Bone marrow microenvironment

Project leaders: Carlijn Voermans

Adult hematopoietic stem and progenitor cells (HSPC) reside in the bone marrow (BM) in so-called niches. Mesenchymal stromal cells (MSC) are an important constituent of this bone marrow microenvironment where they support hematopoiesis via direct cell-cell interactions with hematopoietic stem- and progenitor cells, and by releasing soluble factors (such as Wnt and Slit proteins). However, the mechanism by which the microenvironment regulates hematopoiesis is not completely understood. In the group of Carlijn Voermans PhD the homeostatic and regenerative BM-environment are studied from a HSPC and MSC perspective with a focus on the interactions between the hematopoietic and stromal compartment.

Matrix protein βig-h3 function in hematopoietic and stromal cells
Carlijn Voermans has demonstrated during her stay at the Reya-lab (Durham, NC, USA) that the ECM protein βig-h3 is strongly upregulated in regenerating mouse HSC. Ectopic expression of βig-h3 causes an accelerated differentiation of HSCs and rapid exhaustion of murine, primitive progenitors in vivo as well as in vitro. In the current PPO-C granted project (started Aug 2007) we investigate the role of βigh3 in human hematopoiesis and we postulated that βig-h3 could have a role in HSC biology by being both a homeostatic and regenerative regulator of HSC self-renewal and differentiation. 

We observed high expression of βig-h3 in bone marrow stromal cells, whereas expression in hematopoietic progenitors is low and increases as cells differentiate to monocytes. Immunofluorescence analysis of stromal cells suggests that most βig-h3 is excreted, but we also detect a distinct intracellular staining. The functional relevance of βig-h3 in HSPC differentiation and self-renewal was studied by lentiviral mediated overexpression and knock-down. The overexpression data indicate that βig-h3 functions differentially in distinct hematopoietic lineages, while the knock-down of βig-h3 in HSPC suggest that βig-h3 plays a role in cell proliferation and that reduction of βig-h3 levels maintains renewal capacity of HSPC. This PhD project also involves studying the role of βigh-3 in adhesive interactions and migration of HSPC and its role in hematopoietic malignancies.

Role of mesenchymal stromal cells (MSCs)
MSCs are a potential cell source for cellular therapies, in which recruitment and migration of MSC towards injured tissue is crucial. However, we previously showed that culture-expanded MSC contain only a small percentage of migrating cells in vitro (Maijenburg et al., 2009). To identify genes involved in the process of MSC migration, we generated gene expression profiles of migrating and non-migrating fetal bone marrow MSC (FBMSC) (Maijenburg et al. Stem Cells and Dev 2012). The nuclear receptors Nur77 and Nurr1 showed the highest expression in migratory MSC. Lentiviral expression of Nur77 or Nurr1 enhanced migration of FBMSC toward SDF-1α compared with mock-transduced FBMSC. Further, gain-of-function experiments showed increased hepatocyte growth factor expression and interleukin (IL)-6 and IL-8 production in MSC. Despite the altered cytokine profile, FBMSC expressing Nur77 or Nurr1 maintained the capacity to inhibit T-cell proliferation in a mixed lymphocyte reaction. Our results demonstrate that Nur77 and Nurr1 promote FBMSC migration. Modulation of Nur77 and Nurr1 activity may therefore offer perspectives to enhance the migratory potential of FBMSC which may specifically regulate the local immune response. The current PhD project aims to define and improve migration/homing of relevant MSC subsets and to study the immune-modulatory capacities of these MSC in in vitro and in vivo models.

In addition, we studied the isolation and use of primary MSCs (without ex vivo expansion) and demonstrated that CD271 and CD146 define distinct colony-forming-unit-fibroblasts containing mesenchymal stromal cell subpopulations (Maijenburg et al. Haematologica 2012). Analysis of 86 bone marrow samples revealed that the distribution of CD271brightCD146- and CD271brightCD146+ subsets correlates with donor age. The main subset in adults was CD271brightCD146-, whereas the CD271brightCD146+ population was dominant in pediatric and fetal bone marrow. A third subpopulation of CD271-CD146+ cells contained colony-forming-unit-fibroblasts in fetal samples only. These changes in composition of the mesenchymal stromal cell compartment during development and aging suggest a dynamic system, in which these subpopulations may have different functions.

Interestingly, we observed clear differences in Wnt-(target) gene expression between the primary and cultured adult MSC subsets and between Adult BMSC and Fetal BMSC, which correlated with differences in hematopoietic support. ABMSC and FBMSC differed in Wnt5a expression, but also in the response to exogenous Wnt3a. The distinct response to inhibition of endogenous Wnt-production may be explained by variation in expression of Wnt-inhibitors and Frizzled receptors on the two MSC sources. This seems to lead to a different net balance in autocrine Wnt-signaling between these cells, established through distinct intracellular mechanisms. These last two topics are now studied in the current PhD project.

Key publications

Ongoing projects

Last edited on: 16 April 2013