Sox17-Mediated Maintenance of Fetal Intra-Aortic Hematopoietic Cell Clusters

Abstract

During mouse development, definitive hematopoiesis is first detected around embryonic day 10.5 (E10.5) in the aorta-gonad-mesonephros (AGM) region, which exhibits intra-aortic cell clusters. These clusters are known to contain hematopoietic stem cells (HSCs). On the other hand, it is not clear how the cells in such clusters maintain their HSC phenotype and how they are triggered to differentiate. Here we found that an endodermal transcription factor marker, Sox17, and other F-group (SoxF) proteins, Sox7 and Sox18, were expressed in E10.5 intra-aortic cell clusters. Forced expression of any of these SoxF proteins, particularly Sox17, in E10.5 AGM CD45^low c-Kit^high cells, which are the major component of intra-aortic clusters, led to consistent formation of cell clusters in vitro during several passages of cocultures with stromal cells. Cluster-forming cells with constitutive Sox17 expression retained long-term bone marrow reconstitution activity in vivo. Notably, shutdown of exogenously introduced Sox17 gene expression resulted in immediate hematopoietic differentiation. These results indicate that SoxF proteins, especially Sox17, contribute to the maintenance of cell clusters containing HSCs in the midgestation AGM region. Furthermore, SoxF proteins play a pivotal role in controlling the HSC fate decision between indefinite self-renewal and differentiation during fetal hematopoiesis.

ACKNOWLEDGMENTS

We thank T. Nakano for the OP9 cells, P. Koopman for Sox7 and Sox18 cDNAs and Sox18-deficient mice, K. Nakashima for the pMY-IRES-mCherry vector, T. Kashiwagi for technical support, A. Kudo for help with electron microscopy, and T. Kagawa for valuable discussions. We also thank M. Teramoto and M. Fushimi for secretarial assistance and R. Taguchi, H. Ohnishi, H. Suzuki, and K. Inoue for technical assistance.

This work was supported by the Global COE grant “Cell Fate Regulation Research and Education Unit,” a Grant-in-Aid for Scientific Research (B), and grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The work was partly supported by CREST, JST, and the Takeda Science Foundation. The study was also supported by the Joint Usage/Research project of the Medical Research Institute, Tokyo Medical and Dental University.

I. Nobuhisa performed the experiments, analyzed results, prepared the figures, and cowrote the manuscript. Y. Kishikawa, M. Anani, K. Harada, and K. Saito assisted with experiments, including the culture of AGM-derived cells, and hematopoietic analyses. M. Osawa, H. Takagi, and A. Iwama performed the transplantation analyses. M. Uemura, M. Kanai-Azuma, and Y. Kanai performed in situ hybridization. I. Nobuhisa and T. Taga conceived and directed the project. T. Taga cowrote the manuscript.

We declare no conflicts of interest related to this work.