Browsing by Subject "Hematopoietic Stem Cells"
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Item Aspartate Is Limiting for Hematopoietic Stem Cell Function in vivo(August 2021) Qi, Le; Xu, Jian; Morrison, Sean J.; Hill, Joseph A.; Thomas, Philip J.A key function of the electron transport chain is to promote aspartate synthesis, which is required for cancer cell proliferation. However, it is unclear whether aspartate is also limiting in normal stem cells, which divide intermittently. We found that hematopoietic stem cells (HSCs) do not take up exogenous aspartate. To test if aspartate limits HSC function, we over-expressed the glutamate/aspartate transporter, Glast, or deleted glutamic-oxaloacetic transaminase 1 (Got1). Each increased aspartate levels in hematopoietic stem/progenitor cells, increasing the function of HSCs but not colony-forming progenitors. Conversely, deletion of glutamic-oxaloacetic transaminase 2 (Got2) reduced aspartate levels and HSC function but not colony-forming progenitors. Isotope tracing showed aspartate was used to synthesize asparagine and purines. Both contributed to increased HSC function as deletion of asparagine synthetase (Asns) or treatment with 6-mercaptopurine attenuated the increased function of GLAST over-expressing HSCs. Stem cell function is thus limited by aspartate in some contexts.Item MEIS1: At the Crossroads Between Metabolic and Cell Cycle Regulation(2013-01-17) Kocabas, Fatih; Sadek, Hesham A.Stem cells undergo self-renewal, maintaining themselves in an undifferentiated state while generating differentiated cells that are required for the tissue homeostasis or repair. One intriguing feature of stem cells is their maintenance in their respective hypoxic niche. Survival in this low-oxygen microenvironment requires significant metabolic adaptation. However, little is known about stem cell metabolism, its regulation or its effect on stem cell function. We started our work by focusing on the most comprehensively characterized adult stem cell population, the hematopoietic stem cells (HSCs). We demonstrate that mouse and human HSCs utilize glycolysis instead of mitochondrial oxidative phosphorylation to meet their energy demands. Furthermore, we demonstrate that Meis1 and Hif-1α are markedly enriched in HSCs and that Meis1 functions upstream of the two master redox regulators Hif-1α and Hif-2α, where loss of Meis1 results in a metabolic shift from glycolysis to mitochondrial oxidative metabolism, and increased oxidative stress, and loss of HSC quiescence. These results underscore the critical link between metabolism and cell cycle regulation of HSCs. We then sought to determine whether other stem cell populations share these unique metabolic characteristics. This strategy enabled us to identify the epicardium and the subepicardium of the heart as the cardiac hypoxic stem cell niche, which houses a metabolically distinct, Hif-1α positive population of glycolytic cardiac progenitors. Moreover, our studies indicate that Meis1, which regulates HSC metabolism and quiescence, also induces post-natal cell cycle exit and quiescence of cardiomyocytes through induction of synergistic cyclin dependent kinase inhibitor families. We demonstrate that both embryonic and adult deletion of Meis1 in cardiomyocytes results in widespread cardiomyocyte proliferation in the adult heart. Overall, our studies identify Meis1 as a critical transcriptional regulator of cell cycle and metabolism.Item The Niche for Extramedullary Hematopoiesis in the Spleen(2017-01-13) Inra, Christopher N.; Castrillon, Diego H.; Morrison, Sean J.; Buszczak, Michael; Cleaver, OndineThe ability to regenerate niches or to create new niches after injury is critical to accelerate tissue repair and may underlie the regenerative capacity of mammalian tissues. Despite its physiological importance, almost nothing is known about how mammalian tissues activate facultative niches after injury. The mouse hematopoietic system provides a dynamic example of new stem cell niche activation. After hematopoietic injury, hematopoietic stem cells (HSCs) mobilize from the bone marrow to the spleen and participate in extramedullary hematopoiesis (EMH), which supplements bone marrow hematopoiesis for as long as the hematopoietic stress persists. The induction of hematopoiesis in the spleen involves the creation or activation of a facultative niche in the spleen, yet no niche in this tissue has been characterized. Understanding the nature of the extramedullary niche in the spleen will clarify how the hematopoietic microenvironment regulates HSC and other progenitor function to reestablish homeostasis after injury. The work in this thesis identifies the cell types in the spleen that are physiologically important sources of the niche factors SCF and CXCL12 during extramedullary hematopoiesis. By using fluorescent reporter alleles for each niche factor, I have discovered that spleen endothelial and perivascular stromal cells secrete SCF, and a subset of spleen perivascular stromal cells secretes CXCL12. Conditional deletion of Scf from spleen endothelial or perivascular stromal cells impairs EMH after injury by depleting HSCs and myeloerythroid progenitors from the spleen. Conditional deletion of Cxcl12 from spleen perivascular stromal cells impairs EMH by depleting myeloerythroid progenitors and mobilizing a minority of HSCs from the spleen. This work conclusively demonstrates that spleen endothelial cells maintain EMH by secreting SCF, and spleen perivascular stromal cells maintain EMH by secreting both SCF and CXCL12. These cell types represent the first stromal populations in the spleen shown to maintain HSCs and EMH after injury. Further analyses of these cells during injury may reveal how hematopoietic niches are created.Item Refining Our Understanding of the Hematopoietic Stem Cell Niche(2015-09-24) Peyer, James Gregory; DeBerardinis, Ralph J.; Morrison, Sean J.; Buszczak, Michael; Olson, Eric N.A major therapeutic goal of studying blood-forming hematopoietic stem cells (HSCs) is to understand the mechanisms by which HSCs are maintained in the bone marrow, so that they can be grown outside of the body and used in lieu of or in combination with bone marrow transplantation to treat hematopoietic illnesses. HSCs, as well as other somatic stem cells from different organ systems and organisms, rely on signals from their local microenvironment for their maintenance. However, the identity of the HSC niche is still poorly understood. One new model of the HSC niche is that HSCs, periarteriolar stromal cells, and nerve fibers are closely associated in rare periateriolar niches. Using a novel marker to identify HSCs in three-dimensional confocal images, -catulin-GFP, we measured the distances from thousands of HSCs to various landmarks in the bone marrow. We found that few HSCs are closely associated with either nerve fibers or arterioles. Mice lacking sympathetic nerves exhibit multiple changes in hematopoiesis, especially in response to injury, though all of the studies published so far have systemically ablated sympathetic nerves. This left unresolved the question of whether the changes in hematopoiesis reflect bone marrow denervation itself, or systemic effects of general sympathectomy. To test this, I developed a model for bone marrow-specific neuropathy by conditionally deleting nerve growth factor (Ngf) from bone marrow stromal cells. Using this model, I analyzed the role of bone marrow peripheral nerves in hematopoiesis. I demonstrated that while nerves play no role in bone marrow homeostasis, nerve signaling after bone marrow injury is essential for hematopoietic regeneration. Future studies will build on this work to understand how nerve fibers promote the regeneration of HSCs and bone marrow cells despite not innervating the HSC niche themselves.