Quantitative analysis of mitotic Olig2 cells in adult human brain and gliomas: implications for glioma histogenesis and biology

Novel TCF4:TCF12 heterodimer inhibits glioblastoma growth

TWIST1 (TW) is a pro-oncogenic basic helix-loop-helix (bHLH) transcription factor and promotes the hallmark features of malignancy (e.g., cell invasion, cancer cell stemness, and treatment resistance), which contribute to poor prognoses of glioblastoma (GBM). We previously reported that specific TW dimerization motifs regulate unique cellular phenotypes in GBM. For example, the TW:E12 heterodimer increases periostin (POSTN) expression and promotes cell invasion. TW dimer-specific transcriptional regulation requires binding to the regulatory E-box consensus sequences, but alternative bHLH dimers that balance TW dimer activity in regulating pro-oncogenic TW target genes are unknown. We leveraged the ENCODE DNase I hypersensitivity data to identify E-box sites and tethered TW:E12 and TW:TW proteins to validate dimer binding to E-boxes in vitro. Subsequently, TW knockdown revealed a novel TCF4:TCF12 bHLH dimer occupying the same TW E-box site that, when expressed as a tethered TCF4:TCF12 dimer, markedly repressed POSTN expression and extended animal survival. These observations support TCF4:TCF12 as a novel dimer with tumor-suppressor activity in GBM that functions in part through displacement of and/or competitive inhibition of pro-oncogenic TW dimers at E-box sites.

Dysregulation of inflammasome activation in glioma

Gliomas are the most common brain tumors characterized by complicated heterogeneity. The genetic, molecular, and histological pathology of gliomas is characterized by high neuro-inflammation. The inflammatory microenvironment in the central nervous system (CNS) has been closely linked with inflammasomes that control the inflammatory response and coordinate innate host defenses. Dysregulation of the inflammasome causes an abnormal inflammatory response, leading to carcinogenesis in glioma. Because of the clinical importance of the various physiological properties of the inflammasome in glioma, the inflammasome has been suggested as a promising treatment target for glioma management. Here, we summarize the current knowledge on the contribution of the inflammasomes in glioma and therapeutic insights. Video Abstract.

Heterogeneity and Proliferative and Differential Regulators of NG2-glia in Physiological and Pathological States

NG2-glia, also called Oligodendrocyte Precursor Cells (OPCs), account for approximately 5%-10% of the cells in the developing and adult brain and constitute the fifth major cell population in the central nervous system. NG2-glia express receptors and ion channels involved in rapid modulation of neuronal activities and signaling with neuronal synapses, which have functional significance in both physiological and pathological states. NG2-glia participate in quick signaling with peripheral neurons via direct synaptic touches in the developing and mature central nervous system. These distinctive glia perform the unique function of proliferating and differentiating into oligodendrocytes in the early developing brain, which is critical for axon myelin formation. In response to injury, NG2-glia can proliferate, migrate to the lesions, and differentiate into oligodendrocytes to form new myelin sheaths, which wrap around damaged axons and result in functional recovery. The capacity of NG2-glia to regulate their behavior and dynamics in response to neuronal activity and disease indicate their critical role in myelin preservation and remodeling in the physiological state and in repair in the pathological state. In this review, we provide a detailed summary of the characteristics of NG2-glia, including their heterogeneity, the regulators of their proliferation, and the modulators of their differentiation into oligodendrocytes.

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.

Companion animal models of neurological disease

Clinical translation of novel therapeutics that improve the survival and quality of life of patients with neurological disease remains a challenge, with many investigational drug and device candidates failing in advanced stage clinical trials. Naturally occurring inherited and acquired neurological diseases, such as epilepsy, inborn errors of metabolism, brain tumors, spinal cord injury, and stroke occur frequently in companion animals, and many of these share epidemiologic, pathophysiologic and clinical features with their human counterparts. As companion animals have a relatively abbreviated lifespan and genetic background, are immunocompetent, share their environment with human caregivers, and can be clinically managed using techniques and tools similar to those used in humans, they have tremendous potential for increasing the predictive value of preclinical drug and device studies. Here, we review comparative features of spontaneous neurological diseases in companion animals with an emphasis on neuroimaging methods and features, illustrate their historical use in translational studies, and discuss inherent limitations associated with each disease model. Integration of companion animals with naturally occurring disease into preclinical studies can complement and expand the knowledge gained from studies in other animal models, accelerate or improve the manner in which research is translated to the human clinic, and ultimately generate discoveries that will benefit the health of humans and animals.

Cell type-specific epigenetic links to schizophrenia risk in the brain

BACKGROUND

The importance of cell type-specific epigenetic variation of non-coding regions in neuropsychiatric disorders is increasingly appreciated, yet data from disease brains are conspicuously lacking. We generate cell type-specific whole-genome methylomes (N = 95) and transcriptomes (N = 89) from neurons and oligodendrocytes obtained from brain tissue of patients with schizophrenia and matched controls.

RESULTS

The methylomes of the two cell types are highly distinct, with the majority of differential DNA methylation occurring in non-coding regions. DNA methylation differences between cases and controls are subtle compared to cell type differences, yet robust against permuted data and validated in targeted deep-sequencing analyses. Differential DNA methylation between control and schizophrenia tends to occur in cell type differentially methylated sites, highlighting the significance of cell type-specific epigenetic dysregulation in a complex neuropsychiatric disorder.

CONCLUSIONS

Our results provide novel and comprehensive methylome and transcriptome data from distinct cell populations within patient-derived brain tissues. This data clearly demonstrate that cell type epigenetic-differentiated sites are preferentially targeted by disease-associated epigenetic dysregulation. We further show reduced cell type epigenetic distinction in schizophrenia.

Single-cell transcriptome analysis of lineage diversity in high-grade glioma

BACKGROUND

Despite extensive molecular characterization, we lack a comprehensive understanding of lineage identity, differentiation, and proliferation in high-grade gliomas (HGGs).

METHODS

We sampled the cellular milieu of HGGs by profiling dissociated human surgical specimens with a high-density microwell system for massively parallel single-cell RNA-Seq. We analyzed the resulting profiles to identify subpopulations of both HGG and microenvironmental cells and applied graph-based methods to infer structural features of the malignantly transformed populations.

RESULTS

While HGG cells can resemble glia or even immature neurons and form branched lineage structures, mesenchymal transformation results in unstructured populations. Glioma cells in a subset of mesenchymal tumors lose their neural lineage identity, express inflammatory genes, and co-exist with marked myeloid infiltration, reminiscent of molecular interactions between glioma and immune cells established in animal models. Additionally, we discovered a tight coupling between lineage resemblance and proliferation among malignantly transformed cells. Glioma cells that resemble oligodendrocyte progenitors, which proliferate in the brain, are often found in the cell cycle. Conversely, glioma cells that resemble astrocytes, neuroblasts, and oligodendrocytes, which are non-proliferative in the brain, are generally non-cycling in tumors.

CONCLUSIONS

These studies reveal a relationship between cellular identity and proliferation in HGG and distinct population structures that reflects the extent of neural and non-neural lineage resemblance among malignantly transformed cells.

Twist1 mediated regulation of glioma tumorigenicity is dependent on mode of mouse neural progenitor transformation

Twist1 is a master regulator of epithelial mesenchymal transition and carcinoma metastasis. Twist1 has also been associated with increased malignancy of human glioma. However, the impact of inhibiting Twist1 on tumorigenicity has not been characterized in glioma models in the context of different oncogenic transformation paradigms. Here we used an orthotopic mouse glioma model of transplanted transformed neural progenitor cells (NPCs) to demonstrate the effects of Twist1 loss of function on tumorigenicity. Decreased tumorigenicity was observed after shRNA mediated Twist knockdown in HPV E6/7 Ha-RasV12 transformed NPCs and Cre mediated Twist1 deletion in Twist1 fl/fl NPCs transformed by p53 knockdown and Ha-RasV12 expression. By contrast, Twist1 deletion had no effect on tumorigenicity of NPCs transformed by co-expression of Akt and Ha-RasV12. We demonstrated a dramatic off-target effect of Twist1 deletion with constitutive Cre expression, which was completely reversed when Twist1 deletion was achieved by transient administration of recombinant Cre protein. Together these findings demonstrate that the function of Twist1 in these models is highly dependent on specific oncogenic contexts of NPC transformation. Therefore, the driver mutational context in which Twist1 functions may need to be taken into account when evaluating mechanisms of action and developing therapeutic approaches to target Twist1 in human gliomas.

Low FoxG1 and high Olig-2 labelling indices define a prognostically favourable subset in isocitrate dehydrogenase (IDH)-mutant gliomas

AIMS

Previous data suggest that expression of transcription factors FoxG1 and Olig-2 can separate hotspot histone H3 family member 3A (H3F3A)-mutant tumours in paediatric glioma. We evaluated their prognostic potential and feasibility for identifying H3F3A-mutant tumours among IDH-mutant/wild-type gliomas.

METHODS

Immunohistochemistry of FoxG1/Olig-2 and α-thalassaemia/mental-retardation-syndrome-X-linked gene (ATRX) in 471 cases of diffuse gliomas and molecular determination of IDH, H3F3A, MGMT and 1p/19 codeletion status.

RESULTS

Mean percentage of FoxG1-positive tumour cells increased from 17% in WHO grade II to over 21% in grade III to 37% in grade IV tumours, whereas mean Olig-2 indices decreased from 29% to 28% to 17% respectively. FoxG1 indices were similar in astrocytic and oligodendroglial tumours, whereas Olig-2 indices were increased in oligodendrogliomas compared to astrocytic tumours (n = 451, P < 0.0001). FoxG1-positive nuclei were significantly reduced in IDH and H3F3A K27-mutant tumours, whereas Olig-2-positive nuclei were significantly reduced in IDH-wild-type and H3F3A G34-mutant tumours. Among IDH-mutant tumours, mean Olig-2 index was significantly higher in 1p/19q codeleted tumours (mean: 43%) compared to IDH-mutant tumours with ATRX loss (mean: 23%, P < 0.0001). A significantly better outcome was first suggested for FoxG1^low tumours (n = 212, log rank P = 0.0132) and Olig-2^high tumours (n = 203, log-rank P = 0.0011) based on classification and regression tree determined cutoffs, but this was not confirmed by multivariate analysis including IDH mutation, WHO grade, ATRX status and age.

CONCLUSIONS

While the combined FoxG1/Olig-2 profile may discriminate H3F3A K27- and G34-mutant tumours and define a prognostically favourable subset in IDH-mutant gliomas, our data show that labelling indices of these transcription factors overlap with adult IDH-mutant and wild-type tumour classes.

Simulating PDGF-Driven Glioma Growth and Invasion in an Anatomically Accurate Brain Domain

Gliomas are the most common of all primary brain tumors. They are characterized by their diffuse infiltration of the brain tissue and are uniformly fatal, with glioblastoma being the most aggressive form of the disease. In recent years, the over-expression of platelet-derived growth factor (PDGF) has been shown to produce tumors in experimental rodent models that closely resemble this human disease, specifically the proneural subtype of glioblastoma. We have previously modeled this system, focusing on the key attribute of these experimental tumors-the "recruitment" of oligodendroglial progenitor cells (OPCs) to participate in tumor formation by PDGF-expressing retrovirally transduced cells-in one dimension, with spherical symmetry. However, it has been observed that these recruitable progenitor cells are not uniformly distributed throughout the brain and that tumor cells migrate at different rates depending on the material properties in different regions of the brain. Here we model the differential diffusion of PDGF-expressing and recruited cell populations via a system of partial differential equations with spatially variable diffusion coefficients and solve the equations in two spatial dimensions on a mouse brain atlas using a flux-differencing numerical approach. Simulations of our in silico model demonstrate qualitative agreement with the observed tumor distribution in the experimental animal system. Additionally, we show that while there are higher concentrations of OPCs in white matter, the level of recruitment of these plays little role in the appearance of "white matter disease," where the tumor shows a preponderance for white matter. Instead, simulations show that this is largely driven by the ratio of the diffusion rate in white matter as compared to gray. However, this ratio has less effect on the speed of tumor growth than does the degree of OPC recruitment in the tumor. It was observed that tumor simulations with greater degrees of recruitment grow faster and develop more nodular tumors than if there is no recruitment at all, similar to our prior results from implementing our model in one dimension. Combined, these results show that recruitment remains an important consideration in understanding and slowing glioma growth.

Glia: silent partners in energy homeostasis and obesity pathogenesis

Body weight stability requires homeostatic regulation to balance energy intake and energy expenditure. Research on this system and how it is affected by obesity has largely focused on the role of hypothalamic neurons as integrators of information about long-term fuel storage, short-term nutrient availability and metabolic demand. Recent studies have uncovered glial cells as additional contributors to energy balance regulation and obesity pathogenesis. Beginning with early work on leptin signalling in astrocytes, this area of research rapidly emerged after the discovery of hypothalamic inflammation and gliosis in obese rodents and humans. Current studies have revealed the involvement of a wide variety of glial cell types in the modulation of neuronal activity, regulation of hormone and nutrient availability, and participation in the physiological regulation of feeding behaviour. In addition, one glial type, microglia, has recently been implicated in susceptibility to diet-induced obesity. Together, these exciting new findings deepen our understanding of energy homeostasis regulation and raise the possibility of identifying novel mechanisms that contribute to the pathogenesis of obesity.

Homeobox genes gain trimethylation of histone H3 lysine 4 in glioblastoma tissue

We revealed that in GBM homeobox genes gain tri-methylation on lysine 4 of histone H3 (H3K4me3), whereas the cell–cell adhesion-related cadherin genes lose H3K4me3, suggesting that the H3K4me3 alteration is related to the formation and migration of GBM cells.

Glioblastoma multiforme (GBM) exhibits considerable heterogeneity and associates with genome-wide alterations of the repressed chromatin marks DNA methylation and H3 lysine 27 trimethylation (H3K27me3). Tri-methylation on lysine 4 of histone H3 (H3K4me3) is an activating epigenetic mark that is enriched at promoter and promotes expression. It will be helpful in GBM diagnosis and treatment to identify the alteration of H3K4me3 between human GBM and GBM-surrounding tissues. Here, we performed an analysis using next-generation sequencing techniques to identify H3K4me3 modification in a case of GBM and the GBM-surrounding tissues. The results revealed a global decrease in H3K4me3 in GBM, especially at promoters and CpG islands. In GBM, homeobox genes gain H3K4me3, whereas the cell–cell adhesion-related cadherin genes lose H3K4me3. The products of the homeobox genes are highly connected with Ras-signalling and PI3K-Akt signalling pathways. Using The Cancer Genome Atlas (TCGA) data, we inferred the homeobox-regulated genes’ expression is higher in 548 GBM cases than in 27 lower grade glioma cases giving that OLIG2 expression can be a reference. The results suggested that the H3K4me3 alteration is related to the formation and migration of GBM cells. We also found an extremely high reads count at epidermal growth factor receptor (EGFR) promoter, probably due to an amplification of copy number. Our analysis provides a case study about the change of H3K4me3 during shift to GBM.

Human Traumatic Brain Injury Results in Oligodendrocyte Death and Increases the Number of Oligodendrocyte Progenitor Cells

Oligodendrocyte (OL) death may contribute to white matter pathology, a common cause of network dysfunction and persistent cognitive problems in patients with traumatic brain injury (TBI). Oligodendrocyte progenitor cells (OPCs) persist throughout the adult CNS and may replace dead OLs. OL death and OPCs were analyzed by immunohistochemistry of human brain tissue samples, surgically removed due to life-threatening contusions and/or focal brain swelling at 60.6 ± 75 hours (range 4-192 hours) postinjury in 10 severe TBI patients (age 51.7 ± 18.5 years). Control brain tissue was obtained postmortem from 5 age-matched patients without CNS disorders. TUNEL and CC1 co-labeling was used to analyze apoptotic OLs, which were increased in injured brain tissue (p < 0.05), without correlation with time from injury until surgery. The OPC markers Olig2, A2B5, NG2, and PDGFR-α were used. In contrast to the number of single-labeled Olig2, A2B5, NG2, and PDGFR-α-positive cells, numbers of Olig2 and A2B5 co-labeled cells were increased in TBI samples (p < 0.05); this was inversely correlated with time from injury to surgery (r = -0.8, p < 0.05). These results indicate that severe focal human TBI results in OL death and increases in OPCs postinjury, which may influence white matter function following TBI.

Spontaneously Arising Canine Glioma as a Potential Model for Human Glioma

Human gliomas are malignant brain tumours that carry a poor prognosis and are composed of a heterogeneous population of cells. There is a paucity of animal models available for study of these tumours and most have been created by genetic modification. Spontaneously arising canine gliomas may provide a model for the characterization of the human tumours. The present study shows that canine gliomas form a range of immunohistochemical patterns that are similar to those described for human gliomas. The in-vitro sphere assay was used to analyze the expansion and differentiation potential of glioma cells taken from the periphery and centre of canine tumours. Samples from the subventricular zone (SVZ) and contralateral parenchyma were used as positive and negative controls, respectively. The expansion potential for all of these samples was low and cells from only three cultures were expanded for six passages. These three cultures were derived from high-grade gliomas and the cells had been cryopreserved. Most of the cells obtained from the centre of the tumours formed spheres and were expanded, in contrast to samples taken from the periphery of the tumours. Spheres were also formed and expanded from two areas of apparently unaffected brain parenchyma. The neurogenic SVZ contralateral samples also contained progenitor proliferating cells, since all of them were expanded for three to five passages. Differentiation analysis showed that all cultured spheres were multipotential and able to differentiate towards both neurons and glial cells. Spontaneously arising canine gliomas might therefore constitute an animal model for further characterization of these tumours.

Presence of neural progenitors in spontaneous canine gliomas: A histopathological and immunohistochemical study of 20 cases

Gliomas are the most common primary brain tumours in humans and are associated with a poor prognosis. An accurate animal model of human glioma tumorigenesis is needed to test new treatment strategies. Dogs represent a promising model because they develop spontaneous diffusely-infiltrating gliomas. This study investigated whether spontaneous canine gliomas contain cancer stem cells previously identified in all grades of human gliomas. Twenty spontaneous cases of canine gliomas were graded according to the human WHO classification. The expression of different markers of lineage differentiation was evaluated with immunohistochemistry as follows: nestin and CD133 for neural stem cells, doublecortin for neuronal progenitor cells, Olig2 for glial progenitor cells, glial fibrillary acidic protein, vimentin and S-100 for mature glial cells, and NeuN and βIII-tubulin for mature neurons. Gliomas were characterised as follows: five grade II (oligodendrogliomas); nine grade III (seven anaplastic oligodendrogliomas, one anaplastic astrocytoma, one anaplastic oligoastrocytoma); six grade IV (glioblastomas). Immunohistochemical evaluation revealed that (1) nestin and CD133 were expressed in all grades of gliomas with a higher proportion of positive cells in high-grade gliomas; (2) the expression of S-100 protein and Olig2 did not differ substantially between astrocytic and oligodendroglial tumours, and (3) all gliomas were negative for mature neuron markers. The results demonstrated the presence of undifferentiated neural progenitors in all grades of spontaneous canine gliomas, confirming the relevance of this animal model for further studies on cancer stem cells.

Pre-B-cell leukemia homeobox interacting protein 1 is overexpressed in astrocytoma and promotes tumor cell growth and migration

BACKGROUND

Glial brain tumors cause considerable mortality and morbidity in children and adults. Innovative targets for therapy are needed to improve survival and reduce long-term sequelae. The aim of this study was to find a candidate tumor-promoting protein, abundantly expressed in tumor cells but not in normal brain tissues, as a potential target for therapy.

METHODS

In silico proteomics and genomics, immunohistochemistry, and immunofluorescence microscopy validation were performed. RNA interference was used to ascertain the functional role of the overexpressed candidate target protein.

RESULTS

In silico proteomics and genomics revealed pre-B-cell leukemia homeobox (PBX) interacting protein 1 (PBXIP1) overexpression in adult and childhood high-grade glioma and ependymoma compared with normal brain. PBXIP1 is a PBX-family interacting microtubule-binding protein with a putative role in migration and proliferation of cancer cells. Immunohistochemical studies in glial tumors validated PBXIP1 expression in astrocytoma and ependymoma but not in oligodendroglioma. RNAi-mediated PBXIP1-knockdown in glioblastoma cell lines strongly reduced proliferation and migration and induced morphological changes, indicating that PBXIP1 knockdown decreases glioma cell viability and motility through rearrangements of the actin cytoskeleton. Furthermore, expression of PBXIP1 was observed in radial glia and astrocytic progenitor cells in human fetal tissues, suggesting that PBXIP1 is an astroglial progenitor cell marker during human embryonic development.

CONCLUSION

PBXIP1 is a novel protein overexpressed in astrocytoma and ependymoma, involved in tumor cell proliferation and migration, that warrants further exploration as a novel therapeutic target in these tumors.

NG2-glia and their functions in the central nervous system

In the central nervous system, NG2-glia represent a neural cell population that is distinct from neurons, astrocytes, and oligodendrocytes. While in the past the main role ascribed to these cells was that of progenitors for oligodendrocytes, in the last years it has become more obvious that they have further functions in the brain. Here, we will discuss some of the most current and highly debated issues regarding NG2-glia: Do these cells represent a heterogeneous population? Can they give rise to different progenies, and does this change under pathological conditions? How do they respond to injury or pathology? What is the role of neurotransmitter signaling between neurons and NG2-glia? We will first give an overview on the developmental origin of NG2-glia, and then discuss whether their distinct properties in different brain regions are the result of environmental influences, or due to intrinsic differences. We will then review and discuss their in vitro differentiation potential and in vivo lineage under physiological and pathological conditions, together with their electrophysiological properties in distinct brain regions and at different developmental stages. Finally, we will focus on their potential to be used as therapeutic targets in demyelinating and neurodegenerative diseases. Therefore, this review article will highlight the importance of NG2-glia not only in the healthy, but also in the diseased brain.

Microglial proliferation in the brain of chronic alcoholics with hepatic encephalopathy

Hepatic encephalopathy (HE) is a common complication of chronic alcoholism and patients show neurological symptoms ranging from mild cognitive dysfunction to coma and death. The HE brain is characterized by glial changes, including microglial activation, but the exact pathogenesis of HE is poorly understood. During a study investigating cell proliferation in the subventricular zone of chronic alcoholics, a single case with widespread proliferation throughout their adjacent grey and white matter was noted. This case also had concomitant HE raising the possibility that glial proliferation might be a pathological feature of the disease. In order to explore this possibility fixed postmortem human brain tissue from chronic alcoholics with cirrhosis and HE (n = 9), alcoholics without HE (n = 4) and controls (n = 4) were examined using immunohistochemistry and cytokine assays. In total, 4/9 HE cases had PCNA- and a second proliferative marker, Ki-67-positive cells throughout their brain and these cells co-stained with the microglial marker, Iba1. These cases were termed 'proliferative HE' (pHE). The microglia in pHEs displayed an activated morphology with hypertrophied cell bodies and short, thickened processes. In contrast, the microglia in white matter regions of the non-proliferative HE cases were less activated and appeared dystrophic. pHEs were also characterized by higher interleukin-6 levels and a slightly higher neuronal density . These findings suggest that microglial proliferation may form part of an early neuroprotective response in HE that ultimately fails to halt the course of the disease because underlying etiological factors such as high cerebral ammonia and systemic inflammation remain.

The Olig family affects central nervous system development and disease

Neural cell differentiation and maturation is a critical step during central nervous system development. The oligodendrocyte transcription family (Olig family) is known to be an important factor in regulating neural cell differentiation. Because of this, the Olig family also affects acute and chronic central nervous system diseases, including brain injury, multiple sclerosis, and even gliomas. Improved understanding about the functions of the Olig family in central nervous system development and disease will greatly aid novel breakthroughs in central nervous system diseases. This review investigates the role of the Olig family in central nervous system development and related diseases.

Molecular targeting of TRF2 suppresses the growth and tumorigenesis of glioblastoma stem cells

Glioblastoma is the most prevalent primary brain tumor and is essentially universally fatal within 2 years of diagnosis. Glioblastomas contain cellular hierarchies with self-renewing glioblastoma stem cells (GSCs) that are often resistant to chemotherapy and radiation therapy. GSCs express high amounts of repressor element 1 silencing transcription factor (REST), which may contribute to their resistance to standard therapies. Telomere repeat-binding factor 2 (TRF2) stablizes telomeres and REST to maintain self-renewal of neural stem cells and tumor cells. Here we show viral vector-mediated delivery of shRNAs targeting TRF2 mRNA depletes TRF2 and REST from GSCs isolated from patient specimens. As a result, GSC proliferation is reduced and the level of proteins normally expressed by postmitotic neurons (L1CAM and β3-tubulin) is increased, suggesting that loss of TRF2 engages a cell differentiation program in the GSCs. Depletion of TRF2 also sensitizes GSCs to temozolomide, a DNA-alkylating agent currently used to treat glioblastoma. Targeting TRF2 significantly increased the survival of mice bearing GSC xenografts. These findings reveal a role for TRF2 in the maintenance of REST-associated proliferation and chemotherapy resistance of GSCs, suggesting that TRF2 is a potential therapeutic target for glioblastoma.

SOX2+ cell population from normal human brain white matter is able to generate mature oligodendrocytes

OBJECTIVES

A number of neurodegenerative diseases progress with a loss of myelin, which makes them candidate diseases for the development of cell-replacement therapies based on mobilisation or isolation of the endogenous neural/glial progenitor cells, in vitro expansion, and further implantation. Cells expressing A2B5 or PDGFRA/CNP have been isolated within the pool of glial progenitor cells in the subcortical white matter of the normal adult human brain, all of which demonstrate glial progenitor features. However, the heterogeneity and differentiation potential of this pool of cells is not yet well established.

METHODS

We used diffusion tensor images, histopathology, and immunostaining analysis to demonstrate normal cytoarchitecture and the absence of abnormalities in human temporal lobe samples from patients with mesial temporal sclerosis. These samples were used to isolate and enrich glial progenitor cells in vitro, and later to detect such cells in vivo.

RESULTS

We have identified a subpopulation of SOX2+ cells, most of them co-localising with OLIG2, in the white matter of the normal adult human brain in vivo. These cells can be isolated and enriched in vitro, where they proliferate and generate immature (O4+) and mature (MBP+) oligodendrocytes and, to a lesser extent, astrocytes (GFAP+).

CONCLUSION

Our results demonstrate the existence of a new glial progenitor cell subpopulation that expresses SOX2 in the white matter of the normal adult human brain. These cells might be of use for tissue regeneration procedures.

LGR5 is a proneural factor and is regulated by OLIG2 in glioma stem-like cells

The biological functional roles of LGR5 (leucine-rich repeat containing G protein-coupled receptor 5, also known as GPR49), a novel potential marker for stem-like cells in glioblastoma (GSCs), is poorly acknowledged. Here, we demonstrated that LGR5 was detected in glioblastoma tissues and GSCs. Bioinformatics analysis revealed that LGR5 is closely related to neurogenesis and neuronal functions, and preferentially expressed in Proneural subtype of GBMs. Furthermore, LGR5 is regulated by Proneural factor OLIG2, which is important for both neurogenesis and GSC maintenance. Biological experiments in GSC cells validated the bioinformatics analysis results and revealed that LGR5 regulated the tumor sphere formation capacity, an important stem cell property for GSCs. Therefore, LGR5 expression may be functionally correlated with the neurogenic competence, and be regulated by OLIG2 in GSCs.

Candidate pathways for promoting differentiation or quiescence of oligodendrocyte progenitor-like cells in glioma

Platelet-derived growth factor receptor alpha-positive oligodendrocyte progenitor cells (OPC) located within the mature central nervous system may remain quiescent, proliferate, or differentiate into oligodendrocytes. Human glioblastoma multiforme tumors often contain rapidly proliferating oligodendrocyte lineage transcription factor 2 (Olig2)-positive cells that resemble OPCs. In this study, we sought to identify candidate pathways that promote OPC differentiation or quiescence rather than proliferation. Gene expression profiling conducted in both normal murine OPCs and highly proliferative Olig2-positive glioma cells identified all the transcripts associated with the highly proliferative state of these cells and showed that among the various cell types found within the brain, Olig2-positive tumor cells are most similar to OPCs. We then subtracted OPC transcripts found in tumor samples from those found in normal brain samples and identified 28 OPC transcripts as candidates for promoting differentiation or quiescence. Systematic analysis of human glioma data revealed that these genes have similar expression profiles in human tumors and were significantly enriched in genomic deletions, suggesting an antiproliferative role. Treatment of primary murine glioblastoma cells with agonists of one candidate gene, Gpr17, resulted in a decreased number of neurospheres. Together, our findings show that comparison of the molecular phenotype of progenitor cells in tumors to the equivalent cells in the normal brain represents a novel approach for the identification of targeted therapies.

Adult human progenitor cells from the temporal lobe: another source of neuronal cells

OBJECTIVES

In the adult human brain, neurogenesis occurs in the SVZ and the dentate gyrus of the hippocampus, but it is still unclear whether persistent neural progenitor/stem cells are also present in other brain areas. The present work studies the possibility of obtaining neural progenitor/stem cells from the temporal lobe and investigates their potential to differentiate into neuronal cells.

METHODS

Human biopsies from the temporal lobe of epileptic patients were used to isolate potential neural progenitors. Differentiation was induced in the presence of different agents (NGF, NT3, RA) and immunocytochemistry was then performed for quantitative analysis.

RESULTS

It was shown that a significant number of cells in the temporal lobe are also capable of expansion and multi-potency. These cells can be amplified as neurospheres and have the potential to differentiate naturally in vitro into neurons, astrocytes and oligodendrocytes. Quantitative analyses show that the progenitor cells of the temporal lobe exhibit a better rate of neuronal differentiation in vitro than the cells from the SVZ, particularly in the presence of NGF.

CONCLUSION

This study indicates that neural progenitors are also present in the human temporal lobe. Studying them could be of great interest for cell therapy in neurological disorders.

Glioblastoma cancer stem-like cells: implications for pathogenesis and treatment

Glioblastoma remains one of the deadliest forms of cancer. Infiltrating cancer cells in the surrounding brain prevent complete resection, and tumor cell resistance to chemoradiation results in the poor prognosis of the glioblastoma (GBM) patient. Much research has been devoted over the years to the pathogenesis and treatment of GBM. The tumor stem cell hypothesis, which was initially described in hematopoietic cell malignancies, may explain the resistance of these tumors to conventional therapies. In this model, a certain subset of tumor cells, with characteristics similar to normal stem cells, is capable of producing the variety of cell types, which constitute the bulk of a tumor. As these tumor cells have properties distinct from those constituting the bulk of the tumor, a different approach may be required to eradicate these residual cells within the brain. Here we outline the history behind the theory of GBM cancer stem-like cells, as they are now referred to. We will also discuss the implications of their existence on commonly held beliefs about GBM pathogenesis and how they might influence future treatment strategies.

The cellular origin for malignant glioma and prospects for clinical advancements

Glioma remains incurable despite great advancements in medicine. Targeting the cell of origin for gliomas could bring great hope for patients. However, as a collection of diverse diseases, each subtype of glioma could derive from a distinct cell of origin. To resolve such a complex problem, one must use multiple research approaches to gain deep insights. Here we review current evidence regarding the cell of origin from clinical observations, whole-genome molecular pathology and glioma animal models. We conclude that neural stem cells, glial progenitors (including oligodendrocyte progenitor cells) and astrocytes could all serve as cells of origin for gliomas, and that cells incurring initial mutations (cells of mutation) might not transform, while their progeny cells could instead transform and act as cells of origin. Further studies with multidisciplinary approaches are needed to link each subtype to a particular cell of origin, and to develop effective therapies that target the signaling network within these cells.

Glioblastoma models reveal the connection between adult glial progenitors and the proneural phenotype

BACKGROUND

Tumor heterogeneity is a major obstacle for finding effective treatment of Glioblastoma (GBM). Based on global expression analysis, GBM can be classified into distinct subtypes: Proneural, Neural, Classical and Mesenchymal. The signatures of these different tumor subtypes may reflect the phenotypes of cells giving rise to them. However, the experimental evidence connecting any specific subtype of GBM to particular cells of origin is lacking. In addition, it is unclear how different genetic alterations interact with cells of origin in determining tumor heterogeneity. This issue cannot be addressed by studying end-stage human tumors.

METHODOLOGY/PRINCIPAL FINDINGS

To address this issue, we used retroviruses to deliver transforming genetic lesions to glial progenitors in adult mouse brain. We compared the resulting tumors to human GBM. We found that different initiating genetic lesions gave rise to tumors with different growth rates. However all mouse tumors closely resembled the human Proneural GBM. Comparative analysis of these mouse tumors allowed us to identify a set of genes whose expression in humans with Proneural GBM correlates with survival.

CONCLUSIONS/SIGNIFICANCE

This study offers insights into the relationship between adult glial progenitors and Proneural GBM, and allows us to identify molecular alterations that lead to more aggressive tumor growth. In addition, we present a new preclinical model that can be used to test treatments directed at a specific type of GBM in future studies.

Convection-enhanced delivery of topotecan into a PDGF-driven model of glioblastoma prolongs survival and ablates both tumor-initiating cells and recruited glial progenitors

The contribution of microenvironment to tumor growth has important implications for optimizing chemotherapeutic response and understanding the biology of recurrent tumors. In this study, we tested the effects of locally administered topotecan on a rat model of glioblastoma that is induced by intracerebral injection of PDGF (platelet-derived growth factor)-IRES (internal ribosome entry site)-GFP (green fluorescent protein)-expressing retrovirus, treated the tumors by convection-enhanced delivery (CED) of topotecan (136 μmol/L) for 1, 4, or 7 days, and then characterized the effects on both the retrovirus-transformed tumor cells (GFP(+) cells) as well as the uninfected glial progenitor cells (GFP(-) cells) that are recruited to the tumor. Topotecan treatment reduced GFP(+) cells about 10-fold and recruited progenitors by about 80-fold while providing a significant survival advantage that improved with greater treatment duration. Regions of glial progenitor ablation occurred corresponding to the anatomic distribution of topotecan as predicted by MRI of a surrogate tracer. Histopathologic changes in recurrent tumors point to a decrease in recruitment, most likely due to the chemotherapeutic ablation of the recruitable progenitor pool.

Towards improved animal models of neonatal white matter injury associated with cerebral palsy

Newborn neurological injuries are the leading cause of intellectual and motor disabilities that are associated with cerebral palsy. Cerebral white matter injury is a common feature in hypoxic-ischemic encephalopathy (HIE), which affects full-term infants, and in periventricular leukomalacia (PVL), which affects preterm infants. This article discusses recent efforts to model neonatal white matter injury using mammalian systems. We emphasize that a comprehensive understanding of oligodendrocyte development and physiology is crucial for obtaining new insights into the pathobiology of HIE and PVL as well as for the generation of more sophisticated and faithful animal models.

Neurofibromatosis-1 regulates neuroglial progenitor proliferation and glial differentiation in a brain region-specific manner

Recent studies have shown that neuroglial progenitor/stem cells (NSCs) from different brain regions exhibit varying capacities for self-renewal and differentiation. In this study, we used neurofibromatosis-1 (NF1) as a model system to elucidate a novel molecular mechanism underlying brain region-specific NSC functional heterogeneity. We demonstrate that Nf1 loss leads to increased NSC proliferation and gliogenesis in the brainstem, but not in the cortex. Using Nf1 genetically engineered mice and derivative NSC neurosphere cultures, we show that this brain region-specific increase in NSC proliferation and gliogenesis results from selective Akt hyperactivation. The molecular basis for the increased brainstem-specific Akt activation in brainstem NSCs is the consequence of differential rictor expression, leading to region-specific mammalian target of rapamycin (mTOR)/rictor-mediated Akt phosphorylation and Akt-regulated p27 phosphorylation. Collectively, these findings establish mTOR/rictor-mediated Akt activation as a key driver of NSC proliferation and gliogenesis, and identify a unique mechanism for conferring brain region-specific responses to cancer-causing genetic changes.

NG2-expressing glial precursor cells are a new potential oligodendroglioma cell initiating population in N-ethyl-N-nitrosourea-induced gliomagenesis

Gliomas are the most common primary brain tumor affecting human adults and remain a therapeutic challenge because cells of origin are still unknown. Here, we investigated the cellular origin of low-grade gliomas in a rat model based on transplacental exposure to N-ethyl-N-nitrosourea (ENU). Longitudinal magnetic resonance imaging coupled to immunohistological and immunocytochemical analyses were used to further characterize low-grade rat gliomas at different stages of evolution. We showed that early low-grade gliomas have characteristics of oligodendroglioma-like tumors and exclusively contain NG2-expressing slow dividing precursor cells, which express early markers of oligodendroglial lineage. These tumor-derived precursors failed to fully differentiate into oligodendrocytes and exhibited multipotential abilities in vitro. Moreover, a few glioma NG2+ cells are resistant to radiotherapy and may be responsible for tumor recurrence, frequently observed in humans. Overall, these findings suggest that transformed multipotent NG2 glial precursor cell may be a potential cell of origin in the genesis of rat ENU-induced oligodendroglioma-like tumors. This work may open up new perspectives for understanding biology of human gliomas.

Beta1,4-galactosyltransferase V regulates self-renewal of glioma-initiating cell

Glioma results from unregulated expansion of a self-renewing glioma-initiating cell population. The regulatory pathways which are essential for sustaining the self-renewal of glioma-initiating cells remain largely unknown. Cell surface N-linked oligosaccharides play functional roles in determining cell fate and are associated with glioma malignancy. Previously, we have reported that beta1,4-galactosyltransferase V (beta1,4GalT V) effectively galactosylates the GlcNAcbeta1-->6Man arm of the highly branched N-glycans and positively regulates glioma cell growth. Here, we show that decreasing the expression of beta1,4GalT V by RNA interference in glioma cells attenuated the formation of polylactosamine and inhibited the ability of tumor formation in vivo. Down-regulation of beta1,4GalT V depleted CD133-positive cells in glioma xenograft, and inhibited the self-renewal capacity and the tumorigenic potential of glioma-initiating cells. These data reveal a critical role of beta1,4GalT V in the self-renewal and tumorigenicity of glioma-initiating cells, and indicate that manipulating beta1,4GalT V expression may have therapeutic potential for the treatment of malignant glioma.

Atypical cell clusters expressing both neuronal and oligodendrocytic markers: novel histological pattern of glioneuronal tumors?

Glioneuronal tumors are a group of brain tumors that consist of both neuronal and glial cells. The spectrum of glioneuronal tumors is currently expanding, and many atypical glioneuronal tumors require further characterization. Two patients are described who had an atypical glioneuronal tumor with peculiar pathological features. One patient was a 7-year-old girl with a tumor in the right cerebellar hemisphere. This patient had no recurrence after total resection. The other was a 37-year-old man with a tumor in the spinal cord. He suffered incessant recurrence and received operative treatment four times. Although the clinical features (age at diagnosis, tumor location, and recurrence) were very different in these patients, the tumors had a characteristic common feature of atypical cell clusters. Intriguingly, the tumor cells in the clusters expressed both neuronal and oligodendroglial markers, indicating aberrant differentiation. Furthermore, the cluster-forming cells had modest proliferative indices and CD133 expression, indicating their role in the growth of the tumor. It is believed that these atypical cell clusters are a novel pattern of differentiation of glioneuronal tumors and that they need further investigation.

Recent insights into PDGF-induced gliomagenesis

Gliomas are aggressive and almost incurable glial brain tumors which frequently display abnormal platelet-derived growth factor (PDGF) signaling. Evidence gained from studies on several in vivo animal models has firmly established a causal connection between aberrant PDGF signaling and the formation of some gliomas. However, only recently has significant knowledge been gained regarding crucial issues such as the glioma cell of origin and the relationship between the transforming stimulus and the cellular characteristics of the resulting tumor. Based on recent evidence, we propose that PDGF can bias cell-fate decisions, driving the acquisition of cell type-specific features by the progeny of multipotent neural progenitors, thus determining the shape and direction of the transformation path. Furthermore, recent data about the cellular mechanisms of PDGF-driven glioma progression and maintenance indicate that PDGF may be required, unexpectedly, to override cell contact inhibition and promote glioma cell infiltration rather than to stimulate cell proliferation.

NG2+/Olig2+ cells are the major cycle-related cell population of the adult human normal brain

A persistent cycling cell population in the normal adult human brain is well established. Neural stem cells or neural progenitors have been identified in the subventricular zone and the dentate gyrus subgranular layer (SGL), two areas of persistent neurogenesis. Cycling cells in other human normal brain areas, however, remains to be established. Here, we determined the distribution and identity of these cells in the cortex, the white matter and the hippocampal formation of adult patients with and without chronic temporal lobe epilepsy using immunohistochemistry for the cell cycle markers Ki-67 (Mib-1) and minichromosome maintenance protein 2. Rare proliferative neuronal precursors expressing the neuronal antigen neuronal nuclei were restricted to the SGL. In contrast, the oligodendrocyte progenitor cell markers Olig2 and the surface antigen NG2 were expressed by the vast majority of cycling cells scattered throughout the cortex and white matter of both control and epileptic patients. Most of these cycling cells were in early G1 phase, and were significantly more numerous in epileptic than in non-epileptic patients. These results provide evidence for a persistent gliogenesis in the human cortex and white matter that is enhanced in an epileptic environment.