Wnt-inhibitory factor 1 dysregulation of the bone marrow niche exhausts hematopoietic stem cells

Abstract

The role of Wnt signaling in hematopoietic stem cell fate decisions remains controversial. We elected to dysregulate Wnt signaling from the perspective of the stem cell niche by expressing the pan Wnt inhibitor, Wnt inhibitory factor 1 (Wif1), specifically in osteoblasts. Here we report that osteoblastic Wif1 overexpression disrupts stem cell quiescence, leading to a loss of self-renewal potential. Primitive stem and progenitor populations were more proliferative and elevated in bone marrow and spleen, manifesting an impaired ability to maintain a self-renewing stem cell pool. Exhaustion of the stem cell pool was apparent only in the context of systemic stress by chemotherapy or transplantation of wild-type stem cells into irradiated Wif1 hosts. Paradoxically this is mediated, at least in part, by an autocrine induction of canonical Wnt signaling in stem cells on sequestration of Wnts in the environment. Additional signaling pathways are dysregulated in this model, primarily activated Sonic Hedgehog signaling in stem cells as a result of Wif1-induced osteoblastic expression of Sonic Hedgehog. We find that dysregulation of the stem cell niche by overexpression of an individual component impacts other unanticipated regulatory pathways in a combinatorial manner, ultimately disrupting niche mediated stem cell fate decisions. (Blood. 2011; 118(9):2420-2429) IntroductionHematopoietic stem cells (HSCs) are characterized by their ability to self-renew and differentiate, producing blood cells throughout life. In the adult, the balance of self-renewal and differentiation is tightly regulated by cross-talk between HSCs and specialized cells within the bone marrow (BM) constituting the stem cell niche. This molecular dialogue is beginning to be explored, repeatedly implicating the Wnt signaling pathway. Wnt signaling can be mediated through either canonical ␤-catenin-mediated Lef/Tcf transcriptional activity or other noncanonical pathways. 1,2 Signaling is initiated in most all pathways through binding of Wnts to Frizzled (Fzd) receptors. There are multiple Wnts and Fzds allowing for many ligand/receptor combinations. On the other hand, Wnt signaling can be inhibited by several regulatory molecules. The Dickkopf family (Dkk) actively prevents binding of Wnt to Fzd and its coreceptors low-density lipoprotein receptor-related proteins 5 and 6, inhibiting canonical signaling, whereas secreted Fzd-related proteins (Sfrps) and Wnt inhibitory factor 1 (Wif1) bind Wnt proteins and sequester them in the extracellular space thus inhibiting both pathways. 3 Evidence for a role of Wnt proteins in hematopoiesis arose from experiments demonstrating that multiple Wnts could expand hematopoietic stem/progenitor cells (HSPCs) in culture. 4,5 Subsequently, culture of single HSCs, in the presence of purified Wnt3a, resulted in expansion concomitant with maintenance of phenotype and robust repopulating activity. 6 In addition, retroviral expression of constitutively active ␤-catenin in HSCs allowed their expansion in vitro without loss of rec...

Hematopoietic and Leukemic Stem Cells Have Distinct Dependence on Tcf1 and Lef1 Transcription Factors

“…HSCs lacking both Tcf1 and Lef1 were able to successfully repopulate all non-T blood lineages in primary and secondary hosts, but their capacity for blood regeneration was compromised in the tertiary hosts. This functional deficiency remarkably resembles that observed in transgenic expression of Wnt inhibitors such as Dkk1 and Wif1 and in Wnt3a-deficient fetal liver cells (11)(12)(13). In those studies where Wnt activity is manipulated, a general concern is that such intervention has a side effect on bone structure and/or stromal cells and thus may have altered HSC niches.…”

“…Overexpression of Dkk1 or Wif1, which blocks the interaction between Wnt ligands and their receptors, diminishes HSC quiescence and its repopulation capacity (11,12). Wnt3a deficiency also greatly impaired HSC activity (13).…”

The dual face of parathyroid hormone and prostaglandins in the osteoimmune system

“…In class I MHC Ϫ/Ϫ mice, the anabolic PTH/bone effect was abrogated, highlighting the importance of T cells in trabecular bone formation (126). Furthermore, it is acknowledged that osteoblast-specific overexpression of the Wnt inhibitor DKK1 or WIF caused an impaired HSC self-renewal (26,104). These findings revealed tantalizing scenarios regarding the involvement of different niches controlling PTH effects on trabecular bone resorption/formation and on HSC homing.…”

A stressed niche not Wnted

“…1 A lthough the introduction of reducedintensity conditioning (RIC) has decreased treatment-related mortality associated with myeloablative conditioning, it is currently being actively debated whether patients should be treated with RIC allogeneic stem cell transplantation (RIC alloSCT) with its substantial morbidity and risk of mortality as part of first-line therapy, even if a late survival benefit can be demonstrated. A recent consensus report on alloSCT in MM concluded that new strategies are needed to improve the…”

“…1 H ematopoietic stem cells give rise to our entire blood cell repertoire throughout adult life, and hence the balance of selfrenewal and differentiation of HSCs needs to be tightly controlled. Intrinsic signals and signals from the HSC niche microenvironment, including osteoblasts, are key to maintaining this balance.…”