Recent insights into the transcriptional control of the Tcra/Tcrd locus by distant enhancers during the development of T-lymphocytes

Abstract

Tcra/Tcrd includes 2 genes with distinct developmental programs controlled by 2 distant enhancers, Eα and Eδ. These enhancers work as a developmental switch during thymocyte development and they are essential for generation of αβ and γδ T-lymphocytes. Tcra and Tcrd transit from an unrearranged configuration to a rearranged configuration during T-cell development. Eα and Eδ are responsible for transcription of their respective unrearranged genes in thymocytes but are dispensable for such functions in the context of the rearranged genes in mature T-cells. Interestingly, Eα activates transcription of the rearranged Tcrd in γδ T-lymphocytes but it is inactive in αβ T-lymphocytes.

Keywords: T-cell development; T-cell receptor; T-lymphocytes; chromatin, enhancer, promoter; thymocytes; transcription.

Figure 1.

V(D)J rearrangement at TCR loci during the development and generation of αβ and γδ T-lymphocytes. Schematic diagram of thymocyte maturation depicting the various developmental stages and periods of TCR locus rearrangements. β- and γδ-selection, which depends on the expression of a pre-TCR or a TCRγδ, respectively, are indicated in red. The positive selection, negative selection, and death by neglect events are also indicated in red. Commitment to the T-cell lineage is indicated by the transition from light blue to dark blue (αβ T-lymphocytes) or purple (γδ T-lymphocytes). The approximate percentage of thymocytes present in the different populations is indicated.

Figure 2.

Representation of the genomic structure of the Tcra/Tcrd locus and active Eδ- and Eα-promoter interactions. The V, D, and J segments are represented by yellow rectangles, and the recombination signal sequences are shown as black or white triangles depending on the presence of 23-bp or 12-bp spacers, respectively. The Cδ and Cα regions are represented as green rectangles. The 1.6-Mb 5′-region includes the Vα and Vδ gene segments, and the 0.1-Mb 3′-region includes the Tcrd locus, the Jα gene segments, the Cα region and Eα. A red circle and a blue circle represent Eδ and Eα, respectively. The blue lines represent the chromatin area of Eα influence, whereas the red line represents that of Eδ. Promoters are represented as diamonds: red diamonds represent promoters that are responsive to Eδ, blue diamonds represent promoters that are responsive to Eα, and gray diamonds represent promoters that are not responsive to either Eδ or Eα. Straight arrows represent active sites of transcription associated with promoters. Curved arrows represent enhancer-promoter interactions: red curved arrows represent Eδ-promoter interactions, and blue curved arrows represent Eα-promoter interactions.

Figure 3.

Model for the structure of Eδ and Eα enhanceosomes during T-cell development. The diagrams depict the TFs that are presumably bound to Eδ and Eα in thymocytes and T-lymphocytes. The locations of the defined δE3 and δE4 Eδ elements and of the Tα1, Tα2, Tα3, and Tα4 Eα elements are indicated. The identity of the different TFs is indicated by color-coded ovals. The strength of TF binding is represented by the positions of the colored ovals representing TFs on enhancer DNA: a more centered position represents stronger binding. Different TF-bound complexes are assembled on Eδ and Eα during T-cell development. In DN2/3a thymocytes, Eδ is active and occupied by c-Myb, Runx-1, and GATA-3, whereas Eα is primed and occupied by CREB, TCF-1/LEF-1, Runx-1, Ets-1, Sp1, GATA-3, and E2A/HEB; in DN4 and eDP thymocytes, Eδ becomes inactive and unoccupied by TFs, whereas Eα is activated through the recruitment of the pre-TCR-induced NFAT, Egr-1, and AP-1 TFs; in preselected lDP thymocytes, Eδ remains inactive and unoccupied, whereas Eα is activated through the strong binding of E2A and Ets-1; in SP thymocytes and αβ T-lymphocytes, Eδ remains inactive and presumably unoccupied, whereas Eα is inactive and demonstrates low binding of E2A and HEB TFs; and in γδ T-lymphocytes, Eδ is active and occupied by c-Myb, Runx-3, and GATA-3, whereas Eα is also active and presumably occupied by the same TFs assembled in the preselected lDP thymocyte enhanceosome, with the exception of Runx-3 that it might be exchanged for Runx-1.

Figure 4.

Regulation of Tcra/Tcrd transcription by Eδ and Eα during T-cell development. Schematic diagram of T-cell development depicting the various maturation stages. β- and γδ-selection are indicated. Eδ and Eα are represented a red and blue crossed ovals, respectively. The Tcrd locus and Eδ-activated cell stages are represented in red, the Tcra locus and Eα-activated cell stages are represented in blue, the Eδ- and Eα-activated stage corresponding to γδ T-lymphocytes is represented in purple, and the cell stages during which both Eα and Eδ are inactive is represented in white. Crossed circles represent inactive enhancers. A red empty circle represents deleted Tcrd. Curved arrows represent enhancer-promoter interactions: red curved arrows represent Eδ-promoter interactions, and blue curved arrows represent Eα-promoter interactions. In DN2/3a thymocytes, Eδ drives germline Tcrd transcription and VδDδJδ rearrangements, whereas Eα is primed but inactive. In DN4, eDP, and preselected lDP thymocytes, Eα drives germline Tcra transcription and primary VαJα rearrangements, whereas Eδ becomes inactive. In SP thymocytes and αβ T-lymphocytes, both Eα and Eδ are inactive. In γδ T-lymphocytes, Eα and Eδ are both active to drive the transcription of the rearranged Tcrd locus.