Cristina Hernández-Munain

Regulation of the T-cell receptor delta enhancer by functional cooperation between c-Myb and core-binding factors.

A T-cell-specific transcriptional enhancer lies within the J delta 3-C delta intron of the human T-cell receptor (TCR) delta gene. The 30-bp minimal enhancer element denoted delta E3 carries a core sequence (TGTGGTTT) that binds a T-cell-specific factor, and that is necessary but not sufficient for transcriptional activation. Here we demonstrate that the transcription factor c-Myb regulates TCR delta enhancer activity through a binding site in delta E3 that is adjacent to the core site. Both v-Myb and c-Myb bind specifically to delta E3. The Myb site is necessary for enhancer activity, because a mutation that eliminates Myb binding abolishes transcriptional activation by the delta E3 element and by the 370-bp TCR delta enhancer. Transfection of cells with a c-Myb expression construct upregulates delta E3 enhancer activity, whereas treatment of cells with an antisense c-myb oligonucleotide inhibits delta E3 enhancer activity. Since intact Myb and core sites are both required for delta E3 function, our data argue that c-Myb and core binding factors must cooperate to mediate transcriptional activation through delta E3. Efficient cooperation depends on the relative positioning of the Myb and core sites, since only one of two overlapping Myb sites within delta E3 is functional and alterations of the distance between this site and the core site disrupt enhancer activity. Cooperative regulation by c-Myb and core-binding factors is likely to play an important role in the control of gene expression during T-cell development.

c-Myb and core-binding factor/PEBP2 display functional synergy but bind independently to adjacent sites in the T-cell receptor delta enhancer.

A T-cell-specific transcriptional enhancer lies within the J delta 3-C delta intron of the human T-cell receptor delta gene. We have previously shown that a 30-bp element, denoted delta E3, acts as the minimal TCR delta enhancer and that within delta E3, adjacent and precisely spaced binding sites for core-binding factor (CBF/PEBP2) and c-Myb are essential for transcriptional activity. These data suggested that CBF/PEBP2 and c-Myb synergize to mediate transcriptional activity but did not establish the molecular basis for synergy. In this study, we have examined in detail the binding of CBF/PEBP2 and c-Myb to delta E3. We found that CBF/PEBP2 and c-Myb could simultaneously occupy the core site and one of two overlapping Myb sites within delta E3. However, equilibrium binding and kinetic dissociation experiments suggest that the two factors bind to delta E3 independently, rather than cooperatively. This was found to be true by using isoforms of these factors present in extracts of transfected COS-7 cells, as well as the natural factors present in nuclear extracts of the Jurkat T-cell line. We further showed that CBF/PEBP2 and c-Myb provide unique transactivation functions, since the core-Myb combination cannot be substituted by dimerized core or Myb sites. We propose that spatially precise synergy between CBF/PEBP2 and c-Myb may result from the ability of the two factors to form a composite surface that makes unique and stereospecific contacts with one or more additional components of the transcriptional machinery.

Indistinguishable nuclear factor binding to functional core sites of the T-cell receptor delta and murine leukemia virus enhancers.

We have previously shown that the delta E3 site is an essential element for transcriptional activation by the human T-cell receptor (TCR) delta enhancer and identified two factors, NF-delta E3A and NF-delta E3C, that bound to overlapping core (TGTGGTTT) and E-box motifs within delta E3. In this study, we show that protein binding to the core motif is necessary but not sufficient for transcriptional activation by the delta E3 element. In contrast, protein binding to the E-box motif does not contribute significantly to enhancer activity. A similar core motif present within the enhancers of T-cell-tropic murine retroviruses has been shown to contribute to transcriptional activity of the viral long terminal repeat in T lymphocytes and to viral T-cell tropism. We therefore determined the relationship between the nuclear factors that bind to the TCR delta and Moloney murine leukemia virus core motifs. On the basis of electrophoretic mobility shift binding and competition studies, biochemical analysis of affinity-labeled DNA-binding proteins, and the binding of a purified core binding factor, the proteins that bound to the TCR delta core site were indistinguishable from those that bound to the murine leukemia virus core site. These data argue that DNA-binding proteins that interact with the core site of murine leukemia virus long terminal repeats and contribute to viral T-cell tropism also play an essential role in the T-cell-specific expression of cellular genes.

Chemokine Production by G Protein-Coupled Receptor Activation in a Human Mast Cell Line: Roles of Extracellular Signal-Regulated Kinase and NFAT

Chemoattractants are thought to be the first mediators generated at sites of bacterial infection. We hypothesized that signaling through G protein-coupled chemoattractant receptors may stimulate cytokine production. To test this hypothesis, a human mast cell line (HMC-1) that normally expresses receptors for complement components C3a and C5a at low levels was stably transfected to express physiologic levels of fMLP receptors. We found that fMLP, but not C3a or C5a, induced macrophage inflammatory protein (MIP)-1β (CCL4) and monocyte chemoattractant protein-1 (CCL2) mRNA and protein. Although fMLP stimulated both sustained Ca2+ mobilization and phosphorylation of extracellular signal-regulated kinase (ERK), these responses to C3a or C5a were transient. However, transient expression of C3a receptors in HMC-1 cells rendered the cells responsive to C3a for sustained Ca2+ mobilization and MIP-1β production. The fMLP-induced chemokine production was blocked by pertussis toxin, PD98059, and cyclosporin A, which respectively inhibit Giα activation, mitgen-activated protein kinase kinase-mediated ERK phosphorylation, and calcineurin-mediated activation of NFAT. Furthermore, fMLP, but not C5a, stimulated NFAT activation in HMC-1 cells. These data indicate that chemoattractant receptors induce chemokine production in HMC-1 cells with a selectivity that depends on the level of receptor expression, the length of their signaling time, and the synergistic interaction of multiple signaling pathways, including extracellular signal-regulated kinase phosphorylation, sustained Ca2+ mobilization and NFAT activation.

Cooperation among Multiple Transcription Factors Is Required for Access to Minimal T-Cell Receptor α-Enhancer Chromatin In Vivo

ABSTRACT To understand the molecular basis for the dramatic functional synergy between transcription factors that bind to the minimal T-cell receptor α enhancer (Eα), we analyzed enhancer occupancy in thymocytes of transgenic mice in vivo by genomic footprinting. We found that the formation of a multiprotein complex on this enhancer in vivo results from the occupancy of previously identified sites for CREB/ATF, TCF/LEF, CBF/PEBP2, and Ets factors as well as from the occupancy of two new sites 5′ of the CRE site, GC-I (which binds Sp1 in vitro) and GC-II. Significantly, although all sites are occupied on a wild-type Eα, all sites are unoccupied on versions of Eα with mutations in the TCF/LEF or Ets sites. Previous in vitro experiments demonstrated hierarchical enhancer occupancy with independent binding of LEF-1 and CREB. Our data indicate that the formation of a multiprotein complex on the enhancer in vivo is highly cooperative and that no single Eα binding factor can access chromatin in vivo to play a unique initiating role in its assembly. Rather, the simultaneous availability of multiple enhancer binding proteins is required for chromatin disruption and stable binding site occupancy as well as the activation of transcription and V(D)J recombination.

Developmental regulation of V(D)J recombination at the TCR a/5 locus

Summary: The T‐cell receptor (TCR) α/σ locus includes a large number of V, D, J and C gene segments that are used lo produce functional TCR 8 and TCR a chains expressed by distinct subsets of T lymphocytes. V(D)J recombination events within the locus are regulated as a function of developmental stage and cell lineage during T‐lymphocyte differentiation in the thymus. The process of V(D)J recombination is regulated by cis‐acting elements that modulate the accessibility of chromosomal substrates to the recombinase. Here we evaluate how the assembly of transcription factor complexes onto enhancers, promoters and other regulatory elements within the TCR α/σ locus imparts developmental control to VDJσ and VJα rearrangement events. Furthermore, we develop the notion that within a complex locus such as the TCR α/σ locus, highly localized and region‐specific control is likely to require an interplay between positive regulatory elements and blocking or boundary elements that restrict the influence of the positive elements to defined regions of the locus.

TCERG1 Regulates Alternative Splicing of the Bcl-x Gene by Modulating the Rate of RNA Polymerase II Transcription

ABSTRACT Complex functional coupling exists between transcriptional elongation and pre-mRNA alternative splicing. Pausing sites and changes in the rate of transcription by RNA polymerase II (RNAPII) may therefore have fundamental impacts in the regulation of alternative splicing. Here, we show that the elongation and splicing-related factor TCERG1 regulates alternative splicing of the apoptosis gene Bcl-x in a promoter-dependent manner. TCERG1 promotes the splicing of the short isoform of Bcl-x (Bcl-xs) through the SB1 regulatory element located in the first half of exon 2. Consistent with these results, we show that TCERG1 associates with the Bcl-x pre-mRNA. A transcription profile analysis revealed that the RNA sequences required for the effect of TCERG1 on Bcl-x alternative splicing coincide with a putative polymerase pause site. Furthermore, TCERG1 modifies the impact of a slow polymerase on Bcl-x alternative splicing. In support of a role for an elongation mechanism in the transcriptional control of Bcl-x alternative splicing, we found that TCERG1 modifies the amount of pre-mRNAs generated at distal regions of the endogenous Bcl-x. Most importantly, TCERG1 affects the rate of RNAPII transcription of endogenous human Bcl-x. We propose that TCERG1 modulates the elongation rate of RNAPII to relieve pausing, thereby activating the proapoptotic Bcl-xS 5′ splice site.

Promoter Influences Transcription Elongation TATA-BOX ELEMENT MEDIATES THE ASSEMBLY OF PROCESSIVE TRANSCRIPTION COMPLEXES RESPONSIVE TO CYCLIN-DEPENDENT KINASE 9

Pausing of RNA polymerase II (RNAPII) during transcript elongation is an important mechanism for regulating gene expression at many genes. In this study we investigated the mechanism of regulated elongation of c-myc and human immunodeficiency virus-1 (HIV-1) using an in vitro elongation assay that reproduces the conditional block to elongation. We found that HIV-1 Tat can activate the RNAPII transcription complexes paused on c-myc by enhancing their elongation efficiency. We determined that cyclin-dependent kinase 9 (CDK9), the kinase subunit of positive transcription elongation factor b (P-TEFb) complex, regulates transcriptional elongation of c-myc and is present in transcription pre-initiation complexes formed on the c-myc promoter, which emphasizes a common mechanism of elongation control between HIV-1 and c-myc genes. We also investigated the roles of upstream elements of the HIV-1 and c-myc promoters in CDK9-activated transcriptional elongation. We found that the TATA-box element mediates the assembly of processive transcription complexes responsive to CDK9 and that specific combinations of upstream activation binding sites contribute to the recruitment of these complexes. We propose a common mechanism for elongation control at the c-myc and HIV-1 genes with an essential role for the TATA-box and specific modulatory contribution of upstream regulatory sequences, derived from the unique structure of the promoters, to form a composite surface for efficient recruitment of elongation-competent transcription complexes.

Developmental activation of the TCR α enhancer requires functional collaboration among proteins bound inside and outside the core enhancer

The TCR δ enhancer (Eδ) and TCR α enhancer (Eα) play critical roles in the temporal and lineage-specific control of V(D)J recombination and transcription at the TCR αδ locus, working as a developmental switch controlling a transition from TCR δ to TCR α activity during thymocyte development. Previous experiments using a transgenic reporter substrate revealed that substitution of the 116-bp minimal Eα, denoted Tα1-Tα2, for the entire 1.4-kb Eα led to a premature activation of V(D)J recombination. This suggested that binding sites outside of Tα1-Tα2 are critical for the strict developmental regulation of TCR α rearrangement. We have further analyzed Eα to better understand the mechanisms responsible for appropriate developmental regulation in vivo. We found that a 275-bp Eα fragment, denoted Tα1-Tα4, contains all binding sites required for proper developmental regulation in vivo. This suggests that developmentally appropriate enhancer activation results from a functional interaction between factors bound to Tα1-Tα2 and Tα3-Tα4. In support of this, EMSAs reveal the formation of a large enhanceosome complex that reflects the cooperative assembly of proteins bound to both Tα1-Tα2 and Tα3-Tα4. Our data suggest that enhanceosome assembly is critical for developmentally appropriate activation of Eα in vivo, and that transcription factors, Sp1 and pCREB, may play unique roles in this process.

A new optimized formulation of cationic solid lipid nanoparticles intended for gene delivery: Development, characterization and DNA binding efficiency of TCERG1 expression plasmid

Solid lipid nanoparticles (SLNs) are being considered as a new approach for therapeutics for many known diseases. In addition to drug delivery, their use as non-viral vectors for gene delivery can be achieved by the inclusion of cationic lipids, which provide a positive surface potential that favours binding to the DNA backbone. This work is based on the idea that the optimization of the components is required as the first step in simplifying the qualitative and quantitative composition of SLNs as much as possible without affecting the essential properties that define SLNs as optimal non-viral vectors for gene delivery. We selected the best lipids and surfactants in terms of particle size and zeta potential and characterized the properties of the resulting nanoparticles using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The SLNs had a particle size of approximately 120 nm and a positive surface charge of 42 mV. In addition, we analysed the main physicochemical characteristics of the bulk components of the nanoparticles using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and mass spectrometry (MS). The suitability of the optimized SLNs for DNA binding was evaluated after the lyophilisation process using a carboxyl-terminal region of the TCERG1 gene, a human factor that has been implicated in several diseases. We show that the SLNs presented high efficiency in the binding of DNA, and importantly, they presented no toxicity when assayed in an in vivo system.