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RESEARCH COMMUNICATION
1 Laboratory of Molecular Biology, Howard Hughes Medical Institute, The Rockefeller University, New York, New York 10021, USA; 2 Developmental Genetics Program and the Department of Cell Biology, The Skirball Institute of Biomolecular Medicine, New York University Medical Center, New York, New York 10016, USA; 3 The Jackson Laboratory, Bar Harbor, Maine 04609, USA
| Abstract |
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[Keywords: Neuronalglial interactions; cortex; cerebellum; Bergmann glia]
Received February 2, 2005; revised version accepted March 25, 2005.
The dual role played by radial glia as both migratory scaffolding and neuronal progenitors has suggested that there may be intimate links between the signaling pathways that control radial glial cell development, neuronal generation, and neuronal migration. The fact that migrating neurons regulate morphological and proliferative characteristics of glia (Hatten 1985
) has led to considerable focus on neuronally derived signals, and several cell surface and diffusible molecules have been implicated in neuronalradial glial signaling, including Astrotactin (Zheng et al. 1996
), Neuregulin/ErbB2 (Anton et al. 1997
; Rio et al. 1997
; Patten et al. 2003
; Schmid et al. 2003
), and Notch (Gaiano et al. 2000
; Gaiano and Fishell 2002
; Patten et al. 2003
; Schmid et al. 2003
). In contrast, the genes expressed within radial glia that mediate the radial glial response to neuronal signaling are not well characterized. One candidate is brain lipid-binding protein (Blbp, Fabp7), a gene that is dynamically regulated in radial glia by migrating neurons (Feng et al. 1994
; Kurtz et al. 1994
; Feng and Heintz 1995
). BLBP is a member of the large family of hydrophobic ligand-binding proteins (FABPs), molecules that have been shown to modulate transcription through their interactions with nuclear receptors and to play roles in metabolism (Haunerland and Spener 2004
). Immunoelectron microscopy has demonstrated that BLBP is present in both the cytoplasm and nucleus in vivo, suggesting that similar to other FABPs, BLBP may be involved in the trafficking of a ligand to a nuclear receptor (Feng et al. 1994
). Consistent with its proposed role in mediating neuronalglial signaling, antibody blocking experiments have shown that BLBP function is required for radial glial morphological changes in response to neuronal cues (Feng et al. 1994
; Anton et al. 1997
) and for regulating the morphology and axonal interactions of Schwann cells (Miller et al. 2003
).
Previous analysis in transgenic mice has shown that migrating neurons induce Blbp transcription in glia through elements lying within the first 766 bp of its promoter (Feng and Heintz 1995
). In order to gain insight into the signaling pathways that induce Blbp, we have fine mapped the 766-bp Blbp promoter in transgenic mice. We report here that a single binding site in the 766-bp promoter for the Notch effector CBF1 is required for all Blbp transcription in the developing brain, and that BLBP expression is dramatically reduced in the forebrains of mice lacking the Notch1 and Notch3 receptors. These results identify Blbp as a direct target of Notch signaling in radial glia, and suggest that BLBP may mediate some aspects of glial responses to Notch.
| Results and Discussion |
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-galactosidase, immunostaining to confirm the specificity of the 766-bp promoter was not included. To do this, we generated transgenic mice expressing enhanced green fluorescent protein (eGFP) under the regulation of the 766-bp promoter and immunostained founder mice at E12 and P6 for BLBP and eGFP. At both time points, eGFP expression was highly correlated with BLBP expression, and high magnification merged images demonstrated that expression was in the same cellular populations (Fig. 1). These results establish that the 766-bp Blbp promoter is sufficient to recapitulate endogenous BLBP expression in transgenic mice.
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We next made a series of smaller deletions between 230 and 400 and assayed them in transgenic mice (Fig. 3). The only deletion that ablated reporter activity in both assay locations was the construct lacking the sequence between 230 and 265 (construct 12). Importantly, we noted that although this region is necessary for transcription both embryonically and post-natally, it is sufficient at neither time. This is evident from the fact that each of the other three deletions in the critical 170 bp caused loss of reporter expression at one of the two time points assayed (Fig. 3, constructs 911). These data suggest that a single signaling pathway may act throughout CNS development to induce transcription of Blbp via elements lying between 230 and 265, and that sequences flanking this domain respond to primarily to regionally and/or temporally restricted factors.
|
(Tun et al. 1994
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The identification of Blbp as a Notch target indicates that the role of Notch signaling in neural progenitors varies as development proceeds, and that the spectrum of downstream target genes change. Previous work has shown that Blbp transcription in the neocortex does not begin until the onset of neurogenesis at E10.5, and that significant levels of BLBP protein are not detectable in this region until E12.5 (Anthony et al. 2004
). In contrast, neocortical expression of Hes5 is detectable as early as E9.5 (Petersen et al. 2004
), and high levels of both Notch1 and Hes5 are present by E10.5 (Yun et al. 2002
). These data demonstrate that distinct developmental stages are accompanied by distinct patterns of Notch target gene expression. These shifts in Notch target gene expression appear to be mediated by additional regulatory factors that interact with the CBF1 coactivator complex; the existence of these additional factors is evident from our findings that the CBF1-binding site within the Blbp promoter is necessary but not sufficient for transcription, and that multiple promoter elements mediate transcription of Blbp at different times and places. POU domain transcription factors have previously been implicated in regulating Blbp transcription in the embryonic forebrain (Josephson et al. 1998
), and thus represent one possible class of CBF1-interacting proteins that function in radial glia. Interestingly, Notch and the POU domain protein Nubbin positively interact to promote gliogenesis in certain Drosophila cell lineages (Umesono et al. 2002
). The fact that constitutively active Notch could promote glial fate in many but not all murine radial glia (Gaiano et al. 2000
) may reflect a dependence of Notch signaling on other factors such as POU domain proteins. Additional candidates include factors downstream of Neuregulin and Reelin; these signaling molecules have previously been shown to induce radial glial expression of BLBP (Anton et al. 1997
; Hartfuss et al. 2003
).
Finally, we note that whereas radial glia serve as neuronal progenitors, Bergmann glia do not. This suggests that in addition to its well-documented role as a cell fate regulator, Notch signaling may also function to support neuronal migration. Previous studies have demonstrated that Notch signaling promotes a radial glial phenotype in the forebrain (Gaiano et al. 2000
; Yoon et al. 2004
), radializes cerebellar astrocytes (Patten et al. 2003
), and induces expression of ErbB2, a receptor implicated in radial glial differentiation (Patten et al. 2003
; Schmid et al. 2003
). As antibody blocking experiments have implicated BLBP in regulating glial morphology (Feng et al. 1994
; Anton et al. 1997
; Miller et al. 2003
), the available data suggest that Notch signaling may induce and/or maintain the radial glial scaffold, and that it does so in part through its induction of BLBP. Further insight into the mechanisms regulating radial glial function will likely be gained by the elucidation of BLBP function as well as the identification of additional radial glial Notch target genes.
| Materials and methods |
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All promoter constructs were made by PCRing from 9TM-0.8 (Feng and Heintz 1995
); this plasmid contains 766 to +53 of the Blbp promoter in front of an intron, the LacZ gene, and a polyA cassette (Fig. 1). PCR was done using Expand High Fidelity PCR system (Roche), and products were cloned back into the pNASS
vector (Clontech) between the EcoRI and XhoI sites. The transfer construct was made by placing two 170-bp PCR fragments (from 230 to 400 of the Blbp promoter) in front of the minimal adeno major late promoter (ML), which extends from 40 to +66 of the adenovirus major late promoter (Miyamoto et al. 1984
); in the absence of additional promoter/enhancer sequences, ML does not express at all in transgenic mice (data not shown). All deletions and mutations were confirmed by DNA sequencing. For generation of transgenic mice, conventional transgene DNA was digested to remove vector sequences, isolated from low melting gels using Qiaquick columns (Qiagen), and further purified using Elutip-d columns (Schleicher and Schuell). Transgenic B6CBA mice were generated using standard protocols (Hogan et al. 1994
). All mice were typed by PCR of DNA from yolk sacs (embryos) or tails (post-natal).
Deletion of Notch1 and Notch3
Floxed Notch1 mice were a gift of Freddy Radtke (Ludwig Institute for Cancer Research, Lausanne Branch, University of Lausanne, Epalinges, Sweden) and were genotyped as previously described (Radtke et al. 1999
). Mutant mouse embryos were obtained by crossing homozygous floxed Notch1 mice with mice heterozygous for floxed Notch1 and Foxg1Cre/+. The generation of Foxg1Cre/+ knockin mice was previously published (Hebert and McConnell 2000
) and were maintained as heterozygotes on a Swiss Webster background. The generation of Notch3 null mutant mice was previously described (Krebs et al. 2003
). Conditional Notch1;Notch3 double mutant mice were obtained by breeding double homozygous floxed Notch1;Notch3 null mutant mice with mice heterozygous for floxed Notch1 and Foxg1Cre/+ on a Notch3 null mutant background.
X-Gal histochemistry
Tissues were immersion fixed in 4% paraformaldehyde for 15 min at room temperature and incubated overnight at 37°C in X-Gal solution: 1 mg/mL X-gal (Sigma), 20 mM K3Fe(CN)6, 20 mM K4Fe(CN)6, 2 mM MgCl2, and 0.02% NP-40 in PBS.
Immunofluorescent staining and quantification
Vibratome or cryostat sections were stained with the following antibodies: rabbit
-BLBP (1:2000) (Feng et al. 1994
), mouse
-nestin/rat-401 (1:4, Developmental Studies Hybridoma Bank), rabbit
-
-galactosidase (1:500, ICN), and goat
-GFP (1:500, US Biological). Secondary antibodies used at 1:500 (Cy2 and Cy3) were from Jackson Immunoresearch. Confocal imaging of wild-type and cNotch1;Notch3 tissues was done on an LSM 510 Axioplan (Zeiss) using identical settings; under the imaging parameters used, pixel saturation was not observed. To quantify the reduction of BLBP expression in cNotch1;Notch3 mice, MetaMorph Imaging System software (Universal Imaging Corporation) was used to determine the intensity of immunfluorescent staining present in individual radial glial processes in the GE. Normalization for possible variability in tissue processing (which could affect staining intensities) was done by comparing BLBP immunostaining in radial glia to that in olfactory ensheathing glial cells (OEG); since Foxg1 is not expressed in these cells, Notch1 expression is not affected. Therefore, OEG should contain essentially equivalent levels of BLBP in both genotypes and serve as an internal control. Measurements obtained (in average intensity/µm2 ± standard deviation) were as follows: wild-type OEG, 89.1 ± 3.3 (n = 22); wild-type GE, 84.2 ± 3.9 (n = 21); cNotch1;Notch3 OEG, 88.9 ± 3.4 (n = 20); and cNotch1;Notch3 GE, 31.6 ± 9.1 (n = 18).
| Acknowledgments |
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| Footnotes |
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E-MAIL heintz{at}rockefeller.edu; FAX (212) 327-7878. ![]()
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