Year of Publication

2007

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Medicine

Department

Physiology

First Advisor

Dr. Thomas V. Getchell

Abstract

Olfactory sensory neurons (OSNs) undergo continual degeneration and replacement throughout life, a cycle that can be synchronized experimentally by performing olfactory bulbectomy (OBX). OBX induces apoptosis of mature OSNs, which is followed by an increase in the proliferation of progenitor basal cells. Macrophages, functionally diverse immune effector cells, phagocytose the apoptotic OSNs and regulate the proliferation of basal cells. This provides an advantageous environment to study how macrophages regulate neuronal death, proliferation, and replacement.

The purpose of this dissertation was to identify the cellular and molecular mechanisms by which macrophages regulate the degeneration/proliferation cycle of OSNs. Macrophages were selectively depleted using liposome-encapsulated clodronate (Lip-C). Intranasal and intravenous administration of Lip-C decreased the number of macrophages in the OE of sham and OBX mice by 38% and 35%, respectively, compared to mice treated with empty liposomes (Lip-O). Macrophage depletion significantly decreased OE thickness (22% and 21%, p<0.05), the number of mature OSNs (1.2- and 1.9-fold, p<0.05), and basal cell proliferation (7.6- and 3.8-fold, p<0.05) in sham and OBX mice, respectively, compared to Lip-O mice. Additionally, at 48 h following OBX, OSN apoptosis increased significantly (p<0.05) in the OE of Lip-C mice compared to Lip-O mice.

A microarray analysis was performed to identify the genomic changes underlying the cellular changes associated with macrophage depletion. There were 4,024 genes with either a significant interaction between group (Lip-C vs. Lip-O) and treatment (OBX vs. sham) or a significant main effect. There were a number of significantly regulated immune response and cytoskeletal genes, and genes encoding neurogenesis regulators and growth factors, most of which were expressed at lower levels in Lip-C mice compared to Lip-O mice. Sdf1, the ligand for the chemokine receptor Cxcr4 involved in leukocyte trafficking, axon guidance, and cell migration, was localized to macrophages on the protein level. Additionally, the microarray expression pattern of Hdgf, a growth factor that promotes neuronal survival and proliferation, was validated on the protein level using immunohistochemistry. HDGF appeared to be localized to basal cells and OSNs where it could act as a proliferative or survival factor whose expression is regulated in part by macrophages.

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