The mouse olfactory system offers a unique and powerful model to study how intrinsic molecular identity and sensory experience shape transcriptional plasticity in neurons. Its highly organized structure—where each OSN expresses a single OR gene and projects to a stereotyped glomerulus in the olfactory bulb— enables precise, cell-type-resolved investigation of both intrinsic molecular identity and experience-driven transcriptional remodeling.
In the first part of this thesis, we demonstrate that the OR gene expressed in an OSN is the major driver of its resting transcriptional identity. Using single-cell RNA sequencing, we show that OSNs expressing the same OR converge on highly similar transcriptomic profiles, while OSNs expressing different ORs are transcriptionally distinct. These differences correlate with OR protein sequence, indicating that ORs, and likely their basal activity, shape OSN identity in the absence of stimulation. We also show that prolonged odorant exposure leads to a robust transcriptional reshaping involving hundreds of genes. This adaptation is partially shared across OSN populations, indicating a common, experience-triggered program acting independently of the specific receptor identity.
We next examined the signaling mechanisms required for this activity-induced modulation. By leveraging the modulation of OR gene expression as an in vivo readout of sensory activation, we dissected the olfactory transduction cascade across several mutant lines. We found that OR gene downregulation requires signaling upstream of cAMP production, but occurs even in the absence of CNGA2-mediated neuronal firing. In contrast, other genes, such as Mustn1, require CNGA2 for activity-induced modulation. These findings reveal two independent transcriptional branches downstream of OR activation: one dependent on cAMP, and another dependent on calcium influx and neuronal activity.
To explore how these signaling pathways contribute to both baseline identity and activity-induced plasticity, we performed single-cell transcriptomic profiling of OSNs in WT, Adcy3 KO, and Cnga2 KO mice, before and after odorant exposure. At rest, both AC3 and CNGA2 were found to shape OSN transcriptional identity, with AC3 loss resulting in a more pronounced disruption of OR-defined transcriptomic profiles. This suggests that cAMP signaling and downstream CNGA2-mediated activity contribute separately to the establishment of OR-defined transcriptional programs. Upon stimulation, both WT and Cnga2 KO OSNs displayed broad transcriptional changes, including the activation of a distinct cAMP-dependent gene expression program. This program was completely absent in Adcy3 KO OSNs, underscoring the essential role of AC3 in enabling odorant-evoked transcriptional adaptation. Altogether, this thesis uncovers how OR identity, basal activity, and sensory experience are integrated through parallel signaling pathways to define and reshape OSN transcriptomes. These findings support a multi-layered regulatory model of neuronal identity—where genetic programming, constitutive signaling, and sensory input converge—and provide a conceptual framework applicable to broader principles of neuronal function and adaptation.
