Year of Publication

2009

Degree Name

Doctor of Philosophy (PhD)

Document Type

Dissertation

College

Medicine

Department

Physiology

First Advisor

Dr. Timothy S. McClintock

Abstract

The sense of smell relies upon the detection of odorants by neurons located in the nasal cavity. These neurons, referred to as olfactory sensory neurons (OSNs), line the olfactory epithelium and extend axons that make synaptic connections with mitral/tufted cells in the olfactory bulb. The mechanisms by which these synaptic connections form remain largely unknown. The development of these synaptic connections relies on the axons of immature OSNs innervating the olfactory bulb. The primary goal of this dissertation was to identify components of the mechanisms used by immature OSN axons to innervate the olfactory bulb. To accomplish this goal, a knockout mouse model was used. OSN axons, of Emx2 knockout mice fail to innervate the olfactory bulb. As EMX2 is a transcription factor, this model was used investigate the possible causes of the defective OSN axon growth. To gain a better understanding of OSN axon growth, differences in expression of axon growth and guidance genes in immature and mature OSNs was investigated. This analysis revealed that many axon growth and guidance genes are differential expressed, and helped to identify immature OSN specific genes. The data also revealed a previously unrecognized developmental stage, termed nascent OSNs, identified by the expression of Cxcr4. Analysis of Emx2-/- mice revealed that EMX2 is necessary for OSN survival, odorant receptor expression and expression of the axonogenesis related gene Ablim1. EMX2 is necessary for the expression of many odorant receptor genes; however the loss of odorant receptor expression does not explain the axon growth defects. Apoptosis is increased in Emx2-/- mice, an outcome that may be due to the failed axon growth. Analysis of axon guidance gene expression identified a large reduction in Ablim1 expression in Emx2-/- mice. Ablim1 is expressed by immature OSNs, placing it in the proper cell type to regulate OSN axon growth. The loss of Ablim1 expression in Emx2-/- mice indicates defective signaling in the axon growth cone and a possible mechanism regulating OSN axon growth into the olfactory bulb. The data presented in this dissertation provide new insight into the regulation of odorant receptor gene expression and OSN axon growth.

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