The cellular mechanisms required to ensure homeostasis of the hematopoietic niche and the ability of this niche to support hematopoiesis upon stress remain elusive. We here identify Wnt5a in Osterix+ mesenchymal progenitor and stem cells (MSPCs) as a critical factor for niche-dependent hematopoiesis. Mice lacking Wnt5a in MSPCs suffer from stress-related bone marrow (BM) failure and increased mortality. Niche cells devoid of Wnt5a show defective actin stress fiber orientation due to an elevated activity of the small GTPase CDC42. This results in incorrect positioning of autophagosomes and lysosomes, thus reducing autophagy and increasing oxidative stress. In MSPCs from patients from BM failure states which share features of peripheral cytopenia and hypocellular BM, we find similar defects in actin stress fiber orientation, reduced and incorrect colocalization of autophagosomes and lysosomes, and CDC42 activation. Strikingly, a short pharmacological intervention to attenuate elevated CDC42 activation in vivo in mice prevents defective actin-anchored autophagy in MSPCs, salvages hematopoiesis and protects against lethal cytopenia upon stress. In summary, our study identifies Wnt5a as a restriction factor for niche homeostasis by affecting CDC42-regulated actin stress-fiber orientation and autophagy upon stress. Our data further imply a critical role for autophagy in MSPCs for adequate support of hematopoiesis by the niche upon stress and in human diseases characterized by peripheral cytopenias and hypocellular BM.
Neural stem cells (NSCs) have the ability to self-renew and give rise to neurons, glia and oligodendrocytes; however, the signals that regulate NSCs are not well understood. In order to determine the role of -catenin in neural stem and progenitor cells of the mouse embryonic forebrain, we used Emx1-Cre and Nestin-Cre to restrict inactivation of a floxed β-catenin allele to neural precursor cells in the cerebral cortex and neuroepithelium, respectively. Emx1-Cre-mediated knockout of β-catenin is compatible with viability; however, null mice possess a significantly smaller cerebral cortex, defects in cortical lamination and loss of the hippocampus. Nestin-Cre-mediated knockout of β-catenin also leads to a smaller cerebral cortex and perinatal death. To determine how the neural precursor cell population is affected by loss of β-catenin, we performed an in vitro clonal neurosphere assay on E14.5 striatal germinal zone cells obtained from Nestin-Cre/β-catenin-knockout embryos. Although β-catenin mutant cultures did not develop intact neurospheres, many individual cells were scattered throughout the cultures. This loss of cell adhesion seen in vitro may phenocopy the abnormal cortical morphogenesis that occurs in mutant mice in vivo. We also examined the self-renewal capacity of β-catenin mutant NSCs using a collagen-based, clonal colony forming assay. Although β-catenin mutant cells gave rise to smaller colonies than control cells, the total number of mutant colonies was significantly greater indicating the presence of more NSCs in the β-catenin mutant brain. These mutant colonies were passageable, demonstrating that -catenin is not required for NSC self-renewal. Furthermore, β-catenin mutant colonies retained multipotentiality as demonstrated by their ability to differentiate into neurons, astrocytes and oligodendrocytes. Therefore, we hypothesize that -catenin is required for proper adhesion of neural precursor cells via cadherins, and the proliferation of neural progenitor cells, but is not required for neural progenitor differentiation, and may act to inhibit the symmetric division of NSCs.The molecular basis of organized transition from proliferating neural stem cells to differentiating neurons is a central question in developmental neurobiology. FGF2 exhibits dual functions in maintaining neuroepithelial cells; it keeps the cells in a less differentiated and more proliferative state. It remains to be elucidated how FGF2 signaling links suppression of differentiation and promotion of proliferation. Here, we report that in cultured neuroepithelial cells, FGF2 signals through a PI 3 kinase (PI 3 K)-Akt pathway, leading to inhibition of kinase activity of glycogen synthase kinase 3 (GSK3), which is known as a major component of Wnt signaling pathway. Inactivation of GSK3 increases the nuclear -catenin protein level and subsequent up-regulation of Cyclin D1 expression, promoting cell cycle progression of neuroepithelial cells. We further demonstrated that dominant active GSK3 inhibited secondary neurosphere f...
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