In vivo measurement of gene expression, angiogenesis and physiological function in tumors using multiphoton laser scanning microscopy

Live cell imaging in live animals with fluorescent proteins

The discovery, cloning, and characterization of GFP and related proteins of many colors have enabled live cell imaging to an unprecedented extent and resolution. Essentially, any cellular process can be imaged with a fluorescent protein. These proteins serve as genetic reporters and therefore can be used to follow cellular processes over indefinite periods in vivo as well as in vitro. The brightness and specific spectra of fluorescent proteins allow them to be imaged in vivo, using specific filters, without interference from autofluorescence. This chapter describes the development of live imaging in live animals with subcellular resolution, emphasizing the study of in vivo cell biology of cancer growth, spread, and metastasis.

Line-scanning particle image velocimetry: an optical approach for quantifying a wide range of blood flow speeds in live animals

BACKGROUND

The ability to measure blood velocities is critical for studying vascular development, physiology, and pathology. A key challenge is to quantify a wide range of blood velocities in vessels deep within living specimens with concurrent diffraction-limited resolution imaging of vascular cells. Two-photon laser scanning microscopy (TPLSM) has shown tremendous promise in analyzing blood velocities hundreds of micrometers deep in animals with cellular resolution. However, current analysis of TPLSM-based data is limited to the lower range of blood velocities and is not adequate to study faster velocities in many normal or disease conditions.

METHODOLOGY/PRINCIPAL FINDINGS

We developed line-scanning particle image velocimetry (LS-PIV), which used TPLSM data to quantify peak blood velocities up to 84 mm/s in live mice harboring brain arteriovenous malformation, a disease characterized by high flow. With this method, we were able to accurately detect the elevated blood velocities and exaggerated pulsatility along the abnormal vascular network in these animals. LS-PIV robustly analyzed noisy data from vessels as deep as 850 µm below the brain surface. In addition to analyzing in vivo data, we validated the accuracy of LS-PIV up to 800 mm/s using simulations with known velocity and noise parameters.

CONCLUSIONS/SIGNIFICANCE

To our knowledge, these blood velocity measurements are the fastest recorded with TPLSM. Partnered with transgenic mice carrying cell-specific fluorescent reporters, LS-PIV will also enable the direct in vivo correlation of cellular, biochemical, and hemodynamic parameters in high flow vascular development and diseases such as atherogenesis, arteriogenesis, and vascular anomalies.

Novel insight into stem cell trafficking in dystrophic muscles

Recently published reports have described possible cellular therapy approaches to regenerate muscle tissues using arterial route delivery. However, the kinetic of distribution of these migratory stem cells within injected animal muscular dystrophy models is unknown. Using living X-ray computed microtomography, we established that intra-arterially injected stem cells traffic to multiple muscle tissues for several hours until their migration within dystrophic muscles. Injected stem cells express multiple traffic molecules, including VLA-4, LFA-1, CD44, and the chemokine receptor CXCR4, which are likely to direct these cells into dystrophic muscles. In fact, the majority of intra-arterially injected stem cells access the muscle tissues not immediately after the injection, but after several rounds of recirculation. We set up a new, living, 3D-imaging approach, which appears to be an important way to investigate the kinetic of distribution of systemically injected stem cells within dystrophic muscle tissues, thereby providing supportive data for future clinical applications.

Imaging neocortical neurons through a chronic cranial window

The rich structural dynamics of axonal arbors and neuronal circuitry can only be revealed through direct and repeated observations of the same neuron(s) over time, preferably in vivo. This protocol describes a long-term, high-resolution method for imaging neocortical neurons in vivo, using a combination of two-photon laser scanning microscopy (2PLSM) and a surgically implanted chronic cranial window. The window is used because the skull of most mammals is too opaque to allow high-resolution imaging of cortical neurons. Using this method, it is feasible to image the smallest neuronal structures in the superficial layers of the neocortex, such as dendritic spines and axonal boutons. Because the surface area of the craniotomy is relatively large, this technique is even suitable for use when labeled neurons are relatively uncommon. The surgery and imaging procedures are illustrated with examples from our studies of structural plasticity in the developing or adult mouse brain. The protocol is optimized for adult mice; we have used mice up to postnatal day 511 (P511). With minor modifications, it is possible to image neurons in rats and mice from P2. Most of our studies have used the Thy1 promoter to drive expression of fluorophores in subsets of cortical neurons.

Visualizing dynamics of sub-hepatic distribution of nanoparticles using intravital multiphoton fluorescence microscopy

Nanoparticles that do not undergo renal excretion or in vivo degradation into biocompatible debris often accumulate in the reticuloendothelial system, also know as the mononuclear phagocyte system, with undesired consequences that limit their clinical utility. In this work, we report the first application of intravital multiphoton fluorescence microscopy to dynamically track the hepatic metabolism of nanoparticles with subcellular resolution in real time. Using fluorescently labeled mesoporous silica nanoparticles (MSNs) in mice as a prototypical model, we observed significant hepatocyte uptake of positively charged, but not negatively charged, moieties. Conversely, in vivo imaging of negatively charged, but not positively charged, MSNs reveals an overwhelming propensity for the former's rapid uptake by Kupffer cells in liver sinusoids. Since the only prerequisite for these studies was that nanoparticles are fluorescently labeled and not of a specific composition or structure, the techniques we present can readily be extended to a wide variety of nanoparticle structures and surface modifications (e.g., shape, charge, hydrophobicity, PEGylation) in the preclinical assessment and tailoring of their hepatotoxicities and clearances.

Delivery of molecular and nanoscale medicine to tumors: transport barriers and strategies

Tumors are similar to organs, with unique physiology giving rise to an unusual set of transport barriers to drug delivery. Cancer therapy is limited by nonuniform drug delivery via blood vessels, inhomogeneous drug transport into tumor interstitium from the vascular compartment, and hindered transport through tumor interstitium to the target cells. Four major abnormal physical and physiological properties contribute to these transport barriers. Accumulated solid stress compresses blood vessels to diminish the drug supply to many tumor regions. Immature vasculature with high viscous and geometric resistances and reduced pressure gradients leads to sluggish and heterogeneous blood flow in tumors to further limit drug supply. Nonfunctional lymphatics coupled with highly permeable blood vessels result in elevated hydrostatic pressure in tumors to abrogate convective drug transport from blood vessels into and throughout most of the tumor tissue. Finally, a dense structure of interstitial matrix and cells serves as a tortuous, viscous, and steric barrier to diffusion of therapeutic agents. In this review, we discuss the origins and implications of these barriers. We then highlight strategies for overcoming these barriers by modulating either drug properties or the tumor microenvironment itself to enhance the delivery and effectiveness of drugs in tumors.

Cancer imaging by optical coherence tomography: preclinical progress and clinical potential

The past decade has seen dramatic technological advances in the field of optical coherence tomography (OCT) imaging. These advances have driven commercialization and clinical adoption in ophthalmology, cardiology and gastrointestinal cancer screening. Recently, an array of OCT-based imaging tools that have been developed for preclinical intravital cancer imaging applications has yielded exciting new capabilities to probe and to monitor cancer progression and response in vivo. Here, we review these results, forecast the future of OCT for preclinical cancer imaging and discuss its exciting potential to translate to the clinic as a tool for monitoring cancer therapy.

Nanoscale drug delivery systems for enhanced drug penetration into solid tumors: current progress and opportunities

Poor penetration of anticancer drags into solid tumors significantly limits their efficacy. This phenomenon has long been observed for small-molecule chemotherapeutics, and it can be even more pronounced for nanoscale therapies. Nanoparticles have enormous potential for the treatment of cancer due to their wide applicability as drug delivery and imaging vehicles and their size-dependent accumulation into solid tumors by the enhanced permeability and retention (EPR) effect. Further, synthetic nanoparticles can be engineered to overcome barriers to drag delivery. Despite their promise for the treatment of cancer, relatively little work has been done to study and improve their ability to diffuse into solid tumors following passive accumulation in the tumor vasculature. In this review, we present the complex issues governing efficient penetration of nanoscale therapies into solid tumors. The current methods available to researchers to study nanoparticle penetration into malignant tumors are described, and the most recent works studying the penetration of nanoscale materials into solid tumors are summarized. We conclude with an overview of the important nanoparticle design parameters governing their tumor penetration, as well as by highlighting critical directions in this field.

Intravital imaging

Until recently, the idea of observing life deep within the tissues of a living mouse, at a resolution sufficient to pick out cellular behaviors and molecular signals underlying them, remained a much-coveted dream. Now, a new era of intravital fluorescence microscopy has dawned. In this Primer, we review the technologies that made this revolution possible and demonstrate how intravital imaging is beginning to provide quantitative and dynamic insights into cell biology, immunology, tumor biology, and neurobiology.

A Nanocrystal-based Ratiometric pH Sensor for Natural pH Ranges

A ratiometric fluorescent pH sensor based on CdSe/CdZnS nanocrystal quantum dots (NCs) has been designed for biological pH ranges. The construct is formed from the conjugation of a pH dye (SNARF) to NCs coated with a poly(amido amine) (PAMAM) dendrimer. The sensor exhibits a well-resolved ratio response at pH values between 6 and 8 under linear or two-photon excitation, and in the presence of a 4% bovine serum albumin (BSA) solution.

Multistage nanoparticles for improved delivery into tumor tissue

The enhanced permeability and retention (EPR) effect has been a key rationale for the development of nanoscale carriers to solid tumors. As a consequence of EPR, nanotherapeutics are expected to improve drug and detection probe delivery, have less adverse effects than conventional chemotherapy, and thus result in improved detection and treatment of tumors. Physiological barriers posed by the abnormal tumor microenvironment, however, can hinder the homogeneous delivery of nanomedicine in amounts sufficient to eradicate cancer. To effectively enhance the therapeutic outcome of cancer patients by nanotherapeutics, we have to find ways to overcome these barriers. One possibility is to exploit the abnormal tumor microenvironment for selective and improved delivery of therapeutic agents to tumors. Recently, we proposed a multistage nanoparticle delivery system as a potential means to enable uniform delivery throughout the tumor and improve the efficacy of anticancer therapy. Here, we describe the synthesis of a novel multistage nanoparticle formulation that shrinks in size once it enters the tumor interstitial space to optimize the delivery to tumors as well as within tumors. Finally, we provide detailed experimental methods for the characterization of such nanoparticles.

Multiphoton microscopy applied for real-time intravital imaging of bacterial infections in vivo

To understand the underlying mechanisms of bacterial infections, researchers have for long addressed the molecular interactions occurring when the bacterium interacts with host target cells. In these studies, primarily based on in vitro systems, molecular details have been revealed along with increased knowledge regarding the general infection process. With the recent advancements in in vivo imaging techniques, we are now in a position to bridge a transition from classical minimalistic in vitro approaches to allow infections to be studied in its native complexity-the live organ. Techniques such as multiphoton microscopy (MPM) allow cellular-level visualization of the dynamic infection process in real time within the living host. Studies in which all interplaying factors, such as the influences of the immune, lymphatic, and vascular systems can be accounted for, are likely to provide new insights to our current understanding of the infection process. MPM imaging becomes extra powerful when combined with advanced surgical procedure, allowing studies of the illusive early hours of infection. In this chapter, our intention is to provide a general view on how to design and carry out intravital imaging of a bacterial infection. While exemplifying this using a spatiotemporally well-controlled uropathogenic Escherichia coli (UPEC) infection in rat kidneys, we hope to provide the reader with general considerations that can be adapted to other bacterial infections in organs other than the kidney.

Optical detection of brain function: simultaneous imaging of cerebral vascular response, tissue metabolism, and cellular activity in vivo

It is known that a remaining challenge for functional brain imaging is to distinguish the coupling and decoupling effects among neuronal activity, cerebral metabolism, and vascular hemodynamics, which highlights the need for new tools to enable simultaneous measures of these three properties in vivo. Here, we review current neuroimaging techniques and their prospects and potential limitations for tackling this challenge. We then report a novel dual-wavelength laser speckle imaging (DW-LSI) tool developed in our labs that enables simultaneous imaging of cerebral blood flow (CBF), cerebral blood volume, and tissue hemoglobin oxygenation, which allows us to monitor neurovascular and tissue metabolic activities at high spatiotemporal resolutions over a relatively large field of view. Moreover, we report digital frequency ramping Doppler optical coherence tomography (DFR-OCT) that allows for quantitative 3D imaging of the CBF network in vivo. In parallel, we review calcium imaging techniques to track neuronal activity, including intracellular calcium approach using Rhod2 fluorescence technique that we develop to detect neuronal activity in vivo. We report a new multimodality imaging platform that combines DW-LSI, DFR-OCT, and calcium fluorescence imaging for simultaneous detection of cortical hemodynamics, cerebral metabolism, and neuronal activities of the animal brain in vivo, as well as its integration with microprobes for imaging neuronal function in deep brain regions in vivo. Promising results of in vivo animal brain functional studies suggest the potential of this multimodality approach for future awake animal and behavioral studies.

Evaluation of high frequency ultrasound methods and contrast agents for characterising tumor response to anti-angiogenic treatment

PURPOSE

To compare non-enhanced and contrast-enhanced high-frequency 3D Doppler ultrasound with contrast-enhanced 2D and 3D B-mode imaging for assessing tumor vascularity during antiangiogenic treatment using soft-shell and hard-shell microbubbles.

MATERIALS AND METHODS

Antiangiogenic therapy effects (SU11248) on vascularity of subcutaneous epidermoid-carcinoma xenografts (A431) in female CD1 nude mice were investigated longitudinally using non-enhanced and contrast-enhanced 3D Doppler at 25 MHz. Additionally, contrast-enhanced 2D and 3D B-mode scans were performed by injecting hard-shell (poly-butyl-cyanoacrylate-based) and soft-shell (phospholipid-based) microbubbles. Suitability of both contrast agents for high frequency imaging and the sensitivity of the different ultrasound methods to assess early antiangiogenic therapy effects were investigated. Ultrasound data were validated by immunohistology.

RESULTS

Hard-shell microbubbles induced higher signal intensity changes in tumors than soft-shell microbubbles in 2D B-mode measurements (424 ± 7 vs. 169 ± 8 A.U.; p<0.01). In 3D measurements, signals of soft-shell microbubbles were hardly above the background (5.48 ± 4.57 vs. 3.86 ± 2.92 A.U.), while signals from hard-shell microbubbles were sufficiently high (30.5 ± 8.06 A.U). Using hard-shell microbubbles 2D and 3D B-mode imaging depicted a significant decrease in tumor vascularity during antiangiogenic therapy from day 1 on. Using soft-shell microbubbles significant therapy effects were observed at day 4 after therapy in 2D B-mode imaging but could not be detected in the 3D mode. With non-enhanced and contrast-enhanced Doppler imaging significant differences between treated and untreated tumors were found from day 2 on.

CONCLUSION

Hard-shell microbubble-enhanced 2D and 3D B-mode ultrasound achieved highest sensitivity for assessing therapy effects on tumor vascularisation and were superior to B-mode ultrasound with soft-shell microbubbles and to Doppler imaging.

Cavitation-enhanced extravasation for drug delivery

A flow-through tissue-mimicking phantom composed of a biocompatible hydro-gel with embedded tumour cells was used to assess and optimize the role of ultrasound-induced cavitation on the extravasation of a macromolecular compound from a channel mimicking vessel in the gel, namely a non-replicating luciferase-expressing adenovirus (Ad-Luc). Using a 500 KHz therapeutic ultrasound transducer confocally aligned with a focussed passive cavitation detector, different exposure conditions and burst mode timings were selected by performing time and frequency domain analysis of passively recorded acoustic emissions, in the absence and in the presence of ultrasound contrast agents acting as cavitation nuclei. In the presence of Sonovue, maximum ultraharmonic emissions were detected for peak rarefactional pressures of 360 kPa, and maximum broadband emissions occurred at 1250 kPa. The energy of the recorded acoustic emissions was used to optimise the pulse repetition frequency and duty cycle in order to maximize either ultraharmonic or broadband emissions while keeping the acoustic energy delivered to the focus constant. Cell viability measurements indicated that none of the insonation conditions investigated induces cell death in the absence of a therapeutic agent (i.e. virus). Phase contrast images of the tissue-mimicking phantom showed that short range vessel disruption can occur when ultra-harmonic emissions (nf0/2) are maximised whereas formation of a micro-channel perpendicular to the flow can be obtained in the presence of broadband acoustic emissions. Following Ad-Luc delivery, luciferase expression measurements showed that a 60-fold increase in its bioavailability can be achieved when broadband noise emissions are present during insonation, even for modest contrast agent concentrations. The findings of the present study suggest that drug delivery systems based on acoustic cavitation may help enhance the extravasation of anticancer agents, thus increasing their penetration distance to hypoxic regions and poorly vascularised tumour regions.

Multiphoton microscopy: An introduction to gastroenterologists

Multiphoton microscopy, relying on the simultaneous absorption of two or more photons by a fluorophore, has come to occupy a prominent place in modern biomedical research with its ability to allow real-time observation of a single cell and molecules in intact tissues. Multiphoton microscopy exhibits nonlinear optical contrast properties, which can make it possible to provide an exceptionally large depth penetration with less phototoxicity. This system becomes more and more an inspiring tool for a non-invasive imaging system to realize “optical biopsy” and to examine the functions of living cells. In this review, we briefly present the physical principles and properties of multiphoton microscopy as well as the current applications in biological fields. In addition, we address what we see as the future potential of multiphoton microscopy for gastroenterologic research.

Immobilizing reporters for molecular imaging of the extracellular microenvironment in living animals

We report here an immobilization strategy using a collagen binding protein to deliver and confine synthetic reporters to the extracellular microenvironment in vivo for noninvasively imaging the activity of targets in the microenvironment. We show that the immobilization of reporters on collagens in the local microenvironment is highly efficient and physiologically stable for repetitive, long-term imaging. By using this strategy we successfully developed an immobilized bioluminescent activatable reporter and a dual-modality reporter to map and quantitatively image the activity of extracellular matrix metalloproteinases (MMP) in tumor-bearing mice. The inhibition of MMP activity by chemical inhibitor was also demonstrated in living subjects. We further demonstrated the general applicability of this immobilization strategy by imaging MMP activity at the inflammation site in a mouse model. Our results show that the in vivo immobilization of reporters can be used as a general strategy for probing the local extracellular microenvironment.

Setup and use of a two-laser multiphoton microscope for multichannel intravital fluorescence imaging

Characterizing biological mechanisms dependent upon the interaction of many cell types in vivo requires both multiphoton microscope systems capable of expanding the number and types of fluorophores that can be imaged simultaneously while removing the wavelength and tunability restrictions of existing systems, and enhanced software for extracting critical cellular parameters from voluminous 4D data sets. We present a procedure for constructing a two-laser multiphoton microscope that extends the wavelength range of excitation light, expands the number of simultaneously usable fluorophores and markedly increases signal to noise via 'over-clocking' of detection. We also utilize a custom-written software plug-in that simplifies the quantitative tracking and analysis of 4D intravital image data. We begin by describing the optics, hardware, electronics and software required, and finally the use of the plug-in for analysis. We demonstrate the use of the setup and plug-in by presenting data collected via intravital imaging of a mouse model of breast cancer. The procedure may be completed in ∼24 h.

Dynamics of angiogenesis during wound healing: a coupled in vivo and in silico study

OBJECTIVE

The most critical determinant of restoration of tissue structure during wound healing is the re-establishment of a functional vasculature, which largely occurs via angiogenesis, specifically endothelial sprouting from the pre-existing vasculature.

MATERIALS AND METHODS

We used confocal microscopy to capture sequential images of perfused vascular segments within the injured panniculus carnosus muscle in the mouse dorsal skin-fold window chamber to quantify a range of microcirculatory parameters during the first nine days of healing. This data was used to inform a mathematical model of sequential growth of the vascular plexus. The modeling framework mirrored the experimental circular wound domain and incorporated capillary sprouting and endothelial cell (EC) sensing of vascular endothelial growth factor gradients.

RESULTS

Wound areas, vessel densities and vessel junction densities obtained from the corresponding virtual wound were in excellent agreement both temporally and spatially with data measured during the in vivo healing process. Moreover, by perturbing the proliferative ability of ECs in the mathematical model, this leads to a severe reduction in vascular growth and poor healing. Quantitative measures from this second set of simulations were found to correlate extremely well with experimental data obtained from animals treated with an agent that targets endothelial proliferation (TNP-470).

CONCLUSION

Our direct combination and comparison of in vivo longitudinal analysis (over time in the same animal) and mathematical modeling employed in this study establishes a useful new paradigm. The virtual wound created in this study can be used to investigate a wide range of experimental hypotheses associated with wound healing, including disorders characterized by aberrant angiogenesis (e.g., diabetic models) and the effects of vascular enhancing/disrupting agents or therapeutic interventions such as hyperbaric oxygen.

Intravital microscopy: new insights into metastasis of tumors

Metastasis, the process by which cells spread from the primary tumor to a distant site to form secondary tumors, is still not fully understood. Although histological techniques have provided important information, they give only a static image and thus compromise interpretation of this dynamic process. New advances in intravital microscopy (IVM), such as two-photon microscopy, imaging chambers, and multicolor and fluorescent resonance energy transfer imaging, have recently been used to visualize the behavior of single metastasizing cells at subcellular resolution over several days, yielding new and unexpected insights into this process. For example, IVM studies showed that tumor cells can switch between multiple invasion strategies in response to various densities of extracellular matrix. Moreover, other IVM studies showed that tumor cell migration and blood entry take place not only at the invasive front, but also within the tumor mass at tumor-associated vessels that lack an intact basement membrane. In this Commentary, we will give an overview of the recent advances in high-resolution IVM techniques and discuss some of the latest insights in the metastasis field obtained with IVM.

Role of tumor vascular architecture in drug delivery

Tumor targeted drug delivery has the potential to improve cancer care by reducing non-target toxicities and increasing the efficacy of a drug. Tumor targeted delivery of a drug from the systemic circulation, however, requires a thorough understanding of tumor pathophysiology. A growing or receding (under the impact of therapy) tumor represents a dynamic environment with changes in its angiogenic status, cell mass, and extracellular matrix composition. An appreciation of the salient characteristics of tumor vascular architecture and the unique biochemical markers that may be used for targeting drug therapy is important to overcome barriers to tumor drug therapy and to facilitate targeted drug delivery. This review discusses the unique aspects of tumor vascular architecture that need to be overcome or exploited for tumor targeted drug delivery.

Intratumor administration of the photosensitizer pc 4 affords photodynamic therapy efficacy and selectivity at short drug-light intervals

We evaluated intratumor (IT) versus intravenous (IV) administration of the photosensitizer Pc 4 with respect to tumor photosensitizer concentration, specificity, and responses to irradiation. BALB/c mice bearing intradermal EMT6 tumors were given 0.3 mg/kg Pc 4 injected IT or IV through the tail vein. Photosensitizer concentration was evaluated by chloroform extraction and localization assessed by fluorescence imaging and spectroscopy in vivo. Tumors were irradiated at 667 nm, 50 mW/cm(2), and 100 J/cm(2). Cures were defined as no palpable tumor 90 days after irradiation. Tumor Pc 4 concentrations 1 hour after IT administration were 35,000-fold higher than measured 24 hours after IV administration (0.112 vs 0.317 x 10(-5)microg Pc 4/mg tumor). Exquisite tumor selectivity was observed 1 hour after IT injection. Fluorescence imaging of freshly sectioned tumors revealed no regions devoid of sensitizer at this time point, with pixel intensities in a midline section within a factor of 3 of the peak intensity. For identical photosensitizer doses, IT administration significantly improved tumor responses to irradiation, with more than 70% of tumors cured with IT-Pc 4-PDT. In this model, IT-Pc 4 administration provides improved tumor control, greater selectivity, and opportunity for a short drug-light interval.

Blockade of vascular endothelial growth factor sensitizes tumor-associated vasculatures to angiolytic therapy with a high-frequency ultrashort pulsed laser

Because of high spatial resolution and superior tissue penetration, a femtosecond laser of the near-infrared spectrum has great potential to improve the efficacy of conventional photodynamic therapy; however, the lack of suitable photosensitizers has so far limited its bedside applications. Recently, our group reported that a brief irradiation by femtosecond lasers in the absence of exogenous probes can modulate various cellular behaviors in vitro and in vivo. Here, we demonstrate that targeted irradiation by a femtosecond laser disrupted tumor-associated blood vessels, and the inhibition of vascular endothelial growth factor signaling augmented the efficacy of laser-induced angiolysis. Further, we show that reactive oxygen species (ROS) are generated in response to laser irradiation, and reducing the intracellular levels of ROS rendered endothelial cells resistant to laser-induced cytotoxicity. Collectively, these results indicate that a femtosecond laser can be used as a vascular-disrupting therapeutic modality for cancer treatment, especially when used in combination with conventional anti-angiogenic therapies.

Tumor imaging with multicolor fluorescent protein expression

Imaging with fluorescent proteins has been revolutionary and has led to the new field of in vivo cell biology. Many new applications of this technology have been developed. Green fluorescent protein (GFP)-labeled or red fluorescent protein (RFP)-labeled HT-1080 human fibrosarcoma cells were used to determine clonality of metastasis by imaging of metastatic colonies after mixed implantation of the red and green fluorescent cells. Resulting pure red or pure green colonies were scored as clonal, whereas mixed yellow colonies were scored as nonclonal. Dual-color fluorescent cancer cells expressing GFP in the nucleus and RFP in the cytoplasm were engineered. The dual-color cancer cells enable real-time nuclear-cytoplasmic dynamics to be visualized in living cells in vivo, including mitosis and apoptosis. The nuclear and cytoplasmic behavior of dual-color cancer cells in real time in blood vessels was observed as they trafficked by various means or extravasated in an abdominal skin flap. Dual-color cancer cells were also visualized trafficking through lymphatic vessels where they were imaged via a skin flap. Seeding and arresting of single dual-color cancer cells in the lung, accumulation of cancer-cell emboli, cancer-cell viability, and metastatic colony formation were imaged in real time in an open-chest nude mouse model using assisted ventilation. Novel treatment was evaluated in these imageable models. UVC irradiation killed approximately 70% of the dual-color cancer cells in a nude mouse model. An RFP-expressing glioma was transplanted to the spinal cord of transgenic nude mice expressing nestin-driven green fluorescent protein (ND-GFP). In ND-GFP mice, GFP is expressed in nascent blood vessels and neural stem cells. ND-GFP cells staining positively for neuronal class III-β-tubulin or CD31 surrounded the tumor, suggesting that the tumor stimulated both neurogenesis and angiogenesis. The tumor caused paralysis and also metastasized to the brain. The Salmonella typhimurium A1-R tumor-targeting bacterial strain was administered in the orthotopic spinal cord glioma model. The treated animals had a significant increase in survival and decrease in paralysis. S. typhimurium A1-R was effective against primary bone tumor and lung metastasis expressing RFP in a nude mouse model. S. typhimurium A1-R was effective against both axillary lymph and popliteal lymph node metastases of human dual-color pancreatic cancer and fibrosarcoma cells, respectively, as well as lung metastasis of the fibrosarcoma in nude mice. Imaging with fluorescent proteins will reveal mechanisms of cancer progression and provide visual targets for novel therapeutics.

In search of the Golden Fleece: unraveling principles of morphogenesis by studying the integrative biology of skin appendages

The mythological story of the Golden Fleece symbolizes the magical regenerative power of skin appendages. Similar to the adventurous pursuit of the Golden Fleece by the multi-talented Argonauts, today we also need an integrated multi-disciplined approach to understand the cellular and molecular processes during development, regeneration and evolution of skin appendages. To this end, we have explored several aspects of skin appendage biology that contribute to the Turing activator/inhibitor model in feather pattern formation, the topo-biological arrangement of stem cells in organ shape determination, the macro-environmental regulation of stem cells in regenerative hair waves, and potential novel molecular pathways in the morphological evolution of feathers. Here we show our current integrative biology efforts to unravel the complex cellular behavior in patterning stem cells and the control of regional specificity in skin appendages. We use feather/scale tissue recombination to demonstrate the timing control of competence and inducibility. Feathers from different body regions are used to study skin regional specificity. Bioinformatic analyses of transcriptome microarrays show the potential involvement of candidate molecular pathways. We further show Hox genes exhibit some region specific expression patterns. To visualize real time events, we applied time-lapse movies, confocal microscopy and multiphoton microscopy to analyze the morphogenesis of cultured embryonic chicken skin explants. These modern imaging technologies reveal unexpectedly complex cellular flow and organization of extracellular matrix molecules in three dimensions. While these approaches are in preliminary stages, this perspective highlights the challenges we face and new integrative tools we will use. Future work will follow these leads to develop a systems biology view and understanding in the morphogenetic principles that govern the development and regeneration of ectodermal organs.

Sensitivity of MRI tumor biomarkers to VEGFR inhibitor therapy in an orthotopic mouse glioma model

MRI biomarkers of tumor edema, vascular permeability, blood volume, and average vessel caliber are increasingly being employed to assess the efficacy of tumor therapies. However, the dependence of these biomarkers on a number of physiological factors can compromise their sensitivity and complicate the assessment of therapeutic efficacy. Here we examine the response of these MRI tumor biomarkers to cediranib, a potent vascular endothelial growth factor receptor (VEGFR) inhibitor, in an orthotopic mouse glioma model. A significant increase in the tumor volume and relative vessel caliber index (rVCI) and a slight decrease in the water apparent diffusion coefficient (ADC) were observed for both control and cediranib treated animals. This contrasts with a clinical study that observed a significant decrease in tumor rVCI, ADC and volume with cediranib therapy. While the lack of a difference between control and cediranib treated animals in these biomarker responses might suggest that cediranib has no therapeutic benefit, cediranib treated mice had a significantly increased survival. The increased survival benefit of cediranib treated animals is consistent with the significant decrease observed for cediranib treated animals in the relative cerebral blood volume (rCBV), relative microvascular blood volume (rMBV), transverse relaxation time (T2), blood vessel permeability (K^trans), and extravascular-extracellular space (ν_e). The differential response of pre-clinical and clinical tumors to cediranib therapy, along with the lack of a positive response for some biomarkers, indicates the importance of evaluating the whole spectrum of different tumor biomarkers to properly assess the therapeutic response and identify and interpret the therapy-induced changes in the tumor physiology.

Drug delivery strategies for therapeutic angiogenesis and antiangiogenesis

INTRODUCTION

Angiogenesis is essential to human biology and of great clinical significance. Excessive or reduced angiogenesis can result in, or exacerbate, several disease states, including tumor formation, exudative age-related macular degeneration (AMD) and ischemia. Innovative drug delivery systems can increase the effectiveness of therapies used to treat angiogenesis-related diseases.

AREAS COVERED

This paper reviews the basic biology of angiogenesis, including current knowledge about its disruption in diseases, with the focus on cancer and AMD. Anti- and proangiogenic drugs available for clinical use or in development are also discussed, as well as experimental drug delivery systems that can potentially improve these therapies to enhance or reduce angiogenesis in a more controlled manner.

EXPERT OPINION

Laboratory and clinical results have shown pro- or antiangiogenic drug delivery strategies to be effective in drastically slowing disease progression. Further research in this area will increase the efficacy, specificity and duration of these therapies. Future directions with composite drug delivery systems may make possible targeting of multiple factors for synergistic effects.

Dynamic, long-term in vivo imaging of tumor-stroma interactions in mouse models of breast cancer using spinning-disk confocal microscopy

Tumors contain many components in addition to the cancer cells, including blood vessels, fibroblasts, and immune cells. Genetic studies and tumor biopsies have generated insights into the importance of these stromal components for cancer progression. However, it remains a challenge to reveal the dynamic interactions among the distinct tumor components within live animals. Studies involving multiphoton microscopy allow direct imaging of cellular movement in live mice, but multiphoton microscopy is expensive, complex, and usually relies on a single excitation wavelength for all fluorophores. This article describes a method for intravital imaging using a microlensed spinning-disk confocal microscope. Although tissue penetration with spinning-disk confocal microscopy is lower than with multiphoton microscopy, image acquisition with this method is very rapid, so artifacts from respiratory motion are avoided. Photobleaching and phototoxicity are low, and multicolor acquisition is cheaper and easier than with multiphoton microscopy. This article discusses various aspects of experimental setup, as well as methods for addressing technical barriers, such as generating and working with multiple tumor microenvironments within individual live mice, image collection, and long-term anesthesia.

Multimodality imaging of abnormal vascular perfusion and morphology in preclinical 9L gliosarcoma model

BACKGROUND

This study demonstrates that a dynamic susceptibility contrast-magnetic resonance imaging (DSC-MRI) perfusion parameter may indicate vascular abnormality in a brain tumor model and reflects an effect of dexamethasone treatment. In addition, X-ray computed tomography (CT) measurements of vascular tortuosity and tissue markers of vascular morphology were performed to investigate the underpinnings of tumor response to dexamethasone.

METHODOLOGY/PRINCIPAL FINDINGS

One cohort of Fisher 344 rats (N = 13), inoculated intracerebrally with 9L gliosarcoma cells, was treated with dexamethasone (i.p. 3 mg/kg/day) for five consecutive days, and another cohort (N = 11) was treated with equal volume of saline. Longitudinal DSC-MRI studies were performed at the first (baseline), third and fifth day of treatments. Relative cerebral blood volume (rCBV) was significantly reduced on the third day of dexamethasone treatment (0.65 ± .13) as compared to the fifth day during treatment (1.26 ±.19, p < 0.05). In saline treated rats, relative CBV gradually increased during treatment (0.89 ±.13, 1.00 ± .21, 1.13 ± .23) with no significant difference on the third day of treatment (p>0.05). In separate serial studies, microfocal X-ray CT of ex vivo brain specimens (N = 9) and immunohistochemistry for endothelial cell marker anti-CD31 (N = 8) were performed. Vascular morphology of ex vivo rat brains from micro-CT analysis showed hypervascular characteristics in tumors, and both vessel density (41.32 ± 2.34 branches/mm(3), p<0.001) and vessel tortuosity (p<0.05) were significantly reduced in tumors of rats treated with dexamethasone compared to saline (74.29 ± 3.51 branches/mm(3)). The vascular architecture of rat brain tissue was examined with anti-CD31 antibody, and dexamethasone treated tumor regions showed reduced vessel area (16.45 ± 1.36 µm(2)) as compared to saline treated tumor regions (30.83 ± 4.31 µm(2), p<0.001) and non-tumor regions (22.80 ± 1.11 µm(2), p<0.01).

CONCLUSIONS/SIGNIFICANCE

Increased vascular density and tortuosity are culprit to abnormal perfusion, which is transiently reduced during dexamethasone treatment.

Multistage nanoparticle delivery system for deep penetration into tumor tissue

Current Food and Drug Administration-approved cancer nanotherapeutics, which passively accumulate around leaky regions of the tumor vasculature because of an enhanced permeation and retention (EPR) effect, have provided only modest survival benefits. This suboptimal outcome is likely due to physiological barriers that hinder delivery of the nanotherapeutics throughout the tumor. Many of these nanotherapeutics are ≈ 100 nm in diameter and exhibit enhanced accumulation around the leaky regions of the tumor vasculature, but their large size hinders penetration into the dense collagen matrix. Therefore, we propose a multistage system in which 100-nm nanoparticles "shrink" to 10-nm nanoparticles after they extravasate from leaky regions of the tumor vasculature and are exposed to the tumor microenvironment. The shrunken nanoparticles can more readily diffuse throughout the tumor's interstitial space. This size change is triggered by proteases that are highly expressed in the tumor microenvironment such as MMP-2, which degrade the cores of 100-nm gelatin nanoparticles, releasing smaller 10-nm nanoparticles from their surface. We used quantum dots (QD) as a model system for the 10-nm particles because their fluorescence can be used to demonstrate the validity of our approach. In vitro MMP-2 activation of the multistage nanoparticles revealed that the size change was efficient and effective in the enhancement of diffusive transport. In vivo circulation half-life and intratumoral diffusion measurements indicate that our multistage nanoparticles exhibited both the long circulation half-life necessary for the EPR effect and the deep tumor penetration required for delivery into the tumor's dense collagen matrix.

Scaling rules for diffusive drug delivery in tumor and normal tissues

Delivery of blood-borne molecules and nanoparticles from the vasculature to cells in the tissue differs dramatically between tumor and normal tissues due to differences in their vascular architectures. Here we show that two simple measures of vascular geometry--δ(max) and λ--readily obtained from vascular images, capture these differences and link vascular structure to delivery in both tissue types. The longest time needed to bring materials to their destination scales with the square of δ(max), the maximum distance in the tissue from the nearest blood vessel, whereas λ, a measure of the shape of the spaces between vessels, determines the rate of delivery for shorter times. Our results are useful for evaluating how new therapeutic agents that inhibit or stimulate vascular growth alter the functional efficiency of the vasculature and more broadly for analysis of diffusion in irregularly shaped domains.

Intravital microscopy as a tool to study drug delivery in preclinical studies

The technical developments in the field of non-linear microscopy have made intravital microscopy one of the most successful techniques for studying physiological and pathological processes in live animals. Intravital microscopy has been utilized to address many biological questions in basic research and is now a fundamental tool for preclinical studies, with an enormous potential for clinical applications. The ability to dynamically image cellular and subcellular structures combined with the possibility to perform longitudinal studies have empowered investigators to use this discipline to study the mechanisms of action of therapeutic agents and assess the efficacy on their targets in vivo. The goal of this review is to provide a general overview of the recent advances in intravital microscopy and to discuss some of its applications in preclinical studies.

Whole animal imaging

Translational research plays a vital role in understanding the underlying pathophysiology of human diseases, and hence development of new diagnostic and therapeutic options for their management. After creating an animal disease model, pathophysiologic changes and effects of a therapeutic intervention on them are often evaluated on the animals using immunohistologic or imaging techniques. In contrast to the immunohistologic techniques, the imaging techniques are noninvasive and hence can be used to investigate the whole animal, oftentimes in a single exam which provides opportunities to perform longitudinal studies and dynamic imaging of the same subject, and hence minimizes the experimental variability, requirement for the number of animals, and the time to perform a given experiment. Whole animal imaging can be performed by a number of techniques including x-ray computed tomography, magnetic resonance imaging, ultrasound imaging, positron emission tomography, single photon emission computed tomography, fluorescence imaging, and bioluminescence imaging, among others. Individual imaging techniques provide different kinds of information regarding the structure, metabolism, and physiology of the animal. Each technique has its own strengths and weaknesses, and none serves every purpose of image acquisition from all regions of an animal. In this review, a broad overview of basic principles, available contrast mechanisms, applications, challenges, and future prospects of many imaging techniques employed for whole animal imaging is provided. Our main goal is to briefly describe the current state of art to researchers and advanced students with a strong background in the field of animal research.

Peripheral nerve pericytes modify the blood-nerve barrier function and tight junctional molecules through the secretion of various soluble factors

The objectives of this study were to establish pure blood-nerve barrier (BNB) and blood-brain barrier (BBB)-derived pericyte cell lines of human origin and to investigate their unique properties as barrier-forming cells. Brain and peripheral nerve pericyte cell lines were established via transfection with retrovirus vectors incorporating human temperature-sensitive SV40 T antigen (tsA58) and telomerase. These cell lines expressed several pericyte markers such as α-smooth muscle actin, NG2, platelet-derived growth factor receptor β, whereas they did not express endothelial cell markers such as vWF and PECAM. In addition, the inulin clearance was significantly lowered in peripheral nerve microvascular endothelial cells (PnMECs) through the up-regulation of claudin-5 by soluble factors released from brain or peripheral nerve pericytes. In particular, bFGF secreted from peripheral nerve pericytes strengthened the barrier function of the BNB by increasing the expression of claudin-5. Peripheral nerve pericytes may regulate the barrier function of the BNB, because the BNB does not contain cells equivalent to astrocytes which regulate the BBB function. Furthermore, these cell lines expressed several neurotrophic factors such as NGF, BDNF, and GDNF. The secretion of these growth factors from peripheral nerve pericytes might facilitate axonal regeneration in peripheral neuropathy. Investigation of the characteristics of peripheral nerve pericytes may provide novel strategies for modifying BNB functions and promoting peripheral nerve regeneration.

Intravital imaging of anti-tumor immune response and the tumor microenvironment

Tumor growth, invasiveness, and metastasis are dynamic processes involving cancer interactions with the extracellular matrix, the vasculature, and various types of non-cancerous host cells that form the tumor stroma. An often-present stromal component is the immune cells, such as tumor-associated myeloid and lymphocytic infiltrates, yet endogenous anti-tumor immune responses are typically ineffective in tumor rejection and may even contribute to the progression of some cancers. How exactly cancer cells interact with the stroma and invade healthy tissues while avoiding anti-tumor immune responses, and which interactions should be targeted for anti-tumor therapy, can now be studied by minimally invasive observation using multiphoton and other low impact confocal microscopy techniques and fluorescent animal tumor models. Intravital video microscopy has already been instrumental in defining the roles and modes of cellular motility in the angiogenic process and during tissue invasion at the tumor margin. In the hands of cancer immunologists, intravital video microscopy is beginning to unravel the complexity of effector and suppressory lymphocytic interactions in tumors and in the draining lymphoid organs. As the intravital microscopy approach is beginning to move beyond fundamental description and into analyzing the molecular underpinnings of cell's dynamics, future technical advances will undoubtedly provide yet deeper insight while stitching together a systems dynamics view of cancer-host interactions that will keep on inspiring cancer researchers and therapists.

Evaluation of magnetic resonance vessel size imaging by two-photon laser scanning microscopy

MR vessel size imaging (MR-VSI) is increasingly applied to noninvasively assess microvascular properties of tumors and to evaluate tumor response to antiangiogenic treatment. MR-VSI provides measures for the microvessel radius and fractional blood volume of tumor tissue. However, data have not yet been evaluated with three-dimensional microscopy techniques. Therefore, three-dimensional two-photon laser scanning microscopy (TPLSM) was performed to assess microvascular radius and fractional vessel volume in tumor and muscle tissue. TPLSM data displayed a mazelike architecture of the tumor microvasculature and mainly parallel oriented muscle microvessels. For both MR-VSI and TPLSM, a larger vessel radius and fractional blood volume were found in the tumor rim than in the core. The microvessel radius was approximately six times larger in tumor and muscle for MR-VSI than for TPLSM. The tumor blood volume was 4-fold lower with MR-VSI than with TPLSM, whereas muscle blood volume was comparable for both techniques. Differences between the tumor rim, core, and muscle tissue showed similar trends for both MR-VSI and TPLSM parameters. These results indicate that MR-VSI does not provide absolute measures of microvascular morphology; however, it does reflect heterogeneity in microvascular morphology. Hence, MR-VSI may be used to assess differences in microvascular morphology.

Breast cancer cells attract the migration of adipose tissue-derived stem cells via the PDGF-BB/PDGFR-beta signaling pathway

The origin of vascular cells in tumors is unknown, but it is believed that tumors use cells from the host to build new vessels. To determine whether adipose tissue stem cells (ASCs) could be attracted by cancer cells, we performed migration assays in which ASCs were seeded on a transwell migration system top chamber and tumor-conditioned medium was placed in the bottom chamber. Our data showed that a significant number of ASCs migrated toward the tumor-conditioned medium (p<0.0001), and migration of human ASCs significantly (p<0.0001) increased in response to increased concentrations of recombinant PDGF-BB. In addition, neutralizing antibodies to PDGF receptor (PDGFR)-beta decreased migration of ASCs toward a breast cancer-conditioned medium to the level of serum-free control. These data suggest that tumor cell-derived PDGF-BB is an important factor in governing the microenvironment interaction between tumor cells and local tissue-resident stem cells.

In vivo imaging of tumors

INTRODUCTION

Light microscopy of tumors, as for other thick, scattering tissues such as the brain or the developing embryo, is limited by light penetration and optical access. because of these problems, epifluorescence and confocal microscopy are typically limited to the outer 50-100 microm of the accessible tumor tissue. Most mouse tumors must be exteriorized for examination under the light microscope, a procedure that limits the duration and repeatability of imaging. this protocol describes the generation of chronic window preparations in the mouse. These preparations allow an implanted tumor to grow for several weeks in an optically accessible location in vivo, making it possible to examine the living tumor with high-resolution light microscopy in a repetitive manner. Two chronic window preparations are described: (1) the dorsal skinfold chamber, which allows in vivo imaging of tumors growing in the subcutaneous space, and (2) the cranial window, which allows in vivo imaging of tumors growing on the brain surface.

Tumor microvasculature and microenvironment: novel insights through intravital imaging in pre-clinical models

Intravital imaging techniques have provided unprecedented insight into tumor microcirculation and microenvironment. For example, these techniques allowed quantitative evaluations of tumor blood vasculature to uncover its abnormal organization, structure and function (e.g., hyper-permeability, heterogeneous and compromised blood flow). Similarly, imaging of functional lymphatics has documented their absence inside tumors. These abnormalities result in elevated interstitial fluid pressure and hinder the delivery of therapeutic agents to tumors. In addition, they induce a hostile microenvironment characterized by hypoxia and acidosis, as documented by intravital imaging. The abnormal microenvironment further lowers the effectiveness of anti-tumor treatments such as radiation therapy and chemotherapy. In addition to these mechanistic insights, intravital imaging may also offer new opportunities to improve therapy. For example, tumor angiogenesis results in immature, dysfunctional vessels--primarily caused by an imbalance in production of pro- and anti-angiogenic factors by the tumors. Restoring the balance of pro- and anti-angiogenic signaling in tumors can "normalize" tumor vasculature and thus, improve its function, as demonstrated by intravital imaging studies in preclinical models and in cancer patients. Administration of cytotoxic therapy during periods of vascular normalization has the potential to enhance treatment efficacy.

Simultaneous measurement of RBC velocity, flux, hematocrit and shear rate in vascular networks

Not all tumor vessels are equal. Tumor-associated vasculature includes immature vessels, regressing vessels, transport vessels undergoing arteriogenesis and peritumor vessels influenced by tumor growth factors. Current techniques for analyzing tumor blood flow do not discriminate between vessel subtypes and only measure average changes from a population of dissimilar vessels. We have developed methodologies for simultaneously quantifying blood flow (velocity, flux, hematocrit and shear rate) in extended networks at single capillary resolution in vivo. Our approach relies on deconvolution of signals produced by labeled red blood cells as they move relative to the scanning laser of a confocal or multiphoton microscope and provides fully-resolved three-dimensional flow profiles within vessel networks. Using this methodology, we show that blood velocity profiles are asymmetric near intussusceptive tissue structures in tumors in mice. Furthermore, we show that subpopulations of vessels, classified by functional parameters, exist in, around a tumor and in normal brain.

Guidelines for the welfare and use of animals in cancer research

Animal experiments remain essential to understand the fundamental mechanisms underpinning malignancy and to discover improved methods to prevent, diagnose and treat cancer. Excellent standards of animal care are fully consistent with the conduct of high quality cancer research. Here we provide updated guidelines on the welfare and use of animals in cancer research. All experiments should incorporate the 3Rs: replacement, reduction and refinement. Focusing on animal welfare, we present recommendations on all aspects of cancer research, including: study design, statistics and pilot studies; choice of tumour models (e.g., genetically engineered, orthotopic and metastatic); therapy (including drugs and radiation); imaging (covering techniques, anaesthesia and restraint); humane endpoints (including tumour burden and site); and publication of best practice.