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Theses Doctoral

Axon Development and Synapse Formation in Olfactory Sensory Neurons

Marcucci, Florencia

The olfactory epithelium (OE) possesses the rare capacity among neuronal tissues to regenerate throughout life. As a result, progenitor cells continuously proliferate and differentiate into olfactory sensory neurons (OSNs) that project their axons to the olfactory bulb (OB) where they establish connections to the central nervous system. The olfactory epithelium is therefore an attractive model for the study of axonal growth and synapse formation. The present set of studies attempts to provide insights into synapse formation and axonal development of olfactory sensory neurons. First, I sought to understand the regulation of expression of pre-synaptic molecules in the olfactory epithelium. I established by in situ hybridization that as OSNs mature, they express sequentially groups of pre-synaptic genes. Genes encoding for proteins that play a structural role at the active zone showed an early onset of expression, whereas genes encoding for proteins associated with synaptic vesicles showed a later onset of expression. In particular, the signature molecule for glutamatergic neurons VGLUT2 shows the latest onset of expression. The sequential onset of expression suggests the existence of discrete steps in pre-synaptic development. In addition, contact with the targets in the olfactory bulb is not controlling pre-synaptic protein gene expression, suggesting that olfactory sensory neurons follow an intrinsic program of development. Second, in order to visualize simultaneously OSN axonal arborizations and their pre-synaptic specializations in vivo, I developed a method based on post-natal electroporation of the mouse nasal cavity. This technique allowed me to perform a temporal study where I followed the elaboration of axons and synapses in olfactory sensory neurons at different post-natal ages. The results show that olfactory sensory axons develop with exuberant growth and synapse formation. Exuberant branches and synapses are eliminated to achieve the mature pattern of connectivity in a process likely to be regulated by neural activity. Finally I investigated the consequences of suppressing neural activity in olfactory sensory neuron axonal morphology and synaptic composition. To this end I utilized two anosmic mouse models: cyclic nucleotide gated (CNG) channel and adenylyl cyclase 3 (AC3) knock-out mice. I observed that in the CNG knock-out mice, where OSNs do not generate action potentials after odor stimulation, the morphology of terminal arborizations and the synaptic composition were indistinguishable from wild-type littermates. In sharp contrast, AC3 knock-out mice, where there is no induction of cAMP production after odor stimulation, both the morphology and synaptic composition of OSN axons are severely altered. These results provide evidence that, unlike odor-induced membrane depolarization, odor-induced cAMP signaling events are critical for axonal growth and synapse formation in OSNs.

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More About This Work

Academic Units
Biological Sciences
Thesis Advisors
Firestein, Stuart J.
Degree
Ph.D., Columbia University
Published Here
March 21, 2012
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