Title

Altered mechanisms of signaling and transcription factor regulation in aged B cell development

Date of Award

2004

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Microbiology and Immunology

First Committee Member

Richard L. Riley, Committee Chair

Abstract

Senescence in mice is associated with reductions in bone marrow pre-B cells. These follow developmental steps that are critically dependent on signaling through the pre-B cell receptor (preBCR) and the IL-7 receptor. We have categorized a large panel of aged BALB/c mice into two phenotypes based on their patterns of pre-B/pro-B cell loss. Each phenotype is characterized by distinct responses to IL-7 and capacity for survival in vitro. A "moderate" loss of late-stage pre-B cells (25--80%) coincided with decline in signaling responses to IL-7 as well as IL-7 mediated proliferation. A "severe" loss of pre-B cells (>80%) resulted in a reduced pro-B cell pool which retained normal activation and proliferative responses to IL-7, but increased susceptibility to apoptosis. We suggest that aged mice accumulate B cell precursors that are poorly responsive to IL-7 and progressively eliminate them via apoptosis, selecting for B cell precursors that retain the capacity to respond to IL-7.The E2A-encoded E47 protein is critical for the development of B cell precursors. E47 protein levels increase in the pro-B to pre-B cell transition. We show that the rate of E47 protein turnover is increased in the absence of preBCR expression, suggesting that signaling through the preBCR promotes E47 stability.Freshly isolated bone marrow pro-B/early pre-B cells from aged BALB/c mice expressed lower E47 protein levels than young controls. In contrast, the reduced pool of aged late-stage pre-B cells retained normal E47 expression. Cultured senescent pro-B/early pre-B cells exhibited reduced E47 protein levels, DNA-binding activity and stability as compared to young controls, but E2A mRNA levels and turnover were normal. Reduced E47 protein levels were due to increased protein turnover, likely proteasome mediated. We propose that reduced E47 protein levels in aged B cell precursors result from extrinsic, senescence-associated microenvironmental bone marrow alterations and are further aggravated by defects in signaling mediated via the preBCR. Maintenance of normal E47 protein levels within the highly reduced pre-B cell pool in senescence suggests that pre-B cells undergo selection based on E47 expression. Dysregulation of protein turnover likely constitutes a novel and important defect in B lymphopoiesis during senescence.

Keywords

Biology, Cell

Link to Full Text

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