Functional analysis of the V gamma 3 promoter of the murine gamma delta T-cell receptor

Regulation of TCR Vγ2 gene rearrangement by the helix-loop-helix protein, E2A

V(D)J recombination of Ig and TCR genes is strictly regulated by the accessibility of target gene chromatin in a lineage- and stage-specific manner. In the mouse TCRγ locus, rearrangement of the Vγ2 gene predominates over Vγ3 rearrangement in the adult thymus. This preferential rearrangement is likely due to the differential accessibility of the individual Vγ genes, because the levels of germ line transcription and histone acetylation of the Vγ genes are well correlated with the rearrangement frequency in adult thymocytes. However, factors responsible for the differential regulation of the Vγ gene rearrangement have been largely unknown. In this study, we demonstrated that Vγ2 rearrangement in the adult thymus was substantially reduced in mice deficient for the basic helix-loop-helix protein, E2A. The decreased rearrangement is likely caused by the reduced accessibility of Vγ2 chromatin, since germ line transcription and histone acetylation of the Vγ2 gene were reduced in an E2A dosage-dependent manner. We further showed that E2A bound around the Vγ2 gene in vivo and we identified two canonical E-box sites downstream of Vγ2, to which E2A can bind in vitro. Furthermore, these two E-box sites had the ability to activate transcription upon E2A over-expression. These data suggest that E2A directly binds to and increases accessibility of Vγ2 chromatin, thereby facilitating Vγ2 rearrangement in the adult thymus.

Functional characterization of proximal promoter of gene for human BRAK/CXCL14, a tumor-suppressing chemokine

BRAK/CXCL14 is a chemokine that is expressed in many normal cells and tissues but is absent from or expressed at very low levels in transformed cells and cancerous tissues including head and neck squamous cell carcinoma (HNSCC). We reported previously that the forced expression of BRAK/CXCL14 in HNSCC cells decreased the rate of tumor formation and size of tumor xenografts in athymic nude mice and SCID mice, suggesting that expression level of the gene is important for tumor suppression. In order to study the regulatory mechanisms governing the expression of this gene, we determined the transcriptional start site and promoter motifs of the gene. The major transcriptional start site determined by 5'rapid amplification of cDNA end method was located 283 bp downstream of the first proposed site of the gene. Determination of luciferase activities of reporter gene constructs with various deletions or mutations showed that an atypical TATA-like sequence, TATTAA was essential for the transcription of the gene and that the AP-1 binding sequence and tandem GC box were necessary for stimulating the expression of the gene in human squamous epithelial cells. The human DNA region was highly homologous (95% base identity) to the mouse gene. In addition, okadaic acid, an inhibitor of serine/threonine phosphatases 1, 2A and 2B, stimulated TATTAA sequence and AP-1 binding-sequence dependent promoter activity as well as increased the level of BRAK/CXCL14 mRNA, indicating that these sequences are essential for the regulation of BRAK/CXCL14 gene expression in the cells.

A targeted deletion of a region upstream from the Jkappa cluster impairs kappa chain rearrangement in cis in mice and in the 103/bcl2 cell line

We have shown previously that a mutation of the KI-KII site immediately 5' to J(kappa)1 on the mouse immunoglobulin light chain kappa locus reduces the rearrangement level in cis, although it does not affect transcription. Here we deleted by homologous recombination in mouse embryonic stem cells a 4-kb DNA fragment, located immediately upstream of the KI-KII element, which contains the promoter of the long germline transcript. Analysis of gene-targeted heterozygous mouse splenic B cells showed a strong decrease in rearrangement for the allele bearing the deletion. When both the KI-KII mutation and the 4-kb deletion were present on the same allele, the overall reduction in rearrangement was stronger than with the 4-kb deletion alone underlying the role of these two elements in the regulation of rearrangement. The same deletion was performed by homologous recombination on one allele of the rearrangement-inducible mouse 103/bcl2-hygro(R) pre-B cell line, and resulted in a similar reduction in the induction of rearrangement of the mutated allele. This result validates this cell line as an in vitro model for studying the incidence of gene-targeted modifications of the kappa locus on the regulation of rearrangement.

Positive and negative regulation of V(D)J recombination by the E2A proteins

A key feature of B and T lymphocyte development is the generation of antigen receptors through the rearrangement and assembly of the germline variable (V), diversity (D), and joining (J) gene segments. However, the mechanisms responsible for regulating developmentally ordered gene rearrangements are largely unknown. Here we show that the E2A gene products are essential for the proper coordinated temporal regulation of V(D)J rearrangements within the T cell receptor (TCR) gamma and delta loci. Specifically, we show that E2A is required during adult thymocyte development to inhibit rearrangements to the gamma and delta V regions that normally recombine almost exclusively during fetal thymocyte development. The continued rearrangement of the fetal Vgamma3 gene segment in E2A-deficient adult thymocytes correlates with increased levels of Vgamma3 germline transcripts and increased levels of double-stranded DNA breaks at the recombination signal sequence bordering Vgamma3. Additionally, rearrangements to a number of Vgamma and Vdelta gene segments used predominantly during adult development are significantly reduced in E2A-deficient thymocytes. Interestingly, at distinct stages of T lineage development, both the increased and decreased rearrangement of particular Vdelta gene segments is highly sensitive to the dosage of the E2A gene products, suggesting that the concentration of the E2A proteins is rate limiting for the recombination reaction involving these Vdelta regions.

Developmentally programmed rearrangement of T cell receptor Vgamma genes is controlled by sequences immediately upstream of the Vgamma genes

Distinct subsets of gammadelta T cells expressing different Vgamma and Vdelta chains arise in ordered waves during thymic development. In the murine Jgamma1-Cgamma1 cluster, the Vgamma3 gene segment is utilized earliest in fetal thymic development, in progenitors of dendritic epidermal T cells (DECs). The Vgamma2 gene segment predominates in the late fetal stages and beyond, in cells destined for the secondary lymphoid organs. Using transgenic TCRgamma recombination substrates, we demonstrate that this restricted Vgamma gene usage is determined by developmentally targeted gene rearrangement. We show that sequences immediately upstream of the Vgamma2 and Vgamma3 genes direct the rearrangement pattern in adult thymocytes. Thus, the choice of Vgamma gene for recombination is coordinated with distinct differentiation programs in gammadelta subsets.

Structural and functional analysis of the promoter of the murine V gamma 1.1 T cell receptor gene

The expression of the germ-line gene V gamma 1.1-C gamma 4 of the T cell receptor (TcR) gamma chain depends on interleukin (IL)-3 induction in hematopoietic cells, while in T cells, the rearranged gene is expressed constitutively. To understand the mechanism that controls TcR gamma gene expression, we cloned and characterized the structure and function of the V gamma 1.1-C gamma 4 TcR promoter. IL-3-dependent cell lines and T cell lines utilized the same transcriptional start sites. In chloramphenicol acetyltransferase (CAT) assays, the minimal 70-bp promoter confers strong transcriptional activity which is 50-60% of the Moloney long terminal repeat promoter activity. The 500-bp promoter region linked to the CAT gene exhibits IL-3 dependency similar to the endogenous TcR gamma gene. The immediate 3' and 5' flanking sequences inhibit the promoter activity two- to fourfold. The promoter lacks an obvious TATA box or CAAT box sequences, but contains a GC box in the untranslated region 3' to the promoter. The GC box is the core sequence of the element which binds Sp1-like proteins. Cloning of this Sp1 binding element in front of the thymidine kinase (TK) promoter and mutations generated in this site demonstrate its function as a silencer. Ultraviolet cross-linking analysis with the Sp1 binding site from the TcR gamma promoter revealed binding of a 90-100-kDa protein in a T cell line (EL-4) and 40-50 and 90-100-kDa proteins in FDC-P1 cells. The possible function of the Sp1-like protein in silencing the minimal promoter activity is discussed.