Cranial imaging window implantation technique for longitudinal multimodal imaging of the brain environment in live mice.

Intravital two-photon microscopy of the mouse brain requires visual access without affecting normal cognitive functions, which is crucial for longitudinal imaging studies that may last several months. In this protocol, we describe the surgical implantation of a metal-free cranial imaging window, which can be used to perform two-photon microscopy and magnetic resonance imaging in the same animal. This multimodal imaging platform enables the investigation of dynamic processes in the central nervous system at a cellular and macroscopic level. For complete details on the use and execution of this protocol in the context of brain cancer, please refer to Zomer et al.1.

Multispectral fluorine-19 MRI enables longitudinal and noninvasive monitoring of tumor-associated macrophages.

High-grade gliomas, the most common and aggressive primary brain tumors, are characterized by a complex tumor microenvironment (TME). Among the immune cells infiltrating the glioma TME, tumor-associated microglia and macrophages (TAMs) constitute the major compartment. In patients with gliomas, increased TAM abundance is associated with more aggressive disease. Alterations in TAM phenotypes and functions have been reported in preclinical models of multiple cancers during tumor development and after therapeutic interventions, including radiotherapy and molecular targeted therapies. These findings indicate that it is crucial to evaluate TAM abundance and dynamics over time. Current techniques to quantify TAMs in patients rely mainly on histological staining of tumor biopsies. Although informative, these techniques require an invasive procedure to harvest the tissue sample and typically only result in a snapshot of a small region at a single point in time. Fluorine isotope 19 MRI (19F MRI) represents a powerful means to noninvasively and longitudinally monitor myeloid cells in pathological conditions by intravenously injecting perfluorocarbon-containing nanoparticles (PFC-NP). In this study, we demonstrated the feasibility and power of 19F MRI in preclinical models of gliomagenesis, breast-to-brain metastasis, and breast cancer and showed that the major cellular source of 19F signal consists of TAMs. Moreover, multispectral 19F MRI with two different PFC-NP allowed us to identify spatially and temporally distinct TAM niches in radiotherapy-recurrent murine gliomas. Together, we have imaged TAMs noninvasively and longitudinally with integrated cellular, spatial, and temporal resolution, thus revealing important biological insights into the critical functions of TAMs, including in disease recurrence.

Cancer cell autophagy, reprogrammed macrophages, and remodeled vasculature in glioblastoma triggers tumor immunity.

Glioblastoma (GBM) is poorly responsive to therapy and invariably lethal. One conceivable strategy to circumvent this intractability is to co-target distinctive mechanistic components of the disease, aiming to concomitantly disrupt multiple capabilities required for tumor progression and therapeutic resistance. We assessed this concept by combining vascular endothelial growth factor (VEGF) pathway inhibitors that remodel the tumor vasculature with the tricyclic antidepressant imipramine, which enhances autophagy in GBM cancer cells and unexpectedly reprograms immunosuppressive tumor-associated macrophages via inhibition of histamine receptor signaling to become immunostimulatory. While neither drug is efficacious as monotherapy, the combination of imipramine with VEGF pathway inhibitors orchestrates the infiltration and activation of CD8 and CD4 T cells, producing significant therapeutic benefit in several GBM mouse models. Inclusion up front of immune-checkpoint blockade with anti-programmed death-ligand 1 (PD-L1) in eventually relapsing tumors markedly extends survival benefit. The results illustrate the potential of mechanism-guided therapeutic co-targeting of disparate biological vulnerabilities in the tumor microenvironment.

Multimodal imaging of the dynamic brain tumor microenvironment during glioblastoma progression and in response to treatment.

Tumors evolve in a dynamic communication with their native tissue environment and recruited immune cells. The diverse components of the tumor microenvironment (TME) can critically regulate tumor progression and therapeutic response. In turn, anticancer treatments may alter the composition and functions of the TME. To investigate this continuous dialog in the context of primary brain cancers, we developed a multimodal longitudinal imaging strategy. We combined macroscopical magnetic resonance imaging with subcellular resolution two-photon intravital microscopy, and leveraged the power of single-cell analysis tools to gain insights into the ongoing interactions between different components of the TME and cancer cells. Our experiments revealed that the migratory behavior of tumor-associated macrophages is different in genetically distinct glioblastomas, and in response to macrophage-targeted therapy. These results underscore the importance of studying cancer longitudinally in an in vivo setting, to reveal complex and dynamic alterations in the TME during disease progression and therapeutic intervention.

Cancer cells copy migratory behavior and exchange signaling networks via extracellular vesicles.

Recent data showed that cancer cells from different tumor subtypes with distinct metastatic potential influence each other's metastatic behavior by exchanging biomolecules through extracellular vesicles (EVs). However, it is debated how small amounts of cargo can mediate this effect, especially in tumors where all cells are from one subtype, and only subtle molecular differences drive metastatic heterogeneity. To study this, we have characterized the content of EVs shed in vivo by two clones of melanoma (B16) tumors with distinct metastatic potential. Using the Cre-LoxP system and intravital microscopy, we show that cells from these distinct clones phenocopy their migratory behavior through EV exchange. By tandem mass spectrometry and RNA sequencing, we show that EVs shed by these clones into the tumor microenvironment contain thousands of different proteins and RNAs, and many of these biomolecules are from interconnected signaling networks involved in cellular processes such as migration. Thus, EVs contain numerous proteins and RNAs and act on recipient cells by invoking a multi-faceted biological response including cell migration.

Glycosylated extracellular vesicles released by glioblastoma cells are decorated by CCL18 allowing for cellular uptake via chemokine receptor CCR8.

Cancer cells release extracellular vesicles (EVs) that contain functional biomolecules such as RNA and proteins. EVs are transferred to recipient cancer cells and can promote tumour progression and therapy resistance. Through RNAi screening, we identified a novel EV uptake mechanism involving a triple interaction between the chemokine receptor CCR8 on the cells, glycans exposed on EVs and the soluble ligand CCL18. This ligand acts as bridging molecule, connecting EVs to cancer cells. We show that glioblastoma EVs promote cell proliferation and resistance to the alkylating agent temozolomide (TMZ). Using in vitro and in vivo stem-like glioblastoma models, we demonstrate that EV-induced phenotypes are neutralised by a small molecule CCR8 inhibitor, R243. Interference with chemokine receptors may offer therapeutic opportunities against EV-mediated cross-talk in glioblastoma.

Quantifying exosome secretion from single cells reveals a modulatory role for GPCR signaling.

Exosomes are small endosome-derived extracellular vesicles implicated in cell-cell communication and are secreted by living cells when multivesicular bodies (MVBs) fuse with the plasma membrane (PM). Current techniques to study exosome physiology are based on isolation procedures after secretion, precluding direct and dynamic insight into the mechanics of exosome biogenesis and the regulation of their release. In this study, we propose real-time visualization of MVB-PM fusion to overcome these limitations. We designed tetraspanin-based pH-sensitive optical reporters that detect MVB-PM fusion using live total internal reflection fluorescence and dynamic correlative light-electron microscopy. Quantitative analysis demonstrates that MVB-PM fusion frequency is reduced by depleting the target membrane SNAREs SNAP23 and syntaxin-4 but also can be induced in single cells by stimulation of the histamine H1 receptor (H1HR). Interestingly, activation of H1R1 in HeLa cells increases Ser110 phosphorylation of SNAP23, promoting MVB-PM fusion and the release of CD63-enriched exosomes. Using this single-cell resolution approach, we highlight the modulatory dynamics of MVB exocytosis that will help to increase our understanding of exosome physiology and identify druggable targets in exosome-associated pathologies.

Identity and dynamics of mammary stem cells during branching morphogenesis.

During puberty, the mouse mammary gland develops into a highly branched epithelial network. Owing to the absence of exclusive stem cell markers, the location, multiplicity, dynamics and fate of mammary stem cells (MaSCs), which drive branching morphogenesis, are unknown. Here we show that morphogenesis is driven by proliferative terminal end buds that terminate or bifurcate with near equal probability, in a stochastic and time-invariant manner, leading to a heterogeneous epithelial network. We show that the majority of terminal end bud cells function as highly proliferative, lineage-committed MaSCs that are heterogeneous in their expression profile and short-term contribution to ductal extension. Yet, through cell rearrangements during terminal end bud bifurcation, each MaSC is able to contribute actively to long-term growth. Our study shows that the behaviour of MaSCs is not directly linked to a single expression profile. Instead, morphogenesis relies upon lineage-restricted heterogeneous MaSC populations that function as single equipotent pools in the long term.

Implications of Extracellular Vesicle Transfer on Cellular Heterogeneity in Cancer: What Are the Potential Clinical Ramifications?

The functional and phenotypic heterogeneity of tumor cells represents one of the greatest challenges in the successful treatment of cancer patients, because it increases the risk that certain individual tumor cells possess the ability to, for example, metastasize or to tolerate cytotoxic drugs. This heterogeneity in cellular behavior is driven by genetic and epigenetic changes and environmental differences. Recent studies suggest that an additional layer of complexity of tumor heterogeneity exists, based on the ability of cells to share functional biomolecules through local and systemic transfer of extracellular vesicles (EV), with profound effects on cellular behavior. The transfer of functional biomolecules between various populations of tumor cells and between tumor cells and nontumor cells has large consequences for both the tumor cells and the microenvironment that support the cellular behavior of tumor cells, and therefore for the clinical outcome of cancer. Here, we discuss the latest findings on EV transfer and the potential implications of EV-mediated local and systemic transmission of phenotypic behavior, particularly in the context of tumor heterogeneity, metastatic disease, and treatment response. Cancer Res; 76(8); 2071-5. ©2016 AACR.

Studying extracellular vesicle transfer by a Cre-loxP method.

Extracellular vesicle (EV) transfer is increasingly recognized as an important mode of intercellular communication by transferring a wide variety of biomolecules between cells. The characterization of in vitro- or ex vivo-isolated EVs has considerably contributed to the understanding of biological functions of EV transfer. However, the study of EV release and uptake in an in vivo setting has remained challenging, because cells that take up EVs could not be discriminated from cells that do not take up EVs. Recently, a technique based on the Cre-loxP system was developed to fluorescently mark Cre-reporter cells that take up EVs released by Cre recombinase-expressing cells in various in vitro and in vivo settings. Here we describe a detailed protocol for the generation of Cre(+) cells and reporter(+) cells, which takes ∼ 6 weeks, and subsequent assays with these lines to study functional EV transfer in in vitro and in vivo (mouse) settings, which take up to ∼ 2 months.

In Vivo imaging reveals extracellular vesicle-mediated phenocopying of metastatic behavior.

Most cancer cells release heterogeneous populations of extracellular vesicles (EVs) containing proteins, lipids, and nucleic acids. In vitro experiments showed that EV uptake can lead to transfer of functional mRNA and altered cellular behavior. However, similar in vivo experiments remain challenging because cells that take up EVs cannot be discriminated from non-EV-receiving cells. Here, we used the Cre-LoxP system to directly identify tumor cells that take up EVs in vivo. We show that EVs released by malignant tumor cells are taken up by less malignant tumor cells located within the same and within distant tumors and that these EVs carry mRNAs involved in migration and metastasis. By intravital imaging, we show that the less malignant tumor cells that take up EVs display enhanced migratory behavior and metastatic capacity. We postulate that tumor cells locally and systemically share molecules carried by EVs in vivo and that this affects cellular behavior.

Intestinal crypt homeostasis revealed at single-stem-cell level by in vivo live imaging.

The rapid turnover of the mammalian intestinal epithelium is supported by stem cells located around the base of the crypt. In addition to the Lgr5 marker, intestinal stem cells have been associated with other markers that are expressed heterogeneously within the crypt base region. Previous quantitative clonal fate analyses have led to the proposal that homeostasis occurs as the consequence of neutral competition between dividing stem cells. However, the short-term behaviour of individual Lgr5(+) cells positioned at different locations within the crypt base compartment has not been resolved. Here we establish the short-term dynamics of intestinal stem cells using the novel approach of continuous intravital imaging of Lgr5- Confetti mice. We find that Lgr5(+) cells in the upper part of the niche (termed 'border cells') can be passively displaced into the transit-amplifying domain, after the division of proximate cells, implying that the determination of stem-cell fate can be uncoupled from division. Through quantitative analysis of individual clonal lineages, we show that stem cells at the crypt base, termed 'central cells', experience a survival advantage over border stem cells. However, through the transfer of stem cells between the border and central regions, all Lgr5(+) cells are endowed with long-term self-renewal potential. These findings establish a novel paradigm for stem-cell maintenance in which a dynamically heterogeneous cell population is able to function long term as a single stem-cell pool.

Functional characterization of a STAT3-dependent dendritic cell-derived CD14+ cell population arising upon IL-10-driven maturation.

Interleukin (IL)-10 is a major cancer-related immunosuppressive factor, exhibiting a unique ability to hamper the maturation of dendritic cells (DCs). We have previously reported that IL-10 induces the conversion of activated, migratory CD1a+ DCs found in the human skin to CD14+CD141+ macrophage-like cells. Here, as a model of tumor-conditioned DC maturation, we functionally assessed CD14- and CD14+ DCs that matured in vitro upon exposure to IL-10. IL-10-induced CD14+ DCs were phenotypically characterized by a low maturation state as well as by high levels of BDCA3 and DC-SIGN, and as such they closely resembled CD14+ cells infiltrating melanoma metastases. Compared with DC matured under standard conditions, CD14+ DCs were found to express high levels of B7-H1 on the cell surface, to secrete low levels of IL-12p70, to preferentially induce TH2 cells, to have a lower allogeneic TH cell and tumor antigen-specific CD8+ T-cell priming capacity and to induce proliferative T-cell anergy. In contrast to their CD14+ counterparts, CD14- monocyte-derived DCs retained allogeneic TH priming capacity but induced a functionally anergic state as they completely abolished the release of effector cytokines. Transcriptional and cytokine release profiling studies indicated a more profound angiogenic and pro-invasive signature of CD14+ DCs as compared with DCs matured in standard conditions or CD14- DCs matured in the presence of IL-10. Importantly, signal transducer and activator of transcription 3 (STAT3) depletion by RNA interference prevented the development of the IL-10-associated CD14+ phenotype, allowing for normal DC maturation and providing a potential means of therapeutic intervention.

Intravital imaging of cancer stem cell plasticity in mammary tumors.

It is widely debated whether all tumor cells in mammary tumors have the same potential to propagate and maintain tumor growth or whether there is a hierarchical organization. Evidence for the latter theory is mainly based on the ability or failure of transplanted tumor cells to produce detectable tumors in mice with compromised immune systems; however, this assay has lately been disputed to accurately reflect cell behavior in unperturbed tumors. Lineage tracing experiments have recently shown the existence of a small population of cells, referred to as cancer stem cells (CSCs), that maintains and provides growth of squamous skin tumors and intestinal adenomas. However, the lineage tracing techniques used in these studies provide static images and lack the ability to study whether stem cell properties can be obtained or lost, a process referred to as stem cell plasticity. Here, by intravital lineage tracing, we report for the first time the existence of CSCs in unperturbed mammary tumors and demonstrate CSC plasticity. Our data indicate that existing CSCs disappear and new CSCs form during mammary tumor growth, illustrating the dynamic nature of these cells.

Intravital microscopy through an abdominal imaging window reveals a pre-micrometastasis stage during liver metastasis.

Cell dynamics in subcutaneous and breast tumors can be studied through conventional imaging windows with intravital microscopy. By contrast, visualization of the formation of metastasis has been hampered by the lack of long-term imaging windows for metastasis-prone organs, such as the liver. We developed an abdominal imaging window (AIW) to visualize distinct biological processes in the spleen, kidney, small intestine, pancreas, and liver. The AIW can be used to visualize processes for up to 1 month, as we demonstrate with islet cell transplantation. Furthermore, we have used the AIW to image the single steps of metastasis formation in the liver over the course of 14 days. We observed that single extravasated tumor cells proliferated to form "pre-micrometastases," in which cells lacked contact with neighboring tumor cells and were active and motile within the confined region of the growing clone. The clones then condensed into micrometastases where cell migration was strongly diminished but proliferation continued. Moreover, the metastatic load was reduced by suppressing tumor cell migration in the pre-micrometastases. We suggest that tumor cell migration within pre-micrometastases is a contributing step that can be targeted therapeutically during liver metastasis formation.

Exosomes: Fit to deliver small RNA.

Exosomes are specialized membranous nano-sized vesicles derived from endocytic compartments that are released by many cell types. Microvesicles are distinctive from exosomes in that they are produced by shedding of the plasmamembrane and usually larger in size (>1 µm). Exosome biogenesis involves the tightly controlled process of inward budding from the limiting membrane of multivesicular bodies (MVBs). This results in numerous intraluminal vesicles in the lumen of MVBs that contain distinct protein repertoires. It has been suggested that microvesicles shed by certain tumor cells hold functional messenger RNA (mRNA) that may promote tumor progression. We discovered that purified exosomes contain functional microRNAs (miRNAs) and small RNA, but detected little mRNA. Although a clear and decisive distinction between microvesicles and exosomes cannot be made and different subsets of exosomes exist, we speculate that exosomes are specialized in carrying small RNA including the class 22-25 nucleotide regulatory miRNAs. To demonstrate this we developed a co-culture system and found that exosomes are continuously secreted and transferred from Epstein Barr virus (EBV)-infected cells to uninfected neighboring cells. Throughout exosome transfer, the exogenous EBV-encoded miRNAs were delivered to subcellular sites of miRNA-mediated gene repression. Additionally, we found evidence that mature miRNAs are transferred between circulating cells in humans, since we detected EBV-miRNAs in non-infected cells in the peripheral blood of patients that include monocytes and T cells. In this addendum we discuss these findings in the context of recently published papers that advanced our current knowledge of exosome physiology, (mi)RNA function and intercellular RNA transfer. Based on this information we propose that an intercellular (miRNA-based) mode of signal transmission may be well suited in controlling space-confined processes such as the initiation of immune responses in the secondary (peripheral) lymphoid tissues or in a tumor microenvironment. Deciphering the molecular mechanism(s) that control small RNA loading into exosomes and transfer to recipient cells in vitro will provide new evidence for the physiological relevance of vesicle-mediated intercellular communication in vivo.