Developing thymocytes roll the Dice(r)

Micro-RNAs (miRNAs) and small interfering RNAs (siRNAs) — which are short (21–22-nucleotide) RNAs that are generated from longer precursors by the ribonuclease III enzyme Dicer — regulate gene transcription in many organisms and have been proposed to have a role in controlling haematopoiesis. Now, a report published in The Journal of Experimental Medicine shows that Dicer is required for the generation and survival of αβ-T-cell receptor (αβ-TCR)⁺ T cells.

It is estimated that one in three mRNAs are regulated by miRNAs, and several pieces of evidence indicate that miRNAs regulate haematopoiesis. Mice that are constitutively deficient in Dicer die in utero, so to investigate the role of Dicer-generated small RNAs in T-cell development, Cobb et al. generated mice in which Dicer was specifically eliminated at the early stages of T-cell development. Mice in which Dicer was eliminated at the double negative 3 (DN3) stage of T-cell development (lckCre Dicer^Δ/Δ mice) had markedly reduced levels of the miRNAs miR-181, miR-16 and miR-142s in both total thymocyte populations and double positive (DP) thymocytes. Thymic cellularity in the lckCre Dicer^Δ/Δ mice was also markedly reduced compared with mice expressing functional Dicer. Interestingly, this decrease was mainly a consequence of a reduction in the number of αβ-TCR⁺ thymocytes, whereas there were normal (or even slightly increased) numbers of DN thymocytes and γδ-TCR⁺ thymocytes. Further analysis of thymocytes isolated from lckCre Dicer^Δ/Δ mice showed that they were less able to survive in vitro than thymocytes from mice expressing functional Dicer.

Despite the decreased number of DP thymocytes in the lckCre Dicer^Δ/Δ mice, the frequency of cells being positively selected was normal. Furthermore, those few cells that progressed to the CD4⁺ or CD8⁺ single-positive stage transcribed lineage-specific genes — T-helper-inducing POZ/Krüppel-like factor (Th-POK; also known as cKROX and ZFP76) and cathepsin W, respectively — indicating that CD4/CD8 lineage choice is not regulated by Dicer-generated small RNAs.

These data indicate that functional Dicer is required for the generation and survival of αβ-TCR⁺ T cells, although it is not required for CD4/CD8 lineage choice or for the specification of lineage-specific gene expression. The mechanisms by which Dicer regulates αβ-TCR⁺ T-cell development and survival remain to be determined. However, although Dicer has been shown to mediate heterochromatin silencing through the generation of siRNAs, the authors found no evidence of a defect in either constitutive or facultative heterochromatin silencing in DP thymocytes isolated from the lckCre Dicer^Δ/Δ mice.