Cyclin dependent kinase inhibitors differentially modulate synergistic cytokine responsiveness of hematopoietic progenitor cells

The distinct effects of P18 overexpression on different stages of hematopoiesis involve TGF-β and NF-κB signaling

Deficiency of P18 can significantly improve the self-renewal potential of hematopoietic stem cells (HSC) and the success of long-term engraftment. However, the effects of P18 overexpression, which is involved in the inhibitory effects of RUNX1b at the early stage of hematopoiesis, have not been examined in detail. In this study, we established inducible P18/hESC lines and monitored the effects of P18 overexpression on hematopoietic differentiation. Induction of P18 from day 0 (D0) dramatically decreased production of CD34highCD43- cells and derivative populations, but not that of CD34lowCD43- cells, changed the cell cycle status and apoptosis of KDR+ cells and downregulated the key hematopoietic genes at D4, which might cause the severe blockage of hematopoietic differentiation at the early stage. By contrast, induction of P18 from D10 dramatically increased production of classic hematopoietic populations and changed the cell cycle status and apoptosis of CD45+ cells at D14. These effects can be counteracted by inhibition of TGF-β or NF-κB signaling respectively. This is the first evidence that P18 promotes hematopoiesis, a rare property among cyclin-dependent kinase inhibitors (CKIs).

Enhanced self-renewal of human long-term hematopoietic stem cells by a sulfamoyl benzoate derivative targeting p18INK4C

The use of umbilical cord blood transplant has been substantially limited by the finite number of hematopoietic stem and progenitor cells in a single umbilical cord blood unit. Small molecules that not only quantitatively but also qualitatively stimulate enhancement of hematopoietic stem cell (HSC) self-renewal ex vivo should facilitate the clinical use of HSC transplantation and gene therapy. Recent evidence has suggested that the cyclin-dependent kinase inhibitor, p18INK4C (p18), is a critical regulator of mice HSC self-renewal. The role of p18 in human HSCs and the effect of p18 inhibitor on human HSC expansion ex vivo need further studies. Here we report that knockdown of p18 allowed for an increase in long-term colony-forming cells in vitro. We then identified an optimized small molecule inhibitor of p18, 005A, to induce ex vivo expansion of HSCs that was capable of reconstituting human hematopoiesis for at least 4 months in immunocompromised mice, and hence, similarly reconstituted secondary recipients for at least 4 more months, indicating that cells exposed to 005A were still competent in secondary recipients. Mechanistic studies showed that 005A might delay cell division and activate both the Notch signaling pathway and expression of transcription factor HoxB4, leading to enhancement of the self-renewal of long-term engrafting HSCs and the pool of progenitor cells. Taken together, these observations support a role for p18 in human HSC maintenance and that the p18 inhibitor 005A can enhance the self-renewal of long-term HSCs.

Genome-wide association for testis weight in the diversity outbred mouse population

Testis weight is a genetically mediated trait associated with reproductive efficiency across numerous species. We sought to evaluate the genetically diverse, highly recombinant Diversity Outbred (DO) mouse population as a tool to identify and map quantitative trait loci (QTLs) associated with testis weight. Testis weights were recorded for 502 male DO mice and the mice were genotyped on the GIGAMuga array at ~ 143,000 SNPs. We performed a genome-wide association analysis and identified one significant and two suggestive QTLs associated with testis weight. Using bioinformatic approaches, we developed a list of candidate genes and identified those with known roles in testicular size and development. Candidates of particular interest include the RNA demethylase gene Alkbh5, the cyclin-dependent kinase inhibitor gene Cdkn2c, the dynein axonemal heavy chain gene Dnah11, the phospholipase D gene Pld6, the trans-acting transcription factor gene Sp4, and the spermatogenesis-associated gene Spata6, each of which has a human ortholog. Our results demonstrate the utility of DO mice in high-resolution genetic mapping of complex traits, enabling us to identify developmentally important genes in adult mice. Understanding how genetic variation in these genes influence testis weight could aid in the understanding of mechanisms of mammalian reproductive function.

A Conserved Gammaherpesvirus Cyclin Specifically Bypasses Host p18(INK4c) To Promote Reactivation from Latency

Gammaherpesviruses (GHVs) carry homologs of cellular genes, including those encoding a viral cyclin that promotes reactivation from latent infection. The viral cyclin has reduced sensitivity to host cyclin-dependent kinase inhibitors in vitro; however, the in vivo significance of this is unclear. Here, we tested the genetic requirement for the viral cyclin in mice that lack the host inhibitors p27(Kip1) and p18(INK4c), two cyclin-dependent kinase inhibitors known to be important in regulating B cell proliferation and differentiation. While the viral cyclin was essential for reactivation in wild-type mice, strikingly, it was dispensable for reactivation in mice lacking p27(Kip1) and p18(INK4c). Further analysis revealed that genetic ablation of only p18(INK4c) alleviated the requirement for the viral cyclin for reactivation from latency. p18(INK4c) regulated reactivation in a dose-dependent manner so that the viral cyclin was dispensable in p18(INK4c) heterozygous mice. Finally, treatment of wild-type cells with the cytokine BAFF, a known attenuator of p18(INK4c) function in B lymphocytes, was also able to bypass the requirement for the viral cyclin in reactivation. These data show that the gammaherpesvirus viral cyclin functions specifically to bypass the cyclin-dependent kinase inhibitor p18(INK4c), revealing an unanticipated specificity between a GHV cyclin and a single cyclin-dependent kinase inhibitor.

IMPORTANCE

The gammaherpesviruses (GHVs) cause lifelong infection and can cause chronic inflammatory diseases and cancer, especially in immunosuppressed individuals. Many GHVs encode a conserved viral cyclin that is required for infection and disease. While a common property of the viral cyclins is that they resist inhibition by normal cellular mechanisms, it remains unclear how important it is that the GHVs resist this inhibition. We used a mouse GHV that either contained or lacked a viral cyclin to test whether the viral cyclin lost importance when these inhibitory pathways were removed. These studies revealed that the viral cyclin was required for optimal function in normal mice but that it was no longer required following removal or reduced function of a single cellular inhibitor. These data define a very specific role for the viral cyclin in bypassing one cellular inhibitor and point to new methods to intervene with viral cyclins.

Small-molecule inhibitors targeting INK4 protein p18(INK4C) enhance ex vivo expansion of haematopoietic stem cells

Among cyclin-dependent kinase inhibitors that control the G1 phase in cell cycle, only p18 and p27 can negatively regulate haematopoietic stem cell (HSC) self-renewal. In this manuscript, we demonstrate that p18 protein is a more potent inhibitor of HSC self-renewal than p27 in mouse models and its deficiency promoted HSC expansion in long-term culture. Single-cell analysis indicated that deleting p18 gene favoured self-renewing division of HSC in vitro. Based on the structure of p18 protein and in-silico screening, we further identified novel smallmolecule inhibitors that can specifically block the activity of p18 protein. Our selected lead compounds were able to expand functional HSCs in a short-term culture. Thus, these putative small-molecule inhibitors for p18 protein are valuable for further dissecting the signalling pathways of stem cell self-renewal and may help develop more effective chemical agents for therapeutic expansion of HSC.

MicroRNA 21 (miR-21) and miR-181b couple with NFI-A to generate myeloid-derived suppressor cells and promote immunosuppression in late sepsis

The sepsis initial hyperinflammatory reaction, if not treated early, shifts to a protracted state of immunosuppression that alters both innate and adaptive immunity and is associated with elevated mortality. Myeloid-derived suppressor cells (MDSCs) are myeloid progenitors and precursors that fail to differentiate into mature innate-immunity cells and are known for their potent immunosuppressive activities. We previously reported that murine MDSCs expand dramatically in the bone marrow during late sepsis, induced by cecal ligation and puncture, and demonstrated that they contribute to late-sepsis immunosuppression. However, the molecular mechanism responsible for generating these immature Gr1(+) CD11b(+) myeloid cells during sepsis remains unknown. We show here that sepsis generates a microRNA (miRNA) signature that expands MDSCs. We found that miRNA 21 (miR-21) and miR-181b expression is upregulated in early sepsis and sustained in late sepsis. Importantly, we found that simultaneous in vivo blockade of both miRNAs via antagomiR (a chemically modified miRNA inhibitor) injection after sepsis initiation decreased the bone marrow Gr1(+) CD11b(+) myeloid progenitors, improved bacterial clearance, and reduced late-sepsis mortality by 74%. Gr1(+) CD11b(+) cells isolated from mice injected with antagomiRs were able to differentiate ex vivo into macrophages and dendritic cells and produced smaller amounts of the immunosuppressive interleukin 10 (IL-10) and transforming growth factor β (TGF-β) after stimulation with bacterial lipopolysaccharide, suggesting that immature myeloid cells regained their maturation potential and have lost their immunosuppressive activity. In addition, we found that the protein level of transcription factor NFI-A, which plays a role in myeloid cell differentiation, was increased during sepsis and that antagomiR injection reduced its expression. Moreover, knockdown of NFI-A in the Gr1(+) CD11b(+) cells isolated from late-septic mice increased their maturation potential and reduced their production of the immunosuppressive mediators, similar to antagomiR injection. These data support the hypothesis that sepsis reprograms myeloid cells and thus alters the innate immunity cell repertoire to promote immunosuppression, and they demonstrate that this process can be reversed by targeting miR-21 and miR-181b to improve late-sepsis survival.

An opposite effect of the CDK inhibitor, p18(INK4c) on embryonic stem cells compared with tumor and adult stem cells

Self-renewal is a feature common to both adult and embryonic stem (ES) cells, as well as tumor stem cells (TSCs). The cyclin-dependent kinase inhibitor, p18^INK4c, is a known tumor suppressor that can inhibit self-renewal of tumor cells or adult stem cells. Here, we demonstrate an opposite effect of p18 on ES cells in comparison with teratoma cells. Our results unexpectedly showed that overexpression of p18 accelerated the growth of mouse ES cells and embryonic bodies (EB); on the contrary, inhibited the growth of late stage teratoma. Up-regulation of ES cell markers (i.e., Oct4, Nanog, Sox2, and Rex1) were detected in both ES and EB cells, while concomitant down-regulation of various differentiation markers was observed in EB cells. These results demonstrate that p18 has an opposite effect on ES cells as compared with tumor cells and adult stem cells. Mechanistically, expression of CDK4 was significantly increased with overexpression of p18 in ES cells, likely leading to a release of CDK2 from the inhibition by p21 and p27. As a result, self-renewal of ES cells was enhanced. Our current study suggests that targeting p18 in different cell types may yield different outcomes, thereby having implications for therapeutic manipulations of cell cycle machinery in stem cells.

CD1d expression on and regulation of murine hematopoietic stem and progenitor cells

In the present study, surface CD1d, which is involved in immune cell interactions, was assessed for effects on hematopoiesis. Mouse BM hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) express CD1d. The numbers and cycling status of HPCs in the BM and spleen of different strains of cd1d(-/-) mice were enhanced significantly, suggesting that CD1d is a negative regulator of HPCs. In support of this, CD1d was required for the SCF and Flt3 ligand synergistic enhancement of CSF induction of HPC colony formation and for HPC response to myelosuppressive chemokines. Colony formation by immature subsets of HPCs was greatly enhanced when normal, but not cd1d(-/-), BM cells were pretreated with CD1d Abs in vitro. These effects required the full CD1d cytoplasmic tail. In contrast, long-term, but not short-term, repopulating HSC engraftment was impaired significantly, an effect that was minimally influenced by the presence of a truncated CD1d cytoplasmic tail. Pretreatment of normal BM cells with CD1d Abs greatly enhanced their engraftment of HSCs. The results of the present study implicate CD1d in a previously unrecognized regulatory role of normal and stressed hematopoiesis.