Ablation of Wntless in endosteal niches impairs lymphopoiesis rather than HSCs maintenance

Abstract

Osteoblasts and perivascular stromal cells constitute essential niches for HSC selfrenewal and maintenance in the bone marrow. Wnt signaling is important to maintain HSC integrity. However, the paracrine role of Wnt proteins in osteoblasts-supported HSC maintenance and differentiation remains unclear. Here, we investigated hematopoiesis in mice with Wntless (Wls) deficiency in osteoblasts or Nestin-positive mesenchymal progenitor cells, which presumptively block Wnt secretion in osteoblasts. We detected defective B-cell lymphopoiesis and abnormal T-cell infiltration in the bone marrow of Wls mutant mice. Notably, no impact on HSC frequency and repopulation in the bone marrow was observed with the loss of osteoblastic Wls. Our findings revealed a supportive role of Wnts in osteoblasts-regulated B-cell lymphopoiesis. They also suggest a preferential niche role of osteoblastic Wnts for lymphoid cells rather than HSCs, providing new clues for the molecular nature of distinct niches occupied by different hematopoietic cells.Keywords: B cell r HSC maintenance r Osteoblast r T-cell infiltration r Wntless Additional supporting information may be found in the online version of this article at the publisher's web-site IntroductionHSCs that periodically generate all hematopoietic lineages are mostly stored in vascular or endosteal niches in the bone marrow [1]. Vascular niches consist of sinusoidal endothelium cells [2][3][4] whereas endosteal niches are mainly composed of osteoblasts [5,6]. These niche cells are important to regulate HSCs self-renewal and maintenance. Recent evidence also reveals that perivascular Nestin + mesenchymal progenitor cells form a supportive niche for HSCs [3,7,8]. In addition, studies from Dr. Morrison suggest that HSCs mainly reside in the perivascular niches whereas the early lymphoid progenitor cells prefer to locate at endosteal niches [9,10].Correspondence: Dr. Xizhi Guo e-mail: xzguo2005@sjtu.edu.cn; zjyao@sjtu.edu.cnCompelling evidence reveals that Wnt signaling is an important regulator for HSCs integrity and function. Wnt proteins, which consist of 19 members in mammals, could be transduced through canonical or noncanonical signaling pathways [11]. β-Catenin is the key and obligatory mediator of canonical Wnt signaling. Stabilized β-catenin expression expands the pool of HSCs and leads to deficiency in HSCs repopulation capacity [12,13]. Conversely, deletion of β-catenin or Wnt3a leads to defective HSCs long-term maintenance [14,15]. Nevertheless, several lines of evidence also indicate that β-catenin is dispensable for HSCs maintenance [16,17]. On the other hand, inhibition of environmental canonical Wnt signaling in osteoblasts impairs HSCs self-renewal and quiescence [18]. Collectively, canonical Wnt signaling is revealed to * These authors contributed equally to this work. regulate HSCs self-renewal in a dosage-dependent manner [19]. In addition, noncanonical Wnt signaling is also reported to sustain HSCs maintenance and aging [20,21]. Osteoblasts also support B-cell commitment an...

Niches for Hematopoietic Stem Cells and Their Progeny

“…Depletion of Col2.3 + osteoblastic cells resulted in reduced B lymphopoiesis preceding a reduction in LSKs (Zhu et al, 2007). Disruption of Wntless or Gsα signaling using Col1 -Cre or Osterix ( Osx )-Cre respectively also led to impaired B cell development (Cao et al, 2015, Wu et al, 2008). Depletion of Osteocalcin (OCN) + osteolineage cells or deletion of Dll4 in OCN + cells reduced CLP numbers, T lymphopoiesis without overtly impairing the HSC compartment (Yu et al, 2015).…”

“…Cre recombinase transgenic strains developed to target osteoblasts or osteocytes, such as Ocn -Cre (Yu et al, 2015) or Dmp1 -Cre (Sato et al, 2013), are now known to also target various portions of BM MSCs (Zhang and Link, 2016). However, HSPCs were mostly spared in some of these models despite the severe lymphoid defects at least at the time of analysis, suggesting that the lymphoid progenitors might have a different niche requirement or distinct sensitivity to niche regulations than HSCs (Cao et al, 2015, Yu et al, 2015). Indeed, during inflammation, lymphoid progenitors are the first to be depleted from the BM to allow emergency granulopoiesis, possibly due to their different requirement for niche retention factors such as CXCL12 (Ueda et al, 2005, Ueda et al, 2004).…”

Caught in a Wnt storm: Complexities of Wnt signaling in hematopoiesis

“…Of interest, deletion of the Wntless gene (Wls), which is functionally related to Porcn as it also is required for secretion of Wnt proteins, shows a hematopoietic and lymphopoietic phenotype when deleted from osteoblastic niche cells. (48) Moreover, work in Drosophila has demonstrated that non-secreted Wnts might still have a function, shedding a somewhat different light on the absolute requirement of Wnt secretion (49). Whatever the reason, the levels of Axin2 and other Wnt target genes were not affected by Porcn deletion in hematopoietic cells (46), hence no loss-of function model for Wnt signaling was generated.…”

Microenvironmental cues for T‐cell acute lymphoblastic leukemia development

“…Recent findings show that inactivation of wintless (WLS, a mediator of WNT ligands secretion) in OBs impairs B‐cell lymphopoiesis and affects T‐lineage cells. This mutation led to a reduction in the frequency of ETPs in the thymus and to mature naive T‐cell infiltration in the BM rather than spleen . Thus, although studies have revealed common microenvironmental cues shared between HSCs and progenitors, most of the extrinsic signals regulating lymphoid progenitor commitment and differentiation appear to be specific to this compartment, suggesting the existence of different niches with specialized function in the BM ( Fig.…”