Foxn1 maintains thymic epithelial cells to support T-cell development via mcm2 in zebrafish Dongyuan Ma1, Lu Wang1, Sifeng Wang1, Ya Gao, Yonglong Wei, and Feng Liu2 State Key Laboratory of Biomembrane and Membrane Biotechnology, Institute of Zoology, Chinese Academy of Sciences, Beijing 100101, China Edited by Max D. Cooper, Emory University, Atlanta, GA, and approved November 7, 2012 (received for review September 30, 2012) The thymus is mainly comprised of thymic epithelial cells (TECs), for the specification of lymphoid progenitors toward the T-cell which form the unique thymic epithelial microenvironment essential lineage (18, 19). However, other Foxn1-regulated downstream for intrathymic T-cell development. Foxn1, a member of the forkhead target genes have not yet been reported. transcription factor family, is required for establishing a functional To investigate the function of foxn1 during the development of thymic rudiment. However, the molecular mechanisms underlying thymus and T cells, we have used the zebrafish model to knock the function of Foxn1 are still largely unclear. Here, we show that down foxn1 expression by using antisense morpholinos (MO). Foxn1 functions in thymus development through Mcm2 in the Our data show that foxn1-deficient embryos display impaired zebrafish. We demonstrate that, in foxn1 knockdown embryos, expression of T-cell markers, whereas the expression of early TEC the thymic rudiment is reduced and T-cell development is im- progenitor markers remains relatively unchanged. Expression pro- paired. Genome-wide expression profiling shows that a number filing and functional analysis demonstrate that, besides the pre- of genes, including some known thymopoiesis genes, are dys- viously reported downstream target genes (such as ccl25) in medaka regulated during the initiation of the thymus primordium and and mice, a unique foxn1-mcm2 axis plays a pivotal role during immigration of T-cell progenitors to the thymus. Functional and the development of TECs and T cells in zebrafish. epistatic studies show that mcm2 and cdca7 are downstream of Foxn1, and mcm2 is a direct target gene of Foxn1 in TECs. Finally, Results we find that the thymus defects in foxn1 and mcm2 morphants T-Cell Development Is Impaired in Zebrafish foxn1 Morphants. Foxn1 might be attributed to reduced cell proliferation rather than apo- has been demonstrated to be necessary in thymopoiesis in many ptosis. Our results reveal that the foxn1-mcm2 axis plays a cen- vertebrates (9, 11, 16). To study the role of zebrafish Foxn1, tral role in the genetic regulatory network controlling thymus antisense MOs (16) were used to knock down the expression of development in zebrafish. foxn1 in zebrafish embryos. Then, whole mount in situ hybrid- ization (WISH) and Western blotting were carried out to check he thymus is a central hematopoietic organ that produces ma- the endogenous expression of zebrafish foxn1 mRNA and the Tture T lymphocytes, one of the major players of the vertebrate encoded protein Foxn1. We found that both the levels of foxn1 adaptive immune system (1). In vertebrates, including zebrafish and mRNA and Foxn1 protein were down-regulated in the zebrafish mice, the thymus primordium is derived from the third pharyngeal embryos injected with 4 ng of foxn1 MOs (Fig. 1 A and B). When endodermal pouch and then differentiates into functional cortical foxn1 MOs were injected into a rag2:dsRed transgenic line at the + and medullary thymic epithelial cells (TECs) (1–4). TECs repre- one cell stage, the numbers of the dsRed T cells were signif- sent the primary functional cell type that forms the unique thymic icantly decreased at 5 dpf (Fig. 1C). Compared with the control, epithelial microenvironment supporting T-cell differentiation. the expression of several T-cell markers including rag1, il7r, and Therefore, the thymic epithelial microenvironment must be tightly ikaros was remarkably decreased in the thymus of zebrafish foxn1 controlled by extrinsic signals and intrinsic factors to support morphants at 4 dpf (Fig. 1D). Quantitative RT-PCR (qPCR) fur- T-cell differentiation and maturation (5). Several signaling path- ther confirmed the WISH data (Fig. 1E). Moreover, when foxn1 ways and transcription factors have been demonstrated in thymus was knocked down in a cmyb:GFP transgenic line, the population + and T-cell development during vertebrate embryogenesis (4, 6–10). of the GFP T cells in the thymus was greatly reduced, whereas + Foxn1, Forkhead box protein N1, a winged-helix forkhead the numbers of the GFP hematopoietic progenitors in the pro- transcription factor, occupies a central position in the genetic nephros (the equivalent of bone marrow in mammals) and caudal network(s) that establishes a functional thymic rudiment (9, 11, 12). hematopoietic tissue (the equivalent of fetal liver in mammals) at − − Foxn1 / mice are athymic and hairless (9). Hypomorphic allele 4 dpf were not changed in foxn1 morphants (Fig. S1A) (20). In studies have suggested that Foxn1 is required for TEC development addition, a parathyroid marker, gcm2, was unchanged in foxn1 in both fetal and adult thymus in a dosage-dependent manner morphants (Fig. S1B). These results suggest a role for foxn1 in (13, 14). Conversely, overexpression of foxn1 can improve the thymus, which is consistent with data in mice. Moreover, epcam, reduction in the populations of thymocytes and TECs in aged and hoxa3a, appeared unchanged in foxn1 morphants (Fig. S2A). mice, therefore delaying age-associated thymic involution (15). These data are consistent with the observation that these genes The expression of zebrafish foxn1 is initiated in the thymic pri- act earlier than foxn1 during thymus development in mouse (5, 10). mordium approximately 48 h after fertilization (hpf) and then Previous work showed that thymus homing was defective in zebra- gradually increases with the immigration of T-cell progenitors fish foxn1 morphants because of the down-regulation of chemokine/ marked by rag1 and ikaros (3, 4). Moreover, knockdown of the chemokine receptors (16). Here, expression of chemokine and expression of foxn1 in zebrafish embryos using antisense morpho- linos impairs T-cell development (16). Despite the essential function of foxn1 in the early develop- Author contributions: D.M. and F.L. designed research; D.M., L.W., S.W., Y.G., and Y.W. ment of the thymus, there is limited understanding of its down- performed research; D.M., L.W., and F.L. analyzed data; and F.L. wrote the paper. stream targets and detailed regulatory mechanisms remain elusive. The authors declare no conflict of interest. For example, previous studies have shown that dll4 and chemokine This article is a PNAS Direct Submission. ligand ccl25 might be directly regulated by Foxn1 in mice and 1D.M., L.W., and S.W. contributed equally to this work. medaka (16). Chemokine signaling pathways (ccl25/ccr9, cxcl12/ 2To whom correspondence should be addressed. E-mail: [email protected]. cxcr4) are thought to be important for attracting lymphoid pro- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. genitors (17), whereas the Notch pathway (dll4/notch1) is required 1073/pnas.1217021110/-/DCSupplemental. 21040–21045 | PNAS | December 18, 2012 | vol. 109 | no. 51 www.pnas.org/cgi/doi/10.1073/pnas.1217021110 Downloaded by guest on September 28, 2021 Fig. 1. T-cell development is impaired in zebrafish foxn1 morphants. (A and B) The endogenous expression level of foxn1 transcript and the encoded protein in zebrafish foxn1 morphants at 4 dpf detected by WISH (A) and Western blot (B). Ba, Western blot; Bb, Western blot results were quantified by using Quantity 1 software. (C) The rag2:dsRed expression was abolished in foxn1 morphants at 5 dpf. (D) The expression of lymphocyte markers, including rag1, il7r, and ikaros, was down-regulated in the thymus in zebrafish foxn1 morphants. Anterior to the left and dorsal to the top; circles mark the thymus. (E) qRT-PCR results showing that the expression of rag1, il7r, and ikaros is down-regulated in zebrafish foxn1 morphants (mean ± SD, t test, **P < 0.01, n = 3). IMMUNOLOGY chemokine receptor was also examined. We found that the ex- and Cbfb have been reported to be expressed in mouse T lym- pression of ccr9b and ccl25a was both decreased in foxn1 mor- phocytes (25, 26). The detailed expression pattern of these four phants at 4 dpf (Fig. S2B). Taken together, knockdown of foxn1 genes in zebrafish was examined by using WISH in cloche mu- expression impairs T-cell development in zebrafish embryos. tant, which contains no blood including lymphoid cells, although a few TECs still exist according to the expression of foxn1 (Fig. Expression of mcm2, cdca7, cbfb, and runx3 Is Specifically Down- S4A). Our results show that the absence of runx3 and reduced Regulated in the Thymus of Zebrafish foxn1 Morphants. Although cbfb expression was detected in the thymus, whereas cdca7 and foxn1 is pivotal in establishing a functional thymic rudiment, there mcm2 were still expressed in the thymus of the cloche mutant at is limited understanding of its downstream targets. To further 4 dpf (Fig. S4A). Transverse sections of WISH embryos at 4 dpf study the molecular mechanism of foxn1 in thymopoiesis, micro- clearly demonstrated their expression in the thymus was reduced array experiments were carried out. Zebrafish thymus collected in foxn1 morphants compared with controls (Fig. S4B). To de- at two stages, 2 dpf and 4 dpf, were analyzed because the thymic termine whether these genes were also expressed in TECs and/or anlage forms from the pharyngeal endoderm at 2 dpf and lym- T cells in mammals, we examined their expression in mouse thymus phopoiesis initiates after the expression of rag1 at 4 dpf. According tissue and TEC lines. By RT-PCR, expression of cdca7 and mcm2 to the microarray data, 310 genes were up-regulated, whereas but not runx3 was detected in two mouse TEC lines, 4c18 and 1c6, 466 genes were down-regulated at 2 dpf, and 379 genes were whereas all four genes were expressed in mouse thymus tissue up-regulated, whereas 369 genes were down-regulated at 4 dpf (Fig.