Osteopontin, a key component of the hematopoietic stem cell niche and regulator of primitive hematopoietic progenitor cells

Abstract

Although recent data suggests that osteoblasts play a key role within the hematopoietic stem cell (HSC) niche, the mechanisms underpinning this remain to be fully defined. The studies described herein examine the role in hematopoiesis of Osteopontin (Opn), a multidomain, phosphorylated glycoprotein, synthesized by osteoblasts, with well-described roles in cell adhesion, inflammatory responses, angiogenesis, and tumor metastasis. We demonstrate a previously unrecognized IntroductionHematopoietic stem cell (HSC) engraftment is a multistep process, involving homing, transmarrow migration (TMM), and lodgment within a bone marrow (BM) niche. Homing is the specific recruitment of HSCs to the BM and involves the recognition of HSCs by the BM microvascular endothelium and transendothelial cell migration into the hematopoietic space. Lodgment is defined as the selective migration of HSCs to a suitable niche within the extravascular compartment. In comparison to homing, very little is known about molecules that regulate HSC lodgment and, moreover, the retention of HSCs within these distinct anatomical locations.In accord with the stem cell niche model proposed by Schofield, 1 recent studies within our laboratory demonstrate that HSCs actively migrate toward and reside within the endosteal region at the bone and BM interface. 2,3 This concept is supported by studies reported by Calvi et al, 4 Zhang et al, 5 and Arai et al, 6 which highlight the importance of direct contact and interactions between HSCs and osteoblasts at the endosteal surface in the regulation of HSC proliferation. Further evidence that osteoblasts directly regulate hematopoiesis is provided by studies in which conditional ablation of osteoblasts results in significant reduction of marrow hematopoiesis, 7 although the factors responsible for this profound effect remain to be determined. In addition, in vitro evidence indicates that osteoblastic cells can expand HSC numbers 8 and, when cotransplanted with HSCs, can improve engraftment. 9 Collectively, these findings suggest that osteoblasts are a key cell type within the HSC niche and that molecules expressed by these cells may have previously unrecognized roles in regulating hematopoiesis.One molecule that shows high levels of expression in osteoblasts cells lining bone trabeculae is Osteopontin (Opn), 10 an observation that is not unexpected given its well-described role as a key regulator of bone homeostasis. 11 Opn is a multidomain, phosphorylated glycoprotein synthesized by many cell types and involved in many physiologic and pathologic processes, including cell adhesion, 12 angiogenesis, 13 apoptosis, inflammatory responses, and tumor metastasis. 14 Physiologically, phosphorylation, glycosylation, and cleavage of Opn result in molecular mass variants, ranging from 25 to 75 kDa. The different effects that Opn elicit are attributable to its multiple receptors, binding sites, and its various forms. 15 One of the major serine proteases to cleave Opn is thrombin, giving rise to a 24-kDa and a 45-kDa fragm...

Osteopontin attenuates aging‐associated phenotypes of hematopoietic stem cells

“…Nothing has been reported so for with respect to a likely connection of these cytokines and OPN in BM and aging, though a recent study reported increased IP‐10 serum levels in elderly patients (de Bonfante et al , 2015). As implied by previous studies (Nilsson et al , 2005; Stier et al , 2005; Grassinger et al , 2009), our results further confirm that OPN cleaved by thrombin is biologically active. OPN is a secreted glycoprotein able to bind HSCs through, among others, interactions with CD44 and α 9 β 1 /α 4 β 1 integrins (Stier et al , 2005; Grassinger et al , 2009).…”

“…In both Y→OPN KO and Y→O transplants, a decrease in the overall donor‐derived cell engraftment in PB was observed compared to Y→Y (Fig 3G). Y→OPN KO mice showed an aging‐like increase in the frequency of HSCs in BM that was indistinguishable from the frequency in Y→O, but very distinct from Y→Y controls (Fig 3H) as also already described for early hematopoietic progenitor cells (LSK) cells in OPN −/− animals (Nilsson et al , 2005). Y→O animals presented with an increase in the frequency of myeloid cells over lymphoid T cells, while B‐cell frequencies were not affected (Fig 3I, see also Fig 1).…”

“…Although the overall level of engraftment remained unchanged compared to non‐treated aged controls, treatment of aged HSCs ex vivo with thrombin‐activated OPN fragments resulted in HSCs youthful for polarity and frequency in vivo and also changed the frequency of myeloid and T cell to a youthful level. Thrombin‐treated OPN is known to be able to bind, among others α4β1, α9β1 and αVβ3 receptors on HSCs and by this means to regulate homing, lodging and engraftment (Nilsson et al , 2005; Grassinger et al , 2009). Finally, in order to determine the extent of the engagement of the above listed integrin receptors in the signaling cascade of thrombin‐activated OPN fragment D, old HSCs were treated ex vivo 16 h with OPN fraction D in the presence of previously established and validated peptide inhibitors (Grassinger et al , 2009) for these integrins (Materials and Methods).…”

“…Skewing of aged HSCs toward myeloid differentiation was recently linked to increased secretion of the pro‐inflammatory CC‐chemokine ligand 5 (CCL5; also known as RANTES) in aged stroma (Ergen et al , 2012). Osteopontin (OPN), a secreted matrix glycoprotein, is produced in BM stroma by pre‐osteoblasts, osteoblasts and osteocytes (Nilsson et al , 2005; Grassinger et al , 2009). In young mice, OPN regulates HSC pool size, stem cell homing, trans‐marrow migration and engraftment (Nilsson et al , 2005; Stier et al , 2005; Haylock & Nilsson, 2006; Grassinger et al , 2009).…”

“…Osteopontin (OPN), a secreted matrix glycoprotein, is produced in BM stroma by pre‐osteoblasts, osteoblasts and osteocytes (Nilsson et al , 2005; Grassinger et al , 2009). In young mice, OPN regulates HSC pool size, stem cell homing, trans‐marrow migration and engraftment (Nilsson et al , 2005; Stier et al , 2005; Haylock & Nilsson, 2006; Grassinger et al , 2009). Here, we report that OPN is reduced in aged stroma and that reduced OPN levels confer aging‐associated phenotypes on HSCs.…”