Co-Opting the Unfolded Protein Response to Elicit Olfactory Receptor Feedback

Abstract

Summary Olfactory receptor (OR) expression requires the transcriptional activation of one out of thousands of OR alleles and a feedback signal that preserves this transcriptional choice. The mechanism by which olfactory sensory neurons (OSNs) detect ORs to signal to the nucleus remains elusive. Here, we show that OR proteins generate this feedback by activating the unfolded protein response (UPR). OR expression induces Perk-mediated phosphorylation of the translation initiation factor eif2α causing selective translation of Activating Transcription Factor 5 (ATF5). ATF5 induces the transcription of Adenylyl Cyclase 3 (Adcy3), which relieves the UPR. Our data provide a novel role for the UPR in defining neuronal identity and cell fate commitment and support a two-step model for the feedback signal: first OR protein, as a stress stimulus, alters the translational landscape of the OSN and induces Adcy3 expression; then, Adcy3 relieves that stress, restores global translation and makes OR choice permanent.

Rare event of histone demethylation can initiate singular gene expression of olfactory receptors

“…2). Although the kinetics of this feedback has not been measured, the extremely high transcription levels of ORs and Adcy3 (8), the utilization of unfolded protein response (11), and the intrinsic instability of Lsd1 (rapid precipitation and proteasomal degradation when not bound by cofactors) (16, 17) may help to accelerate the response. Given a typical timescale Δt ∼ 1-2 h for transcriptional regulations (18), the activation of the first ORnamely, the maturation of olfactory sensory neurons-should be ∼5-10 d, which is consistent with observations after olfactory bulbectomy experiments, where the regeneration of sensory neurons occurred 5-10 d after the operation (19).…”

“…With this marker, all OR genes are deeply repressed in nuclear aggregates before activation (9). The existence of negative feedback was also confirmed recently: Once a choice of ORs is made, the histone demethylase Lsd1 (also known as Aof2 or Kdm1a) will be transcriptionally repressed by OR expression (10,11). Lsd1 down-regulation will prevent further activation and freeze the system after the choice because this enzyme is essential for OR activation.…”

“…Notice that here we focus on the activator role of Lsd1; its second role as a repressor is discussed in the last subsection of Results. Once fully activated, OR expression will transcriptionally down-regulate Lsd1 via a recently identified cascade (OR activates the adenylate cyclase Adcy3, which in turn represses Lsd1) (10,11), whose response time is denoted Δt. If the feedback is instant, namely, Δt = 0, singularity will be perfect; however, this is not physiological because Lsd1 depletion takes time (thus Δt > 0).…”

“…Robust singularity can be achieved if OR activation-namely, the complete demethylation of H3K9me3-occurs as a rare, discrete event, as a result of a combination of slow activation and fast feedback. Now that the feedback loop (OR, Adcy3, Lsd1) underlying the maintenance phase has been identified (10,11), the only missing piece in OR singularity is the activation step responsible for the required kinetic bottleneck. It is even possible that both the H3K9me3 and H3K9me2 demethylases are downregulated by the feedback, which may enhance the sensitivity and robustness of the feedback.…”

Activating transcription factor 5 is required for mouse olfactory bulb development via interneuron

“…This stress response leads to ATF5 gene expression, resulting in acceleration of maturation of OSNs. 20) Thus, under the intrinsic stress condition, ATF5 is important for normal cell differentiation and survival of OSNs in the OE. However, it remains to be determined whether ATF5 regulates the formation of OB, to which axons of OSNs form ONL and SVZ-born interneurons migrate.…”