Human cortical glial tumors contain neural stem-like cells expressing astroglial and neuronal markers in vitro

MicroRNA and brain tumors: a cause and a cure?

Malignant brain tumors, including high-grade gliomas, are among the most lethal of all cancers. Despite considerable advances, including multi-modal treatments with surgery, radiotherapy, and chemotherapy, the overall prognosis remains dismal for patients diagnosed with these tumors. With the discovery of RNA interference (RNAi) for target-specific gene silencing via small interfering RNA (siRNA), a novel method to target malignant gliomas has been exposed, an endeavor that is aggressively being carried out in numerous laboratories. However, practical difficulties in tissue- or organ-specific targeting of therapeutic quantities of siRNA still preclude its applicability in a clinical setting. MicroRNA (miRNA), an endogenously expressed form of siRNA, not only presents an alternate method to induce RNAi in a given diseased tissue or organ, but also exposes a unique set of diagnostic markers that can be used to identify, and then differentiate between tumor grades. Thus, miRNA can be considered the cells' answer to siRNA. Discovered over a decade ago, miRNA is fast becoming recognized as crucial in regulating gene expression in cancers. Therein lies the therapeutic potential of miRNA, as it may now be possible to induce or inhibit RNAi in a given diseased cell population by controlling the cells' miRNA expression profile. This review outlines the potential of miRNA as a therapeutic strategy against high-grade gliomas, and also the technological hurdles that need to be addressed before this promising technique can be administered in a clinical setting.

Inhibition of notch signaling induces neural differentiation in Ewing sarcoma

Cells from Ewing sarcoma exhibit cellular features and express markers, suggesting that the tumor is of neuroectodermal origin. Because Notch signaling regulates the differentiation of neuroectodermal cells during development, we examined the role of Notch signaling in Ewing sarcomas. We found that Ewing sarcomas express Notch receptors, ligands, and the Notch target gene HES1. To determine the functional implications of Notch signaling, we expressed tetracycline-regulated constitutively active, dominant-negative (DN), or wild-type Notch-1 receptors in two Ewing sarcoma cell lines, or we treated the cell lines with a gamma-secretase inhibitor. Expression of the constitutively active Notch-1 reduced proliferation and expression of the DN Notch-1 reduced apoptosis in vitro. However, there was only a small difference in the volume of tumors that formed when the cell lines expressing these constructs were implanted in nude mice. Xenograft tumors derived from the cell lines expressing DN Notch-1 exhibited a neural phenotype. Treatment with a gamma-secretase inhibitor caused similar changes as expression of the DN construct. Notch signaling plays a role in cell differentiation, proliferation, and apoptosis in Ewing sarcoma, but its inhibition is only associated with a small change in tumor growth potential.

Brain tumor stem cells

The concept of brain tumor stem cells is gaining increased recognition in neuro-oncology. Until recently, the paradigm of a tumor-initiating stem cell was confined to hematopoietic malignancies where the hierarchical lineages of stem progenitor cells are well established. The demonstration of persistent stem cells and cycling progenitors in the adult brain, coupled with the expansion of the cancer stem cell concept to solid tumors, has led to the exploration of "stemness" within gliomas. Emerging data are highly suggestive of the subsistence of transformed multipotential cells within a glioma, with a subfraction of cells exhibiting increased efficiency at tumor initiation. However, data in support of true glioma stem cells are inconclusive to date, particularly with respect to functional characterization of these cells. Ongoing work aims at the identification of unique pathways governing self-renewal of these putative stem cells and at their validation as ultimate therapeutic targets.

CD133(+) and CD133(-) glioblastoma-derived cancer stem cells show differential growth characteristics and molecular profiles

Although glioblastomas show the same histologic phenotype, biological hallmarks such as growth and differentiation properties vary considerably between individual cases. To investigate whether different subtypes of glioblastomas might originate from different cells of origin, we cultured tumor cells from 22 glioblastomas under medium conditions favoring the growth of neural and cancer stem cells (CSC). Secondary glioblastoma (n = 7)-derived cells did not show any growth in the medium used, suggesting the absence of neural stem cell-like tumor cells. In contrast, 11/15 primary glioblastomas contained a significant CD133(+) subpopulation that displayed neurosphere-like, nonadherent growth and asymmetrical cell divisions yielding cells expressing markers characteristic for all three neural lineages. Four of 15 cell lines derived from primary glioblastomas grew adherently in vitro and were driven by CD133(-) tumor cells that fulfilled stem cell criteria. Both subtypes were similarly tumorigenic in nude mice in vivo. Clinically, CD133(-) glioblastomas were characterized by a lower proliferation index, whereas glial fibrillary acidic protein staining was similar. GeneArray analysis revealed 117 genes to be differentially expressed by these two subtypes. Together, our data provide first evidence that CD133(+) CSC maintain only a subset of primary glioblastomas. The remainder stems from previously unknown CD133(-) tumor cells with apparent stem cell-like properties but distinct molecular profiles and growth characteristics in vitro and in vivo.

Malignant transformation of multipotent muscle-derived cells by concurrent differentiation signals

Recent studies have shown that germ-line determination occurs early in development and that extracellular signaling can alter this fate. This denial of a cell's fate by counteracting its intrinsic signaling pathways through extrinsic stimulation is believed to be associated with oncogenesis. Using specific populations of multipotent skeletal muscle-derived stem cells (MDSCs), we have been able to generate tumors by subjecting cells with specific lineage predilections to concomitant differentiation signals. More specifically, when a stem cell that had a predilection toward osteogenesis was implanted into a skeletal muscle, tumors formed in 25% of implanted mice. When cells predilected to undergo myogenesis were pretreated with bone morphogenetic protein 4 (BMP4) for 4 days prior to implantation, they formed tumors in 25% of mice. These same myogenic predilected cells, when transduced to express BMP4 and implanted into either a long-bone or cranial defect, formed bone, but they formed tumors in 100% of mice when implanted into the skeletal muscle. The tumors generated in this latter study were serially transplantable as long as they retained BMP4 expression. Furthermore, when we impeded the ability of the cells to undergo myogenic differentiation using small interfering RNA to the myogenic regulator MyoD1, we stopped transformation. Based on our findings, we postulate that specific MDSC populations can undergo concomitant signal-induced transformation and that the initial stages of transformation may be due to changes in the balance between the inherent nature of the cell and extrinsic signaling pathways. This theory represents a potential link between somatic stem cells and cancer and suggests an involvement of the niche/environment in transformation.

Neural Stem Cells: Implications for the Conventional Radiotherapy of Central Nervous System Malignancies

Advances in basic neuroscience related to neural stem cells and their malignant counterparts are challenging traditional models of central nervous system tumorigenesis and intrinsic brain repair. Neurogenesis persists into adulthood predominantly in two neurogenic centers: subventricular zone and subgranular zone. Subventricular zone is situated adjacent to lateral ventricles and subgranular zone is confined to the dentate gyrus of the hippocampus. Neural stem cells not only self-renew and differentiate along multiple lineages in these regions, but also contribute to intrinsic brain plasticity and repair. Ionizing radiation can depopulate these exquisitely sensitive regions directly or impair in situ neurogenesis by indirect, dose-dependent and inflammation-mediated mechanisms, even at doses <2 Gy. This review discusses the fundamental neural stem cell concepts within the framework of cumulative clinical experience with the treatment of central nervous system malignancies using conventional radiotherapy.

The hunt for cancer-initiating cells: a history stemming from leukemia

Conventional cancer therapies are plagued by disease relapses due to incomplete eradication of cancer-initiating cells. Evidence for cancer-initiating cells originally arose from studies in hematology and leukemia. Lessons learned from hematopoietic stem cells laid the bedrock for understanding how leukemic cells self-renew and remain in immature states. Decades later, leukemia-initiating cell techniques are now being applied to the field of solid tumors such as brain, breast, bone, colon, pancreas, lung and prostate cancer, with several cancer-initiating cell efforts led by hematologists. Different isolation techniques enriching for primitive cancer-initiating cells have been developed and are described in this review. Although the concept of cancer-initiating cells arose from studies in normal tissue stem cells, differences exist between neoplastic-initiating clones and their normal counterparts. Several efforts have uncovered aberrant molecular pathways and niche interactions unique to cancer-initiating cells. Efforts to exploit these pathways and interactions could ultimately lead to complete eradication of cancers.

Differentiation profile of brain tumor stem cells: a comparative study with neural stem cells

Understanding of the differentiation profile of brain tumor stem cells (BTSCs), the key ones among tumor cell population, through comparison with neural stem cells (NSCs) would lend insight into the origin of glioma and ultimately yield new approaches to fight this intractable disease. Here, we cultured and purified BTSCs from surgical glioma specimens and NSCs from human fetal brain tissue, and further analyzed their cellular biological behaviors, especially their differentiation property. As expected, NSCs differentiated into mature neural phenotypes. In the same differentiation condition, however, BTSCs exhibited distinguished differences. Morphologically, cells grew flattened and attached for the first week, but gradually aggregated and reformed floating tumor sphere thereafter. During the corresponding period, the expression rate of undifferentiated cell marker CD133 and nestin in BTSCs kept decreasing, but 1 week later, they regained ascending tendency. Interestingly, the differentiated cell markers GFAP and beta-tubulinIII showed an expression change inverse to that of undifferentiated cell markers. Taken together, BTSCs were revealed to possess a capacity to resist differentiation, which actually represents the malignant behaviors of glioma.

Symmetric division of cancer stem cells--a key mechanism in tumor growth that should be targeted in future therapeutic approaches

As cancer stem cells (SCs) drive tumor growth, it is only through the elimination of those cancer SCs that a pharmacologic cure can be attained. To study ways to develop drugs that target cancer SC, we investigated changes in cellular mechanisms and kinetics that occur in SC populations during colorectal cancer (CRC) development. We used computer modeling to determine which changes could give rise to exponential increases in both SC and non-SC populations in CRC. Our results show that the only mechanism that can explain how these subpopulations increase exponentially in CRC development involves an increase in symmetric SC cell division. This finding suggests that any systemic therapies designed to effectively treat CRC and other cancers must act to control or eliminate symmetrical cancer SC division in tumors, while minimally affecting normal SC division in non-tumor tissues.

Most C6 Cells Are Cancer Stem Cells: Evidence from Clonal and Population Analyses

Cancer stem cells have been isolated from human gliomas and many other parenchymal tumors. It was previously assumed that many established malignant cell lines also contain a rare subpopulation of stem cells. This study was designed to investigate the fraction of cancer stem cells in the C6 glioma cell line using clonal and population analyses, rather than isolating methods, which are based on specific markers. Interestingly, in the serum-containing medium, each of the 67 single C6 cells plated per miniwell was able to generate a clone and subclones, which subsequently gave rise to a xenograft glioma in the BALB/C-nude mouse. The CD133- C6 cells also possessed clonogenic, self-renewal, and tumorigenic capacities. Moreover, our findings indicated that brief exposure to Hoechst 33342 was harmful to the clonogenicity and proliferation of individual C6 cells. Therefore, the non-side-population cells may be deprived of their stem cell features in the process of Hoechst 33342 staining as a step in isolating a Hoechst-negative side population with flow cytometry. Thus, we concluded that the C6 line was mainly composed of cancer stem cells, although many of them were neither CD133+ nor side population.

Class III β-Tubulin and γ-Tubulin are Co-expressed and Form Complexes in Human Glioblastoma Cells

We have previously shown that the neuronal-associated class III beta-tubulin isotype and the centrosome-associated gamma-tubulin are aberrantly expressed in astrocytic gliomas (Cell Motil Cytoskeleton 2003, 55:77-96; J Neuropathol Exp Neurol 2006, 65:455-467). Here we determined the expression, distribution and interaction of betaIII-tubulin and gamma-tubulin in diffuse-type astrocytic gliomas (grades II-IV) (n = 17) and the human glioblastoma cell line T98G. By immunohistochemistry and immunofluorescence microscopy, betaIII-tubulin and gamma-tubulin were co-distributed in anaplastic astrocytomas and glioblastomas and to a lesser extent, in low-grade diffuse astrocytomas (P < 0.05). In T98G glioblastoma cells betaIII-tubulin was associated with microtubules whereas gamma-tubulin exhibited striking diffuse cytoplasmic staining in addition to its expectant centrosome-associated pericentriolar distribution. Treatment with different anti-microtubule drugs revealed that betaIII-tubulin was not associated with insoluble gamma-tubulin aggregates. On the other hand, immunoprecipitation experiments unveiled that both tubulins formed complexes in soluble cytoplasmic pools, where substantial amounts of these proteins were located. We suggest that aberrant expression and interactions of betaIII-tubulin and gamma-tubulin may be linked to malignant changes in glial cells.

Astrocytic stem cells in the adult brain

The adult mammalian brain harbors a population of neural stem cells (NSCs) that are responsible for persistent neurogenesis in the olfactory system and hippocampus and may also play a role in tumorigenesis. Here, the authors review the evidence that NSCs within the adult brain are a special type of astrocyte. In addition, the authors examine the phylogenetic and ontogenetic relations between this astrocyte stem cell and related members of the astrocyte family. Finally, the authors compare and contrast the functional characteristics of NSCs and hematopoietic stem cells and review the potential oncogenic transformation of astrocyte NSCs that may underlie brain tumorigenesis as seen in glioblastoma and other primary brain tumors.

Brain tumor stem cells: identification and concepts

The study of human brain tumors has characteristically emphasized the molecular and cellular analysis of the bulk tumor. There is increasing evidence in brain tumors and other malignancies that the tumor clone is functionally heterogeneous, however, existing in a cellular hierarchy based on small subpopulations of stem cells. These concepts were first definitively demonstrated in human acute myelogenous leukemia, in which regeneration of a diversely heterogeneous human leukemia cell population in a xenograft mouse model occurred only after injection of a rare relatively homogeneous population of leukemic cells that expressed hematopoietic stem cell markers. Recently, through advances in understanding of normal neural stem cell biology, the use of techniques for cell purification by flow cytometry, and the development of cell functional assays in vivo, the time was made ripe for several groups to characterize brain tumor stem cells (BTSCs). The BTSC resides in the cell fraction expressing the neural precursor cell surface marker CD133.

Notch signaling enhances nestin expression in gliomas

Recent findings suggest that Notch signaling is active in brain tumors and stem cells, and that stem cells or cells with progenitor characteristics contribute to brain tumor formation. These stem cells are marked by expression of several markers, including nestin, an intermediate filament protein. We have studied how the Notch signaling pathway affects nestin expression in brain tumors. We find that Notch receptors and ligands are expressed in vitro and in human samples of glioblastomas, the highest grade of malignant gliomas. In culture, Notch activity activates the nestin promoter. Activation of the Notch pathway also occurs in a glioblastoma multiforme mouse model induced by Kras, with translational regulation playing a role in Notch expression. Combined activation of Notch and Kras in wild-type nestin-expressing cells leads to their expansion within the subventricular zone and retention of proliferation and nestin expression. However, activation of Notch alone is unable to induce this cellular expansion. These data suggest that Notch may have a contributing role in the stem-like character of glioma cells.

The human subventricular zone: a source of new cells and a potential source of brain tumors

The mammalian brain has been perceived as a quiescent organ incapable of postnatal neurogenesis for many years. Most recently, several studies have demonstrated that the adult mammalian brain is indeed capable of neurogenesis and that the process is primarily confined to the subventricular zone (SVZ) of the forebrain and the subgranular zone (SGZ) of the hippocampus. Of these regions, the SVZ is the largest niche of neurogenesis in the adult mammalian brain. Within this niche resides a subpopulation of astrocytes with stem cell-like features of self-renewal and multipotentiality. Interestingly, there is also a subpopulation of cells within brain tumors that possess these same characteristics. Based on these findings, the emerging hypothesis is that brain tumor stem cells may be derived from neural stem cells and that both of these populations may originate from the SVZ. This possible connection stresses the importance of studying and understanding the role that the human SVZ plays in not only harboring neural and brain tumor stem cells, but how this microenvironment may support both neurogenesis and tumorigenesis. Furthermore, the obvious differences in the SVZ between humans and other animals make it important to understand the human model when studying human disease. Such an understanding may lead to novel therapeutic strategies for both neurodegenerative diseases and currently intractable brain tumors.

Ependymomas with neuronal differentiation: a morphologic and immunohistochemical spectrum

The category of mixed glioneuronal tumors of the CNS is rapidly losing its definition as encompassing tumors composed of histologically distinct neuron variants and glia. We encountered five ependymomas with neuronal differentiation seen in two by histology, in two by immunohistochemistry alone, and in one by electron microscopy. Antibodies against GFAP, S-100 protein, neurofilament protein, chromogranin, synaptophysin, Neu-N, and EMA were applied. Ultrastructural studies were also performed. In addition, 33 randomly selected ependymomas of various histologic types were screened for these same antigens. Cases 1 and 2 were anaplastic and showed clearly defined neuropil islands or pale islands as in nodular desmoplastic medulloblastoma, respectively. The tumors affected a 16-year-old male and a 5-year-old female and involved the right frontoparietal lobe and fourth ventricle, respectively. The islands were positive for synaptophysin and Neu-N (cases 1 and 2), and chromogranin (case 1). Cases 3-5, as well as 7 of the 33 screened ependymomas, showed a suggestion of neuronal differentiation by immunohistochemistry alone, including immunoreactivity for Neu-N (n = 8), synaptophysin (n = 4), neurofilament protein (n = 4), and chromogranin (n = 2). Five tumors each were WHO grade II and III. Electron microscopy performed on the two cases with neuronal islands demonstrated microtubule bundles and dense core granules (case 1) and poorly differentiated cells with high nuclear/cytoplasmic ratios, with intermediate filament accumulation and rare cilia (case 2). Cases identified by immunohistochemistry or electron microscopy demonstrated dense core granules (n = 5) and aligned microtubules (n = 3). Neuronal differentiation occurs in ependymomas but is less frequently definitive (histologic, ultrastructural) than merely a limited immunohistochemical finding. The clinical significance of these observations is unknown but deserves further exploration.

Identification of IGF2 signaling through phosphoinositide-3-kinase regulatory subunit 3 as a growth-promoting axis in glioblastoma

Amplification or overexpression of growth factor receptors is a frequent occurrence in malignant gliomas. Using both expression profiling and in situ hybridization, we identified insulin-like growth factor 2 (IGF2) as a marker for a subset of glioblastomas (GBMs) that lack amplification or overexpression of EGF receptor. Among 165 primary high-grade astrocytomas, 13% of grade IV tumors and 2% of grade III tumors expressed IGF2 mRNA levels >50-fold the sample population median. IGF2-overexpressing tumors frequently displayed PTEN loss, were highly proliferative, exhibited strong staining for phospho-Akt, and belonged to a subclass of GBMs characterized by poor survival. Using a serum-free culture system, we discovered that IGF2 can substitute for EGF to support the growth of GBM-derived neurospheres. The growth-promoting effects of IGF2 were mediated by the insulin-like growth factor receptor 1 and phosphoinositide-3-kinase regulatory subunit 3 (PIK3R3), a regulatory subunit of phosphoinositide 3-kinase that shows genomic gains in some highly proliferative GBM cases. PIK3R3 knockdown inhibited IGF2-induced growth of GBM-derived neurospheres. The current results provide evidence that the IGF2-PIK3R3 signaling axis is involved in promoting the growth of a subclass of highly aggressive human GBMs that lack EGF receptor amplification. Our data underscore the importance of the phosphoinositide 3-kinase/Akt pathway for growth of high-grade gliomas and suggest that multiple molecular alterations that activate this signaling cascade may promote tumorigenesis. Further, these findings highlight the parallels between growth factors or receptors that are overexpressed in GBMs and those that support in vitro growth of tumor-derived stem-like cells.

Identification of a subpopulation of cells with cancer stem cell properties in head and neck squamous cell carcinoma

Like many epithelial tumors, head and neck squamous cell carcinoma (HNSCC) contains a heterogeneous population of cancer cells. We developed an immunodeficient mouse model to test the tumorigenic potential of different populations of cancer cells derived from primary, unmanipulated human HNSCC samples. We show that a minority population of CD44(+) cancer cells, which typically comprise <10% of the cells in a HNSCC tumor, but not the CD44(-) cancer cells, gave rise to new tumors in vivo. Immunohistochemistry revealed that the CD44(+) cancer cells have a primitive cellular morphology and costain with the basal cell marker Cytokeratin 5/14, whereas the CD44(-) cancer cells resemble differentiated squamous epithelium and express the differentiation marker Involucrin. The tumors that arose from purified CD44(+) cells reproduced the original tumor heterogeneity and could be serially passaged, thus demonstrating the two defining properties of stem cells: ability to self-renew and to differentiate. Furthermore, the tumorigenic CD44(+) cells differentially express the BMI1 gene, at both the RNA and protein levels. By immunohistochemical analysis, the CD44(+) cells in the tumor express high levels of nuclear BMI1, and are arrayed in characteristic tumor microdomains. BMI1 has been demonstrated to play a role in self-renewal in other stem cell types and to be involved in tumorigenesis. Taken together, these data demonstrate that cells within the CD44(+) population of human HNSCC possess the unique properties of cancer stem cells in functional assays for cancer stem cell self-renewal and differentiation and form unique histological microdomains that may aid in cancer diagnosis.

Bone morphogenetic proteins inhibit the tumorigenic potential of human brain tumour-initiating cells

Transformed, oncogenic precursors, possessing both defining neural-stem-cell properties and the ability to initiate intracerebral tumours, have been identified in human brain cancers. Here we report that bone morphogenetic proteins (BMPs), amongst which BMP4 elicits the strongest effect, trigger a significant reduction in the stem-like, tumour-initiating precursors of human glioblastomas (GBMs). Transient in vitro exposure to BMP4 abolishes the capacity of transplanted GBM cells to establish intracerebral GBMs. Most importantly, in vivo delivery of BMP4 effectively blocks the tumour growth and associated mortality that occur in 100% of mice after intracerebral grafting of human GBM cells. We demonstrate that BMPs activate their cognate receptors (BMPRs) and trigger the Smad signalling cascade in cells isolated from human glioblastomas (GBMs). This is followed by a reduction in proliferation, and increased expression of markers of neural differentiation, with no effect on cell viability. The concomitant reduction in clonogenic ability, in the size of the CD133+ population and in the growth kinetics of GBM cells indicates that BMP4 reduces the tumour-initiating cell pool of GBMs. These findings show that the BMP-BMPR signalling system--which controls the activity of normal brain stem cells--may also act as a key inhibitory regulator of tumour-initiating, stem-like cells from GBMs and the results also identify BMP4 as a novel, non-cytotoxic therapeutic effector, which may be used to prevent growth and recurrence of GBMs in humans.

Cannabinoids induce glioma stem-like cell differentiation and inhibit gliomagenesis

Glioma stem-like cells constitute one of the potential origins of gliomas, and therefore, their elimination is an essential factor for the development of efficient therapeutic strategies. Cannabinoids are known to exert an antitumoral action on gliomas that relies on at least two mechanisms: induction of apoptosis of transformed cells and inhibition of tumor angiogenesis. However, whether cannabinoids target human glioma stem cells and their potential impact in gliomagenesis are unknown. Here, we show that glioma stem-like cells derived from glioblastoma multiforme biopsies and the glioma cell lines U87MG and U373MG express cannabinoid type 1 (CB(1)) and type 2 (CB(2)) receptors and other elements of the endocannabinoid system. In gene array experiments, CB receptor activation altered the expression of genes involved in the regulation of stem cell proliferation and differentiation. The cannabinoid agonists HU-210 and JWH-133 promoted glial differentiation in a CB receptor-dependent manner as shown by the increased number of S-100beta- and glial fibrillary acidic protein-expressing cells. In parallel, cannabinoids decreased the cell population expressing the neuroepithelial progenitor marker nestin. Moreover, cannabinoid challenge decreased the efficiency of glioma stem-like cells to initiate glioma formation in vivo, a finding that correlated with decreased neurosphere formation and cell proliferation in secondary xenografts. Gliomas derived from cannabinoid-treated cancer stem-like cells were characterized with a panel of neural markers and evidenced a more differentiated phenotype and a concomitant decrease in nestin expression. Overall, our results demonstrate that cannabinoids target glioma stem-like cells, promote their differentiation, and inhibit gliomagenesis, thus giving further support to their potential use in the management of malignant gliomas.

Stem Cells, Regenerative Medicine, and Animal Models of Disease

The field of stem cell biology and regenerative medicine is rapidly moving toward translation to clinical practice, and in doing so has become even more dependent on animal donors and hosts for generating cellular reagents and assaying their potential therapeutic efficacy in models of human disease. Advances in cell culture technologies have revealed a remarkable plasticity of stem cells from embryonic and adult tissues, and transplantation models are now needed to test the ability of these cells to protect at-risk cells and replace cells lost to injury or disease. With such a mandate, issues related to acceptable sources and controversial (e.g., chimeric) models have challenged the field to provide justification of their potential efficacy before the passage of new restrictions that may curb anticipated breakthroughs. Progress from the use of both in vitro and in vivo regenerative medicine models already offers hope both for the facilitation of stem cell phenotyping in recursive gene expression profile models and for the use of stem cells as powerful new therapeutic reagents for cancer, stroke, Parkinson's, and other challenging human diseases that result in movement disorders. This article describes research in support of the following three objectives: (1) To discover the best stem or progenitor cell in vitro protocols for isolating, expanding, and priming these cells to facilitate their massive propagation into just the right type of neuronal precursor cell for protection or replacement protocols for brain injury or disease, including those that affect movement such as Parkinson's disease and stroke; (2) To discover biogenic factors--compounds that affect stem/progenitor cells (e.g., from high-throughput screening and other bioassay approaches)--that will encourage reactive cell genesis, survival, selected differentiation, and restoration of connectivity in central nervous system movement and other disorders; and (3) To establish the best animal models of human disease and injury, using both small and large animals, for testing new regenerative medicine therapeutics.

Breast stem cells and cancer

Recent results have increased our understanding of normal stem cells and the signalling pathways which regulate them during the development of the mammary gland. Tumours in many tissues are now thought to develop from dysregulated stem cells and depend on activated stem cell self-renewal pathways such as Notch for their tumourigenic capacity. These cancer stem cells are recognised by specific cell surface proteins that they express and their capacity to grow tumours in vivo or spheres in vitro. We have described human breast DCIS mammospheres grown from cancer stem cells and demonstrated their dependence on the EGF and Notch receptor pathways. Stem cell self-renewal pathways such as these may represent novel therapeutic targets to prevent recurrence of pre-invasive and invasive breast cancer.

Bone morphogenetic proteins regulate tumorigenicity in human glioblastoma stem cells

Human glioblastomas appear to be established and expanded by cancer stem cells, which are endowed with tumour-initiating and perpetuating ability. We report that bone morphogenetic proteins (BMPs), amongst which BMP4 elicits the strongest effect, activate their cognate receptors (BMPRs) and trigger the Smad but not the MAP38 kinase signalling cascade in cells isolated from human glioblastomas (GBMs). This is followed by a reduction in proliferation and increased expression of differentiated neural markers, without affecting cell viability. The concomitant reduction in the clonogenic ability, both in the size of the CD133+ side population and in the growth kinetics of GBM cells, indicates that BMP4 triggers a reduction in the in vitro cancer stem cell (CSC) pool. Accordingly, transient ex vivo exposure to BMP4 abolishes the capacity of transplanted GBM cells to establish intracerebral GBMs. Most important, in vivo delivery of BMP4 effectively blocks the tumour growth and associated mortality which occur in 100% of control mice in less than 12 weeks, following intracerebral grafting of human GBM cells. These findings show that the BMP-BMPR signalling system, which controls the activity of normal brain stem cells, may also act as a key inhibitory regulator of cancer-initiating, GBM stem-like cells and identifies BMP4 as a novel, non-cytotoxic therapeutic effector, which may be used to prevent growth and recurrence of GBMs in humans.

Instructive cross-talk between neural progenitor cells and gliomas

Gliomas are the most common primary brain tumors and offer a poor prognosis in patients because of their infiltrative and treatment-resistant nature. The median survival time after diagnosis is approximately 11-12 months. There is a strong need for novel treatment modalities in targeting gliomas, and recent advances use neural progenitor cells as delivery systems for different therapeutic strategies. In this study, we show that rat embryonic neural progenitor cell (NPC) lines, transplanted at a distant site from a 3-day-preestablished glioma in the striatum, were able to migrate toward and colocalize with tumor isles without general spread into the brain parenchyma. Upon encounter with tumor, neural progenitor cells changed phenotype and became vimentin positive. These results demonstrate that transplanted neural progenitor cells respond to queues from a tumor and home to and exert an antitumor effect on the preestablished glioma, significantly decreasing the tumor volume with approximately 67% compared with control tumors after 1-2 weeks. Moreover, these early effects could be translated into increased survival times of animals treated with neural progenitor cell grafts 3 days after intrastriatal tumor inoculation. In contrast, there was no activation or migration of endogenous subventricular zone (SVZ) neuroblasts in response to an intrastriatal syngeneic tumor. In conclusion, NPC possess the ability to influence tumor growth as well as respond to queues from the tumor or tumor microenvironment, demonstrating a cross-talk between the cells.

HEDGEHOG-GLI1 signaling regulates human glioma growth, cancer stem cell self-renewal, and tumorigenicity

Cancer stem cells are rare tumor cells characterized by their ability to self-renew and to induce tumorigenesis. They are present in gliomas and may be responsible for the lethality of these incurable brain tumors. In the most aggressive and invasive type, glioblastoma multiforme (GBM), an average of about one year spans the period between detection and death [1]. The resistence of gliomas to current therapies may be related to the existence of cancer stem cells [2-6]. We find that human gliomas display a stemness signature and demonstrate that HEDGEHOG (HH)-GLI signaling regulates the expression of stemness genes in and the self-renewal of CD133(+) glioma cancer stem cells. HH-GLI signaling is also required for sustained glioma growth and survival. It displays additive and synergistic effects with temozolomide (TMZ), the current chemotherapeutic agent of choice. TMZ, however, does not block glioma stem cell self-renewal. Finally, interference of HH-GLI signaling with cyclopamine or through lentiviral-mediated silencing demonstrates that the tumorigenicity of human gliomas in mice requires an active pathway. Our results reveal the essential role of HH-GLI signaling in controlling the behavior of human glioma cancer stem cells and offer new therapeutic possibilities.

Neurocytoma is a tumor of adult neuronal progenitor cells

Central neurocytoma (CN) is a rare periventricular tumor, whose derivation, lineage potential, and molecular regulation have been mostly unexplored. We noted that CN cells exhibited an antigenic profile typical of neuronal progenitor cells in vivo, yet in vitro generated neurospheres, divided in response to bFGF (basic fibroblast growth factor), activated the neuroepithelial enhancer of the nestin gene, and gave rise to both neuron-like cells and astrocytes. When CN gene expression was compared with that of both normal adult VZ (ventricular zone) and E/nestin:GFP (green fluorescent protein)-sorted native neuronal progenitors, significant overlap was noted. Marker analysis suggested that the gene expression pattern of CN was that of a proneuronal population; glial markers were conspicuously absent, suggesting that the emergence of astroglia from CN occurred only with passage. The expression pattern of CN was distinguished from that of native progenitor cells by a cohort of differentially expressed genes potentially involved in both the oncogenesis and phenotypic restriction of neurocytoma. These included both IGF2 and several components of its signaling pathway, whose sharp overexpression implicated dysregulated autocrine IGF2 signaling in CN oncogenesis. Both receptors and effectors of canonical wnt signaling, as well as GDF8 (growth differentiation factor 8), PDGF-D, and neuregulin, were differentially overexpressed by CN, suggesting that CN is characterized by the concurrent overactivation of these pathways, which may serve to drive neurocytoma expansion while restricting tumor progenitor phenotype. This strategy of comparing the gene expression of tumor cells to that of the purified native progenitors from which they derive may provide a focused approach to identifying transcripts important to stem and progenitor cell oncogenesis.

Subepithelial myofibroblasts are novel nonprofessional APCs in the human colonic mucosa

The human gastrointestinal mucosa is exposed to a diverse normal microflora and dietary Ags and is a common site of entry for pathogens. The mucosal immune system must respond to these diverse signals with either the initiation of immunity or tolerance. APCs are important accessory cells that modulate T cell responses which initiate and maintain adaptive immunity. The ability of APCs to communicate with CD4+ T cells is largely dependent on the expression of class II MHC molecules by the APCs. Using immunohistochemistry, confocal microscopy, and flow cytometry, we demonstrate that alpha-smooth muscle actin(+), CD90+ subepithelial myofibroblasts (stromal cells) constitutively express class II MHC molecules in normal colonic mucosa and that they are distinct from professional APCs such as macrophages and dendritic cells. Primary isolates of human colonic myofibroblasts (CMFs) cultured in vitro were able to stimulate allogeneic CD4+ T cell proliferation. This process was dependent on class II MHC and CD80/86 costimulatory molecule expression by the myofibroblasts. We also demonstrate that CMFs, engineered to express a specific DR4 allele, can process and present human serum albumin to a human serum albumin-specific and DR4 allele-restricted T cell hybridoma. These studies characterize a novel cell phenotype which, due to its strategic location and class II MHC expression, may be involved in capture of Ags that cross the epithelial barrier and present them to lamina propria CD4+ T cells. Thus, human CMFs may be important in regulating local immunity in the colon.

Cancer stem cells and "stemness" genes in neuro-oncology

The main properties of stem cells include long-term self-renewal and the capacity to give rise to one or more types of differentiated progeny. Recently, much evidence was provided that leukemia and tumor maintenance and growth are sustained by a small proportion of cells exhibiting stem cell properties. In neural tumors, stem cells have been detected in glioblastoma, medulloblastoma and ependymoma. These observations imply that normal stem cells could be the origin of cancer stem cells; alternatively, a more differentiated progeny may revert to a "stem-like" status, and give rise to cancer stem cells. In adult brain residual stem cells are located in the hippocampus, the subventricular zone and possibly the cerebellum. However, evidence for the ability of more differentiated progeny (astroglia, oligodendroglia) to convert into "stem cells" in vitro has also been provided, thus greatly expanding the potential target of oncogenic mutations. In the framework of the cancer stem cell hypothesis, genes originally identified as important for normal neural stem cells may be essential to support cancer stem cells as well. Stem cell genes act in several ways: they stimulate stem cell self-replication, inhibit differentiation, control excessive replication that might lead to "exhaustion" of the stem cell pool. Mutations in man and mouse, in spontaneous or experimental brain tumors, often target stem cell genes or genes lying in their functional pathway, the main examples being the Sonic hedgehog and the Wnt pathways. Interestingly, several stem cell genes are often overexpressed in brain tumors, even if they are not mutated. This suggests that these genes may be important for the generation of cancer stem cells from more differentiated precursors, or for cancer stem cell maintenance. Cancer stem cells partially differentiate in vivo, and in vitro they also give rise to seemingly normal differentiated progeny, like normal stem cells: thus, their main defect, leading to cancer, may lie in the unbalance between self-replication and terminal differentiation of this minority cell population. Knowledge of extrinsic diffusible factors affecting the activity of stem cell genes may help identifying tools for inducing cancer stem cell differentiation, which might be of use in therapy.

Molecular biology of malignant gliomas

Gliomas are the most common primary brain tumours. In keeping with the degree of aggressiveness, gliomas are divided into four grades, with different biological behaviour. Furthermore, as different gliomas share a predominant histological appearance, the final classification includes both, histological features and degree of malignancy. For example, gliomas of astrocytic origin (astrocytomas) are classified into pilocytic astrocytoma (grade I), astrocytoma (grade II), anaplastic astrocytoma (grade III) and glioblastoma multiforme (GMB) (grade IV). Tumors derived from oligodendrocytes include grade II (oliogodendrogliomas) and grade III neoplasms (oligoastrocytoma). Each subtype has a specific prognosis that dictates the clinical management. In this regard, a patient diagnosed with an oligodendroglioma totally removed has 10-15 years of potential survival. On the opposite site, patients carrying a glioblastoma multiforme usually die within the first year after the diagnosis is made. Therefore, different approaches are needed in each case. Obviously, prognosis and biological behaviour of malignant gliomas are closely related and supported by the different molecular background that possesses each type of glioma. Furthermore, the ability that allows several low-grade gliomas to progress into more aggressive tumors has allowed cancer researchers to elucidate several pathways implicated in molecular biology of these devastating tumors. In this review, we describe classical pathways involved in human malignant gliomas with special focus with recent advances, such as glioma stem-like cells and expression patterns from microarray studies.

Glioma stem cells promote radioresistance by preferential activation of the DNA damage response

Ionizing radiation represents the most effective therapy for glioblastoma (World Health Organization grade IV glioma), one of the most lethal human malignancies, but radiotherapy remains only palliative because of radioresistance. The mechanisms underlying tumour radioresistance have remained elusive. Here we show that cancer stem cells contribute to glioma radioresistance through preferential activation of the DNA damage checkpoint response and an increase in DNA repair capacity. The fraction of tumour cells expressing CD133 (Prominin-1), a marker for both neural stem cells and brain cancer stem cells, is enriched after radiation in gliomas. In both cell culture and the brains of immunocompromised mice, CD133-expressing glioma cells survive ionizing radiation in increased proportions relative to most tumour cells, which lack CD133. CD133-expressing tumour cells isolated from both human glioma xenografts and primary patient glioblastoma specimens preferentially activate the DNA damage checkpoint in response to radiation, and repair radiation-induced DNA damage more effectively than CD133-negative tumour cells. In addition, the radioresistance of CD133-positive glioma stem cells can be reversed with a specific inhibitor of the Chk1 and Chk2 checkpoint kinases. Our results suggest that CD133-positive tumour cells represent the cellular population that confers glioma radioresistance and could be the source of tumour recurrence after radiation. Targeting DNA damage checkpoint response in cancer stem cells may overcome this radioresistance and provide a therapeutic model for malignant brain cancers.

Redefining cellular phenotypy based on embryonic, adult, and cancer stem cell biology

Stem cell biology has provided constant alteration if not reversal of dogma related to the understanding of the behaviors of primitive and dynamic cells. This review summarizes recent findings on dynamic changes of phenotype that accompany the in vitro growth and differentiation of not only stem and progenitor cells, but also differentiated cells derived from a variety of normal and pathological tissues. As there are examples of apparent dedifferentiation and transdifferentiation of neural cells that appear to be terminally differentiated, there is a need to reconsider elements of cellular fate choice that have relevance to neurooncology and neural repair. Recent findings of dynamic behaviors and mixed phenotype of both normal and cancer stem cells suggest that some of the diverse lineage attributes of different solid tumors may owe their existence to dynamic cellular phenotypy gone awry.

Evaluation of vascular endothelial growth factor addition in vitro on mouse inner ear progenitor cell cultures

We analyzed progenitor cell cultures of inner ear tissue from newborn mice, and found proliferating cells, morphologically differentiating cells and subpopulations of cells expressing either neuronal or glial markers. In addition, we observed the expression of fetal liver kinase-1, a receptor for the vascular endothelial growth factor in a subpopulation of the cultured cells. Consistent with the expression of fetal liver kinase-1, addition of vascular endothelial growth factor at a dose of 10 ng/ml increased the expansion rate of inner ear-derived progenitor cells. Together with other published data, these results suggest that the vascular endothelial growth factor might be involved in inducing or supporting cochlear repair processes.

Comprehensive DNA methylation profiling in a human cancer genome identifies novel epigenetic targets

Using a unique microarray platform for cytosine methylation profiling, the DNA methylation landscape of the human genome was monitored at more than 21,000 sites, including 79% of the annotated transcriptional start sites (TSS). Analysis of an oligodendroglioma derived cell line LN-18 revealed more than 4000 methylated TSS. The gene-centric analysis indicated a complex pattern of DNA methylation exists along each autosome, with a trend of increasing density approaching the telomeres. Remarkably, 2% of CpG islands (CGI) were densely methylated, and 17% had significant levels of 5 mC, whether or not they corresponded to a TSS. Substantial independent verification, obtained from 95 loci, suggested that this approach is capable of large scale detection of cytosine methylation with an accuracy approaching 90%. In addition, we detected large genomic domains that are also susceptible to DNA methylation reinforced inactivation, such as the HOX cluster on chromosome 7 (CH7). Extrapolation from the data suggests that more than 2000 genomic loci may be susceptible to methylation and associated inactivation, and most have yet to be identified. Finally, we report six new targets of epigenetic inactivation (IRX3, WNT10A, WNT6, RARalpha, BMP7 and ZGPAT). These targets displayed cell line and tumor specific differential methylation when compared with normal brain samples, suggesting they may have utility as biomarkers. Uniquely, hypermethylation of the CGI within an IRX3 exon was correlated with over-expression of IRX3 in tumor tissues and cell lines relative to normal brain samples.

Somatic stem cells and the origin of cancer

Most human cancers derive from a single cell targeted by genetic and epigenetic alterations that initiate malignant transformation. Progressively, these early cancer cells give rise to different generations of daughter cells that accumulate additional mutations, acting in concert to drive the full neoplastic phenotype. As we have currently deciphered many of the gene pathways disrupted in cancer, our knowledge about the nature of the normal cells susceptible to transformation upon mutation has remained more elusive. Adult stem cells are those that show long-term replicative potential, together with the capacities of self-renewal and multi-lineage differentiation. These stem cell properties are tightly regulated in normal development, yet their alteration may be a critical issue for tumorigenesis. This concept has arisen from the striking degree of similarity noted between somatic stem cells and cancer cells, including the fundamental abilities to self-renew and differentiate. Given these shared attributes, it has been proposed that cancers are caused by transforming mutations occurring in tissue-specific stem cells. This hypothesis has been functionally supported by the observation that among all cancer cells within a particular tumor, only a minute cell fraction has the exclusive potential to regenerate the entire tumor cell population; these cells with stem-like properties have been termed cancer stem cells. Cancer stem cells can originate from mutation in normal somatic stem cells that deregulate their physiological programs. Alternatively, mutations may target more committed progenitor cells or even mature cells, which become reprogrammed to acquire stem-like functions. In any case, mutated genes should promote expansion of stem/progenitor cells, thus increasing their predisposition to cancer development by expanding self-renewal and pluripotency over their normal tendency towards relative quiescency and proper differentiation.

The gene expression profile of PDGF-treated neural stem cells corresponds to partially differentiated neurons and glia

We have previously shown that platelet-derived growth factor AA (PDGF-AA) stimulates the expansion of neuronal progenitors from neural stem cells, but is unable to replace fibroblast-growth factor 2 (FGF-2) as a stem cell mitogen. In the present study, we compared gene expression in neural stem cells that were grown in the presence of FGF-2 and in cells cultured with PDGF-AA or in the absence of growth factor, which induces differentiation. The genetic program elicited by PDGF-AA (156 significantly regulated genes) was not unique, but an intermediate between the ones of FGF-2-cultured stem cells and differentiated cells. These observations are compatible with the hypothesis that PDGF-AA induces a partial differentiation of neural stem cells, which retain the ability to proliferate, rather than acting solely as an instructing agent for neuronal differentiation. Finally, the transcriptional signature of stem cells grown with FGF-2 included a large number of genes over-expressed in gliomas and a core set of conserved genes periodically expressed during the eukaryote cell cycle.

Neurosphere assays: growth factors and hormone differences in tumor and nontumor studies

The "no new neuron" dogma that the brain is quiescent throughout adult life has been challenged by the discovery of cells with stem cell-like qualities of self-renewal and multipotency in the subventricular zone of the lateral ventricles and the dentate gyrus of the hippocampus in adults. This self-renewing capacity also makes these neural stem cells a possible source of brain tumors, which was supported by the discovery of self-sustaining brain tumor stem-like cells in cancers such as glioblastoma multiforme. Neurosphere assays are the standard for studying these stem-like cells in both normal and cancer tissues. Despite the importance of these assays, there is no standardized protocol to allow for a comparison of results because several studies use different growth factors and hormones at different concentrations. The primary objective of this study is to review the literature for both nontumor and tumor studies to assess their respective neurosphere assay components. We found significant variation in assay components, namely hormones and growth factors, as well as their respective concentrations. This illustrates the need for a standardized protocol to allow proper comparison among studies and a better assessment of the effects of different factors.

Isolation of multipotent neural crest-derived stem cells from the adult mouse cornea

We report the presence of neural crest-derived corneal precursors (COPs) that initiate spheres by clonal expansion from a single cell. COPs expressed the stem cell markers nestin, Notch1, Musashi-1, and ABCG2 and showed the side population cell phenotype. COPs were multipotent with the ability to differentiate into adipocytes, chondrocytes, as well as neural cells, as shown by the expression of beta-III-tubulin, glial fibrillary acidic protein, and neurofilament-M. COP spheres prepared from E/nestin-enhanced green fluorescent protein (EGFP) mice showed induction of EGFP expression that was not originally observed in the cornea, indicating activation of the neural-specific nestin second intronic enhancer in culture. COPs were Sca-1(+), CD34(+), CD45(-), and c-kit(-). Numerous GFP(+) cells were observed in the corneas of mice transplanted with whole bone marrow of transgenic mice ubiquitously expressing GFP; however, no GFP(+) COP spheres were initiated from these mice. On the other hand, COP spheres from transgenic mice encoding P0-Cre/Floxed-EGFP as well as Wnt1-Cre/Floxed-EGFP were GFP(+), indicating the neural crest origin of COPs, which was confirmed by the expression of the embryonic neural crest markers Twist, Snail, Slug, and Sox9. Taken together, these data indicate the existence of neural crest-derived, multipotent stem cells in the adult cornea.

Glial progenitors in adult white matter are driven to form malignant gliomas by platelet-derived growth factor-expressing retroviruses

To test the gliomagenic potential of adult glial progenitors, we infected adult rat white matter with a retrovirus that expresses high levels of PDGF and green fluorescent protein (GFP). Tumors that closely resembled human glioblastomas formed in 100% of the animals by 14 d postinfection. Surprisingly, the tumors were composed of a heterogeneous population of cells, <20% of which expressed the retroviral reporter gene (GFP). The vast majority of both GFP+ and GFP- tumor cells expressed markers of glial progenitors. Thus, the tumors arose from the massive expansion of both infected and uninfected glial progenitors, suggesting that PDGF was driving tumor formation via autocrine and paracrine stimulation of glial progenitor cells. To explore this possibility further, we coinjected a retrovirus expressing PDGF-IRES-DsRed with a control retrovirus expressing only GFP. The resulting tumors contained a mixture of red cells (PDGF-expressing/tumor-initiating cells) and green cells (recruited progenitors). Both populations were highly proliferative and infiltrative. In contrast, when the control GFP retrovirus was injected alone, the animals never formed tumors and the majority of infected cells differentiated along the oligodendrocyte lineage. Together, these results reveal that adult white matter progenitors not only have the capacity to give rise to gliomas, but resident progenitors are recruited to proliferate within the mitogenic environment of the tumor and in this way contribute significantly to the heterogeneous mass of cells that compose a malignant glioma.

PDGFRα-Positive B Cells Are Neural Stem Cells in the Adult SVZ that Form Glioma-like Growths in Response to Increased PDGF Signaling

Neurons and oligodendrocytes are produced in the adult brain subventricular zone (SVZ) from neural stem cells (B cells), which express GFAP and have morphological properties of astrocytes. We report here on the identification B cells expressing the PDGFRalpha in the adult SVZ. Specifically labeled PDGFRalpha expressing B cells in vivo generate neurons and oligodendrocytes. Conditional ablation of PDGFRalpha in a subpopulation of postnatal stem cells showed that this receptor is required for oligodendrogenesis, but not neurogenesis. Infusion of PDGF alone was sufficient to arrest neuroblast production and induce SVZ B cell proliferation contributing to the generation of large hyperplasias with some features of gliomas. The work demonstrates that PDGFRalpha signaling occurs early in the adult stem cell lineage and may help regulate the balance between oligodendrocyte and neuron production. Excessive PDGF activation in the SVZ in stem cells is sufficient to induce hallmarks associated with early stages of tumor formation.

Transplanted glioma cells migrate and proliferate on host brain vasculature: a dynamic analysis

Glioma cells have a remarkable capacity to infiltrate the brain and migrate long distances from the tumor, making complete surgical resection impossible. Yet, little is known about how glioma cells interact with the complex microenvironment of the brain. To investigate the patterns and dynamics of glioma cell infiltration and migration, we stereotactically injected eGFP and DsRed-2 labeled rat C6 glioma cells into neonatal rat forebrains and used time-lapse microscopy to observe glioma cell migration and proliferation in slice cultures generated from these brains. In this model, glioma cells extensively infiltrated the brain by migrating along the abluminal surface of blood vessels. Glioma cells intercalated their processes between the endothelial cells and the perivascular astrocyte end feet, but did not invade into the blood vessel lumen. Dynamic analysis revealed notable similarities between the migratory behavior of glioma cells and that previously observed for glial progenitor cells. Glioma cells had a characteristic leading process and migrated in a saltatory fashion, with bursts of migration separated by periods of immobility, and maximum speeds of over 100 microm/h. Migrating glioma cells proliferated en route, pausing for as short as an hour to divide before the daughter cells resumed migrating. Remarkably, the majority of glioma cell divisions took place at or near vascular branch points, suggesting that mitosis is triggered by local environmental cues. This study provides the first dynamic analysis of glioma cell infiltration in living brain tissue and reveals that the migration and proliferation of transplanted glioma cells is directed by interactions with host brain vasculature.

Brain tumour stem cells

The dogma that the genesis of new cells is a negligible event in the adult mammalian brain has long influenced our perception and understanding of the origin and development of CNS tumours. The discovery that new neurons and glia are produced throughout life from neural stem cells provides new possibilities for the candidate cells of origin of CNS neoplasias. The emerging hypothesis is that alterations in the cellular and genetic mechanisms that control adult neurogenesis might contribute to brain tumorigenesis, thereby allowing the identification of new therapeutic strategies.

Stem cell properties and epithelial malignancies

The growth and repair of normal tissues depends on a small sub-population of cells termed somatic stem cells whose primary characteristic is an ability for indefinite self-renewal. Epithelial stem cells divide to produce cells, termed transient amplifying cells, that undergo a few rounds of more rapid division before they terminally differentiate. Evidence that the growth of tumours, as for normal tissues, is ultimately dependent on a subpopulation of the proliferatively competent cells was first shown for leukaemias by isolation of small sub-populations of phenotypically distinct 'tumour-initiating cells'. Differing cell surface phenotypes also prospectively identify tumour-initiating sub-populations in solid tumours. Even cell lines derived from tumours retain hierarchical stem cell patterns demonstrable as differing clonogenic abilities related to cellular properties such as size, adhesiveness, dye exclusion, and patterns of gene expression. Malignant stem cells appear to form the primary targets of therapy, but how differences between malignant stem and other cells affect therapeutic responses remains unclear. However, transplantation methods exist for their analysis and the in vitro persistence of stem cell patterns may provide systems for developing new therapeutic approaches.

Human neuroblastoma stem cells

Human neuroblastoma is an embryonic cancer of the neural crest. Cellular heterogeneity is a characteristic feature of both tumors and derived cell lines. Recent studies have revealed that both cell lines and tumors contain cancer stem cells. In culture, these cells are self-renewing, multipotent, and highly malignant; in tumors their frequency correlates with a worse prognosis. Their identification and characterization should now permit a targeted approach to more effective treatment of this often fatal childhood cancer.

Glioma stem cells: evidence and limitation

Gliomas, in particular the high-grade anaplastic glioma and glioblastoma multiforme (GBM), are manifested by morphological, genetic and phenotypic heterogeneity. Most of the studies hitherto have been performed on bulk glioma cells, with limited understanding on the origin and the relative contribution of particular glioma cell populations to glioma growth and progression. Recent studies have demonstrated the existence of a small fraction of glioma cells endowed with features of primitive neural progenitor cells and tumor-initiating function. Such cells have been defined as glioma stem cells. However, questions remain as to whether the currently identified glioma stem cells are the cell-of-origin for glioma initiation and progression, or the results of such processes. In this review, we discuss the current evidence and limitation in identifying glioma stem cells and the potential origin of glioma stem cells in the context of post-natal neural cell regeneration and their transformation mechanisms. The implication of these findings for glioma diagnosis and treatment will also be reviewed.

Brain cancer stem-like cells

Both stem cells and cancer cells are thought to be capable of unlimited proliferation. Moreover, many tumours and cancer cell lines express stem cell markers, including adenosine triphosphate (ATP)-binding cassette transporters, by which the cells pump out specific fluorescent dyes as well as anti-cancer drugs, suggesting either that cancer cells resemble stem cells or that cancers contain stem-like cells. Using the common characteristics of brain tumour cells and neural stem cells, several research groups have succeeded in identifying stem-like cells (cancer stem-like cells) in brain tumours and brain cancer cell lines. The purified cancer stem-like cells, but not the other cancer cells, self-renew and form tumours when transplanted in vivo. Thus, cancer stem-like cells in brain tumours might be a crucial target for anti-brain tumour therapy.

Oligodendroglial-specific transcriptional factor SOX10 is ubiquitously expressed in human gliomas

The two most common types of gliomas: astrocytoma and oligodendroglioma are distinguished based on their morphologic similarities to mature astrocytes and oligodendroglia. Whereas prototypical examples of the tumors have distinct pathogenetic and prognostic differences, the majority of the gliomas falls in the intermediate category and their distinction is problematic. The transcriptional factor SOX10 is one of the key determinants of oligodendroglial differentiation. We applied immunohistochemistry to analyze whether the expression of SOX10 can differentiate astrocytoma and oligodendroglioma. The majority of oligodendrogliomas, but also a large fraction of astrocytomas, including the least differentiated glioblastomas, expressed SOX10, albeit at lower levels. Comparison with 1p and 19q deletion status by FISH analysis also revealed no obvious associations. High levels of expression were also found in pilocytic astrocytoma, consistent with recent studies suggesting that pilocytic astrocytomas have greater overlap with oligodendroglial than astrocytic tumors. Our data raise a possibility that histogenesis of gliomas have more common features than previously anticipated.

In vitro identification and functional characterization of glial precursor cells in human gliomas

Human gliomas including astrocytomas and oligodendrogliomas are defined as being composed of neoplastic astrocytes and oligodendrocytes respectively. Here, on the basis of in vitro functional assays, we show that gliomas contain a mixture of glial progenitor cells and their progeny. We have set up explant cultures from pilocytic astrocytomas, glioblastomas and oligodendrogliomas and studied antigens that characterize glial lineage, from the precursor cells (glial restricted precursors and oligodendrocyte-type2-astrocyte/oligodendrocyte precursor cells expressing the A2B5 ganglioside) to the differentiated cells (oligodendrocyte and type-1 and type-2 astrocytes). All tumoral explants contain A2B5+ cells and can generate migrating cells with distinctive functional properties according to glioma subtypes. In pilocytic astrocytomas, very few migrating cells are dividing and can differentiate in type-2 astrocytes or towards the oligodendrocyte lineage. In glioblastomas, most migrating cells are dividing, express A2B5 or glial fibrillary acid protein (GFAP) and can generate oligodendrocytes and type-1 and type-2 astrocytes in appropriate medium. Oligodendroglioma explants are made by actively dividing glial precursor cells expressing A2B5 or PSA-NCAM. Only few cells can migrate and differentiation towards oligodendrocyte lineage does not occur. Isolated A2B5+ cells from both glioblastomas and oligodendrogliomas showed similar genetic alterations as the whole tumour. Therefore, pilocytic astrocytomas contain slowly dividing oligodendrocyte-type2-astrocyte/oligodendrocyte precursor cells in keeping with their benign behaviour whereas both glioblastomas and oligodendrogliomas contain neoplastic glial restricted precursor cells. In oligodendrogliomas, these cells are trapped in undifferentiated and proliferating state. The precursor cells properties present in gliomas give new insight into their histogenesis and open up new avenues for research in the field of gliomagenesis.

Glioblastoma multiforme with small cell neuronal-like component: association with human neurotropic JC virus

The human polyomavirus JCV, the etiological agent of progressive multifocal leukoencephalopathy, has been associated with primitive neuroectodermal tumors and various glial-derived tumors, including glioblastoma multiforme (GBM). Here we describe the unique clinical case of a 54-year-old man who presented with headaches, hemiparesis and drowsiness. T1 and T2 magnetic resonance images revealed a large solid tumor with a cystic component located in the right temporal lobe, with extension into the parietal lobe. Histologically, the tumor was composed of two areas, a main area of large neoplastic cells with pleomorphic atypical nuclei and abundant cytoplasm, which by immunohistochemistry was reactive for glial fibrillary acidic protein, mixed with several foci of poorly differentiated tumoral cells with elongated nuclei and scant cytoplasm, negative for GFAP, but robustly immunoreactive for synaptophysin and phosphoneurofilaments. Results from PCR in laser capture microdissected cells from both areas of the tumor revealed the presence of DNA sequences corresponding to the early, late and control regions (CR) of the JCV genome and expression of JCV proteins T-antigen and Agnoprotein in both phenotypes. No evidence for capsid protein was observed, excluding productive viral infection. Sequencing demonstrated the presence of the JCV Mad-1 strain with distinct point mutations in the CR of isolates from both, GBM and small cell architectural areas. The presence of JCV DNA sequences and expression of viral proteins further reinforces the role of the widely spread human neurotropic virus in early transformation and in the development of brain tumors.