Porphysome nanovesicles generated by porphyrin bilayers for use as multimodal biophotonic contrast agents

Optically active nanomaterials promise to advance a range of biophotonic techniques through nanoscale optical effects and integration of multiple imaging and therapeutic modalities. Here, we report the development of porphysomes; nanovesicles formed from self-assembled porphyrin bilayers that generated large, tunable extinction coefficients, structure-dependent fluorescence self-quenching and unique photothermal and photoacoustic properties. Porphysomes enabled the sensitive visualization of lymphatic systems using photoacoustic tomography. Near-infrared fluorescence generation could be restored on dissociation, creating opportunities for low-background fluorescence imaging. As a result of their organic nature, porphysomes were enzymatically biodegradable and induced minimal acute toxicity in mice with intravenous doses of 1,000 mg kg(-1). In a similar manner to liposomes, the large aqueous core of porphysomes could be passively or actively loaded. Following systemic administration, porphysomes accumulated in tumours of xenograft-bearing mice and laser irradiation induced photothermal tumour ablation. The optical properties and biocompatibility of porphysomes demonstrate the multimodal potential of organic nanoparticles for biophotonic imaging and therapy.

Facile synthesis of advanced photodynamic molecular beacon architectures

Nucleic acid photodynamic molecular beacons (PMBs) are a class of activatable photosensitizers that increase singlet oxygen generation upon binding a specific target sequence. Normally, PMBs are functionalized with multiple solution-phase labeling and purification steps. Here, we make use of a flexible solid-phase approach for completely automated synthesis of PMBs. This enabled the creation of a new type of molecular beacon that uses a linear superquencher architecture. The 3' terminus was labeled with a photosensitizer by generating pyropheophorbide-labeled solid-phase support. The 5' terminus was labeled with up to three consecutive additions of a dark quencher phosphoramidite. These photosensitizing and quenching moieties were stable in the harsh DNA synthesis environment and their hydrophobicity facilitated PMB purification by HPLC. Linear superquenchers exhibited highly efficient quenching. This fully automated synthesis method simplifies not only the synthesis and purification of PMBs, but also the creation of new activatable photosensitizer designs.

Investigating the specific uptake of EGF-conjugated nanoparticles in lung cancer cells using fluorescence imaging

Targeted nanoparticles have the potential to deliver a large drug payload specifically to cancer cells. Targeting requires that a ligand on the nanoparticle surface interact with a specific membrane receptor on target cells. However, the contribution of the targeting ligand to nanoparticle delivery is often influenced by non-specific nanoparticle uptake or secondary targeting mechanisms. In this study, we investigate the epidermal growth factor (EGF) receptor-targeting specificity of a nanoparticle by dual-color fluorescent labeling. The targeted nanoparticle was a fluorescently labeled, EGF-conjugated HDL-like peptide–phospholipid scaffold (HPPS) and the cell lines expressed EGF receptor linked with green fluorescent protein (EGFR-GFP). Using LDLA7 cells partially expressing EGFR-GFP, fluorescence imaging demonstrated the co-internalization of EGFR-GFP and EGF-HPPS, thus validating its targeting specificity. Furthermore, specific EGFR-mediated uptake of the EGF-HPPS nanoparticle was confirmed using human non-small cell lung cancer A549 cells. Subsequent confocal microscopy and flow cytometry studies delineated how secondary targeting mechanisms affected the EGFR targeting. Together, this study confirms the EGFR targeting of EGF-HPPS in lung cancer cells and provides insight on the potential influence of unintended targets on the desired ligand–receptor interaction.

HDL-mimicking peptide-lipid nanoparticles with improved tumor targeting

Targeted delivery of intracellularly active diagnostics and therapeutics in vivo is a major challenge in cancer nanomedicine. A nanocarrier should possess long circulation time yet be small and stable enough to freely navigate through interstitial space to deliver its cargo to targeted cells. Herein, it is shown that by adding targeting ligands to nanoparticles that mimic high-density lipoprotein (HDL), tumor-targeted sub-30-nm peptide-lipid nanocarriers are created with controllable size, cargo loading, and shielding properties. The size of the nanocarrier is tunable between 10 and 30 nm, which correlates with a payload of 15-100 molecules of fluorescent dye. Ligand-directed nanocarriers targeting epidermal growth factor receptor (EGFR) are confirmed both in vitro and in vivo. The nanocarriers show favorable circulation time, tumor accumulation, and biodistribution with or without the targeting ligand. The EGFR targeting ligand is proved to be essential for the EGFR-mediated tumor cell uptake of the nanocarriers, a prerequisite of intracellular delivery. The results demonstrate that targeted HDL-mimetic nanocarriers are useful delivery vehicles that could open new avenues for the development of clinically viable targeted nanomedicine.

Bcl-XL inhibits membrane permeabilization by competing with Bax

Although Bcl-XL and Bax are structurally similar, activated Bax forms large oligomers that permeabilize the outer mitochondrial membrane, thereby committing cells to apoptosis, whereas Bcl-XL inhibits this process. Two different models of Bcl-XL function have been proposed. In one, Bcl-XL binds to an activator, thereby preventing Bax activation. In the other, Bcl-XL binds directly to activated Bax. It has been difficult to sort out which interaction is important in cells, as all three proteins are present simultaneously. We examined the mechanism of Bax activation by tBid and its inhibition by Bcl-XL using full-length recombinant proteins and measuring permeabilization of liposomes and mitochondria in vitro. Our results demonstrate that Bcl-XL and Bax are functionally similar. Neither protein bound to membranes alone. However, the addition of tBid recruited molar excesses of either protein to membranes, indicating that tBid activates both pro- and antiapoptotic members of the Bcl-2 family. Bcl-XL competes with Bax for the activation of soluble, monomeric Bax through interaction with membranes, tBid, or t-Bid-activated Bax, thereby inhibiting Bax binding to membranes, oligomerization, and membrane permeabilization. Experiments in which individual interactions were abolished by mutagenesis indicate that both Bcl-XL–tBid and Bcl-XL–Bax binding contribute to the antiapoptotic function of Bcl-XL. By out-competing Bax for the interactions leading to membrane permeabilization, Bcl-XL ties up both tBid and Bax in nonproductive interactions and inhibits Bax binding to membranes. We propose that because Bcl-XL does not oligomerize it functions like a dominant-negative Bax in the membrane permeabilization process.

During development and under stress, cells can become committed to die via programmed cell death (apoptosis). In most cases, the permeabilization of the outer mitochondrial membrane is a key component of this commitment. The membrane permeablization step is both positively and negatively regulated by members of the Bcl-2 family of proteins. One member of this protein family with only a BH3 region, such as tBid, activates another family member, Bax, causing it to form large complexes that generate membrane-spanning pores, hence making the membrane permeable. Antiapoptotic members of the Bcl-2 family, such as Bcl-XL, are structurally similar to Bax but inhibit the membrane permeabilization process by an unknown mechanism. Two mutually exclusive models have been proposed to explain how the Bcl-2 family is operating: one states that Bcl-XL binds to tBid, thereby preventing Bax activation, while the second suggests that Bcl-XL binds directly to activated Bax. It has been difficult to sort out which interaction is important in cells, where multiple members of all three protein families are present simultaneously. Here, we describe an in vitro system containing the three recombinant proteins and the use of mutagenesis to selectively remove individual interactions. We show that Bcl-XL inhibits Bax by competing with it for binding to membranes, tBid, and activated Bax. Because Bcl-XL does not form pores, it inhibits apoptosis by acting as if it is a dominant-negative version of Bax.

Bcl-XL and Bax are structurally similar members of the Bcl-2 family of cell-death-related proteins, and they compete for binding to membranes, as well as to Bcl-2 family member tBid and activated Bax. Unlike Bax, Bcl-XL is unable to oligomerize and form pores in membranes, so it inhibits membrane permeabilization--a key step during commitment to apoptosis--by functioning like a dominant-negative Bax.

A tumor mRNA-triggered photodynamic molecular beacon based on oligonucleotide hairpin control of singlet oxygen production

We report a new class of photodynamic molecular beacon (PMB) with tumor specific mRNA-triggered control of singlet oxygen ((1)O(2)) production. The beacon contains a single-stranded oligonucleotide linker that forms a stem-loop structure (hairpin) in which the sequence is an antisense oligonucleotide (AS-ON) complementary to a target mRNA. The stem is formed by the annealing of two complementary arm sequences that are on either side of the loop sequence. A photosensitizer molecule (PS) is attached to the end of one arm and a quencher (Q) is similarly attached to the other end. The conformationally-restricted hairpin forces Q to efficiently silence the photoreactivity of PS. In the presence of target mRNA, the hairpin opens and the PS is no longer silenced. Upon irradiating with light, the PS then emits fluorescence and generates cytotoxic (1)O(2). To show proof of concept, we have synthesized a c-raf-1 mRNA-triggered PMB using pyropheophorbide (Pyro) as PS, carotenoid as Q and c-raf-1 mRNA-targeted AS-ON as the loop sequence. We show that the (1)O(2) production of Pyro is quenched in its native state by 15-fold and is restored 9-fold by the addition of the target RNA. Comparing this to our recently reported self-folding peptide linker-based PMB, the hairpin effect results in an enhanced (1)O(2) quenching efficiency that decreases the residual (1)O(2) production by over 3-fold, thus providing enhanced control of (1)O(2) production upon target-linker interactions. When incubated with c-raf-1 expressing MDA-MB-231 cancer cells, the PMB displayed efficient cellular uptake and subsequently effective PDT activation in targeted cells.