Structural molecules in the niche must also be involved in tightly binding HSC to the bone lining and in releasing HSC from the microenvironment into the circulation. Integrins, which are heterodimeric molecules composed of an α and a β subunit, have long been known to function in engraftment and mobilization of hematopoietic cells [81
]. It was observed that mouse β1−/−
hematopoietic progenitors are unable to seed the fetal liver, but that these progenitors function normally in colony-forming unit assays in vitro
, suggesting that loss of β1 does not cause a defect in hematopoietic cell function, but rather a defect in homing of the cells [82
]. Wagers et al.
investigated the expression of different integrins on Lin−/lo
HSC in the bone marrow and in the circulation after mobilization treatment and found that, in general, HSC express high levels of most β1 integrin subunits and that mobilization downregulated α2 and upregulated α5. Further, mobilized cells did not home as well to the bone marrow as did non-mobilized cells, suggesting that these integrin molecules are indeed important in recruiting cells and retaining them in the bone marrow [83
Zhang et al.
defined the osteoblasts that constitute the support cell of the niche as N-cadherin-positive and claimed that N-cadherin is also expressed in a portion of HSC and forms an adherens complex with β-catenin between the osteoblast and HSC [25
]. However, two aspects of this hypothesis have recently been highly debated. First, these HSC were identified as BrdU-retaining cells based on the theory that stem cells are more quiescent and divide asymmetrically, thus retaining the BrdU longer than other cells [84
], while more recent studies do not support this theory [85
]. In addition, further data have challenged the importance of N-cadherin in maintaining HSC within the niche by suggesting that bone marrow cells with HSC activity do not express N-cadherin [86
]. This latter observation is in contrast to a number of studies which have found N-Cadherin in cells of the hematopoietic lineage [25
Also important in HSC homing to the bone marrow and promoting quiescence are CXCL12 and its receptor, CXCR4. CXCL12 is produced by osteoblasts in the bone marrow [88
]. CXCL12/CXCR4 are important in HSC retention in the niche [41
] and are regulated in osteoblasts by PTH [24
]. Mice lacking CXCL12 have fetal hematopoietic defects [91
]. CXCL12 and CXCR4 are necessary for hematopoietic repopulation, since treatment of primitive CD34+
human bone marrow cells with an antibody to CXCR4 prevented their engraftment in SCID mice [93
]. Further, treatment with a CXCR4 antagonist caused rapid mobilization of mouse long-term repopulating (LTR) cells and human CD34+
cells, along with more mature hematopoietic cells, strongly suggesting that CXCL12/CXCR4 interactions retain HSC in the bone marrow microenvironment [94
]. Interestingly, Sugiyama et al.
illustrated that conditional deletion of CXCR4 decreased HSC and actually slightly increased more mature hematopoietic progenitors, underscoring the specific role that CXCL12 plays in primitive HSC. In addition, they found that an increased proportion of HSC exited G0
and entered the cell cycle in CXCR4 conditionally deficient mice. Intriguingly, chimeric mice generated from donor CD34−
bone marrow cells with the floxed CXCR4 and competitor CD34−
bone marrow cells with wild-type CXCR4 had a decreased percentage of HSC in G0
not only in the CXCR4 deficient donor cells, but also in the wild-type competitor cells, suggesting a microenvironmental effect [95
In contrast to the results of prior studies, Sugiyama et al.
analyzed the expression of CXCL12 in the bone marrow using CXCL12 knock-in mice and found a population of reticular cells within the intertrabecular space that abundantly expressed CXCL12, which they named CXCL12-abundant reticular (CAR) cells. Although it was previously reported that osteoblasts lining the trabeculae expressed CXCL12 [88
], they found only low levels of expression in osteocalcin positive osteoblasts. However, CAR cells were shown to express high levels of Jagged1, which was previously detected in osteoblasts that supported HSC expansion in Col1-caPTH1R mice [24
]. Interestingly, identifying stem cells as Lin−
or with SLAM receptor markers, shown previously to identify cells enriched for HSC activity [37
], demonstrated that most HSC were adjacent to CAR cells. These CAR cells are surrounded by endothelial cells, which could mean that CAR cells are regulating a niche supported by endothelial cells. However, CAR cells have not been fully characterized, nor has their cell of origin been identified.