There are roughly 2600 olfactory receptor (OR) genes in the diploid mouse genome. Each olfactory sensory neuron expresses only a single OR in a monogenic and monoallelic fashion. The molecular mechanisms regulating receptor expression in the mammalian nose are poorly understood. We have identified thetransient expression of histone demethylase LSD1, and the OR-dependent expression of Adenylyl Cyclase 3 (Adcy3) as requirements for initiation andstabilization of OR expression. As a transcriptional co-activator, LSD1 is necessary for de-silencing and initiating OR transcription, but as a transcriptional co-repressor, it is incompatible with maintenance of OR expression and its downregulation is imperative for stable OR choice. Adcy3, a transmitter of an OR-elicited feedback, mediates the downregulation of LSD1 and promotes the differentiation of olfactory sensory neurons (OSNs). This novel, three-node signaling cascade locks the epigenetic state of the chosen OR, stabilizes its singular expression, and prevents the transcriptional activation of additional OR alleles for the life of the neuron.

We have gone on to show that ER stress and specifically PERK kinase likely detects the OR protein and thus serves as a key intermediary in the pathway leading to LSD1 downregulation and OR choice stabilization. The activation of PERK via the UPR triggers the translation of nuclear ATF5, which is required for the upregulation of Adcy3. Thus LSD1 is needed to activate one OR from a repressed chromatin environment which initiates the cascade of molecular events at the ER that lead to its eventual downregulation. This dynamic regulation of LSD1 via ATF5 iscentral to the generation coherent olfactory input to the brain.

We have evidence suggesting that epigenetic silencing, which is mediated by histone H3 lysine 9 methyltransferases G9a and G9a-like protein (GLP), is essential for both the stochastic and the monogenic nature of OR expression. Conditional deletion of both G9a and GLP results in transcriptional domination by a few ORs, loss of monogenic expression, and significant downregulation of the rest of the family members. Therefore, heterochromatin at OR genes creates an epigenetic platform for generating transcriptional and cellular diversity.