The stem cell-intrinsic model of self-renewal via asymmetric cell division (ACD) posits that fate determinants be partitioned unequally between daughter cells to either activate or suppress the stemness state. ACD is a purported mechanism by which hematopoietic stem cells (HSCs) self-renew, but definitive evidence for this cellular process remains open to conjecture. To address this issue, we chose 73 candidate genes that function within the cell polarity network to identify potential determinants that may concomitantly alter HSC fate while also exhibiting asymmetric segregation at cell division. Initial gene-expression profiles of polarity candidates showed high and differential expression in both HSCs and leukemia stem cells. Altered HSC fate was assessed by our established in vitro to in vivo screen on a subcohort of candidate polarity genes, which revealed 6 novel positive regulators of HSC function: Ap2a2, Gpsm2, Tmod1, Kif3a, Racgap1, and Ccnb1. Interestingly, live-cell videomicroscopy of the endocytic protein AP2A2 shows instances of asymmetric segregation during HSC/progenitor cell cytokinesis. These results contribute further evidence that ACD is functional in HSC self-renewal, suggest a role for Ap2a2 in HSC activity, and provide a unique opportunity to prospectively analyze progeny from HSC asymmetric divisions.
"In homeostatic conditions quiescent BM HSCs enter the cell cycle dividing preferentially by asymmetric cell division with the aim to produce at the same time mature cells and to maintain a stem cell pool –, FL HSCs mainly divide by symmetric self-renewing divisions in order to expand the HSC pool , . We show that, in E14.5 FLs, Prep1 deficiency causes a reduction in the number of HSCs and an increase in multipotent progenitors identified as L−S+K+ cells. "
[Show abstract][Hide abstract] ABSTRACT: A hypomorphic Prep1 mutation results in embryonic lethality at late gestation with a pleiotropic embryonic phenotype that includes defects in all hematopoietic lineages. Reduced functionality of the hematopoietic stem cells (HSCs) compartment might be responsible for the hematopoietic phenotype observed at mid-gestation. In this paper we demonstrate that Prep1 regulates the number of HSCs in fetal livers (FLs), their clonogenic potential and their ability to de novo generate the hematopoietic system in ablated hosts. Furthermore, we show that Prep1 controls the self-renewal ability of the FL HSC compartment as demonstrated by serial transplantation experiments. The premature exhaustion of Prep1 mutant HSCs correlates with the reduced quiescent stem cell pool thus suggesting that Prep1 regulates the self-renewal ability by controlling the quiescence/proliferation balance. Finally, we show that in FL HSCs Prep1 absence induces the interferon signaling pathway leading to premature cycling and exhaustion of fetal HSCs.
PLoS ONE 09/2014; 9(9):e107916. DOI:10.1371/journal.pone.0107916 · 3.23 Impact Factor
"Microgravity affects the growth, proliferation, and differentiation of osteoblasts. Since the inhibition of RhoA, observed under microgravity, blocks G1 progression  , this may explain the altered proliferation and differentiation of osteoblastic cells and increased adipogenesis as summarized in Figure 3. Furthermore, several cytoskeletal components, including Rac1 GTPase activating protein 1 (Rac-GAP1) and Tropomodulin 1, segregate asymmetrically during stem cell division , and overexpression of these proteins may enhance MSC commitment, as already proven with asymmetrical divisions of hematopoietic stem cells to progenitor cells  "
[Show abstract][Hide abstract] ABSTRACT: A growing number of studies are revealing that cells reorganize their cytoskeleton when exposed to conditions of microgravity. Most, if not all, of the structural changes observed on flown cells can be explained by modulation of RhoGTPases, which are mechanosensitive switches responsible for cytoskeletal dynamics control. This review identifies general principles defining cell sensitivity to gravitational stresses. We discuss what is known about changes in cell shape, nucleus, and focal adhesions and try to establish the relationship with specific RhoGTPase activities. We conclude by considering the potential relevance of live imaging of RhoGTPase activity or cytoskeletal structures in order to enhance our understanding of cell adaptation to microgravity-related conditions.
BioMed Research International 09/2014; DOI:10.1155/2015/747693 · 2.71 Impact Factor
"Large cortical tensions are generated in cells as they round up and divide during asymmetric division (Sedzinski et al., 2011). Because MYH10 correlates with a half-dozen genes involved in asymmetric division of hematopoietic cells (Ting et al., 2012) (Table S3), confocal imaging and partial knockdown (Figure 3Ai) were used to assess MIIB in asymmetric division of CD34 + cells (Figure 3Aii), which occurs in $30% of cells (consistent with Lordier et al., 2012). MIIB enriches toward the CD34 hi daughter cell, concentrating near the cleavage furrow by $3-fold (Figure 3B), whereas CD34 appears segregated between cells but otherwise locally homogenous, consistent with lateral mobility of this membrane protein. "
[Show abstract][Hide abstract] ABSTRACT: Self-renewal and differentiation of stem cells depend on asymmetric division and polarized motility processes that in other cell types are modulated by nonmuscle myosin-II (MII) forces and matrix mechanics. Here, mass spectrometry-calibrated intracellular flow cytometry of human hematopoiesis reveals MIIB to be a major isoform that is strongly polarized in hematopoietic stem cells and progenitors (HSC/Ps) and thereby downregulated in differentiated cells via asymmetric division. MIIA is constitutive and activated by dephosphorylation during cytokine-triggered differentiation of cells grown on stiff, endosteum-like matrix, but not soft, marrow-like matrix. In vivo, MIIB is required for generation of blood, while MIIA is required for sustained HSC/P engraftment. Reversible inhibition of both isoforms in culture with blebbistatin enriches for long-term hematopoietic multilineage reconstituting cells by 5-fold or more as assessed in vivo. Megakaryocytes also become more polyploid, producing 4-fold more platelets. MII is thus a multifunctional node in polarized division and niche sensing.
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