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

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

Enhanced Abventricular Proliferation Compensates Cell Death in the Embryonic Cerebral Cortex.

Loss of neurons in the neocortex is generally thought to result in a final reduction of cerebral volume. Yet, little is known on how the developing cerebral cortex copes with death of early-born neurons. Here, we tackled this issue by taking advantage of a transgenic mouse model in which, from early embryonic stages to mid-corticogenesis, abundant apoptosis is induced in the postmitotic compartment. Unexpectedly, the thickness of the mutant cortical plate at E18.5 was normal, due to an overproduction of upper layer neurons at E14.5. We developed and simulated a mathematical model to investigate theoretically the recovering capacity of the system and found that a minor increase in the probability of proliferative divisions of intermediate progenitors (IPs) is a powerful compensation lever. We confirmed experimentally that mutant mice showed an enhanced number of abventricular progenitors including basal radial glia-like cells and IPs. The latter displayed increased proliferation rate, sustained Pax6 expression and shorter cell cycle duration. Altogether, these results demonstrate the remarkable plasticity of neocortical progenitors to adapt to major embryonic insults via the modulation of abventricular divisions thereby ensuring the production of an appropriate number of neurons.

Cerebellar cortical lamination and foliation require cyclin A2.

The mammalian genome encodes two A-type cyclins, which are considered potentially redundant yet essential regulators of the cell cycle. Here, we tested requirements for cyclin A1 and cyclin A2 function in cerebellar development. Compound conditional loss of cyclin A1/A2 in neural progenitors resulted in severe cerebellar hypoplasia, decreased proliferation of cerebellar granule neuron progenitors (CGNP), and Purkinje (PC) neuron dyslamination. Deletion of cyclin A2 alone showed an identical phenotype, demonstrating that cyclin A1 does not compensate for cyclin A2 loss in neural progenitors. Cyclin A2 loss lead to increased apoptosis at early embryonic time points but not at post-natal time points. In contrast, neural progenitors of the VZ/SVZ did not undergo increased apoptosis, indicating that VZ/SVZ-derived and rhombic lip-derived progenitor cells show differential requirements to cyclin A2. Conditional knockout of cyclin A2 or the SHH proliferative target Nmyc in CGNP also resulted in PC neuron dyslamination. Although cyclin E1 has been reported to compensate for cyclin A2 function in fibroblasts and is upregulated in cyclin A2 null cerebella, cyclin E1 expression was unable to compensate for loss-of cyclin A2 function.

Cooperative interactions of BRAFV600E kinase and CDKN2A locus deficiency in pediatric malignant astrocytoma as a basis for rational therapy.

Although malignant astrocytomas are a leading cause of cancer-related death in children, rational therapeutic strategies are lacking. We previously identified activating mutations of v-raf murine sarcoma viral oncogene homolog B1 (BRAF) (BRAF(T1799A) encoding BRAF(V600E)) in association with homozygous cyclin-dependent kinase inhibitor 2A (CDKN2A, encoding p14ARF and p16Ink4a) deletions in pediatric infiltrative astrocytomas. Here we report that BRAF(V600E) expression in neural progenitors (NPs) is insufficient for tumorigenesis and increases NP cellular differentiation as well as apoptosis. In contrast, astrocytomas are readily generated from NPs with additional Ink4a-Arf deletion. The BRAF(V600E) inhibitor PLX4720 significantly increased survival of mice after intracranial transplant of genetically relevant murine or human astrocytoma cells. Moreover, combination therapy using PLX4720 plus the Cyclin-dependent kinase (CDK) 4/6-specific inhibitor PD0332991 further extended survival relative to either monotherapy. Our findings indicate a rational therapeutic strategy for treating a subset of pediatric astrocytomas with BRAF(V600E) mutation and CDKN2A deficiency.

Role of Fgf8 signalling in the specification of rostral Cajal-Retzius cells.

Cajal-Retzius (CR) cells play a key role in the formation of the cerebral cortex. These pioneer neurons are distributed throughout the cortical marginal zone in distinct graded distributions. Fate mapping and cell lineage tracing studies have recently shown that CR cells arise from restricted domains of the pallial ventricular zone, which are associated with signalling centres involved in the early regionalisation of the telencephalic vesicles. In this study, we identified a subpopulation of CR cells in the rostral telencephalon that expresses Er81, a downstream target of Fgf8 signalling. We investigated the role of the rostral telencephalic patterning centre, which secretes FGF molecules, in the specification of these cells. Using pharmacological inhibitors and genetic inactivation of Fgf8, we showed that production of Fgf8 by the rostral telencephalic signalling centre is required for the specification of the Er81+ CR cell population. Moreover, the analysis of Fgf8 gain-of-function in cultivated mouse embryos and of Emx2 and Gli3 mutant embryos revealed that ectopic Fgf8 signalling promotes the generation of CR cells with a rostral phenotype from the dorsal pallium. These data showed that Fgf8 signalling is both required and sufficient to induce rostral CR cells. Together, our results shed light on the mechanisms specifying rostral CR cells and further emphasise the crucial role of telencephalic signalling centres in the generation of distinct CR cell populations.

A novel role for Dbx1-derived Cajal-Retzius cells in early regionalization of the cerebral cortical neuroepithelium.

Patterning of the cortical neuroepithelium occurs at early stages of embryonic development in response to secreted molecules from signaling centers. These signals have been shown to establish the graded expression of transcription factors in progenitors within the ventricular zone and to control the size and positioning of cortical areas. Cajal-Retzius (CR) cells are among the earliest generated cortical neurons and migrate from the borders of the developing pallium to cover the cortical primordium by E11.5. We show that molecularly distinct CR subtypes distribute in specific combinations in pallial territories at the time of cortical regionalization. By means of genetic ablation experiments in mice, we report that loss of septum Dbx1-derived CR cells in the rostromedial pallium between E10.5 and E11.5 results in the redistribution of CR subtypes. This leads to changes in the expression of transcription factors within the neuroepithelium and in the proliferation properties of medial and dorsal cortical progenitors. Early regionalization defects correlate with shifts in the positioning of cortical areas at postnatal stages in the absence of alterations of gene expression at signaling centers. We show that septum-derived CR neurons express a highly specific repertoire of signaling factors. Our results strongly suggest that these cells, migrating over long distances and positioned in the postmitotic compartment, signal to ventricular zone progenitors and, thus, function as modulators of early cortical patterning.

A wide diversity of cortical GABAergic interneurons derives from the embryonic preoptic area.

GABA-containing (GABAergic) interneurons comprise a very heterogeneous group of cells that are crucial for cortical function. Different classes of interneurons specialize in targeting specific subcellular domains of excitatory pyramidal cells or other interneurons, which provides cortical circuits with an enormous capability for information processing. As in other regions of the CNS, cortical interneuron diversity is thought to emerge from the genetic specification of different groups of progenitor cells within the subpallium. Most cortical interneurons originate from two main regions, the medial and the caudal ganglionic eminences (MGE and CGE, respectively). In addition, it has been shown that progenitors in the embryonic preoptic area (POA) also produce a small population of cortical GABAergic interneurons. Here, we show that the contribution of the POA to the complement of cortical GABAergic interneurons is larger than previously believed. Using genetic fate mapping and in utero transplantation experiments, we demonstrate that Dbx1-expressing progenitor cells in the POA give rise to a small but highly diverse cohort of cortical interneurons, with some neurochemical and electrophysiological characteristics that were previously attributed to MGE- or CGE-derived interneurons. There are, however, some features that seem to distinguish POA-derived interneurons from MGE- or CGE-derived cells, such as their preferential laminar location. These results indicate that the mechanisms controlling the specification of different classes of cortical interneurons might be more complex than previously expected. Together with earlier findings, our results also suggest that the POA generates nearly 10% of the GABAergic interneurons in the cerebral cortex of the mouse.

A novel transient glutamatergic population migrating from the pallial-subpallial boundary contributes to neocortical development.

The generation of a precise number of neural cells and the determination of their laminar fate are tightly controlled processes during development of the cerebral cortex. Using genetic tracing in mice, we have identified a population of glutamatergic neurons generated by Dbx1-expressing progenitors at the pallial-subpallial boundary predominantly at embryonic day 12.5 (E12.5) and subsequent to Cajal-Retzius cells. We show that these neurons migrate tangentially to populate the cortical plate (CP) at all rostrocaudal and mediolateral levels by E14.5. At birth, they homogeneously populate cortical areas and represent <5% of cortical cells. However, they are distributed into neocortical layers according to their birthdates and express the corresponding markers of glutamatergic differentiation (Tbr1, ER81, Cux2, Ctip2). Notably, this population dies massively by apoptosis at the completion of corticogenesis and represents 50% of dying neurons in the postnatal day 0 cortex. Specific genetic ablation of these transient Dbx1-derived CP neurons leads to a 20% decrease in neocortical cell numbers in perinatal animals. Our results show that a previously unidentified transient population of glutamatergic neurons migrates from extraneocortical regions over long distance from their generation site and participates in neocortical radial growth in a non-cell-autonomous manner.

Multiple origins of Cajal-Retzius cells at the borders of the developing pallium.

Cajal-Retzius cells are critical in cortical lamination, but very little is known about their origin and development. The homeodomain transcription factor Dbx1 is expressed in restricted progenitor domains of the developing pallium: the ventral pallium (VP) and the septum. Using genetic tracing and ablation experiments in mice, we show that two subpopulations of Reelin(+) Cajal-Retzius cells are generated from Dbx1-expressing progenitors. VP- and septum-derived Reelin(+) neurons differ in their onset of appearance, migration routes, destination and expression of molecular markers. Together with reported data supporting the generation of Reelin(+) cells in the cortical hem, our results show that Cajal-Retzius cells are generated at least at three focal sites at the borders of the developing pallium and are redistributed by tangential migration. Our data also strongly suggest that distinct Cajal-Retzius subtypes exist and that their presence in different territories of the developing cortex might contribute to region-specific properties.

A Glial Signature and Wnt7 Signaling Regulate Glioma-Vascular Interactions and Tumor Microenvironment.

Gliomas comprise heterogeneous malignant glial and stromal cells. While blood vessel co-option is a potential mechanism to escape anti-angiogenic therapy, the relevance of glial phenotype in this process is unclear. We show that Olig2+ oligodendrocyte precursor-like glioma cells invade by single-cell vessel co-option and preserve the blood-brain barrier (BBB). Conversely, Olig2-negative glioma cells form dense perivascular collections and promote angiogenesis and BBB breakdown, leading to innate immune cell activation. Experimentally, Olig2 promotes Wnt7b expression, a finding that correlates in human glioma profiling. Targeted Wnt7a/7b deletion or pharmacologic Wnt inhibition blocks Olig2+ glioma single-cell vessel co-option and enhances responses to temozolomide. Finally, Olig2 and Wnt7 become upregulated after anti-VEGF treatment in preclinical models and patients. Thus, glial-encoded pathways regulate distinct glioma-vascular microenvironmental interactions.

A Sequentially Priming Phosphorylation Cascade Activates the Gliomagenic Transcription Factor Olig2.

During development of the vertebrate CNS, the basic helix-loop-helix (bHLH) transcription factor Olig2 sustains replication competence of progenitor cells that give rise to neurons and oligodendrocytes. A pathological counterpart of this developmental function is seen in human glioma, wherein Olig2 is required for maintenance of stem-like cells that drive tumor growth. The mitogenic/gliomagenic functions of Olig2 are regulated by phosphorylation of a triple serine motif (S10, S13, and S14) in the amino terminus. Here, we identify a set of three serine/threonine protein kinases (glycogen synthase kinase 3α/ß [GSK3α/ß], casein kinase 2 [CK2], and cyclin-dependent kinases 1/2 [CDK1/2]) that are, collectively, both necessary and sufficient to phosphorylate the triple serine motif. We show that phosphorylation of the motif itself serves as a template to prime phosphorylation of additional serines and creates a highly charged "acid blob" in the amino terminus of Olig2. Finally, we show that small molecule inhibitors of this forward-feeding phosphorylation cascade have potential as glioma therapeutics.

Cyclin A2 promotes DNA repair in the brain during both development and aging.

Various stem cell niches of the brain have differential requirements for Cyclin A2. Cyclin A2 loss results in marked cerebellar dysmorphia, whereas forebrain growth is retarded during early embryonic development yet achieves normal size at birth. To understand the differential requirements of distinct brain regions for Cyclin A2, we utilized neuroanatomical, transgenic mouse, and mathematical modeling techniques to generate testable hypotheses that provide insight into how Cyclin A2 loss results in compensatory forebrain growth during late embryonic development. Using unbiased measurements of the forebrain stem cell niche, we parameterized a mathematical model whereby logistic growth instructs progenitor cells as to the cell-types of their progeny. Our data was consistent with prior findings that progenitors proliferate along an auto-inhibitory growth curve. The growth retardation inCCNA2-null brains corresponded to cell cycle lengthening, imposing a developmental delay. We hypothesized that Cyclin A2 regulates DNA repair and that CCNA2-null progenitors thus experienced lengthened cell cycle. We demonstrate that CCNA2-null progenitors suffer abnormal DNA repair, and implicate Cyclin A2 in double-strand break repair. Cyclin A2's DNA repair functions are conserved among cell lines, neural progenitors, and hippocampal neurons. We further demonstrate that neuronal CCNA2 ablation results in learning and memory deficits in aged mice.

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

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