Browsing by Subject "Stem Cell Niche"
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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.Item The Response of White Adipose Progenitor Cells to Physiological and Genetic Changes(2013-02-21) Zeve, Daniel; Johnson, Jane E.; Mangelsdorf, David J.; Olson, Eric N.; Graff, Jonathan M.We are in the midst of a dire, unprecedented and global epidemic of obesity and secondary sequelae, most prominently diabetes and hyperlipidemia. Underlying this epidemic are adipocytes and their inherent, dynamic ability to expand and renew. These abilities highlight a newly defined cell population within adipose tissue, the white adipose progenitor cell. These cells have the basic abilities that define a stem/progenitor cell, including the ability to proliferate and differentiate into mature adipocytes, opening up new studies into their involvement in both adipose development and growth. More specifically, interest lies in which physiological and genetic conditions can repress the adipogenic function of these cells, as these findings could lead to possible therapies for obesity and other metabolic diseases. We began our studies by examining the proliferative and adipogenic effect of both high fat diet and exercise on adipose progenitor cells. We found that while high fat diet increased adipose progenitor function, exercise dramatically reduced proliferation of the adipocyte progenitor in addition to diminishing new adipocyte formation during the exercise protocol. One physiological outcome of endurance exercise is the remodeling of skeletal muscle to more of a slow, oxidative fiber type composition. Thus, we hypothesized that type I skeletal muscle may also regulate the adipocyte progenitor. To directly test this hypothesis we analyzed the adipose progenitor cell in two, independent mouse lines that exhibit an increase of Type I fibers. These mice revealed that slow muscle fibers also reduce the activity of the adipocyte progenitor on normal chow and decrease adiposity while on high fat diet. Surprisingly, this effect may be due to non-nutritional factors, as the slow fiber mice exhibit no overt metabolic alterations on normal chow and conditioned media from muscle cell lines reduced pre-adipocyte function. These data suggest Type I fibers directly regulate the adipocyte progenitor cell, which may contribute to the reduced adiposity seen after exercise as well as the reduced adiposity of slow fiber mice in response to high fat diet. We next wanted to examine the genetics that control adipogenesis within the adipose progenitor cells. To do this, we activated Wnt signaling in either adipose progenitor cells or mature adipocytes. Wnt signaling is known to play a role in proliferation and differentiation in multiple stem cell lineages, including intestinal, bone and hematopoietic lineages, and thus we hypothesized that it may also play a role in in vivo adipogenesis and metabolism. Altering canonical Wnt signaling in mature fat tissues in mice had no discernable metabolic effects. In contrast, altering Wnt signaling in fat progenitors led to a depot-specific fate change and a paradoxical murine lipodystrophic syndrome that lacked the expected diabetes and ectopic fatty acid accumulation. Rather, muscle displayed increased glucose uptake and an insulin-independent increase in cell surface glucose transporters along with activation of AMPK and p38 MAPK. Muscle Wnt signaling was unaffected, indicating that these changes resulted from signals derived non-autonomously, which we found to be present in the serum of mutant mice. Thus, this model distinctively dissociates lipodystrophy from dysfunctional metabolism and uncovers a unique and potentially therapeutic method to lower blood glucose and improve metabolism.