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Olfactory bulbSummaryThe olfactory bulb is the region in the brain receiving input from the olfactory neurons in the nasal olfactory epithelium. It is organized in glomeruli (one per smell) and innervates the dendrites of mitral cells that project to other forebrain regions including the olfactory cortex. Local circuits in the glomeruli and plexiform layers finetune the olfactory output information. It consists of the main olfactory bulb and the accessory olfactory bulb, involved in the detection of pheromones. The olfactory bulb is located inferior (bottom) of the human brain, while in most vertebrates it is the most rostral (front) region of the brain. The olfactory bulb is relatively small in humans compared to other vertebrates. Odor molecules are detected by the olfactory sensory neurons in the olfactory epithelium. The signal transduction starts at the olfactory receptors present on the cilia of the sensory neuronal dendrites, covering the surface of the olfactory epithelium. The binding of the odor molecule triggers an electrical response and sensory signals to the olfactory nerve fiber. The olfactory nerve fibers pass through the plate of the bone and connect the epithelium to the olfactory bulbs of the brain.
Figure 1. Schematic overview of the peripheral (olfactory epithelium) and central (olfactory bulb in the brain) olfactory system. The major cells and structures involved in the olfactory system is illustrated and marked with names. Anatomical divisionsThe olfactory bulb is built up by five different layers:
The glomeruli layer is organized into zones and clusters representing different spatial odors and is the first level of synaptic processing. The second level is the external plexiform layer, which mainly includes dendrites from the mitral cells, GABAergic granule cells, astrocytes and interneurons. Mitral cells are the third step in the processing, proving the output to the olfactory cortex, and receiving information from the granule cell layer, located in the center of the olfactory bulb.
Examples of proteins located in glomeruli layer in the mouse olfactory is SLC17A6, ADORA2A and DRD2. Several examples above show positivity in the external plexiform layer, for example CRB2 while C17orf75, ARFGEF1 is located to Mitral cells and their processes in the external plexiform layer. U2AF2 is located in several different cell types, including Mitral cells and DPP6 show a distinct positivity in processes in all the different layers. Regionally elevated protein expression in humanThe current version of the human protein atlas does not contain data on the human olfactory bulb. Regionally elevated protein expression in mouseThe mouse olfactory bulb is similar to the human olfactory bulb, but larger and with a rostral position. The two olfactory bulbs are often seen as coronal sections, with the different layers clearly visible. In Figure 2, TH staining of mouse olfactory bulb indicates strong labeling of the glomerular layer. Mouse protein profiling in the Brain Atlas includes sections of olfactory bulb and 10 separate structures of the olfactory bulb are annotated separately (Glomerular layer, external plexiform layer, mitral cell layer, internal plexiform layer, granule cell layer as well as rostral migratory stream, anterior olfactory nucleus and glomeruli layer, external plexiform layer and granule cell layer of the accessory Olfactory bulb). Table 2: Number of genes within the different categories of regionally elevated expression, in mouse olfactory bulb
The transcriptome analysis shows that 62% (n=12410) of mouse genes with one-to-one orthologue human genes (n=16679) are expressed in the olfactory bulb and 158 genes show an elevated expression in olfactory bulb compared to other regions of the mouse brain. Elevated expression in olfactory bulb compared to other brain regions is divided into three different categories; regionally enriched (at least four-fold higher mRNA levels in olfactory bulb compared to all other regions), group enriched (at least four-fold higher mRNA levels in a group of 2-5 regions) and regionally enhanced (at least four-fold higher mRNA levels in olfactory bulb compared to the average of all regions), The number of genes in the individual category is shown in Table 2. In Table 3, the 12 genes with the highest level of regional specificity among the 79 enriched genes are listed. Table 3: The 12 genes with the highest level of enriched expression in mouse olfactory bulb. "Predicted localization" shows the classification of each gene into three main classes: Secreted, Membrane, and Intracellular, where the latter consists of genes without any predicted membrane and secreted features. RS-score (Regional Specificity score) corresponds to the score calculated as the fold change to the second highest region.
SCGN and DCX are two examples of proteins with an elevated expression in olfactory bulb. The RNA expression of DCX is rather similar in mouse and pig, while human brain shows a more general forebrain expression. In mouse brain, the protein is detected in a limited subpopulation of neuronal cell bodies and their processes in both hippocampus and olfactory bulb. SCGN is a protein with distinct difference between human and mouse, the expression in mouse is restricted to the olfactory bulb and the protein is detected in the granular layer of olfactory bulb, while in human SCGN is expressed in hypothalamus and cerebellum. Regionally elevated protein expression in pigTable 4: Number of genes within the different categories of regionally elevated expression, in pig olfactory bulb.
The pig olfactory bulb is similar to the human olfactory bulb, but larger and with a rostral position. The transcriptome analysis shows that 69% (n=13937) of all pig one-to-one human orthologue genes (n=16614) are expressed in the olfactory bulb, and 89 genes show an elevated expression in olfactory bulb compared to other regions of the brain. Elevated expression in olfactory bulb compared to other brain regions is divided into three different categories; regionally enriched (at least four-fold higher mRNA levels in olfactory bulb compared to all other regions), group enriched (at least four-fold higher mRNA levels in a group of 2-5 regions) and regionally enhanced (at least four-fold higher mRNA levels in olfactory bulb compared to the average of all regions), The number of genes in the individual category is shown in Table 4. In Table 5, the 12 genes with the highest level of regional specificity among the 29 enriched genes are listed. Table 5: The 12 genes with the highest level of enriched expression in pig olfactory bulb. "Predicted localization" shows the classification of each gene into three main classes: Secreted, Membrane, and Intracellular, where the latter consists of genes without any predicted membrane and secreted features. RS-score (Regional Specificity score) corresponds to the score calculated as the fold change to the second highest regional.
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