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.

Developmental origins of astrocyte heterogeneity: the final frontier of CNS development

Astrocytes are the most abundant cell type in the central nervous system, have diverse physiological roles in both health and disease, and exhibit phenotypic heterogeneity. In spite of the overwhelming evidence that astrocytes are a diverse population, there has been relatively little consideration of their molecular heterogeneity. In this review, we will summarize what is known about the heterogeneity of astrocytes and outline challenges that have limited studies understanding their molecular diversity. Approaches that have sought to overcome these limitations will be discussed, with an emphasis on recent progress in the field of developmental gliogenesis, which has revealed that positional identity during embryogenesis is an organizing feature of astrocyte diversity. These recent findings, coupled with emerging technologies that allow for direct isolation of astrocyte populations, have led us to propose that approaches rooted in astrocyte development may be the key to unlocking this immense, untapped diversity.

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.

Sox9 and NFIA coordinate a transcriptional regulatory cascade during the initiation of gliogenesis

Transcriptional cascades that operate over the course of lineage development are fundamental mechanisms that control cellular differentiation. In the developing central nervous system (CNS), these mechanisms are well characterized during neurogenesis, but remain poorly defined during neural stem cell commitment to the glial lineage. NFIA is a transcription factor that plays a crucial role in the onset of gliogenesis; we found that its induction is regulated by the transcription factor Sox9 and that this relationship mediates the initiation of gliogenesis. Subsequently, Sox9 and NFIA form a complex and coregulate a set of genes induced after glial initiation. Functional studies revealed that a subset of these genes, Apcdd1 and Mmd2, perform key migratory and metabolic roles during astro-gliogenesis, respectively. In sum, these studies delineate a transcriptional regulatory cascade that operates during the initiation of gliogenesis and identifies a unique set of genes that regulate key aspects of astro-glial precursor physiology during development.

Daam2 is required for dorsal patterning via modulation of canonical Wnt signaling in the developing spinal cord

The Daam family of proteins consists of Daam1 and Daam2. Although Daam1 participates in noncanonical Wnt signaling during gastrulation, Daam2 function remains completely uncharacterized. Here we describe the role of Daam2 in canonical Wnt signal transduction during spinal cord development. Loss-of-function studies revealed that Daam2 is required for dorsal progenitor identities and canonical Wnt signaling. These phenotypes are rescued by β-catenin, demonstrating that Daam2 functions in dorsal patterning through the canonical Wnt pathway. Complementary gain-of-function studies demonstrate that Daam2 amplifies Wnt signaling by potentiating ligand activation. Biochemical examination found that Daam2 association with Dvl3 is required for Wnt activity and dorsal patterning. Moreover, Daam2 stabilizes Dvl3/Axin2 binding, resulting in enhanced intracellular assembly of Dvl3/Axin2 complexes. These studies demonstrate that Daam2 modulates the formation of Wnt receptor complexes, revealing new insight into the functional diversity of Daam proteins and how canonical Wnt signaling contributes to pattern formation in the developing spinal cord.

Nuclear factor IA is expressed in astrocytomas and is associated with improved survival

Nuclear factor IA (NFIA) is a transcription factor that specifies glial cell identity and promotes astrocyte differentiation during embryonic development. Its expression and function in gliomas are not known. Here, we examined NFIA protein expression in gliomas and its association with clinical outcome in pediatric malignant astrocytomas. We analyzed expression of NFIA by immunohistochemistry in 88 existing glioma specimens from Childrens Hospital Los Angeles and the University of Southern California. Association between NFIA expression and progression-free survival (PFS) was examined in high-grade astrocytomas for which clinical data were available (n = 23, all children). NFIA was highly expressed in astrocytomas of all grades, but only in a minority of cells in oligodendroglial tumors. NFIA was expressed on a higher percentage of tumor cells in low-grade astrocytomas (91 +/- 5% and 77 +/- 14% in World Health Organization [WHO] I and II, respectively) compared with high-grade astrocytomas (48 +/- 18% and 37 +/- 16% in WHO III and IV, respectively; P < .001, low- vs high-grade astrocytomas). There was a significant association between NFIA expression and PFS in children with astrocytoma WHO grade III or IV (Cox regression P = .019; logrank trend test for NFIA tertiles P = .0040 and NFIA quartiles P = .014). The association was not consistently significant in this small series of patients after adjustment was made for WHO grade III or IV. This is the first study to demonstrate expression of NFIA protein in astrocytomas and its association with grades of astrocytoma and PFS, suggesting that NFIA may play a role in astrocytoma biology.

Identification of positionally distinct astrocyte subtypes whose identities are specified by a homeodomain code

Astrocytes constitute the most abundant cell type in the central nervous system (CNS) and play diverse functional roles, but the ontogenetic origins of this phenotypic diversity are poorly understood. We have investigated whether positional identity, a fundamental organizing principle governing the generation of neuronal subtype diversity, is also relevant to astrocyte diversification. We identified three positionally distinct subtypes of white-matter astrocytes (WMA) in the spinal cord, which can be distinguished by the combinatorial expression of Reelin and Slit1. These astrocyte subtypes derive from progenitor domains expressing the homeodomain transcription factors Pax6 and Nkx6.1, respectively. Loss- and gain-of-function experiments indicate that the positional identity of these astrocyte subtypes is controlled by Pax6 and Nkx6.1 in a combinatorial manner. Thus, positional identity is an organizing principle underlying astrocyte, as well as neuronal, subtype diversification and is controlled by a homeodomain transcriptional code whose elements are reutilized following the specification of neuronal identity earlier in development.

The Transcription Factor NFIA Controls the Onset of Gliogenesis in the Developing Spinal Cord

The mechanisms controlling the transition from neurogenesis to gliogenesis in the vertebrate CNS are incompletely understood. We identified a family of transcription factors, called NFI genes, which are induced throughout the spinal cord ventricular zone (VZ) concomitantly with the induction of GLAST, an early marker of gliogenesis. NFIA is both necessary and sufficient for GLAST induction in the VZ. Unexpectedly, NFIA is also essential for the continued inhibition of neurogenesis in VZ progenitors. This function is mediated by the requirement of NFIA for the expression of HES5, a Notch effector. However, Notch effectors are unable to promote glial-fate specification in the absence of NFIA. Thus, NFIA links the abrogation of neurogenesis to a generic program of gliogenesis, in both astrocyte and oligodendrocyte VZ progenitors. At later stages, NFIA promotes migration and differentiation of astrocyte precursors, a function that is antagonized in oligodendrocyte precursors by Olig2.

Olig2+ neuroepithelial motoneuron progenitors are not multipotent stem cells in vivo

Neurons and glia are thought to arise from multipotent and self-renewing stem cells, which comprise the majority of neuroepithelial cells in the ventricular zone (VZ) of the early embryonic CNS. However, this idea remains to be tested rigorously, because CNS stem cells have been identified only by using in vitro assays, from which their abundance in vivo cannot be directly inferred. In the hematopoietic system, stem cells are characterized by using prospective isolation and direct in vivo transplantation. Here we have used this approach to ask whether most VZ progenitors behave as stem cells in vivo. The best-studied region of the embryonic CNS for addressing this problem is, arguably, the ventral spinal cord, within which progenitors in the motoneuron progenitor (pMN) domain sequentially generate motoneurons (MNs) and oligodendrocyte precursors (OPs). Virtually all VZ cells in pMN express the transcription factor Olig2. If most of these cells are stem cells, then they should maintain neurogenic potential, even at later, gliogenic stages. To test this hypothesis, we have prospectively isolated Olig2(+) cells from murine embryonic day (E)9.5 and E13.5 spinal cord and directly transplanted them to E2 chick spinal cord. Transplanted E9.5 cells generate both neurons, including MNs and OPs, whereas E13.5 cells generate. The observation that most Olig2(+) progenitors do not maintain neurogenic potential into the period of gliogenesis argues that they do not self-renew. These results do not support the commonly held view that most neuroepithelial cells in the embryonic CNS VZ are stem cells in vivo.

Functional analysis of the EWS/ETS target gene uridine phosphorylase

The EWS/ETS fusion proteins associated with Ewings family tumors (EFTs) are thought to promote oncogenesis by acting as aberrant transcription factors. Uridine phosphorylase is a gene that is up-regulated by structurally distinct EWS/ETS fusions. Ectopic expression of uridine phosphorylase was able to support anchorage-independent cell growth, indicating that it plays an active role in the oncogenic process. Transcriptional up-regulation of uridine phosphorylase is shown to be mediated in a DNA binding-dependent manner, and reporter gene assays demonstrated that EWS/FLI1 and RAS mediate activation through a single activator protein 1/ETS site located in the uridine phosphorylase promoter. Chromatin immunoprecipitation assays reveal that EWS/FLI1 directly associates with the uridine phosphorylase promoter in vivo. Up-regulation of uridine phosphorylase by EWS/FLI1 sensitizes cells to growth inhibition by the pyrimidine analogue, 5'-deoxy-5'fluorouridine, both in tissue culture and in vivo model systems.

PIM3 proto-oncogene kinase is a common transcriptional target of divergent EWS/ETS oncoproteins

Despite significant structural diversity, present evidence suggests that EWS/ETS fusion proteins promote oncogenesis by transcriptionally modulating a common set of target genes. In order to identify these genes, microarray expression analyses were performed on NIH 3T3 polyclonal populations expressing one of three EWS/ETS fusion genes. The majority of these genes can be grouped into seven functional categories, including cellular metabolism and signal transduction. The biologic significance of these target genes was pursued. The effects of modulating genes involved in metabolism were assessed by flux studies and demonstrated shifts in glucose utilization and lactate production as a result of EWS/FLI1 expression. The proto-oncogene coding for serine/threonine kinase PIM3 was found to one of several genes encoding signal transduction proteins that were up-regulated by EWS/ETS fusions. PIM3 was found to be expressed in a panel of human Ewing's family tumor cell lines. Forced expression of PIM3 promoted anchorage-independent growth. Coexpression of a kinase-deficient PIM3 mutant attenuated EWS/FLI1-mediated NIH 3T3 tumorigenesis in immunodeficent mice.

DNA binding domain-independent pathways are involved in EWS/FLI1-mediated oncogenesis

Specific chromosomal translocations involving the ews gene and one of five members of the ets family of transcription factors create ews/ets fusion genes that are found in approximately 85% of Ewing's family of tumors. ews/ets fusion genes consistently maintain an intact and functional ets DNA binding domain (DBD) in all of these cases. We demonstrate here, however, that EWS/FLI1, the most prevalent EWS/ETS fusion, activates oncogenic pathways independent of its DBD. In in vivo tumor assays, EWS/FLI1 molecules with either point mutations or a large deletion in the ets DBD retain the ability to accelerate tumors in NIH 3T3 cells, whereas they lose the ability to bind DNA in vitro. Additionally, whereas inhibition of DBD functions of EWS/FLI1 with a dominant negative form of FLI1 is sufficient to inhibit anchorage-independent growth in NIH 3T3 cells, it is ineffective in inhibiting tumor growth in SCID mice. Usage of this dominant negative construct in a Ewing's tumor cell line, however, does reduce the rate of tumor formation, supporting the need for a functional DBD in this context. Together, these results suggest that EWS/FLI1 induces both DBD-dependent and DBD-independent oncogenic pathways.

Loss of p16 pathways stabilizes EWS/FLI1 expression and complements EWS/FLI1 mediated transformation

Ewings sarcoma and primitive neuroectodermal tumors (ES/PNET) are characterized by the fusion of the N-terminus of the EWS gene to the C-terminus of a member of the ETS family of transcription factors. While such fusion proteins are thought to play dominant oncogenic roles, it is unlikely that a single genetic alteration by itself will support cellular transformation. Given that EWS/FLI1 is only able to transform immortalized 3T3 fibroblasts and that 30% of ES/PNET tumors contain a homozygous deletion of the p16 locus, it is likely that other genetic events are required for EWS/FLI1 oncogenesis. Here we describe a complementary mechanism utilized in the establishment ES/PNET tumors. EWS/FLI1 has the capacity to induce apoptosis and growth arrest in normal MEFs. Such effects prevent the establishment of stable expression of the protein in these cells. When expressed in p16, p19(ARF), or p53 deficient MEFs, the apoptotic and growth arrest effects are attenuated, creating a environment permissive for stable expression of the protein. While loss of a single tumor suppressor is sufficient to establish expression of EWS/FLI1, cellular transformation requires further genetic perturbation.

Genetic studies of mei-P26 reveal a link between the processes that control germ cell proliferation in both sexes and those that control meiotic exchange in Drosophila

We present the cloning and characterization of mei-P26, a novel P-element-induced exchange-defective female meiotic mutant in Drosophila melanogaster. Meiotic exchange in females homozygous for mei-P26(1) is reduced in a polar fashion, such that distal chromosomal regions are the most severely affected. Additional alleles generated by duplication of the P element reveal that mei-P26 is also necessary for germline differentiation in both females and males. To further assess the role of mei-P26 in germline differentiation, we tested double mutant combinations of mei-P26 and bag-of-marbles (bam), a gene necessary for the control of germline differentiation and proliferation in both sexes. A null mutation at the bam locus was found to act as a dominant enhancer of mei-P26 in both males and females. Interestingly, meiotic exchange in mei-P26(1); bam(Delta)(86)/+ females is also severely decreased in comparison to mei-P26(1) homozygotes, indicating that bam affects the meiotic phenotype as well. These data suggest that the pathways controlling germline differentiation and meiotic exchange are related and that factors involved in the mitotic divisions of the germline may regulate meiotic recombination.

Identification of novel Drosophila meiotic genes recovered in a P-element screen

The segregation of homologous chromosomes from one another is the essence of meiosis. In many organisms, accurate segregation is ensured by the formation of chiasmata resulting from crossing over. Drosophila melanogaster females use this type of recombination-based system, but they also have mechanisms for segregating achiasmate chromosomes with high fidelity. We describe a P-element mutagenesis and screen in a sensitized genetic background to detect mutations that impair meiotic chromosome pairing, recombination, or segregation. Our screen identified two new recombination-deficient mutations: mei-P22, which fully eliminates meiotic recombination, and mei-P26, which decreases meiotic exchange by 70% in a polar fashion. We also recovered an unusual allele of the ncd gene, whose wild-type product is required for proper structure and function of the meiotic spindle. However, the screen yielded primarily mutants specifically defective in the segregation of achiasmate chromosomes. Although most of these are alleles of previously undescribed genes, five were in the known genes alphaTubulin67C, CycE, push, and Trl. The five mutations in known genes produce novel phenotypes for those genes.