Cellular expression of green fluorescent protein, coupled with high-resolution in vivo videomicroscopy, to monitor steps in tumor metastasis

Real-Time Multiphoton Intravital Microscopy of Drug Extravasation in Tumours during Acoustic Cluster Therapy

Optimising drug delivery to tumours remains an obstacle to effective cancer treatment. A prerequisite for successful chemotherapy is that the drugs reach all tumour cells. The vascular network of tumours, extravasation across the capillary wall and penetration throughout the extracellular matrix limit the delivery of drugs. Ultrasound combined with microbubbles has been shown to improve the therapeutic response in preclinical and clinical studies. Most studies apply microbubbles designed as ultrasound contrast agents. Acoustic Cluster Therapy (ACT®) is a novel approach based on ultrasound-activated microbubbles, which have a diameter 5-10 times larger than regular contrast agent microbubbles. An advantage of using such large microbubbles is that they are in contact with a larger part of the capillary wall, and the oscillating microbubbles exert more effective biomechanical effects on the vessel wall. In accordance with this, ACT® has shown promising therapeutic results in combination with various drugs and drug-loaded nanoparticles. Knowledge of the mechanism and behaviour of drugs and microbubbles is needed to optimise ACT®. Real-time intravital microscopy (IVM) is a useful tool for such studies. This paper presents the experimental setup design for visualising ACT® microbubbles within the vasculature of tumours implanted in dorsal window (DW) chambers. It presents ultrasound setups, the integration and alignment of the ultrasound field with the optical system in live animal experiments, and the methodologies for visualisation and analysing the recordings. Dextran was used as a fluorescent marker to visualise the blood vessels and to trace drug extravasation and penetration into the extracellular matrix. The results reveal that the experimental setup successfully recorded the kinetics of extravasation and penetration distances into the extracellular matrix, offering a deeper understanding of ACT's mechanisms and potential in localised drug delivery.

IGFBP1 Sustains Cell Survival during Spatially-Confined Migration and Promotes Tumor Metastasis

Cell migration is a pivotal step in metastatic process, which requires cancer cells to navigate a complex spatially-confined environment, including tracks within blood vessels and in the vasculature of target organs. Here it is shown that during spatially-confined migration, the expression of insulin-like growth factor-binding protein 1 (IGFBP1) is upregulated in tumor cells. Secreted IGFBP1 inhibits AKT1-mediated phosphorylation of mitochondrial superoxide dismutase (SOD2) serine (S) 27 and enhances SOD2 activity. Enhanced SOD2 attenuates mitochondrial reactive oxygen species (ROS) accumulation in confined cells, which supports tumor cell survival in blood vessels of lung tissues, thereby accelerating tumor metastasis in mice. The levels of blood IGFBP1 correlate with metastatic recurrence of lung cancer patients. This finding reveals a unique mechanism by which IGFBP1 sustains cell survival during confined migration by enhancing mitochondrial ROS detoxification, thereby promoting tumor metastasis.

Intravital imaging to study cancer progression and metastasis

Navigation through the bulk tumour, entry into the blood vasculature, survival in the circulation, exit at distant sites and resumption of proliferation are all steps necessary for tumour cells to successfully metastasize. The ability of tumour cells to complete these steps is highly dependent on the timing and sequence of the interactions that these cells have with the tumour microenvironment (TME), including stromal cells, the extracellular matrix and soluble factors. The TME thus plays a major role in determining the overall metastatic phenotype of tumours. The complexity and cause-and-effect dynamics of the TME cannot currently be recapitulated in vitro or inferred from studies of fixed tissue, and are best studied in vivo, in real time and at single-cell resolution. Intravital imaging (IVI) offers these capabilities, and recent years have been a time of immense growth and innovation in the field. Here we review some of the recent advances in IVI of mammalian models of cancer and describe how IVI is being used to understand cancer progression and metastasis, and to develop novel treatments and therapies. We describe new techniques that allow access to a range of tissue and cancer types, novel fluorescent reporters and biosensors that allow fate mapping and the probing of functional and phenotypic states, and the clinical applications that have arisen from applying these techniques, reporters and biosensors to study cancer. We finish by presenting some of the challenges that remain in the field, how to address them and future perspectives.

Intravital microscopy for real-time monitoring of drug delivery and nanobiological processes

Intravital microscopy (IVM) expands our understanding of cellular and molecular processes, with applications ranging from fundamental biology to (patho)physiology and immunology, as well as from drug delivery to drug processing and drug efficacy testing. In this review, we highlight modalities, methods and model organisms that make up today's IVM landscape, and we present how IVM - via its high spatiotemporal resolution - enables analysis of metabolites, small molecules, nanoparticles, immune cells, and the (tumor) tissue microenvironment. We furthermore present examples of how IVM facilitates the elucidation of nanomedicine kinetics and targeting mechanisms, as well as of biological processes such as immune cell death, host-pathogen interactions, metabolic states, and disease progression. We conclude by discussing the prospects of IVM clinical translation and examining the integration of machine learning in future IVM practice.

Primary tumor associated macrophages activate programs of invasion and dormancy in disseminating tumor cells

Metastases are initiated by disseminated tumor cells (DTCs) that colonize distant organs. Growing evidence suggests that the microenvironment of the primary tumor primes DTCs for dormant or proliferative fates. However, the manner in which this occurs remains poorly understood. Here, using the Window for High-Resolution Intravital Imaging of the Lung (WHRIL), we study the live lung longitudinally and follow the fate of individual DTCs that spontaneously disseminate from orthotopic breast tumors. We find that spontaneously DTCs have increased levels of retention, increased speed of extravasation, and greater survival after extravasation, compared to experimentally metastasized tumor cells. Detailed analysis reveals that a subset of macrophages within the primary tumor induces a pro-dissemination and pro-dormancy DTC phenotype. Our work provides insight into how specific primary tumor microenvironments prime a subpopulation of cells for expression of proteins associated with dissemination and dormancy.

Engineering the early bone metastatic niche through human vascularized immuno bone minitissues

Bone metastases occur in 65%-80% advanced breast cancer patients. Although significant progresses have been made in understanding the biological mechanisms driving the bone metastatic cascade, traditional 2Din vitromodels and animal studies are not effectively reproducing breast cancer cells (CCs) interactions with the bone microenvironment and suffer from species-specific differences, respectively. Moreover, simplifiedin vitromodels cannot realistically estimate drug anti-tumoral properties and side effects, hence leading to pre-clinical testing frequent failures. To solve this issue, a 3D metastatic bone minitissue (MBm) is designed with embedded human osteoblasts, osteoclasts, bone-resident macrophages, endothelial cells and breast CCs. This minitissue recapitulates key features of the bone metastatic niche, including the alteration of macrophage polarization and microvascular architecture, along with the induction of CC micrometastases and osteomimicry. The minitissue reflects breast CC organ-specific metastatization to bone compared to a muscle minitissue. Finally, two FDA approved drugs, doxorubicin and rapamycin, have been tested showing that the dose required to impair CC growth is significantly higher in the MBm compared to a simpler CC monoculture minitissue. The MBm allows the investigation of metastasis key biological features and represents a reliable tool to better predict drug effects on the metastatic bone microenvironment.

Biomimetic Microfluidic Platforms for the Assessment of Breast Cancer Metastasis

Of around half a million women dying of breast cancer each year, more than 90% die due to metastasis. Models necessary to understand the metastatic process, particularly breast cancer cell extravasation and colonization, are currently limited and urgently needed to develop therapeutic interventions necessary to prevent breast cancer metastasis. Microfluidic approaches aim to reconstitute functional units of organs that cannot be modeled easily in traditional cell culture or animal studies by reproducing vascular networks and parenchyma on a chip in a three-dimensional, physiologically relevant in vitro system. In recent years, microfluidics models utilizing innovative biomaterials and micro-engineering technologies have shown great potential in our effort of mechanistic understanding of the breast cancer metastasis cascade by providing 3D constructs that can mimic in vivo cellular microenvironment and the ability to visualize and monitor cellular interactions in real-time. In this review, we will provide readers with a detailed discussion on the application of the most up-to-date, state-of-the-art microfluidics-based breast cancer models, with a special focus on their application in the engineering approaches to recapitulate the metastasis process, including invasion, intravasation, extravasation, breast cancer metastasis organotropism, and metastasis niche formation.

Immunological Regulation of Vascular Inflammation During Cancer Metastasis

Metastasis is the predominant cause of cancer-related mortality, despite being a highly inefficient process overall. The vasculature is the gatekeeper for tumor cell seeding within the secondary tissue microenvironment—the rate limiting step of the metastatic cascade. Therefore, factors that regulate vascular physiology dramatically influence cancer outcomes. There are a myriad of physiologic circumstances that not only influence the intrinsic capacity of tumor cells to cross the endothelial barrier, but also that regulate vascular inflammation and barrier integrity to enable extravasation into the metastatic niche. These processes are highly dependent on inflammatory cues largely initiated by the innate immune compartment, that are meant to help re-establish tissue homeostasis, but instead become hijacked by cancer cells. Here, we discuss the scientific advances in understanding the interactions between innate immune cells and the endothelium, describe their influence on cancer metastasis, and evaluate potential therapeutic interventions for the alleviation of metastatic disease. By triangulating the relationship between immune cells, endothelial cells, and tumor cells, we will gain greater insight into how to impede the metastatic process by focusing on its most vulnerable phases, thereby reducing metastatic spread and cancer-related mortality.

Development of an effective protein-labeling system based on smart fluorogenic probes

Proteins are an important component of living systems and play a crucial role in various physiological functions. Fluorescence imaging of proteins is a powerful tool for monitoring protein dynamics. Fluorescent protein (FP)-based labeling methods are frequently used to monitor the movement and interaction of cellular proteins. However, alternative methods have also been developed that allow the use of synthetic fluorescent probes to target a protein of interest (POI). Synthetic fluorescent probes have various advantages over FP-based labeling methods. They are smaller in size than the fluorescent proteins, offer a wide variety of colors and have improved photochemical properties. There are various chemical recognition-based labeling techniques that can be used for labeling a POI with a synthetic probe. In this review, we focus on the development of protein-labeling systems, particularly the SNAP-tag, BL-tag, and PYP-tag systems, and understanding the fluorescence behavior of the fluorescently labeled target protein in these systems. We also discuss the smart fluorogenic probes for these protein-labeling systems and their applications. The fluorogenic protein labeling will be a useful tool to investigate complex biological phenomena in future work on cell biology.

Real-Time Determination of the Cell-Cycle Position of Individual Cells within Live Tumors Using FUCCI Cell-Cycle Imaging

Most cytotoxic agents have limited efficacy for solid cancers. Cell-cycle phase analysis at the single-cell level in solid tumors has shown that the majority of cancer cells in tumors is not cycling and is therefore resistant to cytotoxic chemotherapy. Intravital cell-cycle imaging within tumors demonstrated the cell-cycle position and distribution of cancer cells within a tumor, and cell-cycle dynamics during chemotherapy. Understanding cell-cycle dynamics within tumors should provide important insights into novel treatment strategies.

Dissemination from a Solid Tumor: Examining the Multiple Parallel Pathways

Metastasis can be generalized as a linear sequence of events whereby halting one or more steps in the cascade may reduce tumor cell dissemination and ultimately improve patient outcomes. However, metastasis is a complex process with multiple parallel mechanisms of dissemination. Clinical strategies focus on removing the primary tumor and/or treating distant metastases through chemo- or immunotherapies. Successful strategies for blocking metastasis will need to address the parallel mechanisms of dissemination and identify common bottlenecks. Here, we review the current understanding of common dissemination pathways for tumors. Understanding the complexities of metastasis will guide the design of new therapies that halt dissemination.

MR imaging biomarkers for evaluating therapeutic effects shortly after near infrared photoimmunotherapy

Near infrared photoimmunotherapy (NIR-PIT) is a new cancer treatment that combines the specificity of antibodies for targeting tumors with the toxicity induced by photon absorbers after irradiation with NIR light. The purpose of this study was to determine if MR imaging can detect changes in the MR properties of tumor within several hours of NIR-PIT. A431 cells were injected subcutaneously in the right and left dorsi of 12 mice. Six days later, the mice were injected with a photon absorber, IR700, conjugated to panitumumab, an antibody targeting epidermal growth factor receptor. One day later, only right sided tumor was exposed to NIR light (treated tumor). MRI was performed 1 day before and 1-2 hours after NIR-PIT using gadofosveset for six mice and gadopentetate dimeglumine for another six mice. T2 relaxation times, the apparent diffusion coefficient (ADC) for the following combinations of b-values: 0-1000, 200-1000 and 500-1000 s/mm2 and enhancement indices were compared before and after NIR-PIT using a two-sided paired t-test. For treated tumors, T2 relaxation time increased after NIR-PIT (p < 0.01) and all three ADC values decreased after NIR-PIT (p < 0.01). Moreover, the enhancement area under the curve (AUC) using gadofosveset increased after NIR-PIT (p = 0.02). In conclusion, prolongation of T2, reductions in ADC and increased enhancement using gadofosveset are seen within 2 hours of NIR-PIT treatment of tumors. Thus, MRI can be a useful imaging biomarker for detecting early therapeutic changes after NIR-PIT.

Cancer cell motility: lessons from migration in confined spaces

Time-lapse, deep-tissue imaging made possible by advances in intravital microscopy has demonstrated the importance of tumour cell migration through confining tracks in vivo. These tracks may either be endogenous features of tissues or be created by tumour or tumour-associated cells. Importantly, migration mechanisms through confining microenvironments are not predicted by 2D migration assays. Engineered in vitro models have been used to delineate the mechanisms of cell motility through confining spaces encountered in vivo. Understanding cancer cell locomotion through physiologically relevant confining tracks could be useful in developing therapeutic strategies to combat metastasis.

Real-Time GFP Intravital Imaging of the Differences in Cellular and Angiogenic Behavior of Subcutaneous and Orthotopic Nude-Mouse Models of Human PC-3 Prostate Cancer

There are two major types of mouse xenograft models of cancer: subcutaneous implantation and orthotopic implantation. Subcutaneous transplant models are widely used with both cancer cell lines and human-tumor specimens. Recently, subcutaneous models of patient tumors, termed patient-derived xenographs (PDX) have become highly popular and have acquired such names as "Avatar" and "Xenopatients." However, such s.c. models rarely metastasize and are therefore not patient-like. In contrast, orthotopic models have the capability to metastasize. If intact fragments of tumor tissue are implanted by surgical orthotopic implantation (SOI), the metastatic potential can match that of the donor patient. The present study images in real time, using green fluorescent protein (GFP) expression, the very different tumor behavior at the orthotopic and subcutaneous sites of human prostate cancer PC-3 in athymic nude mice. By day-2 after tumor implantation, the orthotopic tumor is already highly vascularized and the cancer cells have begun to migrate out of the tumor. In contrast, the subcutaneous tumor only begins to be vascularized by day-3 and cells do not migrate from the tumor. Angiogenesis is much more extensive in the orthotopic tumor throughout the 2-week observation period. The orthotopic PC-3-GFP tumor progresses very rapidly and distinct metastasis have appeared in lymph nodes by day-3 which rapidly appear in many areas of the abdominal cavity including portal lymph nodes by day-7. At day-14, no invasion or metastasis was observed with the s.c. tumor even when the animal was extensively explored. These results explain why orthotopic tumors mimimc clinical metastatic tumors in nude mice and why subcutaneous tumors do not. J. Cell. Biochem. 117: 2546-2551, 2016. © 2016 Wiley Periodicals, Inc.

Use of fluorescent proteins and color-coded imaging to visualize cancer cells with different genetic properties

Fluorescent proteins are very bright and available in spectrally-distinct colors, enable the imaging of color-coded cancer cells growing in vivo and therefore the distinction of cancer cells with different genetic properties. Non-invasive and intravital imaging of cancer cells with fluorescent proteins allows the visualization of distinct genetic variants of cancer cells down to the cellular level in vivo. Cancer cells with increased or decreased ability to metastasize can be distinguished in vivo. Gene exchange in vivo which enables low metastatic cancer cells to convert to high metastatic can be color-coded imaged in vivo. Cancer stem-like and non-stem cells can be distinguished in vivo by color-coded imaging. These properties also demonstrate the vast superiority of imaging cancer cells in vivo with fluorescent proteins over photon counting of luciferase-labeled cancer cells.

Quantitative Analysis of Human Cancer Cell Extravasation Using Intravital Imaging

Metastasis, or the spread of cancer cells from a primary tumor to distant sites, is the leading cause of cancer-associated death. Metastasis is a complex multi-step process comprised of invasion, intravasation, survival in circulation, extravasation, and formation of metastatic colonies. Currently, in vitro assays are limited in their ability to investigate these intricate processes and do not faithfully reflect metastasis as it occurs in vivo. Traditional in vivo models of metastasis are limited by their ability to visualize the seemingly sporadic behavior of where and when cancer cells spread (Reymond et al., Nat Rev Cancer 13:858-870, 2013). The avian embryo model of metastasis is a powerful platform to study many of the critical steps in the metastatic cascade including the migration, extravasation, and invasion of human cancer cells in vivo (Sung et al., Nat Commun 6:7164, 2015; Leong et al., Cell Rep 8, 1558-1570, 2014; Kain et al., Dev Dyn 243:216-28, 2014; Leong et al., Nat Protoc 5:1406-17, 2010; Zijlstra et al., Cancer Cell 13:221-234, 2008; Palmer et al., J Vis Exp 51:2815, 2011). The chicken chorioallantoic membrane (CAM) is a readily accessible and well-vascularized tissue that surrounds the developing embryo. When the chicken embryo is grown in a shell-less, ex ovo environment, the nearly transparent CAM provides an ideal environment for high-resolution fluorescent microcopy approaches. In this model, the embryonic chicken vasculature and labeled cancer cells can be visualized simultaneously to investigate specific steps in the metastatic cascade including extravasation. When combined with the proper image analysis tools, the ex ovo chicken embryo model offers a cost-effective and high-throughput platform for the quantitative analysis of tumor cell metastasis in a physiologically relevant in vivo setting. Here we discuss detailed procedures to quantify cancer cell extravasation in the shell-less chicken embryo model with advanced fluorescence microscopy techniques.

Real Time Metastatic Route Tracking of Orthotopic PC-3-GFP Human Prostate Cancer Using Intravital Imaging

The cellular basis of metastasis is poorly understood. An important step to understanding this process is to be able to visualize the routes by which cancer cells migrate from the primary tumor to various distant sites to eventually form metastasis. Our laboratory previously developed single-cell in vivo imaging using fluorescent proteins to label cancer cells. In the present study, using PC-3 human prostate cancer cells labeled with green fluorescent protein (GFP) and orthotopic tumor transplantation, we have imaged in live mice various highly diverse routes by which PC-3 cells metastasize superiorly and inferiorly to distant sites, including in the portal area, stomach area, and urogenital system. Imaging began at day 9, at which time distant metastasis had already occurred, and increased at each imaging point at days 10, 13, 14, and 16. Metastatic cells were observed migrating superiorly and inferiorly from the primary tumor as well as in lymphatic channels and trafficking in various organ systems demonstrating that PC-3 has multiple metastatic routes similar to hormone-independent advanced-stage prostate cancer in the clinic.

RhoC and ROCKs regulate cancer cell interactions with endothelial cells

RhoC is a member of the Rho GTPase family that is implicated in cancer progression by stimulating cancer cell invasiveness. Here we report that RhoC regulates the interaction of cancer cells with vascular endothelial cells (ECs), a crucial step in the metastatic process. RhoC depletion by RNAi reduces PC3 prostate cancer cell adhesion to ECs, intercalation between ECs as well as transendothelial migration in vitro. Depletion of the kinases ROCK1 and ROCK2, two known RhoC downstream effectors, similarly decreases cancer interaction with ECs. RhoC also regulates the extension of protrusions made by cancer cells on vascular ECs in vivo. Transient RhoC depletion is sufficient to reduce both early PC3 cell retention in the lungs and experimental metastasis formation in vivo. Our results indicate RhoC plays a central role in cancer cell interaction with vascular ECs, which is a critical event for cancer progression.

Urokinase receptor promotes ovarian cancer cell dissemination through its 84-95 sequence

The clinical relevance of the urokinase receptor (uPAR) as a prognostic marker in ovarian cancer is well documented. We have shown that the uPAR sequence corresponding to 84-95 residues, linking D1 and D2 domains (uPAR_84-95), drives cell migration and angiogenesis in a protease-independent manner. This study is aimed at defining the contribution of uPAR_84-95 sequence to invasion of ovarian cancer cells. Now, we provide evidence that the ability of uPAR-expressing ovarian cancer cells to cross extra-cellular matrix and mesothelial monolayers is prevented by specific inhibitors of PAR_84-95 sequence. To specifically investigate uPAR_84-95 function, uPAR-negative CHO-K1 cells were stably transfected with cDNAs coding for uPAR D2 and D3 regions and exposing (uPARD2D3) or lacking (uPARΔD2D3) the 84–95 sequence. CHO-K1/D2D3 cells were able to cross matrigel, mesothelial and endothelial monolayers more efficiently than CHO-K1/ΔD2D3 cells, which behave as CHO-K1 control cells. When orthotopically implanted in nude mice, tumor nodules generated by CHO-K1/D2D3 cells spreading to peritoneal cavity were more numerous as compared to CHO-K1/ΔD2D3 cells. Ovarian tumor size and intra-tumoral microvessel density were significantly reduced in the absence of uPAR_84-95. Our results indicate that cell associated uPAR promotes growth and abdominal dissemination of ovarian cancer cells mainly through its uPAR_84-95 sequence.

Application of GFP imaging in cancer

Multicolored proteins have allowed the color-coding of cancer cells growing in vivo and enabled the distinction of host from tumor with single-cell resolution. Non-invasive imaging with fluorescent proteins enabled the dynamics of metastatic cancer to be followed in real time in individual animals. Non-invasive imaging of cancer cells expressing fluorescent proteins has allowed the real-time determination of efficacy of candidate antitumor and antimetastatic agents in mouse models. The use of fluorescent proteins to differentially label cancer cells in the nucleus and cytoplasm can visualize the nuclear-cytoplasmic dynamics of cancer cells in vivo including: mitosis, apoptosis, cell-cycle position, and differential behavior of nucleus and cytoplasm that occurs during cancer-cell deformation and extravasation. Recent applications of the technology described here include linking fluorescent proteins with cell-cycle-specific proteins such that the cells change color from red to green as they transit from G1 to S phases. With the macro- and micro-imaging technologies described here, essentially any in vivo process can be imaged, giving rise to the new field of in vivo cell biology using fluorescent proteins.

Advances and challenges in label-free nonlinear optical imaging using two-photon excitation fluorescence and second harmonic generation for cancer research

Nonlinear optical imaging (NLOI) has emerged to be a promising tool for bio-medical imaging in recent times. Among the various applications of NLOI, its utility is the most significant in the field of pre-clinical and clinical cancer research. This review begins by briefly covering the core principles involved in NLOI, such as two-photon excitation fluorescence (TPEF) and second harmonic generation (SHG). Subsequently, there is a short description on the various cellular components that contribute to endogenous optical fluorescence. Later on the review deals with its main theme--the challenges faced during label-free NLO imaging in translational cancer research. While this review addresses the accomplishment of various label-free NLOI based studies in cancer diagnostics, it also touches upon the limitations of the mentioned studies. In addition, areas in cancer research that need to be further investigated by label-free NLOI are discussed in a latter segment. The review eventually concludes on the note that label-free NLOI has and will continue to contribute richly in translational cancer research, to eventually provide a very reliable, yet minimally invasive cancer diagnostic tool for the patient.

Imaging preclinical tumour models: improving translational power

Recent developments and improvements of multimodal imaging methods for use in animal research have substantially strengthened the options of in vivo visualization of cancer-related processes over time. Moreover, technological developments in probe synthesis and labelling have resulted in imaging probes with the potential for basic research, as well as for translational and clinical applications. In addition, more sophisticated cancer models are available to address cancer-related research questions. This Review gives an overview of developments in these three fields, with a focus on imaging approaches in animal cancer models and how these can help the translation of new therapies into the clinic.

A microfluidic model for organ-specific extravasation of circulating tumor cells

Circulating tumor cells (CTCs) are the principal vehicle for the spread of non-hematologic cancer disease from a primary tumor, involving extravasation of CTCs across blood vessel walls, to form secondary tumors in remote organs. Herein, a polydimethylsiloxane-based microfluidic system is developed and characterized for in vitro systematic studies of organ-specific extravasation of CTCs. The system recapitulates the two major aspects of the in vivo extravasation microenvironment: local signaling chemokine gradients in a vessel with an endothelial monolayer. The parameters controlling the locally stable chemokine gradients, flow rate, and initial chemokine concentration are investigated experimentally and numerically. The microchannel surface treatment effect on the confluency and adhesion of the endothelial monolayer under applied shear flow has also been characterized experimentally. Further, the conditions for driving a suspension of CTCs through the microfluidic system are discussed while simultaneously maintaining both the local chemokine gradients and the confluent endothelial monolayer. Finally, the microfluidic system is utilized to demonstrate extravasation of MDA-MB-231 cancer cells in the presence of CXCL12 chemokine gradients. Consistent with the hypothesis of organ-specific extravasation, control experiments are presented to substantiate the observation that the MDA-MB-231 cell migration is attributed to chemotaxis rather than a random process.

New researches and application progress of commonly used optical molecular imaging technology

Optical molecular imaging, a new medical imaging technique, is developed based on genomics, proteomics and modern optical imaging technique, characterized by non-invasiveness, non-radiativity, high cost-effectiveness, high resolution, high sensitivity and simple operation in comparison with conventional imaging modalities. Currently, it has become one of the most widely used molecular imaging techniques and has been applied in gene expression regulation and activity detection, biological development and cytological detection, drug research and development, pathogenesis research, pharmaceutical effect evaluation and therapeutic effect evaluation, and so forth, This paper will review the latest researches and application progresses of commonly used optical molecular imaging techniques such as bioluminescence imaging and fluorescence molecular imaging.

Imaging metastatic cell trafficking at the cellular level in vivo with fluorescent proteins

Fluorescent proteins have revolutionized biology, allowing what was formerly invisible to be clearly seen. The Nobel Prize in Chemistry was awarded in 2008 for the discovery and early use of green fluorescent protein (GFP) as a genetic reporter. Our laboratory pioneered the use of GFP for in vivo imaging. In this chapter we review the developments within our research on subcellular imaging of metastatic trafficking of cancer cells carried out in real time in mice. Dual-color fluorescent cells, with one color fluorescent protein in the nucleus and another color fluorescent protein in the cytoplasm, enable real-time nuclear-cytoplasmic dynamics to be visualized in living cells in vivo as well as in vitro. In the dual-color cells, red fluorescent protein (RFP) is expressed in the cytoplasm of cancer cells, and GFP is linked to histone H2B and is expressed in the nucleus. Nuclear GFP expression enables visualization of nuclear dynamics, whereas simultaneous cytoplasmic RFP expression allows visualization of nuclear cytoplasmic ratios in addition to simultaneous cell and nuclear shape changes. With the use of dual-color fluorescent cells, it is possible to achieve subcellular real-time imaging of cancer cell trafficking in live mice. Extravasation can also be imaged in real time. Dual-color imaging has shown that cytoplasmic processes of cancer cells exit the vessels first, with nuclei following along the cytoplasmic projections [Yamauchi et al., Cancer Res 66:4208-4214, 2006]. Dual-color in vivo cellular imaging was used to visualize cancer cell trafficking blood vessels, as well as in the lymphatic systems of the mice. The real-time imaging of cancer cell seeding on the lung has now been achieved with dual-color cells. Subcellular in vivo imaging confers great promise for understanding metastasis at the cellular level in vivo.

A microfluidic 3D in vitro model for specificity of breast cancer metastasis to bone

Cancer metastases arise following extravasation of circulating tumor cells with certain tumors exhibiting high organ specificity. Here, we developed a 3D microfluidic model to analyze the specificity of human breast cancer metastases to bone, recreating a vascularized osteo-cell conditioned microenvironment with human osteo-differentiated bone marrow-derived mesenchymal stem cells and endothelial cells. The tri-culture system allowed us to study the transendothelial migration of highly metastatic breast cancer cells and to monitor their behavior within the bone-like matrix. Extravasation, quantified 24 h after cancer cell injection, was significantly higher in the osteo-cell conditioned microenvironment compared to collagen gel-only matrices (77.5 ± 3.7% vs. 37.6 ± 7.3%), and the migration distance was also significantly greater (50.8 ± 6.2 μm vs. 31.8 ± 5.0 μm). Extravasated cells proliferated to form micrometastases of various sizes containing 4 to more than 60 cells by day 5. We demonstrated that the breast cancer cell receptor CXCR2 and the bone-secreted chemokine CXCL5 play a major role in the extravasation process, influencing extravasation rate and traveled distance. Our study provides novel 3D in vitro quantitative data on extravasation and micrometastasis generation of breast cancer cells within a bone-like microenvironment and demonstrates the potential value of microfluidic systems to better understand cancer biology and screen for new therapeutics.

Advanced intravital subcellular imaging reveals vital three-dimensional signalling events driving cancer cell behaviour and drug responses in live tissue

The integration of signal transduction pathways plays a fundamental role in governing disease initiation, progression and outcome. It is therefore necessary to understand disease at the signalling level to enable effective treatment and to intervene in its progression. The recent extension of in vitro subcellular image-based analysis to live in vivo modelling of disease is providing a more complete picture of real-time, dynamic signalling processes or drug responses in live tissue. Intravital imaging offers alternative strategies for studying disease and embraces the biological complexities that govern disease progression. In the present review, we highlight how three-dimensional or live intravital imaging has uncovered novel insights into biological mechanisms or modes of drug action. Furthermore, we offer a prospective view of how imaging applications may be integrated further with the aim of understanding disease in a more physiological and functional manner within the framework of the drug discovery process.

Chemical biology-based approaches on fluorescent labeling of proteins in live cells

Recently, significant advances have been made in live cell imaging owing to the rapid development of selective labeling of proteins in vivo. Green fluorescent protein (GFP) was the first example of fluorescent reporters genetically introduced to protein of interest (POI). While GFP and various types of engineered fluorescent proteins (FPs) have been actively used for live cell imaging for many years, the size and the limited windows of fluorescent spectra of GFP and its variants set limits on possible applications. In order to complement FP-based labeling methods, alternative approaches that allow incorporation of synthetic fluorescent probes to target POIs were developed. Synthetic fluorescent probes are smaller than fluorescent proteins, often have improved photochemical properties, and offer a larger variety of colors. These synthetic probes can be introduced to POIs selectively by numerous approaches that can be largely categorized into chemical recognition-based labeling, which utilizes metal-chelating peptide tags and fluorophore-carrying metal complexes, and biological recognition-based labeling, such as (1) specific non-covalent binding between an enzyme tag and its fluorophore-carrying substrate, (2) self-modification of protein tags using substrate variants conjugated to fluorophores, (3) enzymatic reaction to generate a covalent binding between a small molecule substrate and a peptide tag, and (4) split-intein-based C-terminal labeling of target proteins. The chemical recognition-based labeling reaction often suffers from compromised selectivity of metal-ligand interaction in the cytosolic environment, consequently producing high background signals. Use of protein-substrate interactions or enzyme-mediated reactions generally shows improved specificity but each method has its limitations. Some examples are the presence of large linker protein, restriction on the choice of introducible probes due to the substrate specificity of enzymes, and competitive reaction mediated by an endogenous analogue of the introduced protein tag. These limitations have been addressed, in part, by the split-intein-based labeling approach, which introduces fluorescent probes with a minimal size (~4 amino acids) peptide tag. In this review, the advantages and the limitations of each labeling method are discussed.

Preclinical intravital microscopy of the tumour-stroma interface: invasion, metastasis, and therapy response

Key steps of cancer progression and therapy response depend upon interactions between cancer cells with the reactive tumour microenvironment. Intravital microscopy enables multi-modal and multi-scale monitoring of cancer progression as a dynamic step-wise process within anatomic and functional niches provided by the microenvironment. These niches deliver cell-derived and matrix-derived signals that enable cell subsets or single cancer cells to survive, migrate, grow, undergo dormancy, and escape immune surveillance. Beyond basic research, intravital microscopy has reached preclinical application to identify mechanisms of tumour-stroma interactions and outcome. We here summarise how n-dimensional 'dynamic histopathology' of tumours by intravital microscopy shapes mechanistic insight into cell-cell and cell-tissue interactions that underlie single-cell and collective cancer invasion, metastatic seeding at distant sites, immune evasion, and therapy responses.

Rebuilding cancer metastasis in the mouse

Most cancer deaths are due to the systemic dissemination of cancer cells and the formation of secondary tumors (metastasis) in distant organs. Recent years have brought impressive progress in metastasis research, yet we still lack sufficient insights into how cancer cells migrate out of primary tumors and invade into neighboring tissue, intravasate into the blood or the lymphatic circulation, survive in the blood stream, and target specific organs to initiate metastatic outgrowth. While a large number of cellular and animal models of cancer have been crucial in delineating the molecular mechanisms underlying tumor initiation and progression, experimental models that faithfully recapitulate the multiple stages of metastatic disease are still scarce. The advent of sophisticated genetic engineering in mice, in particular the ability to manipulate gene expression in specific tissue and at desired time points at will, have allowed to rebuild the metastatic process in mice. Here, we describe a selection of cellular experimental systems, tumor transplantation mouse models and genetically engineered mouse models that are used for monitoring specific processes involved in metastasis, such as cell migration and invasion, and for investigating the full metastatic process. Such models not only aid in deciphering the pathomechanisms of metastasis, but are also instrumental for the preclinical testing of anti-metastatic therapies and further refinement and generation of improved models.

In vitro model of tumor cell extravasation

Tumor cells that disseminate from the primary tumor and survive the vascular system can eventually extravasate across the endothelium to metastasize at a secondary site. In this study, we developed a microfluidic system to mimic tumor cell extravasation where cancer cells can transmigrate across an endothelial monolayer into a hydrogel that models the extracellular space. The experimental protocol is optimized to ensure the formation of an intact endothelium prior to the introduction of tumor cells and also to observe tumor cell extravasation by having a suitable tumor seeding density. Extravasation is observed for 38.8% of the tumor cells in contact with the endothelium within 1 day after their introduction. Permeability of the EC monolayer as measured by the diffusion of fluorescently-labeled dextran across the monolayer increased 3.8 fold 24 hours after introducing tumor cells, suggesting that the presence of tumor cells increases endothelial permeability. The percent of tumor cells extravasated remained nearly constant from1 to 3 days after tumor seeding, indicating extravasation in our system generally occurs within the first 24 hours of tumor cell contact with the endothelium.

Microenvironment and tumor cell plasticity: an easy way out

Cancer cells undergo genetic changes allowing their adaptation to environmental changes, thereby obtaining an advantage during the long metastatic route, disseminated of several changes in the surrounding environment. In particular, plasticity in cell motility, mainly due to epigenetic regulation of cancer cells by environmental insults, engage adaptive strategies aimed essentially to survive in hostile milieu, thereby escaping adverse sites. This review is focused on tumor microenvironment as a collection of structural and cellular elements promoting plasticity and adaptive programs. We analyze the role of extracellular matrix stiffness, hypoxia, nutrient deprivation, acidity, as well as different cell populations of tumor microenvironment.

Imaging and tracking of bone marrow-derived immune and stem cells

Bone marrow (BM)-derived stem and immune cells play critical roles in maintaining the health, regeneration, and repair of many tissues. Given their important functions in tissue regeneration and therapy, tracking the dynamic behaviors of BM-derived cells has been a long-standing research goal of both biologists and engineers. Because of the complex cellular-level processes involved, real-time imaging technologies that have sufficient spatial and temporal resolution to visualize them are needed. In addition, in order to track cellular dynamics, special attention is needed to account for changes in the microenvironment where the cells reside, for example, tissue contraction, stretching, development, etc. In this chapter, we introduce methods for real-time imaging and longitudinal tracking of BM-derived immune and stem cells in in vivo three-dimensional (3-D) tissue environments with an integrated optical microscope. The integrated microscope combines multiple imaging functions derived from optical coherence tomography (OCT) and multiphoton microscopy (MPM), including optical coherence microscopy (OCM), microvasculature imaging, two-photon excited fluorescence (TPEF), and second harmonic generation (SHG) microscopy. Short- and long-term tracking of the dynamic behavior of BM-derived cells involved in cutaneous wound healing and skin grafting in green fluorescent protein (GFP) BM-transplanted mice is demonstrated. Methods and algorithms for nonrigid registration of time-lapse images are introduced, which allows for long-term tracking of cell dynamics over several months.

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.

A molecular imaging primer: modalities, imaging agents, and applications

Molecular imaging is revolutionizing the way we study the inner workings of the human body, diagnose diseases, approach drug design, and assess therapies. The field as a whole is making possible the visualization of complex biochemical processes involved in normal physiology and disease states, in real time, in living cells, tissues, and intact subjects. In this review, we focus specifically on molecular imaging of intact living subjects. We provide a basic primer for those who are new to molecular imaging, and a resource for those involved in the field. We begin by describing classical molecular imaging techniques together with their key strengths and limitations, after which we introduce some of the latest emerging imaging modalities. We provide an overview of the main classes of molecular imaging agents (i.e., small molecules, peptides, aptamers, engineered proteins, and nanoparticles) and cite examples of how molecular imaging is being applied in oncology, neuroscience, cardiology, gene therapy, cell tracking, and theranostics (therapy combined with diagnostics). A step-by-step guide to answering biological and/or clinical questions using the tools of molecular imaging is also provided. We conclude by discussing the grand challenges of the field, its future directions, and enormous potential for further impacting how we approach research and medicine.

Do cancer cells undergo phenotypic switching? The case for imperfect cancer stem cell markers

The identification of cancer stem cells in vivo and in vitro relies on specific surface markers that should allow to sort cancer cells in phenotypically distinct subpopulations. Experiments report that sorted cancer cell populations after some time tend to express again all the original markers, leading to the hypothesis of phenotypic switching, according to which cancer cells can transform stochastically into cancer stem cells. Here we explore an alternative explanation based on the hypothesis that markers are not perfect and are thus unable to identify all cancer stem cells. Our analysis is based on a mathematical model for cancer cell proliferation that takes into account phenotypic switching, imperfect markers and error in the sorting process. Our conclusion is that the observation of reversible expression of surface markers after sorting does not provide sufficient evidence in support of phenotypic switching.

Rho GTPases and cancer cell transendothelial migration

Small Rho GTPases are major regulators of actin cytoskeleton dynamics and influence cell shape and migration. The expression of several Rho GTPases is often up-regulated in tumors and this frequently correlates with a poor prognosis for patients. Migration of cancer cells through endothelial cells that line the blood vessels, called transendothelial migration or extravasation, is a critical step during the metastasis process. The use of siRNA technology to target specifically each Rho family member coupled with imaging techniques allows the roles of individual Rho GTPases to be investigated. In this chapter we describe methods to assess how Rho GTPases affect the different steps of cancer cell transendothelial cell migration in vitro.

Subcellular imaging in vivo: the next GFP revolution

The use of fluorescent proteins to differentially label cancer cells in the nucleus and cytoplasm and high-powered imaging technology have been used to visualize the nuclear-cytoplasmic dynamics of cancer-cell in vivo. Nuclear-cytoplasmic dynamics have been imaged in cancer cells trafficking in both blood vessels and lymphatic vessels as well as during seeding on organs and interacting with stroma in the live animal. Fluorescent proteins have also been used to color code the phases of the cell cycle which can now be followed in vivo. This technology has furthered our understanding of the spread of cancer at the subcellular level. Fluorescent proteins thereby provide the basis for the new field of in vivo cell biology.

Assessing cancer cell migration and metastatic growth in vivo in the chick embryo using fluorescence intravital imaging

Cell migration and metastasis are key features of aggressive tumors. These processes can be difficult to study, as they often occur deep within the body of a cancer patient or an experimental animal. In vitro assays are able to model some aspects of these processes, and a number of assays have been developed to assess cancer cell motility, migration, and invasion. However, in vitro assays have inherent limitations that may miss important aspects of these processes as they occur in vivo. The chick embryo provides a powerful model for studying these processes in vivo, facilitated by the external and accessible nature of the chorioallantoic membrane (CAM), a well-vascularized tissue that surrounds the embryo. When coupled with multiple fluorescent approaches to labeling both cancer cells and the embryonic vasculature, along with image analysis tools, the chick CAM model offers cost-effective, rapid assays for studying cancer cell migration and metastasis in a physiologically-relevant, in vivo setting. Here, we present recent developments of detailed procedures for using shell-less chick embryos, coupled with fluorescent labeling of cancer cells and/or chick vasculature, to study cancer cell migration and metastasis in vivo.

Use of in vivo and in vitro systems to select Leishmania amazonensis expressing green fluorescent protein

Various Leishmania species were engineered with green fluorescent protein (GFP) using episomal vectors that encoded an antibiotic resistance gene, such as aminoglycoside geneticin sulphate (G418). Most reports of GFP-Leishmania have used the flagellated extracellular promastigote, the stage of parasite detected in the midgut of the sandfly vector; fewer studies have been performed with amastigotes, the stage of parasite detected in mammals. In this study, comparisons were made regarding the efficiency for in vitro G418 selection of GFP-Leishmania amazonensis promastigotes and amastigotes and the use of in vivo G418 selection. The GFP-promastigotes retained episomal plasmid for a prolonged period and G418 treatment was necessary and efficient for in vitro selection. In contrast, GFP-amastigotes showed low retention of the episomal plasmid in the absence of G418 selection and low sensitivity to antibiotics in vitro. The use of protocols for G418 selection using infected BALB/c mice also indicated low sensitivity to antibiotics against amastigotes in cutaneous lesions.

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.

Metastasis suppressor genes at the interface between the environment and tumor cell growth

The molecular mechanisms and genetic programs required for cancer metastasis are sometimes overlapping, but components are clearly distinct from those promoting growth of a primary tumor. Every sequential, rate-limiting step in the sequence of events leading to metastasis requires coordinated expression of multiple genes, necessary signaling events, and favorable environmental conditions or the ability to escape negative selection pressures. Metastasis suppressors are molecules that inhibit the process of metastasis without preventing growth of the primary tumor. The cellular processes regulated by metastasis suppressors are diverse and function at every step in the metastatic cascade. As we gain knowledge into the molecular mechanisms of metastasis suppressors and cofactors with which they interact, we learn more about the process, including appreciation that some are potential targets for therapy of metastasis, the most lethal aspect of cancer. Until now, metastasis suppressors have been described largely by their function. With greater appreciation of their biochemical mechanisms of action, the importance of context is increasingly recognized especially since tumor cells exist in myriad microenvironments. In this chapter, we assemble the evidence that selected molecules are indeed suppressors of metastasis, collate the data defining the biochemical mechanisms of action, and glean insights regarding how metastasis suppressors regulate tumor cell communication to-from microenvironments.

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.

Distant metastases do not metastasize

Distant metastases (MET) are for most solid cancers decisive life-threatening events. Data about MET-free survival and survival after MET show a strong dependency on the kind of cancer and the prognostic features. Nonetheless, within biological subgroups, the MET process is very homogenous. Therefore, the growth rate can be estimated from initiation of MET to MET diagnosis and to time of death. Based on the known volume doubling time of breast cancer, the time of the first possible dissemination can also be estimated. Important consequences of these MET-initiation estimates are the hypotheses that almost all MET are initiated before removal of the primary tumor and that MET do not metastasize in a clinically relevant magnitude. Although breast cancer data were primarily used to form these hypotheses, the discussed MET process can be generalized to all solid cancers. The impact of these hypotheses on diagnostic, curative and palliative treatment, aftercare, and especially on clinical research would be important.

Optical techniques for tracking multiple myeloma engraftment, growth, and response to therapy

Multiple myeloma (MM), the second most common hematological malignancy, initiates from a single site and spreads via circulation to multiple sites in the bone marrow (BM). Methods to track MM cells both in the BM and circulation would be useful for developing new therapeutic strategies to target MM cell spread. We describe the use of complementary optical techniques to track human MM cells expressing both bioluminescent and fluorescent reporters in a mouse xenograft model. Long-term tumor growth and response to therapy are monitored using bioluminescence imaging (BLI), while numbers of circulating tumor cells are detected by in-vivo flow cytometry. Intravital microscopy is used to detect early seeding of MM cells to the BM, as well as residual cancer cells that remain in the BM after the bulk of the tumor is eradicated following drug treatment. Thus, intravital microscopy provides a powerful, albeit invasive, means to study cellular processes in vivo at the very early stage of the disease process and at the very late stage of therapeutic intervention when the tumor burden is too small to be detected by other imaging methods.