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Phenotypic and Expressional Heterogeneity in the Invasive Glioma Cells12

BACKGROUND: Tumor cell invasion is a hallmark of glioblastoma (GBM) and a major contributing factor for treatment failure, tumor recurrence, and the poor prognosis of GBM. Despite this, our understanding of the molecular machinery that drives invasion is limited. METHODS: Time-lapse imaging of patient-derived GBM cell invasion in a 3D collagen gel matrix, analysis of both the cellular invasive phenotype and single cell invasion pattern with microarray expression profiling. RESULTS: GBM invasion was maintained in a simplified 3D-milieue. Invasion was promoted by the presence of the tumorsphere graft. In the absence of this, the directed migration of cells subsided. The strength of the pro-invasive repulsive signaling was specific for a given patient-derived culture. In the highly invasive GBM cultures, the majority of cells had a neural progenitor-like phenotype, while the less invasive cultures had a higher diversity in cellular phenotypes. Microarray expression analysis of the non-invasive cells from the tumor core displayed a higher GFAP expression and a signature of genes containing VEGFA, hypoxia and chemo-repulsive signals. Cells of the invasive front expressed higher levels of CTGF, TNFRSF12A and genes involved in cell survival, migration and cell cycle pathways. A mesenchymal gene signature was associated with increased invasion. CONCLUSION: The GBM tumorsphere core promoted invasion, and the invasive front was dominated by a phenotypically defined cell population expressing genes regulating traits found in aggressive cancers. The detected cellular heterogeneity and transcriptional differences between the highly invasive and core cells identifies potential targets for manipulation of GBM invasion.


Introduction
Glioblastoma (GBM) is the most frequent and malignant brain cancer. Standard treatment only extends the life of patients with months, and the median survival in unselected patient populations is less than a year [1]. The tumors' ability to invade into the surrounding brain parenchyma is a major challenge as it makes complete resection unachievable. The invasive cells left in the brain after tumor resection are resistant to chemo-and radiotherapy and are thus responsible for the inevitable tumor recurrence [2,3].
GBM cells have the ability to move through the highly packed neuropil, but rarely enter into the circulation [4]. Thus, the invasion of glioma cells is different from the metastatic spread of other cancer cells and is likely dependent on a unique set of molecular pathways [5]. Moreover, GBMs display high levels of inter-and intratumoral heterogeneity, where only a subset of the tumor cells is invasive [5]. To understand the glioma-specific properties of invasion, models must recapitulate the heterogeneous cellular phenotype seen in patients while being simple enough to allow for interpretation.
To experimentally decipher the ability of glioma tumor cells to migrate and invade into the brain, it is essential that the model system retains this key characteristic of GBM. The traditional long term serum cultivated GBM cell lines express markers suggesting neural lineage, but display molecular characteristics more common to other cell lines than the tumor of origin [6]. Upon transplantation to the brain these cells establish rapidly growing tumors, but with sharply delineated borders to the brain parenchymamore resembling brain metastases than glial tumors [7,8]. In contrast, the use of patient-derived GBM tissue allows for isolation of cells with invasive properties. These cells can be propagated as tumorspheres under serum-free, growth factor-enriched media and establish phenocopies of the parent tumor in serial xenotransplantation [7,9,10]. Importantly, these induced tumors are highly invasive, harboring cells that migrate widely throughout the brain [9,[11][12][13].
The specific biological behavior of invasive GBM cells suggests the activation of certain genetic programs that distinguish them from cells in the tumor core [14]. While global expression profiles of glial tumors have been studied extensively, less is known about specific gene expression in the invasive cells [15,16]. The experimental studies exploring transcriptional differences associated with invasion in brain slices [17] or in vivo xenograft models [18] do not use real-time observations that allows for analysis of movement patterns. Thus, an approach that allows transcriptome analysis of invading GBM cells with identified differences in invasive capacity by real time observation has been called for [19].
We have previously described the phenotype and invasive characteristics of invading glioma cells in organotypic brain slices.
Here we present studies on real-time quantification of human GBM cell invasion with comparative analysis of transcriptional profiles in noninvasive and invasive cells. Using a simple collagen 3D matrix system, we demonstrate how this system maintains the invasive characteristics found in more complex systems and how it allows for the detection of intra-and intertumoral heterogeneity to understand mechanisms of glial cell invasion.

Cell Culture
GBM biopsies were obtained from informed and consenting patients after approval by the Norwegian National Committee for Medical Research Ethics (07321b). The biopsies were dissociated into single cells and cultured in a serum-free medium supplemented with bFGF and EGF as previously described [20]. Seven primary cell cultures were established from brain tumor biopsies, all from IDH wild-type, treatment-naïve GBMs, of which one was classified as a giant cell GBM. Two of these cultures have previously been described (T0965, T1008) [12,21,22]. The tumorigenicity of all cultures was confirmed upon xenografting to SCID mice. The tumorspheres in the cultures displayed heterogeneous morphology and growth pattern characteristics, with population doubling times ranging from 2 to 8 days.

Grafting of GBM Cultures for Time-Lapse Microscopy
The plating of tumorspheres on fibronectin-coated plates and into rodent brain slices was performed as previously described [12,23]. For grafting tumorspheres into collagen gel rat collagen I protein (0.5 mg/ml) (Gibco) was prepared according to the manufacturer's recommendation and distributed as 30 μl drops in 24 well plates, before single tumor spheres of 150 to 250 μm were grafted into the gel by a 2 μl pipette. For cell-suspension grafting 2 μl containing approximately 500 cells was used. In experiments where tumorspheres were co-grafted with cell suspension verification of cell origin, whether single cell in suspension or sphere, was done by tracking cells by time-laps imaging starting immediately after grafting. After 30 to 45 min of gelation at 37°C in cell incubator, grafts were supplemented with 200 μl of tumor sphere medium supplemented with 1% foetal bovine serum (FBS).The timelapse imaging was performed on Olympus IX81 inverted fluorescence microscope with a temperature and environmental gas supply control. The time-lapse experiments lasted from one to 5 days with imaging every 20 min. Images were acquired using Olympus Soft Imaging Xcellence software.

Quantification of Invasive Parameters
For quantification of directionality and migratory velocity, postprocessing of the images was performed using the ImageJ package Fiji with a manual cell tracking plug-in. The manual tracking was performed by two independent researches. Directionality is the ratio between the length of a straight line between the start and endpoint of migration to the total accumulated distance and was calculated by Ibidi Chemotaxis and Migration Tool. Total invasive increment is the sum of all distances that invasive cells have moved from the tumorsphere. It was identified by nuclear-stained grafts analyzed by ImageJ software with the "Find Maxima" option. The obtained total number of invasive cells and their X/Y coordinates were transferred to Microsoft Excel.
For immunostaining of membrane surface receptors the GBM spheres were incubated prior to grafting in 4°C for 20 min with fluorescent-

Grafting of Tumorspheres into Collagen Gel for the Isolation of the Invasive Cells
After 2 days in incubator, gels were treated with collagenase (10 mg/ml) (Sigma) in PBS, and visually confirmed for the separation of cores and invasive cells, before the mixture was passed through a 40 μm cell filter to separate the invasive cells from graft cores. The cells were spun down twice before the precipitates were further processed for western blot, qPCR or microarray.

Western Blot
Western blot and quantification of protein expression was performed as previously described [21].

Statistics
GraphPad Prism 6.0 was used for statistics analysis and graphical presentation. Statistical analysis of difference between groups was undertaken using unpaired t-test. The results were considered significant if P b .05. Experiments were done in independent triplicates, unless otherwise stated. Error bars in graphical presentations represent standard error of the mean.

GBM Cellular Invasion is Maintained in a Simplified 3D-Milieue by the Presence of the Core
We have previously demonstrated the phenotype and invasive dynamics of migrating GBM cells in organotypic rat brain-slice cultures [12]. Cells plated on fibronectin are widely used for evaluating migration in the "scratch-assay", while 3D protein matrixes are suggested as better models to study glioma cell invasion [26]. To allow higher resolution imaging on a single cell level and minimize the effects of a heterogeneous extracellular environment, we compared the phenotype and motility pattern of glioma cells in these conditions to cells grafted into brain-slice cultures (Figure 1, Mov. 1). Cells plated on a fibronectin-coated plastic surface displayed sheetlike or fusiform morphology without exhibiting a constant leading process. Their migration was non-directional and associated with random formation of lamellipodia. After attachment of a tumorsphere to the surface, cells migrated out into a confluent monolayer. In contrast, tumorspheres grafted into a 3D collagen matrix developed a halo of invading cells. The majority of invading cells in the collagen matrix adopted a neural progenitor-like phenotype with a round small cell body and a long leading process. This is the same Type I cells that we found to dominate the invasion in organotypic brain-slice cultures [6]. These cells have elongated cell bodies with a diameter of 5 to 25 μm, and a distinct leading process with a length more than 3 to 5 times longer than the cell body. Cellular heterogeneity was present among the invasive cells, and all the cell types described in brain-slice cultures (Type II, giant cells and cells with random morphology) were also found in collagen matrix (data not shown).
We have previously described how signaling cues from the tumorsphere induced and oriented the migratory spread of invasive tumor cells in organotypic brain slice cultures [12]. We verified this in tumorspheres transplanted to collagen matrix using three different experimental setups (Figure 1 and Mov. 2 and 3). First, we dissociated GBM tumorspheres to single cell suspension before grafting into collagen matrix. When no tumorsphere was present, the cells migrated in a non-directional, random manner. Secondly, we performed grafting of single cell suspension to collagen matrix, followed by co-grafting of tumorspheres from the same tumor. After the tumorsphere was cografted, a fraction of the cells located close to the sphere acquired directional, radial movement away from the sphere. However, not all cells reacted to the co-grafting. A fraction of the cells, even close to the spheres, did not respond to the presence of the sphere by initiation of directed migration. These cells continued to move randomly, skipping past the passing stream of invading cells. Thirdly, we grafted tumorspheres into the collagen matrices and then removed the tumor cores by microsurgical resection after 24 hours. The resection of the tumor core interrupted the directional invasive migration of cells, resulting in a reduction of the invasive increment sum and directionality. In tumor T0965 the invasive increment was 281.3 ± 10.7 μm vs. 33.3 ± 2.3 μm (P b .01) and directionality 0.86 ± 0.01 vs. 0.19 ± 0.01 (P b .01), before and after core resection, respectively. In tumor T1402 the invasive increment was 431.7 ± 22.9 μm vs. 101.7 ± 33.6 μm (P b .01) and the directionality 0.78 ± 0.02 vs. 0.24 ± 0.03 (P b .01). In the control grafts, the majority of cells continued the invasive migration away from the core. Thus, without a tumor core, cells migrated randomly, however by introducing a tumorsphere targeted invasion was initiated. Such glioma cell invasion was preserved even in the absence of FBS (Sup. Figure 1).

Glioma Cell Invasion is Cell Culture Specific
Seven different primary established GBM cultures were grafted as single tumorspheres in collagen matrix. For each culture 20 to 70 unique tumorsphere graftings, derived from at least three different passages, were performed. The invasive pattern displayed extensive intertumoral variations between cultures, but was stable for the individual culture (Sup. Figure 2). Although most experiments were done at early passages (b5), the invasive pattern was maintained even after long-time maintenance in vitro (up to passage 20).
We quantified the level of the invasion in five of the cultures using the estimation of the total number of invasive cells and the invasive increment sum to confirm the difference of invasion. The cultures could thus be ranked according to invasiveness (Figure 2, A and B). In five of the seven cultures we found that the invasive increment changed at a culture-specific distance from the tumor core, and this distance was reached within 48 hours. While invasive increment was stable close to the tumor core, cells lost directional movement and started to move randomly after reaching this limit (P b .01 in all analyzed samples). Beyond this, cells moved similar to what we observed in cell suspension grafts and after core resections. Such cells kept forming processes in alternating directions without resulting in directional advancement (Figure 2, C-E, Mov. 4). In the two cultures with the highest invasion (T1402 and T1456) invasive cells continued to migrate radially, accompanied by a gradual decrease of the tumor core density and size, and cells kept on migrating until they reached the border of the collagen gel. Figure 1. GBM cellular invasion is maintained in a simplified 3D-milieue by the presence of the core. A) Comparison between migration/ invasion on 2D-fibronectin coating, 3D-collagen gel and rodent brain slices. The migration pattern of GBM cells in collagen gel grafted as suspension (B), suspension followed by co-grafting of tumorsphere (C), from tumorsphere (D) followed by the resection of the graft core (E). F) Representative GBM cell migration plots before and after graft core resection. BRbefore core resection, ARafter core resection. G) Comparison of migratory directionality and invasive increment in T0965 and T1402 before and after core resection (46 to 87 tracked cells per observation).

The Phenotype of Core and Invasive Cells
In the highly invasive T1402 and T1456 cultures the absolute majority of cells were Type I cells. The less invasive cultures had a much higher diversity of cellular phenotypes ( Figure 3A), similar to our data obtained from GBM invasion in brain slices [12]. Interestingly, in the cell culture derived from a giant cell glioblastoma (T1559), the invasive population was dominated by giant cells. Similar giant cells were also found in other cultures, although rarely (Sup. Figure 2).
To further evaluate the phenotype of the invading glioma cells we performed staining for markers of neural stem cells, astrocytes and neurons (nestin, GFAP and β-III-tubulin/MAP2, respectively). Despite differences in invasion patterns, all grafts were positive for all four markers (Sup. Figure 3, Mov. 5). All tumors had a similar pattern in marker distribution, with the overwhelming majority of cells being positive for nestin, β-III-tubulin and MAP2 both in the invasive front and in the tumor core. In contrast, GFAP staining was strongest in the core, and only a small proportion of the invasive cells were GFAP positive. qPCR-expression analysis verified the variation of GFAP found on immunochemistry (Sup. Figure 4).
To identify the movement pattern of specific cell populations, we used time-lapse imaging of cell cultures before fixation and immunostaining. GFAP + cells could then be backtracked on the time-lapse images to identify movement patterns. Analysis identified that the GFAP + cells had lower velocity than nestin + /GFAP − cells and achieved smaller invasive increment: average velocity 0.12 ± 0.01 vs. 0.39 ± 0.4 μm/min, average invasive increment 86 ± 21 vs. 188 ± 47 μm in GFAP + vs. nestin + /GFAP − invasive cells, respectively (Figure 3, B-D, Mov. 6). qPCR-expression analysis verified the variation of GFAP found on immunohistochemistry. The cultures with the highest invasive increment sum had the lowest expression of GFAP (Figure 3, F-G). In contrast, neither immunostaining nor qPCR could detect correlation between the level of β-III-tubulin expression and migratory capacity although this protein was present in the majority of the invasive cells in all cultures.
We observed cellular divisions and expression of Ki-67 in the invasive front (Sup. Figure 5). Interestingly, invasive cells were often positive for both Ki-67 and MAP2. Both markers were present in invasive cells that entered mitosis. We also used time-lapse recordings of invasion followed by imaging of markers on live or fixed samples (Sup. Figure 6). We could not identify any pattern in the staining distribution of the putative cancer stem/progenitor cell markers CXCR4, CD133, CD166, CD44, CD29 and CD9 related to the invasive properties of the cell cultures.

Grafted Core and Invasive Cells Have Significantly Different Gene Expression Profiles
To identify signaling cues that contribute to the directional cell movement, we analyzed gene expression in the core and the invasive front from three of the tumors grafted. We first investigated core and invasive cells in six grafts from the most invasive GBM culture (T1402), which allowed for a high number of invasive cells to be collected. Unsupervised hierarchical clustering of the samples separated core and invasive cells in two clusters. Rank analysis identified that 391 genes were significantly up-and down-regulated in invasive vs. core cells (P b .01, fold change N1.5). To identify signaling networks across cultures, we additionally included grafts from T1456 and T1008. The global gene expression differences between the cell cultures were larger than the gene expression differences found between invasive and core cells, as the individual cultures clustered separately ( Figure 4A). The results from these microarrays were validated by qPCR of the 10 most dysregulated genes, and a high expression of annexin and CTGF in invasive cells was verified by immunofluorescence (Figures 4, B-C, 5E).
Cultures showed an overall difference in subtype profile using unsupervised hierarchical clustering according to Philips et al. [27]. The highly invasive T1402 culture showed enrichment of genes representing the mesenchymal subtype (CHI3L1, PDPN, FAM20C, SERPINE1) ( Figure 4D). The less invasive cultures were more proneural-like (DLL3, NDRG2, SOX8), where the least invasive culture T1008 was more enriched in proneural genes than the more invasive culture T1456. Lineage markers were differently expressed between the cultures ( Figure 4E). While all cultures expressed high levels of the progenitor markers NES, FABP7 and SOX2, markers of more differentiated neural lineages (GFAP, CSPG4 and MAP2) had highest expression in T1008. Markers of mesenchymal lineage (CHI3L1, PDPN, FAM2OC, SERPINE and CD44) had highest expression in the most invasive culture T1402.

Core Cells Display an Expression Signature of Genes Related to Response from Hypoxia and Include Chemo-Repulsive Signals
As invasive increment was dependent on the presence of a tumorsphere, we further identified a signature of highly expressed genes in the tumor core cells. These genes could be part of a signal to inhibit migration in core cells or a secreted signal to promote the chemo-repulsive effects on the invasive cells. To discern among these possibilities, the two cell cultures that clearly maintained a tumor core were used to establish the signature. Using the Rank product algorithm we identified a highly significant core signature of 73 genes (Q = 0), clearly separating the core and invasive cells ( Figure 5 and Sup. Table 1). This signature could also separate core cells from invasive cells in the highly invasive T1402 culture. Interestingly, T1402 showed a stronger expression of this signature than T1456 and T1008, demonstrating that the signature was more related to chemo-repulsion than inhibition of migration. The signature included several genes known to be involved in response from hypoxia (VEGFA [28][29][30], ALDOC [31], ZNF395 [32], NDRG1 [33], DDIT4 [34]), as well as secreted signaling molecules suggested to play a role in invasion (VEGFA [35][36][37], AGT [38], NMB [39], ANGPTL4 [40], TF [41,42]).

Cells in the Invasive Front are Enriched in Genes Involved in Cell Survival and Aggressive Cancer
Similarly to the chemo-repulsive signature of core cells, we identified a signature of highly expressed genes in invasive cells in T1456 and T1008 by the Rank product algorithm. A significant invasive signature of 99 genes was identified (Q = 0), separating the core and invasive cells in (Figure 5 and Sup. Table 2). Unsupervised hierarchical clustering of these signature genes separated core and invasive cells in T1402, and their expression level in tumorsphere cultures under culturing conditions corresponded with the level of invasion identified in time-lapse experiments (T1402 N T1456 N T1008 N T0965). The invasive signature included genes known to be involved in regulation of cell cycle, cell survival and aggressive cancer such as MELK [43,44], BIRC5 [45], PBK [22], DLGAP5 [46] and CENPA [47]. The TWEAK receptor (TNFRSF12A) and CTGF were among the most dysregulated genes in the signature. We found CTGF to be highly expressed in invasive cells of T1402 on both western blot and immunocytochemistry. TNFRSF12A expression was confirmed on immunochemistry.

Discussion
We have previously used live brain slice cultures to explore invasion by cells expressing fluorescent probes [6]. In this study, we used a collagen matrix, which in contrast to the heterogeneous landscape of the brain-slice cultures allows for evaluation of single-cell movements at higher resolution and tracking of cells without the use of Translational Oncology Vol. 12, No. 1, 2019 Heterogeneity in Invasive Glioma Cells Fayzullin et al.   fluorescent labeling. The stability of the invasive cellular phenotype between different GBM cultures and the presence of a cell culture specific magnitude of invasion suggests that this model recapitulates the variance in tumor cell dispersal seen in patients. The assay is clearly a great simplification, even compared to the brain-slice invasion assay. [48]. Vascular cells, microglia, peripheral immune cells, neural progenitors, and mature neural cells all contribute to the disease evolution [49]. In the present work we strived to limit the number of variables, leaving only the core, invasive cells and 3D matrix as the scaffold for invasion.

The Signaling From the Core of Grafted GBM Tumorspheres Plays a Crucial Role in Promoting Invasion
Several studies have shown that selected proteins guide both neural and glial precursors in the developing brain and play a crucial role in axon guidance: netrins, semaphorins, slit-family proteins, PDGF, HGF and others [50]. Neural stem cells and glioma cells may follow similar topical cues. Glioma tumorspheres release factors that direct cells away from the implanted spheroid in 3D-matrixes by repellent signals [50][51][52]. The magnitude of invasion has been correlated with the hypoxia level in the necrotic center of the implanted spheres [52]. The stressful conditions in the hypoxic core, such as decreased pH, a low level of oxygen and accumulation of metabolic substrates may lead to the generation of repellent factors and a concentration gradient. This results in driving cell invasion from the core and invasion arrest when the gradient is no longer present [50]. Our gene expression signature from tumor core cells support that such signaling is related to long distance migration. By functional time-lapse experiments, we also show that signaling from the tumorsphere is dictating the range and direction of GBM cell invasion. While cellular directionality and speed are affected over distance, enablement of cells to initiate invasion away from the sphere could be regulated though other mechanisms, as, for example, contact inhibition. It has been shown that contact inhibition is involved in switching from restrained to invasive migration of cancer cells [53].
The gene expression data suggests that the repulsive signal produced by the tumorsphere is highest in tumors of the mesenchymal subtype. This does not fit well with a published study on the analysis of tumor samples obtained during surgery, where biopsies from the tumor core showed the highest correlation with the mesenchymal and classical subtypes, whilst biopsies from the invasive area have the highest correlation with the proneural subtypes [16]. This discrepancy might be due technical considerations, as the biopsies from the invasive area would definitely contain normal brain tissue tilting the expression towards a proneural subtype.
The signature of tumor cores encompasses signaling related to hypoxia and neovascularization, where VEGFA was among the most highly regulated genes. Besides playing a crucial role in physiological angiogenesis and tumor neovascularization, VEGF binds to neuropilin/plexin receptorsthe primary receptors for the semaphorin family of proteins. These proteins are central to the control of directed migration of neural progenitor cells [54]. Since VEGF and semaphorins can bind to the same receptor, the VEGF-signaling cascade could be important in orchestrating the chemorepulsive signals in driving GBM migration [55].

The Invasive Glioma Cell Phenotype
Migrating glioma cells display a phenotype similar to migrating neural progenitors. This could be an adaptation to migration through the complex extracellular environment of the brain [56,57]. This phenotype, which we call Type I, can be found at the invasive front in both the complex neuropil of brain slice cultures and in the simple 3D collagen matrix suggests that this phenotype is cell-type intrinsic and not a secondary to the extracellular milieu. The fact that other types of non-glial cancer cells move through collagen 3D matrixes by lamellipodia-based or amoeboid migration [58], without displaying this Type I phenotype, supports this interpretation.
The migratory Type I cells with the highest invasion were GFAP negative. Interestingly, others have shown that GFAP overexpression inhibits glioma cell motility [55] and that down-regulation of GFAP expression is often seen in high-grade gliomas in vivo [59]. Also, cultures with higher invasive increment had lower levels of neural lineage markers, and expressed genes associated with a mesenchymal subtype or lineage. This fits well with a transition towards a mesenchymal expression upon tumor progression [27]. Furthermore, our data highlights the malignancy of the highly invasive cells. Not only do they have the ability to escape tumor resection, but they maintain the ability to proliferate as demonstrated by time-lapse records, Ki-67 staining and overexpressing genes related to the cell cycle and resistance to cell death.
The invasive cells overexpress a number of genes, among which TNFRSF12A and CTGF were most notable. The expression of TNFRSF12A receptor gene is up-regulated in migrating glioma cells in vitro and overexpressed in the invasive rim of cells in surgical glioma specimens [60] and, in particular, in GBMs exhibiting the mesenchymal molecular subtype [61]. As its overexpression is associated with invasion and resistance to chemotherapeutic agents in vitro, TNFRSF12A is suggested as a potential cell surface portal for targeted delivery of anti-GBM therapeutics [61]. CYR61 and CTGF have also previously been shown to be important for glioma motility in vitro [15] and are associated with prognosis, survival and drug resistance in patients with gliomas [62,63].
Clinical experience demonstrates that invasion is present in all glioblastoma tumors, but to a varying degree. This is consistent with the in vitro data we present here. The overwhelming majority of current strategies to influence the cancer invasion aims to inhibit the tumor cells ability to migrate [64]. In GBM this has little sense: at the moment of diagnosis the GBM cells has already spread through the brain due to the extremely high invasive properties. Blocking or reducing the migratory ability of the GBM cellsthat are already at the distant site from the main bulkwould have little effects on tumor spread. Thus, new strategies should selectively target the cells that already invaded in to the brain, eliminate their tumorigenic potential, or force them to change their migratory path in order to make them become more "available" for targeted therapy, such as surgery or focused irradiation [65]. We hope the presented data would add to this understanding.

Conclusion
Although being performed in a simple system, time-lapse imaging of GBM cells in a collagen matrix displays intertumoral variations of invasion pattern. The pro-invasive signaling differs in strength between patient derived cultures, similar to what we see in the clinical setting. We show that glioblastoma tumorsphere core promotes invasion of a defined subpopulation of tumor cells.
Transcriptional differences between invasive and core cells suggest that invasive cells may be manipulated to affect brain tumor behavior with a consequence on treatment outcome. Both the phenotypically