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Background Glioblastomas (GBM) have highly infiltrative growth patterns that contribute to recurrence and poor survival. Despite infiltration being a critical therapeutic target, no clinically useful therapies exist which counter GBM invasion. Here, we report that inhibition of Ataxia telangiectasia and Rad 3 related kinase (ATR) reduces invasion of GBM cells through dysregulation of cytoskeletal networks and subsequent integrin trafficking. Methods GBM motility and invasion was assessed both in vitro and in vivo in response to ATR inhibition (ATRi) and ATR over-expression using timelapse microscopy, two orthotopic GBM models and intravital imaging. Disruption to cytoskeleton networks and endocytic processing were investigated via high throughput, super resolution imaging and intravital imaging. Results High ATR expression was associated with significantly poorer survival in clinical datasets whilst GBM tumour specimens revealed higher ATR expression at infiltrative margins. ATRi opposed invasion of GBM cells due cytoskeletal dysregulation impacting integrin internalisation. Conclusion We demonstrate a novel role for ATR in determining invasion in GBM cells, and propose that pharmacological targeting of ATR could have far reaching clinical benefits.

Supplementary Figures and Figure legends, Supplementary Video legends and supplementary materials and methods

Crossing the blood–brain barrier is a crucial, rate-limiting step of brain metastasis. Understanding of the mechanisms of cancer cell extravasation from brain microcapillaries is limited as the underlying cellular and molecular processes cannot be adequately investigated using in vitro models and endpoint in vivo experiments. Using ultrastructural and functional imaging, we demonstrate that dynamic changes of activated brain microcapillaries promote the mandatory first steps of brain colonization. Successful extravasation of arrested cancer cells occurred when adjacent capillary endothelial cells (EC) entered into a distinct remodeling process. After extravasation, capillary loops were formed, which was characteristic of aggressive metastatic growth. Upon cancer cell arrest in brain microcapillaries, matrix-metalloprotease 9 (MMP9) was expressed. Inhibition of MMP2/9 and genetic perturbation of MMP9 in cancer cells, but not the host, reduced EC projections, extravasation, and brain metastasis outgrowth. These findings establish an active role of ECs in the process of cancer cell extravasation, facilitated by cross-talk between the two cell types. This extends our understanding of how host cells can contribute to brain metastasis formation and how to prevent it.Significance:Tracking single extravasating cancer cells using multimodal correlative microscopy uncovers a brain seeding mechanism involving endothelial remodeling driven by cancer cell–derived MMP9, which might enable the development of approaches to prevent brain metastasis.See related commentary by McCarty, p. 1167

Specific biological properties of those circulating cancer cells that are the origin of brain metastases (BM) are not well understood. Here, single circulating breast cancer cells were fate-tracked during all steps of the brain metastatic cascade in mice after intracardial injection over weeks. A novel in vivo two-photon microscopy methodology was developed that allowed to determine the specific cellular and molecular features of breast cancer cells that homed in the brain, extravasated, and successfully established a brain macrometastasis. Those BM-initiating breast cancer cells (BMIC) were mainly originating from a slow-cycling subpopulation that included only 16% to 20% of all circulating cancer cells. BMICs showed enrichment of various markers of cellular stemness. As a proof of principle for the principal usefulness of this approach, expression profiling of BMICs versus non-BMICs was performed, which revealed upregulation of NDRG1 in the slow-cycling BMIC subpopulation in one BM model. Here, BM development was completely suppressed when NDRG1 expression was downregulated. In accordance, in primary human breast cancer, NDRG1 expression was heterogeneous, and high NDRG1 expression was associated with shorter metastasis-free survival. In conclusion, our data identify temporary slow-cycling breast cancer cells as the dominant source of brain and other metastases and demonstrates that this can lead to better understanding of BMIC-relevant pathways, including potential new approaches to prevent BM in patients.Implications:Cancer cells responsible for successful brain metastasis outgrowth are slow cycling and harbor stemness features. The molecular characteristics of these metastasis-initiating cells can be studied using intravital microscopy technology.

Supplementary Figure 3 shows PKH26 intensity and depth of the cell in relation to the successful establishment of a metastasis. Further, the brain metastatic cascade for PKH26 low (slow cycling) and PKH26 (high) (ultra slow cycling) cells is illustrated.

Supplementary Figure 4 shows IVIS results.

Supplementary Figure 6 shows NDRG1 expression in several cell lines.

Supplementary Figure 1 A-C shows loss of PKH26 over time. Supplementary Figure 1 D shows an additional example of a slow cycling cells with remaining PKH26 staining in vivo

Supplementary Figure 5 shows cell cycle analysis.

Supplementary Figure 2 cell index of PKH26 stained cells, the overlap of PKH26 loss and 2 other methods of cell division analysis as well as the reversion of a slow cycling cell population to a mixed cycling population.

Purpose: The role of blood–brain barrier (BBB) integrity for brain tumor biology and therapy is a matter of debate.Experimental Design: We developed a new experimental approach using in vivo two-photon imaging of mouse brain metastases originating from a melanoma cell line to investigate the growth kinetics of individual tumor cells in response to systemic delivery of two PI3K/mTOR inhibitors over time, and to study the impact of microregional vascular permeability. The two drugs are closely related but differ regarding a minor chemical modification that greatly increases brain penetration of one drug.Results: Both inhibitors demonstrated a comparable inhibition of downstream targets and melanoma growth in vitro. In vivo, increased BBB permeability to sodium fluorescein was associated with accelerated growth of individual brain metastases. Melanoma metastases with permeable microvessels responded similarly to equivalent doses of both inhibitors. In contrast, metastases with an intact BBB showed an exclusive response to the brain-penetrating inhibitor. The latter was true for macro- and micrometastases, and even single dormant melanoma cells. Nuclear morphology changes and single-cell regression patterns implied that both inhibitors, if extravasated, target not only perivascular melanoma cells but also those distant to blood vessels.Conclusions: Our study provides the first direct evidence that nonpermeable brain micro- and macrometastases can effectively be targeted by a drug designed to cross the BBB. Small-molecule inhibitors with these optimized properties are promising agents in preventing or treating brain metastases in patients. Clin Cancer Res; 22(24); 6078–87. ©2016 AACR.See related commentary by Steeg et al., p. 5953

A, Metastasis sizes in the different treatment groups at start of treatment (p=0.596, ANOVA on Ranks). B, Quantifications of the mean grey values of the sodium fluorescein signal, normalized to background signal. Note that all metastases could clearly be attributed to be either permeable vs. non-permeable. There were no statistical significant differences between single metastases within the permeable (p=0.244, ANOVA) and the non-permeable (p=0.106, ANOVA) subgroups, when the different treatment groups were compared to each other. C, Quantification of growth rates per day after / before start of MCT control treatment (p=0.412, Mann-Whitney Rank Sum test).

A, In permeable (red line) vs. non-permeable (green line) brain metastases, the extravasation of sodium fluorescein was significantly different directly after i.v. sodium fluorescein injection, and at the end of the corresponding time series (n=7 permeable metastases from n=6 animals, n=16 non-permeable metastases from n=12 animals; ***p20.000 tumor cells, in red) and permeable (permeability measurement not shown) brain metastasis over one week, showing formation of new blood vessels over time (true angiogenesis); stitches of tilescans (D21 and D25); 3D reconstruction of z-stacks of 237-540µm; scale bars 200µm. All analyses were performed with in vivo data.

Crossing the blood–brain barrier is a crucial, rate-limiting step of brain metastasis. Understanding of the mechanisms of cancer cell extravasation from brain microcapillaries is limited as the underlying cellular and molecular processes cannot be adequately investigated using in vitro models and endpoint in vivo experiments. Using ultrastructural and functional imaging, we demonstrate that dynamic changes of activated brain microcapillaries promote the mandatory first steps of brain colonization. Successful extravasation of arrested cancer cells occurred when adjacent capillary endothelial cells (EC) entered into a distinct remodeling process. After extravasation, capillary loops were formed, which was characteristic of aggressive metastatic growth. Upon cancer cell arrest in brain microcapillaries, matrix-metalloprotease 9 (MMP9) was expressed. Inhibition of MMP2/9 and genetic perturbation of MMP9 in cancer cells, but not the host, reduced EC projections, extravasation, and brain metastasis outgrowth. These findings establish an active role of ECs in the process of cancer cell extravasation, facilitated by cross-talk between the two cell types. This extends our understanding of how host cells can contribute to brain metastasis formation and how to prevent it. Significance: Tracking single extravasating cancer cells using multimodal correlative microscopy uncovers a brain seeding mechanism involving endothelial remodeling driven by cancer cell–derived MMP9, which might enable the development of approaches to prevent brain metastasis. See related commentary by McCarty, p. 1167

Clinically relevant brain metastases (BMs) frequently form in cancer patients, with limited options for effective treatment. Circulating cancer cells must first permanently arrest in brain microvessels to colonize the brain, but the critical factors in this process are not well understood. Here, in vivo multiphoton laser-scanning microscopy of the entire brain metastatic cascade allowed unprecedented insights into how blood clot formation and von Willebrand factor (VWF) deposition determine the arrest of circulating cancer cells and subsequent brain colonization in mice. Clot formation in brain microvessels occurred frequently (>95%) and specifically at intravascularly arrested cancer cells, allowing their long-term arrest. An extensive clot embedded ∼20% of brain-arrested cancer cells, and those were more likely to successfully extravasate and form a macrometastasis. Mechanistically, the generation of tissue factor-mediated thrombin by cancer cells accounted for local activation of plasmatic coagulation in the brain. Thrombin inhibition by treatment with low molecular weight heparin or dabigatran and an anti-VWF antibody prevented clot formation, cancer cell arrest, extravasation, and the formation of brain macrometastases. In contrast, tumor cells were not able to directly activate platelets, and antiplatelet treatments did reduce platelet dispositions at intravascular cancer cells but did not reduce overall formation of BMs. In conclusion, our data show that plasmatic coagulation is activated early by intravascular tumor cells in the brain with subsequent clot formation, which led us to discover a novel and specific mechanism that is crucial for brain colonization. Direct or indirect thrombin and VWF inhibitors emerge as promising drug candidates for trials on prevention of BMs.

Background Malignant gliomas including glioblastomas are characterized by a striking cellular heterogeneity, which includes a subpopulation of glioma cells that becomes highly resistant by integration into tumor microtube (TM)-connected multicellular networks. Methods A novel functional approach to detect, isolate, and characterize glioma cell subpopulations with respect to in vivo network integration is established, combining a dye staining method with intravital two-photon microscopy, Fluorescence-Activated Cell Sorting (FACS), molecular profiling, and gene reporter studies. Results Glioblastoma cells that are part of the TM-connected tumor network show activated neurodevelopmental and glioma progression gene expression pathways. Importantly, many of them revealed profiles indicative of increased cellular stemness, including high expression of nestin. TM-connected glioblastoma cells also had a higher potential for reinitiation of brain tumor growth. Long-term tracking of tumor cell nestin expression in vivo revealed a stronger TM network integration and higher radioresistance of the nestin-high subpopulation. Glioblastoma cells that were both nestin-high and network-integrated were particularly able to adapt to radiotherapy with increased TM formation. Conclusion Multiple stem-like features are strongly enriched in a fraction of network-integrated glioma cells, explaining their particular resilience.

Rationale: Glioblastoma is the most frequent, primary brain tumor that is characterized by a highly immunosuppressive tumor microenvironment (TME). The TME plays a key role for tumor biology and the effectiveness of immunotherapies. Composition of the TME correlates with overall survival and governs therapy response. Non invasive assessment of the TME has been notoriously difficult. Methods: We have designed an in vivo imaging approach to non invasively visualize innate immune cell dynamics in the TME in a mouse glioma model by correlated MRI and multiphoton microscopy (MR-MPM) using a bimodal, fluorescently labeled iron oxide nanoparticle (NP). The introduction of Teflon cranial windows instead of conventional Titanium rings dramatically reduced susceptibility artifacts on MRI and allowed longitudinal MR-MPM imaging for innate immune cell tracking in the same animal. Results: We visualized tumor associated macrophage and microglia (TAM) dynamics in the TME and dissect the single steps of NP uptake by blood-born monocytes that give rise to tumor-associated macrophages. Next to peripheral NP-loading, we identified a second route of direct nanoparticle uptake via the disrupted blood-brain barrier to directly label tissue resident TAMs. Conclusion: Our approach allows innate immune cell tracking by MRI and multiphoton microscopy in the same animal to longitudinally investigate innate immune cell dynamics in the TME.

Glioblastoma multiforme alters healthy tissue vasculature by inducing angiogenesis and vascular remodeling. To fully comprehend the structural and functional properties of the resulting vascular network, it needs to be studied collectively by considering both geometric and topological properties. Utilizing Single Plane Illumination Microscopy (SPIM), the detailed capillary structure in entire healthy and tumor-bearing mouse brains could be resolved in three dimensions. At the scale of the smallest capillaries, the entire vascular systems of bulk U87- and GL261-glioblastoma xenografts, their respective cores, and healthy brain hemispheres were modeled as complex networks and quantified with fundamental topological measures. All individual vessel segments were further quantified geometrically and modular clusters were uncovered and characterized as meta-networks, facilitating an analysis of large-scale connectivity. An inclusive comparison of large tissue sections revealed that geometric properties of individual vessels were altered in glioblastoma in a relatively subtle way, with high intra- and inter-tumor heterogeneity, compared to the impact on the vessel connectivity. A network topology analysis revealed a clear decomposition of large modular structures and hierarchical network organization, while preserving most fundamental topological classifications, in both tumor models with distinct growth patterns. These results augment our understanding of cerebrovascular networks and offer a topological assessment of glioma-induced vascular remodeling. The findings may help understand the emergence of hypoxia and necrosis, and prove valuable for therapeutic interventions such as radiation or antiangiogenic therapy.

Diffuse gliomas, particularly glioblastomas, are incurable brain tumours

Specific biological properties of those circulating cancer cells that are the origin of brain metastases (BM) are not well understood. Here, single circulating breast cancer cells were fate-tracked during all steps of the brain metastatic cascade in mice after intracardial injection over weeks. A novel in vivo two-photon microscopy methodology was developed that allowed to determine the specific cellular and molecular features of breast cancer cells that homed in the brain, extravasated, and successfully established a brain macrometastasis. Those BM-initiating breast cancer cells (BMIC) were mainly originating from a slow-cycling subpopulation that included only 16% to 20% of all circulating cancer cells. BMICs showed enrichment of various markers of cellular stemness. As a proof of principle for the principal usefulness of this approach, expression profiling of BMICs versus non-BMICs was performed, which revealed upregulation of NDRG1 in the slow-cycling BMIC subpopulation in one BM model. Here, BM development was completely suppressed when NDRG1 expression was downregulated. In accordance, in primary human breast cancer, NDRG1 expression was heterogeneous, and high NDRG1 expression was associated with shorter metastasis-free survival. In conclusion, our data identify temporary slow-cycling breast cancer cells as the dominant source of brain and other metastases and demonstrates that this can lead to better understanding of BMIC-relevant pathways, including potential new approaches to prevent BM in patients. Implications: Cancer cells responsible for successful brain metastasis outgrowth are slow cycling and harbor stemness features. The molecular characteristics of these metastasis-initiating cells can be studied using intravital microscopy technology.

Spin dephasing of the local magnetization in blood vessel networks can be described in the static dephasing regime (where diffusion effects may be ignored) by the established model of Yablonskiy and Haacke. However, for small capillary radii, diffusion phenomena for spin-bearing particles are not negligible. In this work, we include diffusion effects for a set of randomly distributed capillaries and provide analytical expressions for the transverse relaxation times T2* and T2 in the strong collision approximation and the Gaussian approximation that relate MR signal properties with microstructural parameters such as the mean local capillary radius. Theoretical results are numerically validated with random walk simulations and are used to calculate capillary radius distribution maps for glioblastoma mouse brains at 9.4 T. For representative tumor regions, the capillary maps reveal a relative increase of mean radius for tumor tissue towards healthy brain tissue of $$128 \pm 23 \%$$ (p < 0.001). The presented method may be used to quantify angiogenesis or the effects of antiangiogenic therapy in tumors whose growth is associated with significant microvascular changes.

Purpose: The role of blood–brain barrier (BBB) integrity for brain tumor biology and therapy is a matter of debate. Experimental Design: We developed a new experimental approach using in vivo two-photon imaging of mouse brain metastases originating from a melanoma cell line to investigate the growth kinetics of individual tumor cells in response to systemic delivery of two PI3K/mTOR inhibitors over time, and to study the impact of microregional vascular permeability. The two drugs are closely related but differ regarding a minor chemical modification that greatly increases brain penetration of one drug. Results: Both inhibitors demonstrated a comparable inhibition of downstream targets and melanoma growth in vitro. In vivo, increased BBB permeability to sodium fluorescein was associated with accelerated growth of individual brain metastases. Melanoma metastases with permeable microvessels responded similarly to equivalent doses of both inhibitors. In contrast, metastases with an intact BBB showed an exclusive response to the brain-penetrating inhibitor. The latter was true for macro- and micrometastases, and even single dormant melanoma cells. Nuclear morphology changes and single-cell regression patterns implied that both inhibitors, if extravasated, target not only perivascular melanoma cells but also those distant to blood vessels. Conclusions: Our study provides the first direct evidence that nonpermeable brain micro- and macrometastases can effectively be targeted by a drug designed to cross the BBB. Small-molecule inhibitors with these optimized properties are promising agents in preventing or treating brain metastases in patients. Clin Cancer Res; 22(24); 6078–87. ©2016 AACR. See related commentary by Steeg et al., p. 5953

Diffuse tumor infiltration into the adjacent parenchyma is an effective dissemination mechanism of brain tumors. We have previously developed correlated high field magnetic resonance imaging and ultramicroscopy (MR-UM) to study neonangiogenesis in a glioma model. In the present study we used MR-UM to investigate tumor infiltration and neoangiogenesis in a translational approach. We compare infiltration and neoangiogenesis patterns in four brain tumor models and the human disease: whereas the U87MG glioma model resembles brain metastases with an encapsulated growth and extensive neoangiogenesis, S24 experimental gliomas mimic IDH1 wildtype glioblastomas, exhibiting infiltration into the adjacent parenchyma and along white matter tracts to the contralateral hemisphere. MR-UM resolves tumor infiltration and neoangiogenesis longitudinally based on the expression of fluorescent proteins, intravital dyes or endogenous contrasts. Our study demonstrates the huge morphological diversity of brain tumor models regarding their infiltrative and neoangiogenic capacities and further establishes MR-UM as a platform for translational neuroimaging.

Metastatic seeding is driven by cell-intrinsic and environmental cues, yet the contribution of biomechanics is poorly known. We aim to elucidate the impact of blood flow on the arrest and the extravasation of circulating tumor cells (CTCs) in vivo. Using the zebrafish embryo, we show that arrest of CTCs occurs mostly in vessels with favorable flow profiles where flow forces control the adhesion efficacy of CTCs to the endothelium. We biophysically identified the threshold values of flow and adhesion forces allowing successful arrest of CTCs. In addition, flow forces fine-tune tumor cell extravasation by impairing the remodeling properties of the endothelium. Importantly, we also observe endothelial remodeling at arrest sites of CTCs in mouse brain capillaries. Finally, we observed that human supratentorial brain metastases preferably develop in areas with low perfusion. Altogether, these results demonstrate that hemodynamic profiles at metastatic sites regulate key steps of extravasation preceding metastatic outgrowth.

Patients with glioblastoma (GBM) have a universally poor prognosis and are in urgent need of effective treatment strategies. Recent advances in sequencing techniques unraveled the complete genomic landscape of GBMs and revealed profound heterogeneity of individual tumors even at the single cell level. Genomic profiling has detected epidermal growth factor receptor (EGFR) gene alterations in more than half of GBMs. Major genetic events include amplification and mutation of EGFR. Yet, treatment strategies targeting EGFR have thus far failed in clinical trials. In this review, we discuss the clonal and functional heterogeneity of EGFRs in GBM development and critically reassess the potential of EGFRs as therapeutic targets.

Early and progressive colonization of the healthy brain is one hallmark of diffuse gliomas, including glioblastomas. We recently discovered ultralong (>10 to hundreds of microns) membrane protrusions [tumor microtubes (TMs)] extended by glioma cells. TMs have been associated with the capacity of glioma cells to effectively invade the brain and proliferate. Moreover, TMs are also used by some tumor cells to interconnect to one large, resistant multicellular network. Here, we performed a correlative gene-expression microarray and in vivo imaging analysis, and identified novel molecular candidates for TM formation and function. Interestingly, these genes were previously linked to normal CNS development. One of the genes scoring highest in tests related to the outgrowth of TMs was tweety-homolog 1 (TTYH1), which was highly expressed in a fraction of TMs in mice and patients. Ttyh1 was confirmed to be a potent regulator of normal TM morphology and of TM-mediated tumor-cell invasion and proliferation. Glioma cells with one or two TMs were mainly responsible for effective brain colonization, and Ttyh1 downregulation particularly affected this cellular subtype, resulting in reduced tumor progression and prolonged survival of mice. The remaining Ttyh1-deficient tumor cells, however, had more interconnecting TMs, which were associated with increased radioresistance in those small tumors. These findings imply a cellular and molecular heterogeneity in gliomas regarding formation and function of distinct TM subtypes, with multiple parallels to neuronal development, and suggest that Ttyh1 might be a promising target to specifically reduce TM-associated brain colonization by glioma cells in patients. SIGNIFICANCE STATEMENT In this report, we identify tweety-homolog 1 (Ttyh1), a membrane protein linked to neuronal development, as a potent driver of tumor microtube (TM)-mediated brain colonization by glioma cells. Targeting of Ttyh1 effectively inhibited the formation of invasive TMs and glioma growth, but increased network formation by intercellular TMs, suggesting a functional and molecular heterogeneity of the recently discovered TMs with potential implications for future TM-targeting strategies.

Combining in vivo imaging with electron microscopy (EM) uniquely allows monitoring rare and critical events in living tissue, followed by their high-resolution visualization in their native context. A major hurdle, however, is to keep track of the region of interest (ROI) when moving from intravital microscopy (IVM) to EM. Here, we present a workflow that relies on correlating IVM and microscopic X-ray computed tomography to predict the position of the ROI inside the EM-processed sample. The ROI can then be accurately and quickly targeted using ultramicrotomy and imaged using EM. We outline how this procedure is used to retrieve and image tumor cells arrested in the vasculature of the mouse brain.

Background Primary and adaptive resistance against chemo- and radiotherapy and local recurrence after surgery limit the benefits from these standard treatments in glioma patients. Recently we found that glioma cells can extend ultra-long membrane protrusions, "tumor microtubes" (TMs), for brain invasion, proliferation, and interconnection of single cells to a syncytium that is resistant to radiotherapy. We wondered whether TMs also convey resistance to the other 2 standard treatment modalities. Methods Patient-derived glioblastoma stemlike cell (GBMSC) lines were implanted under a cranial window in mice. Longitudinal in vivo two-photon laser scanning microscopy was used to follow tumor growth, including the fate of single glioma cells over months. Results After a cylindrical surgical lesion, GBMSCs increasingly extended TMs toward the lesion area, which contributed to the repopulation of this area over many weeks. In fact, an excessive "healing response" was observed in which tumor cell densities significantly exceeded those of unlesioned brain regions over time. Inhibition of TM formation and function by genetic targeting of growth associated protein-43 robustly suppressed this surgery-induced tumor growth reaction, in contrast to standard postsurgical anti-inflammatory treatment with dexamethasone. After one cycle of temozolomide chemotherapy, intra- and intertumoral heterogeneity of TM formation and interconnection was strongly associated with therapy response: when tumor cells were integrated in TM networks, they were more likely to resist chemotherapy. Conclusion TMs can contribute to the resistance against standard treatment modalities in gliomas. Specific inhibition of TMs is a promising approach to reduce local recurrence after surgery and lower resistance to chemotherapy.

Combining in vivo imaging with electron microscopy (EM) uniquely allows monitoring rare and critical events in living tissue, followed by their high-resolution visualization in their native context. A major hurdle, however, is to keep track of the region of interest (ROI) when moving from intravital microscopy (IVM) to EM. Here, we present a workflow that relies on correlating IVM and microscopic X-ray computed tomography to predict the position of the ROI inside the EM-processed sample. The ROI can then be accurately and quickly targeted using ultramicrotomy and imaged using EM. We outline how this procedure is used to retrieve and image tumor cells arrested in the vasculature of the mouse brain.

BACKGROUND: Glioblastoma is the most frequent primary malignant brain tumor characterized by poor survival due to its high resistance to standard therapies. Intra- and intertumoral heterogeneity has been described on many levels. We have recently found that astrocytomas, including glioblastomas, extend tumor microtubes (TMs) to colonize the brain, and allow for long-distance interactions between tumor cells; however, they are heterogeneously formed in the tumor, with about half of the cells staying unconnected. The TM-connected glioma cells resist the adverse effects of radio- and chemotherapy, and repair surgical lesions, being the resistant backbone of the disease. Thus, an important but yet unresolved question is how TM-connected tumor cells differ from the rest. Moreover, it remained unclear, whether cellular heterogeneity in glioblastoma is either plastic or fixed. MATERIAL AND METHODS: We identified sulforhodamine (SR) 101 to specifically label astrocytes and astrocyte-connected glioblastoma cells, and SR101 was injected into mouse model carrying cranial window to distinguish TM-connected vs. TM-unconnected cells in vivo. Both glioblastoma cell subpopulations were separated ex vivo by FACS sorting, and analyzed with high-throughput RNA sequencing. Immunostaining of tumor sections from patients and xenografts was performed to identify stem cell markers, and correlated with TMs. In vivo reporter system based on GFP expression driven by nestin responsive promoter element was applied to dynamically monitor TMs and molecular changes during tumor progression and therapy responses. RESULTS: TM-connected glioma cells with high uptake of SR101 showed gene expression profiles compatible with astrocytic character, cellular stemness, cellular proliferation, and ingenuity profiles enriched for genes involved in cell-to-cell interaction and tissue development. Those cells were particularly nestin-rich, an established marker for glioma cell stemness. Nestin expression as measured with the in vivo reporter system was very high under stem-like in vitro conditions, and decreased over time after tumor implantation, with only a small subpopulation remaining nestin-positive in established gliomas. Nestin expression was up-regulated under stress, and highly correlated with TM-connectivity, TM-proficiency and cellular resistance. Ex vivo experiment revealed that nestin+ population displayed higher sphere formation ability than nestin- cells. Tumor formation assays after re-implantation of connected vs. unconnected cells are ongoing, as well as single cell RNA-Seq analysis. CONCLUSION: Stem-like cellular features are enriched in TM-connected glioma cells, and strongly correlated with the progression and therapy resistance of gliomas. Glioma cells can dynamically change their cellular identity over many days, particularly after stress.

A hypoxic microenvironment induces resistance to alkylating agents by activating targets in the mammalian target of rapamycin (mTOR) pathway. The molecular mechanisms involved in this mTOR-mediated hypoxia-induced chemoresistance, however, are unclear. Here we identify the mTOR target N-myc downstream regulated gene 1 (NDRG1) as a key determinant of resistance toward alkylating chemotherapy, driven by hypoxia but also by therapeutic measures such as irradiation, corticosteroids, and chronic exposure to alkylating agents via distinct molecular routes involving hypoxia-inducible factor (HIF)-1alpha, p53, and the mTOR complex 2 (mTORC2)/serum glucocorticoid-induced protein kinase 1 (SGK1) pathway. Resistance toward alkylating chemotherapy but not radiotherapy was dependent on NDRG1 expression and activity. In posttreatment tumor tissue of patients with malignant gliomas, NDRG1 was induced and predictive of poor response to alkylating chemotherapy. On a molecular level, NDRG1 bound and stabilized methyltransferases, chiefly O6-methylguanine-DNA methyltransferase (MGMT), a key enzyme for resistance to alkylating agents in glioblastoma patients. In patients with glioblastoma, MGMT promoter methylation in tumor tissue was not more predictive for response to alkylating chemotherapy in patients who received concomitant corticosteroids.

A distinct perivascular niche has been implicated in many aspects of tumor biology, particularly stemness, invasion, survival, and therapy resistance of cancer cells. But its overall importance for these factors remains obscure.

The recent discovery of distinct, ultra-long, and highly functional membrane protrusions in gliomas, particularly in astrocytomas, extends our understanding of how these tumors progress in the brain and how they resist therapies. In this article, we will focus on ideas on how to target these membrane protrusions, for which we have suggested the term "tumor microtubes" (TMs), and the malignant multicellular network they form. First, we discuss TM-specific features and their differential biological functions known so far. Second, the connection between 1p/19q codeletion and the inability to form functional TMs via certain neurodevelopmental pathways is presented; this could provide an explanation for the distinct clinical features of oligodendrogliomas. Third, the role of TMs for primary and potentially also adaptive resistance to cytotoxic therapies is highlighted. Fourth, avenues for therapeutic approaches to inhibit TM formation and/or function are discussed, with a focus on disruption (or exploitation) of network functionality. Finally, we propose ideas on how to use TMs as a biomarker in glioma patients. An increasing understanding of TMs in clinical and preclinical settings will show us whether they really are a long-sought-after Achilles' heel of treatment-resistant gliomas.

Neoangiogenesis is a pivotal therapeutic target in glioblastoma. Tumor monitoring requires imaging methods to assess treatment effects and disease progression. Until now mapping of the tumor vasculature has been difficult. We have developed a combined magnetic resonance and optical toolkit to study neoangiogenesis in glioma models. We use in vivo magnetic resonance imaging (MRI) and correlative ultramicroscopy (UM) of ex vivo cleared whole brains to track neovascularization. T2* imaging allows the identification of single vessels in glioma development and the quantification of neovessels over time. Pharmacological VEGF inhibition leads to partial vascular normalization with decreased vessel caliber, density, and permeability. To further resolve the tumor microvasculature, we performed correlated UM of fluorescently labeled microvessels in cleared brains. UM resolved typical features of neoangiogenesis and tumor cell invasion with a spatial resolution of ~5 µm. MR-UM can be used as a platform for three-dimensional mapping and high-resolution quantification of tumor angiogenesis. DOI: http://dx.doi.org/10.7554/eLife.11712.001, eLife digest Blood vessels are the body’s highways that allow blood to transport oxygen, nutrients, hormones and waste products quickly and efficiently around the body. Tumors are made up of particularly active cells and so their growth heavily depends on blood vessels. Indeed, a fundamental hallmark of tumor progression is for nearby blood vessels to form more quickly. Tumor blood vessels also differ in structure from their normal counterparts for reasons that need to be investigated in more detail. Compounds that block the formation of blood vessels have been developed for treating highly malignant brain tumors called gliomas. However, although many of these compounds show promising effects in preclinical trials, clinical trials on humans have been less successful. Having the ability to image the blood vessels in high detail during preclinical trials would help to reveal how treatments that inhibit blood vessel formation work and how tumors might develop resistance to these drugs. However, studying tumor blood vessels remains a challenge due to technical restrictions: techniques that are able to capture how the vessels change over time are unable to show individual cells in much detail, and vice versa. Magnetic resonance imaging is a versatile tool that can monitor how the blood vessel system of a tumor changes over time in living animals. On the other hand, ultramicroscopy is able to determine the structure of single cells of a particular type. By combining these techniques, Breckwoldt, Bode et al. have now developed a imaging platform that allows the formation of tumor blood vessels to be precisely mapped in the setting of a preclinical study. It also enables detailed investigations into how the structure of the blood vessels is altered by treatments that aim to inhibit the formation and growth of new vessels. Using this approach on mice with gliomas, Breckwoldt, Bode et al. demonstrated that drugs that inhibit the formation of the blood vessels that supply tumors also cause the blood vessels to take on a more normal structure. Furthermore, treating the mice with a single inhibitory drug was unable to stop tumor growth, mirroring the situation in humans. Currently, new inhibitors are being developed, offering the possibility of combined treatments that may be more effective than using a single drug on its own. The imaging platform developed by Breckwoldt, Bode et al. will allow the therapeutic effects obtained by these new treatments to be analyzed in detail during preclinical studies. DOI: http://dx.doi.org/10.7554/eLife.11712.002

Metastatic seeding is driven by cell-intrinsic and environmental cues, yet the contribution of biomechanics is poorly known. We aim to elucidate the impact of blood flow on the arrest and the extravasation of circulating tumor cells (CTCs) in vivo. Using the zebrafish embryo, we show that arrest of CTCs occurs in vessels with favorable flow profiles where flow forces control the adhesion efficacy of CTCs to the endothelium. We biophysically identified the threshold values of flow and adhesion forces allowing successful arrest of CTCs. In addition, flow forces fine-tune tumor cell extravasation by impairing the remodeling properties of the endothelium. Importantly, we also observe endothelial remodeling at arrest sites of CTCs in mouse brain capillaries. Finally, we observed that human supratentorial brain metastases preferably develop in areas with low perfusion. Altogether, these results demonstrate that hemodynamic profiles at metastatic sites regulate key steps of extravasation preceding metastatic outgrowth.

For a targeted cancer vaccine to be effective, the antigen of interest needs to be naturally processed and presented on MHC by the target cell or an antigen-presenting cell (APC) in the tumor stroma. The presence of these characteristics is often assumed based on animal models, evaluation of antigen-overexpressing APCs in vitro, or assays of material-consuming immune precipitation from fresh solid tissue. Here, we evaluated the use of an alternative approach that uses the proximity ligation assay (PLA) to identify the presentation of an MHC class II-restricted antigen in paraffin-embedded tissue sections from patients with brain tumors. This approach required a specific antibody directed against the epitope that was presented. We used an antibody that specifically binds an epitope of mutated isocitrate dehydrogenase type 1 (IDH1R132H), which is frequently expressed in gliomas and other types of tumors. In situ PLA showed that the IDH1R132H epitope colocalizes with MHC class II in IDH1R132H-mutated glioma tissue. Moreover, PLA demonstrated colocalization between the class II epitope-containing melanoma antigen New York esophageal 1 and MHC class II. Collectively, our data suggest that PLA may be a useful tool to acquire information on whether an antigen is presented in situ, and this technique has potential to guide clinical studies that use antigen-specific cancer immunotherapy.

Intravital microscopy provides dynamic understanding of multiple cell biological processes, but its limited resolution has so far precluded structural analysis. Because it is difficult to capture rare and transient events, only a few attempts have been made to observe specific developmental and pathological processes in animal models using electron microscopy. The multimodal correlative approach that we propose here combines intravital microscopy, microscopic X-ray computed tomography and three-dimensional electron microscopy. It enables a rapid (c.a. 2 weeks) and accurate (, Highlighted Article: We provide here a novel correlative workflow combining intravital microscopy, microCT and 3D electron microscopy to reveal metastatic events in mouse brain and skin tissue at high resolution.

Astrocytic brain tumours, including glioblastomas, are incurable neoplasms characterized by diffusely infiltrative growth. Here we show that many tumour cells in astrocytomas extend ultra-long membrane protrusions, and use these distinct tumour microtubes as routes for brain invasion, proliferation, and to interconnect over long distances. The resulting network allows multicellular communication through microtube-associated gap junctions. When damage to the network occurred, tumour microtubes were used for repair. Moreover, the microtube-connected astrocytoma cells, but not those remaining unconnected throughout tumour progression, were protected from cell death inflicted by radiotherapy. The neuronal growth-associated protein 43 was important for microtube formation and function, and drove microtube-dependent tumour cell invasion, proliferation, interconnection, and radioresistance. Oligodendroglial brain tumours were deficient in this mechanism. In summary, astrocytomas can develop functional multicellular network structures. Disconnection of astrocytoma cells by targeting their tumour microtubes emerges as a new principle to reduce the treatment resistance of this disease.

Vascular endothelial growth factor (VEGF) is a major driver of blood vessel formation. However, the signal transduction pathways culminating in the biological consequences of VEGF signaling are only partially understood. Here, we show that the Hippo pathway effectors YAP and TAZ work as crucial signal transducers to mediate VEGF-VEGFR2 signaling during angiogenesis. We demonstrate that YAP/TAZ are essential for vascular development as endothelium-specific deletion of YAP/TAZ leads to impaired vascularization and embryonic lethality. Mechanistically, we show that VEGF activates YAP/TAZ via its effects on actin cytoskeleton and that activated YAP/TAZ induce a transcriptional program to further control cytoskeleton dynamics and thus establish a feedforward loop that ensures a proper angiogenic response. Lack of YAP/TAZ also results in altered cellular distribution of VEGFR2 due to trafficking defects from the Golgi apparatus to the plasma membrane. Altogether, our study identifies YAP/TAZ as central mediators of VEGF signaling and therefore as important regulators of angiogenesis.

Angiogenesis is a biological hallmark of malignant gliomas, but antiangiogenic strategies especially targeting the VEGF axis did not show striking antitumor activities in the majority of patients so far. Other pathways may be more relevant in selected tumor entities or patients. Further, it remains unresolved which antiangiogenic combination regimen with standard radio- and/or chemotherapy is most effective. Therefore, we compared the therapeutic effects of anti-VEGF-A, anti-Ang-2, and bispecific anti-Ang-2/VEGF-A antibodies, alone and in combination with radio- or temozolomide (TMZ) chemotherapy in a malignant glioma model using multi-parameter two-photon in vivo microscopy in mice. We demonstrate that anti-Ang-2/VEGF-A leads to strongest vascular changes including vascular normalization, both as monotherapy and when combined with chemotherapy. The latter combination regimen was accompanied by most effective chemotherapy-induced death of cancer cells independent of blood vessel proximity, indicative of a better distribution of TMZ throughout the tumor. Furthermore, the combination of anti-Ang-2/VEGF-A plus TMZ consistently resulted in decreased tumor cell motility, and decreased formation of resistance-associated tumor microtubes (TMs), which finally lead to best tumor growth inhibition. Remarkably, all these parameters were just reverted when radiotherapy was chosen as combination partner. In contrast, when anti-VEGF-A was combined with radiotherapy, vascular normalization was highest, and TM length, nuclear motility and tumor growth were concordantly reduced. In conclusion, while TMZ chemotherapy benefits most from combination with anti-Ang-2/VEGF-A, radiotherapy does from anti-VEGF-A. The findings imply that unexpected, even divergent effects can occur when a specific antiangiogenic therapy is added to chemo- or radiotherapy in glioma, and that uninformed combination regimens should be avoided. Note: This abstract was not presented at the meeting. Citation Format: Gergely Solecki, Matthias Osswald, Weber Daniel, Malte Glock, Miriam Ratliff, Hans-Joachim Müller, Oliver Krieter, Yvonne Kienast, Wolfgang Wick, Frank Winkler. Combining anti-Ang-2/VEGF-A therapy with radio- and chemotherapy in glioma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1803. doi:10.1158/1538-7445.AM2017-1803

Introduction: Glioblastoma is the most common primary malignant brain tumor in adults. It is highly resistant to available treatment options as the invasiveness of the tumor limits complete resection, as well as the dynamic tumor genome, multiple pathways driving the malignant phenotype, and the blood brain barrier, which limits the availability of drugs to the tumor. Evidence suggests a significant role for NF-κB in the gliomagenesis and the mechanism of treatment resistance in glioblastoma making NF-κB a potentially potent target for treatment. However, increasing evidence suggest a great cellular heterogeneity in glioma, making it important to understand which glioma cell types are dynamically driven by certain molecular pathways, and how that relates to their function in glioma progression and resistance. Therefore, we established an in vivo reporter system based on NF-κB responsive promoter elements driving green fluorescent protein (GFP) to monitor the expression of this transcription factor. Materials and Methods: Ten tandem repeats of the NF-κB transcription responsive element were designed, annealed and ligated into the HIV-1 based lentiviral vector pLVXPuro by replacing the CMV promoter with NF-κB elements. GFP was subcloned into the multiple cloning site of pLVXPuro. The integrity of the plasmid was verified by sequencing analysis. Its reliability as a NF-κB reporter was verified in vivo and in vitro. Using a chronic cranial window mouse model two-photon microscopy allows for consecutive in vivo imaging to observe different biological processes such as tumor growth, angiogenesis, and tumor repair after laser ablation and radiotherapy. Results: While cells grown under stem like conditions in vitro were highly NF-κB reporter positive, only few cells were positive in the established tumor in vivo, and that this changed dynamically depending on distinct states of tumor progression, and in response to lesions in the tumor area that were repopulated - “repaired” - by tumor cells. Furthermore, we were able to show that TNFα induced NF-κB reporter activity significantly in vitro. Radiotherapy led to a significant increase in reporter GFP expression in vivo as well as in vitro, to the extent that most cells became reporter positive. Pre-existing reporter-positive cells differed from reporter negative cells with respect to resistance against cytotoxic stress. NF-κB reporter positivity did not colocalize with established “stem cell” markers Mushashi, Nestin and Sox2 on tumor slices grown in our mouse model. DISCUSSION: Our results suggest a role for NF-κB expression during GBM resistance to treatment and progression after radiation in a heterogeneous manner. We will further analyze cell fate and morphology during irradiation and tumor “repair” with regard to their NF-κB positivity, and will include this data in the presentation, including inhibitory strategies of the NF-κB pathway.

High levels of pro-angiogenic factors like VEGF-A and angiopoietin-2 (Ang-2) in the microenvironment of brain tumors lead to an abnormal structure, organization and function of the tumor vasculature. It is of interest whether inhibition of Ang-2 or even dual inhibition of both major angiogenic factors is more efficient in glioma therapy, particularly with respect to vascular normalization. In this context, the ideal combination regimen with chemotherapy and/or radiotherapy is still unclear today. Here, we used an orthotopic glioblastoma model to compare dual blocking of Ang-2 and VEGF-A by a bispecific antibody vs. monospecific Ang-2 and VEGF-A blocking. In addition, we evaluated combinations of both antiangiogenic antibodies with either chemotherapy or radiation therapy.