A feedback mechanism regulates monoallelic odorant receptor expression

Abstract

In olfactory neurons, expression of a single odorant receptor (OR) from a repertoire of >1,000 genes is required for odor coding and axonal targeting. Here, we demonstrate a role for OR protein as an essential regulator in the establishment of monoallelic OR expression. An OR-promoter-driven reporter expresses in a receptor-like pattern but, unlike a native OR, is coexpressed with an additional OR allele. Expression of a functional OR from the identical promoter eliminates expression of other OR alleles. The presence of an untranslatable OR coding sequence in the mRNA is insufficient to exclude expression of a second OR. Together, these data identify the OR protein as a critical element in a feedback pathway that regulates OR selection.T he mammalian olfactory system detects odors with remarkable sensitivity while decoding the complex odorant stimuli present in the environment. Populations of olfactory neurons with similar response profiles generate patterned activity within the olfactory bulb (OB) and lead to the perception of a given odor. These cellular responses are defined by the selective expression of seven-transmembrane odorant receptor (OR) proteins (1).Each olfactory neuron expresses only one allele of an individual OR gene from Ͼ1,000 related genes dispersed throughout the genome, a process termed monoallelic expression (2-4). The neurons expressing a particular OR reside in one of multiple zones defined by their dorsal-medial to ventral-lateral position in the olfactory epithelium (OE) (5, 6). These OR expression zones contribute broad topographic organization for the axons of olfactory neurons as they target the OB (5,7,8). ORs also play an important role in olfactory axon pathfinding (9, 10). Neurons that express the same OR, while interspersed with neurons expressing other ORs in the epithelium, project axons that converge to a small number of stereotyped glomeruli in the OB (11, 12). A transgene comprised of proximal OR promoter sequences is in many cases sufficient to recapitulate normal expression (13,14), although the transgene integration site can significantly influence reporter expression. This variability has confounded the elucidation of the processes that contribute to monoallelic, zonal OR expression.The monoallelic expression of ORs is critical to the establishment of the functionally distinct neuronal populations required for odor discrimination and organization of axonal projections to the OB (15, 16). This highly regulated expression could be achieved through a probabilistic promoter selection mechanism that is independent of the production of functional OR protein (10). Alternatively, OR protein or mRNA could initiate a feedback signal that ensures monoallelic expression.To determine whether monoallelic OR expression depends on feedback regulation, we targeted an OR promoter-driven reporter to a defined, reproducible transgene insertion site amid a cluster of ORs. This approach alleviates transgenic variability while facilitating analysis of potential coexpression with adjacent ORs...

Activity plays a role in eliminating olfactory sensory neurons expressing multiple odorant receptors in the mouse septal organ

“…This result lends support to the second and/or third possibility, but further experiments will be required to distinguish them. In addition to the mechanisms proposed in OR gene selection and stabilization (Fuss et al, 2007;Lewcock and Reed, 2004;Lomvardas et al, 2006;Nguyen et al, 2007;Serizawa et al, 2003;Shykind et al, 2004), the current study reveals an activity-dependent mechanism in ensuring and maintaining that one cell expresses only receptor in a subset of sensory neurons.…”

“…The second possibility is that one of the coexpressed odorant receptors, favorably stimulated by the olfactory cues, becomes dominant in that neuron by suppressing the others via a negative feedback mechanism (Lewcock and Reed, 2004;Serizawa et al, 2003;Shykind et al, 2004). The third possibility is that the OSNs maintain the capability to switch their receptor selection even in adulthood, and the switching only stops when the neuron is functionally activated and one selection is thus stabilized.…”

“…The underlying mechanism is under extensive investigation and likely involves several hierarchical steps from the initial OR selection to silencing other ORs (Fuss et al, 2007;Lomvardas et al, 2006;Nguyen et al, 2007). It has been reported that stabilizing an OR choice in individual neurons requires a negative feedback triggered by the receptor protein, especially encoded by a functional OR gene (Lewcock and Reed, 2004;Serizawa et al, 2003;Shykind et al, 2004). Here we hypothesize that the neuronal activity elicited by sensory stimulation plays a role in ensuring the one neuron-one receptor tenet.…”

Differential development of odorant receptor expression patterns in the olfactory epithelium: A quantitative analysis in the mouse septal organ

“…Each olfactory sensory neuron (OSN) in the main olfactory epithelium (MOE) most likely expresses a single OR gene, although the underlying mechanisms are not fully understood (Serizawa et al, 2003;Lewcock and Reed, 2004;Shykind et al, 2004;Lomvardas et al, 2006;Fuss et al, 2007). All OSNs expressing the same OR converge their axons onto one or a few specific glomeruli in the main olfactory bulb (Mombaerts, 2006).…”

Olfactory receptor genes expressed in distinct lineages are sequestered in different nuclear compartments

“…Consistent with this idea, some OSNs appear biased to express TAARs. OR-expressing OSNs are thought to randomly select one odorant receptor (Olfr) allele for expression, but choose a second Olfr allele to express if the first one fails to produce a functional protein (18)(19)(20). A high proportion of OSNs that express a mutated Taar gene also express a functional Taar allele, suggesting similar mechanisms for Olfr and Taar "gene choice," but one tailored to Taars (21,22).…”