Deconvolving Passive and Active Targeting of Liposomes Bearing LDL Receptor Binding Peptides Using the Zebrafish Embryo Model.

Improving target versus off-target ratio in nanomedicine remains a major challenge for increasing drug bioavailability and reducing toxicity. Active targeting using ligands on nanoparticle surfaces is a key approach but has limited clinical success. A potential issue is the integration of targeting ligands also changes the physicochemical properties of nanoparticles (passive targeting). Direct studies to understand the mechanisms of active targeting and off-targeting in vivo are limited by the lack of suitable tools. Here, the biodistribution of a representative active targeting liposome is analyzed, modified with an apolipoprotein E (ApoE) peptide that binds to the low-density lipoprotein receptor (LDLR), using zebrafish embryos. The ApoE liposomes demonstrated the expected liver targeting effect but also accumulated in the kidney glomerulus. The ldlra-/- zebrafish is developed to explore the LDLR-specificity of ApoE liposomes. Interestingly, liver targeting depends on the LDLR-specific interaction, while glomerular accumulation is independent of LDLR and peptide sequence. It is found that cationic charges of peptides and the size of liposomes govern glomerular targeting. Increasing the size of ApoE liposomes can avoid this off-targeting. Taken together, the study shows the potential of the zebrafish embryo model for understanding active and passive targeting mechanisms, that can be used to optimize the design of nanoparticles.

Stabilin-1 is required for the endothelial clearance of small anionic nanoparticles.

Clearance of nanoparticles (NPs) after intravenous injection - mainly by the liver - is a critical barrier for the clinical translation of nanomaterials. Physicochemical properties of NPs are known to influence their distribution through cell-specific interactions; however, the molecular mechanisms responsible for liver cellular NP uptake are poorly understood. Liver sinusoidal endothelial cells and Kupffer cells are critical participants in this clearance process. Here we use a zebrafish model for liver-NP interaction to identify the endothelial scavenger receptor Stabilin-1 as a non-redundant receptor for the clearance of small anionic NPs. Furthermore, we show that physiologically, Stabilin-1 is required for the removal of bacterial lipopolysaccharide (LPS/endotoxin) from circulation and that Stabilin-1 cooperates with its homolog Stabilin-2 in the clearance of larger (~100 nm) anionic NPs. Our findings allow optimization of anionic nanomedicine biodistribution and targeting therapies that use Stabilin-1 and -2 for liver endothelium-specific delivery.

Stab2-Mediated Clearance of Supramolecular Polymer Nanoparticles in Zebrafish Embryos.

Supramolecular polymers are attractive scaffolds for use as nanocarriers in drug delivery thanks to their modularity and easy fabrication; however, a molecular view into their in vivo behavior is lacking. Herein, we prepare fluorescent squaramide-based supramolecular polymer nanoparticles that range from fibers to spheres while maintaining their surface chemistry and near-neutral surface charge by a co-assembly approach involving a sulfo-cyanine-labeled monomer to track their in vivo biodistribution behavior and clearance in optically transparent zebrafish embryos. Evasion of macrophages, localization of the fibrillar aggregates in the caudal vein, and association with scavenger endothelial cells are observed. The interaction of the fibrillar supramolecular nanoparticles with the caudal vein is abrogated in gene-edited zebrafish lacking Stabilin-2, a receptor analogously found in the mammalian liver, providing a molecular view into their interaction with scavenger endothelial cells. We further show that this interaction can be tuned based on the choice of monomer and its resultant self-assembly.

Structure-function relationship of phase-separated liposomes containing diacylglycerol analogues.

The composition and morphology of lipid-based nanoparticles can influence their overall in vivo behavior. Previously, we demonstrated that phase separation in liposomes composed of DSPC and a diacylglycerol lipid analogue (DOaG) drives the in vivo biodistribution towards a specific subset of endothelial cells in zebrafish embryos. In the absence of traditional targeting functionalities (e.g., antibodies, ligands), this selectivity is mediated solely by the unique liposome morphology and composition, characterized by a DOaG-rich lipid droplet within the DSPC-rich phospholipid bilayer. The phase separation is induced due to the geometry of DOaG lipid and its ability to create non-bilayer phases in lipid membranes. To investigate the underlying principles of phase separation and to optimize the liposome colloidal stability, we performed a structure-function relationship study by synthesizing a library of DOaG analogues with varying molecular properties, such as the number, length and sn-position of the acyl chains, as well as the degree of saturation or carbonyl substituents. We assessed the ability of these lipid analogues to assemble into phase-separated liposomes and studied their morphology, colloidal stability, and in vivo biodistribution in zebrafish embryos. We found that analogues containing unsaturated, medium length (C16-C18) fatty acids were required to obtain colloidally stable, phase-separated liposomes with cell-specific biodistribution patterns. Moreover, we observed that using the pure DOaG isomer, with acyl chains at the sn-1,3 positions, leads to more colloidally stable liposomes than when a mixture of sn-1,2 and sn-1,3 isomers is used. Similarly, we observed that incorporating a DOaG analogue with fatty tails shorter than DSPC, as well as PEGylation, endows liposomes with long term stability while retaining cell-selective biodistribution. Diacylglycerols are known to promote fusion, lipid polymorphism, signaling and protein recruitment on lipid membranes. In this study, we showed that diacylglycerol derivatives can induce phase separation in liposomes, unlocking the potential for cell-specific targeting in vivo. We believe that these findings can be the foundation for future use of diacylglycerols in lipid-based nanomedicines and could lead to the development of novel targeted delivery strategies.

Phase-Separated Lipid-Based Nanoparticles: Selective Behavior at the Nano-Bio Interface.

The membrane-protein interface on lipid-based nanoparticles influences their in vivo behavior. Better understanding may evolve current drug delivery methods toward effective targeted nanomedicine. Previously, the cell-selective accumulation of a liposome formulation in vivo is demonstrated, through the recognition of lipid phase-separation by triglyceride lipases. This exemplified how liposome morphology and composition can determine nanoparticle-protein interactions. Here, the lipase-induced compositional and morphological changes of phase-separated liposomes-which bear a lipid droplet in their bilayer- are investigated, and the mechanism upon which lipases recognize and bind to the particles is unravelled. The selective lipolytic degradation of the phase-separated lipid droplet is observed, while nanoparticle integrity remains intact. Next, the Tryptophan-rich loop of the lipase is identified as the region with which the enzymes bind to the particles. This preferential binding is due to lipid packing defects induced on the liposome surface by phase separation. In parallel, the existing knowledge that phase separation leads to in vivo selectivity, is utilized to generate phase-separated mRNA-LNPs that target cell-subsets in zebrafish embryos, with subsequent mRNA delivery and protein expression. Together, these findings can expand the current knowledge on selective nanoparticle-protein communications and in vivo behavior, aspects that will assist to gain control of lipid-based nanoparticles.

Lipid conjugate dissociation analysis improves the in vivo understanding of lipid-based nanomedicine.

Lipid conjugates have advanced the field of lipid-based nanomedicine by promoting active-targeting (ligand, peptide, antibody), stability (PEGylation), controlled release (lipoid prodrug), and probe-based tracking (fluorophore). Recent findings indicate lipid conjugates dissociating from nanomedicine upon encountering a biological environment. Yet, implications for (pre)clinical outcomes remain unclear. In this study, using the zebrafish model (Danio rerio), we investigated the fate of liposome-incorporated lipid fluorophore conjugates (LFCs) after intravenous (IV) administration. LFCs having a bilayer mismatch and relatively polar fluorophore revealed counter-predictive outcomes for Caelyx/Doxil (clearance vs. circulating) and AmBisome-like liposomes (scavenger endothelial cell vs. macrophage uptake). Findings on LFC (mis)match for Caelyx/Doxil-like liposomes were supported by translational intravital imaging studies in mice. Importantly, contradicting observations suggest to originate from LFC dissociation in vivo, which was investigated by Asymmetric Flow Field-Flow Fractionation (AF4) upon liposome-serum incubation in situ. Our data suggests that LFCs matching with the liposome bilayer composition - that did not dissociate upon serum incubation - revealed improved predictive outcomes for liposome biodistribution profiles. Altogether, this study highlights the critical importance of fatty acid tail length and headgroup moiety when selecting lipid conjugates for lipid-based nanomedicine.

Phase-Separated Liposomes Hijack Endogenous Lipoprotein Transport and Metabolism Pathways to Target Subsets of Endothelial Cells In Vivo.

Plasma lipid transport and metabolism are essential to ensure correct cellular function throughout the body. Dynamically regulated in time and space, the well-characterized mechanisms underpinning plasma lipid transport and metabolism offers an enticing, but as yet underexplored, rationale to design synthetic lipid nanoparticles with inherent cell/tissue selectivity. Herein, a systemically administered liposome formulation, composed of just two lipids, that is capable of hijacking a triglyceride lipase-mediated lipid transport pathway resulting in liposome recognition and uptake within specific endothelial cell subsets is described. In the absence of targeting ligands, liposome-lipase interactions are mediated by a unique, phase-separated ("parachute") liposome morphology. Within the embryonic zebrafish, selective liposome accumulation is observed at the developing blood-brain barrier. In mice, extensive liposome accumulation within the liver and spleen - which is reduced, but not eliminated, following small molecule lipase inhibition - supports a role for endothelial lipase but highlights these liposomes are also subject to significant "off-target" by reticuloendothelial system organs. Overall, these compositionally simplistic liposomes offer new insights into the discovery and design of lipid-based nanoparticles that can exploit endogenous lipid transport and metabolism pathways to achieve cell selective targeting in vivo.

Coating gold nanorods with self-assembling peptide amphiphiles promotes stability and facilitates in vivo two-photon imaging.

Gold nanorods (GNRs) are versatile asymmetric nanoparticles with unique optical properties. These properties make GNRs ideal agents for applications such as photothermal cancer therapy, biosensing, and in vivo imaging. However, as-synthesised GNRs need to be modified with a biocompatible stabilising coating in order to be employed in these fields as the ligands used to stabilise GNRs during synthesis are toxic. An issue is that GNR performance in the aforementioned techniques can be affected by these modified coatings. For example if coatings are too thick then GNR entry into cells, or their sensitivity in sensing applications, can be compromised. Here we show that thiolated peptide amphiphiles (PAs) can act as GNR stabilisers and provide a thin and highly-stable coating under physiologically relevant conditions. Additionally, all tested PAs formed highly ordered (51.8-58.8% ß-content), and dense (2.62-3.87 peptides per nm2) monolayers on the GNR surface. Moreover, the PA-coated GNRs demonstrated no cytotoxicity in vitro and, via injection in zebrafish embryos, the behavior and cellular interactions of such PA-coated GNRs were visualised in vivo, in real time, with two-photon (2P) microscopy.

Anionic Lipid Nanoparticles Preferentially Deliver mRNA to the Hepatic Reticuloendothelial System.

Lipid nanoparticles (LNPs) are the leading nonviral technologies for the delivery of exogenous RNA to target cells in vivo. As systemic delivery platforms, these technologies are exemplified by Onpattro, an approved LNP-based RNA interference therapy, administered intravenously and targeted to parenchymal liver cells. The discovery of systemically administered LNP technologies capable of preferential RNA delivery beyond hepatocytes has, however, proven more challenging. Here, preceded by comprehensive mechanistic understanding of in vivo nanoparticle biodistribution and bodily clearance, an LNP-based messenger RNA (mRNA) delivery platform is rationally designed to preferentially target the hepatic reticuloendothelial system (RES). Evaluated in embryonic zebrafish, validated in mice, and directly compared to LNP-mRNA systems based on the lipid composition of Onpattro, RES-targeted LNPs significantly enhance mRNA expression both globally within the liver and specifically within hepatic RES cell types. Hepatic RES targeting requires just a single lipid change within the formulation of Onpattro to switch LNP surface charge from neutral to anionic. This technology not only provides new opportunities to treat liver-specific and systemic diseases in which RES cell types play a key role but, more importantly, exemplifies that rational design of advanced RNA therapies must be preceded by a robust understanding of the dominant nano-biointeractions involved.

Zebrafish Embryos as a Predictive Animal Model to Study Nanoparticle Behavior in vivo.

A failure to fully understand the complex in vivo behavior of systemically administered nanomedicines has stymied clinical translation. To bridge this knowledge gap, new in vivo tools are needed to rapidly and accurately assess the nearly infinite array of possible nanoparticle designs. Zebrafish embryos are small, transparent, and easily manipulated animals that allow for whole organism visualization of fluorescently labeled nanoparticles in real time and at cellular resolution using standard microscope setups. Furthermore, key nano-bio interactions present in higher vertebrates are fully conserved in zebrafish embryos, making these animal models a highly predictive and instructive addition to the nanomedicine design pipeline. Here, we present a step-by-step protocol to intravenously administer, image, and analyze nanoparticle behavior in zebrafish embryos and highlight key nano-bio interactions within the embryonic zebrafish corresponding to those commonly found within the mammalian liver. In addition, we outline practical steps required to achieve light-triggered activation of nanoparticles within the transparent embryo. Graphic abstract: Zebrafish embryos to study nanoparticle behavior in vivo. Formulation, intravenous administration, imaging, and analysis of nanoparticles.

Light-triggered switching of liposome surface charge directs delivery of membrane impermeable payloads in vivo.

Surface charge plays a fundamental role in determining the fate of a nanoparticle, and any encapsulated contents, in vivo. Herein, we describe, and visualise in real time, light-triggered switching of liposome surface charge, from neutral to cationic, in situ and in vivo (embryonic zebrafish). Prior to light activation, intravenously administered liposomes, composed of just two lipid reagents, freely circulate and successfully evade innate immune cells present in the fish. Upon in situ irradiation and surface charge switching, however, liposomes rapidly adsorb to, and are taken up by, endothelial cells and/or are phagocytosed by blood resident macrophages. Coupling complete external control of nanoparticle targeting together with the intracellular delivery of encapsulated (and membrane impermeable) cargos, these compositionally simple liposomes are proof that advanced nanoparticle function in vivo does not require increased design complexity but rather a thorough understanding of the fundamental nano-bio interactions involved.

Directing Nanoparticle Biodistribution through Evasion and Exploitation of Stab2-Dependent Nanoparticle Uptake.

Up to 99% of systemically administered nanoparticles are cleared through the liver. Within the liver, most nanoparticles are thought to be sequestered by macrophages (Kupffer cells), although significant nanoparticle interactions with other hepatic cells have also been observed. To achieve effective cell-specific targeting of drugs through nanoparticle encapsulation, improved mechanistic understanding of nanoparticle-liver interactions is required. Here, we show the caudal vein of the embryonic zebrafish ( Danio rerio) can be used as a model for assessing nanoparticle interactions with mammalian liver sinusoidal (or scavenger) endothelial cells (SECs) and macrophages. We observe that anionic nanoparticles are primarily taken up by SECs and identify an essential requirement for the scavenger receptor, stabilin-2 ( stab2) in this process. Importantly, nanoparticle-SEC interactions can be blocked by dextran sulfate, a competitive inhibitor of stab2 and other scavenger receptors. Finally, we exploit nanoparticle-SEC interactions to demonstrate targeted intracellular drug delivery resulting in the selective deletion of a single blood vessel in the zebrafish embryo. Together, we propose stab2 inhibition or targeting as a general approach for modifying nanoparticle-liver interactions of a wide range of nanomedicines.

At-Line Cellular Screening Methodology for Bioactives in Mixtures Targeting the α7-Nicotinic Acetylcholine Receptor.

The α7-nicotinic acetylcholine receptor (α7-nAChR) is a ligand-gated ion channel expressed in different regions of the central nervous system (CNS). The α7-nAChR has been associated with Alzheimer's disease, epilepsy, and schizophrenia, and therefore is extensively studied as a drug target for the treatment of these diseases. Important sources for new compounds in drug discovery are natural extracts. Since natural extracts are complex mixtures, identification of the bioactives demands the use of analytical techniques to separate a bioactive from inactive compounds. This study describes screening methodology for identifying bioactive compounds in mixtures acting on the α7-nAChR. The methodology developed combines liquid chromatography (LC) coupled via a split with both an at-line calcium (Ca(2+))-flux assay and high-resolution mass spectrometry (MS). This allows evaluation of α7-nAChR responses after LC separation, while parallel MS enables compound identification. The methodology was optimized for analysis of agonists and positive allosteric modulators, and was successfully applied to screening of the hallucinogen mushroom Psilocybe Mckennaii The crude mushroom extract was analyzed using both reversed-phase and hydrophilic interaction liquid chromatography. Matching retention times and peak shapes of bioactives found with data from the parallel MS measurements allowed rapid pinpointing of accurate masses corresponding to the bioactives.