Function of Gata-2 in thymic epithelial cells : a transcription factor identified from gene expression analysis of endodermal cells committed to thymic epithelial cell fate

The thymus structure composes of clear morphological regions. The T-cell precursors enter the thymus in the cortico-medullary junction and migrate through the cortex towards the sub capsular region and back through to the cortex into the medulla. During this migration process the thymic epithelial cells provide the microenvironment for the maturation and selection of the majority of the peripheral T-cells. The thymic epithelial cells have their origin in the endodermal cells of the ventral aspect of the 3rd pharyngeal pouch while endodermal cells of the dorsal aspect of the 3rd pharyngeal pouch give rise to the parathyroid glands. For a better understanding of genes which might be involved in determination of endodermal cells to the thymic epithelial cell fate, the gene expression profile of the ventral aspect of 3rd pharyngeal pouch was compared to the dorsal aspect of 3rd pharyngeal pouch using microarrays. The analysis revealed 69 genes which were up regulated in the ventral aspect of 3rd pharyngeal pouch. Eleven genes with the largest differential expression values were further assessed (Gata-2, dll-1, C1qdc2, Samd5, Msx2, Msx1, Ehox, Tgfbi, Unc5c, FoxG1, 1110006E14Rik) using RT-PCR and whole mount in situ hybridization. The genes dll-1, Tgfbi, Msx1 and Msx2 are involved in the Notch, Tgf? and Bmp pathways, respectively. All these pathways are associated with thymus development. The role of the genes Ehox, Gata-2, C1qdc2, Samd5 and Unc5c in thymus development is so far undefined.
Gata-2, a transcription factor, known to be involved in hematopoiesis, was the only gene of which its expression was detected by gene chip data, RT-PCR and whole mount in situ hybridization. These results identified Gata-2 as a novel candidate that might be involved in the thymic epithelial cell development. To characterize the function of Gata-2 in thymus development, Gata-2 was specifically deleted in thymic epithelial cells using Foxn1-Cre. The thymi of 3, 6, 13, and 25 weeks old mice were removed and detailed studies were performed. FACS analysis of these thymi revealed an increased thymus cellularity in DN1-DN4, CD4, and CD8 in 6 weeks old thymi and onwards. The thymus architecture which was analyzed by H&E and immunohistochemistry (UEA-1, CK8, CK18, ERTR7) was unaffected when Gata-2 was deleted in TECs. The assessment of TEC population of Gata-2 KO mice did not show any difference. But the gene expression analysis of Gata-2 deficient TECs for the genes c-Jun, CXCL-12, CCL-25, IL-7, c-Fos, c-kit ligand, Edn-1, Edn-Ra, und Edn-Rb showed that CXCL-12 and c-kit ligand were higher expressed. CXCL-12 is involved in homing of T-cell precursors while c-kit L is involved in survival and proliferation of T-cell precursors. 7
In conclusion, Gata-2 might negatively regulate the transcription of CXCL-12 and c-kit ligand. A lack of Gata-2 expression in thymic epithelial cells, therefore, might lead to an increased T-cell precursor attraction and survival/proliferation, thus, explaining the higher cellularity observed in thymus of Gata-2 deficient mice.