Wnt3a deficiency irreversibly impairs hematopoietic stem cell self-renewal and leads to defects in progenitor cell differentiation

Abstract

Canonical Wnt signaling has been implicated in various aspects of hematopoiesis. Its role is controversial due to different outcomes between various inducible Wnt-signaling loss-of-function models and also compared with gain-of-function systems. We therefore studied a mouse deficient for a Wnt gene that seemed to play a nonredundant role in hematopoiesis. Mice lacking Wnt3a die prenatally around embryonic day (E) 12.5, allowing fetal hematopoiesis to be studied using in vitro assays and transplantation into irradiated recipient mice. Here we show that Wnt3a deficiency leads to a reduction in the numbers of hematopoietic stem cells IntroductionHematopoietic stem cells (HSCs) are responsible for the continuous production of blood cells and consequently help to sustain immune function. This is achieved by their unique capacity to self-renew and ability to differentiate into all blood lineages. Several studies have implicated the Wnt-signaling pathway in the regulation of these processes, but its exact role is still not completely understood. 1,2 Upon binding of a Wnt protein to a Frizzled receptor and to a LRP5/6 coreceptor, an elaborate signaling route leads to cytoplasmatic accumulation and subsequent nuclear translocation of ␤-catenin, the key mediator of the Wnt-signaling pathway. In the absence of a Wnt protein, the levels of ␤-catenin are kept very low by the action of the so-called destruction complex consisting of the casein kinase I (CKI) and glycogen synthase kinase 3␤ (GSK-3␤) serine/threonine kinases, the tumor suppressor protein adenomatous polyposis coli (APC) and the scaffolding protein Axin. Phosphorylation of ␤-catenin by CKI and GSK-3␤ leads to its ubiquitination and subsequent breakdown in the proteossome. Activation of the Wnt pathway by a Wnt ligand results in inactivation of GSK-3␤ and consequent translocation of ␤-catenin to the nucleus. In the nucleus, ␤-catenin binds to members of the Tcf/Lef transcription factors family, thereby converting these proteins from transcriptional repressors into transcriptional activators. 3 The first evidence for a role of Wnt proteins in hematopoiesis was reported in studies showing that stromal cell lines transduced with Wnt1, Wnt5a, and Wnt10b have an in vitro stimulatory effect on mouse 4 and human 5 hematopoietic progenitors.Using Tcf1/Lef-GFP reporter assays, Wnt signaling was shown to be active in the highly HSCs enriched Lin Ϫ Sca1 ϩ c-Kit ϩ (LSK) population, both in vivo as well as in vitro after stimulation with purified Wnt3a. 6 Furthermore, Wnt3a treatment in vitro resulted in increased proliferation of LSK cells along with the maintenance of an immature phenotype and led to increased self-renewal as determined by transplantation assays. 7 Retroviral expression of a constitutively active form of ␤-catenin in Bcl2-transgenic LSK cells resulted in augmented multilineage repopulation capacity. In agreement, ectopic expression of the Wnt-signaling inhibitor Axin yielded opposite results. 6 However, subsequent gain-and loss-offunction approaches to fur...

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

“…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].…”

Correction of B-cell development in Btk-deficient mice using lentiviral vectors with codon-optimized human BTK

“…At 20 weeks after transplantation, mice were killed and analyzed for reconstitution in tissues as described previously. 45 …”

Cell cycle regulation in hematopoietic stem cells

“…Deletion of TGF-1 or Indian hedgehog (Ihh) lead to defects in fetal erythropoiesis resulting in embryonic lethality Cridland et al, 2009), whereas Wnt3a-deficient mice show depressed numbers of HSCs in the fetal liver (Luis et al, 2009). However, conclusive in vivo evidence for their role in adult HSC cell cycle regulation has been difficult to generate, as Mx-Cre-driven conditional deletion of Notch, Wnt, and Hh signaling components in adult mice do not reveal clear hematopoietic defects or alterations in HSC cell cycle activity Maillard et al, 2008;Gao et al, 2009).…”