the lipid molecules. (b) Stable lipid bilayer (middle) formed by two opposing lipid monolayers. If the monolayers were not constrained by being in the bilayer, they may want to curve as shown to the left and the right, in which case the middle stable bilayer would suffer from a built-in curvature stress. Courtesy of Dr. Olaf Sparre Andersen. (c) Schematic illustration of the lateral pressure profile, π(z), of a lipid bilayer, revealing regions of expansive (positive) pressures and regions of large tensile (negative) pressures.

Figure 6

Theoretical prediction of changes in the lateral pressure profile of lipid bilayers that become incorporated with 1 mol% unsaturated lipids of different types into a DPPC lipid bilayer. The data are compared to the opposite changes induced by cholesterol incorporation. Courtesy of Dr. Robert S. Cantor.

Figure 7

Fluorescence microscopy image of a 17 µm giant unilamellar liposome composed of native pulmonary surfactants from pig lung displaying fluid–fluid phase separation accompanied by cap formation . From http://www.scienceinyoureyes.com by courtesy of Drs. Jorge Bernadino de la Serna and Luis A. Bagatolli.

Figure 8

Percentage release as a function of time of doxorubicin from 100 nm liposomes composed of POPC lipids upon addition of 20 µM palmitic acid . Courtesy of Dr. Henrik Jespersen.

Figure 9

(a) Schematic illustration of the principle of hydrophobic matching between lipid bilayers and integral membrane proteins. In the case of a mismatch, the deformation in the lipid matrix may induce an indirect, lipid-mediated attraction between the proteins. (b) Schematic illustration of a conformational change in an integral membrane protein induced by changes in the hydrophobic mismatch condition (Adapted from ref. 3). (c) Release of the curvature stress in a lipid bilayer, composed of two lipid monolayers with spontaneous curvature, via the formation of the extended lipid chain conformation. One of the tails of the lipid molecule is captured in a hydrophobic pocket of a peripheral membrane protein, e.g., cytochrome c. Courtesy of Dr. P. K. J. Kinnunen. (d) Curvature stress may induce conformational changes in a membrane channel and hence shift the equilibrium between an open and a closed state. Courtesy of Dr. O. S. Andersen.

Figure 10

Schematic illustration of proteins and peptides that sense membrane curvature and lipid packing. (a) Bar domain that bind to membrane regions with high curvature. (b) Amphiphatic helix that respond to lipid packing. (c) Protein with hydrophobic anchors that sense lipid packing. (d) and (e) Enzymes, such as protein kinase C or PLA₂, with high activity at curved membranes and lower activity at planar membranes. Courtesy of Dr. D. Stamou, reproduced with a permission of the publisher .

Figure 11

Formation of conically shaped molecules by enzymatic action. (a) PLA₂ generates lysolipids and free fatty acids. (b) Phospholipase C generates di-acylglycerol by chopping off the head group of the lipid.

Figure 12

Lag-burst behavior of s-PLA₂ acting on a lipid bilayer substrate. Upon addition of the enzyme a characteristic lag period follows during which very little activity can be discerned. After a lag-time τ a sudden burst of activity sets in. The activity can be monitored, e.g., by detection of intrinsic tryptophan fluorescence.

Figure 13

Schematic illustration of a stealth liposome with a polymer coat of PEG that screens the liposome from the native immune system in the blood stream.

Figure 14

In vivo proof of principle of the efficacy of LiPlasomes loaded with cisplatin applied to a human xenograft MT-3 breast cancer in mice. Data from LiPlasome Pharma A/S.

Figure 15

Liposomal formulation based on a double lipid prodrug with the anti-cancer drug chlorambucil ester-linked in the sn-2 position and where the sn-1 chain is linked by an ether bond. Upon the action of s-PLA₂ the two prodrugs are turned into active drugs. Adapted from with a permission of the publisher.