Activated Notch2 signaling inhibits differentiation of cerebellar granule neuron precursors by maintaining proliferation

Medulloblastoma development: tumor biology informs treatment decisions

Medulloblastoma is the most common malignant pediatric brain tumor. Current treatments including surgery, craniospinal radiation and high-dose chemotherapy have led to improvement in survival. However, the risk for recurrence as well as significant long-term neurocognitive and endocrine sequelae associated with current treatment modalities underscore the urgent need for novel tumor-specific, normal brain-sparing therapies. It has also provided the impetus for research focused on providing a better understanding of medulloblastoma biology. The expectation is that such studies will lead to the identification of new therapeutic targets and eventually to an increase in personalized treatment approaches.

Aryl hydrocarbon receptor deletion in cerebellar granule neuron precursors impairs neurogenesis

The aryl hydrocarbon receptor (AhR) is a ligand-activated member of the basic-helix-loop-helix/PER-ARNT-SIM(PAS) transcription factor superfamily that also mediates the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Increasing evidence suggests that AhR influences the development of many tissues, including the central nervous system. Our previous studies suggest that sustained AhR activation by TCDD and/or AhR deletion disrupts cerebellar granule neuron precursor (GNP) development. In the current study, to determine whether endogenous AhR controls GNP development in a cell-autonomous manner, we created a GNP-specific AhR deletion mouse, AhR(fx/fx) /Math1(CRE/+) (AhR CKO). Selective AhR deletion in GNPs produced abnormalities in proliferation and differentiation. Specifically, fewer GNPs were engaged in S-phase, as demonstrated by ∼25% reductions in thymidine (in vitro) and Bromodeoxyuridine (in vivo) incorporation. Furthermore, total granule neuron numbers in the internal granule layer at PND21 and PND60 were diminished in AhR conditional knockout (CKO) mice compared with controls. Conversely, differentiation was enhanced, including ∼40% increase in neurite outgrowth and 50% increase in GABARα6 receptor expression in deletion mutants. Our results suggest that AhR activity plays a role in regulating granule neuron number and differentiation, possibly by coordinating this GNP developmental transition. These studies provide novel insights for understanding the normal roles of AhR signaling during cerebellar granule cell neurogenesis and may have important implications for the effects of environmental factors in cerebellar dysgenesis.

Not so Fast: Co-Requirements for Sonic Hedgehog Induced Brain Tumorigenesis

The Sonic hedgehog (Shh) pathway plays an integral role in cellular proliferation during normal brain development and also drives growth in a variety of cancers including brain cancer. Clinical trials of Shh pathway inhibitors for brain tumors have yielded disappointing results, indicating a more nuanced role for Shh signaling. We postulate that Shh signaling does not work alone but requires co-activation of other signaling pathways for tumorigenesis and stem cell maintenance. This review will focus on the interplay between the Shh pathway and these pathways to promote tumor growth in brain tumors, presenting opportunities for the study of combinatorial therapies.

Adrenomedullary progenitor cells: Isolation and characterization of a multi-potent progenitor cell population

The adrenal is a highly plastic organ with the ability to adjust to physiological needs by adapting hormone production but also by generating and regenerating both adrenocortical and adrenomedullary tissue. It is now apparent that many adult tissues maintain stem and progenitor cells that contribute to their maintenance and adaptation. Research from the last years has proven the existence of stem and progenitor cells also in the adult adrenal medulla throughout life. These cells maintain some neural crest properties and have the potential to differentiate to the endocrine and neural lineages. In this article, we discuss the evidence for the existence of adrenomedullary multi potent progenitor cells, their isolation and characterization, their differentiation potential as well as their clinical potential in transplantation therapies but also in pathophysiology.

Tumor formation and drug resistance properties of human glioblastoma side population cells

Accumulating evidence has demonstrated that the presence of a subset of cells in several types of brain tumor, termed brain cancer stem cells, are responsible for tumor recurrence following chemotherapy or radiotherapy. The isolation and characterization of side population (SP) cells in several types of solid tumor using Hoechst dye has become a powerful technique for obtaining cancer stem cells (CSCs). In the present study, cancer stem‑like SP cells were isolated from the human glioblastoma cell line MG‑12 using the Hoechst 33342 dye exclusion technique. Flow cytometric analysis revealed that the cell line MG‑12 contained 3.2% SP cells, which was reduced to 0.5% in the presence of verapamil, an inhibitor of ATPase‑binding cassette (ABC) transporters. Reverse‑transcription quantitative polymerase chain reaction analysis revealed that the isolated SP cells exhibited increased expression of stem cell markers, including Nestin, Notch 1, octamer‑binding transcription factor 4 (Oct‑4), epithelial cell adhesion molecule (EpCAM) and also ABC transporter protein ABCG2. Additionally, using western blot analysis it was demonstrated that SP cells exhibit positivity and have a higher expression of CD133, CD44, EpCAM, Oct‑4 and B‑cell lymphoma 2. Furthermore, it was demonstrated that the isolated SP cells undergo rapid proliferation, have a high propensity to form tumor spheres and also have a high survival rate following treatment with 5‑fluorouracil. Therefore, the present findings suggest that SP cells of the glioblastoma MG‑12 cell line share characteristics of CSCs. Therefore, the increased expression of stem cell markers and ABCG2 protein may interact with each other and be responsible for drug and apoptotic resistance, tumor recurrence and invasion.

Notching on Cancer's Door: Notch Signaling in Brain Tumors

Notch receptors play an essential role in the regulation of central cellular processes during embryonic and postnatal development. The mammalian genome encodes for four Notch paralogs (Notch 1–4), which are activated by three Delta-like (Dll1/3/4) and two Serrate-like (Jagged1/2) ligands. Further, non-canonical Notch ligands such as epidermal growth factor like protein 7 (EGFL7) have been identified and serve mostly as antagonists of Notch signaling. The Notch pathway prevents neuronal differentiation in the central nervous system by driving neural stem cell maintenance and commitment of neural progenitor cells into the glial lineage. Notch is therefore often implicated in the development of brain tumors, as tumor cells share various characteristics with neural stem and progenitor cells. Notch receptors are overexpressed in gliomas and their oncogenicity has been confirmed by gain- and loss-of-function studies in vitro and in vivo. To this end, special attention is paid to the impact of Notch signaling on stem-like brain tumor-propagating cells as these cells contribute to growth, survival, invasion, and recurrence of brain tumors. Based on the outcome of ongoing studies in vivo, Notch-directed therapies such as γ-secretase inhibitors and blocking antibodies have entered and completed various clinical trials. This review summarizes the current knowledge on Notch signaling in brain tumor formation and therapy.

Signals that regulate the oncogenic fate of neural stem cells and progenitors

Brain tumors have frequently been associated with a neural stem cell (NSC) origin and contain stem-like tumor cells, so-called brain tumor stem cells (BTSCs) that share many features with normal NSCs. A stem cell state of BTSCs confers resistance to radiotherapy and treatment with alkylating agents. It is also a hallmark of aggressive brain tumors and is maintained by transcriptional networks that are also active in embryonic stem cells. Advances in reprogramming of somatic cells into induced pluripotent stem (iPS) cells have further identified genes that drive stemness. In this review, we will highlight the possible drivers of stemness in medulloblastoma and glioma, the most frequent types of primary malignant brain cancer in children and adults, respectively. Signals that drive expansion of developmentally defined neural precursor cells are also active in corresponding brain tumors. Transcriptomal subgroups of human medulloblastoma and glioma match features of NSCs but also more restricted progenitors. Lessons from genetically-engineered mouse (GEM) models show that temporally and regionally defined NSCs can give rise to distinct subgroups of medulloblastoma and glioma. We will further discuss how acquisition of stem cell features may drive brain tumorigenesis from a non-NSC origin. Genetic alterations, signaling pathways, and therapy-induced changes in the tumor microenvironment can drive reprogramming networks and induce stemness in brain tumors. Finally, we propose a model where dysregulation of microRNAs (miRNAs) that normally provide barriers against reprogramming plays an integral role in promoting stemness in brain tumors.

A marked paucity of granule cells in the developing cerebellum of the Npc1(-/-) mouse is corrected by a single injection of hydroxypropyl-β-cyclodextrin

In this study we show that postnatal development of cerebellar granule neurons (GNs) is defective in Npc1^−/− mice. Compared to age-matched wild-type littermates, there is an accelerated disappearance of the external granule layer (EGL) in these mice. This is due to a premature exit from the cell cycle of GN precursors residing at the level of the EGL. As a consequence, the size of cerebellar lobules of these mice displays a 20%–25% reduction compared to that of age-matched wild-type mice. This size reduction is detectable at post-natal day 28 (PN28), when cerebellar GN development is completed while signs of neuronal atrophy are not yet apparent.

Based on the analysis of EGL thickness and the determination of proliferating GN fractions at increasing developmental times (PN8–PN14), we trace the onset of this GN developmental defect during the second postnatal week. We also show that during this developmental time Shh transcripts undergo a significant reduction in Npc1^−/− mice compared to age-matched wild-type mice. In light of the mitogenic activity of Shh on GNs, this observation further supports the presence of defective GN proliferation in Npc1^−/− mice. A single injection of hydroxypropyl-β-cyclodextrin at PN7 rescues this defect, restoring the normal patterns of granule neuron proliferation and cerebellar lobule size.

To our knowledge, these findings identify a novel developmental defect that was underappreciated in previous studies. This defect was probably overlooked because Npc1 loss-of-function does not affect cerebellar foliation and causes the internal granule layer and molecular layer to decrease proportionally, giving rise to a normally appearing, yet harmoniously smaller, cerebellum.

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Cerebellar lobules of adult Npc1^−/− mice display a 20–25% reduction in size compared to wild-type age-matched mice.

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The proliferation of granule neuron (GN) precursors in the developing cerebellum of Npc1^−/− mice is defective.

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Npc1^−/− GN precursors of the external granule layer (EGL) undergo a premature exit from the cell cycle.

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The EGL of Npc1^−/− mice is thinner and persists for a shorter time.

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A single injection of hydroxypropyl-β-cyclodextrin at PN7 rescues these defects.

Epigenetic silencing of miRNA-9 is associated with HES1 oncogenic activity and poor prognosis of medulloblastoma

BACKGROUND

microRNA-9 is a key regulator of neuronal development aberrantly expressed in brain malignancies, including medulloblastoma. The mechanisms by which microRNA-9 contributes to medulloblastoma pathogenesis remain unclear, and factors that regulate this process have not been delineated.

METHODS

Expression and methylation status of microRNA-9 in medulloblastoma cell lines and primary samples were analysed. The association of microRNA-9 expression with medulloblastoma patients' clinical outcome was assessed, and the impact of microRNA-9 restoration was functionally validated in medulloblastoma cells.

RESULTS

microRNA-9 expression is repressed in a large subset of MB samples compared with normal fetal cerebellum. Low microRNA-9 expression correlates significantly with the diagnosis of unfavourable histopathological variants and with poor clinical outcome. microRNA-9 silencing occurs via cancer-specific CpG island hypermethylation. HES1 was identified as a direct target of microRNA-9 in medulloblastoma, and restoration of microRNA-9 was shown to trigger cell cycle arrest, to inhibit clonal growth and to promote medulloblastoma cell differentiation.

CONCLUSIONS

microRNA-9 is a methylation-silenced tumour suppressor that could be a potential candidate predictive marker for poor prognosis of medulloblastoma. Loss of microRNA-9 may confer a proliferative advantage to tumour cells, and it could possibly contribute to disease pathogenesis. Thus, re-expression of microRNA-9 may constitute a novel epigenetic regulation strategy against medulloblastoma.

Differential patterns of NOTCH1-4 receptor expression are markers of glioma cell differentiation

Background Notch signaling is deregulated in human gliomas and may play a role in their malignancy. However, the role of each Notch receptor in glioma cell differentiation and progression is not clear. We examined the expression pattern of Notch receptors and compared it with differentiation markers in glioma cell lines, primary human cultures, and biopsies of different grades. Furthermore, the effects of a γ-secretase inhibitor (GSI) on cell survival were assessed. Methods Notch receptors and markers of cellular differentiation were analyzed by reverse transcriptase PCR, Western blotting, immunohistochemistry, and immunocytochemistry. GSI sensitivity was assessed in both cell lines and primary cultures grown as monolayers or tumorspheres, by MTT assay. Results In cell lines, Notch1 and Notch2/4 levels paralleled those of glial fibrillary acidic protein (GFAP) and vimentin, respectively. In human gliomas and primary cultures, Notch1 was moderate/strong in low-grade tumors but weak in glioblastoma multiforme (GBM). Conversely, Notch4 increased from astrocytoma grade II to GBM. Primary GBM cultures grown in serum (monolayer) showed moderate/high levels of CD133, nestin, vimentin, and Notch4 and very low levels of GFAP and Notch1, which were reduced in tumorspheres. This effect was drastic for Notch4. GSI reduced cell survival with stronger effect in serum, whilst human primary cultures showed different sensitivity. Conclusion Data from cell lines and human gliomas suggest a correlation between expression of Notch receptors and cell differentiation. Namely, Notch1 and Notch4 are markers of differentiated and less differentiated glioma cells, respectively. We propose Notch receptors as markers of glioma grading and possible prognostic factors.

WNT3 inhibits cerebellar granule neuron progenitor proliferation and medulloblastoma formation via MAPK activation

During normal cerebellar development, the remarkable expansion of granule cell progenitors (GCPs) generates a population of granule neurons that outnumbers the total neuronal population of the cerebral cortex, and provides a model for identifying signaling pathways that may be defective in medulloblastoma. While many studies focus on identifying pathways that promote growth of GCPs, a critical unanswered question concerns the identification of signaling pathways that block mitogenic stimulation and induce early steps in differentiation. Here we identify WNT3 as a novel suppressor of GCP proliferation during cerebellar development and an inhibitor of medulloblastoma growth in mice. WNT3, produced in early postnatal cerebellum, inhibits GCP proliferation by down-regulating pro-proliferative target genes of the mitogen Sonic Hedgehog (SHH) and the bHLH transcription factor Atoh1. WNT3 suppresses GCP growth through a non-canonical Wnt signaling pathway, activating prototypic mitogen-activated protein kinases (MAPKs), the Ras-dependent extracellular-signal-regulated kinases 1/2 (ERK1/2) and ERK5, instead of the classical β-catenin pathway. Inhibition of MAPK activity using a MAPK kinase (MEK) inhibitor reversed the inhibitory effect of WNT3 on GCP proliferation. Importantly, WNT3 inhibits proliferation of medulloblastoma tumor growth in mouse models by a similar mechanism. Thus, the present study suggests a novel role for WNT3 as a regulator of neurogenesis and repressor of neural tumors.

Interaction of PACAP with Sonic hedgehog reveals complex regulation of the hedgehog pathway by PKA

Sonic hedgehog (Shh) signaling is essential for proliferation of cerebellar granule cell progenitors (cGCPs) and its aberrant activation causes a cerebellar cancer medulloblastoma. Pituitary adenylate cyclase activating polypeptide (PACAP) inhibits Shh-driven proliferation of cGCPs and acts as tumor suppressor in murine medulloblastoma. We show that PACAP blocks canonical Shh signaling by a mechanism that involves activation of protein kinase A (PKA) and inhibition of the translocation of the Shh-dependent transcription factor Gli2 into the primary cilium. PKA is shown to play an essential role in inhibiting gene transcription in the absence of Shh, but global PKA activity levels are found to be a poor predictor of the degree of Shh pathway activation. We propose that the core Shh pathway regulates a small compartmentalized pool of PKA in the vicinity of primary cilia. GPCRs that affect global PKA activity levels, such as the PACAP receptor, cooperate with the canonical Shh signal to regulate Gli protein phosphorylation by PKA. This interaction serves to fine-tune the transcriptional and physiological function of the Shh pathway.

Notch signaling in the pathologic adult brain

Along the entire lifetime, Notch is actively involved in dynamic changes in the cellular architecture and function of the nervous system. It controls neurogenesis, the growth of axons and dendrites, synaptic plasticity, and ultimately neuronal death. The specific roles of Notch in adult brain plasticity and neurological disorders have begun to be unraveled in recent years, and pieces of experimental evidence suggest that Notch is operative in diverse brain pathologies including tumorigenesis, stroke, and neurological disorders such as Alzheimer’s disease, Down syndrome, and multiple sclerosis. In this review, we will cover the recent findings of Notch signaling and neural dysfunction in adult human brain and discuss its relevance in the pathogenesis of diseases of the central nervous system.

Differential effects of miR-34c-3p and miR-34c-5p on the proliferation, apoptosis and invasion of glioma cells

Glioblastoma is the most malignant and common intrinsic brain tumor, but the molecular mechanism of glioma pathophysiology is poorly understood. Recent data have shown that microRNAs regulate the expression of several genes associated with human cancer. In the present study, the function of miR-34c in glioma cells was analyzed. It was demonstrated that miR-34c-3p and miR-34c-5p were downregulated in gliomas, by performing qPCR on tumor tissues from glioma patients and glioma cell lines, compared with normal brain tissues and a normal glial cell line. Furthermore, the miR-34c expression was found to be inversely correlated with glioma WHO grades. Overexpression of miR-34c-3p inhibited U251 and U87 cell proliferation; however, miR-34c-5p only had an effect on U251 cells. Transfection with miR-34c-3p or miR-34c-5p in U251 cells and with miR-34c-3p in U87 cells produced S-phase arrest with G0/G1 reduction and induced cell apoptosis, but no significant changes were observed with miR-34c-5p transfection in U87 cells, normal or negative control groups. However, significant inhibition of glioma cell invasion was observed following transfection with miR-34c-3p and miR-34c-5p. Moreover, it was identified that miR-34c-3p overexpression reduced the expression of Notch pathway members, but miR-34c-5p overexpression did not. Therefore, these results suggest differential tumor suppressor roles for miR-34c-3p and miR-34c-5p and provide new insights into the role of miR-34c in glioma, which includes tumor-suppressing effects on proliferation, apoptosis and invasiveness.

Notch1-induced brain tumor models the sonic hedgehog subgroup of human medulloblastoma

While activation of the Notch pathway is observed in many human cancers, it is unknown whether elevated Notch1 expression is sufficient to initiate tumorigenesis in most tissues. To test the oncogenic potential of Notch1 in solid tumors, we expressed an activated form of Notch1 (N1ICD) in the developing mouse brain. N1ICD;hGFAP-cre mice were viable but developed severe ataxia and seizures, and died by weaning age. Analysis of transgenic embryo brains revealed that N1ICD expression induced p53-dependent apoptosis. When apoptosis was blocked by genetic deletion of p53, 30% to 40% of N1ICD;GFAP-cre;p53(+/-) and N1ICD;GFAP-cre;p53(-/-) mice developed spontaneous medulloblastomas. Interestingly, N1ICD-induced medulloblastomas most closely resembled the sonic hedgehog subgroup of human medulloblastoma at the molecular level. Surprisingly, N1ICD-induced tumors do not maintain high levels of the Notch pathway gene expression, except for Notch2, showing that initiating oncogenic events may not be decipherable by analyzing growing tumors in some cases. In summary, this study shows that Notch1 has an oncogenic potential in the brain when combined with other oncogenic hits, such as p53 loss, and provides a novel mouse model of medulloblastoma. Cancer Res; 73(17); 5381-90. ©2013 AACR.

What underlies the diversity of brain tumors?

Glioma and medulloblastoma represent the most commonly occurring malignant brain tumors in adults and in children, respectively. Recent genomic and transcriptional approaches present a complex group of diseases and delineate a number of molecular subgroups within tumors that share a common histopathology. Differences in cells of origin, regional niches, developmental timing, and genetic events all contribute to this heterogeneity. In an attempt to recapitulate the diversity of brain tumors, an increasing array of genetically engineered mouse models (GEMMs) has been developed. These models often utilize promoters and genetic drivers from normal brain development and can provide insight into specific cells from which these tumors originate. GEMMs show promise in both developmental biology and developmental therapeutics. This review describes numerous murine brain tumor models in the context of normal brain development and the potential for these animals to impact brain tumor research.

Testosterone levels influence mouse fetal Leydig cell progenitors through notch signaling

Leydig cells are the steroidogenic lineage of the mammalian testis that produces testosterone, a key hormone required throughout male fetal and adult life for virilization and spermatogenesis. Both fetal and adult Leydig cells arise from a progenitor population in the testis interstitium but are thought to be lineage-independent of one another. Genetic evidence indicates that Notch signaling is required during fetal life to maintain a balance between differentiated Leydig cells and their progenitors, but the elusive progenitor cell type and ligands involved have not been identified. In this study, we show that the Notch pathway signals through the ligand JAG1 in perivascular interstitial cells during fetal life. In the early postnatal testis, we show that circulating levels of testosterone directly affect Notch signaling, implicating a feedback role for systemic circulating factors in the regulation of progenitor cells. Between Postnatal Days 3 and 21, as fetal Leydig cells disappear from the testis and are replaced by adult Leydig cells, the perivascular population of interstitial cells active for Notch signaling declines, consistent with distinct regulation of adult Leydig progenitors.

The octadecaneuropeptide ODN prevents 6-hydroxydopamine-induced apoptosis of cerebellar granule neurons through a PKC-MAPK-dependent pathway

Oxidative stress, induced by various neurodegenerative diseases, initiates a cascade of events leading to apoptosis, and thus plays a critical role in neuronal injury. In this study, we have investigated the potential neuroprotective effect of the octadecaneuropeptide (ODN) on 6-hydroxydopamine (6-OHDA)-induced oxidative stress and apoptosis in cerebellar granule neurons (CGN). ODN, which is produced by astrocytes, is an endogenous ligand for both central-type benzodiazepine receptors (CBR) and a metabotropic receptor. Incubation of neurons with subnanomolar concentrations of ODN (10⁻¹⁸ to 10⁻¹² M) inhibited 6-OHDA-evoked cell death in a concentration-dependent manner. The effect of ODN on neuronal survival was abrogated by the metabotropic receptor antagonist, cyclo₁₋₈ [DLeu⁵]OP, but not by a CBR antagonist. ODN stimulated polyphosphoinositide turnover and ERK phosphorylation in CGN. The protective effect of ODN against 6-OHDA toxicity involved the phospholipase C/ERK MAPK transduction cascade. 6-OHDA treatment induced an accumulation of reactive oxygen species, an increase of the expression of the pro-apoptotic gene Bax, a drop of the mitochondrial membrane potential and a stimulation of caspase-3 activity. Exposure of 6-OHDA-treated cells to ODN blocked all the deleterious effects of the toxin. Taken together, these data demonstrate for the first time that ODN is a neuroprotective agent that prevents 6-OHDA-induced oxidative stress and apoptotic cell death.

Long-term three-dimensional neural tissue cultures in functionalized self-assembling peptide hydrogels, matrigel and collagen I

Designer peptides with self-assembling properties form nanofibers which are further organized to form a hydrogel consisting of up to 99.5% water. We present here the encapsulation of neural stem cells into peptide nanofiber hydrogel scaffolds. This results in three-dimensional (3-D) neural tissue cultures in which neural stem cells differentiate into progenitor neural cells, neurons, astrocytes and oligodendrocytes when cultured in serum-free medium. Cell survival studies showed that neural cells in peptide hydrogels thrive for at least 5 months. In contrast, neural stem cells encapsulated in Collagen I were poorly differentiated and did not migrate significantly, thus forming clusters. We show that for culture periods of 1-2 weeks, neural stem cells proliferate and differentiate better in Matrigel. However, in long-term studies, the population of cells in Matrigel decreases whereas better cell survival rates are observed in neural tissue cultures in peptide hydrogels. Peptide functionalization with cell adhesion and cell differentiation motifs show superior cell survival and differentiation properties compared to those observed upon culturing neural cells in non-modified peptide hydrogels. These designed 3-D engineered tissue culturing systems have a potential use as tissue surrogates for tissue regeneration. The well-defined chemical and physical properties of the peptide nanofiber hydrogels and the use of serum-free medium allow for more realistic biological studies of neural cells in a biomimetic 3-D environment.

Developmental signaling pathways in cancer stem cells of solid tumors

BACKGROUND

The intricate regulation of several signaling pathways is essential for embryonic development and adult tissue homeostasis. Cancers commonly display aberrant activity within these pathways. A population of cells identified in several cancers, termed cancer stem cells (CSCs) show similar properties to normal stem cells and evidence suggests that altered developmental signaling pathways play an important role in maintaining CSCs and thereby the tumor itself.

SCOPE OF REVIEW

This review will focus on the roles of the Notch, Wnt and Hedgehog pathways in the brain, breast and colon cancers. We describe the roles these pathways play in normal tissue homeostasis through the regulation of stem cell fate in these three tissues, and the experimental evidence indicating that the role of these pathways in cancers of these is directly linked to CSCs.

MAJOR CONCLUSIONS

A large body of evidence is accumulating to indicate that the deregulation of Notch, Wnt and Hedgehog pathways play important roles in both normal and cancer stem cells. We are only beginning to understand how these pathways interact, how they are coordinated during normal development and adult tissue homeostasis, and how they are deregulated during cancer. However, it is becoming increasingly clear that if we are to target CSCs therapeutically, it will likely be necessary to develop combination therapies.

GENERAL SIGNIFICANCE

If CSCs are the driving force behind tumor maintenance and growth then understanding the molecular mechanisms regulating CSCs is essential. Such knowledge will contribute to better targeted therapies that could significantly enhance cancer treatments and patient survival. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

Choroid plexus papillomas: advances in molecular biology and understanding of tumorigenesis

Choroid plexus papillomas are rare, benign tumors originating from the choroid plexus. Although generally found within the ventricular system, they can arise ectopically in the brain parenchyma or disseminate throughout the neuraxis. We sought to review recent advances in our understanding of the molecular biology and oncogenic pathways associated with this disease. A comprehensive PubMed literature review was conducted to identify manuscripts discussing the clinical, molecular, and genetic features of choroid plexus papillomas. Articles concerning diagnosis, treatment, and long-term patient outcomes were also reviewed. The introduction of atypical choroid plexus papilloma as a distinct entity has increased the need for accurate histopathologic diagnosis. Advances in immunohistochemical staining have improved our ability to differentiate choroid plexus papillomas from other intracranial tumors or metastatic lesions using combinations of key markers and mitotic indices. Recent findings have implicated Notch3 signaling, the transcription factor TWIST1, platelet-derived growth factor receptor, and the tumor necrosis factor-related apoptosis-inducing ligand pathway in choroid plexus papilloma tumorigenesis. A combination of commonly occurring chromosomal duplications and deletions has also been identified. Surgical resection remains the standard of care, although chemotherapy and radiotherapy may be considered for recurrent or metastatic lesions. While generally considered benign, these tumors possess a complex biology that sheds insight into other choroid plexus tumors, particularly malignant choroid plexus carcinomas. Improving our understanding of the molecular biology, genetics, and oncogenic pathways associated with this tumor will allow for the development of targeted therapies and improved outcomes for patients with this disease.

Single-cell mRNA profiling identifies progenitor subclasses in neurospheres

Neurospheres are widely used to propagate and investigate neural stem cells (NSCs) and neural progenitors (NPs). However, the exact cell types present within neurospheres are still unknown. To identify cell types, we used single-cell mRNA profiling of 48 genes in 187 neurosphere cells. Using a clustering algorithm, we identified 3 discrete cell populations within neurospheres. One cell population [cluster unsorted (US) 1] expresses high Bmi1 and Hes5 and low Myc and Klf12. Cluster US2 shows intermediate expression of most of the genes analyzed. Cluster US3 expresses low Bmi1 and Hes5 and high Myc and Klf12. The mRNA profiles of these 3 cell populations correlate with a developmental timeline of early, intermediate, and late NPs, as seen in vivo from the mouse brain. We enriched the cell population for neurosphere-forming cells (NFCs) using morphological criteria of forward scatter (FSC) and side scatter (SSC). FSC/SSC(high) cells generated 2.29-fold more neurospheres than FSC/SSC(low) cells at clonal density. FSC/SSC(high) cells were enriched for NSCs and Lewis-X(+ve) cells, possessed higher phosphacan levels, and were of a larger cell size. Clustering of both FSC/SSC(high) and FSC/SSC(low) cells identified an NFC cluster. Significantly, the mRNA profile of the NFC cluster drew close resemblance to that of early NPs. Taken together, data suggest that the neurosphere culture system can be used to model central nervous system development, and that early NPs are the cell population that gives rise to neurospheres. In future work, it may be possible to further dissect the NFCs and reveal the molecular signature for NSCs.

Nuclear factor one X regulates the development of multiple cellular populations in the postnatal cerebellum

Development of the cerebellum involves the coordinated proliferation, differentiation, maturation, and integration of cells from multiple neuronal and glial lineages. In rodent models, much of this occurs in the early postnatal period. However, our understanding of the molecular mechanisms that regulate this phase of cerebellar development remains incomplete. Here, we address the role of the transcription factor nuclear factor one X (NFIX), in postnatal development of the cerebellum. NFIX is expressed by progenitor cells within the external granular layer and by cerebellar granule neurons within the internal granule layer. Using NFIX⁻/⁻ mice, we demonstrate that the development of cerebellar granule neurons and Purkinje cells within the postnatal cerebellum is delayed in the absence of this transcription factor. Furthermore, the differentiation of mature glia within the cerebellum, such as Bergmann glia, is also significantly delayed in the absence of NFIX. Collectively, the expression pattern of NFIX, coupled with the delays in the differentiation of multiple cell populations of the developing cerebellum in NFIX⁻/⁻ mice, suggest a central role for NFIX in the regulation of cerebellar development, highlighting the importance of this gene for the maturation of this key structure.

Constitutive Notch2 signaling in neural stem cells promotes tumorigenic features and astroglial lineage entry

Recent studies identified a highly tumorigenic subpopulation of glioma stem cells (GSCs) within malignant gliomas. GSCs are proposed to originate from transformed neural stem cells (NSCs). Several pathways active in NSCs, including the Notch pathway, were shown to promote proliferation and tumorigenesis in GSCs. Notch2 is highly expressed in glioblastoma multiforme (GBM), a highly malignant astrocytoma. It is therefore conceivable that increased Notch2 signaling in NSCs contributes to the formation of GBM. Here, we demonstrate that mice constitutively expressing the activated intracellular domain of Notch2 in NSCs display a hyperplasia of the neurogenic niche and reduced neuronal lineage entry. Neurospheres derived from these mice show increased proliferation, survival and resistance to apoptosis. Moreover, they preferentially differentiate into astrocytes, which are the characteristic cellular population of astrocytoma. Likewise, we show that Notch2 signaling increases proliferation and resistance to apoptosis in human GBM cell lines. Gene expression profiling of GBM patient tumor samples reveals a positive correlation of Notch2 transcripts with gene transcripts controlling anti-apoptotic processes, stemness and astrocyte fate, and a negative correlation with gene transcripts controlling proapoptotic processes and oligodendrocyte fate. Our data show that Notch2 signaling in NSCs produces features of GSCs and induces astrocytic lineage entry, consistent with a possible role in astrocytoma formation.

Signaling in cell differentiation and morphogenesis

All the information to make a complete, fully functional living organism is encoded within the genome of the fertilized oocyte. How is this genetic code translated into the vast array of cellular behaviors that unfold during the course of embryonic development, as the zygote slowly morphs into a new organism? Studies over the last 30 years or so have shown that many of these cellular processes are driven by secreted or membrane-bound signaling molecules. Elucidating how the genetic code is translated into instructions or signals during embryogenesis, how signals are generated at the correct time and place and at the appropriate level, and finally, how these instructions are interpreted and put into action, are some of the central questions of developmental biology. Our understanding of the causes of congenital malformations and disease has improved substantially with the rapid advances in our knowledge of signaling pathways and their regulation during development. In this article, I review some of the signaling pathways that play essential roles during embryonic development. These examples show some of the mechanisms used by cells to receive and interpret developmental signals. I also discuss how signaling pathways downstream from these signals are regulated and how they induce specific cellular responses that ultimately affect cell fate and morphogenesis.

Prenatal NMDA receptor antagonism impaired proliferation of neuronal progenitor, leading to fewer glutamatergic neurons in the prefrontal cortex

N-methyl-D-aspartate (NMDA) receptor is a glutamate receptor which has an important role on mammalian brain development. We have reported that prenatal treatment with phencyclidine (PCP), a NMDA receptor antagonist, induces long-lasting behavioral deficits and neurochemical changes. However, the mechanism by which the prenatal antagonism of NMDA receptor affects neurodevelopment, resulting in behavioral deficits, has remained unclear. Here, we report that prenatal NMDA receptor antagonism impaired the proliferation of neuronal progenitors, leading to a decrease in the progenitor pool in the ventricular and the subventricular zone. Furthermore, using a PCR array focused on neurogenesis and neuronal stem cells, we evaluated changes in gene expression causing the impairment of neuronal progenitor proliferation and found aberrant gene expression, such as Notch2 and Ntn1, in prenatal PCP-treated mice. Consequently, the density of glutamatergic neurons in the prefrontal cortex was decreased, probably resulting in glutamatergic hypofunction. Prenatal PCP-treated mice displayed behavioral deficits in cognitive memory and sensorimotor gating until adulthood. These findings suggest that NMDA receptors regulate the proliferation and maturation of progenitor cells for glutamatergic neuron during neurodevelopment, probably via the regulation of gene expression.

The aryl hydrocarbon receptor contributes to the proliferation of human medulloblastoma cells

The aryl hydrocarbon receptor (AhR), a ligand-activated member of the basic helix-loop-helix (bHLH)/PER-ARNT-SIM (PAS) transcription superfamily, is known to regulate the toxicity of polyaromatic halogenated hydrocarbon environmental chemicals, most notably dioxin. However, the AhR has also been implicated in multiple stages of tumorigenesis. Medulloblastoma (MB), a primary cerebellar brain tumor arising in infants and children, is thought to originate from abnormally proliferating cerebellar granule neuron precursors (GNPs). GNPs express high levels of the AhR in the external germinal layer of the developing cerebellum. Moreover, our laboratory has previously reported that either abnormal activation or deletion of the AhR leads to dysregulation of GNP cell cycle activity and maturation. These observations led to the hypothesis that the AhR promotes the growth of MB. Therefore, this study evaluated whether the AhR serves a pro-proliferative role in an immortalized MB tumor cell line (DAOY). We produced a stable AhR knockdown DAOY cell line [AhR short hairpin RNA (shRNA)], which exhibited a 70% reduction in AhR protein levels. Compared with wild-type DAOY cells, AhR shRNA DAOY cells displayed an impaired G(1)-to-S cell cycle transition, decreased DNA synthesis, and reduced proliferation. Furthermore, these cell cycle perturbations were correlated with decreased levels of the pro-proliferative gene Hes1 and increased levels of the cell cycle inhibitor p27(kip1). Supplementation experiments with human AhR restored the proliferative activity in AhR shRNA DAOY cells. Taken together, our data show that the AhR promotes proliferation of MB cells, suggesting that this pathway should be considered as a potential therapeutic target for MB treatment.

Impaired motor coordination and disrupted cerebellar architecture in Fgfr1 and Fgfr2 double knockout mice

Fibroblast growth factor receptor (FGFR) signaling determines the size of the cerebral cortex by regulating the amplification of radial glial stem cells, and participates in the formation of midline glial structures. We show that Fgfr1 and Fgfr2 double knockouts (FGFR DKO) generated by Cre-mediated recombination driven by the human GFAP promoter (hGFAP) have reduced cerebellar size due to reduced proliferation of radial glia and other glial precursors in late embryonic and neonatal FGFR DKO mice. The proliferation of granule cell progenitors (GCPs) in the EGL was also reduced, leading to reduced granule cell numbers. Furthermore, both inward migration of granule cells into the inner granule cell layer (IGL) and outward migration of GABA interneurons into the molecular layer (ML) were arrested, disrupting layer and lobular morphology. Purkinje neurons and their dendrites, which were not targeted by Cre-mediated recombination of Fgf receptors, were also misplaced in FGFR DKO mice, possibly as a consequence of altered Bergmann glia orientation or reduced granule cell number. Our findings indicate a dual role for FGFR signaling in cerebellar morphogenesis. The first role is to amplify the number of granule neuron precursors in the external granular layer and glial precursor cells throughout the cerebellum. The second is to establish the correct Bergmann glia morphology, which is crucial for granule cell migration. The disrupted cerebellar size and laminar architecture resulting from loss of FGFR signaling impair motor learning and coordination in FGFR DKO mice.

Glial vascular degeneration in CADASIL

CADASIL is a genetic vascular dementia caused by mutations in the Notch 3 gene on Chromosome 19. However, little is known about the mechanisms of vascular degeneration. We characterized upstream components of Notch signaling pathways that may be disrupted in CADASIL, by measuring expression of insulin, IGF-1, and IGF-2 receptors, Notch 1, Notch 3, and aspartyl-(asparaginyl)-β-hydroxylase (AAH) in cortex and white matter from 3 CADASIL and 6 control brains. We assessed CADASIL-associated cell loss by measuring mRNA corresponding to neurons, oligodendroglia, and astrocytes, and indices of vascular degeneration by measuring smooth muscle actin (SMA) and endothelin-1 expression in isolated vessels. Immunohistochemical staining was used to assess SMA degeneration. Significant abnormalities, including reduced cerebral white matter mRNA levels of Notch 1, Notch 3, AAH, SMA, IGF receptors, myelin-associated glycoproteins, and glial fibrillary acidic protein, and reduced vascular expression of SMA, IGF receptors, Notch 1, and Notch 3 were detected in CADASIL-lesioned brains. In addition, we found CADASIL-associated reductions in SMA, and increases in ubiquitin immunoreactivity in the media of white matter and meningeal vessels. No abnormalities in gene expression or immunoreactivity were observed in CADASIL cerebral cortex. In conclusion, molecular abnormalities in CADASIL are largely restricted to white matter and white matter vessels, corresponding to the distribution of neuropathological lesions. These preliminary findings suggest that CADASIL is mediated by both glial and vascular degeneration with reduced expression of IGF receptors and AAH, which regulate Notch expression and function.

Notch signaling and brain tumors

Human brain tumors are a heterogenous group of neoplasms occurring inside the cranium and the central spinal cord. In adults and children, astrocytic glioma and medulloblastoma are the most common subtypes of primary brain tumors. These tumor types are thought to arise from cells in which Notch signaling plays a fundamental role during development. Recent findings have shown that Notch signaling is dysregulated and contributes to the malignant potential of these tumors. Growing evidence point towards an important role for cancer stem cells in the initiation and maintenance of glioma and medulloblastoma. In this chapter we will cover the present findings of Notch signaling in human glioma and medulloblastoma and try to create an overall picture of its relevance in the pathogenesis of these tumors.

Oscillabolastic model, a new model for oscillatory dynamics, applied to the analysis of Hes1 gene expression and Ehrlich ascites tumor growth

This paper introduces a new dynamical model, called the oscillabolastic model, to analyze the dynamical behavior of biomedical data when one observes oscillatory behavior. The proposed oscillabolastic model is sufficiently flexible to represent various types of oscillatory behavior. The oscillabolastic model is applied to two sets of data. The first data set deals with the oscillabolastic modeling of Ehrlich ascites tumor cells and the second one is the oscillabolastic modeling of the mean signal intensity of Hes1 gene expression in response to serum stimulation. A generalized oscillabolastic model is also suggested to accommodate cases in which predictor variables other than time are also involved.

Notch receptor and effector expression in von Hippel-Lindau disease-associated central nervous system hemangioblastomas

OBJECT

Central nervous system hemangioblastomas are the most common manifestation of von Hippel-Lindau (VHL) disease, an autosomal dominant tumor suppressor syndrome that results in loss of VHL protein function and continuous upregulation of hypoxia-inducible factors. These tumors are composed of neoplastic stromal cells and abundant vasculature. Stromal cells express markers consistent with multipotent embryonically arrested hemangioblasts, which are precursors for hematopoietic and vascular lineages. Notch receptors are transmembrane signaling molecules that regulate multiple developmental processes including hematopoiesis and vasculogenesis. To investigate the importance of notch signaling in the development of VHL disease-associated CNS hemangioblastomas, the authors examined the presence of the four notch receptors and downstream notch effectors in this setting.

METHODS

The authors used surgical specimens obtained from confirmed VHL-associated hemangioblastomas. Immunohistochemical analysis for the four notch receptors and the downstream effectors was performed on formalin-fixed paraffin-embedded sections. Western blot analysis for HES1 was performed on frozen specimens.

RESULTS

All four notch receptors are present in hemangioblastomas. NOTCH1 and NOTCH4 receptors were widely and prominently expressed in both the stromal and vascular cells, NOTCH2 receptor expression was limited to primarily stromal cells, and NOTCH3 receptor expression was limited to vascular cells. All 4 receptors displayed a nuclear presence. Immunohistochemical analysis also demonstrated that downstream notch effectors, HES1 and HES5, were uniformly expressed in tumor stromal and vascular cells, but HES3, HEY1, and HEY2 were not. Strong HES1 expression was confirmed by Western blot analysis.

CONCLUSIONS

The presence of all four notch receptors and downstream effector molecules suggests that the notch signaling pathway plays a critical role in the maintenance of the undifferentiated pluripotent phenotype of these tumors and in the associated vascular response. Moreover, the prominent expression of notch receptors in VHL-associated CNS hemangioblastomas reveals a new and possibly potent therapeutic target.

Boc and Gas1 each form distinct Shh receptor complexes with Ptch1 and are required for Shh-mediated cell proliferation

Hedgehog (Hh) proteins regulate important developmental processes, including cell proliferation and differentiation. Although Patched acts as the main Hh receptor in Drosophila, Hh signaling absolutely requires the additional Hh-binding proteins Ihog and Boi. Here we show that, unexpectedly, cerebellar granule neuron progenitors (CGNPs) lacking Boc and Cdon, the vertebrate orthologs of Ihog and Boi, still proliferate in response to Hh. This is because in their absence, Gas1, an Hh-binding protein not present in Drosophila, mediates Hh signaling. Consistently, only CGNPs lacking all three molecules-Boc, Cdon, and Gas1-have a complete loss of Hh-dependent proliferation. In a complementary manner, we find that a mutated Hh ligand that binds Patched1 but not Boc, Cdon, or Gas1 cannot activate Hh signaling. Together, this demonstrates an absolute requirement for Boc, Cdon, and Gas1 in Hh signaling and reveals a distinct requirement for ligand-binding components that distinguishes the vertebrate and invertebrate Hh receptor systems.

Conserved and differential gene interactions in dynamical biological systems

MOTIVATION

While biological systems operated from a common genome can be conserved in various ways, they can also manifest highly diverse dynamics and functions. This is because the same set of genes can interact differentially across specific molecular contexts. For example, differential gene interactions give rise to various stages of morphogenesis during cerebellar development. However, after over a decade of efforts toward reverse engineering biological networks from high-throughput omic data, gene networks of most organisms remain sketchy. This hindrance has motivated us to develop comparative modeling to highlight conserved and differential gene interactions across experimental conditions, without reconstructing complete gene networks first.

RESULTS

We established a comparative dynamical system modeling (CDSM) approach to identify conserved and differential interactions across molecular contexts. In CDSM, interactions are represented by ordinary differential equations and compared across conditions through statistical heterogeneity and homogeneity tests. CDSM demonstrated a consistent superiority over differential correlation and reconstruct-then-compare in simulation studies. We exploited CDSM to elucidate gene interactions important for cellular processes poorly understood during mouse cerebellar development. We generated hypotheses on 66 differential genetic interactions involved in expansion of the external granule layer. These interactions are implicated in cell cycle, differentiation, apoptosis and morphogenesis. Additional 1639 differential interactions among gene clusters were also identified when we compared gene interactions during the presence of Rhombic lip versus the presence of distinct internal granule layer. Moreover, compared with differential correlation and reconstruct-then-compare, CDSM makes fewer assumptions on data and thus is applicable to a wider range of biological assays.

AVAILABILITY

Source code in C++ and R is available for non-commercial organizations upon request from the corresponding author. The cerebellum gene expression dataset used in this article is available upon request from the Goldowitz lab (dang@cmmt.ubc.ca, http://grits.dglab.org/).

CONTACT

joemsong@cs.nmsu.edu

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Enhanced production of neuroprogenitors, dopaminergic neurons, and identification of target genes by overexpression of sonic hedgehog in human embryonic stem cells

Molecular and cellular signaling pathways are involved in the process of neural differentiation from human embryonic stem cells (hESC) to terminally differentiated neurons. The Sonic hedgehog (SHH) morphogen is required to direct the differentiation of hESC to several neural subtypes, for example, dopaminergic (DA) or motor neurons. However, the roles of SHH signaling and the pathway target genes that regulate the diversity of cellular responses arising from SHH activation during neurogenesis of hESC have yet to be elucidated. In this study, we report that overexpression of SHH in hESC promotes the derivation of neuroprogenitors (NP), increases proliferation of NP, and subsequently increases the yield of DA neurons. Next, gene expression changes resulting from the overexpression of SHH in hESC-derived NP were examined by genome-wide transcriptional profiling. Categorizing the differentially expressed genes according to the Gene Ontology biological processes showed that they are involved in numerous cellular processes, including neural development, NP proliferation, and neural specification. In silico GLI-binding sites analysis of the differentially expressed genes also identified a set of putative novel direct target genes of SHH in hESC-derived NP, which are involved in nervous system development. Electrophoretic mobility shift assays and promoter-luciferase assays confirmed that GLI1 binds to the promoter region and activates transcription of HEY2, a NOTCH signaling target gene. Taken together, our data provide evidence for the first time that there is cross-talk between the NOTCH and SHH signaling pathways in hESC-derived NP and also provide significant new insights into transcriptional targets in SHH-mediated neural differentiation of hESC.

Persistent expression of activated Notch inhibits corticotrope and melanotrope differentiation and results in dysfunction of the HPA axis

The hypothalamic-pituitary-adrenal (HPA) axis is an important regulator of energy balance, immune function and the body's response to stress. Signaling networks governing the initial specification of corticotropes, a major component of this axis, are not fully understood. Loss of function studies indicate that Notch signaling may be necessary to repress premature differentiation of corticotropes and to promote proliferation of pituitary progenitors. To elucidate whether Notch signaling must be suppressed in order for corticotrope differentiation to proceed and whether Notch signaling is sufficient to promote corticotrope proliferation, we examined the effects of persistent Notch expression in Pomc lineage cells. We show that constitutive activation of the Notch cascade inhibits the differentiation of both corticotropes and melanotropes and results in the suppression of transcription factors required for Pomc expression. Furthermore, persistent Notch signaling traps cells in the intermediate lobe of the pituitary in a progenitor state, but has no effect on pituitary proliferation. Undifferentiated cells are eliminated in the first two postnatal weeks in these mice, resulting in a modest increase in CRH expression in the paraventricular nucleus, hypoplastic adrenal glands and decreased stress-induced corticosterone levels. Taken together, these findings show that Notch signaling is sufficient to prevent corticotrope and melanotrope differentiation, resulting in dysregulation of the HPA axis.

Sprouty genes prevent excessive FGF signalling in multiple cell types throughout development of the cerebellum

Fibroblast growth factors (FGFs) and regulators of the FGF signalling pathway are expressed in several cell types within the cerebellum throughout its development. Although much is known about the function of this pathway during the establishment of the cerebellar territory during early embryogenesis, the role of this pathway during later developmental stages is still poorly understood. Here, we investigated the function of sprouty genes (Spry1, Spry2 and Spry4), which encode feedback antagonists of FGF signalling, during cerebellar development in the mouse. Simultaneous deletion of more than one of these genes resulted in a number of defects, including mediolateral expansion of the cerebellar vermis, reduced thickness of the granule cell layer and abnormal foliation. Analysis of cerebellar development revealed that the anterior cerebellar neuroepithelium in the early embryonic cerebellum was expanded and that granule cell proliferation during late embryogenesis and early postnatal development was reduced. We show that the granule cell proliferation deficit correlated with reduced sonic hedgehog (SHH) expression and signalling. A reduction in Fgfr1 dosage during development rescued these defects, confirming that the abnormalities are due to excess FGF signalling. Our data indicate that sprouty acts both cell autonomously in granule cell precursors and non-cell autonomously to regulate granule cell number. Taken together, our data demonstrate that FGF signalling levels have to be tightly controlled throughout cerebellar development in order to maintain the normal development of multiple cell types.

RNA-binding protein FXR2 regulates adult hippocampal neurogenesis by reducing Noggin expression

In adult mammalian brains, neurogenesis persists in the subventricular zone of the lateral ventricles (SVZ) and the dentate gyrus (DG) of the hippocampus. Although evidence suggest that adult neurogenesis in these two regions is subjected to differential regulation, the underlying mechanism is unclear. Here, we show that the RNA-binding protein FXR2 specifically regulates DG neurogenesis by reducing the stability of Noggin mRNA. FXR2 deficiency leads to increased Noggin expression and subsequently reduced BMP signaling, which results in increased proliferation and altered fate specification of neural stem/progenitor cells in DG. In contrast, Noggin is not regulated by FXR2 in the SVZ, because Noggin expression is restricted to the ependymal cells of the lateral ventricles, where FXR2 is not expressed. Differential regulation of SVZ and DG stem cells by FXR2 may be a key component of the mechanism that governs the different neurogenic processes in these two adult germinal zones.

Genes expressed in Atoh1 neuronal lineages arising from the r1/isthmus rhombic lip

During embryogenesis, the rhombic lip of the fourth ventricle is the germinal origin of a diverse collection of neuronal populations that ultimately reside in the brainstem and cerebellum. Rhombic lip neurogenesis requires the bHLH transcription factor Atoh1 (Math1), and commences shortly after neural tube closure (E9.5). Within the rhombomere 1-isthmus region, the rhombic lip first produces brainstem and deep cerebellar neurons (E9.5-E12), followed by granule cell precursors after E12. While Atoh1 function is essential for all of these populations to be specified, the downstream genetic programs that confer specific properties to early and late born Atoh1 lineages are not well characterized. We have performed a comparative microarray analysis of gene expression within early and later born cohorts of Atoh1 expressing neural precursors purified from E14.5 embryos using a transgenic labeling strategy. We identify novel transcription factors, cell surface molecules, and cell cycle regulators within each pool of Atoh1 lineages that likely contribute to their distinct developmental trajectories and cell fates. In particular, our analysis reveals new insights into the genetic programs that regulate the specification and proliferation of granule cell precursors, the putative cell of origin for the majority of medulloblastomas.

Upregulation of E2F1 in cerebellar neuroprogenitor cells and cell cycle arrest during postnatal brain development

In the developing cerebellum, proliferation of granular neuroprogenitor (GNP) cells lasts until the early postnatal stages when terminal maturation of the cerebellar cortex occurs. GNPs are considered cell targets for neoplastic transformation, and disturbances in cerebellar GNP cell proliferation may contribute to the development of pediatric medulloblastoma. At the molecular level, proliferation of GNPs is regulated through an orchestrated action of the SHH, NOTCH, and WNT pathways, but the underlying mechanisms still need to be dissected. Here, we report that expression of the E2F1 transcription factor in rat GNPs is inversely correlated with cell proliferation rate during postnatal development, as opposed to its traditional SHH-dependent induction of cell cycle. Proliferation of GNPs peaked at postnatal day 3 (P3), with a subsequent continuing decrease in proliferation rates occurring until P12. Such gradual decline in proliferating neuroprogenitors paralleled the extent of cerebellum maturation confirmed by histological analysis with cresyl violet staining and temporal expression profiling of SHH, NOTCH2, and WNT4 genes. A time course analysis of E2F1 expression in GNPs revealed significantly increased levels at P12, correlating with decreased cell proliferation. Expression of the cell cycle inhibitor p18 ( Ink4c ), a target of E2F1, was also significantly higher at P12. Conversely, increased E2F1 expression did not correlate with either SMAC/DIABLO and BCL2 expression profiles or apoptosis of cerebellar cells. Altogether, these results suggest that E2F1 may also be involved in the inhibition of GNP proliferation during rat postnatal development despite its conventional mitogenic effects.

Notch in the vertebrate nervous system: an old dog with new tricks

The Notch pathway is prominent among those known to regulate neural development in vertebrates. Notch receptor activation can inhibit neurogenesis, maintain neural progenitor character, and in some contexts promote gliogenesis and drive binary fate choices. Recently, a wave of exciting studies has emerged, which has both solidified previously held assertions and expanded our understanding of Notch function during neurogenesis and in the adult brain. These studies have examined pathway regulators and interactions, as well as pathway dynamics, with respect to both gene expression and cell-cell signaling. Here, focusing primarily on vertebrates, we review the current literature on Notch signaling in the nervous system, and highlight numerous recent studies that have generated interesting and unexpected advances.

The Notch effector gene Hes1 regulates migration of hypothalamic neurons, neuropeptide content and axon targeting to the pituitary

Proper development of the hypothalamic-pituitary axis requires precise neuronal signaling to establish a network that regulates homeostasis. The developing hypothalamus and pituitary utilize similar signaling pathways for differentiation in embryonic development. The Notch signaling effector gene Hes1 is present in the developing hypothalamus and pituitary and is required for proper formation of the pituitary, which contains axons of arginine vasopressin (AVP) neurons from the hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON). We hypothesized that Hes1 is necessary for the generation, placement and projection of AVP neurons. We found that Hes1 null mice show no significant difference in cell proliferation or death in the developing diencephalon at embryonic day 10.5 (e10.5) or e11.5. By e16.5, AVP cell bodies are formed in the SON and PVN, but are abnormally placed, suggesting that Hes1 may be necessary for the migration of AVP neurons. GAD67 immunoreactivity is ectopically expressed in Hes1 null mice, which may contribute to cell body misplacement. Additionally, at e18.5 Hes1 null mice show continued misplacement of AVP cell bodies in the PVN and SON and additionally exhibit abnormal axonal projection. Using mass spectrometry to characterize peptide content, we found that Hes1 null pituitaries have aberrant somatostatin (SS) peptide, which correlates with abnormal SS cells in the pituitary and misplaced SS axon tracts at e18.5. Our results indicate that Notch signaling facilitates the migration and guidance of hypothalamic neurons, as well as neuropeptide content.

si-RNA inhibition of brain insulin or insulin-like growth factor receptors causes developmental cerebellar abnormalities: relevance to fetal alcohol spectrum disorder

BACKGROUND

In experimental models of fetal alcohol spectrum disorder (FASD), cerebellar hypoplasia and hypofoliation are associated with insulin and insulin-like growth factor (IGF) resistance with impaired signaling through pathways that mediate growth, survival, plasticity, metabolism, and neurotransmitter function. To more directly assess the roles of impaired insulin and IGF signaling during brain development, we administered intracerebroventricular (ICV) injections of si-RNA targeting the insulin receptor, (InR), IGF-1 receptor (IGF-1R), or IGF-2R into postnatal day 2 (P2) Long Evans rat pups and examined the sustained effects on cerebellar function, structure, and neurotransmitter-related gene expression (P20).

RESULTS

Rotarod tests on P20 demonstrated significant impairments in motor function, and histological studies revealed pronounced cerebellar hypotrophy, hypoplasia, and hypofoliation in si-InR, si-IGF-1R, and si-IGF-2R treated rats. Quantitative RT-PCR analysis showed that si-InR, and to a lesser extent si-IGF-2R, broadly inhibited expression of insulin and IGF-2 polypeptides, and insulin, IGF-1, and IGF-2 receptors in the brain. ELISA studies showed that si-InR increased cerebellar levels of tau, phospho-tau and β-actin, and inhibited GAPDH. In addition, si-InR, si-IGF-1R, and si-IGF-2R inhibited expression of choline acetyltransferase, which mediates motor function. Although the ICV si-RNA treatments generally spared the neurotrophin and neurotrophin receptor expression, si-InR and si-IGF-1R inhibited NT3, while si-IGF-1R suppressed BDNF.

CONCLUSIONS

early postnatal inhibition of brain InR expression, and to lesser extents, IGF-R, causes structural and functional abnormalities that resemble effects of FASD. The findings suggest that major abnormalities in brains with FASD are mediated by impairments in insulin/IGF signaling. Potential therapeutic strategies to reduce the long-term impact of prenatal alcohol exposure may include treatment with agents that restore brain insulin and IGF responsiveness.

Oct-4+/Tenascin C+ neuroblastoma cells serve as progenitors of tumor-derived endothelial cells

Neuroblastoma (NB)-associated endothelial microvessels (EMs) may be lined by tumor-derived endothelial cells (TECs), that are genetically unstable and chemoresistant. Here we have addressed the identification of TEC progenitors in NB by focusing on Octamer-binding transcription factor 4 (Oct-4) as a putative marker. Oct-4(+) cells were detected in primary NB samples (n = 23), metastatic bone marrow aspirates (n = 10), NB cell lines (n = 4), and orthotopic tumors (n = 10) formed by the HTLA-230 NB cell line in immunodeficient mice. Most Oct-4(+) cells showed a perivascular distribution, with 5% of them homing in perinecrotic areas. All Oct-4(+) cells were tumor-derived since they shared amplification of MYCN oncogene with malignant cells. Perivascular Oct-4(+) cells expressed stem cell-related, neural progenitor-related and NB-related markers, including surface Tenascin C (TNC), that was absent from perinecrotic Oct-4(+) cells and bulk tumor cells. TNC(+) but not TNC(-) HTLA-230 cells differentiated in vitro into endothelial-like cells expressing vascular-endothelial-cadherin, prostate-specific membrane antigen and CD31 upon culture in medium containing vascular endothelial growth factor (VEGF). TNC(+) but not TNC(-) HTLA-230 cells formed neurospheres when cultured in serum-free medium. Both cell fractions were tumorigenic, but only tumors formed by TNC(+) cells contained EMs lined by TECs. In conclusion, we have identified in NB tumors two putative niches containing Oct-4(+) tumor cells. Oct-4(+)/TNC(+) perivascular NB cells displayed a high degree of plasticity and served as progenitors of TECs. Therapeutic targeting of Oct4(+)/TNC(+) progenitors may counteract the contribution of NB-derived ECs to tumor relapse and chemoresistance.

Nuclear factor-κB/p65 responds to changes in the Notch signaling pathway in murine BV-2 cells and in amoeboid microglia in postnatal rats treated with the γ-secretase complex blocker DAPT

Microglial cells constitutively express Notch-1 and nuclear factor-kappaB/p65 (NF-kappaB/p65), and both pathways modulate production of inflammatory mediators. This study sought to determine whether a functional relationship exists between them and, if so, to investigate whether they synergistically regulate common microglial cell functions. By immunofluorescence labeling, real-time polymerase chain reaction (RT-PCR), flow cytometry, and Western blot, BV-2 cells exhibited Notch-1 and NF-kappaB/p65 expression, which was significantly up-regulated in cells challenged with lipopolysaccharide (LPS). This was coupled with an increase in expression of Hes-1, tumor necrosis factor-alpha (TNF-alpha), and interleukin-1beta (IL-1beta). In BV-2 cells pretreated with N-[N-(3,5-difluorophenacetyl)-1-alany1]-S-phenyglycine t-butyl ester (DAPT), a gamma-secretase inhibitor, followed by LPS stimulation, Notch-1 expression level was enhanced but that of all other markers was suppressed. Additionally, Hes-1 expression and NF-kappaB nuclear translocation decreased as shown by flow cytometry. Notch-1's modulation of NF-kappaB/p65 was also evidenced in amoeboid microglial cells (AMC) in vivo. In 5-day-old rats given intraperitoneal injections of LPS, Notch-1, NF-kappaB/p65, TNF-alpha, and IL-1beta immunofluorescence in AMC was markedly enhanced. However, in rats given an intraperitoneal injection of DAPT prior to LPS, Notch-1 labeling was augmented, but that of TNF-alpha and IL-1beta was reduced. The results suggest that blocking of Notch-1 activation with DAPT would reduce the level of its downstream end product Hes-1 along with suppression of NF-kappaB/p65 translocation, resulting in suppressed production of proinflammatory cytokines. It is concluded that Notch-1 signaling can trans-activate NF-kappaB/p65 by amplifying NF-kappaB/p65-dependent proinflammatory functions in activated microglia.

Development and cancer of the cerebellum

Medulloblastoma (MB) is the most common malignant pediatric brain tumor and is thought to arise from genetic anomalies in developmental pathways required for the normal maturation of the cerebellar cortex, notably developmental pathways for granule cell progenitor (GCP) neurogenesis. Over the past decade, a wide range of studies have identified genes and their regulators within signaling pathways, as well as noncoding RNAs, that have crucial roles in both normal cerebellar development and pathogenesis. These include the Notch, Wnt/β-catenin, bone morphogenic proteins (Bmp) and Sonic Hedgehog (Shh) pathways. In this review, we highlight the function of these pathways in the growth of the cerebellum and the formation of MB. A better understanding of the developmental origins of these tumors will have significant implications for enhancing the treatment of this important childhood cancer.