The L2a element is a mouse CD8 silencer that interacts with MAR-binding proteins SATB1 and CDP.

Previous transgenic-reporter and targeted-deletion studies indicate that the subset-specific expression of CD8αß heterodimers is controlled by multiple enhancer activities, since no silencer elements had been found within the locus. We have identified such a silencer as L2a, a previously characterized ∼ 220 bp nuclear matrix associating region (MAR) located ∼ 4.5 kb upstream of CD8α. L2a transgenes driven by the E8(I) enhancer showed no reporter expression in thymic subsets or T cells in splenic, inguinal and mesenteric lymph node peripheral T cells. Deletion of L2a resulted in significant reporter de-repression, even in the CD4(+)CD8(+) double positive (DP) thymocyte population. L2a contains binding sites for two MAR-interacting proteins, SATB1 and CDP. We found that that binding of these factors was markedly influenced by the content and spacing of L2a sub-motifs (L and S) and that SATB1 binds preferentially to the L motif both in vitro and in vivo. A small fraction of the transgenic CD8 single positive (SP) thymocytes and peripheral CD8(+) T cells bypassed L2a-silencing to give rise to variegated expression of the transgenic reporter. Crossing the L2a-containing transgene onto a SATB1 knockdown background enhanced variegated expression, suggesting that SATB1 is critical in overcoming L2a-silenced transcription.

Foxp1 is an essential transcriptional regulator for the generation of quiescent naive T cells during thymocyte development.

Proper thymocyte development is required to establish T-cell central tolerance and to generate naive T cells, both of which are essential for T-cell homeostasis and a functional immune system. Here we demonstrate that the loss of transcription factor Foxp1 results in the abnormal development of T cells. Instead of generating naive T cells, Foxp1-deficient single-positive thymocytes acquire an activated phenotype prematurely in the thymus and lead to the generation of peripheral CD4(+) T and CD8(+) T cells that exhibit an activated phenotype and increased apoptosis and readily produce cytokines upon T-cell receptor engagement. These results identify Foxp1 as an essential transcriptional regulator for thymocyte development and the generation of quiescent naive T cells.

Developmental and species-divergent globin switching are driven by BCL11A.

The contribution of changes in cis-regulatory elements or trans-acting factors to interspecies differences in gene expression is not well understood. The mammalian beta-globin loci have served as a model for gene regulation during development. Transgenic mice containing the human beta-globin locus, consisting of the linked embryonic (epsilon), fetal (gamma) and adult (beta) genes, have been used as a system to investigate the temporal switch from fetal to adult haemoglobin, as occurs in humans. Here we show that the human gamma-globin (HBG) genes in these mice behave as murine embryonic globin genes, revealing a limitation of the model and demonstrating that critical differences in the trans-acting milieu have arisen during mammalian evolution. We show that the expression of BCL11A, a repressor of human gamma-globin expression identified by genome-wide association studies, differs between mouse and human. Developmental silencing of the mouse embryonic globin and human gamma-globin genes fails to occur in mice in the absence of BCL11A. Thus, BCL11A is a critical mediator of species-divergent globin switching. By comparing the ontogeny of beta-globin gene regulation in mice and humans, we have shown that alterations in the expression of a trans-acting factor constitute a critical driver of gene expression changes during evolution.

SATB1 is required for CD8 coreceptor reversal.

Intrathymic signals induce the differentiation of immature CD4(+)CD8(+) double positive (DP) thymocytes into mature CD4(+) or CD8(+) single positive (SP) T cells. The transcriptional mechanism by which CD8 lineage is determined is not fully understood. The best evidence, which favors the kinetic signaling/coreceptor reversal model, indicates that signaled DP thymocytes terminate CD8 transcription prior to their subsequent re-initiation of CD8 transcription and ultimate differentiation into CD8SP T cells. We and others have shown that CD8 lineage commitment is severely perturbed in mice in which expression of the transcription factor SATB1 is either conventionally knocked out or T cell-specifically knocked down. Here, we demonstrate that, as with normal thymocytes, cultured SATB1-deficient DP thymocytes inactivate CD8 coreceptor transcription following receipt of signals (PMA plus ionomycin) that mimic TCR-mediated positive selection. However, this terminated CD8 transcription is not re-initiated by signals (IL-7) conducive to CD8 differentiation in SATB1-deficient DP. We show that SATB1 specifically binds to a cis-regulatory element within the CD8 enhancer (E8(III)) known to be required for coreceptor reversal. A requirement in CD8 coreceptor reversal identifies SATB1 as an essential trans-regulator of CD8 lineage fate, whose action may be mediated via recruitment to the E8(III) DP enhancer.

Epitope tagging of proteins at the native chromosomal loci of genes in mice and in cultured vertebrate cells.

Adding epitope tags to proteins is an important method for biochemical analyses and is generally accomplished in metazoan cells using ectopically expressed, tagged trans-genes. In Saccharomyces cerevisiae, the addition of epitope tags to proteins is easily achieved at the genomic locus of a gene of interest due to the high efficiency of homologous recombination in that organism. Most metazoan cells do not exhibit this high homologous recombination efficiency, and therefore trans-genes with in-frame epitope tags are used. Although epitope tagged trans-genes have proven useful, replacing the native promoter with a heterologous promoter introduces numerous artifactual possibilities. These include overexpression, which can lead to promiscuous interactions, and the loss of native transcriptional control, which in live animals often leads to developmental defects and embryonic lethality. We describe an efficient method that overcomes the problems encountered using epitope tagged trans-genes by introducing the epitope tag into the native chromosomal gene locus in vertebrate cells, embryonic stem cells and live mice. These tagged proteins are physically associated with the expected relevant particles, and highly sensitive as shown by co-purification of homologues of the yeast pre-mRNA splicing factors Prp38p and Prp39p, not previously shown to be associated with metazoan snRNPs. These techniques will enhance the validity of conclusions made regarding epitope-tagged proteins and improve our understanding of proteomic dynamics in cultured vertebrate cells and live animals.

A role for SATB1, a nuclear matrix association region-binding protein, in the development of CD8SP thymocytes and peripheral T lymphocytes.

Studies have suggested that binding of the SATB1 protein to L2a, a matrix association region located 4.5 kb 5' to the mouse CD8alpha gene, positively affects CD8 expression in T cells. Therefore, experiments were performed to determine the effect on T cell development of reduced expression of SATB1. Because homozygous SATB1-null mice do not survive to adulthood due to non-thymus autonomous defects, mice were produced that were homozygous for a T cell-specific SATB1-antisense transgene and heterozygous for a SATB1-null allele. Thymic SATB1 protein was reduced significantly in these mice, and the major cellular phenotype observed was a significant reduction in the percentage of CD8SP T cells in thymus, spleen, and lymph nodes. Mice were smaller than wild type but generally healthy, and besides a general reduction in cellularity and a slight increase in surface CD3 expression on CD8SP thymocytes, the composition of the thymus was similar to wild type. The reduction in thymic SATB1 does not lead to the variegated expression of CD8-negative single positive thymocytes seen upon deletion of several regulatory elements and suggested by others to reflect failure to activate the CD8 locus. Thus, the present results point to an essential role for SATB1 late in the development and maturation of CD8SP T cells.

Bop encodes a muscle-restricted protein containing MYND and SET domains and is essential for cardiac differentiation and morphogenesis.

Many transcription factors regulate specific temporal-spatial events during cardiac differentiation; however, the mechanisms that regulate such events are largely unknown. Using a modified subtractive hybridization method to identify specific genes that influence early cardiac development, we found that Bop is expressed specifically in cardiac and skeletal muscle precursors before differentiation of these lineages. Bop encodes a protein containing MYND and SET domains, which have been shown to regulate transcription by mediating distinct chromatin modifications. We show that m-Bop is a histone deacetylase-dependent transcriptional repressor. Targeted deletion of Bop in mice disrupted maturation of ventricular cardiomyocytes and interfered with formation of the right ventricle. Normal expression of Hand2, a transcription factor essential for right ventricular development, in cardiomyocyte precursors is dependent upon m-Bop. These results indicate that m-Bop is essential for cardiomyocyte differentiation and cardiac morphogenesis.