Dynamic colocalization microscopy to characterize intracellular trafficking of nanomedicines

New views and insights into intracellular trafficking of drug-delivery systems by fluorescence fluctuation spectroscopy

Biomaterials in the nanometer size range can be engineered for site-specific delivery of drugs after injection into the blood circulation. However, translation of such nanomedicines from the bench to the bedside is still hindered by many extracellular and intracellular barriers. To realize the concept of targeted drug delivery with nanomedicines, research groups are studying intensively the extra- and intra-cellular mechanisms involved as a response to the physicochemical properties of the nanomedicines. In this review, we highlight the contributions of fluorescence fluctuations spectroscopy techniques to better understand, and in turn to bypass, the major hurdles to therapeutic delivery, focusing mostly on the intracellular dynamics of drug-delivery systems.

Intracellular trafficking and cellular uptake mechanism of mPEG-PLGA-PLL and mPEG-PLGA-PLL-Gal nanoparticles for targeted delivery to hepatomas

The lysosomal escape of nanoparticles is crucial to enhancing their delivery and therapeutic efficiency. Here, we report the cellular uptake mechanism, lysosomal escape, and organelle morphology effect of monomethoxy (polyethylene glycol)-poly (D,L-lactide-co-glycolide)-poly (L-lysine) (mPEG-PLGA-PLL, PEAL) and 4-O-beta-D-Galactopyranosyl-D-gluconic acid (Gal)-modified PEAL (PEAL-Gal) for intracellular delivery to HepG2, Huh7, and PLC hepatoma cells. These results indicate that PEAL is taken up by clathrin-mediated endocytosis of HepG2, Huh7 and PLC cells. For PEAL-Gal, sialic acid receptor-mediated endocytosis and clathrin-mediated endocytosis are the primary uptake pathways in HepG2 cells, respectively, whereas PEAL-Gal is internalized by sag vesicle- and clathrin-mediated endocytosis in Huh7 cells. In the case of PLC cells, clathrin-mediated endocytosis and sialic acid receptor play a primary role in the uptake of PEAL-Gal. TEM results verify that PEAL and PEAL-Gal lead to a different influence on organelle morphology of HepG2, Huh7 and PLC cells. In addition, the results of intracellular distribution reveal that PEAL and PEAL-Gal are less entrapped in the lysosomes of HepG2 and Huh7 cells, demonstrating that they effectively escape from lysosomes and contribute to enhance the efficiency of intracellular delivery and tumor therapy. In vivo tumor targeting image results demonstrate that PEAL-Gal specifically delivers Rhodamine B (Rb) to the tumor tissue of mice with HepG2, Huh7, and PLC hepatomas and remains at a high concentration in tumor tissue until 48 h, properties that will greatly contribute to enhanced antitumor efficiency.

Correlation of dual colour single particle trajectories for improved detection and analysis of interactions in living cells

Interactions between objects inside living cells are often investigated by looking for colocalization between fluorescence microscopy images that are recorded in separate colours corresponding to the fluorescent label of each object. The fundamental limitation of this approach in the case of dynamic objects is that coincidental colocalization cannot be distinguished from true interaction. Instead, correlation between motion trajectories obtained by dual colour single particle tracking provides a much stronger indication of interaction. However, frequently occurring phenomena in living cells, such as immobile phases or transient interactions, can limit the correlation to small parts of the trajectories. The method presented here, developed for the detection of interaction, is based on the correlation inside a window that is scanned along the trajectories, covering different subsets of the positions. This scanning window method was validated by simulations and, as an experimental proof of concept, it was applied to the investigation of the intracellular trafficking of polymeric gene complexes by endosomes in living retinal pigment epithelium cells, which is of interest to ocular gene therapy.

Tracking image correlation: combining single-particle tracking and image correlation

The interactions and coordination of biomolecules are crucial for most cellular functions. The observation of protein interactions in live cells may provide a better understanding of the underlying mechanisms. After fluorescent labeling of the interacting partners and live-cell microscopy, the colocalization is generally analyzed by quantitative global methods. Recent studies have addressed questions regarding the individual colocalization of moving biomolecules, usually by using single-particle tracking (SPT) and comparing the fluorescent intensities in both color channels. Here, we introduce a new method that combines SPT and correlation methods to obtain a dynamical 3D colocalization analysis along single trajectories of dual-colored particles. After 3D tracking, the colocalization is computed at each particle's position via the local 3D image cross correlation of the two detection channels. For every particle analyzed, the output consists of the 3D trajectory, the time-resolved 3D colocalization information, and the fluorescence intensity in both channels. In addition, the cross-correlation analysis shows the 3D relative movement of the two fluorescent labels with an accuracy of 30 nm. We apply this method to the tracking of viral fusion events in live cells and demonstrate its capacity to obtain the time-resolved colocalization status of single particles in dense and noisy environments.

Temporal techniques: dynamic tracking of nanomaterials in live cells

Temporal analytical techniques to track nanoparticles in live cell would provide rich information to well understand the biologic properties of nanoparticles in molecular level. Significant advances in fluorescence microscopy techniques with high temporal and spatial resolution allow single nanoparticles to label biomolecules, ions, and microstructures in live cells, which will address many fundamental questions in cell biology. This review highlights the real time tracking techniques for monitoring the movement of nanomaterials such as carbon nanotubes (CNTs), quantum dots (QDs), metal clusters, upconver-sional nanomaterials, and polystyrene (PS) nanoparticles etc. in live cells. The biological properties of nanoparticles in live cells are also briefly summarized according to fluorescence microscopy studies.

Endocytosis of gene delivery vectors: from clathrin-dependent to lipid raft-mediated endocytosis

The ideal nonviral vector delivers its nucleic acid cargo to a specific intracellular target. Vectors enter cells mainly through endocytosis and are distributed to various intracellular organelles. Recent advances in microscopy, lipidomics, and proteomics confirm that the cell membrane is composed of clusters of lipids, organized in the form of lipid raft domains, together with non-raft domains that comprise a generally disordered lipid milieu. The binding of a nonviral vector to either region can determine the pathway for its endocytic uptake and subsequent intracellular itinerary. Given this model of the cell membrane structure, endocytic pathways should be reclassified in relation to lipid rafts. In this review, we attempt to assess the currently recognized endocytic pathways in mammalian cells. The endocytic pathways are classified in relation to the membrane regions that make up the primary endocytic vesicles. This review covers the well-recognized clathrin-mediated endocytosis (CME), phagocytosis, and macropinocytosis in addition to the less addressed pathways that take place in lipid rafts. These include caveolae-mediated, flotillin-dependent, GTPase regulator associated with focal adhesion kinase-1 (GRAF1)-dependent, adenosine diphosphate-ribosylation factor 6 (Arf6)-dependent, and RhoA-dependent endocytic pathways. We summarize the regulators associated with each uptake pathway and methods for interfering with these regulators are discussed. The fate of endocytic vesicles resulting from each endocytic uptake pathway is highlighted.

Autophagy and formation of tubulovesicular autophagosomes provide a barrier against nonviral gene delivery

Cationic liposome (lipoplex) and polymer (polyplex)-based vectors have been developed for nonviral gene delivery. These vectors bind DNA and enter cells via endosomes, but intracellular transfer of DNA to the nucleus is inefficient. Here we show that lipoplex and polyplex vectors enter cells in endosomes, activate autophagy and generate tubulovesicular autophagosomes. Activation of autophagy was dependent on ATG5, resulting in lipidation of LC3, but did not require the PtdIns 3-kinase activity of PIK3C3/VPS34. The autophagosomes generated by lipoplex fused with each other, and with endosomes, resulting in the delivery of vectors to large tubulovesicular autophagosomes, which accumulated next to the nucleus. The tubulovesicular autophagosomes contained autophagy receptor protein SQSTM1/p62 and ubiquitin, suggesting capture of autophagy cargoes, but fusion with lysosomes was slow. Gene delivery and expression from both lipoplex and polyplex increased 8-fold in atg5^−/− cells unable to generate tubulovesicular autophagosomes. Activation of autophagy and capture within tubulovesicular autophagosomes therefore provides a new cellular barrier against efficient gene transfer and should be considered when designing efficient nonviral gene delivery vectors.

Emerging techniques in proteomics for probing nano-bio interactions

Nanoengineered particles that can facilitate drug formulation and improve specificity of delivery afford exciting opportunities for improved lesion-specific therapy. Understanding and controlling the nano-bio interactions of these materials is central to future developments in this area. Mass-spectrometry-based proteomics techniques, in conjunction with other emerging technologies, are enabling novel insights into the modulation of particle surfaces by biological fluids (formation of the protein corona) and subsequent particle-induced cellular responses. In this Perspective, we summarize important recent developments using proteomics-based techniques to understand nano-bio interactions and discuss the impact of such knowledge on improving particle design.

The role of surface charge in cellular uptake and cytotoxicity of medical nanoparticles

Many types of nanoparticles (NPs) are tested for use in medical products, particularly in imaging and gene and drug delivery. For these applications, cellular uptake is usually a prerequisite and is governed in addition to size by surface characteristics such as hydrophobicity and charge. Although positive charge appears to improve the efficacy of imaging, gene transfer, and drug delivery, a higher cytotoxicity of such constructs has been reported. This review summarizes findings on the role of surface charge on cytotoxicity in general, action on specific cellular targets, modes of toxic action, cellular uptake, and intracellular localization of NPs. Effects of serum and intercell type differences are addressed. Cationic NPs cause more pronounced disruption of plasma-membrane integrity, stronger mitochondrial and lysosomal damage, and a higher number of autophagosomes than anionic NPs. In general, nonphagocytic cells ingest cationic NPs to a higher extent, but charge density and hydrophobicity are equally important; phagocytic cells preferentially take up anionic NPs. Cells do not use different uptake routes for cationic and anionic NPs, but high uptake rates are usually linked to greater biological effects. The different uptake preferences of phagocytic and nonphagocytic cells for cationic and anionic NPs may influence the efficacy and selectivity of NPs for drug delivery and imaging.

Quantification of the internalization patterns of superparamagnetic iron oxide nanoparticles with opposite charge

Time-resolved quantitative colocalization analysis is a method based on confocal fluorescence microscopy allowing for a sophisticated characterization of nanomaterials with respect to their intracellular trafficking. This technique was applied to relate the internalization patterns of nanoparticles i.e. superparamagnetic iron oxide nanoparticles with distinct physicochemical characteristics with their uptake mechanism, rate and intracellular fate.The physicochemical characterization of the nanoparticles showed particles of approximately the same size and shape as well as similar magnetic properties, only differing in charge due to different surface coatings. Incubation of the cells with both nanoparticles resulted in strong differences in the internalization rate and in the intracellular localization depending on the charge. Quantitative and qualitative analysis of nanoparticles-organelle colocalization experiments revealed that positively charged particles were found to enter the cells faster using different endocytotic pathways than their negative counterparts. Nevertheless, both nanoparticles species were finally enriched inside lysosomal structures and their efficiency in agarose phantom relaxometry experiments was very similar.This quantitative analysis demonstrates that charge is a key factor influencing the nanoparticle-cell interactions, specially their intracellular accumulation. Despite differences in their physicochemical properties and intracellular distribution, the efficiencies of both nanoparticles as MRI agents were not significantly different.

High-speed imaging of Rab family small GTPases reveals rare events in nanoparticle trafficking in living cells

Despite the increased application of nanomaterials in diagnostics and therapeutics, methods to study the interactions of nanoparticles with subcellular structures in living cells remain relatively undeveloped. Here we describe a robust and quantitative method that allows for the precise tracking of all cell-associated nanoparticles as they pass through endocytic compartments in a living cell. Using rapid multicolor 3D live cell confocal fluorescence microscopy, combined with transient overexpression of small GTPases marking various endocytic membranes, our studies reveal the kinetics of nanoparticle trafficking through early endosomes to late endosomes and lysosomes. We show that, following internalization, 40 nm polystyrene nanoparticles first pass through an early endosome intermediate decorated with Rab5, but that these nanoparticles rapidly transfer to late endosomes and ultimately lysosomes labeled with Rab9 and Rab7, respectively. Larger nanoparticles of 100 nm diameter also reach acidic Rab9- and Rab7-positive compartments although at a slower rate compared to the smaller 40 nm nanoparticles. Our work also reveals that relatively few nanoparticles are able to access endocytic recycling pathways, as judged by lack of significant colocalization with Rab11. Finally, we demonstrate that this quantitative approach is sufficiently sensitive to be able to detect rare events in nanoparticle trafficking, specifically the presence of nanoparticles in Rab1A-labeled structures, thereby revealing the wide range of intracellular interactions between nanoparticles and the intracellular environment.