Bone morphogenetic proteins promote astroglial lineage commitment by mammalian subventricular zone progenitor cells

Differentiation of mesencephalic progenitor cells into dopaminergic neurons by cytokines

Rat progenitor cells from the germinal region of the fetal mesencephalon were isolated and expanded in media containing the mitogen epidermal growth factor. These cells remained mitotically active (up to 8 months), were immunoreactive for the progenitor cell marker nestin, and were readily infected with the BAG alpha retrovirus. When incubated in complete media containing serum in poly-L-lysine-coated plates, these cells spontaneously converted to neurons and glia but rarely expressed the dopamine (DA) neuron phenotype. Nineteen different cytokines were screened for their ability to induce the DA phenotype and only interleukin (IL)-1 was found to induce the expression of the DA neuron marker tyrosine hydroxylase (TH). The addition of IL-1, IL-11, leukemia inhibitory factor (LIF), and glial cell line-derived neurotrophic factor (GDNF) were found to further increase the number of TH immunoreactive (TH-ir) cells. The addition of mesencephalic membrane fragments and striatal culture-conditioned media along with the cytokine mixture induced the expression of morphologically mature TH-ir cells that were also immunoreactive for dopa-decarboxylase, the DA transporter, and DA itself. The DA neuron cell counts were approximately 20-25% of the overall cell population and 50% of the neurofilament population. Astrocytes and oligodendrocytes were also present. These data suggest that hematopoietic cytokines participate in the development of the DA neuron phenotype. Parallels between the function of hematopoietic cytokines in bone marrow and the central nervous system may exist and be useful in understanding the factors which regulate the differentiation of neurons in the brain.

Neural progenitors and stem cells: mechanisms of progenitor heterogeneity

Heterogeneity among progenitor cells in the vertebrate nervous system has been documented with increasing frequency over the past few years. It has become clear that differences in progenitor cells help to determine when and how they respond to environmental signals. More recent studies have begun to elucidate the molecular basis of the differences in progenitor cell subpopulations that control their developmental potential and responsiveness to environmental signals.

Interactions between glial progenitors and blood vessels during early postnatal corticogenesis: blood vessel contact represents an early stage of astrocyte differentiation

The post-neurogenic period in the mammalian neocortex is characterized by the growth of astrocyte and oligodendrocyte populations and their incorporation into the network of the developing central nervous system (CNS). Many of these glial cells originate as progenitors in the subventricular zone (SVZ) and then migrate into white and gray matter before differentiating. What determines the specific cellular fate of progenitors in vivo is not known, however. In examining the early stages of gliogenesis from progenitors in the SVZ, we noted that interactions with cortical blood vessels took place at what appeared to be an early stage of glial differentiation. We have examined in more detail the interactions of progenitors with blood vessels in the early postnatal rat neocortex after labeling progenitors in vivo with a LacZ-encoding retrovirus. These early interactions are accompanied by an increase in intermediate filament expression, consistent with astrocytic differentiation. Because astrocytes interact with blood vessels and pia, we suggest that such contact represents an early stage in astrocytic differentiation. Furthermore, since angiogenesis and astrogenesis occur over a similar period, the growth of blood vessels may even play a role in the selection of astrocytic fate by a progenitor. As vessel growth slows, fewer progenitors may be directed toward an astrocyte fate, allowing more to differentiate into oligodendrocytes, perhaps explaining the shift from astrocyte genesis to oligodendrocyte genesis during early postnatal cortical development.

Dorsoventral patterning and oligodendroglial specification in the developing central nervous system

While the bulk of oligodendendrocytes are generated postnatally in rodents, it is now clear that the first oligodendrocytes are born during midembryonic development. Recent studies imply that the first oligodendrocytes to appear are specified concurrently with certain neuronal subtypes. In addition, patterning molecules known to confer positional information on neural tissues during development, such as sonic hedgehog and bone morphogenetic proteins, have also been implicated in the specification of glial fate. This review discusses some of the recent advances in our knowledge of how oligodendrocytes are generated and the mechanisms by which this might occur in the developing brain and spinal cord.

Integration of multiple instructive cues by neural crest stem cells reveals cell-intrinsic biases in relative growth factor responsiveness

Growth factors can influence lineage determination of neural crest stem cells (NCSCs) in an instructive manner, in vitro. Because NCSCs are likely exposed to multiple signals in vivo, these findings raise the question of how stem cells would integrate such combined influences. Bone morphogenetic protein 2 (BMP2) promotes neuronal differentiation and glial growth factor 2 (GGF2) promotes glial differentiation; if NCSCs are exposed to saturating concentrations of both factors, BMP2 appears dominant. By contrast, if the cells are exposed to saturating concentrations of both BMP2 and transforming growth factor beta1 (which promotes smooth muscle differentiation), the two factors appear codominant. Sequential addition experiments indicate that NCSCs require 48-96 hrs in GGF2 before they commit to a glial fate, whereas the cells commit to a smooth muscle fate within 24 hr in transforming growth factor beta1. The delayed response to GGF2 does not reflect a lack of functional receptors; however, because the growth factor induces rapid mitogen-activated protein kinase phosphorylation in naive cells. Furthermore, GGF2 can attenuate induction of the neurogenic transcription factor mammalian achaete-scute homolog 1, by low doses of BMP2. This short-term antineurogenic influence of GGF2 is not sufficient for glial lineage commitment, however. These data imply that NCSCs exhibit cell-intrinsic biases in the timing and relative dosage sensitivity of their responses to instructive factors that influence the outcome of lineage decisions in the presence of multiple factors. The relative delay in glial lineage commitment, moreover, apparently reflects successive short-term and longer-term actions of GGF2. Such a delay may help to explain why glia normally differentiate after neurons, in vivo.

Paracrine regulation of colony-stimulating factor-1 in medulloblastoma: implications for pathogenesis and therapeutic interventions

OBJECTIVE

Colony-stimulating factor (CSF)-1, a chemotactic and mitogenic factor for macrophages and microglia, is expressed in a variety of nervous system tumors and when present in nonneural malignancies, is associated with marked inflammatory infiltrates, dissemination, and poorer prognosis. This study investigated the paracrine effects of CSF-1 production by medulloblastoma cells on the macrophage/microglial lineage.

METHODS

A recurrent metastatic desmoplastic medulloblastoma was isolated from a 26-year-old man and propagated in tissue culture. Cellular phenotype and proliferation were assessed by immunocytochemical techniques; transcript expression for CSF-1, granulocyte macrophage-CSF, interleukin-3, and c-fms (the receptor for CSF-1) was examined with reverse transcriptase-polymerase chain reaction; and conditioned media and coculture paradigms were used to study cytokine effects on cellular proliferation.

RESULTS

Serially passaged cells were uniformly immunoreactive for two lineage-independent neuroepithelial markers, nestin and vimentin. A subpopulation of cells with morphological characteristics of early differentiation stained for neurofilament 66 (7%) and microtubule-associated protein (5%) (markers of early neuronal precursors and postmitotic neurons, respectively) and for the Yp subunit of glutathione-S-transferase (3%) (a marker of early oligodendroglial progenitors). Tumor cells expressed transcripts for CSF-1, but not for granulocyte macrophage-CSF, interleukin-3, or c-fms. Treatment of microglia with serum-free medulloblastoma-conditioned media significantly increased proliferation (P < 0.001), suggesting the secretion of CSF-1. Coculture of medulloblastoma cells and microglia significantly increased proliferation of both cell types (each condition, P < 0.01).

CONCLUSION

These observations suggest that CSF-1 mediates important paracrine interactions between transformed cells and the immune system, resulting in increased growth rate and metastatic potential. Future therapeutic goals need to include immunotherapeutic protocols to modulate this interaction.

Isolation of lineage-restricted neuronal precursors from multipotent neuroepithelial stem cells

We have identified a neuronal-restricted precursor (NRP) cell that expresses E-NCAM (high polysialic-acid NCAM) and is morphologically distinct from multipotent neuroepithelial (NEP) cells (Kalyani et al., 1997) and spinal glial progenitors (Rao and Mayer-Proschel, 1997). NRP cells self renew over multiple passages in the presence of fibroblast growth factor (FGF) and neurotrophin-3 (NT-3) and differentiate in the presence of retinoic acid and the absence of FGF into postmitotic neurons. NRP cells can also be generated from multipotent E10.5 NEP cells. Clonal analysis shows that NRP cells arise from a NEP progenitor that generates other restricted CNS precursors. The NEP-derived NRPs undergo self renewal and can differentiate into multiple neuronal phenotypes. Thus, a direct lineal relationship exists between multipotential NEP cells and more restricted neuronal precursor cells present in vivo at E13.5 in the spinal cord.

Role of GGF/neuregulin signaling in interactions between migrating neurons and radial glia in the developing cerebral cortex

During neuronal migration to the developing cerebral cortex, neurons regulate radial glial cell function and radial glial cells, in turn, support neuronal cell migration and differentiation. To study how migrating neurons and radial glial cells influence each others' function in the developing cerebral cortex, we examined the role of glial growth factor (a soluble form of neuregulin), in neuron-radial glial interactions. Here, we show that GGF is expressed by migrating cortical neurons and promotes their migration along radial glial fibers. Concurrently, GGF also promotes the maintenance and elongation of radial glial cells, which are essential for guiding neuronal migration to the cortex. In the absence of GGF signaling via erbB2 receptors, radial glial development is abnormal. Furthermore, GGF's regulation of radial glial development is mediated in part by brain lipid-binding protein (BLBP), a neuronally induced, radial glial molecule, previously shown to be essential for the establishment and maintenance of radial glial fiber system. The ability of GGF to influence both neuronal migration and radial glial development in a mutually dependent manner suggests that it functions as a mediator of interactions between migrating neurons and radial glial cells in the developing cerebral cortex.

Bone morphogenetic proteins: neurotrophic roles for midbrain dopaminergic neurons and implications of astroglial cells

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor beta (TGF-beta) superfamily that have been implicated in tissue growth and remodelling. Recent evidence suggests that several BMPs are expressed in the developing and adult brain. Specifically, we show that BMP 2 and BMP 6 are expressed in the developing midbrain floor of the rat. We studied potential neurotrophic effects of BMPs on the in vitro survival, transmitter uptake and protection against MPP+ toxicity of mesencephalic dopaminergic neurons cultured from the embryonic midbrain floor at embryonic day (E) 14. At 10 ng/ml and under serum-free conditions, most BMPs promoted the survival of dopaminergic neurons visualized by tyrosine hydroxylase immunocytochemistry during an 8-day culture period, but to varying extents (relative potencies: BMP 6 = 12 > 2, 4, 7). BMPs 6 and 12 were as effective as fibroblast growth factor-2 (FGF-2) and glial cell line-derived neurotrophic factor, promoting survival 1.7-fold compared with controls. BMPs 9 and 11 were not effective. Dose-response curves revealed an EC50 for BMPs 2, 6 and 12 of 2 ng/ml. BMPs 2, 4, 6, 7, 9 and 12 also promoted DNA synthesis and astroglial cell differentiation, visualized by 5-bromodeoxyuridine (BrdU) incorporation and glial fibrillary acidic protein (GFAP) immunocytochemistry respectively. Suppression of cell proliferation and subsequent maturation of GFAP-positive cells by 5-fluorodeoxyuridine or aminoadipic acid abolished the neuron survival-promoting effect of BMP 2. This suggests that BMPs, like other non-TGF-beta factors affecting dopaminergic neuron survival, act indirectly, probably by stimulating the synthesis and/or release of glial-derived trophic factors. BMP 6 and BMP 7 also increased the uptake of [3H]dopamine without affecting the uptake of [3H]5-hydroxytryptamine and [3H]GABA, underscoring the specificity of the trophic effect. We conclude that several BMPs share a neurotrophic capacity for dopaminergic midbrain neurons with other members of the TGF-beta superfamily, but act indirectly, possibly through glial cells.

Response diversity and the timing of progenitor cell maturation are regulated by developmental changes in EGFR expression in the cortex

Early cortical progenitor cells of the ventricular zone (VZ) differ from later progenitor cells of the subventricular zone (SVZ) in cell-type generation and their level of epidermal growth factor receptors (EGFRs). To determine whether differences in their behavior are causally related to EGFR number/density, we introduced extra EGFRs into VZ cells with a retrovirus in vivo and in vitro. This results in premature expression of traits characteristic of late SVZ progenitor cells, including migration patterns, differentiation into astrocytes, and proliferation of multipotential cells to form spheres. The choice between proliferation and differentiation depends on ligand concentration and progenitor cell age and may reflect different thresholds of stimulation. The level of EGFRs expressed by progenitor cells in the cortex may therefore contribute to the timing of their maturation and choice of response to pleiotropic environmental signals.

Bone morphogenetic proteins in the nervous system

Bone morphogenetic proteins (BMPs) are a rapidly expanding subclass of the transforming growth factor superfamily. BMP ligands and receptor subunits are present throughout neural development within discrete regions of the embryonic brain and within neural crest-derived pre- and post-migratory zones. BMPs initially inhibit the formation of neuroectoderm during gastrulation while, within the neural tube, they act as gradient morphogens to promote the differentiation of dorsal cell types and intermediate cell types throughout co-operative signaling. In the peripheral nervous system, BMPs act as instructive signals for neuronal lineage commitment and promote graded stages of neuronal differentiation. By contrast, within the CNS, these same factors promote astroglial lineage elaboration from embryonic subventricular zone progenitor cells, with concurrent suppression of the neuronal or oligodendroglial lineages, or both. In addition, BMPs act on more lineage-restricted embryonic CNS progenitor cells to promote regional neuronal survival and cellular differentiation. Furthermore, these versatile cytokines induce selective apoptosis of discrete rhombencephalic neural crest-associated cellular populations. These observations suggest that the BMPs exhibit a broad range of cellular and context-specific effects during multiple stages of neural development.

Bone morphogenetic proteins induce astroglial differentiation of oligodendroglial-astroglial progenitor cells

We have used bipotent postnatal cortical oligodendroglial-astroglial progenitor cells (O-2As) to examine the role of inductive signals in astroglial lineage commitment. O-2A progenitor cells undergo progressive oligodendroglial differentiation when cultured in serum-free medium, but differentiate into astrocytes in medium supplemented with FBS. We now report that the bone morphogenetic proteins (BMPs), a major subclass of the transforming growth factor beta (TGFbeta) superfamily, promote the selective, dose-dependent differentiation of O-2As into astrocytes with concurrent suppression of oligodendroglial differentiation. This astroglial-inductive action is not sanctioned by other members of the TGFbeta superfamily. Astroglial differentiation requires only very brief initial exposure to the BMPs and is accompanied by increased cellular survival and accelerated exit from cell cycle. Dual-label immunofluorescence microscopy documents that O-2A progenitor cells express a complement of BMP type I and type II receptor subunits required for signal transduction. Furthermore, expression of BMP2 in vivo reaches maximal levels during the period of gliogenesis. These results suggest that the BMPs act as potent inductive factors in postnatal glial lineage commitment that initiate a stable program of astroglial differentiation.

Bone morphogenetic proteins (BMPs) as regulators of dorsal forebrain development

Bone Morphogenetic Proteins (BMPs) play crucial roles in a variety of developmental processes, but their functions during early vertebrate brain development are largely unknown. To investigate this problem, we have compared by in situ hybridization the expression of five Bmp genes belonging to the Drosophila Decapentaplegic (Bmp2 and Bmp4) and 60A subgroups (Bmp5, Bmp6 and Bmp7). Striking co-expression of these Bmps is observed within the dorsomedial telencephalon, coincident with a future site of choroid plexus development. Bmp co-expression overlaps that of Msx1 and Hfh4, and is complementary to that of Bf1. The domain of Bmp co-expression is also associated with limited growth of the neuroectoderm, as revealed by morphological observation, reduced cell proliferation, and increased local programmed cell death. In vitro experiments using explants from the embryonic lateral telencephalic neuroectoderm reveal that exogenous BMP proteins (BMP4 and BMP2) induce expression of Msx1 and inhibit Bf1 expression, a finding consistent with their specific expression patterns in vivo. Moreover, BMP proteins locally inhibit cell proliferation and increase apoptosis in the explants. These results provide evidence that BMPs function during regional morphogenesis of the dorsal telencephalon by regulating specific gene expression, cell proliferation and local cell death.

Stem cells in the central nervous system

In the vertebrate central nervous system, multipotential cells have been identified in vitro and in vivo. Defined mitogens cause the proliferation of multipotential cells in vitro, the magnitude of which is sufficient to account for the number of cells in the brain. Factors that control the differentiation of fetal stem cells to neurons and glia have been defined in vitro, and multipotential cells with similar signaling logic can be cultured from the adult central nervous system. Transplanting cells to new sites emphasizes that neuroepithelial cells have the potential to integrate into many brain regions. These results focus attention on how information in external stimuli is translated into the number and types of differentiated cells in the brain. The development of therapies for the reconstruction of the diseased or injured brain will be guided by our understanding of the origin and stability of cell type in the central nervous system.

Isolation, cloning and characterization of a putative type-1 astrocyte cell line

We have established a permanent cell line (1H91) of putative type-1 astrocyte precursor cells that were clonally derived from a single cell isolated from E16 mouse cerebellum. Epidermal growth factor (EGF) and transforming growth factor (TGF alpha) are strong mitogens for 1H91 cells (ED50 of 9.02 + 1.74 ng/ml and 15.98 +/- 2.34 ng/ml, respectively), while basic fibroblast growth factor (bFGF) is only weakly mitogenic and platelet derived growth factor (PDGF) has no mitogenic activity. In the proliferative state, the 1H91 cells are immunohistochemically positive for nestin and vimentin, and negative for A2B5, CNPase, neurofilament (NF), and neuron specific enolase (NSE). The majority of EGF-treated 1H91 cells are not immunoreactive for glial acid fibrillary protein (GFAP). In the presence of 5 ng/ml bFGF, 1H91 cells become non-mitotic and develop a morphology consistent with a fibrous astrocyte. In contrast to the proliferating cultures, the bFGF treated cultures were strongly immunoreactive for GFAP, only mildly immunoreactive for nestin and vimentin, and negative for A2B5, CNPase, NF, and NSE. Type-1 astrocytes are known to proliferate in response to EGF, and are immunohistochemically GFAP positive, A2B5 negative, and CNPase negative [38]. However, type-1 astrocytes only develop a fibrous morphology during the process of reactive gliosis [31]. Since EGF is a strong mitogen for 1H91 cells, and these cells may be differentiated into GFAP positive, A2B5 negative, CNPase negative astrocytes, we conclude that 1H91 cells conform to a type-1 astrocyte precursor phenotype. In addition, the fibrous morphology of the bFGF treated 1H91 cells suggests that these cells follow the process of reactive gliosis. Therefore, the 1H91 clonal cell line may provide an in vitro model for studying the underlying cellular mechanisms of the type-1 astrocyte in reactive gliosis.

A PDGF-regulated immediate early gene response initiates neuronal differentiation in ventricular zone progenitor cells

When exposed to platelet-derived growth factor (PDGF), uncommitted neuroepithelial cells from the developing cortex of embryonic day 14 (E14) rats develop into neurons. Outward signs of the neuronal phenotype are not observed for 4 days following exposure to PDGF. However, only a brief (2-3 hr) period of PDGF receptor activation is required to initiate neuronal development. During the window of receptor activation, RNA synthesis is essential, but protein synthesis is not. These observations indicate that specification of neuronal fate is mediated by an immediate early gene response.

Neural development: instructions for neural diversity

Extracellular signals that can influence the fate of multipotent progenitor cells have been described in recent studies of the vertebrate nervous system, emphasizing the contribution of instructive mechanisms to the generation of cellular diversity.

FGF2 concentration regulates the generation of neurons and glia from multipotent cortical stem cells

The embryonic cerebral cortex contains a population of stem-like founder cells capable of generating large, mixed clones of neurons and glia in vitro. We report that the default state of early cortical stem cells is neuronal, and that stem cells are heterogeneous in the number of neurons that they generate. In low fibroblast growth factor (FGF2) concentrations, most maintain this specification, generating solely neuronal progeny. Oligodendroglial production within these clones is stimulated by a higher, threshold level of FGF2, and astrocyte production requires additional environmental factors. Because most cortical neurons are born before glia in vivo, these data support a model in which the scheduled production of cortical cells involves an intrinsic neuronal program in the early stem cells and exposure to environmental, glia-inducing signals.