ER stress transforms random olfactory receptor choice into axon targeting precision.

Olfactory sensory neurons (OSNs) convert the stochastic choice of one of >1,000 olfactory receptor (OR) genes into precise and stereotyped axon targeting of OR-specific glomeruli in the olfactory bulb. Here, we show that the PERK arm of the unfolded protein response (UPR) regulates both the glomerular coalescence of like axons and the specificity of their projections. Subtle differences in OR protein sequences lead to distinct patterns of endoplasmic reticulum (ER) stress during OSN development, converting OR identity into distinct gene expression signatures. We identify the transcription factor Ddit3 as a key effector of PERK signaling that maps OR-dependent ER stress patterns to the transcriptional regulation of axon guidance and cell-adhesion genes, instructing targeting precision. Our results extend the known functions of the UPR from a quality-control pathway that protects cells from misfolded proteins to a sensor of cellular identity that interprets physiological states to direct axon wiring.

Chemosensory receptor specificity and regulation.

The senses provide a means by which data on the physical and chemical properties of the environment may be collected and meaningfully interpreted. Sensation begins at the periphery, where a multitude of different sensory cell types are activated by environmental stimuli as different as photons and odorant molecules. Stimulus sensitivity is due to expression of different cell surface sensory receptors, and therefore the receptive field of each sense is defined by the aggregate of expressed receptors in each sensory tissue. Here, we review current understanding on patterns of expression and modes of regulation of sensory receptors.

Co-opting the unfolded protein response to elicit olfactory receptor feedback.

Olfactory receptor (OR) expression requires the transcriptional activation of 1 out of 1,000s 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 role for the UPR in defining neuronal identity and cell fate commitment and support a two-step model for the feedback signal: (1) OR protein, as a stress stimulus, alters the translational landscape of the OSN and induces Adcy3 expression; (2), Adcy3 relieves that stress, restores global translation, and makes OR choice permanent.

Affinity purification of microRNA-133a with the cardiac transcription factor, Hand2.

Predictions of microRNA-mRNA interactions typically rely on bioinformatic algorithms, but these algorithms only suggest the possibility of microRNA binding and may miss important interactions as well as falsely predict others. We developed an affinity purification approach to empirically identify microRNAs associated with the 3'UTR of the mRNA encoding Hand2, a transcription factor essential for cardiac development. In addition to miR-1, a known regulator of Hand2 expression, we determined that the Hand2 3'UTR also associated with miR-133a, a microRNA cotranscribed with miR-1 in cardiac and muscle cells. Using a sequential binding assay, we showed that miR-1 and miR-133a could occupy the Hand2 3'UTR concurrently. miR-133a inhibited Hand2 expression in tissue culture models, and miR-133a double knockout mice had elevated levels of Hand2 mRNA and protein. We conclude that Hand2 is regulated by miR-133a in addition to miR-1. The affinity purification assay should be generally applicable for identifying other microRNA-mRNA interactions.