The main objective of the project is to determine the underlying mechanisms that pattern the mitral cell dendrites in the olfactory system. Formation of precise axonal and dendritic pattern is crucial for establishing functional neural circuitry in the brain. In the olfactory system, the guidance mechanisms of the olfactory sensory neuron axons toward their target have been investigated recently. However, studies are still lacking for the molecular mechanisms of the differentiation and the targeting of the mitral cell dendrites, the postsynaptic partner of the olfactory nerve. We propose to investigate the following hypotheses concerning the control of mitral/tufted cell dendritogenesis: 1) Interaction with olfactory nerve is required for the morphogenesis of mitral/tufted cell dendrites. The influence of the olfactory nerve on mitral cell neurite extension will be tested using in vivo and in vitro approaches. The developmental changes of mitral cell dendrites will be characterized. The orientation of the mitral/tufted cell dendrites will be investigated using 3D collagen matrix. The role of the] mature olfactory axons in mitral cell dendritic development will be examined using genetic conditional ablation. 2) Olfactory nerve provides diffusible molecular cues to promote dendritic extension and branching of the mitral/tufted neurons. This hypothesis will be directly tested using dissociated olfactory bulb culture. The presence of the diffusible factors for promoting mitral cell dendritic growth will be examined using olfactory epithelium conditioned medium. Candidate molecules, Semaphorins, neurotrophins and BMPs will be examined for their dendrite growth promoting activity in the dissociated olfactory bulb neuron cultures. Olfactory epithelium derived activity will be characterized by gel filtration and ion exchange chromatography using FPLC. Molecular identity of the OE derived activity will be obtained by peptide sequencing. The long-term goal of our study is to understand the precise cellular and molecular interactions that are critical for the development of the nervous system and to gain insight into the causes of developmental neurological disorder and birth defects.
