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Nuclear transfer to eggs or oocytes provides a potential route for cell-replacement therapies because oocytes directly reprogram transplanted mammalian somatic-cell nuclei such that they have an embryo-like pattern of gene expression. This includes a large increase in the mRNA level of the stem-cell marker gene oct4. We have developed a novel procedure to identify new proteins that greatly increase the level of oct4 mRNA upon nuclear transfer. We have isolated Xenopus oocyte proteins that bind to the regulatory region of the mouse oct4 gene and identified these by mass spectrometry. The proteins include the retinoic-acid-receptor gamma, a known repressor of oct4 transcription, and Tpt1, a cancer-associated factor. The depletion of transcripts of retinoic-acid receptor gamma from oocytes increases oct4 and nanog transcription as expected, and depletion of tpt1 transcripts in oocytes reduces oct4 and nanog transcription in injected HeLa nuclei. An elevation of tpt1 transcripts in oocytes results in an earlier activation of oct4 transcription. Therefore, we identify a novel role for tpt1 in activating pluripotency genes upon nuclear transfer. Our results help to elucidate the mechanism by which somatic-cell nuclei are reprogrammed to have an embryo-like pattern of gene expression.
Figure 1. Xenopus laevis Oocyte Proteins Bind Specifically to the Mouse oct4 Promoter(A) Diagram of the 2.3 kb regulatory region of the mouse oct4 gene (not to scale) [30]. The region contains a distal enhancer (DE), a proximal enhancer (PE), and a promoter (P). Translation is initiated at nucleotide +1. Four methyl-sensitive restriction sites are shown (M). Double-stranded DNA oligo fragments were designed to represent fragments of the oct4 regulatory region, as aligned below the diagram.(B) oct4 promoter fragments (oligos) and their corresponding DNA sequences.(C) Specificity of protein binding to oligos was determined by competition experiments, as shown here for the â166 oligo. Oocyte extract was added to P32-labeled oligos in the presence of an increasing amount (12.5â50 μM) of competitor. The addition of the same but unlabeled oligo resulted in a stronger decrease of the gel-shift signal than the addition of an unspecific oligo of the same length that is not represented in the oct4 upstream region. This illustrates specificity for protein-oligo complex formation, which was found in all oligos.(D) Xenopus oocyte proteins are competed out less efficiently with methylated oligos compared to the same oligos that are unmethylated. Competition with increasing concentrations (12.5â50 μM) of unlabeled oligos (unmethylated or methylated) revealed a protein binding preference for unmethylated oligos (only shown for the â166 oligo). The following abbreviation is used: methylated (Met).
Figure 2. Tpt1 Binds to oct4 DNA, and Its mRNA Can Be Downregulated in Xenopus laevis Oocytes(A) The Tpt1 protein binds to the Sf1 oligo. An antibody bound to the Tpt1-Sf1 protein-oligo complex resulted in a supershift, confirming that Tpt1 binds the Sf1 oligo. As a control, the â166 oligo, which did not bind Tpt1 according to the mass spectrometry results, was used, and the antibody did not shift the extract-oligo complex under these conditions.(B) To confirm Tpt1 binding to the mouse oct4 promoter in vivo, we injected oocytes with HA-tagged tpt1-HA mRNA and an oct4 promoter-containing plasmid, pOct4. A ChIP analysis was performed with an HA antibody and verified Tpt1 binding to the oct4 promoter. An H3 antibody was used as a positive control, and the absence of antibodies served as a negative control. Error bars indicate the standard error of the mean (SEM). The following abbreviation is used: antibody (Ab).(C) Short antisense oligo deoxynucleotides (ODNs) were designed to target Xenopus laevis mRNA-encoding proteins identified by mass spectrometry (only Tpt1, RAR, and GFP control shown). Primers used for monitoring mRNA downregulation are indicated, together with the primers used for detecting the Xenopus laevis housekeeping gene ornithine decarboxylase (ODC). The following abbreviations are used: forward (F), reverse (R).(D) We injected 5 or 10 ng of different ODN oligos (Tpt1 ODN1â4) into oocytes to establish which ODNs are the most successful in downregulating tpt1 mRNA. As a negative control, an ODN was used against gfp (ODN GFP). After 24 hr, the level of tpt1 mRNA was established. Injection of 10 ng Tpt1 ODN3 resulted in the most significant decrease of tpt1 mRNA, down to 9% in comparison to the tpt1-mRNA level of uninjected oocytes (100%). Error bars indicate the SEM.(E) We injected 10 ng of different ODNs (anti-RAR ODN1â3) and the negative ODN GFP into oocytes to establish which ODNs downregulate RAR mRNA most successfully. After 24 hr, the level of RAR mRNA was most downregulated by RAR ODN3, down to 12% in comparison to the RAR mRNA level in control oocytes (100%). Error bars indicate the SEM.(F) ODNs against tpt1, RAR, and gfp do not significantly alter the transcriptional level of the Xenopus housekeeping gene ornithine decarboxylase, suggesting specificity for mRNA downregulation. Error bars indicate the SEM.(G) Antisense oligo against Xenopus tpt1 mRNA results in downregulation of endogenous oocyte Tpt1 protein and mouse-tpt1 (Mtpt1)-mRNA injection in Tpt1 overexpression. Extracts from uninjected oocytes (track 1), oocytes injected with HA-tagged Xenopus tpt1 mRNA (XTpt1-HA, track 2), HA-tagged mouse tpt1 mRNA (MTpt1-HA, track 3), and antisense oligos against Xenopus tpt1 (tracks 4 and 5) were run on a protein gel in the presence of Xtpt1-HA (track 4) and Mtpt1-HA mRNA (track 5). Tpt1 was detected with an HA antibody. Mouse and Xenopus tpt1-HA-mRNA-injected oocytes resulted in an increase of Tpt1 protein (tracks 2 and 3) in comparison to the control oocytes (track 1). XTpt1-HA was successfully depleted by antisense oligos against tpt1 (track 4), but MTpt1-HA was not (track 5). This illustrates that the endogenous Tpt1 protein can be successfully downregulated with antisense oligos but that Mtpt1-HA cannot.
Figure 3. A Functional Assay for Proteins Regulating the Level of oct4 mRNA in Xenopus Oocytes, Illustrating That Tpt1 is Required for oct4 Activation upon Nuclear Transfer(A) Assay identifying proteins that regulate oct4 transcription. Antisense ODN oligos injected into living oocytes result in degradation of their target mRNAs. This is associated with a decrease in the level of the corresponding protein. If this protein is involved in regulating oct4 transcription, HeLa nuclei injected into the living oocytes should show a decrease in the oct4 expression level.(B) Tpt1 binds to the oct4 gene in mouse ES cells, as confirmed by ChIP analysis. Mouse ES cells were transfected with a plasmid containing HA-tagged mouse tpt1 or HA-tagged smad2 (negative control). A cytomegalovirus (CMV) promoter ensured expression of the mRNA in the ES cells. Antibodies against H3 (positive control) or HA were used for determining whether Tpt1-HA binds to the mouse Oct4 or Nanog promoter regions. oct4 was enriched when Tpt1-HA was overexpressed and the HA antibody was used, in comparison to when Tpt1 was not overexpressed or when no or a different antibody was used. In contrast, the nanog signal was not significantly increased, suggesting very weak or no binding of Tpt1 to the nanog promoter region.(C) We injected human HeLa nuclei into oocytes and aimed for their germinal vesicles. Human oct4 reactivation was monitored over time by RT-PCR. An increase of oct4 transcripts was observed with time. When the RAR was downregulated with antisense oligos and HeLa nuclei were injected into the living oocytes, the increase in oct4 mRNA level resulted in accelerated transcriptional activation in comparison to the control. The level of human oct4 mRNA was already higher at 8 hr than the mRNA level detected in control oocytes after 24 hr.(D) tpt1 depletion inhibited the increase of oct4 transcription after nuclear transfer. tpt1 mRNA was depleted with antisense oligos. As a result, the oct4 mRNA level did not increase over time as observed in the control samples. The oct4 mRNA level was much reduced in oocytes depleted of tpt1, showing that tpt1 is required for activation of the pluripotency gene oct4.(E) To show that tpt1 mRNA downregulation is specific, we carried out a rescue experiment. Twenty nanograms of mouse tpt1 mRNA could restore and increase human oct4 expression in HeLa nuclei transplated into oocytes that have been depleted of their endogenous (Xenopus) tpt1 mRNA.(F) Overexpression of tpt1 mRNA resulted in accelerated activation of oct4 transcription. We injected 10 ng and 20 ng of tpt1 mRNA into oocytes before injecting HeLa nuclei. After 8 hr, a significant level of oct4 transcripts was detected, and such a level was not reached even after 24 hr in control oocytes.(G) Tpt1 is required for nanog activation. Upon transfer of HeLa nuclei, nanog is activated, and its level increases with time. Depletion of tpt1 resulted in the lack of nanog activation, as also observed with oct4. RAR depletion did not seem to have a significant effect on nanog, in contrast to oct4. Error bars indicate the SEM.
Figure 4. Knockdown of tpt1 mRNA in Mouse ES Cells Is Associated with a Decrease in the Transcriptional Activity of the oct4 Promoter(A) Illustration of a functional assay for Tpt1 in ES cells. Mouse ES cells containing an additional integrated oct4 promoter driving an egfp reporter were transfected with siRNA against tpt1. Upregulation of egfp mRNA indicates that the targeted mRNA is required for oct4 repression. Conversely, egfp-mRNA downregulation shows that the mRNA is responsible for oct4 activation or maintenance.(B) DNA primers and siRNA target sequences for knockdown are shown. DNA primers were designed to detect reduction of the mRNA of choice. The mouse housekeeping gene β-2-microglobulin was used as a control and reference. siRNAs were designed to target specifically gfp, β-galactosidase, and tpt1. siRNA against tpt1 mRNA resulted in decrease down to 17% of its endogenous level (not shown).(C) FACS analysis of the siRNA experiments shows that Tpt1 siRNA causes a decrease in oct4 transcription in mouse ES cells. Mouse ES cells were transfected with siRNAs against β-galactosidase, gfp, and tpt1, and the GFP level was monitored by FACS analysis after 48 hr, as expected. siRNA targeting β-galactosidase, which is usually not present in mouse ES cells, did not reduce the GFP level (negative control). siRNA against gfp resulted in a decrease of GFP, showing that siRNA can downregulate a message and that this can be detected by FACS analysis. siRNA targeting tpt1 gave rise to a decrease in the GFP level, indicating that tpt1 is also an activator of oct4 transcription in mouse ES cells. The corresponding downregulation of the endogenous oct4 mRNA itself was also confirmed by RT-PCR (data not shown). No differentiation was seen, because the siRNA transfection was transient. The effect of the decrease was restored within 3 days, which is not long enough for ES cells to be differentiated. Finally, we found that the RNAi knockdown experiment was very variable, and different batches of ES cells produced different results.
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