Author's School

Graduate School of Arts & Sciences

Author's Department/Program

Biology and Biomedical Sciences: Molecular Cell Biology

Language

English (en)

Date of Award

January 2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Daniel Link

Abstract

Adult hematopoiesis normally occurs in bone marrow, where hematopoietic stem cells: HSC) reside within a specialized microenvironment. At steady state, hematopoiesis state is regulated such that immature hematopoietic stem and progenitor cells: HSPC) are restricted to the bone marrow and are rarely observed in peripheral blood. Under certain circumstances, however, this regulation is loosened and significant numbers of HSPC are released to the circulation, a process termed "mobilization." Mobilization can be induced pharmacologically by a wide range of agents. Of these, the best characterized and most widely used mobilizing agent is Granulocyte-colony stimulating factor: G-CSF). G-CSF is widely used clinically and several molecular mechanisms have been implicated as mediating its mobilizing action. However, many questions remain as to the relationship between these various pathways. This work begins by focusing on one mobilization pathway, the disruption by G-CSF of signaling between CXCL12, a chemokine expressed in the HSC microenvironment, and its receptor CXCR4, broadly expressed on hematopoietic cells. By examining mice genetically deficient in CXCR4, we show that this mechanism is not only the predominant pathway by which G-CSF induces mobilization, but also a common pathway utilized during treatment with other hematopoietic cytokines. Next, while investigating the mechanism by which G-CSF disrupts CXCL12/CXCR4 signaling, we unexpectedly uncovered a role for osteoblasts in regulating cytokine-induced mobilization. By isolating and sorting different fractions of bone marrow stromal cells we demonstrate that osteoblasts represent a major source of CXCL12 in the bone marrow. In addition, both the number and function of mature osteoblasts declines sharply during cytokine treatment. Subsequent analysis demonstrated that G-CSF both increases the rate of osteoblast apoptosis and blocks osteoblast development. Finally, experiments with G-CSF receptor null chimeras demonstrate that this effect on osteoblasts is not direct but is mediated by the hematopoietic compartment. While the regulation of hematopoiesis by osteoblasts has been well described, the reciprocal regulation of osteoblasts by bone marrow hematopoietic cells has not been widely appreciated. Further work will be required to determine if this regulation occurs not only during the specialized setting of cytokine-induced mobilization but during steady state hematopoiesis in general.

Comments

Permanent URL: http://dx.doi.org/10.7936/K76M34TV

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