Incorporation of a novel photochromic phospholipid molecule into vesicles of dipalmitoylphosphatidylcholine

Postsynthetic Photocontrol of Giant Liposomes via Fusion-Based Photolipid Doping

We report on photolipid doping of giant unilamellar vesicles (GUVs) via vesicle fusion with small unilamellar photolipid vesicles (pSUVs), which enables retroactive optical control of the membrane properties. We observe that vesicle fusion is light-dependent, if the phospholipids are neutral. Charge-mediated fusion involving anionic and cationic lipid molecules augments the overall fusion performance and doping efficiency, even in the absence of light exposure. Using phosphatidylcholine analogs with one or two azobenzene photoswitches (azo-PC and dazo-PC) affects domain formation, bending stiffness, and shape of the resulting vesicles in response to irradiation. Moreover, we show that optical membrane control can be extended to long wavelengths using red-absorbing photolipids (red-azo-PC). Combined, our findings present an attractive and practical method for the precise delivery of photolipids, which offers new prospects for the optical control of membrane function.

Photophosphatidylserine Guides Natural Killer Cell Photoimmunotherapy via Tim-3

Natural killer (NK) cells, in addition to their cytotoxicity function, harbor prominent cytokine production capabilities and contribute to regulating autoimmune responses. T-cell immunoglobulin and mucin domain containing protein-3 (Tim-3) is one of the inhibitory receptors on NK cells and a promising immune checkpoint target. We recently found that phosphatidylserine (PS) binding to Tim-3 can suppress NK cell activation. Therefore, based on the therapeutic potential of Tim-3 in NK-cell-mediated diseases, we developed a photoswitchable ligand of Tim-3, termed photophosphatidylserine (phoPS), that mimics the effects of PS. Upon 365 or 455 nm light irradiation, the isomer of phoPS cyclically conversed the cis/trans configuration, resulting in an active/inactive Tim-3 ligand, thus modulating the function of NK cells in vitro and in vivo. We also demonstrated that reversible phoPS enabled optical control of acute hepatitis. Together, phoPS may be an appealing tool for autoimmune diseases and cytokine storms in the future.

Photolipid Bilayer Permeability is Controlled by Transient Pore Formation

Controlling the release or uptake of (bio-) molecules and drugs from liposomes is critically important for a range of applications in bioengineering, synthetic biology, and drug delivery. In this paper, we report how the reversible photoswitching of synthetic lipid bilayer membranes made from azobenzene-containing phosphatidylcholine (azo-PC) molecules (photolipids) leads to increased membrane permeability. We show that cell-sized, giant unilamellar vesicles (GUVs) prepared from photolipids display leakage of fluorescent dyes after irradiation with UV-A and visible light. Langmuir-Blodgett and patch-clamp measurements show that the permeability is the result of transient pore formation. By comparing the trans-to-cis and cis-to-trans isomerization process, we find that this pore formation is the result of area fluctuations and a change of the area cross-section between both photolipid isomers.

Photochromic coenzyme Q derivatives: switching redox potentials with light

Coenzyme Q is an important redox cofactor involved in a variety of cellular processes, and is thus found in several cell compartments. We report a photochromic derivative of coenzyme Q that combines the molecular structures of the redox active cofactor and a photochromic dye. Light irradiation triggers an electronic rearrangement reversibly changing the redox potential. We used this effect to control the intermolecular redox reaction of the photochromic coenzyme Q derivative with dihydropyridine in solution by light irradiation. On mitochondria, the altered redox properties showed an effect on the respiratory chain. The experiments demonstrate that the redox reactions can be initiated inside the system of interest through irradiation with light and the accompanied photoisomerization.

Photoswitchable fatty acids enable optical control of TRPV1

Fatty acids (FAs) are not only essential components of cellular energy storage and structure, but play crucial roles in signalling. Here we present a toolkit of photoswitchable FA analogues (FAAzos) that incorporate an azobenzene photoswitch along the FA chain. By modifying the FAAzos to resemble capsaicin, we prepare a series of photolipids targeting the Vanilloid Receptor 1 (TRPV1), a non-selective cation channel known for its role in nociception. Several azo-capsaicin derivatives (AzCAs) emerge as photoswitchable agonists of TRPV1 that are relatively inactive in the dark and become active on irradiation with ultraviolet-A light. This effect can be rapidly reversed by irradiation with blue light and permits the robust optical control of dorsal root ganglion neurons and C-fibre nociceptors with precision timing and kinetics not available with any other technique. More generally, we expect that photolipids will find many applications in controlling biological pathways that rely on protein–lipid interactions.

Fatty acids are ancient lipids with numerous functions, from metabolic processes as a source of energy to structural and signalling roles within cell membranes. Here, the authors present azobenzene-modified fatty acids and their application as photoswitchable agonists of the Vanilloid Receptor 1.

H-aggregation of azobenzene-substituted amphiphiles in vesicular membranes

Photochemical switching has been studied of double-tailed phosphate amphiphiles containing azobenzene units in both tails in aqueous vesicular dispersions and in mixed vesicular systems with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Since the ease of switching depends on the strength of the bilayer packing, particular emphasis has been placed on the occurrence of H-aggregation in the hydrophobic core of the vesicles. UV-vis spectrometry was employed to monitor H-aggregation and showed how this process depends on the ionic strength and on the mode of preparation of the vesicles. Two types of H-aggregates were observed in mixed DOPC vesicles with 5 mol % of azobenzene phosphate: one with lambda(max) at around 300 nm and one with lambda(max) at 305-320 nm. Those with lambda(max) at 300 nm could not be trans-cis photoisomerized, whereas those with lambda(max) at 305-320 nm are more loosely packed and can be photochemically switched. The permeability of the vesicular bilayers, as probed with leakage experiments using calcein as a fluorescent probe, was examined as another measure for the strength of bilayer packing. Leakage occurred only for DOPC vesicles containing more than 20 mol % of azobenzenephosphate, irradiated with UV light to induce trans-cis photoisomerization. We contend that detailed information on bilayer packing will be of crucial importance for fine-tuning the lateral pressure in vesicular membranes with the ultimate aim to steer the opening and closing of mechanosensitive protein channels of large conductance.

Thermosensitive polymer-modified liposomes

Temperature-sensitive liposomes are considered to be a promising tool to achieve site-specific delivery of drugs. These liposomes have been prepared using lipids whose membranes undergo a gel-to-liquid crystalline phase transition a few degrees above physiological temperature. However, recently, temperature-sensitization of liposomes has been attempted using thermosensitive polymers. So far, functional liposomes whose contents release behavior, surface properties, and affinity to cell surface can be controlled in a temperature-dependent manner, have been developed according to this strategy. The design and function of these thermosensitive polymer-modified liposomes have been outlined in this review.

Wavelength-programmed solute release from photosensitive liposomes

Liposomes of dipalmitoylphosphatidylcholine containing a photochromic lipid "Bis-Azo PC" release entrapped solutes on exposure to UV light. We have now demonstrated that on addition of cholesterol (up to 25 mol%) to the liposomal membrane the liposomes also release their contents in response to visible light in the region of 470 nm, to which liposomes lacking steroid are insensitive. In a mixed population of liposomes prepared with and without cholesterol, this enables wavelength-dependent release of entrapped solutes on sequential exposure to visible and UV light. Furthermore, the cholesterol-containing liposomes allow stepped partial release of entrapped solute following multiple periods of short visible illumination. It is suggested that the cholesterol-containing liposomes may be potentially useful for drug delivery and for "caging" of reagents.

Photosensitive liposomes as 'cages' for laser-triggered solute delivery: the effect of bilayer cholesterol on kinetics of solute release

Liposomes containing acyl chains incorporating azobenzene chromophores have been investigated as potential 'caging' agents for fast solute release. On photolysis, trapped marker dye can be released from gel-phase liposomes within milliseconds. Solute release is markedly sensitive to the presence of cholesterol in the bilayer. Phospholipids bearing one saturated acyl chain and an azobenzene-substituted chain are ineffective as sensitisers unless cholesterol is present, while doubly substituted phospholipids sensitise release in its absence. Cholesterol markedly affects the temperature profile of solute release depending on the host phospholipid chain length. Solute release is not seen for lipid hosts with unsaturated acyl chains.

Fast solute release from photosensitive liposomes: an alternative to 'caged' reagents for use in biological systems

The kinetics of release of soluble marker trapped in liposomes of gel phase phospholipid containing a photoisomerisable phospholipid analogue have been investigated. Marker release is triggered by UV laser flash photolysis at 355 nm. A markedly temperature-dependent release rate is seen, and above 25 degrees C millisecond release kinetics can be achieved. These results suggest that such liposomes might find application as an alternative to conventional 'caged' reagents for photo-triggered reagent release in biological research.

Liposome fusion and lipid exchange on ultraviolet irradiation of liposomes containing a photochromic phospholipid

A photochromic phospholipid, 1,2-bis[4-4(4-n-butylphenylazo) phenylbutyroyl] phosphatidylcholine (Bis-Azo PC) has been incorporated into liposomes of gel- and liquid-crystalline- phase phospholipids. Liposomes of gel-phase phospholipid are stable in the presence of the trans photostationary state Bis-Azo PC and can encapsulate fluorescent marker dye. On photoisomerization to the cis photostationary state, trapped marker is rapidly released. Liposomes containing Bis-Azo PC can rapidly fuse together after UV isomerization, this process continuing in the dark. Exposure to white light causes reversion of Bis-Azo Pc to the trans form and halts dye leakage and vesicle fusion. Both unilamellar and multilamellar liposomes are able to fuse together on UV exposure. On UV photolysis, liposomes containing Bis-Azo PC do not fuse with a large excess of unlabeled liposomes, but transfer of Bis-Azo PC can be demonstrated spectrophotometrically. Vesicles of pure gel-phase lipid containing trapped marker dye but initially no Bis-Azo PC become leaky as a result of this lipid transfer. Liposomes composed of liquid-crystalline-phase phosphatidylcholine- containing Bis-Azo PC neither leak trapped marker no fuse together on photolysis, nor do liquid-crystalline-phase liposomes fuse with gel-phase liposomes under these conditions. These results are discussed together with some possible applications of liposome photodestabilization.

Characterization of the preparation process and the photochemical control of electrical properties of bilayer lipid membranes containing azobenzene chromophores

We prepared photoresponsive bilayer lipid membranes (BLMs) containing azobenzene derivatives (4'-octylazobenzene-4-oxybutyric acid (AZ)) and observed the rapid and reversible changes in their electrical properties when irradiated with light. The BLMs consist of AZ (8 mol%) and glyceryl monooleate. The changes in capacitance and conductance upon irradiation by light were found to be 10 and 20%, respectively. The changes in the electrical properties of the membrane and in the structure of AZ under light irradiation were analyzed simultaneously by in-situ spectroscopic, electrical and microscopic measurements. These measurements showed that the electrical changes induced by exposure to light resulted from reversible changes in the membrane structure initiated by the photoisomerization reaction of AZ. This structural change in the membrane occurred within 1 s.

Control of pro-oxidant activity of cupric ions by entrapment in unilamellar lipid vesicles

As a demonstration of a potential means of delivering and controlling the biochemical and biological activity of metal ions, cupric ions have been trapped in unilamellar phospholipid vesicles. The activity of these cupric ion-containing vesicles as catalysts of the autoxidation of ascorbate and epinephrine has been investigated. A marked increase in autoxidation rate was observed on release of the cupric ion on addition of detergent. When an azobenzene-containing photochromic lipid was incorporated in the bilayer membrane of the vesicles, the release of cupric ions could be initiated by irradiation with ultraviolet light. In the dark, these vesicles remained stable for at least several weeks. Photo-controlled release of liposomally-entrapped species might find application in areas similar to those where 'caged' reagents are presently used.

Syntheses and properties of circular dichroism active phospholipids

A group of circular dichroism (CD) active phospholipids has been synthesised, in which one or both acyl chains has been replaced with a cinnamoyl or azobenzene chromophore-containing acid. Studies on the structure, CD activity and thermodynamic property of liposome membranes composed of CD active phospholipids were carried out. CD active liposomes were found to be stable, normal liposomes of approximately 550 A diameter based on the electron micrograph and dynamic light scattering, and to have thermodynamic property similar to the conventional phospholipid membranes without serious perturbation by aromatic bulk groups based on DSC. Liposomes composed of phospholipid having two trans-azobenzene chromophores showed an extremely large CD enhancement even well above Tc. This CD enhancement was drastically changed by the presence of cis-azobenzene chromophore and cis-cis isomer content after irradiation was higher than the theoretical value, suggesting the importance of interchromophore interaction in the liposome membranes.

Light-induced fusion of liposomes with release of trapped marker dye is sensitised by photochromic phospholipid

Liposomes have been prepared from dipalmitoylphosphatidylcholine containing small amounts of a synthetic photochromic phospholipid, 'Bis-Azo PC'. In the dark, these are stable at room temperature, and contents do not significantly leak over weeks. Photoisomerisation results in immediate release of trapped marker, and in liposome fusion to form larger structures. Fusion has been detected using a fluorescence polarisation assay, and confirmed by electron microscopy. In mixtures, fusion occurs between 'photochromic' liposomes and those of pure lipid. Bis-Azo PC contains two photochromic acyl chains; analogues bearing a single photochromic chain appear to have little effect on bilayer permeability after isomerisation. Photo-induced leakage and liposome fusion suggest possible applications for localised drug delivery as an adjunct to phototherapy. The ability to non-invasively trigger fusion processes should be useful in fundamental studies of membrane interactions. We believe this to be the first report of photo-induced fusion to date.

The phase behaviour of dispersions of Bis-Azo PC: photoregulation of bilayer dynamics via lipid photochromism

A phospholipid, 1,2-bis(4-(n-butyl)phenylazo-4'-phenylbutyroyl)phosphatidylcholine (Bis-Azo PC), has been synthesised and shown to form stable bilayer vesicles. Light-scattering measurements and differential scanning calorimetry show that a dispersion of the lipid has a cooperative phase transition at a similar temperature to that of dipalmitoylphosphatidylcholine, which Bis-Azo PC resembles in overall size. The phase behaviour of Bis-Azo PC has been investigated by fluorescence spectroscopy and using a series of spin-labelled fatty acid probes. Fluorescence measurements using chlorophyll a as probe sense the onset of the cooperative phase transition, but this is not clearly revealed by any of the spin probes tested. Hysteresis in the phase transition is detected both by light scattering measurements and by fluorescence spectroscopy. No transition is observed for a lipid analogue having a palmitic acid chain and a single azo-containing substituent. Bis-Azo PC is reversibly photochromic, isomerising on exposure to ultraviolet light to a photostationary state mixture where cis isomer predominates. Electron microscopy shows that photoisomerisation decreases average vesicle size, and light scattering and calorimetry demonstrate that the cooperative phase transition is abolished. Illumination with visible light establishes a new photostationary state where trans isomer predominates, and the phase transition is restored. The ability to modulate bilayer phase behaviour reversibly has possible application to relaxation studies of bilayer membrane function, and to drug delivery research.

Lytic effects of melittin and delta-haemolysin from Staphylococcus aureus on vesicles of dipalmitoylphosphatidylcholine

The effects of the lytic peptides, melittin and delta-haemolysin, are compared in vesicles of gel-phase dipalmitoylphosphatidylcholine (DPPC), using calcein as trapped marker. At low concentration, both toxins cause vesicles to lose contents in 5 mM phosphate buffer near neutral pH, with melittin being the more active. As phosphate concentration is increased, the kinetics of melittin-induced leakage change from a slow, sustained loss to a rapid 'burst' of leakage when melittin is present mainly as tetramer in solution, under conditions where it is reported to lose haemolytic activity towards erythrocytes. At low phosphate concentration, the leakage induced by delta-haemolysin is preceded by a lag phase, though fluorescence measurements show that binding of toxin is rapid. At higher phosphate concentration, the toxin binds rapidly to vesicles, but causes no leakage of entrapped calcein. Steady-state fluorescence spectra show no obvious differences in tryptophan emission for delta-haemolysin bound to lipid in high- or low-phosphate buffer. Spin-label fluorescence-quenching studies show that the single tryptophan residue of delta-haemolysin is buried within the lipid bilayer at all phosphate concentrations used. In gel-phase DPPC, delta-haemolysin shows no tendency to cause vesicle aggregation over several hours, as judged by light scattering, though a slow non-linear effect is seen above the lipid phase transition temperature. These effects are contrasted with those of melittin under similar conditions.