Detecting intratumoral heterogeneity of EGFR activity by liposome-based in vivo transfection of a fluorescent biosensor

Despite decades of research in the epidermal growth factor receptor (EGFR) signalling field, and many targeted anti-cancer drugs that have been tested clinically, the success rate for these agents in the clinic is low, particularly in terms of the improvement of overall survival. Intratumoral heterogeneity is proposed as a major mechanism underlying treatment failure of these molecule-targeted agents. Here we highlight the application of fluorescence lifetime microscopy (FLIM)-based biosensing to demonstrate intratumoral heterogeneity of EGFR activity. For sensing EGFR activity in cells, we used a genetically encoded CrkII-based biosensor which undergoes conformational changes upon tyrosine-221 phosphorylation by EGFR. We transfected this biosensor into EGFR-positive tumour cells using targeted lipopolyplexes bearing EGFR-binding peptides at their surfaces. In a murine model of basal-like breast cancer, we demonstrated a significant degree of intratumoral heterogeneity in EGFR activity, as well as the pharmacodynamic effect of a radionuclide-labeled EGFR inhibitor in situ. Furthermore, a significant correlation between high EGFR activity in tumour cells and macrophage-tumour cell proximity was found to in part account for the intratumoral heterogeneity in EGFR activity observed. The same effect of macrophage infiltrate on EGFR activation was also seen in a colorectal cancer xenograft. In contrast, a non-small cell lung cancer xenograft expressing a constitutively active EGFR conformational mutant exhibited macrophage proximity-independent EGFR activity. Our study validates the use of this methodology to monitor therapeutic response in terms of EGFR activity. In addition, we found iNOS gene induction in macrophages that are cultured in tumour cell-conditioned media as well as an iNOS activity-dependent increase in EGFR activity in tumour cells. These findings point towards an immune microenvironment-mediated regulation that gives rise to the observed intratumoral heterogeneity of EGFR signalling activity in tumour cells in vivo

Mapping femtosecond pulse front distortion and group velocity dispersion in multiphoton microscopy

Group velocity dispersion (GVD) and pulse front distortion of ultrashort pulses are of critical importance in efficient multiphoton excitation microscopy. Since measurement of the pulse front distortion due to a lens is not trivial we have developed an imaging interferometric cross-correlator which allows us to measure temporal delays and pulse-widths across the spatial profile of the beam. The instrument consists of a modified Michelson interferometer with a reference arm containing a voice-coil delay stage and an arm which contains the optics under test. The pulse replicas are recombined and incident on a 22×22 lenslet array. The beamlets are focused in a 0.5 mm thick BBO crystal (cut for Type I second harmonic generation), filtered to remove the IR component of the beam and imaged using a 500 fps camera. The GVD and pulse front distortion are extracted from the temporal stack of beamlet images to produce a low resolution spatio-temporal map

Semi-automated software for the three-dimensional delineation of complex vascular networks.

The understanding of tumour angiogenesis is of great importance in cancer research, as is the tumour response to vascular-targeted drugs. This paper presents software aimed at aiding these investigations and other situations where linear or dendritic structures are to be delineated from three-dimensional (3D) data sets. This software application was written to analyse the data from 3D data sets by allowing the manual and semi-automated tracking and delineation of the vascular tree, including the measurement of vessel diameter. A new algorithm, CHARM, based on a compact Hough transform and the formation of a radial map, has been used to locate vessel centres and measure diameters automatically. The robustness of this algorithm to image smoothing and noise has been investigated

Scanning total internal reflection fluorescence imaging

Cell adhesion and focal complex formation require signalling complexes linking cell adhesion molecules to the cytoskeleton. To understand morphogenetic changes associated with tumour cell spreading, migration and tumour cell metastasis, the molecular mechanisms responsible for the regulation, formation and dissolution at the cell-extracellular matrix (ECM) interface need to be identified. In order to achieve this, an improved axial resolution is desirable. We report on the development of a multi-photon (MP) total internal reflection (TIR) fluorescence lifetime imaging (FLIM) system that allows the selective excitation of fluorophores, with such an improved axial resolution. Results from initial experiments are presented. High excitation efficiency is achieved by the use of a Nikon 1.45 NA TIRF objective using annular illumination

Global and pixel kinetic data analysis for FRET detection by multi-photon time-domain FLIM

FLIM/FRET is an extremely powerful technique that can microscopically locate nanometre-scale protein-protein interactions within live or fixed cells, both in vitro and in vivo. The key to performing sensitive FRET, via FLIM, besides the use of appropriate fluorophores, is the analysis of the time-resolved data present at each image pixel. The fluorescent transient will, in general, exhibit multi-exponential kinetics: at least two exponential components arise from both the interacting and non-interacting protein. We shall describe a novel method and computer program for the global analysis of time resolved data, either at the single level or through global analysis of grouped pixel data. Kinetic models are fitted using the Marquardt algorithm and iterative convolution of the excitation signal, in a computationally-efficient manner. The fitting accuracy and sensitivity of the algorithm has been tested using modelled data, including the addition of simulated Poisson noise and repetitive excitation pulses which are typical of a TCSPC system. We found that the increased signal to noise ratio offered by both global and invariance fitting is highly desirable. When fitting mono-exponential data, the effects of a ca. 12.5 ns (ca. 80 MHz) repetitive excitation do not preclude the accurate extraction of populations with lifetimes in the range 0.1 to 10 ns, even when these effects are not represented in the fitting algorithm. Indeed, with global or invariance fitting of a 32×32 pixel area, the error in extracted lifetime can be lower than 0.4 % for signals with a peak of 500 photon counts or more. In FRET simulations, modelling GFP with a non-interacting lifetime of 2.15 ns, it was possible to accurately detect a 10 % interacting population with a lifetime of 0.8 ns

Dynamic imaging of protein-protein interactions by MP-FLIM

The spatio-temporal localization of molecular interactions within cells in situ is of great importance in elucidating the key mechanisms in regulation of fundamental process within the cell. Measurements of such near-field localization of protein complexes may be achieved by the detection of fluorescence (or Förster) resonance energy transfer (FRET) between protein-conjugated fluorophores. We demonstrate the applicability of time-correlated single photon counting multiphoton microscopy to the spatio-temporal localization of protein-protein interactions in live and fixed cell populations. Intramolecular interactions between protein hetero-dimers are investigated using green fluorescent protein variants. We present an improved monomeric form of the red fluorescent protein, mRFP1, as the acceptor in biological fluorescence resonance energy transfer (FRET) experiments using the enhanced green fluorescent protein as donor. We find particular advantage in using this fluorophore pair for quantitative measurements of FRET. The technique was exploited to demonstrate a novel receptor-kinase interaction between the chemokine receptor (CXCR4) and protein kinase C (PKC) a in carcinoma cells for both live and fixed cell experiments