An In Vivo Spectral Multiplexing Approach for the Cooperative Imaging of Different Disease-Related Biomarkers with Near-Infrared Fluorescent Förster Resonance Energy Transfer Probes

Comparison of fluorescence lifetime and multispectral imaging for quantitative multiplexing in biological tissue

Fluorescence lifetime (FLT) multiplexing and multispectral imaging (MSI) are both frequently employed for in vitro and ex vivo biological studies. In vivo applications of MSI for deep seated fluorophores require consideration of diffusive light propagation in biological tissue. We have previously shown that a well-known redshift of fluorescence spectra in diffusive medium induces a fluorophore cross-talk, which cannot be accounted for even with known optical properties of the medium. In contrast, FLT measurements remain largely unaffected by light propagation in tissue, enabling zero cross-talk and accurate relative quantification. While a fully quantitative estimation of fluorophore concentrations requires depth resolved tomographic imaging, this is often not possible due to the difficulty of estimating tissue optical properties and modelling light propagation in complex tissue geometries. Here, we experimentally investigate the performance of planar (non-tomographic) MSI and FLT multiplexing for the quantitative recovery of multiple near-infrared fluorophores embedded in 4-8 mm thick tissue. We show that FLT multiplexing provides a superior quantification accuracy (error < 10%) compared to MSI (error = 20-107%) in tissue. The error rates for MSI increased with tissue thickness and can be directly attributed to the spectral redshift induced cross-talk between emission spectra. Our data indicate that planar FLT multiplexing can provide high quantification accuracy in thick biological tissue without a need for optical property estimation, thereby offering an important validation tool for rapid quantification of fluorophore concentrations in bulk tissue.

Non-invasive synchronous monitoring of neutrophil migration using whole body near-infrared fluorescence-based imaging

Advances in fluorescence imaging coupled with the generation of near infrared probes have significantly improved the capabilities of non-invasive, real-time imaging in whole animals. In this study we were able to overcome a limitation of in vivo fluorescence imaging and have established a dual cell tracking method where two different cell types can be monitored according to the spectral signature of the cell labelling fluorophore. Using a mouse model of acute liver injury, we have characterised the in vivo migration patterns of wild type and transgenic neutrophils with impaired chemotaxis. Here, we were able to demonstrate that IVIS provides a sensitive multiplexing technology to differentiate two different cell populations based on the spectral signature of the cell labelling fluorophores. This spectral unmixing methodology has the potential to uncover multidimensional cellular interactions involved in many diseases such as fibrosis and cancer. In vivo spectral un-mixing provides a useful tool for monitoring multiple biological process in real-time in the same animal.

FRET Microscopy for Real-Time Visualization of Second Messengers in Living Cells

Förster Resonance Energy Transfer (FRET) microscopy is a useful tool in molecular biology and medical research to monitor and quantify real-time dynamics of protein-protein interactions and biochemical processes. Using this well-established technique, many novel signaling mechanisms can be investigated in intact cells or tissues and even in various subcellular compartments. Here, we describe how to perform FRET measurements in living cells expressing FRET-based biosensors and how to evaluate these data. This general protocol can be applied for FRET measurements with various fluorescent biosensors.

Sensitivity and accuracy of hybrid fluorescence-mediated tomography in deep tissue regions

Fluorescence-mediated tomography (FMT) enables noninvasive assessment of the three-dimensional distribution of near-infrared fluorescence in mice. The combination with micro-computed tomography (µCT) provides anatomical data, enabling improved fluorescence reconstruction and image analysis. The aim of our study was to assess sensitivity and accuracy of µCT-FMT under realistic in vivo conditions in deeply-seated regions. Accordingly, we acquired fluorescence reflectance images (FRI) and µCT-FMT scans of mice which were prepared with rectal insertions with different amounts of fluorescent dye. Default and high-sensitivity scans were acquired and background signal was analyzed for three FMT channels (670 nm, 745 nm, and 790 nm). Analysis was performed for the original and an improved FMT reconstruction using the µCT data. While FRI and the original FMT reconstruction could detect 100 pmol, the improved FMT reconstruction could detect 10 pmol and significantly improved signal localization. By using a finer sampling grid and increasing the exposure time, the sensitivity could be further improved to detect 0.5 pmol. Background signal was highest in the 670 nm channel and most prominent in the gastro-intestinal tract and in organs with high relative amounts of blood. In conclusion, we show that µCT-FMT allows sensitive and accurate assessment of fluorescence in deep tissue regions.

FRET imaging approaches for in vitro and in vivo characterization of synthetic lipid nanoparticles

DiI and DiD, two fluorophores able to interact by FRET (Förster resonance energy transfer), were coencapsulated in the core of lipid nanocapsules (LNCs) and nanoemulsions (LNEs), lipophilic reservoirs for the delivery of drugs. The ability of FRET imaging to provide information on the kinetics of dissociation of the nanoparticles in the presence of bovine serum albumin (BSA) or whole serum, or after incubation with cancer cells, and after systemic administration in tumor-bearing mice, was studied. Both microscopic and macroscopic imaging was performed to determine the behavior of the nanostructures in a biological environment. When 2 mg/mL FRET LNEs or LNCs were dispersed in buffer, in the presence of unloaded nanoparticles, BSA, or in whole serum, the presence of serum was the most active in destroying the particles. This occurred immediately with a diminution of 20% of FRET, then slowly, ending up with still 30% intact nanoparticles at 24 h. LNCs were internalized rapidly in cultured cells with the FRET signal decreasing within the first minutes of incubation, and then a plateau was reached and LNCs remained intact during 3 h. In contrast, LNEs were poorly internalized and were rapidly dissociated after internalization. Following their iv injection, LNCs appeared very stable in subcutaneous tumors implanted in mice. Intact particles were found using microscopic FRET determination on tumor sections 24 h after injection, that correlated well with the 8% calculated noninvasively on live animals. FRET investigations showed the potential to determine valid and reliable information about in vitro and in vivo behavior of nanoparticles.

Assessment of rat antigen-induced arthritis and its suppression during glucocorticoid therapy by use of hemicyanine dye probes with different molecular weight in near-infrared fluorescence optical imaging

PURPOSE

Arthritic joints are ideal disease entities to be assessed via optical imaging. Here, we investigated the selective accumulation behavior of two differently sized hemicyanine optical probes in arthritic joints and its modification during glucocorticoid therapy in the course of inflammation.

MATERIALS AND METHODS

The well-standardized preclinical antigen-induced arthritis (AIA) model in rats was used. The animals were divided into 3 groups: arthritic, arthritic and dexamethasone-treated, and immunized only. After intravenous coinjection of DY-752 (size, 800 Da) and DY-682-(rat) IgG (size, 150 kDa) probes, spectrally unmixed near-infrared fluorescence images were acquired and analyzed semiquantitatively. Probe organ distribution, joint swelling, blood cell counts, joint vessel density, and histological scoring of arthritis were determined.

RESULTS

The local joint accumulation kinetics of the DY-752 probe differed from the DY-682-IgG one. In the course of AIA, probe signaling in arthritic joints was similar between each other. Joint swelling and histological scoring were in accordance with signaling. Dexamethasone treatment of rats with AIA significantly reduced both the near-infrared fluorescence signals and severity of arthritis but did not change the joint vascular density or the uptake of the probes by phagocytes. A differential biodistribution of both probes was encountered, but such an accumulation was prevented by dexamethasone treatment.

CONCLUSIONS

Near-infrared fluorescence signaling in the course of AIA closely reflects the pathophysiological events of the arthritic joint and the effects of therapy independently of the molecular size of the probe. The results show the suitability of our hemicyanine probes for imaging of arthritis.

Transfection of VEGF(165) genes into endothelial progenitor cells and in vivo imaging using quantum dots in an ischemia hind limb model

Endothelial progenitor cells (EPCs) were transfected with fluorescently labeled quantum dot nanoparticles (QD NPs) with or without VEGF(165) plasmid DNA (pDNA) to probe the EPCs after in vivo transplantation and to test whether they presented as differentiated endothelial cells (ECs). Bare QD NPs and QD NPs coated with PEI or PEI + VEGF(165) genes were characterized by dynamic light scattering, scanning electron microscopy, and atomic force microscopy. Transfection of EPCs with VEGF(165) led to the expression of specific genes and proteins for mature ECs. A hind limb ischemia model was generated in nude mice, and VEGF(165) gene-transfected EPCs were transplanted intramuscularly into the ischemic limbs. At 28 days after transplantation, the VEGF(165) gene-transfected EPCs significantly increased the number of differentiated ECs compared with the injection of medium or bare EPCs without VEGF(165) genes. Laser Doppler imaging revealed that blood perfusion levels were increased significantly by VEGF(165) gene-transfected EPCs compared to EPCs without VEGF(165). Moreover, the transplantation of VEGF(165) gene-transfected EPCs increased the specific gene and protein expression levels of mature EC markers and angiogenic factors in the animal model.