CRCNS: The Mechanisms of Odorant Receptor Map Formation in Mice - Although much is known about the general flow of information through olfactory networks, we lack fundamental understanding of what features of odorants are represented in the brain. Odorants are sensed by olfactory sensory neurons (OSNs) of the olfactory epithelium (OE). Each OSN expresses one type of the odorant receptor (OR) gene out of hundreds of possibilities. Although OSNs that express the same OR protein type are scattered throughout the anterior-posterior axis of the epithelium, their axons converge in discrete spherical structures on the surface of the olfactory bulb (OB) called glomeruli. In different individual animals, glomeruli innervated by OSNs carrying the same OR type are located at stereotypical positions on the 2D surface of the OB (s.d.~1 glomerular size). The layout of glomeruli on the 2D surface of the OB according to the identity of the OR gene that they receive the primary input from is called the glomerular map. Understanding the organization of the glomerular OR map has been a fundamental challenge in the theory of olfaction: the 2D glomerular map is highly precise and appears to capture an important simplifying principle important to olfactory coding. In this project we will use a unique combination of experimental and theoretical techniques to obtain the complete layout of the glomerular OR map and elucidate the mechanisms of its formation during the first month of life. We will use a next-generation method of assaying gene expression and anatomical projections based on in situ sequencing (Barcoded Anatomy Resolved by sequencing, BARseq) developed in CSHL. BARseq uses in situ sequencing to detect gene expression in hundreds of thousands of neurons in a single animal. Using BARseq, we will map the identity of all ~3,000 glomeruli in the juvenile mouse olfactory bulb according to the OR types from which they receive inputs from. In addition, we will monitor the expression of the major axon guidance molecules that have been implicated in the map formation across different time points during postnatal development. To study map formation, we will use our previously developed theoretical model which successfully explained the formation of retinotopic maps in the visual system. Using gene expression map obtained using BARseq, we will test the coloring hypothesis derived from our theoretical model. This hypothesis is akin to the four-color theorem in mathematics and states that glomerular OR map should be organized to prevent neighboring glomeruli from sharing the expression vectors of sets of axon guidance molecules which can be viewed as `composite color' mosaics Our specific aims (SAs) are D RELEVANCE (See instructions): Our sense of smell helps us enjoy life, serves as a warning system alerting us to danger, such as spoiled food, fire, or a gas leak, and its loss is an early sign of neurodegeneration. This project will build and validate the computational model of development of the mammalian olfactory system. Our studies will greatly expand our ability to predict the patterns of neuronal connections, transform our ability to characterize how these patterns are disrupted in neuropsychiatric and developmental disorders, and give D
