Transformation of quiescent adult oligodendrocyte precursor cells into malignant glioma through a multistep reactivation process

2-Methoxyestradiol, an Endogenous 17β-Estradiol Metabolite, Induces Antimitogenic and Apoptotic Actions in Oligodendroglial Precursor Cells and Triggers Endoreduplication via the p53 Pathway

The abnormal growth of oligodendrocyte precursor cells (OPCs) significantly contributes to the progression of glioblastoma tumors. Hence, molecules that block OPC growth may be of therapeutic importance in treating gliomas. 2-Methoxyestradiol (2ME), an endogenous tubulin-interacting metabolite of estradiol, is effective against multiple proliferative disorders. Based on its anti-carcinogenic and anti-angiogenic actions, it is undergoing phase II clinical trials. We hypothesize that 2ME may prevent glioma growth by targeting OPC growth. Here, we tested this hypothesis by assessing the impact of 2ME on the growth of an OPC line, "Oli-neu", and dissected the underlying mechanism(s). Treatment with 2ME inhibited OPC growth in a concentration-dependent manner, accompanied by significant upregulation in the expression of p21 and p27, which are negative cell-cycle regulators. Moreover, treatment with 2ME altered OPC morphology from multi-arm processes to rounded cells. At concentrations of 1uM and greater, 2ME induced apoptosis, with increased expressions of caspase 3, PARP, and caspase-7 fragments, externalized phosphatidylserine staining/APOPercentage, and increased mitochondrial activity. Flow cytometry and microscopic analysis demonstrated that 2ME triggers endoreduplication in a concentration-dependent fashion. Importantly, 2ME induced cyclin E, JNK1/2, and p53 expression, as well as OPC fusion, which are key mechanisms driving endoreduplication and whole-genome duplication. Importantly, the inhibition of p53 with pifithrin-α rescued 2ME-induced endoreduplication. The pro-apoptotic and endoreduplication actions of 2ME were accompanied by the upregulation of survivin, cyclin A, Cyclin B, Cyclin D2, and ppRB. Similar growth inhibitory, apoptotic, and endoreduplication effects of 2ME were observed in CG4 cells. Taken together, our findings provide evidence that 2ME not only inhibits OPC growth and triggers apoptosis, but also activates OPCs into survival (fight or flight) mode, leading to endoreduplication. This inherent survival characteristic of OPCs may, in part, be responsible for drug resistance in gliomas, as observed for many tubulin-interacting drugs. Importantly, the fate of OPCs after 2ME treatment may depend on the cell-cycle status of individual cells. Combining tubulin-interfering molecules with drugs such as pifithrin-α that inhibit endoreduplication may help inhibit OPC/glioma growth and limit drug resistance.

PDGF-BB overexpression in p53 null oligodendrocyte progenitors increases H3K27me3 and induces transcriptional changes which favor proliferation

Proneural gliomas are brain tumors characterized by enrichment of oligodendrocyte progenitor cell (OPC) transcripts and genetic alterations. In this study we sought to identify transcriptional and epigenetic differences between OPCs with Trp53 deletion and PDGF-BB overexpression (BB-p53n), which form tumors when transplanted in mouse brains, and those carrying only p53 deletion (p53n), which do not. We used unbiased histone proteomics and RNA-seq analysis on these two genetically modified OPC populations and detected higher levels of H3K27me3 in BB-p53n compared to p53n OPCs. The BB-p53n OPC were characterized by higher levels of transcripts related to proliferation and lower levels of those related to differentiation. Pharmacological inhibition of histone H3K27 trimethylation in BB-p53n OPC reduced cell cycle transcripts and increased the expression of differentiation markers. These data suggest that PDGF-BB overexpression in p53 null OPC results in histone post-translational modifications and consequent transcriptional changes favoring proliferation while halting differentiation, thereby promoting the early stages of transformation.

Integrated electrophysiological and genomic profiles of single cells reveal spiking tumor cells in human glioma

Prior studies have described the complex interplay that exists between glioma cells and neurons, however, the electrophysiological properties endogenous to tumor cells remain obscure. To address this, we employed Patch-sequencing on human glioma specimens and found that one third of patched cells in IDH mutant (IDH mut ) tumors demonstrate properties of both neurons and glia by firing single, short action potentials. To define these hybrid cells (HCs) and discern if they are tumor in origin, we developed a computational tool, Single Cell Rule Association Mining (SCRAM), to annotate each cell individually. SCRAM revealed that HCs represent tumor and non-tumor cells that feature GABAergic neuron and oligodendrocyte precursor cell signatures. These studies are the first to characterize the combined electrophysiological and molecular properties of human glioma cells and describe a new cell type in human glioma with unique electrophysiological and transcriptomic properties that are likely also present in the non-tumor mammalian brain.

EZH2 Inhibition Sensitizes IDH1R132H-Mutant Gliomas to Histone Deacetylase Inhibitor

Isocitrate Dehydrogenase-1 (IDH1) is commonly mutated in lower-grade diffuse gliomas. The IDH1R132H mutation is an important diagnostic tool for tumor diagnosis and prognosis; however, its role in glioma development, and its impact on response to therapy, is not fully understood. We developed a murine model of proneural IDH1R132H-mutated glioma that shows elevated production of 2-hydroxyglutarate (2-HG) and increased trimethylation of lysine residue K27 on histone H3 (H3K27me3) compared to IDH1 wild-type tumors. We found that using Tazemetostat to inhibit the methyltransferase for H3K27, Enhancer of Zeste 2 (EZH2), reduced H3K27me3 levels and increased acetylation on H3K27. We also found that, although the histone deacetylase inhibitor (HDACi) Panobinostat was less cytotoxic in IDH1R132H-mutated cells (either isolated from murine glioma or oligodendrocyte progenitor cells infected in vitro with a retrovirus expressing IDH1R132H) compared to IDH1-wild-type cells, combination treatment with Tazemetostat is synergistic in both mutant and wild-type models. These findings indicate a novel therapeutic strategy for IDH1-mutated gliomas that targets the specific epigenetic alteration in these tumors.

BCAS1 defines a heterogeneous cell population in diffuse gliomas

Oligodendrocyte precursor markers have become of great interest to identify new diagnostic and therapeutic targets for diffuse gliomas, since state-of-the-art studies point towards immature oligodendrocytes as a possible source of gliomagenesis. Brain enriched myelin associated protein 1 (BCAS1) is a novel marker of immature oligodendrocytes and was proposed to contribute to tumorigenesis in non-central nervous system tumors. However, BCAS1 role in diffuse glioma is still underexplored. This study analyzes the expression of BCAS1 in different tumor samples from patients with diffuse gliomas (17 oligodendrogliomas; 8 astrocytomas; 60 glioblastomas) and uncovers the molecular and ultrastructural features of BCAS1+ cells by immunostaining and electron microscopy. Our results show that BCAS1+ cells exhibit stellate or spherical morphology with similar ultrastructural features. Stellate and spherical cells were detected as isolated cells in all studied gliomas. Nevertheless, only stellate cells were found to be proliferative and formed tightly packed nodules with a highly proliferative rate in oligodendrogliomas. Our findings provide a comprehensive characterization of the BCAS1+ cell population within diffuse gliomas. The observed proliferative capacity and distribution of BCAS1+ stellate cells, particularly in oligodendrogliomas, highlight BCAS1 as an interesting marker, warranting further investigation into its role in tumor malignancy.

Neuron-oligodendroglial interactions in health and malignant disease

Experience sculpts brain structure and function. Activity-dependent modulation of the myelinated infrastructure of the nervous system has emerged as a dimension of adaptive change during childhood development and in adulthood. Myelination is a richly dynamic process, with neuronal activity regulating oligodendrocyte precursor cell proliferation, oligodendrogenesis and myelin structural changes in some axonal subtypes and in some regions of the nervous system. This myelin plasticity and consequent changes to conduction velocity and circuit dynamics can powerfully influence neurological functions, including learning and memory. Conversely, disruption of the mechanisms mediating adaptive myelination can contribute to cognitive impairment. The robust effects of neuronal activity on normal oligodendroglial precursor cells, a putative cellular origin for many forms of glioma, indicates that dysregulated or 'hijacked' mechanisms of myelin plasticity could similarly promote growth in this devastating group of brain cancers. Indeed, neuronal activity promotes the pathogenesis of many forms of glioma in preclinical models through activity-regulated paracrine factors and direct neuron-to-glioma synapses. This synaptic integration of glioma into neural circuits is central to tumour growth and invasion. Thus, not only do neuron-oligodendroglial interactions modulate neural circuit structure and function in the healthy brain, but neuron-glioma interactions also have important roles in the pathogenesis of glial malignancies.

Amyloids and brain cancer: molecular linkages and crossovers

Amyloids are high-order proteinaceous formations deposited in both intra- and extracellular spaces. These aggregates have tendencies to deregulate cellular physiology in multiple ways; for example, altered metabolism, mitochondrial dysfunctions, immune modulation, etc. When amyloids are formed in brain tissues, the endpoint often is death of neurons. However, interesting but least understood is a close connection of amyloids with another set of conditions in which brain cells proliferate at an extraordinary rate and form tumor inside brain. Glioblastoma is one such condition. Increasing number of evidence indicate a possible link between amyloid formation and depositions in brain tumors. Several proteins associated with cell cycle regulation and apoptotic pathways themselves have shown to possess high tendencies to form amyloids. Tumor suppressor protein p53 is one prominent example that mutate, oligomerize and form amyloids leading to loss- or gain-of-functions and cause increased cell proliferation and malignancies. In this review article, we present available examples, genetic links and common pathways that indicate that possibly the two distantly placed pathways: amyloid formation and developing cancers in the brain have similarities and are mechanistically intertwined together.

Simultaneous Nbs1 and p53 inactivation in neural progenitors triggers high-grade gliomas

AIMS

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive disorder caused by hypomorphic mutations of NBS1. NBS1 is a member of the MRE11-RAD50-NBS1 (MRN) complex that binds to DNA double-strand breaks and activates the DNA damage response (DDR). Nbs1 inactivation in neural progenitor cells leads to microcephaly and premature death. Interestingly, p53 homozygous deletion rescues the NBS1-deficient phenotype allowing long-term survival. The objective of this work was to determine whether simultaneous inactivation of Nbs1 and p53 in neural progenitors triggered brain tumorigenesis and if so in which category this tumour could be classified.

METHODS

We generated a mouse model with simultaneous genetic inactivation of Nbs1 and p53 in embryonic neural stem cells and analysed the arising tumours with in-depth molecular analyses including immunohistochemistry, array comparative genomic hybridisation (aCGH), whole exome-sequencing and RNA-sequencing.

RESULTS

NBS1/P53-deficient mice develop high-grade gliomas (HGG) arising in the olfactory bulbs and in the cortex along the rostral migratory stream. In-depth molecular analyses using immunohistochemistry, aCGH, whole exome-sequencing and RNA-sequencing revealed striking similarities to paediatric human HGG with shared features with radiation-induced gliomas (RIGs).

CONCLUSIONS

Our findings show that concomitant inactivation of Nbs1 and p53 in mice promotes HGG with RIG features. This model could be useful for preclinical studies to improve the prognosis of these deadly tumours, but it also highlights the singularity of NBS1 among the other DNA damage response proteins in the aetiology of brain tumours.

Human endogenous retrovirus K contributes to a stem cell niche in glioblastoma

Human endogenous retroviruses (HERVs) are ancestral viral relics that constitute nearly 8% of the human genome. Although normally silenced, the most recently integrated provirus HERV-K (HML-2) can be reactivated in certain cancers. Here, we report pathological expression of HML-2 in malignant gliomas in both cerebrospinal fluid and tumor tissue that was associated with a cancer stem cell phenotype and poor outcomes. Using single-cell RNA-Seq, we identified glioblastoma cellular populations with elevated HML-2 transcripts in neural progenitor-like cells (NPC-like) that drive cellular plasticity. Using CRISPR interference, we demonstrate that HML-2 critically maintained glioblastoma stemness and tumorigenesis in both glioblastoma neurospheres and intracranial orthotopic murine models. Additionally, we demonstrate that HML-2 critically regulated embryonic stem cell programs in NPC-derived astroglia and altered their 3D cellular morphology by activating the nuclear transcription factor OCT4, which binds to an HML-2-specific long-terminal repeat (LTR5Hs). Moreover, we discovered that some glioblastoma cells formed immature retroviral virions, and inhibiting HML-2 expression with antiretroviral drugs reduced reverse transcriptase activity in the extracellular compartment, tumor viability, and pluripotency. Our results suggest that HML-2 fundamentally contributes to the glioblastoma stem cell niche. Because persistence of glioblastoma stem cells is considered responsible for treatment resistance and recurrence, HML-2 may serve as a unique therapeutic target.

The neuroscience of cancer

The nervous system regulates tissue stem and precursor populations throughout life. Parallel to roles in development, the nervous system is emerging as a critical regulator of cancer, from oncogenesis to malignant growth and metastatic spread. Various preclinical models in a range of malignancies have demonstrated that nervous system activity can control cancer initiation and powerfully influence cancer progression and metastasis. Just as the nervous system can regulate cancer progression, cancer also remodels and hijacks nervous system structure and function. Interactions between the nervous system and cancer occur both in the local tumour microenvironment and systemically. Neurons and glial cells communicate directly with malignant cells in the tumour microenvironment through paracrine factors and, in some cases, through neuron-to-cancer cell synapses. Additionally, indirect interactions occur at a distance through circulating signals and through influences on immune cell trafficking and function. Such cross-talk among the nervous system, immune system and cancer-both systemically and in the local tumour microenvironment-regulates pro-tumour inflammation and anti-cancer immunity. Elucidating the neuroscience of cancer, which calls for interdisciplinary collaboration among the fields of neuroscience, developmental biology, immunology and cancer biology, may advance effective therapies for many of the most difficult to treat malignancies.

Cell of Origin of Brain and Spinal Cord Tumors

A better understanding of cellular and molecular biology of primary central nervous system (CNS) tumors is a critical step toward the design of innovative treatments. In addition to improving knowledge, identification of the cell of origin in tumors allows for sharp and efficient targeting of specific tumor cells promoting and driving oncogenic processes. The World Health Organization identifies approximately 150 primary brain tumor subtypes with various ontogeny and clinical outcomes. Identification of the cell of origin of each tumor type with its lineage and differentiation level is challenging. In the current chapter, we report the suspected cell of origin of various CNS primary tumors including gliomas, glioneuronal tumors, medulloblastoma, meningioma, atypical teratoid rhabdoid tumor, germinomas, and lymphoma. Most of them have been pinpointed through transgenic mouse models and analysis of molecular signatures of tumors. Identification of the cell or cells of origin in primary brain tumors will undoubtedly open new therapeutic avenues, including the reactivation of differentiation programs for therapeutic perspectives.

The Oncogenesis of Glial Cells in Diffuse Gliomas and Clinical Opportunities

Glioma is the most common and lethal intrinsic primary tumor of the brain. Its controversial origins may contribute to its heterogeneity, creating challenges and difficulties in the development of therapies. Among the components constituting tumors, glioma stem cells are highly plastic subpopulations that are thought to be the site of tumor initiation. Neural stem cells/progenitor cells and oligodendrocyte progenitor cells are possible lineage groups populating the bulk of the tumor, in which gene mutations related to cell-cycle or metabolic enzymes dramatically affect this transformation. Novel approaches have revealed the tumor-promoting properties of distinct tumor cell states, glial, neural, and immune cell populations in the tumor microenvironment. Communication between tumor cells and other normal cells manipulate tumor progression and influence sensitivity to therapy. Here, we discuss the heterogeneity and relevant functions of tumor cell state, microglia, monocyte-derived macrophages, and neurons in glioma, highlighting their bilateral effects on tumors. Finally, we describe potential therapeutic approaches and targets beyond standard treatments.

Matricellular protein tenascin C: Implications in glioma progression, gliomagenesis, and treatment

Matricellular proteins are nonstructural extracellular matrix components that are expressed at low levels in normal adult tissues and are upregulated during development or under pathological conditions. Tenascin C (TNC), a matricellular protein, is a hexameric and multimodular glycoprotein with different molecular forms that is produced by alternative splicing and post-translational modifications. Malignant gliomas are the most common and aggressive primary brain cancer of the central nervous system. Despite continued advances in multimodal therapy, the prognosis of gliomas remains poor. The main reasons for such poor outcomes are the heterogeneity and adaptability caused by the tumor microenvironment and glioma stem cells. It has been shown that TNC is present in the glioma microenvironment and glioma stem cell niches, and that it promotes malignant properties, such as neovascularization, proliferation, invasiveness, and immunomodulation. TNC is abundantly expressed in neural stem cell niches and plays a role in neurogenesis. Notably, there is increasing evidence showing that neural stem cells in the subventricular zone may be the cells of origin of gliomas. Here, we review the evidence regarding the role of TNC in glioma progression, propose a potential association between TNC and gliomagenesis, and summarize its clinical applications. Collectively, TNC is an appealing focus for advancing our understanding of gliomas.

The Intricate Epigenetic and Transcriptional Alterations in Pediatric High-Grade Gliomas: Targeting the Crosstalk as the Oncogenic Achilles’ Heel

Pediatric high-grade gliomas (pHGGs) are a deadly and heterogenous subgroup of gliomas for which the development of innovative treatments is urgent. Advances in high-throughput molecular techniques have shed light on key epigenetic components of these diseases, such as K27M and G34R/V mutations on histone 3. However, modification of DNA compaction is not sufficient by itself to drive those tumors. Here, we review molecular specificities of pHGGs subcategories in the context of epigenomic rewiring caused by H3 mutations and the subsequent oncogenic interplay with transcriptional signaling pathways co-opted from developmental programs that ultimately leads to gliomagenesis. Understanding how transcriptional and epigenetic alterations synergize in each cellular context in these tumors could allow the identification of new Achilles’ heels, thereby highlighting new levers to improve their therapeutic management.

Olfactory sensory experience regulates gliomagenesis via neuronal IGF1

Animals constantly receive various sensory stimuli, such as odours, sounds, light and touch, from the surrounding environment. These sensory inputs are essential for animals to search for food and avoid predators, but they also affect their physiological status, and may cause diseases such as cancer. Malignant gliomas-the most lethal form of brain tumour¹-are known to intimately communicate with neurons at the cellular level^2,3. However, it remains unclear whether external sensory stimuli can directly affect the development of malignant glioma under normal living conditions. Here we show that olfaction can directly regulate gliomagenesis. In an autochthonous mouse model that recapitulates adult gliomagenesis^4-6 originating in oligodendrocyte precursor cells (OPCs), gliomas preferentially emerge in the olfactory bulb-the first relay of brain olfactory circuitry. Manipulating the activity of olfactory receptor neurons (ORNs) affects the development of glioma. Mechanistically, olfaction excites mitral and tufted (M/T) cells, which receive sensory information from ORNs and release insulin-like growth factor 1 (IGF1) in an activity-dependent manner. Specific knockout of Igf1 in M/T cells suppresses gliomagenesis. In addition, knocking out the IGF1 receptor in pre-cancerous mutant OPCs abolishes the ORN-activity-dependent mitogenic effects. Our findings establish a link between sensory experience and gliomagenesis through their corresponding sensory neuronal circuits.

Tumor Microenvironment in Glioma Invasion

A major malignant trait of gliomas is their remarkable infiltration capacity. When glioma develops, the tumor cells have already reached the distant part. Therefore, complete removal of the glioma is impossible. Recently, research on the involvement of the tumor microenvironment in glioma invasion has advanced. Local hypoxia triggers cell migration as an environmental factor. The transcription factor hypoxia-inducible factor (HIF) -1α, produced in tumor cells under hypoxia, promotes the transcription of various invasion related molecules. The extracellular matrix surrounding tumors is degraded by proteases secreted by tumor cells and simultaneously replaced by an extracellular matrix that promotes infiltration. Astrocytes and microglia become tumor-associated astrocytes and glioma-associated macrophages/microglia, respectively, in relation to tumor cells. These cells also promote glioma invasion. Interactions between glioma cells actively promote infiltration of each other. Surgery, chemotherapy, and radiation therapy transform the microenvironment, allowing glioma cells to invade. These findings indicate that the tumor microenvironment may be a target for glioma invasion. On the other hand, because the living body actively promotes tumor infiltration in response to the tumor, it is necessary to reconsider whether the invasion itself is friend or foe to the brain.

High replication stress and limited Rad51-mediated DNA repair capacity, but not oxidative stress, underlie oligodendrocyte precursor cell radiosensitivity

Cranial irradiation is part of the standard of care for treating pediatric brain tumors. However, ionizing radiation can trigger serious long-term neurologic sequelae, including oligodendrocyte and brain white matter loss enabling neurocognitive decline in children surviving brain cancer. Oxidative stress-mediated oligodendrocyte precursor cell (OPC) radiosensitivity has been proposed as a possible explanation for this. Here, however, we demonstrate that antioxidants fail to improve OPC viability after irradiation, despite suppressing oxidative stress, suggesting an alternative etiology for OPC radiosensitivity. Using systematic approaches, we find that OPCs have higher irradiation-induced and endogenous γH2AX foci compared to neural stem cells, neurons, astrocytes and mature oligodendrocytes, and these correlate with replication-associated DNA double strand breakage. Furthermore, OPCs are reliant upon ATR kinase and Mre11 nuclease-dependent processes for viability, are more sensitive to drugs increasing replication fork collapse, and display synthetic lethality with PARP inhibitors after irradiation. This suggests an insufficiency for homology-mediated DNA repair in OPCs—a model that is supported by evidence of normal RPA but reduced RAD51 filament formation at resected lesions in irradiated OPCs. We therefore propose a DNA repair-centric mechanism of OPC radiosensitivity, involving chronically-elevated replication stress combined with ‘bottlenecks’ in RAD51-dependent DNA repair that together reduce radiation resilience.

Neural Signaling in Cancer

Nervous system activity regulates development, homeostasis, and plasticity of the brain as well as other organs in the body. These mechanisms are subverted in cancer to propel malignant growth. In turn, cancers modulate neural structure and function to augment growth-promoting neural signaling in the tumor microenvironment. Approaching cancer biology from a neuroscience perspective will elucidate new therapeutic strategies for presently lethal forms of cancer. In this review, we highlight the neural signaling mechanisms recapitulated in primary brain tumors, brain metastases, and solid tumors throughout the body that regulate cancer progression. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Metabolic Features of Tumor Dormancy: Possible Therapeutic Strategies

Simple Summary

Tumor recurrence still represents a major clinical challenge for cancer patients. Cancer cells may undergo a dormant state for long times before re-emerging. Both intracellular- and extracellular-driven pathways are involved in maintaining the dormant state and the subsequent awakening, with a mechanism that is still mostly unknown. In this scenario, cancer metabolism is emerging as a critical driver of tumor progression and dissemination and have gained increasing attention in cancer research. This review focuses on the metabolic adaptations characterizing the dormant phenotype and supporting tumor re-growth. Deciphering the metabolic adaptation sustaining tumor dormancy may pave the way for novel therapeutic approaches to prevent tumor recurrence based on combined metabolic drugs.

Abstract

Tumor relapse represents one of the main obstacles to cancer treatment. Many patients experience cancer relapse even decades from the primary tumor eradication, developing more aggressive and metastatic disease. This phenomenon is associated with the emergence of dormant cancer cells, characterized by cell cycle arrest and largely insensitive to conventional anti-cancer therapies. These rare and elusive cells may regain proliferative abilities upon the induction of cell-intrinsic and extrinsic factors, thus fueling tumor re-growth and metastasis formation. The molecular mechanisms underlying the maintenance of resistant dormant cells and their awakening are intriguing but, currently, still largely unknown. However, increasing evidence recently underlined a strong dependency of cell cycle progression to metabolic adaptations of cancer cells. Even if dormant cells are frequently characterized by a general metabolic slowdown and an increased ability to cope with oxidative stress, different factors, such as extracellular matrix composition, stromal cells influence, and nutrient availability, may dictate specific changes in dormant cells, finally resulting in tumor relapse. The main topic of this review is deciphering the role of the metabolic pathways involved in tumor cells dormancy to provide new strategies for selectively targeting these cells to prevent fatal recurrence and maximize therapeutic benefit.

Cancer stem cells in gliomas: evolving concepts and therapeutic implications

PURPOSE OF REVIEW

Cancer stem cells (CSCs) have been implicated in the hierarchical heterogeneity and treatment resistance of hematologic and solid tumor malignancies, including gliomas, for several decades now but their therapeutic targeting has not been fully realized. Recent studies have uncovered deeper layers of CSC complexity, related to developmental origins, plasticity, cellular states, and interface with the microenvironment.

RECENT FINDINGS

Sequencing and in-vivo lineage-tracing studies in mouse and patient-derived models show evidence of stem and progenitor origin of glioma, at the same time that genomic studies show a relatedness of glioma CSCs with radial glia. The spate of single-cell sequencing analyses demonstrates the diversity of transcriptional cellular states, which are susceptible to transitions, indicating the plasticity of glioma CSCs. The evolution of glioma CSCs and their interactions with niche cells play important roles in CSC treatment resistance and immune evasion, with epigenetic modulation as one of the emerging mechanisms.

SUMMARY

To harness the potential of CSCs for clinical application, there is urgent need to investigate their complex nature and myriad interactions, to better understand the contribution of these self-renewing, stem-like cancer cells in the pathogenesis and therapy resistance of malignant brain tumors.

Old Stars and New Players in the Brain Tumor Microenvironment

In recent years, the direct interaction between cancer cells and tumor microenvironment (TME) has emerged as a crucial regulator of tumor growth and a promising therapeutic target. The TME, including the surrounding peritumoral regions, is dynamically modified during tumor progression and in response to therapies. However, the mechanisms regulating the crosstalk between malignant and non-malignant cells are still poorly understood, especially in the case of glioma, an aggressive form of brain tumor. The presence of unique brain-resident cell types, namely neurons and glial cells, and an exceptionally immunosuppressive microenvironment pose additional important challenges to the development of effective treatments targeting the TME. In this review, we provide an overview on the direct and indirect interplay between glioma and neuronal and glial cells, introducing new players and mechanisms that still deserve further investigation. We will focus on the effects of neural activity and glial response in controlling glioma cell behavior and discuss the potential of exploiting these cellular interactions to develop new therapeutic approaches with the aim to preserve proper brain functionality.

Anticancer Effects of I-BET151, an Inhibitor of Bromodomain and Extra-Terminal Domain Proteins

I-BET151 is an inhibitor of bromodomain and extra-terminal domain (BET) proteins that selectively inhibits BET family members (BRD2, BRD3, BRD4, and BRDT). Over the past ten years, many studies have demonstrated the potential of I-BET151 in cancer treatment. Specifically, I-BET151 causes cell cycle arrest and inhibits tumor cell proliferation in some hematological malignancies and solid tumors, such as breast cancer, glioma, melanoma, neuroblastoma, and ovarian cancer. The anticancer activity of I-BET151 is related to its effects on NF-κB, Notch, and Hedgehog signal transduction pathway, tumor microenvironment (TME) and telomere elongation. Remarkably, the combination of I-BET151 with select anticancer drugs can partially alleviate the occurrence of drug resistance in chemotherapy. Especially, the combination of forskolin, ISX9, CHIR99021, I-BET151 and DAPT allows GBM cells to be reprogrammed into neurons, and this process does not experience an intermediate pluripotent state. The research on the anticancer mechanism of I-BET151 will lead to new treatment strategies for clinical cancer.

The bright and the dark side of myelin plasticity: Neuron-glial interactions in health and disease

Neuron-glial interactions shape neural circuit establishment, refinement and function. One of the key neuron-glial interactions takes place between axons and oligodendroglial precursor cells. Interactions between neurons and oligodendrocyte precursor cells (OPCs) promote OPC proliferation, generation of new oligodendrocytes and myelination, shaping myelin development and ongoing adaptive myelin plasticity in the brain. Communication between neurons and OPCs can be broadly divided into paracrine and synaptic mechanisms. Following the Nobel mini-symposium "The Dark Side of the Brain" in late 2019 at the Karolinska Institutet, this mini-review will focus on the bright and dark sides of neuron-glial interactions and discuss paracrine and synaptic interactions between neurons and OPCs and their malignant counterparts.

Tumor Dormancy: Implications for Invasion and Metastasis

Tumor dormancy refers to a critical stage of cancer development when tumor cells are present, but cancer does not progress. It includes both the concept of cellular dormancy, indicating the reversible switch of a cancer cell to a quiescent state, and that of tumor mass dormancy, indicating the presence of neoplastic masses that have reached cell population equilibrium via balanced growth/apoptosis rates. Tumor dormancy provides the conceptual framework, potentially explaining a major challenge in clinical oncology, tumor recurrence, which may occur years after cancer diagnosis. The mechanisms by which tumors are kept dormant, and what triggers their reawakening, are fundamental questions in cancer biology. It seems that a plethora of intracellular pathways and extracellular factors are involved in this process, rewiring the cells to plastically alter their metabolic and proliferative status. This phenomenon is highly dynamic in space and time. Mechanistic insights into both cellular and tumor dormancy have provided the rationale for targeting this otherwise stable period of cancer development, in order to prevent recurrence and maximize therapeutic benefit.

Rediscovering potential molecular targets for glioma therapy through the analysis of the cell of origin, microenvironment, and metabolism

Gliomas are the most common type of malignant brain tumors. Despite significant medical advances, gliomas remain incurable and are associated with high mortality. Although numerous biomarkers of diagnostic value have been identified and significant progress in the prognosis of the outcome has been made, the treatment has not been parallelly improved during the last three decades. This review summarizes and discusses three aspects of recent discoveries related to glioma, with the objective to highlight the advantages of glioma-specific drugs targeting the cell of origin, microenvironment, and metabolism. Given the heterogeneous nature of gliomas, various cell populations have been implicated as likely sources of the tumor. Depending on the mutation(s) acquired by the cells, it is believed that neuronal stem/progenitor cells, oligodendrocyte progenitor cells, mature neurons, and glial cells can initiate cell transformation into a malignant phenotype. The level of tumorigenicity appears to be inversely correlated with the maturation of a given cell population. The microenvironment of gliomas includes non-cancer cells such as immune cells, fibroblasts, and cells of blood vessels, as well as secreted molecules and the extracellular matrix, and all these components play a vital role during tumor initiation and progression. We will discuss in detail how the tumor microenvironment can stimulate and drive the transformation of non-tumor cell populations into tumor-supporting cells or glioma cells. Metabolic reprogramming is a key feature of gliomas and is thought to reflect the adaptation to the increased nutritional requirements of tumor cell proliferation, growth, and survival. Mutations in the IDH gene can shape metabolic reprogramming and may generate some vulnerabilities in glioma cells, such as abnormal lipid metabolism and sensitivity to endoplasmic reticulum stress (ERS). We will analyze the prominent metabolic features of malignant gliomas and the key pathways regulating glioma metabolism. This review is intended to provide a conceptual background for the development of glioma therapies based on the properties of tumor cell populations, microenvironment, and metabolism.

Novel Tools and Investigative Approaches for the Study of Oligodendrocyte Precursor Cells (NG2-Glia) in CNS Development and Disease

Oligodendrocyte progenitor cells (OPCs), also referred to as NG2-glia, are the most proliferative cell type in the adult central nervous system. While the primary role of OPCs is to serve as progenitors for oligodendrocytes, in recent years, it has become increasingly clear that OPCs fulfil a number of other functions. Indeed, independent of their role as stem cells, it is evident that OPCs can regulate the metabolic environment, directly interact with and modulate neuronal function, maintain the blood brain barrier (BBB) and regulate inflammation. In this review article, we discuss the state-of-the-art tools and investigative approaches being used to characterize the biology and function of OPCs. From functional genetic investigation to single cell sequencing and from lineage tracing to functional imaging, we discuss the important discoveries uncovered by these techniques, such as functional and spatial OPC heterogeneity, novel OPC marker genes, the interaction of OPCs with other cells types, and how OPCs integrate and respond to signals from neighboring cells. Finally, we review the use of in vitro assay to assess OPC functions. These methodologies promise to lead to ever greater understanding of this enigmatic cell type, which in turn will shed light on the pathogenesis and potential treatment strategies for a number of diseases, such as multiple sclerosis (MS) and gliomas.

Many roles for oligodendrocyte precursor cells in physiology and pathology

Oligodendrocyte precursor cells (OPCs) are a fourth resident glial cell population in the mammalian central nervous system. They are evenly distributed throughout the gray and white matter and continue to proliferate and generate new oligodendrocytes (OLs) throughout life. They were understudied until a few decades ago when immunolabeling for NG2 and platelet-derived growth factor receptor alpha revealed cells that are distinct from mature OLs, astrocytes, neurons, and microglia. In this review, we provide a summary of the known properties of OPCs with some historical background, followed by highlights from recent studies that suggest new roles for OPCs in certain pathological conditions.

Premalignant Oligodendrocyte Precursor Cells Stall in a Heterogeneous State of Replication Stress Prior to Gliomagenesis

Cancer evolves from premalignant clones that adopt unusual cell states to achieve transformation. We previously pinpointed the oligodendrocyte precursor cell (OPC) as a cell of origin for glioma, but the early changes of mutant OPCs during premalignancy remained unknown. Using mice engineered for inducible Nf1-Trp53 loss in OPCs, we acutely isolated labeled mutant OPCs by laser-capture microdissection, determined global gene-expression changes by bulk RNA sequencing, and compared with cell-state fluctuations at the single-cell level by stochastic profiling, which uses RNA-sequencing measurements from random pools of 10 mutant cells. At 12 days after Nf1-Trp53 deletion, bulk differences were mostly limited to mitotic hallmarks and genes for ribosome biosynthesis, and stochastic profiling revealed a spectrum of stem-progenitor (Axl, Aldh1a1), proneural, and mesenchymal states as potential starting points for gliomagenesis. At 90 days, bulk sequencing detected few differentially expressed transcripts, whereas stochastic profiling revealed cell states for neurons and mural cells that do not give rise to glial tumors, suggesting cellular dead-ends for gliomagenesis. Importantly, mutant OPCs that strongly expressed key effectors of nonsense-mediated decay (Upf3b) and homology-dependent DNA repair (Rad51c, Slx1b, Ercc4) were identified along with DNA-damage markers, suggesting transcription-associated replication stress. Analysis of 10-cell transcriptomes at 90 days identified a locus of elevated gene expression containing an additional repair endonuclease (Mus81) and Rin1, a Ras-Raf antagonist and possible counterbalance to Nf1 loss, which was microdeleted or downregulated in gliomas at 150 days. These hidden cell-state variations uncover replication stress as a potential bottleneck that must be resolved for glioma initiation. SIGNIFICANCE: Profiling premalignant cell states in a mouse model of glioma uncovers regulatory heterogeneity in glioma cells-of-origin and defines a state of replication stress that precedes tumor initiation.See related articles by Singh and colleagues, p. 1840 and Schaff and colleagues, p. 1853.

Aberrations of Genomic Imprinting in Glioblastoma Formation

In human glioblastoma (GBM), the presence of a small population of cells with stem cell characteristics, the glioma stem cells (GSCs), has been described. These cells have GBM potential and are responsible for the origin of the tumors. However, whether GSCs originate from normal neural stem cells (NSCs) as a consequence of genetic and epigenetic changes and/or dedifferentiation from somatic cells remains to be investigated. Genomic imprinting is an epigenetic marking process that causes genes to be expressed depending on their parental origin. The dysregulation of the imprinting pattern or the loss of genomic imprinting (LOI) have been described in different tumors including GBM, being one of the earliest and most common events that occurs in human cancers. Here we have gathered the current knowledge of the role of imprinted genes in normal NSCs function and how the imprinting process is altered in human GBM. We also review the changes at particular imprinted loci that might be involved in the development of the tumor. Understanding the mechanistic similarities in the regulation of genomic imprinting between normal NSCs and GBM cells will be helpful to identify molecular players that might be involved in the development of human GBM.

Evidence of the Cellular Senescence Stress Response in Mitotically Active Brain Cells-Implications for Cancer and Neurodegeneration

Cellular stress responses influence cell fate decisions. Apoptosis and proliferation represent opposing reactions to cellular stress or damage and may influence distinct health outcomes. Clinical and epidemiological studies consistently report inverse comorbidities between age-associated neurodegenerative diseases and cancer. This review discusses how one particular stress response, cellular senescence, may contribute to this inverse correlation. In mitotically competent cells, senescence is favorable over uncontrolled proliferation, i.e., cancer. However, senescent cells notoriously secrete deleterious molecules that drive disease, dysfunction and degeneration in surrounding tissue. In recent years, senescent cells have emerged as unexpected mediators of neurodegenerative diseases. The present review uses pre-defined criteria to evaluate evidence of cellular senescence in mitotically competent brain cells, highlights the discovery of novel molecular regulators and discusses how this single cell fate decision impacts cancer and degeneration in the brain. We also underscore methodological considerations required to appropriately evaluate the cellular senescence stress response in the brain.

Single-cell analyses reveal YAP/TAZ as regulators of stemness and cell plasticity in Glioblastoma

Glioblastoma (GBM) is a devastating human malignancy. GBM stem-like cells (GSCs) drive tumor initiation and progression. Yet, the molecular determinants defining GSCs in their native state in patients remain poorly understood. Here we used single cell datasets and identified GSCs at the apex of the differentiation hierarchy of GBM. By reconstructing the GSCs' regulatory network, we identified the YAP/TAZ coactivators as master regulators of this cell state, irrespectively of GBM subtypes. YAP/TAZ are required to install GSC properties in primary cells downstream of multiple oncogenic lesions, and required for tumor initiation and maintenance in vivo in different mouse and human GBM models. YAP/TAZ act as main roadblock of GSC differentiation and their inhibition irreversibly lock differentiated GBM cells into a non-tumorigenic state, preventing plasticity and regeneration of GSC-like cells. Thus, GSC identity is linked to a key molecular hub integrating genetics and microenvironmental inputs within the multifaceted biology of GBM.

Genetic Architectures and Cell-of-Origin in Glioblastoma

An aggressive primary brain cancer, glioblastoma (GBM) is the most common cancer of the central nervous system in adults. However, an inability to identify its cell-of-origin has been a fundamental issue hindering further understanding of the nature and pathogenesis of GBM, as well as the development of novel therapeutic targets. Researchers have hypothesized that GBM arises from an accumulation of somatic mutations in neural stem cells (NSCs) and glial precursor cells that confer selective growth advantages, resulting in uncontrolled proliferation. In this review, we outline genomic perspectives on IDH-wildtype and IDH-mutant GBMs pathogenesis and the cell-of-origin harboring GBM driver mutations proposed by various GBM animal models. Additionally, we discuss the distinct neurodevelopmental programs observed in either IDH-wildtype or IDH-mutant GBMs. Further research into the cellular origin and lineage hierarchy of GBM will help with understanding the evolution of GBMs and with developing effective targets for treating GBM cancer cells.

Genome-wide translation patterns in gliomas: An integrative view

Gliomas are the most frequent tumors of the central nervous system (CNS) and include the highly malignant glioblastoma (GBM). Characteristically, gliomas have translational control deregulation related to overactivation of signaling pathways such as PI3K/AKT/mTORC1 and Ras/ERK1/2. Thus, mRNA translation appears to play a dominant role in glioma gene expression patterns. The, analysis of genome-wide translated transcripts, together known as the translatome, may reveal important information for understanding gene expression patterns in gliomas. This review provides a brief overview of translational control mechanisms altered in gliomas with a focus on the current knowledge related to the translatomes of glioma cells and murine glioma models. We present an integrative meta-analysis of selected glioma translatome data with the aim of identifying recurrent patterns of gene expression preferentially regulated at the level of translation and obtaining clues regarding the pathological significance of these alterations. Re-analysis of several translatome datasets was performed to compare the translatomes of glioma models with those of their non-tumor counterparts and to document glioma cell responses to radiotherapy and MNK modulation. The role of recurrently altered genes in the context of translational control and tumorigenesis are discussed.

To Become or Not to Become Tumorigenic: Subventricular Zone Versus Hippocampal Neural Stem Cells

Neural stem cells (NSCs) persist in the adult mammalian brain in two neurogenic regions: the subventricular zone lining the lateral ventricles and the dentate gyrus of the hippocampus. Compelling evidence suggests that NSCs of the subventricular zone could be the cell type of origin of glioblastoma, the most devastating brain tumor. Studies in glioblastoma patients revealed that NSCs of the tumor-free subventricular zone, harbor cancer-driver mutations that were found in the tumor cells but were not present in normal cortical tissue. Endogenous mutagenesis can also take place in hippocampal NSCs. However, to date, no conclusive studies have linked hippocampal mutations with glioblastoma development. In addition, glioblastoma cells often invade or are closely located to the subventricular zone, whereas they do not tend to infiltrate into the hippocampus. In this review we will analyze possible causes by which subventricular zone NSCs might be more susceptible to malignant transformation than their hippocampal counterparts. Cellular and molecular differences between the two neurogenic niches, as well as genotypic and phenotypic characteristics of their respective NSCs will be discussed regarding why the cell type originating glioblastoma brain tumors has been linked mainly to subventricular zone, but not to hippocampal NSCs.

Oncogenic State and Cell Identity Combinatorially Dictate the Susceptibility of Cells within Glioma Development Hierarchy to IGF1R Targeting

Glioblastoma is the most malignant cancer in the brain and currently incurable. It is urgent to identify effective targets for this lethal disease. Inhibition of such targets should suppress the growth of cancer cells and, ideally also precancerous cells for early prevention, but minimally affect their normal counterparts. Using genetic mouse models with neural stem cells (NSCs) or oligodendrocyte precursor cells (OPCs) as the cells-of-origin/mutation, it is shown that the susceptibility of cells within the development hierarchy of glioma to the knockout of insulin-like growth factor I receptor (IGF1R) is determined not only by their oncogenic states, but also by their cell identities/states. Knockout of IGF1R selectively disrupts the growth of mutant and transformed, but not normal OPCs, or NSCs. The desirable outcome of IGF1R knockout on cell growth requires the mutant cells to commit to the OPC identity regardless of its development hierarchical status. At the molecular level, oncogenic mutations reprogram the cellular network of OPCs and force them to depend more on IGF1R for their growth. A new-generation brain-penetrable, orally available IGF1R inhibitor harnessing tumor OPCs in the brain is also developed. The findings reveal the cellular window of IGF1R targeting and establish IGF1R as an effective target for the prevention and treatment of glioblastoma.

Novel guanidine compounds inhibit platelet-derived growth factor receptor alpha transcription and oligodendrocyte precursor cell proliferation

Oligodendrocyte precursor cells (OPCs), also known as NG2 cells or polydendrocytes, are distributed widely throughout the developing and mature central nervous system. They remain proliferative throughout life and are an important source of myelinating cells in normal and demyelinating brain as well as a source of glioma, the most common type of primary brain tumor with a poor prognosis. OPC proliferation is dependent on signaling mediated by platelet-derived growth factor (PDGF) AA binding to its alpha receptor (PDGFRα). Here, we describe a group of structurally related compounds characterized by the presence of a basic guanidine group appended to an aromatic core that is effective in specifically repressing the transcription of Pdgfra but not the related beta receptor (Pdgfrb) in OPCs. These compounds specifically and dramatically reduced proliferation of OPCs but not that of astrocytes and did not affect signal transduction by PDGFRα. These findings suggest that the compounds could be further developed for potential use in combinatorial treatment strategies for neoplasms with dysregulated PDGFRα function.

Oncogenic and Tumor-Suppressive Functions of NOTCH Signaling in Glioma

Although the role of NOTCH signaling has been extensively studied in health and disease, many questions still remain unresolved. Being crucial for tissue homeostasis, NOTCH signaling is also implicated in multiple cancers by either promoting or suppressing tumor development. In this review we illustrate the context-dependent role of NOTCH signaling during tumorigenesis with a particular focus on gliomas, the most frequent and aggressive brain tumors in adults. For a long time, NOTCH has been considered an oncogene in glioma mainly by virtue of its neural stem cell-promoting activity. However, the recent identification of NOTCH-inactivating mutations in some glioma patients has challenged this notion, prompting a re-examination of the function of NOTCH in brain tumor subtypes. We discuss recent findings that might help to reconcile the controversial role of NOTCH signaling in this disease, and pose outstanding questions that still remain to be addressed.

The Emerging Role of Extracellular Vesicles in the Glioma Microenvironment: Biogenesis and Clinical Relevance

Gliomas are a diverse group of brain tumors comprised of malignant cells ('tumor' cells) and non-malignant 'normal' cells, including neural (neurons, glia), inflammatory (microglia, macrophage) and vascular cells. Tumor heterogeneity arises in part because, within the glioma mass, both 'tumor' and 'normal' cells secrete factors that form a unique microenvironment to influence tumor progression. Extracellular vesicles (EVs) are critical mediators of intercellular communication between immediate cellular neighbors and distantly located cells in healthy tissues/organs and in tumors, including gliomas. EVs mediate cell-cell signaling as carriers of nucleic acid, lipid and protein cargo, and their content is unique to cell types and physiological states. EVs secreted by non-malignant neural cells have important physiological roles in the healthy brain, which can be altered or co-opted to promote tumor progression and metastasis, acting in combination with glioma-secreted EVs. The cell-type specificity of EV content means that 'vesiculome' data can potentially be used to trace the cell of origin. EVs may also serve as biomarkers to be exploited for disease diagnosis and to assess therapeutic progress. In this review, we discuss how EVs mediate intercellular communication in glioma, and their potential role as biomarkers and readouts of a therapeutic response.

Sphere-Forming Culture for Expanding Genetically Distinct Patient-Derived Glioma Stem Cells by Cellular Growth Rate Screening

Diffusely infiltrating gliomas (DIGs) are difficult to completely resect and are associated with a high rate of tumor relapse and progression from low- to high-grade glioma. In particular, optimized short-term culture-enriching patient-derived glioma stem cells (GSCs) are essential for customizing the therapeutic strategy based on clinically feasible in vitro drug screening for a wide range of DIGs, owing to the high inter-tumoral heterogeneity. Herein, we constructed a novel high-throughput culture condition screening platform called ‘GFSCAN’, which evaluated the cellular growth rates of GSCs for each DIG sample in 132 serum-free combinations, using 13 previously reported growth factors closely associated with glioma aggressiveness. In total, 72 patient-derived GSCs with available genomic profiles were tested in GFSCAN to explore the association between cellular growth rates in specific growth factor combinations and genomic/molecular backgrounds, including isocitrate dehydrogenase 1 (IDH1) mutation, chromosome arm 1p and 19q co-deletion, ATRX chromatin remodeler alteration, and transcriptional subtype. GSCs were clustered according to the dependency on epidermal growth factor and basic fibroblast growth factor (E&F), and isocitrate dehydrogenase 1 (IDH1) wild-type GSCs showed higher E&F dependencies than IDH1 mutant GSCs. More importantly, we elucidated optimal combinations for IDH1 mutant glioblastoma and lower grade glioma GSCs with low dependencies on E&F, which could be an aid in clinical decision-making for these DIGs. Thus, we demonstrated the utility of GFSCAN in personalizing in vitro cultivation to nominate personalized therapeutic options, in a clinically relevant time frame, for individual DIG patients, where standard clinical options have been exhausted.

CCL5 of glioma-associated microglia/macrophages regulates glioma migration and invasion via calcium-dependent matrix metalloproteinase 2

BACKGROUND

Glioma-associated microglia/macrophages (GAMs) comprise macrophages of peripheral origin and brain-intrinsic microglia, which support tumor progression. Chemokine C-C ligand 5 (CCL5) is an inflammatory mediator produced by immune cells and is involved in tumor growth and migration in several cancers, including glioma. However, the mechanisms detailing how CCL5 facilitates glioma invasion remain largely unresolved.

METHODS

Glioma migration and invasion were determined by wound healing, transwell assay, and 3D µ-slide chemotaxis assay. The expression levels of CCL5, CD68, matrix metalloproteinase 2 (MMP2), phosphorylated Ca2+/calmodulin-dependent protein kinase II (p-CaMKII), p-Akt, and phosphorylated proline-rich tyrosine kinase 2 were determined by cytokine array, quantitative PCR, western blot, or immunohistochemistry. Zymography and intracellular calcium assays were used to analyze MMP2 activity and intracellular calcium levels, respectively.

RESULTS

CCL5 modulated the migratory and invasive activities of human glioma cells in association with MMP2 expression. In response to CCL5, glioma cells underwent a synchronized increase in intracellular calcium levels and p-CaMKII and p-Akt expression levels. CCL5-directed glioma invasion and increases in MMP2 were suppressed after inhibition of p-CaMKII. Glioma cells tended to migrate toward GAM-conditioned media activated by granulocyte-macrophage colony-stimulating factor (GM-CSF) in which CCL5 was abundant. This homing effect was associated with MMP2 upregulation, and could be ameliorated either by controlling intracellular and extracellular calcium levels or by CCL5 antagonism. Clinical results also revealed the associations between CCL5 and GAM activation.

CONCLUSION

Our results suggest that modulation of glioma CaMKII may restrict the effect of CCL5 on glioma invasion and could be a potential therapeutic target for alleviating glioma growth.

Validating indicators of CNS disorders in a swine model of neurological disease

Genetically modified swine disease models are becoming increasingly important for studying molecular, physiological and pathological characteristics of human disorders. Given the limited history of these model systems, there remains a great need for proven molecular reagents in swine tissue. Here, to provide a resource for neurological models of disease, we validated antibodies by immunohistochemistry for use in examining central nervous system (CNS) markers in a recently developed miniswine model of neurofibromatosis type 1 (NF1). NF1 is an autosomal dominant tumor predisposition disorder stemming from mutations in NF1, a gene that encodes the Ras-GTPase activating protein neurofibromin. Patients classically present with benign neurofibromas throughout their bodies and can also present with neurological associated symptoms such as chronic pain, cognitive impairment, and behavioral abnormalities. As validated antibodies for immunohistochemistry applications are particularly difficult to find for swine models of neurological disease, we present immunostaining validation of antibodies implicated in glial inflammation (CD68), oligodendrocyte development (NG2, O4 and Olig2), and neuron differentiation and neurotransmission (doublecortin, GAD67, and tyrosine hydroxylase) by examining cellular localization and brain region specificity. Additionally, we confirm the utility of anti-GFAP, anti-Iba1, and anti-MBP antibodies, previously validated in swine, by testing their immunoreactivity across multiple brain regions in mutant NF1 samples. These immunostaining protocols for CNS markers provide a useful resource to the scientific community, furthering the utility of genetically modified miniswine for translational and clinical applications.

Oligodendrocyte Progenitor Cells in the Tumor Microenvironment

Glioblastoma (GBM) develops from adult brain white matter and is the most common and lethal primary brain tumor, characterized by rapid growth and invasion. GBM tumors frequently spread into the contralateral hemisphere, including in the beginning of tumor development. However, after complete resection of the tumor mass and chemo-radiotherapy, GBM commonly recurs around the tumor removal site, suggesting that the microenvironment at the tumor border provides therapeutic resistance to GBM cells. To improve patient prognosis, understanding the microenvironment at the tumor border is critical. Several microRNAs (miRNAs) show higher expression at the tumor border, with the top three involved in oligodendrocyte differentiation. Oligodendrocyte progenitor cells (OPCs) may induce stemness and chemo-radioresistance in GBM cells, providing a supportive function to promote GBM. This review describes important features of OPCs and insights into the "border niche," a unique microenvironment that allows GBM cells to survive and recur at the tumor border.

An active role for neurons in glioma progression: making sense of Scherer's structures

Perineuronal satellitosis, the microanatomical clustering of glioma cells around neurons in the tumor microenvironment, has been recognized as a histopathological hallmark of high-grade gliomas since the seminal observations of Scherer in the 1930s. In this review, we explore the emerging understanding that neuron‒glioma cell interactions regulate malignancy and that neuronal activity is a critical determinant of glioma growth and progression. Elucidation of the interplay between normal and malignant neural circuitry is critical to realizing the promise of effective therapies for these seemingly intractable diseases. Here, we review current knowledge regarding the role of neuronal activity in the glioma microenvironment and highlight critical knowledge gaps in this burgeoning research space.

Novel concept of the border niche: glioblastoma cells use oligodendrocytes progenitor cells (GAOs) and microglia to acquire stem cell-like features

Glioblastoma (GBM) is a major malignant brain tumor developing in adult brain white matter, characterized by rapid growth and invasion. GBM cells spread into the contralateral hemisphere, even during early tumor development. However, after complete resection of tumor mass, GBM commonly recurs around the tumor removal cavity, suggesting that a microenvironment at the tumor border provides chemo-radioresistance to GBM cells. Thus, clarification of the tumor border microenvironment is critical for improving prognosis in GBM patients. MicroRNA (miRNA) expression in samples from the tumor, tumor border, and peripheral region far from tumor mass was compared, and five miRNAs showing characteristically higher expression in the tumor border were identified, with the top three related to oligodendrocyte differentiation. Pathologically, oligodendrocyte lineage cells increased in the border, but were rare in tumors. Macrophages/microglia also colocalized in the border area. Medium cultured with oligodendrocyte progenitor cells (OPCs) and macrophages induced stemness and chemo-radioresistance in GBM cells, suggesting that OPCs and macrophages/microglia constitute a special microenvironment for GBM cells at the tumor border. The supportive function of OPCs for GBM cells has not been discussed previously. OPCs are indispensable for GBM cells to establish special niches for chemo-radioresistance outside the tumor mass.

Developmental origins and oncogenic pathways in malignant brain tumors

Brain tumors such as adult glioblastomas and pediatric high-grade gliomas or medulloblastomas are among the leading causes of cancer-related deaths, exhibiting poor prognoses with little improvement in outcomes in the past several decades. These tumors are heterogeneous and can be initiated from various neural cell types, contributing to therapy resistance. How such heterogeneity arises is linked to the tumor cell of origin and their genetic alterations. Brain tumorigenesis and progression recapitulate key features associated with normal neurogenesis; however, the underlying mechanisms are quite dysregulated as tumor cells grow and divide in an uncontrolled manner. Recent comprehensive genomic, transcriptomic, and epigenomic studies at single-cell resolution have shed new light onto diverse tumor-driving events, cellular heterogeneity, and cells of origin in different brain tumors. Primary and secondary glioblastomas develop through different genetic alterations and pathways, such as EGFR amplification and IDH1/2 or TP53 mutation, respectively. Mutations such as histone H3K27M impacting epigenetic modifications define a distinct group of pediatric high-grade gliomas such as diffuse intrinsic pontine glioma. The identification of distinct genetic, epigenomic profiles and cellular heterogeneity has led to new classifications of adult and pediatric brain tumor subtypes, affording insights into molecular and lineage-specific vulnerabilities for treatment stratification. This review discusses our current understanding of tumor cells of origin, heterogeneity, recurring genetic and epigenetic alterations, oncogenic drivers and signaling pathways for adult glioblastomas, pediatric high-grade gliomas, and medulloblastomas, the genetically heterogeneous groups of malignant brain tumors. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Gene Networks and Genomics Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cell Differentiation and Reversion Signaling Pathways > Cell Fate Signaling.

In situ 10-cell RNA sequencing in tissue and tumor biopsy samples

Single-cell transcriptomic methods classify new and existing cell types very effectively, but alternative approaches are needed to quantify the individual regulatory states of cells in their native tissue context. We combined the tissue preservation and single-cell resolution of laser capture with an improved preamplification procedure enabling RNA sequencing of 10 microdissected cells. This in situ 10-cell RNA sequencing (10cRNA-seq) can exploit fluorescent reporters of cell type in genetically engineered mice and is compatible with freshly cryoembedded clinical biopsies from patients. Through recombinant RNA spike-ins, we estimate dropout-free technical reliability as low as ~250 copies and a 50% detection sensitivity of ~45 copies per 10-cell reaction. By using small pools of microdissected cells, 10cRNA-seq improves technical per-cell reliability and sensitivity beyond existing approaches for single-cell RNA sequencing (scRNA-seq). Detection of low-abundance transcripts by 10cRNA-seq is comparable to random 10-cell groups of scRNA-seq data, suggesting no loss of gene recovery when cells are isolated in situ. Combined with existing approaches to deconvolve small pools of cells, 10cRNA-seq offers a reliable, unbiased, and sensitive way to measure cell-state heterogeneity in tissues and tumors.

Cell-of-origin susceptibility to glioblastoma formation declines with neural lineage restriction

The contribution of lineage identity and differentiation state to malignant transformation is controversial. We have previously shown that adult neural stem and early progenitor cells give origin to glioblastoma. Here we systematically assessed the tumor-initiating potential of adult neural populations at various stages of lineage progression. Cell type-specific tamoxifen-inducible Cre recombinase transgenes were used to target glioblastoma-relevant tumor suppressors Nf1, Trp53 and Pten in late-stage neuronal progenitors, neuroblasts and differentiated neurons. Mutant mice showed cellular and molecular defects demonstrating the impact of tumor suppressor loss, with mutant neurons being the most resistant to early changes associated with tumor development. However, we observed no evidence of glioma formation. These studies show that increasing lineage restriction is accompanied by decreasing susceptibility to malignant transformation, indicating a glioblastoma cell-of-origin hierarchy in which stem cells sit at the apex and differentiated cell types are least susceptible to tumorigenesis.

Dormancy and cancer stem cells: An enigma for cancer therapeutic targeting

Dormancy occurs when cells remain viable but stop proliferating. When most of a cancer population undergoes this phenomenon, the result is called tumor dormancy, and when a single cancer cell undergoes this process, it is termed quiescence. Cancer stem cells (CSCs) share several overlapping characteristics and signaling pathways with dormant cancer cells, including therapy resistance, and an ability to metastasize and evade the immune system. Cancer cells can be broadly grouped into dormancy-competent CSCs (DCCs), cancer-repopulating cells (CRCs), dormancy-incompetent CSCs and disseminated tumor cells (DTCs). The settings in which cancer cells exploit the dormancy phase to survive and adapt are: (i) primary cancer dormancy; (ii) metastatic dormancy; (iii) therapy-induced dormancy; and (iv) immunologic dormancy. Dormancy, therapy resistance and plasticity of CSCs are fundamentally interconnected processes mediated through mechanisms involving reversible genetic alterations. Niches including metastatic, bone marrow, and perivascular are known to harbor dormant cancer cells. Mechanisms of dormancy induction are complex and multi-factorial and can involve angiogenic switching, addictive oncogene inhibition, immunoediting, anoikis, therapy, autophagy, senescence, epigenetic, and biophysical regulation. Therapy can have opposing effects on cancer cells with respect to dormancy; some therapies can induce dormancy, while others can reactivate dormant cells. There is a lack of consensus relative to the value of therapy-induced dormancy, i.e., some researchers view dormancy induction as a beneficial strategy as it can lead to metastasis inhibition, while others argue that reactivating dormant cancer cells and then eliminating them through therapy are a better approach. More focused investigations of intrinsic cell kinetics and environmental dynamics that promote and maintain cancer cells in a dormant state, and the long-term consequences of dormancy are critical for improving current therapeutic treatment outcomes.