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J. Biol. Chem., Vol. 281, Issue 22, 15073-15081, June 2, 2006
Transcription Factors, cAMP-responsive Element Modulator (CREM) and Tisp40, Act in Concert in Postmeiotic Transcriptional Regulation*
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| ABSTRACT |
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and Tisp40
. Both of these proteins function by binding to UPRE (unfolded protein-response element) but do not recognize CRE motifs. We show here that Tisp40
mRNA is generated under the direct transcriptional regulation of CREM. CREM
and Tisp40 form a heterodimer, which functions through CRE but not through UPRE. Furthermore, binding ability of CREM to CRE is dramatically up-regulated by forming a heterodimer with Tisp40
TM, a truncated form of Tisp40
that lacks the transmembrane domain. We confirmed that Tisp40 and CREM actually bind to the Tisp40 promoter in vivo by chromatin immunoprecipitation assay. Finally, we demonstrate that the Tisp40
TM-CREM
heterodimer acts as a recruiter of HIRA, a histone chaperone, to CRE. Taken together, we propose that Tisp40 is an important transcriptional regulator during spermiogenesis. | INTRODUCTION |
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It is known that the bZip-type transcription factor CREM acts as a master regulator of spermatogenesis (4). CREM generates many alternatively spliced forms. One of these is called ICER (inducible cAMP early repressor), which is an inactive form of CREM that suppresses CRE-mediated gene expression during meiosis. In post-meiotic cells, however, CREM generates active forms called CREM
, -
1, and -
2, which up-regulate many CRE-mediated genes (4, 5). CREM-null mice display male infertility: while the spermatogonia differentiate normally into round spermatids, all the elongating spermatids are eliminated by apoptosis during spermiogenesis, which results in the lack of late spermatids (68). Therefore, it is thought that CREM is an essential factor for spermiogenesis.
Some spermiogenic genes, such as the genes for transition proteins (Tnp)1, Tnp2, protamine (Prm)1, Prm2, and angiotensin-converting enzyme (ACE), have been identified as the direct transcriptional targets of CREM (6, 7). However, CREM probably regulates the expression of many other spermiogenic genes because defects in the Tnp1, Tnp-2, Prm1, Prm2, and ACE genes alone cannot fully explain the male infertility of CREM-null mice (6, 7). For example, mice whose Tnp1 and Tnp2 genes are both knocked out can still produce epididymal sperm, unlike CREM-null mice (6, 7, 9). Thus, identification of other novel target genes of CREM would shed light on the mechanisms regulating spermiogenesis.
CREB (cAMP-response element-binding protein)-binding protein (CBP) acts as a co-activator of many transcription factor through its histone acetyl transferase activity, which is recruited by transcription factors on promoter regions and acetylates histones (10). In case of CREB, recruitment of CBP to the promoter region by CREB is dependent on its phosphorylation (11). Once recruited, CBP acetylates histones to facilitate new transcription. In contrast, CREM is not phosphorylated in male germ cells. Consequently, instead of CBP, ACT (activator of CREM in testis) is believed to function as the trans-activator of CREM in male germ cells (7, 12, 13). Prior to chromatin condensation, ACT is exported by Kif17b, a testis-specific kinesin, independent of its motor domain, and this export is inhibited by leptomycin B (14, 15). Notably, while ACT-null mice display sperm abnormality, they can nevertheless produce offspring (16). This observation reveals that the trans-activation of CREM is retained in ACT-null mice, which indicates that another, as yet unknown, factor can also act as an activator of CREM, probably by complementing ACT function.
Although CREM is an essential factor for spermiogenesis, precisely how CREM regulates the transcription of haploid-specific genes remains unknown. Here we report that a novel regulator, Tisp40, acts as a recruiter of chromatin modifier to CRE by forming heterodimer with CREM during mouse spermiogenesis.
| EXPERIMENTAL PROCEDURES |
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TM, pCMV-6Myc-CREM
(wt and S117A), pCDNA3-CREM
(wt and S117A), and several reporter plasmids, we generated cDNA fragments containing the appropriate open reading frames and the promoter regions by PCR. The lack of artificial point mutations from PCR was confirmed by DNA sequencing of the generated plasmid DNA. p5XATF6 (UPRE)-GL3 is a kind gift from Prof. K. Mori (Kyoto University).
Northern Blot Analysis, in Vitro Pulldown Assay, and Western Blot AnalysisPreparation of RNA from the testes of CREM(+/+) and CREM(/) mice and Northern blot analysis were performed as described previously (3). TISP cDNA inserts cut out of the plasmids by EcoRI/NotI restriction enzymes were radiolabeled by [32P]dCTP with the random primer DNA labeling kit (TaKaRa, Japan) and used as hybridization probes. The Northern blots were probed with radiolabeled glyceraldehyde-3-phosphate dehydrogenase cDNA as a loading control. GST-Tisp40
TM was expressed in Escherichia coli and purified on glutathione-Sepharose 4B. HA-CREM
was expressed by using the TNT expression system (Promega). We mixed GST-Tisp40
TM with HA-CREM
on glutathione-Sepharose 4B and conducted Western blot analysis after extensive washing. HA-CREM
expressed in TNT lysates was used as a positive control whereas GST alone served as a negative control. Western blot analysis was performed as described previously (17).
Cell Culture, Transient Transfection, Luciferase Assay, Co-immunoprecipitation Assay, and EMSAThe culture of HeLa S3 cells, their transient transfection, the luciferase assay and EMSA (electrophoresis mobility shift assay) have been described (17). For the co-immunoprecipitation assay, transfected HeLa cell proteins were extracted in binding buffer (25 mM Hepes-KOH, pH 7.8, 150 mM KCL, 5 mM MgCl2, 2.5 mM CaCl2, 1 mM DTT, 0.05% Nonidet P-40) supplemented with protease inhibitor mixture (Sigma) and 1,000 units micrococcal nuclease for 1 h at 4°C. The lysates were centrifuged by 17,400 x g for 15 min at 4 °C. The supernatant was then incubated with protein G-Sepharose with rotation for 1 h at 4°C and precipitated by centrifugation. The supernatant was subsequently incubated with anti-Myc antibody (PL14;MBL), anti-CREM antibody (Santa Cruz Biotechnology, Santa Cruz, CA), or control IgG with rotation overnight at 4 °C. Protein G-Sepharose was then added to this mixture, incubated with rotation for 1 h at 4°C, and precipitated by centrifugation. After extensive washing, the precipitated proteins were subjected to Western blot analysis using anti-Tisp40 antibody or anti-CREM antibody as the probes. To detect the endogenous interaction between CREM and Tisp40, we used two testes from adult mice and 1 µg of anti-CREM antibody, anti-Tisp40 antibody, or rabbit IgG.
In Vivo Electroporation AssayReporter DNA fragments containing the Tisp40 upstream region, the luciferase gene, and the SV40 poly(A) signal were amplified from the recombinant luciferase constructs by PCR with the following oligonucleotide pair: RV Primer3, 5'-CTAGCAAAATAGGCTGTCCC-3' and RV Primer4, 5'-GACGATAGTCATGCCCCGCG-3'. The control fragment was also amplified from the RL plasmid using the oligonucleotide pair: RL5, 5'-TTTTGCTCACATGGCTCGAC-3' and RL3, 5'-CGCACATTTCCCCGAAAAGT-3'. The in vivo electroporation assay was performed exactly as described (18).
ChIP AssayGerm cells from two testes were fixed in 500 µl of 1% formaldehyde/phosphate-buffered saline() for 5 min at 4 °C. The fixed germ cells were sonicated (Output 7, ON 30 s OFF 60 s, 5 cycles at 4 °C, using handy sonic PR-20, TOMY, Tokyo) after phosphate-buffered saline() washed two times. 200 µl of sonicated chromatin-protein complexes were diluted in 1.8 ml of ChIP dilution buffer (0.01% SDS, 1.1% Triton X-100, 1.2 mM EDTA, and 16.7 mM Tris-HCl, pH 8.0). Diluted chromatin-protein complexes were immune-precipitated by antibody against CREM, Tisp40, or HIRA (mixed WC15 and WC19), respectively, and antibody against rabbit or mouse IgG (negative control) at 4 °C overnight after blocking with 800 µg of single strand DNA. The immune complexes were precipitated with protein G-Sepharose followed by washing with low salt buffer (0.1% SDS, 1% Triton X-100, 2 mM EDTA, 20 mM Tris-HCl, pH 8, and 150 mM NaCl), high salt buffer (0.1% SDS, 1% Triton X-100, 2 mM EDTA, 20 mM Tris-HCl, pH 8.0, and 500 mM NaCl), LiCl buffer (0.25 M LiCl, 1% Np40, 1% deoxycholate, 1 mM EDTA, and 10 mM Tris-HCl, pH 8.0), and TE (10 mM Tris-HCl, pH 8.0 and 1 mM EDTA). The immune-purified complexes of chromatin proteins were eluted in 50 ml of elution buffer (1% SDS, 0.1 M NaHCO3, and 10 mM DTT) twice, and 4 µl of 5 M NaCl were added to eluted mixtures and incubated for 4 h at 65 °C. Then, 0.2 µlof0.5 M EDTA, 4 µl of 1 M Tris-HCl, pH 6.8, and 0.2 µl of 20 mg/ml proteinase K were added and incubated for 1 h at 45°C. These chromatin DNA were purified by PheOH/CHCl3 extractions and EtOH precipitations. Purified chromatin DNA was used as PCR templates. PCR was conducted for 35 cycles at 94 °C for 30 s, 52 °C for 30 s and 72 °C for 1 min (3 min in the final cycle).
Purification of Protein Complexes by Pulldown AssaysGerm cells were suspended in sucrose buffer (0.25 M sucrose, 20 mM Tris-HCl, pH 8.0, and 170 mM KCl) and precipitated by centrifugation at 600 x g for 10 min. The nuclear pellets were resuspended in binding buffer supplemented with protease inhibitor mixture (Sigma), 0.5 mM NaF, 1 mM Na3VO4, and 2,000 units of micrococcal nuclease and rotated for 1 h at 4 °C. These mixtures were centrifuged at 17,400 x g for 15 min at 4 °C, and this supernatant enriched with chromatin-binding proteins was purified using P11 column chromatography for 1 h at 4°C. The mixture was eluted in 5 ml of P11 elution buffer (25 mM HEPES-KOH pH7.6, 1 M KCl, 1 mM DTT, 2.5 mM CaCl2, 0.01% Nonidet P-40, and 10% glycerol) from the P11 column by rinsing with binding buffer. Eluted extracts, which contain many large complexes, were dialyzed in 400 ml of P11 binding buffer (25 mM Hepes-KOH pH7.6, 150 mM KCl, 1 mM DTT, 2.5 mM CaCl2,5mM MgCl2, 0.05% Nonidet P-40, 0.05 mg/ml bovine serum albumin, and 10% glycerol) for 3 h at 4°C. Eluted extracts from P11 column were mixed with the GST-fused trans-regulational regions of Tisp40
/
or CREM
with glutathione-Sepharose overnight at 4 °C after being blocked by glutathione-Sepharose fused to GST alone. These pull-downed complexes were separated by SDS-PAGE, followed by silver staining after extensive rinsing.
| RESULTS |
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mRNA Is Directly Transcribed by CREM
through the CRE2 SiteWe previously showed that TISP40 generates two proteins named Tisp40
and Tisp40
, and that transcription of the mRNAs for these proteins occurs after step 5, which is later than the transcription of the CREM gene (17). Because both of these mRNAs, which will be referred to hereafter as Tisp40
/
mRNAs, were not observed in CREM-null mice, we speculated that the transcription of the Tisp40
/
mRNAs is under the direct control of CREM. To test this assumption, we performed promoter assays. Compared with Tisp40
mRNA, Tisp40
mRNA contains an extra 5'-region that adds 55 amino acids that may act as an acidic trans-activational domain (17). We found two CRE motifs between the transcriptional start sites of Tisp40
and Tisp40
. We named the upstream CRE site, CRE1, and the downstream CRE site, CRE2 (Fig. 2A).
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fusion protein bound to both the CRE1 and CRE2 sites (Fig. 2B, lanes 1, 2, 5, and 6), but not to mutated CRE1 and mutated CRE2 sites (lanes 3, 4, 7, and 8). To confirm that the HA-CREM
-shifted CRE2 band actually contains HA-CREM
, we performed a supershift assay with anti-HA antibody (3F10), where the antibody was allowed to associate with HA-CREM
before the radiolabeled CRE2 probe was added to the reaction mixture. This confirmed that the HA-CREM
-shifted band indeed contained HA-CREM
(Fig. 2C, lane 6). However, we could not detect HA-CREM
bound to the CRE1 site by the supershift assay, probably because the interaction of HA-CREM
with the CRE1 site is very weak (lane 3). Successful competition using 4-, 10-, or 20-fold molar excesses of CRE1 or CRE2 oligonucleotides also confirmed CREM
associates specifically with wild-type CRE1 and CRE2 (Fig. 2D, lanes 15 and 1115). In contrast, competition assays using mutated CRE1 and CRE2 site oligonucleotides did not inhibit the binding between HA-CREM
and CRE very efficiently (Fig. 2D, lanes 610 and 1620). Competition assays to compare the ability of CREM
to bind to CRE1 (wt) and CRE2 (wt) showed that CRE2 (wt) efficiently reduced the binding activity of CREM
to both CRE1 and CRE2, whereas CRE1 (wt) only reduced the binding of CREM
to CRE1 (Fig. 2E). These results indicate that CREM
prefers to bind to the CRE2 site rather than to the CRE1 site (Fig. 2, BE).
We next determined the putative cis-element required for the expression of Tisp40
/
mRNA in vivo. To do so, we performed the in vivo electroporation assay that permits transient expression of reporter plasmids in the testes of mice; this assay was previously used to identify the cis-element in haploid spermatid-specific genes (18). We thus constructed reporter plasmids that harbored a deletion or a point mutation in the Tisp40
/
promoter (Fig. 3A), transferred them into the seminiferous tubules of the murine testes by electroporation and measured the luciferase activity. The results were normalized with full-length Tisp40 promoter-driven Renilla luciferase activity. As shown in Fig. 3A, the relative reporter expression of the 5' deletion mutants 2071
+151, 971
+151, 311
+151, 220
+151, and 101
+151 (lines 1, 2, 4, 5, and 6, respectively) was not significantly reduced when compared with the activity of the full-length Tisp40 promoter. In contrast, the relative reporter expression of the 2071
971 3' deletion mutant (line 3) and the 50
+151 5' deletion mutant (line 7) was reduced to about 70% of the expression driven by the full-length Tisp40 promoter. These data indicate that the region 101
-50 of the Tisp40
promoter region is important for the expression of Tisp40
. This region contains the CRE2 site but not the CRE1 site. Consequently, we then prepared Tisp40
reporter plasmids bearing mutations in the CRE1 and/or CRE2 sites and transferred them into the testes by electroporation. We found that the reporter expression of the
CRE1 promoter was not reduced (line 8), but the reporter expression of the
CRE2 single mutant (line 9) and the
CRE1/2 double mutant (line 10) were reduced to similar levels as the 2071
-971 3' deletion mutant (line 3) or 50
+151 5' deletion mutant (line 7). These data are consistent with the results obtained by EMSA (Fig. 2) and indicate that the CRE2 site is essential for Tisp40
mRNA expression.
Finally, to confirm that CREM
actually up-regulates Tisp40
mRNA, we performed luciferase assays by cotransfecting HeLa cells with a reporter plasmid containing the Tisp40
/
promoter upstream of firefly luciferase cDNA with or without the 6Myc-CREM
-expressing vector. The results were normalized by thymidine kinase promoter-driven Renilla luciferase activity. We found that 6Myc-CREM
-activated transcription through the Tisp40
/
and Tisp40
promoters but not through the Tisp40
promoter (Fig. 3B). From these data, we conclude that Tisp40
is a direct transcriptional target of CREM in vivo.
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TM Associates with CREM
Both in Vitro and in VivoBecause both the Tisp40
and Tisp40
proteins (denoted from here on in as Tisp40
/
proteins) are expressed in the spermatid, we reasoned that Tisp40
/
proteins may form a complex with CREM
. To test this, we performed in vitro pulldown assays using GST-Tisp40
TM and HA-CREM
. We first examined the quality of GST-Tisp40
TM and GST alone by Western blot analysis with anti-GST antibody (Fig. 4A, lanes 2 and 3). Although GST-Tisp40
TM was slightly degraded (indicated by *), full-length GST-Tisp40
TM was also detected (indicated by #) at levels sufficient for pulldown assays. Thus, we mixed the purified GST-Tisp40
TM (or GST alone) with HA-CREM
, which had been expressed by using the in vitro transcription-translation system (TNT). We then conducted Western blot analysis after extensively washing the mixture to remove possible nonspecifically associating proteins. We found that GST-Tisp40
TM associated with HA-CREM
in vitro (lane 6), unlike GST alone (lane 5).
Next, we examined the association between the Tisp40
/
and CREM
proteins in cultured cells by performing co-immunoprecipitation assays after transfecting HeLa cells with pCMV-6Myc-Tisp40
/
TM along with pCDNA3-CREM
or empty vector. We first confirmed the transfection was successful by detecting the bands for 6Myc-Tisp40
/
TM and CREM
in the whole cell extract (WCE) with anti-Myc monoclonal antibody (PL14) and anti-CREM polyclonal antibody, respectively (Fig. 4B, upper two panels). That anti-Myc antibody successfully immunoprecipitated 6Myc-Tisp40
/
TM (lanes 36, second panel from the bottom) was also confirmed by detecting the relevant bands in the Western blot. When the WCE was immunoprecipitated by anti-Myc antibody and probed with anti-CREM antibody, a band for CREM
(indicated by an arrow) was detected only when Tisp40
TM (lane 5, bottom panel) or Tisp40
TM (lane 6, bottom panel) was produced together with CREM
. This indicates that CREM associates with the Tisp40 proteins in HeLa cells. We could not perform reciprocal co-immunoprecipitation experiments, because the anti-CREM antibody nonspecifically recognized 6Myc-Tisp40
/
TM proteins when they were immunoprecipitated with anti-CREM antibody (arrowheads in bottom panel and data not shown).
Finally, we confirmed the physiological association of Tisp40 with CREM in the mouse testis by performing a co-immunoprecipitation assay using whole testis cell extract. As shown in Fig. 4C, CREM proteins were precipitated with anti-CREM antibody but not with control IgG (Fig. 4C, upper panel, lanes 13). When precipitated by anti-CREM antibody, Tisp40 protein was co-immunoprecipitated (Fig. 4C, lower panel, lanes 13). Here, the reciprocal co-immunoprecipitation with anti-Tisp40 antibody was confirmed (lane 4). These data suggest that Tisp40
TM and CREM
form a heterodimer in the mouse testis.
CREM
Inhibits the Trans-activation of Tisp40
through UPREIt has been shown that Tisp40
TM binds to UPRE but not to CRE (17). We thus investigated whether the Tisp40
TM-CREM heterodimer functions through UPRE or CRE. When we cotransfected HeLa cells with the pGL35XUPRE reporter plasmid (kind gift from Prof. K. Mori) together with the CREM
-expressing vector and/or the Tisp40
TM-expressing vector and measured the luciferase activity, we found that Tisp40
TM activated transcription through UPRE but its activity was suppressed by CREM
(wt) (Fig. 5, lanes 13). Furthermore, CREM
(S117A), which is a trans-inactive mutant, also suppressed the trans-activity of Tisp40
TM (lane 4). These data suggest that the CREM-Tisp40
TM heterodimer does not function through UPRE and that the ability of CREM
to inhibit the trans-activity of Tisp40
TM is not because of its transcriptional activity.
The Binding Ability of CREM
with CRE Is Increased by Tisp40
TMMany CREB/CREM family proteins regulate transcription by binding to the CRE motif. However, Tisp40 binds to UPRE but not CRE and regulates transcription through UPRE (17). Because the trans-activation of Tisp40
TM through UPRE was inhibited by CREM
as shown above (Fig. 5), we speculated that the Tisp40
TM-CREM
heterodimer may function as a trans-activator through CRE. To test this, we cotransfected HeLa cells with the pGL35XCRE construct derived from the somatostatin (SOM) gene promoter together with or without vectors expressing CREM
, Tisp40
TM, or Tisp40
TM. Luciferase assays revealed that Tisp40
TM greatly enhanced the trans-activation of CREM (Fig. 6A, lanes 1, 5, 6, and 7). Furthermore, Tisp40
TM, a trans-inactive subtype, also enhanced the trans-activation of CREM
in a dose-dependent manner (lanes 14).
The CRE promoter in the pGL35XCRE construct consists of artificial five tandem promoters. To exclude the possibility of artificial transactivation because of this artificial construct, we examined if Tisp40 enhanced the trans-activation of CRE through the Tisp40
/
TM promoter region that contains the natural CRE sites. As we expected, the trans-activation activity of CREM
cotransfected with Tisp40
/
TM was much greater when the 2071
+151 Tisp40
/
or 971
+151 Tisp40
promoter were co-expressed than when the 2071
971 Tisp40
promoter was co-expressed (Fig. 6B).
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/
to CREM does not require the phosphorylation of the Ser117 residue of CREM. To test this notion, we investigated whether CREM
(S117A) can also be activated by Tisp40
/
TM. Luciferase assays revealed that Tisp40
/
TM activated CREM
(S117A), albeit with slightly lower efficiency (520% lower than that of wild-type CREM
) (Fig. 6C).
The above observations indicate that the Tisp40-mediated enhancement of CREM
trans-activity required the part of the Tisp40
promoter region that contains the CRE2 site. As shown in Fig. 6, A and B, the inactive subtype Tisp40
also enhanced the trans-activation of CREM. We speculated that this activation might be caused by the enhanced interaction of CREM with the CRE site. To test this, we performed EMSA with purified GST-Tisp40
TM and in vitro expressed HA-CREM
. HA-CREM
bound to the CRE site of SOM and the CRE2 site of Tisp40 (Fig. 6D, lanes 2, 6 and Fig. 2), whereas GST-Tisp40
TM could bind to neither CRE site (lanes 3 and 7). However, the interaction of HA-CREM
with both CRE sites was greatly enhanced (lane 4) and the band was slightly shifted (lane 8) when GST-Tisp40
TM was present. These data clearly demonstrate that the heterodimer consisting of CREM
and Tisp40
TM functions through CRE but not through UPRE (Figs. 5 and 6, AD).
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Bind to the CRE Motif in VivoNext, we performed chromatin immunoprecipitation assays to confirm that Tisp40 and CREM actually bind to the TISP40 promoter in vivo. For this purpose, we used the Tisp40 promoter region (220
+151), which contains both CRE1 and CRE2. We found that the TISP40 promoter region was actually co-immunoprecipitated with anti-CREM and anti-Tisp40 antibodies (Fig. 6E). The result suggests that Tisp40
/
TM cooperates with CREM to bind to the CRE at the midstage of mouse spermiogenesis.
Tisp40
TM-CREM Heterodimer Functions as a Recruiter of Histone Chaperone HIRA to CRE MotifRecently, Adham et al. reported that the levels of certain CREM target genes, such as Prms and Tnps, are not reduced in Creb3l4/TISP40-deficient mice that display male fertility (32). We independently generated TISP40-deficient mice and obtained similar results (data not shown). However, we additionally found the reduced levels of histone removal from testis chromatin in TISP40-deficient mice.3 Thus, we think that Tisp40 acts as a recruiter of a chromatin modifier that facilitates histone removal, such as histone (de)acetylase, methylase, or histone chaperone. Indeed, we purified complexes associated with the trans-regulational region of Tisp40 or CREM by performing pulldown assays (Fig. 7, A and B). Then, Tisp40 or CREM-associated proteins were analyzed by nano LC-ESI MS/MS analysis after separated by SDS-PAGE, and several proteins were successfully identified (Fig. 7B and supplemental Table 3). Among them, we focused on HIRA because it is a replication independent H3.3-specific histone chaperone (33). We found that HIRA was not only a partner of the CREM complex but also a component of the present in the Tisp40 complex (Fig. 7C). Interestingly, the intensity of the HIRA band in Tisp40
(lane 2) complex is significantly weaker than those of Tisp40
and CREM complexes (lanes 1 and 3). Next, we tried to demonstrate an endogenous interaction between Tisp40 or CREM and HIRA by performing co-immunoprecipitation assays. Although we could not detect any interactions between HIRA and CREM or Tisp40, we found that HIRA interacted less efficiently with CRE in TISP40(/) testis (Fig. 7D).
Finally, luciferase assay revealed that Tisp40
TM-CREM
-dependent transcriptional activity through CRE is activated by HIRA, but not by HIRA alone (Fig. 7E, lanes 13 and 5). In this assay, to emphasize the effects of Tisp40 and HIRA, expression of CREM
was reduced below the detection level of transcriptional activity induced by CREM
alone through CRE. Consistent with the results obtained by pulldown assays (Fig. 7, C and E), Tisp40
TM-CREM
-dependent transcriptional activity was not enhanced by addition of HIRA (lanes 4 and 6). As described above, HIRA is the histone H3.3-specific chaperone and H3.3 was deposited in the transcriptional active region (34). While their report support HIRA stimulation of transcription (Fig. 7E), it was reported that HIRA could repress the transcription of histones (35). We think HIRA actually represses the transcription of histones, and CRE-mediated transcription stimulated by HIRA seems to be caused by overexpression. Because, HIRA was not recruited to CRE (Fig. 7D) and histone removal was delayed in TISP40(/) testis,3 we speculated that real role of HIRA on CRE is histone removal during midspermiogenesis. From these data, we conclude that Tisp40 and CREM play a pivotal role in the recruitment of HIRA to the CRE in testis.
| DISCUSSION |
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), TISP51 (a kinase anchoring protein 110kDa), TISP52 (AKAP3/4), and TISP61 (actin-like-7
). TISP50 (Shippo1) is localized in both the elongated spermatid and the sperm tail and seems to act as a supporter of sperm conformation (19), while AKAP110 regulates PKA activity and sperm motility (20, 21) and AKAP3/4 is essential for sperm tail conformation (22, 23). Other novel TISP targets of CREM are actin family genes such as TISP19 (actin-like-7
), and TISP61 (actin-like-7
), which are known to be important in maintaining the cytoskeleton. Moreover, TISP69 (sperizin) is a RING finger protein that may function as an E3 ubiquitin ligase to promote the proteasome-mediated degradation of spermatid proteins (24, 25). Indeed, many proteins are poly- or mono-ubiquitinated during spermiogenesis (26). Our discovery that these TISP genes are in the CREM transcriptional pathway explains why male CREM-null mice are infertile: the absence of CREM would lead to poor expression of the cytoskeleton components, supporters of sperm conformation, signal transducers and regulators of proteasome-mediated proteolysis, which would cause abnormal sperm maturation. We believe that further characterization of the roles these TISP genes play in the CREM pathway may unveil the detailed molecular mechanism of spermiogenesis.
It is known that many bZip proteins form not only homodimers but also heterodimers or tetramers. For instance, c-Fos and c-Jun form a heterodimer (AP-1), whereas ATF6
or ATF6
forms a heterotetramer with NF-Y A, B, and C (27, 28). Similarly, it is thought that CREM forms a heterodimer with CREB and ATF1. However, unlike CREM, neither CREB nor ATF1 are expressed in spermatids. Thus, it was thought that CREM functions only as a homodimer in spermiogenesis. In the present study, however, we showed that CREM complexes with Tisp40, a new membrane-bound bZip-type transcription factor that is expressed in postmeiotic male germ cells.
Regulated intramembrane proteolysis (Rip) is a widely conserved process found in species ranging from bacteria to humans (29). SREBPs and ATF6 are well known transcription factors that are activated via the Rip pathway. We previously reported that Tisp40 (17), like other transcription factors such as OASIS (30) and CREBH (31), normally localizes in the endoplasmic reticulum and is activated via the Rip pathway. When cells are exposed to various types of stress, such as sterol depletion or the endoplasmic reticulum stress, these proteins are translocalized into the Golgi apparatus. Subsequently, they are released from the membrane by two step proteolytic cleavage involving first site 1 protease (S1P) and then site 2 protease (S2P). The cleaved portion then enters the nucleus, where it acts as an activated transcription factor that regulates the transcription of its targets gene(s). We previously showed that Tisp40 is subjected to two step cleavage by S1P and S2P, after which it enters the nucleus (17). We show here that the nuclear forms of the Tisp40 proteins (Tisp40
/
TM) associate with CREM
not only in vitro but also in vivo (Fig. 4). Moreover, the trans-activity of Tisp40
through UPRE is inhibited by CREM
(Fig. 5) whereas in contrast, Tisp40
/
TM enhanced the trans-activation of CREM through CRE in cultured cells independent of phosphorylation of CREM
at Ser117 (Fig. 6, AC). This enhancement is caused by an increase in the binding ability of CREM to CRE by Tisp40 (Fig. 6D).
Considering many TISP genes are not expressed in the testis of CREM-null mice (Fig. 1), whereas in ACT-null mice, the expression of these spermatid-specific genes is almost normal (16), we first hypothesized that the CREM-Tisp40 heterodimer functions as a highly active transcription factor in haploid spermatids. However, Adham et al. (32) and our group3 independently found that some of CREM-regulated genes are normally expressed in TISP40/CREB3l4-deficient mice. Moreover, we found that histones were abnormally retained in later stage germ cells of TISP40-deficient testis.3 Thus, we think that Tisp40 acts as a recruiter of histone modifier to CRE site in cooperation with CREM. We here identified HIRA as one of the association partners of Tisp40 and CREM by pulldown and nano LC-ESI MS/MS (Fig. 7). Because HIRA is a histone H3.3 chaperone, we speculated that histone H3 was included in the Tips40 and CREM complexes (33). Actually, histone H3 was included in the Tisp40 complex. Unexpectedly, however, histone H3 was not included in CREM complex (Fig. 7C).
It remains unclear where in chromatin regions H3.3 is deposited during spermatogenesis in mammalian cells. However, in Drosophila, H3.3 is properly deposited in chromatin during first meiotic prophase cells and elongating spermatids cells, and HSP70 genes rapidly lose histone H3 and acquire variant H3.3 histones (36, 37). From these data, H3.3 is thought to be linked to not only transcriptional active regions but also histone replacement, and then deposited and removed by HIRA (38).
Although it remains unclear whether all histones, which are abnormally retained in TISP40(/) testis,3 are directly regulated by HIRA surrounding the CRE, these data imply that both Tisp40 complex and CREM complex include HIRA but Tisp40 complex alone can function as a histone regulator. Considering that binding of Tisp40 with CRE is required for forming a heterodimer with CREM (Fig. 6, AC) that recruitment of HIRA to CRE is Tisp40-dependent (Fig. 7D), that histone H3 (presumably H3.3) is included only in Tisp40 complex, and that histone removal is delayed in TISP40(/) mice,3 we here propose that Tisp40 forms a heterodimer with CREM to bind to CRE site for recruitment of HIRA surrounding the CRE site to deposit and/or remove H3.3, and thus facilitate histone exchange during midspermiogenesis.
| FOOTNOTES |
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The on-line version of this article (available at http://www.jbc.org) contains supplemental Tables. ![]()
1 To whom correspondence should be addressed: Dept. of Molecular Genetics, Research Institute for Microbial Disease, Osaka University, Yamadaoka 3-1, Suita City, Osaka 565-0871, Japan. Tel.: 81-6-6875-3980; Fax: 81-6-6875-5192; E-mail: snj-0212{at}biken.osaka-u.ac.jp.
2 The abbreviations used are: TISP, transcript induced in spermiogenesis; CREM, cyclic AMP-response element modulator; CREB, cyclic AMP-response element-binding protein; CBP, CREB-binding protein; EMSA, electrophoresis mobility shift assay; DTT, dithiothreitol; HA, hemagglutinin; Rip, regulated intramembrane proteolysis; Tnp, transition proteins; Prm, protamine; UPR, unfolded protein response; UPRE, unfolded protein-response element; bZip, basic leucine zipper; ChIP, chromatin immunoprecipitation assay; wt, wild type. ![]()
3 I. Nagamori, K. Yomogida, I. Masahito, M. Okabe, N. Yabuta, and H. Nojima, unpublished data. ![]()
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