T-cell development and selection in the thymus depend on distinct populations of thymic epithelial cells (TECs). The forkhead transcription factor FoxN1 (also known as Whn), which is lacking in athymic nude mice, is required for the growth and differentiation of TECs. However, the signalling pathways that control expression of FoxN1 are not known. In Nature Immunology, Balciunaite and colleagues now report that Wnt signalling pathways regulate the expression of FoxN1 and are crucial for TEC development.

Signals from Wnt glycoproteins are transduced through three intracellular pathways, including the Wnt-β-catenin pathway, which is central to many cell-fate decisions during development. Wnt proteins bind cell-surface receptors composed of Frizzled proteins and low-density lipoprotein receptor-related proteins 5 and 6 (Lrp5 and Lrp6), and downstream signalling inhibits the phosphorylation of β-catenin. Dephosphorylated β-catenin interacts with the high-mobility group (HMG) proteins T-cell factor 1 (Tcf1), Tcf3 and Tcf4, and with lymphoid enhancer binding factor 1 (Lef1), enabling these proteins to activate the transcription of downstream target genes.

Mice that are deficient in Wnt1 and Wnt4 have reduced thymic cellularity, and following experiments that showed that Wnt glycoproteins are expressed by TECs, the authors investigated whether TECs respond to Wnt signalling. TEC lines were transfected with a reporter construct (known as TOP) that contains the luciferase gene under the control of Tcf and Lef1 proteins. These cells were then co-transfected to overexpress various Wnt proteins. Wnt1 and Wnt4 were shown to activate Tcf- and Lef1-dependent transcription of the TOP construct, which indicates that TECs do respond to Wnt signals.

Next, the authors asked whether FoxN1 gene transcription is regulated by Wnt signalling. The level of FoxN1 messenger RNA was increased in TECs cultured with cells that overexpressed Wnt4 or Wnt5. In addition, FoxN1 transcription was blocked in the presence of soluble Frizzled proteins (which inhibit Wnt signalling). The putative promoter region of FoxN1 was shown to be responsive to Wnt4 when it was cloned into a promoter-less luciferase construct.

These results show that Wnt signalling pathways control the transcription of FoxN1 in TECs and that they are implicated in control of the genetic programme of TECs that is required for their development and thymic function.