Exposure to ambient air pollution is a serious and common public health concern associated with growing morbidity and mortality worldwide. In the last decades, the adverse effects of air pollution on the pulmonary and cardiovascular systems have been well established in a series of major epidemiological and observational studies. In the recent past, air pollution has also been associated with diseases of the central nervous system (CNS), including stroke, Alzheimer's disease, Parkinson's disease, and neurodevelopmental disorders. It has been demonstrated that various components of air pollution, such as nanosized particles, can easily translocate to the CNS where they can activate innate immune responses. Furthermore, systemic inflammation arising from the pulmonary or cardiovascular system can affect CNS health. Despite intense studies on the health effects of ambient air pollution, the underlying molecular mechanisms of susceptibility and disease remain largely elusive. However, emerging evidence suggests that air pollution-induced neuroinflammation, oxidative stress, microglial activation, cerebrovascular dysfunction, and alterations in the blood-brain barrier contribute to CNS pathology. A better understanding of the mediators and mechanisms will enable the development of new strategies to protect individuals at risk and to reduce detrimental effects of air pollution on the nervous system and mental health.

SUMMARY The repertoire of ~1200 odorant receptors (ORs) is mapped onto the array of ~1800 glomeruli in the mouse olfactory bulb (OB). The spatial organization of this array is influenced by the ORs. Here we show that glomerular mapping to broad domains in the dorsal OB is determined by two types of olfactory sensory neurons (OSNs), which reside in the dorsal olfactory epithelium. The OSN types express either Class I or Class II OR genes. Axons from the two OSN types segregate already within the olfactory nerve and form distinct domains of glomeruli in the OB. These class-specific anatomical domains correlate with known functional odorant response domains. However, axonal segregation and domain formation are not determined by the class of the expressed OR protein. Thus, the two OSN types are determinants of axonal wiring, operate at a higher level than ORs, and contribute to the functional organization of the glomerular array.

From the approximately 1,200 odorant receptor (OR) genes in the mouse genome, an olfactory sensory neuron is thought to express only one gene. The mechanisms of OR gene choice are not understood. A 2.1 kilobase region (the H element) adjacent to a cluster of seven OR genes has been proposed as a trans- and pan-enhancer for OR gene expression. Here, we deleted the H element by gene targeting in mice. The deletion abolishes expression of a family of three OR genes proximal to H, and H operates in cis on these genes. Deletion of H has a graded effect on expression of a distal group of four OR genes, commensurate with genomic distance. There is no demonstrable effect on expression of OR genes located outside the cluster. Our findings are not consistent with the hypothesis of H as an essential trans-acting enhancer for genome-wide regulation of OR gene expression.

The current consensus model in mammalian olfaction is that the detection of millions of odorants requires a large number of odorant receptors (ORs) and that each OR interacts selectively with a small subset of odorants, which are typically related in structure. Here, we report the odorant response properties of an OR that deviates from this model: SR1, a mouse OR that is abundantly expressed in sensory neurons of the septal organ and also of the main olfactory epithelium. Patch-clamp recordings reveal that olfactory sensory neurons (OSNs) that express SR1 respond to many, structurally unrelated odorants, and over a wide concentration range. Most OSNs expressing a gene-targeted SR1 locus that lacks the SR1 coding sequence do not show this broad responsiveness. Gene transfer in the heterologous expression system Hana3A confirms the broad response profile of SR1. There may be other mouse ORs with such broad response profiles.

First described in 1973, the Grueneberg ganglion (GG) is an arrow-shaped neuronal structure at the anterior end of the nasal cavity. It lines both sides of the nasal septum, within the nasal vestibule, close to the opening of the naris. The functions of the GG and the pattern of projections to the brain are not known. Here, we report that neurons of the mouse GG express olfactory marker protein, which is normally expressed in mature olfactory or vomeronasal sensory neurons. The approx. 500 cells in each GG are arranged in several densely packed cell clusters. Individual cells give rise to single axons, which fasciculate to form a nerve bundle that projects caudally. The axons terminate in glomeruli of the olfactory bulb, one or two large glomeruli associated with a semicircle of up to 10 smaller, somewhat diffusely organized glomeruli that surround the most anterior part of the accessory olfactory bulb. Development of the GG starts around embryonic day 16 and appears to be completed at birth; cell numbers then undergo a minor decrease during postnatal development. The strategic location of the GG, expression of olfactory marker protein, axonal projections to glomeruli at particular locations in the olfactory bulb and early development suggest that this neuronal structure performs specific chemosensory functions at neonatal stages.