The chromatin remodeler CHD8 governs hematopoietic stem/progenitor survival by regulating ATM-mediated P53 protein stability.

The Chd8 gene encodes a member of the chromodomain helicase DNA-binding (CHD) family of SNF2H-like adenosine triphosphate (ATP)-dependent chromatin remodeler, the mutations of which define a subtype of autism spectrum disorders. Increasing evidence from recent studies indicates that ATP-dependent chromatin-remodeling genes are involved in the control of crucial gene-expression programs in hematopoietic stem/progenitor cell (HSPC) regulation. In this study, we identified CHD8 as a specific and essential regulator of normal hematopoiesis. Loss of Chd8 leads to severe anemia, pancytopenia, bone marrow failure, and engraftment failure related to a drastic depletion of HSPCs. CHD8 forms a complex with ATM and its deficiency increases chromatin accessibility and drives genomic instability in HSPCs causing an activation of ATM kinase that further stabilizes P53 protein by phosphorylation and leads to increased HSPC apoptosis. Deletion of P53 rescues the apoptotic defects of HSPCs and restores overall hematopoiesis in Chd8-/- mice. Our findings demonstrate that chromatin organization by CHD8 is uniquely necessary for the maintenance of hematopoiesis by integrating the ATM-P53-mediated survival of HSPCs.

KRasG12D expression in the bone marrow vascular niche affects hematopoiesis with inflammatory signals.

The bone marrow (BM) niche is an important milieu where hematopoietic stem and progenitor cells (HSPCs) are maintained. Previous studies have indicated that genetic mutations in various components of the niche can affect hematopoiesis and promote hematologic abnormalities, but the impact of abnormal BM endothelial cells (BMECs), a crucial niche component, on hematopoiesis remains incompletely understood. To dissect how genetic alterations in BMECs could affect hematopoiesis, we have employed a novel inducible Tie2-CreERT2 mouse model, with a tdTomato fluorescent reporter, to introduce an oncogenic KRasG12D mutation specifically in the adult endothelial cells. Tie2-CreERT2;KRasG12D mice had significantly more leukocytes and myeloid cells in the blood with mostly normal BM HSPC populations and developed splenomegaly. Genotyping polymerase chain reaction revealed KRasG12D activation in BMECs but not hematopoietic cells, confirming that the phenotype is due to the aberrant BMECs. Competitive transplant assays revealed that BM cells from the KRasG12D mice contained significantly fewer functional hematopoietic stem cells, and immunofluorescence imaging showed that the hematopoietic stem cells in the mutant mice were localized farther away from BM vasculature and closer to the endosteal area. RNA sequencing analyses found an inflammatory gene network, especially tumor necrosis factor α, as a possible contributor. Together, our results implicate an abnormal endothelial niche in compromising normal hematopoiesis.

RhoA and Rac1 GTPases Differentially Regulate Agonist-Receptor Mediated Reactive Oxygen Species Generation in Platelets.

Agonist induced generation of reactive oxygen species (ROS) by NADPH oxidases (NOX) enhances platelet aggregation and hence the risk of thrombosis. RhoA and Rac1 GTPases are involved in ROS generation by NOX in a variety of cells, but their roles in platelet ROS production remain unclear. In this study we used platelets from RhoA and Rac1 conditional knockout mice as well as human platelets treated with Rhosin and NSC23767, rationally designed small molecule inhibitors of RhoA and Rac GTPases, respectively, to better define the contributions of RhoA and Rac1 signaling to ROS generation and platelet activation. Treatment of platelets with Rhosin inhibited: (a) U46619 induced activation of RhoA; (b) phosphorylation of p47phox, a critical component of NOX; (c) U46619 or thrombin induced ROS generation; (d) phosphorylation of myosin light chain (MLC); (e) platelet shape change; (f) platelet spreading on immobilized fibrinogen; and (g) release of P-selectin, secretion of ATP and aggregation. Conditional deletion of RhoA or Rac1 gene inhibited thrombin induced ROS generation in platelets. Addition of Y27632, a RhoA inhibitor, NSC23766 or Phox-I, an inhibitor of Rac1-p67phox interaction, to human platelets blocked thrombin induced ROS generation. These data suggest that: (a) RhoA/ROCK/p47phox signaling axis promotes ROS production that, at least in part, contributes to platelet activation in conjunction with or independent of the RhoA/ROCK mediated phosphorylation of MLC; and (b) RhoA and Rac1 differentially regulate ROS generation by inhibiting phosphorylation of p47phox and Rac1-p67phox interaction, respectively.

mTOR disruption causes intestinal epithelial cell defects and intestinal atrophy postinjury in mice.

Intestinal stem cells (ISCs) drive small intestinal epithelial homeostasis and regeneration. Mechanistic target of rapamycin (mTOR) regulates stem and progenitor cell metabolism and is frequently dysregulated in human disease, but its physiologic functions in the mammalian small intestinal epithelium remain poorly defined. We disrupted the genes mTOR, Rptor, Rictor, or both Rptor and Rictor in mouse ISCs, progenitors, and differentiated intestinal epithelial cells (IECs) using Villin-Cre. Mutant tissues and wild-type or heterozygous littermate controls were analyzed by histologic immunostaining, immunoblots, and proliferation assays. A total of 10 Gy irradiation was used to injure the intestinal epithelium and induce subsequent crypt regeneration. We report that mTOR supports absorptive enterocytes and secretory Paneth and goblet cell function while negatively regulating chromogranin A-positive enteroendocrine cell number. Through additional Rptor, Rictor, and Rptor/Rictor mutant mouse models, we identify mechanistic target of rapamycin complex 1 as the major IEC regulatory pathway, but mechanistic target of rapamycin complex 2 also contributes to ileal villus maintenance and goblet cell size. Homeostatic adult small intestinal crypt cell proliferation, survival, and canonical wingless-int (WNT) activity are not mTOR dependent, but Olfm4(+) ISC/progenitor population maintenance and crypt regeneration postinjury require mTOR. Overall, we conclude that mTOR regulates multiple IEC lineages and promotes stem and progenitor cell activity during intestinal epithelium repair postinjury.