Characterization of a promoter within the first intron of the human CD4 gene

A conserved element in the first intron of Cd4 has a lineage specific, TCR signal-responsive, canonical enhancer function that matches the timing of cell surface CD4 upregulation required to prevent lineage choice error

Introduction

The regulation of Cd4 expression during T-cell development and immune responses is essential for proper lineage commitment and function in the periphery. However, the mechanisms of genetic and epigenetic regulation are complex, and their interplay not entirely understood. Previously, we demonstrated the need for CD4 upregulation during positive selection to ensure faithful commitment of MHC-II-restricted T cells to the CD4 lineage. In this study, we investigate whether a conserved region, here called NCE, that is proximal to the Cd4 silencer and contains E4m has the required developmental-stage-specific canonical enhancer function and TCR responsiveness to mediate the CD4 upregulation required to prevent lineage errors.

Methods

To investigate the role of NCE, transient transfection of reporter plasmids was performed in thymoma cell lines arrested at the double-positive (DP, CD4+CD8+) and intermediate (INT, CD4+CD8lo) stages of development. CRISPR/Cas9-mediated deletion of the coreNCE/E4m region was carried out in these cell lines to assess its impact on CD4 surface expression, re-expression rates, and TCR signaling responsiveness. To avoid developmental alterations from direct manipulation of the endogenous Cd4 locus in vivo, BAC-transgenic reporter mice were generated with the locus modified to express EGFP in the presence or absence of NCE. EGFP mRNA levels were measured via RT-qPCR, and EGFP fluorescence was analyzed in post-selection thymocytes.

Results

Our in vitro experiments demonstrate that NCE by itself can function as an enhancer at the INT, but not the DP stage of development. Furthermore, CRISPR/Cas9-mediated deletion of coreNCE/E4m resulted in reduced CD4 surface levels, slower re-expression rates, and reduced TCR signaling responsiveness in INT cells, but not in DP cells. In vivo, NCE-sufficient transgenic mice exhibited upregulation of Cd4 reporter EGFP mRNA levels at the INT stage and a corresponding upregulation of EGFP fluorescence, whereas NCE-deficient mice showed a significant loss of Cd4 reporter EGFP mRNA and no detectable EGFP production in any post-selection thymocytes.

Discussion

This study demonstrates that the canonical enhancer function of coreNCE/E4m is essential for CD4 upregulation following positive selection. The NCE region, with its developmental-stage-specific activity and its known epigenetic regulatory capabilities, ensures faithful lineage commitment to the CD4 lineage.

Porcine CD4 promoters and enhancers can direct foreign gene expression in human cells

Porcine CD4 proximal promoter and enhancer sequences were cloned and aligned with the corresponding human and murine sequences. The alignment showed nucleotide homology between porcine and human sequences was 62.4 % for the CD4 promoter and 56.6 % for the CD4 enhancer. The nucleotide homology between porcine and murine sequences was 42.5 % for the CD4 promoter and 25.4 % for the CD4 enhancer. The proximal enhancer and promoter regions of the CD4 gene from porcine, murine and human cells were compared for their ability to direct foreign gene expression in transiently transfected human cell lines. The results indicated the porcine CD4 promoters and enhancers could effectively direct expression of a foreign gene in human cells. The porcine promoter was equally efficient as CMV and EF-1α in directing gene expression.

Usage of an intronic promoter for stable gene expression in Saccharomyces cerevisiae

AIMS

To construct expression vectors capable of switching promoters under different metabolic circumstances to obtain stable gene expression.

METHODS AND RESULTS

In this study, we designed a series of constructs for the expression of the chicken lactate dehydrogenase (cldh) gene under the control of galactose-inducible GAL1 promoter and the high glucose-inducible HXT1 promoter in Saccharomyces cerevisiae. In one construct, the HXT1 promoter was placed between artificial splicing sequences to function as an intronic promoter. We checked all constructs for the usage of promoters by reverse transcriptional polymerase chain reaction and assayed the expression level of the reporter gene under different culturing conditions. In the presence of galactose, when the GAL1 promoter was linked with the intronic HXT1 promoter, the cldh gene showed 1.5-fold activity compared with single GAL1 promoter, while in the presence of glucose, the construct showed over twofold activity compared with that without splicing sequences.

CONCLUSION

The intronic HXT1 promoter could be induced by the presence of high glucose concentration.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report detailing the use of an intronic promoter in the construction of stable expression vectors and the novel system could serve as a model of expression vectors for fermentation or other purposes.

Deletion of the CD4 silencer element supports a stochastic mechanism of thymocyte lineage commitment

The mechanism of T cell lineage commitment remains controversial; to examine it we deleted the CD4-silencer element in the germ line of a mouse using a combination of gene targeting and Cre/LoxP-mediated recombination. We found that these mice were unable to extinguish CD4 expression either in immature thymocytes or mature CD8+ cytotoxic T cells (CTLs), which resulted in the development of major histocompatibility complex class II-restricted double-positive CTLs in the periphery. This finding strongly supports a stochastic over an instructive mechanism of coreceptor down-regulation.

Distinct regulatory elements are required for faithful expression of human CD4 in T cells, macrophages, and dendritic cells of transgenic mice

To identify the regulatory elements controlling expression of the human CD4 (hCD4) gene in different cell types of the immune system, deletion and chimeric (human/murine) reporter genes were constructed and tested in transgenic (Tg) mice. Regulatory elements required for the proper hCD4 expression in the immature double-positive thymic T cells were identified in the enhancer and in the 3' end of intron 1. Expression of hCD4 in macrophages is controlled by at least 2 sets of regulatory elements: one present in front of exon 1 and the second at the 5' end of intron 1. The hCD4 elements required for expression on both myeloid and lymphoid CD8alpha(+) dendritic cells (DCs) from lymph node and thymus were found to be different from those required for macrophage expression. The results indicate that expression of hCD4 in T cells, macrophages, and DCs is controlled by distinct regulatory elements.

Ets target genes: past, present and future

Ets is a family of transcription factors present in species ranging from sponges to human. All family members contain an approximately 85 amino acid DNA binding domain, designated the Ets domain. Ets proteins bind to specific purine-rich DNA sequences with a core motif of GGAA/T, and transcriptionally regulate a number of viral and cellular genes. Thus, Ets proteins are an important family of transcription factors that control the expression of genes that are critical for several biological processes, including cellular proliferation, differentiation, development, transformation, and apoptosis. Here, we tabulate genes that are regulated by Ets factors and describe past, present and future strategies for the identification and validation of Ets target genes. Through definition of authentic target genes, we will begin to understand the mechanisms by which Ets factors control normal and abnormal cellular processes.

Deletional and mutational analyses of the human CD4 gene promoter: characterization of a minimal tissue-specific promoter

In recent years, considerable interest has arisen in understanding the mechanisms of the CD4 gene transcriptional control which resulted in the identification of a promoter, enhancers and a silencer. While the murine CD4 gene promoter has been well studied, little is known about its human counterpart that we previously identified as a 1.1 kb region. Here, we show that the -170/+20 region represents a minimal tissue-specific promoter with a size compatible with its inclusion in viral vectors for gene therapy. In addition, mutational analyses provided evidence that this fragment contains three regions critical for transcriptional activity: an initiator-like sequence, an Ets consensus site and an ATF consensus site. The latter site is absent in the murine promoter, suggesting that some differences exist between the transcriptional control of the human and murine CD4 genes.

CD4/CD8 lineage commitment: matching fate with competence

The outcome of positive selection of T lymphocytes is that there is a close match between the lineage adopted by a particular cell (CD4+ or CD8+) and the specificity of the T-cell receptor for the class of Major Histocompatibility Complex molecule recognized. How this match is obtained has been a matter of debate. We review the evidence, from recent and older experiments, that indicates that the process follows a selective logic, rather than an instructive one.