Lnk negatively regulates self-renewal of hematopoietic stem cells by modifying thrombopoietin-mediated signal transduction

Laboratory of Stem Cell Therapy, Center for Experimental Medicine, University of Tokyo, 4-6-1 Shirokanedai, Tokyo 108-8639, Japan.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 02/2007; 104(7):2349-54. DOI: 10.1073/pnas.0606238104
Source: PubMed


One of the central tasks of stem cell biology is to understand the molecular mechanisms that control self-renewal in stem cells. Several cytokines are implicated as crucial regulators of hematopoietic stem cells (HSCs), but little is known about intracellular signaling for HSC self-renewal. To address this issue, we attempted to clarify how self-renewal potential is enhanced in HSCs without the adaptor molecule Lnk, as in Lnk-deficient mice HSCs are expanded in number >10-fold because of their increased self-renewal potential. We show that Lnk negatively regulates self-renewal of HSCs by modifying thrombopoietin (TPO)-mediated signal transduction. Single-cell cultures showed that Lnk-deficient HSCs are hypersensitive to TPO. Competitive repopulation revealed that long-term repopulating activity increases in Lnk-deficient HSCs, but not in WT HSCs, when these cells are cultured in the presence of TPO with or without stem cell factor. Single-cell transplantation of each of the paired daughter cells indicated that a combination of stem cell factor and TPO efficiently induces symmetrical self-renewal division in Lnk-deficient HSCs but not in WT HSCs. Newly developed single-cell immunostaining demonstrated significant enhancement of both p38 MAPK inactivation and STAT5 and Akt activation in Lnk-deficient HSCs after stimulation with TPO. Our results suggest that a balance in positive and negative signals downstream from the TPO signal plays a role in the regulation of the probability of self-renewal in HSCs. In general, likewise, the fate of stem cells may be determined by combinational changes in multiple signal transduction pathways.

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    • "Young adult Lnk ) ⁄ ) mice have a 10-fold increase in HSC numbers and show superior multilineage repopulation after BMT (Ema et al., 2005; Buza-Vidas et al., 2006; Bersenev et al., 2008). Lnk controls HSC homeostasis and self-renewal, in part through restricting TPO ⁄ MPL ⁄ JAK2 activity (Buza-Vidas et al., 2006; Seita et al., 2007; Bersenev et al., 2008). Activating mutations in JAK2 as well as loss of function mutations in Lnk have been found in MPNs (Bersenev et al., 2010; Abdel-Wahab, 2011), which are clonal stem cell diseases that arise mostly in older individuals. "
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    ABSTRACT: Upon aging, the number of hematopoietic stem cells (HSCs) in the bone marrow increases while their repopulation potential declines. Moreover, aged HSCs exhibit lineage bias in reconstitution experiments with an inclination toward myeloid at the expense of lymphoid potential. The adaptor protein Lnk is an important negative regulator of HSC homeostasis, as Lnk deficiency is associated with a 10-fold increase in HSC numbers in young mice. However, the age-related increase in functional HSC numbers found in wild-type HSCs was not observed in Lnk-deficient animals. Importantly, HSCs from aged Lnk null mice possess greatly enhanced self-renewal capacity and diminished exhaustion, as evidenced by serial transplant experiments. In addition, Lnk deficiency ameliorates the aging-associated lineage bias. Transcriptome analysis revealed that WT and Lnk-deficient HSCs share many aging-related changes in gene expression patterns. Nonetheless, Lnk null HSCs displayed altered expression of components in select signaling pathways with potential involvement in HSC self-renewal and aging. Taken together, these results suggest that loss of Lnk partially mitigates age-related HSC alterations.
    Aging cell 07/2012; 11(6). DOI:10.1111/j.1474-9726.2012.00862.x · 6.34 Impact Factor
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    • "Although we find no evidence for differential telomeric maintenance, our gene expression profiling indicates that aged LNK ) ⁄ ) HSCs as a cell pool are less proliferative in steady state compared to aged WT HSCs. This was a rather surprising finding not only because it has previously been demonstrated that LNK ) ⁄ ) HSCs proliferate more rapidly in response to cytokine signaling in vitro (Seita et al., 2007), but also because aged HSCs are less proliferative compared to their young counterparts (Attema et al., 2009). Thus, the aged LNK ) ⁄ ) HSC pool can be regarded as 'hyper-quiescent', although it is important to consider that any HSC compartment is heterogeneous, at least in terms of their proliferative status (Nygren & Bryder, 2008 "
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    ABSTRACT: Aging causes profound effects on the hematopoietic stem cell (HSC) pool, including an altered output of mature progeny and enhanced self-propagation of repopulating-defective HSCs. An important outstanding question is whether HSCs can be protected from aging. The signal adaptor protein LNK negatively regulates hematopoiesis at several cellular stages. It has remained unclear how the enhanced sensitivity to cytokine signaling caused by LNK deficiency affects hematopoiesis upon aging. Our findings demonstrate that aged LNK(-/-) HSCs displayed a robust overall reconstitution potential and gave rise to a hematopoietic system with a balanced lineage distribution. Although aged LNK(-/-) HSCs displayed a distinct molecular profile in which reduced proliferation was central, little or no difference in the proliferation of aged LNK(-/-) HSCs was observed after transplantation when compared to aged WT HSCs. This coincided with equal telomere maintenance in WT and LNK(-/-) HSCs. Collectively, our studies suggest that enhanced cytokine signaling can counteract functional age-related HSC decline.
    Aging cell 07/2012; 11(6). DOI:10.1111/j.1474-9726.2012.00863.x · 6.34 Impact Factor
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    • "For example, cytokines that activate STAT5 in the most immature human hematopoietic stem compartment include SCF7 and TPO.8 These cytokines have been shown to promote long-term hematopoiesis in vitro,9 and hypersensitivity to TPO in Lnk−/− mice resulted in elevated stem cell self-renewal, which coincided with increased levels of STAT5 activity.10 Within the erythroid compartment, STAT5 is activated by EPO,11 where STAT5 fulfills an important anti-apoptotic role by upregulating Bcl-Xl,12-15 although a more direct role in initiating erythroid commitment might exist as well.16-18 "
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    ABSTRACT: The level of transcription factor activity critically regulates cell fate decisions such as hematopoietic stem cell self-renewal and differentiation. The balance between hematopoietic stem cell self-renewal and differentiation needs to be tightly controlled, as a shift toward differentiation might exhaust the stem cell pool, while a shift toward self-renewal might mark the onset of leukemic transformation. A number of transcription factors have been proposed to be critically involved in governing stem cell fate and lineage commitment, such as Hox transcription factors, c-Myc, Notch1, β-catenin, C/ebpα, Pu.1 and STAT5. It is therefore no surprise that dysregulation of these transcription factors can also contribute to the development of leukemias. This review will discuss the role of STAT5 in both normal and leukemic hematopoietic stem cells as well as mechanisms by which STAT5 might contribute to the development of human leukemias.
    01/2012; 1(1):13-22. DOI:10.4161/jkst.19316
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