Anovel role of BMP4 in adult hematopoietic stem and progenitor cell homing via Smad independent regulation of integrin-α4 expression

Although it is well established that BMP4 plays an important role in the development of hematopoietic system, it is less well understood whether BMP4 affects adult hematopoiesis and how. Here, we describe a novel mechanism by which BMP4 regulates homing of murine as well as human hematopoietic stem/progenitor cells (HSPCs). BMP4 treatment of murine BM derived c-kit ⁺Lin^-Sca-1⁺ (KLS) and CD150 ⁺CD48^-KLS cells for up to 5 days in vitro prevented the culture-induced loss of Integrin-α4 (ITGA4) expression as well as homing. The effect on ITGA4 expression in response to BMP4 is mediated via SMAD-independent phosphorylation of p38 MAPK, which activates microphthalmia associated transcription factor (MITF), known to induce ITGA4 expression. Elevated ITGA4 expression significantly enhanced HSPC attachment to bone marrow stromal cells, homing and long-term engraftment of the BMP4 treated cells compared with the cells cultured without BMP4. BMP4 also induced expression of ITGA4 on human BM derived Lin^-CD34⁺ cells in culture, which was associated with improved homingpotential. Thus, BMP4 prevents culture-induced loss of ITGA4 expression on HSPCs in a SMAD-independent manner, resulting in improved homing of cultured HSPCs and subsequent hematopoietic reconstitution.