A Role for the Wnt Gene Family in Hematopoiesis: Expansion of Multilineage Progenitor Cells

Abstract

The microenvironment is a key regulator of hematopoietic stem cells (HSCs) and is a likely source of extracellular factors that control stem cell fate. A better understanding of these microenvironmental factors may come from investigations of developmental cell fate determination in which the critical roles of cell-cell interactions of multipotential cells have been shown. The Wnt gene family is known to regulate the cell fate and cell-cell interactions of multipotential cells in a variety of tissues. Expression of Wnts and of their putative receptors encoded by murine homologs of the Drosophila frizzled gene in hematopoietic tissues was examined by reverse transcriptase-polymerase chain reaction. Wnt-5a and Wnt-10b were expressed in day-11 murine yolk sac, day-14 fetal liver, and fetal liver AA4+ cells. The expression profiles of four murine frizzled homologs, Mfz3-7, were nearly identical to that of Wnt-5a and Wnt-10b. Notably, Wnt-10b was expressed in the fetal liver AA4+ Sca+ c-kit+ (flASK) HSC population. A role for Wnts in HSC fate determination was studied by treatment of HSC populations in culture with soluble WNT proteins. The addition of conditioned media from cells transfected with Wnt-1, Wnt-5a, or Wnt-10b cDNAs to cultures of flASK cells stimulated a sevenfold, eightfold, and 11-fold expansion in cell number, respectively, relative to control media. Removal of WNT-5a from this media by immunodepletion depleted the stimulatory activity from the media, whereas addition of a partially purified WNT-5a stimulated a fivefold expansion relative to control cells. Transduction of flASK cells with a retrovirus bearing a Wnt-5a cDNA enhanced proliferation. We conclude that WNTs stimulate the survival/proliferation of hematopoietic progenitors, demonstrating that WNTs comprise a novel class of hematopoietic cell regulators.

Activation of Wnt Signaling in Hematopoietic Regeneration

“…Although we have previously shown that Wnt3a can induce growth of HSCs [36], it was unclear whether this ability is shared with other Wnt family members. Wnt10b is not available in its purified form; however, soluble Wnt10b has been studied in vitro via conditioned media generated from cells expressing Wnt10b [6]. Therefore, to study the effects of Wnt10b in vitro, we generated retroviruses engineered to express Wnt10b and infected HSCs to test their ability to induce HSC expansion in an autocrine/paracrine manner.…”

“…In addition, a role for Wnt signaling during hematopoiesis is beginning to emerge, as modulation of Wnt signaling can affect HSC function. Activation of the Wnt pathway can increase proliferation of HSC‐enriched cells [6] and the ability of HSCs to reconstitute the hematopoietic system of lethally irradiated mice after bone marrow transplantation, whereas overexpression of Axin, a Wnt signaling inhibitor, has the opposite effect [7]. In addition, ectopic activation of the Wnt pathway in committed hematopoietic progenitors can generate cells with stem cell‐like properties [8].…”

Glycogen Synthase Kinase-3β Inhibition Preserves Hematopoietic Stem Cell Activity and Inhibits Leukemic Cell Growth

“…Experimental activation of Wnt was previously demonstrated using recombinant Wnt proteins, bone marrow stroma cells producing Wnt proteins, or by overexpression of the constitutively active β‐ catenin gene, the main downstream effector of Wnt [2, –4]. These studies provide evidence that Wnt signaling positively regulates the self‐renewal of murine hematopoietic stem cells (HSCs) [5, 6]. Understanding the potential role of Wnt signaling in the provision of self‐renewal signals for human HSCs may have potential clinical applications in ex vivo HSC expansion strategies for allogeneic transplantation [7].…”

Analysis of Wnt pathway genes duringex vivoexpansion and neutrophil differentiation of umbilical cord-blood derived CD34+cells

“…WNT 10B expression is decreased in all three differentiated cell populations when compared to the cell populations where G‐CSF was absent. Although addition of this particular Wnt has been found to increase HSC numbers [4], a specific role for the gene product has never been conclusively demonstrated. As WNT proteins can act in an autocrine/paracrine manner, it is possible that WNT10B acts as a general stem cell survival and/or growth factor.…”

“…There are other means of transducing the signals: WNT/Ca 2+ pathway and the planar cell polarity pathway, indeed a fourth pathway which may regulate spindle orientation and asymmetric cell division has been described [2,3]. Evidence has accumulated that the WNT gene pathway is crucial for the maintenance and development of the haematopoietic stem cell (HSC) compartment [4–6]. It has been demonstrated that β‐catenin expression is required for maintaining blood stem cells in the pluripotent state, whereas conditional activation prevents differentiation of HSCs [7,8].…”