Activity-Dependent Gene Expression in the Mammalian Olfactory Epithelium

Abstract

Activity-dependent processes are important to olfactory sensory neurons (OSNs) in several ways, such as cell survival and the specificity of axonal convergence. The identification of activity-dependent mRNAs has contributed to our understanding of OSN axon convergence, but has revealed surprisingly little about other processes. Published studies of activity-dependent mRNAs in olfactory mucosae overlap poorly, but by combining these agreements with meta-analysis of existing data we identify 443 mRNAs that respond to methods that alter OSN activity. Three hundred and fifty of them are expressed in mature OSNs, consistent with the expectation that activity-dependent responses are cell autonomous and driven by odor stimulation. Many of these mRNAs encode proteins that function at presynaptic terminals or support electrical activity, consistent with hypotheses linking activity dependence to synaptic plasticity and energy conservation. The lack of agreement between studies is due largely to underpowered experiments. In addition, methods used to alter OSN activity are susceptible to indirect or off-target effects. These effects deserve greater attention, not only to rigorously identify OSN mRNAs that respond to altered OSN activity, but also because these effects are of significant interest in their own right. For example, the mRNAs of some sustentacular cell enzymes believed to function in odorant clearance (Cyp2a4 and Cyp2g1) are sensitive to unilateral naris occlusion used to reduce odorant stimulation of the ipsilateral olfactory epithelium. Also problematic are odorant receptor mRNAs, which show little agreement across studies and are susceptible to differences in frequency of expression that masquerade as activity-dependent changes in mRNA abundance.

Semiochemical responsive olfactory sensory neurons are sexually dimorphic and plastic

“…Expression analysis revealed an enrichment of S100a5 , Ptprn , Pcp4l1 , and Nrp1 amongst OSNs expressing Olfr910 , Olfr912 , and Olfr1295 . Importantly, enrichment of these genes had been shown to occur in the literature as a function of neural activity ( Figure 8B–C ; Imai et al, 2006 ; Nakashima et al, 2013 ; Wang et al, 2017 ). Since the singly sequenced mature OSNs from Brann et al, 2020 were collected from mature male mice, who self-expose to MTMT and SBT, these results are consistent with the idea that odor experience significantly influences gene expression in OSNs, via an activity-dependent process relying on ligand-receptor interactions, to drive dimorphic representations of Olfr910 , Olfr912 , and Olfr1295 .…”

“…In our analysis of singly sequenced mature OSNs expressing Olfr910 , Olfr912 , and Olfr1295 harvested from mature male mice we observed a clear enrichment in the expression of numerous activity-associated genes (see summary statistics files; Figure 8B-C ). Importantly, the expression of many of these same genes was also previously shown to be changed as a function of OSN activity using surgical strategies like unilateral naris occlusion ( Wang et al, 2017 ), and transgenic strategies like knock-in expression of constitutively active G s ( Imai et al, 2006 ), and knock-in expression of wild-type and activity-deficient mutant non-OR G-protein coupled receptors ( Nakashima et al, 2013 ). Furthermore, these same activity-associated enriched genes were not enriched in singly sequenced mature OSNs expressing Olfr1437 and Olfr235 , consistent with the idea that male mice do not generate agonists for these ORs (see summary statistics files; Figure 8A-C ).…”

Sex separation induces differences in the olfactory sensory receptor repertoires of male and female mice

“…We reasoned that, if the observed receptor changes are driven by olfactory experience, we might find altered expression frequencies of sex-biased ORs on the closed side of the MOE. After confirming UNO efficiency via RNA-FISH analysis of the expression of Kirrel2 5 , 8 , an activity-dependent gene that is expressed more strongly on the open side of the MOE (Fig. 6b ), we quantified expression frequencies on the open and closed sides of the MOE for four ORs that are differentially expressed in SF and SM mice: Olfr912 , Olfr1419 , Olfr1437 , and Ofr235 , as well as a negative control OR ( Olfr867 ) that is not differentially expressed between SF and SM mice.…”

“…The red box corresponds to the approximate region of the images shown in b , d , and f . b Confirmation of UNO efficiency via RNA-FISH analysis of the expression of Kirrel2 5 , 8 , an activity-dependent gene that is more strongly expressed on the open (left) side of the MOE. c , e Quantitation of Olfr912 -expressing ( c ) and Olfr1419 -expressing ( e ) OSNs on the open and closed sides of the MOEs of mice subjected to UNO.…”

“…More recently, novel insights into molecular mechanisms underlying activity-dependent neuronal plasticity have led to a better understanding of these phenomena and their involvement in neurodevelopmental disorders 2 . In the olfactory system, activity plays important roles in both the formation and the refinement of precise connections between olfactory sensory neurons (OSNs) located in the main olfactory epithelium (MOE) and projection neurons within the olfactory bulb (OB) 3 – 8 . Moreover, olfactory experience modulates the abundance of specific OSN subtypes, as defined by the single olfactory receptor (OR) gene that each OSN expresses 9 – 14 .…”