Less is more: unveiling the functional core of hematopoietic stem cells through knockout mice

Abstract

Hematopoietic stem cells (HSCs) represent one of the first recognized somatic stem cell types. As such, nearly 200 genes have been examined for roles in HSC function in knockout mice. In this review, we compile the majority of these reports to provide a broad overview of the functional modules revealed by these genetic analyses and highlight some key regulatory pathways involved, including cell cycle control, Tgf-β signaling, Pten/Akt signaling, Wnt signaling, and cytokine signaling. Finally, we propose recommendations for characterization of HSC function in knockout mice to facilitate cross-study comparisons that would generate a more cohesive picture of HSC biology.

Figure 1. Overview of the hematopoietic hierarchy

At the top of the mammalian blood system is a restricted pool of multipotent long-term HSCs (LT-HSCs), capable of sustaining a continuous supply of blood cells throughout an individual’s lifetime. Long-term maintenance of hematopoiesis depends on HSC quiescence and, at the same time, on their capability to rapidly respond to proliferative stimuli generated by the BM niche or the external macroenvironment. Downstream of LT-HSCs are pools of stem and progenitor cells with increasingly diminished self-renewal potential: short-term HSCs (STHSCs), multipotent progenitors (MPPs), and lineage-restricted progenitors, such as common lymphoid progenitors (CLPs), common myeloid progenitors (CMPs), Granulocyte/macrophage/lymphocyte progenitors (GMLP), megakaryocyte/erythrocyte progenitors (MEPs) and granulocyte/macrophage progenitors (GMPs). By proliferating rapidly, these progenitors generate a large pool of mature cells that eventually exit the BM and enter the peripheral blood. Notably, some details of this hierarchy are still in flux as evidenced by much discussion in the literature, not covered here.

Figure 2. Overview of hematopoietic phenotypes

A, B. Data from Online Table S1 were sorted according to phenotype, duplicate phenotypes of the same gene were removed, and the categories were apportioned as indicated. A few genes are represented in multiple categories (e.g. increased engraftment and cancer – such as Cbl KO), or enhanced engraftment after the primary transplant followed by a total loss of function after a secondary transplant – such as Hif1α)). A. Summary of phenotypes affecting HSCs as per Online Table S1. Phenotypes were ascertained from our interpretation of data in the cited papers. Increased function: Enhanced peripheral blood (PB) production from KO cells after HSC or BM transplantation. B. Summary of different degrees of hematopoietic defects found in KO mice, as per Online Table S1. Categories were arbitrarily defined as: Severe defect: less than 20% of PB production (relative to normal) in both myeloid and lymphoid lineages after deletion or transplantation of KO HSC or bone marrow; moderate defect: reduction in generation of PB to between 20–70% of normal; mild defect: reduction in PB to between 70–100% of normal. After serial: normal hematopoiesis after primary transplant, but defects in PB production emerge after secondary or tertiary transplant; C. Examples of KO phenotypes relative to generalized expression patterns. Breadth of expression gleaned from Unigene EST profiles, Goodell lab data (Chambers 2007); www.bcm.edu/labs/goodell) or the Immunological Genome Project (www.immgen.org). See supplemental Table S1 for details. Readers can add to the database and obtain updated versions of Table S1 at http://www.bonemarrowhsc.com/hscphen/.

Figure 3. Cell cycle regulators in HSCs

A. Cell cycle regulators play a pivotal role in HSC maintenance, by tuning the balance between quiescence and self-renewal. Gene names are enclosed in colored boxes, based on the degree of hematopoietic defect emerged from KO studies. B. Summary of KO phenotypes related to genes involved in cell cycle regulation. All indicated phenotypes are derived from in vivo transplantation experiments. KO: knock-out; cKO: conditional KO; Defect after serial transplant: normal hematopoiesis after primary transplant, but defects emerge after secondary or tertiary transplant; Increased function: KO mice that showed enhanced PB production after HSC or BM transplantation; PMID: PubMed ID number; PD: Perinatal Death, NA: Not Analyzed, NC: No Change, BM: Bone Marrow, FL: Fetal Liver; § proliferation phenotypes in hematopoietic progenitors (not always homogeneous HSCs).

Figure 4. Wnt signaling in HSCs

A. Maintenance of normal HSC function requires a tightly regulated Wnt dosage. While a decrease in Wnt signaling leads to loss of HSC quiescence and self-renewal, mild increases in Wnt pathway activation contribute to HSC maintenance and result in enhanced hematopoietic reconstitution. However – much as its inhibition – the excessive activation of Wnt pathway hinders HSC differentiation, leading to the loss of hematopoietic functions. Both positive and negative regulators contribute to maintaining Wnt-signaling activation within physiological ranges: these include both intrinsic (cell-autonomous) and extrinsic (microenvironmental) factors (the latter indicated in italics in the figure). Gene names are enclosed in colored boxes, based on the degree of hematopoietic defect emerged from KO studies. B. Summary of KO phenotypes related to genes involved in Wnt-signaling. All indicated phenotypes are derived from in vivo transplantation experiments. KO: knock-out; cKO: conditional KO; Defect after serial transplant: normal hematopoiesis after primary transplant, but defects emerge after secondary or tertiary transplant; Increased function: KO mice that showed enhanced PB production after HSC or BM transplantation; PMID: PubMed ID number; PD: Perinatal Death, NA: Not Analyzed, NC: No Change, BM: Bone Marrow, FL: Fetal Liver; § proliferation phenotypes in hematopoietic progenitors (not always homogeneous HSCs).

Figure 5. Overview of recommended steps for determining whether a gene has an impact on HSC function

See Supplemental Table S3 for details.