Identification of a stroma-mediated Wnt/beta-catenin signal promoting self-renewal of hematopoietic stem cells in the stem cell niche

Regulation of CXCL12 expression by canonical Wnt signaling in bone marrow stromal cells

CXCL12 (stromal cell-derived factor-1, SDF-1), produced by stromal and endothelial cells including cells of the bone marrow, binds to its receptor CXCR4 and this axis regulates hematopoietic cell trafficking. Recently, osteoclast precursor cells were found to express CXCR4 and a potential role for the CXCL12-CXCR4 axis during osteoclast precursor cell recruitment/retention and development was proposed as a regulator of bone resorption. We examined the role of canonical Wnt signaling in regulating the expression of CXCL12 in bone marrow stromal cells. In mouse stromal ST2 cells, CXCL12 mRNA was expressed, while its expression was reduced in Wnt3a over-expressing ST2 (Wnt3a-ST2) cells or by treatment with lithium chloride (LiCl). Wnt3a decreased CXCL12 levels in culture supernatants from mouse bone marrow stromal cells. The culture supernatant from Wnt3a-ST2 cells also reduced migratory activity of bone marrow-derived cells in a Transwell migration assay. Silencing of glycogen synthase kinase-3β decreased CXCL12 expression, suggesting that the canonical Wnt signaling pathway regulates CXCL12 expression. In a transfection assay, LiCl down-regulated the activity of a reporter gene, a 1.8kb fragment of the 5'-flanking region of the CXCL12 gene. These results show that canonical Wnt signaling regulates CXCL12 gene expression at the transcriptional level, and this is the first study linking chemokine expression to canonical Wnt signaling.

Proliferation and bone marrow engraftment of AML blasts is dependent on β-catenin signalling

B-catenin is the central effector molecule of the canonical Wnt signalling pathway, which controls self-renewal of haematopoietic stem cells. Deregulation of this pathway occurs in various malignancies including myeloid leukaemias. The present study examined the functional outcome of stable β-catenin down-regulation through lentivirus-mediated expression of short hairpin RNA (shRNA). Reduction of the β-catenin levels in acute myeloid leukaemia (AML) cell lines and patient samples decelerated their in vitro proliferation ability without affecting cell viability. Transplantation of leukaemic cells with control or reduced levels of β-catenin in non-obese diabetic severe combined immunodeficient animals indicated that, while the immediate homing of the cells was unaffected, the bone marrow engraftment was directly dependent on β-catenin levels. Subsequent examination of bone sections revealed that β-catenin was implicated in the localization of AML to the endosteum. Examination of adhesion molecule expression before and after transplantation, revealed down-regulation of CD44 expression, accompanied by reduced in vitro adhesion. Gene expression analysis disclosed the presence of an autocrine compensatory mechanism, which responds to the reduced β-catenin levels by altering the expression of positive and negative pathway regulators. In conclusion, our study showed that β-catenin comprises an integral part of AML cell proliferation, cell cycle progression, and adhesion, and influences disease establishment in vivo.

Concise review: multiple niches for hematopoietic stem cell regulations

Two types of stem cell niches in bone marrow (BM), endosteal osteoblastic, and vascular niches are involved in the microenvironmental regulation of hematopoietic stem cells (HSCs). Recently, redundant features of the two niches were identified, based on their common cellular origins or chemical mediators being produced in each niche. In contrast, studies have also revealed that HSCs are localized differentially in the niches with respect to their distinct functional status, and that the biological activity of each niche is differentially influenced by extrinsic conditions. An important question is, therefore, whether these two niches play distinct roles in regulating HSCs and whether they respond differentially to environmental stimuli/stress for "compartmentalized" niche organization in BM. In this review, recent discoveries related to the characteristics of each type of niche and their common or unique features are discussed, along with the possibility of multiniche regulation of HSCs in BM.

Intramarrow injection of beta-catenin-activated, but not naive mesenchymal stromal cells stimulates self-renewal of hematopoietic stem cells in bone marrow

Bone marrow mesenchymal stromal cells (MSCs) have been implicated in the microenvironmental support of hematopoietic stem cells (HSCs) and often co-transplanted with HSCs to facilitate recovery of ablated bone marrows. However, the precise effect of transplanted MSCs on HSC regeneration remains unclear because the kinetics of HSC self-renewal in vivo after co-transplantation has not been monitored. In this study, we examined the effects of intrafemoral injection of MSCs on HSC self-renewal in rigorous competitive repopulating unit (CRU) assays using congenic transplantation models in which stromal progenitors (CFU-F) were ablated by irradiation. Interestingly, naïve MSCs injected into femur contributed to the reconstitution of a stromal niche in the ablated bone marrows, but did not exert a stimulatory effect on the in-vivo self-renewal of co-transplanted HSCs regardless of the transplantation methods. In contrast, HSC self-renewal was four-fold higher in bone marrows intrafemorally injected with beta-catenin-activated MSCs. These results reveal that naive MSCs lack a stimulatory effect on HSC self-renewal in-vivo and that stroma must be activated during recoveries of bone marrows. Stromal targeting of wnt/beta-catenin signals may be a strategy to activate such a stem cell niche for efficient regeneration of bone marrow HSCs.

How the niche regulates hematopoietic stem cells

The hematopoietic stem cell (HSC) forms all types of blood cells of the hematopoietic system. In the adult, HSC are mainly quiescent, being mostly in G0/G1 phase of cell cycle during steady-state conditions. However, during hematopoietic stress, the stem cells respond quickly to regenerate the damaged hematopoietic system. To understand how environmental signals affect HSC and its progeny, it is essential to know the lineage relationships and transcriptional mechanisms controlling self-renewal, proliferation and differentiation. Because of the high possible output of blood cells from a single HSC, a tight regulation of these processes is extremely important. An essential component for this control is the marrow microenvironment, in this context also referred to as the HSC niche. The niche is heterogeneous and regulates stem cell metabolism through both surface-bound and soluble factors. Several signaling pathways have been shown to take part in these regulation processes, with Notch and especially Wnt signaling being the best studied ones. Dysregulation of the niche, for instance by environmental exposure, has recently been shown to lead to hematopoietic abnormalities. Thus, to understand the effect of the environment on hematopoiesis, it is of importance to study both HSC, its direct progeny and the cellular components of the niche. Detailed knowledge of the regulatory mechanisms operating between hematopoietic cells and their direct surroundings facilitates the study of how such signaling may be disrupted by environmental exposure.