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,
Dina Zand,2,
and
Jeffrey A. Golden1,3*
Departments of Pathology,1 Genetics, The Children's Hospital of Philadelphia, Philadelphia, Pennsylvania,2 Department of Pathology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania3
Received 12 June 2007/ Returned for modification 26 July 2007/ Accepted 12 December 2007
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ABSTRACT |
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 Top ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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BMP signaling plays multiple roles in central nervous system development. Among the established functions are dorsal specifications of the neural tube, including the spinal cord and forebrain (1, 9, 10, 14, 15, 26). Interestingly, in the forebrain, BMPs also play a role in ventral development (5). BMP-7 plays inductive functions in ventral forebrain midline cells in early development (5), and BMP-9, expressed only in the ventral telencephalic neural tube, induces and maintains the expression of septal cholinergic-neuron-specific genes (17, 18). Little is known about how BMP signaling is regulated in a fashion that permits the specification of distinct cell types in spatially and temporally restricted patterns during neural development.
We report the identification of a new modulator of BMP signaling. In contrast to other known BMP signaling modulators, Sizn1 (Smad interacting zinc finger protein) is unique in having developmentally regulated spatial and temporal expression patterns. We show that Sizn1 is expressed in the ventral forebrain and functions as a transcriptional coactivator necessary for BMP-dependent, cholinergic-neuron-specific gene expression. These data provide mechanistic insight into how BMP signaling can be regulated in specific cell subpopulations.
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MATERIALS AND METHODS |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Yeast two-hybrid screening.
The full-length Sizn1 coding sequence was PCR amplified and cloned into pGBKT7 (Clontech) to generate the Sizn1 bait. The yeast cell line AH109(MATa), containing the Sizn1 bait construct, was mated with Y187(MAT
) cells pretransformed with an E11 mouse embryo two-hybrid system Matchmaker3 library (Clontech) to screen for interaction partners of Sizn1, according to the manufacturer's instructions.
Northern blot analysis. Northern blotting was performed with total RNAs (10 µg in each lane) taken from the dorsal or ventral forebrains of E11.5-to-E12.5 mice. For adult tissue Northern blotting, we used mouse multiple-tissue Northern blotting (Clontech), ULTRAhyb (Ambion) (according to the manufacturer's instructions), and the following probes: partial 3' untranslated regions of Sizn1, Dlx5 (the coding region), and GAPDH (glyceraldehyde-3-phosphate dehydrogenase) (Clontech) and the coding region from the vesicular acetyl choline transporter (VaChT; provided by Jan Krzysztof Blusztajn) (17).
In situ hybridization. Whole-embryo in situ hybridization was performed as previously described using a full-length Sizn1 clone to generate the riboprobe (Roche) (10, 11, 28). E9.5-to-E14.5 embryos from CD1 mice were used for all experiments.
DNA constructs. pCMV/Sizn1-GFP was generated by subcloning the mouse Sizn1 coding region into pcDNA3-CTGFP (Invitrogen). pMIWIII/Sizn1-myc was generated by subcloning the PCR product containing the Sizn1 coding region into the HindIII and EcoRV sites of pMIWIII/myc as the myc fusion protein. pGST-Sizn1 was constructed by subcloning the Sizn1 PCR product into pGEX-5T (Amersham). pMIWIII/Gal4DB-Sizn1 was constructed by subcloning the PCR products from pGBK/T7-Sizn1, which has a Sizn1 insert at the EcoRI and XhoI sites of pGBKT7 (Clontech). The FLAG-Smad1 and Gal4DB-Smad1 expression constructs and Smad binding element x4-luciferase (SBEx4-luc) construct were obtained from Kohei Miyazono and Takeshi Imamura (JFCR Cancer Institute, Tokyo, Japan). Smad1(2D) and Smad4 fragments were generated by PCR, inserted into pcDNA3.1D-TOPO (Invitrogen), and fused to the V5 tag. The CBPs were obtained from Gerd Blobel (the Children's Hospital of Philadelphia). Choline acetyltransferase (ChAT)-luc deletion mutants were constructed with PCR products containing each fragment of the ChAT promoter as described in the legend to Fig. 4.
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Immunostaining and antibody production. The predicted amino acid sequences of Sizn1 proteins were used to design a peptide for generating antibody against Sizn1 (Fig. 1A). The C terminus (RARARGAPGEPIGLSE) was synthesized and injected into a rabbit after conjugation with keyhole limpet hemocyanin at Genosys, Inc. This antibody was purified with peptide-conjugated beads. Immunohistochemistry was performed on an E14.5, paraffin-embedded mouse brain. Antigen retrieval was accomplished by microwaving sections for 10 min in EDTA solution (Dako). After being blocked, the primary antibody (anti-Sizn1, 1/100, or anti-acetyl cholinesterase [AChE], 1/500) was applied overnight at 4°C. After being washed in phosphate-buffered saline, the appropriate secondary antibodies, conjugated to fluorescein isothiocyanate or Texas Red, were applied. The cells were washed, counterstained with DAPI (4',6-'-diamidino-2-phenylindole), and viewed by indirect epifluorescence.
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The GST pull-down assay was performed with in vitro-translated Smad1 (labeled with [35S]methionine) and GST-Sizn1 protein from Escherichia coli. The glutathione bead coated with GST-Sizn1 or GST protein (each, 2 µg) was incubated in TNE buffer in the presence of in vitro-translated products of Smad1 for 1 h at 4°C. After washing of the beads five times, the eluated samples were loaded and run on SDS-PAGE gels, followed by autoradiography on X-ray film.
Primary septum culture, shRNA knockdown, and qRT-PCR. Dissociated cells (1.5 x 106 cells per well) obtained from E14.5 mouse septum were treated with trypsin for 20 min, plated onto poly-D-lysine-laminin-coated 24-well plates, and incubated with 10% fetal bovine serum-DMEM in the presence of basic fibroblast growth factor (bFGF) (20 ng/ml). Short hairpin RNA (shRNA) expression constructs against Sizn1 were generated by subcloning the annealed oligonucleotide into pSUPER.neo (Oligoengine). The sequences used included the nontargeting negative control (5'-CAGTCGCGTTTGCGACTGG-3'; Dharmacon), target I (5'-GGAGTCTTGGCCCTTTAAT-3'), target II (5'-GGAGGAAAAGGTCAGACGT-3'), and target III (5'-GGAGACTTTTATTAATCCA-3'). A mouse neuron kit (Amaxa Biosystems) was used to introduce shRNA into primary septal neuron cultures. After incubation for 5 days with bFGF (20 ng/ml) with and without BMP-2 (10 ng/ml) and G418 (240 ng/ml), total RNA was extracted with an RNeasy mini kit (Qiagen) and used for real-time quantitative reverse transcription (qRT)-PCR to measure Sizn1, ChAT, and VaChT mRNA levels. The cultures were treated with G418 (400 µg/ml) to eliminate nontransfected cells. Real-time qRT-PCR was performed with Sybr green PCR reagent (Qiagen) and the Abi Prism 7000 sequence detection system. Total RNA was extracted from two independent experiments for each real-time RT-PCR. Each PCR was done in triplicate. All mRNA levels were normalized to the 18S rRNA level. The mRNA expression levels were calculated by relative quantification. The data were statistically analyzed by analysis of variance, followed by paired Student t tests for individual comparison when the analysis of variance showed significance. The PCR primers included Sizn1 forward (5'-TTTGGTGCAGCTGGTTGTAG-3'), Sizn1 reverse (5'-ACACGAAGTTCAAGGCGTTT-3'), ChAT forward (5'-AGGGCAGCCTCTCTGTATGA-3'), ChAT reverse (5'-GAGACGGCGGAAATTAATGA-3'), VaChT forward (5'-ACACGTCTGGCATTGCCAT-3'), and VaChT reverse (5'-CCACTAGGCTTCCAAAGCTGA-3').
ChIP. Since both the Smad1 and Sizn1 antibodies did not have sufficient affinity for the chromatin immunoprecipitation (ChIP) assays, we used cells transiently expressing these proteins for the ChIP assays. SN56 cells were transfected with expression constructs for Smad1 tagged with V5 and Smad4 or myc-Sizn1. After transfection, the cells were cultured for 2 days, harvested, and fixed with 1% formaldehyde for 10 min. ChIP was performed using an Upstate ChIP kit (Upstate, Inc.), according to the manufacturer's protocols, and anti-V5 (Invitrogen), anti-myc (4A12; Upstate, Inc.), and antihemagglutinin (as a negative control; Roche) antibodies. ChIP DNA was dissolved in 20 µl Tris-EDTA buffer and used as a template for the PCR (1 µl/reaction). Amplification was performed at 95°C for 20 s, 62°C for 30 s, and 68°C for 30 s for 27 cycles or 35 cycles (we tested the amplicons at the higher cycle number [data not shown]). For the two-step ChIP, anti-V5-immunoprecipitated complex was eluted with V5 peptide (100 µg/ml) and then anti-myc antibody (4A12), developed for ChIP, was applied. Southern blot analysis was performed for Sizn1 or the two-step ChIP because of a weak signal after common ethidium bromide staining in the agarose gel. The primer sets used were as follows: from approximately –2534 to –2275, 5'-GGGGCGGTGGGGAGGCAATGTTC-3' and 5'-GCCGCCCCGACTGACTGACTG-3'; from approximately –2275 to –2015, 5'-CGGGGCGGCTGCTGGGATCT-3' and 5'-CCTGGGTCCGCGGGCTCTTCA-3'; and from approximately –375 to 1, 5'-GGGAGTGTGCAACCAGCCCAGCT-3' and 5'-GCTGCCGACCTGGGAACAGAGGA-3'.
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RESULTS |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Sizn1 functions as a transcriptional coactivator in the BMP signaling pathway. We next sought to identify Sizn1-interacting molecules. We first examined whether Sizn1 binds to nucleic acids, given its nuclear localization. However, the direct interaction of Sizn1 with DNA or RNA was not detected by nucleic acid binding studies (data not shown). The absence of nucleic acid binding suggested that Sizn1 may function by interacting with other nuclear proteins. To identify Sizn1-interacting proteins, 5 x 106 clones from an E11 mouse library (GAL4 DB Matchmaker library; Clontech) were screened by using Sizn1 as a bait in a yeast two-hybrid system. Most of the positive clones were nuclear proteins, including members of the Smad family (data not shown). To confirm the yeast two-hybrid system result, we directly tested the interaction between Sizn1 and Smad1. Using a GST pull-down assay with Sizn1 expressed in E. coli and in vitro-translated Smad1, we detected the direct binding of Sizn1 to Smad1 (Fig. 2A; see also the supplemental material). This interaction between Sizn1 and Smad1 was further confirmed by coimmunoprecipitation of Sizn1 with Smad1 in HEK293T cells transiently expressing myc-Sizn1 and FLAG-Smad1 (Fig. 2B). In addition, we tested the interaction of endogenous Sizn1 and Smad1 in mouse septal cell line SN56. In the presence of BMP-2, Sizn1 protein was detected in anti-Smad1 immunoprecipitates (Fig. 2C). To verify the functional interaction between Sizn1 and Smad1 in vivo, Gal4DB-Smad1 was generated and cotransfected with the Gal4x5-luc reporter construct with or without myc-Sizn1 in C2C12 cells. Sizn1 enhanced Gal4DB-responsive Gal4x5-luc reporter gene expression, indicating that it interacts with Smad1 in vivo and enhances Smad1-mediated transcription (Fig. 2D). However, Sizn1 is not able to activate the Gal4x5-luc reporter gene without Gal4DB-Smad1 (see the supplemental material).
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Transcriptional coactivation occurs through multiprotein complexes that dock on transcription factors and recruit chromatin-modifying enzymes, such as CBP/p300 or SWI/SNF family members. BMP-responsive transcriptional activation requires R-Smad-dependent recruitment of CBP (21). Thus, we hypothesized that Sizn1 may function in BMP signaling by associating with CBP in the transcription complex. To test this hypothesis, we cotransfected the CBP expression construct with SBEx4-luc and BMPR1a(QD) into C2C12 cells. Although CBP alone cannot activate the SBEx4-luc expression induced by BMPR1a(QD), the Sizn1-dependent activation of BMP signaling was further enhanced by the presence of CBP (Fig. 3A). Based on these data, we propose that the Sizn1-dependent activation of BMP signaling functions through the association of CBP with the Smad transcription complex and, thus, that Sizn1 is a transcriptional coactivator. To test for a possible interaction between Sizn1 and CBP, we transfected HEK293 cells with Gal4x5-luc and Gal4DB-Sizn1 or Gal4DB, with or without the CBP expression construct. Gal4DB-Sizn1 alone shows slight Gal4x5-luc induction, suggesting that it has inherent transcription coactivator activity (Fig. 3B). In addition, together with CBP, it can exert strong induction of the reporter gene, indicating that Sizn1 can associate CBP with the transcription complex. Consistent with this result, Sizn1 can be coimmunoprecipitated with CBP in C2C12 cells transfected with Sizn1 (Fig. 3C). Thus, at least one function of Sizn1 in the cell is to associate CBP with the transcription complex and, in so doing, to potentiate BMP signaling.
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5-kb sequences upstream of the ChAT start codon as a putative ChAT promoter (17). BMP-2 treatment of C2C12 cells (that do not express Sizn1 [data not shown]) transfected with ChAT-luc exhibited a modest up-regulation of luciferase activity, and this induction was enhanced by the coexpression of Sizn1, indicating that BMP signaling can induce ChAT promoter activity and that Sizn1 can enhance this induction (Fig. 4A; see also the supplemental material). Given the role of Smad1 as a transcriptional activator in BMP signaling, we predict that ChAT promoter activation by BMP-2 is mediated through Smad1 binding to the ChAT promoter. To identify possible Smad binding sites in the ChAT promoter, we generated serial promoter deletions and tested for their induction ability in SN56 cells, immortalized mouse forebrain septal cells (17) that express Sizn1. In this assay, Smad4, required to bind Smad1 for cotranslocation to the nucleus, and the constitutively active mutant Smad1(2D) were used together to induce reporter gene expression with the promoters with serial deletions (22). Deletion of the region from approximately –2534 to –2275 resulted in the loss of Smad1(2D)-induced transcription (Fig. 4B). Based on sequence homology, four other candidate Smad binding sites are present in the region from approximately –4875 to –2875. However, mutations in these sites showed no change in transcription activity (data not shown). These data indicate that a functional Smad binding site exists in the region from approximately –2534 to –2275 of the ChAT promoter.
To confirm a direct interaction between Smad1 and the DNA fragment from approximately –2534 to –2275, a ChIP assay was performed with SN56 cells. V5-tagged Smad1(2D) and FLAG-tagged Smad4 were cotransfected into SN56 cells. The candidate region in the ChAT promoter was interrogated by PCR with anti-V5 ChIP DNA (Fig. 4C). The PCR amplified the fragment from approximately –2534 to –2275 (Fig. 4E), indicating that Smad1 can occupy this site in the ChAT promoter. Based on our model for Sizn1 function, we predict that Sizn1 up-regulates the transcription of the ChAT gene through its interaction with Smad proteins. To test this, we expressed myc-Sizn1 and V5-Smad1(2D)/FLAG-Smad4 in SN56 cells and performed a modified ChIP assay by pulling DNA down with myc antibody (Sizn1). As shown in Fig. 4D, PCR amplified the fragment from approximately –2534 to –2275 from Sizn1-ChIP DNA pools. Since Sizn1 does not directly bind DNA, this result may be due to Sizn1 occupying DNA through its interaction with Smad1. To prove this, we performed a two-step ChIP, sequentially pulling down Smad1 and Sizn1. As expected, the two-step ChIP also resulted in the amplification of the fragment from approximately –2534 to –2275 (Fig. 4E), providing evidence that Sizn1 exists in a protein-DNA complex through its association with Smad1.
Sizn1 is necessary for BMP-dependent, basal-forebrain cholinergic-neuron-specific gene expression. Coimmunostaining of Sizn1 with AChE in an E13.5 mouse brain section reveals that Sizn1 is expressed in the basal-forebrain cholinergic neurons (BFCN) (Fig. 5A). Given that Sizn1 modulates BMP signaling and is expressed in BFCN and that BMP signaling contributes to BFCN development (17, 18), we hypothesized that Sizn1 would play a role in BFCN-specific gene expression. To test this hypothesis, we generated shRNA expression constructs against three target sites in Sizn1. The knockdown efficiency of shRNAs was first tested in SN56 cells. We found that all target shRNAs efficiently inhibited the expression of Sizn1, although target I showed the weakest inhibition (Fig. 5B). Target II shRNA was selected for knocking down primary septum neurons. We next assayed mRNA levels for Sizn1, ChAT, and VaChT, which are preferentially expressed in cholinergic neurons, by real-time qRT-PCR of E13.5 primary septal cell cultures transfected with the target shRNA expression constructs (Fig. 5C; see also the supplemental material). Sizn1 expression in these cultures showed a slight induction by BMP-2. The cultures transfected with the shRNA to Sizn1 showed a repression of Sizn1 expression (Fig. 5C). ChAT and VaChT were up-regulated by BMP, and both transcripts were repressed in the presence of shRNA to Sizn1 (Fig. 5C), indicating that Sizn1 is necessary for the normal BMP-dependent induction of ChAT and VaChT in the cholinergic neurons of the basal forebrain.
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DISCUSSION |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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TGF-β family members, including BMPs and activin, play multiple roles in development, differentiation, and tumorigenesis through signaling (13, 20, 27). Given that TGF-β family members and their receptors show broad temporal and spatial expression patterns during development, the expression patterns of ligands and receptors alone are not sufficient to explain the distinct roles played by TGF-β family members in restricted cell populations. Thus, additional modulators are required to control TGF-β signaling levels. The regulation of pathway activation is found at many levels, including ligand availability, receptor binding and activation, the translocation of the activated Smad complex to the nucleus, and the recruitment of transcription factors and transcriptional coregulators (coactivators or corepressors). Sizn1 can now be added to the list of known transcriptional coregulators. Unlike most ubiquitously expressed transcriptional coregulators that have been reported so far, Sizn1 exhibits restricted spatial and temporal expression patterns in the brain (Msg1 also shows some restricted expression) (24), which can permit further refinement of BMP signaling. Cholinergic-neuron-specific expression of Sizn1 in the septal nucleus is a good example of this postulate. Once its own expression is induced by BMP signaling, Sizn1 can participate in the activation of other genes necessary for inducing and/or maintaining the cholinergic-neuron phenotype.
Although our studies have focused on Sizn1 expression in septal cholinergic neurons, our data also show that Sizn1 is expressed in the developing thalamus, amygdala, putamen (Fig. 1D and E), and brain stem (data not shown) as well as in the septum. Preliminary data indicate that its expression is not restricted to cholinergic neurons in other brain regions. Definition of these areas of expression, the cell types, and the function of Sizn1 in these cells is under active investigation. Outside the nervous system, we know that Sizn1 is also expressed in the testis (Fig. 1C). TGF-β signaling, including through BMP-7, BMP-4, BMP-8a, and BMP-8b, plays multiple roles in spermatogenesis and maintaining epididymal integrity (12). It is reasonable to consider that Sizn1 may also have some role in these processes.
Given the fact that Sizn1 is expressed in the forebrain and hindbrain regions, where cholinergic neurons do not reside, we can postulate that Sizn1 likely has roles in the development/maintenance of other types of neurons. It is also possible that Sizn1 may be involved in other signaling pathways, not only in the BMP pathway. We have some evidence supporting the idea that Sizn1 can modulate Wnt signaling but not Shh signaling (unpublished data). In this manner, Sizn1, along with other transcription factors and transcriptional coregulators, contributes to generating the complexity of neuronal subtypes in the mammalian nervous system. Theoretically, this may also account for the lack of a Sizn1 ortholog, as well as many other transcriptional coactivators/corepressors, in invertebrates, which have simpler nervous systems and fewer neuronal subtypes. It should be noted that Sizn1 is also expressed in the adult brain (Fig. 1C). We speculate that it may have a role in maintaining cholinergic-neuron-specific gene expression. Further studies are obviously required to define the expression of Sizn1 in the adult nervous system and to characterize any functions that it has in the mature brain.
BFCNs project to the cortex and limbic system, where they function in processes such as attention, learning, and memory (7). Previous studies have shown that BMP-9 contributes to the maturation of BFCNs by regulating multiple genes involved in the induction and maintenance of the cholinergic phenotype (17, 18). Our data support these findings and provide the mechanism of how BMP signaling can directly modulate the cholinergic phenotype. In summary, Sizn1 is a novel transcriptional coactivator in the BMP-signaling pathway, functioning by interacting with Smad1 and CBP. It contributes to the BMP-induced enhancement of cholinergic-neuron-specific gene expression, including that of ChAT and VaChT. Given the requirement of ventral-forebrain cholinergic neurons in cognition, future studies with animal models will provide further insight into the potential role of Sizn1 in memory and other related cognitive functions.
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ACKNOWLEDGMENTS |
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This work was supported by Public Health Service grant HD26979 (J.A.G.) from the National Institutes of Health.
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FOOTNOTES |
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Published ahead of print on 26 December 2007.
Supplemental material for this article may be found at http://mcb.asm.org/.
Present address: Department of Pathology and Laboratory Medicine, University of Pennsylvania School of Medicine, 36th and Spruce Streets, Philadelphia, PA 19104-4283.
Present address: Division of Genetics and Metabolism, Children's National Medical Center, George Washington University Medical Center, 111 Michigan Avenue, NW, Washington, DC 20010-2970.
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