The Wnt Signaling Antagonist Kremen1 is Required for Development of Thymic Architecture

Wnt signaling has been reported to regulate thymocyte proliferation and selection at several stages during T cell ontogeny, as well as the expression ofFoxN1in thymic epithelial cells (TECs). Kremen1 (Krm1) is a negative regulator of the canonical Wnt signaling pathway, and functions together with the secreted Wnt inhibitor Dickkopf (Dkk) by competing for the lipoprotein receptor-related protein (LRP)-6 co-receptor for Wnts. Herekrm1knockout mice were used to examinekrm1expression in the thymus and its function in thymocyte and TEC development.krm1expression was detected in both cortical and medullary TEC subsets, as well as in immature thymocyte subsets, beginning at the CD25+CD44+ (DN2) stage and continuing until the CD4+CD8+(DP) stage. Neonatal mice show elevated expression ofkrm1in all TEC subsets.krm1− / −mice exhibit a severe defect in thymic cortical architecture, including large epithelial free regions. Much of the epithelial component remains at an immature Keratin 5+(K5) Keratin 8+(K8) stage, with a loss of defined cortical and medullary regions. A TOPFlash assay revealed a 2-fold increase in canonical Wnt signaling in TEC lines derived fromkrm1− / −mice, when compared withkrm1+ / +derived TEC lines. Fluorescence activated cell sorting (FACS) analysis of dissociated thymus revealed a reduced frequency of both cortical (BP1+EpCAM+) and medullary (UEA-1+EpCAMhi) epithelial subsets, within thekrm1− / −thymus. Surprisingly, no change in thymus size, total thymocyte number or the frequency of thymocyte subsets was detected inkrm1− / −mice. However, our data suggest that a loss of Krm1 leads to a severe defect in thymic architecture. Taken together, this study revealed a new role for Krm1 in proper development of thymic epithelium.