Piwi Genes Are Dispensable for Normal Hematopoiesis in Mice

Mona J Nolde,Shangqin Guo,Haifan Lin,Ee-Chun Cheng,Zilong Wen

doi:10.1371/journal.pone.0071950

Abstract

Hematopoietic stem cells (HSC) must engage in a life-long balance between self-renewal and differentiation to sustain hematopoiesis. The highly conserved PIWI protein family regulates proliferative states of stem cells and their progeny in diverse organisms. A Human piwi gene (for clarity, the non-italicized “piwi” refers to the gene subfamily), HIWI (PIWIL1), is expressed in CD34+ stem/progenitor cells and transient expression of HIWI in a human leukemia cell line drastically reduces cell proliferation, implying the potential function of these proteins in hematopoiesis. Here, we report that one of the three piwi genes in mice, Miwi2 (Piwil4), is expressed in primitive hematopoetic cell types within the bone marrow. Mice with a global deletion of all three piwi genes, Miwi, Mili, and Miwi2, are able to maintain long-term hematopoiesis with no observable effect on the homeostatic HSC compartment in adult mice. The PIWI-deficient hematopoetic cells are capable of normal lineage reconstitution after competitive transplantation. We further show that the three piwi genes are dispensable during hematopoietic recovery after myeloablative stress by 5-FU. Collectively, our data suggest that the function of the piwi gene subfamily is not required for normal adult hematopoiesis.

Highlights

Hematopoiesis is driven by a rare population of cells called hematopoietic stem cells (HSCs)
Using RT-PCR, we detected high levels of murine Miwi2 in undifferentiated cell types, including immunophenotypic hematopoietic stem cells (HSC; Lin-cKit+Sca+CD150+CD105+), multi-potent progenitors (MPP; Lin2cKit+Sca+CD1502CD1052), and granulocyte-macrophage progenitors (GMP; Lin2cKit+Sca2CD1502CD16/32+), as compared to more committed cell types, red blood cells (RBC), macrophages (Mac), neutrophils (Neutr), B-cells, and T-cells (Fig. 1). These data are consistent with previously published results for human hematopoietic cells, and suggests that mouse piwi genes may have a regulatory function in undifferentiated cell types during normal hematopoiesis
Piwi genes are dispensable for steady-state hematopoiesis Given the expression of Miwi2 gene products in the hematopoietic system and the previously published influence of Mili overexpression on hematopoietic cell expansion [6], we investigated the potential function of piwi genes in maintaining HSCs and hematopoietic progenitors during hematopoiesis in mice

Summary

Introduction

Hematopoiesis is driven by a rare population of cells called hematopoietic stem cells (HSCs). In mice, piwi gene expression was found to correlate with increased proliferative capacity in cells of the blood system, as ectopic expression of the PIWIL2 homolog (a.k.a., Mili), one of the three piwi genes in mice, promoted expansion of hematopoetic cells in culture [6]. These interesting, but somewhat conflicting observations, point to the pressing need for a loss-of-function study to clearly define the role, if any, of PIWI proteins in hematopoiesis

Methods

Results

Conclusion