In recent years, the mouse olfactory and vomeronasal systems have emerged as useful models for studying neuronal connectivity in the mammalian brain. These systems relay information about chemical stimuli in their environment in the form of focal patterns of activation in the olfactory bulb of the brain. In mammals, chemical detection is mediated by a large family of odorant and vomeronasal receptors expressed in chemosensory neurons. Current understanding is that odorant and vomeronasal receptors are partially, yet not solely, responsible for the connectivity of sensory axonal projections to the olfactory bulb. In the search for additional guidance molecules, neuropilin-2 (NP2) was identified as having distinct roles in setting up these connections. The major questions raised by these studies, however, concern the ligands for NP2. The first focus of this study is to sort out the contributions of sema3B, sema3C, and sema3F to the axon guidance function of NP2. This will be done by genetically ablating these NP2 ligands from the mouse genome, assaying the resulting pathfinding errors, and correlating these data with observed axon pathfinding defects in NP2 mutant mice. Concurrent to the establishment of sensory innervation, mitral and tufted cells within the main olfactory bulb send out primary axons to the ventral forebrain forming the lateral olfactory tract (LOT). In contrast to sensory axons, mitral and tufted cells innervate their numerous target areas through extensive collateral branching. Yet, the exact timing of this process in mouse remains an open question. The second goal of these studies is to follow the development of the LOT in vivo through genetically-modified fluorescently-labeled mitral and tufted cell axons. Because this process occurs concomitant with sensory innervation, one question raised is the independence of LOT formation from sensory innervation by the olfactory epithelium. This question will be addressed by combining the fluorescently tagged LOT with mutations known to affect sensory innervation. The combination of in vivo fluorescent imaging and genetic ablation will provide powerful tools for investigating axonal guidance in the olfactory systems as well as other areas of the brain.
