Sox4 Is Required for the Formation and Maintenance of Multipotent Progenitors

Introduction

Hematopoiesis is maintained by a hierarchical system, whereas aberrant control of hematopoiesis is the underlying cause of many diseases. Within the hematopoietic hierarchy, hematopoietic stem cells (HSCs) give rise to multipotent progenitors that have lost their self-renewal capacity but remain multipotent to differentiate into mature blood cells. However, the precise molecular mechanisms that modulate this transition are not fully understood yet.

Results

We recently discovered that genetic ablation of SRY sex determining region Y-box 4 gene (Sox4) in the murine hematopoietic system resulted in dramatic loss of multipotent progenitor population (CD48⁺CD150^-Lin^-kit⁺Sca1⁺, or CD48⁺CD150^-LSK) both relatively (to the total LSK population) and in absolute number. Interestingly, the absolute number of HSCs (CD48^-CD150⁺Lin^-kit⁺Sca1⁺, or SLAM⁺LSK) in these conditional Sox4-deficient mice was comparable to their wild-type counterparts. Transcriptional factor Sox4 belongs to the high-mobility group (HMG) domain superfamily which also includes other Sox proteins, TCF-1 (T-cell factor 1) and LEF-1 (lymphoid enhancer factor 1). Sox4 has been implicated in leukemogenesis and may potentially contribute to stem cell properties. Nevertheless, the precise roles of Sox4 in hematopoietic stem/progenitor cells and the underlying mechanisms have not been defined yet. Further analysis of stem/progenitor compartment defined by Flt3 and CD34 expression demonstrated a major loss in lymphoid-primed multipotent progenitors (LMPPs) (CD34⁺Flt3⁺LSK) with relatively normal formation of LT-HSCs (CD34^-Flt3^-LSK) and ST-HSCs (CD34⁺Flt3^-LSK) upon the loss of Sox4, suggesting that Sox4 is essential for the development from HSCs to multipotent progenitors. Such observation is in line with the expression pattern of Sox4. Quantitative PCR (qPCR) analysis of wild-type mice revealed that expression of Sox4 increased from HSCs to multipotent progenitors which expressed Sox4 at the highest level among all the hematopoietic compartments.

Studies of biological behaviors further indicateed that disruption of Sox4 had no effect on proliferative capacity of HSCs and multipotent progenitors, as evidenced by BrdU incorporation assay. However, Annexin V/propidium iodide staining revealed an increased frequency of apoptotic multipotent progenitors, but not that of HSCs upon the ablation of Sox4. In a transplantation setting, although Sox4-deficient LSKs homed appropriately to the bone marrow, they exhibited severely impaired ability to give rise to multipotent progenitors, but contributed normally to HSCs compared to the wild-type donors.

Among a set of genes crucial to the biological properties of stem/progenitor cells, qPCR analysis revealed that upon the loss of Sox4, only the levels of Ikaros1 and Ikaros2, the two major Ikaros isoforms in stem/progenitor cells, were downregulated specifically in multipotent progenitors, but remained normal in HSCs. Intriguingly, in a reminiscent manner of Sox4-deficient mice, mice lacking both Ikaros 1 and Ikaros 2 proteins, also exhibited disrupted B cell development and selectively impaired LMPPs. Previous study identified an enhancer of Ikaros locus as the only cis-regulatory element that was capable of stimulating reporter expression in the LMPPs. Our sequence analysis revealed a highly conserved Sox4 binding motif within this enhancer, therefore potentially connecting Sox4 with the known regulatory networks that modulate the differentiation of HSCs. Currently, we are working on (1) confirming the direct transcriptional regulation of Ikaros by Sox4; (2) assessing whether Ikaros mediates the functions of Sox4 in the formation or maintenance of the multipotent progenitors population in vivo; and (3) delineating the downstream regulatory network of Sox4 in stem/progenitor cells.

Conclusion

In summary, out study reveals a novel role for Sox4 gene in early hematopoiesis and brings important insights into the regulatory mechanisms underlying the commitment of HSCs toward multipotent progenitors.