Astrocyte development and heterogeneity

Astrocytes have many roles within the brain parenchyma, and a subpopulation restricted to germinal niches functions as neural stem cells (NSCs) that produce various types of neuronal progeny in relation to spatiotemporal factors. A growing body of evidence supports the concept of morphological and molecular differences between astrocytes in different brain regions, which might relate to their derivation from regionally patterned radial glia. Indeed, the notion that astrocytes are molecularly and functionally heterogeneous could help explain how the central nervous system (CNS) retains embryonic positional information into adulthood. Here, we discuss recent evidence for regionally encoded functions of astrocytes in the developing and adult CNS to provide an integrated concept of the origin and possible function of astrocyte heterogeneity. We focus on the regionalization of NSCs in the ventricular-subventricular zone (V-SVZ) of the adult mammalian brain and emerging evidence for a segmental organization of astrocytes in the developing spinal cord and forebrain. We propose that astrocytes' diversity will provide fundamental clues to understand regional brain organization and function.

Abstract LB-206: Role of Olig2 phosphorylation in pediatric glioma formation

Malignant gliomas are a leading cause of tumor-related mortality in children. Current therapies often have significant adverse effects on brain development and are largely ineffective against aggressive gliomas. The development of rational therapies has been limited by poor information regarding the cell of origin for pediatric glioma and the relevant underlying genetic alterations. Recent findings now reveal that mutations existing in pediatric glioma are different than those seen in adult gliomas. For example, BRAFV600E activating mutations are associated with deletion of the tumor suppressor gene, p16 (Ink4a-Arf), in about 15% of pediatric gliomas. We have generated a faithful genetic mouse model in which there is concomitant activation of BRAFV600E and deletion of Ink4a-Arf in oligodendrocyte progenitor cells (OPCs) leading to the generation of pediatric high-grade glioma. OPCs have been shown to be a potential cell of origin for glioma and expression of Olig2 is found in 100 percent of human pediatric and adult high-grade gliomas. Given that Olig2 is also required for normal myelin development, targeting all Olig2-expressing cells with small molecule inhibitors would likely have detrimental effects. We have recently found that Olig2 protein is present in a phosphorylated form that promotes proliferation of adult glioma cells. In this study, we demonstrate that phospho-Olig2 is expressed in tumorigenic oligodendrocyte progenitor cells and is required for pediatric glioma generation. In addition, we found that phospho-Olig2 and Olig2 interact with different proteins, some of which are feasible drug targets. This study gives insights into new therapies by understanding key protein interactions in normal oligodendrocyte development as well as in tumorigenic oligodendrocyte progenitor cells.

Citation Format: Amelie GRIVEAU, An-Chi TIEN, David H. Rowitch. Role of Olig2 phosphorylation in pediatric glioma formation. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-206. doi:10.1158/1538-7445.AM2014-LB-206

Abstract SY02-04: Overlapping mechanisms in CNS development and gliomagenesis

Glioblastoma multiforme (GBM) is a lethal brain cancer resistant to therapy in part because of its highly invasive nature. GBMs are a highly vascular tumor with heterogeneous histological features, but they are generally considered to derive from glial lineage progenitors. We have defined Olig2, a bHLH transcription factor, as a critical determinant of oligodendrocyte progenitor identity during development. Olig2 is expressed in human GBMs, raising the possibility of similar features between oligodendrocyte progenitors and glioma-propagating cells, a concept is supported by expression profiling and mouse models. This talk will review functions for OPCs during development, including proliferation, migration into adult parenchymal tissue and angiogenesis. As such, there are provocative similarities between normal OPCs and progenitors for GBM.

Citation Format: David H. Rowitch. Overlapping mechanisms in CNS development and gliomagenesis. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr SY02-04. doi:10.1158/1538-7445.AM2014-SY02-04

Oligodendrocyte-encoded HIF function couples postnatal myelination and white matter angiogenesis

Myelin sheaths provide critical functional and trophic support for axons in white matter tracts of the brain. Oligodendrocyte precursor cells (OPCs) have extraordinary metabolic requirements during development as they differentiate to produce multiple myelin segments, implying that they must first secure adequate access to blood supply. However, mechanisms that coordinate myelination and angiogenesis are unclear. Here, we show that oxygen tension, mediated by OPC-encoded hypoxia-inducible factor (HIF) function, is an essential regulator of postnatal myelination. Constitutive HIF1/2α stabilization resulted in OPC maturation arrest through autocrine activation of canonical Wnt7a/7b. Surprisingly, such OPCs also show paracrine activity that induces excessive postnatal white matter angiogenesis in vivo and directly stimulates endothelial cell proliferation in vitro. Conversely, OPC-specific HIF1/2α loss of function leads to insufficient angiogenesis in corpus callosum and catastrophic axon loss. These findings indicate that OPC-intrinsic HIF signaling couples postnatal white matter angiogenesis, axon integrity, and the onset of myelination in mammalian forebrain.

Olig1 function is required to repress dlx1/2 and interneuron production in Mammalian brain

Abnormal GABAergic interneuron density, and imbalance of excitatory versus inhibitory tone, is thought to result in epilepsy, neurodevelopmental disorders, and psychiatric disease. Recent studies indicate that interneuron cortical density is determined primarily by the size of the precursor pool in the embryonic telencephalon. However, factors essential for regulating interneuron allocation from telencephalic multipotent precursors are poorly understood. Here we report that Olig1 represses production of GABAergic interneurons throughout the mouse brain. Olig1 deletion in mutant mice results in ectopic expression and upregulation of Dlx1/2 genes in the ventral medial ganglionic eminences and adjacent regions of the septum, resulting in an ∼30% increase in adult cortical interneuron numbers. We show that Olig1 directly represses the Dlx1/2 I12b intergenic enhancer and that Dlx1/2 functions genetically downstream of Olig1. These findings establish Olig1 as an essential repressor of Dlx1/2 and interneuron production in developing mammalian brain.

Parallel states of pathological Wnt signaling in neonatal brain injury and colon cancer

In colon cancer, mutation of the Wnt repressor Adenomatous polyposis coli (APC) leads to a state of aberrant and unrestricted “high-activity” signaling. However, relevance of high Wnt tone in non-genetic human disease is unknown. Here we demonstrate that distinct Wnt activity functional states determine oligodendrocyte precursor (OPC) differentiation and myelination. Murine OPCs with genetic Wnt dysregulation (high tone) express multiple genes in common with colon cancer including Lef1, SP5, Ets2, Rnf43 and Dusp4. Surprisingly, we find that OPCs in lesions of hypoxic human neonatal white matter injury upregulate markers of high Wnt activity and lack expression of APC. Finally, we show lack of Wnt repressor tone promotes permanent white matter injury after mild hypoxic insult. These findings suggest a state of pathological high-activity Wnt signaling in human disease tissues that lack pre-disposing genetic mutation.

Evolving concepts of gliogenesis: a look way back and ahead to the next 25 years

Glial cells are present in all organisms with a CNS and, with increasing brain complexity, glial cells have undergone substantive increases in cell number, diversity, and functions. Invertebrates, such as Drosophila, possess glial subtypes with similarity to mammalian astrocytes in their basic morphology and function, representing fertile ground for unraveling fundamental aspects of glial biology. Although glial subtypes in simple organisms may be relatively homogenous, emerging evidence suggests the possibility that mammalian astrocytes might be highly diversified to match the needs of local neuronal subtypes. In this Perspective, we review classic and new roles identified for astrocytes and oligodendrocytes by recent studies. We propose that delineating genetic and developmental programs across species will be essential to understand the core functions of glia that allow enhanced neuronal function and to achieve new insights into glial roles in higher-order brain function and neurological disease.

Expression profiling of Aldh1l1-precursors in the developing spinal cord reveals glial lineage-specific genes and direct Sox9-Nfe2l1 interactions

Developmental regulation of gliogenesis in the mammalian CNS is incompletely understood, in part due to a limited repertoire of lineage-specific genes. We used Aldh1l1-GFP as a marker for gliogenic radial glia and later-stage precursors of developing astrocytes and performed gene expression profiling of these cells. We then used this dataset to identify candidate transcription factors that may serve as glial markers or regulators of glial fate. Our analysis generated a database of developmental stage-related markers of Aldh1l1+ cells between murine embryonic day 13.5-18.5. Using these data we identify the bZIP transcription factor Nfe2l1 and demonstrate that it promotes glial fate under direct Sox9 regulatory control. Thus, this dataset represents a resource for identifying novel regulators of glial development.

Missense mutation in mouse GALC mimics human gene defect and offers new insights into Krabbe disease

Krabbe disease is a devastating pediatric leukodystrophy caused by mutations in the galactocerebrosidase (GALC) gene. A significant subset of the infantile form of the disease is due to missense mutations that result in aberrant protein production. The currently used mouse model, twitcher, has a nonsense mutation not found in Krabbe patients, although it is similar to the human 30 kb deletion in abrogating GALC expression. Here, we identify a spontaneous mutation in GALC, GALCtwi-5J, that precisely matches the E130K missense mutation in patients with infantile Krabbe disease. GALCtwi-5J homozygotes show loss of enzymatic activity despite normal levels of precursor protein, and manifest a more severe phenotype than twitcher, with half the life span. Although neuropathological hallmarks such as gliosis, globoid cells and psychosine accumulation are present throughout the nervous system, the CNS does not manifest significant demyelination. In contrast, the PNS is severely hypomyelinated and lacks large diameter axons, suggesting primary dysmyelination, rather than a demyelinating process. Our data indicate that early demise is due to mechanisms other than myelin loss and support an important role for neuroinflammation in Krabbe disease progression. Furthermore, our results argue against a causative relationship between psychosine accumulation, white matter loss and gliosis.

Neural stem cell engraftment and myelination in the human brain

Pelizaeus-Merzbacher disease (PMD) is a rare leukodystrophy caused by mutation of the proteolipid protein 1 gene. Defective oligodendrocytes in PMD fail to myelinate axons, causing global neurological dysfunction. Human central nervous system stem cells (HuCNS-SCs) can develop into oligodendrocytes and confer structurally normal myelin when transplanted into a hypomyelinating mouse model. A 1-year, open-label phase-1 study was undertaken to evaluate safety and to detect evidence of myelin formation after HuCNS-SC transplantation. Allogeneic HuCNS-SCs were surgically implanted into the frontal lobe white matter in four male subjects with an early-onset severe form of PMD. Immunosuppression was administered for 9 months. Serial neurological evaluations, developmental assessments, and cranial magnetic resonance imaging (MRI) and MR spectroscopy, including high-angular resolution diffusion tensor imaging (DTI), were performed at baseline and after transplantation. The neurosurgical procedure, immunosuppression regimen, and HuCNS-SC transplantation were well tolerated. Modest gains in neurological function were observed in three of the four subjects. No clinical or radiological adverse effects were directly attributed to the donor cells. Reduced T1 and T2 relaxation times were observed in the regions of transplantation 9 months after the procedure in the three subjects. Normalized DTI showed increasing fractional anisotropy and reduced radial diffusivity, consistent with myelination, in the region of transplantation compared to control white matter regions remote to the transplant sites. These phase 1 findings indicate a favorable safety profile for HuCNS-SCs in subjects with PMD. The MRI results suggest durable cell engraftment and donor-derived myelin in the transplanted host white matter.

STAT3-mediated astrogliosis protects myelin development in neonatal brain injury

OBJECTIVE

Pathological findings in neonatal brain injury associated with preterm birth include focal and/or diffuse white matter injury (WMI). Despite the heterogeneous nature of this condition, reactive astrogliosis and microgliosis are frequently observed. Thus, molecular mechanisms by which glia activation contribute to WMI were investigated.

METHODS

Postmortem brains of neonatal brain injury were investigated to identify molecular features of reactive astrocytes. The contribution of astrogliosis to WMI was further tested in a mouse model in genetically engineered mice.

RESULTS

Activated STAT3 signaling in reactive astrocytes was found to be a common feature in postmortem brains of neonatal brain injury. In a mouse model of neonatal WMI, conditional deletion of STAT3 in astrocytes resulted in exacerbated WMI, which was associated with delayed maturation of oligodendrocytes. Mechanistically, the delay occurred in association with overexpression of transforming growth factor (TGF)β-1 in microglia, which in healthy controls decreased with myelin maturation in an age-dependent manner. TGFβ-1 directly and dose-dependently inhibited the maturation of purified oligodendrocyte progenitors, and pharmacological inhibition of TGFβ-1 signaling in vivo reversed the delay in myelin development. Factors secreted from STAT3-deficient astrocytes promoted elevated TGFβ-1 production in cultured microglia compared to wild-type astrocytes.

INTERPRETATION

These results suggest that myelin development is regulated by a mechanism involving crosstalk between microglia and oligodendrocyte progenitors. Reactive astrocytes may modify this signaling in a STAT3-dependent manner, preventing the pathological expression of TGFβ-1 in microglia and the impairment of oligodendrocyte maturation.

Voltage-gated potassium channel EAG2 controls mitotic entry and tumor growth in medulloblastoma via regulating cell volume dynamics

Medulloblastoma (MB) is the most common pediatric CNS malignancy. We identify EAG2 as an overexpressed potassium channel in MBs across different molecular and histological subgroups. EAG2 knockdown not only impairs MB cell growth in vitro, but also reduces tumor burden in vivo and enhances survival in xenograft studies. Mechanistically, we demonstrate that EAG2 protein is confined intracellularly during interphase but is enriched in the plasma membrane during late G2 phase and mitosis. Disruption of EAG2 expression results in G2 arrest and mitotic catastrophe associated with failure of premitotic cytoplasmic condensation. While the tumor suppression function of EAG2 knockdown is independent of p53 activation, DNA damage checkpoint activation, or changes in the AKT pathway, this defective cell volume control is specifically associated with hyperactivation of the p38 MAPK pathway. Inhibition of the p38 pathway significantly rescues the growth defect and G2 arrest. Strikingly, ectopic membrane expression of EAG2 in cells at interphase results in cell volume reduction and mitotic-like morphology. Our study establishes the functional significance of EAG2 in promoting MB tumor progression via regulating cell volume dynamics, the perturbation of which activates the tumor suppressor p38 MAPK pathway, and provides clinical relevance for targeting this ion channel in human MBs.

Evidence that nuclear factor IA inhibits repair after white matter injury

OBJECTIVE

Chronic demyelination can result in axonopathy and is associated with human neurological conditions such as multiple sclerosis (MS) in adults and cerebral palsy in infants. In these disorders, myelin regeneration is inhibited by impaired differentiation of oligodendrocyte progenitors into myelin-producing oligodendrocytes. However, regulatory factors relevant in human myelin disorders and in myelin regeneration remain poorly understood. Here we have investigated the role of the transcription factor nuclear factor IA (NFIA) in oligodendrocyte progenitor differentiation during developmental and regenerative myelination.

METHODS

NFIA expression patterns in human neonatal hypoxic-ischemic encephalopathy (HIE) and MS as well as developmental expression in mice were evaluated. Functional studies during remyelination were performed using a lysolecithin model, coupled with lentiviral misexpression of NFIA. The role of NFIA during oligodendrocyte lineage development was characterized using chick and mouse models and in vitro culture of oligodendrocyte progenitors. Biochemical mechanism of NFIA function was evaluated using chromatin immunoprecipitation and reporter assays.

RESULTS

NFIA is expressed in oligodendrocyte progenitors, but not differentiated oligodendrocytes during mouse embryonic development. Examination of NFIA expression in white matter lesions of human newborns with neonatal HIE, as well active MS lesions in adults, revealed that it is similarly expressed in oligodendrocyte progenitors and not oligodendrocytes. Functional studies indicate that NFIA is sufficient to suppress oligodendrocyte progenitor differentiation during adult remyelination and embryonic development through direct repression of myelin gene expression.

INTERPRETATION

These studies suggest that NFIA participates in the control of oligodendrocyte progenitor differentiation and may contribute to the inhibition of remyelination in human myelin disorders.

Astrocytes and disease: a neurodevelopmental perspective

Astrocytes are no longer seen as a homogenous population of cells. In fact, recent studies indicate that astrocytes are morphologically and functionally diverse and play critical roles in neurodevelopmental diseases such as Rett syndrome and fragile X mental retardation. This review summarizes recent advances in astrocyte development, including the role of neural tube patterning in specification and developmental functions of astrocytes during synaptogenesis. We propose here that a precise understanding of astrocyte development is critical to defining heterogeneity and could lead advances in understanding and treating a variety of neuropsychiatric diseases.

Species-dependent posttranscriptional regulation of NOS1 by FMRP in the developing cerebral cortex

Fragile X syndrome (FXS), the leading monogenic cause of intellectual disability and autism, results from loss of function of the RNA-binding protein FMRP. Here, we show that FMRP regulates translation of neuronal nitric oxide synthase 1 (NOS1) in the developing human neocortex. Whereas NOS1 mRNA is widely expressed, NOS1 protein is transiently coexpressed with FMRP during early synaptogenesis in layer- and region-specific pyramidal neurons. These include midfetal layer 5 subcortically projecting neurons arranged into alternating columns in the prospective Broca's area and orofacial motor cortex. Human NOS1 translation is activated by FMRP via interactions with coding region binding motifs absent from mouse Nos1 mRNA, which is expressed in mouse pyramidal neurons, but not efficiently translated. Correspondingly, neocortical NOS1 protein levels are severely reduced in developing human FXS cases, but not FMRP-deficient mice. Thus, alterations in FMRP posttranscriptional regulation of NOS1 in developing neocortical circuits may contribute to cognitive dysfunction in FXS.

Regional astrocyte allocation regulates CNS synaptogenesis and repair

Astrocytes, the most abundant cell population in the central nervous system (CNS), are essential for normal neurological function. We show that astrocytes are allocated to spatial domains in mouse spinal cord and brain in accordance with their embryonic sites of origin in the ventricular zone. These domains remain stable throughout life without evidence of secondary tangential migration, even after acute CNS injury. Domain-specific depletion of astrocytes in ventral spinal cord resulted in abnormal motor neuron synaptogenesis, which was not rescued by immigration of astrocytes from adjoining regions. Our findings demonstrate that region-restricted astrocyte allocation is a general CNS phenomenon and reveal intrinsic limitations of the astroglial response to injury.

Identification of molecular compartments and genetic circuitry in the developing mammalian kidney

Lengthy developmental programs generate cell diversity within an organotypic framework, enabling the later physiological actions of each organ system. Cell identity, cell diversity and cell function are determined by cell type-specific transcriptional programs; consequently, transcriptional regulatory factors are useful markers of emerging cellular complexity, and their expression patterns provide insights into the regulatory mechanisms at play. We performed a comprehensive genome-scale in situ expression screen of 921 transcriptional regulators in the developing mammalian urogenital system. Focusing on the kidney, analysis of regional-specific expression patterns identified novel markers and cell types associated with development and patterning of the urinary system. Furthermore, promoter analysis of synexpressed genes predicts transcriptional control mechanisms that regulate cell differentiation. The annotated informational resource (www.gudmap.org) will facilitate functional analysis of the mammalian kidney and provides useful information for the generation of novel genetic tools to manipulate emerging cell populations.

Regulated temporal-spatial astrocyte precursor cell proliferation involves BRAF signalling in mammalian spinal cord

Expansion of astrocyte populations in the central nervous system is characteristic of evolutionarily more complex organisms. However, regulation of mammalian astrocyte precursor proliferation during development remains poorly understood. Here, we used Aldh1L1-GFP to identify two morphologically distinct types of proliferative astrocyte precursors: radial glia (RG) in the ventricular zone and a second cell type we call an 'intermediate astrocyte precursor' (IAP) located in the mantle region of the spinal cord. Astrogenic RG and IAP cells proliferated in a progressive ventral-to-dorsal fashion in a tight window from embryonic day 13.5 until postnatal day 3, which correlated precisely with the pattern of active ERK signalling. Conditional loss of BRAF function using BLBP-cre resulted in a 20% decrease in astrocyte production, whereas expression of activated BRAFV600E resulted in astrocyte hyperproliferation. Interestingly, BRAFV600E mitogenic effects in astrocytes were restricted, in part, by the function of p16INK4A-p19(ARF), which limited the temporal epoch for proliferation. Together, these findings suggest that astrocyte precursor proliferation involves distinct RG and IAP cells; is subjected to temporal and spatial control; and depends in part on BRAF signalling at early stages of mammalian spinal cord development.

A small-molecule smoothened agonist prevents glucocorticoid-induced neonatal cerebellar injury

Glucocorticoids are used for treating preterm neonatal infants suffering from life-threatening lung, airway, and cardiovascular conditions. However, several studies have raised concerns about detrimental effects of postnatal glucocorticoid administration on the developing brain leading to cognitive impairment, cerebral palsy, and hypoplasia of the cerebellum, a brain region critical for coordination of movement and higher-order neurological functions. Previously, we showed that glucocorticoids inhibit Sonic hedgehog-Smoothened (Shh-Smo) signaling, the major mitogenic pathway for cerebellar granule neuron precursors. Conversely, activation of Shh-Smo in transgenic mice protects against glucocorticoid-induced neurotoxic effects through induction of the 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) pathway. Here, we show that systemic administration of a small-molecule agonist of the Shh-Smo pathway (SAG) prevented the neurotoxic effects of glucocorticoids. SAG did not interfere with the beneficial effects of glucocorticoids on lung maturation, and despite the known associations of the Shh pathway with neoplasia, we found that transient (1-week-long) SAG treatment of neonatal animals was well tolerated and did not promote tumor formation. These findings suggest that a small-molecule agonist of Smo has potential as a neuroprotective agent in neonates at risk for glucocorticoid-induced neonatal cerebellar injury.

Heparan sulfate sulfatase SULF2 regulates PDGFRα signaling and growth in human and mouse malignant glioma

Glioblastoma (GBM), a uniformly lethal brain cancer, is characterized by diffuse invasion and abnormal activation of multiple receptor tyrosine kinase (RTK) signaling pathways, presenting a major challenge to effective therapy. The activation of many RTK pathways is regulated by extracellular heparan sulfate proteoglycans (HSPG), suggesting these molecules may be effective targets in the tumor microenvironment. In this study, we demonstrated that the extracellular sulfatase, SULF2, an enzyme that regulates multiple HSPG-dependent RTK signaling pathways, was expressed in primary human GBM tumors and cell lines. Knockdown of SULF2 in human GBM cell lines and generation of gliomas from Sulf2(-/-) tumorigenic neurospheres resulted in decreased growth in vivo in mice. We found a striking SULF2 dependence in activity of PDGFRα, a major signaling pathway in GBM. Ablation of SULF2 resulted in decreased PDGFRα phosphorylation and decreased downstream MAPK signaling activity. Interestingly, in a survey of SULF2 levels in different subtypes of GBM, the proneural subtype, characterized by aberrations in PDGFRα, demonstrated the strongest SULF2 expression. Therefore, in addition to its potential as an upstream target for therapy of GBM, SULF2 may help identify a subset of GBMs that are more dependent on exogenous growth factor-mediated signaling. Our results suggest the bioavailability of growth factors from the microenvironment is a significant contributor to tumor growth in a major subset of human GBM.

Ablation of NG2 proteoglycan leads to deficits in brown fat function and to adult onset obesity

Obesity is a major health problem worldwide. We are studying the causes and effects of obesity in C57Bl/6 mice following genetic ablation of NG2, a chondroitin sulfate proteoglycan widely expressed in progenitor cells and also in adipocytes. Although global NG2 ablation delays early postnatal adipogenesis in mouse skin, adult NG2 null mice are paradoxically heavier than wild-type mice, exhibiting larger white fat deposits. This adult onset obesity is not due to NG2-dependent effects on CNS function, since specific ablation of NG2 in oligodendrocyte progenitors yields the opposite phenotype; i.e. abnormally lean mice. Metabolic analysis reveals that, while activity and food intake are unchanged in global NG2 null mice, O₂ consumption and CO₂ production are decreased, suggesting a decrease in energy expenditure. Since brown fat plays important roles in regulating energy expenditure, we have investigated brown fat function via cold challenge and high fat diet feeding, both of which induce the adaptive thermogenesis that normally occurs in brown fat. In both tests, body temperatures in NG2 null mice are reduced compared to wild-type mice, indicating a deficit in brown fat function in the absence of NG2. In addition, adipogenesis in NG2 null brown pre-adipocytes is dramatically impaired compared to wild-type counterparts. Moreover, mRNA levels for PR domain containing 16 (PRDM16) and peroxisome proliferator-activated receptor γ coactivator (PGC)1-α, proteins important for brown adipocyte differentiation, are decreased in NG2 null brown fat deposits in vivo and NG2 null brown pre-adipocytes in vitro. Altogether, these results indicate that brown fat dysfunction in NG2 null mice results from deficits in the recruitment and/or development of brown pre-adipocytes. As a consequence, obesity in NG2 null mice may occur due to disruptions in brown fat-dependent energy homeostasis, with resulting effects on lipid storage in white adipocytes.

Targeted therapy for BRAFV600E malignant astrocytoma

PURPOSE

Malignant astrocytomas (MA) are aggressive central nervous system tumors with poor prognosis. Activating mutation of BRAF (BRAF(V600E)) has been reported in a subset of these tumors, especially in children. We have investigated the incidence of BRAF(V600E) in additional pediatric patient cohorts and examined the effects of BRAF blockade in preclinical models of BRAF(V600E) and wild-type BRAF MA.

EXPERIMENTAL DESIGN

BRAF(V600E) mutation status was examined in two pediatric MA patient cohorts. For functional studies, BRAF(V600E) MA cell lines were used to investigate the effects of BRAF shRNA knockdown in vitro, and to investigate BRAF pharmacologic inhibition in vitro and in vivo.

RESULTS

BRAF(V600E) mutations were identified in 11 and 10% of MAs from two distinct series of tumors (six of 58 cases total). BRAF was expressed in all MA cell lines examined, among which BRAF(V600E) was identified in four instances. Using the BRAF(V600E)-specific inhibitor PLX4720, pharmacologic blockade of BRAF revealed preferential antiproliferative activity against BRAF(V600E) mutant cells in vitro, in contrast to the use of shRNA-mediated knockdown of BRAF, which inhibited cell growth of glioma cell lines regardless of BRAF mutation status. Using orthotopic MA xenografts, we show that PLX4720 treatment decreases tumor growth and increases overall survival in mice-bearing BRAF(V600E) mutant xenografts, while being ineffective, and possibly tumor promoting, against xenografts with wild-type BRAF.

CONCLUSIONS

Our results indicate a 10% incidence of activating BRAF(V600E) among pediatric MAs. With regard to implications for therapy, our results support evaluation of BRAF(V600E)-specific inhibitors for treating BRAF(V600E) MA patients.

Myelin regeneration in multiple sclerosis: targeting endogenous stem cells

Regeneration of myelin sheaths (remyelination) after central nervous system demyelination is important to restore saltatory conduction and to prevent axonal loss. In multiple sclerosis, the insufficiency of remyelination leads to the irreversible degeneration of axons and correlated clinical decline. Therefore, a regenerative strategy to encourage remyelination may protect axons and improve symptoms in multiple sclerosis. We highlight recent studies on factors that influence endogenous remyelination and potential promising pharmacological targets that may be considered for enhancing central nervous system remyelination.

Corridors of migrating neurons in the human brain and their decline during infancy

The subventricular zone (SVZ) of many adult non-human mammals generates large numbers of new neurons destined for the olfactory bulb (OB)^1–6. Along the walls of the lateral ventricles, immature neuronal progeny migrate in tangentially-oriented chains that coalesce into a rostral migratory stream (RMS) connecting the SVZ to the OB. The adult human SVZ, in contrast, contains a hypocellular gap layer separating the ependymal lining from a periventricular ribbon of astrocytes⁷. Some of these SVZ astrocytes can function as neural stem cells in vitro, but their function in vivo remains controversial. An initial report finds few SVZ proliferating cells and rare migrating immature neurons in the RMS of adult humans⁷. In contrast, a subsequent study indicates robust proliferation and migration in the human SVZ and RMS^8,9. Here, we find that the infant human SVZ and RMS contain an extensive corridor of migrating immature neurons before 18 months of age, but, contrary to previous reports⁸, this germinal activity subsides in older children and is nearly extinct by adulthood. Surprisingly, during this limited window of neurogenesis, not all new neurons in the human SVZ are destined for the OB – we describe a major migratory pathway that targets the prefrontal cortex in humans. Together, these findings reveal robust streams of tangentially migrating immature neurons in human early postnatal SVZ and cortex. These pathways represent potential targets of neurological injuries affecting neonates.

Phosphorylation state of Olig2 regulates proliferation of neural progenitors

The bHLH transcription factors that regulate early development of the central nervous system can generally be classified as either antineural or proneural. Initial expression of antineural factors prevents cell cycle exit and thereby expands the pool of neural progenitors. Subsequent (and typically transient) expression of proneural factors promotes cell cycle exit, subtype specification, and differentiation. Against this backdrop, the bHLH transcription factor Olig2 in the oligodendrocyte lineage is unorthodox, showing antineural functions in multipotent CNS progenitor cells but also sustained expression and proneural functions in the formation of oligodendrocytes. We show here that the proliferative function of Olig2 is controlled by developmentally regulated phosphorylation of a conserved triple serine motif within the amino-terminal domain. In the phosphorylated state, Olig2 maintains antineural (i.e., promitotic) functions that are reflected in human glioma cells and in a genetically defined murine model of primary glioma.

The central nervous system-restricted transcription factor Olig2 opposes p53 responses to genotoxic damage in neural progenitors and malignant glioma

High-grade gliomas are notoriously insensitive to radiation and genotoxic drugs. Paradoxically, the p53 gene is structurally intact in the majority of these tumors. Resistance to genotoxic modalities in p53-positive gliomas is generally attributed to attenuation of p53 functions by mutations of other components within the p53 signaling axis, such as p14(Arf), MDM2, and ATM, but this explanation is not entirely satisfactory. We show here that the central nervous system (CNS)-restricted transcription factor Olig2 affects a key posttranslational modification of p53 in both normal and malignant neural progenitors and thereby antagonizes the interaction of p53 with promoter elements of multiple target genes. In the absence of Olig2 function, even attenuated levels of p53 are adequate for biological responses to genotoxic damage.

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.

NIH Consensus Development Conference statement: inhaled nitric-oxide therapy for premature infants

Premature birth is a major public health problem in the United States and internationally. Infants born at or before 32 weeks' gestation (2% of all births in the United States in 2007) are at extremely high risk for death in the neonatal period or for pulmonary, visual, and neurodevelopmental morbidities with lifelong consequences including bronchopulmonary dysplasia, retinopathy of prematurity, and brain injury. Risks for adverse outcomes increase with decreasing gestational age. The economic costs to care for these infants are also substantial (estimated at $26 billion in 2005 in the United States). It is clear that the need for strategies to improve outcomes for this high-risk population is great, and this need has prompted testing of new therapies with the potential to decrease pulmonary and other complications of prematurity. Inhaled nitric oxide (iNO) emerged as one such therapy. To provide health care professionals, families, and the general public with a responsible assessment of currently available data regarding the benefits and risks of iNO in premature infants, the Eunice Kennedy Shriver National Institute of Child Health and Human Development, the National Heart, Lung, and Blood Institute, and the Office of Medical Applications of Research of the National Institutes of Health convened a consensus-development conference. Findings from a substantial body of experimental work in developing animals and other model systems suggest that nitric oxide may enhance lung growth and reduce lung inflammation independently of its effects on blood vessel resistance. Although this work demonstrates biological plausibility and the results of randomized controlled trials in term and near-term infants were positive, combined evidence from the 14 randomized controlled trials of iNO treatment in premature infants of ≤ 34 weeks' gestation shows equivocal effects on pulmonary outcomes, survival, and neurodevelopmental outcomes.

Cerebellar abnormalities following hypoxia alone compared to hypoxic-ischemic forebrain injury in the developing rat brain

Two-day-old (P2) rat pups were subjected to either a global hypoxia or to electrocoagulation of the right carotid artery followed by 2.5 h hypoxia. Cellular and regional injury in the cerebellum (CB) was studied at 1, 2 and 19 days using immunohistology. Following hypoxia and hypoxia-ischemia, all neuronal populations of the CB were damaged in a subset of Purkinje cells. The decrease in the number of interneurons, as well as the thickness of molecular and granular layers was significant following hypoxia. Diffuse white matter damage, with loss of preoligodendrocytes was more severe following hypoxia than hypoxia-ischemia. Global hypoxia in the rat at P2 produces extensive damage to many cell types in different areas of the CB. The addition of unilateral forebrain ischemia does not increase the severity of these changes. Our data provide insight into the mechanisms of the changes observed in the CB of premature newborns.

Oligodendrocyte PTEN is required for myelin and axonal integrity, not remyelination

OBJECTIVE

Repair of myelin injury in multiple sclerosis may fail, resulting in chronic demyelination, axonal loss, and disease progression. As cellular pathways regulated by phosphatase and tensin homologue deleted on chromosome 10 (PTEN; eg, phosphatidylinositol-3-kinase [PI-3K]) have been reported to enhance axon regeneration and oligodendrocyte maturation, we investigated potentially beneficial effects of Pten loss of function in the oligodendrocyte lineage on remyelination.

METHODS

We characterized oligodendrocyte numbers and myelin sheath thickness in mice with conditional inactivation of Pten in oligodendrocytes, Olig2-cre, Pten(fl/fl) mice. Using a model of central nervous system demyelination, lysolecithin injection into the spinal cord white matter, we performed short- and long-term lesioning experiments and quantified oligodendrocyte maturation and myelin sheath thickness in remyelinating lesions.

RESULTS

During development, we observed dramatic hypermyelination in the corpus callosum and spinal cord. Following white matter injury, however, there was no detectable improvement in remyelination. Moreover, we observed progressive myelin sheath abnormalities and massive axon degeneration in the fasciculus gracilis of mutant animals, as indicated by ultrastructure and expression of SMI-32, amyloid precursor protein, and caspase 6.

INTERPRETATION

These studies indicate adverse effects of chronic Pten inactivation (and by extension, activation PI-3K signaling) on myelinating oligodendrocytes and their axonal targets. We conclude that PTEN function in oligodendrocytes is required to regulate myelin thickness and preserve axon integrity. In contrast, PTEN is dispensable during myelin repair, and its inactivation confers no detectable benefit.

CNS-resident glial progenitor/stem cells produce Schwann cells as well as oligodendrocytes during repair of CNS demyelination

After central nervous system (CNS) demyelination-such as occurs during multiple sclerosis-there is often spontaneous regeneration of myelin sheaths, mainly by oligodendrocytes but also by Schwann cells. The origins of the remyelinating cells have not previously been established. We have used Cre-lox fate mapping in transgenic mice to show that PDGFRA/NG2-expressing glia, a distributed population of stem/progenitor cells in the adult CNS, produce the remyelinating oligodendrocytes and almost all of the Schwann cells in chemically induced demyelinated lesions. In contrast, the great majority of reactive astrocytes in the vicinity of the lesions are derived from preexisting FGFR3-expressing cells, likely to be astrocytes. These data resolve a long-running debate about the origins of the main players in CNS remyelination and reveal a surprising capacity of CNS precursors to generate Schwann cells, which normally develop from the embryonic neural crest and are restricted to the peripheral nervous system.

NIH consensus development conference: Inhaled nitric oxide therapy for premature infants

OBJECTIVE

To provide healthcare providers, patients, and the general public with a responsible assessment of currently available data on the use of inhaled nitric oxide in early routine, early rescue, or later rescue regimens in the care of premature infants <34 weeks gestation who require respiratory support.

PARTICIPANTS

A non-Department of Health and Human Services, nonadvocate 16-member panel representing the fields of biostatistics, child psychology, clinical trials, ethics, family-centered care, neonatology, neurodevelopmental follow-up, nursing, pediatric epidemiology, neurobehavior, neurological surgery, neurology, and pulmonology, perinatology, and research methodology. In addition, 18 experts from pertinent fields presented data to the panel and conference audience.

EVIDENCE

Presentations by experts and a systematic review of the literature prepared by the Johns Hopkins University Evidence-based Practice Center, through the Agency for Healthcare Research and Quality. Scientific evidence was given precedence over anecdotal experience.

CONFERENCE PROCESS

The panel drafted its statement based on scientific evidence presented in open forum and on published scientific literature. The draft statement was presented on the final day of the conference and circulated to the audience for comment. The panel released a revised statement later that day at http://consensus.nih.gov. This statement is a report of the panel and is not a policy statement of the NIH or the Federal Government.

CONCLUSIONS

(1) Taken as a whole, the available evidence does not support use of inhaled nitric oxide in early routine, early rescue, or later rescue regimens in the care of premature infants <34 weeks gestation who require respiratory support. (2) There are rare clinical situations, including pulmonary hypertension or hypoplasia, that have been inadequately studied in which inhaled nitric oxide may have benefit in infants <34 weeks gestation. In such situations, clinicians should communicate with families regarding the current evidence on its risks and benefits as well as remaining uncertainties. (3) Basic research and animal studies have contributed to important understandings of inhaled nitric oxide benefits on lung development and function in infants at high risk of bronchopulmonary dysplasia. These promising results have only partly been realized in clinical trials of inhaled nitric oxide treatment in premature infants. Future research should seek to understand this gap. (4) Predefined subgroup and post hoc analyses of previous trials showing potential benefit of inhaled nitric oxide have generated hypotheses for future research for clinical trials. Prior strategies shown to be ineffective are discouraged unless new evidence emerges. The positive results of one multicenter trial, which was characterized by later timing, higher dose, and longer duration of treatment, require confirmation. Future trials should attempt to quantify the individual effects of each of these treatment-related variables (timing, dose, and duration), ideally by randomizing them separately. (5) Based on assessment of currently available data, hospitals, clinicians, and the pharmaceutical industry should avoid marketing inhaled nitric oxide for premature infants <34 weeks gestation.

Conserved role of intragenic DNA methylation in regulating alternative promoters

While the methylation of DNA in 5′ promoters suppresses gene expression, the role of DNA methylation in gene bodies is unclear^1–5. In mammals, tissue- and cell type-specific methylation is present in a small percentage of 5′ CpG island (CGI) promoters, while a far greater proportion occurs across gene bodies, coinciding with highly conserved sequences^5–10. Tissue-specific intragenic methylation might reduce,³ or, paradoxically, enhance transcription elongation efficiency^1,2,4,5. Capped analysis of gene expression (CAGE) experiments also indicate that transcription commonly initiates within and between genes^11–15. To investigate the role of intragenic methylation, we generated a map of DNA methylation from human brain encompassing 24.7 million of the 28 million CpG sites. From the dense, high-resolution coverage of CpG islands, the majority of methylated CpG islands were revealed to be in intragenic and intergenic regions, while less than 3% of CpG islands in 5′ promoters were methylated. The CpG islands in all three locations overlapped with RNA markers of transcription initiation, and unmethylated CpG islands also overlapped significantly with trimethylation of H3K4, a histone modification enriched at promoters¹⁶. The general and CpG-island-specific patterns of methylation are conserved in mouse tissues. An in-depth investigation of the human SHANK3 locus^17,18 and its mouse homologue demonstrated that this tissue-specific DNA methylation regulates intragenic promoter activity in vitro and in vivo. These methylation-regulated, alternative transcripts are expressed in a tissue and cell type-specific manner, and are expressed differentially within a single cell type from distinct brain regions. These results support a major role for intragenic methylation in regulating cell context-specific alternative promoters in gene bodies.

Dexamethasone destabilizes Nmyc to inhibit the growth of hedgehog-associated medulloblastoma

Mouse studies indicate that the synthetic glucocorticoid dexamethasone (Dex) impairs the proliferation of granule neuron precursors in the cerebellum, which are transformed to medulloblastoma by activation of Sonic hedgehog (Shh) signaling. Here, we show that Dex treatment also inhibits Shh-induced tumor growth, enhancing the survival of tumor-prone transgenic mice. We found that Nmyc was specifically required in granule cells for Shh-induced tumorigenesis and that Dex acted to reduce Nmyc protein levels. Moreover, we found that Dex-induced destabilization of Nmyc is mediated by activation of glycogen synthase kinase 3beta, which targets Nmyc for proteasomal degradation. Together, our findings show that Dex antagonizes Shh signaling downstream of Smoothened in medulloblastoma.

Abstract 4186: Development of a novel, genetically faithful model of pediatric glioblastoma

Gliomas are among the most lethal human cancers, with high-grade tumors exhibiting a dismal 5-year survival rate. Mutations and amplifications of receptor tyrosine kinases such as PDGFR and EGFR, which drive Ras-MAPK pathway activity, are among the hallmark mutations of adult gliomas. However, pediatric glioblastomas often do not exhibit the oncogenic mutations found in adult tumors, suggesting that these tumors may have distinct mechanistic origins. The unique genetic characteristics of pediatric brain tumors may also affect their response to conventional treatment. Therefore, there is a need to generate mouse models that faithfully recapitulate the genetics of pediatric glioblastomas to identify potential avenues for improving treatment of these tumors. Recent genetic profiling of pediatric brain tumors has found activating mutations of the BRaf kinase in higher grade tumors but not low-grade juvenile pilocytic astrocytomas. By contrast, BRaf is not frequently altered in adult gliomas, suggesting that cells in the developing brain may be particularly susceptible to this mechanism of Ras pathway activation. In addition, BRaf mutation is closely linked to the loss of the tumor suppressor Ink4a/Arf, indicating a possible requirement for loss of this tumor suppressor in the development of high-grade gliomas.

Here, we describe the generation of a model system allowing expression of oncogenic BRaf in spatially and temporally defined populations of neural stem cells. In BRaf CA/+; R26YFP animals, Cre-mediated recombination induces expression of the oncogenic V600E mutant form of Braf from the endogenous gene locus and simultaneously labels these cells via expression of YFP. By stereotaxically injecting Cre-expressing adenovirus into the neonatal or adult brain in these mice, we can induce oncogene expression in specific populations of neural progenitors and track its effects on infected cells.

Data from multiple tumor types suggests that the activation of Ras-MAPK signaling via BRaf mutation is tightly linked to loss of the Ink4a/Arf tumor suppressor locus. We are also using viral introduction of Cre to activate oncogenic BRaf when Ink4a/Arf expression is absent. The phenotype of these mice will be compared to human pediatric gliomas to determine if this system provides an accurate model of the human disease. Such a model would provide a platform for testing targeted therapies against BRaf - a critical step in designing new treatments for pediatric glioblastoma.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4186.

Oncogenic BRAF mutation with CDKN2A inactivation is characteristic of a subset of pediatric malignant astrocytomas

Malignant astrocytomas are a deadly solid tumor in children. Limited understanding of their underlying genetic basis has contributed to modest progress in developing more effective therapies. In an effort to identify such alterations, we performed a genome-wide search for DNA copy number aberrations (CNA) in a panel of 33 tumors encompassing grade 1 through grade 4 tumors. Genomic amplifications of 10-fold or greater were restricted to grade 3 and 4 astrocytomas and included the MDM4 (1q32), PDGFRA (4q12), MET (7q21), CMYC (8q24), PVT1 (8q24), WNT5B (12p13), and IGF1R (15q26) genes. Homozygous deletions of CDKN2A (9p21), PTEN (10q26), and TP53 (17p3.1) were evident among grade 2 to 4 tumors. BRAF gene rearrangements that were indicated in three tumors prompted the discovery of KIAA1549-BRAF fusion transcripts expressed in 10 of 10 grade 1 astrocytomas and in none of the grade 2 to 4 tumors. In contrast, an oncogenic missense BRAF mutation (BRAF(V600E)) was detected in 7 of 31 grade 2 to 4 tumors but in none of the grade 1 tumors. BRAF(V600E) mutation seems to define a subset of malignant astrocytomas in children, in which there is frequent concomitant homozygous deletion of CDKN2A (five of seven cases). Taken together, these findings highlight BRAF as a frequent mutation target in pediatric astrocytomas, with distinct types of BRAF alteration occurring in grade 1 versus grade 2 to 4 tumors.

Overcoming remyelination failure in multiple sclerosis and other myelin disorders

Protecting axons from degeneration represents a major unmet need in the treatment of myelin disorders and especially the currently untreatable secondary progressive stages of multiple sclerosis (MS). Several lines of evidence indicate that ensuring myelin sheaths are restored to demyelinated axons, the regenerative process of remyelination, represents one of the most effective means of achieving axonal protection. Remyelination can occur as a highly effective spontaneous regenerative process following demyelination. However, for reasons that have not been fully understood, this process is often incomplete or fails in MS. Recognizing the reasons for remyelination failure and hence identifying therapeutic targets will depend on detailed histopathological studies of myelin disorders and a detailed understanding of the molecular mechanisms regulating remyelination. Pathology studies have revealed that chronically demyelinated lesions in MS often fail to repair because of a failure of differentiation of the precursor cell responsible for remyelination rather than a failure of their recruitment. In this article we review three mechanisms by which differentiation of precursor cells into remyelinating oligodendrocytes are regulated-the Notch pathway, the Wnt pathway and the pathways activated by inhibitor of differentiation in myelin debris-and indicate how these might be pharmacologically targeted to overcome remyelination failure.

Dysregulation of the Wnt pathway inhibits timely myelination and remyelination in the mammalian CNS

The progressive loss of CNS myelin in patients with multiple sclerosis (MS) has been proposed to result from the combined effects of damage to oligodendrocytes and failure of remyelination. A common feature of demyelinated lesions is the presence of oligodendrocyte precursors (OLPs) blocked at a premyelinating stage. However, the mechanistic basis for inhibition of myelin repair is incompletely understood. To identify novel regulators of OLP differentiation, potentially dysregulated during repair, we performed a genome-wide screen of 1040 transcription factor-encoding genes expressed in remyelinating rodent lesions. We report that approximately 50 transcription factor-encoding genes show dynamic expression during repair and that expression of the Wnt pathway mediator Tcf4 (aka Tcf7l2) within OLPs is specific to lesioned-but not normal-adult white matter. We report that beta-catenin signaling is active during oligodendrocyte development and remyelination in vivo. Moreover, we observed similar regulation of Tcf4 in the developing human CNS and lesions of MS. Data mining revealed elevated levels of Wnt pathway mRNA transcripts and proteins within MS lesions, indicating activation of the pathway in this pathological context. We show that dysregulation of Wnt-beta-catenin signaling in OLPs results in profound delay of both developmental myelination and remyelination, based on (1) conditional activation of beta-catenin in the oligodendrocyte lineage in vivo and (2) findings from APC(Min) mice, which lack one functional copy of the endogenous Wnt pathway inhibitor APC. Together, our findings indicate that dysregulated Wnt-beta-catenin signaling inhibits myelination/remyelination in the mammalian CNS. Evidence of Wnt pathway activity in human MS lesions suggests that its dysregulation might contribute to inefficient myelin repair in human neurological disorders.

Hedgehog signaling has a protective effect in glucocorticoid-induced mouse neonatal brain injury through an 11betaHSD2-dependent mechanism

Glucocorticoids (GCs) are administered to human fetuses at risk of premature delivery and to infants with life-threatening respiratory and cardiac conditions. However, there are ongoing concerns about adverse effects of GC treatment on the developing human brain, although the precise molecular mechanisms underlying GC-induced brain injury are unclear. Here, we identified what we believe to be novel cross-antagonistic interactions of Sonic hedgehog (Shh) and GC signaling in proliferating mouse cerebellar granule neuron precursors (CGNPs). Chronic GC treatment (from P0 through P7) in mouse pups inhibited Shh-induced proliferation and upregulation of expression of N-myc, Gli1, and D-type cyclin protein in CGNPs. Conversely, acute GC treatment (on P7 only) caused transient apoptosis. Shh signaling antagonized these effects of GCs, in part by induction of 11beta-hydroxysteroid dehydrogenase type 2 (11betaHSD2). Importantly, 11betaHSD2 antagonized the effects of the GCs corticosterone, hydrocortisone, and prednisolone, but not the synthetic GC dexamethasone. Our findings indicate that Shh signaling is protective in the setting of GC-induced mouse neonatal brain injury. Furthermore, they led us to propose that 11betaHSD2-sensitive GCs (e.g., hydrocortisone) should be used in preference to dexamethasone in neonatal human infants because of the potential for reduced neurotoxicity.

Medulloblastoma can be initiated by deletion of Patched in lineage-restricted progenitors or stem cells

Medulloblastoma is the most common malignant brain tumor in children, but the cells from which it arises remain unclear. Here we examine the origin of medulloblastoma resulting from mutations in the Sonic hedgehog (Shh) pathway. We show that activation of Shh signaling in neuronal progenitors causes medulloblastoma by 3 months of age. Shh pathway activation in stem cells promotes stem cell proliferation but only causes tumors after commitment to-and expansion of-the neuronal lineage. Notably, tumors initiated in stem cells develop more rapidly than those initiated in progenitors, with all animals succumbing by 3-4 weeks. These studies suggest that medulloblastoma can be initiated in progenitors or stem cells but that Shh-induced tumorigenesis is associated with neuronal lineage commitment.

Acquisition of granule neuron precursor identity is a critical determinant of progenitor cell competence to form Shh-induced medulloblastoma

Whether the brain tumor medulloblastoma originates from stem cells or restricted progenitor cells is unclear. To investigate this, we activated oncogenic Hedgehog (Hh) signaling in multipotent and lineage-restricted central nervous system (CNS) progenitors. We observed that normal unipotent cerebellar granule neuron precursors (CGNPs) derive from hGFAP(+) and Olig2(+) rhombic lip progenitors. Hh activation in a spectrum of early- and late-stage CNS progenitors generated similar medulloblastomas, but not other brain cancers, indicating that acquisition of CGNP identity is essential for tumorigenesis. We show in human and mouse medulloblastoma that cells expressing the glia-associated markers Gfap and Olig2 are neoplastic and retain features of embryonic-type granule lineage progenitors. Thus, oncogenic Hh signaling promotes medulloblastoma from lineage-restricted granule cell progenitors.

Glioma stem cells: a midterm exam

Several years ago, the discovery of a highly tumorigenic subpopulation of stem-like cells embedded within fresh surgical isolates of malignant gliomas lent support to a new paradigm in cancer biology--the cancer stem cell hypothesis. At the same time, these "glioma stem cells" seemed to resolve a long-standing conundrum on the cell of origin for primary cancers of the brain. However, central tenets of the cancer stem cell hypothesis have recently been challenged, and the cellular origins of stem-like cells within malignant glioma are still contended. Here, we summarize the issues that are still in play with respect to the cancer stem cell hypothesis, and we revisit the developmental origins of malignant glioma. Do glioma stem cells arise from developmentally stalled neural progenitors or from dedifferentiated astrocytes? Five separate predictions of a neural progenitor cell of origin are put to the test.

Expression and function of Nkx6.3 in vertebrate hindbrain

Homeodomain transcription factors serve important functions in organogenesis and tissue differentiation, particularly with respect to the positional identity of individual cells. The Nkx6 subfamily controls tissue differentiation in the developing central nervous system where they function as transcriptional repressor proteins. Recent work indicates that Nkx6.3 is expressed in hindbrain V2 interneurons that co-express Nkx6.1, suggesting the possibility of functional redundancy. Here, we report that Nkx6.3 expression is specific to Chx10+ V2a interneurons but not to Gata3+ V2b interneurons of the hindbrain, and that Nkx6.3 expression appears to mark cells of the prospective medullary reticular formation. Molecular analysis of Nkx6.3 null embryonic mouse hindbrain did not reveal detectable defects in progenitor markers, motor neuron or V2 interneuron sub-types. Forced expression of Nkx6.3 and Nkx6.1 promote V2 interneuron differentiation in the developing chick hindbrain. These findings indicate Nkx6.3 function is dispensable for CNS development and lead to the proposal that absence of overt defects is due to functional compensation from a related homeodomain transcription factor.

Olig2-regulated lineage-restricted pathway controls replication competence in neural stem cells and malignant glioma

Recent studies have identified stem cells in brain cancer. However, their relationship to normal CNS progenitors, including dependence on common lineage-restricted pathways, is unclear. We observe expression of the CNS-restricted transcription factor, OLIG2, in human glioma stem and progenitor cells reminiscent of type C transit-amplifying cells in germinal zones of the adult brain. Olig2 function is required for proliferation of neural progenitors and for glioma formation in a genetically relevant murine model. Moreover, we show p21(WAF1/CIP1), a tumor suppressor and inhibitor of stem cell proliferation, is directly repressed by OLIG2 in neural progenitors and gliomas. Our findings identify an Olig2-regulated lineage-restricted pathway critical for proliferation of normal and tumorigenic CNS stem cells.

Myelin abnormalities without oligodendrocyte loss in periventricular leukomalacia

The cellular basis of myelin deficits detected by neuroimaging in long-term survivors of periventricular leukomalacia (PVL) is poorly understood. We tested the hypothesis that oligodendrocyte lineage (OL) cell density is reduced in PVL, thereby contributing to subsequent myelin deficits. Using computer-based methods, we determined OL cell density in sections from 18 PVL and 18 age-adjusted control cases, immunostained with the OL-lineage marker Olig2. Myelination was assessed with myelin basic protein (MBP) immunostaining. We found no significant difference between PVL and control cases in Olig2 cell density in the periventricular or intragyral white matter. We did find, however, a significant increase in Olig2 cell density at the necrotic foci, compared with distant areas. Although no significant difference was found in the degree of MBP immunostaining, we observed qualitative abnormalities of MBP immunostaining in both the diffuse and necrotic components of PVL. Abnormal MBP immunostaining in PVL despite preserved Olig2 cell density may be secondary to arrested OL maturation, damage to OL processes, and/or impaired axonal-OL signaling. OL migration toward the "core" of injury may occur to replenish OL cell number. This study provides new insight into the cellular basis of the myelin deficits observed in survivors of PVL.