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Molecular and Cellular Biology, February 2006, p. 789-809, Vol. 26, No. 3
0270-7306/06/$08.00+0     doi:10.1128/MCB.26.3.789-809.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Conditional Knockout Mice Reveal Distinct Functions for the Global Transcriptional Coactivators CBP and p300 in T-Cell Development

Lawryn H. Kasper,1 Tomofusa Fukuyama,1 Michelle A. Biesen,1 Fayçal Boussouar,1 Caili Tong,3 Antoine de Pauw,2 Peter J. Murray,2 Jan M. A. van Deursen,3 and Paul K. Brindle1*

Department of Biochemistry,1 Department of Infectious Diseases, St. Jude Children's Research Hospital, 332 N. Lauderdale, Memphis, Tennessee 38105,2 Department of Pediatric and Adolescent Medicine and Department of Biochemistry and Molecular Biology, Mayo Clinic, 200 First Street SW, Rochester, Minnesota 559053

Received 10 July 2005/ Returned for modification 8 August 2005/ Accepted 9 November 2005


   ABSTRACT
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The global transcriptional coactivators CREB-binding protein (CBP) and the closely related p300 interact with over 312 proteins, making them among the most heavily connected hubs in the known mammalian protein-protein interactome. It is largely uncertain, however, if these interactions are important in specific cell lineages of adult animals, as homozygous null mutations in either CBP or p300 result in early embryonic lethality in mice. Here we describe a Cre/LoxP conditional p300 null allele (p300flox) that allows for the temporal and tissue-specific inactivation of p300. We used mice carrying p300flox and a CBP conditional knockout allele (CBPflox) in conjunction with an Lck-Cre transgene to delete CBP and p300 starting at the CD4– CD8– double-negative thymocyte stage of T-cell development. Loss of either p300 or CBP led to a decrease in CD4+ CD8+ double-positive thymocytes, but an increase in the percentage of CD8+ single-positive thymocytes seen in CBP mutant mice was not observed in p300 mutants. T cells completely lacking both CBP and p300 did not develop normally and were nonexistent or very rare in the periphery, however. T cells lacking CBP or p300 had reduced tumor necrosis factor alpha gene expression in response to phorbol ester and ionophore, while signal-responsive gene expression in CBP- or p300-deficient macrophages was largely intact. Thus, CBP and p300 each supply a surprising degree of redundant coactivation capacity in T cells and macrophages, although each gene has also unique properties in thymocyte development.


   INTRODUCTION
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
CREB-binding protein (CBP) (Crebbp) and p300 (Ep300) are ubiquitously expressed ~2,400-residue nuclear phosphoproteins that share 61% overall sequence identity (~80 to 90% in evolutionarily conserved domains). Homologs of CBP and p300 are found in Drosophila melanogaster and Caenorhabditis elegans, but there are no other closely related family members in mammals. These two coactivators typically bind to the activation domains of transcription effectors via five unique protein-binding domains (nuclear hormone receptor binding domain, CH1, KIX, CH3, and SID/IBiD) (64, 92). CBP and p300 appear to function redundantly in many instances, but they also have unique properties, particularly in vivo (92). The lack of other closely related family members or conservation of these protein-binding domains in other mammalian proteins implies that CBP and p300 are indispensable.

Following recruitment by transcription factors, juxtaposition of the adaptor, acetyltransferase, and ubiquitin ligase functions of CBP and p300 at the promoter/enhancer is hypothesized to stimulate target gene transcription (92). Histones are believed to be the main acetylation targets of CBP and p300, thereby alleviating repressive chromatin and potentiating transcription, but the ability of CBP and p300 to regulate transcription factor activity by acetylation of specific lysine residues is also thought to be crucial (27, 176, 184). The importance of CBP and p300 for the transcription of endogenous genes, however, has not been widely investigated.

An astonishingly large fraction of the estimated 1,962 (human) transcriptional regulators (134), as well as other proteins, have been shown to interact physically and functionally with CBP and p300 (described interactions now total at least 312) (Fig. 1; more detailed and up to date information is available at http://www.stjude.org/brindle). This number of interactions puts CBP and p300 at the top end of known "connectivity" in the interactome, as high-throughput screens to identify Drosophila, C. elegans, and mammalian interactome networks indicate that only a very small fraction of proteins have as many as 100 to 200 interacting partners (8, 60, 118). It may even be inferred that CBP and p300 interact with the great majority of transcriptional regulators, as most have not been tested. More than half (>190) of the described CBP/p300 interactors are encoded by essential genes in mice (Fig. 1), thus the common assumption that loss of CBP or p300 will have catastrophic effects on specific cell lineages because of broad alterations in gene expression appears reasonable. Also implicit from the high level of connectivity for CBP and p300 is that cells cannot exist in the absence of both coactivators, although it has not been possible to test this rigorously because a conditional knockout allele for p300 has not been available.


Figure 1
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  		FIG. 1. The CBP and p300 interactome: 312 viral and mammalian proteins that interact physically or functionally with CBP or p300 in vitro. One hundred ninety-six proteins that are encoded by essential genes in mice (i.e., mutation results in a phenotype) are indicated in boldface type, including 56 genes that are essential for T cells (indicated by an asterisk). The source for phenotype information is Mouse Genome Informatics (http://www.informatics.jax.org/). For more detailed and up to date information on the interactome, see http://www.stjude.org/brindle. References are indicated in parentheses.

 
It is generally thought that CBP and p300 together are present in limiting amounts in cells, consistent with the view that they are widely involved in transcription (64, 115, 203). Mice that are homozygous for CBP and p300 null alleles, or doubly heterozygous, die during embryogenesis, providing strong evidence for this notion (190, 223). Mice and humans that are haploinsufficient for CBP have developmental defects, also consistent with models that invoke limiting levels of CBP (203). The fact that CBP–/– and p300–/– mice survive until about embryonic day 9 (E9), however, shows that many cell types can develop and survive with only CBP or p300.

The roles of CBP and p300 in blood cells are of considerable interest because they are thought to be vital for the function of many hematopoietic transcription factors; at least 65 factors that interact with CBP or p300 are encoded by genes that are essential for T and B cells in mice (Fig. 1). Moreover, chromosomal translocations involving CBP or p300 produce gain-of-function fusion proteins in some types of human leukemia (18) (Fig. 1). Both CBP and p300 are necessary for normal definitive hematopoiesis, including lymphopoiesis, in chimeric mice created with CBP–/– or p300–/– embryonic stem cells (164). CBP can also act as a tumor suppressor in hematopoietic cells, including T cells (93), tissue macrophages (histiocytes), and possibly the B-cell lineage (plasmacytomas developed in mice transplanted with CBP heterozygous cells) (109). There is less evidence for p300 acting as a tumor suppressor in hematopoietic cells, although histiocytic sarcoma has been reported in p300–/– chimeric mice (164). Still, peripheral blood T-cell numbers appear to be mostly unaffected in CBP and p300 heterozygous mice (109), even though CBP and p300 have been implicated by a number of studies as being important for the activity of 56 T-cell-critical transcriptional regulators (Fig. 1). In this regard, conditional knockout of CBP in multiple cell lineages that include thymocytes leads to abnormally high levels of CD8+ single-positive (SP) thymocytes (93). In addition, mice that are homozygous for mutations in the CREB- and c-Myb-binding domain (KIX) of p300 have thymic hypoplasia in the context of multilineage hematopoietic defects (95). It is uncertain, however, if CBP and p300 function equivalently in T-cell development. To test the hypothesis that CBP and p300 are each critically limiting in specific cell lineages in vivo, we developed mice with conditional knockout alleles for each gene. We then examined the requirement for CBP and p300 in thymocyte development and their roles in T-cell and macrophage gene expression.


   MATERIALS AND METHODS
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Generation of p300flox mice. The p300flox allele was constructed by flanking exon 9 of the mouse Ep300 gene with LoxP sites. DNA for the targeted region was obtained from an E14 Ola mouse embryonic stem (ES) cell genomic DNA phage library. A LoxP site was inserted in the intron 3' of exon 9 using a Tn7 transposon-based system (GPS mutagenesis system, catalog no. 7101; New England Biolabs) with a modified mini-Tn7 vector (pGPS3-PM30-R1-70-Neo-CAT-TK no. 2; details available upon request) that allowed random insertion of a LoxP site and a selection cassette containing genes for neomycin resistance (Neo), chloramphenicol resistance, and thymidine kinase (TK) flanked by Frt sites (16, 135). A second LoxP site was inserted in the intron 5' of exon 9 using the Tn7 system and another modified mini-Tn7 vector (pGPS3-CAT-R1-70-LoxP no. 2; details available upon request) (16). In addition, a diphtheria toxin A cassette placed at the 3' end of the targeting construct provided negative selection to reduce the number of nonhomologous targeting events. Mouse ES cells were electroporated with linearized targeting construct and cultured with G418 to positively select for clones that had integrated the targeting construct. Clones were screened by PCR across the 3' end of the targeted region, and positive clones were confirmed by Southern blotting using a StuI digest with a 5' external probe as well as an Asp718/SpeI digest with a 3' external probe to check for homologous targeting. To remove the drug selection cassette, correctly targeted clones were subjected to transient expression of Flp recombinase and treated with 2'-fluoro-2'-deoxy-5-iodouracil-ß-D-arabinofuranoside (FIAU) to enrich for clones that had lost the TK gene (169). Recombination of the Frt sites was confirmed by PCR, and positive clones were checked for loss of the selection cassette by Southern blotting using an XbaI digest and a probe for Neo.

Genotyping of mice. p300 and CBP conditional knockout mice were bred to Lck-Cre transgenic mice (69) or lysM-Cre transgenic mice (32). Recombination of the LoxP sites flanking p300 exon 9 in cells expressing Cre was determined by Southern blotting using an XbaI or EcoRI digest and a cDNA probe to the region encoding amino acids 615 to 681 or by semiquantitative PCR using primers p4 (CTCTACATCCTAAGTGCTAGG), p5 (TGGACTGGTTATCGGTTCACC), and p6 (CAGTAGATGCTAGAGAAAGCC), producing a 540-bp wild-type band, a 720-bp p300 flox band and a 1.1-kb p300{Delta}flox band (primer locations shown in Fig. 2). This semiquantitative PCR overestimates the deletion of the p300flox allele in a systematic manner that was corrected for by using a standard curve of samples of known deletion as determined by quantitative Southern blotting. Generation of CBP conditional knockout mice and PCR genotyping for the deletion of the CBPflox allele was reported by Kang-Decker et al. (93). Detection of the flox and {Delta}flox alleles of CBP by Southern blotting was performed using a HindIII digest and a 5' external probe. Thymocyte subpopulations were sorted to check deletion by flow cytometry using anti-CD4 and anti-CD8 antibodies. T cells from the spleen and lymph node were purified to check deletion by positive selection using anti-Thy1.2-conjugated magnetic beads from Miltenyi Biotech.


Figure 2
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  		FIG. 2. Generation of the p300 conditional knockout mouse. (a) Gene targeting strategy for the generation of a p300 conditional knockout allele in mice. Exon number and relative position, selection gene cassettes, positions of external and internal Southern blot probes, PCR primer positions, LoxP sites, Frt sites, and restriction sites are indicated. (b) Southern blot using XbaI digest and internal probe showing detection of wild-type, flox, and {Delta}flox alleles of p300. #1 and #2 are separate p300+/flox; Lck-Cre mice with different deletion efficiencies. (c) Western blot of control and mutant thymocyte nuclear extracts (indicated) using a p300 N-terminal antibody (top panel) and a CBP N-terminal antibody (bottom panel).

 
Indirect immunofluorescent staining, nuclear extracts, and Western blots. Antibodies against CBP (A-22 and C-20) and p300 (N-15 and C-20) for immunofluorescence and Western blotting were from Santa Cruz. Indirect immunofluorescent detection of CBP and p300 was performed as previously described using single-cell suspensions of thymus and anti-p300 (1:800 dilution, C-20) and anti-CBP (1:800, C-20) antibodies (93). Nuclear extract preparations and Western blots were performed as previously described (95).

Flow cytometry and complete blood count. Antibodies for flow cytometry (fluorescence-activated cell sorting [FACS]) analysis (anti-CD3, anti-CD4, anti-CD8, anti-B220) were purchased from Becton Dickenson. FACS was performed on a FACSCalibur system using CellQuest software. Automated complete blood counts were performed using a Hemavet 3700R.

Estimation of total cell counts. Total counts for thymic subpopulations were determined by multiplying the percentage of cells in a subpopulation as measured by FACS by the total manual viable thymocyte count for that mouse. Cell counts in peripheral blood were estimated from automated complete blood count lymphocyte count per microliter multiplied by the percentages of T cells and B cells in peripheral blood as determined from anti-CD3 versus anti-B220 FACS.

Bone marrow-derived macrophages. Bone marrow-derived macrophages (BMDMs) were obtained by flushing bone cavities and isolating macrophages by differentiation in L-cell-conditioned medium as a source of macrophage colony-stimulating factor as described previously (141).

Activation of splenic T cells and qRT-PCR. Splenic T cells were sorted by FACS and cultured at 2 x 106 cells/ml in Dulbecco's modified Eagle’s medium plus 10% fetal bovine serum with 1 ng/ml phorbol myristate acetate (PMA) and 100 ng/ml ionomycin or dimethyl sulfoxide vehicle for 3 h at 37°C in round-bottom plates. cDNA was generated from 100 ng of total RNA in a 20-µl reaction mixture using Superscript II reverse transcriptase (RT; Invitrogen). Quantitative RT-PCR (qRT-PCR) was performed on an Opticon DNA engine (MJ Research) using 1 µl of cDNA per 25 µl PCR with SYBR green dye. qPCR primers were designed using Primer Express software (Applied Biosystems) and confirmed to yield a single product by melting curve analysis. Samples were normalized to GAPDH mRNA.


   RESULTS
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Creation of mice with a p300 Cre/LoxP conditional knockout allele. Mouse genomic DNA harboring p300 exons encoding the CREB-binding domain (KIX) was used in the targeting construct. Two LoxP sites were inserted into the introns flanking exon 9, encoding residues 588 to 626 (Fig. 2a); Cre-mediated recombination results in a frameshift mutation if the flanking exons fortuitously splice and a stable mRNA is produced. The equivalent exon is targeted in the CBPflox allele, so the p300flox allele is a comparable experimental system (93). Neomycin-resistant ES cell clones were confirmed for correct targeting of the p300 locus by Southern blotting using 5' and 3' probes that were external to the targeting construct (Fig. 2b) (L. H. Kasper, data not shown). The Frt site-flanked drug selection cassette was removed in the ES cells by transient expression of Flp recombinase. Chimeric mice were generated by standard methods, and germ line transmission of the p300flox allele was achieved. Importantly, p300flox/flox homozygous mice appeared normal, indicating that the conditional allele is indistinguishable from the wild type in the absence of Cre recombinase. p300flox recombination in the presence of Cre (yielding p300{Delta}flox) was efficient, although it showed considerable variation between mice carrying the Lck-Cre transgene (Fig. 2b). This is likely due to stochastic transgene expression, a relatively common phenomenon in transgenic mice (38).

p300{Delta}flox is a null allele. The deletion efficiency in p300flox/lflox; Lck-Cre mice could be as high as ~90 to 95% and was specific for the thymus (L. H. Kasper, data not shown) (69). Mice with high deletion frequencies were deficient for full-length p300 protein as measured by Western blots using thymic nuclear extracts and antibodies specific for the N terminus of p300 (Fig. 2c). CBP protein levels were not measurably affected in the p300 null thymuses (Fig. 2c; 3a), nor were p300 protein levels altered in thymuses deficient for CBP (Fig. 3a). There was no evidence for truncated forms of p300 appearing in p300flox/lflox; Lck-Cre thymic nuclear extracts as determined by Western blots using an N-terminus-specific antibody following 8% sodium dodecyl sulfate-polyacrylamide gel electrophoresis, although a small amount of material that reacted with a C-terminal p300 antibody ran at the dye front (less than 37 kDa) (P. K. Brindle, data not shown). However, indirect immunofluorescence of thymocytes from p300flox/flox; Lck-Cre and CBPflox/flox; Lck-Cre mice using antibodies specific for the C termini of p300 or CBP confirmed that many cells lacked p300 or CBP, respectively (Fig. 3b). Together, these results indicate that p300{Delta}flox is a null allele.


Figure 3
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  		FIG. 3. Ablation of p300 and CBP protein in mutant thymocytes. (a) Western blot of control and mutant thymocyte whole-cell extracts (indicated) using C-terminal antibodies ({alpha}) to p300 (top panel) and CBP (middle panel) and an antibody against CREB (lower panel). (b) Single-cell suspensions of thymuses from control (p300flox/flox) (upper panels), p300flox/flox; Lck-Cre (middle), and CBPflox/flox; Lck-Cre (lower) mice were stained with 4',6'-diaminido-2-phenylindole (DAPI) to visualize nuclei and antibodies specific for the C termini of p300 or CBP (indicated). Detection was by anti-rabbit immunoglobulin G conjugated with fluorescein isothiocyanate.

 
Thymic development is abnormal in both CBPflox/flox; Lck-Cre and p300flox/flox; Lck-Cre mice. The total number of thymocytes was significantly reduced in both CBPflox/flox; Lck-Cre mice compared to CBPflox/flox mice (0.61 x 108 ± 0.17 x 108 [n = 4] versus 2.16 x 108 ± 0.50 x 108 [n = 4]; t test, P = 0.001) and p300flox/flox; Lck-Cre mice compared to p300flox/flox mice (1.18 x 108 ± 0.63 x 108 [n = 6] versus 2.22 x 108 ± 0.28 x 108 [n = 6]; t test, P = 0.004). As Lck-Cre mice were found to have thymocyte numbers comparable to those of wild-type mice (T. Fukuyama, data not shown), this deficit is likely caused by inactivation of the floxed alleles.

Quantitation of individual thymocyte subtypes showed that double-negative (DN) cell numbers (the least mature major subtype) were relatively unaffected in both mutants (Fig. 4c and 5c), whereas there was a five- to threefold average decrease in double-positive (DP) cell numbers for CBP (Fig. 4d) (n = 4) and p300 (Fig. 5d) (n = 5) mutant mice, respectively, compared to flox controls (n = 4 and 6, respectively). This apparent conflict with the relative increase in the percentage of DN cells seen by FACS (Fig. 4a and 5a) is explained by the decrease in thymus size for both CBP and p300 mutant mice. Since Lck-Cre transgene expression does not initiate until the DN1 stage (the least mature stage in the thymus), DN cell numbers may be normal (Fig. 4c and 5c) because loss of CBP or p300 has no effect on DN cells or, alternatively, because CBP and p300 protein and mRNA levels are not sufficiently depleted by turnover and dilution until the next stage of development (i.e., DP thymocytes).


Figure 4
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  		FIG. 4. CBPflox/flox; Lck-Cre mice have abnormal T-cell subpopulations in the thymus. (a) FACS analysis of thymus, spleen, and peripheral blood from representative control (Lck-Cre, CBPflox/flox) and CBPflox/flox; Lck-Cre mice showing ratios of CD4+ and CD8+ cells. Spleen and peripheral blood were pregated on CD3+ cells. Percentages of cells in relevant quadrants are indicated. (b) Southern blot showing deletion in thymus of CBPflox/flox; Lck-Cre mouse used in panel a. (c to f) Average total counts for the CD4– CD8– DN, CD4+ CD8+ DP, CD4+ SP (CD4 SP), and CD8+ SP (CD8 SP) thymocyte populations in indicated control CBPflox/flox (c to f) (n = 6) and mutant CBPflox/flox; Lck-Cre (c to f) (n = 5) 6-week-old mice (mean ± standard error of the mean).

 

Figure 5
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  		FIG. 5. p300flox/flox; Lck-Cre mice have reduced numbers of the more mature thymocyte subpopulations. (a) FACS analysis of thymus, spleen, and peripheral blood from representative control (Lck-Cre, p300flox/flox) and p300flox/flox; Lck-Cre mice showing ratios of CD4+ and CD8+ cells. Spleen and peripheral blood were pregated on CD3+ cells. Percentages of cells in relevant quadrants are indicated. (b) Southern blot of p300flox/flox; Lck-Cre thymus used in panel a demonstrating efficient deletion. (c to f) Average total counts for the CD4– CD8– DN, CD4+ CD8+ DP, CD4+ SP (CD4 SP), and CD8+ SP (CD8 SP) thymocyte populations in indicated control p300flox/flox (c to f) (n = 4) and mutant p300flox/flox; Lck-Cre (c to f) (n = 4) 6-week-old mice (mean ± standard error of the mean).

 
SP cell numbers were generally decreased about twofold in the mutants compared to controls (Fig. 4e and f and 5e and f), except that CBPflox/flox; Lck-Cre mice had normal numbers of CD8+ cells (Fig. 4f). This indicates that the 10-fold increase in the percentage of CD8+ SP cells in CBPflox/flox; Lck-Cre thymuses (Fig. 4a) was, in fact, due to a decrease in the absolute number of DP and CD4+ SP cells. Given that thymocyte development occurs in a stepwise manner from DN to DP to SP, this would indicate that the "normal" levels of CD8+ SP cells in CBPflox/flox; Lck-Cre mice arise by an altered process, as they are derived from a much smaller pool of DP precursors than in control mice. By contrast, the ratio of CD4+ to CD8+ SP thymocytes in p300flox/flox; Lck-Cre mice was normal (Fig. 5a).

We previously showed that CBPflox/flox; MMTV-Cre mice had an increased percentage of CD8+ SP thymocytes, although it was unclear if this defect was autonomous to the thymus because CBPflox inactivation occurred in multiple tissues (93). Since CBPflox/flox; Lck-Cre mice also had a marked increase in the percentage of CD8+ SP thymocytes compared to Lck-Cre and CBPflox/flox controls (Fig. 4a), and as the Lck-Cre transgene is expressed exclusively in thymocytes (69), the increase in CD8+ SP cells is due to loss of CBP in thymocytes. Together, these results show that CBP and p300 are required for normal thymocyte development, although they have distinct roles in CD8+ SP development.

Modest reduction in peripheral T cells in CBPflox/flox; Lck-Cre mice. Neither CBPflox/flox; Lck-Cre nor p300flox/flox; Lck-Cre mice had obviously abnormal proportions of CD4+ and CD8+ peripheral T cells in the spleen and blood (Fig. 4a; 5a), even when deletion in the thymus was high (~80 to 90%) (Fig. 4b; 5b), but there was a modest ~2-fold reduction in the number of peripheral blood CD4+ (P = 0.028, analysis of variance [ANOVA]) and CD8+ (P = 0.020) T cells in CBPflox/flox; Lck-Cre mice that was not observed in p300flox/flox; Lck-Cre mice (Fig. 6a). Peripheral blood B-cell numbers were comparable between control and mutant mice, demonstrating the lineage specificity of the mutations (P = 0.58, ANOVA) (Fig. 6b). Comparison of the deletion frequency in thymus and affinity-purified splenic T cells revealed that the fraction of cells harboring the nonrecombined alleles increased in peripheral T cells from p300flox/flox; Lck-Cre mice (Fig. 7a) and CBPflox/flox; Lck-Cre mice (Fig. 7b), indicating that late developmental stage or peripheral T cells with p300 or CBP deficiency are at a disadvantage.


Figure 6
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  		FIG. 6. Quantitation of peripheral T-cell subpopulations in CBPflox/flox; Lck-Cre and p300flox/flox; Lck-Cre mice. Average numbers of CD4+ and CD8+ T cells (a) and B220+ B cells (b) per microliter of peripheral blood for the indicated genotypes are shown (6- to 9-week-old mice, n = 3 to 9; mean ± standard error of the mean). WT, wild type.

 

Figure 7
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  		FIG. 7. Moderate selection against p300 and CBP null peripheral T cells in p300flox/flox; Lck-Cre and CBPflox/flox; Lck-Cre mice. (a) Representative Southern blot of genomic DNA from thymus and affinity-purified splenic T cells of a p300flox/flox; Lck-Cre mouse indicating relative abundance of the recombined allele. (b) Representative semiquantitative PCR of genomic DNA from thymus and affinity-purified splenic T cells of a CBPflox/flox; Lck-Cre mouse. Splenic T cells were purified by positive selection using Thy1.2+-conjugated magnetic beads.

 
Loss of p300 in thymocytes does not lead to lymphoma. We previously showed that CBPflox/flox; MMTV-Cre mice develop T-cell lymphoma with high frequency starting at 3 months of age (93). Since CBP and p300 functions are often interchangeable in vitro, we expected to see T-cell lymphoma develop in p300flox/flox; MMTV-Cre mice. Surprisingly, a cohort of 40 p300flox/flox; MMTV-Cre mice did not develop lymphoma out to 104 weeks of age (Fig. 8a). By comparison, a cohort of 37 CBPflox/flox; MMTV-Cre mice developed T-cell lymphoma with high frequency as early as 16 weeks (Fig. 8a). We observed a comparable incidence and onset of T-cell lymphoma in CBPflox/flox; Lck-Cre mice (Fig. 8b) (P = 0.03, one-tailed log rank test, n = 25), although over time, the frequency of lymphoma dropped markedly as the mice were backcrossed, indicating that other genetic or environmental factors contribute to tumor development (T. Fukuyama, data not shown). We did not observe lymphoma in p300flox/flox; Lck-Cre mice out past 1 year of age (Fig. 8c) (n = 21), in agreement with results for the p300flox/flox; MMTV-Cre mice. The CBPflox/flox; MMTV-Cre and p300flox/flox; MMTV-Cre mice were housed in a different facility from the CBPflox/flox; Lck-Cre and p300flox/flox; Lck-Cre colonies, suggesting that environmental factors alone cannot account for the differences in tumor phenotype observed. Together, the data support the view that there are intrinsic differences in the tumor suppressor functions of CBP and p300, even though the contributing environmental and genetic factors required for T-cell lymphomagenesis in the absence of CBP are unclear and somewhat variable. In this regard, it has recently been shown that p300 levels can vary in different mouse strains, suggesting that the combined dosage of CBP and p300 protein may also fluctuate and lead to variable phenotypic penetrance (158).


Figure 8
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  		FIG. 8. CBP but not p300 deficiency in T cells causes lymphoma with incomplete penetrance. (a to c) Percentage of lymphoma-free surviving CBPflox/flox and p300flox/flox mice with MMTV-Cre (a) or Lck-Cre (b and c). Ages and genotypes of mice are indicated. Mice alive at the end of the study or dying of causes other than lymphoma were censored in the analysis (tick marks). (a) n = 37 to 40, P < 0.0001, one-tailed logrank test; (b) n = 13 to 25, P = 0.03; (c) n = 21 to 24.

 
CBP and p300 combined dosage is critical for thymocyte development. The ability of T cells to develop and survive with only CBP or p300 raised the possibility that cells lacking both coactivators could be generated. We tested this by creating CBPflox/flox; p300flox/flox; Lck-Cre mice ("quad" mutants). Recombination of the CBPflox allele could be detected in quad mutant thymuses by semiquantitative PCR (Fig. 9b), albeit at a reduced level compared to CBPflox/flox; Lck-Cre mice. Similar results were obtained for the p300flox allele (Fig. 9b) (PCR quantitation is fairly linear for CBP{Delta}flox but tends to overestimate the relative abundance of the p300{Delta}flox allele; M. A. Biesen, data not shown).


Figure 9
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  		FIG. 9. CBPflox/flox; p300flox/flox; Lck-Cre mice have abnormal thymocyte subpopulations but essentially normal ratios of peripheral T cells. (a) FACS analysis of thymus, spleen, and peripheral blood from representative control and CBPflox/flox; p300flox/flox; Lck-Cre mice (quad mutant) showing ratios of CD4+ and CD8+ cells. Spleen and peripheral blood were pregated on CD3+ cells. Percentages of cells in relevant quadrants are indicated. (b) Semiquantitative PCR of genomic DNA showing deletion in thymus of CBPflox/flox; p300flox/flox; Lck-Cre mice used in panel a. Quad mutant no. 2 has a higher deletion frequency than no. 1. The p300 PCR tends to overestimate the relative abundance of the p300{Delta}flox allele.

 
FACS analysis of quad mutant mice with efficient recombination (~50%) of the CBPflox and p300flox alleles in the thymus revealed about a 10-fold increase in the percentage of DN thymocytes, with a concomitant decrease in DP cells; mice with a lower deletion frequency (~30%) in the thymus had an intermediate immunophenotype (Fig. 9a and b, compare quad mutants 1 and 2). The quad mutant thymuses also lacked the increase in CD8+ SP cells that occurred in the CBPflox/flox; Lck-Cre mice. The proportion of CD4+ and CD8+ T cells in the spleen and blood of the quad mutants were comparable to Lck-Cre and CBPflox/flox control mice, however (Fig. 9a).

We did not observe deletion frequencies greater than ~50% in quad mutant thymuses, suggesting that thymocytes lacking both CBP and p300 are strongly selected against. We confirmed this by quantitating the number of thymocytes per subpopulation in quad mutants that showed both medium (~50%) and low (~10 to 30%) deletion frequencies. Total thymocyte counts decreased with increased deletion frequency, and as a result, DN cell numbers were unaffected in both classes of quad mutants compared to control mice (Fig. 10a), but DP cells were reduced about 10-fold in thymuses displaying medium levels of deletion (Fig. 10b). There was a deficiency of SP thymocytes in quad mutants with medium deletion, although the CD8+ SP thymocytes were more variably affected (Fig. 10c and d). In further support of these data, genomic DNA isolated from FACS-purified quad mutant thymocyte subtypes showed ~30 to 40% deletion of the CBPflox and p300flox alleles in DN cells, but there was a steady drop in the frequency of both recombined alleles in DP and SP thymocytes, with an almost complete absence (~1% deletion) in peripheral T cells purified from the spleen and lymph nodes (Fig. 11a and b). These results indicate that thymocyte development requires either CBP or p300 and that T cells need CBP or p300 to populate the peripheral lymphoid organs. The numbers of T cells in the spleen and blood of quad mutants were not significantly different from those of control mice, however, indicating that there is compensation by homeostatic proliferation of peripheral T cells that did not recombine the alleles (Fig. 10e and f) (P > 0.05, ANOVA, n = 3 to 9).


Figure 10
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  		FIG. 10. Very strong selection against thymocyte subpopulations but not peripheral T cells in CBPflox/flox; p300flox/flox; Lck-Cre mice. Total counts for the CD4– CD8– DN (a), CD4+ CD8+ DP (b), CD4+ SP (CD4 SP) (c), and CD8+ SP (CD8 SP) (d) thymocyte populations of Lck-Cre mice (n = 2), CBPflox/flox; p300flox/flox; Lck-Cre mice (quad mutant) with low deletion (<30%) in whole thymus (n = 3), and CBPflox/flox; p300flox/flox; Lck-Cre mice with medium deletion (30 to 50%, n = 2) in whole thymus (mean ± standard error of the mean). (e) CD3+ T cells per microliter of peripheral blood (mean ± standard error of the mean; n = 4 to 8). Lck-Cre and CBPflox/flox; p300flox/flox (quad) controls are indicated. (f) Ratio of CD3+ to B220+ cells in the spleen as determined by flow cytometry (mean ± standard error of the mean; n = 2 to 3).

 

Figure 11
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  		FIG. 11. Strong selection against thymocyte subpopulations and peripheral T cells lacking both CBP and p300 inCBPflox/flox; p300flox/flox; Lck-Cre mice. Semiquantitative PCR showing deletion of CBPflox (a) and p300flox alleles (b) in FACS-purified CD4– CD8– DN, CD4+ CD8+ DP, CD4+ SP (CD4+ SP), and CD8+ SP (CD8+ SP) thymocyte subpopulations and affinity-purified T cells from spleen and lymph node (LN). T cells from spleen and LN were purified by positive selection using Thy1.2+-conjugated magnetic beads. PCR tends to overestimate the relative abundance of the p300{Delta}flox allele but is fairly quantitative for CBP{Delta}flox. Numbers indicate the approximate deletion percentages; p300 values were corrected by comparison to a standard curve of known deletion frequency derived from Southern analysis.

 
The model that concentrations of CBP and p300 are critically limiting predicts that a modest reduction (~50%) in their combined levels will have catastrophic consequences for cells. In contrast, results from this study suggest a model where CBP and p300 are not extremely limiting and that lesser amounts (between 0 and 50%) of the coactivators can still support many critical functions. We compared these models by determining whether one CBP or p300 wild-type allele is sufficient for thymocyte development and for T cells to populate the periphery. Both CBP+/flox; p300flox/flox; Lck-Cre and CBPflox/flox; p300+/flox; Lck-Cre mice ("triple" mutants) were generated, and semiquantitative PCR analysis of thymic and T-cell genomic DNA was performed. In this instance, we corrected the PCR data by using a standard curve made with samples of known deletion frequency as determined by quantitative Southern blotting. The corrected PCR data demonstrated that triple-mutant cells of both genotypes were selected against in the periphery, although not to the extent seen with the quad mutants (Fig. 12a and b, note that the similarity in deletion frequency for both types of alleles indicates that an individual cell either recombines all the alleles or none; the quad mutant data in Fig. 11 corroborate this). Remarkably, about 20 to 30% deletion was observed for both CBPflox and p300flox in peripheral T cells of CBP+/flox; p300flox/flox; Lck-Cre mice, suggesting that T cells with one wild-type CBP allele may be able to develop and populate the lymphoid organs to some degree (Fig. 12b). The extent of deletion in peripheral T cells does not depend strongly on the level of deletion in the thymus (compare Fig. 12a and c). All told, these results demonstrate that there is a graded effect of the combined dosage of functional CBP and p300 alleles on T-cell development and their ability to populate the periphery. The inactivation of both CBP alleles (along with one p300 allele) was more detrimental to peripheral T cells than the other triple mutation combination (compare Fig. 12a and c with 12b), which is consistent with our observation that CBP is more important for T cells than p300.


Figure 12
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  		FIG. 12. Moderate to strong selection against peripheral T cells lacking three functional alleles of CBP and p300. Percent deletion of CBPflox and p300flox alleles in the thymus and affinity-purified T cells from the spleen and lymph node (LN) of CBPflox/flox; p300+/flox; Lck-Cre (a and c) or CBP+/flox; p300flox/flox; Lck-Cre (b) mice. (c) Deletion frequency in the thymus does not directly impinge on the deletion percentage in the periphery (compare panels b and c). Deletion percentages for CBPflox and p300flox were determined by semiquantitative PCR of genomic DNA corrected for nonlinearity by use of a standard curve made of samples with deletion frequencies determined by quantitative Southern blot.

 
TCR signal-responsive gene expression is defective in CBP and p300 null splenic T cells. To circumvent inadequate gene deletion in peripheral T cells of CBPflox/flox; Lck-Cre and p300flox/flox; Lck-Cre mice, we utilized a reporter transgene, Z/EG that expresses green fluorescent protein (GFP) after Cre-mediated recombination, allowing us to sort for a population of splenic T cells with a high deletion frequency of CBP or p300 (143). We used these cells to elucidate the role of CBP and p300 in gene expression in response to activation of signaling through the T-cell receptor (TCR). We tested two genes, interleukin-2 (IL-2) and tumor necrosis factor alpha (TNF-{alpha}), whose expression in response to TCR signaling is critical for T-cell function and has been reported to involve p300 and CBP (47, 228). IL-2 mRNA expression was reduced less than twofold in CBPflox/flox; Lck-Cre cells and was unaffected in p300flox/flox; Lck-Cre splenic T cells in response to treatment with PMA and calcium ionophore. The decrease in IL-2 may be due to the increased fraction of CD8+ T cells relative to CD4+ cells in the spleen (Fig. 4a), as not all CD8+-T-cell subtypes express IL-2 efficiently (212). TNF-{alpha} mRNA expression, however, was decreased more than twofold in p300flox/flox; Lck-Cre and more than fourfold in CBPflox/flox; Lck-Cre in activated splenic T cells (Fig. 13a and b). This deficit in TNF-{alpha} expression is in agreement with a previous report by Falvo et al. demonstrating that CBP null heterozygous T cells are defective in TNF-{alpha} expression in response to TCR activation with anti-CD3 and suggests that p300 is also important (47).


Figure 13
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  		FIG. 13. Examination of endogenous TCR-signaling-responsive gene expression in splenic T cells lacking p300 or CBP. (a and b) Splenocytes from mice bearing Lck-Cre and Z/EG reporter transgenes were sorted for GFP-expressing control (Lck-Cre; Z/EG), p300 null (p300flox/flox; Lck-Cre; Z/EG), or CBP null (p300flox/flox; Lck-Cre; Z/EG) splenic T cells. Cells were treated ex vivo with 1 ng/ml PMA and 200 ng/ml calcium ionophore (Iono) for 3 h, and qRT-PCR was performed to determine mRNA expression of IL-2 (a) and TNF-{alpha} (b). Expression was normalized to GAPDH expression, and treated control cells were arbitrarily set to 100 in each experiment. Graphs represent the average results from two independent experiments using cells from two different mice of each genotype (n = 2).

 
Gene expression in CBP and p300 null macrophages. We used bone marrow-derived macrophages to further investigate the effects of CBP and p300 deficiency on endogenous signal-responsive gene expression. We chose several cytokine- and Toll-like receptor (TLR)-responsive target genes where either CBP or p300 has been previously implicated as playing a key role in expression or a presumed role based on chromatin immunoprecipitation assays. TNF-{alpha}, arginase (Arg1), inducible nitric oxide synthase (iNOS), and interferon regulatory factor 1 (IRF-1), are regulated by gene-specific combinations of the CBP/p300-interacting transcription factors ATF-2, c-Jun, SP1, Ets, Egr-1, NF-{kappa}B, AP-1, STAT1, PU.1, STAT6, and C/EBPß (Fig. 1) (9, 117, 152). CBPflox/flox; lysMcre mice that express Cre in macrophages yielded cells with an ~80 to 90% reduction in CBP protein when measured by Western blotting of nuclear extracts (Fig. 14a) (32). Northern blot analysis revealed that TNF-{alpha} mRNA was robustly induced in response to lipopolysaccharide (LPS) stimulation in CBPflox/flox; lysMcre macrophages, although not quite as well as in control CBPflox/flox cells (Fig. 14b) (rRNA served as a loading control). Arginase (Arg1) induction in response to the cytokine IL-4 was measured by Western blotting and was comparable between CBPflox/flox; lysMcre and CBPflox/flox macrophages (Fig. 14c) (the nonspecific band serves as a loading control).


Figure 14
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  		FIG. 14. Examination of endogenous gene expression in BMDMs deficient for either CBP or p300. (a) lysMcre-mediated loss of CBP in BMDMs (left panel). Nuclear extracts from BMDMs cultured ex vivo from CBPflox/flox or two different CBPflox/flox; lysMcre mice showed CBP deficiency as measured by immunoblotting with CBP-specific antisera. p300 levels were unaffected. (Right panel) p300 deletion in BMDMs was confirmed by semiquantitative PCR of genomic DNA. (b) Normal CBP levels are not required for the TLR4-mediated induction of TNF-{alpha} expression. Northern blot analysis of TNF-{alpha} mRNA induction in response to stimulation through the TLR4 pathway. BMDMs were plated at 2 x 106 cells per well and stimulated with Escherichia coli LPS at 100 ng/ml for the times indicated. RNA was isolated and probed with a TNF-{alpha} cDNA probe. Ethidium bromide-stained rRNA is the loading control (bottom panels). (c) CBP is not required for the IL-4-mediated induction of arginase 1 (Arg1) expression. BMDMs were stimulated with IL-4 (10 ng/ml) or left unstimulated (–) for the times shown, and lysates were analyzed for Arg1 expression by immunoblotting. The nonspecific band serves as a loading control (asterisk). (d) Normal p300 levels are not required for the TLR4-mediated induction of TNF-{alpha} expression. Total RNA was isolated and analyzed as shown in panel b. (e) Normal p300 levels are not required for the IL-4-mediated induction of Arg1 expression. BMDMs were plated and stimulated with IL-4 as shown in panel c. (f) Normal p300 levels are not required for the IFN-{gamma}-mediated induction of IRF-1 expression but may be required for the normal kinetics of iNOS production. BMDMs were stimulated with LPS and IFN-{gamma} to induce the expression of IRF-1 and iNOS, and lysates were analyzed by immunoblotting. The nonspecific band serves as a loading control (asterisk). Note that although IRF-1 levels are normal in p300flox/flox; lysMcre BMDMs compared to control cells, the induction of iNOS expression is slightly delayed at the 6-h time point but eventually reaches robust levels.

 
We next examined gene expression in macrophages deficient for p300. Recombination of the p300flox allele in p300flox/flox; lysMcre macrophages was confirmed by semiquantitative PCR of genomic DNA (Fig. 14a). TNF-{alpha} mRNA was induced comparably by LPS in p300flox/flox and p300flox/flox; lysMcre macrophages (Fig. 14d, rRNA loading control in bottom panel). Arg1 induction after IL-4 stimulation was also comparable between p300flox/flox and p300flox/flox; lysMcre macrophages (Fig. 14e), although treatment with both gamma interferon (IFN-{gamma}) and LPS revealed a modest deficit in the kinetics of iNOS expression but not IRF-1 (Fig. 14f, nonspecific band acts as a loading control). IRF-1 and iNOS expression in response to the same stimuli in CBPflox/flox; lysMcre macrophages was in the normal range (P. J. Murray, data not shown). Together, these results indicate that CBP or p300 individually is not highly limiting for TNF-{alpha} and Arg1 expression in macrophages in response to LPS and IL-4. Interestingly, iNOS, but not IRF-1, induction in response to IFN-{gamma} plus LPS appears to be partially sensitive to p300 deficiency, consistent with a proposed model that CBP and p300 are limiting for iNOS transcription (117). Induction of high-level iNOS expression requires IRF-1 and NF-{kappa}B activity, so it appears that p300 levels are important for this process but expression levels can be compensated for by CBP or other factors. We did not observe a noticeable defect in TNF-{alpha} induction in macrophages even though both CBP and p300 null T cells showed a clear deficit (Fig. 13b) and B cells deficient for p300 or CBP showed a 30 to 60% decrease in TNF-{alpha} expression in response to B-cell receptor signaling (P. K. Brindle, submitted for publication). This may reflect different transcriptional regulator usage in the three cell types. It is worth noting that the macrophages tested here had less than 100% inactivation of CBPflox or p300flox, so it is possible that contributions to gene expression by CBP or p300 were underestimated in this analysis.


   DISCUSSION
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Inactivation of either CBP or p300 starting at the DN stage of thymocyte development reduced, but did not completely block, the ability of T cells to populate the periphery; whereas, inactivation of all four CBP and p300 alleles had strong negative effects on T-cell development in the thymus. In addition, the aberrant increase in the percentage of CD8+ SP thymocytes seen in CBPflox/flox; Lck-Cre mice was not observed in p300flox/flox; Lck-Cre animals, indicating that CBP and p300 have nonredundant roles in T-cell development. Although the absolute levels of CBP and p300 protein are unknown, thymocytes express both proteins, which implies that large differences in expression do not account for the phenotypes. A provocative possibility is that differences in the biochemical properties of CBP and p300 are fundamental to the production of normal T cells and may also explain why loss of CBP, but not p300, in T cells can result in lymphoma.

Homeostatic proliferative mechanisms largely compensated for any reduction in the number of naive peripheral T cells caused by the mutations by allowing cells that did not recombine the conditional alleles (likely due to stochastic expression of the Lck-Cre transgene) to expand and fill the niche (84). It is somewhat counterintuitive that loss of both CBP and p300 did not cause a significant decrease in peripheral T cells, even though loss of CBP alone caused a modest ~2-fold deficit. There are several possible explanations for this observation. First, T cells lacking CBP alone may function in a non-cell-autonomous manner to suppress homeostatic proliferation of peripheral T cells. For example, it is possible that less IL-7, an essential cytokine for homeostatic proliferation, is available in CBPflox/flox; Lck-Cre mice (84). Since T cells lacking both CBP and p300 are very rare or nonexistent outside the thymus, homeostatic proliferation would function normally in CBPflox/flox; p300flox/flox; Lck-Cre mice in this scenario. Second, homeostatic "cell counting" or "empty space detection" mechanisms may respond more efficiently to a strong deficit in early thymocyte development (as seen in CBPflox/flox; p300flox/flox; Lck-Cre mice) than if cells are less severely affected in the thymus, as with CBPflox/flox; Lck-Cre mice. Third, recombination of the floxed alleles may be more efficient in CBPflox/flox; Lck-Cre mice than in CBPflox/flox; p300flox/flox; Lck-Cre mice, thus homeostatic proliferation may be unable to completely fill the larger empty niche in the former.

It was surprising that loss of either CBP or p300 alone did not result in a more drastic T-cell phenotype given that the two coactivators have been shown to interact in vitro with over 190 proteins encoded by essential genes in mice, of which at least 56 are crucial for T cells (Mouse Genome Informatics) (Fig. 1). It is possible that these two coactivators are not generally limiting in cells to the extent previously believed (64, 115, 203). Indeed, CBP–/– and p300–/– mice are viable through the first half of development (dying at E9 to E11.5), suggestive of three possibilities with regard to the in vivo roles of CBP and p300. (i) In vitro interactions with CBP and p300 may not be biologically relevant. Our findings, however, argue that CBP and p300 interactions based upon in vitro evidence cannot be dismissed out of hand. Although HDM2 appears to aggregate nonspecifically with the unfolded TAZ1 domain of p300 in the presence of EDTA (131), we have found that three essential transcription factors (CREB, c-Myb, and HIF-1{alpha}) with well-characterized in vitro interactions with CBP and p300 show partial loss of activity when CBP and p300 functionality is attenuated by mutation in primary cells or mice (95; Kasper et al., in press). (ii) CBP and p300 may function redundantly in most instances and be generally nonlimiting in many cell types. Support for the second point is self-evident, since many T cells can develop and survive and mice can go through a substantial portion of development with either CBP or p300 alone. In addition, we have not observed increased expression of remaining wild-type alleles as a possible compensatory mechanism in mutant cells, so it appears that CBP and p300 levels are not highly limiting for some of their functions. Nevertheless, there are differences in the phenotypes of the various CBP and p300 mutant mice created to date, making it clear that there are either unique biochemical properties for each coactivator or that some cell types have a skewed ratio of CBP to p300 that makes one of them limiting upon mutation. (iii) Other transcriptional cofactors that are not highly related to CBP and p300 may supply redundant functions (e.g., other proteins that have histone acetyltransferase activity and bromodomains such as Gcn5, P/CAF, and TAF1). This point raises the prospect that coactivator networks are more broadly interconnected and robust than typically understood. But there are obviously limits to this proposition, as our studies show that either CBP or p300 is absolutely essential because T cells lacking both are not viable. So other cofactors may only be able to compensate under a limited number of situations. The identities of such factors are unknown, although recent genetic studies demonstrate that the combined dosage of various histone acetyltransferase proteins harboring bromodomains (p300 and Gcn5, and p300 and P/CAF) are not generally limiting for mouse development (158).

The creation of CBPflox and p300flox mice will now permit researchers to rigorously test whether either member of this small coactivator family is critical in specific cell lineages in vivo and to test their roles in endogenous gene expression ex vivo. Such analyses would be facilitated by the use of a Cre recombination-dependent GFP indicator allele to enable the FACS purification of cells with high frequencies (>90%) of floxed gene inactivation for functional and transcriptional studies ex vivo (143).


   ACKNOWLEDGMENTS
 
We thank S. Lerach and C. Barlow for excellent technical assistance; the Transgenic core; R. Cross, J. Gatewood, J. Smith, and the Mouse Immunophenotyping core; R. Ashmun and the Flow Cytometry and Cell Sorting Shared Resource; the Animal Resource Center facilities at SJCRH; M. Biery, N. Craig, G. Martin, and S. Fiering for plasmids; and J. Opferman for helpful comments. The Hartwell Center at SJCRH provided oligonucleotides and DNA sequencing.

This work was supported by NCI grant RO1 CA076385 (to P.K.B.), the Cancer Center (CORE) support grant P30 CA021765, and by the American Lebanese Syrian Associated Charities of St. Jude Children's Research Hospital.


   FOOTNOTES
 
* Corresponding author. Mailing address: Department of Biochemistry, St. Jude Children's Research Hospital, 332 N. Lauderdale, Memphis, TN 38105. Phone: (901) 495-2522. Fax: (901) 525-8025. E-mail: paul.brindle{at}stjude.org. Back


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 Discussion
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