ISDN2012_0259: Constructing and deconstructing spinal motor neurons

the anatomical locations of OSNs in the OE. Expression levels of D–V guidance molecules, such as Nrp2 and Sema3F, are closely correlated with the expressed OR species. However, unlike Nrp1 and Sema3A which are involved in A–P positioning, the transcription of Nrp2 and Sema3F is not downstream of OR signaling. If D–V guidance molecules are not regulated by OR-derived signals, how are their expression levels determined by the expressed OR species? We assume that both OR gene choice and Nrp2 expression levels are commonly regulated by positional information of OSN cell bodies within the OE. Complementary expression of Nrp2 and Sema3F in OSN axons suggested repulsive interactions among OSN axons. It has been shown that OSNs in D-zone mature earlier than those in Vzone. The olfactory map expands ventrally as the OB grows during embryonic development. These observations indicate an intriguing possibility that Sema3F is secreted by early-arriving D-zone axons and serves as a guidance cue to repel late-arriving V-zone axons that express the Nrp2 receptor. During the process of embryonic development, OSN axons are guided to approximate locations in the OB by a combination of D–V patterning based on anatomical locations of OSNs, and A-P patterning based on OR-derived cAMP signals. After a course map is generated in the OB, further refinement needs to occur in the neonatal period through segregation of glomeruli in an activitydependent manner. To investigate how OR-specific glomerular sorting is regulated, we attempted to locate a group of genes whose transcription profiles correlate with the expressed OR species. Using a transgenic mouse in which the majority of OSNs express a particular OR, such genes were identified. They include those genes that code for homophilic adhesive molecules, such as Kirrel2 and Kirrel3. Mosaic knockouts of these genes generate duplicated glomeruli even though the expressed OR species are the same, suggesting that they play a role in the attraction of “like” axons. Repulsive molecules, such as ephrin-A5 and EphA5, are also expressed in a complementary manner in each subset of OSNs. Therefore, interactions between two subsets of axons, one that is ephrin-A5 high and the other that is EphA5 high, may be important for the segregation of non-“like” axons. We assume that a specific set of adhesive and repulsive molecules, whose expression levels are determined by OR molecules, regulate axonal fasciculation and glomerular segregation for the map refinement. As mentioned earlier, axon-derived guidance molecules alone could organize an olfactory map by axon-axon interactions of OSNs even in the absence of the target OB. However, the map needs to be properly connected with second-order neurons, mitral/tufted (M/T) cells, to make the olfactory map functional. We have found that Sema3F, a repulsive ligand to Nrp2, which is secreted by dorsal OSN axons, guides both Nrp2+ OSN axons and their partner Nrp2+ M/T cells to the ventral region of the OB. This coordinated guidance is an important process for proper alignment and synapse formation of OSN axons with M/T cells during olfactory development. The mammalian main olfactory system mediates various responses, including aversive behaviors to spoiled foods and fear responses to predator odors. Because a particular odorant interacts with many different odorant receptor species, multiple sets of glomeruli are activated in both dorsal (D) and ventral (V) domains of the OB. However, little is known about how the topographical information in the OB is transmitted to and interpreted in the brain to elicit various behaviors. To address these questions, mutant mice, in which the OSNs in a specific area of the OE are ablated with diphtheria toxin, were generated. It was demonstrated that, in D-zone-depleted mice, the D domain of the OB was devoid of glomerular structures. Surprisingly, the mutant mice lacked innate responses to aversive odorants, even though they were capable of detecting them and could be conditioned for aversion using the remaining glomeruli. It was once thought that both D and V domain glomeruli contribute equally to the processing of odor information. However, the mouse main olfactory system appears to be composed of two functional modalities: one for innate odor responses and the other for learned responses. In the olfactory system, the OB may not simply be a projection screen for glomerular map formation. Instead, it may have region-specific functions that are genetically predetermined at least for innate odor responses. Acknowledgement: This work was supported by the Specially Promoted Research Grant from Japanese Government.