CD15 Expression Does Not Identify a Phenotypically or Genetically Distinct Glioblastoma Population

Tools to study neural and glioma stem cell quiescence

Quiescence is a prolonged but reversible state of cell-cycle arrest that is an adaptive feature of most adult stem cell populations. In the brain, quiescence helps to protect adult neural stem cells from stress and supports lifelong neurogenesis. Unfortunately however, entry into a quiescent or a slow-cycling state is also a malignant feature of brain cancer stem cells. In glioblastoma, where the process has been best characterised, quiescent glioma stem cells preferentially survive chemoradiation, and after therapy, reactivate to regrow the tumour and drive recurrence. In this Review, we discuss the in vitro and in vivo models that have been developed for studying neural stem cell quiescence and how these tools may be used to deepen biological understanding and to develop novel therapies targeting quiescent glioma stem cells.

Understanding glioblastoma at the single-cell level: Recent advances and future challenges

Glioblastoma, the most aggressive and prevalent form of primary brain tumor, is characterized by rapid growth, diffuse infiltration, and resistance to therapies. Intrinsic heterogeneity and cellular plasticity contribute to its rapid progression under therapy; therefore, there is a need to fully understand these tumors at a single-cell level. Over the past decade, single-cell transcriptomics has enabled the molecular characterization of individual cells within glioblastomas, providing previously unattainable insights into the genetic and molecular features that drive tumorigenesis, disease progression, and therapy resistance. However, despite advances in single-cell technologies, challenges such as high costs, complex data analysis and interpretation, and difficulties in translating findings into clinical practice persist. As single-cell technologies are developed further, more insights into the cellular and molecular heterogeneity of glioblastomas are expected, which will help guide the development of personalized and effective therapies, thereby improving prognosis and quality of life for patients.

Glioma Stem Cells: Novel Data Obtained by Single-Cell Sequencing

Glioma is the most common type of primary CNS tumor, composed of cells that resemble normal glial cells. Recent genetic studies have provided insight into the inter-tumoral heterogeneity of gliomas, resulting in the updated 2021 WHO classification of gliomas. Thorough understanding of inter-tumoral heterogeneity has already improved the prognosis and treatment outcomes of some types of gliomas. Currently, the challenge for researchers is to study the intratumoral cell heterogeneity of newly defined glioma subtypes. Cancer stem cells (CSCs) present in gliomas and many other tumors are an example of intratumoral heterogeneity of great importance. In this review, we discuss the modern concept of glioma stem cells and recent single-cell sequencing-driven progress in the research of intratumoral glioma cell heterogeneity. The particular emphasis was placed on the recently revealed variations of the cell composition of the subtypes of the adult-type diffuse gliomas, including astrocytoma, oligodendroglioma and glioblastoma. The novel data explain the inconsistencies in earlier glioma stem cell research and also provide insight into the development of more effective targeted therapy and the cell-based immunotherapy of gliomas. Separate sections are devoted to the description of single-cell sequencing approach and its role in the development of cell-based immunotherapies for glioma.

Phenotyping clonal populations of glioma stem cell reveals a high degree of plasticity in response to changes of microenvironment

The phenotype of glioma-initiating cells (GIC) is modulated by cell-intrinsic and cell-extrinsic factors. Phenotypic heterogeneity and plasticity of GIC is an important limitation to therapeutic approaches targeting cancer stem cells. Plasticity also presents a challenge to the identification, isolation, and propagation of purified cancer stem cells. Here we use a barcode labelling approach of GIC to generate clonal populations over a number of passages, in combination with phenotyping using the established stem cell markers CD133, CD15, CD44, and A2B5. Using two cell lines derived from isocitrate dehydrogenase (IDH)-wildtype glioblastoma, we identify a remarkable heterogeneity of the phenotypes between the cell lines. During passaging, clonal expansion manifests as the emergence of a limited number of barcoded clones and a decrease in the overall number of clones. Dual-labelled GIC are capable of forming traceable clonal populations which emerge after as few as two passages from mixed cultures and through analyses of similarity of relative proportions of 16 surface markers we were able to pinpoint the fate of such populations. By generating tumour organoids we observed a remarkable persistence of dominant clones but also a significant plasticity of stemness marker expression. Our study presents an experimental approach to simultaneously barcode and phenotype glioma-initiating cells to assess their functional properties, for example to screen newly established GIC for tumour-specific therapeutic vulnerabilities.

Cancer cell heterogeneity and plasticity: A paradigm shift in glioblastoma

Phenotypic plasticity has emerged as a major contributor to intra-tumoral heterogeneity and treatment resistance in cancer. Increasing evidence shows that glioblastoma (GBM) cells display prominent intrinsic plasticity and reversibly adapt to dynamic microenvironmental conditions. Limited genetic evolution at recurrence further suggests that resistance mechanisms also largely operate at the phenotypic level. Here we review recent literature underpinning the role of GBM plasticity in creating gradients of heterogeneous cells including those that carry cancer stem cell (CSC) properties. A historical perspective from the hierarchical to the nonhierarchical concept of CSCs towards the recent appreciation of GBM plasticity is provided. Cellular states interact dynamically with each other and with the surrounding brain to shape a flexible tumor ecosystem, which enables swift adaptation to external pressure including treatment. We present the key components regulating intra-tumoral phenotypic heterogeneity and the equilibrium of phenotypic states, including genetic, epigenetic, and microenvironmental factors. We further discuss plasticity in the context of intrinsic tumor resistance, where a variable balance between preexisting resistant cells and adaptive persisters leads to reversible adaptation upon treatment. Innovative efforts targeting regulators of plasticity and mechanisms of state transitions towards treatment-resistant states are needed to restrict the adaptive capacities of GBM.

Opportunities and challenges of glioma organoids

Glioma is the most common primary brain tumor and its prognosis is poor. Despite surgical removal, glioma is still prone to recurrence because it grows rapidly in the brain, is resistant to chemotherapy, and is highly aggressive. Therefore, there is an urgent need for a platform to study the cell dynamics of gliomas in order to discover the characteristics of the disease and develop more effective treatments. Although 2D cell models and animal models in previous studies have provided great help for our research, they also have many defects. Recently, scientific researchers have constructed a 3D structure called Organoids, which is similar to the structure of human tissues and organs. Organoids can perfectly compensate for the shortcomings of previous glioma models and are currently the most suitable research platform for glioma research. Therefore, we review the three methods currently used to establish glioma organoids. And introduced how they play a role in the diagnosis and treatment of glioma. Finally, we also summarized the current bottlenecks and difficulties encountered by glioma organoids, and the current efforts to solve these difficulties.

Glioma Stem-Like Cells and Metabolism: Potential for Novel Therapeutic Strategies

Glioma stem-like cells (GSCs) were first described as a population which may in part be resistant to traditional chemotherapeutic therapies and responsible for tumour regrowth. Knowledge of the underlying metabolic complexity governing GSC growth and function may point to potential differences between GSCs and the tumour bulk which could be harnessed clinically. There is an increasing interest in the direct/indirect targeting or reprogramming of GSC metabolism as a potential novel therapeutic approach in the adjuvant or recurrent setting to help overcome resistance which may be mediated by GSCs. In this review we will discuss stem-like models, interaction between metabolism and GSCs, and potential current and future strategies for overcoming GSC resistance.

The adaptive transition of glioblastoma stem cells and its implications on treatments

Glioblastoma is the most malignant tumor occurring in the human central nervous system with overall median survival time <14.6 months. Current treatments such as chemotherapy and radiotherapy cannot reach an optimal remission since tumor resistance to therapy remains a challenge. Glioblastoma stem cells are considered to be responsible for tumor resistance in treating glioblastoma. Previous studies reported two subtypes, proneural and mesenchymal, of glioblastoma stem cells manifesting different sensitivity to radiotherapy or chemotherapy. Mesenchymal glioblastoma stem cells, as well as tumor cells generate from which, showed resistance to radiochemotherapies. Besides, two metabolic patterns, glutamine or glucose dependent, of mesenchymal glioblastoma stem cells also manifested different sensitivity to radiochemotherapies. Glutamine dependent mesenchymal glioblastoma stem cells are more sensitive to radiotherapy than glucose-dependent ones. Therefore, the transition between proneural and mesenchymal subtypes, or between glutamine-dependent and glucose-dependent, might lead to tumor resistance to radiochemotherapies. Moreover, neural stem cells were also hypothesized to participate in glioblastoma stem cells mediated tumor resistance to radiochemotherapies. In this review, we summarized the basic characteristics, adaptive transition and implications of glioblastoma stem cells in glioblastoma therapy.

Cancer stem cells and macrophages: molecular connections and future perspectives against cancer

Cancer stem cells (CSCs) have been considered the key drivers of cancer initiation and progression due to their unlimited self-renewal capacity and their ability to induce tumor formation. Macrophages, particularly tumor-associated macrophages (TAMs), establish a tumor microenvironment to protect and induce CSCs development and dissemination. Many studies in the past decade have been performed to understand the molecular mediators of CSCs and TAMs, and several studies have elucidated the complex crosstalk that occurs between these two cell types. The aim of this review is to define the complex crosstalk between these two cell types and to highlight potential future anti-cancer strategies.

Abuhamad A, Syahir A. Understanding Glioblastoma Biomarkers: Knocking a Mountain with a Hammer

Gliomas are the most frequent and deadly form of human primary brain tumors. Among them, the most common and aggressive type is the high-grade glioblastoma multiforme (GBM), which rapidly grows and renders patients a very poor prognosis. Meanwhile, cancer stem cells (CSCs) have been determined in gliomas and play vital roles in driving tumor growth due to their competency in self-renewal and proliferation. Studies of gliomas have recognized CSCs via specific markers. This review comprehensively examines the current knowledge of the most significant CSCs markers in gliomas in general and in glioblastoma in particular and specifically focuses on their outlook and importance in gliomas CSCs research. We suggest that CSCs should be the superior therapeutic approach by directly targeting the markers. In addition, we highlight the association of these markers with each other in relation to their cascading pathways, and interactions with functional miRNAs, providing the role of the networks axes in glioblastoma signaling pathways.

Personalized therapy and stem cell transplantation for pro-inflammatory modulation of cancer stem cells microenvironment in glioblastoma: Review

Glioblastoma multiforme (GBM) is one of the most aggressive types of brain tumor in humans. The prognosis for patients with GBM is unfavorable and treatment is largely ineffective, where modern treatment regimens typically increase survival by 15 months. GBM relapse and progression are associated with cancer stem cells (CSCs). The present review provides a critical analysis of the primary reasons underlying the lack of effectiveness of modern CSC management methods. An emphasis is placed on the role of the blood-brain barrier in the development of treatment resistance. The existing methods for increasing the efficiency of antitumor genotoxic therapy are also described, and a strategy for personalized regulation of CSC based on post-genome technologies is suggested. The hypothesis that GBM cells employ a special mechanism for DNA repair based on their interactions with normal stem cells, is presented and the function of the tumor microenvironment in fulfilling the antitumor potential of normal stem cells is explained. Additionally, the mechanisms by which cancer stem cells regulate glioblastoma progression and recurrence are described based on novel biomedical technologies.

Stem cell-associated heterogeneity in Glioblastoma results from intrinsic tumor plasticity shaped by the microenvironment

The identity and unique capacity of cancer stem cells (CSC) to drive tumor growth and resistance have been challenged in brain tumors. Here we report that cells expressing CSC-associated cell membrane markers in Glioblastoma (GBM) do not represent a clonal entity defined by distinct functional properties and transcriptomic profiles, but rather a plastic state that most cancer cells can adopt. We show that phenotypic heterogeneity arises from non-hierarchical, reversible state transitions, instructed by the microenvironment and is predictable by mathematical modeling. Although functional stem cell properties were similar in vitro, accelerated reconstitution of heterogeneity provides a growth advantage in vivo, suggesting that tumorigenic potential is linked to intrinsic plasticity rather than CSC multipotency. The capacity of any given cancer cell to reconstitute tumor heterogeneity cautions against therapies targeting CSC-associated membrane epitopes. Instead inherent cancer cell plasticity emerges as a novel relevant target for treatment.

Cancer stem cells (CSCs) comprise a putative population that can drive growth and resistance. Here, in glioblastoma models the authors show that rather than being a distinct clonal entity, the CSC population represents a plastic state adoptable by most cancer cells via reversible state transitions induced by the microenvironment.

Myeloid-Derived Suppressor Cells and Their Potential Application in Transplantation

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immunosuppressive cells of the myeloid lineage upregulated by mediators of inflammation, such as IL-2, granulocyte colony-stimulating factor, and S100A8/A9. These cells have been studied extensively by tumor biologists. Because of their robust immunosuppressive potential, MDSCs have stirred recent interest among transplant immunologists as well. MDSCs inhibit T-cell responses through, among other mechanisms, the activity of arginase-1 and inducible nitric oxide synthase, and the expansion of T regulatory cells. In the context of transplantation, MDSCs have been studied in several animal models, and to a lesser degree in humans. Here, we will review the immunosuppressive qualities of this important cell type and discuss the relevant studies of MDSCs in transplantation. It may be possible to exploit the immunosuppressive capacity of MDSCs for the benefit of transplant patients.

WINDOW consortium: A path towards increased therapy efficacy against glioblastoma

Glioblastoma is the most common and malignant form of brain cancer, for which the standard treatment is maximal surgical resection, radiotherapy and chemotherapy. Despite these interventions, mean overall survival remains less than 15 months, during which extensive tumor infiltration throughout the brain occurs. The resulting metastasized cells in the brain are characterized by chemotherapy resistance and extensive intratumoral heterogeneity. An orthogonal approach attacking both intracellular resistance mechanisms as well as intercellular heterogeneity is necessary to halt tumor progression. For this reason, we established the WINDOW Consortium (Window for Improvement for Newly Diagnosed patients by Overcoming disease Worsening), in which we are establishing a strategy for rational selection and development of effective therapies against glioblastoma. Here, we overview the many challenges posed in treating glioblastoma, including selection of drug combinations that prevent therapy resistance, the need for drugs that have improved blood brain barrier penetration and strategies to counter heterogeneous cell populations within patients. Together, this forms the backbone of our strategy to attack glioblastoma.

Temozolomide-induced increase of tumorigenicity can be diminished by targeting of mitochondria in in vitro models of patient individual glioblastoma

Glioblastoma multiforme (GBM) is a highly heterogeneous and aggressive brain tumor with a dismal prognosis. Development of resistance towards cytostatic drugs like the GBM standard drug temozolomide is a severe problem in GBM treatment. One potential source of GBM relapse could be so called cancer stem like cells (CSCs). These represent an undifferentiated subpopulation of cells with high potential for tumor initiation. Furthermore, it has been shown that differentiated GBM cells can regain CSC properties when exposed to continuous temozolomide treatment in vitro. In this study, treatment of several primary GBM cell lines with clinically relevant doses of temozolomide increased their tumorigenicity as determined by colony formation assays in soft agar. Increased tumorigenicity is a known property of CSCs. Hence, therapy options that specifically target CSCs are under investigation. CSCs appear to be particularly dependent on mitochondria biogenesis which may represent a useful target for CSC elimination. Toxicity towards mitochondria is a known side effect of several antibiotics. Thus, addition of antibiotics like doxycycline may represent a useful tool to inhibit CSCs in GBM. Here, we show that combining temozolomide treatment of primary GBM cells with doxycycline could counteract the increase of tumorigenicity induced by temozolomide treatment.

TGFβ-Responsive HMOX1 Expression Is Associated with Stemness and Invasion in Glioblastoma Multiforme

Glioblastoma multiforme (GBM) is the most common and lethal adult brain tumor. Resistance to standard radiation and chemotherapy is thought to involve survival of GBM cancer stem cells (CSCs). To date, no single marker for identifying GBM CSCs has been able to capture the diversity of CSC populations, justifying the needs for additional CSC markers for better characterization. Employing targeted mass spectrometry, here we present five cell-surface markers HMOX1, SLC16A1, CADM1, SCAMP3, and CLCC1 which were found to be elevated in CSCs relative to healthy neural stem cells (NSCs). Transcriptomic analyses of REMBRANDT and TCGA compendiums also indicated elevated expression of these markers in GBM relative to controls and non-GBM diseases. Two markers SLC16A1 and HMOX1 were found to be expressed among pseudopalisading cells that reside in the hypoxic region of GBM, substantiating the histopathological hallmarks of GBM. In a prospective study (N = 8) we confirmed the surface expression of HMOX1 on freshly isolated primary GBM cells (P0). Employing functional assays that are known to evaluate stemness, we demonstrate that elevated HMOX1 expression is associated with stemness in GBM and can be modulated through TGFβ. siRNA-mediated silencing of HMOX1 impaired GBM invasion-a phenomenon related to poor prognosis. In addition, surgical resection of GBM tumors caused declines (18% ± 5.1SEM) in the level of plasma HMOX1 as measured by ELISA, in 8/10 GBM patients. These findings indicate that HMOX1 is a robust predictor of GBM CSC stemness and pathogenesis. Further understanding of the role of HMOX1 in GBM may uncover novel therapeutic approaches. Stem Cells 2016;34:2276-2289.

The extracellular matrix niche microenvironment of neural and cancer stem cells in the brain

Numerous studies demonstrated that neural stem cells and cancer stem cells (NSCs/CSCs) share several overlapping characteristics such as self-renewal, multipotency and a comparable molecular repertoire. In addition to the intrinsic cellular properties, NSCs/CSCs favor a similar environment to acquire and maintain their characteristics. In the present review, we highlight the shared properties of NSCs and CSCs in regard to their extracellular microenvironment called the NSC/CSC niche. Moreover, we point out that extracellular matrix (ECM) molecules and their complementary receptors influence the behavior of NSCs/CSCs as well as brain tumor progression. Here, we focus on the expression profile and functional importance of the ECM glycoprotein tenascin-C, the chondroitin sulfate proteoglycan DSD-1-PG/phosphacan but also on other important glycoprotein/proteoglycan constituents. Within this review, we specifically concentrate on glioblastoma multiforme (GBM). GBM is the most common malignant brain tumor in adults and is associated with poor prognosis despite intense and aggressive surgical and therapeutic treatment. Recent studies indicate that GBM onset is driven by a subpopulation of CSCs that display self-renewal and recapitulate tumor heterogeneity. Based on the CSC hypothesis the cancer arises just from a small subpopulation of self-sustaining cancer cells with the exclusive ability to self-renew and maintain the tumor. Besides the fundamental stem cell properties of self-renewal and multipotency, GBM stem cells share further molecular characteristics with NSCs, which we would like to review in this article.