HOXB4 can enhance the differentiation of embryonic stem cells by modulating the haematopoietic niche

Haematopoietic differentiation of embryonic stem (ES) cells in vitro has been used as a model to study early haematopoietic development and it is well documented that haematopoietic differentiation can be enhanced by over-expression of HOXB4. HOXB4 is expressed in haematopoietic progenitor cells (HPC) where it promotes self-renewal, but it is also expressed in the primitive streak of the gastrulating embryo. This led us to hypothesise that HOXB4 might modulate gene expression in pre-haematopoietic mesoderm and that this property might contribute to its pro-haematopoietic effect in differentiating ES cells. To test our hypothesis we developed a conditionally activated HOXB4 expression system using the mutant oestrogen receptor (ER(T2)) and showed that a pulse of HOXB4 prior to HPC emergence in differentiating ES cells led to an increase in haematopoietic differentiation. Expression profiling revealed an increase in the expression of genes associated with paraxial mesoderm that gives rise to the haematopoietic niche. We therefore considered that HOXB4 might modulate the formation of the haematopoietic niche as well as the production of haematopoietic cells per se. Cell mixing experiments supported this hypothesis demonstrating that HOXB4 activation can generate a paracrine as well as a cell autonomous effect on haematopoietic differentiation. We provide evidence to demonstrate that this activity is partly mediated by the secreted protein FRZB.