The Hematopoietic Supportive Function of Adult Bone Marrow Mesenchymal Stem Cells Is Lost after Differentiation Induction towards Adipocytic, Osteoblastic and Vascular Smooth Muscle Lineages

The bone marrow (BM) hematopoietic stem cell (HSC) niche is a specialized structure of the microenvironment, which supports survival and regulates HSC function (i.e. the HSC control of the self-renewal/differentiation balance and migration). The supportive cells involved in the HSC niche are usually named as “stromal cells” but their precise nature remains a matter of debate (in particular, to know whether these cells belong to osteoblastic or to vascular smooth muscle lineage). Mesenchymal stem cells (MSCs) that are present into the BM are characterized by a broad differentiation potential including adipocytic (A), osteoblastic (O) and vascular smooth muscle (V) pathways. Although MSCs are believed to be at the origin of stromal cells, their real function within the niche is unknown. The aim of this study was to investigate in vitro the hematopoietic function (HSC support and migration) of cultured adult BM MSCs non-differentiated and during induced differentiation along A, O and V lineages. MSCs were obtained from BM nucleated cells of patients undergoing orthopedic surgery by culture in expansion medium (alpha-MEM medium with 10% FCS and 1 ng/mL FGF-2). The MSCs were tested before (cultured in expansion medium) and during differentiation induction in appropriate medium for A, O or V lineages (from 3 to 21 days). Interestingly, non-differentiated MSCs already co-expressed O (PTH-receptor), A (leptin) and V (ASMA) markers as assessed by Western blotting. Capacity of MSCs to support hematopoiesis was evaluated by long-term cultures (for 5 wks) with BM CD34+ cells in limiting dilution (CAFC assay), and capacity to control CD34+ cell migration by using Transwells seeded with MSCs (trans-stromal migration assay). We showed that non-differentiated MSCs have the most important capacity to support hematopoiesis (5-week CAFC frequency) and that this capacity was quickly and dramatically lost from 3 days of differentiation towards A (36±2% of non-differentiated values), O (40±3%) and V (38±1%) lineages. This capacity was almost abolished after 14 days of A, O and V differentiation (<5%). In parallel, CD34+ cell migration was clearly reduced through 3-day A and O differentiated MSCs, while it was increased through 3-day V differentiated MSCs (5 fold). These results show that MSCs maintained in vitro in non-differentiated state (although already expressing some A, O and V markers), display the strongest hematopoietic supportive activity compared to MSCs induced to differentiate into adipocytic, osteoblastic, or vascular smooth muscle lineages. Therefore, the stromal cell function could be supported by a cell close to a non-differentiated MSC in endosteal or perivascular niches as well. In contrast, vascular smooth muscle differentiated MSCs at advanced stages could be devoted rather to HSC migration control than to HSC support.